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Rapid and low overpotential oxidation of water to dioxygen remains a key hurdle for storage of solar energy. Here, we address this issue by demonstrating that deprotonation of 2-(2'-pyridyl)-imidazole (pimH)-ligated copper complexes promotes water oxidation at low overpotential and low catalyst loading. This improves upon other work on homogeneous copper-based water oxidation catalysts, which are highly active, but limited by high overpotentials. EPR and UV-vis spectroscopic evaluation of catalyst speciation shows that at pH ≥ 12 coordinated pimH is deprotonated and a bis(hydroxide) Cu active catalyst forms. Rapid electrochemical water oxidation (35 s, 0.85 V onset potential) was observed with 150 μM catalyst. These results demonstrate that catalytic water oxidation potentials can be shifted by hundreds of mV in homogeneous metal catalysts bearing an ionisable imidazole ligand.

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Separations based on molecular size (molecular sieving) are a solution for environmental remediation. We have synthesized and characterized two new metal–organic frameworks (MOFs) (Zn 2 M; M = Zn, Cd) with ultramicropores (<0.7 nm) suitable for molecular sieving. We explore the synthesis of these MOFs and the role that the DMSO/H2O/DMF solvent mixture has on the crystallization process. We further explore the crystallographic data for the DMSO and methanol solvated structures at 273 and 100 K; this not only results in high-quality structural data but also allows us to better understand the structural features at temperatures around the gas adsorption experiments. Structurally, the main difference between the two MOFs is that the central metal in the trimetallic node can be changed from Zn to Cd and that results in a sub-Å change in the size of the pore aperture, but a stark change in the gas adsorption properties. The separation selectivity of the MOF when M = Zn is infinite given the pore aperture of the MOF can accommodate CO2 while N2 and/or CH4 is excluded from entering the pore. Furthermore, due to the size exclusion behavior, the MOF has an adsorption selectivity of 4800:1 CO2/N2 and 5 × 1028:1 CO2/CH4. When M = Cd, the pore aperture of the MOF increases slightly, allowing N2 and CH4 to enter the pore, resulting in a 27.5:1 and a 10.5:1 adsorption selectivity, respectively; this is akin to UiO-66, a MOF that is not able to function as a molecular sieve for these gases. The data delineate how subtle sub-Å changes to the pore aperture of a framework can drastically affect both the adsorption selectivity and separation selectivity.

A pair of related metal–organic frameworks (Zn 3 and Zn 2 Cd) developed in our group were incorporated into Pebax 30R51 and PVDF Kynar 761 polymers to fabricate mixed matrix membranes (MMMs). These MOFs were chosen due to the carbon dioxide molecular sieving ability of Zn 3 , and the slightly larger pore aperture of Zn 2 Cd that allows carbon dioxide and larger gases to enter the pores. For Pebax-based MMMs, this work demonstrated an over two-fold and four-and-a-half-fold increase in carbon dioxide permeability for Zn 3 - (15 wt %) and Zn 2 Cd-containing (10 wt %) MMMs over the pristine polymer. Separation selectivity (CO2:N2) of 4.21 and 7.33 were observed for Zn 3 and Zn 2 Cd (10 wt %). For PVDF-based MMMs, the incorporation of Zn 3 and Zn 2 Cd (10 wt %) increased the carbon dioxide permeability approximately two- and three-fold. The CO2/N2 selectivity of the PVDF membranes increased 73% (1.01 to 1.86) and 68% (1.01 to 1.68) when 15 wt % Zn 3 and Zn 2 Cd were incorporated into PVDF. The improved performance of Pebax over PVDF based MMMs is attributed to matching the permeability of the polymer bulk phase (Pebax over PVDF) and the dispersed phase (Zn 3 and Zn 2 Cd). The lower permeability allows the MOF, which has slow kinetics associated with molecular sieving, to participate in the permeation process better. With regards to Zn 3 vs Zn 2 Cd, while Zn 3 acts as a molecular sieve and Zn 2 Cd does not, we hypothesize that the faster diffusion of carbon dioxide gas in Zn 2 Cd can outcompete the lower nitrogen gas permeability and molecular sieving properties of Zn 3 . However, we expect that further increasing the pore aperture would increase the permeabilities of nitrogen gas such that differences in diffusion kinetics due to molecular size would be unimportant.