Mesoporous Silica-Polyethyleneimine Composites as High-Capacity Adsorbents for CO2 Adsorption: Isotherm and Thermodynamic Analysis

In this study, polyethyleneimine-mesoporous silica composite materials were prepared and the effectiveness of the promising sorbents in adsorbing CO₂ was evaluated, along with the impacts of the silica support types (Mesoporous Silica Nanoparticles (MSN) and Mobil Composition of Matter No.48 (MCM-48)), polyethyleneimine (PEI) loading percentages (50 and 70 wt.%), calcination, surface functionalization by alkyl chains (CTMABr), and adsorption temperature (75 and 100 °C). The analysis’s results revealed that the pores of the sorbents were mostly covered with PEI molecules following PEI-functionalization, and the specific surface area and pore volume were also reduced with rising amine content. The highest CO₂ adsorption capacities were achieved for UC-MCM-48–50 and UC-MSN–50 at 2.26 mmol/g and 3.31 mmol/g, respectively. The CO₂ uptake capacities of CC-MSN–50 and CC-MCM-48–50, composed by dispersing CTMABr surfactant with the calcined materials before incorporating PEI, were remarkably similar to those of non-surfactant functionalized adsorbents. When the temperature’s influence on CO₂ adsorption capacity was evaluated, the maximum holding capability adsorbent UC-MSN–50 had a slight increase in adsorption capacity (~ 3.6%), whereas UC-MCM-48–50 had a considerable drop (~ 23.9%) as the temperature elevated to 100 °C. Besides, Langmuir, Freundlich, Dubinin-Radushkevich, and Temkin isotherms were used to model pure CO₂ adsorption data, and a thermodynamic study was applied. In conclusion, a low-cost and more beneficial approach, which included less PEI handling and eliminating the calcination step, was implemented to enhance the CO₂ sorption capacity of composites of PEI with the long alkyl chain template MCM-48 or MSN silica support materials.