J. Phys. Chem. C 2009, 113, 20435–20444
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Comparison of the Transport of Aromatic Compounds in Small and Large MFI Particles O. C. Gobin, S. J. Reitmeier, A. Jentys, and J. A. Lercher* Department of Chemistry, Technische UniVersita¨t Mu¨nchen, Lichtenbergstrasse 4, 85747 Garching, Germany ReceiVed: August 3, 2009; ReVised Manuscript ReceiVed: October 2, 2009
The diffusion of alkyl-substituted aromatic molecules in two H-ZSM5 samples consisting of very small ( 0.8, which is characteristic for macropores larger than 30 nm. These pores can be ascribed to the interparticle voids formed by agglomera-
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Figure 2. Transmission electron micrographs of the sample with (A) small and (B) large particles.
Figure 3. Nitrogen physorption isotherms for the sample with (A) small and (B) large particles.
TABLE 1: Structural Properties of the ZSM5 Materialsa
small large
SBET m2 g-1
Vmi cm3 g-1
Vme cm3 g-1
Vma cm3 g-1
Sext m2 g-1
Vtot cm3 g-1
423 421
0.12 0.12
0.04 0.07
0.22 0.03
65 6
0.38 0.22
a SBET is the surface area according to the BET theory, and Sext the external surface area. Vmi, Vme, Vma, and Vtot are the micropore, mesopore, macropore, and total pore volume, respectively, obtained as described in the experimental section by using the Rs comparative plot.
tion of the primary particles and are, therefore, predominantly present in the small crystalline sample. This is consistent with the particle size obtained by DLS or SEM (360 ( 170 nm), which is significantly larger than the primary particle size by TEM ( 10 s. This is for instance the case for benzene diffusion in ZSM-5 (D ∼ 10-13 m2/s) composed of particles of at least 1-2 µm, or for o-xylene in ZSM-5 (D ∼ 10-16-10-17 m2/s)32 for particles in the nanometer range (10-100 nm). A faster diffusion process or smaller particles will lead to a complex and practically inseparable convolution of multiple kinetic processes masking the intracrystalline diffusion. 6. Conclusions A new model based on frequency response kinetics, able to describe the complex relation between surface transport and diffusion, is proposed. In ZSM5 samples composed of primary particles below 100 nm, the transport of benzene, toluene, and p-xylene is governed by the rate of surface adsorption and pore entering. Diffusion processes cannot be investigated under these conditions, as the rate of diffusion is considerably higher than the one of the surface processes. Although being fast, the diffusion processes still influences the overall transport in a subtle way. In contrast, in particles larger than 1 µm, the diffusion process becomes apparent. Two diffusion pathways for aromatic molecules in ZSM5 are present, leading to diffusion anisotropy in ZSM5 for sterically more demanding aromatic molecules. For any system composed of nanosized particles, we suggest that a complete description of the transport processes requires knowledge of the intrinsic transport properties, that is, elementary steps and rate constants, in order to understand the complex interplay of these transport mechanisms. Any interpretation of the results obtained from a macroscopic method for measuring intracrystalline diffusion without knowledge of these intrinsic properties will suffer from the limitation that the rates of the intracrystalline diffusion processes have to be considerably lower than the ones of the surface adsorption and pore entering. If this is not the case, the intracrystalline diffusion will be masked and only separable from the other kinetic processes if the intrinsic properties are known. Acknowledgment. The financial support from the Deutsche Forschungsgemeinschaft DFG under project JE260-7/1 and
