Cs-doped H₄SiW₁₂O₄₀ catalysts for biodiesel applications

Cs exchanged silicotungstic acid catalysts of general formula Cs_xH_4−xSiW₁₂O₄₀ (x = 0.8–4) have been synthesised and characterised by a range of techniques including elemental analysis, N₂ gas adsorption, XRD, XPS and NH₃ flow calorimetry. Cs substitution promotes recrystallisation of the parent H₄SiW₁₂O₄₀ polyoxometallate to the Cs₄ salt, via a stable intermediate phase formed at compositions between Cs_0.8–2.8. This recrystallisation is accompanied by a pronounced rise and subsequent fall in porosity, with a maximum mesopore volume obtained for materials containing 2.8 Cs atoms per Keggin unit. Calorimetry reveals all Cs_xH_4−xSiW₁₂O₄₀ are strong acids, with ΔH^θ_ads(NH₃) ranging from −142 to 116 kJ mol⁻¹ with increasing Cs content, consistently weaker than their phosphotungstic analogues. Cs_xH_4−xSiW₁₂O₄₀ materials are active catalysts for both C₄ and C₈ triglyceride transesterification, and palmitic acid esterification with methanol. For loadings ≤0.8 Cs per Keggin, (trans)esterification activity arises from homogeneous contributions. However, higher degrees of substitution result in entirely heterogeneous catalysis, with rates proportional to the density of accessible acid sites present within mesopores.