Synthesis and Selective CO2 Capture Properties of a Series of Hexatopic Linker-Based Metal-Organic Frameworks

Article abstract of DOI:10.1021/acs.inorgchem.5b01900

Four crystalline, porous metal-organic frameworks (MOFs), based on a new hexatopic linker, 1′,2′,3′,4′,5′,6′-hexakis(4-carboxyphenyl)benzene (H₆CPB), were synthesized and fully characterized. Interestingly, two members of this series exhibited

Full text of DOI:10.1021/acs.inorgchem.5b01900

Article
pubs.acs.org/IC
Synthesis and Selective CO2 Capture Properties of a Series of
Hexatopic Linker-Based Metal−Organic Frameworks
Phuong T. K. Nguyen,†,§ Huong T. D. Nguyen,†,‡,§ Hung Q. Pham,†,‡ Jaheon Kim,# Kyle E. Cordova,*,†,⊥
and Hiroyasu Furukawa*,⊥,∥
‡
†Center for Molecular NanoArchitecture and Faculty of Chemistry, University of Science, Vietnam National UniversityHo Chi
Minh City (VNUHCM), Ho Chi Minh City 721337, Vietnam
#Department of Chemistry, Soongsil University, 369 Sangdo-Ro, Dongjak-Gu, Seoul 156-743, Republic of Korea
⊥Department of Chemistry and Materials Sciences Division, Lawrence Berkeley National Laboratory, and Center for Global Science
at Berkeley, University of CaliforniaBerkeley, Berkeley, California 94720, United States
∥King Fahd University of Petroleum and Minerals, Dhahran 34464, Saudi Arabia
S
* Supporting Information
ABSTRACT: Four crystalline, porous metal−organic frameworks (MOFs),
based on a new hexatopic linker, 1′,2′,3′,4′,5′,6′-hexakis(4-carboxyphenyl)-
benzene (H6CPB), were synthesized and fully characterized. Interestingly, two
members of this series exhibited new topologies, namely, htp and hhp, which
were previously unseen in MOF chemistry. Gas adsorption measurements
revealed that all members exhibited high CO2 selectivity over N2 and CH4.
Accordingly, breakthrough measurements were performed on a representative
example, in which the effective separation of CO2 from binary mixtures
containing either N2 or CH4 was demonstrated without any loss in performance
over three consecutive cycles.
uer−Emmett−Teller (BET) surface area (7140 m2 g−1) for any
INTRODUCTION
■
known porous material.11 Hexatopic linkers were also utilized
Metal−organic frameworks (MOFs) are constructed via strong
in the synthesis of the ultrahighly porous PCN-69 (also known
bonds between metal-containing clusters and organic linkers,
as NOTT-119), as well as NOTT-112, NU-100, and NU-111.12
both of which are termed secondary building units (SBUs).1
The synthesis of an isoreticular (having the same topology)
Through the use of reticular chemistry, it is well understood
series of MOFs from a different dendritic hexatopic linker,
that MOFs can be designed based on geometrically influenced
choices of inorganic and organic building units.1,2 The resulting
termed JUC-100, JUC-103, and JUC-106, was shown to
combine high porosity with suitable pore sizes for light
crystalline structures have attracted much promise in a wide
hydrocarbon separation.13 Similarly, a series of isoreticular
range of applications, including gas storage and separations,
heterogeneous catalysis, conductivity, and water storage.3−6
MOFs (PCN-61, -66, -68, and -610) was produced from novel
hexatopic linkers, affording structures with high porosity and
One particular direction that has received attention in MOF
synthesis is the use of multicarboxylate organic linkers, which
has afforded new MOF materials with unique structural features
high gas adsorption properties.14 It is noted that a limited
number of MOF architectures built from octatopic and
dodecatopic linkers have also been reported.15,16
and interesting intrinsic properties.7,8 Specifically, when
multicarboxylate linkers are employed, the metrics of the
pore size and shape as well as the pore surface have been shown
to be modifiable to optimize for gas storage and separation
applications. The first example of using a multicarboxylate
linker in MOF synthesis was MODF-1 (where MODF-1 =
metal−organic dendrimer framework-1), in which a dendritic
hexacarboxylate (hexatopic) linker was used to construct a
