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An Indium Layered MOF as Recyclable Lewis Acid Catalyst F. Gándara,† B. Gomez-Lor,† E. Gutiérrez-Puebla,† M. Iglesias,† M. A. Monge,*,† D. M. Proserpio,‡ and N. Snejko† Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid, Spain, and Dipartimento di Chimica Strutturale e Stereochimica Inorganica, UniVersità di Milano, Milano, Italy ReceiVed April 20, 2007. ReVised Manuscript ReceiVed NoVember 5, 2007
By using the bent linker 4,4′-(hexafluoroisopropylidene)bis(benzoic acid) (H2hippb), whose central atom is an sp3 carbon, a new In(III) MOF has been obtained. The structure is described as thick layers with the bent geometry of the ligand leading to the formation of square-shaped channels, which run inside the framework layers. This microporous, thermally stable compound has been proved to be an efficient heterogeneous catalyst for acetalization of aldehydes. Differences in the catalytic activity when a compound with empty or filled channels is used demonstrate that the catalytic reactions take place inside the pores.
Introduction The search for materials with useful properties had led in recent years to intense studies for the synthesis of organoinorganic compounds because they possess advantageous cumulative attributes emerging from both metal and ligand components. Metal-organic frameworks (MOFs) are a subset of this new class of structures, which proved themselves to have immense potential applications,1 including catalysis, gas storage, and separations.2–4 Using principles of coordination chemistry, numerous extended metal-organic from rigid organic linkers networks are reported, some of them with indium.5 The assemblies of extended solids from bent arenedicarboxylate linkers are considerably less studied despite that they would likely offer new topologies with channels and, consequently, a great potential for the design and synthesis of functional materials.6,7
* To whom correspondence should be addressed. † CSIC. ‡ Università di Milano.
(1) (a) Janiak, C. Dalton Trans. 2003, 14, 2781. (b) Mueller, U.; Schubert, M.; Teich, F.; Puetter, H.; Schierle-Arndt, K.; Pastre, J. J. Mater. Chem. 2006, 16, 626. (2) (a) Eddaoudi, M.; Moler, D. B.; Li, H.; Chen, B.; Reineke, T. M.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319. (b) Zaworotko, M. J. Chem. Soc. ReV. 1994, 24, 283. (c) Batten, S. R.; Robson, R. Angew. Chem., Int. Ed. 1998, 37, 1460. (d) Fujita, M.; Umemoto, K.; Yoshizawa, M.; Fujita, N.; Kusukawa, T.; Biradha, K. Chem. Commun. 2001, 509. (e) Kondo, A.; Noguchi, H.; Kajiro, H.; Carlucci, L.; Mercandelli, P.; Proserpio, D. M.; Tanaka, H.; Kaneko, K.; Kanoh, H. J. Phys. Chem. B 2006, 110, 25565. (f) Rowsell, J. L. C.; Yaghi, O. M. J. Am. Chem. Soc. 2006, 128, 1304. (g) Jhung, S. H.; Lee, J.-H.; Cheetham, A. K.; Férey, G.; Chang, J.-S. J. Catal. 2006, 239, 1–97. (h) Hong, M. Cryst. Growth Des. 2007, 7, 10. (i) Vertova, A.; Cucchi, I.; Fermo, P.; Porta, F.; Proserpio, D. M.; Rondinini, S. Electrochim. Acta 2007, 52, 2603. (3) (a) Pan, L.; Huang, X.; Li, J.; Wu, Y.; Zheng, N. Angew. Chem., Int. Ed. 2000, 39, 527. (b) Perles, J.; Iglesias, M.; Martin-Luengo, M.-A.; Monge, M. A.; Ruiz-Valero, C.; Snejko, N. Chem. Mater. 2005, 17, 5837. (c) Barthelet, K.; Marrot, J.; Riou, D.; Ferey, G. Angew. Chem., Int. Ed. 2002, 41, 281. (d) Férey, G.; Walton, R. I. Chem. Commun. 2006, 1518. (4) Eddaoudi, H.; Li, M.; O’Keeffe, M.; Yaghi, O. M. Nature (London) 1999, 402, 276.
