Understanding the Structure–Activity Relationships in Catalytic Conversion of Polyolefin Plastics by Zeolite-Based Catalysts: A Critical Review

Abstract

Polyolefins, the largest used commodity plastics in the world, find extensive application in many fields. However, most end up in landfills or incineration, leading to severe ecological crises, environmental pollution, and serious resource waste problems. As representatives on chemical upcycling of polyolefin plastics polyolefin waste to fuels and bulk/fine chemicals, polyolefin catalytic cracking and hydrocracking based on zeolite or metal/zeolite composite catalysts are considered the most effective paths due to their large capacity and strong adaptability to existing petrochemical equipment. After an overview of the reaction mechanisms of pyrolysis and catalytic cracking, this review aims to comprehensively discuss the influence of zeolite catalyst structure (acidity, pore structure, and morphology) on the catalytic activity, selectivity, and stability of polyolefin cracking, particularly emphasizing the importance for matching acidity and pore structure for target product formation. Subsequently, the structure–activity relationship between the metal site and zeolite’s acid site in polyolefin hydrocracking is also discussed. In the end, emerging opportunities and challenges are proposed to promote a more efficient way for polyolefin chemical upcycling.

Ce-Promoted PtSn-Based Catalyst for Hydrocracking of Polyolefin Plastic Waste into High Yield of Gasoline-Range Products

“…8 Numerous studies have delved into the hydrocracking mechanism, providing guiding principles for optimizing both catalysts and reaction conditions to enhance catalytic performance. 8,16,28,33,69 We believe that the metal−acid balance (MAB) is a crucial aspect, widely considered a fundamental determinant of reaction pathways and product distributions in bifunctional metal−acid site catalysis during hydrocracking. 70 Initially, reactants such as n-alkanes (n-C i H 2i+2 ) undergo dehydrogenation on metal sites, leading to a mixture of nalkenes (n-C i H 2i ).…”

Catalytic hydroconversion processes for upcycling plastic waste to fuels and chemicals

“…In recent years, over twenty excellent reviews have been published in this field. 3–24 These review papers are broad in scope and aim to provide a brief description of each conversion strategy, rather than provide a comprehensive and systematic understanding of each method, including the hydroconversion process. Most of these published reviews focused on different strategic approaches for conversion of plastics including thermal pyrolysis, catalytic pyrolysis, hydrothermal liquefaction, hydroconversion, oxidation, photocatalysis, eletrocatalysis, solvolysis, biological catalysis, microwave- and plasma-assisted catalysis.…”

“…Most of these published reviews focused on different strategic approaches for conversion of plastics including thermal pyrolysis, catalytic pyrolysis, hydrothermal liquefaction, hydroconversion, oxidation, photocatalysis, eletrocatalysis, solvolysis, biological catalysis, microwave- and plasma-assisted catalysis. 3–24 In comparison, this review is a more focused review of only hydroconversion over a broad overview of various technologies and it will be beneficial to the catalysis and sustainability field. For example, Borkar et al provided an excellent review on catalytic conversion of plastics and summarized several approaches, including hydroconversion, solvent treatments, and cross alkane metathesis.…”

Catalytic conversion of polyethylene into aromatics with Pt/ZSM-5: insights into reaction pathways and rate-controlling step regulation

“…14,15 To narrow the product distribution and lower the reaction temperature, catalytic pyrolysis has been used mainly with zeolites, such as ZSM-5, ZSM-11, Hb, HY, USY, REY, FCC, etc., where catalytic cracking mainly proceeds through carbocation intermediates. [16][17][18][19][20][21] It was proposed that PE is rst cracked by the Brønsted acid sites on the external surface of zeolites, and then, the resulting intermediates may enter the zeolite micropore for further reactions over its conned strong Brønsted acid sites to generate light alkenes/alkanes and aromatics. 22 A signicant amount of BTX production from catalytic pyrolysis oen requires 500 °C or higher temperatures.…”