Enhancement of Gas Sorption and Separation Performance via Ligand Functionalization within Highly Stable Zirconium-Based Metal-Organic Frameworks

Article abstract of DOI:10.1021/acs.cgd.7b00090

By using a modulated synthetic strategy, three isostructural Zr-based metal organic frameworks [Zr₆O₄(OH)₄(EDDC)₆] (JLU-Liu34), [Zr₆O₄(OH)₄(EDDC-NO₂)₆] (JLU

Full text of DOI:10.1021/acs.cgd.7b00090

Article
pubs.acs.org/crystal
Enhancement of Gas Sorption and Separation Performance via
Ligand Functionalization within Highly Stable Zirconium-Based
Metal−Organic Frameworks
Published as part of a Crystal Growth and Design virtual special issue on Crystal Engineering of Nanoporous
Materials for Gas Storage and Separation
Jian Zhang,† Shuo Yao,† Shuang Liu,† Bing Liu,† Xiaodong Sun,† Bing Zheng,‡ Guanghua Li,† Yi Li,†
Qisheng Huo,† and Yunling Liu*,†
†State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012,
P. R. China
‡Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation
Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P. R.
China
S
* Supporting Information
ABSTRACT: By using a modulated synthetic strategy, three
isostructural Zr-based metal organic frameworks
[Zr6O4(OH)4(EDDC)6] (JLU-Liu34), [Zr6O4(OH)4-
(EDDC-NO2)6] (JLU-Liu35), and [Zr6O4(OH)4(EDDC-
NH2)6] (JLU-Liu36) (H2EDDC = (E)-4,4′-(ethene-1,2-diyl)-
dibenzoic acid, H2EDDC-NO2 = (E)-2,2′-dinitro-4,4′-(ethene-
1,2-diyl)dibenzoic acid, H2EDDC-NH2 = (E)-2,2′-diamino-
4,4′-(ethene-1,2-diyl)dibenzoic acid) which possess different
substituent groups have been successfully synthesized.
Significantly, the framework and crystallinity of the function-
alized Zr-metal−organic frameworks (Zr-MOFs) are well-
retained even though the nature of the functional group is
different (NO2 is electron-withdrawing, whereas NH2 is
electron-releasing). All of the three analogues display 12-connected fcu topology, and the classical Zr6 clusters in the porous
crystal make the whole framework approach high chemical and thermal stability. As a result of the functionalization effect, the
three analogues show different sorption and separation abilities to small gases, especially for JLU-Liu34 which exhibits a
significant C3H8 uptake capacity of 303 cm3 g−1 at 273 K under 1 bar. Although JLU-Liu36 shows lower Brunauer−Emmett−
Teller surface area than JLU-Liu34, it possesses enhanced CO2 adsorption enthalpy and selectivity for CO2 over CH4 influenced
by the additional amino groups. Ligand modification provides an efficient strategy to design and synthesize porous MOFs
materials for small gases sorption and separation.
