Chemistry Reference
In-Depth Information
targeting high surface area materials. In the case of the pyrochlore hydroxyfluoride, the
addition of a small amount of ether drastically improves the surface area of the solid, from
77 to 140 m
2
.g
1
.
42
Following the work of Kemnitz et al.,
33
ether molecule in the presence
of HF has been suggested to form an oxonium ion giving rise to ion pairs with F
.Sucha
complex should lead to a decrease of the fluorine reactivity known to be a structuring agent
and therefore enable the preparation of nanosized crystallites.
During the synthesis of the HTB-type compound, the surface area could be monitored by
tuning the volume ratio V
¼
V
water
/V
isopropanol
of the used solvents. For V
¼
1, a surface
area of 82 m
2
.g
1
is obtained, whereas for V
¼
6, the value drops to 3 m
2
.g
1
.
Interestingly, both compounds not only differ in their morphology but also by their
chemical composition. Using elemental analysis and high field
27
Al NMR spectroscopy,
the chemical composition of the low and high surface area compounds has been accurately
determined as AlF
2.4
(OH)
0.6
and AlF
2.6
(OH)
0.4
, respectively. Water thus favours the
stabilization of OH groups inside the structure while isopropanol enables the stabilization
of F
ions. This trend is confirmed when using V
>
1, the powder X-ray diffraction pattern
of the corresponding solid displaying some traces of
-AlF
3
, that is a more fluorinated
compound.
2.3.1.1.4 Coupling Sol-gel Alkoxy-fluoride Route and Microwave Irradiation
A more sophisticated route has been proposed by combining nonaqueous sol-gel
fluoride synthesis and microwave solvothermal process.
47
The nonaqueous sol-gel
process developed by Kemnitz et al.,
33
enables the preparation of X-ray amorphous
metal fluorides through two steps. First, an alcoholic solution of a metal alkoxide is
partially fluorinated by anhydrous hydrogen fluoride previously dissolved in an
organic solvent through the reaction: M(OR)
3
þ
xHF
!
MF
x
(OR)
3-x
þ
xROH.
48
After drying, the resulting alkoxy-fluoride is fluorinated using gaseous CFCs or
anhydrous HF, leading to X-ray amorphous metal fluorides with very high surface
area. In a second step, the alkoxy-fluoride sol-gel is subjected to a microwave
solvothermal process at various temperatures ranging from 90 to 200 Cfor1hour
treatment. Finally, the mixture is dried under microwave irradiation but this step has
no impact on the morphology of the final compound. The characteristics of the dry gel
obtained by microwave synthesis are gathered in Table 2.3. While low-temperature
microwave treatment (
<
180 C) leads to X-ray amorphous alkoxy-fluorides, a crystal-
lization process starts at T
¼
180 C with the detection of
-AlF
3
and an unidentified
phase. Such an evolution is clearly related to an increase of the kinetics of fluorination.
At this stage, the resulting compounds exhibit extremely high surface areas (Table 2.3)
reaching 525 m
2
.g
1
for the dry gel obtained at 180 C. At a higher temperature,
200 C, there is clear evidence for the occurrence of a complete crystallization process
through: (i) the XRD identification of
-AlF
3
(HTB), (ii) a large increase of the
fluoride ions substituting the alkoxy ligands and accounting for the very low carbon
content and (iii) a decrease of the surface area dropping from 525 to 125 m
2
.g
1
.A
strong increase in the internal pressure also occurs during the microwave treatment at
200 C and was suggested to arise from the breaking of the Al-OiPr bonds through the
following mechanism: Al-O-CH-(CH
3
)
2
!
Al-OH
þ
CH
2
¼
CH-CH
3
. Such a reaction
has been proposed by analogy with the behaviour of an alkoxy-fluoride upon fluorina-
tion.
49
Another side reaction has been suggested to occur, based on the work from K. G.
