Chemistry Reference
In-Depth Information
4.3.2 Characteristic Reactions and the Detection of Adsorbed Species by a
Radiotracer Method
4.3.2.1
State of the Art
A'probereaction'canbeusedtomakecomparative statements about the nature of
different acidic surfaces. This is an approach of long standing and there are many
applications described in the literature. Two examples will suffice to introduce the
topic, since other chapters in this volume deal specifically with reactions that are
catalysed by solid metal fluorides and related compounds. The dehydration of isopro-
panol to give propene has been used in making comparisons of solid acidic oxides
2
and the isomerization of 1,2-dibromohexafluoropropane, CBrF
2
CBrFCF
3
,to ts
2,2-dibromo-isomer, CF
3
CBr
2
CF
3
, is a reaction diagnostic for very strong solid Lewis
acids such as ACF.
19,23
In the FTIR approach, described above, there is a reasonably
direct link from spectroscopic data to relative acidities and, at least for solid oxides, a
substantial literature linking spectroscopic properties of probe molecules to acid site
strengths. In contrast, the use of probe reactions to make statements about acidity has a
smaller literature and a linkage that is less direct. However, providing comparative
studies are undertaken under identical or near-identical conditions, the probe reaction
approach can provide useful comparative data on catalyst activities and is complemen-
tary to the more widely used FTIR method.
4.3.2.2
Anhydrous Hydrogen Chloride as a Probe
The focus here will be on anhydrous HCl, since, (i) it is a strong Brønsted acid (at least in
water), (ii) in principle it can behave as a Brønsted acid by dissociative adsorption at a
surface that contains oxide (hydroxide) functioning as a Brønsted base and (iii) it could
function as a weak Lewis base as a result of associative adsorption at a surface that contains
(strong) Lewis acid sites. From this analysis, it would be expected that the behaviour of
HCl towards fluorides would be different from its behaviour towards oxides. This com-
parison is less clear cut however if oxyfluorides and partially hydrolysed (hydroxylated or
hydrated) fluoride surfaces are included. Schematic representations of dissociatively
adsorbed HCl at medium strong and weak Lewis acid sites of
-alumina are shown in
Figures 4.6 and 4.7. These are based on spectroscopic observations.
The three possible modes of adsorption of anhydrous HCl at a metal fluoride surface,
physically adsorbed, associatively adsorbed and dissociatively adsorbed (the latter two
cases are chemical adsorption) are represented diagrammatically in Figure 4.8.
Anhydrous HCl can be used as a probe either by direct exposure to a surface or it may be
generated in situ by a dehydrochlorination reaction (XIII).
H
CI
+
HCI
(XIII)
