Improved postsynthesis strategy to Sn-beta zeolites as lewis acid catalysts for the ring-opening hydration of epoxides

Article abstract of DOI:10.1021/cs500891s

Nanocrystalline Sn-Beta zeolites have been successfully prepared via an improved two-step postsynthesis strategy, which consists of creating vacant T sites with associated silanol groups by dealumination of parent H-Beta and subsequent dry impregnation of the resulting Si-Beta with organometallic dimethyltin dichloride. Characterization results from UV-vis, XPS, Raman, and ¹¹⁹Sn solid-state MAS NMR reveal that most Sn species have been successfully incorporated into the framework of Beta zeolite through the postsynthesis process and exist as isolated tetrahedral Sn(IV) in open arrangement. The creation of strong Lewis acid sites upon Sn incorporation is confirmed by FTIR spectroscopy with pyridine adsorption. The Sn-Beta Lewis acid catalysts are applied in the ring-opening hydration of epoxides to the corresponding 1,2-diols under near ambient and solvent-free conditions, and remarkable activity can be obtained. The impacts of Lewis acidity, preparation parameters, and reaction conditions on the catalytic performance of Sn-Beta zeolites are discussed in detail.

Full text of DOI:10.1021/cs500891s

Research Article
pubs.acs.org/acscatalysis
Improved Postsynthesis Strategy to Sn-Beta Zeolites as Lewis Acid
Catalysts for the Ring-Opening Hydration of Epoxides
Bo Tang,† Weili Dai,† Guangjun Wu,† Naijia Guan,† Landong Li,*,† and Michael Hunger‡
†Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, No. 94
Weijin Road, Tianjin 300071, People’s Republic China
‡Institute of Chemical Technology, University of Stuttgart, Pfaffenwaldring 55, 70550 Stuttgart, Germany
S
* Supporting Information
ABSTRACT: Nanocrystalline Sn-Beta zeolites have been
successfully prepared via an improved two-step postsynthesis
strategy, which consists of creating vacant T sites with
associated silanol groups by dealumination of parent H-Beta
and subsequent dry impregnation of the resulting Si-Beta with
organometallic dimethyltin dichloride. Characterization results
from UV−vis, XPS, Raman, and 119Sn solid-state MAS NMR
reveal that most Sn species have been successfully incorpo-
rated into the framework of Beta zeolite through the
postsynthesis process and exist as isolated tetrahedral Sn(IV)
in open arrangement. The creation of strong Lewis acid sites upon Sn incorporation is confirmed by FTIR spectroscopy with
pyridine adsorption. The Sn-Beta Lewis acid catalysts are applied in the ring-opening hydration of epoxides to the corresponding
1,2-diols under near ambient and solvent-free conditions, and remarkable activity can be obtained. The impacts of Lewis acidity,
preparation parameters, and reaction conditions on the catalytic performance of Sn-Beta zeolites are discussed in detail.
KEYWORDS: Sn-Beta, postsynthesis, organometallic, epoxide, ring-opening hydration
have been developed to catalyze the ring-opening hydration of
epoxides. Despite current achievements, efficient and recyclable
catalytic systems that can work under mild reaction conditions
are yet to be explored.
1. INTRODUCTION
Epoxides are important and well-known carbon electrophiles
that can be industrially produced from the corresponding olefin
precursors.1 As a result of the three-membered heterocyclic ring
strain of the epoxides, nucleophiles can attack the electrophilic
carbon atom of the C−O bond and break the bond, thereby
resulting in its ring-opening. Generally, it is susceptible to
attacks by a range of nucleophiles, including nitrogen (e.g.,
ammonia, amines, and azides), oxygen (e.g., water, alcohols,
phenols, and acids) and sulfur-containing compounds, produc-
ing bifunctional molecules for great chemical industry.2,3
Especially, 1,2-diols, produced from the direct ring-opening
hydration of epoxides as the most straightforward approach, are
widely used as intermediates for the manufacture of polyester
resins, antifreezes, cosmetics, medicines, and other products.4
The hydration of epoxides is generally carried out with a large
excess of water (H2O/epoxide = 20−25) at elevated temper-
ature (>413 K) and/or in the presence of solvents to obtain
high substrate conversion as well as desired product selectivity.
Specially, the concentration of the product in the final aqueous
solution is only 10 wt %, and huge energy is consumed for the
distillation of the product from the aqueous solution, making
the epoxide hydration one of the most cost- and energy-
intensive processes in the chemical industry. Various acids and
bases (e.g., cation- and anion-exchange resins,5−8 quaternary
phosphonium halides,9 polymeric organosilane ammonium
salts,10 macrocyclic chelating compounds,11 cyclic amine,12
supported metal oxides,13,14 and silica-based nanoreactors)15
Lewis acids represent a versatile class of catalysts that exhibit
remarkable activity in a number of important chemical
transformations.16,17 Compared with homogeneous analogues,
heterogeneous Lewis acid catalysts offer several advantages for
the development of more sustainable technology in terms of
facile downstream processing and process intensification and,
therefore, attract extensive attention. The exploitation of Sn-
Beta zeolites, Sn(IV) incorporation into the BEA framework
structure, can be viewed as a key breakthrough in the search of
environmental-friendly solid Lewis acid catalysts for sustainable
chemistry. Sn-Beta zeolites have been successfully applied in
several very important reactions (e.g., Baeyer−Villiger
oxidations18 and Meerwein−Ponndor−Verley reductions).19
A most interesting feature of Sn-Beta zeolites is that the Lewis
acid sites processed are tolerant to polar and protic solvents
(e.g., water and alcohols) being essentially different from most
conventional solid Lewis acid catalysts.17 Accordingly, remark-
able catalytic performance of Sn-Beta zeolite in carbohydrate-
related reactions in aqueous media (e.g., sugar isomer-
Received: April 28, 2014
Revised: July 15, 2014
Published: July 23, 2014
© 2014 American Chemical Society
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ization,20−22 epimerization,23 and Cannizzaro-type reac-
that, it was calcined under flowing air at 823 K for 6 h to derive the
final product, denoted as Sn-Beta-X where X indicates the nSi/nSn ratio.
