The Construction Of Silver-Based Catalysts And Its Application For Propylene Electrooxidation

Propylene oxide(PO)is the second largest propylene derivative after polypropylene.As an important industrial intermediate,it has been widely implemented in manufacturing 1,2-propylene glycol,polyether polyols,and various nonionic surfactants.Common industrial methods of PO production include chlorohydrin process,Halcon process,and hydrogen peroxide-based process(HPPO).The chlorohydrin method consumes abundant environmentally hazardous chlorine and produces a large amount of sludge,leading to serious environmental and safety problems.The Halcon method suffers from the complicated procedure.Meanwhile,the PO is just the minor byproduct,which severely restricts the process.Although the HPPO method is more environmentally friendly,it requires a large amount of expensive and explosive hydrogen peroxide,which limits the development of the process from a safety and cost perspective.Therefore,developing a new process for producing PO that is green,environmentally friendly,low-cost,and highly selective is of great significance for promoting the large-scale production and industrial development of PO.Herein,we develop a new pathway for the synthesis of PO via the electrooxidation of propylene under ambient conditions,with pure hydrogen generated on the counter electrode simultaneously.The reaction mechanism of propylene electrooxidation has been investigated on the silver-based catalysts.The facet effect was adopted to explore its influencing factors for carbon-oxygen(C-O)coupling.Subsequently,an indirect electrooxidation method has been designed,which utilizes high-priced silver ions to achieve the electrochemical conversion of propylene into PO at an industrial current density.In addition,we have constructed a dynamic silver-based molecular catalyst,realizing the one-step electrooxidation of propylene into the 1,2-propylene glycol that serves as an important downstream product of PO.Specifically,the main contents of this thesis include the following aspects:Firstly,three types of Ag3PO4 crystals enclosed by(100),(110),and(111)facets were fabricated,including Ag3PO4 cubes,rhombic dodecahedra,and tetrahedra,respectively.The mechanism on the electrooxidation of propylene into propylene oxide was investigated via the crystal face efect.Based on density functional theory calculations and in situ spectroscopy,the adsorbed OH(*OH)is the actual reactive oxygen species during the electrooxidation of propylene into PO.The coupling of adsorbed propylene(*Pr)and*OH is the rate-determining step.The lowered adsorption energies of propylene(Eads.Pr*)and adsorption energies of OH-(Eads.OH*)over Ag3PO4 cubes were favorable to the combination of propylene with*OH,resulting in the enhanced activity relative to Ag3PO4 rhombic dodecahedra and tetrahedra.During propylene electrooxidation,Ag3PO4 cubes exhibited the highest yield rate of 5.3 gpo m2 h-1 at 2.4 V versus reversible hydrogen electrode,which was 1.6 and 2.5 times higher than those over Ag3PO4 rhombic dodecahedra and tetrahedra,respectively.Secondly,we developed an indirect oxidation process,which uses silver(Ⅰ)pyridine as the oxidizing medium to achieve highly efficient electrooxidation of propylene into propylene oxide under industrial-level current density.In-situ characterization shows that silver(Ⅰ)-pyridine is oxidized to silver(Ⅱ)-pyridine during the reaction,which then decomposes into silver(Ⅲ)-oxygen-pyridine.The silver(Ⅲ)oxygen-pyridine served as the reactive oxygen species,reacting with propylene immediately.At the current density of 100 mA cm-2,the Faraday efficiency for propylene oxide is as high as 80%with the selectivity of 90%.Finally,we improved the synthetic process of 1,2-propylene glycol,which served as an important downstream product of PO.The traditional process of propylene oxidation to propylene glycol requires multiple steps.In this study,we constructed the immobilized Ag pyrazole molecular catalyst(denoted as AgPz)as the electrocatalyst for the one-step electrooxidation of propylene into 1,2-propylene glycol.During the propylene electrooxidation,the strong EB(*OH)on AgPz favored the dissociation of H2O into*OH.Subsequently,the AgPz transformed into AgPz-Hvac(the deprotonation of pyrrolic N-H in AgPz)that possessed weak EB(*OH),benefiting to the further combination of*OH with propylene.The dynamically reversible interconversion between AgPz and AgPz-Hvac could break the scaling relationship of EB(*OH),thus greatly lowering the reaction barrier.At 2.0 V versus Ag/AgCl electrode,AgPz achieved a remarkable yield rate of 288.9 mmolPG g-1 h-1,which was more than one order of magnitude higher than the highest value up to date.