What Matters to Fabrication of Type II Porous Liquids: A Case Study on Metallocages and Bulky Ionic Liquid?

Porosity in bulky solvents can be created by the methods of dispersing and dissolving porous hosts or by their chemical adornment. And the ensuing liquids with cavities offer requisite high gas uptakes. Intriguingly, metal-organic cages (MOCs) as discrete nanoporous hosts have been utilized recently as soluble entities to obtain a series of interesting type II porous liquids (PLs). Yet, factors affecting the fabrication of type II PLs have not been disclosed. Herein, three metallocages (NUT-101, ZrT-1-NH₂, and ZrT-1) with the same zirconocene nodes but different organic ligands are chosen as porous hosts and a polyethylene-glycol (PEG) linked bis-imidazolium based IL, IL(NTf₂), is used as a bulky solvent. It is revealed for the first time that the generation of type II PL depends upon the flexibility of MOCs and the interaction between MOCs and solvent molecules. The maximum solubility is observed with NUT-101 (5%) in IL(NTf₂) while ZrT-1-NH₂ and ZrT-1 remain least soluble (0.5% and 0.2%). As a result, PL-NUT-101-5% with most intrinsic cavities shows higher CO₂ uptake (0.576 mmol g^-1) than PL-ZrT-1-NH₂-0.5% and PL-ZrT-1-0.2% as well as those reported type II PLs.