One-step self-assembly of lamellar MWW crystals through intergrowth driven by centrifugal force to form hollownest structure Zeolite
From the considering of simple synthesis step, low cost, and energy saving, one-step synthesis of hierarchical zeolite without involving additional template has obvious advantages compared with post-synthetic modification or one-step templating methods, by which Ti-containing hollownest structure zeolite (Ti-HSZ) has been successfully synthesized. Herein, the origin of Ti-HSZʼs hollownest structure has been investigated by systematical comparison study between the rotating hydrothermal (RH) and static hydrothermal (SH) synthesis routes. It is found that different synthesis routes not only influence the crystallization but also determine the chemical and physical properties of zeolites. Compared with the catalyst prepared by flower-like precursor with larger size obtained by SH route in 12 days, Ti-HSZ catalyst with smaller size formed by RH route in 3.5 days shows higher catalytic activity for the epoxidation of bulky alkenes with H 2 O 2 . On one hand, the rapid and efficient transports of various reaction species in RH environment has accelerated the crystallization rate greatly; on the other hand, the one-step self-assembly of lamellar MWW crystals has happened to facilitate the formation of hollownest structure with hierarchical pores through intergrowth driven by centrifugal force. The different evolution processes of the Si chemical environments during the early stages of crystallization reflected by the percentage of intrinsic defects origin from the different hydrothermal environments. Highly mixed and dispersed reaction species under RH condition accelerate the uniform of gel concentration, and the condensation of smaller clusters with different orientation direction through frequent contacting under dynamic condition is easy to happen to form bigger clusters, which is the basis for the further crystal intergrowth. In contrast, the relatively inhomogeneous gel environments under SH condition has led to the increase of intrinsic defects in the solid phases, and bigger clusters tend to be formed by the condensation of smaller ones with the same orientation direction. This work will be instructive not only for the understanding of zeolites’ crystallization mechanism, but also for the design and synthesis of zeolite materials with unique morphology characteristics.
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