Direct production of biodiesel from waste oils with a strong solid base from alkalized industrial clay ash

Solid base synthesized from SBC ash for biodiesel production from waste oils with 8 cycles and anti-saponification. It was further magnetized for easy separation. [Display omitted] •Direct production of biodiesel at 65 °C from waste oil by solid base was achieved.•Solid base from alkalized SBC ash had basic sites from Na2SiO3 and Na2SiAlO4.•Biodiesel from soybean oil reached >99% yield with 8 cycles (>95%).•Biodiesel yield was 96% from SBC oil (AV 10) at 65 °C (vs. 120 °C with H2SO4).•It was magnetized for separation with biodiesel yield 99% for 3 cycles (87%). Biodiesel was directly one-step produced from waste oils without pretreatment catalyzed by a solid base alkalized from spent bleaching clay (SBC) ash. Optimized conditions were obtained with 99.1% biodiesel yield from soybean oil with an orthogonal design. The base catalyst was stable within 8 cycles (>95% biodiesel yield) and resistant to saponification (AV = 9.7 mg KOH/g, 96.5% biodiesel yield). The base was characterized with XRD, EDX-mapping, FT-IR, XRF and TPD, and it had similar strong basicity to Na2SiO3 (0.21 vs. 0.22 mmol/g for Na2SiO3) with active sites of Na2O and CH3ONa evolved from Na2SiO3 and NaAlSiO4 by reactions of NaOH with oxides (e.g., SiO2, Al2O3) in SBC ash. Furthermore, the base was magnetized with magnetism of 6.86 emu/g by carbonizing residual oil in SBC as carbon support and reductant (of Fe2O3 to magnetic Fe3O4 particles). It catalyzed soybean oil to produce biodiesel with 99.2% yield and blended oil (AV = 5.9) to biodiesel with 91.9% yield without any saponification. The catalyst was magnetically separated and reused for 3 cycles with 87% yield. The non-magnetic base could also efficiently catalyze actual SBC oil for the production of biodiesel with 95% yield at AV of 10. This work realized the full use of inorganics in SBC, and its oil for direct biodiesel production at a low temperature (i.e., 65 vs. 120 °C with sulfuric acid process) without wastes produced and results can easily find practical applications for waste oils.