Ex-situ catalytic upgrading of pyrolysis vapors using mixed metal oxides

Abstract

Ex-situ catalytic fast pyrolysis (CFP) was employed to produce bio-oils from beech wood. The effect of Nb-based mixed oxides (with second metal oxide = W, Al or Mn) in upgrading the pyrolysis vapors was compared to an H-ZSM-5 catalyst and to non-catalyzed pyrolysis. The catalysts acidic properties were assessed by NH3-TPD and FTIR of adsorbed pyridine. The bio-oils obtained were characterized by Karl-Fischer titration, GPC, CHNS analyses, 13C-NMR, GC–MS, GCxGC–MS. In spite their very different acidic properties, the NbxMnyOz catalyst (with essentially Lewis acid sites) exhibited similar performances as H-ZSM-5 (that presents a high density of Brønsted sites), in terms of liquid phase selectivity and reduction of oxygen content in the bio-oils produced. These observations were accompanied by a reduction of compounds such as acids, aldehydes, ketones, ethers and sugars that contribute to the detrimental properties of bio-oils. Finally, the results suggest that the Lewis acid sites of NbxMnyOz are converted into Brønsted sites in the presence of water vapor produced by pyrolysis of wood, whereas the strongest Brønsted sites of H-ZSM-5 have a limited impact on the upgrading process due to their limited accessibility for most components of pyrolysis vapors.