(458b) Moderate Temperature Retro-Aldol Reactions of Hexoses Catalyzed By Molybdates Coupled with Stannosilicate-Catalyzed Production of Lactates

Moderate Temperature Retro-Aldol Reactions of Hexoses Catalyzed by Molybdates Coupled with Stannosilicate-Catalyzed Production of Lactates

Marat Orazov and Mark E. Davis*

Chemical Engineering, California Institute of Technology

 Pasadena, CA 91125

Catalytic conversion of biomass-derived sugars to lactic acid and alkyl lactates has been the focus of numerous studies in the recent years. Lewis acidic zeotypes have been shown to be particularly active and selective for the conversion of trioses, dihydroxyacetone (DHA) and glyceraldehyde (GLA), to alkyl lactates, with nearly-quantitative yields obtained by stannosilicates Sn-Beta and Sn-MFI at temperatures ca. 100 °C.¹ These results indicate that trioses readily undergo dehydration to pyruvaldehyde, alcohol addition to form pyruvaldehyde hemiacetal, and the 1, 2 hydride shift to yield the product alkyl lactate. However, trioses are not abundant in biomass feedstocks, and must be derived from the more common hexoses and pentoses for this strategy of lactate production to be viable. Retro-aldol reactions of ketohexoses are the ideal route for the production of trioses from biomass, and have been explored by many research groups. However, retro-aldol reactions of hexoses are not thermodynamically favored at low temperatures, and have relatively high activation energies even with the best reported catalysts. As a result, most investigations of the retro-aldol-based production of lactates have been carried out at elevated temperatures (160 °C and higher), with long reaction times.²Under these conditions, ketohexoses are prone to side-reactions that can lead to poor yields of lactates.

Recently, we showed that alkali-exchanged Sn-Beta can catalyze glucose epimerization to mannose via 1, 2 intramolecular carbon shift^3,4, known as the Bilik reaction. Reaction of fructose with such catalysts at moderate temperatures (ca. 100 °C) gives, in addition to glucose and mannose, a complex reaction mixture of products including branched sugars (hamamelose, etc.), ketohexoses (sorbose, psicose, and tagatose), and retro-aldol products (DHA and GLA). When fructose is reacted with traditional Bilik catalysts, i.e, molybdates, similar results are obtained. We found that a variety of molybdenum (VI) species (MoO₃, (NH₄)₆Mo₇O₂₄·4H₂O, H₃PMo₁₂O₄₀, and Na₂MoO₄) catalyze retro-aldol reactions of ketohexoses at moderate temperatures in both aqueous and alcoholic solvents. However, under these conditions, the equilibrium of the retro-aldol reaction favors hexoses, and aldol condensation and carbon shift products (ketohexoses and branched hexoses, respectively) begin to form after relatively low concentrations of DHA and GLA are achieved.

Here, we developed a co-catalyst system that involves molybdates to catalyze retro-aldol reactions of ketohexoses, and microporous stannosilicates to promote the formation of lactic acid or alkyl lactates from the established pool of DHA and GLA. With such catalytic systems, we have achieved high yields of lactates (ca. 70% for both methyl- and ethyl- lactate) from fructose at 100 °C. We will report our reaction results as well as results from investigations aimed at understanding the reaction pathways that occur from the use of this co-catalyst system.

References:

[1] Osmundsen, C. M.; Holm, M. S.; Dahl, S.; Taarning, E Proc. R Soc. A, 2012, 468, 2000–2016

[2] Holm, M. S.; Saravanamurugan, S.; Taarning, E. Science 2010, 328, 602–605

[3] Bermejo-Deval, R.; Orazov, M.; Gounder, R.; Hwang, S.; Davis, M. E. ACS Catal. 2014, 4, 2288–2297

[4] Orazov, M.; Bermejo-Deval, R.; Gounder, R.; Hwang, S.; Davis, M. E. 2014 AIChE Annual Meeting, 2014, Atlanta, Georgia, Nov 16-21

*Corresponding author. E-mail: mdavis@cheme.caltech.edu