Cooperative Depolymerization Of Lignin By Br(?)nsted/Lewis Acid Coupled Hydrogenation Sites

Biomass has always been favored by scientific researchers as the only renewable resource that can be directly converted into liquid fuels and chemicals.Lignocellulose is an important biomass resource.Lignin is an important component of lignocellulosic.It is the only renewable resource in nature that can directly provide aromatic compounds.Lignin has the advantages of low oxygen content and high energy density.Depolymerization of lignin into aromatic monomers is an important research content of value-added lignin.However,lignin is a stable three-dimensional network polymer material,and its depolymerization process has problems such as difficulty in breaking bonds,easy polycondensation of reaction intermediates,and low product selectivity.In view of the above problems,this paper proposes a method of Br(?)nsted acid/Lewis acid coupling hydrogenation site synergistic catalysis,in situ depolymerization of corn stover lignin to produce aromatic monomers.Firstly,the research on the synergistic depolymerization of lignin by Lewis acid coupling hydrogenation sites was carried out.A nickel-tin bimetallic catalyst with strong Lewis acidity and hydrogenation activity was prepared.The catalyst was used to depolymerize alkali lignin,the yield of monomer and dimer was 60.4 wt.%at 310℃for 24 h,and the yield of monomer was 9.1 wt.%.Then the catalyst was used to depolymerize corn stover lignin,the yield of monomer was 13.0 wt.%and the removal of lignin was 35.2 wt.%at 235℃ for 3 h.Lewis acid-coupled hydrogenation sites can efficiently depolymerize lignin.The acid sites in the nickel-tin bimetallic catalysts are mainly Lewis acid sites,which are difficult to fully contact with the link bonds between lignin,cellulose/hemicellulose in corn stover.The removal effect is poor,and the monomer yield is not high.In order to improve the ability of the catalyst to remove lignin from corn stover,the NiSnP2O7 catalyst containing Br(?)nsted acid,Lewis acid and hydrogenation sites was prepared by adding phosphorus into the nickel-tin bimetallic catalyst.The catalyst was used to depolymerize corn stover lignin,the yield of monomer was 34.1 wt.%and the removal of lignin was 64.7 wt.%at 235℃ for 3 h.The depolymerization effect was significantly higher than that of nickel-tin bimetallic catalysts.The role of the Brensted acid site in the NiSnP2O7 catalyst is to promote the removal of lignin,the role of the Lewis acid site is to break the C-O bond,and the role of the hydrogenation site is to stabilize the reaction intermediate.In NiSnP2O7 catalyst,nickel species appeared in clusters(>100 nm),and the dispersibility was not good,which affected the activity of the catalyst.To this end,NiAPO-5(H2)catalyst with uniformly dispersed nickel species was prepared by doping method(<10 nm),and the catalyst was used to depolymerize corn stover lignin.The yield of monomer was 35.7 wt.%,and the removal of lignin was 61.9 wt.%.The study of the reaction mechanism showed that the role of the Br(?)nsted acid site in NiAPO5(H2)was also to break the link between lignin and cellulose/hemicellulose to promote the removal of lignin.The removed lignin fragments were depolymerizes into monomers under the action of hydrogen sites coupled with Lewis acid.By analyzing the composition of the residue,it can be found that the removal effect of NiAPO-5 catalyst on lignin needs to be enhanced,and the Br(?)nsted acid sites in the catalyst are generated by hydrogen reduction,which increases the difficulty of catalyst preparation.In order to enhance the strength of Br(?)nsted acid and simplify the generation of Br(?)nsted acid sites,NiSAPO-5 catalyst was prepared by introducing sulfur into NiAPO-5 by grinding method and solvent-free dry heat method.The catalyst was used to depolymerize corn stover lignin.The yield of monomer was 29.4 wt.%,and the removal of lignin was 68.9 wt.%at 235℃ for 3 h.Although the introduction of sulfur can enhance the strength of Br(?)nsted acid,the introduced sulfur will cover the nickel sites to a certain extent,reducing the ability of the catalyst to depolymerize lignin,and the monomer yield and selectivity will decrease.In order to improve the depolymerization ability of NiSAPO-5 catalyst,NiSnSAPO-5 catalyst was prepared by introducing tin element by grinding method and solvent-free dry heat method.The NiSnSAPO-5 catalyst exhibits excellent hydrodeoxygenation ability benefited from the strong Lewis acidity and hydrogenation activity.The catalyst was used to depolymerize corn stover lignin.The yield of monomer was 32.3 wt.%,and the removal of lignin was 69.2 wt.%at 235℃ for 3 h.At the same time,only six aromatic monomers were produced,and the monomer selectivity exceeds all the previous catalysts.Through the prepared nickel-tin bimetallic catalysts,NiSnP2O7,NiAPO-5(H2),NiSAPO-5 and NiSnSAPO-5 catalysts,the paper studied the Br(?)nsted acid/Lewis acid coupling hydrogenation site synergistic catalysis in situ depolymerization of corn stover lignin to aromatic monomers.The role of Br(?)nsted acid sites in the catalyst was mainly to remove lignin from corn stover,and then the removed lignin fragments were depolymerized under the action of Lewis acid coupling hydrogenation sites to generate aromatic monomers.The research results had reference significance for the development of lignin depolymerization methods.