Topotactic Conversion of Alkali-Treated Intergrown Germanosilicate CIT-13 into Single-Crystalline ECNU-21 Zeolite as Shape-Selective Catalyst for Ethylene Oxide Hydration

Article abstract of DOI:10.1002/chem.201900173

The conversion of the alkali-treated intergrowth germanosilicate CIT-13 into the single-crystalline high-silica ECNU-21 (named after East China Normal University) zeolite, with a novel topology and a highly crystalline zeolite framework, has been realized through a creative top-down strategy involving a mild alkaline-induced multistep process consisting of structural degradation and reconstruction. Instead of acid treatment, hydrolysis in aqueous ammonia solution not only readily cleaved the chemically weak Ge(Si)?O?Ge bonds located within the interlayer double four ring (D4R) units of CIT-13, but also cleaved the metastable Si?O?Si bonds therein. This led to extensive removal of the D4R units, and also generated silanol groups on adjacent silica-rich layers, which then condensed to form a novel daughter structure upon calcination. Individual oxygen bridges in the reassembled ECNU-21 replaced the germanium-rich D4R units in CIT-13, thereby eliminating the original intergrowth phenomenon along the b axis. With an ordered crystalline structure of 10-ring (R) channels as well as suitable germanium-related Lewis acid sites, ECNU-21 serves as a stable solid Lewis acid catalyst for the shape-selective hydration of ethylene oxide (EO) to ethylene glycol (EG) at greatly reduced H₂O/EO ratios and reaction temperature in comparison with the noncatalytic industrial process.

Full text of DOI:10.1002/chem.201900173

A Journal of
Accepted Article
Title: Topotactic conversion of alkali-treated intergrowth
germanosilicate CIT-13 into single crystalline ECNU-21 zeolite as
shape-selective catalyst for ethylene oxide hydration
Authors: Xue Liu, Wenting Mao, Jingang Jiang, Xinqing Lu, Mingming
Peng, Hao Xu, Lu Han, Shun-ai Che, and Peng Wu
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To be cited as: Chem. Eur. J. 10.1002/chem.201900173
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Topotactic conversion of alkali-treated intergrown germanosilicate
CIT-13 into single-crystalline ECNU-21 zeolite as shape-selective
catalyst for ethylene oxide hydration
Xue Liu,[a] Wenting Mao,[b] Jingang Jiang,[a] Xinqing Lu,[a] Mingming Peng,[a] Hao Xu,*[a] Lu Han,*[b,c]
Shun-ai Che,[b,c] and Peng Wu*[a]
Abstract: Conversion of alkali-treated intergrowth germanosilicate
CIT-13 into single crystalline high-silica ECNU-21 (named after East
China Normal University) zeolite, with a novel topology and a highly
crystalline zeolite framework, has been realized via a creative top-
down strategy involving mild alkaline treatment-induced multistep
process of structural degradation and reconstruction. Instead of acid
treatment, the hydrolysis in ammonia aqueous solution not only
disconnected readily the chemically weak Ge(Si)-O-Ge bonds located
within interlayer double four ring (D4R) units of CIT-13 but also
cleaved the metastable Si-O-Si bonds therein. This leads to an
extensive removal of D4R units, and also generates the silanol groups
on adjacent silica-rich layers which are then condensed into a novel
daughter structure upon calcination. Individual oxygen bridges in
reassembled ECNU-21 replace the Ge-rich D4R units in CIT-13,
which eliminates the original intergrowth phenomenon along b-axis.
With ordered crystalline structure of 10-ring (R) channels as well as
suitable Ge-related acid sites, ECNU-21 serves as a stable solid-acid
catalyst for the shape-selective hydration of ethylene oxide (EO) to
ethylene glycol (EG) at greatly reduced H2O/EO ratios and reaction
temperature in comparison to non-catalytic industrial process.
zeolites).[2] Beta zeolite (*BEA) is one of the most studied
intergrowth zeolites, the framework of which contains intergrown
stackings of two similar polymorphs A and B with a ratio of 44 :
56.[3] Having the same building layers, other polymorphs C, CH,
D, and E have also been proposed or experimentally verified for
the Beta family.[3b,3c,4] A more complex zeolite ITQ-39 containing
three polymorphs was reported as an effective catalyst for the
alkylation of aromatics.[5] ECNU-5 zeolite built from the polytype
stackings of MWW layers highly resembles SSZ-70 zeolite (*-
SVY).[6] In the field of intergrowth zeolites, the efforts mainly focus
on resolving the crystal structure,[7] exploring the unique catalytic
performance[6a] and synthesizing the pure polymorph.[7] The latter
is of high importance in terms of crystallographic interests and
catalytic applications. As a typical representative, the germanate
(FOS-5)[8] and germanosilicate (ITQ-17)[9] with a pure polymorph
C (BEC) structure in Beta family have been hydrothermally
synthesized via incorporating germanium into the framework to
stabilize the D4R units. Besides, the successful synthesis of CIT-
1 (CON topology) is also another example, which is considered to
be the pure polymorph B of the SSZ-26 and SSZ-33 family (both
SSZ-26 and SSZ-33 are intergrowth of polymorph A and B).[10]
Later, the pure polymorph C of this zeolite family was also
reported, known as ITQ-24 zeolite.[11]
Introduction
Recently,
a novel intergrowth Ge-zeolite CIT-13 (*CTH
topology) has been reported, consisting of the cfi-layers
interconnected by D4R units and intersected 14- and 10-R
channels.[12] Subsequently, two germanosilicates of NUD-2[13] and
SAZ-1[14] were announced to possess the structures analogous to
CIT-13. The 14-R channels running parallel to the crystallographic
c-axis bear a striking resemblance to the UTL topology, while the
arrangement of 10-R channels along b-axis is ill-defined, giving
rise to two or more possible framework structures (AAAA
sequence and ABAB sequence and more), which was revealed
by the serious mosaicity along b-axis in the TEM images.[12b] In
other words, the random or irregular location of D4R subunits
along c direction induced the stacking faults.
