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Issue 28, 2010
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A novel method to measure diffusion coefficients in porous metal–organic frameworks

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Abstract

We present a novel method to determine diffusion constants of small molecules within highly porous metal–organic frameworks (MOFs). The method is based on the recently proposed liquid-phase epitaxy (LPE) process to grow MOF thin films (SURMOFs) on appropriately functionalized substrates, in particular on organic surfaces exposed by thiolate-based self-assembled monolayers (SAMs). By applying the LPE-method to SAM-coated quartz crystals, the time-dependence of the mass-uptake of the MOF when exposing it to a gas is measured by a quartz-crystal microbalance (QCM). The homogenous nature of the SURMOFs together with their well-defined thickness allow to analyze the QCM-data using Fickian diffusion to yield the diffusion constant. We demonstrate the potential of this method for the case of pyridine diffusion within HKUST-1 (Cu3(BTC)2) MOF, for which the diffusion coefficient at room temperature is found to amount to 1.5 × 10−19 m2 s−1. Assuming a Fickian diffusion and a hopping mechanism, we yield a binding energy of 0.78 eV of the pyridine to the Cu2+ sites within the HKUST-1 MOF, a value in good agreement with the results of precise ab initio quantum chemistry calculations.

Graphical abstract: A novel method to measure diffusion coefficients in porous metal–organic frameworks

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Submitted
05 Jan 2010
Accepted
22 Mar 2010
First published
07 Jun 2010

Phys. Chem. Chem. Phys., 2010,12, 8093-8098
Article type
Paper

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

O. Zybaylo, O. Shekhah, H. Wang, M. Tafipolsky, R. Schmid, D. Johannsmann and C. Wöll, Phys. Chem. Chem. Phys., 2010, 12, 8093
DOI: 10.1039/B927601G

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