Role of Recrystallization in Alkaline Treatment on the Catalytic Activity of 1-Butene Epoxidation

Article abstract of DOI:10.1002/cctc.202001480

Two alkylol amines, which were ethanolamine (EOA) and triethanolamine (TEOA), were used to posttreat nanosized titanium silicalite-1 (TS-1) to improve its catalytic activity for 1-butene epoxidation. Comparative studies were conducted to clarify the influences of the EOA and TEOA concentrations on the physicochemical properties through detailed characterization. EOA treatment can dissolve the framework Si, [SiO₄], on the external surface of TS-1, generating some macropores. TEOA serves as structure-directing agent to form MFI topology other than the dissolution of [SiO₄], and promotes the recrystallization of [SiO₄], forming new TS-1 particles over the external surface. The two treatments can both improve the catalytic activity of 1-butene epoxidation, but the samples treated with TEOA exhibit better catalytic stability. This can be attributed to the characteristic recrystallization of [SiO₄] in the case of TEOA treatment, stabilizing the structure of TS-1 and maintaining more framework Ti and intercrystal space.

Full text of DOI:10.1002/cctc.202001480

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Role of recrystallization in alkaline treatment on the catalytic
activity of 1-butene epoxidation
Authors: Yi Zuo, Liqian Yang, Xiao Jiang, Mengtong Ma, Yanli Wang,
Min Liu, Chunshan Song, and Xinwen Guo
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ChemCatChem
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Role of recrystallization in alkaline treatment on the catalytic
activity of 1-butene epoxidation
[
a]
[a]
[b]
[a]
[a]
[a]
[c]
Yi Zuo, Liqian Yang, Xiao Jiang, Mengtong Ma, Yanli Wang, Min Liu, Chunshan Song, and
Xinwen Guo*[a]
Dedication ((optional))
[
a]
Dr. Y. Zuo, L. Yang, M. Ma, Y. Wang, Dr. M. Liu, Prof. Dr. X. Guo
State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, Department of Catalysis Chemistry and Engineering
Dalian University of Technology
Dalian, 116024, P. R. China
E-mail: guoxw@dlut.edu.cn
Dr. X. Jiang
Chemical Sciences Division
Oak Ridge National Laboratory
Oak Ridge, TN 37381, USA
Prof. Dr. C. Song
[
[
b]
c]
Department of Chemistry, Faculty of Science
The Chinese University of Hong Kong
Shatin, NT, Hong Kong, P. R. China
Supporting information for this article is given via a link at the end of the document.
Abstract: Two alkylol amines, which were ethanolamine (EOA) and
triethanolamine (TEOA), were used to posttreat nanosized titanium
silicalite-1 (TS-1) to improve its catalytic activity for 1-butene
epoxidation. Comparative studies were conducted to clarify the
influences of the EOA and TEOA concentrations on the physico-
chemical properties through detailed characterization. EOA treatment
reported for 2.5 mol%.[21] Excessive Ti will exist as
extraframework Ti. Octahedrally coordinated Ti and anatase TiO
are two kinds of extraframework Ti. Recent works have found that
the former was also active for oxidation,[22,23] while the latter can
and water.[24]
2 2 2
decompose H O to O
In the applications of TS-1, the alkene epoxidation, especially
the propene epoxidation to produce propene oxide (PO), is an
extensively studied topic. The hydrogen peroxide to propene
oxide (HPPO) route catalyzed by TS-1 has already been
commercialized in industry.[25-27] Nevertheless, there are still
issues in the synthesis and application of TS-1 that need to
address, including the low activity and short lifetime for large
molecular reactions.
2
can dissolve the framework Si, [SiO
, generating some macropores. TEOA serves as structure-directing
agent to form MFI topology other than the dissolution of [SiO ], and
promotes the recrystallization of [SiO ], forming new TS-1 particles
4
], on the external surface of TS-
1
4
4
over the external surface. The two treatments can both improve the
catalytic activity of 1-butene epoxidation, but the samples treated with
TEOA exhibit better catalytic stability. This can be attributed to the
characteristic recrystallization of [SiO
4
] in the case of TEOA treatment,
Butene oxide (BO) is a homolog of PO, which is
stabilizing the structure of TS-1 and maintaining more framework Ti
and intercrystal space.
manufactured from the byproduct of Chlorohydrin route that
produces PO.[28] This route is gradually abandoned worldwide,
because it produces waste residue and water that cause
environmental concerns. Inspired by HPPO route, an alternative
is to use 1-butene in the epoxidation instead of the propene to
produce BO. However, the catalytic performance for 1-butene
epoxidation is much poorer than that for propene.[29] Therefore,
great efforts have been put forth to improving the catalytic
Introduction
Titanium silicalite-1 (TS-1) is a titanium-containing zeolite with
MFI topology, which was first reported by Taramasso et al.[1] It
contains two types of 10-member ring channels which are straight
channel (0.56 nm × 0.54 nm) and zigzag channel (0.55 nm × 0.51
nm). Due to the excellent catalytic oxidation performance with
performance of 1-butene epoxidation, such as decreasing the
particle size of catalysts,[30] introducing additives,[31] and
optimizing reaction conditions.[32] Nonetheless, the progress is
limited.