Zeolites with well-defined crystalline structures and unique
microporosities of molecular sizes are employed widely as useful
adsorbents, ion-exchangers and in particular heterogeneous
catalysts in petrochemical industry.[1] To date, 239 known
topologies have been recognized by International Zeolite
Association (IZA), including nine partially disordered materials
that consist of variable structures (usually known as intergrowth
[a]
X. Liu, Dr. J. Jiang, Dr. H. Xu, M. Peng, Dr. X. Lu, Prof. P. Wu
Shanghai Key Laboratory of Green Chemistry and Chemical
Processes
In the past years, germanosilicates have set off research
upsurge due to modifiable structure prosperity benefiting from the
relatively weak Ge(Si)-O-Ge bonds in D4R units, since they are
potential precursors useful for postsynthesizing a variety of novel
zeolite structures hardly generated by direct hydrothermal
synthesis. The typical assembly-disassembly-organization-
reassembly (ADOR) strategy[15] has been proved to be highly
effective in selective removing interlayer Ge-rich D4R units in
acidic media and converting 3-dimensional (3D) germanosilicates
to 2D layered intermediates, followed by reconstruction to several
daughter structures with the assistant of ammonia or silane agent.
Though the ADOR strategy has achieved success in the structural
reconstruction of UTL,[15] IWW,[16] ITH,[17] ITR,[17] IWR,[17] SAZ-1[14]
School of Chemistry and Molecular Engineering, East China Normal
University
North Zhongshan Road 3663, Shanghai, 200062 (P.R. China)
E-mail: pwu@chem.ecnu.edu.cn, hxu@chem.ecnu.edu.cn
W. Mao, Prof. L. Han, Prof. S. Che
State Key Laboratory of Metal Matrix Composites
School of Chemistry and Chemical Engineering, Shanghai Jiao
Tong University
Dongchuan Road 800, Shanghai, 200240, (P.R. China)
Prof. L. Han, Prof. S. Che
[b]
[c]
School of Chemical Science and Engineering, Tongji University
Siping Road 1239, Shanghai, 200092, (P.R. China)
Supporting information for this article is given via a link at the end of
the document.
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and UOV[18] germanosilicates, only three highly ordered materials
with the code of OKO, PCR and PCS derived from UTL have been
recognized by IZA. Our group also proposed an acid treatment
strategy to obtain structure derivatives from UTL zeolite via simply
controlling the treatment time.[19] Since the intergrowth of CIT-13
originates from the random or irregular location of D4R subunits
on the (010) plane, we anticipate the disordered phenomenon are
possibly eliminated after fully removing the interlayer D4R units,
forming a layered intermediate and thereafter reconstructing a
novel single crystalline structure. Although the original extra-large
pore system would be sacrificed, the newly constructed daughter
structure could be useful for shape-selective catalysis. For the
purpose of ADOR top-down synthesis, the key step is the
formation of the layered precursor. However, a sufficiently large
amount of Ge atoms are not always enough to unlock the
interlayer linkage completely via acid treatment.[20] And the
metastable Si-O-Si interlayer bonds vertical to the layers in D4R
units are supposed to prohibit the full separation of the layers,
making the framework hardly degradable and changeable against
acid hydrolysis. Unfortunately, Ge-CIT-13 encountered failure to
form a lamellar product in acidic media. The more detailed
discussion of the reasons will be provided (vide post).
zeolite is employing mild alkaline media to cleave not only
chemically weak Ge(Si)-O-Ge bonds but more importantly the
metastable Si-O-Si bonds vertical to the layers in D4R subunits,
inducing disassembly and reconstruction of germanosilicate
frameworks. With moderate Ge-derived Lewis acidity confined in
10-R channels, the ordered ECNU-21 exhibited a promising
shape-selective catalyst in the ethylene oxide hydration reaction.
This is also the first study to ever report the catalytic performance
of germanosilicates serving as solid Lewis acid catalyst in the
hydration reactions.
Results and Discussion
Characterization of high-silica zeolite ECNU-21
We firstly attempted to remove the D4R subunits from *CTH
topology by performing the typical acid-assisted ADOR procedure
on the calcined Ge-rich CIT-13 (Si/Ge
hydrothermally synthesized using
=
3.7) that was
1,2-dimethyl-3-(3-
methylbenzyl) imidazolium hydroxide as organic structure-
directing agent (OSDA) (Figure S1-S3). Irrespective of using
either HCl aqueous solution with varied concentrations (0.1 - 4.0
M) (Figure S4) or identical HNO3 solution via altering the
treatment time (Figure S5), the calcined Ge-rich CIT-13
experienced only limited structural collapse and then rapidly
reconstructed to stable structure analogous to the *CTH topology.
The unique structural property of CIT-13 upon acid treatments is
probably related to the framework composition and the Ge
location in D4R units.[20]
[4Si, 4Ge] D4R
Scheme 1. Structural conversion processes from intergrown CIT-13 into
ordered ECNU-21 via alkali-induced strategy. Mild alkaline media dislodges
D4R units in polymorph CIT-13 completely and further reassembly to single
crystalline high-silica zeolite ECNU-21 upon calcination.
-7.2
[7Si, 1Ge]
D4R
[8Si, 0Ge] D4R
(79%)
-38.1
-17.9
(16%)
(5%)
Herein, we report the systematic removal of the randomly
arranged D4R units in *CTH through a creative top-down strategy
in basic media of ammonia aqueous solution, eliminating the
intergrowth structures of CIT-13 and transferring it into a novel
single-crystalline structure ECNU-21. This process can also be
recognized as an inverse sigma transformation.[21] Scheme 1
depicts the structural degradation and reconstruction processes
for the synthesis of fully ordered ECNU-21 zeolite with intergrowth
CIT-13 as a starting material, involving (i) chemoselective
dislodgement of Si-O-Si bond-containing D4R units in CIT-13 via
alkaline-assisted hydrolysis, thereby resulting in high-silica
precursor ECNU-21P consisting of crystalline monolayers; (ii)
spontaneous condensation of the neighboring cfi-layers upon
calcination to reconstruct the daughter structure ECNU-21 with
1D 10-R channels. The key to achieve highly ordered ECNU-21
10
0
-10
-20
-30
-40
-50
Chemical shift (ppm)
Figure 1. 19F MAS NMR spectrum of the as-synthesized CIT-13 sample. The
numbers in parentheses show the percentages of three major D4R units with
different Si/Ge ratios.
