Inter-connected and open pore hierarchical TS-1 with controlled framework titanium for catalytic cyclohexene epoxidation

Article abstract of DOI:10.1039/c7cy02571h

A post-synthesis method was developed to reduce the extra-framework titanium (Ti) in TS-1 zeolites (Si/Ti ratio = 50), in which tetrapropylamonium hydroxide (TPAOH) aqueous solution was used to promote the dissolution, redistribution and recrystallization processes, and hence to convert amorphous Ti species into zeolitic phases. It was found that TPAOH could effectively convert the extra-framework Ti into framework Ti, and the TPAOH concentration influenced the pore structure significantly. Under lower TPAOH concentration (i.e. 0.05-0.4 M), only closed meso-/macropores (grooves and hollow cavities) can be created in the TS-1 crystals. At an optimum concentration of 0.5 M TPAOH, open and connected hierarchical mesopores and macropores were created in the resulting TS-1 zeolites. Compared with the parent TS-1, the amount of extra-framework titanium was reduced significantly from 14.4% to 0.3% and the meso-/macropore volume was increased from 0.014 to 0.168 cm³ g^-1 accordingly upon TPAOH post treatment. Along with the parent TS-1 zeolite, the developed hierarchical TS-1 zeolites were assessed in the catalytic epoxidation of cyclohexene. It was confirmed that the amount of framework titanium and hierarchical pore structure influenced the catalytic activity considerably. Closed porosity slightly improved the cyclohexene conversion, whereas the open-pore sample shows the optimum catalytic activity in cyclohexene conversion.

Full text of DOI:10.1039/c7cy02571h

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Inter-connected and open pore hierarchical TS-1 with controlled
framework titanium for catalytic cyclohexene epoxidation
Yilai Jiaoa, b, †, Abdul-Lateef Adedigbaa, †, Qian Hea, Peter Miedziaka, Gemma Bretta, Nicholas F.
Dummera, Micheal Perdjona, Jinmin Liu b, Graham J Hutchingsa,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A post-synthesis method was developed to reduce the extra-framework titanium (Ti) in TS-1 zeolites (Si/Ti ratio = 50), in
which tetrapropylamonium hydroxide (TPAOH) aqueous solution was used to promote the dissolution, redistribution and
recrystallization processes, and hence to convert amorphous Ti species into zeolitic phases. It was found that TPAOH could
effectively convert the extra-framework Ti into framework Ti, and the TPAOH concentration influenced the pore structure
significantly. Under lower TPAOH concentration (i.e. 0.05M - 0.4M), only closed meso-/macropore (grooves and hollow
cavities) can be created in the TS-1 crystals. At an optimum concentration of 0.5 M TPAOH, open and connected
hierarchical mesopore and macropore were created in the resulting TS-1 zeolites. Compared with the parent TS-1, the
amount of extra-framework titanium was reduced significantly from 14.4 % to 0.3% and the meso-/macropore volume was
increased from 0.014 to 0.168 cm3/g accordingly upon TPAOH post treatment. Along with the parent TS-1 zeolite, the
developed hierarchical TS-1 zeolites were assessed in the catalytic epoxidation of cyclohexene. It was confirmed that the
amount of framework titanium and hierarchical pore structure influenced the catalytic activity considerably. Closed
porosity slightly improved the cyclohexene conversion, whereas the open-pore sample shows the optimum catalytic
www.rsc.org/
activity
in
cyclohexene
conversion.
the epoxide9-11
.
Introduction
The traditional synthesis method of TS-1 inevitably produces
extra-framework titanium, particularly when synthesizing TS-1
with low silicon to titanium ratios12-14. In addition, due to the
microporous structure of this MFI zeolite, which has large
diffusion resistance in the crystal, only active sites on the
crystal surface and at the pore mouth can participate in many
reactions, which results in a lower utilization of the catalyst
Titanium silicalite-1 (TS-1), where titanium isomorphously
replaces silicon in a tetrahedral site of the MFI silicalite lattice1,
has attracted much attention due to its excellent catalytic
oxidation properties. In many oxidation processes such as
ammoximation of cyclohexanone2, 3, hydroxylation of phenol4,
and epoxidation of olefins5-7
, and under mild reaction
active sites15-19
.
