Interchanged Hysteresis for Carbon Dioxide and Water Vapor Sorption in a Pair of Water-Stable, Breathing, Isoreticular, 2-Periodic, Zn(II)-Based Mixed-Ligand Metal-Organic Frameworks

Article abstract of DOI:10.1021/acs.inorgchem.8b03148

We report the synthesis of two isoreticular mixed-ligand metal-organic frameworks (MOFs), namely, [Zn(μ₂-ia)(μ₂-bpe)]_n·nDMF (1) and [Zn(μ₂-mia)(μ₂-bpe)]_n·nDMF (2), where ia = isophthalate, mia = 5-methoxyisophthalate, bpe = 1,2-bis(4-pyridyl)ethane, and DMF = N,N′-dimethylformamide. Single-crystal X-ray diffraction studies revealed that the structures of 1 and 2 consist of a 2-periodic, layer sql motif. Structures exhibit entanglement through interpenetration of neighboring frameworks to form a two-dimensional bilayer. Variable-temperature powder X-ray diffraction studies confirmed both structures retain crystallinity upon desolvation up to ～500 K. Although structurally similar, activated samples of 1 and 2 showed differing gas and vapor sorption capabilities. Despite activated 2 having the higher actual void space, activated 1 showed significantly higher sorption for carbon dioxide at 195 K, as well as significant hysteresis upon desorption. Empirical evidence points toward weaker bilayer···bilayer interactions, which allow the separation of the bilayers, illustrating that small changes in functional groups within an isoreticular pair of MOFs may have a large tuning effect on sorption properties.

Full text of DOI:10.1021/acs.inorgchem.8b03148

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Interchanged Hysteresis for Carbon Dioxide and Water Vapor
Sorption in a Pair of Water-Stable, Breathing, Isoreticular,
2‑Periodic, Zn(II)-Based Mixed-Ligand Metal−Organic Frameworks
Nolwazi Gcwensa, Nabanita Chatterjee, and Clive L. Oliver*
Centre for Supramolecular Chemistry Research, Department of Chemistry, University of Cape Town, Rondebosch, Cape Town,
South Africa
S
* Supporting Information
ABSTRACT: We report the synthesis of two isoreticular mixed-ligand
metal−organic frameworks (MOFs), namely, [Zn(μ2-ia)(μ2-bpe)]n·nDMF
(1) and [Zn(μ2-mia)(μ2-bpe)]n·nDMF (2), where ia = isophthalate, mia = 5-
methoxyisophthalate, bpe = 1,2-bis(4-pyridyl)ethane, and DMF = N,N′-
dimethylformamide. Single-crystal X-ray diffraction studies revealed that the
structures of 1 and 2 consist of a 2-periodic, layer sql motif. Structures exhibit
entanglement through interpenetration of neighboring frameworks to form a
two-dimensional bilayer. Variable-temperature powder X-ray diffraction
studies confirmed both structures retain crystallinity upon desolvation up to
∼500 K. Although structurally similar, activated samples of 1 and 2 showed
differing gas and vapor sorption capabilities. Despite activated 2 having the
higher actual void space, activated 1 showed significantly higher sorption for
carbon dioxide at 195 K, as well as significant hysteresis upon desorption.
Empirical evidence points toward weaker bilayer···bilayer interactions, which allow the separation of the bilayers, illustrating that
small changes in functional groups within an isoreticular pair of MOFs may have a large tuning effect on sorption properties.
recently been studied for a pair of ligand-originated MOF
isomers, that is, where the two MOFs are constituted of ligands
that are isomers of each other.18 The water stability of MOFs
remain an important challenge in various industrial applica-
tions due to the ubiquitous nature of water in many processes,
for example, the selective sorption of carbon dioxide in flue gas
mixtures (CO2/N2 = 85/15), where water is a major
component.19 Recently, Das et al. reported a Co(II) and
Cu(II)-based water-stable MOF, both of which were shown to
be highly selective toward CO2 sorption.20,21 Factors such as
the absence of coordinatively unsaturated metal centers,
interpenetration, and higher basicity of ligands have been
implicated as structural features that may add to the water
stability of MOFs.22
In this study, we synthesized two isoreticular, Zn-based
mixed-ligand MOFs that have 1,2-bis(4-pyridyl)ethane as the
common ligand and differ only in the introduction of a
methoxy functional group on the isophthalate ligand in the
case of the second MOF. The two novel MOFs have the
formula [Zn(μ2-ia)(μ2-bpe)]n·nDMF (1) and [Zn(μ2-mia)(μ2-
bpe)]n·nDMF (2), respectively, where ia = isophthalate, mia =
5-methoxyisophthalate, bpe = 1,2-bis(4-pyridyl)ethane, and
DMF = N,N′-dimethylformamide. The isophthalate/1,2-bis(4-
pyridyl)ethane combination of ligands with Zn has been
previously employed by Zaworotko et al. in the construction of
INTRODUCTION
■
Metal−organic frameworks (MOFs), which may consist of 1-
periodic, 2-periodic, or 3-periodic coordination networks
comprised of a metal coordinated to organic linkers, have
attracted widespread attention for their porosity and potential
applications in separation chemistry, catalysis, molecular
sensing, and gas storage.1−5 The coordination of a metal ion
with two different ligands, as in the case of mixed-ligand
