Chemistry Reference
In-Depth Information
reactions and some Brønsted acid catalysed reac-
tions, e.g. etherifications (see later).
The Envirocats that are used to catalyse Friedel-
Crafts reactions—EPZG, EPZ10 and EPZE—are
classed as non-toxic and non-corrosive materials
[26]. Unlike homogeneous aluminium chloride, they
are true catalysts. When using Envirocat EPZG in a
benzoylation, for example, there is a tenfold reduc-
tion in the amount of catalyst required (cf. alu-
minium chloride). These catalysts also are totally
inert in water.
They often show improved selectivity in aromatic
substitution and sometimes they can be reused,
although this will depend on the reaction and reac-
tion conditions. They are used normally with a neat
substrate or, if a solvent is required, inert hydrocar-
bon solvents (cf. chlorinated solvents for AlCl
3
).
Once the reaction is complete, the catalyst is filtered
from the reaction mixture, thus eliminating the
aqueous effluent that normally is associated with
these processes. Provided that the spent catalyst has
been washed free of organics, the catalyst can be
safely landfilled.
3.3 Envirocat EPZE
This catalyst is also a 'CLAYZIC' type of supported
reagent but the method of preparation is radically
different to that used for Envirocat EPZ10. This
results in the formation of not only strong Lewis acid
sites but also some strong Brønsted acid sites. Thus,
this catalyst possesses a wider range of applications
than EPZ10, e.g. Friedel-Crafts sulfonylations and, to
a lesser extent, benzoylations.
3.4 Envirocat EPIC
This catalyst is a phosphoric acid, clay-based reagent.
As expected, the active sites display exclusively
strong Brønsted acid characteristics. This supported
reagent was designed primarily to catalyse high-
temperature esterifications, e.g. polyols with fatty
acids. However, it is possible also to catalyse other
reaction types that require Brønsted acid catalysis.
4.2 Esterifications
3.5 Envirocat EPAD
High-temperature esterifications traditionally are
catalysed with an organic acid, e.g. methanesulfonic
acid (MSA)/
p
-toluenesulfonic acid (PTSA) or metal
salts (stannous oxalate). Although these catalysts are
very efficient, they are highly corrosive and toxic and
can be removed from the crude ester only by
aqueous washes. Their use also results in highly
coloured crude products, which must undergo
numerous treatments with a bleaching agent to
remove the colour.
Envirocat EPIC is a non-toxic and non-corrosive
powder. Once the esterification is complete, it can be
filtered from the reaction mixture, negating the need
for aqueous washes and therefore eliminating aque-
ous effluent. Also, unlike its homogeneous counter-
parts, using Envirocat EPIC often can produce a
crude material that does not require any decolouris-
ing treatments.
Figure 13.1 shows the typical colour of the crude
product when PTSA is used to catalyse the esterifi-
cation of pentaerythritol with a fatty acid. Extensive
treatments with activated carbon would be required
to remove the colour from this crude material. In
comparison, the same reaction when catalysed by
Envirocat EPIC produces a white crude solid, thus
This catalyst is a Cr(VI), alumina-based reagent. It
has been designed as a catalyst for the promotion of
aromatic side-chain oxidation to ketones and
benzoic acids. Studies have shown that it can be used
for the oxidation of aliphatic alcohols to their respec-
tive aldehydes, ketones and carboxylic acids [25]. It
should be noted, however, that Envirocat EPAD is
not suitable for oxidations using peroxide. Under
these conditions virtually all of the chromium is
leached from the support as Cr(III).
4 Advantages of Envirocats
4.1 Friedel-Crafts reactions
Aluminium chloride is an extremely powerful and
versatile reagent for the promotion of Friedel-Crafts
reactions. However, it is far from being a green cat-
alyst for chemistry. It is a toxic and corrosive solid
and the only way to remove it from the reaction
mixture is by aqueous quenching. This has a severe
impact on the environment and, as a result of public
pressure and legislation, its use is becoming increas-
ingly unacceptable.
