Toward a CO2-free ethylene oxide process: Homogeneous ethylene oxide in gas-expanded liquids

Among industrial chemical processes, ethylene oxide manufacture emits the largest amount of CO2 (similar to 2-3 million tons/yr), as byproduct from the burning of both the ethylene (feed) and ethylene oxide. Further, the conventional silver-based catalytic process presents safety challenges due to the formation of explosive ethylene oxide/O-2 mixtures in the gas phase. By judicious choice of the catalyst (methyltrioxorhenium), oxidant (H2O2) and reaction medium (methanol/water), a homogeneous liquid phase catalytic system has been demonstrated that eliminates CO2 formation while producing ethylene oxide at > 90% selectivity at near-ambient temperatures. Given its high volatility, the ethylene oxide is easily recovered from the reaction phase by distillation. The vicinity of the gaseous ethylene feed to its critical temperature (9 degrees C) is exploited to significantly increase its solubility in the liquid reaction phase by facile compression beyond the critical pressure of ethylene (similar to 50 bar). Since H2O2 is stable at typical reaction temperatures (40 degrees C or less), potentially explosive ethylene oxide/O-2 mixtures are avoided in the gas phase. In addition to the potential of arresting the carbon footprint of a large-scale industrial process, the demonstrated process concept shows how gas-expanded liquids can be generally exploited in homogeneous catalysis to enhance productivity. (C) 2009 Elsevier Ltd. All rights reserved.