Breaking Structural Energy Constraints: Hydrothermal Crystallization of High-Silica Germanosilicates by a Building-Unit Self-Growth Approach

Article abstract of DOI:10.1002/chem.201802255

Zeolites, a class of crystalline microporous materials, have a wide range of practical applications, in particular serving as key catalysts in petrochemical and fine-chemical processes. Millions of zeolite topologies are theoretically possible. However, to date, only 235 frameworks with various tetrahedral element compositions have been discovered in nature or artificially synthesized, among which approximately 50 topologies are available in pure-silica forms. Germanosilicates are becoming an important zeolite family, with a rapidly increasing number of topological structures having unusual double four-membered ring (D4R) building units and large-pore or extra-large-pore systems. The synthesis of their high-silica analogues with higher (hydro)thermal stability remains a great challenge, because the formation of siliceous D4R units is kinetically and thermodynamically unfavorable in hydrothermal systems. Herein, it is demonstrated that such D4R-containing high-silica zeolites with unexpected crystalline topologies (ECNU-24-RC and IM-20-RC) are readily constructed by a versatile route. This strategy provides new opportunities for the synthesis of high-silica zeolite catalysts that are hardly obtainable by conventional hydrothermal synthesis and may also facilitate a breakthrough in increasing the number and types of zeolite materials with practical applications.

Full text of DOI:10.1002/chem.201802255

A Journal of
Accepted Article
Title: Breaking Structural Energy Constraints: Hydrothermal
Crystallization of High-Silica Germanosilicates via Building Units
Self-Growing Approach
Authors: Mingming Peng, Jingang Jiang, Xue Liu, Yue Ma, Meichen
Jiao, Hao Xu, Haihong Wu, Mingyuan He, and Peng Wu
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201802255
Link to VoR: http://dx.doi.org/10.1002/chem.201802255
Supported by

Chemistry - A European Journal
10.1002/chem.201802255
FULL PAPER
Breaking
Structural
Energy
Constraints:
Hydrothermal
Crystallization of High-Silica Germanosilicates via Building Units
Self-Growing Approach
Mingming Peng, Jingang Jiang*, Xue Liu, Yue Ma, Meichen Jiao, Hao Xu, Haihong Wu, Mingyuan He,
and Peng Wu*
Abstract: Zeolites, a class of crystalline microporous materials,
have a wide range of practical applications, in particular serving as
key catalysts in petrochemical and finechemical processes. Millions
of zeolite topologies are theoretically possible. However, to date,
only 235 frameworks with various tetrahedral element compositions
have been discovered in nature or artificially synthesized, among
which approximately 50 topologies are available in pure silica forms.
Germanosilicates are becoming an important zeolite family, with a
rapidly increasing number of topological structures with unusual
double four-membered-ring (D4R) building units and large-pore or
extra large-pore systems. The synthesis of their high-silica
analogues with higher (hydro)thermal stability remains a great
challenge because the formation of siliceous D4R units is kinetically
structural elucidation and theoretical calculations have provided
solid evidence that the organic structure-directing agents
(OSDAs) and the inorganic Ge ions also with unique structure-
directing ability work synergistically to construct the specific D4R
and/or D3R units that are rarely formed in Ge-free silicate or
aluminosilicate systems.[9,10]
Germanosilicate-type zeolites with
a
large variety of
topologies and pore dimensions have been rapidly developed in
recent years, but they are usually hydrothermally unstable
because of easy hydrolysis of Ge-O bonds. For this feature,
based on a top-down strategy, novel method ADOR (assembly–
disassembly–organization–reassembly) has been proposed to
post-synthesize the new zeolites with reduced pore dimensions
and thermodynamically unfavorable in hydrothermal systems. Herein, by selectively removing the Ge-rich D4R units.[11] On the other
we demonstrate that such D4R-containing high-silica zeolites with
unexpected crystalline topologies (ECNU-24-RC and IM-20-RC) are
readily constructed through a versatile route. This strategy provides
new opportunities for the synthesis of high-silica zeolite catalysts
that are hardly obtained by conventional hydrothermal synthesis and
it would also facilitate a break-through in increasing the number and
types of zeolite materials with practical applications.
hand, as the hydrothermal stabilities of Ge-rich zeolites are
problematic to practical applications, they are subject to
stabilization mainly via post-synthesis strategies. Alumination
under mild acidic conditions incorporates Al ions into the
germanosilicate frameworks, which effectively enhances their
hydrothermal stability.[
12,13]
In addition, quasi-all-silica zeolites
are readily post-synthesized from the UTL[ and ITQ-22 Ge-
containing precursors through the isomorphous substitution of
silicon for germanium.
14]
[15]
Generally, the direct crystallization of D4R- or D3R-unit-
containing materials with high Si/Ge ratios is unfavorable,
although a few examples are known, including hydrothermally
Introduction
Microporous zeolites are widely used in catalysis, adsorption
and separation.[ Because of their extraordinary commercial
impact, new zeolites with attractive topologies and pore
dimensions are urgently desired. Approximately 235 defined
topologies are currently available,[2] but the number is far from
the millions of possible zeolite structures estimated by
[16]
[17]
synthesized pure siliceous HPM-1
and ITQ-7 zeolites with
1]
the same topologies as the STW and ISV germanosilicate
analogues. Why are the high-silica analogues difficult to
synthesize directly?
