Solvent-free heteropolyacid-catalyzed glycerol ketalization at room temperature

Article abstract of DOI:10.1039/c4ra17090c

Currently, glycerol has been produced in large amounts as a biodiesel co-product. Therefore, developing processes to convert it into more valuable chemicals has attracted significant attention. Glycerol ketals are compounds useful as synthesis intermediates, fragrance ingredients, and mainly bioadditives for diesel and gasoline, and have been produced from reactions catalyzed by mineral acids. In this work, we assessed the activity of H₃PW₁₂O₄₀ heteropolyacid on glycerol ketalization with different ketones at room temperature and in the absence of an auxiliary solvent. The effects of the principal reaction parameters such as the reactant stoichiometry, catalyst concentration, reaction temperature, and type of carbonylic reactant were investigated. H₃PW₁₂O₄₀ heteropolyacid was much more active than other Br?nsted acid catalysts evaluated (i.e. H₂SO₄, p-toluenesulfonic acid, H₃PMo₁₂O₄₀ or H₄SiW₁₂O₄₀) and exhibited high selectivity toward five-membered (1,3-dioxolane) cyclic ketals. Although homogeneous, the heteropolyacid catalyst could be recovered and reused without a loss of activity.

Full text of DOI:10.1039/c4ra17090c

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triglycerides + alcohol
O
O
O
O
HO
HO
biofuel
+
H3PW12O40
H3PW12O40
OH
HO
OH
OH
OH
H3PW12O40
H3PW12O40
H3PW12O40
O
O
O
O
OH
O
O

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DOI: 10.1039/C4RA17090C
Journal Name
RSCPublishing
ARTICLE
Solvent free heteropolyacidꢀcatalyzed glycerol
ketalization at room temperature
Cite this: DOI: 10.1039/x0xx00000x
M.J. da Silva*, A. A. Julio and F. C. S. Dorigettoa
Currently, the glycerol has been produced in large amounts as biodiesel coꢀproduct. Therefore,
to develop processes to convert it to more valuable chemicals has attracted significance
attention. Glycerol ketals are compounds useful as synthesis intermediates, and fragrances
ingredient and mainly, bioadditives for diesel and gasoline, and have been produced trough
reactions catalyzed by mineral acids. In this work, we assessed the activity of H3PW12O40
heteropolyacid on glycerol ketalization with different ketones at room temperature and in the
absence of an auxiliary solvent. The effects of the principal reactional parameters such as the
reactants stoichiometry, catalyst concentration, reaction temperature, and type of carbonylic
reactant were investigated. H3PW12O40 heteropolyacid was much more active than other
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Brønsted acid catalysts evaluated (i.e. H2SO4,
pꢀtoluenesulfonic acid, H3PMo12O40 or
H4SiW12O40) and exhibited high selectivity toward fiveꢀmembered (1,3ꢀdioxolane) cyclic
ketals. Although homogeneous, heteropolyacid catalyst could be recovered and reused without
loss activity.
highlight because have acidity and redox properties, which becomes
it useful catalysts for the different reactions [17,18]. Solid matrixes
Introduction
supported heteropolyacids such as niobium, zeolites, zirconium,
silica or active carbon have been used in esterification,
etherification, acetylation, dehydration or ketalization of glycerol
[19ꢀ22].
Despite successful of the solid supported heteropolyacids
catalysts, the synthesis procedures are sometimes laborious and
required additional costs with thermal treatment which in general
consumption long time. In addition, the solidꢀcatalyzed glycerol
ketalization reactions have been performed in the presence of
auxiliary solvents at their respective reflux temperatures resulting in
an increase on process cost [23].
On this regard, da Silva et al circumvented this difficulty
proposing the use of commercial heteropolyacid without any
previous treatment in homogeneous conditions to esterify free fatty
acids or even glycerol [24,25]. On those processes, heteropolyacid
catalyst could be recovered after reaction ends throughout the
solvent distillation followed by the filtration of solid heteropolyacid
remaining.
In general, glycerol ketalization has been carried out
heating reactions to reflux temperature of the solvent, using liquid
mineral acid catalysts as H2SO4, or organic such as pꢀtoluenesulfonic
acid, and ketone excess [26,27]. Alternatively, solid acids have
successfully catalyzed these reactions [28]. However, besides
laborious synthesis, sometimes water produced during reaction may
result in the catalyst leaching [29].
