Tandem Lewis acid catalysis for the conversion of alkenes to 1,2-diols in the confined space of bifunctional TiSn-Beta zeolite

Article abstract of DOI:10.1016/S1872-2067(20)63734-2

The generation of multifunctional isolated active sites in zeolite supports is an attractive method for integrating multistep sequential reactions into a single-pass tandem catalytic reaction. In this study, bifunctional TiSn-Beta zeolite was prepared by a simple and scalable post-synthesis approach, and it was utilized as an efficient heterogeneous catalyst for the tandem conversion of alkenes to 1,2-diols. The isolated Ti and Sn Lewis acid sites within the TiSn-Beta zeolite can efficiently integrate alkene epoxidation and epoxide hydration in tandem in a zeolite microreactor to achieve one-step conversion of alkenes to 1,2-diols with a high selectivity of >90%. Zeolite confinement effects result in high tandem rates of alkene epoxidation and epoxide hydration as well as high selectivity toward the desired product. Further, the novel method demonstrated herein can be employed to other tandem catalytic reactions for sustainable chemical production.

Full text of DOI:10.1016/S1872-2067(20)63734-2

Chinese Journal of Catalysis 42 (2021) 1176–1184
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
journal homepage: www.sciencedirect.com/journal/chinese‐journal‐of‐catalysis
Article
Tandem Lewis acid catalysis for the conversion of alkenes to
1,2‐diols in the confined space of bifunctional TiSn‐Beta zeolite
Qifeng Lei a, Chang Wang a, Weili Dai a,*, Guangjun Wu a, Naijia Guan a,b, Michael Hunger c,
Landong Lia,b
a School of Materials Science and Engineering & National Institute for Advanced Materials, Nankai University, Tianjin 300350, China
b Key Laboratory of Advanced Energy Materials Chemistry of the Ministry of Education, Collaborative Innovation Center of Chemical Science and
Engineering, Nankai University, Tianjin 300071, China
c Institute of Chemical Technology, University of Stuttgart, Stuttgart 70550, Germany
A R T I C L E I N F O
A B S T R A C T
Article history:
The generation of multifunctional isolated active sites in zeolite supports is an attractive method for
integrating multistep sequential reactions into a single‐pass tandem catalytic reaction. In this study,
bifunctional TiSn‐Beta zeolite was prepared by a simple and scalable post‐synthesis approach, and
it was utilized as an efficient heterogeneous catalyst for the tandem conversion of alkenes to
1,2‐diols. The isolated Ti and Sn Lewis acid sites within the TiSn‐Beta zeolite can efficiently inte‐
grate alkene epoxidation and epoxide hydration in tandem in a zeolite microreactor to achieve
one‐step conversion of alkenes to 1,2‐diols with a high selectivity of >90%. Zeolite confinement
effects result in high tandem rates of alkene epoxidation and epoxide hydration as well as high se‐
lectivity toward the desired product. Further, the novel method demonstrated herein can be em‐
ployed to other tandem catalytic reactions for sustainable chemical production.
Received 22 September 2020
Accepted 2 November 2020
Available online 5 March 2021
Keywords:
Tandem catalysis
Confinement effect
Zeolite
Alkene epoxidation
Epoxide hydration
© 2021, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
Ti(IV)‐containing zeolites, such as Ti‐Beta and TS‐1, have been
successfully utilized as efficient catalysts for alkene epoxidation
1,2‐Diols, mainly produced by epoxide hydration, are exten‐
sively utilized as intermediates for producing chemical prod‐
ucts such as antifreezes and polyester resins, medicines, and
cosmetics [1]. Thus far, a variety of acidic and basic catalysts,
including cation‐ and anion‐exchange resins [2,3], supported
metal oxides [4,5], amines [6], and silica‐based reagents for
nanoreactors [7,8], have been utilized in epoxide hydration.
Previously, we reported the excellent catalytic performance of
Lewis acidic Sn‐containing zeolites in epoxide hydration [9,10].
As well‐known carbon electrophiles, epoxides are mainly pro‐
duced from the corresponding alkene precursors.
with hydrogen peroxide [11–13]. Therefore, theoretically,
1,2‐diols can be produced by alkene epoxidation using an ap‐
propriate oxidant over Ti‐containing zeolites, followed by
epoxide hydration over Sn‐containing zeolites (Fig. 1, Route a).
Although desirable activity can be achieved in both alkene
epoxidation and epoxide hydration over Ti‐ and Sn‐containing
zeolite catalysts, respectively, the development of novel routes
for the one‐step conversion of alkene to 1,2‐diols with high
selectivity can be highly effective for decreasing the separation
cost and achieving high energy utilization efficiency.
Tandem catalysis, which integrates multistep sequential re‐
* Corresponding author. Tel/Fax: +86‐22‐85358536; E‐mail: weilidai@nankai.edu.cn
This work was supported by Municipal Natural Science Foundation of Tianjin (18JCJQJC47400, 18JCZDJC37400) and the Fundamental Research
Funds for the Central Universities.
DOI: 10.1016/S1872‐2067(20)63734‐2 | http://www.sciencedirect.com/journal/chinese‐journal‐of‐catalysis | Chin. J. Catal., Vol. 42, No. 7, July 2021

