Evaluation of structural transformation in 2D metal-organic frameworks based on a 4,4′-sulfonyldibenzoate linker: Microwave-assisted solvothermal synthesis, characterization and applications

Article abstract of DOI:10.1039/c4ce01201a

Six 2D metal-organic frameworks containing V-shaped 4,4′-sulfonyldibenzoate (SBA) linkers, [Zn₃(SBA)₂(OH)₂]·EtOH (1 or CYCU-5·EtOH), [M₃(OH)₂(SBA)₂(EtOH)(H₂O)₃]·3.5H₂O (2-4, M = Mg, Ni, and Co), [Mn(SBA)(EtOH)] (5) and [Mn(SBA)(H₂O)] (6), have been synthesized rapidly under microwave-assisted solvothermal conditions. The structural analysis revealed that all compounds display two-dimensional structures containing inorganic motifs with one-dimensional chains (1-5) or dimers of trigonal prisms (6) connected through SBA linkers and forming 2D neutral and porous channels. Compounds 1-4 were further characterized by elemental analysis, thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD) measurements and IR spectroscopy. The desolvated phases of compounds 1-2/3-4 have thermal stabilities up to about 400/350 °C. The desolvated frameworks of 1-4 above 200 °C have similar CYCU-5-like conformations and exhibit gas adsorption properties with N₂, CO₂ and H₂ gases. Interestingly, CYCU-5 displays selective adsorption of CO₂ over N₂ and CH₄ gases at 273 K. In addition, the catalytic ability of CYCU-5 as an initiator for ring-opening polymerization of l-lactide has also been demonstrated. This journal is

Full text of DOI:10.1039/c4ce01201a

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Evaluation of structural transformation in 2D
metal–organic frameworks based on a 4,4′-
sulfonyldibenzoate linker: microwave-assisted
solvothermal synthesis, characterization and
applications†
Cite this: CrystEngComm, 2014, 16,
9308
a
b
c
Cheng-You Wu, Duraisamy Senthil Raja, Chun-Chuen Yang, Chun-Ting Yeh,a
*
a
a
d
a
*
*
Yu-Ru Chen, Chen-Yu Li, Bao-Tsan Ko and Chia-Her Lin
Six 2D metal–organic frameworks containing V-shaped 4,4′-sulfonyldibenzoate (SBA) linkers,
[Zn3(SBA)2(OH)2]·EtOH (1 or CYCU-5·EtOH), [M3(OH)2(SBA)2(EtOH)(H2O)3]·3.5H2O (2–4, M = Mg, Ni, and
Co), [Mn(SBA)(EtOH)] (5) and [Mn(SBA)(H2O)] (6), have been synthesized rapidly under microwave-assisted
solvothermal conditions. The structural analysis revealed that all compounds display two-dimensional
structures containing inorganic motifs with one-dimensional chains (1–5) or dimers of trigonal prisms (6)
connected through SBA linkers and forming 2D neutral and porous channels. Compounds 1–4 were further
characterized by elemental analysis, thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD)
measurements and IR spectroscopy. The desolvated phases of compounds 1–2/3–4 have thermal stabilities
up to about 400/350 °C. The desolvated frameworks of 1–4 above 200 °C have similar CYCU-5-like
conformations and exhibit gas adsorption properties with N2, CO2 and H2 gases. Interestingly, CYCU-5
displays selective adsorption of CO2 over N2 and CH4 gases at 273 K. In addition, the catalytic ability of
CYCU-5 as an initiator for ring-opening polymerization of L-lactide has also been demonstrated.
Received 11th June 2014,
Accepted 18th August 2014
DOI: 10.1039/c4ce01201a
www.rsc.org/crystengcomm
electrochemistry, mechanochemistry, and ultrasonic methods
have been employed.3 Particular attention has been paid to
Introduction
Metal–organic frameworks (MOFs) have been intensively studied
in recent years because of their fascinating structural diversities
and topologies as platforms for various functional applica-
tions.1,2 They are built up of metal ions interconnected by
organic linkers, thus mostly forming two- or three-dimensional
porous frameworks. In order to achieve a diverse range of
structures, various routes such as room temperature syn-
thesis, conventional electronic heating, microwave heating,
microwave-assisted solvothermal reactions, mainly because of
their greener approach to synthesize materials in a shorter
reaction time and with less power consumption than that of
conventional solvothermal synthetic methods.4
It is obvious that the design of MOF materials depends on
careful choice of the building blocks such as metal ions,
organic linkers, co-ligands, and adequate solvents, and their
proper organization in the solid state.5 Among them, the
selection of organic linkers is one of the significant aspects.6
This is because the configurations, rigidities, substituents
and coordination modes of organic ligands have played vital
roles in determining the structural properties of the MOFs.
