Hydrothermal synthesis of a layered-type W-Ti-O mixed metal oxide and its solid acid activity

Article abstract of DOI:10.1039/c6cy02198k

A layered-type W-Ti-O mixed oxide was synthesized by hydrothermal synthesis from an aqueous solution of ammonium metatungstate and titanium sulfate. To avoid the formation of titania, oxalic acid was used as a reductant. Optimized synthesis led to rod-like particles comprised of MO₆ (M = W, Ti) octahedra connected in a corner-sharing fashion in the c-direction and in the form of micropore-containing {W₆O₂₁} units in the a-b plane. The surface area, acidity and acid catalytic activity (alkylation) increased with the amount of the layered-type W-Ti-O phase. Strong Br?nsted acid sites formed due to the thermal release of ammonia from the uncalcined precursor. Calcination at 400 °C led to the highest acidity and alkylation activity. Alkylation of benzyl alcohol and toluene led to heavy product formation due to over-alkylation of the product. The selectivity to the mono-alkylated product was improved by the addition of water, which competes with the selectively formed products for adsorption on the acid sites. FT-IR measurements showed that the layered-type W-Ti-O possesses Br?nsted acid sites and at least two different Lewis acid sites. The stronger Lewis acid sites can be converted into Br?nsted acid sites in the presence of water, and the weaker Lewis acid sites functioned in the presence of water. This water tolerance of Lewis acid sites is an important characteristic of layered-type W-Ti-O, as it allows the bifunctional catalyst to convert 1,3-DHA into lactic acid in water.

Full text of DOI:10.1039/c6cy02198k

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Hydrothermal Synthesis of a Layered-type W-Ti-O Mixed Metal
Oxide and its Solid Acid Activity
Received 00th January 20xx,
Accepted 00th January 20xx
a, b
b, c
b
b, d
e
Toru Murayama, Kiyotaka Nakajima, Jun Hirata, Kaori Omata, Emiel J. M. Hensen and
b, f
Wataru Ueda
DOI: 10.1039/x0xx00000x
A layered-type W-Ti-O mixed oxide was synthesized by hydrothermal synthesis from an aqueous solution of ammonium
metatungstate and titanium sulfate. To avoid the formation of titania, oxalic acid was used as a reductant. Optimized
www.rsc.org/
synthesis led to rod-like particles comprised of MO
6
(M = W, Ti) octahedra connected in a corner-sharing fashion in the c-
21} units in the a–b plane. The surface area, acidity and acid
direction and in the form of micropore-containing {W
6
O
catalytic activity (alkylation) increased with the amount of the layered-type W-Ti-O phase. Strong Brønsted acid sites form
due to the thermal release of ammonia from the uncalcined precursor. Calcination at 400°C led to the highest acidity and
alkylation activity. Alkylation of benzyl alcohol and toluene led to heavy product formation due to over-alkylation of the
product. The selectivity to the mono-alkylated product improved by the addition of water, which competes with the
selective product for adsorption on the acid sites. FT-IR measurements showed that the layered-type W-Ti-O possesses
Brønsted acid sites and at least two different Lewis acid sites. The stronger Lewis acid sites can be converted to Brønsted
acid sites in the presence of water, and the weaker Lewis acid sites functioned in the presence of water. This water
tolerance of Lewis acid sites is an important characteristic of layered-type W-Ti-O, as it allows the bifunctional catalyst to
convert
1,3-DHA
into
lactic
acid
in
water.
required is biomass conversion, involving such reactions as the
formation of 5-hydroxymethylfurfural from hexoses and lactic
acid from trioses. Furfural derivatives are raw materials for
polymers. Accordingly, the conversion from sugars to furfural,
which is the main component of woody biomass, has been
targeted in an effort to make the transition to a sustainable
Introduction
Mixed metal oxides of transition metals are important
inorganic materials which amongst others can serve as
catalysts and ceramics. The demand for solid acid catalysts is
expected to increase because such catalysts are reusable and
can be readily separated from the reaction mixture. Many
inorganic oxides including zeolites and metal oxides, which
have Lewis acid sites, have been found to be nearly inactive for
reactions in water because of the strong coordination of water
3
circular economy . Lactic acid is also a bio-based commodity
chemical with many applications, including the rapidly
4
expanding use of polylactate as a bio-plastic . Lactic acid can
be synthesized from glyceraldehyde or 1,3-dihydroxyacetone
5
–7
1
(DHA) over acid catalysts.
