Article abstract of DOI:10.1016/j.catcom.2018.12.005

Amorphous SiO₂-SO₃H [1] with a small surface area and 1.32-mmol H⁺/g was used for the one-step preparation of solketal from glycerol and acetone; a 20%-w/w catalyst mixture (10% [1] and 10% (Bu₄N)(BF₄) was found to be very efficient for the synthesis of disolketal ether and of oxygenated biofuels fatty acids solketal esters (FASEs), by direct esterification of the caprylic, lauric, stearic, oleic and linoleic acids with solketal in a 4:1 acid:solketal ratio in refluxing toluene. Solketal acetate was also produced in quantitative yields.

Full text of DOI:10.1016/j.catcom.2018.12.005

Catalysis Communications 120 (2019) 76–79
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Oxygenated biofuels: Synthesis of fatty acid solketal esters with a mixture of
sulfonated silica and (Bu N)(BF ) catalyst
4
4
a,⁎
a
a
b
Sandro L. Barbosa , Pamela C. Lima , Wallans T.P. dos Santos , Stanlei I. Klein ,
c
c
Giuliano C. Clososki , Franco J. Caires
a
Department of Pharmacy, Universidade Federal dos Vales do Jequitinhonha e Mucuri-UFVJM, R. da Glória, 187, CEP-39, 100-000 Diamantina, MG, Brazil
Department of General and Inorganic Chemistry, Institute of Chemistry, São Paulo State University-Unesp, R. Prof. Francisco Degni 55, Quitandinha, CEP-14, 800-900
b
Araraquara, SP, Brazil
c
Department of Physics and Chemistry, Faculdade de Ciências Farmacêuticas de Ribeirão Preto, São Paulo University-USP, Av. do Café s/n, CEP-14, 040-903 Ribeirão
Preto, SP, Brazil
A R T I C L E I N F O
A B S T R A C T
2 3
-SO
H [1] with a small surface area and 1.32-mmol H+/g was used for the one-step preparation
Keywords:
Amorphous SiO
of solketal from glycerol and acetone; a 20%-w/w catalyst mixture (10% [1] and 10% (Bu N)(BF ) was found to
Sulfonated silica catalyst
Tetrabutylammonium tetrafluoroborate
catalyst
4
4
be very efficient for the synthesis of disolketal ether and of oxygenated biofuels fatty acids solketal esters
FASEs), by direct esterification of the caprylic, lauric, stearic, oleic and linoleic acids with solketal in a 4:1
(
Crude glycerol
acid:solketal ratio in refluxing toluene. Solketal acetate was also produced in quantitative yields.
Solketal synthesis
Fatty acid solketal esters
Solketal acetate
1. Introduction
esterification of solketal and fatty acids using heterogeneous acid cat-
alysts.
Glycerol becomes a problem because of the large amounts produced
We report herein both a direct preparation of the precursor solketal
using SiO -SO H as catalyst, and its high yield esterifications to the
FASEs of caprylic, lauric, stearic, oleic and linoleic acids using the
sulfonated silica SiO -SO H activated by (Bu N)(BF ) under refluxing
toluene. The short chain acetic acid is easily esterified with solketal
under these reaction conditions. A side reaction to the esterifications
was the etherification of the solketal to the disolketal ether, which is the
key intermediate for the synthesis of the glycerol linear oligomer
in the biodiesel manufacturing [1]. It is, therefore, necessary to valorize
this chemical to lower the costs of production of the associated biofuel.
