(473a) Chemical Upcycling of Waste Polyolefinic Plastics to Low-Carbon Synthetic Naphtha for Closing the Plastic Use Loop

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. Polyolefins, such as high density polyethylene (HDPE), low density polyethylene (LDPE), and propylene (PP), represents the largest amount of discarded waste plastics, accounting for over half of the waste plastic stream.

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. Developing an effective pathway to remove waste plastics from landfill and incineration plants and create a circular economy requires a more appropriate technology beyond the conventional mechanical recycling by melting and re-molding. Pyrolysis has shown the great potential of plastic recycling for energy, fuels, chemicals, and materials production, enabling the plastic wastes to stay in the economy and out of the environment. 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. Notably, relay catalysis of Al₂O₃ followed by ZSM-5 achieved up to 100% conversion into monoaromatics and C5-C12 alkanes/olefins at a catalyst to plastic ratio of 4:1. In order to further mitigate the aromatic formation, Al₂O₃ pillared clay catalyst was developed and tested, resulting over 50% C5-C12 alkanes and less than 10% monoaromatics. The liquid yield can reach over 70 wt.% with 52% of C5-C12 alkanes, 17% of C5-C12 olefins, 14% of C5-C12 aromatics, and 17% of C13+ hydrocarbons from catalytic pyrolysis of polyethylene. More importantly, this process is also suitable for waste plastic mixtures, e.g., LDPE 41%, HDPE 24%, and PP 35%, which represents one of the typical plastic waste streams. Furthermore, we proposed a novel approach (Figure 1) 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.

When just using one catalytic zone with Zn/SBA-15 as a catalyst, 51.67 wt.% of liquid oil can be obtained, which is higher than that of the two catalytic zone (36.65 wt.%). It may be because the second catalytic zone was operated at lower temperature and some unconverted long chain hydrocarbons with high boiling temperature were trapped in this zone. This also highlights the importance of wax cracking in the first catalytic zone. It can be noted that after adding the second catalytic zone, hydrogen content in the gas product experiences a decline. That supports our hypothesis that the olefins produced in the first catalytic zone will be converted into paraffins by consuming the spontaneous hydrogen or hydrogen transfer reaction. The liquid oil composition verifies that the tests were very successful and very promising results were obtained as hypothesized.

The promising results of the two catalytic zone is mainly attributed to the cracking and hydrogenation in the first and second catalytic zone, respectively (Figure 2). The catalytic activity of Al₂O₃ pillared M-clay may be due to the special features of this material, including the large pore size and Al₂O₃-induced active sites (more Brønsted acid sites). On one hand, the large pore size alleviates the steric and diffusional hindrances for entering the catalyst channels because the bulky nature of the polyolefins or long chain hydrocarbon intermediates in the pyrolysis reaction; on the other hand, Brønsted acid sites can be responsible for carbonium ion formation which is cracked into alkanes further. Based on the results above, it is supposed that this catalyst increases the share of C5-C12 alkanes by primarily facilitating the following reactions (Figure 3).

In the HDPE degradation, the long polymer chain is initially cracked into shorter chain olefins, which would then be converted to alkanes and aromatics via the hydrogen transfer reaction over Al2O3 pillared M-clay catalyst.

To advance this technology towards industrial applications, a continuous catalytic microwave-assisted pyrolysis (cMAP) system with a processing capacity of 1 ton/day was developed. The system features a mixing bed of silicon carbide balls enabling fast, uniform and energy-efficient heating for the process. Through this system, 57 wt.% C5-C22 liquid hydrocarbons was obtained from polyethylene, allowing over 32% energy saving compared to production of similar products from virgin materials.

KEYWORDS: Catalysis; Environmental sustainability; Chemical Reaction Engineering