Post-synthesis of Zr-MOR as a robust solid acid catalyst for the ring-opening aminolysis of epoxides

Article abstract of DOI:10.1039/c8nj02449a

Zirconosilicate with the MOR topology (Zr-MOR) was successfully prepared using a two-step post-synthesis strategy from pre-dealumination of a H-MOR zeolite and subsequent dry impregnation of Cp₂ZrCl₂. The incorporated Zr species main

Full text of DOI:10.1039/c8nj02449a

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Post-synthesis of Zr-MOR as robust solid acid catalyst for the
ring-opening aminolysis of epoxides
a
a
a
a
a,b
Bo Tang, WeiꢀChao Song, ShengꢀYang Li, EnꢀCui Yang* and XiaoꢀJun Zhao*
a
Key Laboratory of InorganicꢀOrganic Hybrid Functional Material Chemistry,
Ministry of Education, Tianjin Key Laboratory of Structure and Performance for
Functional Molecules, Tianjin Normal University, Tianjin 300387, People's Republic
of China
b
Department of Chemistry, Collaborative Innovation Center of Chemical Science and
Engineering, Nankai University, Tianjin 300071, People's Republic of China
*Corresponding author
Eꢀmail address: encui_yang@163.com and xiaojun_zhao15@163.com
Abstract: Zirconosilicate with the MOR topology (ZrꢀMOR) was successfully
prepared using a twoꢀstep postꢀsynthesis strategy from preꢀdealumination of HꢀMOR
2 2
zeolite and subsequent dry impregnation of Cp ZrCl . Incorporated Zr species mainly
existed in the form of isolated tetrahedrally coordinated Zr in MOR framework. It was
found that the mechanism of postꢀsynthetic incorporation of Zr involved interaction
2 2
between the Cp ZrCl molecules and silanols at defects generated by steaming and
acid treatment. In comparison with Tiꢀ and Snꢀsilicate analogues, asꢀsynthesized
ZrꢀMOR zeolite showed high activity and regioꢀselectivity in the ringꢀopening
aminolysis of epoxides to the βꢀamino alcohols under ambient and solventꢀfree
conditions, which was related to the impact of Lewis acid sites of catalysts, the
basicity of amines and competitive adsorption of reactant molecules. Moreover,
ZrꢀMOR zeolite could be reused at least five cycles without loss of activity and
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regioꢀselectivity.
Keywords: Zirconosilicate; Ringꢀopening aminolysis; Epoxides; Postꢀsynthesis;
βꢀamino alcohols
1
Introduction
Epoxides are significant important synthetic intermediates, which can be
industrially obtained from the corresponding olefin precursors [1,2]. Due to their
threeꢀmembered heterocyclic ring strain, epoxides are susceptible to attacks by a
range of nucleophiles, such as amines, alcohols, azides and thiols, leading to
bifunctional molecules for great chemical industry. For instance, βꢀamino alcohols,
obtained by the ringꢀopening of epoxides with amines, are used as versatile
intermediates for the synthesis of various biologically active natural products,
unnatural amino acids, βꢀblockers, insecticidal agents, chiral auxiliaries, oxazolines,
etc. [3ꢀ6]. To date, several homogeneous catalytic system including metal triflates,
metal halides, sulfamic acid and ionic liquids have been applied to realize the
ringꢀopening of epoxides [7ꢀ11]. However, most of the homogeneous catalytic process
suffer from several disadvantages, e.g. the use of halogenated and/or toxic solvents,
the difficulty of separation, poor regioꢀselectivity and reusability issue. Therefore, for
potential applications in industry, it is highly desirable and necessary to develop
robust solid catalysts with environmental friendliness, high activity and
regioꢀselectivity as well as good recyclability for this transformation [12ꢀ16].
Zeolites have been widely utilized for diverse applications, such as catalysis,
separation, ionic exchange, etc, because of their crystalline structures, uniform pore
4+
4+
channels and high surface area [17ꢀ21]. Incorporation of metal ions, e.g. Ti , Sn and
4
+
Zr , into zeolites framework endows traditional zeolites with many interesting
properties, making them ideal heterogeneous catalysts for a great many of green
chemical transformations [22ꢀ24]. Recently, mesoporous titanosilicates and
microporous TSꢀ1, as solid Lewis acids, have been found to be active in the
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ringꢀopening aminolysis of epoxides reaction [25ꢀ27]. Attempts are also made to
synthesis nanocrystalline zirconosilicates and applied as promising candidates for the
ringꢀopening of epoxides with amines and alcohols [28]. In spite of the attractive
catalytic performance achieved, the types of metallosilicates for the ringꢀopening of
epoxides reaction were still scarce, and mainly focused on MFI framework structure
[
25,28]. Hence, researches on ringꢀopening of epoxides involving metallosilicates as
catalysts are only in its infancy, and new types of metallosilicates are urgently
required to be further developed.
