The formation of local high temperature regions, or so-called “hot spots”, in heterogeneous reaction systems has been suggested as a critical factor in the enhancement of chemical reactions using microwave heating. In this paper, we report the generation of local high temperature regions between catalyst particles under microwave heating. First, we demonstrated that reaction rate of the dehydrogenation of 2-propanol over a magnetite catalyst was enhanced 17- (250 °C) to 38- (200 °C) fold when heated with microwave irradiation rather than an electrical furnace. Subsequently, the existence of microwave-generated specific local heating was demonstrated using a coupled simulation of the electromagnetic fields and heat transfer as well as in situ emission spectroscopy. Specific high-temperature regions were generated at the vicinal contact points of the catalyst particles due to the concentrated microwave electric field. We also directly observed local high temperature regions at the contact points of the particles during microwave heating of a model silicon carbide spherical material using in situ emission spectroscopy. We conclude that the generation of local heating at the contact points between the catalyst particles is a key factor for enhancing fixed-bed flow reactions under microwave irradiation.

Microwave irradiation can cause high local temperatures at supported metal nanoparticles, which can enhance reaction rates. Here we discuss the temperature of platinum nanoparticles on γ-Al₂O₃and SiO₂supports under microwave irradiation using the Debye–Waller factor obtained from in situ extended X-ray absorption fine structure (EXAFS) measurements. Microwave irradiation exhibits considerably smaller Deby–Waller factors than conventional heating, indicating the high local temperature at the nanoparticles. The difference in the average temperatures between the platinum nanoparticles and the bulk under microwaves reaches 26 K and 132 K for Pt/Al₂O₃and Pt/SiO₂, respectively. As a result, Pt/SiO₂exhibits considerably more reaction acceleration for the catalytic dehydrogenation of 2-propanol under microwave irradiation than Pt/Al₂O₃. We also find microwaves enhance the reduction of PtO_xnanoparticles by using operando X-ray absorption near edge structure (XANES) spectroscopy. The present results indicate that significant local heating of platinum nanoparticles by microwaves is effective for the acceleration of catalytic reactions.

The pyrolysis of lignocellulose facilitates the rapid production of syngas, bio-oil, and biochar. However, its low thermal conductivity and high water content result in its low heat transfer efficiency. In this study, we demonstrate the ultra-fast pyrolysis of lignocellulose using highly tuned microwaves (MWs). The efficient pyrolysis of model biomass (cellulose and alkali lignin) and real biomass (rice straw) was facilitated using a cylindrical cavity resonator and a frequency-auto-tracking solid-state MW generator without any MW susceptors as thermal media. The heating rate of cellulose using 2.45 GHz MWs was 960 °C min⁻¹ at 40 W, which was 8-fold higher than that obtained using conventional conductive heating (120 °C min⁻¹) at 100 W. The highest heating rate of 330 °C s⁻¹ was reached when rice straw was subjected to 915 MHz MWs. Using in situ resonance frequency monitoring, the carbonisation of rice straw occurred within seconds during fast heating under MW irradiation. This study demonstrated that highly tuned MW energy could facilitate the ultra-rapid pyrolysis of lignocellulose by directly applying MW energy without using a thermal medium.

Chemical and mechanical properties of the leaf cuticular membranes (CMs) of a mangrove, Sonneratia alba J. Smith, were analysed at various leaf development stages to evaluate their tolerance to environmental stress. Our analyses demonstrate that the CMs from leaves of S. alba at different growth stages are generally rich in wax (21.5–25.7%) and cutin (52.4–63.4%) which rapidly accumulate at the early stages of leaf growth, while cutan (4.3–10.3%) and polysaccharide (2.3–7.7%) continuously accumulate throughout growth. Immature CMs are physically weak and highly viscoelastic. However, CMs become strengthened and stiffened during leaf expansion and maturation (by factors of about 1.5 and 2.4, respectively) while their flexibility decreases (68–83% decrease). Finally, the CMs lose their strength at the senescent stage (30–43% decreasement). Correlation analysis between chemical composition and mechanical properties revealed that the cutin matrix is mainly responsible for the high viscoelastic properties of CMs, while wax, cutan and polysaccharide contributed to their elasticity. Wax also affected the strength of the CMs, whereas cutan and polysaccharide showed rigidizing effect. Rapid accumulation of wax and cutin in the CMs after bud burst followed by the mechanical supports of cutan and polysaccharide in an isolateral manner contributed to the remarkable environmental tolerance of S. alba.