Study of Coke Formation Mechanism on HZSM-5 Zeolite During Co-cracking of n-Hexane and Alcohols.

The deactivation mechanism of HZSM-5 zeolite in the co-cracking of n-hexane and alcohols was studied. The fresh and spent catalysts were characterized by X-ray diffraction (XRD), N2 adsorption-desorption isotherms, NH3-TPD, gas chromatography-mass spectrometry (GC-MS), high-resolution time-of-flight mass spectrometry (ESI-TOF), thermogravimetric analyzer (TGA), Fourier infrared spectroscopy (FT-IR), 13C MAS NMR, and H2-TPR. The results showed that the internal coke is preferentially formed in the zeolite micropores in the initial reaction stage, leading to the partial deactivation of catalyst. The addition of alcohols to n-hexane significantly promoted the formation of external coke, which hardly had effect on the cracking activity. H2 treatment to the spent catalyst at 800 °C could effectively remove the internal coke of the catalyst, while the external coke with a graphite-like structure still kept stable. Methylated aromatics were considered coke precursors. It was reasonably proposed that methylated aromatics condense to polyaromatic structures as internal coke in the presence of acid sites, but their growth is limited by the catalyst channel structure. This work demonstrated that zeolite deactivation behavior was affected by the location, amount, and type of coke. The formation mechanism of internal and external coke as well as the H2 decoking to generate deactivated catalyst were discussed. [ABSTRACT FROM AUTHOR]