Qifeng Lei et al. / Chinese Journal of Catalysis 42 (2021) 1176–1184
1177
lites and the subsequent introduction of Sn species into the
dealumination [Si]Beta zeolite. The obtained samples were
denoted as Ti‐Beta, Sn‐Beta, and TiSn‐Beta, and the quantity of
Ti or Sn was 5 wt%, unless otherwise specified. Further,
Ti‐Sn‐Beta and Sn‐Ti‐Beta zeolites were prepared using the
same method as that used for preparing TiSn‐Beta zeolite, but
with different introduction sequences of the Ti and Sn species
(TiSn‐Beta: Ti and Sn were co‐introduced; Ti‐Sn‐Beta: Ti was
first introduced, followed by Sn; Sn‐Ti‐Beta: Sn was first intro‐
duced, followed by Ti). Full details of the sample preparation
are provided in the Supporting Information.
Ti‐Beta
Sn‐Beta
a)
HO
R1
OH
R2
O
H2O2
H2
O
C
C
R1
R2
R1
R2
Ti
Sn
TiSn‐Beta
b)
O
HO
R1
OH
R2
H2O2
C
C
H2
O
R1
R2
H2O2
C
R1
C
H2
O
R1
R2
R2
2.2. Characterization techniques
Fig. 1. Routes for the conversion of alkenes to 1,2‐diols. Route a: indi‐
rect route including two processes: alkene epoxidation and epoxide
hydration. Route b: tandem catalytic route of this study.
The physicochemical properties of the as‐prepared samples
were characterized by X‐ray diffraction (XRD), nitrogen ad‐
sorption, transmission electron microscopy (TEM), and
high‐resolution transmission electron microscopy (HR‐TEM).
The precise weight loadings of Sn and/or Ti species were de‐
termined by inductively coupled plasma optical emission spec‐
troscopy (ICP‐OES). The changes in the hydroxyl groups in the
H‐Beta, dealumination [Si]Beta, and Ti‐ and/or Sn‐containing
beta samples were investigated by diffuse reflectance infrared
Fourier transform (DRIFT) spectroscopy. The chemical envi‐
ronment and the nature of the Sn and Ti species within the
[Si]Beta zeolite were verified by diffuse reflectance ultravio‐
let‐visible (UV‐vis) spectroscopy, X‐ray photoelectron spec‐
actions into a single‐pass reaction, provides opportunities to
develop new and efficient catalytic routes [14–22]. Various
types of active sites are required to combine individual steps
into a tandem catalysis reaction. In addition, to achieve high
catalytic efficiency in tandem reactions, the types and distribu‐
tions of the active sites should be controlled for individual re‐
actions, and the reaction conditions should be balanced [23,24].
Lewis acidic zeolites are generally obtained by incorporating
heteroatoms into the zeolite framework via direct hydrother‐
mal synthesis or post synthesis, and several monometallic zeo‐
lite catalysts, such as Ti‐Beta, Sn‐Beta, Zr‐Beta, Ta‐Beta, etc.
[25–29], have been shown to exhibit desirable catalytic per‐
formance in numerous reactions. However, for certain tandem
reactions, it is of significant importance to obtain a large num‐
ber of different types of isolated Lewis acid sites within the
zeolite structure. Li et al. [30] directly synthesized ethylene
glycol (EG) from ethylene over Al‐TS‐1 zeolite, but with a low
EG selectivity of ~35%. Recently, Wu et al. [31] reported that
Ti‐MWW exhibited desirable catalytic activity toward ethylene
to EG conversion.
119Sn magic‐angle spinning
troscopy (XPS), and solid‐state
(MAS) NMR spectroscopy. The Lewis acidic character of Ti
and/or Sn sites in Ti‐ and/or Sn‐containing Beta samples was
qualitatively and quantitatively identified using trime‐
thylphosphine oxide (TMPO) and NH3 adsorption and studied
1
by 31P and H MAS NMR spectroscopy, respectively. Further
equipment parameters and the experimental details of the
abovementioned characterization are provided in the Support‐
ing Information.
Herein, for the first time, we introduce tandem catalysis for
the direct transformation of alkenes to 1,2‐diols by developing
isolated dual Lewis acid sites within the zeolite structure as
molecular reactors (Fig. 1, Route (b)). We demonstrate that
bifunctional TiSn‐Beta zeolite with isolated Lewis acid Ti and
Sn sites obtained using a simple and scalable post‐synthesis
route can efficiently integrate alkene epoxidation and epoxide
hydration in tandem in one reactor to achieve high selectivity
(>90%) for 1,2‐diols. The method of Lewis acid tandem cataly‐
sis in zeolite cages presented in this study can be applied for
other tandem catalytic processes for sustainable chemical pro‐
duction.
2.3. Catalytic performance evaluation
The catalytic performance was tested in a Teflon‐lined
stainless‐steel autoclave. Typically, 5 mmol alkene, 5 mmol
H2O2 (31 wt% aqueous solution), 2.5 mL of solvent, and 0.1 g of
the catalyst were added into the autoclave and mixed vigor‐
ously using a magnetic stirrer. Subsequently, the autoclave was
heated to the desired reaction temperature and maintained for
a certain duration of time at a stirring rate of 500 rpm. After the
reaction, the products were analyzed using a Shimadzu gas
chromatography‐mass spectrometry (GC‐MS) QP2010 SE and
quantitatively determined using a Shimadzu 2010 GC equipped
with an Agilent HP‐5MS column (30 m × 0.25 mm × 0.25 μm),
with the use of chlorobenzene as an internal standard. A carbon
balance of approximately 95% was achieved during all the ex‐
periments.
2. Experimental
2.1. Preparation of zeolite catalysts
Ti‐ and/or Sn‐containing Beta zeolites were prepared via a
two‐step post‐synthesis procedure, as described elsewhere
[9,10], which involves the dealumination of parent H‐Beta zeo‐
3. Results and discussion
3.1. Design of zeolite samples and structural characterization

