(298b) Precise Control of Microwave Assisted Pyrolysis for Waste Plastics Via Fluorescent Nano-Thermometers

In the present study, a microwave assisted catalytic pyrolysis technique was investigated for converting waste high-density polyethylene into liquid fuels. During the microwave-assisted catalytic pyrolysis of waste high-density polyethylene, the selective heating characteristic of microwave irradiation causes the formation of local high-temperature domains (“hot spots”) over the surface of microwave-absorbing catalysts, leading to rapid cracking of polymers near the catalyst particles and hence the generation of hydrocarbon products. This mechanism has been confirmed by investigating the dominant role of the catalysts’ dielectric loss on the composition of produced hydrocarbon. In order to precisely control this “hot spots” phenomenon, we designed thermo-sensitive fluorescent nanoparticles as nano thermometers to probe the intrinsic temperature of iron-based catalyst particles and accordingly derived an energy transfer model to predict the temperature gradient between catalysts and the bulk reactants. The fluorescence results agree with the model predictions that the microwave induced temperature gradient can be enlarged by increasing microwave intensity, as well as the dielectric loss and size of particles. Conversely, the increase in the thermal conductivity and velocity of sweeping gas lowers the temperature gradient. Based on the model, we precisely designed various microwave heating strategies and compared the composition of produced hydrocarbons in these cases. A narrower hydrocarbon product distribution was obtained by using higher-power short-pulse microwave heating strategy to enlarge the catalyst-reactant temperature gradient, exhibiting the unique advantage of microwave irradiation that the instantaneous on-off switch is able to reduce the thermal inertia of processes and the feasibility of the directional conversion of waste plastics to certain high-valued fuel products. Besides, the average chain length of produced hydrocarbons can be flexibly regulated from C₉ to C₃₀ by adjusting the microwave power and the sweeping nitrogen velocity. Results from the present work can serve as the starting point for fine process control and rational development of microwave responsive catalysts for catalytic recycling of plastics.