(The) effects of structural characteristics of Sn-BEA zeolites on catalytic activities for glucose Isomerization

Abstract

Sn-BEA zeolite, a crystalline BEA-type stannosilicate, is one of the most important zeolite-based Lewis acid catalysts. Sn-BEA zeolite having Lewis acidic Sn (IV) centers within the hydrophobic zeolite framework that consists of mainly nonpolar Si–O–Si bonds is usually hydrothermally synthesized in a fluoride (F-) medium. Such hydrophobic Sn-BEA zeolites show high catalytic activities in various liquid-phase Lewis acid catalyzed reactions such as Baeyer-Villiger oxidation, Meerwein-Ponndorf-Verley reduction and Oppenauer oxidation, and glucose isomerization. However, conventional hydrothermal synthesis method for Sn-BEA zeolite requires considerable amount of hazardous chemicals such as F- and long zeolite crystallization time. In this respect, post-synthetic Sn-BEA zeolites via various post-synthetic methods have been intensively investigated. This study investigated the effects of structural properties of Sn-BEA zeolites, such as crystallite sizes, mesoporosity, and defect sites on catalytic activities for glucose isomerization both in water and 1-butanol over a series of Sn-BEA zeolites including one directly crystallized in a F- medium and the others prepared by the post-synthetic method developed in this study. The directly crystallized Sn-BEA showed high fructose yield (34%) in water because of its defect-free hydrophobic nature, which suppressed the inhibition of Lewis acid sites by water. However, in 1-butanol, it showed the lowest fructose yield and fastest deactivation among the catalysts, because of its extra-large zeolite crystallites (ca. 15 µm) causing mass transfer limitation and undesired side reactions. The Sn-BEA catalysts prepared by post-synthetic Sn incorporation showed limited catalytic performance in water because of hydroxyl defects. However, they showed superior performances in 1-butanol because of the much smaller crystallites and enhanced mass transfer. In particular, the hierarchical Sn-BEA having 10–20 nm crystallites and significant intercrystalline mesoporosity showed a very large fructose yield (55%) that are difficult to achieve in typical aqueous-phase glucose isomerization (<35%). The present work would provide important insight into the design of zeolite-based Lewis acid catalysts for biomass conversion.