Operando Raman Spectroscopy of the Microwave-Enhanced Catalytic Dehydration of 2-Propanol by WO₃

Abstract

Operando Raman spectroscopy was performed to understand the mechanism of a microwave (MW)-enhanced fixed-bed catalytic reaction. An MW operando Raman spectroscopy system was constructed by integrating a Raman spectroscopy probe (785 nm) with a single-mode (TM110) MW cavity resonator equipped with a semiconductor MW generator. MW heating enhanced the dehydration of 2-propanol to diisopropyl ether and propylene by +54 to +60% under the air flow condition. In the argon flow condition, the conversion of 2-propanol was further enhanced by +71 to +83% with >99.9% selectivity toward propylene when MW heating was used. Operando Raman spectroscopy revealed that MW irradiation enhances the reduction of the WO3 surface and exhibits intense light emission, especially under the argon flow condition. The enhanced generation of WO3–x species on the surface of the WO3 catalyst should be, therefore, a key factor in the enhancement of the dehydration of 2-propanol under MW irradiation.

Real-Time Facile Detection of the WO3 Catalyst Oxidation State under Microwaves Using a Resonance Frequency

“…From the resonance frequency results, together with Raman spectroscopy and MS, we concluded that the WO 3 surface was reduced in tandem with the enhanced dehydration of 2-propanol. Moreover, these results strongly agreed with our previous report on operando Raman spectroscopy (785 nm) of the same reaction . The Raman peak intensity and resonance frequency profiles strongly coincided with the formation of propylene.…”

“…During the first 10 min, the formation of these products strongly correlated with a decrease in the intensity of the Raman spectra. These behaviors are in line with our previous study, which used GC analysis (every 20 min) . Meanwhile, the present MS system allowed time-resolved product analysis (every 3 s), which was useful for tracking the changes in the product distribution along with the oxidation state of the WO 3 catalyst.…”

“…Figure shows the MW heating system equipped with resonance frequency monitoring, Raman spectroscopy, and MS. Microwave heating was conducted using a TM 010 -mode cylindrical cavity resonator and a GaN semiconductor MW generator (Ryowa Electronics Co. Ltd., Sendai, Japan). The TM 010 -mode cavity resonator equipped with a frequency autotracking MW reactor provided more efficient MW heating than our previous TM 110 -mode reactor, owing to the higher Q factor of the cavity as well as the continuous input of optimum frequency MWs. The cavity resonator was equipped with five windows at the top (1), bottom (1), side (2), and front (1) of the cavity.…”

“…S -parameter measurement by a vector network analyzer (VNA) was used to verify the dependence of the resonance frequency on the oxidation state of the catalyst. Resonance frequency monitoring was applied to detect the oxidation state of the WO 3 catalyst during the dehydration reaction of 2-propanol as a model reaction, in which we have recently reported the rate enhancement effect by MW irradiation …”

“…We previously reported that MWs enhance the dehydration of 2-propanol over the WO 3 catalyst by 83% over that achieved through conventional heating with high selectivity to propylene (99.9%). 14 Microwave operando Raman spectroscopy suggested that the reaction enhancement could be attributed to the surface reduction of WO 3 . Formation of WO 3−x increased the Lewis and Brønsted acid sites at the surface defects and accelerated the dehydration of 2-propanol.…”