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MOF-5 as acid catalyst with shape selectivity properties
Zeolites and Related Materials: Trends, Targets and Challenges Proceedings of 4th International FEZA Conference A. Gédéon, P. Massiani and F. Babonneau (Editors) © 2008 Elsevier B.V. All rights reserved.
467
MOF-5 as acid catalyst with shape selectivity properties Ugo Ravon, Marcelo.E Domine, Cyril Gaudillère, Arnold Desmartin-Chomel, David Farrusseng* IRCELYON, UMR UCBL-CNRS 5256 2 av A.Einstein, 69626 Villeurbanne, France Fax : +33(0)4 72445365 E-mail : [email protected] Keywords: MOF, Alkylation, acid catalyst, shape selectivity
1. Introduction The development of Metal-organic frameworks (MOFs) has already open new perspectives in gas separation and storage applications [1-3]. The ability to tune pore size at the Angstrom scale and to design accessible metallic nanoclusters in a highly porous structure, make these compounds very attractive for Catalysis [4, 5]. Although the potentialities of MOF for catalytic applications are extensively acknowledged, a limited number of successful catalytic studies have been reported [6]. A main reason is that for most of porous MOF structures, the coordination sphere of the metal ions is fully completed by the organic linkers which hampers the activation of reactants [7]. The cubic framework of IRMOF-1 consists of Zn4O nodes which are linked in octahedral arrays of 1,4-benzenedicarboxylic acid (BDC) groups to form a porous material with channel window of about 8 Å [8]. Pore size can be achieved by selecting appropriate dicarboxylic linkers of various length (IRMOF series). For instance, IRMOF-8 is an isoreticular compound with 2,6-naphthalenedicarboxilic acid (2,6NDC) as linker. We report herein the catalytic activity of IRMOF samples having Zn-OH defects for aromatic alkylation with shape selectivity properties. The IRMOF-1 catalyst was prepared following the method developed by Huang et al [9]. This method was extended to the synthesis IRMOF-8 by using 2,6-NDC as ligand.
2. Results and discussion 2.1. Catalyst Characterization Surface areas, micropore volumes and pore sizes correspond to those reported elsewhere with similar preparation procedures for IRMOF-1 [9, 10] and IRMOF-8 [11] (Table 2). Pore diameters are calculated from N2 isotherm data using DFT method and confirm sharp pore size distributions. Measurements are also in good agreement with calculated window diameter (‘‘free-space’’) from structural data. From experimental and theoretical data, there is in principle no steric hindrance for monoalkylated products to diffuse out of all porous solids, including H-BEA. Even very bulky products such as the doubly alkylated product 4,4’-di-t-butylbiphenylene (12.5 x 5.0 Å) can be formed in large amount in H-BEA [12]. Therefore, the shape selectivity properties observed for IRMOF sample can not arise from diffusion hindrance of the most bulky products since pore windows are even larger.
468
U. Ravon et al.
Table 1. N2 physisorption results and comparison of pore sizes with structural data. Sample
Micropore volume / (cm3.g-1)b
Surface Area (m2.g-1)a
Pore diameter / (Å )c
Window diameter / (Å )
450 0.18 8.8 7.8 d 730 0.29 7.3 / 11.6 9.2 d 430 0.10 7.7 6.1 e [a] BET, [b] t-plot, [c] DFT, [d] from ref 8, [e] from ref 13. For the reaction takes place we suggest that the biphenyl is adsorbed in a specific manner to allow the formation of the transitions state for the para oriented product. Once alkylated, it can not be activated in the same manner because of steric hindrance and double alkylation can not proceed. Because of porous structure similarity between IRMOF-1 and IRMOF-8, it is not surprising that similar selectivity is obtained. IRMOF-1 IRMOF-8 H-BEA
Absorbance / a.u.
