Improvement of Methane-Framework Interaction by Controlling Pore Size and Functionality of Pillared MOFs

Article abstract of DOI:10.1021/acs.inorgchem.6b02758

The rational design of functionalized porous metal-organic frameworks (MOFs) for gas adsorption applications has been applied using three spacer ligands H₂DPT (3,6-di(pyridin-4-yl)-1,4-dihydro-1,2,4,5-tetrazine), DPT (3,6-di(pyridin-4-yl)-1,2,4

Full text of DOI:10.1021/acs.inorgchem.6b02758

Article
pubs.acs.org/IC
Improvement of Methane−Framework Interaction by Controlling
Pore Size and Functionality of Pillared MOFs
Sayed Ali Akbar Razavi, Mohammad Yaser Masoomi, Timur Islamoglu, Ali Morsali,*,†,‡ Yan Xu,‡
†
†
‡
§
,‡
,‡
§
Joseph T. Hupp,* Omar K. Farha,* Jun Wang, and Peter C. Junk
†
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, Tehran, Islamic Republic of Iran
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
‡
§College of Science and Engineering, James Cook University, Townsville, Queensland 4811, Australia
*
S Supporting Information
ABSTRACT: The rational design of functionalized porous metal−organic frameworks (MOFs) for gas adsorption applications
has been applied using three spacer ligands H DPT (3,6-di(pyridin-4-yl)-1,4-dihydro-1,2,4,5-tetrazine), DPT (3,6-di(pyridin-4-
2
yl)-1,2,4,5-tetrazine), and BPDH (2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene) to synthesize TMU-34, [Zn(OBA)(H DPT) ] ·
2
0.5 n
DMF, TMU-34(−2H), [Zn(OBA)(DPT) ] ·DMF, and TMU-5, [Zn(OBA)(BPDH) ] ·1.5DMF, respectively. By controlling
0
.5 n
0.5 n
the pore size and chemical functionality of these three MOFs, we can improve the interactions between CO and especially CH
2
4
−1
with the frameworks. Calculated Q (CH ) for TMU-5, TMU-34, and TMU-34(−2H) are 27, 23, and 22 kJ mol , respectively.
st
4
These Q values are among the highest for CH −framework interactions. For systematic comparison, two reported frameworks,
st
4
TMU-4 and TMU-5, have been compared with TMU-34 and TMU-34(−2H) in CO adsorption.
2
1
. INTRODUCTION
and effective way to prepare desired porous frameworks for gas
separation and storage.
Natural gas, which mainly consists of methane, after
hydrogen, is a promising candidate as a clean energy source.
Methane has the minimum number of carbons possible for a
Metal−organic frameworks (MOFs), a class of porous
materials, show great promise for applications in gas storage
and separation, because of their crystalline structure, high
permanent porosity, high surface area, high pore volume, and
adjustable pore size and shape.
or clusters as inorganic nodes which are connected by organic
linkers through coordination bonds. Combination of a large
variety of components, for example, metal ions and function-
alized organic ligands, can afford a broad range of framework
motifs with a large variety of pores. Hybrid materials like
MOFs that combine the rigidity of inorganic clusters with the
flexibility and tunability of organic molecules are promising
candidates for a large variety of applications, especially gas
1
−5
hydrocarbon, so it releases the minimum amount of CO into
2
MOFs consist of metal ions
20−24
the atmosphere.
Mitigating the level of carbon dioxide in
6
,7
the atmosphere, with CO being the major contributor to the
2
greenhouse gas effect, is one of the most important present-day
25,26
environmental concerns.
On the other hand, methane has
8
,9
the maximum ratio of hydrogen to carbon, so it has the
20,27
maximum theoretical octane number.
Hence, the design of
promising and efficient metal−organic frameworks for CH
4
storage and CO capture is considered to be an effective way to
2
10−12
reduce CO emissions.
storage and separation.
2
To date, a great deal of research has been devoted to the
design of solid sorbents for methane storage by controlling a
variety of parameters like surface area and pore volume and
pore size and shape and introducing coordinately unsaturated
metal sites (open metal sites, OMSs) or functional groups in
Mixed ligand MOFs are a subclass of MOFs which are
tunable for targeted applications like gas adsorption. In mixed-
linker MOF structures, usually two-dimensional metal carbox-
ylate layers are pillared by linear linkers for increasing
dimensionality to generate three-dimensional porous frame-
1
3−15
works.
