Intermediate-crystallization promoted catalytic activity of titanosilicate zeolites

Article abstract of DOI:10.1039/c8ta01960f

The exploration of highly efficient catalysts based on nano-sized hierarchical titanosilicate zeolites with controllable active titanium species is of great importance in promoting the oxidative desulfurization of bulky organosulfur compounds. Here we report the anatase-free nano-sized hierarchical TS-1 (MFI) zeolite prepared via a seed-assisted hydrothermal method. Interestingly, it is found that the intermediately crystallized zeolite phase exhibits better performance in oxidative desulfurization of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT), with a turnover number up to 17.9 and 18.0 at 30 min of the reaction, respectively, almost two fold higher than those of the final completely crystallized zeolite phase (8.8 and 6.7, respectively). This superior activity is attributed to the existence of abundant inter- and intracrystalline mesopores, and the surface enriched active titanium species, as well as the absence of anatase species within the nano-sized intermediate phase, which is beneficial for exposing more accessible active sites, enhancing the mass transfer efficiency, as well as avoiding the inefficient decomposition of oxidants. This intermediate crystallization method provides a useful strategy in exploring highly efficient zeolite catalysts.

Full text of DOI:10.1039/c8ta01960f

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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
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Intermediate-Crystallization Promoted Catalytic Activity of
Titanosilicate Zeolites
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
c
a
Risheng Bai, Qiming Sun, Yue Song, Ning Wang, Tianjun Zhang, Fei Wang, Yongcun Zou,
b
b
a, d
Zhaochi Feng, Shu Miao and Jihong Yu*
DOI: 10.1039/x0xx00000x
The exploration of highly efficient catalysts based on nano-sized hierarchical titanosilicate zeolites with controllable active
titanium species is of great importance in promoting the oxidative desulfurization of bulky organosulfur compounds. Here
we report the anatase-free nano-sized hierarchical TS-1 (MFI) zeolite prepared via the seed-assisted hydrothermal
method. Interestingly, it is found that the intermediately crystallized zeolite phase exhibits better performance in oxidative
desulfurization of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT), with turnover number up to
www.rsc.org/
1
7.9 and 18.0 at 30 min of the reaction, respectively, almost two folds higher than the final completely crystallized zeolite
phase (8.8 and 6.7, respectively). This superior activity is attributed to the existence of abundant inter- and in-
tracrystalline mesopores, the surface enriched active titanium species, as well as the absence of anatase species within
nano-sized intermediate phase, which is beneficial for exposing more accessible active sites, enhancing the mass transfer
efficiency, as well as avoiding the inefficient decomposition of oxidants. This intermediate crystallization method provides
a useful strategy in exploring high-efficient zeolite catalysts.
7
-10
sustainable chemistry.
Titanosilicates, such as TS-1 (MFI)
Introduction
zeolite, have been applied in many important catalytic
reactions, including selective oxidation of alkenes and alkanes,
ammoximation of ketones, hydroxylation of phenol, and
Increasingly stringent regulations have been imposed
worldwide to limit the content of sulfur compounds in liquid
fuels to less than 10 ppm for the growing environmental
1
1-15
especially the deep oxidative desulfurization.
The ODS
1
, 2
3
process over titanosilicate zeolite catalysts can be performed
at a relative low temperature (60-90 °C) and atmosphere
pressure. However, the conventional microporous TS-1 zeolite
catalysts are less efficient in removing heterocyclic aromatic
sulfur compounds such as dibenzothiophene (DBT) and 4,6-
dimethyldibenzothiophene (4,6-DMDBT), because of their
steric hindrance arising from alky-groups that makes them
inaccessible to the active sites located inside the zeolite.
