Benzyl benzoate and dibenzyl ether from of benzoic acid and benzyl alcohol under microwave irradiation using a SiO2-SO3H catalyst

Article abstract of DOI:10.1016/j.catcom.2015.04.033

Abstract A sulfonated silica gel of small surface area with large pore diameters was prepared from the sodium silicate obtained from the reaction of construction sand and sodium carbonate, and sulfated with concentrated sulfuric acid. At 7%-w/w it promotes in 5 min the Fischer-Speier esterification of benzoic acid/benzyl alcohol in 93% yields, and at higher concentrations, dibenzyl ether is produced selectively and quantitatively, in solvent free reactions induced by microwave irradiation.

Full text of DOI:10.1016/j.catcom.2015.04.033

Catalysis Communications 68 (2015) 97–100
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Benzyl benzoate and dibenzyl ether from of benzoic acid and benzyl
alcohol under microwave irradiation using a SiO2–SO3H catalyst
a
a
a
a
Sandro L. Barbosa a, , Myrlene Ottone , Maisa C. Santos , Gelson C. Junior , Camila D. Lima ,
⁎
Giuliano C. Glososki b, Norberto P. Lopes b, Stanlei I. Klein c
Pharmacy Department, Federal University of the Jequitinhonha and Mucuri Valleys, Rodovia MGT 367, Km 583, CEP 39100-000 Diamantina, MG, Brazil
Ribeirão Preto Faculty of Pharmaceutical Sciences, São Paulo University, Av. Do Café s/n, CEP 14040-903 Ribeirão Preto, SP, Brazil
Department of General and Inorganic Chemistry, São Paulo State University, R. Prof. Francisco Degni 55, CEP 14800-960 Araraquara, SP, Brazil
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
A sulfonated silica gel of small surface area with large pore diameters was prepared from the sodium silicate ob-
tained from the reaction of construction sand and sodium carbonate, and sulfated with concentrated sulfuric acid.
At 7%-w/w it promotes in 5 min the Fischer–Speier esterification of benzoic acid/benzyl alcohol in 93% yields, and
at higher concentrations, dibenzyl ether is produced selectively and quantitatively, in solvent free reactions in-
duced by microwave irradiation.
Received 16 February 2015
Received in revised form 23 April 2015
Accepted 27 April 2015
Available online 30 April 2015
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Solvent free reactions
Microwave irradiation
Sulfonated silica catalyst
Fischer esterification
Benzyl benzoate
Dibenzyl ether
1. Introduction
under MW irradiation; the new catalyst is water tolerant, and the
catalyst/aromatic alcohol ratio can be tuned to produce dibenzyl ether.
The Fischer–Speier esterification is the direct route for the obtention
of esters. The mechanism involves the protonation of the carboxyl
oxygen of an acid, with the subsequent attack of an alcohol to the posi-
tively polarized carboxylic carbon. This step is hampered by sterically
demanding alcohols and/or acids. To make things worse, the reaction
is reversible, because the product ester is prone to protonation and at-
tack by the water formed. This makes the synthesis of important esters,
such as benzyl benzoate, very difficult, even if the water from the reac-
tion is removed from the equilibrium. To overcome these difficulties,
different authors have suggested indirect routes, involving benzalde-
hyde [1,2], benzyl chloride [3,4], and cross-dehydrogenative coupling
reactions [5]. A Fisher esterification was accomplished using a sulfonic
acid imidazole ionic liquid, in reactions promoted by microwave
(MW) irradiation [6,7].
2. Experimental
SEM micrographs were performed on a Zeiss VEVO 50, EDS with an
IXRF Systems 500. FTIR recorded as KBr pellets in a Varian 640
spectrometer. Reacted contents and yields were determined with a GC/
MS-QP 2010/AOC-5000 AUTO INJECTOR/Shimadzu spectrometer, with a
30 m Agilent J&W GC DB-5 MS column, measured at 70 eV. XRD patterns
collected in a RIGAKU diffractometer at 30 kV and 20 mA using CuKα ra-
diation. DTA carried out using a Perkin Elmer 1700 analyzer.
2.1. Silica gel and sulfonated silica
Our group described the application of Lewis acidic silicas and alu-
minas as catalysts in esterifications promoted by MW irradiation [8], but
they were not able to produce esters when both reactants bore aromatic
rings. The possibility that Bronsted acidic silicas could work, reinforced
after the report of the esterification of fatty acids by silicas treated with
H2SO4 [9], prompted us to develop a protocol using SiO2–SO3H as catalyst
300.0 g of sand and 600.0 g of Na2CO3 were homogenized in porce-
lain crucibles, and heated at 850 °C for 4 h. The hot mixtures were trans-
ferred to a glass frit and washed with 600–900 mL of boiling water. The
filtered solution was acidified to pH = 1 with HCl, the white precipitate
filtered, and dried at 400 °C. The resulting silica was passed through a
24 mesh sieve for standardization. 10.0 g of this silica was mixed with
10.0 mL of H2SO4 and stirred at room temperature for 12 h, filtered
and dried at 150 °C for 4 h, cooled and stored in a desiccator. The acid
⁎
Corresponding author.
E-mail address: sandro.barbosa@ufvjm.edu.br (S.L. Barbosa).
http://dx.doi.org/10.1016/j.catcom.2015.04.033
1566-7367/© 2015 Elsevier B.V. All rights reserved.

