Understanding the Structure-Property Relationships of Highly Well-Defined Metal Nodes over MIL-100(Cr) and its Derivatives

Active site density and homogeneity can significantly impact activity and selectivity. Traditional solid materials like zeolites and metal oxides often exhibit structural diversity, such as the distribution of heteroatom sites, the low percentage of metal loading, or the interference of metal aggregation, which make it difficult to understand the reaction properties and to control over the active site coordination environment within the pore systems. Metal-organic frameworks (MOFs) are a novel class of crystalline, large surface area materials constructed through the combination of isolated transition metal/metal oxide clusters and organic linkers. More specifically, the high degree of uniformity in open-metal coordination environment can be achieved and readily accessed in their native form, allowing for a more rigorous and systematic investigation of structure-property relationship over wide range of applications, such as gas storage, separation, and heterogeneous catalysis. In our first project, we explored CO2 adsorption onto isostructural Al-, Fe-, and Cr-containing MIL-100(M) materials, and show that CO2 adsorption can be amplified through larger open-metal site densities achieved using thermal activation protocols employing higher temperatures. We also tuned CO2 adsorption properties by systematically varying metal identity and oxidation state in MOFs, where isosteric heats of adsorption at zero coverage decrease in the order ((Cr) > (Fe) > (Al)), and CO2 exhibits a much higher propensity to bind to Cr2+ sites compared to Cr3+ sites.

In the next project, we used methanol dehydration as a probe reaction to decipher acid site properties and reaction pathway over MIL-100(Cr). In-situ titration that we employed in the study shows the origin of acid catalysis over MIL-100(Cr) occur over Bronsted acid sites that originate as a consequence of water ligation to open-metal sites. Steady state rate features, kinetic isotope effects, as well as steady state and transient infrared data can be rationalized using a dissociative scheme comprised of rate determining methanol-methanol dimer decomposition steps. The inhibition of dehydration rates by water can be accounted for by invoking the presence of methanol-water dimers that assume greater significance at higher water to methanol ratios.

In this final project, we synthesized novel chromium-based catalysts through one-step thermal decomposition of MIL-100(Cr). Our finding demonstrate that resulting Cr2O3 nanoparticles has showed promising catalytic activity in the CO oxidation reaction compared to bulk Cr2O3, which is rationalized by the higher surface area, hierarchical porosity, accessible, and large volumetric density of adsorbed oxygen species, as well as improved reducibility, that are crucial to the enhancement and effectiveness of catalytic properties that are not commonly seen in the bulk systems.