A review of filamentous carbon nanomaterial synthesis via catalytic conversion of waste plastic pyrolysis products

Waste plastics pose challenging issues to the environment and ecosystems due to their non-biodegradable nature. Aside from mechanical and thermal recycling, thermochemical recycling can convert waste plastics to high-value products. Pyrolysis and gasification processes can convert waste plastics to liquid and gaseous streams that can be used for the synthesis of various products (the waste-refinery concept). Moreover, these same streams can be used to produce filamentous carbon nanomaterials (CNM), that is, carbon nanotubes (CNT) and carbon nanofilaments (CNF). Despite the significant progress made in the field of filamentous CNM synthesis, a full understanding of all aspects of the growth mechanism is still unavailable and, thus, there is no obvious structural control. Consequently, the first section of this article is dedicated to a general summary of CNM, including the current understanding of their growth mechanisms, the difference between the various types, and the conditions which control their growth. The second section aims to synthesize the data published in the scientific literature over the past decade on the production of filamentous CNM from the pyrolysis of waste plastics. The different reactor designs used to convert waste plastics to CNM are considered, and the effects of different parameters, including the catalyst used and process parameters, on the yield and quality of CNM from various types of waste plastics are discussed. Finally, applications for CNM are reviewed.