H
2
O
2
under mild conditions, TS-1 attracted much attention in the
The poor performance is primarily ascribed to the similar
kinetic diameters of 1-butene, BO, and its ramifications (butane
diol (BD) and its monomethyl ethers (MME)) to the TS-1 pore size,
leading to great diffusion resistance in TS-1 channels.
Furthermore, it is widely accepted that the deactivation of TS-1 in
alkene epoxidation stems from the clog of channels by epoxide
oligomers.[33] Due to the analogous diameters of BO ramifications
and TS-1 channels, BO ramifications proceeds to form
dibutanediol monomethyl ethers via oligomerization, resulting in
last three decades. TS-1-catalyzed reactions include alkene
epoxidation,[2-8]
ammoximation,
reactions.[
aromatics
hydroxylation,[9,10]
alkane oxidation,[14,15] and other oxidative
Tetrahedrally coordinated Ti (framework Ti, [TiO ])
ketone
[
11-13]
16-19]
4
20]
is the main active center for selective oxidation in TS-1.[
However, Ti cannot be inserted to the MFI framework unlimitedly,
because of the lattice expansion originated from the longer Ti-O
bond than Si-O bond. The maximum value of framework Ti was
1
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ChemCatChem
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serious channel clog. Thus, the enlargement of TS-1 channels
might be helpful for obtaining a higher catalytic activity and a
longer lifetime.
Treatment with alkaline solution is one of the effective
approaches that may tune the pore structure of zeolites.[34-37] The
influences of organic bases and inorganic bases on the pore
structure are quite different. The treatment with organic bases
of EOA obviously, indicating that the intensities of XRD peaks
were not affected by the particle size. The relative crystallinity
(RC) calculated according to the total intensities of the five peaks
varies significantly, in which the RC decreases with the increased
EOA concentration. When the EOA concentration reaches 1.5
mol/L, the RC is the lowest (TS-1.5EOA, 48%). This is because
more EOA leads to higher basicity of the solution, and thus more
often leads to the dissolution of framework Si ([SiO
4
]) in the zeolite
dissolution of [SiO
framework as structure-directing agent, leading to the poor
recrystallization of dissolved [SiO ].
4
]. EOA likely plays no part in synthesizing MFI
crystals, as well as the neighbored [TiO ], because the cations of
4
bases may adsorb on the external surface of zeolites and protect
4
the [SiO
4
] there.[ When the organic bases can perform as a
38]
structure-directing agent, such as tetrapropylammonium
hydroxide (TPAOH), they will confine the synthesis of new
framework over the external surface by using the dissolved [SiO
4
]
and [TiO ] as silicon and titanium sources, ultimately generating
4
hollows in zeolites. However, the connections between hollows
and external surface are micropores, so the diffusion property is
not significantly improved. In addition, there is a dynamical
balance between the dissolution and recrystallization of [SiO
4
]
and [TiO ]. When the recrystallization rate is faster than the
4
dissolution rate, the crystallinity of TS-1 will increase, and vice
versa. The treatment with inorganic bases such as NaOH usually
results in the dissolution of [SiO ] and [TiO ] on the external
4 4
surface of TS-1, obtaining the mesopores that connect the internal
and external surface.[39] Hence, the treatment with inorganic base
is more conducive to improving the diffusion property. Yet, this
approach is rather challenging, because the dissolved [SiO
4
]
hardly recrystallizes due to the absence of structure-directing
Figure 1. XRD patterns of the TS-1 treated with different concentrated EOA
solutions.
agent. Thus, the crystallinity of TS-1 decreases, and the [TiO
loses seriously.
4
]
Ethanolamine (EOA) and triethanolamine (TEOA) are two
important organic bases. Some researchers have introduced
TEOA to the hydrothermal synthesis gel of TS-1.[40-42] The TEOA
can perform as the mesoporous structure-directing agent, so the
hierarchical TS-1 is obtained. Alternatively, the posttreatment of
microsized TS-1 with EOA has a similar phenomenon to that with
TPAOH.[43,44] Some hollows emerge in TS-1 crystals, which may
enhance the diffusion property of substrates and improve the
catalytic performance.
In this work, different concentrations of EOA and TEOA
solutions were used to treat nanosized TS-1 to improve its
catalytic performance of 1-butene epoxidation. The roles of two
bases in tuning the physicochemical properties of TS-1 catalysts
were studied systematically, and their correlation with the activity
were discussed.
The ultraviolet/visible diffused reflectance (UV/vis) spectra,
which is one of the most widely used techniques for detecting the
Ti coordination states, of the samples are shown in Fig. S1 of
Supporting Information. Three sets of adsorption bands appear in
the UV/vis spectra of TS-1. The bands at 210-220 nm, 250-280
nm, and 310-330 nm are assigned to the characteristic bands of
tetrahedrally coordinated Ti, octahedrally coordinated Ti, and
anatase TiO
concentration, the intensity of the band for anatase TiO
enhanced, indicating the transformation of [TiO ] to anatase TiO
This is analogous to the treatment with the other organic bases.
We know that some [SiO ] dissolve in the process of EOA
treatment, according to XRD results. The [TiO ], bonded to these
SiO ], will then dissociate from the framework and form
octahedrally coordinated Ti or anatase TiO . Since the absence
of structure-directing agent leads to the poor recrystallization of
2
,
respectively.[46] With the increase of EOA
are
2
4
2
.
[38]
4
4
[
4
2
Results and Discussion
TS-1 framework, the reassociation of these [TiO
promoting the generation of extraframework Ti.
