Heterogeneous esterification of fatty acids with methanol catalyzed by Lewis acidic organozirconium complexes with Keggin-type mono-aluminum-substituted polyoxotungstates

Article abstract of DOI:10.1016/j.catcom.2017.03.025

Two Lewis acidic organozirconium complexes with α-Keggin-type mono-aluminum-substituted polyoxotungstates, [(n-C₄H₉)₄N]₆[α-PW₁₁Al(OH)O₃₉ZrCp₂]₂ (Cp = η⁵-C₅H₅^?) (TBA-P-Al-Zr) and [(n-C₄H₉)₄N]₆[α-SiW₁₁Al(OH)₂O₃₈ZrCp₂]₂·2H₂O (TBA-Si-Al-Zr), were used as heterogeneous catalysts for the esterification of various fatty acids with methanol. For the esterification of linoleic acid at 80 ± 2 °C, TBA-P-Al-Zr exhibited 83% conversion after 6 h, approximately six times higher than that of TBA-Si-Al-Zr. TBA-P-Al-Zr also exhibited 69–90% conversion for the esterification of oleic acid, palmitic acid, myristic acid, and lauric acid with methanol at 80 ± 2 °C.

Full text of DOI:10.1016/j.catcom.2017.03.025

Catalysis Communications 96 (2017) 41–45
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Catalysis Communications
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Short communication
Heterogeneous esterification of fatty acids with methanol catalyzed by Lewis
acidic organozirconium complexes with Keggin-type mono-aluminum-
substituted polyoxotungstates
Chika Nozaki Katoa,b,⁎, Tsukasa Ogasawara , Akari Kondo , Daichi Kato
a
a
a
a
Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan
Green Chemistry Research Division, Research Institute of Green Science and Technology, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Two Lewis acidic organozirconium complexes with α-Keggin-type mono-aluminum-substituted polyoxotung-
5
−
Organozirconium complex
Polyoxometalate
Esterification
Fatty acids
Heterogeneous catalysis
states, [(n-C H ) N] [α-PW Al(OH)O ZrCp ] (Cp = η -C H ) (TBA-P-Al-Zr) and [(n-C H ) N] [α-SiW Al
4
9
4
6
11
39
2
2
5
5
4
9
4
6
11
(OH)
2
O
2 2 2
38ZrCp ] ·2H O (TBA-Si-Al-Zr), were used as heterogeneous catalysts for the esterification of various
fatty acids with methanol. For the esterification of linoleic acid at 80 ± 2 °C, TBA-P-Al-Zr exhibited 83%
conversion after 6 h, approximately six times higher than that of TBA-Si-Al-Zr. TBA-P-Al-Zr also exhibited
69–90% conversion for the esterification of oleic acid, palmitic acid, myristic acid, and lauric acid with methanol
at 80 ± 2 °C.
1. Introduction
biodiesel production in heterogeneous systems [14]. Paralleling
the efforts to develop HPA-based acid catalysts, Lewis acidic
Biodiesel consists of a mixture of alkyl (methyl or ethyl) esters of
POMs
have
also
22(OH)10(H
(n-Bu N)
O,
-O)
)}
]·20H
O [20], Na [{Zr
[21], and Cs [(γ-SiW10
been
O) (W
[{W 18Zr(μ-OH)}
15H[Zr(α -P
(H O) ]·10H
]·4H [18,19], Rb
(NH
(μ-OH)
{Zr(H
developed.
10H) (GeW
]·2H
]·25H
[16,17], (n-Bu
Examples
are
]·85-
10[Zr
Na14[Zr
N) H[γ-Si-
O)10(β-
33H) ]·
]·17H
[22].