permanently porous structure with NiAs topology.9 The
reporting of UTSA-62a also demonstrated the use of a
dendritic hexatopic linker, leading to a highly porous
framework with low interactions with CO2.10 These favorable
properties afforded UTSA-62a as an effective adsorbent for H2
purification purposes.10 NU-110E, constructed from an
elongated hexatopic linker, demonstrated the highest Bruna-
To further explore the construction of MOFs based on
multicarboxylate linkers, we describe the synthesis of a new
hexatopic linker, 1′,2′,3′,4′,5′,6′-hexakis(4-carboxyphenyl)-
benzene (H6CPB; Scheme 1).17 It is worth noting that the
H6CPB linker was designed to be highly symmetrical, in which
the local C2 axes of the aryl faces form 60° angles with respect
to one another. With this new linker in hand, we further report
the synthesis and characterization of a series of four new MOFs,
including [Ni2 (CPB)]·4.1H2 O, [Mg2 (H2 CPB)-
(DEF)0.5(EtOH)0.5]·3H2O, [Cu3(CPB)(DMF)0.5]·6H2O, and
[Cu3(CPB)(DEF)0.4]·3.8H2O (termed MOF-888, -889, -890,
Received: August 18, 2015
© XXXX American Chemical Society
A
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within the PLATON software package was used to treat the
unidentified occluded solvent molecules. Full SCXRD analysis details
can be found in the SI, section S3. Powder X-ray diffraction (PXRD)
patterns were recorded using a Bruker D8 Advance diffractometer with
Ni-filtered Cu Kα radiation (λ = 1.54178 Å). The system was also
outfitted with an antiscattering shield that prevents incident diffuse
radiation from hitting the detector. Samples were placed on zero
background sample holders by dropping powders and then leveling the
sample with a spatula. The 2θ range was 3−50° with a scan speed of 1
s step−1 and a step size of 0.02°.
Scheme 1. Hexatopic Linker CPB Used in This Work
Synthesis of MOF-888, [Ni2(CPB)]·4.1H2O. A 0.05 M stock
solution of nickel(II) acetate tetrahydrate in DMF (0.375 mL) was
added to a 2 mL Pyrex tube (o.d. × i.d. = 6.5 mm × 5.5 mm), which
was preloaded with H6CPB (15 mg, 0.019 mmol). This was followed
by the addition of 1.125 mL of deionized water. The tube was then
quickly sealed, sonicated for 5 min, and heated at 150 °C for 48 h to
produce yellow rectangular-shaped crystals. The crystals were then
thoroughly washed with DMF (4 × 5 mL) per day for a total of 3 days.
To yield guest-free material, DMF-washed MOF-888 was subsequently
immersed in DCM (4 × 5 mL) per day over a period of 4 days. The
solvent-exchanged sample was activated under vacuum at ambient
temperature for 24 h, followed by heating at 240 °C under vacuum for
an additional 24 h. EA. Calcd for Ni2C48H32.20O16.10 ([Ni2(CPB)]·
4.1H2O): C, 58.59; H, 3.30. Found: C, 58.55; H, 3.63. FT-IR (KBr,
3500−400 cm−1): 3430 (br), 3066 (w), 2802 (vw), 2361 (vw), 1699
(w), 1609 (m), 1584 (m), 1524 (m), 1427 (s), 1411 (s), 1309 (vw),
1277 (br), 1180 (w), 1150 (vw), 1101 (br), 1018 (w), 866 (s), 758
(s).
Synthesis of MOF-889, [Mg2(H2CPB)(DEF)0.5(EtOH)0.5]·3H2O.
A 0.5 M stock solution of magnesium nitrate hexahydrate in deionized
water (0.12 mL) was added to an 8 mL vial, which was preloaded with
H6CPB (8 mg, 0.01 mmol). This was followed by the addition of DEF
(0.40 mL), EtOH (1.50 mL), and deionized water (0.24 mL) to the
vial. The reaction mixture was quickly sonicated and heated at 120 °C
for 24 h, yielding colorless, block-shaped crystals. The crystals were
then thoroughly washed with DMF (4 × 8 mL) per day for 4 days
total. To yield guest-free material, DMF-washed MOF-889 was
subsequently immersed in DCM (4 × 8 mL) per day for 4 days total.
The solvent-exchanged sample was activated under vacuum at ambient
temperature for 24 h, followed by heating at 180 °C under vacuum for
an additional 24 h. EA. Calcd for Mg2C50.5H37.50O15.50N0.5
([Mg2(H2CPB)(DEF)0.5(EtOH)0.5]·3H2O): C, 63.70; H, 3.99; N,
0.74. Found: C, 63.61; H, 3.60; N, 0.48. FT-IR (KBr, 3500−400
cm−1): 3417 (br), 2975 (vw), 2513 (vw), 1696 (m), 1609 (s), 1541
(m), 1405 (s), 1311 (vw), 1277 (br), 1177 (w), 1149 (vw), 1103 (br),
1018 (w), 866 (w), 744 (s).