Bispyridyl ligands have been employed often as
spacers in order to tune the interlayer distance and chemical
Received: November 18, 2016
1
6−19
functionality.
This approach provides a simple, rational,
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b02758
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Inorganic Chemistry
Article
Scheme 1. Representation of Oxygen and Nitrogen Donor Ligands
2
3
the structure. There are two challenges to consider when
introducing OMSs: (i) the difficulty of introducing and
activating OMSs and (ii) the decrease in deliverable capacity
2.2. Gas Sorption Measurement. A Micromeritics ASAP 2020
surface area analyzer was used to measure gas adsorption isotherms,
and noncryogenic temperatures were controlled with a water
20,27
circulator. N isotherm measurements were collected at 77 and 298
2
of CH in MOFs with OMSs. As deliverable methane
capacity is more important in some cases than total methane
4
K, CO isotherm measurements were carried out at 273, 283, and 298
2
K, also CH isotherm measurements were collected at 273 and 298 K.
27
4
storage capacity, it is necessary to improve the other
parameters such as pore size and functionality rather than
introducing OMSs. Thus, mixed-ligand MOFs which are highly
tunable in pore size and functionality are good candidates for
improving methane−framework interactions.
In this work, our strategy is based on the design of mixed-
linker MOFs with OBA (4,4′-oxybis(benzoic acid)) as the
dicarboxylate linker coupled with functionalized pillared spacers
for optimization of CO and CH adsorption by controlling
2.3. X-ray Collection and Structure Determination. Intensity
data were collected at 100 K on an ADSC Quantum 210r
diffractometer device with a silicon double-crystal diffraction
monochromator by synchrotron radiation type. The structure was
solved by Phi Scan methods. Reflections were merged by SHELXL
according to the crystal class for calculation of statistics and
refinement. The solvent of DMF could not be located. Thus,
PLATON/SQUEEZE has been used to calculate the diffraction
contribution of the solvent molecule, thereby producing a set of
solvent-free diffraction intensities. CCDC-1499123 contains the
supplementary crystallographic data for TMU-34. The data can be
obtained free of charge from the Cambridge Crystallographic Date
Center via www.ccdc.cam.ac.uk/data_request/cif.
2
4
pore size and functionality of the frameworks. OBA is a flexible,
V-shaped oxygen donor ligand (Scheme 1) that we used
previously to generate stable mixed-ligand MOFs.
28−30
The
azine-functionalized TMU-4 and TMU-5 frameworks have
been reported. TMU-5 contains narrow pores that have
2.4. Synthesis of Frameworks. Here, solvothermal methods for
synthesizing appropriate crystals of TMU-34 and the sonochemical
method for synthesizing a powder of all three frameworks are
described. For all gas adsorption measurements, the powder of all
three frameworks have been used which are synthesized by
sonochemical methods.
31
stronger interactions with CO in comparison with TMU-4.
2
Herein, tetrazine-functionalized pillar spacers have been chosen
for a systematic comparison with azine functional groups as
Lewis bases. Tetrazine ligands also are nitrogen rich and hence
Lewis basic, suggesting that (similar to azine groups) they can
Synthesis of [Zn(OBA)(H DPT) ] ·DMF (TMU-34). Solvothermal
2
0.5 n
Method. Single crystals of TMU-34 suitable for X-ray diffraction were
obtained by mixing Zn(NO ) ·6H O (0.06 g, 0.2 mmol), H OBA
polarize CO and to some extent also CH molecules (Scheme
2
4
3
2
2
2
3
2,33
1).
These two properties make tetrazine groups good
(0.05 g, 0.2 mmol), and H DPT (0.048 g, 0.2 mmol) in 12 mL of
2
candidates for gas adsorption. As a consequence, TMU-34 and
TMU-34(−2H) were synthesized which contain tetrazine-
functionalized pores for systematic comparison with TMU-4
and TMU-5. TMU-4 along with TMU-34 and TMU-5 along
with TMU-34(−2H) are isostructures. Furthermore, these
frameworks have been investigated for their interaction with
methane because of their appropriate pore size and
functionality. For all MOFs, similarities (structure and pore
shape) and differences (functionality and pore size) let us
compare the influence of structure, functional groups, and pore
DMF. This mixture was sonicated until all solids were uniformly
dispersed (∼5 min), and it was then heated at 120 °C in a glass vial.
