Flow Semi-continuous Mechanochemistry as a Versatile and Efficient Tool for the Synthesis of Hydrocalumite and the Isomerization of Glucose to Fructose

In this work, hydrocalumite, a layered double hydroxide with formula Ca₂Al(OH)₆Cl·2H₂O, has been prepared for the first time using flow semi-continuous mechanochemistry with a DYNO®-MILL RESEARCH LAB (Willy A. Bachofen AG, Switzerland), with stoichiometric amount of reactants in water, after only 5 min at 25 °C. Hydrocalumite, before and after thermal treatment, was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TG–DTA) and N₂ sorption at − 196 °C. Moreover, calcined hydrocalumite has been evaluated as catalyst for the isomerization of glucose to fructose, a catalytic process which has also been performed in the same flow semi-continuous mechanochemical reactor. This mechanochemical system, unlike conventional ball milling, allows working in semi-continuous and/or continuous mode, using solvents and allowing heating control up to temperatures of 80 °C. The isomerization of glucose to fructose was successfully carried out in this reactor, demonstrating that hydrocalumite prepared by mechanochemistry is more active than that prepared by co-precipitation. The optimization of several experimental variables (reaction temperature and time, glucose/catalyst weight ratio and concentration of glucose in water) has resulted in a 23.5% fructose yield, with a glucose conversion of 38.1%, after 1 h of reaction, at 50 °C, with a 17 wt% glucose and a glucose/catalyst weight ratio of 6. However, the highest fructose productivity was reached under similar experimental conditions, but after only 5 min, with a value of 0.50 kg_fructose L_H2O⁻¹ h⁻¹ (equivalent to 15 kg_fructose Kg_cat⁻¹ h⁻¹), which is susceptible to be improved by implementing a continuous mode, assisted with a liquid pump, in the mechanochemical reactor. Therefore, this work has evidenced the versatility and potential of this new flow semi-continuous mechanochemical reactor for the synthesis of crystalline layered double hydroxides, under sustainable experimental conditions, and to perform catalytic processes with high performance, using water as solvent and atmospheric conditions.