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A highly crystalline layered silicate with
three-dimensionally microporous layers
HAE-KWON JEONG*1, SANKAR NAIR*1, THOMAS VOGT2, L. CHARLES DICKINSON3 AND
MICHAEL TSAPATSIS†1
1
Department of Chemical Engineering,159 Goessmann Laboratory,University of Massachusetts,Amherst,Massachusetts 01003-9303,USA
Physics Department,Brookhaven National Laboratory,Upton,New York 11973-5000,USA
3
Department of Polymer Science and Engineering,Silvio Conte National Center for Polymer Research,Amherst,Massachusetts 01003-4530,USA
*These authors contributed equally to this work
†
e-mail: [email protected]
2
Published online: 22 December 2002; doi:10.1038/nmat795
Layered silicates with three-dimensional microporosity
within the layers have the potential to enable new
applications in catalysis, adsorption and ion-exchange. Until
now no such materials have been reported. However, here
we present the synthesis and structure of AMH-3, a silicate
with three-dimensionally microporous layers, obtained in
high purity and crystallinity. AMH-3 is composed of silicate
layers containing eight-membered rings in all three principal
crystal directions, and spaced by strontium cations, sodium
cations and water molecules. Because of its threedimensional pore structure, acid and thermal stability, this
layered material could find applications in polymer–silicate
composites for membrane applications, for synthesis of
combined microporous–mesoporous materials, and for the
formation of new zeolites and microporous films. Its
existence also opens new possibilities for the synthesis of
other layered silicates with multidimensional microporous
framework layers.
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L
ayered silicates currently find uses in ion-exchange, adsorption,
catalysis and fabrication of nanocomposites1–3. Several layered
silicates have been identified as having a structure that is a
precursor to known zeolites (microporous (alumino)silicate
framework materials)4–7. Potential uses of these layered zeolite
precursors are emerging. For example, delamination of the layered
precursor to the zeolite MCM-22 can lead to a material having a high
surface area, consisting of thin, ordered silicate sheets with improved
access to catalytic sites8. Our interest in these materials stems from their
potential use in the fabrication of permselective membranes using
polymer-layered silicate processing techniques. These include
exfoliation,
delamination,
layer-by-layer
assembly
and
Langmuir–Blodgett deposition, and have been shown to lead to
silicate–polymer nanocomposites with a desirable combination of
mechanical, thermal and/or barrier properties9–13. However, the
straightforward extension of these materials to thin-film membrane
fabrication requires layered silicates with gas-selective pathways along
the silicate layer thickness. Here we report the synthesis and structure of
the first three-dimensionally microporous layered material (which we
call AMH-3),with 8-membered ring (8MR) limiting apertures (rings of
eight ≡Si-O-Si≡units) along the thickness of the silicate layer as well as in
the plane of the layers. Investigations in nanocomposite fabrication are
underway.AMH-3 exhibits good thermal and acid stability,and is also of
fundamental interest in understanding the relationship between twodimensionally (2D) layered materials and 3D framework structures.We
suggest two new zeolite topologies that are directly related to AMH-3.
Based on the structure of the layers, layered oxides or other
compounds can be classified into those with dense non-porous layers
and those that contain pores in the layers.Layered materials with porous
layers can in turn be classified into materials with porous sheet layers and
materials with microporous framework layers, that is, materials with a
porous network within the layer. Figure 1 provides a schematic
representation of this classification along with several examples.
Microporous layers are of particular interest because they can be
considered as framework materials like zeolites with one of their
dimensions in the nanometre scale.In this respect,they are of interest in
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ARTICLES
a
b
c
d
Figure 1 Porous layers in layered materials. Illustrative examples of layered materials are provided along with a representation of the porous structure of the layer.a,Materials with
dense layers such as clays1 and perovskites27.The representative structure of kaolinite is shown.b,Several aluminophosphates have single aluminophosphate sheets with pore openings
(for example,8MR28-30,12MR31,32).Other non-oxide layered materials have similar structures33,34.The structure of a layered aluminophosphate is shown.c,An aluminosilicate (MCM-22P)
has microporous layers with pores running within the layer,but the transport-limiting opening perpendicular to the layer is a 6MR.d,AMH-3 has 3D microporous layers with 8MR openings.
