Polymerized Ionic Liquid Sorbents for CO₂ Separation

Abstract

This contribution describes a method to prepare adsorbent materials for recovery of carbon dioxide from mixed gases. The sorbents comprise a base support material and a covalently attached nanolayer coating of polymerized ionic liquid (pIL), poly([2-(methylacryloyloxy)ethyl]trimethylammonium chloride) (poly[(META) + Cl − ]). Initial measurements were made to determine the thickness evolution of the pIL nanolayers, using a model substrate designed to mimic the structure of the support material. Thereafter, the pIL was grafted from the surface of the support material using surface-initiated atom transfer radical polymerization. Subsequent ion exchange was done to replace Cl − anions with other anions to study the impact of pIL anion on CO 2 adsorption; X-ray photoelectron spectroscopy results confirmed that the ion-exchange reactions were quantitative. Anions that were tested were BF 4 −, CH 3 SO 3 −, and CF 3 SO 3 − . Carbon dioxide and nitrogen adsorption isotherms were measured from 0 to 2.5 bar at temperatures from 20 to 60 °C using a Wicke−Kallenbach cell. Measurements with N 2 showed no measurable adsorption at the highest pressure studied. Measurements on unmodified support material also showed no CO 2 uptake. CO 2 adsorbed to pIL-modified substrates with linear isotherms over the pressure range studied. Using the adsorption isotherm data at low pressures, Henry’s constants and heats of adsorption were measured. From these values, it was found that anions play a marginal role in CO 2 adsorption performance by these trimethylammonium-based pIL sorbents. Despite their high CO 2 /N 2 selectivity, the capacity values of the neat pIL materials were modest. Even for a relatively high CO 2 partial pressure of 0.78 bar, representing a pressure-swing operation used to process flue gas with 13 mol % CO 2 at 6 bar, the poly[(META) + CF 3 SO 3 − ] pIL had a capacity of 2 mmol/g on the basis of per mass of pIL. Opportunities exist for improving sorbent performance by changing polymer surface chemistry, and the synthesis methodology presented in this study can be applied to prepare a wide range of functional materials and yield new materials for CO 2 separations.