Ultrafast synthesis of nanosized Ti-Beta as an efficient oxidation catalyst: Via a structural reconstruction method

Article abstract of DOI:10.1039/c9cy00071b

As a representative selective oxidation titanosilicate catalyst, a Ti-Beta zeolite is less used in comparison with TS-1, Ti-MWW and Ti-MOR, mostly due to its high hydrophilicity originating from a BEA? intergrowth framework. A novel recrystallization method was proposed in the present study to prepare highly hydrophobic Ti-Beta with nanosized crystals (90 nm), high Ti content (Si/Ti = 20) and intercrystal mesoporosity. The fluoride-assisted recrystallization was realized quickly by dissolving extensively a dealuminated Beta zeolite in a mixture of a tetraethylammonium aqueous solution and Ti precursor, producing highly crystalline Ti-Beta in an extremely short time of 1 h. The obtained Ti-Beta zeolite exhibited superior catalytic activity in the liquid-phase epoxidation reactions of bulky alkenes like cyclohexene with hydrogen peroxide or tert-butyl hydroperoxide as an oxidant, compared to those Ti-Beta catalysts prepared by conventional hydrothermal or secondary synthesis routes.

Full text of DOI:10.1039/c9cy00071b

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Page 1 of 36
Catalysis Science & Technology
Ultrafast Synthesis of Nanosized Ti-Beta via Structural Reconstruction MethodView Article Online
DOI: 10.1039/C9CY00071B
as Efficient Oxidation Catalyst
Bowen Wang, Hao Xu*, Zhiguo Zhu, Yejun Guan and Peng Wu*
Corresponding Address:
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry and Molecular Engineering, East China Normal University, North
Zhongshan Road 3663, Shanghai 200062, People’s Republic of China
Tel: +86-21-6223-2292
Fax: +86-21-6223-2292
E-mail address: hxu@chem.ecnu.edu.cn (H. Xu); pwu@chem.ecnu.edu.cn (P. Wu)

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DOI: 10.1039/C9CY00071B
Abstract: As one of representative selective oxidation titanosilicate catalysts, Ti-
Beta zeolite was less used in comparison to TS-1, Ti-MWW and Ti-MOR, mostly due
to its high hydrophilicity originated from BEA* intergrowth framework. A novel
recrystallization method was proposed in present study to prepare highly hydrophobic
Ti-Beta with nanosized crystals (90 nm), high Ti content (Si/Ti = 20) as well as
intercrystal mesoporosity. The fluoride-assisted recrystallization was realized fast by
dissolving extensively dealuminated Beta zeolite in the mixture of tetraethylammonium
aqueous solution and Ti precursor, producing highly crystalline Ti-Beta in an extremely
short time of 1 h. The obtained Ti-Beta zeolite exhibited superior catalytic activity in
the liquid-phase epoxidation reactions of bulky alkenes like cyclohexene with hydrogen
peroxide or tert-butyl hydroperoxide as oxidant, compared to those Ti-Beta catalysts
prepared by conventional hydrothermal or secondary synthesis routes.

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Catalysis Science & Technology
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DOI: 10.1039/C9CY00071B
1. Introduction
The selective catalytic oxidation of organic compounds by hydrogen peroxide or
organic hydroperoxides over Ti-substituted zeolites, has received much attention in the
past decades.1,2 The isolate and tetrahedrally coordinated Ti4+ ions in zeolite framework
are able to activate the hydrogen peroxide molecules under mild condition and generate
the Ti-O-O-H hydroperoxide intermediate, which then oxidize the organic substrates
while giving H2O as the sole by-product. On the other hand, the molecular sieving
property of their subnano micropores makes Ti-zeolites excellent in controlling the
product selectivity.3 The TS-1/H2O2 catalytic system has achieved great success in a
variety of oxidation reactions, such as hydroxylation of aromatics,4 the oxidation of
alcohols to carboxyl compounds,5 propylene epoxidation6 and cyclohexanone
ammoxidation.7 However, the narrow 10-ring (R) pore channel of TS-1 imposes
significant steric constrains on bulk substrates and limit its application in the reaction
involving only small-size substrates. Decreasing the crystal size of TS-1 zeolite to the
level of nanometer, even to 2-3 units cells, was attractive in exposing the active Ti sites
and releasing the diffusion constrains of bulky molecules.8-11 Besides, extensive effort
have been devoted to the development of titanosilicates with large-pore sizes, such as
Ti-MOR,12 Ti-MWW,13-15 Ti-Beta,16 Ti-MCM-68,17 Ti-UTD-1.18 Among them, Ti-
MOR is highly active in the ketone ammoxidation reaction while Ti-MWW exhibited
attractive catalytic performance in both the ketone ammoxidation reactions and alkene
epoxidation. Ti-Beta, with 3-dimension (3D) 12R large-pore channels, possesses a
more open pore system than Ti-MOR and Ti-MWW.19 Actually, Ti-Beta was less used,
although it has shown high conversion in the epoxidation of olefins, monoterpenes,
methyl esters of fatty acids, unsaturated alcohols, and ,-unsaturated carbonyls with
high hydrogen peroxide efficiency.5 Very similar to BEA* aluminosilicate,

