PKU-10: A new 3D open-framework germanate with 13-ring channels

Article abstract of DOI:10.1021/ic100195r

PKU-10, a germanate with the formula [(CH₃)₄N] ₃Ge₁₁O₁₉(OH)₉, is synthesized under hydrothermal conditions, and its structure is determined by single-crystal X-ray diffraction data. PKU-10 possesses 3D intersected 13-ring channels and presents a new 6-connectedness linkage mode of the Ge₇ cluster, T ³P²O, forming a pcu topological network. Each Ge ₇ cluster is, in fact, surrounded by eight Ge₇ clusters in a nearly perfect cube because the hydrogen bonds between Ge₇ clusters are also taken into account. The structure-directing agent tetramethylammonium (TMA⁺) ions, locating in the channels, can be partially exchanged by Li⁺ with retention of the germanate framework. The germanate framework collapses with decomposition of the TMA⁺ ions at temperatures higher than 240 °C.

Full text of DOI:10.1021/ic100195r

Inorg. Chem. 2010, 49, 9765–9769 9765
DOI: 10.1021/ic100195r
PKU-10: A New 3D Open-Framework Germanate with 13-Ring Channels
Jie Su, Yingxia Wang,* Zheming Wang, Fuhui Liao, and Jianhua Lin*
Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Rare Earth Materials Chemistry
and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
Received February 1, 2010
PKU-10, a germanate with the formula [(CH3)4N]3Ge11O19(OH)9, is synthesized under hydrothermal conditions, and
its structure is determined by single-crystal X-ray diffraction data. PKU-10 possesses 3D intersected 13-ring channels
and presents a new 6-connectedness linkage mode of the Ge7 cluster, T3P2O, forming a pcu topological network. Each
Ge7 cluster is, in fact, surrounded by eight Ge7 clusters in a nearly perfect cube because the hydrogen bonds between
Ge7 clusters are also taken into account. The structure-directing agent tetramethylammonium (TMAþ) ions, locating in
the channels, can be partially exchanged by Liþ with retention of the germanate framework. The germanate framework
collapses with decomposition of the TMAþ ions at temperatures higher than 240 °C.
Introduction
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62755538 (Y.W.). Fax: 86-10-62753541.
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2010 American Chemical Society
Published on Web 10/06/2010
pubs.acs.org/IC

9766 Inorganic Chemistry, Vol. 49, No. 21, 2010
Su et al.
via additional building units, forming diversified large-pore
frameworks.11 SU-M, for instance, is a mesoporous germanate
with crystalline pore walls consisting of Ge10 clusters.11b
SU-MB, on the other hand, consists of both Ge10 and Ge7,
in which the Ge7 clusters fill one set of the two gyroidal channels
in SU-M and thus shows channel chirality.11b The Ge7 cluster,
which is built by one octahedron, two trigonal bipyramids,
and four tetrahedra, is, in fact, one of the most frequently
appearing species in various germanate structures, forming 1D
chains,9h,i,10 2D layers,9e,f and 3D frameworks.9a-d
Herein this paper, we report a new germanate, [(CH3)4N]3-
Ge11O19(OH)9 (PKU-10). It is a 3D porous framework struc-
ture constructed by the Ge7 clusters and additional GeO4
tetrahedra. A simplified polyhedron of the Ge7 cluster, where
each vertex corresponds to a Ge atom, is applied to show the
13MR channels of the framework.
Table 1. Crystallographic Data and Structure Refinement Results for PKU-10
chemical formula
fw
Ge11O28C12H45N3
1478.00
298(2)
orthorhombic
P212121
12.4827(12)
17.3115(14)
18.5392(21)
4006.2(16)
4
2.450
8.219
0.20 ꢀ 0.15 ꢀ 0.12
1.97-33.51
38 870
14 944
0.0596
12 648
temperature/K
cryst syst
space group
˚
a/A
˚
b/A
˚
c/A
˚
cell volume/A
3
Z
Dc/g cm-3
3
μ(Mo KR)/mm-1
cryst size/mm3
2θ range/deg
no. of reflns measd
no. of indep reflns
Rint
no. of obsd reflns
final R indices [I > 2σ(I)]
R indices (all data)
GOF
0.0455, 0.1014
0.0578, 0.1067
1.028
Experimental Section
Materials and Methods. All reagents are of analytical grade
and were used as obtained from commercial sources without
further purification. PKU-10 was synthesized under hydrothermal
conditions from a homogeneous mixture of germanium dioxide
(GeO2) and tetramethylammonium hydroxide (TMAOH). Solid
GeO2 and aqueous TMAOH were mixed and stirred until a
clear solution was obtained. The water content was controlled
by evaporation of the mixture at 65 °C in an oven. The molar
ratio of the starting gel was 1:0.6:2.5-4.5 GeO2/TMAOH/H2O.