4
] is difficult,
Treatment with EOA
Due to the indistinct variation of octahedrally coordinated Ti
in UV/vis analysis, ultraviolet resonance Raman (UV-Raman)
spectroscopy was introduced, and resulting UV-Raman spectra of
the EOA-treated TS-1 are shown in Fig. S2 of Supporting
The effects of EOA solution on the physicochemical properties
and catalytic performance of TS-1 were studied. The denotations
of samples are provided in Experimental Section. The X-ray
powder diffraction (XRD) patterns of TS-1 treated with different
concentrations of EOA are shown in Fig. 1. All samples present
five characteristic peaks corresponding to MFI topology (7.8°, 8.8°,
-1
Information. The intensity of the band at 700 cm , the
characteristic band of octahedrally coordinated Ti,[23] increases
with the increased EOA concentration; meanwhile, the intensity of
-1
the characteristic band of MFI topology (~800 cm ) reduces. The
2
3.0°, 23.9° and 24.4°).[45] The full width at half maxima (FWHM)
inverse trends of these two bands corroborate the increment of
octahedrally coordinated Ti and the decrease of RC. These
characterization results (UV/vis, XRD, UV-Raman) demonstrate
that parts of tetrahedrally coordinated Ti transform to octahedrally
of the five characteristic peaks in Fig. 1 were calculated, the
results of which were shown in Table S1 of Supporting Information.
The obtained FWHM values did not vary with the concentrations
2
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coordinated Ti, and the treatment with EOA can destroy part of
the framework of TS-1.
dissolution of [SiO
Furthermore, recrystallization of [SiO ] rarely occurs, according to
4
4
]
originates from the external surface.
Figure S3 in Supporting Information shows the Fourier
transform infrared (FTIR) spectra of TS-1 treated with different
concentrations of EOA. The band at 960 cm-1 is often considered
as the indirect proof for the insertion of titanium to the MFI
framework, while that at 800 cm-1 is the characteristic band of MFI
topology.[47]. Clearly, the intensities of these two bands decrease
with the increased EOA concentration. The relative intensity of the
bands at 960 cm-1 to that at 800 cm-1 is usually used to reflect the
the gradual decreased RC and particle size.
The energy dispersive spectrometer (EDS) analysis of CTS-
1 and EOA-treated TS-1 samples based on the SEM are shown
in Figs. S6-S8 of Supporting Information, while the elemental
composition on the surface thus obtained are listed in Table S2 of
Supporting Information. The molar ratio of Si/Ti (n(Si/Ti)) on
external surface increases from 54.1 to 63.1 with the increased
EOA concentration. This might be due to the increase of Si
content, the decrease of Ti content or both. Because the
variation of [TiO
4
], and the prerequisite is that the RCs of samples
-1
(
or the intensities of the bands at 800 cm ) are similar. However,
4
treatment with EOA leads to the dissolution of [SiO ] from external
different RCs of EOA-treated TS-1 catalysts in this work hinder us
from accurate quantification. Nonetheless, in consideration of the
exactly measured same weight of the TS-1 samples, one still can
surface and EOA does not have structure-directing function, it is
impossible to cause a higher Si content on the surface. Therefore,
we believe that the increased n(Si/Ti) is attributed to the decrease
of Ti content on the surface. This phenomenon also demonstrates
4
deduce that more [TiO ] is lost and more seriously framework is
destroyed. This deduction is also consistent with the results from
UV/vis and XRD spectroscopies.
4 4
that the dissolution of [TiO ] is more serious than that of [SiO ].
Elemental composition analysis in the bulk of the EOA-
treated samples determined by inductively coupled plasma-
optical emission spectrometer (ICP-OES) are listed in Table 1
(Columns 2-4). The Ti content decreases, while the n(Si/Ti)
increases with the increased EOA concentration. The trend of
n(Si/Ti) in the bulk is the same with that on the surface. These
results also indicate that EOA does not have the ability of
protecting framework Ti as TPAOH does, which is probably
accounted for the lacking of structure-directing function.
The morphology of the same EOA-treated TS-1 catalysts
were studied by scanning electron microscope (SEM) and
transmission electron microscope (TEM), the images of which are
shown in Fig. 2. TS-1 particles exhibit spheroidicity with rough
surface. The morphology is hardly influenced by the treatment
when the concentration of EOA is lower than 1.2 mol/L. The
particle size distribution analysis based on the SEM and TEM
observation (Figs. S4 and S5 in the Supporting Information)
shows that the particle sizes are uniform and reduce from 164 nm
to 122 nm with the increase of EOA concentration. This is
2 2
The losing amounts of SiO and TiO during the treatment
were also detected and shown in Table 1 (Columns 5 and 6),
which both increased with the increased EOA concentration,
4
probably accounted for the dissolution of [SiO ] from the external
surface of TS-1 leading to the decrease of particle size. When 1.5
mol/L EOA solution is used, the surface becomes smooth, and
the morphology gradually changes to irregular particles. This
indicates that EOA, unlike the TPAOH, can hardly help preserving
the external surface of zeolites,[38] but can only lead to the
indicating that more SiO
concentration. This agrees with the XRD results.
