J . Phys. Chem. 1991, 95, 7841-1851
7847
Incorporation of Aluminum into Silicate Anions in Aqueous and Methanoiic Solutions of TMA Silicates R. F. Mortlock, 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 (Received: March 29, 1991)
29Siand 27AINMR spectroscopies were used to characterize dilute, highly alkaline, aqueous and methanolic solutions of tetramethylammonium (TMA) aluminosilicates. The solutions contained 2 mol % SOz, a silicate ratio (R = [SiOz]/[M20], M+ = TMA') of 2, Si/Al molar ratios between m and 2, and between 0 and 20 mol % methanol (MeOH). TMA aluminosilicate solutions are characterized by a high concentration of double four membered ring (D4R) anions. 29SiNMR peaks were observed for Si atoms in the dimer, linear trimer, cyclic trimer, branched cyclic trimer, and D4R anions which have one incorporated AI atom. 29SiNMR peak assignments were based on similar assignments in tetrapropylammonium (TPA) aluminosilicate solutions and confirmed by using the correlation between the partial charge of Si atoms and the %i NMR chemical shift. Comparison between the distribution of silicate and aluminosilicateoligomers in TMA and TPA aluminosilicate solutions indicates that the distribution of Si atoms among anions is very different, but the level of A1 incorporation into silicate species is remarkably similar. As a result, the incorporatiton of AI into silicate anions depends strongly on the solution composition and pH and depends only weakly on the nature of the alkylammonium base. Due to a hydrophobic effect, the addition of methanol to TMA aluminosilicate solutions increases the concentration of D4R and D4R( 1Al) anions.
Introduction The synthesis of zeolites normally involves the formation of an aluminosilicate gel. Successful synthesis depends strongly on the gel microstructure, which is determined by the reaction of soluble silicate and aluminate ani0ns.I For this reason there is a growing interest in understanding the chemistry of the reactions between soluble aluminate, silicate, and aluminosilicate anions in alkaline solutions. Studies of silicate chemistry have shown that the silica concentration, aluminum concentration, cation composition, and pH affect the distribution of silicate and aluminosilicate anions in solution but in ways that are not fully understood. "si NMR spectroscopy is well suited for the identification and quantification of silicate anions in dilute alkaline solutions. Both 1-D and 2-D N M R techniques have been used to identify approximately 40 distinct types of silicate anion^.^-'^ The distribution of dissolved silica in various anions depends upon the solution pH, dissolved silica concentration, and the silicate ratio, R = [Si02]/[MzO],where M+ is a an alkali metal cation or alkyl ammonium ~ a t i o n . ~ ~ - lAt * identical silica compositions and silicate ratios, the distribution of silicate species in solution changes markedly with characteristics of the hydroxide cation. In general,
alkylammonium cations are thought to have a structure-directing effect, producing silicate solutions with significant concentrations of high molecular weight, cagelike silicate oligomers such as the double three membered ring (D3R) and double four membered ring (D4R) anions.I6*lez2 For example, tetramethylammonium (TMA) silicate solutions exhibit a high concentration of D4R anions. The addition of methanol (MeOH) to TMA silicate solutions has been observed to force even more of the dissolved silica into the D4R anion.22 The study of aluminosilicate solutions has proven difficult. Evidence for dissolved aluminosilicate anions has come from both 