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J. Phys. Chem. 1996, 100, 4760-4764
Oxygen-17 NMR Study of Aqueous Potassium Silicates Stephen D. Kinrade Department of Chemistry, Lakehead UniVersity, Thunder Bay, Ontario, Canada P7B 5E1 ReceiVed: September 13, 1995X
A quantitative study of the dependence of 17O spectra (67.8 MHz) on the temperature (275-425 K) and alkalinity ([OH-]:[SiIV] ) 1:1 to 2:1) of potassium silicate solutions has yielded six new, albeit tentative, peak assignments. Peak-widths are affected by quadrupolar relaxation (generally more significant for bridging oxygens, tSiOSit) and by rapid intermolecular OsO exchange. At [OH-]:[SiIV] ) 1:1, exchange broadening is approximately uniform for all silicate nonbridging (-OSit) and bridging (tSiOSit) oxygen resonances as well as for the solvent resonance. This implies that intermolecular OsO exchange between silicate anions as well as between silicate and solvent molecules (and aqueous OH-) is controlled by the silicate condensation/ hydrolysis process. Classical band-shape analysis yields chemical exchange lifetimes that are in accordance with rates measured by 29Si NMR; the average exchange lifetime for a potassium silicate solution with [OH-]:[SiIV] ) 1:1 is 0.3 s at 298 K. There was no evidence to support a previous report of very slow OsO exchange.
Introduction Over the past 20 years or so, high-field silicon-29 NMR spectroscopy has provided extensive information on the speciation, thermodynamics, and kinetics of aqueous silicate and metallosilicate systems (see, e.g., refs 1-12). That such information is obtained without perturbation of the solution chemistry is a critical advantage of the NMR method. The 29Si nucleus is characterized by spin quantum number I ) 1/ , 2 a large NMR chemical shift range, and, in silicate solutions at T e 300 K, slow Si-Si site exchange relative to the NMR time scale.2 The resulting fine-lined spectra can be structurally informative, especially for isotopically enriched solutions yielding 29Si-29Si scalar coupling.1 Kinetic information has been obtained from classical band-shape analysis2 and from 1-D2,3 and 2-D4 saturation transfer techniques. On the down side, many hours of data acquisition are typically needed to achieve a reasonable signal to noise ratio, owing to the low natural abundance (4.7%), poor NMR sensitivity (3.7 × 10-4 that of 1H), and slow longitudinal (T ) relaxation5 of 29Si. 1 The only other available nucleus for monitoring silicates is oxygen-17. (Fast H-H exchange precludes 1H NMR.) Because of its quadrupolar moment (Q ) -2.6 × 10-30 m2) and low natural abundance (0.037%), however, 17O seems an impractical alternative to 29Si NMR. Indeed, only one previous 17O study of aqueous silicates is known to the author. Knight et al.,13 in their account, nonetheless argue that 17O NMR has advantages over 29Si NMR for elucidating structure as well as mechanisms of chemical exchange. For a tetramethylammonium (TMA) silicate solution containing only the cubic Si8O208- species, they assigned a broad, high-frequency 17O resonance (72 ppm from H217O) to the anion’s 12 bridging oxygens and a narrower, lowfrequency peak (47.5 ppm) to its 8 nonbridging oxygens.13 By observing the evolution of these peaks following dilution with H217O, they demonstrated that O-O exchange between water and the nonbridging oxygen sites is at least 4 orders of magnitude faster than exchange involving the bridging oxygens. This finding is consistent with 29Si measurements6-8 showing that Si8O208- anions are remarkably resistant to chemical exchange in solutions containing TMA+ cations (ostensibly, X
Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-4760$12.00/0
owing to a protective shell of hydrated TMA+ cations9). In the case of potassium silicates, only the 17O peak which corresponds to silicate monomer was assigned with reasonable certainty. At best, other peaks could only be attributed to generic nonbridging (35-55 ppm) or bridging (45-85 ppm) environments. Knight et al. reported that a potassium silicate solution with [KOH] ) [SiO2], when diluted with 17O-enriched water, took more than a year at ambient temperature to yield an 17O spectrum matching that of an equivalent, preequilibrated solution. This was attributed to “two, very different, bulk water exchange rates, some sites having fully exchanged in a matter of seconds, and