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conventional Ti-Beta contains an intergrowth of polymorph A and B with the ratio of
ca. 44 : 56, and then exhibits a high framework hydrophilicity, which leads to a
relatively lower intrinsic activity per Ti site and also a lower selectivity to desired
products in the epoxidation reaction as a result of easy hydrolysis of oxides. Moreover,
the traditional synthesis procedures of Ti-Beta zeolite are time-consuming (>14 d).
Up to now, several methods have been proposed to prepare Ti-Beta zeolites,
including the conventional hydrothermal crystallization in OH– medium20 and fluoride
medium,21,22 dry-gel conversion23 and post-synthesis method.24-26 The conventional
hydrothermal crystallization in OH– medium and dry-gel conversion can produce small-
size particles (~200 nm). However, the high framework hydrophilicity due to nanosized
crystals and framework defects originated from intergrowth was inevitable. Ti-Beta
hydrothermally synthesized in fluoride medium possessed less defects, however,
leading to the sharp increase of particle size, which caused great diffusion constrain.
The above two methods were also time-consuming procedures, with a crystallization
time of >14 days. Post-synthesis methods, including chemical vapor deposition
(CVD),26 dry impregnation24 and grafting the organometallic precursor titanocene
dichloride (Cp2TiCl2) on Beta frameworks25 have also been reported to prepare Ti-Beta
zeolite in very short time. However, the incorporation of tetrahedral Ti ions was poorly
controlled, tending to form TiO2 easily. Thus, to promote the application of Ti-Beta
zeolite, a fast and easy handle method is highly desired, and meanwhile it is more
preferably to provide hydrophobic nanosized crystals.
Very recently, our group proposed a recrystallization method to synthesized Sn-
Beta zeolite with high Sn content, high framework hydrophobicity and nanosized
crystals within extremely short time of 1 h.27 Inspire by this method, we attempted to
prepare efficient Ti-Beta catalysts with similar strategy in the present study. As

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described in Scheme 1, a commercial Al-Beta aluminosilicate was firstly dealuminated
deeply in HNO3 aqueous solution, which was then used as silicon source and dissolved
in a mixture of titanium butoxide (TBOT) and tetraethylammonium hydroxide
(TEAOH) aqueous solution. The obtained mixture containing primary structural units
were recrystallized rapidly to Ti-Beta with the assistant of F– ions. The recrystallization
process was carefully investigated and the obtained Ti-Beta zeolite was characterized
and evaluated in the cyclohexene epoxidation reaction.
2. Experimental
2.1. Materials
Commercial zeolite with the Si/Al molar ratio of 11 was purchased from Shanghai
Xinnian Petrochemical Additives Co., Ltd. Nitric acid (HNO3, 65 wt% in water),
tetraethylammonium hydroxide (TEAOH, 35 wt% in water), titanium butoxide (TBOT),
and ammonium fluoride (NH4F) were purchased from Sinopharm Chemical Reagent
Co., Ltd. Titanium tetrachloride (TiCl4) was purchased from Tokyo Chemical Industry
Co., Ltd. Ti-MWW and TS-1 were provided by TWRD Co., Ltd.
2.2. Preparation of dealuminated Beta
Typically, the commercial Beta zeolite was treated with 13 mol L–1 HNO3 with a liquid-
to-solid ratio of 50 mL g–1 at 413 K for 24 h. The product was filtered, washed three
times with deionized water, dried at 383 K, and finally calcined at 823 K for 6 h. The
obtained sample was denoted as Beta-DA with a Si/Al ratio of >1900.
2.3. Preparation of Ti-Beta via structural reconstruction
The synthetic gel was prepared by mixing Beta-DA as silica source, TEAOH as organic
structure-directing agent (OSDA), TBOT as Ti source and water together in a Teflon-
lined autoclave. In this dissolution process, the dissolution temperature was set as 413
K and was kept for 1 h to dissolve the Beta-DA. Then, the the autoclave was quenched