After 72 h, brown-orange crystals of TMU-34 were collected. Yield:
−1
0
.06 g (70% based on OBA). IR data (KBr pellet, ν/cm ) selected
bands: 660(m), 777(m), 871(m), 1162(s), 1241(vs), 1391(vs),
606(vs), 1679(s), 2926(m), 3271(m). Anal. Calcd for [Zn-
C O H )(C N H ) ]·(C ONH ): C, 53.8; N, 10.9; H, 3.9.
1
(
1
4
5
8
12
6
10 0.5
3
7
Found: C, 54.1; N, 11.4; H, 4.1.
Sonochemical Method. For synthesis of TMU-34 as a powder,
Zn(CH COO) ·2H O (0.22 g, 1 mmol), H OBA (0.26 g, 1 mmol),
3
2
2
2
and H DPT (0.24 g, 1 mmol) in 30 mL of DMF were mixed together.
2
size on CH −framework binding energy.
The mixture was sonicated for 160 min at ambient temperature and
atmospheric pressure. Then the mixture was centrifuged, and the
resulting powder was washed with DMF and dried at room
temperature. Yield: 0.34 g (78% based on OBA). IR data (KBr pellet,
4
2. EXPERIMENTAL SECTION
−
1
2.1. Material and Characterization. All required chemicals were
ν/cm ) selected bands: 661(m), 778(m), 872(m), 1162(s), 1241(vs),
1407(vs), 1607(vs), 1674(s), 2928(m), 3273(m). Anal. Found for
[Zn(C O H )(C N H ) ]·(C ONH ): C, 54.0; N, 10.8; H, 4.2.
obtained from commercial suppliers and used without further
purification unless otherwise noted.
Melting points were measured on an Electrothermal 9100
apparatus. Infrared spectra were recorded using Thermo Nicolet IR
14
5
8
12
6
10 0.5
3
7
Synthesis of [Zn(OBA)(DPT) ] .DMF (TMU-34(−2H)). Sonochem-
0.5 n
ical Method. A powdered sample of TMU-34(−2H) was synthesized
1
00 FT-IR. Ultrasonication was carried out in an ultrasonic bath
by mixing Zn(CH COO) ·2H O (0.22 g, 1 mmol), H OBA (0.26 g, 1
3
2
2
2
SONICA-2200 EP (frequency of 40 kHz). The thermal behavior was
mmol), and DPT (0.24 g, 1 mmol) in 30 mL of DMF/acetonitrile
(1:1) and sonicating for 60 min at ambient temperature and
atmospheric pressure. The mixture was then centrifuged, and the
resulting powder was washed with DMF and dried at 80 °C for 48 h.
Yield: 0.37 g (85% based on OBA). IR data (KBr pellet, ν/cm )
selected bands: 656(w), 780(w), 874(w), 1091(w), 1160(m),
1231(m), 1399(vs), 1500(s), 1629(s), 1672(s), 3429(m). Anal.
−1
measured with a PL-STA 1500 apparatus with a rate of 10 °C min in
a static atmosphere of argon. X-ray powder diffraction (XRD)
measurements were performed using a Philips X’pert diffractometer
with monochromated Cu Kα radiation. Elemental analyses (carbon,
hydrogen, and nitrogen) were carried out using a high-resolution
Costech ECS 4010 CHNS-O Elemental Analyzer.
−1
B
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Inorganic Chemistry
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Figure 1. (a) Binuclear Zn cluster in TMU-34 (O, red; N, blue; C, gray; Zn, pink). (b) 3D Connolly surface representation of porous TMU-34
2
along the [1 0 1] direction. (c) Representation showing the pore channels and that the network is 2-fold interpenetrated (in brown and blue). (d)
Representation of the pores highlighting the azine groups (blue balls) pointing toward the pores of TMU-34. In the Connolly surface
representations, gray represents outside the surface and blue inside the surface. Hydrogen atoms and DMF molecules are omitted for clarity.
