metal–organic frameworks—prospective industrial applications
DESCRIPTION
Describing examples of different zinc-containing structures, e.g. MOF-2, MOF-5 and IRMOF-8 verified synthesis methods will be given, as well as a totally novel electrochemical approach fortransition metal based MOFs will be presented for the first time.With sufficient amounts of sample now being available, the testing of metal–organicframeworks in fields of catalysis and gas processing is exemplified. Report is given on the catalyticactivation of alkynes (formation of methoxypropene from propyne, vinylester synthesis fromacetylene). Removal of impurities in natural gas (traces of tetrahydrothiophene in methane),pressure swing separation of rare gases (krypton and xenon) and storage of hydrogen (3.3 wt% at2.0 MPa/77 K on Cu-BTC-MOF) will underline the prospective future industrial use of metal–organic frameworks in gas processing. Whenever possible, comparison is made to state-of-artapplications in order to outline possibilities which might be superior by using MOFs.TRANSCRIPT
Metal–organic frameworks—prospective industrial applications{
U. Mueller,* M. Schubert, F. Teich, H. Puetter, K. Schierle-Arndt and J. Pastre
Received 22nd August 2005, Accepted 26th October 2005
First published as an Advance Article on the web 23rd November 2005
DOI: 10.1039/b511962f
The generation of metal–organic framework (MOF) coordination polymers enables the tailoring
of novel solids with regular porosity from the micro to nanopore scale. Since the discovery of
this new family of nanoporous materials and the concept of so called ‘reticular design’, nowadays
several hundred different types of MOF are known. The self assembly of metal ions, which act
as coordination centres, linked together with a variety of polyatomic organic bridging ligands,
results in tailorable nanoporous host materials as robust solids with high thermal and
mechanical stability.
Describing examples of different zinc-containing structures, e.g. MOF-2, MOF-5 and IRMOF-
8 verified synthesis methods will be given, as well as a totally novel electrochemical approach for
transition metal based MOFs will be presented for the first time.
With sufficient amounts of sample now being available, the testing of metal–organic
frameworks in fields of catalysis and gas processing is exemplified. Report is given on the catalytic
activation of alkynes (formation of methoxypropene from propyne, vinylester synthesis from
acetylene). Removal of impurities in natural gas (traces of tetrahydrothiophene in methane),
pressure swing separation of rare gases (krypton and xenon) and storage of hydrogen (3.3 wt% at
2.0 MPa/77 K on Cu-BTC-MOF) will underline the prospective future industrial use of metal–
organic frameworks in gas processing. Whenever possible, comparison is made to state-of-art
applications in order to outline possibilities which might be superior by using MOFs.
1. Introduction
As early as 1965 a first publication by Tomic1 on novel solids
was introduced which, nowadays, would be categorized and
addressed as metal–organic frameworks, coordination poly-
mers or supramolecular structures. Already in the aforemen-
tioned contribution simple syntheses of coordination polymers
based on metals like zinc, nickel, iron, aluminium (but also on
thorium and uranium) employing bi- to tetravalent aromatic
carboxylic acids are described . Interesting features of these
compounds such as high thermal stability and high metal
content were already investigated.
However, decades later interest in the field was stimulated
by the group of O. M. Yaghi, which published the structure of
MOF-5 in late 1999,2 and the concept of reticular design, with
totally different carboxylate linkers, in 2002.3–5 Meanwhile,
numerous reviews have addressed this fast growing research
efforts, the most comprehensive ones given by Kitagawa6 and
Yaghi.7 Structures, properties and possible applications as
BASF Aktiengesellschaft, Chemicals Research & Engineering, D-67056,Ludwigshafen, Germany. E-mail: [email protected]{ Presented at Symposium T: Porous materials for emerging applica-tions, International Conference on Materials for AdvancedTechnologies (ICMAT 2005), Singapore, 3–8 July 2005.
Ulrich Mueller, born 1957 inKatzenelnbogen, Germany.1977: studied chemistry inMainz (thesis on the synthesisof large zeolite crystals andsorption properties) andrecieved his PhD in the groupof Prof. K.K. Unger; researcha c t i v i t i e s a t C N R S‘Tian&Calvet’, Marseille, ILLGrenoble, and with G.T.K o k o t a i l o , U n i v .Pennsylvania. 1989: AmmoniaLaboratory BASF: zeolitesynthesis and application in
catalysis and adsorption. 1999: Senior Scientist, zeolite catalysis:CFC-free polyurethane foams, catalysts for crop protectionagents, chemical intermediates, sorptive olefin feedstream
purification, piloting of pro-pylene epoxidation catalysts.1999: Synthesis, scale-up,modification and testing ofvarious metal–organic frame-work compositions. 2005:BASF Research Director.
Markus M. Schubert, born1971 in Munich, Germany.1991: study of chemistry inUlm and PhD in group ofProf. Behm on catalysis andsurface chemistry. 2000:Postdoc at ETH Zurich with
Prof. Baiker. 2001: Ammonia Laboratory BASF: catalystscarriers, acid–base catalysis. 2004: Scale-up and piloting ofmetal–organic frameworks for gas processing.
