2
S.M.A.H. Siddiki et al. / Journal of Catalysis xxx (2016) xxx–xxx
homogeneous Brønsted acid catalyst forms reverse micelle in
organic layer to promote the hydrolysis of hydrophobic esters on
the surface of the oil-in-water emulsion, resulting in high
conversions for various hydrophobic esters [13]. However, the
method has drawbacks of difficulties in catalyst/products separa-
tion and catalyst reuse. Another example is a octadecylsilane-
treated HZSM5 zeolite [14,15] as a water-tolerant heterogeneous
Brønsted acid catalyst. The immobilization of the alkyl-group on
the outer surface of zeolites increases the hydrophobicity of the
zeolites [16–18] and hence increases the catalytic activity of
HZSM5 for hydrolysis of the esters [14,15]. However, the method
suffers from serious drawbacks of limited substrate scope due to
a narrow pore size of HZSM5, necessity of toluene as a co-solvent
and no report of catalyst reuse.
ZrO
2
and SnO
ꢁnH O and H
40 was prepared by titrating H
Inorganic Color and Chemicals Co.) by aqueous solution of Cs
2
were prepared by calcination (500 °C, 3 h) of
SnO (Kojundo Chemical Laboratory Co., Ltd.).
40 (Nippon
CO
ZrO
2
2
2
3
Cs2.5
H
0.5PW12
O
3
PW12O
2
3
ꢀ
3
(0.10 mol dm ) with vigorous stirring, followed by centrifuging
and drying at 200 °C. Montmorillonite K10 clay and sulfonic resins
(Amberlyst-15 and Nafion-SiO
Sigma-Aldrich. Scandium(III) trifluoromethanesulfonate, Sc(OTf)
and p-toluenesulfonic acid (PTSA) were purchased from Tokyo
Chemical Industry.
Octadecyltriethoxysilane-treated Hb-20 (OD-Hb-20) was pre-
pared by refluxing a mixture of the Hb-20 (1 g, dried at 100 °C)
and octadecyltriethoxysilane (0.2 mmol) in dry toluene (5 mL) for
6 h, followed by filtering and washing with toluene and acetone,
and by drying at 100 °C for 1 h. The number of the octadecyl-
2
composite) were purchased from
3
,
It is well established that aluminosilicate zeolites with a high
Si/Al ratio, so-called high-silica zeolites, have high hydrophobicity
due to nonpolar nature of the Si-O-Si surface [3,4,19–21]. Among
various commercially available zeolites, proton-exchanged ⁄BEA
zeolite (Hb) has relatively large pore size and three-dimensional
pore structure, which arrow rapid diffusion of bulky organic sub-
strates and products. The strong Brønsted acid sites in the
hydrophobic pore of Hb will be effective for acid-catalyzed reac-
tions of hydrophobic substrates. Recently, high-silica Hb zeolites
have been shown to catalyze organic transformations in the pres-
ence of water [22,23]. We hypothesized that hydrophobic and
large pore Hb zeolites may be effective for the catalytic hydrolysis
of hydrophobic esters in the presence of excess amount of water.
We report herein that a high-silica Hb zeolite shows higher
activity than conventional homogeneous acid catalysts and the
water-tolerant Brønsted [3] and Lewis [24–26] acid catalysts for
hydrolysis of hydrophobic esters in the presence of excess amount
of water. Catalytic results show catalyst reusability and wide
applicability of this method. To study critical factors affecting the
catalytic activity, fundamental studies are also carried out for Hb
and HZSM5 zeolites with different Si/Al ratio and alkyl-group
immobilized Hb.
groups determined by the CO
2
formation in the temperature pro-
grammed oxidation analysis (25–500 °C under O
2
flow) was
ꢀ1
0.09 mmol g
.3. In situ IR
The pyridine-adsorption IR (infrared) study was carried with
.
2
JASCO FT/IR-4200 spectrometer equipped with an MCT detector
using a flow-type IR cell connected to a flow reaction system.