Therefore, to increase the catalytic
conditions with hydrogen peroxide as the oxidant, TS-1 is an
effective catalyst.
performance of TS-1 catalyst, two key technical problems
should be addressed. Firstly, TS-1 should be prepared with
more active sites and less extra-framework titanium, and
secondly diffusion and steric hindrances in the crystal should
be minimized.
In TS-1 crystals there exist two types of titanium species, the
framework and the extra-framework titanium. Typically, the
framework titanium is the active site for epoxide production
during epoxidation reactions, whereas the extra-framework
titanium species are responsible for secondary and often
unwanted reactions8. The extra-framework titanium species
include anatase and rutile-type titanium dioxide (TiO2) and
amorphous titanium species, which will decompose H2O2 and
lead to side reactions, such as hydrolysis and ring opening of
At present, the commonly used method for the modification of
TS-1 is the post-synthesis treatment with sodium hydroxide to
create mesopores, which act to improve diffusion and to
reduce steric hindrance. Although sodium hydroxide can
create mesopores in MFI zeolites it also decreases their
relative crystallinity, and in addition increases the amount of
extra-framework titanium in the TS-120-22. Recently, organic
templates such as tetrapropyl ammonium hydroxide
(TPAOH)23-26 and a mixture of TPAOH and NaOH27 have been
used to treat TS-1 after synthesis to obtain hollow TS-1. The
TPAOH acts as an alkaline and dissolves the zeolite crystals to
create mesopores in the TS-1 crystals. Also, the TPAOH can act
as a template to recrystallize the dissolved zeolitic species to
form new crystals. Due to the above dissolution-
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recrystallization process, the relative zeolite crystallinity is crystallization at 160°C for 48 hours. The product was
preserved better than what is obtainable with sodium recovered by centrifugation and washed with deionized water
hydroxide treatment. However, the above methods inevitably – the washing was accomplished by dispersing the solid
break the Si-O-Ti bonds in the crystal and convert part of the products in 100 mL of water followed by separation 3 times –
framework titanium to extra-framework titanium10,
23, 25
.
dried at 110°C and followed by calcination in static air at 550
Furthermore, the above methods produce a closed-pore meso- °C with a ramp rate of 1 °C/min for 6 hours.
/macropore in the zeolites, with a dense shell around the TPAOH Post-treatment
crystal. In the use of such hollow TS-1 in liquid phase reactions
The TPAOH post-treatment was carried out as follows, 2 g of
involving bulky molecules, it is always difficult for the bulky
molecule to easily diffuse through the crystal, which limits
their reactivity to external and pore-mouth active sites.
Therefore, optimization of the TPAOH post-treatment
conditions is highly desirable to simultaneously create an
open-pore (rather than just hollow cavities) and decrease
extra-framework titanium in TS-1 zeolites while preserving
their crystallinity.
the as-prepared TS-1 (AP) was mixed with a 20 mL solution of
TPAOH (0.05–0.5 mol/L) at 400 rpm for 20 minutes to form a
slurry. The slurry was transferred to a Teflon-lined stainless
steel autoclave, and treated under autogenous pressure at 160
°C for 24 hours. Finally, the mixture was filtered, washed with
deionized water, dried at 120 °C for 12 hours, and calcined at
550 °C for 6 hours. The samples obtained were denoted as TT-
x, in which x represented the molar concentration of TPAOH.
Hence, the aim of this work is to produce a hierarchical TS-1
zeolite with connected and open mesopores with minimal
extra-framework titanium species. The procedure includes the
dissolution, recrystallization and redistribution/reinsertion of
any extra framework titanium species in the pristine sample to
the framework positions by the treatment with TPAOH at
elevated temperatures. To achieve the optimal balance
between the amount and structure of the induced porosity,
the reinsertion of the extra-framework species and the
crystallinity of the final material, it is imperative to accurately
obtain a balance between the dissolution and recrystallization
steps. In this work the determination of the optimal TPAOH
concentration to obtain an open-pore hierarchical material,
and to minimize or eliminate any extra-framework titanium in
the samples is reported. Cyclohexene epoxidation was used to
evaluate the catalytic activities of the modified TS-1 materials
and the results are related to the degree of reinserted titanium
Catalytic performance testing
The epoxidation of cyclohexene used as the test reaction was
carried out in a colaver glass reactor. Typically, 100 mg of
catalyst, 6 mmol of reactant, 6 mmol of hydrogen peroxide
(H2O2, 30%), and 10 mL of solvent (acetonitrile) were added
and stirred (800 rpm) at 333 K for 4 hours, after which the
reactor was quenched in an ice bath for 20 minutes. The
catalyst was separated from the product by filtering through a
0.45µm PTFE syringe filter. The products of the reactions were
analyzed using a gas chromatograph (Agilent 7890B) equipped
with a flame ionization detector (FID) and Agilent CP7658I CP-
Wax 52 CB column with dimensions of 25 m*0.53 mm*2µm.