The biodiesel industry has growing around wide world,
producing glycerol surplus, motivating then that new processes will
be developed aiming valorize this strategic feedstock [1]. The
glycerol is a platformꢀmolecule of biorefineries and derivatives as
1,2ꢀpropanediol, acrolein, glyceric and tartronic acids have been
produced at industrial scale since long time ago. However, the
market of these products is enable to absorb the glycerol excess
currently generated as biodiesel coꢀproduct [2]. Ketals, acetais, as
well as glyceryl esters are valuable ingredients for diesel formulation
and gasoline and may answer this demand [3,4]. Nowadays, all these
reactions have been carried out via a homogeneous catalytic process
using mineral acids (i.e. H2SO4, HCl), which are unfriendly to the
environment because are no recyclable and corrosive liquid Brønsted
acids [5]. Therefore, to develop alternative processes to synthesize
glycerol ketals, using catalysts that are less impacting to the
environment is one of the great challenge to be overcomes by
industry.
Although laborious synthesis, solid acids are an attractive
option to the homogenous catalysts. Molybdenum promoted solid
catalysts (i.e. MoO3/ ZrO2 or TiO2) were used in glycerol
acetalization reactions with benzaldehyde carried out in different
solvents (toluene, benzene, dichloromethane) at reflux temperature
[6]. Other solid acids such as Amberlystꢀ15 resin, niobium, zeolite
and niobium oxides has been successfully used on glycerol esters
synthesis [7,8].
In this paper, we investigate the H3PW12O40ꢀglycerol
ketalization reactions with different ketones as carbonylic reactants
in solvent free conditions and at room temperature. The effects of the
ketones in the selectivity of the products, in addition to others
reactional parameters such as the reactants stoichiometry,
temperature, catalyst concentration were studied. The H3PW12O40
Among different solid catalysts, heteropolyacids
incorporated to solid matrixes with high surface area has been also
extensively explored to add value to the glycerol [9ꢀ12].
Heteropolyacids are a versatile class of catalysts that have been
successfully used either in homogeneous or heterogeneous catalysis
conditions [13ꢀ16]. In special, Kegginꢀtype heteropolyacids deserves
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activity has compared with other Brønsted acid catalysts. Noticeably,
H3PW12O40 was the most active catalyst and provided a highly The main spectral data of products obtained are displayed as follows
regioselective conversion of glycerol toward the 1,3ꢀdioxolane in according with the structures shown in the Figures 1ꢀ5.
derivatives, regardless of ketone utilized.
6
O
3
5
Experimental
4
O
2
Chemicals
HO
1
All chemicals were purchased from commercial sources.
The three hydrate heteropolyacids (i.e. H3PW12O40, 221856 sigma
code; H3PMo12O40, P4006 sigmaꢀaldrich code, and H4SiW12O40
H2O (n = 12 to 18), 95395 sigmaꢀaldrich code) were acquired from
SigmaꢀAldrich. Glycerol (99 % w/w) and all the ketones (99 % w/w) 13C NMR (75 MHz, CDCl3): δ = 26.68 (Cꢀ5); 25.24 (Cꢀ6); 62.93
Figure 1. (2,2ꢀdimethylꢀ1,3ꢀdioxolanꢀ4ꢀyl)methanol
n
were acquired from SigmaꢀAldrich.
(Cꢀ1); 65.64 (Cꢀ3); 76.08 (Cꢀ2); 109.39 (Cꢀ4).
FTꢀIR (CH2Cl2 ꢀ νmáx /cmꢀ1): 3428; 2986; 2936; 2884; 1372; 1256;
General ketalization procedure
1214; 1156; 1050; 844.
The reactions were carried out in a stirred glass reactor in a
50ꢀmL threeꢀnecked glass flask. Typically, glycerol and ketone at
adequate molar ratio were magnetically stirred at room temperature
and H3PW12O40 catalyst (0.0923 mmol) was then added to solution
(10.0 mL), being then started the reaction. Blankꢀreactions were
carried out at the same conditions with ketone and glycerol but in the
absence of catalyst at different ketone to glycerol molar ratio.
