MIL-96, a porous aluminum trimesate 3D structure constructed from a hexagonal network of 18-membered rings and μ3-oxo-centered trinuclear units

Article abstract of DOI:10.1021/ja0621086

A new aluminum trimesate Al₁₂O(OH)₁₈(H ₂O)₃(Al₂(OH)₄)[btc] ₆·24H₂O, denominated MIL-96, was synthesized under mild hydrothermal conditions (210 °C, 24 h) in the presence of 1,3,5-benzenetricarboxylic acid (trimesic acid or H₃btc) in water. Hexagonal crystals, allowing a single-crystal XRD analysis, are grown from a mixture of trimethyl 1,3,5-benzenetricarboxylate (Me₃btc), HF, and TEOS. The MIL-96 structure exhibits a three-dimensional (3D) framework containing isolated trinuclear/<3-oxo-bridged aluminum clusters and infinite chains of AlO₄(OH)₂ and AlO₂(OH)₄ octahedra forming a honeycomb lattice based on 18-membered rings. The two types of aluminum groups are connected to each other through the trimesate species, which induce corrugated chains of aluminum octahedra, linked via μ₂-hydroxo bonds with the specific -cis-cis-trans- sequence. The 3D framework of MIL-96 reveals three types of cages. Two of them, centered at the special positions 0 0 0 and 2/3 1/3 1/4, have estimated pore volumes of 417 and 635 A³, respectively, and encapsulate free water molecules. The third one has a smaller pore volume and contains disordered aluminum octahedral species (Al(OH)₆). The solid-state NMR characterization is consistent with crystal structure and elemental and thermal analyses. The four aluminum crystallographic sites are resolved by means of ²⁷Al 3QMAS technique. This product is able to sorb both carbon dioxide and methane at room temperature (4.4 mmol·g^-1 for CO₂ and 1.95 mmol·g^-1 for CH₄ at 10 bar) and hydrogen at 77 K (1.91 wt % under 3 bar).

Full text of DOI:10.1021/ja0621086

Published on Web 07/13/2006
MIL-96, a Porous Aluminum Trimesate 3D Structure
Constructed from a Hexagonal Network of 18-Membered
Rings and µ3-Oxo-Centered Trinuclear Units
Thierry Loiseau,*,† Ludovic Lecroq,† Christophe Volkringer,† Je´roˆme Marrot,†
Ge´rard Fe´rey,†,‡ Mohamed Haouas,§ Francis Taulelle,§ Sandrine Bourrelly,|
Philip L. Llewellyn,| and Michel Latroche
Contribution from the Institut LaVoisier (UMR CNRS 8180), Institut UniVersitaire de France,
Porous Solids Group, Tectospin, UniVersite´ de Versailles Saint Quentin en YVelines, 45, aVenue
des Etats-Unis, 78035 Versailles, MADIREL, UniVersite´ de ProVence (UMR CNRS 6121),
Centre Saint Je´roˆme, 13397 Marseille Cedex 20 and LCMTR (UPR CNRS 209), 2-8, rue Henri
Dunant, 94320 Thiais, France
Received April 6, 2006; E-mail: loiseau@chimie.uvsq.fr
Abstract: A new aluminum trimesate Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6‚24H2O, denominated MIL-96, was
synthesized under mild hydrothermal conditions (210 °C, 24 h) in the presence of 1,3,5-benzenetricarboxylic
acid (trimesic acid or H3btc) in water. Hexagonal crystals, allowing a single-crystal XRD analysis, are grown
from a mixture of trimethyl 1,3,5-benzenetricarboxylate (Me3btc), HF, and TEOS. The MIL-96 structure
exhibits a three-dimensional (3D) framework containing isolated trinuclear µ3-oxo-bridged aluminum clusters
and infinite chains of AlO4(OH)2 and AlO2(OH)4 octahedra forming a honeycomb lattice based on
18-membered rings. The two types of aluminum groups are connected to each other through the trimesate
species, which induce corrugated chains of aluminum octahedra, linked via µ2-hydroxo bonds with the
specific -cis-cis-trans- sequence. The 3D framework of MIL-96 reveals three types of cages. Two of
2
1
them, centered at the special positions 0 0 0 and /3
/
1/4, have estimated pore volumes of 417 and 635
3
Å3, respectively, and encapsulate free water molecules. The third one has a smaller pore volume and
contains disordered aluminum octahedral species (Al(OH)6). The solid-state NMR characterization is
consistent with crystal structure and elemental and thermal analyses. The four aluminum crystallographic
sites are resolved by means of 27Al 3QMAS technique. This product is able to sorb both carbon dioxide
and methane at room temperature (4.4 mmol‚g-1 for CO2 and 1.95 mmol‚g-1 for CH4 at 10 bar) and hydrogen
at 77 K (1.91 wt % under 3 bar).
Introduction
Some of them possess rigid open-frameworks, which are
reminiscent to the functionalities observed in the microporous
In the past decade, there has been an increasing interest in
the synthesis of porous metal-organic framework materials
(MOF) or coordination polymers,1-3 based on the connection
of metal ions (nodes) and organic ligands (linkers), such as
amines or carboxylates, usually including one or several benzene
rings. These solids exhibit novel fascinating three-dimensional
(3D) topologies, which may potentially find applications as
molecular sieves and gas adsorbents (for instance, hydrogen or
methane storage4), ion exchange or heterogeneous catalysis.5,6
aluminosilicates networks (zeolites). The carboxylate family has
attracted more attention since extra-large pore solids with
relative high thermal stability have been synthesized. This series
is well illustrated with the examples of the MOF-n solids (MOF-
5,7 MOF-1778) by Yaghi, or MIL-1009 and MIL-10110 com-
pounds by Fe´rey. Usually, 3d divalent (Ni2+, Cu2+, Zn2+) and
trivalent (Sc3+, V3+, Cr3+, Fe3+) or rare earth metals are used
as metallic centers, but the trivalent p elements such as Al3+
,
Ga3+ or In3+ are much more rarely reported. Recently, a
systematic investigation of the reactivity of indium with
the benzene-based carboxylates was described by different
groups.11-17 The use of aluminum or gallium for the production
† Institut Lavoisier, Porous Solids Group.
‡ Institut Universitaire de France.