MOFs, allows the possibility of direction-dependent pore
variation6,7 and thus has exciting implications for offering
chemists control over the behavioral characteristics of these
MOFs.8−10 Single-crystal-to-single-crystal (SC-SC) transfor-
mation, intriguing solid-state phenomena whereby structural
changes occur without loss of monocrystallinity, may yield
some insight into breathing mechanisms of particular MOFs,
where solvent molecules are exchanged.11−15 Variable-temper-
ature single-crystal X-ray diffraction (VT-SCXRD) may be
used to test directly whether SC-SC transformations occur
upon desolvation at higher temperatures and whether any
structural changes are evident.16,17 The study of isoreticular
MOFs, which have the same network topologies but differ in
their constituent ligands (or metal ions), may prove useful in
not only determining the influence of the ligand on pore
dimensions but also in yielding insight into the influence of
small changes in functional groups on properties such as gas or
vapor sorption. Introducing a methoxy group on a ligand as an
electron-donating group may be advantageous in interacting
with partial positive charges on adsorbed molecules as has
Received: November 8, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b03148
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Single Crystal X-ray Diffraction Studies. For both 1 and 2, a
suitable single crystal was mounted on a cryoloop using Paratone N
oil. Single-crystal X-ray diffraction (SCXRD) data for each compound
were collected using a Bruker Apex II DUO detector. Samples were
irradiated with graphite monochromated Mo Kα radiation of
wavelength λ = 0.71073 Å generated at 50 kV and 30 mA. The
Oxford Cryostream-700 unit maintained temperature control by
streaming N2 gas at a rate of 20 cm3 min−1. Data reduction and unit
cell refinement were performed with SAINT-Plus.26 All intensity data
were scaled and corrected for Lorentz-polarization and absorption
effects using SADABS.27 Structure solution and refinement were
implemented using the crystallographic suite OLEX2.28 The crystal
structures were solved by SHELXT,29 with subsequent refinement
proceeding using the full-matrix least-square method, based on F2
values against all reflections, including anisotropic displacement
parameters for all non-H atoms, as employed in SHELXL-2018/3.30
All hydrogen atoms were placed using riding models (Csp2−H and
Csp3−Hmethylene: 0.93 Å, Uiso(H) = 1.2Ueq(C); Csp3−Hmethyl: 0.96 Å,
Uiso(H) = 1.5Ueq(C)). X-Seed and MERCURY were used for
generating high-quality images using POV-RAY, while MERCURY
was also used to determine void spaces using a probe radius of 1.2
Å.31−33 Hirshfeld analyses of intermolecular interactions were
performed with CrystalExplorer.34
2-periodic, non-interpenetrated and 3-periodic, interpenetrated
nets.23 However, in the case of 1 and 2 the 2-periodic
coordination nets form twofold interpenetrated bilayers, which
are interdigitated with neighboring bilayers. These are similar
to the crystal structures for two Zn(II)-based, threefold
interpenetrated, 2-periodic MOFs constructed with the related
ligands phenylenediacrylate/5-ethoxyisophthalate and 1,2-bis-
(4-pyridyl)ethene.24,25 Despite these similarities, 1 and 2 do
exhibit important differences in terms of the degree of
interpenetration and the location of the solvent molecules,
which impart significantly larger potential void spaces on these
structures. Despite the very close structural similarities of 1 and
2 to each other they display remarkably different sorption
behaviors of carbon dioxide at low temperature with significant
hysteresis observed for 1, a phenomenon that is switched
within the pair in the case of water sorption.
EXPERIMENTAL SECTION
■
Materials and Physical Measurements. All chemicals were
acquired through commercial sources and used without further
purification. Thermal analysis consisted of thermogravimetric analysis
(TGA), differential scanning calorimetry (DSC), and hot stage
microscopy (HSM). TGA was performed using a TGA Q500, and
DSC was performed using a TA DSC25. HSM was performed using a
Linkam THMS600 hot stage and Linkam TP92 control unit fitted to
a Nikon SMZ-10 stereoscopic microscope. Crystals were placed on a
coverslip under silicon oil to visualize solvent release using HSM.
Images of thermal events were monitored with a Sony Digital Hyper
HAD color video camera and visualized on the Soft Imaging System
program analySIS. All thermal analyses were performed at a heating
rate of 10 °C min−1.
RESULT AND DISCUSSION
■
Single Crystal X-ray Diffraction Studies. The unit cell
parameters for MOFs 1 and 2 are very similar, as presented in
Table 1, albeit that these compounds crystallize in different
space groups. MOFs 1 and 2 are isoreticular, and therefore the
description of the main structural features of 1 hold for 2 as
Table 1. Crystal and Refinement Data for MOFs 1 and 2
Powder X-ray diffraction (PXRD) measurements were performed
on a Bruker D8 Advance X-ray diffractometer in the 4−40° 2θ range.
