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

Abstract

A series of insoluble heteropolytungstate (H 3PW 12O 40 HPW) salts, Cs x H 3− x PW 12O 40 ( x = 0.9 – 3 ), were synthesized and characterized using a range of bulk and surface sensitive probes including N 2 porosimetry, powder XRD, FTIR, XPS, 31P MAS NMR, and NH 3 calorimetry. Materials with Cs content in the range x = 2.0 – 2.7 were composed of dispersed crystallites with surface areas ∼100 m 2 g −1 and high Brönsted acid strengths [ Δ H ads 0 ( NH 3 ) = − 150 kJ mol −1 ], similar to the parent heteropolyacid. The number of accessible surface acid sites probed by α-pinene isomerization correlated well with those determined by NH 3 adsorption calorimetry and surface area measurements. Cs x H 3− x PW 12O 40 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 – 2.3 , correlating with the accessible surface acid site density. These catalysts were recoverable with no leaching of soluble HPW.