Dehydration studies of a high-surface-area alumina (Pseudo-boehmite) using solid-state 1H and 27Al NMR

Article abstract of DOI:10.1021/ja970788u

¹H NMR based on the CRAMPS technique has been used to identify and monitor the protons of surface Al-OH groups and 'physisorbed' water associated with a high-surface-area (230 m²/g) pseudo-boehmite material following dehydration in the 110-1100°C temperature range. Three distinguishable ¹H CRAMPS peaks were identified: a broad peak at 4.0 ± 0.2 ppm attributed to the protons of 'physisorbed' water and two peaks at 8.2 ± 0.3 and 2.3 ± 0.2 ppm associated with the protons of structural Al-OR groups. The ¹H CRAMPS results are interpreted in relationship to two important regions of the experimental dehydration weight-loss profile for this material, a lower temperature region (110-300°C), in which desorption of 'physisorbed' water occurs, and an intermediate temperature region (350-550°C), where condensation of adjacent Al-OH groups occurs. The combination of heating between 110 and 300°C and room temperature evacuation were found to eliminate the 'physisorbed' water peak, permitting the observation of the two resonances associated with the structural Al-OH sites. Dipolar dephasing experiments indicate that the 8.2 ppm peak is associated with highly coupled, 'clustered' Al₂OH groups, while the 3.0 ppm resonance is associated with terminal, 'isolated' AlOH groups. ¹H CRAMPS evidence shows that upon heat treatment the Al₂OH groups condense at lower temperatures (350°C) than the AlOH groups (550°C). Three mechanisms are proposed for the condensation of the proton-containing surface Al-OH groups that occur in this material, based on crystalline boehmite as a structural model. In addition to ¹H CRAMPS studies, ²⁷Al MAS NMR spectra at 14 T of samples dehydrated from 100 to 1100°C provide structural information about the aluminums in the high-surface-area pseudo-boehmite. This material dehydrates by condensation of both Al₂OH and AlOH groups to form distorted, hydrogen-bearing 4-, 5-, and 6-coordinate aluminum-containing intermediates in the 350-500°C range. At 1100°C, this hydrogen-bearing γ- or δ-alumina material is converted to a material consisting of primarily α-Al₂O₃.

Full text of DOI:10.1021/ja970788u

7832
J. Am. Chem. Soc. 1997, 119, 7832-7842
Dehydration Studies of a High-Surface-Area Alumina
1
27
(
Pseudo-boehmite) Using Solid-State H and Al NMR
,†
†
‡
‡
John J. Fitzgerald,* Gilberto Piedra, Steven F. Dec, Mark Seger, and
Gary E. Maciel*,‡
Contribution from the Departments of Chemistry, South Dakota State UniVersity,
Brookings, South Dakota 57007, and Colorado State UniVersity, Fort Collins, Colorado 80523
X
ReceiVed March 11, 1997. ReVised Manuscript ReceiVed May 27, 1997
1
Abstract: H NMR based on the CRAMPS technique has been used to identify and monitor the protons of surface
2
Al-OH groups and “physisorbed” water associated with a high-surface-area (230 m /g) pseudo-boehmite material
1
following dehydration in the 110-1100 °C temperature range. Three distinguishable H CRAMPS peaks were
identified: a broad peak at 4.0 ( 0.2 ppm attributed to the protons of “physisorbed” water and two peaks at 8.2 (
1
0
.3 and 2.3 ( 0.2 ppm associated with the protons of structural Al-OH groups. The H CRAMPS results are
interpreted in relationship to two important regions of the experimental dehydration weight-loss profile for this material,
a lower temperature region (110-300 °C), in which desorption of “physisorbed” water occurs, and an intermediate
temperature region (350-550 °C), where condensation of adjacent Al-OH groups occurs. The combination of
heating between 110 and 300 °C and room temperature evacuation were found to eliminate the “physisorbed” water
peak, permitting the observation of the two resonances associated with the structural Al-OH sites. Dipolar dephasing
experiments indicate that the 8.2 ppm peak is associated with highly coupled, “clustered” Al2OH groups, while the
1
3
.0 ppm resonance is associated with terminal, “isolated” AlOH groups. H CRAMPS evidence shows that upon
heat treatment the Al2OH groups condense at lower temperatures (350 °C) than the AlOH groups (550 °C). Three
mechanisms are proposed for the condensation of the proton-containing surface Al-OH groups that occur in this
1
27
material, based on crystalline boehmite as a structural model. In addition to H CRAMPS studies, Al MAS NMR
spectra at 14 T of samples dehydrated from 100 to 1100 °C provide structural information about the aluminums in
the high-surface-area pseudo-boehmite. This material dehydrates by condensation of both Al2OH and AlOH groups
to form distorted, hydrogen-bearing 4-, 5-, and 6-coordinate aluminum-containing intermediates in the 350-500 °C
range. At 1100 °C, this hydrogen-bearing γ- or δ-alumina material is converted to a material consisting of primarily
R-Al2O3.
Introduction
aspore contains two types of Al-O bonding interactions; one
oxygen is approximately coplanar with the three surrounding
aluminums, and the second oxygen forms a pyramid with the
Alumina-based materials are widely used as catalysts, catalyst
supports, adsorbents, coatings, ceramics, and abrasives.
1
-3
The
3
+
three adjacent aluminums. In boehmite, the Al ion exists in
a distorted, edge-sharing octahedral array of oxide ions that
forms a double layer, with the layers being connected by zigzag
state of hydration of a specific alumina material is related to its
stoichiometry and structure. Various aluminum oxide-based
materials of importance include the anhydrous and hydrous
aluminum oxides and hydroxides and a wide range of calcined,
aluminum oxides such as corundum or R-alumina (R-Al2O3),
γ-alumina, and transitional aluminas.1 The hydroxides gibbsite
and bayerite have related crystal structures,1 with double layers
of a hexagonal closely packed array of hydroxyl ions; each
aluminum is coordinated to six anions that form edge-shared
octahedral Al(OH)6 structural units. The double layers in
gibbsite are stacked in the direction of the c axis with repeating
5
chains of hydrogen bonds. Pseudo-boehmite is a poorly
crystallized boehmite with a water content of 1.5-2.5 mol.4
The XRD pattern of pseudo-boehmite shows broad lines that
1
coincide with that of boehmite.
-3
The transitional aluminas, ø-Al2O3, η-Al2O3, γ-Al2O3, κ-Al2O3,
and θ-Al2O3, are formed from oxyhydroxides and hydroxides
under various dehydration or dehydroxylation conditions, where
these two terms represent the formal loss of water by desorption
of “physisorbed” water or by condensation of hydroxyl groups,
2
A and B layers of the sequence ABBAABBA, while in bayerite,
1,3,4
respectively.
The materials, γ-Al2O3 and η-Al2O3, crystallize
the stacking arrangement is ABABAB, with the double layers
held together by hydrogen bonds between adjacent hydroxyl
ions.
Aluminum oxyhydroxide [AlO(OH)] occurs in two distinct
forms, boehmite and diaspore, that exist in a hexagonal closed-
packed arrangement of hydrogen-bonded oxides built up of
in a defect spinel structure, with η-Al2O3 being a more effective
acidic catalyst than γ-Al2O3 for olefin isomerization reactions.4
Strongly dried aluminas chemisorb at least a monolayer of water
5
when exposed to moisture at room temperature. According to
6
De Boer et al., the hydration surface of γ-alumina retains 13
2
molecules of water/100 Å of surface after evacuation at 25 °C
4
edge-sharing AlO4(OH)2 or AlO3(OH)3 octahedral units. Di-
for 100 h, while after drying at 120 °C, γ-Al2O3 still retains
2
6
7
8
.25 molecules of water/100 Å . Peri and Hannan found that
*
To whom correspondence should be addressed.
South Dakota State University.
Colorado State University.
†
γ-alumina adsorbs water at room temperature and after calcina-
‡
X
Abstract published in AdVance ACS Abstracts, August 1, 1997.
(4) Cocke, D. L.; Johnson, E. D.; Merril, R. P. Catal. ReV.sSci. Eng.
1984, 26, 163-231.
(1) Wefers, K.; Misra, C. Oxides and Hydroxides of Aluminum; Alcoa
Technical Paper No. 19, Alcoa Laboratories, 1987.
2) Gitzen, W. H. Alumina as a Ceramic Material; The American
Ceramic Society: Columbus, OH, p 15.
3) Walter, T. H.; Oldfield, E. J. Phys. Chem. 1989, 93, 6744-6752.
