Elucidation of the versatile Brønsted acidity of nanosized ZSM-5 materials

Studying a series of nanosized and microsized H-ZSM-5 zeolites with nSi/nAl ratios of 31–88 revealed systematic differences in the crystallinity, morphology, and Brønsted acidity of these materials. While the aluminium atoms in all samples under study are tetrahedrally coordinated framework species, only 32–34% of these atoms in the nanosized materials contribute to the formation of Brønsted acidic bridging OH groups (Si(OH)Al), which are characteristic for common microsized H-ZSM-5 zeolites. The remaining framework aluminium species cause formation of silanol groups with weakly enhanced Brønsted acidity (Si(OH)⋯Al) comparable with those in amorphous aluminosilicates. These properties of nanosized ZSM-5 materials are explained by the assumption of hybrid particles consisting of internal crystalline ZSM-5 domains containing Si(OH)Al groups covered by X-ray-amorphous surface layers with Si(OH)⋯Al groups. Direct evidence for these surface layers is found with transmission electron microscopy, but the other characterisation methods are also in line with this explanation.