others requiring many months”.13 However, their analysis cannot be reconciled with 29Si kinetic measurements2 showing that alkali metal silicates attain chemical equilibrium exceedingly rapidly, i.e., that the lifetime of any one species in a solution with [OH-]:[SiIV] ) 1:1 is no more than 0.4 s at 298 K. While it is certainly possible for oxygenoxygen chemical exchange to be faster than Si-Si exchange, it cannot be slower. This discrepancy (of at least 6 orders of magnitude!) was the impetus for the present 17O NMR study. Experimental Section Twenty 17O-enriched potassium silicate solutions, with [SiIV] ) 0.1-1 mol kg-1 and [OH-]:[SiIV] ) 0.97:1 to 2.1:1, were prepared10 using amorphous silica (made by hydrolysis of SiCl4 (Aldrich, 99.999%)), KOH (Aldrich, 99.99%), 17O-enriched water (Aldrich, 10 atom %; MSD, 38 atom %), 2H2O (Aldrich, 99.8%), and type-I deionized water. All solutions were prepared directly in Teflon-FEP NMR tube liners and sealed under nitrogen. The solvent in each sample was 9 atom %-enriched in deuterium to provide a lock signal. Oxygen-17 spectra were obtained at 67.8 MHz on a Bruker AMX 500 spectrometer using 90° (20 µs) pulses and a cycling period of 0.33 s. All oxygen17 shifts were internally referenced to the solvent H217O signal. A pressurizable sample tube11 enabled operation at temperatures from 275 to 425 K. The temperature was calibrated ((0.5 K) using the 1H spectrum of neat ethylene glycol.14 The spectra were decomposed using a Marquardt-Levenberg peak-fitting routine,15 while kinetic band-shape analysis was carried out with the program GNMR.2,16 The Qyz symbol is used herein, as by convention, to denote a quadrafunctional Si center with y-coordinated SiO44- tetra© 1996 American Chemical Society
Aqueous Potassium Silicates
Figure 1. (a) 39.7 MHz 29Si spectrum at 300 K of a potassium silicate solution with 0.80 mol kg-1 SiO2 and KOH. (b) 67.8 MHz 17O NMR spectrum at 347 K of the equivalent solution enriched 9 atom in % 17O. (c and d) Computer simulation of part b generated with nine Lorentzian peaks (labeled).
J. Phys. Chem., Vol. 100, No. 12, 1996 4761
Figure 2. Oxygen-17 NMR spectra (67.8 MHz) of a potassium silicate solution (9 atom % 17O) with 0.25 mol kg-1 SiO2 and KOH acquired at T ) 276-425 K. Spectra are not horizontally offset.
hedra, and when applicable, z indicates the number of equivalent Si centers in a totally symmetric anion. Thus, the monomer (H4-qSiO4q-), dimer (H6-qSi2O7q-), and cyclic trimer (H6-qSi3O9q-) are respectively represented by Q0, Q12, and Q23. Results and Discussion Assignment of 17O Spectra. The 17O spectra shown in Figures 1 and 2 are typical of those obtained in the present study. Peaks are poorly resolved at all temperatures, though they are sharpest at ca. 350 K (Figure 2), and are thus quite simple in appearance, i.e., compared with those of the corresponding 29Si spectra (Figure 1). No more than nine fitted peaks were ever required to produce a satisfactory spectral simulation, as exemplified in Figure 1. At least one additional resonancespeak 7asoccurs in the 60-70 ppm region, but because of its small size, it cannot be effectively modeled. Upon increasing solution alkalinity (quantified by the [OH-]: [SiIV] concentration ratio), all resonances shift to higher frequency, owing to silicate deprotonation equilibria.12 (See Figures 3 and 4.) Shifts up to 10 ppm were observed for peak 1 as [OH-]:[SiIV] was increased from 1:1 to 2:1. Somewhat smaller up-frequency shifts are effected by a moderate rise in temperature (Figure 2; ca. 5 ppm for peak 1 from 270-430 K). The magnitude of these shifts tends to decrease from peaks 1 to 6 (peak 4 is an exception, as discussed below) and, again, from peaks 7 to 9. Consequently, the two groups of resonances diverge. Another distinction between the two groups is that, while half-height peak-width ∆ν1/2 tends to increase progressively from peaks 1 to 6, peak-widths apparently decrease from peaks 7 to 9. This difference between group characteristics, combined with the observations of Knight et al.,13 supports the general assignment of resonances 1-6 to nonbridging silicate oxygens (-OSit) and peaks 7-9 (including 7a) to bridging
Figure 3. Oxygen-17 NMR spectra (67.8 MHz) of potassium silicate solutions (9 atom % 17O) with 0.80 mol kg-1 SiO2 and [KOH]:[SiO2] ) 0.97:1 to 2.00:1. Spectra are not horizontally offset.
