Nonaqueous amino-phenolic dual-functionalized ionic liquid absorbents for reversible CO2 capture: Phase change behaviors and mechanism

Abstract

In order to improve the recycling capacity, regenerability and energy efficiency of ionic liquid (IL) based CO2 capture technology, a series of nonaqueous phase change solvents comprising amino-phenolic dual-functionalized ILs and polyethylene glycols (PEGs) were proposed for CO2 capture in this research. The capture performance results demonstrated that the 20 wt% diethylenetriamine-2-bromophenolate(D2Br)-PEG200 and diethylenetriamine-4-bromophenolate(D4Br)-PEG200 solutions with slight phase change could realize high CO2 loading of 1.923 and 1.859 mol/mol IL and CO2-rich phase constituted volume fractions lower than 70%. The absorption ability kept stable at relatively high temperature and the two systems could be rapidly regenerated at 363 K with cyclic regeneration rate above 85% after ten consecutive cycles. The weak thermal stability of the absorption products was proved by TG-DSC analysis, contributing to improved regeneration performance. ATR-IR and 13C NMR analysis confirmed that the amino groups of IL reacted with CO2 to form carbamates, and phenol groups through 1:1 mechanism to form alkyl carbonates, guaranteeing enhanced recycling capacity and regenerability. Spectroscopic analysis and theoretical calculation results revealed the different phase change behaviors presented in PEGs solutions were mainly derived from the product-solvent interaction, which was more influenced by hydroxyl content. The interactions between the product ion pairs could be the main cause of the unstable state of the solutions and phase separation. The different patterns of hydrogen bonding established between IL and PEG molecules could maintain a relatively strong product-solvent interaction in the reacted solution, which would partially overcome the interactions between protonated amine and carbamate, thus delaying the phase transition formation and forming a slight solid–liquid phase change, potentially beneficial for CO2 capture. The two systems experienced a low reaction heat of 1.315 and 0.782 GJ/t CO2, which could significantly reduce regeneration heat load. Therefore, the amino-phenolic dual-functionalized IL solutions are proposed to be a competitive and reversible CO2 trapping agent.