Theoretical Design Of Non-Precious Metal Catalysts And Mechanism Of 2,3,5-Trimethylbenzoquinone Hydrogenation

Vitamin E is a fat-soluble vitamin,and its hydrolyzate is tocopherol,which is one of the most important antioxidants.The 2,3,5-trimethylhydroquinone（TMHQ）is an important intermediate for the synthesis of vitamin E,and it reacts with isophytol to synthesize VE.Traditionally,the method of preparing TMHQ is to directly reduce2,3,5-trimethylbenzoquinone（TMBQ）with sodium thiosulfate.This method has the disadvantages of high reducing agent cost,low yield,and large amount of sewage and other shortcomings.The modern method of preparing TMHQ is hydrogenation reaction of heterogeneous catalyst with H₂as the hydrogen source,because it has the advantages of good yield,environmental protection,and good catalyst cycle performance.In summary,catalytic hydrogenation is a better choice for industrial-scale production of TMBQ.At present,catalysts for catalytic hydrogenation reactions are mainly composed of precious metals Pd,Pt,Ru,etc.,because of their excellent activity and durability.However,due to their high price and low abundance,they are not practical for long-term practical applications.Therefore,it is highly demanding to develop low-cost and effective heterogeneous catalysts supporting transition metals for catalytic hydrogenation reactions.In recent years,researchers have become more and more interested in the development and utilization of non-precious metal catalysts.Iron-based or cobalt-based materials can be used as promising substitutes for precious gold catalysts.In particular,great efforts have been made in synthesizing cobalt-based materials.As we all know,single-metal catalystshave the advantages of maximized atomic efficiency,enhanced selectivity to target products,improved intrinsic activity and good recyclability.The catalyst particle size and dispersion,which are highly related to the catalyst activity,can be changed by introducing a specific carrier.In the past decade,carbon materials with unique electronic properties,large specific surface area,and high porosity have been widely used as catalyst supports.Generally,a large specific surface area facilitates the distribution of nanoparticles.In addition,nitrogen-doped carbon materials have great potential as a new type of carrier due to their relatively small particle size,good dispersibility of metal particles,and reduction of metal leaching.The special properties of nitrogen-doped porous carbon materials provide us an opportunity to develop heterogeneous Co-based catalysts with high activity and stability.In this thesis,the density functional theory was employed for systematic calculations of the electronic structures of Co based catalysts.In addition,the catalytic hydrogenation reaction mechanism of TMBQ on these catalysts was carefully explored.In addition,during the calculation process,the effect of the H coverage was considered and evaluated during the catalytic reaction.The research content and innovation results of this thesis mainly include:（1）Select all transition metals in the fourth period as the central atom to construct the nitrogen-doped carbon-supported single-atom catalyst.With the density functional theory,the stable structures were optimized,and then the thermodynamic behaviors of the adsorption of TMBQ and H₂on the catalyst model were investigated by analyzing their adsorption modes and adsorption energy.In addition,the reaction energy barrier of the first hydrogenation reaction of TMBQ on this series of catalyst models was calculated.The calculated results preliminarily show that the catalyst constructed with Co as the central atom has a better catalytic performance and facilitates the hydrogenation catalytic reaction of TMBQ.（2）With the selected CoN₃single-atom catalyst model,we further designed and calculated the complete reaction pathways of TMBQ hydrogenation on the surface of the catalyst.The reaction mechanism of TMBQ hydrogenation catalysis is mainly divided into two-step hydrogenation modes.The first and second steps represent the reduction of the first carbonyl group and the second carbonyl group,respectively.For the second step of hydrogenation process,an additional H₂molecule was introduced.In addition,it is calculated that the entire reaction is exothermic,the first step is the rate-determining step,and the reaction energy barrier is 0.64 e V.The smaller reaction energy barrier and exothermic reaction for the second step indicate that the intermediate produced during the reduction and hydrogenation of TMBQ can be easily converted into products,which is consistent with the high yield in the experimental phenomenon.（3）Two other small catalyst models,i.e.,metal clusters Co₆and Co（1 1 1）on the metal surface were designed with 60%H coverage The calculations show that the adsorption mode of TMBQ on metal clusters is via the benzene ring.Unlike the Co₆cluster,the TMBQ is adsorbed on the Co（1 1 1）surface via the O-tail adsorption mode.The calculations show that they are also two-step hydrogenation modes.Compared with the previous reaction of TMBQ on CoN₃,it could conclude that the reaction energy barrier gradually increases as the size of the metal catalyst increases.For the CoN₃,Co₆,Co（1 1 1）,the reaction energy barriers are 0.64 e V,0.85 e V,0.86e V,respectively.