Liquid-phase Hydrodeoxygenation Of Waste Lignin-derived Phenolics To High-grade Liquid Fuels

Catalytic upgrading of lignin-derived phenolics is usually required because of its certain undesirable properties.These phenolics can be efficiently converted to transportation fuels（hydrocarbons and oxygenated fuels）by hydrodeoxygenation（HDO）.Therefore,this work focuses on the development of cheap,green reaction systems and optimization of process parameters to regulate the reaction pathway,which realizes the efficient and directional conversion of lignin-derived phenolics to cycloalkanes and oxygenated fuels.Firstly,a metal–solid super acid catalyst,i.e.,Ru(SO₄^2-)/ZrO₂–CeO₂,is used for the HDO of phenolics to produce cycloalkanes in a biphasic n-dodecane/H₂O system.The synergic effect between Ru and water can form the"hydrated"configuration and increase hydrogen concentration on the Ru sites.In addition,ZrO₂–CeO₂ exhibits superior hydrogen storage capacity.Both of them are favorable for the hydrogenation reaction.The strong Br（?）nsted acidity of SO₄^2-/ZrO₂–CeO₂ can benefit the subsequent deoxygenation of the hydrotreated products.Side reactions,induced by strong acidity,can be minimized by using the biphasic system（v/v=1:1）.The yield of cyclohexane from phenol was 98.5%under optimum conditions.However,the yield has a gradual decrease after several runs due to the loss of sulfonic acid groups.To solve the problems of low stability and high cost of catalyst,a highly stable bifunctional catalyst,i.e.,Ni–WO_x/NiAl₂O₄,is developed for the HDO reaction.The Ni⁰ species with small crystallite size and high dispersion are derived from Ni WO₄precursors on NiAl₂O₄,while the W species exhibit strong oxophilicity.The presence of these two species is responsible for high activity.The use of more stable WO_xinstead of common solid acid for the deoxygenation reaction and the presence of highly stable NiAl₂O₄ support account for excellent stability.The stability is also confirmed by constantly high activity for the HDO of guaiacol in five runs and negligible coke formation.The optimized 10Ni–15WO_x/NiAl₂O₄ showed a guaiacol conversion of 97.8%and a high cycloalkane yield of 83.8%under 5 MPa H₂ at250°C for 4 h.The catalytic system also exhibits excellent activity for the HDO of other complex lignin-derived phenolics and bio-oil.After the reaction,the relative content of cycloalkanes in the products from bio-oil is as high as 44.3%.Based on the above results,the effects of different family compounds in bio-oil on the HDO of phenolics are studied.Oxygenated family compounds can occupy active sites of oxophilic catalysts,which lowers the HDO rate of phenolics.The presence of acetic acid can promote the ring-opening of aliphatic ring and esterification.In contrast,gaseous hydrocarbons formed in the HDO of furfural,cyclopentanone,and hydroxylacetone,resulting in carbon loss.Therefore,separation of low-molecular-weight oxygenated compounds followed by graded upgrading of bio-oil is beneficial for reducing carbon loss in the HDO reaction.Besides cycloalkanes,oxygenated fuels are also promising.We innovatively propose a novel strategy for the production of biodiesel from lignin-derived phenolics and plant/animal oils by HDO-in situ esterification.The strategy efficiently utilizes the aromatic units and–OCH₃ functional groups.The HDO of phenolics can produce cyclohexanols and methanol,which further undergo esterification with fatty acids to form esters with high cetane numbers.Their fuel properties meet the standard of biodiesel.Guaiacol and lauric acid are used as the model compounds of lignin and plant oils,respectively.A high ester yield of 112.5 mol%is obtained with an almost complete conversion of guaiacol.This strategy also shows high feasibility for the conversion of other typical lignin-derived phenolics.Alkylphenols,a precursor of fuel antioxidants,can be also obtained by the HDO of phenolics.A new strategy for self-coupling upgrading of lignin is proposed.With lignin-derived biochar as a carbon source,the Mo precursor undergoes carburization to prepare Mo₂C/C catalysts for the HDO reaction.The carburization temperature and activation method can change the crystalline phase of Mo to tune the activity.The formation of active Mo₂C species is favored by high carburization temperatures（800°C）and biochar activated by H₃PO₄,HNO₃,or a two-step process.Optimized12.5Mo₂C/C-H₃PO₄（800）offers a guaiacol conversion of 98.7%with a high yield（66.8%）toward the formation of phenol under the optimum conditions.Furthermore,the catalytic system also exhibited wide adaptability for the conversion of various phenolics.