6
5
7
O
O
3
2
HO
1
Reaction monitoring
Figure 2. (2ꢀethylꢀ2ꢀmethylꢀ1,3ꢀdioxolanꢀ4ꢀyl)methanol
The progress reaction was monitored analyzing aliquots
taken off at periodic time intervals by gas chromatography
(Shimadzu GC 17A instrument), equipped with a flame ionization
detector (i.e. FID) and fitted with Carbowax capillary column (30 m
13C NMR (75 MHz, CD3OD): δ = 7.30 and 7.00 (Cꢀ7); 23.09 and
22.01 (Cꢀ5); 31.22 and 32.14 (Cꢀ6); 62.47 (Cꢀ1); 66.22 and 66.03
(Cꢀ3); 76.79 and 76.03 (Cꢀ2); 110.60 and 110.82 (Cꢀ4).
length
x 0.25 mm i.d. x 0.25 mm film thickness). Gas
IV (CH2Cl2 ꢀ νmáx /cmꢀ1): 3436; 2987; 2978; 2936; 2882; 1376;
1248; 1192; 1138; 1080; 878.
chromatographic conditions were as follows: 80 °C (3 min);
temperature rate: 10 °C/ min; final temperature: 260 °C; injector
temperature: 250 °C; detector temperature: 280 °C. The
concentration of the products in the reaction mixture was calculated
from the corresponding chromatographic peak areas. Toluene was
internal standard. The reaction aliquots were kept at low
temperatures (ca. < 253 K) before GC analysis to avoid reaction
progress.
1
OH
2
3
O
O
4
7
9
8
5
6
The conversion rate of reactions were calculated as shown
in Equation 1;
% conversion = 100 ꢀ (Ci/C0) x 100 (Equation 1)
where Ci = glycerol concentration calculated trough glycerol stander
curve (i.e. glycerol concentration versus GC peak areas of glycerol)
and C0 = initial concentration of glycerol.
Figure 3. (1,4ꢀdioxaspiro[4.5]decanꢀ2ꢀyl)methanol
13C NMR (75 MHz, CD3OD): δ = 23.68 (Cꢀ6); 23.84 (Cꢀ8); 25.11
(Cꢀ7); 34.70 (Cꢀ5); 36.35 (Cꢀ9), 62.87 (Cꢀ1); 65.84 (Cꢀ3); 76.18 (Cꢀ
2); 109.72 (Cꢀ4);
Product chromatographic separation and characterization
Reaction products were separated by column
chromatography (silica) using mixtures of hexane, CH2Cl2 and
methanol as eluents and then identified by GCꢀMS spectrometry
(Shimadzu MSꢀQP 5050A mass spectrometer instrument operating
at 70 eV coupled Shimadzu 17A GC).
FTꢀIR (CH2Cl2 ꢀ νmáx /cmꢀ1): 3420; 2936; 2530; 1100.
1
OH
2
3
The products were characterized through 1H and 13C NMR
spectroscopy analyses. The NMRs spectra were taken in CDCl3
solutions, using a Varian 300 spectrometer at 300.13 and 75.47
MHz, respectively. The chemical shifts were expressed as δ (ppm)
relatively to tetramethylsilane (TMS) as internal stander. FTꢀIR
spectroscopy analysis were performed in Varian spectrometer 660,
in dichloromethane solutions.
O
O
4
8
5
6
7
Figure 4. (1,4ꢀdioxaspiroꢀ[4.4]nonanꢀ2ꢀyl)methanol
Spectral data
2 | J. Name., 2012, 00, 1-3
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13C NMR (75 MHz, CDCl3): δ = 23.69 or 23.28 (C6 e C7); 36.51 or amount ketone at room temperature. Conversely, in the blankꢀ
35.99 (C5 e C8); 63.16 (Cꢀ1); 65.38 (Cꢀ3); 75.78 (Cꢀ2); 119.37 (Cꢀ reactions using low molar ratio ketone to glycerol, the system
4).
became cloudy. This different behavior may be attributed to increase
of ketone polarity provoked by dissolution of heteropolyacid, which
favoring the glycerol solubility.
FTꢀIR (CH2Cl2 ꢀ νmáx /cmꢀ1): 3430; 2958; 2874; 1336; 1206; 1106;
Table 1. Effect of glycerol ketone molar ratio on H3PW12O40ꢀ
catalyzed glycerol ketalization with propanonea
1042; 972; 860.
