A Method for the Selective Quantification of Brønsted Acid Sites on External Surfaces and in Mesopores of Hierarchical Zeolites

Herein, we describe a method for the quantification of Brønsted acid sites located on surfaces and in pores of hierarchical zeolite catalysts. The probe triphenylphosphine (TPP) accesses only pores bigger 0.72 nm. The signal of protonated TPP is baseline separated from other signals and can be directly quantified by 31P MAS NMR spectroscopy. Results are robust and are not affected by the total TPP loading nor by remaining solvent traces. The error of the Brønsted acid site density evaluation is below ±10% for amorphous silica–alumina and below ±5% for probing crystalline materials like MCM-22 or hierarchical zeolites. On amorphous silica–alumina, only 12.5% of all acid sites were accessible by TPP, which binds near tetrahedral and pentahedral aluminum. The 47 ± 2 μmol/g acid sites on the surface and in pore mouths of zeolite MCM-22 represent 12% of the total acidity. On TNU-9, 2% of the total acidity is located on the surface. On commercial zeolite ZSM-5, no surface acidity was found. Desilication of ZSM-5 and TNU-9 zeolites introduced an additional 20 ± 1 and 29 ± 1 μmol/g of Brønsted acid sites, respectively. These additional acid sites are located in introduced mesopores of hierarchical ZSM-5 and TNU-9 zeolites and account for 6–7% of the total sites present. The location in mesopores can cause undesired byproducts in catalysis due to the absence of shape selectivity effects. The techniques described herein will aid the understanding of the acid site density in hierarchical systems and lead to improvements of catalyst synthesis and performance.