Epimerization of glucose to rare sugars using Beta zeolite-supported MoOx catalysts

Abstract

Epimerization of abundant monosaccharides derived from lignocellulosic biomass offers an attractive approach for the synthesis of rare sugars. Molybdenum-based catalysts demonstrate excellent glucose-mannose epimerization performance, but most studies have neglected the synthesis of other sugar epimers and the pathways of their formation. Here, reduced Beta zeolite supported MoO x catalysts (Mo/Beta) with 1.5–10 wt% Mo loading were examined for glucose epimerization in water, specifically focusing on the formation of the rare sugars allose and altrose. The physicochemical structure and the catalytic activity of the catalyst were examined in detail, and the reaction mechanisms for the epimer formation were probed by nuclear magnetic resonance (NMR) spectroscopy. The results demonstrate that 5 wt% Mo/Beta achieved a near-equilibrium mannose yield of 30 % from glucose within 10 min at reaction temperatures below 140 °C, exhibiting a relatively low activation energy (∼67 kJ mol −1 ). At elevated reaction temperatures (≥ 120 °C), the rare sugars allose and altrose accumulated to combined yields of 24 % at 140 °C. An isotope labelling study using NMR spectroscopy corroborated the catalysis to involve 1,2-carbon shifts that elicit the formation of rare sugars at sufficiently high temperatures. Evaluation of catalyst reuse and regeneration indicated that supported MoO x had higher stability than MoO 3 , but the stability of the Mo/Beta catalysts under hydrothermal conditions leaves room for improvement. • Beta zeolite supported MoO x efficient catalyst for aqueous glucose epimerization. • 5 wt% Mo/Beta yields 30 % mannose from glucose at 120 °C with low E a of 67 kJ mol −1 . • Formation of the rare sugars allose and altrose in 24 % yield from glucose at 140 °C. • NMR study with 13 C-glucose shows 1,2-carbon shift elicits allose/altrose formation.

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