Metalloporphyrin As A Biomimetic Catalyst For The Catalytic Degradation Of Lignin To Aromatics Monomers

With the rapid development of industry,the demand for resource and energy is increasingly growing.Seeking alternative biomass resources and energy attracts more and more attentions due to the increasing shortage of energy and environmental pollution.Lignin is the most abundant aromatic polymers on earth.It is very significant to degrade lignin into monophenolic and further to prepare buck chemicals and liquid fuel for facilitating high efficient utilization of biomass and reducing the dependence on fossil resources.In order to provide the basis of high efficient utilization of lignin,metalloporphyrin catalysts were prepared in this work,and the resulting catalyst was used in the degradation process of lignin to produce aromatic monomers.5,10,15,20-tetraphenylporphyrin?TPP?was prepared by using Adler method.The effect of substituent groups?4-methylphenyl,4-methoxyphenyl,4-bromophenyl and4-carboxyphenyl?at the meso-benzene ring of metalloporphyrin,the central metal ion?Co,Mn,Ni and Fe?and the axial ligand?Chlorine,p-Hydroxypyridine,p-Pyridinecarboxaldehyde?of the metalloporphyrin on the degradation of lignin was investigated.It was found that the electron-withdrawing group was beneficial to both reduce the electron cloud density on the benzene ring of metalloporphyrin and promote the formation of[?Porp?Me^IV=O]^+·.Consequently,metalloporphyrin with a strong electron-withdrawing substituent contributed to a high conversion rate of lignin degradation.The manganese and iron ion of metalloporphyrin were easy to form[?Porp?Mn^V=O]^+·and inactive?-oxo dimmers(Fe ^III-O-Fe^III),respectively.Compared to Co,Ni is more difficult to form[?Porp?Ni^IV=O]^+·.By changing the axial ligand of the metalloporphyrin,it is found that the steric hindrance effect of the axial ligand and the change of the metalloporphyrin electron cloud density increase the amount of the active intermediate,and finally obtain a higher YAM.When Meso-5,10,15,20-tetrakis?4-bromophenyl?porphyrin cobalt-4-hydroxypyridine?CoPPyOH?was used the catalyst for degrading lignin,the yield of aromatic monomers increased greatly from 5.6%to 24.3%.Considering the bond between ligand and the central metal are not stable enough,the metalloporphyrins with Chlorine axial ligand was used to improve the strength of the bond between ligand and the central metal.Therefore,Meso-5,10,15,20-tetrakis?4-bromophenyl?porphyrin cobalt-4-chlorine?CoTBrPPCl?contributed to a better lifetime of the catalyst,and the catalytic activity remained almost unchangeable after 5 times-use.The optimal degradation of lignin process variations by using CoTBrPPCl as the catalyst were as follows:0.1 g of lignin,20 mL of NaOH solution?1 mol/L?with 2 mL of isopropyl alcohol,and 0.02 g of catalyst were carried out at 150 ^oC for 60 min under microwave,contributing to a 91.1%of lignin degradation ratio and 20.1%of aromatic monomers yield.Compared to the tranditional catalyst,CoTBrPPCl was favorable for improving the yield of aromatic monomers?YAM?greatly.Combined with the selectivity and yield of aromatic monomers obtained from lignin degradation,changes of functional groups and chemical bonds in lignin degradation residues and the mechanisms obtained from different types of lignin models??-O-4 and 5-5'?degradation,it could be speculated that metalloporphyrins mainly act on lignin to promote the breaking of each connecting bond through reactive intermediates[?Porp?Co^IV=O]^+·to carry out aromatic nucleation of lignin structural units and the oxidation of the side chain,thereby increasing the YAM.The mechanisms provide theoretical guidance for increasing YAM.