Encapsulation of highly viscous CO₂ capture solvents for enhanced capture kinetics: modeling investigation of mass transfer mechanisms

The encapsulation of highly viscous liquid-like Nanoparticle Organic Hybrid Materials (NOHMs) inside a gas permeable polymer to form SIPs (Solvent Impregnated Polymers) significantly enhanced the CO₂ capture kinetics of NOHMs, leading to a remarkable 50-fold increase in CO₂ flux compared to the neat NOHMs. To understand the mechanism for enhanced CO₂ mass transfer within these hybrid materials, kinetic modeling of CO₂ uptake into SIPs containing polyethylenimine functionalized NOHMs, denoted NPEI-SIP, was conducted. CO₂ mass transfer into NPEI-SIP films was found to conform, both qualitatively and quantitatively, with a diffusion-controlled moving front model. The diffusion-controlled model was also used to simulate CO₂ uptake within a fixed bed containing polydisperse NPEI-SIP particles, and this model accurately predicted experimentally measured breakthrough curves at 25 °C and 50 °C. The 50-fold increase in gas flux was shown to be a consequence of the very large CO₂ permeability within the polymer–solvent composite. The increase in gas flux is also dependent on the diffusion–reaction regime in which the chemical solvent operates, and the largest improvement will occur when immobilizing solvents, such as NOHMs, which operate in the instantaneous-reaction regime. The CO₂ capacity (~3 mol CO₂/kg) and saturation time (~5 min) of 430 μm SIP particles were comparable to popular CO₂ chemisorption materials such as amine grafted silicates, in spite of the slow kinetics of NOHM-I-PEI in liquid form (CO₂ saturation time ~24 h for a 1 mm thin film).