decomposition in the case of amines. Physical solvent absorbents also degrade for similar reasons and suffer loss due to evaporation. There is need for new, low-cost sorbent materials that have enhanced stability, are less volatile and less viscous, have higher CO2 capacity per unit of mass, are more environmentally friendly, and require less energy consumption for operation.
Potential candidate materials include but are not limited to the following: molten metal oxides, medium-temperature eutectics, ionic liquids, biphasic materials, and CO2 transfer agents that reversibly form compounds with CO2 (e.g., alkyl carbonate). Some of these materials offer the potential advantages of being stable above 300 °C, are nonvolatile, and have tunable properties. Hybrid materials that possess synergistic effects may offer additional advantages of being multifunctional. Recent developments in experimental methods and computational techniques, such as density-functional theory (DFT) and molecular dynamic methods, provide new tools for designing and synthesizing tailored molecules with unique properties.
Sorbents are used to remove CO2 from a gas stream typically at higher temperatures than those used for absorbents, up to 700 °C or 800 °C in a combustion process. The common sorbents are metal oxides, such as calcium oxide (CaO). These materials chemically react with the carbon dioxide, in the case of CaO by forming carbonates. In most cases, the sorption capacity is limited to about 30 percent—that is, only about 30 percent of the CaO is converted to carbonate. The capacity can be improved by better engineering of the pore structure of the CaO in which case close to 100 percent capacity can be achieved. However, significant improvements in the operational characteristics of the sorbent would make this approach much more attractive. A desirable sorbent should have high CO2 capacity (up to 100 percent of theoretical capacity), function in the presence of water vapor in the gas stream, and have fast reaction and regeneration kinetics, high durability, and the ability to be regenerated with minimal energy consumption. Sorbents that can operate at high temperatures (600 °C to 700 °C) could eliminate the need to cool the gas. The ability to remove other pollutants also is desirable.
High-temperature sorbents can also be applied to the production of hydrogen from fossil fuels. Natural gas or coal can be gasified to a mixture of carbon monoxide and hydrogen (CO/H2). Increased hydrogen production is traditionally achieved by employing the water-gas shift (WGS) reaction. However, the equilibrium of the WGS reaction requires a low reaction temperature in order to achieve high hydrogen concentration. Research is under way to separate hydrogen from high-temperature gas mixtures by means of high-temperature hydrogen separation membranes to shift the equilibrium toward hydrogen formation. Similar results can be achieved by removing CO2 from a high-temperature gas mixture by the reaction of CO2 with high-temperature sorbents, leading to the production of pure hydrogen. Metal oxides can also be effective for multifunctional pollution control. For example, calcium-based sorbents can react with sulfur oxides, hydrogen sulfide, and chlorine to a high extent as well, thus reducing their concentration in effluent streams to parts per million (ppm) levels.
New oxide compositions (e.g., multicomponent oxides, supported oxides) and/or oxides of engineered porosity are candidate materials. Completely novel sorbents, such as a