A kinetic study of heteropolyacid-catalyzed furfural acetalization with methanol at room temperature via ultraviolet spectroscopy

Article abstract of DOI:10.1016/j.cattod.2018.11.071

In this work, the ultraviolet spectroscopy was used as a tool to monitor the kinetics of acetalization reaction of furfural with alkyl alcohols. Among the Bronsted acids evaluated, Keggin heteropolyacids (e.g., H₃PW₁₂O₄₀ and H₃PMo₁₂O₄₀) were the more active and selective in the acetalization of furfural in reactions carried out at room temperature. The ultraviolet spectroscopy, a simple and inexpensive analytic technique was used to quantify the furfural and acetals during the reaction progress. The influence of main reaction parameters such as temperature, time, and catalyst load were assessed. The reaction scope was extended to the other alcohols. The activation energy of heteropolyacid-catalyzed reactions was determined.

Full text of DOI:10.1016/j.cattod.2018.11.071

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A kinetic study of heteropolyacid-catalyzed furfural acetalization with
methanol at room temperature via ultraviolet spectroscopy
Milena Galdino Teixeira, Ricardo Natalino, Márcio José da Silva⁎
Chemistry Department, Federal University of Viçosa, Avenue Peter Henry Rolfs, s/n, Viçosa, Minas Gerais State, 36590-000, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this work, the ultraviolet spectroscopy was used as a tool to monitor the kinetics of acetalization reaction of
furfural with alkyl alcohols. Among the Bronsted acids evaluated, Keggin heteropolyacids (e.g., H3PW12O40 and
H3PMo12O40) were the more active and selective in the acetalization of furfural in reactions carried out at room
temperature. The ultraviolet spectroscopy, a simple and inexpensive analytic technique was used to quantify the
furfural and acetals during the reaction progress. The influence of main reaction parameters such as tempera-
ture, time, and catalyst load were assessed. The reaction scope was extended to the other alcohols. The activation
energy of heteropolyacid-catalyzed reactions was determined.
Keggin heteropolyacids
Furfural
Biomass
Acetalization
1. Introduction
concerns. Among the different solid acid catalysts, heteropolyacids
(HPAs) have attracted the interest not only for their architectures, but
Biomass is an abundant and sustainable source of carbon structures
that can be considered as a valuable starting material to produce che-
micals and fuels, which can potentially replace the fossil derivatives
[1,2]. Monomers present in the lignocellulosic biomass have received
great attention in recent years because can generate platform-molecules
of biorefinery such as furfural and 5-hydroxymethylfurfural [3–5].
These compounds are prepared through the dehydration reactions of
sugar residues and are potential candidates to replacing numerous
chemicals fine that presently petroleum derivatives [6].
also for their catalytic properties that can be easily modified by chan-
ging their size, shape, charge and chemical composition [13–15]. HPAs
are emerging as environmentally benign acidic catalysts for various
organic transformations [16,17].
Keggin heteropolyacids are superacid with anions that have general
formulae [XM12O40
]
n−, where generally X is a heteroatom (i.e., P5+
Si4+) and M is an addenda atom (i.e., Mo6+, W6+) [18–20]. HPAs with
a Keggin structure, such as H3PW12O40 and H3PMo12O40, have received
a lot of attention as promising acid catalysts useful for different che-
mical reactions owing to their purely strong Brønsted acidity, which is
higher than homogeneous mineral acid catalysts [21–24].
Among the different processes to convert the furfural to higher
added value chemicals the acetalization reactions have received high-
light because the furfural acetals are useful as bio- additives and are raw
material to synthesis of perfumes, flavors, pharmaceuticals, solvents
and polymers [7,8]. Furfuryl acetals are ingredients in different fra-
grance formulations because at the time of skin contact undergo hy-
drolysis releasing products with pleasant odors [9].
Due to their strong acidity and environmental benignancy they can
be potentially useful in aldehyde acetalization reactions. Literature has
described several works about acetalization reactions where the focus is
to investigate the properties and activity of the solid catalysts, but only
a few proposed a reaction kinetic study [25–29].
Conventionally, Brønsted acids such as H2SO4 or HCl are the cata-
lysts in acetalization reactions. Although these homogeneous mineral
acids showed satisfactory catalytic performance on these reactions, they
are highly corrosive and require steps of neutralization and products
purification, which generate large amount of residue and effluents
[10–12].
