The Low-temperature, Gas-phase, Seletiveoxidation Of Benzyl Alcohol Over Composite Modiifed HMS Molecular Sieves

The gas-phase selective oxidation of benzyl alcohol to benzaldehyde is extremelyaccorded with an concept of environmental protection and economic “Green Oxidation”. Agcatalysts are widely used in the selective oxidation of alcohols. The catalytic performance wascontrolled by the size and structure of the catalyst. In the paper, a series of catalysts of Agmesoporous molecular sieves were used in the gas-phase selective oxidation of benzyl alcoholon a fixed-bed reactor under atmospheric pressure with molecular oxygen as the oxidant. Theresearch included the following aspects:The K/Ag-HMS mesoporous molecular sieves were synthesized by a facile hydrothermalmethod, with non-ionic surfactant of primary amine as template. The catalysts werecharacterized by XRD, N2adsorption measurement, SEM, TEM, UV-Vis-DRS. The effects ofconditions on appearance and structure were examined, such as n(Si)/n(Ag), n(K)/n(Ag),calcination temperature, calcination time, reaction temperature, reaction time, aged time. Andthe effects were examined on catalytic performance in gas-phase selective oxidation of benzylalcohol to benzaldehyde.According to the above method, the structure and catalytic performance of M/Ag-HMSmolecular sieves were examined, modified by other alkali metal, alkaline metal, rare-earthmetal.The results showed that the moler ratio of Si/Ag and K/Ag, preparation conditionsplayed vital role on the size and dispersity of Ag particles. According to Characterizationresults, nanosized Ag particles mainly existed in active components of catalyst. The growth ofAg particle was restrained by the interaction between alkali metal potassium and Ag to formthe smaller Ag particles, which improved the low-temperature activity. The growth of Agparticle was promoted by the rising of calcination temperature and time to form superior Agparticles not good for oxidizing reation, which controlled the low-temperature activity. Otherconditions have no significant effect on the size of Ag particle and catalytic performance.That is to say, Choosing appropriate synthesis conditions is very important on the structureand catalytic performance of catalyst. The molar ratio of Si/Ag25, the molar ratio of K/Ag2,calcination temperature450℃, calcination time2h, synthesis temperature30℃, synthesistime and aged time24h were identified as the optimum conditions. The reactants contained purified N2(38mL min-1), purified O2(13mL min-1), and benzyl alcohol(0.1mL min-1). Thecatalyst gave92%benzaldehyde yield and close to100%benzaldehyde selectivity at220℃,and the catalyst also had better stability. Through the study of reaction mechanism, the addedalkali metal is thought to effectively act as an electron donor to the silver particles, and thiselectron-rich metallic Ag can interact more easily with gasous molecular oxygen to form theactive oxygen species, which improved the low-temperature oxidation of benzyl alcohol.