Chemistry Reference
In-Depth Information
studies does, however, help to build up a picture of
reactivity in these complex reaction systems.
Sometimes, the most useful information on sur-
face acidity can be obtained by studying the IR char-
acteristic of adsorbed base molecules, such as
pyridine. The Brønsted acid sites (pyridinium ions)
appear at 1540 and 1640 cm
-1
, and correlations of
numbers of acid sites (as a function of certain de-
sorption temperatures) and reaction activity have
been made. An informative discussion of some of the
issues in this approach is given by Kubelkova
et al
.
[10].
Nuclear magnetic resonance (NMR) has been used
extensively to study solid acid catalysts. Proton NMR
is a useful technique for determining the number,
strength and accessibility of acidic protons in zeolites.
This has been reviewed recently by Pfiefer
et al
. [11].
In the case of acidic zeolites, a relationship between
acid strength of different Brønsted acid sites and its
chemical shifts has been reported. The acidic bridg-
ing OH site
1
H chemical shifts can be found in the
range of 3.8-5.2 ppm (from trimethylsilyl) as com-
pared to values for the non-acidic silanol (ca. 2 ppm)
and OH at the non-framework Al (2.5-3.5 ppm). It
is possible to measure the influence of zeolite treat-
ments on the final concentrations of all OH groups,
including the acidic bridged hydroxyls. A good cor-
relation between acidity and catalytic activity of solid
acid catalysts has been described. Hunger
et al
. have
argued that the chemical shifts of bridging hydrox-
yls in zeolites increase with Si/Al ratio to a value of
about 10 [12]. Higher Si/Al ratios produce no
additional chemical shift. It was suggested that all
zeolites with Si/Al ratios greater than 10 have com-
parable proton affinities. This conclusion is implied
also by calorimetric studies.
Nuclear magnetic resonance has been useful also
to investigate and follow the structures and trans-
formations of adsorbed species. One of the goals has
been to identify reaction intermediates. Biaglow
et al
.
[13] have reported the
13
C spectra of acetone, bound
at a stoichiometry of one per Brønsted site, for a
range of zeolites. The results shown in Table 6.2 [13]
illustrate distinguishable isotropic chemical shifts
for a variety of acidic zeolites. These range from
10.1 ppm for SAPO-5 to 18.7 ppm for H-ZSM-22.
These differences result from local structural differ-
ences of the hydrogen-bound complexes that are
related to acidity. Line width measurements also
have been used as a measure of the molecular
Table 6.2
Chemical shift tensor of the C-2 carbon of
chemisorbed acetone relative to the neat solid
Chemical
Peak width
Zeolite
shift at
at 295 K
channel
Sample
125 K (ppm)
(ppm)
size (Å)
H-ZSM-22
1
8
.7
230
5.4
H-ZSM-5A
16.9
200
5.6
H-ZSM-5B
16.
8
200
5.6
H-ZSM-12
16.7
103
6.2
H-Mordenite
15.1
103
7.
8
SAPO-5
10.1
8
4
7.3
HY zeolite
12.9
44
13.4
Scheme 6.1
motion at the site. These kinds of measurements
allow one to understand both site proton affinity and
the interaction energies from the adsorbate/zeolite
framework.
Other techniques are finding increasing use in the
study of adsorbates on the acidic sites of acid cata-
lysts. Fraissard
et al
. have described the use of ultra-
violet (UV) laser photoexcitation of quinoline, an
N
-heteroaromatic base adsorbed on the surface of
acidic Y-zeolite and Nafion
®
(an acidic ion-exchange
material). This has been shown to be a very sensi-
tive fluorescent probe for characterising interactions
with the various acidic sites within these solid acid
catalysts [14].
Reaction chemistry can be used also as a probe for
acidity [15]. For example, the isomerisation of
n
-butane is generally regarded as a reaction that
requires strong acidity, whereas olefin isomerisation
reactions are generally considered to be catalysed by
weak acids. The protonated intermediates involved
in these kinds of reactions include carbenium ions.
For olefins, these form quite readily with a range of
solid acids. The general equilibrium is shown in
Scheme 6.1: the role of the intermediate carbenium
ion can be inferred readily from the reaction
chemistry.
The isomerisation of
n
-butane is thought to go via
a carbonium ion, which is unstable and reacts to give