As an efficient means of characterization, solid-state 19F MAS
NMR spectroscopy has been employed to probe the location and
distribution of Ge atoms in the framework, because F- ions prefer
to locate in the center of D4R units and their chemical
environments varied with the configurations of surrounding Si and
Ge.[20] As-synthesized CIT-13 displayed a major peak at -7.2 ppm
along with two other resonances at -17.9 and -38.1 ppm (Figure
1), assigned to F- ions located in the D4R units with the
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compositions of [4Si, 4Ge], [7Si, 1Ge] and [8Si, 0Ge],
respectively.[20] The relatively stable Si-O-Si bonds vertical to the
layers occupy at least 21% of D4R units arising from the [7Si,
1Ge] and [8Si, 0Ge] groups in CIT-13, not to mention those from
the [4Si, 4Ge] group. Thus, Ge-CIT-13 zeolite was failed to evolve
to new crystalline structures. Moreover, Si-O-Si bonds are known
to be stable in acid solution but unstable in alkaline media. It
would be a rational way to remove extensively the D4R units from
CIT-13 by carrying out the ADOR procedures in alkaline media
like ammonia aqueous solution.
510
200
020
111
040
240
m
l
k
j
130
110
220
i
h
g
f
e
d
c
b
a
200
110
5
10
15
20
25
30
040
020
2Theta (deg.)
c
Figure 3. In-situ PXRD patterns revealing a gradual evolution from two-
dimensional layered structure of ECNU-21P to three-dimensional crystalline
zeolite of ECNU-21 depending on continuous heating at 298 K (a), 323 K (b),
373 K (c), 423 K (d), 473 K (e), 523 K (f), 573 K (g), 623 K (h), 673 K (i), 723 K
(j), 773 K (k), 823 K (l) and 873 K (m) with a heating rate of 30 K min-1 (from
bottom to top).
200
110
b
110
200
a
The reassembly process from 2D ECNU-21P precursor to 3D
ECNU-21 upon calcination was precisely traced via the in-situ
PXRD measurement (Figure 3). Along with the continuous
heating from 298 K to 823 K, the 200 reflection increased in
intensity, accompanied by gradual shift of 2θ position from 8.9° to
10.3º, because of the condensation of neighboring cfi-layers and
the decline of the average interlayer d-spacing (Figure S7). The
drastic structural change upon calcination occurred in the
temperature ranges of 423 - 573 K and 673 - 823 K, as revealed
by the variation trend of the d-spacing of layer-related 200
reflection. The in-situ PXRD measurement also showed a steady
enhancement of the intensity of 110 and 220 reflections, reaching
the maximum value at 823 K. The reassembly process was
supposed to finish at 823 K, after which point the interlayer
distance levelled off (Figure S7). Simultaneously, the slight
intensity enhancement was also observed for the 020, 111, 130,
040, 510 and 240 reflections due to the recovery of a long-range
ordered structure. Besides, the 2θ positions of intralayer-related
020 and 040 reflections almost stayed unchanged independent of
the temperature-induced condensation process, implying the Si-
rich CIT-13 sheets almost kept intact upon calcination. The
reassembled ECNU-21 structure was well preserved and
thermally stable against a further calcination at 873 K. In spite of
the dramatic structural change in the ammonium solution
treatment and subsequent calcination process, the ECNU-21
crystals still preserved the thin platelet morphology similar to the
pristine CIT-13 (Figure S8).
5
10
15
20
25
30
2Theta (deg.)
Figure 2. PXRD patterns of calcined CIT-13 (a), ammonia aqueous solution
treatment-derived layered precursor ECNU-21P (b) and subsequently calcined
sample ECNU-21 (c).
Subsequently, the layered intermediate ECNU-21P was
achieved readily via stirring a suspension of calcined Ge-CIT-13
and NH3•H2O aqueous solution. As demonstrated by the PXRD
pattern of ECNU-21P remarkably distinct from original CIT-13
(Figure 2), the layer-related 200 reflection shifted to a higher 2θ
angle of 8.9° (ca. 6.4° for pristine CIT-13), indicating the
significant narrowing of the interlayer d-spacing. Meanwhile, the
110 reflection also became weaker and moved to higher angle of
2θ ≈ 8.2º. Note that there was a considerable intensity loss of
numerous peaks in the 2θ range of 17º - 30º, implying the decline
of crystallinity as a result of the distinctive structural disassembly.
In contrast, the intralayer structure-related 0k0 reflections
including 020 and 040 were well-preserved, implying that the
long-range structure order within Si-rich layers was well-
maintained during hydrolysis process. Upon subsequent
calcination at 823 K for 6 h, the precursor ECNU-21P underwent
a structural reconstruction, evolving from 2D structure to a new
3D ECNU-21 with a high-silica framework (Si/Ge = 33) and a
comparable crystallinity to the original CIT-13. The well-resolved
200 reflection also became sharper with a further right-shift to 2θ
= 10.3º (Figure 2c). ECNU-21 could be analogous to IPC-15
zeolite,[14] according to the description in the literature.
Nevertheless, the ECNU-21 zeolite derived from Ge-zeolite CIT-
13 via alkali treatment possessed higher crystallinity and more
ordered structure than IPC-15 post-synthesized from the
germanosilicate SAZ-1 through acid-assisted ADOR route.
Moreover, it is worth pointing out that ECNU-21 and IPC-15 are
distinctive in PXRD patterns (Figure S6), indicative of the
differences in both structure and purity.