29Siand 27Al N M R spectroscopies.z3-33 In a study of sodium aluminosilicate solutions, Kinrade and Swaddlez9 successfully identified 29SiN M R peaks due to low molecular weight aluminosilicate anions by acquiring the 29Si spectra a t subambient temperatures. Using a similar approach, Mortlock et al.32 performed a detailed study of tetrapropylammonium (TPA) aluminosilicate solutions. Positive identification was made of the "si NMR peaks due to the dimer, linear trimer, cyclic trimer, and branched cyclic trimer anions with one incorporated AI atom. In
( I ) Barrer, R. M. The Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982. (2) Harris, R. K.; Newman, R. H.J . Chem. Soc., Faraday Trans. 2 1977, 73, 1204. (3) Harrjs, R. K.; Jones, J.; Knight, C. T. G.J . Mol. Srrucr. 1980,69,95. (4) Harris, R. K.; Knight, C. T. G.; Hull, W. E. J . Am. Chem. SOC.1981, 103, 1577. (5) Harris, R. K.; Knight, C. T. G.;Hull, W. E. In Soluble Silicates; Falcone, J. S . , Ed.; ACS Symposium Series 194; Americal Chemical Society: Washington, DC, 1982; p 78. (6) Harris, R. K.; Knight, C. T. G. J . Mol. Struct. 1982, 78, 273. (7) Harris, R. K.; Knight, C. T. G.J . Chem. Soc., Faraday Trans. 2 1983, 79. 1525. (8) Harris, R. K.; Knight, C. T. G. J . Chem. Soc., Faraday Trans. 2 1983, 79. 1539. -(9) Harris, R. K.; Jones, J.; Knight, C. T. G.; Newman, R. H.J . Mol. Liq. 1984, 29, 63. (IO) Harris. R. K.: O'Connor. M. J.: Curzon. E. H.: Howarth. 0. W. J . Magn.. Reson. 1984, 57, 1 IS. ( I I ) Knight, C. T. G.J . Chem. SOC.,Dalton Trans. 1988, 1457. (12) Knight, C. T. G.; Kirkpatrick, R. J.; Oldfield, E. J . Magn. Reson. 1988, 78, 31. (13) McCormick, A. V.; Bell, A. T.; Radke, C. J. Zeolites 1987, 7, 183. (14) McCormick, A. V.; Bell, A. T.; Radke, C. J. J . Phys. Chem. 1989, 93, 1733. ( I S ) McCormick, A. V.; Bell, A. T.; Radke, C. J . J . Phys. Chem. 1989, 93, 1737. (16) McCormick, A. V.; Bell, A. T. Catal. ReuAci. Eng. 1989, 3/ (2), 97. (17) Kinrade, S. D.; Swaddle, T. W. Inorg. Chem. 1988, 27. 4253. (18) Kinrade, S. D.; Swaddle, T. W. Inorg. Chem. 1988, 27, 4259.
(19) Boxhoorn, G.; Sudmeijer, 0.;Kasteren, P. H. G.J . Chem. Soc., Chem. Commun. 1983, 1416. (20) Groenen, E. J. J.; Kortbeck, A. G.T. G.;Mackay, M.; Sudmeijer, 0. Zeolites 1986, 6, 403. (21) Engelhardt, G.; Michel, D. High-Resolution Solid-Stare NMR of Silicates and Zeolites: Wilev: Chichester. U.K.. 1987. (22) Knight, C. T. G.Zeblites 1989, 9,' 448. (23) Dent Glasser, L. S.;Harvey, G. J . Chem. Soc., Chem. Commun. 1984, 1250. (24) Dent Glasser, L. S.;Harvey, G. In Proceedings of the Sixth International Zeolite Conference; Olson,D., Bisio, A., Eds.; Butterworths: Stoneham, MA, 1984; p 925. (25) Harvey, G.;Dent Glasser, L. S.In Zeolite Synthesis; Occelli, M. L., Robson, H. E., Eds.; ACS S y m p i u m Series 398; Americal Chemical Society: Washington, DC, 1989; p 49. (26) Knight, C. T. G.; Kirkpatrick, R. J.; Oldfield, E. J . Am. Chem. Soc. 1987, 109, 1632. (27) Van den Berg, J. P.; de Jong-Versloot, P. C.; Keijsper, J.; Post, M. F. M. In Innmarions in Zeolite Materials Science; Grobet, P. J., Mortier, W. J., Van Sant, E. F., Schultz-Ekloff, G.,Eds.; Elsevier: Amsterdam, 1988; p
1
'
85. (28) McCormick, A. V.; Bell, A. T.; Radke, C. J . J . Phys. Chem. 1989, 93, 1791. (29) Kinrade. S.D.; Swaddle, T. W. Inorg. Chem. 1989, 28, 1952. (30) Thangaraj, A.; Kumar, R. Zeolites 1990, 10, 117. (31) Mueller, D.; Hoebkl, D.; Gessner, W. Chem. Phys. Lett. 1981.84(1). 25. (32) Mortlock, R. F.; Bell, A. T.; Radke. C. J . J . Phys. Chem. 1991. 95, 372. (33) Mortlock, R. F.; Bell, A. T.; Chakraborty, A. K.; Radke, C. J . J . Phys. Chem. 1991, 95, 4501.