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Qifeng Lei et al. / Chinese Journal of Catalysis 42 (2021) 1176–1184
Table 1
‐113
‐116
Physicochemical properties of the samples under study.
Si/Al Ti loading a Sn loading a Surface area b Pore volume c
Samples
‐103
[Si]Beta
ratio a (wt%)
(wt%)
—
—
(m2/g)
590
620
598
605
588
580
(cm3/g)
0.204
0.210
0.199
0.201
0.192
0.191
13.5
>1800
H‐Beta
Si‐Beta
Ti‐Beta
Sn‐/Beta >1800
TiSn‐Beta >1800
TiSn‐Beta d >1800
—
—
Ti‐Beta
>1800
4.8
—
—
4.9
4.7
4.7
Sn‐Beta
4.6
4.5
TiSn‐Beta
a Determined by ICP.b Obtained using the BET method. c Calculated from
the t‐plot. d Sample obtained after four reaction cycles.
‐
‐70
‐140
80 ‐90 ‐100 ‐110 ‐120 ‐130
9Si /ppm
Experimentally, Ti‐ and/or Sn‐containing beta zeolites were
fabricated using a two‐step post‐synthesis approach. First,
de‐aluminated [Si]Beta zeolites were prepared using the nitric
acid treatment of H‐Beta zeolites. Subsequently, Ti‐ and/or
Sn‐Beta zeolites were prepared by the introduction of Sn
and/or Ti precursors into the [Si]Beta zeolites [9,10]. The pos‐
sible textural property changes during the post‐synthesis of
zeolites were studied by XRD, DRIFT, and solid‐state NMR.
Typical diffraction patterns of beta zeolites were observed for
all the samples, which revealed that the structure of the beta
zeolite was well preserved during the post‐synthesis process;
these results are consistent with those reported previously
[9,10]. The textural properties of the beta zeolites utilized in
this study are presented in Table 1. No distinct changes in the
BET areas and micropore volumes occurred after
de‐alumination and Sn and/or Ti introduction. This result im‐
plies that the characteristics of the beta zeolites were not in‐
fluenced by the introduction of Sn and/or Ti. In addition, dis‐
tinct diffraction lines owing to the Ti and/or Sn species were
observed for the Ti‐ and/or Sn‐containing zeolites (Fig. 2(a)).
According to the ICP results (Table 1), 4.5 wt%–4.9 wt% load‐
ings of Ti or Sn can be achieved. This result indicates that a high
dispersion of the Ti and/or Sn species can be obtained using
the Ti‐ and/or Sn‐containing zeolites, which was consistent
with the TEM results.
Fig. 3. 29Si MAS NMR spectra of the as‐prepared samples under study.
2(b). After dealumination, the Si‐OH‐Al group (3600 cm–1) of
H‐Beta disappeared. Moreover, the bands at 3735 and 3520
cm–1 assigned to internal SiOH and the hydrogen‐bonded Si–OH
groups, respectively, appeared for the [Si]Beta zeolite, indicat‐
ing the formation of vacant T sites [9,10,32]. After the introduc‐
tion of the Ti and/or Sn species, a significant decrease was ob‐
served in the intensity of the Si–OH groups at 3735 (shift to
3740 cm–1) and 3520 cm–1, indicating the interactions of the Ti
and/or Sn species with silanol groups from de‐alumination.
The 29Si MAS NMR spectra of the [Si]Beta and Ti‐ and/or
Sn‐containing beta samples indicated numerous similar signals
at –103, –113 and –116 ppm (Fig. 3). The signal at –103 ppm
was due to the Q3 species (Si–OH), while the signals at –113
and –116 ppm were assigned to the Q4 species at different
sites. After the introduction of the Ti or Sn species into the
[Si]Beta zeolite, a significant decrease in the Q3 signal (SiOH)
intensity was observed, indicating an interaction between the
Ti and/or Sn species with SiOH and the formation of the
Si–O–Ti or Si–O–Sn bonds.
The distribution of the Ti and/or Sn sites in the [Si]Beta ma‐
trix was further studied by electron microscopy. The
bright‐field HRTEM image of the TiSn‐Beta sample exhibited
clear lattice fringes of the zeolite matrix; however, there was no
The DRIFT spectra of the zeolite samples are shown in Fig.
(a)
(b)
H‐Beta
TiSn‐Beta
[Si]Beta
Sn‐Beta
Ti‐Beta
[Si]Beta
Ti‐Beta
Sn‐Beta
TiSn‐Beta
H‐Beta
4000
3800
3600
3400
3200
10
20
30
2
40
50
Wavenumber (cm1)
Fig. 2. (a) XRD patterns of the samples; (b) DRIFT spectra of the samples in the hydroxyl stretching vibration region.