(5) Peri, J. B. J. Phys. Chem. 1965, 69, 211-219.
(6) DeBoer, J. H.; Fortuin, J. M. H.; Lippens, B. C.; Meijs, W. H. J.
Catalysis 1963, 2, 1-7.
(
(
(7) Peri, J. B.; Hannan, R. B. J. Phys. Chem. 1960, 64, 1526-1530.
S0002-7863(97)00788-9 CCC: $14.00 © 1997 American Chemical Society

Dehydration Studies of Pseudo-boehmite
J. Am. Chem. Soc., Vol. 119, No. 33, 1997 7833
1
3
NMR signal due to the removal of adjacent hydroxyl groups.
2
7
The Al CP/MAS NMR spectrum of partially dehydroxylated
γ-alumina showed peaks due to surface 4- and 6-coordinated
1
3
14
Al-OH Br o¨ nsted sites. Huggins and Ellis also carried out
2
7
variable-temperature Al MAS NMR studies to examine the
surfaces of transition aluminas. They proposed an alumina
surface model based on three mechanisms of surface dynam-
ics: (1) two neighboring hydroxyl groups condense, with
formation of molecular water that may move or “hop” across
the surface, leave the surface, reassociate to form two adjacent
hydroxyls or cap a Lewis acid site (AlO5); (2) surface proton
movement causes rearrangement of hydroxyl groups to new
positions; and (3) the Lewis sites migrate and the hydroxyl
groups move. The first mechanism occurs on untreated alu-
minas with large concentrations of hydroxyl groups, whereas
the later mechanisms occur on alumina surfaces with or without
1
5
“
physisorbed” water. Coster, Blumenfeld, and Fripiat recently
1
27
used H- Al CP/MAS NMR to examine Lewis acidity in
1
5
pretreated aluminas. Isotopically enriched NH3 was used to
probe the surface sites by magnetization transfer from NH3 to
surface aluminums. Two types of surface Lewis acid sites were
observed for chemisorption of NH3: a 4-coordinate peak at 58
ppm and a 5-coordinate site at 40 ppm. Chemisorption of water
by rehydration converted these sites to 5-coordinate and
6
-coordinate sites, respectively.
Figure 1. (a) Types of surface hydroxyl groups of γ-alumina, as
proposed by Peri. (b) Two types of structural hydroxyl sites of
boehmite.
3
17
9
Walter and Oldfield reported O CP/MAS NMR of bulk and
surface sites on aluminum oxides and transitional aluminas. The
17
17
O NMR of O-enriched boehmite revealed two peaks assigned
1
17
tion at 600 °C and reexposure to moist air. The adsorbed water
is believed to exist in the forms of both “physisorbed” water
and surface hydroxyl groups. At lower temperature heating,
desorbed water molecules react to form “surface” hydroxyl
groups; at higher temperatures, the adjacent surface hydroxyls
condense to form water molecules, which are subsequently
removed by evacuation according to the dehydration reaction
to OAl4 and Al2OH sites, while H- O cross polarization (CP)
selectively enhanced the weak Al2OH signal. Transition alu-
minas contained two types of oxygen sites: tetrahedral OAl4
3
and, to a lesser extent, trigonal OAl3. The range of bond angles
and bond distances for the trigonal sites is due to a range of
1
7
O chemical shifts and quadrupole coupling constants.
1
While extensive solid-state H CRAMPS studies have not
been reported for alumina materials, this approach offers a direct
means to explore the surface proton population on alumina
∼
OH + ∼OH f ∼O∼ + 0 + H O
2
16,17
1
materials. Bronnimann et al.
used H CRAMPS techniques
8
to explore the surfaces of silica-aluminas, γ-alumina, and silica
gel. For alumina-silicas, a sharp 2.0 ppm peak was assigned
to Si-OH groups, while two broad peaks at 3.1 and 4.8 ppm
were assigned to water “physisorbed” onto silica-like regions
and to γ-alumina-like regions, respectively. The peak at 7 ppm
was attributed to bridged Br o¨ nsted sites, although Pfeifer et al.18
assigned it to residual ammonium species.
where 0 represents an oxide vacancy for 4- or 5-coordinated
AlOx sites formed from 6-coordinate sites. From infrared
studies, Peri6 and Kn o¨ zinger and Ratnasamy7 postulated a
surface model for γ-alumina that includes the five different
surface hydroxyl sites shown in Figure 1, thus providing a
framework for discussing and describing the various proton,
oxygen, and aluminum atoms associated with the surface Al-
OH sites. In terms of this model, aluminum oxyhydroxides and
transitional aluminas contain at least one of the five distinct
1
The present work summarizes H CRAMPS NMR investiga-
tions to examine the hydroxyl groups and “physisorbed” water
2
moieties on a high-surface-area (230 m /g) pseudo-boehmite
hydroxyl sites, with the Type III site being the most acidic and
Types IA and IB the most basic.5
(Al O ‚2.05H O) material following various dehydration treat-
2
3
2
1
ments. This report describes the relationship between the H
CRAMPS spectra of this material and its experimental weight-
loss profiles, following low-temperature dehydration (50-110
°C) and higher temperature dehydroxylation (300-1100 °C)
under ambient pressure and in vacuo conditions. Al NMR
spectra are also reported for pseudo-boehmite materials dehy-
Solid-state NMR techniques provide unique capabilities to
obtain information about both the structural nature and dynamics
of the surface chemistry of alumina materials. Morris and Ellis
13
2
7
used solid-state Al CP/MAS NMR to study the surfaces of
27
various γ-aluminas. 27Al CP/MAS NMR spectra of γ-alumina
showed peaks at 6.0 and 55 ppm due to octahedral and
tetrahedral Al-OH surface sites, while calcination to different
degrees of dehydroxylation broadened the 27Al NMR signal at
(
14) Huggins, B. A.; Ellis, P. D. J. Am. Chem. Soc. 1992, 114, 2098-
2108.
(15) Coster, D.; Blumenfeld, A. L.; Fripiat, J. J. J. Phys. Chem. 1994,
8, 6201-6211.
(16) Bronnimann, C. E.; Chuang, I.; Hawkins, B. L.; Maciel, G. E. J.
6
ppm. Heating to 815 °C resulted in a complete loss of the
9
(
8) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic
Processes; McGraw-Hill: New York, 1979; pp 249-260.
Am. Chem. Soc.1987, 109, 1562-1564.
(
9) Peri, J. B. J. Phys. Chem. 1965, 69, 220-230.
(17) Bronnimann, C. E.; Zeigler, R. C.; Maciel, G. E. J. Am. Chem.
Soc.1988, 110, 2023-2026.
(10) Knozinger, H.; Ratnasamy, P. Catal. ReV.sSci. Eng. 1978, 17, 31-
7
6
0.
(18) Pfeifer, H. Applications of NMR Spectroscopy to Surfaces and
Catalysts: Acidic Sites and Absorbed Species. In Nuclear Magnetic
Resonance in Modern Technology; Maciel, G. E., Ed.; NATO ASI Series,
Vol. 447; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1994;
p 504.
(
(
(
11) Parkyns, N. D. J. Phys. Chem. 1971, 75, 526-531.
12) Peri, J. B. J. Phys. Chem. 1965, 69, 231-239.
13) Morris, H. D.; Ellis, P. D. J. Am. Chem. Soc. 1989, 111, 6045-
049.

7834 J. Am. Chem. Soc., Vol. 119, No. 33, 1997
Fitzgerald et al.
drated from 100 to 1100 °C to obtain NMR information on local
structural aluminum site changes. These changes have been
related to changes in the proton populations obtained from the
1
H CRAMPS results.
Experimental Section
1H CRAMPS experiments were taken at 187 and 360 MHz on
modified Nicolet NT-200 and NT-360 spectrometers, using the BR-24
2
0
pulse sequence and “home-built” probes, of samples sealed under
vacuum in thick-walled 5 mm (OD) glass tubes (Wilmad PS241),
following evacuation at 6.5 mTorr for 24 h, unless otherwise specified.
The sealed NMR tubes were spun with a spinning system based on a
1
9
modification of a design by Gay at MAS speeds of 1.2-1.5 kHz.
The 187 MHz CRAMPS spectra were obtained with (τ) spacings of
3
.0 and 1.2 µs π/2 pulses. The 360 MHz CRAMPS spectra were
obtained with τ ≈ 3.1 and 1.4 µs π/2 pulses. All spectra were referred
to the chemical shift of tetrakis(trimethylsilyl)silane (TTMSS) at 0.38
ppm via sample substitution (with liquid tetramethylsilane at 0.00 ppm).