Zr-Beta and Ti-Beta were prepared via procedures similar to that for
Sn-Beta except that the organometallic precursor (CH3)2SnCl2 was
replaced by Cp2ZrCl2 and Cp2TiCl2, respectively. In addition, the Si-
Beta supported oxides (i.e., SnO2/Si-Beta, ZrO2/Si-Beta, and TiO2/Si-
Beta) were prepared via mechanical mixing of Si-Beta with
corresponding metal oxides, followed by calcination at 823 K for 12
h to derive the corresponding products.
2.2. Characterization Techniques. X-ray diffraction (XRD)
patterns of samples were recorded on a Bruker D8 diffractometer with
Cu Kα radiation (λ = 1.54184 Å) from 5° to 40° with a scan speed of
2θ = 6.0°/min.
tions)24,25 has been recently reported.
Generally, Sn-Beta zeolites can be prepared by the direct
incorporation of Sn species into a BEA framework during
hydrothermal synthesis.26 Considerable long time scale of up to
40 days is required to realize a full crystallization to overcome
the negative effect from the existence of Sn(IV), and additive
fluoride is commonly employed to facilitate the crystallization
process,23,27 resulting in both a lack of reproducibility and
feasibility for large-scale synthesis. Accordingly, a much more
rapid hydrothermal strategy has been reported to synthesize
Sn-Beta material in 2 days using a modified seeding method,
but it still involves the environmentally unfriendly HF media
and results in the formation of large crystallites.28 In this
context, alternative postsynthesis strategies have been proposed
for the preparation of Sn-Beta zeolites,29−32 in which a
commercial Beta zeolite is dealuminated to generate silanols
prior to grafting Sn species. Wu et al. conducted the
incorporation of Sn ions into the vacant tetrahedral sites of
Beta by the so-called “atom-planting method” using SnCl4
vapor;29 however, formation of large quantities of inactive bulk
SnO2 could not be avoided. Alternatively, a simple grafting
procedure in isopropanol at elevated temperature was reported
by Sels et al. for preparing a highly active Sn-Beta zeolite,30 in
which the incorporation of a significant amount of Lewis acid is
still challenging. More recently, the exploration of solid-state
ion exchange route involving dealuminated Beta and tin(II)-
acetate by the Hermans group provides a potential strategy to
prepare Sn-Beta with high Sn loading up to 10 wt % without
the formation of bulk SnO2,31 which could be further extended
to the preparation of Ti-Beta and Zr-Beta.32
Scanning electron microscopy (SEM) images of samples were
obtained on a Hitachi S-4800 microscope.
The chemical compositions of samples were determined by
inductively coupled plasma emission spectrometry (ICP-AES) on a
Thermo IRIS Intrepid II XSP atomic emission spectrometer.
The surface areas and pore volumes of the calcined samples were
measured by means of nitrogen adsorption at 77 K on a
QuantachromeiQ-MP gas adsorption analyzer. Before the nitrogen
adsorption, samples were dehydrated at 473 K for 6 h. The total
surface area was calculated via the Brunauer−Emmett−Teller (BET)
equation, and the microporous pore volume was determined using the
t-plot method.
Diffuse reflectance ultraviolet−visible (UV−vis) spectra of dehy-
drated samples were recorded against BaSO4 in the region of 200−700
nm on a Varian Cary 300 UV−vis spectrophotometer.
Raman spectra of dehydrated samples were recorded on a high-
resolution, dispersive Raman spectrometer system (Horiba-JobinY-
vonLabRam-IR) with the He−Cd laser (Kimmon Electric, model
IK57511-G; 441.6 nm output power of 110 mW, sample power of 28
mW).
X-ray photoelectron spectra (XPS) of dehydrated samples were
recorded on a Kratos Axis Ultra DLD spectrometer with a
monochromated Al Kα X-ray source (hν = 1486.6 eV), hybrid
(magnetic/electrostatic) optics, and a multichannel plate and delay
line detector (DLD). All spectra were recorded by using an aperture
slot of 300 × 700 μm. Survey spectra were recorded with a pass energy
of 160 eV and high-resolution spectra with a pass energy of 40 eV.
Accurate binding energies ( 0.1 eV) were determined with respect to
the position of the adventitious C 1s peak at 284.8 eV.
Diffuse reflectance infrared Fourier transform (DRIFT) spectra of
samples were measured on a Bruker Tensor 27 spectrometer with 128
scans at a resolution of 2 cm−1. A self-supporting pellet made of
sample material was placed in the reaction chamber and pretreated in
flowing dry air at 673 K for 1 h. The spectra were recorded in dry air
against KBr as background.
Fourier transform infrared (FTIR) spectra of pyridine adsorption
were also collected on the Bruker Tensor 27 spectrometer. A self-
supporting pellet made of the sample was placed in the flow cell and
evacuated under reduced pressure at 693 K for 4 h. After cooling to
room temperature, the samples were saturated with pyridine vapor and
then evacuated at 473, 573, or 623 K for 30 min. Spectra were
recorded at evacuation temperature in the 4000−650 cm−1 range by
using coaddition of 32 scans. The amount of the Lewis acid sites in
samples was determined from the integral intensity of characteristic
band at ca. 1450 cm−1 using the molar extinction coefficients of
Emeis.33
Inspired by the above-mentioned approaches, we, herein, will
report an improved two-step postsynthesis procedure for the
preparation of Sn-beta zeolites based on the dry impregnation
of dealuminated zeolite Beta with organometallic precursor
dimethyltin dichloride. The formation of isolated Sn(IV) Lewis
acid sites in nanocrystalline Sn-Beta zeolites is guaranteed, and
the obtained Sn-Beta zeolites are, for the first time, applied in
the ring-opening hydration of epoxides to the corresponding
1,2-diols. The excellent catalytic performance obtained
demonstrates the great potential of heterogeneous Lewis acid
(e.g., Sn-Beta) in the ring-opening addition of epoxides.
