In the context of global concern about climate change and greenhouse gas control, a new technology for CO₂ capture, utilization, and storage has attracted broad attention. 

Ionic liquids, composed of only cations and anions, are considered a new type of CO₂ adsorbent due to their ultralow vapor pressure and environmentally friendly features. 

Recently, a group led by Profs ZHANG Suojiang and HE Hongyan from the Institute of Process Engineering (IPE) of the Chinese Academy of Sciences (CAS) has found that two-dimensional ionic liquids show a completely different melting behaviour than the bulk phase, leading to a high CO₂ adsorption capacity and structural robustness during the CO₂ adsorption-desorption process. 

This study was published in Cell Reports Physical Science on July 12 (DOI: 10.1016/j.xcrp.2022.100979). 

The researchers found that ionic liquids can form a two-dimensional-monolayer, ordered, checkerboard structure when supported by a metal surface. The two-dimensional ionic liquids exhibited anomalous stepwise melting processes, involving localized-rotated, out-of-plane-flipped, and fully disordered states, rather than the single melting point for the bulk ionic liquids. 

"Anions and cations are arranged together in a checkerboard manner, thus forming a two-dimensional, ordered Z-bond network. This makes it more likely for the multi-step melting behaviors such as ionic rotation and flip," said Prof. HE. 

The massive molecular dynamics simulation indicates that the two-dimensional ionic liquids show excellent performance for CO₂ capture due to the unsaturated and exposed Z-bonds. The mole fraction of CO₂ adsorbed by two-dimensional [Mmim]PF₆ was improved by at least one order of magnitude compared with the corresponding bulk ionic liquids. 

"The higher CO₂ adsorption capability suggests that such two-dimensional ionic liquids could serve as functional layers of the catalyst to enhance the mass transfer process of CO₂, which is important for the fixation and conversion of CO₂," said Associate Professor WANG Yanlei of IPE. 

This two-dimensional editing technique for ionic liquids is expected to provide a new method for the precise control and functional design of liquids, which is promising for various chemical engineering applications involving solvents, electrolytes, and liquid catalysts. 

The ultrahigh CO₂ adsorption capacity of two-dimensional ionic liquids. (Image by IPE)