Aluminoborate-based molecular sieves with 18-octahedral-atom tunnels.

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Octahedron-Based Molecular Sieves
Aluminoborate-Based Molecular Sieves with 18-
Octahedral-Atom Tunnels**
Jing Ju, Jianhua Lin,* Guobao Li, Tao Yang,
Hongmei Li, Fuhui Liao, Chun-K. Loong, and
Liping You
Microporous frameworks composed of tetrahedra, particu-
larly zeolites, have been extensively studied. These materials
are of technological importance as shape-selective catalysts
and ion-exchange materials and are widely used in various
industrial and technological processes.[1]On the other hand,
although a number of microporous materials consisting of
mixed octahedra/tetrahedra were established in recent
years,[2]microporous frameworks based purely on octahedra
are rare. The todorokite family of manganese oxides,[3]
including pyrolusite, todorokite, hollandite, and romanechite,
is a well-known example among the pure-octahedron frame-
works. In these materials, the octahedra share edges to form
cubic close-packed rock salt (111) layers, which are inter-
connected by sharing corners at 908 8 angles (rutile-like
connection), forming one-dimensional tunnels with different
pore sizes. The large square tunnels and mixed manganese
valences endow these materials with potential applications as
redox catalysts, batteries, and chemical sensors.[4]Here we
report the novel aluminoborate PKU-1, in which AlO6
octahedra share edges to form a three-dimensional porous
framework consisting of 18- and 10-ring windows. The search
for new zeolite-like aluminoborates was carried out several
years ago, and several aluminoborates and aluminum boron
oxide chlorides were identified under hydrothermal condi-
tions, but their stuctures remain unknown.[5]
Aluminoborate PKU-1 was synthesized by reaction of
AlCl3·6H2O with an excess ofH3BO3at 2408 8C in an autoclave
(yield: 95% in Al), whereby the boric acid was used as both
reaction medium and reactant. The product consists of
needle-shaped polycrystallites. X-ray diffraction identified a
trigonal structure (R3¯) with lattice parameters a=22.0381(2)
and c=7.0261(1) ? at room temperature. Analysis of boron
and aluminum by inductively coupled plasma emission
spectroscopy (ICP-ES) gave a B/Al ratio of about 2. The
27Al MAS-NMR,11B MAS-NMR, and IR data indicated that
the Al and B atoms are exclusively in octahedral and
triangular coordination, respectively. The detailed structure
of PKU-1 was established by analysis of the XRD data with
an ab initio method and subsequently by Rietveld refine-
ments with the GSAS program,[6,7]which allowed all non-
hydrogen atoms in the compound to be located.[8]The refined
structural parameters are listed in the Supporting Informa-
tion. The presence of protons in the structure was established
by consideration of charge balance, as well as thermogravi-
metric analysis (TGA), IR spectroscopy, and valence calcu-
lation.
Figure 1 shows the structure of PKU-1 projected along the
c axis. The structure can be considered as a framework of
octahedra that is enveloped by triangular borate groups. The
aluminum atoms are all octahedrally coordinated, and the
octahedra share edges to form a framework with three-
dimensional porosity. The structure contains two crystallo-
graphically independent types of Al atoms [Al1-(18f) and
Al2-(9d)]. The Al1 octahedra share three edges with two Al1
and one Al2 octahedra and act as “nodes” in the framework.
The Al2 atoms, on the other hand, share two opposite edges
with Al1 octahedra, leaving two opposite corners as terminal
oxygen atoms. An alternative description of the framework is
that the Al1 octahedra form one-dimensional edge-sharing
chains along the c axis (threefold helix with an approximate
Al1-Al1-Al1 angle of 1208 8). The Al2 octahedra share two
opposite edges with the Al1 chains to form a three-dimen-
sional porous framework. Thus Al2 octahedra act as “girders”
in the framework. Figure 2 displays a perspective view of the
Figure 1. Projection of the PKU-1 structure along the c axis; AlO6octa-
hedra are shown as polyhedra, and oxygen and boron atoms as dark
and light spheres, respectively; water molecules in the channels are
omitted for clarity.
