Atomic layer deposition modified PIM-1 membranes for improved CO2 separation: A comparative study on the microstructure-performance relationships

The use of atomic layer deposition (ALD) technology has recently been extended to membrane-based gas sep-aration, whereas how the ALD of metal oxides can tailor the membrane microporosity, and the contribution of the interactions between ALD-grown metal oxide clusters and gas molecules to the resultant gas separation performance, remain largely unclear. Here, a comparative study was performed by individually confining three different metal oxides, including Al2O3, ZnO, and TiO2, to the near-surface region of polymers of intrinsic microporosity (PIM-1) membranes in an attempt to clarify the relationships between microstructural properties and CO2 separation performance. Based on experimental characterizations and density functional theory (DFT) calculations, it is suggested that ALD of metal oxides on PIM-1 membranes can not only effectively narrow the micropore sizes, but also enable enhancement of the solubility of CO2 molecules. Meanwhile, gas permeation results revealed that the CO2 permeation behavior through either Al2O3- or ZnO-modified membranes is diffusion-dominated, while that of the TiO2-modified membranes is sorption-dominated, primarily due to the distinct differences in the loading amount of metal oxides and their interaction strength with the CO2 molecules. As a result, the TiO2-modified membranes achieved a significant increase in the CO2 permeability as the ALD cycle numbers increased together with no sacrifice of CO2/N-2 and CO2/CH4 selectivities. This study is expected to lend important insight to the development of ALD-modified membranes for efficient CO2 separation.