A pillared-layer framework with high uptake and selective sorption of light hydrocarbons

Article abstract of DOI:10.1039/c6dt00238b

A new pillared-layer metal-organic framework [Zn₂(NH₂-BTB)(2-nim)] (1) was successfully synthesized based on mixed ligands, where binuclear [Zn₂(CO₂)₃]⁺ cluster-based cationic layers [Zn₂(NH₂-BTB)]⁺ (NH₂-H₃BTB = 1,3,5-(three-benzoic acid)aniline) are connected by 2-nitroimidazoles. Compound 1 shows high uptake and good adsorption selectivities for C₃/C₁ and C₂/C₁. Particularly, the C₂H₂ uptake capacity of 1 is up to 152 cm³ g^-1 at 273 K and the selectivity for C₃H₈/CH₄ is over 85 at room temperature.

Full text of DOI:10.1039/c6dt00238b

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ARTICLE TYPE
A pillared-layer Framework with High Uptake and Selective Sorption of
Light Hydrocarbons
Qing-Rong Ding and Fei Wang*
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
5
DOI: 10.1039/b000000x
A new pillared-layer Metal-Organic Framework [Zn2(NH2-BTB)(2-nim)] (1) was successfully synthesized base on mixed ligands, where
binuclear [Zn2(CO2)3]+ cluster-based cationic layers [Zn2(NH2-BTB)]+ (NH2-H3BTB = 1, 3, 5- (three-benzoic acid) aniline) are
connected by 2-nitroimidazoles. Compound 1 shows high uptake and good adsorption selectivities of C3/C1 and C2/C1. Especially, the
C2H2 uptake capacity of 1 is up to 152 cm3 g−1 at 273 K and the selectivity of C3H8/CH4 is over 85 at room temperature.
10
Introduction
In recent years, porous coordination polymers (PCPs) or metal-
organic frameworks (MOFs) have attracted much attention not
only for their fascinating architectures and intriguing
15 topologies,1 but also for potential applications in selective gas
storage and separation.2 Light hydrocarbons are very important
raw materials for chemical industry and global climate.3
Methane as the principal component of natural gas, is the priori
alternative fuel. C2 hydrocarbons (C2s) play a crucial role in
20 the industrial process. High purity hydrocarbon gas is a
prerequisite for these chemical procedures.4 So it is necessary to
explore materials for high-efficient storage and separation of
hydrocarbons.
Compared to distillation-based technologies, adsorptive
25 separation is widely considered as a more environment-friendly
and cost-efficient alternative.5 Up to date, a variety of porous
adsorbents, such as zeolites, carbon materials, diatomite, have
been investigated for hydrocarbon separations.6 A number of
MOFs have been studied for applications in purification and
30 storage of these C1, C2 and C3 light hydrocarbons.7 However,
the application of these promising materials are limited by the
separation ratio of C3/C1 or C2/C1 compared to the commercial
materials.8 More efforts are needed to improve the selectivity of
light hydrocarbons and enhance gas uptake capacity.
Fig. 1 a) The coordination environment in compound 1; b) 3D pillared-
layer framework of 1 along the a axis (Blue pillar, 2-nitroimidazole;
Gree hexahedron, [Zn2(CO2)3]+ cluster); c); 3D framework of 1 along
the b axis; d) 2-fold interpenetrated (3,5)-connected hms net of 1.
50 Experimental section
Material and instrumentation
All reagents and solvents used in this work were commercially
available. 1,3,5-(tri-benzoic acid)aniline was synthesized via
suzuki reaction.9 Powder X-ray diffraction (PXRD) patterns
55 were collected on a MiniFlex-II diffractometer using Cu (λ =
35
Here, one pillared-layer MOF [Zn2(NH2-BTB)(2-nim)] (1,
NH2-H3BTB = 1, 3, 5-(three-benzoic acid) aniline, 2-nim = 2-
nitroimidazole) was designed and synthesized via the method of
charge balance and mixd ligands. The rigid tritopic aromatic
polycarboxylate ligands coordinate binuclear zinc ions to form
1.54178 Å) radiation with
a
speed of
1
°/min.