long chain free fatty acids and is industrially produced by the
transesterification of oil or fats (triglycerides) with methanol or ethanol
using a stoichiometric amount of a corrosive homogeneous base
catalyst (e.g., KOH, NaOH, or methoxides) [1]. However, this process
is strongly affected by the presence of free fatty acids (FFAs) because of
possible saponification side reactions. Therefore, various types of acid
catalysts, especially solid acid catalysts, have been developed for the
esterification of FFAs with methanol or ethanol. These include sulfated
zirconia [2,3], sulfonated carbon [4,5], Nafion-based composites [6],
sulfonated and incompletely carbonized sugar, starch, or cellulose
Na10
K
22[Zr24
O
2
2
2
O
2
9
O
34
)
O,
4
8
(GeW O
31
)
2
H
2
O
[15],
4
6
5
O
2
2
(Et
2
NH
2
)
(PW11
O
O
39
)
2
]·7H
(μ
36{Al(OH
2
K
2
2
W
17
O
61
)
2
2
O,
4
(P
W
As
2
W
16
59
)
2
3
2
(OH)
(μ-OH)
O,
(H
2
2
4
2
O
4
3
III
10
O
2
2
2
2
O
2
Na
O)10(β-Sb
}(α-1,4-PW10
-O)(μ-OH) ]·26H
2 4 2
[Al (H
III
III
III
W
9
O
33H)
2
2
4
)
2
Na
(μ
O)}
2
[Al
-O)
(μ
4
(H
2
W
9
O
2
20H
2
8
4
2
O)
4
2
3
2
O
37
)
2
2
O
8
O
36
)
2
2
4
4
6
2
O
Although these Lewis acidic POMs are efficient catalysts for various
organic syntheses, few examples of POM-based heterogeneous catalysts
for biodiesel production have been reported; for example, (C16TA)
[
7,8], zirconium-containing metal organic frameworks [9], layered
zinc laurate and zinc stearate [10], and pyrene-based porous organic
polymers [11].
4
H TiPW11O40 (C16TA = cetyltrimethyl ammonium) has both Brønsted
and Lewis acid sites and catalyzes the esterification of palmitic acid
with methanol at 65 °C with 94.7% conversion after 6 h [23].
Recently, we synthesized the two organozirconium complexes with
α-Keggin-type mono-aluminum-substituted polyoxotungstates, [(n-
Polyoxometalates (POMs) are also of interest in the fields of
catalysis, surface science, and materials science because their chemical
properties such as redox potentials, acidities, and solubilities in various
media can be finely tuned by choosing appropriate constituent elements
and countercations [12]. Among various types of POMs, free acidic
POMs—known as heteropolyacids (HPAs)—are well-known acid cata-
lysts because of their strong Brønsted acidity [13]. Supported HPAs
5
−
C
4
H
9
)
4
N]
6
[α-PW11Al(OH)O39ZrCp
2
]
2
(Cp = η -C
38ZrCp
5
H
5
) (TBA-P-Al-Zr)
·2H O (TBA-Si-Al-
[24] and [(n-C
H
4 9
)
4
N] [α-SiW11Al(OH)
6
2
O
2
]
2
2
Zr) [25]. We also demonstrated the catalytic activities of these
compounds for the Meerwein-Ponndorf-Verley reduction of ketones
with 2-propanol in both homogeneous and heterogeneous systems [25].
In this paper, we report the catalytic performances of TBA-P-Al-Zr and
(
3 3
e.g., H PW12O40 and/or H PMo12O40 supported on zirconia, activated
carbon fibers, zeolites, and silica) exhibit excellent catalytic activities or
⁎
Corresponding author at: Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan.
E-mail address: kato.chika@shizuoka.ac.jp (C.N. Kato).
http://dx.doi.org/10.1016/j.catcom.2017.03.025
Received 12 January 2017; Received in revised form 24 March 2017; Accepted 24 March 2017
Available online 29 March 2017
1
566-7367/ © 2017 Elsevier B.V. All rights reserved.

C.N. Kato et al.
Catalysis Communications 96 (2017) 41–45
(a)
(b)
C
Zr
W
Al
O1
O4
O3
O2
Fig. 1. (a) Polyhedral representation of the polyoxoanions P-Al-Zr and Si-Al-Zr. WO
SiO units are represented by the red tetrahedra. The black balls are zirconium atoms. Both P-Al-Zr and Si-Al-Zr are dimeric compounds, in which the two {SiW11AlO40} units are bridged
by two “bent sandwich” Cp
and O2) and at (O1 – O4), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
6 6 4
and AlO units are represented by the white and blue octahedra, respectively. The internal PO or
4
2
+
2
Zr
fragments and (b) the partial structure around the zirconium and aluminum sites. The two and four protons in P-Al-Zr and Si-Al-Zr were located at (O1
TBA-Si-Al-Zr for the esterification of various FFAs with methanol in a
heterogeneous system. The molecular structures of TBA-P-Al-Zr and
TBA-Si-Al-Zr are shown in Fig. 1(a).
