Sn-Beta zeolites with borate salts catalyse the epimerization of carbohydrates via an intramolecular carbon shift

Abstract

Carbohydrate epimerization is an essential technology for the widespread production of rare sugars. In contrast to other enzymes, most epimerases are only active on sugars substituted with phosphate or nucleotide groups, thus drastically restricting their use. Here we show that Sn-Beta zeolite in the presence of sodium tetraborate catalyses the selective epimerization of aldoses in aqueous media. Specifically, a 5 wt% aldose (for example, glucose, xylose or arabinose) solution with a 4:1 aldose:sodium tetraborate molar ratio reacted with catalytic amounts of Sn-Beta yields near-equilibrium epimerization product distributions. The reaction proceeds by way of a 1,2 carbon shift wherein the bond between C-2 and C-3 is cleaved and a new bond between C-1 and C-3 is formed, with C-1 moving to the C-2 position with an inverted configuration. This work provides a general method of performing carbohydrate epimerizations that surmounts the main disadvantages of current enzymatic and inorganic processes.

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

“…The recently reported increase in methyl lactate production by Sn-Beta from fructose in methanol at 170°C (from 16% to 57%) upon alkali carbonate addition (9) is consistent with formation of 1,2-CS sites upon alkali exchange of open sites in Sn-Beta. Sn-Beta systems with added borate and alkali salts were reported to be pH sensitive and are not efficient 1,2-CS catalysts in acidic conditions (11,30). Furthermore, if Sn-MFI is used as a size-dependent 1,2-HS catalyst in conjunction with borate-or alkali-modified Sn-Beta, borate or alkali ions have the capacity to enter the Sn-MFI pores and influence the efficiency of lactate production from trioses.…”

State of the art of Lewis acid-containing zeolites: lessons from fine chemistry to new biomass transformation processes

“…found that the rational combination of Sn-Beta and borate salts cooperatively catalyzes the epimerization of sugars in aqueous media. 45 The intermediate complexes formed by the interaction of the borate salts with the initial sugars react on the Lewis acid sites of Sn-Beta by an epimerization pathway through an 1,2-intramolecular carbon shift (see Figure 7). 45 NMR spectroscopy studies reveal the key role of borate salts to promote the epimer products instead of the isomers.…”

“…45 The intermediate complexes formed by the interaction of the borate salts with the initial sugars react on the Lewis acid sites of Sn-Beta by an epimerization pathway through an 1,2-intramolecular carbon shift (see Figure 7). 45 NMR spectroscopy studies reveal the key role of borate salts to promote the epimer products instead of the isomers. 46 This combined methodology has been successfully described for different monosaccharides (glucose, xylose, or arabinose), mainly producing the epimer products (mannose, lyxose, or ribose, respectively), instead of the isomer product (fructose, xylulose, or ribulose, respectively).…”

Lewis Acid Zeolites for Biomass Conversion: Perspectives and Challenges on Reactivity, Synthesis, and Stability

“…Gunther et al (71,72) demonstrated that although the activation barriers for epimerization and isomerization are virtually identical for reactions catalyzed by Sn-Beta, the presence of substoichiometric amounts of borate induced a drastic selectivity shift in favor of epimerization over isomerization. Similarly, Bermejo-Deval et al (73) hypothesized that exchanging Sn-Beta with Na + increases epimerization selectivity over isomerization because the active site for isomerization is an open site, and exchanging Na + onto the adjacent silanol modifies the site properties needed to carry out the isomerization reaction.…”