Thermogravimetric analysis (TGA) was performed in nitrogen
by using a Netzsch STA449C equipped with a platinum pan
and heated at a rate of 10 °C per minute. Gas sorption
60 experiments were carried out with Accelerated Surface Area
and Porosimetry (ASAP) 2020 system has good crystallinity
after the removal of solvent molecules. The N2 sorption
isotherms were acquired in the pressure rage of P/PO from 0.01
to 0.99 at 77 K in a liquid nitrogen bath. The gas sorption
65 experiments of CO2, CH4, C2H2, C2H4, C2H6, C3H8 and N2 were
conducted at 273 K in a ice-water bath or 297 K. Elemental
analyses were performed on a Vario EL III elemental analyzer.
40 a cationic layer [Zn2(NH2-BTB)]n with special tritopic paddle-
wheel zinc clusters, then the layers are further connected to a
3D neutral pillared–layer framework by 2-nitroimidazole
pillars. The porous material of 1 exhibit high CO2 and light
hydrocarbon storage capacity as well as good C3H8/CH4,
45 C2H6/CH4, C2H4/CH4 and C2H2/CH4 selective separations.
Especially, the selectivity of C3H8/CH4 is over 85 at room
temperature and the C2H2 uptake capacity of 1 is up to 152 cm3
g−1 at 273 K, is the higher one ever reported.
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Synthesis of [Zn2(NH2-BTB)(2-nim)] (1). A mixture of 2-
nitroimidazole (2-nim, 0.0156 g, 0.14 mmol), NH2-H3BTB
(0.0156 g, 0.036 mmol) and Zn(NO3)2·6H2O (0.0416 g, 0.14
mmol), Tetraethylammonium bromide (0.075 mmol, 0.0156 g),
which was used to facilitate the crystallization of product, and
DMF/H2O (v:v = 3:0.5, 3.5 mL) was heated in a 20 mL
scintillation vials at 100°C for 24 hours, followed by cooling to
room temperature. The resulting mixture was washed with
DMF and water, and colorless block crystal were collected and
5
10 dried in air. Calcd for [Zn2(NH2-BTB)][2-nim]·DMF·6H2O: C,
45.32; H, 4.26; N, 8.04; found: C, 44.97; H, 4.54; N, 8.37.
X-ray diffraction analysis. Single-crystal x-ray structural
measurement analysis was performed on a Agilent Super Nava
X-ray diffractometer with Cu‒Kα radiation (λCu-Kα＝1.54178
15 Å) at 100 K. The structure was solved using the direct method
and refined by full-matrix least-squares methods on F2 by using
the SHELX-97 program package.10 Non-hydrogen atoms were
refined anisotropically. The hydrogen atoms of the ligands were
generated theoretically onto the specific atoms and refined
20 isotropically with fixed thermal factors. Part solvent molecules
in the structure were randomly dispersed, and thus their
positions were impossible to refine using conventional discrete-
atom models. The SQUEEZE option of PLATON was used to
eliminate the contribution of disordered guest molecules to the
25 reflection intensities.11 Crystallographic data for compound 1 is
listed in Table S1.
Gas selectivity. The selectivities of C3, C2 over CH4 were
calculated by the ideal adsorption solution theory (IAST).12
IAST is a method for predicting the adsorption equilibria for
Fig. 2 (a) Small hydrocarbons and CO2 sorption isotherms of 1 at 273
K; (b) IAST-predicted adsorption selectivity of 1 at 273 K.
30 components in
adsorption data at the same temperature and on the same
adsorbent.
a mixture using only single-component
samples of 1 have been characterized by powder X-ray
diffraction. As shown in Figure S2, the experimental PXRD
60 pattern is in good agreement with the corresponding simulated
one, implying that the synthesized materials and the measured
single crystals are the same. The after-activated PXRD patterns
also correspond well with the result simulated from the single
crystal data (Fig. S2), indicating the structurally stable of 1.
