Thermal evolution of transitional aluminas followed by NMR and IR spectroscopies

Article abstract of DOI:10.1021/jp983316q

Thermal decomposition of boehmite and bayerite has been studied by X-ray diffraction and by NMR and IR spectroscopies. Coordination and site distortion of Al polyhedra have been estimated by ²⁷Al NMR spectroscopy while IR optical properties of transitional aluminas, η-, γ, δ-, and θ-Al₂O₃ were derived from IR near-normal specular reflectance technique. In the low-temperature phases, aluminum vacancies are located in tetrahedral positions in γ-Al₂O₃ while in η-Al₂O₃ they are distributed at random between tetrahedral and octahedral positions. In addition, a small amount (5%) of pentahedrally coordinated aluminum was found in γ-alumina, which has been tentatively assigned to Al at the external surface of alumina particles. At 800°C a notable increase in structural ordering was detected in both phases; in the case of η-A_l2O₃, this change accelerates the continuous transformation into θ-Al₂O₃ which is totally achieved at 1000°C. On the contrary, γ-Al₂O₃, does not show any sign of transformation until 900°C; at 950deg;C pentahedral cations disappear and formation of θ-Al₂O₃ is detected by IR and NMR techniques. From 900 to 1200°C, γ-, θ-, and α-Al₂O₃ coexist in samples obtained from boehmite. Above 1200°C corundum is the only thermodynamically stable phase in both series of samples.

Full text of DOI:10.1021/jp983316q

6160
J. Phys. Chem. B 1999, 103, 6160-6170
ARTICLES
Thermal Evolution of Transitional Aluminas Followed by NMR and IR Spectroscopies
C. Pecharroma´n,* I. Sobrados, J. E. Iglesias, T. Gonza´lez-Carren˜o, and J. Sanz
Instituto de Ciencia de Materiales, Consejo Superior de InVestigaciones Cientificas,
Cantoblanco 28049, Madrid, Spain
ReceiVed: August 7, 1998; In Final Form: May 10, 1999
Thermal decomposition of boehmite and bayerite has been studied by X-ray diffraction and by NMR and IR
spectroscopies. Coordination and site distortion of Al polyhedra have been estimated by 27Al NMR spectroscopy
while IR optical properties of transitional aluminas, η-, γ-, δ-, and θ-Al2O3 were derived from IR near-
normal specular reflectance technique. In the low-temperature phases, aluminum vacancies are located in
tetrahedral positions in γ-Al2O3 while in η-Al2O3 they are distributed at random between tetrahedral and
octahedral positions. In addition, a small amount (5%) of pentahedrally coordinated aluminum was found in
γ-alumina, which has been tentatively assigned to Al at the external surface of alumina particles. At 800 °C
a notable increase in structural ordering was detected in both phases; in the case of η-Al2O3, this change
accelerates the continuous transformation into θ-Al2O3 which is totally achieved at 1000 °C. On the contrary,
γ-Al2O3, does not show any sign of transformation until 900 °C; at 950 °C pentahedral cations disappear and
formation of θ-Al2O3 is detected by IR and NMR techniques. From 900 to 1200 °C, γ-, θ-, and R-Al2O3
coexist in samples obtained from boehmite. Above 1200 °C corundum is the only thermodynamically stable
phase in both series of samples.
Introduction
different.4 At present, it is admitted that cation ordering increases
with the temperature of treatment, but the structural features of
transitional aluminas are not yet well-known. In the case of
θ-Al2O3, the highest term of the bayerite and boehmite series,
the oxygen network makes up a distorted fcc lattice complex,
but the octahedral and tetrahedral aluminum distribution violates
the 4-fold symmetry rotation axis of the spinel space group.4-6
In this monoclinic structure the relative amounts of tetrahedral
and octahedral aluminum are 50-50%. The formation temper-
ature of this phase depends on the precursor and it can be very
close to that of corundum. The transformation from transitional
aluminas to R-Al2O3, with a hexagonal oxygen sublattice in
which all Al are octahedrally coordinated,8 occurs through a
nucleation and growth process.10
Corundum R-Al2O3 is the ultimate product of hydroxides,
oxyhydroxides, and oxides of aluminum heated at high tem-
peratures. In addition to R-Al2O3, several materials, known as
transition aluminas, can be metastably obtained in the laboratory
and industry during dehydration of tryhydrates Al(OH)3 (bayerite
and gibbsite) and monohydrates AlOOH (boehmite and di-
aspore). The nature of a transitional alumina strongly depends
on its precursor; thus, thermal decomposition of bayerite gives
the sequence η f θ f R, while that of boehmite gives γ f δ
f θ f R. An extensive review about all different aluminum
oxides, hydroxides and oxyhydroxides, and their transformations
can be found in ref 1.
Dehydroxylation reactions preserve the skeleton of the parent
material2 while OH- groups are eliminated. If the oxygen
network of the precursor is close to fcc (boehmite and bayerite),
heating treatments produce spinel-like materials.2-7 γ- and
η-Al2O3 are always obtained with small crystallite size and their
X-ray powder diffraction patterns, showing broad reflections,
correspond to cation-defective spinels (Megaw,8 Greenwood,9
Zhou and Snyder4). The ideal structure of these phases belongs
to space group Fd3hm, but their real structure seems to be
tetragonally distorted.1,2,10 Distortions are different in both
phases and they have been associated with differences in cation
ordering. From 27Al MAS-NMR data,3 cation vacancies have
been reported to be restricted to tetrahedral sites in γ-Al2O3
and to octahedral sites in η-Al2O3; however, after Rietveld
analyses of neutron diffractograms, cation distribution was
The main objective of this article is the determination of
structural properties of transition aluminas. 27Al MAS-NMR and
reflectance IR spectroscopies as well as X-ray diffraction have
been used to monitor structural transformations produced during
the thermal heating of bohemite and bayerite. From MAS-NMR
data, evolution of the coordination of aluminum and distortions
of polyhedra have been analyzed. IR specular reflectance
spectroscopy has been used to determine vibrational modes
(frequencies, intensities, and damping constants) which are
strongly dependent on the crystal structure of alumina phases.
