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 with propane is investigated using Zn/HZSM-5 as the catalyst at 400 degrees C. The presence of methane has a pronounced effect on the product distribution in terms of improved formation of phenyl rings and substitution groups of aromatic species. Isotopic labeling and solid-state NMR spectroscopy studies reveal that the favored site for methane incorporation varies at different stages along the co-aromatization reaction, from the benzylic sites at the initial stage to the phenyl rings and then substitution groups as the reaction proceeds. SIMS spectroscopy of the involved reaction intermediates suggests the formation of C-4 species over the Zn sites that are bonding to the oxygen atoms of the zeolite framework, which is supported by theoretical calculations. The interaction between Zn species and the zeolite framework is also confirmed by XAS and changes in coordination environments are identified through XANES and solid-state NMR spectroscopy. The redistribution of Zn during the reaction is observed on the surface of the zeolite support. Scanning transmission X-ray microscopy images show that Zn is rich in the inner pores compared with the external surface of the catalyst particles in the pristine catalysts. XANES and XPS spectra demonstrate that the Zn concentration increases on the external surface after the reaction, a conclusion that is further supported by the theoretical study. The Zn redistribution might be among the reasons for methane participation pathway alternation during the co-aromatization, since the Zn species located within the inner pores and on the external surface might prefer to catalyze the methane incorporation to the phenyl rings and substitution groups of the formed aromatics, respectively.