Metal–organic frameworks—prospective industrial applications

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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http://dx.doi.org/10.1039/b511962f

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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.

This journal is � The Royal Society of Chemistry 2006 J. Mater. Chem., 2006, 16, 626–636 | 627

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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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Metal–organic frameworks—prospective industrial applications

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