Enhancing gas adsorption and separation capacity through ligand functionalization of microporous metal-organic framework structures

Article abstract of DOI:10.1002/chem.201002818

Hydroxyl- and amino- functionalized [Zn(BDC)(TED)_0.5] ·2DMF·0.2H₂O leads to two new structures, [Zn(BDC-OH)(TED)_0.5]·1.5DMF·0.3H₂O and [Zn(BDC-NH₂)(TED)_0.5]·xDMF·yH₂O (BDC=terephthalic acid, TED=triethylenediamine, BDC-OH=2-hydroxylterephthalic acid, BDC-NH₂=2-aminoterephthalic acid). Single-crystal X-ray diffraction and powder X-ray diffraction studies confirmed that the structures of both functionalized compounds are very similar to that of their parent structure. Compound [Zn(BDC)(TED)_0.5]·2DMF·0.2H ₂O can be considered a 3D porous structure with three interlacing 1D channels, whereas both [Zn(BDC-OH)(TED)_0.5]·1.5DMF·0. 3H₂O and [Zn(BDC-NH₂)(TED)_0.5] ·xDMF·yH₂O contain only 1D open channels as a result of functionalization of the BDC ligand by the OH and NH₂ groups. A notable decrease in surface area and pore size is thus observed in both compounds. Consequently, [Zn(BDC)(TED)_0.5]·2DMF·0. 2H₂O takes up the highest amount of H₂ at low temperatures. Interestingly, however, both [Zn (BDC-OH)(TED)_0.5] ·1.5DMF·0.3H₂O and [Zn(BDC-NH₂)(TED) _0.5] ·xDMF·yH₂O show significant enhancement in CO₂ uptake at room temperature, suggesting that the strong interactions between CO₂ and the functionalized ligands, indicating that surface chemistry, rather than porosity, plays a more important role in CO₂ adsorption. A comparison of single-component CO₂, CH₄, CO, N₂, and O₂ adsorption isotherms demonstrates that the adsorption selectivity of CO₂ over other small gases is considerably enhanced through functionalization of the frameworks. Infrared absorption spectroscopic measurements and theoretical calculations are also carried out to assess the effect of functional groups on CO₂ and H₂ adsorption potentials.

Full text of DOI:10.1002/chem.201002818

FULL PAPER
DOI: 10.1002/chem.201002818
Enhancing Gas Adsorption and Separation Capacity through Ligand
Functionalization of Microporous Metal–Organic Framework Structures
Yonggang Zhao,[a] Haohan Wu,[a] Thomas J. Emge,[a] Qihan Gong,[a] Nour Nijem,[b]
Yves J. Chabal,[b] Lingzhu Kong,[c] David C. Langreth,[c] Hui Liu,[d] Heping Zeng,[d] and
Jing Li*[a]
Abstract: Hydroxyl- and amino-
functionalized [Zn(BDC)-
(TED)0.5]·2DMF·0.2H2O leads to two
new structures, [Zn(BDC-OH)-
(TED)0.5]·1.5DMF·0.3H2O and [Zn-
(BDC-NH2)(TED)0.5]·xDMF·yH2O
(BDC=terephthalic acid, TED=tri-
ethylenediamine, BDC-OH=2-hydrox-
ylterephthalic acid, BDC-NH2 =2-ami-
noterephthalic acid). Single-crystal X-
ray diffraction and powder X-ray dif-
fraction studies confirmed that the
structures of both functionalized com-
pounds are very similar to that of their
parent structure. Compound [Zn-
N
ment in CO2 uptake at room tempera-
ture, suggesting that the strong interac-
tions between CO2 and the functional-
ized ligands, indicating that surface
chemistry, rather than porosity, plays a
more important role in CO2 adsorp-
tion. A comparison of single-compo-
nent CO2, CH4, CO, N2, and O2 adsorp-
tion isotherms demonstrates that the
adsorption selectivity of CO2 over
other small gases is considerably en-
hanced through functionalization of the
frameworks. Infrared absorption spec-
troscopic measurements and theoretical
calculations are also carried out to
assess the effect of functional groups
on CO2 and H2 adsorption potentials.