MORꢀtype zeolite can be facilely synthesized via hydrothermal method even
without any organic structureꢀdirecting agent, and serves as effective solid acid
catalyst in petrochemical industry [29]. However, there is, up to now, no literature
reporting the successful direct synthesis of metallosilicates based on MOR framework.
This is because the synthesis system for MORꢀtype aluminosilicate (alkaline metals
and high Al content) make it difficult to incorporate heteroatom into the MOR zeolite
directly. Postꢀsynthesis strategies have been recently proposed as an alternative choice
for the development of metallosilicates with high catalytic efficiency [30ꢀ35]. Our
previous work reported an improved twoꢀstep postꢀsynthesis strategy for the
preparation of SnꢀBeta zeolite, which was more active than the hydrothermally
synthesized one in the BayerꢀVilliger oxidation and dihydroxyacetone conversion into
methyl lactate reaction [36]. In this context, we, herein, will report a twoꢀstep
postꢀsynthesis procedure for the preparation of ZrꢀMOR zeolite, involving dry
impregnation of dealuminated MOR zeolite with organometallic precursor Cp ZrCl .
2
2
The properties of the ZrꢀMOR zeolite were characterized and the catalytic
performance was investigated in detail for the ringꢀopening of epoxides with amines
reaction.
2
2
Experimental
.1 Sample preparation
ZrꢀMOR zeolite was synthesized via a simple and scalable postꢀsynthetic strategy
previously employed for SnꢀBeta [36], TiꢀBeta [37] and ZrꢀBeta [38], that is,
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dealumination of parent HꢀMOR zeolite and dry impregnation of Zr species into the
zeolite framework. Briefly, commercial HꢀMOR with a n /n ratio of 11 (Nankai
Si Al
University Catalyst Co., Ltd.) was firstly calcined in air at 973 K for 10 h, then
−1
ꢀ1
followed by acid reflux with 13 mol L nitric acid aqueous solution (20 mL g zeolite
)
at 373 K for another 10 h. The mixture was filtrated, thoroughly washed with
deionized water, and dried at 373 K overnight. For dry impregnation of Zr,
dealuminated MOR was preꢀtreated at 473 K overnight under vacuum to remove
physisorbed water. Then, 1.0 g of resultant powder and certain amount of Cp
i.e. 0.024, 0.048, 0.096 and 0.192 g ) were ground in agate mortar for 10 min,
followed by transfer into a tubular reactor and calcined at 823 K for 6 h (heating rate
2 2
ZrCl
(
ꢀ1
at 5 K min ) under flowing air to obtain the final ZrꢀMORꢀX zeolites where X stands
for the n /n ratio of 200, 100, 50 and 25, respectively. It should be noted that
Si Zr
ZrꢀMOR in this study generally refers to ZrꢀMORꢀ100 unless otherwise specified.
2.2 Characterization techniques
Powder Xꢀray diffraction (XRD) was recorded on a Bruker D8 diffractometer with
−1
CuꢀKα radiation (λ = 1.54184 Å) from 5° to 40° with a scan speed of 2θ = 6.0° min .
SEM images of the samples were acquired on a Nova Nano 230 scanning electron
microscope. Surface areas and pore volumes were measured by means of nitrogen
adsorption at 77 K on a Micromeritics ASAP 2020 after activating the samples 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 tꢀplot
method. The chemical compositions were detected by inductively coupled plasma
atomic emission spectrometry (ICPꢀAES) on a Thermo IRIS Intrepid II XSP atomic
emission spectrometer. Fourier transform infrared (FTꢀIR) spectroscopy was recorded
−
1
on a Bruker Tensor 27 spectrometer with 32 scans for a resolution of 4 cm in KBr
medium at room temperature. Diffuse reflectance infrared Fourier transform (DRIFT)
spectra were performed on a Bruker Tensor 27 spectrometer with 128 scans at a
−1
resolution of 2 cm . A selfꢀsupporting pellet made of sample material was placed in
the reaction chamber and calcined in flowing dry Ar at 673 K for 2 h before collecting
the spectra. Diffuse reflectance ultravioletꢀvisible (UVꢀvis) spectra were recorded
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4
against BaSO on a Hitachi Uꢀ4100 UVꢀvis spectrophotometer. Xꢀray photoelectron
spectra (XPS) were recorded on a Kratos Axis Ultra DLD spectrometer with a
monochromated AlꢀKa Xꢀray source (hν = 1486.6 eV), hybrid (magnetic/electrostatic)
optics and a multiꢀchannel plate and delay line detector (DLD). All spectra were
recorded using an aperture slot of 300 × 700 microns. Survey spectra were obtained
with a pass energy of 160 eV and highꢀresolution spectra were obtained with a pass
energy of 40 eV. Accurate binding energies (±0.1 eV) were determined based on the
position of the adventitious C 1s peak at 284.6 eV. CasaXPS software was used for
spectra analysis.
