An alumina‐coated UiO‐66 nanocrystalline solid superacid with high acid density as a catalyst for ethyl levulinate synthesis

Abstract BACKGROUND The zirconium‐containing metal–organic framework (MOF) UiO‐66 was used as a cell to synthesize SO 4 2− /ZrO 2 @Al 2 O 3 , a high‐density superacid with relatively strong Brønsted acidity. The UiO‐66 MOF was first coated with alumina to form ZrO 2 @Al 2 O 3 . Impregnation with ammonium sulfate and calcination at 500 °C afforded the bimetallic composite solid superacid SO 4 2− /ZrO 2 @Al 2 O 3 . RESULTS Scanning and transmission electron microscopy images confirmed that UiO‐66 was successfully coated with aluminium oxide, and its octahedral structure and uniform size (400–600 nm) were retained. SO 4 2− /ZrO 2 @Al 2 O 3 had a Brunauer–Emmett–Teller specific surface area of 301–330 m 2 g −1 and average pore diameter of 9.6–10.7 nm. X‐ray photoelectron spectroscopy and Fourier transform infrared analysis showed that SO 4 2− /ZrO 2 @Al 2 O 3 contained S 6+ and Al 3+ . Temperature‐programmed ammonia desorption analysis showed that SO 4 2− /ZrO 2 @Al 2 O 3 contained super‐strong acid sites. The total volume of desorbed ammonia reached 90–109 cm 3 g −1 . Infrared spectra of adsorbed pyridine indicated that SO 4 2− /ZrO 2 @Al 2 O 3 contained mainly Lewis acid sites and was relatively rich in Brønsted acid sites. Thermogravimetric analysis showed that the thermal stability of SO 4 2− /ZrO 2 @Al 2 O 3 was high. CONCLUSIONS SO 4 2− /ZrO 2 @Al 2 O 3 ‐3M (the impregnation concentration of ammonium sulfate was 3 mol L −1 ) and glucose were used to synthesize ethyl levulinate (EL) in ethanol. The highest EL yield of 37.5 mol% was obtained after reacting the mixture at 200 °C for 5 h. An EL yield of 28.8 mol% was obtained after four consecutive reuses of the SO 4 2− /ZrO 2 @Al 2 O 3 ‐3M catalyst. © 2020 Society of Chemical Industry