Recently, zeolites with extra-large pores, nanoscaled sizes,
layered and hierarchical structures have been developed to
overcome the limitation of the narrow channels of the
traditional microporous zeolites; the highly accelerated mass
transport, and sufficiently accessible active sites can
remarkably improve the catalytic activity of reactions involving
problems. Oxidative desulfurization (ODS) has proven to be
an effective method in removing thiophenic compounds for
the low investment and operating costs, mild operating
4
conditions, as well as high desulfurization rate. Liquid acid,
5
6
oxides, and polyoxometalates have been applied in the ODS
process, however, these catalysts suffer from low oxidant
utilization rate, poor stability or reusability, as well as the
corrosion to equipment during catalyst preparation process or
the catalytic reactions. Therefore, developing ODS catalysts
with high reaction activity and excellent stability is of great
importance.
Zeolites are microporous crystalline materials with well-
defined pores and channels of molecular dimensions (< 2nm),
which are widely used in ion-exchange, gas adsorption-
separation, and catalysis, and play an important role in
1
6-23
bulky molecules.
Up to now, various of hierarchical TS-1
zeolites have been synthesized by the utilization of strategies
including: (i) templating strategies, such as hard templating
2
4
and soft templating; (ii) post-treatment strategies, such as
steaming or acid treatment, desilicated, and chemical
2
5-27
treatment.
These prepared hierarchical titanosilicate
zeolites show enhanced performance in the oxidative
desulfurization reactions for bulky organosulfur compounds
compared to the conventional microporous zeolites. However,
the multi-step synthesis procedures, expensive cost of the
additional mesoporogens, as well as the reduced crystallinity
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of zeolites remarkably limit the large-scale industrial 1(H24), respectively. Typically, the initial gel was prepared by
applications of hierarchical TS-1 zeolites. Particularly, during mixing the TPAOH with distilled water, followed by the
2
the zeolite crystallization process, some anatase TiO species addition of TEOS and tetrabutyl titanate (TBOT), and then
are always formed inevitably, which cause the ineffective stirred for 2 h continuously. Finally, the seeds were added into
decomposition of oxidants and also cover active titanium the solution and stirred for another 2 h continuously. The
2
8
sites. Therefore, development of a simple and cost-effective resulting mixture was charged into autoclaves and crystallized
synthetic strategy for highly efficient anatase-free hierarchical in an oven at 170 °C for 1-24 h. The as-prepared zeolite
titanosilicate zeolite catalysts is full of challenge.
products were washed thoroughly with water and ethanol,
On the other hand, the titanium coordination states have and then dried at 80 °C in the oven overnight, followed by
influence on the catalytic activity of titanosilicate zeolites. It calcination in air atmosphere at 550 °C for 6 h.
was reported that besides the tetra-coordinated titanium sites, Synthesis of conventional nano-sized TS-1. Conventional
the hexa-coordinated titanium species in titanosilicate zeolite nano-sized TS-1 zeolites were synthesized with the same molar
catalysts are also responsible for the enhanced performance in composition of the TS-1(Hx) samples without adding zeolite
2
9-31
alkene epoxidations.
Therefore it is important to construct seeds. The TS-1(con) sample was crystallized for 24 h, and
active titanium species by tuning the titanium coordination some other samples named as TS-1(Cx) were crystallized for 2-
states to improve the catalytic activity of titanosilicate zeolites. 12 h, where x represented the crystallization time in hour.
Note that the completely crystallized zeolite catalysts are Catalytic tests. A certain amount of dibenzothiophene
usually pursued for their good crystallinity, high surface area, (DBT)/4,6-dimethyldibenzothiophene
(4,6-DMDBT)
was
and excellent hydrothermal stability. However, in the present dissolved in n-octane to act as model fuel, and the
work, we find that the intermediately crystallized nano-sized concentration of sulfur in the model fuel was about 500 ppm.
