https://doi.org/10.1016/j.seppur.2022.120786
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Wang et al. [41] described the reaction between NH3(aq) and CO2(aq), including their equilibria dependent species, and the reversible reactions of ammonium carbamate/ carbamic acid formation:(3)CO2aq+H2O→k1←k-1H2CO3(4)CO2aq+OH-→k2←k-2HCO3-(5)CO32-+H+→k3←k-3HCO3-(6)HCO3-+H+→k4←k-4H2CO3(7)OH-+H+→k5←k-5H2O(8)NH3+H+→k6←k-6NH4+
The relative fraction of dissolved CO2 and ammonia as carbonic acid (H2CO3) and protonated ammonium (NH4+) respectively, are dependent on solution pH and temperature (Fig. A1). The formation of carbamate as an intermediate product of reaction, and its protonation to carbamic acid are described by:(9)CO2aq+NH3→k7←k-7NH2COOH(10)NH2CO2-+H+→k8←k-8NH2COOH
Ammonium bicarbonate is formed from the ionic bond between NH4+ and HCO3− [29], which implies an equilibria dependency for the transformation of ammonia and carbon dioxide into crystalline ammonium bicarbonate (NH4HCO3):(11)NH4+(l)+HCO3-(l)→k9←k-9NH4HCO3(s)
Carbon dioxide and ammonia can produce ammonium carbamate (NH2COONH4) in the gas phase, or following reaction between the dissolved gases when in their nascent state:(12)CO2g+2NH3(g)→k10←k-10NH2COONH4(s/l)
Through hydrolysis, NH2COONH4 successively converts into ammonium bicarbonate (NH4HCO3) [21]:(13)NH2COONH4s+CO2(g)+H2O(gl)→k11←k-112NH4HCO3(s)
However, due to the high solubility of ammonium carbamate in water [23], ammonium bicarbonate formation in the gas phase is more likely to proceed by [21], [16]:(14)CO2g+NH3g+H2O(gl)→k12←k-12NH4HCO3(s)
In total, five solid products can form, however, NH4HCO3 is thermodynamically favoured due to the lower solubility of the salt [23], which exhibits slight solubility-temperature dependency (Fig. B1a). Solvent temperature and concentration may also influence the probability for gas-side or liquid side NH4HCO3 crystallisation as this will determine the ammonia vapour pressure (Fig. B1b; [30].
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