| Depolymerization of lignin by chemical routs to generate aromatic compounds is the most direct and economical method for the high-value utilization of lignin.However,the complex structure of lignin makes its catalytic conversion process still a challenge.Reductive depolymerization of lignin,a converse process of lignin biosynthesis,is one of the most efficient methods to produce phenols from lignin,taking advantage of the resulting products in high yields and in a narrow distribution form.In this reaction process,which has been studied for a long time,usually use a heterogeneous catalyst based on late transition metals,including Ru and Pd.The deep understanding of lignin hydrogenolysis mechanism is great of importance and interest for not only the development of novel catalysts but also the regulation of products selectivity.However,due to the lignin units are connected through different C-C and C-O bondsd disorderly as well as the multipaths of the ?-O-4 motif cleavage and subsequent stabilization processes,it is an extreme challenge to elucidate wood hydrogenolysis mechanism in a clear fashion.In this paper,a series of lignin models including deuterated and non-deuterated compounds are selected as the research materials.The mechanism of different late transition metal catalyzed lignin hydrogenolysis has been studied by the qualitative and quantitative analysis of the degradated products.The results of the lignin model cmopounds hydrogenolysis experience catalyzed by Ru/C indicated that the proposed "enol ether" path,the C_? carbonyl path,the radical path,and the consecutive C_?-O and C?-OH bond celeavage paths by previous literature are not account for current reactions.Meanwihile,the mechanism of a concerted hydrogenolysis of C_?-O and C_?-O bonds in lignin ?-O-4 structure synchronously was proposed.The enol ether compound was prepared and used as a substrate for catalytic degradation.The reactivity studies hinted that enol ether compound is not the intermediate for phenolmonomers in the Ru/C catalytic system.During the degradation of lignin models,the solvent(Me OH)served as hydrogen source to supply hydrogen for the cleavage of the C_?-O bond.Adding alkali to the Ru/C system of lignin hydrogenolysis can effectively control the selectivity of propyl and ethyl substituted products,and this system can also be applied to the degradation of native lignin.The reaction starts from methylquinone compounds,which was produced by the reaction of base and pheno compounds.Furthermore,base can promote the deprotonation of C?-OH and the cleavage of C_?-H bonds,which is include in the produce rout of ethyl substituted products.Meanwhile,it was proved that the vinyl ether and enol ether compounds are not the intermediate of the catalytic hydrogenolysis lignin process which contained alkali,and Ru/C can restrain the formation of thes products effectively.The application of this catalytic system to the degradation of natural lignin can lead a monomer selectivity of 44% for ethyl substituted products.An artificial lignin polymer composed of the ?-O-4 substructure,wherein the deuteriums were installed at ?,? and ? positions exclusively was prepared.Then the polymer was applied to the mechanism study of lignin catalytic degradation,which combines the degradation of oligomeric lignin model and real lignin.The results proved that in Ru/C,Pd/C and Pd/Zn/C catalytic systems,C_?-H(D),C_?-H(D)and C?-H(D)are not participate in the lignin degradation process,that is the degradation of lignin is achieved by synchronous breaking of C_?-O and C_?-OH bonds.A C_? methylene dimer was produced by side reactions during the degradation of dimer model catalyzed by Pd/C.This product has high stability and cannot be further degraded.The binding of the O atom in lignin with the metal catalyst is conducive to the catalytic hydrogenolysis of lignin.The addition of acidic metals can effectively enhance the binding of Pd/C with lignin thus promote the degradation of lignin. |
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