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Novel dicarboxylate heteroaromatic metal organic frameworks as the catalyst supports for the hydrogenation reaction
10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.
Novel dicarboxylate heteroaromatic metal organic frameworks as the catalyst supports for the hydrogenation reaction Vera I. Isaeva,*a Olga P. Tkachenko,a Igor V. Mishin,a Elena V. Afonina,a Gennady I. Kapustin,a Ludmila. M. Kozlova,a Wolfgang Grünert,b and Leonid M. Kustov a
N. D. Zelinsky Institute of Organic Chemistry RAS, Leninsky pr. 47, Moscow 119991, Russia b Lehrstuhl Technische Chemie, Ruhr-University Bochum, D-44780 Bochum, Germany
Abstract The novel Zn-derived heteroaromatic metal organic frameworks (MOFs) based on 2(5)pyridinedicarboxylate and 2(5)-pyrazinedicarboxylate ligands were synthesized. In order to elucidate the framework nature effect, a reference sample of aromatic MOF-5 derived from Zn4O clusters and benzene-1,4-dicarboxylate linkers was prepared. The variation of the preparation procedure parameters in respect to the MOF texture (porosity, surface area) was accomplished. The synthesized metal organic frameworks were characterized by the combination of the physicochemical methods: XRD, volumetric N2 adsorption/desorption, DRIFT, and XAS. The catalytic activity of the Pd-containing MOFs in the liquid-phase hydrogenation of cyclohexene (20°C, PH2 1atm) was higher than that of Pd on activated carbon. Keywords: Metal-organic framework, characterization, hydrogenation
1. Introduction The synthesis of metal organic frameworks (MOFs) is a realization of an idea of constructing new materials with tunable physical and chemical properties. However, most of these synthesized systems are being explored from the point of view of its application in adsorption and separation processes with a relatively few citations of the use of MOF materials in catalytic reactions [1-3]. The situation has changed during the recent years. The research work number of the application of the metal organic framework in the catalysis field significantly arose. The phenylenecarboxylate MOFs present a most wide studied group of these materials due to its rigid three dimensional structure characterized by permanent porosity. As a rule these systems are used for the catalysis purposes as the traditional carriers like activated carbons and zeolites. The present contribution deals with the synthesis of the novel porous heteroaromatic dicarboxylate Zn-based MOFs, and its utilization as the supports for the Pd–containing catalysts for the liquid-phase hydrogenation of cyclohexene under mild conditions. Two alternative procedures were used for the synthesis: solvothermal (100°C) and direct mixing methods (20-80°C). In order to compare the MOF texture characteristics as well as the catalytic performance in the hydrogenation tests, a sample of the reference aromatic metal organic framework (MOF-5) derived from Zn4O clusters and benzene1,4-dicarboxylate linkers was prepared.
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2. Experimental 2.1. Synthesis Two alternative procedures were used for the MOF sample preparation: solvothermal and direct mixing methods (Table 1). The syntheses of the samples 2 and 3 were carried out according to the procedure developed by us for the preparation of MOFs based on 2(5)-pyridinedicarboxylic acid and 2(5)-pyrazinedicarboxylic acid respectively. Sample 1. Zn(NO3)2 6H2O (0.350 g, 1.177 mmol) and 2(5)-pyridinedicarboxylic acid (0.197 g, 1.179 mmol) were dissolved in a mixture of N,N’-dimethylformamide (DMF) (9 ml) and toluene (35 ml). The reaction mixture was homogenized (1 h) for solvothermal treatment. The solution was heated in a Teflon-line stain-less autoclave (95°C, 20 h), cooled to room temperature and filtered off in Ar flow. The resulted pale crystals were washed with DMF. The material was evacuated for 6 h (10-3 Hg, 150°C). Sample 2. Zn(NO3)2 6H2O (1.21 g, 4.07 mmol), 2(5)-pyridinedicarboxylic acid (0.33 g, 3.1 mmol), DMF (40 ml) were stirred (80°C, 1.5 ч). The resulting solid was centrifuged repeatedly, washed with DMF (3 x 20 mL) and dried under vacuum (10-3 Hg, 200°C). Sample 3. Zn(NO3)2 6H2O (2.3 g, 7.81 mmol), 2(5)-pyrazinedicarboxylic acid (0.623 g, 3.1 mmol), DMF (85 ml) were stirred (80°C, 1.5 ч). The resulting solid was centrifuged repeatedly, washed with DMF (3 x 20 mL) and dried under vacuum (10-3 Hg, 60°C). Sample 4 was synthesized according to [4]. Table 1. Preparation procedure and composition of MOFs and Pd/MOFs. Sample 1 2 3 4 2a, 3a, 4a