In this context, V-shaped dicarboxylates as linkers for the
construction of MOFs have been in the limelight very
recently. In particular, 4,4′-sulfonyldibenzoic acid (H2SBA) is
a typical example of a V-shaped dicarboxylate linker for the
construction of novel MOFs as it has six potential donor
atoms that allow the formation of variable structural topolo-
gies.7 In this regard, we have recently found and published
the fact that V-shaped dicarboxylate ligands such as 4,4′-
sulfonyldiebenzoate (SBA) and 4,4′-oxybisbenzoate can be
used to construct a variety of MOFs with robust structures.8
a Department of Chemistry, Chung Yuan Christian University, Chungli,
Taoyuan 320, Taiwan. E-mail: chiaher@cycu.edu.tw; Fax: +886 3 2653399;
Tel: +886 3 2653315
b Department of Chemistry, National Tsing Hua University, Hsinchu 300, Taiwan
c Department of Physics, Chung Yuan Christian University, Chungli, Taoyuan 320,
Taiwan. E-mail: chunchuenyang@cycu.edu.tw
d Department of Chemistry, National Chung Hsing University, Taichung, 402,
Taiwan. E-mail: btko@dragon.nchu.edu.tw
† Electronic supplementary information (ESI) available: the ESI file contains
TGA curves and data of 1–4, PXRD patterns at varied temperatures for 1–4,
Rietveld profile fits, the high pressure gas adsorption isotherms and tables
showing selected bond lengths and H-bond lengths. CCDC 943771, 943772,
805748, 998723 and 998724. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4ce01201a
9308 | CrystEngComm, 2014, 16, 9308–9319
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On the other hand, among the MOFs the 3D frameworks
with open channels have largely been known in the recent
past, but relatively few examples of 2D open frameworks are
found in the literature.9 Further, the 2D MOFs have been less
well known for their gas sorption properties, mainly due to
the small interlayer spacings between the 2D layers, which
may not be wide enough for the incorporation of gas mole-
cules.10 Also, the selective adsorption of CO2 using MOFs11
has attracted attention in recent years because CO2 is
believed to be the major greenhouse gas that causes global
warming.12
Synthesis of MOFs
Microwave synthesis of [Zn3(OH)2(SBA)2]·EtOH (1). The
compound 1 was obtained from a reaction mixture of H2SBA
(0.1224 g, 0.4 mmol), Zn(NO3)2·6H2O (0.2360 g, 0.8 mmol),
EtOH (9.0 mL) and H2O (1.0 mL). The mixture was heated at
180 °C for 40 min. The colorless crystals of 1 were filtered
off, washed with EtOH, dried at room temperature, and
collected with the yield of 0.0782 g (88.41%, based on the
H2SBA reagent). Anal. found/calcd.: C, 40.6/40.9; H, 2.5/2.7%
for 1. IR (KBr, cm−1): 3508(m), 3045(w), 2969(w), 1957(w),
1609(s), 1564(m), 1395(s), 1297(m), 1163(m), 1011(w), 838(w),
784(w), 731(s), 619(m), 432(w).
With this above background in mind, and as
a
continuation of our research on H2SBA, herein we report
six 2D MOFs, [Zn3(SBA)2(OH)2]·EtOH (1 or CYCU-5·EtOH),
[M3(OH)2(SBA)2(EtOH)(H2O)3]·3.5H2O (2–4, M = Mg, Ni, and
Co), [Mn(SBA)(EtOH)] (5) and [Mn(SBA)(H2O)] (6), which are
synthesized under microwave-assisted solvothermal condi-
tions. It is to be noted that the structural description of
compound 4 has already been reported,13 but there is no
report on its porous properties. Hence, we report its gas sorp-
tion properties along with the other three new compounds
(1, 2 and 3). Further, we report only the single crystal X-ray
diffraction structural descriptions for compounds 5 and 6
because the yield of both was too low (only very few single
crystals have been obtained) for other studies.
Microwave synthesis of [Mg3(OH)2(SBA)2(EtOH)(H2O)3]·3.5H2O
(2). A mixture of H2SBA (0.1224 g, 0.4 mmol), Mg(NO3)2·6H2O
(0.4100 g, 1.6 mmol), EtOH (5.0 mL) and H2O (1.0 mL) was
heated at 150 °C for 20 min. The colorless crystals of 2 were
filtered off, washed with EtOH, dried at room temperature,
and collected with the yield of 0.0720 g (41.21%, based on the
H2SBA reagent). Anal. found/calcd.: C, 41.77/41.00; H, 3.99/4.24%
for 2. IR (KBr, cm−1): 3531(w), 3268(w), 2978(w), 1620(s),
1564(m), 1413(s), 1291(m), 1165(m), 1102(m), 789(w), 746(s),
698(w), 621(w), 491(w), 442(w).
