An Extra-Large-Pore Pure Silica Zeolite with 16×8×8-Membered Ring Pore Channels Synthesized using an Aromatic Organic Directing Agent

Article abstract of DOI:10.1002/anie.201915232

Extra-large-pore zeolites for processing large molecules have long been sought after by both the academia and industry. However, the synthesis of these materials, particularly extra-large-pore pure silica zeolites, remains a big challenge. Herein we report the synthesis of a new extra-large-pore silica zeolite, designated NUD-6, by using an easily synthesized aromatic organic cation as structure-directing agent. NUD-6 possesses an intersecting 16×8×8-membered ring pore channel system constructed by four-connected (Q⁴) and unusual three-connected (Q³) silicon species. The organic cations in NUD-6 can be removed in nitric acid to yield a porous material with high surface area and pore volume. The synthesis of NUD-6 presents a feasible means to prepare extra-large pore silica zeolites by using assembled aromatic organic cations as structure-directing agents.

Full text of DOI:10.1002/anie.201915232

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Accepted Article
Title: An extra-large-pore pure silica zeolite with 16×8×8-membered
ring pore channels synthesized by using an aromatic organic
directing agent
Authors: Wen-wen Zi, Zihao Gao, Jun Zhang, Bao-Xun Zhao, Xian-Shu
Cai, Hongbin Du, and Fei-Jian Chen
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201915232
Angew. Chem. 10.1002/ange.201915232
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10.1002/anie.201915232
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COMMUNICATION
An extra-large-pore pure silica zeolite with 16 × 8 × 8-membered
ring pore channels synthesized by using an aromatic organic
directing agent
Wen-Wen Zi,†[a] Zihao Gao,†[a] Jun Zhang,[b] Bao-Xun Zhao,[a] Xian-Shu Cai,[a] Hong-Bin Du,*[a] Fei-Jian
Chen*[c]
Dedicated to the 100th anniversary of the School of Chemistry and Chemical Engineering, Nanjing University
Abstract: Extra-large-pore zeolites for processing large molecules
have long been sought after by both the academic and industrial
communities. However, the synthesis of these materials, particularly
extra-large-pore pure silica zeolites, remains a big challenge. Herein
we report the synthesis of a new extra-large-pore silica and
aluminosilicate zeolite, designated NUD-6, by using an easily-
synthesized aromatic organic cation as structure-directing agent.
NUD-6 possesses an intersecting 16×8×8-membered ring pore
channel system constructed by four-connected (Q4) and unusual
three-connected (Q3) silicon species. The organic cations in NUD-6
can be removed in nitric acid to yield a porous material with high
surface area and pore volume. The synthesis of NUD-6 presents a
feasible means to prepare extra-large pore silica zeolites by using
assembled aromatic organic cations as structure-directing agents.
germanosilicates, which are mainly attributed to the formation of
subunits of double-four-membered rings (D4Rs) promoted by Ge
and F. However, these heteroatom-containing extra-large-pore
zeolites are not stable in water or moist atmosphere after
removal of SDAs because of the hydrolysis of Ge-O bonds.[12] In
comparison, pure silica and aluminosilicate zeolites possessing
stable frameworks are highly desirable and can be used as solid
acid catalysts in industry. To the best of our knowledge, there
are only four stable extra-large-pore silica or aluminosilicate
zeolites reported so far, i.e. UTD-1 (14-MR),[3] CIT-5 (14-MR),[4]
SSZ-61 (18-MR),[13] EMM-23 (21-MR).[14] UTD-1 and CIT-5 were
prepared with expensive organic SDAs, while SSZ-61 and EMM-
23 possess severely distorted pores that prevent large
molecules passing through. As a result, the application of these
zeolites has been hindered. Therefore, the synthesis of extra-
large-pore silica zeolites remains a big challenge.
