Tandem catalysis for the production of alkyl lactates from ketohexoses at moderate temperatures

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 112; no. 38; pp. 11777 - 11782
• Main Authors: Orazov, Marat, Davis, Mark E.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 22-09-2015; National Acad Sciences; Proceedings of the National Academy of Sciences
• Subjects: bio-inspired; biofuels (including algae and biomass); Carbon - metabolism; Catalysis; catalysis (heterogeneous); catalysis (homogeneous); Catalysts; Chemical reactions; Hexoses - metabolism; hydrogen and fuel cells; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Isomerism; Ketones - metabolism; Lactates - metabolism; materials and chemistry by design; Molecules; Physical Sciences; synthesis (novel materials); synthesis (scalable processing); synthesis (self-assembly); Temperature; heterogeneous catalysis; retro-aldol reactions; alkyl lactates
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1516466112

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.