11
In previous works, the IR optical properties of η-Al2O3 and
12
the low-resolution NMR spectra of γ-Al2O3 have been
reported. In this article, a more accurate and systematic study
of dehydroxylation and further thermal transformations of
bayerite and boehmite has been undertaken by simultaneous
analysis of data from both spectroscopies.
* To whom all correspondence should be addressed. E-mail: cpg@
icmm.csic.es.
10.1021/jp983316q CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/13/1999

Thermal Evolution of Transitional Aluminas
J. Phys. Chem. B, Vol. 103, No. 30, 1999 6161
calculated15,16 as
Sample Synthesis Procedure
Two different kinds of aluminas have been studied at different
stages of thermal treatment. The first series of data was obtained
by thermal treatment of separate bayerite (Condea) samples,
heated for 2 h at 300, 500, 700, 800, 900, and 1000 °C. In the
second case, a γ-Al2O3 sample (Girdler) with a specific surface
of 208 m2/g, obtained from dehydroxylation of boehmite at 600
°C was used. This sample was accumulatively calcined at
increasing temperatures for periods of 2 h up to 1200 °C. Several
amounts of the original γ-Al2O3 sample were fired at 1200 °C
for short periods of time (several minutes) to increase the
proportion of θ phase.10 After each treatment, the samples were
checked by XRD to follow structural changes.
2
ꢀ ) (1 - f)ꢀm
+
{
[
(1 - Lm) ꢀ + Lmꢀm
2ꢀp
1
+ f
+
]
[
2Lm ꢀ + (1 - 2Lm)ꢀm
(1 - Lp) ꢀ + Lpꢀp
ꢀp
2
(1 - f)
+
} {
]
[
2Lp ꢀ + (1 - 2Lp)ꢀp
/
(1 - Lm) ꢀ + Lmꢀm
1
+
]
2Lm ꢀ + (1 - 2Lm)ꢀm
2
1
f
+
(3)
}
]
[
(1 - Lp) ꢀ + Lpꢀp 2Lp ꢀ + (1 - 2Lp)ꢀp
Experimental and Data Treatment
X-ray Diffraction. X-Ray diffraction powder patterns of
samples were recorded with a PW1050/25 Philips diffractometer
fitted with an automatic divergence slit, a diffracted beam curved
graphite monochromator (Cu KR radiation), a 0.07° receiving
slit and a proportional detector. Data were collected in the 2θ
range 10-70°, with a scanning rate of 2.4°/min.
IR Spectroscopy. For IR reflectance analyses, powders were
compacted in an uniaxial press under a nominal pressure of 1
GPa. A very smooth die, made in zirconia partially stabilized
with yttria, was used to obtain self-supporting powder pellets
with a good quality specular surface. The IR spectra were taken
in a Nicolet 20 SXC FTIR spectrophotometer, from 4000 to
220 cm-1, sampling every 0.48 cm-1, in a near-normal specular
reflectance experimental setup.
IR reflectance spectra of the powder aggregates can be used
to determine the IR complex dielectric constant. In this respect,
reflectance measurements have advantages in comparison with
more common absorbance experiments: they are quantitative,
they do not depend on the preparation method, and they weight
in the same way low- and high-frequency data. In most of the
previously published IR absorbance spectra, low-frequency
bands (from 200 to 500 cm-1) are either missing or else they
display a very poor intensity. In addition, by using an effective
medium approximation, it is possible to accurately reproduce
the experimental reflectance of the powder pellet by calculation,
and hence deduce its intrinsic IR optical parameters.
To solve this equation in ꢀ , it is necessary to know f, the
filling factor, Lp, and Lm, the depolarization factors of particles
and matrix; and ꢀp, the dielectric constant of the particles. The
filling factor can be directly measured by weighing the pellet.
The geometric factors Lp and Lm,17 are related to the particle
shape and the percolation fc threshold.15 In our case, the particles
have irregular shape but they can approximately be taken as
spheres. In the case of spheres, the value of depolarization factor
is Lp
)
1/3. Finally, for ꢀp, a superposition of harmonic
oscillators11,18-20 is assumed.
ωLk2 - ω2 - iγLkω
N
ꢀ ) ꢀ
(4)
∏
p
∞
ωTk2 - ω2 - iγTkω
k)1
where k runs over the vibrational modes, ꢀ∞ is the dielectric
constant at optical frequencies, ωTk and ωLk are the transverse
and longitudinal frequencies, and γTk and γLk are the decay ratio
or damping constant frequencies of the kth mode. In the
dielectric constant function, poles and zeros correspond to
transverse and longitudinal frequencies so high reflectance bands
extend from ωTk to ωLk and the intensity mainly depends on
γTk and γLk. The previous considerations apply to perfect
crystals. In most of our samples the crystallinity is very poor
so that the selection rules relax and the maxima of dielectric
function broaden. Even in these conditions the dielectric constant
can be fitted by eq 4. However, the damping parameters γTk
and γLk do not only represent the decay ratio but also the
broadening of the imaginary part of the dielectric function due
to the lack of crystallinity. In this paper we have fitted these
parameters from experimental spectra and by analogy we have
called them damping parameters. This is an usual convention
among researchers dealing with optical properties of powdered
materials,21-23 although it is clear that this use does not properly
conform to the physical concept of damping.
The normal specular reflectance is related to the dielectric
constant through one of the Fresnel equations:
(
(
ꢀ - 1)
ꢀ + 1)
x
2
R )
(1)
|
|
x
where ꢀ is the average dielectric constant of the aggregate,
defined as
Using eqs 1, 3, and 4 it is possible to calculate the reflectance
as a function of the parameters f, fc, ꢀ∞, ωTk, γTk, ωLk, and γLk.
