https://doi.org/10.1016/j.cogsc.2022.100645
“Over the past 20 years, alkali and alkaline earth metal oxides (MO) have been investigated extensively owing to their high theoretical gravimetric CO2 sorption capacities when converting reversibly into carbonates (MCO3), and the possibility of performing CO2 sorption at high temperatures that would enable better process integration [3]. One such material is magnesium oxide (MgO, CO2 sorption capacity of 1.09 g (24.8 mmol) CO2 per g MgO, ΔH298K = −100.9 kJ/mol for the reaction MgO + CO2→MgCO3), which has been used as a sorbent [4, 5, 6] over a wide temperature range (from ambient temperature to > 300 °C) due to its tunable shape [7,8], surface area [9, 10, 11] and surface basicity [12,13].
Focusing in this review on high reaction temperatures (≥300 °C) that favor the formation of stable carbonates, MgO was proposed as a sorbent for CO2 removal from hot, pressurized gas streams (ca. 300–550 °C, > 10 bar) after coal gasification and sulfur removal in integrated gasification combined cycle (IGCC) plants [14, 15, 16, 17], or water gas-shift reactors [18, 19, 20, 21]. Early on, it was realized that the formation of MgCO3 from MgO without the addition of a promoter suffers from poor kinetics and is limited to a few surface carbonate layers, resulting in only a fraction (<2%) of the theoretical CO2 sorption capacity of MgO to be utilized [22,23]. The slow kinetics is due to the high lattice enthalpy of MgO and the formation of a MgCO3 product layer that functions as a barrier for CO2 molecules [24]. The most efficient promoters for MgO-based sorbents are alkali metal salts (AMS), in particular alkali metal nitrates, which are molten under reaction conditions [25]. Systematic studies using mixtures of alkali metal nitrates (e.g. LiNO3, NaNO3, or KNO3) found that MgO is partially dissolved in the molten nitrates, providing ionic pairs of Mg2+ and O2− that interact with dissolved CO2 forming, in turn, MgCO3 nuclei/clusters [26, 27, 28∗∗]. “