Advances in Hydroxyl Free Radical Assisted Synthesis of Zeolite

Abstract

As one of the most important solid catalysts in chemical industry, zeolites are widely used in different fields. How to prepare zeolites greenly and efficiently has become a research hot issue with the increase of consumption year by year. Hydroxyl free radical is a highly active species, which can promote the depolymerization and repolymerization of aluminosilicates in the zeolite initial gel, and then accelerating the zeolite crystallization process. Besides accelerating the crystallization of zeolite, many studies have shown hydroxyl free radicals have other functions in zeolite synthesis. In this review, the role of hydroxyl free radicals in the synthesis of zeolites is systematically introduced via different generation methods. The challenges of the hydroxyl free radical assisted strategy are put forward, and its development directions are prospected. Keywords zeolite; hydroxyl free radical; structure directing agent; synthesis; acceleration 1 引言 传统沸石是一类由硅氧四面体和铝氧四面体组成 的结晶微孔材料 [1] , 随着沸石工业和制备技术的不断发 展, 沸石分子筛的化学组成也已经扩展到其他四配位原 子, 例如非金属磷和硼, 金属锡、镓和锗, 过渡金属锌、 铁和钛等. 不同 TO 4 四面体的连接方式导致了基于不同 组成的沸石分子筛骨架类型的多样性. 到目前为止, 国 际分子筛协会认定的分子筛拓扑结构已达 255 种. 根据 其孔道窗口, 沸石分子筛被分为小孔分子筛(≤8 元环), 中孔分子筛(10 元环), 大孔分子筛(12 元环)及超大孔分 子筛(＞12 元环) [2] . 因其规则的孔道结构、独特的择形 性、可调节的酸性位点和较高的水热稳定性, 沸石分子 筛材料作为非均相催化剂、吸附剂和离子交换剂等在现 代化学工业和石油化工领域发挥着极其重要的作用 [3] . 近年来, 随着合成方法和表征技术的不断进步, 许多新 型沸石分子筛材料被制备出来, 拓展了分子筛的应用范 围, 如生物质转化 [4] 、燃料电池 [5] 、热能储存 [6] 、二氧化 碳捕获和转化 [7] 、空气污染修复 [8] 和水的净化 [9] 等.20 世纪 40 年代, Barrer [10]