Figure 3 shows the N physisorption isotherms of CTS-1 and
EOA-treated samples. The conventional sample (CTS-1) shows
a type I isotherm, suggesting that the CTS-1 mainly contains
micropores. The isotherms of all the treated samples are still type
I, but an uptake and a hysteresis loop appear at the relative
pressure of 0.9-0.99, which is due to the generation of
macropores by the treatment.
2 2
and TiO were leached at a higher EOA
2
4
dissolution of [SiO ] from the external surface of TS-1 particles.
Of note, TEM images show that there is no irregular hollow
in the EOA treated TS-1 particles. This differs from the results
obtained by TPAOH solution-treated[34-36] or EOA-treated
microsized TS-1.[43,44] The absence of hollow confirms that the
Figure 2. SEM and TEM images of the TS-1 treated with different concentrated EOA solutions.
3
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Table 1. Elemental composition and texture properties of the EOA and TEOA treated TS-1 samples.[a]
Samples
SiO
wt%
2
/
TiO
wt%
2
/
n(Si/Ti)
SiO
(mg/g)
2
/
TiO
(mg/g)
2
/
S
Total
2
/
S
Micro
2
/
S
Ext
2
/
S
Macro
2
/
V
Total
3
/
V
Micro
3
/
V
Ext
/
[
b]
[c]
[d]
3
(m /g)
(m /g)
(m /g)
(m /g)
(cm /g)
(cm /g)
(cm /g)
CTS-1
97.7
98.1
98.2
98.3
98.4
98.2
98.2
98.2
98.2
98.2
98.3
2.3
1.9
1.8
1.7
1.6
1.8
1.8
1.8
1.8
1.8
1.7
56.6
68.8
72.7
77.1
82.0
72.7
72.7
72.7
72.7
72.7
77.1
-
-
483
479
477
475
470
491
498
497
486
484
481
456
448
434
420
406
469
477
472
470
468
444
27
31
43
55
64
23
21
25
16
16
37
12.6
23.2
34.9
45.1
47.3
22.3
20.4
22.6
13.7
12.9
12.8
0.22
0.23
0.28
0.30
0.36
0.23
0.24
0.26
0.24
0.25
0.24
0.21
0.19
0.18
0.17
0.17
0.19
0.19
0.19
0.19
0.19
0.17
0.01
0.04
0.10
0.13
0.19
0.04
0.05
0.07
0.05
0.06
0.07
TS-0.5EOA
TS-1.0EOA
TS-1.2EOA
TS-1.5EOA
TS-0.5TEOA
TS-1.0TEOA
TS-1.2TEOA
TS-1.5TEOA
TS-2.0TEOA
TS-2.5TEOA
4.69
5.60
6.14
6.60
2.73
2.45
2.62
2.84
3.07
3.69
0.132
0.145
0.151
0.162
0.131
0.099
0.117
0.128
0.135
0.154
[
a] The STotal and SMicro stand for the total and microporous surface areas, respectively, and the SExt is the sum of external surface, mesoporous and macroporous
surface area. The VTotal and VMicro represent the total and microporous volumes, respectively, and the VExt is the sum of mesoporous and macroporous volume. [b]
The losing amount of SiO
2
during the treatment. [c] The losing amount of TiO
2
during the treatment. [d] SMacro represents the macroporous surface area provided by
mercury intrusion method.
increased EOA concentration. The untreated CTS-1 has the
largest microporous volume (VMicro) and the smallest meso- and
macroporous volume (VExt), although the results are quite near.
The former decreases and the latter increases with the increased
EOA concentration, indicating that the treatment destroys
micropores and forms macropores.
The pore diameter distribution curves are illustrated in Fig.
S9 of Supporting Information. Two new peaks appear in the
curves of the treated samples at ~50 nm and ~85 nm, indicating
the existence of macropores.
However, considering the unsatisfied accuracy of
N
2
physisorption, mercury intrusion method was adopted to further
study the states of macropores in the EOA-treated TS-1 samples.
The mercury intrusion isotherms of EOA-treated samples are
shown in Figs. S10. The cumulative intrusion volumes increase
sharply after being treated with EOA solution, which agrees with
2
the results of N physisorption.
The pore diameter distribution curves of the EOA-treated
samples are shown in Fig. 4. The main distribution of all the
samples are located at 60-70 nm (macropores), which can be
assigned to the intercrystalline space. All the pore volumes and
pore diameters of the treated samples here are larger than that of
Figure 3.
concentrated EOA solutions.
2
N physisorption isotherms of the TS-1 treated with different
The texture properties of EOA-treated TS-1 obtained from N
2
4
CTS-1, indicating that the dissolution of [SiO ] originating from the
physisorption are shown in Table 1. The total surface area (STotal
,
treatment can generate macropores, which benefits the diffusion
of substances in the catalysts.
Column 7) decreases slightly with the increment of EOA
concentration, primarily due to the sharp reducing of microporous
surface area (SMicro, Column 8). The SExt in Table 1 is considered
to be the sum of external, mesoporous and macroporous surface
The macroporous surface areas (SMacro) obtained from
mercury intrusion method are listed in Column 10 of Table 1. It
shows a similar trend to the total surface area, which is enhanced
by the EOA treatment, promoting more macropores exist in the
samples treated with a higher EOA concentration.
