ab initio structure determination of novel small-pore metal-silicates: knots-and-crosses structures

Inorganica Chimica Acta 356 (2003) 19�/26

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Ab initio structure determination of novel small-pore metal-silicates:knots-and-crosses structures

Artur Ferreira a, Zhi Lin b, Maria R. Soares c, Joao Rocha b,*a ESTGA, University of Aveiro, CICECO, Apartado 473, 3754-909 Aguedo, Portugal

b Department of Chemistry, University of Aveiro, CICECO, 3810-193 Aveiro, Portugalc Laboratorio Central de Analises, University of Aveiro, 3810-193 Aveiro, Portugal

Received 14 October 2002; accepted 9 January 2003

Dedicated to Prof. J.J.R. Frausto da Silva on the occasion of his 70th birthday.

Abstract

Sodium chloride stannosilicate AV-13 (Na2.26SnSi3O9Cl0.26 �/xH2O) and zirconium and hafnium analogues of this material have

been prepared and their structures solved from powder X-ray diffraction data using direct methods, and 23Na, 29Si and 119Sn solid-

state NMR. AV-13 materials are small-pore solids, probably more adequately described as tunnel structures. The AV-13 framework

consists of corner-sharing MO6 (M�/Sn, Zr, Hf) octahedra and SiO4 tetrahedra. The latter form six-membered [Si6O18]12� rings,

which are interconnected by MO6 octahedra. The structure is better understood by considering a three-dimensional knots-and-

crosses lattice. In a given layer, successive distorted-cube M8 cages contain [Na6�x (H2O)x ](H2O,Cl�) octahedra (knots) and

cyclohexasilicate (crosses) units. While the former are extra-framework species, the six-membered rings are, of course, part of the

framework. The cages are accessed via seven-membered [M3Si4O27]26� windows, with free aperture ca. 2.3�/3.2 A, one per each

pseudo-cube face. Pilling up layers generates the structure, with knots-and-crosses alternating. The non-framework five-coordinated

Na cations are disordered.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Silicon; Solid-state structures; Tin; Zirconium; Hafnium; Zeolites

1. Introduction

In conventional zeolites and related microporous

materials such as aluminophosphates, the framework

(Si, P, Al) atoms are in general tetrahedrally coordinated

[1], albeit under certain hydration conditions the alumi-

nium atoms may be penta- or hexa-coordinated [2]. A

small number of rare microporous oxide minerals

exhibiting mixed tetrahedral�/octahedral/pentahedral

frameworks are known, most notably titanium- and

zirconium-silicates [3]. Inspired in no small measure by

the striking structural beauty and diversity of these

natural specimens, in the last decade much research has

been devoted to the laboratory synthesis and character-

* Corresponding author. Tel.: �/351-234-37-0084; fax: �/351-234-

37-0730.

E-mail address: [email protected] (J. Rocha).

0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights re

doi:10.1016/S0020-1693(03)00332-3

isation of novel open-framework solids with structures

formed by linking tetrahedra with (Ti, Zr, Sn, Nb, V)

metal-ion-centred polyhedra such as octahedra and

square pyramids [3�/5].

Dyer and Jafar were the first to report the synthesis

and characterisation of a microporous stannosilicate [6].

Seminal work by Corcoran et al. at Exxon Research in

the late 1980s, produced six stannosilicates, some of

which displaying microporosity [7,8]. They have been

shown to be useful sorbents, for example for the

separation of hydrogen sulphides from gas streams

containing hydrogen contaminated with hydrogen sul-

phides or oxysulphides [8]. Later, Dyer et al. also

reported a microporous sodium stannosilicate and

studied its ion-exchange properties for the replacement

of Na� by a range of monovalent and divalent ions

[9,10]. However, the structures of all these materials

have not been determined. Recently, we reported the

synthesis and structures of microporous stannosilicates

served.

mailto:[email protected]

