Structure-activity relations in Cs-doped heteropolyacid catalysts for biodiesel production

A series of insoluble heteropolytungstate (H₃PW₁₂O₄₀ HPW) salts, Cs_xH_3−xPW₁₂O₄₀ (x=0.9–3x=0.9–3), were synthesized and characterized using a range of bulk and surface sensitive probes including N₂ porosimetry, powder XRD, FTIR, XPS, ³¹P MAS NMR, and NH₃ calorimetry. Materials with Cs content in the range x=2.0–2.7x=2.0–2.7 were composed of dispersed crystallites with surface areas ∼100 m² g⁻¹ and high Brönsted acid strengths [ΔH⁰_ads(NH₃)=−150 kJmol⁻¹], similar to the parent heteropolyacid. The number of accessible surface acid sites probed by α -pinene isomerization correlated well with those determined by NH₃ adsorption calorimetry and surface area measurements. Cs_xH₃−xPW₁₂O₄₀ were active toward the esterification of palmitic acid and transesterification of tributyrin, important steps in fatty acid and ester processing for biodiesel synthesis. Optimum performance occurs for Cs loadings of x=2.0–2.3x=2.0–2.3, correlating with the accessible surface acid site density. These catalysts were recoverable with no leaching of soluble HPW.