Highly selective synthesis under benign reaction conditions of furfural dialkyl acetal using SnCl2 as a recyclable catalyst

Article abstract of DOI:10.1039/c9nj01284b

A new and mild route of furfural acetalization with various alkyl alcohols catalyzed by cheap and simple SnCl₂ has been developed. This process consists of the conversion of furfural to alkyl acetals under benign and mild reaction conditions (i.e., room temperature, without solvent, recyclable catalyst), achieving a very good selectivity (97-100%) and almost complete conversion of furfural. Various tin(ii) salts were used as catalysts for the upgrading of furfural to alkyl acetal in an alcoholic solution at room temperature. SnCl₂ was the most active and selective catalyst toward furfural diethyl acetal. Tin(ii) chloride is a commercially available and water tolerant Lewis acid and was demonstrated to be an efficient and recyclable catalyst for the synthesis of furfural alkyl acetal. The effects of the main variables of the reaction such as the catalyst load, temperature, reaction time and alcohol nature were assessed. SnCl₂ was easily recovered and reused without loss of activity and selectivity.

Full text of DOI:10.1039/c9nj01284b

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The role of atropisomers on the photo-reactivity and fatigue of
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Highly selective synthesis under benign reaction conditions of
furfural dialkyl acetal using SnCl2 as a recyclable catalyst
Márcio José da Silva*, Milena Galdino Teixeira1 and Ricardo Natalino1
Received 00th January 20xx,
Accepted 00th January 20xx
A new and mild route of furfural acetalization with various alkyl alcohols catalyzed by the cheap and simple SnCl2·has been
developed. This process consists in the conversion of furfural to alkyl acetals under benign and mild reaction conditions
(i.e., room temperature, without solvent, recyclable catalyst), achieving a very good selectivity (97–100%) and the almost
complete conversion of the furfural. Various tin(II) salts were used as catalysts to the upgrading of furfural to alkyl acetal in
an alcoholic solution at room temperature. SnCl2 was the most active and selective catalyst toward furfural diethyl acetal.
Tin(II) chloride is a commercially available and water tolerant Lewis acid and demonstrated to be an efficient and
recyclable catalyst for the synthesis of furfural alkyl acetal. Effect of main variables of reaction such as catalyst load,
temperature, reaction time and alcohol nature were assessed. SnCl2 was easily recovered and reused without loss of
activity and selectivity.
DOI: 10.1039/x0xx00000x
of gasoline fractions, and therefore they have antiknock
properties [14].
1. Introduction
Furfural is as an platform molecule of sustainable origin,
Increased energy demand and consciousness of environmental
damages related to the pollutant emissions by the fossil-fuels
burning have encouraged the search for alternative routes to
reduce the dependence of chemicals and fuels fossil origin [1].
The use of bioadditives such as biomass-derived oxygenated
compounds may to reduce these emissions and to improve the
main physicochemical properties of fuels [2,3]. Recently,
extensive efforts have been made to obtain fuel additives from
renewable and sustain
nable biomass resources. Alkoxymethyl furfurals synthesized
from etherification reactions of 5-hydroxymethyl)furfural (5-
HMF) have been proposed as diesel fuel additives and
precursor of the drop-in fuel [4]. Renewable origin alkyl ethers
became an attractive option to replace tetra ethyl lead, which
was prohibited in several countries due to environmental
concerns [5-7].
Acetals are important bioadditives that boost the cetane
number of diesel fuel and reduce toxic exhaust emissions [8,9].
In addition to be a viable route to protect carbonyl
functionalities in organic compounds via reaction with
alcohols, acetalization reactions produce useful compounds in
the multistep synthesis of drugs, cosmetics, food additives,
fragrances and solvents [10-13]. Furfural diethyl acetal is an
analogue of furan that has a boiling point, which is in the range
recognized as one of the most promising raw material for
sustainable production of chemicals and fuels in this century
[15]. The catalytic valorisation of furfural using a simple, low-
cost and recyclable catalyst has been considered as a hot topic
in sustainable chemistry [16,17]. Indeed, in agreement with
Gallezoti, routes to convert biomass-derived such as furfural
should obey the principles of green chemistry to diminish
energy consumption and the waste generation, in addition to
obtain the goal products in high yields through processes with
a low environmental impact [18].
