Full Compositional Flexibility in the Preparation of Mesoporous MFI Zeolites by Desilication

Abstract

We demonstrate that desilication in alkaline medium is a suitable post-synthetic method to introduce intracrystalline mesoporosity in MFI zeolites independent of the Si/Al ratio in the parent material. By systematic screening of the influence of both base concentration (0.1–1.8 M NaOH) and Si/Al ratio (10–1000) on the properties of the treated zeolites, we reveal that effective mesoporosity introduction (>200 m 2 g –1 ) may be achieved in the Si/Al range of 12–200. The use of descriptors like the “indexed hierarchy factor” and the “desilication efficiency” enable the rational categorization of the solids obtained. The highest desilication efficiencies, estimated by correlating the introduced mesoporosity with the yields after NaOH treatment, are obtained in the previously established Si/Al range of 25–50. We identify the crucial role of a subsequent acid treatment for removing amorphous Al-rich debris from alkaline-treated samples in the case of low Si/Al ratios (<20). The latter acid wash uncovers the complete micro- and mesopore network, enabling full compositional flexibility of desilication. The removal of these debris concomitantly enabled restoration of both the acidity and the chemical composition of the hierarchical zeolite to that of the starting (purely microporous) zeolite. Catalytic evaluation of selected Al-rich zeolites in the alkylation of toluene with benzyl alcohol, confirmed the superiority of the mesoporous alkaline-treated samples with respect to the parent material. Hierarchical ZSM-5 after acid washing stands as the most active sample, which stresses the relevance of the additional post-synthetic treatment step.