Sample composition Zn, 2(5)-pyridinedicarboxylate Zn, 2(5)-pyridinedicarboxylate Zn, 2(5)-pyrazinedicarboxylate Zn, 1,4-benzenedicarboxylate Pd-containing MOF samples 2, 3, 4
Preparation method solvothermal direct mixing direct mixing direct mixing incipient wetness impregnation
2.2. Catalyst preparation and catalytic performance Pd-containing MOFs were prepared by impregnation of the parent frameworks with a Pd(OAc)2 solution in dry chloroform (1% wt Pd) analogously to Pd(acac)2 deposition [1]. The solution of Pd(OAc)2 (0.032 - 0.050 g) dissolved in chloroform (0.30 ml) was slowly added to evacuated MOF samples (0.5 g) with formation a light orange paste. The solvent was evaporated under continuous stirring. The Pd(OAc)2/MOFs were dried under reduced pressure (20°C, 4 h). The samples of Pd/heteroaromatic MOFs (2a and 3a) were obtained by heating under vacuum (140°C, 4 h) and Pd/aromatic MOF-5 reference sample (4a) was obtained at 200°C for 4 h. The hydrogenation tests were carried out in absolute 1,4-dioxane (20°C, PH2 1atm, catalyst 0.05 g, cyclohexene 0.2 mL). The reaction products were analyzed by GLC equipped with a FID detector and a capillary column.
2.3. Characterization N2 adsorption data were obtained at -196°C by a volumetric method. Specific surface areas were calculated according to the BET equation. Powder XRD patterns were recorded with a DRON 3M diffractometer using Cu Kα radiation in Bragg-Brentano reflecting and Debye–Sherrer transmission geometry (λ=1.54 Å). The DRIFT spectra were recorded at room temperature with Nicolet 460 Protégé spectrometer with a diffuse reflectance attachment. Before IR study samples were evacuated at 200°C for 1 h. The CD3CN was adsorbed at 20°C and saturated vapour pressure. X-ray absorption spectra
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(Zn K edge at 9659 eV) were measured at the Hasylab X1. The spectra were recorded in the transmission mode at -190°C. The spectrum of a metal foil was registered simultaneously between the second and third ionization chambers for energy calibration. The EXAFS data analysis was performed using the software package VIPER. Reference spectra were taken using standard reference compounds: ZnO and Zn-foil. The fitting was done in the k- and r-spaces.
3. Results and discussion 3.1. Characterization All synthesized MOF samples are characterized by proper crystallinity. The synthesis procedure does not remarkably influence the textural parameters of the resulted metal organic framework. The specific surface areas for the 2(5)-pyridinedicarboxylate samples 1 and 2 are 270-300 m2/g. These values are lower, than for 1,4-benzenedirboxylate (MOF-5) sample 4 (1000 m2/g). Despite the crystallinity retention after evacuation, the 2(5)-pyrazinedicarboxylate sample 3 has no surface area. Tentatively, such difference in surface areas could be explained by lower micropore volume for the samples 1 and 2 or lake the permanent porosity for the 2(5)-pyrazinedicarboxylate framework (sample 3). Figure 1 shows the vibration bands in the region 1400 and 1700 cm-1 (a) corresponding symmetric and asymmetric vibrations of C=O bond in carboxylate ion and aromatic ring, while a few bands in the region ~ 3030 сm-1 (b) belong to valent and combination C-H vibrations of aromatic systems (sample 1 - 4, 4a). These data confirm the retention of heterocyclic or phenylenedicarboxylate bridge fragments in the MOF structures.
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Figure 1. DRIFT spectra of MOFs.
