Titanium-containing Desilicated MCM-41with Bimodal Pore Systemas Green Epoxidation Catalyst

Nowdays, the synthesis of environmental friendly catalysts, such as titanosilicatesieves, have received extensive attention from researchers. Titanosilicate sieves achievehigh epoxidation activity and efficiency of H2O2as an oxidant. In order to enhance theircatalytic activity, the secondary channel is tried to introduce by selective siliconremoval, which may lead to a greatly improved physical transport.In our work, alkaline treatment was carried out through two different routes, socalled post synthesis and direct alkaline treatment. Samples were characterized by XRD,N2adsorption-desorption, FT-IR, UV-vis, TEM and SEM measurement. A novelstrategy on the preparation of titanium-containing desilicated MCM-41with a bimodalmesoporosity system has been developed by alkaline treatment of MCM-41followedwith a post titanation route. Characterization results reveal that secondary largermesopores (8.2nm) created by the desilication of MCM-41framework were mixed withthe small framework mesopores (3.0nm), leading to an interconnected pore system. Thetetrahedral Ti species were successfully incorporated in the framework of the bimodalmesoporous silica via the post titanation route and the resulting bimodal mesoporoustitanosilicate proved to be active for heterogeneous epoxidations of alkenes withaqueous H2O2. Compared with Ti-MCM-41, the bimodal mesoporous titanosilicateshowed higher turnover number (TON) based on the cyclohexene conversion.Furthermore, the advantage of the bimodal mesoporous titanosilicate is morepronounced in the epoxidations of alkenes bulkier than cyclohexene, such ascis-cyclooctene, cyclododecene and trans,trans,cis-1,5,9-cyclododecatriene. Higher siteaccessibility due to the secondary larger mesopores in the bimodal pore system isresponsible for the noteworthy catalytic improvement.Ti-MCM-41was treated directly with sodium hydroxide (NaOH), aqueousammonia(NH3·H2O) or tetrapropylammonium hydroxide (TPAOH) followed by XRD,FT-IR, UV-vis measurement. The directly desilicated Ti-MCM-41materials thusprepared proved to be less active in the epoxidation of cyclohexene, which is due to theloss of Ti species through the alkaline treatment. Ti-MCM-41treated with aqueousammonia need further research.