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A 27Al nuclear magnetic resonance study on the optimalization of the development of the Al13 polymer
]OURNA
Journal of Non-Crystalline Solids 142 (1992) 94-102 North-Holland
L OF
NON-CRYSTALLIN SOLIDS E
A 2~1 nuclear magnetic resonance study on the optimalization of the development of the A113 polymer * J.T. Kloprogge a, D. Seykens b, J.B.H. Jansen
a
and J.W. Geus c
a Institute for Earth Sciences, Department of Geochemistry, University of Utrecht, Budapestlaan 4, P.O. Box 80021, 3508 TA Utrecht, The Netherlands b NMR Spectroscopy, Department of Organic Chemistry, University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands c Department of Inorganic Chemistry, University of Utrecht, P.O. Box 80083, 3508 TB Utrecht, The Netherlands Received 26 June 1991 Revised manuscript received 12 November 1991
Synthesis conditions strongly influence the yield of the tridecameric polymer All3 ([AlO4Al12(OH)24(H20)12] 7+). All amount of 68% tridecamer was achieved by injection of alkali through a capillary tube into a 5 × 10 -2 M A1 solution at a rate of 0.015 m l / s up to an O H / A I ratio of 2.2. Dropwise addition of alkali yielded significantly less tridecameric polymer. During progressive hydrolysis the monomeric Al NMR resonance moved from 0.1 to 0.9 ppm and the linewidth increased from 37 to 112 Hz. Simultaneously the resonance at 63.3 ppm due to tridecameric fourfold coordinated Al changed by 0.02 ppm. During aging the tridecamer rearranged to polymers undetectable by NMR, due to loss of the tetrahedral symmetry of the central AI, which was deduced from the decrease in intensity and the broadening of the 63.3 ppm resonance. The formation of tetrahedral AI(OH)4, due to the inhomogeneous conditions at the point of base introduction, is essential for the synthesis of Al13. Aging over a period of 1 year caused a strong decrease in All3 concentration, which showed that A113 is a metastable polymer.
1. Introduction The
existence of the [A104Al12(OH)24 polymer (All3), first suggested by Johansson [1-4], has been a matter of debate among geochemists and soil chemists. In a favored model the polymer is composed of hexameric rings structurally similar to the rings in the structure of gibbsite (AI(OH) 3) [5,6]. The mineral Zunyite [ A l a 3 ( O H , F)16F2]SisO20C1 has been shown to have a structure with the tridecameric polymers as building blocks [7]. 27Al NMR investigations [8-13] and small angle X-ray scattering [14,15] have provided, finally, unequivocal evidence for the, probably metastable, existence of the All3 polymer. ( H 2 0 ) 1 2 ] 7+
* Publication of The Debye Institute, in cooperation with The Bijvoet Institute, University of Utrecht.
The Al13 polymer is of interest as an important pillaring agent in natural and synthetic smectites, e.g. refs. [16-20], which are used as molecular sieves or shape-selective catalysts. NMR studies have indicated that synthesis conditions, such as OH/A1 molar ratio, rate of neutralization, mixing conditions [12,21], and preparation temperature [22], are important for the genesis and yield of the tridecameric polymer. Also experimental conditions, such as, concentration, pH, and viscosity [23,24] influence the chemical shifts and linewidth of the 27Al NMR resonances of the monomeric and tridecameric species. Furthermore it has been established that the exact position of the NMR signals is to a limited extent dependent on the choice of reference solution and its concentration [23], magnetic field strength, and dilution with D20 [25]. The purpose of this investigation is to evaluate the effects on All3 polymer formation in 0.052M
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
J.T. Kloprogge et al. / Optimalization of the development of the All3 polymer
AI solutions as a function of the following factors: (i) variation of O H / A 1 molar ratio within the range 1.2 to 2.6, (ii) procedure of base addition, (iii) base injection rate, (iv) mixing rate, and (v) aging for a period of 1 year. The effects are assessed by measuring the chemical shifts and the linewidths of the 27A1 N M R resonances. The aim of this study is to optimize the Al13 yield for use as pillaring agent.
2. Experimental method 0.2M A1 stock solutions were p r e p a r e d by dissolving reagent grade A1C13.6H20 (Merck) or AI(NO3) 3. 9 H 2 0 (Merck) in deionized water. Dissolution of reagent grade N a O H (Merck) in deionized water under a N 2 atmosphere provided a 0.2M alkali stock solution. The amount of alkali solution required to provide the desired O H / A 1 molar ratio, ranging from 0.5 to 2.6 in intervals of 0.2, was either injected below the solution surface at various rates using a Gilson p u m p (capillary diameter 0.5 ram), or added dropwise (0.5 m l / d r o p ) to 250 ml of the AI solution, which was vigorously stirred in a vessel described by Vermeulen et al. [26]. The experiments were performed under N 2 atmosphere to exclude effects of carbonate ions. All solutions were adjusted to the same volume to get an identical final A1 concentration of 5.2 × 10-ZM. Additionally some A113 polymer solutions were p r e p a r e d using 0.5M AI(NO3) 3 and N a O H solutions. The A1 concen-
B0.0
60.0
~0.0
E0.0 PPM
95
trations in these solutions were 8.3 × 10 -2 and 15.6 × 10-2M. The 27A1 N M R spectra were recorded with a Bruker WP 200 spectrometer operating at 52.148 M H z (4.6 T) at the D e p a r t m e n t of Organic Chemistry of the University of Utrecht. For comparative purposes chemical shifts and linewidths of some spectra were recorded on a Bruker W M 500 spectrometer operating at 130.321 M H z (11.7 T) at the D e p a r t m e n t of Physical Chemistry, Faculty of Science, University of Nijmegen. An aluminum nitrate solution was used as standard reference with respect to which the chemical shift was measured. The accuracy in the chemical shift m e a s u r e m e n t s is +0.01 ppm. Before measurement the solutions were diluted with D 2 0 (1 : 1).
3. Results T h e 27A1 N M R spectra exhibited distinct resonances at approximately 0.1 and 63.3 p p m due to monomeric sixfold coordinated A1 and to the central fourfold coordinated A1 of the Al13 polymer, respectively (fig. 1). Relative concentrations were based on integrated intensities of the measured resonances. The exact position of both resonances depends on the degree of hydrolysis ( O H / A I molar ratio) and to a limited extent on the reference solution used. Using A12(804) 3 as a reference, 0.5M A1C13 displayed a resonance at - 0 . 1 8 p p m at 4.6 T and at - 0 . 1 9 p p m at 11.7 T, while 0.5M AI(NO3) 3
0.0
-Z0.0
- ~ 0 ~0
Fig. 1.27A1NMR spectra of A1 in 0.05M Al-nitrate solutions with OH/A1 molar ratios 112, 1.6, 2iO~ 2.4 and 2.6 (dropwise addition at 25°C, stirring rate 300 rpm).
