J. Phys. Chem. 1989, 93, 1741-1744
Conclusions Alkali-metal cations are found to influence the rate of Si exchange among monomer, dimer, and cyclic trimer silicate anions. The maximum rate coefficient for Si exchange between dimer and monomer anions occurs for K', whereas the maximum rate of s i exchange between cyclic trimer, dimer, and anions is highest for Na+. The influence of cations on the dynamics of Si exchange is attributed to the formation of cation-anion pairs. Spin-lattice (TI) and spin-spin ( Tz) relaxation rates are also affected by the choice of cation. Spin-lattice relaxation is dom-
1741
inated by the contact of paramagnetic impurities with Si spins in silicate anions. Spin-spin relaxation is governed primarily by scalar coupling.
Acknowledgment. This work was supported by the Director, Office of Basic Energy Sciences, Material Science Division, of the U.S. Department of Energy under Contract DE-AC0376SF0098 and by a grant from w.R. Grace and cO. Registry No. D, 51931-86-3; c-T, 118171-41-8; Mon, 18102-72-2; Li, 7439-93-2; Na, 7440-23-5; K, 7440-09-7; Rb, 7440-17-7; Cs, 7440-46-2.
Multinuclear NMR Investlgation of the Formation of Aluminosilicate Anions A. V. McCormick, A. T. Bell,* and C. J. Radke Center for Advanced Materials, Lawrence Berkeley Laboratory, and Department of Chemical Engineering, University of California, Berkeley, California 94720 (Receiued: March 12, 1988; In Final Form: July 18, 1988)
The structure of simple aluminosilicate anions formed by the reaction of silicate and aluminate anions was investigated by 29Siand 2'Al NMR spectroscopy. The extent of aluminosilicate formation was found to increase with increasing silicate ratio and increasing cation size. The first of these trends is attributed to the redistribution of Si with increasing silicate ratio to more acidic anions. The second trend is attributed to the formation of cation-anion pairs.
Introduction Raman spectroscopy' and 27AlN M R spectroscopyZhave been used to identify the presence of soluble aluminosilicate species formed during the synthesis of zeolites. Since such species may direct the structure of the zeolite, the chemistry occurring in aluminosilicate solutions and gels has been the subject of several recent studies." Though it has been shown that both a decrease in pH and an increase in cation size can produce higher numbers of AlOSi b o n d ~ , ~it, remains ~*~ unclear by what means this is accomplished and what the structure of the product is. To address these issues, we have used N M R spectroscopy to characterize aluminosilicate solutions containing only a small number of species. Our goals were to establish the effects of alkalinity and cation size on the condensation of silicate and aluminate anions. Experimental Section Silicate solutions were prepared by dissolving Baker analyzed SiOz gel and reagent grade N a O H or CsOH in deionized water. A 1 M aluminate solution was prepared by dissolving technical grade sodium aluminate in 3 M N a O H solution. Alkaline solutions of aluminosilicates were prepared by mixing appropriate volumes of the aluminate and silicate solutions. Compositions of the final solutions were chosen so as to avoid immediate formation of a gel or precipitate. For the present work the silicate ratio, R ( R = [SiO,]/[M,O]), was adjusted to 0.1 or 0.4. N M R spectra were recorded on a Bruker AM-500 spectrometer at room temperature unless otherwise noted. Spectra of aluminosilicate solutions were recorded within 15 min of A1 addition. When necessary, the aluminum background free induction decay (FID) was collected with a water-filled sample tube, and the FID (1) Guth, J. L.; Caullet, P.; Jacques, P.; Wey, R. Bull. Chem. SOC.Fr. 1980, 1-121.
(2) Dent Glasser. L. S.: Harvev. G. In Proceedings of the 6th International Zeolite Conferencef Bisio; A., Oison, D. H., Eds.;"Buiterworth: Guildford, U.K., 1984; p 925. (3) Dent Glasser, L. S.; Harvey, G. J. Chem. Soc., Chem. Commun. 1984, 1250. (4) Engelhardt, G.; Hoebbel, D.; Tarmak, M.; Samoson, A,; Lippmaa, A. Z . Anorg. Allg. Chem. 1982, 484, 22. ( 5 ) Hoebel, D.; Garzo, G.; Ujszaszy, K.; Engelhardt, G.; Fahlka, B.; Vargka, A. Z . Anorg. Allg. Chem. 1982, 484, 7. (6) Engelhardt, G.; Michel, D. High Resolution Solid State NMR of Silicates and Zeolites; Wiley: New York, 1987; p 75.
0022-3654/89/2093-1741$01.50/0
was subtracted from the FID of interest. Slight differences between the 27Alchemical shifts reported here and those reported by other authors are probably due to the field adjustment for the field/frequency lock. The 27Alpeak assignments are consistent with peak separations reported by Dent Glasser et aL2 The DANTE pulse sequence7 was used to perform a holeburning experiment, using the approach described by Fukushima and Roeder.8 This experiment investigates whether a spectral feature is homogeneous or is composed of a number of superimposed peaks. A 100-Hz excitation band was produced with 28 1-MUS radio-frequency pulses, and the effect on the Si spectrum was recorded immediately.
