(191b) Sorbent Enhanced Reverse Water Gas Shift Reaction: Maximizing CO Yield

Solar fuels can replace fossil fuels in transportation vehicles. Alcohols or liquid hydrocarbons are preferred solar fuels in view of their combination of energy density and the worldwide existing markets and infrastructure. Hydrogen, which can be produced efficiently from solar energy through direct splitting of water or through combination of solar electricity production with electrolysis of water [1], is a very attractive intermediate as liquid hydrocarbons can be produced through hydrogenation of CO₂. The CO₂ is captured at fossil fuel based power plants or, in the near future, directly from the air [2]. The conversion of CO₂ to transport fuels with hydrogen has the potential of the integration of CO₂ capture and energy storage/conversion. The reaction of hydrogen with CO₂ is initiated by the reverse water-gas shift reaction (RWGS): H₂ + CO₂ => CO + H₂O ΔH₀ 41kJ/mol. The reaction can be combined with Fischer-Tropsch technology to produce liquid hydrocarbon fuels. The use of the RWGS reaction for the production of methanol and dimethyl ether (DME) fuels is another very interesting one. DME is highly volatile and has no C-C bonds. As a consequence, it combusts soot-free in a diesel-type engine, and thus resolves the inherent soot/NOx trade-off that is common for conventional diesel-type fuels. However, the synthesis of methanol and DME directly from CO₂ suffers from very low equilibrium conversion. Synthesis of methanol with CO enriched feed greatly improves the conversion. The RWGS is an endothermic equilibrium reaction, and the equilibrium lies on the product side only at high temperatures. The catalytic conversion at lower temperatures, 473-573K can be enhanced greatly if reaction product water is selectively removed. Carvill et al. [3] showed that pure CO could be produced at 523K in a sorbent enhanced RWGS (COMAX) process. For methanol and DME production it suffices if the COMAX step produces a mixture of CO, CO₂ and hydrogen (synthesis gas). In the present contribution, parameters governing the conversion of CO₂ into CO by in-situ removal of water, the optimization of catalytic materials, the functionalities that selectively form CO and prevent consecutive reaction at higher pressure, as well as the process configuration will be presented.

[1] Blankenship, R. E., Tiede, D. M.,Barber, J.et al. Science 2011, 332 (6031), 805.

[2] Haije,W.G. ,Geerlings, J.J.C. Environ. Sci. Technol.,2011, 45 (20), 8609.

[3] Carvill, B. T., Hufton, J. R., Anand, M., Sircar, S. , AIChE J. 1996, 42, 2765.