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Chapter -1 Introduction
Chapter 1 General Introduction
1
1.0. General Introduction
Heterogeneous catalysts are philosopher’s stones, of fundamental importance for
both the industry and academia in the production of fuels and chemicals, both bulk and fine
globally [1, 2]. Due to rising of world population, the answer for escalating universal
demand for energy and feedstock, a steer towards sustainability, and the cost factor of rare
and noble metals, development of new catalysts is vital. It is necessary to develop new
catalysts which display highest selectivity towards required products at high rates.
Preferably, such development can be achieved through design of particularly made systems
to suit various applications. Present heterogeneous catalysts are based on transition metal
oxide systems and zeolites etc. These catalysts are useful in a wide range of reactions such
as cracking, hydrogenation and oxidation reactions, to produce bulk chemicals at a large
scale. Heterogeneous catalysts offer advantages in terms of easy separation from the
product, high stability and excellent life time compared to homogeneous catalysts.
Homogeneous catalysts, with some exceptions, are mostly applied for special challenging
catalytic problems, e.g., for the production of fine chemicals, as they show very high
selectivity towards desired product. The structural environment can be easily and in some
cases rationally tuned by means of variation of ligands attached to the single atom active
site.
Over the past decades, most of the studies on heterogeneous catalysts focus on
understanding the interaction of the substrate with the catalysts and the parameters deciding
the performance of a heterogeneous catalyst [3, 4]. A classical example of tailor making the
catalytic properties of a heterogeneous catalyst by means of (surface) modification has been
commercially applied for many years; the famous one, in this category is Lindlar catalyst.
Surface modification of Pd/CaCO3 not only selectively hydrogenates triple bonds to double
bonds, but also shows regioselectivity of the resulting double bond [5,6]. Further it was
reported that surface poisoning with quinoline during the processing additionally increases
the selectivity towards the double bond [7]. Even after six decades, the structure of this
catalyst is still under examination and improvements are being proposed. To understand the
structure-activity relationship of heterogeneous catalysts, a deep understanding of all the
parameters which influence the reaction is necessary. It is important to know the
mechanism of a reaction, which involves the information of intermediates during the
Chapter 1 General Introduction
2
catalytic process and the nature of the active sites, i.e. the electronic and geometric
structure of the metal and the support. To gain an insight into deep understanding of all
these issues appropriate individual reactions must be carried out to obtain information
under practical working conditions [4, 8 , 9].
The main aim of this thesis is to develop efficient supported non-noble metal
catalysts for few industrially important hydrogenation reactions, determining the structure
of the catalyst under reaction conditions and predicting the reaction mechanism by spotting
the reaction intermediates. The reactions of interest are the hydrogenation of nitrobenzene
& levulinic acid and the decisive goal is to design a good catalyst systems.
1.1. Supported metal catalysts
Supported metal catalysts are widely employed in many industrial processes such as
petroleum refining and petrochemical industries. These catalysts play an important role in
the conversion of hydrocarbons to synthesis gas via reforming, isomerization of paraffins,
cycloalkanes, reduction of various organic compounds (hydrogenation), oxidation,
reduction of NOx gases, hydrodechlorination and Fischer-Tropsch synthesis. Supported
metal catalysts differ from bulk metal catalysts in the aspect of consisting of smaller metal
particles, which are separated from one other to certain extent. In supported catalysts, the
catalyst usually consists of a support, an active component and promoter (if needed). The
catalytically active material is usually dispersed on a support and the role of support is to
expose maximum amount of the catalyst material, to provide a large interfacial area and to
stabilize the active phase. There are number of advantages in depositing catalytically active
metals on supports like alumina, titania, zirconia, zinc oxide, silica, and mesoporous silica’s
(SBA-15, SBA-16, COK-12 and KIT-6). The use of support is to enhance the mechanical
strength, thermal stability and to disperse the active component in the form of smaller
particles. In most of the catalytic reactions, the combined characteristics of active phase and
support can drastically enhance the catalytic functionalities. Although the support material
only serves as a medium for keeping the catalytically active species supported, these
species interacts to some degree with the support. Due to the active phase-support
interactions, the catalytic performance strongly depends on a complex mix-up of
Chapter 1 General Introduction
3
contributions of the morphology and dispersion of active metal particles. Therefore, besides
the active component and promoter the role of support also needs to be considered. It is
possible to disperse metal particles throughout the porous structure of the support. As a
result, large active metal area is produced relative to the weight of the metal used. The
surface to volume ratio of active component is especially advantageous in case of precious
metals. The support is quite useful in the dissipation of reaction heat and retards the
sintering of metal crystallites, hence maintaining the active surface and increase the poison
resistance, which promotes a longer catalyst life. These advantages make supported metal
catalysts preferable over bulk metal catalysts for chemical processing.
