Adsorption and diffusivity of CO2 in phosphonium ionic liquid modified silica

• For IL modified silica, CO2 adsorption depends on the porous nature of the silica.
• The IL phases act as a selecting film which notably improves CO2/N2 selectivity.
• The grafting method provides a good IL retention with gas cycling.
• Diffusion coefficient in modified silica is in range of 10−8–10−7 m2/s using HSDM.

This work investigates novel hybrid sorbent materials based on ionic liquid (IL) modified porous silica for CO2 capture, aiming to tackle the limitations of the high viscosity of ionic liquids and to improve the selectivity of porous materials. Phosphonium based ionic liquids were covalently attached onto the surface of bare silica. CO2 adsorption isotherms at 273 K were used to characterize the porous structures. The adsorption and desorption behavior of CO2 (at 298, 313 and 333 K) and N2 (at 313 K) up to 2 bar was measured using a gravimetric method. The grafting of ILs on the support surface causes a loss of microporosity, resulting in a slight decrease in CO2 adsorption capacity. CO2/N2 selectivity is however enhanced. Adsorption thermodynamic study using the Freundlich equation suggests a physical sorption mechanism. The covalent grafting method provides a good IL retention with gas sorption/desorption cycling. The homogeneous surface diffusion model (HSDM) was used to estimate the diffusivities. The diffusion coefficients of CO2 in the hybrid adsorbents are level with that of unmodified silica at a level of 10−7–10−8 m2 s−1 and are about two to three orders of magnitude higher than that of neat phosphonium IL. For these IL modified silica, the gas adsorption behavior is dominated by the porous nature of the silica. The IL phases act as a selecting film which notably improves the CO2 sorption selectivity at a marginal expense of CO2 sorption capacity and diffusivity.