Novel Nonaqueous PD/PZ/NMP Absorbent for Energy-Efficient CO2 Capture: Insights into the Crystal-Phase Regulation Mechanism 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 N-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.