§ Institut Lavoisier, Tectospin.
| MADIREL, Universite´ de Provence.
LCMTR, Thiais.
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10.1021/ja0621086 CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 10223-10230
10223

A R T I C L E S
Loiseau et al.
of MOFs materials is less common, despite a rich literature18,19
relating to the chemistry of molecular aluminum carboxylate
complexes for their neurotoxic effects in biological systems or
alumoxanes species resulting from the reaction of carboxylic
acids with boehmite.20-22 On the other hand, the hydrolysis of
aluminum in aqueous solution has been intensively studied for
over 40 years.23 Besides the isolation of large purely inorganic
minum octahedral units interacting with the btc ligand. We
applied an alternative synthesis route for growing well-defined
single crystals of such MOF solids by using the methyl ester
form (trimethyl 1,3,5-benzenetricarboxylate or Me3btc) of the
trimesic acid together with HF and TEOS. The arrangement of
the metallic cation Al3+ in this particular solid is new and
original for the chemistry of aluminum since isolated µ3-oxo-
centered trinuclear units and a two-dimensional (2D) network
of hexagonal 18-membered rings are observed. The MIL-96 (Al)
compound is characterized by means of single-crystal X-ray
24-26
26,27
aggregates such as Al13
or Al30
polycations species,
novel clusters such as the Al1328 and Al429 or Al1530 species are
formed in the presence of the carboxylate ligands heidi (H3heidi
) N(CH2COOH)2(CH2CH2OH)) and hpdta (H5hpdta ) HOCH2-
[CH2N(CH2COOH)2]2), respectively.
analysis, 27Al solid-state MAS NMR and gas adsorption was
reported for CO2, CH4, and H2.
The group in Versailles focuses its attention on the reaction
of aluminum or gallium with aromatic carboxylic acids under
hydrothermal conditions. Up to now, two phases have been
isolated with aluminum, MIL-5331 (with 1,4-benzenedicarbox-
ylic acid or H2bdc) and MIL-6932 (with 2,6-naphthalenedicar-
boxylic acid or H2ndc), and one for gallium, MIL-6133 (with
1,2,4,5-benzenetetracarboxylic acid or H4btec). The MIL-53
compound was found to be a good candidate for H2,34 CH4,
Experimental Section
Synthesis. The aluminum trimesate Al12O(OH)18(H2O)3(Al2(OH)4)-
[btc]6‚24H2O (MIL-96) was hydrothermally synthesized under autog-
enous pressure from a mixture of aluminum nitrate and 1,3,5-
benzenetricarboxylic acid in water. The starting reactants were aluminum
nitrate (Al(NO3)3‚9H2O, Carlo Erba Regenti, 98%), 1,3,5-benzenetri-
carboxylic acid (C6H3(CO2H)3, Aldrich, 95%, or H3btc). In addition,
different synthesis batches used trimethyl 1,3,5-benzenetricarboxylate
(C6H3(CO2CH3)3, 98%, Aldrich, noted Me3btc), diluted hydrofluoric
acid (HF, Normapur, 4.8%) and tetraethylortho silicate (Si(OC2H5)4,
TEOS, Merck, >98%). Typically, the reaction mixture containing the
molar ratio: 1 Al(NO3)3‚9H2O (3.5 mmol, 1.314 g)/0.14 H3btc (0.5
mmol, 0.105 g)/80 H2O (278 mmol, 5 mL) was placed in a 23-mL
Teflon-lined steel Parr autoclave at 210 °C for 24 h (batch 1). The pH
of synthesis was 1. After the hydrothermal treatment, a powdered
product was obtained, which was filtered off, washed with deionized
water, and dried in air at room temperature. Optical microscope analysis
indicated that the sample is composed of a mixture of a fine white
powder (∼1 µm size) of the title compound and large parallelepiped-
shaped crystals of recrystallized H3btc (C9H6O6‚0.83H2O37 form).
Preliminary X-ray powder diffraction pattern showed that the fine
powder is a novel phase. The MIL-96 (Al) phase could be obtained as
a pure phase by using the trimethyl 1,3,5-benzenetricarboxylate as
starting reactant (batch 2). The reaction mixture was 1 Al(NO3)3‚9H2O
(3.5 mmol, 1.314 g)/0.5 Me3btc (1.75 mmol, 0.440 g)/80 H2O (278
mmol, 5 mL), and a pure phase of MIL-96 (Al) is prepared after heating
for 24 h at 210 °C under hydrothermal conditions. Suitable single-
crystals for XRD analysis were obtained from a different mixture (batch
3). In this case, the reaction molar composition was 1 Al(NO3)3‚9H2O
(3.5 mmol, 1.314 g)/0.5 Me3btc (1.75 mmol, 0.440 g)/0.4 HF (1.4 mmol,
0.6 mL)/0.2 TEOS (0.7 mmol, 0.16 mL)/320 H2O (1111 mmol, 20
mL). HF38 and TEOS39 are used for their mineralizing effect under
hydrothermal conditions, inducing the increased crystallinity. These
reactants were placed in a 125-mL Teflon-lined steel Parr autoclave at
210 °C for 24 h, and flat hexagonal-shape crystals of 10-40 µm size
were obtained in the absence of recrystallized H3btc.
35
and CO2 adsorption, and significant uptake values were
observed for these different gases, partly due to the low density
of these materials with aluminum being a relatively light
element. We continue these series with the utilization of the
trimesic acid ligand (1,3,5-benzenetricarboxylic acid or H3btc).
Under some conditions, a ladder-like pure fluoride aluminum
([Al2F8]2-)n species36 intercalated by the triprotonated H3btc
molecules was previously obtained with the HF/pyridine solvent
mixture. Here, this contribution addresses the synthesis and
characterization of a new 3D framework, noted MIL-96 (Al),
Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6‚24H2O involving the alu-
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Single-Crystal X-ray Structure Analysis. A colorless hexagonal-
shaped crystal (0.04 mm × 0.04 mm × 0.04 mm) was selected under
polarizing optical microscope and glued on a glass fiber for a single-
crystal X-ray diffraction experiment. X-ray intensity data were collected
on a Bruker ×8-APEX2 CCD area-detector diffractometer using Mo
KR radiation (λ ) 0.71073 Å). Four sets of narrow data frames (60 s
per frame) were collected at different values of θ for 2 and 2 initial
values of φ and ω, respectively, using 0.3° increments of φ or ω. Data
reduction was accomplished using SAINT V7.03. The substantial
redundancy in data allowed a semiempirical absorption correction
(SADABS V2.10) to be applied, on the basis of multiple measurements
of equivalent reflections. The structure was solved in the space group
P63/mmc by direct methods, developed by successive difference Fourier
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795.
