Structure and Properties of Noncrystalline Aluminum Oxide-Hydroxide Fluorides

Abstract

The thermal treatment of sol−gel derived aluminum hydroxide fluorides leads to new, X-ray amorphous, macroporous aluminum oxide-hydroxide fluorides. Followed by EA, DTA-TG, XPS, FT-IR, NH3-TPD, BET, XRD, and MAS NMR, it can be shown that the simplified composition of these compounds is “AlFO” and “AlF2O1/2”, respectively. These aluminum oxide-hydroxide fluorides exhibit a large number of acid centers. The local structure consists of AlF x Ocn-x units (cn, coordination numbers 6, 5, 4), and all sites are partially fluorinated. It can be shown that bridged and terminal F-sites are related to 5- and 6-fold coordinated Al sites.

Crystalline aluminium hydroxy fluorides—Suitable reference compounds for 19F chemical shift trend analysis of related amorphous solids

“…The 27 Al NMR signals of both compounds lie in the typical region for octahedrally coordinated Al, and a high field shift of the maximum of the observed 27 Al MAS NMR signals from d CS % À6 ppm for fd CS % À12 ppm for g (spectra not shown), clearly confirms the model proposed here. For AlF 5 O (g) a more negative 27 Al NMR shift would be expected in comparison to AlF 4 O 2 [64].…”

“…The corresponding NMR data are given in Table 3. These were prepared here in quite a similar way using dried THF as solvent and aqueous HF as fluorinating agent for another area of interest [64]. However, as these compounds are of course related to the study here, the chemical shift trend analysis may be used for the prediction of local structures.…”

“…The resulting gels were dried in vacuum at 70 8C and handled afterwards in air. Further details are described elsewhere [64].…”

Comparison of acidic site quantification methods for a series of nanoscopic aluminum hydroxide fluorides

“…However, the temperature treatment during TPD experiments, which is necessary for desorption or the Hofmann elimination, can be a serious disadvantage in the quantication of acidic sites. In particular, for catalysts that are active at moderate temperatures, [9][10][11][12] the temperature treatment can lead to dehydroxylation and dehydration and, therefore, decomposition or at least alteration of the catalyst.…”

Nanoscale metal fluorides: a new class of heterogeneous catalysts

“…7) that has been proven in the meantime to be a general synthesis approach for nanoscopic metal fluorides. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] For deeper insights into the mechanism of the fluorolytic sol-gel approach, see ref. 10.…”