Determination of the Exact Microporous Volume and BET Surface Area in Hierarchical ZSM-5

The determination of the exact microporous volume in hierarchical zeolites is of crucial importance as it allows for an accurate description of their textural properties. Standard physisorption procedures based on recording nitrogen and argon adsorption isotherms prove for major limitations for the determination of the amount of micropores and of the BET surface area in such hierarchical systems. We present two methodologies for the precise determination of the microporous volume and BET surface area based on (i) the volume difference in nitrogen physisorption isotherms at 77 K by n-nonane preadsorption and (ii) the interpretation of n-hexane adsorption isotherms measured at 298 K. As far as the n-nonane preadsorption technique is concerned, it allows to decouple the adsorption in the micropores and that on the external (and mesopore) surface, hence allowing to determine the micropore volume, monolayer capacity, and amount of occulted mesopores. In n-hexane physisorption, due to the strong confinement of the sorbate in micropores and the less favored molecule surface interaction of n-hexane, the micropore filling is independently observed at low relative pressures. The adsorption on the external surface, which is at the origin of mesopore filling, occurs only once micropores are completely filled up with the adsorbate. A set of mechanical mixtures of purely microporous ZSM-5 and mesoporous MCM-41 has allowed to assess the accurate of microporous volume by these strategies. While the n-nonane preadsorption strategy is time-consuming, the measurement of n-hexane isotherms at 298 K is easy and could hence be implemented as a standard technique for the accurate determination of textural properties of complex, hierarchical systems.