Depolymerization Of Lignin Over Ni Based Catalysts To Fuel And Chemicals

Lignin is a key component of lignocellulose,which mainly composed of cellulose,hemicellulose and lignin.Different from cellulose and hemicellulose,lignin contains aromatic monomers and hence it is suitable to replace fossil resources for production of aromatic compounds.Corn cob and straw are abundant agricultural by-products.Nowadays,biorefinery strategies achieves the conversion of cellulose and hemicellulose in biomass into ethanol,accompanied with the production of large-volume waste enzymatic hydrolysis lignin（EHL）.This work focuses on the depolymerization of EHL and lignin in native corncob,investigating the process of lignin depolymerization reaction,role of catalyst sites and up-grading of products in detail,and try to promote the utilization of lignin for production of high value-added compounds and high-density fuels.A number of unsupported nickel-based catalysts were prepared from the decomposition of nickel formate,and were employed for the depolymerization of enzymatic hydrolysis lignin（EHL）in a batch reactor at 280 ^oC for 6 h with 2 MPa hydrogen in ethanol.Among the samples,Ni（220H）,prepared from the decomposition of nickel formate at 220 ^oC in hydrogen,achieves the complete liquefaction of EHL and the highest monomer yield of 28.5%.The para-alkyl side chains of phenolic products are independent on the solvent used while the structures of aromatic esters are closely related to the solvent used.The 2D-HSQC NMR analysis of liquid products reveals that both the ether and C-C linkages in EHL are cleaved during the reaction,and the ether linkages are completely cleaved at mild reaction conditions,while the cleavage of C-C linkages needs harsh reaction conditions.Further increase the reaction temperature to 300 ^oC or decrease the initial hydrogen pressure to 0 MPa is conducive to the repolymerization reaction.The depolymerization of enzymatic hydrolysis lignin（EHL）in ethanol over Ni catalysts on different supports,i.e.,SiO₂,Al₂O₃,Mg O,TiO₂ and ZrO₂,were examined in a batch reactor at 280 ^oC for 6 h under 2 MPa H₂,and the roles of catalytic sites were discussed.Ether linkages in EHL are cleaved through non-catalytic homolysis reaction,and the fragments and monomers formed are further converted over the catalyst.Hydrogenation of active monomers and intermediates over the metal sites of catalysts plays the key role for suppressing repolymerization reaction,while the acid and base sites of catalysts both promote alkylation and repolymerization reactions.Among the catalyst samples examined,Ni/SiO₂ shows the highest hydrogenation activity and obtains the highest total monomer yield of 24.7 wt%.The process of reductive catalytic fractionation（RCF）of corncob with Ni/SiO₂as a catalyst in methanol was investigated.The delignification step mainly achieves through non-catalytic solvolysis,while the catalyst is responsible for the lignin depolymerization.Compared with the catalyst prepared from incipient wetness impregnation（i-Ni/SiO₂）,the catalyst prepared from precipitation-deposition method（d-Ni/SiO₂）shows a higher activity for lignin depolymerization.After the RCF reaction with d-Ni/SiO₂at 220 ^oC for 6 h under 2 MPa H₂,90.3 wt%of the entire lignin in corncob is extracted and 24.3 wt%yield of aromatic monomers is obtained,with well-preservation of carbohydrate components in solid residue.The delignification step is basically achieved within 2 h,while lignin depolymerization step gradually undergo during 6 h.The increase of the reaction temperature enhances the delignification step,but also significantly promots the sugar dissolution.Aromatic esters are produced from the cleavage of ester linkages,and are readily formed even without a catalyst,while para-propanol and para-propyl phenols form from the cleavage of ether linkages in lignin and are detected only when a catalyst was present.The hydrodeoxygenation of guaiacol was examined with Ni₂P/SiO₂as a catalyst in n-pentane.At 300 ^oC under 1 MPa H₂for 5 h,the conversion of guaiacol is up to 100mol%,and the yields of cyclohexane,benzene and anisole are 68.2 mol%,13.7 mol%and 1.6 mol%,respectively.When the reaction temperature is reduced,guaiacol conversion decreases but the hydrogenation reaction of benzene rings is promoted.When the H₂ pressure is reduced,guaiacol conversion decreases and the HDO reaction of guaiacol is also suppressed,while the alkylation of benzene rings and etherification reaction of phenolic hydroxyl are promoted.Based on the results of guaiacol conversion at different reaction time and the conversions of phenol,anisole and benzene,we proposed that guaiacol HDO reaction underwent through two reaction processes simultaneously,i.e.,（a）benzene ring in guaiacol is firstly hydrogenated and then the products undergo deoxidation and（b）deoxidation reaction of guaiacol firstly occurs and then the benzene ring in products are hydrogenated.Ni₂P/SiO₂also achieves the completed conversion of syringol,para-ethyl guaiacol and methyl ferulate,yielding benzene,cyclohexane or their alkyl substituted derivatives as the main products,at the same reaction conditions.Nevertheless,Ni₂P/SiO₂ is not efficient for the HDO of lignin oil obtained from EHL depolymerization.