Acetalization of furfural with zeolites under benign reaction conditions

Article abstract of DOI:10.1016/j.cattod.2014.03.004

Acetalization is a viable method to protect carbonyl functionalities in organic compounds and offers a potential synthetic strategy for synthesizing derived chemicals. In this work, several families of commercial zeolites have been employed as solid acid catalysts in the acetalization of furfural to form furfural diethyl acetal at room temperature using ethanol as a renewable solvent. Among the tested catalysts, H-USY (6) provided the highest catalytic activity (79% acetal yield), excellent selectivity and reusability in five consecutive reaction runs. Process parameters such as, e.g. reaction time, catalyst loading and applicability of different lower alcohols were evaluated and optimized.

Full text of DOI:10.1016/j.cattod.2014.03.004

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Catalysis Today
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Acetalization of furfural with zeolites under benign
reaction conditions
Juan Miguel Rubio-Caballeroa, Shunmugavel Saravanamuruganb,
Pedro Maireles-Torresa, Anders Riisagerb,∗
a Departamento de Química Inorgánica, Cristalografía y Mineralogía (Unidad Asociada al ICP-CSIC), Facultad de Ciencias, Universidad de Málaga,
Campus Teatinos, 29071 Málaga, Spain
b Centre for Catalysis and Sustainable Chemistry, Department of Chemistry, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history:
Acetalization is a viable method to protect carbonyl functionalities in organic compounds and offers a
potential synthetic strategy for synthesizing derived chemicals. In this work, several families of commer-
cial zeolites have been employed as solid acid catalysts in the acetalization of furfural to form furfural
diethyl acetal at room temperature using ethanol as a renewable solvent. Among the tested catalysts,
H-USY (6) provided the highest catalytic activity (79% acetal yield), excellent selectivity and reusability
in five consecutive reaction runs. Process parameters such as, e.g. reaction time, catalyst loading and
applicability of different lower alcohols were evaluated and optimized.
Received 15 November 2013
Received in revised form 2 March 2014
Accepted 6 March 2014
Available online xxx
Keywords:
Furfural
Zeolites
© 2014 Elsevier B.V. All rights reserved.
H-USY (6)
Furfural diethyl acetal
Ethanol
1. Introduction
versatile synthetic tool to make new chemicals, integrated in the
so-called biorefinery concept, or may serve as a convenient method
Production of fuels, energy and chemicals from renewable feed-
stocks are attracting worldwide attention in order to minimize
the large dependence on fossil reserves. Development of alter-
native technology based on sustainable resources will counteract
increased fossil energy prices resulting from the depletion as well as
environmental concerns caused by global warming. Lignocellulose,
the second most abundant component of biomass, is positioned as
the most feasible renewable source of organic non-fossil carbon,
available for the production of valued-added chemicals and fuels
[1–3].
Furfural (FUR) can be derived from lignocellulosic biomass and
is a versatile industrial chemical with a wide range of applica-
tions [4]. Accordingly, it has been considered a platform molecule
for the production of fuels (methyltetrahydrofuran – a gasoline
additive), as well as important chemicals such as, e.g. furfuryl
alcohol and tetrahydrofuran [2]. FUR is comprised of a heteroaro-
matic furan ring with an aldehyde group which is a useful handle
for further chemical synthesis. In this connection, acid catalyzed
acetalization of the formyl functionality (to make acetals) can be a
of protection. The stability of acetals under basic conditions and in
the presence of oxidizing and reducing agents, make acetals suit-
able intermediates in multistep organic synthesis [5–7]. Dimethyl,
diethyl and cyclic acetals are the most frequent acetals obtained
through acetalization. Moreover, acetals also present industrial
applications in pharmaceutical, polymer, fragrance and flavour
industries, and they have been used as additive in ethanol fuel to
decrease the autoignition temperature [8].
Acetalization is traditionally carried out with homogeneous
catalysis using strong Brønsted mineral acids such as, e.g. H2SO4
or HCl, which lead to a series of problems associated to prod-
uct separation, catalyst recovery and equipment corrosion. To
overcome these drawbacks focus has recently shifted to the devel-
opment of renewable and environmentally benign heterogeneous
catalyst system instead [9–12]. Many solid catalysts have been
used in the acetalization of carbonyl groups, such as lanthanum
(III) nitrate hexahydrate [13], bismuth oxynitrate [14], anhydrous
cerium(III) chloride [15], phosphotungstic acid [16], sulfated zirco-
nia and titania [17], Ce-exchanged montmorillonite or mesoporous
silica [18], Filtrol-24, Amberlyst-15, dodecatungstophosphoric acid
(DTPA) supported on both ZSM-5 and K-10 clay [19] and melamine-
formaldehyde based polymer [20]. Bhaskar et al. have further
reported the use of zeolites as efficient catalysts for the protec-
tion of active groups in the synthesis of o-isopropylidene acetals
∗
Corresponding author. Tel.: +45 45252233.
