杂原子掺杂beta分子筛的烯烃环氧化催化性能

采用液-固相同晶取代法分别制备了W、Mo、V和Ti掺杂的四种杂原子beta分子筛，并以环己烯和环辛烯为探针底物分子，系统性评价了W-beta、Mo-beta、V-beta和Ti-beta催化烯烃环氧化反应性能。结果表明，所制备的四种杂原子beta分子筛均具有较高的结晶度，且完好继承了母体Al-beta分子筛的微孔结构与纳米颗粒形貌；由于离子半径和M—O键长的差异，W和Mo的骨架掺入量相对较低，V和Ti的掺入量相对较高；催化烯烃环氧化反应中，有限的活性金属含量导致W-beta和Mo-beta催化环烯烃的转化率低于V-beta和Ti-beta，但W-beta、Mo-beta、V-beta的反应转换频率（TOF）较高，说明W、Mo、V金属位点的活性更高；催化环己烯环氧化反应中，V-beta催化剂易引发烯丙基氧化副反应，环氧化物选择性低于其他三种催化剂，且V活性组分氧化物形态含量较高，反应过程中易脱落，导致V-beta催化剂循环再生性能较差；Ti掺杂beta催化剂表现出最佳的烯烃环氧化反应性能，提高Ti含量可有效提高反应转化率，但骨架Ti掺入量存在极值，过多的Ti物种趋向于氧化物形态，降低反应活性（TOF值），因此适宜的骨架Ti掺杂有利于Ti-beta分子筛催化烯烃环氧化反应。

Synthesis of heteroatom-substituted beta zeolites for catalytic epoxidation of cyclic olefins

Four types of heteroatom-substituted beta zeolites were prepared by a post-synthetic method. Nanosized Al-beta was used as the dealumination substrate, followed by the incorporation of 4—6 transition-metal atoms (W, Mo, V, and Ti) into the framework of dealuminated beta. The obtained heteroatom-substituted beta zeolites (W-beta, Mo-beta, V-beta, and Ti-beta ) well inherited the high crystallinity, large micropore volume and nanoparticle morphology from their parent Al-beta. However, the incorporation amounts of various heteroatoms were different due to the discrepancy in ion radiuses and M—O bond lengths, and V-beta, and Ti-beta showed much higher metal contents than W-beta and Mo-beta. The beta zeolites were further used as catalysts for the epoxidation of cyclohexene and cyclooctene. W-beta and Mo-beta revealed lower conversions of cyclic olefins but higher turnover frequency (TOF) values than V-beta and Ti-beta, indicating the higher activities of the W and Mo active sites. V-beta exhibited the highest conversion of cyclohexene due to its high content of V, but the epoxide selectivity over V-beta was much lower than that over the other heteroatom-substituted beta catalysts, which could be ascribed to the excess oxide species that promoted the side reaction of allylic oxidation. Besides, serious leaching of V species were observed for the V-beta catalyst that led to a poor recyclability in liquid-phase epoxidation reaction. Among all the heteroatom-substituted beta catalysts, Ti-beta exhibited the highest conversion and epoxide selectivity in the catalytic epoxidation of cyclic olefins. Moreover, the Ti active species in Ti-beta were quite stable, which further gave a superior recyclability of Ti-beta. In addition, the conversion of cyclic olefins over Ti-beta can be enhanced by increasing the content of Ti, but the excess Ti species tend to form the low-activity oxides and reduce the TOF value.