Qifeng Lei et al. / Chinese Journal of Catalysis 42 (2021) 1176–1184
1179
(a)
(c)
(b)
TiSn‐Beta
TiSn‐Beta
Sn‐Beta
Ti‐Beta
SnO2
TiO
2
475
470
465
460
455
450
500
495
490
485
480
Binding energy (eV)
Binding energy (eV)
Fig. 4. (a) HAADF‐STEM images with the corresponding element mapping of TiSn‐Beta zeolite; Ti 2p (b) and Sn 3d (c) XPS of Ti‐ and Sn‐ containing
samples.
clear evidence of the Ti‐Sn species in the zeolite (Fig. S1). In
addition, a homogeneous distribution of the Ti and Sn species
and their partial close contact can be observed based on the
element mapping results (Fig. 4(a)).
to the framework Sn species with tetrahedral coordination and
in a closed arrangement. In addition, the 119Sn MAS NMR signal
at ˗420 ppm indicated that few Sn species with tetrahedral co‐
ordination but with an open arrangement might be present;
however, this signal was overlapped by the broad signal at
–443 ppm. Upon hydration, these signals disappeared; howev‐
er, new signals appeared at ˗689 ppm, corresponding to the Sn
sites octahedrally coordinated with water molecules [38,39].
Additionally, 119Sn MAS NMR signals (119Sn = ˗605 ppm) at‐
tributed to bulk SnO2 appeared neither for dehydrated nor
The existing states of the Ti and/or Sn species were ana‐
lyzed by XPS. As shown in Fig. 4(b), the binding energies of
460.1 and 465.7 eV ascribed to Ti 2p3/2 and 2p1/2, respectively,
were observed for Ti‐Beta and TiSn‐Beta. These values were
significantly higher than those of bulk TiO2 (458.4 and 464.2
eV) because of Si–O–Ti bond formation. Similarly, the binding
energies of 487.8 and 496.2 eV corresponding to Sn 2p3/2 and
2p1/2 higher than those of bulk SnO2 (486.0 and 494.5 eV) were
observed for the Sn‐Beta and TiSn‐Beta samples (Fig. 4(c)). As
reported previously [9,10,33], the higher binding energies of
the Ti and Sn atoms in the zeolites than those of the corre‐
sponding bulk oxides indicate the possible formation of tetra‐
hedrally coordinated Ti(IV) and Sn(IV) species; however, it
requires further investigation.
‐443
(b)
218
207
(a)
Sn‐Beta
‐685
Dehydration
Ti‐Beta
Sn‐Beta
Hydration
TiSn‐Beta
Dehydration
Further, UV‐vis and 119Sn MAS NMR spectroscopy were em‐
ployed to investigate the nature of the Ti and/or Sn species in
the beta zeolite framework. As shown in Fig. 5(a), the dehy‐
drated Ti‐Beta zeolite exhibited a dominant UV‐vis absorption
band at approximately 218 nm. According to the previous
studies [12,34,35], this band could be due to the framework Ti
species with tetrahedral coordination. The Sn‐Beta sample
exhibited a sharp absorption band centered at approximately
207 nm, which is attributed to the tetrahedrally coordinated Sn
species in the zeolite framework [36,37]. In addition, a strong
UV‐vis band centered at approximately 207 nm with a large
shoulder at approximately 218 nm appeared for the TiSn‐Beta
zeolite. These bands were assigned to the tetrahedrally coor‐
dinated Ti and Sn species in the beta zeolite framework, re‐
spectively. These results indicated that the Ti and Sn species in
the dehydrated TiSn‐Beta zeolite mainly existed in tetrahedral
coordination within the zeolite framework; these results are in
accordance with the results from XPS analyses. As shown in Fig.
5(b), a significantly distinct 119Sn MAS NMR signal at –443 ppm
appeared for the dehydrated Sn‐Beta and TiSn‐Beta zeolites.
Based on the previous studies [38], these signals were assigned
Hydration
TiSn‐Beta
200
300
Wavelength (nm)
1.6
400
500
600
0
‐200 ‐400 ‐600 ‐800 ‐1000