0022.365419 112095-7847%02.50/0 0 1991 American Chemical Society
7848 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Mortlock et al.
TABLE 1: Spectral Peaks in Figure 1 labelled pe&q
1
05
to
-05
Q
b
04s
-3 5
to
-4 5
Q,(lAI)
C
M
-4.5
to
-6.0
Q,OAI)
d
8..
-7.0
to
-7.25
QdIAl)
e
do
-7.25 IO
-7.35
QJW
I
m
-8.25 'to
-8.75
Q,
g
w
-8.75 to
-9.25
QI
h
t=
-9.9
to
-10.1
Q,
I
la \ I
~
-5
Il
I
I
-10
-15
-20
1
-25
.
S I connecuvip
0
i
0
chemical shift ranger (ppm)
a
D3R
la I
spxie\O
-10.2 10
-10.4
-17.2 to
-17.4
-22.4 to
-22.6
-21.25
-27.35
LO
-27.4 io
-27.6
L 1
-30
PPm
Figure 1. (a) 29Sispectrum of TMA silicate solution taken at 10 "C: 2 mol ISO2,R = 2. (b) %i spectrum of TMA aluminosilicate solution taken at IO "C: 2 mol % SO2, R = 2, %/AI molar ratio = 5 . In the schematic structures, a solid circle represents a Si atom and a solid line
represents a bridging oxygen. Letters identifying various peaks are defined in Table l. %i spectra frequencies are referenced to the monomeric peak in (a). a more recent study, the correlation between the partial charge on Si atoms and the 29Sichemical shift was used to confirm the assignment of 2%i NMR peaks due to Si atoms in aluminosilicate anions and to aid in the identification of 29SiN M R peaks due to the D3R(lAI) and D4R(lAI) anions.33 The present study extends the methods used in our earlier ~ o r kto~characterize ~ * ~ ~ aqueous and methanolic solutions of TMA aluminosilicates. Particular emphasis is placed on observing the incorporation of AI into double four membered ring (D4R) anions. Experimental Section TMA aluminosilicate solutions were prepared by dissolving fumed Si02(Cab-0-Sil, grade EH-5) in appropriate amounts of 25 wt 7% aqueous TMAOH (Alfa Products), deionized water, and D 2 0 (Cambridge Isotopes) in polystyrene test tubes. Dissolution of the silica required between 1 day and 3 weeks, yielding clear solutions. Aluminum wire (99.999%, Aldrich) was added to these solutions to achieve the desired %/AI molar ratio. After dissolution of the AI wire (approximately 1 week), the solutions were allowed to equilibrate for no less than 1 week at room temperature (22 "C). Compositions of the final solutions contained 2 mol % S O 2 , a silicate ratio (R = [Si02]/(M20],M+ = TMA+) of 2, and Si/AI molar ratios from m to 2. Methanolic TMA aluminosilicate solutions were prepared in an analogous procedure with the appropriate amount of methanol (MeOH) being added before dissolution of the AI wire. Compositions of the final solutions were chosen to avoid the formation of solid precipitates. NMR spectra were acquired on a Bruker 500-MHz spectrometer with a deuterium lock resonance. 29SiFIDs were collected at 99.356 MHz by using 2000-8000 8-ps pulses (-70") and 12-s recycle delays. A large spectral width was employed to avoid foldback of a background glass peak. Polynomial baseline corrections were used to eliminate this broad peak. 27AlFlDs were collected at 130.290 MHz by using 5000 9.7-ps pulses (~90")
a In the schematic structures, a solid circle represents the Si atom corresponding to the assigned peak, an open circle represents an AI atom, a solid square represents a Si atom resonating in another part of the spectrum, and a solid line represents a bridging oxygen atom.