1.2. Hydrogenation
Selective hydrogenation of aromatic nitro compounds is useful to synthesize
organic chemicals which are key products in fine chemical synthesis. This is an important
and interesting area of research especially if there are two or more C=C bonds. The reaction
becomes challenging if the aromatic compounds possess functional groups, since the
hydrogenation of C=C is thermodynamically more favorable than hydrogenation of the
functional groups. Getting the selectivity towards a particular functionality is not easy and
requires specific reaction conditions and catalyst system.
Chemical reaction between molecular hydrogen and an element or compound,
generally takes place in the presence of a catalyst. The reaction may be one in which
hydrogen simply adds to a double or triple bond connecting two atoms in the structure of
the molecule or one in which the addition of hydrogen results in dissociation (breaking up)
of the molecule (called hydrogenolysis, or destructive hydrogenation). Nearly all organic
compounds containing multiple bonds connecting two atoms can react with hydrogen in the
presence of a catalyst. Especially, hydrogenation of -NO2 an –C=O are of particular interest
in the present thesis.
1.2.1. Common features of hydrogenation reactions
There are two types of hydrogenation reactions i.e., chemeoselective hydrogenation
and regioselective hydrogenation. The chemeoselective hydrogenation catalysts are mostly
Chapter 1 General Introduction
4
based on supported metals involving Pt, Ru, Rh, Cu and Pd. The physico-chemical
properties of a support, like its acid-base character, reducibility and the extent of metal-
support interaction play an important role in the complex chemistry of supported metal
catalysts. Synthesis of catalysts with basic support is an interesting and challenging job. It
is useful to study the nature of metal species present on the support from the point of view
of reduction characteristics, dispersion and metal support interaction.
1.2.2. Industrial applications of hydrogenation reactions
Hydrogenation of organic compounds (hydrogenation and hydrogenolysis) is a
reaction of great industrial importance. The addition of hydrogen is used in the production
of edible fats from liquid oils. In the petroleum industry, plentiful processes involve the
conditioning of gasoline and manufacture of petrochemical products which are based on the
destructive hydrogenation of hydrocarbon. Nickel, platinum, and palladium based catalysts
are commonly used catalysts for hydrogenation reactions. Copper chromate and nickel
supported on kieselguhr are comprehensively used for high pressure hydrogenations
reactions.
Numerous important applications are found in the pharmaceutical and
petrochemical industries. Complete hydrogenation converts unsaturated fatty acids to
saturated ones. In practice the process is not usually carried out to completion. Since the
original oils typically contain more than one double bond per molecule (that is, they are
poly-unsaturated), the result is usually described as partially hydrogenated vegetable oil,
i.e., some, but usually not all, of the double bonds in each molecule have been reduced.
Hydrogenation results in the conversion of liquid vegetable oils to solid or semi-solid fats,
such as margarine. Changing the degree of saturation of the fat changes important physical
properties such as melting point. That’s why semi-solid fats are preferred for baking
because the fat mixes well with flour to produce a more desirable texture in the baked
product. Since partially hydrogenated vegetable oils are much less expensive than most
other fats with similar characteristics, and because they have other desirable characteristics
leading to longer shelf life, they are predominantly used in most commercial baked goods.
Processes accomplishing the reverse are called "dehydrogenation" or "partial
dehydrogenation." A side effect of incomplete hydrogenation that has implications for
Chapter 1 General Introduction
5
human health is the isomerization of the remaining unsaturated carbon bonds. The cis
configuration of these double bonds predominates in the unprocessed fats of most foods.
The catalytic hydrogenation process favors the conversion of cis to trans forms because the
trans conformation is lower energy than the natural cis conformation.
Hydrogenation typically uses hydrogen gas as a reactant and an undissolved (or
"heterogeneous") metal catalyst, such as copper, nickel, palladium or platinum. If not, the
"homogeneous" rhodium-based catalyst commonly known as Wilkinson's catalyst is
frequently used. The reaction is usually carried out at high temperature and pressure. The
French chemist, Paul Sabatier greatly facilitated the industrial use of hydrogenation. In
1897, he discovered that the introduction of a trace of nickel as a catalyst facilitated the
addition of hydrogen to molecules of carbon compounds.
1.2.3. Selective hydrogenation of nitrobenzene
Aniline is the one of the most significant key compound in organic chemistry. Many
commodity chemicals including cyclohexylamine, benzoquinone, akylanilines etc., are
manufactured from aniline. Aniline is mainly produced from the catalytic hydrogenation of
nitrobenzene at 300-475 °C in fixed bed reactor. The hydrogenation of nitrobenzene to
aniline selectively is shown in Scheme 1.1.
Scheme1.1. Hydrogenation of nitrobenzene.