A common explanation is that the
coordination geometry difference between Si and Ge ions is too
large. In the case of siliceous structures, the average Si-O-Si
bonds have a relatively large angle (ca. 148°) and a short length
[
3]
computational enumeration. Various strategies have been
proposed to synthesize zeolites with new crystalline structures.
For example, the introduction of germanium with a structure
directing functionality in the initial gels has proven to be
successful. Zeolite topologies closely depend on the framework
composition, which is controlled by varying the component ratio
in the synthetic gels. The co-existence of Ge ions in gels has
resulted in the synthesis of various zeolitic germanosilicates with
novel structures, such as ITQ, IM and SU series.[4-8] Experiments,
(ca. 1.61 Å for Si-O), which is not favorable for the formation of
D4R or D3R building units. By contrast, the T-O-Ge (T = Si, Ge)
bonds possibly form smaller angles with the help of the O-Ge
bonds with a longer length, which is beneficial for releasing the
geometric stress and thus stabilize D4R and D3R building
[18]
units.
The aforementioned explanation is mostly reasonable
and acceptable; however, it considers only structural issues and
does not take into account the synthetic pathways that are
equally critical to the outcoming zeolites. Once the structural
strains in the D4R or D3R building units can be weakened by
taking unusual synthetic pathways, the synthesis of
corresponding high-silica zeolites would be possible.
[
*] M. Peng, Dr. J. Jiang, X. Liu, Y. Ma, M. Jiao, Dr. H. Xu, Prof. H. Wu,
Prof. M. He, and Prof. P. Wu
Shanghai Key Laboratory of Green Chemistry and Chemical process
School of Chemistry and Molecular Engineering, East China Normal
University North Zhongshan Rd. 3663, Shanghai 200062 (China)
E-mail: jgjiang@chem.ecnu.edu.cn
E-mail: pwu@chem.ecnu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
Here, we put forward a new strategy for hydrothermal
-
synthesis of D4R-containing high-silica zeolites in OH media
containing OSDA and F- anion (Scheme 1). The proposed
concept demonstrates that the chemical compositions of zeolite
products depend on the generation sequence of building units.
The high-silica zeolites were finally formed by weakening or
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Chemistry - A European Journal
10.1002/chem.201802255
FULL PAPER
overcoming the structural strains of specific building units. Two
high-silica zeolites (IM-20-RC and ECNU-24-RC), developed by
this novel method, possess high crystallinity and stability, with
well-defined pore systems composed of small and large pore
windows. This work not only introduces the synthesis strategy
for the high-silica zeolites that were once thought to be
unfavorable but also contributes definitely to increase the
number of available porous solids with practical applications.
readily synthesized using hydrothermal crystallization (Scheme
1). Powder XRD patterns of the products indicate that when the
two calcined parent zeolites (Figure 1a) were treated in a 2 M
HNO
3
solution, amorphization and structural degradation
occurred as a result of the extensive hydrolysis of T-O-Ge bonds
(Figure 1b). Moreover, the acid treatment leached the Ge
species to form the high-silica region and increased the Si/Ge
ratio to greater than 10 (Table S2). When the acid-washed
materials were crystallized under suitable conditions,
hydrothermally stable high-silica zeolites with high crystallinity
and good phase purity were achieved (Figure 1c), and they
maintain a high crystallinity after further calcination (Figure 1d).
UWY
Si/Ge
1
8
d
c
18
19.4
b
3.4
a
Scheme 1. The strategy for synthesizing D4R-containing high-silica zeolites
5
10
15
20
25
30
using UWY-type IM-20 as an example.
UOS
Si/Ge
Results and Discussion
1
0.5
d
c
The germanosilicates constitute an important zeolite family
that exhibits structural diversity in terms of variable pore size
and topology. Usually, these materials crystallize only at
relatively low Si/Ge ratios (e.g., ECNU-24 and IM-20 in Table
S1). The high Ge content in the framework favorable to the
structure construction but decreases the hydrothermal stability of
Ge-zeolites by facilitating the hydrolysis of the Si(Ge)-O-Ge
bond framework (Figure S1). The high-silica analogues of
germanosilicates (e.g., Si/Ge ratio greater than 5) are hardly
obtained by direct hydrothermal synthesis because the lack of
sufficient Ge ions with structure-directing functionality often
leads to poor crystallinity and impurities (Figure S2).
10.5
10.7
b
a
3
.2
5
10
15
20
25
30
2
Theta (degrees)
Figure 1. XRD patterns of various UWY-type IM-20 and UOS-type ECNU-24
zeolites: calcined Ge-rich pristine (823 K, 6 h) (a); after treatment in a 2 M
HNO
3
solution (443 K, 0.5 h; 443 K, 5 h, respectively) (b); as-prepared high-
silica samples (c); after further calcination (823 K, 6 h) (d).
In this work, the high-silica zeolites IM-20-RC and ECNU-24-
RC, which exhibit UWY and UOS topologies, respectively, were
Table 1. The physicochemical properties of as-prepared and treated materials.