J.T. Kloprogge et al. / Optimalization of the development of the All3 polymer
96
Table 1 Influence of the reference solution and its concentration, of the magnetic field strength and of dilution with D 2 0 on the chemical shift 6 Sample
Concentration (mol/1)
A12(804) 3
0.5
AIC13 AI(NO3) 3 Al(NO3) 3
0.5 0.5 0.052
6 (ppm) 4.6 T
6 (ppm) 11.7 T
0.00
0.00
- 0.18 -0.15 - 0.08
- 0.19 -0.15 - 0.10
D20/reference vol. ratio
6 (ppm) 4.6 T
1.000 0.333 0.176
0.02 0.07 0.10
showed a resonance at - 0 . 1 5 ppm at both 4.6 and 11.7 T, respectively. At the same field strengths a 5.2 × 10-~M A I ( N O 3 ) 3 exhibited resonances at - 0 . 8 and at - 0 . 1 0 ppm, respectively (table 1). Dilution with 50 vol.% D 2 0 causes a shift of 0.08 ppm to higher field as compared to dilution with 7.5 vol.% D20. Solutions with an O H / A 1 ratio increasing from 1.2 to 2.6 with an interval of 0.2 displayed a strong continuous decline in the amount of
100
monomeric A1 (figs. 1 and 2). The yield of All3 after addition of alkali up to an O H / A 1 molar ratio of 2.2 displayed a maximum of 56.5% for dropwise addition and 68.0% for injection at a rate of 0.015 m l / s . At O H / A 1 molar ratios higher than 2.2, considerable decreases in the amounts of monomeric A1 and All3 were observed. The amount of undetectable A1 rose more strongly during injection than during dropwise addition. During dropwise addition a faint white amorphous precipitate frequently developed, which redissolved within 12 h of aging. Dropwise neutralization resulted in a less pronounced All3 maximum and a higher monomer content than continuous injection at 0.015 m l / s . Upon progressive hydrolysis a small shift to lower field was observed for the sixfold and the fourfold coordinated A1 resonance (figs. 3 and 4). With the O H / A 1 molar ratio increasing from 1.2 to 2.4 the resonance of sixfold coordinated A1 shifted parabolically from about 0.07 to 0.81 ppm for solutions with A1 concentrations of 5.2 × 10-2M and 8.3 × 10-2M. A solution of an A1 concentration of 15.6 × 10-2M exhibited a linear shift from 0.09 to 0.15 ppm over the same range of O H / A 1 molar ratios, while the linewidth increased from 37 to 112 Hz (fig. 5). The change in
100
dropwise base addition
injected 0.015 ml/s
2
* AI monomer o AI13 polymer 0 undetectoble AI
* monomer o ~ 1 3 polymer
0 undetectoble AI
/ /
80
80
(a)
(b)
,
/
60
/
/
/ -6 E ._c
/
I
60
40
40
20
20
8
0
0 1.0
1.4
1.8
2.2
2.6
1.0
1.4
1.8
2.2
2.6
0H/AI tool ratio Fig. 2. T h e percentages of m o n o m e r AI, All3 and undetectable A1 as analyzed with 27A1 N M R in 0.05M Al-nitrate solution as function of O H / A 1 molar ratio: (a) dropwise N a O H addition at 25°C, stirring rate 300 rpm, (b) N a O H injection, rate 0.015 m l / s (25°C, stirring rate 300 rpm).
Z T. Kloprogge et aL / Optimalization of the development of the All3 polymer
97
115
0.9o AI = 0.052 M • AI = 0.083 M
0.8
105
0.7
0
0.6
-
95"~" 8 5 -
0.5cO
750.4-
i
65-
0,355-
0.2-
45-
0.10.0
1.0
~
,
11,
l
,.8
35
212
.0
1;4
OH/AI rnol ratio
Fig. 3. The influence of O H / A 1 molar ratio on the chemical shift of the monomeric sixfold coordinated A1.
chemical shift for fourfold coordinated A1 amounted _+0.02 ppm, which is much smaller than the shift observed for sixfold coordinated A1. No change in linewidth was observed. The amount of All3 is found to be a function of the injection rate of alkali, while at an O H / A 1 molar ratio of 1.2 the maximum All3 yield was
' 118 ' OH/AI real ratio
212
'
Fig. 5. The variation with O H / A 1 molar ratio of the linewidth at half height of the monomeric sixfold coordinated AI in solution with AI concentration of 0.052M.
obtained at a rate o f injection of 0.020 m l / s , whereas the maximum All3 yield at an O H / A 1 molar ratio of 2.2 was obtained at an injection rate of 0.10 m l / s (fig. 6). At injection rates above 0.035 m l / s , the solution became transiently
63.34 o AI = 0.052 M = At = 0.083 bt
80-
63.32 -
~ . ~ .
70
_____~
= ~ r - - - ~
O H / A I mol ratio o = 1.2 a = 1.6 = 2.0
0
=
2.2
---~
" E6 3 . 3 0 13L
<'-m 60
cO 63.28 -
t,
o
-~ 50 63.26 -
63.24
4.°
1.8
2'.2
OH/AI rnol ratio
Fig. 4. T h e influence of O H / A I molar ratio on the chemical shift of the central fourfold coordinated A1 from the A l l 3 complex.
30 - - ~ 0.005 0.010
l ~ T 0.015 0.020 0.025 NoOH injection (ml/~ec)
T 0.030
0.035
Fig. 6. T h e A l l 3 concentration in 0.05M Al-nitrate solution as function of base injection rate (25°C, stirring rate 300 rpm).
J.T. Kloprogge et aL / Optimalization of the development of the All3 polymer
98
Table 2 Concentrations of monomeric A1, All3 and undetectable AI in a 0.05M AI nitrate solution. The solution was neutralized to an OH/A1 molar ratio of 2.4 as function of the stirring rate (dropwise addition at 25°C) Stirring rate (rpm)
AI monomer (%)
All3 polymer (%)
Undetect. A1 (%)
120 180 240 300
3.7 4.0 4.4 4.0
48.0 49.8 52.0 53.7
48.3 46.2 43.6 42.3
cloudy, due to the formation of a colloidal, amorphous precipitate. The stirring rate during neutralization of AI(NO3) 3 also affected the formation of All3 and monomeric A1 (table 2). At stirring rates lower than 210 rpm, precipitation was observed. At
3.9-
higher stirring rates, the solutions remained clear. The NMR spectra displayed an increase of All3 content from 48.0 to 53.7% without any change in the monomeric resonance upon increasing the stirring rate from 120 to 300 rpm. The amount of undetectable AI decreased simultaneously from 48.3 to 42.3%. After aging for 1 year the NMR spectra of a solution of an OH/A1 ratio of 2.4 showed no All3 and 0.49% of monomeric A1. The pH diminished asymptotically from 4.22 to 3.83. The solutions of O H / A I molar ratios 1.8 and 1.2 decreased in pH from 3.93 and 3.80 to 3.65 and 3.41, respectively. NMR spectra of a solution of an OH/A1 molar ratio of 1.2 taken at aging periods of 176 and 355 days displayed a decrease of All3 from 42.9 to 13.2% and of monomeric A1 from 54.9 to 45.5% (fig. 7). The linewidth of the 63.2 ppm resonance broadened from 20.8 to 41.1 Hz (accuracy 10%), indicating a decrease in the symmetry of the fourfold coordinated A1 in the All3 polymer upon aging.