2
areas, because the N physisorption technique cannot distinguish
the surface areas in crystals or on external surface. It increases
gradually with the increased EOA concentration, which is
probably accounted for the increase of macropores. The variation
of pore volumes (Columns 11-13 in Table 1) is the same with that
of surface areas. The total pore volume (VTotal) increases with the
4
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The catalytic performance of 1-butene epoxidation over
EOA-treated TS-1 catalysts is listed in Table 2. The H
conversions over the treated samples are all higher than that of
the untreated one, except for TS-1.5EOA. The highest H
conversion is obtained when using 1.0 mol/L EOA to treat TS-1.
The improvement of H conversion is attributed to two aspects,
both of which originate from dissolving [SiO ] from the external
2 2
O
2
O
2
2
O
2
4
surface of TS-1. One results from the enlarged pore size and
reduced particle size that allow an easier access of reactants to
interact with active centers; the other attribution is to expose more
active titanium species on the external surface. As shown by ICP
and EDS results, more Ti exists on the external surface rather
than in TS-1 crystals. Therefore, the dissolution shortens the
diffusion pathway of reactants. However, when the EOA
concentration is as high as 1.5 mol/L, excess [SiO
occurs, as well as the neighbored [TiO ] concurrently, leading to
the decrease of crystallinity, [TiO ] content and macroporous
4
] dissolution
4
4
volume. As a result, TS-1.5EOA shows the lowest catalytic activity
of all the samples.
Figure 4. Pore diameter distribution curves of EOA-treated TS-1 samples
determined by mercury intrusion method.
The selectivity of BO over the treated samples increases and
then decreases with the increased EOA concentration slightly.
Such phenomenon is due to the dual characters of diffusion
enhancement. In other words, the treatment leads to a better
diffusion property for both reactants and products. At low EOA
concentrations (e.g., ≤ 0.5 mol/L), the enhancement of diffusion
can promote the diffusion of byproducts out of TS-1 channels
because of their big kinetic diameters; however, it hardly impacts
the main reaction. Thus, the BO selectivity over TS-0.5EOA is
even slightly lower than that over the untreated sample. When the
EOA concentration increases, the main reaction is promoted as
discussed above, leading to more BO being generated.
Table 2. Catalytic performances of 1-butene epoxidation over EOA- and TEOA-
treated TS-1 catalysts.
[
a]
Samples
X(H
2 2
O ) / %
S(BO) / %
97.8
96.8
98.1
97.7
96.5
96.2
98.1
97.6
96.6
96.3
96.3
98.3
98.0
51.9
84.3
93.2
91.1
2 2
U(H O ) / %
CTS-1
61.8
69.6
74.4
73.9
50.0
63.5
69.1
70.5
74.2
69.3
66.3
58.2
74.1
14.6
23.1
65.3
51.6
77.5
68.7
78.1
75.9
76.6
69.2
72.1
77.8
75.3
75.2
75.3
75.4
75.5
50.6
52.5
74.4
69.7
TS-0.5EOA
TS-1.0EOA
TS-1.2EOA
TS-1.5EOA
TS-0.5TEOA
TS-1.0TEOA
TS-1.2TEOA
TS-1.5TEOA
TS-2.0TEOA
The H
The decomposition of H
epoxidation, so a higher conversion of H
2
O
2
utilization shows a similar trend to BO selectivity.
is a competitive reaction with
to epoxides often
2
O
2
2
O
2
leads to a lower decomposition, and thus a higher utilization.
Some other TS-1 catalysts reported in former works were
prepared and evaluated in the epoxidation of 1-butene, including
microsized TS-1 (MTS-1), TPAOH treated microsized TS-1 (MTS-
0.06TPA), TPAOH treated nanosized TS-1 (TS-0.06TPA) and
NaOH treated nanosized TS-1 (TS-0.002NaOH). The preparation
methods were provided in Section S1 of Supporting Information.
MTS-1 shows a quite poor catalytic activity, maybe due to the
large particle size disadvantaging the diffusion of substances.
After the treatment with TPAOH, the H O conversion and BO
2 2
selectivity over MTS-0.06TPA increased obviously. However, the
TPAOH treatment does not work outstandingly on nanosized TS-
TS-2.5TEOA
TS-1.0EOA[b]
TS-1.5TEOA[c]
MTS-1[d]
2 2
1. Furthermore, the NaOH treatment even leads to a lower H O
conversion, which is probably accounted for the leaching of
framework Ti.[35]
MTS-0.06TPA
TS-0.06TPA[e]
TS-0.002NaOH
Treatment with TEOA
TEOA was then used to treat TS-1 as well. XRD patterns of
the mother sample and the ones after TEOA treatment are shown
in Fig. 5, along with the calculated RCs. All treated samples show
the typical MFI topology. The RCs of the treated samples are all
higher than that of the untreated one. This observation differs from
EOA treated samples. Probably, TEOA serves as structure-
directing agent that is conducive to synthesizing MFI topology.
Some amorphous silica in the untreated sample and dissolved
[
2 2
a] Reaction conditions: cat. 0.1 g, H O concentration 1.5 mol/L, 1-butene 0.2
MPa, 50 °C, 1 h. The error of the data is ± 0.1%. [b] The data were obtained on
the TS-1.0EOA reused for three times. [c] The data were obtained on the TS-
1
according to Ref. [48]. MTS-0.06TPA was obtained by treating MTS-1 with 0.06
mol/L TPAOH solution. [e] TS-0.06TPA and TS-0.002NaOH were prepared
by treating CTS-1 with 0.06 mol/L TPAOH solution and 0.002 mol/L NaOH
.5TEOA reused for three times. [d] MTS-1 is a microsized TS-1 synthesized
[
38]
[
35]
solution, respectively.