oxygens (tSiOSit). Specific peak assignments are discussed next (and summarized in Table 1). Of the first group of resonances, peak 1 undergoes the largest pH-dependent chemical shift (Figure 4) and thus corresponds to the most acidic HOSit site. The relative area of this peak grows when temperature is increased (Figure 2), pH is increased (Figure 3), or SiIV concentration is decreased (cf. Figures 2 and 3); i.e., it is favored by factors which induce depolymerization.11 These observations verify Knight et al.’s assignment13 of peak 1 to silicate monomer (H4-qSiO4q-; Q0). Peak 2 is second most favored as pH or T are increased and thus, in accordance with 29Si observations,1-2 can be attributed to terminal oxygens on the silicate dimer (H6-qSi2O7q-; Q1Q1). Peak 3, situated only slightly up frequency, exhibits a pH-shift dependence exactly matching that of peak 2 and probably corresponds to nonbridg-
4762 J. Phys. Chem., Vol. 100, No. 12, 1996
Figure 4. Chemical shift (obtained using peak-fitting routine) vs alkalinity for nine peaks in the 17O spectra of Figure 3. Dashed lines are to signify the inherent uncertainty in estimating δ for very broad resonances. Examination of the experimental and simulated spectra would suggest the existence of additional resonances in the 60-70 ppm regionse.g., peak 7asthat were not included in the fitted spectra.
TABLE 1: Oxygen-17 NMR Assignments for Aqueous Potassium Silicate Solutions peaka
δb/ppm
assignment
(a) Silicate Nonbridging Oxygens (tSisO-) 1 44.4 Q0 monomer 2 47.7 Q1 (-Q1) dimer Q12 3 48.9 Q1 (-Q3) e.g., substituted cyclic trimer 4 54.5 Q2 strainedc e.g., cyclic trimer Q23 5 55.6 Q2 unstrainedc e.g., cyclic tetramer Q24 6 61.6 Q3 c e.g., prismatic hexamer Q36 (b) Silicate Bridging Oxygens (tSisOsSit) 9 73.8 Q3-Q3 c e.g., prismatic hexamer Q36 a See Figure 1 for peak labeling. b Corresponding to solution at 347 K with 0.80 mol kg-1 SiO2 and KOH. c Tentative assignment.
ing oxygens at silicate Q1 centers linked to either Q2 or Q3 (both being less electronegative than Q1). However, silicate species containing Q1Q2 linkages (e.g.: acyclic trimer, Q1Q2Q1; and acyclic tetramer, Q1Q2Q2Q1) are too labile to yield resolvable resonances at 347 K, and therefore, the principal contributors to peak 3 are probably the substituted cyclic trimer (Q1Q3Q22) and substituted cyclic tetramer (Q1Q3Q23). Frequency considerations would indicate that peaks 4 and 5 can reasonably be assigned to Q2 nonbridging oxygens and peak 6 (in part, at least) to Q3 nonbridging oxygens. Indeed, the chemical shift dependence on [OH-]:[SiIV] for peaks 1 to 6sexcluding peak 4sis in accordance with the expected ease of (secondary) deprotonation, i.e.: Q0 > Q1 > Q2 > Q3. The incongruously large pH dependence of peak 4 would suggest that corresponding Q2 centers are more acidic than the Q2 centers which yield peak 5. Similar variationsscorrelating with the extent of Si-O-Si bond strainsare observed in the pH dependence of silicon-29 Q2 resonances.12 Hence, we tentatively assign peak 4 to Q2 nonbridging oxygens of strained silicate oligomers (e.g., cyclic trimer Q23), and peak 5 to nonbridging Q2 oxygens of comparatively unstrained species (e.g., cyclic tetramer Q24).