The gel was transferred to a Teflon-lined stainless autoclave and
heated at 175 °C for 14 days. After cooling to room temperature,
transparent crystals were obtained. The product was filtered,
washed with deionized water and acetone, and then dried at
65 °C. Anal. Calcd for Ge11O28H45C12N3 (fw 1478.00): C, 9.75;
H, 3.08; N, 2.84. Found: C, 9.76; H, 3.17; N, 2.80. The IR
spectrumshowed peaks (cm-1; the intensity character isindicated
following the position) at 3384m, 3221m, 3036m, 2452w, 1640w,
1448m, 1419w, 1384w, 1115w, 1019w, 956m, 847s, 815vs, 787vs,
768vs, 718m, 703m, 583m, 541m, 504m, 488m, and 452m.
The purity of the products was examined by using powder
X-ray diffraction on a Rigaku D/Max-2000 diffractometer with
and refined on F2 with full-matrix least-squares methods using
the SHELX-97 program. All Ge, O, and N atoms were refined
anisotropically, while the C atoms from the structure-directing
agent were refined isotropically. H atoms were added in the
˚
riding model and refined isotropically with C-H = 0.96 A and
˚
O-H = 0.94 A. Crystallographic data and structure refinement
results are summarized in Table 1. The corresponding CIF file is
supplied in the Supporting Information.
Results and Discussion
Structure Description. PKU-10 crystallizes in the ortho-
rhombic space group P212121 with lattice constants a =
˚
˚
˚
12.4827(12) A, b = 17.3115(14) A, and c = 18.5392(21) A.
An asymmetric unit in PKU-10 contains 11 crystallo-
graphically independent Ge atoms, 28 O atoms, and 3
TMAþ ions (12 C and 3 N atoms). As shown in Figure 1a,
a Ge7 cluster and four additional GeO4 tetrahedra form
the fundamental building unit of the germanate frame-
work. The displacement parameters are regular for Ge
atoms but show a considerably wide range for O atoms;
the displacement parameters of the hydroxyl O atom are
about 2 or 3 times larger than those of the bridging O
atoms; the O1 atom, which is at the center of the Ge7
cluster and bonds to three Ge atoms, has very small
displacement parameters. The bond distances and angles
in PKU-10 are all coincident with the previously reported
data (Table S1 in the Supporting Information).9-13 The
˚
a Cu KR radiation source (λ = 1.5418 A) and a graphite
monochromator at the secondary beam. Thermogravimetric
analysis (TGA) of the samples was done on the instrument
TGA Q50 V20.6 with a heating rate of 10 °C min-1 from 25 to
3
900 °C under an air flow. Elemental analysis of carbon, nitrogen,
and hydrogen was carried out with an Elementar Vario EL III
microanalyzer. The elemental contents of GeIV and LiI in the
Liþ-ion-exchanged PKU-10 were measured by using the induc-
tively coupled plasma (ICP) method on an ESCALAB2000
analyzer. IR spectroscopy was performed on a Nicolet Magna-
˚
Ge-O distances are in the range of 1.696(4)-1.764(4) A
IR750 FTIRspectrophotometer inthe regionof4000-400 cm-1
.
˚
in Ge tetrahedra and 1.767(4)-2.059(3) A in octahedra
The Liþ-ion-exchanged sample (Li-PKU-10) was obtained by
heating a mixture of PKU-10 and LiAc 2H2O with the molar
and trigonal bipyramids. The Ge-O-Ge angles can be
divided into two groups: the Ge-O-Ge angles within
one Ge7 cluster are in the range of 91.44-135.25°, which
reflects the relatively “rigid” connection of the groups
inside the cluster, while the Ge-O-Ge angles related to
the linkage of the Ge7 cluster with additional tetrahedra
can vary from 121.05(19) to 140.9(2)°.
