AN EXPERIMENTAL-STUDY OF THE KINETICS OF THE SELECTIVE OXIDATION OF ETHENE OVER A SILVER ON ALPHA-ALUMINA CATALYST

The oxidation of ethene to ethene oxide, carbon dioxide, and water ove r an industrial silver on alpha-alumina catalyst has been studied expe rimentally in an internal recycle reactor. Several sets of rate expres sions were tested, and three of them were found to describe the experi ments with almost the same accuracy. Of these the expressions based on a Langmuir-Hinselwood mechanism in which adsorbed ethene reacts with dissociatively adsorbed oxygen is preferred: r(i) = (k(r)(i) K-E(i) ro ot K-O(i) P-E root P-O)/(1 + K-E(i) P-E + K-E(i) P-E + root (KOPO)-P-i + K-C(i) P-C + K-W(i) P-W + K-EO(i) P-EO)(2). These relations are abl e to describe our experimental data with an average error of 20% for t he epoxidation reaction and 15% for the combustion. Carbon dioxide, et hene oxide, and water were found to inhibit the rates of reaction and affect the selectivity of the reactions. The apparent activation energ y of the combustion reaction is larger than that for the epoxidation r eaction; they are 86 and 70 kJ;r/mol, respectively. Thus the selectivi ty toward ethene oxide decreases with increasing temperature. Deactiva tion of the catalyst used in the kinetic experiments was encountered. This deactivation was corrected for when fitting rate expressions. In a separate set of experiments fluctuations of catalyst activity were f ound during the first 100 h of operation under reacting conditions. Th e final activities for different batches of catalyst varied considerab ly.