Kinrade Assignment of the bridging oxygen resonances is much more difficult. Ignoring structural factors such as ring strain, there are six principal environments for bridging oxygens as defined by the connectivity at adjoining silicate centers, i.e., Q1Q1, Q1Q2, Q1Q3, Q2Q2, Q2Q3, and Q3Q3 (Q1Q3 and Q2Q2 bridging oxygens being similarly shielded). Yet, the chemical shift range is smaller than for nonbridging oxygens, owing to a smaller variation in the combined electronegativity of the substituents. Large peak-widths pose an additional problem (attributed to the smaller quadrupolar coupling constants of bridging oxygens13,17). The combined result is a series of overlapping resonances that are difficult to resolve. Nonetheless, oxygens linking two Q1 centers would be predicted to resonate furthest down frequency (possibly peak 7a), while oxygens bridging Q3 centers resonate furthest up frequency (i.e., peak 9) and exhibit the lowest quadrupolar relaxation rate. Transverse Relaxation and the Dynamics of Chemical Exchange. Since silicate anions yield a series of 17O resonances between 40 and 80 ppm that are distinct from the strong water signal at 0 ppm, the average frequency of O-O exchange must be well below the NMR fast-exchange limit, π∆ν. Thus, for OsO exchange between silicate anions as well as between silicates and water (or aqueous -OH), the average exchange lifetime τ is .10-4 s. Conversely, there was no evidence to support the very slow O-O exchange process reported by Knight et al;13 spectra obtained immediately after sample preparationsor dilution with 17O-enriched waterswere identical to those obtained several months later. Indeed, chemical equilibrium was attained within 2-3 min (i.e., the time required to set up and run a 17O spectrum) for samples with [OH-]:[SiIV] ) 1:1 and 2:1 that had been quenched from boiling by immersion in liquid nitrogen and then quickly warmed to 300 K. All major oxygen exchange processes are therefore characterized by 10-2 , τ-1 , 104 s-1. Quantitative determination of the O-O exchange frequency was accomplished by kinetic band-shape analysis. Application of this technique, however, is complicated by 17O’s quadrupolar moment. The quadrupolar contribution to half-height peakwidth ∆ν1/2,Q ()π-1T2Q-1, where T2Q-1 is the rate of transverse relaxation by electric quadrupole interactions) is inversely proportional to the rate of isotropic tumbling and therefore increases as T is decreased. Spectra were acquired over the widest possible temperature range so that kinetic and quadrupolar peak-width contributions could be quantified. In the case of solutions with [OH-]:[SiIV] ) 1:1, all peak-widths initially decrease as temperature is raised from ambient temperature or lower. (See Figure 2.) They reach a minimum value at about 350 K and subsequently increase at higher temperatures. The H217O signal exemplifies this behavior (Figure 5). The temperature dependence of ∆ν1/2 in Figure 5 shows that quadrupolar relaxation is the dominant factor controlling line-width at T e 330 K for a solution with 0.25 mol kg-1 SiO2 and KOH. A weighted, nonlinear least-squares fit of ∆ν1/2 at T e 330 K yields Arrhenius activation energy Ea ) 16.9 ( 0.6 kJ mol-1 for the isotropic tumbling of water in this solution. Peak-widths at T > 380 K are predominantly affected by exchange broadening. The least broadened spectrum of Figure 2, i.e. at 347 K, was decomposed giving chemical shift, ∆ν1/2, and area of all eight resonances for use as the basis set in kinetic band-shape analysis of spectra at T > 347 K. To compensate for the effects of quadrupolar broadening, the term {∆ν1/2,Q347K - ∆ν1/2,QT} (calculated from Figure 5) was subtracted from each of the basis set peak-widths. As demonstrated in Figure 6, the observed 17O spectra were modeled reasonably well (allowing for temperature effects independent of exchange) by assuming that
Aqueous Potassium Silicates
Figure 5. Temperature dependence of transverse relaxation rate, T2-1 ) τ∆ν1/2 (pseudo-T2 contribution from field inhomogeneity is negligibly small), for the H217O signal of a potassium silicate solution with 0.25 mol kg-1 SiO2 and KOH, i.e., corresponding to Figure 2. The solid circles and line correspond to the quadrupolar relaxation contribution T2,Q-1, which controls peak-widths at T e 330 K. The temperature dependence of T2,Q-1, and thus of the motional correlation time τc, yields Arrhenius activation energy Ea ) -16.9 ( 0.6 kJ mol-1 for isotropic tumbling of water. At higher temperatures, 17O-17O exchange affects T2 relaxation.
