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

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 MoO₃, but the stability of the Mo/Beta catalysts under hydrothermal conditions leaves room for improvement.