Valorization of geothermal sludge into Ni–Cu/SO₄–SiO₂ bifunctional catalysts for renewable oil hydrocracking: catalytic performance and reaction engineering assessment

The valorization of industrial waste into functional catalyst materials is a promising route for supporting renewable fuel production. However, the use of geothermal-sludge-derived silica as a catalyst support for hydrocracking non-edible oils remains insufficiently explored. In this study, silica-rich geothermal sludge was converted into sulfated silica (SO₄–SiO₂) and used as a support for Ni–Cu catalysts in the hydrocracking of castor oil. The catalyst was prepared through sulfation followed by sequential metal impregnation and characterized using XRD, conventional and pyridine-adsorbed FTIR, SEM–EDX, and N₂ adsorption–desorption analysis. Pyridine-FTIR identified predominantly Lewis acidity, with an accessible Lewis acid-site concentration of 0.0744 mmol g⁻¹, whereas the Brønsted-acid contribution was below the quantification limit under the applied measurement conditions.The NiCu/SO₄–SiO₂ catalyst retained a high specific surface area of approximately 199 m² g⁻¹, while SEM–EDX analysis suggested relatively distributed Ni and Cu species on the sulfated silica support. Hydrocracking experiments were performed in a batch reactor at 225–375°C under 20 bar H₂ for reaction times of up to 120 min. Under the investigated conditions, the highest conversion was 72.71 %, with liquid products distributed mainly in the gasoline-, kerosene-, and diesel-range fractions (C₅–C₂₂); diesel-range selectivity reached approximately 49.10 %. The conversion data were reasonably described by a lumped pseudo-first-order model, with apparent rate constants of 0.011–0.012 min⁻¹ and R² values above 0.92. These kinetic parameters should be interpreted as apparent descriptors rather than intrinsic rate constants. Because product selectivity was estimated from normalized GC–MS peak areas without full response-factor correction, the reported product distribution is semi-quantitative. The proposed deoxygenation and hydrocracking pathways are therefore discussed as plausible interpretations rather than confirmed mechanisms. This exploratory study indicates the potential of geothermal-sludge-derived silica as a catalyst-support precursor, while replicated catalytic testing is required to confirm performance reproducibility.