Effect of the degree and type of the dealumination method on the structural, compositional and acidic characteristics of H-ZSM-5 zeolites

A series of H-ZSM-5 zeolites with different framework Si/Al ratios were pre pared by hydrothermal synthesis and post-synthesis dealumination by various methods, i.e., HCl, steaming, steaming/HCl and ammonium hexafluorosilicate (AHFS). The degree of framework dealumination and the amount/type of the e xtra-framework phases formed were greatly dependent on the method of dealum ination and the severity of the experimental conditions. The hydrothermal t reatment of a parent ZSM-5 sample with Si/Al similar to 27 was very effecti ve and produced significant amounts of Si-Al extra-framework phases, while the reaction with HCl in relatively strongly acidic environments resulted i n very low dealumination. Treatment with AHFS was proven to be an appropria te method for preparing moderately dealuminated H-ZSM-5 samples which are f ree from extra-framework Al. The relative crystallinity and the microporosi ty of all dealuminated H-ZSM-5 zeolites were retained to a high degree (> 8 0%), even for the severely steamed samples (similar to 96% dealumination). The morphology of the crystals/particles was not changed significantly; how ever, each dealumination method had a different effect on the partial break down of the crystals and on the formation of extra-framework/amorphous phas es with a mesoporous character. The number of acid sites that corresponded to the high-temperature desorption peak of the ammomia-temperature programm ed desorption spectra was found to be equivalent to the framework Al (FA1) content of all H-ZSM-5 samples, irrespective of the degree of dcalumination and the amount of extra-framework phases. Theses phases had a low capabili ty of adsorbing ammonia compared to the FA1 atoms and were the main source of acidity for the severely steamed samples which had almost no FA1 (D 2001 Elsevier Science B.V. All rights reserved.