3
3
ratio 1:10 at 85 °C for 2 h, in which LiAc 2H2O was in a molten
state. Then the mixture was cooled and washed with ethanol to
move the unreacted LiAc 2H2O, and the product was dried at
3
65 °C. The elemental analysis result (%) for the product Li-
PKU-10 was C 6.99, H 2.62, and N 1.96, while the ICP result
shows that the ratio of Ge to Li is 10.3. We can deduce that some
of the TMAþ ions (about 30%) were exchanged by Liþ ions. The
exchange by Liþ in an aqueous solution was not successful.
Crystallographic Studies. Single-crystal (0.2 ꢀ 0.15 ꢀ 0.12 mm3)
X-ray diffraction data of PKU-10 were collected on a Bruker
SMART X-ray diffractometer, equipped with an APEX-CCD
area detector and using graphite-monochromated Mo KR
The Ge7 cluster (Figure 1b) has seven apical O atoms
that can combine with other atoms. In PKU-10, they all
connect to additional GeO4 tetrahedra: Ge(8)O4, Ge-
(10)O4, Ge(11)O4, and Ge(9)O4. Among these tetrahedra,
Ge(8)O4, Ge(10)O4, and Ge(11)O4 are intergroups that
cross-link two adjacent Ge7 clusters and appear as GeO2-
(OH)2 units, while Ge(9)O4 only connects to one Ge7
cluster through the Ge(6)O4 group and resides as a terminal
˚
radiation (λ = 0.710 73 A) at 298 K. The data absorption correc-
tion was based on symmetry-equivalent reflections using the
ABSOR program. The structure was solved by the direct method

Article
Inorganic Chemistry, Vol. 49, No. 21, 2010 9767
Figure 2. Structures of PKU-10: (a) viewed along [100]; (b) viewed along
[011].
Figure 3. Location of the TMAþ ion in the 13-ring channels of PKU-10.
6-connectedness. The other two known modes are T4P2
and T4PO, which appear together with the Ge9 cluster in
the structures of SU-44 and SU-8.11c
Figure 2 shows the polyhedral presentation of the
PKU-10 structure framework. With hexadentate connec-
tivity, Ge7 clusters are cross-linked by GeO4 tetrahedra,
forming a very open structure with 3D intersecting channel
systems, along the directions [100], [011], and [0-11],
respectively. The 13-ring channel along the [100] direction
(Figure 2a) has an effective open window with the size of
Figure 1. (a) The germanateasymmetricunit ofPKU-10;(b) the Ge7O16
cluster with one GeO6 octahedron (pink), two GeO5 trigonal bipyramids
(orange), and four GeO4 tetrahedra(green); the asterisk symbols indicates
the polyhedra joining to extend the framework; (c) the terminal GeO-
(OH)3 group toward an adjacent Ge7 cluster, forming three hydrogen
bonds.
˚
˚
GeO(OH)3 unit. Interestingly, this GeO(OH)3 unit points
toward another adjacent cluster, forming three hydrogen
5.2 A ꢀ 5.4 A. The other two 13-ring channels are identical,
which are perpendicular to the 13-ring channel along the
[100] direction (Figure 2b). The 13-rings are formed by
the surrounding of eight GeO4, three GeO5, and two
GeO6. The Ge7 cluster works as a node for extension of
the structure and constructs the channels in combination
with the bridging GeO4 tetrahedra. The framework den-
ꢀ
˚
bonds with O-H-O distances of 2.65, 2.75, and 2.95 A,
as shown in Figure 1c. As a consequence, the distance
between the two clusters is nearly the same as the distance
between clusters connected through a GeO2(OH)2 unit.
From the view of the connections by covalent bonds, the
Ge7 cluster in PKU-10 can be regarded as a 6-connected
building unit, which combines with six neighboring Ge7
clusters through GeO(OH)2 groups. However, consider-
ing the Ge7 cluster in the whole structure, the Ge7 cluster
is surrounded by eight clusters, where the two additional
clusters are related by hydrogen bonds. The terminal O
atoms of these four additional tetrahedra, i.e., two on
the cross-linked tetrahedra [Ge(8)O4, Ge(10)O4, and
Ge(11)O4] and three on Ge(9)O4, are all hydroxyl groups,
thus giving rise to a framework formula of [Ge11O19(OH)9]-3
for PKU-10. We can define the connection linkage of the
Ge7 cluster in PKU-10 according to the nomination by
Yu et al.10 Each Ge7 cluster connects to seven GeO4 tetra-
hedra, but there are only six tetrahedra involved in the
extension of the 3D germanate framework. The linkage
mode of the Ge7 cluster in PKU-10 can be assigned as
T3P2O, as shown in Figure 1b, which is a new one with the
3
˚
sity of PKU-10 is very low, 11.0 Ge atoms per 1000 A .