A step size rate of 0.015° sec−1 was used, while X-rays were generated
at 30 kV and 40 mA.
The gas and water vapor sorption experiments were performed
using a Micromeritics 3Flex Surface Area Analyzer. The samples were
prepared using a Micromeritics Flowprep with the flow of nitrogen
over the samples for 12 h with continuous heating at 60 °C.
Thereafter, samples were evacuated by heating at 200 °C for 3 h
before the analysis started. The Brunauer−Emmett−Teller (BET)
method was applied to data points where the slope of the Roquerol
BET graph remained positive.
1
2
ASU formula
formula weight
temperature/K
crystal system
space group
a/Å
b/Å
c/Å
α/deg
C23H23N3O5Zn
486.81
100.0
orthorhombic
Pbca
10.1799(8)
17.9193(17)
23.548(2)
90
C24H25N3O6Zn
516.84
100.0
orthorhombic
Pbcn
23.2656(12)
10.1523(5)
19.7050(10)
90
Preparation of [Zn(μ2-ia)(μ2-bpe)]n·nDMF (1). The metal salt,
Zn(NO3)2·6H2O (100 mg, 0.34 mmol), was dissolved in 3 mL of
H2O and then stirred for 10 min. The organic linkers 1,2-bis(4-
byridyl)ethane (0.062 mg, 0.34 mmol) and isophthalic acid (0.056
mg, 0.34 mmol) were dissolved in a solvent solution of 4 mL of DMF
and 2 mL of diethyl ether then stirred for 10 min. The solution of
organic linkers was mixed with the metal salt solution with continuous
stirring for a further 10 min followed by the addition of 0.2 μL of
H2SO4 acid (3 M) to aid dissolution. The resulting, clear solution was
sealed in a 25 mL vial then heated in an oven at 90 °C for 48 h.
Thereafter, the solution was cooled slowly to 30 °C at a rate of 10 °C
per hour. Clear, block-shaped crystals were obtained within 10 d.
Preparation of [Zn(μ2-mia)(μ2-bpe)]n·nDMF (2). The preparation
followed a similar procedure to that of compound 1. The metal salt,
Zn(NO3)2·6H2O (100 mg, 0.34 mmol), was dissolved in 3 mL of
H2O and stirred for 10 min. The organic linkers 1,2-bis(4-
byridyl)ethane (0.062 mg, 0.34 mmol) and 5-methoxyisophthalic
acid (0.066 mg, 0.34 mmol) were dissolved in 6 mL of DMF and then
stirred for 10 min. The solution of organic linkers was mixed with the
metal salt solution with continuous stirring followed by the addition
of 0.2 μL of H2SO4 acid (3 M). The resulting clear solution was
sealed in a 25 mL vial and then heated in an oven at 90 °C for 48 h.
Thereafter, the solution was cooled slowly to 30 °C at a rate of 10 °C
per hour. Clear, block-shaped crystals were obtained immediately.
β/deg
γ/deg
volume/Å3
Z
ρcalc/g cm−3
μ/mm−1
F(000)
2θ range for data
collection/deg
90
90
90
90
4295.6(6)
8
1.505
1.185
2016.0
4654.3(4)
8
1.475
1.101
2144.0
3.46 to 61.264
3.502 to 52.982
index ranges
−14 ≤ h ≤ 14, −25 ≤ k −29 ≤ h ≤ 29, −12 ≤ k
≤ 25, −33 ≤ l ≤ 33
≤ 12, −24 ≤ l ≤ 24
66 730
4817 [Rint = 0.0395,
Rsigma = 0.0168]
reflections collected
independent
reflections
data/restraints/
parameters
goodness-of-fit on F2 1.055
151 995
6627 [Rint = 0.0903,
Rsigma = 0.0296]
6627/0/291
4817/0/310
1.023
R1 = 0.0261, wR2 =
0.0695
R1 = 0.0321, wR2 =
0.0728
0.33/−0.34
final R indexes [I ≥ R1 = 0.0381, wR2 =
2σ(I)]
0.0978
R1 = 0.0487, wR2 =
0.1039
final R indexes [all
data]
largest diff peak/
0.69/−0.65
hole/e Å−3
B
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Figure 1. (a) Coordination environment of Zn(II) in 1 with ASU atoms labeled. (b) 2-periodic framework of 1. (c, d) Bilayers in (c) 1 and (d) 2
formed by entanglement of neighboring 2-periodic frameworks that stack in an interdigitated fashion. DMF molecules, shown in space-fill mode.
