Catalyst Development And Mechanism Study In Aerobic Oxidation Of P-cresols

The selective oxidative functionalization of C-H is a key technology in industry and fundamental research.From environmental and economic benefits view point,molecular oxygen as the ideal oxidant is widely used in the oxidation processes of laboratory and industrial chemical synthesis.Versatile 4-hydroxybenzaldehydes such as vanillin,syringaldehyde and 3,5-dimethyl-4-hydroxybenzaldehyde are critical raw materials in manufacturing perfumes,pharmaceuticals,polymers and dyes.The direct benzyl ic C(sp3)-H oxyfunctionalization of p-cresols to produce p-hydroxybenzaldehydes is an appealing synthetic approach.Despite the explosion of research in the area of p-cresols oxidation by various catalytic systems,there remain chanllenges,which mainly includes following aspects.First,the research in identification of true catalyst(homogeneous,heterogeneous or nano-particulate catalyst)is limited and overlooked.Second,homogeneously catalyzed oxidations of subtituted p-cresol are common and have many drawbacks such as poor selectivity,catalyst deactivation,and redundant recovery.Designing and constructing of a proper heterogeneous catalyst,which has the strengths of ease of recovery and amenability for continuous processing in p-cresols oxidation,is therefore desired and significant.Third,various mechanisms are proposed and there is still ambiguity as to the mechanism.Based on above problems,work of this paper focus on catalyst development and mechanism study in aerobic oxidation of p-cresols.In the first part,an investigation of aerobic oxidation of 2-methoxy-4-methylphenol to vanillin catalyzed by a number of cobalt salts,complexes,oxides,and hydroxides in the alkaline solution is discussed.The key observations lead to the conclusion that the cobalt salts and complexes act as precatalysts that transformed to cobalt oxide/hydroxide[CoOx(OH)y]nanoparticles which is responsible for the benzyl ic C(sp3)-H oxyfunctionalization of 2-methoxy-4-methylphenol.Additionally,the size of the particles is decisive to catalyst performance.The nature of the active nanoparticles is also studied.In the second part,efficient aerobic oxidation has been developed for the selective preparation of a sequence of valuable p-hydroxybenzaldehydes from corresponding p-cresols,using a new magnetically separable catalyst of nano-structured cobalt-cobalt oxide/nitrogen-doped carbon(CoOx@CN)material.CoOx@CN showed high activity for the 2-methoxy-4-cresol oxidation to vanillin,giving great yield(90%),as well as other p-cresols in good to great yields.The ferromagnetic property of catalyst enables to recycle easily by an external magnetical field.The mechanism of Co-catalyzed aerobic oxidation of p-cresols has been contentious for several decades,especially a debate between benzyl radical and phenol hydroxyl radical mechanism.In the third part,the present work features experimental and sepectroscopic studies of reaction mechanism of p-cresols aerobic oxidation with the CoCl2/NaOH catalyst system,one of the most widely used catalyst.A new reaction mechanism featured with two-electron transfer,which resulted a methide quinoe as active intermediat,is proposed.In summary,a noval magnetically recyclable catalyst of nano-structured cobalt-cobalt oxide/nitrogen-doped carbon(CoOx@CN)material applied in p-cresols oxidation is developed.Furthermore,the true nature(homogeneous,heterogeneous or nano-particulate)of catalyst in aerobic oxidation of 2-methoxy-4-methylphenol to vanillin and the mechanism of aerobic oxidation of p-cresols have been comprehensively studied,which would contribute to understanding of selectively direct C-H oxyfunctionalization,especially catalyzed by Co-based catalyst.