a highly crystalline layered silicate with three-dimensionally

© 2002 Nature Publishing Group

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Layered silicates currently find uses in ion-exchange, adsorption,catalysis and fabrication of nanocomposites1–3. Several layeredsilicates have been identified as having a structure that is a

precursor to known zeolites (microporous (alumino)silicateframework materials)4–7. Potential uses of these layered zeoliteprecursors are emerging. For example, delamination of the layeredprecursor to the zeolite MCM-22 can lead to a material having a highsurface area, consisting of thin, ordered silicate sheets with improvedaccess to catalytic sites8.Our interest in these materials stems from theirpotential use in the fabrication of permselective membranes usingpolymer-layered silicate processing techniques. These includeexfoliation, delamination, layer-by-layer assembly andLangmuir–Blodgett deposition, and have been shown to lead tosilicate–polymer nanocomposites with a desirable combination ofmechanical, thermal and/or barrier properties9–13. However, thestraightforward extension of these materials to thin-film membranefabrication requires layered silicates with gas-selective pathways alongthe silicate layer thickness.Here we report the synthesis and structure ofthe first three-dimensionally microporous layered material (which wecall AMH-3),with 8-membered ring (8MR) limiting apertures (rings ofeight ≡Si-O-Si≡units) along the thickness of the silicate layer as well as inthe plane of the layers. Investigations in nanocomposite fabrication areunderway.AMH-3 exhibits good thermal and acid stability,and is also offundamental interest in understanding the relationship between two-dimensionally (2D) layered materials and 3D framework structures.Wesuggest two new zeolite topologies that are directly related to AMH-3.

Based on the structure of the layers, layered oxides or othercompounds can be classified into those with dense non-porous layersand those that contain pores in the layers.Layered materials with porouslayers can in turn be classified into materials with porous sheet layers andmaterials with microporous framework layers, that is, materials with aporous network within the layer. Figure 1 provides a schematicrepresentation of this classification along with several examples.Microporous layers are of particular interest because they can beconsidered as framework materials like zeolites with one of theirdimensions in the nanometre scale.In this respect,they are of interest in

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 three-

dimensional 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.

nature materials | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials 1

A highly crystalline layered silicate with three-dimensionally microporous layersHAE-KWON JEONG*1, SANKAR NAIR*1, THOMAS VOGT2, L. CHARLES DICKINSON3 AND MICHAEL TSAPATSIS†1

1Department of Chemical Engineering,159 Goessmann Laboratory,University of Massachusetts,Amherst,Massachusetts 01003-9303,USA2Physics Department,Brookhaven National Laboratory,Upton,New York 11973-5000,USA3Department 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]

Published online:22 December 2002; doi:10.1038/nmat795


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2 nature materials | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials

catalysis, adsorption and ion exchange because they will allow fasttransport while preserving desirable properties of framework materialssuch as well-defined catalytic sites and ion-exchange capability. It hasalso been shown previously that non-microporous 2D-layered silicatessuch as kanemite can be converted to mesoporous materials14,15 byintercalation of surfactant molecules between the layers, followed by achange in pH that induces the bending of the silicate sheets around thesurfactant phase to form a mesostructured material. Microporouslayered silicates open up the possibility of an elegant route towardscombined microporous–mesoporous materials. A 3D-microporouslayered structure is expected to possess unique properties, owing to theexistence of micropores both parallel and perpendicular to the layers.The layered silicate AMH-3 described below possesses these structuralfeatures, in addition to good thermal and acid stability,and we considerit to be the first example of a class of materials that may enable novelcatalytic, adsorption and synthetic applications. Moreover, polymercomposites with materials like AMH-3 will combine the demonstratedmechanical strength of layered silicate–polymer composites withzeolite-like molecular sieving properties,and the ability to be processedas a thin film enabling fabrication of mixed-matrix membranes.

