Thermal evolution of the CI--LiAl2 layered double hydroxide: A multinuclear MAS NMR and XRD perspective

Article abstract of DOI:10.1021/ic010671d

Layered double hydroxides (LDHs) with a cation composition of LiAl₂ have a wide range of potential applications as catalysts, catalyst supports, and precursors for refractory oxide materials, including several industrially important lithium aluminate phases. The understanding of the calcination behavior of this group of LDH phases is essential to advancing these applications, and the research described here focuses on the thermal decomposition and structural evolution of LiAl₂(OH)₆Cl·nH₂O in the temperature range of 20-1100 °C. ²⁷Al, ³⁵Cl, and ^6.7Li magic angle spinning nuclear magnetic resonance spectroscopy, powder X-ray diffraction, thermal analysis (including thermogravimetric and differential scanning calorimetry), and compositional analysis provide a highly consistent picture of the thermally induced phase formation and transformations of this LDH. The loss of the surface and interlayer water can begin as low as room temperature, depending on the relative humidity. Beginning at about 300 °C, the simultaneous volatilization of H₂O and HCl and the exsolution of crystalline LiCl result in the formation of amorphous Li-Al-O-OH. By at least 500 °C, volumes with the structures of α-LiAlO₂ and LiAl₅O₈ appear, and these phases become progressively more ordered with increasing temperature. LiCl begins to volatilize by 850 °C and is present only in trace amounts above ca. 1000 °C. α-LiAlO₂ converts to γ-LiAlO₂ between 970 and 1100 °C. Because of the delithiation due to LiCl volatilization, the final products are dominated by LiAl₅O₈, in contrast to the calcination products of previously studied LiAl₂ LDHs which are dominated by LiAIO₂.

Full text of DOI:10.1021/ic010671d

Inorg. Chem. 2001, 40, 6397-6404
6397
Thermal Evolution of the Cl--LiAl2 Layered Double Hydroxide: A Multinuclear MAS
NMR and XRD Perspective
Xiaoqiang Hou* and R. James Kirkpatrick
Department of Geology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
ReceiVed June 25, 2001
Layered double hydroxides (LDHs) with a cation composition of LiAl2 have a wide range of potential applications
as catalysts, catalyst supports, and precursors for refractory oxide materials, including several industrially important
lithium aluminate phases. The understanding of the calcination behavior of this group of LDH phases is essential
to advancing these applications, and the research described here focuses on the thermal decomposition and structural
evolution of LiAl2(OH)6Cl‚nH2O in the temperature range of 20-1100 °C. 27Al, 35Cl, and 6,7Li magic angle
spinning nuclear magnetic resonance spectroscopy, powder X-ray diffraction, thermal analysis (including
thermogravimetric and differential scanning calorimetry), and compositional analysis provide a highly consistent
picture of the thermally induced phase formation and transformations of this LDH. The loss of the surface and
interlayer water can begin as low as room temperature, depending on the relative humidity. Beginning at about
300 °C, the simultaneous volatilization of H2O and HCl and the exsolution of crystalline LiCl result in the formation
of amorphous Li-Al-O-OH. By at least 500 °C, volumes with the structures of R-LiAlO2 and LiAl5O8 appear,
and these phases become progressively more ordered with increasing temperature. LiCl begins to volatilize by
850 °C and is present only in trace amounts above ca. 1000 °C. R-LiAlO2 converts to γ-LiAlO2 between 970 and
1100 °C. Because of the delithiation due to LiCl volatilization, the final products are dominated by LiAl5O8, in
contrast to the calcination products of previously studied LiAl2 LDHs which are dominated by LiAlO2.
