A sustainable redox-mediated pathway for improved transition metal organic framework activation and CO2 uptake performance

Abstract

Metal organic frameworks (MOFs) have yet to appreciably become ubiquitous, manufacturable chemical products, despite their impressive functional versatility, particularly in the fields of separation, adsorption, and catalysis. Processing demands and high energetic costs contribute to suppressed material adoption. Eco-friendly substitutions at the process level must focus on the preserved performance (e.g. maximized storage capacity or conversion yield) of sustainably fabricated MOFs, with special consideration paid to the material defect character and metal cation active site valency. In this work, redox engineering was used as a strategy to tune the metal active site valency and, as a result, defect amount, for the well-known transition metal MOF, MIL-100(Fe). Tuning of redox equilibria, achieved via pH and reaction temperature reciprocity, permitted control of Fe²⁺/Fe³⁺, as verified by UV-vis and XPS analyses. With increasing amounts of Fe²⁺ framework cations, MOF defect content was likewise observed to increase, imbuing advantageous performance metrics when within an identified optimal range. CO₂ adsorption capacity served to evaluate active site identity and accessibility. 19% improved CO₂ uptake was achieved with optimum Fe²⁺/Fe³⁺ ratio, attributed to the presence of beneficial defects which expanded the material micro- and mesoporous area for accessible CO₂ coordination. Framework structural degradation, leading to decreased BET surface area and depressed CO₂ adsorption and selectivity, was observed in samples with the highest Fe²⁺ amounts, highlighting the narrow separation between productive and detrimental defects. Uniquely, defects originated at the metal node itself, rather than arising from heterogeneous mixed-linker or modulated approaches. Our redox pathway preserves green product quality by balancing valency/defect tradeoffs, thus alleviating industrial scaling demands and culminating in a flexible, more energetically conscious MOF generation pathway.