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9
10224 J. AM. CHEM. SOC. VOL. 128, NO. 31, 2006

Construction of a New Aluminum Trimesate Compound
A R T I C L E S
Table 1. Crystal Data and Structure Refinement for
Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6‚24H2O (MIL-96)
Solid-State NMR. All spectra were collected with a Bruker Avance
500 spectrometer using a 2.5-mm triple resonance MAS probe at room
temperature. Powder samples were packed into 2.5-mm ZrO2 rotors.
27Al (130.3 MHz) spectra were referenced to external alum ((NH)4Al-
(SO4)2‚12H2O) at -0.6 ppm relative to 1 M aqueous Al(NO3)3 as
secondary reference. The radio frequency field strength was determined
from nutation experiments on both cubic solid alum (liquidlike behavior)
and solid nutation of berlinite AlPO4. The single-pulse 27Al spectrum
was collected at a spinning rate of 30 kHz. The experiment was recorded
after excitation with π/12 pulses of 86 kHz rf power and repetition
time of ∼100 ms. The 3QMAS 2D NMR experiment40,41 was achieved
at a spinning rate of 25 kHz using a z-filter sequence42 with two hard
pulses of 4.1 µs (127°) and 1.6 µs (50°) and one selective pulse of
12.2 µs (90°). Additional 3QMAS experiment with rf pulses of higher
power (∼170 kHz) and fast amplitude modulation (FAM-II) method
for conversion of observable coherence was run to optimize MQMAS
efficiency for sites with larger quadrupolar interactions.43 Simulations
of spectra were done with the Bruker dmFit software package.
Adsorption Microcalorimetry. The apparatus used for microcalo-
rimetry experiments is able to measure both isotherms and experimental
enthalpies of adsorption in the temperature region from 25 to 150 °C.44
The Tian-Calvet type microcalorimeter used to measure the enthalpies
consists of two thermopiles mounted in electrical opposition. Each
thermopile comprises ∼500 chromel-alumel thermocouples. The
adsorption isotherms are obtained using a manometric device built to
withstand pressures up to 100 bar. The pressure gauge measures
pressures up to 50 bar. A point-by-point introduction of gas is most
adapted to this system. Each introduction of adsorbate to the sample is
accompanied by an exothermic thermal effect, until equilibrium is
attained. This peak in the curve of energy with time has to be integrated
to provide an integral (or pseudodifferential) molar enthalpy of
adsorption for each dose. The calorimetric cell (including the relevant
amounts of adsorbent and gas) is considered as an open system. In
this procedure, it is important to consider that the gas is introduced
reversibly. Under these conditions it is possible to determine the
differential enthalpy of adsorption ∆adsh, via the following expression:
identification code
empirical formula
formula weight
MIL96 (Al)
C6H2Al1.56O7.33
233.38
temperature
293(2) K
wavelength
0.71073 Å
crystal system, space group
unit cell dimensions
hexagonal, P63/mmc
a ) 14.2074(2) Å
b ) 14.2074(2) Å
c ) 31.2302(9) Å, γ ) 120°
5459.27(19) Å3
18, 1.278 Mg/m3
0.219 mm-1
volume
Z, calculated density
absorption coefficient
F(000)
2104
crystal size
θ range for data collection
limiting indices
0.04 × 0.04 × 0.04 mm
1.30-24.99°
-16 e h e 16
-16 e k e 16
-37 e l e 34
65893/1858
reflections collected/unique
[R(int) ) 0.0910]
100.0%
completeness to θ ) 24.99
refinement method
full-matrix least-squares on F2
1858/125
data/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I )]
R indices (all data)
1.139
R1 ) 0.0509, wR2 ) 0.1578
R1 ) 0.0717, wR2 ) 0.1807
0.930 and -0.856 e‚Å-3
largest diff. peak and hole
syntheses, and refined by full-matrix least-squares on all F2 data using
SHELXTL V6.12. Hydrogen atoms were included in calculated
positions and allowed to ride on their parent atoms. Three unique
aluminum atoms (Al1, Al2, and Al3) were first revealed, and the
remaining atoms (O, C, N) were placed from successive Fourier map
analyses. At this stage, electron density residues (∼8 e-‚Å3) were
observed nearby the terminal atoms O5 (bonded to Al2) at ∼1.80 Å,
suggesting the presence of an additional aluminum atom on the 12k
site (Al4). However, due to its steric hindrance a partial occupancy
was assumed for this aluminum and it was fixed to 1/3 in agreement
with the balance of the electronic charges of the structure. Electron
residues assigned to the oxygen atoms O11, O12, and O13 (with
occupancy fixed to 1/3) are found in the surrounding of Al4 with
unrealistically short Al-O distances, and this would reflect average
position due to the aluminum disorder. The hydrogen atoms were placed
using geometrical constraints for the trimesate species. The population
ratio between these four aluminum sites Al1:Al2:Al3:Al4 was as
follows: 3:6:3:2, respectively. The final refinement including aniso-
tropic thermal parameters of all non-hydrogen atoms converged to R1
) 0.0509 and wR2 ) 0.1578 All the calculations were performed using
the SHELX-TL program on the basis of F2. The crystal data are given
in Table 1. The chemical formula deduced from the X-ray diffraction
analysis is the following: Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6‚2H2O.
Thermogravimetric Analysis. The thermogravimetric analysis
(under O2, 2 °C‚min-1, TA Instrument 2050) of the pure phase of MIL-
96 (batch 2) shows a two-step weight loss. The first event is assigned
to the continuous removal of water (free molecules trapped within the
cavities and water bonded to aluminum atoms). It corresponds to 19%
at 200 °C and ∼24 free water molecules per Al12 unit. The second
weight loss is attributed to the decomposition of the structure with the
departure of the trimesate species at 300 up to 580 °C (obs: 50.5%;
calc: 50.8%). The final residue is Al2O3 with a remaining weight of
30.5% of the original value (calc: 28.9%). The deduced chemical
formula for MIL-96 (Al) is Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6‚24H2O.