Introduction
analysis provides a far more complete picture of the phase
formation, phase transformations, and compositional changes
The understanding of the structural and compositional evolu-
tion of layered double hydroxides (LDHs) with progressive
calcination at elevated temperatures is essential to advancing
their use as catalysts, catalysts precursors, and precursors for
refractory oxide materials.1-3 We present here comprehensive
X-ray diffraction (XRD), thermogravimetric/differential scan-
ning calorimetry (TG/DSC), and multinuclear NMR data that
explore the thermal evolution of the chloride containing the
LiAl2 LDH compound LiAl2(OH)6Cl‚nH2O. The end products
of its thermal decomposition, R- and γ-LiAlO2 and LiAl5O8,
are important ceramic phases which have potential applica-
tions in nuclear fusion reactors4 as electrolyte matrixes,4 in
plant illumination5,6 as catalyst supports, and in the prepara-
tion of delithiated alumina compounds.1-3 The thermal evolution
of similar LiAl2 LDH phases, LiAl2(OH)6NO3‚nH2O,7 LiAl2-
(OH)6(CO3)0.5‚nH2O,8 and LiAl2(OH)6OH‚nH2O,1,7 has been
previously examined using primarily XRD. Here, the combina-
occurring at elevated temperatures. Most importantly, the 27Al,
7
6Li, Li, and 35Cl NMR data provide detailed local structural
information for both the well-crystallized phases and the
amorphous and poorly crystallized intermediate phases. The
results define the temperature ranges over which evolutionary
events including dehydration, dehydroxylation, dechlorination,
delithiation, and the crystallization and structural reorganization
of R- and γ-LiAlO2 and LiAl5O8 occur. Because of the
delithiation and dechlorination specific to the LiAl2(OH)6Cl‚
nH2O compound, the amount of LiAl5O8 in the final calcination
products is about 3.1 times that of γ-LiAlO2, which is the
dominant product in the calcined nitrate, carbonate, and hydroxyl
forms.1,7,8
LDHs are layer-structured hydroxides that have a permanent,
positive layer charge and, thus, a significant anion-exchange
capacity due to isomorphous substitution, most commonly in
the hydroxide layers.9 The crystal structures of most LDHs
consist of positively charged hydroxide sheets separated by
interlayers containing exchangeable anions and water mole-
cules.9-11 The LiAl2 LDH compounds develop a positive charge
because of the substitution of Li+ for the vacancies of a
dioctahedral gibbsite sheet, and the Li and Al are well ordered.11
At room temperature, the interlayer Cl- is well ordered in the
anhydrous phase, but in the hydrated phase, the Cl- and water
molecules are positionally disordered.12
6
7
tion of 27Al, Li, Li, and 35Cl NMR spectroscopies at a very
high field (H0 ) 17.6 T), XRD, TG/DSC, and compositional
* To whom correspondence should be addressed. Phone: (217) 244-
2355. Fax: (217) 244-4996. E-mail: xhou@uiuc.edu.
(1) Nayak, M.; Kutty, T. R. N.; Jayaraman, V.; Periaswamy, G. J. Mater.
Chem. 1997, 7 (10), 2131-2137.
(2) Jimenez-Becerril, J.; Bosch, P.; Bulbilian, S. J. Nucl. Mater. 1991,
185, 304-307.
(3) Poeppelmeier, K. R.; Chiang, C. K.; Kipp, D. O. Inorg. Chem. 1988,
27, 4523-4526.
(4) Alessandrini, F.; Alvani, C.; Casasio, S.; Mancini, M. R.; Nannetti,
C. A. J. Nucl. Mater. 1995, 224, 236-244.
(9) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173.
(10) Serna, C. J.; White, J. Clays Clay Miner. 1977, 25, 384-391.
(11) Serna, C. J.; Rendon, J. L.; Iglesias, J. E. Clays Clay Miner. 1982, 30
(3), 180-184.
(12) Besserguenev, A. V.; Fogg, A. M.; Francis, R. J.; Price, S. J.; O’Hare,
D. Chem. Mater. 1997, 9, 241-247.
(5) Kutty, T. R. N.; Nayak, M. Mater. Res. Bull. 1999, 34 (2), 249-262.
(6) Kutty, T. R. N.; Nayak, M. J. Alloys Compd. 1998, 269, 75-87.
(7) Poeppelmeier, K. R.; Hwu, S.-J. Inorg. Chem. 1987, 26, 3297-3302.
(8) Hernandez, M. J.; Ulibarri, M. A. Thermochim. Acta 1985, 94, 257-
266.
10.1021/ic010671d CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/30/2001

6398 Inorganic Chemistry, Vol. 40, No. 25, 2001
Hou and Kirkpatrick
a similar Larmor frequency, and the 6Li pulse length for the solids was
2 µs. The 6Li recycle time for the spectra reported here was 60 s. There
is no significant difference in the relative intensities of the 6Li spectral
components and their peak shapes between spectra collected with
recycle times of 20 and 60 s. MAS spinning frequencies were 9.5-11
kHz. Normally, 80-300 scans were acquired for each spectrum. An
exponential line broadening of 15 Hz was used to process the data
with the NUTs processing package. Line simulation and integration
Experimental Section
Sample Preparation. LiAl2(OH)6Cl‚nH2O samples were prepared
by the direct reaction of gibbsite with a LiCl solution.13 For each sample,
9 g of gibbsite and 15 g of LiCl were mixed with 25 mL of boiled
deionized (DI) water, and the mixture was reacted at 90 °C for 14 h in
a tightly sealed polyethylene bottle under strong stirring. The suspension
was centrifugally separated and ultrasonically washed for three cycles
using boiled DI water to remove the excess LiCl. The sample used
here (LiAlCl8) was then dried in a vacuum at room temperature for
several weeks. A high concentration of LiCl (14 M in our case) is
critical in the preparation of LiAl2(OH)6Cl‚nH2O by this method. Fogg
and O’Hare13 reported that the rate of the reaction is half-order with
respect to the initial concentration of LiCl. Our experience shows that
a significant formation of LiAl2(OH)6Cl‚nH2O using 0.5 M LiCl under
conditions similar to those described previously does not occur.
Calcination was conducted using the following methods. For
temperatures of 720 °C and below, aliquots of dried LiAlCl8 were
placed in a furnace at the desired temperature and heated for 6 h in air.
For temperatures of 850 °C and above, samples were placed in the
furnace at 850 °C, the furnace was ramped to the desired calcination
temperature over a few minutes, and the sample was held at the run
temperature for 3 h. In all of the cases, the samples were heated in
open Pt crucibles in air, allowed to cool to 200 °C in the furnace, and
then quickly removed to a desiccator with P2O5. After being cooled to
room temperature, five aliquots of the sample were stored in separate
vials in a glovebag for parallel XRD, NMR, and elemental analysis.
Sample Examination. Samples were examined by elemental
7
were also performed using the NUTs software. Li has a spin of I )
3/2, a large natural abundance (92.6%) and nuclear receptivity, and a
relatively small electric quadrupole moment.14,15 For our samples, only
7
one scan is required for good S/N ratios in the Li spectra.
Results and Interpretation
Chemical Analysis. The structural formula of the sample
used for the calcination experiments (LiAlCl8) based on
compositional analysis is Li0.970Al2.030(OH)6Cl0.977‚0.644H2O.
This composition conforms closely to the desired stoichiometric
composition of LiAl2(OH)6Cl‚H2O except that the analyzed
water content depends significantly on the drying and storage
conditions. Although LiCl was greatly in excess in the initial
mixture, the Al/Li molar ratio of the final product is close to 2,
consistent with a strict Al/Li ordering in the octahedral sheet.