Results Figure 1 shows the z9Sispectrum of a sodium silicate solution having only monomeric anions and the 29Siand z7Al spectra of aluminosilicate solutions taken immediately after mixing the appropriate quantities of sodium aluminate and sodium silicate solutions. The large z7Al peak centered near 80 ppm can be assigned to monomeric Al(OH),-; the smaller peak may be assigned to an aluminum bound through an oxygen bridge to one silicon.z The features of the 29Sispectrum cannot be interpreted as readily. Changes in the z9Siand 27Alspectra take place very slowly, and gelation does not occur for several hours. As a consequence, the spectra in Figure 1 represent a quasi-equilibrium distribution of structures that is rapidly established between the silicate and aluminate species. The broadening of the z9Siand z7Alpeaks and the progressive shift of the 29Sifeature as the [Si02]/[A1203]ratio decreases were reproduced with a variety of A1 sources and degrees of protection from contamination. Therefore, we are confident that these observations are not due to the addition of paramagnetic impurities. Neither can they be attributed to the slight dilution or the very small change in N a + concentration upon doping. The results of hole-burning experiments* on the Si resonance of the aluminosilicate solution for which [SiOZ]/[Al2O3]= 2.0 are shown in Figure 2. It is apparent that the shape of the spectrum changes depending on the frequency at which the Si spins (7) Morris, G. A,; Freeman, R. J . Magn. Reson. 1978, 29, 433. (8) Fukushima, E.; Roeder, S. G. W. Experimental Pulse NMR; Addison-Wesley: Reading, MA, 1981.
0 1989 American Chemical Society
1742 The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
McCormick et al. 0
I mol% SiO,
I
SiOp/NopO = 0 I
No A1203
I
,
3 mol% SIO, Si02/No20 = 0 4
No Aluminate
11
0 (ppm)
-5 -10 from Si(OH), AI ( 0 SI)
- -90, "2'3
0
-5
I20
-IO
8s,(ppm) from Si(OH), '
SI02
--
6,,
80
40
3+ (ppm) from Al(H20),
Figure 3. (a) 29Sispectrum of a Na silicate solution of the composition 3 mol % SOz,R = 0.4. (b) 29Siand 27A1spectra of a Na aluminosilicate solution of the composition 3 mol % SO2,R = 0.4, [SiO2]/[AI2O3]=
I
10.0
- 2
3 mol% S l o p
I20
80
40
Si02/No20 = 0 4
Figure 1. (a) 29Sispectrum of a monomeric Na silicate solution of the composition 1 mol % SO2,R = 0.1, and 27A1spectrum of a monomeric Na aluminate solution of the composition 1 M NaA102. (b)-(d) %i and 27Alspectra of aluminosilicatesolutionswith increasing AI concentration. 29Sispectral frequencies are referenced to .%(OH),, and 27Alspectral frequencies are referenced to the octahedral AI3+ ion in an aqueous solution of AIC13.
Si02/AIg03: IO
b
I\
b
normal
/Aburned
IO
8
6
4
2
0 -2 SNo I w m )
-4
-6
-8
-IC
Figure 4. 23Naspectra (a) of a Na silicate solution of the composition 3 mol % SO2,R = 0.4, and (b) of a Na aluminosilicate solution of the composition 3 mol % SO2,R = 0.4, [SiO2]/[AI2O3]= 10.0.
1
1
0
I
I
-2 Ss, ( w m )
1
-3
I
-4
1
0
,
-I
I
I
-2
-3
-4
Ss, ( w m )
Figure 2. Conventional and selectively inverted 29Sispectra of a Na aluminosilicate solution of the composition 1 mol % S O z , R = 0.1, [SiOz]/[Ali03]= 2.0. The two selective inversion spectra were recorded following excitation of a 100-Hz band on the left (a) and on the right (b), respectively, of the spectral feature. are excited. This response suggests that the resonance is very likely a composite of several peaks rather than a single peak. Figure 3 shows a 29Sispectrum for a sodium silicate solutions containing 3 mol % Si02a t R = 0.4. The three peaks present can be assigned to monomer, dimer, and cyclic trimer silicate anions9 Upon addition of sodium aluminate there is a slight increase in the shielding of the monomer and dimer peaks and the intensity of the cyclic-trimer peak decreases substantially. The 27Alspectrum, which is also shown in Figure 3, exhibits three peaks. These features correspond to A1 bound to zero, one, and two Si atoms through an oxygen bridge (denoted Al(OSi), Al( lSi), and Al(ZSi), respectively). 23Na spectra for the sodium silicate and the sodium aluminosilicate solutions are shown in Figure 4. In both cases the spectrum is referenced to a 0.1 M N a O H solution, for which the Na+ ion is present primarily in its fully (9) Harris, R. K.; Knight, C. T . G.J. Chem. Soc.,Faraday Tram. 2 1983, 79, 1525.
