In situ sulfonic acid-functionalized MIL-101(Cr) catalyzed liquid-phase Beckmann rearrangement of cyclohexanone oxime

We prepared in situ sulfonic-acid-functionalized MIL-101(Cr) catalysts using different chromium sources and mineralizers. We characterized these catalysts by X-ray diffraction, N2-physical adsorption, Fourier-transform infrared, scanning electron microscopy, X-ray photoelectron spectroscopy, NH3 temperature-programmed desorption, and inductively coupled plasma measurements. We subsequently used the solid acid catalysts to catalyze the liquid-phase Beckmann rearrangement of cyclohexanone oxime to caprolactam for the first time. The results showed that when the reaction was carried out at 130 °C for 4 h with benzonitrile as the solvent, the MIL-101(Cr)-S1, prepared using chromium trioxide (CrO3) as the chromium source and hydrochloric acid as the mineralizer, showed the highest conversion of 35.1% and selectivity of 65.9%. This was better than that of MIL-101(Cr)-S2, which was prepared using chromium (III) nitrate nonahydrate [Cr(NO3)3·9H2O] as the chromium source and hydrofluoric acid as the mineralizer. This occurred because the MIL-101(Cr)-S1 had a higher acid content and strength than those of the MIL-101(Cr)-S2. Additionally, the MIL-101(Cr)-S1 could be reused three times in succession without loss of its catalytic activity.