Probing Self-Diffusion of Guest Molecules in a Covalent Organic Framework: Simulation and Experiment

Area of Science:

• Materials Science
• Physical Chemistry
• Chemical Engineering

Background:

• Covalent Organic Frameworks (COFs) are porous materials primarily studied via physisorption.
• Understanding guest molecule self-diffusion in COFs is crucial for applications like molecular separation and catalysis.
• Confinement effects and transport limitations within COF pores require detailed investigation.

Purpose of the Study:

• To quantify the self-diffusion of acetonitrile and chloroform within the 1D channels of imine-linked COFs.
• To investigate the influence of crystallinity and porosity on molecular diffusion in COFs.
• To correlate experimental diffusion data with molecular dynamics simulations.

Main Methods:

• Pulsed field gradient nuclear magnetic resonance (PFG NMR) measurements.
• Molecular dynamics (MD) simulations.
• Utilized two imine-linked COFs (PI-3-COF) with varying crystallinity and porosity.

Main Results:

• Higher crystallinity/porosity COF showed anisotropic acetonitrile diffusion (D_par = 6.1(3) × 10⁻¹⁰ m² s⁻¹), indicating 1D transport and a 7.4-fold reduction vs. bulk.
• Lower porosity COF exhibited significantly reduced, isotropic diffusion (D_B = 1.4(1) × 10⁻¹¹ m² s⁻¹) due to diffusion barriers.
• Diffusion coefficients for chloroform followed similar trends in both high- and low-porosity samples.

Conclusions:

• Real structure effects (stacking faults, grain boundaries) significantly influence long-range molecular diffusivity in COFs.
• COF porosity and crystallinity critically dictate guest molecule transport behavior.
• Efficient intracrystalline transport can occur in COFs at short diffusion times despite structural imperfections.