Chemical Reaction under Highly Precise Microwave Irradiation

Abstract

Chemists sometimes experience enhancements in the rates and selectivity of the products in chemical reactions conducted under microwave irradiation, but these enhancements are not always reproducible. This could be due to the unstable irradiation of chemical systems by microwaves. To achieve experimental reproducibility of chemical reactions under microwave irradiation, a solid-state microwave source with an ultra-precise voltage-controlled surface-acoustic-wave oscillator (VCSO), a high-power amplifier (HPA) module, and an elliptical applicator are developed. The HPA has an average power of up to 141 W and generates a 2.45 GHz sine signal. Although the output power is 100 W, the phase noise of the developed microwave source is still small, reaching as low as-80 dBc/Hz at a 1-kHz offset. This good phase noise characteristic originates from that of the VCSO. An elliptical applicator generates the TM110 mode and can concentrate the electric field of the microwaves at the focal point. The signal purity is very important for microwave irradiation to achieve high efficiency for heating and chemical processes. It is demonstrated boiling of water in a shorter time using the HPA with a VCSO and a 40% reduction in the water-boiling time compared with a magnetron under 150-W MW irradiation was observed because of the 2.45 GHz sine signal of the HPA with a VCSO. With these features, reproducible reactions can be conducted. Methanol decomposition as a solid-gas reaction with a Pd/C catalyst under microwave irradiation using this HPA and applicator is also shown. Before comparing with electrical furnace heating, the temperatures of the catalyst bed were thoroughly investigated by experiment and simulation, and there were no hot spots in the catalyst bed. The reaction rate under microwave irradiation was three times greater than under electric furnace heating.