(substitution method) consisting of 85% H
Chemical shifts for the 31P NMR spectra are reported as negative on the
PO
was carried out with 0.010 mol/L tetra-n-butylammonium hydroxide as
a titrant under an Ar atmosphere [29]. TBA-P-Al-Zr and TBA-Si-Al-Zr
3 4
PO in a sealed capillary.
δ scale for resonances upfield of H
3
4
(δ 0). Potentiometric titration
2. Experimental
(
0.010 mmol) were dissolved in acetonitrile (20 mL) at 25 °C, and the
2
.1. Materials
solution was stirred for approximately 5 min. The titration data were
obtained with a pH meter (Mettler Toledo). Data points were obtained
in millivolts. The tetra-n-butylammonium hydroxide solution
(0.010 mol/L) was syringed into the suspension in 0.10-equivalent
intervals. The Brønsted acidities of TBA-P-Al-Zr, TBA-Si-Al-Zr, and
HPW in acetonitrile were evaluated using Hammett indicators (dicin-
TBA-P-Al-Zr [24], TBA-Si-Al-Zr [25], [(n-C
4
H
9
)
4
N]
4
[α-PW11{Al
(
(
[
OH
OH
24], (n-Bu
2
)}O39
)}O39]⋅H
N)
{Zr(H
]·25H
PW12 40⋅23H
were obtained from Nippon Inorganic Colour & Chemical Co., Ltd.
Japan) and Aldrich, respectively. The number of water molecules in
HPW was determined by TG/DTA analysis. HPW supported on ZrO
abbreviated as HPW/ZrO ) was prepared according to the literature
28]. The loading of HPW on ZrO was 20 wt%. Linoleic acid (> 88%),
]
(TBA-P-Al) [26], [(n-C
O (TBA-Si-Al) [25], Cp Zr(OTf)
H[γ-SiW10 36{Al(OH )} (μ-OH)
O)} (μ -O)(μ-OH) ]·26H [22], and
O [27] were synthesized as described in the litera-
O (abbreviated as HPW) and ZrO (99%, 5 μm)
4
H
9
)
4
N]
·THF (OTf = CF
]·4H O [18], Cs
15H[Zr(α
4 0.5
K H0.5[α-SiW11{Al
−
2
2
2
2
3
SO
3
)
4
3
O
2
2
2
2
8
[(γ-
SiW10
P
ture. H
O
36
)
2
2
4
4
6
2
O
K
2
-
namalacetone; pK
a
value of the protonated indicator is −3.0). The
+
−5
2
W
17
O
61
)
2
2
concentrations of the indicator and H were adjusted to 3.5 × 10
−
3
3
O
2
2
and 4.9 × 10
M, respectively [30]. Specific surface areas and pore
sizes were obtained through adsorption-desorption experiments in the
BELSORP-max (MicrotracBEL Co., Ltd. Japan). Before analysis, the
samples were degassed for 0.5 h under vacuum at 80 °C. The measure-
ments were performed at −196 °C, and the specific surface area was
calculated from the adsorption isotherm using the Brunauer-Emmett-
Teller (BET) method.
(
2
(
[
2
2
oleic acid (> 99%), and palmitic acid (> 95%) were obtained from
Wako Pure Chemical Industries, Ltd. Myristic acid (> 99%) and lauric
acid (> 98%) were obtained from Tokyo Chemical Industry Co., Ltd.
All reagents and solvents were obtained and used as received from
commercial sources.