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to ambient temperature, and then a desired amount of ammonium fluoride (NH4F) waVsiew Article Online
DOI: 10.1039/C9CY00071B
added into the gel, forming a homogeneous mixture with the composition of 1 SiO2 :
1/x TBOT : (0.3 - 0.5) TEAOH : (4 - 7.5) H2O : 0.1 NH4F. Subsequently, the
recrystallization process was performed at 413 K for 20 min to 24 h. The product was
filtered, washed three times, dried at 353 K and calcined at 873 K for 6 h. The obtained
sample was denoted as Ti-Beta-Re-x, where Re indicated structural reconstruction and
x suggested the Si/Ti ratio in the synthetic gel. The Ti-free Beta zeolites were also
synthesized similarly but without adding TBOT in synthetic gels, giving rise to Beta-
Re-ꝏ samples.
2.4. Preparation of Ti-Beta by conventional method and post-synthetic method
For control experiment, Ti-Beta zeolites were also prepared via hydrothermal method
and post-synthetic method. Hydrothermal synthesis of Ti-Beta zeolite in both OH-
medium and F– medium was performed strictly according to the literatures21 and were
labeled as Ti-Beta-OH-y and Ti-Beta-F-y, respectively, where y indicated the Si/Ti
ratio. For post-synthesis approach, the Beta-DA sample was placed into a quartz tube
and pretreated at 823 K for 2 h in a dry N2 flow (50 ml min-1). Then, the TiCl4 vapor
was brought by N2 to plant Ti ions into the framework defects of Beta-DA. After 2 h,
the sample was purged with N2 for another 2 h to remove the residual TiCl4 and
produced HCl gas during solid-gas reaction. After cooling to the room temperature,
the sample was washed with deionized water, dried at 353 K overnight and calcined
at 873 K for 4 h, named as Ti-Beta-PS. Ti-MOR was prepared from dealuminated
MOR zeolite also via the same post-synthetic gas-solid reaction.28,29
2.5. Characterization methods
The X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku Ultima
IV X-ray diffractometer with CuK radiation (= 1.5406 Å) with a scan speed of 2 =

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10 o min–1. Scanning electron microscopy (SEM) images were obtained using a Hitachi
S-4800 microscope. The chemical composition of the samples was determined by
inductively coupled plasma emission spectrometry (ICP-AES) on a thermo IRIS
intrepid Ⅱ XSP atomic emission spectrometer. The samples were dissolved in the HF
aqueous solution before measurements. The nitrogen sorption isotherms were measured
at 77 K on a Quantachrome iQ-MP gas adsorption analyzer. Before the nitrogen
absorption, the samples were dehydrated at 573 K at least for 3 h. The total surface area
was calculated via the Brunauer Emmett Teller (BET) equation and the micropore
volume was determined with the t-plot method. Diffuse reflectance ultraviolet-visible
(UV-vis) spectra were recorded against BaSO4 reference in the region of 190 – 500 nm
under ambient conditions on a Shimadzu UV-2700 spectrophotometer. Diffuse
reflectance infrared Fourier transform (DRIFT) spectra were measured on a Nicolet
Nexus 670 FT-IR spectrometer with a spectral resolution of 2 cm–1 in absorbance mode.
Self-supporting pellets made of sample was set in a quartz cell and sealed with CaF2
windows. The cell was connected to a vacuum system. After the sample was dehydrated
at 723 K for 2 h to remove any water in the material, the FT-IR spectra in the hydroxyl
stretching vibration region were then measured at room temperature. Solid-state nuclear
magnetic resonance (NMR) experiments were performed on a VARIAN VNMRS-
400WB spectrometer using [(CH3)3SiO]8SiO12 as the chemical shift reference at a
frequency of 79.43 MHz, a spinning rate of 3 kHz, and a recycling delay of 60 s.
2.6. Catalytic reactions
The catalytic performance was evaluated the epoxidation of cyclic alkene in a 25 mL
flake linked to a water condenser under appropriate stirring. In a typical run,
cyclohexene (10 mmol), hydrogen peroxide (10 mmol, 30 wt %), acetonitrile (10 mL),
and the catalyst (50 mg) were stirred at 333 K for 60 min. And the tert-butyl

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hydroperoxide in water or decane was also used as the oxidant to investiDgOaIt:e10.t1h03e9/C9CY00071B
catalytic performance. Once the reaction finished, the catalyst was removed from the
reaction system by centrifugation, and the reaction mixture was analyzed using a gas
chromatograph (Shimadzu GC-14B, FID detector) equipped with a 30 m DB-1
capillary column to determine the conversion and selectivity.
3. Result and discussion
3.1. Synthesis of Ti-Beta-Re via structural reconstruction method
The preparation of Ti-Beta-Re zeolite included the deconstruction of the dealuminated
Beta framework and subsequent recrystallization process to Ti-containing Beta
(Scheme 1). The deconstruction of Beta framework was achieved in the alkaline
solution containing TEAOH. As TEAOH was the only source to provide alkaline
medium, its concentration was critical for the successful decomposition of Beta
framework. Thus, the effects of TEAOH/Si and the H2O/Si ratio on the deconstruction
process of Beta-DA were firstly investigated in the present study (Fig. S1). The
dealuminated Beta zeolite showed well-resolved diffraction peaks characterized by
*BEA topology (Fig. S1Aa and S1Ba). With the increase of TEAOH/Si ratio, the
diffraction peaks obviously decreased in intensity and almost disappeared when the
TEAOH/Si ratio was larger than 0.3, suggesting enough TEAOH amount was needed
for the successful decomposition of Beta framework (Fig. S1A). On the other hand, the
H2O/Si ratio was varied in the range of 1-7.5 while the TEAOH/Si ratio was fixed at
0.3, and the full dissolution of Beta framework was observed with the H2O/Si ratio
varying in the range of 4 - 7.5 (Fig. S1B). Subsequently, the recrystallization process
was performed with the assistant of F– ions. The TEAOH served as OSDA in the
recrystallization process. Thus, the amount of TEAOH was also critical for directing