Furfural diethyl acetal can be synthesized in the absence of
a catalyst at room temperature using a small excess of alcohol
(3:1); nonetheless, the process requires long reaction time (ca.
90 days) [19,20]. Currently, acetalization has been
conventionally carried out with homogeneous Brønsted acid
catalysts (i.e., H2SO4, HCl), which lead to a series of problems
associated to the purification of products, a large generation
of effluents and neutralization residues, besides the
equipment corrosion [21,22]. Moreover, these soluble
catalysts are not reusable. To overcome the drawbacks a lot of
attention has been recently focused on the development of
environmentally friendly heterogeneous catalysts [23,24].
Several solid acids have been used as catalysts in
acetalization reactions of furfural; cerium(III) chloride [25],
propyl phosphonic anhydride [26], porous aluminosilicate [27],
melamine-formaldehyde based polymer [28] and cerium
phosphate [29] are some examples. Although benign to the
environment, several supported solid catalysts require a
laborious synthesis and sometimes are deactivated or suffer
leaching problems, mainly in reactions in which water is a by-
product, as in the furfural acetalization.
a. Chemistry Department, Federal University of Viçosa, Viçosa, Minas Gerais state,
Brazil. 36590-0001. E-mail: silvamj2003@ufv.br; Fax: +55-31-3899-3065; Tel: +55-
31-3899-3210
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Therefore, the use of available and affordable commercial Reaction conversions were monitored by UV/ Vis VsipewecAtrtricolesOconlpinye
catalysts may to be an alternative to simplify the reactions and analyses recorded in beam UV/Vis
doubDleOI: 10.1039/C9NJ01284B
a
reduce costs of the routes for transformation of biomass spectrophotometer (Micronal, model AJX-6100PC) [35].
derived to more value chemicals. On this sense, Peng et al. Selectivity and the identification of the products were
assessed the catalytic activity of various Al-salts in the determined by GC-MS analysis (Shimadzu MS-QP 2010 ultra,
synthesis of alkyl levulinates from esterification of furfuryl mass spectrometer, electronic impact mode at 70 eV, coupled
alcohol [30]. Those authors have found that although soluble, to a Shimadzu 2010 plus, GC).
AlCl3 was easily recovered and reused without loss activity.
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Soluble Sn(II) salts have been used as catalysts in
transformations of biomass derivatives (i.e., esterification of
3. Results and Discussion
glycerol and fatty acids) [31,32]. On the other hand, Sn(II)
3.1. General aspects
halides were also active in ketalization reactions of glycerol
In general, the acetalization of furfural follows the line of the
carried out at room temperature. In these last processes,
typical Brønsted acid catalyzed-reactions between alcohol and
although soluble the SnCl2 catalyst could be easily recovered
aldehyde (Scheme 1) [36]. All the reaction steps are reversible,
and reused [33,34].
meaning that acetals are highly unstable. Strategies such as
In this work, several Sn(II) salts were used as acid catalysts
the water removal or using an alcohol excess may to shift the
to acetalize furfural with alkyl alcohols. Ethyl alcohol was
reaction equilibrium toward products.
chosen as a reaction medium because it has a renewable origin
and leads to the useful and green derivative furfural diethyl
acetal. Effects of main reaction parameters were investigated.
H
O
O
O
O
Catalyst
Among the various catalysts, SnCl2 was the most active and
selective toward furfural acetal. We paid special attention to
assess the reusability of the SnCl2 catalyst. Finally, the
efficiency of our developed method was further established
for the synthesis of a variety of furfural alkyl acetal by
performing the reaction with different alcohols.
+
2
+ H2O
OH
O
furfural diethyl acetal
furfural
Scheme 1. Acetalization reaction of furfural with ethyl alcohol
Figure 1 shows an example of how furfural acetalization
reactions were monitored by UV spectroscopy. It is possible to
observe the decay of the furfural absorption band at 272 nm
wavelength and the increase of the characteristic band of the
product at 221 nm. At the end of each period an aliquot was
analyzed by CG-MS spectrometry to characterize the formed
product. In all the reactions the acetal was always the only
product observed.