2.3. Esterification of FFAs with methanol
A sample catalyst was placed in a 60 mL Schlenk tube in air. FFAs
(
linoleic acid, oleic acid, palmitic acid, myristic acid and lauric acid)
and methanol (49.3 mmol) were added using a micropipette. The
reaction mixture was heated in an oil bath at 60 ± and
2.2. Instrumentation/analytical procedures
2
Infrared spectra were recorded on a Perkin Elmer Spectrum100 FT-
80 ± 2 °C. The reaction solution was analyzed by liquid chromato-
graphy (Shim-pack VP-ODS column, 4.6 mm × 150 mm). Values of the
products were assigned by comparing the obtained results with the
analysis results obtained from analyzing authentic samples under the
same conditions. The conversion (%) and turnover number (TON) was
IR spectrometer in KBr disks at room temperature. Thermogravimetric
TG) and differential thermal analysis (DTA) data were obtained using a
Rigaku Thermo Plus 2 series TG/DTA TG 8120. TG/DTA measurements
(
−
1
were performed in air with a temperature increase of 4 °C min
1
13
between 20 and 500 °C. Solution H (600.17 MHz), C (150.92 MHz),
and P (242.95 MHz) NMR spectra were recorded in 5-mm-outer
diameter tubes on a JEOL ECA-600 NMR spectrometer. The H and
calculated as {[mol of substrate]
0
− [mol of substrate]
t
}/[mol of
3
1
substrate] × 100 and [mol of corresponding product]
0
t
/[mol of cata-
1
lyst], respectively.
1
3
3
C NMR spectra were measured in acetonitrile-d with reference to
tetramethylsilane (TMS). Chemical shifts are reported as positive for
3. Results and discussion
3
1
resonances downfield of TMS (δ 0). The
P NMR spectra were
measured in acetonitrile-d with reference to an external standard
3
We examined the catalytic activities of TBA-P-Al-Zr and TBA-Si-Al-
4
2

C.N. Kato et al.
Catalysis Communications 96 (2017) 41–45
90
80
70
60
50
40
30
20
10
0
(a)
16
(b)
1
1
1
4
2
0
8
6
4
2
0
0
1
2
3
4
5
6
0
1
2
3
4
5
6
Fig. 2. Time course for the esterification of linoleic acid with methanol catalyzed by (a) TBA-P-Al-Zr and (b) TBA-Si-Al-Zr. Reaction conditions: TBA-P-Al-Zr and TBA-Si-Al-Zr
19.1 μmol), linoleic acid (0.28 mmol), methanol (49.3 mmol), and reaction temperature 80 ± 2 °C.
(
Zr during the esterification of linoleic acid with methanol at
0 ± 2 °C. The molar ratio of linoleic acid to methanol was 1:176.
Time courses are shown in Fig. 2. The catalytic activities are summar-
ized in Table 1. Methyl linoleate was the main product, with > 99%
selectivity. Only a slight amount of methyl linoleate was observed in the
absence of the catalyst. For TBA-P-Al-Zr, the conversion after 6 h was
TBA-P-Al-Zr and TBA-Si-Al-Zr were found to be insoluble in the
mixture of linoleic acid and methanol, and they did not leach into the
reaction solution. During the esterification reactions, water was formed
as a by-product; however, the Cp ligands were not released from the
8
2
+
zirconium atoms, and Cp
surface of [α-PW11{Al(OH
2
Zr
)}O39 4
(Si-Al). This was also confirmed by the following experiments:
fragments were maintained on the
−
2
]
2
(P-Al) and [α-SiW11{Al(OH )}
5
−
83%, reaching 99% after 12 h. At 60 ± 2 °C, the reaction rates
39
O ]
decreased; however, the conversion after 6 h and 48 h reached to 8.8
and 95%, respectively, as shown in Fig. S1. In contrast, TBA-Si-Al-Zr
showed a slower initial rate and lower conversion (15% after 6 h) than
those of TBA-P-Al-Zr even at 80 ± 2 °C. The catalytic activities of
TBA-P-Al-Zr were approximately six times higher than those of TBA-Si-
Al-Zr, suggesting that the internal atoms in the Keggin units signifi-
cantly affected the catalytic activities. After stirring the reaction
solutions at 80 ± 2 °C for 6 h, TBA-P-Al-Zr and TBA-Si-Al-Zr were
TBA-P-Al-Zr or TBA-Si-Al-Zr (19.1 μmol) was stirred at 80 ± 2 °C for
6 h in methanol (2 mL) containing water (0.28 mmol), followed by
1
filtration through a membrane filter (JG 0.2 μm) immediately. The
H
NMR measurements in acetonitrile-d
3
of the filtrates did not detect the
fragments. Furthermore, to confirm
2
+
2
signals of Cp ligands and Cp Zr
whether TBA-P-Al-Zr and TBA-Si-Al-Zr acted as heterogeneous cata-
lysts, we also carried out the experiments as follows: TBA-P-Al-Zr or
TBA-Si-Al-Zr (19.1 μmol) was stirred at 80 ± 2 °C for 6 h in methanol
(2 mL), followed by filtration through a membrane filter (JG 0.2 μm)
immediately. Then, linoleic acid (0.28 mmol) was added to the filtrates,
and stirred the solutions at 80 ± 2 °C. After 6 h, the conversions were
0.4% and 0%, respectively.
