CuO–ZnO/LTA Zeolite for Catalytic Ozonation of Methylene Blue: Calcination and DFT Insights

In this work, LTA-type zeolite synthesized from kaolin was successfully modified via ion exchange with Zn2+ and Cu2+ ions followed by thermal treatment at 550 °C and 650 °C, leading to the formation of ZnO–CuO/zeolite composites (ZA550ZnCu and ZA650ZnCu). The structural and physicochemical properties of the catalysts were investigated using XRD, SEM, EDS, XPS, TEM, and zeta-potential analysis, confirming the successful incorporation of metal oxides while preserving the zeolitic framework, particularly at moderate calcination temperature. The catalytic performance of the prepared materials was evaluated in the heterogeneous catalytic ozonation of methylene blue (MB) in aqueous solution. The results demonstrated a strong synergistic effect between adsorption on the zeolite surface and ozone activation by ZnO and CuO species, leading to the generation of reactive oxygen species (•OH, O2•−) responsible for rapid methylene blue degradation. Among the tested catalysts, ZA550ZnCu exhibited superior activity compared to ZA650ZnCu, attributed to better dispersion of active phases and higher structural stability. Operational parameters, including pH, catalyst dosage, and initial MB concentration, significantly influenced the decolorization efficiency. Alkaline conditions (pH 11) strongly enhanced MB removal due to increased hydroxyl radical formation. Kinetic analysis showed that the degradation process follows a pseudo-first-order model, with higher rate constants in the presence of the catalyst. The electronic structure and local reactivity of methylene blue were investigated through DFT calculations and used to support the identification of potentially reactive sites and to propose plausible initial oxidation steps. The most reactive sites were identified as the dimethylamino nitrogen atoms, the central sulfur and nitrogen centers, and certain carbon atoms of the conjugated framework using frontier molecular orbital analysis, global reactivity descriptors, Hirshfeld charges, condensed Fukui functions, and the dual descriptor. These results are consistent with an initial N-demethylation, oxidative attack on the heterocyclic chromophore, C–N and C–S bond cleavage, hydroxylation, and aromatic-ring opening followed by mineralization. The study highlights the effectiveness of natural-source zeolite-based bimetallic catalysts as promising and sustainable materials for wastewater treatment via catalytic ozonation.