Aluminum Distribution on the Microporous and Hierarchical ZSM-5 Intracrystalline and Its Impact on the Catalytic Performance.

Abstract

In our previous work, the control of aluminum distribution on microporous ZSM-5 with and without the addition of sodium (Na) was conducted. In the current research, adjustment of the aluminum distribution on hierarchical ZSM-5 synthesized using a surfactant as a mesoporogen has been carried out. The investigation of aluminum distribution was based on ²⁷Al MAS NMR, constraint index (CI) value, and Co(II) ion-adsorbed UV-vis. The aforementioned characterizations revealed that the hierarchical ZSM-5 with Na exhibited a more concentrated aluminum distribution in the channel intersections than the hierarchical ZSM-5 without Na did. The opposite trend was observed with microporous ZSM-5. Furthermore, the influence of the hydrothermal synthesis time on the formation of the hierarchical structure and the arrangement of aluminum within the framework was also investigated. The prolongation of the hydrothermal synthesis time to 144 h was found to be an optimal period for the formation of a well-hierarchical structure, as demonstrated by the observed increase in the hierarchy factor. Moreover, this process resulted in an increase in the strength of the acid sites and a change in the crystal morphology of the hierarchical ZSM-5 from a coffin-like morphology to a coral reef-like or a flower-like morphology. Additionally, the influence of the alteration in the aluminum distribution on the catalytic performance was also investigated. In the case of the n-hexane cracking and methanol conversion reactions, hierarchical ZSM-5 with Na was observed to produce bulkier molecules (≥C5s) than that without Na. On the other hand, it was observed that the hierarchically structured ZSM-5 exhibited enhanced performance in the production of lower olefins, particularly propene, in comparison to the microporous ZSM-5.