Synthesis of MOF-890, [Cu3(CPB)(DMF)0.5]·6H2O. A 0.165 M
stock solution of copper nitrate trihydrate in DMF (0.5 mL) was
added to an 8 mL vial, which was preloaded with H6CPB (20 mg,
0.025 mmol). This was followed by the addition of deionized water (4
mL) to the mixture, which was subsequently heated at 100 °C for 18 h
to yield blue, block-shaped crystals. The crystals were then thoroughly
washed with DMF (3 × 10 mL) per day for 3 days total. To yield
guest-free material, DMF-washed MOF-890 was immersed in MeOH
(3 × 10 mL) per day for a total of 4 days. The solvent-exchanged
sample was activated under vacuum at ambient temperature for 24 h,
followed by heating at 180 °C under vacuum for an additional 24 h.
EA. Calcd for Cu3C48H33.40O16.70 ([Cu3(CPB)(DMF)0.5]·6H2O): C,
52.71; H, 3.53; N, 0.62. Found: C, 52.10; H, 2.71; N, 0.63. FT-IR
(KBr, 3500−400 cm−1): 3429 (br), 1608 (s), 1540 (m), 1404 (br),
1277 (vw), 1180 (w), 1017 (m), 860 (m), 752 (m).
Synthesis of MOF-891, [Cu3(CPB)(DEF)0.4]·3.8H2O. A 0.02 M
stock solution of copper nitrate trihydrate in DEF (0.5 mL) was added
to a 8 mL vial, which was preloaded with H6CPB (20 mg, 0.025
mmol). This was followed by the addition of deionized water (3 mL)
to the mixture, which was then heated at 85 °C for 16 h to yield blue,
block-shaped crystals. The crystals were then thoroughly washed with
DMF (3 × 10 mL) per day for 3 days total. To yield guest-free
material, DMF-washed MOF-891 was immersed in MeOH (3 × 10
mL) per day over a total of 4 days. The solvent-exchanged sample was
and -891, respectively, where DEF = N,N-diethylformamide,
EtOH = ethanol, and DMF = N,N-dimethylformamide), and
describe their gas adsorption properties. Within this series, the
structural frameworks were shown to vary from two-dimen-
sional (2D) layers (MOF-888) to three-dimensional (3D)
architectures (MOF-889 to -891). Finally, we demonstrate that
MOF-890 is an effective adsorbent for the dynamic separation
of CO2 from binary mixtures containing N2 or CH4.
EXPERIMENTAL SECTION
■
Materials and General Procedures. Full synthetic and character-
ization details can be found in the Supporting Information (SI),
sections S1 and S2. Copper(II) nitrate trihydrate, nickel(II) acetate
tetrahydrate, magnesium nitrate hexahydrate, anhydrous DMF, and
DEF were obtained from Sigma-Aldrich. Anhydrous dichloromethane
(DCM), methanol (MeOH), and EtOH were purchased from Acros
Organics. Deionized water was obtained from a Barnstead Easypure II
(17.8 MΩ·cm resistivity). All chemicals, unless otherwise specified,
were used without further purification.
Elemental microanalyses (EA) were performed in the Micro-
analytical Laboratory of the College of Chemistry at University of
CaliforniaBerkeley, using a PerkinElmer 2400 series II CHNS
elemental analyzer. Fourier transform infrared (FT-IR) spectra were
measured using KBr pellets on a Bruker Vertex 70 system, and the
output signals are described as follows: vs, very strong; s, strong; m,
medium; sh, shoulder; w, weak; vw, very weak; br, broad. Thermal
gravimetric analysis (TGA) was performed on a TA Q500 thermal
analysis system with the sample held in a platinum pan in a continuous
airflow. Low-pressure N2, CO2, and CH4 adsorption isotherms were
recorded on a Micromeritics 3Flex. A liquid-N2 bath was used for
measurements at 77 K, and a water circulator was used for
measurements at 273, 283, and 298 K. For all sorption measurements,
He was used to estimate the dead space. Breakthrough measurements
were performed using a L&C Science and Technology PSA-300-LC
analyzer. The bed column dimensions were l × i.d. = 14 cm × 0.635
cm. The gaseous effluent from the sample bed was monitored for CO2,
CH4, N2, H2O, and O2 by a ThermoStar GSD320 mass spectrometer.
Ultrahigh-purity-grade N2, CH4, and He gases (99.999% purity) as
well as high-purity-grade CO2 (99.995%) were used throughout the
experiments.