One research in that direction is the reaction of glycerol with ketones to
form ketal additives for biodiesel, such as solketal [2]. The transfor-
mation of the glycerol ketals into esters of long chain fatty acids is also
promising; when mixed to biodiesel, itself a collection of fatty acid
methyl or ethyl esters, these esters increase the oxygen content of the
biofuel, which is desirable [3]. Importantly, Solketal Fatty Acid Esters
2
3
2
3
4
4
2 2 2
((HO)CH -CH(OH)-CH ) O; oligomers of glycerol are important to the
(
FASEs) can be converted back to Fatty Acid Glycerol Esters (FAGEs) by
food and cosmetics industries and are very difficult to produce by direct
procedures [6a]. We therefore optimized our two-component catalyst
system for the synthesis of that ether as well.
the opening of the ketal rings, and biofuel formulations based on
mixtures of biodiesel with FAMEs or/and FAGEs have been described as
very efficient [7,8], and therefore, methods for the production of FASEs
are desirable. So far, FASEs have been obtained by the transesterifica-
tion of mixtures of fatty acid methyl esters with solketal using sodium
carbonate [5] or oxides derived from calcium rich natural materials [6]
as catalysts, or by the esterification of fatty acids with solketal using
enzymatic processes [3], or catalysts such dicyclohexylcarbodiimide
2. Experimental
2.1. Instrumentation
IR spectra of solid samples were recorded as KBr pellets in the
−
1
[
4a], acid resins [4b] and p-toluene sulfonic acid [4c]. To the best of
4000–400 cm range on a Varian 640 spectrometer. FASEs and acetic
our knowledge, there is no report of a method for the direct
acid solketal ester contents and yields were determined with a
⁎
Corresponding author.
E-mail addresses: sandro.barbosa@ufvjm.edu (S.L. Barbosa), fjcaires@usp.br (F.J. Caires).
https://doi.org/10.1016/j.catcom.2018.12.005
Reçu le 13 septembre 2018; Reçu en forme révisée le 8 novembre 2018; accepté le 8 décembre 2018
Available online 10 December 2018
1566-7367/ © 2018 Published by Elsevier B.V.

S.L. Barbosa et al.
Catalysis Communications 120 (2019) 76–79
Shimadzu GC/MS-QP 2010 equipped with an AOC-5000 auto injector
and a 30-m Agilent J&W GC DB-5 MS column. Direct insertion spectra
were measured at 70 eV. Quantitative analyses were performed on a
Shimadzu GC-2010 gas chromatograph equipped with a flame ioniza-
2.4.4. Synthesis of acetic acid solketal ester
In a 500 mL, two-necked round bottom flask, 200 mL of toluene
were mixed to 0.1321 g of SiO -SO H, 0.1321 g of (Bu N)(BF ),
6.9680 g of acetic acid (40 mmol) and 1.321 g of solketal (10 mmol),
20% catalyst mixture ratio to the solketal. The reaction was heated for
30-min, and the final temperature of the slurry did not exceed 120 °C.
2
3
4
4
1
13
tion detector. H and C NMR spectra were recorded on Bruker Avance
00 and Avance 500 spectrometers. All reactions were performed under
4
atmospheric pressure, using heating mantle and monitored by TLC with
pre-prepared plates (Silica Gel 60 F 254 on aluminum). Merck silica gel
The vessel was cooled to room temperature, the SiO
and toluene was distilled off. The organic residue was diluted with
20 mL diethyl ether and filtered in silica to remove (Bu N)(BF ). The
ether fraction washed with 100.0 mL of saturated NaHCO , dried over
anhydrous MgSO and concentrated under reduced pressure. The re-
sidue was purified on a chromatographic column using CH Cl and
ethyl acetate as eluents to obtain the pure product as a colorless oil in
2 3
-SO H was filtered
(
230–400-mesh) was used for the purification of products by flash
4
4
column chromatography using CH
ents.
2
Cl
2
and ethyl acetate (8:2) as elu-
3
4
2
2
2.2. Raw materials and chemicals
1
8
6% yield. Acetic acid solketal ester was identified by GC/MS, H and
1
3
C NMR spectroscopy. The catalyst mixture was recovered as before.
Glycerol was obtained from the transesterification of waste cooking
oil with methanol [9]. All other chemicals were used as purchased from
VETEC.
