Heterobimetallic Ta–Nb MOF offering moderate Lewis/Brønsted acidity expedites glucose isomerization to fructose under microwave conditions†

Abstract

Fructose is considered a key intermediate in the preparation of green energy chemicals, especially 5-hydroxymethylfurfural. Herein, we report the highest fructose production using glucose over a heterobimetallic metal–organic framework (MOF) catalyst. The catalyst was designed by employing tantalum and niobium species as combinatorial metal nodes that can offer favorable Lewis acid centers for glucose isomerization. To bridge the metal nodes, we introduced SO₃H groups by employing a conventional sulfuric acid treatment. It can also improve the catalytic activity through modulation of the Lewis/Brønsted acidic density and influence the catalyst's intrinsic characteristics that can be beneficial for the reaction. The Nb@S-Ta MOF catalyst comprising Ta, Nb and sulfur (S) species exhibited favorable microporous and acidic characteristics, and it afforded a maximum fructose yield (40%) and selectivity (73%) using glucose under microwave conditions within 7 min at 100 °C in a water medium. The conversion was determined to follow first-order kinetics (k_G = 3.82 × 10⁻⁵ s⁻¹) and was temperature-dependent (E_a = 39.99 kJ mol⁻¹). Furthermore, theoretical DFT modeling verified the favorable interaction between glucose and metal nodes towards isomerization (as sulfur bridges both Ta and Nb), with a binding energy E_B of −3.95 eV for Nb@S-Ta MOF + glucose. However, the catalyst exhibited a less fair durability for recycling, which was caused by extended leaching of Ta (up to 24% after the 4th cycle) and acidic centre's deactivation through possible humin deposition.