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Improved synthesis, thermal stability and catalytic properties of the metal-organic framework compound Cu3(BTC)2
Microporous and Mesoporous Materials 73 (2004) 81–88 www.elsevier.com/locate/micromeso
Improved synthesis, thermal stability and catalytic properties of the metal-organic framework compound Cu3(BTC)2 Klaus Schlichte, Tobias Kratzke, Stefan Kaskel
*
Max-Planck-Institut f€ur Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 M€ulheim an der Ruhr, Germany Received 9 November 2003; received in revised form 22 December 2003; accepted 23 December 2003 Available online 8 June 2004
Abstract The catalytic properties of the metal-organic framework compound Cu3 (BTC)2 (H2 O)3 Æ xH2 O (BTC ¼ benzene 1,3,5-tricarboxylate) were explored. Cu2 O-free powder samples of Cu3 (BTC)2 (H2 O)3 Æ xH2 O were obtained using an improved synthesis at 393 K under hydrothermal conditions. The microporous material has a high specific pore volume of 0.41 cm3 g1 and a pore diameter of (Horvath-Kawazoe). Removal of the three copper-bound water molecules allows to access the Lewis acid copper sites. The 10.7 A exchange of coordinated water by substrates or solvent molecules is recognized from the color change of the compound. For chemisorbed benzaldehyde, the IR stretching frequency m(C@O) is decreased from 1702 to 1687 cm1 . The chemisorption results in an activation of benzaldehyde for the liquid phase cyanosilylation with a reasonable yield of 50–60% after 72 h (313 K) and a high selectivity. Filtration experiments demonstrate that the reaction mechanism is heterogeneous. Coordinating solvents such as THF completely block the Lewis acid sites of the catalyst. Solvents such as CH2 Cl2 or higher reaction temperatures (353 K) cause decomposition of the catalyst. 2004 Elsevier Inc. All rights reserved. Keywords: Metal-organic frameworks; Copper; Catalysis; Cyanosilylation
1. Introduction In recent years, metal-organic frameworks (MOFs) have received considerable attention as potentially valuable gas storage and catalyst materials [1–7]. They are composed of an organic linker, for example a dicarboxylic acid and a transition metal complex or cluster, both units being building blocks that are designed to assemble a three-dimensional open framework. High micropore volume, large pore sizes, crystallinity and a high metal content offering potentially valuable active sites are the key features of this new and emerging class of porous materials. Especially large pore size within the crystalline framework and well defined pore architecture is a pre-requisite for shapeselective transformation of larger molecules and is important in fine chemical syntheses [8]. However, lim* Corresponding author. Present address: Inst. fur Anorganische Chemie, Technische Univ. Dresden, Mommsenstr 13, 01062 Dresden, Germany. Tel.: +49-351-46333632; fax: +49-351-46337287. E-mail address: [email protected] (S. Kaskel).
1387-1811/$ - see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2003.12.027
itations are mostly due to the low thermal stability of the organic linker. Redox reactions of the linker with the transition metal typically limit the applicability to temperatures lower than 600 K. Thus, reactions catalyzed at low temperature, such as liquid phase transformations, seem to be more promising as compared with high temperature gas phase reactions. So far, only few catalytic applications of metalorganic frameworks have been reported. Some of the potential applications of metal-organic frameworks were outlined recently [9]. For example, Zn-containing frameworks were used for the addition of alcohol or carboxylic acids to acetylene and propyne. Thus, the addition of methanol to propyne produces 2-methoxypropene with a propyne conversion of 30% and 80% selectivity [9]. Our interest is to explore the catalytic properties of metal-organic frameworks. From the structural and esthetical point of view, MOFs are engineered by linking cluster building blocks at their coordination sites via multifunctional linkers into perfect and esthetic architectures, the topology being pre-defined by the
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arrangement of the coordination sites and the orientation and number of the binding groups in the linker. However, an aspect often overlooked is that connecting the coordination sites to the linker also blocks such sites for catalytic transformations. A change in coordination geometry is often required for dissociation-association mechanisms, that are responsible for the high catalytic activity of molecular transition metal complexes in organometallic chemistry. However, such a structural rearrangement is not desirable for the application of metal-organic frameworks in heterogeneous catalysis, because dissociation of the cluster–linker bond will inevitably destroy the framework. Instead, the sites must be oriented towards the pore interior and be accessible for molecules. The high metal content alone is not a guarantee for catalytic activity. We have therefore screened a variety of known MOFs and selected a compound in which residual coordination sites are accessible from the pore interior. A candidate fulfilling these requirements is Cu3 (BTC)2 (H2 O)3 . The crystal structure of Cu3 (BTC)2 (H2 O)3 was reported by Chui et al. [10]. Recently, Wang et al. have studied the potential use of Cu3 (BTC)2 (H2 O)3 for gas purification and separation [11]. Monte Carlo simulations in conjunction with high-resolution low-pressure argon adsorption measurements were carried out by Vishnyakov et al. [12].
2. Experimental 2.1. Synthesis of Cu3 (BTC)2 (H2 O)3 xH2 O In a typical synthesis, 0.875 g (3.6 mmol) Cu(NO3 )2 Æ 3H2 O were dissolved in 12 ml de-ionized water and mixed with 0.42 g (2.0 mmol) of trimesic acid dissolved in 12 ml ethanol. The solution was filled in a 40 ml Teflon liner, placed in an autoclave, and heated to 393 K for 12 h. 2.2. Characterization Nitrogen physisorption measurements were carried out using a micromeritics ASAP 2000 instrument. X-ray powder diffraction patterns were recorded on a STOE diffractometer, equipped with a position sensitive detector (Braun, 6) and a germanium primary beam monochromator in the transmission mode using CuKa1 For the XRD measurements, the radiation (1.54051 A). dried powder was filled into capillaries (0.5 mm diameter) and sealed under argon if necessary. 2.3. Catalytic tests The catalytic test reactions were carried out in solution using various solvents and temperatures (Table 1).