APPLICATION www.rsc.org/materials | Journal of Materials Chemistry
626 | J. Mater. Chem., 2006, 16, 626–636 This journal is � The Royal Society of Chemistry 2006
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storage media were again studied by Rowsell, from Yaghi’s
group.7,8 Comparisons with oxides, molecular sieves, porous
carbon and heteropolyanion salts has been filed by Barton
and coauthors.9 Nowadays several hundred different MOFs
are known. The self assembly of metal ions, which act as
coordination centres, linked together with a variety of
polyatomic organic bridging ligands, results in tailored
nanoporous host materials as robust solids with high thermal
and mechanical stability (Fig. 1). Interestingly, unlike other
solid matter, e.g. zeolites, carbons and oxides, a number of
coordination compounds are known to exhibit high frame-
work flexibility and shrinkage/expansion due to interaction
with guest molecules.6 The most striking difference to state-of-
art materials is probably the total lack of non-accessible bulk
volume in metal–organic framework structures. Although high
surface areas are already known from activated carbons and
zeolites as well, it is the absence of any dead volume in MOFs
which principally gives them (on a weight-specific basis) the
highest porosities and world record surface areas (Fig. 2),
especially with MOF-177, for which values of 4500 m2 g21 are
reported.5 Of course, properties like the drastically increased
velocity of molecular traffic through these open structures
are closely related to the regularity of pores in nanometer size
as well.
Thus the combination of so far unreached porosity, surface
area, pore size and wide chemical inorganic–organic composi-
tion recently brought these materials to the attention of many
researchers in both academia and industry, with about 1000
publications on ‘coordination polymers’ per annum.6
This paper, however, aims to describe how MOF-materials
can be synthesized using verified synthetic methods as well as
by a totally novel electrochemical approach.10
With a large range of samples now available, the testing of
metal–organic frameworks in fields of catalysis and gas
processing is enabled. A report is given on the catalytic
activation of alkynes (the formation of methoxypropene
from propyne, vinylester synthesis from acetylene).11 Further
examples like olefin polymerization, Diels–Alder reaction,
transesterification6 or cyanosilylation12 are referenced in the
literature.
The removal of impurities in natural gas (i.e. traces of
tetrahydrothiophene in methane), pressure swing separation of
Friedhelm Teich, born 1955 inTrier, Germany. 1973: studyof Chemical Engineeringat Karlsruhe (PhD on gasprocessing). 1986: Dyestuff &Pigments laboratory BASF.2 0 0 4 : B A S F C h e m i c a l sResearch & Engineering,20 years experience in design-ing and developing processesfor the production of pigmentsand fine chemicals. 2004:process simulation for applica-tion of metal–organic frame-work materials.
Hermann Putter, born 1944 in Duesseldorf, Germany. 1951–1964: school in Duesseldorf. 1964–1972: study of chemistry inWuerzburg (thesis on the preparation and elucidation of theelectrochemical properties of squaric acid derivatives). 1969:summer/autumn: electroanalytical studies at the HeyrovskyInstitute in Prague. 1973–1985: Main Laboratory BASF,
Ludwigshafen, development ofdirect and indirect organicelectrosyntheses, commerciali-sation of the first electro-dialysis processes in ourcompany. 1985–1992: plantmanager of a chloralkali plantin Ludwigshafen. 1990: estab-lishment of the first chlorineb a s e d ‘ ‘ c h e m i s t r e e ’ ’ o fGermany for VCI. During thistime: papers, public discussionsand lectures on chlorinec h e m i s t r y . S i n c e 1 9 9 3 :Manager of R&D activities on
all electrochemical processes of our company. Since 1994:lectures on environmental and sustainability aspects of chemistry.1999: BASF innovation award for the first technical pairedelectrosynthesis, a process with high atom efficiency that avoidsemissions and halves the energy demand of an electrosynthesis.2001: BASF Research Fellow. 2003, Synthesis of metal–organicframeworks using electrochemistry.
Kerstin Schierle-Arndt, born1971 in Koln, Germany. 1990:study of chemistry in Bonn;PhD in organic electro-chemistry. 1998: AmmoniaLaboratory BASF: electro-chemical research. 2003:C h e m i c a l s R e s e a r c h &E n g i n e e r i n g , H e a d o fContro l l ing . 2005: NewBusiness development atBASF’s Inorganic Specialties.
Joerg Pastre, born 1970, inGroß-Gerau, Germany. 1977:s t u d y o f c h e m i s t r y i nDarmstadt. 2000: PhD onChemical Engineering at ETHZu r ich. 2000: Ammonialaboratory BASF: processdeve lopment department.2005: New business develop-ment at BASF InorganicSpecialties.
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rare gases (krypton and xenon) and storage of hydrogen
(3.3 wt% at 2.5 MPa/77 K on Cu-BTC-MOF) will underline
the prospective industrial use of metal–organic frameworks.
Whenever possible, comparison is made to state-of-art applica-
tions in order to outline possibilities of processes which might
be beneficially run by using MOFs.
