Diffusivity in MFI zeolites

Positron Emission Profiling a Study of Hydrocarbon Diffusivity in MFI Zeolites 279... 

Lin, D., Ducarme, V., and Vedrine, J.C. Adsorption and diffusion of different hydrocarbon in MFI zeolite of varying crystallite sizes. in Zeolites as Catalysts, Sorbents and Detergent Builders (H.G. Karge and J. Weitkamp, Eds.), Elsevier, Amsterdam, p. 615, 1989. 

Studies of adsorption, diffusion and molecular simulation of cyclic hydrocarbons in MFI zeolites... 

Figure 13 displays the self-diffusivities of n-hexane and 2-methylpentane in silicalite-1 and H-ZSM-5 as a function of the ratio of the hydrocarbons. The self-diffusivities of both hexanes linearly decrease with increasing gas-phase fraction of the branched hexane in the gas phase for the non-acidic and acidic zeolite. In H-ZSM-5, the mobility of alkanes is approximately two times slower than in silicalite-1. Obviously, the presence of acid sites strongly affects the molecular transport due to stronger interactions with the n-hexane molecules. A similar effect of Bronsted sites on the single component diffusion of aromatics was observed in MFI zeolites with different concentration of acid sites [63-65]. The frequency response (FR) technique provided similar results... 

Figure 5. Arrhenius plots of diffusivities of aromatics in MFI type zeolite crystal. (Reproduced with permission from reference 20. Copyright 1991 Elsevier.)...

Diffusion of alkanes in zeolites is widely studied. The experimentally obtained difliisivities, however, vary by orders of magnitude. In many cases noble gasses are used and it is generally believed that they neither influence the adsorption rate nor the adsorption equilibrium. We studied the influence of argon and helium on the adsorption rate and equilibrium of n- and iso-butane in the zeolites BEA, MFI and FER at 383 K and found a pronounced effect on the adsorption equilibrium as well as on the adsorption rate. 

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