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J. Phys. Chem. 1995,99, 15138-15141
Quantification of Aluminum Coordinations in Amorphous Aluminas by Combined Central and Satellite Transition Magic Angle Spinning NMR Spectroscopy G. Kunath-Fandrei,?T. J. Bastow: J. S. Hall: C. Jlger: and M. E. Smith*,+Jl Friedrich-Schiller Universitat Jena, PATF, 0-07743 Jena, Germany; CSIRO Division of Materials Science and Technology, Private Bag 33, Rosebank MDC Clayton, Victoria 3169, Australia; CSIRO Division of Mineral and Process Engineering, P.O. Box 312, Rosebank MDC Clayton, Victoria 3169, Australia; Max Planck Institut f i r Polymerforschung, Postfach 3148, 0-55021 Mainz, Germany; and Physics Laboratory, University of Kent, Canterbury, Kent, Cn 7NR, U.K. Received: March 8, 1995; In Final Form: May 30, 1995@
An amorphous alumina formed by partial dehydration of gibbsite is characterized by observation and simulation of the central and satellite transitions of 27Almagic angle spinning Nh4R spectra. The improved resolution f ' / 2 ) transitions allows the individual sideband manifolds to of the Mod, AlOs, and A106 sites in the (f3/2, be simulated so that the NMR interaction parameters and the distribution of the quadrupolar interaction of each coordination can be deduced. Based on these parameters, simulation of the centerband provides the most accurate information to date on the distribution of aluminum between the three coordinations in an amorphous alumina.
Introduction With the advent of routine magic angle spinning (MAS) NMR' and its application to inorganic materials, one of the first popular nuclei as a result of its high NMR sensitivity was 27Al. In particular, it allowed elucidation of the aluminum distribution between A 1 0 4 and AlO6 sites in transitional al~minas.*-~ The ability of NMR to characterize different local structural units for 27Alextended its application to a wide range of materials including zeolites, ceramics, cements, and amorphous films (see ref 4 for review). Although NO4 and AlO6 coordinations could be distinguished in the earliest MAS NMR spectra, there were problems from both slow MAS rates (3-4 kHz), resulting in extensive overlap of centerbands and spinning sidebands, and rf excitation conditions producing distorted spectral intensities. Improvements in both NMR technology, including MAS rates routinely in excess of 14 kHz, high applied magnetic fields, and reduced recovery times of probes, together with much better understanding of the underlying theory have led to accurate quantification of the aluminum distribution in materials that produce well-defined second-order quadrupole perturbed NMR spectra. Quantification has been demonstrated for crystalline materials even when sites with widely differing quadrupole frequencies are presenP (VQ = 3e2qQ/21(21 - l)h, where eq is the maximum component of the electric field gradient (efg), eQ is the nuclear electric quadrupole moment, and I is the nuclear spin quantum number). In disordered materials there remains the problem that the 27AlMAS NMR centerbands are largely featureless except for asymmetric tails to low frequency7 which are a result of there being a distribution of quadrupolar interactiom8 There have been previous simulations of 27Al MAS NMR centerbands in transitional aluminas9 but these are difficult make unambiguous and the approach here extends such work.