[
a]
Yield[c]
(%)
S
BET
2
V
total
3
V
micro
3
V
meso
3 -1
Si/Ge[b]
Crystallinity[d] (%)
Sample
-1
-1
-1
(
m g )
(cm g )
(cm g )
(cm g )
ECNU-24
ECNU-24-A
ECNU-24-RC
IM-20
244
199
397
423
396
481
0.16
0.21
0.24
0.34
0.36
0.35
0.09
0.06
0.14
0.15
0.14
0.18
0.07
0.15
0.10
0.19
0.22
0.17
3.2
-
100
32
10.7
10.5
3.4
75
89
-
112
100
26
IM-20-A
19.4
18
75
75
IM-20-RC
139
[
a] Obtained using a t-plot method. [b] Measured using ICP. [c] Determined from the weight of the solid product after calcination. [d] The relative crystallinity
of the pristine Ge-rich samples is assumed to be 100%.
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Chemistry - A European Journal
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zeolite crystals, and then disintegrated debris crystallized (rather
than a simple structural mending).
2
2
1
1
5
0
5
0
5
0
a
c
b
d
Figure 3A shows the IR spectra in the framework vibration
-1
region for calcined IM-20 and IM-20-RC. The band at 1023 cm ,
which is ascribable to the Si-O-Ge asymmetric stretching
vibration,[20] nearly disappeared in the spectrum of IM-20-RC
because of its much lower Ge content. The parent zeolite IM-20
2
4
6
8
10 12 14
Particle size (nm)
-
1
-1
exhibited vibrational bands at approximately 525 cm , 551 cm
-1
and 567 cm , which were assigned to the Ge-rich D4R units,
whereas the bands for Ge-less IM-20-RC were blue-shifted to
42 cm , 563 cm , 588 cm and 620 cm . Similar changes in
the vibrations of the D4R units were also observed after ECNU-
was converted into ECNU-24-RC (Figure S8). The
-
1
-1
-1
-1
5
24
aforementioned IR bands were assigned on the basis of DFT
calculations according to the combined Si/Ge compositions in
the D4R units. The calculated band positions were in good
agreement with the experimental values (Figure 3B, Tables S3-
S6). This consistency suggests that the synthesis of high-silica
zeolites essentially involved the reorganization of D4R units
when the Ge content was decreased.
Figure 2. TEM images of calcined ECNU-24 (a), acid-treated sample ECNU-
4-A (b, c) and zeolite ECNU-24-RC (d). The inset shows the size distribution
-
The presence of F ions in the synthetic sol-gels effectively
2
expands the range of possible chemical compositions, structure
topologies and preparation conditions for the crystallization of
zeolites.[21-23] The F ions may act as mineralizing agents and
of the nanoparticles in ECNU-24-A.
Table 1 and Figure S3 show the physicochemical properties
and N adsorption/desorption isotherms. Compared to the parent
2
-
and acid-washed samples, the zeolites IM-20-RC and ECNU-24-
RC exhibited higher Si/Ge ratios (10.5 – 38), higher crystallinities
and larger surface areas. Moreover, their micropore volumes
were close to the theoretical values,[19] mostly because these
zeolites in calcined form were stable against hydrolysis when
exposed in moisture-laden air. Notably, the novel method
significantly enhanced the hydrothermal stability of the resultant
Ge-less germanosilicates. The crystalline structure of IM-20-RC
1.5
Si
O
588 542
620 563
A
1.2
b
a
5
25
551
67
0
0
0
0
.9
.6
.3
.0
5
1
023
6
50 600 550 500-1
Wavenumber (cm )
tolerated three harsh treatments with a 2 M HNO
3
solution at
443 K for 24 h (Figure S4A). Similarly, ECNU-24-RC was
b
a
hydrothermally stable in a 2 M HNO
temperatures (Figure S4B).
3
solution at different
To clarify the crystallization mechanism, we traced the
reinforcement process of zeolites IM-20 and ECNU-24. The
changes in the zeolite crystalline morphology were investigated
using TEM and SEM techniques. The SEM images indicated
that as-prepared IM-20 and ECNU-24 were composed of the
crystals with a smooth surface (Figure S5 and Figure S7),
whereas the crystal surface of the corresponding acid-washed
materials appeared rough, possibly because of an extensive
demetallization. The morphologies of high-silica zeolites were
recovered. The TEM images further confirmed these morphology
changes. The acid treatment disassembled the crystals of as-
prepared ECNU-24 into nanoparticles with an average diameter
of 6 – 10 nm (Figure 2b and 2c) because of the corrosion that
followed the hydrolysis of the T-O-Ge bonds in the acidic
solution. The plate crystals of the ECNU-24-RC zeolite
resembled the morphology of the parent sample even though it
possessed cavities, indicating that the dissolved small particles
were crystallized in the presence of OSDA. Similar
morphological changes were observed for IM-20 (Figure S6).