a
b
0.8
0.4
0.0
-0.4 4000
3600
3200
2800 -1
Wavenumber / cm
16
14
12
10
8
6
4
2
0
-2
-4
δ 1H / ppm
Figure 1. IRMOF-1 characterization showing Zn-OH moiety; (a) Diffuse IRTF after desorption at 573°K (b) 1H solid NMR. Structural (XRD, IR) and thermal (TGA) characterisation of IRMOF-1 are in very good agreement with results reported by Huang et al [9] and Havizovic et al (namely, MOFZn) [10]. The sharp band at 3602 cm-1, which can be assigned to Zn–OH species [14], may be responsible of the catalytic activity (Figure 1a). Also, we observe an excess of 10% of Zn, which is confirmed by elemental analysis, and which leads to a global composition of Zn4O(BDC)2.H2O + Zn(OH)2. According to Havizovic et al [10], zinc hydroxide nanoclusters which partly occupied the cavities are responsible of the structure distortion evidenced by the splitting of the diffraction peak at 9.7°. On the other hand, 1H NMR on different IRMOF-1 samples indicate the presence of Zn-OH-Zn species (very small peak at = -0.4 ppm) like those encountered in MOF-69 series [13a] and Zn3(OH)2(DBC)2.2DEF (MOF-69c) which are isostructural and contain Zn-(μ3OH)-Zn chains (Figure 1b). The peak integration gives a ratio of 1 μ3-OH for 25 BDC linkers. The later phase can be formed during the synthesis of IRMOF-1 and its kinetic is favoured in moisture conditions as it is the case in this study [14]. Therefore, in addition to Zn hydroxides clusters, IRMOF-1 samples also contain Zn-(μ3-OH) chains in minor quantity which could be either crystalline default of the cubic structure or XRD not detectable MOF-69c microcrystallites. Finally, it was recently demonstrated that pure IRMOF-1 undergoes rapid phase transformation in moisture conditions to lead to hydroxilated Zn. In summary, catalytic centres may arise either from Zn hydroxide
469
MOF-5 as acid catalyst with shape selectivity properties
clusters or from the presence of MOF-69c, or structural defects as Zn-OH species formed at the synthesis step or upon water adsorption. In contrast to results on toluene benzylation over MIL-100(Fe), the reaction takes place in the first minutes with IRMOFs indicating that the structure do not required profound modifications to become catalytic (such as creation of Zn-Cl bonds). After reaction, we do not observe major change in XRD patterns although carboxylic acid groups are detected in 1H NMR which should arise from a partial hydrolysis of the framework in acid conditions [6]. Further experiments are undergoing in order to identify exact catalytic centres. 2.2. Alkylation The activity and selectivity of MOF samples were studied for the alkylation of toluene and biphenyl with t-butylchloride (tBuCl) in batch reactor. Because of the size matching between alkylbiphenyl compounds and large pore zeolites, the alkylation of biphenyl is an appropriate model reaction to highlight pore shape selectivity properties for acid catalysis 7. The acid form of beta zeolite (denoted hereafter H-BEA) and AlCl3 were used as reference catalysts. As results, IRMOF samples are as active as H-BEA and AlCl3 and reactions are completed within 2 hours at 443°K. Although all catalysts are active at much lower temperatures, selectivity data are presented in Table 2 at 443°K in order to compare catalyst performances in similar tBuCl conversions ranging from 60 to 80%. For toluene alkylations, IRMOF samples exhibit good selectivity to the less bulky para oriented-products in the same range than H-BEA whereas AlCl3 produce mixtures of ortho- and para-compounds in equivalent amount. On the other hand, the alkylation of biphenyl reveals exceptional selectivity to p-isomer 4-tert-butylbiphenyl for IRMOF samples with respect to H-BEA and AlCl3. Indeed, only traces of doubly alkylated products are detected for IRMOFs whereas H-BEA and AlCl3 show much higher selectivity to the ortho- isomer and about 15% to di-alkylated products. For 1hr of reaction at 373°K, 100% selectivity is obtained in the para product with conversion of 20% for IRMOF-1 whereas H-BEA shows much lower selectivity of 55% (for 35% conversion). Those results point out outstanding pore-shape selectivity properties of IRMOFs for large polyaromatics. Table 1. Selectivity of tert-butylation reaction at 170°C. Toluene
Biphenyl
Catalyst
paraa
ortho
dic
parab
ortho
dic
IRMOF-1
82
18
0
96
3
1
IRMOF-8
84
16
0
95
3
2
H-BEA
72
28
0
55
22
23
AlCl3
46
54
0
51
38
11
Conversions are ranging from 60 to 80%. [a] p-tert-butyltoluene, [b] 4-tert-butylbiphenyl, [c] sum of dialkylated products. Molecules drawing for biphenyl alkylation are shown in scheme 1.
