372
J . Phys. Chem. 1991, 95, 372-378
NMR Investigations of Tetrapropylammonium Aluminosilicate and Borosilicate Solutions R. F. Mortlock, A. T. Bell,* and C. J. Radke Department of Chemical Engineering, University of California at Berkeley, Berkeley, California 94720 (Received: April 23, 1990) 29Si,27AI,and "B N MR spectroscopies were used to characterize dilute, highly alkaline tetrapropylammonium (TPA) aluminosilicate and borosilicate solutions. The compositions of the solutions range from 0.1 to 2 mol 5% SO2, silicate ratios ( R = [Si02]/[M20];M+ = TPA') from 0.1 to 2, and Si/AI or Si/B molar ratios from 0.25 to 10. 29Siand 27AINMR spectra provide evidence for the formation of aluminosilicate anions. Si is observed with Q,(IAI), Q2A(1Al),Q2(1Al) + Qsa(1 AI), and Q3(1AI) connectivity. Several new 29SiNMR peaks are assigned to specific aluminosilicate anions based on chemical shifts. 27AlNMR spectra of aluminosilicate solutions reveal peaks corresponding to AI bound to zero, one, two, and three siloxane bonds. The distribution of aluminosilicate anions is affected by the concentration of dissolved Si and AI, as well as the solution pH. The distribution of aluminosilicatespecies is described in terms of simple chemical equilibria. 29Siand "B NMR spectra of borosilicate solutions indicate that borate anions are less reactive than aluminate anions. IIB NMR spectra reveal only a small fraction of the dissolved B appears to form borosilicates.
Introduction
Zeolites are crystalline aluminosilicates which consist of a regular three-dimensional system of pores and cavities. The most useful applications of zeolites are as adsorbents, ion exchangers, and catalysts. In recent years, increased attention has been given to isomorphous substitution of such elements as B, Ga, Ge, Cr, Fe, V, Ti, Ce, and Zr for both Si and AI in the zeolite lattice.'-5 Elemental incorporation modifies both the acidic and shape selective properties of the final but, to date, only boronsubstituted pentad derivatives, called boralites, have found industrial application.'+l* Despite the great interest in modified zeolites, knowledge of zeolite synthesis is empirical, gained through experimentation and experience. Thus, the kinetics and reaction mechanisms of zeolite nucleation and crystal growth remain unclear. It is generally accepted that silicate anions in solution play a key role in determining the final zeolitic structure. Many studies have supported the view first proposed by Barrer that zeolitic nucleation and crystal growth occur in the solution phase.I3 Zeolites have been synthesized directly from the solution phase,I4 prompting considerable emphasis on the identification and study of silicate anions in dilute, highly alkaline, alkali-metal, and alkylammonium silicate solution^.^^-^^ Both 1-D15-21*24-33 and ( I ) Barrer, R. M. In Proceedings of the Sixth International Zeolite Conference; Olson, D., Bisio, A., Eds.; Butterworths: London, 1984; p 870. (2) Jansen, J. C.; Biron, E.; van Bekkum, H. In Innooations in Zeolite Materials Science; Grobet, P. J., Mortier, W. J., Van Sant, E. F., SchulzEkloff, G., Eds.; Elsevier: Amsterdam, 1988; p 133. (3) Ruren, X.;Wenqin, P. In Zeolites: Synthesis, Structure, Technology and Application; Drzaj, B., Hocevar, S., Pejovnik, S., Eds.; Elsevier: Amsterdam, 1985; p 27. (4) Borade, R. B.; Halgeri, A. B.; Prasada Rao, T. S. R. In Proceedings of the 7th International Zeolite Conference; Murakami, Y . ,Iijima, A,, Ward, J. W., Eds.; Elsevier: Amsterdam, 1986; p 851. (5) Chu, C. T. W.; Chang, C. D. J . Phys. Chem. 1985, 89, 1569. (6) Coudurier, G.; Vedrine, J. C. Pure Appl. Catal. 1986,58 (IO), 1389. (7) Coudurier, G.; Auroux, A.; Vedrine, J. C.; Farlee, R. D.; Abrams, L.; Shannon, R. D. J . Card. 1987, 108, I . (8) Ratnasamy, P.;Hedge, S. G.; Chandadkar, A. J. J. Catal. 1986,102, 467. (9) Auroux, A.: Sayed, M. B.; Vedrine, J. C. Thermochim. Acta 1985.93, 557. (IO) Datka, J.; Piwowarska, Z . J. Chem. SOC.,Faraday, Trans. 1 1989, 85 ( I ) , 47. ( I I ) Taramasso, M.; Perego, G.; Notari, B. In Proceedings ofthe Fifth International Conference on Zeolites; Rees, R. V. C., Ed.; Heyden: London, 1980; p 40. (12) Chandawar. K. H.: Kulkarni, S. B.; Ratnasamy, P. Appl. Catal. 1982, 4 , 287. ( I 3) Barrer. R. M. The Hydrothermal Chemistry . of. Zeolites; Academic Press: London, 1982. (14) Ueda, S.; Kadeyama, N.; Koizumi, M. In Proceedings of the Sixth International Zeolite Conference; Olso. D..Bisio. A,. Eds.; Butterworths: London, 1984; p 905. (15) McCormick, A. V.; Bell, A. T.; Radke, C. J. Zeolites 1987, 7 , 183. (16) McCormick, A. V.; Bell, A. T.; Radke, C. J. J. Phys. Chem. 1989, 93. 1737.