A. Ferreira et al. / Inorganica Chimica Acta 356 (2003) 19�/2620

AV-6 and AV-7, analogues of, respectively, zirconosili-

cate minerals umbite [11] and kostylevite [12]. The

structure of a third material, AV-10, was determined

by ab initio methods from powder X-ray diffraction(XRD) data [13]. The powder XRD patterns of AV-6

and AV-10 are similar to the patterns of, respectively,

phase G and A reported by Corcoran et al. [8]. In the

course of our comprehensive synthesis work on micro-

porous tin silicates we have now obtained a solid (AV-

13, Aveiro material no. 13), which exhibits a powder

XRD pattern similar to the Corcoran’s phase B pattern

[8]. Zirconium and hafnium analogues have also beenprepared and are reported here.

Scanning electron microscope (SEM) images show

that microcrystalline Sn-AV-13 consists of spherical

particles with an average size of ca. 12 mm, while Zr-

AV-13 and Hf-AV-13 samples consist of cubic crystal-

lites with ca. 5�/8 and 1 mm, respectively (Fig. 1). Hence,

the structure was solved from powder XRD data in

space group Pa/3 (No. 205) (Fig. 2), using directmethods, and solid-state NMR.

Fig. 1. SEM images of Sn-, Zr- and Hf-AV-13 materials.

2. Experimental

Sn-AV-13 was synthesised as follows. An alkaline

solution was made by dissolving 20.0 g of sodium

metasilicate (Na2SiO3 �/5H2O, BDH) into 22.27 g H2O.

A solution of 11.02 g SnCl4 �/5H2O (98 m/m%, Riedel-deHaen) in 11.73 g H2O was added to the alkaline

solution while stirring thoroughly. The formed gel, with

a molar composition 3.0Na2O:3.0SiO2:1.0SnO2:80H2O,

was transferred to a Teflon-lined autoclave and treated

at 230 8C for 10 days under autogenous pressure,

without agitation. Zr- and Hf-AV-13 analogues were

obtained by using 7.33 g ZrCl4 and 10.08 g HfCl4, in

place of the Sn source, and allowing 14 days at 230 8C.The product was filtered off, washed at room tempera-

ture with distilled water, and dried at 70 8C overnight.

Within experimental error, bulk chemical analysis gives

the formulae Na2.26SnSi3O9Cl0.26, Na2.27ZrSi3O9Cl0.27

and Na2.29HfSi3O9Cl0.29.

SEM images were recorded on a Hitachi S-4100

microscope. Powder XRD, example for Hf-AV-13.

Data were collected on a X’Pert MPD Philips difract-ometer (Cu Ka X-radiation) with a curved graphite

monochromator, a fix divergence slit of 0.58, and a flat

plate sample holder, in a Bragg�/Brentano para-focusing

optics configuration. Intensity data were collected by the

step counting method (step 0.038 and time 10 s) in the

range 2u 11�/1408. The powder XRD pattern of AV-13

was auto-indexed with the POWDERX programme pack-

age [14] using the first 31 well-resolved lines. A cubicunit cell with a�/12.6845 A was indicated by the

TREOR90 indexing programme [15] with high figures of

merit (M31�/364 and F31�/649). The space group Pa/3

(No. 205) was unambiguously determined from the

systematic absences. The ab initio crystal structure

determination from powder XRD data was carried out

with the package EXPO [16]. Firstly, the structure factor

amplitudes were extracted by the Le Bail method from

the powder pattern [17]. The structure factors of 557

reflections were obtained. The structures were then

solved by direct methods. Although all atoms were

located simultaneously, re-labelling of atoms was neces-

Fig. 2. Experimental, calculated and difference powder XRD pattern of Hf-AV-13.