This observation is in contrast to Mg/Al LDHs (hydrotalcites)
for which the final Al/Mg ratio can vary from 1/5 to 1/2 depending
on the initial solution Al/Mg ratio and the synthesis route.9 Here,
the analyzed Al/Li ratio is slightly larger than 2 because of a
small amount of gibbsite, as observed by XRD (Figure 1).
LiAlCl8 contains the least amount of gibbsite of all of the
samples that we synthesized and has an Al/Li ratio closest to
2. The gibbsite present could be unreacted starting material or
could have formed by the leaching of the LDH product during
washing.7 No carbonate was detected in LiAlCl8, although
contamination of LDHs by atmospheric CO2 is common and
was observed for our other samples.
Compositional analysis shows a significant decrease in the
Cl content and an increase in the Al/Li molar ratio in the cal-
cined samples. For the 850 °C sample, the elemental composi-
tion (wt %) is 43.01% Al, 3.88% Li, 2.54% Cl, 0.13% C, and
0.24% H and, for the 1100 °C sample, it is 49.25% Al, 2.80%
Li, 1.09% Cl, 0.05% C, and 0.04% H. On the basis of these
analyses, the atomic Al/Li ratios of these two samples are 2.85
and 4.52, respectively. These values are in good agreement with
the line fits of the 6Li NMR data, as discussed in the following
paragraphs.
TG and DSC. The TG and DSC curves obtained in Ar and
air are essentially identical, and only those under Ar are
presented here (Figure 2). For the sample LiAlCl8 (Figure 2a),
there are four well-defined endotherms with maxima at about
125, 355, 535, and 1020 °C, and each of these events has an
associated weight loss except for that at 535 °C. The three
observed weight loss events are readily assigned to the loss of
physically adsorbed and structural (interlayer) water at 125 °C,
dehydroxylation and partial dechlorination at 355 °C (see the
discussion of NMR data that follows), and volitalization of LiCl
at 1020 °C. The LiAl2 LDH compounds are well-known to lose
surface and interlayer water at quite low temperatures, and our
peak near 125 °C is in good agreement with the previous data.16
35Cl NMR and relative humidity (RH) controlled IR data (data
not shown) also show that samples equilibrated over P2O5 for
several days or that samples equilibrated in RH ) 0% air for 2
h lose most of their surface and interlayer water. The 355 °C
feature is relatively narrow for the dehydroxylation of an LDH,
analysis, XRD, TG, DSC, and 27Al , Li, Li, and 35Cl MAS NMR.
For elemental analysis, Li and Al and were determined using inductively
coupled plasma-atomic emission spectroscopy (ICP-AES), C and H
were determined with a CHN analyzer, and Cl- was determined by
titration. Dissolution of the refractory high-temperature calcination
products was difficult, and these samples were dissolved in a mixture
of phosphoric and sulfuric acids at ∼250 °C for analysis. TG and DSC
data were recorded from room temperature to 1225 °C at a heating
rate of 10 °C min-1 in both Ar and air using a NETZSCH Simultaneous
Thermal Analyzer STA 409. Powder XRD patterns were recorded with
a Rigaku diffractometer using Cu KR radiation at a scanning rate of
1° 2θ min-1 and a step size of 0.02° 2θ. No special precautions were
taken to prevent a preferred orientation during sample loading or to
prevent the absorption of moisture. The samples were exposed to
ambient atmosphere during XRD data collection. The only apparent
effect of this was in the hydration of crystalline LiCl. KGa-1 kaolinite
(Clay Minerals Repository, University of Missouri, Columbia) was used
as an external XRD standard.
27Al and 35Cl MAS NMR spectra were collected at room temperature
using both a Varian 750 spectrometer (H0 ) 17.62 T) and a home-
built 500 MHz spectrometer (H0 ) 11.74 T) equipped with Doty
Scientific fast MAS probes. The calcined samples were quickly loaded
from their sealed containers into capped rotors just prior to data
collection to avoid moisture absorption. Aqueous solutions of 1 M AlCl3
and 1 M NaCl were used as external chemical-shift standards for 27Al
and 35Cl, respectively, and their chemical shifts were set at 0 ppm.
The measured 90° pulses for both of the solutions, 27Al and 35Cl, were
8 µs. Shorter pulses of 1 µs for 27Al and 3 µs for 35Cl were used for
the MAS NMR spectra of the solid samples to get better quantification.
Typically, a recycle time of 1 s was used for both 27Al and 35Cl.
6Li and 7Li MAS NMR spectra were recorded using the Varian 750
instrument only. A 1 M LiCl aqueous solution was used as an external
chemical-shift reference and set at 0 ppm. 6Li has a spin of I ) 1, low
relative abundance (7.4%), and a surprisingly long T1 but the smallest
electric quadrupole moment of any quadrupolar nuclide.14 Thus, its
spectral lines are very narrow, and it often provides higher resolution
than 7Li.14,15 This is the case here. The estimated 90° pulse for solution
6Li is 8.2 µs on the basis of the pulse length of 2H in 2H2O which has
6
7
(13) Fogg, A. M.; O’Hare, D. Chem. Mater. 1999, 11, 1771-1775.
(14) Xu, Z.; Stebbins, J. F. Solid State Nucl. Magn. Reson. 1995, 5, 103-
112.
(15) Alam, T. M.; Conzone, S.; Brow, R. K.; Boyle, T. J. J. Non-Cryst.
Solids 1999, 140-154.
(16) Mascolo, G. Thermochim. Acta 1986, 102, 67-73.

