(158g) Exploiting Mesoporosity for the Design of Novel Materials
Zeolite crystallization occurs in the presence of inorganic and/or organic structure-directing agents (SDAs) that facilitate the formation of microporous crystals with various pore geometries. One of the challenges with ZSM-5 synthesis is to identify new and inexpensive organic SDAs that can tailor the physicochemical properties of the final product. The most frequently used SDA in ZSM-5 synthesis is tetrapropylammonium (TPA); however, recent studies have shown that the surfactant cetyltrimethylammonium (CTA) can be used as an alternative SDA and mesoporogen. Here, we will present the effects of using CTA and TPA as organic SDAs in combination with a variety of alkali metals as inorganic SDAs. Our findings reveal that the selection of SDA combinations has a significant impact on the kinetics of ZSM-5 crystallization, as well as the properties of the resulting crystals. Notably, we show that TPA/Na and CTA/K are optimal combinations of SDAs that can markedly alter the size, morphology, and aluminum distribution in ZSM-5. Using a combination of experiments and molecular modeling, we explore the use of CTA as an alternative organic SDA for zeolite MFI and show that we can achieve smaller crystals (ca. 600 nm) in similar time (< 24 h) as syntheses employing TPA. The ability of dual SDAs to alter the physicochemical properties of zeolite crystallization is a potentially useful approach to rational design. Moreover, CTA and other amphiphilic molecules can be used in surfactant-templating to generate hierarchical zeolites with a range of pore sizes that enhance the performance of zeolites in commercial processes. Garcia-Martinez and coworkers have pioneered a unique process wherein CTA can be used post-synthesis as a âmesoporogenâ of microporous structure, leading to the generation of uniformly-sized mesopores in USY (FAU) and other zeolites. Here, we will present our collaborative work with Prof. Garcia-Martinez that seeks to elucidate the mechanism of surfactant-templating at a molecular level.