Water-tolerant zeolite catalyst for the acetalisation of glycerol

Article abstract of DOI:10.1039/b813564a

We studied the acid-catalysed reaction of glycerol with aqueous formaldehyde and acetone in absence of solvents and using heterogeneous catalysts. The reactivity of acetone was usually higher than formaldehyde and the glycerol conversion was over 90% within 40 min of reaction time for all the heterogeneous acid catalysts studied. With aqueous formaldehyde solution, the glycerol conversion was within 60 to 80%, depending upon the acid catalyst used (Amberlyst-15, K-10 montmorillonite, p-toluene-sulfonic acid), due to the high amount of water in the reaction medium, which shifts the equilibrium and weakens the acid sites. However, the use of zeolite Beta, with Si/Al ratio of 16, leads to a conversion of over 95% within 60 min of reaction time. The hydrophobic character of the zeolite, due to the high silicon content, prevents the diffusion of the water to the interior of the pore, preserving the strength of the acid sites. In addition, the water formed during the acetalisation is expelled off from the pore environment, impairing the reverse reaction, and avoiding the use of hazard solvents, commonly employed to distil off the water formed.

Full text of DOI:10.1039/b813564a

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www.rsc.org/greenchem | Green Chemistry
Water-tolerant zeolite catalyst for the acetalisation of glycerol
Carolina X. A. da Silva, Valter L. C. Gon c¸ alves and Claudio J. A. Mota*
Received 6th August 2008, Accepted 14th October 2008
First published as an Advance Article on the web 30th October 2008
DOI: 10.1039/b813564a
We studied the acid-catalysed reaction of glycerol with
aqueous formaldehyde and acetone in absence of solvents
and using heterogeneous catalysts. The reactivity of acetone
was usually higher than formaldehyde and the glycerol
conversion was over 90% within 40 min of reaction time
for all the heterogeneous acid catalysts studied. With
aqueous formaldehyde solution, the glycerol conversion
was within 60 to 80%, depending upon the acid catalyst used
acetalisation with acetone and aqueous formaldehyde solution,
under acid catalysis conditions, showing that the proper choice
of a heterogeneous acid catalyst might prevent the use of hazard
solvents.
The reactions were carried out under batch conditions.
Typically, 5 g (54.3 mmol) of glycerol was stirred with 4.8 ml
(65.5 mmol) of acetone or 5.3 ml (65.5 mmol) of aqueous
formaldehyde solution (37%) and a specific mass of the catalyst,
◦
at 70 C (Fig. 1,2). The weight of the catalyst varied to maintain
(
Amberlyst-15, K-10 montmorillonite, p-toluene-sulfonic
1
.5 mmol of acid sites in every reaction. Table 1 shows some
acid), due to the high amount of water in the reaction
medium, which shifts the equilibrium and weakens the acid
sites. However, the use of zeolite Beta, with Si/Al ratio of 16,
leads to a conversion of over 95% within 60 min of reaction
time. The hydrophobic character of the zeolite, due to the
high silicon content, prevents the diffusion of the water to
the interior of the pore, preserving the strength of the acid
sites. In addition, the water formed during the acetalisation
is expelled off from the pore environment, impairing the
reverse reaction, and avoiding the use of hazard solvents,
commonly employed to distil off the water formed.
characterization data, as well as the pre-treatment temperature
used to activate the catalysts. The molar ratio of glycerol to
acetone or formaldehyde was 1 : 1.2. The kinetics of glycerol
conversion was estimated by withdrawing samples at specific
time intervals, followed by gas chromatography coupled with
mass spectrometry analysis. In all experiments, 1,4-dioxane was
used as an internal standard.
Glycerol is formed as a byproduct in the biodiesel production
1
from vegetable oils and animal fat. The forecast for 2010 points
2
out to a global production of 1.2 million tons of glycerol, mostly
coming from biodiesel production. Therefore, it is imperative to
develop new uses for glycerol to prevent environmental problems
and to add value to the biodiesel production chain.
The chemistry of glycerol has attracted the interest of
the scientific community and several reviews have appeared
2,3
in the literature. Most of the studies are concentrated on
4
5
6
7
etherification, hydrogenation, oxidation, and dehydration.
