Nonframework tetrahedral alumina

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... 

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 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. 

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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