Influence of the desilication process on the acidity of HZSM-5 zeolite

The influence of the alkaline treatment on the acidity of zeolite ZSM-5 (SiO2/Al2O3 = 23, 50 and 80) modified by desilication was investigated. The samples were characterized by X-ray diffraction, low temperature adsorption of nitrogen and solid-state 27Al MAS NMR. Acidity of the samples was investigated using microcalorimetry-volumetry of ammonia adsorption at 423 K. Acid sites accessibility and intracrystalline diffusion were estimated by the thermokinetic parameter. The desilication process was shown to be Si/Al ratio dependant. Relative enhancement of mesopore surface decreased in the following order of SiO2/Al2O3: 50 > 8 0 >23. Pore size distribution broadened and mesopore sizes increased with increasing Si/Al ratio. For samples with SiO2/Al2O3 = 50 and 80 almost full preservation of acid sites strength and distribution was achieved. Significant alteration of the acidity was found for the mesoporous sample with SiO2/Al2O3 = 23. Thermokinetic parameter values indicate facilitated intracristalline diffusion for mesoporous samples.