27Al NMR spectra were recorded on a Bruker AM-600 spectrometer,
using home-built probes or a Chemagnetics Infinity 600 spectrometer,
using 3.2 and 4.0 mm Chemagnetics MAS systems, with evacuated
samples that were loaded into the spinner in a drybox. To ensure
minimum error in the quantitation of the observed NMR peak
intensities, short excitation pulses (<22° tip angles) were used. MAS
speeds of 16-17 kHz were employed. The chemical shift reference
Figure 2. Weight-loss profiles for the high-surface-area pseudo-
2 3 2
boehmite (Al O ‚2.05H O) material. (a) Gravimetric analysis of samples
equilibrated in aqueous suspensions over the pH range 3-12 (curve
A). (b) Gravimetric analysis of “as received” samples (curve B). (c)
Thermogravimetric (TGA) analysis of an “as received” sample in
ambient atmosphere at 20 °C/min (curve C).
was an aqueous solution of 1 M AlCl
3 2
‚6H O, assigned a chemical shift
of 0.0 ppm. Higher chemical shifts correspond to larger resonance
2
7
frequencies and lower shielding constants. For Al spin counting
Results and Discussion
3
9
experiments, based on a previously reported method, the spinning
angle was set approximately 1° off the magic angle to broaden into
the baseline the extensive spinning sideband arrays due to the noncentral
transitions. This facilitated integration of the centerband signal as a
function of the pulse width in this single-pulse experiment. An
authentic sampe of kaolin of known water content served as a spin
counting standard.
Experimental Weight-Loss Data For High-Surface-Area
Pseudo-boehmite. The high-surface-area (HSA) pseudo-boe-
hmite was obtained from Norton (lot 08061, surface area 230
m /g, average particle size 46 µm, and pore volume > 0.5 mL/
g). Three series of pseudo-boehmite samples were prepared in
order to obtain the weight loss at various heating temperatures
2
(
weight-loss profiles), as given in Figure 2. In the experiments
(
(
(
19) Gay, I. D. J. Magn. Reson. 1984, 58, 413.
20) Burum, D. P.; Rhim, W. K. J. Phys. Chem. 1979, 71, 944-956.
21) Christoph, G. G.; Corbat o´ , C. E.; Hofmann, D. A.; Tettenhorst, R.
represented in curve A, high-surface-area pseudo-boehmite
samples were suspended in 0.02 M NaCl for 1 h, followed by
equilibration in aqueous solutions at pH 3.0, 5.0, 6.5, 7.5, 9.0,
T. Clay Clay Miner. 1979, 27, 81-86.
(
(
22) Hill, R. J. Clay Clay Miner. 1981, 29, 435-445.
23) Christensen, A. N.; Lehman, M. S.; Convert, P. Acta Chem. Scand.
1
1.0, and 12.0 solutions for 24 h. Adjustment of the pH was
carried out by addition of either 1.0 M HCl or 1.0 M NaOH
solutions. The resulting solid materials obtained from the
slurries were pressure filtered under 95 psi of N2, using a
Gelman pressure filtration funnel with a 45 µm Nylaflo nylon
membrane filter. Following desiccation over Drierite for 72 h
at atmospheric pressure (725 Torr), the solids were heated at
atmosphere pressure in the temperature range 110-1100 °C until
a constant weight was obtained. A weight-loss profile was
obtained by plotting the average weight loss of these materials
at the various temperatures (50, 110, 150, 200, 250, 300, 350,
1
982, A36, 303-308.
24) Corbat o´ , C. E.; Tettenhorst, R. T.; Christoph, G. G. Clay Clay Miner.
985, 33, 71-75.
25) Haase, J.; Freude, D.; Frlich, T.; Himpel, G.; Kerbe, F.; Lippmaa,
E.; Pfiefer, H.; Sarv, P.; Schfer, H.; Seiffert, B. Chem. Phys. Lett. 1989,
(
1
(
1
3
1
1
1
56, 328-332.
26) John, C. S.; Alma, N. C.; Hays, G. R. Appl. Catal. 1982, 6, 341-
46.
(
(27) Dec, S. F.; Maciel, G. E.; Fitzgerald, J. J. J. Am. Chem. Soc. 1990,
12, 9069-9077.
(28) Fitzgerald, J. J.; Dec, S. F.; Hamza, A. I. Am. Mineral. 1989, 74,
405-1408.
29) Lambert, S. F.; Millman, W. S.; Fripiat, J. J. Am. Chem. Soc. 1989,
11, 3517-3522.
30) Cruickshank, M. C.; Dent Glasser, L. S.; Barri, A. I.; Poplett, I. J.
(
5
50, 850, and 1100 °C) versus the heating temperature (curve
A, Figure 2).
(
F. J. Chem. Soc., Chem. Commun. 1986, 23-24. Alemany, L. B.; Kirker,
In the experiments represented in curve B, the effects of
dehydration/dehydroxylation were examined on samples pre-
pared from “as received” material by heating the samples at
ambient pressure at the same temperatures and during the same
periods of time indicated above for the samples related to curve
A. A weight-loss profile was also obtained as shown in curve
B of Figure 2.
In the work represented in curve C, two TGA thermograms
were obtained (from Hazen Research Inc. in Golden, CO) on a
“as received” pseudo-boehmite sample heated in ambient
atmosphere (∼725 Torr) at 20 °C/min to a limit of 1100 °C
and on an analogous sample heated to 1100 °C at 20 °C/min
under 152 Torr vacuum. The TGA thermogram obtained in
ambient atmosphere is shown as curve C of Figure 2, which is
G. W. J. Am. Chem. Soc. 1986, 108, 6158-6162.
(31) Dec, S. F.; Fitzgerald, J. J.; Frye, J. S.; Shatlock, M. P.; Maciel, G.
E. J. Magn. Reson. 1991, 93, 403-406.
(32) Gilson, J.-P.; Edwards, G. C.; Peters, A. W.; Rajagopalan, K.;
Wormsbecker, R. F.; Roberie, T. G.; Shatlock, M. P. J. Chem. Soc., Chem.
Commun. 1987, 91-92.
(
33) Lippens, B. C.; de Boer, J. H. Acta Crystallogr. 1964, 17, 1312-
1
321.
(
34) O’Reilly, D. E. AdV. Catal. 1960,12, 31.
(35) Pearson, R. M.; Schramm, C. M. Colloids Surf. 1990, 45, 323-
3
34 and references therein.
(
(
(
36) Haase, J.; Oldfield, E. J. Magn. Reson. A 1993, 104, 1-9.
37) Schmitt, K. D.; Haase, J.; Oldfield, E. Zeolites 1994, 14, 89-100.
38) Maciel, G. E.; Bronnimann, C. E.; Hawkins, B. L. High-Resolution
1
H Nuclear Magnetic Resonance in Solids by CRAMPS. In AdVances in
Magnetic Resonance; The Waugh Symposium, Vol. 14; Warren, S., Ed.;
Academic Press: San Diego, CA, 1990; pp 125-150.

Dehydration Studies of Pseudo-boehmite
J. Am. Chem. Soc., Vol. 119, No. 33, 1997 7835
Table 1. Average Weight Losses (as mol of water/mol of Al)
Figure 2 also shows the weight-loss profile for “as received”
samples of high-surface-area alumina obtained by gravimetric
analysis (curve B) and by thermogravimetric analysis (curve
C), in the same temperature range (50-1100 °C). Curve B
corresponds to the average gravimetric weight-loss profile and
depicts three regions: (1) an initial region from 110 to 200 °C
associated with the removal of “physisorbed” water, (2) a second
region in the 200-350 °C temperature range where a sharp
increase in the weight loss is observed, attributed to dehydration
via the condensation of hydroxyl groups, and (3) a final region
from 350 to 1100 °C, showing further dehydration/condensation
behavior similar to that observed in the final region of curve
A. The TGA thermogram of the “as received” samples heated
at 20 °C/min in ambient atmosphere (curve C) also depicts three
regions: (1) an initial low-temperature region (50-150 °C) in
which a sharp increase in weight loss occurs due to the removal
of “physisorbed” and bulk molecular water, (2) a second region
in the 150-500 °C range that is associated with dehydration
due to condensation of hydroxyl groups, and (3) a third
dehydration/dehydroxylation region (500-1100 °C), probably
due to additional dehydration by condensation of hydroxyl
groups, where the weight-loss increases only slightly. The TGA
thermogram for the “as received” sample heated at 20 °C/min
in vacuo shows results (not shown) similar to those obtained
for the “as received” sample heated at 20 °C/min in ambient
atmosphere (curve C).
