(116b) Mechanistic Understanding of Surface Barriers in Transport through MFI Structured Catalysts
Mechanistic understanding of surface barriers in transport through MFI structured catalysts
Zeolites are widely used in the conversion and upgrading during traditional petrochemical processes as well as biomass to renewable fuels and chemical pathways.1 . The combined high surface areas, shape selectivity, and catalytic functionality of the zeolites have made them ideal for the high temperature thermal pathways. 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.) in an attempt to reduce transport time. This driving force has led to the development of hierarchical materials that have been the focus of recent synthesis advances. Traditionally, bulk crystals have been synthesized on the order of one to one hundred microns, however mesoporous materials have now been developed with the specific aim of decreasing microporous transport lengthscales.
Diffusion in MFI has presented significant challenges. Zeolite particles with the MFI structure are known to exhibit significant structural complications including intergrowths, subcrystals, and surface defects. A combination of the diffusion anisotropy and structural effects has led to a complex transport scenario, as exhibited in the apparent diffusivities which have been experimentally observed to vary over three orders of magnitude. 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 Experimental evidence is presented with two independent systems that measure both the desorption profiles (Zero Length Chromatography), and sinusoidal steady state of the uptake/desorption system (Frequency Response). 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 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.