choosing metal sources and ligands.8−13 Recent reports indicate
that Zr-based MOFs, in particular those with Zr6O4(OH)4
INTRODUCTION
■
Metal−organic frameworks (MOFs), as a class of porous
architectures composed of diverse organic and inorganic
clusters, exhibit dramatically improved chemical and mechanical
stabilities.14 Most UiO-66-type zirconium MOFs possess highly
building blocks, have developed rapidly in the fields of
chemistry and material science.1−3 Because of their high
porous frameworks and comprise large octahedral cages and
smaller tetrahedral cages. Therefore, the porous frameworks
porosity, well-defined open channels, structural diversity, and
rich functionalities, MOFs play significant roles in many
different applications such as gas storage and separation,
and high chemical stabilities of Zr-based MOFs make them
promising candidates for gas sorption and separation.15
catalysis, drug delivery, and sensing.4−7 However, because of
Recently, a large number of porous MOFs with enhanced gas
uptake have been designed and synthesized through predesign-
their relatively low chemical and thermal stabilities, the
ing the organic ligands.16,17 Among them, utilizing linkers with
potential applications of these materials are limited. To
overcome this limitation, diverse MOFs materials with
reinforced secondary building units (SBUs), such as Zn4O-
(CO2)6, Cu2(CO2)4, Cr3O(CO2)6, Al8(OH)8(CO2)16, and
Ti8O8(OH)4(CO2)6, have been constructed by rationally
Received: January 20, 2017
Revised: February 22, 2017
Published: March 10, 2017
© XXXX American Chemical Society
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similar length but distinct functionalities in the construction of
isostructural MOFs is a succinct method to construct
functionalized MOF materials for adjusting gas adsorption
and separation performance.18−21 One of the representative
examples is the successful synthesis of multivariate MOFs
(MTV-MOFs) presented by Yaghi and co-workers.22 These
isoreticular Zn4O(CO2)6 SBUs-based MOF materials exhibit
different gas adsorption performances for their different grafted
substituent groups. Owing to diverse fascinating features,
functionalized MOFs have been shown to exhibit great
potential over the conventional adsorbents for mixture
separation. For instance, Fe-MIL-101-NH2 exhibits an exciting
CO2 adsorption capacity,23 and UiO-66(Zr)-COOH presents
high selectivity of CO2 from a CO2/CH4 gas mixture.24 Among
them, UiO-66-type modified MOF materials possess not only
pore tunability and structural diversity, but also high thermal
and chemical stabilities. The remarkable stability makes
modified Zr-based MOFs an ideal platform for further
investigations of the functionalization effect on the small
molecular gases capture.
Herein, we employ a series of linear dicarboxylic acids ligands
with different substituent groups (H2EDDC = (E)-4, 4′-
(ethene-1,2-diyl)dibenzoic acid, H2EDDC-NO2 = (E)-2,2′
dinitro-4,4′-(ethene-1,2-diyl)dibenzoic acid, H2EDDC-NH2 =
(E)-2,2′ diamino-4,4′-(ethene-1,2-diyl)dibenzoic acid) to fab-
ricate MOFs materials with different functional groups and
explore the effect of the functional groups in the gas sorption
and separation. By using modulated synthetic strategy, different
functionalized linear linkers have been introduced to form a
class of porous isostructural Zr-MOFs with fcu topologies,
JLU-Liu34 (Zr-H2EDDC), JLU-Liu35 (Zr-H2EDDC-NO2),
and JLU-Liu36 (Zr-H2EDDC-NH2), respectively. All the
materials contain a highly stable Zr6O4(OH)4 cluster, which
leads to a strong resistance toward water or even hydrochloric
acid solution. Unfortunately, only compounds JLU-Liu34 and
JLU-Liu36 exhibit a high BET surface area after the removal of
guest molecules. Compound JLU-Liu34 exhibits high C3H8
adsorption behavior, and compound JLU-Liu36 possesses high
adsorption enthalpy of CO2 as well as selectivity of CO2 over
CH4. Guest molecules in compound JLU-Liu35 cannot be
removed absolutely by solvent exchange or heat treatment.
Scheme 1. Synthesis Route of Modified Linear Ligands,
H2EDDC-NO2 and H2EDDC-NH2
is cooled through a water and ice bath to a temperature of 0 °C. At this
point, 10 g (0.0580 mol) of reagent are added, small portions at each
time. The reaction continues for about 90 min, in which it is important
that the reagent is completely dissolved inside the acid mixture. Then
the mixture was poured into about 700 mL of water and ice. Solid
white substance precipitates were formed, which are filtered off and
subsequently washed in a beaker in order to remove the acid residues.
The crystallization occurs in toluene.
Synthesis of (E)-2,2′-Dinitro-4,4′-(ethene-1,2-diyl)dibenzoic Acid
(H2EDDC-NO2) (Step 2). 45 mL of absolute ethanol are poured into a
250 mL beaker and 5.47 g (0.0970 mol) of KOH are slowly dissolved
therein. At this point, 5.00 g (0.023 mol) of 3-nitro-(4-chloromethyl)
benzoic acid are added. The precipitation of a brownish powder in the
system immediately starts, being the potassium salt of dinitro-
stilbenedicarboxylic acid. The system is left to react at room
temperature for a time of about 45 min. The salt is filtered off
under a vacuum and is dissolved in about 70 mL of water; it is
necessary to increase the temperature until complete dissolution
occurs. At this point, the acid ligand is precipitated with dilute
hydrochloric acid, until pH = 1 is reached. The solid product is
recovered, then dried in the stove. 1H NMR (300 MHz, DMSO-d6, 25
°C) δ (ppm): 12.97 (s, 2H), 8.62 (d, J = 3.0 Hz, 2H), 8.27 (d, J = 1.5
Hz, 2H), 8.06 (d, J = 8.4 Hz, 2H), 7.69 (s, 2H).
