Gas Phase Selective Oxidation Of Benzyl Alcohol Over Mesoporous Metal Oxide Catalysts

The oxidation of alcohols to aldehydes is an important reaction for basic research and chemical industrial processes. Traditional alcohol-to-aldehyde conversion is achieved by chloride hydrolysis process or stoichiometric oxidation by heavy metals that causes serious environmental pollution problems. New green catalysts are requested to accomplish selective oxidation of alcohols to aldehydes using oxygen as oxidant. Many green catalysts have been developed to catalyze the alcohol-to-aldehyde conversion in liquid phase and gas phase. Gas phase alcohols oxidation is often proceeded at high reaction temperature even using those noble metal catalysts (eg. Au, Pt, Pd). In this thesis, two transition oxides (mesoporous K-Cu-TiO2and nanocrystal CeO2) are synthesized and used as catalysts for highly selective gas-phase benzyl alcohol oxidation. The main conclusions are listed as follows:(1) Mesoporous multi-component metal oxide K-Cu-TiO2materials with ordered mesostructure, high surface area and highly dispersed component have been synthezed according to original AcHE method which is hydrochloric acid, acetic acid and ethanol system to control the growth rates of composite metal oxide precursor particles and cooprerative assembly processes with oganic template molecules. A series of characterizations such as XR.D, TEM, XPS, TPO and EPR etc. demonstrated that copper species existed as an isolated Cu+ions state in TiO2matrix at low copper doping content when the Cu/Ti ratio is below1:50. DFT calculation results explained that the stronger electron transfer in Cu-O-Ti bonds leads to highly dispersed stable Cu+ions at low Cu/Ti ratio only.(2) The mesoporous K-Cu-TiO2catalysts revealed an excellent catalytic capability for low temperature (203℃as b.p. of benzyl alcohol), highly selective (～100%) oxidation reaction of benzyl alcohol to benzaldehyde. all results such as the conversion from Cu+to Cu2+,the synchronous production of benzaldehyde with hydrogen gas and the detection of hydrogen atoms through spin trapping tests in anaerobic condition etc. convinced us that Cu+ions are the active centers within dehydrogenation process of benzyl alcohol molecules and entire catalytic reaction fellows oxidative dehydrogenation mechanism. The oxygen molecules play crucial role that combine hydrogen atom to yield water and release dehydrogenation centers Cu+sites to accelerate reaction process. Furthermore by controlling the flow rate of oxygen (Po2:PBA=0.5), the over oxidation of Cu+ions can be avoided and selective oxidation temperature window can be extended to above310℃from203℃thus enhances the industrial application potential of mesoporous K-Cu-TiOi catalysts.(3) Nanocrystal CeO2catalysts have been synthezed by AcHE method and the catalytic performances of CeO2catalysts with different particle sizes in gas-phase, low temperature, selective oxidation reaction of benzyl alcohol have been also studied. The effects of oxygen molecules including the supplement on CeO2lattice oxygen and suppression on surface carboxylic acid species also have been investigated. By XRD, BET and TEM analysises, CeO2catalyst prepared by AcHE method showed nanoparticle size less than10nm and disordered accumulation mesopore structure. For gas phase selective oxidation of benzyl alcohol, CeO2-AcHE catalyst exhibited the highest activity than other two commercial CeO2particle catalysts (25nm and micrometer level). After long time reaction with benzyl alcohol in anaerobic condition. CeO2catalyst exhibited a distinct catalytic selectivity which is same as product distribution of commercial Ce2(CO3)3sample and main products became benzene and CO2via splitting reaction. By employing IR, XPS and in-situ Raman spectrums, we confirmed that, in this catalytic reaction process, the carboxylic acid species deposited on catalyst surface in anaerobic condition and did not occur in aerobic condition. In addition in-situ Raman spectrums clearly showed that the consumption process of lattice oxygen of CeO2through intereaction with benzyl alcohol molecules in anaerobic condition and supllement process of lattice oxygen in O2condtion.