A highly stable multifunctional three-dimensional microporous framework: Excellent selective sorption and visible photoluminescence

Article abstract of DOI:10.1039/c4dt00230j

A Cd(ii)-MOF [Cd(L)(DMF)] (1) (H₂L = 5-(4-pyridyl)-isophthalic acid) featuring high thermal and chemical stability has been solvothermally synthesized and characterized. Complex 1 is a robust multifunctional three-dimensional (3D) microporous framework possessing an unusual (4,5)-connected topological net. Activated 1 shows high-efficiency for the selective capture of CO₂ over N₂ and CH₄ under ambient conditions, which ranks it among MOFs with the highest selectivity. In addition, desolvated 1 exhibits good separation ratios of H₂ over N₂ at lower temperature, as well as alcohols from water. In particular, desolvated 1 demonstrates significantly higher adsorption enthalpies for CO₂ and CH₄ gas molecules, which are in the range of the highest Q_st values reported to date. Furthermore, complex 1 displays visible photoluminescence in the solid state under UV irradiation. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00230j

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
A highly stable multifunctional three-dimensional
microporous framework: excellent selective
sorption and visible photoluminescence†
Cite this: Dalton Trans., 2014, 43,
6811
Yumei Huang,a Xiaofang Zheng,a Jingui Duan,b Wenlong Liu,c Li Zhou,a
Chenggang Wang,a Lili Wen,*a Jinbo Zhaoa and Dongfeng Li*a
A Cd(II)–MOF [Cd(L)(DMF)] (1) (H2L = 5-(4-pyridyl)-isophthalic acid) featuring high thermal and chemical
stability has been solvothermally synthesized and characterized. Complex 1 is a robust multifunctional
three-dimensional (3D) microporous framework possessing an unusual (4,5)-connected topological net.
Activated 1 shows high-efficiency for the selective capture of CO2 over N2 and CH4 under ambient con-
ditions, which ranks it among MOFs with the highest selectivity. In addition, desolvated 1 exhibits good
separation ratios of H2 over N2 at lower temperature, as well as alcohols from water. In particular, de-
solvated 1 demonstrates significantly higher adsorption enthalpies for CO2 and CH4 gas molecules, which
are in the range of the highest Qst values reported to date. Furthermore, complex 1 displays visible photo-
luminescence in the solid state under UV irradiation.
Received 22nd January 2014,
Accepted 23rd February 2014
DOI: 10.1039/c4dt00230j
www.rsc.org/dalton
more strongly with the absorbed gas molecule, and moreover,
could selectively separate certain gas mixtures due to their dis-
Introduction
Microporous metal–organic frameworks (MOFs) have attracted tinct polarizability and/or quadrupole moment. On the other
considerable attention in view of their structural diversity hand, a ligand-based strategy to the design of new luminescent
and adjustable porosity that allow almost unlimited ways to MOFs is commonly used,2 arising from π-conjugated skeleton
optimize their properties and functionality, making them very and rigidity of the organic molecules. In our effort to develop
promising for
a wide range of important applications, multifunctional MOFs combined with selective adsorption and
especially in capture and separation of small gases or hydro- strong luminescence, we incorporated the dianionic form of
carbons,1 as well as luminescence.2 More remarkably, these tripodal 5-(4-pyridyl)-isophthalic acid (H2L)5 into a cadmium
physical properties might be integrated into individual archi- MOF based on the following considerations: (1) the relatively
tectures to generate multifunctional MOFs.3
large Cd(II) cation may enhance the presence of OMSs via
Among many ongoing efforts to manipulate high-perform- uptake of solvent molecules (e.g. H2O, DMF) as terminal
ance MOF materials, one of the effective approaches to ligands, which can be readily detached following desolvation
improve their affinity and gas selectivity is to build structures at elevated temperatures and/or under vacuum; (2) the robust
bearing open metal sites (OMSs) on the pore surfaces.4 The rigid organic moiety facilitates the preparation of highly stable
OMSs serving as charge-dense binding sites could interact porous materials; (3) the collaborative interaction between the
d10 metal ion of Cd(II) and the rigid organic ligand is favorable
for enhancing fluorescence of the resulting architecture.
aKey Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of
Chemistry, Central China Normal University, Wuhan, 430079, P. R. China.
E-mail: wenlili@mail.ccnu.edu.cn, dfli@mail.ccnu.edu.cn; Fax: +86 27 67867232;
Tel: +86 27 67862900
bInstitute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University,
Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
cCollege of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou,
Herein, we present a microporous framework, [Cd(L)(DMF)]
(1), which has been solvothermally synthesized and character-
ized. Complex 1 is a robust multifunctional three-dimensional
(3D) microporous framework possessing an unusual (4,5)-con-
nected topological net. The open Cd(II) metal sites and small-
sized pores enable desolvated 1 to exhibit excellent selective
capture of CO2 over N2 and CH4 at 298 K, H2 over N2 at lower
temperature, as well as alcohols from water. In addition, the
desolvated form of 1 possesses high adsorption enthalpy for
CO2 and CH4. It is worth noting that complex 1 displays visible
green-blue photoluminescence in the solid state under UV
225002, P. R. China
†Electronic supplementary information (ESI) available: X-ray crystallographic
file (CIF), additional crystal figures, TGA data, PXRD patterns, Qst and CO2
selectivity calculation details and decay lifetime fitting for complex 1. CCDC
971223. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4dt00230j
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 6811–6818 | 6811

View Article Online
Paper
Dalton Transactions
irradiation. So far, a combination of significantly stable, highly a Quantachrome IQ2 system. A solvent-exchanged sample was
selective absorption of small molecules and intense visible obtained by soaking the as-made sample in methanol for 3
fluorescence in Cd-MOF materials is uncommon in the days with methanol, refreshing every 8 h. A completely de-
literature.
solvated sample (about 100 mg) was afforded by heating the
solvent-exchanged bulk at 473 K under vacuum overnight.
