Chemistry Reference
In-Depth Information
bimolecular conversion of alcohols to ethers in excel-
lent yields [74]. The method provides excellent yield,
easy isolation of the product and ready regeneration
of the catalyst in the case of primary alcohols. The
mechanism for the reaction involves the in situ for-
mation of an oxonium ion, where Nafion
®
converts
the hydroxyl group of the alcohol into a better
leaving group by protonation. The ether then is
formed by the nucleophilic attack of alcohol on
the oxonium ion in a bimolecular reaction (S
N
2) as
shown in Scheme 6.16.
The water was removed by azeotropic distillation
using toluene as an inert solvent. Dihexyl ether
was formed in about 97% yield at a temperature of
145°C, with similar yields for longer chain alcohols
leading to di(
n
-octyl) ether (95%).
An eco-friendly catalytic route for the preparation
of perfumery-grade methyl anthranilate from
anthranilic acid and methanol has been reported by
Yadav & Krishnan [75]. Among the catalysts studied,
Amberlyst
®
-15 and Indion-130 resins were found to
be the most effective. Other heterogeneous catalysts
such as ZSM-5, Filtrol-24 and dodecatungstophos-
phoric acid were found to be totally ineffective. Ion-
exchange resins (macroreticular) were found to be
the catalysts of choice in the selective synthesis of
monoglycerides from glycerol and oleic acid in the
presence of solid acid catalysts. Ion-exchange resins
were found to give greater selectivity (ca. 90%) com-
pared with zeolites and clays [76]. A number of
studies have appeared recently on the process opti-
misations for a variety of esterification reactions.
Amberlite
®
XH 2071 has been shown to be a very
effective catalyst for the manufacture of methacrylic
acid esters using methacrylic acid with alkyl
t
-butyl
ethers at 55°C, with a high selectivity of 98% [77].
High yields have been reported also in the following
esterification reactions: methyl esterification of
L
-
phenylalanine (and methanol) [78], the direct ester-
ification of 1-butene with acrylic acid [79], acetic
acid and butanol [80], the reaction of glycol ether
acetate with acetic acid [81], esterification of acetic
acid and propyl alcohol [82] and, finally, the
trans
-
esterification of cyclohexyl acrylate with
n
-butanol
and 2-ethylhexanol [83].
We strongly encourage the readers of this chapter
to look at the excellent reviews of Sharma
et al
. and
also the work of Olah
et al
., who have extensively
reviewed the use of ion-exchange resins (macro-
porous, polystyrene based) and perfluorinated resin
sulfonic acids, respectively [55-57].
3.5 Acidic and pillared clays
Very recent accounts of the synthesis and catalytic
applications of acidic and pillared clays have been
described [84,85] and so only a brief description
of these materials will be presented. The reader is
referred also to a number of excellent earlier re-
views [86-88]. Clays have a layered-type structure
wherein the silicates (SiO
4
) form tetrahedral sheets
linked via an octahedral sheet of alumina. Charge
compensation effects (replacing Si with Al) lead to
a net negative charge that is compensated by ions
in-between the sheets (Fig. 6.7).
These interlamellar cations are generally
exchangeable and their amounts indicate the cation-
exchange capacity (CEC) of the clay. Natural mont-
morillonite (one of the most common clays) has
limited activity. These clays are often acid activated
by treatment with acids such as sulfuric acid. This
results in a change in the surface area, porosity and
the type and concentrations of the ions in the
exchange sites. The acidity is due to either the free
acid, e.g. in acid-treated clays, or the dissociation of
O
Si
O
O
O
Si
O
CH
2
CH
2
CH
2
SO
3
H
O
Scheme 6.15
Scheme 6.16
