Nazarov cyclization, a conrotatory 4π electrocyclization reaction of a divinyl ketone through the formation of the pentadienyl cation intermediate, is promoted by a Lewis acid to provide cyclopentenone. Our group has reported an enhancement of a reaction rate based on a microwave specific effect. An activation of an equilibrium of conformers was hypothesized and the microwave irradiation could formally increase a frequency of a formation of the suitable conformer, resulting in the acceleration of the reaction rate. In this work, our strategy for the acceleration of the ring-closure in Nazarov cyclization is the microwave-assisted activation of the equilibrium between conformers of the pentadienyl cation intermediate. When a copper-catalyzed Nazarov cyclization using aryl vinyl ketone derivative containing 1,3-dicarbonyl moiety was carried out under microwave irradiation conditions, the Nazarov cyclization was dramatically accelerated and the kinetic rate by the microwave irradiation was 5.8 times higher than that by a conventional heating conditions. We discussed about the microwave specific effect in terms of a reaction dynamics and the hypothesis of the conformational equilibrium activation by the microwave specific effect was theoretically supported.
Although interfacial modification by microwave selective heating has been reported in previous study, it was difficult to estimate the thermal energy concentration of the absorption at liquid-liquid interface. In this study, optimum operation of microwave pulse mode was investigated to enhance the effect of surfactant desorption by using our dimensionless number of the local heating. According to the results, it was found that power is an important factor for the quick thermal response of interfacial tension. Finally, dimensionless number is expected to be a good tool for controlling heat transfer at the interface.
Microwaves selectively heat solid catalysts and allow enhancement of fixed-bed flow reactions. In this study, microwave in situ and operando Raman spectroscopy and resonance frequency were performed for real-time monitoring of the oxidation state of the tungsten oxide (WO3) catalyst during microwave irradiation. First, the enhancement of catalytic dehydration and dehydrogenation over WO3 catalyst was demonstrated under microwave irradiation. Operando Raman spectroscopy revealed that the remarkable reduction occurred at the surface of the WO3 catalyst. We further applied in situ resonance frequency for real-time monitoring of oxidation state of WOx during model oxidation reaction of WO2 to WO3. We found that the oxidation state of WOx determined by the in situ resonance frequency was comparable to that obtained by XRD. However, the changes in the resonance frequency should be considered carefully because it is also affected by the volume and temperature of the samples.
Luminescent and mono-dispersed quantum dots have been expected for the industrial applications as a useful visible- light emission source. Particularly, carbon quantum dot (CQD), composed of organic or inorganic carbon sources, has attracted much attention from the viewpoint of environmental adaptability. However, unlike existing metal/semiconductor quantum dots, CQD still has critical tasks; low quantum yields at 570−750 nm and uncertainty of microwave superiority on CQD synthesis. In this research, we attained high quantum yields (61.1 % at 580 nm) CQD using microwave solvothermal heating under low pressure. Moreover, the obvious differences in quantum yields and morphology were observed between microwave and conventional solvothermal heating conditions, as the polymerization degree/amount of polyethylene glycol was changed. We assume that this apparent difference comes from selectively heated CQD or their intermediates enhanced by microwave solvothermal heating, causing uniform pyrolysis of CQD.
This paper summarizes the effects of microwave heating for formation of local high temperature at the supported metal nanoparticles. We demonstrated the enhancement of catalytic dehydrogenation of 2-propanol over supported Pt catalysts. Then, we discussed the mechanism of microwave heating caused by supported Pt nanoparticles using a substrate system. Furthermore, microwave in situ XAFS was used to estimate the local temperature of Pt nanoparticles. We found that the local temperature of Pt was 26-132 K higher than that of the bulk. From these results, we concluded that the microwave irradiation causes localized high tempretaure at the supported nanoparticles and enhance heterogeneous catalytic reactions.