The Contrallable Synthesis Of Titanosilicate Beads With Bimodal Mesopore And Its Application As Green Epoxidation Catalysts

Since the first report of mesoporous MCM-41material, there has been intenseactivity in the design and synthesis of a variety of porous solids with high surface areasand uniform and unimodal framework mesopores due to their potential applications asadvanced catalysts, adsorbents, optical guides and sensors. As extensively discussedelsewhere, the accessibility to the active sites could be effectively improved via theuniform mesopore systems in many catalytic applications of bulky molecules. Withoutprejudice to the above, however, it has been noted that the principal feature of MCM-41and the related solids could not always offer expected advantages in certain catalyticapplications. Even more, precisely owing to a typical uniform pore system, unimodalporous materials might suffer from poor accessibility to active sites because of poreblocking.In our work, bimodal mesoporous titanosilicate beads (BMB-TiSil) were preparedby using aqueous ammonia or tetrapropylammonium hydroxide (TPAOH) as alkalisource via a one-pot dual-templating method, where cetyltrimethylammonium bromide(CTAB) acted as a template for framework mesopores and anion-exchange resin(Amberlite IRA-900) beads as a shape-and large-mesopore-directing agent. BMB-TiSilwere characterized by XRD, FT-IR, UV-vis, N2adsorption-desorption, SEM and TEMmeasurement. The method presented here provides an approach to synthesize bimodalmesoporous titanosilicate beads with framework mesopores at size of3.5nm andcontinuous large mesopores at size of30.7nm. In addition, we also investigate somesynthetic conditions deeply, such as resin content, the hydrothermal temperature and thetype and contention of titanium source. The bimodal pore character results in highaccessibility to the active tetrahedral Ti species and, in turn, enhances catalytic activity.In practice, BMB-TiSil showed higher turnover frequency. The cyclohexene conversionand epoxide selectivity in the green epoxidation of cyclohexene with aqueous H2O2was33.7and21.3%, respectively. In the cases of epoxidation of alkenes bulkier thancyclohexene, such as cyclooctene, cyclododecene and trans, trans, cis-1,5,9-cyclododecatriene, the advantage of BMB-TiSil on catalytic activities is morepronounced, confirming the role of the continuous large mesopore system on theimproved accessibility. Importantly, BMB-TiSil is a stable catalyst immune to titaniumleaching, and can be reused in three successive catalytic cycles without significant lossof activity.