2. EXPERIMENTAL SECTION
2.1. Sample Preparation. Epichlorohydrin (99%), styrene oxide
(>98%), cyclohexene oxide (>98%), ethylene oxide (99%), propylene
oxide (>99%), epoxybutane (>98%), dimethyltin dichloride (98%),
titanocene dichloride (>99%), and zirconocene dichloride (>99%)
were purchased from Alfa Aesar and used as received without further
purification.
Sn-Beta zeolites were prepared through a two-step postsynthesis
procedure, which consisted of the dealumination of parent H-Beta and
then incorporation of Sn species into the framework of dealuminated
Si-Beta via dry impregnation and calcination. Briefly, commercial Beta
zeolite with nominal nSi/nAl ratio of 13.5 (Sinopec Co.) was stirred in a
13 mol L−1 nitric acid aqueous solution (20 mL gzeolite−1) at 373 K
overnight to obtain a dealuminated Beta (Si-Beta). The powder was
filtered, washed thoroughly with deionized water, and dried at 353 K
overnight. Before the incorporation of Sn, the sample was pretreated at
473 K overnight under vacuum to remove physisorbed water.
Afterward, 1.0 g of the solid powder was finely ground with
appropriate amount of (CH3)2SnCl2 in the glovebox to achieve an
intimate mixture with the nSi/nSn ratio of 40, 50, and 100, respectively.
The solid mixture was put into a tubular reactor, then sealed and
heated to 823 K for 6 h (heating rate at 5 K/min) under vacuum. After
The 119Sn solid-state magic angle spinning nuclear magnetic
resonance (MAS NMR) experiments were performed on a Bruker
Avance III spectrometer at resonance frequencies of 149.5 MHz and
with a sample spinning rate of 8 kHz. 119Sn MAS NMR spectra of Sn-
Beta were recorded on dehydrated samples, which were obtained by
heating the samples at 673 K at a pressure below 10−2 Pa for 6 h.
2.3. Catalytic Evaluation. The catalytic processes for epoxides
hydration were performed in a 25 mL round-bottom glass flask with a
cryogenic-liquid condenser under atmospheric pressure. The vessel
was charged with the mixture of 10 mmol of epoxide, 20 mmol of
H2O, and 0.1 g of catalyst, and mixed vigorously by a magnetic stirrer.
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After the reaction, samples of the reaction were qualitatively analyzed
by Shimadzu 2010 GC (Agilent HP-5MS column, 30 m × 0.25 mm ×
0.25 μm; FID detector) with octanol as the internal standard. GC
peaks were identified by comparison with the retention time of known
standard samples, and also by means of Shimadzu GC-MS QP2010 SE
equipped with the Agilent HP-5MS column. Carbon balance of over
95% was obtained for all experiments.
A hot-filtration test was performed to confirm the heterogeneous
nature of the catalyst under the identical conditions employed for the
catalytic hydration reaction. The solid was removed from the reaction
mixture after 1 h by centrifugation and subsequently filtered over a
syringe filter. Then, the filtrate was allowed to react for another 5 h.
To verify the potential recyclability of Sn-Beta catalyst, recycling
tests were performed. After reaction completion, the sample was
centrifuged at 6000 rpm to deposit the solid catalyst. The solid catalyst
was directly reused for next cycle. After the fourth cycle, the solid
catalyst was washed with 5 mL of acetone for three times, dried at 393
K overnight, and subjected to calcination at 823 K for 6 h for reuse in
the next cycle.
Si-Beta (2θ = 22.80°) to 3.927 Å for Sn-Beta-100 (2θ = 22.58°)
and then to 3.945 Å for Sn-Beta-50 (2θ = 22.52°)). This is a
clear evidence of BEA framework expansion, indicating the
successful incorporation of Sn species into the framework of
Beta zeolite.
The well-preserved miroporous BEA structure after deal-
umination and Sn incorporation can be further confirmed by
the nitrogen physisorption (Figure S1), in which similar and
typical type I adsorption/desorption isotherms can be observed
for all the samples. On the other hand, the enhanced nitrogen
uptake is clearly observed at high relative pressures p/p0 > 0.9,
which can be ascribed to the contributions of a large amount of
interparticle voids originating from the disordered agglomer-
ation of small crystallites of the zeolite Beta, and further
confirmed by SEM (Figure S2). The postsynthesized Sn-Beta-
50 samples appear as uniform nanoparticles with the average
size of 80 nm, over 1 order of magnitude lower than the typical
Sn-Beta samples prepared via hydrothermally route in the
presence of HF.27,29 Such small crystallites of the postsynthe-
sized Sn-Beta zeolites are presumed to encounter less diffusion
limitation in transferring bulky substrate molecules.
3. RESULTS AND DISCUSSION
3.1. Incorporation of Sn into Zeolite Framework. To
verify the possible structure changes during the postsynthesis
procedures, the XRD patterns of the parent H-Beta and post-
treated samples are collected in Figure 1. All the samples show
Table 1 summarizes the physicochemical properties of H-
Beta, Si-Beta, and Sn-Beta samples. All the zeolite materials
Table 1. Physicochemical Properties of H-Beta, Si-Beta, and
Metal-Containing Zeolites
b
c
surface area
micropore volume
a
a
catalyst
H-Beta
Si/Al
13.5
Si/M
(m2 g−1
)
(cm3 g−1
)
590
620
600
580
570
573
575
0.204
0.210
0.202
0.200
0.198
0.197
0.198
Si-Beta
>1800
>1800
>1800
>1800
>1800
>1800
Sn-Beta-100
Sn-Beta-50
Sn-Beta-40
Zr-Beta-50
Ti-Beta-50
97.0
51.3
41.8
51.9
52.1
a
b
Determined by ICP. Specific surface area obtained by BET method.
c
Calculated from t-plot.
have the similar BET surface area (570−590 m2 g−1) and
micropore volume (0.198−0.204 cm3 g−1), suggesting the
textural properties of Beta zeolite are well preserved after
dealumination and Sn incorporation. Upon treatment with
concentrated HNO3, the nSi/nAl ratio of H-Beta sharply
increases from 13.5 to over 1800 of dealuminated Si-Beta,
suggesting the latter is essentially free of Al. After incorporation
of Sn, the actual nSi/nSn ratio determined by ICP is identical to
the desired expected value, confirming the so-called dry
impregnation method is an efficient strategy for the preparation
of Sn(IV)-containing Beta zeolites.
