Selective CO2 adsorption by a triazacyclononane-bridged microporous metal-organic framework

Article abstract of DOI:10.1002/chem.201003680

Metal-organic frameworks constructed by self-assembly of metal ions and organic linkers have recently been of great interest in the preparation of porous hybrid materials with a wide variety of functions. Despite much research in this area and the large choice of building blocks used to fine-tune pore size and structure, it remains a challenge to synthesise frameworks composed of polyamines to tailor the porosity and adsorption properties for CO₂. Herein, we describe a rigid and microporous three-dimensional metal-organic framework with the formula [Zn₂(L)(H₂O)]Cl (L=1,4,7-tris(4-carboxybenzyl)-1,4,7-triazacyclononane) synthesised in a one-pot solvothermal reaction between zinc ions and a flexible cyclic polyaminocarboxylate. We have demonstrated, for the first time, that a porous rigid framework can be obtained by starting from a flexible amine building block. Sorption measurements revealed that the material exhibited a high surface area (1350 m² g^-1) and was the best compromise between capacity and selectivity for CO₂ over CO, CH₄, N ₂ and O₂; as such it is a promising new selective adsorbent for CO₂ capture. Copyright

Full text of DOI:10.1002/chem.201003680

FULL PAPER
DOI: 10.1002/chem.201003680
Selective CO2 Adsorption by a Triazacyclononane-Bridged Microporous
Metal–Organic Framework
Guillaume Ortiz, Stꢀphane Brandꢁs, Yoann Rousselin, and Roger Guilard*[a]
Abstract: Metal–organic frameworks
constructed by self-assembly of metal
ions and organic linkers have recently
been of great interest in the prepara-
tion of porous hybrid materials with a
wide variety of functions. Despite
much research in this area and the
large choice of building blocks used to
fine-tune pore size and structure, it re-
mains a challenge to synthesise frame-
works composed of polyamines to
tailor the porosity and adsorption prop-
erties for CO2. Herein, we describe a
rigid and microporous three-dimen-
sional metal–organic framework with
boxylate. We have demonstrated, for
the first time, that porous rigid
a
the formula [Zn2(L)
N
framework can be obtained by starting
from a flexible amine building block.
Sorption measurements revealed that
the material exhibited a high surface
area (1350 m2 gꢀ1) and was the best
compromise between capacity and se-
lectivity for CO2 over CO, CH4, N2 and
O2; as such it is a promising new selec-
tive adsorbent for CO2 capture.
1,4,7-tris(4-carboxybenzyl)-1,4,7-triaza-
cyclononane) synthesised in a one-pot
solvothermal reaction between zinc
ions and a flexible cyclic polyaminocar-
Keywords: adsorption · gas separa-
tion · metal–organic frameworks ·
microporous materials
dioxide
·
carbon
Introduction
rials were shown to be among the best sorbents for CO2 sep-
aration close to 1 atm.[27,30,31] By taking into account the
Lewis acid properties of CO2 itself, strong interactions
through chemisorption processes can occur with primary
and secondary alkyl amines, both in solution[32] and in the
solid state.[33–35] The choice of the nitrogen-functionalised or-
ganic linker is of critical importance for the required appli-
cation and it is still a challenge to develop new porous
MOFs, containing polyamine linkers that are able to interact
with CO2.[36–39] Our contribution was to use an N-functional-
ised triazamacrocycle as a spacer ligand to build a new
MOF with the aza ligand integrated into the framework of
the adsorbent. These materials, including cyclic polyamino-
carboxylates, were aimed at enhancing CO2 capture by
strong interactions (coulombic, van der Waals) with the
amine or the Lewis acid metals of the framework and also
to increase the selectivity for CO2 towards CO, CH4, O2 and
N2 thanks to the polarity of the surface. Herein, a micropo-
rous coordination polymer incorporating the cyclic amine
1,4,7-triazacyclononane (TACN) was synthesised and char-
acterised by single-crystal and powder X-ray diffraction, po-
rosity and gas adsorption measurements. The high adsorp-
tion capacity and selectivity for CO2 versus N2, O2, CH4 and
CO was clearly demonstrated.