The carbon balance is above 95%.
Characterization
and the nature of the induced porosity. All the samples were Powder X-ray diffraction (XRD) was recorded using
a
characterized to determine the crystallinity, the PANalytical X'Pert Pro system fitted with a CuKα1 X-ray source
extra/framework titanium content, mesopore size and operated at 40 kV and 40 mA. Each sample was scanned from
distribution, and the crystal size.
2θ = 5° to 35° for 30 minutes. A JEOL 7401 high-resolution field
emission scanning electron microscope with an energy
dispersive X-ray spectrometer (EDX) was used to image and
analyze the silicon to titanium ratio of the catalysts. A JEOL
1010 and FeiTecnaiF20 Transmission Electron microscope was
used to carry out HR-STEM. The samples were prepared for
TEM by placing a droplet of the powder dispersed in ethanol
(sonicated for 5 min in an ultrasonic bath) on a carbon-coated
mesh grid. The specific surface area and pore structure of the
as-prepared and modified samples were determined by
nitrogen (N2) adsorption–desorption measurements at the
liquid nitrogen temperature (77 K) using a Micromeritics 3flex
Surface Area and pore size analyzer. The micropore size
distribution was calculated by the Horvath-Kawazoe (HK)
method, and mesopore and macropore distribution was
determined from the adsorption branch of the isotherms by
the Barrett-Joyner-Halenda (BJH) method. The meso-
/macropore sizes of the catalysts were also analyzed using
Experimental methods
Synthesis of TS-1
Tetraethyl orthosilicate (TEOS), titanium (IV) ethoxide (TEOT),
and tetrapropyl ammonium hydroxide (TPAOH, 25 wt. % in
water) – used without further purification – acted as the silica,
titanium, and the organic structure directing agents,
respectively. The molar composition of the synthesis solution
was SiO2: 0.02TiO2: 0.3TPAOH: 19H2O. The typical preparation
procedures were as follows: Firstly, titanium (IV) ethoxide was
added to TEOS under vigorous stirring, this solution was
continually stirred for 1 hour. Then, a solution composed of
the desired amount of TPAOH and deionized water was added
drop-wise into the above solution under vigorous stirring. The
mixture was stirred for 40 minutes at room temperature, then
heated to 313 K under continuous stirring to remove alcohol
and to obtain the synthesis gel. The synthesis gel was
transferred into a Teflon-lined stainless steel autoclave for
mercury (Hg) intrusion porosimetry on
a Micromeritics'
AutoPore IV 9510 (pressure range: 0.10 to 60000 psia). The
UV–Vis and infrared (FTIR) spectra were recorded on a
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Shimadzu UV-2550 340 spectrophotometer and a Nicolet isotherm, which is characteristic of a hierarchical porous
NEXUS 670 FT-IR spectrometer respectively.
system with a combination of both micro- and mesoporosity.
The details of the textural properties of the sample treated
with different TPAOH concentration are summarized in Table
1. With increasing TPAOH concentration, the mesopore
surface area and mesopore volume can be seen to increase.
The highest micropore surface area (442 m2g-1) and total pore
volume (0.338 cm3g-1) were obtained when the sample was
treated with 0.5M TPAOH. In addition, the as-prepared TS-1
and TPAOH modified samples have similar micropore
distribution, which confirms the process of dissolution-
recrystallization (Figure S1a, b). The BJH pore size distribution
of the TPAOH treated samples shows random mesopores and
macropores in the range 10 nm to 160 nm (Figure 2b and
Figure S1c), and the pore size increase with the concentration
of TPAOH.
