(625a) Elucidating the Role of Defects within Chromium MOFs for Carbon Transformations and Capture

Potential energy landscapes govern ad/de-sorptive and reactive elementary steps that dictate observed rates within catalytic systems and uptakes in gas capture applications. Defects within a material can impact these landscape topologies via perturbations to species adsorption/desorption energies, influencing energy barriers that determine species surface coverages. To investigate defects, metal-organic frameworks are used as they can be systematically tuned to incorporate differing defect densities, often using monodentate modulating agents. This work utilizes two distinct crystalline MIL-101(Cr) phases that contain disparate densities of undercoordinated Cr sites from missing linkers defects. For a probe catalytic transformation, styrene oxidation by hydrogen peroxide at mild reaction conditions (323 K, 1 h) was employed; the higher defect density phase MIL-101(Cr)-ρ_high demonstrates oxidation turnover rates (6.30±0.36 mM oxygenates s^-1 mmol Cr^-1) that are an order of magnitude larger than those for the lower defect density phase MIL-101(Cr)-ρ_low (0.19±0.02 mM oxygenates s^-1 mmol Cr^-1). Here, defective undercoordinated Cr sites have an intrinsically higher reactivity for oxygenate production because of lower barriers for oxidant activation to form reactive intermediates or for the formation of metallocycle transition state structures. Defects within MIL-101(Cr) also influence barriers for CO₂ adsorption within capture applications. MIL-101(Cr)-ρ_high shows a 33 % higher uptake capacity per gram than MIL-101(Cr)-ρ_low at 308 K from simulated flue gas compositions (10 % CO₂ in He), consistent with increased availability of undercoordinated Cr adsorption sites originating from missing linker defects. Further, Cr defect sites can effectively tether confined aminopolymer guest species, granting higher composite thermal and oxidative stability during uptake-regeneration cycling processes. Overall, deviations from nominal MOF structures perturb Cr site reactivities for styrene oxidation and serve as additional sites for interactions with CO₂ and amine adsorbents for capture applications.