Study On Particle Size Effect And Support Effect On Gold Catalyst For Selective Oxidation Of Alcohols

The selective oxidation of alcohols to aldehydes is an important reaction in fine chemical production.Using molecular oxygen as the oxygen source for selective oxidation of alcohols has attracted widespread attention in recent years due to its environmentally friendly characteristics.The supported gold catalysts exhibit good activity and high selectivity in aerobic oxidation of alcohols.However,their catalytic performances strongly depend on the preparation method,the size of gold particles,and the nature of the supports.Besides,the inherent of catalytic activity is still unclear.Therefore,choosing a suitable support to controllable synthesis of gold nanocatalysts with certain size distributions is of great significance to further explore the catalytic performances and influencing factors of gold catalysts.In this thesis,based on the synthesis of supported gold clusters catalysts on different oxides,the support effect was systematically studied.In addition,the supported gold catalysts prepared by different methods have various size distributions,which was used to investigate the size effect.The above studies were beneficial for understanding the key factors that determining the catalytic performances.Specifically:1.Using chloroauric acid as the precursor and cysteine as the protective agent,the precisely atomic Au₂₅ nanoclusters were prepared by sol-gel method and loaded on a series of different oxide supports?including MgO,Zn O,Al₂O₃,SiO₂,MgAl-HT,ZnAl-HT?for the selective oxidation of benzyl alcohol.The Catalytical activities decreased with the following order:Au₂₅/MgAl-300>Au₂₅/Al₂O₃-300>Au₂₅/ZnAl-300>Au₂₅/SiO₂-300>Au₂₅/MgO-300>Au₂₅/ZnO-300.Among them,the MgAl-HT and Al₂O₃ supported Au₂₅cluster catalysts displayed the best performances,with the turnover frequency?TOF?values of 2927 h^-1 and 2892 h^-1,respectively.The TEM?XPS and CO₂/NH₃-TPD results revealed that the key factor determining their catalytic performances mainly derived from the acid-base property of the supports,which greatly promotes the reaction.The comparison experiment suggested the support itself has no any activity for the reaction,indicating the gold with the support synergistically catalyzed the reaction.Therewith,the acid-base promotion mechanism of aerobic oxidation of benzyl alcohol was proposed.2.Gold catalysts with different particle sizes were prepared on the same MgAl-HT support by the impregnation method,the deposition-precipitation method and the sol-gel method.Their catalytic performances showed that the particle sizes significantly impacted on the catalytic activity.That is,the smaller the particle sizes,the higher the activities.When the gold catalyst prepared with various loadings,it exhibited the similar consequence.That implied the small gold particles were beneficial for the activation of alcohol or oxygen molecules.The Au₂₅/MgAl-300 catalyst prepared by the sol-gel method was tested in five cycles.The results showed that the selectivity was basically unchanged,whereas the conversion was obviously reduced,which indicated the stability of Au₂₅/MgAl-300 catalyst need to be improved.By doping a small amount of magnesium into the ZnAl-HT,the catalytic performance of the gold catalyst was further improved without dramatically influencing the dispersion of the gold particles.The stability of these catalysts will be studied in the following work.The above results provided a good method for rational design of supported gold catalysts with controllable sizes,which affords an in-depth understanding of the role of support effect and particle size effect in selective oxidation of alcohols over the supported gold catalysts.