2.3 Catalytic evaluation
Catalytic ringꢀopeing aminolysis of epoxides was carried out in a 10 mL
roundꢀbottom glass flask. The vessel was charged with a mixture of equimolar
quantities of epoxide and amine (5 mmol) and 25 mg of catalyst, and mixed
vigorously using a magnetic stirrer. After each reaction, aliquots of the reaction
medium were taken out and diluted with a known quantity of dichloromethane, then
subjected to a Agilent GC 7890B (Agilent HPꢀ5 column, 30 m × 0.25 mm × 0.25 ꢁm;
FID detector) to monitor the reaction process. The GC peaks were identified by
comparison with the retention times of the known standard samples, and by means of
a Shimadzu GCꢀMS QP2010 SE equipped with an Agilent HPꢀ5MS column.
3
3
Results and discussion
.1 Incorporation of Zr into zeolite framework
The XRD patterns of parent HꢀMOR and the corresponding postꢀtreated
deAlꢀMOR and ZrꢀMOR samples were shown in Fig. 1a. It was obvious that similar
diffractions attributed to a typical MOR topology with the comparable intensity could
be observed for the three samples, indicating no destruction of zeolite framework
during the dealumination and Zr incorporation procedures, in accord with previous
literatures [39,40]. The steaming and acid treatment led to an obvious shift of the
diffraction peak (200) to higher angel (Fig. 1b), revealing the contraction of the MOR
matrix upon relatively larger Al ions removal (in comparison to Si ions). Conversely,
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the subsequent Zr incorporation process resulted in a shift of (200) peak to lower
angel compared with the dealuminated sample. This was a clear evidence of MOR
framework expansion due to much larger inoic radius of Zr ions, thus indicating the
successful incorporation of Zr species into the framework of MOR zeolite. There was
no characteristic reflections of crystalline zirconia (JCPDS# 04ꢀ001ꢀ7279) in the
patterns of asꢀprepared ZrꢀMOR, suggesting that Zr was homogeneously dispersed
during the dry impregnation procedure. The wellꢀpreserved microporous MOR
structure after dealumination and Zr incorporation was further confirmed by
possessing the same type I isotherms for parent HꢀMOR and the postꢀtreated samples
(Fig. S1).
b
(200)
Zr-MOR
deAl-MOR
H-MOR
9.0
9.2
9.4
9.6
9.8
10.0 10.2 10.4
2Theta / deg.
Fig. 1 Wide angle XRD patterns (a) and enlarged (200) reflections (b) of HꢀMOR,
deAlꢀMOR and ZrꢀMOR.
SEM investigation was made to confirm the morphology of postꢀtreated samples in
this study. As shown in Fig. 2, parent HꢀMOR showed aggregates of small pseudoꢀrod
shaped crystallites, and the same morphology were observed for the deAlꢀMOR and
asꢀprepared ZrꢀMOR, demonstrating the preservation of zeolite framework after the
dealumination and Zr incorporation processes. Atomic contents in ZrꢀMOR sample
was obtained using EDX spectroscopy (Fig. S2), which confirmed the presence of Si,
O, Al and Zr atoms. For a detailed distribution of atomic contents inside the
nanocrystals, elemental mapping of Si and Zr was performed. The elemental mapping
showed fairly homogeneous dispersion of Zr species in the case of ZrꢀMOR; no
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particles of bulk zirconia were detected, in line with the XRD characterization.
Fig. 2 SEM images and EDS mapping of Si and Zr for ZrꢀMOR.
The textural properties of HꢀMOR, deAlꢀMOR and ZrꢀMOR were listed in Table 1.
The steaming and nitric acid treatment led to a distinct decrease in the nSi/nAl ratio
from 11 for HꢀMOR to 160 for deAlꢀMOR, as a result of substantial Al from MOR
framework. Interestingly, there was no obvious change in BET surface areas (370‒397
2
ꢀ1
3 ꢀ1
m g ) and microporous volumes (0.148‒0.153 cm g ), demonstrating that the
dealumination and Zr incorporation procedures did not strongly affect the textural
properties of MOR framework. The actual nSi/nZr ratio of asꢀsynthesized ZrꢀMOR was
determined to be 97.0, in good agreement with the theoretical value of 100.