O/mol of Ald
weight loss as %wt
weight loss as H
2
temp, °C SEa ARb TGAc
SEa
ARb
TGAc
5
0
3
4
8
10
12
15
19
18
20
24
1
7
0.11
0.28
0.31
0.37
0.45
0.56
0.74
0.70
0.72
0.91
0.04
0.29
0.40
0.44
0.50
0.54
0.60
0.99
1.06
1.08
1
1
2
2
3
3
5
8
10
50
00
50
00
50
50
50
9
11
13
15
19
26
29
32
38
0.36
0.42
0.51
0.56
0.72
1.01
1.11
1.23
1.44
10
12
13
14
16
26
27
28
1
100
a
Solution-equilibrated material, corresponds to curve A in Figure
2
. b “As received” material, corresponds to curve B in Figure 2.
Thermogravimetric analysis, corresponds to curve C in Figure 2.
Maximum moles of water in 2.05.
c
d
virtually identical with the profile for the sample dehydrated
under vacuum.
In this article, unless different conditions are specified
explicitly, the term “dehydrated” or “heated” signifies heating
the sample at ambient pressure (∼725 Torr), and the term
“
evacuated” or “evacuation” signifies subjecting the sample to
some specified vacuum (or 6.5 mTorr, if not otherwise speci-
fied), at room temperature.
Weight-Loss Profiles for Dehydration/Dehydroxylation.
Two processes have been postulated5 to occur during the
dehydration of alumina materials: (1) loss of water molecules
by desorption of physically adsorbed or bulk molecular water
and (2) condensation of adjacent surface or internal hydroxyl
groups. Both processes are associated with the weight-loss
profiles observed upon heating alumina materials. The des-
orption of “physisorbed” water has been postulated to occur at
lower temperatures (100-120 °C),6 while condensation of
hydroxyl groups occurs above 110 °C. Removal of hydroxyl
groups during water condensation conditions has been postulated
to give rise to strained Al-O-Al linkages on the alumina
surface, imparting catalytic properties to the alumina materials.
Since the pseudo-boehmite material is composed exclusively
of octahedral AlO6 sites (Figure 1b), it is possible that the
oxygen vacancies produced during the condensation of hydroxyl
groups reduce the aluminum coordination number to 5, thereby
forming Lewis acid AlO5 structural moieties.
Table 1 summarizes the average gravimetric weight losses
of solution-equilibrated and “as received” solid samples of a
pseudo-boehmite material (Norton, Al2O3‚2.05H2O) heated at
various temperatures from 50 to 1100 °C at ambient pressure.
Some weight-loss data obtained from the TGA thermogram
obtained in ambient atmosphere are also included in Table 1
for comparison. The weight-loss profiles, plotted as mol of
H2O/mol of Al (maximum equals 2.05 H2O/Al) versus dehydra-
tion/dehydroxylation temperature, for the various high-surface-
area alumina materials studied are given in Figure 2. Curve A
shows the average weight loss for the series of samples
equilibrated in aqueous solutions with pHs ranging from 3.0 to
Both the gravimetric curves (A and B) and TGA curve (C)
are comparable at low temperature during the first dehydration
stages, where the desorption of “physisorbed” water occurs;
however, at high temperatures, the weight-loss profile derived
from the gravimetric measurements for the solution-equilibrated
material is less steep than the weight-loss profiles derived from
both the gravimetric and TGA measurements with “as received”
materials. The lower weight-loss values observed for the
solution-equilibrated materials may be due to readsorption of
the water onto the alumina material following cooling to room
temperature. The weight-loss values obtained for samples from
the TGA analysis are intermediate between the two sets of
gravimetric measurements. In the TGA analysis, the alumina
sample is systematically heated from 50 to 1100 °C under
conditions that do not permit water readsorption, but the heating
time period is much shorter than that utilized in the gravimetric
measurements (a minimum of 24 h). The gravimetric measure-
ments with the “as received” samples show the highest weight-
loss values, probably due to the longer time periods of heating
at these temperatures.
1
1
H CRAMPS NMR Results. The H CRAMPS spectra,
obtained at frequencies of 187 and 360 MHz, on the high-
surface-area alumina sample equilibrated at pH 6.5, then heated
at 110 °C at ambient pressure, and then evacuated at 6.5 mTorr
at 25 °C for 24 h, are shown in Figure 3. The two resonances
centered at ca. 3.0 and 8.2 ppm are tentatively assigned on the
basis of patterns observed in this work (vide infra) to two
distinguishable structural Al-OH bulk moieties, an octahedral
AlOH site and an octahedral Al2OH site, respectively, as
depicted in Figure 1b. These two different types of structural
Al-OH groups may be either internal or external groups, based
on the microporous nature of this material (vide infra). These
two 6-coordinate aluminum sites of the pseudo-boehmite
material are probably similar to the Type IB and Type IIB sites
1
2.0. This gravimetric weight-loss curve shows three distin-
guishable regions: (1) an initial region of weight loss from 50
to 200 °C corresponding to a dehydration process that can be
related to the removal of “physisorbed” and/or bulk molecular
water prior to the condensation of hydroxyl groups on the
alumina material, (2) a temperature region from 200 to 350 °C,
in which significant weight-loss is observed that is associated
with the first stage of dehydration by condensation of Al-OH
groups, and (3) a final region of weight loss in the 350-1100
9
,10
of the surface model proposed by Peri
for γ-alumina, as
shown in Figure 1a. These two types of Al-OH sites are known
to exist in crystalline boehmite in a 1:1 ratio based on its crystal
21-24
1
°
C temperature range that is attributed to a second stage of
structure, as shown in Figure 1b.
The H CRAMPS results
dehydration by condensation of hydroxyl groups.
show that an increase in the magnetic field from 4.39 to 8.45 T

7836 J. Am. Chem. Soc., Vol. 119, No. 33, 1997
Fitzgerald et al.
1
1
Figure 4. H- H dipolar dephasing CRAMPS results (360 MHz) of
pseudo-boehmite (Al ‚2.05H O) material equilibrated at pH 6.5,
O
2 3
2
followed by evacuation at 6.5 mTorr for 24 h at 25 °C, showing dipolar
dephasing time (2τ).
at 3.0 ppm requires 80 µs of dephasing to completely disappear.
Dipolar dephasing preferentially attenuates the transverse
magnetization of protons involved in stronger dipolar interac-
1
6
tions with other protons.
The rapid disappearance of the
proton signal at 8.2 ppm at a smaller dephasing time indicates
that the protons associated with this species are involved in
Figure 3. 1H CRAMPS NMR spectra (187 and 360 MHz) of a pseudo-
boehmite (Al O) material equilibrated in an aqueous suspen-
sion at pH 6.5, filtered, heated at 110 °C for 5 h, and evacuated for 24
2 3
O
‚2.05H
2
1
1
stronger H- H dipolar coupling, probably due to hydrogen
1
1
bonding, than the H- H dipolar coupling exhibited by the
protons resonating at 3.0 ppm. On the basis of this behavior,
the 8.2 ppm resonance is assigned to closely associated,
“clustered” hydroxyls, while the signal at 3.0 ppm is assigned
to “isolated” hydroxyls. On the basis on the internuclear
distances implied from Figure 1b, we identify the former with
Al2OH groups and the latter with AlOH groups.
h at 6.5 mTorr.
increases the resolution of the proton peaks, probably because
of a narrowing of the observed resonances as a result of
decreased quadrupolar interference with MAS averaging of the
1
27
25
H- Al dipolar interactions. The presence of two aluminums
close to the protons in the Al2OH sites should be expected to
cause the proton peak of the Al2OH group to be broader than
that of the corresponding proton peak of the Al-OH; however,
the chemical shift dispersion due to a wide range of different
proton environments in both the AlOH and Al2OH sites is
probably the dominant line broadening influence at higher fields,
e.g., 360 MHz.