119Sn/ppm
1.3
41
52
(d)
(c)
Ti‐Beta
Ti‐Beta
+ NH3
+ NH3
*
*
difference
54
53
Sn‐Beta
Sn‐Beta
*
*
difference
TiSn‐Beta
TiSn‐Beta
+ NH3
+ NH3
*
*
*
*
difference
[Si]Beta
[Si]‐Beta
difference
0
‐5
10
5

150
100
50
31P/ppm
0
‐50
1H/ppm

Fig. 5. (a) UV‐vis spectra of the dehydrated samples; (b) 119Sn MAS NMR
spectra of the Sn‐Beta and TiSn‐Beta samples before and after hydra‐
tion; (c) H MAS NMR spectra of the dehydrated samples before (top)
and after (middle) ammonia loading. The difference spectra (bottom)
were obtained by subtracting the top spectra from the middle spectra;
(d) 31P MAS NMR spectra of TMPO‐loaded samples.
1

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Qifeng Lei et al. / Chinese Journal of Catalysis 42 (2021) 1176–1184
hydrated Sn‐Beta and TiSn‐Beta samples, excluding the for‐
mation of extra‐framework SnO2 species. Based on these ex‐
perimental results, we concluded that most of the Ti and Sn
species are incorporated into the beta zeolite framework and
exist in a highly isolated state in the TiSn‐Beta zeolite obtained
from post‐synthesis modification.
cant decrease in the epoxide yield with an increase in the
1,2‐diol yield was observed, indicating that the sequential reac‐
tions of alkene epoxidation and epoxide hydration were
achieved to a certain extent in the presence of Sn‐Beta. Moreo‐
ver, it was revealed that the Lewis acidic Sn species are the
main active sites for epoxide hydration, and this result is also
supported by the results of cyclohexene oxide hydration (Fig.
S2). However, a large number of by‐products originating from
the self‐condensation of 1,2‐diol or the excessively oxidized
species, as shown in Fig. 6(a), were also obtained over the
mixed catalysts simultaneously. For the bifunctional Ti‐ and
Sn‐containing beta zeolites, TiSn‐Beta, 1,2‐diol was observed to
be the main product, indicating that the close distance between
the two functional sites in the confined space of the zeolite
leads to its desirable activity [43]. Therefore, the tandem catal‐
ysis of cyclohexene to 1,2‐cyclohexanediol can be efficiently
achieved in the confined cages of the TiSn‐Beta zeolite.
Moreover, the introduction sequences of the Ti and Sn spe‐
cies were observed to exhibit a significant influence on their
catalytic activity. TiSn‐Beta zeolite with the co‐introduction of
the Ti and Sn species exhibited the highest catalytic perfor‐
mance, while Ti‐Sn‐Beta and Sn‐Ti‐Beta zeolites with the step‐
wise introduction of the Ti and Sn species exhibited relatively
low catalytic activities (Fig. 6(a)). This can be explained by the
formation of inert extra‐framework SnO2 or TiO2 species during
stepwise introduction, as shown in the UV‐vis results (Fig. S3).
These results confirm that co‐introduction ensures the for‐
mation of isolated, tetrahedrally coordinated Ti and Sn sites in
the zeolite framework, while stepwise introduction leads to the
extra‐framework oxide formation of the second element, the
SnO2 or TiO2 species, which was significantly less active in the
conversion of cyclohexene to 1,2‐cyclohexanediol. To investi‐
gate the effect of the catalyst support on the performance of the
TiSn‐loaded catalysts in the conversion of cyclohexene to
1,2‐cyclohexanediol, we prepared the TiSn‐SiO2 and
TiSn‐SBA‐15 catalysts and compared their catalytic activities
with that of the TiSn‐Beta catalyst (Fig. S4). The results clearly
indicate that the supports exhibited a strong impact on the
performance of the TiSn‐modified catalysts during the conver‐
sion of cyclohexene to 1,2‐cyclohexanediol. Because
TiSn‐SBA‐15 and TiSn‐SiO2 possess large pore windows but do
not possess confined cages, the cyclohexene molecules can
easily diffuse out of the catalyst; therefore, they exhibit signifi‐
cantly lower catalytic activity. With TiSn‐Beta, the cyclohexene
