Catalytic selective partial oxidations using O2 in supercritical water: the continuous synthesis of carboxylic acids

A variety of different alkylaromatic compounds having a range of alkyl groups (methyl, ethyl and isopropyl), varying aromatic groups (benzene, naphthalene and pyridine), and largely different ring electron densities (toluene and 3,4-dimethoxytoluene) have been successfully oxidized selectively to aromatic aldehyde and carboxylic acids in sub- and supercritical water using a continuous reactor under a wide range of experimental conditions. The estimated difference in initial reactivities of these substrates is approximately 10 000. Selected yields include toluene [rightward arrow] benzoic acid (83%), isopropylbenzene [rightward arrow] benzoic acid (46%), ethylbenzene [rightward arrow] benzoic acid (68%), 2,6-dimethylnaphthalene [rightward arrow] 2,6-naphthalenedicarboxylic acid (25%; co-oxidized with p-xylene) and 3,4-dimethoxytoluene [rightward arrow] 3,4-dimethoxybenzoic acid (60%). 2-Nitrotoluene gave very poor yields, consistent with previous attempts to oxidize this substance homogeneously. For the first time, the reactivities and carboxylic acid yields of all three isomers of methylpyridine (picoline) have been determined. The initial reactivities are 3-isomer 2-isomer 4-isomer, the thermal decarboxylation rates are 2-isomer [dbl greater-than] 3-isomer 4-isomer, and the best yields are 3-isomer 4-isomer 2-isomer, with maximum yields of 50, 33, and 18 mol% respectively.