Low-cost diatomite supported binary transition metal sulfates: an efficient reusable solid catalyst for biodiesel synthesis

Using a simple method of impregnation and then calcination, diatomite supported binary transition metal sulfates (Fe and Zr, designated as Fe₂(SO₄)₃&Zr(SO₄)₂@diatomite) were prepared and used as a catalyst in the preparation of renewable biofuels. The synthesised Fe₂(SO₄)₃&Zr(SO₄)₂@diatomite catalyst (Fe₂(SO₄)₃ : Zr(SO₄)₂ : diatomite = 1 : 2 : 6, mass ratio) was thoroughly characterised using transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, microbeam X-ray fluorescence (μ-XRF) spectroscopy and thermogravimetric analysis (TG). The results demonstrated that the sulfate was successfully loaded onto the diatomite with a uniform distribution. The N₂ adsorption/desorption analysis indicated that the catalyst's specific surface area was 1.54 m² g^-1. The catalyst exhibited outstanding performance in the preparation of renewable biofuel (biodiesel) from waste fatty acids and the optimal parameters were methanol-to-oil 1.25 : 1, reaction temperature 70 °C, catalyst concentration 10 wt%, reaction time 4 h. The conversion was found to reach 98.90% under optimal parameters, which is better than that of Fe₂(SO₄)₃·xH₂O, Zr(SO₄)₂·4H₂O, Fe₂(SO₄)₃@diatomite and Zr(SO₄)₂@diatomite. Moreover, the catalyst can be recycled by simple filtration and reused for three cycles after regeneration without noticeable reduction in catalytic activity.