Synthesis of the Hexatopic Linker 1′,2′,3′,4′,5′,6′-Hexakis(4-
carboxyphenyl)benzene (H6CPB). H6CPB was synthesized via a
one-pot Sonogashira coupling reaction, followed by a Co-catalyzed
cyclotrimerization, and subsequent hydrolysis to produce the resulting
H6CPB linker (Scheme 1).17 Full synthetic details are described in the
SI, section S2.
X-ray Diffraction Analysis. Single-crystal X-ray diffraction
(SCXRD) analysis was performed on a Bruker D8 Venture
diffractometer using monochromatic fine-focus Cu Kα radiation (λ =
1.54178 Å). The measurements were operated at 50 kV and 1.0 mA
using a Photon 100 CMOS detector at 100 K for MOF-888, -890, and
-891 and at 298 K for MOF-889. The data were collected with ϕ and
ω scans using a narrow-frame algorithm. The frames were integrated
using the Bruker SAINT software package. The reflection data were
corrected for absorption by using Bruker SADABS, solved with the
Bruker SHELXTL software package using direct methods and refined
by full-matrix least-squares techniques against F2 with the SHELXL
program package. To improve the refinement, the SQUEEZE routine
B
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Table 1. Crystal Structure Data and Refinement for MOF-888, MOF-889, MOF-890, and MOF-891
MOF-888
MOF-889
MOF-890
MOF-891
empirical formula
C48H24O12Ni2
910.09
C59H55NO16Mg2
1082.66
monoclinic
P21/n
C57H47N3O16Cu3
1220.59
orthorhombic
P212121
11.540(3)
16.778(6)
28.468(3)
90
C63H61N3O17Cu3
1322.76
fw (g mol−1
cryst syst
space group
a (Å)
)
monoclinic
C2/c
triclinic
P1
̅
16.2264(3)
29.4379(6)
12.4371(3)
90
11.8582(6)
28.0640(12)
16.7331(8)
90
12.6325(4)
17.4726(6)
27.9667(9)
91.275(2)
92.433(2)
106.100(2)
5921.7(3)
4
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
29.4379(6)
90
96.876(2)
90
90
90
4980.83(19)
4
5528.5(5)
4
5512(3)
4
ρcalcd (g cm−3
)
1.214
1.301
1.471
1.484
μ(Cu Kα) (mm−1
)
1.405
0.984
1.956
1.878
Rint
0.0855
0.0404
0.0910
0.1289
0.3090
0.0727
a
R1
0.0550
0.0977
0.1072
b
wR2
0.1363
0.2409
0.2100
a
b
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑w(Fo2 − Fc2)2/w(Fo)2]1/2 for 3397 reflections satisfying I > 2σ(I) (MOF-888), for 2590 reflections satisfying I
> 2σ(I) (MOF-889), for 2194 reflections satisfying I > 2σ(I) (MOF-890), and for 3397 reflections satisfying I > 2σ(I) (MOF-891).
activated under vacuum at ambient temperature for 24 h, followed by
heating at 180 °C under vacuum for an additional 24 h. EA. Calcd for
Cu3C63H61O17N3 ([Cu3(CPB)(DEF)0.4]·3.8H2O): C, 54.98; H, 3.32;
N, 0.51. Found: C, 54.67; H, 2.97; N, <0.2. FT-IR (KBr, 3500−400
cm−1): 3433 (br), 2921 (vw), 1609 (m), 1542 (w), 1405 (br), 1179
(vw), 1153 (vw), 1104 (w), 1017 (w), 865 (m), 792 (m).
RESULTS AND DISCUSSION
■
1. Crystal Structures of MOF-888−MOF-891. All bonds
in the single-crystal structures of MOF-888−MOF-891 were
rigorously elucidated using bond-valence theory before the
topological type was determined.18 Special attention was paid
to hydrogen and metal−O bonds because these played a
significant role in determining the underlying net of the
resulting MOF frameworks. The TOPOS4.0 package was
employed to simplify the structures and to identify the
respective underlying net. Details are described in the SI,
section S4.19
MOF-888. SCXRD analysis revealed that MOF-888
crystallizes in the monoclinic C2/c space group with a =
16.226 Å, b = 29.438 Å, and c = 12.437 Å (Table 1). The
framework of MOF-888 is composed of single-metal Ni units
with three points of extension resulting from bidentate-
Figure 1. Crystal structure of MOF-888. (A) Linking of hexagonal
CPB and triangular Ni(CO2)3 SBUs resulting in (B) MOF-888. (C)
coordinated carboxylate functionalities of the CPB linkers
(Figure 1A). The overall extended structure can be described as
2D layers that adopt the kgd topology as a result of linking
triangular (Ni units) and hexagonal (CPB linkers) SBUs
together (Figure 1B,C). Interestingly, charge-balance analysis
on the Ni SBUs indicates that the oxidation state of Ni must be
3+, which is rather uncommon but not unprecedented (SI,
section S4).20 It is noted that a carboxylate functionality in the
CPB linker can potentially exist in protonated form to afford
the Ni atoms with the more common oxidation state of 2+.