Glycerol
ketone
molar
ratio
1
OH
Products
selectivityb
(%)
Blankꢀ
reaction
Conversionb
(%)
3
5
2
Run
conversiond
O
O
8
6
9
4
nic
7
10
Solke
tal
(%)
Figure 5. (2,2ꢀdiisopropilꢀ1,3ꢀdioxolanꢀ4ꢀyl)methanol
13C NMR (75 MHz, CD3OD): δ = 16.46 (Cꢀ6, Cꢀ7, Cꢀ9 or Cꢀ10);
33.51 or 34.54 (Cꢀ5 or Cꢀ8); 62.75 (Cꢀ1); 68.93 (Cꢀ3); 78.38 (Cꢀ2);
116.42 (Cꢀ4)
1:1
1:4
38
44
58
63
83
85
98
97
97
96
98
97
2
3
3
4
2
3
1
0
<5
5
2
3
4
5
6
1:10
1:15
1:20
1:30
FTꢀIR (CH2Cl2 ꢀ νmáx /cmꢀ1): 3394; 2967; 2937; 2159; 2046; 1471;
1380; 1218; 1118; 1079; 1025; 929.
8
Procedures to recovery and reuse of H3PW12O40 catalyst
10
15
The reactions were carried out in a scale five times greater
than that described in previous section, in order to facilitate the
catalyst recovery. Additionally, H3PW12O40 catalyst concentration
was increased of 1 to 5 mol % aiming maximize the glycerol
conversion. Typically, a solution of glycerol (ca. 31.6 mmol, 2.3
mL) and propanone (ca. 633.2 mmol, 45.9 mL) at 1:20 molar ratio
was magnetically stirred at room temperature; toluene (1.8 ml) and
H3PW12O40 catalyst (5 mol %) were then added to solution (50.0
mL), being then started the reaction. The reaction solution was
evaporated almost until dryness heating in oil bath and under
reduced pressure by using rotatory evaporator. After removing the
ketone and solketal, the solid H3PW12O40 was filtered, washed with
small portions of dichloromethane aiming eliminate the unreacted
glycerol. Afterward, we dried H3PW12O40 under vacuum and heating
to 333 K, to be reutilized it in further catalytic runs. This overall
procedure has repeated for three times.
aReaction conditions: H3PW12O40 (1.0 mol %), room temperature,
magnetic stir, 2 hours.
bDetermined by GC analyses.
cni: complex mixture of not identified minority products.
dReactionsꢀblank carried out in the absence of catalyst
When carrying out reactions in absence of catalyst, we
have noted that even not significant, an increase on ketone amount
resulted in an increase in the glycerol conversion. However, poor
conversions (i.e. lower than ca. 5 %) were achieved regardless
ketone excess (Table 1).
100
Results and discussion
glycerol : ketone
molar ratio
90
1:1
1:4
1:10
1:15
1:20
1:30
80
Effect of the glycerol ketone molar ratio on H3PW12O40ꢀ
catalyzed glycerol ketalization with propanone
70
60
50
40
30
20
10
0
The glycerol molecule is highly functionalized and
consequently the selectivity control is always hard. On the other
hand, the low polarity of some ketones sometimes compromises the
glycerol solubility, requiring that an auxiliary solvent or heating
source have be used. We investigate H3PW12O40 catalytic activity on
ketalization of glycerol with propanone ranging molar ratio glycerol
to ketone of 1:1 to 1: 30. The main results are summarized in Table
1.
0
20
40
60
80
100
120
time (min)
We find out that the presence of H3PW12O40 in the solution
drastically changes the solubility of glycerol in the ketone. In all
catalytic runs, the system was completely homogeneous regardless
Figure 6. Kinetic curves of H3PW12O40ꢀcatalyzed glycerol
ketalization with propanone using different reactants stoichiometrya
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aReaction conditions: H3PW12O40 (1.0 mol %); toluene (7.06 mmol); 343 K during 4 hours [9]. In that case, as observed herein, the
volume (15 mL); 298 K.
H3PW12O40 was also the most active catalyst. However, those
authors verified that regardless heteropolyacid nature, the supported
silica catalysts lost activity after successive cycles of reuse,
provoked by high polarity of reaction medium. Herein, as will be
described in the next section, although homogeneous, we recovered
and reused the soluble heteropolyacid without activity loss.
Therefore, we can neglect this minor effect herein. On
the other hand, the reaction selectivity remained unchanged and did
not affected by ketone excess. Unlikely, the addition the H3PW12O40
catalyst to solution provoked a rapid conversion of glycerol to
solketal (Figure 6).
The profile of kinetic curves shows that maximum
conversion was attained within the first reaction hour. After this
period, only slight increase on conversion is noted. Moreover, the
higher initial rate of reaction were achieved when a higher amount of
ketone was present at reaction beginning.