Table 1 Yields for the Cu3 (BTC)2 -catalyzed liquid phase cyanosilylation of benzaldehyde and acetone Substrate
Solvent
Temperature/ time
Yield/%
Benzaldehyde Benzaldehyde Benzaldehyde Benzaldehyde Benzaldehyde Acetone Acetone
CH2 Cl2 Pentane Heptane Toluene THF Toluene Heptane
313 313 333 353 313 333 333
50 40 55 20 2 12 25
K/48 K/48 K/48 K/24 K/72 K/48 K/48
h h h h h h h
The solvents were dried prior to use using standard drying techniques. The as made framework material (129 mg, 0.21 mmol) was activated in vacuum (1 · 104 mbar, 393 K) either prior to the test in the flask, or directly after the synthesis. In the latter case, appropriate amounts of the activated powder were transferred to a reaction vessel using a vacuum line or a glove box. However, handling in a glove box is not necessary, since the catalyst can be activated (dehydrated) in vacuum or in an inert gas stream in the reactor prior to use. After introduction of the solvents and reactants (trimethylsilylcyanide (TMSCN), 8 mmol, Aldrich, 98%; benzaldehyde, 4 mmol, Fluka, distilled, 99.9%) the suspension was stirred and the products were analyzed using GC/MS (n-octane and n-hexadecane used as internal standard). For the filtration test, the catalyst was separated after 8 h and the solution was divided into two equal portions. Hereafter, one portion was stirred with and the other one without the catalyst.
3. Results and discussion 3.1. Improved synthesis and up-scaling Typical synthesis procedures described by Chui et al. [10] produce Cu3 (BTC)2 (H2 O)3 Æ xH2 O at 453 K under hydrothermal conditions in the form of single crystals, however, significant amounts of Cu2 O are obtained as a by-product, using these high temperature syntheses. High temperatures are beneficial for single crystal work but impurities can significantly affect the catalytic properties of solids and thus it is necessary to develop synthesis methods for metal-organic frameworks that do not yield side products. We have therefore optimized the synthesis conditions, obviously parallel to another group [11], in order to obtain pure Cu3 (BTC)2 (H2 O)3 Æ xH2 O samples for catalytic tests without Cu2 O impurities. According to our results, Cu2 O-free samples are obtained under hydrothermal conditions after 12 h at 393 K. The lower synthesis temperature allows to suppress the formation of Cu2 O since the reduction of the Cu2þ ions is avoided. A powder X-ray diffraction pat-
83
(111)
(311)
Intensity /a.u.
K. Schlichte et al. / Microporous and Mesoporous Materials 73 (2004) 81–88
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
2Theta
Fig. 1. Observed (top) and calculated (bottom) X-ray diffraction pattern of Cu3 (BTC)2 (H2 O)3 Æ xH2 O.
tern of a typical sample is shown in Fig. 1 and compared with a pattern calculated from crystallographic data. The overall agreement is good, however some deviation is observed in the relative intensities due to variations in the degree of hydration (see below).
Fig. 2. Structure of the Cu2 -paddle-wheel in Cu3 (BTC)2 (H2 O)3 .
3.2. Structure The crystal structure of Cu3 (BTC)2 (H2 O)3 Æ xH2 O was reported by Chui et al. [10]. As compared to other metal-organic frameworks, with most of the coordination sites blocked by the frameworkconstituting ligand, Cu3 (BTC)2 (H2 O)3 Æ xH2 O has the advantage that a Lewis acid coordination site is on the interior of the pore wall, and thus, copper sites are accessible for catalytic conversions. It is therefore important to understand the network structure. In the cubic network, Cu2 -clusters are coordinated by carboxylate groups to give a so-called paddle-wheel unit in which four carboxylate groups are arranged in a square (Fig. 2). The binuclear cluster is a common feature of Cu2þ carboxylates. The Cu2þ ions are connected through a weak bond and the residual axial coordination site is filled by a weakly bound water molecule. These primary building blocks are connected through the BTC ligand (trimesic acid) into a three-dimensional cubic network with an open pore system (Fig. 3). The weakly bound water molecules point towards the center of the pore. This hydrophilic interior, with 12 water molecules per pore, is of high symmetry and the oxygen atoms of these water ligands are arranged to form a cuboctahedron (Fig. 4). The vertical and diagonal O–O respectively. The same distances are 8.25 and 11.67 A, geometry is obtained for the copper atoms but the corresponding Cu–Cu distances across the pore are 11.3 respectively. and 16.0 A,
Fig. 3. View inside the pore of Cu3 (BTC)2 (H2 O)3 . The axially bound water molecules point towards the pore center.
3.3. Thermal stability The thermal stability was studied using TG/DTA and high temperature X-ray diffraction. The as made samples show a continuous mass loss of 32% up to a temperature of 523 K due to the dehydration of the material (Fig. 5). The weight change corresponds to more than 3 mol H2 O per Cu3 (BTC)2 . The calculated weight change for 3, 6 and 9H2 O molecules per Cu3 unit is 8.2%, 15.2%, and 21.1%, respectively. The observed weight loss heavily
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Cu3(BTC)2(H2O)3•xH2O
873 K 773 K 673 K 573 K 473 K 373 K 20
25
30
35
40
45
(420)
(331)
15
Fig. 4. Polyhederal arrangement of the axially bound water molecules inside the pore.
50
2θ
Cu3(BTC)2 100 90
dried in air (363 K)
Weight /%
80 as made
70 60
dried in
873 K
vacuum
773 K
(363 K)
673 K 573 K 473 K
rehydrated
373 K
50 15
40 30 273
20
25
30
35
40
45
50
2θ 373
473 573 Temperature / K
673
Fig. 5. Thermogravimetric analysis of Cu3 (BTC)2 (H2 O)3 Æ xH2 O with variations in the degree of hydration.
depends on the pre-treatment of the material. If the material is dried in high vacuum (1 · 104 mbar, 373 K), a weight loss of only 1–2% is observed. For a material dried in air (363 K), the weight loss is 12.2%. If the material dried in high vacuum is re-hydrated in a stream of moist argon, the weight loss is 36.4%. For re-hydration in air, the weight loss is typically lower (27%, not shown in Fig. 5). Re-hydrated samples show a clear step at 373 K, whereas for the as made sample, desolvation covers a wider region due to residual amounts of ethanol physisorbed inside the pore system. In Chuis crystal structure, additional H2 O molecules, that do not bind to copper, occupy the 96 j position (Fm 3 m) in the pore interior [10]. Full occupancy of the 96 j position would give the fully hydrated composition Cu3 (BTC)2 (H2 O)3 Æ 6H2 O. However, for re-hydrated samples, the experimentally determined water content is higher than 21.1%, demonstrating the structures’ capacity for more solvent molecules in the pore interior. A second weight change is observed at 623 K due to the decomposition of the network, but the weight loss is significantly lower as expected for a complete transformation into CO2 and Cu2 O or Cu metal.
Fig. 6. In situ X-ray diffraction analysis of Cu3 (BTC)2 (H2 O)3 Æ xH2 O and the dehydrated Cu3 (BTC)2 .