2. Experimental
2.1. Chemicals
Solvents and reactants used in this work were analytical grade
(ca. 99 wt. %); methyl, ethyl, propyl, isopropyl and butyl
alcohols, which were acquired from Vetec. Furfural was
purchased from Sigma and was freshly distilled under reduced
pressure and stored under an atmosphere of nitrogen prior to
use. Tin(II) salts (i.e., SnCl2, SnBr2, SnF2, Sn(OAc)2) were
acquired from Sigma-Aldrich (ca. 99 wt. %). All the catalysts
and alcohols were used directly without any pretreatment.
3.0
20 min
60 min
100 min
180 min
40 min
80 min
120 min
240 min
2.5
2.0
1.5
1.0
0.5
0.0
2.2. Catalytic runs
Catalytic runs were carried out in a glass reactor (25 mL) fitted
with sampling septum, under magnetic stirrer. Typically, tin(II)
catalyst was stirred and solved in alkyl alcohol, and the reactor
temperature was adjusted to 298 K. After to add the furfural
to the solution, the reaction was started.
To recovery and reuse of catalyst, a simple procedure was
performed. After the end of the reaction, ethyl alcohol was
distillated under vacuum at 323 K and 20 mL of water was
added to the neat containing the suspended catalyst. The
resulting mixture was three times extracted with
dichloromethane (3 x 20 mL). The aqueous phase was
evaporated under heating plate to near dryness and then dried
at room conditions giving the solid catalyst. It was collected,
weighted and reused.
200
250
300
wavelength / nm
350
400
Figure 1. UV spectra obtained along the SnCl2-catalyzed furfural acetalization with ethyl
alcohola
aReaction conditions: Furfural (0.62 mmol); reaction volume (10 mL); catalyst (2 mol %);
temperature (298 K)
3.2. Effect of catalyst on the furfural acetalization with ethyl
alcohol
2.3. Method and techniques
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Thus, based on results of Table 1 we accomplish that either
DOI: 10.1039/C9NJ01284B
acidity of medium or catalyst solubility played a pivotal role in these
The catalytic activity of tin(II) salts (SnCl2, SnBr2, SnF2, Sn(OAc)2)
was evaluated in reactions of furfural acetalization with ethyl
alcohol. In the absence of a catalyst, regardless alcohol excess,
no furfural conversion was observed (Figure 2). Conversely,
SnCl2 or SnBr2-catalyzed reactions achieved a high conversion
(ca. 80 and 70 %, respectively) (Figure 2). For a comparison, a
catalytic test was done with p-toluenesulfonic acid (i.e., a
Bronsted acid). The p-toluenesulfonic acid-catalyzed reactions
presented the highest initial rate and achieved a maximum
conversion within the first 25 min of run. Nonetheless, it is not
a recyclable catalyst.
Previously, we have found that transition metal salt
catalysts may react with acetic acid or alcohols releasing H+
ions in solution [37,38], which can itself to catalyze the furfural
acetalization [35]. To investigate if the SnCl2 catalyst might
react with ethyl alcohol and consequently produce H+ ions, we
carried out pH measurements in all the reactions (Table 1).
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reactions. However, may be possible that there is another key
aspect, that also affect this reaction; remarkably, notwithstanding
the p-toluenesulfonic acid ethanolic solution has presented higher
acidity than SnCl2 solution, the conversion of the reaction catalyzed
by this salt was higher.
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Table 1. Effect of catalyst nature on the acetalization of furfural with ethyl alcohola
Run
Catalyst
Solubility
pH valueb
Conversion
(%)
80
70
21
18
80
76
< 2
1
2
3
4
5
6
7
SnCl2
SnBr2
SnF2
Sn(OAc)2
SnCl4
p-TSA
soluble
partially
insoluble
insoluble
soluble
soluble
-
1.9
1.6
5.9
6.5
1.2
1.3
7.1
No catalyst
aReaction conditions: furfural (0.62 mmol); catalyst concentration (4 mol%);
temperature (298 K); ethanol (10 mL); reaction time (4 h).
105
p-TSA
SnCl
SnBr
2
2
bThe measurements of pH were done only in the presence of alcohol and catalyst.
90
75
60
45
30
15
0
SnF
Sn(OAc)
blank-reaction
2
2
It is suggestive that SnCl2 itself contribute of anyway to favor
the reaction. To better describe the participation of the Sn2+
and H+ cations in this reaction, a catalytic cycle is depicted in
the Scheme 2.