The structure models are created using the Cerius2 program (Accelrys).
catalysis, adsorption and ion exchange because they will allow fast
transport while preserving desirable properties of framework materials
such as well-defined catalytic sites and ion-exchange capability. It has
also been shown previously that non-microporous 2D-layered silicates
such as kanemite can be converted to mesoporous materials14,15 by
intercalation of surfactant molecules between the layers, followed by a
change in pH that induces the bending of the silicate sheets around the
surfactant phase to form a mesostructured material. Microporous
layered silicates open up the possibility of an elegant route towards
combined microporous–mesoporous materials. A 3D-microporous
layered structure is expected to possess unique properties, owing to the
existence of micropores both parallel and perpendicular to the layers.
The layered silicate AMH-3 described below possesses these structural
features, in addition to good thermal and acid stability, and we consider
it to be the first example of a class of materials that may enable novel
catalytic, adsorption and synthetic applications. Moreover, polymer
composites with materials like AMH-3 will combine the demonstrated
mechanical strength of layered silicate–polymer composites with
zeolite-like molecular sieving properties, and the ability to be processed
as a thin film enabling fabrication of mixed-matrix membranes.
The layered silicate AMH-3 was crystallized hydrothermally at
473 K from an alkaline, aqueous, sodium–strontium–titanosilicate
reactant mixture.The crystalline material was obtained in yields close to
25% and subsequently purified by decantation and centrifugation (see
Methods). Figure 2a shows an SEM image of the purified crystalline
product.The inset shows an individual prismatic crystal of the material.
Temperature-resolved in situ X-ray powder diffraction (XRD) patterns
collected from the material (see Supplementary Fig. S1a) indicate that
AMH-3 remains crystalline up to 723 K with little lattice contraction.
2
The structure appears to collapse at a temperature of 773K,although the
lowest angle reflection retains considerable X-ray intensity. The
structure of AMH-3 was solved (see Supplementary Information for
complete details of structure determination) from a powder XRD
pattern collected in-house. The structure of the silicate layers and the
positions of the inter-layer strontium cations were elucidated from the
preliminary structure determination, and it became evident that
AMH-3 was a 3D layered silicate with a novel topology. A highresolution synchrotron XRD pattern was then used for structure
refinement by the Rietveld technique. The sodium ions and water
molecules were located by a combination of Fourier difference electron
density maps, bond valence considerations, and Grand Canonical
Monte Carlo (GCMC) simulations. The final structure of AMH-3 is in
accordance with ICP-OES chemical composition analysis, chemical
bond valence and bond geometry considerations16–18, and
thermogravimetric water-loss measurements (see Supplementary
Information for details). The unit cell formula for AMH-3 is
Na8Sr8Si32O76.16H2O (monoclinic,space group C2/c,a=22.7830(60)Å,
b = 6.9395(18) Å, c = 13.5810(40) Å, β = 92.5935(13)°,
V = 2145.0(10) Å3). The occupancy of the metal cations is large enough
to provide a charge-balanced structure. This means that the
concentration of geminal silanol (Si-OH) groups is expected to be very
low, and that the terminal oxygen anions in the silicate layers are
coordinated with the strontium and sodium cations, or with protons of
the water molecules. Figure 2b shows the 29Si-MAS NMR spectrum of
AMH-3 (see discussion below),and Fig.2c shows the experimental and
calculated synchrotron XRD patterns.
