Adsorption, reactivity, and deactivation mechanisms on metal-organic framework catalysts and sorbents

Apr 4, 2024, 1:30 pm3:00 pm
A210 E-Quad (Lapidus Lounge)



Event Description

Metal-organic frameworks (MOFs) are crystalline solids that exhibit exceptional physicochemical tunability through metal node and organic ligand species design. Reticular chemistry advancements enable synthetic routes to control isolated metal active site identities and their surrounding coordination environment, yielding a system that can be well-characterized and utilized in fundamental studies.

This dissertation focuses on Fe and Cr carboxylates MIL-100 and MIL-101, which are investigated via spectroscopic, kinetic, and numerical methods to understand how material properties correlate with activity and at a molecular-scale, to define reaction, adsorption, and deactivation mechanisms for the liquid-phase oxidation of styrene by hydrogen peroxide (H2O2) and for CO2 capture.

Selective oxidation is central to produce modern chemicals and is sensitive to metal identity and the coordination environment around active metal sites. Comparison of isoreticular MIL-101(Fe) and MIL-101(Cr) in batch kinetics for styrene oxidation by H2O2 demonstrates ≥2× oxygenate production rates and higher selectivity to primary product benzaldehyde than to styrene oxide over Fe compared to Cr. Metal electron affinities govern the stabilization and distribution of metal-bound H2O2-derived intermediates (-OH, -OOH, =O) that determine oxygenate selectivities. Isometallic Fe-based MIL-100, MIL-101, and NH2-MIL-101 oxygenate production rates suggest that metal valency and inductive linker effects control reactivity while outer-sphere coordination effects, including H-bonding and defect densities, modulate selectivity. Finally, metal leaching is quantified as a dominant deactivation mode for MIL-101(Cr) but not for the Fe-based frameworks.

Beyond catalysis, MOF and aminopolymer-MOF composites demonstrate notable CO2 uptake capacities, though molecular-scale insights are lacking. Poly(ethyleneimine) (PEI) and poly(propyleneimine) (PPI) are incorporated into MIL-101(Cr) and are contextualized with Zr-based terephthalate UiO-67(Zr). Confinement reduces oxidative degradation in air for PEI/UiO-67(Zr), relative to unconfined polymer with amine efficiencies maintained/improved through storage in methanol rather than in air or other common solvents due to polymer conformational changes. In contrast to UiO-67(Zr), open Cr site densities in MIL-101(Cr), increased through missing linker defects, tether PPI and PEI, reducing amine efficiencies by decreasing polymer dynamics, but confer enhanced resistance to oxidative degradation during regeneration.

Overall, developing structure-function relationships for MOFs within probe catalytic and gas capture systems will inform future material formulations for more sustainable processes.