hierarchical TS-1 zeolite prepared via the seed-assisted The reaction was performed in a 50 mL three neck glass flask
hydrothermal method exhibits better performance in oxidative with an oil bath under vigorous stirring (600 rap/min). In a
desulfurization of DBT and 4,6-DMDBT than the completely standard run, 10 mL of model fuel and proper amount of
crystallized zeolite phase. The excellent catalytic activity is catalyst were added to the reactor, with the molar ratio of
attributed to the easily accessible active sites and the reactant to titanium in zeolite catalysts was fixed at 10. The
enhanced mass transfer benefitted from abundant inter- and tert-butylhydroperoxide (TBHP) was used as oxidant, and the
intracrystalline mesopores in nano-sized crystals, as well as the molar ratio of TBHP to the sulphide was 2. The reaction was
surface enriched active titanium species and the absence of carried at 60 °C for 30 min. Then, filtering the reacted solution
anatase species in the intermediately crystallized catalysts. and analyzing the TBHP conversion by titration with Ce(SO4)2
This discovery provides a useful guidance for the preparation and Ferroin as the indicator. The products were analyzed by
of high-efficient zeolite catalysts utilized in catalytic reactions.
Gas chromatography-mass spectrometry (GC-MS, Thermo
Fisher Trace ISQ, equipped with TG-5MS column, 60 m × 320
µm × 25 µm). Mass balances were accurate to within 5%.
Experimental section
Synthesis of nano-sized silicalite-1 zeolite seeds. The nano-
Results and discussion
sized silicalite-1 zeolite seeds were synthesized with
tetrapropylammonium hydroxide (TPAOH) as the organic The nano-sized hierarchical zeolites (denoted as TS-1(H1), TS-
structure directing agent with the molar composition of 1.0 1(H3) and TS-1(H24), and the 1, 3, and 24 represent for
SiO
template TPAOH was mixed with distilled water and stirred for silicalite-1 zeolite seeds into the synthetic gel with the molar
5 min, followed by the addition of tetraethylorthosilicate composition of 1.0 SiO : 0.025 TiO : 0.4 TPAOH: 50 H O: 10
TEOS). After a continuous stirring for 2 h, TEOS was fully wt% Seed under hydrothermal conditions. The product yields
2 2
: 0.4 TPAOH: 30 H O. As a typical procedure: the organic crystallization time in hour) were synthesized by introducing
1
2
2
2
(
hydrolyzed. Then the reaction mixture was transferred into a of the TS-1(H1), TS-1(H3), and TS-1(H24) are 8.4%, 75.9%, and
Teflon-lined stainless steel autoclave and then transferred into 96.3%, respectively. Scanning electron microscope (SEM) and
a pre-heated oven at 170 °C for 4 days under static conditions. transmission electron microscopy (TEM) images show that the
The as-synthesized zeolites were washed thoroughly with
water and ethanol, and then dried at 80 °C in the oven
overnight, followed by calcination in air atmosphere at 550 °C
for 6 h.
Synthesis of nano-sized hierarchical TS-1 zeolites. The nano-
sized hierarchical TS-1 zeolites were synthesized with the
molar composition of 1.0 SiO
0 wt% Seeds, under hydrothermal conditions at 170 °C with
the crystallization time of 1-24 h, and the resulting products
were named as TS-1(Hx), where represented the
crystallization time in hour. The zeolite samples crystallized for
2 2 2
: 0.025 TiO : 0.4 TPAOH: 50 H O:
1
2
Figure 1. (a) XRD patterns and (b) N adsorption and desorption isotherms of the
x
conventional nano-sized TS-1(con) and the nano-sized hierarchical TS-1(H1), TS-1(H3)
and TS-1(H24) zeolite catalysts.
1h, 3h, and 24h are denoted as TS-1(H1), TS-1(H3) and TS-
2
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spectroscopy (XPS) are 119.2, 64.9. 75.8 for the three TS-1(Hx)
samples, which are higher than the corresponding ICP results,
indicating that the titanium species in the intermediately
2
9
crystallized zeolites are much more surface enriched. Si MAS
NMR spectra of these TS-1(Hx) samples all give resonance
bands mainly centered at -114 ppm (Figure S6), attributed to
32
the tetrahedral framework silicon.