Results and Discussion
Compound 1 crystallizes in space group P21/m and features a
35 3D neutral framework with dinuclear Zn2(CO2)3 units. As
shown in Figure 1, the asymmetric unit of 1 contains one Zinc
atom, a half NH2-BTB and a half 2-nitroimidazole molecule.
Two Zinc cations are eight-coordinated by six carboxylate
oxygen atoms from three NH2-BTB3- ligands and two nitrogen
40 atoms from two 2-nitroimidazoles ligands, forming a similar
paddle-wheel building unit Zn2(CO2)3. Compoud 1 is a typical
layered-pillar structure, the axial coordination sites are ligated
by μ2-im groups which acts as a pillar inter-connecting the
Zn2((NH2-BTB)-layers in a axis direction to form a three
45 dimension neutral framework [Zn2(NH2-BTB)(2-nim)] (Fig.
1b). Due to the large size of NH2-BTB3- ligand, the pores are
large enough for a second net, two such frameworks are
mutually interpenetrated (Fig. 1c). From a topological point of
view, the [Zn2(CO2)3]+ paddle-wheel-unit can be considered as
50 a 5-connected node, the NH2-BTB3- anion ligand can be
reduced as 3-connected node and the 2- nitroimidazole ligand
can be reduced as line. Therefore, framework of 1 can be
simplified as a 2-fold interpenetrated hexagon-planar (3,5)-
connected hms net with Point (Schläfli) symbol of (63) (69.8)
55 (Fig. 1d).
65
Thermogravimetric analysis (TGA) in N2 atmosphere with a
heating rate of 10 oC/min was performed on polycrystalline
sample to determine their thermal stability from 25 to 800 C.
o
The TGA curve of 1 indicated that a weight loss of 20%
occurred in the temperature range of 100
- 200 °C,
70 corresponding to the loss of H2O and DMF guest molecules
(expected 20.7%) and it began to decompose from 430 oC
(Figure S3). The TGA curve of 1-activated indicates that the
host framework has no weight loss occurred before 410 oC,
then the framework decomposed. The results show clearly that
75 there is nothing remained in the framework after the activation.
The rigidity of these porous frameworks as well as the
accessibility of guest molecules into the porous framework of 1
was assessed by solid–gas sorption experiments with N2 and
CO2 as probe molecules. Prior to measurement, as-synthesized
80 crystalline samples were activated as follows: immersing in
methanol for three days (solvent was exchanged nine times),
following by the thermal/vacuum activation at 303 K, 8 h to
generate activated compound 1. It has a larger pore void
comprising ~37% of the cell volume as estimated by
85 PLATON.11 The N2 sorption isotherms show type-I behaviors,
characterized the typical of crystalline microporous materials
In order to confirm the crystal structures are truly
representative of the crystal materials, the as-synthesized

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DOI: 10.1039/C6DT00238B
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30 frameworks of type [Zn2(fu–bdc)2(dabco)]n, which composed
of two-dimensional square grids, consisting of paddle-wheel
building units, then interconnected to 3D architecture by pillars
with dangling different chemical side groups, shows the
excellent effect and hysteresis adsorption on the affinity of CO2
35 via an combination of functionalized linkers.17 The isosteric
enthalpy of adsorption (Qst) of light hydrocarbons were
calculated based on the virial method at zero coverage. The Qst
values of CH4, C2H2, C2H4, C2H6, C3H8 and CO2 are 16.22,
19.72, 19.81, 21.52, 28.16 and 22.38 kJ/mol for 1 (Note: the
40 hysteresis of C3H8 adsorption isotherms is tiny, the
adsorption isotherms of C3H8 was fitted as typeⅠcurve).