E
ACHTUNGTRENNUNG
G
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
R
ACHTUNGTRENNUNG
A
ACHTUNGTRNE(NUNG BDC)-
ACHTUNGTRENNUNG
U
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG(TED)0.5]·2DMF·0.2H2O can be
Keywords: adsorption · functionali-
zation · gas separation · gas storage ·
metal–organic framework
three
whereas
interlacing
both
1D
[Zn
channels,
ACHTUNGTRENNUNG
Introduction
bustion of fossil fuels, forest clearing, and other biomass
burning, has been recognized as the driving force of climate
change and global warming.[1] To prevent the situation from
getting worse, considerable effort has been made to develop
effective methods and materials capable of capturing and se-
questering CO2.[2] Current techniques proposed to capture
CO2 include adsorption, membrane separation, and chemi-
cal absorption.[3] Among them, adsorption using porous ma-
terials may be energetically efficient and economically com-
petitive.[4] Recent work on microporous metal–organic
frameworks (MMOFs) has demonstrated their potential for
storing and/or separating CO2.[5] However, despite numerous
interesting results and findings,[6] a great challenge remains
in the design of porous materials with exceptionally high ca-
pacity and selectivity toward CO2 adsorption. This is be-
cause most of the materials that strongly adsorb CO2 also
exhibit high adsorption of other small gases, such as CH4,
CO, N2, and O2. One of the ongoing endeavors to effectively
enhance the performance of these materials is to functional-
ize organic linkers.[7] Through this process, the pore proper-
ties (e.g., pore size and shape, pore volume and surface
area) of the MMOF materials and the host–guest interac-
tions (surface chemistry) can be systematically tuned to en-
Rapid CO2 accumulation, predominantly arising from the
sharp increase of anthropogenic activities, for example, com-
[a] Dr. Y. Zhao, H. Wu, Dr. T. J. Emge, Q. Gong, Prof. Dr. J. Li
Department of Chemistry and Chemical Biology, Rutgers
The State University of New Jersey Piscataway
New Jersey 08854 (USA)
Fax : (+1)732-445-5312
E-mail: Jingli@rutgers.edu
[b] N. Nijem, Prof. Dr. Y. J. Chabal
Department of Materials Science & Engineering
University of Texas at Dallas, Richardson, TX 75080 (USA)
[c] Dr. L. Kong, Prof. Dr. D. C. Langreth
Department of Physics and Astronomy, Rutgers
The State University of New Jersey, Piscataway
New Jersey 08854 (USA)
[d] H. Liu, Prof. Dr. H. Zeng
Institute of Functional Molecules
School of Chemistry and Chemical Engineering
South China University of Technology
Guangzhou, 510641 (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002818.
hance their uptake capacity and selectivity toward
a
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targeted gas. Modification of the ligands with functional
groups prior to the formation of MOFs (pre-synthesis func-
tionalization) is one of the two main approaches for such
functionalization. Studies have shown that the introduction
of functional groups into MMOFs could generate extraordi-
nary advantages over their parent frameworks. These in-
clude, but are not limited to, control of pore size and shape
without changing the structure topology, manipulation of
the level of interpenetration to tune the porosity, addition of
reactive sites to improve affinity and selectivity towards cer-
tain gas species, and introduction of a targeted environment
to adjust hydrophilicity/hydrophobicity and/or acidity/basici-
ty.[8] Another way to chemically functionalize a framework
is by post-synthesis modification, in which functional groups,
for example, an amine or aldehyde, are brought to the sur-
face of the pores after the MOF synthesis.[9]
solvothermal synthesis and structure characterization of
three MOFs, their gas adsorption and separation properties,
and the effect of functional groups on these properties.
Results and Discussion
Structure characterization: Compounds 1–3 were all pre-
pared by solvothermal synthesis. Single-crystal X-ray diffrac-
tion analysis of compound 2 revealed its jungle-gym-type
3D framework, as shown in Figure 1. Compounds 1 and 3
Among numerous organic ligands, terephthalate ligands
are a highly studied family and can be easily functionalized
with chemical groups of various polarities, hydrophilicities,
and acidities.[10] Examples include a series of isoreticular
MOF (IRMOF) structures possessing large pores and high
surface areas,[11] MIL-53 and related compounds,[12] and
many others.[13]
To better understand how selected functional groups may
affect the adsorption properties of a given structure type, we
have
(BDC)
(TED)0.5]·1.5DMF·0.3H2O
targeted
(TED)0.5]·2DMF·0.2H2O
(2),
three
microporous
MOFs,
[Zn(BDC-OH)-
[Zn(BDC-NH2)-
[Zn
G
G
(1),
and
ACHTUNGTRENNUNG
E
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG(TED)0.5]·xDMF·yH2O (3) in this study (BDC=terephthalic
acid, TED=triethylenediamine, BDC-OH=2-hydroxylter-
ephthalic acid, BDC-NH2 =2-aminoterephthalic acid). The
three structures are closely related in such a way that
1) they are isostructural and 2) they differ only in the func-
tional group (-R) of the terephthalate, BDC-R (Scheme 1).
We report the pre-synthesis functionalization of ligands, the
Figure 1. View of the crystal structure of compound 2. Zn (black), O
(dark grey), N (grey), and C (light grey). For clarity, solvent molecules
and hydrogen atoms are omitted.
are very similar to 2. All three structures contain paddle-
wheel dinuclear Zn clusters [Zn2ACTHNUTRGNEUNG(COO)4] (or secondary
building unit, SBU) that are bridged by BDC-R ligands
(R=H, OH, and NH2 for 1, 2, and 3, respectively), to form
a two-dimensional (2D) 44 grid. The BDC-R ligands in the
2D nets bend significantly, making the grid highly nonsym-
metric. The neighboring 2D grids are pillared by TED mole-
cules and extended to a three-dimensional (3D) framework.