UV-Visible spectroscopy is utilized to investigate the
coordination states of titanium in these samples. As shown in
Figure 3a, the adsorption bands of TS-1(H1) and TS-1(H3)
mainly appear at around 220 and 270 nm, which are attributed
3
3
to the tetra-coordinated titanium and hexa-coordinated
titanium species, respectively. Note that the characteristic
peak at about 330 nm for anatase species cannot be observed
in these two samples. However, a strong absorption peak at
about 330 nm can be obviously detected in TS-1(H24) sample,
indicating the existence of anatase species. The conventional
TS-1(con) zeolite shows the absence of hexa-coordinated
titanium (270 nm) and the existence of the anatase species
(
330 nm). It is clear that by modulating the crystallization time,
Figure 2. TEM images and selected area electron diffraction patterns of nano-sized
the formation of anatase species in the TS-1 samples can be
avoided. The probable reason is that the extra-framework
titanium can be stabilized as hexa-coordinated titanium
species in the intermediately crystallized zeolite phase, while
with the crystallization time prolonging, the surface titanium
hierarchical TS-1(H1) (a-c), TS-1(H3) (d-f), and TS-1(H24) (g-i) zeolite catalysts. Scale bar,
2
0 nm.
silicalite-1 seeds have a hexagonal morphology with the size of
around 250 nm (Figure S1). For comparison, conventional
nano-sized TS-1 zeolites were also synthesized with the same
molar composition of the TS-1(Hx) samples without adding
zeolite seeds (Figure S2).
3
4
species may form the anatase species upon calcination.
Anatase TiO is sensitive to the ultraviolet Raman resonance
2
spectroscopy (UV-Raman) with the excitation wavelength of
3
20 nm. The bands at 144, 390, 516, and 637 cm are
5
−1
3
Powder X-ray diffraction (XRD) patterns of the prepared TS-
3
attributed to the anatase TiO . As shown in Figure 3b, a
6
1
zeolites show the typical diffraction peaks of the MFI
2
−1
strong band at 144 cm appears in the spectra of both the
conventional nano-sized TS-1 and the TS-1(H24) samples,
indicating the existence of anatase TiO2, but these
structure, indicating the phase purity of the samples (Figure 1a
and Table S2). SEM images reveal that all of these nano-sized
hierarchical TS-1 zeolite samples crystallized for 1 h and 3 h are
single particles with a regular ellipsoid morphology (100-250
nm), and the sample crystallized for 24 h (TS-1(H24)) as well as
the conventional nano-sized TS-1 zeolite (TS-1(con)) exhibit a
regular hexagonal morphology (150-300nm) (Figure S3).
Figure 2 and Figure S4 show the TEM images of the
samples.TS-1(H1) consists of aggregated small crystals with
dimensions of 5-10 nm. TS-1(H3) displays abundant
intracrystalline mesopores, as well as the intercrystalline
mesopores formed by the stacking of the TS-1 nanocrystals.
The completely crystallized TS-1(H24) sample also shows
abundant uniform intracrystalline mesopores. The selected
area electron diffraction (SAED) patterns of TS-1(Hx) all show
discrete spots indexed to the (002) and (020) planes of the MFI
phase, indicating the single-crystalline nature of these
prepared samples. The STEM-energy-dispersive X-ray
spectrometry (EDX) analysis for the TS-1(H3) sample shows
that the titanium atoms are distributed uniformly in the zeolite
crystals (Figure S5).
characteristic peak of anatase TiO is absent in both TS-1(H1)
2
and TS-1(H3) samples. These results are in accordance with the
UV-Vis spectroscopic analyses.
The Fourier-transform infrared spectroscopy (FT-IR)
measurements are performed to investigate the state of
titanium in the TS-1 samples. All the spectra show the
-1
adsorption peaks at 960 cm assigned to the stretching
vibration of the Si–O bond with titanium atoms at coordination
3
7
sites (Figure S7).
