Investigating the Influence of Hexanuclear Clusters in Isostructural Metal-Organic Frameworks on Toxic Gas Adsorption

The efficient capture of toxic gases, such as ammonia (NH3) and sulfur dioxide (SO2), can protect the general population and mitigate widespread air pollution. Metal-organic frameworks (MOFs) comprise a tunable class of adsorbents with high surface areas that can meet this challenge by selectively capturing these gases at low concentrations. In this work, we explored how modifying the metal ions in the node of an isostructural MOF series from a transition metal to a lanthanide or actinide influences the electronic environment of the node-based active site. Next, we investigated the adsorption properties of each MOF toward the relatively basic NH3 and relatively acidic SO2 gases. Within the NU-907 family of MOFs, we found that Zr6-NU-907 exhibits the best uptake toward NH3 at low pressures, while Th6-NU-907 demonstrates the best low-pressure performance for SO2 adsorption. Tracking the infrared (IR) stretching frequency of the node-based μ3-OH groups provides insights into the electronegativity of the metal ion and suggests that the most electronegative metal ion (Zr) affords the node with the best NH3 uptake at low pressures. In contrast, the Th6 node contains additional coordinated water groups relative to the other M6 nodes, which appears to yield the MOF with the greatest affinity for SO2 uptake that occurs predominately through reversible physisorption interactions. Finally, in situ NH3 IR spectroscopic studies indicate that both NH4+ and Lewis-bound NH3 species form during adsorption. Combined, these results suggest that tuning the electronic properties and structure of the node-based active site in an MOF presents a viable strategy to change the affinity of an MOF toward toxic gases.