Found for [Zn(C O H )(C N H ) ]·(C ONH ): C, 54.0; N, 10.9;
from three OBA ligands and one N atom (N6) from the
14
5
8
12
6
8
0.5
3
7
H, 3.9.
Synthesis of [Zn(OBA)(BPDH) ]·1.5DMF (TMU-5). Sonochemical
H DPT ligand (Figure 1a). The distance between Zn1 and Zn2
2
0
.5
2
9
is 3.432 Å. Each binuclear Zn unit is further linked to four
2
Method. TMU-5 was synthesized according to a reported procedure.
equivalent neighbors through four OBA ligands to give 2D
sheets. The 2D sheets are connected through the linear
A powdered sample of TMU-5 was synthesized by mixing
Zn(CH COO) ·2H O (0.22 g, 1 mmol), H OBA (0.26 g, 1 mmol),
3
2
2
2
and BPDH (0.17 g, 0.7 mmol) in 30 mL of DMF and sonicating for 60
min at room temperature and pressure. The mixture was then
centrifuged, and the resulting powder was washed with DMF and dried
at 80 °C for 48 h. Yield: 0.35 g (80% based on OBA). IR data (KBr
H DPT, extending the structure in three dimensions (Figures
2
1b). TMU-34 is doubly interpenetrated as shown in Figure 1c
and shows one-dimensional 5.4 × 6.2 Å pores including the van
der Waals radius. The internal surface of these pores is
−1
pellet, ν/cm ) selected bands: 652(s), 779(m), 873(m), 1021(m),
functionalized with the tetrazine groups of the H DPT ligands,
2
1
3
092(m), 1162(s), 1233(vs), 1397(vs), 1499(m), 1631(vs), 1671(vs),
which are highlighted in Figure 1d. For calculation of solvent-
accessible volume and porosity in the rigid framework of TMU-
414(w-br). Anal. Found for [Zn(C O H )(C N H )0.5]·
1
4
5
8
14
4
14
(
C ONH ) : C, 54.8; H, 4.2; N, 8.8.
3
7 1.5
3
4, void space and pore volume have been calculated by
2
.5. Activation Method. For activating TMU-34 and TMU-
3
−1
3
4(−2H) frameworks before gas adsorption measurements, both
samples were soaked in acetonitrile for 2 days to exchange the DMF.
Then all samples were outgassed under high dynamic vacuum at 120
PLATON to give 25% (nitrogen probe) and 0.27 cm ·g ,
respectively. Thermogravimetric analysis (TGA) of TMU-34
34
revealed a first weight loss in the range of 50−300 °C (calcd
2
9
°
C (Figures S1−S6). TMU-5 was activated as previously reported.
1
4.4%, found 15.7%), which is attributed to the removal of
DMF molecules inside the framework. Then a second weight
loss was observed in the range of 315−500 °C, corresponding
to the decomposition of the framework. The final residual
weight for TMU-34 corresponds to that of ZnO (Figure S2).
By applying BPDB, 1,4-bis(4-pyridyl)-2,3-diaza-1,3-buta-
3
. RESULTS AND DISCUSSION
3.1. Synthesis and Characterization. The MOFs, TMU-
5
, TMU-34, and TMU-34(−2H), were synthesized via
sonochemical reaction as detailed in the Experimental Section
diene, instead of H DPT, TMU-4 is generated, which has
using zinc acetate, H OBA, and a pillared spacer ligand. PXRD
2
2
31
been reported before. Similar to TMU-34, TMU-4 contains
one-dimensional pores (6.8 × 7.8 Å, including van der Waals
radii) which are functionalized with azine groups along with a
patterns of all synthesized MOFs show that the sonochemical
syntheses gave rise to TMU-5, TMU-34, and TMU-34(−2H),
which are structurally identical to those prepared via
solvothermal methods (Figure S5 and S6). Here, we used
sonochemical methods for synthesizing all three frameworks
instead of solvothermal methods, because the former is easy,
fast, and scalable.
A new MOF, TMU-34, was synthesized via solvothermal
method and characterized by X-ray crystallography. TMU-34
crystallizes in the monoclinic crystal system and P2 /c space
2
-fold interpenetrated framework (Figure 2).