Synthesis of (E)-2,2′-Diamino-4,4′-(ethene-1,2-diyl)dibenzoic
Acid (H2EDDC-NH2) (Step 3). 100 mL of distilled water are poured
into a 250 mL beaker and 6.90 g (0.0288 mol) of sodium sulfide are
dissolved therein. Once the salt is dissolved, 5.00 g (0.0138 mol) of
dinitro-stilbenedicarboxylic acid are added. The system is kept boiling
for a time of 15 min. At this point, under stirring, dilute hydrochloric
acid is added up to a pH = 4−5. (E)-2,2′-diamino-4,4′-(ethene-1,2-
diyl)dibenzoic acid is precipitated, which is redissolved and
reprecipitated in aqueous solution so to remove possible residues
containing sulfur. The product is dried in an oven at 120 °C. 1H NMR
(300 MHz, DMSO-d6, 25 °C) δ (ppm): 12.97 (s, 2H), 8.62 (d, J = 1.8
Hz, 2H), 8.49 (d, J = 1.2 Hz, 4H), 8.3 (d, J = 1.2 Hz, 4H), 8.2 (d, J =
1.2 Hz, 4H), 7.69 (s, 2H).
Synthesis of Isostructural Zr-MOFs on the Basis of
Modulated Synthetic Strategy. Synthesis of [Zr6O4(OH)4(EDDC)6]
(JLU-Liu34, Zr-H2EDDC). A mixture of H2EDDC (0.009 g, 0.034
mmol), ZrCl4·4H2O (0.007 g, 0.034 mmol), acetic acid (175 μL), and
DMF (2 mL) was sealed in a 20 mL vial and heated to 115 °C for 50
h, and then cooled to room temperature. The colorless octahedral
crystals were collected and air-dried. The agreement between the
experimental and simulated PXRD patterns indicated the phase-purity
of as-synthesized product (Supporting Information, Figure S2).
Synthesis of [Zr6O4(OH)4(EDDC-NO2)6] (JLU-Liu35, Zr-H2EDDC-
NO2). A mixture of H2EDDC-NO2 (0.012 g, 0.034 mmol), ZrCl4·
4H2O (0.007 g, 0.034 mmol), benzoic acid (0.240 g, 0.002 mol), and
DMF (2 mL) were sealed in a 20 mL vial and heated to 115 °C for 50
h, and then cooled to room temperature. The yellow octahedral
crystals were collected and soaked in DMF solution. The agreement
between the experimental and simulated PXRD patterns indicated the
phase-purity of as-synthesized product (Supporting Information,
Figure S2).
EXPERIMENTAL SECTION
■
Materials and Physical Characterizations. Apart from two
functionalized ligands, H2EDDC-NO2 and H2EDDC-NH2, all the
other reagents were obtained from commercial sources and used
without further purification. Powder X-ray diffraction (PXRD) data
were collected on a Rigaku D-Max 2550 diffractometer working with
Cu−Kα radiation (λ = 1.5418 Å) at room temperature. Thermogravi-
metric analyses (TGA) were carried out with the TA Q500
thermogravimetric analyzer under a heating rate of 10 °C min−1 to
800 °C in air. The infrared spectra were recorded within a Bruker IFS-
66v/S FTIR spectrometer in the range of 400−4000 cm−1 using a KBr
pellet. The N2 sorption experiments were measured by a Micrometrics
ASAP 2040 instrument at 77 K. The CO2, CH4, C2H6, and C3H8
adsorption isotherms were measured at 273 and 298 K with a
Micrometrics 3-Flex instrument.