These molecules are readily
to Lewis acid sites. Therefore, the development of a Lewis
produced by the oxidation of glycerol, the main by-product in
biodiesel production. Therefore, an efficient catalytic system
that converts GLA and DHA into lactic acid would be promising
acid catalyst that exhibits excellent catalytic activity and
selectivity for various organic reactions in the presence of
2
water is desirable.
8
for lactic acid production.
An area in which solid acid catalysts operating in water are
One of the typical solid acid catalysts is niobium oxide,
which has Lewis acidity and Brønsted acidity. Niobium oxide
(
or hydrated niobium oxide) has been used as a water-tolerant
1
,9–12
solid acid catalyst.
Titania is also known for its combined
Lewis and Brønsted acid properties. Kasuga et al. reported that
nanotube-type titania can be easily obtained by heating titania
1
3
in a concentrated aqueous alkali solution . Titania nanotubes
have been shown to function as acid catalysts. The structural
distortion of the nanotube structure is thought to give rise to
solid acidity, a feature which is absent in the nanosheet
1
4,15
31
structure
. The presence of acid sites was confirmed by
P
MAS NMR and COads IR measurements, demonstrating the
enhanced Brønsted acidity of the bridging Ti-OH-Ti groups in
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the nanotube structure. These catalysts showed already good with a ZB-1 column. To study hydrolysis, 5 wt% of an ethyl
performance in the Friedel-Crafts alkylation reactions with acetate aqueous solution (30 g, 17 mmol ethyl acetate) or 1
toluene and benzyl chloride at room temperature.
wt% o-toryl acetate aqueous solution (60 g, 4.03 mmol methyl
We have studied the relationship between the crystalline phenyl acetate) with acetonitrile as an internal standard was
structure of W-based oxide mixed metal oxides (W-M-O introduced into the reactor. The reaction temperature was
1
6–22
(
M=Nb, Ta, V) and their catalytic activity.
were found to function as solid acids, and the acidic and the reactant and product were measured by GC-FID with a TC-
structural properties have been discussed from the viewpoint WAX column. For pyruvaldehyde and 1,3-DHA
These catalysts 60°C and 0.8 g of a catalyst was used. The concentrations of
of crystalline metal oxide structures. Tungsten oxides are a transformations, an aqueous solution (2 mL) of the substrate
class of solid acids that form tungsten bronze based on W- (100 mM) and 0.1 g of catalyst were used at 120°C for 6 h by
2
3
centered oxygen octahedra. The formation of a mixed oxides using glass-type pressure tight vial (3mL). The catalyst was
between Ti and W or the substitution of W in WO by Ti will removed after the reaction, and the solution was diluted with
as strong aqueous H SO . The concentrations of the reactant and
Brønsted acid sites, attributed to bridging hydroxyl groups M- product were measured by HPLC (JASCO, LC-2000 plus)
3
2
4
enhance acidity and catalytic performance,
2
4
25–
(
OH)-M’ (M = Ti, Nb or Ta, M’= W or Mo) in previous studies
7
equipped with Aminex HPX-87 H column (diameter: 300 mm x
-1
7.8 mm, eluent: 0.005M H SO 0.5 mL min , temperature:
2 4
2
and W-(OH)-Ti in this study, will appear in such materials.
We report herein the synthesis of a W-Ti-O mixed oxide by 35°C), with refractive index (RI) and photodiode array (PDA)
a hydrothermal process from ammonium metatungstate and detectors.
titanium (III) sulfate precursors. The W-Ti-O mixed oxide is
6
synthesized by the assembly of MO (M = W, Ti) units, which Characterization.
gives rise to octahedra-based layered materials. These layered
mixed W-Ti-O contain Brønsted acid and Lewis acid sites at
their surface. The promise of the W-Ti-O as a Lewis acid
catalyst was demonstrated in a water-mediated reaction.
The catalysts were characterized by the following techniques.
