Study of diffusion and counter-diffusion of para- and ortho-xylene in H-SSZ-24 and H-ZSM-11 zeolites

Abstract

Fourier transform infra-red spectrometry was applied to the study of the sorption kinetics of para- and ortho-xylene in single component diffusion and counter-diffusion experiments in H-SSZ-24 and H-ZSM-11. A solution of the Fick's second equation was applied for the evaluation of the kinetic data and the diffusion and counter-diffusion coefficients for p- and o-xylene in the considered zeolites were determined. In addition, single file diffusion (SFD) was considered as a possible significant process in the interpretation of diffusion and counter-diffusion in zeolites with structures consisting of one-dimensional channel networks. It was also deduced a SFD partial differential equation for the description of this type of transport. The Fickean diffusion coefficients of p- and o-xylene in single component diffusion, and the effective diffusivity for counter-diffusion were measured and the results discussed on the basis of the diffusion selectivity induced by the size and the geometry of the pores of the framework structures of SSZ-24 and ZSM-11. For H-SSZ-24 was shown that p-xylene and o-xylene relatively freely move in its framework. It was also verified that the jump-diffusion mechanism was not accomplished during single component diffusion of p- and o-xylene in H-SSZ-24. Besides, was concluded that ordinary diffusion is the dominant transport process for single component and counter-diffusion experiments. For H-ZSM-11, during single component diffusion of p- and o-xylene the transport process was described by ordinary diffusion and the jump-diffusion mechanism is satisfied. Besides, the diffusion of o-xylene is evidently hindered by geometric factors. Likewise, was measured the effective diffusivity for the counter-diffusion of p+ o-xylene and was established that ordinary diffusion is the dominant transport process. The SFD mechanism was not obeyed during diffusion of p- and o-xylene and counter-diffusion of p+ o-xylene mixtures in H-SSZ-24, notwithstanding the fact that the framework of this zeolite is composed of a one-dimensional channel network.