Controllable Synthesis Of Molybdenum Carbide Catalysts And Its Application In Catalytic Cleavage Of Carbon-Oxygen Bonds In Lignin

Lignin is one of the key components of biomass and also an aromatic renewable resource in nature.Extensive research and development programs on catalytic conversion of lignin have been provoked in the last few decades.Over eighty kinds of chemicals consisting of C₆-C₁₀0 alcohols,esters,arenes and phenols can be obtained from lignin depolymerization over molybdenum carbide catalyst.However,the poor control on the particle size and fine structure of catalysts results in a highly dispersed distribution of liquid products and low selectivities for phenols.Here,a reverse microemulsion is used to synthesize a kind of highly dispersed molybdenum carbide catalysts,which can selective cleavage of C-O bonds lignin and lignin model compounds in the subcritical alcohol solvent,and result in improved conversion of lignin and selectivities for phenols.The main research contents are as follows:1.A inverse microemulsion is used to create molybdenum oxide encapsulated within silica nanospheres.The resulting composite particles are subjected to a reduction and carburization process and a highly dispersed molybdenum carbide supported on the porous silica?Mo2C/SiO2?is obtained.The prepared catalysts were characterized using various techniques such as X-ray diffraction?XRD?,nitrogen adsorption and desorption,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,and X-ray photoelectron spectroscopy?XPS?.The Mo2C particle size can be adjusted by the content of Mo in the catalyst,and the average particle size of Mo2C in the catalyst could be controlled at around 1.8 nm.Moverover,some Mo active centre were detected in the form of V valence in the catalyst,which is much different from the results in the traditional impersion method.2.Benzyl phenyl ether?BPE?,phenethyl phenyl ether?PPE?and diphenyl ether?DPE?,which contained?-O-4,?-O-4 and 4-O-5 bonds,were used as lignin model compounds to study the activities of the synthesized Mo₂C/SiO₂ catalysts on the cleavage of C-O bonds.When BPE was used as the reaction substrate,the cleavage of C_alkyl-O bonds and the C_aryl-O bonds occurred and resulted in two kinds of products,i.e.toluene and phenol,benzene and benzyl alcohol.The catalytic activity and the cleavage rate of C_aryl-O bonds decreased with the increase of Mo₂C particle size.Complete conversion of BPE could be achieved by increasing the reaction time and reaction temperature over 1 wt%Mo₂C/SiO₂ catalyst.However,with the increase of initial hydrogen pressure,the conversion of BPE and the selectivities for benzene and benzyl alcohol decreased;When water was used as the reaction solvent,Calkyl-O bond,C_aryl-O bond hydrogenolysis reaction and C_alkyl-O bond hydrolysis reaction occurred in the presence hydrogen and water solvent over 1 wt%Mo₂C/SiO₂ catalyst and the product distribution was much different from the result in ethanol solvent.The highest conversion of PPE and DPE were only 59.2%and 25.8%over 1 wt%Mo₂C/Si O₂catalyst,which can be attributed to the high C-O bonds energy,while the product distribution in PPE was similar with the result of BPE.3.Enzymatic hydrolysis lignin was depolymerized in the ethanol over Mo2C/SiO2 catalysts with different Mo2C content.Qualitative and quantitative analysis of liquid product functional groups,linkage types,molecular weight distribution,and liquid products were carried out using Fourier transform infrared spectroscopy,Gel permeation chromatography,Gas chromatography,and Gas chromatography-mass spectrometry.The enzymatic hydrolysis lignin can be fully liquefied to the liquid products with a weight average molecular weight of 943,polydispersity coefficient of 1.82 and a liquefaction ration of 93%over1.0 wt%Mo₂C/SiO2.The total selectivity for monophenols was up to 75%.The yield of liquid products and the selectivity for monophenols decreased with the increase of Mo₂C content in the catalyst.