Synthesis and Characterization of Organic Ligands (Scheme
1). Synthesis of (E)-2,2′-Dinitro-4,4′-(ethene-1,2-diyl)dibenzoic Acid
and (E)-2,2′-Diamino-4,4′-(ethene-1,2-diyl)dibenzoic Acid. There
are three steps in the sythesis process, which were described in detail,
respectively.
Synthesis of 3-Nitro-(4-chloromethyl) Benzoic Acid (Step 1). 230
mL of 96% sulfuric acid are poured into a 500 mL flask, and 130 mL of
fuming nitric acid are slowly added under stirring. The above solution
B
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Synthesis of [Zr6O4(OH)4(EDDC-NH2)6] (JLU-Liu36, Zr-H2EDDC-
NH2). A mixture of H2EDDC-NH2 (0.010 g, 0.034 mmol), ZrCl4·
4H2O (0.007 g, 0.034 mmol), benzoic acid (0.180 g, 0.0015 mol), and
DMF (2 mL) was sealed in a 20 mL vial and heated to 115 °C for 50
h, and then cooled to room temperature.. The orange octahedral
crystals were collected and soaked in DMF solution. The agreement
between the experimental and simulated PXRD patterns indicated the
phase-purity of as-synthesized product (Supporting Information,
Figure S2).
Fm3m space group with the cell of 29.99 Å. Each Zr atom is
̅
eight-coordinated by oxygen atoms in the as-synthesized
materials and six Zr atoms form a 12-connected Zr6O4(OH)4
metal center. Similar to the recently reported ZrMOF materials,
the triangular faces of the Zr6 octahedron are alternatively
capped by μ3-O and μ3-OH groups.29,32 As shown in Figure 1,
X-ray Structure Determination. Single-crystal X-ray diffraction
data for suitable crystalline JLU-Liu 34 were recorded on a Bruker
Apex II CCD diffractometer. The structures were solved by direct
methods using the SHELXS program of the SHELXTL package and
refined by full-matrix least-squares analyses on F2 (SHELXTL-97).25
Unfortunately, because of the small crystal size and atom vibration,
only a general structure can be determined by single-crystal X-ray
diffraction data. Therefore, powder XRD patterns for JLU-Liu 34 were
recorded in 2θ degree ranging from 4 to 140° with a 0.016 step size.
To obtain accurate crystalline information, the General Structure
Analysis System (GSAS)-EXPGUI was used to refine the XRD
patterns.26 For these isostructural compounds, carbon−carbon double
bonds in the ligand cause a strong vibration in the framework, so we
use both powder X-ray diffraction data and crystallographic data to
complete the final structure simulation. All the XRD patterns of as-
synthesized samples match well with the simulation model as we
expected.
Figure 1. Illustration for the construction of MOF-fcu.
all the polyhedron edges are bridged by the linear carboxylates
(μ2-COO) originating from the dicarboxylic acids to form a
Zr6O4(OH)4(CO2)12 cluster. There are also two types of
polyhedral cages in these analogous UiO-66 type frame-
works.33−35 Every six hexanuclear Zr cluster is linked to each
other by ligands to form an octahedral cage with a distance of
24.6 Å between the opposite Zr6 clusters, and another
tetrahedral cages were consisted of five Zr6 clusters with the
side length of polyhedron is 17.4 Å (Figure 2). Different from
other previous works which are focused on the moducation of
ethynyl and ethenyl,36,37 two substituents were grafted to
benzene rings to achieve isostructural functionalized com-
pounds JLU-Liu35 and JLU-Liu36.
FT-IR Spectroscopy Characterizations of JLU-Liu34−36.
