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The growing interest in biomass as a renewable feedstock drives research toward innovative processes and performance-advantaged bioproducts. Homogeneous metal salt solutions are promising media owi...

Metal chlorides are demonstrated to behave as bifunctional acid catalysts in organic media in the one-pot reductive etherification of 5-hydroxymethylfurfural (HMF) in 2-propanol toward production of biodiesel. Two competing reaction pathways, direct etherification to 5-(isopropoxymethyl)furfural and reductive etherification to 2,5-bis(alkoxymethyl)furan, are proposed with the selectivity depending on the metal ion. Furfural and furfuryl alcohol are used as model compounds to investigate each pathway individually. The roles of Lewis/Bronsted acidity of metal chlorides solution are elucidated by kinetic studies in conjunction with salt speciation using electrospray soft ionization mass spectrometer. Bronsted acidic species, generated from alcoholysis of the metal chlorides, are the predominant catalytically active species in etherification. On the other hand, partially hydrolyzed metal cations produced by alcoholysis/hydrolysis are responsible Lewis acid centers for furfural reduction to furfuryl alcohol. I...

While the mechanisms of glucose transformation via heterogeneous metal-substituted BEA zeolites have recently been elucidated, understanding of the same chemistry in homogeneous metal salt catalysts in water is still limited. Here, we investigate the mechanisms of various Lewis acid metal(III) chlorides in glucose isomerization, epimerization, and other interconversions using nuclear magnetic resonance spectroscopy (13C NMR and 1H NMR). We show that the metal chlorides isomerize glucose to fructose via a C2–C1 intramolecular hydride transfer, despite their wide range of catalytic activity. Glucose epimerization to mannose proceeds via two parallel mechanisms, a reverse C2–C1 hydride transfer, and a C1–C2 intramolecular carbon shift (the Bilik reaction), with the hydride transfer being predominant. We hypothesize that the relative activity for the Bilik reaction correlates with the ionic radius of the catalyzing metal aqua cation, with an optimum at ∼1 A for lanthanide cations. In addition, we show that th...