Extra-framework aluminum acidity characterization

The microcalorimetry of NH3 adsorption coupled with infrared spectroscopy was used to study the effect of the synthesis medium (OH or F ) on the nature and amount of acid sites present in Al,Si-MFl zeolites [103]. Both techniques revealed that H-MFl (F ) with Si/Al < 30 contained extra-framework aluminum species. Such species were responsible for the presence of Lewis acid sites and poisoning of the Brpnsted acidity. In contrast, MFl (F ) characterized by Si/Al > 30 presented the same behavior as H-MFl (OH ). 

The different performances of catalysts (A) and (B) was elucidated by characterization of these materials [29]. Al and Si MAS NMR showed that after treatment with 0.01 M HCl most of the amorphous silica material is removed from the parent catalyst (A), leaving extra-framework aluminum species also created by the steaming procedure [29]. This can be readily understood, because the solubility of silica is maximum at pH 2 [34]. It is believed that the silica species blocked most of the catalytically active centers, i. e. the highly dispersed Lewis acidic, extra-framework alumina sites, which seem to be partly bonded to the zeo-litic framework of the starting material (A). The EFA species are not, therefore, leached out. 

However, this statement was obviously derived only from Al MAS NMR spectroscopic results, and the bulk aluminum content was not considered. Since it is well known [161] that part of the extra-framework aliunimun may be invisible for Al MAS NMR spectroscopy, it may be possible that intracrystaUine or intercrystalline oxidic aliuninum was overlooked. A recent paper by Stockenhuber and Lercher [162] focused on the characterization of extra-framework species and add sites in Y zeoHtes subsequent to dealumination with SiC. Extra-frame-work alumimun species were found to be present after the treatment with SiC, but partially extractable from the product with ammonia solutions. Besides this type of species, a second one, rich in silica and located mainly outside the zeoUte channels, was also suggested. The effect of acid leaching subsequent to partial dealumination of NaY zeoHte with SiCl4 at 450has also been studied [163]. 

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