Before the gas/vapor measurement, the samples were evacu-
ated again by using the “degas” function of the surface area
analyzer for 10 h at 473 K.
Experimental section
Materials and methods
All chemicals and solvents were commercially available and
used as received. Elemental microanalyses (C, H and N) were
carried out using a Perkin-Elmer 240 elemental analyzer. IR
spectra were recorded on a Bruker Vector 22 spectrometer as
dry KBr discs in the 400–4000 cm−1 region. Thermogravimetric
analyses (TGA) were measured on a Netzsch STA449C appar-
atus under a N2 stream (heating rate of 10 °C min−1). All
powder X-ray diffraction (PXRD) data were collected on a
Siemens D5005 diffractometer using Cu Kα radiation (λ =
1.5418 Å) and 2θ ranging from 5 to 50°. Solid-state fluo-
rescence spectra were recorded on an Edinburgh Analytical
instrument FLS920.
Synthesis of [Cd(L)(DMF)] (1). A mixture of Cd(NO3)2·6H2O
(30.8 mg, 0.10 mmol), H2L(24.3 mg, 0.10 mmol), 5-amino-
benzene-1,2,3-tricarboxylic acid (22.5 mg, 0.10 mmol), and
DMF (3 mL) was sealed in a parr Teflon-lined stainless steel
vessel (25 cm3) and heated at 120 °C for 2 days. After slowly
cooling to room temperature, light brown crystals of 1 were iso-
lated (yield: 75% based on Cd). Anal. Calcd for C16H14CdN2O5:
C, 45.04; H, 3.31; N, 6.57%; found: C, 45.08; H, 3.39; N, 6.51%.
IR spectrum (cm−1): 3411m, 1658s, 1613s, 1566s, 1504w,
1443s, 1416s, 1368s, 1294w, 1102w, 1072w, 840w, 779m, 731s,
644m, 511w.
Results and discussion
Each asymmetric unit of complex 1 includes one crystallo-
graphically independent Cd(II) atom, one L2− anion and one
coordinated DMF molecule. As displayed in Fig. 1a, Cd(II)
exhibits distorted pentagonal bipyramidal character, the equa-
torial positions being occupied by two chelating carboxylate
groups and one L2− nitrogen atom. The axial positions are co-
ordinated by a monodentate carboxylate group and a terminal
DMF. The Cd–ODMF bond distance is 2.369 Å and somewhat
shorter than the average Cd–Ocarboxylate bond length (2.415 Å)
in 1, thereby implying stronger interaction between DMF mole-
cules and the central Cd(II) ions. Obviously, the unsaturated
X-Ray crystallography study
The diffraction data for 1 were collected on a Bruker Smart
Apex DUO CCD equipped with graphite-monochromated Mo
Kα radiation (λ = 0.71073 Å) at 293(2) K. Data reductions and
absorption corrections were respectively performed with the
SAINT6 and SADABS7 software packages. The structure was
solved by direct methods, and refined anisotropically on F2 by
the full-matrix least-squares technique using the SHELXL-978
except for the disordered atoms. Disordered segments were
subjected to geometric restraints during the refinements. The
coordinated DMF is disordered over two positions with equal
site occupancy factors, the C–N bonds of DMF were restrained
by DFIX, and equal C15 and C15′, C16 and C16′ displacement
parameters were obtained by EADP instructions. Crystallo-
graphic data and refinement information for complex 1 are
provided in Table S1.† Relevant selected parameters are given
in Table S2.†
Sorption measurements
Fig. 1 (a) The coordination environment of Cd1 atom with hydrogen
atoms for clarity of 1 with ellipsoids drawn at the 30% probability level.
(b) 3D structure of 1 with microporous cavity. (c) Schematic represen-
All gases used are of 99.999% purity throughout the gas
sorption measurements. Low-pressure nitrogen (N2), carbon
dioxide (CO2), methane (CH4), hydrogen (H2), water and
tation of the (4,5)-connected 2-nodal 3D network of
1 with
alcohol sorption experiments (up to 1 bar) were performed on (3·65)(32·4·67) topology: purple, L2− ligand; black, Cd1.
6812 | Dalton Trans., 2014, 43, 6811–6818
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
metal binding sites are accessible upon removal of co-
ordinated solvent molecules.
In 1, each L2− ligand links four Cd(II) atoms via two carboxy-
late groups respectively adopting μ1–η1:η1 and μ2–η1:η2 coordi-
nation modes, as well as one pyridyl donor, while each Cd(II)
connects four L2− anions (Fig. S1†), giving rise to a non-inter-
penetrated three-dimensional (3D) porous structure. Viewed
along the a axis in 1, there exist micropore windows of approxi-
mately 6.0 × 4.0 Å2, taking into account van der Waals dis-
tances (Fig. 1b). The solvent accessible volume per unit cell for
1 is calculated to be ca. 24.0% (472 Å3) using PLATON
program,9 where the ligated DMF molecules were excluded
from the framework. From a topological point of view,10 for
the structure of 1, each L2− ligand connects four Cd(II) atoms
to serve as a 4-connected node, whereas each Cd(II) center in
turn links four L2− and another Cd(II) atom to act as a 5-con-
nected node (Fig. S1†). Therefore, the overall structure of 1 can
be simplified as quite a rare binodal (4,5)-connected tcj/hc net
with the Schläfli symbol (3·65)(32·4·67) (Fig. 1c). The topology
of the network is identified by the transformation symbol11
tcj/hc; P63/mmc–P21/c (a − b, c, −2a; 1/2, 0, 0); bond sets:
5,6,7,9,14:tcj in the TOPOS binodal database. This symbol
suggests that the net is derived from the binodal net tcj/hc by
decreasing its space-group symmetry from P63/mmc to P21/c
with transformation of the unit cell by (a − b, c, −2a; 1/2, 0, 0)
vector.