The ability to design an adsorbent with fine-tuning of the
pore size, pore shape and chemical functionality would pro-
vide a significant advance in areas such as gas separation
and catalysis. Metal–organic frameworks (MOFs) are an at-
tractive alternative to the use of zeolites for gas adsorption
processes because of the large number of possible structures
that can be effectively elaborated.[1–4] In the last decade,
many crystalline porous coordination polymers have led to a
new class of porous materials with the potential for a range
of applications,[2,5–10] and they have also provided flexible
platforms for developing well-designed gas molecule adsorb-
ents,[1,4,11,12] in particular, for CO2 capture with high selectivi-
ty.[13–24] To enhance the adsorption capacity, microporous
materials with open metal sites, such as Cu
(BTC=1,3,5-benzenetricarboxylate) and M2ACHTNUGRTENNU(G DOTP) (CPO-
[25]
3ACHTUNGERTN(NUNG BTC)2
27M; M=Ni, Mg, DOTP=2,5-dioxyterephtalate),[26,27] were
studied and their high capacity for CO2 adsorption was
shown thanks to strong electrostatic interactions with the ex-
posed metallic ions.[26,27] The Lewis acid character of the co-
ordinatively unsaturated metal ions was taken advantage of
by using zeolites and Cu3ACTHNUTRGNE(UNG BTC)2 for the separation of gas
mixtures with high selectivity,[28,29] and the CPO-27M mate-
[a] G. Ortiz, Dr. S. Brandꢀs, Dr. Y. Rousselin, Prof. R. Guilard
Institut de Chimie Molꢁculaire de lꢂUniversitꢁ de Bourgogne
UMR 5260, CNRS, Universitꢁ de Bourgogne
9 avenue Alain Savary, 21078 Dijon Cedex (France)
Fax : (+33)380-39-61-17
Results and Discussion
Synthesis and X-ray structure of [1ACTHNUTRGNEUNG(H2O)]Cl: We have de-
signed and synthesised a new MOF composed of a tris-car-
boxybenzyl TACN (L) ligand, which possesses a high porosi-
ty. The crude cyclic triamine was first isolated in its penta-
E-mail: Roger.Guilard@u-bourgogne.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003680.
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ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6689

A
arm ligand. Zn1 is located 1.436(1) ꢄ from the mean plane
formed by the three nitrogen atoms. A tetracoordinated
Zn2 metal is located above Zn1 and presents Td symmetry.
The tetrahedron was formed by three carboxylate oxygen
atoms and one water molecule in the apical position. The
ꢀ
contrast to the frameworks constructed by the reaction of
metallo-tetraazacycloalkane with pillared ligands,[40] this ma-
Zn2 O3ACTHNUTRGNEUNG(H2O) distance (2.183(10) ꢄ, Table S2 in the Sup-
porting Information) is comparable to the distance found
ꢀ
for
a
complex with Td geometry (Zn O (aqua)=
2.092(61) ꢄ).[41] The two zinc ions are bridged by three car-
boxylate groups and are arranged in close proximity to each
ꢀ
ꢀ
other (Zn1 Zn2: 3.517(1) ꢄ). The Zn1 N1 bond lengths of
ꢀ
2.201(5) ꢄ are similar to the average Zn N tertiary amine
bond length of 2.147(68).[41] The most noteworthy feature of
the structure is the trigonal distortion of the octahedron.
The N1-Zn1-N1 angles of 81.1(2)8 are constrained by the
ring size of the macrocycle. As a consequence of this distor-
tion, the O1-Zn1-O1 bite angle is higher (96.51(19)8) than
that expected for a regular octahedron. The structure owes
its robustness to this dinuclear zinc cluster despite the ab-
sence of hydrogen bonding.
Scheme 1.
terial was prepared by self-assembly of the zinc–TACN com-
plex without adding an exogenous ligand. Reaction of an
2+
excess of ZnACHTUNGTRENNUNG(NO3)2·6H2O with LH5 in a mixture of DMF,
EtOH and Et3N at 353 K for 24 h under autogeneous pres-
sure afforded crystals of [Zn2(L)(H2O)]Cl·DMF (=[1-
(H2O)]Cl·DMF), which was colourless and had a trigonal
G
The diameters of the channels are between 6.9 and 8.7 ꢄ
and interconnected through circular and rectangular win-
dows, which can be seen along a, b or c crystallographic
ACHTUNGTRENNUNG
prismatic-like morphology with curved edges (Figure S1 in
the Supporting Information).
