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

Abstract

Presented here is an investigation of the activity and recovery of the homogeneous manganese/bromide catalyst during the continuous flow oxidation of o-xylene, as model substrate, with molecular oxygen (O 2 ) in supercritical water (scH 2 O). Two strategies are discussed for preventing catalyst precipitation, mainly in the form of oxides such as manganese(IV) oxide, The first strategy involves varying the manganese:bromide ratio using either manganese(II) acetate or manganese(II) bromide in the presence of hydrobromic and other acids. The results show that the effect of acidity and bromide concentration plays an important role in preventing the manganese/bromide catalyst from precipitating. The second strategy uses aromatic carboxylic acids in combination with the manganese/bromide catalyst, particularly benzoic acid, which improves the catalyst recovery dramatically over a certain range of acid concentrations. Our studies show how the presence of an organic acid and/or its precursors is important in stabilising the catalyst. Our results are rationalised on the basis of a tentative reaction mechanism.

Ultrasound‐assisted deacetylation of beta‐chitin: influence of processing parameters

“…A comparably high selectivity has so far been observed only in the case of multinary bulk molybdates phases such as AgMoVO 6 22a, 23. Typically the selectivity is limited to less than 40% with materials such as MnO 2 , multinary metal polyphosphates or borosilicates 24. Most often the oxidation of o‐ xylene is even observed not to stop at o‐ tolyl aldehyde as an intermediate product, but to proceed under formation of phytalic anhydride 21, 25…”

Supercritical synthesis and characterization of SWNT-based one dimensional nanomaterials

“…They can diffuse like a gas with zero surface tension and dissolve materials like a liquid, and their physicochemical properties tend to be adjusted with temperature and pressure. 16,17 In addition, supercritical fluids can lead to reactions which are difficult or even impossible to achieve under conventional conditions. Therefore, supercritical fluids should easily take filling materials into the nanocavity of SWNTs in high yields, and more materials can be inserted if we choose appropriate supercritical fluids as media or different supercritical reactions to synthesize new resultants according to the properties of the filling materials.…”

Catalytic Oxidation of Biosourced 3‐Methylphtalic Anhydride under O2: One‐Pot Hemimellitic Acid Synthesis and Novel Example of Biomass Valorization.

“…Despite the efficiency of such methods for 1,4‐ and 1,3‐dimethylbenzene oxidation, very few examples for more challenging hydrocarbon oxidation, i. e. 1,2‐dimethylbenzene or higher substituted benzenes which would give access to 1,2‐dicarboxylic aromatics or various tricarboxylic derivatives are described . Oxidation of 1,2‐dimethylbenzene using HNO 3 , MnBr 2 /O 2 and KMnO 4 have been described but generally give yields around 75%. The formation of adjacent COOH groups on benzene rings is clearly defined as a limitation of these systems and leads in most cases to catalyst deactivation.…”