Insights Into The Co(?)and Cu(?)Catalytic Oxidation Of Cresols In Excess Alkali

Selective oxidations of cresol compounds,whether in academic research,or for industrial production,are very important chemical reactions.The aldehydes by oxidation of cresol compounds are a very important class of chemical raw materials.For a long time,cobalt compounds are used as catalysts in the catalytic oxidation of cresol compounds.Such as p-cresol is catalyzed by Co3O4;4-methyl guaiacol is catalyzed by CoCl2 or porphyrin cobalt,etc..In these reactions,whether its chemical-selectivity or stereo-selectivity,cobalt shows special activity from other transition metals.Another transition metal copper is also used for the oxidation of cresols,but more for ortho-disubstituted cresols,such as 2,6-dimethoxy-4-methyl phenol oxidation.However,the catalytic mechanism is still controversial,especially cobalt ion catalysis.Some researchers thought that the key was the interconversion between Co(?)and Co(?),some believed it involved the structure of Co-O-O.It was also suggested that the rate-determining step is disproportionation.To know the catalyst-catalyzed differences,we have further explored the mechanism by using transition metal-catalyzed 4-methyl guaiacol and its analogues,such as copper and cobalt,as a comparison to find the transition metals catalyze such reactions and the essential reason for the difference of catalytic effect.The experimental results showed that cobalt ions were favorable for the oxidation of 4-methyl guaiacol to vanillin in excess alkali,whereas copper ions were favorable for the formation of dimers and oligomers.When cobalt and copper ions were used together,the reaction was still oxidized towards vanillin.In addition,other metals have little effect on cobalt catalysis.Once cobalt ions were added,the reaction was controlled by cobalt.By means of DLS,ESR and UV-Vis,we have determined that the two kinds of reactions are heterogeneous catalyzed reactions,and the coordination structure of cobalt-copper catalyst in the reaction solution,and obtained the different structural information of Co(?)/NaOH and Cu(?)/NaOH system.On the basis of these,we give the reaction mechanism of each catalyst by using some logic experiments.It is suggested that the specificity of cobalt catalysis comes from cobalt with tetravalent.