| Selective oxidation of alcohols into aldehydes,ketones,and acids is necessary during many organic compounds’ synthesis process,which is also one of the most widely studied reactions.Since carbonyl and carboxyl groups are widely found in the molecules of drugs,vitamins,spices,polymers,and fine chemicals.Traditionally,the oxidation of alcohols often relies on heavy metal oxidants and has many disadvantages.From the view of the environment,the development of green and sustainable catalytic oxidation of alcohol has become the focus of attention.Compared with the traditional thermal catalytic oxidation method,photocatalytic oxidation is a highly efficient and clean chemical reaction process under non-toxic and harmless conditions,reducing and eliminating environmental pollution of industrial production.However,there are still some problems in photocatalytic alcohol oxidation,such as low selectivity and low conversion.Perovskite has been widely used in photocatalytic reactions due to its good chemical stability and photoelectric properties.Since strontium titanate(STO)can only respond to ultraviolet light and has the shortcoming of a high photogenerated electron-hole recombination rate,introducing oxygen vacancies through metal or non-metal doping is an effective method to realize visible-light response and inhibit the recombination of photogenerated electron-hole.Bismuth vanadate(BVO),as a narrow-bandgap semiconductor with good visible-light response,has broad application prospects in the field of photocatalysis.The catalytic activity of BVO can be further improved by noble metal loading and surface morphology regulation.Based on the above background,this thesis aims to improve the conversion and selectivity of alcohol oxidation.From the perspective of catalyst design and reaction conditions adjustment,a series of efficient nano metal oxide catalysts were synthesized and the optimum solvent reaction system was explored for converting the simple alcohols to high value-added chemicals with high selectivity.Furthermore,the reaction mechanism of the alcohol oxidation was investigated at the same time.The specific work and conclusions are as follows:(1)Based on polymer-complexation method(also known as modified sol-gel method),some Ti sites in STO were substituted by transition metal Fe,and iron-doped strontium titanate(Fe-STO)nanomaterial with visible-light response was obtained.Pt nanoparticles were loaded on the surface of Fe-STO(Pt/Fe-STO)by chemical reduction method,which was used to test the performance of anaerobic photocatalytic selective oxidation of benzyl alcohol(BA)into benzaldehyde(BAD).The results show that the optimal catalytic performance of Pt/Fe-STO is 130.3 μmol/6 h,and the same amount of H2 is produced as the only by-product.By comparing STO and Fe-STO to explore the reaction mechanism,it was found that the surface oxygen vacancies(OVs)induced by Fe doping and the finely adjusted edge position of valence band not only promoted the activation of α-C-H bond in alcohols,but also avoided the overoxidation of carbonyl compounds,thus showing potential value in the field of photocatalytic selective conversion of organic compounds.(2)Based on the above investigation,in order to further improve the photocatalytic oxidation efficiency of alcohol and to seek more cheap metals as dopants to reduce the cost,the polymer-complexation method was used to successfully synthesize nickel-doped strontium titanate(Ni-STO).Using water as a green solvent,the highly selective conversion of benzyl alcohol and simultaneous hydrogen production were achieved under anaerobic conditions and visible light irradiation over Ni-STO decorated with Pt nanoparticles.The study shows that the apparent quantum efficiency(AQE)of benzyl alcohol conversion into benzaldehyde reaches 19.4%at 420 nm,which is 5.8%higher than that of Fe-STO.Combined with a variety of characterization methods,it was proved that chemisorbed isopropoxy is the main activated intermediate in photocatalytic dehydrogenation,and the existence of OVs is conducive to the formation of the intermediate and accelerates the dehydrogenation process.This work provides an effective method for preparing visible-light responsive oxide photocatalysts on a large scale,and proves the applicate prospect of non-noble metal-doped STO for selective alcohol oxidation under green environment and mild conditions.(3)Based on the pursuit of the principle of green synthesis,we further explore the effects of solvent on the benzyl alcohol dehydrogenation process catalyzed by Pt loaded defective STO,in order to establish the optimal solvent system for alcohol oxidation reaction.Combined with batch adsorption experiment and photocatalytic reaction,it is found that the catalytic performance in the system of water as solvent is far better than that of organic solvent,and the best solvent system is water mixed with a small amount of DMF or acetonitrile(The concentration of DMF or acetonitrile is 20%-40%of the benzyl alcohol content).The adsorption of benzyl alcohol and benzaldehyde on the STO catalyst greatly affected the photoactivity of the catalytic oxidation of alcohol,and the positive effect of substrate adsorption was the premise of effective benzyl alcohol photocatalytic oxidation.In conclusion,this work provides direct and basic evidence for the importance of selecting effective solvents for alcohol oxidation reactions.Solvent system with water as the main solvent with the addition of a small amount of organic matter ensures the catalytic efficiency as well as the requirement of green and harmless conditions.(4)Conversion of fatty alcohol into corresponding carboxylic acid is an essential synthesis procedure in the laboratory and industry,the current photocatalytic oxidation of fatty alcohol exists problems such as low efficiency.BVO was successfully prepared by a modified polymer-complexation method and used for photothermal co-catalytic oxidation of 1-octanol under visible light and room temperature conditions to improve the conversion and selectivity of 1-octanol oxidation.After 6 hours of visible light irradiation,the conversion rate of n-octanol and the selectivity of n-octanoic acid were 90.6%and 96.2%respectively in air atmosphere and 15℃.Characterization analysis showed that the modified BVO surface formed a porous structure,and the increased specific surface area and surface hydrophobic structure promoted the adsorption of 1-octanol on the surface of BVO.By comparing the light reaction and dark reaction,it was found that visible-light irradiation significantly improved the efficiency and selectivity of 1-octanol oxidation.Combined with the radical quenching experiment and EPR technology,the free radical species produced in the reaction were determined.This work provides a novel strategy for the activation of inert α-C-H bonds and opens up a new field of organic conversion.In conclusion,metal oxide nanoparticle catalysts are modified by transition metal doping and surface porous manufacturing in the thesis.The structure regulation of catalysts,catalytic reaction optimization and photo-thermal catalytic synergy function were also conducted to modulate the substrate adsorption,light absorption,photogenerated carrier separation and other important stages in order to obtain the best photocatalytic alcohol oxidation performance.Combined with experiments and advanced characterization methods,the efficient and highly selective conversion of alcohol by modified nano-metal oxides was verified from multiple angles,and the mechanism of the reaction process was thoroughly verified and discussed,which provides a practical theoretical basis for the development of high-activity photocatalytic organic conversion system,and offers certain guiding significance for industrial application. |
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