Quantitative Evaluation of the Effect of the Hydrophobicity of the Environment Surrounding Br?nsted Acid Sites on Their Catalytic Activity for the Hydrolysis of Organic Molecules

Article abstract of DOI:10.1021/jacs.8b11471

Sulfo-functionalized siloxane gels with a variety of surface hydrophobicities were fabricated to elucidate the effect of the environment surrounding the Br?nsted acid site on their catalytic activity for the hydrolysis of organic molecules. A detailed structural analysis of these siloxane gels by elemental analysis, X-ray photoelectron spectroscopy, Fourier-transformed infrared (FT-IR), and ²⁹Si MAS NMR revealed the formation of gel catalysts with a highly condensed siloxane network, which enabled us to quantitatively evaluate the hydrophobicity of the environment surrounding the catalytically active sulfo-functionality. A sulfo group in a highly hydrophobic environment exhibited excellent catalytic turnover frequency for the hydrolysis of acetate esters with a long alkyl chain, whereas not only conventional solid acid catalysts but also liquid acids showed quite low catalytic activity. Detailed kinetic studies corroborated that the adsorption of oleophilic esters at the Br?nsted acid site was facilitated by the surrounding hydrophobic environment, thus significantly promoting hydrolysis under aqueous conditions. Furthermore, sulfo-functionalized siloxane gels with a highly hydrophobic surface showed excellent catalytic activity for the hydrolytic deprotection of silyl ethers.

Full text of DOI:10.1021/jacs.8b11471

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Article
Quantitative Evaluation of the Effect of the Hydrophobicity of
the Environment Surrounding Brønsted Acid Sites on Their
Catalytic Activity for the Hydrolysis of Organic Molecules
Hiroki Miura, Shutaro Kameyama, Daiki Komori, and Tetsuya Shishido
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11471 • Publication Date (Web): 28 Dec 2018
Downloaded from http://pubs.acs.org on January 2, 2019
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Quantitative Evaluation of the Effect of the
Hydrophobicity of the Environment Surrounding
Brønsted Acid Sites on Their Catalytic Activity for
the Hydrolysis of Organic Molecules
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Hiroki Miuraa,b,d*, Shutaro Kameyama , Daiki Komori , Tetsuya Shishido
a
a
a,b,c,d*
a
Department of Applied Chemistry for Environment, Graduate School of Urban Environmental
Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397,
Japan
b
Research Center for Hydrogen Energy-based Society, Tokyo Metropolitan University, 1-1
Minami-Osawa, Hachioji, Tokyo 192-0397, Japan
c
Research Center for Gold Chemistry, Tokyo Metropolitan University, 1-1 Minami-Osawa,
Hachioji, Tokyo 192-0397, Japan
d
Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Katsura, Nishikyo-
ku, Kyoto 615-8520, Japan
*
Corresponding authors: Tel: +81-42-677-2850, Fax: +81-42-677-2850 (T. Shishido)
E-mail address: miura-hiroki@tmu.ac.jp (H. Miura)
E-mail address: shishido-tetsuya@tmu.ac.jp (T. Shishido)
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Abstract
Sulfo-functionalized siloxane gels with a variety of surface hydrophobicities were fabricated
to elucidate the effect of the environment surrounding the Brønsted acid site on their catalytic
activity for the hydrolysis of organic molecules. A detailed structural analysis of these siloxane
gels by elemental analysis, X-ray photoelectron spectroscopy, Fourier-transformed infrared
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(FT-IR) and Si MAS NMR revealed the formation of gel catalysts with a highly condensed
siloxane network, which enabled us to quantitatively evaluate the hydrophobicity of the
environment surrounding the catalytically active sulfo-functionality. A sulfo group in a highly
hydrophobic environment exhibited excellent catalytic turnover frequency for the hydrolysis
of acetate esters with a long alkyl chain, whereas not only conventional solid acid catalysts but
also liquid acids showed quite low catalytic activity. Detailed kinetic studies corroborated that
the adsorption of oleophilic esters at the Brønsted acid site was facilitated by the surrounding
hydrophobic environment, thus significantly promoting hydrolysis under aqueous conditions.
Furthermore, sulfo-functionalized siloxane gels with a highly hydrophobic surface showed
excellent catalytic activity for the hydrolytic deprotection of silyl ethers.