Table 1 Physicochemical properties of HꢀMOR, deAlꢀMOR, and ZrꢀMOR samples
a
a
b
c
Sample
n
Si/nAl
n
Si/nZr
Surface area
Pore volume
2
ꢀ1
3 ꢀ1
(
m g )
(cm g )
0.148
HꢀMOR
11
―
370
397
383
deAlꢀMOR
160
158
―
0.153
ZrꢀMOR
97.0
0.148
a
b
c
Determined by ICP.
Specific surface area obtained by the BET method.
Calculated from tꢀplot.
3.2 Existence states of Zr species in Zr-MOR
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Diffuse reflectance UVꢀvis spectroscopy was widely utilized to disclose the site
isolation and the coordination states of the transition metal species in zeolite. The
UVꢀvis spectrum of ZrꢀMOR in Fig. 3 showed a strong adsorption band at ca. 205 nm.
2
ꢀ
This band can be ascribed to ligandꢀtoꢀmetal charge transitions from O to dispersed
4
+
Zr ions, specially as the catalytically active isolated tetrahedrally coordinated Zr(IV)
in zeolite framework [41ꢀ43]. Moreover, no bands related to bulk ZrO reference
2
could be examined in the ZrꢀMOR spectrum, suggesting the exclusion of aggregated
2
ZrO crystalline, which was in consistent with the characterization result from XRD.
2
Fig. 3 UVꢀvis spectra of ZrꢀMOR and ZrO .
For further insights into the local environment of the incorporated Zr species, XPS
measurements were applied. As shown in Fig. 4, two signals referent to the Zr 3d3/2
and Zr 3d5/2 photoelectrons located at 185.1 and 182.7 eV, respectively, were emerged
2
for ZrꢀMOR, much higher than that of the bulk ZrO reference (184.6 and 182.2 eV).
According to the previously reported work [44], the binding energy was shifted
obviously to higher values could be seen as an indicator of the formation of Si–O–Zr
bonds in the zeolite structure together with transformation of geometry from
octahedral to tetrahedralꢀcoordination. This result suggested that the incorporated Zr
species in MOR framework mainly existed in the form of tetrahedrally coordinated
state, in accordance with the UVꢀvis analysis. Similar results has also been reported
for BEAꢀ and MFIꢀtype metallosilicates due to heteroatom incorporation into their
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frameworks [44ꢀ46].
2
Fig. 4 Zr 3d XPS of ZrꢀMOR and ZrO .
3.3 The mechanism of Zr incorporation
On the basis of the characterization results discussed above, we assumed that the
mechanism of postꢀsynthetic incorporation of Zr atom into the framework of MOR
zeolite involved creating defect sites associated with silanol groups through
dealumination and subsequent interaction of these silanol groups with the
corresponding metal precursor, thus resulting in tetrahedral metal sites incorporated
into the zeolite framework, as graphically described in Scheme 1. Here, such a
hypothesis was investigated based on infrared spectroscopy.
Scheme 1 Schematic representation of the incorporation of tetrahedral Zr(IV) into
MOR framework.
Fig. 5a showed the FTꢀIR spectra of the parent HꢀMOR, deAlꢀMOR and ZrꢀMOR
ꢀ1
in the skeletal vibration region of 700‒1400 cm . A newly formed band located at 955
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ꢀ1
cm was observed for deAlꢀMOR sample. According to the early study, it was
associated with asymmetric vibration of the SiꢀOH bonds at defect sites [40]. An
analogous phenomenon was also been observed for BEA zeolite due to the extraction
of framework Al ions [36,37]. A distinct decline in the band intensity occurred as for
the following dry impregnation with Cp ZrCl . Simultaneously, a minor vibration
2
2
ꢀ1
band at 965 cm appeared, which belonged to an asymmetric stretching mode of
[O
3
Si–O–Zr] unit [28,48,49]. These results probably gave the support for the binding
Zr at the defect sites and thus incorporation of Zr into the zeolite MOR framework.
To further investigate into this issue, the DRITF spectra in the hydroxyl stretching
ꢀ1
region of 3400‒4000 cm of parent HꢀMOR, deAlꢀMOR and ZrꢀMOR were also
performed and presented in Fig. 5b. In the HꢀMOR spectrum, three major bands at
ꢀ1
3
735, 3650 and 3605 cm , corresponding to external silanol groups, nonꢀframework
Al and bridging hydroxyls (SiꢀOHꢀAl), respectively, could be distinguished [39,50].
