Transformation of α-Pinene and Limonene Over Water Treatment Sludge-Derived Catalysts

The mud provided by the process of purification of water contains metals such as aluminum, silicon and iron which have significant potential for catalytic applications. Sludge after thermal (LS600) and chemical treatment with HCl (LS600-HCl) and subsequently with iron (LS600-HCl-Fe7%) were used as catalysts in the transformation of α-pinene and limonene. The thermal treatment removed organic matter and concentrated the metal oxides, while the HCl treatment improved the acidity that was enhanced with the addition of iron. The LS600-HCl catalyst favored the isomerization of α-pinene with 85% conversion and 39% selectivity towards camphene, while over LS600-HCl-Fe7% catalyst, complete conversion of limonene was reached with 44% selectivity towards p-cymene. The treated materials showed increases in acidic sites, with Lewis acidity associated with metals such as Al and Fe. LS600-HCl and LS600-HCl-Fe7% materials had surface areas of 103 and 84 m²/g, respectively, dominated by mesopores, with slight morphological changes but greater roughness. Mössbauer analysis of the LS600-HCl-Fe7% material showed 36% magnetite and maghemite, and 64% paramagnetic phases such as hematite or goethite.