and 0. I-s recycle delays. The standard Lorentz-Gauss transformation was applied to all 27Al FIDs for resolution enhanceEach NMR spectrum was internally scaled to the tallest peak in that spectrum. Therefore, peak heights between spectra cannot be compared quantitatively. Results and Discussion Figure 1 presents the 29SiN M R spectra of a TMA aluminosilicate solution in the absence (a) and presence (b) of AI. In the absence of AI, the %i NMR spectrum is characterized by narrow lines and an intense peak at -27.3 ppm due to dissolved silica in the D4R anion. Due to the dominance of the D4R anion peak, the spectra in Figure 1 are vertically expanded, and the D4R anion peak is not fully displayed in order to closely observe other peaks. The range of chemical shifts corresponding to a given type of Si connectivity is designated in Figure la by Qi and Qia (where i refers to the Si atom connectivity and A designates Si in threemembered rings).35 Principal features labeled above Figure l a are assigned to silicate anions and identified in Table I on the basis of previous In the schematic structures in Figure 1, a solid circle represents a Si nucleus and a solid line represents a bridging oxygen bond. When AI is added to the TMA silicate solution to achieve a Si/Al molar ratio of 5, some of the 29SiN M R peaks broaden. The broadening of peaks can be attributed to chemical exchange between Si and AI in aluminosilicate species. A similar chemical exchange broadening has been observed with increasing temperature in silicate s o l ~ t i o n s ~and J ~in~ studies ~~ of sodium and TPA aluminosilicate solution^.^^^^^ Because these experiments are in the near slow exchange limit, the line width is a measure of the minimum site lifetime.9 Figure 1b shows that the minimum site lifetimes of nearly all species decrease with the addition of AI. Along with peak broadening, the addition of AI causes new peaks to appear in regions of the spectrum that can be attributed to Si atoms in aluminosilicate species. The connectivity of the (34) Ferige, A. G.; Lindon, J. C. J. Magn. Reson. 1978, 31, 337. (35) Engelhardt, V. G.; Zeigcn, D.; Janke, H.;Hoebbel, D.; Wieker, W. 2.Anorg. Allg. Chem. 1975, 418, 17.
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7849
Incorporation of AI into Silicate Anions
25
Y -12 0
- s 2 d - 8 E
- 10 -
-
-
(b)
J
~
~
U
=
j
A
I I
0.2 Ai I
0
-5
.lo
I
I
I
-1.5
-20
-25
-30
PPm
Figure 2. 29Sispectra of TMA aluminosilicate solutions taken at IO O C : 2 mol % Si02,R, = 2. (a) Si/AI molar ratio = -, (b) Si/AI = 5 , (c) Si/AI = 4, (d) Si/AI = 3, (e) Si/AI = 2. 29Sispectral frequencies are referenced to the monomeric peak in (a).