* Address correspondence to this author at Physics Laboratory, University of Kent, Canterbury, Kent, CT2 7NR, U.K. Friedrich-Schiller Universitat Jena. CSIRO Division of Materials Science and Technology. 5 CSIRO Division of Mineral and Process Engineering. Max Planck Institut fur Polymerforschung. University of Kent. 'Abstract published in Advance ACS Abstracts, August 1, 1995. +
0022-3654/95/2099-15 138$09.00/0
Calcination of aluminum hydrates such as gibbsite is of considerable commercial interest. Refining of bauxite to alumina proceeds via a thermal dehydration of alumina trihydrate. Partially dehydrated products are themselves useful in the manufacture of low calcia refractory cements and related materials and via rehydration to form strong porous catalyst supports. The reactivity of activated, partially calcined alumina hydrates toward rehydration appears to be linked to the structure of the intermediate phase. Dehydration of alumina hydrates often forms complex phase mixtures with some components amorphou~.~ Some disordered transitional phases contain Al04, AIOs, and A106 sites (e.g., x- and e-AlzO3) with the separate resonances showing extensive overlap in the centerband. It is vital to understand variations of the phase content and more importantly changes in the underlying structure as particle size and dehydration conditions including heating rate and pressure are varied. For characterizing such materials, particularly the amorphous components, an atomic scale structural probe such as NMR is ideal. A variety of NMR techniques have been applied including cross-polarization and double angle rotation (DOR). A combination of NMR techniques was used recently to examine a partially calcined superfine alumina (PCSA) and revealed a-Al2O3, gibbsite, and an amorphous transitional alumina." DORI2 is a technique that removes both first- and second-order broadening improving spectral resolution over conventional MAS in certain cases. In the PCSA sample the more crystalline components, gibbsite and a-AlzO3, were better resolved by DOR but the transitional alumina component did not appreciably narrow, a consequence of its NMR line width being determined by a distribution of both chemical shifts and quadrupole interactions. Hence, for most calcination products MAS is likely to provide resolution comparable with other approaches, yet in MAS spectra extensive overlap of asymmetric centerband resonances makes accurate quantitative analysis difficult. With fast stable MAS now available, routine observation of the spinning sidebands from the satellite transitions (Le. (*3/2, k1/2) inner and (H2, f3/$ outer) is possible. For I = 5/2 nuclei the inner satellite transition shows much reduced second-order quadrupole effects compared to the central transition with its isotropic position being much closer to the isotropic chemical 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 41, 1995 15139
Quantification of A1 Coordinations in Amorphous Aluminas shift value and the width of each sideband only 30% (see ref 13 for a review). Hence, resolution of the different coordinations is often better for 27Al in the inner satellite transition sidebands and there is the added advantage that when chemical shift dispersion and second-order quadrupole effects are the major contributors to the line width as the chemical shift contribution increases in relative importance for the inner satellite transition compared to the central transition the resonances associated with the satellite transition become more closely Gaussian. The envelope of the satellite transitions is almost entirely determined by first-order quadrupole effects and the sideband intensities can thus be used to determine CQ,17, and the distribution of components of the efg tensor. Quantification of A104, AlOs, and A106 environments using satellite transition spectroscopy has already been demonstrated in some glasses (e.g., aluminoborates).8 This paper demonstrates the utility of this approach to the much more widely studied amorphous aluminas and highlights the details of this approach to be able to accurately quantify such 27Al NMR spectra of aluminas. The results here provide the most accurate quantification of aluminum environments in an amorphous alumina and records the largest variation in intensity between different sites (-9:1) from an amorphous material by this method to date. The importance and industrial relevance of aluminas will make the specific result reported here of widespread interest. Also now that accurate quantification is really possible in such materials their characterization in terms of the populations of the aluminum coordinations is a genuinely viable alternative for the first time.
75
50
25
0
-25
[ppm]
Figure 1. 27Al MAS N M R centerband of an amorphous alumina showing (a) the experimental data, (b) complete simulation and the individual components (c) A106, (d) A104, and (e) AlOs.
Experimental Details An amorphous partially dehydrated alumina was formed by calcining gibbsite. The sample was X-ray amorphous and the heat treatment corresponds to conditions necessary to produce ~ - A l 2 0 3 . ' ~The water content was determined as 1.2 mol % H20. 27AlMAS NMR spectra were accumulated on a Bruker ASX500 spectrometer operating at 130.32 MHz with a 4 mm doublebearing MAS probe. Short pulses (-0.7 ps) corresponding to a tip angle of 115' were used with recycle delays of 1 s and MAS rates of ca. 14 kHz which completely removes spinning sidebands from the centerband region. Spectra were referenced against a secondary standard of the AlO6 resonance of Y3Al5O12 at 0.7 ppm (with respect to primary shift scale reference of (Al(H20)6)3+). Spectra were accumulated with frequency ranges covering 400 kHz for the centerband and around 2 MHz for the satellite transitions. The strategy adopted here was to accumulate the satellite transition data at 1 s recycle delays, since although this causes some differential saturation of the different coordinations it is vital here to achieve good signalto-noise in the sidebands so that the quadrupole parameters can be accurately extracted. However, for the quantitative information from the centerbands a recycle delay of 15 s was employed which gave fully relaxed spectra since no change was observed on using longer delays. To get the true satellite intensities that are free from instrumental distortions, various offsets were recorded (at least two) and the sidebands subsequently corrected for the finite bandwidth of the probe. Initial probekystem deadtime of -8 ps occurred which led to baseline roll. For spectra where the spinning sidebands from different sites are sufficiently resolved that the baseline can be seen, a simple cubic spline correction is possible and has been applied to all spectra here. Simulations of the centerband and satellite transitions were performed on a 486 PC8 using the theory of Skibsted et al.I4
1
900
1
I
1
1
1
,
I
800 700 600 500 400 300 200
I
[kHz]
Figure 2. 27Al MAS N M R sideband manifold showing (a, upper) simulation and (b, lower) experiment.
