Adsorptive Removal of Dichloromethane Vapor on FAU and MFI Zeolites: Si/Al Ratio Effect and Mechanism

Abstract

MFI zeolites (ZSM-5) with different Si/Al ratios were synthesized to assess their adsorptive removal of dichloromethane vapor, compared with FAU zeolites (NaX and NaY). All adsorbents were characterized by means of N2 adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The adsorption and desorption performance, water vapor tolerance, and regeneration ability were fully examined. FAU zeolites exhibited better equilibrium capacity than MFI zeolites, while MFI zeolites had superior dynamic capacity. The kinetic uptake of FAU zeolites was significantly depressed by low humidity levels in the mixture. Regardless of the type of zeolite, high Si content was not only beneficial to equilibrium adsorption but also helpful for water vapor resistance. ZSM-5 (200) (SiO2/Al2O3 = 200) with the highest Si/Al ratio was the best candidate in this study for removal of dichloromethane. In addition, it could be reused without a significant decrease in uptake after several regeneration cycles. With the aid of in situ diffuse reflectance Fourier transform infrared spectroscopy and density functional theory simulation, adsorbed dichloromethane was found to be in intimate contact with the pore window of FAU zeolites but was surrounded by the pore walls of MFI zeolites. Experimental and theoretical results were consistent with each other and indicated that MFI zeolites show better adsorptive selectivity of dichloromethane (DCM) over water than FAU.