The traditional approach to acid catalysis is changing rapidly to reusable and highly tolerant catalysts, such as heterogeneous Lewis acid. In this context porous MOFs offer an interesting alternative for their intrinsic zeolite mimicking size- and shape-selective properties. On the other hand, indium salts have been used as highly effective water-tolerant Lewis acid catalysts in a variety of synthetic transformations;8 additionally, they offer interesting advantages in the field of the green chemistry, such as low toxicity and stability in water. Previously, we have reported some In(III) heterogeneous catalysts using rigid linkers. In them due to the small pore size, the activity took place in the surface.9,10 Here we present a new In(III) MOF, in which a bent arenedicarboxylate linker gives rise to the formation of nanochannels and, thus, to the diffusion of reactants through the tunnels during the catalytic processes.
(5) (a) Sun, J.; Weng, L.; Zhou, Y.; Chen, J.; Chen, Z.; Liu, Z.; Zhao, D. Angew. Chem., Int. Ed. 2002, 41, 4471. (b) Lin, Z.-Z.; Jiang, F.-L.; Chen, L.; Yuan, D.-Q.; Zhou, Y.-F.; Hong, M.-C. Eur. J. Inorg. Chem. 2005, 77. (c) Anokhina, E. V.; Vougo-Zanda, W. X.; Jacobson, A. J. J. Am. Chem. Soc. 2005, 127, 15000. (d) Wang, Y.-L.; Liu, Q.-Y.; Zhong, S.-L. Acta Crystallogr. 2006, 62, 395. (e) Au-Yeung, A. S.-F.; Sung, H. H.-Y.; Cha, J. A. K.; Siu, A. W.-H.; Chui, S. S.-Y.; Williams, I. D. Inorg. Chem. Commun. 2006, 9, 507. (f) Pang, W. Chem. Mater. 2006, 18, 2950. (g) Lin, Z.; Chen, L.; Yue, C.; Yuan, D.; Jiang, F.; Hong, M. J. Solid State Chem. 2006, 179, 1154. (h) Volkringer, C.; Loiseau, T. Mater. Res. Bull. 2006, 41, 948. (i) Liu, Y.; Eubank, J. F.; Cairns, A. J.; Eckert, J.; Kravtsov, V. Ch.; Luibke, R.; Eddaoudi, M. Angew. Chem., Int. Ed. 2007, 46, 3278. (6) (a) Pan, L.; Sander, M. B.; Huang, X.; Li, J.; Smith, M.; Bitner, E.; Brockrath, B.; Johnson, K. J. J. Am. Chem. Soc. 2004, 126, 1308. (b) Pan, L.; Olson, D. H.; Ciemnolonski, L. R.; Heddy, R.; Li, J. Angew. Chem., Int. Ed. 2006, 45, 616. (7) Monge, A.; Snejko, N.; Gutiérrez-Puebla, E.; Medina, M.; Cascales, C.; Ruiz-Valero, C.; Iglesias, M.; Gómez-Lor, B. Chem. Commun. 2005, 1291. (8) Fringuelli, F.; Piermatti, O.; Pizzo, F.; Vaccaro, L. Curr. Org. Chem. 2003, 7, 1661. (9) Gomez-Lor, B.; Gutierrez-Puebla, E.; Iglesias, M.; Monge, M. A.; Ruiz-Valero, C.; Snejko, N. Inorg. Chem. 2002, 41, 2429. (10) Gomez-Lor, B.; Gutierrez-Puebla, E.; Iglesias, M.; Monge, M. A.; Ruiz-Valero, C.; Snejko, N. Chem. Mater. 2005, 17, 2568.
