Abstract
A family of mesoporous SBA-15 supported H3PW12O40 (HPW) catalysts were synthesized by wet-impregnation and compared with fumed silica analogues for the solventless isomerization of α-pinene under mild conditions. Structural and acidic properties of supported HPW materials were characterized by powder XRD, HRTEM, XPS, TGA, N2 porosimetry, DRIFTS, and ammonia and propylamine chemisorption and TPD. The high area, mesoporous SBA-15 architecture facilitates the formation of highly dispersed (isolated or low dimensional) HPW clusters and concomitant high acid site densities (up to 0.54mmolg−1) relative to fumed silica wherein large HPW crystallites are formed even at low HPW loadings. α-Pinene exhibits a volcano dependence on HPW loading over the SBA-15 support due to competition between the number and accessibility of acid sites to the non-polar reactant, with the superior acid site accessibility for HPW/SBA-15 conferring a 10-fold rate enhancement with respect to HPW/fumed silica and pure HPW. Monocyclic limonene and terpinolene products are favoured over polycyclic camphene and β-pinene by weaker polyoxometallate analogues over SBA-15.
Highlights
Bio-derived chemicals and fuels are urgently sought as sustainable alternatives to those currently derived from petrochemical refineries and related fossil fuel transformations
HPW/SBA-15 materials only exhibit the parent support reflection at the lowest W loadings, consistent with isolated Keggin clusters, with reflections characteristic of crystalline HPW emerging for loadings >9 wt%, presumably due to the nucleation of individual Keggin units, whose intensities increase and peakwidths decrease with W loading
The parent Keggin unit is preserved over both supports, the higher area SBA-15 support permits the stabilisation of high concentrations of isolated and/or low dimensional polyoxometallate clusters throughout the mesopores, which present a high density of accessible Brönsted acid sites
Summary
Bio-derived chemicals and fuels are urgently sought as sustainable alternatives to those currently derived from petrochemical refineries and related fossil fuel transformations Catalytic routes to such products from lignocellulosic biomass, and plant or microorganisms derived lipids and carbohydrates, have shown great recent promise in this regard. HPW supported over MCM41 has shown promise in ␣-pinene isomerization [43], albeit the small mesopore diameter of 2.5 nm and low maximum acid loading of 0.25 mmol g−1, necessitated a high reaction temperature of 100 ◦C and poor catalyst:substrate ratio to achieve significant activity. The superior surface area and pore architecture of SBA-15 significantly increases the polyxometallate dispersion and concomitant rate of isomerization over that attainable over a non-porous fumed silica support, while selectivity towards monocyclic limonene and terpinolene products is inversely proportional to acid strength
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