immediately removed off by filtration through a membrane filter (JG
1
0
.2 μm). The H NMR spectra in acetonitrile-d
3
of the filtrates did not
fragments, and tetra-n-
2
+
2
observe signals due to Cp ligands, Cp Zr
butylammonium ions. For TBA-P-Al-Zr, P NMR spectrum in acetoni-
3
1
trile-d
3
of the filtrate also showed no signal. These results indicated that
Next, the solids collected by filtration were washed with methanol,
water, and ethanol. The 31P NMR spectrum in acetonitrile-d
of the
solid of TBA-P-Al-Zr showed three signals at −11.89, −12.25, and
3
Table 1
Esterification of linoleic acid with methanol catalyzed by various compounds.
−
−
12.34 ppm with 0.4:1:1 intensities. The two signals at −12.25 and
12.34 ppm were assigned to the original TBA-P-Al-Zr (δ = −12.25
a
Catalyst (μmol)
System
Selectivity of
Conversion (%)b TONc
and −12.33). A new signal at −11.89 ppm would be due to the species
generated by the interaction of TBA-P-Al-Zr with methanol; this was
methyl linolate
(
%)
consistent with the result that 1H NMR spectrum in acetonitrile-d
3
of
the solid showed a signal of the methyl group of methanol at 3.28 ppm.
The signals of Cp ligands were also observed at 6.86, 6.67, and
6.57 ppm, in which the two signals at 6.67 ppm and 6.57 ppm were
assigned to the original TBA-P-Al-Zr (δ = 6.67 and 6.57). A new signal
TBA-P-Al-Zr
Heterogeneous
Heterogeneous
> 99
> 99
83
15
12
2
(
19.1)
TBA-Si-Al-Zr
19.1)
(
TBA-P-Al (19.1)
TBA-Si-Al (19.1)
Heterogeneous
Homogeneous
Homogeneous
> 99
> 99
> 99
2.3
1.9
99
< 1
< 1
12
at 6.86 ppm would also be generated by the interaction of TBA-P-Al-Zr
1
with methanol. In contrast, the solid of TBA-Si-Al-Zr showed four
H
Cp
(
(
2
Zr
OTf)
23.7)
ZrCl
2
⋅THF
NMR signals of Cp ligands at 6.69, 6.64, 6.62, and 6.59 ppm, which
were the same as those of the original TBA-Si-Al-Zr (δ = 6.69, 6.63,
Cp
2
2
(23.9)
Homogeneous
Homogeneous
Heterogeneous
Hetero- and
homogeneous
−
> 99
> 99
> 99
> 99
98
99
15
99
11
18
−
110e
6.62, and 6.59). Thus, the molecular structures of these compounds
HPW (15.2)
d
were maintained after the esterification reaction although the methanol
interacted with TBA-P-Al-Zr was not completely removed off even after
washing with water and ethanol. When a solid sample of TBA-P-Al-Zr
was used as a catalyst in the second run for the esterification of linoleic
acid with methanol at 60 ± 2 °C, the conversion after 48 h was
approximately 97%, demonstrating that TBA-P-Al-Zr is recyclable. As
control experiments, TBA-P-Al and TBA-Si-Al, which were used as
molecular supports for the syntheses of TBA-P-Al-Zr and TBA-Si-Al-Zr,
ZrO
HPW/ZrO
2
d
2
No catalyst
> 99
3.9
−
a
Reaction conditions: catalyst (15.2–23.9 μmol), linoleic acid (0.28 mmol), methanol
(
49.3 mmol), 80 ± 2 °C, and reaction time 6 h.
b
Conversion (%) = {[mol of linoleic acid]
× 100.
Turnover number (TON) = [mol of product]/[mol of catalyst].
0 t
− [mol of linoleic acid] }/[mol of linoleic
acid]
0
c
showed no reaction at 80 ± 2 °C for 6 h. Cp
Cp ZrCl , which were also used as starting materials for the syntheses
of TBA-P-Al-Zr and TBA-Si-Al-Zr, were soluble in the reaction solution
2 2
Zr(OTf) ⋅THF and
d
e
5
0 mg of sample was used as a catalyst.
