synthesis and activity of heterogeneous lewis acidic sn catalysts
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Research Collection
Doctoral Thesis
Synthesis and Activity of Heterogeneous Lewis Acidic SnIVCatalysts
Author(s): Conrad, Sabrina
Publication Date: 2015
Permanent Link: https://doi.org/10.3929/ethz-a-010636811
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ETH Library
DISS. ETH NO. 23185
Synthesis and Activity ofHeterogeneous Lewis Acidic SnIV Catalysts
A thesis submitted to attain the degree of
DOCTOR OF SCIENCES of ETH ZURICH
(Dr. sc. ETH Zurich)
presented by
SABRINA CONRAD
Dipl. Chemist, Karlsruhe Institute of Technology, Germanyborn on 24.10.1986citizen of Germany
accepted on the recommendation of
Prof. Dr. C. Copéret, examinerProf. Dr. I. Hermans, co-examinerProf. Dr. C. Müller, co-examiner
2015
Life is what happens while you are busy making other plans.
- John Lennon
Acknowledgements
Firstly and mostly, I would like to thank Prof. Dr. Ive Hermans for giving me the opportunity
to conduct my doctoral thesis in a unique research environment. I would like to thank him in
particular for the fascinating insights into industrially related research, the exceptional facilities
and his impressive strive for excellence that has helped me to push things forward. I am also
very grateful for the opportunity to spend the second half of my Ph.D. at the University of
Wisconsin-Madison in the freezing but homey Midwest of the US. This invaluable experience
definitely broadened my mind in many different directions. Ive, thank you for always providing
me with space to try out new ideas, and at the same time for your scientific impulses; this
together greatly contributed to my personal and professional development throughout this
Ph.D. Thank you for letting me be part of an unmemorable trans-atlantic experience, and for
your warm welcome in West Lawn Avenue.
Secondly, I would like to thank Prof. Dr. Christophe Copéret for kindly taking over the
responsibility of being my ETH supervisor when the Hermans lab moved to the US. I am
also very thankful for the scientific discussions I had with him at ETH and UW and for the
opportunity to complete my thesis in his group at ETH.
Prof. Dr. Christoph Müller is kindly acknowledged for agreeing to act as co-examinator to
my thesis.
Special thanks go to the Hermans lab from ETH (Muppets Part I) for creating a very
positive and enjoyable working environment. I want to thank Ceri for his invaluable advice and
support, Philipp for introducing me into the secrets of grafting, and Camila for her tremendous
help in discussions about my research. I will also always remember the unforgettable moments
we shared outside of the lab, such as the fabulous cakes from Natascia, our very individual
group hit parade ("Ceri Ceri lady..."), and the special kind of Hermans humour that no one
can resist. I also want to thank the Hermans lab from Madison (Muppets Part II) for being
such great labmates and making the work in a new place very soon very enjoyable. I want
to thank Florian for his advice with my Sn project, Phil for his help with IR measurements
iv
whenever needed, and Alyssa for the amazing contributions from her Design Department. In
particular, I want to thank Patrick, my longest Ph.D.-mate and "727" roommate - for always
being a motivating and reliable labmate, for his Franconian way of cheering-up and for making
the move to Madison much easier than I thought. I also want to thank a list of people for their
support with logistical, technical and electronical questions throughout this Ph.D. As such, my
thanks go to René Verel, Max Wohlwend, Andreas Dutly, Erol Dedeoglu, Roland Walker and
Urs Krebs (Zurich), as well as Kat Myhre, Kristi Heming, Jeff Nielsen, Tracey Drier and Matt
Martin (Madison).
Lastly but truly, I want to thank my beloved family and friends. Nina, Janne, Franzi and
Amaia (ETH-Grazien) for turning my time at ETH into much more fun and for always being
there to chat. Hanna, for being my first and most sincerest consultant throughout this Ph.D.
My mom (Jutta), my dad (Reinhard) and Felix for their unconditional support and care, and
for just being there when I needed it most. Michael (Mr Mbughuni), who has stepped into
my life very recently, for genuinely sharing my academic ups-and-downs, for his thoughtful
encouragement, and for showing me day by day what is important.
Table of Contents
Acknowledgements iii
Table of Contents v
Summary vii
Zusammenfassung xi
1 Introduction 1
2 Simple and Scalable Preparation of Highly Active Lewis Acidic Snβ 15
3 Insights into the Baeyer-Villiger Oxidation of Cyclohexanone withH2O2 catalyzed by Snβ 25
4 Silica-Grafted SnIV Catalysts in Hydrogen-Transfer Reactions 39
5 Confinement Effects in Hydrogen-Transfer Reactions on Sn Sitesin Porous Silica Materials 57
6 Conclusions and Outlook 63
Bibliography 69
Appendix A Annexes 87
Appendix B List of Publications 99
Appendix C Presentations 101
Appendix D Cover Gallery 103
Appendix E Curriculum Vitae 109
Summary
The discovery of novel catalytic solutions for Lewis acid catalyzed reactions is and will be an
object in the focus of academical and industrial research efforts. This is primarily stimulated
by the broad array of catalytic applications of Lewis acids, and the need to re-conceptualize
existing homogeneous processes on the way to a more sustainable chemical industry. In
this regard, particularly promising results have lately been obtained by incorporating isolated
metal sites into framework positions of microporous zeolite structures. However, due to the
complexity of zeolitic frameworks, a fundamental understanding of the intrinsic reactivity,
and the subsequent tailored material engineering presents a significant challenge. In addition,
conventional synthesis procedures for such materials (i.e., hydrothermal syntheses) are in many
cases time- and skill-demanding, and may furthermore pose environmental issues, which thus
far restrict their application to laboratory scale.
Taking Sn-doped β-zeolite (Snβ) as a relevant example for the seminal class of Lewis
acid solids with appealing applications, the first part of this thesis (Chapter 2) addresses the
general interest to translate the preparation of this material to a commercial level. This is
approached by the development of a straightforward synthesis strategy, consisting of the post-
synthetic incorporation of Sn atoms into dealuminated, commercially available β-zeolite, which
overcomes several of the practical hurdles given by the conventional hydrothermal synthesis.
The obtained material possesses similar structural characteristics to conventional Snβ, and
demonstrated similar, or even higher activities and selectivities in the Baeyer-Villiger oxidation
of cyclohexanone with hydrogen peroxide and a triose sugar isomerization. This, in combination
with the increased amount of incorporated metal based on this novel procedure, is expected to
significantly facilitate the industrial utilization of this or a similar material.
The chemoselective interaction between the Lewis acidic SnIV-sites of Snβ and carbonyl
groups gives this catalyst unprecedented selectivity in reactions with carbonyl substrates.
At the same time, the chemoselectivity of the SnIV-sites exposes Snβ to potential catalyst
inhibition caused by the competitive adsorption of other Lewis basic molecules that may
viii
be present as solvent or product molecules in the reaction mixture to the active sites. In
Chapter 3 we demonstrate that such inhibition occurs for the Baeyer-Villiger oxidation of
cyclohexanone with hydrogen peroxide by measuring catalytic activities as a function of
lactone and water concentrations in the reaction solution. Aiming toward improving the
activity of Snβ in Baeyer-Villiger oxidations, we then modify our novel post-synthetic synthesis
method and prepare different Snβ catalysts with varying amounts of framework silanols (i.e.,
varying hydrophilicities), while monitoring changes in activity. We find that catalyst activity
goes through a maximum as a function of hydrophilicity, which indicates that a hydrophilic
framework aids the adsorption of the ketone substrate to the active SnIV-sites, and that this
effect is outweighed by competitive adsorption of solvent (water) and product (lactone) at higher
hydrophilicities. Hence, this work clearly illustrates that zeolite hydrophilicity influences the
activity of Snβ in Baeyer-Villiger oxidations with hydrogen peroxide, and that flexible synthesis
methods, such as our post-synthetic metal incorporation, allow optimizing activities through
targeted structural modifications.
In the second part of this thesis (Chapter 4) we study site-isolated silica-grafted SnIV-
sites as a catalytic model-system for SnIV/SiO2-based catalysts. The surface-anchored SnIV-
sites are stepwise functionalized, while implications of these modifications on catalyst structure
and activity are followed by in-depth material characterization and catalytic tests. With this
methodology, we distinguish different material properties that add to the activity of SnIV/SiO2-
based catalysts in the Meerwein-Ponndorf-Verley reduction of cyclohexanone with 2-butanol
(active site speciation, hydrophilicity, confinement effects). We furthermore demonstrate the
feasibility of model-systems as a tool to investigate which features contribute to the performance
of an active Lewis acid catalyst. Not least, the results presented in this chapter, indicate that
the grafting of our amine-containing Sn precursor leads to opening of siloxane bridges on the
surface of thermally pretreated silica; an observation that raises interesting questions for future
studies involving the grafting of various metal precursors and ligands.
The results, which were drawn from our catalytic model-study, indicate confinement effects
as one underlying reason for the difference in activity between benchmark Snβ and silica-grafted
SnIV-sites. With the aim of assessing this contribution in more detail, we extend our study by
ix
grafting SnIV-sites on to the surface of mesoporous MCM-41 (Chapter 5). In line with our
findings presented in Chapter 4, we find a strong dependence of activity on the pore size of
the support. We propose that this trend in reactivity can be attributed to an adsorption-
based confinement, which most likely consists of a suppressed reverse reaction in confined
environments.
Overall, this thesis describes decisive steps on the long path toward an improved
understanding and industrial realization of novel solid Lewis acid catalysts. We demonstrate
that innovative strategies, such as the utilization of model-systems or unconventional synthesis
approaches, can bring outstanding advancements. The results presented in this thesis also
expose the current challenges in the field, in particular, the complexity that zeolite catalysis
still represents, despite the insights gained to date.
Zusammenfassung
Die Entdeckung neuer katalytischer Lösungsansätze fur Lewissäure-katalysierte Reaktionen
ist und wird im Schwerpunkt akademischer und industrieller Forschung stehen. Dies liegt
vorwiegend am breitgefächerten Anwendungsbereich von Lewissäuren und dem dringenden
Bedarf existierende homogene Prozesse durch heterogene Konzepte zu ersetzen um den Weg
zu einer nachhaltigeren chemischen Industrie zu ebnen. In diesem Zusammenhang wurden
mit dem Einbau von isolierten Metallzentren in Gerüstpositionen von mikroporösen Zeolithen
vor kurzem äußerst vielversprechende Ergebnisse erzielt. Allerdings stellt ein fundamentales
Verständnis der intrinsischen Reaktivität dieser Materialien und die darauf folgende gezielte
Entwicklung neuer Materialen durch die Komplexität von zeolithischen Strukturen eine große
Herausforderung dar. Hinzu kommt, dass die konventionellen Syntheseverfahren derartiger
Materialien (d.h., hydrothermale Synthesen) in der Regel ein hohes Maß an synthetischem
Geschick sowie Zeit erfordern, und nicht selten belastend für die Umwelt sind. Dies hat die
Anwendung dieser Materialien bisher auf Synthesen im Labormaßstab beschränkt.
Vor dem Hintergrund eines außerordentlichen Vertreters der zukunftsträchtigen
Materialklasse von heterogenen Lewissäuren – Sn-dotiertem β-Zeolithen (Snβ) – widmet sich
der erste Teil dieser Dissertation (Kapitel 2) dem allgemeinen Interesse die Herstellung dieses
Materials auf kommerzieller Ebene zu ermöglichen. Dies geschieht durch die Entwicklung
einer simplen Synthesestrategie, bestehend aus der post-synthetischen Inkorporierung von
Sn-Atomen in dealuminierten, kommerziell erhältlichen β-Zeolithen, wodurch einige der
praktischen Schwierigkeiten, die mit der konventionellen hydrothermalen Synthese verbunden
sind, umgangen werden können. Das erhaltene Material weist ähnliche strukturelle
Eigenschaften zu konventionellem Snβ auf und demonstrierte ähnliche oder sogar höhere
Aktivitäten und Selektivitäten in der Baeyer-Villiger-Oxidation von Cyclohexanon mit
Wasserstoffperoxid sowie in einer Dreifachzucker-Isomerisierung. Mit Betonung auf der
gesteigerten Menge an inkorporiertem Metal ist zu erwarten, dass unsere neue Synthesestrategie
die industrielle Verwendung dieses oder eines ähnlichen Materials wesentlich erleichtern wird.
xii
Die chemoselektive Wechselwirkung zwischen den Lewissauren SnIV-Zentren in Snβ und
Carbonylgruppen verleiht diesem Katalysator seine beispiellose Selektivität in Reaktionen mit
Carbonylsubstraten. Gleichzeitig wird Snβ durch die Chemoselektivität seiner SnIV-Stellen
möglicher Katalysatorinhibierung ausgesetzt, da andere Lewisbasische Moleküle, die in der
Reaktionsmischung vorhanden sind (in Form von Lösungsmittel- oder Produktmolekülen),
um die Adsorption an den aktiven Stellen konkurrieren können. In Kapitel 3 zeigen
wir, dass solch eine Inhibierung für die Baeyer-Villiger-Oxidation von Cyclohexanon mit
Wasserstoffperoxid zu ε-Caprolakton auftritt, indem wir die katalytische Aktivität von Snβ in
Abhängigkeit von unterschiedlichen Lakton- undWasserkonzentrationen in der Reaktionslösung
aufzeichnen. Mit dem Ziel die Aktivität von Snβ in Baeyer-Villiger-Oxidationen zu verbessern,
modifizieren wir im Weiteren unsere neue post-synthetische Synthesestrategie und präparieren
eine Reihe von Snβ-Katalysatoren mit unterschiedlichen Mengen an Gerüstsilanolgruppen
(d.h., unterschiedlichen Hydrophilitäten). Wir stellen fest, dass die Katalysatoraktivität als
Funktion der Materialhydrophilität durch ein Maximum geht, wodurch angezeigt wird, dass ein
hydrophiles Framework die Adsorption des Ketonsubstrates unterstützt, und dass dieser Effekt
bei höheren Hydrophilitäten durch kompetitive Adsorption von Lösungsmittel (Wasser) und
Produckt (Lakton) überwogen wird. Diese Studie illustriert daher eindeutig, dass Hydrophilität
die Aktivität von Snβ beeinflussen kann, sowie dass flexible Synthesemethoden, wie zum
Beispiel unsere post-synthetische Metallinkorporierung, Aktivitätsoptimierung durch gezielte
strukturelle Abänderungen ermöglicht.
Im zweiten Teil dieser Dissertation (Kapitel 4) untersuchen wir isolierte SnIV-Stellen auf
einer Silikaoberfläche als katalytisches Modellsystem für SnIV/SiO2-basierte Katalysatoren.
Die SnIV-Stellen, die mittels einer Grafting-Technik auf die Silikaoberfläche aufgebracht
werden, werden schrittweise funktionalisiert. Diese Modifikationen und deren Auswirkungen
auf die Struktur des Katalysators werden mittels eingehender Charakterisierung und
katalytischen Tests verfolgt. Hierdurch können wir unterschiedliche Materialeigenschaften,
die zur Aktivität von SnIV/SiO2-basierten Katalysatoren in der Meerwein-Ponndorf-Verley-
Reaktion von Cyclohexanon mit 2-Butanol beitragen, unterscheiden (Struktur der aktiven
Stellen, Hydrophilität, Begrenzungeffekte). Darüber hinaus veranschaulicht unsere Studie die
xiii
Möglichkeit Modellsysteme als Hilfsmittel zu instrumentalisieren, um die unterschiedlichen
Eigenschaften, die zur Aktivität eines Lewissäurekatalysators beitragen, zu identifizieren. Nicht
zuletzt deuten die Ergebnisse diese Kapitels an, dass das Grafting von einem Amingruppen-
tragenden Sn-Ausgangsstoff zum Öffnen von Siloxanbrücken auf der Oberfläche des thermisch
vorbehandelten Silikaträgers führt. Diese Beobachtung wirft interessante Fragen für mögliche
Folgestudien auf, die das Grafting von unterschiedlichen Metalausgangsstoffen sowie -liganden
untersuchen.
Die Resultate, die wir aus unserer katalytischen Modellstudie gewonnen haben,
deuten darauf hin, dass Begrenzungseffekte eine zu Grunde liegende Ursache für den
Aktivitätsunterschied zwischen dem derzeitigen Benchmark-Material Snβ und SnIV-Stellen
auf einer Silikaoberfläche sind. Mit dem Ziel diesen Beitrag genauer bemessen zu
können, haben wir unsere Modellstudie in Kapitel 5 erweitert, indem wir SnIV-Stellen
auf mesoporösem MCM-41-Trägermaterial aufbringen. In Übereinstimmung mit unseren
Erkenntnissen aus Kapitel 4 stellen wir eine starke Abhängigkeit der Aktiviät von der
Porengröße des Silikaträgermaterials fest. Wir schlagen vor, dass dieser Aktivitätstrend auf
einem adsorptionsbasierten Begrenzungseffekt beruht, welcher höchstwahrscheinlich auf eine
unterdrückte Rückreaktion in stärker begrenzten Umgebungen zurükzuführen ist.
Gesamtheitlich beschreibt diese Dissertation entscheidende Entwicklungen auf dem Weg
zu einem verbesserten Verständnis sowie zur industriellen Implementierung von neuen festen
Lewissäurekatalysatoren. Wir demonstrieren, dass innovative Strategien, wie die Verwendung
von Modellsystemen und nicht-konventionelle Syntheseansätze bedeutende Fortschritte erzielen
können. Darüber hinaus legen die Ergebnisse dieser Dissertation bestehende Herausforderungen
des bearbeiteten Forschungsfeldes dar, insbesondere die Komplexität, die Zeolithkatalyse trotz
der bislang gewonnenen Erkenntnisse nach wie vor mit sich bringt.
Chapter 1
Introduction
In this chapter, a short introduction to heterogeneous Lewis acid catalysis is given. First of
all, the on-going shift from homogeneous to more sustainable heterogeneous catalytic systems
is illustrated and several potential industrial applications are presented. In the following, a
short overview of different approaches to synthesize solid Lewis acid catalysts is given. In the
end, the activity of solid Lewis acid catalysts, with the focus set on metal-containing zeolites,
is discussed by assessing the different material features contributing to their reactivity.
1.1. Sustainable Heterogeneous Lewis Acid Catalysis
Lewis acid catalyzed reactions constitute one of the most thoroughly investigated catalytic
systems, in which an organic molecule undergoes diverse chemical transformations with a
nucleophilic reagent in the presence of a Lewis acid catalyst.[1–3] The catalytic activity of Lewis
acids is rooted in the formation of an acid-base adduct between the catalyst and one of the
two reaction substrates in order to enhance their relative reactivity.[4] The process of activation
involves the transfer of electron-density from a substrate into the empty orbitals of the Lewis
acid, making it more prone to a nucleophilic attack.
Within the sector of industrial chemistry, Lewis acids have been found to accelerate an
array of pivotal chemical transformations,[5,6] including (but not limited to) olefin epoxidations
and isomerization reactions (Figure 1.1; A, C and D). Traditionally, homogeneous Lewis
acids, for instance those based on AlCl3 or ZnCl3 have been used for industrial applications
(e.g., for Friedel-Crafts alkylations).[7,8] However, for large-scale processes, their heterogeneous
counterparts are highly desirable both from an environmental and process point of view. Indeed,
the utilization of solid catalysts facilitates catalyst recovery, reuse and continuous production
technologies, and generally leads to reduced quantities of metal waste (i.e., inorganic salts
dissolved in wastewaters). Additionally, the emerging interest in the exploration of biomass
2 Chapter 1
(A) (B)
(D) (C)
(E) (F)
Figure 1.1. Examples of Lewis acid catalyzed reactions (with particular attention to SnIV catalyzedtransformations). (A) Epoxidation of propylene. (B) Baeyer-Villiger oxidation of cyclohexanone withhydrogen peroxide. (C) Isomerization of glucose to fructose. (D) Isomerization of the C-3 sugarsdihydroxyacetone (DHA) or glycerinaldehyde (GLA) to lactic acid in aqueous media. (E) Meerwein-Ponndorf-Verley reduction of cyclohexanone with 2-butanol. (F) Rearrangement of β-pinene oxideinto myrtanal.
as a sustainable carbon source,[9–13] and an environmentally driven shift toward non-toxic and
non-oxidizable solvents[14,15] has stimulated the search for well-performing Lewis acid catalysts,
which are stable in aqueous media.[16,17] The use of conventional homogeneous metal complexes
for such purposes is problematic due to prevailing deactivation caused by catalyst hydrolysis.
