Kinetic Impacts of Defect Sites in Metal–Organic Framework Catalysts under Varied Driving Forces

Abstract

Defects in metal–organic frameworks (MOFs) primarily manifest as missing linkers or metal nodes induced through synthesis, post-synthetic modification, and/or exposure to reaction conditions. By changing the nature of active site(s) and perturbing crystalline frameworks, defects confer physico­chemical alterations to MOF catalysts that may promote or inhibit intrinsic reactivity, electron transfer and excitation, and mass transport. However, the complexity and dynamic character of defects often obfuscate the structure–function relations needed to permit rational catalyst design. Here, highlights of recent studies examining the impact of MOF defects in thermo-, photo-, and electro­catalytic systems pertinent to energy applications demonstrate progress toward identifying defect impacts on MOF catalysis, particularly for widely studied zirconium-based frameworks. Moreover, the combination of ex situ and operando/in situ defect identification and quantification will be paramount in future research to improve the mechanistic understanding of MOF-catalyzed systems for energy conversion but also extends to MOF energy storage, photovoltaics, and gas separations/storage applications.

Stability and kinetics of iron‐terephthalate MOFs with diverse structures in aqueous pollutant degradation

“…This material exhibits an intermediate conversion profile under identical catalyst mass loading and initial reactant concentrations, but a higher Fe‐normalized pseudo‐first order rate constant of 16 ± 3 mol Fe −1 s −1 (Figure S13) than the rate constant for crystalline MIL‐101. Incomplete coordination with terephthalate ligands may lead to crystallization with lack of long‐range order and Fe sites that have higher reactivity; such defects can have varied effects on reactions and have demonstrated rate enhancement in some cases 54–57 . Indeed, water content (via TGA) is greater in this material than crystalline MIL‐101 (4.4 ± 0.4 vs. 2.4 ± 0.2 mmol/g, respectively), suggesting a higher defect density since water binds to hydroxyl moieties from missing metals or linkers and node sites missing a capping ligand 55 .…”

“…Incomplete coordination with terephthalate ligands may lead to crystallization with lack of long-range order and Fe sites that have higher reactivity; such defects can have varied effects on reactions and have demonstrated rate enhancement in some cases. [54][55][56][57] Indeed, water content (via TGA) is greater in this material than crystalline MIL-101 (4.4 ± 0.4 vs. 2.4 ± 0.2 mmol/g, respectively), suggesting a higher defect density since water binds to hydroxyl moieties from missing metals or linkers and node sites missing a capping ligand. 55 While more detailed structurefunctional analysis of this framework is out-of-scope for this text, here it is observed that the presence of less ordered networks promotes reactivity.…”

MOFganic Chemistry: Challenges and Opportunities for Metal–Organic Frameworks in Synthetic Organic Chemistry

“…The structure of defects and thus their contribution to catalysis can also vary over the course of a reaction. Hence, computational and mechanistic studies guided by ex situ and operando/in situ ,,, characterization of defects, catalytic sites, and reaction intermediates are necessary to clarify the origin of catalytic activity and inform rational catalyst design.…”

MOFganic Chemistry: Challenges and Opportunities for Metal-Organic Frameworks in Synthetic Organic Chemistry

“…The structure of defects-and thus their contribution to catalysis-can also vary over the course of a reaction. Hence, computational and mechanistic studies guided by ex situ and operando/in situ 49,50,54,157 characterization of defects, catalytic sites, and reaction intermediates are necessary to clarify the origin of catalytic activity and inform rational catalyst design.…”