Leaching issues were investigated in order to check that the reaction takes place in heterogeneous conditions. Thus, after the completion of reactions, the mixtures were filtrated and both catalyst and liquid were separately recovered. First, reuse of the recovered catalyst with fresh reactants leads to similar alkylated products yields. Second, any additional conversion was observed when the recovered liquid was reacted again, even when fresh tBuCl was added. Second any trace of Zn was found in filtrates. Finally, we have observed that the reaction takes place at 50°C with IRMOF-1 at a very
470
U. Ravon et al.
slow rate, however. In these soft conditions, IRMOF samples are not soluble in decane indicating that the reaction does not proceed in homogeneous phase.
3. Conclusion We have demonstrated that MOFs can undergo very shape selective catalytic alkylations of large molecules which shall open new perspective for C-C coupling of polyaromatics or biomolecules which are too large to be addressed by zeolites. We further believe that the control of structural defects in MOF materials may lead to the generation of a new class of shape selective catalysts with different acidity and hydrophilic properties than encountered in zeolites.
References [1] Rosseinsky, M. J., Microporous and Mesoporous Materials, 73 (2004) 15. [2] Yaghi, O. M.; O'Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J.,. Nature, 423, (6941),( 2003) 705. [3] Cheetham, A. K.; Ferey, G.; Loiseau, T., Angewandte Chemie-International Edition, 38, (22) (1999) 3268. [4] Schlichte, K.; Kratzke, T.; Kaskel, S., Microporous and Mesoporous Materials, 73 (1-2) (2004) 81. [5] Alaerts, L.; Seguin, E.; Poelman, H.; Thibault-Starzyk, F.; Jacobs, P. A.; De Vos, D. E., Chemistry-a European Journal, 12, (28) (2006) 7353. [6] Mueller, U.; Schubert, M.; Teich, F.; Puetter, H.; Schierle-Arndt, K.; Pastre, J., Journal of Materials Chemistry, 16, (7) (2006) 626. [7] Llabre´s i Xamena, F.; Corma, A.; Garcia, H., J. Phys. Chem. C, 111 (2007) 80. [8] Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O'Keefe, M.; Yaghi, O. M., Science, 295, (5554) (2002) 469. [9] Huang, L. M.; Wang, H. T.; Chen, J. X.; Wang, Z. B.; Sun, J. Y.; Zhao, D. Y.; Yan, Y.S Microporous and Mesoporous Materials 2003, 58, (2), 105-114. [10] Hafizovic, J.; Bjorgen, M.; Olsbye, U.; Dietzel, P. D. C.; Bordiga, S.; Prestipino, C.; Lamberti, C.; Lillerud, K. P., Journal of the American Chemical Society 2007, 129, (12), 3612-3620. [11] Li, Y. W.; Yang, R. T., Journal of the American Chemical Society, 128, (3) (2006) 726. [12] Mravec, D.; Zavadan, P.; Kaszonyi, A.; Joffre, J.; Moreau, P., Applied Catalysis, A, 257, (1), (2004) 49. [13] Foster, M. D.; Rivin, I.; Treacy, M. M. J.; Friedrichs, O. D., Microporous and Mesoporous Materials, 90, (1-3) (2006) 32. [14] Liao, J. H.; Lee, T. J.; Su, C. T., Inorganic. Chemistry. Communications., 9, (2) (2006) 201.
467
MOF-5 as acid catalyst with shape selectivity properties Ugo Ravon, Marcelo.E Domine, Cyril Gaudillère, Arnold Desmartin-Chomel, David Farrusseng* IRCELYON, UMR UCBL-CNRS 5256 2 av A.Einstein, 69626 Villeurbanne, France Fax : +33(0)4 72445365 E-mail : [email protected] Keywords: MOF, Alkylation, acid catalyst, shape selectivity
1. Introduction The development of Metal-organic frameworks (MOFs) has already open new perspectives in gas separation and storage applications [1-3]. The ability to tune pore size at the Angstrom scale and to design accessible metallic nanoclusters in a highly porous structure, make these compounds very attractive for Catalysis [4, 5]. Although the potentialities of MOF for catalytic applications are extensively acknowledged, a limited number of successful catalytic studies have been reported [6]. A main reason is that for most of porous MOF structures, the coordination sphere of the metal ions is fully completed by the organic linkers which hampers the activation of reactants [7]. The cubic framework of IRMOF-1 consists of Zn4O nodes which are linked in octahedral arrays of 1,4-benzenedicarboxylic acid (BDC) groups to form a porous material with channel window of about 8 Å [8]. Pore size can be achieved by selecting appropriate dicarboxylic linkers of various length (IRMOF series). For instance, IRMOF-8 is an isoreticular compound with 2,6-naphthalenedicarboxilic acid (2,6NDC) as linker. We report herein the catalytic activity of IRMOF samples having Zn-OH defects for aromatic alkylation with shape selectivity properties. The IRMOF-1 catalyst was prepared following the method developed by Huang et al [9]. This method was extended to the synthesis IRMOF-8 by using 2,6-NDC as ligand.
2. Results and discussion 2.1. Catalyst Characterization Surface areas, micropore volumes and pore sizes correspond to those reported elsewhere with similar preparation procedures for IRMOF-1 [9, 10] and IRMOF-8 [11] (Table 2). Pore diameters are calculated from N2 isotherm data using DFT method and confirm sharp pore size distributions. Measurements are also in good agreement with calculated window diameter (‘‘free-space’’) from structural data. From experimental and theoretical data, there is in principle no steric hindrance for monoalkylated products to diffuse out of all porous solids, including H-BEA. Even very bulky products such as the doubly alkylated product 4,4’-di-t-butylbiphenylene (12.5 x 5.0 Å) can be formed in large amount in H-BEA [12]. Therefore, the shape selectivity properties observed for IRMOF sample can not arise from diffusion hindrance of the most bulky products since pore windows are even larger.
468
U. Ravon et al.
Table 1. N2 physisorption results and comparison of pore sizes with structural data. Sample
Micropore volume / (cm3.g-1)b
Surface Area (m2.g-1)a
Pore diameter / (Å )c
Window diameter / (Å )
450 0.18 8.8 7.8 d 730 0.29 7.3 / 11.6 9.2 d 430 0.10 7.7 6.1 e [a] BET, [b] t-plot, [c] DFT, [d] from ref 8, [e] from ref 13. For the reaction takes place we suggest that the biphenyl is adsorbed in a specific manner to allow the formation of the transitions state for the para oriented product. Once alkylated, it can not be activated in the same manner because of steric hindrance and double alkylation can not proceed. Because of porous structure similarity between IRMOF-1 and IRMOF-8, it is not surprising that similar selectivity is obtained. IRMOF-1 IRMOF-8 H-BEA
Absorbance / a.u.