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the reconstruction of *BEA framework. As exhibited in Fig. S2A, highly crystalline Ti-
Beta-Re could be obtained readily at TEAOH/Si ratios of 0.3 - 0.5, which indicated Ti-
Beta could be crystallized with lower SDA amount via the recrystallization process in
comparison to conventional hydrothermal synthesis. Similarly, the H2O/Si ratio was
also investigated in the recrystallization process (Fig. S2B). The dissolved Beta-DA
could be well-restructured the H2O/Si ratio range of 4 - 7.5. The recrystallization
temperature of Ti-Beta also varied from 413 K to 463 K. According to the XRD patterns
of the products (Fig. S3), Ti-Beta could be well restructured in a wide range of
recrystallization temperature.
The whole dissolution and recrystallization process of Beta zeolite was traced by the
XRD patterns (Fig. 1A) depending on the recrystallization time, with the TEAOH/Si
and H2O/Si ratio fixed as 0.3 and 7.5, respectively. The highly crystalline Beta-DA (Fig.
1Aa) was dissolved in the mixture of TEAOH aqueous solution and TBOT, and the
reflection peaks disappeared after 1 h at the dissolution temperature of 413 K (Fig. 1Ab).
Once the F– ions were added into the mixture, the recrystallization process was triggered
with the diffraction peaks attributed to *BEA topology appeared and increased in
intensity along with the reconstruction time (Fig. 1A, c-f). The intensity of these
diffraction peaks remain constant after 60 min, indicating that the reconstruction of
*BEA framework was extremely fast. As shown in Fig. S4, the crystallization rate of
Ti-Beta-Re-50 showed a sharp increase after 10 min and the crystallinity of Ti-Beta-Re
was > 96 % after 60 min. The time-dependent UV-vis spectra of Ti-Beta-Re-50 were
also collected (Fig. S5). The spectrum of the starting recrystallization gel showed
obvious bands around 212 nm, 260 nm and 330 nm, which indicated that a large amount
of Ti species exited in the form of pentacoordinate Ti species, oligomeric octahedral Ti
species and anatase phase. Prolonging the recrystallization time, the bands shifted to

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the high frequency zone (205 nm), which indicated that the amorphous Ti specDieOsI: 1w0.e10r3e9/C9CY00071B
incorporated in the framework forming the isolated tetrahedral Ti species gradually.
Combining the crystallinity curve, the crystallization process was accompanied by the
incorporation of Ti. Accordingly, the SEM images showed only amorphous phase after
the Beta-DA was dissolved in the mixture of TEAOH and TBOT (Fig. S6a). Prolonging
the reconstruction time, the nanoparticles appeared and the amorphous phase
disappeared gradually (Fig. S6, b-d). The amorphous phase originated from Beta-DA
served as nutrition for the crystallization of Ti-Beta zeolites. Using this effective
synthetic strategy, the Si/Ti ratio was tailored in the range of 10 - 100 (Fig. 1B). All the
Ti-Beta-Re zeolites with different Ti contents showed well-resolved diffraction patterns
attributed to *BEA topology. However, the intensity of these diffraction peaks
decreased with increasing the Ti content, suggesting that the addition of Ti ions in the
synthetic system hindered the recrystallization of Beta framework. It should be noted
that an obvious peak around 2θ = 35, ascribe to TiO2,30 could be observed for the
sample with Si/Ti ratio of 10 in the synthetic gel, implying that too many Ti ions could
cause the aggregation of Ti species. Thus, highly crystalline Ti-Beta-Re with a pure
BEA* phase could be obtained at Si/Ti ratios >20.
3.2. Physicochemical properties of Ti-Beta-Re
The SEM images of Ti-Beta zeolites synthesized from different methods are shown in
the Fig. 2. Evidently, among the four Ti-Beta zeolites, HF-mediated Ti-Beta-F
possessed the largest primary crystal size of ~ 5 μm, while Ti-Beta-Re had the smallest
ones with the size of ~ 90 nm. Considering Ti-Beta-PS and Ti-Beta-Re both originated
from Beta-DA, the atom-plating process with TiCl4 vapor did not change the crystal
size (180~ 200 nm), but the recrystallization process decreased the crystal size to ~ 90
nm. The TEM images of Ti-Beta-Re further revealed the highly crystalline zeolite