Powder XRD patterns were measured with a diffractometer
(
RINT Ultima+, Rigaku) using Cu-K
α radiation (tube voltage: 40
kV, tube current: 20 mA). Diffractions were recorded in the
−1
range of 4–60° with 5° min . The morphology was investigated
by using a transmission electron microscope (JEM-2100F, JEOL)
at 200 kV. The samples were dispersed in ethanol by ultrasonic
treatment for several minutes, and drops of the suspension
were placed on a copper grid for TEM observations. Raman
spectra were obtained using a spectrometer (in Via Reflex,
Experimental
Preparation of W-Ti-O samples.
W-Ti-O oxides were synthesized by a hydrothermal method
from ammonium metatungstate (AMT, (NH
Nippon Inorganic Colour & Chemical) and titanium sulfate
Ti (SO ·nH O, Mitsuwa Chemical) precursors. Typically,
Ti (SO ·nH O (1.27 mmol based on Ti) was added to 25 mL of
water and stirred for about 20 min until it was dissolved. AMT
5 mmol W) was dissolved in 20 mL of deionized water, and
4 6 2 12 2
) H W O40·nH O,
-1
Renishaw, a spectral resolution of 2 cm ) under the conditions
of a wavelength of 532 nm and collection time of 10 s. N
adsorption isotherms at liquid N temperature were measured
by using an automatic physisorption apparatus (BELSORP MAX,
BEL JAPAN) for the samples. Prior to N adsorption, the
(
2
4
)
3
2
2
2
4
)
3
2
2
(
2
catalysts were evacuated under vacuum at 300 °C for 2 h. The
external surface area was calculated by the Braunauer-
Emmett-Teller (BET) method. Temperature-programmed
then the W precursor solution was added to the Ti-containing
solution. Oxalic acid (5 mmol, Wako) was added to the
solution, and the mixture was stirred for 5 min. Then the
solution was sealed in a 60 mL Teflon liner stainless-steel
autoclave. Hydrothermal reaction was carried out at 175°C for
3
desorption (TPD) of ammonia (NH -TPD) was used to
determine the acidity. These measurements were done in an
automated chemisorption apparatus (BEL JAPAN). The
experimental procedure was as follows. The catalyst (ca. 50
mg) was placed between two layers of quartz wool in a quartz
24 hours. The obtained solid was filtered, washed thoroughly
with deionized water, and dried at 80 °C overnight and then
calcined at 400°C for 2 h.
-1
reactor and pre-heated in a helium flow (50 mL min ) at 400°C
°
for 1 h. Then ammonia was introduced at 100 C for 30 min.
Catalytic activity measurements.
°C
The desorption profile from 100 to 700 was recorded with a
A 50 mL round-bottom three-neck flask equipped with a reflux
condenser was used as a stirred bed reactor to evaluate
catalytic performance of the solids. For alkylation, a mixture of
benzyl alcohol (10 mmol), anisole or toluene (100 mmol), and
an internal standard (decane, 5 mmol) was added to the
reactor and the reaction temperature was adjusted to 100°C.
Then 0.2 g of a catalyst and a Teflon-coated magnetic stir bar
were loaded into the reactor. Aliquots (each 0.1 mL) were
collected at regular intervals. The concentrations of the
reactant and product were measured by gas chromatography
using a flame ionization detector (GC-FID, GL science GC390B)
-1
mass spectrometer in
Temperature-programmed decomposition mass spectrometry
TPD-MS) measurements were performed from 40 to 700°C at
a helium flow (50 mL min ).
(
-1
-1
a heating rate of 10°C min in helium flow (50 mL min ). The
decomposed gas molecules were monitored by a mass
spectrometer (ANELVA, Quadrupole Mass Spectrometer, M-
1
00QA, BEL Japan), collecting several mass fragments: CO
44), O (32), CO and N (28), H O (18, 17, 16) and NH (17, 16,
5). The acidity of the catalysts was measured by FT-IR
2
(
2
2
2
3
1
spectroscopy (JASCO) of adsorbed pyridine or CO using a
2
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furnace cell with CaF
2
windows containing a self-supporting type W-Ti-O mixed oxide based on MO
6
(M= W, Ti) octahedra
disk of a sample. The samples were pretreated in a vacuum at connected in the c-axis direction. This conclusion is based on
50°C. Pyridine was adsorbed onto the samples at 100°C, and our earlier report on a similar W-based mixed oxide having a
the adsorption spectrum was recorded after evacuation at layered-type structure based on MO octahedra and
50°C for 1 h. The spectrum of adsorbed pyridine on a sample pentagonal bipyramidal {M 21} (M =W or Nb) units
in the presence of water vapor (4.6 Torr) was also recorded. The oxidation state of Ti in the precursor (Ti (SO
CO was adsorbed at -183°C and the adsorption spectrum was 3+, which changed to 4+ during mixed oxide formation as
3
6
1
9–22,28
2
6
O
.