Results and discussion
Crystallinity
The XRD patterns of the as-prepared and TPAOH treated
samples are shown in Fig. 1. The five characteristic peaks of
the MFI topology at 7.8
identified in the as-prepared and TPAOH treated samples, and
the peak at 24.4 , which is a singlet also indicates the crystal is
°, 8.8°, 23.0°, 23.9° and 24.4° were
°
typical of TS-1. The relative crystallinity, calculated by
comparing the total intensity of the peaks from 22.5° to 24.6°
with that of each sample is given in Table 1. Following the
post-synthesis treatment with TPAOH, the relative crystallinity
increased with increasing TPAOH concentration. The highest
relative crystallinity was obtained for the sample treated with
0.3M TPAOH, TT-0.3M. Further increasing the TPAOH
concentration results in
crystallinity.
a slightly decreased relative
Figure 1. XRD pattern of as-prepared and samples treated with different TPAOH
concentrations
The treatment of the zeolite with a base can cause the
dissolution of zeolite, and in the presence of TPAOH and heat
treatment recrystallisation can be induced. The dissolution and
recrystallisation process can lead to the re-insertion of some
amorphous silica and extra-framework titanium species into
the zeolite framework, so that the relative crystallinity is
enhanced. Although, increasing the TPAOH concentration will
speed up the above process, at higher TPAOH concentrations
the silica and titanium concentration in the solution will be
significantly high, and result in a slight decrease in the relative
crystallinity.
Figure 2. N2 adsorption–desorption isotherms (a) and BJH pore size distribution (b) of
as-prepared and TPAOH treated samples
In liquid phase reactions, the diffusivity of species is generally
five orders of magnitude slower than that in the gas phase28
,
Textural properties
hence, open porosity (mesopore and macropore) is
advantageous as it will facilitate molecular diffusion and
The type I isotherms of the as-prepared sample measured by
N2 adsorption (Fig. 2a) confirm their microporous character.
The as-prepared samples treated in TPAOH solution under
hydrothermal conditions show a combined type I and type IV
increase the accessibility to the active sites that are located in
29,
the microporous channels from the outside
30. As N2
sorption only provides the total porosity – including both open
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and close porosity – a complementary method is required to measurements were performed. The Hg intrusion technique
discriminate between the open and closed mesopore and was selected to distinguish between the open and close pores
macropores.
To complement the porosity of samples AP, TT-0.4M and TT-
0.5M obtained by N2 sorption, mercury porosimetric
properties
Table
1.
Summary
of
the
of
as-prepared
(AP)
and
TPAOH
modified
(TT-XM)
b
TS-1
samples
Relative
crystallinity
[%]
a
a
a
SBET
[m2g-1]
Smicro
[m2g-1]
Sexternal
[m2g-1]
Vmicro
[cm3g-1]
Vt-m
Vt
[cm3mg-1]
[cm3g-1]
ro
Sample
Si/Ti
AP
91.0
97.0
46
42
495
495
462
414
33
81
0.168
0.174
0.014
0.088
0.182
0.262
TT-0.05M
TT-0.1M
97.9
44
500
416
84
0.173
0.089
0.262
TT-0.3M
TT-0.4M
TT-0.5M
100
99.6
99.1
42
42
44
497
506
549
413
423
442
84
83
0.172
0.175
0.170
0.104
0.123
0.168
0.276
0.298
0.338
107
at-plot
method;
bCalculations
were
based
on
total
pore
volume
minus
micropore
volume
the internal meso/macropores are open through to the
exterior from where the Hg intrudes. The Hg intrusion and
pore size distribution curves are shown in Fig. 3. From figure
3a, an obvious increase from 10 nm to 50 nm can be seen in
sample TS-0.5M, confirming that the mesopore and macropore
in TT-0.5M are open to the external zeolite surface. Also, an
obvious peak in this pore range can be seen in figure 3b for
sample TT-0.5M, whereas sample TT-0.4M only displays
random peaks in the same pore region, which implies that the
Hg cannot intrude into the crystals of the sample TT-0.4M
thereby resulting in the collapse.
Morphology
The SEM images of the as-prepared and TPAOH modified
samples are shown in Figure 4. There are no obvious
differences between the AP sample and the TPAOH modified
samples. All the samples have almost uniform particle size of
approximately 500 nm. The silicon to titanium ratio of the
samples before and after TPAOH treatment evaluated by EDS
is given in Figure S2. From the EDS analysis, after TPAOH
treatment, the silicon to titanium ratio decreased slightly from
46 to 42 (Table S1), which further confirms the dissolution-
recrystallisation process with TPAOH treatment.
