Deoxygenation of propionic acid on heteropoly acid and bifunctional metal-loaded heteropoly acid catalysts: Reaction pathways and turnover rates

Reaction pathways of the gas-phase deoxygenation of propionic acid in the presence of heteropoly acid and bifunctional metal-loaded heteropoly acid catalysts were investigated in a fixed-bed reactor at 250-400 °C in flowing H₂ or N₂. Silica-supported H₃PW ₁₂O₄₀ (HPW) and bulk acidic salt Cs_2.5H _0.5PW₁₂O₄₀ (CsPW), both in H₂ and in N₂, exhibited ketonisation activity between 250 and 300 °C to yield 3-pentanone, CsPW being more selective than HPW. At 400 °C, HPW and CsPW were active for decarbonylation and decarboxylation of propionic acid to yield ethene and ethane, respectively. Loading Pd or Pt onto CsPW greatly enhanced decarbonylation in flowing H₂ but had little effect in N₂. Similar performance exhibited Pd/SiO₂ and Pt/SiO ₂, giving almost 100% selectivity to ethene in H₂. These results are consistent with hydrodeoxygenation of propionic acid on Pd and Pt, suggesting that hydrogenolysis of CC bond plays essential role. In contrast to the Pd and Pt catalysts, Cu catalysts, Cu/CsPW and Cu/SiO₂, were both active in hydrogenation of CO bond to yield propanal and 1-propanol. Turnover rates of propionic acid conversion on metal catalysts followed the order Pd > Pt > Cu for both CsPW-supported and silica-supported metal catalysts.