Synthesis and thermal behaviour of gallium-substituted aluminosilicate inorganic polymers

Article abstract of DOI:10.1039/c0dt00941e

A combined sol-gel and solid-state method reported for the synthesis of gallium silicate analogues of aluminosilicate inorganic polymers has also been extended to the formation of related compounds with a range of Al-for Ga substitutions. Homogeneous, robust products were obtained at an optimum composition of SiO₂:(Ga₂O₃ + Al ₂O₃) = 7. After curing at 40 °C, all the products were typically X-ray amorphous, and the Al and Ga was shown by ²⁷Al and ⁷¹Ga MAS NMR spectroscopy to be in solely tetrahedral coordination. The ²⁹Si MAS NMR spectra were as expected for silicate inorganic polymers, but also indicated the presence of some unreacted silica. Electron microscopy in conjunction with EDS elemental mapping showed that the Ga, Al and Si was homogeneously distributed in the products. Thermal treatment of these compounds results in endothermic water loss at about 75-160 °C followed by an exothermic event at about 950 °C corresponding to crystallization of KGaSi₂O₆ in the gallium end-member. By contrast, the Al-substituted compounds never fully crystallised, but melted at 1200 °C to an X-ray amorphous product.

Full text of DOI:10.1039/c0dt00941e

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PAPER
Synthesis and thermal behaviour of gallium-substituted aluminosilicate
inorganic polymers
Andrew T. Durant,a,b Kenneth J. D. MacKenzie*a,c and Hideki Maekawad
Received 2nd August 2010, Accepted 8th February 2011
DOI: 10.1039/c0dt00941e
A combined sol–gel and solid-state method reported for the synthesis of gallium silicate analogues of
aluminosilicate inorganic polymers has also been extended to the formation of related compounds with
a range of Al-for Ga substitutions. Homogeneous, robust products were obtained at an optimum
◦
composition of SiO
2
:(Ga
2
O
3
+ Al
2
O
3
) = 7. After curing at 40 C, all the products were typically X-ray
2
7
71
amorphous, and the Al and Ga was shown by Al and Ga MAS NMR spectroscopy to be in solely
2
9
tetrahedral coordination. The Si MAS NMR spectra were as expected for silicate inorganic polymers,
but also indicated the presence of some unreacted silica. Electron microscopy in conjunction with EDS
elemental mapping showed that the Ga, Al and Si was homogeneously distributed in the products.
Thermal treatment of these compounds results in endothermic water loss at about 75–160 C followed
by an exothermic event at about 950 C corresponding to crystallization of KGaSi
◦
◦
2
6
O in the gallium
◦
end-member. By contrast, the Al-substituted compounds never fully crystallised, but melted at 1200 C
to an X-ray amorphous product.
9
Introduction
virtue of the favourable bandgap of Ga, the substitution of this
element for aluminium in aluminosilicate inorganic polymers is of
interest, both for their own sake and as potential precursors for
the formation of crystalline gallium aluminosilicate compounds
by thermal treatment.
Aluminosilicate inorganic polymers, sometimes called geopoly-
mers, are materials that cure and harden at ambient temperatures
and consist of X-ray amorphous randomly arranged tetrahedral
aluminate and silicate units, with charge balance provided by
the presence of alkali metal cations. Conventional synthesis
of these materials is by reaction of a solid aluminosilicate
such as a dehydroxylated 1 : 1 aluminosilicate clay with sodium
silicate solution under highly alkaline conditions, although other
synthesis methods have been reported. In one such alternative
sol–gel synthesis, the use of a solid aluminosilicate is avoided by
reacting an alkali aluminate solution with alkali silicate prepared
The synthesis of gallium-substituted inorganic polymers by the
conventional method used for the aluminosilicate compounds is
militated against by the lack of natural gallium analogues of the
1,2
1
: 1 layer-lattice aluminosilicates, necessitating the development
1,2
of a new synthesis method based on a combination of sol–gel and
solid-state syntheses. The resulting intermediates and products
with a range Ga-for-Al substitutions were characterised by X-ray
diffraction and multinuclear solid-state MAS NMR spectroscopy
and their thermal behaviour was determined by thermal analysis
and X-ray powder diffraction.