Keywords
Brønsted acid catalyst, siloxane gel, hydrophobic surface, hydrolysis of organic molecules
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1. Introduction
Acid catalysis is the most fundamental tool for promoting chemical reactions in the
petrochemical industry1–5 and fine-chemical synthesis.6–9 Recently, considerable attention has
been paid to the up-grading of biomass-derived materials by Lewis and Brønsted acid catalysis
in aqueous media.10–16 However the presence of water in acid-catalyzed reactions is generally
considered to result in a decrease in efficiency, since water acts not only as a nucleophile in
catalytic reactions but also as an inhibitor by adsorbing at the acid site due to its Lewis-basic
nature. Hence, control of the adsorption of water and organic molecules at the acid site is
anticipated to be one of the most important factors for taking full advantage of their potential
for acid catalysis. Among various solutions for overcoming this obstacle, the introduction of a
hydrophobic environment to the surroundings of the acid site is considered to be a promising
strategy.17,18 In fact, proton-exchange organic resins19–21 and acidified-carbonaceous
materials22–26 have been reported to function as water-tolerant solid acid catalysts for the
hydrolysis of esters. The organic frameworks of these materials provide a hydrophobic
environment around the acid site, which prevents deactivation of the catalytic active site caused
by the strong coordination of water. High-silica zeolites with a highly condensed siloxane
network in their microporous cages are also known as water-tolerant acid catalysts for
promoting a series of reactions under aqueous conditions.27–32 As described above, the positive
effect of a hydrophobic environment around acid sites on catalytic reactions under aqueous
conditions is now widely accepted. However, solid catalysts generally have a variety of
functional groups on their surface, which complicates the quantitative evaluation of the effect
of the degree of hydrophobicity of the region surrounding the catalytically active acid site. A
better understanding of this effect may lead to novel guidelines for designing highly active,
durable and water-tolerant acid catalysts.
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In this context, we focused on the use of alkyl group-functionalized siloxane gels developed
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by Nakanishi and co-workers as acid catalysts. These materials exhibit a scaffold with highly
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condensed siloxane networks. Thus, their surface is expected to consist only of alkyl groups
for hydrophobic sites and some other functionality for hydrophilic acid sites. The presence of
limited functional groups around acid sites should simplify the evaluation of the effect of
hydrophilicity on their catalytic activity. Furthermore, with microporous zeolite catalysts, the
heterogeneity of the acid site complicates the effect of hydrophobicity on their catalytic activity.
However, this concern can be avoided with siloxane gels due to their non-porous structure.
In this paper, sulfo-functionalized siloxane gels with a variety of surface hydrophobicities were
fabricated to elucidate the effect of the environment surrounding the Brønsted acid site on their
catalytic activities for the hydrolysis of organic molecules. A detailed analysis of the
relationship between structure and acid catalysis revealed that the hydrophobicity of the
environment surrounding the acid sites significantly dominated their catalytic turnover
frequency. While siloxane gel catalysts with hydrophobic surface properties exhibited excellent
activity for the hydrolysis of acetate esters with a long alkyl chain, not only conventional solid
acid catalysts but also liquid acids showed quite low catalytic activity. Furthermore, the present
hydrophobic gel catalysts also showed high activity for the hydrolytic deprotection of silyl
ethers.
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2. Experimental Section
.1. Materials
Acetone, methanol, ethyl acetate, acetic acid, urea, aqueous H
2
2
O
2
30wt%, and H
2
SO were
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purchased from FUJIFILM Wako Pure Chemical Co. Ltd. (Japan). n-
Hexadecyltrimethylammonium chloride (CTAC), 3-mercaptopropyltrimethoxysilane
(
MPTMS), trimethoxymethylsilane (TMMS), 3-mercaptopropyldimethoxymethylsilane
MPDMS), and dimethoxydimethylsilane (DMDMS) were obtained from Tokyo Chemical
(
Industry, Ltd. (Japan). Amyl acetate, octyl acetate, and dodecyl acetate were purchased from
Tokyo Chemical Industry, Ltd. Amberlyst-15 and Nafion-NR50 were obtained from Tokyo
Chemical Industry, Ltd and FUJIFILM Wako Pure Chemical Co. Ltd., respectively. H-ZSM-5
(JRC-Z5-90H) and H-Beta (JRC-HB-150) were kindly supplied by the Catalysis Society of
Japan. All of the chemical regents were used as received.
