The Study On Catalytic Synthesis Of N-Methylamines Using CO₂ As C1Building Block

The increase of atmospheric CO₂ concentration has led to serious environmental issues such as global warming,ocean acidification,and rising sea levels.As a nontoxic,renewable,and abundant carbon source,CO₂ can be turned into treasure through chemical conversion to synthesize highly valuable chemicals,which can not only effectively alleviate the greenhouse effect,but also reduce dependence on fossil fuels.Recently,great developments have been made on CO₂ transform into methanol,carbonates,formic acid and its derivatives in the chemical industry.In addition to these transformations,one of the important advances in this research area is the utilization of CO₂ as a methylation reagent for the production of N-methylated compounds.Replacing traditional toxic methylation reagents with CO₂,such as formaldehyde,methyl iodide and dimethyl sulfate,has important scientific significance and research value.The N-methylation of amines to produce methyl-substituted amines is an important reaction in the organic synthesis and chemical industry,which can produce pharmaceuticals,agrochemicals,dyes,perfumes,and so on.In this paper,CO₂ as methylated carbon source,H₂ as reducing agent and amines for N-methylation reaction,which mainly focuses on the preparation of the catalyst,the influence of the crystal phase structure of TiO₂,the formation of the reaction intermediate and the reaction pathway are discussed.The main research contents and innovative results are as follows.A non-precious metal Cu/TiO₂?P25?catalyst was prepared by a traditional deposition precipitation method.The effect of different calcination temperatures of TiO₂on the catalytic performance was explored,the physical and chemical properties of the catalysts were investigated by BET,XRD,TPR,TEM,XPS,FTIR etc.BET results show that the support at different calcination temperatures has a significant effect on the structure of the catalyst.compared with the catalyst prepared by calcined TiO₂,the specific surface area,pore size and pore volume of the catalyst prepared by TiO₂ without calcination treatment are increased.TEM and XRD analysis results suggested that the Cu particle size increases from 2.3 nm to 6.1 nm,and the rutile/anatase ratio also gradually increases with increasing the calcination temperature of the carrier,but catalytic activity is decreased,which indicates that the metal particle size and crystal ratio can affect the catalytic activity of the catalyst.In addition,XPS results proved that the reactivity enhances with the amount of Cu⁺and Cu⁰ exposed on the catalyst surface,indicating that the exposed Cu⁺and Cu⁰ are the active sites of the reaction.In situ diffuse reflection Fourier transform infrared spectroscopy further elucidates the reaction path of N-methylation:CO₂ reacts with H₂ to form the active intermediate CHO*species,CHO*formed reacts with N-methylaniline to form N-methylformanilide intermediate product,which are finally hydrogenated to give N,N-dimethylaniline products.Moreover,we found that the deactivated Cu/TiO₂ catalyst can be regenerated and recovered by calcined in air at 450°C for 4 h and reduced at 250°C in H₂ atmosphere for 2 h,and the particle size of Cu is also decreased to 2.4 nm,which is similar to the metal particle size of the newly prepared catalyst,which proves that the catalyst has good reproducibility.The Cu/TiO₂ heterogeneous catalyst can be recycled in the N-methylation of amine compounds.The conversion of N-methylaniline was 82%and the selectivity of N,N-dimethylaniline was as high as 98%under optimized reaction conditions.