2. Experimentals
In order to enable readers to look into the properties of MOFs,
some easy-to-follow recipes are listed hereafter. They were all
repeatedly checked and scaled-up to the order of kg. A typical
scheme of a semi-technical process is given in Fig. 3, indicating
the different steps of preparation, as well as recycling of
the solvent and further processing of the dried powders into
shaped particles.
2.1 Verified lab-scale synthesis recipes of MOF samples
MOF-2. A glass reactor equipped with a reflux condenser
and a teflon-lined stirrer was filled with 24.9 g of terephthalic
acid (BDC) and 52.2 g of zinc nitrate tetrahydrate (Merck)
were dissolved in a mixture of 43.6 g of N-methyl-2-
pyrrolidone, 8.6 g of chlorobenzene and 24.9 g of dimethyl-
formamide (Merck) and heated up to 70 uC for a total of 3 h.
After about 60 min, 30 g triethylamine was added. The white
precipitate formed was filtered off, dried at room temperature
and finally heated at 200 uC for 8 h. The molar yield based on
zinc amounted to 87%.
MOF-5. Uniformly large crystals of MOF-5 were synthe-
sized by using an optimized procedure starting from
terephthalic acid, zinc nitrate and diethylformamide as organic
solvent.
In a glass reactor equipped with a reflux condenser and a
teflon-lined stirrer, 41 g of terephthalic acid (BDC) and 193 g
of zinc nitrate tetrahydrate (Merck) were dissolved in 5650 g of
diethylformamide (BASF AG; ,100 ppm water) and heated
up to 130 uC for 4 h. After about 45 min, crystallization started
and the formerly clear solution turned slightly opaque. After a
total of 4 h, the reaction product was cooled down to room
temperature. The solid was filtered off, washed three times
with 1 L of dry acetone and dried under a stream of flowing
nitrogen. Finally the product was activated at 60 uC for at least
3 h under a reduced pressure of ,0.2 mbar.
The wet chemical analysis of the thus obtained solid
yielded 33 wt% Zn, equivalent to 91% molar yield of MOF-5
calculated as Zn4O(BDC)3. The concentration from residual
nitrate amounted to 0.05 wt% N. Cubic shaped crystals in
between 50–150 mm size are to be observed by scanning
micrographs (Fig. 4). The PXRD pattern is shown in Fig. 5.
Specific surface area measurements with nitrogen at 77 K were
determined as 3400 m2 g21.
Fig. 3 Principle flowsheet scheme of industrial MOF synthesis
procedure. Cost efficiency and sustainability requires solvent recycling
and zinc oxide rather than zinc nitrate.Fig. 1 Illustration of metal–carboxylate building units of MOFs
(upper left: MOF-5 with a Zn4O-cluster linked to the terephthalic acid
molecules, upper right: IRMOF-8 with a Zn4O-cluster attached to the
2,6-naphthalene dicarboxylic acid, lower left: Cu-BTC with a dimeric
Cu-cluster terminated by 1,3,5-benzenetricarboxylic acid, lower right:
MOF-2 showing the paddle-wheel of a dimeric Zn-cluster linked to the
terephthalic acid units).
Fig. 2 Framework of MOF-5 displaying free access to nanosized
voids and the absence of non-accessible bulk-volume. The picture
shows a MOF-5 particle of about 500 nanocells with a cube edge
length of about 100 nm.
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Using argon adsorption at liquid argon temperature (87 K)
the adsorption properties were compared to state-of-art
materials like zeolite X and activated carbon (Ceca, carbon
AC40). As depicted in Fig. 6 the outstanding uptake behaviour
of argon on MOF-5 is obvious and clearly exceeds zeolites and
carbon components.
IRMOF-8. Large crystals of IRMOF-8 were grown from a
starting mixture containing 2,6-naphthalenedicarboxylic acid,
zinc nitrate tetrahydrate, diethylformamide and N-methyl-2-
pyrollidone as organic solvent.
In a glass reactor equipped with a reflux condenser and a
teflon-lined stirrer 7.5 g of naphthalenedicarboxylic acid
(NDC) and 67.4 g of zinc nitrate tetrahydrate (Merck) were
dissolved in 883 g of diethylformamide (BASF AG) and heated
up to 130 uC for 4 h. After about 75 min crystallization started
and the formerly clear solution became turbid. After the reac-
tion the product was cooled down to room temperature and
the precipitate was filtered off, washed three times with 1 L of
dry chloroform and dried under a stream of flowing nitrogen.
Finally the product was activated at 60 uC for 3 h under
reduced pressure of ,0.2 mbar giving 9.5 g of the final product.
Wet chemical analysis of the solid yielded 27.3 wt% Zn
equivalent to 15% molar yield of IRMOF-8 calculated on zinc
and 92% on NDC (calculated as Zn4O(NDC)3). Concentration
from residual nitrate amounted to 0.78 wt% nitrogen. The
Langmuir specific surface area reached 1750 m2 g21. All
crystals of IRMOF-8 had a cubic shape of about 100 mm size,
however, the scaly morphology indicated a high degree of
intergrowth.
Cu-MOF. For the first time, to our knowledge, MOFs are
synthesized using an electrochemical route.
