Dye adsorption selectivity in pristine and aluminum-doped MCM-41 mesoporous silica: A density functional theory investigation

Abstract

The adsorption behaviors of cationic (methylene blue), neutral (neutral red), and anionic (methyl orange) dyes on pristine MCM-41 and aluminum-doped MCM-41 (Al-MCM-41) mesoporous silica represented as surface slab cluster model were investigated using ONIOM2(revPBE-D4/def2-SVP:GFN-FF) geometry optimizations and CPCM-revPBE-D4/def2-TZVP energy calculations. This study aims to elucidate molecular-level interactions and the impact of aluminum doping on dye adsorption selectivity. Methyl orange exhibited the strongest average adsorption energy on both MCM-41 (−42.17 ± 8.28 kcal/mol) and Al-MCM-41 (−38.49 ± 4.76 kcal/mol), likely due to multiple hydrogen bonds and reduced steric hindrance arising from its molecular shape and orientation. An independent t-test analysis was carried out to assess the statistically significant changes in the average adsorption energy upon doping. Aluminum substitution enhanced the averaged adsorption energy of cationic methylene blue (from −20.08 ± 6.24 to −23.31 ± 5.45 kcal/mol), while reducing that of anionic methyl orange. The adsorption of the neutral red dye remained statistically unchanged. These findings suggest that aluminum doping of MCM-41 can selectively enhance cationic dye adsorption from aqueous solutions, primarily through modified electrostatic interactions at the surface.