Nitrogen adsorption-desorption isotherms of nano-sized
hierarchical TS-1(H1) sample shows a typical type-I isotherm as
with conventional nano-sized TS-1, while the isotherms of the
As shown in Table 1, Inductively Coupled Plasma-Optical
Emission Spectrometry (ICP-OES) gives the Si/Ti ratios 160.5,
83.6, and 77.8 for the TS-1(H1), TS-1(H3), and TS-1(H24),
Figure 3. UV-Vis spectra (a) and UV-Raman spectra (b) of conventional nano-sized TS-
respectively, indicating that more titanium elements are
1
(con) and TS-1(Hx) zeolite catalysts. The wavelength of the excitation light in UV-
incorporated into the crystals with the crystallization time Raman is 320 nm.
prolonged. The Si/Ti ratios measured by X-ray photoelectron
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Scheme 1. Synthesis of the intermediately and completely crystallized
zeolite catalysts
To investigate the formation process of the nano-sized
hierarchical TS-1 crystals, the samples with different
crystallization time are synthesized. According to the TEM
measurements, we propose a possible crystallization process
as shown in Scheme 1. After adding the nano zeolite seeds into
the initial gel, the seeds are dissolved as silicalite-1 structural
3
8, 39
fragments in the gel.
At the beginning of the crystallization
stage (Figure S9a), TS-1 zeolite crystals grow on the surface of
the silicalite-1 structure fragments from the seed, forming
nano TS-1 crystals of about 5-10 nm to give rise to the
intercrystalline mesopores by aggregation. With the
crystallization process continuing, the small nano crystals
develop into a single nano crystal with both the inter- and
intracrystalline hierarchical pores (Figure S9b-d). Finally, the
TS-1(H3) and TS-1(H24) samples give large hysteresis loops intermediately crystallized zeolite crystal evolves into a well-
near saturation pressure (0.8 < P/P < 0.99), arising from the crystallized crystal with regular hexagonal morphology and
aggregation of small crystallites and the existence of abundant intracrystalline mesopores via ripening mechanism
0
3
1
mesopores (Figure 1b). The nano-sized hierarchical samples (Figure S9e-f). Note that the intermediately crystallized
TS-1(H1), TS-1(H3), and TS-1(H24) exhibit larger external crystals, and completely crystallized crystals are all single
2
surface areas (118-215 m /g) and mesopore volumes (0.18- crystals as proved by EM analyses (Figure 2).
3
.23 cm /g) than those of traditional nano-sized TS-1 sample
0
It is believed that the zeolite seeds introduced into the
105 m /g and 0.11 cm /g). TS-1(H1) exhibits the highest BET synthesis gel not only play a crucial role in the formation of
2
3
(
2
surface area and the external surface area (464 m /g, 215 hierarchical structures, but also accelerate the crystallization
2
m /g), which may arise from the aggregated nano TS-1 crystals processes and improve the crystallinity and yields of zeolite
(
5-10 nm). Besides, the high external surface area of TS-1(H3) products. As shown in Figure S10, TEM images of the
2
149 m /g) ascribes to the ultra-small crystallites as well as the conventional nano-sized TS-1 zeolites synthesized without
(
inter- and intracrystalline mesopores. As for the completely seeds clearly reveal that there are no intracrystalline
crystallized TS-1(H24), the microporous surface area increases mesopores in the crystals crystallized for 2-24 h. Besides,
2
to 295 m /g, and the external surface area decreases to 118 according to the XRD analyses, both the crystallinity and the
2
m /g. The detailed surface area and pore volume data are yields of hierarchical TS-1 zeolites synthesized with seeds are
summarized in Table S1. The density functional theory (DFT) higher than those of the counterpart synthesized without
pore size distributions of these prepared samples further seeds (Figures S11-S12 and Tables S2-S3).
demonstrate the existence of mesopores (Figure S8).