3.7-
4. D i s c u s s i o n
3.5
4.1. Reference chemical shift 3.3-
OH/A[ rotlo = 1.2
*
70-
monomer
[
undetectable AI 6O-
J
50-
I
40
// i
3020-
...IV
100
0
50
100
~ 150 200 250 Time (days)
300
350
400
Fig. 7. The decrease of pH and corresponding changes in concentration of monomeric A1, All3 and undetectable A1 as function of aging time. The concentrations are analyzed with NMR in a partially neutralized A1 nitrate solution with OH/A1 molar ratio of 1.2.
Most studies performed so far have been based on the assumption that the chemical shift of the reference does not depend on the concentration and the A1 salt used. Recent recordings by Akitt and Elders [23] as well as the present results have demonstrated that the chemical shift varies to a small extent. Dilute aluminum chloride or aluminum perchlorate solutions are assumed to be the most appropriate references. The 5.2 × 10-2M aluminum nitrate reference used in this study, instead of aluminum chloride, exhibits a resonance shift of approximately 0.10 ppm to high field in comparison with aluminum chloride. The chemical shift at a high magnetic field strength of 11.7 T is increased by another 0.10 ppm to high field as compared to the shift at 4.6 T. The dilution with D 20 instead of H 2 0 [27] finally shifts the resonance by about 0.08 ppm. The same effect has been observed on organosilanes and siloxanes with 29Si NMR [25]. The
J.T. Kloprogge et aL / Optimalization of the development of the All3 polymer
effect of the anion, the concentration, the magnetic field strength, and the dilution with D20 can account for the chemical shift of the six and fourfold coordinated A1 measured in this work differing from the shifts published in most papers concerning A1 hydrolysis (table 3).
4.2. Relation between O H / A I molar ratio and chemical shift and linewidth The shift of a resonance can be explained by a change in magnetic susceptibility and ionic strength. In our solutions the ionic strength increases during hydrolysis due to the addition of NaOH. The small shift for the fourfold coordinated AI resonance is due to the shielding by the 12 sixfold coordinated AI ions, forming a cage-like structure around the central fourfold coordinated AI in the Al13 complex (Keggin structure). Akitt et al. [28] pointed out that the linewidth of quadrupolar nuclei is directly proportional to the bulk viscosity of the solution. Upon hydrolysis the NaOH concentration increases, while the AI nitrate concentration remains the same in all solutions. The ratio of the viscosity of the NaOH solution and that of pure water increases from 1.000 to approximately 1.025, if no polymerization takes place. The polymerization and aggregation of Al13 polymers [29] may increase the viscosity even more. The linewidth of the monomer resonance is not only influenced by the viscosity of the solution but also by the pH [24]. The increase in linewidth observed during forced hydrolysis may be partly explained by the reaction [Al(H20)6] 3 = [AI(H20)5(OH)] 2+ + [H] +
99
Assuming that this is the only effective reaction, the increase in linewidth can be calculated with the expression of Akitt and Eiders [24], resulting in a broadening of 17.5 Hz. This is only a fraction of the observed linewidth, viz. 37 to 112 Hz, which indicates that this reaction can only play a minor r61e.
4.3. Quadrupole relaxation The isotropic chemical shifts and linewidths of resonances from quadrupolar nuclei such as A1 are largely influenced by the magnitude of the quadrupole coupling constant, e2qQ/h. Deviations introduced by the field-dependent secondorder quadrupole-induced shift from the isotropic value, can be calculated according to the following relation [30]: 6(ppm) = 6(1 + ~72/3)(e2qQ/hvo) 2 × 103,
(l)
where v 0 is the Larmor frequency, e 2 q Q / h the nuclear quadrupole coupling constant and ~7 the asymmetry parameter. In solutions under conditions of rapid, isotropic tumbling the spin-lattice (T 1) and spin-spin (T2) relaxation times for 27A1 are given by:
1 / T 1 = 1 / T 2 = (12ar2/125)(1 + r/2/3) 2
2
× (e q Q / h ) %,
(2)
where ~'c is the rotational correlation time. The linewidth at half height provides the opportunity to calculate T2 with the relation T2 = l/,rru1/2, where vl/2 is the linewidth at half-height in Hz. The fourfold coordinated A1 at 63.3 ppm has at half-height a linewidth of 18.2 Hz, which results
Table 3 Variation in chemical shift 8 as published in previous articles concerning All3 Authors
Frequency (MHz)
Reference
6(AI vl) (ppm)
8(AI TM) (ppm)
Kloprogge et al., 1991 Akitt and Farthing, 1978 Akitt and Elders, 1988 Bertsch et al., 1986b Bottero et al., 1980 Denney and Hsu, 1986 Thompson et al., 1987
52.142 23.45 104.2 52.1 23.45 20.727 130.3
AI(NO3) 3 A1C13 AI(OD) 4 A1 Fisher AI(OH) 4 AI(OH) 4 AIC13
0.1-0.9
63.3 62.5 62.5 62.5 63 63 62.8
0 0 0.1 0 0
100
J.T. Kloprogge et al. / Optimalization of the development of the All3 polymer
in a T 2 of 1.75 × 10 -2 s. This spin-spin relaxation time is of the same order of magnitude as the value reported by Thompson et al. [30]. Substitution in expression (2) using rc = 1.3 × 10-10 s [30] and assuming r / = 0 , gives for e2qQ/h a value of 0.68 MHz. Deviation from the isotropic value can be derived by substitution of these values in eq. (1). The calculated result is a shift of 1 ppm from the isotropic value. The T 2 value for monomeric A1 is calculated to be approximately 0.9 × 10 -2 s for an O H / A 1 molar ratio of 1.2 and changes to 0 . 3 × 10 .2 for an O H / A 1 molar ratio of 2.2. The T 1 values of 5 . 3 × 10 .2 s for fourfold coordinated A1 and 1.2 × 10 .2 for sixfold coordinated A1 reported by Bertsch et al. [13] for an O H / A 1 molar, ratio of 2.25 are much higher than the values in this. paper and those reported by Thompson et al. [30].