The nature and structure of the mesopore in the treated
samples was further evaluated by the TEM, and is shown in
Figure 5. The as-prepared microporous TS-1 crystal displayed a
dark image (Fig. 5a), and as expected, some large voids can be
seen in the inner part of the zeolite crystals treated with 0.1 M
of TPAOH (Fig. 5b). Compared to the samples treated with
0.1M TPAOH, a higher quantity of voids can be seen in the
sample that was treated with 0.4M TPAOH solution (Fig. 5c).
However, from the higher magnification image (Figure 5d), we
can see that the voids in the sample was covered by a dense
shell. On the contrary, the pore structure in the sample that
was treated with 0.5 M TPAOH solution was open to the
surface (Figure 5e, f). Furthermore, the mesopore and
macropore in TT-0.5M are interconnected and exposed to the
Figure 3. Mercury porosimetric measurements for sample AP, TT-0.4M and TT-0.5M (a)
cumulative intrusion, (b) pore size distribution
because Hg cannot significantly penetrate pores smaller than 3
nm31. Hence, for Hg to successfully intrude the inner pores
from the exterior, the external pore through which it accesses
the interior must be large enough (> 3nm) and it will imply that
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crystals external surface (Figure S3b). The above results are treatment conditions on the titanium distribution, UV-Vis
complimentary to the results of the mercury intrusion, and analysis was used (Fig. 7). The samples show a transition at
confirm the open hierarchical porosity in TT-0.5M. The approximately 210 nm characteristic of framework titanium
selected area electron diffraction (SAED) patterns of 0.1M, species, which correlates with the results in Fig. 6. In addition
0.4M and 0.5M TPAOH treated samples (Figure S4b, c and d) to the 210 nm transition, all the samples also exhibit an
are similar to that of the as-prepared sample (Fig. S4a), which additional transition at around 300 nm, which is characteristic
also support the conclusion that the TPAOH treatment do not of extra-framework titanium species8. Importantly, the
destroy the crystallinity of the as-prepared sample contrary to intensity and shape of the peak at about 300 nm for the
those of desilication by NaOH22.
different treated samples differs. The as-prepared sample
To determine the onset of the open pore structure and to displayed the highest and broadest peak at 300 nm in the
assess if the open pore can be sustained beyond 0.5M, the as- series, and with increasing TPAOH concentrations, the
prepared sample was treated with 0.45M and 0.6M solutions transition at 300 nm reduces, which suggests that the extra-
of TPAOH. Figure S3a and S3e show the TEM images of these framework titanium content in the samples is reduced, which
two samples and confirm that concentrations of TPAOH higher demonstrates
the
dissolution,
recrystallization
and
than 0.4M were required to induce the open-pore structure. redistribution of the titanium species. The titanium amount in
The TT-0.6M maintains an open pore structure and the parent the as-prepared sample and TPAOH modified samples was
morphology beyond 0.5M TPAOH concentrations.
calculated from the peak area of the UV-Vis curve. The as-
prepared sample comprised of approximately 85.6%
framework titanium, whereas upon treatment with TPAOH the
framework titanium content increased gradually to 92.1%
(0.05M), 98.5% (0.1M), 99.5% (0.3M), 97.8% (0.4M) and 99.7%
(0.5M), another evidence to support the dissolution,
recrystallization and redistribution of the titanium species.
Figure 4. SEM images of as prepared and TPAOH treated samples (a) as prepared TS-1,
(b) TT-0.05M, (c) TT-1-0.1M, (d) TT-0.3M, (e) TT-0.4M and (f) TT-0.5M
The nature of the Titanium composition
The FTIR and the diffuse reflectance UV-Vis were used to
evaluate the nature of the titanium species on both the
original and modified zeolite samples. As shown in Fig. 6, all
the samples including those treated with TPAOH retain the
band at 960cm-1, which is attributable to titanium in the
framework position. The presence of titanium in the
framework position can be inferred from the FTIR analysis of
TS-1 samples as represented in Fig. 6, however, the existence
of extra-framework species cannot be detected with the FTIR.