The pronounced hydrophobic character that has been found for several solid Lewis acids thus
offers unique opportunities for the design of novel water-tolerant catalytic systems.
Immobilized complexes of multiple main group and transition elements such as Al, Sn, Ti,
V, Cr and Fe, have proven to feature distinctive Lewis acidic properties.[5] Amongst these, Ti
is by far the most studied representative, given its unprecedented activity and selectivity in
oxidation reactions (e.g., epoxidations), which are the tool for the synthesis of huge quantities
of intermediates and monomers for the polymer industry. In this regard, the development of
TS-1 (a TiIV-doped MFI-type zeolite) is, for instance, considered as one of the most significant
Introduction 3
material innovations during the past decades.[18,19] Due to the high specificity of TS-1 toward
hydrogen peroxide, several peroxide-based reactions, such as the hydroxylation of phenol, the
ammoximation of cyclohexanone and cyclododecanone, and the epoxidation of propene, are
now industrial processes.[20] Propene epoxidation is an example of liquid phase oxidations that
have been the focus of studies striving for a more sustainable technology, which was realized
with the commercial implementation of the HPPO process, using aqueous hydrogen peroxide
as the oxidant for propene (largest HPPO plant: Antwerp/Belgium, BASF/Dow, 300 000 t
a-1).[20]
Besides Ti, Sn is a critical component that has recently received increasing attention due
to its widespread applicability.[21] Amongst other Lewis acids, it appears that Sn possesses the
unique capability to activate carbonyl functional groups without activation of the oxidant itself
(such as hydrogen peroxide), thus avoiding undesirable side-reactions and poor selectivities
in the overall reaction. One example is the Baeyer-Villiger oxidation, which converts
ketones (readily available building block compounds) into value-added esters or lactones for
the production of polymers (Figure 1.1 B).[22,23] Traditionally, peracids have been used as
stoichiometric oxidants for this reaction, resulting in one equivalent of acid by-product. The
heterogeneously catalyzed version of the Bayer-Villiger oxidation, utilizing hydrogen peroxide
as oxidant, is hence very appealing due to the simplification of the process conditions and the
elimination of stoichiometric waste.[24,25]
An additional emerging field for Lewis acid Sn catalysts is the conversion of renewable
carbohydrates to chemicals and fuels as alternate pathways to the "classical" chemical industry,
which is currently highly dependent on crude oil. Attractive routes are the isomerization
of glucose to fructose (Figure 1.1 C) for the production of high-fructose corn-syrups (HFCS,
8 x 106 tons/yr),[26–29] as well as the isomerization of triose sugars (dihydroxyacetone DHA,
glyceraldehyde GLA) in order to synthesize lactate-derivatives, such as lactic acid (Figure
1.1 D),[30–33] which are employed for the production of biodegradable polymers and solvents.
Moreover, furan derivatives such as 5-hydroxymethylfurfural (HMF) and furfural (FUR) can
be obtained from the selective dehydration of monosaccharides, which have been stated to be
relevant compounds for fuel, polymer and pharmaceutical industries.[34–36] As an example, the
4 Chapter 1
selective oxidation of HMF leads to 2,5-furandicarboxylic acid (FDCA), which is a potential
replacement for terephthalic acid that is widely used in synthetic polyesters such as those found
in soft drink bottles.[37,38]
Other applications are Meerwein-Ponndorf-Verley-Oppenauer redox reactions (Figure
1.1 E), which convert aldehydes and ketones to their corresponding alcohols,[39,40] various
rearrangements, such as the transformation of β-pinene oxide into myrtanal, which is used
as antiseptic (Figure 1.1 F),[41–43] and Carbon-Carbon Coupling Reactions.[44,45]
1.2. Synthesis of Solid Lewis Acid Catalysts
In general, there are two main approaches to synthesize solid Lewis acid catalysts as depicted
schematically in Figure 1.2: (A) The direct incorporation of the active species during the
synthesis of the material, e.g., via sol-gel chemistry, or (B) the immobilization of the active
species at the surface of pre-synthesized materials, e.g., via grafting.
The foremost process (Figure 1.2 A), also known as isomorphous substitution, is exemplified
by Lewis acid doped zeolites, in which a heteroatom substitutes the Si or Al in the framework of
a microporous crystalline solid (i.e., a zeolite). A pioneer in this field is the aforementioned TS-
1 (Ti-silicate-1), a siliceous material containing low amounts of Ti (< 2 wt%), isomorphously
substituted into the framework of MFI-zeolite. Stimulated by the successes achieved with TS-1,
interesting sites for catalytic applications were evolved when other metal atoms, such as Sn, were
embedded into framework positions of the large-pore β-zeolite (pore diameter ca. 0.7 nm).[46,47]
Firstly synthesized in 1998, this material soon turned out to be a chemoselective catalyst with
exceptional activities in carbonyl-based transformations (Figure 1.1 B-F).[26,30,39,48]
The preparation of metal-doped zeolites such as TS-1 or Snβ resembles the synthesis of
zeolites with the difference that an appropriate amount of metal precursor is added to the
synthesis mixture.[49,50] Usually, the syntheses are carried out in acidic or basic media from gels
containing a silica source, a metal precursor, a suitable template (e.g., tetraethylammonium
hydroxide, tetrapropylammonium hydroxide), fluoric acid and water. The materials are
subsequently crystallized at high-temperature (> 150 °C) and autogenic pressure over a period
of 24 – 240 hours. Depending on the utilized template and the exact synthesis conditions,
Introduction 5
SiO2
M+X$
(A) direct incorporation e.g., via sol-gel chemistry
(B) immobilization on pre-synthesized supports e.g., via grafting on to silica
M$ M$ M$
M$
silicon'source'
M+X$
addi-ves'
Figure 1.2. Schematic outline of different synthesis approaches to synthesize solid Lewis Acidcatalysts.
a broad spectrum of materials with pore diameters from the micro- to the mesopore range
(micropores: < 2 nm, mesopores: 2 – 50 nm) with metal sites incorporated in the framework
may be prepared with this methodology.
Despite the promising developments in the preparation of solid Lewis acids through
isomorphous substitution, not a few of the obtained materials suffer from a number of
drawbacks, which complicate their industrial realization on a commercial level and necessitates
the exploration of alternative synthetic procedures. Many syntheses, for example, still rely on
the traditional fluoride method, which has the advantage of generally leading to higher product
yields and increasing the hydrophobicity of the obtained zeolite.[51] However, this route impedes
large-scale production due to the corrosive reaction conditions and the enormous environmental
impact of fluoride systems. Moreover, the incorporation of large Lewis acid centers, such as
SnIV, typically entails retardation of the zeolite nucleation, resulting in large crystals (approx.
0.5 – 2 µm), and therefore poor molecular diffusion throughout the zeolitic channels. Not
least, only restricted quantities of metal may commonly be incorporated into the framework
structures (< 2 wt%) via this route.
Owing to these difficulties and aiming at extending the usage of metal-doped zeolites
to reactions with bulkier molecules, the active metals were thereafter incorporated into the
framework of zeolites with larger porosity and into mesoporous structures. In this regard, Ti-
MWW exemplifies a successful attempt to overcome the pore restrictions of the medium-sized
6 Chapter 1
pores of TS-1 (approx. 0.55 nm).[52,53] The unique topology of the MWW-zeolite, with its 12
membered-ring "supercages" and pockets on the exterior has indeed proved to be more useful
for oxidations of larger molecules compared to TS-1.[52] Similar efforts have been undertaken
with SnIV as Lewis acid, leading to novel materials such as the mesoporous stannosilicates Sn-
MCM-41,[54–57] Sn-MFI[28,58] and Sn-SBA-15[59]. Despite their advantageous (more spacious)
pore architecture these catalysts can barely match the activities and selectivities of Snβ, which
cannot simply be explained by differences in diffusion properties but rather points to additional
changes in the material when shifting to a different type of framework structure (e.g., a different
local environment of the active sites or the amorphous structure of the silica walls).[59,60]
The immobilization of the active species at the surface of pre-synthesized materials (Figure
1.2 B) can be carried out by impregnation and grafting. To the best of our knowledge,
impregnation techniques are not thoroughly explored for the synthesis of solid Lewis acid
catalysts. One reason for this fairly poor amount of impregnation studies is the fact that
these techniques may lead to oligomeric species if a slight excess of precursor is added during
the impregnation step. Site-isolation (viz., well isolated species) is, however, a crucial trait to
obtain a highly active solid Lewis acid, since it provides that the individual metal centers on the
catalyst surface do not interact with each other.[61,62] There is one fairly well studied catalytic
material prepared by a wet impregnation technique. In this work, a commercial silica support
was co-impregnated with aqueous solutions of both a Pt and a Sn chlorine salt solution, to give
a Pt-Sn/SiO2 catalyst which was subsequently tested for the isomerization of β-pinene oxide
to naturanol (Figure 1.1 F).[41,42]
Grafting methods, on the other hand, are significantly better understood, and have
been investigated on several support materials (oxides, porous materials, clays).[63–69] In this
approach, metal precursors react with the surface functional groups of pre-synthesized supports,
which are in most of the cases SiO2- or Al2O3-based materials. In this regard, TiCl4-
grafted silica is an example for an industrially implemented catalyst prepared via grafting,
which is used for the epoxidation of propylene with ethylbenzene hydroperoxide (styrene
monomer/propylene oxide process, SMPO).[20] More grafting materials are currently researched
and can be found in the literature. As such, Corma and co-workers grafted different Sn-alkyl
Introduction 7
Figure 1.3. Atomplanting method as demonstrated by Wu and co-workers.[70] Sn atoms are post-synthetically incorporated into nanosized Snβ zeolites by exposing them with SnCl4 vapors at elevatedtemperatures.
precursors (RnSnIVX4-n) on to mesoporous MCM-41 and demonstrated that the precursors
with one or two alkyl substituents (e.g., BuSnCl3) reacted most efficiently with the silanol
groups of the support while avoiding the formation of inactive polymeric tin-oxo species.[64] A
comparison of the catalytic activity with SnIV-sites incorporated during the synthesis of MCM-
41 showed, however, that the catalysts prepared by direct synthesis gave some higher activity.
Another straightforward post-synthetic synthesis approach based on grafting was demonstrated
by Wu and co-workers, who treated highly dealuminated β-zeolite with SnCl4 vapor at elevated
temperatures (773 K) (Figure 1.3).[70] In this case, the Sn sites were introduced into the
framework via the reaction between silanols of the hydroxyl nests created during dealumination
and the SnCl4 molecules. The level of metal incorporation achieved with this synthesis (6.2 wt%
Sn) clearly exceeded the typical metal loading of Snβ synthesized via isomorphous substitution
(< 2 wt% Sn), indicating promising opportunities for industrial applications. Stimulated by this
work, many similar studies were undertaken, aiming toward the optimization of post-synthetic
methodologies for preparing heterogeneous Sn catalysts (see also Chapter 2).[71–75]
In studies involving the preparation of solid Lewis acids via grafting, the support material
is often carefully pretreated prior to contact with the metal precursor, which sets up unique
pathways that allow to control the exact structure of the immobilized metal sites and thus to
conduct rational structure-property studies. In this regard, silica presents an extensively studied
support for the preparation of isolated well-defined surface metal complexes by reaction with an
appropriate (organo-)metallic precursor.[76–85] In such a synthesis, a thermal treatment (under
vacuum) may initially be applied to reduce the amount of vicinal (hydrogen-bonded) silanol
8 Chapter 1
groups on the surface (Figure 1.4 A2) by condensation and elimination of water (Figure 1.4
B). At high temperatures (> 500 °C), this dehydration process results in low silanol densities
on the silica surface and an average distance between individual silanols that is sufficiently
enough to exclude interactions.[86,87] Upon reaction with a metal precursor ML4 that holds at
least on hydrolysable ligand isolated ≡Si-O-ML3 sites can then easily be introduced to the
surface (Figure 1.4 C).[88–96] Even though this strategy is often far from commercialization
due to the complex synthesis conditions and the costly metal precursors, it may lead to an
enhanced fundamental understanding given by the possibility to form uniform active sites,
which makes the establishment of structure-activity relations possible. This is exemplified by
several catalytic systems (e.g., alkane metathesis) that have been discovered and/or studied
using the conception and the techniques of surface organometallic chemistry.[97–100]
1.3. Activity of Solid Lewis Acid Catalysts
Having the superior performance of metal-doped zeolites in mind, it is not entirely surprising
that this class of materials has lately been the object of extensive computational and
experimental studies, striving for a better understanding of their catalytic behaviour. As
outcome from these works, researchers propose that the observed catalytic performance is
rooted in the combination of numerous material properties: (i) the exact local structure of the
active metal sites (i.e., site speciation),[21,88,89,101–105] (ii) the exact crystallographic site in the
lattice where the metal substitution with Si or Al during the synthesis has occurred,[106,107]
(iii) physical properties of the material, including material hydrophilicity,[16,17,108,109] pore size
distributions and crystallinity, (iv) confinement effects induced by the pores of the zeolite.[110–114]
Amongst these material properties, the identification of the nature of the active sites in solid
Lewis acids has recently been given highest priority. Nevertheless, the accurate determination
remains in many cases a topic of much debate. Indeed, two different types of active sites
have been proposed for Snβ.[101,111] Earlier data from 119Sn MAS NMR,[115] XPS,[47] TEM[47,106]
and EXAFS[47,106] suggested the presence of fully-encapsulated tetrahedral framework SnIV-
sites (Sn(–O–Si)4). More recent investigations, involving IR spectroscopy of adsorbed
acetonitrile,[101,111] pointed toward the existence of a second type of site, namely partially
Introduction 9
M"
M"M"M"
(A)
(B)
(C)
1
2
3
Figure 1.4. Schematic illustration of the immobilization of isolated metal sites on thermallypretreated silica. (A) Untreated silica surface consisting of isolated (1), vicinal (hydrogen-bonded)(2) and geminal (3) silanols. (B) Silica after thermal treatment at 700 °C with remaining isolated andgeminal silanols, as well as siloxane bridges. (C) Silica surface after reaction of (B) with an appropriatemetal precursor that has reacted with isolated and geminal silanols.
hydrolyzed Sn ((–Si–O)3Sn–OH), which was also reported in several NMR studies.[116–121]
Adsorption studies indirectly probe the active sites of a solid Lewis acid by studying shifts
in vibrational bands of adsorbed probe molecules, and correlate the extent in shift with the
Lewis acidity of the metal sites. In addition, comparison of experimentally observed shifts and
calculated ones are employed to draw conclusions about the active site structure.[122] In some
recent studies, individual bands are even used to quantify the amount of individual sites.[101]
However, not only the nature of the active site but also multiple adsorbed probe molecules might
cause broad IR shifts.[123] Moreover, these studies exclusively adsorb one probe compound and
neglect superimposed effects from additional molecules (solvent, product or other substrate
molecules) that may be present in the reaction mixture (i.e., under realistic conditions). Thus,
10 Chapter 1
there is a necessity to critically assess such studies by thorough consideration of potential effects
from co-adsorbed molecules, and to careful interpret the obtained experimental data.
Besides active site speciation, confinement effects have been demonstrated to impact
zeolite-catalyzed reactions.[110–114] These effects, which were captured as shape selectivity in
the early history of zeolite catalysis, are traced back to Van der Waals interactions between
reactants, products or intermediates and the walls of the zeolite cavities. This may result
in the stabilization of confined intermediates or transition states and significantly alter the
dynamics of elementary steps within a catalytic cycle. In addition, confinement effects may
influence the adsorption behavior of molecules in zeolites.[124,125] Indeed, it has been found that
the free enthalpies of adsorption of simple hydrocarbons in zeolites raise with an increase in
alkyl chain length, which was ascribed to an increase in the Van der Waals interactions of the
alkanes with every additional -CH2- group.[126] These findings have significant consequences for
the understanding of hydrocarbon-based zeolite-catalyzed reactions and ought to be carefully
considered.
Lastly, a key determinant of metal-doped zeolites is the encapsulation of the Lewis acid sites
inside a water-resistant framework (as found for highly siliceous zeolites), which prevents the
hydrolysis and subsequent deactivation of the Lewis acid, and thereby allows these promising
materials to be utilized for aqueous phase reactions.[16,17,109,127] This feature might make Lewis
acid doped zeolites become the material of choice for biomass conversions in water and biphasic
water-organic mixtures. Moreover, hydrophobic surroundings can weaken the binding of water
(or other hydrophilic solvent molecules such as alcohols) to the actives sites, which aids
in attenuating catalyst inhibition due to competitive adsorption.[17] Therefore, hydrophobic
environments as found in solid Lewis acid zeolites impart the material with exceptional stability
that generally leads to enhanced activities for such hydrophobic materials.
Introduction 11
1.4. Scope of the Thesis
The goal of this thesis is, on one hand, to enhance the large-scale applicability of Lewis acid
heterogeneous catalysts with tremendous catalytic potential. This necessitates the design of
a novel synthetic approach and targeted customization of the synthesis procedure depending
on the chosen application. In particular, this includes detailed material characterization and
catalytic testing in order to assure that the newly developed materials meet the desired material
properties and activities. Attention will be placed on industrially relevant test reactions and
commercially available materials. On the other hand, the goal of this thesis is to enhance the
understanding of activity of current benchmark Lewis acid heterogeneous catalysts, and hereby
aid the rational design of similar catalytic systems. With this in mind, a catalytic model-
system will be created, which enables the identification of explicit material properties that
influence activity by gradual modifications to the material. We expect that improved synthesis
and rationalization of activity will enable the optimized development of catalytic systems and
promote the introduction of sustainable industrial processes based on Lewis acid heterogeneous
catalysts.
1.5. Outline of the Thesis
The results of this thesis are presented in four chapters (Chapter 2 – 5) followed by the
conclusions and outlook of the challenges ahead (Chapter 6). The research presented in this
thesis has been carried out in the frame of an ETH Grant (ETH-38 12-1) titled "Synthesis and
Characterization of Sn-containing materials for sustainable Baeyer-Villiger oxidations".
Currently, Lewis acid doped zeolites are typically prepared in complicated hydrothermal
syntheses procedures, which have several hurdles that complicate or prevent their industrial
implementation. In Chapter 2 we address these limitations for the novel and state-of-the-art
heterogeneous Sn-catalyst Snβ, which to date has been restricted to the laboratory scale and is
prepared in gram quantities under precisely controlled conditions. Aiming toward the industrial
utilization of this or a similar material, we design a straightforward synthesis strategy, applying
a convenient post-synthetic route that consists of the incorporation of the metal into a pre-
synthesized zeolite. Extrapolation of the properties of the prepared material is undertaken by
12 Chapter 1
spectroscopic and catalytic comparison with hydrothermally synthesized Snβ.
The chemoselective interaction between Lewis acidic SnIV-sites and carbonyl groups gives
Snβ its unrivalled selectivity in a wide range of industrially relevant reactions, such as the
Baeyer-Villiger oxidation of cyclohexanone with hydrogen peroxide. At the same time, the
chemoselectivity of the SnIV-sites exposes Snβ to activity-restricting interactions with further
Lewis basic molecules that are present in the reaction mixture, such as the product lactone
and water in Baeyer-Villiger oxidations, as we show in Chapter 3. Based on the synthesis
knowledge gained in Chapter 2, we explore the opportunity to reduce catalyst inhibition
through co-adsorbed molecules to the active sites by modifying the surface properties of post-
synthetically prepared Snβ. This is attained by varying the amount of framework silanols
through modification of the synthesis protocol of the parent zeolite, subsequent quantifying of
the obtained surface hydrophilicities and monitoring of activity changes in the Baeyer-Villiger
oxidation of cyclohexanone with hydrogen peroxide. Furthermore, initial catalytic tests are
carried out in order to assure that all following investigations are undertaken in the kinetic
regime of the reaction.
