Preparation Of Phenolic Compounds By Catalytic Depolymerization Of Lignin With Nickel-Based Oxide

Non-renewable fossil fuels are still the main source of energy worldwide,and lignin-based biomass has the advantage of being renewable and widely distributed,so it is particularly important to produce valuable biomass fuels to partially supplement or replace fossil energy.Although many scholars have conducted many studies on lignin,its utilization rate is still difficult to be improved due to its complex molecular structure and the relative lack of research on the depolymerization mechanism.In this paper,NiO-Fe₂O₃/CeO₂-Al₂O₃ metal oxide catalysts with certain acidity and catalytic depolymerization ability were synthesized by chemical reduction,and the lignin was depolymerized under mild conditions,and the catalysts and depolymerization products were characterized and analyzed.Specific studies and conclusions are as follows:（1）TheλNiO/CeO₂ catalysts with different metal loadings were prepared by chemical reduction and lignin depolymerization experiments were carried out in the composite solvent catalytic system.The results showed that the optimum metal loading was 10 wt.%,at which the bio-oil yield was 48.5 wt.%.In the same way,10Fe₂O₃/CeO₂ and 10Zn O/CeO₂ catalysts were prepared and used for lignin depolymerization,and the bio-oil yield were 46.5 wt.%and 39.6wt.%,respectively.Elemental and calorific analyses of the initial lignin and bio-oil showed that the high heating value（HHV）of the depolymerized bio-oil was greater than that of the original lignin（17.5 MJ/kg）,and the H/C ratio increased and the O/C ratio decreased.It indicates that the lignin underwent an effective hydrodeoxygenation process in the depolymerization reaction with the participation of catalyst.（2）To investigate the effect of carrier on lignin depolymerization,NiO/Al₂O₃ and NiO/Mg O catalysts were prepared and used for lignin depolymerization experiments.The catalytic performance of NiO/CeO₂>NiO/Al₂O₃>NiO/Mg O was demonstrated,and x CeO₂-y Al₂O₃composite supports with different mass ratios and NiO loading were synthesized by co-impregnation to investigate the effects of different catalyst addition,reaction temperature and reaction time on the macroscopic yield of bio-oil and the microscopic yield and distribution of phenolic monomers.The experimental results showed that the optimal reaction conditions were0.1 g NiO/1CeO₂-3Al₂O₃ catalyst addition,240℃and 8 h at 0.5 g of lignin,when the highest lignin conversion,bio-oil yield and quantitative yield of phenolic compounds were achieved,74.0wt.%,52.8 wt.%and 7.2 wt.%,respectively.Scanning electron microscopy-energy dispersive spectroscopy（SEM-EDS）of the catalyst for 5 cycles and the recycled old catalyst showed good stability and recyclability.（3）Metal Fe was introduced into the catalysts to prepare m NiO-n Fe₂O₃/1CeO₂-3Al₂O₃bimetallic catalysts with different metal mass ratios.The catalyst characterization analysis revealed that the addition of metal Fe resulted in the formation of a unique new phase NiFe₂O₄with porous structure and the acidity value dominated by moderately acidic sites increased from0.36 mmol NH₃/g up to 0.62 mmol NH₃/g.The depolymerization experiments showed that the highest lignin conversion,bio-oil yield,and quantitative yield of phenolic compounds were achieved at NiO/Fe₂O₃=1/1,with 82.0 wt.%,67.1 wt.%and 8.8 wt.%,respectively.The catalyst was more selective for guaiacol-type phenolics（G-type）with 82.8%.Fourier transform infrared spectroscopy（FTIR）analysis of the residual lignin showed that the peaks at 1268 cm^-1 and 1213cm^-1 disappeared with the depolymerization experiment,and these two peaks were related to the C-O bond vibrations of the guaiacol and clove rings.This indicates that guaiacol and eugenol were effectively shed during lignin depolymerization and the lignin underwent an effective depolymerization process.1NiO-1Fe₂O₃/1CeO₂-3Al₂O₃ catalysts cycling experiments and SEM-EDS and X-ray diffraction（XRD）analyses of the recycled catalysts were consistent in showing that the catalyst surface still had some catalytic activity after 7 cycles,although the amount of active metal decreased.The catalytic activity of the catalysts was still certain.