Due to the high oxygen functionality of biomass, the valorization of renewable substrates to valuable chemicals typically requires deoxygenation reactions, such as dehydration. We have shown that the ionic liquid tetrabutylphosphonium bromide (Bu4PBr) is an excellent dehydration catalyst for various substrates. Using 1,2-hexanediol as a model compound, we discovered an unusual mechanism consisting of two steps, in both of which the bromide anion plays an important role. Next, we broadened the substrate scope to other diols. For all tested diols, typical acid catalyzed side products such as aldehydes and cyclic ethers were largely avoided, and conjugated dienes were formed in good to excellent yields. Regarding industrial potential, the most promising reaction is the dehydration of 1,4-butanediol to butadiene. As 1,4-butanediol can be produced from waste sugar streams through fermentation, this enables a green route towards butadiene. Optimizing the conditions for this substrate led to an excellent 94% yield of butadiene at 100% conversion after only 2 hours. Another promising reaction for biomass valorization is the dehydration of lactic acid to acrylic acid. The fermentative production of lactic acid from biomass is very mature, while acrylic acid is an important starting compound for a plethora of polymers. Most research so far has focused on gas phase reaction over solid acid catalysts. However, important issues such as high temperatures, dilute lactic acid feeds and mediocre yields (around 60%) persist. The Bu4PBr system proves to be an interesting alternative. By using a combination of gas and liquid phase analysis, we managed to identify and quantify some so far unreported volatile side products. As a result, we could close the mass balance, which up to now was problematic in literature. A reaction network was mapped out and the insights it provided allowed us to increase the acrylic acid yield to 70% at 98% conversion of the cyclic dimer lactide. Finally, we are working on the combined dehydration and hydrogenation of sugar alcohols, by using Bu4PBr together with a transition metal catalyst. Although we are only in an exploratory phase of this research, we have achieved olefin yields over 70%. In conclusion, the ionic liquid Bu4PBr is a highly performant catalyst for the valorization of various renewable substrates. This system combines a high solubility for bio-based compounds, excellent stability, high reaction rates, and easy product separation.

Tetabutylphosphonium bromide: A versatile ionic liquid catalyst for the biobased production of butadiene and acrylic acid

255th National Meeting (2018)

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