Hydrophobicity enhancement of Ti-MWW catalyst and its improvement in oxidation activity

Article abstract of DOI:10.1016/j.apcata.2015.07.003

The effect of the preparation conditions of the Ti-MWW catalyst on the hydrophobicity of the catalyst and its catalytic activity was investigated. Our findings demonstrated that the condensation of interlayer hydroxyl groups was greatly affected by the preparation conditions, in particular washing conditions of the as-synthesized lamellar precursor of Ti-MWW, then further controlling the final hydrophobicity and oxidation properties. Using organic solvents, especially EAOH, instead of water to wash the wet lamellar precursor would synchronize the interlayer hydroxyl condensation with the decrease of interlayer distance mainly caused by the leaching of piperidine, which was used as structure-directing agent (SDA). After acid-treatment, less SDA was kept in the EAOH washed sample and 3D-MWW with less defects was formed by calcination. Both drying temperature and acid-treatment would also affect the amount of SDA occluded in the interlayer void space of the acid-treated samples and then further affect the final interlayer hydroxyl condensation upon the following calcination. The lower both drying temperature and acid-treatment temperature were favorite to unequal interlayer dehydroxylation to form MCM-56, while higher drying temperature such as 150 C not only caused the anatase phase in the calcined samples but also occluded more SDA molecules in the acid-treated samples which greatly affect the further interlayer hydroxyl condensation upon the calcination. Ti-MWW-OH-100 containing smallest amount of silanols and less defect sites showed the best hydrophobicity and the highest catalytic activity in 1-hexene oxidation.

Full text of DOI:10.1016/j.apcata.2015.07.003

Applied Catalysis A: General 503 (2015) 156–164
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Hydrophobicity enhancement of Ti-MWW catalyst and its
improvement in oxidation activity
Hong Zhaoa,b, Toshiyuki Yokoia,∗, Junko N. Kondoa, Takashi Tatsumia
a Chemical Resources Laboratory, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503, Japan
b Shanghai Advanced Research Institute, Chinese Academy of Sciences, 100 Haike Road, Zhangjiang Hi-Tech Park, Pudong, Shanghai 201210, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of the preparation conditions of the Ti-MWW catalyst on the hydrophobicity of the catalyst
and its catalytic activity was investigated. Our findings demonstrated that the condensation of interlayer
hydroxyl groups was greatly affected by the preparation conditions, in particular washing conditions of
the as-synthesized lamellar precursor of Ti-MWW, then further controlling the final hydrophobicity and
oxidation properties. Using organic solvents, especially EAOH, instead of water to wash the wet lamellar
precursor would synchronize the interlayer hydroxyl condensation with the decrease of interlayer dis-
tance mainly caused by the leaching of piperidine, which was used as structure-directing agent (SDA).
After acid-treatment, less SDA was kept in the EAOH washed sample and 3D-MWW with less defects
was formed by calcination. Both drying temperature and acid-treatment would also affect the amount
of SDA occluded in the interlayer void space of the acid-treated samples and then further affect the final
interlayer hydroxyl condensation upon the following calcination. The lower both drying temperature
and acid-treatment temperature were favorite to unequal interlayer dehydroxylation to form MCM-56,
while higher drying temperature such as 150 ◦C not only caused the anatase phase in the calcined sam-
ples but also occluded more SDA molecules in the acid-treated samples which greatly affect the further
interlayer hydroxyl condensation upon the calcination. Ti-MWW-OH-100 containing smallest amount of
silanols and less defect sites showed the best hydrophobicity and the highest catalytic activity in 1-hexene
Received 14 April 2015
Received in revised form 25 June 2015
Accepted 3 July 2015
Available online 11 July 2015
Keywords:
Ti-MWW
Preparation conditions
Hydrophobicity
Catalytic activity
oxidation.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
there is no doubt that the outstanding hydrophobicity of TS-1
plays a greatly important role in its unique behavior [26–28]. It
Titanium silicate-1 molecular sieve (TS-1), an extremely effi-
cient and environment benign redox catalyst, has been considered
to be a milestone in zeolite catalysis in 80th last century due
to its unique properties in oxidation of small organic molecules
with dilute hydrogen peroxide solutions under mild conditions
[1–6]. During the past decades great efforts have therefore been
concentrated on the synthesis and application of a variety of
titanium zeolites and Ti-containing mesoporous materials with dif-
ferent structures and pore dimensions including TS-2 [7], Ti-MWW
[8,9], Ti-MOR [10], Ti-MCM-38 [11], Ti-Beta [12–14], Ti-MCM-
41/48 [15–20], Ti-SBA-15 [21–23] and Ti-containing polyhedral
oligomeric silsesquioxanes (POSS) [24,25] and so on.
is reported that TS-1 synthesized by microwave heating exhibited
the higher selectivity in the epoxidation of styrene as well as the
higher catalytic activity in the epoxidation reactions of 1-hexene
and styrene than that synthesized by conventional heating [29]. The
superiority of the former is considered to be directly caused by the
enhanced surface hydrophobicity. Similar to TS-1, Camblor et al.