Within the range of molar ratio studied, the highest
conversion was obtained using 1:30 of glycerol to ketone. However,
a very close conversion was reached using 1:20, consequently, it was
the molar ratio selected to study the other reaction parameters.
Regardless of glycerol to ketone molar ratio, the selectivity
of products remained almost constant during the whole reaction.
Outstandingly, solketal was always the major product in all the
reactions. For this reason, we omitted selectivity curves of products
versus time.
Recovery and reuse of H3PW12O40 catalyst
The catalyst has retained its activity after three successive
recycles (Figure 8). Even after recycling the catalyst three times, the
conversion and reactions selectivity were practically identical to
those observed when fresh catalyst was used (ca. 90%). However,
the washing of catalyst with dichloromethane was inefficient to total
removing of the unreacted glycerol. Consequently, we did not
measuring the recovery rate of catalyst due to its increase of weight.
To circumvent this problem, before recycle runs, we solubilize the
recycled catalyst in propanone. After this procedure, a content of
glycerol lower than 5 % was detected in all the reactions. Thus, we
adjusted the initial amount of glycerol to be added discounting
glycerol already present in solution. We believes that an inevitable
loss of catalyst could occur during recovery steps; nevertheless, this
did not compromises the catalytic activity.
The glycerol ketalization is an acidꢀcatalyzed reversible
reaction that may give two main products: (2,2ꢀdimethylꢀ1,3ꢀ
dioxolanꢀ4ꢀyl)ꢀmethanol (i.e. solketal, 1a) and 1,3ꢀdioxaneꢀ5ꢀ
methanol (1b). These products are resulting of the nucleophilic
attack of the primary and secondary hydroxyl group of glycerol,
respectively, on the carbonylic carbon of propanone (Figure 7).
H+
O
O
OH
O
O
O
+
HO
OH
+
HO
OH
1b
1
1a
convesrion
solketal selectivity
100
Figure 7. Glycerol ketals with propanone.
Products with 1,3ꢀdioxolaneꢀtype carbon skeletal (i.e. 1a)
80
60
40
20
0
are formed throughout reaction between neighboring hydroxyl
groups present on glycerol (i.e. 1 and 2 positions) and the carbonyl
group of the ketone. Otherwise, product 1b (i.e. 1,3ꢀdioxaneꢀtype
carbon skeletal) are formed via reaction of glycerol hydroxyl groups
placed at 1 and 3 positions and carbonyl of the ketone. Different of
the reactions of glycerol acetalization with aldehydes, which
commonly result in an equimolar mixture of five and sixꢀmembered
cyclic products, the glycerol ketalization provided preferentially 1,3ꢀ
dixolane fiveꢀmembered ring product (i.e. 1a).
fresh
first
second
recycles
third
The lower stability of sixꢀmembered (1,3ꢀdioxane)
compared to fiveꢀmembered (1,3dioxolane) is attributed to repulsion
between the methyl group in the axial position, and the two
hydrogen atoms in the others axial positions of the six members ring.
Conversely, when has a fiveꢀmembered ring, this repulsion is
significantly lower [26].
Figure 8. Conversion and selectivity achieved after recovery and
reuse of H3PW12O40 catalyst.a
aReaction conditions: glycerol (31.6 mmol); propanone (633.2
mmol); catalyst concentration (5 mol %); toluene (1.8 mL); volume
(50 mL).
If we compare these results to ones achieved in solid
supported heteropolyacidꢀcatalyzed glycerol acetalization reactions,
we may realize some positive points of this homogeneous process.
Undoubtedly, the reusability of heterogeneous catalyst is an aspect
relevant, because it makes the process lesser pollutant than
homogeneous reactions in addition to be more attractive
economically. However, sometimes it is require more drastic
reaction conditions or longer reaction times. For instance,
Castanheiro et al assessed the activity of supported silica Keggin
heteropolyacids (i.e. H3PW12O40, H4SiW12O40 and H3PMo12O40), and
they found that the best result was attained heating the reaction to
Effect of the H3PW12O40 concentration on glycerol ketalization
with propanone
The effect of the catalyst concentration on conversion and
reaction selectivity was investigate within the range of 0.10 to 1.00
mol % (Table 2). Indeed, the catalyst concentration do not affect the
conversion reached in the reaction equilibrium. However, our
intention herein was assess what the minimum concentration
required to achieve maximum conversion within reaction time equal
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to 2 hour. To accurately determining the effect of catalyst
concentration on reaction rate it is require carry out the
4
1.00
90
97
3
aReaction conditions: glycerol (9.23 mmol); propanone (184.6
mmol); toluene (7.06 mmol; 0.75 mL); volume (15 mL); room
temperature (ca., 298 K); reaction time (2 h).
measurements within initial period of the process, where
a
decreasing on reactant concentration should not affect the reaction
rate. Nevertheless, as showed in the Figure 9, the glycerol
ketalization is a fast reaction and within the first 30 minutes, a high
conversion is achieved, regardless catalyst concentration employed.