The IR disk of the sample (40 mg, 20 mm /) was first dehydrated
under He flow at 500 °C, and then a background spectrum was
taken under He flow at 200 °C. Then, pyridine (0.3 mmol g ) was
introduced to the sample, followed by purging by He for 600 s,
ꢀ1
and by IR measurement of adsorbed pyridine at 200 °C.
2.4. Catalytic tests
The heterogeneous catalysts, stored under ambient conditions,
were used for catalytic reactions without any pretreatment. Typi-
cally, ester (1 mmol), 1 mL H O and 10 mg of catalysts and a mag-
2
netic starter bar were added to a reaction vessel (Pyrex pressure
tube, 13 mL), and the mixture was heated at 130 °C under air with
stirring at 300 rpm. For the catalytic tests in Table 1 and kinetic
study, conversions and yields were determined by GC-FID using
n-dodecane as an internal standard as follows. After completion
of the reaction, acetone (7 mL) was added to the mixture, and
the catalyst was separated by centrifugation. Then, n-dodecane
(0.2 mmol) was added to the reaction mixture, and the mixture
was analyzed by GC-FID and GC–MS. The GC-FID sensitivities of
the products were determined using commercial carboxylic acids
or the isolated products after the reaction. For some of the products
in Tables 2 and 3, we determined isolated yields of the carboxylic
acids as follows. After the filtration of the catalyst, followed by
washing the catalyst with acetone (6 mL), and by evaporation,
the product was isolated by column chromatography using silica
2
. Experimental
2.1. General
Commercially available organic compounds (from Tokyo Chem-
ical Industry or Aldrich) were used without further purification. GC
Shimadzu GC-2014) and GCMS (Shimadzu GCMS-QP2010)
(
+
analyses were carried out with Ultra ALLOY -1 capillary column
Frontier Laboratories Ltd.) with N and He as the carrier. Column
chromatography was performed with silica gel 60 (spherical,
3–210 m, Kanto Chemical Co. Ltd.). Molecular sieves 4 Å
MS4Å) were dehydrated at 100 °C.
(
2
6
l
(
gel 60 (spherical, 63–210
lm, Kanto Chemical Co. Ltd.) with hex-
2
.2. Catalyst preparation
ane/ethyl acetate (60/40–80/20) as the eluting solvent, followed
1
13
by analyses by H NMR, C NMR and GC–MS equipped with the
same column as GC-FID.
The zeolites used in this study are designated as Hb-x, where x
denotes the Si/Al ratio of the Hb zeolite. Hb-75 (JRC-Z-HB150,
originally supplied from Clariant), HZSM5-45 (JRC-Z5-90H(1)),
HMOR-45 (JRC-Z-HM90, originally supplied from Clariant), TiO
JRC-TIO-4), CeO (JRC-CEO-3), amorphous SiO -Al (JRC-SAL-2,
Al content = 13.75 wt%, surface area = 560 m g
supplied from Catalysis Society of Japan. Hb-20 (HSZ-940HOA),
Hb-255 (HSZ-980HOA), HZSM5-20 (HSZ-840HOA) and HY-50
2
2.5. Adsorption experiments
(
2
2
2 3
O
2
ꢀ1
2
O
3
)
were
Liquid phase adsorption experiments (Figs. 7, 10 and 1) were
carried out as follows. Organic molecules (1 mmol of 1-pentanol,
methyl 3-phenylpropionate, ethyl acetate and acetic acid) with
1 mL water were stirred with solid adsorbent (0.1 g) for 24 h at
room temperature, followed by centrifugation to separate the
solids from the liquid. Then, acetone (6 mL) and the internal stan-
dard (0.2 mmol n-dodecane) were added to the liquid, which was
analyzed by GC-FID.
(
HSZ-385HUA) were commercially purchased from Tosoh Co.
HZSM5-75 and HZSM5-300 were supplied from N.E. CHEMCAT
Co. Niobic acid (HY-340) was kindly supplied by CBMM, and
Nb
2
O
5
was prepared by calcination of the niobic acid at 500 °C
(Q-10) was supplied from Fuji Silysia Chemical Ltd.
for 3 h. SiO
2
(