[30] reported that aluminum-free Ti-Beta zeolite synthesized in
fluoride had less Si–OH connectivity defects as opposed to that syn-
thesized in OH− media. This enhanced hydrophobicity of Ti-Beta
(F) determines its higher activity and selectivity in the oxidation
of organic substrates containing a polar moiety. Except from Ti-
containing zeolites, Tatsumi report that silylated Ti-MCM-41 and
Ti-MCM-48 exhibited enhanced catalytic activity in the epoxida-
tion of olefins, as a result of increase in hydrophobicity [31]. Since
then, several activity-improved Ti-containing mesoporous mate-
rials and even FAU zeolite supported TiO2 photocatalyst [32] are
reported by hydrophobicity enhancement through the direct or
secondary incorporation of organic group [21,33].
Although it is still a challenge to thoroughly understand why
different Ti-catalysts seem to have different catalytic properties,
∗
Corresponding author.
E-mail address: yokoi.t.ab@m.titech.ac.jp (T. Yokoi).
http://dx.doi.org/10.1016/j.apcata.2015.07.003
0926-860X/© 2015 Elsevier B.V. All rights reserved.

H. Zhao et al. / Applied Catalysis A: General 503 (2015) 156–164
157
Ti-MWW, which possesses a unique pore structure comprised
of two independent 10-membered-ring (MR) channels and 12-MR
cups on the crystal exterior, has proved to be much more active than
TS-1 and Ti-Beta in the epoxidation of linear alkenes with H2O2
[8] and it was used in various organic reactions [34–37]. However,
MWW lamellar precursor is very flexible and condition-sensitive.
Through different post treatments it is readily converted into MCM-
22, MCM-36 [38], MCM-56 [39], ITQ-2 [40] and IEZ-MWW [41]. It is
also these futures of MWW precursor that makes MWW sheets very
difficult to stack together regularly and orderly and to form more
rigid and perfect 3D MWW structure with T-O-T linkages upon the
calcination. Ti-MWW with more hydroxyl groups was caused by
the incomplete interlayer dehydroxylation. This means that there
is much space for us to improve the Ti-MWW catalytic properties
by hydrophobicity enhancement.
Through the incorporation of organic groups into the porous
materials can effectively improve the property of Ti-containing
mesoporous catalysts, but this method is not necessarily the most
suitable one for Ti-MWW due to the relatively smaller pore chan-
nel. Wang et al. report that the post treatment of 3D Ti-MWW
with PI/HMI can induce the reversible structural rearrangement of
firstly deboroned Ti-MWW and efficiently improve the hydropho-
bicity and oxidation activity of Ti-MWW [42]. Nevertheless, this
post hydrothermal treatment with PI/HMI not only increases the
cost but also complicates the preparation process.
summarized the compositions and physical properties of the pre-
pared catalysts under the different conditions.
The X-ray diffraction (XRD) patterns were recorded on a
MAC Science M3X 1030 X-ray diffractometer with Cu source
˚
˚
(ꢀ˛1 = 1.54056 A, ꢀ˛2 = 1.54439 A, 40 kV, 20 mA) to identify the
crystalline phase and calculate unit-cell parameters. The titanium
coordination states of the as-synthesized and calcined materials
were investigated by diffuse reflectance (DR) UV–vis spectroscopy
(JASCO V-550 UV–vis spectrophotometer). OH-region infrared (IR)
spectra were measured on a PE-1600 FTIR spectrometer. Before
recording the spectra in the OH stretching vibration region, the
samples were first evacuated at 500 ◦C for 2.5 h under high-
vacuum conditions and then cooled to room temperature. 29Si
MAS NMR measurements were performed on a JEOL ECA-400
nuclear magnetic resonance spectrometer at ambient temperature.
N
2 adsorption at −196 ◦C and H2O adsorption at 25 ◦C were carried
out on Belsorp 28SCA and Belsorp 18SCA instruments, respectively.
TG/DTA was measured on an ULVAC-Rigaku TGD 9600 thermal
analysis system. The temperature was ramped to 800 ◦C at a heat-
ing rate of 10 ◦C min−1. The amounts of Ti and B of the samples were
determined by inductively coupled plasma-atomic emission spec-
trometry (Shimadzu ICPS-8000E). Scanning electron microscopy
(SEM) was performed on a JEOL JSMT220 instrument.