However, the fact that all six Ni−O bond lengths are virtually
equal excludes such a possibility.
MOF-889. MOF-889 crystallizes in the monoclinic P21/n
space group with a = 11.858 Å, b = 28.064 Å, and c = 16.733 Å
(Table 1). A combination of SCXRD and charge-balance
analyses indicates that only four of the six carboxylic acid
moieties of the CPB linker are fully deprotonated. Of these four
carboxylates, three form bridging coordinate covalent bonds
Structure of MOF-888 adopting the kgd topology. Color code: Ni,
blue polyhedra; C, black, O, red. All H atoms are omitted for clarity.
with two Mg atoms. The fourth carboxylate binds in a
monodentate fashion and participates in hydrogen bonding
(center-to-center oxygen distance in the range of 2.617−2.847
Å) with the remaining two carboxylic acid functionalities of the
CPB linker and two EtOH ligands. Taken together, this
produces hydrogen-bonded discrete Mg2(CO2)4(CO2H)2-
(EtOH)2 clusters that arrange in infinite [Mg2(CO2)4(CO2H)2-
(EtOH)2]∞ rod-shaped SBUs (Figure 2A). It is noted that one
DEF ligand is also present to complete the coordination sphere,
but does not play a role in the formation of the rod-shaped
SBUs. Topological analysis reveals that the 3D MOF-889
framework adopts the yav topology because of the linking of
infinite binodal rod-shaped SBUs with hexagonal CPB linker
SBUs (Figure 2B,C and SI, section S4).
C
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Figure 2. Crystal structure of MOF-889. (A) Linking of hexagonal
CPB and infinite [Mg2(CO2)4(CO2H)2(EtOH)2]∞ (red and blue
binodal rods) SBUs resulting in (B) MOF-889. (C) Structure of
MOF-889 adopting the yav topology. Color code: Mg, blue polyhedra;
C, black, O, red. All H atoms, except for those participating in
hydrogen bonding, are omitted for clarity.
Figure 3. Crystal structure of MOF-890. (A) Linking of hexagonal
CPB and trigonal-prismatic Cu3(CO2)6 SBUs resulting in (B) MOF-
890. (C) Structure of MOF-890 adopting the htp topology. This is the
first framework with a topology composed of both hexagonal and
trigonal-prismatic SBUs. Color code: Cu, blue polyhedra; C, black, O,
red. All H atoms are omitted for clarity.
MOF-890. SCXRD analysis revealed that MOF-890
crystallizes in the orthorhombic P212121 space group with a =
11.540 Å, b = 16.778 Å, and c = 28.468 Å (Table 1). In this
structure, all carboxylate groups of the CPB linker are fully
deprotonated to yield discrete Cu3(CO2)6 SBUs (Figure 3A).
Three carboxylate functionalities coordinate in a bridging
fashion to two Cu atoms, and one carboxylate functionality
bridges these two Cu atoms to the third Cu atom, which has
the final two carboxylate atoms coordinated in a monodentate
fashion. DMF and water molecules complete the coordination
spheres of the Cu atoms. Considering that the CPB linkers are
the points of extension, the Cu3(CO2)6 SBUs can be viewed as
trigonal prisms (Figure 3A). Accordingly, the resulting 3D
framework adopts a new (6,6)-connected htp topology. MOF-
890 is the first framework with a topology composed of both
hexagonal and trigonal-prismatic SBUs (SI, section S4).
Although the infinite rod and discrete cluster SBUs have similar
chemical formulas, their geometries are distinctly different.
Accordingly, a 3D framework with the new hhp topology
results from the linking of infinite rods with trigonal-prismatic
(discrete Cu3 clusters) and hexagonal (CPB linker) SBUs
(Figure 4B,C). This is the first framework with a topology
derived from a combination of discrete and rod-shaped SBUs in
one structure (SI, section S4).