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framework as well as the presence of intercrystal mesopores (Fig. S7).
The N2 sorption isotherms of Ti-Beta-Re-50, Ti-Beta-F-50, Ti-Beta-PS and Ti-
Beta-OH-50 are shown in Fig. S8. The hysteresis loops could be observed in the
isotherms of nanosized Ti-Beta-Re and Ti-Beta-PS due to the presence of intercrystal
mesopores. The surface areas and pore volumes of various Ti-Beta zeolites are
summarized in Table 1. The Ti-Beta-Re samples with different Ti contents exhibited
comparable micropore volumes (0.19 - 0.23 cm3 g-1) as those of Ti-Beta zeolites
prepared by other methods, confirming the highly crystalline frameworks. However,
the surface areas, external surface areas and mesopore volumes of Ti-Beta-Re zeolites
were much larger than those of other Ti-Beta zeolites prepared by conventional or post-
synthesis methods, as the intergranular mesopores resulted from the aggregation of
nanosized were present in Ti-Beta-Re crystals. The results were consistent with the
SEM (Fig. 2) and TEM (Fig. S7) images. The Si/Ti ratios were also measured for all
the Ti-Beta samples. As for the Ti-Beta-Re and Ti-Beta-OH samples, the Si/Ti ratios in
the product were comparable to that in the synthetic gels, indicating that the two
methods were effective in incorporating the Ti ions into the Beta framework. In contrast,
the Si/Ti ratio in the product was only 80 for Ti-Beta-F when the Si/Ti ratio in the gel
was set at 50, indicating a large amount of Ti source was not fully utilized.
In general, tetrahedrally coordinated Ti species were considered to serve as the
active sites for the selective oxidation reactions. Before being used as catalysts, the
obtained Ti-Beta-Re samples were firstly characterized with UV-Vis and IR spectra to
confirm the coordinated state of Ti species. The UV-vis spectra of Ti-Beta-Re samples
with different Ti contents are displayed in Fig. 3. It has been widely reported that Ti
species give characteristic adsorption bands in the UV-vis spectrum due to the specific
coordination states, including isolated tetrahedral Ti at 205 nm, isolated

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pentacoordinate Ti at 212 nm, isolated octahedral Ti at 225 nm, oligomeric octahedral
Ti-O-Ti at 260 nm, and anatase phase at 330 nm.24 As can be seen in Fig. 3a, the
adsorption peak of Ti-Beta-Re-ꝏ sample was extremely weak, due to the absence of Ti
species in the sample. And other Ti-Beta-Re samples (Fig. 3, b-e) showed a main band
around 205 nm, indicating the Ti atoms were mainly tetrahedrally coordinated in the
framework. However, the band became broad with the Ti content increase, especially
for the one with Si/Ti ratio of 20, indicating the Ti species with higher coordinated
number were developed. The minor band around 330 nm in the spectrum of Ti-Beta-
Re-20 indicated the presence of very few amount of anatase phase. Although the
attribution of the band around 960 cm–1 is still a debate, it could be considered as a
proof for the incorporation of Ti ions in the zeolite framework.31,32 As shown in Fig. 4,
the band of 960 cm–1 was absent for the Ti-free Beta-Re-ꝏ sample, while all the Ti-
Beta-Re samples exhibited the characteristic vibration band around 960 cm–1, indicating
the Ti ions were well-incorporated into the Beta framework in the recrystallization
process. Moreover, the intensity of the 960 cm–1 band was enhanced with increasing Ti
content.
In
addition
to
the
coordination
state
of
Ti
species,
the
hydrophilicity/hydrophobicity natures are also critical to the catalytic performance of
the Ti-zeolite catalysts.3,33,34 Thus, the FT-IR spectra in the hydroxide stretching region
were collected to characterize the hydrophilicity/hydrophobicity of the Ti-Beta samples
synthesized with four different methods (Fig. 5). All the samples show the band around
3740 cm–1, ascribed to the terminal silanol groups. Ti-Beta-OH and Ti-Beta-PS
developed an additional band at 3540 cm–1, showing a broad feature probably due to
hydrogen-bonded silanol groups on defect sites such as hydroxyl nests resulting from
the stacking fault. The minor 3697 cm–1 band shown by Ti-Beta-F was also relate to the

