The Research Of Conversion Of Lignin And Its Derivatives To Aromatics Catalyzed By Copper/polybenzoxazine Composites

Lignin,the second most abundant biomass resource in nature after cellulose,is the most important crude renewable source of massive aromatic compounds.As important platform chemicals for industrial synthesis,aromatics are currently mainly obtained from fossil fuels such as petroleum.With energy crisis and environmental pollution becoming more and more serious,the efficient conversion of lignin into aromatic compounds will be helpful to reduce the dependence on fossil fuels,cut down carbon emissions,mitigate the greenhouse effect and preserve the ecological environment.However,most previously reported processes bear high energy consumption,low efficiency and serious environmental pollution.Hence,a mild,clean efficient conversion of lignin into aromatic compounds was conducted using highly selective copper/polybenzoxazine composites catalysts from the perspective of green chemistry.?1?The CuCl₂/Polybenzoxazine composites?CuCl₂/PBOZ?were synthesized with CuCl₂ and as-prepared benzoxazine,and their morphology,structure and thermal stability were characterized by SEM,FTIR,XPS,TGA and so on.The results showed that CuCl₂/PBOZ is composed of many nonuniform spheres with a diameter of about tens of nanometers to several micrometers.These micronanospheres of different sizes are interconnected with each other and exhibit a coral-like three-dimensional?3D?structure.Thus the composites possess a rough surface and a Brunauer-Emmett-Teller?BET?specific surface area of 7.08 m2/g.The stable coordination bonds exist in structure between Cu2+and N/O atoms in benzoxazine.Furthermore,the composite exhibits much better thermal stability than pure polybenzoxazine?PBOZ?,which is more suitable for practical industrial application.Study on the synthesis mechanism of the composites suggests that a micelle-like prepolymer is formed first,and then molded via polymerization curing.?2?Benzyl alcohol and cyclohexanol were efficiently oxidized to corresponding carbonyl compounds catalyzed by CuCl₂/PBOZ under mild conditions.Under the optimum conditions,the conversion of benzyl alcohol reach 98.00%,and the yield ofmain product benzoic acid is up to 88.00%,while the conversion of cyclohexanol is as high as 99.40%,and the yields of cyclohexanone and dicarboxylic acid were0.09% and 62.25%,respectively.The analysis of reaction products show that malonic acid was the main dicarboxylic acid from the oxidation reaction of cyclohexanol and it accounts for 96%⁹9%.At the same time,it was found from the comparative analysis of these oxidation products that the aromatic ring of benzyl alcohol was well preserved while most of the six-carbon rings of cyclohexanol were ring-opened.?3?Aliphatic C-C bonds in lignin model chalcone were selectively oxidatively cleaved catalyzed by CuCl₂/PBOZ at room temperature and pressure.Under the optimum conditions,the conversion of chalcone and the selectivity to main product benzoic acid reach 72.00% and 97.22%,respectively.It was found in further reactions that a electron-donating group?such as methoxy?on the aromatic ring of lignin is more likely to promote its oxidative depolymerization and the oxidative cleavage of chemical bonds mainly occurs on C-C bonds on aliphatic chains between aromatic rings.It can be also concluded that C-O bonds can be more easily cleaved than C-C bonds in the catalytic oxidation of a given ether model substrate.The study on oxidative depolymerization of natural lignin showed that the lignin sample was almost completely degraded within 16 h at room temperature and pressure,and most of aromatic rings were preserved.The main degradation products were dimer-trimeric aromatic acids,aldehydes and phenols et al.Kinetic studies show that the catalytic aliphatic C-C bonds cleavage reaction appears as a first-order reaction and its apparent activation energy is calculated as 80.20 kJ/mol.Research shows that the catalyst action is of heterogeneous nature and the total selectivity to aromatics remained above 96.00% during all the three consecutive cycles.Moreover,the catalytic activity and morphology of the composite also kept stable.Mechanism study shows that the selectivity of the CuCl₂/PBOZ composite rooted in its whole structure,and the selectivity to aromatics in the catalytic cleavage reaction indicates that the degradation of lignin undergoes a two-step process,that is oxidation-then-cleavage.