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Figure 2. DRIFT spectra of CD3CN.
The DRIFT spectra of adsorbed acetonitrile-d3 on the samples 3, 4, 4a (Figure 2) show the presence of strong Zn2+ Lewis acid sites at the MOF surface. The C≡N stretching vibrations frequency shift relative to the gas phase of acetonitrile-d3 (2253 cm-1) is 45-55 cm-1. The XANES (Figure 3a) evidence that zinc exists as Zn2+ ions in all synthesized organic frameworks. EXAFS spectra exhibit pronounced differences (Figure 3b) in the position and the intensity of the second peak. The analysis of the EXAFS oscillations shows that the nearest neighbors of the central Zn atom in all synthesized frameworks are O atoms with an average coordination number (CN) 3-4 and with Zn-O real distance ~ 1.95-2.04 Ǻ. Next neighbors in the sample 4 (MOF-5) are Zn atoms with average coordination numbers of 3 and 3.20-3.22 Ǻ real distance. The presence of Zn neighboring atoms in this sample suggests the presence in our samples of some Zn species and/or MOF frameworks interpenetrating each other like that observed in [5]. The introduction of Pd in this sample results in the increase of the amount of interweaved cells, the average Zn-Zn coordination number in 4a sample is 4.
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3.2. Hydrogenation reaction As it was mentioned above (see Experimental section) incipient wetness impregnation was used for the preparation of Pd-containing MOF samples. It should be noted that the solvent quantity needed for impregnation is lower, than in case of the reference sample 4 (MOF-5). Probably it could be connected with the different framework dimensionality (2D and 3D) and various pore openings of the synthesized heteroaromatic (sample 2, 3) and aromatic (sample 4) MOFs. Despite the differences in specific surface areas and framework nature the catalytic activities of synthesized frameworks in cyclohexene hydrogenation are similar. The specific surface area does not influence in the activity that indicates the localization of Pd mainly in outer surface of MOF microcrystals, i.e. all Pd species are accessible to cyclohexene. The leaching of Pd from the metal organic support is not detected. The XRD patterns indicate the retention of the crystallinity of Pd/MOFs systems both in the impregnation course and during the reaction. Using Pd/MOFs this reaction proceeds much faster, than on 5%Pd/C (Figure 4). Cyclohexene is hydrogenated selectively to cyclohexane over Pd/MOFs. Any traces of benzene due to disproportionation reaction into benzene and cyclohexane are found in hydrogenation course unlike the hydrogenation reaction on Pd/C. Probably above mentioned observations indicate the advantage of MOFs as the highly ordered and crystalline catalytic systems as compared to activated carbon with an irregular structure [1]. 100
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Figure 3. Zn K XANES (a) and FT EXAFS (b) of MOFs and ZnO.
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Figure 4. Cyclohexene hydrogenation.
4. Conclusions The novel Zn-derived metal organic frameworks based on heterocyclic 2(5)pyridinedicarboxylate and 2(5)-pyrazinedicarboxylate bridging ligands were synthesized. The heteroaromatic MOFs were utilized as the supports of the Pd catalysts for the liquid-phase hydrogenation of cyclohexene. Pd/MOF systems showed the higher catalytic activity in this reaction in comparison with Pd supported on activated carbon.
References [1] Sabo M., Henschel A., Frode H., Klemm E., Kaskel S., J. Mater. Chem., 17(2007) 38273832. [2] Xamena, F. X. L. i.; Abad, A.; Corma, A.; Garcia, H J. Catal., 250 (2)(2007) 294-298. [3] Opelt S., Turk S., Dietzsch E., Sabo M., Henschel A., Kaskel S., Klemm E., Catal. Commun., 9(2008)1286-1290. [4] Isaeva V.I., Tkachenko O.P., Mishin I.V., Kostin A.A., Brueva T.R., Klementiev K.V., Kustov L.M., Topics in Chemistry and Materials Science. Advanced Micro and Mesoporous Materials, 1(2007) 155-162. [5] J. Havicovic, M. Bjorgen, U. Olsbye, P.D.C. Dietzel, S. Bordiga, C. Prestipino, C. Lamberti and K.-P. Lillerud J., Am. Chem. Soc., 129 (2007) 3612-3620.