5
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[
SiO
process.
Notably, the RCs of the treated samples increase and then
4
] thus generates new MFI structure during the treatment
This can be attributed to the relationship between crystallization
and generation of anatase TiO
2
, wherein increasing crystallinity
leads to less generation of anatase TiO
2
.
decrease with the increased TEOA concentration. When the
concentration is lower than 1.0 mol/L, the basicity of the solution
The FTIR spectra of the TEOA-treated samples are also
acquired and shown in Fig. S12. The intensity of the bands at 800
cm-1 increases at different extents after the treatment, indicating
the increased crystallinity. This coincides with the XRD results.
The intensity of the band at 960 cm-1 of the treated samples
increases and then decreases with the increased TEOA
is weak. Thus, a very small amount of [SiO
surface of the TS-1. However, the crystallization rate of
amorphous silica and dissolved [SiO ] depends on the amount of
4
] dissolves from the
4
structure-directing agent, and increases with the increased TEOA
amount in this work. Thus, the combinational effect of the above
two aspects results in the increase of RC. When the TEOA
concentration is higher than 1.0 mol/L, the solution becomes more
concentration, proving that anatase TiO
[TiO ].
SEM and TEM images of the TEOA treated samples are
2
was transformed from
4
basic, resulting in a higher dissolution rate of [SiO
4
]. At the same
shown in Fig. 6. The morphologies of the six samples are similar
in the SEM images, but they are quite different in TEM images.
Both TS-0.5TEOA and TS-1.0TEOA show a monodispersed
spheroidal particle, while those treated with TEOA concentrated
higher than 1.2 mol/L reveal aggregated particles. The
aggregation degree enhances with the increased TEOA
concentration. In addition, the particle sizes (cf. the size
distributions in Figs. S13 and S14 of Supporting Information) are
hardly affected by the treatment, which are located at ~180 nm.
time, the crystallization rate remains unchanged because of the
excessively high concentration of TEOA. Therefore, the
difference between dissolution and crystallization reduces, and
the RC decreases.
This indicates that the dissolved [SiO
the external surface of TS-1. In sum, both crystallization of
amorphous silica and recrystallization of dissolved [SiO ] occur
4
] would recrystallize along
4
during the treatment with TEOA, which is different from that with
EOA. TEM images also confirm that there is no hollow in the
treated samples, which is similar to those EOA-treated samples.
The EDS analysis of TEOA-treated samples are shown in Figs.
S15 and S16, and the obtained n(Si/Ti) on the surface are also
listed in Table S2. The n(Si/Ti) on the surface reduce with the
increment of TEOA concentration, indicating that more Ti exists
on the surface.
The elemental compositions in the bulk of TEOA-treated
samples are listed in Table 1. The contents of silica and titania,
and the molar ratio of Si/Ti nearly keep constant after being
treated with different concentrated TEOA solution. This might be
Figure 5. XRD patterns of the TS-1 treated with different concentrated TEOA
solutions.
due to the recrystallization of dissolved [SiO
directing of TEOA, promoting the recrystallization of dissolved
TiO ].
2
The losing amounts of SiO and TiO after the treatment with
4
] under the structure-
[
4
Figure S11 shows the UV/vis spectra of the same TEOA-
treated catalysts, along with the untreated one as reference. The
treatment with TEOA also leads to the transformation of
framework Ti (~215 nm) to extraframework Ti (~265 nm & ~320
2
TEOA solution were also determined (Table 1). They decrease
first, and then increase with the increased TEOA concentration.
The trend is different from that of EOA-treated TS-1, but still
coincide with the XRD results obtained from TEOA-treated
samples.
nm). Interestingly, the intensity of the band for anatase TiO
2
decreases and then increases with the increased TEOA
concentration, which exhibits an opposite trend to that of RC value.
Figure 6. SEM and TEM images of the TS-1 treated with different concentrated TEOA solutions.
6
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By comprehensively analyzing the elemental compositions of
EOA and TEOA-treated samples, we conclude that the two
EOA treatment, leading to the preservation of more mesopores
and macropores.
treatments lead to the dissolution of [SiO
the dissolution of [TiO ] from the surface and in the crystals. EOA
does not have the structure-directing effect, so that the dissolved
SiO ] and [TiO ] were hardly recrystallized, leading to the
4
] from the surface and
4
[
4
4
increase of n(Si/Ti) both on the surface and in the bulk. On the
other hand, TEOA have the ability to structure-directingly
synthesize MFI topology. The dissolved [TiO ] in the crystals and
4
on the surface were both recrystallized on the surface, which
caused the decrease of n(Si/Ti) on the surface. However, some
4
dissolved [TiO ] would remain in the mother liquor of treatment.
Thus, the n(Si/Ti) in the bulk of TEOA-treated samples are higher
than that of CTS-1.
2
The N physisorption isotherms of TEOA-treated samples are
shown in Fig. 7. The uptake and hysteresis loop also appear in
the treated samples, proving the generation of macropores.
However, unlike the trend obtained from EOA-treated samples,
the height of uptake and area of hysteresis loop are nearly the
same. This is also probably contributed to the recrystallization of
[
4
SiO ], restraining the formation of larger macropores.
