Electrochemical H₂ Production Coupled With Selective Oxidation Of Aromatic Alcohols

Hydrogen production from electrolytic water,driven by renewable energy generation,is an important way to obtain“green hydrogen”However,hydrogen production from electrolytic water still faces the challenges of low electrolysis efficiency and high energy consumption,where the slow anodic oxygen evolution（OER）kinetics process severely limits the efficiency of hydrogen production and the value added of the oxygen product is low.The use of organic molecule oxidation instead of OER is expected to significantly reduce the overpotential of the electrolytic water reaction,while obtaining high value-added chemicals at the anode,providing new ideas to solve the“efficiency and cost reduction”problem of hydrogen production from electrolytic water.The coupling of electrolytic water to hydrogen production is the key to the commercialisation of organic oxidation reactions in this field.The selective oxidation of aromatic alcohols is a very important class of reactions that can be used to produce high-value oxygenated chemicals such as aromatic acids and aromatic aldehydes through catalytic oxidation.Aromatic acids are widely used in pharmaceuticals,food and dyestuffs,while aromatic aldehydes are important intermediates for dyestuffs,fragrances and pharmaceuticals.In addition,aromatic aldehydes can be condensed by hydroxyl aldol condensation（C-C bond coupling）to giveα,β-unsaturated ketone products with higher added value.Although electrocatalytic oxidation of aromatic alcohols has been extensively studied,it still faces problems such as high overpotential and low current density.In addition,the direct oxidation of aromatic alcohols by electrocatalytic methods to obtain aromatic aldehydes and the in situ preparation ofα,β-unsaturated ketones by hydroxyl aldol condensation has not been reported in the literature.This thesis focused on the electrolytic hydrogen production coupled with aromatic alcohol selective oxidation reaction system.Using benzyl alcohol oxidation as a model reaction,we firstly realized the electrolytic hydrogen production coupled with benzyl alcohol oxidation to benzoic acid at high current density by constructing Au/Cu O composite electrocatalysts,and revealed the reaction mechanism of Au/Cu O promoted benzyl alcohol oxidation by a series of experimental means.In addition,we proposed a new idea of electrocatalytic aromatic alcohol oxidation coupled with hydroxyl aldol condensation reaction for the preparation ofα,β-unsaturated ketones,which further broadened the anode product species.The main research contents and conclusions of the thesis are as follows:（1）Previous work in our group and previous studies have demonstrated that Au can activate H₂O/OH^-to produce reactive oxygen species（Au-OH*）at low voltages（～0.85 V vs.RHE）for the oxidation of alcohol molecules such as benzyl alcohol,but pure Au catalysts for benzyl alcohol oxidation still face low current density.And the use of pure Au catalysts for benzyl alcohol oxidation still faces the problem of low current density and the use of pure Au can lead to high catalyst costs.In this work,we proposed the construction of Au-based composite catalysts to improve the intrinsic catalytic activity by exploiting the synergistic effect between Au and the carrier,while reducing the Au loading.Based on this idea,we constructed Au-based composite catalysts by electrodeposition of Au nanoparticles using Cu O,Co（OH）₂,Ni（OH）₂and Mn O₂as carriers,and investigated their oxidation performance on benzyl alcohol.The experimental results demonstrated that the Au/Cu O composite catalyst had the optimal electrocatalytic oxidation activity of benzyl alcohol,with a conversion rate of 2.24 mmol cm^-2h^-1at a constant voltage of 1.3 V vs.RHE.And the conversion of benzyl alcohol was 100%and the yield of benzoic acid was as high as 99%after a long time reaction.In addition,the current density of the Au/Cu O catalyst reached 300 m A cm^-2at 1.3 V vs.RHE by constructing Cu O nanosheet array structures.The results show that there is electron transfer between Au and Cu O NSs,where Au favours positive valence,which is more favourable for the adsorption of OH^-,thus enhancing its ability to generate reactive oxygen species.In addition,the adsorption capacity of the Au/Cu O NSs for benzyl alcohol is stronger than that of Au and Cu O NSs,thus substantially enhancing the electrocatalytic oxidation performance of benzyl alcohol.（2）In order to further broaden the variety of aromatic alcohol electrocatalytic oxidation products and increase their added value,we proposed an innovative idea of electrocatalytic aromatic alcohol oxidation in tandem with hydroxyl aldol condensation reaction for the preparation ofα,β-unsaturated ketones.Firstly,benzyl alcohol oxidation was used as the model reactant,and the benzyl acetone product was successfully prepared by selective oxidation of benzyl alcohol using Au/Cu O NSs composite catalyst as the anode and adding acetone inside the electrolyte.At a reaction voltage of 1.2 V vs.RHE,the benzyl alcohol conversion rate was 0.42 mmol cm^-2h^-1and the benzylidene acetone selectivity reached 84%.The conversion rate of benzyl alcohol and benzylidene acetone reached 98%and 84%after a long time reaction.We also demonstrate the general applicability of this electrocatalytic oxidative tandem hydroxyl aldol condensation strategy for the preparation ofα,β-unsaturated ketones from the oxidation of series of aromatic alcohols.Moreover,in addition to cathodic hydrogen production,we also designed an aromatic alcohol electrocatalytic oxidation for the preparation ofα,β-unsaturated ketones coupled with cathodic hydrogenation reaction system and successfully obtained benzylacetone from the anodic preparation of benzylidene acetone by cathodic hydrogenation,which provides a new idea for the development of an efficient electrolytic water hydrogenation coupled with oxidation reaction system.