2
4 3 2
) ·nH O) is
III
recorded with degassing.
confirmed by XPS analysis. The Ti species acted as a reductant
for the formation of the pentagonal bipyramidal {W 21} units,
which are essential in the formation of the layered-type
structure. Hexagonal WO consists of WO octahedra. The
presence of a reductant is essential for the formation of the
pentagonal bipyramidal {W units. Accordingly, the
6
O
Results and discussion
3
6
Synthesis of Octahedra-layered W-Ti-O Mixed Metal Oxide.
6 21
O }
In order to discuss the relationship between the crystalline
structure of W-Ti-O catalyst and catalytic activity, various types
presence of a reductant will suppress the formation of the
hexagonal structure of tungsten oxide. The formation of
3
of W-Ti-O mixed metal oxide and WO were prepared. The
pentagonal bipyramidal {W
Raman spectroscopy. Fig. S5 shows Raman spectra of the
layered-type W-Ti-O and hexagonal WO samples. For
6
O21} units was confirmed by
synthesis conditions were optimized in an attempt to obtain as
pure as possible samples of a single phase. Fig. 1 shows the
XRD patterns of the most important W-Ti-O samples obtained
in this study (further XRD patterns are shown in the supporting
information (Fig. S1-4)). The W/Ti ratio in the precursor
solution was first investigated (Fig. S1). The XRD pattern of W-
Ti-O mixed oxides showed weak diffraction peaks at 2θ = 23°
and 47°, when the atomic W/Ti ratio was between 3/2.84 and
3
-1
-1
hexagonal WO
3
, Raman bands at 642 cm , 688 cm , and 817
-1
cm are assigned to the symmetric stretching modes of the
29
−1
W–O–W linkage . The Raman band at 781 cm of the layered-
type W-Ti-O is assigned to the symmetric stretching mode of
polyhedra, supporting the formation of pentagonal
6
bipyramidal {W O21} units.
3
3/1.42 (Fig. S1a-c). Pure hexagonal WO was obtained when
Although the formation of the hexagonal tungsten oxide
III
phase was increasingly by the addition of more Ti precursor,
there was no Ti in the precursor solution (Fig. S1f). When Ti
was added to the synthesis solution (W/Ti = 3/0.5), hexagonal
W-Ti-O mixed oxides were obtained without the formation of
it led to the formation of anatase titania when there was too
much Ti in the precursor solution. In attempting to avoid the
formation of titania, we added oxalic acid as an additional
reducing agent (Fig. S2, S3). This greatly suppressed formation
of the crystalline hexagonal structure already by the addition
anatase TiO
2
(Fig. S1e). When the W/Ti ratio was lower than
was formed (Fig. S1c) and the intensity of
3/1.42, anatase TiO
2
the diffraction peaks of the hexagonal phase decreased and
broadened. The diffraction peaks at 2θ = 23° and 47° observed
in W-Ti-O mixed oxides when the atomic W/Ti ratio was
between 3/1.42 and 3/2.84 were different from the peak
IV
of 5 mmol of oxalic acid (Fig. 1b). Ti can form a four-
coordinate structure with a carboxyl group. The Ti precursor
IV
composed of T coordinated with four oxalic acid accelerates
deriving from the (001) crystal plane of hexagonal WO
3
(d001=
the self-assembly of a mixed oxide based on octahedra under
hydrothermal conditions. Fig. S4 shows the influence of the
concentration of the precursor solution on the phases
obtained. The W-Ti-O samples obtained by the higher
precursor concentration (W/Ti ratio between 4/0.925 and 15/
3.898 Å) and hexagonal W-Ti-O mixed oxide (d001= 3.896 Å).
