Hydrogen and terephthalic acid production from polyester-polyolefin mixed waste plastics by low-temperature hydrothermal conversion process

<sec><p indent="0mm">Hydrogen, as a clean energy with high energy density and zero carbon emissions, is considered an ideal alternative to fossil fuels, and it has an extremely high potential application value in industries, such as transportation, manufacturing, and chemical industries. The carbon content in waste plastics is ~35%–85%, and the hydrogen content is ~4%–15%; therefore, waste plastics have great potential for conversion into hydrogen. However, in domestic waste and other industrial organic solid waste, waste plastics mainly exist in the form of various plastic mixtures. Therefore, the development of advanced technologies that can achieve the direct and efficient conversion of mixed waste plastics is important to solve the problem of organic solid waste treatment in actual industrial applications. </sec><sec> The thermochemical process can enable the efficient conversion of waste plastics to hydrogen-rich syngas in high-temperature reaction systems. However, because this traditional process used for hydrogen production generally involves high-temperature and harsh reaction conditions, it inevitably requires a large amount of energy input; additionally, the reaction involved in the conversion process is complex and easily yields a large number of byproducts, which will have a serious impact on the catalytic system and reaction equipment. </sec><sec> Meanwhile, the aqueous-phase reforming process enables hydrogen production from organic matter under mild conditions in a low-temperature hydrothermal environment. However, due to the difference and incompatibility of the physical and chemical properties of each component in mixed waste plastics, it is difficult to directly decompose and transform them via the liquid-phase reforming process. To address this problem, this study considered the depolymerization of mixed waste plastics into small molecular organic compounds suitable for the liquid-phase reforming process via a universal pretreatment process. </sec><sec> In this study, a hydrothermal depolymerization coupled with aqueous-phase reforming reaction system to produce hydrogen and terephthalic acid from polyester–polyolefin mixed waste plastics was developed. Typical single waste plastics, such as polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), and polyvinyl chloride (PVC), were first efficiently depolymerized into small molecular organic acids in this reaction system, which were then converted into hydrogen-rich syngas. The mixture ratio of waste plastics has a significant effect on the gas yield and concentration of the PET-PE-PP mixed waste plastics employed in the conversion process. The effect of PVC on the Ru-20ZnO/NiAl₂O₄ catalyst was further investigated. The results show that Cl-containing compounds in the liquid-phase products of PET-PE-PP-PVC oxidation form strong acidic sites on the catalyst surface, interacting with the NiAl₂O₄ support and active metal components and thus forming ClO₂Ni(ClO₄)₃ and Ni(ClO₄)₂. As a result, the structure of the Ru–20ZnO/NiAl₂O₄ catalyst is damaged, and its catalytic performance for hydrogen production is decreased. Finally, the feasibility of low-temperature hydrothermal semi-continuous hydrogen production from mixed waste plastics was verified using a semi-continuous experimental platform. The proposed mixed waste plastics treatment technology avoids the formation of byproducts, such as tar and coke, to a certain extent and thus provides a new path for the high-value utilization of mixed waste plastics. </sec>