Transport Phenomena: Challenges and Opportunities for Molecular Catalysis in Metal-Organic Frameworks.

Ben A Johnson,Sascha Ott,Brian D Mccarthy,Anna M Beiler

doi:10.1021/jacs.0c02899

Ben A Johnson, Sascha Ott + Show 2 more

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https://doi.org/10.1021/jacs.0c02899

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Abstract

Metal–organic frameworks (MOFs) are appealing heterogeneous support matrices that can stabilize molecular catalysts for the electrochemical conversion of small molecules. However, moving from a homogeneous environment to a porous film necessitates the transport of both charge and substrate to the catalytic sites in an efficient manner. This presents a significant challenge in the application of such materials at scale, since these two transport phenomena (charge and mass transport) would need to operate faster than the intrinsic catalytic rate in order for the system to function efficiently. Thus, understanding the fundamental kinetics of MOF-based molecular catalysis of electrochemical reactions is of crucial importance. In this Perspective, we quantitatively dissect the interplay between the two transport phenomena and the catalytic reaction rate by applying models from closely related fields to MOF-based catalysis. The identification of the limiting process provides opportunities for optimization that are uniquely suited to MOFs due to their tunable molecular structure. This will help guide the rational design of efficient and high-performing catalytic MOF films with incorporated molecular catalyst for electrochemical energy conversion.

Highlights

Metal−organic frameworks (MOFs) are emerging as widely popular support structures to incorporate molecular catalysts for the electrochemical activation and conversion of small molecules.[1−3] Composed of discrete molecular building blocks,[4,5] MOFs have a wide range of synthetic tunability.[6,7]
We postulate in this Perspective that one critical, but often overlooked, aspect of utilizing MOFs for the molecular catalysis of electrochemical reactions is transport
A notable barrier to this realization and to the eventual application of these materials to catalytic processes at industrial scale is the coupling of transport phenomena to traditional chemical kinetics, as a consequence of immobilization of the molecular species in a finite 3D porous structure

Summary

■ INTRODUCTION

Metal−organic frameworks (MOFs) are emerging as widely popular support structures to incorporate molecular catalysts for the electrochemical activation and conversion of small molecules.[1−3] Composed of discrete molecular building blocks (organic linkers and metal nodes),[4,5] MOFs have a wide range of synthetic tunability.[6,7] Varying the microscopic molecular structure produces materials with different pore sizes and chemical environments, perhaps allowing for augmentation of the active site of molecular catalysts.[8]. Typical surface concentrations and electronhopping transport parameters reported in literature show that catalytic MOF films are able to reach the recognized minimum 10 mA cm−2 needed for large-scale applications[85] with a moderately active catalyst (25 turnovers per second); with a typical film thicknesses df = 1 μm the efficiency is rather low (see the shaded region in Figure 10 indicating the size of the reaction-diffusion layer). This assumes facile substrate transport through s−1) more typical of a freely the film The first and most straightforward strategy is to modify the film to an optimal thickness where df

≈ 1.15,67,68 Practical methods to make

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES