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1302
J.
Phys. Chem. 1994,98,
1302-1305
ESCA Studies
of
Framework Silicates with the Sodalite Structure.
1.
Comparison
of
Purely
Siliceous Sodalite and Aluminosilicate Sodalite
Bruno Herreros, Heyong He, Tery
L.
Barr,*’+ and Jacek
Klinowski’
Department
of
Chemistry, University
of
Cambridge, Lensfield Road, Cambridge CBl2
IEW,
U.K.
Received: June 29, 1993;
In
Final Form: October 29, 1993’
The first detailed X-ray photoelectron spectroscopy (ESCA) study of materials with the sodalite structure shows
how the purely siliceous sodalite prepared in a nonaqueous medium differs from the conventional aluminosilicate
sodalite and from various zeolites and clay minerals. The purely siliceous sodalite gives a unique set of binding
energies and valence band patterns similar
to
those for silica, while the ESCA pattern
of
an aluminosilicate
sodalite is similar to that of zeolite Na-A. The small differences in binding energies for aluminosilicate sodalite
are due to the presence of some
K+
and Mg2+ and the fact that the framework Si/Al ratio is slightly higher
than unity. While the binding energies for zeolite Na-A and aluminosilicate sodalite form a common set, their
ESCA patterns differ substantially from that for the layered aluminosilicate kaolinite. ESCA is thus shown
to be sensitive to structure as well as composition.
Introduction
In 1887 Sir William Thomson (later Lord Kelvin) demon-
stratedl that the truncated octahedron (more correctly a tetra-
kaidodecahedron), the simplest space-filling polyhedron apart
from the cube, divides space with minimum partitional area. The
tetrakaidodecahedron is thus the three-dimensional equivalent
of a regular hexagon, the most economical divisor of a plane. The
frameworkof sodalite is a periodic array of such [4686] polyhedra
which are known in mineralogy as sodalite or
@
cages. The cages
are
in
turn built from corner-sharing SiO4”- and A10P tetrahedra.
The negative charge of the framework brought about by the
presence of the aluminate tetrahedra is neutralized by charge-
balancing cations, typically Na+. In addition, sodalite cages may
accomodate a variety of guest species, such as inorganic salts,
water, or organic molecules. Many natural and synthetic
materials with the sodalite structure and a variety of enclathrated
species have been described.2 Sodalite may have a range of
compositions2 but generally has almost equal amounts of
tetrahedral Si and A1 in its framework. A typical unit cell formula
is Na6A16Si6024*2NaCl. Because all tetrahedral sites in sodalite
are crystallographically equivalent and only one kind of cages is
present, sodalite is an archetypal molecular sieve.
Sodalite is normally synthesized from strongly basic media
under mild
hydro thermal condition^.^
However, by using ethylene
glycol as solvent, Bibby and Dale4 prepared a purely siliceous
sodalite. The unit cell composition of the product is Si] 2024-2C2H4-
(OH)2, and the glycol is encapsulated in the sodalite
cage^.^^^
We
report an X-ray photoelectron spectroscopy (ESCA) study of
sodalites including its purely siliceous analogue. Our aim was
(i) to carry out the first detailed ESCA study of sodalite, (ii) to
see
how the purely siliceous sodalite differs from its aluminosilicate
analogue, and
(iii)
to compare the results with those for various
zeolites and clays.6 This work is part of a concerted effort to
extend the range of application of the technique to cover all types
of silicates.
Experimental Section
Synthesis
of
Purely Siliceous Sodalite.
The starting materials
were ethylene glycol, Cab-0-Si1 fused silica, and sodium hydroxide
in the molar ratio 4 SiOz:NaOH:40 CzHd(OH)z, slightly different
from that originally de~cribed.~ NaOH was first added to the
Permanent address: Department
of
Materials and Laboratory for Surface
@
Abstract published in
Adoance
ACS
Abstracts,
January 1, 1994.
Studies, University
of
Wisconsin-Milwaukee, Milwaukee, WI
53201.