Table 1

Conditions of X-ray data collection and refinement for Sn-, Hf-, Zr-AV-13

Data collection

Diffractometer, geometry Philips MPD, Bragg-Bren-

tano

Radiation Cu Ka2u Range (8) 11.00�/140.00

Step scan 0.038(2u )

Time per step (s) 10

Na2.26SnSi3O9Cl0.26 �/2.5H2O

Na2.29HfSi3O9Cl0.29 �/2.5H2O

Na2.27ZrSi3O9Cl0.27 �/2.5H2O

Results of Rietveld refinement in Pa/3 (n 205) space group by the

FULLPROF program

Cell parameters

a (A) 12.4600(5) 12.6800(1) 12.7140(1)

Volume (A3) 1934.5(1) 2038.71(3) 2055.15(4)

Formula units/cell (Z ) 8 8 8

Formula mass (g) 437.94 497.72 410.45

Calculated density (g cm�3) 3.01 3.24 2.65

Independent reflections/parameters 650/40 573/40 671/40

Zero point 0.013(2) �/0.0552(6) �/0.0297(7)

Peak shape function: Pseudo-Voigt ([PV�/hL�/(1-h)G])

h 0.841(7) 0.599(8) 0.568(9)

U 0.18(2) 0.025(1) 0.021(1)

Caglioti law parameters

V 0.01(1) �/0.018(1) �/0.014(1)

W 0.072(2) 0.0137(3) 0.0135(3)

Asymmetry parameters (up to 358 2u)

0.052(3) 0.071(2) 0.073(3)

0.0131(5) 0.0520(8) 0.0547(9)

Reliability factors (conventional: background excluded)

For points with Bragg contribution

cRP 7.94 11.2 13.4

cRwp 10.8 15.5 18.6

cRexp 5.27 7.24 8.06

Chi2 4.19 4.57 5.35

Structure reliability factors

RB 4.73 4.30 6.04

RF 3.33 3.31 4.52

A. Ferreira et al. / Inorganica Chimica Acta 356 (2003) 19�/26 21

Fig. 3. Polyhedra representation of the AV-13 structure viewed down

the a axis. MO6 (M�/Sn, Zr, Hf) octahedra and SiO4 tetrahedra are

depicted in green and yellow, respectively. For clarity, Na�, Cl� and

H2O molecules are omitted.

Fig. 5. Polyhedra representation of the AV-13 M8 cages (M�/Sn, Zr,

Hf, green). These cages are accessed via the seven-membered

[M3Si4O27]26� windows shown.


sary, coupled with changes in bond distances and bond

angles. This procedure was alternated with least-squares

refinements. The coordinates of atoms obtained from

direct methods were used in the Rietveld refinement of

the structure by the FULLPROF program [18]. The final

profile analysis refinement was carried out in the range

11�/126.38 2u for the occurring 573 independent reflec-

tions and involved the following parameters: structural,

17 atomic coordinates; 8 isotropic temperature factors;

Fig. 4. (a) Schematic representation of the knots-and-crosses structure of AV

(b) [Na6�x (H2O)x ](H2O,Cl�) octahedra (knots, red) and cyclohexasilicate (

profile, one scale factor, three halfwidth and the h

parameters (a Pseudo-Voigt peak shape function was

used), one cell parameter, two peak asymmetry para-

meters; global, one zero point, six coefficients of

polynomial background. Soft constraints to some of

the bond distances were applied. Table 1 gives the final

crystallographic data for AV-13 materials. Bond dis-

tances and selected bond angles are collected in Tables 2

and 3.23Na, 29Si and 119Sn NMR spectra were recorded at

105.85, 79.49 and 149.09 MHz, respectively, on an

Avance (9.4 T, wide-bore) Bruker spectrometer. 29Si

MAS NMR spectra were recorded with 408 pulses, a

-13 showing two layers of M8 cages (M�/Sn, Zr, Hf, green) containing

crosses, yellow) units.