2. Structural Robustness and Permanent Porosity.
The successful synthesis of highly crystalline and phase-pure
MOF-888, MOF-889, MOF-890, and MOF-891 was confirmed
by PXRD analysis of the as-synthesized materials in comparison
with the simulated pattern generated from the single-crystal
structures (SI, section S5, Figures S9−S12). The as-synthesized
MOFs were subsequently washed with DMF to remove any
unreacted species and then exchanged with either DCM (for
MOF-888 and -889) or MeOH (for MOF-890 and -891). In
order to yield guest-free materials, occluded solvent molecules
were removed under reduced pressure at room temperature,
followed by heating at 80 °C for 6 h, at 120 °C for 6 h, and
finally at 180 °C for 24 h. Structural maintenance after
activation was proven by PXRD analysis, in which the peak
positions of the activated samples were coincident to both the
as-synthesized and simulated patterns (SI, Figures S9−S12).
The thermal robustness of MOF-888−MOF-891 was
demonstrated by performing TGA (SI, section S6). For
activated MOF-888 and -889, the TGA curves under airflow
exhibited no appreciable weight loss up to 350 and 400 °C, thus
highlighting the high thermal stability of these materials (SI,
Figures S13 and S14). In the TGA curves for MOF-890 and
-891, a weight loss of 3.06 and 4.52% was observed from room
MOF-891. SCXRD analysis revealed that MOF-891
crystallizes in the triclinic P1 space group with unit cell
̅
parameters a = 12.633 Å, b = 17.473 Å, and c = 27.967 Å
(Table 1). In MOF-891, there exist two types of Cu3 SBUs, one
discrete and one that can be described as an infinite rod (Figure
4A). The discrete Cu3 cluster, Cu3(CO2)6(H2O)2, is formed
through hydrogen bonding of two smaller charged clusters,
notably, [Cu2(CO2)3(H2O)]+ and [Cu(CO2)3(H2O)]− (Figure
4A). Topological analysis reveals that these discrete clusters
have six points of extension and can be viewed as trigonal
prisms. In a similar fashion, the infinite Cu3 rods,
[Cu3(CO2)6(H2O)2]∞, are also formed via hydrogen bonding
of two discrete charged clusters, [Cu2(CO2)3(H2O)2]+ and
[Cu(CO2)3]−. Here, hydrogen bonding occurs between water
molecules and monodentate carboxylate functionalities from
the CPB linker (center-to-center oxygen distance in the range
of 2.563−2.740 Å; Figure 4A). It is noted that DEF ligands are
present in both the discrete cluster and infinite rod SBUs.
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MOF-888, MOF-889, MOF-890, and MOF-891 were esti-
mated to be 40/70, 140/160, 300/340, and 200/240 m2 g−1,
respectively. These measurements are in line with the
theoretical accessible surface areas calculated for MOF-888,
MOF-889, MOF-890, and MOF-891 (60, 125, 450, and 290 m2
g−1, respectively).
3. Gas Adsorption Properties. Thermodynamic Uptake
Capacity. Single-component gas adsorption isotherms (CO2,
N2, and CH4) were measured at 273, 283, and 298 K for all
members of this series (SI, Figures S18−S29). As shown in
Table 2, MOF-890 and MOF-891 display the highest CO2
capacities (58 cm3 g−1) at 800 Torr and 298 K. This is in
contrast to the N2 and CH4 uptake capacities observed for
MOF-890 (6.0 and 24 cm3 g−1, respectively) and MOF-891
(6.4 and 30 cm3 g−1, respectively) at the same temperature and
pressure, which demonstrates the potential of these MOFs for
CO2 storage and separation (Figure 5 and SI, section S7).
Figure 4. Crystal structure of MOF-891. (A) Linking of hexagonal
CPB, trigonal-prismatic Cu3(CO2)6(H2O)2, and infinite
[Cu3(CO2)6(H2O)2]∞ (red and blue binodal rods) SBUs resulting
in (B) MOF-891. (C) Structure of MOF-891 adopting the hhp
topology. This is the first framework with a topology derived from a
combination of discrete and rod-shaped SBUs in one structure. Color
code: Cu, blue polyhedra; C, black, O, red. All H atoms, except for
those participating in hydrogen bonding, are omitted for clarity.
Figure 5. CO2 (blue), CH4 (red), and N2 (green) isotherms for MOF-
890 at 298 K. Filled and open symbols represent adsorption and
desorption branches, respectively. The connecting curves are guides
for the eye.
temperature to 100 °C, corresponding to exchanged solvent
MeOH molecules in the channels, with framework decom-
position occurring at 300 °C (SI, Figures S15 and S16). Finally,
analysis of the weight percent of residual metal oxide for MOF-
888 (13.5%), MOF-889 (8.99%), MOF-890 (22.5%), and
MOF-891 (23.8%) was found to be consistent with the
calculated value derived from EA (16.5, 8.45, 22.2, and 22.8%,
respectively).
