Chemistry Reference
In-Depth Information
Scheme 6.12
H
2
Scheme 6.13
H
H
Spillover
H
H
+
Pt
Like many other solid acid catalysts, the exact
nature of the acid sites is complex and depends very
strongly upon the synthesis procedure. This in part
may explain the variation in the numbers of differ-
ent structural models within these catalysts. This
does imply that these very interesting materials merit
greater study in order to produce greater and yet
more controlled superacidity.
As we discuss briefly below, these materials have
found wide applications for a variety of reactions.
In many cases, however, these materials deactivate
quickly. To solve this problem, sulfated zirconia cata-
lysts modified with various metals such as Pt, Pd, Ir,
Fe and Mn have been found to exhibit better cata-
lytic activity. It has been shown, for example, that
Pt-promoted sulfated zironia has higher rates for
alkane isomerisation than non-promoted systems.
The stability of the catalysts was increased towards
deactivation during the isomerisation of alkanes.
One possible role for the supported Pt is the cleans-
ing of sites by hydrogenation of the coke that causes
deactivation. Hattori has proposed a possible mecha-
nism of protonic acid site generation on Pt-modified
sulfated zirconia in the presence of hydrogen [54].
He has proposed a dissociative adsorption of hydro-
gen on the Pt particles to form H atoms, which sub-
sequently undergo spillover on the support. The H
atom migrates on the surface to Lewis sites [54]
where it loses an electron to form a proton stabilized
on the surface O atoms near the Lewis acid sites
(Fig. 6.5).
Sulfated zirconia has been used for a range of
catalytic reactions, such as Friedel-Crafts alkylation,
SO
4
2
-
- ZrO
2
e
-
Fig. 6.5
Mechanism of hydrogen spillover.
acylation, condensation, esterification, etherifica-
tion, isomerisation, nitration, cracking, dehydration,
oligomerisation, etc. The reader is referred to the lit-
erature for a more detailed account of each of these
processes [45-49]. As far as the authors are aware,
there are relatively few examples of industrial
applications of these types of catalysts; although
with further process optimisation these materials
inevitably will expand into more commercial use.
Much of the chemistry that is described, such as in
the alkylation of phenol with MTBE, MTBE synthe-
sis and acylation-type chemistry, shows that these
catalysts are very active, although other catalysts
such as HPAs or ion-exchange resins tend (but not
always) to give higher activity. In the etherification
of beta-naphthol with methanol, HPAs gave the
highest conversion (93%); Amberlyst
®
-15 (92%)
with sulfated zirconia gave about 18%. In the case
of toluene nitration, sulfated zirconia has similar
activity to supported HPAs. Many of the reactions
described require Brønsted acidity and, under the
condition of catalyst synthesis, in many cases opti-
misation of Lewis sites rather than the Brønsted sites
may have taken place.
