Quantification of enhanced acid site accessibility in hierarchical zeolites – The accessibility index

An accessibility index (ACI) was derived from infrared spectroscopy of substituted alkylpyridines with different size (pyridine: 0.57 nm, 2,6-lutidine: 0.67 nm, and 2,4,6-collidine: 0.74 nm) over hierarchical porous ZSM-5 crystals. The samples were prepared by selective silicon extraction of a commercial sample in NaOH (desilication), and they contained different degrees of intracrystalline mesoporosity. Our results demonstrate the enhanced accessibility of Brønsted acid sites in the hierarchical zeolites. For example, a relatively bulky molecule such as collidine, which probes practically no acid site of the parent medium-pore MFI structure, can access up to 40% of the acid sites in the mesoporous sample. The ACI is a powerful tool to standardize acid site accessibility in zeolites, and can be used to rank the effectiveness of synthetic strategies to prepare hierarchical zeolites.

The accessibility index (ACI) derived from infrared spectroscopy of substituted alkylpyridines with different size is a powerful tool to quantify the enhanced accessibility of acid sites in hierarchical porous zeolites.Figure optionsDownload high-quality image (79 K)Download as PowerPoint slide