Transport into Metal–Organic Frameworks from Solution Is Not Purely Diffusive

Abstract

In virtually all applications of metal-organic frameworks (MOFs), [1] it is important to understand and quantify the transport through their nanoscopic channels. [2] To date, most studies have focused on gas-phase systems and their modeling using molecular dynamics (MD) simulations. This effort has provided a wealth of information on the self-, corrected, and transport diffusivities of molecules from the gas phase into the MOF bulk. [3,4] Significantly fewer studies have examined molecular transport into solvent-filled MOFs [1b, 5] which is becoming increasingly important in heterogeneous catalysis. In the vast majority of these works, the transport has been described as simple Fickian diffusion. [5a, 6] Here, we combine experiment with theory to show that such a pure diffusion approach is a gross oversimplification and, instead, transport equations should be of the reaction-diffusion (RD) form. The RD description is appropriate at both the macroscopic scale (studied using the dye diffusion profiles) and the nanoscale (studied using fluorescence correlation spectroscopy).Currently available techniques for the quantification of transport into nano-or microporous materials (e.g., zeolites, [7] MOFs) include quasi-elastic neutron scattering (QENS), [8] pulsed field gradient nuclear magnetic resonance (PFG NMR), [9] interference microscopy (IFM) and infrared microscopy (IRM). [6a-b, 10] In particular, IFM and IRM, recently developed by Kärger's group, are two techniques which can monitor diffusion in individual nanoporous crystallites, though they cannot easily extract full, three-dimensional concentration profiles [11] -this difficulty might explain why both techniques have not yet been widely used. Other techniques, such as luminescence quenching [5a] and quartzcrystal microbalance (QCM), [6c] provide diffusivity data that are averaged over one or more MOF particles. As mentioned above, the concentration profiles observed during infusion of the MOF are usually explained using pure or effective diffusion models. While in some cases this approach seems to fit the data reasonably well, [5a, 6] there are other instances where it does not-for example, in the separations of liquid mixtures over MOF-crystal "columns" we recently described. [1b] The motivation for the present work is therefore two-fold. First, we wish to develop an experimental method in which the diffusion profiles into MOFs could be studied directly and without ambiguity and, importantly, using equipment that is available to most research groups. Second, we wish to provide a general theoretical framework with which to study transport into MOFs from solution. To meet these objectives, we combine the ability to grow large, millimeter-sized MOF crystals [1b, 12] (here, MOF-5) with confocal laser scanning microscopy (CLSM) [13] and fluorescence correlation spectroscopy (FCS) [14] which have been previously applied to measure the transport of dyes or fluorescently labeled molecules into protein crystal and other materials. The concentration pro...

Confinement of pyridinium hemicyanine dye within an anionic metal-organic framework for two-photon-pumped lasing

“…It should be pointed out that bio-MOF-1 significantly enriches DMASM cations into the pores from the solution (Supplementary Fig. S3), attributed to the energetically favourable interactions between the dye molecules and the MOF scaffold39. Owing to the strong electrostatic interactions between cationic dyes and the anionic framework within bio-MOF-1⊃DMASM , 0.8 mmol cm −3 DMASM cations can be loaded into the pores, indicating that most Me 2 NH 2 cations in the as-synthesized bio-MOF-1 have been replaced by DMASM cations in bio-MOF-1⊃DMASM .…”

Three-dimensional superdiffusive chemical waves in a precipitation system

“…Subdiffusion is encountered in highly heterogeneous media where surfaces tend to be disordered or porous thus trapping particles, like in gels 8 or metal-organic frameworks (MOFs). 11 In chemical systems, the dynamics of fronts and waves are governed by the interplay between mixing in the system (transport phenomena) and the interaction (e.g., chemical reactions) between the different species. Usually the mixing is merely due to molecular diffusion as a result of underlying Brownian motion.…”

A Metal–Organic Framework Stabilizes an Occluded Photocatalyst

“…In the UV/Vis spectra (Figure 2 b) the loaded catalysts exhibit an adsorption band centered at approximately 450 nm, similar to-but broader than-the free [Ru- ). In this case, the transport through the MOF [22] and the exchange of ligands with the outside solution are hampered by the tight fitting (cf. Most importantly, the occluded [RuA C H T U N G T R E N N U N G (bpy) 3 ]Cl 2 remains catalytically active and can photoreduce various metal salts to the corresponding nanoparticles.…”

“…In sharp contrast, irradiation of [Ru(bpy) 3 ]Cl 2 occluded in the CD‐MOF has marginal effect on its optical properties (Figure 2 e). In this case, the transport through the MOF22 and the exchange of ligands with the outside solution are hampered by the tight fitting (cf. above) of the catalysts occluded in the MOF’s cavities.…”