Bulk copper plates, thickness 5 mm, are arranged as the
anodes in an electrochemical cell with the carboxylate linker,
viz. 1,3,5-benzenetricarboxylic acid, dissolved in methanol as
solvent and a copper cathode. Details are to be found in.10
During a period of 150 min at a voltage of 12–19 V and a
Fig. 5 PXRD of large MOF-5 crystals from laboratory preparation indicating superior crystallinity.
Fig. 4 SEM-picture of large MOF-5 crystals from laboratory
preparation (scale bar: 1 mm).
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currency of 1.3 A, a greenish blue precipitate was formed.
After separation by filtration and drying at 120 uC overnight
pure Cu-MOF was obtained. The surface area determination
yielded 1649 m2 g21 and after activation at 250 uC finally
1820 m2 g21 giving a dark blue coloured solid of octahedral
crystals from 0.5 to 5 mm size (Fig. 7)
The experimental setup of the electrochemical cell in a
laboratory glass reactor being under operation is depicted in
Fig. 8.
2.2 Analysis and characterization
Adsorption measurements were performed with commercially
available equipment (Autosorb 6, Quantachrome Corp.) using
nitrogen sorption at 77 K. Prior to measurement, samples
were activated down to 1024 mbar, first at 60 uC and finally
at 120 uC, until a constant vacuum was achieved for 14 h.
Surface area values were calculated according to the Langmuir
equation.
X-Ray powder diffraction was monitored on sealed samples
stored under dry nitrogen using Cu Ka radiation (D 5000,
Siemens). Scanning electron micrographs were taken at a 10 kV
electron beam using field emission cathode arrangement (JSM
6400F; Jeol).
Selected samples of electrochemically prepared Cu-MOF
were compared to conventionally synthesized materials by
EXAFS and XANES. Spectra were collected on beamline E4
at HASYLAB in the DESY synchrotron, Hamburg. Single
crystal data of monoclinic tenorite mineral (Cc-symmetry)
served as model basis. Although the X-ray powder diffraction
pattern of both Cu-BTC-MOF conventionally synthesized and
electrochemically prepared are very close to each other, the
samples offer clear differences with regard to the local fine-
structure of the copper. The electrochemically prepared
Cu-MOF gives Cu–K edge spectra which nicely agree with
the spectra of the mineral tenorite having as model a fourfold
coordination of Cu, whereas the Cu-BTC-MOF following
literature recipes12–14 is indicative of having a disturbed
Cu-environment and an additional peak at 8.98 keV photon
energy. The latter one is presumably due to the existence of
occluded nitrate moieties close to the open metal copper
ligand site, which could also explain the lower surface areas
of only 917 m2 g21.14 Beneficially, this does not occur on the
electrochemically prepared material.10 The latter is a very
useful adsorbent with a strong possibility to attract electron-
rich molecules on the open copper-sites, as is illustrated in our
gas purification example in section 2.5, below.
In order to measure intracrystalline self-diffusion, pulsed
field gradient NMR measurements were performed on the
NMR spectrometer FEGRIS 400 NT using the 13-interval
stimulated spin echo pulse sequence with two pairs of
alternating pulsed magnetic field gradients at the group of
Karger.15 The results were compared to literature data
obtained on NaX zeolite crystals and will be discussed in
detail elsewhere.15
Nevertheless, for ethane and benzene, in summary from
Fig. 9, it can be seen that diffusion in MOF-5 is clearly orders
Fig. 8 Synthesis cell for electrochemical preparation of MOFs with
Cu-plates as electrode material (laboratory scale).
Fig. 6 Adsorption isotherms of argon at 87 K on MOF-5 compared
to activated carbon and zeolite X.
Fig. 7 SEM-picture of Cu-MOF crystals from novel electrochemical
preparation (scale bar: 1 mm).
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of magnitude faster than in zeolite NaX. This is considered to
be a consequence of the difference between the diameters of the
two large nanoporous cavities in the MOF-5 (1.1–1.3 nm2)
over the smaller NaX supercage (1.2 nm) with an even lower
entrance window size (0.7 nm). The effective self-diffusion
coefficient of benzene in the MOF-5 structure is only slightly
smaller than the self-diffusion coefficient in the neat liquids at
the same temperature (C6H6: 2.5 61029 m2 s21). For both
catalysis and gas processing, this is an important observation
and promises fast molecular transport and low mass transfer
resistance as additional benefit when using MOF materials
rather than zeolites. In industrial applications of MOFs this
might contribute to permanent benefits in variable energy cost
over state-of-art solids.
2.3 Catalysis
For testing the catalytic activity in methoxypropene formation,
a differentially-running reactor (volume 200 ml) filled with
55 g of MOF-2 tablet material, obtained according to the
preparation, using pyrrolidone as solvent, were packed in a
fixed bed basket-type arrangement. The reactor was heated to
250 uC and fed with a mixture of methanol–cyclohexane at a
10 : 1 ratio and at a rate of 1.5 g h21. Propyne was added at a
flux of 6 g h21. Product analysis was done on line using gas
chromatography.