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S.L. Barbosa et al. / Catalysis Communications 68 (2015) 97–100
strength of 1.44 mmol of H+/gram of catalyst was determined by poten-
tiometric titration.
final temperature 69 °C. CG/MS analysis of the cold mixture showed
total conversion of the alcohol.
b) In the absence of benzoic acid: SiO2–SO3H (0.0216 g–20%-w/w) and
the standard amount of benzyl alcohol were irradiated for 5 min,
final temperature 70 °C. The crude mixture showed total conversion
of the alcohol (by CG/MS).
2.2. Typical procedures
In all reactions involving SiO2–SO3H the amount of reagents benzoic
acid (0.6839 g–5.6 mmol) and benzyl alcohol (0.1081 g–1.0 mmol) was
used as standard. The amount of catalyst on each run was adjusted to
maintain a weigh-to-weigh relation to the alcohol. All reactions were
irradiated in an unmodified MW oven (900 GHz)/360 W [8] using an
unstoppered 125 mL two-necked round bottom flask, and the final
temperature of the slurries did not exceed 73 °C.
3. Results and discussion
3.1. Silica gel and catalyst characterization
The non-crystalline nature of the silica gel and of the sulfonated cat-
alyst was confirmed by XRD, which presented broad peaks centered at
2θ = 23o.
2.3. Synthesis of benzyl benzoate
a) H2SO4 catalyst: In this reaction, the amount of H2SO4 was adjusted so
that the total H+ content (as a monoprotic acid) corresponded to the
protons in SiO2–SO3H 20%-w/w catalyst/alcohol; the required stan-
dard quantities were scaled up 100-fold.
The infrared spectrum of the silica (Fig. 1) is also representative of an
amorphous gel. The characteristic bands around 3400 and 1630 cm−1
are associated with the stretching and bending modes of molecular
water, with the shoulder at 3200 and the weak absorption at
960 cm−1 related to the –OH and Si–OH vibrations of the silanols.
Also characteristics are the bands at 1090 and the associated shoulder
at 1190 cm−1, relative to the asymmetric stretchings of the Si–O–Si
bonds. The absorption at 800 cm−1 of the ring structured SiO4 tetrahe-
dra (Si–O–Si symmetric stretchings), the Si–O–Si bending vibrations at
470 cm−1, and the observable overtones typical of the amorphous sil-
icas in the region of 1800–1900 cm−1 complete the spectrum of the
gel. The IR spectrum of the sulfonated silica showed remarkable differ-
ences. For instance, the H2O bands at 3400 cm−1 become much broader,
with additional intramolecular hydrogen bond features from 2900 to
2100 cm−1, with the concomitant increase of the H2O bending at
1637 cm−1. These changes reflect the accommodation of additional
water molecules via hydrogen bonds to the –SiOH and –O–SO3H groups.
The splitting of the absorptions from 1210 to 1040 cm−1 in that spec-
trum is noteworthy, which indicates a profound modification of the
overall surface structure of the Si–O–Si original arrangement, also
Benzoic acid (68.39 g–0.56 mol), benzyl alcohol (10.81 g–0.10 mol)
and 0.080 g of concentrated H2SO4 (7.8 × 10−4 mol) were irradiated
for 5 min, with a final temperature of 110 °C. The reaction vessel
was cooled to room temperature, and the total mixture analyzed
by CG/MS, which evidenced total conversion of the alcohol to benzyl
benzoate.
b) SiO2–SO3H catalyst: in the reaction flask it was added 0.0076 g of the
SiO2–SO3H (7%-w/w) and the standard quantities of both reagents.
After 5 min of irradiation the temperature of the reaction rose to
70 °C. The vessel was cooled to room temperature, 30 ml of diethyl
ether was added, and the mixture filtered. The organic extract was
washed with 10.0 mL of saturated NaHCO3, dried over anhydrous
MgSO4 and concentrated under reduced pressure. The residue was
purified by chromatographic column using hexane and ethyl acetate
as eluent to obtain the pure benzyl benzoate as colorless oil. Yield.
0.2080 g and 98.97%. The filtered catalyst was washed with 10 mL
diethyl ether, and dried at 150 °C for 2 h before reuse.
reflected in the further weakening of the Si–OH band at 960 cm−1
.
The contribution of the groups –O–SO3H to the absorptions of the IR
spectrum of the catalyst is ambiguous, because of the close proximity
of the atomic weights of silicon and sulfur oxygen tetrahedra. However,
one might assume that the enhancement of the Si–O–Si longitudinal-
transverse optical vibration (LO) absorption at 1210 cm−1 and the
2.4. Synthesis of dibenzyl ether
a) In excess benzoic acid: SiO2–SO3H (0.0216 g–20%-w/w) and the
standard quantities of both reagents were irradiated for 5 min,
transversal optical (TO) rocking of the Si–O–Si bonds at 592 cm−1
,
Fig. 1. IR spectrum of silica (red) and catalyst (blue).