Depending on the scale of the
synthesis, the crystals had sizes
between 10 and 30 mm. Energy-
dispersive X-ray spectroscopy
(EDS) and elemental analyses
supported a twofold excess of
zinc relative to TACN and re-
vealed the presence of a chlo-
ride anion in approximately
half the amount of the zinc
(Table S1 in the Supporting in-
formation).
Single crystals of [1ACTHNUTRGNEUNG(H2O)]Cl
were analysed by X-ray diffrac-
tion and the coordination poly-
hedra of the dinuclear cluster
and its porous framework are
depicted in Figure 1. The com-
plex crystallises in
a
cubic
system (space group: I43d) with
unit cell parameter a=
¯
a
25.8644(10) ꢄ and unit cell
volume of 17302.4(12) ꢄ3. Un-
expectedly, two zinc ions are
coordinated to three nitrogen
atoms of TACN and six oxygen
atoms of the carboxylate func-
tions. Zn1 is hexacoordinated
with Oh symmetry; it is chelat-
ed to the three nitrogen atoms
of the macrocycle and bound
with three carboxylate oxygen
Figure 1. a) ORTEP[60] view of the dinuclear zinc cluster in [1
ty. The thermal ellipsoids are drawn at probability level of 50%. b) Packing view of [1AHCNUTGTRENNUNG
tallographic ab plane. c) 3D pore channel along the crystallographic ab plane. Green spaces represent the
inside of the channel. d) View of the channels along the crystallographic ab plane showing the coordination
ACHTUNGTRENNUNG
atoms from another pendant polyhedra (tetrahedron and octahedron). H atoms were omitted for clarity.
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Triazacyclononane-Bridged Metal–Organic Framework
FULL PAPER
axes (Figure 1d). The space-filling model shows clearly that
pore accessibility is not restricted (Figure 1b). The simula-
tion of a Connolly void surface with a 1.4 ꢄ probe size
shows that channels are connected by narrower necks (Fig-
ure 1c). The solvent molecules and chloride counterion are
disordered within the pores and their contributions to the
X-ray diffraction patterns were estimated by using Platon/
Squeeze.[42] The electroneutrality of the structure is fulfilled
when considering one negative charge (chloride) in the
pores in accordance with the EDS analysis. By using Platon/
Solv,[42] the accessible voids in the desolvated structures
were estimated to be 47.5% of the total volume.
Gas adsorption and structural properties: Prior to gas ad-
sorption measurements, the thermal stability of [1-
ACHTUNGTRENNUNG(H2O)]Cl·DMF was studied by thermogravimetric analysis
(TGA) and X-ray powder diffraction at different steps of
the synthesis. TGA analysis performed on the as-synthesised
samples showed the loss of solvent molecules (DMF guest)
from 373 to 543 K and a further mass decrease occurred up
to 903 K as a result of ligand calcination (Figure S2 in the
Figure 2. PXRD patterns (CuKa1) showing the stability of the framework:
a) simulated from the X-ray single-crystal diffraction data of [1ACHTUNGTRENNUNG
b) experimental patterns for as-synthesised [1CAHTUNGTRENNUNG
353 K under 10ꢀ3 torr; c) after degassing at 463 K under 10ꢀ3 torr, and
d) experimental patterns after chloroform exchange and degassing at
353 K under 10ꢀ3 torr.
Supporting Information). Soaking [1
(H2O)]Cl·DMF in
in contrast to that described previously with tetraazacycloal-
kanes.[40] To the best of our knowledge, rigid frameworks ob-
tained from flexible building blocks have not been described
in the literature.