The treatment of HꢀMOR with steaming and concentrated nitric acid resulted in a
ꢀ1
great decrease in the intensity of framework Alꢀrelated 3650 and 3610 cm bands.
This indicated that a significant amount of Al ions were eliminated from the MOR
framework, as confirmed by the ICP analysis (Table 1). Meanwhile, the appearance
ꢀ1
of two new bands at 3725 and 3578 cm derived from the internal silanol groups were
observed after the dealumination process [36,51,52], confirming the formation of
ꢀ1
defect sites in the zeolite framework. Specifically, the broad feature of the 3578 cm
band indicated that it was attributed to hydrogenꢀbonded silanols, which located at the
defect sites of dealuminated MOR framework in the form of hydroxyl nests. After dry
ꢀ1
impregnation of Cp
2 2
ZrCl , the bands at 3725 and 3578 cm associated with internal
silanols decreased in intensity, hinting that Cp
2
2
ZrCl molecules reacted with the
internal silanols to incorporate the framework Zr species, in consistent with the FTꢀIR
spectra.
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Fig. 5 FTIR (a) and DRIFT (b) spectra of HꢀMOR, deAlꢀMOR and ZrꢀMOR.
.4 Catalytic activity of Zr-MOR in the ring-opening of epoxides with amines
In order to assess the catalytic performance of postꢀsynthesized ZrꢀMOR, the
3
ringꢀopening of styrene oxide with aniline was initially investigated under
solventꢀfree conditions (Table 2). In the present study, two regioꢀisomer βꢀamino
alcohols,
i.e.
2ꢀphenylꢀ2ꢀ(phenylamino)ethanol
and
1ꢀphenylꢀ2ꢀ(phenylamino)ꢀethanol, were observed as the major and minor products,
respectively, as illustrated in scheme 2. Blank experiment showed very low styrene
oxide conversion (< 7%) with nearly the same selectivity to regioꢀisomer A (56.3%)
and B (43.7%). Negligible increase in the product was gained when using HꢀMOR or
deAlꢀMOR as catalyst. Dealuminated zeolite supported oxides like ZrO
SnO /deAlꢀMOR and TiO /deAlꢀMOR worked in the epoxide ringꢀopening process
but gave poor reactivity. Interestingly, the incorporation of metal ions (Sn , Ti or
2
/deAlꢀMOR,
2
2
4
+
4+
4
+
Zr ) into the MOR framework resulted into considerable styrene oxide conversion as
well as high regioꢀisomer A selectivity. This could be due to the fact that these ions in
zeolite framework could serve as Lewis acid sites to adsorb and activate the epoxide
substrates [27,28]. Compared with SnꢀMOR and TiꢀMOR analogues, ZrꢀMOR
showed a much higher catalytic performance, achieving 90.2% styrene oxide
conversion and 93.1% regioꢀisomer A selectivity. This result indicated the unique
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activity of frameworkꢀincorporated Zr sites for the ringꢀopening aminolysis of
epoxides reaction. In our previous work, it was found that the ringꢀopening aminolysis
of reaction reactivity depended largely on the nature of active sites of catalysts [38].
4+
4+
Both the strong Lewis acidity of Sn sites and Ti sites with weak Lewis acidity led
to a stronger adsorption of aniline over styrene oxide, which was fatal for the
4
+
activation of epoxide, a key step in the ringꢀopening process. As a result, Zr sites
with moderate Lewis acidity represented the advantage on the catalytic ringꢀopening
of epoxide. This maybe a plausible explanation for the reactivity order of ZrꢀMOR >
TiꢀMOR > SnꢀMOR in this study. Moreover, the TON value of ZrꢀMOR was
estimated to be 1050, even higher than our previously reported hierarchical ZrꢀBeta
zeolite (TON = 939).
a
Table 2 Aminolysis of styrene oxide with aniline over various zeolite catalyst
b
b
c
Catalyst
Styrene oxide conv. βꢀAmino alcohols select. TON
ꢀ1
(%)
(%)
A
M
(molEpomol )
B
None
< 7
9.0
8.3
56.3
51.8
52.3
83.8
84.1
88.0
90.5
91.9
93.1
43.7
48.2
47.7
16.2
15.9
12.0
9.5
―
HꢀMOR
deAlꢀMOR
―
―
SnO
TiO
ZrO /deAlꢀMOR 15.6
2
/deAlꢀMOR 9.8
117
147
182
563
767
1050
2
/deAlꢀMOR 12.5
2
SnꢀMOR
TiꢀMOR
48.5
65.8
90.2
8.1
ZrꢀMOR
6.9
a
Reaction conditions: 5 mmol styrene oxide; 5 mmol aniline, 25 mg catalyst,
b
temperature = 313 K, reaction time = 4 h. Experimental accuracy of ± 2% from GC
c
analysis. TON = moles of styrene oxide converted per mole of the metal center.
