Is it Possible to Achieve Highly Selective Oxidations in Supercritical Water? Aerobic Oxidation of Methylaromatic Compounds

Abstract

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.

Prevention of Manganese Precipitation during the Continuous Selective Partial Oxidation of Methyl Aromatics with Molecular Oxygen in Supercritical Water

“…All the experiments were conducted using a tubular continuous flow reactor, similar to the one that has been previously described. [11,12] Experimental work was carried out on the continuous oxidation of o-xylene by O 2 in scH 2 O at about 380 8C and 230 to 240 bar with a combined catalyst and acid solution. In a typical experiment, 56 mL of 100 volume H 2 O 2 solution (> 30% w/v) commercially available from Fisher Scientific UK Ltd. was mixed with 760 mL deionised H 2 O.…”

“…[11,12] The resulting aqueous solution of products exiting the reactor via the back pressure regulator was then sampled for periods of 2 min every 10 min and analysed by HPLC and atomic absorption.…”

“…The first involves designing a flow reactor for the best possible mixing between the methyl aromatic, dioxygen and the catalyst. [11,12] One can also apply one or more of the methods used currently in acetic acid/water solvents. 1) By increasing the bromide concentration by addition of a bromide source to manganese, i.e., increasing the ratio Br/Mn so that the rate of reduction of MnA C H T U N G T R E N N U N G (III) to Mn(II) by the bromide ions [Eq.…”

Supercritical Water Gasification of Municipal Sludge: A Novel Approach to Waste Treatment and Energy Recovery

“…Some catalyst regeneration was evident with the flushing of sulfur-poisoned catalysts with subcritical water (at 250-300°C), which removed up to 75% of the sulfur (Elliott, 2008). Water can activate or deactivate metal-catalyzed reactions via autoxidation (García-Verdugo et al, 2004). Regeneration effects can be augmented via the addition of an oxidant, such as H2O2 (Elliott, 2008).…”

“…Direct observations of small-scale, transparent reactors (e.g., diamond anvil cells or quartz capillary tubes) allow reactions to be seen, photographed, and quickly halted if necessary (Azadi & Farnood, 2011;Fang et al, 2008;Hashaikeh et al, 2007;Maharrey & Miller, 2001;Peterson et al, 2008a;Sasaki et al, 2000;Vogel et al, 2005). Larger scale systems have been directly observed via optical, laser Raman spectroscopy through sapphire reactor viewing ports in order to capture finite details of the reaction progress, fluid mechanics, reactant destruction completeness, and oxidation efficiencies (Chuntanapum & Matsumura, 2010;García-Verdugo et al, 2004;Hunter et al, 1996;Koda et al, 2001;Rice et al, 1996). Indirect, nuclear radiography accomplishes the same result as optical Raman spectroscopy, but does not require viewing-port reactor modifications (Peterson et al, 2008a(Peterson et al, , 2008b(Peterson et al, , 2010.…”