molecules can be subjected to diffusion resistance within the
zeolite channels or cages, which can significantly improve cy‐
clohexene conversion.
Further, the hydroxyl groups in the Ti‐ and Sn‐containing
1
beta samples were studied by H MAS NMR spectroscopy. As
1
shown in Fig. 5(c), a high‐intensity H MAS NMR signal in the
range 1.3–1.6 ppm, attributed to the Si–OH groups, appeared
for all the samples. Upon ammonia loading, additional 1H MAS
NMR signals were not observed in any of the samples. Howev‐
1
er, the intensities of the H MAS NMR signals at 1.3–3.0 ppm
were enhanced after ammonia loading. According to the previ‐
ous studies [40−42], the enhanced 1H MAS NMR signals are due
to Lewis acid sites coordinated with the ammonia probe mole‐
cules. In addition, no Brønsted acid sites that could produce
ammonium ions (1H = 6–7 ppm) after ammonia protonation
were observed. These results reveal that only Lewis acid sites
are formed after introducing Ti and/or Sn species into the
[Si]Beta matrix. The densities of the strong Lewis acid sites, as
evaluated based on the intensities of the 1H MAS NMR signals at
1.3–3.0 ppm, were 0.25, 0.13, and 0.36 mmol/g for the Ti‐Beta,
Sn‐Beta, and TiSn‐Beta samples, respectively. The results indi‐
cated that the amount of Lewis acid sites in the TiSn‐Beta zeo‐
lite was close to the sum of the amounts of acid sites available
in the Ti‐Beta and Sn‐Beta zeolites, thereby revealing the suc‐
cessful preparation of TiSn‐Beta zeolite. As further proof of the
formation of Lewis acid sites in the Ti‐Beta, Sn‐Beta and
TiSn‐Beta samples under study, the 31P MAS NMR spectra of the
dehydrated samples loaded with TMPO probe molecules were
examined. As shown in Fig. 5(d), a strong signal at 1P = 41 ppm
ascribed to the TMPO molecules adsorbed on the weak acidic
Si‐OH groups appeared for all the samples. Additional 31P MAS
NMR signals at 52–54 ppm attributed to the TMPO molecules
coordinated with the Lewis acid sites appeared [42]; this result
is consistent with the 1H MAS NMR results.
3.2. Catalytic performance and stability
The catalytic performance of the as‐prepared zeolites was
examined for the direct conversion of cyclohexene to
1,2‐cyclohexanediol using hydrogen peroxide solution (i.e.,
H2O2), which functions as an oxidant, for alkene epoxidation as
well as the nucleophile (i.e., H2O) for epoxide hydration. As
shown in Fig. 6(a), almost no products could be observed over
Sn‐Beta zeolite, indicating that Sn‐Beta is inactive in the con‐
version of cyclohexene to 1,2‐cyclohexanediol. In contrast, con‐
siderable cyclohexene conversion with different product dis‐
tributions could be obtained over Ti‐containing beta zeolite
catalysts (i.e., Ti‐Beta, Ti‐ and Sn‐Beta, Ti‐Sn‐Beta, Sn‐Ti‐Beta,
and TiSn‐Beta). For Ti‐Beta zeolite, a high epoxide yield with a
low amount of 1,2‐diols could be achieved, indicating that Ti
species are beneficial for alkene epoxidation; this result is con‐
sistent with the results in previous studies [31]. Additionally,
the corresponding catalytic performance of the mixed Ti‐Beta
and Sn‐Beta zeolites was evaluated for comparison. A signifi‐
Owing to its outstanding activity, the bifunctional TiSn‐Beta
is regarded as a promising solid catalyst for the direct conver‐
sion of alkene to 1,2‐diol. Further, the effects of the reaction
parameters on the catalytic activity of TiSn‐Beta zeolite were
examined (Table S1, Figs. 6(b,c)). Table S1 presents the effects
of solvents on the catalytic activity of TiSn‐Beta catalysts in the
direct conversion of cyclohexene to 1,2‐cyclohexanediol. Apro‐
tic solvents, such as acetone, 1,4‐dioxane, and γ‐valerolactone,
with the lone pairs of electrons are beneficial for the formation
of 1,2‐diols with acetone being the most suitable. Further, the