SnCl2/H
O
SnCl2/H
O
O
H
(II)
ROH
O
H
O
H
O
O
H
R
(I)
Prototropism
(III)
or H+
SnCl2
0
50
100
150
Time / min
200
250
H
SnCl2/H
O
H2O
+
Figure 2. Acid-catalyzed furfural acetalization with ethyl alcohol at room temperaturea.
aReaction conditions: Furfural (0.62 mmol); reaction volume (10 mL); catalyst (4 mol%);
temperature (298 K). Selectivity to furfural diethyl acetal was 100 % in all runs,
determined by GC-MS analyses.
(VI)
O
OR
OR
OR
H
O
H
OR
OR
- HOSnCl2
or
(IV)
O
OR
- H2O
H
O
It is noteworthy that although pH measurements have not
been corrected to an aqueous medium, they are useful to
compare the medium acidity. Table 1 clearly shows that
reactions with lower pH values gave a higher conversion
(entries 1, 2, 5; Table 1). Nonetheless, while the acidity of SnCl2
ethanolic solution was lower than ones SnBr2, the furfural
conversion was higher. It may be assigned to the different
solubility of these catalysts (i.e., SnCl2, soluble; SnBr2, partially
soluble). Indeed, since the SnBr2-catalyzed reaction have
achieved a reasonable conversion, we carried out reactions
trying to recover the solid catalyst by filtration. However, the
recovery rate was very low and therefore it was impracticable.
Table 1 included a catalytic run where SnCl4 was the catalyst
(run 5, Table 1). Although active and equally selective to the
SnCl2, the SnCl4 catalyst is very unstable; the formation of
hydrochloric gas was visible during the reaction, an aspect that
compromise its use.
(V)
H
H
HOSnCl2
ROH
Scheme 2. Catalytic cycle proposed for acetalization of furfural with alcohol in the
presence of cations H+ and SnCl2
Stannous chloride is recognized a Lewis acid and thus we
suppose that Sn2+ cations may coordinate to the carbonyl
group of furfural favoring its attack by the hydroxyl group of
alcohol. Alternatively, the carbonyl may be also protonated,
making the carbon atom more electrophilic and more reactive
to the attack of ethyl alcohol. Thus, we supposed that both
SnCl2 and H+ might be active catalytically species in this
reaction, generating the intermediates depicted in Scheme 2.
The acetalization of furfural is described as a reaction that
proceeds typically through steps of a pseudo-first order
mechanism, where carbocation intermediates are formed,
however, by simplification, they will not be shown in the
Scheme 2 [39,40]. Therefore, after the protonation or
3.3 Mechanistic insights
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aThe amount of SnCl2 was equal in all of systems (ca. 0.0248 mmol)
coordination of SnCl2 to the furfural carbonyl group, an
oxonium ion is then generated (step I). This step result in the
formation of a carbocation, which undergoes a nucleophilic
attack of an alcohol molecule giving a positively charged
tetrahedral intermediate (step II). This intermediate suffers a
prototropism (step III), releasing water or OHSnCl2 (step IV).
This intermediate with an oxygen atom positively charged
View Article Online
DOI: 10.1039/C9NJ01284B
Measurements of hydrogenionic potential of the pure
compounds (i.e., H2O and C2H5OH) and of the solutions (i.e.,
H2O/ SnCl2 and C2H5OH/ SnCl2) were performed (Figure 3a).
Interesting, we have found that the reaction of SnCl2 with ethyl
alcohol releases a greater amount of H+ ions than the reaction
with water. The HSAB theory may explain this result [42]. Since
the Sn2+ cations have a slight character of hard acid due to the
positive charge and its ionic radium, it tends to react with a
hard base. Indeed, the electron donating effect of the ethyl
group makes the C2H5OH a base harder than H2O. Therefore,
SnCl2 react more favorably with C2H5OH, producing a higher
amount of H+ ions and giving a lower pH value. This higher
affinity between SnCl2 and C2H5OH can also explain why the pH
value measured after to add water to the SnCl2 ethanolic
solution was almost constant (Figure 3b).
generate
a carbocation (not shown), which will be
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nucleophilically attacked by another alcohol molecule (step V).
