Chemistry Reference
In-Depth Information
homogeneity of the surface can be obtained via a postfluorination treatment, as a
sample thermal treatment under CF
3
H. The number of strong acid sites so obtained
depends on the probe molecule used to measure them (CO: 0.2-0.4 site per nm
2
; py:
1.2-1.5 site per nm
2
).
45, 48, 53
These results for HTB-AlF
2.6
(OH)
0.4
will be reconsidered below, in the light of com-
plementary radiotracer experiments, to discuss in a complete way the surface behaviour of
this fluoride using a variety of methods.
4.4.2 Using HCl as a Probe with Detection via [
36
Cl]-Labelling
4.4.2.1
Aluminium Chlorofluoride
The benchmark compound for this technique is aluminium chlorofluoride (ACF), some of
whose properties were described in Section 4.2.2 above. There are many pieces of
evidence for this compound being a strong, solid Lewis acid.
19, 23
Exposure of ACF to
successive aliquots either of anhydrous H
36
Cl or of [
36
Cl]-Bu
t
Cl leads to rather similar
behaviour, shown in Figure 4.14. This describes the [
36
Cl] surface counts resulting from
H
36
Cl (a) or [
36
Cl]-Bu
t
Cl additions (b). Although numerical comparisons between (a) and
(b) are not possible, since the [
36
Cl] specific activities of the H
36
Cl and [
36
Cl]-Bu
t
Cl used
differ, the very similar qualitative relationship observed suggests that the surface count
data observed in (b) are due to adsorbed H
36
Cl, which results directly from dehydrochlor-
ination of [
36
Cl]-Bu
t
Cl at the ACF surface. Removal of volatile material by condensation
after count No. 9 in Figure 4.14 (a), leads to a reduction in the surface count, No. 10, and a
further reduction after one day, count No. 11. However, the count is still well above
background, indicating that a significant fraction of H
36
Cl is strongly adsorbed. A small
increase in the [
36
Cl] surface count derived from exposures of [
36
Cl]-Bu
t
Cl to ACF, count
No. 9 in Figure 4.14 (b), is observed after removal of all volatiles, i. e. after count No. 8.
This can be accounted for by the assumption that some dehydrochlorination sites with the
accompanying adsorption sites for H
36
Cl are located at grain boundaries within the solid
ACF rather than being limited to the exterior surface. The isotope [
36
Cl], in common with
all b
emitters, is subject to self-absorption of its radiation.
42
It will be detected by the end-
window G. M. counters used (Figure 4.10) only when located at the exterior surface of the
sample under investigation. Evidently in the case of ACF, some time must elapse for
migration of the [
36
Cl] species to occur from the bulk to the exterior surface. This
phenomenon is observed also for the other compounds described in this section.
4.4.2.2 HS-Aluminium Trifluoride
As explained above HCl can be used directly or as a product from the dehydrochlorination
of Bu
t
Cl. The latter reaction is easily observed over HS-AlF
3
at room temperature
57
and
comparisons between the behaviour of HS-AlF
3
with those of fluorinated (by SF
4
)
-alumina and b-AlF
3
(HTB structure) can be made.
41,44
They indicate that the greatest
reactivity is found for HS-AlF
3
, either because it has the greatest site density (specific
surface areas of samples of this compound as determined by the BET method are usually in
the range 200-300 m
2
g
1
)
45
or because it has uniquely strong Lewis acid sites. The