Qifeng Lei et al. / Chinese Journal of Catalysis 42 (2021) 1176–1184
1181
60
40
20
0
60
(a)
(b)
Epoxide
Diol
Others
Epoxide
Diols
Others
40
20
0
100
80
60
40
20
0
(c) 100
(d)
80
60
40
20
0
Cyclohexene conversion
Diols selectivity
Epoxide selectivity
313
323
333
343
353
0
1
2
3
4
5
6
Temperature (K)
Time‐on‐stream (h)
Fig. 6. (a) Product yields over the different catalysts (TiSn‐Beta: Ti and Sn were co‐introduced; Ti‐Sn‐Beta: Ti was first introduced followed by Sn;
Sn‐Ti‐Beta: Sn was first introduced followed by Ti). (b) Product yields during cyclohexene conversion over TiSn‐Beta zeolite with different Ti and Sn
weight loadings (2.5%:7.5%, 5%:5%, and 7.5%:7.5%). (c) Effect of reaction temperature on cyclohexene conversion and 1,2‐cyclohexanediol selec‐
tivity catalyzed by TiSn‐Beta zeolite. (d) Cyclohexene conversion and diol selectivity during cyclohexene conversion over TiSn‐Beta zeolite, plotted as
a function of time‐on‐stream. Reaction conditions: 5 mmol cyclohexene; 2.5 mL of MeCN ((a) and (b)), MeCOMe ((c) and (d)); 5 mmol H2O2; 0.1 g of
catalyst; temperature 333 K ((a), (b), and (d)), 313−353K (c); and time 2 h ((a), (b), and (c)), 0−6 h ((d)).
reaction temperature exhibited a significant influence on the
catalytic performance. With an increase in the reaction tem‐
perature, cyclohexene conversion gradually increased with
decreased selectivity toward 1,2‐diol (Fig. 6(c)). This may be
due to the occurrence of the side reactions (e.g.,
self‐condensation) of 1,2‐diol at the external surface of the
TiSn‐Beta catalyst at high reaction temperatures. The effect of
the amount of H2O2 on the catalytic performance of the
TiSn‐Beta zeolites was also investigated, and the results are
shown in Fig. S5. With an increase in the amount of H2O2, cy‐
clohexene conversion gradually increased with a simultaneous
decrease in the selectivity toward diols. Considering the re‐
quirements of high yield and high selectivity for diols, the initial
substrate/H2O2 ratio was set to 1.
served (Fig. 6(d)). Simultaneously, a slight decrease in 1,2‐diol
selectivity was observed, which might be because the Ti and/or
Sn active sites were covered by bulky by‐products, which can
be verified by recycling experiments. After the reaction, the
catalyst was separated from the reaction mixture by centrifu‐
gation, washed with acetone, and dried at 353 K overnight. The
recycled catalyst was used for the next cycle under the same
reaction conditions. A sharp decrease in cyclohexene conver‐
sion from 74.6% to 50.1% occurred after four reaction cycles
(Fig. 7(a)). This decrease clearly indicates that the bulky
by‐products cannot be completely removed by simple washing.
However, when the spent TiSn‐Beta catalyst was regenerated
by calcination at 873 K in flowing air, the complete recovery of
the catalytic activity with almost no changes in cyclohexene
conversion and 1,2‐diol selectivity was achieved after four cy‐
cles (Fig. 7(b)). In addition, the possibility of Ti and Sn leaching
as well as structural and textural property changes of the zeo‐
lite catalyst during the reaction and regeneration could be ex‐
cluded according to the ICP, BET, XRD, and UV‐vis results (Ta‐
Under identical reaction conditions, that is, at a reaction
temperature of 333 K and with acetone as the solvent, the ki‐
netic plots of cyclohexene conversion exhibited enhanced cy‐
clohexene conversion within the short time‐on‐stream of 3 h;
thereafter, a stable cyclohexene conversion of ~80% was ob‐

1182
Qifeng Lei et al. / Chinese Journal of Catalysis 42 (2021) 1176–1184
100
(b)
100
80
60
40
20
0
(a)
80
60
40
20
0
1
2
3
4
1
2
3
4
Cycle number
Cycle number
(c)
(d)
Fresh sample
Fresh sample
Sample after 4 cycles
Sample after 4 cycles
10
20
30
40
50
200
300
400
500
600
2
Wavelength (nm)
Fig. 7. Recycling tests of cyclohexene conversion catalyzed by TiSn‐Beta zeolite without (a) and with calcination (b). Reaction conditions: 5 mmol
cyclohexene, 2.5 mL of MeCOMe, 5 mmol H2O2, 0.1 g of catalyst, temperature 333 K, and time 2 h. XRD patterns (c) and UV‐vis spectra (d) of the fresh
and spent TiSn‐Beta catalyst after 4 cycles.
ble 1, Figs. 7(c,d)).
zeolite can efficiently integrate alkene epoxidation and epoxide
hydration in tandem in one reactor. The zeolite confinement
Bifunctional TiSn‐Beta zeolite was observed to be an effi‐
cient solid Lewis acid catalyst for the direct conversion of cy‐
clohexene to 1,2‐cyclohexanediol. The excellent catalytic per‐
formance, originating from the bifunctional Lewis acid Ti and
Sn sites confined in zeolite cages, as well as the desirable recy‐
clability demonstrate the potential of this material for the tan‐
dem conversion of alkenes to 1,2‐diols. As expected, high con‐
version can be achieved for most cyclic alkene substrates, while
the selectivity for 1,2‐diols differs significantly (Table 2). For
unsaturated ketones, the C=O groups can adsorb on the strong
Lewis acidic Sn sites and make them inaccessible for further
epoxide hydration; thus, high selectivity for epoxide could be
achieved. Furthermore, a decrease in the conversion of the
substrates with substituted methyl groups was observed, im‐
plying that steric constraints on the substrate should be con‐
sidered.
Table 2
Catalytic performance of TiSn‐Beta for the conversion of various al‐
kenes.
Selectivity (%)
Conversion
Substrate
(%)
Epoxide
1,2‐Diol
Others
6.2
74.6
0.4
93.4
57.5
79.8
1.2
3.7
78.6
70.2
20.2
26.1
46.1
71.4
6.7
95.8
91.5
32.6
4.2
7.4
0
4. Conclusions
1.1
In summary, a tandem Lewis acid catalysis procedure with‐
in zeolite cages was successfully developed as a novel approach
for the one‐pot conversion of alkenes to 1,2‐diols. While hy‐
drogen peroxide solution can function as the oxidant (H2O2) for
alkene epoxidation and nucleophile (H2O) for epoxide hydra‐
tion, the isolated Lewis acid Ti and Sn sites in the TiSn‐Beta
22.1
45.3
Reaction conditions: 5 mmol substrate, 2.5 mL of MeCOMe, 5 mmol
H2O2, 0.1 g of catalyst, 333 K, and 2 h.

