Nano-gold Catalyst Used For The Gas-phase Selective Oxidation Of Benzyl Alcohol

The oxidation of alcohols to aldehyde is one of the most basic organic reactions and it is of great meaning for both academic research and industrial applications,in which the oxidation of benzyl alcohol to benzaldehyde is one of the representative reactions.As one of the members of the simplest and high-value aromatic aldehydes,benzaldehyde is also one of the chemical intermediates in huge demand and its downstream products have been widely used in the synthesis of perfumes and food additives.The traditional synthetic methods of benzaldehyde have the disadvantages of low yield,the residue of organic chlorine,and the use of complex equipment;thus this not only limit its application in pharmaceuticals and food,in which high quality benzaldehyde was required,but also lead to some severe environmental pollution problems.The gas-phase oxidation of benzyl alcohol to prepare high quality benzaldehyde using air or oxygen is the eco-friendly synthetic process route and it is suitable for the large-scale industrial production due to the advantage of the free of the product separation and continuous production operation.The supported metal catalysts have been widely researched,in which the nano-gold catalysts exhibited excellent performance in the oxidation reactions.However,for the gas-phase reaction,the nano-gold particles are easier to aggregate than the liquid-phase reaction due to the high-temperature,resulting in rapid deactivation of the catalysts.Thus,the development of catalysts with excellent performance and stability is still a huge challenge for the gas-phase oxidation of benzyl alcohol.In the thesis,for solving the problems of nano-gold catalysts in gas-phase oxidation of benzyl alcohol,the mesoporous silica nanoparticles?MSNs?with three-dimensional dendritic structure were chosen as the support and a series of supported nano-gold catalysts with uniform size were synthesised by the pore physical confinement effect of the MSNs and the chemical environment after surface modification,which showed superior catalytic performance and stability in the gas-phase oxidation of benzyl alcohol.The reaction mechanism was also studied.The main research contents are as follow:?1?The functionalization of different organic groups on the surface of MSNs was carried out by post-grafting organosilane method,and the nano-gold catalysts were prepared by the coordination interaction between the organic ligands and gold.The reactivity of nano-gold catalysts coordinated with different ligands was tested.The results showed that the reactivity and stability of nano-gold catalysts coordinated with ligands depend on the type of ligand,in which the nano-gold catalyst coordinated with SH group exhibited higher reactivity and remarkably longer life-time than the NH₂counterpart.The reactivity and the life-time performance of nano-gold catalysts modified by the ligands were ascribed to the strength of affinity between ligand groups and nano-gold.Furthermore,the reactivity and life-time of nano-gold catalysts can be adjusted by grafting double ligands.The prepared catalysts achieved excellent reactivity and life-time of 600 h.?2?The gold particles were loaded on MSNs support with the coating of Co₃O₄and the grafting of ligand groups,and the catalytic activity with different type of ligands was investigated.The catalytic results showed that the gold catalyst supported on MSNs with the coating of Co₃O₄ and the grafting of amino ligand groups exhibited the most outstanding catalytic performance.The conversion of benzyl alcohol and the selectivity of benzaldehyde can be maintained over 96%and 98%for 1200 h respectively.This is the catalyst with longest life-time for the gas-phase selective oxidation of benzyl alcohol that have been reported.The role of each component in catalyst has been clarified by the gas switching experiments and the analysis of characterization data,and the reaction mechanism for the oxidation of benzyl alcohol by the nano-gold catalyst with the coexistence of Co₃O₄ has been proposed.?3?A series of different gold catalysts modified with metal oxides were synthesised and their catalytic performance were tested.The relationship curve between metal oxides and corresponding activities was plotted.The noble metal catalysts supported on the Co₃O₄-doped MSNs were prepared and their catalytic activities were tested.