Zoned aluminium distribution in synthetic zeolite ZSM-5

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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“…Surface zone enrichment in Al and Si excess in the bulk was reported by several authors e.g. [27,28].…”

Section: Chemical Analysis Of Bulk and Surfacementioning

confidence: 95%

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

Sadowska

Góra‐Marek

Drozdek

et al. 2013

Microporous and Mesoporous Materials

132

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“…[2,3] In addition, the control of crystal size is important to extend the use of zeolites in areas such as medicine, electronics, and optics, which are not traditional fields of application for this type of materials. For instance, the synthesis of high-quality, large, single-crystal zeolite is essential for structural determinations [4,5] and advanced applications related to host-guest chemistry. [6][7][8][9] On the other hand, the synthesis of nanocrystalline zeolite particles pertains to applications such as membrane and thin-film preparation and the development of devices related to microelectronics, thermoelectricity, and ionic conductivity.…”

Section: Introductionmentioning

confidence: 99%

Factors That Control Zeolite L Crystal Size

Itani

Bozhilov

Clet

et al. 2011

Chemistry A European J

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Time-series hydrothermal syntheses from two organic-cation-free gels with different compositions were employed to study the factors that control the final size of zeolite L crystals. The first gel had a starting K/Al ratio of 10, whereas in the second one it was three times lower. The relatively simple chemical composition of the starting gels and the combination of complementary characterization methods allowed us to track down the different stages of transformation of the initial amorphous gels into zeolite crystals and the factors that control the nucleation and growth processes. The role of the starting mixture components in the formation of the primary amorphous particles was explored. It was found that the profoundly different reaction kinetics in the two systems are caused by the difference in diffusion rates, which in turn are controlled by the extent of the polymerization reactions at room temperature during mixing of the starting components prior to hydrothermal treatment. As a consequence, nucleation is fast and ubiquitous in the first system with higher water content and K/Al ratio, whereas it is slow and sporadic in the second system with lower water content and K/Al ratio. Ultimately, these differences in the kinetics lead to the formation of two distinctly different patterns of crystal-size distribution, with a large number of small nanocrystals in the first sample and fewer large crystals in the second sample. The new findings put zeolite crystal growth on a rational basis that would enable the control of zeolite crystal size in similar organic-template-free systems.

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“…As the synthesis of the large and well-defined zeolite crystals is difficult to control on the chemical composition level, numerous studies have reported the presence of the Al zoning within the zeolite material. [27][28][29] A gradient in the Al (or Si) content within the material volume would have a significant impact on the zeolites local susceptibility towards the steam treatment and obscure the understanding of the micropore orientation influence on the dealumination of ZSM-5. Therefore it is important to underline that the parent ZSM-5 crystals discussed here contain an equal amount of Al throughout the crystal, [30] allowing the mesoporosity to be related to the crystals intergrowth architecture.…”

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confidence: 99%