Biomass Liquefaction Process And Catalytic Mechanism Catalyzed By Composite Metal Oxides

Catalytic liquefaction of biomass is an important approach for the efficient use of biomass resources.The main products usually have high oxygen content,which require further selective hydrogenation/deoxidation and conversion to liquid fuels and fine chemicals.At present,the main source of hydrogen is still fossil resources.How to avoid the use of precious metal catalysts andreduce the direct use ofhydrogen to achieve controlled hydrogenation/deoxidation of biomass liquefaction processes has important practical significance for improving the quality of biomass liquefaction products.In view of current problems in catalytic liquefaction and its derived unsaturated compounds in the selective catalytic hydrogenation/hydrogenolysis process,such as low selectivity of the target product,low liquefaction efficiency,easy coking,and harsh reaction conditions;high catalyst cost,easy to deactivate,poor stability,easy loss of active components and agglomeration,etc.CuNiMgAl composite metal oxides?Cu/Ni-CMO?derived from hydrotalcite-like compound were used as catalysts in methanol medium through in-situ Hydrogenation enables the liquefaction of biomass under mild conditions and the selective catalytic hydrogenation of liquefied products into renewable liquid fuels and high-value chemicals.1.Cu/Ni-CMO catalysts was prepared by co-precipitation method with hydrotalcite-like as precursors.The structure and properties of the catalysts were characterized by ICP,XRD,BET,H₂-TPR and CO₂-TPR.XRD analysis showed that CuO and NiO were uniformly dispersed in the catalyst;TPR and N₂O titration analysis showed that the doping of Ni contributed to the reduction of CuO,increased the dispersion of Cu and the amount of basic sites on the catalyst.2.The effect of reaction conditions on the liquefaction of bamboo catalyzed by Cu/Ni-CMO in methanol was investigated using bamboo as raw material.The results showed that the liquefaction products of bamboo were dominated by alcohols,ketones,and hydrocarbons(using Cu_0.5Ni_0.5Mg₃Al-O as a catalyst and reacted at 280°C for 8h with a total yield of 71.7%).Cu/Ni-CMO catalyst is more capable than Cu-CMO for promoting the directional distribution of liquefied products and exhibits more excellent hydrogenolysis and hydrogenation activity.The supercritical temperature of methanol is a key factor affecting the composition and distribution of liquefaction products.Alcohols and hydrocarbons have their maximum at a Cu/Ni ratio of 1:1,while the amounts of phenolic and ester products are minimal.From the perspective of promoting the directional distribution of bamboo liquefaction products,co-doping of Cu and Ni shows a certain synergistic catalytic effect.GC analysis of the gas product shows that the Cu/Ni-CMO catalyst can promote the cracking reaction of methanolandprovideahydrogensourceforthehydrogenolysisand hydrogenation/deoxygenation reactions in the liquefaction process of bamboo.3.Using microcrystalline cellulose as raw material,the effects of different reaction conditions on the depolymerization of Cu/Ni-CMO-catalyzed cellulose were studied.The results showed that the yield of the alcohol product in the cellulose depolymerization product was above 80%,the amount of oligomer was significantly reduced,and almost no coke was generated.The basicity of the surface of the catalyst and the solvation effect of supercritical methanol are the main reasons leading to the breaking of the glycosidic bonds in the macromolecules of cellulose.The Cu⁰ and Ni⁰ active species formed in the catalyst can effectively catalyze the cleavage of C-C and C-O in the glucose and oligomers formed by the depolymerization of cellulose,followed by further hydrogenation to form alcohols or polyols.4.For the complex and variable characteristics of lignin structure,we first studied the depolymerization characteristics of industrial alkali lignin.The Cu/Ni-CMO catalyst can effectively promote the depolymerization of lignin in supercritical methanol.Through GC-MS analysis,it was known that the depolymerized monomer product of alkali lignin mainly contained C₆-C₉ phenols,alcohols and hydrocarbons,among which monocyclic phenols and aromatic alcohols were the major products.In order to further investigate the role of methanol in the depolymerization of lignin,the homogenization of P1000 lignin was investigated.The depolymerization behavior of P1000 lignin catalyzed by Cu/Ni-CMO was investigated.The results show that the use of methanol as a solvent is more effective than ethanol in promoting the relevant reactions in the depolymerization of lignin and obtaining a higher yield of the monomer product.The Cu/Ni-CMO catalyst exhibits excellent hydrogenation/deoxidation and lower ring hydrogenation activity in the lignin depolymerization reaction.Based on the influence of the progress of the reaction,it was found that the repolymerization occurred mainly in the early stage of the reaction,and then the depolymerization reaction dominated.5.Comprehensive analysis using ¹H-NMR,2D HSQC NMR,GPC,and GC×GC-MS techniques followed the structural changes of lignin in the depolymerization process.The results showed that the depolymerization and repolymerization of lignin occurred in the reaction process.occurs,producing depolymerized lignin fragments and repolymerized lignin fragments,respectively.The phenolic hydroxyl group was found to be a major player in repolymerization and coke formation.Methanol not only acts as a hydrogen donor solvent but also acts as a blocking agent,protecting the highly reactive phenol intermediates via O-alkylation of the hydroxyl group or C-alkylation of the aromatic ring,thus inhibiting the repolymerization reaction.6.Based on the CMO catalyzed methanol cleavage reaction and DFT theoretical analysis,it was indicated that the generation of H₂ was the result of Cu⁰ and Cu⁺species co-action:Cu⁰species causes the O-H in methanol to be cracked into CH₃O and further lysed to generate HCHO.The Cu⁺species decomposed HCHO derived from CuO into CO and H₂.7.Based on the catalytic reaction of phenol as a model reactant and the depolymerization of cellulose and lignin,the catalytic activity of the active metal in the catalyst is clarified.The Cu and Al active sites are Lewis acid active sites that constitute the alkylation of aromatic compounds.The combination of Cu and basic sites has the activity to catalyze the cleavage and dehydrogenation of methanol,and it is also the active site to catalyze the aldol condensation and esterification reactions.Our work provides a reliable experimental basis for the establishment of liquefaction/depolymerization of biomass in the methanol in-situ hydrogen supply system,and provides a theoretical reference for a comprehensive elucidation of the reaction mechanism in the process of biomass liquefaction.