chloroform at reflux led to a total exchange of the guest
molecules, as revealed by FTIR spectroscopy of the material
1
and H NMR spectroscopy of the decomposed framework in
a mixture of [D6]DMSO/DCl (Figures S3 and S4 in the Sup-
porting Information). Evacuation at 393 K led to removal of
the solvent and water molecules in the porous matrix, as
shown by the TGA trace because no abrupt weight loss oc-
curred before 623 K. TGA curves indicated that after evacu-
ation of the sample to 393 K for 12 h and re-exposition to
humid air for few minutes, a weight loss of 2% assigned to
water release was observed below 423 K. This loss was due
to the re-binding of water on the zinc ion during exposition
to air and was in perfect agreement with the expected theo-
retical value. This result demonstrates that evacuation of the
material leads to a framework with open zinc sites. Accord-
ingly, the CHCl3-exchanged material was activated at 393 K
under reduced pressure before adsorption measurements in
order to remove traces of guest molecules in the pores and
to create unsaturated metal sites.
In addition, PXRD analyses of the dehydrated [1]Cl have
displayed a partial collapse of the framework in humid air
over a few weeks (Figure S11 in the Supporting Informa-
tion). However, the material is stable for several weeks in a
pre-dried inert gas containing less than 10 ppm of water.
Another important point is related to the stability of the as-
synthesised material. PXRD analyses have shown that
before elimination of DMF by thermal activation or after
chloroform exchange, the material is stable for months. The
persistence of the porous framework after solvent removal
was confirmed by the N2 adsorption data. The N2 isotherm
[1]Cl at 77 K (Figure 3) exhibits a type I profile characteris-
tic of the presence of micropores and the desorption profile
shows no hysteresis. The BET and apparent Langmuir sur-
Powder X-ray diffraction (PXRD) provided a measure of
the phase purity and the stability of the framework. The
PXRD patterns of the as-synthesised crystals were found to
be identical to those simulated from single-crystal structures
of [1ACHTUNGTRENNUNG(H2O)]Cl·DMF, thus indicating the similarity of the
bulk sample with the single crystal even after exchange with
CHCl3 (Figure 2). After drying of the DMF-exchanged solid
at temperatures up to 463 K under vacuum, a weak phase
change or collapse had barely begun, and the XRD patterns
matched the simulated ones. Moreover, variable-tempera-
ture PXRD experiments showed that the material kept its
crystallinity up to 433 K with no shift of the diffraction
peaks (Figure S5 in the Supporting Information). This indi-
cates that no shrinkage occurs and the porous framework is
still robust after removal of free guest molecules in the
channels. It also demonstrates the rigidity of the framework,
Figure 3. N2 adsorption–desorption isotherm of [1]Cl at 77 K after activa-
tion under 10ꢀ5 torr at 393 K. The inset displays the pore size distribution
using the Horvath–Kawazoe method (Saito Foley) calculation. STP=
standard temperature and pressure.
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6691

R. Guilard et al.
face areas were estimated to be 1199 and 1350 m2 gꢀ1, re-
spectively. The small discrepancies between the two values
lies in the pore-filling mechanism, which is not valid for the
BET assumption, but a fairly good agreement was obtained
by using a proper pressure range.[43,44] The micropore
volume was estimated by using the Dubinin–Radushkevich
equation to give 0.45 cm3 gꢀ1; a value very close to the exper-
imental one (0.48 cm3 gꢀ1). These data are totally consistent
with the accessible porosity calculated geometrically from
the crystal structure (0.46 cm3 gꢀ1). The excellent agreement
between adsorption and crystallographic data rules out any
collapse of the crystalline structure and demonstrates the
high stability of the porous framework after solvent remov-
al. It is noteworthy than when DMF is removed by heating
without chloroform exchange, a lower porosity was ob-
tained, whatever the activation temperature (Figure S6 in
the Supporting Information). The onset pressure was ana-
lysed by using the Horvath–Kawazoe method and the pore
diameter was shown to be 8.2 ꢄ, in accordance with the
crystallographic results (mean pore size 7.8 ꢄ). The adsorp-
tion–desorption of CO2 at 195 K (Figure S7 in the Support-
ing Information) displayed a capacity close to 251 cm3 gꢀ1 at
1 atm and the Langmuir surface area estimated from the
CO2 isotherm was 1245 m2 gꢀ1, very close to that for N2 at
77 K. The isotherm shows a large hysteresis that is generally
assigned to guest-directed framework rearrangement leading
to a gate-opening structure[40,45] due its flexibility. It is un-
likely that this mechanism operates in the case described
herein because [1]Cl keeps its crystallinity up to 433 K with-
out structural change. The hindrance of some counteranions
at the pore entrance can lead to partial pore blocking.