4.4. Relation between OH / AI molar ratio and All 3 concentration Bottero e t a l . [11] and Bertsch et al. [12,13] have studied the effect of the O H / A 1 molar ratio on the development of All3 complexes. The results of the present study, in which solutions of a final A1 concentration of 5.2 × 10 .2 and 8.3 × 10-2M were investigated, are comparable With those of Bertsch et al. [12,13,21]. The latter authors had a final AI concentration of 3.34 × 10 2M. The amorphous precipitate formed during dropwise addition does not contribute to the formation of Al13 during neutralization Or aging, causing a lower Al13 yield as compared to the alkali injection experiments. The optimal All3 yield of 68.9% is reached at an O H / A 1 molar ratio of 2.2, which is significantly lower than the theoretical effective degree of hydrolysis, 2.46 (32 O H / 1 3 A1), based o n [Al13(OH)32] 7+ [9]. Bottero et al. [11,15] obtained with a 1 0 × 10-2M A1 solution an optimum All3 yield of 95.8% at an O H / A I molar ratio of 2.1, which is 0.1 lower than the O H / A 1 molar ratio at which we observed the maximum yield. Bertsch et al. [12,13] observed a maximum All3 concentration at an O H / A 1 molar ratio of approximately 2.25 with a 0.03 × 10-2M A1 (69.7%) and a 3 . 3 4 × 10-2M A1 (68.6%) solution. Akitt and Elders [31] deter-
mined a maximum Al13 concentration of 100% using 50-80 × 10-2M A1 salt solutions rapidly neutralized at 90°C to an O H / A 1 molar ratio of 2.5. This agrees with unpublished data from this laboratory.
4.5. Alkali solution injection and mixing conditions The Al13 yield is a function of the base injection rate, which has an optimum depending on the O H / A 1 molar ratio. The optimum injection rate shifts from 0.020 m l / s for solutions with O H / A 1 molar ratio = 1.2 to approximately 0.010 m l / s for solutions with O H / A 1 molar ratio 2.2. At higher rates the fraction of undetectable A1 rapidly increased and the solution became cloudy. Bertsch et al. [12] and Bottero et al. [11] suggested that at high injection rates more AI(OH)2 is generated than can be consumed by the formation of Al13. The excess AI(OH) 4 re-equilibrates with the bulk solution resulting in colloidal or precipitated AI(OH) 3. At addition rates below 0.010 m l / s amounts of AI(OH) 4 created are too small and re-equilibrate with the solution before Al13 can be formed, which represents the other limitation. An alternative is a series of sequential reactions during the slow injection of the alkali solution
[AI(H20)6] 3+-~ [AI(H20)5(OH)] 2+ .
[Al(H20)4(OH)2
] + - - All3.
It is assumed by several authors [10,21] that AI(OH) 4 formed at the point of alkali introduction is needed as a precursor for the formation of All3. Akitt and Farthing [10] hydrolyzed A13+ via solid Na2CO 3. They assumed a very rapid AI(OH) 4 production under the inhomogeneous conditions at the interface of solid Na2CO 3 and the solution. As discussed above several equilibria exist in partly neutralized A1 solutions, all depending on the rate of alkali injection. Low O H / A 1 molar ratios and slow injection favors the formation of mono- and dimers. Fast injection favors precipitation instead. Slow stirring rates causes the AI(OH) 4 to re-equilibrate with the acid solution. The All3 and monomer concentrations are similar up to an O H / A 1 molar ratio of
J.T. Kloprogge et a L / Optimalization of the development of the All3 polymer
2.6 for the neutralization reaction of AIC13 and AI(NO3) 3 solutions, as confirmed by Akitt and Farthing [10].
10l
at Nijmegen. W. Veeman, A.M.J. van der Eerden, P. Buining and J. Van Beek are thanked for critically reviewing this article.
4.6. Aging The decrease of the Al13 content upon aging confirms that the A113 is actually a metastable complex in solution. The Al13 polymer rearranges with time into large polymers unobservable with the NMR technique. Formation of other polymers is evident from the large increase of the amount of undetectable A1, the disappearance of Al13, and the decrease of monomeric A1 in the solution. Several authors have suggested that these polymers consists of hexameric rings [12,13,15,32]. The constant pH upon aging suggests a structural rearrangement without changing the degree of hydrolysis of the polymer [33]. Small angle X-ray scattering data suggest that the alumina tetrahedra present in Al13 are squeezed between growing octahedral layers and that tetrahedra are totally lost at the end of the aging process [15,29]. The broadening of the 63.3 ppm resonance upon aging, indicating a decrease in tetrahedral symmetry, support this structural rearrangement.
5. Conclusions
(1) Chemical shifts are influenced by (i) the choice of reference solution and its concentration, (ii) magnetic field strength and (iii) dilution with D20. (2) The increasing chemical shift and linewidth of the monomer resonance upon hydrolysis is mainly caused by increasing ionic strength and viscosity. (3) The maximum amount of All3 formed is influenced by the hydrolysis conditions, such as OH/A1 molar ratio, injection rate and mixing conditions. (4) The All3 rearranges into large polymers with a hexameric ring structure upon aging. The authors wish to thank G. Nachtegaal for the technical assistance at the HF-NMR facility
References [1] [2] [3] [4l [5]
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J.T. Kloprogge et aL / Optimalization of the development of the All3 polymer
[27] J,W. Akitt, Prog. Nucl. Magn. Resort. Spectrosc. 21 (1989) 1. [28] J,W. Akitt, W. Gessner and M. Weinberger, Magn. Reson. Chem. 26 (1988) 1047. [29] J,Y. Bottero, M. Axelos, D. Tchoubar, J.M. Cases, J.J. Fripiat and F. Fiessinger, J. Coll. Interf. Sci. 117 (1987) 47.
[30] A.R. Thompson, A.C. Kunwar, H.S. Gutowsky and E. Oldfield, J. Chem. Soc. Dalton Trans. (1987) 2317. [31] J.W. Akitt and J.M. Elders, J. Chem. Soc. Dalton Trans. (1988) 1347. [32] D.Z. Denney and P.H. Hsu, Clays Clay Minerals 34 (1986) 604. [33] P.P. Tsai and P.H. Hsu, Soil Sei. Soc. Am. J. 48 (1984) 59.
Journal of Non-Crystalline Solids 142 (1992) 94-102 North-Holland
L OF
NON-CRYSTALLIN SOLIDS E
A 2~1 nuclear magnetic resonance study on the optimalization of the development of the A113 polymer * J.T. Kloprogge a, D. Seykens b, J.B.H. Jansen
a
and J.W. Geus c
a Institute for Earth Sciences, Department of Geochemistry, University of Utrecht, Budapestlaan 4, P.O. Box 80021, 3508 TA Utrecht, The Netherlands b NMR Spectroscopy, Department of Organic Chemistry, University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands c Department of Inorganic Chemistry, University of Utrecht, P.O. Box 80083, 3508 TB Utrecht, The Netherlands Received 26 June 1991 Revised manuscript received 12 November 1991
Synthesis conditions strongly influence the yield of the tridecameric polymer All3 ([AlO4Al12(OH)24(H20)12] 7+). All amount of 68% tridecamer was achieved by injection of alkali through a capillary tube into a 5 × 10 -2 M A1 solution at a rate of 0.015 m l / s up to an O H / A I ratio of 2.2. Dropwise addition of alkali yielded significantly less tridecameric polymer. During progressive hydrolysis the monomeric Al NMR resonance moved from 0.1 to 0.9 ppm and the linewidth increased from 37 to 112 Hz. Simultaneously the resonance at 63.3 ppm due to tridecameric fourfold coordinated Al changed by 0.02 ppm. During aging the tridecamer rearranged to polymers undetectable by NMR, due to loss of the tetrahedral symmetry of the central AI, which was deduced from the decrease in intensity and the broadening of the 63.3 ppm resonance. The formation of tetrahedral AI(OH)4, due to the inhomogeneous conditions at the point of base introduction, is essential for the synthesis of Al13. Aging over a period of 1 year caused a strong decrease in All3 concentration, which showed that A113 is a metastable polymer.