Upon the removal of solvent molecules, the BDC-R ligand
in all three guest-free frameworks 1’, 2’, and 3’ becomes
linear and the 44 grid becomes symmetric. Intercrossing
channels of two different sizes are found in compound 1:[13a]
À
the cross section of 7.5ꢁ7.5 ꢂ (measured between the C C
atoms in the two lateral BDC ligands excluding the van der
Waals radius of carbon) along the c axis; and of 4.8ꢁ3.2 ꢂ
À
À
along the a and b axes (measured between C C and H H
atoms, excluding van der Waals radius of carbon or hydro-
gen atoms). In such an interconnected porous structure, a
very large void (61.3%) is accessible for solvent and guest
molecules. Differing from 1, the two small windows along
the a and b axes in compound 2 (4.8ꢁ2.6 ꢂ measured from
the single-crystal structure) become inaccessible for most
Scheme 1. Modified terephthalate ligands and schematic representation
of the modification on the MOF pore surface.
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Ligand Functionalization of Microporous Metal–Organic Frameworks
FULL PAPER
small gas molecules due to the grafting of the OH group
onto the benzene ring. Hydroxyl groups were found almost
parallel to the benzene ring without protruding into the
channel along the c axis, thus compound 2 features a 1D
surface area in 1’. The Brunauer–Emett–Teller (BET) and
Langmuir surface areas are estimated to be 1937/2057, 1023/
1111 and 1081/1183 m2 gÀ1 for compounds 1’, 2’, and 3’, re-
spectively (Table 1). The pore volumes calculated by the
channel structure along the
c axis. As calculated by
PLATON, 53.6% of the unit cell becomes accessible to
guest molecules upon the removal of the solvent from 2. De-
spite the unsuccessful attempt to produce a single crystal
suitable for structure analysis, the phase and purity of com-
pound 3 were identified by powder X-ray diffraction analy-
sis. Both compounds 2 and 3 have very similar PXRD pat-
terns to that of compound 1 (Figure 2), confirming that the
Table 1. Summary of surface area, pore volume, H2, and CO2 adsorption
data for 1’, 2’, and 3’.
H2
Qst
CO2
Qst
Surface area
[m2 gÀ1
Langmuir
(BET)
Pore
Uptake
Uptake
[wt%]
298 K,
]
volume [wt%]
[kJmolÀ1
(0.01–
]
[kJmolÀ1
(1–
]
[cm3 gÀ1
]
77 K,
1 atm
0.7 wt%) 1 atm
ꢀ5.0 7.4
5.5–4.9 13.1
4.4–3.9 9.5
6 wt%)
1’ 2057 (1937)
2’ 1111 (1023)
3’ 1183 (1081)
0.75
0.56
0.46
2.1
1.8
1.5
19.8–20.3
24.2–26.9
22.8–22.9
Horvath–Kawazoe (HK) method are 0.75, 0.56, and
0.46 cm3 gÀ1 for 1’, 2’, and 3’, respectively.[14] Apparently, the
significantly lower surface areas and the pore volumes
found in the functionalized compounds 2’ and 3’ are a direct
result of occupation by the functional groups (OH and NH2)
in the voids.
Hydrogen adsorption: As shown in Figure 3, the hydrogen
adsorption–desorption isotherms were measured at 77 and
87 K as a function of pressure ranging from 10À4 to 1 atm.
Figure 2. Powder X-ray diffraction (PXRD) patterns of compound
1
(bottom), 2 (middle), and 3 (top).
pillared 3D structure is retained upon ligand functionaliza-
tion. Considering the size of the amino group in 3, a similar
1D channel structure to compound 2 is expected for 3.
The thermal stability measurements by thermogravimetric
analysis (TGA) indicate that all solvent molecules trapped
in compound 1 (32.2%) can be removed at about 1508C.
The resulting framework was stable to 2608C, followed by a
decomposition at about 3008C. For compound 2, 29.5% of
weight loss was observed upon heating to 1808C, which cor-
responds well to the amount of solvent molecules. The
guest-free structure was stable up to 2508C. TGA data show
all guest molecules of compound 3 (30.3%) can be removed
at about 2008C, and the decomposition of the guest-free
framework occurred at about 2308C. The experimental mass
losses are in good agreement with the solvent weight calcu-
lated from single-crystal data: 34.4% for 1 and 27.6% for 2.
The PXRD analysis indicates that upon removal of solvent
molecules, all three compounds (1’, 2’, and 3’) remain highly
crystalline.
Figure 3. H2 adsorption–desorption isotherms at 77 K for 1’ (square), 2’
(circle), and 3’ (triangle); adsorption branch: open symbols, desorption
branch: filled symbols.
