Effect Of The Capping Agent And Lignin Structural Characteristics On The Hydrogenolysis Of Lignin

With the rapid development of the global economy,energy crisis has become one of the major problems faced by human society.Therefore,renewable biomass resources become the necessary requirement of the sustainalbe development.Lignocellulose has the advantages of wide distribution,renewable,abundant reserves and low price,which attracts much attention to researchers all over the world.Compared with cellulose,lignin is mainly used as a low heating value fuel by direct combustion because of its complex structure.It is of great significance to explore lignin depolymerization to prepare monophenols for high-valued utilization of lignin.The recondensation of depolymerized small molecular compounds from lignin is one of the key reasons for the low yield of monophenols.Therefore,a method to improve the yield of monophenols(YMC)was investigated from the point of the capping agent,and optimal process variables were explored by changing the dosage of boric acid,reaction temperature and time with Pd/C as the catalyst,formic acid as the hydrogen donor and ethanol/water as mixed solvent for hydrogenolysis under the protection of nitrogen gas.The depolymerization ratio(DD)and the YMC reached 89.39% and 17.45%,respectively,under the optimal conditions of the weight ratio of boric acid/lignin 1:1.The results showed that boric acid could significantly improve the DD and the YMC.Four lignin fractions were obtained by ultrafiltration with cutoff membrane,including less than 2500(WAL-1),2500-5000(WAL-2),5000-10000(WAL-3),more than 10000(WAL-4).All lignin fractions were characterized by using elemental analysis,FT-IR,and 2D-NMR,potentiometric titration method.The effect of structure of lignin fraction and the dosage of capping agent on the degradation ratio of lignin and the YMC were investigated.It was found that the lignin with low molecular weight containing more phenolic hydroxyl,carboxyl,methoxyl,β-O-4 and β-5 linkages reached high depolymerization efficiency,whereas lignin with high molecular weight containing more double bond and β-β linkages was hard to be depolymerized,and hence the YMC was low.The DD and the YMC were 85.17% and 12.82% for WAL-1,while 69.16% and 4.37% for WAL-4,respectively.Lignin fraction with low molecular weight was favorable for depolymerization into syringyl and alkyl syringyl,while the high molecular weight was prone to depolymerize into guaiacyl and alkyl guaiacyl.The optimal dosage of boric acid for lignin hydrogenolysis was determined by contents of both hydroxyl and phenolic hydroxyl groups of lignin.