The synthesis of zeolites with a new structure has drawn great
attention owing to their potential industrial applications in
adsorption, separation and catalysis. Particularly of interest are
extra-large-pore zeolites possessing pores with more than 12-
membered ring (12-MR) opening, as these materials may serve
as ideal catalysts or catalyst supports for processing large
molecules such as heavy oils and biomass.[1] However, the
syntheses of these materials have not been easy, because they
usually require the use of expensive and difficult-to-prepare
bulky organic cations of appropriate rigidity and hydrophobicity
as structure-directing agents (SDAs).[2] Nonetheless, by using
the bulky organic cations, together with the use of heteroatoms
(i.e. other than Al and Si) and fluoride, a number of extra-large-
pore zeolites have been synthesized. Examples include UTD-1
(14-MR),[3] CIT-5 (14-MR),[4] SSZ-53 (14-MR),[5] ITQ-33 (18-
MR),[6] ITQ-44 (18-MR),[7] NUD-1 (18-MR),[8] SYSU-3 (24-MR),[9]
ITQ-37 (28-MR),[10] ITQ-43 (30-MR),[11] etc. Most of them are
Recently, a class of aromatic organic cations has been found
to be effective in the synthesis of large- and extra-large-pore
zeolites.[15] These cations can form larger supramolecular
assemblies through interactions between the aromatic rings
during the synthesis, which then act as SDAs for the formation
of large- and extra-large-pore zeolites. This method allows the
use of small organic cations to direct the formation of extra-
large-pore or -cage germanosilicate zeolites, such as ITQ-29,[15a]
ITQ-37,[16] NUD-1,[8] NUD-2 (14-MR),[17] etc. Herein, we report
the synthesis of an extra-large-pore pure silica zeolite, denoted
as NUD-6, by using an easily-synthesized aromatic organic
cation as SDA. NUD-6 possesses a one-dimensional (1-D) 16-
MR pore channel, inside which the assembled organic SDAs
reside. Unlike most silica zeolites, NUD-6 consists of both three-
(Q3) and four-connected (Q4) Si atoms (Qn represents
Si(OSi)n(OH)4-n), which renders the framework thermally
unstable at high temperature. However, the SDAs in NUD-6
could be removed in nitric acid to produce a microporous zeolite
material with large specific surface area and pore volume.
NUD-6 was prepared by using N,N-dimethyl-(2-
methyl)benzimidazolium as a structure-directing agent. The
crystalline and pure samples of NUD-6 were hydrothermally
made from the gel with molar ratios of 1 SiO2: 0.5 SDA(OH): 0.5
HF: (5-10) H2O at 160°C for 15 d (Table S1). Powder X-ray
diffraction (PXRD) patterns of the as-synthesized samples are in
good agreement with the simulated patterns derived from the
single-crystal structure data, suggesting the as-synthesized
samples are pure (Figure 1). Scanning electron microscopy
(SEM) images showed that the as-synthesized samples consist
of long stick-shaped crystals, free of impurities (Inset in Figure 1).
[a]
W.-W. Zi, Z. Gao, B.-X. Zhao, X.-S. Cai, Prof. H.-B. Du
State Key Laboratory of Coordination Chemistry,
School of Chemistry and Chemical Engineering,
Nanjing University,
Nanjing, 210023, China.
E-mail: hbdu@nju.edu.cn.
[b]
[c]
Prof. J. Zhang
School of Materials and Chemistry Engineering,
Anhui Jianzhu University,
Hefei, 230601, China.
Prof. F.-J. Chen
Department of Chemistry,
Bengbu Medical College,
Bengbu, 233030, China.
E-mail: cfj-cumt@163.com.
† The authors contribute to the work equally.
Supporting information for this article is given via a link at the end of the
document.
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10.1002/anie.201915232
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O atom. The Q3:Q4 ratio in NUD-6 is 1:2. The presence of
unusually large percentage of Q3 Si species in NUD-6 is
confirmed by the distinct resonance peak at -101 ppm in 29Si
MAS and 1H-29Si cross-polarization MAS NMR spectra (Figure
[13]
S1). It is noted that extra-large-pore silica zeolites SSZ-61
and EMM-23 [14] also consist of well-defined Q3 or Q2 species.
NUD-6 possesses a very open structure with a framework
density of 12.0 T/1000 Å3, which is one of the lowest among the
known zeolites.[18] The straight extra-large 16-MR pore channels
run along the (100) direction, which are intersected by two 8-MR
pore channels running along the (111) and (11-1) directions
(Figures 2 and 3), respectively, to form a three-dimensional pore
system. The 16-MR pore channels are elliptical, and the longest
interatomic distances between the axial O atoms across the
pore channel are 11.3 Å × 9.0 Å (taking account of an oxygen
radius of 1.35 Å). However, four terminal O atoms protrude into
the 16-MR pore channel, shrinking the pore opening to 9.0 Å ×
6.4 Å. The 8-MR pore channels are also elliptical, with the
longest interatomic distances between the O atoms across the
pore channel of 4.7 Å × 3.8 Å.
Figure 1. PXRD pattern of the as-synthesized NUD-6 (a), compared with the
calculated NUD-6 (b) derived from the single-crystal structure data. Insert
shows the SEM image of NUD-6.
