Textural Characterization of Mesoporous Zeolites

Abstract

Zeolites and related zeolite-analog materials have found major industrial applications as catalysts and adsorbents, especially in refinery and petrochemical processes where zeolite catalysts exhibit unique shape selectivity endowed by their crystalline yet highly porous structures with well-defined channels of molecular dimensions [1–5]. However, as one side effect of their microporous structure, zeolite catalysts often suffer from restricted diffusion of guest species [6, 7]. Mass transport to and from the active sites inside the micropores (known as configurational diffusion) is much slower than that of molecular and Knudsen diffusion, which has led to lower catalyst utilization, and sometimes fast deactivation due to coke formation. Different approaches have been proposed to alleviate the diffusion limitation and enhance the accessibility of the internal sites [8–21]. One strategy is to synthesize novel zeolitic structures with a larger pore size. Various large-pore zeolites and zeolite analogs have been obtained, for example, VPI-5 [22], UTD-1 [23], SSZ-53 and SSZ-59 [24], ITQ-15 [25], ITQ-21 [26], ITQ-33 [27], and ITQ-37 [28]. However, despite the fact that a considerable amount of knowledge has been gained on the formation mechanisms of zeolites and numerous types of theoretical structures have been proposed for predicative synthesis, synthesis by design of novel zeolite structure is still a challenging topic, and most new structures are discovered by trial-and-error processes.