Ex-situ 29Si MAS NMR spectra were employed to obtain more
information for the structural change in the respect of silicon
coordination during the structural degradation and reconstruction
processes. To confirm the accurate position of (SiO)3Si(OH)
1
groups (Q3) in the spectra, H-29Si cross-polarization MAS NMR
spectrum of ECNU-21 was firstly performed (Figure S9) and the
resonance band at about -101.8 ppm was ascribed to Q3 groups.
The pristine Ge-rich CIT-13 zeolite showed a resonance signal at
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for the resultant ECNU-21 zeolite, which are attributed to Ge-
containing groups nSi(4-n)Ge.[23]
Q4
Q3
The IR technique is also a sensitive method for characterizing
structural change. As displayed in Figure 5, the bands around
1000 and 910 cm−1, attributed to the asymmetric stretching
vibration of Si-O-Ge and symmetric vibration Ge-O-Ge in the
framework,[24] disappeared for ECNU-21 (Si/Ge = 33 or 0.48 mmol
g-1 Ge content), due to the extensive removal of Ge atoms by
84.3% from the parent CIT-13 (Si/Ge = 3.7 or 3.06 mmol g-1 Ge
content). Meanwhile, the extraction of Ge species also made the
band around 550 cm−1 of CIT-13, attributed to the asymmetric Ge-
O-Ge vibration,[24] blue shift to 580 cm−1 for Ge-poor ECNU-21.
The blue shift of 30 cm−1 was caused by the major loss of heavier
Ge atoms to result in a high-silica ECNU-21 zeolite, similar to that
seen for high-silica zeolite CIT-13S (0.52 mmol g-1 Ge content).
Moreover, the pristine 580 cm−1 band of CIT-13, related to the
vibration of D4R units,[23] was absent for ECNU-21, indicating the
deconstruction and removal of D4R units.
f
6.3%
13.2%
18.5%
22.1%
e
d
c
24.6%
24.9%
b
a
-90
-100
-110
-120
-130
Chemical shift (ppm)
Figure 4. Ex-situ 29Si MAS NMR spectra (dotted lines) and fitting results (solid
lines) of as-made ECNU-21P samples (a), and after calcination at 423 K (b),
523 K (c), 623 K (d), 723 K (e) and 823 K (f) for 6 h, respectively.
δ = -101.9 ppm, which is assigned to the Q3 and 1Si3Ge
groups,[22] accounting for 8.1% population (Figure S10 and Table
S2). The hydrolysis in ammonia aqueous solution led to a rapid
increase of Q3 groups to 24.9% (Figure 4), as a result of an
extensive removal of Ge-containing D4R units located between
the cfi-layers. At the initial heating stage (T < 523 K) of reassembly
process, a slight decrease of Q3 groups together with a minor
increase of signals in the range of -112 to -114 ppm arising from
Si(OSi)4 (Q4) groups was observed, indicative of the condensation
of adjacent layers. In the higher temperature region, the
population of Q3 species was sharply decreased to a minimum
value of 6.3% at 823 K, meaning that defects still existed in the
final ECNU-21 zeolite. At this temperature, the d-spacing of layer-
related 200 reflection (d200) reached a minimum value of ca. 8.7 Å
Structure analysis of ECNU-21 zeolite
As has been revealed above, the Si-rich cfi-layers have been well-
preserved in the hydrolysis and reassembly processes. Hence,
the structural model of ECNU-21 zeolite was constructed by
reconnecting adjacent ECNU-21P layers with oxygen linkage in a
periodic manner, which resulted in the formation of a 3D zeolite
with 1D 10-R channel system parallel to c-axis. The structure of
the formed ECNU-21 zeolite was optimized fully (optimizing
fractional coordinates and the unit-cell volume and shape) using
Materials Studio. Figure 6 shows the simulated PXRD pattern
ECNU-21 Exp.
ECNU-21 Sim.
difference
580
Bragg position
0.3
c
550
b
580
609
a
640 600 560 520
c
1000
910
b
a
20
40
60
80
100
120
2Theta (deg.)
Figure 6. Rietveld refinement of the PXRD data for ECNU-21. Experimental
(black) and calculated (red) PXRD patterns, as well as the difference profile, are
shown (blue). The short tick marks (green) below the patterns give the positions
of the Bragg reflections.
1200
1000
Wavenumber
800
600
400
(
cm-1
)
Figure 5. IR spectra in the framework vibration region of CIT-13 (a), CIT-13S
(b) and ECNU-21 (c) after calcination. The inset shows the enlarged region of
640 - 520 cm-1.
matched well with the experimental data, indicating the daughter
structure ECNU-21 was highly crystalline. According to the
refined structure, ECNU-21 possessed the unit-cell parameters of
a = 17.1516 Å, b = 13.7371 Å and c = 5.0289 Å with the space
group Cmmm (65) (Table S3-S7). The refinement of the unit-cell
exhibited that the lattice parameters a decreased from ~27.4 Å for
reported CIT-13 (a = 27.4374 Å, b = 13.8000 Å, c = 10.2910 Å) to
at 823 K. Moreover, the extensive removal of Ge-rich D4R
subunits made the resonances from -102 to -110 ppm disappear
(Figure S7). Both PXRD and 29Si MAS NMR investigations
suggested a full condensation of adjacent layers was completed
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~17.2 Å for ECNU-21. Thus, the difference of interlayer distance
between CIT-13 and ECNU-21 was ca. 5.1 Å (half of a value
represents the interlayer spacing, Scheme 1), in good agreement
with the length of D4R linkage removed in the alkaline media. The
b value was almost identical with that of CIT-13, meaning the
silica-rich layers were well-preserved in the structural
transformation process. As the random distribution of D4R
subunits along c-axis was selectively removed by alkaline
treatment, the lattice parameters c was reduced by half.
