In Operando Analysis of Diffusion in Porous Metal‐Organic Framework Catalysts

Abstract

The potential to exert atomistic control over the structure of site‐isolated catalyst sites, as well as the topology and chemical environment of interstitial pore spaces, has inspired efforts to apply porous metal‐organic frameworks (MOFs) as catalysts for fine chemical synthesis. In analogy to enzyme‐catalyzed reactions, MOF catalysts have been proposed as platforms in which substrate confinement could be used to achieve chemo‐ and stereoselectivities that are orthogonal to solution‐phase catalysts. In order to leverage the tunable pore topology of MOFs to impact catalyst selectivity, catalysis must proceed at interstitial catalyst sites, rather than at solvent‐exposed interfacial sites. This Minireview addresses challenges inherent to interstitial MOF catalysis by 1) describing the diffusional processes available to sorbates in porous materials, 2) discussing critical factors that impact the diffusion rate of substrates in porous materials, and 3) presenting in operando experimental strategies to assess the relative rates of substrate diffusion and catalyst turnover in MOF catalysis. It is anticipated that the continued development of in operando tools to evaluate substrate diffusion in porous catalysts will advance the application of these materials in fine chemical synthesis.

Deciphering the Effect of Microbead Size Distribution on the Kinetics of Heterogeneous Biocatalysts through Single-Particle Analysis Based on Fluorescence Microscopy

“…Hence, the effect of the bead size on the enzyme performance under aqueous conditions is more dramatic when using bulky and hydrophobic substrates (TFA and benzaldehyde) than using small and polar ones (acetone and formic acid). In porous and functionalized materials, both the polarity and size of the substrates affect their diffusion through the carriers where the active phase is immobilized [4,33]. Furthermore, the nature of the solid materials (hydrophilic or hydrophobic) can also hamper the diffusion of the substrates through their porous microstructure [14].…”

“…For this reason, enzyme technologists are encouraged to develop 2 of 16 highly active and stable heterogeneous biocatalysts [2]; however, molecular characterization of the supported enzymes is rather limited. Thus far, techniques for the molecular characterization of solid-supported enzymes are scarce, unlike chemical catalysis, where molecular characterization drives the design and optimization of heterogeneous catalysts [3,4]. Recently, several authors have deeply reviewed different techniques for the advanced characterization of heterogeneous biocatalysts [5][6][7].…”

Influence of Pore Size on Carbon Dioxide Diffusion in Two Isoreticular Metal–Organic Frameworks

“…where equation [1] gives the chemical shift (δ) of CO2 located in a crystal at angle θ relative to B0, given residual chemical shift anisotropy parameters δ// and δ⊥. Equation [2] gives the effective diffusion coefficient for CO2 in a given crystal obtained from PFG NMR measurements, where the magnetic field gradient is applied along the z-direction (Deff,z). Equations [3] and [4] give the signal decay observed in pulsed-field gradient NMR experiments performed with the 13-interval sequence, 46 where I(b) is the integrated signal intensity for a given b value, g is the applied magnet field gradient strength, b0 is the b value at the smallest g used, and γ is the gyromagnetic ratio of the studied nucleus (see also Figure S13).…”

Nanoscale metal–organic frameworks as key players in the context of drug delivery: evolution toward theranostic platforms

“…These key features responsible for its success as DDS are the following: i) nanoscale materials; nowadays there are diverse synthetic methods for the preparation of MOFs at the nanoscale [13,[37][38][39][40], and these nanosized MOFs (below 200 nm) are superior as DDS due to their higher cellular uptake (internalization rates), longer bloodcirculation times, and enhanced permeability and retention (EPR) effect to increase the drug concentration at tumor regions [43, 52, 61-]. Moreover, the smaller the size (i.e., shorter diffusion paths [62,63]) the faster the delivery kinetics are.…”