Hence, the hampered diffusion of the gas molecules is likely
to be the cause of such hysteresis. We also speculate that
such behaviour is a result of the CO2–anion interactions be-
cause no hysteresis was observed with N2 at 77 K.
Figure 4. Adsorption–desorption isotherms for [1]Cl at 298 K with CO2
~
&
*
*
^
(ads: , des: &), CH4
( ), CO ( ), N2 ( ), and O2 ( ). The thin solid
lines are related to the calculated dual-site Langmuir isotherm model for
CO2 and single-site Langmuir-type isotherms for the other gases. Dashed
and dotted lines indicate the two contributions for the calculated CO2
isotherm.
observed for MIL-53 and MIL-101 than that for [1]Cl at low
pressure (ꢁ1 atm). In the higher coverage region, the heat
of adsorption slightly decreases to ꢀ23 kJmolꢀ1, which
shows that the interaction energies are almost homogene-
ously distributed on the surface and relatively weak interac-
tions of CO2 with the metal sites occur even after water
molecule decoordination. The quasi-independence of the
heat of adsorption with the coverage clearly indicates that
zinc is slightly accessible to CO2. The relatively low value of
the enthalpy also indicates that the free anions in the pores
do not participate in the adsorption process. In fact, material
with a higher heat of adsorption is unlikely to be satisfactory
in practical applications because of the large energy re-
quired to desorb CO2 from the porous solid.
The ability for selective CO2/CO and CO2/CH4 separation
was further examined for post- and pre-combustion capture
technologies. Indeed, the adsorption selectivity for CO2 over
CO, CH4, O2 and N2 is important for applications of the ma-
terial as a CO2 sorbent.[32,50] [1]Cl did not show any signifi-
cant uptake of N2, O2 or CO at 298 K.
The permanent porosity for [1]Cl allowed us to examine
its potential application as a selective sorbent. The CO2,
CO, CH4, O2 and N2 adsorption isotherms at 298 K are illus-
trated in Figure 4 and the selective adsorption of CO2 is
clearly evidenced. The material uptake of CO2 is 88 cm3 gꢀ1
at 1 atm, which corresponds to 3.92 mmolgꢀ1 or 17.8 wt%.
A perfect adjustment of the adsorption branch for CO2 was
obtained by using a dual-site Langmuir-type isotherm model
and a single Langmuir isotherm for the other gases (see
Eqs. (1) and (2) in the Experimental Section and Table S3 in
the Supporting Information).[46–49]
The selectivity for CO2 towards other gases was calculated
from the Henry constants [see Eq. (4) in the Experimental
Section] and values were 21.1, 4.2, 26.0 and 22.6 over CO,
CH4, O2 and N2, respectively. The selectivity over CH4 is
quite similar to that described for ZIF materials.[22] To the
best of our knowledge, the selectivity over CO[18,22,25] is one
of the highest ever obtained for microporous materials and
is significantly higher than for the ZIF family[18,22,51] or Cu3-
The heat of CO2 adsorption was determined with the
Clausius–Clapeyron analysis by recording isotherms at 273,
288 and 298 K (Figures S8 and S9 in the Supporting Infor-
mation). As expected, the Henry constant increases as the
temperature decreases and the enthalpy at zero coverage is
ꢀ26 kJmolꢀ1, corresponding clearly to a physisorption pro-
AHCTUNGTRENNUNG
a
low selectivity over CO was observed because of its coordi-