In this work, we report a kinetic study of the acetalization reaction
of furfural with different alcohols under mild conditions, employing
H3PW12O40 and H3PMo12O40 as catalysts. UV spectroscopy was the
used to obtain the kinetic data. As far we know, there has been no
attempt to use ultraviolet spectroscopy for the kinetic investigation of
furfural acetalization catalyzed by Keggin HPAs. The activity of these
HPAs was compared to the mineral (H2SO4 and HCl) and organic (p-
toluene sulfonic acid) acid catalysts. HPA catalysts were the most active
and achieved high yields. The main reaction parameters were assessed.
On this sense, to develop highly active solid acid catalysts that are
environmentally friendly is one of the main challenges to be overcome
and become important for economic reasons and environmental
⁎ Corresponding author.
E-mail address: silvamj2003@ufv.br (M.J. da Silva).
https://doi.org/10.1016/j.cattod.2018.11.071
Received 21 May 2018; Received in revised form 1 November 2018; Accepted 30 November 2018
0920-5861/©2018ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:DaSilva,M.,CatalysisToday,https://doi.org/10.1016/j.cattod.2018.11.071

M.G. Teixeira et al.
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2. Material and methods
reacts with another alcohol molecule giving the intermediate (V),
which upon deprotonation forms the acetal (VI) and regenerates the H+
cation (Scheme 1).
2.1. Chemicals
Acetal formation is strongly affected by electronic and steric factors,
but generally it has been accepted that the rate determining step is the
formation of the cation from the protonated hemiacetal [31]. To com-
pensate the low rate of hemiacetal formation, the reaction media must
be sufficiently acidic to promote an effective protonation of any
hemiacetal formed, and sufficiently polar to allow the stabilization of
the cationic intermediate. However, the amount of acid must be con-
trolled since an excess of acid could also protonate the alcohol, dras-
tically reducing its nucleophilicity. In addition, an excess of acid favors
the hydrolysis of the acetal to aldehyde.
All the chemicals were of analytical reagent grade and were used as
such without any further purification, except for furfural. Hydrated
heteropolyacids (e.g. H3PW12O40 and H3PMo12O40; 99 wt. %) were
acquired from Sigma-Aldrich. Alcohols (e.g. methanol, ethanol, pro-
panol, propran-2-ol and butanol) having purity between 99.5 and
99.8 wt. %, were also purchased from Sigma-Aldrich. Sulfuric, hydro-
chloric and p-toluene sulfonic acids (98 wt. %) were Vetec. Furfural was
purchased from Sigma and was freshly distilled under reduced pressure
and stored under an N2 atmosphere prior to use.
As can be observed in the mechanism of acetal formation (Scheme
1), all the steps are reversible; to avoid shifting equilibrium back to
reactants, a strategy commonly used is to remove the water by using
physical and chemical means [32].
2.2. Catalytic runs and products identification
Catalytic tests were carried in a glass reactor (25 mL) fitted with
sampling septum, under magnetic stirrer. In a typical procedure, acid
catalyst and the adequate alcohol were dissolved, and the reactor
temperature was adjusted to 298 K. Then, the reaction was started by
adding of furfural. The reactions were monitored taking aliquots at
regular time intervals (1, 3, 5, 10, 15, 20, 25 and 30 min) and analyzing
them by ultraviolet spectroscopy, in a double beam UV/vis spectro-
photometer (Micronal, model AJX-6100PC).
In this work, we have found that HPA-catalyzed furfural acetaliza-
tion with methanol quickly provided dimethyl acetal (ca. 30 min); since
they were stable products at room temperature, there was no need to
remove water during the reaction.
3.2. Ultraviolet spectroscopy and GC–MS analysis
The main reaction products were identified by GC/MS analyses
(Shimadzu MS-QP 2014 ultra mass spectrometer instrument) operating
at 70 eV, coupled with a Shimadzu 2014 GC.
According to Crowel et al., the absorption bands of ultraviolet
spectra of acetals are significantly different from the spectra of the
parent aldehyde and methanol; therefore, these bands can be used to
quantify these species in solution [33]. The UV spectrum of furfural
have two absorption bands; the more intense at wavelength 272 nm and
the second at 222 nm (Fig. 1sp, Supplemental material). Herein, we
follow the reaction progress measuring the decrease of the absorption
band at 272 nm.
2.3. Kinetic measurements
To quantify the furfural during the reaction, standard solutions
(furfural in alcohol- 0.5–22 ppm) were prepared and their characteristic
absorptions were determined by ultraviolet spectroscopy. For the ki-
netic measurements, an aliquot of the reaction was withdrawn and
diluted 250 times and the absorbance was measured in the UV/vis
spectrophotometer.