High temperature X-ray diffractograms reveal the structural changes during the heat treatment (Fig. 6). We have studied the decomposition of as made Cu3 (BTC)2 (H2 O)3 Æ xH2 O and the dehydrated Cu3 (BTC)2 . Both samples start to decompose at temperatures exceeding 533 K to form metallic copper. Whereas the decomposition in air typically produces Cu2 O, the reason for copper metal formation in our experiments is the protective atmosphere of argon that prevents oxidation. In that case, the ligand itself reduces the metal and is pyrolyzed to form carbonic deposits. Some small differences are observed in the two experiments. Whereas in the dehydrated sample only one peak appears at 14.5, on the other hand, in the hydrated sample, two small peaks are observed around 14.5, the (3 3 1) and (4 2 0) reflection. However, when the hydrated sample is heated to 473 K, the two peaks are merged into one peak and the pattern is identical to that of the dehydrated sample. In Fig. 7a, the calculated diffraction patterns are shown for three idealized structures: The completely dehydrated framework (bottom), the partially hydrated and the fully hydrated form. The minute changes detected using in situ X-ray diffraction are partially understood: The intensity ratio of I3 3 1 =I4 2 0 decreases,
K. Schlichte et al. / Microporous and Mesoporous Materials 73 (2004) 81–88
85
(b)
(a)
Cu3 (BTC)2 (H2 O)3 •xH2 O as made
Cu3 (BTC)2 (H2 O)3 •6H2O
Cu3 (BTC)2 (H2 O)3 •xH2 O rehydrated, 29 % water
(200)
(200)
(222)
Cu3 (BTC)2 (H2 O)3
Cu3 (BTC)2
6.0
8.0
10.0
14.0
16.0
2Theta
(331) (420)
(111)
(440)
(333)
(422)
(400) 12.0
(331) (420)
(311)
(111) 4.0
(220)
(220)
Cu3 (BTC)2
4.0
6.0
8.0
10.0
12.0
14.0
16.0
2Theta
Fig. 7. (a) Calculated and (b) observed X-ray diffractograms of Cu3 (BTC)2 (H2 O)3 Æ xH2 O with variations in the degree of hydration.
when the naked framework is hydrated with 3H2 O molecules. These differences due to variations in the degree and structure of the hydrated forms are even more obvious in ex situ analyzed samples (Fig. 7b): (1) The I3 3 1 =I4 2 0 ratio is higher for the completely dehydrated form. (2) The (1 1 1) reflection is present in the as made and in the dehydrated sample, but not in the rehydrated sample. This observation is in agreement with the calculated patterns, but the water content of the samples cannot be correlated to the intensity of the (1 1 1) reflection, since the occupation of the physisorbed water positions does not necessarily result in long range ordered physisorbed molecules. (3) The I2 0 0 =I2 2 0 ratio is significantly higher for the dehydrated sample. The changes in the observed powder patterns roughly reproduce the differences predicted in the calculated patterns (Fig. 7a) and demonstrate that the small changes in the diffraction patterns can be attributed to variations in the degrees of hydration.
3.4. Nitrogen physisorption results A typical nitrogen physisorption isotherm of Cu3 (BTC)2 is shown in Fig. 8. A type I isotherm is observed for the microporous network with a specific micropore volume of 0.41 cm3 g1 and a Horvath (cylinder model). The Kawazoe (HK) pore size of 10.7 A sharp increase of nitrogen adsorption at a relative pressure of 104 mbar is recognized in logarithmic plots (Fig. 8, inlay) and indicates a narrow pore size distribution as expected for a porous crystalline framework. 3.5. Exchange of coordinated water As indicated above, physically and chemically bound water molecules are easily removed from the host material by heating the compound in vacuum. The dehydration makes the copper coordination sites accessible for other molecules. The dehydration of the compound is accompanied by pronounced color changes. Whereas the as made material is blue-turquoise, the dehydrated material shows a dark-violet color. Re-adsorption of acetone, THF, ethanol, acetonitrile, benzaldehyde, smaller amines, and ammonia restore the original color indicating coordination to the copper
K. Schlichte et al. / Microporous and Mesoporous Materials 73 (2004) 81–88
Volume adsorbed /cm3g-1
86
400
300
200
100
-log(P/P0) 6
5
4
3
2
1
0
0 0.1
0.2
0.3
0.4
0.5
P/P0 Fig. 8. Nitrogen physisorption isotherm of Cu3 (BTC)2 at 77 K.
C@O stretching frequency of pure benzaldehyde in solution is located at 1702 cm1 . When benzaldehyde is adsorbed from a diluted solution, the chemisorbed aldehyde is only barely detected as a shoulder in the band of the free aldehyde. In this case, the aldehyde is located inside the pores but only part of it is coordinated to the copper sites. The physically bound aldehyde is partially removed by heating to 373 K in vacuum (1 · 104 mbar), which allows to detect the position of the chemisorbed aldehyde at 1687 cm1 . In situ adsorption of benzaldehyde from the gas phase to an activated sample in an IR microscope gave the same results, and according to adsorption measurements carried out at room temperature, the line position of the chemisorbed benzaldehyde is located at 1686 cm1 . Thus the shift of the C@O stretching frequency to lower wave numbers caused by the coordination of the C@O group to the copper site is 16 cm1 . Whereas other substrates such as water can be removed from the pore by activation (1 · 104 mbar, 373 K), benzaldehyde is bound to the pore and cannot be removed in vacuum without decomposition of the copper network. 3.7. Catalytic properties
Fig. 9. Color changes during the dehydration (activation) of Cu3 (BTC)2 (H2 O)3 Æ xH2 O to give Cu3 (BTC)2 , and subsequent readsorption of the aldehyde to give Cu3 (BTC)2 (C6 H5 CHO)x .
ions (Fig. 9). Whereas for most of the substrates (carbonyl compounds, amines) the network structure remains intact, for some substrates, such as gaseous ammonia, the adsorption results in an irreversible transformation of the network into another crystal structure.
The cyanosilylation of aldehydes and ketones is a convenient route to cyanohydrin derivatives and can be used under thermal and catalytic conditions using molecular or solid Lewis acid catalysts [13,14]. Cyanosilylation of aldehydes was reported to occur with high selectivity over Cd(4,40 -BPY)2 (NO3 )2 , a square network material (Scheme 1) [15]. The framework is highly shape-selective for the inclusion of ortho-dihalogenobenzenes (halogen ¼ chlorine, bromine), whereas the meta- and para-isomers do not form clathrates. Shape specificity, similar to that in clathration was observed in cyanosilylation [15]. For example the conversion of 2tolualdehyde (40%) is significantly higher as compared to 3-tolualdehyde (19%). We have used the liquid phase cyanosilylation of benzaldehyde as a test reaction to study selectivity and activity of the metal-organic framework compound Cu3 (BTC)2 . For all tests, the material was dehydrated in high vacuum at 373 K in order to remove physically and chemically bound water molecules, before the material is used as a catalyst. Thus, the Lewis acid copper(II) sites of the Cu2 -paddle-wheel become accessible for the coordination of the aldehyde (Fig. 9).