2.2. Catalytic reactions
Up to now, Ti-MWW is necessarily prepared from the very flex-
ible and condition-sensitive lamellar precursor through washing,
drying, acid-treatment and the following calcination. Inevitably,
the preparation conditions, which affect the condensation of
hydroxyl groups, control the final hydrophobicity followed by the
oxidation properties of Ti-MWW. This means that it is entirely pos-
sible for us to control the hydrophobic/hydrophilic properties of
Ti-MWW by controlling the preparation conditions. This assump-
tion is well applied by our current work.
Here we show a very convenient and efficient way to enhance
the hydrophobicity and improve the property of Ti-MWW. Use of
EAOH instead of pure water to wash wet crystals after crystalliza-
tion was the key to prepare highly hydrophobic Ti-MWW catalysts.
The following drying and acid-treatment also have great effect on
the Ti-MWW hydrophobicity. Choosing 1-hexene epoxidation as
the probe reaction, a detailed study describing the effect of prepa-
ration conditions on the physiochemical properties and catalytic
behavior of Ti-MWW materials is presented.
The epoxidation of 1-hexene with H2O2, which was used as
the probe reaction, was performed by batchwise in a 25-mL flask
equipped with a magnetic stirrer and a condenser. Typically,
the mixture containing 50 mg Ti-MWW catalyst, 10 mL of MeCN,
10 mmol of 1-hexene and 10 mmol of H2O2 was stirred vigorously
at 60 ◦C for 2 h. After the catalyst was removed, the products were
analyzed on a gas chromatograph (Shimadzu 14B, FID detector)
equipped with a 50 m DB-1 capillary column. The amount of uncon-
verted H2O2 was quantified by standard titration method with
0.1 M Ce(SO4)2 solution.
3. Results
3.1. Ti-MWW synthesis and characterization
3.1.1. Washing solution
The influence of washing solution on the structure was investi-
gated using the samples dried at 100 ◦C as representatives. Fig. 1
showed the XRD patterns of the precursors washed by water
or EAOH and the correspondingly calcined samples. Ti-MWW
(P)-EAOH-100 showed the same XRD patterns as Ti-MWW (P)-
H2O-100 and the both calcined samples exhibited the typical MWW
structure, indicating that washing solution had no significant influ-
ence on the structure and crystallinity of both the precursor and
the corresponding calcined Ti-MWW samples. Scanning electron
micrographs (SEM) (not shown here) and N2 adsorption results
(shown in Table 1) revealed that the washing solution had little
effect on the hexagonal morphology and crystal size distribution
or surface area and pore volume of the calcined samples.
2. Experimental
2.1. Catalyst preparation and characterization
The layered precursor of the MWW titanosilicate, Ti-MWW (P),
was hydrothermally synthesized using piperidine (PI) as structure-
directing agent (SDA) and boric acid as crystallization-supporting
agent according to the literature with our slight modifications [8].
The gels with the molar compositions of 1.0 SiO2:0.033 TiO2:1.4
PI:0.67 B2O3:19 H2O were crystallized under rotation (20 rpm) at
170 ◦C for 7.5 days. After the hydrothermal treatment, the wet crys-
tals were washed by three times volume of water or EAOH three
times and then dried at 30–150 ◦C for 24 h. This sample was denoted
as Ti-MWW (P)-X-Y, where X and Y were the washing solution and
the drying temperature, respectively. For example, when the wash-
ing solvent was EAOH and the drying temperature was 100 ◦C, the
sample was designated as Ti-MWW-EAOH-100. Ti-MWW (P)-X-Y
was acid-treated at 100 ◦C for 20 h and further calcined at 550 ◦C
for 10 h to obtain Ti-MWW-X-Y, which has 3D MWW structure.
The series of the Ti-MWW catalysts were prepared from the same
batch of the Ti-MWW lamellar precursor (Ti-MWW (P)). Table 1
3.1.2. Drying temperature
In order to investigate the influence of the drying tempera-
ture on the structure of Ti-MWW, the samples washed with EAOH
dried at 30, 70, 100 and 150 ◦C were used as representatives. The
XRD patterns of precursors were showed in Fig. 1. Clearly, the
patterns of Ti-MWW(P)-EAOH-30, Ti-MWW(P)-EAOH-70 and Ti-
MWW(P)-EAOH-100 were totally consistent with that of the typical
lamellar precursor of the MWW topology. However the pattern of
Ti-MWW(P)-EAOH-150 was a very close approximation assigned

158
H. Zhao et al. / Applied Catalysis A: General 503 (2015) 156–164
Table 1
Preparation conditions and physicochemical properties of Ti-MWW catalysts.*
b
Sample
Structure
Si/Tia
Si/Ba
SBET (m2/g)b
Sext (m2/g)b
Vmicro
Ti-MWW-EAOH-30
Ti-MWW-EAOH-70
Ti-MWW-H2O-100
Ti-MWW-EAOH-100
Ti-MWW-EAOH-150
MCM-56
MWW
MWW
MWW
MWW
57
63
59
62
65
44
36
35
41
39
451
546
532
536
520
151
142
138
137
124
0.13
0.16
0.17
0.16
0.17
*
All samples were prepared from the same batch of wet lamellar crystals.
Given by ICP.
Given by N2 adsorption for calcined samples.
a
b
Fig. 1. (A) XRD patterns of (a) Ti-MWW(P)-EAOH-30, (b) Ti-MWW(P)-EAOH-70 ◦C, (c) Ti-MWW(P)-EAOH-100 ◦C, (d) Ti-MWW (P)-H2O-100 and (e) Ti-MWW(P)-EAOH-150 ◦C;
(B) XRD patterns of (a) Ti-MWW-EAOH-30, (b) Ti-MWW-EAOH-70, (c) Ti-MWW-EAOH-100, (d) Ti-MWW-H2O-100 and (e) Ti-MWW-EAOH-150.
to the (0 0 1) and (0 0 2) diffraction lines of lamellar MWW pre-
cursor nearly disappeared, while the peaks due to the indexes of
h00 and hk0 remained practically unchanged, indicating the struc-
tural change evolved along the c-axis and the loss of the lamellar
structure.
Fig. 2(A) and (B) showed the UV spectra of the Ti-MWW cata-
lysts prepared at different conditions. It can be seen that Ti-MWW
(P)-EAOH-100 and Ti-MWW (P)-H2O-100 exhibited a main band at
260 nm together with a weak shoulder around 220 nm, which were
attributed to the octahedral and/or dimeric Ti derivatives and the
tetrahedrally coordinated Ti species highly dispersed in the frame-
work, respectively. After the acid-treatment at 100 ◦C for 20 h, the
band at about 260 nm disappeared and both Ti-MWW-EAOH-100
and Ti-MWW-H2O-100 showed only a narrow band at 220 nm. This
result implied that the washing solution had no obvious effect on
the Ti species coordination in the precursors or the acid-treated
and further calcinated samples. However, it should be noted that
the precursor dried at 150 ◦C showed another obvious band at about
330 nm which was assigned to an anatase-like Ti phase. After the
acid-treatment and further calcination, the band at about 260 nm
disappeared but the obvious band at around 330 nm was still kept.
These findings indicated that the anatase formed in the precursor
dried at higher temperature could withstand the acid extraction
and was hardly removed and that high drying temperature was
harm to prepare the anatase-free samples.
Upon acid-treatment and further calcination, the XRD pattern
of Ti-MWW-EAOH-70, Ti-MWW-EAOH-100 and Ti-MWW-EAOH-
150 were shown in Fig. 1(B), indicating the characteristic
diffractions due to the 100, 101, and 102 planes of the MWW
structure but with the absence of the 001 and 002 peaks due to
the layered structure, which was fully consistent with that of the
well-known 3D MWW structure. However, the XRD patterns of
Ti-MWW-EAOH-30 (trace a) showed that there was a marked dif-
ference in the 101 and 102 diffractions associated with the c-axis.
Clearly, the intensity of the 101 peak decreased and the 102 peak
broadened seriously. It was well established that the broadband in
the region of 7.5–11◦ can be taken as evidence for the formation of
a structure analog to MCM-56 which was a disordered collection of
the MWW sheets apparently along c direction [39]. It suggested
that the MWW sheets of Ti-MWW-EAOH-30 were at least not
an ordered arrangement as that of typical 3D-MWW structure.
Moreover, the sample prepared by washing and followed direct
acid-treatment and calcination but not drying showed a typical
XRD pattern of MCM-56 (not shown). The XRD results of these sam-
ples matched well with the textual characteristics obtained from N2
adsorption isotherm shown in Table 1. These results implied that
the drying process before acid-treatment was essential to prepare
the 3D MWW structural sample and that lower drying temperature
such as 30 ◦C, similar to the reported lower acid-treatment tem-
perature [43], resulted in a disordered collection of MWW sheets
apparently along c direction to form MCM-56.
3.2. Hydrophobicity of the prepared Ti-MWW
The IR spectra in the region of the hydroxyl stretching
vibration, 29Si MAS NMR and water adsorption measurements
were employed to correlate the preparation conditions and
hydrophobicity using Ti-MWW-EAOH-30, Ti-MWW-EAOH-100,
Ti-MWW-EAOH-150 and Ti-MWW-H2O-100 as representatives.
Fig. 2(C) showed the IR spectra in the region of hydroxyl stretch-
ing and Table 2 showed the band area integrated in the range
of 3000–3900 cm−1. The band at 3745 cm−1 was assigned to the

H. Zhao et al. / Applied Catalysis A: General 503 (2015) 156–164
159
Fig. 2. UV–visible spectra of (A) as-synthesized and (B) calcined Ti-MWW-H2O-100 (a) and Ti-MWW-EAOH-X. X = 30 (b), 70 (c), 100 (d) and 150 (e); (C) IR spectra in region
of hydroxyl stretching vibration of Ti-MWW samples.
Table 2
that it had the most silanols in the framework. Ti-MWW-EAOH-30
Hydrophobicity of Ti-MWW catalysts.
gave a very intensive band at 3745 cm−1, which should be firmly
a
b
Sample
SIR (3850–3100 cm
V [cm−3(STP) g−1
]
related to an opener structure of MCM-56. This was in good agree-
ment with the XRD patterns in Fig. 1(B) (trace a). Both MCM-56
prepared from lower drying precursor and that prepared at lower
acid-treatment temperature were presumed to be a disordered col-
lection of the MWW sheets along the c-axis. Hence, we can conclude
that lower drying and lower acid-treatment temperatures made for
the incomplete and unequal interlayer dehydroxylation.
−1
)
Ti-MWW-EAOH-30
Ti-MWW-EAOH-100
Ti-MWW-H2O-100
Ti-MWW-EAOH-150
131
82
117
159
101
79
95
114
a
SIR (3850–3100 cm
:
determined from the OH stretching in IR spectra
−1
)
(3900–3000 cm−1).
b
V [cm−3(STP) g−1]: given by H2O adsorption isotherms at 25 ◦C.
To further probe the hydrophobicity of Ti-MWW catalysts, 29Si
MAS NMR spectroscopy was performed and showed in Fig. 3. The
Q4 sites, where Qn is Si(OM)n(OH)4−n, M = Si or Ti, in the region
of −105 to −130 ppm were deconvoluted into five lines, which
can be assigned to several distinctive crystallographic T sites in
the MWW framework but with overlapping resonances [38,39]. In
addition, an obvious resonance at −98 ppm attributed to the Q3 site,
Si(OH)(OSi)3 or Si(OH)(OTi)(OSi)2. Ti-MWW-EAOH-100 showed
very similar resonances due to the Q4 sites but a less intensive
−98 ppm resonance due to the Q3 site compared to Ti-MWW-
H2O-100. This agreed well with the above IR spectra to verify
the less defect sites in Ti-MWW-EAOH-100. Moreover, the spectra
of Ti-MWW-EAOH-150 and Ti-MWW-EAOH-30 showed another
well-resolved resonance in the downfield Q2 site (−93 ppm), which
was absent in the spectra of Ti-MWW-H2O-100 and Ti-MWW-
EAOH-100, being in good agreement with the above IR spectra that
the strong band at 3520 cm−1 was observed for both samples.
The amount of water adsorbed on samples was listed in Table 2
based on the water adsorption isotherms measurements. The
isolated external or terminal silanols on the crystal surface. The
band at 3720 cm−1 probably due to asymmetric hydrogen-bonded
silanols located inside the crystals, the band at 3690 cm−1 assigned
to vicinal silanols and the broad band at around 3520 cm−1 due
to hydrogen-bonded silanols nests were also observed. Com-
pared with Ti-MWW-H2O-100, Ti-MWW-EAOH-100 showed much
weaker bands at 3745 cm−1, 3690 cm−1 and 3520 cm−1. The lat-
ter two bands were due to the internal silanols of Ti-MWW which
originate from the defect sites as a result of deboronation and/or
incomplete interlayer dehydroxylation [42]. Note that the boron
amount in Ti-MWW-H2O-100 was a little higher than that in Ti-
MWW-EAOH-100 (Table 1). This implied that a large number of
internal silanols in the Ti-MWW-H2O-100 were mainly caused by
the incomplete interlayer dehydroxylation.
Drying temperature also showed great influence on the OH
stretching vibrations. Ti-MWW-EAOH-150 showed the greatest
band area integrated in the range of 3000–3900 cm−1, suggesting

160
H. Zhao et al. / Applied Catalysis A: General 503 (2015) 156–164
Fig. 3. 29Si MAS NMR spectra of (a) Ti-MWW-H2O-100, (b) Ti-MWW-EAOH-100, (c) Ti-MWW-EAOH-30 and (d) Ti-MWW-EAOH-150.
amount of the absorbed water on the Ti-MWW-EAOH-100 was the
lowest compared with other four samples. This demonstrated that
Ti-MWW-EAOH-100 was the most hydrophobic than other sam-
ples.
of the EAOH-washed samples were almost twice as high as the
H2O-washed ones.
As a control, besides EAOH, other solutions such as acetonitrile
and alcohols with different carbon atoms were also investigated
to be used as washing solution. The Ti-MWW sample prepared by
reversible structural rearrangement also used for comparison. The
reaction results are summarized in Table 3, suggesting that the
use of the organic solutions, except C4-C6OH which cannot wet
the crystal precursor, in substitution for water can also improve
the catalytic activities of the samples. Because EAOH was the most
efficient, cheapest and environment-friendly, it was used as rep-
resentative and was investigated detailedly. As verified by the IR
spectra in the region of the hydroxyl stretching vibration, 29Si MAS
NMR and water adsorption measurements, the superior catalytic
activity of the EAOH-washed samples was mainly caused by the
enhanced hydrophobicity.
