Enhanced selective oxidation of dimethyl ether to formaldehyde by MoO3-Fe2(MoO4)3 interaction over iron-molybdate catalysts

Abstract

The efficient catalytic conversion of fossil-based low-carbon small molecules to oxygen-containing chemicals is an attractive research topic in the fields of energy and chemical engineering. The selective oxidation of dimethyl ether (DME), which is derived from fossil resources, represents a promising approach to producing high-concentration formaldehyde with low energy consumption. However, there is still a lack of catalysts achieving satisfactory conversion of DME with high selectivity for formaldehyde under mild conditions. In this work, an efficient iron-molybdate (FeMo) catalyst was developed for the selective oxidation of DME to formaldehyde. The DME conversion of 84% was achieved with a superior formaldehyde selectivity (77%) at 300 °C, a performance that is superior to all previously reported results. In an approximately 550 h continuous reaction, the catalyst maintained a conversion of 64% and a formaldehyde selectivity of 79%. Combined X-ray diffraction  (XRD), Transmission electron microscope (TEM), Ultraviolet–visible spectroscopy (UV–Vis), Hydrogen temperature-programmed reduction (H₂-TPR), Fourier transform infrared (FT-IR) analyses, along with density functional theory (DFT) calculations, demonstrated that the excellent FeMo catalyst was composed of active Fe₂(MoO₄)₃ and MoO₃ phases, and there was an interaction between them, which contributed to the efficient DME dissociation and smooth hydrogen spillover, leading to a superior DME conversion. With the support of DME/O₂ pulse experiments, in-situ Raman, in-situ Dimethyl ether infrared spectroscopy (DME-IR) and DFT calculation results, a Mars-van Krevelen (MvK) reaction mechanism was proposed: DME was dissociated on the interface between Fe₂(MoO₄)₃ and MoO₃ phases to form active methoxy species firstly, and it dehydrogenated to give hydrogen species; the generated hydrogen species smoothly spilled over from Fe₂(MoO₄)₃ to MoO₃ enhanced by the interaction between Fe₂(MoO₄)₃ and MoO₃; then the hydrogen species was consumed by MoO₃, leading to a reduction of MoO₃, and finally, the reduced MoO₃ was re-oxidized by O₂, returning to the initial state. These findings offer valuable insights not only for the development of efficient FeMo catalysts but also for elucidating the reaction mechanism involved in the oxidation of DME to formaldehyde, contributing to the optimized utilization of DME derived from fossil resources.