Only the filling factor is experimentally fixed, while the rest
are adjusted by a nonlinear least-squares refinement. The starting
value for ꢀ∞ is obtained from the reflectance in the IR
transparency region (4000-1200 cm-1), and the percolation
threshold is assumed to be fc = 0.9f. Initial values of the
remaining parameters are judiciously chosen. The goodness of
the fit is judged from the estimator.
D(br) dV
∫
V
ꢀ )
)
ꢀ
E(br) dV
V
∫
0
ꢀ (br)E (br) dV + V ꢀp(br)Ep(br) dV
∫
∫
Vm
m
m
p
(2)
E (br) dV + V Ep(br) dV
∫
∫
Vm
m
p
We suppose that this aggregate is composed by a mixture of
isotropic spheroidal powder (“p” subscript) and spheroidal
matrix (“m” subscript) particles. In this case the matrix is air.
Under these approximations and introducing an “effective
medium” hypothesis,13,14 the average dielectric constant can be
2
1/2
(R - R
)
exp
∑
th
R )
(5)
2
exp
[
]
R
∑

6162 J. Phys. Chem. B, Vol. 103, No. 30, 1999
Pecharroman et al.
Figure 1. XRD powder patterns obtained from thermal evolution of
(a, top) transitional aluminas obtained from bayerite samples (R-Al-
(OH)3) taken for the same initial batch, heated for 2 h at the specified
temperatures (initial bayerite diffractogram not shown); (b, bottom)
transitional aluminas obtained from an original sample of γ-Al2O3
prepared from bohemite (γ-AlOOH) after dehydroxylation at 600 °C
and subjected to accumulative thermal treatments of 2 hours at the
indicated temperatures. The pattern labeled as “T ) 1200 °C (6′)”
corresponds to the original sample heated for only 6 min at 1200 °C.
Figure 2. (a, top) Experimental IR reflectance spectra (s) and fitted
curve (O) of aluminas obtained from thermal decomposition of bayerite
(samples as described under Figure 1a). Horizontal lines including the
abscissa axis are the zero-reflectance lines of each spectrum, which
are all drawn to the same indicated scale. (b, bottom) Logarithm of the
imaginary part of the dielectric constant deduced from fitting to
experimental reflectance. Horizontal lines are the zero lines for the
logarithm of the imaginary part of the relative dielectric constant for
each curve.
NMR Spectroscopy. 27Al MAS-NMR spectra were recorded
at 104.26 MHz, with a Bruker MSL-400 spectrometer. The
external magnetic field was 9.4 T. All measurements were
carried out at 295 K, and the samples were spun around an axis

Thermal Evolution of Transitional Aluminas
J. Phys. Chem. B, Vol. 103, No. 30, 1999 6163
TABLE 1: Vibrational Parameters Obtained from Least-Squares Fit on Bayerite Series Reflectance Data
T (°C)
300
ωT
γT
ωL
γT
T (°C)
900
ω
γ
ωL
γL
61.8
64.6
35.4
45.2
74.6
68.4
51.9
81.5
88.8
31.3
53.3
R ) 2.6%
247.2
281.0
316.7
356.3
441.6
505.5
564.9
604.6
681.5
766.0
774.4
76.1
53.1
20.4
23.3
64.1
56.6
60.2
108
126
35.3
258.2
299.9
340.2
375.2
453.3
549.1
590.0
652.9
743.2
770.4
945.0
310.0
458.0
592.8
737.5
786.8
245
103
111
81.6
308
438.9
654.2
550.8
745.5
937.6
112
148
370
85.2
87.1
R ) 2.5%
ꢀ∞ ) 4.13
500
351.9
482.3
594.7
736.8
801.9
221
109
82.4
79.2
292
434.0
670.1
598.7
740.1
942.5
132
269
90.3
82.1
82.5
R ) 2.9%
89.2
ꢀ∞ ) 3.44
1000
ꢀ∞ ) 3.15
700
256.2
282.4
317.9
355.5
441.4
507.8
566.1
603.6
684.0
767.0
785.4
75.6
57.0
21.7
22.7
54.3
41.0
56.8
104
115
31.6
80.7
258.9
297.0
343.0
379.4
451.5
546.2
589.8
652.5
743.8
772.3
945.7
59.8
54.8
30.8
34.4
63.0
69.3
45.4
70.1
93.0
30.6
36.6
R ) 2.6%
322.5
363.9
487.2
753.7
791.9
150
42.0
122
78.9
363.7
403.0
655.1
756.9
947.4
49.4
126
254
74.9
50.1
R ) 2.2%
300
ꢀ∞ ) 3.36
800
313.5
357.4
496.7
768.2
778.2
117.3
35.4
111.8
72.7
264
356.5
386.9
640.2
770.6
946.4
45.2
123
209
ꢀ∞ ) 3.44
62.3
48.4
ꢀ∞ ) 3.27
R ) 3.3%
inclined 54° 44′ with respect to the magnetic field, at a spinning
rate of 12 kHz (MAS technique). NMR spectra were obtained
after a π/8 excitation (2 µs) and intervals between successive
accumulations of 5 s. Chemical shift values are given relative
to an 1 M AlCl3 aqueous solution. In all cases the mean error
on position of the components was lower than 1 ppm. The
sampling of the NMR signal was done with dwell times of 0.2
and 4 µs in order to analyze the whole spectrum (2.5 MHz)
and the central transition (125 kHz). Special attention was given
to properly choosing the origin of the FID signal (free induction
decay) and to remove the points associated with the radiofre-
quency pulse, both necessary for a good determination of the
baseline and phasing of NMR components. The baselines of
the transformed spectra were finally corrected with a sin(kx)/kx
expression to compensate for the lack of information in the
neighborhood of the excitation pulse.
The analysis of 27Al MAS-NMR spectra was done with the
WINFIT24 program. In this program, intensities of side bands
are calculated with Herzfeld and Berger’s method,25 but
quadrupolar CQ and η constants must be determined by a trial
and error procedure. In all cases, 27Al MAS-NMR spectra were
fitted by considering only first-order quadrupolar interactions,
but in some favorable cases, quadrupolar constants were also
estimated from the central transition shape (1/2, -1/2), by
considering that second-order quadrupolar interactions were
significant. In more disordered samples, a dispersion of electric
field gradients produces a strong asymmetric broadening of the
central (1/2, -1/2) transition. This asymmetric broadening was
described by introducing two symmetric I1 and I2 components.