3.3. Catalytic properties of the prepared Ti-MWW
The oxidation of 1-hexene with H2O2 was used as the probe
reaction to explore the Ti-MWW catalytic properties. The reaction
was considered to be able to take place on all Ti species in Ti–MWW
because of the relatively small molecular sizes of both the sub-
strate and the oxidant without significant diffusion problems in the
Ti-MWW structure [8,42]. Fig. 4(A) compared the turnover num-
bers (TON) of the Ti-MWW-EAOH and Ti-MWW-H2O samples. In
order to eliminate the effect of Ti content on TON, the Si/Ti ratio of
the calcined samples was controlled at 60–65. Clearly, the EAOH-
washed samples showed a higher TON than the H2O-washed ones
prepared under the same other preparation conditions; the TONs
Fig. 4(A) also depicted the influence of the drying temperature
on the catalytic activity of Ti-MWW. The TON for the 1-hexene oxi-
dation showed a volcanic behavior over both Ti-MWW-OH and

H. Zhao et al. / Applied Catalysis A: General 503 (2015) 156–164
161
Fig. 4. (A) The comparison of TON between Ti-MWW-EAOH-Y (-᭹-) and Ti-MWW-H2O-Y (-᭿-) (Y = 30, 50, 70, 100, 120 and 150); (B) unit-cell parameter c values of the
outgassed Ti-MWW (P)-EAOH (-᭹-), Ti-MWW (P)-H2O (-᭿-), the air-exposed Ti-MWW (P)-EAOH after outgas (-ꢀ-) and the air-exposed Ti-MWW (P)-H2O after outgas (-ᮀ-).
Ti-MWW-H2O. As described above, the samples prepared from
the precursors dried at 30 ◦C were Ti-MCM-56 analogs and the
activities of 1-hexene oxidation were very low. When the drying
temperature was as high as 50 ◦C, the Ti-MWW samples after the
acid-treatment followed by the calcination the TON values were
increased greatly. The TON value over the both series was increased
with an increase in the drying temperature and simultaneously
reached the maximum values at about 100 ◦C. Further increasing
the drying temperature, the hydrophobicity of the samples was
decreased and the TON also decreased. Especially when the drying
temperature was as high as 150 ◦C, anatase phase appeared and the
activities of samples fell down sharply.
calcined MCM-22 [45], was used during the refinement process
from 101 and 102 diffraction lines which were very intense and
easy to accurately determine their positions and d-spacing values.
Table 4 summarized the unit-cell parameters of these samples, and
Fig. 4(B) showed the effect of vacuum-heating temperatures on the
unit-cell parameter c. In order to clearly illustrate the change in the
c values with the vacuum temperature, the start points in Fig. 4(B)
were the values of the precursors but not the data got from the
samples vacuumed at 100 ◦C. All samples showed similar unit-cell
parameter a and b but different c values. Clearly, c values of both
EAOH-washed samples and H2O-washed samples were decreased
with an increase in the vacuum-heating temperature, but the lat-
ter changed more sharply and acutely than the former. In detail, c
value of EAOH-washed sample was decreased gradually along with
the vacuum-heating temperature, and when the vacuum-heating
4. Discussion
◦
˚
temperature reached 200 C, c value decreased to 24.62 A and was
much closed to that of calcined 3D-strutural sample. However,
as for H2O-washed samples, the c value was dec◦reased sharply
In order to investigate the influence of organic washing solution
on the transformation of the layered solid into Ti-MWW, we pre-
pared a series of comparable samples from Ti-MWW (P)-EAOH-100
and Ti-MWW (P)-H2O-100 by vacuumed at different tempera-
tures for 16 h. The organic leaching upon vacuum-heating was
detected by TG analysis and the unit-cell parameters were checked
by XRD analysis. The XRD data were analyzed via Rietveld’s refine-
ment method [44] using the JADE 6.0 software. The background
was fit manually and the line profiles were fit using pseudo-Voigt
functions. P6/mmm structure, determined by Leonowicz et al. for
˚
to 24.83 A when the outgas temperature was 125 C, and further
raising vacuum temperature the c value changed little. Since the
crystalline structure within layers is rigid, it suggested that the
interlayer spacing of both the H2O-washed and EAOH-washed sam-
ples was decreased with an increase in the vacuum temperature,
and that the former changed more rapidly and acutely than the
latter.