2.4.5. Synthesis of fatty acid solketal esters
2
All the reactions involved 200 mL of toluene, 0.1321 g of SiO -
SO
amount of catalyst on each run was adjusted to maintain a constant
0%- 10% SiO -SO H and 10% (Bu N)(BF ) mass-to-mass ratio to the
solketal. 40.0 mmol of each of the fatty acids were used: 5.7685 g of
caprylic acid, 8.01271 g lauric acid, 11.3792 g stearic acid, 11.2179 g
linoleic acid and 11.2988 g of oleic acid, respectively. The reactions
were heated for 2 or 3-h depending of the fatty acid (Table 1) using a
3 4 4
H, 0.1321 g of (Bu N)(BF ), and 1.3216 g solketal (10 mmol). The
2
2 3
.3. Preparation of the silica gel and sulfonated silica (SiO -SO H)
2
2
3
4
4
The preparation of silica gel and the sulfonated silica SiO
catalyst have been reported [2,10].
2 3
-SO H
2
2
.4. Typical procedures
500 mL two-necked round bottom flask equipped with a reflux con-
.4.1. Synthesis of solketal from crude glycerol and acetone with SiO
H catalyst
Acetone (23.2320 g; 400.0 mmol), glycerol (20.0 mmol; 18.4160 g)
and the catalyst SiO -SO H (3.6832 g, 20%-w/w glycerol) were mixed
2
-
denser. The final temperature of the slurries did not exceed 120 °C. The
vessels were cooled to room temperature, the mixtures were filtered
and toluene was eliminated by distillation. The organic extracts were
diluted with 20 mL diethyl ether and filtered in silica to retired the
SO
3
2
3
in a 500 mL round bottom flask equipped with a reflux condenser and
refluxed for 8-h. The mixture was then cooled and the solid catalyst
filtered. The organic layer was extracted with 20.0 mL of a saturated
(
Bu
4
N)(BF
4
). The ethereal phase was washed with 100.0 mL of a sa-
, dried over anhydrous MgSO and
turated, aqueous solution of NaHCO
3
4
concentrated under reduced pressure. The residue was purified on a
chromatographic column using hexane and ethyl acetate as eluents to
obtain the pure FASEs as colorless oils very good yield (CG/MS,
Table 1). The catalysts were recovered as before. The FASEs were
identified by GC/MS, ESI-TOF MS and 1H and C NMR spectroscopy.
3 4
solution of NaHCO , dried with MgSO and concentrated; CG/MS
analysis of the extract pointed to 99% yield. The solketal was purified
by distillation (86-87 °C/13-Torr), to obtain the pure product as a col-
orless oil. The catalyst was transferred to a muffle and heated for 2-h at
13
2
00 °C, cooled and stored in a desiccator before reuse.
(
Voir Scheme 1.)
2
2 3
.4.2. Synthesis of disolketal ether from solketal with SiO -SO H catalyst
3
. Results and discussion
In a similar apparatus, solketal (1.3216 g; 10.0 mmol), the catalyst
-SO H (0.2643 g; 20%-w/w) and 100 mL of toluene were main-
SiO
2
3
In a previous work [10], a sulfonated silica was prepared from the
tained under reflux for 5-h. The mixture was cooled, the solid catalyst
filtered, and the solution extracted with 20 mL of a concentrated NaCl
treatment of a silica gel with sulfuric acid, the silica itself being made in
the laboratory, starting from fine construction sand. Both gel and cat-
alyst were non-crystalline, showing bell-shaped x ray diffraction pat-
terns centered at 2θ = 23 . However, the overall structure of the silica
was remarkably different from that of the catalyst, as indicated by the
appearance, upon sulfonation, of a splitting of the infrared absorptions
of the Si-O-Si groups in the 1210–1040 cm
vealed a catalyst with a specific surface area of 115 m /g and pore
solution and dried with MgSO
The pure product was obtained as a colorless oil by distillation
119–120 °C/7-Torr), 90% recovered yield. The catalyst was recovered
as before.
