Reversible chemisorbents for carbon dioxide and their potential applications

Physisorbents like zeolites, activated carbons, silica, and alumina gels have been traditionally used for separation of bulk or trace carbon dioxide from a gas mixture using a pressure or a thermal swing adsorption process. These processes are generally operated at near ambient or at moderate temperatures (say <100 °C) because the equilibrium sorption capacity and selectivity of sorption of CO₂ on the physi-sorbents rapidly decrease at higher temperatures. Recently, a variety of reversible CO₂ chemisorbents have been developed which offer decent sorption capacity and high selectivity for CO₂ at relatively higher temperatures (∼150 to 500° C). Some of these chemisorbents even exhibit high selectivity of CO₂ sorption in the presence of H₂O which is mechanistically impossible for a physisorbent. Consequently, these chemisorbents can be used in (a) sorption enhanced reaction (SER) concepts for production of fuel-cell grade H₂ from natural gas by low temperature steam-methane reformation reaction at ca. 400-500° C or from synthesis gas by water-gas shift reaction at ca. 200-400° C, and (b) removal and recovery of CO₂ from a flue gas at a temperature of 150-200° C without precooling, predrying, and precompression. A comprehensive review of this subject is presented.