A conventional hydrocarbon feedstock that is of particular interest for CO2utilization is ethylene, which is used to make ethylene oxide. Ethylene oxide is an important feedstock for the chemical industry and is used to make many useful products such as polyurethanes, polyols, glycols, nitriles, alcoholamines, and ethers. Ethylene oxide is manufactured by several closely related industrial processes by major chemical makers worldwide (e.g., DOW Chemical, Japan Catalytic Company, Shell International Chemicals, Sumitomo Chemical, BASF, Scientific Design).
The chemical methodology by which ethylene oxide is currently produced is the partial oxidation of ethylene using oxygen (this process is called epoxidation). A silver-based catalyst is used, and either air or preferably oxygen is the oxidant. Ethylene epoxidation is performed at lower temperatures (220 to 280°C) compared with other selective oxidation processes and at high pressure (10 to 30 bar). 1
The Captured CO2 Catalyst for the Production of Ethylene oxide (C3-PEO), a technology being developed at RTI International, aims at producing ethylene oxide—a high-value chemical—while consuming CO2—a greenhouse gas. This technology is based on novel catalysts utilizing the following key discoveries:
1) Abstraction of oxygen from CO2using the reduced mixed-metal oxide catalysts
2) Transfer of the abstracted oxygen from CO2to react with hydrocarbons to form desired products
3) Operation at temperatures which make these catalysts commercially practical to produce ethylene oxide
When considering the thermal stability of ethylene or ethylene oxide, both of these are highly unstable at high temperatures (i.e., greater than 300°C). Our proposed process is a two-step process detailed in Figure 1. In the first step, an oxygen atom is abstracted from CO2 producing CO in the reducing zone of the reactor. In the second step, this highly reactive oxygen atom reacts with ethylene to produce ethylene oxide in the oxidizing zone of the reactor as shown in Figure 1 . One reason for this arrangement is the inherent reactivity between CO2 and ethylene oxide, which could lead to the formation of undesirable ethylene carbonate in a co-feed system. Another reason for having separate reaction zones is that oxygen abstraction from CO2typically requires higher temperature than ethylene oxidation, which is a relatively low temperature process.
The proposed ethylene oxide process will have a concentrated CO byproduct stream, which could be used for the manufacture of many products such as methanol, dimethyl ether, acetic acid, acetic anhydride, vinyl acetate, styrene, terephthalic acid, formic acid, n-butanol, 2-methylpropanal, acrylic acids, neopentylacids, propanoic acid, dimethyl formamide, and Fischer-Tropsch hydrocarbons.2 Therefore, the two marketable product streams from the proposed process are ethylene oxide and CO, which are both valuable intermediates for the petrochemical industry.
RTI has improved on its previous catalyst formulations for CO2 utilization3 and developed families of catalysts which can both remove oxygen from CO2 and transfer the oxygen to ethylene to make ethylene oxide. The catalyst families are based on metal oxide phases which were found to be similar to iron in terms of reacting with CO2, but are more selective than iron for ethylene epoxidation. Improvements on the production of ethylene oxide have been made by the use of promoters, probing the catalyst support to identify a correlation with support acidity, and observing the impact of surface area on dispersion. Ethylene oxide has been produced at small scale in the fixed-bed catalyst test reactor in both a continuous co-feed and a transport mode of operation.
(1) Chongterdtoonskul, A.; Schwank, J. W.; Chavadej, S. Journal of Molecular Catalysis A: Chemical 2013, 372.
(2) Kolb, K. E.; Kolb, D. Journal of Chemical Education 1983, 60, 57.
(3) Shen, J. P.; Mobley, P. D.; Douglas, L. M.; Peters, J. E.; Lail, M.; Norman, J. S.; Turk, B. RSC Advances 2014, 4, 45198.