Study On The Performance Of Selective Electrocatalytic Hydrogenation Of Oxalic Acid To Glycolic Acid

Glycolic acid（GA）is an essential building block to produce biodegradable polymers（polyglycolic acid;PGA）.Meanwhile,glycolic acid can also be used as drug intermediates,dyes and food additives,which is widely used in chemical industry,pharmaceutical and other fields with considerable industrial interest.Nowadays,the main protocols for glycolic acid production is the catalytic hydrogenation of dimethyl oxalate,but it requires high temperature,high pressure reaction conditions and consumes a lot of hydrogen.Electrocatalytic hydrogenation technology provide a green route for glycolic acid production,and it is powered by clean energy（e.g.,solar energy,wind）and used water as a hydrogen source,providing a new reaction path for produced glycolic acid,which has the characteristics of good atomic economy,less pollution emission,high resource utilization rate and low pressure on the environment.In addition,as a reaction raw material,oxalic acid can be obtained by the oxidation of waste biomass（such as glucose,sucrose,cellulose）or reduction of carbon dioxide,which has the advantages of a wide range of sources and low price.Although,there are many studies have been reported for oxalic acid reduction to glycolic acid on the Ti O₂（cathode）,still exist many problems,such as low current density,poor glycolic acid selectivity and Faradaic efficiency and so on.It is urgent to develop efficient catalysts for oxalic acid hydrogenation to glycolic acid.This thesis focuses on the design of high-performance catalysts for oxalic acid electrocatalytic hydrogenation to glycolic acid.Titanium dioxide（Ti O₂）is selected as the cathode catalyst.Firstly,the performance of different crystalline Ti O₂for the electrocatalytic hydrogenation of oxalic acid is compared,proving that anatase Ti O₂has better selectivity for glycolic acid synthesis.Furthermore,the performance of oxalic acid electrocatalytic hydrogenation was enhanced by designing anatase Ti O₂nanosheet array.In addition,in view of the limited mass transfer of oxalic acid in the reaction process,the metal cation modification strategy（namely the construction of adsorption sites on the catalyst surface）was adopted to promote the adsorption and enrichment of oxalic acid on the surface of Ti O₂catalyst,and further improve the current density of oxalic acid electrocatalytic hydrogenation and the selectivity of glycolic acid.Finally,several electrochemical/in situ characterization methods were used to reveal the mechanism of oxalic acid hydrogenation.The specific research content and conclusion of this thesis are as follows:（1）In order to explore the influence of Ti O₂catalyst crystal type on its catalytic performance,two types of commercial Ti O₂crystal type（rutile type and anatase type）were respectively supported on the titanium paper substrate by spray method,and their performances for the electrocatalytic hydrogenation of oxalic acid were compared.The electrochemical test results show that anatase type Ti O₂（A-Ti O₂）has higher glycolic acid generation rate(0.29 mmol cm^-2h^-1)and selectivity（45.1%）at p H=1,-0.74 V vs.RHE applied voltage.Based on the above research results,nano-sheet array anatase Ti O₂catalyst（A-Ti O₂-NA）was prepared by hydrothermal etching method,which further improved the electrocatalytic hydrogenation performance of oxalic acid,and the glycolic acid yield was increased by 2.24 times(0.65mmol cm^-2h^-1)compared with powder A-Ti O₂.Finally,a variety of electrochemical testing methods（electrochemical impedance spectroscopy test,electrochemical active area test,electrochemical adsorption desorbed experiment,etc.）were used to explore the reaction mechanism of A-Ti O₂-NA catalyst promoting the hydrogenation of oxalic acid to produce glycolic acid.The results showed that the nanosheet array structure could reduce the material resistance,increase the electrochemical active area,and promote the adsorption of reactants.Thus,the catalytic activity can be improved.（2）Due to the negative charge of oxalic acid itself,the electrostatic repulsion effect exists in the electrocatalytic hydrogenation of cathode,which leads to the limited mass transfer of oxalic acid to the catalyst surface and hindering the reaction.In order to improve the reaction rate of electrocatalytic oxalic acid hydrogenation,an innovative idea was proposed to strengthen the oxalic acid adsorption process and improve the reaction rate through metal cationic modification strategy（i.e.constructing adsorption sites on the catalyst surface）.Based on this idea,Ti O₂(Ti O₂-Al³⁺)catalyst for surface adsorption of Al³⁺ions was prepared.Compared with Ti O₂nanosheet array catalyst,the current density of Ti O₂-Al³⁺was increased by 1.9 times,and the production rate of glycolic acid was increased by 2 times(1.31 mmol cm^-2h^-1).Faraday efficiency of glycolic acid reached 85.0%.The mechanism of catalytic hydrogenation of oxalic acid to glycolic acid on Ti O₂-Al³⁺surface was investigated by means of electrochemical adsorption,in situ infrared and electron paramagnetic resonance spectroscopy.The results show that the modification of Al³⁺can promote the adsorption of oxalic acid on the catalyst surface,and promote the production of active hydrogen,and improve the rate of oxalic acid hydrogenation.Finally,we designed a reaction system of oxalic acid hydrogenation at cathode coupled with glycol oxidation at anode to produce glycolic acid.At a battery voltage of 1.5 V,the glycolic acid reaction rate is 1.31 mmol h^-1,and the apparent Faraday efficiency reaches 186%.Compared with the traditional oxalic acid electrocatalytic hydrogenation coupled oxygen precipitation system,the system can save 53%of energy consumption,and greatly improve the electronic economy of electrocatalytic glycolic acid preparation.