Selective oxidation of cyclohexane in supercritical carbon dioxide

Abstract

A feasibility study into the novel concept of using molecular oxygen to carry out one-step catalytic oxidation of cyclohexane to adipic acid in supercritical carbon dioxide over two types of catalysts, namely Co 2þ =Mn 2þ =NaBr and Ag polyoxometallate, the silver decamolybdodivanadophosphate ðAg 5 PMo 10 V 2 O 40 Þ was carried out. Poor activity and selectivity towards adipic acid were initially noted over the aqueous micellar Co 2þ =Mn 2þ =NaBr catalyst for the cyclohexane oxidation in supercritical carbon dioxide while under comparable conditions, the same catalyst gave a high activity for alkylaromatics oxidation to corresponding acids. It was later found that the adipic acid, being the extremely polar oxidised products, was virtually insoluble in the supercritical phase, which was rapidly degraded to carbon oxides after its prolonged contact with catalyst and O 2 . Thus, the one-step cyclohexane oxidation to adipic acid with good selectivity can only be achieved by modifying the solvent with acetic acid or methanol, which enabled isolation of the acid from further oxidation. On the other hand, Ag 5 PMo 10 V 2 O 40 , in methanol modified supercritical carbon dioxide gave an impressive selectivity for cyclohexane conversion to other oxygenates.

Selective Catalytic Oxidation of CH Bonds with Molecular Oxygen

“…So far, the highest selectivities for cyclohexanol/cyclohexanone and diacids of 28 % and 40 %, respectively, at 10 % conversion at 180 °C and 14.0 MPa were obtained for the polyoxometallate catalyst Ag 5 PMo 10 V 2 O 40 in a methanol‐modified carbon dioxide solvent 255. In its proton form, this polyoxometallate as a quasi‐heterogenous catalyst is also active for the partial oxidation of activated arenes in supercritical CO 2 reaching yields up to 70 % (80 °C, 17.0 MPa) 256.…”

Selective Oxidation of Cyclohexanone to Adipic Acid Using Molecular Oxygen in the Presence of Alkyl Nitrites and Transition Metals as Catalysts

“…Importantly, the oxidation of cyclohexane to AA in the absence of solvent has been studied in the presence of heterogeneous catalysts such as Fe(III)AlPO-31 [ 31 ], NHPI/Fe(BTC) [ 32 ], Fe(III)T(O-Cl)PP [ 33 ] , and salts as well as complexes of transition metals in combination with a lipophilic 4-dodecyloxycarbonyl- N -hydroxyphthalimide (C 12 -NHPI) derivative [ 34 ]. Oxidation of cyclohexane has been also achieved using a AcOH/H 2 O mixture in the presence of Co/Mn [ 35 ] and Mn(III)T(p-Cl)PP [ 36 ] clusters in PhCOOH/H 2 O [ 36 ] and AcOH/scCO 2 [ 37 ].…”

Oxidation of cycloalkanes catalysed by N-hydroxyimides in supercritical carbon dioxide

“…To date, limited number of studies on the oxidation of cyclic hydrocarbons in supercritical carbon dioxide have been reported. In previous report (Kerry et al 2007), the oxidation of cyclohexane using NHPI/Co(II)/Mn(II) in scCO 2 was described (155 °C, 2.0 MPa O 2 − , 14.0 MPa CO 2 , 8 h). It was determined that it is necessary to add polar AcOH to the system, and without this additive, the reaction did not proceed.…”