Date of Award

Winter 3-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Shengnian Wang

Abstract

Plastics have been an irreplaceable component of modern technology as well as everyday life. They have brought much convenience to us with their characteristics of great malleability, durability, and stability. The versatile and low-cost nature of plastics also enables their wide engagement in many modern industries including automobile, medical, communication as well as aerospace. Polyethylene (“PE”), made from polymerization of ethylene, is one of the most widely used plastics in the world. Being cheap, flexible, and long-lasting, they are extensively used in the packaging industry, especially for plastic bags and other sorts of containers. However, the durability of plastics, on the other hand, leads to increasing environmental problems. Annually, over 400 million tons of plastics are manufactured globally and only less than 9% of them are recycled. The majority of plastic waste ends up in landfills or other natural environments including the ocean, leading to severe concern for both animal and human health. The main cause of the low plastic recycling rate is due to the limitations of current recycling methods. Many challenges and obstacles, including plastic diversity, contamination, and downcycling, lead to a low incentive for plastic recycling, as they are not economically profitable. Efforts have been made to explore the possibilities of upcycling plastics into value-added products for a better economic drive in plastic recycling. One promising solution is catalytical recycling, which breaks down the polymer chain through less energy-intensive processes to form basic building blocks, wax, fuel, or other valuable materials. Zeolites are crystalline aluminosilicates that are widely used in the chemical industry. They have specified pores that are defined by their crystal structures, which make them highly selective in many catalytic reactions. The solid acidic sites of Bronsted and Lewis acid also help to effectively break down polymer chains as catalysts. Given their desired porous structure and solid acidic properties, there is much potential for catalytical recycling of polyethylene using zeolites. Yet, the current use of zeolites in polyethylene recycling still suffers from several drawbacks like high temperature (energy intensive), and low selectivity towards desired products (not enough value for recycling). Not to mention that conventional zeolites with micropores often encounter diffusion issues which would reduce catalytical efficiency and lead to coking. New methods are needed to make better and cheaper zeolites as well as improve the upcycling effectiveness to make the process more economical. Hierarchical zeolites with mesopores have better diffusion for large molecules such as polyethylene. Traditionally hierarchical zeolites are made either using expensive templates involving hydrothermal treatment, which produces liquid waste and is energyintensive, or through complicated post-synthetic procedures. We here report a new solidstate method to synthesize hierarchical ZSM-5 without using any mesoscale template. The hierarchical zeolites we synthesized have a particle size of around 300-400 nm and larger pore volume and surface area compared to commercial ZSM-5. The synthesis conditions for these hierarchical zeolites were also optimized by changing aging conditions, SDA/TEOS ratio, reaction temperature, and time. To evaluate the catalytical properties over PE upcycling of our hierarchical zeolites, we performed the depolymerization experiments of low-density polyethylene (LDPE) with both solid-solid reaction and solvothermal systems. Reaction results were compared between conventional microporous zeolites and our newly synthesized mesoporous zeolites. Higher conversion and selectivity towards liquid products were observed using our mesoporous zeolites (Meso-ZSM-5 in particular). Detailed analysis of liquid and solid products was made for the upcycling process at a temperature range of 240-320 ℃ . We also investigated the reaction results using different solvents (hexane, cyclohexane, and petroleum ether) to further explore the possibilities of solvent-assisted depolymerization of LDPE using zeolites.

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