Table 3
Oxidation of 1-hexenea with H2O2 over Ti–MWW.a
Sample
Conversion (%)
Selectivity (%)
H2O2
Conversion (%)
Efficiency (%)b
Ti-MWW-CH3OH
Ti-MWW-C2H5OH
Ti-MWW-C3H7 OH
Ti-MWW-C(CH3)3OH
Ti-MWW-C8H17OH
Ti-MWW-CH3CH2CHO
Ti-MWW-CH3CN
Ti-MWW-H2O
53.6
55.6
49.6
31.3
26.2
50.7
51.9
36.3
58.7
99.2
99.5
99.4
98.9
99.1
99.1
99.4
99.4
99.3
65.6
62.5
59.5
35.3
39.7
58.8
58.1
44.3
69.3
81.7
89.0
83.4
88.7
66.0
86.2
89.3
81.9
84.7
cRe-Ti-MWW
a
Reaction conditions: catalyst (0.05 g), cyclohexene (10 mmol), H2O2 (10 mmol), acetonitrile (10 mL), temperature (60 ◦C), reaction time (2 h). The by-product was mainly
diol checked by GC-MAS.
b
Ti-MWW samples were prepared from the same batch of lamellar crystals and Si/Ti in gel was 40. Drying temperature was 100 ◦C, acid-treatment temperature was
100 ◦C.
c
Re-Ti-MWW prepared according to [36], Si/Ti is 66.

162
H. Zhao et al. / Applied Catalysis A: General 503 (2015) 156–164
Table 4
Refined unit-cell parameters of the vacuum-heated samples.
Vacuum temperature (◦C)
EAOH washed sample
H2O washed sample
a = b (Å)
c (Å)
a = b (Å)
c (Å)
a
–
13.90
13.95
14.11
14.08
14.10
27.09
26.17
25.56
25.03
24.62
13.88
14.09
14.03
13.99
14.17
27.01
25.43
24.83
24.51
24.47
110
125
150
200
a
Precursors dried at 100 ◦C.
Table 5 showed the TG results of the above vacuumed sam-
ples. The weight losses, in the range of 200–700 ◦C derived from
the total removal of PI molecules as SDA and in the range of
200–400 ◦C derived from the removal of PI molecules located at
the void space between layers, were decreased with an increase in
the vacuum-heating temperatures. However the much less weight
loss occurred in the above two ranges of the H2O-washed samples
than in that of EAOH-washed samples. This implied that the more
organic molecules were removed from the interlayer space and
other place of water-washed samples than EAOH-washed samples
at the same vacuum-heating temperature.
with intermediate c values between that of precursor and the 3D
structure should be an intermediate situation between the layered
precursor and the condensed 3D structure. Similar inference was
also drawn by Luca and co-workers using a combined XRD and com-
putational study [46]. This meant that part of organic molecules
were eliminated from the interlayer channels to cause the decrease
in the interlayer distance upon vacuum-heating, and simulta-
neously Si-O-Si bands were partially formed between the layers.
Based on Table
5 and Fig. 4(B) we deduced that upon
vacuum-heating the interlayer distance of the water washed pre-
cursor should reduce more rapidly than the condensation of the
interlayer hydroxyl groups which was closely associated with
vacuum-heating temperature, and the interlayer space was easy
to reversibly expand when the samples were exposed to air
atmosphere again. Inversely, the interlayer spacing decrease of
EAOH-washed precursor was much slower and could synchro-
nize with interlayer hydroxyl condensation upon vacuum-heating,
which would make for the complete condensation of the interlayer
silanols over the following calcination.
However, it was really important to note in Fig. 4(B) that c val-
ues were dramatically changes when the above outgassed samples
were re-exposed to air atmosphere for 3 days at r.t. After expo-
sure to air, the c values of water-washed samples were increased
reversibly except that of the sample vacuum-heated at 200 ◦C,
while the c values of almost all EAOH-washed samples were
unchanged except for the one vacuumed at 150 ◦C, of which c value
˚
was increased from 25.03 to 25.24 A. The difference between c val-
ues of H2O-washed samples and EAOH-washed samples became
very obscure and were only a function of vacuum-heating temper-
atures no matter with what washing solution were used or what
the c values were before exposure to air atmosphere. Apparently,
the interlayer spacing decrease upon vacuum-heating was mainly
caused by the progressive elimination of organic molecules located
between the layers. However, if there was no force to maintain the
shrink of the interlayer space caused by organic molecules leaching,
it was certainly easy to re-expand once the air and water molecules
enter the interlayer space again.