The X-ray thermodiffraction experiment (in air) indicated that the MIL-
96 structure is stable upon water removal and collapsed at 320 °C.
The chemical analyses gave 25.5% (calc: 26.0%) for C and 3.1%
(calc: 3.7%) for H and are in good agreement with the chemical formula
of MIL-96.
dQrev
dna
dp
∆
adsh )
+ Vc
(
)
( )
T
dna
T
Here dQrev is the heat reversibly exchanged with the surrounding
environment at temperature T, as measured by the calorimeter, dna is
the amount adsorbed after introduction of the gas dose, dp is the increase
in pressure and Vc is the dead space volume of the sample cell within
the calorimeter itself (thermopile). The term Vcdp can be obtained via
blank experiments. Prior to each adsorption experiment, the samples
were outgassed using sample controlled thermal analysis, SCTA.45,46
Around 0.3 g of each sample was thus heated under a constant residual
vacuum pressure of 0.02 mbar up to a final temperature of 100 °C
which was maintained until the residual pressure was less than 5 ×
10-3 mbar. The carbon dioxide and methane were obtained form Air
Liquide (Alphagaz, France) and are of 99.998% purity. These green-
house gases have been chosen with respect to their interest in several
applications. From a more fundamental point of view, a comparison
between a probe molecule with a significant quadrupole moment and
one without any permanent moment can be beneficial for the
characterization of adsorbents.
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9
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A R T I C L E S
Loiseau et al.
Figure 2. View of the µ3-oxo-centered trinuclear Al3(µ3-O)[(O2C-C6H3-
(CO2)2]6(H2O)3 core in MIL-96, showing the three AlO5(H2O) octahedral
units (site Al1) chelated by the trimesate groups (black circles: carbon;
open circles: hydrogen; gray circles: oxygen).
Figure 1. Projection of the structure of MIL-96 (Al) along the c axis,
showing the hexagonal network of the aluminum octahedra (gray) containing
18-membered rings connected to the µ3-oxo-centered trinuclear units, via
the trimesate ligand. Water molecules: black.
moiety with the aluminum octahedra AlO5(H2O), corner-sharing
a µ3-oxo centered group (corresponding to the crystallographic
site Al1 (6h)). The four equatorial Al-O distances are 1.889-
(2) Å while the Al-µ3-O is shorter (1.834(2) Å). The remaining
oxygen atom belongs to a coordinated water molecule, which
is trans to the Al-µ3-O bond. It corresponds to a longer Al-O
distance (1.945(5) Å), and this is in good agreement with valence
bond calculations52 (expected value for H2O: 0.4, calculated:
0.45). This type of trimeric oxo-centered carboxylate-bridged
cluster formulated as [M3O(O2CR)6L3]+ (L ) H2O, N, ...) is
quite well-known for many trivalent transition metals such as
V, Cr, Mn, Fe, Co, Ru, Rh, Ir, and Pt.53 Most of these phases
have been isolated using the acetate species, but more recently,
such triangular metal-centered units have been encountered in
phases crystallizing with aromatic polycarboxylates.9,10,54-57
However, to our knowledge, this is the first time that such a
trinuclear configuration is observed in aluminum chemistry.
Indeed, only one trimeric complex was reported with aluminum.
It is a cluster stabilized by citric acid58,59 exhibiting a different
configuration; it consists of two aluminum octahedra sharing
an edge (µ2-OH) connected to a third aluminum octahedron by
corner µ2-O bridges. On the other hand, trimer of aluminum
can be extracted from the structural description of the brucite-
Hydrogen Storage Measurements. Hydrogen storage measurements
were made using a volumetric device (Sievert’s method) equipped with
calibrated and thermalized volumes and pressure gauges. About 1 g of
powder was transferred into a tight stainless steel sample holder. The
sample was then outgassed at 150 °C during 18 h under primary
vacuum. At the end of the experiment, the sample holder was transferred
into a glovebox under purified argon and weighed again to measure
the mass loss from the dehydrated sample. All weight capacities refer
here to outgassed samples. For adsorption measurements at 77 K, the
sample holder was immersed in liquid nitrogen and high-purity
hydrogen (Alphagaz H22) was introduced step by step in the container
up to 0.41 MPa. The pressure variations due to both gas cooling and
hydrogen adsorption were measured as a function of time. Under these
thermodynamic conditions (0 < P < 1 MPa; T ) 77 K), the ideal gas
law is no longer valid, and a different equation of state was used for
this pressure and temperature range.47
Results and Discussion
Structure Description. The MIL-96 (Al) phase exhibits a
3D framework (Figure 1) consisting of the connection of
octahedrally coordinated aluminum, which are linked through
the trimesate ligand, [btc]3-. The presence of the [btc]3- ligand
in the structure indicates that the trimethyl 1,3,5-benzenetricar-
boxylate ester is hydrolyzed into the carboxylate form, which
further reacted with the aluminum cations under hydrothermal
conditions. Here, the use of the methyl ester form of the
carboxylic acid has a drastical effect on the synthesis conditions
since no recrystallization of the trimesic acid is observed and a
pure form of the MIL-96 (Al) is obtained. The method of the
in situ carboxylate ligand synthesis was previously reported for
the formation of 3D coordination polymers, and it was reported
that the hydrothermal in situ slow hydrolysis reaction improved
the crystallization process for the crystal engineering of such
compounds.48,49 Using this strategy, analogous compounds of
MIL-96 have been prepared with gallium50 and indium51 and
will be discussed elsewhere.
25
24
like polycation flat-Al13 or the Keggin-type species ꢀ-Al13
26
and δ-Al13 for which each face of the motif is composed of
three aluminum octahedral sharing edges.
The second inorganic part consists of a 2D network containing
chains of aluminum AlO2(OH)4 and AlO4(OH)2 octahedra,
interconnected with each other to generate a hexagonal 18-
membered ring in the (a,b) plane (Figure 1). There are two
crystallographically inequivalent types of aluminum, Al2 and
Al3, which are respectively coordinated to four and two oxygen
atoms coming from the carboxylate groups of the trimesate
molecule and four and two bridging µ2-hydroxo moieties: the
(52) Brese, N. E.; O’Keeffe, M. Acta Crystallogr. B 1991, 47, 192.
