Synthesis and Characterization of CIT-13, a Germanosilicate Molecular Sieve with Extra-Large Pore Openings

Article abstract of DOI:10.1021/acs.chemmater.6b02468

The synthesis of the germanosilicate CIT-13, a molecular sieve that is the first to have a two-dimensional (2D) pore system possessing pores that are bounded by 14- and 10-rings, is accomplished using a family of monoquaternary, benzyl-imidazolium organic

Full text of DOI:10.1021/acs.chemmater.6b02468

Article
pubs.acs.org/cm
Synthesis and Characterization of CIT-13, a Germanosilicate
Molecular Sieve with Extra-Large Pore Openings
Jong Hun Kang, Dan Xie, Stacey I. Zones, Stef Smeets, Lynne B. McCusker, and Mark E. Davis*,†
†
‡
‡
§
§,⊥
†
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
‡
Chevron Energy Technology Company, 100 Chevron Way, Richmond, California 94802, United States
§
̈
Department of Materials and Environmental Chemistry, University of Stockholm, Svante Arrhenius vag 16C, SE-106 91 Stockholm,
Sweden
⊥
Department of Materials, ETH Zurich, CH-8093 Zurich, Switzerland
S Supporting Information
*
ABSTRACT: The synthesis of the germanosilicate CIT-13, a
molecular sieve that is the first to have a two-dimensional (2D)
pore system possessing pores that are bounded by 14- and 10-
rings, is accomplished using a family of monoquaternary,
benzyl-imidazolium organic structure-directing agents
(
OSDAs) in aqueous media containing fluoride. CIT-13 is
prepared using either hydrogen fluoride (HF) or ammonium
fluoride (NH F). The structure refinement suggests that most
4
of the Ge atoms are located in the d4r (double-4-rings) units,
and that there are framework disorders in the arrangement of
29
those d4r units. Other characterizations of CIT-13 such as Si
MAS NMR spectra, Ar-adsorption isotherms, and so forth are presented and compared to those of IM-12 (UTL), a previously
reported germanosilicate with 14- and 12-ring pores.
1
. INTRODUCTION
inverse sigma transformation to create high-silica molecular
sieves with new frameworks that are not accessible by
Zeolites, a class of open framework aluminosilicates, and their
analogs are popular heterogeneous catalysts in the petrochem-
ical industry because of their high thermal stability and shape-
selective properties. The latter arise from the well-defined pore
systems in these crystalline materials, whose channel- and pore-
22−24
conventional synthesis methods.
The germanosilicate
IM-12 with the UTL framework type (3-letter codes are
assigned to designate framework types by the Structure
25
Commission of the International Zeolite Association) has
been a prime starting material for exploring such trans-
formations via the ADOR (Assembly-Dissembly-Organization-
Reassembly) process, yielding the IPC-2 (OKO)/IPC-4
1
−3
wall compositions are tunable.
The extra-large-pore zeolite
frameworks with channels defined by more than 12 Si or other
tetrahedrally coordinated framework atoms (T atoms) are of
particular interest, because they can accommodate and process
larger feedstock molecules.
Many such extra-large-pore frameworks are found in the
23
26
(
PCR) and IPC-9/IPC-10 pairs.
4
−6
As part of a screening program to synthesize germanosilicate
molecular sieves using mono and diquaternary ammonium
structure-directing agents derived from benzyl imidazolium, the
3
,4
germanosilicate (Si/Ge ratio >3) family.
Germanium is
27
new molecular sieve, CIT-13, was produced. The framework
structure found in that study can be described in terms of dense
layers, similar to those found in the CIT-5 (CFI) framework
known to play a stabilizing role in the formation of small
composite building units (CBUs) with high geometric
constraints such as the double 4-ring (d4r) or double 3-ring
28
6
−8
structure (hereafter called cfi-layers), connected via d4r units
(d3r).
This concept led to a synthesis route for producing a
to form a 2-dimensional channel system delimited by 14- and
large array of new molecular sieves with low framework
densities and extra-large pores.
the mineralizing agent, that further stabilizes the formation of
d4r units.
structural weakness that reduces the thermal and hydrothermal
stability of the molecular sieve, so it is desirable to minimize its
content. To this end, some researchers have developed
experimental methods to replace Ge with Al, Si, or Ti
in a postsynthesis treatment, while others have exploited the
9
−16
1
0-ring pore openings (the term n-ring stands for the number
Typically, fluoride is used as
25
of tetrahedral atoms defining the size of a channel) between
the layers (Figure 1). The cfi-layers contain cas, mtt, and ton
CBUs. For comparison, adjacent layers in the CFI framework,
are shifted by half a unit cell along the 14-ring channel and are
1
6−18
However, the inclusion of Ge also introduces a
19
20
21
Received: June 17, 2016
Revised: August 17, 2016
Published: August 18, 2016
preferential occupation of Ge in the d4r units to invoke an
©
2016 American Chemical Society
6250
DOI: 10.1021/acs.chemmater.6b02468
Chem. Mater. 2016, 28, 6250−6259

Chemistry of Materials
Article
imidazole (98%, Sigma-Aldrich) was dissolved in 300 mL of toluene
and heated up to 45 °C. While vigorously stirring, 21.1 g (150 mmol)
of 3-methylbenzyl chloride (98%, Sigma-Aldrich) was added dropwise.
