Multiple-Quantum and Cross-Polarized 27Al MAS NMR of

School of Chemistry, UniVersity of Exeter, Stocker Road, Exeter EX4 4QD, U.K., Physical Chemistry. Laboratory, UniVersity of Oxford, South Parks Road,...
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6408

J. Phys. Chem. B 2000, 104, 6408-6416

Multiple-Quantum and Cross-Polarized of Kaolinite and Gibbsite

27Al

MAS NMR of Mechanically Treated Mixtures

Sharon E. Ashbrook,†,‡ Jamie McManus,† Kenneth J. D. MacKenzie,§ and Stephen Wimperis†,* School of Chemistry, UniVersity of Exeter, Stocker Road, Exeter EX4 4QD, U.K., Physical Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford OX1 3QZ, U.K., and Department of Materials, UniVersity of Oxford, Parks Road, Oxford OX1 3PH, U.K. ReceiVed: January 27, 2000; In Final Form: April 25, 2000

Mixtures of the clay mineral kaolinite with gibbsite, when heated, form mullite, a high-temperature ceramic. The 27Al multiple-quantum magic angle spinning (MQMAS) NMR technique is used to study the local Al environment in mixtures of kaolinite and gibbsite that have been ground together for varying amounts of time. This mechanical treatment forms new chemical species that are amorphous in nature and, therefore, unsuited to analysis by X-ray diffraction. Both novel and established cross-polarization techniques are employed to characterize the starting materials and mixtures, confirm the spatial proximity of 1H and 27Al nuclei, and “edit” the two-dimensional 27Al MQMAS NMR spectra, thereby allowing overlapping peaks corresponding to octahedral Al sites to be studied.

Introduction The commercial importance of mullite (Al6Si2O13) as a hightemperature ceramic1 has led to much interest in its synthesis from mixtures of clay minerals with materials such as aluminas and aluminum hydroxides. Kaolinite (Al2Si2O5(OH)4) is a layerlattice clay mineral which, when heated, forms a mixture of mullite and cristobalite (SiO2). If the stoichiometry is adjusted by addition of a reactive alumina such as gibbsite (γ-Al(OH)3), the mixture will form monophase mullite when heated.2 If less reactive forms of alumina, such as R-Al2O3, are used, temperatures of 1500-1700 °C are necessary for complete formation of mullite.3 It has been shown that the temperature of mullite formation from aluminosilicate precursors also depends on the preparative methods used and that, in particular, mechanochemical treatment of mixtures may cause significant changes in the temperature dependence of the formation reaction.4 It has been suggested that grinding mixtures not only enhances the rate of reaction by decreasing the particle size but also introduces the possibility of the formation of new chemical species by mechanochemical reaction.5-7 Temuujin et al.8 have postulated that grinding of kaolinite and gibbsite mixtures results in polymerization of Al-O-Si bonds, thereby creating new precursors from which mullite can be formed upon heating. The amorphous precursors formed by mechanochemical treatment of kaolinite and gibbsite mixtures are not wellcharacterized by X-ray diffraction, which is preferentially sensitive to domains exhibiting long-range order. Unlike X-ray diffraction, 27Al NMR spectroscopy is, in principle, equally sensitive to Al atoms within both amorphous and crystalline domains. However, the ability of solid-state 27Al NMR to yield structural information is often hindered by the presence of significant second-order broadening of the 27Al resonances. Standard line-narrowing methods, such as magic angle spinning * To whom correspondence should be addressed. Fax: +44-1392263434. E-mail: [email protected]. † School of Chemistry, University of Exeter. ‡ Physical Chemistry Laboratory, University of Oxford. § Department of Materials, University of Oxford.

(MAS),9 may improve resolution by removing quadrupolar broadening to first order but are unable to average this broadening fully. As a result, much attention has been focused on the newly developed multiple-quantum magic angle spinning (MQMAS) technique, which is able to remove the second-order contribution to the quadrupolar broadening in a two-dimensional experiment, thus yielding high-resolution spectra.10-12 Many applications of the 27Al MQMAS NMR technique have been demonstrated, notably on aluminosilicates and amorphous systems.13-17 The technique would seem to be particularly appropriate for studying mixtures of kaolinite and gibbsite because both starting materials contain Al atoms in octahedral coordination with hydroxyl groups, while new octahedral Al sites may be formed on grinding, and the distinction between all of these will be subtle. The purpose of this paper is to study the local environment of Al atoms in kaolinite, gibbsite, and their mechanochemically treated mixtures by using 27Al NMR spectroscopy to investigate the newly formed chemical species. We combine MQMAS with recently developed multiple-quantum cross-polarization techniques to provide information unavailable from the simple MAS spectrum alone. Cross-polarization is a technique used frequently in the study of spin I ) 1/2 systems, both to enhance the sensitivity of low-γ nuclei and to determine the spatial proximity of nuclei.18,19 Although cross-polarization involving a quadrupolar nucleus is a much less well understood process, with signal enhancements observed only rarely, it remains a useful tool for obtaining structural information and spectral simplification. The combination of cross-polarization with the MQMAS experiment to “edit” the high-resolution spectrum of a quadrupolar nucleus has recently been proposed20 while the feasibility of crosspolarization directly to the multiple-quantum coherences of a spin I g 3/2 nucleus has also been demonstrated.21-23 In the present work, the use of these novel cross-polarized MQMAS techniques allows further insight into the nature of mechanochemical change.

