Synergistic effect between CaCl2 and γ-Al2O3 for furfural production by dehydration of hemicellulosic carbohydrates

[Display omitted] •CaCl2 increases the α-xylopyranose/β-xylopyranose ratio favoring the xylose dehydration with respect to other salts.•γ-Al2O3 promotes the furfural production due to their Lewis acid sites.•The synergistic effect between CaCl2 and γ-Al2O3 gives full conversion and furfural yield of 55% after 50 min at 150 °C.•γ-Al2O3 can be reused at least for 10 catalytic runs without regeneration.•CaCl2 and γ-Al2O3 provided a furfural yield of 83% by liquors obtained from olive stones. γ-Al2O3 has successfully been used as solid acid catalyst for xylose dehydration in a water:toluene biphasic system. The addition of alkaline earth metal chlorides improved the partition coefficient, thus facilitating the extraction of furfural from the aqueous phase. In this sense, the best catalytic performance was attained by using CaCl2 and γ-Al2O3 with almost full xylose conversion and a furfural yield of 55% after 50 min, at 150 °C. A 1H NMR study has demonstrated that CaCl2 shifts the α-xylopyranose/β-xylopyranose anomeric equilibrium toward the α-form, more favorable for initiating the dehydration process, and γ-Al2O3 promotes the furfural production. γ-Al2O3 can be reused during ten catalytic cycles without any pretreatment. Finally, the synergistic effect between CaCl2 and γ-Al2O3 could also be inferred from the increase in furfural yield, until a value of 83% after 50 min at 175 °C, by using a hemicellulosic liquor obtained from olive stones.