nation to the nickel ions.[53]
cess. This value is significantly lower than for Cu
N
Since the diameter and pore entrance are close to 8 ꢄ,
the selectivity for CO2 cannot be attributed to small pores
restricting the adsorption of gases thanks to a molecular
sieving process, giving rise to a diffusion barrier. In other
words, the diffusion barrier is not the limiting factor. Ac-
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Triazacyclononane-Bridged Metal–Organic Framework
FULL PAPER
K2CO3 (25.9 g, 188.1 mmol) in CH3CN (200 mL). After heating at reflux
for 24 h, the salts were removed by filtration and the yellow solution was
evaporated to dryness. Water (40 mL) was added and the yellow powder
was extracted with CH2Cl2 (3ꢅ50 mL). The combined extracts were
dried (MgSO4) and evaporated to give a yellow oil, which precipitated in
Et2O/pentane 2/1. The crude product was filtered and dried in vacuo
(8.7 g, 18.3 mmol, yield 87.8%). 1H NMR (300 MHz, CDCl3, 300 K): d=
7.57 (d, 3J=8.1 Hz, 6H; Ar), 7.41 (d, 3J=8.2 Hz, 6H; Ar), 3.64 (s, 6H;
NCH2Ph), 2.75 ppm (s, 12H; NCH2CH2N); 13C{1H} NMR (75 MHz,
CDCl3, 300 K): d=145.8 (CArCH2), 132.2 (CHAr), 129.4 (CHAr), 119.1
(CN), 110.7 (CArCN), 62.7 (CH2-Ph), 55.5 ppm (NCH2CH2N); FTIR
(ATR): n˜ =2229 cmꢀ1 (CN); MS (m/z): 474.98 [M+H]+; elemental anal-
cordingly, a size exclusion effect could not be evoked be-
cause the pore size was significantly higher than the kinetic
diameter of CO2. On the other hand, [1]Cl has shown a
higher adsorption capacity for CH4, but it is still lower than
that for CO2. The gas–solid interaction of the MOF materi-
als results from coulombic and van der Waals interactions
between gas molecules and the microporous surface. Be-
cause the material is composed of dinuclear zinc clusters
and chloride anions, which are supposed to be non-coordi-
nating and remain free in the pores, a large adsorption of
gas molecules with large quadrupole moments, such as CO2
(1.34ꢅ10ꢀ39 Cm2), is expected thanks to strong electrostatic
interactions between the field gradient induced by Zn2+ and
Clꢀ. Hence, the lower CO and CH4 adsorption is due to
their smaller quadrupole moment and lower polarisability
than that of CO2.
Multicycle adsorption realised in dynamic conditions by
gravimetric measurements also showed the reversibility of
the CO2 adsorption process at 298 K (Figure S10 in the Sup-
porting Information), while no loss of activity was observed
through six cycles, indicating that the material is able to
withstand repeated cycles of CO2 stream. The material was
then recovered totally free of CO2 by switching the gas
stream to Ar. The weight change under a CO2 flow was
15.5% in an 80% CO2/Ar mixture over repeated cycles and
this value agreed with the static measurements described
above. Moreover, the adsorption kinetic is high because the
half-saturation capacity is reached in only 30 s under dynam-
ic conditions. This result thus shows that the material is a
suitable adsorbent candidate in a vacuum swing adsorption
(VSA) or pressure swing adsorption (PSA) process for CO2
removal like amine-containing silica adsorbents.[54]
AHCNUTRTGEGyNNUN sis calcd (%) for C30H30N6·0.23H2O: C 75.26, H 6.41, N 17.55; found: C
75.29, H 6.31, N 17.39.