To identify extra-framework titanium in the as-prepared and
TPAOH treated samples, and to monitor the role of the post-
Figure 5. TEM images of as prepared and TPAOH treated samples (a) as prepared TS-1,
(b) TT-0.1M, (c, d) TT-0.4M and (e, f) TT-0.5M
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Cyclohexene epoxidation performance
The influence of the titanium distribution and the induced
hierarchical porosity of the treated samples were evaluated
with cyclohexene epoxidation. Table 2 shows the results of the
catalytic activity and the product distribution over each
sample. As expected the as-prepared sample exhibited the
least activity compared to the TPAOH treated samples, and the
sample treated with 0.5M TPAOH showed the highest activity,
while the other TPAOH treated samples display similar activity,
which is higher than the pristine sample. The activity of the
sample treated with 0.5M TPAOH doubles (19%) that of the
other treated samples (10%). The marked difference in the
activity between TT-0.5M and other treated samples can be
correlated with the structure of the induced meso-
/macropores in the samples. At concentrations of TPAOH
lower than 0.5M, only closed mesopores and macropores or
hollow cavities were present, which means the active sites are
only those on the catalyst surface and at a few distances below
the pore mouth. However, and as observed in Fig. 3 and 5, TT-
0.5M have open and inter-connected meso-/macropores
offering access to more active sites both on the surface and
within the inner pores. In addition, the steric and diffusion
limitation on TT-0.5M would be lower than the others due to
the inter-connectivity and the open nature of its pores.
Figure 6. FTIR spectra of as-prepared and TPAOH modified samples. The arrow indicate
the 960 cm-1 position that indicates framework titanium species
These results differ from those of previous studies where the
relative crystallinity decreased and the amount of framework
titanium increased after TPAOH treatment27, 32, 33. The reason
for such disparity we reckon is due to the low TPAOH
concentration used in those studies, typically 0.005M to 0.1M.
Similarly, such low TPAOH concentration were effective in
creating groves and hollow cavities in the TS-1 but were not
able to initiate open-pore meso-/macropore and that could be
the reason why some reseachers33 have noticed no influence
of the created mesopore on activity. It shows that the post
treatment condition, namely TPAOH concentration must be
carefully selected to obtain open-pore hierarchical TS-1.
The product distribution of the as-prepared and modified
samples is also given in table 2. Generally, there are two
pathways to convert cyclohexene according to the proposed
scheme by Kwon et al.26 – given in scheme 1. Firstly,
cyclohexene (
can be hydrolysed to 1,2-cyclohexanediol (
conditions26. The other pathway is the allylic oxidation of
cyclohexene to cyclohexen-1-one ( ) and 2-cyclohexen-1-ol
). Compared with the as-prepared sample, the TPAOH
modified samples have comparable selectivity to
1) is epoxidized to cyclohexene oxide (
2), which
4) under acidic
3
(
5
a
cyclohexene oxide, while the TPAOH modified samples have
higher selectivity of 1,2-cyclohexanediol and lower selectivity
of cyclohexen-1-one and 2-cyclohexen-1-ol. This implies that
more cyclohexene is oxidized to cyclohexene oxide and 1,2-
cyclohexanediol via Pathway 1 over the TPAOH modified
samples when compared to over the as-prepared sample.
These results clearly demonstrate the positive role of
framework titanium in promoting Pathway 1 as the primary
reaction route.
Figure 7. UV-Vis analysis of as prepared and TPAOH modified samples. The spectra are
normalised to the same scale to ensure accurate and representative direct comparison
Table 2. Cyclohexene conversion and product selectivity over the as-prepared and TPAOH treated samples
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Selectivity [%]
Cyclohexene
Sample
conversion [%]
Cyclohexene
Cyclohexen-1-one and 2-cyclohexe-
1,2-cyclohexanediol
10.2
epoxide
1-ol
AS
6.5
82.3
7.5
TT-0.05M
TT-0.1M
10.2
9.6
78.3
81
16.4
14.7
5.3
4.3
TT-0.3M
10
77
18.2
4.8
TT-0.4M
TT-0.5M
11
19
82
78
15
17
3
5
Reaction conditions; Cyclohexene:
6
mmol, 30 wt% H2O2:
6
mmol, Catalyst: 100 mg, temperature: 60 C, reaction time:
4
h
to the external surface, which promoted a high cyclohexene
conversion. The TPAOH treated samples all show selectivity to
epoxide similar to that of the as-prepared samples, which
demonstrate the efficiency of this technique in mitigating
against the extra-framework titanium species.
Conflicts of interest
There are no conflicts to declare
Acknowledgements
YJ thanks the China Scholarship Council (CSC) for his fellowship
in the UK (201604910181). YL also thanks Dr Greg Shaw for
UV-Vis training, Dr Thomas Davies for TEM training.
Scheme 1. Schematic of cyclohexene (1) oxidation with hydrogen peroxide26
Conclusions
References
In conclusion, open-pore hierarchical TS-1 type zeolites with
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