The continuous selective oxidation of p-xylene (p-X) by O 2 (generated by thermal decomposition of aqueous H 2 O 2 ) catalyzed by MnBr 2 in supercritical H 2 O at ca. 400 °C is reported for the first time. The selectivity for terephthalic acid (TA) is > 90%. Compared to existing industrial processes, the reaction has the potential for a significant increase in energy efficiency and a substantial reduction in waste. This reaction is significant because the presence of H 2 O lowers the catalytic activity of MnBr 2 in the conventional route to TA via oxidation of p-X in CH 3 COOH.

The objective of this research was to conduct constructive organic chemistry in water and to achieve yields that were comparable to, or better than, those in conventional media. The synthesis of 2-phenylbenzimidazole from 1,2-phenylenediamine and benzoic acid was chosen as a benchmark reaction. The reaction parameters, such as temperature, density and reaction time, have been systematically studied to understand whether the solvent properties of high-temperature water can have a positive effect on the chemistry. The reaction was performed in a new design of batch-type autoclave and was also monitored in situ by UV-vis spectroscopy. By tuning the parameters, the yield has been optimised to around 90%. The optimised conditions were then applied to related benzimidazoles, some of which crystallised from solution in situ to yield single crystals that were sufficiently pure to be analysed directly by X-ray diffraction, without any further purification. JHC This journal is

Abstract:We have demonstrated that different methylaromatic compounds [1,4-dimethylbenzene (p-xylene), 1,3-dimethylbenzene (m-xylene), 1,2-dimethylbenzene (o-xylene), 1,3,5-trimethylbenzene (mesitylene) and 1,2,4-trimethylbenzene (pseudocumene)] can be aerobically oxidized in supercritical water (scH 2 O) using manganese(II) bromide as catalyst to give corresponding carboxylic acids in the continuous mode over a sustained period of time in good yield. No partially oxidized intermediates (i.e., toluic acid and benzaldehydes) were detected for the dimethylbenzenes and mesitylene reactions. By fine tuning pressure and temperature, scH 2 O becomes a solvent with physical properties suitable for single-phase oxidation since both organic substrate and oxygen are soluble in scH 2 O. There is a strong structural similarity of metal/bromide coordination compounds in the active oxidation solvents (acetic acid and scH 2 O) which does not exist in the much less active H 2 O at lower temperatures. This may account for the successful catalysis of the reactions reported herein. Aromatic acids produced by the loss of one methyl group occurred in all of these reactions, i.e., 3 ± 6% benzoic acid formed during the oxidation of the dimethylbenzenes. Part of this loss is thought to be due to thermal decarboxylation. The thermal decarboxylation process is monitored via Raman spectroscopy.

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A high-temperature high-pressure ultraviolet-visible (UV-Vis) cell is described. The cell has been designed specifically for use with the UV spectrophotometer Hewlett–Packard 8453 but it could work up to 780 K at 100 MPa with any other UV-Vis spectrophotometer, as well as for near infrared (NIR) experiments. Three features of the cell make it convenient for experiments with supercritical fluids: the possibility to choose an optimal path length, the presence of three interchangeable ports into the cell, and a movable thermocouple in the working zone of the cell. The cell has been used to study a range of chemical reactions in water under near-critical and supercritical conditions, as well as for measuring the kinetics of such reactions. Some examples illustrating the performance of the cell are given.