Conversion Of Lignin To Alkane Fuel Catalyzed By Rhenium-based Catalyst

Lignin is the only renewable natural carbon resource that possesses abundant production of aromatics and alkane fuel,since lignin is an amorphous aromatic polymer widely existing in plants,and its molecular structure contains oxyphenylpropanol or its derivative structural units.The efficient transformation of lignin into aromatic chemicals and alkane fuel not only remain huge challenge but have important scientific value and practical significance for the full utilization of biomass resources due to the variability and the complexity of its irregular structure.In view of the stubborn chemical properties of lignin,a new catalytic system of selective depolymerization of lignin and lignocellulose to aromatic chemicals or liquid alkane fuel over Ir-ReO_x/Si O₂ and Ni-ReO_x/CeO₂ catalysts was studied in this study.The main results are shown as followed:First,different lignin model compounds were converted to naphthenes at 260 ^oC and 2 MPa using Ir-ReO_x/Si O₂ as catalyst and n-hexane as solvent,which is prepared by two-steps wetness impregnation.Tranformation of realistic lignin(organosolve lignin,industrial enzymolysis lignin and commercial alkaline lignin)to obtain hydrocarbons with carbon yield of 23%to 40%.BET,TEM,NH₃-TPD,H₂-TPR,Pyridine-IR,XPS and XAFS were used to characterize the catalyst,revealing acid site,reactive species and the synergistic effect of Re and Ir in the bimetallic catalyst.The changes of C,H,O elements and linkage in lignin before and after the reaction were determined by elemental analysis and 2D HSQC NMR,which indicated that lignin could be convered to naphthenes by Ir-ReO_x/Si O₂.Second,Ni-ReO_x/CeO₂ was prepared by wetness impregnation,this catalyst showed excellent activity for the swichablely catalyzing various lignocellulose to aromatic compounds and hydrocarbons under different reaction conditions.The yield of bio-oil was 14-21%and the monomer yield was 2-6%under mild conditions.In addition,under relatively harsh conditions,realistc lignin was hydrodeoxygenation to alkanes with monocycle yield of 9-12%,and other liquid products yield was 2-3%.In order to verify that most products of lignocellulose under mild conditions are from lignin,we transformed pure realistic lignin under the same conditions.The yield of bio-oil was 72-89%and the monomer yield was 7-17%.In addition,under relatively harsh conditions,realistic lignin was hydrodeoxygenation to alkanes with a carbon yield of 27-41%and a monomer yield of 8-17%.Lignin model compounds(?-O-4,?-O-4)were used as substrates to explore the reaction pathways and mechanism under the two pathways.STEM,NH₃-TPD,H₂-TPR,CO pulse and XPS were used to characterize the catalyst,revealing acid site,reactive species and the synergistic effect of Re and Ni in the bimetallic catalyst.The changes of C,H,O elements and linkage in lignocellulose and lignin before and after the reaction were determined by elemental analysis and 2D HSQC NMR,which indicated that the bimetallic Ni-ReO_x/CeO₂ can swichablely catalyze lignocellulose and lignin to aromatic compounds and hydrocarbons under different reaction conditions.