On the other hand, the acetal group −CH(OCH3)2 is a totally in-
active chromophore in the ultraviolet spectrum region. Acetals of the
furanoid aldehydes have residual chromophore systems comparable to
that of the methyl furan or its derivatives. For instance, the UV spec-
trum of 2-methylfuran has only one absorption band at wavelength
221 nm (Fig. 1sp).
The absorption band of furfural used to its quantification was placed
at wavelength λ = 272 nm. The absorbance of furfural solutions with
different concentrations was used to build the calibration curve, which
allowed quantify the furfural during the reaction.
Fig. 1 shows demonstrate how the furfural acetylation reactions can
be monitored by UV spectroscopy. It is possible to observe the reduction
in the intensity of the furfural absorption band at 272 nm and the in-
crease of the characteristic band of the acetal, placed at 221 nm wa-
velength. After the end of each reaction, an aliquot was analyzed by
GC–MS to identify the compounds in the solution. The acetal was al-
ways the only product formed (ca. 100% acetal selectivity).
The kinetic curves of acetalization reactions of furfural in the pre-
sence of H3PW12O40 or H3PMo12O40 heteropolyacid catalysts are dis-
played in Fig. 2.
2.4. Recovery and reuse of the catalyst
After the end of the reaction, excess alcohol was removed under
reduced pressure and 20 mL of water was added. The resulting mixture
was three times extracted with dichloromethane. The aqueous phase
was evaporated under heating plate to near dryness and then dried at
room conditions. The solid catalyst was weighted and reused in another
catalytic run.
The behavior of tungsten and molybdenum HPAs was almost the
same. Within first 20 min of reaction, a virtually complete conversion
was attained by both catalysts, with 100% selectivity of acetal furfural.
3. Results and discussion
3.1. General aspects
3.3. Effect of catalyst on the furfural acetalization with methanol
Furfural acetalization is accomplished through the reaction me-
chanism shown in Scheme 1 [30]. Acetal formation occurs through an
acid-catalyzed reversible reaction, where the role of catalyst is to in-
crease the electrophilicity of the carbonyl carbon of furfural for that the
reaction proceed.
The acetalization of furfural with different acidic catalysts was
performed in methanol at room temperature and the results are pre-
sented in Fig. 3. It is important to note that all the acid catalysts were
used at the same H+ cations concentration.
In the absence of the catalyst, even though a large excess of alcohol
to furfural (555:1), no reaction progress was observed (Fig. 3). Con-
versely, the presence of acidic catalysts improved significantly the
conversion of reactions. The catalysts that achieved the highest con-
version percentages were H3PW12O40 and H3PMo12O40 (Fig. 3,
ca. 96%). The catalytic activity obeys the trends H3PW12O40
≥ H3PMo12O40 > H2SO4 > HCl > p-toluenesulfonic acid. This ten-
dency agrees with the pKa values of these acids, which were measured
In according with mechanism proposed in Scheme 1, on the first
reaction step, the oxygen of carbonyl group of the furfural (I) is pro-
tonated, providing an intermediate (II), which undergoes a nucleophilic
attack by the alcohol molecule, giving a protonated tetrahedral inter-
mediate, that after releasing the proton gives the hemiacetal (III). The
intermediate (III) is then protonated and after suffering a dehydration
step generating the carbocation (IV). Finally, the intermediate (IV)
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Scheme 1. Mechanism proposal of the acid-catalyzed acetalization (adapted from ref. [30]).
in HOAc [34]. To confirm this tendency, we performed the titration
with n-butylamine of main catalysts evaluated herein (Fig. 4).
The acid strength of the catalysts was determined by potentiometric
titration using n-butylamine (Fig. 4). According to Pizzio et al., the acid
strength of a catalyst is measured by the initial electrode potential (E.I),
being classified as very strong (Ei > 100 mV), strong (0 < Ei < 100
mV), weak, Ei < 0 mV) and very weak (Ei < −100 mV) [35]. We can
conclude that although all the catalysts have been characterized as
strong acids, the trends observed agree with the efficiency of catalysts
in reaction studied. Therefore, as described in Scheme 1, if there is a
higher amount of H+ cations available, a higher amount of substrate
molecules can be protonated and consequently a higher conversion will
be achieved.
It is known that in organic solvents such methanol, probably the
leveling effect can be less effective. Therefore, despite the acids have
been used at the same concentration, probably the stronger acids pro-
vided higher protons amount. However, the size of anion present in the
acid may be important too. Larger anions should be more efficient to
stabilize the protonated intermediates formed along the reaction as
suggested in the Scheme 1. Thus, this effect may also favor the HPA-
catalyzed reactions, because these catalysts have bigger anions.