[*] Prof. Dr. J. Lin, Dr. J. Ju, Dr. G. Li, T. Yang, H. Li, F. Liao
College of Chemistry and Molecular Engineering
Peking University
Beijing 100871 (P. R. China)
Fax: (+ +86)10-6275-1708
E-mail: jhlin@chem.pku.edu.cn
Dr. C.-K. Loong
Intense Pulsed Neutron Source Division
Argonne National Laboratory
Argonne, IL 60439 (USA)
L. You
Electron Microscopy Laboratory
College of Physics
Peking University
Beijing 100871 (P. R. China)
[**] We thank Profs. Xuanwen Li, Youchang Xie, and Xiyao Yang at PKU
for stimulating discussions and Prof. Feng Deng at Wuhan Institute
of Physics and Mathematics for his support with MAS NMR
analysis. This work was supported by the Natural Science
Foundation of China and by the State Key Basic Research Program
of China. Work performed at Argonne National Laboratory was
supported by the U.S. DOE, Basic Energy Sciences, under contracts
No. W-31-109-ENG-38.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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framework of PKU-1, which clearly shows the connectivity of
octahedra in the structure. The cavity of the framework is
three-dimensional and contains 18-ring hexagonal channels
along the [001] direction and 10-ring rectangular channels
along each of the {100} directions. The structural model of
PKU-1 was also supported by high-resolution TEM data
(Figure 3). The HREM image (central inset), simulated by
using the structural parameters of PKU-1, agrees reasonably
well with the observed pattern. In addition, electron diffrac-
tion (insert left) also confirms the trigonal cell of the
structure.
The framework of edge-sharing Al octahedra in PKU-1 is
highly negatively charged ([Al3O10]11?). This is a common
feature of three-dimensional porous frameworks of octahedra
and may also be the reason that these frameworks are rare. To
stabilize the octahedral framework, other positively charged
species are needed. As shown in Figure 1, all of the oxygen
atoms of the octahedral framework are further shared by
surrounding borate groups [i.e., BO2(OH) and B2O4(OH)]
thatcompensatemostof
([Al3B6O12(OH)4]?). The remaining negative charge could
be compensated by countercations in the channels. In this
case, it is likely that protons act as the countercations. The IR
spectrum of a sample after calcination at 2008 8C under
vacuum clearly shows three distinct OH vibrations at 3180,
3440, and 3650 cm?1for the hydroxy groups (see Supporting
Information). The overall composition of PKU-1 can be
expressed as HAl3B6O12(OH)4.
The borate groups partially block the channels of the
framework and thus reduce the porosity of the structure.
Figure 4 shows the structures of the 18- and 10-rings. The
B2O4(OH) groups are located within the 10-ring rectangular
thenegativecharges
windows and almost completely block the channels along the
{100} directions. The BO2(OH) groups are located within the
18-ring windows and narrow the channels along the [001]
direction. The O?O distances of about 5.8 ? between the
terminal oxygen atoms on borate groups and about 9.5 ?
between those on the Al octahedra correspond to columns
6.7 ? and 11.0 ? in diameter, respectively. The nitrogen
adsorption isotherm of a PKU-1 sample degassed at 2008 8C
under vacuum was characteristic of a microporous material,
with a maximum uptake of about 71.4 cm3of nitrogen per
gram of sample.
Figure 2. Perspective view of the framework of octahedra in PKU-1
showing the large, parallel hexagonal channels (18-ring) along [001]
and the smaller rectangular channels (10-ring) along {100}.
Figure 3. HREM image of PKU-1 in the basal plane; the electron dif-
fraction pattern (top, left inset) shows a typical trigonal symmetry. The
inset at the center is a simulated HREM image based on the structural
parameters of PKU-1. The basal axes a and b are shown in the figure,
and the length of the axis bars is about 44 ?.
Figure 4. a) 18- and b) 10-ring windows in PKU-1; the borate groups in
the channel are also shown. The distances of 5.8 and 9.5 ? shown in
the 18-ring are between crystallographically equivalent terminal oxygen
atoms and oxygen atoms of the octahedra, which result in diameters
of 6.7 and 11.0 ?, respectively, for the channel.
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The structure refinement also indicated that the as-
synthesized sample contains a considerable number of
partially ordered water molecules in the 18-octahedron
channels. These water molecules are mobile and can be
removed completely without affecting the integrity of the
framework structure. Structure refinement carried out on a
PKU-1 sample calcined at 2008 8C showed that the residual
electron density in the channels was completely removed (see
Supporting Information). Furthermore, TGA shows two-
stage weight loss. The weight loss below 2008 8C (ca. 8.1 wt%)
originates from removal of water, and that between 300 and
6008 8C (ca. 10.1 wt%) is related to dehydration of the borate
hydroxy groups. Above 6008 8C the framework eventually
collapses.
