Catalytic Epoxidation Of Propylene Over Mesostructured Cellular Foam Supported Schiff Base Molybdenum Complex

Propylene oxide is an important chemical raw material for the production of waterproof materials,coatings and other products.PO/TBA process attributed to co-oxidation method is a propylene epoxidation technology currently being encouraged in China,during which epoxidation of propylene with tert butyl hydrogen peroxide?TBHP?came from the oxidation of isobutane was carried out to produce propylene oxide.Compared with traditional chlorohydrin method,this process has advantages of less pollution and higher value of co-products.Homogeneous molybdenum complexes are the most mature catalysts used in the current PO/TBA process.However,such catalysts have difficulties in recovery and are easily deposited in reactors.Therefore,it is important to construct an efficient supported molybdenum complex for heterogeneous catalysis of propylene epoxidation.Firstly,Ti-salen and MoO₂-salen were respectively prepared from isopropyl titanate and molybdenum acetylacetonate and the corresponding coordination structures were confirmed by means of FT-IR,UV-Vis,XPS,ICP and elemental analysis.According to the evaluation of the epoxidation of propylene with the complex,MoO₂-salen was selected as the active component in this research.The results of process condition investigation showed that when 1,2-dichloroethane is used as the solvent,70%TBHP aqueous solution as oxidant,the ratio of propylene and TBHP is 10:1,catalyst dosage is 2mol%,reaction time is 1h,the conversion of TBHP was 51.7%,the selectivity of PO was 90.4%,and the selectivity of TBA was 100%.The reaction evaluation and characterization analyses confirmed that the presence of H₂O in the tert-butyl hydroperoxide oxygen source destroyed the chemical structure of MoO₂-salen.In order to improve the catalytic performance and stability of MoO₂-salen,mesostructured cellular foam?MCF?silica immobilized molybdenum complexes MoO₂-salen@MCF was prepared by impregnation,in-situ and grafting method respectively.Compared with impregnation and grafting method,MoO₂-salen@MCF prepared in situ has better catalytic performance.Process conditions inspection results showed that the best performance of the catalyst is obtained when 70%TBHP aqueous solution as oxidant and 1,2-dichloroethane as solvent,the ratio of propylene and TBHP is 8:1,catalyst dosage is 1.69wt.%and the reaction is carried out at 110°C for 1 h.The corresponding TBHP conversion was 43.4%,the PO selectivity was94.9%,and the TBA selectivity was 100%.In order to eliminate the effect of H₂O on the catalytic performance of MoO₂-salen and MoO₂-salen@MCF,azeotropically removing water to obtain a1,2-dichloroethane solution of TBHP as oxygen source.The results showed that non-water system can greatly improve the catalytic performance of MoO₂-salen and MoO₂-salen@MCF.Under the same reaction conditions,the TBHP conversion rate of MoO₂-salen is 97.8%,the corresponding PO and TBA selectivity are 77.7%and 81.6%respectively.The catalytic performance of MoO₂-salen@MCF prepared in situ is better than that of MoO₂-salen,and the optimal ratio of Si to Mo is 4.5:1.The corresponding TBHP conversion rate can reach 99.7%and the PO selectivity is 84.5%,the TBA selectivity is 81.3%,TOF value is 161.6 h^-1.Further hydrophobic modification of MoO₂-salen@MCF catalyst by liquid silanization were carried out.MoO₂-salen@MCF-S prepared by post-grafting silanization with HMDS as the silylation reagent had the best catalytic performance.The corresponding TBHP conversion rate was 100%,PO selectivity increased to94.7%,and TBA selectivity was 84.6%.After 3 times recovery,the selectivity of PO for MoO₂-salen@MCF-S was decreased to 59.4%,which is due to the loss of active ingredient Mo?VI?.