| Epoxy cyclohexane is a very important organic intermediate, which can react with various substances to obtain products with high added value. Compared with the conventional chlorohydrin method, the direct epoxidation of cyclohexene to epoxy cyclohexane has the characteristics of environmental friendly. Because cyclohexene oxidation reaction can occur at a position of double bond and allyl at the same time, how to improve the selectivity of epoxy cyclohexane from double bond oxidation is a difficult problem. In our work, organic-inorganic hybrid Ti-functionalized mesostructured materials were constructed to explore the structure-activity relationship of the catalyst, aimed at getting the cyclohexene epoxidation catalytic reaction system with high activity and selectivity.Firstly, we prepared titanium-modified hybrid periodic mesoporous silicas(Ti-SBA-15) from tetraethoxysilane(TEOS) and tetrabutyl titanate using P123 as astructure directing agent in acidic media by the hydrothermal method. Factors affect the structure of catalysts were investigated, including ratio of silicon/titanium, acid concentration, H2O2 as hydrolysis inhibitors of titanium source, hydrolysis time of template agent. The samples were characterized by the techniques of N2 physical adsorption, UV-Vis, FT-IR and the like. The results showed the optimal process conditions were as follows: silicon/titanium ratio 25, hydrochloric acid concentration 2 mol/l, hydrolysis time of template agent 1 h, adding H2O2 as the hydrolysis inhibitors of titanium source. Ti-SBA-15 has well-organized mesoporous structure and tetravalent titanium prepared under such condition. Used Ti-SBA-15 as catalyst, cyclohexene epoxidation reaction with tert-butyl hydroperoxide(TBHP) as oxidant proceed at temperature 70℃, molar ratio of cyclohexene to TBHP 2:1. The cyclohexene conversion is 17.5%, the selectivity toward the epoxide is 11.6%, and the selectivity toward 1-(tert-buty1peroxy)-2-cyclohexe is 50.6%.Secondly, aimed at the construction of catalysts with interior hydrophobic environment, the Ti-functionalized benzene-bridged mesoporous organosilicas was prepared with direct hydrothermal method by aserting the phenyl groups into the pore wall of SBA-15 molecular structure. Effects of conditions such as the type of organic silicon source type, the molar ratio of inorganic silicon source and organic silicon source, the hydrolysis time interval between inorganic silicon and organic silicon, crystallization time were discussed. The results of characterization of multiple methods and epoxidation reaction of cyclohexene showed the optimal preparation conditions as follows: the organic silicon 1,4-(double ethoxy silane) benzene(1,4-BTEB), the molar ratio of inorganic silicon source to organic silicon source 3:1, the hydrolysis time interval between inorganic silicon and organic silicon 2 h, crystallization time 48 h. The Ti-PMOs samples in cyclohexene epoxidation reaction exhibit excellent catalytic activity, the cyclohexene conversion is 18.2%, and the selectivity toward the epoxide is 99.7%.Finally, silylation modifications were conducted on surface of Ti-PMOs to further improve the hydrophobic properties. Using hexamethyl-disilazane(HDMS) as silylation reagent, we adopted the post-grafting method to make the surface silicon hydroxyl of molecular sieve react with silylating reagent. The characterization results showed that organic-inorganic hybrid Ti-PMOs catalysts after both pore wall and the outer surface modification can maintain good mesoporous structure, the much better hydrophobic properties and the more easily exposed tetravalent titanium in the mesoporous wall. Based on the further optimization of the cyclohexene epoxidation reaction conditions, organic-inorganic hybrid Ti-SBA-15 as the catalyst, the amount of catalyst is 1.57% of the weight of cyclohexene, reaction temperature 80°C, reaction time 5h, the molar ratio of cyclohexene to TBHP 2:1, the cyclohexene conversion is 23.4%, the selectivity toward the epoxide is up to 100%. |
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