At 77 K, the uptake of hydrogen corresponds to 2.1, 1.8, and
1.5 wt% at 1 atm for 1’, 2’, and 3’, respectively. Compound
1’ shows the highest H2 adsorption capacity at 77 K, which is
consistent with its large surface area and pore volume. The
isosteric heats of H2 adsorption (Qst), derived from the
modified Clausius–Clapeyron equation were used to esti-
mate the extent of interactions between H2 and the frame-
works (Figure 4).[15] The higher uptake amount of H2 in 1’
over 2’ suggests that the pore size (and thus pore volume
and surface area) of framework rules, since the isosteric
Pore characterization: To determine the surface areas and
porosity of the three compounds, argon and nitrogen ad-
sorption–desorption experiments were carried out. The
evacuated compound 1’ shows much higher N2 adsorption
capacity than 2’ and 3’, confirming a larger pore size and
Chem. Eur. J. 2011, 17, 5101 – 5109
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J. Li et al.
(1 atm), respectively. Note the adsorbed amount of CO2 is
nearly doubled in 2’ with respect to that by its parent struc-
ture 1’ at room temperature.
The isosteric heats of CO2 adsorption were calculated
based on the adsorption raw data collected at 278, 288, and
298 K without fitting (Figure 6). The highest Qst values for
Figure 4. Isosteric heats of H2 adsorption (Qst) for 1’ (square), 2’ (circle),
and 3’ (triangle) calculated by Virial method.
heats of adsorption (Qst) are generally comparable for the
two compounds but with higher values for 2’ at low load-
ings: for 2’ they range from 4.9 to 5.5 kJmolÀ1 between 0.01
and 0.7 wt% and are about 5.0 kJmolÀ1 for 1’ in the same
loading range. Compared with those of 3’ (3.9 to
4.4 kJmolÀ1 in the same loading range), the Qst values indi-
cate stronger H2-MOF interactions in 2’ than in 3’. This is
also consistent with the observation that 2’ takes up more
H2 although it has a comparable surface area to 3’.
Figure 6. Isosteric heats of CO2 adsorption (Qst) values for 1’ (square), 2’
(circle), and 3’ (triangle).
compound 2’ are 24.2–26.9 kJmolÀ1 at low loadings (1–
6 wt%), in comparison with those of 1’ (19.8–20.3 kJmolÀ1)
and 3’ (22.8–22.9 kJmolÀ1), respectively. The trend in the Qst
values is clearly in accordance with the uptake amount.
Both follow the order: 2’>3’>1’ and override the trend in
surface area values (1’@3’ꢀ2’). The higher adsorption en-
thalpies of CO2 in 2’ and 3’ that have functionalized BDC li-
gands compared with those of 1’ suggest stronger interac-
tions between the functionalized frameworks and CO2. For
compound 2’, the primary interaction between CO2 and the
framework is considered to be the electron donor–acceptor
reaction, in which the oxygen atom of the hydroxyl group
serves as the electron-donor center, whereas the C atom of
CO2 as the electron-acceptor center. In addition, the dipolar
or quadrupolar interactions between the hydroxyl group and
CO2 molecule also contribute to the observed trend. Ac-
cordingly, a similar but weaker electron donor–accepter in-
teraction may be expected in the CO2 adsorption by amino
group functionalized compound 3’, due to the lower elec-
tron-donating ability of NH2 compared with the OH group.
Consequently, the enhanced CO2 adsorption can be ach-
ieved by functionalizing MOF structures to increase gas–
framework interactions.
Carbon dioxide adsorption and separation: The CO2 adsorp-
tion isotherms for evacuated compounds 1’, 2’, and 3’ were
collected at 278, 288, and 298 K, and the results are plotted
in Figure 5. In contrast to the adsorbed amount of H2, an
opposite trend is obtained for the CO2 uptake in 1’ and 2’,
which is clearly not coincident with the order of surface
area and pore size. As seen from the isotherm data, com-
pound 2’ has a maximum CO2 uptake of 23.7 and 13.0 wt%
at 278 and 298 K (1 atm), respectively, corresponding to
about 16 and 10 CO2 molecules per unit cell. For 1’, the CO2
uptake decreased to 18.4 and 7.4 wt% at 278 and 298 K
The enhancement of CO2 uptake found in functionalized
MOF structures prompted us to further investigate their
CO2 selectivity over other important gases, including CH4,
CO, N2, and O2. The adsorption isotherms of CO2, CH4, CO,
N2, and O2 on compounds 1’ and 2’ were performed under
the same conditions (298 K, up to 1 atm), and are shown in
Figure 7. Experimental data reveals that both compounds
possess significantly higher affinity to CO2 than to CH4, CO,
N2, and O2. Compared with compound 1’, slight decreases in
Figure 5. CO2 adsorption–desorption isotherms at 298 K for 1’, 2’, and 3’;
adsorption branch: open symbols, desorption branch: filled symbols.