3.6. IR measurements The chemisorption of substrates is detected ex situ using FT-IR spectroscopy in the transmission mode, for example by treating activated samples in the bulk with the suitable substrate either in solution or with a gas phase and subsequent preparation of KBr pellets. The
OSiMe3 CHO
Me3SiCN
CN
Scheme 1. Cyanosilylation of benzaldehyde.
K. Schlichte et al. / Microporous and Mesoporous Materials 73 (2004) 81–88
100
Mol%
80
CHO
60 OSiMe3
40
CN
20 0 0
10
20
30
40
50
60
70
t/h Fig. 10. Educt and product concentration of the Cu3 (BTC)2 -catalyzed cyanosilylation of benzaldehyde (solvent: pentane, temperature: 313 K).
Initial tests using the reaction conditions reported in [15] gave only little conversion at 293 K (below 5% after 24 h). At higher temperature, 313 K, the conversion is significant. Fig. 10 shows the concentration of benzaldehyde and the product for a reaction carried out at 313 K in pentane in the presence of Cu3 (BTC)2 . After 72 h, the yield is 57% (88.5% selectivity). For comparison, a blank experiment was carried out under the same conditions without catalyst. The thermal conversion was below 10% after 72 h. 3.8. Impact of the solvent In general, solvents of low polarity are beneficial for the use of Cu3 (BTC)2 because of the low solubility in such solvents, but also due to the competing adsorption of donor type solvents at the Lewis acidic copper site. For example, tetrahydrofuran (THF), and other ethers that are coordinated, block the acidic sites. A typical example is shown in Fig. 11. Here the reaction was carried out in THF and practically no conversion is
87
observed after 72 h of reaction time. The latter further supports the reaction mechanism and the role the catalyst plays i.e. the coordination of the aldehyde to the copper center being responsible for the enhanced reaction rate. We have used pentane, heptane, toluene, and CH2 Cl2 as solvents (Table 1). They are quite suitable, but the reaction temperature has to be kept as low as 313 K, because at higher temperature the catalyst decomposes. In heptane, at 333 K, 55% yield are achieved after 48 h, but this higher reaction temperature also causes decomposition of the framework. Typically, the decomposed solid shows a white color indicating reduction of copper (II) by the aldehyde. In toluene, at 353 K, decomposition of the framework is observed only after 24 h, and the yield is only 20%. These observations demonstrate the limited range of solvents and temperatures applicable for using the copper framework in combination with aldehydes. The aldehyde activation is probably not the best test reaction for metal-organic frameworks containing reducible transition metal ions. The use of CH2 Cl2 as solvent is disadvantageous, since it leads to decomposition of the catalyst even at room temperature within days. The decomposition causes a reddish-brown appearance of the solid material. At 313 K after 48 h the color of the catalyst changes to reddish-brown which indicates partial decomposition of the material. The solvent effects demonstrate the limited compatibility of the framework with organic solvents and substrates. However, the catalyst is capable of catalyzing the reaction to a yield of up to 57%. In none of the experiments we were able to enhance the yield. Thus, the yield is considerably lower as compared with other solid catalysts such as Montmorillonite-K10 (90% yield under comparable conditions [14]). The heating experiments indicate that the rate of the reaction is enhanced at higher temperatures, but above 313 K, decomposition (reduction) occurs. The reason for the incomplete conversion below the decomposition temperature could be product inhibition or deactivation due to partial decomposition of the solid. 3.9. Heterogeneity
Fig. 11. Educt and product concentration of the Cu3 (BTC)2 -catalyzed cyanosilylation of benzaldehyde (solvent: tetrahydrofurane, THF, temperature: 313 K).
In order to demonstrate that the reaction mechanism is heterogeneous and not homogeneous, a filtering test was carried out. After 8 h (8.8% conversion), 50% of the solution (solvent: pentane) was separated from the catalyst by filtration and the reaction was followed by GC/MS in the remaining suspension containing the solid catalyst, and the filtrate (Fig. 12). In the filtrate, the reaction does not proceed further, whereas in the catalyst-containing suspension, the conversion of the substrate continues. Thus, the catalytic activity of Cu3 (BTC)2 originates from the presence of the solid
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mild solid Lewis acid. Liquid phase cyanosilylation of benzaldehyde gives a yield of up to 57%. However, the thermal stability, especially in the presence of substrates such as aldehydes, is limited to 313 K in solution. Thus, the applicability of metal-organic frameworks to applications in catalysis is very sensitive to the choice of reaction conditions and further enhancement of the activity is necessary.
100 Filtrate
Mol%
80
CHO
60 Cu3(BTC)2
OSieM3
40
CN
20 0
0
12
24
36 t/h
48
60
72
Fig. 12. Educt and product concentration during the cyanosilylation of benzaldehyde in the filtration test. The catalyst was separated after 8 h and the reaction was continued and analyzed in the filtrate (broken line) and the remaining suspension containing the catalyst (solid line).
catalyst and is not caused by molecular species that dissolve into solution. The filtering experiment indicates that the mechanism is heterogeneous and not homogeneous. Dissolution of copper complexes is also unlikely, due to the low polarity (pentane) of the solvent and the filtrate is typically colorless and does not contain copper. 3.10. Application to other substrates The application to other substrates is limited. Cyanosilylation of acetone gave only low conversions (Table 1). We have also studied aldol-type reactions of aldehydes with silyl enol ethers over Cu3 (BTC)2 , a reaction which is also typically catalyzed by Lewis acids. However, the reaction of benzaldehyde with 1-phenyl-1-(trimethylsiloxy)ethene and 1-trimethylsiloxy-cyclohexene at 293 K in heptane gave no significant conversion, whereas with a reference catalyst, Montmorillonite-K10, under the same conditions, a conversion of 70–80% is observed [16].
4. Conclusion We have studied the catalytic properties of a metalorganic framework compound, Cu3 (BTC)2 for applications in catalysis. The orientation of the copper dimers towards the pore center and the weakly bound water ligand render this material as an ideal catalyst. Water removal allows to coordinate aldehydes which results in an activation of the substrate. The network acts as a
Acknowledgements We thank W. Brijoux and C. Weidenthaler for the in situ-IR and -XRD measurements.
