Understanding Nitrogen-Induced Catalyst Deactivation in Plastic Waste Pyrolysis from Lab to Pilot Scale

To increase the recycling rates of plastic waste, thermochemical recycling processes have been investigated for decades as a complement to mechanical recycling. Previous studies at the KIT have shown that the catalytic pyrolysis of polyolefin-rich plastic waste using microporous zeolites presents a promising pathway, as it yields high amounts of C2 to C4 olefins and valuable by-products such as BETX.1 However, when heterogeneous plastic waste streams containing polyamides are pyrolyzed, the catalytic pyrolysis products begin to resemble those of non-catalytic pyrolysis, accompanied by a pronounced increase in coke formation. This behavior is attributed to catalyst deactivation. To develop effective strategies mitigating this deactivation, it is crucial to elucidate the underlying mechanisms.
In this study, these deactivation mechanisms were investigated through systematic and comparative catalytic pyrolysis experiments conducted on the milligram, gram, and kilogram scale. Low-density polyethylene (LDPE) was pyrolyzed in a gram scale batch reactor at 500 °C, either on its own or with the addition of 5 wt% polyamide-6 (PA-6) or ammonia as an inorganic basic nitrogen compound. ... mehrThe resulting pyrolysis vapors were subsequently catalytically cracked in a downstream fixed bed loaded with a microporous zeolite with a catalyst-to-plastic ratio of 0.08, and gaseous and liquid products were separated in a condenser. The gaseous products were identified and quantified using a gas chromatograph (GC) coupled with thermal conductivity (TCD) and flame ionization detector (FID), as well as a mass spectrometer (MS), while the liquid products were identified and quantified using proton nuclear magnetic resonance spectroscopy (1H-NMR) and a two-dimensional-GC coupled with a FID and MS.
The resulting mass balances and the detailed gas and liquid product compositions reveal distinct shifts in product distribution toward non-catalytic products associated with catalyst deactivation when 5 wt% PA-6 or ammonia was added.
Additionally, thermogravimetric analyses were carried out in which the PA-6 content in LDPE varied from 0 to 100 wt% along with the catalyst loading. A correlation between the PA-6 content and the coke deposited on the catalyst was observed. PA-6 also altered the decomposition behavior of LDPE, resulting in a three-step decomposition.
The combined findings from both the gram-scale experiments and the thermogravimetric analyses reveal that coke formation alone cannot account for the observed catalyst deactivation. The results further indicate that nitrogen-containing species, which are formed by the decomposition of PA-6 are adsorbed and retained on the catalyst surface, providing an additional deactivation pathway that significantly contributes to the deterioration of catalytic performance. The findings were validated using a screw pyrolysis pilot plant operating at the kilogram scale with an integrated downstream fixed-bed reactor, demonstrating the relevance of these deactivation pathways under practical process conditions.