Mechanisms of competitive adsorption and diffusion of ethyl sulfide and n-butyl mercaptan with cyclohexene in FAU: MC and MD

Abstract

An in-depth understanding of the competition mechanism between olefins and different types of sulfides in gasoline is essential to improve the desulfurization selectivity of the adsorption desulfurization process (ADS). In this study, the competitive adsorption and diffusion mechanism of two systems, diethyl sulfide/cyclohexene and n-butyl mercaptan/cyclohexene, with different adsorption amounts in siliceous faujasite zeolite (FAU) were investigated by Monte Carlo (MC) and molecular dynamics (MD). The systems exhibited a two-stage loading-dependent competitive adsorption and diffusion mechanism, with an inflection point of 32 molecule/UC (moleculers per microcoulomb). Before the inflection point (4–32 molecule/UC), the competition mechanism of the two systems was the “optimal-displacement” mechanism. After the inflection point, the mechanism of the diethyl sulfide/cyclohexene changed to “relocation-displacement”, while that of the n-butyl mercaptan/cyclohexene system changed to “dominant-displacement”. Compared to ether functional groups, the alcohol functional group has higher polarity and stronger adsorption stability, thus occupying more favorable adsorption sites within the supercages (SCs), while ethyl sulfide shifts outward to other sites within other SCs. In addition, the diffusion performance of adsorbent is related to the adsorption energy. The lower the adsorption energy, the weaker the diffusion ability. Meanwhile, the diffusion performance of adsorbates is better at high temperatures and low adsorption capacity. The effect of temperature on the desulfurization selectivity was determined. A lower temperature is favorable for the adsorption capacity of the two systems and the removal selectivity of sulfides. This study provides fundamental insights into the competitive adsorption and diffusion mechanisms among sulfides, mercaptans and olefins, offering theoretical guidance for adsorbent design and reaction temperature optimization.