Further single-crystal X-ray diffraction analyses revealed
disordered SDA cations were located inside the 16-MR pore
channels. As shown in Figure 4, the benzimidazolium planar
rings of SDAs are nearly perpendicular to the 16-MR pore
channel axis, only slightly tilted ca. ±3.5°. The two neighboring
benzimidazolium cations form an inverted, eclipsed face-to-face
dimer, which then stack over each other to form a 1D array
along the 16-MR pore channels (Figure S2). The
benzimidazolium planes parallel with each other, with interplanar
distances of ca. 3.4 Å and 4.0 Å, respectively, indicating the
presence of π interactions between the neighboring SDA cations.
This observation implied that the benzimidazolium cations
possibly formed supramolecular assemblies through aromatic
interactions during the synthesis, and the assemblies might act
as SDA to direct the formation of extra-large-pore zeolite NUD-6.
The hypothesis is supported by photoluminescence studies
(Figure S3). The photoluminescent spectra of the dilute solution
of SDAI (1.0 × 10-3 M) showed a broad emission band at ca. 375
nm, attributed to the intra-molecular transitions. However, the
concentrated solution of the SDAI salt (0.3 M, c.f. the SDA
concentration of the zeolite synthesis gel was in the range of 1.7
– 14 M) showed the main emission peak at ca. 450 nm owing to
Single-crystal X-ray diffraction analyses reveal that NUD-6
crystallizes in the orthorhombic space group Imma, with a =
7.4356(14) Å, b = 15.528(3) Å, c = 17.361(3) Å, and V = 2004.5
Å3 (see Tables S2-S4 in the Supporting information for details).
As depicted in Figure 2, NUD-6 is built from a composite building
unit (CBU) called bea, i.e. a [435261] cage, which consists of
three 4-MRs, two 5-MRs and one 6-MR. The bea CBU is found
in several zeolites, notably Beta and SSZ-26.[18] Each bea CBU
in NUD-6 shares two of its three 4-MRs in an alternate manner
to form infinite 1D chains running along the (100) direction. Each
bea chain then connects to four neighboring chains via sharing
common oxygen vertexes on 5-MRs to form a porous 3D
framework with straight 16-MR pore channels in the (100)
direction and 8-MR pore channels in the (111) directions.
From another point of view, NUD-6 can be constructed from
a CBU double crankshaft chain (dcc) (Figure 2c) and a pair of
SiO4 tetrahedra. The dcc CBU is an edge-sharing 4-MR chain,
which has been found in zeolite AlPO-21, UTD-1, etc.[18] The dcc
chains in NUD-6 are aligned parallel to the a-axis direction, and
linked to four neighboring dcc chains by dimers Si2O7 to form an
open framework. The two Si atoms in every dimer Si2O7 are
three-connected Si (Q3), each having an uncoordinated terminal
Figure 2. Stick views of the structure of NUD-6. (a) the bea SBU. (b) the bea chain. (c) the dcc SBU. (d) 3D structure of NUD-6 along the (100) direction, showing
the 16-MR pore channels. (e) 3D structure of NUD-6 along the (111)-direction, showing the 8-MR pore channels. The O atoms and SDAs have been omitted for
clarity. The dashed circle and retangle show the bea and Si2O7 units, respectively.
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Figure 4. Ar adsorption and desorption isotherms of the acid-treated NUD-6.
Insert shows the DFT pore size distribution curves.
NUD-6 with occluded SDAs was stable when heated at
450°C in vacuum or 350°C in air atmosphere (Figure S6).
However, after calcination at 550°C in air to remove organic
SDAs, the framework of NUD-6 collapsed into a non-porous
amorphous material, as confirmed by PXRD and N2 adsorption
measurements (Figures S7 and S8). This could be attributed to
condensation of large amount of Q3 silanol groups in NUD-6 at
high temperature. Consistently, 29Si MAS NMR spectra of the
calcined sample showed the Q3 Si peak almost disappeared
compared with that of the as-synthesized sample (Figure S9).
