From kaolin to catalyst: Ni on quartz-amorphous silica for carboxylic acid-free green fuel production

Ni/SiO₂ catalysts were synthesized from kaolin as a sustainable silica source using a low CTAB-assisted method (0.005–0.04 mol) to investigate the effect of surfactant concentration on structure and catalytic performance in waste cooking oil (WCO) deoxygenation. The results show that CTAB plays a crucial role in tuning mesoporosity and pore architecture. An optimal catalyst was obtained at 0.02 mol CTAB, exhibiting balanced mesoporous properties (S_meso = 27.6 m²/g, pore diameter = 3.26 nm) and the highest surface Ni content (13.27%), leading to superior selectivity toward diesel-range hydrocarbons (C₁₁–C₁₈) and improved conversion (~70%). In contrast, excessive CTAB (0.04 mol) increased surface area and pore size (~7.34 nm) but weakened confinement effects and metal-support interactions, limiting catalytic selectivity. The absence of carboxylic acids confirms effective deoxygenation via decarboxylation and decarbonylation pathways. TGA analysis reveals that the optimal catalyst forms predominantly soft and hard coke without very hard coke, indicating good coke tolerance. XRD results confirm structural stability after reaction. These findings highlight that controlling CTAB concentration is essential to achieve an optimal balance between pore structure and surface Ni content, enabling efficient and sustainable catalyst design for green diesel production.