From Fig. 4(B), the c values of both H2O and EAOH washed sam-
ples outgassed at 200 ◦C were little changed after exposure to air
and were very similar to that of calcined 3D samples. The XRD
patterns of these two samples (not shown here) had no signifi-
cant changes before and after exposure to air and were the typical
3D MWW patterns. Those results implied that the MWW zeolitic
frameworks finally evolved from the layered precursor when the
samples was outgassed at 200 ◦C, although a majority of struc-
turally ordered PI molecules was still occluded in the zeolite at
this time, as suggested by thermogravimetric analysis (TGA) in
Table 5. Certainly, other samples outgassed at lower temperatures
Fig. 5(A) compared the TGA results of Ti-MWW(P)-H2O-100,
Ti-MWW(P)-EAOH-100 and the corresponding acid-treated sam-
ples. The weight loss of Ti-MWW(P)-H2O-100 was slightly smaller
than that of Ti-MWW(P)-EAOH-100. The exothermic peak, usually
believed to be attributed to the removal of PI molecules sited at
the interlayer space, was around 380 ◦C for Ti-MWW(P)-H2O-100
and about 405 ◦C for Ti-MWW(P)-EAOH-100. The exothermic peak
at lower temperature suggested that the combustion of organic
molecules in interlayer space of Ti-MWW(P)-H2O-100 should be
easier than Ti-MWW(P)-EAOH-100. This result matched well with
the conclusion drawn from the analysis of the vacuum-heated sam-
ples above.
The weight loss of the corresponding acid-treated samples
greatly reduced compared with the corresponding precursors. It
meant that a large part of SDA molecules were removed by acid-
treatment from both the water washed precursor and the EAOH
washed precursor. Moreover, the TG curves showed that the weight
loss of water washed samples was much greater than that of EAOH
washed samples. This implied that the amount of PI molecules
occluded in the former was much larger than in the latter. This part
of PI molecules which was difficult to be removed by acid washing
Table 5
Weight loss of the vacuumed-heated samples.
Vacuum temperature (◦C)
Weightlossin200–700 ◦C(w%)
Weightlossin200–400 ◦C(w%)
Weight loss by vacuum-heating in
200–400 ◦C (w%)
EAOHa
H2Ob
EAOHa
H2Ob
EAOHa
H2Ob
c
–
14.5
14.5
14.1
13.3
12.0
14.1
13.8
12.8
11.7
10.8
9.9
9.6
9.1
8.0
7.1
9.2
8.4
7.6
6.8
5.8
0
3.3
8.5
19.5
28.2
0
8.8
17.2
26.5
37.7
110
125
150
200
a
Sample prepared from Ti-MWW(P)-EAOH-100.
Sample prepared from Ti-MWW(P)-H2O-100.
Precursor dried at 100 ◦C.
b
c

H. Zhao et al. / Applied Catalysis A: General 503 (2015) 156–164
163
Fig. 5. (A) TG/DTA profiles of precursors Ti-MWW (P)-H2O-100 (a, aꢁ) and Ti-MWW (P)-EAOH-100 (b, bꢁ) and corresponding acid-treated samples Ti-MWW(A)-H2O-100 (c),
Ti-MWW(A)-EAOH-100 (d); (B) TG/DTA profiles of the acid-treated samples prepared from EAOH-washed precursors dried at 50 ◦C (a and aꢁ), 100 ◦C (b and bꢁ) and 150 ◦C (c
and cꢁ).
and tightly occluded in interlayer void space of the water washed
sample would inevitably interrupt the further condensation of the
interlayer hydroxyl groups upon the following calcination.
Fig. 4(B) compared the TGA results of the acid-treated samples
prepared from the Ti-MWW(P)-OH dried at 50, 100 and 150 ◦C.
The total weight loss of these three acid-treated samples was Ti-
MWW(A)-OH-100 < Ti-MWW(A)-OH-50 < Ti-MWW(A)-OH-150,
although the weight loss in the corresponding precursors was
Ti-MWW(P)-OH-50 > Ti-MWW(P)-OH-100 > Ti-MWW(P)-OH-150
(not shown here) as expected. This result meant that the drying
temperature would affect the PI amount occluded in the acid-
treated samples and the lowest amount of PI was kept in the
100 ◦C dried sample. It was most noteworthy that very obvious
peaks in the range of 300–400 ◦C were observed in the DTA curves
of Ti-MWW(A)-OH-50 and Ti-MWW(A)-OH-150 but disappeared
in the curve of Ti-MWW(A)-OH-100. This implied that after acid-
treatment very few PI molecules were occluded in the interlayer
void space of Ti-MWW(A)-OH-100, which certainly made for the
completely condensation of interlayer hydroxyl groups and the
formation of the perfect 3D structure.
form MCM-56, while higher drying temperature such as 150 ◦C not
only cause the formation of anatase phase in the calcined sam-
ples but also occluded more SDA molecules in the acid-treated
samples which greatly affect the further interlayer hydroxyl con-
densation upon the calcination. Ti-MWW-OH-100 containing the
lowest amount of silanols and less defect sites showed the highest
catalytic activity in 1-hexene oxidation.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2015.07.
003
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