Design rationale of thermally responsive microgel particle films that reversibly absorb large amounts of CO₂: fine tuning the pK_a of ammonium ions in the particles

Herein we revealed the design rationale of thermally responsive gel particle (GP) films that reversibly capture and release large amounts of CO₂ over a narrow temperature range (30-75°C). The pK_a value of ammonium ions in the GPs at both the CO₂ capture temperature (30°C) and release temperature (75°C) is found to be the primary factor responsible for the stoichiometry of reversible CO₂ capture by the amines in the GP films. The pK_a values can be tuned by the properties of GPs such as volume phase transition temperature (VPTT), size, swelling ratio, and the imprinted microenvironment surrounding the amines. The optimal GP obtained according to the design rationale showed high capture capacity (68 mL CO₂ per g dry GPs, 3.0 mmol CO₂ per g dry GPs), although the regeneration temperature was as low as 75°C. We anticipate that GP films that reversibly capture other acidic and basic gases in large amounts can also be achieved by the pK_a tuning procedures.