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Improved synthesis, thermal stability and catalytic properties of the metal-organic framework compound Cu3(BTC)2 Klaus Schlichte, Tobias Kratzke, Stefan Kaskel
*
Max-Planck-Institut f€ur Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 M€ulheim an der Ruhr, Germany Received 9 November 2003; received in revised form 22 December 2003; accepted 23 December 2003 Available online 8 June 2004
Abstract The catalytic properties of the metal-organic framework compound Cu3 (BTC)2 (H2 O)3 Æ xH2 O (BTC ¼ benzene 1,3,5-tricarboxylate) were explored. Cu2 O-free powder samples of Cu3 (BTC)2 (H2 O)3 Æ xH2 O were obtained using an improved synthesis at 393 K under hydrothermal conditions. The microporous material has a high specific pore volume of 0.41 cm3 g1 and a pore diameter of (Horvath-Kawazoe). Removal of the three copper-bound water molecules allows to access the Lewis acid copper sites. The 10.7 A exchange of coordinated water by substrates or solvent molecules is recognized from the color change of the compound. For chemisorbed benzaldehyde, the IR stretching frequency m(C@O) is decreased from 1702 to 1687 cm1 . The chemisorption results in an activation of benzaldehyde for the liquid phase cyanosilylation with a reasonable yield of 50–60% after 72 h (313 K) and a high selectivity. Filtration experiments demonstrate that the reaction mechanism is heterogeneous. Coordinating solvents such as THF completely block the Lewis acid sites of the catalyst. Solvents such as CH2 Cl2 or higher reaction temperatures (353 K) cause decomposition of the catalyst. 2004 Elsevier Inc. All rights reserved. Keywords: Metal-organic frameworks; Copper; Catalysis; Cyanosilylation
1. Introduction In recent years, metal-organic frameworks (MOFs) have received considerable attention as potentially valuable gas storage and catalyst materials [1–7]. They are composed of an organic linker, for example a dicarboxylic acid and a transition metal complex or cluster, both units being building blocks that are designed to assemble a three-dimensional open framework. High micropore volume, large pore sizes, crystallinity and a high metal content offering potentially valuable active sites are the key features of this new and emerging class of porous materials. Especially large pore size within the crystalline framework and well defined pore architecture is a pre-requisite for shapeselective transformation of larger molecules and is important in fine chemical syntheses [8]. However, lim* Corresponding author. Present address: Inst. fur Anorganische Chemie, Technische Univ. Dresden, Mommsenstr 13, 01062 Dresden, Germany. Tel.: +49-351-46333632; fax: +49-351-46337287. E-mail address: [email protected] (S. Kaskel).
1387-1811/$ - see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2003.12.027
itations are mostly due to the low thermal stability of the organic linker. Redox reactions of the linker with the transition metal typically limit the applicability to temperatures lower than 600 K. Thus, reactions catalyzed at low temperature, such as liquid phase transformations, seem to be more promising as compared with high temperature gas phase reactions. So far, only few catalytic applications of metalorganic frameworks have been reported. Some of the potential applications of metal-organic frameworks were outlined recently [9]. For example, Zn-containing frameworks were used for the addition of alcohol or carboxylic acids to acetylene and propyne. Thus, the addition of methanol to propyne produces 2-methoxypropene with a propyne conversion of 30% and 80% selectivity [9]. Our interest is to explore the catalytic properties of metal-organic frameworks. From the structural and esthetical point of view, MOFs are engineered by linking cluster building blocks at their coordination sites via multifunctional linkers into perfect and esthetic architectures, the topology being pre-defined by the
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arrangement of the coordination sites and the orientation and number of the binding groups in the linker. However, an aspect often overlooked is that connecting the coordination sites to the linker also blocks such sites for catalytic transformations. A change in coordination geometry is often required for dissociation-association mechanisms, that are responsible for the high catalytic activity of molecular transition metal complexes in organometallic chemistry. However, such a structural rearrangement is not desirable for the application of metal-organic frameworks in heterogeneous catalysis, because dissociation of the cluster–linker bond will inevitably destroy the framework. Instead, the sites must be oriented towards the pore interior and be accessible for molecules. The high metal content alone is not a guarantee for catalytic activity. We have therefore screened a variety of known MOFs and selected a compound in which residual coordination sites are accessible from the pore interior. A candidate fulfilling these requirements is Cu3 (BTC)2 (H2 O)3 . The crystal structure of Cu3 (BTC)2 (H2 O)3 was reported by Chui et al. [10]. Recently, Wang et al. have studied the potential use of Cu3 (BTC)2 (H2 O)3 for gas purification and separation [11]. Monte Carlo simulations in conjunction with high-resolution low-pressure argon adsorption measurements were carried out by Vishnyakov et al. [12].
2. Experimental 2.1. Synthesis of Cu3 (BTC)2 (H2 O)3 xH2 O In a typical synthesis, 0.875 g (3.6 mmol) Cu(NO3 )2 Æ 3H2 O were dissolved in 12 ml de-ionized water and mixed with 0.42 g (2.0 mmol) of trimesic acid dissolved in 12 ml ethanol. The solution was filled in a 40 ml Teflon liner, placed in an autoclave, and heated to 393 K for 12 h. 2.2. Characterization Nitrogen physisorption measurements were carried out using a micromeritics ASAP 2000 instrument. X-ray powder diffraction patterns were recorded on a STOE diffractometer, equipped with a position sensitive detector (Braun, 6) and a germanium primary beam monochromator in the transmission mode using CuKa1 For the XRD measurements, the radiation (1.54051 A). dried powder was filled into capillaries (0.5 mm diameter) and sealed under argon if necessary. 2.3. Catalytic tests The catalytic test reactions were carried out in solution using various solvents and temperatures (Table 1).