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Scheme 2 Ringꢀopening of styrene oxide with amines for synthesis of βꢀamino
alcohols.
The influence of reaction parameters (e.g. role of solvent, nSi/nZr ratio, reaction
temperature and reaction time) were investigated in this study. As given in Table 3,
catalytic activity of ZrꢀMOR zeolite was found to be rather higher when reaction was
conducted in the absence of solvent. Decrease in the catalytic activity was observed
once solvent was used, and more evident in polar aprotic solvent than nonꢀpolar
solvent. This difference in reactivity could be related to the adsorption of solvent
molecules on the catalytic active sites (i.e. Lewis acid sites). In the absence of solvent,
only reactant molecules were accessible to the active sites and activated, while solvent
molecules, especially polar solvent solvents could mask the active sites ascribed to the
competitive adsorption of reactant and solvent molecules, which hindered the
interaction between the catalytic sites and the reactant molecules, thus leading to the
lowered catalytic activity.
a
Table 3 Influence of solvent on the aminolysis of styrene oxide with aniline
b
b
c
Solvent Styrene oxide conv. (%) βꢀAmino alcohols select. (%) TON
ꢀ1
A
B
(molEpomolM
1050
949
)
None
90.2
93.1
93.4
93.0
93.1
93.8
92.8
6.9
6.6
7.0
6.9
6.2
7.2
Toluene 81.5
CH
CHCl
CCl
2
Cl
2
68.3
69.6
83.7
795
3
810
4
974
CH
3
CN 40.5
471
a
Reaction conditions: 5 mmol styrene oxide; 5 mmol aniline, 5 mL solvent; 25 mg
b
catalyst, temperature = 313 K, reaction time = 4 h. Experimental accuracy of ± 2%
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c
from GC analysis. TON = moles of styrene oxide converted per mole of the metal
center.
The nSi/nZr ratio exhibited great impact on the catalytic performance of ZrꢀMOR in
the ringꢀopening aminolysis of styrene oxide reaction (Table 4). A big improvement
in conversion of styrene oxide from 40.5 to 90.2% was gained when increasing the
n
Si/nZr ratio in ZrꢀMOR from 200 to 100. However, further increasing the Zr loading
only resulted in a marginal increase in the styrene oxide conversion (91.6% for
ZrꢀMORꢀ50 and 92.0% for ZrꢀMORꢀ25, respectively). To reveal the intrinsic activity
of asꢀprepared ZrꢀMOR zeolites, the productivity based on Zr were taken into
ꢀ1
ꢀ1
consideration. Similar productivity was obtained for ZrꢀMORꢀ100 (447.2 g
A
g
Zrh )
ꢀ1
ꢀ1
A
and ZrꢀMORꢀ200 (402.4 g g Zrh ), which was distinctly higher than those of
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ZrꢀMORꢀ50 (226.3 gAg h ) and ZrꢀMORꢀ25 (113.9 g g h ). This result indicated
Zr
A
Zr
the fact that it was approaching saturation of framework Zr in MOR zeolite at nSi/nZr
ratio of 100, and more Zr species would be forced to locate nonꢀframework positions
and form oxides, lowering the catalytic activity.
a
Table 4 Influence of nSi/nZr ratio on the aminolysis of styrene oxide with aniline
b
b
c
n
Si/nZr Styrene oxide conv.
βꢀAmino alcohols select. (%) Productivity TON
ꢀ1
ꢀ1
ratio
(%)
A
B
(g g h )
(molEpo
A
Zr
ꢀ
1
mol
M
)
2
1
5
00
00
0
40.5
90.2
91.6
92.0
93.3
93.1
92.8
93.0
6.7
6.9
7.2
7.0
402.4
447.2
226.3
113.9
943
1050
533
2
5
268
a
Reaction conditions: 5 mmol styrene oxide; 5 mmol aniline, 5 mL solvent; 25 mg
b
catalyst, temperature = 313 K, reaction time = 4 h. Experimental accuracy of ± 2%
c
from GC analysis. TON = moles of styrene oxide converted per mole of the metal
center.