10.1021/cm071079a CCC: $40.75 2008 American Chemical Society Published on Web 12/06/2007
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Table 1. Crystal Data and Structure Refinement [In(OH)L] empirical formula formula weight crystal system space group unit cell dimensions volume Z density (calculated) absorption coefficient crystal size independent reflections data/restraints/parameters final R indices [I > 2sigma(I)] R indices (all data)
[In(OH)(C17H8F6O4)] 522.06 orthorhombic Fddd a ) 7.1145(4) Å b ) 25.857(2) Å c ) 43.551(3) Å 8011.7(8) Å3 16 1.731 mg/m3 1.259 mm-1 0.10 × 0.10 × 0.02 mm3 1844 [R(int) ) 0.067] 1844/0/134 R1 ) 0.0720, wR2 ) 0.1435 R1 ) 0.0849, wR2 ) 0.1478
Experimental Section General Information. All reagents were commercially available from Aldrich [In(OAc)3 98%, 4,4′-(hexafluoroisopropylidene)bis (benzoic acid) (H2hfipbb, H2L from now on) 98%, and pyridine 99%] and were used as received. The FT-IR spectra were measured from KBr pellets in the range 4000–400 cm-1 on a Perkin-Elmer spectrometer. Thermogravimetric and differential thermal analyses (TGA-DTA) were performed using a SEIKO TG/DTA 320 apparatus in the temperature range between 25 and 1000 °C in N2 (flow of 50 mL min-1) and static air atmosphere and at a heating rate of 5 °C/min. Microanalyses were performed at the Microanalyses Service of the Universidad Autonoma de Madrid. Synthesis of [In(OH)L] · 0.5Py. A mixture of In(OAc)3 (0.1 g, 0.34 mmol), H2hfipbb (0.15 g, 0.38 mmol), 8 mL of distilled water, and 0.5 mL of pyridine was sealed in a Teflon-lined autoclave (mixture pH ) 6) and heated for 48 h at 170 °C. After cooling to room temperature, colorless crystals were filtered and washed with distilled water and acetone (yield ∼68% based on In; %C: 41.69 calculated, 40.52 obtained; %H: 2.05 calculated, 2.65 obtained). Synthesis of [In(OH)L]. A mixture of In(OAc)3 (0.1 g, 0.34 mmol), H2L (0.20 g, 0.51 mmol), 10 mL of distilled water, and 3 mL of EtOH (96%) was sealed in a Teflon-lined autoclave (mixture pH ) 7) and heated for 72 h at 170 °C. After cooling to room temperature, the product was filtered and washed with distilled water and acetone. Colorless crystals of the product were obtained (∼72% yield based on In; %C: 38.53 calculated, 39.35 obtained; %H: 1.72 calculated, 1.90 obtained). Optical microscopic analysis indicated that the sample [In(OH)L] · 0.5Py is composed of large colorless parallelepiped-shaped crystals, while sample [In(OH)L] is a microcrystalline phase. The purity of each preparation was tested by comparison of the XRD powder patterns of the bulk products with those simulated with atomic coordinates derived from single-crystal X-ray diffraction. X-ray Structure. Colorless parallelepiped-shaped crystals of [In(OH)L] · 0.5Py and a carefully selected suitable small needle crystal of [In(OH)L] were glued on a glass fiber for X-ray diffraction experiment. The main crystallographic and refinement data for [In(OH)L] are given in Table 1, while that for [In(OH)L] · 0.5Py can be found in the Supporting Information. X-ray intensity data were collected in a Bruker SMART CCD diffractometer equipped with a normal focus, 2.4 kW sealed tube X-ray source (Mo KR radiation ) 0.710 73 Å). Data were collected at room temperature over a hemisphere of the reciprocal space by a combination of three sets of exposures. Each exposure of 20 s covered 0.3° in ω. Unit cell dimensions were determined by a least-squares fit of 60 reflections with I > 20σ(I). The structures were solved by direct methods. The final cycles of refinement were carried out by fullmatrix least-squares analyses with anisotropic thermal parameters
Figure 1. In(OH)(L): top, atomic numeration showing more than the asymmetric unit; middle, chains of InO6 octahedra along the a direction via µ2-OH; bottom, complete structure. Indium in green, carbon in gray, oxygen in red, and fluorine in light green.
for all non-hydrogen atoms, with exception of those belonging to the occluded pyridine molecules in the [In(OH)L] · 0.5Py crystal. Hydrogen atoms of the hydroxyl groups were located in difference Fourier maps. Calculations were carried out with SMART software for data collection and data reduction and SHELXTL.11 Thermogravimetric Analysis. TGA was performed on crystalline samples of the compound in the range 20–1000 °C. The weight losses of approximately 6.25% and 2.8% between 100 and 380 °C correspond to the loss of pyridine (calculated 7.03% for 0.5Py), when [In(OH)L] · 0.5Py and to a small amount of ethanol used in the synthesis of the empty [In(OH)L], respectively. The framework remains stable up to ∼450 °C, and the final residue at 1000 °C is In2O3. Catalytic Experiments. Activation of catalysts was performed by heating the solids at 280 °C for 3 days and 100 °C for 12 h for (11) Software for the SMART System V5.04 and SHELXTL V 5.1; BrukerSiemens Analytical X-ray Instrument Inc., Madison, WI, 1998.