2
.2. Preparation of siloxane gel catalysts
Sulfo-functionalized siloxane gels were prepared through a three-step sequence: synthesis of
a thiol-functionalized siloxane gel, oxidation and protonation of thiol to sulfo. Thiol-
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functionalized siloxane gels were prepared as reported by Nakanishi and co-workers. CTAC
(0.80 g) and urea (5.0 g) were dissolved in 5 mM aqueous acetic acid (15 mL). To the solution
were added alkyl trimethoxysilane (MPTMS and TMMS, 21 mmol) and dialkyl
dimethoxysilane (MPDMS and DMDMS, 14 mmol) at the same time under vigorous stirring
at ambient temperature, and stirring was continued to give a clear solution. Gelation and aging
of the obtained sol were performed in an oven at 90 °C for 6 h. After the formed siloxane gel
was washed with methanol and distilled water to remove the residual surfactant and other
chemicals, the obtained powder was dried in an oven at 80 °C. Subsequently, 2.1 g of the
obtained siloxane gel with thiol and methyl groups was treated with 12 mL of aqueous H
2
O
2
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0wt% in 18 mL of methanol at ambient temperature. After 24 h, the resulting powder was
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washed with methanol and water. Finally, the powder was stirred in a mixture of H O and
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methanol (v/v = 1/1) containing 1.8 mL of conc. H
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SO at ambient temperature for 4 h. The
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thus-obtained gel was washed with methanol and water, and then dried in an oven at 80 °C to
form sulfo-functionalized siloxane gel catalysts. The contents of surface functional groups on
the gel were controlled by changing the molar ratio of S and Me (S/Me) in the starting silicon
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materials, and gel catalysts with S/Me=XX were designated as Gel cat.(XX). In the present
study, S/Me was varied from 0.17 to 2.50.
2.3. Physical and analytical measurements
The products of the catalytic runs were analyzed by GC-MS (Shimadzu GCMS-QP2010,
CBP-10 capillary column, i.d. 0.25 mm, length 30 m, 50250 °C) and gas chromatography
(Shimadzu GC-2014, CBP-20 capillary column, i.d. 0.25 mm, length 30 m, 50250 °C). Liquid
1
NMR spectra were recorded on a JMN-ECS400 instrument (FT, 400 MHz ( H), 100 MHz
(13
C)). Chemical shifts () of H and C{ H} NMR spectra are referenced to SiMe
1
13
1
. The
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catalysts were analyzed by nitrogen gas and water vapor adsorption, C CP/MAS NMR, Si
MAS NMR, elemental analysis, XPS, FT-IR, water contact angle and acid-base titration. The
Brunauer–Emmett–Teller (BET) specific surface area was estimated from N isotherms
2
obtained using a BELSORP-mini II (MicrotracBEL) at 77 K. The analyzed samples were
evacuated at 423 K for 15 h prior to measurement. Water vapor adsorption was evaluated using
BELSORP-max at 25 °C. The analyzed samples were evacuated at 353 K for 15 h prior to
measurement. Solid NMR spectra were recorded on a JMN-ECA500 instrument (FT, 100 MHz
(13
C), 98.37 MHz ( Si)). The rate of sample-spinning was set at 6 kHz, and the number of
29
scans was 3000. Elemental analyses were performed using an EAI CE-440 CHN/S Elemental
Analyzer (Exeter Analytical, Inc.). XP spectra were recorded using a JPS-9010 MX instrument.
The spectra were measured using MgK radiation (15 kV, 400 W) in a chamber at a base
α
-7
pressure of ~10 Pa. All spectra were calibrated using O1s (532.0 eV) as a reference. FT-IR
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spectra were recorded with FT/IR-4200 (JASCO) by a transmission method using a KBr disc
containing the sample at 1 wt%. Water contact angles were measured with a Drop Master DM-
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00 (Kyowa Interface Science Co., Ltd., Japan). The volume of a water droplet was fixed at
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.0 μL and the contact angle was determined at 30 seconds after attachment to the gel surface.