Novel dicarboxylate heteroaromatic metal organic frameworks as the catalyst supports for the hydrogenation reaction Vera I. Isaeva,*a Olga P. Tkachenko,a Igor V. Mishin,a Elena V. Afonina,a Gennady I. Kapustin,a Ludmila. M. Kozlova,a Wolfgang Grünert,b and Leonid M. Kustov a
N. D. Zelinsky Institute of Organic Chemistry RAS, Leninsky pr. 47, Moscow 119991, Russia b Lehrstuhl Technische Chemie, Ruhr-University Bochum, D-44780 Bochum, Germany
Abstract The novel Zn-derived heteroaromatic metal organic frameworks (MOFs) based on 2(5)pyridinedicarboxylate and 2(5)-pyrazinedicarboxylate ligands were synthesized. In order to elucidate the framework nature effect, a reference sample of aromatic MOF-5 derived from Zn4O clusters and benzene-1,4-dicarboxylate linkers was prepared. The variation of the preparation procedure parameters in respect to the MOF texture (porosity, surface area) was accomplished. The synthesized metal organic frameworks were characterized by the combination of the physicochemical methods: XRD, volumetric N2 adsorption/desorption, DRIFT, and XAS. The catalytic activity of the Pd-containing MOFs in the liquid-phase hydrogenation of cyclohexene (20°C, PH2 1atm) was higher than that of Pd on activated carbon. Keywords: Metal-organic framework, characterization, hydrogenation
1. Introduction The synthesis of metal organic frameworks (MOFs) is a realization of an idea of constructing new materials with tunable physical and chemical properties. However, most of these synthesized systems are being explored from the point of view of its application in adsorption and separation processes with a relatively few citations of the use of MOF materials in catalytic reactions [1-3]. The situation has changed during the recent years. The research work number of the application of the metal organic framework in the catalysis field significantly arose. The phenylenecarboxylate MOFs present a most wide studied group of these materials due to its rigid three dimensional structure characterized by permanent porosity. As a rule these systems are used for the catalysis purposes as the traditional carriers like activated carbons and zeolites. The present contribution deals with the synthesis of the novel porous heteroaromatic dicarboxylate Zn-based MOFs, and its utilization as the supports for the Pd–containing catalysts for the liquid-phase hydrogenation of cyclohexene under mild conditions. Two alternative procedures were used for the synthesis: solvothermal (100°C) and direct mixing methods (20-80°C). In order to compare the MOF texture characteristics as well as the catalytic performance in the hydrogenation tests, a sample of the reference aromatic metal organic framework (MOF-5) derived from Zn4O clusters and benzene1,4-dicarboxylate linkers was prepared.
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2. Experimental 2.1. Synthesis Two alternative procedures were used for the MOF sample preparation: solvothermal and direct mixing methods (Table 1). The syntheses of the samples 2 and 3 were carried out according to the procedure developed by us for the preparation of MOFs based on 2(5)-pyridinedicarboxylic acid and 2(5)-pyrazinedicarboxylic acid respectively. Sample 1. Zn(NO3)2 6H2O (0.350 g, 1.177 mmol) and 2(5)-pyridinedicarboxylic acid (0.197 g, 1.179 mmol) were dissolved in a mixture of N,N’-dimethylformamide (DMF) (9 ml) and toluene (35 ml). The reaction mixture was homogenized (1 h) for solvothermal treatment. The solution was heated in a Teflon-line stain-less autoclave (95°C, 20 h), cooled to room temperature and filtered off in Ar flow. The resulted pale crystals were washed with DMF. The material was evacuated for 6 h (10-3 Hg, 150°C). Sample 2. Zn(NO3)2 6H2O (1.21 g, 4.07 mmol), 2(5)-pyridinedicarboxylic acid (0.33 g, 3.1 mmol), DMF (40 ml) were stirred (80°C, 1.5 ч). The resulting solid was centrifuged repeatedly, washed with DMF (3 x 20 mL) and dried under vacuum (10-3 Hg, 200°C). Sample 3. Zn(NO3)2 6H2O (2.3 g, 7.81 mmol), 2(5)-pyrazinedicarboxylic acid (0.623 g, 3.1 mmol), DMF (85 ml) were stirred (80°C, 1.5 ч). The resulting solid was centrifuged repeatedly, washed with DMF (3 x 20 mL) and dried under vacuum (10-3 Hg, 60°C). Sample 4 was synthesized according to [4]. Table 1. Preparation procedure and composition of MOFs and Pd/MOFs. Sample 1 2 3 4 2a, 3a, 4a
