(154ar) Microwave-Assisted Chemical Recycling of Polyolefinic Plastics for the Production of Monomers

Current global plastic production is around 380 million tons/year, only 14% of which is recycled. Most of them ends in the landfill, the environment, or the incineration plant, polluting our ecosystem and leading to micro/nano-plastic issue. 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. However, plastic recycling through catalytic pyrolysis currently suffers from a range of limitations, including: unreliable pyrolysis/catalysis reactor system, short catalyst lifetime, and low economic feasibility. We have proposed to recycle plastic waste based on our catalytic microwave-assisted pyrolysis (CMAP) technology, where the plastic waste is converted into monomers for new plastic manufacturing.

Developing a novel CMAP technology to convert waste plastics to monomers for new plastic production requires highly efficient and reliable system and catalysts. In the past few years, we developed a novel system of CMAP featuring a mixing SiC ball bed for monomer production from polyolefinic plastics with promising economic and environmental outlook. In terms of catalysts, zeolites are considered the most promising cracking catalyst. It has been reported that the increase in acidity and accessibility of acid sites can decrease the degradation temperature and the activation energy, and improve the selectivity of desirable products. However, no clear descriptions for the structure vs performance relationship during catalytic cracking of waste plastics have been developed. Clearly, extending catalyst lifetime without affecting the cracking performance is one of the most important objectives in developing new catalysts for plastic cracking. In our study, we present a comprehensive investigation on the effect of BAS and pore structure on the service lifetime of ZSM-5 zeolites in the plastic cracking. By developing different ZSM-5 zeolites with gradient BAS concentrations, and a range of meso-porosity, we have attempted to establish a correlation between BAS structure and catalyst lifetime, which could be applied to fast catalyst screening in the booming plastic waste pyrolysis industry. Results indicated that the plastic conversion presents significant declining trends with time on stream, indicative of the catalyst deactivation. The experimental data show that both the lifetime and product selectivity of the catalysts changed when their Bronsted acid site (BAS) density and pore structure were altered through the modifications. To quantitatively describe the relationship between them, we developed a BAS surface density (C_B) factor by normalizing the BAS concentration (nBAS) by the BET area (SBET). The relationship between C_B and high density polyethylene (HDPE) throughput is not a linear correlation. Instead, both high and low C_B lead to relatively short catalyst lifetime. Only when BAS densities are located at the middle range (around 0.5-0.9), can zeolite perform the best in terms of lifetime. High BAS densities would mean closer proximity for acid sites, thereby resulting in faster deactivation due to higher coking rates. However, too low BAS densities also result in short lifetime because BAS plays a vital role during cracking reactions.

To improve the catalyst lifetime, a hierarchically ordered macro-meso-microporous nano-sized zeolite catalyst has been developed for catalytic cracking of polyolefinic plastics for monomers (arenes and light olefins) production. The catalyst we developed is able to last for more than 30 h in this reaction at the weight hourly space velocity of 20 /h. Our efforts will advance this technology to a larger commerical operation and help resolve the plastic waste issue facing the society.