1
Figure 5 shows the H CRAMPS spectra of four alumina
samples dehydrated at room temperature and 110 °C: one placed
in a dessicator over Drierite at room temperature, one heated at
110 °C at atmospheric pressure (725 Torr) for 5 h, one heated
at 110 °C at atmospheric pressure (725 Torr) for 5 h followed
by evacuation at 25 °C 6.5 mTorr for 24 h, and one heated at
1
1
The 360 MHz H CRAMPS NMR spectra (not given here)
110 °C under vacuum (8 mTorr) for 24 h. The H CRAMPS
for pseudo-boehmite materials equilibrated in aqueous suspen-
sions at various pH values (3.0, 6.5, and 12.0), followed by
simultaneous heating and evacuation at 110 °C and at 6.5 mTorr
show two peaks centered at 8.2 and 3.0 ppm similar to those of
Figure 3 associated with structural Al2OH and AlOH sites,
respectively. The relative intensity of the 3.0 ppm peak is
drastically decreased at the lower pH of 3.0, probably due to
the protonation of AlOH sites with concomitant formation of
surface Al(III)sOH2 species. These protonated species would
account for the presence of a peak at about 4.0 ppm associated
with water molecules (vide infra) physically adsorbed onto the
pseudo-boehmite material. Higher pH values did not drastically
change the population of either Al2OH or AlOH sites on the
pseudo-boehmite material, although it seems that at the very
high pH values the number of Al2OH sites is increased.
spectra of the dessicator-dried sample (Figure 5a) and the sample
heated at 110 °C under ambient pressure (Figure 5b) exhibit a
broad band centered at about 4.0 ppm attributed to “physisorbed”
water (vide infra; this material is a porous material that likely
contains some internal, trapped water and “physisorbed” surface
water; in addition, both interanl and exteranl structural Al-
OH groups are present) and a less intense shoulder at 8.2 ppm
associated with the Al2OH sites; the 3.0 ppm resonance due to
1
AlOH sites is absent. The H CRAMPS spectrum corresponding
to the pseudo-boehmite material that was heated at 110 °C in
ambient pressure exhibits a decreased intensity for the water
proton peak (4.0 ppm) in comparison with the proton peak of
the Al2OH group, indicating that partial removal of “phys-
isorbed” water may be achieved by simply heating the pseudo-
1
boehmite solid at 25 °C at 110 °C. In their H NMR studies of
1
1
H- H dipolar dephasing experiments of the type described
the dehydration of the silica gel and silica-alumina surfaces,
16
16,17
previously for the study of silica gel to qualitatively determine
Bronnimann et al.
observed for nonevacuated samples a
1
1
the relative strengths of the H- H dipolar coupling between
broad feature at ca. 3.5 ppm that was attributed to physically
adsorbed water on the surfaces of silica regions and silica-like
regions. On the basis of the similarity between the behavior
protons of the two types of silanols on silica were used here.
1
The H CRAMPS dipolar dephasing results obtained with
1
dephasing times from 0 to 80 µs are shown in Figure 4 for the
alumina sample that was heated at ambient pressure 110 °C,
followed by evacuation at 6.5 mTorr for at 25 °C 24 h. These
results show that the resonance at 8.2 ppm is dephased beyond
observation after a dephasing time of 20 µs, whereas the peak
observed for the pseudo-boehmite studied here and the H
CRAMPS results for silica-alumina or silica materials, the
broad peak at 4.0 ppm in Figure 5a and 5b is tentatively assigned
to “physisorbed” water on the pseudo-boehmite alumina mate-
rial. The proton signal for “physisorbed” water apparently

Dehydration Studies of Pseudo-boehmite
J. Am. Chem. Soc., Vol. 119, No. 33, 1997 7837
Figure 6. 1H CRAMPS spectra (360 MHz) of pseudo-boehmite
Figure 5. 1H CRAMPS spectra (360 MHz) of pseudo-boehmite
(
6
Al
2
O
3
‚2.05H
2
O) material equilibrated in aqueous suspension at pH
(
Al
2
O
3
2
‚2.05H O) materials prepared following various dehydration
.5, filtered, heated in the 110-1100 °C temperature range (as specified
procedures: (a) desiccation over Drierite at ambient pressure (725 Torr)
for 72 h, (b) heating at 110 °C (in ambient pressure) for 5 h, (c) heating
at 110 °C (in ambient pressure) for 5 h, followed by evacuation at 6.5
mTorr for 24 h, and (d) heating at 110 °C under vacuum (8 mTorr) for
in the figure), followed by evacuation at 6.5 mTorr for 24 h. 2τ values
of 2 µs for all samples, except 1100 °C (2τ ) 0 µs).
evacuated samples (Figure 5c,d) show that the Al2OH (8.2 ppm)
and AlOH (3.0 ppm) resonances are clearly resolved.
2
4 h.
1
A series of H CRAMPS spectra of pseudo-boehmite samples
masks the AlOH peak at 3.0 ppm, possibly because of proton
spin exchange between AlOH protons and “physisorbed” water
protons; this could occur in a fashion similar to the mechanism
depicted in Scheme 1 of the Huggins and Ellis model for
equilibrated at pH 6.5, heated at 725 Torr over the 110-1100
°
°
C temperature range, and then evacuated at 6.5 mTorr at 25
C for 24 h, are given in Figure 6. The results summarized in
1
Figure 5, as well as other H CRAMPS measurements not shown
here, indicate that such heating/evacuation procedures with
dehydration temperatures up to 350 °C remove nearly all the
14
aluminas. The protons of “physisorbed” water molecules may
also experience spin-spin flip-flop exchange with the AlOH
protons on the surface, rendering the spin equilibration between
the AlOH protons and “physisorbed” water protons fast, at least
on the relevant time scale of these experiments.
“physisorbed” water protons. After sample heating temperatures
up to 300 °C, the peaks for both the Al2OH and AlOH sites are
observed for interior and/or surface hydroxyl groups. However,
in the samples dehydrated at 725 Torr at temperatures in the
Further water removal is achieved by heating the pseudo-
boehmite solid at 110 °C followed by evacuation at 6.5 mTorr,
300-350 °C region, the proton peak at 8.2 ppm due to Al OH
2
1
as shown in Figure 5c. The H CRAMPS results show the
groups shows a drastic reduction in intensity (Figure 6b,c); this
appearance of the 3.0 ppm peak, associated with AlOH sites,
and the nearly complete disappearance of the “physisorbed”
water peak (4.0 ppm). The presence of spectral intensity
reduction in peak intensity is interpreted as due to a dehydration
process as a result of condensation of adjacent, “clustered” Al2-
OH surface hydroxyls. For the sample heated at 350 °C, a sharp
artifact peak at about 12 ppm is also observed due to a rotor
1
between the 3.0 and 8.2 ppm H NMR signals is due to a low-
39
intensity “physisorbed” water peak, indicating that some traces
of water are still present on or within the pseudo-boehmite
material; therefore, complete removal of water is not achieved
line (Figure 6c). Further increases in the heating temperature
above 350 °C result in a gradual reduction in the intensity of
hydroxyl groups of the 3.0 ppm signal as a consequence of
further loss of hydroxyl groups due to condensation of the
“isolated” AlOH groups. However, the broad, low-intensity
1
utilizing this procedure. The H CRAMPS spectrum (Figure
7
d) corresponding to a pseudo-boehmite solid that was dehy-
1
proton peak in the H CRAMPS spectra of the 550 and 1100
drated by heating at 110 °C in vacuo (8 mTorr) depicts only
the two peaks at 8.0 and 3.0 ppm associated with surface and/
or interior Al2OH and AlOH sites, respectively, in a nearly 1:1
ratio. This result indicates that this dehydration procedure
removed all the “physisorbed” water present in the pseudo-
°
C samples (Figure 6d,e) indicates that complete removal of
all the Al-OH protons does not occur even after heating at
100 °C at ambient pressure, followed by evacuation at 25 °C
1
(39) Fitzgerald, J. J.; Kohl, S. D.; Piedra, G.; Dec, S. F.; Maciel, G. E.;
1
boehmite material. Thus, the H CRAMPS spectra of both the
Chem. Mater. 1994, 6 (11), 1915-1917

7838 J. Am. Chem. Soc., Vol. 119, No. 33, 1997
Fitzgerald et al.
boehmite samples equilibrated in aqueous suspension (pH 6.5),
heated at 725 Torr in the 110-550 °C temperature range, and
then evacuated at 6.5 mTorr at 25 °C for 24 h, are shown in
Figure 7. The dipolar dephasing spectra of the sample heated
at 110 °C (Figure 7a) show two peaks at 8.2 and 3.0 ppm
assigned to protons associated with the structural Al2OH and
AlOH sites, respectively. The 8.2 ppm peak dephased com-
pletely at a dephasing time of 20 µs, whereas the 3.0 ppm peak
required 80 µs to completely disappear. The dipolar dephasing
spectra (Figure 7b) of the sample heated at 725 Torr at 300 °C
also show peaks due to the protons associated with the Al2OH
(8.2 ppm) and AlOH (3.0 ppm) sites, the dephasing time
required to deplete these peaks (20 and 80 µs, respectively) being
the same values as obtained with the sample heated at 110 °C.