The details of how disorder is introduced into the quadrupolar parameters have been described previously.* Two components of the efg tensor are given a Gaussian distribution (AVzZbeing its full width at half-height converted into frequency units) together with a mean starting value of V,. By taking combinations of the two varying components of efg the corresponding distributions of V Q and 17 can be deduced. It should be emphasized that a Gaussian distribution of V Q is not taken.
Results and Discussion The 27AlMAS NMR spectrum is dominated by the centerband of the central (I/z, -'/*) transition (Figure 1). There are clearly three resonances associated with A104, AlOs, and 40.5 resonances that peak around 65, 28, and 7 ppm, respectively. However, the peaks strongly overlap and unambiguous spectral deconvolution would be difficult, particularly given the asymmetric nature of the resonances, manifested as a low-frequency tail. After recording the spinning sideband manifold at different offsets and then correcting for the response of the probe, the true intensity envelope is produced. Figure 2a shows the resulting experimental data after a cubic spline fit has been
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15140 J. Phys. Chem., Vol. 99, No. 41, 1995
30.00 t 25.00
i
20.00 t 15.00
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10.00 10
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30 40 Spinnlng aideband number
50
IkHzl
80
40
0
M)
Figure 3. Experimental sideband intensities of the first 60 sidebands of the A106 site in an amorphous alumina with four possible sets of simulation parameters (inset shows the corresponding centerbands). applied to the baseline. The correction is imperfeot but these errors introduce only small overall errors into the manifold, particularly the extent of the manifold. These errors do not markedly change the NMR interaction parameters deduced from the simulation and for this approach to be practically robust it should be insensitive to such errors. The intensity of the sideband envelope is dominated by the AlO6 site, and the intensities for these sidebands can be read directly from the experimental data. The intensities of the first 60 sidebands are shown in Figure 3. The intensities of these sidebands are then simulated for various combinations of YQ, AVz,, and 7, and four possibilities are shown in Figure 3. Although variation in AVzz was investigated, for Figure 3 a constant value (420 kHz) has been shown. V Q = 750 kHz and 7 = 1 produces a very poor fit, while making 7 = 0 simulates well the higher-order (large n) sidebands but the intensity variation at lower values is poor, and as this is where signalto-noise is best any simulation should reproduce this region. V Q = 630 kHz and 7 = 1 fits well for the higher-order sidebands but below number 15 and between numbers 30 and 40 fits poorly. The best fit obtained was with V Q = 680 kHz and 7 = 0.3. The accuracy of these numbers is f 3 0 kHz in YQ and f 0 . 2 in 7,Le., 7 is neither 0 nor 1 but has some intermediate value, which can perhaps be anticipated given the amorphous nature produces a distribution in 7 that is unlikely to have a mean value at either of the extremes (0 or 1). The corresponding simulations of the centerbands using the same parameters are shown as an inset in Figure 3. This figure illustrates that the satellite transitions are more sensitive than the central transition (CT) to changes in YQ and 7. The actual peak position may vary little over a range of Y Q and 7, reflecting their interconnection in determining the isotropic quadrupolar shift.4 A similar approach is adopted for the N O 4 and NO5 sideband manifolds except that the sidebands have to be simulated to extract the intensity because of the overlap of the peaks, particularly of A105 with AlO6. There are increased errors in the parameters determined for the lower coordinations as their lower intensities result in lower signal to noise. A selection of intermediate sidebands, their simulation, and individual A104, A105, and A106 contributions is shown in Figure 4. The much more symmetric nature of the resonances from the inner satellite transition compared to the central transition is clearly evident. The noise present means that although the A105 site cannot be clearly resolved the A105 site is necessary to reproduce the line shape at the base of the A106 resonance and the intensity
Figure 4. Expansion of some sidebands showing (a) the experimental data, (b) full simulation and the individual contributions from (c) A104, (d) AlOs, and ( e ) A106. TABLE 1: NMR Interaction Parameters Deduced from the Satellite and Central Transitions of an Amorphous Alumina and the Intensities Required To Simulate the Centerband= site ~ 1 0 ~ A105 A104 400 i40 750 1 55 VQ (mz) 680 1 35 400 1 40 420 rt 20 AV:: (kHz) 420 =k 20 0.3
7
0.3
0.3
38.0 f 1.0 71.5 1 1.0 11.5 rt 0.5 22.5 1 1.5 70.0 rt 2 7.5 i 1 I (%) a 6,,, is the isotropic chemical shift and I the fractional centerband intensity. Errors in 7 have not been formally calculated but are likely to be 1 0 . 2 and this has little effect on the other parameters, particularly the intensities. 6so
(PPm)
