Ab initio structure determination of novel small-pore metal-silicates: knots-and-crosses structures
Transcript of 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.
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.
A. Ferreira et al. / Inorganica Chimica Acta 356 (2003) 19�/2622
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.
A. Ferreira et al. / Inorganica Chimica Acta 356 (2003) 19�/26 23
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.
A. Ferreira et al. / Inorganica Chimica Acta 356 (2003) 19�/2624
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.
A. Ferreira et al. / Inorganica Chimica Acta 356 (2003) 19�/26 25
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.
References
[1] (a) R.M. Barrer, Zeolites and Clay Minerals, Academic Press,
London, 1978;
(b) W.M. Meyer, D.H. Olsen, C. Baerlocher, Atlas of the Zeolite
Structure Types, Elsevier, London, 1996.
[2] (a) S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan, E.M.
Flanigen, J. Am. Chem. Soc. 104 (1982) 1146;
A. Ferreira et al. / Inorganica Chimica Acta 356 (2003) 19�/2626
(b) B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R.
Cannan, E.M. Flanigen, J. Am. Chem. Soc. 106 (1984) 6092.
[3] J. Rocha, M.W. Anderson, Eur. J. Inorg. Chem. (2000) 801.
[4] D.M. Poojary, R.A. Cahill, A. Clearfield, Chem. Mater. 6 (1994)
2364.
[5] X. Wang, L. Liu, A.J. Jacobson, J. Am. Chem. Soc. 214 (2002)
7812.
[6] A. Dyer, J.J. Jafar, poster presented at the Innovations in Zeolite
Materials Science International Conference, Nieuwpoort, Bel-
gium, 1987.
[7] E.W. Corcoran, Jr., D.E.W. Vaughan, Solid State Ionics 32/33
(1989) 423.
[8] E.W. Corcoran Jr., D.E.W. Vaughan, P.E. Eberly Jr., US Patent,
5 110 568, May 5, 1992.
[9] A. Dyer, J.J. Jafar, J. Chem. Soc., Dalton Trans. (1990)
3239.
[10] A. Dyer, J.J. Jafar, J. Chem. Soc., Dalton Trans. (1991) 2639.
[11] Z. Lin, J. Rocha, A. Valente, Chem. Commun. (1999)
2489.
[12] Z. Lin, J. Rocha, J.D. Pedrosa de Jesus, A. Ferreira, J. Mater.
Chem. 10 (2000) 1353.
[13] A. Ferreira, Z. Lin, J. Rocha, C. Morais, M. Lopes, C.
Fernandez, Inorg. Chem. 40 (2001) 3330.
[14] C. Dong, J. Appl. Cryst. 32 (1999) 838.
[15] P.E. Werner, L. Eriksson, M. Westdahl, J. Appl. Cryst. 18 (1985)
367.
[16] A. Altomare, M.C. Burla, M. Carmalli, B. Carrozzini, G.L.
Cascarano, C. Giacovazzo, A. Guagliardi, A. Moliterni, G.
Polidori, R. Rizzi, J. Appl. Cryst. 32 (1999) 339.
[17] A. Le Bail, H. Duroy, J.L. Fourquet, Math. Res. Bull. 23 (1988)
447.
[18] J. Rodriguez-Carvajal, FULLPROF Program for Rietveld Refine-
ment and Pattern Matching Analysis; Abstracts of the Satellite
Meeting on Powder Diffraction of the XVth Congress of the
International Union of Crystallography, Toulouse, France, 1990,
p. 127.
[19] Z. Lin, J. Rocha, Studies in surface science and catalysis 142
(2002) 319.