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Figure 7. Temperature dependence of the spin-site residence times from Figure 6 for a solution with 0.25 mol kg-1 SiO2 and KOH. A least-squares fit to the Eyring equation gives ∆H* ) 46 ( 3 kJ mol-1 and ∆S* ) -82 ( 8 J K-1 mol-1 for 17O-17O exchange.
Figure 8. Half-height peak-width ∆ν1/2 (obtained using peak-fitting routine) vs alkalinity for the three low-frequency resonances in Figure 3.
τ-1 ) (kBT/h) exp[(∆S*/R) - (∆H*/RT)]
Figure 6. Experimental (a) and GNMR-simulated (b) 17O spectra of a solution with 0.25 mol kg-1 SiO2 and KOH (see Figure 2). The simulations are based on the assumption that the inverted spin-site lifetime τ-1 at 347 K is 53 s-1, as calculated from data in ref 2. Simulated spectra do not take into account the equilibrium shift toward Q0 (i.e., from high to low frequency) that occurs as temperature is increased. Other than the low-frequency Q0 resonance, the simulated peaks also are not corrected for up-frequency shifts caused by increased deprotonation as temperature is increased.
all spin sitessi.e., bridging and nonbridging oxygensshave equal exchange lifetimes (τ). The uniformity of exchange broadening exhibited by silicate and water resonances indicates that intermolecular O-O exchange between silicate anions as well as between silicate and solvent molecules (and aqueous OH-) is controlled entirely by the silicate condensation/ hydrolysis process. Because there are insufficient data from which to extrapolate the lower temperature limit of 17O exchange broadening, the expected value of τ-1 at 347 K (53 s-1) was calculated from data in ref 2. A weighted, nonlinear leastsquares fit of τ to the Eyring equation (Figure 7)
gives, for silicate polymerization, enthalpy of activation ∆H* ) 45 ( 3 kJ mol-1, entropy of activation ∆S* ) -83 ( 8 J K-1 mol-1, and, at 298.2 K, τ-1 ) 3.8 s-1. Within experimental uncertainty limits, these figures are in agreement with values obtained using 29Si NMR. As the ratio [OH-]:[SiIV] was raised for a fixed SiIV concentration, the width of the 17O peaks decreased, despite an anticipated rise in ∆ν1/2,Q owing to increased viscosity. (See Figure 8.) Therefore, as noted previously,2 the rate of chemical exchange between silicate anions must decrease with increased alkalinity. For solutions with [OH-]:[SiIV] > 1:1, kinetic broadening cannot be modeled using the assumption of equal spin-site lifetimes (Figure 9). Peak 1 broadens less than all other resonances as T is increased under these conditions. This is because the pKa of silicate anions ranges significantly, pKa being lowest for the monomer. Q centers with charge > -1 are comparatively inert to exchange.2 Conclusions Six broad 17O NMR signals between 40 and 62 ppm are assigned to nonbridging oxygens of the silicate monomer (Q0), the dimer (Q12), other Q1 centers, strained and nonstrained Q2 centers, and Q3 silicate centers. Bridging oxygens are responsible for a series of even broader 17O bands between 60 and 80
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Kinrade found to support the existence of a very slow O-O exchange process in such systems. Acknowledgment. The experimental assistance of David Probizanski and Jacob Nyati is gratefully acknowledged, as are discussions with Dr. C. T. G. Knight. This work was supported by the Lakehead University Senate Research Committee and the Natural Sciences and Engineering Research Council of Canada. References and Notes
Figure 9. Experimental 17O spectra (a) and the attempted GNMR simulations (b) for a solution (9 atom % 17O) with 0.25 mol kg-1 SiO2 and 0.50 mol kg-1 KOH. The simulations were generated assuming uniform site lifetimes, as in Figure 6, and set up to give a “best fit” for the low-frequency peak. However, the kinetic broadening of all other resonances was subsequently underestimated.