Figure 3a–c shows views of the crystal structure down the [010],
[001] and [100] axes respectively. The presence of 8MRs in all three
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ARTICLES
a
–80
–85
–90
–93.5
–90.8
–89.4
b
–95
–100
p.p.m
c
1.5
Observed intensities
0.5
Counts ( × 10–3)
1.0
Rietveld structure fit
0
Possible C 2/c reflections
Difference curve
10
20
30
40
Degrees
Figure 2 Characterization of AMH-3. a,Purified sample of AMH-3 showing plate-like crystals (scale bar:10 µm).Inset,an individual crystal of AMH-3 (scale bar:5 µm).b, 29Si-MAS
NMR spectrum of AMH-3 showing three resonances.The chemical shifts are with respect to tetramethylsilane.c,Powder synchrotron X-ray pattern of AMH-3 (λ = 0.690911 Å).
directions is evident. The layers are stacked along [100], with chargebalancing cations and water molecules in the interlayer space. The
structure also contains 10MRs along the [011] direction. The
asymmetric unit of the silicate layers is a 4MR containing the silicon
atoms SI1, SI2, SI3 and SI4, and which is clearly visible in Fig. 3c. From
the structure of the layer, it is apparent that the atoms SI3 and SI4 are
topologically identical and have the same coordination environment.
These could possibly be related by a symmetry operation if the crystal
had orthorhombic symmetry. However, the monoclinic angle deviates
significantly from 90°,and the diffraction patterns could not be indexed
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adequately with an orthorhombic unit cell. AMH-3 also contains
composite Na-O/Sr-O octahedral sheets in between the silicate layers.
These are formed by the coordination of strontium (SR1) and sodium
(NA1) cations to water molecules and oxygens in the silicate layers, the
SR1:NA1 molar ratio in the octahedral sheet being 2:1. The sodium
cation NA1 is octahedrally coordinated by O6, O10 and Ow1 (a water
molecule), whereas SR1 is in a distorted octahedral coordination with
the O4, O5, O6 and O10 atoms. The other sodium (NA2) cations are
occluded in the pore space of the silicate layers, and are sevencoordinated by O1, O3, O8 and Ow2 (water molecule). The strong
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ARTICLES
a
c
b
NA2
Ow2
SI1
[001]
SI2
SI3
SI4
SR1
[010]
NA1
[100]
[100]
Ow1
[001]
[010]
Figure 3 Ortep35 views of the AMH-3 structure along three crystallographic directions. Colour coding:red = Si,blue = O,green = Na,gold = Sr.The size of the spheroids is
proportional to their isotropic Debye–Waller factors.AMH-3 consists of layers with 8MRs in all three crystallographic directions,spaced by Sr cations,Na cations and water molecules.
[100] projection of sheets
from a single layer of AMH-3
Rotate about
[001]
[010]
[010]
[100] projection of sheets
from two adjacent layers
(identical to lovdarite)
[001]
[001]
Tilt about
[010]
[100]
Tilt about
[010]
[001]
[010]
[010]
AMH-3 structure
[101]
[101]
[10-1] projection of AMH-3 layer
showing inter-layer microporosity
[10-1] projection of adjacent layers
showing inter-layer microporosity
Figure 4 Projections of the AMH-3 structure omitting cations and water molecules.Top left:Projection of a single AMH-3 layer down [100].Bottom left:Projection of the same
layer along [10-1] showing 8MRs in the layer.Top right:Projection down [100] of two sheets from adjacent layers.Bottom right:Projection of the same down [10-1] showing an interlayer
transport path through 8MRs. Red = Si,blue = O.
4
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Translation along
[011]
Layer 1
Layer 2
+ Layer 1
New zeolite type 1
180 ° rotation about [100] +
translation along [011]
Layer 3
+ Layer 1
New zeolite type 2
Figure 5 Construction of two new zeolite frameworks from AMH-3 layers. Left:
Translation of Layer 1 along [011] leads to Layer 2.Bonding of layers 1 and 2 leads to a new
zeolite (Type 1).Right:Translation and rotation of Layer 1 leads to Layer 3.Bonding of layers
1 and 3 leads to another framework (Type 2).Red = Si,blue = O.
be visualized down approximately the [10-1] direction (bottom left).