After this period, the reaction rate decrease due to glycerol
consumption.
High regioselectivity toward 1,3ꢀdioxolane product (i.e. solketal)
was attained in all reactions, regardless catalyst concentration used.
In addition, as displayed in the Fig. 9, highest reaction initial rate
was obtained when 1.0 mol % of H3PW12O40 was employed.
Effect of the catalyst nature on glycerol ketalization with
propanone
100
90
80
70
60
To compare the activity of the H3PW12O40 with other
Brønsted acid catalysts, the reactions of glycerol ketalization with
propanone were carried out using equal hydrogenionic concentration
(ca. 3 mol % H+) at glycerol to ketone molar proportion 1 to 20 at
room temperature (Table 3).
Regardless of acid catalyst, a high solketal selectivity was
obtained in all catalytic runs. Otherwise, the conversion achieved
was highly dependent of kind of catalyst. The acid strength of
heteropolyacid catalysts depends on their composition; to replace
W(VI) by Mo(VI), or P(V) by Si(IV) result in a decrease of the
acidity strength [13,14]. Therefore, it was expected that this aspect
might affect the activity of these catalysts. On the other hand, it has
known that species present in the reaction solution may affect the
ionization of these acid catalysts. For instance, the water provokes a
levelling effect on these catalysts that make them totally ionizable;
however, their pKa values are drastically changed when other
solvents are employed [31].
50
H3PW12O40 concentration
40
0.10 mol %
0.25 mol %
0.50 mol %
1.00 mol %
30
20
10
0
0
20
40
60
80
100
120
time (min)
Figure 9. Kinetic curves of H3PW12O40ꢀcatalyzed glycerol
ketalization with propanonea
aReaction conditions: glycerol (9.23 mmol); propanone
(184.6 mmol); toluene (7.06 mmol); volume (15 mL); 298 K.
In general, notwithstanding Brønsted acid catalyst, the
reactions rates obtained after the 60 minutes of reaction were
significant lower than ones measured within initial period. This fact
can be once more attributed to decreasing on glycerol concentration.
Results showed in Table 3 and Figure 10 indicate that heteropolyacid
It is important to highlight that, although the kinetic curves
of Figure 9 were achieved by using different initial concentration of
catalyst in relation to the glycerol, this later not remained constant
throughout the whole reaction. However, as glycerol is the limiting
reactant and the ketone was used in large excess, the change in the
proportion glycerol/ catalyst was higher than ketone/ catalyst. For
these reasons, the profile of kinetic curves show that after the first 30
minutes reaction, the reaction rate becomes significantly lower than
ones observed at initial period initial of process. We expected that a
linear dependence of reaction rate, as described in heteropolyacidꢀ
catalyzed esterification reactions [30]. However, it is not clear in
Figure 9.
catalysts activity follow the sequence: H3PW12O40 > H3PMo12O40
>
H4SiW12O40. This result is in according with acid strength reported
in literature [13,14]. Isumi et al by using Pope’s method reported
that the acidity strength of these heteropolyacids accompany the
same sequence [32]. They verified that when the heteropolyanion
structure is unchanged (i.e. Keggin structure), a decreases on charge
of the anion result on increases of acid strength of the catalyst. Thus,
we initially think that heteropolyanion type may affect these
reactions.
On the other hand, Misono et al discussed a possible
formation of complexes between cationic organic intermediates and
heteropolyanions [33]. They verified that in reactions involving
alcohol substrate, the heteropolyanion softness that could have
essential role on stabilization of the charged intermediates is
insignificant. Consequently, only acid strength of heteropolyacids
may explain their catalytic activity.
