CO2 Adsorption by Amine-Functionalized MCM-41: A Comparison between Impregnation and Grafting Modification Methods

Modifications by impregnation and grafting are commonly used for the preparation of amine-functionalized MCM-41. A comprehensive evaluation of the advantages and disadvantages of the two methods was performed in this work. MCM-41 was synthesized by the hydrothermal method, setting the amine-loading mass fraction at 40, 50, and 60 wt %. Three amine-modified adsorbents were prepared by impregnating polyethylenimine (PEI), and the three other adsorbents were prepared by grafting 3-aminopropyltriethoxysilane (APTS) onto MCM-41. The as-prepared adsorbents were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, and N2 adsorption–desorption techniques. CO2 adsorption capacities were measured, and the experimental data were fitted with adsorption kinetic models. The cyclic stability of the adsorbents prepared by the two kinds of amine-modified methods was compared using the cyclic adsorption–desorption experiments. The characterization results showed that the target adsorbents were prepared successfully. The thermal stability of the adsorbents modified by grafting was better than the thermal stability of the adsorbents modified by the impregnation. Maximum CO2 adsorption capacities of 3.53 mmol g–1 (50% PEI–MCM-41) and 2.41 mmol g–1 (50% APTS–MCM-41) could be reached at 25 °C and 1 atm, which were 4.7 and 3.2 times greater than that of MCM-41. The Avrami model fitted the experimental data well, indicating a variety of interactions between the adsorbents and CO2. CO2 adsorption capacity after 5 adsorption–desorption cycles decreased by 14.22 and 5.19% for the adsorbents prepared by impregnation and grafting, respectively. It was concluded that MCM-41 modified by impregnation and grafting followed the same kinetic model. The absorbents modified by impregnation showed higher CO2 adsorption capacity and amine-loading efficiency, while those prepared by grafting had better thermal and cyclic stabilities.