Synthesis of LH52+ =1,4,7-tris(4-carboxybenzyl)-1,4,7-triazacyclononane-
2HCl-2H2O: A suspension of L’ (8.5 g, 17.9 mmol) was heated at reflux
in HCl (12m, 80 mL) for 48 h. The crude white solid was filtered and
washed with water (3ꢅ100 mL). The product was stirred for 24 h in a so-
lution of NaOH (4m). Concentrated HCl was added to the cooled solu-
tion until pH 1 was reached. The white solid formed was filtered, washed
with water (3ꢅ40 mL) and the crude product was dried in vacuo (70.3%,
8.1 g, 12.6 mmol). 1H NMR (600 MHz, D2O, NaOD, dioxane as an inter-
nal reference, 300 K): d=7.91 (d, 3J=7.9 Hz, 6H; Ar), 7.15 (d, 3J=
7.9 Hz, 6H; Ar), 3.66 (s, 6H; CH2Ph), 2.75 ppm (s, 12H; NCH2CH2N);
13C{1H} NMR (150 MHz, D2O, NaOD, dioxane as an internal reference,
300 K): d=174.9 (CO2H), 138.6 (CArCO2H), 137.2 (CArCH2), 130.6
(CHAr); 130.1 (CHAr), 59.4 (CH2Ph), 49.5 ppm (NCH2CH2N); FTIR
(ATR): n˜ =1706 (C=O), 2996 cmꢀ1 (CH); TGA: 5.5% weight lost for
water (loss of two water molecules); MS (m/z): 531.9 [M+H]+; elemental
analysis calcd (%) for C30H33N3O6·2HCl·2H2O: C 56.25, H 6.14, N 6.56;
found: C 56.35, H 6.10, N 6.51.
Synthesis of [1
ACHUTNGTREN(NUG H2O)]Cl ([Zn2(L)ACHUTGNTREN(NUGN H2O)]Cl): The material was prepared
by using the solvothermal method. A solution of the protonated ligand
2+
LH5
(1 g, 1.56 mmol) in DMF (60 mL) and a solution of Zn-
AHCTNUGTREN(GNNU NO3)2·6H2O (4.2 g, 14 mmol) in DMF (45 mL) was added. A solution of
triethylamine (0.158 g, 1.56 mmol) in EtOH (12 mL) was added and the
colourless solution was sealed in a 150 mL glass autoclave and heated at
353 K for 24 h. Trigonal prismatic shaped crystals were thus obtained.
DMF was removed and fresh DMF (50 mL) was added to the crystals
and they were left overnight. After removal of DMF, the crystals were
heated at reflux in CHCl3 (50 mL) for 6 h without stirring. The yield was
96% after drying in vacuum for 14 h at 293 K. The absence of residual
solvent was shown by 1H NMR spectroscopic analysis after material de-
composition in [D6]DMSO/DCl (see Figure S4 in the Supporting Infor-
mation) or by FTIR analysis. FTIR (ATR): n˜ =1610 cmꢀ1 (C=O for
CO2ꢀ); elemental analysis calcd (%), after drying in vacuum for 6 h at
393 K under 10ꢀ5 torr, for C30H32N3O7Zn2Cl·3H2O: C 46.99, H 4.99, N
5.48; found: C 46.92, H 4.09, N 5.95; FTIR (ATR) before chloroform ex-
change: n˜ =1655 (C=O free DMF), 1614 cmꢀ1 (C=O for CO2ꢀ); elemen-
tal analysis calcd (%), before chloroform exchange, for
C30H32N3O7Zn2Cl·0.8DMF·1.9H2O: C 48.31, H 5.18, N 6.61; found: C
48.21, H 4.71, N 6.77.
Conclusion
We have synthesised a new porous MOF [1ACTHNUTRGNE(NUG H2O)]Cl com-
posed of a cyclic triamine and zinc ions. The material dis-
played a high surface area and a high adsorption capacity
close to 1 atm. The crystallinity and the ordered channels
were retained after solvent removal from the porous struc-
ture. The originality of the material lies in the high rigidity
of the framework despite the flexibility of the cyclic poly-
ACHTUNGTRENNUNGaminocarboxylate building block. The polarisation of the
Single-crystal X-ray crystallography: Single crystals were grown by using
the previous procedure in the absence of triethylamine and without DMF
exchange. Diffraction data were collected on a Nonius Kappa Apex II
CCD diffractometer equipped with a nitrogen jet stream low-tempera-
ture system (Oxford Cryosystems). The X-ray source was graphite-mono-
chromated MoKa radiation (l=0.71073 ꢄ) from a sealed tube. The lattice
parameters were obtained by least-squares fit to the optimised setting
angles of the entire set of collected reflections. No significant tempera-
ture drift was observed during the data collections. Data were reduced by
using Denzo[55] software and the structure was solved by direct methods
using the SIR97[56] program. Refinements were carried out by full-matrix
least-squares on F2 using the SHELXL97[57,58] program on the complete
set of reflections. Anisotropic thermal parameters were used for non-hy-
drogen atoms. All hydrogen atoms, on carbon atoms, were placed at cal-
surface led to a high adsorption capacity of CO2 up to
88 cm3 gꢀ1 at 1 atm with a high selectivity over CO, CH4, N2
and O2. The combination of these two properties makes
such a microporous material very competitive as a CO2-se-
lective sorbent for capture applications.
