(560dx) Towards Safe Process Intensification of Partial Oxidation Reactors: Theoretical Insights into Selective Ethylene Oxide Formation on Ag Catalysts

Process intensification (PI) is a design approach that leads to significantly cleaner, safer, and more energy-efficient process technology. Catalytic partial oxidation processes are excellent candidates for PI efforts. These chemical conversions are responsible for a wide range of industrial chemicals, plastics, and intermediates used at extremely large scales. Moreover, these processes are relatively expensive to perform, and typically operate at high thermodynamic inefficiencies because these reactions are very temperature sensitive and also non-selective when even minor temperature excursions occur within the reactor. Specifically, the selective oxidation of ethylene to ethylene oxide (EO) will be the focus of our study. Approximately 26 million tons of ethylene oxide have been produced globally by selective oxidation of ethylene in 2013, and the scale of production increased to 34.5 million tons in 2016. [1] EO is mainly used to produce ethylene glycol, and it is produced commercially via an exothermic, vapor-phase reaction of ethylene and oxygen over Ag catalysts. There exists a strong driving force toward unselective reaction to complete combustion, and conversion must be kept low to ensure high selectivity to EO.

In the recent years, the experimental studies on the surface catalytic mechanism of EO process has attracted much attention. However, these experimental methods are still seeking the support from theoretical analysis. [2] In our study, periodic plane-wave Density Functional Theory (DFT) methods are used to analyze the reaction mechanism of the EO process on the Ag(111) surface facet with low coverage. Binding energies, reaction energies, and activation barriers have been calculated for both desired and undesired reaction pathways to allow for rational catalyst design efforts to improve selectivity and yield of commercial catalysts. Our DFT analysis will be discussed in the context of microkinetic modeling techniques which provide detailed information on surface coverage and gas phase species concentrations as a function of reactor process conditions and feed ratios. Key reactive surface species have been identified which are responsible for the branching ratio of ethylene oxide and acetaldehyde, which ultimately decides the selectivity of ethylene partial oxidation. Our results are consistent with previous kinetic modeling efforts in the literature that did not employ DFT analysis. [2] Lastly, our study demonstrates how fundamental theoretical investigations and multi-scale modeling techniques are currently impacting rational catalyst design and process intensification efforts in the chemicals industry.

[1] Transparency Market research (2011). Global Ethylene Oxide and Ethylene Glycol Market by Applications, by Geography, Raw Materials, Price Trends and Forecasts. Transparency Market research, TMRGL431.

[2] C. Stegelmann, N.C. Schiødt, C.T. Campbell, P. Stoltze, Microkinetic modeling of ethylene oxidation over silver, Journal of Catalysis 221 (2004) 630-649.