Fig. 6 showed the influence of reaction temperature and reaction time on the
ringꢀopening aminolysis reaction. As the temperature increased from 303 to 333 K,
14

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the styrene oxide conversion increased from 54.9 to 100%. The reaction proceeded
much faster with higher temperature, however, adversely affecting the product
proportion. There was a margin decrease in the product A selectivity, indicating at
higher temperature product A could isomer into B on the acidic sites (Fig. 6a). It was
worth noting that, different from most traditional process, the ringꢀopening aminolysis
of styrene oxide occurred even under ambient temperature (303 K) with high yield
and selectivity for product A, suggesting the high catalytic efficiency of
asꢀsynthesized ZrꢀMOR zeolite. As represented in Fig. 6b, the styrene oxide
conversion increased steadily with prolonging the duration time in 4 h, and the
maximum conversion was observed at ca. 91.0% with the product A selectivity of
93.1%. Further increase in the reaction time could not exert obvious influence on the
ringꢀopening aminolysis process.
Fig. 6 Influence of reaction temperature (a) and reaction time (b) on the ringꢀopening
aminolysis of epoxides reaction. Reaction conditions: styrene oxide, 5 mmol; aniline,
5
mmol; catalyst amount, 25 mg; reaction time, 4h for a; reaction temperature, 313 K
for b.
After optimisation of the reaction parameters, the ringꢀopening aminolysis reaction
was further investigated to broad the substrate scope by using different epoxide and
amine reactants with ZrꢀMOR as the catalyst, and the results were summarized in
Table 5. Amongst the amine substrates investigated, aniline was found to be the most
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active one, achieving 90.2% styrene oxide conversion with 93.1% βꢀamino alcohol
selectivity, which showed much higher activity than the substituted amines. In
contrast to aniline, very low styrene oxide conversion and unsatisfactory
regioꢀselectivity were afforded with aliphatic primary amine, such as nꢀbutylamine.
Similar phenomenon has also been reported in the titanosilicates catalytic system
[
26,27]. To explain such a result, a good correlation between the aminolysis rate and
the basicity, i.e. pKa value, of the amine was plotted in Fig. 7. As can be seen, the
epoxide ringꢀopening rate decreased with increasing pKa value of amine. It was
accepted that ringꢀopening aminolysis of epoxide was a bimolecular reaction, in
which there should be an optimal bonding between the active sites of catalyst and the
two reactant molecules [27]. However, when amine with higher pKa value was used
4
+
as the substrate, it would be strongly bonded to the Zr sites, thus lowering their
ability to further activate epoxide molecules. Taking nꢀbutyl amine (pKa = 10.6) as an
example, it just showed a very low conversion (7.1%), much lower than
4ꢀchloroaniline (pKa = 4.0) with the conversion high up to 72.5%. On the other hand,
the reactivity was determined to be in the sequence of styrene oxide >
epichlorohydrin > propene oxide. Trends in the catalytic activity could be ascribed to
the higher epoxide/amine adsorption ratio for styrene oxide than the other two
epoxides [28]. From the results discussed above, it could be inferred that preferential
adsorption and activation of epoxide were the key step in ringꢀopening aminolysis of
epoxide reaction.
a
Table 5 Aminolysis of epoxides with amines ZrꢀMOR zeolite
c
TON
b
b
Epoxide
Amine
βꢀamino alcohol Epoxide conv. Select.
(
molEpo
(%)
(%)
ꢀ1
mol
M
)
9
0.2
8.3
93.1
1050
679
5
91.2
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4
7
7
9.4
4.1
2.5
91.5
94.8
93.1
575
863
844
6
5.8
91.0
26.8
766
83
7
.1
6
3.4
1.6
92.6
80.8
738
601
5
a
Reaction conditions: 5 mmol epoxide; 5 mmol amine; 25 mg catalyst, temperature =
b
c
3
13 K, reaction time = 4 h. Experimental accuracy of ± 2% from GC analysis.
TON = moles of styrene oxide converted per mole of the metal center.
Fig. 7 Correlation between the catalytic activity and the pKa value of amines in the
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ringꢀopening aminolysis of styrene oxide reaction. Reaction conditions: styrene oxide,
5
mmol; aniline, 5 mmol; catalyst amount, 25 mg; reaction temperature, 313 K;
reaction time, 4h.
To confirm the true heterogeneity of asꢀsynthesized ZrꢀMOR, a hotꢀfiltration
experiment was conducted to determine if active components could leach into the
reaction mixture and possibly participated in the catalytic ringꢀopening aminolysis
reaction. As shown in Fig. 8, the βꢀamino alcohol yield reached ca. 34% after 1 h,
then the catalyst was immediately removed from the reaction system, and the mother
liquor was allowed to proceed for another 3 h under the identical reaction conditions.