On the other hand, glycerol acetals and ketals have received
considerably less attention. They can find applications as fuel
8
9
10
◦
additives, surfactants and flavours. The glycerol formal,
produced upon the reaction of glycerol with formaldehyde,
finds applications as a disinfectant and solvent for cosmetics
Fig. 1 Kinetics of the glycerol reaction with acetone at 70 C over
various acid catalysts. (ꢀ) Amberlyst-15, (ꢁ) zeolite Beta, (●) K-10,
(
᭺) PTSA, (᭢) HUSY, (ꢀ) zeolite ZSM-5.
11
and medical usage. A drawback in the glycerol acetalisation
is the formation of water, which weakens the acid strength
of the catalyst and favors the reversibility of the reaction.
The use of solvents to distil off the water formed has been
The reactions with acetone afforded only one ketal
Scheme 1), with a five-membered ring, as already reported
(
2
in the literature. Amberlyst-15 acid resin showed the best
performance among the catalysts tested, achieving over 95% of
glycerol conversion within 15 min of reaction time. Zeolite Beta
and K-10 montmorillonite behaved similarly in this reaction,
showing a glycerol conversion of about 90% after 40 min.
All these heterogeneous catalysts showed a better performance
than p-toluene-sulfonic acid (PTSA), a model for homogeneous
catalysis. The ZSM-5 zeolite showed a glycerol conversion of
about 20% after 40 min of reaction time, whereas USY converted
approximately 60% of the glycerol within the same period of
12
employed to increase the yield of the glycerol ketals and
acetals, but this practice is not environmentally recommended,
since most of these solvents, such as benzene, toluene, chlo-
roform and dichloromethane, are hazards to humans. In this
communication, we wish to present the results of glycerol
Universidade Federal do Rio de Janeiro, Instituto de Qu ´ı mica, Av Athos
da Silveira Ramos 149, CT Bloco A, 21941-909, Rio de Janeiro, Brazil
3
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Table 1 Characterization data and pre-treatment temperature of the
with a six-membered ring. The reaction proceeded slower than
catalysts
with acetone, when a same catalytic system was considered. For
instance, in the reaction with acetone catalysed by Amberlyst-15,
the glycerol conversion was over 95% after 15 min. Nevertheless,
after the same period of time, the glycerol conversion was about
Pre-treatment
temperature ( C) (mmol/g) Area (m /g)
Acidity
◦
a
b
2
Catalyst
c
Amberlyst-15
K-10 Montmorillonite 150
Zeolite Beta (16)
Zeolite ZSM-5 (14)
120
4.2
0.5
1.6
1.2
1.9
50
240
633
374
566
5
% in the reaction with formaldehyde solution, using the same
d
catalyst and conditions.
300
300
300
Contrarily to what was found in the reactions with acetone,
Amberlyst-15 acid resin and K-10 Montmorillonite showed
a worse performance than PTSA. They showed a glycerol
conversion around 60% after 60 min of reaction time, whereas
PTSA converted about 80% of the glycerol in 50 min of reaction.
The best catalyst studied for the reaction of glycerol with
formaldehyde solution was zeolite Beta, affording over 95%
conversion after 60 min. This result might be explained in
terms of the water-tolerance of this zeolite. Due to its high
siliceous composition, the zeolite environment is hydrophobic
and prevents the diffusion of the water molecules to the interior
of the pores, where most of the acid sites are located. In the
same way, the water formed upon the acetalisation is expelled
off from the pores, thus preserving the integrity and strength of
the acid sites and minimizing the reverse reaction. This behavior
is typical of high-silica zeolites and has already been observed in
other water-sensitive reactions, such as esterification and olefin
e
Zeolite USY (2.8)
a
◦
b
Rate = 10 C/min. Time in activation temperature = 2 h. Measured
◦
15 c
by n-butylamine adsorption at 150 C.
Rohm and Haas). Brackets stand for the Si/Al ratio. Total Si/Al
Informed by the producer
e
d
(
ratio = 2.8; framework Si/Al ratio = 4.5.
13
hydration. In opposition to this behavior, USY showed almost
no conversion after 60 min of reaction. Although the pore
aperture of USY is as wide as zeolite Beta, its Si/Al ratio is
much lower, leading to a hydrophilic character for this zeolite.
Hence, the water molecules might diffuse and remain adsorbed
inside the pores, weakening the acid sites and explaining the
low conversion observed in the experiments. The behavior of
ZSM-5 might be understood in terms of its pore structure.