The layered silicate AMH-3 was crystallized hydrothermally at473 K from an alkaline, aqueous, sodium–strontium–titanosilicatereactant mixture.The crystalline material was obtained in yields close to25% and subsequently purified by decantation and centrifugation (seeMethods). Figure 2a shows an SEM image of the purified crystallineproduct.The inset shows an individual prismatic crystal of the material.Temperature-resolved in situ X-ray powder diffraction (XRD) patternscollected from the material (see Supplementary Fig. S1a) indicate thatAMH-3 remains crystalline up to 723 K with little lattice contraction.

The structure appears to collapse at a temperature of 773K,although thelowest angle reflection retains considerable X-ray intensity. Thestructure of AMH-3 was solved (see Supplementary Information forcomplete details of structure determination) from a powder XRDpattern collected in-house. The structure of the silicate layers and thepositions of the inter-layer strontium cations were elucidated from thepreliminary structure determination, and it became evident that AMH-3 was a 3D layered silicate with a novel topology. A high-resolution synchrotron XRD pattern was then used for structurerefinement by the Rietveld technique. The sodium ions and watermolecules were located by a combination of Fourier difference electrondensity maps, bond valence considerations, and Grand CanonicalMonte Carlo (GCMC) simulations. The final structure of AMH-3 is inaccordance with ICP-OES chemical composition analysis, chemicalbond valence and bond geometry considerations16–18, andthermogravimetric water-loss measurements (see SupplementaryInformation for details). The unit cell formula for AMH-3 isNa8Sr8Si32O76.16H2O (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 enoughto provide a charge-balanced structure. This means that theconcentration of geminal silanol (Si-OH) groups is expected to be verylow, and that the terminal oxygen anions in the silicate layers arecoordinated with the strontium and sodium cations,or with protons ofthe water molecules. Figure 2b shows the 29Si-MAS NMR spectrum ofAMH-3 (see discussion below),and Fig.2c shows the experimental andcalculated 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

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 withdense 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).

a

c

b

d


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directions is evident. The layers are stacked along [100], with charge-balancing cations and water molecules in the interlayer space. Thestructure also contains 10MRs along the [011] direction. Theasymmetric unit of the silicate layers is a 4MR containing the siliconatoms SI1, SI2, SI3 and SI4, and which is clearly visible in Fig. 3c. Fromthe structure of the layer, it is apparent that the atoms SI3 and SI4 aretopologically identical and have the same coordination environment.These could possibly be related by a symmetry operation if the crystalhad orthorhombic symmetry. However, the monoclinic angle deviatessignificantly from 90°,and the diffraction patterns could not be indexed

adequately with an orthorhombic unit cell. AMH-3 also containscomposite 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, theSR1:NA1 molar ratio in the octahedral sheet being 2:1. The sodiumcation NA1 is octahedrally coordinated by O6, O10 and Ow1 (a watermolecule), whereas SR1 is in a distorted octahedral coordination withthe O4, O5, O6 and O10 atoms. The other sodium (NA2) cations areoccluded in the pore space of the silicate layers, and are seven-coordinated by O1, O3, O8 and Ow2 (water molecule). The strong

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-MASNMR 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).

1.5

1.0

0.5

0

Coun

ts (

× 10

–3)

–80 –85 –90 –95 –100

a b

c

10 20 30 40

p.p.m

–89.

4–9

0.8

–93.

5

Degrees

Possible C2/c reflections

Difference curve

Observed intensities

Rietveld structure fit


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[001]

[100

]

[010]

[100

]

[010]

[001

]

NA2

NA1

SI3SI1

SI4

SI2

Ow2

Ow1

SR1

a b c

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 isproportional 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 sheetsfrom a single layer of AMH-3

[10-1] projection of AMH-3 layer showing inter-layer microporosity

[100] projection of sheetsfrom two adjacent layers

(identical to lovdarite)

[010

]

[001]

[010

]

[001]

[100

]

[001]

[010

]

[101]

[10-1] projection of adjacent layers showing inter-layer microporosity

[010

]

[101]

Tilt about[010]

Tilt about[010]

Rotate about[001]

AMH-3 structure

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 interlayertransport path through 8MRs. Red = Si,blue = O.


