Microwave Magnetic Field Heating Offers a Highly Selective and Intensive Method of Heating for Catalytic Decomposition of Plastic Waste

Abstract

Plastic waste poses an escalating global environmental challenge. One promising mitigation strategy demonstrated in the literature is the catalytic microwave-assisted decomposition of waste plastics to yield hydrogen gas and carbon-rich solids. This work investigates iron oxide catalysts for use in magnetically driven microwave heating, a technique with potential for more selective and rapid thermal decomposition than electromagnetic microwave approaches. Magnetite powder was characterized using microwave cavity perturbation from room temperature to 200 °C, revealing real permeability values of 0.8–1.5 with strong negative frequency dependence and limited temperature sensitivity. Imaginary permeability ranged from 0.48 to 0.68 and exhibited moderate temperature dependence. The substantial magnetic loss of magnetite enabled efficient microwave heating: a 0.4 g sample repeatedly reached >620 °C with <100 W of microwave input power in an inert atmosphere without performance degradation. When mixed with post-consumer HDPE powder, magnetite induced rapid partial decomposition of the plastic, generating primarily light alkane gases ethane and propane (42.9 and 14.9 %vol.) within a mixture of other olefins, hydrogen, CO and CO2 (0.2, 9.7 and 4.0 %vol. respectively). The advantageous effects of microwave heating in catalysis are rooted in the selective and rapid generation of heating at local sites. Magnetically driven heating is significant in this regard as a potential mechanism for more targeted catalytic heating, for example in mixed waste streams. A carefully chosen magnetically lossy catalyst material can enhance heating rates leading to faster reactions, improved yields and different product distributions. These findings demonstrate the potential of magnetically driven microwave heating to further leverage these advantages and as a route for catalytic plastic-to-hydrogen conversion.