TON was calculated based on the number of moles of H
(50 mg).
2
2
3
2
PW12O40⋅23H O (2.53 μmol)
contained in HPW/ZrO
2
4
3

C.N. Kato et al.
Catalysis Communications 96 (2017) 41–45
at 80 ± 2 °C, and exhibited > 98% conversion in a homogeneous
(O1, O2, O3, and O4) (see Fig. 1(b)), respectively. To determine
whether the protons act as Brønsted acid sites, we measured the
Brønsted acidity by using dicinnamalacetone (pK value of the proto-
a
2
+
system. These results indicate that the grafting reactions of Cp
fragments onto the surfaces of TBA-P-Al and TBA-Si-Al were critical to
generating the active centers. H PW12 40⋅23H O (abbreviated as
2
Zr
3
O
2
nated indicator is −3.0) as a Hammett indicator [30]. When a solution
containing dissolved TBA-P-Al-Zr and TBA-Si-Al-Zr in acetonitrile (the
HPW) was soluble in the solution at 80 ± 2 °C, and exhibited > 99%
conversions after 6 h in a homogeneous system. HPW supported on
−
3
concentration of protons was 4.9 × 10
M) was added to an acetoni-
−
5
ZrO
2
(abbreviated as HPW/ZrO
2
) also showed > 99% conversions after
trile solution of dicinnamalacetone (3.5 × 10
M) at approximately
31
6
h; however, HPW was leached into the solution, as observed by
P
25 °C, an acidic color (red) was not observed. As a control experiment,
we used a Brønsted acid catalyst, HPW, which yielded an acidic red
color under the same reaction conditions. Here, the two and four
protons in TBA-P-Al-Zr and TBA-Si-Al-Zr, respectively, can be depro-
3
NMR spectroscopy in acetonitrile-d (Fig. S2). This result suggests that
HPW leached into the solution and HPW grafted onto the surface of the
ZrO acted as homogeneous and heterogeneous catalysts, respectively.
In contrast, ZrO acted as a heterogeneous catalyst; however, the
conversion after 6 h was 15%, which was lower than that of TBA-P-
Al-Zr. Some Lewis acidic POMs (n-Bu N) 36{Al(OH )}
15H[Zr(α ]·25H
(μ-OH) (μ }(α-1,4-
{Zr
O (10.0 μmol) [22] showed no reaction
under the reported reaction conditions.
2
−
2
tonated by the addition of OH , as shown in Fig. S3 and as previously
reported [25]. The deprotonated solids were obtained by the addition
of two and four equiv. of tetra-n-butylammonium hydroxide aqueous
solution to acetonitrile solutions containing dissolved as-prepared TBA-
P-Al-Zr and TBA-Si-Al-Zr, respectively, followed by precipitation from
diethyl ether in air. These solids showed no deactivation of their
catalytic activities for the esterification of benzoic acid with methanol
in acetonitrile at 80 ± 2 °C (conversions after 96 h were 82% and 20%,
respectively). These results indicate that the protons in TBA-P-Al-Zr
and TBA-Si-Al-Zr did not act as Brønsted acid sites, and the Lewis acidic
centers are responsible for the catalysis of the carboxylic acid ester-
ification reaction with methanol under the reported reaction condi-
tions.