Figure 1. XRD patterns of H-Beta, Si-Beta, Sn-Beta, and SnO2
samples.
similar diffractions peaks characteristic of typical BEA topology
with the comparable intensity, ruling out the obvious collapse
of zeolite framework during the dealumination and Sn
incorporation, in line with earlier reports.29−31 The deal-
umination and incorporation of Sn species into the zeolite
framework of Beta should be accompanied by the contraction/
expansion of the framework and therefore result in detectable
changes in the position of the diffraction peak (302) at 2θ =
22.5°, as reported by Dzwigaj et al.,34,35 considering the
difficulty in determining the unit cell parameters in Beta zeolite
due to the coexistence of several polytypes.36 As shown in
Figure 1, the d302 spacing, obtained from the corresponding 2θ
value, decreases from 3.941 Å (H-Beta, 2θ = 22.54°) to 3.897 Å
(Si-Beta, 2θ = 22.80°) through dealumination, revealing the
contraction of the BEA matrix. The d302 spacing of zeolite
samples enlarges after the incorporation of Sn and the spacing
increases with increasing the Sn loadings (i.e., from 3.897 Å for
On the basis of our previous work,37 the dealumination of H-
Beta and incorporation of Ti(IV) species were associated with
the evolution of the silanols related to the vacant sites during
postsynthesis. This phenomenon is also observed for preparing
the Sn(IV)-containing zeolites using the dry impregnation
method. The DRIFT spectrum (Figure 2) of parent H-Beta
exhibits several characteristic bands in hydroxyl stretching
region: one band at 3740 cm−1 ascribed to isolated external Si−
OH groups, one band at 3600 cm−1 ascribed bridging hydroxyls
Si−OH-Al, two bands at 3775 and 3655 cm−1 ascribed to Al−
OH groups of extra-framework aluminum species, and one
broad weak band centered at 3520 cm−1 ascribed to hydrogen-
bonded Si−OH groups.38,39 The dealumination procedure in
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coordination states of Sn species incorporated. As shown in
Figure 3, Sn-Beta-100 and Sn-Beta-50 samples both exhibit a
Figure 2. DRIFT spectra in the hydroxyl stretching vibration region of
H-Beta, Si-Beta, Sn-beta-100, and Sn-Beta-50 samples.
Figure 3. UV−vis spectra of dehydrated Sn-Beta and SnO2/Si-Beta
concentrated HNO3 solution is a well-established means to
remove aluminum atoms from the framework of, in particular,
Beta zeolite.29−31,37 The treatment of H-Beta zeolite with
concentrated HNO3 solution results in the complete
disappearance of three bands at 3775, 3655, and 3600 cm−1
associated with Al species (Figure 2), evidencing the complete
elimination of Al from the framework, consistent with the ICP
analysis results (nSi/nAl > 1800). Simultaneously, the
intensification of the band at 3735 cm−1 due to isolated
internal SiOH groups and the band at 3520 cm−1 related to
hydrogen-bonded silanol groups are clearly observed, indicating
the formation of vacant T atom sites (Scheme 1, dealumination
step), in accordance with earlier assignments.34,38,40 Dry
impregnation of the obtained Si-Beta sample with Sn precursor
(i.e., (CH3)2SnCl2) leads to a decrease in the intensity of OH
bands at 3735 and 3520 cm−1, located in the vacant T atom
sites, suggesting that Sn species react with these silanol groups
and thus are incorporated into the Beta zeolite framework
(Scheme 1, Sn incorporation step).
samples.
strong absorption band centered at ca. 48300 cm−1 (207 nm),
arising from the ligand-to-metal charge transfer from O2− to
Sn4+. This band has been generally assumed to be the
catalytically active isolated Sn species in the framework with
tetrahedral coordination,27,29−31 and the intensity of this band
increases distinctly with increasing Sn loading. For Sn-Beta-100,
no distinct bands could be observed below 40 000 cm−1 (>250
nm), ruling out the existence of extra-framework bulk SnO2
species.29,30 For Sn-Beta-50, a very small shoulder could be
observed at 35 000−40 000 cm−1, probably due to the existence
of small quantities of SnO2 species. These results indicate that
most Sn species could be incorporated into the framework of
BEA via postsynthesis and exist in the highly isolated form, in
great contrast to SnO2/Si-Beta reference sample. The absence
of crystalline SnO2 aggregates in Sn-Beta samples is further
evidenced by the invisible characteristic XRD diffraction peaks,
for example, (110) at 2θ = 26.67° (Figure 1), and Raman
vibrational modes, for example, A1g at 632 cm−1 (Figure S3).
3.2. Existence States of Sn Species in Sn-Beta. Diffuse
reflectance UV−vis analysis is first employed to understand the
Scheme 1. Schematic Representation of the Incorporation of Tetrahedrally Coordinated Sn(IV) Species into Beta Zeolite
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The existing states of Sn species in dehydrated Sn-Beta
samples are investigated by XPS, and the results are shown in
Figure 4. In the Sn 3d region, two signals positioned at 487.4
Figure 5. 119Sn MAS NMR spectra of dehydrated and hydrated Sn-
Beta and SnO2/Si-Beta samples.
Sn species in open arrangement in our postsynthesized Sn-Beta
might be active Lewis acid sites for various corresponding
catalytic applications. On the basis of the characterization
results, we further specify the scheme for the incorporation of
tetrahedrally coordinated Sn(IV) species into Beta zeolite in
open arrangement (Scheme 1).
Figure 4. XPS of Sn-Beta and SnO2/Si-Beta samples.
and 495.8 eV can be observed for both Sn-Beta-100 and Sn-
Beta-50 samples, which should be ascribed to 3d5/2 and 3d3/2
photoelectrons of tetrahedrally coordinated framework Sn
species.41,42 In contrast, binding energy values of 486.0 and
494.4 eV, arising from 3d5/2 and 3d3/2 photoelectrons of
octahedral Sn species, are clearly observed for SnO2/Si-Beta,
which is characteristic of Sn species existing in the form of
SnO2.41,42 The XPS results reveal that Sn species in Sn-Beta
samples are tetrahedrally coordinated in BEA framework via the
dry impregnation method, in accordance with the results from
UV−vis analysis.
