(530b) Catalytic Microwave-Assisted Pyrolysis of Plastic Waste for Fuels and Carbon Material

Plastic materials are extremely popular all over the world and used in the different walks of life due to the inherent properties of being strong, lightweight, and easily shaped. However, the vast majority of waste plastics ever produced enters into landfills or our ecosystems, creating a plastic waste crisis. Waste plastic conversion for the production of green fuels, chemicals, and hydrogen is able to reduce the consumption of fossil energy, thereby alleviating the climatic effects and achieving the 1.5 ºC Paris climate goal. Currently, the most common solution to recycle the plastic waste is mechanical method that is limited by the corresponding challenges. The recycled plastics after melting and remolding generally have poorer properties than those of the virgin plastics, enabling the recycled plastics to be only applicable to lower quality products. This downcycling process makes the waste plastic recovery not attractive in the industry. In this regard, we proposed a novel catalytic microwave-assisted pyrolysis (CMAP) technology that is designed to achieve the conversion of waste plastics to high quality naphtha, that are injected to new plastic manufacturing, with the hope of creating a circular economy and minimizing greenhouse emission in mind. The byproduct, non-condensable gas, ever produced from the pyrolysis-reforming process will be recovered for carbon material and hydrogen via chemical vapor deposition technique.

Recently, we have screened different catalysts to improve the yield and quality of the liquid hydrocarbons from plastic waste pyrolysis, with the aim of maximizing naphtha fractions for new plastic production and extending the catalyst lifetime as much as possible. First, we proposed a novel approach of combining two catalytic reforming zones, where the first catalytic reforming zone was intended to improve the cracking of polyolefins into short chain olefins, and the second, lower-temperature catalytic reforming zone was intended for the hydrogenation process to convert these olefins to C5-C12 alkenes. The tests were very successful and very promising results (60-75% C5-C12 alkanes, 3-5% C5-C12 olefins, 5-15% mono-aromatics) were obtained as hypothesized. Unfortunately, the catalysts for plastic cracking are prone to be deactivated easily. The relationship between the catalyst structure and catalyst lifetime was studied comprehensively by fine-tuning the acidity and pore structure of zeolites. It was confirmed that proper acid density and larger pore size significantly improved the catalyst lifetime. Considering that the pore size of conventional ZSM-5 catalyst is too small for plastic pyrolysis intermediates entering into the pore system, which will limit the diffusion of intermediates and block the pore opening and result in the fast deactivation of catalyst, the hierarchically macro-meso-microporous high Si/Al ratio ZSM-5 zeolite was developed and tested. It was confirmed the catalyst lifetime of hollow ZSM-5 is five times longer than the conventional ZSM-5 due to the better diffusion channels and improved accessibility of acid sites inside catalysts. After producing fuels or chemicals, 30~40 wt.% non-condensable gases (mainly composed of hydrogen and C1-C4 hydrocarbons) will be left, to be further treated. So, we synthesized a metal oxide coating material to catalytically decarburize the non-condensable gases for carbon material and hydrogen production. When the non-condensable gas products pass through a high temperature reactor packed with an effective Ni-Fe/Al₂O₃ catalyst, high yield of hydrogen (88% hydrogen selectivity) can be achieved, with carbon products being produced. In summary, our studies indicated that the CMAP is a low-cost and highly efficient technology convert waste plastics to naphtha and hydrogen.