Phase-Change Mechanism for Capturing CO ₂ into an Environmentally Benign Nonaqueous Solution: A Combined NMR and Molecular Dynamics Simulation Study

Understanding the phase-change mechanism is vital to the development and application of novel biphasic solvents for CO ₂ capture. However, this mechanism remains poorly investigated. Triethylenetetramine-polyethylene glycol (TETA-PEG200) solution is a typical solution that may exhibit liquid-solid phase-change phenomena after CO ₂ absorption. Results from ¹³ C NMR and FTIR analyses show the existence of same types of monocarbamates, dicarbamates, and alkylcarbonate in both the liquid and solid phase after the phase change occurs. In addition, monocarbamate is a main product that precipitates out from the solution. The spectroscopic analyses also indicate that there were hydrogen bonds between âNH ₃ ⁺ and âCOO ^- in the solid-phase products. Experimental work reveals that when the TETA concentration is in the range of 1-2 M, precipitation will happen as CO ₂ loading reaches â1 mol/mol TETA, and the process of precipitation is reversible by adding extra TETA. Based on experimental observations, a phase-change mechanism was then proposed, in which zwitterionic monocarbamate (ZM) is the main form of precipitates. Evidence from molecular dynamics simulations further confirmed that ZM had a tendency to form intermolecular and intramolecular hydrogen bonds, while reducing the number of hydrogen bonds with the solvent as CO ₂ loading increases, leading to self-aggregation.