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J. Phys. Chem. 1992,96, 10928-10933
(26) Smith, J. V.; Blackwell, C. S. Nature (London) 1984, 308, 521. (27) Thomas, J. M.; Kennedy, J.; Ramdas, S.; Hunter, B. K.; Tennakoon, D.T.Chem. Phys. Lett. 1983, 102, 158. (28) Ramdas, S.; Klinowski, J. Nature (London) 1984. 308, 521. (29) Engelhardt, G.; Radeglia, R. Chem. Phys. Lett. 1984, 108, 271. (30) Radeglia, R.; Engelhardt, G. Chem. Phys. Lett. 1985, 114, 28. (31) Smith, K. A.; Kirkpatrick, R. J.; Oldfield E.; Henderson, D. M. Am. Mineral. 1983, 68, 1206. (32) Grimmer, A.-R.; Radeglia, R. Chem. Phys. Lett. 1984, 106, 263. (33) Grimmer, A.-R. Chem. Phys. Lett. 1985, 119, 416. (34) Hartman, J. S.; Sherriff, B. L. J . Phys. Chem. 1991, 95, 7575. (35) Olivieri, A. C. J . Magn. Reson. 1989, 81, 201. (36) VanderHart, D. L.; Earl, W. L.; Garroway, A. N. J . Magn. Reson. 1981, 44, 361.
(37) Alla, M.; Lippmaa, E. Chem. Phys. Lett. 1982, 87, 30. (38) Ganapathy, S.; Schramm, S.; Oldfield, E. J . Chem. Phys. 1982, 77, 4360. (39) Engelhardt, G.;Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites; John Wiley & Sons: New York, 1987; p 140. (40) Abragam, A. The Principles of Nuclear Magnetism; Oxford University: London, 1961; p 108. (41) Powles, J. G.; Carazza, B. In Magnetic Resonance; Coogan, C. K., Ham, N . S., Stuart, S. N., Pilbrow, J. R., Wilson, G. V. H., Eds.; Plenum: New York, 1970; p 133. (42) Yesinowski, J. P.; Eckert, H.; Rossman, G.R. J . Am. Chem. Soc. 1988, 110, 1367. (43) Freude, D.; Hunger, M.; Pfeifer, H. 2. Phys. Chem. (Munich) 1987, 152, 171.
NMR Study of Kaollnlte. 2. ‘H, 27AI, and 28SlSpin-Lattice Relaxations Shigenobu Hayashi,* Takahiro Ueda,Kikuko Hayamizu, and Etsuo Akiba National Chemical Laboratory for Industry, Tsukuba, Ibaraki 305, Japan (Received: May 26, 1992; In Final Form: September 1, 1992)
Spin-lattice relaxation times of ’H, 27Al,and %i spins in kaolinites with various origins have been measured at room temperature. Relaxation by paramagnetic impurities is dominant in those spins. Spin diffusion plays an important role in the IH and 27Alrelaxations, and their relaxations belong to the diffusion-limited case. 29Sispins relax through the dipole-dipole interaction with electron spins directly, and the contribution of the spin diffusion is negligible. A part of the electron spins having an ESR signal at g = 4.1 play the role of relaxation centers. We have analyzed the relaxation times theoretically and have estimated the concentrations of paramagnetic impurities and the spin-lattice relaxation time of the electron spins.
Introduction High-resolution solid-state NMR is a valuable method to analyze the structures of silicates. Especially, 29SiNMR is widely used. However, the 2%i spin-lattice relaxation time (TI)is often so long that quantitative interpretation of the spectra becomes very difficult. Barron et a1.I have measured 29Sirelaxation times of some layer aluminosilicates and found that the TI varied over 3 orders of magnitude, ranging from 4 to 5000 s. Watanabe et a1.2 have reported TIvalues of several clay minerals which were less than 1 s. Rocha and Klinowski3 have measured the T Ivalue (about 17 s) of kaolinite by the saturation recovery method. Hartman and S h e e have studied the relaxation in kyanite and found that the relaxation m e did not follow an exponential decay. Although most studies have ascribed the relaxation mechanism to paramagnetic impurities, no quantitative discussion was made to validate the mechanism. In contrast to 29Si,27Alrelaxation has rarely been studied in layered aluminosilicates. Since the 27Alspin is quadrupolar, relaxation by the quadrupole effect is expected to be dominant. Indeed, Haase et al.5*6 have reported that the 2’Al spins in hydrated zeolites are relaxed by modulation of the time-dependent electric field gradient due to motion of water molecules in the zeolite pores. However, kaolinite has no water molecules between the layers, so that the relaxation mechanism is not known. IH spins can be relaxed easily by the fluctuation of the dipole-dipole interaction between them if they undergo some kind of motion. Therefore, ‘HT I is a valuable benchmark to check the presence of motion. Stone and Torres-Sanchez’ have studied the IH spin-lattice relaxation in a collection of kaolinites, concluding that paramagnetic impurities play a dominant role. In the previous paper,* we have reported 29Si,27Al,and IH NMR spectra of kaolinites with various origins, which were interpreted quantitatively. In the present work, we have studied the relaxation behavior in kaolinite. We have measured IH,27Al, and 29Sispin-lattice relaxation times of kaolinite samples with various origins at rmm temperature. The relaxation mechanism
is concluded to be paramagnetic impurities, and the relaxation rates are analyzed theoretically.