Chapter 1 General Introduction
6
Reactions involving catalysts for hydrogenation seems to fall into four groups based
on nature of the catalyst, pressure and temperature ranges, economy and selectivity. These
are (i) Platinum-palladium series, (ii) Nickel series, (iii) Copper-mixed oxide series and (iv)
Molybdenum-Tungsten sulfide series. Platinum–palladium catalysts are useful at low
temperatures and pressures. Nickel catalysts prepared by varied processes are particularly
helpful. Raney nickel, prepared by leaching a nickel-aluminium alloy with caustic, finds
widespread application in pressure reductions; Copper chromium oxide is an exceptionally
smart catalyst for the reduction of nitro group. The nickel catalysts were applied to
hydrogenation of aromatic compounds such as nitrobenzene to aniline, benzene to
cyclohexane etc.
Around 85% of aniline is obtained by catalytic hydrogenation of nitrobenzene.
However, the transformation is extremely facile and is administrated under relatively mild
conditions. For this reason, hydrogenation occurs rapidly over most of the metals and is
often used as a standard reference reaction for scrutinizing the activities of other
hydrogenation catalysts [10 -15]. The hydrogenation of nitrobenzene has been reported to
occur over carbon nanotube supported platinum catalyst [16]. Platinum nanoparticle core-
polyaryl ether trisacetic acid ammonium chloride dendrimer shell nanocomposites were
employed for hydrogenation of nitrobenzene to aniline with petro chemically derived H2
under mild conditions [17]. Active carbons were used as supports for palladium in the
liquid phase hydrogenation of nitrobenzene to aniline [18]. Hydrogenation of nitrobenzene
was studied over Pt/C catalyst in supercritical carbon dioxide and ethanol [19]. Polymer
anchored metal complex catalyst has been used for the hydrogenation of nitrobenzene [20].
Liquid phase hydrogenation of nitrobenzene was studied over Pd-B/SiO2 amorphous
catalyst [21].
1.2.3.1. Synthesis of Aniline
Aniline is an important chemical that is used extensively in laboratory organic
synthesis and possesses larger scale industrial applications [22-24]. A reliable method of
aniline production provides a constant feed to many wide ranging industries including
pharmaceutical, automotive and construction industries.
Chapter 1 General Introduction
7
There are several methods of synthesizing aniline using a variety of starting
materials but nitrobenzene is the classical and the most frequently used feedstock. On the
small scale, aniline can be produced by the chemical reduction of nitrobenzene in a process
known as the Bechamp reaction [25-27] (Scheme 1.2). This traditional method involves the
use of iron turnings and water in presence of hydrochloric acid to reduce the nitro-group to
amine. This was also the first method used in the industrial production of aromatic amines
way back in 1854 and had the advantage of getting iron oxide pigments from the iron oxide
residues, as a side product during the reaction [27]. The Bechamp reaction is still currently
utilized by Bayer to produce a wide range of iron oxide pigments in batch processes but not
aimed for the commercial production of aniline.
NO2
Nitrobenzene (NB)
Fe
HCl
NH2
Aniline (AN) Scheme 1.2. The Bechamp Reaction
More recently, the Bechamp reaction was surpassed by a more economically viable
route, by a catalytic hydrogenation of nitrobenzene. In this process the aromatic nitro group
is reacted with three mole equivalents of hydrogen gas, in the presence of suitable catalyst
to produce the amine and water. This hydrogenation is very easily, carried out under
relatively mild conditions and produces only low level of by-products. This reaction occurs
rapidly over most of the metals and is often employed as a reference reaction to compare
the activity of other hydrogenation catalysts in different reactor systems [28-39]. Currently
using heterogeneously catalysed hydrogenation of nitrobenzene, aniline can be produced
with greater than 99% selectivity. Details of commercial reactions in operation and
industrial catalysts are provided in the subsequent sections.
Although utilized to a lesser extent, three other catalytic routes have been used in
the production of aniline. The first route involves the amination of chlorobenzene [25]. For
instance, the Kanto Electrochemical Co. Process involves of the ammonolysis of
chlorobenzene using aqueous ammonia over a Niewland catalyst: a mixture of copper (I)
Chapter 1 General Introduction
8
chloride and ammonium chloride, to produce aniline and hydrochloric acid as reaction
products. This process yields 91% selectivity towards amine and was employed by Bayer
until 1986 when it was discontinued for economic reasons [25].
A second route towards aniline uses phenol as a starting material and again involves
ammonia in the amination process [25, 27]. The hydroxyl group on phenol reacts with
gaseous ammonia over an A12O3-SiO2 catalyst to produce aniline and water as products.
