Catalytic Performances Of GQDs/Au And NGO/CoMn₂O₄ Nanocomposites In Oxidative Cleavage Of β-o-4 Constitutional Unit In Lignin Models

Lignin is a kind of bio-macro-molecules that is generated by plants to maintain their macroscopic structure.In general,lignin consists of three kinds of substituted phenylpropanol units that are cross linked by C-C and C-O bonds.As biomass resources for aromatic compounds,lignin has been widely studied recently.However,the aromatic rings in lignin are linked by strong covalent bonds,and it is hard to generate small aromatic molecules which are even useful.On the other hand,billion tons of underutilized lignin is returned to nature each year in waste liquid or solid,causing certain environmental problems.It has been illustrated that catalytic oxidation should be one of the most efficient protocol to cleaving the linkages in lignin,which is the key to its valorization.Herein,the complexes of GQDs/Au and NGO/CoMn₂O₄ are assembled and their catalytic activities for the oxidation cleavage of the most commonβ-O-4 linkage within lignin models are studied.The main results are as follows:1.Preparation and purification of GQDs.The GQDs used in the work is prepared through photo-Fenton reaction of graphene oxide sheets.The as-obtained GQDs contain a large number of peripheral carboxylic groups which ensure them to move towards the anode in electrostatic field in agarose or polyacrylamide gel.Thus,the GQDs are separated within the gels and form certain color stripes under UV light.The agarose or polyacrylamide gel is cut in different stripes according to their colors under UV light,and the GQDs within are then recovered.The GQDs from different stripes of the gel are of different lateral size.As a result,GQDs with different sizes are acquired,which are also purified in the process.2.Preparation of GQDs/Au and its catalytic activities for selective oxidation of 1-phenylethane-1,2-diol.GQDs are first neutralized by NaOH and then introduced to gold nanoparticles（AuNPs）colloid to form GQDs/Au as complex.AuNPs can selectively oxidize the C_α-OH of1-phenylethane-1,2-diol,a monomer model of lignin,and the selectivity of the product 2-hydroxy-1-phenylethan-1-one is up to 92.5%under certain temperature,pH and O₂ pressure.In contrast to AuNPs,GQDs/Au is even more effective.On the other hand,it is also illustrated that the yield of benzoic acid is no more than 30%when O₂ is applied as oxidant,but can reach 85%when H₂O₂ is applied,indicating that H₂O₂ might also play a role in the C-C bond cleavage reaction.3.Oxidative cleavage ofβ-O-4 linkage with GQDs/Au as catalysts.Using GQDs/Au as catalysts,theβ-O-4 linkage in a series of lignin models can be cleaved with H₂O₂ as oxidant under certain pH and temperature.It is also found that the oxidative cleavage undergoes stepwisely.First,the C_α-OH bond is oxidized by AuNPs;next,both AuNPs and GQDs catalyze the oxidative cleavage of C_α-C_βbond.Since GQDs not only stabilize AuNPs but also participate in the catalysis process,playing multiple roles in the whole reaction,GQDs/Au are more effective than some other AuNPs based nanocomposites.GQDs/Au complex should be practically useful for catalyzing the oxidation cleavage ofβ-O-4 linkage.4.Oxidative cleavage ofβ-O-4 linkage by NGO/CoMn₂O₄ composite as catalysts.NGO/CoMn₂O₄ composite is designed and fabricated through two hydrothermal reactions.It is further demonstrated that NGO/CoMn₂O₄can catalyze effectively the oxidation cleavage ofβ-O-4 linkage under O₂atmosphere in neutral（pH 7）buffer.It is also found that during the C_α-OH oxidation,CoMn₂O₄ and NGO show significant synergistic effects.Next,for the oxidative C_α-C_βbond cleavage,it is mainly catalyzed by NGO.Additionally,in contrast to NGO/Co₃O₄,NGO/CoMn₂O₄ is more effective,and it should be beneficial for developing novel catalytic systems for lignin valorization.