Investigation of silver catalyst for propylene epoxidation: promotion and reaction mechanism

Bedolla-Pantoja, Marco
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University of Delaware
The gas-phase epoxidation of propylene on silver catalysts has been examined. Drawing parallels from the successful catalysts used in ethylene epoxidation, several silver-based catalysts have been tested for propylene epoxidation. However, no catalyst proved to be selective to the desired product, propylene oxide. Cadmium, copper, cesium and rhenium were tested as promoters to silver, but no catalyst achieved more than 5% selectivity to propylene oxide under the conditions tested. When the catalysts were tested at 267 °C with a feed consisting of a gas mixture of propylene, oxygen and nitrogen at volume ratio of 1:1:8, the unpromoted silver catalyst was the least selective. The most selective catalyst was the copper-silver catalyst, which exhibited 4.3% selectivity to propylene oxide. The main limitation to selectivity is the high degree of combustion of propylene to carbon dioxide. By running the catalysts at different reaction conditions, important conclusions were drawn regarding the the kinetics of the reaction. The conversion of propylene increased as the volume ratio of oxygen to propylene fed to the reactor increased. As the oxygen fed to the reactor was increased with respect to propylene, the conversion of propylene increased but the selectivity to propylene oxide remained nearly constant for all but the rhenium-silver catalyst. For the rhenium-silver catalyst the selectivity decreased as the oxygen to propylene ratio was increased. The reaction order with respect to oxygen for propylene oxidation was deduced from these data. For the silver, cadmium-silver and rhenium-silver catalysts, the reaction order was greater than 1. However, for the cesium-silver and copper-silver catalysts the reaction order with respect to oxygen was 0.4 and 0.5, respectively. The propylene effect of the activity of the cesium-silver and copper-silver catalysts was also studied. The conversion of propylene decreased as the propylene fed to the reactor was increased with respect to oxygen. These results indicate that oxygen, and not propylene, controls the rate for propylene oxidation. This hypothesis was confirmed by measuring the apparent activation energy for the reaction. For all the catalysts tested the apparent activation energy was 14-17 kcal/mol, in agreement with the energy of oxygen dissociation on silver. Thus, it was concluded that oxygen dissociation is the rate determining step of propylene oxidation. The decomposition of the desired product, propylene oxide, was also analyzed. Propylene oxide was introduced to an unpromoted silver catalyst that had been previously tested for propylene oxidation. Acetone, acrolein, allyl alcohol and propanal were detected on the effluent stream from the reactor. However, propanal was produced in larger amounts than any other compound regardless of the conditions at which the reaction was run. At 267 °C, for example, the selectivity to propanal was 73 %, to acrolein 14.8 % and for acetone and allyl alcohol 7.0% and 5.2%, respectively. As the reaction temperature was decreased, propanal selectivity increased at about the same rate as acrolein selectivity decreased. The selectivities to acetone and allyl alcohol remained constant. However, further experiments reveal that all of the products from PO decomposition are primary products. By probing this reaction under different conditions, a mechanism of propylene epoxidation has been proposed. The proposed mechanism assumes an oxametallacycle as the crucial intermediate from which acetone, propylene oxide, propanal, acrolein and allyl alcohol are formed. All of these are products of competing reactions of surface oxametallacycles. Acrolein and allyl alcohol are formed through dehydrogenations of the oxametallacycle while acetone and propanal are formed through 1,2 H-shifts. PO is formed through simple ring closing of the oxametallacycle. The kinetics and mechanism investigated in this work provides important insights that will help guide the search for a selective catalyst for propylene epoxidation. However, future work needs to address the need to learn more about the relative ease with which the oxametallacycle undergoes ring closing against alternative reactions. This can be accomplished through Density Functional Theory calculations or careful desorption experiments.