Table 2

List of selected bond distances for Sn-, Hf-, Zr-AV-13

Na2.26SnSi3O9Cl0.26 �/2.5H2O Na2.29HfSi3O9Cl0.29 �/2.5H2O Na2.27ZrSi3O9Cl0.27 �/2.5H2O

Bond distance (A)

M�/O2 2.027(14) 2.0670(12) 2.0726(9)

M�/O2#1 2.0247(14) 2.0670(12) 2.0726(9)

M�/O2#2 2.0247(14) 2.0670(12) 2.0726(9)

M�/O1#1 2.0507(13) 2.079(3) 2.1127(14)

M�/O1#2 2.0507(13) 2.079(3) 2.1127(14)

M�/O1 2.0507(13) 2.079(3) 2.1127(14)

Si�/O3 1.5523(14) 1.5920(14) 1.5858(9)

Si�/O2#3 1.5527(12) 1.5951(12) 1.5912(10)

Si�/O1 1.6197(13) 1.601(3) 1.5960(14)

Si�/O3#3 1.6260(12) 1.6152(12) 1.6169(8)

Na�/Ow1 2.313(4) 2.471(5) 2.466(4)

Na�/O2#4 2.538(5) 2.559(7) 2.585(4)

Na�/O1 2.596(5) 2.667(5) 2.697(5)

Na�/O3#5 2.602(5) 2.747(6) 2.710(4)

Na�/Ow2#6 2.870(4) 2.805(5) 2.830(4)

Symmetry transformations used to generate equivalents atoms: #1 z ,x ,y ; #2 y ,z ,x ; #3 �/y�/1/2,z�/1/2,x ; #4 x ,�/y�/1/2,z�/1/2; #5 �/x�/1/2,�/

y�/1,z�/1/2; #6 �/x ,y�/1/2,�/z�/1/2.


spinning rate of 5.0 kHz and 60 s recycle delays.

Chemical shifts are quoted in ppm from TMS. 119Sn

MAS NMR spectra were recorded with a 408 pulse, a

spinning rate of 14 kHz and a recycle delay of 100 s.

Chemical shifts are quoted in ppm from Sn(CH3)4. 23Na

MAS NMR spectra were measured using short andpowerful radio-frequency pulses (0.6 ms, equivalent to a

158 pulse angle), spinning rates of 15 kHz and a recycle

delay of 2 s. Chemical shifts are quoted in ppm from 1

M aqueous NaCl. The triple-quantum 23Na MAS NMR

spectrum was recorded using the z-filter three-pulse

sequence. The lengths of the first and second hard pulses

were 3.5 and 1.4 ms, respectively. The length of the third

soft pulse (y1�/10 kHz) was 12.5 ms. The MAS rate was14.5 kHz. 170 data points were acquired in the t1

dimension in increments of 18 ms. The recycle delay

was 2 s. The ppm scale of the sheared spectra was

referenced to yo frequency in the y2 domain and to 3.78

yo in the y1 domain (reference 1 M aqueous NaCl).

Thermogravimetric (TGA) curves were measured

with a TGA-50 analyser. The samples were heated

under air at a rate of 5 8C min�1.

3. Results and discussion

The three-dimensional framework structure of AV-13

consists of corner-sharing MO6 (M�/Sn, Zr, Hf)

octahedra and SiO4 tetrahedra. The latter form six-

membered [Si6O18]12� rings, which are interconnected

by MO6 octahedra (Fig. 3). The structure is betterunderstood by considering a three-dimensional knots-

and-crosses lattice (Fig. 4(a)). In a given layer, successive

distorted-cube M8 cages contain [Na6�x(H2O)x ]

(H2O,Cl�) octahedra (knots) and cyclohexasilicate

(crosses) units (Fig. 4(b)). While the former are extra-

framework species, the six-membered rings are, of

course, part of the framework (Fig. 4(b)). The cages

are accessed via seven-membered [M3Si4O27]26� win-

dows, with free aperture ca. 2.3�/3.2 A, one per each

pseudo-cube face (Fig. 5). Pilling up layers generates the

structure, with knots-and-crosses alternating.

Each sodium cation is five-coordinated to three

framework oxygens, one water molecule (which con-

nects to two other sodium cations) and a fifth ligand,

which may be a second water molecule or a chloride

anion (Fig. 6). Thus, the sodium local environment is

disordered. Indeed, considering at the first coordination

sphere we have two possible sodium environments,

Na[O3(Ow1)(Ow2)] and Na[O3(Ow1)(Cl�)], the pre-

sence of which is clearly indicated by the two peaks

observed in the 23Na 3Q MAS NMR spectrum (Fig. 7).