To better understand and optimize the reactivity of current benchmark Lewis acid doped
zeolites, advanced characterization tools are used to build a detailed understanding of the
structural organization of the catalyst, and probe reactions are employed to link the obtained
structural information with the reactivity of the materials. However, the structural complexity
of zeolites complicates unequivocal distinction between the diverse material characteristics
that add to the performance of the catalyst and hence to rationally improve the catalyst.
Thus, in Chapter 4, the potential of a catalytic model-system is assessed in order to provide
complementary information to studies based on the direct investigation of the zeolitic materials.
This is approached by the preparation of silica-supported SnIV-sites, deploying chemical vapor
deposition as immobilization technique and step-wise thermal and chemical functionalizations.
A set of characterization techniques and catalytic tests are brought in to follow structural
modifications and alterations in activity, respectively. This in combination with the final
comparison to benchmark Snβ allows conclusions about the different contributions to reactivity
in SnIV/SiO2-based catalysts.
Introduction 13
In Chapter 4 we learn that confinement effects influence the activity of SnIV/SiO2-
based catalysts in the Meerwein-Ponndorf-Verley reaction of cyclohexanone with 2-butanol
significantly. Based on these findings, the next step encompasses targeted experimental
approaches in order to confirm the existence of this effect. Hence, in Chapter 5 we extend our
previous model-studies to SnIV-sites supported on mesoporous silica (MCM-41), representing a
pore size range that stands in between the micropores of β-zeolite and the external surface of
amorphous silica, which we used in Chapter 4.
Chapter 6 summarizes the key results of the research performed throughout this thesis
and identifies challenges ahead.
Chapter 2 – 4 of this thesis were written based on one publication and can be read
independently. Accordingly, some overlap between the chapter introductions occurs.
Chapter 2
Simple and Scalable Preparation of HighlyActive Lewis Acidic Snβ
In this chapter FT-IR and UV-Vis measurements, as well as the triose isomerization reactions
were performed by the author. Material synthesis, Raman measurements and the Baeyer-
Villiger reactions were performed by Ceri Hammond.
2.1. Introduction
Lewis acids are a versatile class of catalysts that exhibit remarkable activity for a number of
essential transformations, including oxidations and isomerizations.[16] Although homogeneous
analogues are well-established, heterogeneous catalysts offer several advantages for the
development of more sustainable technologies in terms of facile downstream processing and
process intensification. Of particular interest are Lewis acid-doped zeolites, some of which
exhibit remarkable activity, selectivity and lifetime for a number of processes.[39,52,128] The
development of TS-1 (a TiIV-doped MFI-type zeolite) is for instance viewed as one of the
greatest breakthroughs in sustainable chemistry over the recent decades, having resulted in a
"greener" process for the epoxidation of propylene,[20] amongst others. Promising results have
also been obtained in the development of SnIV-doped zeolite β, which has shown unparalleled
activity and selectivity for the isomerization of glucose to fructose and the Baeyer-Villiger
oxidation of ketones to lactones using H2O2 as green oxidant.[20,27,48,115]
Lewis acid-doped zeolites are typically obtained by direct framework incorporation during
hydrothermal synthesis.[20,27,39,48,52,115,128] It remains, however, a challenge to obtain a significant
amount of isolated sites within the structures without the undesirable formation of metal
oxide particles which are significantly less active. Moreover, even under optimized conditions,
the incorporation of large Lewis acidic centres, such as SnIV, typically leads to a significant
retardation of the zeolite nucleation, and hence long synthesis timescales (up to 40 days). This
16 Chapter 2
results in unfavourable large crystals. In order to facilitate the crystallisation of such materials,
additives such as HF are commonly added to the synthesis gel, posing additional practical
and environmental limitations. This, in combination with the limited amount of active metal
that can be incorporated into the structures (viz., < 2 wt%) currently limits the large scale
applicability of these otherwise promising materials.
With these limitations in mind, we aimed to develop a convenient post-synthetic route for
the incorporation of various Lewis acid centres into zeolitic frameworks. An attractive route
involves the incorporation of the desired transition metal ions into the vacant tetrahedral (T)-
sites of a pre-dealuminated zeolite. Not only does this avoid the long synthesis times associated
with the conventional hydrothermal synthesis routes, but it also allows for the synthesis of a
material with significantly smaller crystallite sizes than possible through direct synthesis. As
a proof of concept, this communication focusses on the preparation of Snβ. This material
has tremendous potential,[27,48,115] equivalent to TS-1, but the industrial implementation is
hampered by the tedious synthesis procedure.
2.2. Experimental
2.2.1. Material Synthesis
Commercial Zeolite H-β (ZeoChem) was dealuminated by treatment in HNO3 solution (13 M)
at 100 °C for 20 h (20 mL g−1zeolite). SSIE was performed by grinding the appropriate amount
of Sn(II)acetate with the required amount of dealuminated zeolite for 15 min. Samples were
calcined in an air flow at 550 °C.
Conventional Snβ was synthesized according to the original procedure of Corma et al.[48]
TEOS was added to a TEAOH solution under stirring. After a single phase was obtained, the
desired amount of Sn (SnCl4 · 5 H2O, 98 %, STREM) dissolved in H2O was added drop-wise.
The solution was stirred open to evaporate ethanol and water until a viscous gel was obtained.
Addition of hydrofluoric acid resulted in a solid gel with the molar composition 1.0 SiO2 : 0.01
SnCl4 : 0.55 TEAOH : 0.55 HF : 7.5 H2O. Lastly, a solution of dealuminated Beta seeds in water
was added and the mixture homogenized with a Teflon spatula. Crystallization was carried out
at 140 °C in 45 mL scale teflon-lined stainless-steel autoclaves, tumbled at 60 rpm for 14 days.
Simple and Scalable Preparation of Highly Active Lewis Acidic Snβ 17
After cooling down, the samples were filtered and subsequently washed with deionized water
and acetone before drying them in an oven at 110 °C over night. To remove the structure
directing agent, samples were calcined at 580 °C under a steady air flow for 6h.
2.2.2. Characterization Methods
The Sn-content was quantified with ICP-OES (Ultima 2 from Horiba Jobin Yvon) after digestion
of the samples with HF.
FT-IR spectroscopy was performed on a self-supporting wafer using a Bruker Alpha
spectrometer in transmission mode (resolution of 2 cm-1). Intensities were normalized to the
Si-O-Si overtones of the silica framework. Diffuse Reflectance UV-Vis spectra were recorded
with a Maya 200 spectrometer (Ocean Optics) equipped with a UV-Vis deuterium/halogen
light source (DH-2000-BAL from Mikropack) using BaSO4 as background. Both FT-IR and
UV-Vis analysis was carried out inside a glove box (< 1 ppm H2O and O2).
Porosimetry measurements were performed on a Micromimetics 2000 apparatus. The
samples were degassed prior to use (275 °C, 3 h). Adsorption isotherms were obtained at
77 K and analyzed using BET and t-plot methods.
2.2.3. Catalytic Experiments
Baeyer-Villiger oxidation of cyclohexanone was carried out in a 50 mL round bottomed flask
equipped with a reflux condenser. The vessel was charged with the reactant solution (5 mL,
0.33 M cyclohexanone in 1,4-dioxane) and the desired amount of catalyst (corresponding to
1 mol% Sn relative to ketone). The vessel was heated to the reaction temperature (90 °C)
for 15 min, prior to the addition of H2O2 at a final concentration of 0.5 M (H2O2/ketone =
1.5) and stirred vigorously for the required reaction period. Samples were taken periodically
and quantified by GC-FID against a biphenyl internal standard (30 m FFAP column). H2O2
concentrations were determined by titration against Ce4+. The conversion of dihydroxyacetone
(DHA) to ethyl lactate was performed in an autoclave reactor at 10 bar N2 pressure. The vessel
was charged with 25 mL of DHA solution (0.4 M in EtOH plus 0.3 mol% Sn, relative to DHA),
and the reaction performed at 100 °C. The reactant and product were quantified against a
biphenyl internal standard by means of GC-FID (30 m FFAP column) and HPLC (nucleodur
18 Chapter 2
100-5 NH2-RP column).
2.3. Incorporation of SnIV into Dealuminated β Zeolite
Preliminary work focussed on the efficient dealumination of a parent Al-β zeolite. Although
steaming is a well-known method for removing framework Al3+, it has the disadvantage
of leaving behind ill-defined extra-framework Lewis acidic Al3+ species, which could affect
the catalytic performance of the material. In view of this, an acidic pre-treatment with
HNO3 (13 M, 100 °C, 20 h, 20 mL g-1) was performed in order to extract and remove Al
quantitatively.[129] The results in Table 2.1 show indeed that nearly all Al can be removed
without destruction of the BEA framework, or significant alterations to its textural properties.
The removal of framework Al is exemplified by the loss of the Brønsted acidity, i.e., the sharp IR
signal at 3610 cm-1 (Figure 2.2 A). In its place, a broad absorbance around 3500 cm-1 appears,
confirming the successful formation of silanol nests and vacant T-sites for the incorporation of
SnIV (Figure 2.1 and 2.2 B).[130]
Subsequently, methods of introducing SnIV into the vacant T-sites were considered.
Although liquid-phase routes (e.g., impregnation) offer a convenient and scalable route for
the deposition of Sn, the solvation shell surrounding the Sn cations could potentially lead
to significant diffusion limitations within the zeolite micropores, negatively affecting the
incorporation of tin and resulting in poor dispersion. Furthermore, hydrolysis of the Sn-
precursor may also result in the partial formation of bulk oxide species in the final material.
Although traditional gas-solid deposition (e.g., chemical vapour deposition) offers exciting
possibilities of incorporating ‘naked’ metals into the T-sites, the low volatility of typical Sn-
Figure 2.1. Post-synthetic synthesis route to solid Lewis acidic Snβ zeolite. In a first stepcommercially available β-zeolite is dealuminated in an acidic treatment, in a second step SnIV isincorporated into the vacant silanols nests created during the dealumination by solid-state ion-exchange (SSIE).
Simple and Scalable Preparation of Highly Active Lewis Acidic Snβ 19
Table 2.1. Physicochemical properties of the materials.
Entry Catalyst Treatment SBETa Vmicro
b SiO2/Al2O3c SiO2/SnO2
c
[m2 g−1] [cm3 g−1]
1 H-βd – 600 0.17 25 –2 deAl-βe H+ 620 0.18 > 1900 –3 Sn/deAl-βf H+/SSIE 610 0.17 > 1900 324 Sn/deAlβf H+/SSIE 600 0.17 > 1900 16a Brunauer-Emmett-Teller surface area.[131] b Micropore volume, t-plot method.[132]
c Molar ratio in solid, determined by AAS. d Commcercial H-β zeolite. e Dealuminatedβ zeolite. f Snβ zeolite.
precursors (e.g., SnCl4) results in difficulties grafting sufficient quantities of Sn into the final
material. Moreover, previous work has demonstrated that a multitude of Sn-species are formed
by SnCl4 grafting, the majority of which appear to be extra-framework species or bulk tin
oxides.[70] The catalytic activity of such materials is therefore rather poor.
Solid-state ion-exchange (SSIE) was therefore considered to be an interesting route for
incorporating the desired amount of metal into the structure, with both high dispersion and
homogeneity. The procedure simply involves mechanical grinding of dealuminated zeolite and
the appropriate precursor, in this case Sn(II)acetate, prior to calcination at 550 °C for removal
of the residual organic species.
Figure 2.2. IR spectra of (A) H-β, (B) dealuminated β (deAlβ), (C) 5 wt% Sn/deAlβ, and (D)10 wt% Sn/deAlβ (see Table 2.1).
20 Chapter 2
wavenumbers / cm-1
(1)
(2)
(3)
(4)
SnO2
Figure 2.3. UV-Vis spectra of (A) conventional Snβ (1.5 wt%), (B) 10 wt% Sn/deAlβ, (C) 10 wt%Sn/(O2)/deAlβ. The inset shows Raman spectra of (1) deAlβ, (2) 5 wt.% Sn/deAlβ, (3) 10 wt%Sn/deAlβ, (4) bulk SnO2.
2.4. Characterization of the Prepared Materials
A combination of spectroscopic techniques was used to determine the nature of the post-
synthetically prepared Sn-species. The incorporation of Sn into the dealuminated framework
can be observed through closure of the silanol nests by IR spectroscopy (Figure 2.2, plot C
and D). Diffuse reflectance spectroscopy also reveals a sharp UV-absorbance around 216 nm,
even for very high loadings (Figure 2.3 B); this signal is characteristic of isolated, tetrahedral
SnIV-species within the zeolite framework. No SnO2 could be detected with UV-Vis or Raman
spectroscopy (Figure 2.3). It should be noted that the UV-Vis spectra of the post-synthetically
prepared samples are almost identical to that of a reference Snβ sample prepared by direct
hydrothermal synthesis (Figure 2.3 A). This spectroscopic data indicates that very similar
SnIV-sites are obtained even at ca. eight-times the total loading.
2.5. Catalytic Performance of Post-Synthetically Prepared Snβ
Zeolite
The catalytic efficiency of the prepared materials was evaluated for the Baeyer-Villiger oxidation
of cyclohexanone with H2O2 (Figure 1.1 B and Table 2.2). As can be seen, SSIE of SnIV into
dealuminated β (Table 2.2, Entries 1 and 2) results in the formation of an active catalyst,
comparable in both yield and TON to that previously reported for hydrothermally-synthesized
Simple and Scalable Preparation of Highly Active Lewis Acidic Snβ 21
Table 2.2. Catalytic Activity of various Sn-containingsamples for the Baeyer-Villiger oxidation of cyclohexanonewith aqueous H2O2.a
Entry Catalyst Conversion Selectivity Yield[%] [%] [%]
1 5 wt% Sn/deAlβ 37 75 262 10 wt% Sn/deAlβ 42 93 383 10 wt% Sn(O2)/deAlβ 0 – 04 10 wt% Sn/β 0 – 0a Experimental conditions: cyclohexanone in 1,4-dioxane (0.33 M),H2O2 (30 wt% solution, H2O2/ketone = 1.5), Sn content (relativeto ketone) = 1 mol%, 90 °C, 4 h.
materials.[48] Interestingly, the 5 and 10 wt% Sn/deAlβ show comparable activity per SnIV-
site, but lead to a different lactone selectivity. We attribute this to the presence of un-closed
silanol nests in the 5 wt% sample, resulting in the presence of residual Brønsted acid sites.
Such sites are capable of catalyzing the formation of 6-hydroxycaproic acid via acid-catalyzed
hydrolysis, as confirmed by lactone stability studies. At higher Sn loadings, a larger fraction
of the silanol nests are closed, thus decreasing the number of available Brønsted acid sites,
resulting in increased selectivity.
To substantiate the hypothesis that isolated, tetrahedral SnIV-species are indeed the active
sites formed in the post-synthetic material, we verified that dispersion of SnO2 into/onto the
dealuminated framework (Table 2.2, Entry 3) leads to zero activity. Similarly, the dispersion of
Sn(II)acetate into/onto a non-dealuminated framework (Table 2.2, Entry 4) leads to an inactive
material; clearly, when the formation of framework SnIV-sites is prohibited (either due to an
inappropriate precursor or the lack of vacant framework sites), an inactive catalyst is obtained.
In order to confirm the heterogeneous nature of the catalytic reaction, a hot-filtration test
was performed,[133] demonstrating that removal of the catalyst after 1 hour (i.e., after ca. 15 %
conversion) leads to complete termination of the reaction (Figure 2.4 B).
Comparing the SSIE synthesized material with the hydrothermally-made material, it is
clear that along with proceeding at comparable levels of lactone and H2O2-based selectivity
(Table 2.3), the Sn/deAlβ is significantly more productive, given the ca. three-fold increase
in space-time-yield. This is predominantly due to the incorporation of such high quantities
of SnIV, without causing significant unwanted side effects such as agglomeration, increased
22 Chapter 2
Figure 2.4. (A) Time-dependent formation of ε-caprolactone in the presence of 10 wt% Sn/deAlβ,and (B) hot-filtration test for the catalyst (conditions, see Table 2.2).
crystallite sizes and leaching. This improved reaction efficiency, in combination with the facile
and scalable synthesis procedure is expected to catalyze the industrial applicability of Snβ.
To explore the general applicability of the post-synthetic material, we studied the conversion
of the triose sugar dihydroxyacetone to ethyl lactate (Figure 2.5), a convenient building
block towards biorenewable and biodegradable solvents and polymers.[30] Recently, it has been
reported that SnIV-containing compounds are amongst the most active and selective materials
for these reactions,[59] and that a combination of Lewis acidity and mild Brønsted acidity is
advantageous for this reaction.[32] As can be observed from the results in Table 2.4, along with
proceeding at exceptional levels of selectivity, the 10 wt% Sn/deAlβ catalyst is significantly
more productive than the benchmark materials in terms of turnover numbers and space-time-
yield.
2.6. Conclusions
In conclusion, a convenient route for the preparation of Lewis acidic Snβ has been developed.
In addition to exhibiting comparable or higher levels of catalytic activity and selectivity to the
state-of-the-art materials, significantly higher space-time-yields can be obtained through the
Figure 2.5. Additionally applied probe reaction to test the catalytic activities of the post-synthetically prepared Snβ catalysts: Conversion of dihydroxyacetone to ethyl lactate.
Simple and Scalable Preparation of Highly Active Lewis Acidic Snβ 23
Table 2.3. Comparison of post-synthetic and hydrothermal routes.a
Entry Catalyst Selectivity(lactone)b Selectivity(H2O2)c STYd Ref.[%] [%] [g kg-1 h-1]
1 1.5 wt% Snβ > 98 > 95 374 [48]2 10 wt% Sn/deAlβ 93 > 95 1075 this worka Comparison under identical experimental conditions as previously described: cyclohexanone in1,4-dioxane (0.33 M), H2O2 (30 wt% solution, H2O2/ketone = 1.5), Sn content (relative to ketone)= 1 mol%, 90 °C, 4 h. b Calculated as mole ε-caprolactone formed per mole cyclohexanoneconverted. c Calculated as mole ε-caprolactone formed per mole H2O2 converted. d Calculatedas glactone kg−1
zeolite h-1.
Table 2.4. Conversion of dihydroxyacetone to ethyl lactate.a
Entry Catalyst Selectivity(lactate)b TONc STYd Ref.[%] [–] [g kg-1 h-1]
1 3.9 wt% Sn/MCM-41 98 30 195 [59]2 1.6 wt% Snβ > 99 120 70 [134]3 Sn-carbon-silica 100 350 200 [32]4 10 wt% Sn/deAlβ > 99 250 1050 this worka Experimental conditions: dihydroxyacetone in ethanol (0.4 M), Sn content (relative todihydroxyacetone) = 0.3 mol%, 100 °C, 10 bar N2, 24 h. b Calculated as mole ethyllactate formed per mole dihydroxyacetone converted. c Calcuated as mole ethyl lactateformed per mole of Sn. d Calculated as glactone kg−1
zeolite h-1.
preparation of a high metal-content material. Furthermore, the developed procedure requires
significantly less synthesis time and produces no toxic waste in comparison to the benchmarked
process. We expect such a straightforward approach to facilitate the utilization of the same or
similar materials on a large scale.
Chapter 3
Insights into the Baeyer-Villiger Oxidation ofCyclohexanone with H2O2 catalyzed by Snβ
In this chapter FT-IR, UV-Vis and BET measurements, as well as the catalytic tests to study
the influence of water and lactone concentration and the effect of hydrophilicity on Snβ were
performed by the author. Material synthesis and water adsorption studies were performed by
Patrick Wolf, the initial kinetic studies were performed by Hailey Orsted.