The presence of the introduced functional groups on the linear
ligands and tagged Zr-MOFs was further evidenced by IR
spectroscopy. The presence of the nitro group on the linkers is
confirmed by the presence of typical spectroscopic features
arising from aromatic nitro compounds. In particular, the nitro
group shows absorption because of asymmetric and symmetric
N−O stretching modes.38 Asymmetric modes typically result in
a strong band in the 1550−1500 cm−1 region, symmetric
modes absorption peaks in 1360−1290 cm−1 region. JLU-
Liu35 shows the presence of two additional bands in the above-
mentioned regions. The first is centered at 1340 cm−1 and the
second appears as a shoulder peaks at 1420 and 1525 cm−1,
which can be reasonably ascribed to the -NO2 group.39 As
shown in Figure S3, the IR spectra of the JLU-Liu36 have two
medium absorptions which is different from the spectra of JLU-
Liu34; one is centered at 3355 cm−1 and the other one is
centered at 3450 cm−1. These peaks stand for the asymmetric
and symmetric N−H stretching modes, respectively.40
RESULTS AND DISCUSSION
■
Introducing functional groups into pores of MOFs through
ligand modification provides an efficacious strategy for tuning
gas adsorption and separation performances without altering
the original topologies.27−29 Therefore, we design and
synthesize two functionalized ligands, H2EDDC-NO2 and
H2EDDC-NH2, which have the same linear molecular
geometry as H2EDDC. On the basis of the modulated synthetic
strategy,30 two kind monocarboxylic acids (acetic acid or
benzoic acid) were added to the Zr-MOFs synthetic system as
modulators, and three isostructural crystalline Zr-MOFs with or
without functional groups were successfully synthesized. All the
resulting crystalline compounds exhibit the same crystal
structure with fcu topologies and contain different substituent
groups in the pores. The structures of these isostructural
compounds have been described in detail, together with their
chemical stabilities and gas sorption properties.
Structure Descriptions. Crystal Structure of JLU-Liu 34−
36. Selective single-crystal X-ray diffraction data for crystalline
JLU-Liu 34 were recorded and analyzed. Because of the low
diffraction intensity and carbon−carbon double bond atom
vibrations, only a general structure can be determined. To
further confirm and construct the simulated structure proposed
by single-crystal X-ray diffraction data, the PXRD patterns of
JLU-Liu34 are measured and refined by the GASA program.31
All the Rw and Rwp of the data are under 10% and 12%,
respectively. Rietveld refinement of PXRD patterns for
desolvated JLU-Liu34 could be obtained with satisfactory R
factors using simulated models that predict biased Zr−O and
Zr−Zr distances. On the basis of the results of XRD refinement
and crystallographic data, we successfully calculated and
constructed the structural models for these isostructural
compounds. The experimental PXRD patterns (Figure S2,
Supporting Information) obtained from the as-synthesized
samples reveal that all the compounds are isostructural,
although the modified functional groups of the ligand are
different. The crystal structure of JLU-Liu34 was representa-
tively illustrated in detail. JLU-Liu34 crystallizes in the cubic
Thermal Stability and Chemical Stability of JLU-Liu 34−
36. Thermogravimetric analysis (TGA) was performed on
crystalline samples under atmosphere from 30 to 800 °C
(Figure S4) which indicates that all of isostructural Zr-MOFs
display high thermal stability due to their stable Zr6 clusters.41
It is worth mentioning that even the electrons of the functional
groups are different; the NH2-modified JLU-Liu36 is electron
rich and NO2-modified JLU-Liu35 is electron poor, and there
is not any loss for the thermal stabilities. JLU-Liu34 is selected
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Figure 2. Two types of cages in isostructural Zr-MOFs with different sizes.
Figure 3. (a) The different functional groups in the section and PXRD patterns of as-synthesized tagged Zr-MOFs, showing the framework stability
of upon treatments in water, pH = 1, and pH = 12 aqueous solutions for 3 days, (b) JLU-Liu34; (c) JLU-Liu35; (d) JLU-Liu36, respectively.
as an example to discuss the thermal stability of the three
compounds. The TG curve shows a mass loss of 35.7% below
500 °C due to the release of nomadic DMF molecules,
unbonded acetic acid, and the coordinated DMF molecules.