This method has become greater importance in recent years, once the as phenol technology
was commercially established. The development of a novel single-stage phenol
transformation of benzene to phenol has promoted to produce large quantities of phenol as
a feedstock [40]. In addition, Du Pont has explored a third route for aniline production
using benzene as starting material to produce aniline directly without the need of producing
nitrobenzene intermediate [25]. Benzene is reacted with concentrated ammonia over a
NiO/Ni catalyst promoted with zinc oxide. An aniline selectivity of 97 % was observed in
this route at a maximum benzene conversion of only 13 % and hence proved to be a barrier
to the industrial scale production. Despite the intensive investigations into these routes, the
method of choice for the production of aniline is the hydrogenation of nitrobenzene using a
heterogeneous catalyst because of its simplicity in operation. A summary of all four
methods can be seen in the scheme. 1.3
(i) The catalytic hydrogenation of nitrobenzene
NO2 NH2
3 H2 (g) 2 H2OCatalyst
(ii) The amination of chlorobenzene
Chapter 1 General Introduction
9
Cl
NH3
NH2
HClCatalyst
(iii) The amination of phenol
OH
NH3
NH2
H2OCatalyst
(iv) The amination of benzene
NH3
NH2
Catalyst
H2O
Scheme 1.3. Various routes for the synthesis of aniline
1.2.3.2. Commercial production of aniline
As already stated the commercial production of aniline is based on using a
heterogeneous catalyst through the effective hydrogenation of nitrobenzene. However the
reaction conditions widely vary depending on the type of catalyst used.
Chapter 1 General Introduction
10
Figure-1.1. Aniline capacity share by company in Western Europe
During 2001, the global capacity for aniline stood at 3.0 million tonne/year [27].
The majority of global aniline production is centered in the U. S, Western Europe and Asia.
Western Europe and Asia alone contributes 42.2 % to the world’s aniline stocks [41-43]. In
recent years the major companies producing aniline are Bayer, Hustsman, BASF, DOW
and Quimigal and the percentage contribution by each company is shown in Figure-1.1.
The global aniline market is projected to reach 6.2 million tons by the year 2015,
due to the increasing demand of aniline from various end-user markets. In particular, the
rising demand for methylene diphenyl diisocyanate (MDI), the chief ingredient in
polyurethane foam, prompts to consume more quantities of aniline.
The nitrobenzene hydrogenation process can be carried out in either the gas phase
or liquid phase and both are used in commercial production. Of the aniline producers using
a gas phase process, Bayer utilizes a fixed-bed of NiS catalyst that has been activated using
copper or chromium [26]. Reactions are performed at temperatures ranging from 573-748
K and aniline selectivity’s greater than 99 % are achieved. The catalyst bed is prone to
catalyst deactivation due to carbon deposition but can be regenerated in air at 250-350 °C
followed by H2 (g) passivation. On the other hand, a fluidized bed is utilized for the
hydrogenation process operated by BASF using catalysts containing copper - chromium,
barium and zinc oxides on a SiO2 support [23]. Reaction conditions are in the temperature
range of 270-290 °C and 1-5 bar pressure with a large excess of hydrogen. This method
exhibits a high selectivity of 99.5% towards aniline but needs regeneration of deactivated
catalyst in air at regular intervals throughout production process.
33%
27%
19%
11%
10%
BayerHuntsmanBASFDOW chemicalsQuimigal
Chapter 1 General Introduction
11
The liquid phase hydrogenation of nitrobenzene is also performed industrially with
Huntsman technology employing a semi-continuous batch process using a stirred tank
reactor to produce aniline [41]. A typical catalyst consists of 55 % by weight of nickel
supported on Kieselguhr that has been pre-reduced and stabilized before use [45]. The
catalyst is re-activated by hydrogen when the reactor vessel reaches reaction temperature
(usually between 70- 150 °C). Hydrogenation is generally performed under a hydrogen
pressure of 20-40 bar and a nitrobenzene conversion in excess of 99.7 % can be achieved.
1.2.3.3. Uses of Aniline
Aniline is used as a feedstock in a number of different industries leading to a wide
range of applications.
Figure-1.2. Uses of Aniline in Global Market
Aniline is primarily used in MDI foams for the automotive and construction
industries. Many chemicals can be made from Aniline, including:
Isocyanaates for the urethane industry
Antioxidants, activators, accelerators, and other chemicals for the rubber industry
Indigo, acetoacetanilide, and other dyes and pigments for a variety of applications
Diphenylamine for the rubber, petroleum, plastics, agricultural, explosives, and
chemical industries
Various fungicides and herbicides for the agricultural industry
MDI Pharmaceuticals Rubber Dyes others
Chapter 1 General Introduction
12
Pharmaceutical, organic chemical, and other products
In the synthesis of sildenafil (Viagra), the antibiotic linezolid (Zyvox) and the HIV
ptotease inhibitor amprenavir (agenerase) etc.
(i) Synthesis of Methylene di- phenylene diisocyanate (MDI)
The huge amount of global aniline ( 80-85%) is consumed in the production of
methylene di-phenylene diisocyanate (MDI) [26,27,42]. MDI is synthesized in a two step
process, as shown in Scheme 1.4 [46,47], which includes condensation with formaldehyde
to produce a, dimeric species followed by phospenation to transform the amine
functionality to isocyanate groups. MDI is then polymerized and used to synthesize
extremely versatile materials known as polyurethanes. This is a growing industry and
presently, the demand for MDI is increasing steadily at the rate of 6-8% per year [27].