E-mail address: ar@kemi.dtu.dk (A. Riisager).
http://dx.doi.org/10.1016/j.cattod.2014.03.004
0920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: J.M. Rubio-Caballero, et al., Acetalization of furfural with zeolites under benign reaction conditions,
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from monosaccharides [21]. Application of zeolites takes advan-
tage of their lack of toxicity, non-corrosiveness, easy recovery and
possibility of reuse. Furthermore, their regular pore size as well as
pore architecture provides them with shape-selective properties.
An important advantage lies further in their tunable acid strength,
which can be controlled by modification of the Si/Al ratio, that along
with their thermal and chemical stability make them largely versa-
tile as catalysts for a wide variety of organic reactions [22] in both
the petrochemical industry and in biomass conversion.
adsorbed/entrapped compounds, where after the catalyst was cal-
cined overnight at 550 ◦C with a ramp of 1 ◦C min−1 in air. Finally,
the catalyst was collected and placed into a fresh test tube and the
reaction started.
2.3. Product analysis
Aliquots of the reaction mixtures were subjected to GC-
FID analysis (Agilent 6890N instrument, HP-5 capillary column
In connection with our continuous work on biomass valoriza-
tion, we have recently shown that commercially available Y and
beta zeolites are very promising solid acid catalysts for the isom-
erization of glucose to fructose and for the direct conversion of
glucose to alkyl levulinates in lower alcohols [23,24]. In the present
work, we have explored the use of the commercial Y, beta, ZSM-5,
and mordenite zeolites for the benign conversion of FUR to furfural
diethyl acetal (FDA) by acetalization in ethanol at room tempera-
ture. The hydrolysis of FDA to yield FUR was further studied with
the catalyst H-USY (6) by water addition as a mean of deprotection
of the acetal.
30.0 m × 320 ␮m × 0.25 ␮m) after filtering off the catalyst.
A
GC–MS system (Agilent 6850 GC system coupled with an Agilent
5975C mass detector) was used for qualitative analysis. Authentic
samples of FUR and FDA were used as standards to obtain quan-
titative calibration curves. The following formulae were employed
to calculate FUR conversion and FDA yield:
mol of FURinitial − mol of FURfinal
FUR Conversion =
× 100
mol of FURinitial
mol of FDAproduced
FDA Yield =
× 100
mol FURinitial
2. Experimental
3. Results and discussion
2.1. Materials
The acetalization of FUR to FDA was performed in ethanol at
room temperature and the results are presented in Table 1. In the
absence of catalyst, no reaction was observed (Table 1, entry 13).
Moreover, no conversion to FDA was observed when Na-mordenite
was used as catalyst (Table 1, entry 12), implying that an acid cata-
lyst was indeed required to perform the reaction as expected. Next,
experiments were carried out over H-forms of commercial beta,
ZSM-5 and mordenite zeolites with different Si/Al ratios. The results
demonstrate that in all cases, FDA was essentially the only reaction
product formed (>95% selectivity). All Y and beta large-pore zeo-
lites yielded between 71 and 79% of FDA (Table 1, entries 1–6) with
H-USY (6) giving the highest yield (79%) among the employed zeo-
lites. The total number of acid sites for the H-zeolites was estimated
by using NH3-TPD and reported in our previous work [23,24] and
the results are given in Table 1. Interestingly, H-beta (150) – hav-
ing a relatively low number of acid sites (Table 1) – gave also a
good FDA yield of 75%. On the other hand, H-ZSM-5 (medium-pore
size) gave FDA yields below 50% (Table 1, entries 9–12). When com-
paring the same zeolite structure with different Si/Al molar ratios,
the only appreciable differences were observed for H-ZSM-5 with
Si/Al ratios of 40 and 140, respectively. Moreover, the formation
of FDA did not depend on the number of acid sites present in the
zeolites, but pore size of the zeolites played a major role. However,
H-mordenite (large-pore size) gave only 33% FDA.
All chemicals in the study were of analytical grade and
used without further purification. 2-furaldehyde diethyl acetal
(97%), furfural (99%), naphthalene (99%, internal standard), ethanol
(99.9%) were purchased from Sigma–Aldrich. All the commer-
cially zeolites used were provided from Zeolyst International and
obtained in pure NH4+-forms without added binder material. Prior
to use, the zeolites were calcined at 550 ◦C for 6 h with a heating rate
of 1 ◦C min−1 in static air in order to convert them to their H-forms.
