Two Li-Zn cluster-based metal-organic frameworks: Strong H2/CO2 binding and high selectivity to CO2

Article abstract of DOI:10.1021/acs.inorgchem.6b02407

Two metal-organic frameworks (MOFs) {(Me₂NH₂)[ZnLi(PTCA)(H₂O)]}nn{3DMFC₄H₈O₂ 4H₂O} (1) and {(Me₂NH₂)[ZnLi(PTCA)]}_nn{3DMF 5H₂O} (2) have been constructed from Li-Zn clusters and pyrene-1,3,6,8-tetracarboxylic acid (H₄PTCA) under solvothermal conditions. Gas sorption measurements have revealed that the pore of desolvated 2 (2d) can strongly interact with H2 and CO₂, with high H₂ and CO₂ adsorption heats of 15.3 and 51.9 kJ/mol, respectively. Furthermore, 2d can selectively adsorb CO₂ over N₂and CH₄, with high adsorption selectivity of CO₂/N₂ and CO₂/CH₄.

Full text of DOI:10.1021/acs.inorgchem.6b02407

Communication
pubs.acs.org/IC
Two Li−Zn Cluster-Based Metal−Organic Frameworks: Strong H2/CO2
Binding and High Selectivity to CO2
Yong-Liang Huang,§ Di-Chang Zhong,*,†,‡ Long Jiang,§ Yun-Nan Gong,‡ and Tong-Bu Lu*,†,§
†Institute of New Energy Materials & Low Carbon Technology, School of Material Science & Engineering, Tianjin University of
Technology, Tianjin 300384, China
‡School of Chemistry and Chemical Engineering, Gannan Normal University, Ganzhou 341000, China
§MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen
University, Guangzhou 510275, China
S
* Supporting Information
and a tetra-carboxylate ligand of pyrene-1,3,6,8-tetracarboxylic
ABSTRACT: Two metal−organic frameworks (MOFs)
{(Me2NH2)[ZnLi(PTCA)(H2O)]}n·n{3DMF·C4H8O2·
4H2O} (1) and {(Me2NH2)[ZnLi(PTCA)]}n·n{3DMF·
5H2O} (2) have been constructed from Li−Zn clusters
and pyrene-1,3,6,8-tetracarboxylic acid (H4PTCA) under
solvothermal conditions. Gas sorption measurements have
revealed that the pore of desolvated 2 (2d) can strongly
interact with H2 and CO2, with high H2 and CO2
adsorption heats of 15.3 and 51.9 kJ/mol, respectively.
Furthermore, 2d can selectively adsorb CO2 over N2 and
CH4, with high adsorption selectivity of CO2/N2 and
CO2/CH4.
acid (H4PTCA). Their syntheses, crystal structures, and gas
sorption properties have been thoroughly investigated and
discussed.
A solvothermal reaction of Zn(NO3)2·6H2O and LiCl with
H4PTCA in DMF/dioxane/H2O (2:1:1) at 110 and 140 °C for
48 h led to the formation of yellow crystals of 1 and 2,
respectively (Supporting Information). The result of single-
crystal X-ray diffraction analysis has revealed that 1 crystallizes in
a monoclinic crystal system, with a space group of P21/c (Table
S1). The asymmetric unit of 1 contains one crystallographically
independent Zn(II), one independent Li(I), and one PTCA4−
+
ligand, as well as one Me2NH2 counterion, three DMF, one
dioxane, and four H2O molecules. Zn1 is four-coordinated by
four carboxylate O atoms from four individual PTCA4−, forming
a tetrahedral coordination geometry; Li1 is also four-coordinated
with a tetrahedral coordination geometry. The coordinated
atoms around Li1 are three carboxylate O atoms from three
individual PTCA4− and one O from one coordinated H2O
(Figure 1a). PTCA4− employs two types of coordination modes.