0022-365419412098-1302$04.50/0
glycol, and the mixture was stirred for
6
h until the base dissolved.
Fused silica was then added under stirring, producing a thick gel
which was transferred into a Teflon-lined stainless steel autoclave
and heated under autogeneous pressure at 180 OC for 3 days. The
crystalline product was finally filtered, washed withdistilled water,
and dried at room temperature. It crystallizes in the cubic space
group Im3m, with a cell parameter
a
=
8.83
A
(as compared with
8.94
A
for a conventional aluminosilicate sodalite).
A synthetic aluminosilicate sodalite, made using standard
procedure^,^
was also examined in the as-prepared state. Other
results referred to in this study were described in earlier
publications.”l
I
Solid-state
NMR.
Magic-angle-spinning (MAS) NMR
spec-
tra were acquired at 9.4 T using a Chemagnetics CMX-400
spectrometer with rotors 4 mm in diameter spun in nitrogen gas
at 8-10 kHz. 29Si spectra were measured at 79.5 MHz with 45O
radiofrequency pulses and 30-s recycle and A1 spectra
at 104.3 MHz with very short,
0.6-ps
(less than loo), radiof-
requency pulses and 0.3-s recycle delays. IH-W cross-polar-
ization (CP) MAS NMR spectra were measured at 100.63 MHz
with 4-ms contact times. The length of the IH and I3C u/2
pulses was 3
ps,
recycle delay
10
s,
and MAS rate
8
kHz. IH-
29Si CP/MAS spectra were recorded at 7.05
T
using a Bruker
MSL-300 spectrometer with a single contact pulse sequence,
8-ms
contact time, 4.8-ps IH 90° pulse, and 10-s recycle delay. The
Hartmann-Hahn condition for ‘H-27A1 and lHJ9Si CP/MAS
was established using kaolinite12 and for IH-I3C CP/MAS using
hexamethylbenzene. Chemical shifts of 27Al are given in parts
per million from external Al(H2O)p and the shifts for 29Si and
I3C from external tetramethylsilane (TMS).
ESCA
Measurements.
ESCA measurements were carried out
using a Vacuum Generators ESCALAB system at the University
of Wisconsin-Milwaukee in the conventional mode with A1
Ka
X-rays. Powdered samples were either sprinkled onto double-
sided adhesive Scotch tape and the excess shaken off or pressed
as a wafer onto a thin indium foil. The tape or the foil was then
mounted onto a stainless steel specimen stub.
No
problems with
differential charging were encountered
(see
below). In order to
remove charging shifts and deal with Fermi edge coupling
problems, binding energies were scaled against the C( 1s) peak
(set to 284.6 eV) of the alkane part of the ethylene glycol retained
in the sodalite cage and also against any adventitious car-
bon.6,10.11
,I
3
0
1994 American Chemical Society
Framework Silicates with Sodalite Structure
The Journal
of
Physical Chemistry, Vol.
98,
No.
4, 1994
1303
TABLE 1: Core-Level Binding Energies and
Line
Widths
(in
Parentheses) in eV, Based on C(ls) at
284.6
eV.
Also
Included are Select Values
for
the Width at Half Maximum
of Various Valence Bands
’H
-
29Si
CP
1
MAS
purely
siliceous sodalite
-40
-60
-80
-100 -120 -140 -160
(b)
spinning
sample
-40
-60
-60
-100
-120
-140
-160
ppm
from
TMS
Figure
1.
1H-29Si CP spectra
of
purely siliceous sodalite: (a) static
sample, (b) with MAS.
1H-13CCPIMAS
purely
siliceous
sodalite
64
19
ppm
1
65
80
75
70
65 60 55
50
45
40
ppm
from
TMS
Figure
2.
lH-l3C CP/MAS spectrum
of
purely siliceous sodalite.
Results
and Discussion
Purely
Siceous
Sodalite.