NH2
CH2O (aq)
HCl
Aniline Formaldehyde
H2CH2N NH2
H2COCN NCO
COCl2
Methylene di-para-phenylene-isocyanate (MDI)
Scheme 1.4 Synthesis of MDI from aniline.
(ii) Polyurethanes
R OHHO R NCOOCN
Dihydroxy compound Diisocyanate
R O CO
NH R OCO
HN
Polyurethane
Scheme 1.5 Formation of polyurethane
Polyurethanes are synthesized by diisocyanate addition polymerization, a method
discovered by Bayer in 1937 [46]. This involves a reaction of a diisocyanate with a
polyhydroxyl compound in the presence of a suitable catalyst and additives [24, 46, 48, and
49]. A variety of diisocyanates and hydroxylated compounds can be used in the reaction
leading, to a range of polymeric products with different physical properties. The most
common commercial methods utilize a diisocyanate and hydroxyl terminated polyester or
polyether. The production of polyurethane is shown in scheme 1.5. MDI polyurethanes are
Chapter 1 General Introduction
13
versatile polymers used in the manufacture of rigid and semi- rigid foams, elastomers and
coating resins. These materials are in general used in the construction industry for building
insulation, white goods industry for insulation in the automotive industry for car interiors
and in the sportswear industry for flexible trainer soles [47, 50 and 51].
The remainder of the aniline is used in the production of a number of other products
namely dyes, rubber additives and pharmaceuticals. In addition, it is also utilized in
agricultural industry to synthesize pesticides, herbicides, photographic developers,
explosives and speciality fibres [52].
1.2.3.4. Production of aniline from nickel catalysts
An assortment of catalytic systems was prepared for the synthesis of aniline from
the hydrogenation of nitrobenzene. All the heterogeneous catalytic systems require under
dynamic conditions such as high pressure, high temperatures and solvent systems etc.
NO2 NH2
3 H2
Ni/SBA-15 2 H2O
Nitrobenzene (NB) Aniline (AN)
Scheme 1.6 Hydrogenation of nitrobenzene over Ni/SBA-15 catalysts
However synthesis of aniline from nitrobenzene can be operated at normal
conditions like atmospheric pressure, solvent free conditions over nickel based catalysts.
Scheme.1.6 shows the hydrogenation of nitrobenzene over Ni/SBA-15 catalyst, which
appears to be a highly economically viable procecess [53].
Chapter 1 General Introduction
14
1.2.4. Conversion of biomass to levulinic acid
1.2.4.1. Methods and catalytic system
Figure 1.9. Conversion of lignocellulosic biomass to levulinic acid [54, 55].
Extensive R & D work to convert renewable, reliable and abundant lignocelulosic
biomass to promising building blocks have been conducted by National Renewable Energy
Laboratory (NREL) and Pacific Northwest National Laboratory (PNNL) [54, 55]. Among
the promising candidates, levulinic acid resides at the top 12 building blocks which is
obtained through agriculture waste and forestry [54, 55].
Levulinic acid (LA) is a water-soluble acid (pKa = 4.59) with a high-boiling point
(248 °C), that crystallizes at room temperature (melting point 38 °C). The molecular
structure of LA contains two reactive functional groups (–C=O and –COOH) that affords
the prospect for a range of synthetic transformations [55, 56].
Isomerisation of D-glucose to D-fructose in the presence of acidic species produces
a number of compounds such as 5-hydroxymethylfurfural (HMF), formic acid, including
LA [57-59] (Figure 1.10). HMF is prone to recombine with sugars or itself through aldol-
condensation in acidic solution, resulting in polymers with undefined structures and
stoichiometry [60, 61]. It is thus expected that by changing the solvent as well as the
Chapter 1 General Introduction
15
amount and/or the nature of the employed catalyst, the reaction rates of diverse steps can be
altered to obtain variety of products [62-65].
Figure 1.10. Possible reaction pathways from cellulose to levulinic acid
To take advantage of the large potential applications of LA as key platform
chemical, chemical industry firms and researchers devoted numerous activities to address
the above mentioned issues. The first pioneering study performed by Mulder in early 1840s
on the preparation of LA was reported [66]. His group tried to prepare LA by heating a
mixture of sucrose with mineral acid such as HCl. However the details on the reaction
conditions and the LA yield are unknown. The first commercial scale was set up for the
production of levulinic acid in an autoclave in United States by Stanley in 1940 [66].
Hanna [67] and Heeres [68] along with their co-workers have performed the reaction by
using kernel grain sorghum and starch as starting material for the manufacture of levulinic
acid. Particularly, the maximum yield of levulinic acid obtained was above 50%. Hawley
and co-workers [57] have been studied the conversion of HMF into levulinic acid in the
eighties wherein a LZY zeolite catalyst was selected and later on Heeres [68] reported a
maximum yield of 60% by using sulphuric acid as a catalyst. Horváth and fellow workers
[69] reported 54% yield of levulinic acid by the conversion of sucrose and rehydration of
HMF to levulinic acid in 2008. For these processes they have used sulphuric acid or Nafion
NR50 – a solid acid catalyst with water as a reaction medium which facilitates easy
separation.