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Ligand Functionalization of Microporous Metal–Organic Frameworks
FULL PAPER
pore and channel may also help the enhancement of selec-
tivity.[17] The selectivity found in compound 2’ is comparable
to those reported for zeolite and carbon adsorbents (e.g.,
zeolite 13X: CO2/CH4, 2–24; CO2/N2, 18), as well as some
other MOFs (e.g., Cu-BTC: CO2/CH4, 6; CO2/N2, 20) under
similar conditions.[18] The relatively high CO2 uptake capaci-
ty and selectivity of 2’ place it in the category of qualified
candidates for further development of CO2 capture and se-
questration.
IR study: To characterize and understand interactions of H2
and CO2 within the pores of 1’, 2’, and 3’, we performed IR
absorption spectroscopy measurements. Figure 8 shows the
Figure 7. Comparison of CO2 (square) and selected gases (CH4, circle;
CO, triangle; N2, rhombus; O2, hexagon) adsorption–desorption iso-
therms of compound 1’ (top) and 2’ (bottom) at 298 K; adsorption
branch: open symbols, desorption branch: filled symbols.
the uptake capacity for other small gases, including CH4,
CO, N2, and O2, are found in compound 2’. These changes
coincide with the reduction of void size and surface area
due to the space occupation by functional groups. Moreover,
in virtue of the significant enhancement of CO2 uptake in
compound 2’, its selectivity of CO2 to CH4, CO, N2, and O2
will therefore be further improved. The single-component
separation ratios of CO2/CH4, CO2/CO, CO2/N2, and CO2/O2
at 298 K and 1 atm are calculated to be 4:1
G
ACHTUNGTRENNUNG
Figure 8. IR absorption spectra of H2 at 55 bar in 2’ (top) and 3’ (bottom)
at room temperature (gas-phase subtracted).
15:1
23:1
(v/v), and 14:1
R
H
U
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
the selectivity will be further increased at higher pressure.
In addition to the stronger electron donor–acceptor interac-
tions between the CO2 molecule and hydroxyl group, anoth-
er two factors should also be taken into account for the high
CO2 selectivity in 2’. First, since the higher molecular polari-
ty can lead to stronger interactions and faster sorption rates,
it is expected that the introduction of polar OH groups will
be in favor of the attraction of CO2, which has the largest
quadrupole moment (13.4 Cm2) and polarizability (2.93)
among all small gases investigated (the quadrupole moments
of CO, N2, and O2 are 8.3, 4.7, and 1.3 Cm2, respectively,
and CH4 is nonpolar).[16] Second, the confinement by the
IR absorption spectra of H2 at 55 bar in 2’ and 3’ referenced
to the MOFs in vacuum, after subtracting the contribution
from high pressure H2 gas, as described earlier.[19] An IR red
shift of about À38 cmÀ1 from the unperturbed ortho-H2 at
4155 cmÀ1 is observed for n˜(H2) in 2’ as shown in the left
panel of Figure 8. This shift is similar to shifts that were ob-
served for n˜ (H2) in 1’.[19–20] A slightly larger red shift of
about À41 cmÀ1 is observed for n˜ (H2) in 3’ as shown in the
right panel of Figure 8. This slightly larger shift in 3’ indi-
cates that the incorporation of amine groups in the pores
leads to a relatively small perturbation of the adsorption po-
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5105

J. Li et al.
tential. Moreover the integrated area of the H2 band for
both 2’ and 3’ is similar. This indicates a similar amount of
adsorbed H2 for both MOFs, which is again consistent with
the low loading regime (<0.012 wt%), shown in Figure 3.
The measured IR shifts are not inconsistent with the mea-
sured Qst values, since previous IR work has clearly shown
that IR shifts do not correlate with binding energies.[19] For
example, the binding energy of H2 in the [Ni3ACHTNUGRTENUNG(COOH)6]
MOF is 8.3 kJmolÀ1 with an IR shift of À30 cmÀ1, whereas
the corresponding results for H2 in 1’ are a binding energy
of about 5 kJmolÀ1 with an IR shift of À38 cmÀ1.
The IR absorption spectroscopy measurements of CO2 ad-
sorption were also carried out on all three compounds. Plot-
ted in Figures 9 and 10 are the IR absorption spectra of CO2
Figure 10. IR absorption spectrum of CO2 trapped in the pores of 2’ at
room temperature.
The similarities in the magnitude of the IR shifts indicate
that the adsorption environment of CO2 is not greatly affect-
ed by the incorporation of hydroxyl or amine in [Zn
ACHTUNGTRENNUNG(BDC)-
AHCTUNGTREN(GUNN TED)0.5]. More specifically, the shapes of the potential are
similar in the three cases. These results support the conclu-
sions of earlier studies with H2 that IR shifts of molecules
incorporated in MOFs do not to have a direct correlation
with binding energies. First principles calculations are neces-
sary to determine whether the variation in isosteric heats of
adsorption shown in Figure 6 are due to variations in bind-
ing energies or variations in kinetic barriers for CO2 remov-
al.
