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

Abstract

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, sorbitol) to mono-alkenes 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.

From crude industrial waste glycerol to biopropene via Ru-mediated hydrodeoxygenation in ionic liquids

“…When applying the existing system to analytically pure glycerol, a significant propene yield of 57 % was obtained after 1 h at 210°C. 20 The proposed main reaction pathway for the complete HDO of glycerol is illustrated in Scheme 1, based on previous observations in the HDO of sugar alcohols and glycerol. 12,15,20 However, the selectivity was not maintained when the reaction time was increased to convert the remaining hydroxy compounds in full; in an attempt to boost propene yields, an olefin selectivity drop was observed with a 10% increase in propane yield after 2 h (Scheme 1 shows all observed products).…”

“…Previous reports on dehydration reactions in HBr/Bu4PBr showed activity preserved over multiple cycles, despite the significant water build-up. 20,21 Glycerol-water simulated blends were evaluated, ranging from pure glycerol to 50 wt% glycerol in water. The latter feed contains over 5 equivalents of water, in addition to the 3 equivalents potentially formed by the dehydration.…”

“…Extensive experiments with C4-C6 polyols showed that temperature plays a crucial role in balancing the dehydration with the hydrogenating activity of the in situ formed Ru species, converting carbonyl compounds to alcohols. 20 In the case of erythritol, the dehydration by the IL proved to be more rate limiting at low temperature, while at elevated temperature the hydrogenation of butanone was identified as the kinetically slowest step. 20 High temperatures also increase to risk of overreduction of the desired olefin product to the alkane.…”

“…20 In the case of erythritol, the dehydration by the IL proved to be more rate limiting at low temperature, while at elevated temperature the hydrogenation of butanone was identified as the kinetically slowest step. 20 High temperatures also increase to risk of overreduction of the desired olefin product to the alkane.…”

“…More recently, our group discovered a novel approach to perform the HDO of biobased polyols using a combination of a homogeneous hydrogenation catalyst and a dehydrating solvent of the ionic liquid type (IL; tetrabutylphosphonium bromide, Bu4PBr). 20 Our previous work focused on the conversion of higher polyols (i.e. erythritol, xylitol and sorbitol).…”