羟基自由基在沸石分子筛合成中的作用

Roles of Hydroxyl Radicals in Zeolite Synthesis

沸石分子筛由于具有独特的形选催化作用及可调的酸性,已成为化学工业中最重要的固体催化材料.沸石分子筛的合成主要基于碱性条件下的水热晶化,OH‒被认为起到催化硅铝物种的解聚及聚合作用.近年来,研究者发现了羟基自由基加速分子筛的水热晶化机制.通过利用紫外光照射或芬顿反应等物理或化学方法向分子筛合成体系引入羟基自由基,可以实现沸石分子筛的加速晶化及高硅沸石分子筛的合成.理论计算结果表明,羟基自由基可以促进Si—O—Si键的断裂和重新生成,从而显著加快分子筛成核并促进硅原子进入骨架.本综述介绍了羟基自由基在沸石分子筛晶化方面的最新研究进展,探讨了羟基自由基的主要作用和优势,并对沸石分子筛合成的羟基自由基路线发展前景进行了展望.

Zeolites have been widely utilized in chemical industry as the most important solid catalysts because of their tunable acidities and shape selective catalysis.Synthesis of zeolites is usually performed via hydrothermal crystallization in a strong basic medium.Hydroxide ions(OH‒)are considered to catalyze the depolymerization and polymerization of the silicate and aluminate species.Recently,it was found that the hydroxyl radicals(·OH)existed in the hydrothermal synthesis system.By using physical or chemical methods such as ultraviolet irradiation and Fenton’s reagent,·OH radicals can be introduced into the zeolite synthesis system,which remarkably accelerated the zeolite crystallization and promoted the formation of zeolites with higher SiO2/Al2O3 ratio.Density functional theory calculations demonstrated that·OH radicals significantly facilitated the depolymerization of the aluminosilicate gel by breaking the Si—O—Si bonds and the polymeriza⁃tion of the aluminosilicate anions around the hydrated cation species by remaking the Si—O—Si bonds,thus speeding up the nucleation and favoring Si incorporation into the zeolite frameworks.This review summarizes the recent progress in the application of·OH radicals in the synthesis of zeolites,with an emphasis on the role and advantage of·OH radicals in zeolite crystallization.The future perspective of the·OH radical route for the synthesis of zeolites is pointed out.