3.1. Introduction
The Baeyer-Villiger (BV) oxidation of ketones to esters or lactones is of high synthetic utility
in the pharmaceuticals and fine chemicals industry.[22,23,135,136] So far, mostly peracids such as
3-chloroperbenzoic acid (m-CPBA) have been used as oxidizing agent, complicating the general
applicability of this process through the formation of undesired by-product (carboxylic acid)
and the high explosiveness of the oxidant itself (Figure 3.1).[25,137] In an alternative approach,
hydrogen peroxide (H2O2) is utilized as oxidant, which imposes less environmental (water is
the only by-product) and less safety issues (Figure 3.1).[24,138–141]
Figure 3.1. Baeyer-Billiger oxidation of cyclohexanone to ε-caprolactone. The traditional route(top), using a percarboxylic acid as oxidizing agent, leads to the production of one equivalent oforganic acid as by-product. Alternative routes (bottom), using H2O2 as "green" oxidant, have wateras the only by-product.
Since H2O2 is kinetically inert, it requires catalytic activation in order to perform
as required. Initially, homogeneous transition metal complexes,[142–146] Brønsted acid
26 Chapter 3
catalysts[147–152] and Ti-silicalites[153] have been reported to catalyze BV oxidations by activating
H2O2. However, these catalysts showed low selectivities due to product hydrolysis and/or
preferred epoxidation reactions if additional functional groups are present. Corma and co-
workers showed that SnIV-sites incorporated into the framework of β-zeolite (Snβ) show hitherto
unrivalled catalytic activities and selectivities for the BV oxidation of various ketones and
aldehydes with H2O2.[48,115,122,154]
In order to explain the high chemoselectivity of Snβ, it was suggested that unlike previous
catalysts, which activate H2O2, Snβ activates the carbonyl substrate, hereby making it more
reactive toward an attack by the peroxide.[48] Indeed, IR adsorption studies have demonstrated
that the carbonyl oxygen of cyclohexanone interacts with the SnIV-centers incorporated in the
β-zeolite framework, based on an observed shift in the carbonyl IR stretch when cyclohexanone
is adsorbed on to Snβ.[48,108,122]
The selective interaction between Lewis acidic SnIV-centers and Lewis basic carbonyl groups
give Snβ its unprecedented chemoselectivity in BV oxidations with H2O2. However, it also
exposes Snβ to interfering interactions with other basic molecules that are present in the
reaction mixture. As such, there is the product carbonyl (a lactone or an ester), which can be
expected to compete with cyclohexanone for coordination to the SnIV-centers, thus restricting
the conversion of substrate.[122] In addition, the utilization of H2O2 as oxidant poses the difficulty
of water as co-solvent (and product). Water may, similar to the product carbonyl, hinder the
substrate to effectively interact with the active SnIV-sites and/or the peroxide, hereby restricting
the performance of the catalyst.[17,122,155]
Recently, it was shown that Snβ catalysts which are prepared following different synthesis
protocols, have different capacities to adsorb water (i.e., hydrophilicities), resulting from
different amounts of framework silanols.[156,157] The possibility to synthesize the parent Al-
β-zeolite in various media (fluoride or hydroxyl) prior to dealumination and post-synthetic Sn
incorporation indeed allows controlling the concentration of silanols in Snβ catalysts. It has
not yet been studied if such changes in hydrophilicity influence the catalytic performance of
Snβ in the BV oxidation with aqueous H2O2.
In this work, we look at the influence of the presence of water and ε-caprolactone on the
Insights into the Baeyer-Villiger Oxidation of Cyclohexanone with H2O2 catalyzed by Snβ 27
activity of Snβ in the BV oxidation of cyclohexanone with H2O2. We prepare a series of Snβ
catalysts with different hydrophilicities, both by traditional hydrothermal synthesis and by post-
synthetic incorporation of SnIV into dealuminated β-zeolites. We quantify the hydrophilicities
of the catalysts by recording water adsorption isotherms as well as by analyzing the metal
hydroxyl IR region of the materials. Finally, we test the obtained catalysts in the BV oxidation
of cyclohexanone with H2O2 in order to investigate the influence of catalyst hydrophilicity on
the efficiency of Snβ in this reaction.
3.2. Experimental
3.2.1. Material Synthesis
Al-β in fluoride media was prepared via hydrothermal synthesis according to a literature
procedure.[158] First Aluminum powder (99.99 %, Acros) was dissolved in an aqueous solution of
tetraethyl ammonium hydroxide (TEAOH; 35 %, SACHEM). After complete dissolution of the
aluminum the solution was added to a tetraethyl orthosilicate (TEOS; 98 %, Sigma-Aldrich)
plus TEAOH solution. The resulting mixture is stirred until complete evaporation of ethanol,
formed upon TEOS hydrolysis. To the resulting viscous gel, hydrofluoric acid (48 %, Sigma-
Aldrich) was added to result in a gel with the following composition: 1 SiO2 : x Al2O3 : (0.54
+ 2x) TEAOH : (0.54 + 2x) HF : (7 + 2x) H2O. Crystallization was carried out at 140 °C in
45 mL scale teflon-lined stainless-steel autoclaves, tumbled at 60 rpm for 7 days. After cooling
down, the samples were filtered and subsequently washed with deionized water and acetone
before drying them in an oven at 110 °C over night. To remove the structure directing agent,
samples were calcined at 580 °C under a steady air flow for 6 h.
Post-synthetic incorporation of Sn was performed with an improved synthesis method
originating from the procedure that we described in Chapter 2.[159] Dealumination of the
parent Al-β zeolites was done by acid leaching (13 M HNO3, 20 mL g-1, 100 °C, 20 h).
Sn was incorporated via solid-solid ion-exchange by grinding the dealuminated β with the
appropriate amount of the Sn(II) acetate precursor (Sigma Aldrich) followed by subsequent
3 h heat treatments under N2 and air at 550 °C. 10SnβOH25, 1SnβOH25 and 1SnβOH300
represent the post-synthetic materials starting from commercial Al-β zeolite (SiO2/Al2O3 =
28 Chapter 3
25, Zeochem and SiO2/Al2O3 = 300, Zeolyst) with 10 and 1 wt% Sn, respectively. 1SnβF30,
1SnβF200 and 1SnβF400 (SiO2/Al2O3 = 30, 200, 400) are synthesized from Al-β synthesized
in fluoride media vide supra.
Snβ zeolite via direct incorporation of Sn during hydrothermal synthesis was synthesized
as described in Chapter 2.
3.2.2. Characterization Methods
The Sn-content was quantified with ICP-OES (Perkin Elmer Optima 2000) after digestion of
the samples with HF (48 %, Sigma-Aldrich).
FT-IR and UV-Vis analysis was performed as described in Chapter 2.
Water sorption experiments were performed on a Micromeritics 3Flex instrument at 298 K.
Water was purified by three freeze and thaw cycles. Microporous water uptake was determined
at the relative pressure p/p0 that showed complete filling of the micropores in the N2 adsorption.
N2 sorption measurements were performed on a Micromeritics 3Flex apparatus at 77 K. Samples
were degassed under vacuum at 350 °C for 3 h prior to every sorption analysis. The surface
area was calculated using the Brunauer-Emmett-Teller (BET) theory.
3.2.3. Catalytic Experiments
The experiments were carried out in 10 mL thick wall tube reactors capped with a
PTFE/silicone seal, capable of holding 15 bar over pressure. For reactions under standard
conditions, the vessel was charged with the reactant solution (4 mL, 0.33 M cyclohexanone
in 1,4-dioxane) and the desired amount of catalyst. The vessel was heated to the required
reaction temperature for 10 min prior to the addition of H2O2 (H2O2/ketone = 1.5) and stirred
vigorously for the required reaction period. Aliquots were taken periodically and quantified
against the internal standard decane by GC-FID (30 m FFAP column).
Insights into the Baeyer-Villiger Oxidation of Cyclohexanone with H2O2 catalyzed by Snβ 29
3.3. Results and Discussion
3.3.1. Initial Kinetic Studies
Initial catalytic tests focused on the determination of the kinetic regime for the BV oxidation of
cyclohexanone with H2O2. For this we used a 10 wt% Snβ catalyst prepared via post-synthetic
incorporation of SnIV into dealuminated β-zeolite with a SiO2 to Al2O2 ratio of 25 (material
denoted as 10Snβ-OH25). We performed the reactions under similar conditions to those that
we previously employed (see Chapter 2), using 50 instead of 30 wt% aqueous H2O2 in order to
minimize the concentration of water in the reaction mixture. We studied initial reaction rates
as a function of temperature and catalyst concentration to test for diffusion limitations.
kinetic regime EA = 13.9 kcal/mol
diffusion limitations EA = 5.2 kcal/mol
65°C
45°C
90°C
Figure 3.2. Arrhenius plot of the BV oxidation of cyclohexanone with aqueous H2O2 in thetemperature range between 45 and 90 °C. Experimental conditions: cyclohexanone in 1,4-dioxane(0.33 M), H2O2 (50 wt% aq. solution, H2O2/ketone = 1.5), 10Snβ-OH25.
Figure 3.2 shows initial reaction rates as a function of temperature (Arrhenius plot) in
the range from 45 to 90 °C. The plot shows a curved shape, pointing toward mass-transfer
limitations at higher reaction temperature. An activation energy of ca. 14 kcal mol-1 proofs that
the system is under chemical control between 45 and 65 °C. This value is in good agreement with
computational predictions by Corma and co-workers, which resulted in an activation energy of
14.8 kcal mol-1 for hydrothermally synthesized Snβ.[122] Increasing the temperature to about
90 °C, leads to a drop in activation energy to ca. 5 kcal mol-1, similar to the experimentally
determined activation energy of 7.2 kcal mol-1 by Sels and co-workers for post-synthetically
30 Chapter 3
synthesized Snβ in a similar temperature range.[104] Although Sels and co-workers exclude
mass-transfer limitations, our data clearly suggests the presence of diffusion limitations at
temperatures above 65 °C and a Sn loading of 1.0 mol% (relative to cyclohexanone).
As can be seen in Figure 3.3 (80 °C), the oxidation reaction is first order in Snβ up to
a Sn loading of around 0.6 mol% (relative to ketone), after which the reaction rate levels off.
This observation indicates mass transfer limitations at higher catalyst loadings. In order to
exclude that the observed leveling-off is caused by a change in reaction order, going from low
to high catalyst loadings, we performed a similar study at a lower temperature. Figure 3.3
(60 °C) shows that the leveling off of the initial reaction rates is less pronounced than at 80 °C
suggesting that at Sn loadings higher than 0.6 mol% diffusion limitations are present. Despite
of the clear indication for diffusion limitations, it is not possible to distinguish between extra-
and intragranular mass transfer limitations based on our observations.
mass-transfer limitations
first order in Snβ
Figure 3.3. Initial reaction rates as a function of the employed Sn loading (relative to ketone) forthe BV oxidation of cyclohexanone with aqueous H2O2 at 80 and 60 °C. Experimental conditions seeFigure 3.2.
Diffusion limitations are typical for heterogeneously catalyzed reactions.[160,161] However,
their occurrence has barely been considered when trying to optimize the BV oxidation with
Snβ. This lack of attention is surprising since it is well established that mass-transfer limitations
may cause a lowering of the selectivity for exothermic reactions. In order to avoid diffusion
limitations in our following tests, we hence employed a Sn loading of 0.5 mol% at a reaction
temperature of 80 °C.
Insights into the Baeyer-Villiger Oxidation of Cyclohexanone with H2O2 catalyzed by Snβ 31
3.3.2. Influence of Water and Lactone Concentration
To study the impact of water on the performance of Snβ, we performed reactions with different
initial water concentrations in the reaction solution. For this purpose, different amounts of
concentrated aqueous H2O2 (10, 30 and 50 wt% H2O2) were added to the reaction solution,
while keeping the final peroxide concentration constant. Additional data points were obtained
by adding pure water to the standard amount of 50 wt% aqueous H2O2.
As depicted in Figure 3.4, the performance of Snβ decreases with increasing amounts of
water in the reaction mixture, as evidenced by a gradual decline in lactone formation (red
arrow indicates increasing water content). Initial reaction rates plotted as a function of the
water concentration (inset in Figure 3.4) show an exponential decrease with increasing water
concentration. Interestingly, selectivity values at same levels of ketone conversion are similar
for the different reactions, which indicates that the loss in activity does not arise from enhanced
hydrolysis of the lactone (to hydroxycaproic acid), but rather from catalyst inhibition through
active site blocking.
The best performance is reached if 50 wt% aqueous H2O2 is used, given by the lowest amount
of water introduced to the system, which minimizes catalyst inhibition. This observation is of
practical relevance since it suggests the usage of the highest possible concentration of aqueous
H2O2 for BV oxidations with Lewis acid catalysts.
Figure 3.4. ε-Caprolcatone yield over time for different initial water concentrations in the BVoxidation of cyclohexanone with aqueous H2O2 at 80 °C. Experimental conditions see Figure 3.2.
32 Chapter 3
Figure 3.5. Initial reaction rates as a function of the initial lactone and water concentration in thereaction solution for the BV oxidation of cyclohexanone with aqueous H2O2 at 80 °C. Experimentalconditions see Figure 3.2.
Analogous reactions with different initial lactone concentrations were performed by adding
different amounts of ε-caprolactone shortly before initiating the reaction through H2O2
addition. A decline in the formation of lactone was observed (Figure A.1). In addition, similarly
to the influence of water, an exponential decrease of the initial reaction rate with increasing
lactone concentration was noticed (Figure 3.5, top curve). However, the drop in activity based
on the presence of lactone compared to water is not as strong (Figure 3.5), which indicates that
catalyst inhibition through the lactone is less detrimental than through water.
3.3.3. Effect of Hydrophilicity on the Catalytic Efficiency of Snβ
From our preliminary catalytic tests, we learned that an increase in both water and lactone
concentration in the reaction mixture negatively impacts the performance of Snβ. An open
question in this context is if the catalyst hydrophilicity (i.e., the silanol density) may influence
the catalyst efficiency in BV oxidations with H2O2. A less hydrophilic zeolite surface might
indeed reduce the affinity of water and product carbonyl to the zeolite, and hereby lower the
local concentration of water and lactone at the active SnIV-sites. In general, we expect a
preferential adsorption of our substrate carbonyl (cyclohexanone) compared to our product
carbonyl (ε-caprolactone) due to the higher polarity of ketone- compared to ester-compounds.
To investigate the impact of surface polarity on the activity of Snβ in BV oxidations more
Insights into the Baeyer-Villiger Oxidation of Cyclohexanone with H2O2 catalyzed by Snβ 33
closely, we prepared a series of Snβ catalysts with different hydrophilicities by following various
synthesis protocols. We quantified the hydrophilicities of the obtained materials by recording
water adsorption isotherms. We also analyzed the metal hydroxyl IR regions of the catalysts
and utilized the peak integrals of the SiO–H signal to quantify the silanol groups of the zeolite.
Lastly, we tested their catalytic activities in the BV oxidation of cyclohexanone with H2O2,
using our optimized experimental conditions (0.5 mol% Sn; 50 wt% aq. H2O2).
The Snβ catalysts with a Sn loading of 1.0 ± 0.2 wt% were prepared by both post-synthetic
incorporation of the metal into dealuminated β-zeolites (five materials) and by traditional
hydrothermal synthesis (one material, denoted as 1Snβ-HT). The post-synthetic samples were
prepared from commercial Al-β zeolites that were synthesized in hydroxide media (SiO2/Al2O2
ratios of 25 and 300; materials denoted as 1Snβ-OH25 and 1Snβ-OH300) and from Al-β zeolites
that were synthesized in fluoride media (SiO2/Al2O2 ratios of 30, 200 and 400; materials denoted
as 1Snβ-F30, 1Snβ-F200 and 1Snβ-F400).
In a next step, we recorded water adsorption isotherms in order to quantify the
hydrophilicities of the Snβ catalysts. Table 3.1 summarizes the total uptake of water per surface
area for the different catalysts. As some of us reported recently, the uptake of water is biggest
for the catalysts prepared from Al-β zeolites synthesized in hydroxide media (1Snβ-OH25
and 1Snβ-OH300), compared to those prepared from zeolites synthesized in fluoride media
(1Snβ-F30, 1Snβ-F200, 1Snβ-F400) and the Snβ from direct hydrothermal Sn incorporation
(1Snβ-HT).[157] This is not surprising, since it is known that zeolites prepared in hydroxide
media have more defect sites, leading to a higher intrinsic hydrophilicity.[158]
Another observation we make is that the hydrophilicity decreases with increasing SiO2
to Al2O3 ratio for the materials prepared from Al-β zeolites in fluoride media. This is most
likely caused by different amounts of residual silanol nests, which are not being filled with Sn
during metal incorporation. For 1Snβ-F400, for instance, almost all the vacant T-sites ought
to be filled with SnIV, resulting in a low hydrophilicity. For 1Snβ-F30, however, only around
10 % of the silanol nests created during dealumination are being occupied, leading to a higher
hydrophilicity compared to 1Snβ-F400. The very similar water adsorption for 1Snβ-OH25 and
1Snβ-OH300 indicates that the defect sites of these materials constitute the majority of silanols.
34 Chapter 3
Table 3.1. Water adsorption results and catalytic activities ofdifferent Snβ catalysts in the BV oxidation of cyclohexanone withaqueous H2O2.a
Entry Catalyst H2Ob,cads TOFd,e Conversionf Selectivityf
[µmol m-2] [h-1] [%] [%]
1 1Snβ-F30 11.0 141 ± 21 50 852 1Snβ-F200 3.98 112 ± 17 52 703 1Snβ-F400 4.15 106 ± 16 48 754 1Snβ-HT 2.73 18 ± 3 13 855 1Snβ-OH25 19.5 128 ± 19 41 856 1Snβ-OH300 15.9 99 ± 15 42 84a Experimental conditions: cyclohexanone in 1,4-dioxane (0.33 M), H2O2
(50 wt% aq. solution, H2O2/ketone = 1.5), Sn content (relative to ketone)= 0.5 mol%, 80 °C, 4 h. b Quantity of water adsorbed at p/p0 = 0.82,normalized by the BET surface areas of the catalysts.[162] c See Table A.1for BET surface areas. d Defined as mole ε-caprolactone produced per moleSn per hour at the initial stage of the reaction. e Error based on ICP-OESdetermined Sn-loading. f After 2.5 h of reaction.
In addition to our water adsorption measurements, we determined the peak areas of the
silanol IR signals of the different catalysts in order to quantify their hydrophilicities. Figure 3.6
B shows the metal hydroxyl IR region of all our prepared Snβ catalysts (see Figure A.2 for
full spectra). The broad absorbance around 3500 cm-1 arises from unclosed silanol nests, the
small signal at ca. 3660 cm-1 from Sn hydroxyl groups, and the sharp peak at ca. 3750 cm-1
from isolated silanols. The Sn hydroxyl vibration is only detected for 1HT-Snβ, 1Snβ-F400
and 1Snβ-F200, while it is overlapped by more intense silanol signals for the other catalysts.
As can be seen in Figure 3.6 A, the quantities of adsorbed water, as determined by our water
adsorption measurements, show a linear correlation with the peak areas of the silanol IR signals.
This indicates that the hydrophilicity of Snβ correlates with the amount of framework silanols.
To link the results from our water adsorption studies and IR analysis with the activity of
the catalysts, we performed BV oxidations of cyclohexanone with H2O2. As can be seen in
Table 3.1, the different Snβ catalysts show different activities. The highest turnover frequency
(TOF) is obtained with 1Snβ-F30, closely followed by 1Snβ-OH25. The lowest activities are
observed for 1Snβ-OH300, which has still a ca. four times higher activity compared to 1Snβ-
HT.