Microwave synthesis of [Ni3(OH)2(SBA)2(EtOH)(H2O)3]·3.5H2O
(3). The compound 3 was obtained from a reaction mixture of
H2SBA (0.0612 g, 0.2 mmol), NiCl2·6H2O (0.0475 g, 0.2 mmol),
DMF (3.0 mL), (EtOH, 7.0 mL), and H2O (1.0 mL). The mixture
was heated at 180 °C for 20 min. The green crystals of 3 were
filtered off, washed with EtOH, dried at room temperature,
and collected with the yield of 0.0425 g (43.41%, based on
the H2SBA reagent). Anal. found/calcd.: C, 36.92/36.84; H,
3.48/3.53% for 3. IR (KBr, cm−1): 3525(m), 3226(b), 1680(s),
1608(s), 1569(s), 1482(s), 1285(s), 1156(s), 1104(s), 1011(s),
743(s), 684(s), 628(s), 490(s), 422(s).
Microwave synthesis of [Co3(OH)2(SBA)2(EtOH)(H2O)3]·3.5H2O
(4). The compound 4 was obtained from a reaction mixture of
H2SBA (0.0612 g, 0.2 mmol), CoCl2·6H2O (0.0475 g, 0.2 mmol),
DMF (3.0 mL), EtOH (7.0 mL), and H2O (1.0 mL). The mixture
was heated at 150 °C for 20 min. The pink crystals of 4 were
filtered off, washed with EtOH, dried at room temperature, and
collected with the yield of 0.0503 g (51.23%, based on the H2SBA
reagent). Anal. found/calcd.: C, 36.79/36.67; H, 3.82/3.80% for 4.
IR (KBr, cm−1): 3529(m), 3224(b), 1682(s), 1601(s), 1572(s),
1480(s), 1159(s), 1101(s), 741(s), 624(s), 493(s), 425(s).
Microwave synthesis of [Mn(SBA)(EtOH)] (5). The
compound 5 was obtained from a reaction mixture of H2SBA
(0.1224 g, 0.4 mmol), Mn(NO3)2·4H2O (0.2000 g, 0.8 mmol),
EtOH (5.0 mL) and H2O (1.0 mL). The mixture was heated at
180 °C for 20 min. Only very few light-pink crystals of 5 were
collected.
Experimental section
Materials and general methods
Chemicals of reagent grade or better were used as received.
Microwave-assisted reactions were done in a 100 mL Teflon
autoclave placed in a microwave oven (START D, Milestone,
maximum power of 1200 W, 2.45 GHz) in which the precur-
sor mixtures were heated for several minutes. All products’
phase purity was examined by powder X-ray diffraction
(PXRD) before further characterization. Elemental analyses
were carried out to confirm the composition of the organic
part of the prepared samples. Thermogravimetric analyses
(TGA), using a DuPont TA Q50 analyzer, were performed
on powder samples under flowing N2 with a heating rate of
10 °C min−1. FT-IR spectra were recorded in the range of
400–4000 cm−1 on a JASCO FT-IR-460 spectrophotometer
using KBr pellets. The gas sorption isotherms have been mea-
sured at 77 K for N2 and H2, and for CO2 at 273 and 298 K by
using a Micromeritics ASAP 2020 adsorption apparatus.
About 100 mg of adsorbent was used for the gas sorption
studies. The high pressure gas sorption studies were mea-
sured on a Quantachrome iSorpHP adsorption system with
about 500 mg of each sample. The initial gas removal process
was performed under vacuum at 423 K for 12 h. The free
space of the system was determined by using He gas.
Ultrahigh purity CO2, N2, CH4, H2 and He gases were used as
received in all the gas sorption experiments. Ring-opening
polymerization (ROP) of L-lactide (L-LA) catalyzed by CYCU-5
was carried out by following our previous study.14
Microwave synthesis of [Mn(SBA)(H2O)] (6). The
compound 6 was obtained from a reaction mixture of H2SBA
(0.2448 g, 0.8 mmol), Mn(NO3)2·4H2O (0.2000 g, 0.8 mmol),
EtOH (7.0 mL), and H2O (3.0 mL). The mixture was heated at
180 °C for 20 min. Only very few colorless crystals of 6 were
collected.
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Preparation of desolvated phases, [M3(OH)2(SBA)2] (CYCU-5
Radiation Research Center (NSRRC) in Taiwan. The ring
energy of NSRRC was operated at 12 keV with a typical cur-
rent of 300 mA. The wavelength of the incident X-rays was
1.0332 Å, delivered from the superconducting wavelength-
shifting magnet, and a Si(111) double-crystal monochroma-
tor. The diffraction patterns were recorded with a Mar345
imaging plate detector approximately 280 mm from sample
positions and typical exposure duration of 5 min. The pixel
size of Mar345 was 100 μm. The one-dimensional powder dif-
fraction profile was converted with the program FIT2D and
cake-type integration. The diffraction angles were calibrated
according to Bragg positions of Ag-benhenate and Si powder
(NBS640b) standards. The powder sample was sealed in a
capillary (0.7 mm diameter) and heated in a stream of hot
air. The Rietveld refinements of all powder samples were car-
ried out with the GSAS program.17 Crystallographic details of
the compounds determined from synchrotron PXRD data are
given in Tables 2 & 3 and S4 (ESI†). Selected bond lengths
are listed in Tables S5 and S6 (ESI†). The Rietveld plot corre-
sponds to the crystal structure model and profile factors are
displayed in Fig. S7 and S8 (ESI†). Crystallographic data (CIF)
for the powder samples structural refinements are published
as ESI.†
and 2a–4a). The heating of as-synthesized compounds 1–4
at 200 °C for 10 h resulted in the desolvated phases (CYCU-5
and 2a–4a).
