Self-Diffusion of Acetonitrile in a Covalent Organic Framework: Simulation and Experiment

Covalent Organic Frameworks have emerged as a new class of porous materials whose sorption properties have so far been studied primarily with physisorption techniques. Quantifying the self-diffusion of guest molecules in the interior of their nanometer-sized pores allows for a better understanding of confinement effects or transport limitations and is thus vital for various applications ranging from molecular separation to catalysis. Using a combination of pulsed field gradient nuclear magnetic resonance (PFG NMR) measurements and molecular dynamics (MD) simulations we have probed the self-diffusion of acetonitrile in the 1.7 nm diameter pore channels of two imine-linked COFs (PI-3-COF) featuring different levels of crystallinity and porosity, between 270 K and 300 K. In the sample showing higher crystallinity and porosity, we observe clear evidence for anisotropic diffusion parallel to the pore channel direction as characterized by a diffusion coefficient of D_par = 6.1 × 10^-10 m2/s at T = 300 K, consistent with 1D transport. Self-diffusion in the pores vs. bulk liquid is thus reduced by a factor of 7.4, in good agreement with MD simulations which predict a reduction of the self-diffusion coefficient by a factor of 5.4 compared to the bulk liquid value, assuming an offset-stacked COF layer arrangement. In contrast, more frequent diffusion barriers give rise to isotropic, yet significantly reduced diffusivities in the low-porosity sample (D_B = 1.4 × 10^-11 m2/s at T = 300 K). Our multimodal study thus highlights the significant influence of real structure effects such as stacking faults and grain boundaries on the long-range diffusivity of molecular guest species, while suggesting efficient intracrystalline transport at short diffusion times.