Obviously, removal of the catalyst led to the complete termination of the reaction,
verifying the truly heterogeneous nature of the ringꢀopening aminolysis process over
ZrꢀMOR.
Fig. 8 βꢀAmino alcohol yield in the ringꢀopening of styrene oxide with aniline over
ZrꢀMOR. Reaction conditions: styrene oxide, 5 mmol; aniline, 5 mmol; reaction
temperature, 313 K.
Catalyst stability and the feasibility of recycling was a very important factor for
heterogeneous catalysis. To this end, the ringꢀopening of styrene oxide with aniline
reaction was investigated in successive runs over ZrꢀMOR (the catalyst was
preꢀcalcinated for the next use), and the results were presented in Fig. 9. As can be
seen, there was no obvious change in the catalytic activity and product selectivity
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even after six successive recycling. The possible leaching of Zr species into reaction
medium as well as change in the texture properties of ZrꢀMOR after cycles were
excluded by ICP analysis (Table S1), XRD and surface area characterization (Fig. S3
&
Table S1), respectively. These results emphasized the fact that ZrꢀMOR prepared
by the postꢀsynthesis dry impregnation strategy in this study serves as a potential
heterogeneous catalyst for the ringꢀopening aminolysis of epoxides.
Fig. 9 Recycling test for the ringꢀopening of styrene oxide with aniline over ZrꢀMOR.
Reaction conditions: 5 mmol; aniline, 5 mmol; catalyst amount, 25 mg; reaction
temperature, 313 K; reaction time, 4h.
4
Conclusions
In the present study, ZrꢀMOR zeolite has been prepared using a twoꢀstep
postꢀsynthesis strategy, in which the incorporation of Zr atom into the framework of
MOR zeolite involved creating defect sites associated with silanol groups via
dealumination and subsequent interaction of these silanol groups with the
2 2
corresponding Cp ZrCl precursor. Characterization results from UVꢀvis and XPS
analysis confirmed that the incorporated Zr species were in tetrahedrally coordinated
configuration. For the ringꢀopening aminolysis of epoxides reaction, asꢀprepared
ZrꢀMOR exhibited much higher catalytic activity in comparison with Snꢀ and
Tiꢀsilicate analogues. The catalytic aminolysis activity depended much on the pKa
value of amines and the adsorption abilities of reactant molecules. The hotꢀfiltration
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and recycling experiments revealed the true heterogeneity and perfect stability of
ZrꢀMOR prepared by postꢀsynthesis procedures.
Acknowledgment
This work was financially supported by the NSFC (Grants 21531005, 21401140) and
the 973 program (2014CB845601). The support from the doctoral program foundation
of Tianjin Normal University (52XB1509) and the program for Innovative Research
Team in University of Tianjin (TD12ꢀ5038) were also acknowledged.
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Figures caption
Fig. 1 Wide angle XRD patterns (a) and enlarged (200) reflections (b) of HꢀMOR,
deAlꢀMOR and ZrꢀMOR.
Fig. 2 SEM images and EDS mapping of Si and Zr for ZrꢀMOR.
Fig. 3 UVꢀvis spectra of ZrꢀMOR and ZrO2.
2
Fig. 4 Zr 3d XPS of ZrꢀMOR and ZrO .
Fig. 5 FTIR (a) and DRIFT (b) spectra of HꢀMOR, deAlꢀMOR and ZrꢀMOR.
Fig. 6 Influence of reaction temperature (a) and reaction time (b) on the ringꢀopening
aminolysis of epoxides reaction. Reaction conditions: styrene oxide, 5 mmol; aniline,
5
mmol; catalyst amount, 25 mg; reaction time, 4h for a; reaction temperature, 313 K
for b.
Fig. 7 Correlation between the catalytic activity and the pKa value of amines in the
ringꢀopening aminolysis of styrene oxide reaction. Reaction conditions: styrene oxide,
5
mmol; aniline, 5 mmol; catalyst amount, 25 mg; reaction temperature, 313 K;
reaction time, 4h.
Fig. 8 βꢀAmino alcohol yield in the ringꢀopening of styrene oxide with aniline over
ZrꢀMOR. Reaction conditions: styrene oxide, 5 mmol; aniline, 5 mmol; reaction
temperature, 313 K.
Fig. 9 Recycling test for the ringꢀopening of styrene oxide with aniline over ZrꢀMOR.
Reaction conditions: 5 mmol; aniline, 5 mmol; catalyst amount, 25 mg; reaction
temperature, 313 K; reaction time, 4h.
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Post-synthesis of Zr-MOR as robust solid acid catalyst for the
ring-opening aminolysis of epoxides