Sample composition Zn, 2(5)-pyridinedicarboxylate Zn, 2(5)-pyridinedicarboxylate Zn, 2(5)-pyrazinedicarboxylate Zn, 1,4-benzenedicarboxylate Pd-containing MOF samples 2, 3, 4
Preparation method solvothermal direct mixing direct mixing direct mixing incipient wetness impregnation
2.2. Catalyst preparation and catalytic performance Pd-containing MOFs were prepared by impregnation of the parent frameworks with a Pd(OAc)2 solution in dry chloroform (1% wt Pd) analogously to Pd(acac)2 deposition [1]. The solution of Pd(OAc)2 (0.032 - 0.050 g) dissolved in chloroform (0.30 ml) was slowly added to evacuated MOF samples (0.5 g) with formation a light orange paste. The solvent was evaporated under continuous stirring. The Pd(OAc)2/MOFs were dried under reduced pressure (20°C, 4 h). The samples of Pd/heteroaromatic MOFs (2a and 3a) were obtained by heating under vacuum (140°C, 4 h) and Pd/aromatic MOF-5 reference sample (4a) was obtained at 200°C for 4 h. The hydrogenation tests were carried out in absolute 1,4-dioxane (20°C, PH2 1atm, catalyst 0.05 g, cyclohexene 0.2 mL). The reaction products were analyzed by GLC equipped with a FID detector and a capillary column.
2.3. Characterization N2 adsorption data were obtained at -196°C by a volumetric method. Specific surface areas were calculated according to the BET equation. Powder XRD patterns were recorded with a DRON 3M diffractometer using Cu Kα radiation in Bragg-Brentano reflecting and Debye–Sherrer transmission geometry (λ=1.54 Å). The DRIFT spectra were recorded at room temperature with Nicolet 460 Protégé spectrometer with a diffuse reflectance attachment. Before IR study samples were evacuated at 200°C for 1 h. The CD3CN was adsorbed at 20°C and saturated vapour pressure. X-ray absorption spectra
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(Zn K edge at 9659 eV) were measured at the Hasylab X1. The spectra were recorded in the transmission mode at -190°C. The spectrum of a metal foil was registered simultaneously between the second and third ionization chambers for energy calibration. The EXAFS data analysis was performed using the software package VIPER. Reference spectra were taken using standard reference compounds: ZnO and Zn-foil. The fitting was done in the k- and r-spaces.
3. Results and discussion 3.1. Characterization All synthesized MOF samples are characterized by proper crystallinity. The synthesis procedure does not remarkably influence the textural parameters of the resulted metal organic framework. The specific surface areas for the 2(5)-pyridinedicarboxylate samples 1 and 2 are 270-300 m2/g. These values are lower, than for 1,4-benzenedirboxylate (MOF-5) sample 4 (1000 m2/g). Despite the crystallinity retention after evacuation, the 2(5)-pyrazinedicarboxylate sample 3 has no surface area. Tentatively, such difference in surface areas could be explained by lower micropore volume for the samples 1 and 2 or lake the permanent porosity for the 2(5)-pyrazinedicarboxylate framework (sample 3). Figure 1 shows the vibration bands in the region 1400 and 1700 cm-1 (a) corresponding symmetric and asymmetric vibrations of C=O bond in carboxylate ion and aromatic ring, while a few bands in the region ~ 3030 сm-1 (b) belong to valent and combination C-H vibrations of aromatic systems (sample 1 - 4, 4a). These data confirm the retention of heterocyclic or phenylenedicarboxylate bridge fragments in the MOF structures.
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Figure 1. DRIFT spectra of MOFs.
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Figure 2. DRIFT spectra of CD3CN.
