Controlling transport and sorption rates of binary mixtures of alkanes and aromatics by surface modification of zeolites
Final Report Abstract
Within this project the control of the transport processes by rational tailoring of the pore sizes and the external surface of zeolites by synthesis or post-synthetic modification was investigated. The molecular sieving properties of zeolites allow to separate aromatic and aliphatic molecules via their minimum kinetic diameters. It was shown that in hierarchical zeolites, created by the generation of mesopores, the effective pore length inside the microporous domains is reduced, while the acid/base properties of the microporous domains of the zeolites are not affected. This allows to optimize the selectivity of the materials via the transport properties without affecting their reactivity. The transport processes of aromatic and aliphatic molecules as well as of binary mixtures between them were studied by time resolved IR spectroscopy. The characteristic differences in the molecular extinction coefficients for the C-H stretching and bending vibrations of alkanes on the outer surface and inside the pores of the zeolite allowed to follow the transport network from a pre-adsorbed state on the external surface via a (rate determining) reorientation and pore entrance step followed by the diffusion to the sites inside the pores. The preferred sorption of alkanes over aromatic molecules led to a blockage of the sorption sites close to the pore entrances, which significantly slowed down the transport of the aromatic molecules in binary mixtures between alkanes and aromatics. The effect could not be compensated by a modification of the outer surface, which only improved the situation for the smallest alkane molecules. The existence of the weakly adsorbed state on the surface and the subsequent rate determining pore entrance step elucidates why the modification of the outer surface zeolite of particles by deposition of an amorphous layer can enhance the overall transport. Based on this description it becomes evident that this concept can only be applied to nano-sized zeolite crystals, where the pore entrance is the rate determining step. In lager crystals the diffusion inside the pores is the slowest step, and therefore, a modification of the surface will not have any positive effect on the overall transport rate.
Publications
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A comparative study of diffusion of benzene/p-xylene mixtures in MFI particles, pellets and grown membranes, Catal. Today 168, 147-157 (2011)
R. Kolvenbach, N. Al-Yassir, S. S. Al-Khattaf, O. C. Gobin, J. H. Ahn, A. Jentys and J. A. Lercher
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On the location, strength and accessibility of Brønsted acid sites in hierarchical ZSM-5 Particles, Catal. Today 198 (1), 3-11 (2012)
D. Tzoulaki, A. Jentys, J. Pérez-Ramírez, K. Egeblad, J. A. Lercher
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Enhancing shape selectivity without loss of activity – novel mesostructured ZSM5 catalysts for methylation of toluene to p-xylene, Chem. Commun. 49, 10584-10586 (2013)
J. H. Ahn, R. Kolvenbach, S. S. Al- Khattaf, A. Jentys and J. A. Lercher
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Methanol usage in toluene methylation with medium and large pore zeolites, ACS Catalysis 3, 817-825 (2013)
J. H. Ahn, R. Kolvenbach, S. S. Al-Khattaf, A. Jentys and J. A. Lercher
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Molecular understanding of sorption in mesoscale organized zeolites with MFI structure, Catal. Lett. 143 (11), 1116-1122 (2013)
R. Kolvenbach, L. F. Gonzalez-Peňa, A. Jentys and J. A. Lercher
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Diffusion of mixtures of light alkanes and benzene in nano-sized H-ZSM5, J. Phys. Chem. C. 118 (16), 8424–8434 (2014)
R. Kolvenbach, L. F. Gonzalez Peña, A. Jentys and J. A. Lercher
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Tailoring mesoscopically structured H-ZSM5 zeolites for toluene methylation, J. Catal. 311, 271-280 (2014)
J. H. Ahn, R. Kolvenbach, C. Neudeck, S. S. Al-Khattaf, A. Jentys and J. A. Lercher