3
in situ by reaction of silica with alkali hydroxide solution or
by reacting aluminium hydroxide with alkali silicate solution.
In another synthesis method, undehydroxylated aluminosilicate
clay is reacted in the solid state at about 550 C with solid
4
◦
alkali hydroxide; the solid product is then set by hydrating with a
minimum amount of water.
Experimental
5
The substitution of gallium for aluminium in these compounds
has not previously been reported, although crystalline gallium-
substituted zeolites and mullite-structured compounds have
been reported. Since gallium-containing compounds are known
to demonstrate useful electronic and opto-electronic properties by
Synthesis of potassium gallium silicate inorganic polymers
Based on the sol–gel synthesis previously developed for alumi-
6,7
8
3
nosilicate inorganic polymers, the synthesis of the analogous
potassium gallosilicate compound should be possible from a
reaction mixture of SiO
proportions. The KGaO
a method reported for the formation of b-gallates, employing
2
, KGaO
2
, KOH and H O in the correct
2
2
was prepared in a preliminary step by
a
School of Chemical and Physical Sciences, Victoria University of Welling-
10
ton, New Zealand
b
the solid-state reaction of Ga
Drug Houses (BDH) reagent grade K CO at 900 C for 5 h:
2
O
3
(Aldrich 99.99+%) with British
Industrial Research Ltd., Lower Hutt, New Zealand
◦
c
MacDiarmid Institute for Advanced Materials and Nanotechnology, Victo-
2
3
ria University of Wellington, New Zealand
d
Tohuku University, Japan
Ga
2
O
3
+ K
2
CO
3
→ 2KGaO
2
+ CO
2
↑
(1)
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The product of this reaction was shown by XRD to be crystalline
KGaO (Powder Diffraction File no. 01-070-2402) but was highly
2
hygroscopic, absorbing 43–52% of its mass to form a solution
when exposed to the atmosphere for 2–3 h. This solution was
used to prepare the inorganic polymer, by dissolving silica fume
(
(
Doral Fused Materials Pty) in a concentrated solution of KOH
Pure Science) with continuous mixing, then adding the KGaO
: Ga O
2
solution with thorough mixing. The optimal molar SiO
2
2
3
composition of the samples was found to be 7; at higher ratios
the samples separated into layers and cracked, while at lower
ratios the samples were weak and in some cases did not set. The
reaction mixture was poured into a greased mould, sealed in an
airtight container and cured at room temperature for 48 h. Room-
temperature curing was found to be preferable to the more usual
◦
curing temperature for inorganic polymers (40–60 C) since even
in samples of the optimum composition, the higher temperature
caused the product to settle into two layers, possibly due to
the settling of the heavier gallate species in the low-viscosity
mixture, this viscosity effect becoming even more apparent at
higher temperatures.
A series of gallium-aluminium silicate inorganic polymers was
synthesised in a similar manner, with the addition of varying
proportions of KAlO
2
solution to the above mixture. The sequence
of these reactions is shown schematically in Fig. 1. The KAlO
2
was synthesised by solid-state reaction of Al(OH)
3
(British Drug
◦
Houses reagent grade) with K
2
CO
3
at 900 C for 17 h, the product
being shown by XRD to be crystalline and monophase (Powder
Diffraction File no. 53-0809). Five gallium aluminosilicate inor-
ganic compounds were prepared, with compositions ranging from
Fig. 1 Schematic diagram of the synthesis steps in the formation of
potassium gallium and potassium gallium alumium silicate inorganic
polymers.
KGaO
2
: KAlO
2
= 90 : 10 to 10 : 90, and SiO
2
:(Ga
2
O
3
+ Al
2
O ) =
3
Ga and Al silicate end-members and the 50% substituted Ga–Al
compound were heated at 1000 C, then at 1200 C for 20 h in air
and the products examined by XRD and MAS NMR as above.
7
, the optimum ratio for the gallium end-member compound.
