LOW PRESSURE HYSTERESIS IN THE NITROGEN ADSORPTION ISOTHERM OF ZSM-5: EFFECT OF ACID TREATMENT

摘要

Zeolites have long been of interest~(1-3) in academic but also applied and industrial research, because of their significant adsorptive, catalytic, and ion-exchange properties. In particular, ZSM-5 has been of interest to our group~(4,5) because of the unusual low-pressure hysteresis present in the nitrogen adsorption isotherm for this solid, at certain Si:Al ratios but not others. These solids have the general formula Na_n[Si_(96n)Al_2O_(192)].16H_2O (n<8)~(6,7) and can be synthesized with a wide range of possible Si:Al ratios. This ratio has a profound effect on the surface solid state properties of the solid, affecting among others, the number and strength of surface acid sites, and the charge of the framework and thus the number of exchangeable cations present in the channels. With increase in the Si:Al ratio, the thermal stability of the solid is reduced, and the nature of the surface changes from hydrophilic to hydrophobic. The catalytic behaviour of the solid is therefore influenced~8, and can at some extent be fine-tuned by adjusting the Si:Al ratio.