Single-crystal structure analysis
The diffraction measurements were performed on a Bruker AXS
SMART APEX II diffractometer (Mo-Kα radiation, graphite
monochromator, λ = 0.71073 Å). All data were corrected for
Lorentz and polarization effects, and the program SADABS in
APEX2 (ref. 15) was used for the absorption correction. On the
basis of systematic absences and statistics of intensity distribu-
tion, all the single crystal structures were solved with metal
atoms being located first, followed by the atoms O, N, and C
located on successive difference Fourier maps. The H atoms
were added in idealized positions and constrained to ride on
their parent atoms. All calculations were performed by using
SHELXTL programs in APEX2.16 While the framework structure
of 1 was refined easily, the solvent molecules in 1 could not be
refined because of their highly disordered structure. Therefore,
the SQUEEZE function of the PLATON program was used to
eliminate the contribution of the electron density in the solvent
region from the intensity data. Crystallographic data are given
in Table 1. Selected bond lengths are listed in Table S1 (ESI†).
Crystallographic data (CIF) for the single crystal structures
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre (CCDC) as supplementary publica-
tion numbers CCDC 943771, 943772, 805748, 998723, and
998724 for the compounds, 1, CYCU-5, 2, 5, and 6, respectively.
Results and discussion
Structural description of [Zn3(OH)2(SBA)2]·EtOH
(1 or CYCU-5·EtOH)
Single crystal X-ray diffraction structure analysis shows that
the structure of the compound 1 possesses an extended 2D
framework. The asymmetric unit consists of two crystallo-
graphically independent Zn centers and two SBA ligand units
(Fig. 1a). The Zn(1) center is six-coordinated and bonded to
four oxygen atoms of the carboxylate groups belonging to
Structural matching and Powder X-Ray Diffraction (PXRD) analysis
The powder X-ray diffraction (PXRD) patterns were also
recorded at the BL01C2 beamline of the National Synchrotron
Table 1 Single crystal crystallographic data
1
CYCU-5
2
5
6
Formula
Fw
Space group
a (Å)
b (Å)
c (Å)
C15H12Zn1.5O7.5
442.36
P21/n
14.0847(4)
5.9953(2)
19.3347(6)
90
93.729(2)
90
1629.20(9)
4
S
C14H9Zn1.5O7S
419.33
P21/n
14.1168(2)
6.0531(10)
19.3678(3)
90
93.627(10)
90
1651.67(4)
4
C30H37Mg3O21.5S2
878.65
P2/c
28.1637(9)
6.441(2)
24.8679(8)
90
115.445(2)
90
4073.6(2)
4
C16H13MnO7S
404.26
P21/n
10.1610(5)
7.3197(3)
22.3295(10)
90
93.6990(10)
90
1657.31(13)
4
C14H10MnO7S
377.22
P1
¯
5.998(2)
11.8959(4)
13.0287(7)
112.118(3)
102.502(3)
99.816(2)
807.49(6)
2
α (°)
β (°)
γ (°)
V (Å3)
Z
Dcalc (g cm−3
)
1.803
2.393
1.686
2.353
1.433
0.28
1.620
0.959
1.551
0.978
μ (mm−1
)
Reflections collected
Independent reflections
R(int)
26 234
4029
0.0337
1.147
0.0459
0.1146
0.0508
0.1167
943771
15 157
4098
0.0300
0.979
0.0301
0.0709
0.0470
0.0785
943772
45 162
10 210
0.0806
1.037
0.0541
0.1468
0.1118
0.1672
805748
14 612
4090
0.0381
1.165
0.0572
0.1548
0.0630
0.1573
998723
13 815
3960
0.0344
1.082
0.0492
0.1241
0.0601
0.1282
998724
Goodness-of-fit on F2
R1 [I > 2σ(I)]
wR2 [I > 2σ(I)]
R1 [all data]
wR2 (all data)
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Table 2 Crystallographic details of 3 and 4 at 303 K, as determined from
synchrotron PXRD data
of atoms) without including the water and EtOH molecules.
The solvent accessible volume (SAV) calculated without
including ethanol and water molecules by PLATON19 analysis
is 301.7 Å3 which is 18.5% of the unit cell volume.