These phenomena imply that the hydrothermal crystallization for
high-silica zeolites involved the disintegration of the original
1
350 1200 1050
900
750
600
450
-
1
Wavenumber (cm )
6
60
B
Blue shift
640
6
6
20
00
580
5
5
60
40
520
3
Si, 5Ge 6Si, 2Ge 7Si, 1Ge 7Si, 1Ge 8Si, 0Ge
IM-20
IM-20-RC
Figure 3. (A) IR spectra in the framework vibration region of pristine IM-20
(Si/Ge = 3.4) (a) and IM-20-RC (Si/Ge = 18) (b) both in calcined form. The
inset shows an enlarged area of the breathing vibrations of the D4R units; (B)
the DFT calculated IR vibration frequencies depending on the Si/Ge
distributions of D4R units for IM-20 and IM-20-RC.
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Chemistry - A European Journal
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catalysts for the formation of Si-O-Si bonds during the nucleation, resonances at -37.8 ppm and -39.5 ppm for the (8Si, 0Ge)-D4R
crystallization and growth of zeolite crystals.[24,25] In some cases,
units. The resonances of ECNU-24-R were the same as those of
pristine ECNU-24, however, the resonance at -39.5 ppm, which
was attributed to the (8Si, 0Ge)-D4R units, substantially
increased in intensity, which indicates an increase in the quantity
of Ge-free (8Si, 0Ge)-D4R units.
-
F ions possess unique structure-directing roles through strong
interactions with the zeolite therein.[26,27] When the IM-20-A was
-
crystallized in the absence of F ions, only a material with an
2
extremely low framework obtained, the excess H O or less
OSDA were all have very negetive influences to synthesize high
crystalline IM-20-R (Figure S9 and Table S7). Meanwhile,
thermogravimetric (TG) analysis indicated that the OSDA
species participated in the crystallization process (Figure S10).
Thus, a cooperative interaction of both F- ions and organic
cations with the silicate framework was crucial for the
construction of high-silica zeolite structures.
Rietveld refinements were carried out to investigate the phase
purity and crystalline structures of the high-silica zeolites
synthesized via the novel method. The technical details and
results of the structure refinement are summarized in Tables S8
- S16. The refined structures of the high-silica IM-20-RC and
ECNU-24-RC are shown in Figure 5. The Rietveld results with
acceptable and reasonable R-factors (Rwp = 11.1 %, Rp =
10.7 %, Rexp = 2.90 % for IM-20-RC; Rwp = 8.23 %, Rp =
8.35 %, Rexp = 4.74 % for ECNU-24-RC) indicate that the
simulated powder patterns fit well with the experimental data. In
For the D4R-containing zeolites synthesized in a fluoride
19
medium, F is a useful NMR probe to elucidate the local
composition in the framework because its chemical shift strongly
[
20,28]
depends on the Ge content in the D4R units.
Thus, owing to
the preferential locations of both F and Ge in D4R units,[29] 19
-
4+
F
Si
Ge
O
(
8Si, 0Ge)-D4R
MAS NMR spectroscopy was used to provide further evidence
that the high-silica frameworks were successfully reconstructed.
(
7Si, 1Ge)-D4R
19
F
(4Si, 4Ge)-D4R
The F MAS NMR spectrum of as-prepared IM-20 (Si/Ge = 3.4)
exhibited a unique broad resonance at -9.1 ppm (Figure 4a).
After the Ge content was reduced by acid washing and further
crystallization was carried out, three new resonances appeared
in the spectrum of IM-20-R (Si/Ge = 18) at -20.4 ppm, -37.8 ppm
and -39.5 ppm (Figure 4b). However, compared with the
spectrum of the as-prepared ECNU-24, the spectrum of ECNU-
-
39.5
-
21.2
-
10.9
d
c
-
37.8
-
20.4
-20
b
a
2
4-R exhibited changes only in the relative intensities of the
three resonances (Figure 4c and 4d), that is, the resonance
intensity increased with increasing magnetic field strength.
-9.1
-10
19
Normally, F resonances with chemical shifts at approximately -
10
0
-30
-40
-50
-
3
8, -20 and -9 ppm correspond to F ions located at the center of
Chemical Shift (ppm)
D4R units with chemical compositions of (8Si, 0Ge), (7Si, 1Ge),
and (4Si, 4Ge), respectively.[20,28] Thus, the high-silica zeolites
mainly contained D4R units composed of 8Si. The UWY
topology possesses a 3D pore system and two structurally
different D4R cages (Figure S11A).[19] Consistent with these
crystallographic inequivalent features, IM-20-R exhibited two
Figure 4. 19F MAS NMR spectra of as-prepared IM-20 (a), IM-20-R (b), as-
prepared ECNU-24 (c) and ECNU-24-R (d). The Si, Ge, F and O ions in the
D4R units presented in yellow, green, purple and red, respectively. The novel
method efficiently generated Ge-less or pure-silica D4R units in high-silica
products.
Yobs
Yobs
A
Ycal
Yobs-Ycal
Bragg_position
B
Ycal
Yobs-Ycal
Bragg_position
X 6
X 3
1
0
20
30
40
50
1
0
20
30
40
2
Theta (degree)
2
Theta (degree)
3
20
40
60
80
100
120
3
20
40
60
80
100
120
2
Theta (degree)
2 Theta (degree)
Figure 5. Observed (black), calculated (red) and difference (blue) profiles for the Rietveld refinement of IM-20-RC (A) and ECNU-24-RC (B). The patterns in the
high angle region have been scaled up by a factor of 6 and 3, respectively, to show more detail. The short-tick marks (green) below the patterns indicate the
Bragg reflection positions.
