title: Tandem catalysis for the production of alkyl lactates from ketohexoses at moderate temperatures.
abstract: Retro-aldol reactions have been implicated as the limiting steps in catalytic routes to convert biomass-derived hexoses and pentoses into valuable C2, C3, and C4 products such as glycolic acid, lactic acid, 2-hydroxy-3-butenoic acid, 2,4-dihydroxybutanoic acid, and alkyl esters thereof. Due to a lack of efficient retro-aldol catalysts, most previous investigations of catalytic pathways involving these reactions were conducted at high temperatures (≥160 °C). Here, we report moderate-temperature (around 100 °C) retro-aldol reactions of various hexoses in aqueous and alcoholic media with catalysts traditionally known for their capacity to catalyze 1,2-intramolecular carbon shift (1,2-CS) reactions of aldoses, i.e., various molybdenum oxide and molybdate species, nickel(II) diamine complexes, alkali-exchanged stannosilicate molecular sieves, and amorphous TiO2-SiO2 coprecipitates. Solid Lewis acid cocatalysts that are known to catalyze 1,2-intramolecular hydride shift (1,2-HS) reactions that enable the formation of α-hydroxy carboxylic acids from tetroses, trioses, and glycolaldehyde, but cannot readily catalyze retro-aldol reactions of hexoses and pentoses at these moderate temperatures, are shown to be compatible with the aforementioned retro-aldol catalysts. The combination of a distinct retro-aldol catalyst with a 1,2-HS catalyst enables lactic acid and alkyl lactate formation from ketohexoses at moderate temperatures (around 100 °C), with yields comparable to best-reported chemocatalytic examples at high temperature conditions (≥160 °C). The use of moderate temperatures enables numerous desirable features such as lower pressure and significantly less catalyst deactivation.
authors.affiliation: Chemical Engineering, California Institute of Technology, Pasadena, CA 91125.
authors.forename: Marat
authors.initials: M
authors.lastname: Orazov
authors.affiliation: Chemical Engineering, California Institute of Technology, Pasadena, CA 91125 mdavis@cheme.caltech.edu.
authors.forename: Mark E
authors.initials: ME
authors.lastname: Davis
mesh_terms: D002244:Carbon / Q000378:metabolism; D002384:Catalysis; D006601:Hexoses / Q000378:metabolism*; D007536:Isomerism; D007659:Ketones / Q000378:metabolism*; D007773:Lactates / Q000378:metabolism*; D013696:Temperature
chemical_list: D006601:Hexoses; D007659:Ketones; D007773:Lactates; D002244:Carbon
keywords: alkyl lactates; heterogeneous catalysis; retro-aldol reactions