Transport, Zwitterions, and the Role of Water for CO2 Adsorption in Mesoporous Silica-Supported Amine Sorbents

The uptake of CO2 in highly loaded, silica-supported, polyethylenimine (PEI)-impregnated sorbents was investigated in a reaction-diffusion model of the CO2 adsorption process. The model successfully replicated the pseudoequilibrium behavior experimentally observed in thermogravimetry (TGA) experiments. A parametric study and sensitivity analysis of the model revealed that the stability and mobility of diffusive intermediates?assumed in the model to be zwitterions?effectively control the observable capacity of the sorbent. A subsequent quantum chemical study called into question the stability of zwitterions in PEI but suggested that physically bonded moieties involving water, amines, and CO2 may be better candidates for diffusive intermediates. The implications are a strong dependence of the observable CO2 capacity of the sorbent on the presence of water in the gas stream, which was found to be consistent with TGA results.