(53) Cotton, F. A.; Wilkinson, G. in “AdVanced Inorganic Chemistry”, Fifth
Edition, John Wiley & Sons: 1988.
(54) Barthelet, K.; Riou, D.; Ferey, G. Chem. Commun. 2002, 1492.
(55) Serre, C.; Millange, F.; Surble´, S.; Fe´rey, G. Ang. Chem., Int. Ed. 2004,
43, 6285.
Two distinct inorganic blocks have been found in the MIL-
96 framework. The first one (Figure 2) is a discrete trinuclear
(47) Younglove, B. A. J. Phys. Chem. Ref. Data 1982, 11, 1.
(48) Evans, O. R.; Lin, W. Acc. Chem. Res. 2002, 35, 511.
(49) Zhang, X.-M. Coord. Chem. ReV. 2005, 249, 1201.
(50) Volkringer, C.; Loiseau, T.; Fe´rey, G.; Morais, C.; Taulelle, F.; Montouil-
lout, V.; Massiot, D. 2006, in preparation.
(56) Sudik, A. C.; Coˆte´, A. P.; Yaghi, O. M. Inorg. Chem. 2005, 44, 2998.
(57) Fe´rey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F. Acc. Chem. Res.
2005, 38, 217.
(58) Feng, T. L.; Gurian, P. L.; Healy, M. D.; Barron, A. R. Inorg. Chem. 1990,
29, 408.
(51) Volkringer, C.; Loiseau, T. Mater. Res. Bull. 2006, 41, 948.
(59) Malone, S. A.; Cooper, P.; Heath, S. L. Dalton Trans. 2003, 4572.
9
10226 J. AM. CHEM. SOC. VOL. 128, NO. 31, 2006

Construction of a New Aluminum Trimesate Compound
A R T I C L E S
Figure 3. View of the corrugated chains of corner-sharing aluminum
octahedral, with the -cis-cis-trans sequence corresponding to the -Al2-
OH-Al2-OH-Al3- bonding scheme, in MIL-96. The hydrogen-bond
interaction scheme is shown between the water molecule and the µ2-OH
groups bridging the aluminum atoms Al2 and Al3.
Figure 4. View of the cavities A and B in MIL-96. The light gray spheres
(centered on position 0 0 0 for A and /3
space of the cage.
two oxo groups are in cis positions for Al2, whereas the hydroxo
groups are in trans positions for Al3, with typical Al-O
distances. The lengths of the Al2-OH bonds are in the range
1.837(1)-1.893(1) Å, and the Al2-O bond is longer at 1.906-
(2) Å (Al2-O8)). The Al3-O6 distances correspond to the
value of 1.862(1) Å, whereas the length of the Al3-OH bond
is shorter at 1.836(1) Å (Al3-O2). The hydroxo groups are in
a bridging position between two aluminum atoms. The valence
bond calculations52 give values in the range 1.189-1.210 for
oxygen atoms bridging aluminum, which are close to the
expected value (1.2) for hydroxo species. This µ2-OH connection
generates original corrugated infinite chains of aluminum
octahedra, with a -cis-cis-trans- corner-sharing sequence
(Figure 3). The sinusoidal shape of the octahedra file is induced
by the ternary geometry of the trimesate molecule. Such a
configuration was already reported in other metal carboxylates
synthesized with the same tribenzoic acid. For instance in a
cobalt-based solid,60 infinite chains of edge- and corner-sharing
octahedra occur. In MIL-96, the particular corner-sharing
octahedra connection mode differs from those previously
observed in the tancoite-like solids61,62 (straight chains with trans
corner-sharing linkage [see for instance the MIL-5331/MIL-6932
topologies]) or KTP-like materials63,64 (zigzag chains with -cis-
trans- corner-sharing linkage). However, an identical arrange-
ment of such corrugated corner-sharing chains was recently
encountered in the titanium phosphate, Ti3P6O27‚5(C2H10N2)‚
2H3O,65 containing a 1D chiral chain. A further aspect is the
corner-sharing octahedra connection observed in MIL-96, which
is in contrast with the edge-sharing mode occurring in the
/
1/4 for B) indicates the empty
3
2
1
and the structure can be described from the hexagonal network
of 18 aluminum centers.
The cohesion of the structure is ensured by the connection
of the oxo-centered trinuclear aluminum units with the 2D
network of aluminum octahedra through the trimesate molecules.
This results in a 3D framework with the compact packing of
three types of cavities. The first one (noted A, Figure 4) is
delimited by two trinuclear units along the c axis connected to
the 18-membered ring in the (a,b) plane via six btc ligands and
is centered on the special position 0 0 0 (2a). The corresponding
cavity-free diameter is estimated to be 8.8 Å (based on the ionic
radius of 1.35 Å for oxygen) with a pore volume of ∼420 Å3
(calculations using the PLATON69 software, option CALC
SOLV). A water molecule was clearly located within the period
of one aluminum sinusoidal chain, bordered by one trimesate
motif. This molecule is hydrogen-bonded to two bridging µ2-
OH groups with short H2O‚‚‚OH distances (2.917(1) Å). A
second cage (noted B, Figure 4) is located at the special position
2
1
/
3
/
3
1/4 (2d). It is delimited by the three trimeric µ3-oxo-bridged
aluminum units in the (a,b) plane connected to two triangular
units composed of aluminum octahedra Al2 and Al3 via six
btc species. It results in an elongated cavity along the c axis
with a larger pore volume (∼635 Å3).69 For both cages A and
B, the XRD analysis does not indicate any significant electronic
residue, which might be assigned to trapped species such as
water for instance, as observed by TG analysis. Thus, this empty
space could be occupied by water molecules with a static or
motional disorder, which is hardly observable by the XRD
technique. The pore-opening diameters of these large cavities
are rather small and were estimated in the range 2.5-3.5 Å
(based on the ionic radius of 1.35 Å for oxygen). The third
molecular species,19 large polycationic forms such as Al8,66
Al13,24,25 Al15,30 or Al30
or dense infinite networks such as
26,27
boehmite or gibbsite.67 This situation is also reported in the
aluminum borate PKU-1,68 which exhibits a porous framework
with 18-membered ring channels. In this compound, all the AlO6
octahedra are connected to each other through a common edge,
1
cavity (noted C, Figure 5) is centered around the position /3
2
/
1/4 (2c) and situated between three trinuclear units in the
3
(60) Livage, C.; Guillou, N.; Marrot, J.; Fe´rey, G. Chem. Mater. 2001, 13, 4387.
