Study On Preparation Of Methyl Levulinate From Biomass Directed Depolymerization

With the rapid development of society and science and technology,human’s demand for energy products increases sharply.Biomass energy is known as the "fourth largest energy source" in the world.In order to alleviate the increasingly exhausted fossil energy crisis and deteriorating environmental problems,it is urgent to prepare high value-added platform chemicals from lignocellulosic biomass to serve human life.Due to its excellent physical and chemical properties,methyl levulinate(ML)can be used as a high value-added chemical in different industries of life,such as pharmaceuticals,fragrances,coatings,adhesives,organic solvents and fuel additives.Pine wood resources are very abundant in biomass and are often used to produce various types of furniture.Therefore,in this study,the processing residues of wood processing plants,pine wood powder and cellulose with relatively simple structure were selected as raw materials to prepare ML.Starting from exploring the best reaction conditions,transition metal sulfides was selected as solid acid catalysts,and modified catalysts with rich catalytic active sites were prepared through doping methods to maximize the yield of target products and explore their catalytic mechanism.Finally,a two-step efficient preparation of ML was achieved through simple pre-treatment of pine.First of all,cellulose with simple structure and high purity was selected as raw material to prepare ML by directional depolymerization.Meanwhile,the transition metal sulfide materials were prepared as the solid acid catalysts by a simple one-step hydrothermal method.It was found that the optimum experimental conditions for the preparation of ML by directional depolymerization of cellulose were to choose cobalt sulfide(Co S2)as catalyst,methanol as solvent,high-purity hydrogen as reaction atmosphere,reaction temperature of 200 ℃,constant temperature reaction time of 3 h,initial gas pressure of 2 MPa,and feed ratio of raw material to catalyst of 10:1.Under the optimal experimental conditions mentioned above,the yield of cellulose depolymerization to prepare ML reached 30.5 wt%.In addition,to address the limitations of the acidic sites and catalytic activity of cobalt sulfide materials,doping with heteroatoms was used to optimize the performance of the modified catalyst.Based on this,Ni doped and P doped cobalt sulfide catalyst materials were prepared respectively.It can be seen from the results of NH3-TPD that the introduction of Ni atoms by doping can increase the acid strength and acidity sites of the catalyst.When the molar ratio of Ni: Co is 1,the abundance of acidity sites of the Ni0.50Co0.50S2 catalyst is 7678,which is higher than that of Co S2(5587).According to the XPS results,the minimum Co3+/Co2+ value in Ni0.50Co0.50S2 catalyst is 0.51,indicating that there is more chemical valence states of Co2+ in the material,which is conducive to promoting the depolymerization of cellulose to prepare ML,and the yield of ML prepared with Ni0.50Co0.50S2 as the catalyst reached 39.6 wt%.The introduction of P atoms by doping can change the crystal structure by triggering lattice distortion,thus regulating the electronic structure and the active site.When the ratio of P: S moles is 1,the abundance of acidity sites of Co S0.50P0.50 is 10947,much higher than Co S2.The highest yield of ML was 48.9 wt% with Co S0.50P0.50 as catalyst.At the same time,the differential charge density,density of states,electron localization function and the adsorption free energy on the intermediate of various catalysts were calculated by density functional theory(DFT),and the reason for the performance difference among Co Sx Py catalysts was understood.The results showed that the synergistic interaction between P and S atoms weakened the electron binding force of the bonding bond formed between them and Co atoms,activated electrons,promoted the transfer of electrons to Co atoms,and thus weakened the free energy required by the catalyst to adsorb intermediates,and then affected the yield of the target product ML.The raw material of pine has the characteristics of very complex structure,various components and anisotropy,resulting in harsh reaction conditions and low efficiency for the direct preparation of ML from pine.Simple pretreatment methods such as pickling,pre-oxidation of H2O2 and alkali extraction of Na OH were adopted to change the dense and complex cross-winding structure of pine,thereby improving the yield of the subsequent ML preparation.After 24 h of pickling with 1 mol/L H2SO4,the yield of ML prepared from pine increased by 2.6 wt%;After 1 h pre-oxidation with 5 wt% H2O2 solution at 150 ℃,the yield of ML prepared from pine reached 7.3 wt%.After 2 h extraction of 5 wt%Na OH solution at120 ℃,the yield of the pine to prepare ML reached a high level(over 17 wt%).However,the main source of ML is cellulose component,and excessive concentrations of oxidized solution and alkali solution can not only destroy the structure of lignin components,but also cause significant damage to the holocellulose component in raw materials,which is not conducive to the subsequent preparation of the target product ML.Therefore,it is crucial to use the pretreatment method with appropriate concentration for the directional preparation of ML from pine by two-step method.