10.1021/jp000316t CCC: $19.00 © 2000 American Chemical Society Published on Web 06/17/2000

27Al

MAS NMR of Mechanically Treated Mixtures

Figure 1. (a) Conventional, (b) and (c) single-quantum cross-polarized and (d) triple-quantum cross-polarized 27Al MAS NMR spectra of kaolinite. 27Al spin-locking field ω1S/2π ≈ 25 kHz in (b) and ω1S/2π ≈ 100 kHz in (c) and (d). 1H spin-locking field ∼77 kHz in each case. (e) Conventional, (f) and (g) single-quantum cross-polarized, and (h) and (i) triple-quantum cross-polarized 27Al MAS NMR spectra of gibbsite. 27Al spin-locking field ω1S/2π ≈ 25 kHz in (f), ω1S/2π ≈ 40 kHz in (h), and ω1S/2π ≈ 100 kHz in (g) and (i). 1H spin-locking field ∼77 kHz in each case. Single-quantum cross-polarized spectra recorded with contact time, τCP, of 1 ms; triple-quantum cross-polarized spectra recorded with τCP ) 0.5 ms. 240 transients were averaged with a recycle interval of 3 s. Displayed spectral width is 7 kHz. MAS rate was 6.5 kHz.

Experimental Procedure Mixtures of commercial kaolinite (Kaolin und Quarzsandwerke GmbH & Co., Germany) and gibbsite (Merck, Germany) with a stoichiometric mullite composition (3Al2O3:2SiO2) were dry ground in a planetary ball mill at a rate of 250 rpm. The effect of grinding time was investigated by grinding sample mixtures for 5 min and 4 h. 27Al MAS NMR spectra were obtained on a Bruker MSL 400 equipped with a 9.4 T wide-bore magnet, yielding a 27Al Larmor frequency of 104.3 MHz. Powdered samples were packed inside 4-mm MAS rotors. The strength and duration of radio frequency pulses for cross-polarization experiments are given in the text and figure captions. All spectra were recorded by using 1H decoupling, with a decoupling field strength, ω1I/ 2π, of ∼77 kHz, and are referenced to Al(NO3)3 (aq). 27Al

NMR of Kaolinite and Gibbsite

Kaolinite is an aluminosilicate with a dioctahedral 1:1 layer structure consisting of a sheet of AlO2(OH)4 octahedra and a tetrahedral silica sheet.24 Figure 1a presents the 27Al MAS NMR spectrum of kaolinite, which shows a broadened peak centered

J. Phys. Chem. B, Vol. 104, No. 27, 2000 6409

Figure 2. Pulse sequences and coherence transfer pathway diagrams for (a) triple-quantum MAS NMR experiment (with z filter) and (b) triple-quantum cross-polarized MAS NMR experiment (with z filter). In (b), 27Al triple-quantum coherences are created directly by crosspolarization from 1H during the spin-lock period, as signified by the curved coherence pathways. The experiment makes use of a “flip-back” pulse to aid recovery of the 1H magnetization.

at ∼1 ppm characteristic of octahedrally coordinated Al but displays few of the features typical of a second-order broadened line shape. The lack of distinct features may be due to disorder in the Al positions in the crystal, leading to small distributions in isotropic chemical shifts and quadrupolar interactions. The basic MQMAS experiment yields a two-dimensional spectrum that has both a triple-quantum dimension (F1) and a conventional single-quantum MAS dimension (F2). The secondorder quadrupolar broadening appears along a “ridge” with a gradient characteristic of the spin system and the multiplequantum coherence involved, i.e., 19/12 for triple-quantum MAS experiments on a spin S ) 5/2 nucleus such as 27Al. An “isotropic spectrum” may be obtained from a projection of the two-dimensional spectrum onto an axis orthogonal to this gradient, thereby allowing remarkable enhancements in resolution over conventional MAS experiments. The pulse sequence used in this paper for triple-quantum MAS experiments, proposed by Amoureux et al.,25 is shown in Figure 2a. Triplequantum coherences are excited and precess during an interval, t1. At the end of this evolution period, they are converted into observable single-quantum coherences, using a z-filter26 to ensure pure phase line shapes. The free induction decay is then acquired in a final time interval, t2. Figure 3a shows the two-dimensional 27Al triple-quantum MAS NMR spectrum of kaolinite recorded by using the sequence in Figure 2a. In contrast to the simple ridge described above, the single line shape in Figure 3a is more complex, despite still being broadened predominantly along a gradient of 19/12. Distributions of quadrupolar and chemical shift interactions have been shown to yield line shapes that exhibit broadenings along gradients of 3 (distribution of isotropic chemical shifts) and 3/4 (distribution of isotropic quadrupolar

6410 J. Phys. Chem. B, Vol. 104, No. 27, 2000

Ashbrook et al. TABLE 1: Mean 27Al Isotropic Chemical Shifts (δCS), Quadrupolar Products (PQ), Quadrupolar Coupling Constants (CQ) and Asymmetries (η) for the Al Species in Kaolinite, Gibbsite, and Mixtures Ground for 5 min (KG5) and 4 h (KG240)a sample

site

δCS (ppm)

PQ/MHz

kaolinite gibbsite

1 1 2 1 2 3 1 2 3

7(1 11 ( 1 12 ( 1 11 ( 1 13 ( 1 7(1 10 ( 1 40 ( 1 69 ( 1

3.6 ( 0.2 2.2 ( 0.2 4.7 ( 0.1 2.3 ( 0.2 4.8 ( 0.1 3.4 ( 0.3 3.6 ( 0.2 3.7 ( 0.2 3.6 ( 0.2

KG5 KG240

CQ/MHz

η

4.5 ( 0.1

0.45 ( 0.05

4.5 ( 0.1

0.45 ( 0.1

a Only measurement errors in the peak position are quoted and (for KG240 in particular) do not reflect the true range of values of δCS and PQ present.

