Functionalization of Ti₃C₂ MXene with cholinium amino acid ionic liquids for enhanced CO₂ adsorption performance

The 2D structure and unique properties of MXenes have attracted significant research interest. In this study, Ti₃C₂ MXene was functionalized with cholinium amino acid ionic liquids ([Cho][AA]s) to synthesize IL@Ti₃C₂ MXene, aiming to expand interlayer spacing, enhance surface functionality, and improve CO₂ adsorption performance. The modification effectively increased the interlayer spacing, facilitating superior adsorption properties compared to pristine Ti₃C₂ MXene. The IL@Ti₃C₂ MXene composites were characterized using X-ray Diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Thermogravimetric Analysis (TGA), Raman Spectroscopy, Scanning Electron Microscopy (SEM) with Energy-Dispersive X-ray Spectroscopy (EDX), and Brunauer-Emmett-Teller (BET) surface area analysis, while CO₂ adsorption was evaluated via Quartz Crystal Microbalance (QCM) at 288.15-308.15 K and pressures up to 1 bar. Adsorption isotherms followed the Langmuir model, suggesting a predominant chemisorption mechanism involving amine functionalities. At 298.15 K and 1 bar, Ti₃C₂ MXene exhibited a CO₂ adsorption capacity of 1.95 mmol/g, while IL@Ti₃C₂ MXene composites displayed enhanced adsorption: [Cho][Arg]@Ti₃C₂ (3.25 mmol/g), [Cho][His]@Ti₃C₂ (2.92 mmol/g), and [Cho][Tyr]@Ti₃C₂ (2.64 mmol/g). These improvements were resulted from electrostatic interactions, hydrogen bonding, Lewis acid-base interactions, and interlayer expansion. The exothermic nature of adsorption, confirmed by negative enthalpy (ΔH) values, indicates an energetically favorable process.