Si atoms in these regions is indicated above Figure 1b. Here, Qi (nAl) refers to Si nuclei with n aluminosiloxane linkages. Assignments of the peaks due to specific aluminosilicate and silicate oligomers are labeled above Figure 1b and are identified in Table I. The assignment of these regions is based on studies of solid aluminosilicates2' and studies of sodium and TPA aluminosilicate ~oIutions.~~J~J3 As already discussed, TMA silicate and aluminosilicate solutions are characterized by a high concentration of D4R anions. When AI is added to the solution, two very distinct peaks appear at -22.5 and -27.5 ppm in Figure 1b. These two new features are assigned to the D4R(IAl) anion, the double four membered ring with one AI atom, on the basis of a correlation between 2?3 NMR chemical shift and the partial charge of the Si nucleus.33 The larger the partial charge on a Si nucleus, the lower the chemical ~ h i f t . ~ ' J ~ In the D4R anion, all eight Si atoms have a partial charge of 1.64 and a chemical shift of -27.3 ppm." When an AI atom is substituted for one of the eight Si atoms, the three Si atoms nearest the incorporated AI have a partial charge of 1.62 and the remaining four Si atoms have a partial charge of 1 .65.33 Therefore, the peak at -22.5 ppm in Figure 1b is assigned to the Si atoms nearest the incorporated AI atom and the remaining four Si atoms are assigned to the peak at -27.5 ppm. Such assignments are supported by the observed ratio of 4 to 3 in the experimental intensities of the peaks. The method used to synthesize the aluminosilicate solution is important for successful incorporation of AI into the D4R anion. For example, Engelhardt and Miche12' reported that attempts to prepare aluminosilicate solutions by mixing concentrated TMA silicate solutions and TMA aluminate solutions yielded no aluminosilicate anions. In the present investigation, dissolving pure AI wire in concentrated TMA silicate solutions resulted in not only a wide variety of low molecular weight aluminosilicate anions but also a high concentration of D4R( 1Al) anions. These observations indicate that stable aluminosilicate oligomers in solution can easily be formed by the incorporation of AI atoms into a silicate species and that formation of larger aluminosilicate species (greater than a dimer) in solution does not occur by the reaction of aluminate and small silicate anions.
.
T0.4 Mol% AI I
V
I
6p
- 4 2
0.6
Figure 3. Percentage of total silica present in various connectivity environments of the spectra presented in Figure 2 as a function of the mole percent of AI. The Qo, QI,QU, and Q2+ Q 3 A regions are associated with the lower right hand ordinate axis. The Q3(lAl),Q,(lAl) + Q3A(1Al), Ql(lAl), QZA(1Al). and Q3(2AI)regions are associated with the lower left hand ordinate axis. The Q3 region is associated with the upper left hand ordinate axis. Error bars are approximately twice the size of graph symbols.
Figure 2 presents the %i NMR spectra for a series of solutions in which the concentrations of S i 0 2 and TMAOH were held constant as the Si/AI ratio was decreased from 01 to 2. It is evident that the 29SiNMR peaks due to the monomer, dimer, and cyclic trimer anions uniformly broaden with increasing AI content. The peak width of the D4R anion is broadened to a lesser degree, and the peak width of the D3R anion is unaffected by the presence of AI until Si/AI ratios of less than 2 are reached. These trends indicate that aluminate chemical exchange is greater with smaller silicate species, resulting in shorter site lifetimes and broader lines. On the other hand, D3R anions do not interact appreciably with aluminate anions at these solution concentrations. It is also seen that with increasing AI content a greater proportion of the Si is present in low molecular weight silicate and aluminosilicate species. These latter trends are particularly evident in Figure 3, which shows the percentage of Si in a given connectivity class as a function of the mole percent of AI in solution. The depolymerization of silicate anions caused by the introduction of AI into TMA aluminosilicate solutions is similar to that reported recently for TPA silicate sol~tions.'~ The 27AlNMR spectra of the same solutions for which the 29Si N M R spectra were presented in Figure 2 are shown in Figure 4. The spectra are characterized by four distinct bands. The bands appearing at 73-77, 68-71, 62-66, and 57-62 ppm are assigned to AI(OSi), AI( ISi), A1(2Si), and A1(3Si), respectively (where nSi refers to the number of siloxane linkage^).^^,^^ The 27AINMR spectra in Figure 4 show that the distribution of AI connectivities is relatively unaffected by the Si/AI ratio, whereas the %i NMR spectra of these same solutions (presented in Figure 2) show that the average connectivity of Si decreases with decreasing Si/AI ratios. This point will be explained below. Smaller peaks at 71-72,66-67, and 65.5-66 ppm are resolvable in Figure 4 as a direct consequence of the Lorentz to Gauss resolution enhancement technique. Assignment of these bands is complicated because the solutions that yield the most distinct 27AI peaks yield corresponding 2ySiNMR spectra that exhibit broad overlapping peaks due to a large number of low molecular weight aluminosilicate species and fast exchange of Si and AI sites
7850 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
'
I
d0
IbO
I
I
8b
i'0
'
6b PPm
I
I
" ' I '
5b
4b
1
30
Figure 4. 27Alspectra of TMA aluminosilicate solutions taken at 22 OC: 2 mol 7'% SO2,R = 2. (a) Si/AI molar ratio = 5 , (b) Si/AI = 4, (c) Si/AI = 3, (d) Si/AI = 2. 27Al spectral frequencies are referenced to the AI3+ ion in a 0.1 M AICI, aqueous solution. ""
5 50 2
z
2 400 3 30-
: 65 20I
0
&
10-
0
0.2
0.4 Mol% AI
0.6
Figure 5. Estimates of the number of aluminosiloxane bonds calculated from the integrated areas of the spectral regions of the %i and 27AI spectra in Figures 2 and 4. Error bars are approximately twice the size of graph symbols.