Theoretical calculations: To have a better understanding of
H2-MOF interactions, we performed first-principles calcula-
tions for H2 in 1’ and 2’. As shown in reference [20], there
are two types of H2 adsorption channel in 1’. The first type
is labeled a and is near the [Zn2ACHTUNRTGNEUNG(COO)4] cluster and runs
along the c axis. The other one is near the face center of the
BDC–TED plane, labeled as b in Figure 11. The two chan-
nels have similar binding energies of around 10–11 kJmolÀ1
before zero-point energy corrections and each channel can
accommodate 2 hydrogen molecules so a total of 12 sites
Figure 9. Room temperature IR absorption spectra of CO2 in 3’ (top) and
1’ (bottom).
trapped in the pores of the MOFs upon loading at 4 torr
and subsequent evacuation. A shift of about À13 cmÀ1 in n˜as-
ACHTUNGTRENNUNG(O=C=O) was observed for amine modified 3’, as shown in
Figure 9 (left), which is similar to that observed for n˜as-
ACHTUNGTRENNUNG(O=C=O) in unmodified 1’ (Figure 9, right). A slightly
smaller shift (ꢀÀ11 cmÀ1) was found for n˜as
ACHTUNGTRENUN(NG O=C=O) in 2’
as shown in Figure 10. The red-shifted absorption for n˜as-
ACHTUNGTRENNUNG
(CO2) from the unperturbed gas-phase value (2349 cmÀ1)
further supports the finding that CO2 acts as an electron ac-
ceptor[21] and that the interaction is involving the carbon of
the CO2.
Figure 11. Two types of H2 adsorption site in 1’ (light-grey dumbbells)
and 2’ (dark-grey dumbbells). The H2 bond at the b site is perpendicular
to the plane and the hydrogen atom in the back is not shown.
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Ligand Functionalization of Microporous Metal–Organic Frameworks
FULL PAPER
per primitive cell (each unit cell contains four primitive cells
in the simplified structure) are predicted. These calculated
results are consistent with the experimental H2 isotherms at
77 and 87 K. Each of the two isotherms can be fitted to a
single Langmuir isotherm and the number of adsorption
sites extracted from the fit is around 13. The calculated fre-
quency shift is also consistent with IR measurements con-
firming the location of the adsorption sites.
When 1’ is functionalized by replacing one hydrogen atom
in the benzene ring with a OH group, the binding energies
of the sites near [Zn2ACTHNUTRGNEUNG(COO)4] change only slightly. The face
center sites, however, are significantly affected and the bind-
ing energy decreases from about 11 to around 8 kJmolÀ1, as
shown in Table 2. The reduction can be attributed to the re-
Conclusion
We have carried out a systematic study to investigate and
understand the effect of (pre-synthesis) ligand functionaliza-
tion in the gas adsorption and separation processes on three
isostructural microporous metal–organic framework com-
pounds. Hydroxyl and amino groups were used for such
functionalization. The amount of H2 uptake correlates to
both pore properties of the MOF structures and the extent
of sorbate–sorbent interactions. The introduction of hydrox-
yl and amino groups in compounds 2 and 3 leads to a signifi-
cant decrease in the pore size and surface area of the frame-
work structure, and consequently lower H2 adsorption ca-
pacity than that of compound 1. In contrast to the H2 ad-
sorption, our study demonstrates that the CO2 adsorption is
primarily affected by the chemical properties of the frame-
work. Upon functionalization of the frameworks by hydrox-
yl and amino groups, the modified compounds 2 and 3 show
greatly enhanced CO2 uptake than that of 1 at room temper-
ature, in the order of 2>3>1. Such a significant enhance-
ment can be attributed primarily to the strong electron
donor–acceptor interaction between the framework and
CO2 molecule, in which the O and N atoms containing lone
pairs of electrons from hydroxyl and amino groups serve as
the electron donor centers. Due to the high CO2 affinity
through functionalization, considerable improvement of
CO2 selectivity to CH4, CO, N2, and O2 is also observed in
both 2 and 3. This study helps to better understand and ex-
plain the functionalization effect of MOFs, and will provide
useful information toward future design and synthesis of
new MOFs with improved gas adsorption and separation ca-
pacity through functionalization.
