Modification of MCM-41 type structures by carbon deposition and acid washing for CO₂ adsorption

A modification of the MCM-41 mesoporous materials through carbon deposition followed by acid washing has been studied for application in carbon dioxide adsorption. Two routes were used to prepare pure silica (Si-MCM-41) and Al-containing (Al-MCM-41) samples, through hydrothermal synthesis at 150 °C for 24 h. The post-synthesis treatment took place in two stages: carbon deposition using commercial glucose, in the mass ratio of 1 g (support): 2.2 g (glucose): 3.5 g (water), followed by washing with 0.2 M hydrofluoric acid solution at 65 °C for 2 h. The samples were characterized by X-ray diffraction, dispersive energy X-ray spectrometry, nitrogen adsorption–desorption, Fourier-transform infrared spectroscopy, thermal analyzes, and scanning electron microscopy. CO₂ adsorption tests were performed on a thermogravimetric balance at 40 °C under atmospheric pressure. Carbon deposition led to a drastic reduction in the surface area and pore volume values of MCM-41 samples, possibly due to deposits of carbonaceous species blocking part of the mesopores and defect sites in the pore wall as self-assembled monolayers. After acid washing, part of these deposits was removed, as well as the occurrence of structural silicon leaching. The post-synthesis treatment allowed the generation of structures with a bimodal pore system, providing a substantial increase in the CO₂ adsorption capacity compared to the as-synthesized mesoporous materials (~66% for pure silica and 44% for Al-containing MCM-41 sample), representing a significant improvement in the adsorbents performance for CO₂ capture.