Characterization results from UV−vis, XPS, and 119Sn MAS
NMR reveal that most Sn species have been successfully
incorporated into the framework of Beta zeolite through the
postsynthesis process and exist as isolated tetrahedral Sn in
open arrangement (in dehydrated sample). Here, the use of
organometallic dimethyltin dichloride as Sn precursor instead
of tin(II)acetate, as employed by Hermans et al.,31 could ensure
a better incorporation of Sn into BEA structure, because the
unsaturated Sn(II) species are very susceptible to oxygen
during the preparation process and the formation of oxide-like
species could be observed with a NMR signal at −604 ppm
(Figure S4). The formation of oxidelike species in Sn-Beta
would probably result in the low activity in various catalytic
applications (vide infra, section 3.4). Compared with direct
hydrothermal synthesis, the postsynthesis strategy appears to be
simpler and easier to reproduce. More importantly, it is possible
to obtain Sn-Beta nanocrystallines with high Sn loading via
postsynthesis (Typical Sn loading in Sn-Beta prepared by direct
hydrothermal synthesis is below 2.0 wt %.), which is crucial for
catalytic applications.
119Sn MAS NMR spectroscopy is further employed to
provide a direct proof on the coordination states of Sn species
in dehydrated Sn-Beta samples, and the results are shown in
Figure 5. SnO2/Si-Beta sample gives a main resonance signal at
−604 ppm, corresponding to typical SnO2 species in octahedral
coordination. The NMR resonance signals of dehydrated Sn-
Beta-100 and Sn-Beta-50 samples are observed at ca. −420
ppm, which should be originated from Sn species in a lower
coordination state (i.e., tetrahedral Sn). The tetrahedral Sn
species in the framework of BEA, Sn-Beta prepared via direct
hydrothermal synthesis, was reported to give a resonance at
−443 ppm by Corma et al.18,43 Davis et al. further reported two
resonance signals at −420 and −443 ppm in dehydrated Sn-
Beta also prepared via direct hydrothermal synthesis: the
former being ascribed to tetrahedral Sn in open arrangement
and the latter to tetrahedral Sn in close arrangement.44
Accordingly, the signal at −420 ppm observed for Sn-Beta in
this study is ascribed to framework tetrahedral Sn in open
arrangement, and the absence of signal at −604 ppm indicates
that most Sn species have been successfully incorporated into
BEA framework. It should be noted that hydration of
tetrahedrally coordinated Sn species resulted in the formation
of octahedral Sn species, as revealed by the NMR signal at ca.
−685 ppm.44 Considering that the open framework Sn species
in Sn-Beta were proposed to be the active sites in Baeyer−
Villiger oxidation of cyclic ketones,45 the framework tetrahedral
3.3. Acidic Properties of Sn-Beta. FTIR analysis with
pyridine adsorption allows a clear distinction between Brønsted
and Lewis acid sites. As a good Lewis base, pyridine molecules
can interact with the Brønsted acid sites forming pyridiniu-
mions, which are characterized by bands in the FTIR spectrum
from 1512 to 1567 cm−1. Pyridine molecules can also adsorb on
the surface of Lewis acid sites through their isolated electron
pair on nitrogen atoms, characterized by bands in the FTIR
spectrum from 1423 to 1472 cm−1.39,46,47 Both Brønsted acid
sites and Lewis acid sites can be observed for H-Beta samples,
as shown in Figure S5. Although all the acid sites could be
removed after complete dealumination (Si-Beta). The sub-
sequent incorporation of metal ions (i.e., Sn4+, Zr4+, and Ti4+)
into Beta framework could create some Lewis acid sites, as
confirmed by the FTIR bands at ∼1445 and 1605 cm−1 due to
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pyridine adsorption (Figure 6). The pyridine adsorbed on
Lewis sites could survive high-temperature evacuation (i.e., 573
the results reported by Corma et al. from FTIR spectra of
cyclohexanone adsorption.19,48
3.4. Catalytic Properties of Sn-Beta in Various
Reactions. The catalytic performance of as-obtained Sn-Beta,
together with some other Sn-Beta samples, is initially
investigated in two typical model reactions, that is, Baeyer−
Villiger oxidation of cyclohexanone with H2O2 and conversion
of triose sugar dihydroxyacetone (DHA) into methyl lactate
(LA), and the results are shown in Tables S1 and S2.
For the Baeyer−Villiger oxidation of cyclohexanone, our Sn-
−1
−1
−1
Beta-50 exhibited a similar productivity (1.08 g
like 10 wt % Sn-Beta prepared by SSIE (1.07 g
g
h )
lactone
cat
−1
g
h ),
lactone
cat
much higher than the hydrothermally synthesized one (0.37
glactoneg−1 h−1, Table S1). As far as the productivity based on
cat
Sn is concerned, Sn-Beta-50 should be the most active one
−1
−1
(31.7 g
g
h ), followed by the hydrothermally
lactone
Sn
synthesized one (24.6 glactone g−1 h−1) and then the 10 wt %
Sn-Beta prepared by SSIE (10.7 g
Sn
−1
−1
g
h ). These results
lactone
Sn
confirm the remarkable activity of our Sn-Beta sample in the
Baeyer−Villiger oxidation reaction. The lower lactone
selectivity can be ascribed to the existence of remaining silanol
nests in the Sn-Beta-50 zeolite and such residual weak acid sites
are capable to catalyze the hydrolysis of formed lactone to 6-
hydroxycaproic acid.31 For the conversion of DHA to LA, dry
impregnated Sn-Beta-50 also exhibited very good activity as
well as perfect product selectivity, and the productivity based
on catalyst weight was comparable with the best results ever
reported (Table S2). As far as productivity based on Sn is
Figure 6. FTIR spectra of Si-Beta, Sn-Beta, Ti-Beta, and Zr-Beta
samples after pyridine adsorption and evacuation at 473 (black line),
573 (dark gray line), and 623 K (gray line). B: Brønsted acid sites; L:
Lewis acid sites.
concerned, Sn-Beta with remarkable productivity of 23.8 gLA
−1
g−1
h
was several times more active than other Sn-zeolites
Sn
prepared by different strategies.