Table 1 Yields for the Cu3 (BTC)2 -catalyzed liquid phase cyanosilylation of benzaldehyde and acetone Substrate
Solvent
Temperature/ time
Yield/%
Benzaldehyde Benzaldehyde Benzaldehyde Benzaldehyde Benzaldehyde Acetone Acetone
CH2 Cl2 Pentane Heptane Toluene THF Toluene Heptane
313 313 333 353 313 333 333
50 40 55 20 2 12 25
K/48 K/48 K/48 K/24 K/72 K/48 K/48
h h h h h h h
The solvents were dried prior to use using standard drying techniques. The as made framework material (129 mg, 0.21 mmol) was activated in vacuum (1 · 104 mbar, 393 K) either prior to the test in the flask, or directly after the synthesis. In the latter case, appropriate amounts of the activated powder were transferred to a reaction vessel using a vacuum line or a glove box. However, handling in a glove box is not necessary, since the catalyst can be activated (dehydrated) in vacuum or in an inert gas stream in the reactor prior to use. After introduction of the solvents and reactants (trimethylsilylcyanide (TMSCN), 8 mmol, Aldrich, 98%; benzaldehyde, 4 mmol, Fluka, distilled, 99.9%) the suspension was stirred and the products were analyzed using GC/MS (n-octane and n-hexadecane used as internal standard). For the filtration test, the catalyst was separated after 8 h and the solution was divided into two equal portions. Hereafter, one portion was stirred with and the other one without the catalyst.
3. Results and discussion 3.1. Improved synthesis and up-scaling Typical synthesis procedures described by Chui et al. [10] produce Cu3 (BTC)2 (H2 O)3 Æ xH2 O at 453 K under hydrothermal conditions in the form of single crystals, however, significant amounts of Cu2 O are obtained as a by-product, using these high temperature syntheses. High temperatures are beneficial for single crystal work but impurities can significantly affect the catalytic properties of solids and thus it is necessary to develop synthesis methods for metal-organic frameworks that do not yield side products. We have therefore optimized the synthesis conditions, obviously parallel to another group [11], in order to obtain pure Cu3 (BTC)2 (H2 O)3 Æ xH2 O samples for catalytic tests without Cu2 O impurities. According to our results, Cu2 O-free samples are obtained under hydrothermal conditions after 12 h at 393 K. The lower synthesis temperature allows to suppress the formation of Cu2 O since the reduction of the Cu2þ ions is avoided. A powder X-ray diffraction pat-
83
(111)
(311)
Intensity /a.u.
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5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
2Theta
Fig. 1. Observed (top) and calculated (bottom) X-ray diffraction pattern of Cu3 (BTC)2 (H2 O)3 Æ xH2 O.
tern of a typical sample is shown in Fig. 1 and compared with a pattern calculated from crystallographic data. The overall agreement is good, however some deviation is observed in the relative intensities due to variations in the degree of hydration (see below).
Fig. 2. Structure of the Cu2 -paddle-wheel in Cu3 (BTC)2 (H2 O)3 .
3.2. Structure The crystal structure of Cu3 (BTC)2 (H2 O)3 Æ xH2 O was reported by Chui et al. [10]. As compared to other metal-organic frameworks, with most of the coordination sites blocked by the frameworkconstituting ligand, Cu3 (BTC)2 (H2 O)3 Æ xH2 O has the advantage that a Lewis acid coordination site is on the interior of the pore wall, and thus, copper sites are accessible for catalytic conversions. It is therefore important to understand the network structure. In the cubic network, Cu2 -clusters are coordinated by carboxylate groups to give a so-called paddle-wheel unit in which four carboxylate groups are arranged in a square (Fig. 2). The binuclear cluster is a common feature of Cu2þ carboxylates. The Cu2þ ions are connected through a weak bond and the residual axial coordination site is filled by a weakly bound water molecule. These primary building blocks are connected through the BTC ligand (trimesic acid) into a three-dimensional cubic network with an open pore system (Fig. 3). The weakly bound water molecules point towards the center of the pore. This hydrophilic interior, with 12 water molecules per pore, is of high symmetry and the oxygen atoms of these water ligands are arranged to form a cuboctahedron (Fig. 4). The vertical and diagonal O–O respectively. The same distances are 8.25 and 11.67 A, geometry is obtained for the copper atoms but the corresponding Cu–Cu distances across the pore are 11.3 respectively. and 16.0 A,
Fig. 3. View inside the pore of Cu3 (BTC)2 (H2 O)3 . The axially bound water molecules point towards the pore center.
3.3. Thermal stability The thermal stability was studied using TG/DTA and high temperature X-ray diffraction. The as made samples show a continuous mass loss of 32% up to a temperature of 523 K due to the dehydration of the material (Fig. 5). The weight change corresponds to more than 3 mol H2 O per Cu3 (BTC)2 . The calculated weight change for 3, 6 and 9H2 O molecules per Cu3 unit is 8.2%, 15.2%, and 21.1%, respectively. The observed weight loss heavily
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Cu3(BTC)2(H2O)3•xH2O
873 K 773 K 673 K 573 K 473 K 373 K 20
25
30
35
40
45
(420)
(331)
15
Fig. 4. Polyhederal arrangement of the axially bound water molecules inside the pore.
50
2θ
Cu3(BTC)2 100 90
dried in air (363 K)
Weight /%
80 as made
70 60
dried in
873 K
vacuum
773 K
(363 K)
673 K 573 K 473 K
rehydrated
373 K
50 15
40 30 273
20
25
30
35
40
45
50
2θ 373
473 573 Temperature / K
673
Fig. 5. Thermogravimetric analysis of Cu3 (BTC)2 (H2 O)3 Æ xH2 O with variations in the degree of hydration.
depends on the pre-treatment of the material. If the material is dried in high vacuum (1 · 104 mbar, 373 K), a weight loss of only 1–2% is observed. For a material dried in air (363 K), the weight loss is 12.2%. If the material dried in high vacuum is re-hydrated in a stream of moist argon, the weight loss is 36.4%. For re-hydration in air, the weight loss is typically lower (27%, not shown in Fig. 5). Re-hydrated samples show a clear step at 373 K, whereas for the as made sample, desolvation covers a wider region due to residual amounts of ethanol physisorbed inside the pore system. In Chuis crystal structure, additional H2 O molecules, that do not bind to copper, occupy the 96 j position (Fm 3 m) in the pore interior [10]. Full occupancy of the 96 j position would give the fully hydrated composition Cu3 (BTC)2 (H2 O)3 Æ 6H2 O. However, for re-hydrated samples, the experimentally determined water content is higher than 21.1%, demonstrating the structures’ capacity for more solvent molecules in the pore interior. A second weight change is observed at 623 K due to the decomposition of the network, but the weight loss is significantly lower as expected for a complete transformation into CO2 and Cu2 O or Cu metal.
Fig. 6. In situ X-ray diffraction analysis of Cu3 (BTC)2 (H2 O)3 Æ xH2 O and the dehydrated Cu3 (BTC)2 .