From the fitted intensities of the two bands, a parameter of
heterogeneity h ) I2/(I1 + I2), was defined, where I1 corresponds
to the band centered at the maximum and I2 to that which takes
into account the upfield asymmetry. This parameter must be
only considered as a qualitative guide due to the presence of
second-order quadrupolar effects that have not been considered
in this analysis.
Experimental Results
X-ray Diffraction. X-ray diffractograms have been used to
identify the samples, following criteria established by Lippens
and De Boer.2 Diffraction patterns of samples obtained by
heating bayerite are given in Figure 1a. In the diffractogram
corresponding to 300 °C, narrow peaks of residual bayerite
coexist with broad maxima of newly formed η-Al2O3. At 500
°C bayerite is eliminated altogether and the X-ray pattern
corresponds to that of η-Al2O3 (see refs 1 and 2). Heating up
to 800 °C does not substantially modify the diffractogram.
However, at 900 °C, diffraction peaks narrow down, and a
pattern similar to that of δ-Al2O3 previously reported6 is
obtained. At 1000 °C, the X-ray diffraction pattern corresponds
to that of the θ-Al2O3 phase.5,7 Finally, at 1100 °C corundum
is formed (not shown).
The X-ray diffraction pattern of γ-Al2O3 is obtained by
thermal treatment of boehmite during 2 h at 600 °C (Figure
1b). From 600 to 800 °C the XRD patterns do not change much,
but above this temperature, δ-Al2O3 is clearly detected. Above
950 °C, variable proportions of θ- and R-phases in addition to
δ-alumina were identified in diffractograms. From this temper-
ature up to 1200 °C, prolonged thermal treatments increase the
amount of R-Al2O3 at the expense of that of δ- and θ-Al2O3
phases. However, a short calcination of the γ-Al2O3 starting
material (1200 °C during 6 min) gives a XRD pattern with a
higher proportion of θ-Al2O3 (Figure 1b).
IR Reflectance Measurements. In order to keep a consistent
notation for all samples, we have assumed that the vibrational
spectra of transitional aluminas are assimilable to those of
spinel26 (space group Fd3hm). Under this assumption, the four
detected modes will be identified as modes I to IV of the spinel
structure. According to ab initio calculations of Ishii et al.,27
these modes, in order of increasing frequency, correspond to I,
stretching of octahedral against tetrahedral aluminum; II,
bending of oxygen against aluminum ions; III, stretching of

6164 J. Phys. Chem. B, Vol. 103, No. 30, 1999
Pecharroman et al.
anions against octahedral cations; and IV, stretching of anions
against octahedral and tetrahedral cations.
The spectrum of the bayerite sample heated at 300 °C
corresponds to a mixture of η-Al2O311 with some remaining
amount of bayerite, in good agreement with XRD observations.
The IR reflectance spectrum presents the four expected bands
(Figure 2a and Table 1) plus one additional band around 600
cm-1, which is probably due to the presence of traces of bayerite.
Small peaks due to the presence of OH- groups were also found
(see for example, the small feature around 1100 cm-1). The
imaginary part of the dielectric constant was fitted with 5
modes: modes I, III, and IV are located at 310, 737, and 787
cm-1 and mode II is split into two peaks at 458 and 593 cm-1
(Figure 2b). At 500 °C, the high-frequency peak of this doublet
remains unchanged, while the low-frequency one, which is now
clearly visible, shifts 24 cm-1 to higher frequencies, and mode
I shifts up to 352 cm-1. Hydroxyl groups are eliminated at 700
°C, and the mode around 600 cm-1 also vanishes. Above 700
°C, all bands narrow down and new small peaks are visible; in
particular, a new band around 322 cm-1 is detected. These
observations can be assigned (see the discussion below) to the
onset of θ-Al2O3 formation. The new peaks gradually become
better resolved up to 1000 °C, where the only phase detected
by XRD is θ-Al2O3. For this phase the spectrum was fitted with
a model of 11 oscillators (Table 1).
Spectra of transition aluminas obtained by calcination of
boehmite (Figure 3 and Table 2) are similar to those obtained
from bayerite. The resulting imaginary part of the dielectric
constant deduced from reflectance IR data, plotted in Figure
3b, was adjusted with four modes at 371, 465, 776, and 819
cm-1 at 600 °C. At 700 °C, stretching bands corresponding to
OH- groups are eliminated (not shown). In samples heated at
800 °C the frequencies of modes I to III shift to lower values
and, with the exception of mode II, the damping values decrease.
From 900 to 950 °C, spinel modes start to split as a consequence
of symmetry lowering. Upon further heating, peaks of the
θ-Al2O3 phase are detected, and they are best resolved in the
sample heated at 1200 °C for 6 min. In this sample as well as
in that heated at 1000 °C, reflectance spectra were reproduced
by using a 12-mode dielectric constant model, at least one (637
cm-1) being a resolved mode of R-alumina. Finally, the
spectrum of the sample heated at 1200 °C, 2 h shows the
formation of R-Al2O3.
NMR Measurements. The 27Al MAS-NMR spectra of the
starting aluminas is formed by two broad asymmetric compo-
nents placed around 70 and 0 ppm associated with tetra- and
octahedral aluminum and their corresponding side bands. Central
bands are asymmetrically broadened with respect to sidebands,
suggesting that most of the Al atoms participate to the central
lines; however, only part of them are detected in sidebands.
The relative amounts of tetrahedral and octahedral Al were
determined from the centerbands once subtracted the sideband
intensities of the (5/2, 3/2) and (3/2, 1/2) transitions.28
The 27Al MAS-NMR spectrum of η-Al2O3 phase, obtained
by calcination of bayerite at 500 °C is formed by two
components at 65 and 55 ppm (Figure 4a). Relative amounts
of tetra- and octahedrally coordinated aluminum are respectively
28 and 72% (See Table 3). In samples heated at increasing
temperatures, the octahedral component becomes narrower and
the tetrahedral one broadens, increasing its integrated intensity.
