Effect of Al Distribution in MFI Framework Channels on the Catalytic Performance of Ethane and Ethylene Aromatization

The relationship between the formation of aromatics in catalytic ethane/ethylene aromatization and Al distribution in Mobil-type five (MFI) channel was investigated using ZSM-5 zeolites with controlled Al distribution, which were obtained by employing different organic structural directing agents (OSDAs) during the ZSM-5 synthesis step. ZSM-5 samples with framework Al mainly distributed in the MFI straight/sinusoidal channels or intersections were obtained using pentaerythritol or tetrapropylammonium hydroxide as OSDAs, respectively. Samples with Al distributed in both channel and intersection were synthesized using n-butylamine as a template. Ethane aromatization results showed that Pt-modified ZSM-5 with acid sites located in the channel intersection tended to produce more aromatic compounds and less methane than that of a catalyst with acid sites located in the straight and/or sinusoidal channels under the same conversion of ethane. Ethylene aromatization was performed to understand how the intermediates evolve within acid sites. The results indicated that acid sites located in the intersection with more space void were conducive for the geometry transformation of intermediates, resulting in a much faster hydrogen transfer reaction rate than that in the straight and/or sinusoidal channels with bulky bimolecular reaction intermediates involved. Hence, ZSM-5 with acid site mainly located in the channel intersection would be more favorable for the formation of aromatics over the other two types of catalysts in ethylene aromatization.