Issue 37, 2023

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 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 (Mw) and catalysts of varying accessibility showed that low Mw model polymers can be cracked below 275 °C, while PP of higher Mw 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 Mw polymers, which prohibits efficient catalyst–polymer contact. Additionally, the high Mw 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 Mw 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.

Graphical abstract: Transport limitations in polyolefin cracking at the single catalyst particle level

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Article information

Article type
Edge Article
24 Jun 2023
14 Aug 2023
First published
16 Aug 2023
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY license

Chem. Sci., 2023,14, 10068-10080

Transport limitations in polyolefin cracking at the single catalyst particle level

S. Rejman, I. Vollmer, M. J. Werny, E. T. C. Vogt, F. Meirer and B. M. Weckhuysen, Chem. Sci., 2023, 14, 10068 DOI: 10.1039/D3SC03229A

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