The d-spacing corresponding to this value of the new phase W-
Ti-O mixed oxides was 3.940 Å, indicative of a new layered-
3
.56) showed intense and sharp peaks at 2θ = 23° and 47°,
evidence for the formation of the layered W-Ti-O without
formation of hexagonal WO and titania. The higher precursor
○
: h-WO
3
3
(a)
concentration increases the rate of nucleation during
hydrothermal synthesis and improves phase purity by avoiding
the formation of other hexagonal (WO
phases.
3 2
) and anatase (TiO )
(
b)
A representative TEM image of the layered-type W-Ti-O
mixed oxide is shown in Fig. S6. The layered-type W-Ti-O
mixed oxide appeared as rod-like crystals with a diameter of 5-
10 nm and lengths of 30-50 nm.
(c)
○
○
○
By varying the concentrations in the precursor, we were
able to obtain a phase-pure layered-type structure based on
○
○
○
(
d)
○
○
○
○ ○○
○
○
octahedra MO
= 23 and 47°, without the formation of hexagonal WO
titania (Fig. 1). The crystalline motif of the layered-type W-Ti-O
in the a-b plane is due to the interconnection of MO (M= W,
6
(M = W, Ti), showing two diffraction peaks at
2
θ
3
and
10
20
30
θ / ° (Cu Kα)
40
50
60
2
6
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6
Ti) octahedra and {M O21} (M= W) pentagonal units. However, that all ammonium cations have decomposed. Thus, the
the atomic ordering in the a-b plane is not sufficient to give
increase in alkylation activity going from calcination at 300°C
Table 1. Physical properties of catalysts and results of alkylation.
NH
surface desorbed
3
Alkylation
1
anisole
of
2
Alkylation of toluene
Crystalline
phase
BET
Catalyst
d(001)
2
area / m g
-1
amount
/
4
3
Conv. /%
Yield
3
Conv. /% Yield /%
4
-
1
mmol g
/%
W-Ti-O
Layered-type 3.940
107
79
0.451
0.277
0.036
48
6
43 (5)
5 (1)
2
78
32
-
32 (46)
12 (20)
-
h-W-Ti-O
hexagonal
hexagonal
3.896
3.898
1h-WO
3
11
4 2
o
Reaction conditions: benzyl alcohol (10 mmol), anisole (100 mmol), catalyst (0.2 g), 100 C, 10 min. Reaction conditions: toluene (10 mmol), anisole (100 mmol),
o
catalyst (0.2 g), 100 C, 20 min toluene. Conversion of benzyl alcohol. Yield of benzyl anisole or benzyl toluene. Parentheses show the yield of dibenzyl ether.
3
4
rise to strong diffraction peaks of the sample, which is in line to 400°C is due to ammonia release, generating Brønsted acid
with the nanosized morphology of the rod-shaped particle sites. A possible reason for the decreasing catalytic activity
visible by TEM.
observed for calcination temperatures higher than 400 °C is
the phase transition of tungsten oxide to monoclinic WO . This
3
Characterization of W-Ti-O sample.
is clear from XRD patterns shown in the Supporting
Information. Therefore, the layered W-Ti-O sample was
calcined at 400°C for 2 h.
We then proceeded to characterize the optimal phase-pure
layered-type W-Ti-O sample, which was prepared at an atomic
W/Ti ratio of 5/1.27 in the presence of oxalic acid. Fig. S7
shows the nitrogen adsorption-desorption isotherm and the
corresponding Barrett-Joyner-Halenda (BJH) plot of the
layered-type W-Ti-O mixed oxide. The adsorption isotherms
displayed type IV hysteresis, suggesting the presence of
We also compared the performance of the layered W-Ti-O
3
sample to that of hexagonal W-Ti-O and hexagonal WO . The
hexagonal W-Ti-O mixed oxide sample was obtained by using
W/Ti = 3/ 0.5 without oxalic acid. Table 1 shows the results of
alkylation reactions of anisole (or toluene) and benzyl alcohol.
