Introduction of MgO and the necessary of adding alkali metal salts

https://doi.org/10.1039/D2TA02897B

“Magnesium oxide-based sorbents that capture CO₂ reversibly through the reaction MgO + CO₂ ↔ MgCO₃ at intermediate temperatures (300–500 °C) have shown great potential for CCS owing to its high theoretical gravimetric uptake (1.09 gCO₂ gMgO⁻¹) and low energy requirement for sorbent regeneration (2.68 kJ g⁻¹ CO₂) when compared to other metal oxide sorbents (e.g. 4.04 kJ g⁻¹ CO₂ for CaO).^4,5 However, bulk MgO exhibits sluggish carbonation kinetics resulting in low experimentally obtained uptakes of only 0.02 gCO₂ gMgO⁻¹ after 1 hour of exposure to CO₂.^4,5 To accelerate the kinetics of CO₂ uptake of MgO-based sorbents, different approaches, such as the increase of surface area, have been reported.^4,6,7 However, the addition of alkali metal salts e.g. NaNO₃, KNO₃, LiNO₃ or mixtures thereof have resulted in the largest increases in the CO₂ uptake kinetics.^8–11 It is important to notice, that an enhancement in the CO₂ uptake only occurs when the salt is in the molten state and for an optimized system containing 20 wt% NaNO₃ an CO₂ uptake of 0.31 gCO₂ gMgO⁻¹ (after 60 min of carbonation) has been reported.^9,10,12 Although there is consensus on the promoting effect of alkali metal salts, there is an ongoing debate on the mechanism(s) through which alkali metal salt promotes carbonate formation with accelerated rates.^{1,9,10,13–16} One hypothesis considers alkali metal salts (AMS) as phase transfer catalysts that dissolve MgO yielding solvated ionic [Mg²⁺…O²⁻] pairs that have weaker bonds compared to the strong ionic bond of bulk MgO. According to density functional theory calculations by Zhang et al. the dissociation of MgO into a [Mg²⁺…O²⁻] pair requires 7.1 eV (in vacuum) while only 5.3 eV is required in NaNO₃ due to solvation effects.⁹ In addition AMS have the ability to dissolve CO₂ promoting further its reaction with solvated [Mg²⁺…O²⁻] pairs.^9,17 Besides the mechanism itself, currently also the location of MgCO₃ growth and the rate limiting step is under debate, i.e. the dissolution of MgO or the dissolution of CO₂.^1,9,10“