One bridges four pairs of Zn1 and Li1 by four carboxylate groups
(Scheme S2, Mode I), and the other coordinates with four Zn1
and two Li1, where two pairs of Zn1 and Li1 are bridged by two
carboxylate groups and the other two Zn1 are bonded to the
other two carboxylate groups (Scheme S2, Mode II). Zn1 and
Li1 are bridged by three μ2-COO from two PTCA4− in Mode I
and one PTCA4− in Mode II to form a heterobimetallic cluster of
[ZnLi(CO2)3]. The distance of Zn1···Li1 is 3.411(3) Å. Each
[ZnLi(CO2)3] cluster connects with four PTCA4−, and each
PTCA4− links to four [ZnLi(CO2)4] clusters, generating a 3D
(4,4)-connected framework with 1D channels along the c axis
(Figures 1b and S1). The size of the channel is 6.24 Å × 9.63 Å
(the distances of Li1···O8 and Zn1···O6, respectively). The
he interests in simple fabrication and structure inves-
T
tigations of metal−organic frameworks (MOFs) have
evidently decreased during the past several years. However,
research enthusiasm on extending MOF applications is rapidly
increasing. Particularly, ultrafine tuning of the pore size and
delicate modification of the pore surface to endow MOFs with
unique functions have attracted more attention.1 For instance, a
class of “pillared square grids” MOFs named SIFSIX,2−6 which
are constructed from two-dimensional (2D) nets as layers and
hexafluorosilicate (SiF62−) anions as pillars,3 has exhibited
exceptional performance for separation of gas molecules, such
as CO2 from CH4 and N2,4 acetylene from ethylene,5 and
propylene from propane,6 due to their tunable pore sizes and
SiF62−-functionalized pore surfaces. Additionally, a number of
MOFs with pore surfaces decorated with coordinatively
unsaturated metal sites,7 amino groups,8 and exposed N
atoms9 have shown strong binding affinities to gas molecules
such as H2, CO2, and so on, and improved H2 storage or CO2
capture capacities. By this token, untrafine design of the pore
structure is a significant approach to thoroughly develop MOF
applications.
Recently, MOFs with Li-lined pore surfaces, obtained by ion-
exchange, have attracted considerable attention, as Li atoms on
pore surfaces can significantly enhance gas storage.10 However,
Li-MOFs or Li-participated MOFs have been sporadically
reported.11 In this Communication, we report two 3D MOFs,
{(Me2NH2)[ZnLi(PTCA)(H2O)]}n·{3DMF·dioxane·4H2O}n
(1) and {(Me2NH2)[ZnLi(PTCA)]}n·{3DMF·5H2O}n (2),
which are constructed from heterobimetallic Li−Zn clusters
+
Me2NH2 counterions, originating from the decomposition of
DMF molecules, fill in the pore together with the disordered
1
DMF, dioxane, and H2O molecules, which is confirmed by H
NMR analysis (Figure S2a). About 42.2% of the solvent
accessible volume is estimated by the PLATON procedure.12
Compound 2 crystallizes in the space group of C2/m (Table
S1). As shown in Figure 2a, the crystallographically independent
Received: October 9, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b02407
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Figure 1. (a) Coordination environments of Zn(II) and Li(I) and
coordination modes of PTCA4− in 1 (symmetry codes: (i) 1 − x, −1/2 +
y, 1/2 − z; (ii) −x, −1/2 + y, 1/2 − z). (b) 3D porous structure of 1 with
1D triangular channels along the c axis.
Figure 2. (a) Coordination environments of Zn(II) and Li(I) and
coordination modes of PTCA4− in 2 (symmetry codes: (i) x, y, 1 − z; (ii)
1/2 − x, 1/2 − y, −z; (iii) x, y, 1 + z). (b) 3D structure of 2 with 1D
trapezium channels along the b axis.
Zn(II) and Li(I) also assume four-coordinated tetrahedral
coordination geometries. All the coordinated atoms around
Zn1/Li1 come from the carboxylate O atoms of PTCA4−. The
PTCA4− also adopts two coordination modes. One coordinates
with four Zn and six Li through four carboxylate groups (Scheme
S2, Mode III), and the other only bonds to four Zn by four
carboxylate groups (Scheme S2, Mode IV). Six carboxylate
groups of six PTCA4− in Mode III connect two Zn1 and two Li1
together to form a planar heterotetranuclear cluster of
[Zn2Li2(CO2)6]. The distance of Zn1···Li1 is 2.953(2) Å,
much shorter than that in 1. Each [Zn2Li2(CO2)6] cluster is
surrounded by eight PTCA4−, and each PTCA4− is encircled by
four [Zn2Li2(CO2)6] clusters, resulting in a 3D (4,8)-connected
porous framework with 1D trapezium channels (Figures 2b and
S3). The size of the channel is 8.58 Å × 7.63 Å (the distances of
C20···O4 and Zn1···O5, respectively), which is filled with
H2O molecules (Calcd 11.2%). Further heated to 360 °C, the
three DMF molecules lose (Calcd 27.3%), then followed the
decomposition of the rest of 2 (Figure S5). To examine the
framework stabilities of 1 and 2, freshly prepared 1/2 was soaked
in methanol/acetonitrile to exchange the guest molecules in the
channels of 1/2. The results of the XRD measurements have
revealed that the framework of 1 collapses after being exchanged
by methanol/acetonitrile (Figure S4a); the framework of 2 can
remain stable after being exchanged by acetonitrile (Figure S4b).