The alkylation activity of the layered-type W-Ti-O sample was
much higher for both alkylation reactions than that of the
hexagonal W-Ti-O sample. Fig. S11 shows the IR spectrum of
pyridine adsorbed at 100°C followed by evacuation at 250°C
for 1 h. Clearly, the layered W-Ti-O sample contains Brønsted
and Lewis acid sites, whereas these sites are not observed in
the hexagonal W-Ti-O sample. These results indicate that the
amount of acid sites for the hexagonal W-Ti-O was much
smaller than that for the layered W-Ti-O sample. The
2
-1
mesopores. The BET surface area was 107 m g . The TEM
image suggests that the layered-type W-Ti-O mixed oxide
sample seemed to be composed of the aggregation of
nanosized crystal particles, and the mesopores are formed by
the intraparticle voids. The insert highlights adsorption at low
-7
pressure. Uptake starts at P/P
0
= 10 , showing that the
1
2,30
layered-type W-Ti-O sample has micropores
. Thus, by
combining results of physisorption, XRD, Raman spectra and
TEM, we demonstrate that the layered-type W-Ti-O sample
contains nanosized rod-like crystals comprised of a crystalline
motif of W–Ti–O in the a–b plane made up of interconnected
units of MO (M =W, Ti) octahedra and {W O } pentagonal
hexagonal WO was the least active among the three tested
3
6
6 21
units that hold micropore channels (Fig. S8).
In order to understand the solid acid properties of the
layered W-Ti-O sample, we determined the influence of the
calcination temperature on the amount of ammonia released
during NH -TPD. The calcined samples were also evaluated in
3
alkylation reactions that require Brønsted acid sites. The
results of these experiments are shown in Fig. 2. The highest
activity for the alkylation of benzyl alcohol and anisole to
benzylanisole was observed for the sample calcined at 400°C.
At lower calcination temperature (300°C), ammonium cations
remained in the sample originating from the precursor. This
leads to a higher than expected ammonia release during NH3-
TPD. This is supported by TPD patterns of the uncalcined
layered W-Ti-O sample and the sample after calcination at
4
00°C shown in Fig. S9. The desorption peak of the ammonia
derived from the ammonium cation precursor was around
90°C for the uncalcined sample. We argue that Brønsted acid
3
sites are formed due to desorption of ammonia from the
layered W-Ti-O sample. From the TPD of the sample calcined
at 400°C, which shows no further release of ammonia, we infer
4
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samples.
dibenzyltoluene and tribenzyltoluene were observed. The
strong Brønsted acid sites of the layered-type W-Ti-O catalyst
adsorb the alkylation products on the surface of the catalyst
and promote their successive alkylation. On the other hand,
the addition of water to the reactant solution leads to
competitive adsorption with the reactants and in this way
decreases by-product formation. The other function of the
addition of water would be the shift of the equilibrium
Solid acid reactions in the presence of water.
The prospect of using the W-Ti-O mixed metal oxides as
catalysts in aqueous phase reactions was investigated. First,
alkylation of toluene with benzyl alcohol was investigated for
the layered W-Ti-O sample. Fig. 3 shows the influence of water
on the time course of the benzyltoluene and dibenzyl ether
yields. In spite of the addition of water (5 mmol) into the
mixture of reactant solution with the catalyst, the substrate
between benzyl alcohol and dibenzyl ether (C14
CH OH), resulting in higher alkylation rate.
