Unraveling the active states of WO₃-based catalysts in the selective conversion of cellulose to glycols

Tungsten-based catalysts, including crystalline WO₃ and H₂WO₄, have been reported to be efficient for the selective cleavage of C−C bonds of sugar intermediates involved in the conversion of cellulose to ethylene glycol and propylene glycol in combination with hydrogenation catalysts under H₂ atmosphere. However, there is still not a consensus on the actual states of these catalysts during the reaction. Herein, we demonstrate that both WO₃ and H₂WO₄ tended to undergo reduction to hydrogen tungsten bronze (H_xWO₃) species in the cellulose reaction, consisting of H_0.23WO₃ and H_0.33WO₃, which were then re-oxidized to WO₃ upon exposure to ambient air after the reaction. Both WO₃ and H_xWO₃ were insoluble in water and acted as the heterogeneous catalysts in the cellulose reaction, as further validated by the strong dependence of the catalytic activity of WO₃ on its crystallite size and surface area. Such identification of the heterogeneous H_xWO₃ species, albeit exemplified here only from the in situ reduction of WO₃ and H₂WO₄ in the cellulose reaction, unravels the active state of the tungsten-based catalysts under reductive reaction conditions.