Chemistry Reference
In-Depth Information
microwave-promoted solventothermal route referred to above,
47
will be described using
data from both techniques.
4.4.1 Using FTIR Spectroscopy
Despite the great interest in fluoride compounds for catalysis, their surface properties have
been little studied up to now, compared with the corresponding metal oxides. Moreover,
the published studies deal mainly with material having specific surface areas less than
20 m
2
g
1
. Recently we have investigated well crystallized and amorphous iron, chromium
and aluminum fluorides, all presenting a high specific surface area, up to 300 m
2
g
1
.We
characterized first crystalline hexagonal HTB structure fluorides, synthesized in
Bordeaux. These solids are constituted by MF
6
octahedra linked by corners.
50
They can
be considered as microporous materials, having monodirectional channels of 3-4
˚
in
diameter, which can contain water or ammonia. Their thermal stability varies between 423
and 623 K according to the nature of the cation (Fe, Cr, Al or Ga). In addition, we have
studied materials having the pyrochlore structures, synthesized in Bordeaux, as well as
amorphous compounds of a new type, synthesized using a sol-gel method in Berlin and
having a very high specific surface area. The identities of the compounds to be discussed
below are summarized in Table 4.1.
Table 4.1 Formulation, method of synthesis and specific surface area of the samples synthe-
sized by the ICMCB team in Bordeaux and the HU-Berlin Institut f¨ r Chemie team
Composition and
structure
Synthesis
Specific surface
area/m
2
g
1
Reference
HTB-AlF
2.2
(OH)
0.8
a
nitrate route
15
51, 52
HTB AlF
3-x
(OH)
x
nitrate (microwave activation)
82
47
HS-AlF
3
post
fluorinated (F
2
)
alkoxide route (microwave
activation)
330
48
HS-AlF
3
post
fluorinated (CFC)
alkoxide route (sol-gel)
180-310
53 (see
also 45)
Pyrochlore
AlF
1.8
(OH)
1.2
alkoxide route (microwave
activation)
137
49
a
Study extended to MF
3-x
(OH)
x
, with M ¼ Fe, Cr, Ga.
4.4.1.1 Chemical Impurities and Hydroxyl Groups
Fluorides often present surface impurities as residuals from synthesis.
47-53
Their identifi-
cation can elucidate the reaction mechanisms; in particular, it has been demonstrated by
IR spectroscopy that aluminium hydroxy fluorides having the HTB structure contain
coordinated or protonated ammonia.
47
Curiously, their synthesis involves a nitrate
Al(NO
3
)
3
,9H
2
O precursor and ammonia is absent as a reagent. Therefore we have deduced
that precursor decomposition implies nitrate reduction to ammonia by isopropanol during
the reaction, thus explaining the presence of both ammonia inside the material and acetone
