Nitro functionalized chromium terephthalate metal-organic framework as multifunctional solid acid for the synthesis of benzimidazoles

Abstract

In the present work, nitro functionalized chromium terephthalate [MIL-101(Cr)-NO2] metal- organic framework is prepared and characterized by powder X-ray diffraction (XRD), elemental analysis, infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) surface area. The inherent Lewis acidity of MIL-101(Cr)-NO2 is confirmed by FT-IR spectroscopy using CD3CN as a probe molecule. The performance of MIL-101(Cr)-NO2 as bifunctional catalyst (acid and redox) promoting the synthesis of wide range of benzimidazoles has been examined by catalyzed condensation on acid sites and subsequent oxidative dehydrogenation. The catalytic activity of MIL-101(Cr)-NO2 is found to be superior than analogues catalysts like MIL-101(Cr)-SO3H, MIL-101(Cr)-NH2, UiO-66(Zr), UiO-66(Zr)-NO2, MIL-100(Fe) and Cu3(BTC)2 (BTC: 1,3,5-benzenetricarboxylate) under identical reaction conditions. The structural stability of MIL-101(Cr)-NO2 is supported by leaching analysis and reusability tests. MIL-101(Cr)-NO2 solid is used five times without decay in its activity. Comparison of the fresh and five times used MIL-101(Cr)-NO2 solids by powder XRD, SEM and elemental analysis indicate identical crystallinity, morphology and the absence of chromium leaching, respectively.