The full spectra of bayerite samples heated at different tem-
peratures are given in Figure 4b. It is observed that the sideband
pattern of octahedral aluminum spreads over a large spectral
region and that the extent of its occupied region decreases with
Figure 3. (a) (a, top) Experimental IR reflectance spectra (s) and
fitted curve (O) of aluminas obtained from thermal decomposition of
boehmite (samples as described under Figure 1a). Horizontal lines
including the abscissa axis are the zero-reflectance lines of each
spectrum, which are all drawn to the same indicated scale. (b, bottom)
Logarithm of the imaginary part of the dielectric constant deduced from
fitting to experimental reflectance. Horizontal lines are the zero lines
for the logarithm of the imaginary part of the relative dielectric constant
for each curve.
increasing temperature of treatment. On the contrary, the region
covered by sidebands of tetrahedral aluminum slightly increases

Thermal Evolution of Transitional Aluminas
J. Phys. Chem. B, Vol. 103, No. 30, 1999 6165
TABLE 2: Vibrational Parameters Obtained from Least-Squares Fit on Boehmite Series Reflectance Data
T (°C)
600
ω
γ
ωL
γT
T (°C)
ω
γ
ωL
γL
59.0
371.1
464.9
776.2
819.1
265
137
151
231
425.2
712.2
784.9
940.9
127
296
134
1200 (6′)
260.2
290.3
321.1
357.6
444.3
510.5
555.4
627.5
678.90
724.7
757.3
802.9
65.8
55.4
27.6
27.6
68.2
54.6
63.7
58.6
87.2
16.4
36.6
88.5
266.2
303.5
341.5
376.7
472.8
538.4
619.0
659.5
737.9
724.1
766.7
945.0
50.5
36.1
55.0
73.4
72.7
64.9
84.2
113.6
21.5
49.4
52.7
77.5
R ) 1.2%
ꢀ∞ ) 3.39
800
331.2
334.3
497.6
762.6
804.6
62
121
126
119
262
332.3
404.4
663.5
768.2
948.7
47.4
153
308
100
67.6
ꢀ∞ ) 3.3
900
R ) 2.6%
ꢀ∞ ) 3.34
1000
R ) 1.4%
329.6
354.1
447.6
499.6
525.3
624.9
665.4
686.0
721.9
737.7
812.3
96.2
25.0
117
100
57.7
69.1
38.8
48.3
31.6
72.9
349.6
376.0
462.4
610.2
525.7
656.2
674.1
710.0
724.1
778.3
948.2
28.3
60.3
82.1
79.5
49.3
57.6
40.8
61.6
34.5
96.6
73.5
R ) 3.0%
330.8
337.9
426.6
492.8
505.7
536.9
636.7
656.4
689.0
723.2
740.7
811.8
118
341.4
384.9
472.7
496.0
535.6
640.3
642.1
683.9
711.7
730.4
789.4
934.7
21.4
51.9
41.5
32.2
29.2
42.9
36.7
43.2
56.3
35.8
104
34.1
64.0
33.2
74.9
29.6
23.0
43.6
38.4
33.6
58.8
95.0
115
ꢀ∞ ) 3.4
57.0
R ) 5.0%
ꢀ∞ ) 3.45
950
331.5
338.2
439. 6
502.1
525.4
618.8
667.0
686.1
724.2
739.0
797.1
63.3
55.9
104.6
60.8
60.9
64.5
37.5
43.6
38.4
67.3
102
339.4
384.0
476.0
605.7
525.7
656.5
676.0
704.5
729.6
778.3
946.1
21.7
51.5
74.6
66.2
49.1
55.0
36.2
53.7
35.2
75.9
47.5
ꢀ∞ ) 3.5
R ) 3.6%
with treatment temperature. From a fitting of sideband patterns,
CQ and η values were deduced and from these values, the
isotropic chemical shifts were calculated (Table 3). It can be
seen that the quadrupolar constant CQ of tetrahedral aluminum
slightly increases and that of octahedral aluminum appreciably
decreases with increasing temperature. When θ-Al2O3 is formed,
the intensities of tetrahedral and octahedral Al are approximately
the same (47-53% at 1000 °C, see Table 3), and the quadru-
polar constant of tetrahedral Al (5.5 MHz ) is clearly higher
than that of the octahedral one (3.1 MHz) (Figure 4b). At the
same time, asymmetry η values of tetrahedral and octahedral
aluminum are 0.8 and 0.4. In the case of the tetrahedral
component of θ-Al2O3 phase, the shoulder at 65 ppm suggests
the presence of second-order quadrupolar effects. The quadru-
polar constants CQ determined by fitting the centerbands is only
a 10% higher than that deduced from fitting the sidebands
pattern.
MAS-NMR spectrum of the sample heated at 1000 °C, the
amount of octahedral aluminum increases as a consequence of
R-Al2O3 formation. From analysis of sidebands patterns (Table
3), it was concluded that CQ of tetrahedral aluminum does not
change very much above 900 °C, but that of the octahedral Al
decreases clearly with temperature. In the spectrum of the
sample heated at 1000 °C, the octahedral central line shifts
toward more positive values and maxima of the R-Al2O3 phase
are detected in the sideband pattern (Figure 5b, labeled with
asterisks).
Discussion
In order to get information about the thermal evolution of
transition aluminas, NMR, IR, and XRD techniques have been
brought to bear. Information deduced from NMR spectra
concerns aluminum coordination, but that obtained from IR
spectroscopy refers to the atomic arrangement at the scale of a
few unit cells. In the case of X-ray diffraction technique, a longer
range order is required, and structural information refers to
regions of several hundred angstro¨ms.