3 (83%)
3a (17%)
4
Chemical formula C30H29Ni3O18S2 C28H18Ni3O14S2 C30H29Co3O18S2
Formula weight
Crystal system
Temperature (K) 303
Space group
a (Å)
b (Å)
914.89
Monoclinic
815.8
Monoclinic
303
917.88
Monoclinic
303
Structural description of [M3(OH)2(SBA)2(EtOH)(H2O)3]·3.5H2O
(2–4, M = Mg, Ni, Co)
P2/c
P21/n
P2/c
27.051(7)
6.2522(14)
24.945(8)
115.191(12)
3817.66(183)
4
1.0332
6.332
5.31
12.738(6)
6.240(6)
21.545(8)
89.61(5)
1712.47(219)
2
1.0332
6.332
5.31
28.1888(19)
6.3545(3)
24.9584(15)
115.737(4)
4027.2(4)
4
1.0332
2.346
4.64
Single crystal X-ray structure analysis of compound 2 and PXRD
fitting of compounds 3 and 4 displayed that compounds 2
to 4 have similar conformations to [Co3(OH)2(SBA)2(EtOH)
(H2O)3]·3.5H2O,13 which was synthesized by conventional
heating. For 2, all of the Mg–O bond distances range from
2.049(2) to 2.124(2) Å (Fig. 2a, Table S1, ESI†). The layer struc-
ture of 2 (Fig. 2b) consists of [Mg3(μ3-OH)2]n chains linked by
SBA ligands. In the chain, the Mg atoms, Mg(1), Mg(2) and
Mg(3) are first linked by one μ3-OH. Along the chain direc-
tion (b-axis) these three magnesium centers are further
connected by another μ3-OH. Within the 1D chains, the
Mg(2)O6 and Mg(3)O6 octahedra are edge-sharing, and they
are apex-shared with the Mg(1)O6 octahedra (Fig. 2c). The
layers are further connected by hydrogen bonds between the
coordinated water molecules and oxygen atoms on the sul-
fone group (Table S2, ESI†). Each layer contains rhomboidal
channels with a free diameter of 5.2 × 4.7 Å2 (considering the
van der Waals radii of atoms) without including the water
and EtOH molecules. The SAV calculated without including
ethanol and water molecules by PLATON19 analysis is 1319.4 Å3,
which is 32.4% of the unit cell volume. The comparison of
this hypothetical calculation with the results of CYCU-5
clearly indicates that if the solvent molecules of 2 are
removed without structural transformation or distortion, this
leads to a larger free pore volume.
c (Å)
β (°)
V (Å3)
Z
Radiation (Å)
χ2
Rp (%)
Rwp (%)
7.53
7.53
7.08
four SBA units, and the remaining two octahedral coordina-
tion sites, which are ‘trans’ to each other, have been occu-
pied by two oxygen atoms (O3) of the two hydroxide ions. On
the other hand, the tetrahedrally four-coordinated Zn(2) atom
is coordinated with two oxygen atoms of the carboxylate
groups belonging to two SBA ligand units, and two oxygen
atoms (O3) of the two hydroxide ions. The hydroxide ions act
as a bridge to connect the Zn(1) and Zn(2) centers. The SBA
ligand is tetradentate, bridging to four Zn centers through
each of its four carboxylate oxygen atoms (Fig. 1b). All of the
Zn–O bond distances range from 1.909(3) to 2.170(3) Å
(Table S1, ESI†). As shown in Fig. 1c, the inorganic motif
displays 1D chains formed by the edge-sharing Zn2O10 and
corner-sharing ZnO6. The one-dimensional inorganic build-
ing unit in compound 1 has been reported previously for a
zinc terephthalate open framework.18 These chains are linked
together by the μ4-links of SBA ligands in two directions to
generate the 2D metal–organic framework of 1 (Fig. 1d). The
layers interact further by hydrogen bonds (Table S2, ESI†).
The framework contains rhomboidal channels with a free
diameter of 5.1 × 4.7 Å2 (considering the van der Waals radii
Structural description of [Mn(SBA)(EtOH)] (5)
Single crystal structure analysis shows that the structure of
the compound 5 also possesses an extended 2D framework.
The asymmetric unit consists of one crystallographically
independent Mn center, one SBA ligand, and one EtOH
Table 3 Crystallographic details of CYCU-5 and 2a–4a at 473 K, as determined from synchrotron PXRD data
CYCU-5
2a
3a
4a
Chemical formula
Formula weight
Crystal system
Temperature (K)
Space group
a (Å)
C28H18Zn3O14S2
833.8
Monoclinic
473
C28H18Mg3O14S2
714.0
Monoclinic
473
C28H18Ni3O14S2
815.8
Monoclinic
473
C28H18Co3O14S2
818.8
Monoclinic
473
P21/n
P21/n
P21/n
P21/n
13.9323(6)
6.0385(2)
19.3839(8)
93.2021(22)
1628.3(2)
2
14.307(21)
6.082(5)
18.816(17)
89.325(11)
1637.16(313)
2
13.9069(32)
6.1607(10)
19.288(7)
90.30(4)
1652.5(8)
2
13.881(14)
6.1930(26)
20.006(13)
90.0(1)
1719.82(219)
2
b (Å)
c (Å)
β (°)
V (Å3)
Z
Radiation (Å)
1.0332
2.653
1.0332
3.425
1.0332
6.479
1.0332
4.447
χ2
Rp (%)
2.49
6.74
5.55
6.818
Rwp (%)
4.54
9.90
9.03
9.90
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Fig. 2 (a) The coordination spheres of magnesium atoms in 2. (b) The
2D structure view of 2 with the channels along the b-axis. (c) The
edge-sharing and corner-sharing 1-D inorganic chain in 2.