The 29Si CP/MAS spectrum of
purely siliceous sodalite
consists
of a single very sharp single peak
at -1 17.3 ppm from TMS (see Figure lb). Even the spectrum
of a static sample (Figure la) is relatively narrow and symmetric,
reflecting the strictly cubic environment of the silicon atoms. The
IH-l3C CP/MAS spectrum (Figure
2)
consists of a single sharp
peak at 64.19 ppm corresponding to the encapsulated ethylene
glycol, indicating that all glycol molecules are located
on
equivalent
sites in the structure.
The ESCA results are given in Table 1 and Figures 3-7. Also
given in Table 1 is a representative set of results for several
material Si(2p)
O(
1s)
valence band A1 (2p)
Si02 (silica) 103.5 532.9 10.4
(1.8) (1.9)
purely siliceous sodalite 103.2 532.45 10.4
(2.6) (2.2)
zeolite
EM-5
103.1 532.45 10.4 14.5
zeolite Na-Y 102.55 532.0 10.2 14.4
zeolite Na-X 102.0 531.1 9.8 13.95
(1.8) (1.65) (1.6)
zeolite Na-A 101.1 530.5 9.1 13.5
(1.7) (1.75) (1.6)
montmorillonite 102.75 532.0 10.2 74.8
(2.05) (2.4)
(2.0)
kaolinite 102.45 531.5 10.2 14.3
(2.3) (2.45) (2.2)
(1.7) (2.05) (1.55)
aluminosilicate sodalite 101.5 530.9
a
13.6
Line width obscured by interference from
In(4d).
(b)
aluminosilicate sodalite
350
300 250
200
150
100
50
0
(a)
purely
siliceous sodalite
200
160
120
80
40
0
Binding energy (eV)
Figure
3.
(a) ESCA survey scan of purely siliceous sodalite; (b) ESCA
survey scan
of
aluminosilicate sodalite confirming a significant presence
of
C,
K,
and Mg.
reference materials. A “survey” ESCA scan for the purely
siliceous sodalite is shown in Figure 3a, and Figure 4 documents
the C( 1
s)
region, demonstrating that the principal elemental peaks
detected are due to Si and
0
and that substantial amounts of C
and moderate amounts of Na are also present. The carbon signal
comes from the glycol molecules and the adventitious carbon
species which are always present
on
the outer surface of air-
exposed material~.~J~ The intense peak at
288.5
eV is the C(1s)
signal primarily from the hydrocarbon parts of these species (with
a charging shift of 3.9 eV). The size and shape of the shoulders
in this C( 1s) spectrum indicate the presence of various types of
carbon-xygen units, most of them C-OH groups of the ethylene
glycol. Thevarious sodiumpeaks indicate that someof the sodium
1304
The Journal
of
Physical Chemistry,
VoI.
98.
No.
4, 1994
(b)
aluminosilicate sodalite
c
(1s)
.-
0
E
2
(a) purely siliceous sodalite
-
295 290 285 280 275
Binding energy (eV)
Figure
4.
C(1s) region
of
a high-resolution ESCA spectrum
of
purely
siliceous sodalite.
I\
(b)
aluminosilicate
sodalite
108 106 104 102
100
98 96 94 92
90
Binding energy (eV)
Figure
5.
Key Si(2p) peak binding energies adjusted to C(1s)
=
284.6
eV.
(which is abundant in the aluminosilicate sodalite) is retained in
the purely siliceous material. The sizes of these peaks indicate
that the amount of sodium is small. On the basis of the binding
energies, we attribute the majority of this sodium to residual
NaOH. This is supported by the absence
of
any other anions.I4
The Si(2p) and
O(1s)
spectra in Figures
5
and
6
are the
ESCA
“signatures” of the purely siliceous sodalite. This conclusion is
based on (i) the relative narrowness and the Lorentzian-Gaussian
shape
of
the lines, (ii) their relative intensities, (iii) the binding
Herreros et al.
r
18
536 534 532 530 528 526 524 522 520
Binding energy (eV)
Figure
6.
Key
O(1s)
peak binding energies adjusted to C(1s)
=
284.6
eV
.
