H2/CO2 separations in multicomponent metal-adeninate MOFs with multiple chemically distinct pore environments.

Metal-organic frameworks constructed from multiple (≥3) components often exhibit dramatically increased structural complexity compared to their 2 component (1 metal, 1 linker) counterparts, such as multiple chemically unique pore environments and a plurality of diverse molecular diffusion pathways. This inherent complexity can be advantageous for gas separation applications. Here, we report two isoreticular multicomponent MOFs, bMOF-200 (4 components; Cu, Zn, adeninate, pyrazolate) and bMOF-201 (3 components; Zn, adeninate, pyrazolate). We describe their structures, which contain 3 unique interconnected pore environments, and we use Kohn-Sham density functional theory (DFT) along with the climbing image nudged elastic band (CI-NEB) method to predict potential H2/CO2 separation ability of bMOF-200. We examine the H2/CO2 separation performance using both column breakthrough and membrane permeation studies. bMOF-200 membranes exhibit a H2/CO2 separation factor of 7.9. The pore space of bMOF-201 is significantly different than bMOF-200, and one molecular diffusion pathway is occluded by coordinating charge-balancing formate and acetate anions. A consequence of this structural difference is reduced permeability to both H2 and CO2 and a significantly improved H2/CO2 separation factor of 22.2 compared to bMOF-200, which makes bMOF-201 membranes competitive with some of the best performing MOF membranes in terms of H2/CO2 separations.

Designing Open Metal Sites in Metal-Organic Frameworks for Paraffin/Olefin Separations.

Incorporating open metal sites (OMS) into metal-organic frameworks allows design of well-defined binding sites for selective molecular adsorption, which has a profound impact on catalysis and separations. We demonstrate that Cu(I) sites incorporated into MFU-4l preferentially adsorb olefins over paraffins. Density functional theory (DFT) calculations show that the OMS are independent, with no dependence of binding energy on olefin loading up to one olefin per Cu(I). Experimentally, increasing Cu(I) loading increased olefin uptake without affecting the binding energy, as predicted by DFT and confirmed by temperature-programmed desorption. The potential of this material for olefin/paraffin separation under ambient conditions was investigated by gas adsorption and column breakthrough experiments for an equimolar ratio of olefin/paraffin. High-grade propylene and ethylene (>99.999%) can be generated using temperature-concentration swing recycling from a Cu(I)-MFU-4l packed column with no measurable paraffin breakthrough.