Thermal Evolution of Layered Double Hydroxide
Inorganic Chemistry, Vol. 40, No. 25, 2001 6399
Figure 1. Powder XRD patterns of LiAl2(OH)6Cl‚nH2O (LiAlCl8) and its products calcinated under the conditions indicated. The data were
collected under ambient atmospheric conditions after the samples were cooled. Miller indexes in a, g, and j are for LDH, R-LiAlO2, and LiAl5O8,
respectively. Figure a is for the vacuum-dried nonheated sample. Legend: (@) LDH, (!) gibbsite, (+) LiCl, (#) LiCl‚H2O, (O) R-LiAlO2, (|)
γ-LiAlO2, (*) LiAl5O8.
implying energetically uniform hydroxyl and chloride environ-
ments as expected from the well-ordered Al/Li arrangement.
The comparable peak for gibbsite is at about 330 °C (Figure
2b), indicating that the incorporation of LiCl into gibbsite
increases the dehydroxylation temperature by ∼25 °C and that
the LDH is slightly more stable than gibbsite, even in the
absence of a LiCl solution. The highest-temperature weight loss
actually starts near 850 °C and ends at 1050 °C. Its assignment
to the volatilization of LiCl exsolved from the LDH is well
confirmed by the XRD and 35Cl and 6,7Li MAS NMR data as
shown in the following paragraphs. LiCl has a melting point of
610 °C and a boiling point of 1383 °C.17 It readily volatilizes
in air at temperatures lower than its boiling point, as indicated
by the complete loss of LiCl salt when heated at 900 °C in an
oven for 3 h.
we describe in the following paragraphs. A feature for gibbsite
in the same temperature range is associated with a weight loss
and has been assigned to the dehydration of boehmite AlOOH,
an intermediate phase in the formation of γ-Al2O3.18 For our
LDH sample, this feature is probably not due to an impurity in
gibbsite because it is too large for the amount of gibbsite present.
XRD. The powder XRD patterns of our room temperature
LDH samples are essentially identical to those previously
published11,19 (Figure 1). The data for the calcined samples show
a significant breakdown of the LDH to both an amorphous or
poorly crystalline material and to crystalline LiCl at 300 °C
and also the formation of crystalline Li-aluminate phases at
higher temperatures. Table 1 summarizes the phases identified
at each temperature.
(18) Yudin, R. N.; Ershova, K. S.; Umnova, E. G. In Izuch. Str. FazoVogo
SostaVa Miner. Ob’ektoV Kompleksom Fiz. MetodoV Anal. Resheniya
Geol. Zadach; Solntseva, L. S., Krivokoneva, G. K., Eds.; VNII Miner
Syr’ya: Moscow, Russian Federation, 1978; pp 63-81.
(19) Sissoko, I.; Iygba, E. T.; Sahai, R.; Biloen, P. J. Solid State Chem.
1985, 60, 283-288.
The endothermic event at 540 °C is not associated with a
weight loss and is probably due to a phase transition, which
(17) Lide, D. R. CRC Handbook of Chemistry and Physics, 79th ed.; CRC
Press: Boca Raton, FL, 1998.

6400 Inorganic Chemistry, Vol. 40, No. 25, 2001
Hou and Kirkpatrick
1c). There is a broad hump from 20° to 50° 2θ and centered
near 30° 2θ because of the development of a large fraction of
an amorphous phase. Comparison to the DSC/TG data indicates
that this phase change is due to partial dehydroxylation. The
basal spacing of the residual LDH decreases to 7.20 Å, and its
(00l) peaks broaden. Broad peaks are present near 46° and 66.6°
2θ. These two peaks are characteristic of Li/Al oxides, which
we discuss later. The small peaks near 30° and 35° 2θ are due
to LiCl (JCPCD 4-664) and those at 33° and 23° 2θ to its
hydrate LiCl‚H2O (JCPCD 22-1142). Thus, the exsolution of
LiCl from the LDH structure occurs simultaneously with
dehydroxylation. The resulting LiCl is very hygroscopic17 and
easily adsorbs water from air to form the monohydrate phase.
(b) 400-600 °C. Heating at 400-500 °C for 6 h (parts d
and e of Figure 1) causes the full destruction of the layer
structure of the original LDH and the formation of additional
crystalline LiCl. The broad band from 20° to 50° 2θ decreases
in intensity with increasing temperature, and peaks near 38°,
46°, 62°, and 67° 2θ become more pronounced and sharper. It
is difficult, however, to unambiguously identify the ill-crystal-
lized phases giving rise to these reflections because the peaks
are broad and several potential Li-alumnate phases have similar
d spacings. Both LiAl5O8 and R-LiAlO2 have peaks near 38°,
46° (45.4° 2θ for R-LiAlO2 and 45.9° 2θ for LiAl5O8, as shown
in parts g-i of Figure 1), and 67° 2θ. The peak near 61° appears
to be characteristic of LiAl5O8 (511),20 and volumes with a
structure similar to it first come into existence at least by 500
°C. At 600 °C (Figure 1f), a broad peak at 19° unambiguously
representing R-LiAlO2 (JCPCD 19-713) first becomes well
resolved. The observed DSC endotherm at 540 °C (Figure 2a-
c) may be related to the formation of the R-LiAlO2 and LiAl5O8
phases, which could occur at lower temperatures during static
heating for 6 h.
(c) 720-1100 °C. All of the observed Bragg peaks in this
temperature range can be assigned unambiguously, although
there is still some broad intensity from 20° to 50° 2θ at 720
and 850 °C. The reflections at 19° 2θ representing R-LiAlO2
are well-defined, reach their maximum relative intensity at 720
°C (Figure 1g), decrease at higher temperatures, and are not
present at 1100 °C. Well-resolved reflections for LiAl5O8 at
39.5° (222) and 45.9° (400) 2θ (JCPCD 38-1425) first appear
at 720 °C (Figure 1g). The relative intensities of these reflections
increase with increasing temperature, and at 1100 °C, LiAl5O8
is the dominant phase (Figure 1j). Its (110) and (210) peaks
near 16° and 26° 2θ are superlattice reflections and signify an
ordered distribution of 1Li and 3Al over the octahedral sites of
this inverse-spinel structure.6,21 They appear here only for the
samples heated at 970 °C and above. At 1100 °C, a minor new
phase, γ-LiAlO2, with characteristic reflections at 22.3°, 33.4°,
and 34.7° 2θ appears (JCPCD 38-1464).