PQ,29 is given by

PQ ) CQx(1 + η2/3)

Figure 3. Two-dimensional 27Al triple-quantum MAS NMR spectra of (a) kaolinite and (c) gibbsite recorded with the z-filtered experiment in Figure 2a. Two-dimensional 27Al triple-quantum cross-polarized MAS NMR spectra of (b) kaolinite and (d) and (e) gibbsite, recorded with the pulse sequence in Figure 2b. Cross-polarization performed with 27Al spin-locking field ω /2π ≈ 100 kHz in (b) and (d) and with ω / 1S 1S 2π ≈ 40 kHz in (e), with ω1I/2π ≈ 77 kHz in each case. Contact pulse duration was 0.5 ms. 96 transients were averaged for each of 384 t1 increments of 16.67 µs. MAS rate was 6.5 kHz. Displayed F1 and F2 spectral widths are 7 kHz with contour levels drawn at 8, 16, 32 and 64% of maximum intensity. Negative contours drawn with dashed lines.

shifts)27 and some evidence for these, particularly the former, may be seen in Figure 3a. Because the F1 and F2 positions of the line shape are dependent upon both the isotropic chemical shift, δCS, and the isotropic second-order quadrupolar shift, δQ, it is possible to determine these parameters from a single two-dimensional MQMAS experiment. For a triple-quantum MAS experiment on a spin S ) 5/2 nucleus, the position of the center of gravity of a line shape is given by28

4 16 (δ1, δ2) ) 3δCS - δQ, δCS - δQ 5 15

(

)

(1)

The second-order shift parameter, δQ (in ppm), for a spin S ) 5/2 nucleus, can be expressed as

δQ ) (75 PQ/ν0)2

(2)

where ν0 is the Larmor frequency. The quadrupolar product,

(3)

where CQ ) e2qQ/h is the quadrupolar coupling constant and η is the asymmetry parameter. Values of these parameters may be extracted for kaolinite in this manner and are shown in Table 1, although these must be considered as mean values of the small distributions in PQ and δCS. The values are consistent with those known from the literature.30 To investigate the spatial relationship of 27Al and 1H nuclei, cross-polarization from 1H to 27Al was performed by using a conventional cross-polarization sequence. An initial 90° pulse creates 1H magnetization, which can be transferred to neighboring 27Al nuclei during a period of simultaneous spin locking, the “contact time”, τCP. The 27Al signal, from only those spins close to 1H nuclei, is then observed while applying 1H decoupling. For the case of cross-polarization between two nuclei with I ) S ) 1/2, magnetization is transferred when the nutation rates of the two spin systems are equal, i.e., at the Hartmann-Hahn condition,18

ω1I ) ω1S

(4)

Cross-polarization to a quadrupolar nucleus is inherently a more complex process, owing to the presence of multiple nutation rates,31-33 several of which may be matched by the 1H nutation rate. As a consequence, many Hartmann-Hahn “matching conditions” are found experimentally, all with differing relative intensities. Figures 1b and 1c show cross-polarized 27Al NMR spectra of kaolinite recorded with 27Al radio frequency field strengths, ω1S/2π, of ∼25 kHz and ∼100 kHz, respectively, while the 1H radio frequency field strength, ω1I/2π, is ∼77 kHz in both cases. The ability to obtain a cross-polarized spectrum for kaolinite indicates that the 27Al and 1H nuclei are spatially related, as perhaps may be expected from the crystal structure. It should be noted that the spectra in Figures 1b and 1c have only 6% and 15%, respectively, of the signal intensity of the conventional spectrum in Figure 1a. Unlike the situation often found in 1H/ 13C systems, a signal enhancement has not been achieved, which indicates, perhaps, that the potential of cross-polarization involving quadrupolar systems may be the spectral editing capability that the technique offers. Figure 4a shows the crosspolarized signal intensity obtained for kaolinite as a function of the contact time, τCP, for the two matching conditions described above. The plot shows behavior typical of cross-

27Al

MAS NMR of Mechanically Treated Mixtures

Figure 4. 27Al MAS NMR signal intensity plotted as function of contact time, τCP, for (a) and (c) single-quantum cross-polarization and (b) and (d) triple-quantum cross-polarization in (a) and (b) kaolinite and (c) and (d) gibbsite. Signal intensity expressed as percentage of that obtained in conventional 27Al MAS NMR. Single-quantum crosspolarization performed with 27Al spin-locking fields ω1S/2π ≈ 100 kHz and 25 kHz, signified by open circles and open triangles, respectively, and triple-quantum cross-polarization with ω1S/2π ≈ 100 kHz and 40 kHz, signified by filled circles and filled triangles. 1H spin-locking field ω1I/2π ≈ 77 kHz. MAS rate was 6.5 kHz.

polarization, a multiexponential-like dependence on τCP, peaking at 0.3 ms and 0.75 ms, respectively. The match at lower ω1S field strength, which has significantly lower intensity, decays less rapidly with increasing τCP than the match at the higher ω1S field strength. It has recently been shown that it is possible to cross-polarize directly to the multiple-quantum coherences of a half-integer quadrupolar nucleus under MAS conditions in a similar way to conventional single-quantum cross-polarization.22,23 Phase cycling34 is used to select the multiple-quantum coherence after the period of simultaneous spin locking and a further pulse is required to convert this into observable single-quantum coherences. Figure 1d shows the MAS NMR spectrum of kaolinite recorded with a triple-quantum cross-polarization sequence with ω1S/2π ≈ 100 kHz and ω1I/2π ≈ 77 kHz. The spectrum contains only 6% of the signal intensity of the conventional MAS spectrum, an intensity similar to that observed for the singlequantum match at a radio frequency field strength of ω1S/2π ≈ 25 kHz. It should be noted, however, that the spectrum contains 35% of the intensity of a conventional triple-quantum filtered MAS spectrum. This indicates that, at the matching conditions we are using, triple-quantum cross-polarization is relatiVely more efficient than single-quantum cross-polarization. The dependence of this triple-quantum cross-polarization on the contact time, τCP, is shown in Figure 4b. As with singlequantum cross-polarization, typical multiexponential behavior is observed with a maximum in signal intensity occurring at τCP ) 0.5 ms. The signal intensity then decays rapidly with increasing τCP, a more rapid decay than was observed for singlequantum cross-polarization using the same field strengths (Figure 4a, open circles). It should be noted, however, that the similarity of the profiles observed for single- and triple-quantum cross-polarization in kaolinite in Figures 4a and 4b may not necessarily indicate similar dynamics for the two processes. Spin locking a quadrupolar system under MAS conditions often leads to a rapid decay of the cross-polarized signal as the spin locking time increases. This effect may dominate the shape of the curves, which would ultimately lead to similar profiles.