Figure 3.7 shows the TOFs of the different Snβ catalysts as a function of their
Insights into the Baeyer-Villiger Oxidation of Cyclohexanone with H2O2 catalyzed by Snβ 35
(A) (B)
Figure 3.6. (A) Quantities of adsorbed water obtained from water adsorption measurements plottedagainst the peak areas of the SiO–H IR signal between 3750 and 3000 cm-1 (both values are normalizedby the BET surface areas of the catalysts). (B) Metal hydroxyl region taken from FT-IR spectra ofthe different Snβ catalysts (see Figure A.2 for full IR spectra).
hydrothermal
fluoride media post-synthetic hydroxide media
post-synthetic
Figure 3.7. Catalytic activities of the different Snβ catalysts as a function of their hydrophilicities,as determined by the peak areas of the SiOH IR signals between 3750 and 3000 cm-1 (see Table 3.1and Figure 3.6 B).
hydrophilicities (as given by the peak areas from their metal hydroxyl IR regions). It can
be seen, that the activities of the Snβ catalysts increase with increasing hydrophilicity up to a
certain level, after which they decrease again. For the catalysts prepared from parent zeolites
synthesized in fluoride media, we observe an increase in activity with increasing hydrophilicity.
For the catalysts prepared from zeolites synthesized in hydroxide media we observe, however,
36 Chapter 3
a small drop in activity with increasing hydrophilicity. The Snβ prepared via traditional
hydrothermal synthesis is, on the other hand, about four times less active than 1Snβ-F400,
although the hydrophilicities of these two materials are similar.
These observations demonstrate that the activity of Snβ in the BV oxidation of
cyclohexanone with H2O2 is influenced by the hydrophilicity of the zeolite framework, and that
higher hydrophilicity (i.e., higher silanol density) has a positive effect on the reaction rate up to
a certain level, beyond which it leads to a lowering of the reaction rate (c.f., Sabatier principle).
This behavior most likely shows that a certain amount of silanols aids the performance of the
catalyst by providing an optimal surface character for the ketone to approach and adsorb to
the active SnIV-sites. At higher silanol amounts this effect is offset by increasingly strong
interactions between the silanols and water, substrate or product molecules, which leads to a
reduced local concentration of substrate at the active SnIV-sites and a lowering of activity.
The data presented here indicates that the activity of Snβ in the Baeyer-Villiger oxidation of
cyclohexanone with H2O2 is highly dependent on the hydrophilicity of the material. However, to
better understand the observed trend in reactivity, also other physical and chemical properties of
the Snβ catalysts have to be considered. As such, BET surface areas between 545 and 718 m2 g-1
were obtained from N2 adsorption measurements at 77 K (see Table A.1), which do not show a
clear correlation with our measured activities, even though the high surface area of 1Snβ-F30 is
possibly related with the high activity of this best-performing catalyst. Furthermore, we know
from a previous study that 1Snβ-F400 and 1Snβ-HT show significantly lower external surface
areas compared to the other Snβ catalysts as a result of their prolonged crystallization process
in fluoride media.[157] However, this does not fall in line with the catalytic activities we observe,
since 1Snβ-F400 and 1Snβ-HT exhibited significantly different activities in our catalytic tests.
We also learned in a previous study that the materials with a more hydrophobic framework
have higher crystallinities, even though no direct correlation between crystallinities and our
observed catalytic activities in the Baeyer-Villiger oxidation of cyclohexanone with H2O2 can
be established.[157]
Lastly, also differences in the nature of the active sites between the various Snβ catalysts
might contribute to their different activities. Indeed, based on the data presented here, we
Insights into the Baeyer-Villiger Oxidation of Cyclohexanone with H2O2 catalyzed by Snβ 37
cannot exclude that active site speciation has an influence on the reactivity of our different Snβ
catalysts in the BV oxidation of cyclohexanone with H2O2. In a previous combined experimental
and computational study some of us investigated the influence of the active site structure
of a series of Snβ catalysts on their activities in the aqueous phase isomerization of glucose
to fructose.[157] Their experiments suggest that post-synthetically prepared Snβ catalysts are
mostly represented by closed (T8) and open (T5/T9) sites incorporated into the hydrophilic
framework. Hydrothermally synthesized Snβ, on the other hand, mainly consists of closed (T6,
T5, T7) sites, which can be hydrolyzed under reaction conditions. Based on our data, open
sites (i.e., 1Snβ-OH25, 1Snβ-OH100, 1Snβ-F30), as well as closed sites (i.e. 1Snβ-F200, 1Snβ-
F400) are active in the Baeyer-Villiger oxidation of cyclohexanone with H2O2, which does not
indicate that site structure has a prevailing influence on activity in this reaction system.
Interestingly, Snβ samples obtained from post-synthetic modification (i.e., 1Snβ-OH25,
1Snβ-OH300, 1Snβ-F30, 1Snβ-F200, 1Snβ-F400) showed a ca. 3-5 times lower activity per Sn
in the isomerization of glucose to fructose (in terms of TOF) compared to hydrothermally
synthesized Snβ catalyst (i.e., 1Snβ-HT),[157] which is the opposite trend in reactivity to
what we observe for the BV oxidation of cyclohexanone with H2O2. Indeed, hydrothermally
synthesized Snβ zeolite shows lowest activity in our study. This indicates that the relation
between catalyst properties and catalytic activities is strongly dependent on the individual
reaction system. The disclosure of such contrary trends, may, however, strongly contribute
to the understanding of structure-activity-relationships for a material in different catalytic
systems.
3.4. Conclusions
In this work, we determine the kinetic regime of the BV oxidation of cyclohexanone with
aqueous H2O2 for post-synthetically prepared Snβ zeolite. For this we study initial reaction
rates as a function of metal loading and temperature and define reaction conditions that exclude
diffusion limitations for following investigations.
We also study the impact of the water and lactone concentration in the reaction solution
on the catalytic activity of Snβ and find that both lower the performance of the catalyst,
38 Chapter 3
which we assign to active-site inhibition through these compounds. We furthermore prepare a
series of Snβ catalysts with different hydrophilicities by following different synthesis protocols.
The hydrophilicities of the prepared materials are quantified with water adsorption studies and
by analysis of the silanol IR region, which results in a good correlation between these two
quantities.
The catalytic performance of the different Snβ catalysts in the BV oxidation of
cyclohexanone with aqueous H2O2 is different and is influenced by the zeolite hydrophilicities.
We observe that an optimum in hydrophilicity exists, which indicates that a hydrophilic
framework aids the adsorption of the ketone substrate to the active SnIV-sites, and that this
effect is outweighed by active site inhibition through solvent (water) and product molecules
(lactone) at higher hydrophilicities.
Our work demonstrates that hydrophilicity may influence the catalytic performance of
Snβ, and that flexible synthesis methods, such as our post-synthetic metal incorporation, allow
optimizing activities through targeted structural modifications.
Chapter 4
Silica-Grafted SnIV Catalysts inHydrogen-Transfer Reactions
In this chapter all catalytic experiments, material synthesis and characterization was conducted
by the author. NMR measurements were performed with the assistance of René Verel. The
synthesis of the reference material Snβ was done by Patrick Wolf.
4.1. Introduction
A wide range of important chemical transformations, including isomerizations and oxidations,
are catalyzed by heterogeneous catalysts with isolated Lewis acid sites.[5,16,61] Most notable
amongst them are metal-doped zeolites, some of which have demonstrated outstanding activity
and selectivity, as well as high stability and water tolerance.[20,52,128,163] A contemporary example
is SnIV-containing β-zeolite (Snβ).[46,164] This catalyst excels in activating carbonyl groups and
has been explored for various reactions, such as Baeyer-Villiger oxidations,[48,115,154] Meerwein-
Ponndorf-Verley (MPV) hydrogen-transfer reactions,[39,108] sugar isomerizations[26,29,30] and C-C
bond formations.[45,165]
It is expected that various material properties contribute to the overall performance of
Snβ: (i) the molecular environment of the active sites,[21,101] (ii) confinement effects induced
by the micropores of the zeolite,[110,166] and (iii) the hydrophobicity of the framework (as found
for highly siliceous zeolites).[16,17,108,109] In particular, the exact site structure has recently
received significant attention and initiated characterization work using 119Sn NMR,[116–121]
EXAFS,[47,106] TEM[47,106] and XPS.[47] These studies yielded valuable structural information
and have been complemented by IR probe studies, which are more closely related to the
actual reaction conditions. In these investigations, interaction-induced shifts in vibrational
bands of adsorbed probe molecules can indirectly provide information regarding the catalytic
transformation.[60,64,101,111,115,123,127]
40 Chapter 4
One important probe molecule in this context is cyclohexanone,[64,115,127] a widely used
substrate in Snβ catalyzed reactions.[26,29,30,39,45,48,108,115,154,165] It is assumed that the observed
shifts of the carbonyl band correlate with the Lewis acidity of the SnIV-sites,[64,115,127] and
is hence a good measure for the catalytic performance of the site in question. Additionally,
the comparison of experimental and computed shifts allowed researchers to draw structural
conclusions about the active sites.[122] Some studies also correlated measured catalytic activities
with the individual activities of different types of sites in Snβ.[111] Therefore, the intensities of
the different bands are used to estimate the amounts of individual sites present in the catalyst.
However, not only the nature of the active site, but also multiple adsorption to one site might
induce similar shifts of the IR bands.[123] This leads to difficulties for an unambiguous assignment
of the active site(s), based solely upon IR bands. Moreover, exclusively cyclohexanone is
adsorbed in these studies, while a mixture of molecules is present under reaction conditions.
It is not entirely clear whether these molecules (co-reactants, solvent or product molecules)
competitively adsorb to certain types of active centers.
Besides the structure of the active site, also the nature of the zeolite (pore architecture and
polarity) might contribute to the outstanding performance of Snβ. It is therefore challenging
to distinguish between the different contributions in a zeolite system. An appealing approach
to separate the individual factors is to use a model-system.
One possibility to form isolated active sites on a silica surface (where confinement effects
are absent) is grafting.[67,76–79] In this approach, a metal precursor[88–90,93,167] reacts, after
careful pretreatment of the support, with surface silanol groups,[86] which leads to a uniform
distribution of highly dispersed sites. By applying post-grafting treatments, the coordination
of the active centers can be modified.[91,92,102,168] For instance, the metal may form additional
bonds to the surface during a thermal treatment (i.e., multipodal anchoring).[88,89] All these
synthetic steps can be monitored by various characterization techniques (FT-IR, UV-Vis, solid-
state NMR) and often change the activity of the catalyst.[88,89,92,102,168]
In this work (Figure 4.1), we form isolated SnIV-sites on amorphous silica upon grafting (I)
and apply a thermal and a chemical post-treatment (II), leading to a set of three Sn-model-
catalysts. We characterize the active sites with different spectroscopic techniques and test
Silica-Grafted SnIV Catalysts in Hydrogen-Transfer Reactions 41
Figure 4.1. To investigate the effect of the active site structure and surface hydrophobicity onthe activity of SnIV-based catalysts, we form isolated SnIV-species on amorphous silica via chemicalvapour deposition of Sn(NMe2)4 (I). Subsequently we perform post-synthetic treatments to changethe bonding environment of the sites (II), and test the catalytic activities of the materials in MPVreductions (III). We finally relate this to results obtained from IR adsorption studies (IV).
their performance in the MPV reduction of cyclohexanone with 2-butanol (III), a hydrogen-
transfer-reaction. We study the adsorption of cyclohexanone with FT-IR (IV) and relate this
to their catalytic activities. Comparing our data with Snβ then allows to draw conclusions
about different contributions to the activity of SnIV/SiO2 catalysts.
4.2. Experimental
4.2.1. Material Synthesis
Silica powder (Aerosil 200® from Degussa, specific surface area: 200 m2 g-1) was impregnated
with water and dried overnight at 100 °C in a vacuum oven. Afterwards, the material was
dehydrated at 700 °C (16 h) at 15 µbar dynamic vacuum and stored inside of a glove box
(< 1 ppm O2 and H2O) in order to avoid re-adsorption of water. Sn(NMe2)4 (Sigma-Aldrich,
two times distilled before use, colorless) was then deposited onto 300 mg of dried silica at ca.
20 µbar dynamic vacuum (5 equivalents based on initial silanol content). The transfer phase
(45 min) was followed by a reaction phase at RT (30 min) and a mild thermal post-treatment at
50 °C and 15 µbar dynamic vacuum (1 h) in order to evaporate excess of transferred Sn(NMe2)4
(material denoted as Sn/SiO2(700)).
Samples were calcined in an air flow at 550 °C for 3 h (material denoted as Sn/SiO2(700)-
O2) and dehydrated for further use (300 °C, 3 h, 15 µbar dynamic vacuum). N,N -
bis(trimethylsilyl)methylamine (Sigma Aldrich, two times distilled before use) was used for
the silylation of the dried silica (material denoted as TMS-SiO2(700)). The silylating agent was
42 Chapter 4
contacted with the material in the same way as Sn(NMe2)4 and thermally post-treated at 250 °C
(5 equivalents based on initial silanol content). Afterwards, Sn(NMe2)4 was deposited on the
silylated material to obtain Sn/TMS-SiO2(700) (2 equivalents based on initial silanol content).
N,N -bis(trimethylsilyl)methylamine was also used for the silylation of the dehydrated
calcined material Sn/SiO2(700)-O2 (material denoted as Sn/SiO2(700)-O2-TMS) using the same
deposition procedure (5 equivalents based on initial silanol content).
4.2.2. Characterization Methods
ICP-OES, FT-IR and UV-Vis analysis was performed as described in Chapter 2.
The nitrogen content was obtained with a thermal conductivity detector. The combustion
products (CO2, H2O) were quantitatively analyzed by infrared spectroscopy. The nitrogen
content was obtained with a thermal conductivity detector.
The DRIFT spectra were recorded by averaging 32 scans with a resolution of 8 cm-1. The
DiffusIR accessory (PIKE Technologies) was flushed with synthetic air at a flow rate of ca.
20 mL min-1 and heated at a heating rate of 20 °C min-1 up to a temperature of 550 °C.
Solid-state 13C-NMR spectra were acquired on an Avance NMR spectrometer (Bruker,
Karlsruhe, Germany) operating at a 1H Larmor frequency of 700 MHz. The samples were spun
around the Magic Angle with a rate of 10 kHz at room temperature using a double resonance
4 mm probe (using ca. 40 mg sample). The two channels of the probehead were tuned to the
resonance frequencies of 1H (700.13 MHz) and 13C (176.06 MHz). The ppm scale of the spectra
was calibrated using the 13C signal of adamantane as an external secondary reference. The 13C
spectrum was acquired using a Cross Polarization (CP) sequence with higher power TPPM
decoupling of the protons during detection of the 13C signal.
Thermogravimetric analysis of the calcination step was performed using a TGA-DSC
Thermobalance (Mettler Toledo) in combination with an OmniStarTM massspectrometer
(Pfeiffer Vacuum).
4.2.3. Adsorption Studies
The samples were initially heated to 300 °C at ca. 15 µbar dynamic vacuum (3 h) in order
to remove any water from the surface of the material. After cooling the samples to room
Silica-Grafted SnIV Catalysts in Hydrogen-Transfer Reactions 43
temperature, cyclohexanone vapors were exposed to the samples using a closed evacuated
system (ca. 20 µbar static vacuum) consisting of a sample holder (10 mL tube reactor)
connected to a capillary reactor filled with cyclohexanone. The exposure time was complete
when the filling level of the ketone in the capillary reactor was stable. After recording an
FT-IR spectrum of the material inside a glove box (< 1 ppm O2 and H2O) the sample holders
were evacuated several times in order to gradually re-adsorb the probe molecule. After each
desorption, an FT-IR spectrum was measured.
4.2.4. Catalytic Experiments
The Meerwein-Ponndorf-Verley reactions of cyclohexanone were carried out in a 20 mL round
bottomed-flask fitted with a reflux condenser, using 2-butanol as the hydrogen transfer agent.
In a typical reaction, 1 mmol of cyclohexanone and 60 mmol of 2-butanol (corresponds to 5.5 mL
of a 0.18 M reaction solution of cyclohexanone in 2-butanol) were added to the appropriate
amount of catalyst (corresponding to 0.5 mol% Sn relative to the ketone). The vessel was
heated at a constant temperature (90 °C) for 6 h under stirring (500 rpm). The reactant
and product were quantified against the internal standard biphenyl by GC-FID analysis (30 m
FFAP column).
4.3. Grafting of Sn(NMe2)4 to Thermally Treated Silica
The first step in our synthesis procedure is the pretreatment of the silica support (Aerosil 200®)
at 700 °C under vacuum in order to desorb physisorbed water and to condensate hydrogen-
bonded vicinal silanols (material denoted as SiO2(700)). Figure 4.2 shows the FT-IR spectrum of
SiO2(700) (B), together with a spectrum of silica treated at 200 °C (SiO2(200), (A)). Both spectra
exhibit Si-O-Si overtones of the silica framework (ca. 1500 – 2100 cm−1), as well as a signal at
higher wavenumbers in the IR region characteristic of SiO-H vibrations. While the O-H band
of SiO2(200) features a low-energy shoulder, stemming from residual hydrogen-bonded silanols
(blue area), the O-H band of SiO2(700) is sharp and symmetrical (red area).1 This indicates
that the surface of SiO2(700) consists predominantly of non-interacting silanols.[86]
To anchor SnIV to these isolated silanols, we then transferred Sn(NMe2)4 onto Sn/SiO2(700)
1Under these conditions, the presence of minor amounts (≤ 5 %) of geminal diols cannot be excluded.[86]
44 Chapter 4
SiO2(200)
SiO2(700)
Sn(NMe2)4
Sn/SiO2(700)
(A)
(B)
(C)
(D)
Figure 4.2. FT-IR spectra of silica dehydrated at (A) 200 and (B) 700 °C, (C) Sn(NMe2)4 and (D)Sn/SiO2(700). The thermal treatment induced the condensation of hydrogen-bonded silanols (blue)and left isolated surface silanols behind (red). Grafting of Sn(NMe2)4 led to the conversion of allisolated silanols and the appearance of C-H precursor stretches (green).
in the gas phase. After evacuating under mild heating, we obtained the grafted material
Sn/SiO2(700). The IR spectrum in Figure 4.2 D indicates that all silanols were quantitatively
consumed during grafting. In addition, intense IR bands could be observed in the C-H stretching
(2800 – 3100 cm−1) and bending (1400 – 1470 cm−1) range (green area). Comparison of the IR
bands of Sn/SiO2(700) with those of the Sn(NMe2)4 precursor (Figure 4.2 C) suggests that only
minor changes in the precursor structure took place upon grafting. We believe that all these
observations indicate a surface reaction between silanols and Sn(NMe2)4 to yield HNMe2 and
≡SiOSn(NMe2)3 species. This hypothesis was supported by the unambiguous identification of
HNMe2 in the liquid-nitrogen trapped effluent of the synthesis reactor with 1H- and 13C-NMR
after Sn(NMe2)4 deposition (Figure A.4). In addition, we performed ICP- and CHN-analyses
to obtain molar ratios (C/Sn, H/Sn, N/Sn). These ratios are in good agreement with the
proposed monopodal SnIV-sites (≡SiOSn(NMe2)3) (Table A.2).
Surprisingly, the Sn-loading, that we obtained from bulk analysis (4.0 ± 0.1 wt%,
0.33 mmol Sn g−1cat, corresponds to a site density of 1 Sn site per nm2. This exceeds the
site density of silanols on SiO2(700)[78] by around 25 %. Therefore, we suspect that silanols are
Silica-Grafted SnIV Catalysts in Hydrogen-Transfer Reactions 45
Figure 4.3. Possible pathways for the reaction of surface siloxanes with Sn(NMe2)4. Indirect route:(A) opening of siloxanes through HNMe2 (liberated during the grafting); (B) reaction of the formedsilanols with Sn(NMe2)4. The direct route (C) immediately yields a 1:1-mixture of Sn and aminespecies through cleavage of siloxanes by Sn(NMe2)4.
not the only surface functional group that can react with Sn(NMe2)4.
