Surface Barrier Transport Limitations in MFI-Structured Zeolites

Zeolites are widely used to catalyze chemical reactions and improve selectivity to desired products through ordered, shape selective microporous channels [1]. As microporous materials are synthesized with length scales approaching that of a single lattice (pillared, nanosheets, membranes, etc.), the need to understand mass transfer limitations within small particles becomes paramount. Experimental evidence accumulated over the past several decades 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. 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] C.Luke Williams, Chun-Chih Chang, Phuong Do, Nima Nikbin, Stavros Caratzoulas, Dionisios G. Vlachos, Raul F. Lobo, Wei Fan, Paul J. Dauenhauer, "Cycloaddition of Biomass-Derived Furans for Catalytic Production of p-Xylene," ACS Catalysis 2012, 2(6), 935-939.