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.