Nonframework alumina

Surface areas were determined by nitrogen sorption at -196°C using a 5-point (p/pQ - 0.06-0.21) BET calculation method. Because HDY s can contain large amounts of nonframework alumina, as-measured surface areas tend to underestimate the concentration of the zeolitic constituent. For this reason, surface areas are reported on both an as-measured (m /g product) and a "m /g framework1 basis. For the latter, framework mass computations assumed that sll... 

The pH 10 data permit some generalisation about the relationship between zeolite composition and stability in NH4N03. in particular, structural stability decreases as either framework or chemical eilica/alutnlna ratio increases. While mechanistic inferences (e g r intracrystalline buffering or "propping" by nonframework alumina) cannot be drawn from these data, several points are... 

ABSTRACT. This contribution reviews the most recent progress on the development of novel FCC catalyst. The following subjects are considered catalyst preparation and structure (matrix, binder, zeolite component), catalytic selectivity (stability, mesoporosity, nonframework alumina, hydrogen transfer), isobutene production for reformulated gasolines and FCC additives (CO promoters, 80, transfer catalysts, and octane additives). 

In both cases post dealumination chemical treatments are necessary. In the case of the use of silicon hexafluoride dealumination, it is necessary to remove every trace of fluoride in order to preserve stability (15). In the case of hydrothermal dealumination, additional sodium is removed, and nonframework alumina may or may not be removed as desired. 

In the following sections the catalytic effects of the three major structural changes in the operating dealuminated zeolite will be discussed. These structural changes include the formation of nonframework alumina, the formation of mesoporosity, and a reduction in the number of aluminum acid sites in the framework. 

The catalytic activity of these materials has been the subject of considerable discussion. A prominent role has been proposed for nonframework alumina in generating strong acidity... 

In summary, the role of nonframework alumina is not clear. There are claims for a positive role and claims of a more negative impact on selectivity. If there is an effect, it is probably associated with the tetrahedral species. 

Pentacoordinate and octahedral nonframework aluminas have no significant effect on activity or selectivity. 

Table 3. Selectivities of catalyst containing nonframework alumina observed at 55 ppm by PA MASNMR.

Biaglow AI, Gorte RJ, White D. NMR studies of acetone in dealuminated faujasites - a probe for nonframework alumina. J Catal 1994 150 221-4. 

Nonframework alumina can also be removed from USY-zeolite catalysts by solution in ammonium nitrate, at pH 2.5, and washing out the aluminum nitrate formed. This does lead to further dealumination and, although defects remain, the zeolite is still very crystalline. The NFA-free catalysts have increased stability, produce higher-octane gasoline, have better coke selectivity, and are widely used. Most catalyst suppliers can provide dealuminated USY-zeolite with reduced nonframework alumina. 

NMR-characterization. 27 Corbin et al. (35) were able to show by a systematic study that Z/A1 MAS NMR gives the true Si/Al ratio with a mean error of 10 %, if two conditions are met a) The amount of paramagnetic species is less than 0.05 % and b) the sample does not contain "NMR-invisible" aluminium. Chemical analyses of the samples under study showed that condition one is fulfilled. If samples contain "NMR-invisible" aluminium a difference between the concentration determined by chemical analysis and the framework aluminium concentration determined by NMR should be observed. From the absence of such a difference we conclude that "NMR-invisible" aluminium species do not exist in our samples. Also a line at the position of about 0 ppm due to octahedrally coordinated non-framework aluminium and a broad line at about 30 ppm due to tetrahedrally coordinated nonframework aluminium (36) could not be observed. The values for the concentration of framework aluminium atoms derived from the intensities of the line at about 60 ppm (see below) are in good agreement with those corresponding to the amount of alumina used in the synthesis mixtures. In conclusion, through the Al MAS NMR measurements it was possible to show that all aluminium atoms are incorporated in tetrahedrally oxygen coordinated framework positions. 

In ion exchange, ions are exchanged between two different environments so, zeolites, hydrotalcite-like materials, and clays can exchange nonframework ions when suspended in aqueous solutions of other ions in some cases, the process can also be performed in molten salts at moderate temperatures. Sodium/silver substitution in p-alumina takes place at 300 °C according to the reaction ... 

A nonframework tetrahedral alumina (NFTA) species is a more promising candidate since tetrahedral aluminum oxides are acidic in other systems. The NFTA has been associated with... 

A1 characterization is the quantitative determination of its tetrahedral vs. octahedral coordination (Fig. 3). Since pure alumina phases prefer octahedral A1 coordination, it is expected that nonframework A1 prefers octahedral sites. Thus, the determination of tetrahedral vs. octahedral A1 provides information characterizing the A1 in the zeolite framework and aiding in the determination of the framework Si/Al ratio. In addition, it reveals the formation of extra-framework alumina phases formed upon steaming or following other chemical treatment [15-19]. 

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