The effect of zeolite particle size on the activity of Mo/HZSM-5 in non-oxidative methane dehydroaromatization

Abstract

The effect of the particle size of HZSM-5 on the activity of Mo/HZSM-5 in the non-oxidative CH 4 dehydroaromatization was studied. For the purpose five 5 wt%Mo/HZSM-5 catalysts based on one highly crystalline parent HZSM-5 of an average particle size of 4 μm and four ball-milled samples with their average particle sizes being 1.9, 1.0, 0.5 and 0.2 μm, respectively, were prepared using conventional impregnation method, characterized using XRD, BET, 27Al MAS NMR, NH 3-TPD, H 2-TPD, TPO and TG techniques, and tested at 1073 K and a wide range of space velocities from 5000 to 20,000 ml/h/g. XRD and NH 3-TPD measurements revealed that both crystallinity and Brønsted acidity of the ball-milled sample became lower and lower as the average particle size decreased in the range of <1.9 μm. For this reason the catalysts based on the milled samples with the average particle sizes of 1.0, 0.5 and 0.2 μm, respectively, all showed lower Mo dispersions and fewer free acid sites than the parent zeolite-based catalyst, predicting their lower activities in CH 4 dehydroaromatization. Activity evaluation tests, however, demonstrated that the maximum benzene formation activities of the catalysts based on the milled samples having the average particle sizes of 1.0 and 0.5 μm, respectively, were higher than that observed over the parent zeolite-based catalyst at 1073 K and 10,000 ml/h/g, and the differences in the maximum benzene formation activity between the former two catalysts and latter one enlarged with increasing space velocity to 20,000 ml/h/g. Simultaneously, NH 3-TPD measurements also revealed that little reduction in population and strength of Brønsted acid sites occurred for the zeolite sample softly milled and having the relatively large size of 1.9 μm. Consequently, the catalyst prepared with this sample came to be fully comparable to that based on the parent zeolite in both Brønsted acidity and Mo dispersion. While this certainly suggests comparability of the activities of these two catalysts, activity evaluation tests showed again that there existed large differences between their activities, and the maximum benzene formation activity of the former catalyst based on the milled zeolite sample always kept higher than that of the latter based on the parent zeolite. All these indicate that use of the zeolites of small particle sizes with less resistance towards diffusion of formed benzene out of their short channels is certainly effective in making Mo/HZSM-5 catalyst to show its intrinsic maximum benzene formation activity. Hence, detailed discussions were made on an assumption that channel diffusion limitation existed at the tested conditions and all observations were reasonably explained.