Metal-organic frameworks (MOFs) have high metal contents and a proposed high degree of site homogeneity, making them attractive for rigorous kinetic and mechanistic investigation in metal-catalyzed reactions like selective formation and conversion of oxygenates. Here, iron-based MIL-100(Fe) is examined for liquid-phase reactions of styrene and its products in acetonitrile. Styrene oxidation over MIL-100(Fe) leads to the formation of styrene oxide and benzaldehyde, with further secondary reactions of the epoxide (to styrene glycol and phenylacetaldehyde) and benzaldehyde (to benzoic acid). These selectivities were altered by oxidant identity (hydrogen peroxide (HP) vs. tert-butyl hydroperoxide (TBHP)), where TBHP was more selective than HP for styrene oxide formation (52% vs. 4%, respectively). HP exhibited a higher extent of secondary ring opening of styrene oxide to styrene glycol, resulting from higher MIL-100(Fe) defect densities generated by iron and linker leaching during reaction. The mechanism of glycol formation, which is proposed to be catalyzed by either the Lewis acid (Fe) sites or Brønsted acid (defect) sites in the presence of water, was further investigated using in-situ titrations with various pyridine analogs. Product formation rates normalized by total Fe content decreased to a greater extent with pyridine than with the sterically hindered pyridines (2,4,6-trimethylpyridine and 2,6-di-tert-butylpyridine), suggesting that coordinatively undersaturated Fe sites were the predominant contributor to activity since bulky pyridines bind selectively to Brønsted acid sites. Additional characterizations like NH3 temperature programmed desorption, Mössbauer spectroscopy, and in-situ FTIR spectroscopy were used to gain further insights into site densities, Fe oxidation state, and nodal coordination environment before, during, and after reaction. Altogether, this work utilizes kinetic and spectroscopic techniques to investigate the local structure of MOF catalysts during relevant liquid-phase reactions.