4
3
H[γ-SiW10
-P
-O)
36 2
[(γ-SiW10O )
O
2
2
(μ-
2
O
OH)
10.6 μmol)
PW10 ]·17H
O)} -O)(μ-OH)
2
]·4H
2
O
(27.6 μmol)
[18],
Na
K
[{Zr
2
2
W
17
O
61
)
2
(
[16,17,27],
8
4
2
3
2 2 4
(H O)
O
37
)
2
2
O
(8.5 μmol) [21], and Cs
8
(
H
2
4
(μ
4
6
]·26H
2
TBA-P-Al-Zr was also observed to catalyze the esterification of oleic
acid (the conversion after 6 h was 90%), palmitic acid (86%), myristic
acid (75%), lauric acid (69%), and benzoic acid (the conversion after
69 h was 54%) to the corresponding ester products with selectivities
of > 99% at 80 ± 2 °C in a heterogeneous system, as shown in
Table 2. Even in the presence of acetonitrile (where the system was
homogeneous), the conversion after 96 h was 83% for the esterification
of benzoic acid with methanol at 80 ± 2 °C, which was also higher
than that of TBA-Si-Al-Zr (the conversion after 96 h was 22%). TBA-P-
Al-Zr and TBA-Si-Al-Zr were non-porous compounds, and both BET
To investigate the Lewis acid character, we observed the interac-
3
1
tions of Lewis acidic sites in TBA-P-Al-Zr with benzoic acid using
P
NMR spectroscopy. When 1, 2, 4, or 5 equiv. of benzoic acid were added
to a solution containing dissolved TBA-P-Al-Zr, followed by stirring for
3–93 h at approximately 25 °C, a new 31P NMR signal was observed at
around −11.46 ppm, a slight shift from those of as-prepared TBA-P-Al-
Zr (δ = −12.25 and −12.33), and no signal due to TBA-P-Al
(δ = –12.5) was observed (Fig. S4). The intensities of the signals did
not change over 93 h. As a control experiment, we added 50 equiv. of
benzoic acid to HPW and found that the signal (δ = –14.16) did not
shift, as shown in Fig. S5. This also suggests that the Lewis acid sites in
TBA-P-Al-Zr interacted with benzoic acid. For the 13C NMR spectrum in
2
surface area was approximately 1.0 m /g. Thus, the texture of the
catalyst is not the origin of the differences in catalytic activity.
Although it is difficult to draw a simple comparison, the catalytic
activities of TBA-P-Al-Zr were by no means inferior to some current
solid catalysts, e.g., sulfated zirconia (the conversions after 8 h were
7
4.7–98% for the esterification of lauric, myristic, palmitic, stearic,
oleic, and sebacic acids with methanol at 60 °C) [2], sulfonated carbon
70% after 10 h for esterification of oleic acid with ethanol at 80 °C)
4], Nafion-based composites (ca. 80% after 40 h for esterification of
(
[
3
acetonitrile-d of TBA-P-Al-Zr in the presence of 5 equiv. of benzoic
acid, two new signals were observed at 130.91 and 129.33 ppm, which
are likely due to the interaction of benzoic acid with the Lewis acid sites
in TBA-P-Al-Zr, as shown in Fig. S6. When 100 equiv. of methanol was
added to the mixture of TBA-P-Al-Zr and 5 equiv. of benzoic acid,
followed by stirring at 60 °C for 72 h, a signal due to the methyl group
of methyl benzoate was observed at 52.77 ppm. A signal due to the
palmitic acid in sunflower oil with methanol at 60 °C) [6], and
zirconium-containing metal organic frameworks (99% after 2 h for
lauric acid esterification with methanol or ethanol at 60 °C) [9]. In
particular, pyrene-based porous organic polymers exhibit high catalytic
activities (88–94% of conversion after 10 h in the esterification of
various FFAs even at room temperature) [11].
1
methyl group of methyl benzoate was also observed in the H NMR
31
As determined by X-ray crystallography [24,25], TBA-P-Al-Zr and
TBA-Si-Al-Zr have the two and four protons located at (O1 and O2) and
spectrum at 3.87 ppm. In the
−11.46 ppm completely disappeared, and
12.10 ppm appeared; this was observed for a mixture of TBA-P-Al-
P NMR spectrum, the signal at
a
new signal at
−
Zr and methyl benzoate, as shown in Fig. S7. These results suggest that
Table 2
31
the signal observed at −11.46 ppm in the P NMR spectrum was a
Esterification of various carboxyl acids with methanol catalyzed by TBA-P-Al-Zr in a
31
reaction intermediate. In contrast, the signals in the P NMR spectrum
heterogeneous system.a
3
in acetonitrile-d of TBA-P-Al-Zr did not shift in the presence of 100
Substrate (mmol)
Selectivity of product (%)
Conversion (%)b
TONc
equiv. of methanol, followed by 3 h stirring at approximately 25 °C (Fig.