2.2. Catalytic reactions
The catalytic acetalizations were carried out at room temper-
ature (25 ◦C) in 15 ml Ace vials equipped with magnetic stirring.
In a typical procedure, 80 mg (0.8 mmol) of furfural (2 wt.% with
respect to ethanol), 75 mg of zeolite, 25 mg (0.2 mmol) of naphtha-
lene (internal standard) and 4 g (87 mmol) of anhydrous ethanol
were charged into a reactor and treated under vigorous stirring for
1 h, where after the reaction mixtures were analyzed.
In recycling experiments the same experimental conditions
as above were applied. The reaction time for each run was
1 h (when the maximum FDA yield was achieved). To perform
consecutive acetalizations, the reaction mixture along with the
catalyst were initially dried for a few hours in order to remove
Table 1
Acetalization of FUR to FDA over various zeolites.a
Entry
Pore size
Large
Zeolite
Si/Al ratio
Acid sitesb (␮mol/g)
Conversion (%)
FDA yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
H-USY
H-USY
H-beta
H-beta
H-beta
H-beta
H-mordenite
Na-mordenite
H-ZSM-5
H-ZSM-5
H-ZSM-5
H-ZSM-5
–
6
30
12.5
19
25
150
10
6.5
11.5
25
40
140
–
835
347
855
806
n.a
147
1656
n.a
1394
n.a
451
n.a
79
75
76
75
71
79
33
<1
40
35
29
8
79
74
72
72
71
75
33
<1
37
35
28
8
Medium
–
<1
<1
a
Reaction conditions: 80 mg (0.8 mmol) FUR, 75 mg zeolite, 25 mg (0.2 mmol) naphthalene (internal standard), 4 g (87 mmol) ethanol, 25 ◦C, 1 h.
n.a = not available.
b
Please cite this article in press as: J.M. Rubio-Caballero, et al., Acetalization of furfural with zeolites under benign reaction conditions,
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3
From the data of Table 1, it can be inferred that the pore size of
the zeolites had a stronger influence on the acetalization reaction
than the concentration of acid sites. Climent et al. [25] and Thomas
et al. [18] have observed a similar effect in the acetalization of alde-
hydes and ketones employing zeolites as catalysts. Since the most
active and selective catalyst was H-USY (6), exhibited a yield of
79% FDA, this catalyst was chosen for further study and optimiza-
tion of other reaction conditions, such as, e.g. reaction time, catalyst
amount and catalyst reusability.
Methanol is the lower alcohol most commonly employed as
solvent in acetalization of aldehydes and ketones due to its
high reactivity [9,17,26,27]. However, to make the process more
environmentally sustainable, ethanol was here used instead of
methanol since it can be easily produced from renewable agricul-
tural feedstocks. Moreover, it was found that reactions with higher
alcohols such as, e.g. 1-propanol or 1-butanol, resulted in lower
conversion to the respective acetals (70 and 25%, respectively)
when using H-USY (6) as catalyst under identical reaction condi-
tions. Probably, this decrease in catalytic activity can be attributed
to diffusional limitations in the zeolite, which hinders the larger
molecular sized acetals to readily diffuse out of the micropores
when formed.
Fig. 1. The influence of H-USY (6) catalyst loading on the yield of FDA. Reaction
conditions: 80 mg (0.8 mmol) FUR, 4 g (87 mmol) ethanol, 25 ◦C, 1 h.
The acetalization of FUR is accomplished through the reaction
mechanism shown in Scheme 1. The first step is the formation of a
protonated intermediate (2) via protonation of the carbonyl group
of FUR (1) by a Brønsted acid site of the zeolite. Then, ethanol reacts
with the intermediate (2) forming the hemiacetal (3) after removal
of a proton, which then re-protonates and dehydrates to the inter-
mediate (4). Afterwards, the intermediate (4) reacts with another
ethanol molecule giving the intermediate (5), which upon depro-
tonation forms the acetal FDA (6) and regenerates the acid site.
Although the synthesis of FDA requires initial formation of the cor-
responding hemiacetal (3), this was not detected as intermediate in
any of the examined reactions. This suggests that the acetal forma-
tion was fast compared to hemiacetal formation, as also previously
reported for other systems.
It is noteworthy that acetalization is a reversible reaction and
Moreau et al. have shown that the hydrolysis of acetals and thioac-
etals of FUR can be carried out in water in the presence of strong
Brønsted acids, through a pseudo-first order mechanism [28].