The [100] projection of two sheets from adjacent layers (top right) is
very similar to that of the beryllosilicate zeolite lovdarite20. The [10-1]
projection (bottom right) shows alignment of the 8MRs providing a
possible transport path from one layer to the next.
In Fig. 5, we show the close relationship of AMH-3 with two new
zeolite topologies, resulting from translation or rotation of individual
layers followed by bonding of the layers with each other. No additional
silicate species need be added to perform these conversions. For
example,translation of one layer approximately along [011] (Fig.5,left)
brings the Si-O– groups of adjacent layers into close proximity,allowing
a possible condensation into a new ‘small-pore’zeolite type, containing
8MRs. Translation of the layer along [011] followed by a 180° rotation
about the [100] axis (Fig. 5, right) leads to a second small-pore zeolite
type. Because these two framework types are formed by bonding of
individual layers, a number of disordered intergrowths between these
two hypothetical materials are also possible.
Because the layers of AMH-3 have a unique 3D microporosity, the
material can find applications in high-performance composite
polymer–silicate membrane materials for gas separations.The material
is thermally stable up to 773 K,and also remained stable after 24 hours in
a nitric acid solution at pH 2.7. Hence, it can be subjected to posttreatment procedures such as those leading to ion-exchange and
intercalation of organic species. Apart from its potential applications,
AMH-3 is of fundamental interest in exploring the connection between
layered silicate materials and framework silicates such as zeolites.
METHODS
SYNTHESIS AND CHARACTERIZATION
electrostatic effect of the divalent strontium cations could account for
the high crystallinity and thermal stability of the material.
The 29Si-MAS NMR spectrum (Fig. 2b) shows three resonances at
–93.5, –90.8 and –89.4 p.p.m. (with respect to a tetramethylsilane
reference sample).The ratio of the integrated intensities of these peaks is
1.00:1.28:2.11, obtained by fitting of the spectrum with three gaussian
peaks. The crystallographic structure reveals that SI2 is connected to
four silicon atoms (and is designated a Q4 silicon atom),whereas SI1,SI3
and SI4 are connected to three silicon atoms with the fourth Si-O bond
directed into the interlayer space (and are designated Q3 silicon atoms).
We may associate the highest chemical shift (at –93.5 p.p.m.) with the
SI2 Q4 species,the next highest (at –90.8 p.p.m.) with the SI1 Q3 species,
and the intense resonance at –89.4p.p.m.with the SI3 and SI4 Q3 species,
both of which have a similar coordination environment. This
assignment should lead to an intensity ratio of approximately 1:1:2 for
the three resonances,which is close to that obtained experimentally.The
assignment of the two low-field resonances to Q3 species is also
consistent with the approximate chemical shift (–88.5 p.p.m.) predicted
by the empirical correlation of Proshko19. This correlation is based on
data from layered silicates and aluminosilicates (having exclusively Q3
silicon coordination), and relates the 29Si chemical shifts in these
materials to the type and number of cations in the octahedrally
coordinated cation layers.The assigned chemical shift of the Q4 silicon is
lower by several p.p.m. than those found in zeolite materials, consistent
with the low Si–O–Si angles involving the SI2 site (these range between
134.4° to 140.7°, see Supplementary Table S2). 29Si-MAS NMR spectra
collected with cross-polarization were not significantly different from
the spectrum shown in Fig. 2b. This is in agreement with the chemical
formula of AMH-3, which indicates absence of a significant
concentration of silanol groups in the crystal.
In Fig.4,we show the layers of the AMH-3 structure,with the cations
and water molecules removed for clarity. Each layer is formed by
bonding together two silicate sheets containing 4MRs and 8MRs (Fig.4,
top left). The transport path of a species diffusing through the layer can
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The synthesis solution had a molar composition of 1 TiO2: 10 SiO2: 14 NaOH: x SrCl2: 675 H2O, where
2<x<14. In a typical experiment, NaOH was dissolved in deionized water, and SrCl2.6H2O was added.