K for Ti-Beta and 623 K for Sn-Beta and Zr-Beta), indicating
the high strength of Lewis acid sites created. A quantitative
analysis on the Lewis acid site densities of Sn-Beta, Ti-Beta, Zr-
Beta probed by FTIR spectroscopy of pyridine adsorption and
evacuation is shown in Figure 7. The Lewis acidity (i.e., the
density and strength) created by metal ions incorporation could
be determined as Sn-Beta > Zr-Beta > Ti-Beta, consistent with
In light of the above-mentioned results, it is clear that
postsynthesized nanocrystalline Sn-Beta is a good solid Lewis
acid catalyst for typical model reactions. The catalytic activities
of postsynthesized nanocrystalline Sn-Beta zeolites, together
with Zr-Beta and Ti-Beta zeolites, are further investigated in the
ring-opening of cyclohexene oxide with stoichiometric mole of
H2O. In addition, the corresponding Si-Beta supported oxides
(i.e., SnO2/Si-Beta, ZrO2/Si-Beta, and TiO2/Si-Beta) are also
prepared and evaluated for comparison. Under our reaction
conditions, no product is obtained in the absence of catalyst,
and very little product (<1%) is obtained using Si-Beta as
catalyst, as shown in Table 2. Si-Beta supported metal oxides
like SnO2/Beta, ZrO2/Beta, and TiO2/Beta also exhibit very
low activities in epoxide hydration. When Beta zeolites with
framework incorporated metal ions (i.e., Sn4+, Zr4+, or Ti4+) are
employed as catalysts, considerable cyclohexene oxide con-
version as well as high 1,2-diol selectivity can be obtained
(ether from the dimerization as major byproduct). It is clear
that the incorporation of metal ions in BEA framework is
crucial to obtain the high catalytic activity in epoxide hydration.
Under identical reaction conditions, Sn-Beta zeolites exhibit
distinct higher catalytic activity than Zr-Beta and Ti-Beta with
the same molar amount of metal ions. The activity difference
should be correlated with the Lewis acid strength of the metal-
containing zeolites in the sequence of Sn-Beta > Zr-Beta > Ti-
Beta. The stronger Lewis acid is employed, the substrate
molecule (i.e., cyclohexene oxide) is more efficiently activated
on the Lewis acid sites, therefore leading to high catalytic
activity. Moreover, Sn-Beta is obviously more active than the
conventional Brønsted acid (Amberlite IR120) and Lewis base
(Mg/Al-LDH) catalyst. Homogeneous Lewis acid catalyst
SnCl4·5H2O is more active in the hydration of cyclohexene
Figure 7. Lewis acid site densities of Sn-Beta, Ti-Beta, Zr-Beta probed
by FTIR spectroscopy of pyridine adsorption and evacuation at
different temperatures.
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nSn of 50 and more Sn species will be forced to locate at the
nonframework positions and form oxides, which show very low
catalytic activity. Besides the nSi/nSn ratio, both the preparation
route and Sn precursor show great impact on the catalytic
performance of Sn-Beta. It is seen that the outstanding catalytic
performance of Sn-Beta can only be obtained through the
precise combination of preparation route (DI, WI, or SSIE) and
Sn precursor (SnCl4 or (CH3)2SnCl2) employed, demonstrat-
ing the superiority of the two-step postsynthesis strategy
developed for the preparation of Sn-Beta in the present study.
Table 4 shows the effects of solvent employed on the
catalytic performance of Sn-Beta-50 catalyst in the ring-opening
Table 2. Hydration of Cyclohexene Oxide over Various
Catalysts
a
d
conversion selectivity
reaction rate−1
b
c
catalyst
Si/M
(%)
(%)
(mmol h−1 gcat
)
Sn-Beta-50
Zr-Beta-50
Ti-Beta-50
SnO2/Si-Beta
ZrO2/Si-Beta
TiO2/Si-Beta
SnCl4·5H2O
H-Beta
51.3
51.9
52.1
50.1
49.8
50.1
/
90.2
34.5
13.7
2.5
92.7
91.0
95.0
56.6
60.2
53.8
80.5
89.5
94.0
15.0
5.8
2.3
0.4
0.4
0.5
12.3
6.4
3.5
2.4
2.8
73.5
45.9
21.0
<1.0
<1.0
0
13.5
Amberlite IR120
Mg/Al-LDH
Si-Beta
Table 4. Influence of Solvent on the Cyclohexene Oxide
a
Hydration over Sn-Beta-50
>1800
c
no catalyst
conversion
selectivity
(%)
reaction rate−1
b
(mmol h−1 gcat
)
a
solvent
none
(%)
Reaction conditions: 10 mmol cyclohexene oxide, 20 mmol H2O, 0.1
b
g catalyst, temperature = 313 K, reaction time = 6 h. Determined by
ICP. Experimental accuracy of 2% from GC analysis. Average
reaction rate within reaction time of 6 h.
90.2
78.0
63.8
45.5
12.6
92.7
92.1
90.6
92.9
89.5
15.0
13.0
10.6
7.6
c
d
acetone
1,4-dioxane
tetrahydrofuran
acetonitrile
2.1
oxide and high reaction rate of 12.3 mmol h−1 gcat−1, and a high
1,2-diol selectivity of 80.5% can be obtained; however, the Sn
specific activity of SnCl4·5H2O is calculated to be 4294.6 mmol
h−1 molSn−1, much lower than postsynthesized Sn-Beta-50 (46
272.6 mmol h−1 molSn−1). In addition, SnCl4·5H2O will
hydrolyze during reaction in aqueous media and cannot be
recycled, which is a common fatal shortcoming of such kind of
Lewis acid catalysts.