To estimate the adsorption selectivity of light hydrocarbons,
the adsorption selectivities of C3/C1, C2/C1 and CO2/CH4
(equimolar binary mixtures) were calculated by the ideal
45 adsorption solution theory (IAST) at 273 and 297 K (Fig. 2b,
3b). The IAST calculations indicated that the hierarchy of
adsorption capacity is commonly CH4 < C2H4 < C2H6 < C2H2 <
C3H8, making the feasibility of this material for the separation
of light hydrocarbons. This framework display of great
50 separation ratio of C2H6/CH4, exceeding 16 at 297 K, the values
of 1 is higher than that for ZIF-8(10),18 Mg-MOF-74(7),19 while
is close to C2H6/CH4 selectivity of Fe-MOF-74(18).19
Noteworthily, the selectivity of C3H8/CH4 for 1 is over 85 at
room temperature, much higher than the very high values of
55 UTSA-35a (80).20 Such high selectivities further confirm that
this material has a great potential in separation application of
light hydrocarbons.
Fig. 3 (a) Small hydrocarbons and CO2 sorption isotherms of 1 at 297 K;
(b) IAST-predicted adsorption selectivity of 1 at 297 K.
(Fig. S4). The BET surface area is 893.83 m2 g-1 and the total
pore volume is 0.450 cm3 g−1 (Fig. S4). The CO2 uptakes for 1
are 82.40 cm3 g-1 at 273 K, and 37.55 cm3 g-1 at 297 K,
respectively (Fig. 2a, 3a).
Compound 1 exhibits different adsorption capacities upon
response to C3H8, C2H6, C2H4, C2H2 and CH4 at 273 and 297 K
(Fig. 2a, 3a). In general, MOFs have an adsorption capacity
with the following trend: C3 > C2 > C1. Nevertheless, the
impressive C2H2 uptake of 152 cm3 g−1 at 273 K in compound 1
10 (Fig. 2a), surpassing those of the previously reported porous
adsorbents, such as UMCM-150 (150),13 NOTT-102 (142),14 is
a higher one ever reported.1c More interestingly, a so-called
gate-opening effect is observed in the C3H8 adsorption isotherm
of 1, which displays a tiny hysteresis phenomenon at 273 K
15 (Fig. 2a), indicating framework has some degree of flexibilty.
As shown in Figure 1, 2-nitroimidazole ligand as pillar that
plays a important role in the this framework. The pore property
Conclusion
In summary, we report a 2-fold interpenetrated pillared-layer
60 compound by paddle-wheel building [Zn2(CO2)3] units and
linear 2-nitroimidazole ligands. The adsorption and separation
of the carbon dioxide and small hydrocarbons were
characterized, the C2H2 uptake capacity of 1 is up to 152 cm3
g−1 at 273 K and it also shows good adsorption selectivities of
65 C3/C1 and C2/C1. Especially, the selectivity of C3H8/CH4 is over
85 at room temperature. These results demonstrate that this
compound may be of great value as a promising material for the
fuel gas purification and the separation of light hydrocarbons.
5
70 Acknowledgements
This work is supported by NSFC (21573236).
can be tuned via rotation of nitro group, thereby inducing gate-
Notes and references
15
opening type adsorption.7,
However, other kinds of
State Key Laboratory of Structural Chemistry, Fujian Institute of
Research on the Structure of Matter, the Chinese Academy of Sciences,
75 Fuzhou, Fujian 350002, P. R. China. E-mail: wangfei04@fjirsm.ac.cn
Electronic Supplementary Information (ESI) available: [addition figures,
powder X-ray patterns, adsorption isotherms and cif files (CCDC-
1440822)]. See DOI: 10.1039/b000000x/
20 hydrocarbons are used as adsorptive probes, the phenomenon
can not occur. Take many factors into consideration, the
molecular polarizability and molecular size are the dominant
effects.16 First, compared with CH4, C2H2, C2H4, C2H6, the
steep isotherm of C3H8 indicates that C3H8 has the highest
25 affinity to the framework, consistent with the polarizability of
this molecule. Second, the C3H8 molecule with the largest
molecular size has more opportunities to interact with the 2-nim
pillar, which can cause the rotation of 2-nim to generate the
open effect. A typically example of the family of pillared-layer
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TOC
A pillared-layer Metal-Organic Framework was successfully synthesized, which shows high
storage capacity for C2 light hydrocarbons and good adsorption selectivity of C3/C1 and C2/C1 at
room temperature.