Figure 2. Thermal gravimetric (TG) analysis and differential scanning
calorimetry (DSC) data for the LiAl2(OH)6Cl‚nH2O sample (LiAlCl8)
and the gibbsite used to prepare it: (a) LiAlCl8; (b) gibbsite.
Table 1. Phases Identified in the Calcination of
6
LiAl2(OH)6Cl‚nH2O and Their Observed Li NMR Chemical Shiftsa,
fwhh’s, and Temperature Ranges
observed
temperature
range in
6Li MAS
chemical
shift (ppm)
Li-containing
phase
peak width,
fwhh (Hz)
this study
LiCl solution
0 (reference)
62 (static)
N/A
(1 M)
LiCl solid
LiCl‚H2O
-1.10
-0.60
36
52
300-970 °C
hydration
product
during
XRD only
LiAl2(OH)6Cl‚nH2O -0.05
53-100
71-95
<100 °C
LiAl2(OH)6Cl
amorphous
Li-Al-O-OH
R-LiAlO2
-0.07
100-300 °C
300-500 °C
-0.04 to -0.4 78-98
-0.4
-0.06
0.31
59-82
55
46-93
500-970 °C
∼1100 °C
γ-LiAlO2
LiAl5O8
500-1100 °C
a Peak maxima in MAS spectra at H0 ) 17.6 T.
NMR Spectra. (a) 27Al MAS NMR Spectra. The 27Al MAS
NMR spectra of the samples are in good agreement with the
TG/DSC and XRD data. The spectrum of a room-humidity
sample of LiAl2(OH)6Cl‚nH2O shows a narrow symmetric peak
at both high field (H0 ) 17.6 T) and lower field (H0 ) 11.74
T, data not shown), with a maximum at 8.6 ppm at H0 ) 17.6
T (Figure 3a). There are no singularities that would allow for
the determination of the quadrupolar coupling parameters. This
chemical shift is typical of six-coordinated Al (Al[6]),22 in
(a) 300 °C and Below. For the vacuum-dried, unheated
sample and the aliquot heated at 200 °C (parts a and b of Figure
1), the XRD patterns show the expected features of LiAl2-
(OH)6Cl‚nH2O with good crystallinity. Bragg peaks include
sharp and intense (00l) basal reflections at low 2θ values and
less intense (hk0) and (hkl) reflections at higher 2θ values. The
indexing of the peaks here is based on a comparison to the
reported refined patterns of a two-layer hexagonal superlat-
tice.11,19 The d spacing of the (002) reflection, which corresponds
to the thickness of a single structural layer, is 7.80 Å for the
initial wet paste, 7.56 Å for the sample dried over P2O5, and
7.42 Å for the sample heated at 200 °C for 6 h.
(20) Famery, R.; Queyeoux, F.; Gilles, E. J.-C.; Herpin, E. P. J. Solid State
Chem. 1979, 30, 257-263.
(21) Nageswara Rao, E.; Ghose, J. Mater. Res. Bull. 1986, 21, 55-60.
(22) Kirkpatrick, R. J. In Spectroscopy Methods in Mineralogy and Geology;
Hawthorne, F. C., Ed.; Mineralogical Society of America: Washington,
DC, 1988; Vol. 18, pp 341-403.
The powder XRD pattern of our 300 °C sample is signifi-
cantly different than those for lower-temperature samples (Figure

Thermal Evolution of Layered Double Hydroxide
Inorganic Chemistry, Vol. 40, No. 25, 2001 6401
Figure 5. Molar ratios of Al[6]/Al[4] as a function of the heating
temperature as determined by the curve fitting of the 17.6 T MAS NMR
spectra.
different quadrupolar products (PQ) of 2.2 and 4.7 MHz.23 These
arise from the two crystallograpically distinct Al sites in
gibbsite.24 These observations indicate that Li+ for vacancy
substitution leads to a more uniform distribution of a positive
charge within the octahedral sheet, thereby reducing the
differences among individual Al-O bond lengths and O-Al-O
bond angles. In the Reitveld refinements of powder XRD data,12
the Al-O bond lengths in the hydrated LiAl2(OH)6Cl‚nH2O
phase range only from 1.898 to 1.911 Å, and those in the
dehydrated LiAl2(OH)6Cl phase are all 1.888 Å.12 In contrast,
in gibbsite, they range from 1.862 to 1.947 Å.24 The 27Al MAS
NMR spectra of samples heated at 200 °C and below, corre-
sponding to the fully dehydrated form, are similar to those of
ambient samples (Figure 3b, only 200 °C spectrum shown),
indicating that the hydroxide-layer structure remains unchanged
in this temperature range, consistent with the XRD and TA data
discussed previously. The peak maximum is displaced only
about 1-7.6 ppm due to dehydration.
At 300 °C (Figure 3c), a low and broad resonance with a
peak maximum at 71.5 ppm representing four-coordinated Al
(Al[4])22 appears, and an additional Al[6] resonance occurs as
a shoulder at about 13 ppm to the left of the main peak at 7.8
ppm. These resonances are consistent with the XRD data,
indicating the formation of an amorphous phase at this tem-
perature along with the persistence of some LiAl2(OH)6Cl. As
is often observed for amorphous hydrous aluminate materials,25
this amorphous phase contains both Al[4] and Al[6]. No Al[5]
is observed for any of our samples at any temperature. At 400
°C, only the broad 73 and 13 ppm peaks are present, and there
is no signal for residual LDH, consistent with the XRD data.