at these concentrations. These smaller resonances are tentatively assigned to end-substituted AI atoms in anions of the type Al-(O-Si)n. The absence of features associated with these anions in the 29SiNMR spectra can be attributed to a fast exchange of A1 and Si atoms in these species, which results in %i NMR peaks that are too broad to distinguish from the background noise. The proportion of Si associated with aluminosilicate structures can be estimated from both 27AIand 29SiNMR spectra. The triangles in Figure 5 indicate the number of AI-O-Si bonds per 100 Si atoms calculated from the 27AINMR spectra shown in Figure 4. The corresponding values calculated from the %i NMR spectra shown in Figure 2 are given by the circles and squares. The squares are calculated on the basis of the assumption that all the aluminosilicate species contain only one AI atom per anion, whereas the circles are calculated by assuming that 50% of the Si resonating in the region from -1 5 to -19 ppm is assigned to Si atoms with Q2 + Q3,, connectivity and 50% is assigned to Q3(2AI) connectivity. The species presumed to have Q3(2A1) connectivity are doubly AI substituted derivatives of the D4R anion. The existence of such D4R derivatives has been postulated by Knight" from 2-D ?Si COSY experiments of potassium silicate solutions. As can be seen from Figure 5 , the second assumption provides close agreement with the calculated numbers of AI-O-Si bonds derived from 27AIspectra.
Mortlock et al. As mentioned earlier, the %i and 27AlN M R peak assignments of TMA aluminosilicate solution species are based on the assignment of similar peaks that appear in 29Siand 27AI N M R experiments of TPA aluminosilicate solution^.^^*^^ The most significant difference between TMA and TPA aluminosilicate solutions of similar compositions is the large concentration of D4R and D4R( 1Al) anions in TMA aluminosilicate solutions and the absence of D4R anions in TPA aluminosilicate solutions. This can be attributed to a cation-crowding effect.36 In TPA aluminosilicate solutions, a TPA cation does not have sufficient free volume space per cation to promote the formation of an oligomer the size of the D4R or D4R( 1Al) anions. The stabilization of the D4R and D4R( 1Al) anions in TMA aluminosilicate solutions leads to very low concentrations of all other solution species. Although most of the dissolved Si in TMA aluminosilicate solutions is forced into D4R and D4R( 1Al) anions, the distribution of Si nuclei among other anions is very similar to the distribution of Si nuclei in anions in TPA aluminosilicate solutions. For example, the ratio of the intensities of the D3R to the monomeric peak are identical for TMA and TPA aluminosilicate solutions of identical composition^.