To verify the presence of highly ordered structure in ECNU-21,
the investigation has been carried out using 3D electron diffraction
tomography (3D-EDT).[25] The 3D-EDT data set was collected in
the range of -60.4° to +52.2° with a beam-tilt step of 0.1°. From
the acquired data set, 3D reciprocal space was reconstructed
using the software EDT Process (Figure 7A). The unit-cell
parameters calculated from 3D-EDT were a = 17.00 Å, b = 13.57
Å and c = 4.96 Å, which are consistent with the results from PXRD
data. The observed reflection conditions were summarized as hkl:
h + k = 2n; 0kl: k = 2n; h0l: h = 2n; hk0: h + k = 2n; h00: h = 2n;
0k0: k = 2n, which suggests four possible space groups—C222
(21), Cmm2 (35), Cm2m (38) and Cmmm (65). According to the
plane groups of the high-resolution transmission electron
microscopy (HRTEM) images (vide post), C222 and Cmmm are
both consistent. However, considering the original structure,
synthesis process, and PXRD data, the space group Cmmm (65)
is chosen. The cuts of reconstructed reciprocal lattice from three
main directions contain sharp diffraction spots, showing high
crystal-structural coherence (Figure 7B-D). The selected-area
electron diffraction (SAED) pattern taken along [001] direction
shown in Figure 7E is consistent with the 3D-EDT results. The
corresponding HRTEM image has been taken for further
confirmation of the structure, which allows the observation of well-
ordered 10-R channels in the sample (Figure 7F). The averaged
structural feature was obtained from Fourier transform of the raw
HRTEM image using the crystallographic image processing
software CRISP,[26] for which c2mm symmetry is chosen. In an
attempt to gain some insight to the degree of ordering in ECNU-
21, we further acquired the SAED pattern (Figure 7G) and
HRTEM (Figure 7H) from [010] direction. Both show a highly
ordered framework structure with single crystal feature. The
averaged structure was also obtained by the crystallographic
image processing with p2mm symmetry applied. The structural
model of ECNU-21 matches well with the averaged HRTEM
image. Therefore, it further confirmed that the D4R units has been
completely removed from the structure. There is no disorder in the
projection along b-axis. Thus, it can be concluded that ECNU-21
comprises only O-linkages between exactly the same layers as
the parent CIT-13 zeolite.
N2 adsorption in Figure 8A manifested that the ECNU-21 zeolite
was a typical microporous material with lower specific surface
area and micropore volume than the pristine CIT-13 zeolite (376
m2 g-1 and 0.06 cm3 g-1 vs. 579 m2 g-1 and 0.17 cm3 g-1; Table S8),
caused by the pore shrinkage and framework density increase
upon the removal of D4R units. Intergrowth CIT-13, composed of
intersected 14- and 10-R channels, exhibited one board pore size
distribution around 6.5 Å according to Horvath-Kawazoe model
(Figure 8B). After extensive removal of the D4R units and
interlayer condensation process, the resultant ECNU-21 zeolite
Figure 7. (A) Projection of three-dimensional reciprocal space reconstructed from the 3D-EDT data. Three cuts of reconstructed reciprocal lattice giving two-
dimensional slices of 0kl (B), h0l (C) and hk0 (D), respectively. (E) The SAED pattern of the crystal taken from [001] direction. (F) The corresponding HRTEM image.
(G) The SAED pattern of the crystal taken from [010] direction. (H) The corresponding HRTEM image. Inserts are the corresponding averaged TEM images obtained
by crystallographic image processing with c2mm and p2mm symmetry applied, respectively. The structural models are both superimposed.
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150
1.5
1.0
0.5
0.0
A
B
a
100
50
0
b
a
b
0.0
0.2
0.4
0.6
0.8
1.0
5
6
7
8
9
10 11
Relative Pressure (P/P0)
Pore width (Å)
Figure 8. N2 adsorption isotherms (A) at 77 K and pores size distribution (Horvath-Kawazoe method) (B) based on Ar adsorption at 87 K of calcined CIT-13 (a) and
ECNU-21 (b).
possessed a smaller pore size centered at 5.2 Å corresponding to
the 10-R channels running along c-axis.
temperatures (423 - 493 K), which is highly energy-consuming in
the separation of EG product. Using unstable acidic resins as the
catalysts, the H2O/EO ratio was possibly decreased to 8, while
keep a EG selectivity high as 95.6% at 378 K.[28] Recently, a
heterogeneous catalyst Sn-SSZ-13 with active Sn sites confined
in Chabazite cages has made a key breakthrough for EO
hydration at approaching stoichiometric H2O/EO ratio.[29] As Sn
and Ge are the congeners, they are presumed to have similar
chemical and catalytic characteristics.
The reaction parameters of the EO hydration over ECNU-21
zeolite were firstly examined in detail. A significant temperature
effect on the catalytic performance of ECNU-21 zeolite was
observed when the reaction was carried out in the temperature
range of 293 - 353 K (Figure 9A). Increasing the reaction
temperature promoted the EO conversion from 13.5% to 62.1%,
but resulted in a distinct decline of the EG selectivity by > 8%,
because higher reaction temperatures may induce the further
condensation of EG to diethylene glycol (DEG) at the outer
surface of ECNU-21 zeolite.[29] Elevating H2O/EO ratio played a
weak positive role on the EO conversion but did not affect EG
Catalytic performance of ECNU-21 in epoxide hydration
The newly formed pure crystalline ECNU-21 zeolite was
characteristic of high-silica nature and hydrothermal stability
compared to the pristine Ge-rich CIT-13 zeolite. Nevertheless,
ECNU-21 still possessed a Ge content of 0.48 mmol g-1,
corresponding to Si/Ge = 33. The framework Ge ions exhibited
Lewis acidity as evidenced by pyridine-IR spectra (Figure S11).
