Hierarchical MFI Zeolite Catalysts: How to Determine Their Textural Properties?-A Comparative Study

It is important to elucidate the role of the surface areas and pore volumes of hierarchical zeolites to understand their behavior as catalysts. Micro-and mesopore surface areas and volumes of the hierarchical MFI (Mobil Five zeolites) were assessed following several methods: (<em>i</em>) N₂ adsorption at 77 K using classical and corrected <em>t</em>-plot analyses methods, (<em>ii</em>) pre-adsorption of <em>n</em>-nonane was followed by the study of the N₂ adsorption at 77 K, and (<em>iii</em>) non-local-density functional theory (NLDFT) analysis using either the N₂ adsorption method at 77 K or the Ar adsorption method at 87 K. In order to assess the viability of each method, a set of hierarchical MFI-type zeolites was prepared by different approaches: alkaline treatment (desilication), synthesis in clear solution (nanocrystals), synthesis in the presence of bifunctional organic surfactant (nanosheets), and micelle-templating assisted alkaline treatment. NLDFT methods could not be used to accurately determine the micro-and mesopore surface areas, as larger surface areas compared to those obtained using the BET equation were obtained. This overestimation is even more pronounced with Ar at 87 K. Results from the classical <em>t</em>-plot analysis performed under conditions of N₂ adsorption at 77 K for mechanical mixtures of MFI and MCM-41 revealed the underestimation of the micropore volumes and the overestimation of the mesopore surface areas. Corrections were provided for <em>t</em>-plot analysis. The results obtained using the corrected <em>t</em>-plot method were in good agreement with the results obtained using the NLDFT method in the presence of Ar at 87 K during the calculation of the micro-and mesopore volumes of hierarchical MFI-type zeolites. Micropore and mesopore surface areas calculated by the corrected <em>t</em>-plot method were in good agreement with those calculated using the <em>n</em>-nonane pre-adsorption method for the hierarchical MFI-type zeolites characterized by the presence of large zeolite domains. The NLDFT method, in the presence of Ar, can be used to assess the micro-and mesopore volumes of the hierarchical zeolites at 87 K. However, it cannot be used to determine the surface areas. The corrected <em>t</em>-plot method can be used to efficiently calculate both the volumes and surface areas.