2-D34-37FT liquid-line NMR spectroscopies have proven to be powerful, nonintrusive, and quantitative in situ tools for studying silicate anions in dilute solution. Although silicate solutions have been characterized extensively, metallosilicate solutions have received less attention. Most of the metallosilicate solution work has focused on aluminosilicate solution^.^^-^* The evidence for
(17) McCormick, A. V.; Bell, A. T.; Radke, C. J. J . Phys. Chem. 1989, 93, 1733. (18) McCormick, A. V.; Bell, A. T. Catal. Reu. 1989, 3 (1 & 2), 97. (19) Kinrade, S. D.; Swaddle, T. W. Inorg. Chem. 1988, 27,4253. (20) Kinrade, S. D.; Swaddle, T. W. Inorg. Chem. 1988, 27, 4259. (21) Kinrade, S. D.; Swaddle, T. W. 1.Am. Chem. Soc. 1986,108,7159. (22) Dent Glasser, L. S.; Lachowski, E. E. J . Chem. SOC.,Dalton Trans. 1980, 393. (23) Dent Glasser, L. S.; Lachowski, E. E. J . Chem. SOC.,Dalton Trans. 1980, 399. (24) Engelhardt, G.; Hoebbel,D. J . Chem. SOC.,Chem. Commun. 1984, 514. (25) Harris, R. K.; Knight, C. T. H.; Hull, W. E. In Soluable Silicates; Falcone, J. S., Ed.; American Chemical Society: Washington, DC, 1982; ACS Symp. Ser. No. 194; p 78. (26) Harris, R. K.; Knight, C. T. G.; Hull, W. E. J . Am. Chem. Soc. 1981, 103, 1577. (27) Harris, R. K.; Newman, R. H. J . Chem. Soc., Faraday Trans. 2 1977, 73, 1204. (28) Harris, R. K.; Jones, J.; Knight, C. T. G.; Pawson, D. J . Mol. Struct. 1980, 69, 95. (29) Harris, R. K.; Knight, C. T. G. J . Mol. Struct. 1982, 78, 273. (30) Harris, R. K.; Knight, C. T. G. J . Chem. SOC.,Faraday Trans. 2 1983, 79, 1525. (31) Harris, R. K.; Knight, C. T. G. J . Chem. SOC.,Faraday Trans. 2 1983, 79, 1539. (32) Harris, R. K.; Jones, J.; Knight, C. T. G.; Newman, R. H. J . Mol. Liq. 1984, 29, 63. (33) Groenen, E. J. J.; Kortbeek, A. G. T. G.; Mackay, M.; Sudmeijer, 0. Zeolites 1986, 6, 403. (34) Harris, R. K.; OConner, M. J. J . Mogn. Reson. 1984, 57, 115. (35) Knight, C. T. G.; Kirkpatrick, R. J.; Oldfield, E. J. Am. Chem. Soc. 1987, 109, 1632. (36) Knight, C . T. G.; Kirkpatrick, R. J.; Oldfield, E. J. Magn. Reson. 1988, 78, 31. (37) Knight, C. T. G. J. Chem. SOC.,Dalton Trans. 1988, 1457. (38) McCormick, A. V.; Bell, A. T.; Radke, C. J. J . Phys. Chem. 1989, 93, 1741. (39) Kinrade, S. D.; Swaddle, T. W. Inorg. Chem. 1989, 28, 1952. (40) Dent Glasser, L. S.; Harvey, G. In Proceedings of the Sixth International Zeolite Conference;Olson, D., Bisio, A., Eds.; Butterworths: London, 1984; p 925. (41) Harvey, G.; Dent Glasser, L. S. In Zeolite Synthesis; Occelli, M. L., Robson, H. E., Eds.; American Chemical Society: Washington, DC, 1989; ACS Symp. Ser. No. 398; p 49.
0022-365419 112095-0372$02.50/0 0 1991 American Chemical Society
Tetrapropylammonium Aluminosilicate and Borosilicate
dissolved anions containing aluminosiloxane linkages has come from direct observation of resonances due to silicon nuclei connected to aluminum through a bridging oxygen39and indirectly from 29Siand 27Al N M R peak broadening and ~ h i f t i n g . ~ * - ~ . ~ ' The purpose of the present work is to provide evidence for the presence of dissolved silicate oligomers containing Si-O-AI and Si-0-B linkages and to illustrate the usefulness of FT 29Si,27Al, and "B liquid-line N M R techniques for the study of aluminosilicate and borosilicate anions through a detailed study of dilute, highly alkaline tetrapropylammonium (TPA) metallosilicate solutions.