hydrated state. Before aluminate addition, the 23Na peak is centered at 2.5 ppm. After aluminate addition the peak shifts to 2.0 ppm. In the absence of dissolved S O 2 , the 23Napeak of a sodium aluminate solution with the same A1 concentration is centered near 0 ppm. Figure 5 shows the 29Siand 27Alspectra of a cesium aluminosilicate system with a molar composition similar to the monomeric sodium aluminosilicate system. The 29Sispectrum of the silicate solution shows that, before the addition of aluminate solution, some dimer and cyclic trimer anions exist in solution as well as monomer anions. Similar to the sodium silicate case in Figure 3, the addition of aluminate ions results in the disappearance of the cyclic trimer peak and the appearance in the 27Al spectrum of Al(OSi), Al( lSi), and Al(2Si) peaks. The fraction of linked A1 and the average number of AlOSi linkages are even larger for the Cs silicate solutions than for the Na silicate solutions for which R = 0.4. Shown in Figure 6 are 133Cs spectra for the cesium silicate and the cesium aluminosilicate solutions. In both cases the spectrum is referenced to a 0.1 M CsOH solution, for which the Cs' ion is present primarily in its fully hydrated state. Before aluminate addition, the I3%s peak is centered at 25 ppm. After aluminate addition the peak shifts to 22.5 ppm. Discussion Condensation of Monomer Silicate Anions with Aluminate low. Aluminosilicate anions are formed by the condensation of Al(OH).,- anions with silicate anions. In very basic solutions where
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 1743
Formation of Aluminosilicate Anions
aluminosilicate concentration should increase with increasing aluminate concentration and with decreasing pH. Reaction 1 can be used to interpret the behavior of the 29Si spectrum shown in Figure 1. This figure shows only a single spectral feature which shifts in the direction of increased shielding as increasing amounts of aluminate solution are added to the silicate solution. We propose that the presence of only a single feature in the 29Sispectrum is due to the coalescence of peaks associated with Si in the silicate and the aluminosilicate anions, the coalescence being caused by very rapid exchange of Si between these environments. The argument behind this statement is as follows. When the frequency of exchange exceeds the frequency separation of the distinct N M R peaks, the chemical shift of the coalesced peak is a monotonic function of the concentration of Si in the product environment.I0 Thus
0.5mol% Si02 Si02/Cs20 = 0.I
a
No A1203
6= SiO2/AI2O3 = 2.0
b
b
I
I
I
I
I
I
2
0
-2
-4
-6
-8
I
I
-12
-10
BSi ( w m )
IO0
80
20
40
60
B,,
0
(ppm
Figure 5. (a) 29Sispectrum of a Cs silicate solution of the composition 0.5 mol '7% Si02, R = 0.1. (b) 29Siand 27AI spectra of a Cs aluminosilicate solution of the composition 0.5 mol '7% S O 2 ,R = 0.1, [ S O 2 ] / [AI2031 = 2.0.
i
0.5 mol% SiOe
Si02/A1203 = 2 0
/I
27
26
25
24
23 22 SCr I P P ~ I
21
,
20
Figure 6. 133Cs spectra (a) of a Cs silicate solution of the composition 0.5 mol % SO2,R = 0.1, and (b) of a Cs aluminosilicate solution of the composition 0.5 mol '7% SO2, R = 0.1, [Si02]/[A1203] = 2.0.
most of the silicon is present in the form of (SiOx(OH)+x)X-anions, the formation of aluminosilicate anions can be represented by the following reaction: KI
AI(OH),-
+ n(SiOx(OH)4~x)x-=
[AISi,04(,+1)-yHl(n+l)-4x-y]
(nx-v)-
+ YOH- (11
It is evident from reaction 1 that, for a fixed silicate concentration,
EJy, i
(1)
where hi is the chemical shift for Si in environment i andf, is the fraction of Si in environment i. We will assume that i = 1 corresponds to Si in monomeric silicate anions and i I2 corresponds to Si in aluminosilicates. As discussed below, the chemical shift of Si in aluminosilicates is smaller than that for Si in a monomeric silicate anion, and as a consequence 6 will decrease a s h decreases. This trend is consistent with the observations from Figure 1 that increasing the A1 concentration decreases 6. The structure of the aluminosilicate anion can be deduced in the following manner. It is noted from the 27A1spectrum that, although most of the A1 in solution resides in the Al(OH); anion, a small amount of A1 is bonded to one Si atom. Though deconvolution of the 27Alspectrum is complicated by the width of the peaks, the distinct AI(0Si) and AI( 1%) signals can be distinguished since the frequency separation between the 27Alfeatures is larger than the corresponding separation between the 29Sisignals. A liberal estimate of the fraction of the total 27Alsignal in the Al( 1Si) position (Le., in the aluminosilicate) at [SiO,)] / [AI2O3] = 2.0 is only 10%. Clearly, then, K,