Taking the advantage of unique 10-R channels and Ge-related
Lewis acidity, ECNU-21 was expected to serve as a promising
shape-selective catalyst for processing important petrochemical
reactions, e.g. the hydration of ethylene oxide (EO) to ethylene
glycol (EG), a bulk chemical with a world market of 28 million tons
per year by 2015.[27] Conventionally, the EO hydration was
conducted in non-catalytic way in industrial process, but with an
extremely high H2O/EO molar ratio of 20 - 25 at elevated
100
100
80
60
40
20
0
100
80
60
40
20
0
B
C
A
80
60
40
20
0
5
10
H2O/EO
15
20
1
S
293
313
333
353
-5
-2
NU
lank
UTL
CIT-13
M
S
M-5
BEC
EC
b
-GS-1
-GS-1
T (K)
Z
H
-ZS
-
CIT-13
Na
H
Na
Figure 9. (A) Effect of reaction temperature on the EO conversion (blank) and EG selectivity (solid) for the EO hydration catalyzed by ECNU-21 catalyst. Reaction
conditions: cat., 0.3 g; EO, 10 mmol; H2O, 150 mmol; time, 8 h. (B) Effect of H2O/EO ratio on the EO conversion (blank) and EG selectivity (solid) for the EO
hydration catalyzed by ECNU-21 catalyst. Reaction conditions: cat., 0.3 g; EO, 10 mmol; temp., 333 K; time, 8 h. (C) EO conversion (blank) and EG selectivity
(solid) in the EO hydration catalyzed by different zeolite catalysts. Reaction conditions: cat., 0.3 g; EO, 10 mmol; H2O, 150 mmol; temp., 333 K; time, 8 h.
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10.1002/chem.201900173
Chemistry - A European Journal
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selectivity keeping at ca. 96% (Figure 9B). Hence, ECNU-21 was
capable to catalyze EO hydration reaction, providing a high EO
conversion (52.8%) and EG selectivity (96.6%) at 333 K with
H2O/EO ratio of 15, that is, under extremely mild conditions in
comparison to conventional non-catalytic process. Under the
identical conditions, the catalytic performance of as-designed
zeolite ECNU-21 in the EO hydration was compared to other
germanosilicates and aluminosilicates (Figure 9C and Table S9).
No products could be detected without any catalyst, while
considerable EO conversions (5.2% - 52.8%) were obtained with
the catalysts investigated in present study. Moreover, the ECNU-
21 zeolite exhibited the highest EG selectivity of 96.6% at 8 h,
followed by H-ZSM-5 (~92.3%) and H-GS-1 (~91.7%) catalysts.
In contrast, the selectivity obtained by large and extra-large pores
germanosilicates (UTL, CIT-13, CIT-13S, BEC) were only ~70%.
The by-product was DEG with larger molecular dimension than
EG, suggesting that the open pore system favored the further
condensation of EG and EO. Furthermore, the sodium-type
zeolites, Na-GS-1 and Na-ZSM-5, exhibited an apparent
decrease in catalytic activity than H-GS-1 and H-ZSM-5 as a
result of losing a large amount of the acid sites. The framework
structure was well-preserved after the reaction for ECNU-21, CIT-
13S and MFI-type zeolites (Figure S12).
analysis and in-situ IR. EG/ECNU-21 and EG/H-ZSM-5 both
showed weight loss in the temperature range of 298 - 1073 K
(Figure S13). In the case of EG/ECNU-21, the first stage (298 -
400 K) corresponded to the removal of the physically adsorbed
water, and the second one, in the temperature range of 400 - 450
K, was assigned to the combustion of the physically absorbed EG.
With respect to EG/H-ZSM-5, it displayed wider temperature
range (400 - 600 K) including a small shoulder peak around 520
K in the second stage, indicating that the removal of adsorbed EG
from H-ZSM-5 was more difficult in comparison to ECNU-21. A
similar phenomenon has also been observed in the EG adsorption
of Ti-MWW and H-MCM-22.[30] As shown in Figure S14, two
bands emerged at 2885 and 2951 cm-1 in the in-situ IR spectra
A100
ECNU-21
80
60
40
20
0
CIT-13S
100
80
60
40
20
0
100
95
90
85
80
75
0
4
8
12 16 20 24 28
Time (h)
ECNU-21
ECNU-21
Conv.
water washing
Sel.
B 100
calcined in air
1.0
0.8
0.6
0.4
0.2
0.0
H-ZSM-5
H-ZSM-5
80
60
40
20
0
0
2
4
6
8
Time (h)
Figure 10. Dependence of EO conversion (blank) and EG selectivity (solid) on
reaction time in the EO hydration over ECNU-21 and H-ZSM-5. Reaction
conditions: cat., 0.3 g; EO, 10 mmol; H2O, 150 mmol; temp., 333 K.
1
2
3
4
5
6
Recycle times
Figure 11. (A) Dependence of EO conversion (blank) and EG selectivity (solid)
on reaction time in the EO hydration over ECNU-21 and CIT-13S catalysts.
Reaction conditions: cat., 0.3 g; EO, 10 mmol; H2O, 150 mmol; temp., 333 K.
(B) EO conversion (blank), EG selectivity (solid) and the change of Ge content
in the recycling test for EO hydration over ECNU-21 catalyst. Reaction
conditions: cat., 0.3 g; EO, 10 mmol; H2O, 150 mmol; temp., 333 K; reaction
time for reuse, 24 h. The used ECNU-21 catalyst was regenerated by washing
with deionized water or further calcination in air at 823 K for 6 h
Despite of the comparable 10-R pore channels,
germanosilicate ECNU-21 with weaker acidity showed a higher
ability for converting EO to EG in comparison to aluminosilicate
H-ZSM-5 with stronger acidity (Figure 9C). In order to clarify this
phenomenon, we have compared the time course of EO hydration
over ECNU-21 and H-ZSM-5 (Figure 10). H-ZSM-5 exhibited a
higher initial EG conversion than ECNU-21 during the first 2 h
possibly due to its stronger acidity. And the conversion was
maintained at ~27% irrespective of prolonging the time, indicating
the extremely fast deactivation of H-ZSM-5 after 2 h. In contrast,
the EO conversion over ECNU-21 displayed a linear relationship
with reaction time, reaching 52.8% at 8 h. In order to illustrate the
significantly different resistance to the deactivation in the
hydration of EO between ECNU-21 and H-ZSM-5, the removal of
EG adsorbed in ECNU-21 and H-ZSM-5 were compared by TG
that are assigned to symmetrical and asymmetrical stretching CH2
groups, respectively.[31] These bands should be closely related to
EG and decreased in intensity with raising the desorption
temperature for EG/ECNU-21. On the contrary, these bands are
more resistant against evacuation for EG/H-ZSM-5, indicative of
a stronger interaction between EG and H-ZSM-5. Hence, it was
reasonable that H-ZSM-5 suffered a faster deactivation than
ECNU-21, because the product of EG could not desorb easily
from H-ZSM-5 and then possibly caused a carbon deposition on
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FULL PAPER
Table 1. Hydration of various epoxides over ECNU-21 catalyst[a]
.
