| Cyclohexanol and cyclohexanone both are important petrochemical raw materials andintermediates, they were generally produced by cyclohexane oxidation process in industry.Cyclohexane oxidation reaction is a typical consecutive reaction, its intermediatecyclohexanol and cyclohexanone are easily further oxidized and generate by-products acidand ester. Therefore, how to keep the selectivity of cyclohexanol and cyclohexanone at therelative high conversion of cyclohexane is still a challenge. So far, developping a catalystwith higher activity and selectivity is still a hot spot in today’s academic research. Ourpreliminary work showed that the immobilized catalyst Cosalen/MCM–41prepared byN,N’-Bis(salicylidene)ethylenediamine cobalt salt (Cosalen) with mesoporous moleculersieve MCM-41as carrier had good catalytic activity in cyclohexane oxidation reaction, andconfirmed that the hydrophobic environment of MCM-41carrier was conducive to improvethe selectivity of cyclohexanol and cyclohexanone. In this paper, on the basis of existingwork, a comparative study of two hydrophobic modification methods, namelyafter-grafting method and cohydrolysis polycondensation method, were carried out in orderto choose the optimum way of hydrophobically modification, the obtained catalyst withrelative higher hydrophobicity was expected to improve the selectivity of cyclohexanol andcyclohexanone in the cyclohexane molecular oxidation.At first, MCM-41was hydrophobically modified by after-grafting method, thenfunctionalized with amino groups, after which Cosalen was immobilized on the modifiedMCM-41carrier. The modification efficiency was evaluated by characterization methodssuch as FT-IR, the adsorption of water and cyclohexane and catalytic performance ofcyclohexane oxidation reaction. The results showed that the efficiency of silanizationhydrophobic modification for MCM-41using dimethyldiethoxylsilane was better than theother three methyl silane reagents, from which prepared MCM-41’s hydrophobiccoefficient (Adsorption amount of cyclohexane/Water on one unit of specific surface areas)was1.10. To activate the carrier of MCM-41, hydrochloric acid pretreatment method isbetter than that of high temperature calcination pretreatment method, the hydrophobiccoefficient of the sample after processing is1.43, the catalysts further prepared was appliedto catalyze cyclohexane oxidation reaction, the cyclohexane conversion was reached9.8%,the total selectivity of cyclohexanol and cyclohexanone was70.0%.As for after-grafting method, some problems, such as complicated preparationprocess, low level silanization degree and unstable catalytic activity of catalyst were existed. So in the following work, Silica mesoporous molecular sieve MCM-41functionalized with amino groups and hydrophobically modified with methyl groups wasprepared by cohydrolysis polycondensation method in one step, and then Cosalenimmobilized on the modified MCM-41. The effects of quantity of the silane reagent, thetype of alkali source, and quantity of alkali source on silanization efficiency were studied.The results showed that the quantity of γ-aminopropyl methyl diethoxysilane (AMS) hadsignificance influence on the structure of the silica mesoporous molecular sieve, with theintroduction amount of AMS reduced to5mol%, the average pore diameter of preparedAMS-MCM-41was gradually increasing from1.68nm to2.19nm. Compared withammonia and tetrapropylammonium hydroxide, NaOH was more suitable for using as thealkali source, along with reducing the amount of NaOH, the average pore diameter ofprepared AMS-MCM-41was gradually increased to2.76nm, but its hydrophobicitydecreased. In general, at room temperature, with the material molar ratio of0.12CTAB:0.05NaOH:0.05AMS:0.95TEOS:130H2O, the better hydrophobicity mesoporous structuremolecular sieve AMS-MCM-41bifunctionalized with amino groups and methyl wassuccessfully prepared, the catalysts further prepared was applied to catalyze cyclohexaneoxidation reaction, the cyclohexane conversion was reached10.0%, the total selectivity ofcyclohexanol and cyclohexanone was67.5%.Compared the two methods, the catalyst prepared by after-grafting method had abetter hydrophobicity and selectivity of cyclohexanol and cyclohexanone. On the contrary,the catalyst prepared by cohydrolysis polycondensation method had a higher activity oncyclohexane oxidation reaction, while it couldn’t use calcination method to remove thetemplate agent, the surface silicon hydroxyl concentration was increased by acid alcoholextraction methods instead, which resulted in the effect of hydrophobic modification bysilanization was weakened. |
PDF Full Text Request