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Chemistry - A European Journal
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the case of IM-20-RC, the four Ge-containing T sites yielded a
total of 3.22 Ge sites per unit cell, which corresponds to a Si/Ge
ratio of 17.6. This ratio is close to the Si/Ge ratio of 18.0
determined by chemical analysis. As a result of the substitution
of Si for Ge, the lattice parameters of IM-20-RC decreased
compared to those of Ge-rich, pristine IM-20. In the case of
ECNU-24-RC, the occupancy factors were refined to a Si/Ge
ratio of 11.0, which agrees well with the experimental value
TEM and SEM images (Figure 2 and Figure S12). In the initial
crystallization stage, the basic OSDA molecules played the main
role in dissolving the amorphous debris in the acid-washed
materials, whereas in the case of a prolonged crystallization time,
they functioned as structure-directing agents to construct the
zeolite frameworks. The participation of OSDA was clearly
confirmed from the weight loss measured using TG analysis of
the as-prepared zeolites (Figure S13), which indicated that the
contents of occluded OSDA species increased with increasing
crystallization time.
(Si/Ge = 10.5), and the lattice parameters also decreased
compared to those of the Ge-rich parent ECNU-24.
3
2
The time-dependent crystallization course was traced by XRD
investigations. Based on the time-dependent patterns (Figure 6),
the acid treatment caused an extensive amorphization to pristine
Ge-rich zeolites, whereas the subsequent crystallization readily
restored the zeolite structure, whose crystallinity dramatically
increased with prolonged crystallization time. Fully crystallized
high-silica zeolites were obtained within relatively short time
periods (one day for ECNU-24-RC and less than three days for
IM-20-RC). Specifically, the diffraction intensity of IM-20-RC
remained unchanged within the first 2 h (or even decreased
compared to that of IM-20-A) (Figure 6B), and then considerably
increased after 8 h. Additionally, similar results were obtained for
ECNU-24-RC (Figure 6A). These results are consistent with the
The ascription of Q and Q groups for zeolites in MAS NMR
spectra by using the H - 29Si cross-polarization (CP) technique30,
which can selectively enhance the corresponding NMR
resonances (Figure S14). The acid washing generated a large
quantity of Ge-depleted vacancies, which supported by the
1
3
2
intensive Q and Q resonances at approximately -101 and -92
ppm, respectively. The quantitatively reliable spectra then
obtained to trace the changes in structural defect sites (Figure 7).
The deconvoluted spectra are shown using individual lines. The
crystallization processes were clearly accompanied by the
gradual disappearance of defect sites and the construction of
3
the siliceous framework, because the Q resonance at -101 ppm
2
and the Q resonance at -92 ppm decreased in intensity, which
4
gave rise to a relative increase in the Q (Si(4Si)) resonance at
A
approximately -109 ppm. The quantitative results obtained by
deconvolution are summarized in Tables S17 and S18.
A
B
g
g
f
2
4
h
g
f
1
2
e
6
d
2
c
0
.5
b
Time (h)
0
.2
a
ECNU-24-A
5
10
15
20
25
30
f
2
Theta (degrees)
e
B
e
d
d
c
g
c
9
6
f
6
5
e
1
6
d
b
b
8
c
2
b
Time (h)
0.5
a
IM-20-A
5
10
15
20
25
30
a
a
2
Theta (degrees)
-90
-100 -110 -120 -130
-90 -100 -110 -120 -130
Chemical shift (ppm)
Chemical shift (ppm)
Figure 6. A: XRD patterns of ECNU-24-A (a), ECNU-24-RC crystallized for 0.2
h (b), 0.5 h (c), 2 h (d), 6 h (e), 12 h (f) and 24 h (g). B: XRD patterns of
calcined IM-20-A (a), IM-20-RC crystallized for 0.5 h (b), 2 h (c), 8 h (d), 16 h
Figure 7. A: Non-CP 29Si MAS NMR spectra of calcined ECNU-24 (a), ECNU-
24-A (b), ECNU-24-RC crystallized for 0.5 h (c), 2 h (d), 6 h (e), 12 h (f) and 24
h (g). B: 29Si MAS NMR spectra of calcined IM-20 (a), IM-20-A (b), IM-20-RC
(e) 65 h (f) and 96 (g).
crystallized for 0.5 h (c) 2 h (d), 8 h (e), 16 h (f), 65 h (g) and 96 h (h).
