Depolymerization Of Preoxidized Lignin In Supercritical Hydroconversion System

Lignin is one of the three major components of biomass and the only renewable aromatic resource in nature.However,due to its complex network structure and carbon-based inactive property,lignin is difficult to be transformed and utilized,resulting in a great waste of lignin resources.Therefore,effective valorization of industrial lignin becomes a research hotspot.Among the various methods for lignin conversion and utilization,catalytic hydrogenolysis is a promising method for lignin depolymerization to obtain oriented aromatic products,where hydrogen-donating solvents shows great effect in lignin conversion without external hydrogen.In addition,studies have shown that the selective oxidation of C_?-OH within the?-O-4 structure to the C_?=O can not only promote further transformation of lignin by drcreaseing bond dissociation enthalpy(BDE)of the C-O-aryl ether bond in?-O-4 linkage,but also shield the active sites to inhibit the condensation in the lignin depolymerization process.However,it's complicated in the practical application of preoxidized lignin depolymerization.In this paper,preoxidized lignin samples were hydroconverted in supercritical ethanol catalyzed by Cu/Cu Mg Al O_x,giving a study on the applicability of preoxidation to different kinds of lignin.Products yields prediction formula of preoxidized lignin were further applied in cellulose jet fuel residue lignin hydroconversion in order to produce phenolic aromatics used as jet fuel precursor in a more efficient way.Preoxidation was first applied in two different kinds of lignin,milled-wood birch lignin(MWL)and industrial hot-water extracted eucalyptus lignin(HEL),followed by hydroconversion in supercritical ethanol.According to 2D HSQC NMR analysis,C_?-OH within the?-O-4 structure was converted to the C_?=O by preoxidation,while the?-O-4 structures were degraded and aromatic rings were conjucted at the same time.Product yields of HELO-15%reached the highest value of 38.02 C%in 240 min,which was higher than most of the reported product yields from industrial lignin hydroconversion.The product yields of MWLO was determined by preoxidation degree and hydroconversion reaction time,which reaches the highest value of 54.91 C%in 120 min from MWLO-15%.Preoxidation promoted the breaking of C-O bonds in lignin and also led to the degradation of?-O-4 linkages,therefore,the competitive relationship between these two transformation trends determines the distribution and yield of depolymerized products form preoxidized lignin.The single factor influence on the depolymerization of six different preoxidized lignin was studied,showing that hydroconversion reaction time(A),?-O-4 linkages content(B),hydroconversion reaction temperature(C)and oxidation equivalent(D)would affect the product distribution and yields.The prediction and evaluation formula of product yields was established by Box-Behnken central composite experiment based on response surface method,among which?-O-4 linkages content,hydroconversion reaction time and temperature was highly significant factors and showed linear relationship.According to the fitting evaluation formula of total product yields?=^{-97.17804+0.13899A+0.2928B+0.37591C-10.77426D},the increase rate of total product (?)yields after preoxidation can be calculated and the best reaction conditions were achieved at the same time,which provided guidance for the practical application of preoxidation.Based on the evaluation formula of product yields of preoxidized cellulose jet fuel residue lignin,the optimal reaction conditions were calculated and the actual yields of products were verified by experiments.The results show that the depolymerization yield of preoxidized cellulose jet fuel residue lignin increased most at 280°C in 180 min with the DDQ dosage of 7wt%,which was 29.02 C%compared with 18.82 C%of lignin without preoxidation and the increase rate was 1.54 times.The identified monomer products were mainly composed of saturated cyclics and phenols,which could be used as the precursor of bio jet fuel.Preoxidation promoted the hydroconversion of cellulose jet fuel residue lignin and inhibited the condensation of intermediate products during lignin depolymerization.