(61) Hawthorne, F. C. Tschermaks Mineral. Petrogr. Mitt. 1983, 31, 121.
(62) Attfield, M. P.; Morris, R. E.; Burshtein, I.; Campana, C. F.; Cheetham,
A. K. J. Solid State Chem. 1995, 118, 412.
(63) Tordjman, I.; Masse, R.; Guitel, J. C. Z. Kristallogr. 1974, 139, 103.
(64) Kirkby, S. J.; Lough, A. J.; Ozin, G. A. Z. Kristallogr. 1995, 210, 956.
(65) Guo, Y.; Shi, Z.; Yu, J.; Wang, J.; Liu, Y.; Bai, N.; Pang, W. Chem. Mater.
2001, 13, 203.
(66) Casey, W. H.; Olmstead, M. M.; Phillips, B. L. Inorg. Chem. 2005, 44,
4888.
(67) Euzen, P.; Raybaud, P.; Krokidis, X.; Toulhoat, H.; Le Loarer, J.-L.; Jolivet,
J.-P.; Froidefond, C. in “Handbook of Porous Solids”, Schu¨th, F., Sing,
K., Weitkamp, J. Eds.; Wiley-VCH Verlag GmbH: Weinheim 2002, 1591.
(68) Ju, J.; Lin, J.; Li, G.; Yang, T.; Li, H.; Liao, F.; Loong, C.-K.; You, L.
Ang. Chem., Int. Ed. 2003, 42, 5607.
(a,b) plane and two trimeric motifs containing three AlO2(OH)4
octahedra along the c axis. Each of these trimers corresponds
to one of the hexagonal nodes of the 18-membered ring of
aluminum octahedra. In fact, the Fourier map examination
showed the positions of several residual peaks, with distances
of 1.789 Å, suggesting the presence of an additional aluminum
atom, noted Al4. This latter is surrounded by three residues
(noted O11, O12, and O13) with very short unrealistic distances
(69) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.
9
J. AM. CHEM. SOC. VOL. 128, NO. 31, 2006 10227

A R T I C L E S
Loiseau et al.
Figure 5. View of the cavity C with the additional aluminum species (Al4,
pale gray) disordered on the different sites and surrounded by the different
oxygen atoms (O11, O12, and O13, dark gray). (a) view along the (b,c)
planes; (b) view along the c axis.
(1.246-1.425 Å) for Al-O, but it could be assigned to a
disordering situation, for which Al is assumed to occupy the
1
Al4 site with a factor of /3. Indeed, within the cavity, the
Figure 6. Experimental (a) and simulated (b) 2D 27Al 3QMAS NMR
spectrum of Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6‚24H2O (MIL-96). Four
components are simulated in (b) MQMAS and slices. In (b) Al1 site, very
broad, is drawn in gray with amplified intensity for sake of visualization.
Its low intensity is due to the lack of efficiency of the excitation/reconversion
of the triple-quantum coherences for this large QCC. Its quadrupolar
parameters were estimated from an additional MQMAS experiment
optimized for higher QCC, degrading consequently the low QCC sites (see
Supporting Information) and from the MAS simulation (see Figure 7).
distances between the residues are quite short (2.7 Å), and two
aluminum atoms on the Al4 site could not be face to face with
each other because of the electrostatic repulsion. This involves
the shift of the aluminum Al4 on one site over the three possible
ones of the special position 12k. In accordance with the
electroneutrality of the structure, the aluminum species Al4
would be surrounded by six hydroxo ligands. With the presence
of the additional aluminum atom Al4, no empty space occurs
in this third cavity.
NMR Investigations. The 27Al MAS NMR spectrum of
Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6‚24H2O (MIL-96) exhibited
overlapped signals in the chemical shift range (from 20 to -40
ppm) for which aluminum is in an octahedral environment.70
This is in perfect agreement with the XRD structure determi-
nation, which indicated that all aluminum atoms were exclu-
sively located on hexacoordinated sites. To gain more spectral
resolution, MQMAS spectroscopy has been applied. This
technique combines MAS and multiquantum spectroscopy that
can remove completely the second-order quadrupolar interac-
tion.71 Two experiments with different conditions and methods
were performed. The first using the classical z-filter three-pulse
sequence with moderate rf power (∼85 kHz) yielded three well-
resolved signals with pure-absorption line shapes, shown in
Figure 6. The 2D map is analyzed by modeling the quadrupolar
line shape of the observed experimental signals. The second
experiment used fast amplitude modulation (FAM) pulse
sequence under strong rf power (∼170 kHz) conditions.
Significantly better sensitivity was afforded by the FAM
technique enabling evidence of a fourth site with much larger
quadrupolar coupling constant (see Supporting Information). The
efficiency of multiple-quantum excitation and conversion is
strongly dependent on the quadrupolar frequency (νQ), resulting
in spectra with intensities that may not quantitatively represent
the relative site population. However, the resulting anisotropic
line shapes that were severely distorted may be a result of the
use of strong multiple-pulse rf irradiation. In this case, the lack
of a full powder pattern prevented obtaining the quadrupolar
parameters accurately via numerical fitting of the anisotropic
line shape. It was thus possible to extract meaningful quadrupole
parameters by the two combined MQMAS methods, which were
used as a starting point for the simulation of the 1D MAS NMR
Figure 7. Experimental (red) and simulated (black) single-pulse 27Al NMR
spectra of Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6‚24H2O (MIL-96). The four
components used in the decomposition are shown.
spectrum to obtain site occupancy (Figure 7). Therefore, 27Al
NMR resolved four distinct lines, which account for the four
crystallographic sites Al1, Al2, Al3, and Al4.
The single-pulse 1D spectrum obtained with 30 kHz MAS
(Figure 7) can be very well simulated with virtually identical
δCS, e2qQ/h, and η values used to simulate the MQMAS spectra.
