No abstract

Dealuminated zeolite samples prepared by different methods are characterized by different techniques, and the structural and acidic characteristics are investigated in an effort to identify the parameters that decide their structural and acidic characteristics. The investigation gave evidence that the structure breakdown of the parent type-Y zeolite is mainly dependent on the degree of dealumination, but the nature of extraframework aluminum (EFAl) species formed is dependent on the type of dealumination procedure. Monomeric Al species and oligomeric Si,Al species were formed by steam treatment at relatively low and high temperatures, respectively. Aluminosilicate phases of Si/Al ratios 2.0−4.0 and 8.0 were found in SiCl4-treated and ammonium hexafluorosilicate (AHFS)-treated samples, respectively. EFAl species of octahedral and tetrahedral coordination were observed by NMR spectra in steam and SiCl4-treated samples. High degrees of dealumination within each type of treatment resulted in loss of microporosity of the crystalline material, but the increase of mesoporous and/or macroporous structure was dependent on the type of dealumination. The increase of macroporous structure was the result of crystallite agglomerates formed during the procedure. The number of acid sites determined by the ammonia temperature-programmed desorption method, under appropriate experimental conditions, matched with the FAl content of the dealuminated H−Y samples that do not contain a significant amount of EFAl species. The loss of strong acid sites with the degree of dealumination is evidenced to be dependent on the method of dealumination. Dealumination 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 decrease almost linearly with a FAl decrease. At higher degrees of dealumination achieved by steaming or SiCl4-treatment, both weak/medium and strong acid sites 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 EFAl species produced by the SiCl4 method result in more acidic H−Y zeolite catalysts compared to the high-temperature steamed samples.

No abstract

No abstract

No abstract