Fig. 2 In situ temperature-dependent PXRD profiles for complex 1.
Simulated spectrum was calculated from the single crystal data.
Fig. 3 N2 (77 K) and CO2 (195 K) sorption isotherms for desolvated 1,
where filled and open shapes represent adsorption and desorption,
respectively.
Although the crystal data of complex 1 has also been
reported by other groups,12 the unsaturated metal binding
sites and micropore windows of the compound responsible for
the potential selective adsorption were not detected and men-
tioned completely in the literature.
decomposition of MOFs typically occurs at moderate tempera-
ture (423–573 K). Currently, only limited MOFs have been
reported to be stable beyond 673 K.13
It is interesting to note that 5-amino-benzene-1,2,3-tricarb-
oxylic acid may have a template effect in the preparation of 1,
although it is not incorporated into the final network struc-
ture. In the absence of 5-amino-benzene-1,2,3-tricarboxylic
acid, only undetermined polycrystalline samples were attained.
The thermal stability and bulk identity of 1 were verified by
PXRD and TGA. As revealed in Fig. S2,† the TGA curve of 1 at
298–443 K shows a 17.45% weight loss, which corresponds
well to the release of one coordinated DMF molecule (calcd
17.13%) per formula unit. The absence of a plateau after 673 K
suggests that the organic ligands start to decompose. Addition-
ally, activated sample 1 exhibits a similar TGA curve as that of
the as-synthesized compound (Fig. S2†), indicative of the good
thermal stability of the architecture after evacuation of co-
ordinated DMF molecules.
In situ temperature-dependent PXRD studies were recorded
for the as-synthesized sample of 1 under a N2 atmosphere
from room temperature to 673 K (Fig. 2). The peak positions of
the as-synthesized sample at ambient temperature agree well
with the simulated PXRD profiles, indicating the phase purity
of the products (Fig. S3†). Notably, there is no significant
change of the peak positions except only a slight broadening
of some diffraction peaks when the sample is heated up to
673 K, further suggesting that the framework is robust upon
removal of coordinating DMF molecules. Generally, the
The similarity of simulated PXRD forms of as-synthesized,
methanol-exchanged, and activated 1 means that the activated
sample maintains the same structure as that of 1. In addition,
complex 1 is air-stable even after several months (Fig. S4†).
This excellent stability is primarily assigned to the strongly co-
ordinated nitrogen atoms from L2− rather than purely co-
ordinated oxygen atoms of carboxylate, which is believed to be
fragile with respect to water molecules.14 Further, the chemical
stability of 1 is attested by suspending crystal samples of 1 in
boiling toluene, ethanol, DMF and water for 24 h. The resem-
bling PXRD patterns (Fig. S5†) confirm that the solid samples
retain structural integrity after the treatments.
The porosity as well as highly thermal and chemical stabi-
lity makes complex 1 a very promising potential gas storage
material. The desolvated 1 only exhibits external surface
adsorption of N2 at 77 K. In contrast, CO2 adsorption of acti-
vated 1 measured at 195 K and 1 bar shows a fully reversible
type-I isotherm with an adsorption amount of 76.7 cm3 g−1
,
thus indicating that 1 possesses permanent porosity (Fig. 3).15
Fitting the CO2 adsorption isotherm gives an apparent BET
(Brunauer–Emmett–Teller) surface area of 231.31 m2 g−1. More
importantly, the gas sorption and surface area of complex 1
after boiling it in different solvents were examined (Fig. S6†).
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 6811–6818 | 6813

View Article Online
Paper
Dalton Transactions
Table 1 CO2 enthalpy of adsorption for selected MOFs and 1
MOF
Qst (CO2) (kJ mol−1
)
Reference
1
44.5
62
44
47
42
35
21
This work
MIL-100(Cr)
MIL-101(Cr)
Mg-MOF-74
Ni-MOF-74
HKUST-1
CuBTTri
19
19
20
21
22
14a
that of MIL-100(Cr),19 quite close to those found for represen-
tative compounds bearing open metal sites, such as MIL-101-
(Cr), Mg-MOF-74 and Ni-MOF-74,19–22 but considerably higher
than those determined for well-studied compounds featuring
open metal sites, such as HKUST-1 and CuBTTri (Table 1). The
Qst decreased to ∼33.7 kJ mol−1 at the maximum measured
loading. Here, a relatively high initial adsorption enthalpy of
CH4 for 1 of 29.03 kJ mol−1 has been achieved (Fig. 4b), com-
parable to that of SUN-50′ (26.8 kJ mol−1),23 which is among
the best values reported thus far for MOF materials.24 It
should be pointed out that the high enthalpy of CO2 and CH4
adsorption of 1 stems principally from the presence of exposed
metal Cd(II) sites on the pore surface, which is easily polarized
due to the relatively large ionic radius and therefore could
interact strongly with the absorbed gas molecules.
Fig. 4 (a) Sorption isotherms for CO2, CH4, and N2 at 273 and 298 K of
desolvated 1 (adsorption and desorption branches are shown with filled
and empty shapes, respectively). (b) The isosteric heats of CO2 and CH4
adsorption (Qst) for desolvated 1.