J. Phys. Chem. B, Vol. 104, No. 27, 2000 6411 Cross-polarization directly to the multiple-quantum coherences of a quadrupolar nucleus would appear to represent the most simple and direct method of combining cross-polarization with the MQMAS experiment,22 although other methods are also possible.20 As can be seen from the pulse sequence and coherence transfer pathway diagram of the cross-polarized MQMAS experiment shown in Figure 2b, triple-quantum 27Al coherences are created by cross-polarization from 1H and are allowed to evolve in a period, t1. The multiple-quantum coherence is then converted to observable single-quantum coherences via a z-filter, thereby ensuring purely absorptive line shapes. The two-dimensional 27Al triple-quantum MAS NMR spectrum of kaolinite, recorded by using this pulse sequence, is shown in Figure 3b. Apart from the fact that the ridge line shape is slightly narrower, this spectrum is similar to the conventional triple-quantum MAS spectrum in Figure 3a. The 27Al MAS NMR spectrum of the other component of the ground mixtures, gibbsite (γ-Al(OH)3), is shown in Figure 1e and is consistent with the known crystal structure.35 There is a relatively narrow peak at ∼8 ppm and, partly underlying it, a broader peak centered at slightly lower frequency, both corresponding to octahedrally coordinated Al. The twodimensional 27Al triple-quantum MAS NMR spectrum of gibbsite, recorded with the pulse sequence in Figure 2a, is shown in Figure 3c. The spectrum allows resolution of the two Al sites in the octahedral region, with one site consisting of a broad ridge line shape along the expected 19/12 axis, while the other resonance is much narrower. The values for PQ and δCS for both Al sites, extracted as described previously, are displayed in Table 1. It can be seen that the two sites possess quite distinct PQ parameters, with that of the broader ridge (∼4.7 MHz) being slightly larger than that found in kaolinite (∼3.6 MHz), while that of the narrow peak is much smaller (∼2.2 MHz). The high-resolution isotropic spectrum may be obtained from the two-dimensional spectrum of Figure 3c by means of a shearing transformation.36 This transformation then allows extraction of the isotropic projection, orthogonal to the original 19/12 axis, which is shown in Figure 5a. Two distinct Al sites are visible with approximately equal amplitudes. The crosssection along the ridge line shape of the site with the larger PQ, shown in Figure 5b, reveals a distinctive second-order broadened MAS line shape, slightly distorted by the multiple-quantum excitation and conversion processes. This line shape may be fitted to give values for the quadrupolar coupling constant, CQ, and the asymmetry, η, both of which are given in Table 1. The values are consistent with those in the literature37 and also with the value of PQ determined from the position of the line shape in the two-dimensional spectrum. Figures 1f and 1g display the MAS NMR spectra of gibbsite recorded by using a conventional cross-polarization sequence with 27Al field strengths of ω1S/2π of ∼25 kHz and ∼100 kHz, respectively. The ω1I/2π (1H) radio frequency field was ∼77 kHz. It appears that it is possible to cross-polarize to both 27Al sites, which indicates that both are relatively close to 1H. It should be noted, however, that the line shapes at the two matching conditions do show distinct differences as a result of distortions that are characteristic of the cross-polarization process. Again, a signal enhancement is not observed, with the spectra in Figures 1f and 1g showing 5% and 15% of the signal intensity in the conventional MAS spectrum in Figure 1e. The variation of this cross-polarized signal intensity with τCP is displayed in Figure 4c. As found with kaolinite, the dependence is multiexponential with maxima at 0.5 and 0.8 ms for matches at low and high ω1S field strengths, respectively. However, it

6412 J. Phys. Chem. B, Vol. 104, No. 27, 2000

Ashbrook et al. This experiment illustrates an important point that must be considered when performing cross-polarization to a quadrupolar nucleus. Two sites with distinct PQ values may display markedly different cross-polarization characteristics and exhibit matching conditions at very different field strengths. The nonappearance of a site may not, therefore, be taken as positive proof of its nonproximity to 1H nuclei until other cross-polarization matching conditions have also been carefully considered. For example, both Al sites appear in the two-dimensional triple-quantum cross-polarized MAS NMR spectrum in Figure 3d, where the cross-polarization is performed at a higher ω1S field strength. The spectral editing capability offered by variation of the spinlocking radio frequency field strength, although not based primarily on the spatial proximity of nuclei, has great potential as a method for separating peaks that are closely spaced or, indeed, overlapping in a two-dimensional MQMAS spectrum. As will be shown in the next section, this approach is particularly powerful in mixtures where, as in the present case, the field strengths for this selective cross-polarization may be calibrated on the pure components. The isotropic projection of the spectrum in Figure 3d is shown in Figure 5c. As with the projection of the conventional MQMAS spectrum in Figure 5a, two Al sites are clearly visible, but the intensities are now unequal (and, as with kaolinite, the lines appear narrower). It appears that multiple-quantum crosspolarization at this matching condition has produced a relative enhancement of the intensity of the Al site with the larger PQ value. Although, since both 27Al peaks are still present, this cannot be viewed as full spectral editing, relative enhancements such as those observed in Figures 3 and 5 may aid greatly in the investigation of sites that are of relatively low intensity in a conventional MQMAS spectrum.

