Pathways to Aromatics in the Catalytic Pyrolysis of a Polyvinylchloride Model Compound Revealed by Operando Photoelectron Photoion Coincidence Spectroscopy

Dechlorination channels and pathways to olefins and aromatics in the catalytic pyrolysis of the polyvinylchloride (PVC) model compound 1,3-dichlorobutane are revealed using operando photoelectron photoion coincidence (PEPICO) spectroscopy. Experimental and computational results agree that the primary pathway involves double dehydrochlorination producing 1,3-butadiene and HCl. Minor radical channels are evidenced by the detection of chloromethyl, methyl, and propargyl radicals in thermal decomposition, while chlorine radicals are absent. HZSM-5 zeolites lower the reaction temperature and facilitate 1,3-butadiene association reactions producing C₅-C₁₂ olefins. Further reaction steps, detected experimentally and in part isomer-selectively, mimic previously postulated cross-linking pathways to aromatics in PVC catalytic pyrolysis. This study identifies CC coupling as well as Diels-Alder dimerization of butadiene to yield polymethylated cyclopentadienes. These are central precursors to aromatics, for example, benzene, toluene, and xylenes (BTX). Ring expansion and contraction as well as transmethylation reactions are found to be dominant routes to aromatic products. The mechanisms during thermocatalytic conversion of PVC are applicable to other plastics and resemble the chemistry upon methanol- and methylchloride-to-hydrocarbon and aromatics conversion, which will inspire new strategies to enhance selectivity towards aromatics and mitigate coke formation.