After 30 min of additional stirring, the temperature was increased to
1
05 °C and the reaction was allowed to proceed for 24 h. After that,
the reaction mixture was cooled in a dry ice bath since this
imidazolium salt exists as a liquid salt at room temperature. A cold
filtration was performed to isolate the product in solid form. The
chloride salt obtained was washed repeatedly with 4 L of cold diethyl
Figure 1. Idealized framework structure of CIT-13. (A) cfi-layer in the
y−z plane with the cas (red), mtt (yellow), and ton (green) composite
building units highlighted, (B) view along the [001] direction showing
the 14-ring pore, and (C) view along the [010] direction showing the
ether and dried under vacuum for 12 h. The ion-exchange was
1
conducted as described above. H NMR (500 MHz, CDCl ): δ 10.54
3
(
(
s, 1H), 7.64 (t, 1H), 7.32 (m, 1H), 7.14 (m, 3H), 7.05 (m, 1H), 5.42
s, 2H), 3.97 (s, 3H), 2.22 (s, 3H). 13C NMR (125 MHz, CDCl ): δ
3
10-ring pore. In (B) and (C), the d4r units connecting the cfi-layers
1
5
39.14, 137.42, 133.02, 130.04, 129.28, 129.12, 125.77, 123.92, 121.78,
3.07, 36.46, 21.21.
(
blue) have been highlighted in green. Bridging O atoms have been
omitted for clarity.
2
.1.3. Preparation of OSDA 3: 1,2-Dimethyl-3-(3,5-
dimethylbenzyl)imidazolium Hydroxide. First, 12.3 g (150 mmol)
of 1,2-dimethylimidazole (98%, Sigma-Aldrich) was dissolved in 300
mL of toluene in an ice bath. While vigorously stirring, 29.9 g (150
mmol) of 3,5-dimethylbenzyl bromide (98%, Alfa Aesar) was added.
After 30 min of additional stirring, the temperature was slowly
increased to 105 °C and the reaction was allowed to proceed for 15 h.
After that, the reaction mixture was cooled and filtered. The chloride
salt obtained was washed repeatedly with 4 L of diethyl ether and dried
connected by double zigzag chains to form 1-dimensional 14-
ring channels. The CIT-13 framework structure bears a striking
resemblance to that of IM-12, where dense layers similar (but
not identical) to those in the FER framework are connected via
d4r units to create a 14- and 12-ring pore system between the
layers. We anticipate, therefore, that CIT-13 will exhibit a
chemistry similar to that of the very versatile IM-12, and are
currently exploring this possibility.
under vacuum for 12 h. The ion-exchange was conducted as described
1
above. H NMR (500 MHz, CDCl ): δ 7.77 (d, 1H), 7.49 (d, 1H),
3
Here, we report synthetic methods for preparing CIT-13, its
structural refinement and the characterization of its properties.
The crystallization of silica-based molecular sieves is typically a
very complex process that depends on a large number of
variables, such as alkalinity, structure of the organic structure-
directing agent (OSDA), water content, temperature, and
6.99 (m, 1H), 6.89 (m, 2H), 5.41 (s, 2H), 4.03 (s, 3H), 3.19 (s, 3H),
2
.31 (s, 6H). 13C NMR (125 MHz, CDCl ): δ 144.29, 139.19, 132.55,
3
1
30.81, 125.71, 122.92, 121.61, 52.57, 36.19, 21.23, 11.38.
2
.1.4. Preparation of OSDA 4: 1-Methyl-3-(3,5-dimethylbenzyl)-
imidazolium Hydroxide. First, 12.3 g (150 mmol) of 1,2-
dimethylimidazole (98%, Sigma-Aldrich) was dissolved in 300 mL of
toluene in an ice bath. While vigorously stirring, 29.9 g (150 mmol) of
2
8,29
mineralizer used.
Therefore, we performed a systematic
3
,5-dimethylbenzyl bromide (98%, Alfa Aesar) was added. After 30
investigation of the synthetic conditions that form CIT-13, and
developed a synthesis protocol using ammonium fluoride
min of additional stirring, the temperature was slowly increased to 105
°C and the reaction was allowed to proceed for 15 h. After that, the
reaction mixture was cooled and filtered. The bromide salt obtained
was washed repeatedly with 4 L of diethyl ether and dried under
(
(
NH F) to avoid the use of concentrated hydrogen fluoride
HF). To better understand the synthesis and potential
4
vacuum for 12 h. The ion-exchange was conducted as described above.
applications of CIT-13, the samples were characterized by
1
2
9
H NMR (500 MHz, CDCl ): δ 10.46 (td, 1H), 7.53 (t, 1H), 7.27 (d,
3
thermogravimetric analysis (TGA), Si magic-angle spinning
MAS) nuclear magnetic resonance (NMR) and cryogenic
1
3
1
H), 7.00 (dm, 3H), 5.44 (s, 2H), 4.09 (s, 3H), 2.28 (s, 6H).
C
(
NMR (125 MHz, CDCl ): δ 139.23, 137.34, 132.55, 131.17, 126.59,
3
argon-adsorption, and the results compared with the
corresponding data from IM-12 (UTL). To confirm the
framework structure of CIT-13, and to investigate the
distribution of Ge in the framework and the location of the
OSDA in the pores, we also undertook a full structure analysis
using synchrotron X-ray powder diffraction (XPD) data.
1
23.65, 121.71, 53.44, 36.78, 21.18.
.2. Synthesis of Germanosilicate CIT-13 in Fluoride Media.
The synthesis of CIT-13 was performed in the same way as reported
2
2
7
previously. Specifically, the gel compositions used were x/(x + 1)
+
−
SiO :1/(x + 1) GeO :y(OSDA) OH : yHF:5−15 H O where x is the
2
2
2
Si/Ge molar ratio of the gel. The values of y were tested within 0.5−
0
.75. (The condition with x = 4, y = 0.5, and T = 160 °C is referred to
2. EXPERIMENTAL SECTION
as the reference condition in this work.) The desired amount of
germanium(IV) oxide (GeO , Strem, 99.999%) was dissolved in the
2
2
.1. Synthesis of Organic Structure-Directing Agents
OSDA). 2.1.1. Preparation of OSDA 1: 1,2-Dimethyl-3-(3-
methylbenzyl)imidazolium Hydroxide. First, 14.4 g (150 mmol) of
,2-dimethylimidazole (98%, Sigma-Aldrich) was dissolved in 300 mL
of toluene and heated up to 45 °C. While vigorously stirring, 21.1 g
150 mmol) of 3-methylbenzyl chloride (98%, Sigma-Aldrich) was
desired amount of OSDA aqueous solution and tetraethyl orthosilicate
(
(TEOS, Alfa Aesar, 98%) in a 23 mL PTFE liner (Parr Instrument).
