Vanadium-substituted Keggin heteropolycompounds as catalysts for ecofriendly liquid phase oxidation of 2,6-dimethylphenol to 2,6-dimethyl-1,4-benzoquinone

摘要

Molybdovanadophosphate heteropolycompounds,H_4PMo_(11)VO_(40) and H_9PMo_6V_6O_(40) and the corresponding Cs salts,were synthesized.The influence of thermal treatment on their Keggin primary structures and their acidic properties were studied.The presence of Keggin units and the V incorporation into these units were observed by TGA,DTA,DRS and FT-IR.H_4PMo_(11)VO_(40) possesses an acid strength higher than that of the V-free sample and H_9PMo_6V_6O_(40),at room temperature.Heating above 200 deg C leads to considerable structural changes and acidity decrease.The Cs salts from H_4PMo_(11)VO_(40) possesses stronger acidic sites than those from H_9PMo_6V_6O_(40).Moreover when the Cs number increases,the acidic properties decrease.The salts with one,two,and three Cs are more unstable with temperature than the salt with 2.5 Cs.The specific surface area for the Cs salts depends on the temperature and Cs number.The prepared heteropolycompounds were used as catalysts for the liquid phase reaction of synthesis of 2,6-dimethyl-p-benzoquinone from 2,6-dimethylphenol,using hydrogen peroxide,with high yields.A selectivity to the p-benzoquinone of 79-59% was obtained when the reaction was catalyzed by Cs salts from the H_4PMo_(11)VO_(40) acid.