Experimental Section
Synthesis of L’=1,4,7-tris(4-cyanobenzyl)-1,4,7-triazacyclononane (see
Scheme S1 in the Supporting Information): A solution of a-bromo-p-
tolunitrile (12.32 g, 62.8 mmol) in CH3CN (100 mL) was added to a sus-
pension of trihydrochloride 1,4,7-triazacyclononane (5 g, 20.9 mmol) and
ꢀ
culated positions using a riding model with C H=0.95 ꢄ (aromatic),
0.99 ꢄ (methylene) with Uiso(H)=1.2U
U
Chem. Eur. J. 2011, 17, 6689 – 6695
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6693

R. Guilard et al.
In [1
(H2O)]Cl material, the hydrogen atoms of the coordinated water
Fitting of the adsorption branch for CO2, CO, CH4, N2 and O2 based on
the Langmuir isotherm models allowed us to calculate the Henry con-
molecule have not been located and were not included in the formulae.
The distance between C1 and C2 was restrained by using DFIX re-
straints.[57] The components of the anisotropic displacement parameters,
of O3 and Zn2, in the direction of the bond are restrained by using
DELU restraint[57] to be equal within an effective standard deviation to
maintain a reasonable model. The unit cell includes a large region of dis-
ordered solvent and counterion (Clꢀ) molecules and cannot be modelled
as discrete atomic sites. We employed Platon/Squeeze[59] to calculate the
diffraction contribution of these molecules and, thereby, to produce a set
of solvent-free diffraction intensities. CCDC-786902 contains the supple-
mentary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
stants for CO2, HCO =K1V1 +K2V2, and for the other gases, Hi =KiVi.
2
The Henry law selectivity (=upper limit selectivity) for CO2 over gas i is
then expressed by Equation (4):
H
Hi
CO2
ð4Þ
SCO
¼
2
It is noteworthy that selectivity was also calculated by using the Toth iso-
therm model and similar values were obtained.
Gas sorption measurements: Gas adsorption isotherms on evacuated
nanoporous materials were performed by using the volumetric method
recording the adsorption isotherms on a Micromeritics ASAP 2010 or
2020 analyser. Samples were previously degassed by heating at 393 K in
vacuo (10ꢀ5 torr) for at least 6 h. The equilibrium of the solid–gas re-
Acknowledgements
This work was supported by the ANR “IMCAT” project (ANR-06-CO2-
007-03). G.O. gratefully acknowledges the CNRS and the “Rꢁgion Bour-
gogne” for financial support. We thank E. Wenger for PXRD analysis, C.
Josse for EDS measurements and C. Loup for his help in the precursor
synthesis.
ACHTUNGTRENNUNGaction was determined for several relative pressures (P/P0, P0 =saturated
vapour pressure) to record the adsorption isotherm at a controlled tem-
perature in a large range of P/P0 (10ꢀ6 to 1). The specific surface area
were determined by physisorption of N2 in the channels at 77 K by using
the BET and the apparent Langmuir equations assuming a monolayer
coverage of N2 and a cross-sectional area of 16.2 ꢄ2 per molecule. To
avoid large discrepancies between the two values due to the invalid BET
assumption for microporous solids over the usual range 0.05<P/P0 <
0.25, care was taken to choose a suitable pressure range for which the
BET analysis could be applied. As required by the groups of Snurr[43] and
Rouquerol,[44] the pressure range selected should give increasing values
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Received: December 20, 2010
Revised: February 24, 2011
Published online: April 28, 2011
Chem. Eur. J. 2011, 17, 6689 – 6695
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6695