The mixture was stirred for 12 h in order to hydrolyze all TEOS and
dried under a continuous air flow to evaporate excess water and
ethanol until the gel became very viscous. The equivalent amount of
concentrated hydrofluoric acid (HF, Sigma-Aldrich, 48 wt %) was
added dropwise and thoroughly mixed using a PTFE spatula. After this
HF-mixing, the mixture became powdery. After an additional 2 days of
drying, the desired amount of distilled water and seed material (2−3
wt % of total mixture, optional) was added and the mixture was mixed
thoroughly. The seed material was as-synthesized CIT-13 from the
reference condition. The PTFE liner containing the mixture was firmly
clad with a Parr still reactor and put in a convection oven. The
crystallization temperature was typically 160 °C, but other temper-
atures in a range from 140 to 175 °C have also been used. The
crystallization was performed under static conditions and monitored
for at least 1 month. The resultant CIT-13 powder was washed
carefully with distilled water, methanol, and acetone, and dried in a 70
°C convection oven before being characterized.
1
(
added dropwise. After 30 min of additional stirring, the temperature
was increased to 105 °C and the reaction was allowed to proceed for
24 h. After that, the reaction mixture was cooled and filtered. The
chloride salt obtained was washed repeatedly with 4 L of diethyl ether
and dried under vacuum for 12 h. The chloride anions were exchanged
with hydroxyl anions using OH-form styrene-divinylbenzene (DVB)-
1
matrix ion-exchange resin (DOWEX MARATHON). H NMR (500
MHz, CDCl ): δ 7.79 (d, 1H), 7.58 (d, 1H), 7.29 (m, 1H), 7.19 (m,
3
1
3
1
2
H), 7.11 (m, 1H), 7.10 (m, 1H), 5.49 (s, 2H), 4.03 (s, 3H), 2.81 (s,
H), 2.37 (s, 3H). 13C NMR (125 MHz, CDCl ): δ 144.29, 139.37,
3
32.79, 129.92, 129.29, 128.60, 125.10, 122.96, 121.73, 52.46, 35.90,
1.36, 10.90.
2
.1.2. Preparation of OSDA 2: 1-Methyl-3-(3-methylbenzyl)-
imidazolium Hydroxide. First, 14.4 g (150 mmol) of 1-methyl-
6
251
DOI: 10.1021/acs.chemmater.6b02468
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Chemistry of Materials
Article
2
.3. Synthesis of Germanosilicate CIT-13 Using Ammonium
Fluoride (NH F). The gel composition for the NH F-protocol was the
4
4
same as for the HF-protocol described above, except that the molar-
equivalent amount of NH F was used in place of HF. The desired
4
amounts of GeO and NH F were dissolved in the OSDA aqueous
2
4
solution and TEOS in a 23 mL PTFE liner. The mixture was stirred
for 12 h in order to hydrolyze all TEOS, and dried under a continuous
air flow to evaporate excess water, ammonia, and ethanol until the gel
became completely powdery. Then, water was added to the desired
level. The PTFE liner containing the mixture was firmly clad with a
Parr still reactor and put in a convection oven at 160 °C. The rest of
the protocol is the same as above.
Figure 2. Organic structure-directing agents (OSDAs) used in this
study to produce CIT-13 in a germanosilicate system and fluoride
medium.
this set of crystallization parameters (hereafter the reference
conditions), an impurity-free CIT-13 sample can be obtained
after 14−21 days of crystallization time. The morphology of the
CIT-13 crystals resembles that of UTL-framework materials
2
.4. Synthesis of Germanosilicate IM-12 in Hydroxide
Media. The germanosilicate IM-12 (UTL) was also synthesized for
comparison with CIT-13. A spiro-quaternary ammonium, (6R,10S)-
16
6
,10-dimethyl-5-azaspiro[4.5]decanium hydroxide, was used as the
(Figure S1−S3).
OSDA for the synthesis of IM-12. This OSDA was prepared according
to the protocol described in literature.
was synthesized solvothermally using hydroxide (OH ) medium. The
gel composition was 0.667 SiO :0.333 GeO :0.25 (OSDA) OH :30
Other crystallization conditions also yielded excellent CIT-13
products and several of the XPD profiles from these syntheses
are displayed in Figure 3. The CIT-13 samples synthesized
under these conditions showed good thermal stability (Figure
S4).The Si/Ge ratio of the gel was varied from 2 to 16, and
crystallization kinetics studied using XPD results (Figure S5).
The lower the Si/Ge ratio of the gel, the faster CIT-13
crystallized (and the higher the germanium content of the
crystals). The CIT-13 crystals typically showed Si/Ge ratios
higher than those of the parent gels as shown in Figure S6 (also
true for the IM-12 sample, Figure S3). For instance, when the
gel Si/Ge ratios were 2, 3, and 4, the Si/Ge ratios of the
product, determined using EDS, were 3.84 ± 0.59, 4.72 ± 0.55,
and 5.73 ± 0.89, respectively. The amount of water in the gel
also affected the crystallization process of CIT-13. We showed
16,30
The IM-12 germanosilicate
−
+
−
2
2
H O. The gel mixture was prepared by dispersing the desired amounts
2
+
−
of silica (Cab-o-Sil) and GeO in (OSDA )OH solution and the
2
water content was adjusted by simply adding the equivalent amount of
distilled water. The crystallization was performed at 175 °C for 14 day
in a 23 mL Parr reactor. The rinsing step of the crystalline IM-12
material was the same as that described above for CIT-13.
