(600bz) Mechanistic Insights into Rate-Dominant Transport Barriers in MFI Structured Zeolites

Zeolites are used to catalyze chemical reactions and improve selectivity to desired products through well ordered, shape selective microporous channels. MFI structured ZSM-5 has widely been used for catalytic fast pyrolysis of biomass and subsequent upgrading.¹ To maximize turnover frequencies, microporous and mesoporous materials are now being synthesized with length scales approaching that of a single lattice (pillared, nanosheets, membranes, etc.). However, in small particles, mass transfer becomes dominant and the apparent diffusivity appears to be orders of magnitude smaller than through large particles.^{2, 3} This has led to the general phenomenon referred to as 'surface barriers,' which describes the hindrance to mass transfer at the surface of zeolite particles. While this timescale is often negligible when compared to the diffusion or reaction times, we show that it does contribute significantly to overall turnover in small particles.² Chemical species exhibiting differing kinetic diameters and adsorption affinities were used to probe diffusion within MFI-structured zeolites to elucidate the role of guest molecule size and binding energy. Kinetic Monte Carlo simulations are used to mechanistically probe the effect of physical structures on overall transport timescales, including internal grain boundaries, regular surface restrictions and total pore blockages. The relative trends with particle size are compared to experimental measurements and potential mechanisms are proposed.

1.         Williams, C. L.; Chang, C.-C.; Do, P.; Nikbin, N.; Caratzoulas, S.; Vlachos, D. G.; Lobo, R. F.; Fan, W.; Dauenhauer, P. J., Cycloaddition of Biomass-Derived Furans for Catalytic Production of Renewable p-Xylene. ACS Catalysis 2012, 2, (6), 935-939.

2.         Teixeira, A. R.; Chang, C.-C.; Coogan, T.; Kendall, R.; Fan, W.; Dauenhauer, P. J., Dominance of Surface Barriers in Molecular Transport through Silicalite-1. The Journal of Physical Chemistry C 2013, 117, (48), 25545-25555.

3.         Chang, C.-C.; Teixeira, A. R.; Li, C.; Dauenhauer, P. J.; Fan, W., Enhanced Molecular Transport in Hierarchical Silicalite-1. Langmuir 2013, 29, (45), 13943-13950.