Three TMAþ ions, which act as structure-directing agents
as well as counterions, are located within the 13-ring
channels (Figure 3). Two cations (N1 with C1-C4 and
N3 with C9-C12) arrange alternately in the middle of the
channels along the a axis, while the third one (N2 with
C5-C8) locates in the cross of the channels of the [011]
and [0-11] directions.
Though the structure of PKU-10 has been described
above by the polyhedral expression, it is still somehow
difficult to get a straightforward view of the structure
feature, owing to the complexity of the Ge7 cluster. The
connections of Ge7 clusters via the GeO4 tetrahedra in
PKU-10 can be expressed concisely with the aid of the
scale chemistry concept.5,6 We can define a simplified poly-
hedron (or pseudopolyhedron), which only takes the geom-
etrical configuration of the center atoms (Ge) into account.

9768 Inorganic Chemistry, Vol. 49, No. 21, 2010
Su et al.
Figure 5. TGA of PKU-10.
symmetrical stretch of the Ge-O bonds, and the absorp-
tions at 504, 488, and 452 cm-1 are related to Ge-O bend-
ing vibrations.9d
Thermal Stability. TGA for PKU-10 shows a weight
loss of 22.03% between 250 and 850 °C, as shown in
Figure 5, which agrees well with the expected value
(22.16%) of the removal of all TMAþ ions from the
structure. Interestingly, an unexpected peak occurs at
about 780 °C, indicating an intermediate phase change
during decomposition. To understand the reaction process,
the powder X-ray diffraction patterns were recorded after
heating treatment at different temperatures (Figure 6).
The results clearly show that the structure of PKU-10
collapses with degradation of the TMAþ ions at about
250 °C. An amorphous phase forms at 250-630 °C, and it
transforms to GeO2 after further treatment at 700, 780,
and 900 °C under atmospheric condition. Although we do
not know the exact composition of the amorphous phase,
the unexpected weight increase indicates the formation of
the low-valent germanium species. It was reported that GeO
might be formed in an amorphous state, which is volatile
but could be caught by charcoal.15 The removal of organic
species and dehydration of the hydroxyl groups from
PKU-10 are complex, giving out various species including
charcoal, CO, CO2, and H2O by combining oxygen from
the air flow. Apparently, GeIV can be reduced by C or CO
in situ. Thus, we can infer that GeO was formed in the
decomposition process of the TMAþ ions at ∼300-500 °C.
GeO is caught by charcoal from decomposition of the
TMAþ cations. With an increase of the temperature, GeO
can be reoxidized by oxygen to GeO2, which is accom-
panied by a weight increase. Meanwhile, the residual
charcoal also can be oxidized to gaseous CO and/or
CO2, which corresponds to a weight loss. In the tempera-
ture range of 630-780 °C, reoxidation of GeO to GeO2
dominates, while at the stage of 780-850 °C, the removal
of carbon is the main action. To further confirm the
reduction of GeIV in the process by C or CO, we treated
PKU-10 at 900 °C under a nitrogen atmosphere, which
led to the formation of elementary germanium as shown
in Figure 6b.
Figure 4. (a) Ge7 cluster representedasa simplified polyhedron in which
each vertex corresponds to a Ge atom. (b) Framework structure of PKU-
10 by simplified polyhedra, showing the 13MR channels. (c) Each Ge7
cluster surrounded by eight neighbors in a cubic environment. (d)
Interaction between GeO(OH)3 tetrahedra and Ge7 clusters by hydrogen
bonds as indicated by blue dashed lines.
The Ge7 cluster can be drawn out as a convex polyhedron
composed of a triangular prism and a tetragonal pyramid
by sharing faces, as shown in Figure 4a. The drawing
of the structure by the simplified polyhedra is shown in
Figure 4b. It clearly shows that the hexadentate connec-
tivity of the Ge7 cluster enables it to extend in 3D space in
a pcu topology,14 although the irregularness of the “brick”
results in a seriously twisted network. By a simplified
polyhedral presentation, the 13-ring channel along the
[100] direction can be recognized, and it is also easy to see
that each Ge7 cluster is surrounded by eight neighbors in
a nearly perfect cube, as shown in Figure 4c. The inter-
action between Ge7 clusters and GeO(OH)3 groups by
hydrogen bonds is drawn by blue dashed lines. Thus, the
Ge7 cluster can be regarded as an 8-connected species where
the GeO6 octahedron provides two branches (Figure 4c):
one is connected to a bridging GeO(OH)2 group, and the
other is linked to a GeO(OH)3 group by hydrogen bonds.
