Olefins from Biobased Sugar Alcohols via Selective, Ru-Mediated Reaction in Catalytic Phosphonium Ionic Liquids

Hydrodeoxygenation (HDO) of sugar alcohols could be a valuable reaction for the renewable production of alkenes. However, reports on deoxygenation of longer polyols (e.g., erythritol, xylitol, and sorbitol) to monoalkenes are scarce, especially when using H2 as reducing agent. Here we design a conceptually different catalyst system for polyol conversion to olefins, containing a selective Ru hydrogenation function (e.g., RuBr3) in an ionic liquid that dehydrates alcohols (tetrabutylphosphonium bromide, Bu4PBr). This system performs HDO of erythritol to butenes in yields up to 69%. Strikingly, overreduction to less valuable butane is effectively suppressed. We show that in situ formed CO is crucial in the formation and stabilization of the active and selective catalyst. The actual hydrogenation catalyst is identified as a Ru carbonyl bromide species, as demonstrated by FTIR and XANES. Finally, we show that this HDO also works for more economically relevant polyols: glycerol, xylitol, and sorbitol, leading to propene, pentenes, and hexenes as main products, respectively.