In situ reductive transformation of indium oxides on zeolites towards efficient microwave heating

The interaction of incident microwaves with heterogeneous catalysts presents unique opportunities for selective and energy-efficient heating in catalytic and materials processes. Recently, microscopic selective heating phenomena have emerged as a promising strategy to revolutionize microwave-assisted systems by enabling targeted energy absorption and improved control over local reaction environments. However, since the interaction between catalysts and microwaves is highly dependent on the temperature, catalysts that induce such localized heating at elevated temperatures do not necessarily absorb microwave energy efficiently at room temperature. Typically, catalysts that do not absorb microwaves well at room temperature require the addition of microwave susceptors such as SiC to initiate heating. However, under high-temperature reaction conditions, these susceptors can interfere with the efficient delivery of microwave energy directly to the catalyst. In this work, to propose a new material heating method that leverages structural transformations during microwave irradiation, the structural transformation of indium oxide (In2O3) deposited MFI-type zeolites under H2 condition was investigated. At lower temperature (<200 °C), In2O3 played a role of moderate microwave absorber. Once the sample reached 200 °C, In2O3 was reduced to form In+ cations, which migrate into zeolite pores via reductive solid-state ion exchange. This results in a sharp increase in heating efficiency, reaching 500 °C with only 40 W of microwave power. In contrast, thermally pretreated samples showed minimal heating under the same conditions. These findings reveal that In+ cations act as atomic-scale microwave hotspots at elevated temperatures, potentially applicable to thermally-effective catalysis. This discovery provides a new strategy for the design of practical catalysts tailored for microwave heating, enabling more efficient and selective catalytic processes without relying on external susceptors.