At 600 °C, the spectrum is essentially the same as that at 400
°C except that the apparent Al[6]/Al[4] ratio determined from
peak areas in the 17.6 T MAS spectra decreases from 2.3 to
1.9 (Figure 5).
Figure 3. 27Al MAS NMR spectra (H0 ) 17.6 T) of LiAl2(OH)6Cl‚
nH2O (LiAlCl8) and its products calcinated under the conditions
indicated (ss, spin sidebands).
At 850 °C, the Al[6] and Al[4] resonances are narrower and
better resolved. Al[6] resonances with peak maxima at 17.3 and
14 ppm can be distinguished, and there is a poorly resolved
structure in the Al[4] peak. At 1100 °C, the peaks are even
narrower, the resonance at 17 ppm is no longer present, and,
additionally, there is a small peak at 79.3 ppm and a very small
shoulder at 73 ppm. From 850 to 1100 °C, the overall Al[6]/
Figure 4. 27Al MAS NMR spectra of starting gibbsite collected at (a)
H0 ) 11.7 T and (b) H0 ) 17.6 T.
agreement with the known structure of this phase.11,12 The
narrow, symmetric peak indicates that Al occupies highly or-
dered and symmetric octahedral positions. In contrast, the 27Al
spectra of the gibbsite used to synthesize the LDH are broader
(Figure 4), and previous two-dimensional multiple-quantum
27Al experiments show that gibbsite yields two resonances with
similar isotropic chemical shifts (ICSs) of 11 and 12 ppm but
(23) Ashbrook, S. E.; McManus, J.; MacKenzie, K. J. D.; Wimperis, S. J.
Phys. Chem. B 2000, 104, 6408-6416.
(24) Saalfeld, H.; Wedde, M. Z. Kristallogr., Kristallgeom., Kristallphys.,
Kristallchem. 1974, 139, 129-35.
(25) Yokoyama, T.; Nishui, K.; Torii, S.; Ikeda, Y.; Watanabe, T. J. Mater.
Res. 1997, 12, 2111-2116.

6402 Inorganic Chemistry, Vol. 40, No. 25, 2001
Hou and Kirkpatrick
Al[4] ratio decreases from 1.65 at 850 °C to 1.43 at 1100 °C
(Figure 5), indicating that Al redistribution is still occurring in
this temperature range.
The observed 27Al MAS NMR behavior reflects the structural
evolution of the phases present and is consistent with the XRD
and TA data. Dehydroxylation begins near 300 °C, and after
heating at this temperature for 6 h, a significant amount of the
OH- is lost and much of the LiCl is exsolved. Dehydroxylation
results in the formation of both Al-O tetrahedra and octahedra
but in contrast to the dehydration of aluminosilicate minerals
such as kaolinite and pyrophyllite not Al[5].26,27 At the
intermediate stages, all of the Al-containing phases are amor-
phous or poorly crystalline and, thus, yield broader peaks than
both the ambient LDH samples and the high-temperature phases.
At higher temperatures, the peak at 17 ppm at 850 °C (Figure
3f) is due to R-LiAlO2, which has a distorted NaCl-like structure
containing cross-linked sheets of alternating AlO6 and LiO6
octahedra,3 in agreement with the single Al[6] peak for it. The
other features are due to LiAl5O8 and have an Al[6]/Al[4] ratio
of ∼1.5, consistent with its structure.20,28 LiAl5O8 has an inverse-
spinel structure, with 3Al and 1Li occupying the octahedral sites
and the remaining 2Al occupying tetrahedral sites.20 The signal
in the 60-85 ppm range for the 1100 °C sample (Figure 3g)
can be best simulated with two components with ICSs, quad-
rupole coupling constants (QCCs), and quadrupolar asymmetry
parameters (η) of 82.5 ppm, 6.1 MHz, and 0.1 and of 72.6 ppm,
4.05 MHz, and 0.0, respectively. The first component is due to
γ-LiAlO2, which has a tetragonal structure containing Al[4] and
Li[6].1 The large QCC indicates that the Al tetrahedra are
significantly distorted, consistent with XRD refinement data29
and IR data.5 The second component is due to Al[4] in LiAl5O8.
The ICS and quadrupolar parameters for this phase found here
are very similar to those of Stewart et al.30 except that our ICS
is 2 ppm more positive. Previously reported 27Al MAS NMR
spectra for γ-LiAlO21,5 were interpreted to indicate the presence
of two sites, but this interpretation is inconsistent with the XRD
data that indicate a single Al site.29 The previously reported
spectra were taken at low field (7.05 T) and actually showed a
well-defined second-order quadrupolar powder pattern.
(b) 6Li and 7Li MAS NMR. The 6Li MAS NMR spectra of
all of the samples have a very high resolution, and all of the
observed spectral components are well simulated with sym-
metrical Lorenzian/Gausian peaks (Figure 6). The room-
temperature samples and those heated at 200 °C and below have
similar 6Li spectra, with peak maxima between -0.04 and -0.06
ppm (parts a and c of Figure 6) and a full width at half-height
(fwhh) of ∼90 Hz. For the ambient sample in paste form, the
resonance (Figure 6b) remains at the same frequency but
narrows to a fwhh of ∼53 Hz. These observations indicate that
the local Li structural environments are not significantly affected
by hydration state and temperature in this range, as is also
observed for Al[6]. This chemical shift is in the range expected
for Li[6],14,15 consistent with the structure of LiAl2(OH)6Cl‚
nH2O.10-12 The 300 °C sample (Figure 6d) yields this peak, a
resolvable component at -0.40 ppm, and a narrow peak with a
Figure 6. 6Li MAS NMR spectra of LiAl2(OH)6Cl‚nH2O (LiAlCl8)
and its products calcinated under the conditions indicated. Spectra l
and m are for the LiCl salts. See the text for a detailed description.
maximum at -1.10 ppm and a fwhh of 35 Hz. The latter is
readily assigned to the crystalline LiCl observed in the XRD
patterns (Figure 1) and is present at all of the temperatures up
to 970 °C, where its relative intensity decreases significantly.
