Hydrothermal synthesis of metallosilicate SSZ-24 from metallosilicate beta as precursors

Abstract

Aluminosilicate version of SSZ-24 ([Al]-SSZ-24) zeolite ( AFI topology) was synthesized by a new route including phase-transformation, wherein aluminosilicate analog of beta zeolite ([Al]- BEA) was used as a precursor and N(16)-methylsparteinium hydroxide (MeSPA +OH −) was used as structure-directing agent (SDA). [Al]- BEA precursors synthesized by various routes, mainly hydrothermal synthesis (HTS) and steam-assisted crystallization (SAC) methods, were employed for the synthesis. In general, [Al]-SSZ-24 samples with the SiO 2/Al 2O 3 ratios comparable to the input SiO 2/Al 2O 3 ratio were readily synthesized by using the precursor route when the SiO 2/Al 2O 3 was as high as 200. The limit of aluminum introduction depended on the origin of BEA precursor. After optimization, pure [Al]-SSZ-24 with SiO 2/Al 2O 3 = 109 was successfully obtained. During crystallization, a phase transformation from [Al]- BEA to [Al]-SSZ-24 was observed, suggesting dissolution of beta phase to a minimum structural unit followed by its redirection towards SSZ-24 phase in the presence of the SDA. Field emission scanning electron microscopy (FE-SEM) showed hexagonal rod-shaped crystals of about 1 μm in size. NH 3-TPD measurement showed the presence of enough amounts of acidic sites as compared to the expected amount based on the elemental analysis. The catalytic performance of [Al]-SSZ-24 synthesized from [Al]- BEA was compared with that of the [Al]-SSZ-24 synthesized via [B]-SSZ-24 by isomorphous substitution of Al for B. The [Al]-SSZ-24 synthesized by the former method from [Al]- BEA showed comparable or better shape-selectivity than that synthesized by the latter method in the isopropylation of biphenyl. The applicability of this “ BEA precursor method” to other metallosilicates (e.g. titanosilicate) was also demonstrated.