Chapter 1 General Introduction
16
Mascal [70] recently reported the dehydration of glucose to levulinic acid with 79%
yield using hydrochloric acid as a catalyst and dimethyl chloride as a solvent. As well, 5-
(chloromethyl) furfural was dehydrated into LA at 190 °C in water toget 91% yield [70].
Jin and coworkers converted carbohydrate biomass to HMF and levulinic acid. The highest
yield of LA was 55%, which was obtained with HCl at a pH of 1.5 in 5 min reaction time
[71]. Zhuang and coworkers investigated the conversion of cellulose to levulinic acid by
different metal chlorides including alkali metals (Li, Na and K), alkaline earth metals (Mg
and Ca), transition metals (Cr, Mn, Fe, Co, Cu and Zn) and Al as a group IIIA metal.
Among those metal chlorides, chromium chloride was found to be exceptionally effective
for the conversion of cellulose to levulinic acid, affording yield of 67 mol% after a reaction
time of 180 min at 200 °C [72]. Lucht investigated the conversion of cellulose to glucose
and LA by means of a solid catalyst system based on Nafion SAC 13 or FeCl3/silica and
obtained 5% yield of LA [73].
Likewise, a comprehensive overview of biomass based synthetic protocols for LA
formation, representative studies were cited. Based on literatures, HMF was traditionally
thought as an intermediate for the formation of levulinic acid under acidic conditions. A
large number of researches have been focused on the production of HMF using different
solvents and different catalysts including homogeneous and heterogeneous catalysts. In
order to better understand the production of HMF and levulinic acid, the representative
results of HMF are also cited in the literatures and patents.
Although major achievements in HMF or LA production was reached, the main
disadvantages still exists, especially for production of LA, which can be shortly
summarized as following:
(1) Often mineral acids such as hydrochloric acid or sulphuric acid were used which
enables serious drawbacks such as corrosiveness leading to deterioration of steel based
high pressure equipment.
(2) The usage of high boiling point solvents such as DMF or DMSO might be an alternative
because it allows normal pressure applications. However the necessary separation and
isolation of the gained products is tedious and not cost effective.
(3) If bio-catalysts are used for the transformation, their separation from the reaction
mixture is difficult. Overall, the need for more selective and active catalysts, which are
Chapter 1 General Introduction
17
easily separable, robust, and which are compatible with earlier and following
transformation steps is still a major goal in this field.
1.2.4.2. Mechanistic aspects of the formation of levulinic acid
Numerous studies have contributed to investigate possible reaction mechanisms of
sugar dehydration to LA. However, until now a fully comprehensive reaction network able
to explain the variety of product distributions obtained, is still not clearly understood [74-
78]. The available information implies that C6-sugars initially would be dehydrated to form
5- hydroxymethylfurfural (HMF) as an intermediate which was subsequently hydrated to
give the final products levulinic and formic acid. Figure 1.11 shows the proposed acidic
mechanism for the conversion of C6 sugars, such as D-fructose to HMF. A study by Antal
and coworkers suggested that HMF is formed from dehydration of fructose in its furanose
form and occurs through a series of cyclic furan intermediates (Figure1.11, pathway b)
[75]. Moreau [59, 78] and coworkers postulated that HMF is formed via an enediol
pathway (Figure 1.11, pathway a) in which the enediol is the decisive intermediate in the
isomerization of glucose to fructose. The conversion of HMF results the formation of
water in addition to the C-2 and C-3 bond of the furan ring to give the final products
levulinic and formic acid (see Figure 1.12) [59, 78].
Figure-1.11. Possible dehydration mechanisms for formation of HMF [55]. The
acyclic route is labeled with an “a,” the cyclic route with a “b”[57].
Chapter 1 General Introduction
18
O
O O
OHOO
OHOO
O
O
OH
OH OH
OO
OH
OH
O O O
CH(OH)2O
O
OH
O
+H2O/H+ -H2O +H2O
HCOOH 5,5- dihydroxypent-3-ene-2-one
HMF
-H2O
Figure 1.12 Proposed mechanisms for formation of levulinic acid from HMF [79].
1.2.5. Potential applications of LA and its derivatives
1.2.5.1. Building blocks derived from levulinic acid
Figure-1.13. Overview of important LA derivatives. This figure was adapted from PNNL
report [55].
LA can be generated at least in principle from almost all C6 sugars manufactured in
the biorefinery, and for that reason, have frequently been suggested as a starting material
Chapter 1 General Introduction
19
for a wide number of compounds [56]. Reductions, oxidations and condensations reactions
could give access to potential derivatives (Figure-1.13).
1.2.5.2. Economical and ecological considerations
The family of compounds available from LA is quite broad, and addresses a number
of large volume chemical markets [56].
