Precipitating Absorbents for Postcombustion CO ₂ Capture: A Comprehensive Review

The anthropogenic CO2 release is a primary driver of climate change. Postcombustion CO2 capture using chemical absorbents remains one of the most mature mitigation options, with cyclic absorption using aqueous alkanolamines, notably MEA 30 wt %, serving as the benchmark. However, these systems suffer from a significant energy penalty during regeneration. Precipitating absorbents have emerged as a promising alternative, maintaining the cyclic concept while introducing phase transitions that reshape expectations. This review provides a comprehensive, chemistry-centered overview of the four main classes of precipitating absorbents reported in the literature: amines, amino acid salts, hydrogen-bonded frameworks, and ionic liquids. It aims to provide an understanding of their fundamental principles, including the rationale behind their development and research trends. Particular attention is given to reaction and precipitation mechanisms, the nature of the precipitate, and key design considerations related to absorption, regeneration, and phase-transition behavior. A quantitative comparison with the benchmark MEA 30 wt % is also presented.