(217a) Thermochemical Co-Conversion of Waste Polyolefins with Low-Rank Aromatic-Rich Hydrocarbons into an Intermediate of High-Quality Anisotropic Pitch

As primary packaging materials, polyolefin plastics (PP, HDPE, and LDPE) are the most widely used plastics in daily life. As a result, polyolefin plastics commonly have a useful life shorter than one month and account for more than 40% of the total waste plastic amount. However, current recycling and discharge methods (energy recovery, fractions recycled, and landfill) of these waste polyolefin plastics are considered neither economical nor environmentally friendly, because they have a high recycling cost and low financial output, and have led to a great waste of resources and serious secondary pollution. Therefore, a total thermochemical conversion of waste plastics into value-added chemicals and materials can be more attractive than conventional waste plastic recycling and discharge methods.

Even so, due to the composition of polyolefins, current thermochemical products derived from polyolefins are more like liquid fuels, which makes the process less competitive than petroleum-derived liquid fuels, especially when the environmental impacts are not considered. In contrast, thermochemical co-conversion of polyolefins and other low-rank aromatic-rich hydrocarbons (polystyrenes, lignite, and coals) provides a different opportunity. The co-conversion is similar to a thermochemical liquefaction process, where the melted hydrogen-rich polyolefins play the role of reactants, self-generated solvents, and possibly hydrogen sources, while the heated aromatic-rich hydrocarbons crack valuable aromatic-containing free radicals.

In the past, research on this type of co-thermochemical process was limited, because the co-conversion liquid products contained a high content of undesired heavy asphaltene and pre-asphaltene fractions, while the process was designed to produce light oil fractions. However, these heavy fractions were partially-hydrogenated yet still maintained abundant aromatic structures. Therefore, the co-conversion products can be used as an intermediate to generate a pitch material with a unique anisotropic texture. The type of pitch is named mesophase pitch and can be used as an excellent intermediate for the manufacture of value-added high-performance carbon materials such as carbon fibers, carbon foam, needle coke, and graphene supercapacitors.

In the research presented here, direct/catalytic thermochemical co-conversion of waste polyolefins (HDPE and LDPE) and aromatic-rich hydrocarbons (polystyrene, low-rank coal, metallurgical tar) under pressurized conditions was carried out in a 100 mL high-pressure stirring reactor. The effects of multiple operating parameters (types of aromatic-rich materials, reaction temperature, reaction time, and feedstock mass ratio) on the composition and yield of the targeted product intermediate were investigated. The obtained intermediates were further thermally converted into mesophase pitch, and the effects of the intermediate compositions and thermal treatment conditions on the carbonization performance and anisotropic texture development level were studied and will be discussed. The products were analyzed via solvent fractionation, FTIR, NMR, TGA, DMA (Dynamic Mechanical Analysis), and polarizing microscopy. Based on the characterization results, the proton transferability of the intermediates and the possibility of whether the polyolefins are a hydrogen vehicle or a hydrogen donor will be discussed. The co-conversion method produced a precursor with a high conversion yield and ideal composition, which demonstrated an encouraging potential for converting waste plastics and low-rank hydrocarbon resources into an anisotropic pitch with high quality for further manufacturing of high-value high-performance carbon materials. This pathway provides a bright prospect for simultaneously handling recycled waste plastics and manufacturing value-added chemicals and materials in the U.S.