The proton conductivity in the gel catalysts was measured by electrochemical impedance
spectroscopy over the frequency range from 50 Hz to 50 kHz (Hioki 3532-50, Tokyo, Japan)
under dry conditions. A pellet of the gel catalyst (0.5cm in diameter and 0.1 cm in thickness)
and two blackened platinum plate electrodes were placed in a Teflon cell. The titration
technique was used to determine the acid amount of the siloxane gel catalysts by using a
mixture of aqueous saturated sodium chloride and methanol as exchange agents.35
2
.4 General procedure for the catalytic hydrolysis of esters
A typical reaction procedure was as follows: 0.5 mL of ethyl esters and 2.5 mL of H
2
O were
added to a Schlenk tube containing solid acid catalyst (100 mg) under an Ar atmosphere. The
reaction was carried out at 60 °C (in the case of ethyl acetate) or 90 °C (in the case of other
esters) for 3 h with a fixed stirring speed of 500 rpm. After the reaction solution was cooled to
room temperature, to the solution were added decane (1 mmol) as an internal standard and 9
mL of acetone to homogenize the organic and aqueous phases of the solution. The activity of
the catalysts was evaluated in terms of the initial formation rate of alcohols as determined by
gas chromatography.
2
.5 General procedure for the hydrolytic desilylation of silyl ether
A typical reaction procedure was as follows: 0.25 mmol of silyl ether and 4.0 mL of H
2
O were
added to a Schlenk tube containing solid acid catalyst (50 mg) under an Ar atmosphere. The
reaction was carried out at 100 °C for 24 h under a fixed stirring speed of 500 rpm. After the
reaction solution was cooled to room temperature, to the solution were added decane (0.5
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mmol) as an internal standard and 20 mL of acetone to homogenize the organic and aqueous
phases of the solution. The activity of catalysts was evaluated in terms of the amount of formed
alcohols as determined by gas chromatography.
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3. Results and discussion
Characterization of siloxane gel catalysts
The amounts of functional groups actually incorporated in the synthesized siloxane gel
catalysts are summarized in Table 1. The total amount of carbon, hydrogen and sulfur atoms
was estimated by elemental analysis, and the sum weight of these elements is about from 35 to
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0 wt% regardless of S/Me ratio. TG analysis of Gel cat.(0.26) under air flow condition (Figure
S3 in supporting information) shows about 40% weight loss by increasing the temperature,
which suggests that the parts of siloxane gel catalysts other than C, H and S were composed by
6
+
2-
Si-O-Si unit. XP spectra show the presence of S and S species which originated on the
3
6
surface sulfo and thiol groups on siloxane gels, respectively (Figure S1). The molar ratio of
6+
2-
S and S species was calculated from the intensity ratio of these peaks, which quantified the
amount of sulfo groups on the siloxane gel catalysts. Furthermore, the acid amount on siloxane
gels estimated by acid-base titration technique was also summarized in Table 1. Notably, the
6
+
S ratio and acid amount decreased with a decrease in the S/Me ratio. This is probably due to
the difficultly of hydrophilic H accessing a thiol group tethered on the hydrophobic surface
of siloxane gel, which contributed to why the S/Me ratio did not directly reflect the ratio of
2
O
2
6
+
S /Me actually incorporated in the synthesized siloxane gel catalysts. On the other hand, the
2
-1
present gel catalysts possess very small BET surface area (<10 m g , Figure S2 and Table 2
in Supporting Information) estimated by N -adsorption under dry condition regardless of their
2
S/Me ratio. Thus, the surface methyl and sulfo group are present with high density under dry
condition. However, the characteristics of the present siloxane gel is that they can swell by
adsorbing organic molecules in several times of their dry weight.31 Thus, real surface
configuration of the methyl and sulfo group during catalytic reactions would probably be
changed from that under dry conditions.
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Table 1. Composition of functional groups and acid amount in siloxane gel catalysts
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
6+
6+
2-
S6+
/ mmol g-1
Me group
S
S /(S +S )
(%)b
Acid amount /
-1 c
mmolg
S/Me C(wt%)a H(wt%)a S(wt%)a
/ mmol g-1 / mmol g-1
0
0
.17
.26
22.99
23.91
23.41
21.13
20.95
21.48
21.58
21.15
5.58
5.50
5.47
5.20
4.60
4.88
4.77
4.61
5.50
9.48
14.00
11.04
10.15
6.58
1.71
2.96
3.12
3.67
4.20
4.83
5.35
5.04
11.4
14.4
28.2
44.4
52.7
65.1
59.5
68.0
0.19
0.43
0.88
1.63
2.21
3.14
3.18
3.43
0.04
0.20
0..56
1.36
1.54
1.78
1.90
2.32
0.40
9.99
0
0
.53
.75
11.77
13.47
15.49
17.15
4.84
1.33
3.39
1
2
.88
.50
1.93
16.15
2.50
a
b
c
Determined by elemental analysis. Determined by XPS analysis. Determined by acid titration
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Figure 1 shows the relationship between the S/Me ratio of siloxane gels and the amounts of
6
+
Me and S species. The decrease in the S/Me ratio of siloxane gels led to an increase in the
6+
amount of methyl group and a decrease in S species. Although the amount of SO H group
3
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1
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6
7
8
9
0
6
+
was slightly lower than S species estimated by XPS and EA, similar association with S/Me
6
+
ratio in gel catalysts to that of the amount of S species was observed. This result indicated
that the concentration of acid site and methyl group on the surface of siloxane gels could be
controlled by changing the S/Me ratio. If we consider the hydrophobic character of a methyl
group and the hydrophilic character of a sulfo group, the surface of siloxane gel catalysts with
a low S/Me ratio is anticipated to be hydrophobic.