ppm that, for the most part, cannot be readily assigned. All chemical shifts are sensitive to changes in pH, temperature, and concentration. Because of the quadrupolar moment and small chemical shift range of 17O, it is doubtful that 17O NMR will contribute significantly to the speciation of dissolved silicate oligomers. Oxygen-17 kinetic band-shape analysis yields silicate condensation rates that are in accordance with previous 29Si NMR results for aqueous alkali metal silicates. No evidence was
(1) Harris, R. K.; Knight, C. T. G. J. Mol. Struct. 1982, 78, 273; J. Chem. Soc., Faraday Trans. 2 1983, 79, 1525, 1539. Knight, C. T. G.; Kirkpatrick, R. J.; Oldfield, E. J. Chem. Soc., Chem. Commun. 1989, 919. (2) Kinrade, S. D.; Swaddle, T. W. Inorg. Chem. 1988, 27, 4259. (3) Creswell, C. J.; Harris, R. K.; Jageland, P. T. J. Chem. Soc., Chem. Commun. 1984, 1261. (4) Knight, C. T. G. J. Chem. Soc., Dalton Trans. 1988, 1457. (5) Kinrade, S. D.; Swaddle, T. W. J. Am. Chem. Soc. 1986, 108, 7159. (6) Knight, C. T. G.; Kirkpatrick, R. J.; Oldfield, E. J. Chem. Soc., Chem. Commun. 1986, 66; J. Magn. Reson. 1988, 78, 31. (7) Keijsper, J. J.; Post, M. F. M. In Zeolite Synthesis; Occelli, M. L., Robson, H. E., Eds.; ACS Symposium Series 398; American Chemical Society: Washington, DC, 1989; p 28. (8) Syvitski, R. T. M.Sc. Thesis, Lakehead University, 1994. (9) Knight, C. T. G.; Syvitski, R. T.; Kinrade, S. D. Stud. Surf. Sci. Catal. 1995, 97, 483. (10) Kinrade, S. D.; Pole, D. L. Inorg. Chem. 1992, 31, 4558. (11) Kinrade, S. D.; Swaddle, T. W. J. Magn. Reson. 1988, 77, 569. (12) Kinrade, S. D.; Swaddle, T. W. Inorg. Chem. 1988, 27, 4253. Sjo¨berg, S.; O ¨ hman, L.-O.; Ingri, N. Acta Chem. Scand., Ser. A 1985, A39, 93. Svensson, I. L.; Sjo¨berg, S.; O ¨ hman, L.-O. J. Chem. Soc., Faraday Trans. 1 1986, 82, 3635. (13) Knight, C. T. G.; Thompson, A. R.; Kunwar, A. C.; Gutowsky, H. S.; Oldfield, E.; Kirkpatrick, R. J. J. Chem. Soc., Dalton Trans. 1989, 275. (14) Ammann, C.; Meier, P.; Merbach, A. E. J. Magn. Reson. 1982, 46, 319. (15) Press, W. H.; Flannery, B. F.; Tenkolsky, S. A.; Vetterling, W. T. Numerical Recipies in Pascal, the Art of Scientific Computing; Cambridge University Press: Cambridge, 1989. (16) Program GNMR is derived from exchange-modified Bloch equations and is listed in: Kinrade, S. D. Ph.D. Thesis, The University of Calgary, 1987. (17) Timken, H. K. C.; Schramm, S.; Kirkpatrick, R. J.; Oldfield, E. J. Phys. Chem. 1987, 91, 1054.
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