Yuebing Xu

Experimental evidence for three rate-controlling regions of the non-oxidative methane dehydroaromatization over Mo/HZSM-5 catalyst at 1073 K

Mo/HZSM-5 catalyst offers high selectivity for the non-oxidative dehydroaromatization of methane to benzene. This strongly suggests the deep involvement of the zeolite channels in controlling of the overall reaction rate. Therefore, two 5 wt% Mo/HZSM-5 catalysts based on two zeolite samples of different average crystal sizes were tested for the methane dehydroaromatization reaction over a wide range of space velocities (from 3500 to 60 000 mL g−1 h−1) at 1073 K to examine the effect of superficial velocity on the benzene formation rates. Additionally, a recently developed on-line sampling and off-line analysis approach was employed to follow the maximum outlet benzene concentrations reached over very short time frames. Multiplying the obtained maximum outlet benzene concentrations by the corresponding inlet gas flow rates that were corrected for a temperature factor allowed approximate estimation of the maximum benzene formation rates. Consequently, two rate–space velocity curves were obtained to reveal that three rate-controlling regions exist for the title reaction: 40 000 mL g−1 h−1. Moreover, some specifically designed tests and detailed data analysis were performed to further reveal that these three rate-controlling regions correspond, respectively, to the external mass transfer, intracrystalline diffusion and kinetic desorption controlling steps of the reaction.

Comparison of the activities of binder-added and binder-free Mo/HZSM-5 catalysts in methane dehydroaromatization at 1073 K in periodic CH4-H2 switch operation mode

Three industry-supplied, well-shaped Mo/HZSM-5 catalysts, two binder-added and one binder-free, were tested for the first time in methane dehydroaromatization to benzene at 1073 K and 10000 mL/(gh) in periodic CH4-H2 switch operation mode, and their catalytic performances were compared with those of three self-prepared, binder-free powder Mo/HZSM-5 catalysts. XRD, 27Al NMR, SEM, BET and NH3-TPD characterizations of all the catalysts show that the zeolites in the two binder-added catalysts are comparable to those in the three binder-free powder catalysts in crystallinity, crystal size, micropore volume and Bronsted acidity. The test results, on the other hand, show that the catalytic performances of the two binder-added catalysts are worse than those of the four binder-free catalysts on both catalyst mass and zeolite mass bases. Then, TPO and BET measurements of all spent samples were conducted to get a deep insight into the negative effects of binder addition, and the results suggest that the binder additives functioned mainly to enhance the polyaromatization of formed aromatics to coke on their external surfaces and consequently lower the benzene formation activity and selectivity of the catalyst.

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

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.

Mechanism of Fe additive improving the activity stability of microzeolite-based Mo/HZSM-5 catalyst in non-oxidative methane dehydroaromatization at 1073 K under periodic CH4–H2 switching modes

The catalytic stabilities of Fe-modified and -unmodified 5% Mo/HZSM-5 catalysts in non-oxidative methane dehydroaromatization were compared at 1073 K and three reaction/H2-regeneration cycle periods: 5 min CH4–5 min H2, 5 min CH4–10 min H2, and 5 min CH4–20 min H2. Fe addition proved capable of remarkably increasing the catalyst stability over the cycles of 5 min CH4–20 min H2 but was hardly effective over the cycles of 5 min CH4–5 min H2. On the other hand, SEM observation of all spent samples revealed that Fe addition causes a massive accumulation of carbon nanotubes under the latter cyclic condition but little in the former. Thus several sets of comparative tests were specially designed and performed to gain an insight into the role of Fe-catalyzed cyclic formation of carbon nanotubes in stabilizing the activity under the cyclic condition of 5 min CH4–20 min H2. The results further confirmed that at this condition, cyclic formation of carbon nanotubes enhances cyclic evolution of H2 and increases the H2 concentration of the system, which is thermodynamically beneficial for suppression of formation of the activity-deactivating surface coke. Finally, it was further confirmed that at least a 20 min H2 exposure is required to remove most of the carbon nanotubes and surface coke formed during a 5 min CH4 exposure and reactivate most of the Fe nanoparticles and make them available again for formation of catalytic carbon nanotubes with an enhanced H2 evolution, i.e., with a controlled formation of the surface coke in the next CH4 exposure.