Magnetic core–shell catalysts based on mesoporous titanosilicates for limonene epoxidation
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- In the context of an unprecedented ecological crisis, improving the management of highly polluting systems is urgent. Food waste significantly contributes to global greenhouse gas emissions, thus its management via green chemistry is essential. Limonene, a monoterpene abundant in orange waste, emerges as an ideal substrate. Its epoxidation yields limonene-1,2-epoxide, a valuable alternative to traditional, polluting monomers used in polycarbonate synthesis, and a promising anti-tumoral compound. In this work a magnetic core-shell catalyst featuring a mesoporous titanosilicate shell was synthesized in three steps using the sol-gel method assisted by hydrothermal synthesis. The catalyst's performance was improved through three strategies: firstly, enhancing limonene accessibility by enlarging pore sizes; secondly, optimizing active titanium incorporation in the mesoporous shell by employing tetra propylammonium hydroxide (TPAOH) and hydrothermal treatment; and lastly, increasing the affinity for limonene through hydrophobic functionalization of the mesoporous shell. The catalyst was characterized by different techniques including FTIR-ATR, XRD, TEM, and SEM analyses. Hydrophobicity was assessed using FTIR-ATR and supported by TGA analysis. Surface titanium incorporation (active and inactive) was quantified by XPS, while bulk titanium content was measured by ICP-OES. Porosity characterization was performed through nitrogen adsorption. The hydrophobized catalyst CCoF@Ti-SiO₂ 15% MTES exhibited the highest catalytic activity in limonene epoxidation, achieving a limonene conversion of 41% with a selectivity towards limonene-1,2-epoxide of 59%. This catalyst demonstrated excellent structural and textural properties, including a framework titanium (FW-Ti) incorporation percentage of 79% and a specific surface area (SSA) reaching 501 m²/g. Additionally, the catalyst offered notable advantages, such as easy recovery due to its magnetic properties, high recyclability among other due its resistance to leaching, and robust chemical stability.