Hierarchical Porous Metal-Organic Frameworks via Solvent Assisted Interpenetration and Linker Extraction

Abstract

Two different pillaring linkers, 4,4’- azopyridine (azopy) and bis(4-pyridyl) acetylene (bpa), were introduced to two dimensional sheets of [Ni(HBTC)(DMF)2] (where H3BTC = benzene-1,3,5-tricarboxylic acid and DMF = N,N-dimethylformamide) to form two isoreticular hms topological MOFs. Azopy was used to improve CO2 capturing ability since the interaction between the azo group and CO2 molecules had a potential for the amount of CO2 uptake. Bpa was selected to increase the stability of the structure due to the rigid alkyne group in the linker. Although bpa-hms [Ni(HBTC)(bpa)] had a larger porosity, azo-hms [Ni(HBTC)(azopy)] had a greater CO2 adsorption amount at the room temperature because of the amino group, interacting with CO2. For the pore size control, the hms structures were heated so to form interpenetrated structures (hms-c). Under the solvent-assisted environment, the neutral ligands were easily detached and then attached to the sheets. It was the first time to demonstrate the interpenetration in the complicated topology, besides pcu-net, through the post-synthetic heat treatment. After the interpenetration, thereby it was possible to tune the pore size, and enhance the stability of the frameworks. The reduced pore size also increased the interaction between the guest molecules and the frameworks. Meanwhile, to compensate the extremely reduced porosity of interpenetrated MOFs, the defect-engineering strategy was implemented. The neutral pillars in hms MOFs were able to be removed systematically under the thermal vacuum condition. The vacant sites formed the mesopores, and they deserved additional porosity. They can be utilized as active sites for chemical reactions and accelerating the mass transportation of guest molecules. Therefore, these hierarchical interpenetrated MOFs (hms-d) may be effectively applied to catalytic performance.