High temperature X-ray diffractograms reveal the structural changes during the heat treatment (Fig. 6). We have studied the decomposition of as made Cu3 (BTC)2 (H2 O)3 Æ xH2 O and the dehydrated Cu3 (BTC)2 . Both samples start to decompose at temperatures exceeding 533 K to form metallic copper. Whereas the decomposition in air typically produces Cu2 O, the reason for copper metal formation in our experiments is the protective atmosphere of argon that prevents oxidation. In that case, the ligand itself reduces the metal and is pyrolyzed to form carbonic deposits. Some small differences are observed in the two experiments. Whereas in the dehydrated sample only one peak appears at 14.5, on the other hand, in the hydrated sample, two small peaks are observed around 14.5, the (3 3 1) and (4 2 0) reflection. However, when the hydrated sample is heated to 473 K, the two peaks are merged into one peak and the pattern is identical to that of the dehydrated sample. In Fig. 7a, the calculated diffraction patterns are shown for three idealized structures: The completely dehydrated framework (bottom), the partially hydrated and the fully hydrated form. The minute changes detected using in situ X-ray diffraction are partially understood: The intensity ratio of I3 3 1 =I4 2 0 decreases,
K. Schlichte et al. / Microporous and Mesoporous Materials 73 (2004) 81–88
85
(b)
(a)
Cu3 (BTC)2 (H2 O)3 •xH2 O as made
Cu3 (BTC)2 (H2 O)3 •6H2O
Cu3 (BTC)2 (H2 O)3 •xH2 O rehydrated, 29 % water
(200)
(200)
(222)
Cu3 (BTC)2 (H2 O)3
Cu3 (BTC)2
6.0
8.0
10.0
14.0
16.0
2Theta
(331) (420)
(111)
(440)
(333)
(422)
(400) 12.0
(331) (420)
(311)
(111) 4.0
(220)
(220)
Cu3 (BTC)2
4.0
6.0
8.0
10.0
12.0
14.0
16.0
2Theta
Fig. 7. (a) Calculated and (b) observed X-ray diffractograms of Cu3 (BTC)2 (H2 O)3 Æ xH2 O with variations in the degree of hydration.
when the naked framework is hydrated with 3H2 O molecules. These differences due to variations in the degree and structure of the hydrated forms are even more obvious in ex situ analyzed samples (Fig. 7b): (1) The I3 3 1 =I4 2 0 ratio is higher for the completely dehydrated form. (2) The (1 1 1) reflection is present in the as made and in the dehydrated sample, but not in the rehydrated sample. This observation is in agreement with the calculated patterns, but the water content of the samples cannot be correlated to the intensity of the (1 1 1) reflection, since the occupation of the physisorbed water positions does not necessarily result in long range ordered physisorbed molecules. (3) The I2 0 0 =I2 2 0 ratio is significantly higher for the dehydrated sample. The changes in the observed powder patterns roughly reproduce the differences predicted in the calculated patterns (Fig. 7a) and demonstrate that the small changes in the diffraction patterns can be attributed to variations in the degrees of hydration.
3.4. Nitrogen physisorption results A typical nitrogen physisorption isotherm of Cu3 (BTC)2 is shown in Fig. 8. A type I isotherm is observed for the microporous network with a specific micropore volume of 0.41 cm3 g1 and a Horvath (cylinder model). The Kawazoe (HK) pore size of 10.7 A sharp increase of nitrogen adsorption at a relative pressure of 104 mbar is recognized in logarithmic plots (Fig. 8, inlay) and indicates a narrow pore size distribution as expected for a porous crystalline framework. 3.5. Exchange of coordinated water As indicated above, physically and chemically bound water molecules are easily removed from the host material by heating the compound in vacuum. The dehydration makes the copper coordination sites accessible for other molecules. The dehydration of the compound is accompanied by pronounced color changes. Whereas the as made material is blue-turquoise, the dehydrated material shows a dark-violet color. Re-adsorption of acetone, THF, ethanol, acetonitrile, benzaldehyde, smaller amines, and ammonia restore the original color indicating coordination to the copper
K. Schlichte et al. / Microporous and Mesoporous Materials 73 (2004) 81–88
Volume adsorbed /cm3g-1
86
400
300
200
100
-log(P/P0) 6
5
4
3
2
1
0
0 0.1
0.2
0.3
0.4
0.5
P/P0 Fig. 8. Nitrogen physisorption isotherm of Cu3 (BTC)2 at 77 K.
C@O stretching frequency of pure benzaldehyde in solution is located at 1702 cm1 . When benzaldehyde is adsorbed from a diluted solution, the chemisorbed aldehyde is only barely detected as a shoulder in the band of the free aldehyde. In this case, the aldehyde is located inside the pores but only part of it is coordinated to the copper sites. The physically bound aldehyde is partially removed by heating to 373 K in vacuum (1 · 104 mbar), which allows to detect the position of the chemisorbed aldehyde at 1687 cm1 . In situ adsorption of benzaldehyde from the gas phase to an activated sample in an IR microscope gave the same results, and according to adsorption measurements carried out at room temperature, the line position of the chemisorbed benzaldehyde is located at 1686 cm1 . Thus the shift of the C@O stretching frequency to lower wave numbers caused by the coordination of the C@O group to the copper site is 16 cm1 . Whereas other substrates such as water can be removed from the pore by activation (1 · 104 mbar, 373 K), benzaldehyde is bound to the pore and cannot be removed in vacuum without decomposition of the copper network. 3.7. Catalytic properties
Fig. 9. Color changes during the dehydration (activation) of Cu3 (BTC)2 (H2 O)3 Æ xH2 O to give Cu3 (BTC)2 , and subsequent readsorption of the aldehyde to give Cu3 (BTC)2 (C6 H5 CHO)x .
ions (Fig. 9). Whereas for most of the substrates (carbonyl compounds, amines) the network structure remains intact, for some substrates, such as gaseous ammonia, the adsorption results in an irreversible transformation of the network into another crystal structure.
The cyanosilylation of aldehydes and ketones is a convenient route to cyanohydrin derivatives and can be used under thermal and catalytic conditions using molecular or solid Lewis acid catalysts [13,14]. Cyanosilylation of aldehydes was reported to occur with high selectivity over Cd(4,40 -BPY)2 (NO3 )2 , a square network material (Scheme 1) [15]. The framework is highly shape-selective for the inclusion of ortho-dihalogenobenzenes (halogen ¼ chlorine, bromine), whereas the meta- and para-isomers do not form clathrates. Shape specificity, similar to that in clathration was observed in cyanosilylation [15]. For example the conversion of 2tolualdehyde (40%) is significantly higher as compared to 3-tolualdehyde (19%). We have used the liquid phase cyanosilylation of benzaldehyde as a test reaction to study selectivity and activity of the metal-organic framework compound Cu3 (BTC)2 . For all tests, the material was dehydrated in high vacuum at 373 K in order to remove physically and chemically bound water molecules, before the material is used as a catalyst. Thus, the Lewis acid copper(II) sites of the Cu2 -paddle-wheel become accessible for the coordination of the aldehyde (Fig. 9).
