Dealuminated H−Y Zeolites:  Influence of the Degree and the Type of Dealumination Method on the Structural and Acidic Characteristics of H−Y Zeolites

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.

Hierarchically Nanoporous Zeolites and Their Heterogeneous Catalysis: Current Status and Future Perspectives

“…In order to retain the crystal structure of dealuminated zeolite crystal, the post dealumination can also be carried out in aqueous solutions containing ammonium hexafluorosilicate [20,21] and SiCl 4 [22][23][24]. This can heal the dealuminated framework sites with silicate species during the dealumination treatment.…”

“…This can heal the dealuminated framework sites with silicate species during the dealumination treatment. The resultant pore textural properties and Al distribution depend very much on the types of dealumination procedure [21]. The dealumination process involves an inevitable change of acid property, which is generally difficult to control intentionally.…”

Influence of steam-calcination and acid leaching treatment on the VGO hydrocracking performance of faujasite zeolite

“…At higher steaming temperature the amount of polymerized EFAl species increases. The difficulty in removing EFAl species can therefore be related to their higher degree of agglomeration [18].…”

“…The removal of framework Al atoms by hydrothermal treatment generates extraframework aluminum species (EFAl). The amount and nature of the EFAl species formed depend on the severity of the hydrothermal treatment step [18]. The presence of large amounts of EFAl has a negative effect on the catalytic and transport properties.…”

Formation of Mesopores in USY Zeolites: A Case Revisited

“…Much work has been done to characterize mesopores in ultra-stable Y zeolites (USY) [1][2][3] since the first reports on their dealumination by steam treatment which have been published almost four decades ago [4,5]. However, debates over the origin of these mesopores are still continuing.…”