2
.5. Characterization. Lab XPD data were collected on all
samples using a Rigaku Miniflex II diffractometer with Cu Kα radiation
wavelength = 1.5418 Å). Synchrotron XPD data were collected on an
(
as-synthesized sample of CIT-13 from the reference condition in 0.3
mm capillaries on the MS-Powder beamline at the Swiss Light Source
31
in Villigen, Switzerland using a wavelength of 0.7764 Å and a
Mythen II detector. Three-dimensional electron diffraction data were
also collected on 3 crystals of CIT-13 using the rotation electron
previously that a gel with low water content (H O/(Si + Ge) =
2
32
27
diffraction (RED) technique. The RED software was installed on a
JEOL 2010 microscope operating at 200 kV, and data were collected
over a tilt range of ±50° with a tilt step of 0.50° for the three sets, the
exposure time is 2 s per tilt step.
5) crystallized into CIT-13. Here, three examples of higher
water levels (H O/(Si + Ge) = 7.5, 10 and 15) were tested, and
2
all resulted in faster crystallization and higher crystallinity. As
shown by the data listed in Table 1 and Figure S7, CIT-13
crystallizes with high purity in this range of water content, and
Scanning electron microscopy (SEM) was conducted on a FE-SEM
microscope (ZEISS 1550VP), and the elemental analysis of the crystals
was performed using an Oxford X-Max SDD X-ray Energy Dispersive
the ratios H O/(Si + Ge) = 7.5 and 10 appear to be optimal in
2
1
13
terms of purity and crystallization rate. The effect of the
concentration of the OSDA in the gel on the CIT-13
crystallization was also studied (the amount of HF was also
changed molar-equivalently considering the neutralization
process, Figure S8). OSDA/(Si + Ge) = 0.5 and 0.625 gave
relatively pure CIT-13, but some impurities are discernible as
extra peaks in the X-ray diffraction patterns when the OSDA-to-
T-atom ratio became higher than 0.75.
The influence of temperature on the crystallization process of
CIT-13 was also investigated. As illustrated by the data
provided in Figure 3, pure CIT-13 can be synthesized at all
temperatures studied: 140, 150, 160, and 175 °C. The
crystallization was faster at higher temperatures. At 175 °C,
pure CIT-13 was synthesized within 1 week without seeding,
whereas at least 4 weeks were required at 140 and 150 °C
(Figure S9). Furthermore, an increased crystallization temper-
ature resulted in larger crystals (Figure S10).
Spectrometer (EDS) system. The H and C solution-phase NMR
spectra of the four OSDAs were obtained using a Varian 500 MHz
Spectrometer with auto-x pfg broadband probe with the carrier
frequencies 500 and 125 MHz, respectively. The organic salts were
dissolved in CDCl in their halide forms for the solution-phase NMR.
3
The 13C and Si MAS solid-state NMR spectra were obtained using a
29
Bruker Avance 500 MHz spectrometer. The powdered samples of
CIT-13 were charged in a 4 mm Bruker zirconia (ZrO ) rotor, and the
2
MAS rate was 8−12 kHz. The cryogenic argon adsorption isotherms
were obtained using a Quantachrome Autosorb iQ at 87.45 K.
3
. RESULTS
.1. Synthesis Conditions. We recently reported the first
synthesis of CIT-13 using a benzylimidazolium-derived OSDA
3
27
(
OSDA 1) and fluoride medium. With the same OSDA, the
crystallization conditions for CIT-13 were systematically tested,
and the crystallization kinetics were obtained for various Si/Ge
ratios of the gel, water content, presence or absence of seeding
3.2. OSDAs for the Crystallization of CIT-13. Previously,
we also showed that the position of the methyl group on the
benzene ring of benzylimidazolium-derived monoquaternary
OSDAs plays an important role in determining which
+
−
material, amounts of OSDA OH /HF, and crystallization
temperatures. A family of OSDAs with closely related structures
(
3
Figure 2) was also explored with varying levels of Ge. In total,
3 sets of conditions were tested and these are summarized in
27
germanosilicate framework is formed. The use of ortho-
substituted benzylimidazolium-derivatives resulted in IWS-type
materials and impure CIT-13, and meta- and para-mono-
methyl-substituted benzylimidazolium-derivatives yielded pure
CIT-13, and LTA-type materials, respectively. Here, the
crystallization processes of CIT-13 using three OSDAs that
Table 1. The purity of the samples obtained was determined
qualitatively from the XPD profiles.
Crystallization conditions that produce high quality CIT-13
product reproducibly were determined to be 0.8 SiO :0.2
2
+
−
GeO :0.5 (OSDA 1) OH :0.5 HF:10 H O at 160 °C. With
2
2
6
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Chemistry of Materials
Article
Table 1. Summary of Synthesis Conditions Tested in This Study, Where All Ratios Are Molar Ratios
OSDA
Si/Ge (gel)
H O/T (gel)
2
(SDA)OH/T and HF/T (gel)
seeding
temperature (°C)
time (days)
major phase
b
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
3
3
3
3
3
4
4
4
4
4
4
4
15
15
10
10
10
10
10
10
7.5
7.5
5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.625
0.625
0.75
0.75
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
O
X
X
X
O
X
X
X
O
X
O
X
X
X
O
X
O
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
160
160
160
160
160
160
160
160
160
160
160
140
150
175
160
160
160
160
160
160
160
160
160
160
160
160
160
160
160
160
160
160
28
35
14
14
7
CIT-13
b
CIT-13
b
2
CIT-13
a
3
CIT-13
a
4
CIT-13
a
4
21
21
35
14
14
21
28
28
7
CIT-13
c
8
CIT-13
c
16
4
CIT-13
b
CIT-13
a
4
CIT-13
b
4
CIT-13
a
4
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
CIT-13
a
4
CIT-13
a
4
CIT-13
b
4
14
14
14
14
14
14
14
28
28
21
21
21
56
56
7
CIT-13
b
4
CIT-13
c
4
CIT-13
c
4
CIT-13
c
2
CIT-13 /MFI
c
4
CIT-13 /MFI
8
MFI
MFI
MFI
16
50
2
c
CIT-13
c
4
CIT-13
c
8
CIT-13
c
16
50
2
CIT-13
amorphous
a
CIT-13
a
4
7
CIT-13
b
8
14
21
49
CIT-13
b
16
50
CIT-13
160
amorphous
a
b
c
CIT-13 purity index: very pure (∼100%). CIT-13 purity index: acceptably pure (>95%). CIT-13 purity index: not pure but major phase; the
purity was determined qualitatively based on the XPD patterns.
