Bio-oil upgrading via vapor-phase ketonization over nanostructured FeO_x and MnO_x: catalytic performance and mechanistic insight

In this study, nanostructured FeO_x and MnO_x were prepared by two synthetic routes, nanocasting and hydrothermal, and evaluated for bio-oil upgrading via vapor-phase ketonization. Catalytic performance measurements in the ketonization of representative model compounds, acetic and propionic acid, at 335 °C showed high activity for the hydrothermal MnO_x and nanocast FeO_x (conversion >90%) with high selectivity to the respective ketones. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies followed by temperature-programmed thermogravimetric analysis (TGA) and MS showed that the reactive intermediates are bidentate acetate species that desorb as acetone over FeO_x and unreacted acetic acid over MnO_x (in contradiction to its associated catalysis). Powder X-ray diffraction and X-ray photoelectron spectroscopy analysis of used samples revealed that MnO₂ was reduced to MnO during reaction. The relative surface concentrations of adsorbed acetate for the used MnO_x catalysts (from DRIFTS) correlated with their corresponding acetic acid conversion (from ketonization studies), indicating that MnO is the active phase for acetic acid ketonization, with MnO₂ a precursor which is reduced in situ at temperatures >300 °C. Vapor-phase ketonization of the aqueous phase of a real thermal bio-oil, produced from the fast pyrolysis of lignocellulosic biomass, was demonstrated successfully over MnO_x prepared by the hydrothermal route, highlighting this as an attractive approach for the upgrading of pyrolysis bio-oils.