Investigating the Process and Mechanism of Molecular Transport within a Representative Solvent-Filled Metal–Organic Framework

Abstract

Effective permeation into, and diffusive mass transport within, solvent-filled metal–organic frameworks (MOFs) is critical in applications such as MOF-based chemical catalysis of condensed-phase reactions. In this work, we studied the entry from solution of a luminescent probe molecule, 1,3,5,7-tetramethyl-4,4-difluoroboradiazaindacene (BODIPY), into the 1D channel-type, zirconium-based MOF NU-1008 and subsequent transport of the probe through the MOF. Measurements were accomplished via in situ confocal fluorescence microscopy of individual crystallites, where the evolution of the fluorescence response from the crystallite was followed as functions of both time and location within the crystallite. From the confocal data, intracrystalline transport of BODIPY is well-described by one-dimensional diffusion along the channel direction. Varying the chemical identity of the solvent revealed an inverse dependence of probe-molecule diffusivity on bulk-solvent viscosity, qualitatively consistent with expectations from the Stokes–Einstein equation for molecular diffusion. At a more quantitative level, however, measured diffusion coefficients are about 100-fold smaller than expected from Stokes–Einstein, pointing to substantial channel-confinement effects. Evaluation of the confocal data also reveals a non-negligible mass transport resistance, i.e., surface barrier, associated with the probe molecule leaving the solution and permeating the exterior surface of the MOF. Permeation by the probe entails displacement of solvent from the MOF channels. The magnitude of the resistance increases with the size of the solvent molecule. This work draws attention to the importance of MOF structure, external-surface barriers, and solvent molecule identity to the overall transport process in MOFs, which should assist in understanding the performance of MOFs in applications such as condensed-phase heterogeneous catalysis.

Rapid Quantification of Mass Transfer Barriers in Metal–Organic Framework Crystals

“…However, both methods require specialized and expensive facilities. Methods for determining transport diffusivities include microscopy, in which concentration profiles within individual crystals are imaged as a function of time, ,,− and zero-length column (ZLC), a chromatographic technique in which a small sample amount is exposed to the adsorbate in a carrier gas until equilibrated and then the pure carrier gas is flowed through during which the concentration is recorded over time. − In principle, a straightforward way to determine transport diffusivities in nanoporous materials is to record the uptake as a function of time upon exposing the material to a step change in the surrounding fluid-phase concentration. This can be done using either a gravimetric or a volumetric adsorption system.…”

MOF-enabled confinement and related effects for chemical catalyst presentation and utilization

“…360 Once adsorbed, transport tends to be slow, with movement achieved mainly during interludes of desorption. A curious consequence of reactant adsorption to polarizable linkers is that apparent diffusivities can increase with increase channel loading; 264,361,362 once high-affinity adsorption sites are largely occupied, subsequently introduced reactants tend to spend greater fractions of their time in mobile, desorbed form. Other considerations are missing-linker type defects, which tend to enhance transport, 363 and crystallite permeation barriers, also called (external) surface resistances.…”

“…Other considerations are missing-linker type defects, which tend to enhance transport, 363 and crystallite permeation barriers, also called (external) surface resistances. 361 These can be caused, for example, by structural damage to pores at the crystallite perimeter or by partial collapse of channels at their termini. 364 Depending on its magnitude, surface resistance can replace transport diffusion as the rate-limiting step in delivering reactants to internal sites such as catalysts.…”

Stability and Degradation of Metal–Organic‐Framework Films under Ambient Air Explored by Uptake and Diffusion Experiments

“…These surface barriers hinder the access to the bulk part of the MOF film and slows down the uptake process. [18][19][20][21][22][25][26][27] In addition, some domains of the MOF material may be fully inaccessible, decreasing the uptake amount. When such defects act as a transport resistance for the molecular uptake in addition to the (intracrystalline) diffusion, the mass transfer is best described by a thin layer at the external MOF surface which has a reduced permeability α, referred to as surface permeability.…”