1. Introduction The
existence of the [A104Al12(OH)24 polymer (All3), first suggested by Johansson [1-4], has been a matter of debate among geochemists and soil chemists. In a favored model the polymer is composed of hexameric rings structurally similar to the rings in the structure of gibbsite (AI(OH) 3) [5,6]. The mineral Zunyite [ A l a 3 ( O H , F)16F2]SisO20C1 has been shown to have a structure with the tridecameric polymers as building blocks [7]. 27Al NMR investigations [8-13] and small angle X-ray scattering [14,15] have provided, finally, unequivocal evidence for the, probably metastable, existence of the All3 polymer. ( H 2 0 ) 1 2 ] 7+
* Publication of The Debye Institute, in cooperation with The Bijvoet Institute, University of Utrecht.
The Al13 polymer is of interest as an important pillaring agent in natural and synthetic smectites, e.g. refs. [16-20], which are used as molecular sieves or shape-selective catalysts. NMR studies have indicated that synthesis conditions, such as OH/A1 molar ratio, rate of neutralization, mixing conditions [12,21], and preparation temperature [22], are important for the genesis and yield of the tridecameric polymer. Also experimental conditions, such as, concentration, pH, and viscosity [23,24] influence the chemical shifts and linewidth of the 27Al NMR resonances of the monomeric and tridecameric species. Furthermore it has been established that the exact position of the NMR signals is to a limited extent dependent on the choice of reference solution and its concentration [23], magnetic field strength, and dilution with D20 [25]. The purpose of this investigation is to evaluate the effects on All3 polymer formation in 0.052M
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
J.T. Kloprogge et al. / Optimalization of the development of the All3 polymer
AI solutions as a function of the following factors: (i) variation of O H / A 1 molar ratio within the range 1.2 to 2.6, (ii) procedure of base addition, (iii) base injection rate, (iv) mixing rate, and (v) aging for a period of 1 year. The effects are assessed by measuring the chemical shifts and the linewidths of the 27A1 N M R resonances. The aim of this study is to optimize the Al13 yield for use as pillaring agent.
2. Experimental method 0.2M A1 stock solutions were p r e p a r e d by dissolving reagent grade A1C13.6H20 (Merck) or AI(NO3) 3. 9 H 2 0 (Merck) in deionized water. Dissolution of reagent grade N a O H (Merck) in deionized water under a N 2 atmosphere provided a 0.2M alkali stock solution. The amount of alkali solution required to provide the desired O H / A 1 molar ratio, ranging from 0.5 to 2.6 in intervals of 0.2, was either injected below the solution surface at various rates using a Gilson p u m p (capillary diameter 0.5 ram), or added dropwise (0.5 m l / d r o p ) to 250 ml of the AI solution, which was vigorously stirred in a vessel described by Vermeulen et al. [26]. The experiments were performed under N 2 atmosphere to exclude effects of carbonate ions. All solutions were adjusted to the same volume to get an identical final A1 concentration of 5.2 × 10-ZM. Additionally some A113 polymer solutions were p r e p a r e d using 0.5M AI(NO3) 3 and N a O H solutions. The A1 concen-
B0.0
60.0
~0.0
E0.0 PPM
95
trations in these solutions were 8.3 × 10 -2 and 15.6 × 10-2M. The 27A1 N M R spectra were recorded with a Bruker WP 200 spectrometer operating at 52.148 M H z (4.6 T) at the D e p a r t m e n t of Organic Chemistry of the University of Utrecht. For comparative purposes chemical shifts and linewidths of some spectra were recorded on a Bruker W M 500 spectrometer operating at 130.321 M H z (11.7 T) at the D e p a r t m e n t of Physical Chemistry, Faculty of Science, University of Nijmegen. An aluminum nitrate solution was used as standard reference with respect to which the chemical shift was measured. The accuracy in the chemical shift m e a s u r e m e n t s is +0.01 ppm. Before measurement the solutions were diluted with D 2 0 (1 : 1).
3. Results T h e 27A1 N M R spectra exhibited distinct resonances at approximately 0.1 and 63.3 p p m due to monomeric sixfold coordinated A1 and to the central fourfold coordinated A1 of the Al13 polymer, respectively (fig. 1). Relative concentrations were based on integrated intensities of the measured resonances. The exact position of both resonances depends on the degree of hydrolysis ( O H / A I molar ratio) and to a limited extent on the reference solution used. Using A12(804) 3 as a reference, 0.5M A1C13 displayed a resonance at - 0 . 1 8 p p m at 4.6 T and at - 0 . 1 9 p p m at 11.7 T, while 0.5M AI(NO3) 3
0.0
-Z0.0
- ~ 0 ~0
Fig. 1.27A1NMR spectra of A1 in 0.05M Al-nitrate solutions with OH/A1 molar ratios 112, 1.6, 2iO~ 2.4 and 2.6 (dropwise addition at 25°C, stirring rate 300 rpm).