The peak at -0.40 ppm must be due to Li in the amorphous
Li-aluminate phase detected by XRD and 27Al NMR. At 400
°C (Figure 6e), only a broad peak for the amorphous phase
centered at 0 ppm and the LiCl peak at -1.1 ppm occurs.
From 500 to 970 °C, all of the spectra can be well simulated
with two components with chemical shifts of 0.31 and -0.4
ppm, in addition to the peak for LiCl at -1.1 ppm. On the basis
of their variation in relative intensity with calcination temper-
ature and a comparison to the XRD data previously stated, the
(26) Lambert, J. F.; Millman, W. S.; Fripiat, J. J. J. Am. Chem. Soc. 1989,
111, 3517-3522.
(27) Fitzgerald, J. J.; Dec, S. F.; Hamza, A. I. Am. Mineral. 1989, 74, 1405-
1408.
(28) Kriens, M.; Adiwidjaja, G.; Guse, W.; Klaska, K. H.; Lathe, C.;
Saalfeld, H. Neues Jahrb. Mineral., Monatsh. 1996, 8, 344-350.
(29) Bertaut, F.; Delapalme, A.; Bassi, G.; Durif-Varambon, A.; Joubert,
J. C. Bull. Soc. Fr. Mineral. Cristallogr. 1965, 88 (1), 103-8.
(30) Stewart, F. F.; Stebbins, J. F.; Peterson, E. S.; Farnan, I.; Dunham, S.
O.; Adams, E.; Jennings, P. W. Chem. Mater. 1995, 7, 363-367.

Thermal Evolution of Layered Double Hydroxide
Inorganic Chemistry, Vol. 40, No. 25, 2001 6403
resonance at -0.4 ppm is readily assigned to R-LiAlO2 and
that at 0.31 ppm to LiAl5O8 (Table 1). Their fitted peak widths
(fwhh) decease by about a factor of 2 with an increase in the
heating temperature, paralleling their progressively improving
crystallinity observed by XRD. The line fit of the 850 °C
spectrum (Figure 6i) yields a molar ratio of LiAl5O8/R-LiAlO2/
LiCl of 3.13/1/1.27, implying a molar Al/Li ratio of 3.08 if these
are the only phases present and if they are stoichiometric. This
value is close to that from the compositional analysis, 2.85. The
1100 °C spectrum (Figure 6k) can be fairly well simulated with
two components, one at 0.31 ppm for LiAl5O8 and one at -0.06
ppm for γ-LiAlO2 (Table 1), although there still might be a
small amount of signal at -0.4 ppm from residual R-LiAlO2.
The molar ratio of LiAl5O8/γ-LiAlO2 in the 1100 °C sample
6
determined from the Li peak areas is about 3.09, yielding a
molar Al/Li ratio of 4.02 which is again in reasonably good
agreement with the compositional analyses (molar Al/Li ) 4.52).
These values are also in agreement with the simulation of the
27Al NMR spectrum of the two samples, although the precision
of this simulation is probably poorer because of peak overlap
and quadrupolar effects.
Hydrated lithium chloride, LiCl‚H2O, which was observed
in the XRD patterns, resonates at -0.6 ppm (Figure 6m) but is
not observed in the NMR samples because they were not
6
significantly exposed to air. Figure 6m shows the Li MAS
NMR spectrum of heated LiCl salt (Figure 6l) exposed to air
for ∼60 min. It contains three components representing solution-
like LiCl at 0 ppm (trace), LiCl‚H2O at -0.6 ppm, and LiCl at
1.1 ppm.
The 6Li chemical shifts indicate that essentially all of the Li
in the calcined samples retain the sixfold coordination of the
LDH. 6Li has a very small quadruple moment; thus, the observed
peak maxima are very close to the isotropic chemical shifts.14,15
Figure 7. 35Cl MAS NMR spectra of LiAl2(OH)6Cl‚nH2O (LIALC8)
and its products calcinated under the conditions indicated. The spectrum
g (top) is for the as-purchased LiCl salt.
6
The correlation between coordination number (CN) and Li
chemical shifts has been established for Li-containing silicate/
aluminosilicate14 and lithium phosphate glasses15 but may not
fully apply to aluminate systems. On the basis of the known
effects of Al, Si, and P on the chemical shifts of other cations
to which they are connected through M-O-M bonds,22 the
6Li ICSs should be slightly more positive for aluminate systems
than for aluminosilicate or phosphate systems. The isotropic
chemical shifts of 0.31 ppm for LiAl5O8, -0.06 ppm for
γ-LiAlO2, and -0.4 ppm for R-LiAlO2, all of which contain
Li in only sixfold coordination,3,20,28,30 are in the range or are
deshielded relative to the -0.3 to -1.3 and -0.7 to -1.5 ppm
chemical shift range of Li[6] in aluminosilicates14 and phosphate
glasses,15 consistent with this idea. This result implies that Li
in the amorphous phase present at 300 and 400 °C is also six-
coordinated because its peak maxima are in the range -0.04 to
-0.40 ppm.
