Conversion of Polypropylene into Light Hydrocarbons and Aromatics by Metal Exchanged Zeolite Catalysts.

Abstract

Polyolefins can be converted into C2-C5 hydrocarbons and benzene-toluene-xylene (BTX) aromatics as high-demand petrochemical feedstocks via catalytic pyrolysis on acidic zeolites. Bro̷nsted and Lewis acid sites are responsible for cracking polyolefins into olefins and subsequent aromatic formation. In this study, we have subjected the parent HZSM-5 zeolite to postsynthetic partial metal exchange with Fe, Co, Ni, Cu, and Ce cations to perturb Bro̷nsted/Lewis acidity. We have investigated these metal-modified HZSM-5 on the catalytic pyrolysis of polypropylene (PP) in a micropyrolyzer connected to a two-dimensional gas chromatograph coupled to a time-of-flight mass spectrometer and flame ionization detector (Tandem Pyrolyzer-GC × GC-TOF-MS/FID setup). Whereas Fe-, Co-, Cu-, and Ce-exchanged zeolites (with 2.5, 2.3, 1.9, and 0.8 wt % metal, respectively) had comparable product yields with the parent zeolite, Ni-exchanged zeolites with Ni content of 0.5 to 2 wt % were associated with enhanced BTX formation (28-38 wt %) compared to that of the parent zeolite (22 wt %). Pyridine-FTIR indicated that the Bro̷nsted/Lewis acid ratio of the parent zeolite decreased upon metal ion exchange. According to Pyridine-TPD, the parent zeolite's medium-strength acid sites were redistributed into weak and strong acid sites in Ni-exchanged zeolites. The higher amount of carbon deposits on Ni-exchanged zeolites compared to the parent and other metal ion exchanged zeolites was attributed to the enhanced aromatization activity by the simultaneous decrease in the Bro̷nsted/Lewis acid ratio and emergence of strong acid sites.