Ruhr-Universität Bochum, Lehrstuhl für Physikalische Chemie 1, 44780 Bochum, Germany.
Phys Chem Chem Phys. 2010 Jul 28;12(28):8092-7. doi: 10.1039/b927601g. Epub 2010 Jun 7.
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 (Cu(3)(BTC)(2)) MOF, for which the diffusion coefficient at room temperature is found to amount to 1.5 x 10(-19) m(2) s(-1). Assuming a Fickian diffusion and a hopping mechanism, we yield a binding energy of 0.78 eV of the pyridine to the Cu(2+) sites within the HKUST-1 MOF, a value in good agreement with the results of precise ab initio quantum chemistry calculations.
我们提出了一种新的方法来确定小分子在高度多孔的金属有机骨架（MOF）内的扩散常数。该方法基于最近提出的液相外延（LPE）工艺，在适当官能化的衬底上生长 MOF 薄膜（SURMOF），特别是在由硫醇基自组装单分子层（SAM）暴露的有机表面上。通过将 LPE 方法应用于 SAM 涂覆的石英晶体，通过石英晶体微天平（QCM）测量当暴露于气体时 MOF 的质量吸收的时间依赖性。SURMOF 的均匀性质及其明确的厚度允许使用菲克扩散分析 QCM 数据，以得出扩散常数。我们展示了这种方法在吡啶在 HKUST-1（Cu(3)(BTC)(2)）MOF 内扩散的情况下的潜力，在室温下发现吡啶的扩散系数达到 1.5 x 10(-19) m(2) s(-1)。假设菲克扩散和跳跃机制，我们得出吡啶与 HKUST-1 MOF 中 Cu(2+)位点的结合能为 0.78 eV，这与精确从头算量子化学计算的结果非常吻合。