Hydrogen atoms are omitted for clarity.
well. The asymmetric unit (ASU) of 1 consists of a fully
occupied Zn2+ cation, one neutral 1,2-bis(4-pyridyl)ethane
ligand, one isophthalate ligand (5-methoxyisophthalate in the
case of 2), and one fully occupied, uncoordinated DMF
molecule (Figure 1a). The 2+ charge on the metal cation is
balanced by the 2− charge on the doubly deprotonated ia
ligand (mia ligand in 2). The Zn2+ cations are linked, as part of
a 2-periodic framework (Figure 1b), through tetrahedral
coordination via the two nitrogen atoms (N1 and N2) of
the bpe ligands along the longest axis of 23.548 Å (23.266 Å in
2) as well as a single oxygen atom (O1 and O4) on of each the
carboxylate groups of the isophthalate ligands along the
shortest axis of 10.180 Å (10.152 Å in 2). A single 2-periodic
framework is entangled with its neighbor to form an
interpenetrated bilayer (Figure 1c) with neighboring bilayers
stacked in an interdigitated manner along the 17.919 Å axis
(19.705 Å axis in 2).
The topology of the nets in both MOFs, formed when the
metal ions and centroids of the aromatic ia and mia rings are
taken as nodes, may be described sql nets that are, according
to terminology used by Carlucci, Proserpio and Blatov et al., 2-
periodic and two-dimensional (2D), the latter indicating that
there is no finite coordination in the third dimension.35
Topologically, the entanglement is also referred to as two-
C
DOI: 10.1021/acs.inorgchem.8b03148
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Table 2. Crystal and Refinement Data for Water-Exchanged and Dehydrated Forms of 1 and 2
1W
1′ (195 K)
1′ (298 K)
2W
2′ (195 K)
2′ (298 K)
ASU formula
formula weight
temperature/K
crystal system
space group
a/Å
C20H20N2O6Zn
449.75
C20H16N2O4Zn
413.72
C20H16N2O4Zn
413.72
C21H22N2O7Zn
479.77
C21H18N2O5Zn
443.74
C21H18N2O5Zn
443.74
100(2)
orthorhombic
Pbca
195(2)
orthorhombic
Pbca
298(2)
orthorhombic
Pbca
100(2)
195(2)
298(2)
orthorhombic
Pbcn
orthorhombic
Pbcn
orthorhombic
Pbcn
10.113(2)
17.913(3)
23.299(4)
90
10.154(3)
17.786(4)
23.447(6)
90
10.134(3)
17.966(5)
23.416(7)
90
23.1978(11)
10.1635(5)
19.3156(10)
90
23.2246(13)
10.1622(6)
19.4663(12)
90
23.236(2)
10.1511(9)
19.5507(19)
90
b/Å
c/Å
α/deg
β/deg
90
90
90
90
90
90
γ/deg
90
90
90
90
90
90
volume/Å3
4220.8(13)
8
4234.6(18)
8
4263(2)
8
4554.1(4)
8
4594.3(5)
8
4611.4(7)
8
Z
ρcalc/g cm−3
μ/mm−1
F(000)
1.416
1.298
1.289
1.400
1.283
1.278
1.201
1.184
1.176
1.121
1.100
1.095
1856.0
1696.0
1696.0
1984.0
1824.0
1824.0
2θ range for data
4.944 to 52.644
3.474 to 56.458
3.478 to 52.68
3.512 to 52.812
3.508 to 52.872
3.506 to 52.882
collection/deg
index ranges
−5 ≤ h ≤ 12, −22 ≤ k −7 ≤ h ≤ 13, −14 ≤ k −12 ≤ h ≤ 6, −14 ≤ k −20 ≤ h ≤ 29, −12 ≤ −20 ≤ h ≤ 29, −10 ≤ −29 ≤ h ≤ 20, −10 ≤
≤ 14, −28 ≤ l ≤ 14 ≤ 23, −13 ≤ l ≤ 31 ≤ 22, −29 ≤ l ≤ 12 k ≤ 10, −24 ≤ l ≤ 17 k ≤ 12, −21 ≤ l ≤ 24 k ≤ 12, −24 ≤ l ≤ 21
reflections collected 11 967 13 315 11 366 15 985 16 181 16 305
independent
reflections
4284 [Rint = 0.0692,
Rsigma = 0.0754]
5199 [Rint = 0.1098,
Rsigma = 0.1278]
4325 [Rint = 0.1186,
Rsigma = 0.1232]
4674 [Rint = 0.0369,
Rsigma = 0.0370]
4702 [Rint = 0.0441,
Rsigma = 0.0453]
4725 [Rint = 0.0506,
Rsigma = 0.0504]
data/restraints/
parameters
4284/0/265
5199/0/244
4325/0/244
4674/0/287
4702/0/263
4725/0/263
goodness-of-fit
on F2
1.186
0.911
0.919
1.014
1.043
0.928
final R indexes [I ≥ R1 = 0.1044, wR2 =
2σ(I)] 0.2162
R1 = 0.0702, wR2 =
0.1533
R1 = 0.0707, wR2 =
0.1548
R1 = 0.0415, wR2 =
0.1319
R1 = 0.0418, wR2 =
0.1068
R1 = 0.0443, wR2 =
0.1301
final R indexes [all R1 = 0.1421, wR2 =
R1 = 0.1833, wR2 =
0.2053
R1 = 0.1906, wR2 =
0.2108
R1 = 0.0565, wR2 =
0.1442
R1 = 0.0638, wR2 =
0.1172
R1 = 0.0751, wR2 =
0.1521
data]
0.2300
largest diff peak/
1.81/−1.38
0.57/−0.49
0.45/−0.35
1.47/−0.63
0.56/−0.38
0.58/−0.30
hole/e Å−3
dimensional; that is, 2D bilayers are formed (Figures S1 and
S2). Channels, occupied by DMF molecules, are formed down
the shortest axis of both 1 and 2.