The sharp weight loss of 41.4% from 500 to 700 °C
corresponds to the decomposition of the ligand. Thus, JLU-
Liu34 could undergo decomposition at a rather high
temperature at ∼500 °C. As for the other two modified
MOFs, JLU-Liu35 and JLU-Liu36 also maintain their
frameworks above 450 °C. Moreover, the release rate of
nomadic solvent molecules slow down at a lower temperature
which is caused by the steric hindrance in the functionalized
materials. This phenomenon also attests to the existence of
functional groups and their strong intermolecular interaction
with solvents. In spite of these variations, it is worth mentioning
that all of these isostructural Zr-MOFs with or without
functional groups exhibit better thermal stability than UiO-66
and other MOFs materials.33,42
The stabilities of these compounds toward water, acid (HCl
aqueous solution, pH = 1), and base (NaOH aqueous solution,
pH = 12) were also tested and analyzed. A series of Zr-
H2EDDC samples were immersed in the corresponding solvent
for 3 days at room temperature. As it is shown in Figure 3, all of
these materials revealed quite high resistance toward these
chemical treatments, and the chemical stabilities of modified
Zr-H2EDDC materials were similar to the untagged JLU-Liu34.
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Figure 4. (a) The N2 adsorption and desorption isotherms of JLU-Liu34 and JLU-Liu36 at 77 K; (b) pore size distribution of JLU-Liu34 and JLU-
Liu36 calculated using the DFT method.
Figure 5. (a) CO2, (b) CH4, (c) C2H6, (d) C3H8 gas sorption isotherms for JLU-Liu34 and JLU-Liu36 at 273 and 298 K under 1 bar.
In addition, even in water and HCl solution, all of them
maintain their good crystallinity. However, JLU-Liu35 showed
the best structure stability in high concentrations of NaOH
aqueous solution which is better than JLU-Liu34 and -NH2
grafted JLU-Liu36. The high water stability of JLU-Liu35 can
be ascribed to the low hydration number of the nitro group.15,43
The results demonstrate that thermal and chemical stability of
Zr6-based MOFs are completely retained even after function-
alization. These high stable materials show potential
applications in different fields, such as postsynthesis, catalysis,
and biochemistry.44−47
Gas Adsorption Behaviors. Inspired by the large void
volume of the frameworks, gas adsorption properties of these
isostructural materials were studied. The as-synthesized samples
of JLU-Liu34, JLU-Liu35, and JLU-Liu36 were respectively
solvent exchanged with CH3CN, EtOH, and CH2Cl2 several
times, and all of them were activated for 10 h under 150 °C
before gas adsorption measurement (Figures S5−S7). In order
to evaluate the permanent porosity of these isostructural
MOFs, N2 adsorption experiments were detected at 77 K. As
shown in Figure 4, JLU-Liu34 and JLU-Liu36 exhibit similar
Type I nitrogen gas adsorption isotherms, and Brunauer−
Emmett−Teller (BET) surface area were calculated to be ca.
2496 and 2619 m2 g−1, respectively. The pore size distributions
of JLU-Liu34 and JLU-Liu36 calculated by using a nonlocal
density functional theory (NLDFT) method show a micro-
porous feature with a pore size of about 1.2 nm, which is
homologous to the crystalline data (Figure 4b). As shown in
Figure S12, JLU-Liu35 exhibits less BET surface area which
was calculated to be ca. 1103 m2 g−1. This is because the guest
molecules cannot be removed absolutely by solvent exchange
or evacuation, which could be confirmed by PXRD and TGA
(Figures S10 and S11). Recently, similar structural material
UBMOF-9 with high hydrogen storage capacity has been
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Figure 6. (a) Adsorption enthalpies of CO2 for JLU-Liu34 (red) and JLU-Liu36 (blue), (b) CO2/CH4 mixture adsorption selectivity is predicted by
IAST at 298 K and 100 kPa for JLU-Liu36.
Figure 7. CO2, CH4, C2H6, and C3H8 adsorption isotherms at 298 K along with the dual-site Langmuir Freundich (DSLF) fits (a, c); gas mixture
adsorption selectivity are predicted by IAST at 298 K and 100 kPa for JLU-Liu34 (b, d).
reported, but a systematic study of the other small molecular
gas storage performance has not been carried out.15 Therefore,
the CO2 and small hydrocarbon gas adsorption isotherms of
JLU-Liu34 and JLU-Liu36 were measured to investigate the
roles of amino group in gas sorption and separation properties.
