Selective Oxidation Of Biomass Platform Molecules Over Cobalt-based Catalysts

Nowadays,social development relies too much on non-renewable fossil resources,which leads to energy shortage and environmental deterioration,which is not conducive to the long-term development of human society.In this situation,we urgently need to build a sustainable development system of resource and energy,and in a series of relevant measures,reasonable development and effective use of renewable resources is particularly important.Biomass resource is a renewable source of organic carbon.Rational utilization of biomass resources can produce various clean fuels and clean chemicals,and has broad application prospect.Lignocellulosic biomass,the most widely available and cheapest,is rich in oxygen-containing groups(hydroxyl and carbonyl groups,etc.).To take full use of this advantage of structure,we can oxidize it into renewable organic carboxylic acids and their derivatives(carboxylic esters,etc.)to improve atomic economy.Among numerous heterogeneous metal-based catalysts used for oxidation reactions,low-cost cobalt-based catalysts are more capable of activating oxygen and thus contributed to activate substrates.On this basis,we designed and synthesized cobalt-based catalysts for the oxidation of lignocellulosic biomass-based molecules to obtain organic carboxylic acids and carboxylic esters with high yields.At the same time,we used a series of characterization methods to clarify the composition,structure and electron interaction of catalyst,and further clarified the relationship between the structure and performance of the catalyst to provide reference for related research work.In the first work,we prepared metal oxide supported cobalt oxide hydrate(Co(OH)x)for the C(OH)-C bond oxidative cleavage in alcohols.It is well known that the structure of lignin contains a large number of C(OH)-C bonds,which can be converted into renewable aldehydes,carboxylic acids and ester compounds through oxidative cleavage of C(OH)-C bonds.This work provided an available catalytic system.85%benzoic acid was obtained at 150℃ under atmospheric oxygen for 12 h without any additives,when diphenylethanol was the model substrate and Co(OH)x/ZnO was the catalyst.According to the characterization results,we gave the reasons for the better performance of Co(OH)x/ZnO:(1)the actual loading of cobalt was consistent with the theoretical value and the distribution was uniform;(2)the vast majority of cobalt species were high-priced cobalt with better oxidability;(3)the basicity was strong,which promoted the process of dehydrogenation oxidation.In addition,the catalytic system,was suitable for lignin β-O-4 model compounds,which had the potential of practical application.In the second work,we synthesized N-doped carbon modified with cobalt and manganese catalyst CoMn@NC for the oxidation of 5-hydroxymethylfurfural(HMF)to dimethyl furan-2,5-dicarboxylate(DMFDCA).Renewable DMFDCA was an ideal substitute for the petroleum-based compound terephthalic acid(TPA)when used as polymer monomer.85%DMFDCA was obtained at 50℃ under atmospheric oxygen for 12 h,when Co3Mn2@NC-800 was the catalyst.According to the characterization results,we gave the reasons for the better performance of Co3Mn2@NC-800:(1)the specific surface area of catalyst was very large,which improved the mass transfer efficiency of the reaction;(2)metal coordinated to nitrogen to form M-Nx,which had strong adsorption and activation capacity for O2 and was one of the main active sites;(3)the electron-deficient metal NPs tended to capture hydrogen atoms in the substrate structure and activate O2 to superoxide radical anion(·O2-),thus promoting the process of dehydrogenation oxidation;(4)appropriate amount of pyrrole nitrogen and pyridine nitrogen promoted the process of hydroxyl condensation and dehydrogenation oxidation respectively;(5)there were more defects in the carbon matrix,which increased the active sites.The catalyst was stable and could be expanded to various substituted aromatic alcohols.This catalytic system had applicable potential for the production of carboxylic esters for polymers.