The mixture was stirred for 1 hour in a silicone oil bath at 353 K. Sodium silicate solution (27% SiO2,
14% NaOH, 59% H2O, Aldrich) was then added to the above solution and stirred for 30 minutes. Finally,
titanium(III) trichloride (20% TiCl3, 20% HCl, 60% H2O, Aldrich) was added very slowly under vigorous
stirring. The mixture was then homogenized by stirring for 30 minutes. The resulting solution was then
introduced into a Teflon-lined stainless steel autoclave (Parr) and crystallized at 473 K with varying
crystallization times. The product was washed with deionized water to neutral pH, and dried at 363 K
overnight. A powder XRD pattern of the as-synthesized solid product was obtained (Supplementary
Information, Fig. S1b). The crystals were separated from amorphous material by repeated precipitation
from suspension. Figure 2 shows the final pure crystalline product. The synthesis has been carried out
reproducibly with the method described above; however, up to now we are unable to make the material in
the absence of titanium. Temperature-resolved powder XRD patterns were collected on a well-ground
sample, using a Philips X’Pert diffractometer equipped with a Paar high-temperature attachment. The
chamber was swept with a 50 cm3 min–1 helium flow to maintain an inert atmosphere. The sample was
equilibrated for 1 hour at each temperature before data collection. Solid-state 29Si NMR spectra were
obtained on a Bruker DSX300 with a magic-angle spinning (MAS) probe at room temperature, using a
Bloch decay pulse program with a 3 µs 45° pulse and 1H decoupling at 50 kHz for an acquisition time of
23 ms. The recycle delay was 15 s, with approximately 500 scans required for a full 7 mm rotor spinning at
3 kHz. The chemical shift scale was externally set to zero for the 29Si signal of tetramethylsilane.
STRUCTURE DETERMINATION
For initial structure determination, powder X-ray data was collected from a well-crushed sample at room
temperature. A well-aligned Philips X’Pert diffractometer operating in a Bragg–Brentano geometry was
used, with λ (Cu Kα1) = 1.5406 Å and a Kα2:Kα1 intensity ratio of 0.523. Data were collected from
5–100° 2θ with an angular step size of 0.02° 2θ, and a data collection time of 125 s per step. Divergence
and receiving slits of 1/32° were used. Powder synchrotron X-ray data for structure refinement were
obtained at room temperature in a Debye–Scherrer geometry on beamline X7A of the National
Synchrotron Light Source (Brookhaven National Laboratory), with λ = 0.690911 Å, an angular range of
3–42° 2θ, and an angular step size of 0.01° 2θ. ICP-OES chemical analysis of the material was carried out
by Galbraith Laboratories (Knoxville, TN). Structure solution and structure refinement were carried out
with the aid of the EXPO21,22 and GSAS23 packages, respectively (see Supplementary Information for
details of structure solution and structure refinement). The background was fitted using a shifted
Chebyshev24 polynomial with 22 coefficients. A pseudo-Voigt22 function with an asymmetry correction
(7 parameters in all) was used to model the profile shape. The final number of structural parameters
(fractional atomic coordinates, lattice parameters, and isotropic atomic displacement factors) was 74.
Including the profile-scaling factor, the total number of refinable parameters was thus 104, with 3,900
observations. Also, the number of observed unique hkl reflections was 1,251, so that the fit of 74 structural parameters is sufficiently overdetermined. The adsorption simulations were carried out with the
Cerius2 program (Accelrys). The Universal Force Field25 was used for representing the van der Waals interactions between the water molecules and the crystal. The electrostatic charges on all atoms were calculated by the charge equilibration26 technique.
Received 7 July 2002; accepted 22 November 2002; published 22 December 2002.
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Acknowledgments
We acknowledge support from NASA-Microgravity (98-HEDS-05-218), NSF (CTS 0091406) and
Engelhard Co.
Correspondence and requests for materials should be addressed to M.T.
Supplementary Information is available on the website for Nature Materials
(http://www.nature.com/naturematerials)
Competing financial interests
The authors declare that they have no competing financial interests.
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