Dealumination H-Y zeolites

Keywords Two-Dimensional correlation, Infrared spectroscopy, Dealuminated H-Y zeolite, Bronsted acidity, MQ-MAS NMR. 

As a numerical example, consider a partially dealuminated H-Y zeolite that contains 32 Alf/u.c., all of which are isolated, and 8 extraframework Al cations/u.c. This example is similar to the case of the partially dealuminated H-ZSM-20 zeolite in Figure 6 if one assumes that most of the extraframework Al is present in the cationic form. If the extraframework Al is complexed such that each Al has an equivalent charge of +2, 16 protons would be required to balance the framework charge. Here it is assumed that the cationic extraframework... 

In our research group the glycosylation reaction between D-glucose and n-butanol was investigated over a dealuminated H-Y zeolite with a Si/Al ratio of 15.[45, 46] In this way, butyl-D-glucofuranosides and glucopyranosides are readily synthesized, at temperatures from 90 to 110 °C, with 6 wt% of catalyst and with a butanol/glucose ratio from 5 to 40. 

At the same time it has been shown that certain zeolite compositions do not exhibit uniform acid strength. Lunsford and coworkers extended the acidity-aluminum content correlation to the high Si/Al range of dealuminated H-Y zeolites and found that above approximately 35 Al/unit cell (Si/Al 4) the overall acidity of the zeolites decreased, presumably due to next nearest neighbor interactions, i.e., the intensive factor of acidity of the zeolite is diminished. It has also been established that certain dealumination procedures can affect the intensive factor of acidity. This was clearly demonstrated by Lago et al. on H-ZSM-S catalysts, in which dealumination by mild steaming resulted in a four-fold increase in the cracking activity for n-hexane. ... 

Figure 10 Correlation between Brensted acid site density as determined by MAS>NMR and nonneutrallzed lattice Al atoms for a series of dealuminated H>Y zeolites. ...

A comprehensive study has been made to probe the spatial proximities among different acid sites in modified dealuminated H-Y zeolites by using a variety of different solid-state NMR techniques, including multinuclear MAS NMR and two-dimensional H DQ MAS NMR spectroscopy. ... 

Remy, M.J., Stanica, D., Poncelet, G., Feijen, E.J.P., and Grobet, P. Dealuminated H-Y zeolites relation between physicochemical properties and catalytic activity in heptane and decane isomerization. J. Phys. Chem. 1996, 100, 12440-12447. 

High amounts of 24 were also obtained by the use of H-US-Y (96) followed by H-US-Y (96)-HCl. The H-US-Y (96)-HCl used, a modified highly dealuminated ultrastable Y zeolite, was pretreated with diluted acid according to the method described before (28, 31). This zeolitic catalyst, unlike many others, remains active at lower temperatures and also at high loading, as was previously demonstrated in the isomerization of alpha-pinene oxide using this heterogeneous catalyst (28, 31). 

In the present study, a commercial H-Y zeolite was dealuminated via the procedure described by Skeels and Breck [3,4] using ammonium hexafluorosilicate (AHFS) as the dealuminating agent under closely controlled conditions. The fluorosilicate method is attractive because it allows to produce silicon-enriched zeolites which are in principle perfectly microporous and exempt from framework defects and non-ffamework A1 species. Typically, the AHFS treatment differs from many of the dealumination methods in that it is carried out in aqueous media under relatively mild conditions [5]. 

The complete nitrogen isotherms of dealuminated Y zeolites are reported in Fig. 1. The curve of the parent H-Y zeolite corresponded to type I in the Brunauer classification, which was typical for the crystalline microporous materials [17]. As expected, the starting material showed no evidence of mesopores. Fig. 1 shows that the AHFS-treated samples with dealumination levels equal or lower than 50% were characterised by a very flat adsorption-desorption isotherm with nearly no hysteresis loop [18]. 

The objective of this work is to evaluate the dealumination via ammonium hexafluorosilicate treatment as an effective method for enhancing the catalytic performance of H-Y zeolite for oxidative destruction of chlorinated VOC. A series of Y zeolites with various Si/Al ratios was prepared from a commercial sample, then characterised and tested for the catalytic decomposition of chlorinated VOC (1,2-dichloroethane and trichloroethylene). In general, these modified Y zeolites exhibited a higher activity with respect to that of the parent material, the zeolite subjected to 50% dealumination resulting in the most active catalyst. This increase in activity was associated with the development of strong Bronsted acidity due to dealumination. 

Bronsted acidity. In the present study, an H-Y zeolite was dealuminated via the procedure described by Skeels and Breck [7,8] using ammonium hexafluorosilicate (AHFS) as the dealuminating agent under closely controlled conditions. 

P-23 - Comparative properties of modified HEMT and H Y zeolites from the FTIR study of CO adsorption effect of the dealumination and amorphous debris on the Brdnsted acidity... 

Calcination changes the acid site strength distribution, and high-temperature calcination is a method of reducing total acidity via dehydroxylation and dealumination, while increasing the number of Lewis acid sites. Shannon et al. [89] have shown that dehydroxylation of H-Y zeolite at 923 K results in the destruction of most of the acid sites of medium strength (75-140 kJ mol ) and their replacement by a smaller population of stronger sites (150-180 kJ mol ). 

Influence of Dealumination on H-Y Zeolites. . . Acidity in Fluid Cracking Catalysts (FCCs). . . . ... 

Macedo et aL [151], also studying H-Y zeolites dealuminated by steaming, foimd that the strength of intermediate sites decreased with increased dealumination for Si/Al ratios from 8 to greater than 100. For comparison, isomorphously substituted H-Y, which is free of extra-framework cationic species, possesses more acid sites than conventionally dealuminated solids with a similar framework Si/Al ratio [151]. This is because some of the extra-framework aluminum species act as charge-compensating cations and therefore decrease the number of potential acid sites. 

Shi et al. [158] measured the strength of H-Y zeolites dealuminated hy treatment with SiCU for Si/Al ratios ranging from 4.2 to 37.1, and they also found a decrease in the number of sites possessing intermediate strength with increasing dealumination. As the Si/Al ratio increased, the initial heat of adsorption, presumably corresponding to a few Lewis sites present in the samples, first increased and then passed through a maximum at an Si/Al... 

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