◦
◦
Characterisation of the samples
The cured samples were powdered and examined by XRD (Philips
Results and discussion
PW1730 diffractometer with graphite monochromator and CoKa
2
9
27
39
radiation) and by multinuclear solid-state Si, Al and K MAS
On mixing, all the Ga-containing samples were less viscous than
the aluminosilicate analogues, and, in the case of the gallosil-
icate end-member, tended to separate into two layers as noted
above. This problem was overcome in the samples of optimum
NMR at 11.7 T (Bruker Avance 500 spectrometer with a Doty
7
1
MAS probe spun at 6–10 kHz) and Ga MAS NMR at 14.1 T
JEOL ECA-600 spectrometer) as follows:
(
2
9
Si: 6 ms (p/10) pulse with a 30 s delay, the spectra referenced
to TMS,
SiO
2
: Ga
2
3
O composition by curing at ambient temperature,
or by conditioning the mixture for several short periods at
2
7
◦
Al: 1 s (p/10 pulse for solution) with a 1 s delay, the spectra
50 C before placing in the mould at room temperature. The
3
+
referenced to Al(H
2
O)
6
,
viscosity of mixtures containing potassium aluminate increases
with increasing Al content, allowing these compounds to be cured
3
9
K: Hahn spin-echo pulse sequence with 16-step recycling, 0.1
◦
s recycle time with a refocusing interval of 100 ms and a spectral
conventionally at 50 C. The effect of Al on the curing properties
width of 400 Hz. The spectra were referenced to 1M KCl solution.
of these systems may be related to the relative rates of reaction,
the aluminate component reacting more rapidly with the silicate
in solution than the gallate species.
7
1
◦
Ga: 1.5ms 90 pulse with a 2 s delay, the spectra referenced to
GaCl solution. 4000 transients were acquired for each spectrum.
3
Scanning electron microscopy was carried out at 15 kV using
a JEOL JSM-6500F microscope fitted with a field emission gun
and an EX-23000BU X-ray analyser. The powdered samples were
coated with a 8 nm platinum coating to prevent surface charging.
The XRD traces of the products (Fig. 2) show them all to be
typically X-ray amorphous.
The Al MAS NMR spectra (Fig. 3) show all the Ga-substituted
aluminosilicate samples contain predominantly tetrahedral Al
2
7
(
58.3–60.2 ppm), as is the case with conventional inorganic
polymers. This tetrahedral shift is less positive than the major
resonance of the KAlO starting material (75.2 ppm, Fig. 3E),
Thermal reactions of the compounds
2
The behaviour of the samples on heating was investigated by
combined TG-DSC (TA Instruments SDT Q600) at a heating rate
of 10 C min in flowing air (50 mL min ). Small pieces of the
confirming that Al has been incorporated into an inorganic
polymer environment. Small broad features at 8–9 ppm arise from
the presence of traces of Al in octahedral coordination.
◦
-1
-1
11
4
866 | Dalton Trans., 2011, 40, 4865–4870
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Fig. 2 Representative XRD traces of the as-synthesised inorganic poly-
mers. A. Ga silicate end member, B. Ga : Al = 75 : 25, C. Ga : Al = 50 : 50.
2
9
Fig. 4 shows a selection of typical Si MAS NMR spectra
of the products. The spectrum of the Ga end-member (Fig. 4A)
contains a substantial resonance from unreacted SiO (-108 ppm,
2
Fig. 4C), in addition to resolvable overlapping features at -81
and -86 ppm, the former from Si(4Ga) and the latter from either
Si(3Ga) or Si(2Ga) sites. Progressive substitution of Al for Si in
2
7
Fig. 3 Representative 11.7 T Al MAS NMR spectra of gallium
aluminium silicate inorganic polymers, both as-synthesised and heated.
A. Gibbsite starting material, B. KAlO
0 : 10, D. Ga : Al = 75 : 25, E. Ga : Al = 25 : 75, F. Ga : Al – 50 : 50, fired at
11
2
starting material, C. Ga : Al =
the compounds results in a shift of the (Ga,Al)-Si resonance from
9
1
◦
-
89 ppm in the 75 : 25 Ga : Al sample (Fig. 4B) to a broadened peak
200 C.
with a centre-of-gravity of about -95 ppm in the 25 : 75 Ga : Al
sample (Fig. 4C). This shift arises from differences in the shielding
effect of the Ga and Al. The presence of the resonance at -108
polymers (typically -97 ppm for a pure aluminosilicate compound
heated to 1400 C), clearly indicating that that in the present
polymers the cation is hydrated.