J.T. Kloprogge et al. / Optimalization of the development of the All3 polymer
96
Table 1 Influence of the reference solution and its concentration, of the magnetic field strength and of dilution with D 2 0 on the chemical shift 6 Sample
Concentration (mol/1)
A12(804) 3
0.5
AIC13 AI(NO3) 3 Al(NO3) 3
0.5 0.5 0.052
6 (ppm) 4.6 T
6 (ppm) 11.7 T
0.00
0.00
- 0.18 -0.15 - 0.08
- 0.19 -0.15 - 0.10
D20/reference vol. ratio
6 (ppm) 4.6 T
1.000 0.333 0.176
0.02 0.07 0.10
showed a resonance at - 0 . 1 5 ppm at both 4.6 and 11.7 T, respectively. At the same field strengths a 5.2 × 10-~M A I ( N O 3 ) 3 exhibited resonances at - 0 . 8 and at - 0 . 1 0 ppm, respectively (table 1). Dilution with 50 vol.% D 2 0 causes a shift of 0.08 ppm to higher field as compared to dilution with 7.5 vol.% D20. Solutions with an O H / A 1 ratio increasing from 1.2 to 2.6 with an interval of 0.2 displayed a strong continuous decline in the amount of
100
monomeric A1 (figs. 1 and 2). The yield of All3 after addition of alkali up to an O H / A 1 molar ratio of 2.2 displayed a maximum of 56.5% for dropwise addition and 68.0% for injection at a rate of 0.015 m l / s . At O H / A 1 molar ratios higher than 2.2, considerable decreases in the amounts of monomeric A1 and All3 were observed. The amount of undetectable A1 rose more strongly during injection than during dropwise addition. During dropwise addition a faint white amorphous precipitate frequently developed, which redissolved within 12 h of aging. Dropwise neutralization resulted in a less pronounced All3 maximum and a higher monomer content than continuous injection at 0.015 m l / s . Upon progressive hydrolysis a small shift to lower field was observed for the sixfold and the fourfold coordinated A1 resonance (figs. 3 and 4). With the O H / A 1 molar ratio increasing from 1.2 to 2.4 the resonance of sixfold coordinated A1 shifted parabolically from about 0.07 to 0.81 ppm for solutions with A1 concentrations of 5.2 × 10-2M and 8.3 × 10-2M. A solution of an A1 concentration of 15.6 × 10-2M exhibited a linear shift from 0.09 to 0.15 ppm over the same range of O H / A 1 molar ratios, while the linewidth increased from 37 to 112 Hz (fig. 5). The change in
100
dropwise base addition
injected 0.015 ml/s
2
* AI monomer o AI13 polymer 0 undetectoble AI
* monomer o ~ 1 3 polymer
0 undetectoble AI
/ /
80
80
(a)
(b)
,
/
60
/
/
/ -6 E ._c
/
I
60
40
40
20
20
8
0
0 1.0
1.4
1.8
2.2
2.6
1.0
1.4
1.8
2.2
2.6
0H/AI tool ratio Fig. 2. T h e percentages of m o n o m e r AI, All3 and undetectable A1 as analyzed with 27A1 N M R in 0.05M Al-nitrate solution as function of O H / A 1 molar ratio: (a) dropwise N a O H addition at 25°C, stirring rate 300 rpm, (b) N a O H injection, rate 0.015 m l / s (25°C, stirring rate 300 rpm).
Z T. Kloprogge et aL / Optimalization of the development of the All3 polymer
97
115
0.9o AI = 0.052 M • AI = 0.083 M
0.8
105
0.7
0
0.6
-
95"~" 8 5 -
0.5cO
750.4-
i
65-
0,355-
0.2-
45-
0.10.0
1.0
~
,
11,
l
,.8
35
212
.0
1;4
OH/AI rnol ratio
Fig. 3. The influence of O H / A 1 molar ratio on the chemical shift of the monomeric sixfold coordinated A1.
chemical shift for fourfold coordinated A1 amounted _+0.02 ppm, which is much smaller than the shift observed for sixfold coordinated A1. No change in linewidth was observed. The amount of All3 is found to be a function of the injection rate of alkali, while at an O H / A 1 molar ratio of 1.2 the maximum All3 yield was
' 118 ' OH/AI real ratio
212
'
Fig. 5. The variation with O H / A 1 molar ratio of the linewidth at half height of the monomeric sixfold coordinated AI in solution with AI concentration of 0.052M.
obtained at a rate o f injection of 0.020 m l / s , whereas the maximum All3 yield at an O H / A 1 molar ratio of 2.2 was obtained at an injection rate of 0.10 m l / s (fig. 6). At injection rates above 0.035 m l / s , the solution became transiently
63.34 o AI = 0.052 M = At = 0.083 bt
80-
63.32 -
~ . ~ .
70
_____~
= ~ r - - - ~
O H / A I mol ratio o = 1.2 a = 1.6 = 2.0
0
=
2.2
---~
" E6 3 . 3 0 13L
<'-m 60
cO 63.28 -
t,
o
-~ 50 63.26 -
63.24
4.°
1.8
2'.2
OH/AI rnol ratio
Fig. 4. T h e influence of O H / A I molar ratio on the chemical shift of the central fourfold coordinated A1 from the A l l 3 complex.
30 - - ~ 0.005 0.010
l ~ T 0.015 0.020 0.025 NoOH injection (ml/~ec)
T 0.030
0.035
Fig. 6. T h e A l l 3 concentration in 0.05M Al-nitrate solution as function of base injection rate (25°C, stirring rate 300 rpm).
J.T. Kloprogge et aL / Optimalization of the development of the All3 polymer
98
Table 2 Concentrations of monomeric A1, All3 and undetectable AI in a 0.05M AI nitrate solution. The solution was neutralized to an OH/A1 molar ratio of 2.4 as function of the stirring rate (dropwise addition at 25°C) Stirring rate (rpm)
AI monomer (%)
All3 polymer (%)
Undetect. A1 (%)
120 180 240 300
3.7 4.0 4.4 4.0
48.0 49.8 52.0 53.7
48.3 46.2 43.6 42.3
cloudy, due to the formation of a colloidal, amorphous precipitate. The stirring rate during neutralization of AI(NO3) 3 also affected the formation of All3 and monomeric A1 (table 2). At stirring rates lower than 210 rpm, precipitation was observed. At
3.9-
higher stirring rates, the solutions remained clear. The NMR spectra displayed an increase of All3 content from 48.0 to 53.7% without any change in the monomeric resonance upon increasing the stirring rate from 120 to 300 rpm. The amount of undetectable AI decreased simultaneously from 48.3 to 42.3%. After aging for 1 year the NMR spectra of a solution of an OH/A1 ratio of 2.4 showed no All3 and 0.49% of monomeric A1. The pH diminished asymptotically from 4.22 to 3.83. The solutions of O H / A I molar ratios 1.8 and 1.2 decreased in pH from 3.93 and 3.80 to 3.65 and 3.41, respectively. NMR spectra of a solution of an OH/A1 molar ratio of 1.2 taken at aging periods of 176 and 355 days displayed a decrease of All3 from 42.9 to 13.2% and of monomeric A1 from 54.9 to 45.5% (fig. 7). The linewidth of the 63.2 ppm resonance broadened from 20.8 to 41.1 Hz (accuracy 10%), indicating a decrease in the symmetry of the fourfold coordinated A1 in the All3 polymer upon aging.
3.7-
4. D i s c u s s i o n
3.5
4.1. Reference chemical shift 3.3-
OH/A[ rotlo = 1.2
*
70-
monomer
[
undetectable AI 6O-
J
50-
I
40
// i
3020-
...IV
100
0
50
100
~ 150 200 250 Time (days)
300
350
400
Fig. 7. The decrease of pH and corresponding changes in concentration of monomeric A1, All3 and undetectable A1 as function of aging time. The concentrations are analyzed with NMR in a partially neutralized A1 nitrate solution with OH/A1 molar ratio of 1.2.
