Tuning the balance between CO2 absorption capacity and kinetics in diol-based low transition temperature mixtures: The dual role of 1,2-hexanediol

The ability of low transition temperature mixtures (LTTMs) composed of potassium hydroxide, boric acid, 1,2-diols of varying chain lengths, and water to serve as environmentally friendly solvents for CO2 capture was systematically investigated. CO2 solubility was measured at 35 and 50 °C under pressures ranging from 1 to 4 MPa. Among the tested systems, the ethylene glycol (EG)-based LTTM exhibited the highest CO2 absorption capacity, while the 1,2-hexanediol (1,2-HD)-based LTTM showed the fastest absorption kinetics, despite a lower overall uptake. Blends of EG or 1,2-propanediol (1,2-PD) with 1,2-HD demonstrated a synergistic kinetic enhancement, accelerating CO2 uptake compared with single-diol systems. This kinetic advantage came at the expense of reduced absorption capacity relative to the parent EG- and 1,2-PD-based LTTMs, although total uptake remained higher than that of the 1,2-HD system. Notably, in the 1,2-HD-based LTTM, a shift in the CO2 absorption mechanism from chemical to physical was observed, indicating its role as a kinetic rather than thermodynamic promoter. Preliminary computational studies support this observation, suggesting that the butyl chains of 1,2-HD molecules may (i) shield the OH groups, hindering CO2 chemical capture, and (ii) form non-polar pockets capable of hosting CO2 molecules, favouring their physical absorption. These findings emphasise the importance of short-chain diols in maintaining adequate absorption capacity. Overall, this work highlights the potential of compositional tuning in LTTMs to optimise kinetic and thermodynamic performance in CO2 capture, offering valuable insights for the rational design of green, high-performance sorbents tailored to specific process conditions.