Preparation Of Nitrogen-doped Porous Carbon Supported Metal Catalyst And Its Application In Hydrogenolysis Of Lignin

Lignin is a renewable aromatic polymer composed of phenylpropane structural units,which is regarded as the most important candidates to substitute foosil feedstock for the production of aromatic monomers due to its abdundant sources.However,it has been not fully utilized because of its complex structure and lack of effective degradation methods.The liquid-phase catalytic depolymerization is regarded as the most effective way to convert lignin into aromatic monomer compounds.Therefore,it is of importance to develop metal-based catalysts with high efficiency,low cost and good stability to depolymerize lignin by selectively cleaving C-O bonds for facilitating the high valued utilization of lignin.The metal-based catalysts have been applied in the lignin depolymerization,whereas the process suffers from more or less drawbacks such as catalyst deactivation and low yield.In this work,the N-doped carbon supported Ru and Ni-N-C single atom catalyst were designed to address these issues.Combined with desity functional theory(DFT)calculation,the mechanism of C-O bond cleavage in lignin was deeply analyzed.The high-performance N-doped carbon supported Ru catalyst was prepared in situ by a feasible two-stage pyrolysis of a mixture of glucosamine hydrochloride(GAH),ruthenium trichloride and melamine for the hydrogenolysis of lignin.The sandwich-like structure of g-C₃N₄ and the carbon skeleton,which were formed from the thermal condensation of melamine(nitrogen precursor)and GAH(carbon precursor)at low temperature,respectively,inhibiting the growth of Ru NPs and thus enabled to the formation of well-dispersed ultrafine Ru NPs.The thermal decomposition of carbon nitride at high temperature could generate the well-wrinkled,defect-rich,mesoporous carbon structure,where the N heteroatoms were incorporated into the graphitic carbon texture and thus significantly increased the specific surface area of the catalyst.The strong metal-support interaction due to the formation of Ru-Nx chemical bonds also contributed to the formation of the highly dispersed Ru NPs and the change of the Ru electronic structure.Hence,the high activity of Ru@NCM-800 catalyst for lignin depolymerization originated from a combination of high specific surface area,defect-rich mesoporous structure,and highly dispersed and small-sized Ru NPs.This high-performance Ru@N-doped carbon catalyst displayed a record high yield of 30.5%aromatic monomers from the hydrogenolysis of lignin in aqueous ethanol solvent under relatively mild condition(300oC,120 min and 1.0 MPa of H₂).Despite that the commercial Pd-or Ru based catalysts exhibited fair performance in lignin depolymerization,the high price and overhydrogenation of aromatic ring restrict their applications.Instead,with the facile synthesis and relatively low cost,the N-doped carbon supported Ru catalyst developed in this work has the potential for large-scale production of aromatic monomers from lignin.N-doped carbon support was obtained by direct pyrolysis of melamine and glucosamine hydrochloride.The morphology,specific surface area,nitrogen-dopant species and content could be regulated by varying the weight ratio of melamine to GAH.The nitrogen content increased and the pyridinic N became dominant by increasing the weight ratio of melamine to GAH.When the weight ratio of melamine to GAH reached 10:1,the highly-dispersed and small-sized Ru NPs immobilized N-doped carbon catalyst(Ru/NCM-10)was synthesized.The optimal catalyst(Ru/NCM-10)could obtain 40.7%aromatic monomers for the hydrogenolysis of lignin.The result showed that there was an obvious electronic interaction between N and Ru NPs.The presence of pyridinic N not only served as metal coordination sites to stabilize and disperse Ru NPs but also enhanced the proportions of Ru⁰.Density functional theory(DFT)calculation showed that the pyridinic N could strongly adsorbed PPE molecule and reduced the dissociation energy of C_alkyl–O bond in PPE.The superior activity of Ru/NCM-10 could be attributed to a combination of strong adsorption,reduction of the dissociation energy of C_alkyl-O bond in PPE and the synergistic effects between N-doped carbon support and Ru NPs,leading to the high aromatic monomers yield.A N-doped hierarchical porous carbon support(LNPC)was prepared by directly pyrolyzing alkali lignin(carbon source),potassium hydroxide(acticator)and ammonium oxalate(nitrogen source)in order to solve the problems that lignin macromolecules cannot effectively contact the active sites of catalyst.It was found that the LNPC afforded a large specific surface area(1824 m² g^-1),and the micro-meso-macroporous structure was interconnected with the marcropore size of 50 nm to 10?m.The yield of aromatic monomer compounds reached 45.6%when the catalyst that 1.5 wt%Ru content loading on the LNPC was used in the hydroegnolysis of lignin.And the catalytic mechanism of Ru supported on hierarchical porous carbon was proposed by comparing the relationship between the pore size of the macropore in the LNPC and the particle size of the lignin.Lignin macromolecules preferred to enter macropores of support and contact the metallic active site,breaking the peripheral ether bonds of lignin to generate lignin fragments with smaller molecular weight or some soluble oligomers.Therefore,the resulting lignin fragments could enter the mesopores and micropores,improving the rate of mass transfer,facilitating the contact between lignin fragments and metallic active sites.Consequently,lignin depolymerization was enhanced to generate aromatic monomers via the cleavage of C-O bonds.Considering the high cost of noble metallic catalyst,a Ni-N-C single atom catalyst(Ni-N-C SAC)with high Ni loading was developed for the efficient conversion of lignin in order to reduce the coat of lignin hydrogenolysis.The Ni-N-C SAC was synthesized by a chelation-anchored strategy,where the nickel ion was first chelated with a chelating agent containing oxygen-containing species or amino groups to form Ni-GAH complexes.The high-temperature pyrolysis resulted in the the coordination with Ni and N,forming atomically dispersed Ni active sites.Based on the results of synchrotron radiation and DFT calculations,the catalytically active site was identified as the Ni-N₄ structure.Compared with Ni nanoclusters,the atomically dispersed Ni sites have a better activation capacity for hydrogenation.The Ni-N-C SAC has good catalytic activity for lignin depolymerization,reaching 36.5%of yield aromatic monomers.