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Chemistry - A European Journal
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2
2
1
1
0
.5
.0
.5
.0
.5
.0
A
A
541
07
3
25
1
021
460
6
g
3
73
4
236
5
13
f
e
d
c
b
a
c
478
42
9
71
320
b
a
6
61
0
2
1
1
1
0
0
0
0
1
350 1200 1050
900
750
600
450
2
00
400
600
800
1000
.1
.8
.5
.2
.9
.6
.3
.0
B
B
360
4
44
244
498
542
88
5
70
1
023
5
f
e
d
c
b
816
72
c
7
961
b
541
685
a
a
1
350 1200 1050
900
750
600
1
450
2
00
400
600
800
1000
-
Wavenumber (cm )
-1
Wavenumber(cm )
Figure 8. A: IR spectra of ECNU-24 (a), and ECNU-24-RC obtained by
crystallizing for 0.2 h (b), 0.5 h (c) 2 h (d), 6 h (e), 12 h (f) and 24 h (g). B: IR
spectra of IM-20 (a), IM-20-RC obtained by crystallizing for 0.5 h (b), 2 h (c) 8
h (d), 16 h (e) and 65 h (f). The spectra were all collected using calcined
samples.
Figure 9. A: UV–Raman spectra of IM-20 (a), IM-20-A (b) and IM-20-RC (c). B:
UV–Raman spectra of ECNU-24 (a), ECNU-24-A (b) and ECNU-24-RC (c).
The spectra were all obtained using calcined samples.
A previous NMR investigation attributed the resonance at-106
ppm to the Q configuration in D4R units.[31] Unfortunately, this
signal could not be isolated from the spectra with sufficient NMR
line resolution. Nevertheless, the IR spectra provided
crystallization-time-dependent information regarding chemical
composition changes for the D4R units (Figure 8). The
intensities of the bands attributed to all-silica D4R units for
ECNU-24-RC at 607 cm-1 and IM-20-RC at 588 cm-1 gradually
increased with crystallization time, whereas the band intensity of
the other building units showed almost no change.
UV-Raman spectroscopy is a sensitive tool for investigating
the building units and can provide more detailed local structural
change information during the stabilization. The entire crystalline
structure of IM-20-RC was reconstructed through the connection
of the basic building units of 6MRs, 5MRs, 4MRs and D4R
still existed after the acid treatment. This result indicates that
zeolite building units were still preserved although the long-
range-ordered crystalline structures were extensively degraded.
Similar results were obtained in the case of ECNU-24 (Figure
9B). The basic building unit of ECNU-24 also remained after
acid washing.
4
To compare with the seeding method, we adopt the gel
compositions according to the synthetic conditions of IM-20-RC
through adding different amount of the calcined IM-20 as seeds.
The results illustrated that MFI and UWY mixed crystalline was
easy to obtained (Figure S15 and Table S19). On the basis of
the aforementioned results, a possible formation mechanism of
high-silica zeolites is proposed. The germanosilicates with Ge-
rich D4R units were first hydrothermally synthesized,[37,38] which
can be explained according to the framework energy of building
units (Scheme 2). The theoretical calculations indicated that with
an increase in the Si/Ge ratio or a reduction in the amount of Ge,
the framework energy of the D4R units rapidly increased,
whereas the energies were almost same for the D4R units with
the same Si/Ge ratio but different Ge locations (Table S20).
Compared with other building units, the Ge-rich D4R unit, (e.g.,
(4Si, 4Ge)) had the lowest framework energy. The synthesis of
zeolites follows such a rule, the lower the framework energy, the
easier the synthesis. Consequently, the high-silica zeolites
composed of the Ge-poor D4R (e.g., (8Si, 0Ge)) units cannot be
synthesized directly via a hydrothermal method. Interestingly,
the formation of the Ge-rich D4R units facilitate the preparation
of high-silica zeolites. The existing high-silica building units of
mtw, lau and stf, which possess a lower framework energy than
all-silica D4R units, served as the main silicon source for
-
1
(
3
Figure 9Ac), as indicated by the UV-Raman bands at 325 cm ,
-
1
-1
-1
73 cm , 460 cm , and 513 cm , respectively, which were
attributed to the corresponding building units.[32] Compared with
IM-20-RC, the bands of pristine IM-20 exhibited a red shift
[
33]
(
Figure 9Aa), which is related to the Ge-rich framework. The
-
1
band at 661 cm , which was ascribed to Si-O-Ge deformation
vibrations,[34] disappeared after the hydrolysis of the Si-O-Ge
bonds and extensive removal of Ge (Figure 9Ab). Furthermore,
-
1
a new band emerged at 971 cm , which was assigned to the
[
35,36]
asymmetric stretching vibrations of Si-OH bonds.
This result
is consistent with the formation of Q and Q3 groups, as
indicated by the aforementioned 29Si NMR investigation. The
bands that were attributed to other basic building units became
weaker, broader and blue-shifted after the degermanation but
2
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1
00
80
60
40
20
0
Beta-60
ZSM-5-60
IM-20-RC-60
IM-20-RC-30
0
2
4
6
8
10 12 14 16 18
Scheme 2. The theoretic framework energy relation between the D4R units
and composite-building units (CBUs).
Time (h)
synthesizing high-silica zeolites with the assistance of an OSDA
1
00
propanol
pentanol
henanol
-
and
F
ions. The governing factor for the crystallization
mechanism is that the integrity of the building units (except D4R)
is fully retained after acid washing. The existence of rigid
building units could weaken or overcome the framework tension
induced by the Si-rich D4R units. In combination with other
conditions, the synergetic effects finally gave rise to the
synthesis of high-silica zeolites.