The uptake capacities at approximate pressures of 0.15 bar
for CO2 and 0.75 bar for N2 have been used to effectively
perform the selectivity calculation for CO2 over N2, which is
Obviously, the CO2 adsorption isotherms at 195 K/1 bar before more favorable compared to the value based on the adsorbed
and after treatment are almost the same, and the estimated amounts of both CO2 and N2 at 1 bar. The latter substantially
apparent BET surface area is 224.4 m2 g−1 (after treatment), overestimates the fraction of CO2 in post-combustion flue gas
very close to that of as-made samples, which again strongly and the entire pressure of the binary mixture system.1d At
corroborates the high stability of 1.
298 K, the uptake amount of CO2 is 13.19 cm3 g−1 (2.59 wt%)
Type-I CO2 isotherms of the desolvated 1 are also probed at at 0.15 bar, which outperforms those of well-known MOFs
273 and 298 K (1 bar), both of which are completely reversible with open metal sites under similar conditions, such as
and exhibit a steep rise at the low-pressure region. Compound PCN-68 (0.8 wt%) and SNU-21S (2.25 wt%).25 In contrast, its N2
1 shows a maximum CO2 uptake of 46.9 cm3 g−1 (2.09 mmol adsorption is 1.6 cm3 g−1 (0.2 wt%) at 298 K and 0.75 bar.
g−1, 9.21%) at 273 K and 32.5 cm3 g−1 (1.45 mmol g−1, 6.38%) Therefore, the selectivity of CO2/N2 is 54 at 298 K, which is
at 298 K, respectively. This value is moderately high, compar- evaluated from the pure-component isotherms according to
able to that of earlier reported MOFs.16,17 Comparatively, the eqn (S1) (ESI†). As far as we know, such a high selectivity of
N2 uptake is only 3.8 cm3 g−1 (0.17 mmol g−1, 0.48%) and CO2/N2 has only been reported for several most promising
1.6 cm3 g−1 (0.071 mmol g−1, 0.2%) at 273 and 298 K (1 bar), MOFs,26 apparently illustrating that 1 may be a feasible candi-
respectively. Furthermore, the low-pressure CH4 absorptions date for the post-combustion capture of CO2. Also, the selec-
on evacuated 1 are also examined. As seen from the isotherm tivities were calculated using Henry’s law constants27 from the
data (Fig. 4a), the desolvated 1 has a maximum CH4 uptake of adsorption isotherms in the linear low pressure regime (<0.1
19.7 cm3 g−1 (0.879 mmol g−1, 1.41%) and 10.1 cm3 g−1 bar) and the Ideal Adsorbed Solution Theory (IAST)28 for the
(0.879 mmol g−1, 0.72%) at 273 and 298 K (1 bar). Clearly, CH4 CO2/N2 mixture in the ratio of 0.15/0.85 and the equimolar
sorption isotherms under ambient conditions show higher CO2/CH4 mixture. Therefore, the ratios for the initial slopes of
uptakes than that for N2, but much less compared to CO2.
CO2, N2 and CH4 adsorption isotherms were also used to esti-
To better understand these observations, the adsorption mate the adsorption selectivity for CO2 over N2 and CH4
enthalpies (Qst) were calculated by a virial method.18 Notably, (Fig. S8a†).29 From these data, the calculated CO2/N2 and CO2/
compound 1 exhibits a strong binding affinity for CO2 (44.5 kJ CH4 Henry’s selectivity is 45 : 1 and 15 : 1 at 298 K, respectively,
mol−1) at zero coverage (Fig. 4b), placing it at the third highest which are close to the superior values of MOFs with significant
record of MOFs with accessible metal sites so far, despite a sorption affinity for CO2 over N2 and CH4, such as ZIF-7829
surface area of only 202 m2 g−1. The value is just lower than (CO2/N2, 50.1; CO2/CH4, 10.6), Mg-MOF-7430 (CO2/N2, 49),
6814 | Dalton Trans., 2014, 43, 6811–6818
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
en-Cu-BTTri31 (CO2/N2, 44) and NJU-Bai832 (CO2/N2, 58.3; CO2/
CH4, 15.9). However, the CO2/N2 selectivity is apparently lower
than that of bio-MOF-11, which features the highest selectivity
value (75 : 1) under the same conditions.33 Additionally, as
illustrated in Fig. S8b,† the IAST selectivity of CO2 relative to
N2 is very sensitive to loading and demonstrates two steps in
the change of selectivity: a sharp decrease of CO2 selectivity at
the lower pressure region, and a slow increase at higher press-
ures (77.3–116.3). The predicted IAST value of CO2 over CH4
reaches 10.9–34.4 in the region of 0–1 bar at 298 K.
The significant CO2 sorption selectivity over CH4 and N2
under ambient conditions 1 can primarily be related to the
open metal cation sites, which function as charge-dense
binding sites and interact more strongly with CO2 because of
its greater quadrupole moment (CO2, 13.4 × 10−40 C m2; N2,
4.7 × 10−40 C m2) and polarizability (CO2, 29.1 × 10−25 cm−3
;
CH4, 25.9 × 10−25 cm−3; N2, 17.4 × 10−25 cm−3) compared with
CH4 and N2. In addition, the kinetic sieving effect may be
responsible for the high selectivity, where the small windows
(6.0 × 4.0 Å2) limit the diffusion of larger N2 (3.64 Å) and CH4
(3.8 Å) molecules into pores, consequently resulting in rela-
tively lower adsorption, whereas smaller CO2 molecules (3.3 Å)
are allowed to enter into the pore under the given conditions.
Taking into consideration that CH4 and N2 molecules are close
in size, the distinct uptakes of CH4 and N2 could not be simply
ascribed to the size-sieving effect. In fact, the different polariz-
ability of CH4 and N2 could be the reason why CH4 could inter-
act with the polar void more significantly.
