Supported Silver Catalysts for Ethylene Epoxidation

Abstract

12or to increase the stability of the catalyst. [74][75][76] The activity of the catalyst is less important as high conversions decrease the ethylene oxide selectivity due to the secondary reaction (r 3 in Figure 1.2, the full oxidation of ethylene oxide).The presence of chlorine on the surface suppresses the full oxidation of ethylene, 74,77,78 as the presence of chlorine on the catalyst surface prevents the formation of certain surface species responsible for the unselective pathway, leading to an increase in the intrinsic selectivity. 75 The competitive adsorption of chlorine on the silver surface indiscriminately suppresses oxygen adsorption on the silver surface and therefore decreases the overall activity by suppressing both the selective (r 1 ) and unselective (r 2 ) pathways. 47,77,79,80 A secondary effect of chlorine promotion leads to a suppression of the secondary, unselective reaction (r 3 ); the decomposition of ethylene oxide to form carbon dioxide and water. 79,81 In a typical epoxidation reactor a feed containing oxygen, ethylene and ppm-levels of the chlorine moderator in an inert diluent gas is fed over the reactor at 200 -280 °C at 15 bar. 55,72 The reactor is designed to have good control over the temperature as to prevent hot-spots. 82 The oxygen content is important to maintain an oxide overlayer on the silver, which is required to obtain high selectivities. However, the oxygen content is kept below 10 % per volume to prevent the formation of explosive mixtures. 55,72,83 When the oxygen content is below this value, the ethylene concentration can be maintained at a relatively high value of 30 -40 vol% safely. 55,72,83 As selectivity decreases with conversion, the (ethylene) conversion is kept low to maintain a high yield. Commercially, a yield of 1 -3 vol% EO is viable. 55 Modern industrial catalysts obtain selectivities toward ethylene oxide of 90 % 77,84 compared to about 50-70 % for early epoxidation catalysts. 85,86 Besides (promoted) silver catalysts, also other catalytic systems are being investigated for this reaction. [87][88][89][90] High selectivities towards ethylene oxide are reported by Jankowiak and Barteau for CuAg bimetallic catalysts, 89,91 and later also for AgPd bimetallic catalysts. 87 Due to the bimetallic nature of these catalysts, the environment during reaction can greatly influence composition of the catalyst surface, as for example in the CuAg catalysts segregation of the metals occurs under reaction conditions. 92,93 Depending on the metal ratios and reaction conditions, alloying, but also promotion, additional metals can therefore be either beneficial or unfavorable for the catalytic performance of the silver catalyst. 88,92,93 As silver is capable of selectively oxidizing ethylene, the simplest olefin, it seems straightforward that this system is also an interesting candidate for the epoxidation of other olefins. However, silver appears to be unable to selectively oxidize higher olefins in gas phase, due to activation of the β-hydrogen (one of the hydrogen atoms b...