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silanol groups.13,14,32,35 The absorption band around 3740 cm–1 were deconvoluted into
3740 and 3731 cm–1 bands, which are attributed to the external and internal-type
terminal silanols, respectively.36 And the corresponding results were summarized in Fig.
6. The amount of silanol groups in Ti-Beta-Re and Ti-Beta-F were comparable but far
less than those in Ti-Beta-OH and Ti-Beta-PS. Compared to the other Ti-Beta zeolites,
the particle size of Ti-Beta-F was larger, which significantly cut down the amount of
terminal silanols. Ti-Beta-Re with the smallest nanosized crystals possessed the lowest
amount of internal silanols, probably due to less structural defects evolved in the
reconstruction process, which may be favorable for the catalytic performance.
Considering both the terminal (3740 cm-1) and hydrogen-bonded (3540 cm-1) silanols,
Ti-Beta-Re and Ti-Beta-F were more hydrophobic than Ti-Beta-OH and Ti-Beta-PS.
The thermogravimetric analysis was also performed to assess the
hydrophilicity/hydrophobicity of these materials. As shown in the Fig. S9, the weight
loss in the range of 35-150 oC, attributed to the physically adsorbed water,27,37 was 1.5,
3.3, 9.1 and 10.2 wt % for Ti-Beta-Re-50, Ti-Beta-F-50, Ti-Beta-OH-50 and Ti-Beta-
PS, respectively, which indicated that Ti-Beta-Re-50 was most hydrophobic than other
Ti-Beta zeolites, consistent with the results from above IR spectra (Fig. 6).
To further demonstrated the excellent hydrophobicity of Ti-Beta-Re, the 29Si MAS
NMR spectra also was conducted. The 29Si MAS NMR spectra of all the Ti-Beta
samples could be deconvoluted into three resonance bands at -116, -113 and -103 ppm.
The signals at -116 and -113 ppm correspond to the framework Si(OSi)4 (Q4) species in
two different crystallographic sites, while the signal at -103 ppm results from the
framework Si(OSi)3(OH) (Q3) species.4,36 Ti-Beta-Re possessed the least Q3 groups of
2.8 % population, again proving fewer structural defects and higher hydrophobicity
than other Ti-Beta zeolites (Table S1). It could be concluded here that the

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deconstruction and reconstruction method was effective in producing Ti-BetaDzOeI:o10li.1t0e3Vs9ie/wC9ArCtiYcl0e0O0n7li1nBe
with smaller crystal size and at the same time high framework hydrophobicity due to
less terminal silanols.
3.3. Catalytic performance of Ti-Beta-Re in the epoxidation reactions
In general, solvent was used to homogenize the substrate and oxidation in the
epoxidation of cyclohexene with hydrogen peroxide (H2O2, 30 wt %), in order to
promote the activity retarded by the mass-transfer problem.24 As reported by former
studies, the aprotic/protic nature and polarity of solvent have great effects on the
activity.3 It was widely accepted that the Ti species and solvent would form different
intermediates in protic (Species Ⅰ) and aprotic (Species Ⅱ) solvents.3,15,38 As shown
in the Scheme 2, the size of Specie Ⅰ is far larger than that of species Ⅱ, leading to
lower epoxidation activity. Fig. 8 shows the catalytic results of cyclohexene
epoxidation over Ti-Beta-Re-50 in different solvents. When chlorobenzene (C6H5Cl)
was used as the solvent, the conversion of cyclohexene (3.2 %) was lower than that in
other solvents, because C6H5Cl was immiscible with aqueous H2O2. The catalytic
activity of Ti-Beta-Re in the protic solvent of i-PrOH, was also very poor, attributing
to the formation of larger species Ⅰ (Scheme 2), which would hinder the activation of
H2O2 in the channel of zeolite. Higher catalytic activity of Ti-Beta-Re was achieved
with the aprotic solvents of MeCN and acetone, with MeCN giving the highest
cyclohexene conversion of 11.7 % and cyclohexene oxide selectivity of 78.1 %.
According to the previous studies,3 the polarity of acetonitrile is larger than acetone,
which indicated larger solvent polarity favored the cyclohexene epoxidation. Moreover,
the weak basic nature of MeCN could neutralize the weak acid sites arising from the
stacking fault and structural defects in Ti-Beta-Re zeolite.24
Table 2 provides the catalytic performance of Ti-Beta zeolites prepared by

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different methods in the cyclohexene epoxidation reaction. For Ti-Beta-Re samDpOleI:s10,.t1h03e9/C9CY00071B
turnover frequency (TOF) value decreased gradually with the Ti content increasing, due
to the uneven distribution of Ti atoms in the framework, which was consistent with
previous study.39 On the other hand, the coordination status of Ti species changed with
the increase of Ti content, as described in Fig. 3, which could also result in the decrease
of TOF for cyclohexene epoxidation. The TOF values of Ti-Beta-Re-50, Ti-Beta-OH-
50 and Ti-Beta-PS were also compared at comparable Si/Ti ratio. Ti-Beta-Re-50
showed the highest TOF value of 54 h–1, which was mainly related to the higher
hydrophobicity. Ti-Beta-Re-100 and Ti-Beta-F-50 possessed the Ti contents at
comparable level, but the former showed higher TOF value than Ti-Beta-F-50,
attributed to the smaller crystal size of Ti-Beta-Re-100. From the view of cyclohexene
oxide selectivity, Ti-Beta-Re and Ti-Beta-F showed higher selectivity than Ti-Beta-OH
and Ti-Beta-PS, which was associated to the lower content of hydroxyl groups in these
two catalysts.
The epoxidation of cyclohexene with TBHP in different solvent of H2O and decane
were processed to illustrate the importance of hydrophobicity (Table 3). As anticipated,
the usage of TBHP led to the decrease of cyclohexene conversion (> 5 %), due to its
larger steric hindrance than H2O2. All the Ti-Beta zeolites showed higher TOF value
with the TBHP in decane than that in water. Compared to the TBHP in decane, the
water molecules would bond with the silanol groups through the hydrogen bonds, when
aqueous TBHP was used as oxidation, which would retard the activity of Ti-Beta
severely. However, the differences coming from the aqueous and organic solvents of
TBHP was more obvious for Ti-Beta-OH-50 and Ti-Beta-PS than Ti-Beta-F-50 and Ti-
Beta-Re-50, due to higher hydrophobicity of the lattes.
The reusability of heterogeneous catalysts is an important property, which would