Table 2. Effect of catalyst concentration on conversion and
selectivity of glycerol ketalization with propanonea
Entry
H3PW12O40
concentration
(mol %)
Conversion
Products selectivity
(%)
(%)
The mechanism of Brønsted acidꢀcatalyzed glycerol
ketalization involve an initial protonation of carbonyl group by acid
catalyst. The attack of terminal hydroxyl group of glycerol on
carbonylic carbon atom of protonate ketone, result in the formation
of a hemicetal (i.e. protonate oxonium intermediate), which after a
rearrangement produce a tertiary carbocation intermediate. The
neighbor hydroxyl group attacks the tertiary carbocation giving a
cyclic ketal and eliminating water. We believe that in glycerol
ketalization, there are no complexes between heteropolyanions and
Solketal
98
Others
1
2
3
0.10
0.25
0.50
65
68
75
2
3
2
97
98
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C4RA17090C
these charged intermediates and that the catalyst efficiency depends aReaction conditions: glycerol (9.23 mmol); propanone (184.6
on its acid strength.
mmol); catalyst concentration (3 mol % H+); toluene (7.06 mmol);
volume (15 mL); 298 K.
Conversely, the low activity of sulfuric acid may be
attributed to the use of an H2SO4 aqueous solution (i.e. 97ꢀ99 w/w),
what resulted in the introduction of a water additional amount who
might to shift the reaction equilibrium toward the reactants. Early,
we have verified the high activity of pꢀtoluenesulfonic acid in
solketal synthesis [34].
Effect of the temperature on H3PW12O40ꢀcatalyzed glycerol
ketalization with propanone
In general, we find out that these reactions are little
sensible to temperature variation of 298 to 338 K (Figure 11).
Although of low difference between final conversions obtained, it
can be noted that a decrease on temperature seem to enhance
efficiency of reaction, resulting in slightly superior conversions.
Table 3. Effect of catalyst nature on conversion and selectivity of
glycerol ketalization with propanonea
100
80
Entry
Catalyst
Conversion
Products selectivity
(%)
(%)
Solketal
Others
60
288 K
298 K
318 K
338 K
1
2
H3PW12O40
H3PMo12O40
H4SiW12O40
H2SO4
83
41
40
31
76
98
98
98
98
98
2
2
2
2
2
40
20
0
3
0
20
40
60
80
100
120
time (min)
4
5b
PTSA
Figure 11. Kinetic curves of H3PW12O40ꢀcatalyzed glycerol
ketalization with propanonea
aReaction conditions: glycerol (9.23 mmol); propanone (184.6
mmol); catalyst concentration (3 mol % H+); toluene (7.06 mmol);
volume (15 mL); room temperature (ca. 298 K); reaction time (2 h).
aReaction conditions: glycerol (9.23 mmol); propanone (184.6
mmol); catalyst concentration (3 mol % H+); toluene (7.06 mmol);
vol. (15 mL).
bPTSA = pꢀtolenesulfonic acid
To attempt verify if the reaction conversions may be increased
with a decreasing on temperature, we carry out reactions at
temperatures lower than room temperature (i.e. 288 K).
Although the initial rate of reaction has been lower if compared
the those obtained at higher temperatures, after two hours
reaction carried out at 288 K was achieved the highest
conversion. It is suggestive that the reaction has an exothermic
character.
In organic solvents, the pꢀtoluenesulfonic acid is weaker
than sulfuric acid or Keggin heteropolyacids [14]. Nevertheless, it is
possible that different of other reactions studied, in pꢀtoluenesulfonic
acidꢀcatalyzed reactions may exist van der Walls strengths between
the substrates (i.e. ketone, glycerol) and organic catalyst molecule
favoring interactions that resulting in a high glycerol conversion.
H3PW12O40
p
-toluenesulfonic acid
Effect of the ketone nature on solventless H3PW12O40ꢀcatalyzed
glycerol ketalization at room temperature
100
90
80
70
60
50
40
30
20
10
0
H3PMo12O40
H2SO4
H4SiW12O40
Acidꢀfunctionalised solid catalysts materials have been
used in the solventꢀfree acetalisation of glycerol with propanone to
produce
2,2ꢀdimethylꢀ1,3ꢀdioxolaneꢀ4ꢀmethanol
(solketal),
nevertheless, heating reaction to propanone reflux temperature
[35,36]. Herein, this reaction has extended to other ketones. Unlike
of cyclic ketones, aliphatic ketones with carbon chain containing
more than five carbon atoms are immiscible with glycerol.
Consequently it is difficult determine the reaction conversion and
build the kinetic curves. For this reason, their results were omitted in
the Figure 12.
