Preparation of MIL-88B(Fex,Co1−x) catalysts and their application in one-step liquid-phase methanol oxidation to methyl formate using H2O2

Abstract

The selective oxidation of methanol to methyl formate is one of the most attractive processes to obtain value-added methanol-downstream products. The development of highly efficient and stable catalysts is critical for this transformation. In this study, a series of MIL-88B(Fex,Co1–x) bimetallic catalysts with different Fe/Co molar ratios were prepared through a one-pot hydrothermal method. X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, energy dispersive spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, N2 adsorption-desorption, and inductively coupled plasma-mass spectrometry characterization were performed to elucidate the structure of the catalysts. The activity of the catalysts were assessed in the one-step oxidation of methanol to methyl formate with H2O2 in a liquid-phase batch reactor. The results show that the MIL-88B(Fex,Co1–x) catalysts exhibit uniform needle-like morphologies with an average length and width of 400–600 nm and 100–150 nm, respectively. Co2+ is incorporated into the framework by partially replacing Fe3+ in MIL-88B. Moreover, the catalyst efficiently promoted the conversion of methanol to methyl formate. When MIL-88B(Fe0.7,Co0.3) catalyst was used with a molar ratio of H2O2 to methanol of 0.5 at 80 °C for 60 min, 34.8% methanol conversion was achieved, and the selectivity toward methyl formate was 67.6%. The catalysts also showed great stability with a steady conversion and selectivity even after four cycles. The preliminary oxidation mechanism was also studied. It was determined that H2O2 is first adsorbed on the Fe3+ sites and subsequently activates these sites. Methanol is adsorbed by the O atoms of the framework through hydrogen bonding and is gradually oxidized to formic acid. Subsequently, formic acid reacts with the residual methanol at the Fe3+ and Co2+ Lewis acid sites to form methyl formate.