(683d) Surface Barriers in Diffusion Through MFI Structured Microporous Materials

Understanding diffusion in both microporous and mesoporous materials is critical to the development of 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 self-diffusion of single components within zeolite crystals [1,2]. Moreover, diffusion through one and three dimensional pore networks has been assumed to be independent of particle size, despite the crystals having inherent structural differences at the surface. Using zero length chromatography (ZLC), we have characterized the diffusivity of cyclohexane through various sizes of silicalite-1 to show that diffusivity (D) can vary by as much as three orders of magnitude depending on particle size. Furthermore, this work reveals insight into the mechanistic transport by exploring the intracrystalline diffusional pathways and the effect of surface barriers. For all cases, activation energy is observed to be 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 with identical activation energy. 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.