The permanent porosity of all members of this series was
evaluated by measuring N2 adsorption isotherms at 77 K using
activated materials. The resulting N2 isotherms exhibited typical
type I behavior, indicative of microporosity (SI, Figure S17).
From these isotherms, the BET/Langmuir surface areas for
MOF-889 also displayed relatively moderate uptake capacities
for CO2 and CH4 (55 and 26 cm3 g−1, respectively) in
comparison with a low N2 capacity (5.3 cm3 g−1) at 800 Torr
and 298 K (Table 2 and SI, section S7). Furthermore, for
MOF-888, MOF-889, MOF-890, and MOF-891, the initial
uptake in the low-pressure region of the CO2 isotherms at 298
K is much steeper than those observed for N2 or CH4. This is
indicative of the MOF frameworks’ higher affinity to CO2
(Figure 5 and SI, section S7).
Coverage-Dependent Isosteric Heat of Adsorption and
Gas Selectivity. Motivated by these observations, we sought to
Table 2. Surface Area, Thermodynamic CO2 Uptake Capacity, and CO2/N2 and CO2/CH4 Selectivities for MOF-888, -889,
-890, and -891
ABET
CO2 uptake
N2 uptake
CH4 uptake
isosteric heat of CO adsorption
CO2/N
CO2/CH4
d
a
b
b
b
2c
2d
MOF
(m2 g−1
)
(cm3 g−1
)
(cm3 g−1
)
(cm3 g−1
)
(kJ mol−1
)
selectivity
selectivity
MOF-888
MOF-889
MOF-890
MOF-891
38
144
295
200
24
55
58
58
1.8
5.3
6.0
6.4
12
26
24
30
34
28
31
34
57
25
63
56
6.8
3.9
9.0
7.2
a
b
c
d
Calculated by the BET method. At 800 Torr and 298 K. Calculated by a virial-type expansion equation at near-zero CO2 coverage. Calculated
from pure-component isotherms by Henry’s law.
E
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Article
analyze the applicability of MOF-889, -890, and -891 in the
industrially relevant flue and natural gas purification pro-
cesses.21 Accordingly, the coverage-dependent isosteric heat of
adsorption (Qst) for CO2, N2, and CH4 were calculated by
fitting the respective isotherms at 273, 283, and 298 K using a
virial-type expansion equation (SI, Figures S30 and S31).22 As
shown in Table 2, all members of this series exhibited relatively
high CO2 adsorption enthalpies at zero coverage (34, 28, 31,
and 34 kJ mol−1 for MOF-888, -889, -890, and -891,
respectively; SI, Figure S30). For perspective, the Qst values
calculated for MOF-888, -890, and -891 are greater than other
well-known MOFs, such as NOTT-140 and HKUST-1
(hydrate; 25 and 30 kJ mol−1, respectively) but slightly less
than those of MIL-53(Al) and Co-MOF-74 (35 and 37 kJ
mol−1, respectively).22 Unsurprisingly, these values were
substantially higher than the values calculated for N2 and
CH4 (SI, Figures S30 and S31). These results led, in turn, to
estimation of the CO2/N2 and CO2/CH4 selectivities using
Henry’s law (Table 2 and SI, section S7). MOF-890 exhibits
the highest selectivity in this series toward CO2 in binary
mixtures containing either N2 or CH4 (63 and 9.0 for CO2/N2
and CO2/CH4, respectively). The other members of this series
also demonstrate high selectivities for such binary mixtures, as
depicted in Table 2.23 To prove the validity of these
calculations and definitively demonstrate the CO2 separation
properties of these materials, dynamic breakthrough experi-
ments were subsequently performed.21
Table 3. Dynamic CO2 Uptake Capacity for MOF-890
a
b
CO2/N2
CO2/CH4
uptake
capacity
(cm3 g−1
uptake
capacity (wt
%)
uptake
uptake
capacity (wt
%)
capacity
material
)
(cm3 g−1
)
MOF-890
BPL
31
18
6.1
3.5
25
11
4.9
2.1
a
b
Composition: 16:84, v/v. Composition: 20:80, v/v.
prove the recyclability of MOF-890, breakthrough measure-
ments were performed over three consecutive cycles (SI,
Figures S32 and S34). Remarkably, there was no observable
loss in the separation performance of MOF-890 in either the
CO2/N2 or CO2/CH4 measurements. Furthermore, it was
demonstrated that, by simply flowing pure N2 or CH4 gas,
respectively, through the MOF-loaded bed, MOF-890 could be
fully regenerated.