TMU-5 is constructed from paddlewheel Zn (COO)
2
4
clusters that are pillared by BPDH spacer ligands. TMU-5 is
shown in Figure 3a and functionalized with azine groups
(Figure 3b). By using a DPT spacer ligand instead of BPDH,
TMU-34(−2H) is generated and isostructural to TMU-5
31,35
(Figure S9, Table S3).
Both structures contain 3-dimen-
1
group. TMU-34 is based on a binuclear Zn unit (Zn1 and
Zn2). Zn1 is tetrahedrally coordinated to three carboxylate O
atoms (O2, O4, and O6) from three OBA ligands and one N
sional and interconnected pores (4.4 × 8.1 Å for TMU-
34(−2H) and 3.8 × 5.6 Å TMU-5, including van der Waals
radius) which have identical pore shape but different pore size
and functionality. The TMU-34(−2H) framework is shown in
Figure 3c and is functionalized with tetrazine groups (Figure
2
atom (N1) from the H DPT ligand. Zn2 is tetrahedrally
2
coordinated to three carboxylate O atoms (O1, O7, and O10)
C
DOI: 10.1021/acs.inorgchem.6b02758
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Figure 4. N adsorption properties of TMU-5, TMU-34, and TMU-
2
3
4(−2H) at 77 K.
hysteresis type H4 which is often associated with narrow pores
36
in microporous materials. TMU-5, TMU-34, and TMU-
2
−1
3
4(−2H) have BET surface areas of 580, 540, and 670 m g ,
respectively. The results show that the calculated pore volumes
from N adsorption, 0.30, 0.28, and 0.31 cm g at 77 K and p/
3
−1
Figure 2. Comparison of TMU-4 and TMU-34 structure. (a) Two-
fold interpenetrated framework of TMU-4. (b) Azine-functionalized
pore of TMU-4. (c) Two-fold interpenetrated framework of TMU-34.
2
p° = 0.98, are higher than the static pore volumes of 0.24, 0.27,
3
−1
and 0.25 cm g for TMU-5, TMU-34, and TMU-34(−2H),
31
(
d) Tetrazine-functionalized pore of TMU-34.
respectively. TMU-4 is nonporous toward N molecules.
2
3.2.2. CO2 Adsorption and Separation. The sorption
behavior of TMU-34 and TMU-34(−2H) toward CO revealed
2
3
−1
that they can adsorb 29.6 (3.7% wt) and 43.5 cm ·g (5.44%
wt), respectively, at 298 K and 1 bar (Figures 5a, S12, and S13).
The isosteric heat of adsorption (Q ) for CO as a function of
st
2
relative pressure was calculated for evaluating the affinity of
tetrazine-functionalized frameworks, TMU-34 and TMU-34-
(
−2H), toward CO . CO adsorption single-component
2
2
isotherms at 273, 283, and 298 K have been fitted by the
virial method. The calculated magnitudes of Qst at zero
coverage for TMU-34 and TMU-34(−2H) are 23 and 30 kJ
−1
mol , respectively (Figure 5b). In all pressure ranges, the
magnitude of Q for TMU-34(−2H) is higher than TMU-34.
st
These data reveal that TMU-34(−2H) has a higher affinity
toward CO in comparison with TMU-34 (Table S4). To
2
evaluate the sorption performance of these two MOFs toward
CO in a binary mixture of CO /N (15%/85%), we calculated
2
2
2
the CO /N2 selectivity factor. The selectivity factor was
2
calculated at 298 K based on the IAST method and single-
component isotherm of CO and N proposed by Myers and
2
2
37
Prausnitz (Figure 5c). The selectivity factors for TMU-34 and
TMU-34(−2H) initially were calculated to be 15.5 and 29.5,
which increase up to 18.1 and 33.1 at higher pressure
respectively (Table S9).
