Density Functional Theory Analysis of the Reaction Pathway for Methane Oxidation to Acetic Acid Catalyzed by Pd~(2+) in Sulfuric Acid

摘要

Density functional theory has been used to investigate the thermodynamics and activation barriers associated with the direct oxidation of methane to acetic acid catalyzed by Pd~(2+) cation in concentrated sulfuric acid.Pd~(2+) cations in such solutions are ligated by two bisulfate anions and by one or two molecules of sulfuric acid.Methane oxidation is initiated by the addition of CH_4 across one of the Pd-O bonds of a bisulfate ligand to form Pd(HSO_4)(CH_3)(H_2SO_4)_2.The latter species will react with CO to produce Pd-(HSO_4)(CH_3CO)(H_2SO_4)_2.The most likely path to the final products is found to be via oxidation of Pd-(HSO_4)(CH_3)(H_2SO_4)_2 and Pd(HSO_4)(CH_3CO)(H_2SO_4)_2 to form Pd(eta~2-HSO_4)(HSO_4)_2(CH_3)(H_2SO_4) and Pd(eta~2-HSO_4)(HSO_4)_2(CH_3CO)(H_2SO_4),respectively.CH_3HSO_4 or CH_3COHSO_4 is then produced by reductive elimination from the latter two species,and CH_3COOH is then formed by hydrolysis of CH_3COHSO4.The loss of Pd~(2+) from solution to form Pd(0) or Pd-black is predicted to occur via reduction with CO.This process is offset,though,by reoxidation of palladium by either H_2SO_4 or O_2.