Modified Mg–Al hydrotalcite: a highly active heterogeneous base catalyst for
cyanoethylation of alcohols
Pramod S. Kumbhar,† Jaime Sanchez-Valente and Fran c` ois Figueras*‡
Institut de Recherches sur la Catalyse, 2 Av. A. Einstein, 69626 Villeurbanne, France
Modified Mg–Al hydrotalcite (Mg:Al = 3:1) prepared by
thermal decarbonation followed by rehydration of a conven-
tional Mg–Al hydrotalcite is found to be a highly active,
reusable and air stable catalyst for cyanoethylation of
alcohols.
flow of nitrogen saturated with water at room temperature in a
controlled fashion.
The catalytic reactions were carried out using 10 ml of
alcohol and 4 mmol of acrylonitrile. The catalyst after
rehydration was transferred to the reactor containing the alcohol
followed by addition of acrylonitrile. The samples were
analysed by gas-liquid chromatography.
Cyanoethylation of alcohols is a widely used reaction for the
synthesis of drug intermediates and organic compounds of
industrial interest.1 Acrylonitrile undergoes cyanoethylation
with a number of monohydric alcohols to give alkoxypropio-
nitriles which, after hydrogenation, give industrially important
amines. The reaction is catalysed by homogeneous base
catalysts such as alkali hydroxides2 and alkoxides3 and
tetraalkyl ammonium hydroxide. However, these catalysts need
to be neutralised before purification of the product, resulting in
the generation of waste, loss of catalyst and reduced product
yields. Alternatively, use of anion exchange resins as heteroge-
Table 1 shows a comparison of the modified hydrotalcites
rehydrated for different periods of time with fresh and calcined
6
hydrotalcites and MgO (from the results of Hattori ) for the
reaction of acrylonitrile with MeOH in which methoxypropio-
nitrile is the only product. The modified hydrotalcites are much
more active than MgO, which also needs a very high activation
temperature (800 °C in vacuo). The hydrotalcite as such shows
only marginal activity as it contains very few basic sites. It is
interesting to note that even though the calcined hydrotalcite has
a high Lewis basicity, it is only marginally active. This shows
that this reaction is catalysed by weak Br o¨ nsted basic sites
4,5
neous catalyst have also been reported. Recently, Hattori and
6
2
Kabashima reported use of alkaline earth oxides, hydroxides
(OH sites).
and alumina supported KF and potassium hydroxide supported
on alumina as heterogeneous catalysts for this reaction. High
activities were reported for high temperature activated MgO
catalyst (800 °C in vacuo). The general reaction scheme for
cyanoethylation of alcohols is shown in Scheme 1.
Similar to our earlier results for the aldol condensation
reaction, for cyanoethylation the catalytic activity also depends
7
upon the rehydration time. In the present case optimum activity
is observed for the catalyst rehydrated for 6 h (catalyst = 0.1 g,
2
1
2
water saturated N flow = 80 ml min ).
Previously we had reported that modified hydrotalcite,
prepared by thermally decarbonating the conventional Mg–Al
hydrotalcite followed by controlled rehydration, is highly active
for aldol and Knoevenagel condensation reactions.7 The high
activity of this catalyst is attributed to the presence of a large
The reusability of the catalyst was studied by using the same
catalyst after allowing the catalyst to settle, decanting the
supernatanat reaction mixture and continuing the reaction by
introducing fresh reactants (acrylonitrile and MeOH). The
results are sumarised in Table 2. The catalyst was found to be
reusable without appreciable loss in activity.
,8
2
number of OH groups, generated during rehydration of the
thermally activated hydrotalcite, which act as Br o¨ nsted basic
sites. The use of modified hydrotalcites as heterogeneous
Br o¨ nsted basic catalysts instead of homogeneous catalysts such
as alkali hydroxides and alkoxides has a number of advantages,
viz. ease of separation, reusability, no waste, and higher
activities and selectivities (depending on the reaction).
The study was further extended to other monohydric alcohols
i
t
such as EtOH, Pr OH, BuOH and Bu OH. The results are
t
summarised in Table 3. Except for Bu OH, the catalysts are
found to be highly active for all the alcohols, which is in
agreement with the results for the homogeneous catalyst. The
selectivity for the corresponding alkoxypropionitrile was 100%,
In continuation of our above mentioned work here we report
that the modified hydrotalcites having formula [Mg(1 2x)Al
x
-
Table 1 Comparison of various catalysts for cyanoethylation of acrylonitrile
(
OH) ](OH) ·yH O show unprecedented high activity for the
2
x
2
with MeOHa
cyanoethylation of alcohols with acrylonitrile. The activity of
this catalyst is the highest so far reported in the literature for any
heterogeneous catalyst. The catalysts were found to be reusable
without significant loss in activity. The other interesting aspect
of this work is that these catalysts were found to be active even
after exposure to air in this reaction, a rare phenomena for a
solid basic catalyst.
Conditions for
activation of the
catalyst
Conversion of
acrylonitrile
t/min (%)
Catalyst
Mg–Al Hydrotalcite
Mg–Al Hydrotalcite
Modified Mg–Al
Hydrotalcite
used as was
120
120
90
2.5
20
98
N
N
2
flow at 450 °C
at 450 °C rehydration
2
Mg–Al hydrotalcites having Mg:Al = 3:1 was synthesised
for 3 h in wet N
temp.
2
at room
9
using the procedure reported by Miyata et al. The presence of
pure hydrotalcite structure was confirmed by powder X-ray
diffraction. The preparation of modified hydrotalcite includes
Modified Mg–Al
Hydrotalcite
N
2
at 450 °C rehydration
45
99.8
for 6 h in wet N
temp.
2
at room
activation of the hydrotalcite in carbon dioxide free N
2
at
450 °C, followed by cooling and hydrating the material in a
Modified Mg–Al
Hydrotalcite
N at 450 °C rehydration
45 100
2
for 12 h in wet N
room temp.
2
at
Base
MgO
b
800 °C in vacuo
120
98.7
RO
ROH
+
CN
CN
a
Reaction conditions: acrylonitrile (0.04 mol), MeOH (10 ml), 50 °C,
Catalyst (0.1 g). b Results from ref. 6.
Scheme 1
Chem. Commun., 1998
1091