Experimental Section Tetrapropylammonium aluminosilicate solutions were prepared in polystyrene test tubes by dissolving fumed silica, Si02(Cab0-Sil, grade EH-5), in 40 wt % aqueous TPAOH (Alfa Products) for approximately 1 week. Aluminum wire (99.999%, Aldrich) was added to these solutions to achieve the desired Si/AI molar ratio. After dissolution of the AI wire, the appropriate amounts of deionized water and DzO(Cambridge Isotopes) were added, and the solutions were allowed to equilibrate for several days. Borosilicate solutions were prepared by an analogous procedure with orthoboric acid (99.999%, Aldrich) dissolved in a H 2 0 / D z 0 mixture. Subsequently, this aqueous borate solution was added to the TPA silicate solution. The compositions of the aluminosilicate and borosilicate solutions range from 0.1 to 2 mol % Si02, silicate ratios ( R = [Si02]/[M20]; M+ = TPA+) from 0.1 to 2, and Si/AI or Si/B molar ratios from 0.25 to IO. Compositions of the final solutions were chosen to avoid gels or solid precipitates. The NMR spectra were recorded on a Bruker 500 MHz spectrometer with a deuterium lock resonance. 29Sifid's were collected at 99.356 MHz, using 8 - ~ spulses (=70°) and 12-s recycle delays. The number of scans varied from 2000 to 8000 depending on the silica and aluminum concentrations. The largest T,'s observed in this study were between 1 and 2 s. Therefore, a 12-s recycle delay was considered sufficient to ensure relaxation after a 70° pulse. Proton decoupling was not necessary due to rapid proton exchange, and no Si-Si splitting was observed at 29Si natural abundance. A large spectral width was employed to avoid foldback of a background glass peak. Standard polynomial baseline corrections were used to eliminate this broak peak. 27AI fid's were collected at 130.290 MHz, using 2000 10-ps pulses and 0 5 s recycle delays. "B fid's were collected at 160.460 MHz, using 2000 9.7-ps pulses and 0.1-s recycle delays. The standard Lorentz-to-Gauss transformation was applied to all z7AIand IlB fid's for resolution e n h a n ~ e m e n t . ~Each ~ N M R spectrum was internally scaled to the tallest peak in that spectrum. Therefore, peak heights between spectra cannot be compared quantitatively. Results and Discussion
All of the solutions studied were stable over periods of up to several months. None of the solutions developed gels or precipitates. NMR spectra of solutions aged 3-6 months were identical with the spectra obtained after 1-day aging. As a consequence, the distribution of species observed by N M R spectroscopy is assumed to be an equilibrium distribution. Aluminosilicate Solutions. Figure 1 shows 29SiN M R spectra of TPA silicate solutions prepared with (b) and without (a) aluminum. In the absence of aluminum, narrow lines are observed, corresponding to Si in different chemical environments. The Si connectivity, designated by Qo, QI, Qza, Q2 + Q3*, and Q:, is indicated above spectrum a. (The symbol A designates Si in three-membered rings.) The principal features in this spectrum are assigned to specific silicate structures on the basis of previous studies. 15-21.24-37 When aluminum is added to the TPA silicate solution to achi ve a Si/AI molar ratio of 4, many new peaks appear, and some of the peaks present in the original spectrum broaden. These new (42) Dent Glasser, L. S.; Harvey, G.J . Chem. Soc., Chem. Commun. 1984, 1250. (43) Ferige, A. G.;Lindon, J. C. J . Magn. Reson. 1978, 31, 337.
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The Journal of Physical Chemistry, Vol. 95, No. 1 , 1991 313 0 0
01
n
&A
Or + & A
01
PI I' I ',
I
I
0
I
-5
I
I
I
I
1
-10
-15 PPm
-20
-25
-30
Figure 1. (a) 29Sispectrum of a TPA silicate solution of composition 2 mol % SO,; R = 2 at 5 OC. (b) 29Sispectrum of a TPA aluminosilicate solution of composition 2 mol %; R = 2 and Si/AI molar ratio = 4 at 5 "C. In the schematic structures, a closed circle represents the Si atom corresponding to the assigned peak, an open circle represents an AI atom, a closed square represents a Si atom resonating in another part of the spectrum, and a solid line represents a bridging oxygen atom. 29Si spectral frequencies are referenced to the Qopeak in the AI-free solution.
peaks are associated with the formation of aluminosilicate anions. The regions in which Si resonances attributed to aluminosilicates appear are designated as Ql(lAl), QzA(lAl),Qz(lAl) + Q3a(lAI), and Q3(1AI). These connectivity assignments are based on previous studies of aluminosilicate solutions39and on chemical shift ranges determined from solid-state N M R analyses of well-defined aluminosilicates.44 As discussed below, the proposed assignments of Si connectivity are internally consistent with the interpretation of 27Al N M R spectra obtained in this study. Regions of the spectrum where one would expect to observe peaks for Si atoms bonded to two AI atoms are indicated above Figure 1b as Q2(2AI) + Q3,(2Al) and Q3(2Al). Assignments of the new features in spectrum b to specific aluminosilicate species are presented schematically in Figure I . In these schematic structures, a closed circle represents the Si atom corresponding to the assigned peak, an open circle represents an AI atom, a closed square represents a Si atom resonating in another part of the spectrum, and a solid line represents a bridging oxygen atom. These assignments are based purely on the location of the chemical shift and are consistent with the assignments made recently by Kinrade and Swaddle39 for similar features observed in sodium aluminosilicate solutions. The effects of temperature on the spectrum of an aluminosilicate solution are presented in Figure 2. As the temperature at which the fid is collected increases, the 29SiNMR lines in the resulting spectrum broaden and coalesce. Similar effects of temperature on 29SiNMR line widths have been observed p r e v i o ~ s l yand ~~~~*~~ have been attributed to the presence of paramagnetic impurities3z or chemical e x ~ h a n g e ,both ~ ~ *of~which ~ cause peak broadening (44) Engelhardt, G.;Michel, D. High-Resolution Solid-state NMR of Silicafes and Zeolites; Wiley: Chichester, 1987.