5.9 × 5.3 Å2 (considering the van der Waals radii of atoms)
without including the water and EtOH molecules. The SAV
calculated without including ethanol molecules by PLATON19
analysis is 374.2 Å3, which is 22.6% of the unit cell volume.
This value is comparable to that of CYCU-5.
Fig. 1 (a) The coordination spheres of zinc atoms in 1. (b) The
coordination environment of the SBA ligand. (c) The edge-sharing and
corner-sharing 1D inorganic chain in 1. (d) The 2D structure view of 1
with the channels along the b-axis.
Structural description of [Mn(SBA)(H2O)] (6)
Single crystal X-ray diffraction shows that the structure of the
compound 6 also possesses an extended 2D framework. The
asymmetric unit consists of one independent Mn center, one
SBA ligand, and one coordinated water molecule (Fig. 4a).
The Mn(II) ion is six-coordinated and binds with six oxygen
atoms from the carboxylate groups belonging to five H2SBA
ligands, and one oxygen atom from the coordinated water
molecule. The H2SBA ligand overall is pentadentate, bridging
with five Mn(II) ions through each of its five carboxylate
oxygen atoms (Fig. 4b). The Mn–O bond distances range from
2.126(2) to 2.284(2) Å (Table S1, ESI†). As shown in Fig. 4c,
the inorganic motif displays 1D chains, which are formed by
the edge-sharing Mn2O10 that are connected by SBA ligand
units. These chains are further linked together by the μ5-links
(Fig. 3a). The Mn(II) ion is six-coordinated and bonded with
five oxygen atoms from the carboxylate groups belonging to
four SBA ligands, and one oxygen atom from the coordinated
EtOH molecule. The SBA ligand has acts as pentadentate and
is coordinated with four Mn centers (Fig. 3b) through its car-
boxylate oxygen atoms. The Mn–O bond distances range from
2.085(3) to 2.369(3) Å (Table S1, ESI†). As shown in Fig. 3c,
the inorganic motif displays 1D chains, formed by the
corner-sharing MnO6 polyhedra. These chains are linked
together by the SBA ligands in two directions to generate the
2D metal–organic framework of 5 (Fig. 3d). The framework of
5 contains rhomboidal channels with a free diameter of
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Fig. 3 (a) The coordination sphere of Mn in 5. (b) The coordination environment of the SBA ligand in 5. (c) The corner-sharing 1D inorganic chain
in 5. (d) The 2D structure view of 5 with the channels along the b-axis.
of SBA ligands in two directions to generate the 2D metal–
organic framework of 6 (Fig. 4d). The framework contains
rhomboidal channels with a free diameter of 5.6 × 5.0 Å2
(considering the van der Waals radii of atoms) without
including the water molecules. The SAV calculated without
including water molecules by PLATON19 analysis is 129.6 Å3,
which is 16.1% of the unit cell volume. The lower number of
coordinated solvent molecules in the structure reduces the
possible free volume in compound 6.
values after 423 K. The unit cell volume generally expanded
when the temperature was increased. As shown in Table S6
(ESI†), two bond lengths (Zn2–O2 and S1–O4) and one dis-
tance (O1⋯O4) are consistent with the variation of the c-axis.
Both tetrahedral distortion and hydrogen bonding could
explain the structural transformation well.
All desolvated samples of 2a–4a are isostructural and have
the same structure as in activated 1, with a monoclinic crystal
system and P21/n space group (Fig. 7). In 2–4, the first two
peaks in the PXRD pattern transfer to 2a–4a with one main
peak at 473 K, which may be corresponding to the solvent
removal and the structural transformation to CYCU-5-like
frameworks (Fig. S3–S5, ESI†). However, a few extra phases
(intermediate phases) have been observed in the varied tem-
perature PXRD patterns of 3 at 303–473 K and 4 at 373 K
(Fig. S4 and S5, ESI†), which were structurally identified as
as-synthesized and desolvated phases. The Rietveld profile
fits to synchrotron PXRD data for 3 and 4 showed that the
percentage ratio of as-synthesized and desolvated phases are
83 : 17 and 67 : 33 respectively. For 2a–4a, as shown in
Table S7 (ESI†), the bond length (M2–O2) relative to tetrahedral
Evaluation of structural transformation
It is interesting to note that the various temperature PXRD
patterns of 1 displayed similar patterns up to 673 K (Fig. S2,
ESI†), which indicates the SBUs of inorganic chains remain
stable with ZnO4 tetrahedra and ZnO6 octahedra under high
temperature (673 K). All these PXRD patterns were evaluated
in detail by carrying out Rietveld profile fitting (Fig. 5 and S7,
ESI†), which displayed that the lattice parameters change
upon activation (Fig. 6). The results showed a trend for
increasing lattice parameter values along the a- and b-axes as
temperature increased, while the c-axis showed decreasing
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Fig. 4 (a) The coordination sphere of the Mn atom in 6. (b) The coordination environment of the SBA ligand in 6. (c) The edge-sharing and
corner-sharing 1D inorganic chain in 6. (d) The 2D structure view of 6 with the channels along the b-axis.