When the system had reached steady state conditions the
conversion of propyne was calculated to be 30% with a
selectivity of 80% into 2-methoxypropene.11
Using zinc silicate as catalyst in a comparative example the
selectivity towards 2-methoxypropene was found to be 77%,
however, full conversion at temperatures of only 180–200 uCwas obtained.21
In a different reaction, the esterification of 4-tert-butylben-
zoic acid was performed in a batch type test configuration. An
autoclave was filled with 2.5 g of MOF-2 material suspended
in 100 g of N-methyl-2-pyrrolidone and 40 g of 4-tert-
butylbenzoic acid. After inflating to 0.5 MPa of nitrogen, the
reactor vessel was heated to 180 uC and 2 MPa of acetylene
were introduced. The acetylene pressure was kept constant for
24 h. The final liquid product after cooling down was analyzed
by means of gas chromatography.11 In a repeated trial MOF-5
was used instead of MOF-2. Table 1 collects the results.
Interestingly, although both MOF catalyst materials reach
at almost the same conversion of tert-butylbenzoic acid, the
selectivity towards the vinyl ester is expressed far more by
the zinc paddle-wheel containing MOF-2, with about a
tenfold lower surface area.16 MOF-5 with fully saturated
zinc-coordination obviously is quite poor in performing in the
selective esterification.
Although a clear understanding of the underlying
mechanism is not yet available, there is without any doubt
proof that MOFs can act as catalysts. Successful catalysis on
zinc-containing MOFs in the activation of alkoxides and
carbon dioxide into polypropylene carbonate has already
been reported.17,18 Even enantioselective conversions with
an enatiomeric excess of 8% on homochiral metal–organic
POST-1 have been addressed by Kim and for heterogeneous
asymmetric catalysis by Lin19,20 towards chiral secondary
alcohols.
Further catalytic reactions from other research groups on
MOFs were recently reviewed and collected by Kitagawa.6
Ziegler–Natta-type polymerization, Diels–Alder-reaction,
transesterification, cyanosilylation of aldehydes, hydrogena-
tion and isomerization reactions were reported.
Future research is now dedicated to find out if the metal
centers, the ligands or functionalized ligands, or even metal–
ligand interactions or differences in particle size, may cause
unusual catalytic properties and if MOFs can compete with
well-known heterogeneous industrial catalysts. Beyond activity
and selectivity, also the questions of time-on-stream behaviour
and leaching stability will be crucial.
2.4 Gas purification
Cu-BTC-MOF, prepared as described above, was used in
demonstrating the removal of sulfur odorant components from
natural gas. In a fixed bed reactor vessel (inner diameter of
10 mm) about 10 g of granular Cu-MOF of a particle size
fraction of 1–2 mm were thoroughly packed. At a temperature
of 25 uC a gas stream of methane odorized with 13 ppm of
tetrahydrothiophene (THT) was fed over the packing and
analyzed in the reactor effluent by means of gas chromato-
graphy until breakthrough occurred (Fig. 10).
The used catalyst was removed from the reactor and
analyzed for residual sulfur content using bulk wet chemical
analysis. For comparison, the trials were repeated using
two commercially available activated carbon materials as
adsorbents, viz. Norit (type RB4) and CarboTech (type C38/4).
Taking the breakthrough curve of tetrahydrothiophene as
depicted in Fig. 10, it can be clearly seen that the sulfur
odorant is completely omitted from the natural gas in the
effluent and even detection by smelling is not any longer
indicative. The analysis of the used Cu-MOF after the test
cycle reveals an volume specific uptake of 70 g THT L21MOF
Table 1 Vinyl group esterification on MOFs
CatalystSurfaceArea/m2 g21
Conversion(mol% acid)
Selectivity(mol% ester)
MOF-2 330 94 83MOF-5 3400 91 56
Fig. 9 Comparison of diffusivity of ethane and benzene molecules in
MOF-5 over zeolite X experimentally determined from PFG-NMR
measurements.15
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which is considerably higher than 0.5 g THT L21 for Norit-
carbon or 6.5 g THT L21 for the CarboTech sample.
Interestingly, the colour of the Cu-MOF had changed from
a deep blue into a light greenish one.
Obviously, Cu-MOF is a powerful material for the separa-
tion of polar components from nonpolar gases. Looking at
the structure, the special arrangement of channels in
Cu-BTC-MOF together with open metal ligand sites offers a
dual type sorption behaviour.12–14 Bulk adsorption, on one
hand, exemplified e.g. during nitrogen loading at 77 K, aiming
at pore filling to deduce pore volumes, and secondly, the
predominant existence of open ligands at the copper
paddle wheel-cluster (cf. Fig. 1, lower left structure unit), give
rise to possible chemisorptive sites, the latter being nicely
kept free and unaffected during BASF’s electrochemical
synthesis starting from bulk metal. The anion-free preparation
omits the unwanted occlusion of nitrates into the metal
organic framework that is usually applied in state-of-the-art
preparations.13,14,22,23
Further electron-rich molecules that are successfully
removed by Cu-MOF in a similar manner are amines and
ammonia, water traces, alcohols and oxygenates.