314
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The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 T
42 'C
J
(e)
(d)
T = 35 'C
(b)
T = 10 'C
(a)
T.5.C I
lb0
I
i'0
d0
$0
I
6b
I
I
50
i
30
4b
PPm I
0
-5
-10
-15
-20
-25
-30
PPm Figure 2. 29Sispectrum of a TPA aluminosilicate solution of composition 2 mol % ' SO2;R = 2 and Si/AI molar ratio = 5 at (a) 5, (b) IO, (c) 22, (d) 35, and (e) 42 OC. 29Sispectral frequencies are referenced to the Qo peak in the 5 "C spectrum.
as the temperature increases. Since special efforts were made in the present study to avoid impurities, it is assumed that chemical exchange is the principal cause of broadening. In their study of sodium aluminosilicate solutions, Kinrade and Swaddle39 reported that resonances due to dissolved species containing Si-O-AI linkages were observed at 5 O C , where silicate and aluminate exchange is slow. They noted that the rate of chemical exchange increases with temperature and, above 25 O C , becomes so rapid relative to the NMR acquisition time scale that Si resonances in Si-O-AI structures broaden into the background noise. The results of the present study indicate that the effects of temperature on chemical exchange are much weaker in TPA aluminosilicate solutions than i n Na aluminosilicate solutions. Figure 2 illustrates that even at temperatures as high as 42 O C the resonances due to species containing aluminosiloxane bonds are still apparent. These observations concur with the results of Knight et al.,45showing that exchange reactions in tetraalkylammonium silicate solutions are slower than in alkali-metal silicate solutions. Also relevant are the results of silica dissolution studies which show slower gel dissolution rates in alkylammonium hydroxide solutions than in alkali-metal hydroxide solutions. This effect is attributed to both the hydrophobicity and water-structuring effects of alkylammonium base^.^^,^' Figure 3 illustrates 27AIN M R spectra taken at progressively higher temperatures of the same aluminosilicate solutions for which the 29SiNMR spectra were presented in Figure 2. Each of the spectra presented in Figure 3 contains four principal features located at 74-77,69-72,64-67, and 58-61 ppm. In agreement with previous s t u d i e ~ , ~these ~ , ~ 'peaks are assigned to AI(OSi), AI(lSi), A1(2Si), and AI(3Si), respectively. The spectra in Figure 3 also exhibit small peaks at 71-72,6648, and 62-63 ppm. Such peaks have not been observed in previous studies of aluminosilicate solutions. Resolution of the latter features is a direct consequence (45) Knight, C. T. G . ; Kirkpatrick, R. J.; Oldfield, E. J . Chem. Soc., Chem. Commun. 1986,66. (46) Wijnen, P. W. J. G.; Beelen, T. P. M.; de Haan, J. W.; Rummens, C . P. J.: van de Ven, L. J. M.; van Santen, R. A. J . Non-Cryst. Solids 1989,
109, 85.
(47) Wijnen, P.W. J. G.; Beelen, T. P. M.; de Haan, J. W.; van de Ven, L. J. M.; van Santen, R. A. Colloids Surf. 1990, 45, 2 5 5 .
Figure 3. 27Alspectrum of a TPA aluminosilicatesolution of composition 2 mol % SO2;R = 2 and Si/AI molar ratio = 5 at (a) 5, (b) 10, (c) 22, (d) 35, and (e) 42 OC. 27AI spectral frequencies are referenced to octahedral AI3+ ion in an AIC13 aqueous solution. (0)
SJAl s 3
0
-5
-10
-15
-20
-25
-30
PPm
Figure 4. 29Sispectrum of a TPA aluminosilicate solution at 5 OC of composition 2 mol % SO,; R = 2 and (a) Si/AI (molar ratio) = m, (b) Si/AI = 10, (c) Si/AI = 5 , (d) Si/AI = 4, and (e) Si/AI = 3. 29Si spectral frequencies are referenced to the Qopeak in (a).
of using the Lorentz-Gauss transformation. As the temperature is raised above 5 OC,some of the 27AlNMR peaks in Figure 3 broaden and shift due to chemical exchange. The peak for free aluminate ions, AI(OSi), broadens and shifts upfield toward the AI( 1Si) peak, which in turn broadens slightly and shifts downfield. This trend might be attributed to an exchange of AI between environments in which AI is bonded to zero and one nearest Si atoms. The Al(2Si) peak is broad at 5 O C probably due to viscous effects, as suggested by Kinrade and Swaddle.39 As the temperature increases, the Al(2Si) peak sharpens and shifts downfield slightly. The Al(3Si) peak does not shift with increasing temperature, indicating that the rate of exchange of AI from such environments is not strongly affected by temperature. In addition to causing peak shifts, raising the temperature causes a significant change in the distribution of peak intensities. Of particular note is the observation that the proportion
Tetrapropylammonium Aluminosilicate and Borosilicate
The Journal of Physical Chemistry, Vol. 95, No. I , 1991 375
-
-
o 2 9 ~ NMR i results A2 0
0.4
0.2
0.2
0
0.6
0.6
0.4 mole% AI
mole% AI
Figure 5. Percentage of total silica present in the connectivity environments as a function of the mol 7% of AI in a TPA aluminosilicate solution of composition 2 mol 7% S O 2and R = 2 at 5 OC. Error bars are smaller than the graph symbols.