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Figure 2. Polyhedral representation of the layer (top) and the structure (down).
compounds with and without pyridine, respectively. After this time, solutions of the carbonyl compound (2.54 mmol) and trimethyl orthoformate (TMOF, 1.30 mmol) in tetrachloromethane (3 mL) were added onto the activated catalysts. The resulting suspensions were magnetically stirred at reflux temperature for 3.5 h. Samples were taken from each reaction at intervals, and the reaction products were analyzed by gas chromatography. Specific Surface Area. Samples were previously evacuated for 16 h at 250 °C. Experimental data of adsorption–desorption of N2 at 77 K were measured in an ASAP2010 from Micromeritics and used to apply the Langmuir theory to obtain the specific surface area (SSA) values.
Results Single-crystal X-ray diffraction studies revealed the composition of the new indium MOF, In(OH)L (L ) C17F6O4H8) and an extended structure composed of the building units shown in Figure 1 (right). Indium atoms, which are situated on 2-fold axes, are octahedrically coordinated to two µ2-OH groups in apical positions (In–O distance ) 2.088(3) Å) and to four different fully deprotonated linking ligands L2– (In–O lengths ) 2.156(4), 2.162(4) Å). Chains of sharing vertex In(O)6 octahedra run along the a direction via µ2-OH (In–OH–In angle ) 119.30(2)°) and bridging carboxylate groups (In–In distance )2.592(1) Å) (Figures 1 (middle) and 2). The L2- counterion does not act as chelating, but it is coordinated to four different indium atoms (two of each
chain). Linkages among chains in the b direction give rise to thick layers perpendicular to the c direction. Because of the geometry of the ligand, its central atom being a sp3 carbon, square-shaped channels of ∼10 × 10 Å2 running down a are formed inside each layer with the fluorine atoms pointing outside of them. A computation of the voids using Platon12 gives a value of 18% of empty space in the unit cell. The voids are all located inside the square-shaped channels within the layers, and the accessible window is for a sphere of diameter 5.4 Å. These thick layers stack in such a way that each InO6 octahedron faces four CF3 groups (two from each neighboring layer). As a result, there are no voids left in the undulated interlamellar space (Figures 1 and 2). The shortest interlayer contacts are weak hydrogen bonds from the fluorine and OH hydrogen atoms that are located pointing in between adjacent layers (F · · · H–O 2.58 Å, F · · · O 3.34 Å, ∠FHO ) 144°). One half of pyridine per formula fits inside the nanochannels in [In(OH)L] · 0.5Py, while in [In(OH)L], the compound synthesized with ethanol instead of pyridine, they are empty. The topology of the layer may be described as a binodal (6-c)(4-c) 2-periodic 3D net formed by 6-connected (6-c) indium nodes, 4-c bent ligand, and 2-c OH. In other words, it is a network, with periodicity in only two directions (layers), that exists only in a 3D space (its embedding is (12) Spek, A. L. PLATON A Multipurpose Crystallographic Tool; Utrecht University, Utrecht, The Netherlands, 2007.
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Figure 3. Topological study on In(OH)L structure. Table 2. Acetalization of Benzaldehyde with Trimethyl a Orthoformate
catalyst [In2(OH)3(BDC)1.5]c [In2(BDC)3(bipy)2]d [In2(OH)2(BDC)2(phen)2]d [In(BTC)(H2O)(bipy)]d [In(BTC)(H2O)(phen)]d [In(OH)(L)] · 0.5Py [In(OH)(L)] · 0.3Pye [In(OH)(L)] c
t 2h 4h 2h 2h 2h 1h 0.5 h 15 min
conv (%)b
TOF (h-1)
68 22 56 60 76 100 90 100
281 75 230 300 380 480 540 1200
a Catalyst 10 mol %. b Total conversion was achieved after 6 h. Reference 9. d Reference 10. e Reaction performed at 100 °C.
