Chemistry Reference
In-Depth Information
activity of a selection of solid acid catalysts for the
vapour-phase reaction of acetic acid with ethylene to
form ethyl acetate. The supported HPA is the most
active catalyst.
In order to achieve a commercially viable process,
a thorough investigation of reactor design and
process engineering, including the use of supported
catalysts, was undertaken. Some process options
include operation with a high ethylene/acetic acid
feed ratio, addition of 3-8 mol.% steam (which leads
to ethanol and diethyl ether, which can be recycled
in the reactor) and protection of the catalyst from
ingress of corrosive metal impurities. By judicious
choice of catalysts and optimising process conditions,
a previously non-viable catalyst system has been
transformed into a commercially viable and envi-
ronmentally attractive process.
been overcome to a large extent by modification,
using various transition metals such as platinum and
refining the synthetic methodology.
In terms of the synthesis [47-49], it is found that
catalysts obtained by sulfating the amorphous
hydrated ZrO
2
possess a significantly higher activity
than that obtained by sulfating microcrystalline
samples [45]. The effects of different sulfating agents
and their post-treatment have been studied in detail.
With the use of microscopy and diffraction tech-
niques, it has been found that the nature of the acid
surface sulfates grafted by any of the techniques on
amorphous or precrystallised zirconia is similar,
although local acid site identification is complicated
to define.
A number of models have been proposed to iden-
tify the nature of the active sites. A few of these pro-
posed structures are shown in the schemes below,
however the reader is encouraged to refer to more
detailed descriptions found in the literature.
Kumbhar
et al
. [50] had speculated the structure of
sulfated zirconia as shown in Scheme 6.11.
A similar model has been proposed by Yamaguchi
[51]. The dried surface has been described as having
a highly covalent character, with Lewis acid-type
sites. Partial hydration (water acting as a weak Lewis
base) of the catalyst initially tends to convert the
surface sulfates to a lesser covalent form and then
into an ionic form. This results in the transformation
of the strong Lewis sites to Brønsted acid sites.
Another model has been proposed by Arata & Hino
[52] for the structure of the active site, wherein the
sulfate bridges across two zirconium atoms (see
Scheme 6.12). The formation of Brønsted sites
results from the uptake of water molecules as a weak
Lewis base on the Lewis acid sites.
Kustov
et al
. [53] have proposed schemes for both
an ionic structure, with a proton forming a multi-
centre bond with the sulfate anion, and a covalent
structure, with hydrogen-bonded hydroxyl groups
(see Scheme 6.13).
3.3 Sulfated zirconia
The synthesis of sulfated zirconia, its structural char-
acterisation and its catalytic properties have been
described in an excellent review by Yadav & Nair
[45]. Sulfated zirconia is generally described as a
solid superacid, although the performance is strongly
related to the method of preparation. These kinds of
materials are normally prepared from zirconium
hydroxide, which then is treated with either ammo-
nium sulfate or sulfuric acid, followed by calcination
(typically to 500-700°C). The precise nature of the
acidity is the subject of considerable controversy,
including whether these materials are mainly Lewis
acids or Brønsted acids. Owing to the high activity of
these catalysts for
n
-butane isomerisation, these cat-
alysts are at the very least strongly acidic catalysts.
Clark
et al
. [46] have shown recently that these kinds
of catalysts under normal use conditions pick up suf-
ficient moisture to lead to a performance that is more
akin to the behaviour of Brønsted acid sites. Two
shortcomings of these kinds of acids are deactivation
due to coke formation and acid leaching. These have
Scheme 6.11