3.6. IR measurements The chemisorption of substrates is detected ex situ using FT-IR spectroscopy in the transmission mode, for example by treating activated samples in the bulk with the suitable substrate either in solution or with a gas phase and subsequent preparation of KBr pellets. The
OSiMe3 CHO
Me3SiCN
CN
Scheme 1. Cyanosilylation of benzaldehyde.
K. Schlichte et al. / Microporous and Mesoporous Materials 73 (2004) 81–88
100
Mol%
80
CHO
60 OSiMe3
40
CN
20 0 0
10
20
30
40
50
60
70
t/h Fig. 10. Educt and product concentration of the Cu3 (BTC)2 -catalyzed cyanosilylation of benzaldehyde (solvent: pentane, temperature: 313 K).
Initial tests using the reaction conditions reported in [15] gave only little conversion at 293 K (below 5% after 24 h). At higher temperature, 313 K, the conversion is significant. Fig. 10 shows the concentration of benzaldehyde and the product for a reaction carried out at 313 K in pentane in the presence of Cu3 (BTC)2 . After 72 h, the yield is 57% (88.5% selectivity). For comparison, a blank experiment was carried out under the same conditions without catalyst. The thermal conversion was below 10% after 72 h. 3.8. Impact of the solvent In general, solvents of low polarity are beneficial for the use of Cu3 (BTC)2 because of the low solubility in such solvents, but also due to the competing adsorption of donor type solvents at the Lewis acidic copper site. For example, tetrahydrofuran (THF), and other ethers that are coordinated, block the acidic sites. A typical example is shown in Fig. 11. Here the reaction was carried out in THF and practically no conversion is
87
observed after 72 h of reaction time. The latter further supports the reaction mechanism and the role the catalyst plays i.e. the coordination of the aldehyde to the copper center being responsible for the enhanced reaction rate. We have used pentane, heptane, toluene, and CH2 Cl2 as solvents (Table 1). They are quite suitable, but the reaction temperature has to be kept as low as 313 K, because at higher temperature the catalyst decomposes. In heptane, at 333 K, 55% yield are achieved after 48 h, but this higher reaction temperature also causes decomposition of the framework. Typically, the decomposed solid shows a white color indicating reduction of copper (II) by the aldehyde. In toluene, at 353 K, decomposition of the framework is observed only after 24 h, and the yield is only 20%. These observations demonstrate the limited range of solvents and temperatures applicable for using the copper framework in combination with aldehydes. The aldehyde activation is probably not the best test reaction for metal-organic frameworks containing reducible transition metal ions. The use of CH2 Cl2 as solvent is disadvantageous, since it leads to decomposition of the catalyst even at room temperature within days. The decomposition causes a reddish-brown appearance of the solid material. At 313 K after 48 h the color of the catalyst changes to reddish-brown which indicates partial decomposition of the material. The solvent effects demonstrate the limited compatibility of the framework with organic solvents and substrates. However, the catalyst is capable of catalyzing the reaction to a yield of up to 57%. In none of the experiments we were able to enhance the yield. Thus, the yield is considerably lower as compared with other solid catalysts such as Montmorillonite-K10 (90% yield under comparable conditions [14]). The heating experiments indicate that the rate of the reaction is enhanced at higher temperatures, but above 313 K, decomposition (reduction) occurs. The reason for the incomplete conversion below the decomposition temperature could be product inhibition or deactivation due to partial decomposition of the solid. 3.9. Heterogeneity
Fig. 11. Educt and product concentration of the Cu3 (BTC)2 -catalyzed cyanosilylation of benzaldehyde (solvent: tetrahydrofurane, THF, temperature: 313 K).
In order to demonstrate that the reaction mechanism is heterogeneous and not homogeneous, a filtering test was carried out. After 8 h (8.8% conversion), 50% of the solution (solvent: pentane) was separated from the catalyst by filtration and the reaction was followed by GC/MS in the remaining suspension containing the solid catalyst, and the filtrate (Fig. 12). In the filtrate, the reaction does not proceed further, whereas in the catalyst-containing suspension, the conversion of the substrate continues. Thus, the catalytic activity of Cu3 (BTC)2 originates from the presence of the solid
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mild solid Lewis acid. Liquid phase cyanosilylation of benzaldehyde gives a yield of up to 57%. However, the thermal stability, especially in the presence of substrates such as aldehydes, is limited to 313 K in solution. Thus, the applicability of metal-organic frameworks to applications in catalysis is very sensitive to the choice of reaction conditions and further enhancement of the activity is necessary.
100 Filtrate
Mol%
80
CHO
60 Cu3(BTC)2
OSieM3
40
CN
20 0
0
12
24
36 t/h
48
60
72
Fig. 12. Educt and product concentration during the cyanosilylation of benzaldehyde in the filtration test. The catalyst was separated after 8 h and the reaction was continued and analyzed in the filtrate (broken line) and the remaining suspension containing the catalyst (solid line).
catalyst and is not caused by molecular species that dissolve into solution. The filtering experiment indicates that the mechanism is heterogeneous and not homogeneous. Dissolution of copper complexes is also unlikely, due to the low polarity (pentane) of the solvent and the filtrate is typically colorless and does not contain copper. 3.10. Application to other substrates The application to other substrates is limited. Cyanosilylation of acetone gave only low conversions (Table 1). We have also studied aldol-type reactions of aldehydes with silyl enol ethers over Cu3 (BTC)2 , a reaction which is also typically catalyzed by Lewis acids. However, the reaction of benzaldehyde with 1-phenyl-1-(trimethylsiloxy)ethene and 1-trimethylsiloxy-cyclohexene at 293 K in heptane gave no significant conversion, whereas with a reference catalyst, Montmorillonite-K10, under the same conditions, a conversion of 70–80% is observed [16].
4. Conclusion We have studied the catalytic properties of a metalorganic framework compound, Cu3 (BTC)2 for applications in catalysis. The orientation of the copper dimers towards the pore center and the weakly bound water ligand render this material as an ideal catalyst. Water removal allows to coordinate aldehydes which results in an activation of the substrate. The network acts as a
Acknowledgements We thank W. Brijoux and C. Weidenthaler for the in situ-IR and -XRD measurements.
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