Atom-planting synthesis of MCM-36 catalyst to investigate the influence of pore structure and titanium coordination state on epoxidation activity

Abstract

Abstract The borosilicate ERB-1 was modified as titanosilicate with microporous and mesoporous composite MCM-36 structure accommodated as epoxidation catalyst for large size molecule of reactants in the aqueous solution. Ti-MCM-36 was synthesized by swelling and pillaring the MWW layer structure with SiO2 and atom-planting method with TiCl4. The 2D 1H double quantum NMR and in-situ infrared spectroscopy confirm that the swelling process with cetyltrimethylammonium bromide and tetrapropylammonium hydroxide for the MCM-36 synthesis causes the deboronation of the ERB-1 and formation of hydroxyl nest. During the atom-planting treatment, the TiCl4 reacts with the generated hydroxyl group and consumes the hydroxyl nest to form the tetrahedral titanium in the zeolite framework as Ti–O–Si. The UV–Vis spectra show that the incorporated titanium atoms are mainly existed in the form of tetracoordinated titanium. The titanosilicate pillared Si/Ti-MCM-36 with coexistence of tetrahedral and octahedral coordination titanium species were compared with Ti-MCM-36 to investigate the coordination state of Ti species for the liquid phase epoxidation of cyclohexene, cyclooctene with tert-butyl hydroperoxide aqueous solution, decane solution or H2O2 solution. Moreover, compared with the extra-framework octahedral coordination Ti species, the framework tetrahedral coordination titanium species are more efficient for epoxidation, especially with H2O2 as oxidant. By comparing with the performance of microporous Ti-ERB-1, the microporous and mesoporous composite Ti-MCM-36 is more efficient with the increase of the reactant molecule size from 1-hexene, cyclohexene, cyclooctene and soybean oil, which shows good epoxidation activity for soybean oil within H2O2 solution.