Advancing Plastic Recycling: A Review on the Synthesis and Applications of Hierarchical Zeolites in Waste Plastic Hydrocracking

Abstract

The extensive use of plastics has led to a significant environmental threat due to the generation of waste plastic, which has shown significant challenges during recycling. The catalytic hydrocracking route, however, is viewed as a key strategy to manage this fossil-fuel-derived waste into plastic-derived fuels with lower carbon emissions. Despite numerous efforts to identify an effective bi-functional catalyst, especially metal-loaded zeolites, the high-performing zeolite for hydrocracking plastics has yet to be synthesized. This is due to the microporous nature of zeolite, which results in the diffusional limitations of bulkier polymer molecules entering the structure and reducing the overall cracking of plastic and catalyst cycle time. These constraints can be overcome by developing hierarchical zeolites that feature shorter diffusion paths and larger pore sizes, facilitating the movement of bulky polymer molecules. However, if the hierarchical modification process of zeolites is not controlled, it can lead to the synthesis of hierarchical zeolites with compromised functionality or structural integrity, resulting in reduced conversion for the hydrocracking of plastics. Therefore, we provide an overview of various methods for synthesizing hierarchical zeolites, emphasizing significant advancements over the past two decades in developing innovative strategies to introduce additional pore systems. However, the objective of this review is to study the various synthesis approaches based on their effectiveness while developing a clear link between the optimized preparation methods and the structure-activity relationship of the resulting hierarchical zeolites used for the hydrocracking of plastics.