(1) Conversion of LA to 2-methyltetrahydrofuran (2-MTHF) [80,81] and various levulinate
esters in the Cluster of Excellence ˝Tailor-Made Fuel from Biomass (TMFB)˝ which
address ˝fuel markets as gasoline and biodiesel additives, respectively˝[82]. 2-MTHF is
a highly flammable mobile liquid which currently acts as a replacement for THF in
special applications. In comparison with THF, 2-MTHF dissolves only small amounts
of water which allows easier separations [83,84].
(2) Levulinate esters have been considered as potentially renewable diesel fuels [85-87].
Besides, these ketoesters are good substrates for a variety of condensation and addition
reactions [88-90]. A levulinate ester can also be efficiently converted into a glassy
polymer via reaction with primary alkyl amines. This polymer finds application in
coatings and films [87-91].
(3) γ-Amino-levulinic acid is ˝a herbicide and a precursor for porphyrins and hemoglobin.
It targets a market of 90 – 140 thousand tons per year and is produced at a cost in the
range of 4 to 6 $/kg.˝ This material find application in the production of new acrylate
polymers [92], ˝at a market size of 1.1 billion t/a with production costs of about 2.8
$/kg˝ [55].
(4) Diphenolic acid is of particular interest because it can serve as a replacement for
bisphenol A in the production of polycarbonates [55, 93]. It can be prepared by the
condensation reaction between phenol and levulinic acid in the presence of
hydrochloric acid. ˝The polycarbonate resin market is almost 2 million t/a, at a rate of
about 5 $/kg˝ [94].
(5) Novel technology also suggests oxidative processes for production of acrylic acid from
LA [95-96].
(6) LA is also a potential starting material for production of succinic acid, which is now
used within ˝the food and beverage industry, primarily as a sweetener. Global
Chapter 1 General Introduction
20
production is estimated at 16.000 to 30.000 tons a year, with an annual growth rate of
10%.˝ [97].
(7) Production of LA derived lactones offers the opportunity to enter a large solvent
market, as these materials could be converted into analogs of N-methylpyrrolidinone
[98, 99]. Reduction of LA leads to 1,4-pentanediol [98], which could be used for
production of new polyesters [100,101].
1.2.6. Uses of Gamma valerolactone
Gamma valerolactone is a promising candidate for production of fuels and
chemicals. Various products are manufactured from the gamma valerolactone such as 2
methyl tetrahydrofuran,1,4 pentane diol, α-methylene γ-valerolactone, methyl pentenoate,
dimethyl adepate, butene, C8 alkanes, pentenoic acid, pentanoic acid, pentanoate esters, 5-
nonanone. Furthermore, Gamma valerolactone is used as a solvent due to its renewable
nature. The following Figures represent the transformation of GVL to variety of chemicals (
Figure-1.15) and as a solvent for many reactions (Figure-1.14).
Figure-1.14. Lignocellulosic biomass derived product obtained using GVL as solvent.
Chapter 1 General Introduction
21
Figure-1.14. Reaction pathways for the conversion of GVL into fuels, fuel additives and
chemicals [102].
1.3. Aims & Objectives
Reactions of hydrogen play a vital role in the chemical reactions with about 40% of
the industry directly depends on hydrogen in the synthesis of bulk and fine chemicals. Most
part of hydrogen is utilized in the production of NH3 and the remaining of it is utilized in
various hydrogenation reactions counting from hydrogenation of oils to synthesis of fuels,
from the manufacture of bulk chemicals such as methanol to making of various drugs and
pharmaceuticals, in the transformation of nitro compounds to amines compounds, levulinic
acid to γ-valerolactone. Hydrogen can also be produced in small way in reactions like
secondary alcohols to carbonyl compounds. Based on the importance of hydrogen, the
following reactions have been selected for study in the present work.
Chapter 1 General Introduction
22
a) Hydrogenation of Nitro benzene to aniline
b) Hydrogenation of levulinic acid to γ-valerolactone
Nickel catalysts supported on various supports are used for hydrogenation reactions.
The support plays an important role in dispersing the active metal and influences the
electronic and adsorption properties of the metal thereby changing the catalytic properties.
Various conventional supports such as Al2O3, SiO2, and MgO are commonly used in
dispersing the non-noble metals. Even though, many inorganic materials have been used for
dispersing the metal; the metal support interaction is also an important aspect for increasing
the activity. So the search for new and improved supports is still on, for better dispersion of
the metal, strong metal-support interaction which helps in enhancing the catalytic activity
involving the metal function. Using these catalysts for the selective synthesis of chemicals
and chemical intermediates at optimum conversion levels is, of course, the final objective.
The aim of the present investigation is to study systematically and exploit the
various supports such as conventional to latest mesoporous materials as supports for Ni.
Many variables can affect the structural and surface properties, which in turn affect the
adsorption, and catalytic properties of the supported metals. By controlling these variables
one can tailor the catalyst to suit a particular reaction.