◦
17
11
ppm in all these samples indicates that unreacted SiO
2
is present,
in approximately the same amount.
Fig. 5 shows a selection of Ga MAS NMR spectra. The Ga
starting material (Fig. 5A) shows two second-order quadrupolar
lineshapes corresponding to the tetrahedral and octahedral sites
known to be present in the crystal structure. Formation of the Ga
inorganic polymer results in the conversion of all the octahedral
In summary, the multinuclear NMR spectra of these X-ray
amorphous compounds indicate that they are all conventional
potassium inorganic polymers in which the Ga or (Ga+Al) is in
the expected tetrahedral coordination, the silicon is associated
with either 2 or 3 Ga or (Ga+Al) (but some unreacted silica is
present) and the potassium charge-balancing cation is hydrated.
Secondary electron images of the gallium silicate and gallium
aluminium inorganic polymers (Fig. 7) show similar morphology,
with amorphous gel-like particles and 1–3 mm spherical particles
of unreacted silica. EDS elemental maps of these regions (not
shown) indicate an even distribution of Ga, Al and Si throughout
7
1
2
O
3
11
7
1
Ga to tetrahedral coordination, evidenced by the single Ga
resonance at 166 ppm (Fig. 5B). The position of this resonance is
typical of tetrahedral Ga in a variety of Ga-substituted zeolites
12–16
(
150–174 ppm
), confirming that the target Ga inorganic
polymer has indeed been formed. Substitution of Al for Ga in
7
1
these compounds does not change their tetrahedral Ga spectra
Fig. 5C).
the sample, apart from the Si-rich SiO spheres. These results
2
(
are consistent with conclusions drawn from the X-ray and NMR
results.
3
9
The K NMR spectra of two Ga–Al inorganic polymers of
differing Ga : Al ratios were also acquired (Fig. 6). Both spectra
show a single resonance, consistent with uniform distribution
of the potassium in a single site throughout the samples, but
the positions of these resonances are different from the shift of
Thermal behaviour of these compounds
The TG/DSC trace of the gallium silicate inorganic polymer is
shown in Fig. 8. The thermograms of samples of all composi-
tions follow similar trends to those of aluminosilicate inorganic
polymers, endothermically losing mass in overlapping stages up
17
-
44 ppm reported for a pure aluminosilicate inorganic polymer,
possibly reflecting the different SiO contents of these samples.
2
3
9
The K chemical shifts of the present compounds are very
different from those of heated potassium aluminosilicate inorganic
◦
to about 600 C, with the most rapid mass loss occurring below
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29
Fig. 4 Representative 11.7 T Si MAS NMR spectra of potassium
gallium and potassium gallium-aluminium silicate inorganic polymers,
both as-synthesised and heated. A. gallium silicate end member, B.
7
1
Fig. 5 Typical 14.1 T Ga MAS NMR spectra of potassium gallium
and potassium gallium aluminium silicate inorganic polymers, both
as-synthesised and heated. A. Ga
end member, C. Ga : Al = 75 : 25, D. gallium end member, fired at 900 C.
2
O
3
starting material, B. gallium silicate
Ga : Al = 75 : 25, C. Ga : Al = 25 : 75, D. SiO
2
fume starting material, E.
◦
◦
Ga : Al = 50 : 50, fired at 1200 C.
◦
about 200 C. The small apparent mass change at the end
of the experiment is an artefact corresponding to the furnace
switching off. The thermal behaviour of all the compositions
was investigated by sequentially firing both powder and fragment
◦
samples at temperatures between 500–1000 C for 18 h and
determining the crystalline products formed at each temperature
by XRD. All the inorganic polymer samples remained amorphous
◦
up to 800 C, above which temperature they crystallised; the Ga
silicate compound formed KGaSi
2
O (Powder Diffraction File no.