Most studies performed so far have been based on the assumption that the chemical shift of the reference does not depend on the concentration and the A1 salt used. Recent recordings by Akitt and Elders [23] as well as the present results have demonstrated that the chemical shift varies to a small extent. Dilute aluminum chloride or aluminum perchlorate solutions are assumed to be the most appropriate references. The 5.2 × 10-2M aluminum nitrate reference used in this study, instead of aluminum chloride, exhibits a resonance shift of approximately 0.10 ppm to high field in comparison with aluminum chloride. The chemical shift at a high magnetic field strength of 11.7 T is increased by another 0.10 ppm to high field as compared to the shift at 4.6 T. The dilution with D 20 instead of H 2 0 [27] finally shifts the resonance by about 0.08 ppm. The same effect has been observed on organosilanes and siloxanes with 29Si NMR [25]. The
J.T. Kloprogge et aL / Optimalization of the development of the All3 polymer
effect of the anion, the concentration, the magnetic field strength, and the dilution with D20 can account for the chemical shift of the six and fourfold coordinated A1 measured in this work differing from the shifts published in most papers concerning A1 hydrolysis (table 3).
4.2. Relation between O H / A I molar ratio and chemical shift and linewidth The shift of a resonance can be explained by a change in magnetic susceptibility and ionic strength. In our solutions the ionic strength increases during hydrolysis due to the addition of NaOH. The small shift for the fourfold coordinated AI resonance is due to the shielding by the 12 sixfold coordinated AI ions, forming a cage-like structure around the central fourfold coordinated AI in the Al13 complex (Keggin structure). Akitt et al. [28] pointed out that the linewidth of quadrupolar nuclei is directly proportional to the bulk viscosity of the solution. Upon hydrolysis the NaOH concentration increases, while the AI nitrate concentration remains the same in all solutions. The ratio of the viscosity of the NaOH solution and that of pure water increases from 1.000 to approximately 1.025, if no polymerization takes place. The polymerization and aggregation of Al13 polymers [29] may increase the viscosity even more. The linewidth of the monomer resonance is not only influenced by the viscosity of the solution but also by the pH [24]. The increase in linewidth observed during forced hydrolysis may be partly explained by the reaction [Al(H20)6] 3 = [AI(H20)5(OH)] 2+ + [H] +
99
Assuming that this is the only effective reaction, the increase in linewidth can be calculated with the expression of Akitt and Eiders [24], resulting in a broadening of 17.5 Hz. This is only a fraction of the observed linewidth, viz. 37 to 112 Hz, which indicates that this reaction can only play a minor r61e.
4.3. Quadrupole relaxation The isotropic chemical shifts and linewidths of resonances from quadrupolar nuclei such as A1 are largely influenced by the magnitude of the quadrupole coupling constant, e2qQ/h. Deviations introduced by the field-dependent secondorder quadrupole-induced shift from the isotropic value, can be calculated according to the following relation [30]: 6(ppm) = 6(1 + ~72/3)(e2qQ/hvo) 2 × 103,
(l)
where v 0 is the Larmor frequency, e 2 q Q / h the nuclear quadrupole coupling constant and ~7 the asymmetry parameter. In solutions under conditions of rapid, isotropic tumbling the spin-lattice (T 1) and spin-spin (T2) relaxation times for 27A1 are given by:
1 / T 1 = 1 / T 2 = (12ar2/125)(1 + r/2/3) 2
2
× (e q Q / h ) %,
(2)
where ~'c is the rotational correlation time. The linewidth at half height provides the opportunity to calculate T2 with the relation T2 = l/,rru1/2, where vl/2 is the linewidth at half-height in Hz. The fourfold coordinated A1 at 63.3 ppm has at half-height a linewidth of 18.2 Hz, which results
Table 3 Variation in chemical shift 8 as published in previous articles concerning All3 Authors
Frequency (MHz)
Reference
6(AI vl) (ppm)
8(AI TM) (ppm)
Kloprogge et al., 1991 Akitt and Farthing, 1978 Akitt and Elders, 1988 Bertsch et al., 1986b Bottero et al., 1980 Denney and Hsu, 1986 Thompson et al., 1987
52.142 23.45 104.2 52.1 23.45 20.727 130.3
AI(NO3) 3 A1C13 AI(OD) 4 A1 Fisher AI(OH) 4 AI(OH) 4 AIC13
0.1-0.9
63.3 62.5 62.5 62.5 63 63 62.8
0 0 0.1 0 0
100
J.T. Kloprogge et al. / Optimalization of the development of the All3 polymer
in a T 2 of 1.75 × 10 -2 s. This spin-spin relaxation time is of the same order of magnitude as the value reported by Thompson et al. [30]. Substitution in expression (2) using rc = 1.3 × 10-10 s [30] and assuming r / = 0 , gives for e2qQ/h a value of 0.68 MHz. Deviation from the isotropic value can be derived by substitution of these values in eq. (1). The calculated result is a shift of 1 ppm from the isotropic value. The T 2 value for monomeric A1 is calculated to be approximately 0.9 × 10 -2 s for an O H / A 1 molar ratio of 1.2 and changes to 0 . 3 × 10 .2 for an O H / A 1 molar ratio of 2.2. The T 1 values of 5 . 3 × 10 .2 s for fourfold coordinated A1 and 1.2 × 10 .2 for sixfold coordinated A1 reported by Bertsch et al. [13] for an O H / A 1 molar, ratio of 2.25 are much higher than the values in this. paper and those reported by Thompson et al. [30].
4.4. Relation between OH / AI molar ratio and All 3 concentration Bottero e t a l . [11] and Bertsch et al. [12,13] have studied the effect of the O H / A 1 molar ratio on the development of All3 complexes. The results of the present study, in which solutions of a final A1 concentration of 5.2 × 10 .2 and 8.3 × 10-2M were investigated, are comparable With those of Bertsch et al. [12,13,21]. The latter authors had a final AI concentration of 3.34 × 10 2M. The amorphous precipitate formed during dropwise addition does not contribute to the formation of Al13 during neutralization Or aging, causing a lower Al13 yield as compared to the alkali injection experiments. The optimal All3 yield of 68.9% is reached at an O H / A 1 molar ratio of 2.2, which is significantly lower than the theoretical effective degree of hydrolysis, 2.46 (32 O H / 1 3 A1), based o n [Al13(OH)32] 7+ [9]. Bottero et al. [11,15] obtained with a 1 0 × 10-2M A1 solution an optimum All3 yield of 95.8% at an O H / A I molar ratio of 2.1, which is 0.1 lower than the O H / A 1 molar ratio at which we observed the maximum yield. Bertsch et al. [12,13] observed a maximum All3 concentration at an O H / A 1 molar ratio of approximately 2.25 with a 0.03 × 10-2M A1 (69.7%) and a 3 . 3 4 × 10-2M A1 (68.6%) solution. Akitt and Elders [31] deter-
mined a maximum Al13 concentration of 100% using 50-80 × 10-2M A1 salt solutions rapidly neutralized at 90°C to an O H / A 1 molar ratio of 2.5. This agrees with unpublished data from this laboratory.