Next, solid acid activity was examined by several catalytic
reactions in water by using typical model reactions of
hydrolysis of ethyl acetate and -toryl acetate (Table 2 and
2
H14O + H O →
2C
6
H
5
2
solution apparently homogeneous because of the bare
o
solubility of benzyl alcohol to water (7.15 g/100 g-H O at 90 C)
2
-
1
o
and an adsorptive property of catalyst (2.35 mmol g ). For the
alkylation of toluene and benzyl alcohol without the addition
of water, dibenzyl ether was only obtained directly after the
start of the reaction, followed by an increase of the
benzyltoluene yield with reaction time. The benzyltoluene and
dibenzyl ether yields stabilized after 1 h reaction when water
was not present. On the other hand, in the presence of water
3
1
Table S1). Hydrolysis of ethyl acetate by using the layered-
type W-Ti-O catalyst resulted in the formation of ethanol at a
-
1
-1
rate of 18.7
ꢀmol g min for 2 h. ZSM-5 and WO /TiO2
3
prepared by the impregnation method were tested for
comparison in the same reaction conditions. The formation
-
1
-1
mol g min for ZSM-5 and 2.1
rates of ethanol were 19.8
-1
ꢀ
mol g min for WO /TiO . For the hydrolysis of the more
-1
ꢀ
bulky ester o-toryl acetate, the formation rates of o-cresol
3
2
(5 or 10 mmol), the yield of dibenzyl ether decreased only
after 30 min and the yield of benzyltoluene increased as
reaction time proceeds. In another experiment shown in Fig. 3,
we added 10 mmol of water after 70 min of reaction. This led
to a decrease of the dibenzyl ether yield and an increase of the
benzyltoluene yield. We speculate that water promotes
desorption of the products from the acid sites on the layered
W-Ti-O catalyst. From the TPD spectrum (Fig. S12) for the
layered-type W-Ti-O catalyst used in the absence of water, it is
clear that the surface contains heavy products, containing
toluene groups. The peak at low temperature is due to
toluene, the peak at high temperature due to decomposition
of heavier products, which likely originate from multiple
alkylations. From the analysis of the reactant solution by GC-
FID and GC-MS (Fig. S13), heavy alkylation products such as
-1
-1
were 8.3 ꢀm- ol g min for the layered-type W-Ti-O catalyst,
.3 ꢀmol g min for WO /TiO and 0.3 ꢀmol g min for
1
-1
-1
-1
3
3 2
ZSM-5. The catalytic activities based on both catalytic weight
and acid site of the layered-type W-Ti-O catalyst were
competitive compared to the other typical solid acids such as
2
-
31
Cs2.5H0.5PW O , SO /ZrO and Nb O . The bulkiness of the
2 5
12
40
4
2
reactant, in this case, prevents diffusion into the small zeolite
micropores. Yet, the layered-type W-Ti-O can act as a solid acid
in an aqueous solution for the hydrolysis of both large and
small esters. The reactant solution was analyzed by ICP-AES to
check for possible leaching of the layered-type W-Ti-O catalyst.
The results of ICP-AES showed that the amounts of leaching of
the layered-type W-Ti-O catalyst were negligible.
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ARTICLE
Table 2. Catalytic activity over the layered-type W-Ti-O sample in the presence of water .
Catalyst Reaction
Journal Name
1
Reaction
Conv. /%
Yield /%
/g
temp./ °C
60
-1
-1
CH COOC
3
2
H
5
+ H
2
O→ CH
3
COOH + C
2
H
5
OH
0.8
(18.7 ꢀmol g min )
-
1
-1
mol g min )
0
.2
.1
60
(8.3
ꢀ
+
H2O→
+CH COOH
3
2
0
100
18
70
18
40
OH
3
OH
0.1
120
O
3
4
0.1
120
>99
37 (3)
1
2
3
4
Details of reaction conditions are shown in the experimental section, Reaction time, 6 h. reaction time, 2 h. Parentheses show the yield of pyruvaldehyde.
Then the layered-type W-Ti-O catalyst was applied to rate based on acid density. This result shows that the layered-
pyruvaldehyde conversion to lactic acid. The formation rate of type W-Ti-O catalyst contains Brønsted acid sites and Lewis
lactic acid was slow at 100°C. The conversion after 2 h was acid sites suitable for transformation of 1,3-DHA. The Lewis
1
8%, with nearly 100% lactic acid selectivity. The reaction at acid sites are effective in water for pyruvaldehyde
3
2, 33
1
20°C enhanced the rate of pyruvaldehyde conversion at the transformation.
The conversion of this sample contains such sites.
pyruvaldehyde was 70% with 40% lactic acid yield for 2 h at
20°C. By-products of the reaction were dimers and trimers of Solid acidity analysed by FT-IR.
Below, we will show by pyridine IR that
expense of lactic acid selectivity.
