Synthesis Modulation of Metal-Organic Frameworks: Structural Defect Effects on CO₂ Adsorption and Liquid-Phase Catalysis

Abstract

Metal-organic frameworks (MOFs) have emerged as a promising class of porous materials for various chemical applications due to their high surface areas, tunable pore structures, and accessible metal active sites. However, whether MOF samples of the same chemical identity can have inherently distinct site environments induced by differing synthetic protocols and how those structural differences map onto differences in reactivity is not well-established. This project elucidates structural differences in three MIL-100(Fe) samples synthesized using two HF-free synthetic protocols and how these differences influence CO2 adsorption and styrene oxidation reactions. Batch A employed Protocol A, an extensive procedure using Fe(III) precursor under elevated temperatures, while Batch B1 and B2 utilized Protocol B, a simpler procedure using an Fe(II) precursor under ambient temperature. Structural characterization via TGA, FTIR, nitrogen physisorption, and XRD revealed that Protocol B batches had higher iron content and more missing linker defects. CO2 adsorption analysis demonstrated that Batch B2 (with the lowest BET surface area and total pore volume) reported higher CO2 capacity values and more coordinatively unsaturated sites (CUS) than Batch A. Hydrogen peroxide exposure increased CUS, while ethylenediamine functionalization distorted the local coordination environment. Styrene oxidation with tert-butyl hydroperoxide showed the Batch B2 exhibited higher turnover numbers and lower apparent activation barrier energies. Overall, these results demonstrate that MIL-100(Fe) samples with structural differences can form intrinsically different active sites with clear differences in reactivity and catalytic behavior, which can be used to inform optimized and rational design of MOFs for catalytic and gas separation applications.