Accessibility of Acid Sites in Hierarchical Zeolites: Quantitative IR Studies of Pivalonitrile Adsorption

Accessibility studies of acid sites in zeolites involving quantitative IR measurements with hindered pivalonitrile as probe molecule were performed. The extinction coefficients of the diagnostic bands of pivalonitrile interacting with Bronsted and Lewis acid sites were determined by using several zeolites structures of different concentrations and acid strength of sites. The accessibility factor (AF) was defined as the ratio of the concentrations of acid sites accessible to pivalonitrile and sites interacting with pyridine. The AF can be applied to both Bronsted and Lewis sites, whereas the methods proposed in previous works were limited only to Bronsted sites. Moreover, this method can also be applied to the accessibility studies of transition-metal cations being active sites in redox reactions. The AF suitability for accessibility studies was investigated for desilicated zeolites HZSM-5 of medium and high Si/Al ratio treated with NaOH and NaOH/tetrabutylammonium hydroxide. The correlation between the values of mesopore surfaces and AFs for both Bronsted and Lewis sites in highly siliceous zeolites was observed. In more extensively desilicated zeolites, all of the acid sites were accessible to pivalonitrile, that is, AF = 1. Thereby, we assume that pivalonitrile can react not only with sites on external surfaces and in micropore mouths, as generally accepted, but small amounts of pivalonitrile can also migrate inside micropores. In desilicated zeolites with the extended mesopore system smaller fragments of micropores between two mesopores are preserved, which results in shorter average length of micropores. It facilitates the migration of pivalonitrile, and the sites inside micropores became accessible to bulky molecule. The accessibility of Lewis acid sites in desilicated zeolites is even more enhanced than that of Bronsted sites. Taking into consideration the fact that in desilicated zeolites the majority of Lewis sites originate from dehydroxylation of the Si-OH-Al groups, previously formed by the reinsertion of Al extracted from zeolite during alkaline treatment, their high accessibility is reasonable. Those newly formed Lewis sites are situated on mesopore surfaces, which facilitates accessibility to bulky molecules.