Sorption and Diffusion Properties of H- and Modified Forms of ZSM-5 Zeolites

摘要

The sorption properties of samples of H-ZSM-5 zeolite and their forms modified with P, B and Mg were determined by the physical adsorption of nitrogen and evaluated by BET isotherm and t-plot methods. The diffusivities of the samples were measured using a new flow gravimetric method involving the sorption of/;- and u-xylenes. The BET specific surface area decreased from 358 m2/g to 35-90 m2/g mainly because of a decrease in micropore volume as determined by the t-plot method. The rate of sorption of/>xylene on all the samples investigated followed Fick's second law of diffusion, leading to an effective diffusivity of ca. 1.7 X 10"" m2/s, irrespective of whether the H-form or the modified forms were investigated. The sorption isotherms for o-xylene showed some unusual steps. Values for the effective diffusivities decreased from 2.6 X 1O12 m2/s for the H-form to 2.0-2.3 X 10~12 m2/s for the modified forms. The equilibrium concentration of o-xylene at P/P° = 0.1 was only 0.38-0.68 mol/unit cell, while that of />xylene was 4.40-5.98 mol/unit cell.