Transport limitations in polyolefin cracking at the single catalyst particle level

Catalytic cracking is a promising approach to chemically recycle polyolefins by converting them into smaller hydrocarbons like naphtha, and important precursors of various platform chemicals, such as aromatics. Cracking catalysts, commonly used in the modern refinery and petrochemical industry, are...

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Vydané v:Chemical science (Cambridge) Ročník 14; číslo 37; s. 168 - 18
Hlavní autori: Rejman, Sebastian, Vollmer, Ina, Werny, Maximilian J, Vogt, Eelco T. C, Meirer, Florian, Weckhuysen, Bert M
Médium: Journal Article
Jazyk:English
Vydavateľské údaje: Cambridge Royal Society of Chemistry 27.09.2023
The Royal Society of Chemistry
Predmet:
Addition polymerization
Catalysts
Catalytic converters
Catalytic cracking
Chemistry
Contact melting
Covalent bonds
Fluid catalytic cracking
Macromolecules
Mass transport
Microscopy
Naphtha
Optical microscopy
Polymer matrix composites
Polymers
Polyolefins
Refineries
Viscosity
ISSN:2041-6520, 2041-6539
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Shrnutí:Catalytic cracking is a promising approach to chemically recycle polyolefins by converting them into smaller hydrocarbons like naphtha, and important precursors of various platform chemicals, such as aromatics. Cracking catalysts, commonly used in the modern refinery and petrochemical industry, are tailored to process gaseous or liquid feedstock. Polyolefins, however, are very large macromolecules that form highly viscous melts at the temperatures required to break their backbone C-C bonds. Therefore, mass transport is expected to limit the performance of traditional cracking catalysts when applied to the conversion of polymers. In this work, we study these effects during the cracking of polypropylene (PP) over catalysts utilized in the fluid catalytic cracking (FCC) process. Thermogravimetric experiments using PP of varying molecular weight ( M w ) and catalysts of varying accessibility showed that low M w model polymers can be cracked below 275 °C, while PP of higher M w required a 150 °C higher temperature. We propose that this difference is linked to different degrees of mass transport limitations and investigated this at length scales ranging from milli- to nanometers, utilizing in situ optical microscopy and electron microscopy to inspect cut open catalyst-polymer composites. We identified the main cause of transport limitations as the significantly higher melt viscosity of high M w polymers, which prohibits efficient catalyst-polymer contact. Additionally, the high M w polymer does not enter the inner pore system of the catalyst particles, severely limiting utilization of the active sites located there. Our results demonstrate that utilizing low M w polymers can lead to a significant overestimation of catalyst activity, and suggest that polyolefins might need to undergo a viscosity reducing pre-treatment in order to be cracked efficiently. Catalytic cracking could enable low temperature conversion of hard-to recycle polyolefin plastics. However, traditional cracking catalysts suffer from macro and microscopic mass transport limitations, which call for plastic pre-treatment.
Bibliografia:optical microscopy. Additional data and experimental details. See DOI
Electronic supplementary information (ESI) available: Videos of
https://doi.org/10.1039/d3sc03229a
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ISSN:2041-6520
2041-6539
DOI:10.1039/d3sc03229a