Theoretical Backgrouad If a nuclear spin I relaxes through the dipolar coupling with an electron spin S on paramagnetic impurities, the magnetization recovers exponentially. The relaxation time, TIP, of the I spin can be expressed by9
where re is a distance between the I and S spins, y r and ys are gyromagnetic ratios of the I and S spins, reapectively, h is P W s constant, S is a spin quantum number of the electron spin (S = I/J, wI is a Zeeman frequency of the I spin, and T, is a spin-lattice relaxation time of the electron spin. The relaxation time TIP becomes shorter with decrease in the distance between the nucleus and the electron spin. However, nuclei with too short distances cannot be observed because of a severe line broadening and a large frequency shift. For nuclei far away from the electron spin, spin diffusionplays an important role in the relaxation. If the relaxation is diffuson-limited, the relaxation curve approaches exponential m e r y for a sufficiently large time. The time constant can be written as9 1 / T , , = 8.80N,C1/4DD,3/4
(3)
where N p is the concentration of the paramagnetic impurities. D, is a spin diffusion coefficient, and
D, = (MI,1/2/30)u2
(4)
where u is the distance between an I spin and its nearest-neigh-
0022-365419212096-10928503.00/0 0 1992 American Chemical Sccietv
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 lM29
NMR Study of Kaolinite. 2
12 .-0 1
TABLE I: Spin-Lattice Relaxation T i m
'H' TII sample
6)
(a)
N1
2.5 1.3 0.25 0.88 0.075 0.075 0.54 1.18
(62) (40) (63) (61) (57) (68) (59) (40)
N2
N3 N4
N5 0.0
1.0
2.0
3.0
t (SI Figure 1. IH magnetization recovery curve for the sample N1,measured by the inversion recovery method. The solid line indicates the diffusion-limited relaxation.
boring I spin. MI] is a second moment of the interaction between the I spins
where r, is the distance between a given I spin and the other I spins. If the spin diffusion is inhibited, the magnetization at a given nuclear site recovers exponentiallywith a time constant expressed by eq 1. However, each site has its own relaxation time, and thus the relaxation curve for the total spins does not follow the commonly observed exponential decay.l*I4 When the concentration of the paramagnetic impurities is low enough, the magnetization recovers as exp(-(t/Tlp)l/2), where" 1= 16 -TIP g"3N,2c Experimental Section
Materklo. Kaolinite samples were the same as those previously studied: and thus the same notations were used. To test the effects of water molecules and oxygen atmosphere on the relaxation, the atmosphere of the sample N1 was replaced by O2and N2gases after evacuating for 4 h at room temperature. NMR Measurements. Spin-lattice relaxation times were measured at room temperature, using a Bruker MSL400 spectrometer with a static magnetic field strength of 9.4 T. &man frequencia were 400.136,104.263, and 79.496 MHz for 'H, 27Al, and 29si spins, respectively. IH TI was measured by the inversion recovery method for a static sample. The r / 2 pulse width was 1.1 MS. 27Al TI was measured by using a saturating comb sequence for nonselective saturation. A hundred */2 pulses with 5 MS spacings were used to saturate the spin energy levels. A 9r/2 pulse was given to pick up the signal after a delay time, r. The x / 2 pulse length was 3.5 p, which was calibrated using the kaolinite samples. ?3i TI was measured by the Torchia sequence.I5 Cross polarization (CP) was necessary, since the TI was very long. lH dipolar decoupling was carried out during the signal acquisition for 27Aland 29Si. The effects of the magic angle spinning (MAS)on the spin-lattice relaxation were checked in 27Aland 29Si. ESR MeuunmeaQ. Electron spin resonance (ESR) spectra at X-band (9.3 GHz) were traced by a JEOL JES-RE1X ESR spectrometer at room temperature. The microwave power was set at 4 mW. The signals were not saturated under the low-power conditions, being confinned by the power dependence of the signal intensity. The field modulation amplitude was 0.5 mT. Results md Discussion 'HRelaxation. The 'H spectra of kaolinites have two components with different line shapes and widths: a broad Gaussian line and a narrow Lorentzian.8 The broad component is ascribed to the hydroxyl group in the kaolinite structure. Figure 1 shows the magnetization mavery curve for the broad component in the sample N1, The curve deviates slightly from an exponential decay.