0
20
40
60
80
100
120
time (min)
Figure 10. Kinetic curves of Brønsted acidꢀcatalyzed glycerol
ketalization with propanonea
6 | J. Name., 2012, 00, 1-3
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Journal Name
View ArticAle ROnTliInCeLE
DOI: 10.1039/C4RA17090C
Ketal
select.b
(%)
Ketalc
Conv.b
(%)
Run
1
Ketone
100
90
80
70
60
50
40
30
20
10
HO
O
O
propanone
90
97
propanone
butanone
cyclopentanone
cyclohexanone
HO
O
2
3
butanone
74
98
99
99
O
HO
O
0
0
cyclohexa
none
20
40
60
80
100
120
O
Reaction time (min)
Figure 12. Kinetic curves of H3PW12O40ꢀcatalyzed glycerol
HO
O
ketalization with different ketonesa
cyclopent
anone
O
4
88
95
47
aReaction conditions: glycerol (9.23 mmol); propanone (184.6
mmol); catalyst concentration (3 mol % H+); toluene (7.06 mmol);
volume (15 mL).
HO
O
Cyclohexanone was highly reactive and was almost totally
converted to ketal (i.e. 1,4ꢀdioxaspiro [4.5] decaneꢀ2ꢀyl)methanol)
(Table 4). It may be attributed to low hindrance steric on carbonylic
carbon. Although cyclopentanone has also the same effect and
consequently display high reactivity, the cyclohexanone has an extra
destabilizing effect (Figure 12).
O
4ꢀmethylꢀ
2ꢀ
pentanone
(trans)
5d
ꢀ
HO
O
There is a torsional strain triggered by hydrogens on
carbon atoms neighbor to carbonyl carbon (i.e. C5 and C9 carbon
atoms) in the chair conformation of cyclohexane ring, which are
eclipsed by the carbonyl oxygen [37]. This effect do not exist in the
cyclopentanone.
45
78
O
(cis)
HO
O
2,4ꢀ
dimethylꢀ
3ꢀ
6d
ꢀ
O
Table 4. Effect of ketone nature in the solvent free H3PW12O40ꢀ
catalyzed glycerol ketalizationa
pentanone
aReaction conditions: glycerol (9.23 mmol); H3PW12O40 catalyst
(0.0923 mmol); ketone (184.76 mmol); 298 K.
kind of carbonylic substrate were assessed. It was observed that
conversion was dependent on the catalyst concentration and the
amount of ketone used. Emphasis was given to the study of solketal
synthesis using different catalysts. Among catalysts studied,
H3PW12O40 heteropolyacid was the most active. The reactions were
carried out at room temperature and without solvent. The
combination of inexpensive raw material (i.e. glycerol) and mild
reactions conditions (i.e. room temperature), in addition to absence
of solvent are positive aspects of these synthesis procedure.
Although used in homogeneous conditions, H3PW12O40 catalyst was
recovered and reused without loss activity.
bDetermined by GC analyses; in addition to ketal, it was formed a
variable amount of not identified minority products.
cDetermined by GCꢀMS, NMR and FTꢀIR spectroscopy analyses.
dThe glycerol was insoluble in the ketone, it was not possible assure
that reagent molecules were in contact; therefore, we do not
measured the glycerol conversion.
The butanone was lesser reactive than propanone, that may
be consequence of higher carbonic chain, however, the catalytic tests
with linear ketones that could demonstrate this hypothesis were
compromised by solubility problems with the glycerol. Although
conversion has not been measured, the ketals formed in these
reactions are soluble and can be detected by GC or GCꢀMS analyses
(Table 4). In general, regardless structure of ketone, the reaction
selectivity was always directed toward 1,3ꢀdioxolane fiveꢀmembered
products.
Acknowledgements
The authors are grateful for the financial support from CAPES,
CNPq, and FAPEMIG (Brazil).
Notes and references
aGrupo de Catalise Homogênea
e Heterogênea, Departamento de
Química, CCE, Universidade Federal de Viçosa, Viçosa, MG, Brazil,
36570ꢀ900.
Conclusions
In conclusion, H3PW12O40 heteropolyacid was an efficient
and regioselective catalyst for the synthesis of glycerol ketals (i.e.
1,3ꢀdioxolane fiveꢀmembered ring products) from cyclic and acyclic
ketones. Effects of reactants stoichiometry, catalyst concentration,
*Corresponding author: silvamj2003@ufv.br; Fax: +55 31 3899 3065,
Tel: +55 31 3899 3210.
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C4RA17090C
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