Si–OH Nest and Al Distribution of Silicalite-1 Core/Al-ZSM-5 Shell Zeolites for Methane Dehydroaromatization Reaction

Abstract

Silicalite-1 core/Al-ZSM-5 shell zeolite crystals were prepared in various sizes for use as catalysts in methane dehydroaromatization (MDA), and the growth kinetics and corresponding physicochemical properties of this core–shell zeolite were investigated. Al-ZSM-5 was grown on silicalite-1 seeds at various Si/Al ratios. Core–shell catalysts of all size variations exhibited similar deactivation trends in the MDA reaction, with minor changes in aromatic yields despite clear differences in reaction channel lengths and acid-site properties. This outcome was shown to originate from the unique growth kinetics of the Al-ZSM-5 layer on silicalite-1 seeds, in which the Al species in the sol used in the synthesis were consumed quickly during the early aggregative growth period. This led to an interesting spatial distribution of Al in the Al-ZSM-5 layer, in that the inner layer was relatively Al-rich. This distribution is advantageous because it can inhibit coke deactivation, which often occurs at the catalyst surface during MDA. However, a substantial quantity of Si–OH nests, which inhibit the effective loading of Mo species at the acid sites of the crystals, were detected in the microstructural analysis of large crystals. Therefore, this study shows that silicalite-1 core/Al-ZSM-5 shell zeolites can be prepared for use as coke-resistant catalysts for the MDA reaction. Further work is required, however, to design a synthesis method which reduces the number of Si–OH nests formed.

Coke relocation and Mo immobilization in donut-shaped Mo/HZSM-5 catalysts for methane dehydroaromatization

“…Different ways of catalyst improvement have been reported, such as Mo/Al balance tuning, 7–9 variation of the molybdenum loading approach, 10,11 pretreatment, 12–14 metallic promoter addition, 15–17 or crystal engineering. 18–21 The latter represents an extensive research topic for hydrocarbon processing catalyzed by acidic-microporous materials since the diffusional limits in zeolites lead to coke accumulation and mitigate the efficiency of the crystal core. 22 In this sense, the decrease of the diffusion pathway in MDA catalysts has been studied through hierarchization and crystal nanosizing.…”

“…A selective dissolution of the silicon-rich part of the silicalite-1-Core/HZSM-5-Shell material or HZSM-5 zeolites synthesized by the seed-assisted method through post-treatments has been reported. 20,27 Core/shell materials made with silicalite-1 and HZSM-5 allow tuning of catalyst reactivity. The diffusion pathway is not shortened, but the absence of Brønsted acid sites in the silicalite-1 part reduced the probability of further transforming the products into heavier compounds.…”

Size controllable synthesis of ZSM-5 zeolite and its catalytic performance in the reaction of methanol conversion to aromatics

“…The template and seed play a synergistic role in the ZSM-5 crystallization. Lee and co-workers [ 33 ] prepared a series of ZSM-5 nanocrystals using this method and found the size of ZSM-5 nanocrystals was influenced by the amount of seeds, crystallization time and Si/Al ratio. Tang and co-workers [ 29 , 34 , 35 ] used silicalite-1 as seed to prepare the ZSM-5 zeolites with the controllable size of 270–1100 nm without adding the templates.…”