Because the resolution is better, the MQMAS results provide
the most meaningful values for the second-order quadrupolar
powder parameters. Table 2 summarizes data of quadrupolar
patterns fits from 3QMAS experiments as well as results of
MAS spectrum fits. Because of the highly contrasted NMR
parameters, the structural NMR assignment of the aluminum
sites can easily be done. It is based on a prior knowledge of the
structural description. The signal, which has the smallest
quadrupole-coupling constant, is assigned to Al4 site. Indeed,
this aluminum is only partially attached to the framework, and
due to its flexibility within the structure, the corresponding
quadrupolar interaction would be consequently reduced. The
signal with the largest quadrupole-coupling constant, which
indicates the highest distortion of the Al complexes, is assigned
to Al1 site of the trimeric cluster. These aluminum atoms have
fewer regular Al-O distance distributions than the other Al sites
due to the presence of Al-µ3-O bonds. Moreover, the coordina-
tion of water molecules to these trimeric cluster aluminums
(70) Alemany, L. B.; Kirker, G. W. J. Am. Chem. Soc. 1986, 108, 6158.
(71) Frydman, L.; Harwood: J. S. J. Am. Chem. Soc. 1995, 117, 8367.
9
10228 J. AM. CHEM. SOC. VOL. 128, NO. 31, 2006

Construction of a New Aluminum Trimesate Compound
A R T I C L E S
Figure 8. Isotherms (left) and differential enthalpies (right) obtained for the adsorption of carbon dioxide and methane on MIL-96 (Al) at 30 °C.
1.95 mmol‚g-1 for CH4) is below many materials including
Table 2. Data of Al Sites Present in Al12O(OH)18(H2O)3(Al2(OH)4)
[btc]6‚24H2O (MIL-96) from Simulation of the 27Al MAS NMR
zeolites,72-75 activated carbons,76-78 and other MOF35,79 materi-
Spectra Obtained in Both 3QMAS (Figure 6) and Single Pulse
als.
The initial enthalpies of adsorption of carbon dioxide are very
Experiments (Figure 7)
d
NMR experiment
site
intensitya
QCCb
ηc
δiso
similar to those already observed for previous experiments on
MOF samples35 where initial values were obtained of 35
kJ‚mol-1 for MIL-53 (Al) and 32 kJ‚mol-1 for MIL-53 (Cr).
These adsorption enthalpies observed are significantly lower
than those obtained with aluminosilicate zeolites72,73 (50-70
kJ‚mol-1) and in the same range as those observed for pure
silica zeolites such as silicalite.72 This highlights the relative
electronegativity of the MIL-96 framework as the quadrupole
moment of the CO2 molecule would engender stronger interac-
tions, closer to those observed with the aluminosolicate zeolites.
The initial enthalpies of adsorption observed for methane on
MIL-96 are similar to those previously reported for MIL-53
(17 kJ‚mol-1) as well as for the adsorption of methane in the
aluminosilicate zeolite NaY75,78 (17-17.8 kJ‚mol-1). These
values are all quite similar, emphasizing that the methane
molecule, having no permanent moment, is not affected by
framework charge. One would expect differences due to strong
confinement effects which do not seem to be the case.
Following the return to the baseline of the calorimetric signal
gives an indication of the adsorption equilibrium. For the
adsorption of carbon dioxide, complete adsorption equilibrium
takes ∼1 h for each point. It is noteworthy that for methane
adsorption, >5 h were required for the equilibrium of each point.
For most porous solids, methane equilibrium is attained after
20 min per point. Such a long time to reach equilibrium is the
first time, to our knowledge, that such behavior is observed and
should be compared to the hydrogen experiments. Furthermore,
the adsorption at room temperature can be compared with
nitrogen adsorption at 77 K, and interestingly, almost no uptake
3QMAS
Al1
Al2
Al3
Al4
Al1
Al2
Al3
Al4
-
-
7.9
4.2
2.8
1.7
8.2
4.7
4.0
1.9
0.49
0.03
0.30
0.56
0.49
0.03
0.30
0.56
3.7
2.2
0.3
6.5
3.1
2.4
1.8
6.4
-
-
single pulse
23%
45%
22%
10%
a Accuracy: (4%. b Quadrupole coupling constant QCC ) (e2qQ)/h,
with the z-component, Vzz ) eq, of the electric field gradient, the electric
quadrupole moment, eQ, of 27Al nuclei, and Planck’s constant, h. Accuracy:
(0.6 MHz. c Asymmetry parameter η ) (Vxx - Vyy)/Vzz as a function of
the x-, y-, and z-components of the electric field gradient. Accuracy: (0.03.
d Isotropic chemical shift. Accuracy: (0.8 ppm.
contributes to more disorder within their local environments.
The most intense signal is attributed to the Al2 site since this
aluminum is the most abundant crystallographic site. Finally,
the Al3 site would present a relatively moderate quadrupole
coupling constant as it is sitting on a special crystallographic
position. According to these signal attributions, line integrals
agree with the crystallographic stoichiometry of Al12O(OH)18-
(H2O)3(Al2(OH)4)[btc]6‚24H2O. In the chemical formula of
MIL-96, 14 aluminums are associated to four different sites:
Al1:A2:Al3:Al4, respecting the ratio 3:6:3:2, respectively. The
corresponding ratio determined by simulation of the NMR
spectrum was as follows: 3.2:6.3:3.1:1.4, very close to the
expected values. Nevertheless, the amount of the Al4 appears
somewhat underestimated, presumably due to the fact that the
stoichiometry of this site within the structure varies from a
sample to sample. Indeed, its relative NMR signal intensity
changed significantly for different samples. Since XRD and
NMR analyses have been done on two samples from different
batches, the stoichiometry of Al4 site could be different.
CO2 and CH4 Adsorption Microcalorimetry. The dif-
ferential enthalpies and isotherms at 30 °C for the adsorption
of carbon dioxide and methane are shown in Figure 8,
respectively. The adsorption of carbon dioxide occurs essentially
below 10 bar with an enthalpy of around 33 kJ‚mol-1. The
adsorption isotherm of methane is far more linear with an initial
interaction of 16 kJ‚mol-1 which rapidly drops. The amount
adsorbed for each gas at 10 bar (4.4 mmol‚g-1 for CO2 and
(72) Dunne, J. A.; Mariwala, R.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A. L.
Langmuir 1996, 12, 5888.
(73) Dunne, J. A.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A. L. Langmuir
1996, 12, 5896.