A possible explanation for this is the participation of (strained) siloxane bridges (≡SiOSi≡),
formed during the dehydration. A fraction of these siloxanes is part of strained ring-structures
((SiO)n, n=2).[86,87,169] Earlier studies demonstrated the reaction of these strained sites with
molecules containing O-H or N-H bonds.[170–172] If water and ammonia are used, such a surface
reaction results in adjacent ≡SiOH groups or a 1:1-mixture of ≡SiOH and ≡SiNH2 groups,
respectively.[170–172] We expect that the leaving group of our grafting reaction (HNMe2) and/or
the Sn-precursor itself (Sn(NMe2)4) can react with the strained siloxanes of SiO2(700) in a similar
manner, which would explain the unexpected high Sn-loading.
Based on this hypothesis, we propose two possible pathways for the reaction of the surface
siloxanes of Sn(NMe2)4 with HNMe2 and/or Sn(NMe2)4 under grafting conditions (Figure 4.3).
One pathway ((A)-(B) in Figure 4.3) consists of two consecutive steps. In a first step, siloxane
bridges are opened by HNMe2, yielding a 1:1-mixture of new silanols and N-functionalized
silicon atoms (≡SiNMe2). The formed silanols can then, similar to isolated silanols, react with
Sn(NMe2)4 to form SnIV-sites and HNMe2 (which leaves the surface or reacts with additional
siloxanes). The second pathway ((C) in Figure 4.3) consists of the direct cleavage of siloxanes
by Sn(NMe2)4. This immediately yields a 1:1-mixture of Sn- and amine-sites.
To verify if siloxanes can directly react with Sn(NMe2)4 (via step (C) in Figure 4.3) we
protected the silanols of SiO2(700) via silylation with N,N -bis(trimethylsilyl)amine (viz., ≡SiOH
46 Chapter 4
δ / ppm
Sn/SiO2(700)
Sn/TMS-SiO2(700)
(A)
(B)
Figure 4.4. 13C-NMR spectra of Sn(NMe2), grafted on to (A) SiO2(700) and (B) TMS-SiO2(700). Twosignals in the region of amine-based methyl-groups were observed (green and orange area), which givesevidence for the proposed pathways in Figure 4.3. The silylated material also showed an expectedTMS-related peak at 0 ppm (pink area).
→ ≡SiOSi(CH3)3; material denoted as TMS-SiO2(700)), prior to bringing it into contact with
Sn(NMe2)4. The resulting material contained 0.9 ± 0.02 wt% Sn, which exactly corresponds
to the amount of Sn exceeding the exclusive reaction with silanols during the synthesis of
Sn/SiO2(700) (Sn-content of 4.0 ± 0.1 wt%). This confirms the participation of siloxanes in the
grafting reaction. We further performed solid-state 13C-NMR measurements (Figure 4.4) to
investigate the nature of the organic groups we observed with FT-IR. The 13C-NMR spectrum
of Sn/SiO2(700) (Figure 4.4 A) shows two signals in the chemical shift region of amines (green
and orange area). The 13C spectrum of Sn/TMS-SiO2(700) (Figure 4.4 B) showed signals at the
exact same shifts (albeit lower intensity due to the smaller relative site concentration), together
with an expected TMS-related peak at 0 ppm (pink area). This experiment indicates that there
are two distinct amine species on Sn/SiO2(700) and Sn/TMS-SiO2(700), which are most likely of
the type ≡SiOSn(NMe2)3 (42 ppm) and ≡SiNMe2 (36 ppm). We based our assignment on the
higher expected amount of Sn-coordinated amine groups compared to Si-bound amine groups.
To obtain additional evidence for the existence of these two sites, we analyzed the C-H
stretch region in the IR spectrum of Sn/TMS-SiO2(700) (Figure A.5 and Figure A.6). Since the
theoretical ratio between our hypothesized amine (≡SiNMe2) and SnIV-sites (≡SiOSn(NMe2)3)
is 1:1 in Sn/TMS-SiO2(700), but 1:5 in Sn/SiO2(700), we expected that signals arising from
the ≡SiNMe2 sites should be (relatively) more pronounced in Sn/TMS-SiO2(700). Compared
Silica-Grafted SnIV Catalysts in Hydrogen-Transfer Reactions 47
to Sn/SiO2(700), Sn/TMS-SiO2(700) revealed similar signals, along with two additional bands,
which might be attributed to ≡SiNMe2 sites. We emphasize that our silylating agent (N,N -
bis(trimethylsilyl)amine) has been reported to also open a small fraction of siloxanes to form
≡SiN(TMS)Me sites.[89] Computationally predicted methyl C-H vibrational frequencies for
≡SiNMe2 and ≡SiN(TMS)Me (Table A.3) allow us to assign the higher values to the TMS-
holding site, in line with our experimental observations. We believe this demonstrates the
presence of grafted amine species, which are likely amine sites of the type ≡SiNMe2.
To verify route (A) in Figure 4.3, we exposed TMS-SiO2(700) also to HNMe2. We obtained a
material with a small fraction of nitrogen, indicating that the direct reaction between siloxanes
and pure HNMe2 is negligible. In order to test if the siloxane-oxygen atom first needs to be
activated by a Lewis acid (i.e., Sn(NMe2)4) to react with amines, we simultaneously deposited
HNEt2 and Sn(NMe2)4 on to SiO2(700). By using diethyl- instead of dimethylamine, it was
possible to differentiate Sn(NMe2)4- and HNEt2-related organic fragments. This experiment
yielded a material with significantly more intense signals in the C-H IR stretch region compared
to Sn/SiO2(700) (Figure A.7). Since the material revealed the exact same Sn-loading as
Sn/SiO2(700), this increase in intensity did not arise from additional SnIV-sites but from grafted
diethylamine groups and their extra secondary C-H groups. By comparing this to the negligible
reaction between HNMe2 and TMS-SiO2(700), we consider this as clear indication of the Lewis
acid-activation of siloxane-oxygens through Sn(NMe2)4, which then allows HNEt2 (and therefore
most likely also HNMe2) to react with them. Accordingly, we believe that both pathways (A)-
(B) and (C) in Figure 4.3 are possible.
4.4. Post-Synthetic Functionalizations
At this stage, the monopodal SnIV-species are coordinated by three –N(CH3)2 ligands as
illustrated by bulk analysis, NMR and FT-IR. We continued the synthesis by subjecting
Sn/SiO2(700) to a calcination in an air flow in order to obtain SnIV-centers that are exclusively
coordinated by oxygen atoms (material denoted as Sn/SiO2(700)-O2). This is expected to
increase the Lewis acidity of the SnIV-species by avoiding electron-donating amine ligands.
We monitored the calcination process in situ with DRIFT spectroscopy (Figure 4.5). A
48 Chapter 4
gradual decrease of all C-H stretches (region 1: 2800 – 3100 cm−1) confirmed the complete
removal of all organic groups. In parallel, a signal in the IR region typical of N-H stretches
(signal 2: 3290 cm−1) increased in intensity, before it decreased again at around 150 °C and
disappeared above 350 °C. We assume that this signal arises from an intermediate ammonium
species that is formed on the silica surface during the calcination. Furthermore, we observed
two peaks appearing in the IR region characteristic of metal hydroxyl O-H stretches (signal
3: 3664 cm−1; signal 4: 3740 cm−1), which we attribute to Sn hydroxyls and isolated silanol
groups, respectively. The build-up of a SnOH signal suggests that the initial Sn-NMe2 bonds
have been replaced by Sn-OH groups, while the appearance of a silanol band (4 in Figure 4.5)
shows that isolated silanols are generated during the calcination.
We emphasize that no Sn is released from the surface during the calcination (as confirmed
by bulk analysis). To test if water impurities in the air flow could open siloxanes under our
calcination conditions and could, thus, be responsible for the formation of new silanols, we
exposed SiO2(700) to a humid air stream. By monitoring the hydroxyl region with DRIFT
spectroscopy while heating, we observed a signal of hydrogen-bonded silanols (Figure A.8). This
indicates the presence of adjacent ≡SiOH groups, due to opening of siloxanes by water. Since
the silanol band of Sn/SiO2(700)-O2 (1 in Figure 4.5) possesses no such low-energy shoulder, we
conclude that it is not water that induces the release of silanols during the thermal treatment.
Another possibility is that the Sn hydroxyl groups, which are formed during the calcination
4 3
2 1
1 2
3 4
Figure 4.5. In situ DRIFTS study of the air calcination of Sn/SiO2(700). Initially, the C-H bandsof the amine ligands (1) were visible. At rising temperatures an intermediate ammonium species (2),SnO-H (3) and SiO-H (4) stretches appeared.
Silica-Grafted SnIV Catalysts in Hydrogen-Transfer Reactions 49
Figure 4.6. Thermal reconstruction of (A) ≡SiO-Sn(OH)3 and (B) (≡SiO)3Ti-Cl species.[88]
(3 in Figure 4.5) react with nearby siloxane bridges to form bipodal Sn-sites ((≡SiO)2Sn(OH)2)
plus silanols. This would provide the Sn with additional (stabilizing) anchoring points to
the silica surface and, at the same time, explain the generation of new isolated silanols (4 in
Figure 4.5). To gain evidence for this site-restructuring we estimated the amount of silanols
that were released during the calcination by comparing the silanol peak areas in the IR spectra
of Sn/SiO2(700)-O2 and SiO2(700). It turned out that relative to the Sn-loading of Sn/SiO2(700)-
O2, 50 % (± 5) new silanols are formed. This is consistent with the quantification of the SnOH
IR signal, which resulted in a SnOH-to-Sn ratio of 2.5 ± 0.3 (Figure A.9). We thus assume
that about half of the monopodal Sn species (≡SiO-Sn(OH)2) react with nearby siloxanes
and restructure to bipodal Sn species ((≡SiO)2Sn(OH)2), while generating isolated silanols
(Figure 4.6 A). Conclusively, our calcination step yields a mixture of mono- and bipodal
hydrolyzed SnIV-sites, which have been suggested to be more reactive than closed SnIV-sites
(i.e., fully anchored to a material framework (≡SiO)4Sn).[45,60]
Conversely, earlier work within our group demonstrated that similar site-restructuring of
monopodal Ti-Cl-species (≡SiO-TiCl3) resulted in quantitative conversion to tripodal sites
((≡SiO)3Ti-Cl; Figure 4.6 B), whereas no tripodal species were observed on our materials
based on two quantification methods. More work is necessary to confirm and understand the
molecular reasons behind this behavior.[88] In this respect, DQ/TQ 1H MAS NMR at high
spinning rates is one envisaged technique in order to differentiate and quantify various types of
Sn hydroxyl species.
50 Chapter 4
wavelength / nm
Kub
elka
Mun
k
(A)
(B)
(C)
(D)
(E)
(F)
wavelength / nm
Sn-β
SnO2 Sn/SiO2(700)-O2
Sn/SiO2(700)
Sn(NMe2)4 Sn/SiO2-O2(700)-TMS
Figure 4.7. Comparing the UV-Vis spectra of (A) Sn(NMe2)4, (B) Sn/SiO2(700), (C) Sn/SiO2(700)-O2
and (D) Sn/SiO2(700)-O2-TMS with (E) Snβ and (F) SnO2 indicated the absence of SnOx-species in ourmaterials (for the UV/Vis spectra of Sn/TMS-SiO2(700) and Sn/TMS-SiO2(700)-O2, see Figure A.11).
Having the hydrophobic character of the highly siliceous cavities of Snβ in mind, our
second functionalization consisted of the capping of all hydroxyl groups on the surface (SiOH
and SnOH) with trimethylsilyl-units using N,N -bis(trimethylsilyl)methylamine. The infrared
spectrum of the obtained material (denoted as Sn/SiO2(700)-O2-TMS) confirms the conversion
of both types of hydroxyls together with the introduction of C-H stretches from the organic
substituents of the silylating agent (3100 – 2700 cm−1 and 1500 – 1400 cm−1; Figure A.10).
In order to show that no SnOx species are formed during the post-synthetic treatments
we performed Diffuse Reflectance UV-Vis measurements. According to the obtained spectra
(Figure 4.7), there is no indication of SnOx species, given by the lack of absorbance above
280 nm. As expected, we also observed a blue-shift, going from our Sn-precursor (Sn(NMe2)4:
233 nm) along the different synthesis steps (Sn/SiO2(700)-O2-TMS: 226 nm), since four nitrogen
atoms are gradually replaced by four oxygen atoms. Compared to the absorption band of Snβ
with a maximum at 216 nm (Figure 4.7 E), which is indicative of tetrahedral isolated SnIV-
sites in a zeolite framework, the bands of our materials appear at slightly higher wavelengths
(i.e., Sn/SiO2(700): 228 nm; Sn/SiO2(700)-O2: 222 nm; Sn/SiO2(700)-O2-TMS: 226 nm). One
possible explanation for this is the lower coordinative strain on the more flexible silica support,
compared to the rigid crystalline zeolite framework.
Silica-Grafted SnIV Catalysts in Hydrogen-Transfer Reactions 51
We also performed deuterated acetonitrile adsorption studies with our Sn materials and
reference Snβ (Figure A.13). Both Sn/SiO2(700)-O2 and Sn/SiO2(700)-O2-TMS show signals at
around 2310± 2 cm−1, which is characteristic of Lewis acid sites, as shown before for Snβ.[101,111]
This observation points towards the similarities between our silica-supported SnIV-sites and the
framework SnIV-sites in Snβ.
4.5. Catalytic Reactivity of the Supported Species
To test the catalytic activities of our prepared materials we performed Meerwein-Ponndorf-
Verley (MPV) reductions of cyclohexanone with 2-butanol (Figure 4.8).[48,64,173,174] Bulk
analyses of the Sn-loading before and after the reactions confirmed that no leaching of the
active metal takes place.
Figure 4.8. Catalytic reaction to probe the activities of our materials: Meerwein-Ponndorf-Verleyreaction of cyclohexanone with 2-butanol (0.18 M cyclohexanone, 90 °C, 0.5 mol% Sn). For moredetails see Experimental Part.
The catalytic performances of our materials follow the trend Sn/SiO2(700) � Sn/SiO2(700)-
O2 < Sn/SiO2(700)-O2-TMS, with turnover frequencies (TOF) for Sn/SiO2(700), SnO2/SiO2(700)
and SiO2(700) being close to zero (see Table 4.1 and Figure 4.9).
Figure 4.9. Catalytic activities of various Sn-containing materials and SiO2(700) in the MPV reductionof cyclohexanone with 2-butanol (reaction conditions see Figure 4.8 and Experimental Part).
52 Chapter 4
Table 4.1. Catalytic activities in the performed MPV reduction and amounts ofadsorbed cyclohexanone (after exposure to 2-butanol) from IR adsorption studiesfor our materials and Snβ.a
Entry Material TOFinitb,c TON6h
d Areac
[h-1] [–] [abs. cm-1 mol−1Sn ]
1 Sn(NMe2)4 < 1 < 1 N/Af
2 Sn/SiO2(700) < 1 < 1 36 ± 4.53 Sn/SiO2(700)-O2 6 ± 0.2 10 60 ± 7.04 Sn/SiO2(700)-O2-TMS 8 ± 0.2 28 85 ± 9.45 Snβ zeolitef 140 ± 4 181 35 ± 0.83a Reaction conditions see Figure 4.8; b Defined as mole product generated per mole Snper hour extrapolated to 0 % conversion; c Errors are estimated using the Sn loadingdetermined with ICP; d Defined as the mole product generated per mole Sn after 6hreaction time; e The area of chemisorbed cyclohexanone is estimated with the peak areasof shifted C=O signals (marked in grey in Figure 4.10), normalized to the Sn-loadings.All deconvoluted spectra are given in Figure A.12. Errors correspond to the standarderrors of the peak fit given by OriginPro 8.5.1.; f No adsorption studies were performedwith the Sn precursor, which is a liquid at RT; g Snβ zeolite with a Sn-loading of 10 wt%was prepared in a two-step post-synthetic method.[164]
To rationalize the observed trend in reactivity we performed an IR adsorption study of
cyclohexanone, a well-established technique to probe interactions between metal sites and probe
molecules.[60,64,101,111,115,123,127]
For all three catalysts, we found significant IR signals in the carbonyl region (between
ca. 1600 and 1730 cm−1) after the adsorption of the ketone to the catalysts (red curves
in Figure 4.10). However, the three samples show very different signals. While the
signal of chemisorbed cyclohexanone (around 1650 cm−1) is most dominant for Sn/SiO2(700),
Sn/SiO2(700)-O2 has a main feature in the region of physisorbed carbonyl (around 1700 cm−1).
In Sn/SiO2(700)-O2-TMS both features are present with similar intensities.
In the literature, the presence and extent of shifted bands (relative to the band of
physisorbed cyclohexanone) has been linked to the activity of the SnIV-catalysts.[64,115,127]
However, in this study we do not find such a correlation, since the catalyst with the highest
intensity in this region (viz., Sn/SiO2(700)) does not show any catalytic activity.
One possible reason is that not only cyclohexanone, but also 2-butanol is present under
reaction conditions. Therefore, we also studied the adsorption of cyclohexanone after the
catalyst was exposed to the alcohol co-reagent. This pretreatment induced significant changes in
the observed spectra (blue curves in Figure 4.10). Almost all the intensity in the chemisorption
Silica-Grafted SnIV Catalysts in Hydrogen-Transfer Reactions 53
wavenumbers / cm-1
Sn/SiO2(700) Sn/SiO2(700)-O2 Sn/SiO2(700)-O2-TMS
Figure 4.10. FT-IR spectra of cyclohexanone adsorbed on to our three SnIV-catalysts. Red curvesrepresent the adsorption of only cyclohexanone, blue curves the adsorption of ketone after exposureto 2-butanol. (Spectra were background corrected.)
region disappeared for Sn/SiO2(700) and a distinct physisorption feature appeared. This
spectrum was very similar to the spectrum of Sn/SiO2(700)-O2 without exposure to the alcohol,
which indicates a ligand exchange at the SnIV-center. CHN analysis confirmed this hypothesis
(Table A.2). For Sn/SiO2(700)-O2 the shape of the spectrum remains unchanged and only
the overall intensity decreased, while the spectra with or without alcohol are very similar for
Sn/SiO2(700)-O2-TMS.
In a next step we deconvoluted these spectra in order to link the data with the catalytic
activity results (Figure A.12). As shown in Table 4.1 (Entry 3 and 4), it is possible to correlate
the peak areas of the activated carbonyl features with the catalytic activity. This allows a
qualitative understanding of the performance for Sn/SiO2(700)-O2 and Sn/SiO2(700)-O2-TMS,
but it does not explain the inactivity of Sn/SiO2(700).
Although our data does not unambiguously allow identifying the underlying reasons of the
observed catalytic behavior, we believe that possible explanations include differences in the
structure of the SnIV-sites and in surface hydrophobicity.
4.6. Discussion
Our observations in the IR study of cyclohexanone adsorption after exposure to alcohol clearly
indicate that it is the ability of the ketone to adsorb to the SnIV-centers that semi-quantitatively
describes the activity of the catalyst. However, we could not establish a similar correlation for
the ketone adsorption in the absence of alcohol. This is not entirely surprising, since the
presence of alcohol mimics realistic conditions in the MPV reduction.
54 Chapter 4
For MPV reductions on Snβ, the current benchmark material for Sn-catalyzed
heterogeneous reactions,[26,29,30,39,45,48,108,115,154,165] several computational studies suggested a
reaction pathway.[175,176] These mechanisms propose that both the ketone and the alcohol are
initially adsorbed to the SnIV-sites (Figure 4.11). Our results confirm this, since they imply
that under reaction conditions both the alcohol and the ketone simultaneously interact with
the SnIV-centers.
Figure 4.11. Proposed way of adsorption of cyclohexanone and 2-butanol on a partially hydrolysedSnIV-site in Snβ.[175]
In order to put the absolute activities of our materials in perspective, we also measured
the activity of Snβ in the MPV reduction (Figure A.14). It turned out that Snβ has a TOF
that is ca. 18 times higher compared to the TOF of our silylated material. This is remarkable
since the peak area of the perturbed carbonyl stretch in the IR spectrum of Snβ is significantly
smaller compared to the silylated material with co-adsorption of alcohol (Table 4.1, Entries 4
and 5; for spectra of Snβ see Figure A.12). This adsorption behavior does not explain the high
activity of Snβ and strongly suggests that we need to take additional parameters into account
such as (i) different active site structures and (ii) confinement effects in the pores of Snβ.