The differences between the dephasing times for the 3.0 and
8
.2 ppm peaks are interpreted as due to structural differences
of the “clustered” Al2OH and “isolated” AlOH groups (vide
supra) but may also be explained by the differences in the proton
exchange dynamics.
Figure 7c shows the series of dipolar dephasing spectra
corresponding to the pseudo-boehmite sample heated at 725 Torr
at temperatures up to 350 °C, followed by evacuation at 6.5
mTorr at 25 °C for 24 h. The spectra depict a broad feature
centered at about 4.0 ppm, probably associated with protons of
water and both internal and external structural AlOH sites, which
dephases beyond observation after 320 µs of dipolar dephasing.
Figure 7d (sample heated at 550 °C, and then evacuated) also
shows the same peak, although it dephases completely at 160
µs, probably due to a lower population of protons as a result of
the dehydration/dehydroxylation procedures.
1
The 360 MHz H CRAMPS spectra were obtained for “as
received” pseudo-boehmite materials heated at 350 °C in
ambient pressure, followed by equilibration over Drierite for
24 h, and the same material heated at 350 °C in vacuo (8 mTorr)
for 24 h. The spectrum (not shown here) of the sample heated
at 350 °C at ambient pressure is very similar to that shown in
Figure 7c and shows a broad peak centered at 4.0 ppm due to
a wide variety of protons present on the surface, including the
remaining Al2OH sites that have not condensed yet, some traces
of physisorbed water, and the “isolated” AlOH sites. The
simultaneous heating/evacuation procedure is more effective in
eliminating the “physisorbed” water from the pseudo-boehmite
material but also drastically reduces the number of structural
Al-OH sites, as can be concluded from the decreased signal-
1
to-noise ratio in the H CRAMPS spectrum (not shown). This
in vacuo dehydration/dehydroxylation method at 350 °C com-
pletely removes both the “physisorbed” water and the majority
of the Al2OH site protons and also greatly reduces the proton
population associated with the AlOH sites.
Bulk and Surface Protons of High-Surface-Area Pseudo-
boehmite. The various dehydrated/dehydroxylated alumina
materials examined in this work have been derived from a high-
2
surface-area (230 m /g) pseudo-boehmite material with a
chemical formula of Al2O3‚2.05H2O. Calculations based on the
particle size of this material (a mean diameter of 46.25 µm),
assuming spherical geometry, indicate that about 4.2% of the
total protons are within the first five AlO6 layers from the
surface. Calculations based on the surface area of this material
indicate that the Al2O10 octahedra located in the first monolayer
contain ca. 30% of the total protons, assuming a H:Al ratio of
1
1
Figure 7. H- H (360 MHz) dipolar dephasing CRAMPS results of
a pseudo-boehmite (Al ‚2.05H O) material equilibrated at pH 6.5,
O
2 3
2
filtered, heated from 110 to 1100 °C, followed by evacuation at 6.5
mTorr for 24 h: (a) 110 °C, (b) 300 °C, (c) 350 °C, and (d) 550 °C,
showing dipolar dephasing time (2τ).
1
:1 in this monolayer. The difference between 4.2% and 30%
is probably due to the porous nature of the pseudoboehmite
material employed here, which contains channels or pores within
the particles that are ill-defined at present. The microporous
nature of this material suggests the existence of both internal
at 6.5 mTorr for 24 h. A broad distribution of different types
of residual Al-OH protons is still observed.
A series of dipolar dephasing spectra obtained on pseudo-

Dehydration Studies of Pseudo-boehmite
J. Am. Chem. Soc., Vol. 119, No. 33, 1997 7839
and external structural Al-OH groups for the “clustered” Al2-
OH and “isolated” AlOH groups.
1
The assignment of the H CRAMPS resonances correspond-
ing to the Al2OH and AlOH sites have been based on the
assumption that these “aluminol” groups are present both on
the surface (external) and within (internal) the pseudo-boehmite
material and are structurally similar to the Al2OH and AlOH
sites of crystalline boehmite.2
1-24
According to this model, the
pseudo-boehmite structure consists of three different types of
aluminol” groups: (a) interlayer Al2OH sites, (b) “clustered”
“
Al2OH surface and/or internal sites, and (c) surface and/or
internal AlOH sites. Interlayer and “clustered” Al2OH sites
differ in the nature of their hydrogen bonding. In the interlayer
Al2OH sites, the hydrogen bonding occurs between the oxygen
of one layer and the hydrogen of an adjacent layer, whereas in
the “clustered” Al2OH groups, the hydrogen bond is formed
between an oxygen atom and a hydrogen atom coordinated to
an adjacent oxygen in the same layer. The dipolar dephasing
1
1
results indicate stronger H- H dipolar interactions of the Al2-
OH protons, which suggest the existence of strong hydrogen
bonding; however, the dipolar dephasing data do not provide
enough evidence to make a clear distinction between the two
types of Al2OH sites. In the AlOH groups, each oxygen is only
2-coordinate, with available electron pairs that might be utilized
in the formation of hydrogen bonds with neighboring water
molecules. This latter situation would account for the proton
exchange process between AlOH groups and water molecules.
There are three possible combinations of hydroxyl sites by
which condensation might occur: (1) the combination of pairs
of Al2OH sites, (2) the combination of pairs of AlOH sites,
and (3) the combination of Al2OH sites with AlOH sites. The
reported internuclear O-O distance between a pair of octahedral,
edge-sharing Al2OH sites in crystalline boehmite is 2.535 Å,
the same as the O-O distance between a pair of AlOH sites;
the corresponding O-O distance within a Al2OH-AlOH pair
is 2.558 Å, whereas the distance within the interlayered Al2-
OH-Al2OH pair is 2.709 Å.4
,21-24
On the basis of these
Figure 8. Proposed schemes for the condensation of structural hydroxyl
groups of pseudo-boehmite: (a) condensation of “isolated” AlOH
parameters, Al2OH-Al2OH condensation and AlOH-AlOH
condensation between octahedral sites are expected to be favored
over the condensation of Al2OH-AlOH pairs and should
definitively be preferred over the condensation of interlayered
Al2OH pairs. Figure 8a depicts a proposed scheme by which
the condensation of axially distributed surface/internal AlOH
groups (oriented along the a axis) may occur. The condensation
of such axial AlOH groups would result in the formation of
some highly distorted, “external” 6-coordinate AlO4(OH)2 sites
and “internal” 6-coordinate AlO4(OH)2 sites. The condensation
of equatorial Al2OH sites (located on the b-c plane) would
give rise to some “external” 5-coordinate AlO4OH sites and
groups, (b) condensation of “clustered” Al
2
OH groups, (c) condensation
OH groups. Some of the Al-O bonds were
of interlayered Al
2
deliberately omitted in the figure for clarity.
be energetically more favorable than the condensation of axially
distributed AlOH hydroxyl groups. Structurally, the order in
which the different “aluminol” groups condense upon heating
may be predicted to be the following: first, the equatorial,
“
clustered” Al2OH sites, second, the axial, “isolated” AlOH
sites, and third, the interlayer Al2OH sites.
27
MAS Al NMR Studies of the Dehydration of High-
“
internal” 5-coordinate AlO5 sites, as shown in Figure 8b.
27
Surface-Area (HSA) Pseudo-boehmite. MAS Al NMR
Condensation of the interlayer Al2OH groups results in cross-
linking of adjacent layers with concomitant formation of
channels or pores throughout the material, as depicted in Figure
studies of the dehydration products of bayerite, boehmite,
sodium aluminate dihydrate, Na2O‚Al2O3‚3H2O, and the miner-
als pyrophyllite and kaolinite have been successfully used to
determine and assign different 4-, 5-, and 6-coordinate aluminum
sites that occur in these materials during their dehydration in
8c, where highly distorted, 6-coordinate AlO6 sites are generated.
The simultaneous condensation of AlOH and Al2OH groups,
which would occur by a simultaneous combination of the
schemes described in Figure 8a,b, would also generate highly
distorted, 4-coordinate AlO4 sites (not shown here).
26-29
27
the solid state.