To further confirm the governing factors for the mechanism,
the effect of acid-washing time was investigated (Table S21).
Extension of the acid-washing period adversely affected the
method. The framework, including D4R units, was extensively
damaged with prolonged acid-washing time. Additionally, the
crystallinity of high-silica zeolites was more tenuous, as shown
by the powder XRD patterns. To demonstrate the importance of
the tension generated by D4R units, we also investigated the
effect of the Si/Ge ratio. A small amount of residual Ge species
that remained in the acid-washed materials played an important
role in achieving high-crystalline high-silica zeolites. At a Si/Ge
ratio greater than 20, the high-silica zeolites were difficult to
obtain (Figure S16).
80
60
40
20
0
Beta-60
ZSM-5-60
IM-20-RC-60IM-20-RC-30
Figure 10. Time-dependent 1-propanol conversion (upper), and conversion of
various alcohol substrates (lower) in the tetrahydropyranylation of 3,4-2H-
dihydropyran (DHP) over different zeolites. Reaction conditions: catalyst. 50
mg; alcohol: 9 mmol; 3,4-2H-dihydropyran: 15 mmol; pseudocumene (as an
internal standard): 0.1 g; time: 12 h; temperature: 298 K. For various zeolite-x
catalysts, x indicates the Si/Al molar ratio (60 or 30).
Two approaches are usually adopted to incorporate the Al
ions into the framework with the purpose to endow the Ge-
zeolites zeolite with solid Brønsted activity. One approach is to
Compared with ZSM-5, beta exhibited a lower activity,
presumably because it exhibits stronger hydrophilic character.
Compared to these two representative zeolite catalysts, Al-
containing IM-20-RC with an open pore system[19] exhibited a
higher catalytic activity for alcohols with various molecular
lengths. Thus, the novel method is expected to be useful for
preparing active and stable zeolite catalysts.
The aforementioned studies verified that a novel and general
route for achieving high-silica zeolites was exhibited. In addition
to the acid washing, we found that the Ge species in pure water
were also effectively removed from the Ge-rich zeolite
framework, which resulted in a Si/Ge ratio with the same
regulation (Table S21). In this method, the reutilization of Ge
was accomplished by recycling the filtrate after solution
evaporation (Figure S21A). Furthermore, the new Ge-rich
zeolites were obtained using the recycled Ge species as the Ge
source (Figure S21B). Thus, combined with water treatment, it is
expected to reduce the manufacturing costs associated with
expensive Ge and to avoid the problems of waste disposal and
corrosion which is related to acidic solutions.
generate the tetrahedral Al ions via direct hydrothermal
synthesis.[
39]
The resultant zeolites thus obtained, however,
unavoidablely face the problem of a low hydrothermal stability.
The other one is to introduce the Al ions into the defects that are
+
created after removing the Ge ions under H media, which has
been well addressed by Shamzhy et al.[40-43] Here, our method
verified that that the Al ions could be readily incorporated into
the
zeolite
framework
during
OH-
media-assisted
recrystallization when the Al source and the acid-washed Ge-
zeolites were combined together. As shown in Figure S17, the
Al-containing IM-20-RC had a better crystallization than the
calcined parent IM-20. The 27Al NMR spectra show that Al
existed mainly in tetrahedral sites (Figure S18). The
physicochemical properties of Al-containing zeolite are shown in
the Figure S19, S20 and Table S22. The catalytic performance
of the materials was investigated using a reaction that protects
hydroxyl groups, tetrahydropyranylation of alcohols (Figure 10).
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Conclusions
2.3. Recycling Utilization of Noble Germanium Source. For the
calcined IM-20 zeolite, a simple H O treatment was conducted as follows.
2
One gram of calcined IM-20 was dispersed in a 50 mL aqueous solution.
The solution was then stirred at room temperature for 0.5 h, filtered,
washed with deionized water and dried at 353 K. Next, the dried sample
was transferred to a Teflon-lined autoclave and heated at 443 K for 4 d
under agitation using the method. Subsequently, the Ge species in the
filtrate were gathered for recycled synthesis. After the water was
evaporated and the sample was dried, a solid mixture of Ge and Si was
obtained, which contained mainly noble Ge (Ge/Si molar ratio = 7.7). This
solid, named GexSiyOz, was used as the germanium source for re-
synthesizing Ge-rich IM-20.
The present research provides
a versatile method for
synthesizing novel high-silica zeolites. More importantly, it
proposes the new idea that Si-rich germanosilicate structures
previously considered unfavorable can be constructed through a
new synthetic pathway that effectively avoids the structural
limitations of D4R units. The explored novel method is believed
to provide a general route for the practical synthesis of useful
zeolite catalysts with required active sites and stability. The
crystallization mechanism enables the design of reasonable
OSDAs capable of changing the synthesis sequence of building
units, it also enables the meaningful synthesis of high- or all-
silica zeolites.
The initial gel was prepared by dissolving recycled GexSiyOz in an
aqueous
solution
of
3-butyl-1-methylimidazolium
hydroxide.
