Study On Bifunctional Catalysts And Their Catalytic Performance For The Reaction Of CO₂ And Amine/H₂

CO2 is a gas that has a greenhouse effect on the atmosphere,and it is also a cheap carbon source.CO2 can be converted into methane,methanol,formic aid and other organic compounds under the action of the catalyst.Under the action of bifunctional catalyst,CO2 and amine/H2 can be converted into N-formamide(or N-methylamine)and other high value-added organic compounds by N-formylation(or N-methylation)reaction.It can greatly improve the use value of CO2 and also has important significance for reducing the greenhouse effect of CO2.In this paper,four kinds of bifunctional catalysts were prepared,which were catalysts with different active components supported on different acid supports,Au/MOF-808(Zr)catalysts,Au/MOF-808(Hf)catalysts,and Au/UiO-66(Hf)catalysts.The structure of the catalysts was characterized by modern analytical testing methods such as XRD,SEM,HRTEM,FT-IR,XPS,N2 desorption-desorption and NH3-TPD.In a liquid-solid reactor,the catalytic performance of the catalyst for the N-formylation(or N-methylation)reaction of CO2 with amine/H2 to form N-formamide(or N-methylamine)compounds was evaluated.The reaction mechanism of N-formylation(or N-methylation)of CO2 and amine/H2 on bifunctional catalyst was discussed.The important experimental results obtained are as follows:(1)A series of supported bifunctional catalysts were prepared with acidic oxides and MOFs as supports,and Au and Pd as active components.By comparing the evaluation results of the reaction performance of CO2 and aniline/H2 on these catalysts,it was found that Au/TiO2 bifunctional catalyst had the best reaction performance,among which 3.0wt%Au/TiO2 bifunctional catalyst had the best performance.Under optimal reaction conditions,the conversion of aniline(AN)was 27.1%(CAN),the selectivity of N-methylaniline(MA)was 71.9%(SMA),and the selectivity of N,N-dimethylaniline(DMA)was 27.2%(SDMA).When different types of amines are used for the reaction of CO2 with amines/H2,the primary amines are easy to produce N-methyl products,and the secondary amines are easy to produce N-formyl products.The structural characterization of Au/TiO2 bifunctional catalysts shows that the particle size of the catalytic active component Au is 1-10 nm,and is evenly dispersed on the surface of TiO2.The average pore size of Au/TiO2 is 1.94 nm.The weak acid site concentration and strong acid site concentration are 334.6?mol·g-1 and 614.4 ?mol·g-1,respectively.Large pore size and high acid site concentration are favorable for the reaction.(2)Using the synthesized MOF-808(Zr)with coordinated unsaturated(CUS)Zr4+sites as the support and Au as the active component,Au/MOF-808(Zr)bifunctional catalysts with different Au loadings were prepared.The structural characterization of catalysts shows that the Au/MOF-808(Zr)retains the complete MOF-808(Zr)structure.The particle size of Au nanoparticles is 2-20 nm,which is evenly dispersed on the surface of MOF-808(Zr).The weak acid site concentration and strong acid site concentration of catalyst are 100.5?mol·g-1 and 523.2 ?mol·g-1,respectively.The acid sites can absorb and activate amines and CO2,which is conducive to the reaction.By comparing the evaluation results of the reaction performance of CO2 and aniline/H2 on Au/MOF-808(Zr)with different Au loading,it was found that 3.0wt%Au/MOF-808(Zr)had the best catalytic performance.After optimizing the reaction conditions,CAN was 17.9%,SMA and SDMA were 80.9%and 19.1%,respectively.The catalyst was used 5 times and still maintained good catalyst performance.When different types of amines are used for the reaction of CO2 with amines/H2,the primary amines are easy to produce N-methyl products,and the secondary amines are easy to produce N-formyl products.The mechanism study shows that N-formamide,N-methyl-N-formamide and formic acid may be intermediate substances in the reaction.Au nanoparticles are hydrogenation active centers,and the acid-base sites of MOF-808(Zr)are N-formylation active centers.H2 is adsorbed and activated on Au nanoparticles.The CO2 is adsorbed on O2-basic sites and CUS Zr4+ acidic sites,then reacts with activated H2 to form HCOOH.The amines are adsorbed on the CUS Zr4+ acidic sites and then react with HCOOH to form N-formamide and water.Subsequently,the formed N-formamide react with activated H2 to yield N-methylamine.When the primary amines are used as reactants,N-methylamine could continue to react with HCOOH to form N-methyl-N-formamide and water.Subsequently,the formed N-methyl-N-formamide react with activated H2 to yield N,N-dimethylamine.The two reaction active centers and three active sites play synergistic roles in the N-formylation(or N-methylation)reaction of CO2 with aniline and H2.(3)Using the synthesized MOF-808(Hf)with CUS Hf4+sites as the support and Au as the active component,Au/MOF-808(Hf)bifunctional catalysts with different Au loadings were prepared.By comparing the evaluation results of the reaction performance of CO2 and aniline/H2 on these catalysts,it was found that 3.0wt%Au/MOF-808(Hf)had the best catalytic performance.Under optimal reaction conditions,CAN was 28.8%,SMA and SDMA were 77.0%and 23.0%,respectively.Compared with Au/MOF-808(Zr),CAN increases,SMA decreases,and the conversion rate of different types of amines also increases.The structural characterization results of catalysts show that the formed Au/MOF-808(Hf)retains the complete MOF-808(Hf)structure.The particle size of Au nanoparticles is 2-16 nm,which is uniformly dispersed on the surface of MOF-808(Hf).The weak acid site concentration of the catalyst is 425.5?mol·g-1,and the strong acid site concentration of the catalyst is 752.8 ?mol·g-1.The average pore size of the catalyst is 1.78 nm.Compared with Au/TiO2 and Au/MOF-808(Zr),it has a higher acid site concentration and a larger pore size,which makes it easier to adsorb and activate amines and CO2,and to facilitate the transfer of reaction substrates and products.This results in the catalyst having higher activity for amines and lower selectivity for N-methylaniline.(4)UiO-66(Hf)with CUS Hf4+sites was successfully synthesized using formic acid as a modulator and vacuum heat treatment by thermal reflux method.Using UiO-66(Hf)as the support and Au as the active component,Au/UiO-66(Hf)bifunctional catalysts with different Au loadings were prepared.By comparing the evaluation results of the reaction performance of CO2 and aniline/H2 on these catalysts,it was found that 3.0wt%Au/UiO-66(Hf)had the best reaction performance.Under optimal reaction conditions,CAN was 25.3%,SMA and SDMA were 83.3%and 16.7%,respectively.Comparing the reaction results of Au/UiO-66(Hf)and Au/MOF-808(Zr/Hf),SMA of the former is higher than the latter,and CAN and the conversion rates of different types of amines of the former are lower than the latter.The structural characterization of Au/UiO-66(Hf)bifunctional catalyst displays that the Au/UiO-66(Hf)retains the complete UiO-66(Hf)structure.The particle size of Au nanoparticles is 2-22 nm,which is uniformly dispersed on the surface of UiO-66(Hf).The average pore size of Au/UiO-66(Hf)is 1.59 nm,the weak acid site concentration of the catalyst is 255.1 ?mol·g-1,and the strong acid site concentration of the catalyst is 173.8?mol·g-1.The average pore size,strong acid site concentration and total acid site concentration are lower than that of Au/MOF-808(Zr/Hf).Lower acid site concentration will weaken the activation ability of amines and CO2,and reduce the activity of the catalyst.The smaller pore size is not conducive to the transfer of macromolecular reaction substrates and products,and improves the selectivity of N-methylamine.