have one or two methyl groups only at the meta-position
amounts indicate that a unit cell (64 T atoms) of CIT-13
contains approximately four OSDA molecules (corresponding
to 18% weight loss). The refined structure has an OSDA
occupancy of only 0.8, giving 3.2 OSDA molecules per unit cell.
This corresponds to a weight loss of 15%, which is rather close
to the observed amount, and does not include any water that
may be occluded in the pores. The less-than-complete
occupancy is probably a result of the disorder of the d4r in
the framework structure, and this can vary somewhat from
sample to sample. Similar reduced occupancies for the OSDA
(OSDA 2−4, Figure 2) were studied for gel Si/Ge ratios
ranging from 2 to 50. The XRD profiles in Figure S11−S13
indicate that all four OSDAs promoted the crystallization of
CIT-13. Therefore, it can be concluded that methyl group(s)
substituted at the meta-position of the OSDA-benzene ring is
an important condition for CIT-13 production. Specifically,
OSDA 1 and OSDA 4 yielded much purer CIT-13 than did the
other two (Figure 4 and S13). OSDA 2 resulted in a mixture of
CIT-13 and MFI. In a Ge-rich system, CIT-13 was the major
phase, whereas MFI was the major phase when the gel Si/Ge
was high (Figure S11). OSDA 3 also resulted in CIT-13, but
one or more impurity phase(s) were observed at all tested gel
Si/Ge ratios (Figure S12).
33
were also observed in a recent study of SSZ-55 and SSZ-59.
3.3. Use of NH F Instead of HF. CIT-13 was also prepared
4
using NH F to see if the use of HF could be avoided. CIT-13
4
samples were prepared using the reference conditions and the
The as-prepared CIT-13 products from OSDA 1 and OSDA
molar-equivalent amount of NH F salt in place of HF.
4
1
3
4
were examined using C NMR and TGA (Figure 5). The
Ammonia was removed by drying the gel thoroughly using
an air flow until the appearance of the gel became very
powdery. The crystallization was performed statically in a 160
°C oven. CIT-13 samples prepared in this way were compared
with those obtained using HF with XPD, SEM, EDS, TGA, and
argon adsorption at 87.45 K. The results are summarized in
1
3
resonances from the solution-phase C NMR spectra of OSDA
1
and OSDA 4 match those from the solid-state 13C MAS
NMR of as-prepared CIT-13 germanosilicate incorporating
each OSDA (Figure 5A), thus showing that the OSDAs were
incorporated intact during the synthesis. The TGA curves of as-
prepared CIT-13 shown in Figure 5B give approximately 16.5%
Figure 6. CIT-13 from the NH F-protocol is essentially
4
(
OSDA 1) and 15.6% (OSDA 4) weight loss during the
identical to CIT-13 from the HF-protocol. The XPD profiles
temperature ramping from room-temperature to 900 °C. These
(Figure 6A) for both samples show pure CIT-13 with no
6
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Figure 3. XPD patterns from selected crystallization conditions. The
top pattern is from a sample produced under the reference conditions
of 0.8 SiO :0.2 GeO (Si/Ge = 4):0.5 SDAOH:0.5 HF:10 H O at 160
2
2
2
°
C without seeding. The effect of varying a specific condition in the
synthesis with respect to the reference conditions can be seen in the
following patterns.
impurity peaks. The SEM images displayed in Figure 6B and
6
C illustrate that the morphologies of the CIT-13 crystals are
similar; intergrowth is observed in both samples. The Si/Ge
ratios of CIT-13/HF and CIT-13/NH F are 5.03 ± 0.48 and
4
Figure 4. XPD patterns of the samples resulting from different Si/Ge
ratios in the gel with (A) OSDA 1 and (B) OSDA 4 for gel
compositions x/(x + 1) SiO :1/(x + 1) GeO :0.5 (OSDA)OH:0.5
5
.19 ± 0.15, respectively. The TGA studies also reveal that the
weight loss data are very similar: 16.5% for CIT-13/HF and
6.1% for CIT-13/NH F (Figure 6D). The argon adsorption
2
2
1
4
HF:10 H O where x is the gel Si/Ge ratio. Asterisks (*) denote
2
isotherms of the two CIT-13 samples after calcination at 580
C for 6 h are shown in Figure 6E and are the same. The
impurity peaks.
°
micropore volumes of CIT-13/NH F characterized using the t-
4
3
plot method and the Saito−Foley (SF)-method are 0.191 cm
−
1
3
−1
g
and 0.215 cm g , respectively.
and a second d4r was added and constrained to the position of
the existing d4r with z + 0.5. All Si positions were redefined as
mixed Ge/Si positions with a maximum shared occupancy of
1.0 (0.5 in the case of T atoms in the d4r). This showed most
Ge to be located in the d4r units. A subsequent difference map
revealed a large smeared out cloud of residual electron density
in the center of the channel.
