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

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 H-1 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.

Automated summary

The study demonstrates that Ti-MCM-36 is more effective for epoxidation of large molecule reactants, particularly when using hydrogen peroxide as an oxidant. Compared to extra-framework octahedral coordination titanium species, framework tetrahedral coordination titanium species are more efficient for epoxidation reactions. The deboronation of ERB-1 and formation of hydroxyl nest are crucial for the synthesis of Ti-MCM-36.