CO adsorption of TMU-4 and TMU-5 has been reported
2
Figure 3. (a) Connolly representation of TMU-5. (b) Azine-
functionalized pore of TMU-5. (c) Connolly representation of
TMU-34(−2H). (d) Tetrazine-functionalized pore of TMU-34(−2H).
previously where TMU-5 has higher CO adsorption and Q
rather than TMU-4. Similar to TMU-4 and TMU-5 behavior
2
st
31
in CO adsorption, TMU-34(−2H), which is an isostructure
2
with TMU-5, has a higher amount of adsorption and affinity
3
d). For TMU-5 void space and pore volume (PLATON) are
toward CO rather than TMU-34, which is an isostructure with
2
3
−1
calculated to be 24.1% (nitrogen probe) and 0.24 cm ·g ,
TMU-4. This observation demonstrates that azine and tetrazine
groups in 3D interconnected pores of TMU-5 and TMU-
34(−2H) frameworks are more accessible rather than these
Lewis basic groups in one-dimensional pores of TMU-4 and
TMU-34 frameworks.
respectively, while for TMU-34(−2H) these data are 24.6%
3
−1
34
(
nitrogen probe) and 0.25 cm ·g , respectively.
3
.2. Gas Adsorption Measurements. 3.2.1. N Adsorp-
2
tion at 77 K. The permanent porosity of TMU-5, TMU-34,
and TMU-34(−2H) synthesized by sonochemical methods was
verified by N adsorption at 77 K, Figure 4. For all frameworks,
In two isostructural frameworks TMU-5 and TMU-34(−2H)
−1
31
the Q (CO ) are 43 and 30 kJ mol , respectively. This
2
st 2
isotherms reveal a type-I behavior which is a characteristic of a
microporous material, but TMU-34(−2H) and TMU-5 show a
noticeable difference is related to the different pore size and
functionality of these two frameworks. The pores of TMU-5
D
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Figure 5. CO adsorption properties of TMU-34 and TMU-34(−2H): (a) CO at 298 K, (b) isosteric heat of CO adsorption, and (c) CO /N
2
2
2
2
2
selectivity.
Figure 6. CH adsorption of TMU-5, TMU-34, and TMU-34(−2H): (a) CH at 298 K and (b) isosteric heat of CH adsorption.
4
4
4
contain methyl groups, and the presence of this group leads to a
narrower pore size in TMU-5 rather than TMU-34(−2H).
Also, methyl groups simply can increase the polarizability of the
framework. Hence, narrower pore size and improved polar-
izability of the framework lead to higher affinity of CO2
molecules toward TMU-5 framework.
For evaluating the binding energy of CH −framework
4
interactions, we calculated the Q for CH adsorption in
st
4
TMU-5, TMU-34, and TMU-34(−2H) at 273 and 298 K based
on the virial method (Supporting Information). The Q for
st
CH adsorption was calculated at 1 bar, because errors in high-
4
pressure isotherm measurements are more significant than at
2
7
On the other hand, in comparison of another two
low pressure. Calculated Q for TMU-5, TMU-34, and TMU-
st
−1
isostructure frameworks, TMU-4 and TMU-34, with one-
34(−2H) are 28, 24, and 23 kJ mol at zero coverage,
respectively (Figure 6b), which are among highest reported
−1
dimensional pores, calculated Q (CO ) are 25 and 23 kJ mol .
st
2
In contrast to TMU-5 and TMU-34(−2H), there is no
significant difference in affinity of TMU-4 and TMU-34 toward
values for Qst of CH adsorption (Table 1).
4
CO molecules. This phenomenon may be attributed to the
Table 1. Comparison for Q Values of Methane Uptake
2
st
one-dimensional pores of these two structures, which leads to
Qst (kJ mol−1) zero
limited accessibility of CO molecules toward Lewis basic sites
2
compound
PCN-14
coverage
functionality
anthracene
ref
of these two structures.
30
40
27
41
3
.2.3. CH Adsorption. For CH storage, it is necessary to
4
4
PCN-14
18.7
29
anthracene
tetrazole
optimize capacity and CH −framework interactions simulta-
4
NENU-520
TMU-5
2
7
neously. Here, we focused on CH −framework interactions
by combining the optimization of pore size and chemical
functionality.
