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Williams, Howard
University of Huddersfield
in Cooperation with on an Cooperation-Score of 37%
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article
Cs-doped H4SiW12O40 catalysts for biodiesel applications
Abstract
<p>Cs exchanged silicotungstic acid catalysts of general formula Cs<sub>x</sub>H<sub>4-x</sub>SiW<sub>12</sub>O<sub>40</sub> (x = 0.8-4) have been synthesised and characterised by a range of techniques including elemental analysis, N<sub>2</sub> gas adsorption, XRD, XPS and NH<sub>3</sub> flow calorimetry. Cs substitution promotes recrystallisation of the parent H<sub>4</sub>SiW<sub>12</sub>O<sub>40</sub> polyoxometallate to the Cs<sub>4</sub> salt, via a stable intermediate phase formed at compositions between Cs<sub>0.8-2.8</sub>. 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<sub>x</sub>H<sub>4-x</sub>SiW<sub>12</sub>O<sub>40</sub> are strong acids, with ΔH<sup>θ</sup><sub>ads</sub>(NH<sub>3</sub>) ranging from -142 to 116 kJ mol<sup>-1</sup> with increasing Cs content, consistently weaker than their phosphotungstic analogues. Cs<sub>x</sub>H<sub>4-x</sub>SiW<sub>12</sub>O<sub>40</sub> materials are active catalysts for both C<sub>4</sub> and C<sub>8</sub> 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.</p>