Oxidative desulfurization of dibenzothiophene (DBT) and
,6- dimethyldibenzothiophene (4,6-DMDBT) are performed to
4
a
Table 1. Oxidative desulfurization of DBT and 4,6-DMDBT with TBHP as the oxidizing agent over Various Titanosilicate Catalysts.
i
i
Conv. (%)
Conv. (%)
Yield (%)
Yield (%)
b
c
g
h
Samples
Si/Ti
Si/Ti
TON
TON
d
d
e
f
DBT
4,6-DMDBT
TBHPDBT
TBHP4,6-DMDBT
DBSO DBSO
2
DMDBSO DMDBSO
2
TS-1(H1)
TS-1-(H3)
TS-1(H24)
TS-1(C3)
TS-1(con)
a
160.5
83.6
77.8
97.3
73.2
119.2
64.9
75.8
86.6
79.4
100
100
100
100
91.7
92.9
91.6
73.8
58.9
30.2
18.0
17.9
8.8
18.1
18.0
6.7
18.8
19.4
22.2
50.3
23.6
81.2
17.9
19.1
33.3
46.5
16.4
b
82.1
80.9
17.1
7.3
90.8
80.6
33.3
10.4
3.7
55.5
60.7
27.3
50.4
53.8
17.7
76.8
56.4
7.1
6.1
33.4
3.1
1.8
1.3
Reaction conditions: 10mL 500 ppm model fuels, 60 °C, the molar ratio of n(sulphide)/n(Ti in zeolite) was fixed at 10, n(sulphide)/n(TBHP) was 0.5. Measured by
c
d
e
inductively coupled plasma (ICP). Measured by X-ray photoelectron spectroscopy (XPS). After 30 min of the catalytic reaction. Conversion of TBHP in the oxidation
f
g
of DBT, after 30 min of the reaction. Conversion of TBHP in the oxidation of 4,6-DMDBT, after 30 min of the reaction. Turnover number (moles of converted DBT and
h
i
DBSO per mole of Ti site). Turnover number (moles of converted DMDBT and DMDBSO per mole of Ti site). Product yield based on the DBT or 4,6-DMDBT after 30
min of the reaction.
4
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investigate the catalytic activity of nano-sized hierarchical reactions, we further apply this intermediately crystallization
zeolites with TBHP as the oxidizing agent (Table 1 and Figures method in improving the catalytic activity of conventional
(S13-S14). As compared with the conventional nano-sized TS- nano-sized TS-1 and Ti-Beta zeolite catalysts (Figures S20-S22).
1(con) catalyst, all of the TS-1(Hx) catalysts show an Strikingly, both the intermediately crystallized catalysts are
outstanding performance in removing the bulky organosulfur enriched with hexa-coordinated titanium, exhibit higher
compounds. Significantly, the intermediately crystallized TS- external surface area than the completely crystallized ones
1(H3) catalyst exhibit the superior catalytic performance in and free of anatase phase. As expected, the intermediately
oxidative desulfurization of DBT and 4,6-DMDBT, which can be crystallized titanosilicates show enhanced catalytic activity
completely converted in just 15 minutes, much higher than the towards the desulfurization of DBT with the TON of 7.1 and
completely well-crystallized zeolite catalyst TS-1(H24) (49.1% 16.1 at 30 min respectively, compared with the completely
for DBT and 36.5% for 4,6-DMDBT at 15 min) as well as other crystallized catalysts (3.1 and 13.9) (Figure S23 and Table S6).
.