An indication for structural differences is given by the differences found in the UV-Vis
spectra of our grafted materials compared to Snβ, pointing towards more strain (i.e., higher
reactivity) for the zeolite-based SnIV/SiO2 catalyst. Interestingly, our results show that open
(SiO)xSn(OH)4-x sites are not necessarily more active than closed sites as is the current
hypothesis for Snβ. However, based on our data, we cannot exclude that SiOH groups adjacent
to open sites in Snβ could play a major role in activating the substrate(s). Nevertheless, it is
remarkable, that our calcined material (with Sn hydroxyls) is able to activate two times more
Silica-Grafted SnIV Catalysts in Hydrogen-Transfer Reactions 55
ketone but is 23 times less active compared to Snβ.
In addition to the nature of the active sites, confinement effects have been shown to
be important for various zeolite-catalyzed reactions. Most often they are traced back to a
greater stabilization of the transition state, relative to the reactants by weaker van der Waals
forces.[110,166] Most likely, this effect is complemented by an adsorption-based confinement
effect. Adsorption to active sites inside zeolite pores is composed of two components:[124,125]
(i) chemisorption of the molecule to the active site and (ii) van der Waals interactions with
the cavity. Clearly, this confinement effect increases the local concentration of the reactants.
Additionally, it needs to be considered that both reactants and products in our reaction
consist of an alcohol and a ketone. Based on the arguments above, the adsorption of ketone
to the active site is possible in the presence of an alcohol. We expect the chemisorption
interactions for the reactant and product ketone to be of similar magnitude. However, the van
der Waals interactions increase with the number of C-atoms in the molecule.[124–126] Since the
product ketone (2-butanone) contains less C-atoms than the reactant ketone (cyclohexanone),
the concentration of butanone at the SnIV-sites will be far lower than the concentration of
cyclohexanone compared to the silica materials. Therefore, the active sites are less likely to
be covered by the product ketone and the reverse reaction is less likely in Snβ. Both factors
should increase the net forward reaction rate significantly.
4.7. Conclusions
In this work we formed isolated SnIV-sites on the surface of amorphous silica via a simple
grafting procedure using Sn(NMe2)4 as volatile Sn-precursor. We investigated the structure of
the prepared materials with UV-Vis and FT-IR spectroscopy, as well as with solid-state 13C-
NMR experiments. Our results indicate that opening of strained siloxane bridges is initiated
by the amine-containing Sn-precursor. After a thermal treatment in air, the resulting Sn-OH
and Si-OH groups were modified with trimethylsilyl groups in order to increase the surface
hydrophobicity.
We furthermore measured the activity of our materials in the Meerwein-Ponndorf-Verley
reduction of cyclohexanone with 2-butanol. Only the calcined and silylated material showed
56 Chapter 4
activity, which we attribute to changes in the structure of the SnIV-sites and hydrophobicity,
respectively. Additionally, we established a semi-quantitative relationship between the IR
spectra of adsorbed cyclohexanone in the presence of 2-butanol. This clearly indicates that the
activity of the catalyst is related to the ability to chemisorb ketone under reaction conditions.
When comparing the activity of our silica-based materials to Snβ, the state-of-the-art
material for this reaction, we find a significantly lower activity for our materials. The underlying
reasons for this behavior are not yet unambiguously established. Our work demonstrates that
the development of model-catalysts is a convenient and feasible route to investigate which
features contribute to the performance of an active Lewis acid catalyst. Based upon this work
we suggest that confinement effects might significantly influence catalytic activity in the studied
system. Even though it is not possible to quantitatively asses this contribution to activity, more
targeted experimental approaches will help to further elucidate this effect in the future.
Chapter 5
Confinement Effects in Hydrogen-TransferReactions on Sn Sites in Porous Silica Materials
All experiments in this chapter were conducted by the author.
5.1. Introduction
In zeolite catalysis reactions take place inside micropores and the confining environment plays
a key role in understanding the activity as well as the selectivity of the catalyst for a series of
reactions.[113,177] While early ideas revolve around steric constraints of the pore, in recent years
a more detailed picture of these effects has emerged.[110,114,178–182] It involves the adsorption
of the molecule into the pore, where it is initially stabilized by van der Waals interactions,
its chemisorption to the active site and finally the reaction itself. Thermodynamically these
processes are governed by their respective free energies and depending on the molecules,
catalysts and the studied reactions confinement can affect reaction rates differently.
A reaction that has been demonstrated to be catalyzed by zeolite-based materials
is the Meerwein-Pondorff-Verley (MPV) reduction (see Figure 4.8), which is a promising
transformation for several biomass-relevant reaction schemes, and a frequently used tool in
organic synthesis.[39,108,175,176,183,184] In Chapter 4 we separated the contributions to the activity
of benchmark Sn-doped β-zeolite (Snβ) in the MPV reduction of cyclohexanone with 2-butanol
by grafting SnIV-sites, similar to those encountered in the zeolite framework, to silica.[185]
However, the obtained sites showed activities of more than an order of magnitude lower
compared to Snβ even after targeted thermal and chemical post-treatments, which led to slight
activity increases. Besides possible differences in the local structure of the active sites and
differences in hydrophobicity of the materials, we suggested that confinement effects in the
zeolite pores are responsible for the significantly higher activity of Snβ in this reaction. To
confirm this contribution, we extend our activity studies to SnIV-sites grafted to a mesoporous
58 Chapter 5
support (MCM-41), which represents a pore system that stands in between microporous β
zeolite and amorphous silica (i.e., a surface support where confinement effects can be excluded).
We prepare two MCM-41 supports with different pore sizes and expect that the resulting Sn
catalysts show activities that are higher than our Sn-silica-system, but lower than Snβ.
5.2. Experimental
5.2.1. Material Synthesis
Post-synthetic incorporation of Sn was performed in two steps as described in Chapter 3.[159]
MCM-41 was synthesized according to a procedure described elsewhere.[186] n-
Alkyltrimethylammonium bromides of different alkyl chain lengths from C12 and C16 were
used as template. The template was dissolved in 120 g of deionized water to yield a 0.055 M
solution, and 8.2 g of aqueous ammonia (28 wt%, 0.14 mol) was added to the solution. While
stirring, 10 g of tetraethoxysilane (0.05 mol) was added slowly to the surfactant solution over
a period of 15 min resulting in a gel with the following molar composition: 1 TEOS : 0.152 n-
alkyltrimethylammonium bromide : 2.8 NH2 : 141.2 H2O. The mixture was stirred for one
hour, then the white precipitate was filtered and washed with 100 mL of deionized water. After
drying at 363 K for 12 h, the sample was heated to 823 K (rate: 1 K min-1) in air and kept at
this temperature for 5 h to remove the template.
The grafting of Sn to MCM-41 was performed as reported in Chapter 4. The support was
dehydrated at 700 °C (16 h) at 15 µbar dynamic vacuum and stored inside of a glove box
(< 1 ppm O2 and H2O) in order to avoid re-adsorption of water. Sn(NMe2)4 (Sigma-Aldrich,
two times distilled before use, colorless) was then deposited onto 300 mg of dried support at ca.
20 µbar dynamic vacuum. The transfer phase (45 min) was followed by a reaction phase at RT
(30 min) and a mild thermal post-treatment at 50 °C and 15 µbar dynamic vacuum (1 h) in
order to evaporate excess of transferred Sn(NMe2)4 (material denoted as Sn/MCM-41). Samples
were calcined in an air flow at 550 °C for 5 h (material denoted as Sn/MCM41-O2).
Confinement Effects in Hydrogen-Transfer Reactions on Sn Sites in Porous Silica 59
5.2.2. Characterization Methods
The Sn-content was quantified with ICP-OES (Perkin Elmer Optima 2000) after digestion of
the samples with HF (48 %, Sigma-Aldrich). FT-IR and UV-Vis analysis was performed as
described in Chapter 2.
Powder diffraction patterns were recorded on a Bruker D8 advance diffractometer using
Cu-Kα1 radiation and a Lynxeye detector.
N2 sorption measurements were performed on a Micromeritics Gemini VII (Version 2.00) at
77 K. Samples were degassed under vacuum at 350 °C for 3 h prior to every sorption analysis.
The surface area was calculated using the Brunauer-Emmett-Teller (BET) theory.
5.2.3. Catalytic Experiments
The Meerwein-Ponndorf-Verley reductions of cyclohexanone were performed in 10 mL thick
wall tube reactors capped with a PTFE/silicon seal capable of holding 15 bar over-pressure. In
a typical reaction, 1 mmol of cyclohexanone and 60 mmol of alcohol (ethanol, 2-butanol, iso-
propanol and 2-pentanol) were added to the appropriate amount of catalyst (corresponding to
0.5 mol% Sn relative to the ketone). The vessel was heated at a constant temperature (80 °C)
for 6 h under vigorous stirring (500 rpm). Aliquots were taken periodically and the reactant
and product were quantified against the internal standard biphenyl by GC-FID analysis (30 m
FFAP column).
5.3. Results and Discussion
Snβ was prepared via post-synthetic incorporation of SnIV into dealuminated β-zeolite (see
Chapter 3). For the MCM41-grafted catalysts, we first synthesized two mesoporous MCM-41
materials with different pore sizes (2.5 and 3.3 nm; Table A.4), applying a procedure by Grün
and co-workers. In order to form SnIV-sites on these materials, we then followed the preparative
steps that we reported for the synthesis of silica-grafted SnIV-sites (see Chapter 4). We
dehydrated the materials at high temperatures (700 °C) to provide a surface that predominantly
consists of non-interacting isolated silanols, prior to contacting it with an appropriate amount
of Sn-precursor (materials obtained denoted as Sn/MCM41-25 and Sn/MCM41-33). We then
60 Chapter 5
subjected the materials to a calcination in air in order to remove the organic parts of the
precursor and obtain SnIV-sites that are exclusively coordinated by oxygen atoms. Opposed
to previous work, we omitted the silylation step to avoid blocking of the pores. Comparing
the UV-Vis spectra of Sn/MCM41-25 and Sn/MCM41-33 with SnO2 and Snβ shows that
the MCM41-grafted Sn catalysts predominantly contain tetrahedrally coordinated isolated Sn
centers (Figure 5.1).
Figure 5.1. UV-Vis spectra of different Sn-silica materials. (A) Snβ, (B) SnO2, (C) Sn/MCM41-25,(D) Sn/MCM41-36, (E) Sn/SiO2.
Figure 5.2. Catalytic activity measurements of different Sn-silica materials in the Meerwein-Ponndorf-Verley reduction of cyclohexanone with 2-butanol. Experimental conditions: 1 mmol ofcyclohexanone, 60 mmol of 2-butanol, 0.5 mol% Sn (relative to ketone), 80 °C, 6 h reaction time.
To probe the catalytic activities of the prepared materials we then performed MPV-
Confinement Effects in Hydrogen-Transfer Reactions on Sn Sites in Porous Silica 61
reductions of cyclohexanone with 2-butanol. We observe that for 2-butanol as reducing agent,
the activity of Sn/MCM41-33 is very similar to silica-grafted SnIV-sites, i.e., a surface catalyst
(Figure 5.2). However, for the same alcohol the activity more than doubles when the pore
diameter decreases to 2.5 nm (i.e., for Sn/MCM41-25) and is more than an order of magnitude
higher for microporous Snβ (pore diameter approx. 0.7 nm).
This observation clearly confirms that the pore size of the catalyst, (i.e., confinement)
influences the reaction rate of SnIV/SiO2-based catalysts in the Meerwein-Ponndorf-Verley
reduction of cyclohexanone with 2-butanol. In line with the arguments in our previous work
with silica-grafted SnIV-sites, we propose that an enhanced reverse reaction at active sites in
less-confined environments is one possible explanation for the observed smaller reaction rates for
MCM41- and silica-grafted SnIV-sites compared to Snβ. The underlying reason are stabilizing
Van der Waals forces in confining environments, which stabilize the larger substrate ketone
(cyclohexanone, C6) more strongly than the product ketone (2-butanone, C4), which would
lead to a higher coverage of the active sites with the reactant than with the product, and
hereby to an acceleration of the forward reaction and a slowdown of the reverse reaction. In
addition, other confinement effects could play a role. As such, we can not exclude an impact
of confinement on other thermodynamic contributions, such as the reaction entropy or reaction
enthalpy. Lastly, differences in the nature of the active sites or changes in the hydrophilicity of
the support material might also cause the observed trend in reactivity.
5.4. Conclusions
The results presented in this work indicate that confinement effects are one determinant
that contributes to the high activity of Snβ in the Meerwein-Ponndorf-Verley reduction of
cyclohexanone with 2-butanol. This is demonstrated by an increase in activity with decreasing
pore diameters for SnIV/SiO2-based catalysts (Sn/SiO2, Sn/MCM-41, Snβ). We attribute the
observed confinement effect with a possible change of the adsorption behaviour of substrate
and product molecules in confining environments. In the future we want to arrive at a solid
theoretical framework to describe these interactions quantitatively. We expect similar effects to
be present in different zeolite catalyzed reactions, and that more targeted approaches stimulated
62 Chapter 5
by the ideas presented here might help to rationally improve operating conditions for various
catalytic system
Chapter 6
Conclusions and Outlook
Accompanied by the statement of the principles of green or sustainable chemistry, a logical
transition from homogeneous to heterogeneous Lewis acid catalysts has been initiated, providing
catalysis researchers with new scientific challenges to overcome. As such, the exploration of
novel material solutions and an enhanced understanding of the obtained catalysts is necessary
and will sustain a long-lasting stream of academic research looking into basic aspects of
heterogeneous catalyst preparation, characterization, and testing.
Prior to the commencement of this thesis, the commercial realization of the state-of-the-art
solid Lewis acid Sn catalyst – Snβ zeolite – was hampered by its tedious and lengthy synthesis
procedure, and the identification of the activity characteristics of Snβ was complicated by its
complex structure. In order to fully utilize the commercial potential of advanced materials such
as Snβ, a few critical tasks need to be addressed. First, is the design of simple and scalable
synthesis procedures resulting in materials with comparable or higher activity, and second, is
the need for an in-depth understanding of the material reactivity in order to provide a rationale
for the design of materials with improved activity.
6.1. Conclusions
Simple and Scalable Synthesis. In this thesis, a simpler and "greener" route for the preparation
of Lewis acidic Snβ zeolite is described, consisting of the post-synthetic incorporation of Sn
atoms into dealuminated β-zeolite via solid-state-ion-exchange (Chapter 2). The obtained
material demonstrates similar to higher activity and selectivity compared to Snβ prepared via
hydrothermal synthesis, and the increased amount of incorporated metal significantly raises
the achieved space-time-yield (defined as gproduct kg−1catalyst h-1). In addition, the developed
procedure requires less synthesis time and avoids the formation of undesired toxic waste. As
such, the work presented in this thesis starts addressing one of the key challenges in shifting
64 Chapter 6
from homogeneously to heterogeneously based Lewis acid catalysts, that is the design of simpler
and environmentally benign synthesis routes to active materials. Based on this work, we expect
that the industrial realization of Snβ or a similar material (e.g., with different types of metals
or zeolite frameworks) will be strongly promoted.
Hydrophilicity. The Baeyer-Villiger oxidation of cyclohexanone with hydrogen peroxide is
one out of many potential industrial applications of Snβ zeolite. It is known from literature that
the high activity and selectivity of Snβ in this reaction arises from the selective interaction of the
active SnIV-sites of the catalyst with the carbonyl groups of substrate molecules. However, this
chemoselectivity also causes catalyst inhibition, most likely as a result of competitive adsorption
of solvent (water) and product molecules (lactone) to the active SnIV-sites, as we demonstrate
in Chapter 3. With this background, we modify the synthesis protocol of our newly developed
post-synthetic synthesis route in order to alter the amount of framework silanols, and hereby the
hydrophilicity of Snβ, aiming toward improving the activity in Baeyer-Villiger oxidations. We
learn that an optimum in hydrophilicity exists, reflecting the need to allow strong coordination
of the substrate (cyclohexanone) to the active SnIV-sites but to avoid inhibition by solvent and
product molecules. As such, the work presented in this chapter shows that flexible synthesis
protocols, such as the one from our post-synthetic synthesis route, allow targeted modifications
to the catalyst that may lead to optimized overall activities. Furthermore, our work clearly
identifies the hydrophilicity of the β-framework as one determinant of the activity of Snβ, and
therefore gives fundamental insights on material reactivity.
Catalytic Model-System. In the second part of this thesis (Chapter 4), we choose the
utilization of a catalytic model-system for SnIV/SiO2-based catalysts as an alternate pathway
to enhance our understanding of the reactivity of complex catalytic systems such as Snβ zeolite.
This is approached by grafting isolated SnIV-sites on to silica and gradually modifying the
surface-anchored metal sites with a thermal and a chemical post-treatment. Implications of
these treatments on catalyst structure and activity are followed with various characterization
techniques and catalytic tests, respectively. With this methodology we are able to distinguish
different contributions to the reactivity of SnIV/SiO2-based catalysts (including active site
speciation, material hydrophilicity and confinement effects), and demonstrate that catalytic
Conclusions and Outlook 65
model-systems are a feasible tool to untangle the material properties that determine the
performance of a Lewis acid catalyst. Moreover, our results indicate that the grafting of
our amine-containing Sn precursor (Sn(NMe2)4) leads to opening of siloxane bridges on the
thermally pretreated silica surface, which presents an interesting observation in view of surface
organometallic synthesis concepts.
Confinement. The results from our catalytic model-study (Chapter 4) point towards
confinement effects as one reason for the difference in activity between benchmark Snβ and
silica-grafted SnIV-sites. In order to confirm this contribution we extend our model-study by
grafting SnIV-sites on to a mesoporous MCM-41-support (Chapter 5). In line with our previous
findings, we show that the activity of SnIV/SiO2-based catalysts in the Meerwein-Ponndorf-
Verley reduction of cyclohexanone with 2-butanol strongly depends on the pore size of the
employed SiO2-based-support. We attribute this trend in reactivity with an adsorption-based
confinement effect, which most probably results from a suppressed reverse reaction in more
confined environments.
6.2. Outlook
The work described in this thesis has led to achievements in the large-scale applicability of
state-of-the-art Snβ zeolite, and the distinction of material properties that govern the activity
of Lewis acid SnIV/SiO2-based catalysts. Nonetheless, there are several aspects that need
further dedication.
For example, the synthesis of a catalyst for industrial purposes is always done in the context
of the overall process optimization, giving consideration to the utilization of the catalyst in a
flow reactor in order to facilitate continuous operation and to reduce catalyst recuperation
cost. For systems, where consecutive reactions of the product cause a selectivity decrease
at increasing conversion, the overall performance of the catalyst can be expected to increase
under continuous flow conditions. Emphasizing the industrial importance of the Baeyer-Villiger
oxidation of cyclohexanone to ε-caprolactone (a momomer intermediate) and stimulated by
the promising results with our post-synthetically prepared Snβ, we believe that it is of general
interest to include aspects of reaction engineering into future work on this system. A preliminary
66 Chapter 6
experimental test investigating the Baeyer-Villiger oxidation with 10 wt% Snβ under continuous
operation shows stability of the catalyst over 4 days (Figure 6.1), although at lower conversion
and yield levels than those achieved under batch conditions, which requires further optimizing
studies. In addition, the possibility to utilize binders as catalyst carrier would be an interesting
aspect to include into future work, aiming toward higher mechanical stability.
Figure 6.1. Cyclohexanone conversion and ε-caprolactone yield in a continuous Baeyer-Villigerexperiment. Experimental conditions: cyclohexanone in 1,4-dioxane (0.33 M), 80 °C, H2O2 (50 wt%aq. solution, H2O2/ketone = 0.8), 10 wt% Snβ.