The proton present in this last intermediate will be then
abstracted by OHSnCl2 (i.e., previously formed), generating the
reaction products (i.e., acetal and water) (step VI) and
regenerating the catalyst (i.e., H+ or SnCl2).
It is noteworthy that a Lewis acid center may at the same
time activates both ROH and RCHO molecules (by
coordination) and as consequence, a step of alcohol addition
to the carbonyl carbon may occur as
a very fast
We realize that SnCl2 plays two pivotal roles on the
acetalization of furfural with alcohol; it activates the furfural
carbonyl group favoring the reaction between the two
reactants, as well as it also react with alcohol to release H+ ion,
which is also a catalyst for this reaction.
quasiintramolecular concerted reaction [41]. Nonetheless, as
far we know, no detection of evidence that support this
mechanism was done until now.
Two hypothesis might to explain the presence of H+ ions in
solution; the first, the SnCl2 catalyst might reacts with the
water generated throughout the reaction of acetalization; the
second one, it may react with ethyl alcohol releasing H+ ions.
Undeniably, this last hypothesis was demonstrated by strong
3.4 SnCl2-catalyzed reactions with ethyl alcohol: Effect of catalyst
concentration
decreasing of pH value after the addition of the tin salt to the To verify the effect of catalyst load, it was varied over a range
ethyl alcohol (ca. 7.1 to 1.9, Table 1). The most important is of 0.25 to 4 mol %. The reaction conversions were affected by
that both alcohol and water are present in the reaction the catalyst concentration, meaning that the equilibrium was
medium, and we would like to know how they will to compete not reached. Figure 4 shows that the lowest concentration
by SnCl2 catalyst. Therefore, to check this point, two additional provided the highest conversion at the shortest reaction time.
experiments were performed by us (Figure 3).
4.0 mol %
1.0 mol %
3.0 mol %
0.5 mol %
2.0 mol %
0.25 mol %
90
75
60
45
30
15
0
7
6
5
4
3
2
1
0
H
O
H
O/SnCl
2
C
H OH C
H OH/SnCl
5
2 5 2
2
2
2
(a)
3.0
2.5
2.0
0
50
100
150
200
250
Time/ min
Figure 4. Effects of catalyst load on the SnCl2-catalyzed acetalization of furfural with
C2H5OHa
1.5
1.0
aReaction conditions: Furfural (0.62 mmol); reaction volume (10 mL); temperature (298
K)
An increase of the catalyst concentration enhanced the
initial rate of reaction, however, we observed that up to 2 mol
%, no significant difference was observed in the conversion.
Hence, 2 mol % concentration was chosen to continue our
investigation (Figure 4). Though have been obtained at room
temperature, our results were superior than those obtained in
0
10 20 30 40 50 60 70 80 90 100
Volume of added water / L
(b)
Figure 3. Measurements of hydrogenionic potential in different systems (a) and
variation of the pH value with the addition of water to the SnCl2 ethanolic solution (b)a
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several solid-catalyzed reactions; for instance, in CePO4-
catalyzed reactions carried out at reflux temperature, 79 % reactions was affected by the increaseDoOfI: 1t0e.m10p39e/rCa9tNuJr0e1,28th4Be
yield of furfural dimethyl acetal was achieved after 6 h [29]. final conversions were very close (ca. 62 to 67 %). When
The main observation done is that while the initial rate of
View Article Online
Figure 5 describes how the pH value was affected by the carried at a higher temperature, the effective collisions
increase of the catalyst concentration and what was the effect number is enhanced, consequently, the formation of products
on the furfural conversion. Its reasonable suppose that after (i.e., furfural diethyl acetal and water) is more favored.
1.9 mol % concentration, only a slight excess of H+ ions were Nevertheless, it is possible that in the presence of a higher
generated, which was insufficient to cause a decrease of pH amount of water, the reaction equilibrium may be shifted
value, although the conversions have continued to show a toward reactants. It can explain because at higher conversions,
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slight growth trend.
the temperature effect was less pronounced.