S.L. Barbosa et al. / Catalysis Communications 68 (2015) 97–100
99
Fig. 2. FE-SEM images of the amorphous silica gel.
Fig. 4. N2 adsorption–desorption isotherms of the catalyst.
may be due to the presence of those –OSO3H groups of the catalyst [10,
11].
catalyst/alcohol, 18.97% of benzyl benzoate was formed (by CG/MS).
Different ratios of sulfonated silica were tested, and, surprisingly, at
7.0% catalyst/alcohol, the maximum yield of benzyl benzoate was
achieved (98.97%, entry 8 in Table 1), but at this concentration, a
novel spot in the TLC monitoring plates, and its corresponding new
peak in CG/MS, indicated the formation of dibenzyl ether (1.03%). Fur-
ther experiments indicated that higher yields of ether could be obtained
with increased amounts of catalyst; at 20%-w/w, only the ether was
formed. The maximum yield of benzoate free from ether is 93.61%,
with a catalyst load of 6%; this yield is superior to that obtained by us
using NbCl5 catalyst [12] or by others using ionic liquids [6,7]. Similar
yields can be obtained in about 4 h using conventional heating at 80 °C.
The differences between the original silica and the sulfonated mate-
rial are also evident from the FE-SEM images shown in Figs. 2–3. The
even distributions of the particles of the silica, responsible for its large
surface area, form aggregates upon sulfonation, which are well separat-
ed by macropores several nanometers large.
BET analysis reinforced the changes that occurred upon sulfonation.
The lowering of the specific surface area was found from 37.86 to
23.10 m2 g−1, and the widening of the pore diameters to 3.73 nm. The
N2 adsorption–desorption isotherms of the gel and catalyst (Fig. 4),
were typical for mesoporous materials.
TG/DTG/DTA showed the decomposition of the silica gel in synthetic
air taking place in one stage, related to the endothermic elimination of
water molecules. The decomposition of the catalyst takes place in two
stages, from room temperature to 169.39 °C, related to the elimination
of water molecules, and up to 240 °C, corresponding to the endothermic
loss of –SO3H groups.
3.3. Mechanism and the reusability of SiO2–SO3H
In the Fischer–Speier mechanism, it is the acid partner who is acti-
vated by protonation from a Bronsted acid. Accordingly, in our laborato-
ries, when an excess of benzoic acid, benzyl alcohol and concentrated
H2SO4 were irradiated in a microwave oven, the only product obtained
is benzyl benzoate. However, the same alcohol to acid ratio of reactants
shows the interesting shift, moving from ~100% benzyl benzoate to
~100% dibenzyl ether with the increase of the w/w ratio of the catalyst
(Table 1). We had noted this shift in concurring esterification/
etherification reactions, the etherification taking over as the w/w ratio
of the catalyst to alcohol increased [10], and recently, Chang et al.
noted this effect in the acetal formation versus esterification of acetalde-
hyde and butanol-1 promoted by their SiO2–SO3H catalyst (from
chlorosulfonic acid and silica) [13]. These evidences indicate a relation
of the catalysts Lewis basic sites to an “activated” alcohol, and the fol-
lowing reaction scheme can be proposed (Fig. 5; ** refers to catalyst
bound species; Ph = phenyl, and Bz = benzyl):
3.2. Reactions
To evaluate the action of our synthetic silicas for the specific reaction
of condensing benzoic acid and benzyl alcohol, we run a blank experi-
ment using the silica gel as the catalyst. Reagents and gel were irradiat-
ed for 5 min, and no products were detected. However, when reagents
were irradiated for 5 min in the presence of only 1.0%-w/w sulfonated
Table 1
Esterification/etherification reactions.
Catalyst (w/w%)
Temp.(°C)
% yield Benzyl ether
% yield Benzyl Benzoate
1.0
2.0
4.0
6.0
7.0
8.0
9.0
10.0
20.0
73
72
72
72
70
71
70
71
69
18.97
40.29
85.47
93.61
98.97
94.11
93.61
85.36
1.03
5.89
6.39
14.64
100.00
Reaction conditions: benzoic acid, 5.60 mmol; benzyl alcohol, 1.00 mmol; MW 360 W, 5
min.
Fig. 3. FE-SEM images of the sulfonated silica.