associated with the A105 site can be unambiguously extracted. For the A106 simulation (Figure 4e) there is a tail to high frequency from the main sidebands which is intensity from the much broader outer satellite transitions (6.3 times broader compared to the inner satellite transition). The sideband intensities deduced for the A104 and AlO5 sites are then analyzed in the same way as for the A106 site so that the quadrupole interactions can be extracted and are summarized in Table 1. The parameters in Table 1 provide the basis for the complete simulation of the central transition centerband with the individual components (Figure 1). The simulation provides a good reproduction of the centerband. It should be stressed that the interaction parameters deduced from the spinning sideband manifolds are not adjusted in the centerband simulation only the relative intensities are varied until the best simulation is anived at (Table 1). This approach is more rigorous than simulations of the centerband alone. The interaction parameters deduced for the different sites make interesting comparison. The more commonly encountered AlO6 and A104 coordinations have larger interaction parameters than the AlOs site. However, the distribution of the quadrupolar interaction for the A105site is larger (as a fraction) than for the other two sites. This is clearly reflected in the more asymmetric appearance of the A105 centerband. The smaller Y Q indicates that the distortion from uniformly distributed charges is smallest for A105, yet the variation in the A105site is much greater than for the other two sites. For crystalline materials, A105 sites typically have larger coupling constants than for A104 and AlO6, in contrast to this amorphous alumina. This observation probably reflects an interesting difference for A105 sites in crystalline and amorphous materials in that it is more difficult to accommodate A105sites in a regular crystalline lattice whereas an amorphous structure is much more able to
J. Phys. Chem., Vol. 99, No. 41, 1995 15141
Quantification of A1 Coordinations in Amorphous Aluminas accommodate such a coordination and can result in the five oxygens being quite symmetrically arranged about the aluminum. The isotropic chemical shifts for the three sites are typical for the given coordinations. The relative intensities deduced from the centerbands of AlO6, AlO5, and A104 are 70, 7.5, and 22.5%, respectively. These intensities are those deduced directly from the centerband simulation which strictly should be corrected for the fraction of the total central transition lying in the centerband which depends on VQ~/V,V,, where vo is the Larmor frequency and vr the rotation rate.5 For the three sites in this sample this parameter has values of 0.25, 0.09, and 0.31 which mean that at least 97% of the (I/*, - I h ) transition appears in the centerband for each site. Consequently, correction and renormalization make less than 1% difference to the intensities observed directly, a much smaller contribution than other errors. A question that should be addressed is, although the quantitative analysis of the detected signal is accurate, to what extent are sites severely quadrupole broadened so that they are not recorded in the signal. The conditions used here are similar to those applied in a former centerband study of andalusite6 where VQ’S of up to 4.65 MHz were detected quantitatively. Based on this observation, the much smaller mean VQ and the width of the distribution determined here it would seem physically reasonable that the fraction of sites that will not contribute to the intensity is very small. Hence it is believed that the spectra are accurately quantitative in an absolute sense. It is interesting to compare the interactions and intensities deduced for the aluminum sites here with other aluminumcontaining compounds. Crystalline aluminas and related hydrates typically have VQ’ s for octahedrally coordinated sites in the range 300-500 kHz, indicating that the A106 sites are more distorted here. VQ deduced for A105 here is below the values quoted from multiple-field peak position analysis of aluminas9 and aluminosilicate g1a~ses.l~ However, given the large degree of overlap of the resonances in such materials peak, position analysis, particularly at lower magnetic fields, is difficult. A previous study that looked at the intensity distribution between different sites simply divided the centerband into different regions by dropping perpendiculars. For ~-A1203the AlO6: A105:A104 ratio was 73%:7%:20%, in remarkable agreement with Table 1. This intensity distribution clearly distinguishes this phase from other intermediate amorphous aluminas such as Q where the corresponding ratio is 55%:20%:25%. The distribution derived here for aluminum among the different coordinations is the most accurate to date for an amorphous alumina. Characterization by X-ray diffraction shows this phase to be amorphous and the ability to distinguish differences with variations in calcination conditions are very difficult by XRD. However, the atomic scale approach offered
by NMR based on accurate quantitative determination of the different coordinations is highly promising for examining subtle structural changes with calcination conditions in amorphous intermediate aluminas. This approach should be widely applicable for characterization of aluminum in aluminosilicate glasses, gel, and mineral calcines.