The thermal evolution of transition aluminas has two different
stages, one extending up to 800 °C, and the second one from
this point to 1000 °C. Above this temperature, transitional
aluminas irreversibly transform into corundum in a broad
temperature range from 1000 to 1200 °C.
The 27Al MAS-NMR spectra of γ-Al2O3 heated at increasing
temperatures are given in Figure 5a. The spectrum at 600 °C is
formed by two broad asymmetric components at 65 and 10 ppm,
associated with Al in tetrahedral and octahedral coordination,
and a small component at 30 ppm that is due to pentahedral Al
(Figure 5a). The relative intensities of tetrahedral and octahedral
components are 21 and 76%. When the sample is heated
between 800 and 900 °C, the octahedral component becomes
narrower and the tetrahedral one slightly broader. Above 950
°C, pentahedral aluminum is totally eliminated and the relative
amount of tetrahedral aluminum increases. Finally, in the 27Al
Dehydroxylation of bayerite and boehmite proceeds in a
similar way, only the temperatures are slightly different. Bayerite

6166 J. Phys. Chem. B, Vol. 103, No. 30, 1999
Pecharroman et al.
bohemite heated at 600 °C (γ-Al2O3) also shows a small amount
of OH- groups and a small shoulder around 600 cm-1, both
precursor features that are also eliminated at 700 °C.
By separate study of the parameter γT deduced from fitting
a model of oscillators to the reflectance spectra, it is possible
to obtain information about the local order of the atoms which
participate in each mode. Thus, the so-called “damping constant”
of mode I mainly depends on short-range cationic order, while
modes II, III, and IV also depend on the order of the oxygen
sublattice. In addition, the damping of mode III only refers to
octahedral disorder. The evolution of the damping constant with
temperature is then a good order parameter. On the other hand,
the heating process induces symmetry lowering and, as a
consequence, the number of IR modes increases. High-temper-
ature phases, δ- or θ-Al2O3, are structurally related with low
temperature ones, η- and γ-Al2O3. Then the damping constants
of all modes included in each [ωTk, ωLk] interval, corresponding
to a particular transverse-longitudinal splitting in the spinel
structure, can be averaged because of their weak frequency
dependence.18,20 Thus, the averaged γTk damping values of
the components inside each spectral regions should only depend
on the crystal order of the atoms involved in those vibrations.
These values have been plotted in Figure 6 as a function of
temperature. This parameter is very sensitive to structural order
in the range 20-100 Å; in the case of large crystals, the values
of γT are approximately 10 cm-1 (data for corundum can be
found in refs 20 and 29), but in amorphous or disordered
materials they can be one order of magnitude higher. The same
analysis could be carried out on γL; however, this parameter
strongly correlates with neighbor modes in the least square
reflectance fit, and hence, has a larger error.
The analysis of damping values for both series of samples
shows that those of modes II to IV remain nearly unchanged
between 500 and 800 °C, but that of mode I suffers a continuous
reduction with temperature. We think that this change corre-
sponds to a cationic rearrangement inside the rigid skeleton of
the fcc oxygen framework. This process occurs at lower
temperatures in the bayerite than in the boehmite series.
In the ideal spinel structure the octahedral cations are twice
as many as the tetrahedral ones, which gives relative proportions
of 67-33% of octahedral and tetrahedral cations in this
structure. In the defective spinel Al2O3, there is 1 Al missing
for every 9 cations according to the formula Al80O12. If
aluminum is preferentially located on octahedral sites as in a
defect-free spinel, 75% (6/8) of the aluminum will be octahedral,
while the remainder 25% will occupy tetrahedral positions,
leaving 1/3 of the spinel tetrahedral sites vacant. On the contrary,
if aluminum fully occupies the tetrahedral positions of the ideal
spinel, octahedral and tetrahedral aluminum will amount to 62.5
and 37.5% of the total. Usually, literature refers to these two
cases as defective spinel structures with tetrahedral or octahedral
vacancies. In the case of η-Al2O3 at 300 °C, 78% of aluminum
cations are in octahedral coordination while at 700 °C, once
the dehydroxylation process has finished, only 69% of them
remain in octahedral positions (Table 3 and Figure 7). These
values indicate a mixed character between the above two spinel
types, so that it appears that cationic defects could be equally
distributed between octahedral and tetrahedral positions.1,3 On
the contrary, the NMR spectrum of γ-Al2O3 exhibits three peaks
(Figure 5), which correspond to octahedral, pentahedral, and
tetrahedral coordination of aluminum. At 600 °C, the occupation
of the different sites are 76, 3, and 21%, respectively (Table 3
and Figure 7). These values agree with a model of preferential
location of vacancies at tetrahedral sites. In this sample there
exists a significant proportion of pentahedral aluminum30,31 (3-
Figure 4. Evolution of (a, top) the central -1/2,1/2 transition and (b,
bottom) the whole 27Al MAS-NMR spectra of aluminas obtained from
bayerite. Spectra were recorded at 12 kHz (samples as described under
Figure 1a). In the case of the spectrum of the sample heated at 300 °C
in (a), the symmetric components used on the fitting have been plotted.