7 NMR ~ ~ results
Figure 7. Estimates of the number of aluminosiloxane bonds calculated from the assignments presented in Figure 1 and the integrated areas of spectral regions of the 29Siand 27AI NMR results in Figures 4 and 6. Error bars are smaller than graph symbols.
R
=
1.75
Si/Al
I
lb)
r~
100
k
1
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70
1
1
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'
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I
I
40
I
R
=
=
4.375
1.25
I
30
PPm
R = l Si/AI = 2.5
Figure 6. 27AI spectrum of an aluminosilicatesolution at 22 OC of the composition 2 mol % SiO,; R = 2 and (a) Si/AI (molar ratio) = 10, (b) Si/AI = 5, (c) Si/AI = 4, and (d) Si/AI = 3. 27AIspectral frequencies referenced to octahedral AI" ion in an AIC13aqueous solution.
of AI in the Al(2Si) environment increases at the expense of AI in the AI( ISi) environment as the temperature increases. The effects of aluminum content on the 29SiNMR spectra of aluminosilicate solutions is presented in Figure 4. The silica concentration is 2 mol %, and the silicate ratio, R ( R = [Si02]/[(TPA),0]), is 2 in each case. As the Si/AI ratio decreases from infinity to 3, there is a progressive broadening of the peaks associated with silicate structures and a growth in the intensity of the broad features assigned to Si in aluminosilicates. It is also interesting to observe that the width of the tall peak at -1 7.3 ppm assigned to Si atoms in the double three-membered ring (D3R)appears to be unaffected by the presence of Al, suggesting that this species does not interact with aluminate ions in TPA-aluminosilicate solutions. This observation is consistent with the work of Engelhardt and M i ~ h e Iwho , ~ ~observed, only after heat treatment at elevated temperatures, three new features in the 29SiNMR spectrum assigned to Si in a D3R(IAI) structure in TEA aluminosilicate solutions. Engelhardt and Michel postulate that the D3R anion is very stable in TEA aluminosilicate solutions and that incorporation of AI occurs only after destruction of the cage at elevated temperatures. Van den Berg et a1.4" also suggest that high temperature cause destruction of the D3R cage, resulting in the formation of aluminosilicates. We find similar results in (48) Van den Berg, J. P.; de Jong-Versloot, P. C.; Keijsper, J.; Post, M. F. M. In Innovations in Zeolite Materials Science; Grobet, P.J., Mortier, W. J., Van Sant, E. F., Schulz-Ekloff, G., Eds.; Elsevier: Amsterdam, 1988; p 85.
1
0
1
I
-5
!
1
-10
'
1
-15
1
I
-20
I
I
-25
1
1
-30
PPm
Figure 8. 29Sispectrum of a TPA aluminosilicate solution at 5 OC of composition 0.4 mol 7% AI, 1 mol 7% (TPA)20 and (a) R = 2, Si/AI (molar ratio) = 2.5; (b) R = 1.25, Si/A1 = 3.125; fc) R = 1.5, Si/AI = 3.75; (d) R = 1.75, Si/AI = 4.375; and (e) R = 2, Si/AI = 5. 29Si spectral frequencies are referenced to the Qopeak in (e).
TPA aluminosilicate solutions. The peak assigned to the D3R anion at -17.3 ppm is unaffected by the presence of AI (Figure 4) but broadens at higher temperatures (Figure 2). Figure 5 plots the percentage of the total silica present in different connectivity environments as a function of the mole percent AI added to the solution. As the mole percent of AI increases, the fraction of Si with Q2 + Q3a and Qs connectivity declines as the fraction of Si with Q,(IAI), Q2a(1A1), Q2 Q3*( 1AI), and Q3(1Al) connectivity increases. The aluminate anions appear to react readily with small silicate anions to produce aluminosilicates and cause a depolymerization of high molecular weight silicate structures. We will return to this point when the solution equilibria are discussed. 27AIN M R spectra corresponding to the 29SiNMR spectra in Figure 4 are shown in Figure 6 . It is apparent that, as the mole percent of A1 increases, the signal-to-noise ratio improves, but the relative intensities of the peaks do not change. Thus, the AI environments remain the same as the AI concentration increases even though silicate oligomers are breaking down into lower molecular weight silicate structures.
+
376 The Journal of Physical Chemistry, Vol. 95, No. I , 1991
+
n
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Mortlock et al.
+
=&OH =Si-O-Si= H20). For simplicity we refer to monomeric silicate anions as SIx-and oligomeric silicate anions as S/-. The equilibrium between SIr and Sn* can be represented by nSlr Snx- + (n - 1)OH(2)
-
Aluminate anions, AI(OH)4-, represented by AI-, react with silicate anions in the following fashion: A,-
+mSP
F!