Conclusions Satellite transitions of an 27AlMAS NMR spectrum have been recorded from an amorphous partially dehydrated transitional alumina and simulated to deduce the NMR interaction parameters and their distribution for the A104, A105 and A106 sites. Simulation of centerbands based on these parameters allows the quantitative distribution of aluminum between these sites to be deduced for x-Al2O3. This provides a novel, accurate characterization method for amorphous aluminas that should allow the effect of calcination conditions on alumina hydrates to be better understood and even offer the possibility of better process control.
Acknowledgment. M.E.S. and C.J. thank the British Council and DAAD for funding the collaboration between Kent and Mainz under the ARC program. C.J. and G.K.F. also thank the Deutsche Forschungsgemeinschaft for financial support and C.J. thanks Prof H. W. Spiess, in whose laboratory some of this work was performed, for his support. References and Notes (1) Andrew, E. R. Int. Rev. Phys. Chem. 1981, 1 , 195. (2) Mastikhin, V. M.; Krivoruchko, 0.P.; Zolotovskii, B. P.; Buyanov, R. P. React. Kinet. Catal. Lett. 1981, I , 117. (3) John, C. S.; Alma, N. C. M.; Hays, G. R. Appl. Catal. 1983, 6, 341. (4) Smith, M. E. Appl. Magn. Reson. 1993, 4, 1. ( 5 ) Massiot, D.; Bessada, C.; Coutures, J. P.; Taulelle, F. J. Magn. Reson. 1990, 90, 231. (6) Alemany, L. B.; Massiot, D.; Shemff, B. L.; Smith, M. E.; Taulelle, F. Chem. Phys. Lett. 1991, 177, 301. (7) Kohn, S. C.; Dupree, R.; Smith, M. E. Geochim. Cosmochim.Acta 1989, 53, 2925. ( 8 ) Kunath, G.; Losso, P.; Schneider, H.; Steuemagel, S.; Jager, C. Solid State NMR 1992, 1, 261. (9) Meinhold, R. H.; Slade, R. C. T.; Newman, R. H. Appl. Magn. Reson. 1993, 4, 121. (10) Wefers, K.; Misra, C. Alcoa Tech. Paper, No. 19, 1987, Alcoa Labs. (11) Bastow, T. J.; Hall, J. S.; Smith, M. E.; Steuemagel, S. Mater. Lett. 1994, 18, 197. (12) Samoson, A.; Lippmaa, E.; Pines, A. Mol. Phys. 1988, 65, 1013. (13) Jager, C. In NMR Basic Principles and Progress; Blumich, B., Kosfeld, R., Eds.; Springer-Verlag: Berlin, 1994; Vol. 31, p 134. (14) Skibsted, J.; Nielsen, N. C.; Bildst~e,H.; Jakobsen, H. J. J . Magn. Reson. 1991, 95, 88. (15) Sato, R. K.; McMillan, P. F.; Dennison, P.; Dupree, R. J. Phys. Chem. 1991, 95, 4483.
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