No.
Substrate
Conv. / %
52.8
Product
Sel. / %
96.6
1
2
3
4
5
35.7
34.1
30.2
28.4
96.1
96.7
96.9
97.4
[a] Reaction conditions: cat., 0.3 g; substrate, 10 mmol; H2O, 150 mmol; temp., 333 K; time, 8 h.
the Al active sites.
selective Lewis acid catalysis confined in the 10-R pores of
Especially, ECNU-21 also showed a gradually increased
activity with prolonging reaction time and reached a remarkable
catalytic performance of 97.1% EO conversion at 28 h together
with a steady EG selectivity of 95.5% (Figure 11A). The main by-
product was DEG with a selectivity merely 4.5%. On the contrary,
the framework-stabilized CIT-13S, with the same building layers
as that of ECNU-21 but larger 14×10-R channels, gave a lower
EO conversion of 87.1% and much lower EG selectivity of 67.5%.
The extra-large pores are non-shape selective and could not
suppress the dimerization of EG, with the DEG selectivity up to
32.5%. An obviously decrease in activity (e.g., EO conversion
from 94.9% to 75.9%) was observed within four cycles when the
used ECNU-21 catalyst was just washed with deionized water for
reuse (Figure 11B). However, the catalytic activity could be fully
recovered when the used catalyst was further calcined to remove
any organic species. The deactivation of ECNU-21 is presumed
to be caused mainly by coke formation. Besides, the Ge content
was almost constant during six reaction-regeneration cycles.
Inspired by the great success of zeolite ECNU-21 in EO
hydration reaction, we further expanded to other epoxide
substrates (epoxypropane, 1,2-epoxybutane, 1,2-epoxypentane
and 1,2-epoxyhexane) producing the corresponding 1,2-diols
(Table 1). Along with the increase of the substrate size, ECNU-21
lost its hydration activity gradually because of steric restrictions.
When using bulky 1,2-epoxyhexane as substrate, the conversion
dropped to 28.4%. It is worth highlighting that in the hydration of
various epoxide substrates, the expected high 1,2-diol selectivity
(> 96%) could be maintained by ECNU-21 catalyst due to a 10-R
channel system. These results confirmed the feasibility of shape-
ECNU-21 zeolite.
Conclusions
In conclusion, an intergrowth Ge-CIT-13 with a large amount of
Si-O-Si interlayer bonds (> 21%) in D4R units encountered failure
to form a lamellar product under acidic circumstance. For the first
time, a facial alkali treatment strategy was proposed to efficiently
cleave the metastable Si-O-Si interlayer bonds vertical to layers,
realizing the purification of polymorph CIT-13 into single
crystalline ECNU-21 zeolite with a novel topology. With moderate
Lewis acidity confined in ordered 10-R channels, ECNU-21
served as a promising selective catalyst in the EO hydration
reaction with the EO conversion of > 97% and EG selectivity ca.
96%. The profound work deepens the knowledge in hydrolyzing
Ge-containing zeolites and provides brand new ideas for
extending the practical applications of germanosilicates.
Experimental Section
Details for synthesizing the parent germanosilicate CIT-13, partial
characterization, catalytic reaction and additional discussion are described
in the Supplementary Information.
Synthesis of high-silica zeolite ECNU-21. The powder of calcined Ge-
CIT-13 (Si/Ge molar ratio of 3.7) was immersed in 1.0 wt% NH3•H2O
aqueous solution with a solid-to-liquid weight ratio of 1 : 100 at 298 K for
24 h in a Teflon-lined autoclave. This treatment induced pronounced
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10.1002/chem.201900173
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structural changes to the CIT-13 framework. The solid was recovered by
filtration, extensively washing with deionized water and drying overnight at
353 K, forming a layered precursor, denoted as ECNU-21P. The product
was further calcined at 823 K for 6 h in air, inducing full interlayer
condensation and then giving rise to stable ECNU-21 zeolite with 3D
crystalline structure and novel high-silica framework (Si/Ge = 33).
Plus 400 WB spectrometer using a 2.5 mm HX MAS probe. The chemical
shift was referenced to trifluoroacetic acid at -76.55 ppm.
Structure analysis
The powder diffraction patterns of ECNU-21 was indexed with the program
X-Cell[32] and the software of Materials Studio[33]. The crystalline structure
was refined with FULLPROF.[34] Structure refinement of the diffraction data
of ECNU-21 was started in Cmmm (65) space group. All the atoms were
refined isotropically, while neutral scattering factors were employed for
them. The background curve was fitted by using the linear interpolation
between a set background points with refinable heights. Soft restraints
were placed on the bond distances of the framework throughout the
refinement. On the basis of the HRTEM images of ECNU-21, a structure
model was derived and subjected to geometry optimization using the
quantum mechanical code Dmol3 in the Materials Studio suite of programs.
The exchange-correlation functional was expressed by the generalized
gradient corrected (GGA) functional by Perdew-Burke-Ernzerh
parametrization. Crystallographic image processing on HRTEM images
was carried out using the program CRISP.[26] The simulations of the TEM
images were performed using the MacTempas software.
