Desilication of highly siliceous zeolite ZSM-5 with NaOH and NaOH/tetrabutylamine hydroxide

Abstract

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.

Combined desilication and phosphorus modification for high-silica ZSM-5 zeolite with related study of hydrocarbon cracking performance

“…Typically1 g Z5 was uniformly dispersed into the pre-heated alkaline solution containing both NaOH and tetraethylammonium hydroxide (TEAOH) with a molar ratio of TEAOH to NaOH equal to 0.25 [28]. After stirring for 30 min, the mixture was cooled down rapidly in ice-water bath.…”

A new method to construct hierarchical ZSM-5 zeolites with excellent catalytic activity

“…Different treatments conditions had distinct effects on crystallinity of zeolites [23,24], so we investigated every possible factor which could have influence on ZSM-5 zeolites. The percent crystallinity was calculated using the total crystal-line peak area and the total area of the diffraction pattern between 22.5°and 25°using a straight line background.…”

Catalytic cracking performance of alkaline-treated zeolite Beta in the terms of acid sites properties and their accessibility

“…Alkaline leaching, performed in aqueous NaOH and tetraalkylammonium hydroxides mixtures, leads to the dissolution of both Si and smaller amounts of Al species from the framework. Nevertheless, most of these extracted Al species are able to realuminate on the mesopore surface, resulting in the lowering the Si/Al ratio of the hierarchical [9,10]. Zeolite desilication has been successfully applied to produce a large number of hierarchically structured zeolites MFI [11,12]MTW [13], MOR [14,15], FER [16,17], FAU [18], and Beta [19,20].…”