Catalytic Transformation Of Cellulose Into Gamma-valerolactone Over Ni/HZSM-5 Catalysts

With increasing demands for chemicals and fuels,it is urgent to find sustainable alternatives energy resources.The catalytic conversion of biomass to high value-added chemicals and liquid fuels has attracted much attentions because of its abundance reserves and renewability.As an important biomass platform molecular compound,gamma-valerolactone（GVL）can be used as the green solvents or the additive of fuels and foods.A series of Ni/HZSM-5 catalysts with different Ni-loading capacities and Si/Al ratios were prepared by incipient wetness impregnation.The catalytic performances of the prepared Ni/HZSM-5 catalysts for the transformation of levulinic acid（LA）and cellulose to GVL were investigated,and the relevant reaction mechanism was also proposed.The surface morphology and physicochemical properties of the catalysts were characterized by inductively coupled plasma mass spectrometry（ICP-MS）,scanning electron microscope（SEM）,transmission electron microscopy（TEM）,nitrogen adsorption-desorption,X-ray diffraction（XRD）,X-ray photoelectron spectrometer（XPS）,temperature-programmed desorption of ammonia（NH₃-TPD）,temperature-programmed reduction of hydrogen（H₂-TPR）,and Fourier transform infrared（FTIR）spectra of pyridine adsorption.The results show that nickel particles were uniformly distributed on the surface of HZSM-5,and the addition of nickel particles increased the amount of mesoporous in the support,which facilitates the diffusion of reactants and products.Meanwhile,there exist a strong interaction between active component and the support,while the MFI structure of support has been unchanged.The maximum GVL yield was 100%over 5 wt%Ni/HZSM-5-50 under the optimized reaction conditions of 210 ^oC,2 h,and 3 MPa initial H₂ pressure.The amount of strong acid in catalysts determines the amount of active sites and the activity of catalysts.Meanwhile,high weak acid intensity of catalysts also is in favor of the conversion of LA to GVL.Moreover,the coexistence of B and L acid sites in catalysts efficiently improved the catalytic activity of the catalyst.The possible reaction mechanism for the transformation of LA to GVL is as follows.Hydrogen is firstly adsorbed and dissociated on the metal surface,afterwards LA is adsorbed on the acid sites and hydrogenated to 4-hydroxypentanoic acid（HPA）,then HPA is converted to GVL by dehydration.Under the optimized reaction conditions mentioned above,the conversion,yield and selectivity for the transformation of cellulose to GVL are 32.2%,31.9%and 98.9%,respectively.The possible reaction mechanism of cellulose to GVL is as follows.The H⁺form the split of water molecules on the surface of the catalyst promotes the hydrolysis of cellulose into cellobiose.Afterwards,the B acids on the surface of the catalyst promote the deformation and cleavage ofβ-1,4 glycosidic bond to produce glucose,then the produced glucose is converted into 5-hydroxymethylfurfural（HMF）catalyzed by the L acid sites.Subsequently,HMF is converted to LA through a series of reactions including hydrogenation,dehydration and decarboxylation,then LA is hydrogenated to GVL catalyzed by the B acid sites.