(74) Mentasy, L.; Woestyn, A. M.; Basaldella, E.; Kikot, A.; Zgrablich, G. Ads.
Sci. Techn. 1994, 11, 209.
(75) Papp, H.; Hinsen, W.; Do, N. T.; Baerns, M. Thermochim. Acta 1984, 82,
137.
(76) Buss, E. Gas Separation & Purification 1995, 9, 189.
(77) Guillot, A.; Follin, S.; Poujardieu, L. Royal Soc. Chem. - Characterisation
of Porous Solids 1997, 213, 573.
(78) Himeno, S.; Komatsu, T.; Fujita, S. J. Chem. Eng. Data 2005, 50, 369.
(79) Wang, Q. M.; Shen, D.; Bu¨low, M.; Lau, M. L.; Deng, S.; Fitch, F. R.;
Lemcoff, N. O.; Semanscin, J. Microporous Mesoporous Mater. 2002, 55,
217.
9
J. AM. CHEM. SOC. VOL. 128, NO. 31, 2006 10229

A R T I C L E S
Loiseau et al.
53 (3.8% under 1.6 MPa)34 or some metal-organic framework
compounds under higher pressures.80-82 Nevertheless, it is
similar to the storage capacity of other microporous MOF solids
such as Ni(dhtp)(H2O)2‚8H2O,83 Prussian blue analogues,84 or
the NaY zeolite.85
Conclusions
Hydrothermal reaction of aluminum nitrate with 1,3,5-
benzenetricarboxylic acid (or the methyl ester form, trimethyl
1,3,5-benzenetricarboxylate) in water led to a new aluminum
trimesate compound: Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6‚
24H2O. The structure, solved by means of single-crystal X-ray
diffraction, consists of a 3D framework containing isolated tri-
nuclear µ3-oxo-bridged aluminum clusters and infinite chains
of AlO4(OH)2 octahedra forming a layerlike hexagonal network
based on an 18-membered ring. As far as we are aware, this is
the first time that such a µ3-oxo-centered trinuclear configuration
is observed in aluminum chemistry. The organic molecules
interact strongly with both the oxo-centered trimeric aluminum
units and the 2D network of aluminum octahedra ensuring the
cohesion of the structure. Solid-state NMR characterization
agrees well with the structure description. The four crystal-
lographic Al sites are resolved in 27Al spectra using 3QMAS.
The compound exhibits a hydrogen storage capacity up to 1.91
wt % at 77 K under 3 bar and is able to sorb carbon dioxide
(4.4 mmol‚g-1 at 10 bar) and methane (1.95 mmol‚g-1 at 10
bar).
The remarkable feature is the occurrence of the µ3-oxo-
centered trinuclear aluminum motif in the structure of MIL-96.
Such a trimeric building block was previously described in very
promising MOF materials based on super-tetrahedral units for
the construction of zeolite-type frameworks with very large
pores in MIL-1009 and MIL-10110 or solids exhibiting very large
swelling effects in the MIL-88 series.86 Future work will be
focused on the isolation of such a triangular unit and its
reactivity with the carboxylate ligands. The synthesis of the
MIL-100 compound incorporating aluminum is under progress
and will be reported soon.87
Figure 9. Isotherm curves for the adsorption and desorption of hydrogen
on MIL-96 (Al) at liquid nitrogen temperature (the lines are only guides
for eyes). (Inset) Hydrogen pressure variation as a function of time for the
second adsorption step (see arrow). The final equilibrium pressure (0.159
MPa) corresponds to a capacity of 1.73 wt %. The dashed lines delimit the
pressure range between the adsorption processes taking place.
was observed in the latter case, certainly due to the temperature
differences of the experiments.
The above-mentioned behavior can be explained by the very
small apertures of the pores. Carbon dioxide may be able to
enter the apertures due to its smallest dimension of 2.5 Å and
due to relatively favorable attraction to the free hydroxyls groups
inside the pores. The adsorption of methane is probably hindered
by its dimension of 3.8 Å which is very close to that of the
pore apertures. It may also be limited by the residual water inside
the cages. This presence of water may also explain the limited
free volume for both of these probe molecules. Unfortunately,
a higher heat treatment leads to a collapse of the structure and
no significant adsorption. This is the first of a series of studies
with MOF-type materials in which the presence of water either
is beneficial or can significantly alter the adsorption of these
gases. Nevertheless, it should be noted that saturation is not
observed for carbon dioxide and methane adsorption, and a
higher adsorption capacity can be expected at higher pressures.
Hydrogen Adsorption. Before the hydrogen adsorption
experiments, the MIL-96 (Al) sample was thermally outgassed
to remove the water molecules lying within the cages. After a
thermal treatment at 150 °C overnight, a weight loss of 18.5%
is observed in good agreement with the 19% measured by TGA
at 200 °C. The hydrogen uptake was then measured at 77 K.
Surprisingly, an unexpected behavior for physisorption was
observed with a rather slow adsorption process, and the
thermodynamic equilibrium was reached only after 6 to 7 h
(Figure 9). This might be due to the small pore opening size of
the cavity which would slow the hydrogen diffusion kinetic
within the 3D network of the different cages in MIL-96. The
adsorption isotherm shows a significant hydrogen uptake at
moderate pressure and increases rapidly to reach 1.91 wt %,
and a saturation plateau is observed above 0.3 MPa. The
desorption branch was tentatively measured down to low
pressure, but the reversibility is rather poor, and the sample was
not desorbed below 1.82 wt % (Figure 9). Again, the slow
desorption kinetic might be attributed to poor hydrogen diffusion
kinetics within the framework. The H2 uptake value is lower
than that observed in the porous aluminum terephthalate MIL-
Acknowledgment. T.L. and C.V. thank Dr. Thomas Devic
(Versailles) for fruitful discussions. Dr. Claudia Morais is greatly
acknowledged for her contribution to this study. T.L., G.F., P.L.,
and S.B. acknowledge the financial support of the European
Community STREP project “DeSANNS” (Nï. FP6-SES6-
020133)).
Supporting Information Available: Crystallographic data in
CIF format and figures showing the FAM-II-MQMAS 27Al
spectrum, IR spectrum, and TG curve. This material is available
free of charge via the Internet at http://pubs.acs.org.
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