Although this zeolite has a high Si/Al ratio, and should be
considered hydrophobic, its pore structure, with a diameter in
Fig. 2 Kinetics of the glycerol reaction with aqueous formaldehyde at
◦
7
0
C over various acid catalysts. (ꢀ) Amberlyst-15, (ꢁ) zeolite Beta,
(
●) K-10, (᭺) PTSA, (᭢) HUSY, (ꢀ) zeolite ZSM-5.
˚
the range of 5.6 A, is too narrow to allow the formation of
the glycerol acetals and ketals. This property is known as shape
14
selectivity, and is typical of medium-pore zeolites. As observed
in the reaction with acetone, ZSM-5 presented a low conversion
for the acetalisation of glycerol with aqueous formaldehyde. This
reinforces the hypothesis that the reaction takes place at the
external surface of this zeolite, and therefore, the acid sites are
weakened by the presence of the water molecules, explaining the
low conversion observed.
We also tested the effect of catalyst loading. For the reactions
with acetone, an increase of the catalyst loading did not
significantly affect the performance, because the conversions
were high enough, especially with Amberlyst-15. However,
in reactions with formaldehyde solution the loading has an
impact in the conversion. After 60 min, the conversion was
Scheme 1 Acid-catalysed acetalisation of glycerol with acetone.
time. The poor catalytic activity of the ZSM-5 zeolite can be
explained in terms of the narrow pore structure, which probably
does not permit the acetalisation to occur inside the pores.
Thus, the reaction takes place at the external surface or at
the pore entrance, leading to a poor yield of the ketal. USY
is an aluminium-rich zeolite, having a hydrophilic character.
Thus, the water formed during the reaction might remain inside
the pores, weakening the acid sites and explaining its worse
performance, compared with zeolite Beta, Amberlyst-15 and
K-10 Montmorillonite.
6
1
5% and 72% for 100% and 150% increase in the Amberlyst-
5 loading, respectively, whereas with the standard loading
The reactions with formaldehyde solution afforded two ac-
etals (Scheme 2), one with a five-membered ring and another
the conversion was 61%. Notwithstanding, this increase in the
number of active sites was not sufficient to level the catalytic
activity of Amberlyst-15 and zeolite Beta. The conversion,
using about one third of zeolite Beta loading, was significantly
higher than that observed with Amberlyst-15. This is another
additional proof of the importance of choosing a proper
zeolite catalyst, with hydrophobic character, to perform this
reaction.
Scheme 2 Acid-catalysed acetalisation of glycerol with aqueous
formaldehyde.
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Scheme 3 Possible mechanistic scheme to explain the product selectivity in the acetalisation of glycerol with formaldehyde and acetone.
The distribution of the two acetals formed in the reactions of
glycerol with aqueous formaldehyde did not change very much
with the catalyst used, being around 70% for the six-membered
ring and 30% for the five-membered ring acetal. We did not
find in the literature a more systematic study to account for the
difference in selectivity between acetone and formaldehyde in
the acetalisation of glycerol. A possible explanation is shown in
Scheme 3. Upon the formation of the hemiacetal or hemiketal,
there could be two different pathways for the two systems.
For acetone/glycerol hemiketal, dehydration yields a tertiary
carbenium ion, also stabilized by resonance with the non-
bonded electron pairs of the adjacent oxygen atom. Then, there
occurs a rapid nucleophilic attack of the secondary hydroxyl
group to form the five-membered ring ketal. As the lifetime
of the carbenium ion in the reaction medium is supposed
to be short compared with the lifetime of the hemiketal, the
product distribution is governed by kinetics, which favors the
formation of the less thermodynamically stable five-membered
ring transition state, as already observed in other cyclisation
The results of glycerol acetalisation with acetone and
formaldehyde solution shows that the choice of the proper
heterogeneous catalyst might prevent the use of hazardous
solvents, such as benzene and chloroform, normally used to distil
off the water formed and shift the equilibrium. This procedure
might be employed in other reactions, where water is present in
the reaction medium or is formed during the reaction and affects
the catalyst activity.
Acknowledgements
Authors thank Repsol/YPF, FINEP, CNPq, FAPERJ and
PRH/ANP for financial support.
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