Fig. 5 (a) Sorption isotherms for H2 at 77 and 87 K of desolvated 1,
where filled and open shapes represent adsorption and desorption,
respectively. (b) Isosteric heats of H2 adsorption (Qst) for 1.
Table 2 H2 enthalpy of adsorption for selected MOFs and 1
MOF
Qst (H2) (kJ mol−1
)
Reference
Low-pressure H2 sorption isotherms were recorded at 77
and 87 K as well to evaluate its H2 adsorption performance,
which were also both sharp in the low-pressure regime
and completely reversible, as shown in Fig. 5. A moderate
1
8.25
5.68–6.70
6.65
7.1
6.3
6.3
6.0
6.0
6.36
6.5
This work
NOTT series
SNU-21S
SNU-50′
UMCM-150
MIL-100
MIL-102
PCN-6′
34
35
23
36
37
38
39
40
41
H2-uptake of 0.73 and 0.66 wt% (81.6 and 73.5 cm3 −1) was
g
achieved at 77 and 87 K (1 bar) for 1, respectively. At zero
loading, Qst of H2 adsorption was calculated to be ∼8.25 kJ
mol−1, which is significantly greater than those of “benchmark
MOFs” with both larger surface area and exposed metal cation
sites (Table 2).23,34–41 In addition, the value surpasses the
average value (ca. 7.8 kJ mol−1) of isosteric heats of H2 adsorp-
tion for MOFs with vacant metal sites reported hitherto. The
H2 adsorption enthalpy of complex 1 is also higher than
the typical van der Waals-type interactions (4–6 kJ mol−1).42
The Qst decreased slowly with increasing H2 loading as
expected and reached ∼6.88 kJ mol−1 at the maximum cover-
age. Obviously, this high adsorption enthalpy unveiled a sig-
nificant interaction between H2 molecules and the framework,
which is possibly associated with the presence of accessible
open metal sites and narrow pores (6.0 Å) since an optimal
pore size of just slightly over twice the kinetic diameter of the
H2 molecules (2.9 Å) strengthens the interactions between
hydrogen molecules and pore walls.43 In our case, the prefer-
ential adsorption of H2 over N2 might be due to the size-exclu-
sion effect.
PCN-46
soc-MOF
framework displays preferential adsorption of H2 over N2 at
lower temperature. Apparently, 1 may be applied in capturing
CO2 from flue gases, natural gas purification, and hydrogen
enrichment of the H2/N2 exhaust in ammonia synthesis.
In order to further evaluate the porosity and adsorption
capacity of 1, we have also carried out vapor adsorption
measurements comparing water, methanol, ethanol, and i-pro-
panol at 298 K (Fig. 6). For 1, the isotherms of water, methanol
and ethanol show typical type-I adsorption. In addition, in the
low pressure region, the adsorptions of methanol and ethanol
reached saturation quickly, thereby demonstrating strong
host–guest interactions. The adsorption isotherms of water
and light alcohols exhibit obvious hysteresis loops, indicating
that desorption of water and alcohol molecules from the
micropores is difficult. This is presumably related to the
hydroxyl groups of water and alcohol molecules binding at
the open Cd(II) sites and forming H-bonds with the L2− ligand.
The systematic sorption studies clearly reveal that the de-
solvated 1 exhibits excellent selective adsorption of CO2 over
N2 and CH4 under ambient conditions. In addition, the
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 6811–6818 | 6815

View Article Online
Paper
Dalton Transactions
Conclusions
We have synthesized and structurally characterized a robust
multifunctional 3D microporous framework [Cd(L)(DMF)] (1)
with high thermal and chemical stability, and carried out a
systematic study to investigate and understand the effect of
open Cd(II) metal sites and small-sized pores on the selective
small adsorption of the compound. The activated 1 showed
high-efficiency for the selective capture of small molecules of
CO2 over N2 and CH4 under ambient conditions, which ranks
it among MOFs with the highest reported values of selectivity.
In addition, the desolvated 1 exhibits good separation ratios of
H2 over N2 at lower temperature, as well as alcohols from
water. In particular, the desolvated 1 demonstrates signifi-
cantly higher adsorption enthalpy for CO2 and CH4 gas mole-
cules, which are in the range of the highest Qst value reported
to date. More interesting, complex 1 displays intense green-
blue photoluminescence in the solid state under UV
irradiation at room temperature, which mainly originates from
the collaborative interaction between the d10 metal ion of Cd(II)
and the rigid organic ligand. The permanent porosity and
collaborative luminescent properties of complex 1 are particu-
larly useful to exploit potential luminescent detecting
materials.
Fig. 6 Water and alcohol vapor adsorption–desorption isotherms of
the desolvated 1: water, methanol, ethanol and i-propanol at 298 K,
where filled and open shapes represent adsorption and desorption,
respectively.
Fig. 7 The solid-state fluorescent spectra of 1 and free ligand L at room
temperature. The inset displays visible photoluminescence manifested
by 1 under UV irradiation.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (no. 21171062, 21371065, 21371150
and 21301148), Program for Chenguang Young Scientists of
Wuhan (2013070104010020). L.-L. Wen thanks Prof. Thomas
C. W. Mak of the Chinese University of Hong Kong for reading
the manuscript.
The adsorption isotherms also suggest that the saturated
uptake amount decreases with increasing kinetic diameter of
water and alcohol molecules (water: 2.64–2.9 Å; methanol:
3.626–4.0 Å; ethanol: 4.3–4.53 Å; i-propanol: 4.7 Å). The
maximum uptake of water into the pores was approximately
188 cm3 g−1, 74 cm3 g−1 for methanol, 38 cm3 g−1 for ethanol,
and 10 cm3 g−1 for i-propanol, which indicates the possibility
of selective separation of alcohols from water.