1
the carbonyl compounds formed by the aldol condensation
reaction. This performance was much better than that with
sulfuric acid, which is a strong Brønsted acid (10% conversion
with 6% selectivity to lactic acid at 120°C for 2 h). Lactic acid
synthesis from 1,3-DHA proceeds by the dehydration reaction
of 1,3-DHA to pyruvaldehyde followed by hydride transfer of
pyruvaldehyde. The layered-type W-Ti-O catalyst showed full
conversion with 38% selectivity to lactic acid. This values were
Fig. 4 shows IR spectra of CO adsorbed at -183°C on the
-1
layered-type W-Ti-O sample. The negative feature at 3660 cm
due to OH groups becomes weaker with increasing CO
coverage and is replaced by a shift OH stretch band at 3450
-1
cm . This OH frequency shift upon interaction with CO
indicates that the OH groups are strongly Brønsted acidic. The
-1
absorption at 2170 cm is attributed to the vibration of CO
adsorbed on these Brønsted acid sites. The absorption band at
-1
179 - 2205 cm is attributed to the vibrations of CO adsorbed
2
-1
-1
corresponding to approximately 238 mol (m mol-acid ) h of
2
2
-1
-1
the conversion rate and 88 mol (m mol-acid ) h of formation
(
150, 150, vacuum)
150, 150, water)
(
(250, 250, vacuum)
(250, 250, water)
(250, 150, water)
(300, 300, vacuum)
(300, 300, water)
(300, 150, water)
(350, 350, vacuum)
(350, 350, water)
(350, 150, water)
(400, 400, vacuum)
(400, 400, water)
(400, 150, water)
A
1450b(L acid)
(B acid)
A
1540
a
0
1
2
3
4
5
-
1
Integral absorbance of IR spectra peak / cm
6
| J. Name., 2012, 00, 1-3
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Cat Pa ll ey as si es dS oc ni eo nt ca ed j &u s Tt me ca hr gn i on ls ogy
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Journal Name
ARTICLE
-1
on Lewis acid sites. The adsorption at around 2180 cm
due to its strong adsorption on the catalytic surface. The
observed at 4 Torr shifted to a higher wavenumber with the selectivity could be improved by the addition of water, which
decrease in CO pressure. The presence of various absorption competes with the products for the acid sites. FT-IR
bands shows that the layered-type W-Ti-O sample possesses at measurements showed that the layered-type W-Ti-O
least two kinds of Lewis acid sites of different strengths.
possesses Brønsted acid sites and at least two different Lewis
Fig. 5 shows the ratio of Brønsted acid sites and Lewis acid acid sites. The stronger Lewis acid sites can be converted to
sites calculated by the integral IR absorbance of pyridine Brønsted acid sites in the presence of water, and the weaker
adsorbed on the layered-type W-Ti-O sample. The absorption Lewis acid sites functioned in the presence of water. This
-1
-1
bands at 1450 cm and 1540 cm were calculated as Lewis water tolerance of Lewis acid sites is an important
acid sites and Brønsted acid sites, respectively. After pyridine characteristic of layered-type W-Ti-O, as it allows the
was adsorbed onto the samples at 100°C, the parameters of conversion of 1,3-DHA into lactic acid in water.
the desorbed temperature of pyridine, measurement
temperature of FT-TR and measurement atmosphere (in
Acknowledgements
vacuum or presence of water) were controlled and shown as
(
desorbed temperature/ °C, measurement temperature/ °C,
This work was financially supported by the European Union
FP7 NMP project NOVACAM (Novel cheap and abundant
materials for catalytic biomass conversion, FP7-NMP-2013-
EUJapan-604319). And this study was supported by the
Cooperative Research Program of Institute for Catalysis,
Hokkaido University (13A1002) and the Cooperative Research
Program of Materials and Structures Laboratory, Tokyo
Institute of Technology (2013-53).
measurement atmosphere), respectively. A comparison of the
condition (150, 150, vacuum) with the condition (150, 150,
-
water) shows that the area of the adsorption peak at 1450 cm
1
-1
decreased and that of the peak at 1540 cm increased by the
addition of water. This demonstrates that part of the Lewis
acid sites were converted to Brønsted acid sites at 150°C upon
interaction with water. Tentatively, this can be expressed by
M(L) + M=O + H
2
O → 2M-OH(B). The amount of Lewis acid
sites that changed to Brønsted acid sites increased with
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