Development of the controllable solid–liquid biphasic solvent for CO2 capture: Molecular configurations and regulation mechanisms

Abstract

Solid-liquid phase change controllable absorbents can effectively regulate the solution absorption products as crystalline solids at the same time, but also to avoid the crystalline solid products in the early stage of the reaction throughout the continuous production, which can lead to equipment clogging, fouling and other problems. However, the structure-effect relationship of the regulator in the absorption solution is not clear at this stage. In this study, multistage amine/AMP/NMF solid–liquid phase change controllable adsorbents were fabricated to regulate the phase change nodes and product morphology of AMP/NMF-based solid–liquid phase change adsorbents using different structural multistage amines as regulators. Meanwhile, the effects of organic amines with different structural features as phase change regulators on solid–liquid phase change controllable absorbents were investigated by comparing the structural features (straight-chain amines, cycloalkyl amines, and symmetry of organic amines) and amine group types (primary and secondary amines). The results showed that ethylenediamine (EDA) and piperazine (PZ) could be used as solid–liquid phase change controllable absorbents for phase change modulation, however, the absorbents also underwent solid–liquid phase change near saturation and the products were in the form of crystal powder. Therefore, the influence mechanism of the regulators on the occurrence of phase change nodes and the formation state of the products is also described. The primary amino group tended to react with carbon dioxide (CO₂) to form carbamates and the secondary amino group tended to deprotonate. However, the protonated secondary amine group would weaken the interaction force between the products. During the reaction, mono-carbamates reduced the interaction strength of the process products and enhanced their solubility in the solvent making the reaction process homogeneous. The final product dicarbamate enhanced the interaction between the products and caused the products to self-aggregate and precipitate as crystalline solids. Straight chain amine EDA as a regulator, the final product was mainly through the strong hydrogen bonding effect, so that the product aggregation was crystalline; while the cycloalkyl amine PZ as a regulator, the final product could be not only through the strong hydrogen bond linked, but also formed a strong steric hindrance effect.