The microporous materials known so far are mostly based
on corner-sharing tetrahedra, and octahedron-based molec-
ular sieves are rare. One reason for this might be the high
negative charge of the three-dimensional porous frameworks
of octahedra for common MIIIand MIVcations. Two different
approaches may reduce the negative charge: sharing the
octahedral edges more extensively throughout the framework
or compensating with other positively charged species. The
OMS (octahedral molecular sieves) family of manganese
oxides[4]exemplifies extensive sharing of octahedral edges, in
this case to form rock salt (111) layers. The oxygen atoms in
this family all exhibit threefold coordination by Mn4+ +and
Mn2+ +. PKU-1 is a representative in which the negative charge
is compensated by borate groups. In the structure of PKU-1,
most of the oxygen atoms on the framework of octahedra of
PKU-1 are also threefold coordinated by Al and B (except for
the opposite vertices on the Al2-octahedron, which are two-
coordinate). Such a mode of compensation allows the
formation of a three-dimensional framework of octahedra
containing tunnels.
It is noteworthy that this octahedron-based molecular
sieve was synthesized in boric acid flux without using any
organic template. Although the reaction mechanism is not yet
understood, the formation of the microporous structure
implies the assembly of borate groups and water molecules,
perhaps mediated by hydrogen bonding during the reaction.
In fact, in addition to PKU-1, many new polyborates,
including rare-earth polyborates and transition-metal poly-
borates, were obtained.[9]Moreover, some other metal ions
can also be accommodated at the octahedral sites in the PKU-
1 structure. Galloborate (HGa3B6O12(OH)4) also crystallizes
in the PKU-1 structure under the same reaction conditions.
The lattice constants are significantlylarger than thanthose of
the aluminum analogue, as shown in Table 1. We also applied
similar reaction conditions to the transition metal salts and
found that most transition metals do not themselves form
PKU-1 structures, but can partially replace Al to form solid
solutions such as HAl2.4Fe0.6B6O12(OH)4 and HAl2.5Cr0.5-
B6O12(OH)4. As shown in Table 1, substitution by transition-
metal ions modifies the lattice parameters and the porosity of
the frameworks. In terms of potential applications, the
octahedral molecular sieves containing d-block elements are
considerably more interesting because they may provide a
means of tailoring not only chemical and physical properties
such as catalysis, ion exchange, and molecular sieving, but also
electronic properties.
Experimental Section
PKU-1 was synthesized by direct reaction of aluminum salts with
boric acid in a flux of boric acid in a closed autoclave. In a typical
reaction AlCl3·6H2O (5 mmol) and H3BO3(100 mmol) were charged
to a 50 cm3Teflon autoclave, and the mixture was heated at 2408 8C for
four days. After cooling to room temperature, the solid (containing
PKU-1 and residual H3BO3) was washed with hot water (508 8C) until
the residual boric acid was completely removed. The yield was about
95% in Al. Other aluminum sources, such as Al2O3and Al(NO3)3,
also yielded the same product. Ga-PKU-1 and transition-metal-
substituted PKU-1 (AlCr-PKU-1 and AlFe-PKU-1) were synthesized
from gallium oxide and the transition-metal nitrates by a similar
method at 2208 8C for 10–14 days. Table 1 summarizes the reaction
conditions for these compounds. The degree of substitution by
transition-metal cations was analyzed by the ICP method.
Received: June 30, 2003 [Z52263]
.
microporous materials · zeolite analogues
Keywords: aluminoborates · aluminum · boron ·
[1] M. E. Davis, Nature 2002, 417, 813.
[2] a) M. W. Anderson, O. Terasaki, T. Ohsuna, A. Philippou, S. P.
Mackay, A. Ferrreira, J. Rocha, S. Lidin, Nature 1994, 367, 347;
b) R. C. Haushalter, K. G. Strohmaier, F. W. Lai, Science 1989,
246, 1289; c) S. M. Kuznicki, V. A. Bell, S. Nair, H. W. Hillhouse,
R. M. Jacubinas, C. M. Braunbarth, B. H. Toby, M. Tsapatsis,
Nature 2001, 412, 720; d) C. H. Lin, S. L. Wang, K. H. Lii, J. Am.
Chem. Soc. 2001, 123, 4649; e) N. Guillou,Q. Gao, P. M. Forster, J.
Chang, M. Nogu?s, S. Park, G. F?rey, A. K. Cheetham, Angew.
Chem. 2001, 113, 2913; Angew. Chem. Int. Ed. 2001, 40, 2831; f) S.
Ekambaram, S. C. Sevov, Angew. Chem. 1999, 111, 384; Angew.
Chem. Int. Ed. 1999, 38, 372.
Table 1: Reaction and lattice parameters for PKU-1.[a]
CompoundSynthesis conditions
Reactants
Lattice parameter
T [8 8C]t [d]a [?] c [?]
