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Characterisation and catalytic testing of heteropoly acids on porous silica supports

Jones, Christopher David 2014. Characterisation and catalytic testing of heteropoly acids on porous silica supports. MPhil Thesis, Cardiff University.
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The aim of this investigation was to rationalise the catalytic activity of heteropoly acid H3PW12O40 (HPW) on porous silicas. To avoid leaching, activities were evaluated for a reaction involving non-polar reagents: the isomerisation of neat alpha-pinene. Activities were found to depend on the nature of the support: per unit mass, the mesoporous silicas SBA-15 and KIT-6 deliver higher rates of conversion than fumed silica at HPW concentrations below 50 wt.%, but become less effective at higher loadings. These trends were attributed to changes in porosity, as thermogravimetric analysis and inverse gas chromatography studies showed little variation in silanol density or hydrophobicity. For each catalyst, the density of HPW crystallites was measured by powder X-ray diffraction; their loading, by energy-dispersive X-ray spectroscopy; and the pore volume and area, by nitrogen porosimetry. From these values, an estimate was obtained for the support area rendered inaccessible by pore blocking. When normalised for accessibility and support area, reaction rates were found to be independent of the support at loadings below 40 wt.%. The plots appear linear, with gradients corresponding to a turnover frequency (expressed per quantity of accessible catalyst) of 1700 mol molHPW-1 h-1. Thus, the activity of HPW on these and similar supports should be predictable from loading and porosimetry measurements alone. Catalyst particle growth appears not to affect accessibility, suggesting that most HPW exists as monolayer deposits. Further evidence for such structures is supplied by the strong “interfacial” W 4f signals in X-ray photoelectron spectra of the catalysts, representing HPW in direct contact with the support. Studies were also undertaken to assess the suitability of the catalysts for biodiesel synthesis. UV-vis and EDX measurements showed near-total leaching of HPW in methanol under typical reaction conditions. However, exchanging a fraction of protons for caesium ions almost totally prevented leaching, with only partial loss of catalytic activity.

Item Type: Thesis (MPhil)
Status: Unpublished
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Date of First Compliant Deposit: 30 March 2016
Last Modified: 19 Mar 2016 23:39

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