Molecular dynamics study on the diffusion of organosulfur compounds in porous solids

Desulfurization of organosulfur compounds in petrochemical fuels is mainly limited by diffusion in porous solids. Design and optimization of such heterogeneous process need basic knowledge of molecular diffusion inside porous solids. In this work, molecular dynamics method was applied to study the intrinsic pore diffusion and surface diffusion of some common organosulfur compounds of thiophene, benzothiophene and dibenzothiophene in silica pores. Based on the molecular dynamics data, the effects of temperature and pore size on the intrinsic pore diffusion and surface diffusion were investigated by an established mathematic model considering the molecular size effect on diffusion. It was found that the widely used Knudsen diffusion model might overestimate the pore diffusion as much as a few dozens folds, but could be improved by incorporating molecular interaction and molecular size effect into diffusion model. The proportion of mass transfer by surface diffusion in total mass transfer in different pores was estimated. The results exhibited herein convey some basic instruction not only for design pores of porous solids used in hydrodesulfurization or in adsorption desulfurization, but also for optimization of operation conditions for desulfurization processes.