Application Of Cerium-Based Catalyst In Catalytic Depolymerization Of Lignin

As a natural resource rich in aromatic units,lignin can be considered as a substitute for aromatic hydrocarbons produced by non-renewable petroleum.The new catalytic oxidation and hydrogenolysis routes was developed based on the conversion of lignin model compounds?dimer ?-O-4 model?into aromatic compounds;Catalysts were designed for lignin to oxidize and deoxidize by liquid reforming under mild conditions,and the two-step depolymerization strategy for lignin was proposed.This article contains the following parts:???Under mild conditions,catalytic conversion of lignin model molecules and lignin into aromatic monomersIn the chapter 3,heterogeneous catalysts?Au/CeO₂?utilizing ceria supported gold nanoparticles?average particle size is about 5 nm?are designed.Further experiments show that Au/CeO₂ can divide lignin model molecule?2-phenoxy-l-phenylethanol denoted as PP-ol?into phenol,methyl benzoate and some acetals under mild conditions.The morphology,structure and composition of Au/CeO₂ were tested and analyzed by TEM,SEM,XRD,BET and XPS.Combined with the above test characterization results,the relationship between structure and performance was analyzed,and the reaction mechanism of PP-ol catalytic depolymerization was further speculated.?1?First,an evaluation experiment was carried out on the oxidative depolymerization of model molecule PP-ol,and CeO₂ was selected as the best catalyst carrier.Then,this reaction path was applied to other lignin model molecules.The test results show that the lignin model molecule containing methoxy group?CH₃O-?has a higher conversion rate of catalytic depolymerization.Further analysis revealed that the presence of electron-donating groups?such as CH₃O-?weakened the strength of ?-O-4 link located in lignin organic structure,promoted the depolymerization of lignin,and favored the formation of aromatic compounds.It is of research significance that natural lignin is usually rich in various electron-donating groups,especially CH₃O-.?2?In addition,for giving the optimal loading of precious metal Au in Au/CeO₂ catalyst,Abundant experimental research were also conducted in this paper.The results show that the catalytic effect is the best when the Au loading is 0.88 wt%.?3?In the Au/CeO₂ catalytic system,lignin was depolymerized at 180? for 4 hours using methanol as the solvent.Finally,vanillin,methyl vanillate and methyl syringate were obtained and their corresponding monomer yields were 10.5 wt%,6.8 wt%and 3.4 wt%.?4?Through the research in this part,not only were innovated the reaction path and catalyst performance evaluation methods,but the key reaction steps and mechanisms in this reaction path were studied as well.First,molecular oxygen is activated by negatively charged gold nanoparticles to form a peroxide or superoxide,which promote the activates of C-H bond.Subsequently,the oxidation process is completed through a series of deprotonation and elimination steps.That is,the adsorbed molecule O₂ obtains electrons from Au/CeO₂ to generates Au/CeO₂-O₂·-,and then reacts with PP-ol to form 2-phenoxy-1-phenylethanone?abbreviated as PP-one?.???Developing the cheap,stable and efficient catalyst for converting of ligninIn the chapter 4,a series of cyclic tests and characterizations were performed using different rare earth oxides as active sites.The results show that carbon-supported cerium oxide is the best catalyst to improve the selectivity and stability of the aromatics among all products,and CeO₂/C shows the highest reactivity.On the one hand,due to the high surface effect of CeO₂/C,the effective mass transfer process is enhanced to exhibit high reactivity.On the other hand,as electron-rich carrier,the activated carbon is more beneficial to electron transfer and thus exhibits high activity.?1?A two-step strategy was explored in the conversion of lignin and model compounds.Considering that ?-O-4 ketone has a lower bond energy of about 40-50 kJ/mol than ?-O-4 alcohol,the pre-oxidizing C_?-OH of ?-O-4 alcohol to C_?=O can more easily achieve the depolymerization of lignin.In the fourth chapter,during the subsequent hydrogenation process of C_?-O-ph ether bond,it was found that the prepared CeO₂/C catalyst can achieve reductive cleavage of C_?-O-ph ether bonds selectively under the inert gas atmosphere.Moreover,it is suitable for a variety of substituted C_?-O-ph ether bond compounds,showing high activity and selectivity.?2?In the meanwhile,the study on mild acid hydrolysis of lignin from different sources?pine,cotton stalk,birch and poplar?has been performed.The results show that:under this catalytic system,the carbon balance is maintained at about 95%,and shows a high monomer yield;lignin from different sources shows different yields,and the total monomer yield reaches 12.1-32.6 wt.%.By discussing the changes of the reaction temperature,solvent system and reaction time separately,it was found that in the methanol system,as the reaction time increases or reaction temperature rise,the monomer yield all increases first and then decreases.And when the reaction time reached 6 hours,unsaturated aromatic compounds showed the highest selectivity.?3?For the complex lignin system,FT-IR,GPC and GC-MS were used to analyze the structure,molecular weight and product quality of lignin.By means of the evaluation experiments on lignin and its models,the reaction path of lignin is analyzed,which provide a reference for the subsequent product up-gradation of lignin.