Dealuminated H-Y zeolites: Influence of the degree and the type of dealumination method on the structural and acidic characteristics of H-Y zeolites

Dealuminated zeolite samples prepared by different methods are characterize d by different techniques, and the structural and acidic characteristics ar e investigated in an effort to identify the parameters that decide their st ructural and acidic characteristics. The investigation gave evidence that t he structure breakdown of the parent type-Y zeolite is mainly dependent on the degree of dealumination, but the nature of extraframework aluminum (EFA l) species formed is dependent on the type of dealumination procedure. Mono meric Al species and oligomeric Si,Al species were formed by steam treatmen t at relatively low and high temperatures, respectively. Aluminosilicate ph ases of Si/Al ratios 2.0-4.0 and 8.0 were found in SiCl4-treated and ammoni um hexafluorosilicate (AHFS)-treated samples, respectively. EFAl species of octahedral and tetrahedral coordination were observed by NMR spectra in st eam and SiCl4-treated samples. High degrees of dealumination within each ty pe of treatment resulted in loss of microporosity of the crystalline materi al, but the increase of mesoporous and/or macroporous structure was depende nt on the type of dealumination. The increase of macroporous structure was the result of crystallite agglomerates formed during the procedure. The num ber of acid sites determined by the ammonia temperature-programmed desorpti on method, under appropriate experimental conditions, matched with the FAl content of the dealuminated H-Y samples that do not contain a significant a mount of EFAl species. The loss of strong acid sites with the degree of dea lumination is evidenced to be dependent on the method of dealumination. Dea lumination by AHFS up to ca. 50% does not affect the number of strong acid sites considerably; in contrast to the weak/medium acid sites which decreas e almost linearly with a FAl decrease. At higher degrees of dealumination a chieved by steaming or SiCl4-treatment, both weak/medium and strong acid si tes decrease steeply with FAl decrease. There is evidence that the produced EFAl-species by the different methods have different acidic properties and result in low stoichiometries of adsorbing ammonia probe molecules. The EF Al species produced by the SiCl4 method result in more acidic H-Y zeolite c atalysts compared to the high-temperature steamed samples.