Sorption and diffusion properties of H- and modified forms of ZSM-5 zeolites

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 p- and o-xylenes. The BET specific surface area decreased from 358 m(2)/g to 35-90 m(2)/g mainly because of a decrease in micropore volume as determined by the t-plot method. The rate of sorption of p-xylene on all the samples investigated followed Fick's second law of diffusion, leading to an effective diffusivity of ca. 1.7 x 10(-11) m(2)/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 10(-12) m(2)/s for the H-form to 2.0-2.3 x 10(-12) m(2)/s for the modified forms. The equilibrium concentration of o-xylene at P/P-0 = 0.1 was only 0.38-0.68 mol/unit cell, while that of p-xylene was 4.40-5.98 mol/unit cell.