Adsorption of ionic liquids onto activated carbons: Effect of pH and temperature

Adsorption isotherms and kinetics of three ionic liquids (1-methyl-3-octylimidazolium chloride, 1-butyl-3-methylimidazolium chloride and N-octylpyridinium bromide) were studied on several types of activated carbon (AC) – a microporous granular AC (from China), a microporous–mesoporous AC fabric (Zorflex from Calgon) and an AC prepared from Artichoke leaves using phosphoric acid activation. ACs were characterized by N2 adsorption measurement at 77 K (DFT pore size distribution), acido-basic titrations (Boehm method) and point of zero charge measurement. Isotherms were studied, using UV–visible spectrometry, in buffer solutions at pH = 2, 7 and 9, in the temperature range 20–50 °C. Kinetics models were found to be dependent on adsorbent type. Adsorption uptakes were correlated to the AC porous structures. The kind of interactions between organic cations and the carbon surface were found to be related to the amount of oxygenated groups and to the ionic liquid type. Presence of oxygen groups promoted electrostatic interactions which were stronger for the more hydrophilic cation (1-butyl-3-methylimidazolium). The calculated thermodynamic parameters showed that as compared to methylimidazolium, the adsorption of longer chain cations and pyridinium like cations was more favorable.

Difference between maximum adsorption uptakes (Qm) for BMIm+ (▴), OMIm+ (♢) and OPy+ (■) measured at pH = 9 and 2 as a function of the oxygen content of activated carbons (Chinese, Fabric and Artichokes).Figure optionsDownload as PowerPoint slideHighlights
► Adsorption of IL cations in ultramicropores is suggested.
► Adsorption interactions related to the amount of oxygenated groups and the IL type.
► Oxygen groups promote electrostatic interactions.
► Electrostatic interactions are stronger for the more hydrophilic cation.
► Adsorption of longer chain cations and pyridinium like cations more favorable.