Novel NonaqueousPD/PZ/NMP Absorbent for Energy-EfficientCO₂ Capture: Insights into the Crystal-Phase RegulationMechanism of the Powdery Product

Solid–liquid biphasic absorbents are a promising solution for overcoming the high-energy consumption challenge faced by liquid amine-based CO₂ capture technologies. However, their practical applications are often hindered by difficulties in separating viscous solid-phase products. This study introduces a novel nonaqueous absorbent system (PD/PZ/NMP) composed of 4-amino-1-methylpiperidine (PD), piperazine (PZ), and <i>N</i>-methyl-2-pyrrolidone (NMP), engineered to produce easily separable powdery products. The PD/PZ/NMP absorbent achieves a CO₂ loading of 0.86 mol-CO₂/mol-amine, with 91% of CO₂ concentrated in the solid phase. It demonstrates excellent cyclic stability, maintaining a regeneration efficiency of 91% after five regeneration cycles, and reduces energy consumption by 52% compared with the conventional monoethanolamine absorbent. Remarkably, PZ plays a crucial role in regulating the crystal composition of solid-phase products, transforming them from a viscous state to a crystalline powder. Characterization and density functional theory analysis explain the crystal-phase regulation mechanism: PD absorbs CO₂ to form zwitterions (PDH⁺COO^–), affording viscous products, and PZ forms protonated amines (PZH⁺) and monocarbamates (PZCOO^–), which interact with PDH⁺COO^– via hydrogen bonding to form a crystalline powder. These findings demonstrate the CO₂ capture efficacy of the PD/PZ/NMP absorbent and provide a robust theoretical framework for fabricating solid–liquid biphasic absorbents tailored for practical applications.