^"^ In contrast to the %i NMR spectra, the 27AIN M R spectra indicate that the connectivities of A1 to Si atoms in both TMA and TPA aluminosilicate solutions of identical compositions are very similar. Thus, although the distribution of Si among oligomers varies greatly between TMA and TPA aluminosilicate solutions, the numbers of aluminosiloxane bonds per 100 Si atoms (estimated from both 2%i and 27AlNMR spectra) as a function of AI solution concentration are virtually identical. This trend also holds true for tetraethylammonium (TEA) and tetrabutylammonium (TBA) aluminosilicate solut i o n ~ . ~This ~ indicates that the connectivity of AI atoms in alkylammonium silicate solutions is dependent on the solution composition and pH but very weakly dependent on the characteristics of the alkylammonium cation. Another point worth noting is the absence of D3R(lAl) anions in TMA aluminosilicate solutions in the present study. Again, a comparison to TPA aluminosilicate solutions is helpful. In TPA aluminosilicate solutions at 2 mol 3'% S O 2 , R values of less than 1.5 lead to stabilization of the D3R(lAl) anion.33 On the other hand, TMA aluminosilicate solutions at 2 mol 3'% Si02 and R values of less than 1.5 were found to crystallize solid TMA silicates, probably of the form TMA,,-[SiBO2,,]"-. The results presented in Figures 2-4 can be explained in terms of the equilibria governing the distribution of silicate, aluminate, and aluminosilicate anion^.^^.'^ The distribution between monomeric silicate anions, Sl-,and oligomeric silicate anions, Sf, is governed by the reaction nSf-
F?
Sf
+ (n - 1)OH-
(1)
Aluminate anions, AI(OH),, represented by A;, react with silicate anions to form aluminosilicate anions in accordance with the reaction AT
+ mSf
~t
(AI(Sn),,Jx-
+ OH-
(2)
The effects of Si02and TMAOH concentrations on the distribution of AI connectivities can be deduced from reaction 2. At equilibrium, the ratio of concentrations of aluminosilicate anions to aluminate anions can be written [(A~(Sn)m)~-l / [Ail = K2[S~-lm/[OH-l
(1)
It is apparent from eq 1 that the distribution of AI connectivities to Si should not be affected by the concentration of AI. This explains the results seen in Figure 5. Since aluminate anions react preferentially with small silicate anion^,)^^^^ reactions 1 and 2 predict that as more AI is added to an equilibrated silicate solution, the distribution of silicate oligomers will shift to lower molecular weights. (36) Hendricks, W. M.;Bell, A. T.; Radke, C. J. J . Phys. Chem., in press. (37) Hendricks, W. H. Ph.D. Thesis, University of California at Berkeley, CA. 1991.
The Journal of Physical Chemistry, Vol. 95, NO. 20, 1991 7851
Incorporation of AI into Silicate Anions
20moM MeOH
h 1OmolC MeOH
NO
(1)
0
-5
.lo
.15
-20
-25
m m
-30
$0
8b
I
I
I
IbO
+O
6b
'
5b
1 '
I
4b
1
30
PPm Figure 7. 27A1spectra of TMA aluminosilicate solutions taken at 22 OC: 2 mol 9% SO2,R = 2, Si/AI molar ratio = 5. (a) 0.0 mol % MeOH, (b) IO mol % MeOH, (c) 15 mol 9% MeOH, (d) 20 mol % MeOH. 27AI spectral frequencies are referenced to the AI3+ ion in a 0.1 M AlCl3
aqueous solution.