Notes and references
The solid state photoluminescent properties of the as-syn-
thesized 1 were explored at room temperature (Fig. 7). Upon
excitation by a UV lamp at 360 nm, 1 shows intense green-blue
luminescence (observed by naked eyes) with a maximum at
500 nm, whereas the free organic ligand exhibits a weaker
emission at 452 nm. Indeed, owing to the formation of the
network structure, an apparent fluorescent enhancement
along with a red-shift for 1 compared with that of organic
linkers has been observed. Obviously, the overall 3D architec-
ture has endowed the rigidity of the aromatic backbones and
maximized the intramolecular/intermolecular interactions
among the organic moieties for energy transfer, and hence
effectively reduced the intraligand HOMO–LUMO energy gap.
Moreover, the emission decay lifetimes of compound 1 were
monitored and the curves have been best fitted by biexponen-
tials in the solid.44 The emission decay lifetime of compound
1 is as follows: τ1 = 4.961 ns (25.35%) and τ2 = 1.450 ns (8.11%)
(Fig. S9†).
1 (a) S. D. Burd, S. Q. Ma, J. A. Perman, B. J. Sikora,
R. Q. Snurr, P. K. Thallapally, J. Tian, L. Wojtas and
M. J. Zaworotko, J. Am. Chem. Soc., 2012, 134, 3663–3666;
(b) P. Nugent, Y. Belmabkhout, S. D. Burd, A. J. Cairns,
R. Luebke, K. Forrest, T. Pham, S. Q. Ma, B. Space,
L. Wojtas, M. Eddaoudi and M. J. Zaworotko, Nature, 2013,
495, 80–84; (c) K. Sumida, D. L. Rogow, J. A. Mason,
T. M. McDonald, E. D. Bloch, Z. R. Herm, T. H. Bae and
J. R. Long, Chem. Rev., 2012, 112, 724–781; (d) J. R. Li,
J. Sculley and H. C. Zhou, Chem. Rev., 2012, 112, 869–932;
(e) S. H. Yang, X. Lin, W. Lewis, M. Suyetin,
E. Bichoutskaia, J. E. Parker, C. C. Tang, D. R. Allan,
P. J. Rizkallah, P. Hubberstey, N. R. Champness,
K. M. Thomas, A. J. Blake and M. Schröder, Nat. Mater.,
2012, 11, 710–716; (f) H. B. T. Jeazet, C. Staudt and
C. Janiak, Chem. Commun., 2012, 15, 2140–2142.
2 (a) S. Pramanik, C. Zheng, X. Zhang, T. J. Emge and J. Li,
J. Am. Chem. Soc., 2011, 133, 4153–4155; (b) Y. J. Cui,
6816 | Dalton Trans., 2014, 43, 6811–6818
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Y. F. Yue, G. D. Qian and B. L. Chen, Chem. Rev., 2012, 112,
1126–1162; (c) D. Tanaka, S. Horike, S. Kitagawa, M. Ohba,
M. Hasegawa, Y. Ozawac and K. Toriumi, Chem. Commun.,
2007, 3142–3144; (d) J. H. Cui, Z. Z. Lu, Y. Z. Li, Z. J. Guo
and H. G. Zheng, Chem. Commun., 2012, 48, 7967–7969;
(e) A. L. Lan, K. H. Li, H. H. Wu, D. H. Olson, T. J. Emge,
Chem. Mater., 2010, 22, 3316–3322; (b) S. Q. Ma,
X. S. Wang, D. Q. Yuan and H. C. Zhou, Angew. Chem., Int.
Ed., 2008, 47, 4130–4133; (c) V. Colombo, S. Galli,
H. J. Choi, G. D. Han, A. Maspero, G. Palmisano,
N. Masciocchi and J. R. Long, Chem. Sci., 2011, 2, 1311–
1319.
W. Ki, M. C. Hong and J. Li, Angew. Chem., Int. Ed., 2009, 14 (a) A. Demessence, D. M. D’Alessandro, M. L. Foo and
48, 2334–2338.
J. R. Long, J. Am. Chem. Soc., 2009, 131, 8784–8786;
(b) H. J. Choi, M. Dinca, A. Dailly and J. R. Long, Energy
Environ. Sci., 2010, 3, 117–123; (c) K. A. Cychosz and
A. J. Matzger, Langmuir, 2010, 26, 17198–17102.
3 (a) H. L. Jiang, Y. Tatsu, Z. H. Lu and Q. Xu, J. Am. Chem.
Soc., 2010, 132, 5586–5587; (b) Y. Takashima,
V. M. Martínez, S. Furukawa, M. Kondo, S. Shimomura,
H. Uehara, M. Nakahama, K. Sugimoto and S. Kitagawa, 15 (a) M. S. Chen, M. Chen, S. Takamizawa, T.-a. Okamura,
Nat. Commun., 2011, 2, 168; (c) B. L. Chen, Y. Yang,
F. Zapata, G. Lin, G. D. Qian and E. B. Lobkovsky, Adv.
Mater., 2007, 19, 1693–1696; (d) B. L. Chen, L. B. Wang,
Y. Q. Xiao, F. R. Fronczek, M. Xue, Y. J. Cui and G. D. Qian,
J. Fan and W. Y. Sun, Chem. Commun., 2011, 47, 3787–3789;
(b) M. S. Chen, M. Chen, T.-a. Okamura, W. Y. Sun and
N. Ueyama, Microporous Mesoporous Mater., 2011, 139,
25–30.
Angew.
Chem.,
Int.
