Acidic and catalytic properties of hierarchical zeolites and hybrid ordered mesoporous materials assembled from MFI protozeolitic units

Abstract

Two types of MFI zeolitic materials with improved accessibility, hierarchical zeolites and hybrid ordered mesoporous materials are here compared in terms of textural, acidic and catalytic properties. In both cases, protozeolitic units were employed as starting building blocks, the growth of which was perturbed by a silanization agent (hierarchical ZSM-5) or controlled by assembling around surfactant micelles (hybrid ordered mesoporous material, HZM). Likewise, two standard samples (nanocrystalline ZSM-5 and Al-MCM-41) were used as reference. Nitrogen adsorption measurements confirmed the presence of a secondary porosity and a high share of non-microporous surface area in hierarchical ZSM-5, whereas no significant amount of micropores was detected in the hybrid HZM sample. However, 27Al MAS NMR measurements indicated that zeolitic fragments are present within the walls of the latter sample. The acidic properties of the materials under study were extensively investigated by adsorption–desorption of different base probe molecules. Specifically, FTIR spectra taken after pyridine adsorption show the presence of a high population of strong Lewis acid sites in both hierarchical ZSM-5 and HZM samples. This fact, along with a high accessibility to the active sites, is proposed to be mainly responsible for the enhanced catalytic activity exhibited by these materials in 1,2-epoxyoctane rearrangement and anisole acylation. The results obtained clearly indicate the strong and positive influence of the textural and acidic features of the hierarchical materials on their catalytic properties.