4
. CG/MS analysis pointed to 98% yield.
o
(
−1
region. BET analysis re-
2.4.3. Synthesis of disolketal ether from solketal using the mixture SiO
2
-
2
SO
3
H and (Bu
In a similar apparatus, solketal (1.3216 g; 10.0 mmol), the catalyst
-SO H/(Bu N)(BF ) (0.2642 g; 20%-w/w 10%-w/w SiO -SO H and
N)(BF ), 0.1321 g each) and 100 mL of toluene were
4
N)(BF
4
) as catalyst
3 −1
volume of 0.38 cm g . The catalyst was very effective for the ester-
ification and etherification of benzyl alcohol under microwave irra-
diation [10].
SiO
1
2
3
4
4
2
3
0%-w/w (Bu
4
4
When a large excess of acetone is refluxed with glycerol for 8 h in
maintained under reflux for 1-h. The mixture was cooled, the solid
catalyst filtered, and the toluene was then eliminated by distillation.
The residue was mixed with 30 mL of diethyl ether and filtered in silica
2 3
the presence of SiO -SO H (20%w/w-glycerol), the result is the sole
Table 1
gel for the removal of (Bu
4 4
N)(BF ). The ether extract was washed with
Synthesis yields (%) of solketal esters.
2
0 mL of a concentrated aqueous NaCl solution and dried with MgSO .
4
Solketal esters
Time, h
Yield, %
CG/MS analysis pointed to 96% yield. The ether was distilled off, and
the pure product was obtained as a colorless oil by distillation
Acetic acid SE
Caprylic FASE
Lauric FASE
Stearic FASE
Oleic FASE
0.5
2
2
3
3
99.10
92.03
93.44
92.35
93.50
97.08
(
119–120 °C/7-Torr), in 89% recovered yield. The silica catalyst was
recovered as before. The ammonium salt was recovered by washing the
silica gel used for the second filtration step with 20 mL of ethyl acetate.
(Bu
4 4
N)(BF ) was dried in an oven at 80 °C for 2-h after removal of the
Linoleic FASE
3
solvent.
77

S.L. Barbosa et al.
Catalysis Communications 120 (2019) 76–79
Scheme 1. Synthesis of solketal and solketal esters.
Table 2
esterification reaction is not completely clear yet. It was important to
note that when we sought to synthesize the linoleic FASE using linoleic
acid and solketal in the ratio of 4:1, toluene as solvent and only the
Dependence of the molar ratio linoleic acid: solketal for the production of li-
noleic acid FASE.
(
Bu
after 48-h of reflux, 10% yield of linoleic solketal ester and 15% yield of
disolketal ether. These findings demonstrate that (Bu N)(BF ) isn't very
4 4
N)(BF ) in the ratio 10%-w/w solketal as catalyst, we obtained,
Linoleic acid/Solketal
0.5:1
1:1
2:1
3:1
4:1
Linoleic FASE
Di-Solketal ether
5%
95%
17%
83%
63%
37%
72%
28%
100%
0
4
4
effective, but nonetheless it can promote the condensation reactions of
etherification and esterification in solution. In a previous work [10] we
have pointed out that esterifications, over the SiO
pended on the fact that the reactive species R-C(OH)
2 3
-SO H catalyst, de-
formation of 2,2-dimethyl-1,3-dioxolan-4-yl methanol, known as solk-
etal, with no traces of its 6-membered ring isomer. The typical yield of
such preparations is 99%, the remaining 1% being unreacted glycerol.