Ed.,
2009,
48,
500–503; 16 (a) R. R. Yun, J. G. Duan, J. F. Bai and Y. Z. Li, Cryst. Growth
(e) B. V. Harbuzaru, A. Corma, F. Rey, P. Atienzar,
J. L. Jordá, H. García, D. Ananias, L. D. Carlos and J. Rocha,
Angew. Chem., Int. Ed., 2008, 47, 1080–1083.
Des., 2013, 13, 24–26; (b) L. Hou, L. N. Jia, W. J. Shi,
Y. Y. Wang, B. Liu and Q. Z. Shi, Dalton Trans., 2013, 42,
3653–3659.
4 (a) M. P. Suh, H. J. Park, T. K. Prasad and D. W. Lim, Chem. 17 (a) C. Hou, Q. Liu, T.-a. Okamura, P. Wang and W. Y. Sun,
Rev., 2012, 112, 782–835; (b) Z. J. Zhang, Y. G. Zhao,
Q. H. Gong, Z. Li and J. Li, Chem. Commun., 2013, 49, 653–
CrystEngComm, 2012, 14, 8569–8576; (b) Y. P. He, Y. X. Tan
and J. Zhang, Cryst. Growth Des., 2013, 13, 6–9.
661; (c) T. A. Makal, J. R. Li, W. G. Lu and H. C. Zhou, 18 J. L. C. Rowsell and O. M. Yaghi, J. Am. Chem. Soc., 2006,
Chem. Soc. Rev., 2012, 41, 7761–7779; (d) J. G. Duan, 128, 1304–1315.
M. Higuchi, S. Horike, M. L. Foo, K. P. Rao, Y. Inubushi, 19 P. L. Llewellyn, S. Bourrelly, C. Serre, A. Vimont, M. Daturi,
T. Fukushima and S. Kitagawa, Adv. Funct. Mater., 2013, 23,
3525–3530.
5 (a) H. H. Li, W. Shi, K. N. Zhao, Z. Niu, X. T. Chen and
L. Hamon, G. D. Weireld, J.-S. Chang, D.-Y. Hong,
Y. K. Hwang, S. H. Jhung and G. Férey, Langmuir, 2008, 24,
7245–7250.
P. Cheng, Chem.–Eur. J., 2012, 18, 5715–5723; (b) X. Tan, 20 S. R. Caskey, A. G. Wong-Foy and A. J. Matzger, J. Am.
J. X. Zhan, J. Y. Zhang, L. Jiang, M. Pan and C. Y. Su, Cryst- Chem. Soc., 2008, 130, 10870–10871.
EngComm, 2012, 14, 63–66; (c) J. H. Jia, H. S. Athwal, 21 P. D. C. Dietzel, R. E. Johnsen, H. Fjellvag, S. Bordiga,
A. J. Blake, N. R. Champness, P. Hubberstey and
M. Schröder, Dalton Trans., 2011, 40, 12342–12349;
E. Groppo, S. Chavan and R. Blom, Chem. Commun., 2008,
5125–5127.
(d) S. J. Liu, L. Xue, T. L. Hu and X. H. Bu, Dalton Trans., 22 Q. M. Wang, D. Shen, M. Bülow, M. L. Lau, S. Deng,
2012, 41, 6813–6819; (e) S. L. Xiang, J. Huang, L. Li,
J. Y. Zhang, L. Jiang, X. J. Kuang and C. Y. Su, Inorg. Chem.,
2011, 50, 1743–1748.
F. R. Fitch, N. O. Lemcoff and J. Semanscin, Microporous
Mesoporous Mater., 2002, 55, 217–230.
23 T. K. Prasad, D. H. Hong and M. P. Suh, Chem.–Eur. J.,
2010, 16, 14043–14050.
6 SAINT, version 6.2, Bruker AXS, Inc., Madison, WI, 2001.
7 G. M. Sheldrick, SADABS, University of Göttingen, Göttin- 24 (a) S. Q. Ma, D. F. Sun, J. M. Simmons, C. D. Collier,
gen, Germany, 1997.
8 G. M. Sheldrick, SHELXTL, version 6.10, Bruker Analytical
X-ray Systems, Madison, WI, 2001.
D. Q. Yuan and H. C. Zhou, J. Am. Chem. Soc., 2008, 130,
1012–1016; (b) Y. Peng, V. Krungleviciute, I. Eryazici,
J. T. Hupp, O. K. Farha and T. Yildirim, J. Am. Chem. Soc.,
2013, 135, 11887–11894.
9 A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7–13.
10 A. T. Balaban, From Chemical Topology to Three-Dimensional 25 (a) D. Q. Yuan, D. Zhao, D. F. Sun and H. C. Zhou, Angew.
Geometry, Plenum Press, New York, 1997.
11 (a) V. A. Blatov, TOPOS, A Multipurpose Crystallochemical
Chem., Int. Ed., 2010, 49, 5357–5361; (b) T. K. Kim and
M. P. Suh, Chem. Commun., 2011, 47, 4258–4260.
Analysis with the Program Package, Russia, 2004; 26 (a) J. A. Mason, K. Sumida, Z. R. Herm, R. Krishna and
(b) V. A. Blatov and D. M. Proserpio, Acta Crystallogr., Sect.
A: Found. Crystallogr., 2009, 63, 329–343.
