Effect of superficial velocity on the coking behavior of a nanozeolite-based Mo/HZSM-5 catalyst in the non-oxidative CH4 dehydroaromatization at 1073 K

Abstract

The lifetime benzene and naphthalene productivity of a nanosized zeolite-based 5%Mo/HZSM-5 catalyst in the non-oxidative CH₄ dehydroaromatization has been investigated at 1073 K and three different space velocities (4500, 10 000 and 30 000 mL g⁻¹ h⁻¹, corresponding to the superficial velocities 1.3, 3.0, and 9.0 cm s⁻¹, respectively). The aim is to demonstrate that diffusion limitations do exist and have a strong influence on the coking behaviour and the lifetime aromatics productivity. The results showed that the catalyst deactivation rate increased with increasing CH₄ superficial velocity, leading to decreased productivity. On the other hand, TPO and BET measurements of the spent samples revealed that the amount of coke accumulated in the spent samples decreased with increasing velocity, whereas their microporosity varied essentially depending upon the coke amount. Putting these observations together thus leads to a fact that less coke forms at a higher velocity but results in a more rapid deactivation and reduced productivity. As this may suggest the occurrence of non-uniform coke formation across catalyst particles, physical powdering of all spent samples and re-evaluation of the activity of all powdered samples were conducted to confirm this. It was found that the benzene formation activities of all powdered samples exhibited considerable but different degrees of recovery, and the most rapidly deactivated sample provided the greatest recovery. Thus, it is reasonably concluded that the preferential coke formation in the near-surface outer layers of catalyst particles and/or of crystal agglomerates inside the particles does occur in the nanozeolite-based catalyst at the test superficial velocities, particularly at 30 000 mL g⁻¹ h⁻¹. The essential cause for this occurrence is discussed in detail in the article.