Figure 5. (a) Isotropic projection of 27Al triple-quantum MAS NMR spectrum of gibbsite in Figure 3c. (b) Cross-section along ridge line shape in Figure 3c, displaying second-order broadened central-transition line shape. (c) Isotropic projection of 27Al triple-quantum cross-polarized MAS NMR spectrum in Figure 3d. In (a) and (c), the shearing transformation used has scaled the spectral width by a factor of 12/31 to 2.7 kHz.

should be remembered that the contact time dependence of the signal intensity shown for gibbsite is in fact a sum of the τCP dependence observed for the two individual Al sites. The triple-quantum cross-polarized MAS NMR spectra of gibbsite, with 27Al field strengths of ω1S/2π of ∼40 kHz and ∼100 kHz and with a 1H field strength of ω1I/2π ≈ 77 kHz, are shown in Figures 1h and 1i. The corresponding dependence of this triple-quantum cross-polarized signal on τCP, with maxima at ∼0.5 ms, is shown in Figure 4d. As with singlequantum cross-polarization, distortions are apparent compared with the conventional MAS line shape, particularly in the case of Figure 1h, where the triple-quantum cross-polarized MAS line shape is noticeably narrower and less structured than the conventional line shape. It is also important to note that Figure 1h has been inverted because the cross-polarization signal obtained at this matching condition has the opposite sign to that obtained at the higher field strength.23 Two-dimensional triple-quantum cross-polarized MAS spectra, recorded by using the pulse sequence in Figure 2b, with 27Al field strengths of ω /2π of ∼100 kHz and ∼40 kHz and 1S with a 1H field strength of ω1I/2π ≈ 77 kHz, are shown in Figures 3d and 3e. The spectrum in Figure 3e shows only a single Al site, with the nonappearance of the broader ridge line shape accounting for the narrow line shape found in Figure 1h.

27Al

NMR of Ground Mixtures of Kaolinite and Gibbsite

Mullite precursors were formed from a 1:2 stoichiometric mixture of kaolinite and gibbsite ground together for varying lengths of time, i.e., 5 min (KG5) and 4 h (KG240). As mentioned previously, it has been suggested that the effects of grinding are 2-fold: first, a reduction in particle size, leading to an increase in further reaction rate; and, second, the formation of new chemical species. By studying mixtures ground for different amounts of time, it should be possible to investigate the structural changes taking place on mechanical treatment and, possibly, to gain insight into the mechanism by which they occur. A previous study of these mixtures,8 based on 27Al and 29Si MAS spectra and XRD and DSC results, has indicated that a longer grinding time may be necessary to promote significant chemical change. However, the lack of resolution in the MAS spectra limits the ability of NMR to show if any chemical change occurs at shorter grinding times. The conventional 27Al MAS NMR spectrum of the sample ground for the shorter length of time, KG5, is shown in Figure 6a and retains the principal spectral characteristics common to the kaolinite and gibbsite starting materials. The presence of more than one distinct Al site is confirmed by the twodimensional triple-quantum MAS NMR spectrum, recorded by using the pulse sequence in Figure 2a, shown in Figure 7a. The spectrum appears to be a combination of the two-dimensional MQMAS spectra of kaolinite (Figure 3a) and gibbsite (Figure 3c), with peaks corresponding to both gibbsite sites (designated 1 and 2 in Table 1) and with a kaolinite-like ridge (designated Site 3 in Table 1) overlapping at slightly lower frequency. Owing to the overlap of the peaks of Sites 1 and 3, the isotropic projection of the spectrum in Figure 7a, displayed in Figure

27Al

MAS NMR of Mechanically Treated Mixtures

Figure 6. (a) Conventional, (b) and (c) single-quantum cross-polarized, and (d) and (e) triple-quantum cross-polarized 27Al MAS NMR spectra of KG5 (displayed spectral widths 7 kHz). (f) Conventional, (g) and (h) single-quantum cross-polarized, and (i) and (j) triple-quantum crosspolarized 27Al MAS NMR spectra of KG240 (displayed spectral widths 27 kHz). 27Al spin-locking field ω1S/2π ≈ 25 kHz in (b) and (g), ω1S/ 2π ≈ 40 kHz in (d) and (i), and ω1S/2π ≈ 100 kHz in (c), (e), (h), and (j). 1H spin-locking field was ∼77 kHz. Single-quantum cross-polarized spectra recorded with τCP ) 1 ms; triple-quantum cross-polarized spectra recorded with τCP ) 0.5 ms. 240 transients were averaged with a recycle interval of 3 s. MAS rate was 6.5 kHz.

8a, shows only two 27Al resonances, although the intensity of the peak corresponding to both Sites 1 and 3 is significantly higher than that of the peak corresponding only to Site 2. A further indication that this sample contains Al sites related to both kaolinite and gibbsite is found in the cross-section along the ridge line shape of Site 2, shown in Figure 8b. This is a second-order MAS line shape very similar to that observed in gibbsite (Figure 5b). Table 1 shows the PQ and δCS parameters obtained from the positions of centers of gravity of the three line shapes and the CQ and η extracted from Figure 8b, and these are all closely related to the values found in kaolinite and gibbsite. Any differences arise from overlap of the twodimensional line shapes and a small broadening of these peaks compared with those of the pure materials, which hinders the accuracy of the location of the center of gravity, a feature which is reflected in the larger errors associated with some of these parameters. The additional broadening observed in KG5 compared with the starting materials, as evidenced by the isotropic projection in Figure 8a and the cross-section in Figure 8b, would seem to indicate that grinding has introduced some additional disorder, which leads to slightly increased ranges of chemical shift and quadrupolar interactions.

