Study On The Preparation And Catalytic Performance Of Au@ZIF-8 Composite Catalyst

Nano-gold(Au)catalysts,as a new type of metal nanomaterials,have been widely used in the fields of heterogeneous catalysis,homogeneous catalysis and photocatalysis in recent years.However,the recovery of Au catalysts is challenging due to the aggregation or loss of Au nanoparticles(Au NPs),so it is very important to select suitable supports to stabilize Au NPs.In order to improve the stability and catalytic performance of Au catalyst,three composite catalysts Au@ZIF-8,Au@ZIF-8/Al₂O₃,Au@ZIF-8/ZSM-5 were prepared by selecting ZIF-8,which has better thermal stability in MOFs materials.,the stability and catalytic performance of the composites were studied,and the main conclusions were as follows:(1)A series of Au NPs with different contents were encapsulated into ZIF-8 by the gold glue method,and the interaction mechanism between Au NPs and ZIF-8 was investigated.The catalyst showed high hydrogenation activity to 3-nitrophenol(3-NP)(conversion>99 wt%)and high selectivity to 3-aminophenol(3-AP)(selectivity>97 wt%).In addition,Au@ZIF-8 catalyst also showed high conversion and target product selectivity in the selective hydrogenation of 4-nitrophenol(4-NP)to 4-aminophenol(4-AP).The experiments demonstrated that the coating of Au NPs by ZIF-8 prevented the migration and aggregation of Au NPs,and kept the Au@ZIF-8 catalyst with high stability.(2)ZIF-8 was introduced into traditional Au/Al₂O₃ surface to prepare ZIF-8 modified Au/Al₂O₃ composite Au@ZA(Z refers to ZIF-8,A refers to Al₂O₃).The synergistic catalytic mechanism between ZIF-8 and Al₂O₃ was investigated.In the hydrogenation of cinnamaldehyde(CAL),the Au@ZA composite catalyst combines high chemical stability and high selectivity for cinnamyl alcohol(COL).When the CAL conversion is close to100%,the selectivity of the Au@ZA catalyst for COL can still reach?85%.And the conversion of CAL and the selectivity of COL did not decrease significantly after the catalyst was recycled for 8 times.Compared with Au/Al₂O₃,the presence of ZIF-8 helps to anchor Au and avoid the loss of tiny Au NPs from the Al₂O₃ channel,while ZIF-8 also provides steric hindrance for the catalytic reaction and improves the selectivity of COL.Compared with Au@ZIF-8,the presence of Al₂O₃ improves the thermal and chemical stability of the material and avoids the agglomeration and loss of Au NPs caused by the decomposition of ZIF-8.(3)ZIF-8 was introduced into traditional Au/ZSM-5 surface to prepare ZIF-8modified Au/ZSM-5 composites.The effects of different catalysts on the selective oxidation of ethanol were investigated by changing the loading of ZIF-8,the ratio of SiO₂/Al₂O₃(Si/Al)and the size of the ZSM-5 support.The results show that the addition of ZIF-8 can significantly improve the selectivity of Au/ZSM-5 for acetal(DEE).When the Si/Al ratio is 37,The Au@5%ZIF-8/ZSM-5 composite catalyst achieved 58%ethanol conversion and 78%acetal selectivity in 6 h in the selective oxidation of ethanol.The acidic support is beneficial to the oxidation reaction.By comparing the influence of the size of the support and the ratio of Si/Al on the oxidation reaction,it is concluded that the micron-sized ZSM-5 support pores can promote the contact of the reactants with more acidic sites and improve the ethanol conversion.Properly increasing the Si/Al ratio of the carrier can improve the acid strength and promote the aldol condensation process.