3.6 Effect of alcohol on the SnCl2-catalyzed furfural acetalization
90
75
60
45
The performance of SnCl2 catalyst in furfural acetalization with
different alcohols was assessed at room temperature (ca. 298
K, Figure 7). The size of alcohol carbon chain as well as the
steric hindrance on hydroxyl group are the main characteristics
that may affect the reactivity of alcohol in acetalization
reactions [35,36]. Figure 7 shows that even at room
temperature the methyl alcohol quickly react with furfural,
while the reactions of other alcohols were slower (Figure 7).
pH 1.9 pH 1.9
pH 1.9
pH 2.1
pH 2.2
30
pH 2.4
15
90
75
60
45
30
15
0
methyl alcohol
0
2.00
SnCl2 catalyst concentration/ mol %
0.25
1.00
3.00 4.00
0.50
ethyl alcohol
Figure 5. Furfural conversion obtained with different SnCl2 catalyst load and the
pH values measureda
aReaction conditions: Furfural (0.62 mmol); reaction volume (10 mL);
temperature (298 K)
propyl
butyl alcohol
isopropyl
3.5. Effect of temperature on the SnCl2- catalyzed furfural
acetalization with ethyl alcohol
The catalytic performance of SnCl2 at different temperatures is
shown in Figure 6. To do cleaner the effect of temperature, we
carried out the reactions with a lower catalyst concentration
(ca. 0. 25 mol %).
0
30
60
90 120 150 180 210 240
Time/ min
Figure 7. Effects of alcohol on the conversion of SnCl2-catalyzed furfural acetalizationa
aReaction conditions: Furfural (0.62 mmol); reaction volume (10 mL); SnCl2 (2 mol %);
temperature (298 K)
70
60
50
Although the two aspects previously mentioned (i.e., steric
hindrance and size of carbon chain) might affect the alcohol
reactivity in furfural acetalization, as demonstrated the
tendency observed in Figure 7 (i.e., methyl > ethyl > propyl >
butyl > isopropyl), we also verified if the pH value may be
correlated with the conversions obtained (Table 2).
40
298 K
303 K
308 K
313 K
30
20
318 K
10
0
0
25 50 75 100 125 150 175 200 225 250
Time/ min
Figure 6. Effects of temperature on the SnCl2-catalyzed furfural acetalization with ethyl
alcohola
aReaction conditions: Furfural (0.62 mmol); reaction volume (10 mL); SnCl2 (0.25 mol%)
This journal is © The Royal Society of Chemistry 20xx
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aReaction conditions: furfural (0.62 mmol); SnCl2 concentraVtiioewn A(r2ticmleoOlnl%in)e;
Table 2. Effect of pH on the SnCl2-catalyzed furfural acetalization with different
alcoholsa
DOI: 10.1039/C9NJ01284B
temperature (298 K); alcohol (10 mL); reaction time (4 h)
5
6
7
8
Run
Alcohol
pH
Valu
ec
8.0
7.1
3.2
4.9
3.3
Conv. Run
(%)
Alcohol
pH
Val
uec
1.5
1.9
1.6
1.9
1.7
Conv.
(%)
Benzaldehyde was more reactive than furfural; however,
although after 4 h of reaction the conversions have been very
close, the initial rate of benzaldehyde reaction was higher than
ones of furfural. In this reaction, benzaldehyde ethyl acetal
was the only product selectively formed.
1b
2b
3b
4b
5b
Methyl
Ethyl
Propyl
Isopropyl
Butyl
< 5
< 2
< 2
0
6
7
8
Methyl
Ethyl
Propyl
Isopropyl
Butyl
100
79
74
22
26
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10
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Conversely, the presence of a hydroxyl group on the
furfural carbon skeletal (i.e., 5-HMF) had an unexpected effect
on his reactivity. Although 5-HMF has quickly reacted with the
ethyl alcohol, achieving a high conversion within first 30 min of
reaction (ca. 60 %), no reaction progress was observed after
this time. This apparent deactivation of catalyst may be
explained by the reaction selectivity. Two main products were
detected in this reaction; 5-(ethoxymethyl)furan-2-
carbaldehyde (5-EMF) and (2) 2-(diethoxymethyl)-5-
(ethoxymethyl)furan (2-DEM-5-EMF). The formation of these
products resulted in a higher water formation (i.e., a co-
product), which may has shifted the equilibrium toward
reactants. In addition, we have found that 5-HMF underwent a
condensation after longer reaction periods; it was confirmed
checking the mass balance of reaction.