Figure 8. Pore diameter distribution curves of TEOA-treated TS-1 samples
determined by mercury intrusion method.
The catalytic performance of 1-butene epoxidation over the
TS-1 treated with different concentrations of TEOA is compared
in Table 2. The H
TEOA concentration, until the concentration reaches 1.5 mol/L.
2 2
The higher concentration leads to the decrease of H O
2 2
O conversion increases with the increased
conversion. These results may be due to the lower amount of
active Ti species, according to the UV/vis and FTIR spectra.
Although the highest H
TEOA-treated TS-1 are similar, their reusability differs
significantly. After recycled for three times, the H conversion
2 2
O conversions over EOA- and
2
O
2
over EOA-treated sample drops to 58.4%, while that over TEOA-
treated one remains stable.
The spent catalysts which were denoted as TS-1.0EOA-s
and TS-1.5TEOA-s, corresponding to the spent TS-1.0EOA and
TS-1.5TEOA, respectively, were characterized by XRD (Fig. S19),
Figure 7.
concentrated TEOA solutions.
2
N physisorption isotherms of the TS-1 treated with different
2
UV/vis (Fig. S20), and N physisorption (Fig. S21) without any
further treatment. The macropores and framework Ti amount
reduced in the spent EOA-treated TS-1, but kept nearly
unchanged in spent TEOA-treated TS-1. Therefore, the lower
crystallinity of EOA-treated TS-1 leading to the unstable structure
and framework Ti is believed to be the primary reason for its lower
stability. The better reusability of TS-1.5TEOA relates to the
The microporous surface areas of TEOA-treated samples
see Table 1) show a same trend as its RCs, while the variation
of external surface areas is indistinct. Thus, the total surface
areas increase and then decrease with the increased TEOA
concentration.
Figure S17 shows the pore diameter distribution curves of
2
TEOA-treated TS-1 obtained from N physisorption. There is only
one wide peak at 70-100 nm in the curves, which is quite different
from the curves of EOA-treated samples.
The pore diameter distribution of TEOA-treated samples
obtained from mercury intrusion method (Fig. 8) demonstrates
that the macroporous volume of TS-0.5TEOA is the largest, and
it decreases and then increases with the increment of TEOA
concentration. This phenomenon is also due to the
(
4
characteristic recrystallization of [SiO ] in TEOA treatment.
For the BO selectivity, the results before and after the
treatment are not significantly different, indicating that the
intercrystal space can also improve the diffusion property.
In summary, the catalytic performance is affected by two
aspects, which are the diffusion property and intrinsic catalytic
activity (type or number of active centers). The relative crystallinity
of TS-1.5TEOA is much higher than that of TS-1.0EOA, indicating
that the more recrystallization occurs in the former. We know that
the recrystallization leads to the formation of micropores, but
reduces the numbers of macropores. This plays a negative role
recrystallization extent of [SiO
coincides with those obtained from N
4
] and amorphous silica, which
physisorption. Besides, the
2
4
on the diffusion property. Although more [SiO ] recrystallized in
macroporous diameters of the TEOA-treated samples are all
smaller than those EOA-treated ones at the same concentration.
This again corroborates the less recrystallization in the case of
TS-1.5TEOA, generating more tetrahedral Ti (active center), the
approach of substances to active centers is more difficult.
7
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Therefore, the catalytic performances over TS-1.0EOA and TS-
fluorescence and has
coordinated Ti.[50,51]. N
liquid N
sorption apparatus. Total surface area and pore volume were calculated
according to the BET and t-plot method, respectively. The relative pressure
calculating microporous surface area and volume was 0.4-0.6. The c-
values of the treated samples are all higher than 300, and that of CTS-1 is
a
very high sensitivity to the octahedrally
2
physisorption measurements were performed at
temperature on a Quantachrome QUADRASORB SI physical
1.5TEOA are similar accounted for the synergetic effect of active
2
center and diffusion property. However, the recrystallization may
stabilize the active centers, which leads to a better catalytic
stability.
640. Mercury intrusion method was used to determine the macroporous
Conclusion
distribution of the samples, which was operated on a Micromeritics
AutoPore IV 9500 instrument. The appearances of samples were
observed on a FEI QUANTA 450 SEM equipped with an EDS, and a
Tecnai G220 S-Twin TEM. The particle size distribution was analyzed by
the Nano Measurer software. The average particle sizes of all the samples
were illustrated in the distribution charts, accompanied by the standard
deviation (R) values. The number of particles used to establish the particle
size distribution is more than 50. The elemental composition in the bulk
and the losing amount of silica and titania were provided by a Perkin Elmer
OPTIMA 2000DV ICP-OES. The losing amount was calculated by the
silica and titania contents in the mother liquid obtained from the treatment.
Nanosized TS-1 was treated with EOA and TEOA solutions at
different concentrations. They both caused the dissolution of
[
4
SiO ] from the external surface of TS-1, forming macropores.
TEOA behaves as structure-directing agent to generate new MFI
framework, which, however, is absent in the case of EOA
treatment. Both treatments can improve the catalytic activity of 1-
butene epoxidation, and the optimal concentrations of EOA and
TEOA are 1.0 mol/L and 1.5 mol/L, respectively. The reusability
of TEOA-treated TS-1 is more excellent than that of EOA-treated
one, which can be associated with the TEOA-induced
Epoxidation of 1-butene
4
characteristic recrystallization of [SiO ].