SUMMARY
■
In this work, we have demonstrated the synthesis and full
characterization of a series of MOFs constructed from a novel,
highly symmetric hexatopic linker. Interestingly, the underlying
topologies of two members of this series, notably MOF-890
with htp topology and MOF-891 with hhp topology, were
found to be new. MOF-888, MOF-889, MOF-890, and MOF-
891 were shown to have moderately high CO2 uptake capacities
at 800 Torr and 298 K (55, 58, and 58 cm3 g−1, respectively)
with correspondingly high isosteric heats of CO2 adsorption at
zero coverage (28, 31, and 34 kJ mol−1, respectively).
Motivated by these findings, breakthrough measurements
were performed on MOF-890, as a representative example, to
demonstrate its capability of separating CO2 from binary gas
mixtures containing either N2 or CH4. As expected, MOF-890
was an effective adsorbent for such separations (6.1 and 4.9 wt
% CO2 capacity for CO2/N2 and CO2/CH4 separations,
respectively) and was shown to be effective over three
consecutive cycles with minimal energy input needed for
regeneration (simple N2 or CH4 flow).
Dynamic Adsorption Capacity via Breakthrough Measure-
ments. Breakthrough measurements were performed on MOF-
890 as a representative example (SI, section S8). In a typical
experiment, a bed packed with MOF-890 was exposed to binary
gas streams containing dry mixtures of CO2/N2 (16:84, v/v) or
CO2/CH4 (20:80, v/v) at room temperature (Figure 6). In
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b01900.
■
S
Linker synthesis and characterization, full MOF synthetic
details and characterization (including PXRD, TGA
curves, and adsorption isotherms), and the experimental
conditions for breakthrough measurements (PDF)
Crystallographic data in CIF format (CIF)
Crystallographic data in CIF format (CIF)
Crystallographic data in CIF format (CIF)
Figure 6. Binary mixture of CO2/N2 (A) or CO2/CH4 (B) flowed
through a fixed bed of MOF-890. The breakthrough time is indicated
by the dashed line.
Crystallographic data in CIF format (CIF)
both cases, it was clearly observed that only CO2 was retained
in the pores, while either N2 or CH4 passed through. From
these measurements, the dynamic CO2 uptake capacities in the
CO2/N2 and CO2/CH4 separation were 31 (6.1) and 25 (4.9)
cm3 g−1 (wt %), respectively (Table 3). In comparison with that
of BPL carbon, which is widely used in industry because of its
ease of desorption and regeneration, MOF-890 exhibited a
longer CO2 retention time for both measurements (Table 3
and SI, Figures S32−S35), indicating higher CO2 capacities. To
AUTHOR INFORMATION
Corresponding Authors
*E-mail: kcordova@berkeley.edu.
*E-mail: furukawa@berkeley.edu.
■
Author Contributions
§P.T.K.N. and H.T.D.N. contributed equally.
Notes
The authors declare no competing financial interest.
F
DOI: 10.1021/acs.inorgchem.5b01900
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Inorganic Chemistry
Article
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O'Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2001, 123, 11482−11483.
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ACKNOWLEDGMENTS
■
Prof. O. M. Yaghi is gratefully acknowledged for founding and
supporting MANAR. We acknowledge T. T. Ho and A. M.
Osborn at MANAR for valuable discussion and assistance on
this work. We thank Dr. H. T. C. Ho, Dr. Q. T. Ton, and Prof.
H. T. Nguyen at the University of Science (VNUHCM) for
their valuable input. We are indebted to Prof. M. O’Keeffe
(Arizona State University) for useful discussion on topological
analysis. The work was supported for the synthesis and general
adsorption characterization by VNUHCM (Nos. B2011-50-
01TĐ and A2015-50-01-HĐ-KHCN), and the dynamic
breakthrough measurement was supported by the United
States Office of Naval Research Global: Naval International
Cooperative Opportunities in Science and Technology
Program (No. N62909-15-1N056). J.K. acknowledges support
from the Mid-Career Researcher Program of the National
Research Foundation of Korea funded by the Ministry of
S c i e n c e , I C T , a n d F u t u r e P l a n n i n g ( N R F -
2014R1A2A1A11054190).
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G
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at ∼55 Torr (20 cm3 g−1) is almost twice as high as that observed for
MOF-889 at the same pressure, it is reasonable for MOF-890 to
exhibit greater CO2/N2 and CO2/CH4 selectivities than MOF-889.
H
DOI: 10.1021/acs.inorgchem.5b01900
Inorg. Chem. XXXX, XXX, XXX−XXX