The influence of preparation parameters (e.g., nSi/nSn ratio
and preparation methods) on the activity of Sn-Beta in
cyclohexene oxide hydration reaction is investigated. As shown
in Table 3, the nSi/nSn ratio shows great impact on the catalytic
performance of Sn-Beta in the ring-opening hydration of
cyclohexene oxide. With increasing Sn loading in Sn-Beta (nSi/
nSn from 100 to 50), the conversion of cyclohexene oxide
dramatically increases from 36.0 to 90.2%. However, only a
margin increase in the cyclohexene oxide conversion (from 90.2
to 91.5%) can be achieved with further increasing Sn loading
(nSi/nSn from 50 to 40). The turnover frequency (TOF) is
further calculated to reveal the intrinsic activity of catalyst
samples. It is seen that similar TOF could be observed for Sn-
Beta-50 (110.0 h−1) and Sn-Beta-100 (105.9 h−1), distinctly
higher than that of Sn-Beta-40 (88.4 h−1). This indicates the
approaching saturation of framework Sn in Beta zeolite at nSi/
a
Reaction conditions: 10 mmol cyclohexene oxide, 20 mmol H2O, 20
mmol solvent, 0.1 g of catalyst, temperature = 313 K, reaction time = 6
b
c
h. Experimental accuracy of 2% from GC analysis. Average reaction
rate within reaction time of 6 h
hydration of cyclohexene oxide. Generally, the role of the
solvents is to homogenize the liquid phase, thus reducing the
mass-transfer problems to accelerate the interaction of reactants
with catalyst. However, catalytic activity of Sn-Beta-50 catalyst
is significantly suppressed when polar aprotic solvents (i.e.,
acetone, 1,4-dioxane, tetrahydrofuran, and acetonitrile) are
employed in the reaction process. This should be due to the
competing adsorption of solvent molecules on the catalytic
active sites (i.e., Lewis acid sites), which might hinder the
interaction between the catalytic active sites and the reactants
molecules. In this context, the higher basicity of solvent
employed, the higher hinder effects could be observed. To
confirm this speculation, different solvents were added into the
reaction system after reaction of 1 h, and their negative impacts
could be clearly observed (Figure S6).
To assess the true heterogeneity of the Sn-Beta catalyst, hot-
filtration based leaching test is performed to determine if Sn4+
ions can leach from the catalyst into the reaction mixture and
a
Table 3. Influence of nSi/nSn Ratio and Preparation Method on the Hydration of Cyclohexene Oxide
b
c
d
e
f
g
Si/Sn Lewis acid site (μmol g−1
)
conversion (%) selectivity (%) reaction rate (mmol h−1 gcat
)
TOF (h−1
)
−1
catalyst
preparation method
Sn-Beta-40
Sn-Beta-50
Sn-Beta-100
Sn-Beta-50
Sn-Beta-50
Sn-Beta-50
Sn-Beta-50
Sn-Beta-150
DI; Me2SnCl2
DI; Me2SnCl2
DI; Me2SnCl2
DI; SnCl4
41.8
51.3
97.0
49.8
51.2
49.3
49.5
148.7
51
49
34
46
38
19
53
21
91.5 (33.5)
90.2 (30.6)
36.0 (16.8)
22.8 (11.3)
13.1 (7.2)
16.4 (8.9)
44.2 (17.3)
32.5 (14.2)
92.5
92.7
92.3
80.8
84.4
84.1
90.6
91.8
15.3
15.0
6.0
88.4
110.0
105.9
40.6
3.8
WI; Me2SnCl2
WI; SnCl4
2.2
25.9
2.7
31.9
SSIE; Sn(OAc)2
HT; SnCl4
7.4
62.2
h
5.4
126.7
a
b
Reaction conditions: 10 mmol cyclohexene oxide, 20 mmol H2O, 0.1 g of catalyst, temperature = 313 K, reaction time = 6 h. DI: dry impregnation,
c
d
WI: wet impregnation, SSIE: solid-state ion-exchange, HT: hydrothermal. Determined by ICP. Determined by FTIR spectroscopy of pyridine
adsorption followed by evacuation at 473 K. Values in parentheses indicate the conversion after reaction of 1 h, based on which the TOF is
e
f
g
calculated. Experimental accuracy of 2% from GC analysis. Average reaction rate within reaction time of 6 h. Calculated as mole of cyclohexene
oxide converted per hour per mole of Sn. Synthesized according to Corma’s method and kindly provided by Prof. Peng Wu.29
h
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possibly participate in the catalytic reaction. The catalyst is
removed from the reaction mixture by centrifugation after the
reaction time of 1 h, and the filtrate proceeds by itself for
another 5 h under identical conditions. As shown in Figure 8,
next four cycles. The possible leaching of Sn species during
reaction can be excluded from element analysis, and the
possible changes in the textural properties of Sn-Beta after
recycles could be ruled out according to the XRD and surface
area analysis. In this context, the decrease in the catalytic
activity should be due to the block of active sites by bulky
byproducts, and the removal of these species can completely
regenerate the catalyst. On the whole, postsynthesized Sn-Beta
is a potential heterogeneous catalyst for future application in
the ring-opening of epoxides if a suitable regeneration process
is employed.
3.5. Ring-Opening Hydration of Epoxides Catalyzed
by Lewis Acid. The ring-opening hydration of epoxides
catalyzed by Brønsted acid has been well studied in the past. In
the present study, it is verified that the ring-opening hydration
of epoxide can also be catalyzed by water-tolerant Lewis acid. A
direct comparison between H-Beta and Sn-Beta is made to
disclose the superiority of solid Lewis acid to conventional
Brønsted acid in this reaction. The apparent activation energies,
calculated using the Arrhenius equation, are 34.8 and 55.5 kJ
mol−1 catalyzed by Sn-Beta and H-Beta, respectively (Figure
10). The much lower apparent activation energy obtained on
Figure 8. 1,2-Diol yield in the ring-opening of cyclohexene oxide on
Sn-Beta-50, plotted as a function of reaction time. Reaction conditions:
10 mmol cyclohexene oxide, 20 mmol H2O, 0.1 g of catalyst,
temperature = 313 K.
the removal of the catalyst leads to complete termination of the
reaction, confirming the heterogeneous nature of the Sn-Beta
catalyst in the ring-opening hydration of cyclohexene oxide.