6
+
Figure 1. Amounts of Me (○), S species (■) and acid site (□) in gel catalysts
with various S/Me ratios.
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6
The surface hydrophobicity of sulfo-functionalized gel catalysts was evaluated in terms of the
water contact angle (Figure 2). Water droplets were not maintained on the surface of gel
catalysts with S/Me ratios of higher than 0.90. In contrast, catalysts with S/Me ratios lower
than 0.75 kept water droplets on their surface, and the contact angles increased with a decrease
in the S/Me ratio of the siloxane gel catalysts.
0
1
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9
0
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2. Water contact angles of siloxane gel catalysts with different S/Me ratios (A) and
their images (B) (a) S/Me=0.17; (b) S/Me=0.26; (c) S/Me=0.40; (d) S/Me=0.53; (e)
S/Me=0.75; (f) S/Me=0.90; (g) S/Me=1.04; (h) S/Me=1.23; (i) S/Me=1.33; (j) S/Me=1.88;
(k) S/Me=2.50
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The hydrophobicity of the gel catalysts was further analyzed by water vapor adsorption
(
Figures 3 and S4). The amount of adsorbed water vapor at a relative pressure (p/p ) of 0.9 in
0
water vapor isotherms decreased as the S/Me ratio decreased. These results indicated that the
surface hydrophobicity of the siloxane gel catalysts could be controlled by varying the S/Me
ratio in the starting silicon materials, and siloxane gel catalysts with a low S/Me ratio had a
highly hydrophobic surface.
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0
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0
1
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3
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5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 3. Water adsorption at p/p =0.9 in the isotherm of
0
gel catalysts with different S/Me ratios.
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6
On the other hand, the hydrophobic nature of siloxane gels without oxidative treatment, i.e.,
thiol-functionalized siloxane gels, was evaluated in terms of the water contact angle and water
vapor adsorption (Figure 4). In sharp contrast to sulfo-functionalized siloxane gels, a negligible
amount of water vapor was adsorbed on thiol-functionalized gels. Furthermore, water droplets
were maintained on their surfaces with high contact angles. These results strongly suggest that
the resulting thiol groups did not exhibit a hydrophilic nature.
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0
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9
0
1
2
3
4
5
6
7
8
9
0
Figure 4. Water contact angles (●) and water adsorption at p/p =0.9 in
0
the isotherm (□) of gel catalysts without oxidative treatment.
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The origin of the surface hydrophilic nature of the present siloxane gels was also examined by
3
7
FT-IR. Figure 5 shows FT-IR spectra of siloxane gel catalysts with different S/Me ratios and
a siloxane gel without oxidative treatment. In the spectra for all of the siloxane gels, distinct
peaks due to asymmetric stretching vibrations of the Si–O–Si bridges appeared at around 1000
1
1
1
1
1
1
1
1
1
1
2
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2
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3
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3
4
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6
0
1
2
3
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6
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9
0
1
2
3
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8
9
0
1
2
3
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6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-1
-1
to 1100 cm . Furthermore, peaks at 750 to 900 cm , which can be attributed to CH
in Si-CH and C-Si asymmetric stretching in C-Si-O units, were also confirmed in all of the
spectra. On the other hand, in the spectrum of a siloxane gel without oxidative treatment, a
3
rocking
3
-1
quite weak band appeared at around 3000 to 3700 cm , and was mainly attributed to a surface
sulfonyl group. In contrast, a remarkable broad band appeared in the spectra of sulfo-
functionalized siloxane gels, and their intensity decreased with a decrease in the S/Me ratio.