heated at 500 °C is almost completely transformed into η-Al2O3
but total elimination of OH- groups (they can be detected by a
small band around 1100 cm-1) and bayerite bands (ω ) 592
cm-1) is only accomplished at 700 °C. The IR spectrum of

Thermal Evolution of Transitional Aluminas
J. Phys. Chem. B, Vol. 103, No. 30, 1999 6167
TABLE 3: NMR Parameters of Bayerite and Boehmite Seriesa
(A) Bayerite Series
Tet
Oct
T (°C)
δTet (ppm)
δ
Oct (ppm)
CQ (MHz)
CQ (MHz)
ηTet
ηOct
ITet (%)
IOct (%)
hTet (%)
hOct (%)
300
500
77.5
76.5
77.3
79
15.1
14.5
14.2
15
4.7
4.5
4.9
5.2
5.5
5.5
3.9
3.9
3.8
4.0
3.0
3.1
0.7
0.7
0.5
0.5
0.5
0.8
0.4
0.4
0.4
0.4
0.4
0.4
22
28
31
33
43
47
78
72
69
67
57
53
29
35
33
31
20
4
45
50
50
45
32
18
700
800
900
80
11
1000
80
11.3
(B) Boehmite Series
Tet
Oct
T (°C)
δTet (ppm)
δ
Oct (ppm)
CQ (MHz)
CQ (MHz)
ηTet
ηOct
ITet (%)
IPent (%)
IOct (%)
hTet (%)
hOct (%)
600
800
900
78.3
78.5
77.7
77
79
74
14
4.9
4.7
4.7
5.1
5.5
5.3
3.7
3.6
3.7
3.3
3.0
3.1
0.7
0.7
0.7
0.7
0.7
0.7
0.4
0.4
0.4
0.4
0.4
0.4
21
24
26
37
37
25
3
6.5
4
3
0
76
69
70
60
62
75
30
35
33
30
15
12
33
40
44
45
25
15
15.2
13.5
14
12.2
14
950
1200b
1000
0
a The estimated errors on δ are 0.7 ppm. and those on CQ and η values are 0.2 MHz and 0.1, respectively. Relative errors on integrated intensities
are 5% while those for h parameters are 10%. b 6 minutes.
6.5%), which is probably due to aluminum atoms at the surface
of the particles. This analysis differs considerably from those
previously reported3,12 on spectra recorded with a lower spinning
rate (4 kHz), in which detection of pentahedral and tetrahedral
components was hindered by the presence of octahedral
sidebands. Similar conclusions were deduced by Slade et al.32
during the study of thermal transformation of alumina hydrates.
On the other hand, we see no hint of any abnormal aluminum
coordination in NMR spectra of η-alumina, contrary to reports
by Zhou and Snyder.4 Quadrupolar constants associated with
the two main types of aluminum coordination in γ- and η-Al2O3
phases are similar (Figure 8). The quadrupolar constants, CQ
are significantly different from zero, which would indicate a
clear departure from the ideal cubic symmetry. Quadrupolar
constants are larger for tetrahedra (CQ ) 4.7-4.9 MHz) than
they are for octahedra (CQ ) 3.6-3.9 MHz). Similar conclusions
were obtained by Kunath-Fandrei et al.;33 however, in that case,
the CQ constants were slightly higher (5 and 4.5 MHz,
respectively). Additionally, deduced asymmetry values, η, are
different from zero, suggesting that the symmetry at Al sites
may be lower than axial (Table 3). As a consequence of the
structural disorder, central components (-1/2/1/2 transitions) are
very asymmetric. Although it could be possible to perform an
analysis of the asymmetric broadening of centerbands by
assuming sites with different quadrupolar constants,33,34 the
existence in this case of several possible solutions with different
distributions of CQ and η made us to desist from using this
procedure. In order to describe the asymmetry of these
components, only two bands were finally introduced and the
parameter h described in the experimental section has been
adopted. This parameter, related to Al-site heterogeneity, is
approximately 40% for octahedra (Figure 10), but it takes on a
value around 30% for the tetrahedral component in both fases.
predicted by group theory (Table 1).
The average damping constant for each spinel mode decreases
for modes II and IV around 800 °C while the already low value
of mode I only slightly diminishes. The damping constant of
mode III does not change over the whole range of temperatures
(Figure 6). These data can be explained through the increase of
long-range order in the oxygen sublattice that goes parallel to
textural changes detected by TEM. It should be noted that the
X-ray diffraction peaks only start to narrow at 900 °C, i.e. 100
°C above the temperature detected by IR. This could mean that
from 800 to 900 °C the crystallite size would be of the order of
some tens of angstro¨ms.
From 800 to 1000 °C, octahedral and tetrahedral aluminum
fractions tend toward the theoretical limit of 50-50% which
corresponds to θ-Al2O3 (53-47% at 1000 °C). The thermal
evolution of CQ for tetrahedral aluminum consists of a gradual
increase toward 5.7 MHz, while for octahedral aluminum, a
steep decrease from 4 to 3 MHz happens near 900 °C. Based
on these facts, the heating of bayerite samples produces higher
distortions in tetrahedra and lower distortions in octahedra. In
θ-Al2O3 heated at 1000 °C, (Table 3) the chemical shift values,
corrected for second-order quadrupolar effects, are respectively
80 and 11.3 ppm. for tetrahedral and octahedral coordination
of aluminum. These observations indicate the formation of a
well-defined phase (θ-Al2O3) that is difficult to detect when
lower spinning rates are used, because the presence of tetra-
and octahedral side bands in the analyzed spectral region. In
this phase, quadrupolar constants are 5.5 and 3.1 MHz.
respectively, indicating that tetrahedra and octahedra are
considerably distorted. An analysis of distortions of polyhedra
is difficult because of the different influence of O-Al-O angles
and Al-O distances in CQ constants. In general, a modification
on the shorter Al-O tetrahedral distances should affect more
the CQ constants than that produced in the longer octahedral
distances. This fact has been experimentally confirmed.
In a similar way as in η-Al2O3, the process of heating γ-Al2O3
samples at temperatures higher than 800 °C also produces a
notable increase in order. In Figure 6 it can be observed that
the damping constants of modes II and IV behave in the same
way as in the bayerite series. However, at 900 °C, neither the
X-ray pattern nor the IR spectrum shows any sign of formation
From 800 to 1000 °C, large changes have been observed by
IR and NMR spectroscopies, as well as by X-ray diffraction.
In this range of temperatures, the evolution of structural
parameters in both series of samples notably differs. In the first
case, the transformation of η-Al2O3 into θ-Al2O3 is a gradual
process. The XRD pattern of the sample heated at 800 °C
corresponds to that of η-Al2O3, but at 900 °C it notably changes
displaying at 1000 °C the pattern of θ-Al2O3. In these samples,
narrow bands are observed in reflectance spectra corresponding
to the IR spectrum of θ-Al2O3, which is fully developed at 900
°C. This experimental spectrum displays 11 out of the 12 bands

6168 J. Phys. Chem. B, Vol. 103, No. 30, 1999
Pecharroman et al.