Qifeng Lei et al. / Chinese Journal of Catalysis 42 (2021) 1176–1184
1183
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Chin. J. Catal., 2021, 42: 1176–1184 doi: 10.1016/S1872‐2067(20)63734‐2
Tandem Lewis acid catalysis for the conversion of alkenes to
1,2‐diols in the confined space of bifunctional TiSn‐Beta
zeolite
HO
R1
OH
R2
C
R1
C
R2
Qifeng Lei, Chang Wang, Weili Dai *, Guangjun Wu, Naijia Guan,
Michael Hunger, Landong Li
Nankai University, China; University of Stuttgart, Germany
O
R1
R2
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tion in tandem to achieve the selective conversion of alkenes into
1,2‐diols with a high selectivity of >90%.
Ti
Sn

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双功能TiSn-Beta分子筛限域的串联Lewis酸催化烯烃生成1,2-二醇
雷琦锋a, 王 畅a, 戴卫理a,*, 武光军a, 关乃佳a,b, Michael Hunger c, 李兰冬a,b
a南开大学材料科学与工程学院, 国家新材料研究院, 天津300350, 中国
b南开大学先进能源材料化学教育部重点实验室, 化学化工协同创新中心, 天津300071, 中国
c斯图加特大学化学技术研究所, 斯图加特70550, 德国
摘要: 1,2-二醇主要由环氧化合物水合产生、广泛用于防冻剂, 聚酯树脂和医药等化学品中间体的生产. 研究表明, 具有
Lewis酸性的含锡沸石分子筛在环氧化物的水合反应中表现出优异的催化性能. 环氧化合物是众所周知的碳亲电体之一,
主要由烯烃环氧化生成. 含Ti(IV)沸石, 如Ti-Beta和TS-1, 是烯烃环氧化反应的高效催化剂. 串联催化可以将多步反应整合
为一次反应, 无需分离中间体, 有效缩短合成路线, 提高生产效率. 因此, 双功能Ti和Sn分子筛有望应用于烯烃制二醇的串
联催化反应中, 即烯烃在Ti活性位点上发生环氧化反应, 随后在Sn位点上发生水合反应, 进而串联一步生成1,2-二醇. 在沸
石分子筛中产生孤立的多功能活性位点是将多步反应整合成串联催化反应的一种有吸引力的设计策略.
本文通过简单且可规模制备的后合成路线构建了双功能TiSn-Beta分子筛, 并用于烯烃串联催化制1,2-二醇反应. 一方
面, 该反应中的过氧化氢溶液既能提供氧化剂(如H2O2)用于烯烃环氧化反应, 又能提供环氧化合物水合反应的亲核试剂(如
H2O); 同时TiSn-Beta作为一种高效双功能催化剂, 孤立的Ti和Sn活性中心可以有效地将烯烃环氧化反应和环氧化物水合
反应串联在沸石微反应器中, 实现烯烃一步转化为1,2-二醇. 另一方面, 沸石的限域效应较好地保证了烯烃环氧化和环氧
化物水合的高串联速率和目标产物的高选择性, 在最优的反应条件下, 1,2-二醇选择性高达90%以上, 收率接近70%. 本文
结果表明沸石分子筛是构筑多功能孤立活性位点的理想载体, 为其他串联反应催化剂的设计提供了良好的借鉴.
关键词: 串联催化剂; 限域效应; 分子筛; 烯烃环氧化; 环氧化合物水合
收稿日期: 2020-09-22. 接受日期: 2020-11-02. 上网时间: 2021-03-05.
*通讯联系人. 电话/传真: (022)85358536; 电子信箱: weilidai@nankai.edu.cn
基金来源: 天津市自然科学基金(18JCJQJC47400, 18JCZDJC37400); 中央高校基本科研业务经费.
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/journal/chinese-journal-of-catalysis).