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0
1
2
3
4
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6
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9
0
1
2
3
4
5
6
7
8
9
0
Figure 5. FT-IR spectra of gel catalysts with various S/Me ratios:
(a) 0.17; (b) 0.26; (c) 0.40; (d) 0.53; (e) 0.75; (f) 1.33; (g) 1.88;
(h) 2.50; (i) 2.50 without oxidative treatment.
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33,37
The structure of the siloxane scaffold was analyzed by Si MAS NMR
(Figure 6). The
most important characteristic of the present siloxanes gel is their highly condensed siloxane
31
2
network. In all of the spectra of siloxane gels with different S/Me ratios, peaks due to D and
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0
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9
0
3
1
2
T silicon units dominantly appeared, and remarkable peaks due to D and T silicon units ,
which indicate the presence of silanol groups (Si-OH) , were not observed. Despite the fact that
the surface hydrophilicity over amorphous SiO is mainly derived from surface Si-OH groups,
2
the hydrophilicity of the present siloxane gel catalysts predominantly originated in surface
sulfo groups formed through the oxidation of thiol groups. These unique features of the present
siloxane gel catalyst simplified the consideration of the effect of surface hydrophobicity on
Brønsted catalysis.
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1
2
T2 T3
D D
a
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9
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b
c
d
e
f
g
h
chemical shift / ppm
2
9
Figure 6. Si MAS NMR spectra of gel catalysts with various S/Me
ratios: (a) S/Me=0.17; (b) S/Me=0.26; (c) S/Me=0.40; (d) S/Me=0.53; (e)
S/Me=0.75; (f) S/Me=1.33; (g) S/Me=1.88 (h) S/Me=2.50.
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Catalytic activity of siloxane gel catalysts for the hydrolysis of acetate esters
The catalytic activity of siloxane gel catalysts for the hydrolysis of acetate esters with alkyl
chains of different lengths was investigated. Figure 7 shows the initial formation rate of the
corresponding alcohols in the reactions with siloxane gel catalysts with different S/Me ratios.
As for the hydrolysis of ethyl acetate, gel catalysts with a high S/Me ratio exhibited a high
formation rate of ethanol, and this rate gradually decreased with a decrease in the S/Me ratio
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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0
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9
0
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(Figure 7A). In contrast, Gel cat.(1.33) showed the highest activity for the hydrolysis of 1-
pentanol (Figure 7B). Furthermore, hydrolysis of esters with a longer alkyl chain proceeded
efficiently in the presence of gel catalysts with a low S/Me ratio, and the highest initial
formation rates of 1-octanol and 1-dodecanol were observed with the use of Gel cat.(0.40) and
Gel cat.(0.26), respectively. As summarized in Table 1, the amount of acid sites that involved
sulfo groups per catalyst weight decreased with a decrease in the S/Me ratio. Nevertheless, gel
catalysts with a low S/Me ratio showed high catalytic performance for the hydrolysis of acetate
esters with long alkyl chains, suggesting that the hydrophobicity around sulfo groups should
be an important factor for determining the catalytic activity for the hydrolysis of acetate esters.
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0
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9
0
Figure 7. Initial formation rate of ethanol (◇), 1-pentanol (△), 1-octanol
(
□) and 1-dodecanol (○) upon the hydrolysis of alkyl acetate by gel
catalysts with various S/Me ratios.
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TOF values based on the sulfo group in the siloxane gels for the hydrolysis of each acetate ester
are summarized in Figure 8(a). Importantly, high values were obtained for all ester hydrolyses
by gel catalysts with a high surface hydrophobicity, and these values drastically increased with
1
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1
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1
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2
2
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1
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3
4
5
6
7
8
9
0
6
+
a S/Me ratio below 0.5. Figure 8(b) shows the ratio of methyl and sulfo groups (Me/S ) that
were actually incorporated in the siloxane gels. The Me/S6+ value reflects the degree of
hydrophobicity surrounding a sulfo group, and a high value indicates that Brønsted acid sites
are located in a highly hydrophobic environment. The close association between these two
different factors clearly suggests that as the environment surrounding a Brønsted acid site
becomes more hydrophobic, the site exhibits higher catalytic turnover for the hydrolysis of
esters. On the other hand, water contact angle and water vapor adsorption are often applied to
evaluate the hydrophobicity of catalyst surface. Nevertheless, there is no clear correlation
between these two factors and the turnover frequency of sulfo-functionality for hydrolysis as
shown in Figures 2 and 3. This result suggests that the effect of hydrophobic environment on
the catalytic activity for hydrolysis of esters should be considered from molecular-level
viewpoint as we demonstrated in Figure 8. Note that the full understanding of the origin of the
hydrophobicity and hydrophilicity of the present siloxane gel catalysts led accurate evaluation
of the effect of the hydrophobicity of the environment surrounding acid sites on their catalytic
activity.