(Al(Sn),,Jx-
+ OH-
(3)
From reaction 2 it is evident that the oligomerization of silicate anions is favored by decreasing the pH of the solution. This relationship is well-documented in the literature and has been observed previously by 29Si N M R spectroscopy.'5~18~1g~z6~2g~30 Decreasing pH also enhances the formation of aluminosilicate anions, as indicated by reaction 3. When metallic AI is dissolved in a TPA silicate solution, aluminate ions are formed via the reaction
R = 0.5 SilAl i 1.25 R s 0.25 SilAl = 0.525
( i ' R i 0.1 0.25 SilAl
AI
NO Si
I
90
'
1
80
I
I
70
I '
I
60 PPm
I
'I
50
" ' I
40
'
1
30
Figure 9. (a) 27AI spectrum of a 0.093 M aluminate solution. 27Al spectrum of a TPA aluminosilicate solution at 22 'C of composition 0.4 mol 5% AI, I mol 5% (TPA)20 and (b) R = 0.1, Si/AI (molar ratio) = 0.25; (c) R = 0.25. Si/AI = 0.625; (d) R = 0.5, %/AI = 1.25; (e) R = 0.75, Si/AI = 1.875; (f) R = I , Si/AI = 2.5; (g) R = 1.25, %/AI = 3.125; (h) R = 1.5, %/AI = 3.75; (i) R = 1.75, Si/AI = 4.375; and 0 ) R = 2, Si/AI = 5. 27Alspectral frequencies are referenced to octahedral AI3+ ion in a n AICI, aqueous solution.
To check the consistency of the assignments of peaks in the 29Si and 27AINMR spectra, the number of AI-0-Si bonds was calculated as a function of mole percent AI in solution. Figure 7 shows the results of the calculation obtained from the 29Siand 27AINMR spectra. It is immediately apparent that the proposed assignments for 29Siand 27AIpeaks lead to equivalent values for the percentage of Si involved in aluminosiloxane bonds for a given mole percent of AI in solution. The acquisition temperature for 29Sifid's (5 "C)and 27AIfid's (22 "C)did not affect the calculation between 5 and 22 O C . The results of experiments in which increasing amounts of Si02 are dissolved in solutions of constant AI and TPAOH concentrations are shown in Figures 8 and 9. As the amount of dissolved SiO, increases, Figure 8 illustrates that the distribution of Si peaks shifts from environments with low to high connectivity. The proportion of Si in aluminosiloxane bonds decreases with increasing Si concentration, as evidenced by the decrease in the relative intensities of peaks assigned to aluminosilicates oligomers. *'AI N MR spectra corresponding to the 29SiN M R spectra in Figure 8 are presented in Figure 9. In the absence of dissolved silica, a single narrow peak is observed at 80 ppm characteristic of AI(OH)4- in basic aqueous solutions. As increasing a'mounts of Si02are dissolved, the position of the aluminate peak shifts upfield and new peaks appear for AI bonded to one, two, and three siloxane linkages. The distribution of AI shifts progressively from AI(0Si) toward Al(3Si) as the AI/Si ratio increases, and small upfield shifts of 5.5, 0.25, and 0.25 ppm are observed for the AI(OSi), AI( ISi), and AI(2Si) peaks, respectively. The Al(3Si) peak changes intensity but does not shift significantly. The results presented in Figures 4,6,8, and 9 can be interpreted in the following manner. When SOzdissolves in basic solutions, monomeric silicate species are produced. This process can be represented by SiO,
+ xOH- + (2 - x ) H 2 0 G (SiO,(OH),-,)X-
(1)
The monomers can polymerize to form oligomeric silicates via condensation of OH groups and release of HzO (e.g., ESi-OH
+ OH- + 3H20 + AI(OH), + 1.5H2
(4) The consumption of OH- anions due to reaction 4 is partially offset by the formation of aluminosilicate anions according to reaction 3. For the experiments presented in Figures 4 and 6 , the 27Al N M R spectra show that most of the AI is present in aluminosilicate species. As a consequence, for these experiments, it is reasonable to conclude that the OH- concentration in solution is not strongly perturbed by the addition of AI. The effects of AI concentration on the distribution of AI connectivities can be deduced from reaction 3. At equilibrium, the following relationship can be written: [(AI(Sn)m)'-l
= K3- [Snrl"'
(5)
[OH-] We discover that the distribution of A1 connectivities is not influenced directly by the concentration of dissolved AI. However, this distribution is affected by the concentration of silicate and hydroxyl anions. Since the total concentration of Si is constant for the experiments presented in Figures 4 and 6, and since the OH- concentration is not expected to vary significantly with the Si/AI ratio (see above), we argue that the distribution of AI connectivities should not strongly depend on the Si/AI ratio. This is in fact what is observed in Figure 6. Figures 4 and 5 show that as the mole percent of AI in solution increases, the fraction of si in Q2 Q3Aand Qs connectivities decreases monotonically. These changes are offset primarily by a rise in Si atoms attached to one AI atom. As seen in Figure 5, the distribution of connectivities for such Si atoms decreases in order QI(1Al) = Qz(lAl) > QtA(lAl) > Q3(1Al). These trends suggest that aluminate anions react preferentially with small silicate oligomers. The consumption of the smaller silicate species drives reaction 2 to the left, thereby causing a decrease in the population of larger silicate oligomers. The trends observed in Figures 8 and 9 can also be explained with the aid of reactions 1-3. As indicated by reaction I , addition of SiO, to a TPA aluminate solution causes a reduction in the pH. The observed changes in the distribution of silicate and aluminosilicate species can then be ascribed to the effects changing