Synthesis And Application Of Catalysts For Selective Oxidation Of Glycerol In Base-free Aqueous Solution

Biodiesel is an important renewable resource with the advantages in the area of raw materials,excellent performance in combustion,degradable,renewable and environmentally friendly.At present,biodiesel is mainly produced by transesterification,and the by-product glycerol is also produced.The amount of glycerol increased quickly with the rapid expanding of biodiesel,and the conversion of glycerol to value-added products is currently a research hotspot.Glycerol can be converted into a series of important chemicals via oxidation,hydrogenolysis,dehydration,esterification,oxidative carbonylation,transesterification,polymerization.Some important C₃ oxygenated chemicals,such as dihydroxyacetone,glyceraldehyde and glyceric acid,can be obtained from the selective oxidation of glycerol.According to previous work,glycerol can be oxidized over Pt catalyst with high efficiency and selectivity under base-free conditions.However,Pt catalyst is easily deactivated because of the weak interaction between the support and Pt,agglomeration and loss of supported Pt,over-oxidation of surface Pt,and/or the strong adsorption of acid or ketone on the active center.In this work,modifying the properties of carbon support by introducing N atoms,optimizing the precursor,alloying Pt with a second metal(Sb and Ru)and using metal oxide(WO₃)as catalyst support were applied in order to enhance the interaction between Pt and catalyst support,improve the dispersion of Pt and modify the electronic properties of Pt,thereby improving the activity and stability of Pt catalyst in oxidation of glycerol.The main results are as follows:A series of N-doped carbon film coated active carbon(N_xC-XC-72)support Pt catalyst was prepared via direct pyrolysis of mixed vulcan XC-72,polyvinyl pyrrolidone and urea,and followed by microwave reduction of Pt.The experimental results showed that additional carbon materials(carbon film)formed during the pyrolysis of polyvinyl pyrrolidone,urea and vulcan XC-72,and this carbon film would bring increased surface area and meso pore channels,and provide more sites for Pt loading.At the same time,the introduction of N into catalyst enhanced the interaction between Pt and carbon support,which prevented the agglomeration of Pt during the reduction process,thereby improving the dispersion of Pt.N in the catalyst could donate electrons to Pt,thereby protecting Pt from being over-oxidized,and enhancing the stability of the Pt catalyst in the oxidation of glycerol.Raising the reaction temperature could significantly increase the conversion of glycerol,but also promoted the cleavage of C-C and decreased the selectivity of glyceric acid.Changing the molar ratio of glycerol to Pt and increasing the oxygen pressure had little influence on the selectivity of glyceric acid.[Pt(?-SC₈H₁₇)₂]_n was synthesized by n-octanethiol and H₂Pt Cl₆,and used as the precursor for preparation of reduced graphene oxide(r GO)supported Pt catalyst.The experimental results showed that-SC₈H₁₇in[Pt(?-SC₈H₁₇)₂]_n precursor could enhance the interaction between Pt and the carbon support,and conduct the adsorption and deposition of Pt,thereby controlling the dispersion and growing of Pt nanoparticles.Enhanced interaction could also prevent the agglomeration and leaching of Pt during the hydrogen reduction.The catalysts prepared with[Pt(?-SC₈H₁₇)₂]_n as the precursor had higher Pt dispersion and load efficiency,and had significantly enhanced activity and stability in the oxidation of glycerol.An N-doped carbon supported Sb@PtSb₂ hybrid with core-shell structure was fabricated facilely via the direct pyrolysis of mixed glucose,melamine and Sb Cl₃,and followed impregnation of Pt.The experimental results showed that introducing of Sb could increase the surface area and pore volume of the catalyst.And the formation of PtSb₂ alloy shell in Sb@PtSb₂ hybrid promoted the adsorption and activation of oxygen atoms,thereby improving the activity of catalysts.At the same time,the formation of PtSb₂ alloy could depress the further oxidation of dihydroxyacetone,thereby improving the selectivity of dihydroxyacetone.The strong interaction between Pt and Sb enhanced the stability of catalyst during the reaction.An N-doped carbon supported Pt Ru alloy catalyst was fabricated facilely via the direct pyrolysis of mixed glucose,melamine and RuCl₃,and followed impregnation of Pt.The experimental results showed that introducing Ru into the catalyst could change the structure of the catalyst,and increase the surface area and pore volume of the catalysts.The strong interaction between Ru and Pt prevented the agglomeration of Pt during the hydrogen reduction,which significantly improved the dispersion of Pt and provided more active sites for the reaction.The introducing of Ru could also enhance the transfer of electrons between the catalyst support and Pt,which improved the activity of catalyst.The electrons transferred to Pt could also protect Pt from being over-oxidized and suppress the deactivation of the catalyst.Three types of WO₃ with different structures(rod-like,lamellar and cuboid)were prepared,and used as the support for preparation of Pt catalysts.We characterized the structure and properties of these catalysts,and revealed the deposition sites and bonding mode of Pt on the support.The experimental results showed that the first deposited Pt mainly bond to the W=O bond of WO₃,which brought the strong interaction between Pt and WO₃.The rod WO₃ had higher surface area,and formed vertical pore channels and exposed plane(100)of rod WO₃ promoted the adsorption,storage and transfer of oxygen,which significantly improved the activity of the catalysts in oxidation of glycerol.