to those calculated from the single-crystal structures (Figure
S9). Desolvated crystals of 1 and 2 (hereinafter 1′ and 2′,
respectively) for structure determination, obtained through
SC-SC transformation, were accessed via slightly different
methods. Crystals of 1 were heated on a hot stage to 573 K,
with SCXRD analysis after 24 h (Table 2) revealing that water
was absorbed from the atmosphere (1W hereinafter).
Subsequent in situ heating of the same crystal on the single-
crystal X-ray diffractometer to 393 K removed the adsorbed
water from the crystals to produce 1′. Heating crystals of 2 on
a hot stage to 573 K with subsequent structure determination
revealed the presence of significant electron density corre-
sponding to DMF; thus, another method of obtaining 2′ was
investigated. Instead, as-synthesized crystals of 2 were placed
in water, which underwent solvent exchange after two weeks
(hereinafter 2W). SCXRD confirmed the water exchange, after
which in situ removal of the adsorbed water in the same crystal
was achieved by heating it to 393 K (hereinafter 2′).
Interestingly, the water-exchange experiment on 1, by
immersing crystals of 1 in water, showed that 1W is obtained
after 1 d. For both 1′ and 2′, SCXRD data were collected at
195 and 298 K for direct comparison with sorption studies at
these temperatures (Table 2). There were no significant
connectivity or conformational changes observed for the
processes 1 → 1W → 1′ and 2 → 2W → 2′ (Figure 2).
Void Space Analyses. Void space analyses were
performed on potential void spaces of 1, 1W, 2, and 2W,
that is, when the solvent coordinates are deleted, as well as on
the actual void spaces of the desolvated structures (1′ and 2′)
DESOLVATION STUDIES−THERMAL ANALYSIS
AND VARIABLE-TEMPERATURE X-RAY
DIFFRACTION ANALYSES
■
Thermal analysis and variable-temperature powder X-ray
diffraction were employed to characterize MOFs 1 and 2
and also to assess the suitability of these compounds for
sorption studies. Both compounds were found to be highly
thermally stable. The TGA thermogram of 1 shows a mass loss
of 13.72% in the broad temperature range of 292−558 K,
which starts off gradually and peaks by ∼398 K, representing
the loss of one DMF molecule (calculated 14.91%), followed
by a plateau over a nearly 75 K range before decomposition
starts at 573 K (Figure S3). The TGA thermogram of 2 shows
a mass loss of 13.60% (calculated 14.47%) for the DMF mass
loss in the broad range of 290−540 K, which also starts
gradually and peaks by 418 K, followed by a plateau over a
nearly 50 K range before the onset of decomposition starts at
558 K (Figure S4). This mass loss was followed by
decomposition of the compound with continued heating to
723 K. DSC and HSM analyses were used to confirm the
thermal events described in the TGA thermograms (Figures
S5−S8), while VT-PXRD analyses indicate that the MOF
framework structures of the bulk material of 1 and 2 remain
largely intact upon solvent removal and up to >500 K, inferred
from the close agreement of the experimental PXRD patterns
D
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Figure 2. Crystal structures (from left to right) of the as-synthesized MOFs 1 and 2 (at 100 K), their water-exchanged (at 100 K) and subsequent
dehydrated structures (at 195 K with mapped void spaces shown in orange).
Table 3. Void Space Analyses of Potential Void and Actual Void Spaces of MOFs 1 and 2
a
a
a
a
1
2
1W
2W
1′ (195 K)
2′ (195 K)
1′ (298 K)
2′ (298 K)
void space percent per unit cell (%)
absolute void space per unit cell (Å3/unit cell)
volume of unit cell (Å3)
atomic mass unit (amu) per unit cell
void space per atomic mass unit (Å3/amu)
void space per gram of MOF (cm3/g)
18.6
801
4296
3309.8
0.242
0.145
18.3
851
4654
3549.9
0.240
0.143
14.5
614
4221
3309.8
0.186
0.111
14.2
646
4554
3549.9
0.182
0.109
14.9
631
4235
3309.8
0.191
0.114
15.5
712
4594
3549.9
0.201
0.120
15.4
655
4263
3309.8
0.198
0.118
15.6
719
4611
3549.9
0.203
0.121
a
Solvent molecule coordinates are deleted.
determined at 195 and 298 K. The void space analyses on 1′
and 2′ were performed for direct comparison with sorption
studies of the activated forms at these two temperatures, while
the potential void space analysis served as a reference for
conclusions that may be reached from the as-synthesized
structures. The potential void spaces per unit cell for 1 (18.6%
and 801 Å3) and 2 (18.3% and 851 Å3) are bigger than those of
1W (14.5% and 614 Å3) and 2W (14.2% and 646 Å3),
respectively, indicating that the respective MOFs undergo a
degree of “breathing” upon solvent exchange with water. This
is further supported by smaller changes in the absolute void
spaces when the MOFs are dehydrated. The void space in 2′ is
larger than in 1′, which is counterintuitive, since at cursory
inspection of the ligand one would expect the substitution of
an H atom (in ia) by a bulkier methoxy group (in mia) to
reduce the cavity size. Instead, the methoxy group is oriented
in such a manner as not to protrude into the cavity, but instead
it causes the bilayers in 2′ to be further apart, as indicated by
its ∼19 Å axis, an increase of ∼2 Å over the common axis in 1′.