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determine whether the catalysts is suitable for the industrial application. The reusability
of Ti-Beta prepared by different methods in the epoxidation of cyclohexene is shown
in Fig. 9. The cyclohexene conversion over Ti-Beta-Re decreased from 12.4 % to 6.5 %
slowly after 3 runs when the used catalyst was only washed by acetonitrile. After
calcination, the activity of used catalytic was almost restored to the level of fresh
catalyst. Similarly, the Ti-Beta-F could also achieve the level of fresh catalyst after
calcination, which indicated that the decrease of activity resulted from pore blocking
other than the loss of active sites. In contrast, significant activity decrease was observed
for Ti-Beta-OH-50 (> 9 %) and Ti-Beta-PS (> 6 %) even after calcination, probably
due to the loss or chemical state change of active sites. Thus, the contrast between fresh
and used catalysts after calcination was shown Fig. S10. For Ti-Beta-Re-50 and Ti-
Beta-F-50 catalysts, the intensity of the band around 205 nm, attributed to the
framework tetrahedrally coordinated Ti species, was slightly decreased after the
catalytic reactions, indicating that very few amount of framework Ti species was
extracted during the reaction. Meanwhile, minor bands around 260 and 330 nm
appeared in the used catalysts, suggesting that the extracted framework Ti species were
converted to extraframework Ti species or even anatase phase. However, for
deactivated Ti-Beta-OH-50 and Ti-Beta-PS, the adsorption band around 205 nm
severely decreased, with very broad adsorption band appearing around 260 and 330 nm.
Thus, the Ti active sites were more stable in the Ti-Beta-Re and Ti-Beta-F samples than
those in hydrophilic Ti-Beta-OH and Ti-Beta-PS. The Ti sites in hydrophobic
framework are less affected by water attacking.
The catalytic performance of Ti-Beta-Re was also compared to other Ti-zeolites,
including TS-1, Ti-MWW, and Ti-MOR in the epoxidation of cyclohexene (Fig. 10).
As expected, the TS-1 and Ti-MWW exhibited poor catalytic performance, owing to

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the greater steric hindrance of the cyclohexene in the narrow 10-R pore cDhOaIn: 1n0e.10ls3.9/C9CY00071B
Though Ti-MOR possesses a 12-R pore system, its one-dimensional pore channels
imposed negative influence on the accessibility of active sites. Additionally, Ti-MOR
prepared by post-synthetic method, contained residual Al ions, resulting in the high
hydrophilicity and thus low activity. Ti-Beta-Re possessed 3-dimensional 12-R pore
system, favoring the transport of substrates, so that its catalytic performance was
superior to the other three catalysts.
4. Conclusions
A novel and convenient destruction and recrystallization method was developed
to prepare Ti-Beta zeolite in extremely short time of 1 h, with nanosized crystals, high
framework hydrophobicity and Ti content. The Ti-Beta-Re catalysts showed higher
activity in the cyclohexene epoxidation reactions than the Ti-Beta zeolites synthesized
by conventional hydrothermal method and post-synthetic method, with both the H2O2
and bulky TBHP as the oxidants. Moreover, the high framework hydrophobicity
resulted in more stable Ti active sites, endowing Ti-Beta-Re catalysts high reusability.
The available commercial Al-Beta and TEAOH, the facile synthetic procedure, and the
high activity in epoxidation reactions made Ti-Beta-Re a promising catalyst for
industrial applications.
Acknowledgments
We gratefully acknowledge the financial supports from the NSFC of China (21533002，
21603075 and 21872052), and China Ministry of Science and Technology
(2016YFA0202804).