Table 2. Calculated H2 stretch frequency shift and binding energy in 1’
and 2’.[a]
Site a
Binding
Site b
Binding
Dn˜
[cmÀ1
Dn˜
[cmÀ1
]
energy
]
energy
[kJmolÀ1
]
[kJmolÀ1
]
[Zn
[Zn
N
R
À32
À29
10.4
10.6
À25
À14
10.9
8.0
ACHTUNGTRENNUNG
[a] The zero-point energies, which are not included, are typically
ꢀ2.5 kJmolÀ1 for H2 in MOFs,[22] but have not been explicitly calculated
in these cases. The sites are illustrated in Figure 11.
pulsion from the OH group since the b site moves upwards
and away from OH in 2’ compared with that in 1’, as can be
seen in Figure 11. The equilibrium position for site a also
changes slightly. However, the binding energies remain
almost constant, which is consistent with the fact that the
measured Qst values are comparable for 1’ and 2’. The
change in binding energies also shows that the potential well
along the moving direction is very flat for the a site, but is
very deep for the b site. Now the number of adsorption sites
Furthermore, the IR data show that the adsorption envi-
ronment for both H2 and CO2 changes little upon function-
alization of the pores. For H2 the incorporation of NH2 and
OH groups modifies the H2 potential very little (there is a
slightly larger shift with NH2). For CO2, the situation is simi-
lar with a very slightly smaller shift with OH. The IR shifts
for H2 in 2’ are consistent with the theoretical calculations
of the IR shifts for site a, which is least affected by the func-
tionalization. These results also confirm earlier findings that
IR shifts do not correlate with binding energies, as evi-
denced by the measured isosteric heats of adsorption.
in 2’ at low pressure arises mainly from the [Zn2ACTHNUTRGNEN(UG COO)4]
sites and a total of 8 is predicted. These are consistent with
the surface area and pore volume reduction discussed
above. Furthermore, the fitting of the isotherms of 2’ with
Langmuir isotherms gives 9.4 adsorption sites per cell for
77 K and 10.5 for 87 K, whereas the fit of the isotherm of 1’
implies around 13. Both the total number of sites and the
decrease are consistent with the calculations.
The results from vdW–DF calculations for H2 adsorption
in 1’ and 2’ indicate that both the binding energy and the vi-
The change in frequency shift upon OH functionalization
is similar to that in binding energy, as shown in Table 2. The
shift at site b is much smaller for 2’. Unfortunately, this site
has a high symmetry so that the H2 stretch vibration is
almost inactive to IR compared with the corner sites a.[19]
For site a in Figure 11, the frequency shift is comparable for
1’ and 2’, consistent with the small change in binding ener-
gies and also with IR measurements. We should point out
brational frequency for the [Zn2ACTHNGUTERNNU(G COO)4] sites are only
slightly changed by the OH functionalization. The BDC-
TED face center sites, however, are significantly affected
and the binding energy decreased by about 3 kJmolÀ1 reduc-
ing the predicted adsorption sites from around 12 to 8 at
low pressures. These results are consistent with the mea-
sured H2 uptake, Qst, and IR frequency as well.
here that the four corners around [Zn2ACTHNUTRGNE(UNG COO)4] are not com-
pletely equivalent due to the 3-fold symmetry in the TED
unit. The range of the variation in binding energy and fre-
quency shift are about 1 kJmolÀ1 and 5 cmÀ1, respectively.[19]
Chem. Eur. J. 2011, 17, 5101 – 5109
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5107

J. Li et al.
sample between experiments for about 0.5–1 h. Pore properties (e.g.,
pore volume, pore size, and surface area) were analyzed using Autosorb
v1.50 software. The N2 adsorption–desorption isotherms for 1’–3’ are plot-
ted in Figure S5. The hydrogen adsorption–desorption isotherms were
collected in the pressure range from 10À4 to 1 atm at 87 and 77 K, respec-
tively. Results for 1’–3’ are plotted in Figure S6. Temperature-dependent
adsorption–desorption isotherms of CO2 (278, 288, and 298 K) on 1’–3’
were collected from 10À3 to 1 atm and are depicted in Figure S7. Plots in
Figures S8 and S9 are isotherms of CO2 in 1’ and 2’ compared with those
of other selected small gases (298 K and up to 1 atm).
Experimental Section
Materials: All chemicals were purchased from commercial sources (Alfa
Aesar, Acros, Aldrich, or TCI America) and used as received. The 2-hy-
droxyterephthalic acid was prepared according to the literature.[23]
2-Hydroxyterephthalic acid: 2-Bromoterephthalic acid (2.45 g, 10 mmol)
and NaOH (1.20 g, 30 mmol) were mixed in water (40 mL). NaOAc
(2.05 g, 25 mmol) and Cu powder (0.02 g) were then added while stirring.
The solution was heated to reflux for 3 d. Aqueous NaOH (1m) solution
was added occasionally during the reaction to keep the solution alkaline
and a few drops of phenolphthalein were used here as an indicator. After
cooling to room temperature, the solution was filtered and the filtrate
was acidified to pH 1 using HCl (1m). The light gray product was collect-
ed by filtration and used without further purification (1.80 g, 99% yield).
