Study On Cleavage Of 4-O-5 Linkage In Lignin Via Uranyl Photocatalysis

Lignocellulosic biomass,an organic polymer formed by photosynthesis of green plants,is a rich renewable resource in nature.The recovery of aromatic compounds from lignin,the only natural plant resource containing benzene rings,not only makes the best of renewable resources,but also helps alleviate environmental pollution.Lignin is polymerized from aromatic monomers through C-C bonds and C-O bonds.Among C-O bonds,cleavage of 4-O-5 linkage is most challenging owing to its higher bond dissociation energy（BDE=77.74 kcal/mol）in comparison toα-O-4（BDE<57.28kcal/mol）andβ-O-4（BDE<69.35 kcal/mol）.Two primary methods to recover aromatic compounds from 4-O-5 linkage are hydrogenolysis and hydrolysis.A temperature above 120℃and strong base are mostly required when executing the former one,and transition metal catalysis is also needed in some cases.Meanwhile,hydrogen as a typical source applied widely in early hydrogenolysis is accompanied by danger.Hydrolysis of 4-O-5 linkage is thermodynamically challenging with the reaction energy of-7.1 kcal/mol compared with-14.2 kcal/mol in hydrogenolysis,which explains why most of the reported hydrolysis occurred at temperatures above300℃with the assistance of acid or base,and some hydrolysis even required high pressure.Limitations in safety,energy saving,environmental protection and economic efficiency lie in both hydrogenolysis and hydrolysis.Based on previous research,uranyl-photocatalyzed hydrolysis of diaryl ethers has been developed under normal temperature and pressure,with the unique properties of the excited uranyl cation including high oxidation potential（E^o=+2.60 V vs SHE）and gently water-splitting ability.Mechanistic studies with radical quenching experiments,ultraviolet-visible absorption experiments and Stern-Volmer analysis demonstrated that the excited state of uranyl cation(*UO₂²⁺)produced under blue light was quenched by diaryl ether via single electron transfer.Oxygen labeling experiment showed that the extra oxygen in phenol products came from water,which was splitted during the process of self-polymerization of uranyl cation to uranyl peroxide complex.And the following depolymerization promoted the oxygen atom transfer from water to the phenols.Guided by this strategy,we successfully broke the C_sp2-O bond of two 4-O-5 lignin template substrates,as well as nitrofen and triclosan,marking the establishment of a mild and effective method for the degradation of 4-O-5 lignin structure and pesticides with similar structures.The potential industrial application of this strategy was further demonstrated by flow operation.