Catalytic oxidation of olefins over modified copper oxide catalysts.

Abstract

The vapor phase air oxidation of propene and 2-methylpropene to acrolein and methacrolein was investigated in an isothermal integral flow reactor at atmospheric pressure between 300 and 500°C. The skeleton catalyst (copper oxide) supported on pumice stone or inactivated alumina was modified in a constant and continuous supply of trace amounts of selenium, sulfur, bromine, chlorine and iodine, all having the electronegativities higher than that of copper. The effect of various process variables, namely the weight ratio of modifier to olefin in the feed, the feed ratio of oxygen to olefin, reaction temperature and the reciprocal of space velocity, on the conversion of olefin and the product distribution was determined by gas chromatography. It was found that in the presence of small amounts of these modifiers, the selectivity for unsaturated aldehydes increased very substantially. The promotional effects of different modifiers are compared and discussed. It was observed that the weight ratio of modifier to olefin in the feed is the most important variable in the selective oxidation of olefin, and that its optimum value is in the range of 0.0005 to 0.0015 for the oxidation of 2-methylpropene. The catalytic oxidation of methacrolein over the same catalyst in the presence and absence of a chlorine modifier was also studied. The results indicate that the promotional effect of the modifier is mainly due to its successfully suppressing the further oxidation of aldehyde to the undesired products. The active phase of the catalyst for the selective oxidation of olefin was found to be cuprous oxide by X-Ray diffraction studies. The Electronic Theory of Catalysis has been modified to explain the results and a mechanism is proposed. It has been suggested that the charged oxygen and olefin are selectively adsorbed on the catalyst surface and that the partial oxidation of olefin is a p-type reaction, and its further oxidation to carbon dioxide is an n-type reaction under the optimum conditions. Out of the several models proposed for the oxidation of 2-methylpropene over chlorine modified copper oxide catalyst, the most suitable one appears to be the one in which the rate-determining step is the surface reaction between adsorbed olefin and oxygen on different active sites. The rate of reaction can be expressed by the equation r=KSKHPH 1+KHPH+KMPM ˙KOPO1+K OPO Where KS, KH and KM are temperature dependent constants.