20
15
io
b
0
Binding
energy
(eV)
Figure 7.
Valence band ESCA spectra: (a) Purely siliceous sodalite;
(b)
a-silica; (c) aluminosilicate sodalite; (d) zeolite Na-A. The dotted line
removes the In(4d) satellite peak, providing a more accurate rendition
of
the true valence band spectrum.
energies (see below), and (iv) the nature of the corresponding
valence band features (see Figure
7).
The widths and shapes of
the peaks are those of a singular species. The one exception is
in the
O(1s)
spectra because of the presence
of
a small amount
of NaOH. However, NaOH is not detected in the
O(1s)
ESCA
spectrum because its
O(
1s)
binding energy
is
apparently almost
identical to those for the purely siliceous sodalite, resulting in a
O(
1s)
line which is narrow (2.2 eV) and symmetric.
Framework Silicates with Sodalite Structure
The Journal
of
Physical Chemistry, Vol.
98,
No.
4,
1994
1305
aluminosilicate sodalite is not unexpected since the %/A1 ratio
of the latter is higher. As expected, the shift is to higher Si(2p)
and Al(2p) binding energies for the sodalite (see Table
1
and
Figures
5
and
6).
Theother notable feature is that the core-level binding energies
for aluminosilicate sodalite are not at all similar to those for
kaolinite (see Table
1).
In this case, ESCA is apparently sensitive
to thestructuraldifferences (frameworkversus sheet) ofthe species
as well as to the Si/Al ratio.
Some of the other features of these ESCA results become
apparent from the analysis of the binding energies of the core
lines and of the features of the valence band.8 Thus, we see from
Table 1 and Figures 5 and
6
that the binding energies of Si(2p)
and
O(
1s)
lines from the purely siliceous sodalite are almost the
same as from pure silica.6 The slightly lower values for sodalite
may be due to the different structures of the two materials. It has
been our contention that there are readily detectible shifts in the
binding energy positions for different and that these
shifts are primarily due to the nature of the immediate chemical
bonding environment adjacent to the Si-0 bonds.9 Thus, for
example, as more A1-0 units are inserted into a silica-type
framework, the binding energy of the Si(2p) peak progressively
decreases to reflect the enhanced covalency induced into the Si-0
bonds by the more ionic A1-0 unit~.~J~ As a result, the Si(2p)
and
O(
1s)
binding energies in the purely siliceous sodalite and
silica are higher than those in ZSM-5.6,8 We note from Table
1
and Figure 7a that the structure and character of the valence
band for the purely siliceous sodalite are very similar to those for
silica (see Figure 7b).
Aluminosilicate Sodalite.
Studies of many zeolites and clays
allow
us
to suggest that the binding energies and valence band
patterns for aluminosilicate sodalite should be very similar to
those for zeolite Na-A (see Figure
7
and Table 1). This is based
on
the shift of ESCA binding energies and valence bands for
similar system~.~J~ These shifts are found to reflect the localized
bonding chemistry in each structural type. Thus the ESCA
features of aluminosilicate sodalite and zeolite Na-A, two
frameworksilicates with Si/Al
=
1
which obey the “Loewenstein
rule” (which forbids AI-0-A1 linkages), should be very similar,
while kaolinite, a clay silicate also with Si/A1
=
1
but built of
octahedral aluminate and tetrahedral silicate units, should give
substantially different binding energies (see Table
l).6,7,9
The
structural difference between zeolite Na-A and sodalite may not
be resolved by ESCA, except perhaps as subtle shifts at very high
resolution. At the same time, we note that if all the aluminate
units in framework silicates are located in tetrahedral environ-
ments, essentially substituted for some of the silicate units (and
obeying the Loewenstein rule), then both the A1 and Si binding
energies will progressively increase as the Si/Al ratio is
in~reased.~-~ Structural variations have been found to influence
these binding energies, but far less than the change from silicate
to aluminate bonding chemistry within the same structural unk6
The binding energies for aluminosilicate sodalite should therefore
be similar to those of zeolite Na-A, differing only slightly due to
the structural differences, just as is the apparent result in
comparing the ESCA results for the purely siliceous sodalite with
silica.