Fig. 2. Kinetic curves of HPA-catalyzed acetalization reaction of the furfural
with methanola.
aReaction conditions: Furfural (0.62 mmol); reaction volume (10 mL); catalyst
(0.5 mol% of H+ cations); temperature (298 K).
a
Fig. 1. Kinetic monitoring by ultraviolet spectroscopy of acetalization reaction of the furfural with methanol in the presence of (A) H3PW12O40 and (B)
a
H3PMo12O40
.
aReaction conditions: furfural (0.62 mmol); reaction volume (10 mL); catalyst (0.5 mol % of H+ cations); temperature (298 K).
3

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Table 1
Linear plots of ln ([furfural]/[furfural]initial) versus time, rate constant and
linear correlation coefficient for different Bronsted acids-catalyzed furfural
acetalization reactions.
Catalyst
Rate constants (s−1
)
R2
H3PMo12O40
H3PW12O40
H2SO4
5.0 × 10−3
6.4 × 10−3
1.7 × 10−3
2.3 × 10−3
7.0 × 10−4
0.9944
0.9989
0.9880
0.9891
0.9852
HCl
p-toluenesulfonic acid
The high linearity coefficient presented in Table 1 assure that the reactions
were the first-order in relation to the furfural concentration. The rate constants
obtained confirm that heteropolyacids were the most efficient catalysts.
were far from equilibrium. In Table 1, the rate constants of these re-
actions are presented.
Fig. 3. Brønsted acid-catalyzed furfural acetalization with methanol at room
temperaturea.
aReaction conditions: furfural (0.62 mmol); reaction volume (10 mL); catalyst
(2 mol % of H+ cations); temperature (298 K).
3.4. H3PW12O40 or H3PMo12O40-catalyzed furfural acetalization reactions
with methanol: Effect of catalyst concentration
The catalyst concentration should not affect the equilibrium reac-
tion; nonetheless, it can contribute to it be more quickly reached. To
assess this effect, the catalyst loading was varied over a range of
0.1–2 mol % of H+ cations (Fig. 6a and b).
Fig. 6 shows that when the reactions were carried out with
concentrations equal or lower than 0.25 mol % of H+ cations, the
conversions were significantly low. Conversely, the runs with
0.50–2.00 mol % of catalyst achieved conversions virtually complete
after thirty minutes reaction. Although the conversions have been very
close to this concentration range, the initial rate of reactions was en-
hanced when the catalyst load was increased. These observations are
equally valid for two HPA catalysts because both had a very similar
performance.
3.5. Effect of alcohol on the H3PW12O40 or H3PMo12O40 -catalyzed
furfural acetalization
The reactivity of alcohols is affected by carbon chain size as well as
the hindrance steric on hydroxyl group in ketalization reactions [36].
This effect was investigated herein, and the kinetic curves obtained in
reactions with two HPAs catalysts are displayed in Fig. 7.
Fig. 4. Potentiometric titration with n-butylamine of the Bronsted acid catalysts
used in furfural acetalization reactions.
The HPAs have a very comparable activity on the acetalization re-
actions of alkyl alcohols. The trends of reactivity of the alcohols ob-
served in reactions in the presence of H3PW12O40 or H3PMo12O40 was
as follow: methanol > ethanol > propanol > butanol > propan-2-
ol. The lowest reactivity of propan-2-ol can be explained by the steric
hindrance on hydroxyl group. On the other hand, an increase on carbon
chain size resulted in a lower reactivity. It can be attributed to pKa of
alcohols, which decrease as the same way, decreasing the reactivity of
alcohols in the furfural acetalization reactions.
3.6. Effect of temperature on H3PW12O40 and H3PMo12O40-catalyzed
furfural acetalization with methanol
To investigate the effect of temperature on the rate of reaction, plots
of furfural conversion versus time were constructed for each
H3PW12O40 and H3PMo12O40-catalyzed reaction at different tempera-
tures (Fig. 8). On these experiments, the reactions were carried out
0.25 mol % of H+ concentration; under these conditions, the maximum
conversion obtained at room temperature was ca. 37% (Fig. 6).
The kinetic curves of the reactions carried out the different tem-
peratures was very similar, although the H3PW12O40-catalyzed reac-
tions have been more sensitive to the increase on temperature and
presented rate reactions slight higher. An increment on temperature
resulted in an equal effect on kinetic curves. In both cases, for that the
Fig. 5. Plots of ln (instantaneous concentration of furfural)/ (initial con-
centration of furfural) versus reaction time obtained from Brønsted acid-cata-
lyzed furfural acetalization with CH3OH.