3.4. Structure Analysis of As-Synthesized CIT-13. For
structure analysis, a sample of CIT-13 produced using the
reference conditions was used. The topology of CIT-13 was
32
obtained from rotation electron diffraction (RED) data using
34
the zeolite-specific program FOCUS. Rietveld refinement was
initiated in the space group Cmmm (a = 13.77 Å, c = 27.32 Å)
using the coordinates of the structure for CIT-13 proposed
To locate the OSDA in this cloud, a systematic approach that
2
7
35
35
previously and the program TOPAS. The background was
subtracted manually, and continuously adjusted during the
course of the refinement. Framework restraints were put in
place, and the framework coordinates were optimized using a
geometrical refinement against the restraints. Initial scaling was
performed by refining just the scale factor using only the higher
angle XPD data, and keeping all other parameters fixed. In a
difference electron density map, we expected to find large
electron density clouds in the channels corresponding to the
position of the OSDA, but instead noticed eight very strong
residual peaks reminiscent of a d4r and holes on the atoms of
the existing d4r, indicating that the framework might be
disordered. The occupancy of the existing d4r was set to 0.5,
we developed recently was applied. A model of the OSDA
was generated and optimized using the energy minimization
36
routine in the program Jmol and then added as a rigid body to
the TOPAS input file. An initial location for the OSDA was
found by using the simulated annealing algorithm, by allowing
free rotation and translation of the OSDA. The occupancy of
the OSDA was allowed to refine as part of this process. The
OSDA settled on a position of 8-fold symmetry in the center of
the channel intersection with an occupancy of 0.12. This
corresponds to a total of almost four OSDA molecules per unit
cell, or two per channel intersection. However, with this model,
it was not possible to find two OSDA molecules that could be
present simultaneously without overlapping.
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simulated annealing using geometric restraints only. Out of
these trials, one model with the imidazole rings facing one
another around the inversion point, and the benzyl rings on the
other side pointing into the 10-ring windows stood out. The
position of the OSDA was transferred to the cell with the
original Cmmm symmetry, which resulted in a surprisingly good
fit to the data.
It is likely that two charged imidazole groups facing one
another could give rise to unfavorable interactions, so a similar
arrangement with the two benzyl groups facing each other and
the imidazole groups pointing into the 10-ring windows was
also tested. It was difficult to find a satisfactory position of the
OSDA that did not result in very close (<2.5 Å) distances
between the framework and the OSDA, or between the
OSDAs. A short search of the literature showed that in fact
37
imidazole stacking is not uncommon, so the original position
was retained.
Line broadening was treated by applying an anharmonic
38
model. A difference map revealed residual electron density in
−
the middle of the d4r, and that was interpreted as F . The rigid-
body model for the OSDA was changed to a geometrically
39
restrained one using the expected bond angles and distances.
Antibump restraints were applied to keep the imidazole rings
from creeping closer together, and restraints to keep the
imidazole and benzyl rings flat were added. Only in the final
stages of the refinement was it possible to remove these
restraints.
Figure 5. (A) 1H-decoupled 13C solid-state MAS NMR spectra
(
Black) of CIT-13 samples prepared using OSDA 1 and OSDA 4 with
13
The refinement resulted in a good fit to the data (Figure 7)
the C liquid NMR spectrum (red) of the corresponding OSDA
superimposed. (B) TGA profiles of as-prepared CIT-13 synthesized in
the presence of OSDA 1 and OSDA 4.
with agreement values R = 0.013 and R = 0.077 (Rexp
=
I
wp
Figure 7. Observed (blue), calculated (red), and difference profiles
black) for the Rietveld refinement of as-prepared CIT-13.
(
Figure 6. Comparison of the HF and NH F protocols for preparing
CIT-13. (A) XPD patterns, (B) SEM micrograph images of CIT-13
0.0015). The main differences seem to arise from an anisotropic
peak shape that affects some of the reflections. We were unable
to find a model to describe this effect completely. The
refinement yields a Si/Ge ratio of 5.63. The Ge is located
primarily on T7 in the d4r, about half of which is Ge. The total
occupancy of the OSDAs refined to 0.103, giving a total of 3.26
OSDA molecules per unit cell (out a possible total of 4). Only
4
resulting from the HF (left) and the NH F (right) protocols, (C)
4
TGA profiles, and (D) argon adsorption and desorption isotherms at
8
7.45 K.
It seemed that the most likely case would be two OSDA
molecules related by inversion symmetry. Therefore, the
−
2 F were found, so we expect the remaining difference in the
symmetry of the structure was lowered to P1, and the
̅
charge to balance the positively charged OSDA is made up by
−
1
.26 OH disordered in the channel system.
simulated annealing routine was repeated, allowing an addi-
+
tional rotation around the connecting C−N bond. However,
4
. DISCUSSION
the simulated annealing routine did not generate any diffraction
profiles that matched the observed one. In the next attempt, we
set up antibump restraints between the framework oxygen
atoms and the C and N atoms of the OSDA, as well as between
the C and N atoms of neighboring OSDA molecules, and ran
4.1. Structure. The results of the Rietveld refinement show
that the previously proposed framework structure of CIT-13 is
partially correct, in that the framework consists of cfi-layers
interconnected via d4r units to form 10- and 14-ring channels
6
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Chemistry of Materials
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between the layers (Figure 1). In addition, the structure
refinement shows that the d4r units connecting the cfi-layers
can adopt either of two positions that are related by a
translation of 0.5 along the z axis. However, their arrangement
along the x axis is ill-defined, resulting in a disordered
framework structure. Two ordered representations (AAAA
sequence and ABAB sequence) are shown in Figure 8. The
Figure 9. (A) Projection of the structure along x showing a possible
arrangement of the OSDA in the refined structure, taking into account
that one in five OSDA molecules may be missing. (B) Projection of a
theoretically possible framework structure in the y−z-plane with
alternating rows of d4r’s shifted by one-half along z. Three
arrangements of pairs of the OSDA molecules based on the refined
structure are shown, but none is chemically reasonable.
