Tuning CO₂ separation of Type I UiO-66 porous liquids by varying the size of porous hosts

Porous liquids (PLs), combine the high adsorption capacity of porous solids with the fluidity of liquids, show great promise in selective CO₂ capture. However, current studies on Type I PLs lacks systematic investigations into the regulatory role of porous host particle size and its impact on in-depth analysis of the gas sorption-separation process. Herein, we successfully fabricated PLs by grafting polyether amine oligomer chains onto the surface of size-tailored UiO-66-OH as the porous host, which endow the PLs with good thermal stability, transparency, low viscosity, and promising CO₂/N₂ separation. UiO-66-OH-PLs were fabricated by grafting polyether amine oligomer steric solvent onto UiO-66-OH via dehydrative condensation. The effect of porous host size on CO₂ separation behavior of UiO-66-OH-PLs and the selective sorption-separation mechanism were systematically investigated using sorption isotherm and kinetic models. UiO-66-OH-PLs exhibit markedly enhanced CO₂ sorption capacity relative to the pure steric solvent, confirming the preservation of accessible cavities in porous hosts. The PLs further exhibit distinct size-dependent physicochemical and sorption properties as decreasing the particle size of UiO-66-OH porous hosts increases oligomer grafting density, reduces bulk viscosity and enhances both CO₂ sorption capacity and CO₂/N₂ separation selectivity. CO₂ adsorption isotherms are best fitted by the SS-Langmuir-Freundlich model, indicating that CO₂ sorption by UiO-66-OH-PLs is single-component sorption on heterogeneous surfaces, integrating Langmuir monolayer sorption and Freundlich non-ideal sorption characteristics. Kinetic analysis reveals that the optimal pseudo-first-order kinetic model, demonstrating that CO₂ sorption rate is dominated by the sorption-desorption process between adsorbate molecules and adsorbent surface-active sites, rather than intraparticle or liquid film diffusion. This work opens up a promising avenue for the rational design of high-performance PLs that can be tailored to meet the demands of practical industrial chemical separation processes.