Even with further activation by removal of acetonitrile, the
framework of 2 can remain stable (Figure S4b). The variable-
temperature PXRD for 1 and 2 have given similar results. The
diffraction peaks of 1 disappear when the sample is heated to 120
°C (Figure S6a), illustrating the collapse of the framework; the
diffraction peaks of 2 show no obvious change from 30 to 200 °C
(Figure S6b), demonstrating that the framework of 2 is stable up
to 200 °C. Although both 1 and 2 are constructed from Li−Zn
clusters and the PTCA4− ligand, they show obviously different
framework stabilities. This may be ascribed to the different
structures of the Li−Zn clusters within 1 and 2. The tetranuclear
[Zn2Li2(CO2)6] clusters in 2 are stronger than the bimetallic
[ZnLi(CO2)3] clusters in 1 to strut such large frameworks after
removing the guests inside the cavities.
To further evaluate the permanent porosity in desolvated 2
(2d), N2, H2, and CO2 sorption measurements were carried out
(Supporting Information). The results have shown that 2d
selectively adsorbs H2 and CO2, rather than N2 (Figure 3a). The
isotherm of the CO2 adsorption at 195 K shows a typical I curve,
with the amount of CO2 adsorption increasing abruptly at low
pressure and gradually approaching 133 cm3 (STP)/g at 1.0 atm
(Figure 3a). Fitting the Brunauer−Emmett−Teller (BET)
equation to the sorption isotherm at 195 K gives the estimated
surface area of 602.5 m2/g. The H2 adsorption isotherm for 2d
shows an adsorption capacity of 135 cm3 g−1 (∼1.2 wt %) at 77 K
+
counterion Me2NH2 , disordered DMF, and lattice H2O
1
molecules, confirmed by H NMR analysis (Figure S2b). The
calculated porosity per unit cell volume is 43.8%.12
Compounds 1 and 2 were obtained by tuning reaction
temperature, illustrating that temperature tuning is a useful
approach for constructions of function coordination polymers.13
Single-crystal structural analyses have clearly given the Li/Zn
molar ratio of 1:1 within 1 and 2, which is further evidenced by
the results of ICP-AES analyses for 1 and 2 (Table S2). The bulk
phase purity of 1 and 2 were examined by powder X-ray
diffraction. The results have shown that all the peaks displayed in
the measured patterns closely match with those in the simulated
ones generated from single-crystal diffraction data (Figure S4),
indicating that single phases of 1 and 2 have been formed.
The TG curve of 1 shows a weight loss of 19.5% in the
temperature range from 30 to 180 °C (Figure S5), corresponding
to the removal of one dioxane and five water molecules (Calcd.
20.0%). Then from 180 to 360 °C, a weight loss of 23.3% is
observed, corresponding to the removal of three DMF molecules
(Calcd 24.6%). Upon further heating, the desolvated 1 begin to
decompose. From 30 to 100 °C, the TG curve of 2 shows a
weight loss of 10.5%, corresponding to the removal of five lattice
B
DOI: 10.1021/acs.inorgchem.6b02407
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Inorganic Chemistry
Communication
with high values of adsorption selectivity, demonstrating that 2
may have applictions in natural gas and flue gas purification.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02407.
■
S
Description of syntheses and structure characterizations of
H4PTCA, 1, and 2; gas sorption measurements and data
handling; additional tables and figures. (PDF)
CIF file for 1. (CIF)
CIF file for 2. (CIF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: zhong_dichang@hotmail.com (D.-C. Z.).
*E-mail: lutongbu@mail.sysu.edu.cn (T.-B.L.).
■
Figure 3. Gas adsorption isotherms of 2d for (a) N2 at 77 K, H2 at 77 and
87 K, and CO2 at 195 K and (b) CO2, N2, and CH4 at 273 and 298 K,
respectively. (c) Adsorption heat plots of 2d for H2 and CO2. (d)
Adsorption selectivity plots of 2d for CO2/N2 and CO2/CH4 at 273 K.
ORCID
Di-Chang Zhong: 0000-0002-5504-249X
Notes
The authors declare no competing financial interest.
and 1 atm (Figure 3a). Moreover, the adsorption isotherms
present rare steep slopes at low pressure at either 77 or 87 K,
suggesting that the pores in 2d can strongly bind with H2,14
which is supported by the high adsorption heat of 15.3 kJ/mol at
the initial pressure, calculated using the modified Clausius−
Clapeyron equation15 (Figure 3c and Supporting Information).
To the best of our knowledge, this adsorption heat is comparable
to those of the reported MOFs.16
ACKNOWLEDGMENTS
■
This work was supported by the 973 Program of China
(2012CB821706, 2014CB845602), NSFC (21331007,
21363001, 21401026), and NSF of Guangdong Province
(S2012030006240).
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■
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DOI: 10.1021/acs.inorgchem.6b02407
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