Preparation Of Supported Ni-based Catalyst And Its Catalytic Hydroconversion Of Lignin

Lignins,as by-products of paper industry,are high-volume products in China.Most of them are directly burned or abandoned as industrial wastes,resulting in wasting of resources and environmental pollutions.With increasing shortage of resources and awareness of human environmental protection,how to effectively utilize lignin as a natural and renewable resource has become a subject of great economic value and far-reaching social significance.In the process of lignin utilization,the research and development of good performance catalyst is an important way to optimize the lignin conversion technology.In this study,four supported Ni-based catalysts（Ni/AC,Ni/HZSM-5,Ni/Hβ,and Ni/HY）were prepared by impregnating Ni（CO）₄ onto activated carbon（AC）,HZSM-5,Hβ,and HY followed by decomposing Ni（CO）₄,respectively.A series of characterizations for the above four catalysts were performed,including analyses with transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）,X-ray diffraction（XRD）,N₂ adsorption/desorption and NH₃ temperature programmed desorption（NH₃-TPD）.The characterization results show that metal Ni was effectively loaded on the surface of four kinds of supports.Compared with AC,HZSM-5,and HY,Hβhas a large specific surface area and a highest total pore volume,which facilitates the dispersion of metal Ni on its surface.There exists interaction between Hβand metal Ni.Hydrodeoxygenation of phenol in n-hexane and catalytic cracking of benzyloxybenzene（BOB）in methanol were used as probe reactions to investigate the activities of the four Ni-based catalysts under the same reaction conditions.The results show that Ni/Hβis much more active for hydrodeoxygenation of phenol in n-hexane and catalytic cracking of BOB in methanol than other three Ni-based catalysts.Based on the characteristics of large molecular structure of lignin,two groups of compounds were selected as lignin-related model compounds to investigate the hydrodeoxygenation mechanism over Ni/Hβ.One group compounds are monocyclic phenolic compounds,including phenol,p-cresol,anisole,and guaiacol.The other group compounds are bicyclic phenolic compounds,including BOB,phenethoxybenzene（POB）,oxydibenzene（ODB）,and benzofuran.Meanwhile,bicyclic phenolic compounds,including BOB,POB,and ODB,were selected as lignin-related model compounds to investigate catalytic cracking mechanism over Ni/Hβ.The results show that both monocyclic and bicyclic phenolic model compounds can be effectively converted into cyclanes by hydrodeoxygenation reaction.Hydrogenation of benzene ring catalyzed by Ni/Hβis a precondition for hydrodeoxygenation of model compounds.The protons released from Ni/Hβattack on the oxygen atom,leading to the cleavage of C_alk-O bond,which could be the important step for hydrodeoxygenation of model compounds.The main products are monocyclic cyclanes from hydrodeoxygenation of monocyclic phenolic compounds.The hydrodeoxygenation of monocyclic phenolic compounds to monocyclic cyclanes over Ni/Hβmainly proceeds via benzene ring hydrogenation,dehydroxylation or demethoxy,and abstraction of H^-from Ni/Hβsurface.The main products are bicyclic cyclanes from hydrodeoxygenation of BOB,while the main products are monocyclic cyclanes from hydrodeoxygenation of POB,ODB and benzofuran.The hydrodeoxygenation of POB and ODB to monocyclic cyclanes proceeds via hydrogenation of benzene ring,C_alk-O bond cleavage induced by H⁺addition,dehydroxylation,and H^-abstraction.The hydrodeoxygenation of benzofuran to monocyclic cyclanes proceeds via full hydrogenation of benzene ring and oxygen heterocycle,C_alk-O bond cleavage induced by H⁺addition,dehydration,and H^-abstraction.The hydrodeoxygenation of BOB to bicyclic cyclanes proceeds via C_alk-O bond cleavage induced by H⁺addition to produce phenol and benzylium,the attack of benzylium on phenol to produce benzylphenol,hydrogenation of benzene ring in benzylphenol,dehydration,and H^-abstraction.Both POB and BOB can be completely cracked by catalytic cracking reaction except ODB.The protons released from Ni/Hβattack on the oxygen atom,leading to the cleavage of C–O bond,which could be the crucial step for catalytic cracking of model compounds.The stability of cations generated from the cleavage of C–O bond determines the difficulty of catalytic cracking of model compounds.The protons transfer to the oxygen atom in POB and BOB lead to the cleavage of C_alk-O bond to produce benzylium,2-phenyleth-1-ylium,and phenol.Benzylium and 2-phenyleth-1-ylium can abstract H^-from Ni/Hβsurface to produce toluene and ethylbenzene.They can also attack phenol to produce benzylphenol and phenethylphenol.Catalytic hydrodeoxygenation of lignin over Ni/Hβwas used to produce cyclanes.The effects of reaction conditions on the conversion of lignin hydrodeoxygenation and distribution of liquid products were investigated.Increasing the temperature and prolonging the reaction time are beneficial to the improvement of the conversion of lignin hydrodeoxygenation and the increase of the content of cyclanes detected in liquid products.Possible pathways for catalytic lignin hydrodeoxygenation were revealed based on the mechanism for hydrodeoxygenation of monocyclic and bicyclic phenolic model compounds and the composition analysis of liquid products from lignin hydrodeoxygenation,which providing provides a reference for the directional conversion of lignin to naphthenic hydrocarbon fuels.Catalytic craking of lignin over Ni/Hβwas used to produce aromatic compounds.The effects of reaction conditions on yields of lignin craking products and the distribution of liquid products were investigated.Increasing the IHP reduces the total yield of aromatic compounds and the yield of liquid products.Increasing the temperature and prolonging the reaction time properly are beneficial to the improvement of total yield of aromatic compounds and the yield of liquid products.Possible pathways for catalytic craking of lignin were revealed based on the mechanism for catalytic cracking of bicyclic phenolic model compounds and the composition analysis of liquid products from lignin craking,providing a reference for the directional depolymerization of lignin to aromatic fine chemicals.