Study On Fabrication And Regulation Of Active Copper Species Over CuY Catalyst For Methanol Oxidation Carbonylation To Dimethyl Carbonate

Dimethyl carbonate is a widely used green chemical product.Methanol oxidation carbonylation to dimethyl carbonate reaction with favorable thermodynamics and high atomic efficiency has attracted the attention of the industrial and scientific circles.The development of high activity catalyst is the key to industrialization of the process.As an important catalyst for this reaction,the control of copper species on the surface of chlorine-free CuY is the key to improve the performance of methanol oxidation carbonylation reaction.It has been found that ionic Cu⁺can provide the active site for reaction,and the surface CuO_X provides lattice oxygen for forming intermediate formation.Therefore,when preparing highly active CuY,it is necessary to promote the speciation of exchangable Cu⁺and highly dispersed CuO_X.In view of the competitive adsorption of methanol and carbon monoxide on the surface of CuY,excessive CO adsorption is not conducive to the formation of methoxyl group.In addition to trying to control the state of active copper species on the surface of the catalyst,the adsorption of CO on the catalyst should also be appropriately weakened to further optimize the reaction performance.Therefore,in this work,CuY catalyst was prepared by ion exchange method of copper ammonia solution.By regulating the concentration of copper ammonia solution and the activation temperature of catalyst,the state of copper species on the catalyst surface was adjusted to clarify the formation rule of copper species on the catalyst surface and its influence mechanism on the carbonylation reaction of methanol oxidation.In addition,to further adjust the copper species state on the catalyst surface and optimize the adsorption performance of catalyst to reactant molecules and the final reaction performance,the catalyst was prepared by doping S-1 molecular sieve into Y molecular sieve as composite support.The main conclusions are listed as follow:（1）CuY catalysts were prepared by adjusting the concentration of cu-Ammonia exchange solution.It was found that increasing the concentration of cu-ammonia exchange solution reduced the porosity of the catalyst,but significantly increased the copper load from 2.21%to9.95%,and kept the high dispersion of copper species,with the copper particle size less than 4nm.The solution exchange could destroy the surface acid structure and reduce the surface acid level,inhibited side reactions and improved the selectivity of DMC.The copper species of the low-load catalyst were mainly ionic Cu⁺.Increasing the copper content could improve the ionic Cu⁺content,but also significantly increased the nano-CuO_X on the surface,which could rapidly improve the catalytic performance,methanol conversion and DMC yield reached 9.07%and396.27 mg·g^-1·h^-1,respectively.（2）The activation process of copper species was studied by controlling the activation temperature of CuY.It was found that the ionic copper species with low load of CuY could not be fully reduced at 400℃,and it was difficult to contact and polymerization to form nano-CuO_X due to the small number of copper species on the surface.When it was active at 600℃,more Cu⁺and a small amount of CuO_X were obtained and the yield of DMC increased from21.91 mg·g^-1·h^-1 to 94.76 mg·g^-1·h^-1.The catalyst with high Culoading formed more Cu⁺when activated at 400℃,and more Cuspecies on the surface were easily contacted to form nano-CuOx,resulting in higher catalytic performance.The space-time yield of DMC reached 368.92mg·g^-1·h^-1.Although it was beneficial to the formation of Cu⁺at high activation temperature of600℃,the catalytic performance did not change significantly due to the slight agglomeration of Cuspecies.In addition,the appropriate activation temperature could not only promote the diffusion and exchange of Cuspecies to the pore,but also reduced the adsorption strength of methanol,which was beneficial to the improvement of performance.（3）By comparing the structures and properties of Si O₂ nanospheres,S-1 and Y molecular sieve supported copper catalysts,it was found that Y and S-1 molecular sieve with rich microporous structure showed better copper dispersion ability.Especially the Na Y with rich cation exchange sites supported copper catalyst had both ionic Cu⁺and highly dispersed CuO_X.In contrast,the surface copper species of Cu/Si O₂ were mainly CuO_X,but the dispersion was low.The adsorption properties of the reactants on the catalyst surface showed that Cu/Si O₂catalyst did not have the adsorption capacity of methanol and CO,and had no catalytic activity.Cu/S-1 showed certain energy absorption properties for methanol and CO,and the DMC yield reached 61.66 mg·g^-1·h^-1,and the selectivity of DMC reached 68.66%due to the minimal surface acid sites.In contrast,the adsorption capacity of methanol and CO on Cu/Y catalyst was significantly stronger,and the DMC yield reached 226.52 mg·g^-1·h^-1.The selectivity of DMC was 65.03%,which was slightly lower than that of CuS-1 due to the large number of surface acid sites.（4）The catalyst was prepared by mixing S-1 with Y zeolite as a composite support to load copper.I It was found that the adsorption of methanol on the surface of the catalyst was weakened,especially for CO,and the adsorption of methanol and CO became weaker with the increase of the incorporation of S-1.Combined with the activity data,it was found that Cu/（0.12S-1+1.88Y）catalyst prepared by mixing S-1 and Y molecular sieve at a ratio of0.12:1.88 had the best activity,methanol conversion and DMC yield reached 6.96%and 303.15mg·g^-1·h^-1,which were significantly higher than 5.30%and 226.52 mg·g^-1·h^-1 of Cu/Y.In addition,the catalyst prepared by the composite support doped with S-1 obstructs the diffusion of copper species into the internal pores of the molecular sieve,making more copper species located on the surface of the support,which was conducive to the improvement of activity.