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Elsevier Sadowska, K.; Góra-Marek, K.; Drozdek, M.; Kustrowski, P.; Datka, J.; Martínez-Triguero, J.; Rey Garcia, F. (2013). Desilication of highly siliceous zeolite ZSM-5 with NaOH and NaOH/tetrabutylamine hydroxide. ABSTRACTThe results of both chemical and XPS analysis pointed out that desilication of highly siliceous ZSM-5 of Si/Al = 164 was more effective in the surface zone than in the bulk, contrary to zeolite ZSM-5 of Si/Al = 31.6. According to the IR studies in parent zeolite the concentration of protonic sites was very close to the concentration of Al indicating that all Al atoms can form Si-OH-Al. The results of our quantitative IR studies strongly support the realumination thesis, i.e. some Al atoms extracted in basic solutions are subsequently reinserted forming new acidic hydroxyls. In desilicated zeolites all Al atoms were able to form protonic sites, however part of them dehydroxylated during the activation of zeolite producing Lewis acid sites according to the stoichiometry: one protonic site was transformed into one Lewis site. Low temperature nitrogen adsorption revealed that the treatment of highly siliceous zeolite with 0.2 M NaOH/TBAOH mixture produced mesopores of smaller diameter and narrower pore size distribution than in the case of zeolite of medium Si/Al ratio. This result can be explained by low concentration of Al which similarly as TBA + cations plays the role of pore directing agents (PDA). Contrary to TBA + , the presence of Al in desilication mixture, led to the formation of larger pores. Therefore, in highly siliceous zeolite TBA + played dominant role as PDA producing narrower pores. Highly siliceous zeolite with uniform distribution of relatively narrow pores may be useful catalyst or catalyst support. The influence of desilication temperature on porosity development was also investigated. The increase of desilication temperature from 338 K to 353 K resulted in both more extensive demetalation (more Si and Al is extracted) and the distinct increase of the volume and surface of mesopores. Both lower concentration of protonic sites and higher concentration of Lewis 2 sites confirmed partial zeolite destruction upon desilication at elevated temperature. The experiments of pivalonitrile sorption followed by IR spectroscopy showed a significant increase of accessibility of acid sites to bulky pivalonitrile molecules.

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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.

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