A porous material was obtained after the as-synthesized
NUD-6 was immersed in concentrated nitric acid at 120°C for 22
h to remove the SDAs. TG and elemental analyses suggested
that most SDA cations have been removed after acid treatment
(Figure S10). The morphology of the acid-treated NUD-6
remained intact (Figure S11). PXRD studies showed that the
acid-treated NUD-6 retained its framework with only partial loss
of its crystallinity owing to the acid leaching (Figure S12). The
removal of SDAs resulted in the framework distortion, which led
to partial changes in the PXRD pattern. As shown in Figure 4,
the acid-treated NUD-6 after activation at 300°C displayed a
steep Ar gas uptake at very low p/p0 pressures, suggesting the
presence of micropores. At higher relative pressures, the
adsorption curve displayed a type II isotherm with an H4
desorption hysteresis loop, implying the presence of micro-
mesoporous network.[19] The Brunauer–Emmett–Teller (BET)
surface area and pore volume were calculated to be 434 m2/g
and 0.346 cm3/g, respectively (Table S5). The t-plot analyses
showed the acid-treated NUD-6 had a large external surface
area of 214 m2/g, which likely resulted from the damage to the
zeolite crystals by acid. The pore size distribution analyses
based on density functional theory (DFT) revealed that the acid-
treated NUD-6 possessed narrowly-distributed micropores at 6.8
Å and 8.0 Å, which could be related to the elliptical 16-MR pore
channels (9.0 Å × 6.4 Å, Figure 3). The presence of narrow
micropores implies the pore structure in NUD-6 was retained
after acid treatment and subsequent activation at 300°C. In
addition, there exist widely-distributed mesopores with pore
Figure 3. Polyhedral presentations of NUD-6 (a) along the a-axis, showing the
SDAs in 16-MR pore channels, (b) along the (111)-direction, showing the 8-
MR pore channels. Only one possible set of SDAs are shown. The H and F
atoms have been omitted for clarity.
the inter-molecular transitions, suggesting the formation of
excimers, i.e. aggregates of the SDA cations. The spectra of the
as-synthesized NUD-6 and the solid SDAI salt are very similar to
that of the concentrated SDAI solution. These results implied
that the aggregates of the SDA cations possibly played a role
during the crystallization of NUD-6.
The presence of the SDA benzimidazolium cations in
NUD-6 was confirmed by elemental analyses and solid state 13
C
MAS NMR studies. Elemental analyses revealed the C/N molar
ratio of 4.9 in the as-synthesized NUD-6, close to the theoretical
value (5.0) of SDA. The 13C NMR peaks of the benzimidazolium
cations in as-synthesized NUD-6 matched well with those
dissolved in the solvent (Figure S4), indicating the SDA cations
in NUD-6 were intact. Thermogravimetric (TG) analyses showed
that NUD-6 lost 34.7% of its weight from 30°C to 1000°C (Figure
S5), attributed to the loss of SDA (theoretical 32.3%) and
hydroxyl groups (theoretical 3.2%). These results suggest there
are four SDA cations in a unit cell of NUD-6, consistent with the
single crystal X-ray diffraction results. Based on the above
results, the formula of the as-synthesized NUD-6 can be
deduced as Si6O11(OH)2·(C10H13N2F).
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diameters ranging between 2 and 20 nm in the acid-treated
NUD-6 (Figure S13). The formation of these mesopores could
be attributed to partial dissolution of the framework atoms during
acid treatment of NUD-6. The mesopores are beneficial and
could facilitate mass transport during catalysis. These results
suggest that NUD-6 could be potentially used as adsorbents or
catalyst supports.
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In summary, a new extra-large-pore silica zeolite NUD-6 was
synthesized by using N,N-dimethyl-(2-methyl)benzimidazolium
as a structure-directing agent. NUD-6 possesses an open
framework with intersecting 16×8×8-MR pore channels. Inside
the 16-MR pore channels, the stacked planar organic aromatic
cations are located, which form supramolecular assemblies.
There exists high proportion of Q3 versus Q4 Si atoms in NUD-6,
making NUD-6 thermally unstable during calcination at high
temperature. The organic cations in NUD-6 can be removed in
nitric acid to yield a porous zeolitic material with high surface
area and pore volume, making it a promising candidate for
adsorption and for shape-selective catalysis. This work further
demonstrates that it is feasible to prepare extra-large-pore silica
zeolites by using small, easily-synthesized aromatic organic
cations as structure-directing agents.
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Acknowledgements
The authors are grateful for financial support from the National
Natural Science Foundation of China (grant numbers 21673115
and 21601004). They also thank the staffs from BL17B beamline
of National Facility for Protein Science in Shanghai (NFPS) at
Shanghai Synchrotron Radiation Facility, for assistance during
data collection.
Keywords: extra-large pores • organic structure directing agents
• structure elucidation • zeolites • synthesis design
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
An extra-large-pore silica zeolite
was prepared by using a small,
easily-synthesized aromatic
organic cations as structure-
directing agent. It possesses an
intersecting 16×8×8-membered
ring pore system, and shows
large specific surface area and
pore volume after removal of
organic templates.
Wen-Wen Zi,†[a] Zihao Gao,†[a] Jun
Zhang,[b] Bao-Xun Zhao,[a] Xian-Shu
Cai,[a] Hong-Bin Du,*[a] Fei-Jian
Chen*[c]
Page No. – Page No.
An extra-large-pore pure silica
zeolite with 16×8×8-membered
ring pore channels synthesized
by using an aromatic organic
directing agent
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