An Experimental Study of the Kinetics of the Selective Oxidation of Ethene over a Silver on α-Alumina Catalyst

摘要

The oxidation of ethene to ethene oxide, carbon dioxide, and water over an industrial silver on α-alumina catalyst has been studied experimentally in an internal recycle reactor. Several sets of rate expressions were tested, and three of them were found to describe the experiments 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 (K_O~i)~(1/2) P_E (P_O)~(1/2) +(1+K_E~i P_E + (K_O~iP_O)~(1/2)+ K_c~i P_c+ K_w~iP_W + K_(EO)~i P_(EO))~2. These relations are able to describe our experimental data with an average error of 20% for the epoxidation reaction and 15% for the combustion. Carbon dioxide, ethene oxide, and water were found to inhibit the rates of reaction and affect the selectivity of the reactions. The apparent activation energy of the combustion reaction is larger than that for the epoxidation reaction; they are 86 and 70 kJ/mol, respectively. Thus the selectivity toward ethene oxide decreases with increasing temperature. Deactivation 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 found during the first 100 h of operation under reacting conditions. The final activities for different batches of catalyst varied considerably.