IR experiment: Infrared absorption spectroscopy measurements were
performed using a Thermo 6700 FTIR spectrometer equipped with a
high pressure cell and a liquid-N2-cooled InSb detector. The MOF pow-
ders were gently pressed onto a KBr support and measurements were
performed in transmission. Hydrogen uptake measurements were carried
out at room temperature at a pressure of 55 bar. CO2 measurements
were performed by sequential loading to a pressure of 4 torr into the acti-
vated MOFs and subsequent evacuation to remove the gas phase CO2,
thereby providing information on the CO2 molecules that are most
strongly bound within the pores and their stability.
Compound 1: [Zn
lowing the same procedure reported previously.[13a]
Compound 2: [Zn(BDC-OH)(TED)0.5]·1.5DMF·0.3H2O (2) was prepared
by solvothermal reaction of [Zn(NO3)]·6H2O (0.089 g, 0.3 mmol),
ACHTUNGTRENNUNG(BDC)ACHUTTGNRNEN(UGN TED)0.5]·2DMF·0.2H2O (1) was synthesized fol-
G
ACHTUNGTRENNUNG
a
ACHTUNGTRENNUNG
BDC-OH (0.055 g, 0.3 mmol), and TED (0.022 g, 0.2 mmol) in DMF
(10 mL). The mixture was sealed in a Parr reaction vessel and heated at
373 K for 2 d. After naturally cooling to room temperature, the colorless
rod-like single crystals of 2 (0.080 g, 64% yield based on metal) were iso-
lated by filtration and washed with DMF.
Theoretical calculations: Calculations were performed within the plane-
wave implementation of the density functional theory in the ABINIT
package,[24] which we have adapted to incorporate the van der Waals in-
teraction.[25] The basic structure was taken from single-crystal data. For
1’, we used the simplified structure eliminating the disorder of C in TED
and O in BDC.[13c] For 2’, we started from 1’ and replaced one hydrogen
atom in the benzene by a OH group which was relaxed for optimal orien-
tations while keeping all the other MOF atoms fixed. The H2 adsorption
sites in 1’ were established in a previous paper.[20] We used these sites as
our initial estimate for H2 binding positions in 2’ and then relaxed it for
final positions. After these equilibrium positions and orientations were
obtained, we preformed a series of total energy calculations with differ-
ent H2 bond lengths while keeping the center of H2 and the MOF atoms
fixed. The resulting total energies were used in the Schrodinger equation
to obtain the vibrational frequencies. An energy cutoff of 50 Ry and 2ꢁ
2ꢁ2 Monkhorst–Pack grids were used throughout the calculations.
Compound 3: [Zn
ACHTUNGTRENNUNG(BDC-NH2)ACHTUGNTRNE(NUNG TED)0.5]·xDMF·yH2O (3) was synthesized
from [Zn(NO3)]·6H2O, BDC-NH2, and TED by using the same procedure
described above for 2. The reaction yielded 0.075 g of 3 as grey needle
ACHTUNGTRENNUNG
crystals.
Crystal structure analysis: Single-crystal X-ray diffraction analysis was
performed at 100 K on a Bruker-AXS smart APEX CCD system with
graphite-monochromatized Mo Ka radiation (l=0.71073 ꢂ). For com-
pound 2, a total number of 22338 reflections were collected (2764
unique, RACHTUNGTRENNUNG(int)=0.0228) between a q angle of 1.72 to 28.288. The structure
was solved by direct methods and refined by full-matrix least-squares on
F2 using the Bruker SHELXTL package. Crystal data for 2:
C
15.50H21.12ZnN2.50O6.81; FW=416.84 gmolÀ1; tetragonal; I4m (No. 87); a=
15.0208(8), b=15.0208(8), c=19.217(2) ꢂ; V=4335.9 ꢂ3; Z=8; 1calcd
=
1.277 gcmÀ3; R1=0.0380 (2764 data with I>2s(I)); wR2=0.1001 (all
data); GoF=1.001 (all data). CCDC-792747 contains the supplementary
crystallographic data for 2. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
Acknowledgements
The authors would like to thank the Department of Energy (DOE) for
the financial support through Grant No. DE-FG02–08ER46491. J.L. is a
Cheung Kong Scholar associated with SCUT.
PXRD analysis: Powder X-ray diffraction experiments were conducted
using a D/M-2200T automated system (Ultima+, Rigaku) with Cu Ka ra-
diation (l=1.5406 ꢂ). The PXRD patterns were collected between 2q
angles of 3 to 508 at a scan rate of 5 degminÀ1. A graphite monochroma-
tor was used and the generator power settings were at 40 kV and 40 mA.
The PXRD patterns of the simulated, as-made, and outgassing samples
of 1–3 are shown in Figures S1–S3, respectively.
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Chem. Eur. J. 2011, 17, 5101 – 5109

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Received: September 30, 2010
Revised: January 7, 2011
Published online: March 23, 2011
Chem. Eur. J. 2011, 17, 5101 – 5109
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5109