Natural gas sweetening, especially CO₂ cleaning, is an important prerequisite in many different applications and industrial areas for meeting increased environmental requirements. Ionic liquids as green solvents have attracted great attention due to the unique properties, e.g., extremely low vapor pressure, non-flammable, excellent solvent power for organic and inorganic compounds and easy to be modified structurally to elicit desired physical properties. As a result, ionic liquid membranes are emerging as promising candidates rivaling with conventional amine scrubbing. This brief review presents a survey of the most recent development in ionic liquid membranes for natural gas sweetening. Membranes based on all kinds of ionic liquids, e.g., conventional ionic liquids, task-specific ionic liquids, polymeric ionic liquids and ionic liquid mixtures have been reviewed. The properties of ionic liquids are greatly affected by structures, such as alkyl chain length on cation, anion size and functionalization of both cations and anions. In addition to the above effects, performances of ionic liquid membranes are also involved in pore size and hydrophobicity/hydrophilicity of supporting membranes, humidity/water content and free ionic liquids in poly ionic liquids. Furthermore, the future developments of ionic liquid membranes for gas sweetening are suggested aiming to expand these membranes to large scale processes.

Contents
1 Introduction
2 Kinds of ionic liquids
3 Ionic liquid membranes for acid gas capture
3.1 Membranes based on conventional ionic liquid
3.2 Membranes based on task-specific ionic liquid
3.3 Membranes based on polymerized ionic liquid
3.4 Membranes based on ionic liquid mixture
4 Conclusions and outlook