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0
1
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3
4
5
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9
0
1
2
3
4
5
6
7
8
9
0
Figure 8. Relationship between TOFs for the hydrolysis of ethyl acetate (●), amyl acetate
(
■), octyl acetate (◆), and dodecyl acetate (▲) with the S/Me ratios of gel catalysts (a),
6
+
and their Me/S ratios (○) (b)
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Table 2 compares the activity of Gel cat.(0.26) with those of a series of conventional solid and
liquid acid catalysts for the hydrolysis of dodecyl acetate. Gel cat.(0.26) showed the highest
values among the solid acid catalysts listed in Table 2 with regard to both activity per catalyst
weight and TOF per acid site. Furthermore, liquid acid and water soluble organo-sulfonic acid
were not effective for the reaction. In contrast, p-dodecylbenzenesulfonic acid functioned as an
effective acid catalyst to show high catalytic turnover. Kobayashi and co-workers reported that
DBSA functioned as a surfactant-type acid catalyst that can encapsulate organic molecules in
aqueous media, leading to show high efficiency for acid catalyzed reactions in aqueous
media.38,39 These results clearly indicated that microenvironment surrounding acid site is much
more important factor than acid strength for dominating the catalytic activity for hydrophobic
substrates. On the other hand, In the case of hydrophilic organic molecules, such as ethyl
acetate that can dissolve into water, H-β and Nafion-NR50 which have strong Brønsted acid
site showed higher catalytic turnover than the siloxane gel catalysts (Table S5 in Supporting
Information). This is probably because relatively strong adsorbing ability at acid site of ethyl
acetate decrease the necessity of the aid from hydrophobic environment, and the catalytic
activity would be dominated not by hydrophobic effect but rather by acid strength. Thus, we
assume that the promoting effect of hydrophobic environment should be positively reflected in
the hydrolysis of hydrophobic substrates, such as dodecyl acetate.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
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3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 2. Hydrolysis of dodecyl acetate with various acid catalysts
0
1
2
3
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5
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7
8
9
0
1
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a
b
SiO
2
/Al
2
O
3
ratio of zeolites. Values from T. Okuhara et al. Appl. Catal. A, 1997,
c
d
167, 227. Values based on the SiO
2
/Al
O
2 3
ratio. para-dodecylbenzenesulfonic acid.
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A detailed kinetic study of the hydrolysis of esters was carried out to elucidate the effect of
hydrophobicity of the siloxane gel catalysts. Initially, we examined the effect of the amount of
ethyl acetate or water on the initial formation rate of ethanol in the hydrolysis of ethyl acetate
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(EA) with a hydrophilic gel catalyst (Gel Cat.(2.50)) (Figure 9). When the amount of water
was fixed at 2.5 mL, the formation rate of ethanol increased with an increase in the amount
ethyl acetate. In contrast, when the amount of ethyl acetate was fixed at 0.50 mL, the rate of
formation was constant even though the amount of water was varied. These results clearly
suggest that the rate-limiting step is the protonation of esters by the Brønsted acid site.
Figure 9. Effect of the amount of ethyl acetate or water on the hydrolysis
of ethyl acetate with Gel cat.(2.50)
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On the other hand, a reverse tendency was observed in the hydrolysis of dodecyl acetate (DA)
by hydrophobic siloxane gel catalysts (Gel cat.(0.26)) (Figure 10). The formation rate of 1-
dodecanol was almost constant even when the amount of DA was varied. In contrast, the
formation rate of 1-dodecanol increased with an increase in the amount of water. If we consider
these facts and quite poor solubility of water into DA, the active site should be saturated with
DA and the reaction rate have zeroth-order dependence on DA. This indicates that the rate-
limiting step in the hydrolysis of hydrophobic ester by hydrophobic siloxane gel catalysts is
nucleophilic attack of water to protonated esters.