a
b
0.8
0.4
0.0
-0.4 4000
3600
3200
2800 -1
Wavenumber / cm
16
14
12
10
8
6
4
2
0
-2
-4
δ 1H / ppm
Figure 1. IRMOF-1 characterization showing Zn-OH moiety; (a) Diffuse IRTF after desorption at 573°K (b) 1H solid NMR. Structural (XRD, IR) and thermal (TGA) characterisation of IRMOF-1 are in very good agreement with results reported by Huang et al [9] and Havizovic et al (namely, MOFZn) [10]. The sharp band at 3602 cm-1, which can be assigned to Zn–OH species [14], may be responsible of the catalytic activity (Figure 1a). Also, we observe an excess of 10% of Zn, which is confirmed by elemental analysis, and which leads to a global composition of Zn4O(BDC)2.H2O + Zn(OH)2. According to Havizovic et al [10], zinc hydroxide nanoclusters which partly occupied the cavities are responsible of the structure distortion evidenced by the splitting of the diffraction peak at 9.7°. On the other hand, 1H NMR on different IRMOF-1 samples indicate the presence of Zn-OH-Zn species (very small peak at = -0.4 ppm) like those encountered in MOF-69 series [13a] and Zn3(OH)2(DBC)2.2DEF (MOF-69c) which are isostructural and contain Zn-(μ3OH)-Zn chains (Figure 1b). The peak integration gives a ratio of 1 μ3-OH for 25 BDC linkers. The later phase can be formed during the synthesis of IRMOF-1 and its kinetic is favoured in moisture conditions as it is the case in this study [14]. Therefore, in addition to Zn hydroxides clusters, IRMOF-1 samples also contain Zn-(μ3-OH) chains in minor quantity which could be either crystalline default of the cubic structure or XRD not detectable MOF-69c microcrystallites. Finally, it was recently demonstrated that pure IRMOF-1 undergoes rapid phase transformation in moisture conditions to lead to hydroxilated Zn. In summary, catalytic centres may arise either from Zn hydroxide
469
MOF-5 as acid catalyst with shape selectivity properties
clusters or from the presence of MOF-69c, or structural defects as Zn-OH species formed at the synthesis step or upon water adsorption. In contrast to results on toluene benzylation over MIL-100(Fe), the reaction takes place in the first minutes with IRMOFs indicating that the structure do not required profound modifications to become catalytic (such as creation of Zn-Cl bonds). After reaction, we do not observe major change in XRD patterns although carboxylic acid groups are detected in 1H NMR which should arise from a partial hydrolysis of the framework in acid conditions [6]. Further experiments are undergoing in order to identify exact catalytic centres. 2.2. Alkylation The activity and selectivity of MOF samples were studied for the alkylation of toluene and biphenyl with t-butylchloride (tBuCl) in batch reactor. Because of the size matching between alkylbiphenyl compounds and large pore zeolites, the alkylation of biphenyl is an appropriate model reaction to highlight pore shape selectivity properties for acid catalysis 7. The acid form of beta zeolite (denoted hereafter H-BEA) and AlCl3 were used as reference catalysts. As results, IRMOF samples are as active as H-BEA and AlCl3 and reactions are completed within 2 hours at 443°K. Although all catalysts are active at much lower temperatures, selectivity data are presented in Table 2 at 443°K in order to compare catalyst performances in similar tBuCl conversions ranging from 60 to 80%. For toluene alkylations, IRMOF samples exhibit good selectivity to the less bulky para oriented-products in the same range than H-BEA whereas AlCl3 produce mixtures of ortho- and para-compounds in equivalent amount. On the other hand, the alkylation of biphenyl reveals exceptional selectivity to p-isomer 4-tert-butylbiphenyl for IRMOF samples with respect to H-BEA and AlCl3. Indeed, only traces of doubly alkylated products are detected for IRMOFs whereas H-BEA and AlCl3 show much higher selectivity to the ortho- isomer and about 15% to di-alkylated products. For 1hr of reaction at 373°K, 100% selectivity is obtained in the para product with conversion of 20% for IRMOF-1 whereas H-BEA shows much lower selectivity of 55% (for 35% conversion). Those results point out outstanding pore-shape selectivity properties of IRMOFs for large polyaromatics. Table 1. Selectivity of tert-butylation reaction at 170°C. Toluene
Biphenyl
Catalyst
paraa
ortho
dic
parab
ortho
dic
IRMOF-1
82
18
0
96
3
1
IRMOF-8
84
16
0
95
3
2
H-BEA
72
28
0
55
22
23
AlCl3
46
54
0
51
38
11
Conversions are ranging from 60 to 80%. [a] p-tert-butyltoluene, [b] 4-tert-butylbiphenyl, [c] sum of dialkylated products. Molecules drawing for biphenyl alkylation are shown in scheme 1.