In the 14 T Al NMR work described
herein for the dehydration products of the pseudo-boehmite
materials produced over the temperature range 110-1100 °C,
structural information regarding changes in the coordination
environments of aluminum sites in these materials has been
obtained. The relationship between these aluminum site changes
and the observed proton population changes that occur in these
On the basis of the nature of the species formed following
the condensation of the two different types of structural
“aluminol” sites (both internal and external), one could envision
that the formation of the less-distorted 5-coordinated AlO4OH
sites should be favored over the formation of the more distorted
1
dehydrated materials as identified from the H CRAMPS results
6
-coordinated AlO4(OH)2 sites. Hence, it seems reasonable to
are also discussed.
propose that the condensation of equatorial Al2OH sites will
A series of 156.38 MHz 27Al NMR spectra of the pseudo-

7840 J. Am. Chem. Soc., Vol. 119, No. 33, 1997
Fitzgerald et al.
Figure 9. 27Al MAS NMR spectra (14 T) of various aluminum oxide materials: (a) pseudo-boehmite heated at 110 °C, (b) pseudo-boehmite
heated at 300 °C, (c) pseudo-boehmite heated at 350 °C, (d) pseudo-boehmite heated at 550 °C, (e) γ-alumina, (f) γ-alumina, heated 3 h at 1280
°
C, (g) R-alumina, (h) pseudo- boehmite heated 1100 °C.
boehmite material and its dehydration products obtained over
the temperature range 110-1100 °C at ambient pressure,
followed by evacuation at 6.5 mTorr at 25 °C for 24 h, are
given is Figure 9, together with various reference spectra of
R-Al2O3, γ-Al2O3 and γ-Al2O3 dehydrated at ambient pressure
at 1280 °C, followed by similar room temperature evacuation
procedures. The series of pseudo-boehmite materials are
identical with those samples used to obtain the dehydration
and is similar to the 27Al NMR peak observed for cyrstalline
boehmite and diaspore (spectra not shown here). In the material
produced at a dehydration temperature of 300 °C at ambient
pressure, followed by evacuation at 25 °C at 6.5 mTorr, the
2
7
Al NMR spectrum (Figure 9b) shows a low-intensity peak at
69 ppm due to newly formed 4-coordinate AlO4 sites, in addition
to the 6-coordinate peak at 8.8 ppm. Dehydration at ambient
pressure at 350 °C, followed by evacuation at 6.5 mTorr at 25
1
27
profile shown in Figure 2A and to obtain the H CRAMPS NMR
°C for 24 h, produces an Al NMR spectrum (Figure 9c) that
results shown in Figures 6 and 7. The 27Al NMR spectrum
shows three peaks, a more intense 4-coordinate peak at 67.8
ppm, a broad but intense peak at 9.0 ppm due to 6-coordinate
aluminum, and a weak but discernible NMR signal at 37.0 ppm
that is assigned to 5-coordinate aluminum sites. The 27Al NMR
(Figure 9a) of pseudo-boehmite dehydrated at 110 °C, followed
by evacuation, exhibits a sharp peak at 8.8 ppm similar to that
of the “as received” material, which is assigned to AlO6 sites

Dehydration Studies of Pseudo-boehmite
J. Am. Chem. Soc., Vol. 119, No. 33, 1997 7841
Table 2. Summary of 27Al Spin Counting Experiments Based on
NMR and TGA Measurements for Pseudo-Boehmite Samples
Heated at the Temperatures Noted
spectrum of the 350 °C sample is similar to the spectrum of a
commercial γ-alumina sample (Figure 9e, peaks at 9.0 and 68.8
ppm), indicating that the pseudo-boehmite material is dehydrated
to a transition alumina.
dehydration
sample wt
(mg ( 0.1 mg)
%Al
by NMR
%Al
by TGA
temp (°C)
The spectrum (Figure 9c) of the sample dehydrated at ambient
pressure at 550 °C, followed by room temperature evacuation
at 6.5 mTorr for 24 h, is very similar to that of the sample
dehydrated at 725 Torr at 350 °C, showing the same three
resonances at 9.0, 36.5, and 68.4 ppm. The three spectra of
the samples dehydrated at 725 Torr at 300, 350, and 550 °C
given in Figure 9 all show the presence of this 36-37 ppm
aluminum peak. This 27Al NMR peak in the chemical shift
range 36-37 ppm observed in these spectra of the dehydrated
samples is thus assigned to 5-coordinate AlO5 sites, consistent
with similar assignments of 5-coordinate AlO5 sites observed
in the 2 Al NMR of barium aluminum glycolate, andalusite,
pyrophyllite dehydroxylate, dehydrated kaolinite (metakaolinite),
1
3
3
5
1100
1280
10
00
50
50
10.1
5.3
4.7
7.7
13.6
12.3
44 ( 2%
45 ( 2%
51 ( 3%
52 ( 2%
53 ( 1%
53 ( 1%
42%
46%
48%
51%
52%
54%
from 4 to 14 mg), with an uncertainty of (0.1 mg; hence, the
smallest samples show largest uncertainties.) Comparison of
2
7
the aluminum content determined by Al NMR spin counting
to the values obtained from TGA analysis (Table 2) indicates
that (to within experimental uncertainty) all of the aluminum
nuclei in the dehydrated materials are being observed at very
short pulse lengths at 14 T magnetic field strength. Therefore,
the major changes in the Al NMR spectra of the dehydrated
transition aluminas obtained by calcination of pseudo-boehmite
in this study indicate that all of the aluminums are NMR
observable and that the integrated peak intensities accurately
reflect the 4-, 5-, and 6-coordinate aluminum compositions in
these samples. These results may be contrasted with the only
work reported on the relative contents of 4- and 6-coordinate
7
28-30
27
and selected silica-alumina solids.
The reference Al
NMR spectrum (Figure 9e) of the commercial γ-alumina also
shows these three NMR peaks, which are consistent with these
assignments for the products of pseudo-boehmite dehydration
at intermediate temperatures (Figure 9b-d).
2
7
At a higher temperature of ambient pressure dehydration,
1
100 °C (followed by similar room temperature evacuation),
2
7
the Al NMR spectrum of the dehydrated pseudo-boehmite
material shows only a single 6-coordinate AlO6 resonance at
26
aluminas in transition aluminas by John et al., where not all
of the aluminum is observable under the conditions used in those
1
(
1
4.3 ppm (Figure 9h), similar to that in the spectra of R-alumina
Figure 9g, 13.9 ppm) and the γ-alumina sample (Figure 9f,
4.1 ppm) that has been heated at 1280 °C at ambient pressure.
early studies.
Relationship between 1H and 27Al NMR Results. The
These results provide conclusive evidence that the transition
aluminas produced from the pseudo-boehmite material by
heating at ambient pressure in the 300-350 °C temperature
range (followed by room temperature evacuation) undergo a
solid-state thermal conversion to R-alumina at 1100 °C, similar
to the R- to γ-alumina conversion observed for the commercial
major changes in the aluminum coordination environment
displayed in the Al NMR results of the dehydration products
of the HSA pseudo-boehmite material may be interpreted in
relation to the information obtained on the proton populations
of these same dehydrated materials from the H CRAMPS
27
1
results. In the sample dehydrated at ambient pressure at 110
C, followed by room temperature evacuation, the Al NMR
2
6
γ-Al2O3 material.
27
°
A fundamental question regarding the 27Al NMR spectra is
whether these spectra show intensities that properly represent
the amounts of aluminum in the dehydrated samples. This is
particularily true for 27Al NMR signals associated with alumi-
num sites with very low symmetry and, consequently, very large
electric field gradients that produce broad NMR resonances.
The question of “invisible aluminum” has been a common
problem noted in the literature for solid-state 27Al NMR studies
shows a single 6-coordinate peak at 8.8 ppm due to Al(OH)6
sites, while the H CRAMPS spectrum of a sample heated at
110 °C under vacuum (6.5 mTorr) for 24 h shows two
resonances due to the “isolated” AlOH (2.3 ppm) and “clus-
1
tered” Al OH (8.2 ppm) groups, indicating that the disordered
2
pseudo-boehmite structure is intact following these treatments.