3D), since the net cannot be flattened on a plane without crossing edges/bonds. A different, and simpler, topological analysis is illustrated in Figure 3 taking as 4-connected node the midpoint between two indium atoms along the –(In– OH–In)– chain. Now also the bent linker became 2-c, and the net is described as 4-c uninodal with Schläfli symbol (42.63.8). The layers stack in ABCD sequence along c, the In atoms are coplanar, and the linkers connect them above and below the plane of the metals producing unavoidable crossings, hence the 3D embedding of the net illustrated in Figure 3, top right. A topological study was done with TOPOS.13 In(OH)L is a nanoporous and thermically stable compound for which some tests in heterogeneous catalysis have been performed. (13) (a) Blatov, V. A.; Shevchenko, A. P.; Serezhkin, V. N. J. Appl. Crystallogr. 2000, 33, 1193. (b) Blatov, V. A.; Carlucci, L.; Ciani, G.; Proserpio, D. M. CrystEngComm. 2004, 6, 377.
Figure 4. Acetalization catalyzed by [In(OH)L] · 0.5Py (white columns) and [In(OH)L] (black columns).
Catalytic Properties. In order to test the new materials as acid catalysts, [In(OH)L] · xPy and In(OH)L compounds were tested as heterogeneous catalysts in acetalization of aldehydes (benzaldehyde and R-methylbenzeneacetaldehyde) with trimethyl orthoformate. The results revealed that both In compounds are active and selective catalysts for acetalization of aldehydes. When using [In(OH)L] · 0.5Py as catalyst, the reaction of benzaldehyde with trimethyl orthoformate proceeds with relatively high conversion, and the corresponding dimethylacetal is obtained in up to 97% yield, in relatively short reaction time (1 h) and under mild conditions (60–70 °C) using 10 mol % of In catalyst. The bulkier R-methylbenzeneacetaldehyde was also transformed but after longer reaction time (72 h), showing the hindrance to the diffusion of the reactants inside the catalyst pore. After
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Figure 5. Recycling experiments for catalyst [In(OH)L] (left). On the right, three XRPD patterns of the [In(OH)L] catalyst: simulated (bottom), measured before (middle), and after being used (top).
several thermal treatments to eliminate the pyridine molecules and empty the channels, little improvement was achieved in the catalytic behavior of this compound (Table 2). In fact, after each heating procedure single crystal structure determinations showed a diminution in the pyridine population factors, but not the total channel emptiness. It is for this reason that the synthesis of the compound without pyridine in the reaction media was accomplished. The measurements of the specific surface area (SSA) of both empty and filled compounds effectively show as the pyridine molecules make inaccessible the pores to the reactants diffusion. The pyridine-filled compound exhibits an SSA of only 27 m2, while the empty shows the much bigger value of 215 m2. The same catalytic reactions under identical conditions with new empty In(OH)L compound as catalyst were performed, leading to higher conversions; 100% in 15 min and 24 h for the benzaldehyde and R-methylbenzeneacetaldehyde, respectively (Figure 4). The results of the performed reactions using two differently filled catalysts [In(OH)L] · 0.5Py and [In(OH)L] · 0.3Py (obtained after a thermal treatment of the initial with 0.5py: 48 h at 200 °C under vacuum) and the empty [In(OH)L], together with those reported in our
previous papers9,10 are shown in Table 2. Looking at these results, it is clear that the reaction in the new framework takes place inside the channels since the TOF (h-1) value for[In(OH)L] is nearly 3 times that of [In(OH)L] · 0.5Py and remarkably higher than that previously reported. This fact is also confirmed by the higher conversion of the bulkier R-methylbenzeneacetaldehyde when using the empty [In(OH)L] as catalyst. These new Lewis heterogeneous catalysts are stable in both water and organic solvents, being easily recovered by filtration and reused at least in four cycles without loss of yield or selectivity. The stability of the solid catalyst was checked by XRPD before and after reaction (Figure 5). Acknowledgment. F.G. acknowledges a FPI fellowship from Spanish Ministry for Education and Science (MEC) cofunded by Fondo Social Europeo. This work has been supported by the Spanish MCIT project: MAT 2004-2001, CTQ2004-02865/ BQU, and CONSOLIDER-INGENIO 2010CSD2006-0015. Supporting Information Available: Crystallographic information files (CIF), crystallographic and refinement data for [In(OH)L] · 0.5Py, TG curves, and gas chromatography details. This material is available free of charge via the Internet at http://pubs.acs.org. CM071079A