Supplementary TEOS was then added dropwise into the Ge-containing
solution. The mixture was continually stirred at 333 K to evaporate the
ethanol formed from the hydrolysis of TEOS and excess water. Finally,
hydrofluoric acid (40% aqueous solution) was added to obtain a synthetic
gel with
OSDAOH : 0.5 HF : 10 H
following the normal procedure for synthesizing IM-20.
a
chemical composition of 0.73 SiO
2 2
: 0.27 GeO : 0.5
Experimental Section
2
O. The crystallization was then performed
Details for synthesizing the parent germanosilicate zeolites (IM-20 and
ECNU-24) with UWY and UOS topologies are described in Section 1 of
the Supplementary Materials.
2
.4. Theoretical Calculation. The structural models for calculating infra-
red (IR) spectra were obtained from the Rietveld refinement. By using the
models, the calculated values are in good agreement with the
experimental values. The framework energy of building units (mtw, lau,
stf and D4R) was obtained from optimized structure. The acid treatment
disassembled the crystals into debris which reassembled into ordered
crystalline framework, so the building units were isolated with the
hydroxyl group terminating the building units. The structural optimization
and IR spectra were calculated with density functional theory (DFT),
2
.1. Acid Treatment of Parent Materials. A typical acid treatment for
the calcined Ge-zeolite IM-20 (Si/Ge = 3.4) was carried out as follows.
IM-20 (1.0 g) powder was dispersed in 50 mL of a 2 M HNO aqueous
3
solution, stirred at room temperature for 0.5 h, then transferred into a
Teflon-lined autoclave and heated at 443 K for 0.5 h under agitation. The
treated sample was subsequently filtrated, washed with deionized water
and oven-dried at 353 K. The acid-treated product is denoted as IM-20-A.
This treatment increased the Si/Ge ratio from 3.4 of the pristine sample
to 19.4 for IM-20-A, which implies that at least 76% of the Ge species
were removed.
3
which was performed with the DFT quantum mechanical code Dmol .
The Double Numerical plus d-functions (DND) atomic orbital basis set
was used with orbital cutoff 4.7Å in the calculation. The exchange-
correlation functional is expressed by the generalized gradient corrected
For zeolite ECNU-24 structurally analogous to IM-167,
a similar
(GGA) functional with the Perdew-Burke-Ernzerh parameterization. The
treatment was conducted using the aforementioned process. The pickling
time was extended to 5 h at 443 K. The treatment increased the Si/Ge
ratio from 3.2 to 10.7, which implies that the Ge species were removed
approximately by 60%. The obtained sample was denoted by ECNU-24-
A.
convergence threshold for the maximum force was set to 0.054 eV/Å.
The convergence threshold for the maximum energy change was set to
2
.7 × 10-4 eV. The convergence threshold for the maximum displacement
-3
was set to 5 × 10 Å. The powder diffraction patterns of IM-20-RC (Si/Ge
molar ratio of 18.0) and ECNU-24-RC (Si/Ge molar ratio of 10.5) with the
UWY and UOS topology respectively were refined with software
FULLPROF.
2
.2. Hydrothermal Synthesis of High-Silica Zeolites from Acid-
Washed Ge-Zeolites. The UWY-type high-silica zeolite was
hydrothermally synthesized using the corresponding acid-washed sample
IM-20-A as a silicon source. IM-20-A was added to the solution of NH
and 3-butyl-1-methylimidazolium hydroxide as an OSDA to form a gel
with a final chemical composition of 0.951 SiO : 0.049 GeO : 1.0
OSDAOH : 1.0 NH F : 10 H O. The gel was heated in a 100 mL Teflon-
lined stainless steel autoclave under agitation (10 rpm) at 443 K for 96 h.
The solids were isolated by filtration, washed with distilled water, dried at
4
F
Acknowledgements
2
2
4
2
The authors gratefully acknowledge the financial support from
China Ministry of Science and Technology under contract of
2016YFA0202804 and NSFC of China (21403069, 21533002
3
53 K, and then calcined at 823 K for 6 h at a temperature ramp of 1 K
-
1
and 21603075).
min . The obtained material was designated as IM-20-RC. The Al-
containing IM-20-RC samples were similarly prepared by incorporating
additional aluminum isopropoxide into the synthetic gel. These samples
were designated as IM-20-RC-x, where x represents the Si/Al ratio of 30
or 60.
Keywords: germanosilicates • hydrothermal crystallization •
high-silica zeolites • D4R • CBUs
The high-silica zeolite ECNU-24 was synthesized using the same method
while using the acid-treated sample ECNU-24-A as the silicon source.
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Chemistry - A European Journal
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FULL PAPER
Mingming Peng, Jingang Jiang*, Xue
Liu, Yue Ma, Meichen Jiao, Hao Xu,
Haihong Wu, Mingyuan He, and Peng
Wu*
Page No. – Page No.
Breaking Structural Energy
Constraints: Hydrothermal
Crystallization of High-Silica
Germanosilicates via Building Units
Self-Growing Approach
Highly crystalline and thermally stable high-silica zeolites with various topologies
and a wide range of porosity were synthesized through a unique process. The lower
the framework energy of building units, the easier it is to synthesize, which is the
key factor determining the structure construction in this process. This study would
bring about a breakthrough in increasing the number and types of practically usable
zeolite materials.
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