(488c) Surface and Pore Structure Assessment of Hierarchical Zeolites by Advanced Water and Argon Sorption Studies

Alkaline treatment is a widely-applied post synthesis method for the preparation of hierarchical zeolites. The selective dissolution of framework Si leads to the creation of intra-crystalline mesopores, which have proven to be highly beneficial in improving the zeolite utilization efficiency for catalytic applications. Due to its lower solubility, the framework Al remains undissolved in basic solution, leading to a decrease in the Si/Al ratio of the mesoporous zeolites and possible modification of the surface properties. Desilicated zeolites are known to have increased Lewis acidity, which may be largely removed by subsequent acid washing, which is thought to restore the acidic properties of the parent zeolite. Characterization  methods which permit surface sensitivity will be advantageous in understanding the compositional alterations occurring in zeolites upon alkaline treatment.

 Our data reveal that the assessment of the pore and surface properties of such hierarchical zeolites is enhanced by the application of H₂O adsorption in combination with Ar sorption. Argon adsorption at 87 K provides an accurate understanding of the pore structure, being insensitive to the surface chemistry. Complimentary information with respect to the surface properties (e.g. hydrophobicity/hydrophilicity) is gained from the H₂O adsorption isotherms (at 298 K), which are found to be sensitive both to the Si/Al framework ratio and Al distribution of the zeolite. Our findings are not only important for a comprehensive surface and pore structural characterization of hierarchical zeolites in particular with regard to optimizing their application in catalysis, but also in general for further advancing the characterization of nanoporous materials.