Figure 6. Damping of vibrational modes of alumina samples obtained
from thermal decomposition of bayerite (O) and bohemite (b).
Figure 7. Temperature evolution of structural sites occupancies
deduced from the intensity of the central band of 27Al NMR spectra of
calcined aluminas. Open symbols stand for bayerite series and the full
ones for that of boehmite. Diamonds (]) stand for octahedral aluminum,
triangles (3) for tetrahedral aluminum, and stars (*) for pentahedral
aluminum.
using the triple spinel cell of δ-Al2O3,6 group theory analysis
predicts the existence of 104 IR modes:
Experimental spectra only show 11 modes at frequencies very
close to those of γ- and θ-Al2O3. Taking into account that for
a similar material, γ-Fe2O3, in which the triple cell has been
proved correct by different diffraction techniques,35,36 up to 28
modes were clearly observed,37 it must be concluded that the
IR spectrum of this sample does not agree with the hypothesis
of a trebled spinel cell. Instead, the IR reflectance spectrum
can be explained as a mixture of γ- and θ-Al2O3. NMR data
also confirms this hypothesis. CQ and chemical shift values are
similar to those of γ-Al2O3 in the 600-900 °C temperature range
(see Table 3). In this range of temperature, the occupancy ratio
Figure 5. Evolution of (a, top) the central -1/2,1/2 transition and (b)
the whole 27Al MAS-NMR spectra of aluminas obtained from boehmite.
Spectra were recorded at 12 kHz (samples as described under Figure
1b). In the case of the spectrum of the sample heated at 600 °C in the
(a), the symmetric components used on the fitting have been plotted.
of a lower symmetry phase. Furthermore, the sample heated at
950 °C presents an X-ray pattern similar to that of δ-Al2O3
reported in the literature2,6 with small amounts of θ-Al2O3. By

Thermal Evolution of Transitional Aluminas
J. Phys. Chem. B, Vol. 103, No. 30, 1999 6169
Figure 8. Temperature evolution of CQ deduced from 27Al NMR
sideband patterns of calcined aluminas. Labeling is the same as Figure
7.
Figure 10. Temperature evolution of the heterogeneity parameter
deduced from the shape of the central -1/2,1/2 transition of 27Al MAS-
NMR spectra of calcined aluminas. Labeling is the same as in Figure
7.
950 °C most of pentahedral aluminum has been eliminated. It
is well-known that the addition of defects, as sodium cations,2,4
stabilizes the spinel structure of γ-Al2O3, even at temperatures
as high as 1100 °C. We think that in this case, pentahedral
aluminum, probably located at crystallite surfaces, could play
the same role in γ-Al2O3. When the temperature is sufficiently
high, aluminum defects become unstable and cations rearrange
to form θ-Al2O3. The presence of pentahedral aluminum in
γ-Al2O3 could also be responsible for the strong catalytic activity
as compared to that of η-Al2O3. Finally, the sample heated at
1200 °C for only 6 min, where pentahedral aluminum was also
removed, presents IR, XRD, and NMR parameters very similar
to those of θ-Al2O3.
R-Al2O3 formed by a process of nucleation and growth of
the oxygen sublattice begins to appear, in the boehmite series,
at temperatures higher than 950 °C, in such a way that in the
sample heated at 1000 °C for 2 h, three alumina phases, γ-, θ-,
and R-Al2O3, can be detected by XRD, IR, and NMR. The
formation of θ-Al2O3 and corundum in the temperature range
from 900 to 1200 °C follows kinetics with different time
constants, so that in a short thermal process, the θ phase tends
to form, while in case of long firing times corundum is always
obtained.10 When R-Al2O3 is formed, chemical shift values and
quadrupolar constants of octahedral Al components tend toward
those of corundum (δiso ) 16 ppm, and CQ ∼ 2.38 MHz);38
and η vanishes, which is typical of axial symmetry.
Figure 9. Temperature evolution of chemical shift of 27Al MAS-NMR
components. Positions were corrected for second-order quadrupolar
shifts. Labeling is the same as Figure 7.
of octahedral, tetrahedral, and pentahedral aluminum does not
notably change. At the same time, the asymmetry broadening
of the NMR components, the h parameters, reach maxima of
45% and 33% at 950 °C. This suggests that at this temperature
considerable amounts of γ-Al2O3 transform into θ-Al2O3 in such
a way that the material can be seen as a binary mixture. This is
also confirmed by close observation of CQ, chemical shift and
Al coordination. All these magnitudes have intermediate values
between those of γ- and θ-Al2O3. It should be remarked that at
Conclusions
From the IR and NMR analysis of samples obtained from
thermal decomposition of boehmite and bayerite, new insights
on the structural characteristics of transitional aluminas, defined
by their XRD patterns, have been obtained.
From bayerite dehydroxylation, η-Al2O3 is formed in which
aluminum vacancies are randomly distributed over spinel
octahedral and tetrahedral sites. The transition from η- to

6170 J. Phys. Chem. B, Vol. 103, No. 30, 1999
Pecharroman et al.
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of cations is detected, while above 800 °C, all atoms participate
in θ-Al2O3 formation. The structure of θ-Al2O3 is made of
distorted octahedra and tetrahedra in proportions close to 1:1.
Distortion of the Al coordination polyhedra is the origin of
quadrupolar constants measured for tetrahedra (5.5 MHz) and
octahedra (3.1 MHz).
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In γ-Al2O3 formed from boehmite, cation vacancies are
preferentially located at tetrahedral sites and 5% of Al is
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particle surface, and responsible for the origin of the catalytic
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at the crystallite surfaces that are eliminated above 900 °C.
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phase. In samples heated between 900-1200 °C, θ-Al2O3
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Acknowledgment. We acknowledge financial support of
DGICYT (Spain) through Research Project PB 95-0225, and
thank S. Medioroz and J. C. Conesa for kindly furnishing the
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