4
27
azine, methyl
this
work
MIL-120
27
Al-centered
octahedral
42
CH adsorption was performed for TMU-5, TMU-34, and
4
SNU-50
TMU-34
26.8
23
iimide, OMSs
43
TMU-34(−2H) at 298 K and 1 bar (Figure 6a). The data show
dihydrotetrazine
this
work
that TMU-5, TMU-34, and TMU-34(−2H) can adsorb 20.3
3
−1
(
2.54 wt %), 7.4 (0.93 wt %), and 15.5 cm g (1.96 wt %) of
TMU-34(−2H)
22
22
tetrazine
OMSs
this
work
CH , respectively. TMU-4 did not adsorb CH at all. By
4
4
comparing the two isostructural frameworks, TMU-5 and
Ni (dhtp)
2
44
TMU-34(−2H), the latter has higher pore size and BET surface
area, but the former adsorbs more CH at low pressure and
4
Optimal pore sizes for CH4 uptake are in very good
agreement with the kinetic diameter of one (4 Å) and two (8
Å) methane molecules. In this case, the pore sizes of TMU-5
and TMU-34(−2H) are well matched with the kinetic diameter
of one and two methane molecules, respectively. Hence, by
controlling the pore size and functionality in these three MOFs,
room temperature. This may due to a stronger framework−
20
CH in TMU-5 compared to TMU-34(−2H). This behavior of
4
TMU-5 and TMU-34(−2H) toward CH is completely
4
identical to the behavior of these two frameworks toward
CO molecules.
2
E
DOI: 10.1021/acs.inorgchem.6b02758
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 2. Comparison of BET Surface Area and CH Uptake with Some of the Other MOFs
4
condition
2
3
compound
BET (m /g)
pore volume (cm /g)
T (K)
P (bar)
CH uptake (wt %)
ref
4
SNU-25
CUK-1
Cu(tip)
795
630
810
1386
582
667
536
0.368
0.26
0.34
0.58
0.24
0.25
0.27
298
298
298
298
298
298
298
1.01
1.01
1.01
1.0
0.7
45
46
47
48
0.6
1.6
Co (ndc)(HCOO) (μ -OH)
3
1.1
3
3
TMU-5
1.06
1.06
1.06
2.54
1.94
0.93
this work
this work
this work
TMU-34(−2H)
TMU-34
we succeeded to improve the interaction between CH and the
CO , and CH isotherms, Q and selectivity calculations
4
2
4
st
framework.
(PDF)
(CIF)
Despite the moderate BET surface area and pore volume, the
high Q values and improved methane uptake capacity (wt %)
st
at room temperature and pressure (Table 2) can be attributed
to the azine- and tetrazine-functionalized narrow pores in the
MOFs discussed. This is especially the case for TMU-5, which
AUTHOR INFORMATION
Corresponding Authors
E-mail: morsali_a@modares.ac.ir. Phone: (+98) 21-82884416.
E-mail: j-hupp@northwestern.edu.
E-mail: o-farha@northwestern.edu.
■
*
*
*
is well matched with the kinetic diameter of CH . Also, the
4
pores of TMU-5 consist of methyl groups which simply add to
the polarizability of the framework and thereby increase
ORCID
London dispersion interactions with CH molecules. Such
4
behavior has been reported in which a methyl group can
increase the interaction between a framework and CH4.
Timur Islamoglu: 0000-0003-3688-9158
Ali Morsali: 0000-0002-1828-7287
Joseph T. Hupp: 0000-0003-3982-9812
Omar K. Farha: 0000-0002-9904-9845
38
On the other hand, in comparison of two tetrazine-
functionalized frameworks, TMU-34(−2H) has higher CH4
adsorption in the entire range of pressure. Again, this higher
adsorption may be attributed to differences in the structure of
two frameworks whose 3D interconnected pores of TMU-
Notes
The authors declare no competing financial interest.
3
4(−2H) cause to effectively interact with the framework with
4
ACKNOWLEDGMENTS
Support of this investigation by Tarbiat Modares University
and Northwestern University is gratefully acknowledged.
■
CH molecules compared with limited accessibility of tetrazine
groups to gas molecules in TMU-34. However, a slightly higher
Q (CH ) for TMU-34 in low pressures and zero coverage may
be related to the more basic N(sp ) of H DPT pillar spacers.
These results reveal that controlling the pore size and
chemical functionality in two tetrazine-functionalized frame-
works, TMU-34 and TMU-34(−2H), and azine-functionalized
MOF, TMU-5, can enhance the affinity of MOF toward CH4
gas.
st
4
2
39
2
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