On SiO₂-TiO₂ Composite Materials Supported Au Active Centers:Preparation And Catalytic Studies In Benzyl Alcohol Oxidation

Benzaldehyde is a kind of aromatic aldehyde with widespread applications in medicine,fine chemical engineering and so on.Compared with the traditional process for preparing benzaldehyde by the benzyl chloride hydrolysis method,the benzyl alcohol oxidation method has the advantages of environmental friendliness and simple process,and can synthesize chlorine-free benzaldehyde that has a large market demand.The efficiency of the benzyl alcohol oxidation process is directly determined by the performance of the catalyst,so obtaining a catalyst with excellent performance is the core of the research in benzyl alcohol oxidation process.Compared with other catalysts,Au catalysts with well catalytic activity are widely used in the oxidation of benzyl alcohol.The performance of Au catalysts is often affected by the size,morphology,and dispersion of the active metal centers.In the preparation and design of the catalyst,the support material and preparation method determine the morphology and distribution of the catalyst active centers.Therefore,controlling the preparation method of the catalyst and regulating the active center of the catalyst are the keys to the benzyl alcohol oxidation process.At the same time,using SiO₂-TiO₂composite material as supporter can combine the characteristics of TiO₂and mesporous SiO₂to investigate the distribution of active centers in supported-Au-catalyst to catalytic performance in benzyl alcohol oxidation.In this paper,SiO₂-TiO₂（R）with pure rutile TiO₂only and SiO₂-TiO₂（A）with pure anatase TiO₂only were synthesized.Such composite materials have advantages in possesing a typical mesoporous structure and high specific surface.Au _P/SiO₂-TiO₂,Au _DP/SiO₂-TiO₂,Au _G/SiO₂-TiO₂catalysts with SiO₂-TiO₂composite material supporter were prepared via photo-deposition method,deposition-precipitation method,grafting method,respectively.Combined with characterization technique,the physicochemical properties of both SiO₂-TiO₂composite material and Au/SiO₂-TiO₂catalyst were investigated and the effects of the structure and composition of Au/SiO₂-TiO₂catalyst on the catalytic performance of benzyl alcohol oxidation were explored.The main conclusions were obtained as follow:The Au/SiO₂-TiO₂catalysts prepared by different methods have different distributions of the Au active species contacting with the support material.The Au active centers of Au _P/SiO₂-TiO₂catalyst are mainly distributed on TiO₂.Au particles of Au _DP/SiO₂-TiO₂catalyst are dispersed on TiO₂and SiO₂.The Au particles of Au _G/SiO₂-TiO₂catalysts prepared by grafting method are mainly distributed on SiO₂.With the difference of the crystalline phase of the TiO₂species in the SiO₂-TiO₂material,the particle size and dispersion degree of the Au particles of the Au/SiO₂-TiO₂catalyst are different.The size of the Au particles of the Au/SiO₂-TiO₂（A）catalyst prepared by photo-deposition method and deposition-precipitation method,is generally smaller than the Au particles’of the Au/SiO₂-TiO₂（R）catalyst,and the dispersibility is better.Au/SiO₂-TiO₂catalysts showed nearly 100%benzaldehyde selectivity in the selective oxidation of benzyl alcohol using hydrogen peroxide as the oxidant and acetonitrile as the solvent.Among them,the Au _DP/SiO₂-TiO₂（A）catalyst obtained the highest benzyl alcohol conversion.The crystalline phase of the TiO₂species contained in the catalyst,the preparation method of the catalyst,the size of the Au particles and other factors have significant effects on the catalytic activity of the benzyl alcohol oxidation.