A novel method to measure diffusion coefficients in porous metal-organic frameworks

Area of Science:

• Materials Science
• Chemical Engineering
• Physical Chemistry

Background:

• Metal-organic frameworks (MOFs) are highly porous materials with tunable properties.
• Accurate determination of guest molecule diffusion within MOFs is crucial for applications.
• Current methods for measuring diffusion in MOFs can be complex and time-consuming.

Purpose of the Study:

• To present a novel, precise method for determining diffusion constants of small molecules in MOFs.
• To utilize the liquid-phase epitaxy (LPE) process for growing MOF thin films (SURMOFs).
• To quantify the diffusion of pyridine within the HKUST-1 MOF and calculate its binding energy.

Main Methods:

• Growing MOF thin films (SURMOFs) on quartz crystals coated with self-assembled monolayers (SAMs) using LPE.
• Measuring mass-uptake kinetics using a quartz-crystal microbalance (QCM).
• Analyzing QCM data with Fickian diffusion models to extract diffusion constants.

Main Results:

• Successfully determined the diffusion coefficient of pyridine in HKUST-1 MOF at room temperature to be 1.5 x 10(-19) m(2) s(-1).
• Calculated a binding energy of 0.78 eV for pyridine to Cu(2+) sites in HKUST-1, consistent with quantum chemistry calculations.
• Demonstrated the efficacy of the SURMOF-QCM method for diffusion studies.

Conclusions:

• The LPE-based SURMOF-QCM method provides a reliable approach for measuring diffusion constants in MOFs.
• The method offers insights into guest-host interactions and binding energies within MOF structures.
• This technique has significant potential for characterizing MOF materials for gas separation and storage applications.