Thermal and catalytic conversion of used tyre rubber and its polymeric constituents using Py-GC/MS

This paper investigates the effect of various acid catalysts in the conversion of tyre rubber into hydrocarbon products. A used tyre rubber and three pure elastomers (cis-1,4-polyisoprene, polystyrene-co-butadiene and cis-polybutadiene) were characterised for their chemical composition and thermal behaviour in order to determine the main constituent of the waste material. Five acid solids of comparable aluminium contents but varying textural and/or acid characteristics were used as catalysts, including three zeolites (standard ZSM-5, nanocrystalline n-ZSM-5 and beta) and two mesostructured materials (Al-MCM-41 and Al-SBA-15). The presence of catalysts did not affect the degradation temperature of waste tyre rubber, although significant differences were observed in the nature of the resulting hydrocarbons. All zeolitic materials showed a marked selectivity towards the formation of aromatic species, particularly toluene (up to 19.82%), m/p-xylene (up to 16.91%) and benzene (up to 10.28%). This behaviour was slightly more marked in zeolites with stronger acid properties and reduced steric impediments, like nanocrystalline n-ZSM-5. Mesostructured catalysts (Al-MCM-41 and Al-SBA-15) not only exhibited strong aromatisation properties but also a marked benzene alkylation capacity, which resulted in the formation of a wider range of alkylaromatic products. This behaviour was associated with their weaker Lewis type acid character and also to their larger pore sizes, which posed no impediment to the formation of larger molecular size products.