Structure Regulation Of Copper-based Catalysts Derived From Hydrotalcites And The Study On 5-HMF Selective Hydrogenation

2,5-dimethylfuran(DMF)is a emerging biomass fuel with high specific energy and high explosion resistance.Nowadays,Pd,Rh,Pt,Ru and other noble metals with homogeneous acid or solid acid are commonly used to catalyze the hydrogenation of 5-hydroxymethylfuran(5-HMF)to synthesize DMF.However,due to the limited resources and the high cost,the further development of noble metal catalysts is limited.Therefore,researchers begin to focus on non-noble metal catalysts in order to find alternatives with excellent performance,simple preparation and low cost.5-HMF is a multi-functional compound and the key to obtain DMF efficiently from 5-HMF is to promote the hydrogenation of C=O and C-O bond with inhibiting the hydrogenation of C=C bond.The d orbital of Cu atom is fully occupied by electrons so there is stronger repulsion between C=C bonds and Cu,which can help inhibit the hydrogenation of C=C bond.At the same time,the Cu-based catalyst has good adsorption and activation ability of C=O bond.In addition,it is reported that the oxidation state of metals can work as Lewis acid sites and show a significant impact on C-O cleavage.However,the traditional methods of preparing Cu catalysts for 5-HMF hydrogenation show some problems,such as poor activation ability of C-O bond and low availability of active components.Therefore,it is necessary to design a kind of Cu catalyst with high dispersion of active components and optimized activity of C-O bond hydrogenation.Layered double hydroxides(LDHs)is a kind of emerging inorganic functional materials,whose unique lattice confinement effect and adjustable properties of matels in the layer can help promote active components to be highly dispersed while stably controlling the valence state.In this thesis,in order to prepare highly-dispersed Cu-based catalysts and stably controll Cu+species,with the help of lattice confinement effect and adjustable properties of LDHs,a series of Cu/xZnO-Al2O3 catalysts were prepared by coprecipitation method.The catalysts were used in 5-HMF hydrogenation with H2 as a hydrogen donor.XRD,XPS and in situ FT-IR were used to characterize the crystal structure,surface metal valence and active sites of the catalysts for adsorption of different functional groups.The effects of Cu+-Cu0 sites regulation on the selective hydrogenation of 5-HMF were investigated.The results showed that the catalytic performance of Cu/ZnO-Al2O3 was the best and the selectivity of DMF reached 90.05%at 100%conversion of 5-HMF.The high activity of Cu/ZnO-A12O3 is attributed to the interaction between Cu and ZnO,which enables the stable existence of Cu+species in the reaction system.For a series of Cu-based catalysts,the Cu+sites play the role of C-O bond adsorption and activation sites,and the Cu0 sites play the role of C=O bond adsorption and activation sites with hydrogen adsorption dissociation as well.Only under the synergy of Cu+and Cu0 species,the high activity of 5-HMF hydrogenation and the high selectivity of DMF can be achieved.At the same time,the hydrogenation of C-O bond on 5-HMF is obviously promoted due to the presence of Cu+sites,resulting in a large number of intermediate products 5-methylfurfural,which changes the hydrogenation pathway of 5-HMF.On the basis of Cu/ZnO-A12O3 catalyst,the fourth element was introduced to prepare Cu/ZnZrAlOx catalyst.A highly dispersed copper-based catalyst with abundant C-O bond adsorption sites was obtained and used for selective hydrogenation of 5-HMF using isopropanol as hydrogen source.The experimental results showed that the catalytic performance of C u/ZnZrAlOx catalyst was the best.DMF selectivity could reach 94.35%at 100%conversion of 5-HMF.Its high activity is attributed to the introduction of ZrO2,which can further increase the amount of Cu+sites and work as an adsorption site for C-O bond.A large number of ZrO2 sites adjacent to Cu+sites on the support can help adsorb isopropanol and provide hydrogen for furan compounds adsorbed on Cu+ sites and thus the hydrogenation activity of 5-HMF is further enhanced.The C=O bond of 5-HMF molecule is adsorbed and activated at Cuo sites on the catalyst surface and isopropanol molecule is adsorbed and activated at Cu+ sites or ZrO2 site.Because of the existence of a large number of isopropanol,the Cu+sites on the catalyst surface is occupied by the C-O bond of isopropanol,and the hydrogenation gives priority to the C=O bond of 5-HMF reaction to produce the intermediate product 5-methyl fufural,which makes the reaction system have only one hydrogenation reaction pathway.