Abstract
We evaluated bismuth doped cerium oxide catalysts for the continuous synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide in the absence of a dehydrating agent. BixCe1−xOδ nanocomposites of various compositions (x = 0.06–0.24) were coated on a ceramic honeycomb and their structural and catalytic properties were examined. The incorporation of Bi species into the CeO2 lattice facilitated controlling of the surface population of oxygen vacancies, which is shown to play a crucial role in the mechanism of this reaction and is an important parameter for the design of ceria-based catalysts. The DMC production rate of the BixCe1−xOδ catalysts was found to be strongly enhanced with increasing Ov concentration. The concentration of oxygen vacancies exhibited a maximum for Bi0.12Ce0.88Oδ, which afforded the highest DMC production rate. Long-term tests showed stable activity and selectivity of this catalyst over 45 h on-stream at 140 °C and a gas-hourly space velocity of 2,880 mL·gcat−1·h−1. In-situ modulation excitation diffuse reflection Fourier transform infrared spectroscopy and first-principle calculations indicate that the DMC synthesis occurs through reaction of a bidentate carbonate intermediate with the activated methoxy (−OCH3) species. The activation of CO2 to form the bidentate carbonate intermediate on the oxygen vacancy sites is identified as highest energy barrier in the reaction pathway and thus is likely the rate-determining step.
Highlights
Carbon dioxide (CO2) is a major greenhouse gas, contributing to climate change and global warming [1, 2]
The surface properties of the BixCe1–xOδ nanocomposites were investigated by Raman spectroscopy, Electron paramagnetic resonance (EPR), Transmission electron microscopy (TEM), energydispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and Temperature programmed desorption (TPD) of CO2, CH3OH, and NH3
The BixCe1–xOδ nanocomposites were synthesized by an aqueous-phase co-precipitation method and characterized by X-ray diffraction (XRD), TEM, EDX, EPR, XPS, TPD, and DRIFTS
Summary
Carbon dioxide (CO2) is a major greenhouse gas, contributing to climate change and global warming [1, 2]. The strong beneficial effect of in-situ removal of water from the reaction mixture has recently been demonstrated for a ceria catalyst, a high methanol conversion of 95% with ~ 99% DMC selectivity was achieved using a continuous fixed-bed reactor and 2-cyanopyridine as a dehydrating agent [18]. Monolithic catalysts exhibit good interphase mass and heat transfer, low pressure drop at relatively large surface area, and could facilitate more efficient removal of the formed by-product (water), which unfavorably affects the thermodynamic equilibrium and limits the catalytic efficiency of the DMC synthesis [27]. A strong correlation of oxygen vacancies and catalytic performance was observed Among these monolithic catalysts those based on Bi0.12Ce0.88Oδ nanocomposite exhibited the best performance in the DMC synthesis, i.e., a promising 20.8% methanol conversion and a high DMC selectivity of 83.5% without using any dehydrating agents. In-situ modulation excitation spectroscopy in tandem with diffuse reflectance Fourier transform spectroscopy and first-principle calculations were employed to shed some light on the molecular surface processes of the DMC synthesis on this catalyst
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