Targeted Synthesis of a Zeolite with Pre-established Framework Topology

Article abstract of DOI:10.1002/anie.201909336

Given their great potential as new industrial catalysts and adsorbents, the search for new zeolite structures is of major importance in nanoporous materials chemistry. However, although innumerable theoretical frameworks have been proposed, none of them have been synthesized by a priori design yet. We generated a library of diazolium-based cations inspired from the organic structure-directing agents (OSDAs) recently reported to give two structurally related zeolites (PST-21 and PST-22) under highly concentrated, excess-fluoride conditions and compared the stabilization energies of each OSDA cation in ten pre-established hypothetical structures. A combination of the ability of the OSDA selected in this way with the excess-fluoride approach has allowed us to crystallize PST-30, the targeted aluminosilicate zeolite structure. We anticipate that our approach, which aims to rationally couple computational predictions of OSDAs with an experimental setup, will advance further development in the synthesis of zeolites with desired properties.

Full text of DOI:10.1002/anie.201909336

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Accepted Article
Title: Targeted Synthesis of a Zeolite with Pre-Established Framework
Topology
Authors: Donghui Jo and Suk Bong Hong
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Targeted Synthesis of a Zeolite with Pre-Established Framework
Topology
Donghui Jo and Suk Bong Hong*
2
4
Abstract: Given their great potential as new industrial catalysts and
adsorbents, the search for unprecedented zeolite structures is of
major importance in nanoporous materials chemistry. However,
although an innumerable number of theoretical frameworks have
been proposed, none of them have been synthesized by a priori
design yet. Here, we have generated a library of diazolium-based
cations inspired from the organic structure-directing agents (OSDAs)
recently reported to give two structurally related zeolites PST-21 and
PST-22 under highly concentrated excess fluoride conditions and
compared the stabilization energies of each OSDA cation in ten pre-
established hypothetical structures. A combination of the ability of the
OSDA selected in this way with the excess fluoride approach has
allowed us to crystallize PST-30, the targeted aluminosilicate zeolite
structure. We anticipate that our scheme, which aims to rationally
couple the ability of computationally predicted OSDAs into an
experimental setup, will take a step forward in the synthesis of
hypothetical zeolites with desired properties.
structures can be constructed exclusively from bre ([4 .5 ]) units
by linking the corners of this small building unit by edges. Thus,
by definition,[2] the bre unit is a secondary building unit (SBU) of
PST-21 and PST-22. This stimulated us to generate ten
chemically feasible hypothetical zeolite structures constructed
from the same SBU (Figure S1).[
6a,7]
Considering the SBU as a vertex, on the other hand, all
derived theoretical frameworks have the same net topology
(primitive cubic; pcu)[8] as that of PST-21 and PST-22 (Figure
1a,b and Table S1). Consequently, most of them have only one
type of ‘18-hedral’ natural building units[9] (NBUs) surrounded by
eight bre units. We therefore thought that organic compounds with
similar sizes and shapes compared to the experimentally
validated OSDAs in the synthesis of PST-21 and PST-22 would
direct the formation of cavities towards the predicted hypothetical
frameworks. Here we report the synthesis of one of ten
hypothetical zeolites proposed in our recent study.[6a] The
successful design of this new zeolite, denoted PST-30, arose from
our insights into structural relationships between the existing
PST-21 and PST-22 and the hypothetical structures, combined
with energy calculations of intrazeolitic OSDA stabilization. This
led us to rationally identify a suitable organic cation for the
targeted framework.
Owing to their outstanding shape- and surface-selective
properties, zeolites and molecular sieves are finding commercial
applications in catalysis and separation.[1] While about 250
distinct framework types are now recognized,[2] a combination of
theory and experiment suggests that the number of chemically
feasible hypothetical structures is practically inexhaustible.[3]
Although many innovative synthetic concepts, such as organic
structure-directing agent (OSDA) design, charge density
match/mismatch, Ge introduction in OSDA-mediated zeolite
synthesis and assembly-disassembly-organization-reassembly,
have been developed,[4] the discovery of new zeolite structures is
mostly achieved by trial and error. The synthesis of materials with
known framework topologies, e.g., DAF-5 (framework type CHA),
HPM-1 (STW), SSZ-39 (AEI), SSZ-52 (SFW), etc., can be
accomplished using computationally predicted OSDAs.[5]
However, no attempt has yet been made to synthesize a
hypothetical zeolite structure a priori.
