(600bi) Surface Barriers: Understanding Diffusion Limitations in MFI Structured Zeolites

Diffusion of molecules in both microporous and mesoporous materials is a critical process needed to develop the next generation of catalysts for traditional hydrocarbon processing as well as biomass pyrolysis and upgrading. Despite decades of research in zeolite diffusion, there exists considerable disagreement between publications for diffusion of single components within zeolite crystals [1, 2]. Diffusion through one and three dimensional pore networks has historically been assumed to be independent of particle size, despite the crystals having inherent structural differences at the surface. We have used zero length chromatography (ZLC) to characterize the diffusivity of cyclohexane through various sizes of silicalite-1, examining the role of particle size on intraparticle diffusion. Specifically, we have shown that diffusivity (D) can vary by as many as three orders of magnitude depending primarily on crystal size. Moreover, this work offers insight into the mechanistic transport by exploring the intracrystalline diffusional pathways and the effect of surface barriers. For all crystals, activation energy is observed to remain constant, independent of particle size. Variations in diffusivity are examined by modeling residence time as an additive timescale comprised of both bulk diffusion and diffusion across a surface barrier. These experiments provide evidence suggesting a surface barrier effect which hinders overall intrapatricle diffusion in crystals up to one micron in diameter.

1.  Ruthven, D., Diffusion in zeolites—a continuing saga. Adsorption, 2010. 16(6): p. 511-514.

2.  Kärger, J., Measurement of Diffusion in Zeolites—A Never Ending Challenge? Adsorption, 2003. 9(1): p. 29-35.