(500e) Co-Aromatization of Methane with Propane over Zn/HZSM-5: The Methane Reaction Pathway and the Effect of Zn Distribution

The co-aromatization of methane and propane over the Zn/HZSM-5 catalyst under a condition similar to the desired reaction condition for industrial applications is investigated. The methane participation demonstrates different reaction pathways and incorporation sites at different stages of the reaction, which might be related to the Zn catalytic site redistribution on the catalyst surface as well as the product molecules including benzene and toluene. Initially, methane favors the benzylic sites of the product molecules. Then methane mainly contributes to the phenyl ring formation as the reaction continues. As the reaction proceeds further, the methane incorporation site in the co-aromatization product molecules is on the substitution groups of the formed aromatics. These phenomena are witnessed by GC-MS analysis of the products, and confirmed by a ¹³C isotope labeling study and SSNMR spectroscopy. The reaction intermediates formed on the catalyst surface are studied by DRIFT, SIMS and theoretical calculations. The reaction between methane and propane in the co-aromatization reaction over Zn/HZSM-5 might be closely related to the interaction between methane and the unsaturated species upon propane activation, particularly the reaction between the C₁ and C₃ pieces produced from methane and propane activation. Zn sites bonding to the zeolite framework are critical in the co-aromatization process by stabilizing the intermediates. XAS, XPS, XANES, and ⁶⁷Zn SSNMR spectroscopies provide further support to the SIMS results on the catalytic zeolite and the importance of Zn. Theoretical calculations predict a lower energy for Zn in the inner pores compared with the external surface in the pristine catalyst, resulting in a higher Zn concentration in the inner pores, which is confirmed by STXM images. The XANES spectra of the pristine catalyst and spent catalysts after the co-aromatization reaction reveal that the Zn concentration on the external surface is increased after the reaction. This phenomenon is further confirmed by EFTEM and XPS analyses, indicating that the Zn species is transferred to the external surface in the reaction and exhibits an interaction with the oxygen sites exposed on the external surface of the zeolite framework, where the space on the external surface is sufficient to hold intermediates and product molecules with large molecular sizes. The migration of Zn is attributed to the decreased energy gap between inner pore Zn sites and those on the external surface. The methane participation pathway is consequently altered to the incorporation to alkyl substitution groups of the formed aromatics at the later stage of the reaction.