Homogeneous Catalysts for the Hydrodeoxygenation of Biomass-Derived Carbohydrate Feedstocks

The use of homogeneous rather than heterogeneous catalysts for the hydrodeoxygenation of sugars, sugar alcohols, and their condensates such as furfural, 5-hydroxymethylfurfural, levulinic acid, and isosorbide may offer reaction pathways that have distinct advantages, notably with respect to catalyst deactivation by coking and fouling as observed on many heterogeneous systems with these highly reactive and polar substrates. Homogeneous systems, however, also face unique challenges in ligand, catalyst, and process design. The catalyst systems employed will have to be stable to the required aqueous acidic high-temperature (T > 150 °C) reaction conditions while exhibiting activities that make them kinetically competent over acid-catalyzed decomposition and oligo- and polymerization reactions leading to humin formation. For each of the hydrodeoxygenation reaction cascades for the C3 (glycerol), C4 (erythritol), C5 (xylose and derivatives or levulinic acid), and C6 (glucose and derivatives) value chains, comparatively few homogeneous catalyst systems have been evaluated to date. Key issues remain the thermal and redox stability of the complexes employed against decomposition and reduction to bulk metal acting as a heterogeneous catalysts and the recovery and recycling of the catalyst from the often very complex reaction and product mixtures.