The sample of aluminosilicate sodalite contained OH- instead
of the C1- which is normally present in the sodalitecages.2 ESCA
detected
no
evidence of the presence of chlorine. The type of
anion has
no
effect
on
the sodalite framework2 and thus does not
influence the critical ESCA shifts. We note that the relative
quantification of the Si(2p) to Al(2p) and the Na(2s) to the
Al(2p) in aluminosilicate sodalite also agrees with the result of
chemical analysis (Si/Al
=
1.05)
and suggests that the Na/A1
ratio is slightly lower than unity. The latter feature is explained
by the presence, detected by ESCA, of
K+
and Mg2+ cations (see
Figure 3b).
The core-level binding energies andvalence band patterns from
aluminosilicate sodalite (see Figures 5-7c and Table
1)
shift
dramatically with respect to the corresponding features for the
purely siliceous structural analogue. Substantial shifts of the
core lines of the aluminosilicate sodalite also occur with respect
to the silica, whereas the key binding energies for the former are
almost equal to those for zeolite Na-A (see Table
1
and Figures
5
and
6).6
In addition, thevalence band pattern for aluminosilicate
sodalite is also almost identical to that for zeolite Na-A (see
Figure 7c and d).8 The fact that there are small differences
between the binding energy values for zeolite Na-A and
Conclusions
ESCA core-level shifts and valence band patterns of sodalite-
type framework silicates are summarized in Table
1
and Figures
5-7.
These show the following:
(1)
The purely siliceous sodalite gives a unique set of binding
energies and valence band patterns which resemble those for silica.
The small variations may be indicative of the detection by the
ESCA of the structural differences.
(2) The ESCA core-level binding energies and the valence
band pattern for the purely siliceous sodalite and the alumino-
silicate sodalite are very different.
(3) The ESCA patterns of aluminosilicate sodalite and zeolite
Na-A are similar. The small difference in binding energies may
be due to the presence of
K+
and Mg2+ and the higher Si/Al ratio
of the aluminosilicate sodalite.
(4)
In
some cases ESCA is as sensitive to structure as it is to
composition. Thus, while zeolite Na-A and aluminosilicate
sodalite form
a
common binding energy set (with Si/Al-
1
and
all tetrahedral Si-0 and A14 alternating according to the
Loewenstein rule), their ESCA patterns differ substantially from
that of kaolinite which also has Si/Al=
1
but in which all A1-0
units are in octahedrally-bonded subsheets linked via apical
oxygens to tetrahedral Si-0 subsheets.13
This study of the surface/near-surface chemistry of sodalites
leaves many aspects unexamined. Although we have demon-
strated that ESCA may differentiate between substantially
different members of this group, there are other members of the
sodalite-ultramarine family the differences between which are
much more subtlee2 An ESCA study of these materials will be
described in a future paper.
Acknowledgment.
We are grateful to Dr.
P.
J.
Barrie,
University College, London, for acquiring an NMR spectrum,
the Fulbright Commission for a Professorial Fellowship for T.L.B.,
the Oppenheimer Fund for a Research Studentship for
H.H.,
and Unilever Research, Port Sunlight, for a Research Studentship
for B.H.
References and Notes
(1)
Thomson, Sir William.
The London, Edinburgh, and Dublin
Philosophical Magazine and Journal
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(2)
Deer, W. A.; Howie,
R.
A.; Zussman,
J.
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pp
496-502.
(3)
Barrer, R. M.
Hydrothermal Chemistry
of
Zeolites;
Academic
Press: London,
1982.
(4)
Bibby, D. M.; Dale, P.
Nature
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157.
(5)
Richardson,
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W.;
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(6)
Barr, T. L.
In
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Chapter
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(7)
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T.
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Barr, T. L.
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(10)
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Barr, T.
L.
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