Figure 4d shows the germanate framework including
extension by the hydrogen bonds. The GeO(OH)3 groups
and its connections to adjacent Ge7 clusters block the
channels along the b direction.
IR Spectrum. Figure S1 in the Supporting Information
shows the IR spectrum of PKU-10. The peaks at 3400-
3200 and ∼1000 cm-1 correspond to the stretching and
bending vibrations of the -OH groups, respectively. The
absorption bands at 1488 and 1419 cm-1 correspond to the
C-H bending vibrations of the TMAþ ions. The absorption
bands at 847, 815, 787, 768, 718, and 703 cm-1 can be assigned
to the asymmetric stretching vibrations of the Ge-O bonds.
The peaks at 583 and 541 cm-1 are attributed to the
Ion-Exchange Property. The structure-directing-agent
TMAþ cations in the channels can be partially exchanged
(15) Bailar, J. C.; Emeleus, H. J.; Nyholm, Sir R.; Trotman-Dickenson,
A. F. Comprehensive inorganic chemistry, 1st ed.; Pergamon Press: Oxford,
1973; Vol. 2.
(14) Friedrichs, O. D.; O’Keeffe, M.; Yaghi, O. M. Acta Crystallogr. 2003,
A59, 22.

Article
Inorganic Chemistry, Vol. 49, No. 21, 2010 9769
Figure 6. Powder X-ray diffraction patterns of PKU-10 calcined under atmospheres except the one marked with “in N2”: (a) from the room temperature
(RT) to 400 °C; (b) from 630 to 900 °C. The peaks marked by asterisks are from the impurity Li3HGe7O16
.
by Liþ in a melt of LiAc 2H2O at 85 °C. Elemental
work as structure-directing agents and reside in the channels.
The Ge7 cluster adopts a new 6-connected linkage mode,
T3P2O, forming a pcu topological network. The simplified
polyhedral presentation, where the Ge7 cluster is drawn as a
convex polyhedron composed of a triangular prism and a
tetragonal pyramid by sharing faces, shows the porous struc-
ture concisely. Some of the TMAþ ions can be exchanged by
Liþ ions with retention of the germanate framework. The
germanate framework collapses with decomposition of the
TMAþ ions at temperatures higher than 240 °C. The removal
of organic species and dehydration of the hydroxyl groups
from PKU-10 is complex, and the weight increase at about
630-780 °C in the TGA curve is assigned as reoxidation of
GeO formed from the reduction of GeIV in situ by the reductive
species from TMAþ cations.
3
analysis and the ICP result revealed that nearly 30% of
the TMAþ ions were exchanged by the Liþ ions. The
reaction time needs to be controlled carefully. With the
prolonging of the exchange time to 4 h, the framework of
PKU-10 collapsed and transferred partly to pharmacosi-
derate Li3HGe7O16. The higher temperature also accel-
erated the formation of Li3HGe7O16 and caused the loss
of crystal water molecules from LiAc 2H2O, which then
3
affected the exchange reaction activity. The powder X-ray
diffraction profile of Li-PKU-10 is shown in Figure S2
in the Supporting Information. Profile fitting reveals
that the lattice constants of the exchanged product are
˚
˚
˚
a = 12.487(1) A, b = 17.309(2) A, and c = 18.529(1) A,
almost no change compared with those of the as-synthe-
sized PKU-10. This means that the partial exchange of
TMAþ ions by Liþ ions has little effect on the germanate
framework, while the remaining ∼70% of the TMAþ ions
still support the structure well.
Acknowledgment. This work was financially supported
by National Natural Science Foundation of China (NSFC
20571004 and 20221101) and the State Science and Tech-
nology Commission of China (Grant 2009CB939902).
Conclusion
PKU-10 is a porous germanate with a low framework
density, 11.0 Ge per 1000 A . The structure of PKU-10 is
Supporting Information Available: Crystallographic data in
CIF format, selected bond distances and angles, an IR spectrum,
and the peak profile fitting result of Li-PKU-10. This material is
available free of charge via the Internet at http://pubs.acs.org.
3
˚
built by the connections of Ge7 clusters with additional tetra-
hedra, possessing 3D intersecting 13-ring channels. TMAþ ions