| Palladium catalysts with high catalytic activity have been widely used in catalytic coupling and hydrogenation reactions,but there are many problems in the process of catalytic reaction:large metal particles lead to low utilization of atoms,many side reactions,loss or aggregation of metal species,etc.Metal clusters are generally composed of a finite number of atoms with a small size(less than 2 nm),which can significantly improve the utilization of atoms.Meanwhile,their unique geometric and electronic structures show excellent performance in catalytic reactions.Therefore,it is of great significance to design and develop palladium cluster catalysts with high activity and selectivity,which are economical,environmental protection.In order to prepare high performance palladium cluster catalysts,carbon nanospheres(CS)with multiple radial channels were prepared by co-assembly method.At the same time,nitrogen and sulfur atoms were introduced into the carbon material for modification.The doping of heteroatoms improves the spatial geometry and electronic effect of the support,improves the dispersion and atomic utilization of palladium,and successfully prepares highly active cluster catalysts.(1)Precise synthesis of phenolic resin/silica composites and their derived carbon nanospheres(CS)with radial mesoporous pores by a quantitative co-assembly method.Then the carbon nanospheres were modified with nitrogen atoms to become N-doped porous carbon spheres(CS-N),and Pd nanoclusters were anchored to the CS-N carrier by wet chemical impregnation-reduction strategy.The abundant radial pore structure physically isolates the metal sites and improves the stability of the catalyst.The introduction of N heteroatoms not only provides abundant anchoring sites,but also improves the structure and catalytic performance of PdX/CS-N cluster catalysts.HR-TEM images confirmed the successful implementation of materials from design to precise synthesis,and confirmed the successful preparation of highly dispersed Pd clusters.The large surface area and abundant pore structure accelerate the mass transfer efficiency,expose more active sites,and improve the catalytic efficiency.Compared with the traditional nano-particle catalysts,the PdX/CS-N cluster catalyst greatly improves the utilization of noble metal Pd,which greatly reduces the cost of the application of noble metal supported catalyst.The PdX/CS-N catalyst showed excellent catalytic activity in the selective hydrogenation of phenol and benzoic acid under mild conditions.The general applicability of PdX/CS-N catalytic selective hydrogenation of aromatic rings was confirmed by substrate expansion reactions of various phenols and aromatic carboxylic acid derivatives.In addition,the catalyst showed good stability and recyclability after many cycles of catalytic reaction.(2)In this thesis,the quantitative co-assembly strategy was continued to accurately synthesize S-doped carbon spheres(CS-S)with radial mesoporous pores,and then the metal Pd nanoclusters were loaded on the carrier by wet chemical impregnation-heat treatment reduction strategy.A novel S-doped Pd cluster catalyst was prepared for the selective conversion of furfural and acetophenone.The interaction between doped sulfur atoms and metal species enables the sulfur doped radial mesoporous carbon spheres CS-S to anchor Pd metal nanoclusters stably,which can effectively prevent the aggregation of Pd nanoclusters.Meanwhile,the selectivity of unsaturated aldehyde/ketone hydrogenation can be effectively improved by regulating the structural properties of metal clusters with doped sulfur atoms.In this work,Pd/CS-S cluster catalyst was used to catalyze the hydrogenation of furfural and acetophenone,and the highly selective target product was obtained under extremely mild reaction conditions.The yield of tetrahydrofurfuryl alcohol and 1-phenyl ethanol could reach more than 95%.In addition,Pd/CS-S catalyst maintained good stability and recyclability after several catalytic cycles. |
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