Polypropylene Waste Valorization via Pyrolysis: Assessing the Interchangeability of Low-Cost Montmorillonite Clay Catalysts

Numerous efficient catalysts have been developed for plastic pyrolysis; however, their relatively high cost limits their practical application in waste valorization. Natural clays are among the most promising low-cost catalytic materials because of their abundance and availability. Their wider application is limited by concerns regarding the reproducibility of catalytic performance. The current study was designed to answer the question of whether montmorillonite-rich clays from different deposits are interchangeable in polypropylene pyrolysis. Pyrolysis of polypropylene-based plastic was carried out in a flow fixed-bed reactor in inert atmosphere. Four samples of natural montmorillonite-containing clays from deposits in Russia and Kazakhstan with similar chemical composition were used as catalysts. Liquid products of the pyrolysis were analyzed using one-dimensional (1H, 13C) and two-dimensional (COSY and HSQC) NMR spectroscopy to assess the similarity of their molecular compositions. NMR analysis revealed that all pyrolysis oils consisted mainly of aliphatic hydrocarbons, accompanied by olefins and only minor aromatic fractions. The relative proportions of the major groups of compounds as well as research octane numbers were highly similar for all clay catalysts. The results indicate that the use of montmorillonite-containing clays with comparable mineralogical and chemical characteristics can ensure reproducible product composition, providing flexibility in catalyst selection for polypropylene pyrolysis.