U,
N6 S1 S2
2'Alb TI,
0.45 0.23 0.058 0.15 0.019 0.011 0.15 0.35
U,
(%I
(38) (60) (37) (39) (43) (32) (41) (60)
TI! (8)
17.5 11.3 5.7 11.6 1.9 2.0 6.8 10.9
TI,
(4
2.5 1.4 0.80 0.50 0.25 0.30 1.2 1.8
29Sic TI (SI 2300 360 51 240 3.3 5.3 72 390
For static samples. f is the fraction of the component. Under MAS conditions at a rate of 3.5 kHz. CUnderMAS conditions at a rate of 3.0 kHz.
We deconvolute the relaxation curve into two components: long (TII)and short (TI,) components. Table I summarizes the 'H T, values of all the samples. Stone and Tom-Sanchez7 have observed the third component with a second moment of about 910 kHz2 at Zeeman frequencies of 90 and/or 24 MHz for samples containing at least 0.5 wt 7% Fe203. This component is considered to be broadened by the bulk magnetic susceptibility effect. The ahsence of the third component in our work might be caused by the use of the high magnetic field. Since the magnetic susceptibility broadening is proportional to the magnetic field (in hertz), the thid component might be broadened too much at the Zeeman frequency of 400.136 MHz. The TI values markedly depend on the sample, in spite that all the samples have the kaolinite structure. This can be explained by the relaxation due to paramagnetic impurities. The concentration of paramagnetic impurities is largely dependent on the sample origin. The 'H line width demonstrates that the IH spins of the hydroxyl group are in a rigid lattice state,* and the contribution of a motion to the relaxation is considered to be negligible. The observed nonexponential recovery is consistent with the relaxation due to the paramagnetic impurities. The recovery curve does not fit to exp(-(t/Tl,)1/2), indicating that spin diffusion plays a role in the relaxation. The relaxation curve approaches an exponential decay for a sufficiently large time where the relaxation is diffusion-limited. Therefore, the Tllcorresponds to the time constant expressed by eq 3. Stone and Torres-Sanchez7 analyzed the relaxation curve by the spin-diffusion vanishing case14 and by the direct dipolar coupling with electron spins, neglecting the contribution of spin diffusion. As indicated by eq 6, the direct relaxation rate is inversely proportional to the square of the applied magnetic field >> 1. The use of the high magnetic field in the present when oHrC work makes the contribution of the direct relaxation much smaller. They reported the exp(-(t/ Tlp)l/2)behavior only for a relatively short time. The initial part of the relaxation curve is governed by the direct relaxation prccess, indicating the exp(-(t/ Tlp)e1/2) behavior.1° In the present work, we have got the relaxation time for a sufficiently large time region where the relaxation is considered to be diffision limited. They concluded that the recovery curve for their purest sample is exponential from the relatively short time behavior. A nonexponential behavior is slightly recognized in their data at 90 MHz. The D, value they estimated was unexpectedly small (about cm2/s), since they analyzed the data using the spin-diffusion vanishing case. Replacements of air atmosphere by N2 and O2 gases after evacuation have no effects on the relaxation curve of the sample N 1. These facts indicate that water molacules and O2atmosphere have negligible contributions to the relaxation. The narrow components have very short relaxation times, ranging from 3 to 11 ms. These components can be ascribed to water molecules adsorbed on the outer surface. The motion of the water molecule is considered to be the origin of the fast relaxation. nAl Relaxation. Figure 2 shows 27Alrelaxation curves of the sample N 1, measured by the saturating comb pulse sequence. The magic angle spinning influences the relaxation behavior, as shown
10930 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 1
l.Ob
0
Hayashi et al.
20
40
t (SI Figure 2. 27Almagnetization recovery curves for the sample N1 with the magic angle spinning at 3.50kHz (e) and without spinning (A). The solid lines indicate the diffusion-limited relaxation.
t V 2 (s"2 )
Figure 4. 29Simagnetization recovery curves for the sample N1, plotted as a function of square root of time, and measured with the magic angle spinning at 3.0 WIz (e) and without spinning (A). The solid lines show
least-squares fits.