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S.L. Barbosa et al. / Catalysis Communications 68 (2015) 97–100
Table 2
Catalyst reusability.
Run
Catalyst acidity (mmol H+/g)
%Yield
1
2
3
4
5
1.44
0.35
0.22
0.20
0.11
98.97
97.63
96.61
95.11
82.07
Catalyst load 7%; MW 360 W, 5 min. Products quantified by CG/MS.
The amenable conditions of the reported preparations of benzyl ben-
zoate and dibenzyl ether facilitate the catalyst recovery, which is filtered
from the reaction mixture, washed with ether and dried at 150 °C. In
assessing the reusability, a sample of the spent catalyst was titrated be-
fore each run, (Table 2). Note that the quantity of sulfonic groups on the
catalyst enough for sustaining 0.20 mmol H+/g (run 4, Table 2), still pro-
motes a very good esterification yield, probably influenced by the high
water tolerance of the catalyst, which is characteristic of its textural
parameters.
4. Conclusion
A low cost silica gel produces a highly efficient sulfonated catalyst for
microwave-induced preparation of benzyl benzoate in 93%, by the es-
terification of the otherwise unreactive benzoic acid and benzyl alcohol.
The small surface area and large pores of the catalyst also facilitate
quantitative and selective preparation of dibenzyl ether.
Fig. 5. Suggested reaction scheme for the esterification and etherification of aromatic sub-
strates by SiO2–SO3H.
Reaction 1 represents the classical protonation of the acid by the
SiO2–SO3H Bronsted sites. The protonated species can react with the ex-
cess acid forming an anhydride, but this would be too reactive towards
hydrolysis (reaction 2) or decomposition by the alcohol (reaction 3);
the net effect of (2 and 3) would appear as an increase of the ester con-
tent in the mixture. Reactions 4–10 should be highly dependent on the
reagents/catalyst w/w ratio; at low catalyst ratios, reaction 4 would be
responsible for the retention of water, favoring the production of the
ester. At high catalyst ratios, ours [10] and others' [13] results point to
the increase of the role of the adsorbed alcohol. Proton exchange be-
tween solvated species or direct protonation from the catalyst and sub-
sequent adsorption of the cationic alcohol intermediate (reactions 5 and
6) could bring about the formation of ester (reaction 7) and ether
(reaction 8) or, more likely, provide a mechanism through which the
adsorbed species could interact at the level of the surface of the catalyst
(reactions 9 and 10), therefore producing high quantities of ether, de-
spite the enormous quantity of organic acid used in the reactions (mol
ratio acid/alcohol 5.6:1); in this particular, the textural properties of
this SiO2–SO3H catalyst, such as small surface area with large pore diam-
eters, should facilitate water adsorption and benzyl alcohol activation.
Acknowledgments
This work was supported by the CNPq, Fapesp and Fapemig.
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