In addition, in all cases a dominant color change readily
allows visible detection of breakthrough and contaminant
saturation on the MOF. During removal of the contaminant
by vacuum treatment or heating, the original color reappears,
indicating a possible regeneration of the adsorbent. Depending
on time and temperature applied, chemical analysis of the
sulfur content as well as the final weight of the used Cu-MOF
versus the fresh sample helps to monitor the degree of
regeneration.
2.5 Gas storage
Gas storage both at room temperature and 77 K up to 10 MPa
was measured on a homebuilt gravimetric prototype equip-
ment comprising two highly sensitive balances A and B
(Mettler, PG 5002-S, ¡0.01 g displayed). Balance A carried a
stainless steel (material type 1.4541) fabricated container A of
about 460 ml content which was filled up to the neck with
about 150 g of ‘in-situ’ activated MOF sample sitting in
a thermostatted Dewar vessel. This experimental setup
guaranteed a sufficiently high resolution of storage capacity
at a sensitivity of about 104. The reproducibility usually was
better than ¡2%.
Balance B monitored the weight of a high pressure hydrogen
feed flask B (e.g. 20 MPa). By controlled stepwise opening of
valves in the connecting piping steel system between containers
A and B, there was a redundant checking of weight and
pressure increase in the MOF-containing vessel, whereas
weight and pressure drop was indicated in the hydrogen
feeding container B. Of course uptake on one side and release
data on the other side had to fit to each other in terms of
proving a leak-free mass balance. Blank runs without MOF-
sample were repeated prior to each measurement. In order to
obtain weight-specific uptake the data was corrected for bulk
volume using the helium density of the MOF-sample.
As for the MOF-5 sample adsorption isotherms with either
argon, krypton or xenon at room temperature were registered
and compared to their conventional compression curve up to
about 5 MPa.
Obviously, as shown in Fig. 11, for all rare gases under
investigation (Ar, Kr, Xe) the volume specific uptake is
higher in case of a gas cylinder being filled with MOF-5. This
effect is becoming even more expressed from argon over
krypton to xenon. In the latter case, at about 10 bar pressure,
the amount of xenon stored in a MOF-filled container is
about fourfold higher (if one compares the curves vertically).
Of course such an enhanced uptake of one gas over another
can be exploited to separate gas mixtures of e.g. krypton and
xenon. Details on the performance of such a process are given
in section 2.6.
Looking at a storage level in the xenon curve of about
100 g Xe L21 in Fig. 11, it becomes clear from a horizontal
comparison of the isotherms at a constant uptake value that
with a MOF-filled container the pressure which is needed
for holding a gas in a given volume is reduced from about 17
to 2 bar.
Thus, storage of a gas in MOF-filled canisters can be used
either way to enhance capacity in a given volume or to
transport an equivalent amount of gas at a far lower pressure.
Fig. 11 Compression curves of rare gases Ar, Kr, Xe and comparison
over inflation curves into MOF-5-filled gas containers (room
temperature, up to 60 bar; lecture bottles).
Fig. 10 Breakthrough-curve of continuous tetrahydrothiophene-
removal from natural gas using Cu-BTC-MOF out of electrochemical
synthesis.
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Similarly, other gases like methane and hydrocarbons can be
held at a denser state in the same manner.22,24
The same finding holds for propane as well. In Fig. 12 are
depicted the curves for shaped MOF-5 pellets in gas containers
over non-filled containers. Again the inflation curves are
markedly different. The non-MOF-filled container represents
the almost linear uptake behaviour, whereas the MOF-filled
can is far higher and with a steeper increase at the beginning.
Taking the pressure at about 10 bar it becomes obvious that
one MOF-filled container substitutes the amount of three
state-of-art pressurized cylinders.
Already today much interest is attributed to the storage of
hydrogen for mobile and portable fuel-cell application by
using hydrogen as a carrier for electric power supply.25 Due to
the need for alternative fuel sources and energy carriers, the
target values of US Department of Energy were recently
reviewed,8 compiling storage data of some 0.2–3.8 wt% of
hydrogen on MOFs, the maximum on a weight-specific
calculation being found by Ferey on MIL-537 derived from
aluminium salts and BDC. Pillaring with secondary amine
(triethylenediamine) linkers as strategy was employed by
Kim27 and Seki22 and MOF-505 again by Yaghi-group28
with Cu paddle-wheels connected by 3,39,5,59-biphenyltetra-
carboxylic acid ranged up to more than 2 wt%. Doubly
interpenetrated nets of zinc frameworks built by NTB-linkers
(4,49,40-nitrilotrisbenzoic acid) by Suh29 were reported to reach
1.9 wt%. of hydrogen uptake at 77 K. However, it is not
yet possible to foresee if large surface area materials like
MOF-177,29 MIL-10032 or MOF-5 and isoreticular mem-
bers,2–4 or materials with an average surface of between 1000
to 1500 m2 g2126,29 or even small pore MOFs like30,33 will be
the most promising storage media. Neither can it be concluded
if divalent or trivalent metal-clusters34 are the most favourable
ones. Simulations obviously support metal–organic frame-
works as being favorable over zeolites.31
From results of our prototype equipment (77 K tempera-
ture, up to 40 bar) it can be seen, how different MOF-materials
contribute differently to volume-specific hydrogen storage
(cf. Fig. 13).