The recycling ability of Sn-Beta-50 catalyst is investigated,
and the results are shown in Figure 9. A gradual decrease in the
activity (e.g., cyclohexene oxide conversion from 90.2 to 59.5%)
could be observed within four cycles if Sn-Beta catalyst was just
centrifuged for reuse. However, if the catalyst sample was
washed with acetone and further calcined for reuse, its catalytic
activity could be fully recovered, and no significant changes in
the catalytic activity or idol selectivity could be observed in the
Figure 10. Plots of apparent activation energy in the hydration of
cyclohexene oxide catalyzed by H-Beta and Sn-Beta.
Sn-Beta than that on H-Beta reveals that solid Lewis acid site
created by Sn incorporation into BEA structure is better
candidate for the ring-opening hydration of epoxide.
On the basis of the above-mentioned results, we further
applied Sn-Beta in the ring-opening hydration of different
epoxide substrates (Table 5). It is seen that high activity can be
obtained for the hydration of all epoxide substrates, whereas the
selectivity to 1,2-diols is somewhat different. Generally, epoxide
molecules with small dynamic sizes can accumulate in the
channel of BEA zeolite and react with each other to produce
undesired byproducts, resulting in the decrease in selectivity
toward 1,2-diols to some extent. In contrast, epoxide substrates
with large dynamic size are subject to severe stereoconfinement,
which effectively suppresses the formation of byproducts, and,
therefore, improves the yield of 1,2-diols.
Figure 9. Recycling test for cyclohexene oxide hydration catalyzed by
Sn-Beta-50. Reaction conditions: 10 mmol cyclohexene oxide, 20
mmol H2O, 0.1 g of catalyst, temperature = 313 K, reaction time = 6 h.
It is also found that the epichlorohydrin undergoes catalytic
reaction more slowly than styrene oxide under the same
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a
Table 5. Hydration of Different Epoxide Substrates Catalyzed by Sn-Beta-50 Catalyst
a
b
c
Reaction conditions: 10 mmol epoxide, 20 mmol H2O, 0.1 g of catalyst. Experimental accuracy of 2% from GC analysis. Reaction was
performed in a 10 mL autoclave under N2 pressure (1.0 MPa).
reaction conditions. Similar results have been observed for the
epoxide ring-opening reaction using mesoporous SBA-15
supported Fe(III) catalysts.49 The difference in reactivity can
be attributed to the steric and electronic difference among the
substrate molecular, which accordingly affects the ring-opening
mechanism (see discussion in the Supporting Information).
catalytic activity as well as high 1,2-diol selectivity could be
achieved in the ring-opening hydration of cyclohexene oxide
with a H2O/epoxide molar ratio of 2:1 at near ambient and
solvent-free conditions. Lewis acid Sn-Beta exhibits a distinct
higher catalytic activity than H-Beta in the ring-opening
hydration of cyclohexene oxide with a much lower apparent
activation energy of 34.8 kJ mol−1 compared with 55.5 kJ mol−1
of H-Beta, whereas the catalytic activities of different metal ion
incorporated Si-Beta samples (i.e., Sn-Beta, Zr-Beta, and Ti-
Beta, are determined by their Lewis acidity. The hot-filtration
and recycling experiments reveal the true heterogeneity and
perfect stability of Sn-Beta zeolites during reaction. Sn-Beta
zeolites also exhibit good catalytic performance in the ring-
opening hydration of different epoxide substrates. All these
issues demonstrate the great potential of Sn-Beta Lewis acids
for various catalytic applications. Moreover, the preparation
parameters of Sn-Beta zeolites are most important, and the
combination of dry impregnation route and organometallic
(CH3)2SnCl2 as Sn precursor appears to be the unique and
feasible strategy to obtain the robust Sn-Beta catalysts.
4. CONCLUSION
In this study, a simple and scalable two-step synthesis
procedure has been developed to prepare Sn-Beta zeolites.
Briefly, in the first step, commercial H-Beta zeolites are treated
in concentrated nitric acid solution to result in Si-Beta, and
vacant T sites with associated silanol groups are created. In the
second step, the organometallic Sn precursor (CH3)2SnCl2
interacts with vacant T sites in the Si-Beta zeolite, and Sn
species are subsequently incorporated into the framework of
Beta zeolite upon calcination. The preparation procedure is
monitored by means of XRD and DRIFT analysis. Exclusive
tetrahedral coordinated Sn species in the BEA framework in
open arrangement are clearly evidenced by characterization
results from UV−vis, XPS, and 119Sn MAS NMR analysis. The
postsynthesis procedure developed here is considered as an
attractive strategy to fabricate Sn-Beta nanocrystallines with
high Sn loading.
The acidity of parent H-Beta and metal ions incorporated Si-
Beta is investigated by FTIR spectroscopy with pyridine
adsorption. Lewis acid sites are created on the basis of the metal
ion incorporation, and the Lewis acidity is observed to be Sn-
Beta > Zr-Beta > Ti-Beta. The obtained Sn-Beta zeolites are
studied as potential catalysts in the ring-opening hydration of
epoxides to the corresponding 1,2-diols. Remarkably high
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional characterization and catalytic results. This material is
available free of charge via the Internet at http://pubs.acs.org/
AUTHOR INFORMATION
Corresponding Author
*E-mail : lild@nankai.edu.cn. Fax/Tel.:+ 86-22-23500341.
■
Notes
The authors declare no competing financial interest.
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(29) Li, P.; Liu, G. Q.; Wu, H. H.; Liu, Y. M.; Jiang, J. G.; Wu, P. J.
Phys. Chem. C 2011, 115, 3663−3670.
ACKNOWLEDGMENTS
■
This work is financially supported by the National Natural
Science Foundation of China (21373119) and the Ministry of
Education of China (NCET-11-0251, IRT13022, IRT13R30).
The support from 111 Project (B12015) and the Collaborative
Innovation Center of Chemical Science and Engineering
(Tianjin) are also acknowledged. Furthermore, M.H. wants to
acknowledge financial support by Deutsche Forschungsgemein-
schaft.
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