because the additional AI incorporates into a broad spectrum of aluminosilicate species, none of which are present in high concentration. The absence of any Q4Si environments suggests that the Al(4Si) peak in Figure 7 is due to AI atoms connected to four Si atoms, each of which may have connectivities of Q,, Qz, QZA, Methanolic TMA Aluminosilicate Solutions. Previous studies Q3, and Q3A. Due to this isotropic distribution of silicate species have shown that the addition of methanol (MeOH) to silicate bonded to the central AI atom, no one silicate species is bonded solutions causes the stabilization of large ring silicate structures to the central AI atom in high enough concentrations to observe due to increases in the hydrophobic character of the s o l u t i ~ n . ~ ~the ~ ~29Si ~ N M R Qi(lAI) or QiA(1Al) peaks. As part of the present investigation, MeOH was added to TMA Conclusions. The present study provides convincing evidence aluminosilicate solutions to determine the influence of MeOH on for the stabilization of high concentrations of the D4R and the formation of aluminosilicate anions. Figure 6 presents the D4R( 1Al) anions in aqueous and methanolic solutions of TMA z9SiNMR spectra of a series of TMA aluminosilicate solutions aluminosilicates. Use was made of a correlation between the prepared with increasing amounts of MeOH. For each of these partial charge of a Si nucleus and its 29Sichemical shift to verify experiments, concentrations were maintained at 2 mol % Si02, the assignment of the D4R( 1Al) peak. In addition to the D4RR = 2, and %/AI = 5. The addition of MeOH forces the dissolved (1AI) peak assignments in TMA aluminosilicate solutions, 29Si Si into ring structures. At 20 mol % MeOH, only four species NMR peaks due to the formation of the dimer (lAl), linear trimer are observed in the 29Sispectrum: D4R, D4R( 1AI), D3R, and (1AI), cyclic trimer (1AI), and branched cyclic trimer (1Al) anions monomeric silicate anions. The percentage of total dissolved Si were also assigned. Due to the formation of D4R and D4R( 1AI) in D4R and D4R(IAI) anions increases from 57% in the anions in TMA aluminosilicate solutions, the distribution of Si MeOH-free solution to 69% in the 10% MeOH solution to 80% among silicate and aluminosilicate species is very different from in the 20% MeOH solution. Although the concentrations of D4R that observed in TPA aluminosilicate solutions, but the level of and D4R(lAI) anions both increase with increasing MeOH AI incorporation into silicate species is very similar. This indicates concentrations, it appears that the proportion of D4R anions to that the nature of the alkylammonium cation strongly influences D4R( 1Al) anions remains fairly constant, indicating that the the distribution of silicate oligomers and only weakly affects the presence of MeOH does not preferentially stabilize the D4R( I AI) incorporation of AI into silicate anions. Thus, the level of AI anions over the D4R anions. incorporation into silicate species depends almost solely on the Figure 7 presents the 27AINMR spectra of the same solutions solution composition and pH. At identical silicate ratios, TMA for which the 29SiN M R spectra are presented in Figure 6. At and TPA aluminosilicate solutions behave similarly to changes 10 mol % MeOH, a band appears at 56 ppm due to AI(4Si)."sZ5 in the AI concentration. Both solutions exhibit a depolymerization The intensity of this band increases with increasing MeOH content of silicate and aluminosilicate anions to lower molecular weight and eventually masks the Al(3Si) band centered at 58.5 ppm. The species as the AI concentration is increased. The addition of remaining features in the spectrum are similar to those observed methanol to TMA aluminosilicate solutions increases the proin the absence of MeOH. The principal change in these bands portion of Si in D4R and D4R( 1Al) anions and suppresses the caused by the addition of MeOH is an increase in the line width. formation of all other silicate and aluminosilicate oligomers. A very broad peak centered at 80 ppm peak is observed as the Acknowledgment. This work was supported by the Director, MeOH concentration increases. This feature is a background peak Office of Energy Research, Office of Basic Energy Sciences, due to tetrahedral AI environments in the probe and sample tube. Materials Sciences Division of the US. Department of Energy, The fraction of AI atoms connected to Si atoms increases as the under Contract No. De-AC03-76SF00098, and by a grant from MeOH concentration increases from 0 to 20 mol %. A similar W. R. Grace and Co. Fellowship support for R.F.M. was provided level of AI incorporation into aluminosilicate species cannot be by Upjohn Co. deduced from the r)si NMR spectra shown in Figure 6, very likely Figure 6. %i spectra of TMA aluminosilicate solutions taken at 10 ' C : 2 mol % Si02, R =: 2, Si/AI molar ratio = 5. (a) 0.0 mol % MeOH, (b) 10 mol 76 MeOH. (c) 20 mol % MeOH. 2%i spectral frequencies are referenced to the monomeric peak in (a).