Si and OH- concentrations. As more Si02is dissolved, the Si/Al ratio increases and the OH- concentration declines. These changes cause an increase in the proportion of silicate oligomers and aluminosilicates, as dictated by reactions 2 and 3. The changes in AI connectivity with increasing %/AI ratio are also readily explained. As is evident from reaction 3, the AI connectivity is expected to increase as the pH decreases and the concentration of dissolved Si increases. Borosilicate Solutions. Figure 10 illustrates the effects of adding increasing amounts of H$O, to a TPA silicate solution. As the Si/B ratio decreases, all of the 29Siresonances shift upfield slightly and broaden, and a progressive change is observed in peak area toward species with higher Si connectivity. However, in contrast to aluminosilicate solutions, no new features appear in 1'41-1
+
Tetrapropylammonium Aluminosilicate and Borosilicate
-5
0
-10
-I5
-20
-25
The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 311
iI
-30 I
PPm
-14
29Sispectrum of a TPA borosilicate at 5 OC of composition 3 mol 7% SiO,; R = 2 and (a) Si/B (molar ratio) = a,(b) Si/B = 10, (c) Si/B = 5 , (d) Si/B = 4, and (e) Si/B = 3. 29Sispectral frequencies are referenced to the Qopeak in (a). Figure 10.
I
Si/B
E
5
I
-18
= lo
si'B I
-22
PPm
spectrum of a TPA borosilicate solution at 5 OC of composition 3 mol 7% SiO,; R = 2 and (a) Si/B (molar ratio) = IO, (b) Si/B = 5 , (c) Si/B = 4, and (d) Si/B = 3. I'B spectral frequencies are referenced to the B(OH)3peak in a 0.1 M (aqueous) boric acid solution. Figure 12. "B
lines are considerably broader when boron is present. It is postulated that the large line widths observed for the borosilicate solutions may be due to relaxation of 29Sinuclei through chemical exchange with borate ions. Evidence for chemical interactions between borate and silicate anions can be drawn from the IIB N M R spectra shown in Figure 12. These spectra are taken with the same solutions for which I Si/HCI = 5 29Si N M R spectra were shown in Figure IO. The "B N M R spectra exhibit a large peak at -18 ppm due to B(0H); ani0ns.4~ Two small peaks are also observed at -18.8 and -19.2 ppm. These features are assigned to B( 1Si) and B(2Si) environments, respectively. Similar assignments have been made by Irwin et al.so for acidic borosilicate solutions. The B( 1Si) and B(2Si) peaks grow with increasing boric acid concentration as expected, if a reaction similar to reaction 3 (with B replacing AI) is assumed to describe the formation of borosilicate species. The growth of these features is partially masked by the intensity and width of the B(OH),- peak as the boron concentration increases. As the H3B03 concentration increases in solution, the IlB N M R peaks 0 -5 -IO -I5 -20 -25 -30 broaden as a result of increased chemical exchange between borate ppm anions and Si oligomers, and the borate peak shifts slightly Figure 11. 29Sispectrum of a TPA borosilicate solution at 5 OC of downfield as a result of the decrease in the pH of the solution. composition 3 mol o/o SiO,; R = 2 and (a) no boron or HCI; (b) Si/HCI Most significant, however, is the observation that the proportion molar ratio = 5 , no B; and (c) Si/B molar ratio = 5 , no HCI. 29Si of B bonded to Si is very small. This very likely explains the spectral frequencies are referenced to the Qopeak in (a). absence of peaks in the 29SiN M R spectrum (Figure IO) attributable to borosilicate species. Although the proportion of Si the 29SiN M R spectrum that can be attributed to the formation involved in borosiloxane bonds is low, the 29Sipeak line widths of borosilicate species. appear very sensitive to the concentration of boron in solution, The shift toward higher Si connectivity as the Si/B ratio deindicating that the borate-silicate exchange rate is significant in creases is attributed to the decrease in pH caused by the addition transverse ( T2)relaxation. of H3BO3. This point is illustrated in Figure 1 I , which shows 29Si Figure 13 shows the effects of adding progressively more Si02 N M R spectra taken when HCI rather than H3BO3 is added to to a solution containing a fixed amount of H3B03and TPAOH. the silicate solutions such that Si/HCl = Si/B = 5. It is evident As the Si concentration increases, the distribution of 29SiN M R in Figure 1 1 that the distribution of peak areas in virtually the peaks shifts in the direction of higher Si connectivities. This trend same regardless of whether the pH is lowered by HCI or H3BO3 is due to the reduction in pH caused by the dissolution of Si02 addition. It is also evident that addition of HCl and H3BO3 cause (reaction 1). The corresponding IiB N M R spectra, shown in an equivalent upfield shift in the 29Siresonances. Shifts of this 14, indicate that the intensities of the B(1Si) and B(2Si) type have been observed p r e v i o ~ s l yand ~ ~can ~ 'be~ attributed ~ ~ ~ ~ ~ ~ ~Figure ~ to increased shielding of 29Sinuclei occurring as a consequence (49) Noth, H.; Wrackmeyer, B. Nuclear Magnetic Resonance Spectrosof a decrease in the degree of ionization of SiOH groups due to copy of Boron Compounds; Diehl, P.,Fluck, E., Kosfeld, R.,Eds.; Springa lowering of the solution pH. er-Verlag: Berlin, 1978; Vol. 14. Comparison of the 29SiN M R peak widths in the spectra of the (50) Irwin, A. D.; Holmgren, J. S.; Zerda, T. W.; Jonas, J. J . Non-Cryst. solutions containing borate and chloride anions shows that the Solids 1987, 89, 191.