< 2
10
aReaction conditions: furfural (0.62 mmol); SnCl2 concentration (2 mol %);
temperature (298 K); alcohol (10 mL); reaction time (4 h)
bIn the absence of catalyst
cThe measurements of pH were done only in the presence of alcohol and catalyst
It is important to highlight that these hydrogenionic potential
measurements are not strictly a way to quantify H3O+ ions,
because the reactions were not performed in aqueous solution
and the values were not corrected. Nonetheless, similarly to
the others pH values reported herein, the glass electrode is
sensible to the H+ ions, regardless the solvent used, therefore,
a comparison of values measured before and after to add SnCl2
for each system is still valid.
The decrease of pH value triggered by the addition of SnCl2
obeyed the sequence; methyl > ethyl > isopropyl > propyl ≈
butyl. Thus, we can conclude that the reactivity of these
alcohols with SnCl2 follows this same tendency. Thus,
comparing the conversion of the reactions and this last trend
we realized that the only exception was isopropyl alcohol,
which was very reactive with SnCl2 but gave only conversion
lower than that reached in reaction of furfural with butyl
alcohol. This behavior is a consequence of high hindrance
steric on the hydroxyl group in the isopropyl alcohol.
3.8. Recovery and reuse of SnCl2 catalyst on the furfural
acetalization with ethyl alcohol
After the removal of ethyl alcohol by distillation under vacuum
at moderate temperature, a neat phase was obtained, which
contain the SnCl2 catalyst and ethyl furfural acetal. A special
careful with temperature avoids that furfural can be
oligomerized. Indeed, it is critical point; at temperatures
higher than 323 K, the neat obtained became dark brown,
indicating the formation of oligomers.
3.7 Effect of aldehyde on the SnCl2-catalyzed acetalization reaction
of ethyl alcohol
recovery rate
furfural conversion
The efficiency of the SnCl2 catalyst was investigated on the
acetalization reactions with ethyl alcohol of other aromatic
aldehydes
80
60
40
20
0
(Figure
8).
Benzaldehyde
and
5-
hydroxymethylfurfural (5-HMF) were the selected aldehydes.
furfural
5-HMF
benzaldeyde
90
75
60
45
30
15
0
1
2
3
number of recycles
Figure 9. Recovery rates and conversion of furfural after successive reuse of SnCl2
catalyst in furfural acetalization with ethyl alcohola
aReaction conditions: furfural (0.62 mmol); SnCl2 concentration (2 mol %); temperature
0
40
80
120
160
200
240
time / h
(298 K); alcohol (10 mL); reaction time (4 h)
Figure 8. Effects of aldehyde on the conversion of SnCl2-catalyzed acetalization
reactionsa
Consequently, the suspension was solved in a mixture
dichloromethane: ethyl acetate, which give the solid catalyst.
6 | J. Name., 2012, 00, 1-3
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13 Y. Sasaki, T. Endo, N. Tanaka and H. Inoue, Green Chem.,
After the steps of filtration, washing and drying, it is already to
be reused. Figure 8 shows that SnCl2 catalyst was successfully
recovered and reused without loss activity (Figure 9).
View Article Online
2009, 11, 27.
DOI: 10.1039/C9NJ01284B
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4. Conclusion
Several tin(II) salts were assessed in furfural acetalization with
ethyl alcohol at room temperature. SnCl2 was the only soluble
tin salt, and was the most active catalyst, achieving a
performance higher than a strong Brønsted acid (i.e., p-
tolunenesulfonic acid). We demonstrated that either Lewis
acidity of SnCl2 and its ability in generate H+ ions from reaction
with alcohols are essential aspects for the performance of
catalyst on these reactions. Based on experimental data, a
mechanism was proposed where both catalysts (i.e., H+ cation
or SnCl2) may promote the key steps of reaction. The SnCl2 was
recoverable from the reaction mixture and three times reused
without loss of activity. The reactivity of other alkyl alcohols
was also studied, and we verified that its reactivity depends on
the steric hindrance of hydroxyl group as well as the size of
carbon chain. This method is based on the inexpensive,
noncorrosive, reusable and commercially available SnCl2
catalyst, which was efficient to convert furfural to alkyl acetals
at room temperature.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors are grateful for the financial support from CNPq
and FAPEMIG (Brazil). This study was financed in part by the
Coordenação de Aperfeiçoamento de Pessoal de Nível
Superior - Brasil (CAPES) - Finance Code 001.
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