Among the many technologies used to separate gases, membrane-based separations are the most energetically-efficient option. Significant progress has been made recently in the development of highly efficient porous membranes to tackle challenging gas separations, such as CH4/CO2, CH4/CO, and olefin/paraffin separations. Isolating greenhouse gases such as CH4 and CO2 is essential in the fight against global warming. Many industrial applications, therefore, require membranes with even greater selectivity. A novel way to achieve an order of magnitude improvement in membrane selectivity is to exploit an anomalous molecular transport mode known as single-file diffusion (SFD), i.e., diffusion in narrow one-dimensional channels where gas molecules cannot pass each other in the channels. Recent results of molecular dynamics (MD) simulations indicate that the transport rate of a selected component in the nanochannels can be reduced by many orders of magnitude if the diffusion mechanism of this component is changed from normal diffusion to SFD.
To achieve this it is essential to have an understanding of mechanism of SFD in gas mixtures. However, currently there are no experimental studies that explore molecular SFD of gas mixtures. Here, we report the observation of single-file diffusion of pure CO and CH4 as well as their mixture in Ala-L-Val (AV) nanochannels by pulsed field gradient (PFG) NMR. 13C PFG NMR was applied to study diffusion of these 13C-labeled molecules. High magnetic field gradients up to 23T/m were used to study diffusion for root MSD as small as one micrometer. The reported PFG NMR data will be discussed in the context of relevant molecular simulation studies that have investigated SFD of mixtures.