On the Determining Role of the External Surface Area of Zeolite in Maximizing the Activity and Selectivity of Mo/HZSM-5 Catalyst in the Nonoxidative Methane Dehydro-Aromatization at 1073 K

Three series of 2-8 % Mo/HZSM-5 catalysts were prepared using zeolites with different crystal sizes and Si/Al ratios. These catalysts were characterized by XRD, SEM, BET, and NH3-TPD techniques and tested for methane dehydro-aromatization at 1073 K. Characterization results indicated that, with an identical Mo loading, two nanozeolites retain more Mo species on their external surface and fewer Mo species within their channels than the microsized zeolite. Catalytic performance tests showed that the optimal Mo loadings for the two nanozeolitebased catalysts differ. While one loading is identical to that for the microzeolite-based catalyst, the other is two percentage points higher. However, at their respective optimal loadings, the nanozeolite-based catalysts exhibited lower benzene formation activities and selectivities than the microzeolite-based catalyst. TG measurements of spent catalysts and catalytic pyrolysis of benzene over the three Mo/HZSM-5 catalysts revealed that the nanozeolite-based catalysts exhibit a higher activity for pyrolysis of benzene to external coke than that of the microzeolite-based one. These findings suggest that the zeolite external surface area significantly influences Mo distribution, optimal Mo loading, external coke capacity, and the aromatic selectivity and catalytic stability of the catalysts.