The second coordination sphere is also disordered

because the sodium site is partially occupied by water

molecules. The considerable degree of disorder in the

sodium local environment is clearly reflected in the 23Na

3Q MAS NMR spectrum: the two peaks observed are

broadened due to distributions of isotropic chemical

shifts and quadrupole parameters.29Si and 119Sn solid-state NMR data support the

structure proposed for AV-13. The 29Si MAS NMR

spectra (not shown) display a broad resonance (full-

width-at-half-maximum, FWHM of 2.2 ppm) for Sn-

AV-13, and a relative sharp resonance (FWHM of 1.0

ppm) for Zr-AV-13 at ca. �/87.1 and �/87.2 ppm,

respectively. Hf-AV-13 also gives a relative sharp

resonance (FWHM of 1.0 ppm), slightly shifted to ca.

�/84.4 ppm. The previously reported Sn�/B sample

Table 3

List of selected bond angles for Sn-, Hf-, Zr-AV-13

Na2.26SnSi3O9Cl0.26 �/2.5H2O Na2.29HfSi3O9Cl0.29 �/2.5H2O Na2.27ZrSi3O9Cl0.27 �/2.5H2O

Bond angle (8)O2�/M�/O2#1 88.79(6) 89.89(6) 90.56(4)

O2�/M�/O2#2 88.79(6) 89.89(6) 90.56(4)

O2#1�/M�/O2#2 88.79(6) 89.89(6) 90.56(4)

O2�/M�/O1#1 91.41(4) 88.17(8) 176.15(5)

O2#1�/M�/O1#1 90.19(7) 92.87(9) 88.13(5)

O2#2�/M�/O1#1 178.96(5) 176.62(8) 93.07(5)

O2�/M�/O1#2 178.96(5) 176.62(8) 93.07(5)

O2#1�/M�/O1#2 91.41(4) 88.17(8) 176.15(5)

O2#2�/M�/O1#2 90.19(7) 92.87(9) 88.13(5)

O1#1�/M�/�/O1#2 89.61(5) 89.16(12) 88.33(6)

O2�/M�/O1 90.19(7) 92.87(9) 88.13(5)

O2#1�/M�/O1 178.96(5) 176.62(8 93.07(5)

O2#2�/M�/O1 91.41(4) 88.17(8) 176.15(5)

O1#1�/M�/O1 89.61(5) 89.16(12) 88.33(6)

O1#2�/M�/O1 89.61(5) 89.16(12) 88.33(6)

O3�/Si�/O2#3 111.28(8) 109.06(8) 109.67(6)

O3�/Si�/O1 114.32(7) 112.97(12) 110.24(6)

O2#3�/Si�/O1 108.82(6) 109.72(11) 111.84(6)

O3�/Si�/O3#3 106.02(6) 106.09(6) 108.60(6)

O2#3�/Si�/O3#3 107.33(8) 107.73(8) 105.47(5)

O1�/Si�/O3#3 108.78(6) 111.08(12) 110.88(7)

Ow1�/Na�/O2#4 92.98(15) 92.48(15) 92.29(13)

Ow1�/Na�/O1 80.76(13) 76.34(16) 75.63(15)

O2#4�/Na�/O1 107.30(15) 109.33(17) 105.08(14)

Ow1�/Na�/O3#5 77.39(15) 74.44(18) 75.99(12)

O2#4�/Na�/O3#5 103.30(18) 104.11(19) 108.11(16)

O1�/Na�/O3#5 143.03(19) 136.0(3) 136.67(17)

Ow1�/Na�/Ow2#6 175.52(19) 176.5(2) 174.4(2)

O2#4�/Na�/Ow2#6 82.87(13) 85.6(2) 84.97(12)

O1�/Na�/Ow2#6 98.83(15) 101.51(17) 109.80(13)

O3#5�/Na�/Ow2#6 105.15(13) 108.87(14) 100.19(14)

Si�/O1�/M 131.66(9) 135.20(16) 134.11(9)