0.6
? 0.4 ul Y
0.0
-
0
5
1/&\(H)
10
15
(S')
Figure 3. Correlation bttween relaxation rates of IH and 27Al.The solid line shows a least-squares fit.
in the figure. The curves are deconvoluted into two components: TI,= 17.5 s and TI, = 2.5 s. The two TI values do not change by the use of MAS. MAS only increases the fraction of the TI1 component. The change of the spinning rate from 3.5 to 2.5 kHz has no effect on the relaxation curve. Table I summarizes the TIvalues of all the samples, measured under the spinning conditions. The fraction of each component is omitted from the table, since it depends on the use of MAS. As seen in the table, the 27AlT , correlates well with the 'H TI, suggesting that the paramagnetic impurities play an important role in the 27Alrelaxation. Since the natural abundance of 27Al is 10096, spin diffusion plays a role in the relaxation path to the paramagnetic centers. The recovery curve does not fit to e x p (-(r/Tlp)1/2),except for the samples N5 and N6. Thus, the relaxation is considered to be diffusion limited for a sufficiently large time in the samples containing relatively low Fe concentrations. For the relatively impure samples N5 and N6, the relaxation through the direct dipolar coupling with electron spins might have considerable contribution. If eq 6 is applied, the relaxation times are 0.34and 0.39 s for the samples N5 and N6, respectively. However, we cannot conclude which mechanism is dominant between the two contributions at the present stage. Temporarily, the relaxation is treated as the diffusion-limited case, and we will comment on it later. Two possible origins can give the nonexponential recovery in the case of 27Al. First, a nonexponential recovery is observed at a short time even when the paramagnetic relaxation is diffusion-limited at a sufficientlylarge time. The initial part is similar to the case of no spin diffusion, and the direct dipolar coupling with the electron spin is the dominant relaxation mechanism. MAS might reduce the dipolar coupling to some extent. Second, 27Alspin is quadrupolar, and perturbation by the levels other than m = gives the nonexponential behavior. If the second case is true, MAS might reduce the perturbation by separating the center peak from the sidebands originating from the other levels. However, suppression of the perturbation from the outer levels , ~ is inverse to the MAS should make the TI value ~ h o r t e rwhich effects. Consequently, the fmt origin is considered to be important.
-85
-95
-90 ppm
Figure 5. 29SiMAS NMR spectra of the sample N1. (A) CP/MAS with the contact time of 8 ms and the repetition time of 10 s. (B, C) Single pulse with DD/MAS and the r/2 pulse flip angle. The repetition times are (B) 100 s and (C)5 s. Spinning rates of the sample are all 3.02 kHz. The numbers in the figure are full widths at half-maximum.
Both the 'Hand 27Alrelaxations are considered to be diffusion limited, and then the TI,values can be used as the relaxation time in eq 3. Figure 3 shows a correlation between the relaxation rates of 27Aland 'H.The following relation is obtained experimentally by a least-squares calculation: (7)
This good linear correlation confirms that the paramagnetic impurities are the dominant mechanism in the relaxations of both IH and 27Al. The constant term on the right-hand side is considered to be the 27Alrelaxation by other mechanisms such as quadrupole relaxation by spin-phonon c o ~ p l i n g . ~ 2gSiRelaxation. Figure 4 shows 29Simagnetization recovery curves of the sample N1,measured by Torchia's method. The commonly used methods such as the inversion recovery and the saturation recovery are practically impossible because of the very long relaxation times. The relaxation curve is not exponential, and it can be fit to exp(-(t/Tle)1/2) very well. Thus, the magnetization is plotted as a function of the square root of time in the figure. The T Ivalues are estimated from the slope: 2300 and 870 s with and without MAS, respectively. The square root time dependence demonstrates that the magnetiztion relaxes through the dipolar coupling with electron spins on paramagnetic
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10931
NMR Study of Kaolinite. 2
TABLE I 1 ESR Sign81 Intensity ESR relative intensity
0.4
T5 0.3
6
sample N1 N2 N3 N4 N5 N6 s1
0.2
'I0 . 1
s2
0.0
l/Tt(H)
(S-'
Figure 6. Correlation between relaxation rates of 'Hand 29Si. The solid line shows a least-squares fit.
g = 4.1 (total) 1 16 40 2.1 69 69 6.3 1.9
g = 4.1 (inner) 0.88 2.1 6.9 0.88 22 20 2.4 0.98
content of Fe20,b (wt %)