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Scheme 1. Strategy for the synthesis of hydrophobic nanosized Ti-Beta

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R
O
H
O
H
H
Ti
Ti
O
OH
O
OH
Species Ⅰ
Species Ⅱ
Scheme 2. Structure of proposed intermediate Ti species

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Table 1 Textural properties of various Ti-Beta samples.
SSA (m2 g–1)
Pore volume b (cm3 g–1)
Si/Ti
No.
Sample
c
d
d
Gel
20
30
50
100
50
50
–
Product a
S BET
633
656
643
660
537
553
487
S ext
V total
0.69
0.79
0.70
0.62
0.24
0.26
0.60
V micro
Vmeso
0.49
0.57
0.47
0.43
0.03
0.05
0.40
1
2
3
4
5
6
7
Ti-Beta-Re-20
Ti-Beta-Re-30
Ti-Beta-Re-50
Ti-Beta-Re-100
Ti-Beta-F-50
Ti-Beta-OH-50
Ti-Beta-PS
22
31
48
98
80
47
57
191
0.20
0.22
0.23
0.19
0.21
0.21
0.20
207
195
191
58
89
113
a Determined by ICP analysis.
b Given by N2 adsorption at 77 K.
c Calculated by BET method.
d Calculated by t-plot method.

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Table 2 Catalytic activity of various catalysts for epoxidation of cyclohexene with
hydrogen peroxide.a
OH
O
O
O
Ti-Beta
H2O2
H2O
+
O
+
+
+
+
OH
OH
333 K
Alnene
conv. (%)
14.7
Oxide sel.
(%)
No.
Catalyst
Si/Tib
TOF (h –1)
1
2
3
4
5
6
7
Ti-Beta-Re-20
Ti-Beta-Re-30
Ti-Beta-Re-50
Ti-Beta-Re-100
Ti-Beta-F-50
Ti-Beta-OH-50
Ti-Beta-PS
22
31
48
98
80
47
57
79.7
33
37
54
66
16
36
45
12.4
77.3
11.7
78.1
7.2
76.7
2.2
74.8
10.1
62.2
9.2
69.1
a Reaction condition: cat., 50 mg; cyclohexene, 10 mmol; H2O2 (30 wt %), 10 mmol;
CH3CN, 10 mL; temp., 333 K; time, 60 min.
b Determined by ICP analysis.

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Table 3 Catalytic activity of various catalysts for epoxidation of cyclohexene with TBHP.a
OH
O
O
O
Ti-Beta
333 K
O
OH
+
+
+
+
+
OH
OH
O
OH
TBHP in water
Oxide
TBHP in decane
Alkene Oxide
Increase
No.
Catalyst
Si/Tib
Alkene
ratioc (%)
TOF (h-1)
TOF (h–1)
conv. (%)
6.5
sel. (%)
59.8
conv. (%)
7.9
sel. (%)
60.0
1
2
3
Ti-Beta-Re-50
Ti-Beta-F-50
48
80
47
23
13
1
28
18
12
21.5
39.1
600
2.3
60.0
3.2
58.4
Ti-Beta-OH-50
0.5
46.5
3.5
57.4
4
Ti-Beta-PS
57
0.9
50.8
3
2.6
57.9
11
189
a Reaction condition: cat., 50 mg; cyclohexene, 10 mmol; TBHP, 10 mmol; CH3CN, 10 mL; temp., 333 K;
time, 60 min.
b Determined by ICP analysis.
퐶
.
푑푒푐푎푛푒 ― 퐶표푛푣.푤푎푡푒푟
onv
c
increase ratio =
× 100 %
퐶표푛푣.푤푎푡푒푟

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B
A
f
e
d
e
c
d
c
b
b
a
a
5
10
15
20
25
30
35
10
20
30
40
2 Theta (deg.)
2 Theta (deg.)
Fig. 1 (A) XRD patterns of Beta-DA (a) and Ti-Beta-Re obtained in recrystallization
process at Si/Ti ratio of 50 for 0 min (b), 20 min (c), 40 min (d), 60 min (e), 24 h (f).
(B) XRD patterns of Ti-Beta-Re synthesized with Si/Ti ratio of 100 (a), 50 (b), 30 (c),
20 (d), 10 (e). Other conditions: NH4F/SiO2 = 0.5; H2O/SiO2 = 7.5; TEAOH/SiO2 =
0.3; temp., 413 K; time, 24 h

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Fig. 2 Scanning electron micrograph images of Ti-Beta-PS (a), Ti-Beta-F (b), Ti-Beta-
OH (c), and Ti-Beta-Re (d) samples. Insets show the size distribution primary crystals.

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205 nm
e
d
c
b
a
200
300
400
500
Wavelength (nm)
Fig. 3 UV-vis spectra of Ti-Beta-Re synthesized at Si/Ti molar ratio of ꝏ (a), 100 (b),
50 (c), 30 (d) and 0 (e).

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960 cm1
e
d
c
b
a
1200 1150 1100 1050 1000 950 900 850 800
Wavenumber (cm-1)
Fig. 4 FT-IR spectra of calcined Ti-Beta-Re samples prepared at the Si/Ti molar ratio
of ∞ (a), 100 (b), 50 (c), 30 (d), 20 (e) under the conditions of TEAOH/Si= 0.3; NH4F/Si
= 0.5; H2O/Si = 7.5; temp., 413 K; time, 24 h.