The DRIFT spectra of adsorbed acetonitrile-d3 on the samples 3, 4, 4a (Figure 2) show the presence of strong Zn2+ Lewis acid sites at the MOF surface. The C≡N stretching vibrations frequency shift relative to the gas phase of acetonitrile-d3 (2253 cm-1) is 45-55 cm-1. The XANES (Figure 3a) evidence that zinc exists as Zn2+ ions in all synthesized organic frameworks. EXAFS spectra exhibit pronounced differences (Figure 3b) in the position and the intensity of the second peak. The analysis of the EXAFS oscillations shows that the nearest neighbors of the central Zn atom in all synthesized frameworks are O atoms with an average coordination number (CN) 3-4 and with Zn-O real distance ~ 1.95-2.04 Ǻ. Next neighbors in the sample 4 (MOF-5) are Zn atoms with average coordination numbers of 3 and 3.20-3.22 Ǻ real distance. The presence of Zn neighboring atoms in this sample suggests the presence in our samples of some Zn species and/or MOF frameworks interpenetrating each other like that observed in [5]. The introduction of Pd in this sample results in the increase of the amount of interweaved cells, the average Zn-Zn coordination number in 4a sample is 4.
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3.2. Hydrogenation reaction As it was mentioned above (see Experimental section) incipient wetness impregnation was used for the preparation of Pd-containing MOF samples. It should be noted that the solvent quantity needed for impregnation is lower, than in case of the reference sample 4 (MOF-5). Probably it could be connected with the different framework dimensionality (2D and 3D) and various pore openings of the synthesized heteroaromatic (sample 2, 3) and aromatic (sample 4) MOFs. Despite the differences in specific surface areas and framework nature the catalytic activities of synthesized frameworks in cyclohexene hydrogenation are similar. The specific surface area does not influence in the activity that indicates the localization of Pd mainly in outer surface of MOF microcrystals, i.e. all Pd species are accessible to cyclohexene. The leaching of Pd from the metal organic support is not detected. The XRD patterns indicate the retention of the crystallinity of Pd/MOFs systems both in the impregnation course and during the reaction. Using Pd/MOFs this reaction proceeds much faster, than on 5%Pd/C (Figure 4). Cyclohexene is hydrogenated selectively to cyclohexane over Pd/MOFs. Any traces of benzene due to disproportionation reaction into benzene and cyclohexane are found in hydrogenation course unlike the hydrogenation reaction on Pd/C. Probably above mentioned observations indicate the advantage of MOFs as the highly ordered and crystalline catalytic systems as compared to activated carbon with an irregular structure [1]. 100
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Figure 3. Zn K XANES (a) and FT EXAFS (b) of MOFs and ZnO.
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Figure 4. Cyclohexene hydrogenation.
4. Conclusions The novel Zn-derived metal organic frameworks based on heterocyclic 2(5)pyridinedicarboxylate and 2(5)-pyrazinedicarboxylate bridging ligands were synthesized. The heteroaromatic MOFs were utilized as the supports of the Pd catalysts for the liquid-phase hydrogenation of cyclohexene. Pd/MOF systems showed the higher catalytic activity in this reaction in comparison with Pd supported on activated carbon.
References [1] Sabo M., Henschel A., Frode H., Klemm E., Kaskel S., J. Mater. Chem., 17(2007) 38273832. [2] Xamena, F. X. L. i.; Abad, A.; Corma, A.; Garcia, H J. Catal., 250 (2)(2007) 294-298. [3] Opelt S., Turk S., Dietzsch E., Sabo M., Henschel A., Kaskel S., Klemm E., Catal. Commun., 9(2008)1286-1290. [4] Isaeva V.I., Tkachenko O.P., Mishin I.V., Kostin A.A., Brueva T.R., Klementiev K.V., Kustov L.M., Topics in Chemistry and Materials Science. Advanced Micro and Mesoporous Materials, 1(2007) 155-162. [5] J. Havicovic, M. Bjorgen, U. Olsbye, P.D.C. Dietzel, S. Bordiga, C. Prestipino, C. Lamberti and K.-P. Lillerud J., Am. Chem. Soc., 129 (2007) 3612-3620.