J. Phys. Chem. B, Vol. 104, No. 27, 2000 6413

Figure 7. (a) Two-dimensional 27Al triple-quantum MAS NMR spectrum of KG5 recorded with the z-filtered pulse sequence in Figure 2a. (b) and (c) Two-dimensional 27Al triple-quantum cross-polarized MAS NMR spectra of KG5 recorded with the pulse sequence in Figure 2b. Displayed F1 and F2 spectral widths are 7 kHz. Contour levels drawn at 8, 16, 32, and 64% of maximum intensity in (a) and (b) and at 16, 32, and 64% in (c). (d) Two-dimensional 27Al triple-quantum MAS NMR spectrum of KG240 recorded with the z-filtered pulse sequence in Figure 2a. The three centerband peaks are labeled 1, 2, and 3; other peaks are spinning sidebands. (e) and (f) Two-dimensional 27Al triplequantum cross-polarized MAS NMR spectra of KG240 recorded with the pulse sequence in Figure 2b. Displayed F1 and F2 spectral widths are 30 and 24 kHz, respectively. Contour levels drawn at 6, 10, 15, 20, 40, 60, and 80% of maximum intensity in (d) and (e) and at 20, 40, and 60% in (f). Negative contours drawn with dashed lines. Crosspolarization performed with 27Al spin-locking field ω1S/2π ≈ 100 kHz in (b) and (e) and ω1S/2π ≈ 40 kHz in (c) and (f). 1H spin-locking field ω1I/2π ≈ 77 kHz and τCP ) 0.5 ms. In (a), (b), and (c), 96 transients were averaged for each of 384 t1 increments of 16.67 µs; in (d), (e), and (f), 192 transients were averaged for each of 256 t1 increments of 10 µs. MAS rate was 6.5 kHz.

The single-quantum cross-polarized MAS NMR spectra of KG5, with 27Al field strengths of ω1S/2π ≈ 25 kHz and 100 kHz and with a 1H field strength of ω1I/2π ≈ 77 kHz, are shown in Figures 6b and 6c, respectively. Although both line shapes occur in the octahedral region of the 27Al NMR spectrum, it is impossible to say whether cross-polarization to all Al sites is observed owing to the lack of resolution in the MAS spectrum. The signal intensities observed reflect those in kaolinite and gibbsite, with Figures 6b and 6c having 6.5% and 16.5% of the intensity of the conventional MAS spectrum in Figure 6a. The contact time dependence of this signal, a combination of that obtained from all the Al sites, is shown in Figure 9a for both matching conditions. The behavior observed is very similar to that seen previously for kaolinite and gibbsite, although the

6414 J. Phys. Chem. B, Vol. 104, No. 27, 2000

Figure 8. (a) Isotropic projection of 27Al triple-quantum MAS NMR spectrum of KG5 in Figure 7a. (b) Cross-section along ridge line shape in Figure 7a, displaying second-order broadened central-transition line shape. (c) Isotropic projection of 27Al triple-quantum cross-polarized MAS NMR spectrum in Figure 7b. In (a) and (c), the shearing transformation used to obtain the isotropic spectrum has scaled the spectral width by a factor of 12/31 to 2.7 kHz.

maximum in signal intensity at the lower ω1S field occurs at slightly shorter τCP, ∼0.2 ms. To establish if cross-polarization to all Al sites is achieved, as indeed would be expected if the sample were a combination of kaolinite and gibbsite, we performed multiple-quantum crosspolarization experiments. The triple-quantum cross-polarized MAS spectra of KG5, recorded with 27Al field strengths of ω1S/ 2π ≈ 40 kHz and 100 kHz and with a 1H field strength of ω1I/ 2π ≈ 77 kHz, are shown in Figures 6d and 6e, respectively, and display only ∼0.2% and 5.5% of the signal intensity of the conventional MAS spectrum (Figure 6a) but have ∼1% and 33% of the intensity of a conventional triple-quantum filtered MAS spectrum. Line shape distortions are more noticeable than in the case of single-quantum cross-polarization, particularly in the spectrum in Figure 6d, which, it is important to note, has been inverted. The dependence on τCP of the triple-quantum cross-polarization, again a combination of all three sites, is shown in Figure 9b and is very similar to that observed in Figures 4b and 4d. The two-dimensional triple-quantum cross-polarized MAS NMR spectra of KG5, recorded with the pulse sequence in Figure 2b, are shown in Figures 7b and 7c for 27Al field strengths of ω1S/2π ≈ 100 kHz and 40 kHz, respectively. The spectrum in Figure 7c provides an indication that no chemical change

Ashbrook et al.