HAl3B6O12(OH)4
HGa3B6O12(OH)4
HAl2.4Fe0.6B6O12(OH)4
5 mmol AlCl3·6H2O
5 mmol Ga2O3
4 mmol AlCl3·6H2O
1 mmol Fe(NO3)3·6H2O
4 mmol AlCl3·6H2O
1 mmol Cr(NO3)3·6H2O
100 mmol H3BO3
200 mmol H3BO3
100 mmol H3BO3
240
220
220
422.0381(2)
22.6212(9)
22.2130(5)
7.0261(1)
7.2441(8)
7.1024(3)
10
14
HAl2.5Cr0.5B6O12(OH)4
100 mmol H3BO3
220 1422.2041(9)7.0716(1)
[a] The lattice constants were determined by refinement with EXTRA.
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[3] a) F. V. Chukhrov, A. I. Gorshkov, A. V. Sivtsov, V. V. berezov-
skaya, Nature 1979, 278, 631; b) S. Turner, P. R. Buseck, Science
1981, 212, 1024; c) R. G. Burns, V. M. Burns, H. W. Stockman,
Am. Mineral. 1983, 68, 972.
[4] a) Y. F. Shen, R. P. Zerger, R. N. DeGuzman, S. L. Suib, L.
McCurdy, D. I. Potter, C. L. O'Young, Science 1993, 260, 511;
b) X. Chen, Y. Shen, S. L. Suib, C. L. O'Young, Chem. Mater.
2002, 14, 940; c) Y.-C. Son, V. D. Makwana, A. R. Howell, S. L.
Suib, Angew. Chem. 2001, 113, 4410; Angew. Chem. Int. Ed. 2001,
40, 4280.
[5] a) J. Wang, S. Feng, R. Xu, J. Chem. Soc. Chem. Commun. 1989,
265; b) J. Yu, R. Xu, Y. Xu, Y. Yue, J. Solid State Chem. 1996, 122,
200; c) J. Yu, R. Xu, J. Chen, Y. Yue, J. Mater. Chem. 1996, 6, 465.
[6] a) C. Dong, Appl. Crystallogr. 1999, 32, 838; b) A. Altomare,
M. C. Burla, G. Cascarano, G. Giacovazzo, A. Guagliardi,
A. G. G. Moliterni, G. Polidori, J. Appl. Crystallogr. 1995, 28, 842.
[7] A. C. Larson, R. B. von Dreele, Report LAUR 86-748, Los
Alamos National Laboratory, Los Alamos, NM, 1985.
[8] Powder XRD data of PKU-1 were collected at room temperature
on a Bruker D8 diffractometer in the Bragg–Brentano geometry
with a curved germanium primary monochromator (CuKa1, l=
1.5406 ?). Tube voltage and current were 50 kVand 40 mA. Step-
scan size and time: 0.028 8 (2q) and 30 s. The patterns were indexed
to trigonal unit cells, a=22.0392(2) and c=7.02643(8) ? by using
TREOR90-PowderX,[6]and the analysis of the observed system-
atic absences narrowed the possible space groups to R3¯, R3, R32,
R3m, and R3¯m. PKU-1 does not show nonlinear optical behavior,
so the space group was restricted to R3¯or R3¯m. The crystal
structure was established by using EXPO with space group R3¯
and was further developed by difference Fourier analysis during
the Rietveld refinement.[6,7]All non-hydrogen atoms in the
structure were refined with geometric constraints of triangular
borate groups. The first reflection peak at about 8.08 8 was not used
in the structure refinement. The number of contributing reflec-
tions was 421 with 29 structure and profile variables in the final
refinement. The residual factors of the refinement were Rwp=
0.069 and Rp=0.049. The Rietveld refinement was also carried
out for a PKU-1 sample calcined at 2008 8C. The structure
parameters are almost the same as those of the as-synthesized
sample (with Rwp=0.048 and Rp=0.036), but the disappearance
of all of the electron density in the channels indicated complete
removal of the mobile water from the channels. Crystallographic
data, atomic parameters, and selected bond lengthsand angles are
givenin the SupportingInformation.Further detailson thecrystal
structure investigations may be obtained from the Fachinforma-
tionszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Ger-
many (fax: (+ +49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.
de), on quoting the depository number CSD-413233.
[9] a) L. Y. Li, P. C. Lu, Y. Y. Wang, X. L. Jin, G. B. Li, Y. X. Wang,
L. P. You, J. H. Lin, Chem. Mater. 2002, 14, 4963; b) L. Y. Li, X. L.
Jin, G. B. Li, Y. X. Wang, F. H. Liao, G. Q. Yao, J. H. Lin, Chem.
Mater. 2003, 15, 2253; c) P. C. Lu, Y. X. Wang, L. P. You, J. H. Lin,
Chem. Commun. 2001, 1178.
Communications
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