We first carried out stabilization energy calculations[10] for five
methyl-substituted imidazolium derivatives in PST-21, PST-22,
and ten hypothetical frameworks based on the bre unit as SBU
+
(
Figure S2 and Table S2). It was calculated that 123TMI has a
stabilization energy of -13.7 kJ (mol Si)-1 in PST-21, whereas
1
(
1
34TMI+ and 1234TMI+ provide energies of -13.4 and -13.8 kJ
mol Si) in PST-22, respectively. Thus, we set -13.0 kJ (mol Si)-
-1
as a threshold energy to assess whether a particular OSDA
could play a structure-directing role in formation of a targeted
structure. We also calculated the stabilization energies of a series
of methylated pyrazolium cations with two adjacent nitrogen
atoms in the aromatic ring, given the structural similarity between
these diazolium species (Figure S2 and Table S2). Among them,
Under the so-called excess fluoride conditions where the F-
concentration in zeolite synthesis mixtures is higher than that of
the OSDA, we have recently synthesized PST-21 (PWO) and
PST-22 (PWW), two novel high-silica (Si/Al ~ 10) zeolites, using
the
1,2,3-trimethylpyrazolium
(123TMP+),
and
1,2,4-
+
trimethylpyrazolium
(124TMP ),
1,2,3,5-
+
tetramethylpyrazolium (1235TMP ) ions, with geometric shapes
and sizes similar to those of 123TMI , 134TMI , and 1234TMI ,
respectively, gave stabilization energies less than -13.0 kJ (mol
+
+
+
1
,2,3-trimethylimidazolium
(123TMI+)
and
1,3,4-
+
trimethylimidazolium (134TMI ) or 1,2,3,4-tetramethylimidazolium
-1
Si) not only in PST-21 and PST-22 but also in some hypothetical
+
[6]
(
1234TMI ) ions as OSDA, respectively. As-made PST-21 and
structures like C1_L1_1.
PST-22 were both characterized to possess one OSDA molecule
To minimize the effects of inorganic synthesis parameters, we
performed zeolite synthesis using the above-mentioned three
pyrazolium-based OSDAs under the same conditions as those of
PST-21 and PST-22 syntheses (Table S3).[6a] While no
hypothetical zeolites were found, PST-21, ferrierite (FER), and
PST-22 were obtained using 123TMP , 124TMP , and 1235TMP
as OSDA, respectively. It is worth noting that 124TMP , despite
its similarity to 134TMI , produced ferrierite instead of PST-22,
on average per the 18-hedral cavity, being either the t-pwo
6
8
4
6
8
2
2
(
[4 .5 .9 ]) or t-pww ([4 .5 .8 .10 ]) unit (Figure S1). Their
+
+
+
[*]
Dr. D. Jo, Prof. S. B. Hong
Center for Ordered Nanoporous Materials Synthesis
Division of Environmental Science and Engineering
POSTECH, Pohang 37673 (Korea)
+
+
E-mail: sbhong@postech.ac.kr
demonstrating a non-negligible effect of differences in the charge
distribution of OSDAs on the synthesis product.[11]
Supporting information for this article is given via a link at the end of
the document.
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molecular shape very difficult to mimic
using an imidazolium-like nitrogen
distribution, crystalizes the targeted
structure BRE_L1_1 (PST-30), as
computationally predicted (Table S5 and
Figure S3), although the HF/OSDA2+ and
Si/Al ratios in the synthesis mixture
leading to the successful formation of
PST-30 are very narrow (ca. 4 and 10,
respectively), similar to the case of PST-
21 and PST-22 (Table S7).
The successful synthesis of the
hypothetical BRE_L1_1 structure can be
demonstrated by comparing its simulated
powder X-ray diffraction (XRD) pattern
with the powder XRD pattern of the
proton form (H-PST-30) of PST-30 and
was further confirmed by Rietveld
analysis (Figure 3a,b and Tables S8-
S10). The integrity and stability of PST-
30 were verified by scanning electron
Figure 1. Zeolite structures built from the bre unit as SBU and OSDAs used for their synthesis. a) pcu net
2
microscopy (SEM), N adsorption, and
topology. Vertices, red; edges, black; tiling, yellow. b) PST-21 (left) and BRE_L1_1 (right) structures with the
27Al and 29
Si MAS NMR (Figures 3a, S4,
2
4
pcu net of bre ([4 .5 ]) units. The bridging O atoms have been omitted for clarity, and the bonds
interconnecting bre units are represented by blue dotted lines. c) Eight quaternizable methyl-substituted
and S5). While 1H- C CP MAS NMR
13
2
+
2
+
indicates that 13DMP-C
in as-made
diazole molecules (left) and 13DMP-C
4
(right), one of the 32 candidate OSDAs obtained by combining two
4
2+
19
diazoles and one methylene chain. d) Energy minimized-conformation of 13DMP-C
4
in the BRE_L1_1
PST-30 remains intact, its F MAS NMR
spectrum is characterized by a prominent
resonance at -128 ppm (Figure 3c,d).