I
J
378 The Journal of Physical Chemistry, Vol. 95, No. 1, 1991
R = 0.67 Si/B = 3.33
I
0
I
-5
I
-10
I
-15
-20
-25
-30
ppm
Figure 13. 29Sispectrum of a TPA borosilicate solution at 5 OC of composition 0.3 mol 7% boric acid, 1.5 mol 5% (TPA)20, and (a) R = 0.67, Si/B (molar ratio) = 3.33; (b) R = 1.0, Si/B = 5 ; (c) R = 1.33, Si/B = 6.67; and (d) R = 2, Si/B = IO. 29Sispectral frequencies are referenced to thc Qopcak in (a).
Mortlock et al. It was found for borosilicate solutions that the distribution of 29Siconnectivities has no noticeable dependence on temperature between 5 and 22 OC. However, the IIB NMR spectra do show a temperature dependence. IlB N M R peaks that are assigned to B(1Si) and B(2Si) are observed only at temperatures below 10 OC. These peaks broaden due to chemical exchange and become indistinguishable from the background noise at temperatures above 10 "C. Releuance to Zeolite Synthesis. It is clear from these results that aluminate anions are far more reactive than borate anions toward reaction with dissolved silicate anions. Consistent with this observation, studies of zeolite synthesis show that the incorporation of boron into the lattice occurs only when extreme precautions are used to eliminate aluminum from the synthesis batch because aluminate ions preferentially react with silicate oligomer^.^^-^^ The results of the present experiments provide direct evidence that TPA aluminosilicate solutions contain a large number of aluminosilicate oligomers existing in solution. The identification of a single silicate or aluminosilicate anion or a small group of oligomers acting as precursors for nucleation and crystal growth from the solution phase of zeolite synthesis remains a formidable task. Species that might play a key role in zeolite syntheses involving TPAOH are the lower molecular weight silicate and aluminosilicate anions identified in Figure 1 and also the D3R silicate anion which is very stable in alkaline TPA silicate solut i o n ~ . ~ ~ ~ ~ ~
Conclusions
R i 0.67 Si/B = 3.33
-20 -21 -22 -23 PPm Figure 14. "B spectrum of a TPA borosilicate solution at 5 O C of composition 0.3 mol % boric acid, 1.5 mol % (TPA)20, and (a) R = 0.67, Si/B (molar ratio) = 3.33; (b) R = 1.0, Si/B = 5 ; (c) R = 1.33, Si/B = 6.67; and (d) R = 2, Si/B = IO. "B spectral frequencies are refer-16
-17
-18
-19
enced to the B(OH), peak in a 0.1 M (aqueous) boric acid solution. Spcctra arc offsct for clarity.
peaks grow as the concentration of Si in solution increases due to an increase in the formation of borosilicate species. The broadening of the B(OH)4- peak as the Si concentration is increased is due to the increased interaction of borate anions with silicate oligomers.
The present study provides convincing evidence for the formation of aluminosilicate anions through the reaction of aluminate and silicate anions. Silicon is observed with Q I (1AI), 02,( 1Al), Q2 + Q3A(1A1),and Q3(1A1) connectivity. Several new peaks observed in 29SiNMR spectra of aluminosilicate solutions are assigned to specific anions. 27Al N M R spectra reveal evidence for A1 bound to zero, one, two, and three Si atoms through oxygen atoms. The proportion of Si and A1 present in aluminosilicate species is affected by the concentration of dissolved Si and AI, as well as the solution pH. The distribution of aluminosilicate species can be described in terms of simple chemical equilibria. 29Siand "I3 NMR spectra of borosilicate solutions indicate that borate anions are less reactive than aluminate anions. Only a very small fraction of the dissolved B appears to react to form borosilicates.
Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the US.Department of Energy, under contract No. DE-AC03-76SF00098 and by a grant from W. R. Grace and Co. Fellowship support for Robert F. Mortlock was provided by The Upjohn Co. The authors thank D. M. Ginter for valuable discussions. Registry No. TPAOH, 4499-86-9.
(51) Bodart, P.;Nagy, J. B.; Gabelica, Z.; Derouane, E. G . Appl. Catal. 1986, 24, 315. ( 5 2 ) Howden, M. G. Zeolites 1985, 5, 334. (53) Chu, C. T. W.; Kuehl, G . H.; Lago, R. M.; Chang, C. D. J . Catal. 1985, 93, 451.