Figure 8. View along y showing two stacking possibilities, where the
only difference between A and B is the location of the d4r (pink and
light blue). Model A shows an ABAB sequence of d4r’s while Model B
shows an AAAA sequence.
initially proposed structure corresponds to an AAAA sequence
of d4r units. Of course, all other sequences of A and B
connections are also possible and this is the cause of the
disorder that we observed. However, the XPD data did not
show any signs of diffuse scattering or line broadening. We
hypothesize that the material consists of medium sized domains
(
10−20 unit cells) of either AAAA or ABAB along x; however, a
definitive answer cannot be given based on XPD data alone.
Another arrangement, in which alternate rows of d4r units
are shifted within the interlayer region, can also be imagined,
but this layout is very unfavorable for the OSDA (Figure 9B).
The organic cation is either too close to the framework, too
close to its neighboring OSDA, or too far from its neighboring
OSDA, so that the average occupancy of 0.8 can no longer be
reached. Therefore, we believe that disorder within the
interlayer region is much less likely than the stacking disorder
shown in Figure 8, but it cannot be ruled out completely.
Although the OSDA molecule is also disordered, it seems
that two OSDA molecules adopt a supramolecular arrangement
at the center of the 14-ring, to carry out their structure-
directing effect. The imidazole rings of an OSDA pair are
parallel to one another, with a centroid distance of 3.54(1) Å.
Each pair can adopt one of four different symmetry-related
positions. The methylbenzyl groups on either end of an OSDA
pair point into the 10-rings (Figure 9A). The occupancy of the
OSDA refines to 0.8, and this allows neighboring OSDA pairs
to adopt a different orientation occasionally. The closest
distances between the OSDA and the framework are 3.05(1) Å
for O10C13, 3.08(1) Å for O3C2, 3.09(1) Å for O4C9,
and 3.10(1) Å for O10C10. All other distances are above 3.1
Å. This has been illustrated with a Hirshfeld surface (Figure
Figure 10. 14-ring channel of CIT-13 (along the z axis) viewed down
the 10-ring channel (y axis), showing the Hirshfeld surface of one
OSDA and the arrangement of OSDAs at the intersections of the
channels.
contact distances to the framework are either short (red), long
(blue), or about equal to the sum of the van der Waals radii
(white).
4.2. Comparison between CIT-13 and UTL. Table 2
shows a comparison of the physicochemical data related to the
channel and pore systems of IM-12 and CIT-13. The
micropore volume found for IM-12 is consistent with the
6
,23,40
previously reported results.
The micropore volume of
CIT-13 is slightly smaller than that of IM-12. This is a
reasonable value for CIT-13, because the framework density for
−3
CIT-13 (16.41 T atoms nm ) is higher than that of IM-12
−3
(15.60 T atoms nm ). The physisorption isotherm of CIT-13
shows two distinct pore filling phenomena in the micropore
adsorption region (Figure 11B). We believe that the first pore
−
4
−3
filling at p/p ∼ 10 −10 arises from the high eccentricity of
0
−2
the 10-ring channels and the second one at p/p = 10 from
0
10). The color scheme used on this surface indicates that the
the 14-ring channels. The pore size distribution of CIT-13
6
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Chemistry of Materials
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Table 2. Comparison of Channel and Pore Systems of IM-12 and CIT-13
b
IM-12
CIT-13
a
sample Si-to-Ge ratio
space group
4.5
5.0
C 2/m (monoclinic)
Cmmm (orthorhombic)
3
framework density (T atoms per nm )
material density (g cm−3)
15.60
1.56
16.41
pure silica
1.64
a
Germanosilicate
1.77
1.84
channel dimension
14-ring
14-ring
[9.1 × 7.2 Å]
10-ring
[6.2 × 4.5 Å]
0.352
[
9.5 × 7.1 Å]
2-ring
8.5 × 5.5 Å]
1
[
micropore volume (cm g−1)
3
theoretically available
t-plot method
a
0.376
0.177
0.205
0.182
Saito−Foley
0.222
a
b
Calculated based on germanosilicate of the Si/Ge ratio characterized using EDS. Calculated from the crystallographic data from RED (the CIF
files are given as Supporting Information).
Figure 12. 29Si MAS Solid-state NMR spectra of calcined CIT-13 and
IM-12.
4
resonances than CIT-13 in this Q Si-region, indicating that the
UTL framework has more crystallographically unique T atoms
than does ideal CIT-13. Also, there are shoulder-signals in the
downfield region from −105 to −110 ppm that can be assigned
20,42
as either Si atoms residing in the d4r units
Ge silicon atoms.
or as nSi-(4-n)
However, we have not yet been able to
make definitive peak-assignments.
43,44
5
. CONCLUSIONS
The synthesis of the extra-large-pore molecular sieve, CIT-13,
with perpendicular intersecting 14- and 10-ring channels was
achieved over a wide range of synthetic conditions. Four
monoquaternary OSDAs belonging to a family of methyl-
benzylimidazolium cations produced CIT-13 as the major
phase. The optimized conditions for CIT-13 are as follows: the
Figure 11. Ar physisorption isotherms at 87 K of calcined CIT-13 and
IM-12: (A) linear and (B) log-scale.
41
+ −
according to the cylindrical Saito−Foley model is shown in
Figure S14A, and shows two distinct pore-filling steps. This can
be attributed to the large size and shape difference between the
gel composition at Si/Ge = 3−8, H O/T = 5−7.5, OSDA F /
2
T = 0.5 using 1-methyl-3-(3,5-dimethylbenzyl)imidazolium or
1,2-dimethyl-3-(3-methylbenzyl)imidazolium as the OSDA in a
static/rotating autoclave in a 140 °C-175 °C oven for 1−3
weeks.