In our attempts to relate pore volumes to sorption studies
(which are typically reported in amount or volume of adsorbed
gas per unit mass of activated material), we determined the
void space per unit mass for each structure, since the molar
masses of 1′ and 2′ are slightly different (Table 3). The table
shows that the void space in 2′ is large enough to offset its
slightly higher molar masses so that the void space per unit
mass of sample is also larger than that of 1′ (5.3% larger at 195
K and 2.5% at 298 K). Thus, it would be expected that 2′
would have a slightly higher sorption by mass for a given gas or
vapor.
GAS AND VAPOR SORPTION STUDIES
■
At 298 K 1′ and 2′ both show appreciable CO2 sorption, both
exhibiting type-I isotherms (Figure 3). Despite an initial lag, 2′
has the expected higher sorption with 39.3 cm3 (standard
temperature and pressure (STP)) g−1 (0.78 CO2 molecules per
ASU) compared to 29.8 cm3 (STP) g−1 (0.55 CO2 molecules
per ASU) for 1′ at 795 mmHg pressure. Initially, the same
trend is observed at 195 K in terms of 2′ having a higher
uptake (64.4 cm3 (STP) g−1 at a pressure of 236 mmHg
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Figure 3. Adsorption and desorption isotherms for carbon dioxide sorption for 1 and 2.
Figure 4. C···C isolated interactions for curvedness property plotted onto Hirshfeld surface of (a) 1′ (195 K) and (b) 2′ (195 K) and fingerprint
plots for (c) 1′ (195 K) and (d) 2′ (195 K).
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corresponding to 1.28 CO2 molecules per ASU) than 1′ (53.6
cm3 (STP) g−1 at 236 mmHg corresponding to 0.99 CO2
molecules per ASU). We note that sorption ratio of 2′ to 1′
(1.32) is significantly higher than the ratio of void space per
unit mass of 2′ to 1′ (1.05), indicating that there are additional
factors aiding the sorption of CO2 in 2′, the presence of the
methoxy group possibly being one. However, after 275 mmHg
pressure the isotherm of 1′ has an inflection point and shows a
marked increase in adsorption of CO2 over a relatively short
range up to 444 mmHg, after which it plateaus with an
eventual uptake of 99.9 cm3 (STP) g−1 (1.84 CO2 per ASU) at
709 mmHg pressure. This represents a near doubling of the
amount of CO2 that can be adsorbed by 1′ over 2′ at this
temperature. Inflection points are usually indicative of
structural changes, and a possible explanation is that the
bilayers in 1′ separate to accommodate, effectively, a second
CO2 molecule per ASU. However, this is difficult to justify in
terms of isolated, intermolecular interactions alone for the
structures of 1′ and 2′ at 195 K, since both structures have a
C−H···O interaction in common between neighboring bilayers
(C1−H1···O4 and C5−H5···O4 in 1′ and 2′, respectively),
while 1′, in fact, has an additional C−H···π interaction (C19−
H19···N1−C1−C2−C3−C4−C5) between neighboring bi-
layers, which is absent in 2′ (Tables S3 and S4). Hirshfeld
surface analysis may provide some clue in that a greater
percentage of C···H interactions (Table S5), which are
generally attractive, are found in 2′ than 1′ (23.3% vs
22.4%), while 1′ has a greater percentage of H···H interactions
(36.9% vs 35.4%), which are generally repulsive. Furthermore,
the mapping of the curvedness property onto the Hirshfeld
surfaces indicate a red region over the phenyl ring of mia in 2′,
which is an indication of closer π−π stacking interactions
(Figure 4a,b). In both structures, the phenyl rings are the
sections of the bilayers that interdigitate with those of
neighboring bilayers. In 2′ and 1′, the centroids of the closest
phenyl rings of neighboring bilayers are 3.843 and 4.054 Å
apart, respectively. Also, in the fingerprint plots of the
Hirshfeld surfaces, 2′ has a “lighter blue” color at di and de
of ∼2.0 Å, indicating a larger percentage of interactions at
distances associated with the π−π stacking regions (Figure
4c,d). The absence of these π−π interactions in 1′ may allow
the relative movement of bilayers thus causing a second
molecule of CO2 per ASU to be adsorbed.