Based on kinetic curves displayed in Fig. 3, we built the plots of first
order presented in Fig. 5. It is important to know that only the initial
points were used to obtain these curves, to assure that the reactions
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Fig. 6. Effects of catalyst load in the kinetic curves acetalization reactions of furfural with CH3OH in the presence of (a) H3PW12O40 or H3PMo12O40 (b)a.
aReaction conditions: Furfural (0.62 mmol); reaction volume (10 mL); temperature (298 K).
Fig. 7. Effects of alcohol on (a) H3PW12O40 and (b) H3PMo12O40-catalyzed furfural acetalization reactiona.
aReaction conditions: Furfural (0.62 mmol); reaction volume (10 mL); HPAs catalyst (0.5 mol% of H+ cations); temperature (298 K).
(B)
(A)
Fig. 8. Kinetic curves of (a) H3PW12O40 or (b) H3PMo12O40-catalyzed furfural acetalization with CH3OH at different temperaturesa.
aReaction conditions: furfural (0.62 mmol); reaction volume (10 mL); HPAs catalyst (0.25 mol% of H+ cations).
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Fig. 9. Plots of ln (instantaneous concentration of furfural)/ (initial concentration of furfural) versus reaction time obtained from (a) H3PW12O40 or (b) H3PMo12O40
-
catalyzed furfural acetalization with CH3OH at different temperatures.
indicating a dependence of first-order in relation to furfural con-
centration.
Table 2
Linear plots of ln ([furfural]/[furfural]initial) versus time, rate constants and
linear correlation coefficients (R2) obtained from Figs. 6 and 7.
Temperature (K)
All the linear equations resulting had a high correlation coefficients
(R2), indicating that the reaction is of first order in relation to the
furfural concentration at any temperature. The rate constant at each
temperature are listed in Table 2.
H3PW12O40
H3PMo12O40
Rate constant (s−1
)
R2
Rate constant (s−1
)
R2
The Arrhenius plot of the reactions of furfural acetalization with
methanol catalyzed by H3PW12O40 and H3PMo12O40 are shown in
Fig. 10a and b, respectively.
298
303
308
313
318
2.0 × 10−4
7.0 × 10−4
1.2 × 10−3
2.0 × 10−3
2.8 × 10−3
0.9938
0.9802
0.9942
0.9954
0.9976
2.0 × 10−4
6.0 × 10−4
1,3 × 10−3
1.4 × 10−3
2.4 × 10−3
0.9573
0.9688
0.9883
0.9964
0.9973
From angular coefficient (−Ea/R) obtained by plotting ln k × 1/T
(i.e. linear Arrhenius equation and the k values determined at different
temperatures), the activation energy value (Ea) of H3PW12O40 and
H3PMo12O40-catalyzed furfural acetalization with methanol was de-
termined (Fig. 10). The activation energy of the H3PW12O40 and
H3PMo12O40-catalyzed acetalization reactions was found to be 95 kJ
mol−1 and 97 kJ mol−1, respectively.
catalytic reactions achieve the maximum conversion, it was required to
increase the temperature to 318 K.
3.6.1. Dependence of reaction rate in relation to furfural concentration
From data of Fig. 8, the kinetic parameters such as activation energy
and order of the reaction with respect to the furfural concentration
were obtained. To do it, we selected only the data of first 15 reaction
minutes; it is a guarantee that the reaction is far from equilibrium, an
essential requirement to obtain consistent kinetic data.
3.7. Catalyst reuse
Due to alike behavior, only one phosphotungstic acid was selected
to be reused. The H3PW12O40 is a solid catalyst but completely soluble
in the reaction medium (methanol). However, we developed a simple
procedure to recover the catalyst (see Section 2.4). We achieved a rate
recovery of 95% for the H3PW12O40 catalyst. This high recovery sug-
gests that procedure used was efficient. Therefore, it was possible to
From data of Fig. 8, the Fig. 9 was constructed and starting from the
data obtained at each reaction temperature, it was possible to de-
termine the specific reaction rate constant (k) using the pseudo-first-
order rate law. How can be seen, the curves presented high linearity,
Fig. 10. Arrhenius plot of (a) H3PW12O40 or (b) H3PMo12O40-catalyzed furfural acetalization with methanol.
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Fig. 11. Ultraviolet spectra obtained of aliquots take off during the acetalization reaction of the furfural with methanol catalyzed by recovered HPW (a) and kinetic
curve plota.
aReaction conditions: Furfural (0.62 mmol); reaction volume (10 mL); catalyst (0.5 mol% of H+ cations); temperature (298 K).
reuse this solid catalyst in another reaction. The results of reuse of the
catalyst are shown in Fig. 11a and b.
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