Creating Accessible Active Sites in Hierarchical MFI Zeolites for Low-Temperature Acid Catalysis

A versatile desilication design strategy for the creation of hierarchical H–ZSM-5 zeolite catalysts with different Si/Al ratios has been demonstrated. The nature, strength, and the accessibility of the acid sites after the alkaline treatment was elucidated by employing a range of physico-chemical characterization tools; notably probe-based FTIR spectroscopy along with Solid State (SS) MAS NMR spectroscopy. In addition, structural and textural properties of the hierarchical zeolites were also explored and compared to their corresponding microporous analogues. CO was used to probe the acidic properties of the hierarchical zeolites with the concomitant deployment of a bulky molecular probe, 2,4,6-trimethylpyridine (collidine), which is too large to access the micropores, to specifically investigate the enhanced accessibility of the active sites. The hierarchical zeolites were evaluated in the industrially relevant, acid-catalyzed Beckmann rearrangement of cyclohexanoneoxime to ϵ-caprolactam, the precursor for Nylon-6, in liquid phase and at low temperatures. The catalytic findings with the hierarchical catalysts reveal a significant enhancement in the production of ϵ-caprolactam, compared with the parent microporous H–ZSM-5 zeolite, thereby highlighting the merits of our design approach in facilitating enhanced diffusion and mass transfer.