The epoxidation of 1-butene was carried out in a stainless-steel batch
reactor. The aqueous H
1
0
2
O
2
(30 wt%) was mixed with methanol to form a
.5 mol/L solution, and 34 mL of the solution was fed into the reactor. Then,
.1 g of TS-1 catalyst (60 mesh) was dispersed in the solution. 1-Butene
Experimental Section
(0.2 MPa) was finally introduced into the reactor and replaced the air for
Preparation of nanosized TS-1
three times. The reactor was sealed and heated at 50 °C for 1 h with
magnetic stirring. After that, liquid products were separated from the
2 2
suspension and analyzed. The residual H O was checked by iodometric
titration. The products were obtained on an Agilent 7890B gas
chromatograph equipped with a FID and a HP-INNOWAX capillary column
The nanosized TS-1 powder was synthesized in a TPAOH hydrothermal
system referred to the literature.[49] Tetraethyl orthosilicate and tetrabutyl
titanate were used as silicon and titanium sources, respectively. TPAOH
was the template and base. The silicon and titanium sources were
hydrolyzed separately, and then were mixed together, followed by heating
at 90 °C for 40 min to remove the alcohols. The synthesis gel with a molar
(
50 m × 0.25 mm × 0.5 μm). BO was the main product, and BD and MME
were the byproducts. The conversion of H (X(H )), selectivity of BO
)) were calculated with the Eqs. (1),
O
2 2
2 2
O
(
(
2 2 2 2
S(BO)) and utilization of H O (U(H O
2) and (3), respectively.
2 2 2
composition of SiO : TiO : TPAOH : H O = 1 : 0.025 : 0.33 : 47, was
transferred to a Teflon-lined autoclave and crystallized at 170 °C for 48 h.
The suspension containing TS-1 was separated by centrifugation. The
solid was washed with distilled water, dried at 80 °C overnight and calcined
it at 540 °C for 6 h, after which the nanosized TS-1 powder (denoted as
CTS-1) was obtained.
X(H
S(BO) = n(BO) / (n(BO) + n(MME) + n(BD))
2 2 2 2
U(H ) = (n(BO) + n(MME) + n(BD)) / (n (H O ) · X(H O ))
2
O
2
) = (n
0
(H
2
O
2
) - n(H
2
O
2
)) / n
0
(H
2
O
2
)
(Eq. 1)
(Eq. 2)
(Eq. 3)
2
O
2
0
Treatment of TS-1
0 2 2 2 2
The n (H O ) and n(H O ) represent the initial and final molar
The CTS-1 powder was ground to 60 mesh and dispersed into EOA
or TEOA solutions with the ratio of 1.0 g of TS-1 to 10 mL of solution. The
mixture was then transferred to a Teflon-lined autoclave and heated at
numbers of H O , respectively. The n(BO), n(MME) and n(BD) stand for
the molar numbers of BO, MME and BD, respectively.
2
2
170 °C for 48 h. The obtained solid was washed, dried and calcined by the
same means with those mentioned in above section. The concentrations
of EOA in the solutions are 0.5, 1.0, 1.2 and 1.5 mol/L, while those of TEOA
are 0.5, 1.0, 1.2, 1.5, 2.0 and 2.5 mol/L. The treated samples were denoted
as TS-xEOA and TS-yTEOA, corresponding to the samples treated with
EOA and TEOA, respectively. The x and y stand for the concentration of
EOA and TEOA, respectively. For instance, TS-1.5TEOA stands for the
TS-1 treated with 1.5 mol/L TEOA.
Acknowledgements
This work was financially supported by the National Key Research
and Development Program of China (2016YFB0301704), the
National Natural Science Foundation of China (21506021), the
Fundamental Research Funds for the Central Universities
(
DUT19LK61), and the Program of Innovative and
Characterization of TS-1
Entrepreneurial Talent of Jiangsu, China. X.J. Acknowledges
support from the Center for Understanding and Control of Acid
Gas-Induced Evolution of Materials for Energy (UNCAGE-ME),
an Energy Frontier Research Center funded by U.S. Department
of Energy, Office of Science, Basic Energy Sciences.
XRD patterns were determined by a Rigaku Corporation SmartLab 9
X-ray diffractometer equipment using Cu Kα radiation. UV/vis spectra were
recorded on a Jasco UV-550 spectrometer from 190 to 800 nm. Pure
4
BaSO was used as the reference. FTIR spectra were collected on a
-
1
Bruker EQUINOX55 spectrometer from 4000 to 400 cm , using the KBr
pellet technique. UV-Raman spectra were performed on a DL-1 UV-
Raman spectrometer made by Dalian Institute of Chemical Physics. The
UV light with a wavelength of 266 nm was used as the excitation light
source, because it could effectively eliminate the interference of
Keywords: heterogeneous catalysis • zeolites • titanium
silicalite-1 • posttreatment • 1-butene epoxidation
8
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Entry for the Table of Contents
Insert graphic for Table of Contents here.
Ethanolamine (EOA) and triethanolamine (TEOA) were used to treat nanosized TS-1 to improve its catalytic activity in 1-butene
epoxidation. Both treatments lead to the dissolution of framework Si and the generation of macropores. TEOA can help the
recrystallization of framework Si, but EOA cannot.
1
0
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