Structural modification of CIT-13 zeolite by acid treatment. Following
the traditional acid-assisted ADOR procedures, the structural change
through hydrolysis of D4R units in CIT-13 framework was also attempted
for control experiment. The acid treatment of the calcined CIT-13 sample
was performed in HCl aqueous solution with changeable concentrations
(0.1 - 4.0 M) at a solid-to-liquid weight ratio of 1 : 100 and at 368 K for 16
h. After being filtrated, washed and dried, the obtained solids were calcined
at 823 K for 6 h.
To further investigate the structural change of CIT-13, hydrolysis
treatment was carried out by following our previously reported
procedures.[19] The CIT-13 zeolite in calcined form was soaked in 1 M
HNO3 aqueous solution with a solid-to-liquid weight ratio of 1 : 100, and
the mixture was heated in a Teflon-lined autoclave at 443 K for a different
period of times (15 min - 24 h). Hydrolysed samples were then recovered
with the same abovementioned procedures. Note that the sample obtained
by HNO3 treatment for 2 h exhibited a PXRD pattern same as pristine CIT-
13 zeolite, implying the HNO3 treatment only stabilized the *CTH
framework but did not cause effective structural degradation as reported
before.[19] The resultant material is denoted as CIT-13S (Si/Ge = 30).
Catalytic reactions
The catalytic hydration of various epoxides (ethylene oxide, epoxypropane,
1,2-epoxybutane, 1,2-epoxypentane and 1,2-epoxyhexane) were
performed under vigorous stirring in an autoclave reactor equipped with a
Teflon-inner. In a typical experiment, a mixture of catalyst (0.3 g), epoxides
(10 mmol) and H2O (150 mmol) was vigorously stirred at 333 K. The effect
of both reaction temperature (293 - 353 K) and H2O/EO molar ratio (5 - 20)
on the catalytic performance of ethylene oxide (EO) hydration on ECNU-
21 were also investigated.
Characterization methods
Powder X-ray diffraction (PXRD) patterns were collected on a Rigaku
Ultima IV X-ray diffractometer using CuKα radiation (λ = 1.5406 Å, 35 kV,
25 mA). High resolution data for determining the ECNU-21 structure by
Rietveld refinement were recorded on a Bruker D8 Advance X-ray
diffractometer using CuKα radiation (35 kV, 25 mA) with an incident
monochromatic X-ray wavelength of 1.5406 Å. To improve accuracy, the
sample was continuously rotated in a 0.5 mm glass capillary. The in-situ
PXRD measurements were also carried out on Bruker D8 Advance X-ray
diffractometer with a wavelength 1.5406 Å. The ECNU-21P sample was
packed in air at ambient pressure inside ceramic plate and heated from
298 to 873 K with a ramp rate of 30 K min-1. The high-resolution
transmission electron microscopy (HRTEM) images were collected with a
FEI G2F30 with an accelerating voltage of 300 kV. The 3D-EDT data
collected on a JEOL JEM-2100 microscope that was equipped with a LaB6
gun operated at 200 kV (Cs 1.0 mm, point resolution of 2.3 Å). A TENGRA
CCD camera (2304 × 2304 pixels with a 2:1 fiber-optical taper and an
effective pixel size of 18 μm2) was used for digital recording of the
diffraction pattern. The collected dataset was processed using the EDT
Process software. N2 adsorption isotherms were recorded at 77 K on a
BELSORP-MAX instrument after activating the calcined samples at 573 K
for at least 10 h under vacuum conditions. The pore size distribution was
calculated by the Horvath-Kawazoe (HK) method based on the data
collected by Ar adsorption at 87 K on a Micromeritics ASAP2020
adsorption instrument. The IR spectra in the zeolite framework vibration
region were collected on a Nicolet Nexus 670 FT-IR spectrometer in
absorbance mode using KBr technique (6 wt% wafer). The Si, Ge, Al and
Na contents were measured with inductively coupled plasma (ICP)
The recycling tests were performed to investigate the recyclability of
ECNU-21 catalyst. After each reaction, the used ECNU-21 catalyst was
centrifuged, washed with deionized water and dried at 353 K overnight for
reuse in the next cycle. To restore the initial activity level, the used catalyst
was also regenerated with further calcination in air at 823 K for 6 h.
The products were separated from the reaction mixture by centrifugation
and determined by a gas chromatograph (Shimadzu 2014) equipped with
an Rtx®-Wax capillary column (30 m × 0.25 mm × 0.25 μm) and FID
detector using isopropanol (or cyclohexanone) as internal standard. The
products formed were identified by a GC-MS (Agilent 6890 series GC
system, 5937 network mass selective detector). The results of the
hydration of EO were calculated as follows.
Acknowledgements
spectroscopy on
a Thermo IRIS Intrepid II XSP atomic emission
spectrometer. Solid-state 29Si MAS NMR spectra were measured on a
VARIAN VNMRS 400WB NMR spectrometer using single-pulse method at
a frequency of 79.43 MHz, a spinning rate of 3 kHz, and a recycling delay
of 60 s. The chemical shift was referred to [(CH3)3SiO]8SiO12. The 19F MAS
NMR spectrum of pristine CIT-13 was acquired at 9.4 T on a Varian Infinity
The authors gratefully acknowledge the financial support from
NSFC of China (21533002, 21603075 and 21571128), China
Ministry of Science and Technology (2016YFA0202804), National
Excellent Doctoral Dissertation of China (201454), Shanghai
Rising-Star Program (17QA1401700).
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The authors declare no conflict of interest.
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10.1002/chem.201900173
Chemistry - A European Journal
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Structural conversion from polymorph
germanosilicate CIT-13 to novel
ECNU-21 zeolite as shape-selective
catalyst for ethylene oxide hydration
Xue Liu, Wenting Mao, Jingang Jiang,
Xinqing Lu, Mingming Peng, Hao Xu,*
Lu Han,* Shunai Che, Peng Wu*
Page No. – Page No.
Topotactic conversion of alkali-
treated intergrowth germanosilicate
CIT-13 into single crystalline ECNU-
21 zeolite as shape-selective catalyst
for ethylene oxide hydration
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