This resonance can be attributed to the
structure. C, grey; N, blue; H, white; Si, yellow; O, red. The regions marked in translucent green indicate the
accessible surfaces calculated using a solvent radius of 1.4 Å .
-
free F ions within the PST-30 channels
We then moved our attention to diquaternary OSDAs, built
and/or the anions in fluorosilicate species, where Si is
from methylated diazolium moieties connected by methylene
chains with different lengths, in a manner similar to Camblor and
other groups who extended the OSDAs for zeolite syntheses from
monoquaternary imidazolium ions to diquaternary analogs.[5b,12] It
was reasoned that their diazolium rings could still organize the
pcu net of bre units, whereas the central methylene chain could
arrange a 10-ring window connecting neighboring cavities. We
established a library of a total of 32 diquaternary candidate
OSDAs by combining eight representative methyl-substituted
diazole molecules and methylene linkers and calculated their
stabilization energies not only in PST-21 and PST-22 but also in
ten hypothetical structures (Figures 1c,d and 2 and Table S4).
Here, when combining the same two diazoles and a methylene
chain, we designated the resulting diquaternary OSDA as ‘diazole
-
hexacoordinated, but not to the F ions occluded in a small
cage.[ A combination of elemental, thermal, and F MAS NMR
12f]
19
analyses on as-made PST-30 gave a unit cell composition of
|
(13DMP-C 2
4
+)0.5F-0.2(H
2
O)0.4|[Al0.8Si9.2
O20] (Figures 3d and S3).
This indicates that each 18-hedral cavity is occupied by one
pyrazolium moiety of 13DMP-C
computational screening analysis of the OSDA library (Figure 1d).
The PST-30 structure is built by stacking the bre_L1 layer, a
hypothetical layer proposed in our recent paper,[6a] along the b
axis, without imposing any interlayer symmetry operation (Figure
2
+
4
,
as assumed in the
1b). This layer is made up of a building chain nonjointly
connecting bre units, denoted bre chain, which can be obtained
by deconstructing the layer in the BRE structure. Consequently,
PST-30 has a two-dimensional (2D) pore system consisting of 10-
ring (4.7 × 6.7 Å) channels interconnected by 8-ring (4.2 × 4.4 Å)
2
+
n
acronym-C ’ (n: methylene chain length, 3-6; Figure 1c). Seven
members of this library were calculated to have stabilization
energies below -13.0 kJ (mol Si)-1 in at least one hypothetical
6
8
2
2
channels, which comprises unprecedented [4 .5 .8 .10 ] cavities
(
Figures 3e and S1). The framework density (defined as the
2
+
structure. For example, while 13DMP-C
4
gives a stabilization
3
number of tetrahedral atoms per 1000 Å ) of PST-30 was
calculated to be 17.1, which is essentially the same as the values
-1
energy of -13.6 kJ (mol Si) in BRE_L1_1, the other six OSDAs
4 5 6 4 4
, 1MP-C
2+, 1MP-C 2+, 14DMP-C 2+, 15DMP-C 2+,
2
+
(
i.e., 1MP-C
(
17.1 and 17.2, respectively) of PST-21 and PST-22. The n-
2
+) are characterized by energies ranging from -
and 12DMI-C
4
hexane/3-methylpentane cracking test reveals that H-PST-30 has
a constraint index of 9.0, which is within the range for medium-
pore zeolites (Figure S6).[14] It should be noted that H-PST-30
shows a comparable isobutene yield compared to H-ferrierite, one
of the best 1-butene skeletal isomerization catalysts (Figure
S7),[ probably due to the similarity in their pore architecture.
1
3.0 to -14.3 kJ (mol Si)-1 in PST-21_H1.
It is remarkable that among the latter six OSDAs described
above, all of them, except 1MP-C 2+, experimentally produced
6
PST-22 rather than PST-21_H1, in which all the OSDAs display
more favorable stabilization energies (Figure 2 and Tables S4 and
S5). This indicates that the framework energy[13] of targeted
hypothetical structures can affect the phase selectivity of the
15]
-1
crystallization (14.8 (PST-22) vs. 17.7 (PST-21_H1) kJ (mol Si) ;
Table S6). More interestingly, we found that 13DMP-C 2+, i.e.,
,1'-(1,4-butanediyl)bis(2,5-dimethyl-1H-pyrazol-2-ium), with
4
1
a
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Figure 2. Stabilization energies (SEs) of a total of 32 candidate OSDAs in
PST-21, PST-22, and ten hypothetical structures. Each OSDA consists of
methyl-substituted diazole moieties connected by methylene chains with 3-6
carbon atoms.
