Isomorphous dealumination solids

Macedo et al. [227] studied HY zeolites dealuminated by steaming, and found that the strength of intermediate sites decreased with increasing dealumination for Si/Al ratios varying from 8 to greater than 100. For comparison, isomorphously substituted HY, which is free of extra-framework cationic species, possesses more acid sites than conventionally dealuminated solids with a similar framework Si/Al ratio [227], This is because some of the extra-framework aluminum species act as charge-compensating cations and therefore decrease the number of potential acid sites. 

In agreement with previous studies, microcalorimetry confirms that in steamed products, most of the strong acid sites are poisoned by cationic extra-framework A1 species. These sites can be recovered by an optimized post-steaming acid leaching. Isomorphously substituted HY which is free of extra-framework cationic species possesses more acid sites than conventional dealuminated solids with similar framework Si/Al ratio. 

Y zeolites with dealumina ion levels lower than about 50% were, according to N2 adsorption, exempt from mesopores. Moreover, since no trace of amorphous material was apparent in these solids, it could be concluded in agreement with other physico-chemical properties, such as an excellent X-ray crystallinity that the isomorphous Si-Al substitution took place successfully. For solids with higher dealumination levels (>50%) the isomorphous substitution was no longer ideal resulting in the formation of a secondary pore system. 

At present, this liquid-solid substitution approach has been extended to the secondary synthesis of zeolites containing Si, Fe, Sn, Ti, Cr, and other heteroatoms. An exception is that BF4 cannot be used for liquid-phase dealumination and boron-addition of zeolites to prepare boron-containing zeolites.[33] In the following galliation of NH4Y will be taken as an example to discuss the isomorphous substitution secondary synthesis technique. 

It is important at this stage to emphasize that the isomorphic substitution by (NH J SiF gives solids with limited dealumination levels up to about 60 %, which corresponds to unit cell parameters not smaller than about 2.442 nm (26,27), whereas a total framework dealumination could be easily achieved by conventional treatments (unit cell parameter down to 2.423 nm (4)). It is not entirely clear at the moment, whether this limited dealumination level for the (IS) solids results from the experimental conditions we used or is specific to the fluorosilicate technique. Nevertheless this situation renders difficult the detailed comparison between the acidic properties of solids with identical framework dealumination levels prepared by the two techniques. However, as will be seen later, important trends can be derived. 

In this work, using H2S and C2H4 as probes, we aim to characterize the strongest Bronsted acid sites of HY solids, dealuminated either by conventional treatment (steaming followed by acid leaching) or isomorphous substitution (by fluorosilicate) and to compare the results with those obtained with pyridine. 

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