40
reported hierarchical titanosilicate zeolites (Table S4). Since Above studies demonstrate that the intermediate
Si/Ti ratios of the samples are different, the Turnover Number crystallization is beneficial for the formation and stabilization
(TON) values were calculated to illustrate the intrinsic activity of active hexa-coordinated titanium and hierarchical structures
of the catalysts after 30 min of the reaction (Table 1). The with higher external surface area, as well as inhibiting the
intermediately crystallized TS-1(H3) gives the TON of DBT and formation of anatase phase. These features are favorable for
4,6-DMDBT up to 17.9 and 18.0, respectively, almost two folds enhancing the oxidative desulfurization of bulky
higher than those of the completely well-crystallized zeolite organosulphides.
TS-1(H24) (8.8 for DBT and 6.7 for 4,6-DMDBT). The fact that
the intermediately crystallized TS-1 catalysts give higher
Conclusions
selectivity toward the final product sulfone than the
completely crystallized phases as well as the conventional
nano-sized TS-1 catalyst suggests their enhanced oxidative
activities. The remarkably improved catalytic activity of the
intermediately crystallized catalysts can be ascribed to the
abundant inter- and intracrystalline mesopores, the surface
enriched active hexa-coordinated titanium sites, as well as the
absence of anatase species. These could reduce the transport
limitation of bulky molecules, and avoid the inefficient
decomposition of oxidants.
In summary, we present a facile intermediate crystallization
strategy for the synthesis of nano-sized hierarchical TS-1
zeolites free of anatase species with the seed-assisted method.
The catalytic properties of oxidation of DBT and 4,6-DMDBT
are greatly dependent on the catalysts prepared at different
crystallization time. The intermediately crystallized catalyst
with abundant inter- and intracrystalline mesopores, the
surface enriched active titanium species, as well as the
absence of anatase species, give rise to significantly higher
catalytic activity than the completely crystallized analogue.
This suggests that the completely crystallized zeolite catalysts
are not always expected to have the best catalytic
performance. This work provides a useful guidance to explore
highly efficient catalysts by modulating the crystalline states.
As shown in Figure S15, the catalyst regenerated with
centrifugation shows a decreased activity compared to the
fresh one (67% vs. 100%) in the catalytic oxidative
desulfurization of DBT, ascribing to the adsorption of reaction
product molecules, which may cover some of the surface
active sites of the catalyst. However, the catalytic activity can
be almost regenerated by calcination in air. After seven
reaction–regeneration cycles, the TS-1(H3) catalyst retains Conflicts of interest
~
2
92% of its initial activity (Figure S16). The XRD, N adsorption
There are no conflicts to declare.
desorption and TEM of the TS-1(H3) after the durability test
indicate that the catalyst remains the high crystallinity, with
3
-1
slightly decreased mesopore volume (from 0.23 to 0.17 cm g )
and crystallinity (from 100% to 93.2%) (Figures S17-S18 and
Tables S1 and S5). UV-vis spectrum shows that the titanium in
the used catalyst remains as tetra-coordinated (220 nm) and
hexa-coordinated states (270 nm), without the formation of
anatase species. ICP measurement indicates that there is slight
leaching of the titanium, with ~92% of the initial Ti remained in
the TS-1(H3) sample recycled for seven times (Table S1).
However, the activity decreases to ~85% when the catalyst is
recycled for ten times, which may be ascribed to the obviously
reduced crystallinity and titanium content (Figure S19 and
Tables S1 and S5).
Acknowledgements
The authors thank the Nation Key Research and Develop-ment
Program of China (Grant No. 2016YFB0701100), Na-tional
Natural Science Foundation of China (Grant Nos.
2
1320102001, 21621001), the 111 Project (Grant No. B17020),
Jilin Province/Jilin University Co-construction Project – Funds
for New Materials (SXGJSF2017-3) and Graduate Innovation
Fund of Jilin University (Grant No. 2017021) for supporting this
work.
Notes and references
With the understanding that the intermediately crystallized
zeolite catalysts can enhance the oxidative desulfurization
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H. Zheng, Z. Sun, X. Chen, Q. Zhao, X. Wang and Z. Jiang,
Appl. Catal. A, 2013, 467, 26-32.
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