Si#7�/O2�/M 137.45(8) 139.44(8) 139.86(6)

Si�/O3�/Si#7 140.43(7) 137.82(7) 138.54(5)

Symmetry transformations used to generate equivalents atoms: #1 z ,x ,y ; #2 y ,z ,x ; #3 �/y�/1/2,z�/1/2,x ; #4 x ,�/y�/1/2,z�/1/2; #5 �/x�/1/2,�/

y�/1,z�/1/2; #6 �/x ,y�/1/2,�/z�/1/2; #7 z ,�/x�/1/2,y�/1/2.


displays a single, broad resonance at �/89.2 ppm [7]. In

accord with this observations, the crystal structure of

AV-13 calls for the presence of a single Si(2Si,2Sn) site.

Framework stannosilicates with a Si(2Si,2Sn) environ-

ments resonate between �/84 and �/89 ppm [11�/13].

NMR data on 29Si environments in hafnosilicates are

scanty; the hafnium analogue of mineral umbite displays

a peak 2�/3 ppm down field relatively to the resonance of

the zirconium form [19]. The 119Sn MAS MNR spec-

trum of Sn-AV-13 (not shown) displays a single broad

(FWHM of 15.3 ppm) peak at ca. �/705.4 ppm,

ascribed to the single Sn(6Si) environment present.

Sn�/B material gives a single resonance at �/708 ppm

[7]. Other framework stannosilicates with Sn(6Si) envir-

onments give resonances between �/688 and �/709

ppm, respectively [11�/13].

The total mass loss of as-prepared Sn-AV-13, ascer-

tained by TGA between 25 and 700 8C, is ca. 8.5%. The

water is lost in two steps: between 30 and 125 8C, and

between 125 and 550 8C (Fig. 8). A sample calcined at

550 8C for 4 h and rehydrated (in a saturated ammoni-

um chloride solution atmosphere at room temperature)

always looses less water (7.0%) than the parent Sn-AV-

13 sample. After the first rehydration�/dehydration cycle

the sample looses water in a reversible way. The

difference between the water losses of these materials

is probably due to the fact that the removal of some

O3w (the water molecule sharing the sodium site) is

irreversible because this is non-coordinated water. The

total TGA mass losses, between 25 and 700 8C, of as-

prepared (or rehydrated after calcination at 550 8C for 4

h) Zr- and Hf-AV-13 are, respectively, ca. 9.0 (8.0)% and

Fig. 6. Local disordered Na environment in AV-3. Large solid

circles*/Na� or H2O (Ow3); small solid circle*/Cl� or H2O

(Ow2); white open circles*/framework oxygen atoms; small grey

circle*/H2O (Ow1).

Fig. 7. 23Na 3Q MAS NMR spectrum of AV-13.

Fig. 8. TGA curves of Sn-, Zr- and Hf-AV-13 materials.


7.2 (6.6)%. Powder XRD confirms that the framework

of AV-13 materials is preserved after dehydration (up to

550 8C for 4 h).

4. Conclusion

Sodium stanosilicate AV-13, exhibiting a powder

XRD pattern similar to Corcoran’s phase B pattern,

and zirconium and hafnium analogues of this material

have been prepared and their structures solved from

powder XRD data by direct methods, and solid-stateNMR. AV-13 materials are small-pore solids, probably

more adequately described as tunnel structures. It is

interesting to note that the new sodium stanosilicates

possess both cations and anions in the pores, in contrast

with conventional zeolites where the extra-framework

species are cationic. We are currently investigating the

possibility that the chloride anions may be ion-ex-

changed for other (e.g. nitrate) anions. Alternatively, itmay be possible to enclose other types of anions in the

pores of AV-13 by introducing these species in the

parent synthesis gel.

5. Supplementary material

Further details on the crystal structure investigation

may be obtained from the Fachinformationszentrum

Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Ger-

many (e-mail: [email protected]) on quoting

the depository numbers CS-391172, CS-391173, CS-

391174.

Acknowledgements

This work was supported by FCT, POCTI and

FEDER.

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