Chemistry Reference
In-Depth Information
1.2.2.6
Structural Characterization of Sol-Gel Derived High-surface Fluorides
1.2.2.6.1 Vibrational Spectroscopy of HS-AlF
3
The IR transmission spectra of HS-AlF
3
were recorded and compared with those of
aluminium isopropylate and of dry aluminium fluoride gel - i.e. the Al moieties involved
in HS-AlF
3
synthesis (Figure 1.17) [8], as well as of crystalline AlF
3
phases (Figures 1.18
and 1.19) [42]. In Figure 1.17 the CH
3
andCHaswellOHabsorptionbandsofisopropoxide
and/or isopropanol can be seen at wave numbers higher than 2800 cm
-1
in the spectra of
aluminium isopropylate and of dry aluminium fluoride gel, whereas the broad band in the
HS-AlF
3
spectrum can be attributed (i) to water adsorbed in course of sample preparation
due to the high Lewis acidity, and (ii) to water residues present in the bulk as a consequence
of the formation of diisopropylether at postfluorination. At low wave numbers there are the
absorption bands of Al-O and Al-F valence vibrations (360-700 cm
-1
) and of C-C frame
vibrations of the organic constituents. These spectra show the disappearance of organic
components converting the dry gel, the 'precursor', into HS-AlF
3
.
HS-AlF
3
precursor
Al(O
i
Pr)
3
4000
3500
3000
2500
2000
1500
1000
500
Wave number (cm
-1
)
Figure 1.17 IR spectra of Al(O
i
Pr)
3
, the AlF
2.3
OiPr
0.7
.
xiPrOH gel, and HS-AlF
3
(Reprinted
with permission from [8] Copyright (2007) Springer Science þ Business Media.)
The amorphous, highly disordered state of HS-AlF
3
follows convincingly from a
comparison of its IR spectra with those of
-AlF
3
shown in Figure 1.18. Since crystalline
-AlF
3
also gives a well resolved Raman spectrum (Figure 1.19) all the distorted X-ray
amorphous phases of HS-AlF
3
do not yield any useful information.
The very broad peaks in the IR-spectrum of HS-AlF
3
are obviously the consequence of
its amorphous state and can be interpreted as superposition of many unresolved peaks, also
covering the range of vibration bands of the
- and
-AlF
3
phases.
1.2.2.6.2 X-ray Diffraction and TEM Investigations of HS-AlF
3
In contrast to vibrational spectroscopy, the X-ray diffraction patterns of HS-AlF
3
and of its
precursor, the AlF
2.3
OiPr
0.7
.
xiPrOH dry gel, show no peak and therefore only indicate the