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Figure 10. Effect of the amount of dodecyl acetate or water on the hydrolysis of
dodecyl acetate with Gel cat.(0.26)
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Figure 11 shows pictures of siloxane gel catalysts dispersed in an ester/water two-phase system.
Hydrophilic Gel cat.(2.50) was dispersed in the aqueous phase, whereas hydrophobic Gel
cat.(0.26) was dispersed in the organic phase. Note that although the molecular concentration
in reaction solution should be a critical parameter for estimating the reaction order to gain
mechanistic insight, such biphasic reaction system complicate to calculate these values. Thus,
the amount of the substrates and water are alternatively associated with the reaction rate to
evaluate the catalytic performance in Figures 9 and 10.
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Figure 11. Photos of siloxane gels in an ester/water two-phase system (a) ethyl
acetate and H
cat.(0.26).
2
O with Gel cat.(2.50) and (b) dodecyl acetate and H O with Gel
2
Based on these results, the effect of the surface hydrophobicity of the siloxane gel catalysts on
the catalytic activity of Brønsted acid is discussed as follows. In the case of gel catalysts with
a hydrophilic surface, water is mainly adsorbed on the region surrounding a sulfo group as a
Brønsted acid site, whereas hydrophobic molecules such as esters with a long alkyl chain can
hardly access the acid site. Therefore, the rate-limiting step should be protonation of the
carbonyl moiety. In contrast, for gel catalysts with a hydrophobic surface that can disperse in
an organic phase, hydrophobic surroundings of acid sites remarkably promote the adsorption
of hydrophobic organic molecules onto the acid site. Thus, the facile protonation of esters can
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occur on the surface acid site over hydrophobic gel catalysts. This step is anticipated to be
much faster than the rate-limiting step of protonation of esters in hydrolysis over hydrophilic
siloxane gel catalysts. Thus, the hydrophobic environment surrounding the Brønsted acid site
is believed to significantly enhance the reaction rate of ester hydrolysis.40,41 The increase of
hydrophobicity at acid site should decrease the accessibility of water to protonated esters. Thus,
the nucleophilic attack of water to the activated hydrophobic esters should mainly take place
at the water/organic phase interface where water can contact organic molecules with high
frequency.
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Next, we applied the catalytic effects of the present siloxane gels with a hydrophobic surface
to another hydrolytic transformation of organic molecules. The hydrolysis of silyl ether, i.e. ,
the deprotection of silyl ethers, to form the corresponding silanols and alcohols is an important
reaction in multi-step organic synthesis requiring protection-deprotection sequences.42,43
Fluoride salts, such as n-tetrabutylammonium fluoride (TBAF), are generally used as efficient
reagents for cleaving the Si-O bond. However, these reactions require a stoichiometric amount
of expensive fluoride reagent. Furthermore, the strong basic nature of TBAF limits the
application of acidic substrates in silyl-deprotection. On the other hand, the use of acid catalysis
in this reaction can realize an environmentally benign transformation. Particularly, the use of
water as a proton source is significantly important due to its safety, low cost and easy separation
from organic molecules. The acid-catalyzed hydrolytic deprotection of silyl group can be
anticipate to proceed following two elemental reactions; 1) protonation of O atom of silyl ether
by Brønsted acid 2) nucleophilic attack of water to Si atom to give silanol and alcohol. As the
case with ester hydrolysis under acid catalysis, highly hydrophobic surface of siloxane gel
catalysts with low S/Me ratio are expected to facilitate the protonation of O atom of silyl ether,
which can promote the cleavage of Si-O bond in aqueous media. Thus, we initially compared
the catalytic activity of hydrophobic siloxane gel with those of conventional solid acid catalysts
for the hydrolytic deprotection of octyl silyl ethers (Figure 12). The bulkiness of the silyl group
significantly affects the stability of silyl ether under acidic conditions, and a less-hindered
trimethylsilyl group (TMS) was readily deprotected under both catalytic and non-catalytic
conditions. In contrast, the reaction efficiency was greatly reduced in the reaction of
triethylsilyl-(TES) or tert-butyldimethylsilyl-(TBS) ethers by amberlyst-15 or H-ZSM-5.
Furthermore, these two solid acid catalysts did not participate in the deprotection of more stable
triisopropylsilyl-(TIPS) or tert-butyl diphenylsilyl (TBDPS) groups. In sharp contrast,
hydrophobic siloxane gel catalysts (Gel Cat.(0.26)) exhibited excellent activity for all silyl
ethers with different protecting groups.
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