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

Abstract

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 Brønsted 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 Brønsted and Lewis sites, whereas the methods proposed in previous works were limited only to Brønsted 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 Brønsted 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 Brønsted 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.

Probing zeolites by vibrational spectroscopies

“…Molecular sieving can influence reactants and probe molecules. 90,91,97,[629][630][631][632][633][634]636,637,641,653 This section is divided into five subsections. This question is also valid for all sites, and not only for those that are catalytically active.…”

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

“…FTIR spectroscopy of adsorbed probe molecules is a powerful tool for systematic studies of acidity of hierarchical zeolites providing information on the influence of desilication conditions on location, content, acid strength and accessibility of both Brønsted and Lewis sites. [21][22][23] In this study, CO was used to assess the acidic properties of the hierarchical zeolites and a larger basic molecule such as 2,4,6-trimethylpyridine (collidine), which cannot assess the micropores, was used to get information on the enhanced accessibility to the active sites. The hierarchical H-ZSM-5 zeolites were evaluated in the industrially relevant acid-catalyzed Beckmann rearrangement of cyclohexanone oxime to -caprolactam in the liquid-phase and at low temperatures, specifically to avoid the formation of large molecular weight condensation products and other by-products that are generated in the conventional gas-phase process.…”

Selective production of propylene from methanol over high-silica mesoporous ZSM-5 zeolites treated with NaOH and NaOH/tetrapropylammonium hydroxide

“…Nevertheless, the application of desilication by an aqueous NaOH solution is limited to zeolites with a Si/Al ratio in the range of 25-50 [17][18][19][20][21][22][23][24][25][26][27][28]. Recently, the desilication of high-silica ZSM-5 and silicalite-1 has attracted much attention.…”