Another example is the remaining question if the different proposed types of active SnIV-sites
for Snβ (i.e., open or closed) correlate with different individual activities. Indeed, our catalytic
model-study (Chapter 4) suggests that open sites ((SiO)xSn(OH)4-x) are not necessarily more
active than closed sites as is the current hypothesis for Snβ. Our Baeyer-Villiger study
(Chapter 3), on the other hand, indicates that both open and closed sites are active, which
questions that site structure has a prevailing influence on activity in this reaction system. The
relation between active site structures and catalytic activities is, hence, strongly dependent on
the characteristics of individual reaction systems. We believe that this topic, which has already
been the object of several investigations, will continue interesting academic research in order to
improve the rational design of novel materials. One promising approach in this context is the
conjunction of experimental and computational studies based on different Snβ catalysts and
probe reactions. The comparison of structure-activity information gained for various reaction
systems will reveal contrary and/or similar trends, which will help to further elucidate the
catalytic behavior of Snβ in different types of reactions and improve our understanding of the
Conclusions and Outlook 67
impact of site structure on catalytic activities for this catalyst. Another intriguing approach in
this field would be to deploy a methodology introduced by Notestein and co-workers, involving
the "counting" of active sites by selective poisoning of TiIV-sites with hydroxyl groups, which
results in "true turn over frequencies" that are normalized on an active-site-basis.[187] With an
appropriate poisoning agent, this could be applied to SnIV-sites with hydroxyl groups (i.e., open
sites) in different Snβ catalysts with varying active site distributions and activities, respectively,
and allow the establishment of structure-activity-relationships.
Moreover, in terms of understanding the influence of catalyst hydrophilicity on catalyst
activity in the Baeyer-Villiger oxidation of cyclohexanone with hydrogen peroxide, this thesis
has only been exploring the implications on the observed performance, but not in view to
possible underlying mechanistic reasons. Thus, it could be envisaged to expand the scope of
future research considering this aspect. As such, it is conceivable that silanol groups do not only
steer the hydrophilicity, and hereby influence occuring catalyst inhibition but also participate
in the mechanistic cycle, possibly by activating the peroxide substrate.
Not least, our work demonstrates various pathways by which a metal precursor can react
with the functional groups found on silica surfaces (i.e., via reaction with silanol groups
and/or via reaction with siloxane bridges of the type ≡Si–O–Si≡). An important question
in this regard is which aspects of the metal precursor govern such surface anchoring and
subsequent restructuring mechanisms. We believe that targeted experimental studies with
various metal precursors (and ligands) will help to further elucidate this interesting material
synthesis question and result in a better understanding of catalyst preparation by means of
grafting and impregnation chemistry.
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Appendix A
Annexes
Chapter 3
Figure A.1. ε-Caprolactone yield over time for different initial lactone concentrations in the BVoxidation of cyclohexanone with H2O2. Experimental conditions: cyclohexanone in 1,4-dioxane(0.33 M), H2O2 (50 wt% aq. solution, H2O2/ketone = 1.5), 10Snβ-OH25, desired amount of ε-Caprolactone.
Table A.1. Physicochemical properties of differentSnβ catalysts.
Entry Catalyst SBETa Si/Snb H2O
cads
[m2 g−1] [–] [mmol g-1]
1 1Snβ-F30 718 200 7.902 1Snβ-F200 570 173 2.273 1Snβ-F400 545 192 2.264 1Snβ-HT 610 182 1.675 1Snβ-OH25 616 217 12.06 1Snβ-OH300 637 227 10.1a Brunauer-Emmett-Teller surface area.[131]
b Determined by ICP-OES. c Quantity of wateradsorbed at p/p0 = 0.82.
88 Chapter A
Figure A.2. FT-IR spectra of different Snβ catalysts, normalized to the Si-O-Si overtones of thesilica framework between 2200 and 1400 cm-1.
Figure A.3. Diffuse Reflectance UV-Vis measurements of different Snβ catalysts and SnO2 as areference.
Annexes 89
Chapter 4
1H NMR 13C NMR
Figure A.4. The formation of HNMe2 during the grafting was confirmed by analyzing the liquidnitrogen-trapped effluent of the synthesis reactor with 1H- and 13C-NMR. 1H NMR (500 MHz, CDCl3):δ 2.39 (6H, s); 13C NMR (500 MHz, CDCl3): δ 38.65 (2C, s), 77.01 (1C, t; solvent).
TMS νCH
Figure A.5. After grafting Sn(NMe2)4 on to TMS-SiO2(700) (i.e., silica with capped silanols of thetype ≡SiOSiMe3, –SiMe3 = TMS), we observed additional bands in the C-H IR region (see Sn/TMS-SiO2(700)). In combination with the measured Sn loading of Sn/TMS-SiO2(700), this confirms theparticipation of siloxanes in the grafting reaction. Additionally, by comparing the C-H bands ofSn/TMS-SiO2(700) (after subtraction of TMS-SiO2(700); see red curve) with Sn/SiO2(700), a strongoverlap could be noted. This suggests that similar species are formed when Sn(NMe2)4 reacts withsilanols or siloxanes, respectively.
90 Chapter A
Table
A.2.
Elementalanalysis
dataofthe
differentprepared
materials. a
EntryMaterial
SnH
CN
C/Sn
H/Sn
N/Sn
[wt%
][wt%
][wt%
][wt%
]exp.(calc.)
exp.(calc.)exp.(calc.)
1Sn/SiO
2(700) b,c4.0±
0.12.4±
0.10.7±
0.02d
1.6±
0.04d
6.0/5.6(6)
21.5(20.2)
3.4/3.2(3)
2TMS-SiO
2(700) dN/A
1.0±
0.030.2±
0.0050.05
±0.001
3.1(3)
10.3(9)
0.2(0)
3Sn/T
MS-SiO
2(700) e0.9±
0.021.8±
0.10.5±
0.010.5±
0.019.0
(8)42.6
(24)4.5
(4)4
Sn/SiO2(700) -O
24.0±
0.10.06
±0.002
0.04±
0.001(0.40)
0.05±
0.001–
––
5Sn/SiO
2(700)+
2-butanol f4.0±
0.11.0±
0.031.0±
0.030.5±
0.019.5
(12)26.4
(27)1.1
(0)aC
HN
andIC
P-OES
analyseswere
performed
inorder
todeterm
ineexperim
entalmolar
ratios((C
,H,N
)/Sn).These
valuesare
compared
with
calculatedratios
basedon
ourhypothesized
activesite
structures.bT
hecalculated
valuesare
basedon
thehypothesis
thatSn(N
Me2 )4
isbond
tothe
silicasurface
byone
singleSn–O
bondwhile
threeam
inegroups
(–NMe2 )
areattached
tothe
Snatom
.cT
he(C
,H,N
)/Snratios
aredeterm
inedunder
theassum
ptionthat
only≡Sn(N
M2 )3
speciesare
presentOR
thatalso≡SiN
Me2
speciesare
presentwith
amolar
amount
of0.071mmolg
-1(derived
fromthe
amount
ofSnthat
isgrafted
tosilylated
silica).dM
olarratios
correspondto
(C,H
,N)/≡
SiOH
ratios.e(C
,H,N
)/Snratios
arecalculated
aftersubtracting
themolar
amounts
ofC,H
,Nmeasured
onTMS-SiO
2(700) .fSn/SiO
2(700)after
theadsorption
of2-butanol.Calculated
ratiosare
basedon
theassum
ptionthat
amine
ligandsare
replacedby
2-butoxyligands.
Annexes 91
*
*
**
(A) (B)
(C)
Figure A.6. In addition to our 13C NMR measurements, the presence of grafted amine species≡SiNMe2 (see B) is indicated by two C-H bands in the IR spectrum of Sn/TMS-SiO2(700) (markedwith blue star in (A)). These sites are formed when Sn(NMe2)4 reacts with siloxane bridges. However,their C-H bands are not clearly visible in the IR spectrum of Sn/SiO2(700), due to the lower relativeconcentration of such sites in this material. More evidence for this was found in the IR spectrum ofTMS/SiO2(700). During the silylation step (C) mostly ≡SiOTMS, but also ≡SiN(TMS)Me speciesare formed in small amounts. Computationally predicted νCH frequencies for these sites appear athigher wavenumbers compared to ≡SiNMe2. This agrees with the position of C-H bands observedfor TMS-SiO2(700) (marked with red star in (A)).
Table A.3. Asymmetric and symmetric C–H stretch vibrations calculated withB3LYP/6-31 G8(d,p).a
Entry Structure Asymmetric stretch ν(C,H) Symmetric stretch ν(C,H)[cm-1] [cm-1]
1 ≡SiNMe2 2985 30402 ≡SiN(TMS)Me 3000 3069a For absolute values a scaling factor of 0.9614 has to be applied.
92 Chapter A
Figure A.7. More intense signals in the IR region of C-H stretches when HNEt2 is grafted togetherwith Sn(NMe2)4 (see red curve) indicates that HNEt2 reacts with siloxanes. The more intense bandsdo not arise from a higher amount of grafted precursor, since the Sn loading of the material is identicalto Sn/SiO2(700). This observation shows that both of our hypothesized pathways for the opening ofsiloxanes ((A)-(B) or (C)) are possible. However, pure HNMe2 does not react with SiO2(700). Thus,we believe that under our grafting conditions, amines such as HNR2 (R = Me, Et), only react withsiloxanes when they are Lewis acid activated by Sn(NMe2)4.
!!
(A) (B)
(C)
Figure A.8. DRIFT spectra from the thermal treatment of pure SiO2(700) with (black curves) andwithout (red curves) humidity in the air stream (gas stream was passed through a water trap, ca.3.2 % water in the stream) indicate opening of siloxanes by water, as shown by the shoulder at ca.3660 cm-1 (B). Additionally, a broad signal appears at around 3550 cm-1 (C) at lower temperatures,which signalizes the presence of physisorbed water on the surface that is released during heating.
Annexes 93
Figure A.9. Quantification of the SnOH IR signal of Sn/SiO2(700)-O2 (red) with the SnOH IRsignal of a reference Sn hydroxyl compound – tricyclohexyltinhydroxide (blue) (IR spectrum taken ofa physical (homogeneous) mixture with SiO2(700) to allow normalization to the Si-O-Si overtones ofthe silica framework).
Figure A.10. Our second functionalization step consist of the silylation of Sn/SiO2(700)-O2 with N,N -bis(trimethyl)silylamine (TMS2NMe). The IR spectrum of Sn/SiO2(700)-O2-TMS shows the conversionof both types of hydroxyls (SiOH, SnOH), together with the introduction of C-H stretches from theorganic substituents of the silylating agent.
94 Chapter A
Figure A.11. In the UV-Vis spectra of Sn/TMS-SiO2(700) and Sn/TMS-SiO2(700)-O2 there is no signof the formation of SnOx species.
Sn/SiO2(700) Sn/SiO2(700)-O2
Sn/SiO2(700)-O2-TMS
Sn-β zeolite
Figure A.12. Deconvolutions of the IR regions of adsorbed cyclohexanone after exposure to 2-butanol(performed with origin 8.5.1). The fitted curves are shown in black (individual and cumulative peakfits), the original curves are depicted in red. All y-axes are scaled equally. Areas from peaks between1600 and 1690 cm-1 were used for the analysis. Sn/SiO2(700), R2 = 0.99898; Sn/SiO2(700)-O2, R2 =0.99988; Sn/SiO2(700)-O2-TMS, R2 = 0.99796; Snβ zeolite, R2 = 0.99932.
Annexes 95
Figure A.13. FT-IR spectra of deuterated acetonitrile adsorbed on to our three SnIV-catalysts andSnβ (spectra are background corrected). Sn/SiO2(700)-O2 and Sn/SiO2(700)-O2-TMS, our catalyticallyactive materials (see Figure 4.9 and Table 4.1, Entries 3 and 4), show signals at around 2310 cm-1,which is characteristic of Lewis acidic metal sites as shown before for Snβ. This observation pointstowards strong similarities between our SnIV-sites and the sites in Snβ.[101,111]
Figure A.14. Catalytic activity measurements of all Sn-silica materials (A) – (C) and Snβ zeolite(D) for the MPV reduction of cyclohexanone with 2-butanol. The Turnover Number (TON) is definedas the moles of product generated per mole Sn.
Annexes 97
Chapter 5
Table A.4. Properties of MCM-41 samples prepared in heterogeneousmedium with n-alkyltrimethylammonium bromides as templates, followinga procedure from Grün and co-workers.[186]
Entry Template SBETa SBET
b Pore diameterc,e Pore diameterd,e
[m2 g−1] [m2 g−1] [nm] [nm]
1 C12TMABr 1549 1424 2.6 2.52 C16TMABr 1400 1269 3.6 3.3a Brunauer-Emmett-Teller surface area.[131] b Brunauer-Emmett-Teller surfacearea after a thermal treatment at 700 °C under vacuum (approx. 20 µbar).[131]
c Pore diameter. d Pore diameter after a thermal treatment at 700 °C undervacuum (approx. 20 µbar). e Determined from X-ray diffraction data, ascalculating the pore size from nitrogen sorption data leads to an underestimationof the pore sizes for MCM-41 materials.[186] Calculated using the d100-value andassuming a pore wall thickness of 1.0 nm: d = 2d100√
3 − 1.0 with d100 = 1.542sinθ .
Appendix B
List of Publications
Doctoral Publications
F. Göltl, S. Conrad, P. Wolf, P. Müller, J. Wheeler, R.J. Hamers, G. Kresse, I. HermansExperiment to Understand the Large Stoke Shift for Isolated Copper Centers in Zeolitesto be submitted
S. Conrad,* P. Wolf,* P. Müller, H. Orsted, C. Hammond, I. Hermans* these authors contributed equallyInsights into the Baeyer-Villiger Oxidation of Cyclohexanone with H2O2 catalyzed by Snβsubmitted (Chapter 3)
S. Conrad, R. Verel, C. Hammond, P. Wolf, F. Göltl, I. HermansSilica-Grafted SnIV-Catalysts in Hydrogen-Transfer ReactionsChemCatChem 2015, 7, 3188-3403 (front cover) (Chapter 4)
P. Wolf, C. Hammond, S. Conrad, I. HermansPost-Synthetic Preparation of Sn-, Ti-, Zr-β: A Facile Route to Water-Tolerant, Highly ActiveLewis Acidic ZeolitesDalton Trans. 2014, 43, 4515-4519
C. Aellig, D. Scholz, S. Conrad, I. HermansIntensification of TEMPO-Mediated Aerobic Alcohol Oxidations under Three-Phase-FlowConditionsGreeen Chem. 2013, 15, 1975-1980
P. Mania, S. Conrad, R. Verel, C. Hammond, I. HermansThermal Restructuring of Silica-Grafted –CrO2 and –VOCl2 speciesDalton Trans. 2013, 42, 12725-12732
C. Hammond, M. Schümperli, S. Conrad, I. HermansHydrogen Transfer Processes Mediated by Supported Iridium Oxide NanoparticlesChemCatChem 2013, 5, 2983-2990
100 Chapter B
C. Hammond, S. Conrad, I. HermansSimple and Scalable Preparation of Highly Lewis Acidic SnβAngew. Chem. Int. Ed. 2012, 51, 11736-11739 (back cover) (Chapter 2)
C. Hammond, S. Conrad, I. HermansOxidative Methane UpgradingChemSusChem 2012, 9, 1668-1686
Pre-Doctoral Publications
A. Gänzler, M. Casapu, A. Boubnov, O. Müller, S. Conrad, H. Lichtenberg, R. Frahm, J.-D.GrunwaldtOperando Spatially and Time-Resolved X-Ray Absorption Spectroscopy and InfraredThermography during Oscillatory CO OxidationJ. Catal. 2015, 328, 216-224
A. Boubnov, A. Gänzler, S. Conrad, M. Casapu, J.-D. GrunwaldtOscillatory CO Oxidation over Pt/Al2O3 Catalysts studied by In Situ XAS and DRIFTSTop. Catal. 2013, 56, 333-338
Appendix C
Presentations
International Symposium on Activation of Dioxygen and Homogeneous Oxidation CatalysisMadison (WI), United States, 2015Poster: "Silica-Grafted SnIV-Catalysts in Hydrogen-Transfer Reactions"S. Conrad, R. Verel, C. Hammond, P. Wolf, F. Göltl, I. Hermans
125th International Summer Course for Chemists and EngineersBASF-SE, Ludwigshafen, Germany, 2014Talk: "Toward a Better Understanding of the Activity of Site-Isolated Lewis Acid Catalysts"S. Conrad, C. Hammond, I. Hermans
Advisory Board Meeting - Chemistry DepartmentUniversity of Wisconsin-Madison, Madison (WI), United States, 2014Poster: "Influence of Surface Polarity and Cavity Effects on Hydrogen-Transfer-Reactions"S. Conrad, C. Hammond, I. Hermans
Catalysis Club of Chicago - Spring SymposiumBP Naperville Campus, Chicago (IL), United States, 2014Poster: "Influence of Surface Polarity and Cavity Effects on Hydrogen-Transfer-Reactions"S. Conrad, C. Hammond, I. Hermans
Appendix D
Cover Gallery
Based on the work in this thesis, two covers were published, which are presented on the followingpages. Each cover comes along with a short explanation to illustrate the scientific background.
104 Chapter D
This (back) cover relates to a communication published in the Angewandte Chemie, and isbased on Chapter 2. This publication is about the heterogeneous catalyst Snβ, which iscurrently prepared in a complicated hydrothermal synthesis, which has several hurdles thatprevent its industrial implementation. A new convenient preparation of Snβ by solid-state-ion-exchange is reported in this communication, which profits from less time and synthetic skillsdemanded. Additionally, the produced catalyst has more favorable catalytic properties, givingspace-time-yields over one order of magnitude higher than previously observed.
Cover Gallery 105
The heterogeneous catalyst Sn-b is currently prepared in a complicated hydro-thermal synthesis, which has several hurdles that prevent its industrial implemen-tation. In their Communication (10.1002/anie.201206193), Hermans and co-work-ers report a convenient preparation of Sn-b by solid-state ion exchange. It requiresmuch less time and synthetic skill, and the product has more favorable catalyticproperties, giving space-time-yields over one order-of-magnitude higher thanpreviously observed.
Dateiname: W208116e Pagina: 2Pfad: l:/daten/verlage/vch/ach/hefte/pool/ Seite: 1 te von 1Status Neusatz Umfang (Seiten): 1Datum: 42 KW., 12. Oktober 2012 (Freitag) Zeit: 7:14:41 Uhr
106 Chapter D
This (front) cover, based on results presented in Chapter 4, illustrates silica-supported SnIV
sites that are tuned to be active in the Meerwein-Ponndorf-Verley reaction of cyclohexanonewith 2-butanol. In the corresponding article published in ChemCatChem, the catalyticperformance of three silica-grafted SnIV model-catalysts is studied to differentiate betweenthe various parameters that contribute to to the activity of SnIV/SiO2-based catalytic systems.The differences in performance observed highlight that the activity of such systems significantlydepends on active site speciation and the hydrophobicity of the framework. Comparison tobenchmark Snβ further suggests that confinement effects inside zeolite cavities play a key role.
Cover Gallery 107
ISSN 1867-3880 · Vol. 7 · No. 20 · October, 2015
20/2015Front Cover:
I. Hermans et al.Silica-Grafted SnIV Catalysts in Hydrogen-Transfer Reactions
www.chemcatchem.org
A Journal of
Appendix E
Curriculum Vitae
Name Sabrina Conrad
Date of birth October 24th, 1986
Place of birth Stuttgart, Germany
Nationality German
Education
2014 – 2015 External continuation of ETH Doctoral Studies under the supervisionof Prof. Dr. Ive Hermans and Prof. Dr. Christophe Copéret at theUniversity of Wisconsin-Madison, USA
2012 – 2013 Ph.D. under the supervision of Prof. Dr. Ive Hermans at ETH Zurich,Switzerland
2007 – 2011 Diploma studies in Chemistry, Karlsruhe Institute of Technology,Germany