Lignin Hydrogenolysis To Produce Monophenolic Compounds In Ethyl Acetate/Water System

Lignin is the most abundant aromatic compounds in nature.It has a significance to depolymerize lignin into monophenolic compounds as bulk chemicals and advanced liquid fuel by the further hydrodeoxygenation for substituting fossil resources and energy and realizing sustanable development of society.However,most lignins are utilized by directly burning due to the complicated structure of lignin,resulting in the waste of biomass resources.Therefore,the high-valued and effiecient use of lignin become the one of the hot topics all over the world.In this work,Pd/C was used as catalyst,and formic acid was used as an in-situ hydrogen donor.The influence of the biphasic system composed of ethyl acetate and water on the hydrogenolysis of lignin was explored.It was found that the lignin depolymerization ratio and the yield of monophenolic compounds in the biphasic systerm were significantly higher than those in the single-phase medium.The existence of ethyl acetate stabilized the depolymerized products which were dissolved in ethyl acetate.At the same time,ethyl acetate was decomposed into acetic acid and ethanol under acidic condition,in which acetic acid provided with hydrogen donors collaborating with formic acid,and ethanol together with water are favorable for the increase of solubility of lignin in medium.Consequently,the yield of monophenols increased obviously.The effect of ratio of ethyl acetate to water,reaction time,temperature and the amount of Pd/C on lignin depolymerization were also investigated in the biphasic system.The results showed that the biphasic system made the depolymerization process occurr for a long time without recondensation,increasing the depolymerizaiton ratio and yield of monophenols.However,an excessive depolymerization time still resulted in the recondensation,decreasing the yield of monophenols.The low reaction temperature caused the low efficiency of depolymerization,but the too higher reaction temperature resulted in a stronger recondensation,decreasing the depolyemrization ratio and yield of monophenols.The addition of mineral salt facilitated transferring the depolymerized products into ethyl acetate phase.When potassium pyrophosphate was added into the biphasic system,the yield of monophenols increased efficiently,obtaining 15.87%under the optimal conditions,whereas the yield was 7.89%was obtained in the aqueous phase system.Three kinds of lignin were used for the hydrogenolysis reaction of the biphasic system.Elemental analysis,Infrared spectroscopy,~1H-NMR,Py-GC-MS,GPC tests were used to analyze the structure of the three lignin.The results indicated that eucalyptus lignin and lignin provided by Sigma CO.were mainly composed of S and G type monomer,respectively.However,the contents of S and G monomers have the similar value in bamboo pulp lignin.The yield of monophenolic compounds was higher in biphasic system than in the aqueous system alone for all the three kinds of lignin.While the hydrogenolysis of S-type lignin in biphasic system produced a relatively high yield of monophenols,which was 2.20 times higher than that in the aqueous phase alone.