4.5. Alkali solution injection and mixing conditions The Al13 yield is a function of the base injection rate, which has an optimum depending on the O H / A 1 molar ratio. The optimum injection rate shifts from 0.020 m l / s for solutions with O H / A 1 molar ratio = 1.2 to approximately 0.010 m l / s for solutions with O H / A 1 molar ratio 2.2. At higher rates the fraction of undetectable A1 rapidly increased and the solution became cloudy. Bertsch et al. [12] and Bottero et al. [11] suggested that at high injection rates more AI(OH)2 is generated than can be consumed by the formation of Al13. The excess AI(OH) 4 re-equilibrates with the bulk solution resulting in colloidal or precipitated AI(OH) 3. At addition rates below 0.010 m l / s amounts of AI(OH) 4 created are too small and re-equilibrate with the solution before Al13 can be formed, which represents the other limitation. An alternative is a series of sequential reactions during the slow injection of the alkali solution
[AI(H20)6] 3+-~ [AI(H20)5(OH)] 2+ .
[Al(H20)4(OH)2
] + - - All3.
It is assumed by several authors [10,21] that AI(OH) 4 formed at the point of alkali introduction is needed as a precursor for the formation of All3. Akitt and Farthing [10] hydrolyzed A13+ via solid Na2CO 3. They assumed a very rapid AI(OH) 4 production under the inhomogeneous conditions at the interface of solid Na2CO 3 and the solution. As discussed above several equilibria exist in partly neutralized A1 solutions, all depending on the rate of alkali injection. Low O H / A 1 molar ratios and slow injection favors the formation of mono- and dimers. Fast injection favors precipitation instead. Slow stirring rates causes the AI(OH) 4 to re-equilibrate with the acid solution. The All3 and monomer concentrations are similar up to an O H / A 1 molar ratio of
J.T. Kloprogge et a L / Optimalization of the development of the All3 polymer
2.6 for the neutralization reaction of AIC13 and AI(NO3) 3 solutions, as confirmed by Akitt and Farthing [10].
10l
at Nijmegen. W. Veeman, A.M.J. van der Eerden, P. Buining and J. Van Beek are thanked for critically reviewing this article.
4.6. Aging The decrease of the Al13 content upon aging confirms that the A113 is actually a metastable complex in solution. The Al13 polymer rearranges with time into large polymers unobservable with the NMR technique. Formation of other polymers is evident from the large increase of the amount of undetectable A1, the disappearance of Al13, and the decrease of monomeric A1 in the solution. Several authors have suggested that these polymers consists of hexameric rings [12,13,15,32]. The constant pH upon aging suggests a structural rearrangement without changing the degree of hydrolysis of the polymer [33]. Small angle X-ray scattering data suggest that the alumina tetrahedra present in Al13 are squeezed between growing octahedral layers and that tetrahedra are totally lost at the end of the aging process [15,29]. The broadening of the 63.3 ppm resonance upon aging, indicating a decrease in tetrahedral symmetry, support this structural rearrangement.
5. Conclusions
(1) Chemical shifts are influenced by (i) the choice of reference solution and its concentration, (ii) magnetic field strength and (iii) dilution with D20. (2) The increasing chemical shift and linewidth of the monomer resonance upon hydrolysis is mainly caused by increasing ionic strength and viscosity. (3) The maximum amount of All3 formed is influenced by the hydrolysis conditions, such as OH/A1 molar ratio, injection rate and mixing conditions. (4) The All3 rearranges into large polymers with a hexameric ring structure upon aging. The authors wish to thank G. Nachtegaal for the technical assistance at the HF-NMR facility
References [1] [2] [3] [4l [5]
G. Johansson, Acta Chem. Scand. 14 (1960) 771. G. Johansson, Acta Chem. Scand. 16 (1962) 403. G. Johansson, Ark. Kemi 20 (1963) 305. G. Johansson, Ark. Kemi 20 (1963) 321. C. Brosset, G. Biedermann and L.G. Sill~n, Acta Chem. Scand. 8 (1954) 917, [6] P.H. Hsu, in: Minerals in Soil Environments, ed. J.B. Dixon and S.B. Weed (Soil Science Society of America, Madison, WI, 1977) p. 99. [7] F. Lampe, D. Miiller, W. Gessner, A.-R. Grimmer and G. Scheler, Z. Anorg. Allg. Chem. 489 (1982) 16. [8] J.W. Akitt, N.N. Greenwood, B.L. Khandelwal and G.D. Lester, J. Chem. Soc. Dalton Trans. (1972) 604. [9] J.W. Akitt and A. Farthing, J. Magn. Reson. 32 (1978) 345. [10l J.W. Akitt and A. Farthing, J. Chem. Soc. Dalton Trans. (1981) 1617. [11] J.Y. Bottero, J.M. Cases, F. Fiessinger and J.E. Poirier, J. Phys. Chem. 84 (1980) 2933. [12] P,M. Bertseh, G.W. Thomas and R.I. Barnhisel, Soil Sci, Soc. Am. J. 50 (1986) 825. [13] P.M. Bertsch, W.T. Layton and R.I. Barnhisel, Soil Sci. Soc. Am. J. 50 (1986) 1449. [14] W.V. Rausch and H.D. Bale, J. Chem. Phys. 40 (1964) 3391. [15] J.Y. Bottero, D. Tchoubar, J.M. Cases and F. Fiessinger, J. Phys. Chem. 86 (1982) 3667. [16] J. Shabtai, M. Rosell and M. Tokarz, Clays Clay Minerals 32 (1984) 99. [17] D. Plee, L. Gatineau and J.J. Fripiat, Clays Clay Minerals 35 (1987) 81. [18] A. Schutz, W.E.E. Stone, G. Poncelet and J.J. Fripiat, Clays Clay Minerals 35 (1987) 251. [19] J. Sterte and J. Shabtai, Clays Clay Minerals 35 (1987) 429. [20] J.T. Kloprogge, J.B.H. Jansen and J.W. Geus, Clays Clay Minerals 38 (1990) 409. [21] P.M. Bertsch, Soil Sci. Soc. Am. J. 51 (1987) 825. [22] J.T. Kloprogge, D. Seykens, J.W. Geus and J.B.H. Jansen, J. Non-Cryst. Solids, this issue, p. 87. [23] J.W. Akitt and J.M. Elders, J. Magn. Reson. 63 (1985) 587. [24] J.W. Akitt and J.M. Elders, J. Chem. Soc. Faraday Trans. 81 (1985) 1923. [25] F. Berchier, Y.-M. Pai, W.P. Weber and K.L. Servis, Magn. Reson. Chem. 24 (1986) 679. [26] A.C. Vermeulen, J.W. Geus, R.J. Stol and P.L. de Bruyn, J. Coll. Interf. Sci. 51 (1975) 449.
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J.T. Kloprogge et aL / Optimalization of the development of the All3 polymer
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