S8). These results suggest that the coordination of benzoic acid to the
Lewis acid sites in TBA-P-Al-Zr was more preferential than that of
methanol. Thus, the esterification process is initiated by the interaction
of Lewis acid sites in TBA-P-Al-Zr with carboxylic acids, thus generat-
ing the intermediate; subsequently, the nucleophilic attack of methanol
on the intermediates resulted in the formation of the corresponding
esters, as reported for Lewis acidic zirconium-containing metal organic
frameworks [9] and layered zinc laurate and zinc stearate [10].
When excess water was added to a mixture of TBA-P-Al-Zr and
benzoic acid (50 equiv.), a yellow solid was precipitated (the solid was
oleic acid (0.31)
methyl oleate (> 99)
90
86
75
69
15
13
12
12
palmitic acid (0.28)
myristic acid (0.30)
lauric acid (0.34)
benzoic acid (0.41)
benzoic acid (0.82)e
methyl palmitate (> 99)
methyl myristate (> 99)
methyl laurate (> 99)
methyl benzoate (> 99)
methyl benzoate (> 99)
d
d
54
11
83
99
a
Reaction conditions: catalyst (19.1 μmol), substrates (0.28–0.34 mmol), methanol
(
49.3 mmol), 80 ± 2 °C, and reaction time 6 h.
b
Conversion (%) = {[mol of substrate]
× 100.
0
− [mol of substrate]
t
}/[mol of sub-
strate]
0
c
Turnover number (TON) = [mol of corresponding product]
Reaction time 69 h.
This system is homogeneous. Reaction conditions: catalyst (6.8 μmol), benzoic acid
t
/[mol of catalyst].
31
abbreviated as TBA-P-Al-Zr-S). The P NMR spectrum in acetonitrile-
d
e
d
−
3
of TBA-P-Al-Zr-S contains signals at (−12.33 and −12.25 ppm) and
11.46 ppm, which were assigned to the original TBA-P-Al-Zr and the
(
0.82 mmol), methanol (24.7 mmol), acetonitrile (4 mL), 80 ± 2 °C, and reaction time
1
3
9
6 h.
reaction intermediate, respectively (Fig. S9). Although the C NMR
4
4

C.N. Kato et al.
Catalysis Communications 96 (2017) 41–45
2
13–220.
3
spectrum in acetonitrile-d of TBA-P-Al-Zr-S was quite noisy, some
[
[
[
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in Fig. S10(a). In contrast, TBA-Si-Al-Zr-S prepared by the addition of
excess water to a mixture of TBA-Si-Al-Zr and benzoic acid (50 equiv.)
1
3
showed no signals in the C NMR spectrum corresponding to benzoic
acid (Fig. S10(b)). Thus, the interaction of benzoic acid with the Lewis
acid sites in TBA-P-Al-Zr was stronger than that in TBA-Si-Al-Zr. These
results indicate that Lewis acidity of TBA-P-Al-Zr was higher than that
of TBA-Si-Al-Zr; this is consistent with the activities of the catalysts for
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. Conclusion
[
[
Organozirconium complexes with α-Keggin-type mono-aluminum-
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39ZrCp
OH) 38ZrCp
polyoxotungstates,
(TBA-P-Al-Zr)
[(n-C
4
H
9
)
4
N]
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[α-PW11Al(OH)
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esterification of linoleic acid at 80 ± 2 °C, TBA-P-Al-Zr exhibited 83%
conversion after 6 h, approximately six times higher than that of TBA-
Si-Al-Zr; thus, the internal atoms in the Keggin units affected the
catalytic activities significantly. TBA-P-Al-Zr also exhibited 69–90%
conversions for the esterification reactions of oleic acid, palmitic acid,
myristic acid, and lauric acid with methanol at 80 ± 2 °C. The protons
located at the oxygen atoms around the zirconium centers in TBA-P-Al-
Zr and TBA-Si-Al-Zr did not act as Brønsted acid sites, and TBA-P-Al-Zr
and TBA-Si-Al-Zr acted as Lewis acid catalysts. In the esterification
reaction mechanism, the carboxylic acids interact with the Lewis acid
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[
III
6−
III
III
We acknowledge Prof. Kenji Nomiya (Kanagawa University, Japan)
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O
2 2
-based oxidations with [Al
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(H
2
O)10(β-XW
9
O33H)
2
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(X = As , Sb ),
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a
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(μ-OH)
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(μ
3 2 2 4
-O) (H O) }(α-1,4-
[
[
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2
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0−
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and [(γ-SiW10
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5