(c) 35Cl MAS NMR Data. The 35Cl MAS NMR spectra
(Figure 7) provide important information about the Cl- sites
present in the various samples. The spectrum of the dehydrated
200 °C sample (Figure 7a) contains a single, symmetrical,
narrow peak with a maximum at 9.6 ppm, as expected for the
well-ordered structure of this phase.12 The 300 °C sample
(Figure 7b) yields this peak (maximum at 10.8 ppm) and a
broader peak with a maximum at 2.7 ppm. The latter peak is
the only one for the 400 °C sample (Figure 7c). At 600 °C
(Figure 7d), the peak is narrower, has a maximum at 4.6 ppm,
and is slightly skewed to less positive values. At 850 °C (Figure
7e), the peak is narrow and symmetrical and has a maximum
of 5.1 ppm, identical to that of crystalline LiCl (Figure 7g).
The 1100 °C sample (Figure 7f) yields no 35Cl signal, consistent
7
6
The Li MAS NMR spectra for samples heated at 400 °C
with the absence of LiCl indicated by the Li NMR data. By
6
and below are very similar to the Li spectra except that the
comparison with the XRD data, we interpret the peaks with
maxima varying from 2.7 to 5.1 ppm to be due to Cl- in LiCl
crystallites. At the lower temperatures, the peak maximum is
probably displaced to the right because of quadrupolar effects
associated with disorder or small particle size.
peaks are much broader (e.g., ∼900 Hz fwhh for the RH ) 0%
LDH sample, 568 Hz for the RH ) 75% sample, and 446 Hz
for LiCl formed from LDH (spectra not shown)). The chemical
6
shifts (peak maxima) are almost identical to those of Li (e.g.,
-1.06 ppm for 7LiCl and -0.03 ppm for room humidity LDH).
7
Thus, we do not discuss them in detail. Simulation of the Li
Structural Transformations
MAS NMR spectra of the 300 and 400 °C samples yields
6
Taken together, the compositional, XRD, TG/DSC, and NMR
data indicate the following set of transformations for the
LiAl2(OH)6Cl‚nH2O LDH phase.
(1) Dehydration (loss of surface and interlayer water mol-
ecules) via the reaction LiAl2(OH)6Cl‚nH2O f LiAl2(OH)6Cl
relative resonance intensities similar to those from Li MAS
NMR. Because the 7Li spectra were obtained with a single scan,
there was no spin saturation, and the similarity of the relative
intensities provides confidence in the relative intensities from
the 6Li data, even though the T1 values are long and unknown.

6404 Inorganic Chemistry, Vol. 40, No. 25, 2001
Hou and Kirkpatrick
+ nH2O begins near room temperature, depending on RH, and
culminates near 125 °C in our DSC/TG scans.
(2) Decomposition of the dehydrated LDH occurs via the
to LiAl5O8 at 720 °C suggests that R-LiAlO2 becomes better
ordered at a lower temperature. TG data indicate a progressive
weight loss to temperatures of above 600 °C associated with
this reorganization. This weight loss is probably due to the loss
of OH groups or water molecules. The appearance of the (110)
peak for LiAl5O8 at 970 °C is the result of the ordering of Li
and Al on the octahedral sites.
reaction LiAl2(OH)6Cl f amorphous Li1-xAl2O6-yH6-2y-z
+
xLiCls + yH2Og + zHClg beginning at temperatures of less than
300 °C. At 400 °C, most of the water and HCl loss is complete.
The XRD and NMR data indicate that the Li/Al phase is quite
amorphous at this temperature. Our DSC/TG unit is not
equipped with a mass spectrometer; thus, the volatile HCl was
not positively identified. However, without the occurrence of
redox reactions for which there is no evidence, there is no other
way to fully remove the Cl without also removing all of the Li
as LiCl. The Li/Cl ratio of the starting LDH is 1/1, and because
Li is present in the high-temperature oxides, some of the Cl
must be lost as HCl.
(5) Volatilization of LiCl begins at about 850 °C and
culminates between 1000 and 1048 °C at the 10 °C min-1
heating rate of our DSC/TG scan. Because of this delithiation,
the final products of the breakdown of the LiAl2(OH)6Cl‚nH2O
LDH phase studied here are dominated by LiAl5O8. This
contrasts with previous studies of the breakdown of LiAl2 LDHs
intercalated with OH-,1,7 NO3-,7 and CO3
for which de-
2-
8
lithiation does not occur because the volatile products are only
H2O, NOx, and CO2. For these phases, the calcination products
are dominated by LiAlO2 and have a LiAlO2/LiAl5O8 molar
ratio of 3/1.1,7,8
(6) There is a transformation of R-LiAlO2 to γ- LiAlO2
between 970 and 1100 °C. This transition is well resolved in
the XRD and NMR data and may contribute to the DSC signal
near 1000 °C.
(3) Formation of domains with the R-LiAlO2 and LiAl5O8
structures begins by at least 500 °C in the samples heated for
6 h and probably gives rise to the 540 °C peak in the DSC
6
data. This transformation is best seen in the Li NMR spectra.
These domains are probably initially small and highly disor-
dered.
(4) There is a progressive ordering and recrystallization of
the R-LiAlO2 and LiAl5O8 phases with increasing temperature.
These processes are clearly seen in the progressive narrowing
of the XRD peak widths, in the resolution of the Bragg peaks
for both of the phases at 720 °C, and in the decreased peak
widths and increased resolution in the 6Li and 27Al NMR spectra.
The better resolution of the XRD pattern for R-LiAlO2 compared
Acknowledgment. This research is supported by DOE Grant
DEFG02-00ER15028 (R.J.K., Principal Investigator). Reviews
by two anonymous reviewers substantially improved the
manuscript.
IC010671D