Preparation and characterization of novel CO₂ "molecular basket" adsorbents based on polymer-modified mesoporous molecular sieve MCM-41

Novel CO₂ "molecular basket" adsorbents were prepared by synthesizing and modifying the mesoporous molecular sieve of MCM-41 type with polyethylenimine (PEI). The MCM-41-PEI adsorbents were characterized by X-ray powder diffraction (XRD), N₂ adsorption/desorption, thermal gravimetric analysis (TGA) as well as the CO₂ adsorption/desorption performance. This paper reports on the effects of preparation conditions (PEI loadings, preparation methods, PEI loading procedures, types of solvents, solvent/MCM-41 ratios, addition of additive, and Si/Al ratios of MCM-41) on the CO₂ adsorption/desorption performance of MCM-41-PEI. With the increase in PEI loading, the surface area, pore size and pore volume of the PEI-loaded MCM-41 adsorbent decreased. When the PEI loading was higher than 30 wt.%, the mesoporous pores began to be filled with PEI and the mesoporous molecular sieve MCM-41 showed a synergetic effect on the adsorption of CO₂ by PEI. At PEI loading of 50 wt.% in MCM-41-PEI, the highest CO₂ adsorption capacity of 246 mg/g-PEI was obtained, which is 30 times higher than that of the MCM-41 and is about 2.3 times that of the pure PEI. Impregnation was found to be a better method for the preparation of MCM-41-PEI adsorbents than mechanical mixing method. The adsorbent prepared by a one-step impregnation method had a higher CO₂ adsorption capacity than that of prepared by a two-step impregnation method. The higher the Si/Al ratio of MCM-41 or the solvent/MCM-41 ratio, the higher the CO₂ adsorption capacity. Using polyethylene glycol as additive into the MCM-41-PEI adsorbent increased not only the CO₂ adsorption capacity, but also the rates of CO₂ adsorption/desorption. A simple model was proposed to account for the synergetic effect of MCM-41 on the adsorption of CO₂ by PEI.