Therefore, water produced in the acetalization process together
with the Brønsted acid sites present on the H-USY (6) zeolite
could potentially hydrolyze FDA to reform FUR during the reac-
tion (Scheme 1). To suppress the concurrent hydrolysis reaction and
obtain high FUR conversion, a large excess of alcohol in comparison
to the stoichiometric amount (i.e. EtOH:FUR molar ratio = 100:1)
was accordingly used for the acetalization reactions. The hydroly-
sis reaction was, however, facile in the presence of the same zeolites
with added water resulting in complete hydrolysis of FDA to FUR
after 1 h of reaction at room-temperature with a H2O:EtOH weight
ratio of 1:2 or larger.
of added catalyst augmented from 2 to 5 mg. With higher catalyst
loadings (12.5–75 mg) the FDA yield remained unchanged, indicat-
ing that excess of acid needed to facilitate the reaction was present.
When using a low amount of catalyst (2 mg), the FDA yield attained
a value of 65% after 2 h of reaction time and increased further to
77% after 3 h of reaction (Fig. 2). This confirmed that the catalysts
remained active and longer reaction time was just required to pro-
duce almost the same FDA yield as found for higher amount of
catalyst at shorter reaction time.
Catalyst reusability in the acetalization of FUR was also evalu-
ated for H-USY (6) by performing five consecutive runs (see Section
2.2 for regeneration procedure applied after each run). Fig. 3 shows
that the catalyst maintained a good performance providing a simi-
lar product distribution after each run with a FDA yield of 76–84%.
These results corroborate that the H-USY (6) zeolite can be regener-
ated with the applied procedure without structural damage, since
such structural alteration would be expected to result in loss of
performance. Similarly, leaching of Al from the zeolite is not likely
to occur as this also would be expected to alter the catalyst per-
formance due to the associated loss of acid sites. Such a change in
catalyst performance has already been reported by Thomas et al.
in their study on acetalization of carbonyl compounds with cation
exchanged montmorillonites [9].
The influence of the concentration of FUR in ethanol was also
studied with 2–10 wt.% FUR with H-USY (6) under identical condi-
tions as shown in Table 1. The yield to FDA decreased from 79 to
74% when the concentration of FUR was increased from 2 to 5 wt.%.
However, as the amount of FUR increased to 10 wt.% the FDA yield
(73%) remained almost unchanged, implying that a high yield of
FDA could also be achieved at a higher concentration of FUR.
The effect of catalyst loading on the FUR acetalization was also
studied over H-USY (6) under typical experimental conditions (see
Section 2.2). In general, an increase in catalyst loading implies an
increase of the number of acid active sites available for the cat-
alytic process, and consequently the conversion (i.e. reaction rate)
is expected to increase if the reaction is kinetically controlled. Nev-
ertheless, the data displayed in Fig. 1 reflect that the FDA yield (and
thus also the FUR conversion) only increased when the amount
Fig. 2. The formation of FDA over H-USY (6) as a function of time. Reaction condi-
tions: 80 mg (0.8 mmol) FUR, 4 g (87 mmol) ethanol, 2 mg catalyst, 25 ◦C.
Please cite this article in press as: J.M. Rubio-Caballero, et al., Acetalization of furfural with zeolites under benign reaction conditions,
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H
O+
CH2CH3
O+H
H
OCH2CH3
O
+
O
+
CH CH OH
O
Zeolite, H
3
2
-H
O
O
OH
OH
H
(3)
(1)
(2)
+
H
OCH2CH3
O+H2
O
O
-H
O
2
O+CH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
O
O
CH CH OH
3
2
O+CH2CH3
H
(6)
(5)
(4)
Scheme 1. The mechanism of the acid-catalyzed acetalization of FUR to FDA.
10-081991) and The Catalysis for Sustainable Energy initiative
funded by the Danish Ministry of Science, Technology and Inno-
vation.
100
80
60
40
20
0
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A series of commercial microporous zeolites were applied for
room-temperature acetalization of furfural with ethanol to form
furfural diethyl acetal. The hemiacetal was not observed as reac-
tion product with the catalysts under the applied conditions. The
best acetal yield close to 80% was found using H-USY (6) zeolite
catalyst. This yield was also obtained with low loading of the cat-
alyst (2.5 wt.% with respect to the substrate) at prolonged reaction
time. The H-USY (6) catalyst proved further reusable in five consec-
utive runs with unchanged performance, thus demonstrating it to
be an excellent recyclable solid acid catalyst for acetalization with
potential for industrial application.
Acknowledgements
The work was supported by the Danish Council for Indepen-
dent Research – Technology and Production Sciences (project no.
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Please cite this article in press as: J.M. Rubio-Caballero, et al., Acetalization of furfural with zeolites under benign reaction conditions,
Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.03.004