Study On Surfactant-assisted Preparation Of Copper Catalysts For Low-Temperature Methanol Synthesis

Methanol is an important chemical raw material and energy storage carrier.As the energy market changes methanol will play more role.Low temperature is conducive to methanol synthesis,that is,the theoretical conversion rate of the feedgas can be as high as above 95%at low reaction temperature.The challenge is the inadequate reaction activity and low extent of real transformation.Cu/ZnO is the prime active component of traditional methanol synthesis catalyst(220-280℃).Applied to low-temperature process,however,the real conversion of the feedgas is far from the equilibrium one.In this paper,the performance of Cu/ZnO catalyst prepared using surfactant-assisted homogeneous precipitation method for low-temperature methanol synthesis was studied.A series of Cu/ZnO were prepared and used to catalyze the synthesis of methanol at 150℃ and 4 MPa from CO/CO2/H2 through the gas-liquid-solid triphasic reaction.Their physicochemical properties were characterized by XRD(X-ray diffraction),SEM(scanning electron microscope),XPS(X-ray photoelectron spectroscopy),CO/CO2-TPD(temperatureprogrammed desorption)and N2O-titration.The effects of Cu/ZnO molar ratio,surfactant-assisted preparation and reaction solvent on the physicochemical properties and catalytic activity of the catalysts were studied.The catalyst structure,active site and catalytic performance were correlated.Cu/ZnO with molar ratio of X=0.5,1,2 and 3 were prepared using homogeneous precipitation method.When the Cu/ZnO molar ratio was 1:1,the catalytic activity for low temperature methanol synthesis was the best.CO and CO2 were converted,the total carbon conversion in batch reaction was 36.9%,and the methanol selectivity was 94.9%.Side-product was ethyl formate.Higher Cu specific surface area,Cu dispersion and surface basicity improved the adsorption-activation and hydrogenation performance of the feedgas,promoting the formation of formate species and further hydrogenation to methanol.Three kinds of surfactants,polyethylene glycol(PEG),Tween-20 and sodium dodecyl sulfate,were used to assist the homogeneous precipitation of Cu/ZnO(1:1).The activity of the catalyst assisted by PEG showed best,with the total carbon conversion of 40.8%in batch reaction,and the CO conversion of more than 15%in semi-continuous reaction(W/F=75 g·h mol-1).The selectivity of methanol reached above 99.5%.Surfactant-assisted preparation markedly changed the surface structure of the catalyst and raised the catalytic activity toward low-temperature methanol synthesis,which was related to the structural properties of the surfactant.The spatial structure of PEG promoted the dispersion of Cu species in the catalyst precursor,which made the precursor easy reduced,possessing larger Cu surface area,higher surface Cu+/Cu0 ratio and more basic sites.Pd with mass fraction of 0,1%and 5%was respectively added in the PEG-assisted preparation of Cu/ZnO(1:1).With the increase in the Pd content,the catalytic activity first increased and then decreased.When the mass fraction of Pd was 1%,the activity was the best,with the total carbon conversion of 39.3%in batch reaction.The use of[Bmim][OAc]ionic liquid evidently affected the the synthesis reaction.The conversion of CO2 in the feedgas increased significantly attributed to the changed reaction pathway.