At 273 K and atmospheric pressure, the amount of CO2 uptake
of JLU-Liu34 and JLU-Liu36 was close and reaches 58.9 and
54.7 cm3 g−1 (ca. 2.63 and 2.46 mmol g−1), respectively. To
compare the affinity of JLU-Liu34 and functionalized JLU-
Liu36 to CO2, the isosteric heat (Qst) was also calculated
(Figure 6). At zero-coverage, the Qst value of CO2 for JLU-
Liu34 and JLU-Liu36 is ca. 22.2 and 26.8 kJ mol−1,
respectively. The increased Qst of -NH2 modified JLU-Liu36
could be due to favorable interactions between adsorbed CO2
molecules and the Lewis basic sites in amine groups decorated
in the pores.48,49 The adsorption isotherms of CH4, C2H6 and
C3H8 are also measured at 273 and 298 K under 1 bar,
respectively. The maximum adsorption of JLU-Liu34 for CH4
is 16.0 and 9.1 cm3 g−1, C2H6 is 118.8 and 59.4 cm3 g−1, and
C3H8 is 303.2 and 240.0 cm3 g−1, respectively (Figure 5). It is
worth mentioning that the maximum adsorption of JLU-Liu34
for C3H8 is higher than many reported MOF materials.50,51 At
zero loading, Qst of CH4, C2H6, and C3H8 adsorption of JLU-
Liu34 is 23.0, 22.2, and 30.2 kJ mol−1, respectively (Figure 7).
For comparison, the maximum adsorption of JLU-Liu36 for
CH4 is 14.7 and 8.6 cm3 g−1, C2H6 is 109.5 and 55.6 cm3 g−1,
and C3H8 is 291.1 and 223.2 cm3 g−1, respectively. The Qst of
CH4, C2H6, and C3H8 adsorption of JLU-Liu36 was calculated
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to be 15.3, 22.0, and 29.6 kJ mol−1, respectively. These results
demonstrate that JLU-Liu36 can inherit most small hydro-
carbon gas adsorption capacities of JLU-Liu34, due to their
similar BET surface area and pore sizes.
size distribution for JLU-Liu35, CO2, CH4, C2H6, and
C3H8 adsorption isotherms at 298 K along with the dual-
site Langmuir Freundich (DSLF) fits and gas mixture
adsorption selectivity for JLU-Liu36, and isosteric heat
of CH4 and C2H6 for JLU-Liu34 and JLU-Liu36 (PDF)
The CO2/CH4 (50% and 50%, 5% and 95%) adsorption
selectivity was also calculated by ideal adsorption solution
theory (IAST).52,53 All of results were obtained by fitting the
experimental single-component isotherms through the dual site
Langmuir Freundlich (DSLF) model at 298 K and 1 atm.54 The
selectivity of CO2 over CH4 for JLU-Liu34 according to the
experimental data is 5.2 and 5.9 (Figure 7). In comparison, the
selectivity of CO2/CH4 is 5.8 and 6.2 for JLU-Liu36 (Figure
6). It is highlighted that the selectivity of CO2/CH4 for JLU-
Liu36 is comparable to the higher values for many reported
MOFs, such as JLU-Liu15,55 JLU-Liu18,56 BUT-10,57 and
MOF-508b (Table S2).58 The selectivity of CO2/CH4 for JLU-
Liu36 is much higher than isostructural JLU-Liu34, demon-
strating that functionalized amino groups of the ligands are able
to efficaciously enhance the CO2 adsorption capacities.
Furthermore, the selectivity of equimolar mixtures C2H6/CH4
and C3H8/CH4 for JLU-Liu34 is 8.7 and 45.9, respectively
(Figure 6). The C2H6/CH4 and C3H8/CH4 adsorption
selectivity for JLU-Liu36 were also calculated and fitted
(Figure S13). Because of their similar structures and pore
sizes, the results for separation of C2H6/CH4 and C3H8/CH4
were close to that of JLU-Liu34. Therefore, these results show
that introducing the amino groups in the pores is indeed useful
in decorating MOFs for CO2 capture and selectivity.
Accession Codes
CCDC 1533694 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*Fax: +86-431-85168624. E-mail: yunling@jlu.edu.cn.
■
ORCID
Yunling Liu: 0000-0001-5040-6816
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the financial support of the
National Natural Science Foundation of China (Nos.
21373095, 21371067, and 21621001).
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CONCLUSION
■
In summary, with the aim of investigating the role of the
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