The synthesized solketal was mixed to acetic acid in a 1:4 mixture,
+
2
, derived from
the proton transfer from the catalyst surface to the acids, would be
dispersed into the solution, where it would react with the alcohol
partner to yield the required ester product. In the same article, we also
pointed out that the concurring etherification reactions with this cata-
lyst would arise from the catalyst surface-bound alcohol-derived species
2 3
no solvents, with a catalyst load of 10% SiO -SO H w/w to solketal; the
acetic acid solketal ester [11] was obtained in 96.8% yield after 5 h of
reaction at the reflux temperature of the mixture; attempts to perform
this transformation with lower ratios solketal:acid led to an increased
formation of disolketal ether. A similar attempt to esterify solketal with
[
4 4
10]. It is possible that the addition of (Bu N)(BF ) or related species
may alter the catalyst-toluene interface to a condition somewhat similar
to an ionic liquid state, which would be beneficial for the stabilization
linoleic acid in a 1:4 mixture of reagents, no solvents, showed that SiO
2
-
+
of the reactive R-C(OH)
2
intermediates. In any case, the data in
3
SO H was inactive, the attempt leading to the recovery of the catalyst
Table 1 show a lowering of the polarity of the R end of the acids tested,
from caprylic −8 carbon atoms- to linoleic acid −18 carbons and two
insaturations. The high yield of the ester synthesis shows therefore the
importance of the phase-transfer characteristic of the tetra-
butylammonium salt. The dependence of the acid to solketal ratio in the
preparation of FASEs is presented in Table 2, using linoleic acid as an
example.
and of the pure acid even after 48 h at 120 °C. It is possible that the
strong polarity of the highly hydrophilic catalyst surface might repel
the long apolar carbon chain of the fatty acid, effectively reducing the
possibility of formation of reactive intermediates. Nonetheless, the low
affinity of the reactant by the catalyst seemed to be reduced by the
addition of toluene as solvent, in the ratio of 1.0 mmol of solketal to
20 mL of toluene. Therefore, a new reaction process was attempted,
composed of linoleic acid 4:1 solketal, catalyst load 10%-w/w solketal,
and toluene as solvent. This mixture was left reacting for 72-h under
reflux (circa 120 °C), by which time TLC monitoring revealed the total
consumption of the solketal. Under these conditions, the major product
of the reaction was the disolketal ether, which was obtained in 66.50%
yield, the desired linoleic FASE comprising the remaining 33.50% of the
product.
4
. Conclusions
2 3
The amorphous catalyst SiO -SO H produces solketal from acetone
and glycerol quantitatively. It can be activated with quaternary am-
monium transfer phase agents, to catalyze either the quantitative pro-
duction of disolketal ether, or, in a 4:1 fatty acid to solketal, the
synthesis of fatty acid solketal esters of the lauric, caprylic, oleic, li-
noleic, and stearic acids in excellent yields, within 2–3-h in refluxing
toluene; in a 4:1 mixture of acetic acid and solketal, solketyl acetate can
be formed using the mixed catalyst within 30-min, no solvents. Both
catalyst and quaternary ammonium salt can be recovered for further
use.
The beneficial use of toluene as solvent for the esterification of
solketal using SiO
the synthesis of the acetic acid solketal ester using acetic acid and
solketal in a 4:1 mixture, SiO -SO H 10%-w/w to solketal as catalyst,
2 3
-SO H as catalyst became more evident by repeating
2
3
and 20 mL toluene per mmol of solketal as solvent. Under these con-
ditions, TLC monitoring and CG-MS data indicated the formation of the
acetic acid solketal ester in 97.31% yield in only 2 h reflux. More im-
portant was the observation that aliphatic solvents like heptane, for
instance, failed to promote the reaction, making it clear that a solvent
with increased polarity, such as the mixture toluene-acetic acid, was
important for the stabilization of the reactive intermediates of the es-
terification process.
The problem of increasing the polarity of the aromatic solvent by
the addition of a non-interfering polar solute seemed to be nicely solved
by the addition of tetra n-butylammonium tetrafluorborate to the re-
action mixture. For instance, the 4:1 mixture of acetic acid and solketal,
Acknowledgements
The authors acknowledge the financial support by Fapemig and
CAPES.
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