Leaching issues were investigated in order to check that the reaction takes place in heterogeneous conditions. Thus, after the completion of reactions, the mixtures were filtrated and both catalyst and liquid were separately recovered. First, reuse of the recovered catalyst with fresh reactants leads to similar alkylated products yields. Second, any additional conversion was observed when the recovered liquid was reacted again, even when fresh tBuCl was added. Second any trace of Zn was found in filtrates. Finally, we have observed that the reaction takes place at 50°C with IRMOF-1 at a very
470
U. Ravon et al.
slow rate, however. In these soft conditions, IRMOF samples are not soluble in decane indicating that the reaction does not proceed in homogeneous phase.
3. Conclusion We have demonstrated that MOFs can undergo very shape selective catalytic alkylations of large molecules which shall open new perspective for C-C coupling of polyaromatics or biomolecules which are too large to be addressed by zeolites. We further believe that the control of structural defects in MOF materials may lead to the generation of a new class of shape selective catalysts with different acidity and hydrophilic properties than encountered in zeolites.
References [1] Rosseinsky, M. J., Microporous and Mesoporous Materials, 73 (2004) 15. [2] Yaghi, O. M.; O'Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J.,. Nature, 423, (6941),( 2003) 705. [3] Cheetham, A. K.; Ferey, G.; Loiseau, T., Angewandte Chemie-International Edition, 38, (22) (1999) 3268. [4] Schlichte, K.; Kratzke, T.; Kaskel, S., Microporous and Mesoporous Materials, 73 (1-2) (2004) 81. [5] Alaerts, L.; Seguin, E.; Poelman, H.; Thibault-Starzyk, F.; Jacobs, P. A.; De Vos, D. E., Chemistry-a European Journal, 12, (28) (2006) 7353. [6] Mueller, U.; Schubert, M.; Teich, F.; Puetter, H.; Schierle-Arndt, K.; Pastre, J., Journal of Materials Chemistry, 16, (7) (2006) 626. [7] Llabre´s i Xamena, F.; Corma, A.; Garcia, H., J. Phys. Chem. C, 111 (2007) 80. [8] Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O'Keefe, M.; Yaghi, O. M., Science, 295, (5554) (2002) 469. [9] Huang, L. M.; Wang, H. T.; Chen, J. X.; Wang, Z. B.; Sun, J. Y.; Zhao, D. Y.; Yan, Y.S Microporous and Mesoporous Materials 2003, 58, (2), 105-114. [10] Hafizovic, J.; Bjorgen, M.; Olsbye, U.; Dietzel, P. D. C.; Bordiga, S.; Prestipino, C.; Lamberti, C.; Lillerud, K. P., Journal of the American Chemical Society 2007, 129, (12), 3612-3620. [11] Li, Y. W.; Yang, R. T., Journal of the American Chemical Society, 128, (3) (2006) 726. [12] Mravec, D.; Zavadan, P.; Kaszonyi, A.; Joffre, J.; Moreau, P., Applied Catalysis, A, 257, (1), (2004) 49. [13] Foster, M. D.; Rivin, I.; Treacy, M. M. J.; Friedrichs, O. D., Microporous and Mesoporous Materials, 90, (1-3) (2006) 32. [14] Liao, J. H.; Lee, T. J.; Su, C. T., Inorganic. Chemistry. Communications., 9, (2) (2006) 201.