Figure 5. Adsorption and desorption isotherms for water vapor
sorption for 1′ and 2′.
sorption occurs with the next recorded point at P/P0 = 0.14
showing an adsorption of 53.7 cm3 (STP) g−1 (0.99 H2O
molecules per ASU). Subsequently, the sorption starts to
plateau, but an inflection point occurs at P/P0 = 0.18 and
adsorption of 66.5 cm3 (STP) g−1 (1.23 molecules of H2O per
ASU) indicates a structural change occurring allowing a rapid
increase in sorption with the maximum sorption being 115.7
cm3 (STP) g−1 (2.14 molecules of H2O) at P/P0 = 0.81. The
desorption isotherm is virtually superimposable on the
adsorption isotherm, showing no signs of hysteresis. The
water sorption for 2′ shows a similar trend to that of 1′ but
with some distinct differences. The first significant accelerated
increase in sorption occurs at P/P0 = 0.10, where 2′ has
adsorbed 17.2 cm3 (STP) g−1 (0.34 H2O molecules per ASU).
Thus, the pores of 2′ are more hydrophilic than those of 1′, as
the same amount of water is adsorbed at a lower relative
pressure than in 1′, and therefore an indication that the pores
of 2′ are more polar.19,36 This could also be the reason that
CO2 sorption in 2′ is higher than expected based on the void
space analysis alone (except at 195 K, where structural changes
in 1′ allow the sorption of a second molecule of CO2 per
ASU), the methoxy group in 2′ creating a more polar
environment. Unlike, in the case of 1′ the increase in sorption
is more modest with the isotherm inflection point at P/P0 =
0.17 indicating a sorption of 29.4 cm3 (STP) g−1 (0.58 H2O
molecules per ASU). However, after this point the accelerated
increase in sorption is significant over a short range, with the
isotherm starting to plateau at P/P0 = 0.24, the maximum
sorption of 105.6 cm3 (STP) g−1 (2.09 H2O molecules per
ASU) occurring at P/P0 = 0.82. Unlike in the case of 1′, 2′
shows a significant degree of hysteresis with significant
desorption only occurring at P/P0 = 0.10 and sorption of
84.2 cm3 (STP) g−1 (1.67 H2O molecules per ASU), the next
desorption point at P/P0 = 0.09 corresponding to a sorption of
24.9 cm3 (STP) g−1 (0.49 H2O molecules per ASU).
Upon desorption, a very large hysteresis step was evident for
1′ with a sharp decrease in sorption to the lower plateau only
occurring as from 83 mmHg (92.2 cm3 (STP) g−1
corresponding to 1.70 molecules of CO2 per ASU) to 55
mmHg (53.6 cm3 (STP) g−1 corresponding to 0.99 molecules
of CO2 per ASU), which is ∼200 mmHg (or P/P0 ≈ 0.25)
lower than the sharp increase on the adsorption run. The
isosteric heats (Qst) of CO2 adsorption were also determined
for 1′ and 2′ from isotherms obtained from 273 to 298 K
(Figures S12). The Qst values for 1′ were in the range of 27.8−
28.2 kJ mol−1 up to a coverage of 29.7 cm3 (STP) g−1, while
those for 2′ started lower at 25.1 kJ mol−1 but increased to
29.7 kJ mol−1 at 29.7 cm3 (STP) g−1 coverage, which is
consistent with the 2′ sorption isotherms at different
temperatures initially lagging behind, but eventually surpassing,
those of 1′.
Post water vapor sorption PXRD patterns prove that both
MOFs are water stable (Figure S13). After the water sorption
experiments the same samples were tested for their affinities
for atmospheric water by in situ TGA and VT-PXRD
experiments. The samples were desolvated on the TGA
equipment and exposed to the atmosphere, which showed that
these rehydrated to their original water content after ∼10 min
(Figure S14). This rapid water sorption guided the in situ VT-
PXRD experiments, where a sample of 1W and 2W were
heated to 373 K and subsequently cooled to 298 K while being
1′ and 2′ both displayed interesting water vapor sorption
behaviors (Figure 5). For 1′ the sorption is moderate up to P/
P0 = 0.13, where it has adsorbed 18.7 cm3 (STP) g−1 (0.34
H2O molecules per ASU), after which a sudden increase in
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exposed to the atmosphere. Because of the rapid sorption of
water, only the range of 7−12.5° 2θ was measured over a scan
period of ∼1 min to visualize the peaks at ∼9.9° (1W) and
∼9.2° (2W), which were the most affected by dehydration and
rehydration (Figure S15). These peaks are either absent or
greatly diminished at 373 K; however, they return close to their
original intensities after 10 min of cooling. These experiments
lead us to believe that with the inevitable exposure of samples
to the atmosphere after initial evacuation of the DMF
molecules, the as-synthesized crystals of 1 and 2 proceed
through a hydrated phase even before being activated for
sorption, that is, 1/2 → 1W/2W → 1′/2′.
