Composition of tungsten oxide bronzes active for hydrodeoxygenation

We explore the composition of reduced tungsten oxide bronze materials as a new class of heterogeneous catalysts for hydrodeoxygenation of bio-fuels. We demonstrate a method using thermogravimetric analysis (TGA) and mass spectrometry to determine both the oxygen substoichiometry and hydrogen content of the HyWO3−z catalytic phase that is formed during hydrogen pretreatment at temperatures from 200 to 500 °C. Results for three WO3 materials show that the composition depends on crystallinity and surface area, as well as pretreatment temperature and time. The optimal pretreatment conditions for hydrogenation of acrolein to allyl alcohol at 50 °C occur over a narrow temperature range around 350 °C. We report for the first time that at higher reaction temperatures the catalysts are active for hydrodeoxygenation of allyl alcohol to propene and 1,5-hexadiene. We have proposed a mechanism similar to the Mars–van Krevelen cycle in which an alcohol adsorbs on an oxygen vacancy, forming a tungsten–oxygen bond and breaking the carbon–oxygen bond. Our TGA and reaction kinetics data indicate that the composition of the active catalyst is between H0.9WO2.9 and H1.3WO2.7 and that the rate of hydrodeoxygenation is comparable to the oxygen vacancy creation rate.

Figure optionsDownload high-quality image (102 K)Download as PowerPoint slideResearch highlights▶ Pretreatment of WO3 in hydrogen produces a reduced tungsten oxide bronze. ▶ A TGA/MS method determined hydrogen and oxygen content in HyWO3−z reduced bronzes. ▶ Allyl alcohol is hydrodeoxygenated to propene and 1,5-hexadiene above 150 °C. ▶ Surface oxygen vacancies adsorb the alcohol, forming W–O bonds, breaking C–O bonds.