Figure 3. a) Experimental and simulated (λ = 1.541874 Å ) XRD patterns of H-
PST-30 and BRE_L1_1, respectively, and SEM image (inset) of as-made
We also carried out powder XRD and Rietveld analysis on as-
made PST-30, as well as as-made PST-21 and PST-22
2+
PST-30 synthesized using 13DMP-C
4
as an OSDA. b) Rietveld plot of H-
PST-30 (λ = 1.5167 Å ): observed data (+), calculated fit (solid line), and
+
+
synthesized using 123TMI and 1234TMI , respectively, to
determine the OSDA locations in each zeolite. (Figure S8 and
Tables S11-S14). All diazolium rings in OSDAs were found to be
located around the center of 18-hedral cavities, which is in
excellent agreement not only with the positive difference electron
density maps after initial scaling [16] (Figure S9), but also with the
theoretically calculated OSDA locations (Figures S10-S12). This
again illustrates the usefulness of the computational method in
the search for a suitable OSDA to direct the synthesis of a given
pre-established zeolite framework.
Finally, we note that most of the zeolites including our PST-30,
which have been synthesized using computationally predicted
OSDAs, possess only one type of NBU cavities with occluded
OSDA molecules (Table S15). In addition to these zeolite
structures, there are at least another 16 framework types having
such a type of large cages when the number of types of NBUs
present is limited for simplicity to three or lower (Table S16). Thus,
once there are hypothetical zeolites that are structurally related to
the above zeolites, the scheme described here, which integrates
the known synthesis information into the computer-aided design
of synthesis experiments, may be a possible way to synthesize
such theoretical frameworks.
difference plot (lower trace). The tick marks represent the positions of allowed
1
13
19
reflections. c) H- C CP and d) F MAS NMR spectra of as-made PST-30.
The solution 13C NMR spectrum of the dibromide salt of 13DMP-C
2+
4
2
in D O
(red) is given for comparison. e) The H-PST-30 structure with 10-ring and 8-
ring channels viewed along the a (left) and c (right) axes. Si, blue; O, red.
the corresponding framework under excess fluoride conditions.
We believe that our integrated approach can be extended to the
synthesis of other hypothetical structures. To make it possible to
routinely crystallize hypothetical zeolites with desired pore
structures, however, it is necessary to better understand the
effects of inorganic synthesis parameters, most notably the
concentration of water and hydroxide or fluoride anions and the
type of alkali metal cations and heteroatoms other than Si in
zeolite synthesis mixtures, on the product selectivity.[6b]
Experimental Section
The details of the computational methods, experimental
procedures, characterization, and structural analysis are provided
in the Supporting Information. CCDC 1919622–1919625 contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
In conclusion, the targeted synthesis of
a specified
hypothetical structure was found to be possible when the
structure-directing ability of the organic additives computationally
predicted to give this targeted structure is suitably combined with
the experimental insights obtained from the synthesis of existing
zeolites. BRE_L1_1 is a pre-established theoretical framework,
which contains a 2D medium-pore system with intersecting 10-
and 8-ring channels, and its synthesis has been achieved by the
use of an OSDA with the largest calculated stabilization energy in
Acknowledgements
This work was supported by the National Creative Research
Initiative Program (2012R1A3A2048833) through the National
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Research Foundation of Korea and by the National Research
Council of Science & Technology (CRC-14-1-KRICT) grant from
the Korea government (MSIP). We thank K. C. Kemp (POSTECH)
for helpful comments and PAL (Pohang, Korea) for synchrotron
diffraction measurement at the 9B beamline (D. Ahn). PAL is
supported by MSIP and POSTECH.
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Conflict of interests
The authors declare no conflict of interest.
Keywords: aluminosilicates • excess fluoride approach •
hypothetical zeolites • microporous materials • targeted
synthesis
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1
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Synthesis of a hypothetical
structure: A targeted hypothetical
zeolite structure has been
D. Jo, S. B. Hong*
Page No. – Page No.
successfully synthesized by
combining theoretical results with
experimental insights.
Targeted Synthesis of a Zeolite with
Pre-Established Framework
Topology
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