Gobin et al. JE260-8/1 is acknowledged. Stephan J. Reitmeier acknowledges the “Studienstiftung des Deutschen Volkes” for the financial support by a Ph.D. scholarship. For the HR-SEM measurements, the authors thank Ferdi Schu¨th and Hans Bongard. For the possibility to perform DLS measurements the authors are grateful to Michael Paul from the Schu¨th group. The authors are also grateful to Martin Neukamm for conducting the SEM measurements and for the fruitful discussions in the framework of the network of excellence IDECAT and the Bavarian graduate school NanoCat (S.R.). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Davis, M. E. Nature 2002, 417, 813. (2) Reitmeier, S. J.; Mukti, R. R.; Jentys, A.; Lercher, J. A. J. Phys. Chem. C 2008, 112, 2538. (3) Jentys, A.; Tanaka, H.; Lercher, J. A. J. Phys. Chem. B 2005, 109, 2254. (4) Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meier, W. M. J. Phys. Chem. 1981, 85, 2238. (5) Kokotailo, G. T.; Lawton, S. L.; Olson, D. H.; Meier, W. M. Nature 1978, 272, 437. (6) Corma, A. Chem. ReV. 1995, 95, 559. (7) Gobin, O. C.; Reitmeier, S. J.; Jentys, A.; Lercher, J. A. Microporous Mesoporous Mater. 2009, 125, 3. (8) Yasuda, Y. Heterogen. Chem. ReV. 1994, 1, 103. (9) Kruk, M.; Jaroniec, M.; Choma, J. Carbon 1998, 36, 1447. (10) Gregg, S. J.; W., S. K. S. Adsorption Surface Area and Porosity, 2nd ed.; Academic Press Inc.: New York, 1982. (11) Zheng, S. R.; Heydenrych, H. R.; Jentys, A.; Lercher, J. A. J. Phys. Chem. B 2002, 106, 9552. (12) Hansen, N. The CMA evolution strategy: a comparing review. In Towards a new eVolutionary computation. AdVances on estimation of distribution algorithms; Lozano, J. A., Larranaga, P., Inza, I., Bengoetxea, E., Eds.; Springer: New York, 2006; p 75. (13) Sun, L. M.; Bourdin, V. Chem. Eng. Sci. 1993, 48, 3783. (14) Sun, L. M.; Meunier, F.; Karger, J. Chem. Eng. Sci. 1993, 48, 715. (15) Song, L. J.; Sun, Z. L.; Rees, L. V. C. Microporous Mesoporous Mater. 2002, 55, 31. (16) Song, L. J.; Rees, L. V. C. Microporous Mesoporous Mater. 2000, 35-6, 301. (17) Snurr, R. Q.; Bell, A. T.; Theodorou, D. N. J. Phys. Chem. 1993, 97, 13742. (18) Snurr, R. Q.; Bell, A. T.; Theodorou, D. N. J. Phys. Chem. 1994, 98, 11948. (19) Derouane, E. G.; Gabelica, Z. J. Catal. 1980, 65, 486. (20) Muller, G.; Narbeshuber, T.; Mirth, G.; Lercher, J. A. J Phys. Chem. 1994, 98, 7436. (21) Mirth, G.; Cejka, J.; Lercher, J. A. J. Catal. 1993, 139, 24. (22) Tzoulaki, D.; Heinke, L.; Lim, H.; Li, J.; Olson, D.; Caro, J.; Krishna, R.; Chmelik, C.; Karger, J. Angew. Chem., Int. Ed. 2009, 48, 3525. (23) Tzoulaki, D.; Heinke, L.; Schmidt, W.; Wilczok, U.; Karger, J. Angew. Chem., Int. Ed. 2008, 47, 3954. (24) Tzoulaki, D.; Schmidt, W.; Wilezok, U.; Karger, J. Microporous Mesoporous Mater. 2008, 110, 72. (25) Reitmeier, S. J.; Gobin, O. C.; Jentys, A.; Lercher, J. A. Angew. Chem., Int. Ed. 2009, 48, 533. (26) Yasuda, Y.; Sugasawa, G. J. Catal. 1984, 88, 530. (27) Jentys, A.; Mukti, R. R.; Lercher, J. A.; Ruren Xu, Z. G. J. C. a. W. Y. The Energetic and Entropic Contributions Controlling the Orientation of Alkyl Substituted Aromatic Molecules in the Pores of MFI Zeolites. In Studies in Surface Science and Catalysis; Elsevier, New York, 2007; Vol. 170, Part 1; p 926. (28) Ruthven, D. M. Adsorption. 2007, 13, 225. (29) Yasuda, Y. Bull. Chem. Soc. Jpn. 1991, 64, 954. (30) Sun, L. M.; Meunier, F.; Grenier, P.; Ruthven, D. M. Chem. Eng. Sci. 1994, 49, 373. (31) Jordi, R. G.; Do, D. D. Chem. Eng. Sci. 1993, 48, 1103. (32) Klemm, E.; Emig, G. Chem. Eng. Sci. 1997, 52, 4329.
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