Chemistry Reference
In-Depth Information
More recently,
- and b-AlF
3
3H
2
O and AlF
3
9H
2
O were studied by Kemnitz et al.
[80] (Table 5.1). The structures of AlF
3
9H
2
O and
-AlF
3
3H
2
O both consist of isolated
octahedral AlF
6
x
(H
2
O)
x
complexes with x
¼
3 on average.
All these studies show that both
19
F and
27
Al
iso
of AlO
6
x
F
x
environments increase
with the oxygen content. These findings are essential for the NMR investigations of
disordered Al-based fluoride materials discussed in the following.
5.3.3
Fluorinated Aluminas and Zeolites, HS AlF
3
In situ X-ray diffraction (XRD) and NMR methods were used to follow the structural
changes that occur during the dismutation reaction of CHClF
2
over
-alumina [68].
19
F/
27
Al CP experiments of
-Al
2
O
3
activated at 300 CshowedthatAlF
3
had already
begun to form at this temperature. By 400 C, resonances from a phase that resembles
-AlF
3
dominate both the
19
Fand
27
Al NMR spectra of the used catalyst (Table 5.1).
Considering that heterogeneous reaction rates scale with surface area, new synthetic routes
to HS aluminium fluoride are currently being sought. The fluorination of H-SSZ-32 zeolite
(H
1.3
[Al
1.3
Si
22.7
O
48
]) with NF
3
plasma led to the formation of a HS AlF
3
phase (190 m
2
g
1
)
[81] (Table 5.1). It was shown by
27
Al NMR that the AlF
6
octahedra in this amorphous
material are greatly distorted, even in comparison with the catalytically active b-AlF
3
.The
structures of a series of pristine and Zn
2
þ
-impregnated aluminas following fluorination with
HF were also investigated using both solid-state NMR and X-ray powder diffraction
methods [75].
19
F/
27
Al HETCOR spectra for samples with different fluorine and aluminium
environments show that the fluorine atoms with
iso
between
140 and
160 ppm are
correlated with the aluminium sites with
iso
between 0 and
15 ppm. The
19
F
iso
values
become more negative as the
27
Al
iso
shift toward more negative values. For example,
19
F
iso
of
150 and
160 ppm correlate with
27
Al
iso
shifts of
5and
15 ppm, respectively.
This is consistent with a later work on the mechanism for the fluorination reaction of CHClF
2
on alumina determined by various 1D and 2D solid-state NMR methods [82]. Moreover an
additional group of
19
F resonances between
180 and
220 ppm (Table 5.1), observed on
fluorinated aluminas for the first time, was assigned to NBF atoms. This range of
19
F
iso
values for NBF atoms seems, at first sight, surprising compared to the values presented in
Table 5.1 for this kind of fluorine atoms (
120,
160 ppm). Furthermore, the explanation
given by Chupas et al. [82], referring to the fluorine atoms bonded to a single aluminium in
Na
5
Al
3
F
14
showing
iso
values in the -200 ppm region [3], is not convincing because in this
compound, these NBF atoms have several sodium atoms in their neighbouring.
Nevertheless, the so-called superposition model (see Section 5.4.1.1) for the
19
F isotropic
chemical shift [1,16,70] shows that
iso
increases with the number of cations surrounding the
fluorine ion (the F-M distances remaining equal). In this way, the values gathered in
Table 5.1 for NBF atoms (
120,
160 ppm) suggest that these NBF atoms are H-bonded
to hydrogen atoms from hydroxyl or water groups. This assumption is strengthened by
several studies on fluoroaluminate complexes, AlF
6
x
(H
2
O)
x
, in aqueous solution [83,84],
fluoride-promoted dissolution of bayerite b-Al(OH)
3
(s) and boehmite
-AlOOH [85] and
fluorination of the e-Keggin Al
13
polycation [86] (Table 5.1).
It was also shown by Chupas et al. [82] that the initial stages of fluorination lead to the
formation of terminal F-Al groups and that five-coordinate aluminium sites are initially