Comparing to the pressurizing of an empty container with
hydrogen MOF-5, IRMOF-8 and Cu-BTC-MOF increasingly
take up higher amounts of hydrogen, all of them exceeding the
standard pressure–volume–temperature (PVT) uptake curve of
the empty container and with steepest incline below 10 bar. At
40 bar the PVT-relation of hydrogen in an empty canister is
registered as 12.8 g H2 L21, whereas Cu-BTC-MOF filled into
containers reach a plus 44% capacity of up to 18.5 g H2 L21.
For comparison, the volume-specific density of liquid hydro-
gen at its boiling point (20 K) is 70 g H2 L21.
Above 10 bar, the curves run mostly parallel to the
conventional H2-pressure–volume relation. On a per weight-
calculation it becomes clear that the saturation of MOFs with
hydrogen is already achieved at pressures of less than 15 bar
(Fig. 14). For electrochemically-prepared Cu-BTC-MOF this
attributes to about 3.3 wt% H2-uptake.
This data from our large scale prototype compares reason-
ably well with the literature.8
It should be mentioned here, that for many volume-limited
fuel-cell applications, i.e. the mobile and portable cases, it
will be industrially much more relevant to compare storage
data on a volume-specific rather than on a weight-specific
storage capacity basis. Typically the packing densities of
MOF powders are around 0.2 to 0.4 g cm23 increasing to
0.5–0.8 g cm23 when shaped into tablets or extrudates. So far,
this material density is so low that weight limitation in an
application is non-existent, in contrast of course to the
alternative use of metal hydride as storage media. Much
more importantly, however, there is only a strictly confined
volume available in both mobile automotive as well as in
Fig. 13 Volume-specific storage curves of hydrogen on different
MOF-materials in comparison to compression curve of hydrogen into
empty gas containers (77 K, 40 bar; measurement from BASF
prototype setup as described in section 2.5).
Fig. 14 Volume-specific versus weight-specific storage curve of
hydrogen on Cu-BTC-MOF from electrochemical preparation.
Fig. 12 Compression of propane into gas container with and without
MOF-filling (MOF-5 tablets in lecture bottles, room temperature).
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portable electronic device applications. Hence, storage data
communicated as weight-specific rather than volume-specific
numbers can be misleading by merely focusing on only one of
them. Depending on the intended application and the given
boundary conditions, it might be necessary in future to
indicate both types of values.
Anyhow, the most important issue is the amount of
hydrogen which, in a reasonable time-scale, can be discharged
from storage media. Here, MOFs really do have a fully
reversible uptake and release behaviour. As the storage
mechanism is predominately physisorption, there are no huge
activation energy barriers to be overcome as compared to e.g.
metal hydrides when trying to liberate the stored hydrogen.
Simple pressure reduction by controlled valve opening is
sufficient to draw off hydrogen from MOFs within a few
seconds.
Energy density values of 1.1 kW h L21, as requested in the
European Hydrogen and Fuel Cell Strategic Research Agenda
and Deployment Strategy,25 are equivalent to a volumetric
hydrogen storage capacity of about 33 g H2 L21. As
demonstrated, almost two thirds of this value can be reached
by storing hydrogen in MOFs at 77 K and a moderate pressure
of 40–50 bar. Increasing this volumetric capacity by shaping
MOF-powders into tablets and extrudates is proved to be
feasible. Presently, a high amount of storage at room
temperature has not been attained.
Further research concepts are dedicated to increase the
storage capacity and to shift it to higher temperatures. Once it
is clarified where the hydrogen molecules are favourably
adsorbed and attached in the different MOFs, it will be easy to
browse through the huge variety of several hundred (and
constantly still-increasing) numbers of structures and to
identify the most promising examples. In this respect,
molecular modelling tools might become as important as
elaborate experimental synthesis efforts.31 It should be kept in
mind that depending on the temperature required for a
possible application, highly porous MOFs might be favourable
for low temperatures, whereas rather small pore materials,30,33
or highly attractive and flexible ones,26,35 could be favourites
for room temperature storage. New mechanisms bridging
chemisorption (as in hydrides) and physisorption (as in metal–
organic frameworks) might be also requested to meet future
challenges.
2.6 Gas separation
Already during the monitoring of the diverse uptake behaviour
of rare gases on MOF-5 (Fig. 11) it became obvious that
this property could possibly be used to separate mixtures
of rare gases by adsorption on MOFs. Such mixtures are
usually to be found technically in cryogenic air separation
units and once the gases are separated, xenon and krypton
can be marketed separately, e.g. xenon as narcotic medical
gas and krypton as filler for lamp industry. Unlike cryogenic
distillation, the following experiments indicate a far simpler
process of pressure–swing adsorption to separate rare gases
mixtures.
