A Solid–Liquid Phase-Change Absorbent Based on a Synergistic Combination of PZ and DMEA for Efficient CO2 Capture

Solid–liquid phase-change absorbents exhibit remarkable potential for CO2 capture owing to their significant energy-saving merits. Nevertheless, their industrial implementation is constrained by limited CO2 loading capacity and the high viscosity of solid products, which complicates solid–liquid separation. To address these issues, an efficient phase-change absorbent was developed using piperazine (PZ) as the active component and N,N-dimethylethanolamine (DMEA) as a dual-functional additive serving simultaneously as an auxiliary proton acceptor and a phase splitter. The proposed absorbent demonstrated high CO2 loading and formed readily separable crystalline solid products. The optimized 15 wt % PZ-DMEA absorbent achieved a CO2 loading of 1.24 mol CO2·mol–1 PZ, exceeding the theoretical limit predicted by the zwitterion mechanism. Notably, 99% of the absorbed CO2 was enriched in the solid phase, which accounted for only 56.7% of the total solution volume. Compared to the benchmark MEA absorbent, the developed system reduced regeneration energy consumption by 64%, mainly due to the substantial reduction in sensible and latent heat requirements. Additionally, the PZ-DMEA system displayed excellent cyclic stability, maintaining a CO2 loading of 0.72 mol·mol–1 after five absorption–desorption cycles. Characterization analyses and quantum chemical calculations were performed to elucidate the enhanced absorption and phase transition mechanisms. PZ initially reacted with CO2 to form a zwitterionic intermediate, which was subsequently deprotonated by free PZ and DMEA molecules to yield carbamates. The participation of DMEA in the deprotonation process enabled more PZ to engage in CO2 absorption, thereby boosting the overall PZ’s CO2 loading. The highly polar carbamate products formed strong intermolecular interactions with protonated amines, producing polar ion pairs that precipitated from the low-polarity DMEA-rich phase, ultimately driving the spontaneous solid–liquid phase transition.