Fig. 5 Rietveld profile fits to synchrotron PXRD data for CYCU-5 at 473 K and 673 K. The measured values are in red, the calculated intensity is in
green and the difference plot is in pink.
distortion and both the bond length (S1–O4) and distance
(O1⋯O4) relative to hydrogen bonding are the main consid-
erations during the structural transformation. Finally, the
PXRD patterns of CYCU-5 and 2a–4a at 473 K were well fitted
and reveal that the activated frameworks should have similar
conformations.
Thermogravimetric analysis
Thermogravimetric analyses (TGA) were performed to deter-
mine the thermal stabilities of the studied compounds 1–4
(Fig. S1, ESI†), and their corresponding weight losses of
solvent molecules at the different temperatures with their
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Fig. 6 Final Rietveld refinement lattice parameters for 1 at various temperatures.
calculated weight losses are tabulated (Table S3, ESI†). The
TGA curve of 1 showed the first gradual weight loss of about
6% up to 300 °C, corresponding to the release of the crystal-
lized ethanol molecule (ca. 5.2%). So, the CYCU-5 was ther-
mally stable up to about 400 °C. The further weight loss up
to 800 °C might correspond to the decomposition of organic
ligands, which form zinc oxide (Fig. S7, ESI†). The TGA curve
of 2 revealed that all the water and ethanol molecules are eas-
ily removed in two steps slightly above 150 °C, and the mass
loss of 18.1% agrees well with the calculated 16.8%; it might
lose its lattice water molecules in the first step, followed by
the loss of coordinated water and ethanol molecules in the
second step. Further, the desolvated phase, [Mg3(OH)2(SBA)2]
was fairly stable up to 400 °C, then the framework of 2
started to decompose. The TGA curves of compounds 3 and 4
have shown similar thermal stability behaviors to those seen
in 2. That is, the water and ethanol molecules removal
occurred in two steps at around 200 °C and 120 °C for the
compounds 3 and 4, respectively. It is noteworthy to mention
that the desolvated phases of 3 and 4 are thermally stable up
to 350 °C. Further heating of the compounds up to 800 °C
results in decomposition of framework structures. As
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Gas sorption properties
To verify the porosity of the guest-free phase of compounds
1–4 (CYCU-5, 2a–4a), gas sorption isotherms have been mea-
sured with N2, CO2 and H2. The N2 gas adsorption isotherm
at 77 K for the compounds shows type-I behavior with a steep
rise in the very low pressure region, suggesting a micropo-
rous nature. The N2 gas sorption results of compounds
CYCU-5, 2a–4a are tabulated in Table 4. From the results
obtained from the N2 gas sorption experiments (Fig. 8), it has
been found that compound 4a has better N2 gas sorption
capacity than that of the other three (CYCU-5, 2a and 3a).
In the case of the CO2 adsorption isotherms for the com-
pounds at 273 and 298 K (Fig. 9), the compounds showed
moderate CO2 adsorption behavior and their corresponding
values are depicted in Table 5. All the compounds showed
better CO2 uptake at low temperature (273 K) than at room
temperature (298 K). Compound 4a showed the highest CO2
uptake among the four as we have seen in the N2 case also.
The results of the CO2 uptake for the compounds are compa-
rable with some of the reported compounds, Zn2(tetrakis[4-
(carboxyphenyl)oxamethyl]methane)(4,4′-bipyridine)20 and
MOF-5/IRMOF-1 (ref. 21), under similar conditions.
On the other hand, the H2 gas sorption properties of the
compounds have been studied at 77 K (Fig. 10). All four
compounds showed some amount of H2 uptake at 1 atm,
given in Table 5. Like in the previous two cases, compound
4a exhibited better H2 storage capacity among the four com-
pounds studied. Surprisingly, CYCU-5 displayed almost no
Table 4 The N2 gas sorption data of CYCU-5, 2a–4a
BET surface area
Langmuir surface area
Compound
(m2 g−1
)
(m2 g−1
)
CYCU-5
2a
3a
19.6
44.5
9.7
24.1
59.9
11.7
92.6
4a
71.5
Fig. 7 Final Rietveld-plot for 2a–4a at 473 K. The measured values are in
red, the calculated intensity is in green and the difference plot is in pink.
displayed in Fig. S8 (ESI†), the Rietveld profile fits of synchro-
tron PXRD data for 4 at 673 K show the existence of three
types of cobalt oxides.