12 (a) C. Hou, Y. Zhao, T.-a. Okmura, P. Wang and W. Y. Sun,
J. Coord. Chem., 2012, 65, 4409–4418; (b) F. Wang,
X. M. Jing, B. Zheng, G. H. Li, G. Zeng, Q. S. Huo and
Y. L. Liu, Cryst. Growth Des., 2013, 13, 3522–3527.
J. R. Long, Energy Environ. Sci., 2011, 4, 3030–3040;
(b) P. Aprea, D. Caputo, N. Gargiulo, F. Iucolano and
F. J. Pepe, Chem. Eng. Data, 2010, 55, 3655–3661;
(c) T. M. McDonald, D. M. D’Alessandro, R. Krishna and
J. R. Long, Chem. Sci., 2011, 2, 2022–2028.
27 (a) A. L. Myers and J. M. Prausnitz, AIChE J., 1965, 11, 121–
127; (b) Y.-S. Bae, O. K. Farha, A. M. Spokoyny,
C. A. Mirkin, J. T. Hupp and R. Q. Snurr, Chem. Commun.,
13 (a) M. Gustafsson, A. Bartoszewicz, B. Martín-Matute,
J. L. Sun, J. Grins, T. Zhao, Z. Y. Li, G. S. Zhu and X. D. Zou,
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 6811–6818 | 6817

View Article Online
Paper
Dalton Transactions
2008, 4135–4137; (c) Y.-S. Bae, O. K. Farha, J. T. Hupp and 37 M. Latroche, S. Suble, C. Serre, C. Mellot-Draznieks,
R. Q. Snurr, J. Mater. Chem., 2009, 19, 2131–2134.
P. L. Llewellyn, J.-H. Lee, J.-S. Chang, S. H. Jhung and
28 (a) H. A. Patel, S. H. Je, J. Park, D. P. Chen, Y. Jung,
G. Férey, Angew. Chem., Int. Ed., 2006, 45, 8227–8231.
C. T. Yavuz and A. Coskun, Nat. Commun., 2013, 4, 1357; 38 S. Surblé, F. Millange, C. Serre, T. Düren, M. Latroche,
(b) J. G. Duan, Y. Zhen, J. F. Bai, B. S. Zheng, Y. Z. Li and
S. H. Li, Chem. Commun., 2012, 48, 3058–3060.
S. Bourrelly, P. L. Llewellyn and G. Férey, J. Am. Chem. Soc.,
2006, 128, 14889–14896.
29 R. Banerjee, H. Furukawa, D. Britt, C. Knobler, M. O’Keeffe 39 H.-C. Zhou, S. Q. Ma, D. F. Sun, M. Ambrogio, J. A. Fillinger
and O. M. Yaghi, J. Am. Chem. Soc., 2009, 131, 3875–
3877.
30 (a) S. R. Caskey, A. G. Wong-Foy and A. J. Matzger, J. Am.
and S. Parkin, J. Am. Chem. Soc., 2007, 129, 1858–1859.
40 D. Zhao, D. Q. Yuan, A. Yakovenko and H.-C. Zhou, Chem.
Commun., 2010, 46, 4196–4198.
Chem. Soc., 2008, 130, 10870–10871; (b) H. H. Wu, 41 Y. Liu, J. F. Eubank, A. J. Cairns, J. Eckert, V. C. Kravtsov,
R. S. Reali, D. A. Smith, M. C. Trachtenberg and J. Li,
Chem.–Eur. J., 2010, 16, 13951–13954.
R. Luebke and M. Eddaoudi, Angew. Chem., Int. Ed., 2007,
46, 3278–3283.
31 K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, 42 (a) W. Zhou, H. Wu, M. R. Hartman and T. Yildirim,
E. D. Bloch, Z. R. Herm, T. H. Bae and J. R. Long, Chem.
Rev., 2012, 112, 724–781.
32 L. T. Du, Z. Y. Lu, K. Y. Zheng, J. Y. Wang, X. Zheng, Y. Pan,
J. Phys. Chem. C, 2007, 111, 16131–16137; (b) A. G. Wong-
Foy, O. Lebel and A. J. Matzger, J. Am. Chem. Soc., 2007,
129, 15740–15741.
X. Z. You and J. F. Bai, J. Am. Chem. Soc., 2013, 135, 562– 43 (a) J. Luo, H. Xu, Y. Liu, Y. Zhao, L. L. Daemen, C. Brown,
565.
T. V. Timofeeva, S. Ma and H.-C. Zhou, J. Am. Chem. Soc.,
2008, 130, 9626–9627; (b) S. Barman, H. Furukawa,
O. Blacque, K. Venkatesan, O. M. Yaghi and H. Berke,
Chem. Commun., 2010, 46, 7981–7983; (c) B. Chen, X. Zhao,
A. Putkham, K. Hong, E. B. Lobkovsky, E. J. Hurtado,
A. J. Fletcher and K. M. Thomas, J. Am. Chem. Soc., 2008,
130, 6411–6423.
33 J. An, S. J. Geib and N. L. Rosi, J. Am. Chem. Soc., 2010, 132,
38–39.
34 X. Lin, I. Telepeni, A. J. Blake, A. Dailly, C. M. Brown,
J. M. Simmons, M. Zoppi, G. S. Walker, K. M. Thomas,
T. J. Mays, P. Hubberstey, N. R. Champness and
M. Schröder, J. Am. Chem. Soc., 2009, 131, 2159–2171.
35 T. K. Kim and M. P. Suh, Chem. Commun., 2011, 47, 4258– 44 (a) X. Li, H. L. Sun, X. S. Wu, X. Qiu and M. Du, Inorg.
4260.
Chem., 2010, 49, 1865–1871; (b) J. S. Hu, L. F. Huang,
X. Q. Yao, L. Qin, Y. Z. Li, Z. J. Guo, H. G. Zheng and
Z. L. Xue, Inorg. Chem., 2011, 50, 2404–2414.
36 A. G. Wong-Foy, O. Lebel and A. J. Matzger, J. Am. Chem.
Soc., 2007, 129, 15740–15741.
6818 | Dalton Trans., 2014, 43, 6811–6818
This journal is © The Royal Society of Chemistry 2014