A mixture of Kr (ca. 94% mol) and Xe (ca. 6% mol) was fed
continuously to an isothermal tubular reactor (2 m length,
1.6 cm diameter) filled with 193 g Cu-MOF at a given
temperature of 55 uC and 40 bar (Fig. 15). The flow rate
(60 L h21) and reactor dimensions were chosen appropriately
in order to prevent complete saturation of the MOF material
during the time-span of the measurement. The pressure
was adjusted manually by a needle valve at the reactor outlet.
The gas composition leaving the adsorber was detected by
an on-line mass spectrometer (Pfeiffer Vacuum OmniStar2
QMS-200).
In the gas stream leaving the adsorber, Xe was reduced to a
level of ca. 50 ppm. After more than 100 min on-stream the
MOF became saturated with Xe and a rapid breakthrough was
observed. The calculated capacity of the Cu-MOF for Xe was
more than 60 wt%. This is almost twice as much Xe as a high
surface active carbon (Ceca, AC 40, ca. 2000 m2 g21) could
take up under identical conditions.
Due to the fact that MOFs exhibit a gas molecule mobility
of about two to three orders higher than state-of-art molecular
sieves or active carbons, a faster swing operation period
between adsorption and desorption cycle seems possible. In
terms of economic consideration, this contributes to fairly
reduced purge time, energy consumption and overall variable
costs thus rendering MOFs beneficial over established
technology. Again novel material properties of metal–organic
frameworks, viz. their porosity with nanometer size pores in
regular arrays and the absence of blocked bulk volume
contribute to principal differences in performance.
Looking into the literature where some zeolite-like (MTN
topology) metal–organic frameworks have already been
observed, it will be interesting to watch if, with the small pore
materials, separations based on molecular sieving might
become possible.30,33,36 Given this possibility, MOFs would
grow to be competitors to zeolitic molecular sieves, which
currently have a market volume of some hundred thousand
tons capacity per year.
3. Conclusion and prospects
In this article, we have shown that metal–organic frameworks
or coordination polymers are not merely a new class of porous
materials for combining inorganic and organic chemistry
classifications. From an industrial point of view, they offer,
Fig. 15 Gas separation of Kr–Xe mixture by continuous adsorption
on electrochemically produced Cu-BTC-MOF.
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in principle, many interesting and promising features over
prior art, viz.
N world records in surface area
N ultimate porosity with absence of blocked volume in solid
matter
N combined flexible and robust frameworks
N full exposure of metal sites
N high mobility of guest species in regular framework
nanopores
N fast growing number of novel inorganic–organic chemical
compositions.
Obviously, many applications might (and surely will) be
tested once verified synthesis recipes of MOFs are available.
The recipes given in the Experimental section will allow the
reader to prepare these new compounds in laboratory-scale
amounts. However, industrial synthesis at BASF is understood
to be far more advanced, already into barrel-size pilot scale,
and additional issues need to be taken into account during the
manufacturing procedures, which of course are beyond the
scope of this paper.
Unlike many other novel materials, e.g. carbon polymorphs,
fullerenes, bucky-balls, CNT, the metal–organic framework
materials’ preparation and fabrication does not necessarily
need additional capital investment into a totally new synthesis
technology. Simply adaptation of conventionally available
precipitation and crystallisation manufacturing methods needs
to be done. Shaping of metal–organic framework powders
into industrially widespread geometries of mm-sized tablets,
extrudates, honeycombs, etc. can be performed on MOFs as
well without any major obstacle.
The examples which we gave for catalysis as well as for gas
processing and storage already indicate that there is much
room left for many future research efforts (viz. storage of
alternative energy carriers like small hydrocarbons, odour
removal in both stationary—e.g. household—as well as
mobile (bus, train, subway, ship) environments or the adverse
odorization in carrying perfumes, etc. Pick-up of liquids
without swelling of solids could be of interest as well, e.g. for
food packaging or removal of hazard liquids like organic
solvents, oils, brake fluids and the like).
It is worthwhile to mention that, unlike state-of-art
heterogeneous catalysts, the metal sites in MOFs are usually
fully exposed, therefore giving an ultimately high degree of
metal-dispersion. From supplementary work we already
know that these metal sites usually behave differently from
bulk metals. Compared to zeolites the amount of metals in
MOFs are by almost a factor of ten higher and many of the
metal species belong for chemists to the interesting class of
transition metals.
In summary and perspective, all this might lead to a fast
growing, prosperous and widespread innovation in materials
science, both in academia and industry. However, one
certainly has to keep attention to find superior performance
by applying MOFs over state-of-art technologies. It will
never be sufficient just to find a ‘me-too’ solution instead
of looking for considerable improvement of the best existing
one. Only the latter approach will finally contribute to true
innovation and value-added growth of industrial companies
and society.
Acknowledgements
Technical assistance of Dr O. Metelkina-Schubert, Dr Cox,
S. Lutter, W. Kippenberger, U. Diehlmann, R. Hess, R. Ruetz,
I. Schwabauer, R. Senk, and H. Sichler is gratefully acknowl-
edged. U.M. thanks O.M. Yaghi and S. Kitagawa for many
stimulating discussions on metal–organic frameworks (MOFs)
and coordination polymers (CPLs).
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