Fig. 8 Nitrogen adsorption isotherms for the compounds, CYCU-5,
2a–4a.
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Fig. 9 The CO2 adsorption isotherms for the compounds CYCU-5 and 2a–4a, measured at 273 K and 298 K.
Table 5 The CO2 and H2 uptake data of CYCU-5, 2a–4a
CO2 uptake in cm3 g−1
(mmol g−1) at 1 atm
H2 uptake at 77 K,
Compounds 273 K
298 K
1 atm in mmol g−1 (wt.%)
CYCU-5
2a
3a
29.39 (1.31) 21.87 (0.98) 0.44 (0.09)
38.52 (1.72) 25.28 (1.13) 1.63 (0.32)
28.54 (1.27) 25.46 (1.14) 1.49 (0.30)
41.62 (1.86) 32.11 (1.43) 1.96 (0.39)
4a
Fig. 11 CO2, N2, and CH4 gas uptake of CYCU-5 at 273 K.
273 K, which is much higher than those of N2 (0.51 wt.%),
and CH4 (0.58 wt.%). Hence, the above values clearly suggest
that CYCU-5 has selective CO2 gas adsorption over N2 and
CH4 gases at similar conditions. The selective CO2 adsorption
over the other gases for CYCU-5 can be ascribed to the quad-
rupole moment of CO2 (−1.4 × 10−39 C m2),26 which induces
an interaction with the framework.27 By fitting the CO2
adsorption data of CYCU-5 to the Dubinin–Radushkevich
(DR) equation, the surface area was determined to be
36.4 m2 g−1.
Fig. 10 The H2 adsorption–desorption isotherms of CYCU-5, 2a–4a, at
77 K.
Further, the high pressure CO2 and CH4 adsorption
isotherms of CYCU-5 at 273 and 298 K have been measured
(Fig. S10, ESI†). The results indicate that CYCU-5 has a maxi-
mum CO2 adsorption of 94.8 cm3 g−1 (at 33.5 bar, and 273 K)
and 58.5 cm3 g−1 (at 39.4 bar, and 298 K). However, the maxi-
mum CH4 uptake of CYCU-5 at 89 bar pressure, is 110 cm3 g−1
(at 273 K) and 49.9 cm3 g−1 (at 298 K). In order to see the sta-
bility of the compounds after gas sorption experiments, PXRD
was carried out for the compounds before and after gas sorp-
tion studies. The PXRD patterns of the compounds after gas
H2 gas sorption capacity. The H2 gas sorption capacity of 4a,
3a and 2a are comparable with the reported compounds
[Cu(hfipbb)(h2hfipbb)0.5],22 PCN-13,23 [Mn(NDC)],24 and
[Cd2(Tzc)2]25 under similar conditions.
In order to explore the potential gas separation properties
of CYCU-5 under ambient conditions, the adsorption iso-
therms of CO2, N2, and CH4 for CYCU-5 at 273 K were mea-
sured (Fig. 11). The CO2 uptake value at 1 atm is 5.00 wt.% at
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Table 6 Results of the ROP of L-LA using compound CYCU-5 as a catalyst
Conv.b (%)
Mn (calcd.)
Mn (obsd.)
PDIe
c
d
Entry
[L-LA]0/[Cat.]0
Toluene (mL)
1a
2a
3a
100/1
100/1
100/1
0
5
1
78
34
47
11 278
4942
6814
40 100 (23 300)
3900 (2262)
15 600 (9048)
1.70
1.06
1.15
a
b
c
[Cat.]0 = 88 mg (0.1 mmol), 140 °C, 72 h. Obtained from 1H NMR determination. Calculated from the molecular weight of lactic acid ×
d
[L-LA]0/[Cat.]0 × conversion yield. Obtained from GPC analysis and calibrated by polystyrene standard. Values in parentheses are the values
obtained from GPC × 0.56. Obtained from GPC analysis.
e
sorption studies (Fig. S6, ESI†) indicated that the frameworks
of all the compounds were stable after the gas sorption
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Conclusions
In summary, we have successfully synthesized six 2D
metal–organic frameworks under microwave-assisted
solvothermal reaction conditions, based on a rigid V-shaped
4,4′-sulfonyldibenzoic acid (H2SBA) ligand. The compounds
contain 1D open channels inside the 2D frameworks, which
are constructed from 1D inorganic chain SBUs. The various
temperature PXRD patterns of 1–4 suggested that the
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CYCU-5-like conformations with high thermal stabilities.
Compounds 1–4 demonstrated impressive gas adsorption
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CYCU-5 displayed selective CO2 gas adsorption over N2 and
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Financial assistance received from the Ministry of Science
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