Upon heating the pseudo-boehmite material up to 300 °C,
27
followed by evacuation, its Al NMR spectrum exhibits a
of minerals such as andalusite and dehydrated clays such as
single, intense 8.8 ppm resonance, with the appearance of very
low signal intensity in the 4- and 5-coordinate Al chemical shift
region (30-70 ppm), indicating the initial stages of the
pyrophyllite dehydroxylate and metakaolin,2
8-32
and surface
aluminum sites on transition aluminums that have reduced
coordination numbers less than 6.1
3,14,26,34-36
To address this
1
condensation of Al-OH groups. The H CRAMPS spectrum
signficant question, spin counting experiments were carried out
on the pseudo-boehmite material and its dehydration products
formed over the temperature range 300-1280 °C. A plot of
of this sample shows that the resonances due to the two AlOH
and Al OH proton populations are virtually unchanged in
2
comparison with those of the sample heated at ambient pressure
2
7
the integrated Al NMR centerband intensity for pseudo-
boehmite and its dehydrated products as a function of pulse
length (0.10-1.0 µs), shows a complex relationship (related to
the relative size of the quadrupolar interactions and the rf field
at 110 °C (followed by room temperature evacuation), except
for the appearance of a low-intensity, broad signal between these
two peaks (Figure 6b), possibly due to residual “physisorbed”
water protons. Proton signals due to OH groups coordinated
on aluminums in both the samples dehydrated at 110 and 300
°C, followed by evacuation at 6.5 mTorr, are not obscured by
this weak proton peak at 4.0 ppm due to “physiosorbed” water.
3
5-37
strength).
Comparison to the integrated signal for the
centerband of an authentic sample of kaolin (containing a known
2
7
number of Al spins) gave the NMR-detectable aluminum
content of each of the dehydrated materials. The integrated
intensities after a 0.10 µs pulse (the shortest achievable with
the available spectrometer hardware and software) are shown
by comparison to the TGA results (Table 2) to be proportional
to the number of Al spins in the sample. (Uncertainties were
estimated by repeat analysis and are dominated by the uncer-
tainty in the determination of sample mass (the samples ranged
1
It is proposed on the basis of the H CRAMPS spectrum of the
sample dehydrated at 300 °C under ambient pressure, and the
subsequent changes in the spectra of the samples dehydrated at
350 and 550 °C under ambient pressure, to be described below,
that these early stages of condensation of aluminum-coordinated
hydroxyl groups probably occur in the absence of “physisorbed”
water and that the loss of water at 300 °C is principally due to

7842 J. Am. Chem. Soc., Vol. 119, No. 33, 1997
Fitzgerald et al.
the initiation of condensation of adjacent hydroxyl groups
associated with the “clustered” Al2OH groups of the pseudo-
boehmite material.
drated at 1100 °C, followed by evacuation, are markedly reduced
relative to the spectrum of the sample dehydrated at 550 °C, as
evidenced by the observed decrease in the signal-to-noise ratio
The 27Al NMR spectra (Figure 9c,d) of the samples dehy-
drated at 350 and 550 °C under ambient pressure, followed by
room temperature evacuation at 6.5 mTorr for 24 h, show 4-,
of the H CRAMPS signal for the sample dehydrated at 1100
1
°C. The observed broad proton peaks in the spectra of both
the samples dehydrated at 550 and 1100 °C at ambient pressure,
followed by room temperature evacuation, are likely a result
of a wide range of chemically different hydrogen-bearing AlO4
and AlO6 aluminum polyhedra in the 550 °C sample and a
similar wide distribution of AlO6 octahedra for the sample
dehydrated at 1100 °C. Another possible contribution to the
5
-, and 6-coordinate peaks with fractional areas of 0.23, 0.03,
and 0.74, respectively. The fractional peak areas for the 4-, 5-,
and 6-coordinate aluminum peaks in the spectrum of γ-alumina
2
6
(
Figure 9e) are 0.27, 0.04, and 0.69. John, Alma, and Hays
27
have reported 11.4 T Al NMR results which show that the
various transition aluminas have the following fractional peak
areas for the 4-coordinate aluminums, γ (0.25), δ (0.25), θ
1
broadened H CRAMPS resonances is the occurrence of “proton
1
4
hopping”, as proposed by Huggins and Ellis, from their
2
7
(
0.00), and η (0.35), with the remaining peak areas being due
variable-temperature Al NMR studies of partially or fully
dehydrated transition aluminas.
2
6
to 6-coordinate aluminums. The works of John et al. and
Lippens33 have also shown that crystalline boehmite undergoes
dehydration to γ-alumina (0.27 Td) at 450 °C, then to δ-alumina
Conclusions
1
(
0
°
0.25 Td) at 750 °C, then to a mixture of R- and θ-alumina (both
The H CRAMPS results and weight-loss data indicate that
.00 Td) at 1000 °C, and finally to R-alumina (0.00 Td) at 1200
most of the “physisorbed” water protons are lost at 110 °C (with
27
1
C. The Al NMR results obtained here for the pseudo-
or without evacuation). In addition, the H CRAMPS results
boehmite material substantiate the view that the formation of
either γ- or δ-alumina begins initially at 300 °C and is essentially
complete, from a 27Al NMR viewpoint, in the 350-550 °C
temperature range, where a fractional 4-coordinate composition
in the range 0.25-0.27 is observed.
and weight-loss data indicate that water protons from aluminum-
coordinated hydroxyls in the “clustered” Al2OH sites are
removed from the pseudo-boehmite material at temperatures in
the 200-350 °C range, whereas protons from aluminum-
coordinated hydroxyls in the “isolated” AlOH sites begin to
condense at higher temperatures (350-1100 °C). The favored
condensation of “clustered” Al2OH groups over the condensation
of the “isolated” AlOH groups is presumably associated with
the less distorted nature of the 5-coordinated AlO4OH species
that is formed by such processes and/or the relative proximity
of the “clustered” Al2OH groups to one another. In the case of
the formation of a “less” distorted AlO4OH species or the
proximity of Al2OH groups to one another, the condensation
of these surface Br o¨ nsted acid sites gives rise to the formation
of strained Al-O-Al linkages throughout the alumina surface
as a result of oxygen vacancies (5-coordinate Al) or oxygen
defects (distorted 6-coordinate Al). The Al NMR evidence
shows the appearance of minor 5-coordinate species in addition
to an approximately 3:1 ratio of hydrogen-bearing 6-coordinate/
4-coordinate aluminum oxyhydroxide species distribution present
in a γ- or δ-transition alumina material formed following
dehydration of the pseudo-boehmite material in the 350-550
°C range at ambient pressure (followed by room temperature
evaucation at 6.5 mTorr). With a heating temperature of 1100
°C, followed by evacuation, dehydration of the proton-bearing
γ- or δ-transition alumina to R-alumina containing AlO6
structural moieties with a very low hydrogen content occurs.
1
The H CRAMPS results provide additional insight into the
formation of transition alumina (either γ or δ) in the 350-550
°
C temperature region from the high-surface-area pseudo-
boehmite. The formation of principally 4-coordinate and
6
5
-coordinate aluminums, in addition to a minor population of
-coordinate aluminums, occurs by condensation of both the
“
5
“
clustered” Al2OH and “isolated” AlOH groups in the 350-
50 °C temperature region. The rate of condensation of the
clustered” Al2OH groups is greater than that of the “isolated”
AlOH groups and occurs at lower temperature, as evidenced
1
by the more significant changes in the 8.2 ppm peak in the H
2
7
CRAMPS spectrum for the lower temperature (350 °C) sample
(Figure 6c) in comparison with the spectrum of the sample
dehydrated at 550 °C (Figure 6d). The various schemes for
the condensation of the AlOH groups, as shown in Figure 8,
indicate that the adjacent “clustered” Al2OH groups may
condense to 5-coordinate aluminums, with the formation of
distorted 6-coordinate sites as well. Simultaneous condensation
of Al2OH and AlOH groups would lead to the formation of
appreciable amounts of distorted, 4-coordinate aluminums, as
27
is observed from the Al NMR spectra of the samples
dehydrated at 350 and 550 °C at ambient pressure, followed by
room temperature evacuation at 6.5 mTorr for 24 h. The
presence of a low population of 5-coordinate aluminums in these
spectra suggest that the AlO5 species is a likely intermediate
formed prior to the formation of the more abundant 4-coordinate
aluminums.
Acknowledgment. The authors gratefully acknowledge
support from the National Science Foundation Grants No. RII-
8
902066 and CHE-9021003 and the technical assistance of Dr.
Bruce Hawkins, Dr. Herman Lock and Dr. Mark F. Davis of
the Colorado State University NMR Center, Fort Collins, CO.
J.J.F. acknowledges support of the Office of the President, South
Dakota State University.
In the samples dehydrated in the 550-1100 °C temperature
1
range, the H CRAMPS spectra show a very broad resonance
extending over approximately a 10 ppm chemical shift range.
However, the relative proton populations of the sample dehy-
JA970788U