Catalytic Oxidation of Ethylbenzene to Acetophenone Using Alumina-Supported Dichromate:  Process Optimisation and Development of a Continuous Process

Potassium dichromate supported on neutral alumina is a heterogeneous catalyst for the liquid phase oxidation of hydrocarbons. The material has been used to catalyse the oxidation of ethylbenzene to acetophenone using air as the consumable oxidant with high selectivity and is truly catalytic in the metal, unlike homogeneous Cr(VI) systems. Optimisation of reaction conditions has been achieved in terms of air flow rate, agitator speed, and catalyst quantity. An induction period prior to achievement of maximum catalytic turnover, which is proportional to both temperature and the concentration of catalyst, may be reduced by doping the substrate with 5−15% acetophenone (w/w) and eliminated by doping with 30% (w/w) acetophenone. A rate of conversion of ca. 3.8% h-1 (0.39 turnover s-1) may be achieved at a reaction temperature of 130 °C for a period of ca. 10 h before catalyst deactivation occurs. However, this rate may be maintained for periods in excess of 24 h by continuous addition of substrate to the reaction vessel in order to maintain a constant molar ratio of substrate to product. It may thus be concluded that, at high agitator speeds, the rate-limiting factor is adsorption/desorption at the catalyst surface (and is therefore affected by the overall polarity of the medium) rather than mass transfer between solid−liquid−gas interfaces. The catalyst has been successfully recovered and reused with no induction period, and initial activity is comparable with that for fresh catalyst, although deactivation occurs readily thereafter.