Hydrogenation reactions are generally studied on Ru , Pd and Ni based catalysts as
these materials are active components for these reactions. However, the expensive nature of
Ru and Pd catalyst limit their application in industry. It is therefore important to design and
develop inexpensive non-noble metal catalysts such as Ni supported on various supports for
these reactions.
The following are the objectives/highlights of the thesis:
i) A comparative study on the oxide carriers like SBA-15 and MgO as supports for
Ni catalysts for aniline synthesis from nitrobenzene hydrogenation.
ii) A detailed study of the aniline synthesis over Ni supported on ZrO2 and TiO2
prepared by reductive deposition method and comparison of their activity with
impregnated catalysts. Ni dispersion studies to confirm the activity differences
between the catalysts prepared by reductive deposition and impregnation
methods.
Chapter 1 General Introduction
23
iii) A detailed study of the γ-valerolactone synthesis over Ni supported on HZSM-5
in vapour phase at atmospheric pressure. Studies on optimization of Ni loading
to get good LA conversion and GVL selectivity.
iv) A comparative study on hydrogenation of levulinic acid using various supported
Ni catalysts like N/iAl2O3, Ni/MgO, Ni/SiO2,Ni/TiO2, Ni/ZnO and Ni/ZrO2
prepared by impregnation method.
v) A comparative and detailed study of hydrogenation of levulinic acid using Ni
supported on mesoporous silica (SBA-15) with different Ni loadings.
vi) A study on the hydrogenation of levulinic acid using various types of
mesoporous silica’s with 2D and 3D architechtures supported Ni catalysts, with
more emphasis on the influence of nature of porosity on the catalytic
performance.
1.4. Scope of thesis
1. The Ni/SBA-15 catalyst is more active in nitrobenzene hydrogenation than
the Ni/MgO catalyst. Influence of water released during the nitrobenzene
hydrogenation is studied for the first time which is not reported in the
literature earlier.
2. TiO2 supported Ni catalyst prepared by reductive deposition method is
found to be more active in aniline synthesis than impregnated catalyst.
3. The vapour phase hydrogenation of levulinic acid over non-noble metal
catalysts such as Ni/HZSM-5 prepared by conventional impregnation
method is a highly efficient catalyst under additive free conditions.
4. Influence of support on the levulinic acid hydrogenation over Ni catalysts
prepared by impregnation method, reveal that Ni/SBA-15 is a better catalyst.
5. Porosity plays crucial role in hydrogenation of levulinic acid over Ni
supported on various mesoporous silica’s with 2D and 3D architectures than
the conventional supports.
There is a lot of scope to extend these studies to other hydrogenation reactions.
Chapter 1 General Introduction
24
1.5. Organization of Thesis
The thesis has been organized into six chapters:
Chapter-I: It presents the introduction part which contains the importance of
hydrogenation, in particular, hydrogenation of Nitrobenzene and levulininc acid. The aims,
objectives and scope of thesis in carrying out the hydrogenation of Nitrobenzene and
levulininc acid over various supported Ni catalysts are presented in this chapter.
Chapter-II: This chapter deals with the literature review on hydrogenation of Nitro
benzene and levulinic acid.
Chapter-III: This chapter describes the different experimental methods employed for the
preparation, characterization and activity evaluation of the catalysts.
Results and Discussion: Chapter- IV and V present the results and discussion of the work
over different catalysts studied for the hydrogenation under different conditions. The
structure-activity relationship has been discussed with the help catalyst characteristics
derived from the characterization techniques such as XPS, SEM-EDAX, TEM-SAED,
XRD, TPR, BET-surface are measurements, H2 chemisorption, TG-DTA, AAS, FT-IR,
pyridine adsorbed FT-IR and NH3 TPD.
Chapter-IV: This chapter has been divided into two sections.
Section-A: This section deals with the advantage of support for the nitrobenzene
hydrogenation and also role of water on its catalytic activity released during the reaction.
The catalysts employed for NB hydrogenation are Ni/MgO and Ni/SBA-15.
Section-B: A comparative study of hydrogenation of nitrobenzene hydrogenation over
Ni/ZrO2 and Ni/TiO2 catalysts prepared by conventional impregnation method and
reductive deposition precipitation methods is discussed.
Chapter-V: This chapter has been divided into four sections highlighting the
hydrogenation of levulinic acid to γ-valerolactone.
Chapter 1 General Introduction
25
Section-A: Studies on the hydrogenation of levulinic acid hydrogenation using
Ni/HZM-5 catalysts prepared by conventional impregnation method have been discussed.
Section-B: This section deals with the influence of various oxidic supports for the
levulinic acid hydrogenation over Ni catalysts.
Section-C: This section describes hydrogenation of levulinic acid using SBA-15 supported
Ni catalysts prepared by impregnation method with various Ni loadings.
Section-D: This section deals with the advantage of support having 2D and 3D
architectures for the LA hydrogenation over Ni catalysts prepared by impregnation method.
Chapter-VI: This chapter concludes and summarizes all the observations made in the
results and discussion chapters viz, chapter- IV, and V.
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