Figure 9. 27Al MAS NMR signal intensity plotted as function of τCP for (a) single- and (b) triple-quantum cross-polarization in KG5. Signal intensity expressed as percentage of that obtained in conventional 27Al MAS NMR. Single-quantum cross-polarization performed with 27Al spin-locking fields ω1S/2π ≈ 100 and ω1S/2π ≈ 25 kHz, signified by open circles and open triangles, respectively, and triple-quantum crosspolarization with 27Al spin-locking fields ω1S/2π ≈ 100 kHz and 40 kHz, signified by filled circles and filled triangles. (c) Plot of intensity of each KG5 site in two-dimensional 27Al triple-quantum cross-polarized MAS NMR spectra recorded with varying values of contact time, τCP, in pulse sequence in Figure 2b. Cross-polarization performed with 27Al spin-locking field ω1S/2π ≈ 100 kHz. Filled triangles, diamonds and squares signify KG5 sites 1, 2, and 3, respectively. (d) and (e) 27Al MAS NMR signal intensity of each KG240 site plotted as function of τCP for (d) single- and (e) triple-quantum cross-polarization. Signal intensity expressed as percentage of that obtained in conventional 27Al MAS NMR. Cross-polarization performed with 27Al spin-locking field ω1S/2π ≈ 100 kHz. KG240 sites 1, 2, and 3 denoted by squares, diamonds, and triangles, respectively. In all experiments in (a) to (e): 1H spin-locking field ω /2π ≈ 77 kHz; MAS rate 6.5 kHz. 1I

has occurred during the grinding process and that the sample is indeed a mixture of kaolinite and gibbsite. The spectrum contains only a single 27Al resonance (Site 1) as was observed in the gibbsite sample (Figure 3e), with the kaolinite-like resonance (Site 3) missing from the spectrum, thereby establishing that the two overlapping resonances seen in Figure 7a are indeed separate. The two-dimensional spectrum in Figure 7b, recorded with the cross-polarization performed at the higher ω1S field strength, provides evidence of cross-polarization to all three 27Al sites, as is expected if the sample has undergone no chemical change. The spectrum shows enhancement of both Sites 2 and 3 relative to Site 1, but this results in no change in the isotropic spectrum shown in Figure 8c. As two-dimensional methods show the existence of three separate 27Al resonances, it is possible to consider the dependence on τCP of the multiple-quantum cross-polarization to each site separately by performing the two-dimensional experiment at varying contact times. This allows the intensities of the three individual sites to be monitored separately and plotted against τCP, which gives plots such as those shown in Figures 4b, 4d, and 9b. Because these “build up” plots are dependent upon the

27Al

MAS NMR of Mechanically Treated Mixtures

27Al/1H

distance, it is possible that important structural information could be obtained from a detailed mathematical analysis. Figure 9c displays the dependence on τCP of triple-quantum cross-polarization to the three distinct Al sites in KG5, obtained from a series of two-dimensional triple-quantum cross-polarized spectra recorded with cross-polarization conditions of ω1S/2π ≈ 100 kHz and ω1I/2π ≈ 77 kHz. All three sites can be seen to possess very similar dependencies with maxima between 0.4 and 0.6 ms, as was observed for the kaolinite and gibbsite separately. The conventional MAS spectrum of the second mixture of kaolinite and gibbsite, KG240, is shown in Figure 6f. It is immediately obvious that grinding for 4 h has produced substantial changes in the 27Al MAS NMR spectrum. In addition to a peak in the octahedral region of the spectrum at ∼4 ppm, broad peaks are also apparent at ∼55 ppm, which is characteristic of Al in a tetrahedral coordination, and at ∼30 ppm. The latter has been attributed previously to either pentahedral Al or to the distorted tetrahedral tricluster unit that occurs in mullite.8,17 All three line shapes lack obvious second-order quadrupolar features yet possess long tails to low frequency characteristic of distributions of quadrupolar and chemical shift interactions. Figure 7d shows the 27Al two-dimensional triple-quantum MAS NMR spectrum of KG240, recorded by using the pulse sequence shown in Figure 2a. The three peaks (labeled 1, 2 and 3 in Figure 7d) differ markedly from the characteristic ridge line shapes observed in crystalline systems. Moreover, the three line shapes are quite distinct, although all appear to possess components lying along gradients of 3 and 3/4, which indicates the presence of ranges of both chemical shift and quadrupolar interactions. Although it is possible to extract PQ and δCS parameters from the F1 and F2 positions of maximum peak height, as given in Table 1 for the three sites, the presence of significant ranges of these interactions severely limits the use of this method. The three values of PQ obtained are similar, perhaps indicating a similar mean PQ, but the range of this parameter (not shown in the table) seems to vary between the three sites. The octahedral site shows a wide range of PQ, which we can estimate very roughly to be 0 to 7 MHz, and also a range of approximately (5 ppm in δCS. The tetrahedral site displays a slightly smaller range of PQ but a similar distribution in δCS, while the center site displays smaller ranges of both parameters. The changes in the 27Al MAS and MQMAS NMR spectra appear to indicate that structural change has occurred in this sample and has created new Al sites possessing a large amount of amorphous character. The conventional cross-polarized 27Al MAS spectra, shown in Figures 6g and 6h, show that it is possible to cross-polarize to all three of the Al sites in KG240, which indicates that all are relatively close to 1H nuclei. These spectra were recorded with 27Al radio frequency field strengths of ω1S/2π ≈ 25 kHz and 100 kHz and with a 1H field strength of ω1I/2π ≈ 77 kHz and contain 7% and 12%, respectively, of the signal intensity of the conventional spectrum in Figure 6f. The dependence on contact time of the signal intensity of each site is shown in Figure 9d for the cross-polarization match where ω1S/2π ≈ 100 kHz. Each site possesses a multiexponential dependence with a similar rate of decay of the signal, although the cross-polarization is slightly more selective for the center site, which reaches 16% of its intensity in the conventional MAS compared with the 14% and 10% achieved by the octahedral and tetrahedral sites, respectively. The signal maximum for each site is found at approximately the same τCP value, i.e., 0.5 ms.

