Deoxygenation and hydrogenation of biomass-derived molecules over multifunctional catalysts

The aim of this work was to investigate heterogeneous catalysis of the deoxygenation of renewable feedstocks for value-added chemicals and fuels using heteropoly acids. The main focus was on H3PW12O40 (HPW) and its Cs acidic salt Cs2.5H0.5PW12O40 (CsPW), which have sufficiently high thermal stability, with decomposition temperatures of 450 and > 500 °C, respectively. These compounds have very strong Brønsted acidity and are well documented as acid catalysts. They were used for the deoxygenation and hydrogenation of propionic acid, methyl isobutyl ketone (MIBK) and diisobutyl ketone (DIBK) in the gas phase. For comparison, zeolite catalysts doped with Pt were examined for the deoxygenation of MIBK. CsPW was doped with Pd, Pt, Cu and Ru metals using the impregnation method, while Pt was doped on zeolites by ion exchange. The catalysts under study were characterised using various physical and chemical techniques. CsPW and CsPW-supported Pd, Pt and Cu catalysts were found to be stable in the deoxygenation of propionic acid. They retained the Keggin structure of their polyanion (primary structure), as well as the CsPW crystal structure (secondary structure), after reaction at 400 °C in H2, whereas HPW decomposed above 350 °C in N2. The reaction was found to involve several pathways including ketonisation, decarbonylation, decarboxylation and hydrogenation, leading to the partial or total deoxygenation of propionic acid. HPW/SiO2 and 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 the 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 was exhibited by Pd and Pt on SiO2, giving almost 100% selectivity to ethene in H2. These results are consistent with the hydrodeoxygenation of propionic acid on Pd and Pt, suggesting that hydrogenolysis of the C-C bond plays an essential role. Cu catalysts were active in the hydrogenation of the C=O bond to yield propanal and 1-propanol. Turnover rates of propionic acid conversion on metal catalysts followed the order Pd > Pt > Cu. Pt/CsPW was found to be a very efficient catalyst for the selective one-step hydrodeoxygenation of biomass-derived aliphatic ketones MIBK and DIBK to yield 2-methylpentane (MP) and 2,6-dimethylheptane (DMH) under mild conditions at 100 °C and 1 bar pressure without isomerisation of the carbon backbone via a metal-acid bifunctional mechanism. For MIBK hydrogenation, the mechanism involves MIBK hydrogenation to MP-ol on metal sites followed by MP-ol dehydration on acid sites to form olefin and finally olefin hydrogenation to 2MP on metal sites. Pt/ZSM-5 matched the catalytic performance of Pt/CsPW at 200 °C, but considerable isomerisation of MP took place at this temperature. This shows that the strong acidity of CsPW is essential for the high efficiency of the Pt/CsPW catalyst