10- and 14-ring channels of CIT-13. For IM-12, the dimensions
of the 12- and 14-ring channels are not too different from one
another, and thus these separated pore-filling steps were not
observed.
The CIT-13 product was characterized by 29Si MAS NMR
and argon adsorption experiments at 87 K and the results were
compared with those of IM-12 (UTL). Calcined CIT-13
The 29Si MAS NMR spectra (Figure 12) from the two
materials show multiple peaks that significantly overlap with
one another in the region from −110 to −120 ppm. The
absence of any peaks above −105 ppm indicates that both
29
showed a Si NMR spectrum largely similar to that of calcined
IM-12, with no silanol groups detected. However, the calcined
CIT-13 sample produced a two-step argon adsorption isotherm
in the low pressure range, indicating that the adsorption
behavior inside the 10-ring channel is very different to that of
4
calcined CIT-13 and IM-12 are composed of Q Si with no
significant amounts of silanol groups. IM-12 shows more
6
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Chemistry of Materials
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the 14-ring channel of IM-12. This type of isotherm was not
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Gap between Micro and Mesoporous Structures. Angew. Chem., Int. Ed.
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observed for IM-12. The micropore volume from the t-plot
3
−1
method was 0.18−0.19 cm g .
(
5) Maesen, T. The zeolite scene  an overview. In Stud. Surf. Sci.
̌
Cejka, H. v. B. A. C., Ferdi, S., Eds.; Elsevier, 2007; Vol. 168,
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4
Catal.; Jirí
Ch. 1, pp 1−12.
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́
6) Shvets, O. V.; Zukal, A.; Kasian, N.; Zilkova, N.; Cejka, J. The
̌
developed and the material from that protocol characterized.
The CIT-13 solids obtained via the two protocols were
essentially the same, thus effectively providing a way of avoiding
the use of concentrated hydrogen fluoride.
We analyzed the structure of CIT-13 using high-resolution
synchrotron XPD data. Using Rietveld refinement, we were
able to sort out the disorder present in the framework structure,
to determine the positions of Ge in the framework, and to
locate the extra-framework species in the channels and cavities.
The OSDA adopts a pairwise supramolecular arrangement at
the center of the 14-ring. As in other studies of
germanosilicates, we found that Ge is located primarily in the
d4r unit, making it a suitable starting material for trans-
formations via the ADOR process.
̌
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ASSOCIATED CONTENT
Supporting Information
■
Moliner, M. High-throughput synthesis and catalytic properties of a
molecular sieve with 18- and 10-member rings. Nature 2006, 443,
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemma-
ter.6b02468.
8
(
42−845.
11) Sun, J.; Bonneau, C.; Cantin, A.; Corma, A.; Diaz-Cabanas, M.
J.; Moliner, M.; Zhang, D.; Li, M.; Zou, X. The ITQ-37 mesoporous
Schematic diagrams of the frameworks structures,
supplementary XPD patterns and SEM images, adsorp-
tion isotherms, additional crystallographic information
on CIT-13. (PDF)
Crystal Data of As-Synthesized CIT-13 from Rietveld
Refinement (CIF)
Idealized Model of As-Synthesized CIT-13 (for visual-
ization) (CIF)
Crystal Data of Framework-Only CIT-13 from Rota-
tional Electron Diffraction (CIF)
chiral zeolite. Nature 2009, 458, 1154−1157.
̃
(12) Corma, A.; Díaz-Cabanas, M. J.; Jiang, J.; Afeworki, M.; Dorset,
D. L.; Soled, S. L.; Strohmaier, K. G. Extra-large pore zeolite (ITQ-40)
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Synthesis of an Extra-Large-Pore Zeolite with Double Three-Ring
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2
010, 49, 4986−4988.
AUTHOR INFORMATION
■
(15) Jiang, J.; Yun, Y.; Zou, X.; Jorda, J. L.; Corma, A. ITQ-54: a
multi-dimensional extra-large pore zeolite with 20 [times] 14 [times]
Corresponding Author
1
(
2-ring channels. Chem. Sci. 2015, 6, 480−485.
16) Paillaud, J.-L.; Harbuzaru, B.; Patarin, J.; Bats, N. Extra-Large-
*E-mail: mdavis@cheme.caltech.edu.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final manuscript.
Pore Zeolites with Two-Dimensional Channels Formed by 14 and 12
Rings. Science 2004, 35, 990−992.
(17) Guth, J. L.; Kessler, H.; Wey, R. New Route to Pentasil-Type
Zeolites Using a Non Alkaline Medium in the Presence of Fluoride
Ions. In Studies in Surface Science and Catalysis; Y. Murakami, A. I.,
Ward, J. W., Eds.; Elsevier B.V.: Amsterdam, 1986; Vol. 28, pp 121−
Notes
The authors declare no competing financial interest.
1
28.
ACKNOWLEDGMENTS
■
(18) Corma, A.; Puche, M.; Rey, F.; Sankar, G.; Teat, S. J. A Zeolite
Structure (ITQ-13) with Three Sets of Medium-Pore Crossing
Channels Formed by9- and 10-Rings. Angew. Chem., Int. Ed. 2003,
We thank Chevron Energy and Technology Company for
support of this research. We thank Dr. Joel Schmidt for useful
comments throughout the research study. We thank Antonio
Cervellino for his assistance with the powder diffraction
measurement on the Materials Science Beamline at the SLS
in Villigen, Switzerland. This work was supported also by the
Swiss National Science Foundation.
4
(
2, 1156−1159.
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Starzyk, F.; Valtchev, V. Ultra-fast framework stabilization of Ge-rich
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