Furthermore, complete water exchange with the as-synthesized
crystals 2, to produce 2W, occurred over two weeks, whereas
the same experiment on 1 (to produce 1W) was complete after
1 d. We have on a previous occasion shown that low-
temperature CO2 dioxide isotherms can reveal different
sorption behaviors when compared to room-temperature
sorption, possibly due to increased interaction strengths at
lower temperatures.40 The low-temperature sorption experi-
ments show the utility of investigating these possibilities in the
absence of high-pressure, room-temperature carbon dioxide
sorption experiments. The carbon dioxide and water-sorption
hysteresis behaviors are interchanged in the isoreticular MOF
pair, possibly pointing to different modes of interaction of
water and carbon dioxide molecules when adsorbed into the
MOFs.
This work highlights that sorption behavior of isoreticular
MOFs, even in the case of such closely related MOFs as
described in this work, can be very different and may depend
on a multitude of factors such as chemical functionality and
“breathability”, which may come into effect at different
pressures. Therefore, continued investigation of subtle changes
in MOF structures and their effects on properties such as gas
sorption is warranted.
N2 sorption isotherms at 77 K for 1′ and 2′ were not
deemed appropriate to determine internal surface areas by
BET analysis, as the shape of the isotherms did not conform to
a type-I isotherm even though SC-SC transformations, SCXRD
analyses, and sorption experiments with water and carbon
dioxide indicate that 1′ and 2′ are microporous. This is
supported by the relatively low BET analysis surface areas
calculated from N2 sorption data, which were 40 m2 g−1 and 72
m2 g−1 for 1′ and 2′, respectively. Bae et al. showed that, in the
absence of reliable N2 sorption data, CO2 sorption at 273 K
could be used to determine BET surface areas of ultra-
microporous MOFs with pore diameters of less than 7 Å.37 In
this work, the CO2 sorption isotherms at 273 K revealed
virtually identical BET surface areas of 187 and 188 m2 g−1 for
1′ and 2′, respectively. The 298 K single-crystal structures of
the desolvated forms (taken as the SCXRD analyses closest in
temperature to those of the BET isotherms used), revealed
channel diameters of 3.8 × 4.9 Å2 and 4.9 × 4.6 Å2 for 1′ and
2′, respectively.38 There is some controversy around the use of
CO2 sorption for determining BET surface areas;39 however,
we include these results in this work to compare sorption
values 1′ and 2′ but also to show that CO2 is more able to
penetrate the structures than N2. H2 sorption was low for both
MOFs, with 2′ performing slightly better than 1′ at 0.17 versus
0.11 wt % (Figure S16).
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b03148.
Structural and thermal analysis of MOFs such as
thermogravimetric analysis (TGA), differential scanning
calorimetry (DSC) and hot stage microscopy (HSM)
and variable temperature powder X-ray diffraction (VT-
PXRD) are included along with several tables of
supramolecular hydrogen bonding interactions in the
frameworks. Adsorption and desorption isotherms are
provided for gases N2, H2, and CO2 (at different
temperatures), as well as water vapor. The CO2 heat of
adsorption plots for both the frameworks are also
provided (PDF)
CONCLUSION
■
In conclusion, two isoreticular, mixed-ligand, 2-periodic MOFs
were synthesized using Zn(NO3)2·6H2O, 1,2-bis(4-pyridyl)-
ethane and either isophthalic acid (in the case of 1) or 5-
methoxyisophthalic acid (in the case of 2). Single-crystal
structures of the activated, desolvated forms (1′ and 2′) could
be obtained through an intermediate, hydrated phase in each
case. Despite their similar structures, 1′ and 2′ showed marked
differences in their carbon dioxide sorption behaviors at 195 K.
At this temperature, 1′ adsorbs a second CO2 molecule per
ASU attributed to structural changes as indicated by an
inflection point in its isotherm. We suggest that interbilayer
separation, that is, through a “breathing” effect, may be the
type of structural change that aids the sorption of this second
CO2 molecule per ASU. Void space analyses indicate that pore
volumes, as well as unit cell axes along the bilayer stacking
direction, change through the 1/2 → 1W/2W → 1′/2′
transformations, suggesting that inter-bilayer separation is a
plausible explanation. The methoxy group may initially aid
carbon dioxide and water sorption in 2′ (initial sorption is
higher for 2′ than for 1′ in both cases); however, it may be
indirectly responsible for retarding inter-bilayer separation in
2′. That transformations are more difficult in 2 is further
supported by the empirical observations that direct SC-SC
desolvation of 2 was not possible, whereas it was for 1.
Accession Codes
CCDC 1877607−1877614 contain the supplementary crys-
tallographic data for this paper. This data can be obtained free
of charge via www. ccdc.cam.ac.uk/data_request/cif [or from
the Cambridge Crystallographic Data Centre (CCDC), 12
Union Road, Cambridge CB2 1EZ, U.K.; fax: + 44(0)1223−
336033; e-mail: deposit@ccdc.cam.ac.uk.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: clive.oliver@uct.ac.za.
ORCID
Clive L. Oliver: 0000-0001-7946-6492
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
N.Z.G., N.C., and C.L.O. thank the South African National
Research Foundation and Univ. of Cape Town for financial
support.
H
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Highly Selective CO2 Separation under Ambient Conditions. Chem. -
Eur. J. 2018, 24, 5982−5986.
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