J. Phys. Chem. B, Vol. 104, No. 27, 2000 6415 Figures 6i and 6j show the triple-quantum cross-polarized MAS spectra of KG240, recorded with 27Al fields strengths ω1S/ 2π ≈ 40 kHz and 100 kHz and with a 1H field strength ω1I/2π ≈ 77 kHz in both cases. The spectrum in Figure 6i, recorded with field strengths at which the corresponding gibbsite and KG5 spectra demonstrated selective matches for the Al site with lowest PQ only, shows very little signal, with only a tiny amount of the octahedral site visible. This spectrum has also been inverted because the signal again has negative intensity. Figure 6j, however, demonstrates that multiple-quantum cross-polarization is possible in KG240 and contains 4% of the intensity of the conventional MAS spectrum but 28% of that of a conventional triple-quantum filtered spectrum. The dependence on τCP of the multiple-quantum cross-polarized signal obtained at this matching condition is demonstrated in Figure 9e and shows that each site possesses a similar profile, peaking at ∼0.5 ms, although it appears that at this matching condition crosspolarization is relatively slightly more selective for the octahedral site. The two-dimensional 27Al triple-quantum cross-polarized MAS NMR spectrum of KG240, recorded with the pulse sequence in Figure 2b with ω1S/2π ≈ 100 kHz and ω1I/2π ≈ 77 kHz, is shown in Figure 7e. All three peaks are similar to those observed in the conventional triple-quantum MAS spectrum in Figure 7d. This similarity of line shape indicates that, although the distinct 27Al nuclei may exhibit differing quadrupolar and chemical shift interactions as a result of the distribution of these parameters, all show cross-polarized signals and are, therefore, relatively close to 1H nuclei. Figure 7f shows the corresponding two-dimensional triplequantum MAS NMR spectrum of KG240, with the crosspolarization performed with a 27Al field strength of ω1S/2π ≈ 40 kHz and a 1H field strength of ω1I/2π ≈ 77 kHz. The spectrum contains very little signal but displays a peak in the octahedral region, clearly much narrower than the resonance observed in the same region previously. The position of the center of gravity of this peak yields values of PQ ≈ 2.6 MHz and δCS ≈ 8 ppm, values similar to those observed for the Al site in gibbsite (Figure 3e) and KG5 (Figure 7c). One explanation for the appearance of this small signal involves the presence of residual gibbsite in the sample, despite the extended mechanical treatment, the narrower peak of which is observed at this crosspolarization matching condition. However, the XRD pattern of KG240 does not reveal any indication that residual gibbsite may be present.8 Another explanation might be that triple-quantum cross-polarization at this field strength is more selective for 27Al nuclei with lower PQ values and, therefore, selects only these nuclei out of the full distribution. The counter-argument here, however, is that only a peak in the octahedral region is observed in Figure 7f, with no sign of similar peaks in the other spectral regions. Conclusions We have employed a range of modern 27Al MAS NMR techniques to characterize an aluminosilicate (kaolinite) and a hydrated alumina (gibbsite) and to study mixtures of the two that have been ground together for varying times. We used the 27Al MQMAS NMR experiment to increase spectral resolution and to determine quadrupolar and chemical shift parameters; single- and triple-quantum cross-polarization to confirm the spatial proximity of 1H and 27Al; and a combination of crosspolarization and MQMAS for two-dimensional spectral editing (based on radio frequency field strengths and quadrupolar parameters rather than on internuclear distances), which allowed

6416 J. Phys. Chem. B, Vol. 104, No. 27, 2000 overlapping peaks to be studied. By means of these techniques, we have been able to establish that mechanical treatment of the kaolinite-gibbsite mixture for a short period does not produce any significant structural changes, although there is some evidence that the mixture is more amorphous at the microscopic level. Any increase in the reaction (mullite formation) rate of this mixture is likely, therefore, to be due only to the decreased particle size. More extended grinding of these mixtures, however, leads to substantial changes in structure and the formation of new Al sites with varying coordination number. The amorphous nature of this mullite precursor is immediately apparent from 27Al MQMAS spectra and the technique is shown to be a useful tool in studying distributions of quadrupolar and chemical shift interactions. Acknowledgment. We are grateful to EPSRC for generous support (grant no. GR/M12209) and for the award of a studentship to S. E. A. We would also like to thank Dr. J. Temuujin for providing the ground samples and Dr. P. Hodgkinson for writing the two-dimensional Fourier transform program. K. J. D. M. is indebted to the Royal Society of New Zealand for a James Cook Research Fellowship. References and Notes (1) Schneider, H.; Okada, K.; Pask, J. A. Mullite and Mullite Ceramics; J. Wiley and Sons: Chichester, U.K., 1994. (2) Hamano, K.; Okada, S.; Nakajima, H.; Okuda, H. Abstracts of the Annual Meeting of the Ceramic Society of Japan, 1988, Paper 2F05. (3) Rezaie, H. R.; Rainforth, W. M.; Lee, W. E. Br. Ceram. Trans. 1997, 96, 181. (4) Kawai, S.; Yoshida, M.; Hashizume, G. J. Ceram. Soc. Jpn. 1990, 98, 669. (5) Nikaido, M.; Yoshizawa, Y.; Saito, F. J. Chem. Eng. Jpn. 1996, 29, 456. (6) Temuujin, J.; Okada, K.; MacKenzie, K. J. D. J. Eur. Ceram. Soc. 1998, 18, 831. (7) Temuujin, J.; Okada, K.; MacKenzie, K. J. D. J. Mater. Res. 1998, 13, 2184. (8) Temuujin, J.; MacKenzie, K. J. D.; Schmu¨cker, M.; Schneider, H.; McManus, J.; Wimperis, S. J. Eur. Ceram. Soc. 2000, 20, 413. (9) Ganapathy, S.; Schramm, S.; Oldfield, E. J. Chem. Phys. 1982, 77, 4360.

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