(754a) Microscopic View on Different Types of Transport of Light Gases in Mixed-Matrix Membranes: A Diffusion NMR Study

Mixed-matrix membranes (MMMs) are promising gas-separation membranes which consist of molecular sieve particles dispersed throughout a polymer matrix. Recently, reports in the literature have communicated examples of MMMs with remarkable sorption and diffusion selectivities which exceed the intrinsic properties of the polymer matrix. While macroscopic transport measurements are important for industrial application, gaining a complete microscopic understanding of the transport properties of MMMs from only macroscopic observations is not feasible due to the complexity of MMM structure.  

Here we report an application of a pulsed field gradient (PFG) NMR technique to resolve different types of gas diffusion in MMMs. Such resolution was made possible by employing PFG NMR with a high field (17.6 T) and high field gradients (30 T/m).  The diffusion types that were resolved, viz. studied separately, by this microscopic technique include gas diffusion inside the molecular sieve particles, transport in the polymer matrix between the particles and exchange of gas molecules between the molecular sieve particles and the polymer matrix. As an example, we present microscopic diffusion results for ethylene sorbate in a MMM composed of 6FDA/DAM polymer and ZIF-8 particles. It will be shown that a detailed knowledge of the different types of microscopic diffusion revealed by PFG NMR enhances understanding of long-range gas transport, i.e. transport on the length scales of displacements comparable with the thickness of MMMs.