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electrostatic effect of the divalent strontium cations could account forthe 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 tetramethylsilanereference sample).The ratio of the integrated intensities of these peaks is1.00:1.28:2.11, obtained by fitting of the spectrum with three gaussianpeaks. The crystallographic structure reveals that SI2 is connected tofour silicon atoms (and is designated a Q4 silicon atom),whereas SI1,SI3and SI4 are connected to three silicon atoms with the fourth Si-O bonddirected into the interlayer space (and are designated Q3 silicon atoms).We may associate the highest chemical shift (at –93.5 p.p.m.) with theSI2 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. Thisassignment should lead to an intensity ratio of approximately 1:1:2 forthe three resonances,which is close to that obtained experimentally.Theassignment of the two low-field resonances to Q3 species is alsoconsistent with the approximate chemical shift (–88.5p.p.m.) predictedby the empirical correlation of Proshko19. This correlation is based ondata from layered silicates and aluminosilicates (having exclusively Q3

silicon coordination), and relates the 29Si chemical shifts in thesematerials to the type and number of cations in the octahedrallycoordinated cation layers.The assigned chemical shift of the Q4 silicon islower by several p.p.m. than those found in zeolite materials, consistentwith the low Si–O–Si angles involving the SI2 site (these range between134.4° to 140.7°, see Supplementary Table S2). 29Si-MAS NMR spectracollected with cross-polarization were not significantly different fromthe spectrum shown in Fig. 2b. This is in agreement with the chemicalformula of AMH-3, which indicates absence of a significantconcentration of silanol groups in the crystal.

In Fig.4,we show the layers of the AMH-3 structure,with the cationsand water molecules removed for clarity. Each layer is formed bybonding together two silicate sheets containing 4MRs and 8MRs (Fig.4,top left). The transport path of a species diffusing through the layer can

be visualized down approximately the [10-1] direction (bottom left).The [100] projection of two sheets from adjacent layers (top right) isvery similar to that of the beryllosilicate zeolite lovdarite20. The [10-1]projection (bottom right) shows alignment of the 8MRs providing apossible transport path from one layer to the next.

In Fig. 5, we show the close relationship of AMH-3 with two newzeolite topologies, resulting from translation or rotation of individuallayers followed by bonding of the layers with each other. No additionalsilicate species need be added to perform these conversions. Forexample,translation of one layer approximately along [011] (Fig.5,left)brings the Si-O– groups of adjacent layers into close proximity,allowinga possible condensation into a new ‘small-pore’zeolite type, containing8MRs. Translation of the layer along [011] followed by a 180° rotationabout the [100] axis (Fig. 5, right) leads to a second small-pore zeolitetype. Because these two framework types are formed by bonding ofindividual layers, a number of disordered intergrowths between thesetwo hypothetical materials are also possible.

Because the layers of AMH-3 have a unique 3D microporosity, thematerial can find applications in high-performance compositepolymer–silicate membrane materials for gas separations.The materialis thermally stable up to 773K,and also remained stable after 24 hours ina nitric acid solution at pH 2.7. Hence, it can be subjected to post-treatment procedures such as those leading to ion-exchange andintercalation of organic species. Apart from its potential applications,AMH-3 is of fundamental interest in exploring the connection betweenlayered silicate materials and framework silicates such as zeolites.

METHODS

SYNTHESIS AND CHARACTERIZATIONThe 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 DETERMINATIONFor 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 structur-

al 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 inter-

actions between the water molecules and the crystal. The electrostatic charges on all atoms were calculat-

ed by the charge equilibration26 technique.

Received 7 July 2002; accepted 22 November 2002; published 22 December 2002.

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 newzeolite (Type 1).Right:Translation and rotation of Layer 1 leads to Layer 3.Bonding of layers1 and 3 leads to another framework (Type 2).Red = Si,blue = O.

Layer 1

Layer 2

+ Layer 1 + Layer 1

Layer 3

Translation along[011]

180° rotation about [100] +translation along [011]

New zeolite type 1 New zeolite type 2


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AcknowledgmentsWe 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 interestsThe authors declare that they have no competing financial interests.

a highly crystalline layered silicate with three-dimensionally

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