6
◦
37-0349) above 800 C, while the unsubstituted aluminosilicate
polymer begins to form leucite, KAlSi
2
O (Powder Diffraction File
6
◦
no. 71-1147) at 1000 C. Samples with 50% Ga–for–Al substitution
◦
appear to crystallise at 900 C to a mixture of these two phases,
but the similarity in their XRD patterns militates against their
unambiguous differentiation. Since the Al silicate end member
◦
is still relatively amorphous even at 1000 C, it is likely that
◦
at 900 C the 50 : 50 compound consists of a mixture of an
amorphous Al-rich component and crystalline potassium gallium
◦
silicate. On further heating to 1200 C for 20 h, the Ga sample
39
Fig. 6 Representative 11.7 T K MAS NMR spectra of potassium
gallium aluminium silicate inorganic polymers. A. Ga : Al = 75 : 25, B.
Ga : Al = 25 : 75.
remained unchanged, but the sample in which all of the Ga was
replaced by Al melted to an X-ray amorphous form. A sample
containing 50 : 50 Ga : Al substitution also melted and became
X-ray amorphous. Gallium silicate systems typically display a
slightly higher melting temperature than the analogous aluminium
mixed Ga–Al compounds may be driven by the Al component
which causes the Ga component to melt rather than crystallise.
18
silicate systems, suggesting that the melting behaviour of the
4
868 | Dalton Trans., 2011, 40, 4865–4870
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2
9
changes to the Si MAS NMR spectra, it produces little change in
the Ga or Al environments, which remain in four-fold coordination
with oxygen.
Conclusions
Gallium silicate analogues of aluminosilicate inorganic polymers
were successfully synthesised by a combination of sol–gel and
solid-state techniques. The same method could also be used to
produce compounds with a range of Al-for Ga substitutions.
The optimum composition of these compounds was SiO
2
:(Ga
2
O
3
+
Al ) = 7; samples in which this ratio was greater or less
2
O
3
than 7 either separated into layers and cracked, or were weak
and in some cases did not set. The cured and hardened prod-
ucts all showed typical inorganic polymer characteristics, being
X-ray amorphous with the Al and Ga shown by MAS NMR
2
9
spectroscopy to be in solely tetrahedral coordination. Si MAS
NMR and electron microscopy showed that although the Ga,
Al and Si was homogeneously distributed in the products, they
contained some unreacted silica.
Fig. 7 SEM micrograph of a potassium gallium aluminium silicate
inorganic polymer, Ga : Al = 50 : 50. Arrows indicate unreacted spherical
silica particles.
Thermal treatment of these compounds results in an over-
◦
lapping two-stage endothermic water loss at about 75 C and
◦
◦
1
60 C followed by a broad exothermic event at about 950 C
corresponding to crystallization of the amorphous compound.
◦
The gallium end-member crystallises to KGaSi
2
O
6
above 800 C,
whereas compounds containing Al never fully crystallise but melt
◦
at 1200 C to an X-ray amorphous product. Thermal treatment
causes the unreacted silica in all these compounds to fully react,
2
9
as evidenced by the disappearance of the characteristic Si NMR
resonance at -108 ppm.
Acknowledgements
This work was partly funded by the MacDiarmid Institute
for Advanced Materials and Nanotechnology, and by contract
C08 ¥ 0302 from the NZ Foundation for Research Science
and Technology, as a subcontract from Industrial Research Ltd.
ATD acknowledges financial support from a William Georgetti
Scholarship, the Todd Foundation, a J.L Stewart Scholarship,
Freemasons NZ University Scholarship and a Curtis–Gordon
Scholarship. We are indebted to D. Flynn for assistance with the
electron microscopy.
Fig. 8 Thermogram of potassium gallium silicate inorganic polymer.
2
9
The Si NMR spectrum of the 50 : 50 Ga : Al silicate inorganic
◦
polymer fired at 1200 C (Fig. 4E) shows that all the unreacted
silica (the resonance at -108 ppm in the original compound) has
reacted, forming a single resonance, the position of which suggests
a change of Si(3Al) and Si(3Ga) in the original inorganic polymer
to Si(1Al) and Si(1Ga) in the fired product. The position of this
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