Porous Metal-Organic Polyhedral Frameworks with Optimal Molecular Dynamics and Pore Geometry for Methane Storage

Article abstract of DOI:10.1021/jacs.7b05453

Natural gas (methane, CH₄) is widely considered as a promising energy carrier for mobile applications. Maximizing the storage capacity is the primary goal for the design of future storage media. Here we report the CH₄ storage properties in a family of isostructural (3,24)-connected porous materials, MFM-112a, MFM-115a, and MFM-132a, with different linker backbone functionalization. Both MFM-112a and MFM-115a show excellent CH₄ uptakes of 236 and 256 cm³ (STP) cm^-3 (v/v) at 80 bar and room temperature, respectively. Significantly, MFM-115a displays an exceptionally high deliverable CH₄ capacity of 208 v/v between 5 and 80 bar at room temperature, making it among the best performing metal-organic frameworks for CH₄ storage. We also synthesized the partially deuterated versions of the above materials and applied solid-state ²H NMR spectroscopy to show that these three frameworks contain molecular rotors that exhibit motion in fast, medium, and slow regimes, respectively. In situ neutron powder diffraction studies on the binding sites for CD₄ within MFM-132a and MFM-115a reveal that the primary binding site is located within the small pocket enclosed by the [(Cu₂)₃(isophthalate)₃] window and three anthracene/phenyl panels. The open Cu(II) sites are the secondary/tertiary adsorption sites in these structures. Thus, we obtained direct experimental evidence showing that a tight cavity can generate a stronger binding affinity to gas molecules than open metal sites. Solid-state ²H NMR spectroscopy and neutron diffraction studies reveal that it is the combination of optimal molecular dynamics, pore geometry and size, and favorable binding sites that leads to the exceptional and different methane uptakes in these materials.

Full text of DOI:10.1021/jacs.7b05453

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Article
Porous Metal–Organic Polyhedral Frameworks with Optimal
Molecular Dynamics and Pore Geometry for Methane Storage
Yong Yan, Daniil I Kolokolov, Ivan da Silva, Alexander G Stepanov, Alexander J.
Blake, Anne Dailly, Pascal Manuel, Chiu C Tang, Sihai Yang, and Martin Schröder
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05453 • Publication Date (Web): 03 Aug 2017
Downloaded from http://pubs.acs.org on August 3, 2017
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Porous Metal–Organic Polyhedral Frameworks with Optimal Molecular
Dynamics and Pore Geometry for Methane Storage
Yong Yan,1† Daniil I. Kolokolov,2,3† Ivan da Silva,4† Alexander G. Stepanov,2,3 Alexander J. Blake,5 Anne
Dailly,6 Pascal Manuel,4 Chiu C. Tang,7 Sihai Yang1* and Martin Schröder1,8*
1. School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom.
Sihai.Yang@manchester.ac.uk; M.Schroder@manchester.ac.uk
2. Boreskov Institute of Catalysis, Siberian Branch of Russian Academy of Sciences, Prospekt Akademika
Lavrentieva 5, Novosibirsk 630090, Russia.
3. Novosibirsk State University, Pirogova Street 2, Novosibirsk 630090, Russia.
4. ISIS facility, Science and Technology Facilities Council (STFC), Rutherford Appleton Laboratory, Didcot,
OX11 0QX, United Kingdom.
5. School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, United
Kingdom.
6. Chemical and Environmental Sciences Laboratory, General Motors Corporation, Warren, Michigan 48090,
United States.
7. Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United
Kingdom.
8. Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, 3 Acad.
Lavrentiev Ave., Novosibirsk, 630090, Russia.
†These authors contributed equally to this work.
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Abstract
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Natural gas (methane, CH4) is widely considered as a promising energy carrier for mobile
applications. Maximising the storage capacity is the primary goal for the design of future storage media.
Here we report the CH4 storage properties in a family of isostructural (3,24)ꢀconnected porous materials,
MFMꢀ112a, MFMꢀ115a and MFMꢀ132a with different linker backbone functionalisation. Both MFMꢀ112a
and MFMꢀ115a show excellent CH4 uptakes of 236 and 256 cm3 (STP) cm–3 (v/v) at 80 bar and room
temperature, respectively. Significantly, MFMꢀ115a displays an exceptionally high deliverable CH4 capacity
of 208 v/v between 5 and 80 bar at room temperature, making it among the best performing MOFs for
methane storage. We also synthesized the partially deuterated versions of the above materials and applied
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solidꢀstate H NMR spectroscopy to show that these three frameworks contain molecular rotors which
exhibit motion in fast, medium and slow regimes, respectively. In situ neutron powder diffraction studies on
the binding sites for CD4 within MFMꢀ132a and MFMꢀ115a reveal that the primary binding site is located
within the small pocket enclosed by the [(Cu2)3(isophthalate)3] window and three anthracene/phenyl panels.
The open Cu(II) sites are the secondary/tertiary adsorption sites in these structures. Thus, we obtained direct
experimental evidence showing that a tight cavity can generate a stronger binding affinity to gas molecules
than open metal sites. Solidꢀstate 2H NMR and neutron diffraction studies reveal that it is the combination of
optimal molecular dynamics, pore geometry and size, and favourable binding sites that leads to the
exceptional and different methane uptakes in these materials.
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Introduction
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Natural gas (comprised primarily of 95% methane, CH4) has become an important fuel for mobile
applications owing to its potentially reduced carbon emission at the point of use in comparison to gasolineꢀ
based hydrocarbon fuels.1,2 Another key advantage of utilising CH4 as fuel lies on its abundant reserves on
the earth and the widespread and mature infrastructure for its costꢀviable recovery.3 However, the extensive
implementation of CH4 as transportation fuel is restricted because of the underdevelopment of safe, efficient
and high capacity storage systems. Therefore, challenges exist in the development of practical CH4 storage
materials that can outperform the stateꢀofꢀtheꢀart technologies mainly based upon liquefaction in cryogenic
tanks or compression in heavyꢀwalled pressure vessels.4
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Porous metal–organic framework (MOF) materials have attracted increasing research interest
because of their potential applications in gas storage,5–8 separation,9,10 carbon capture,11 small molecule
sensing12 and heterogeneous catalysis,13 among others.14,15 Constructed by bridging metal ions/clusters with
organic ligands, crystalline MOFs have the unique advantage of versatile structural diversity and
tunability.16,17 The crystalline nature of MOFs allows advanced crystallographic analysis of the structural
change upon external stimuli and/or the host‒guest interactions.18 These structural insights at a molecular
level can effectively direct the design of future MOFs showing optimized capability of binding guest
molecules with the porous host for enhanced storage/separation performance. In the context of CH4 storage,
the primary target for materials design is to maximise the adsorption capacity and more importantly the
deliverable capacity with the aim to approach the DOE target for onꢀboard CH4 storage.19
The synthesis of MOFs based on Cu(II) with isophthalate ligands20 has been proved to be an
effective strategy to achieve high CH4 storage capacity.21 A family of robust (3,24)ꢀconnected networks with
ultraꢀhigh porosity incorporating hexacarboxylate ligands and [Cu2(O2CR)4] paddlewheels have been
reported to show exceptional gas adsorption properties.21–25 Open metal sites can provide strong affinity to
CH4, as evidenced by previous structural studies on several MOFs such as Cu3(BTC)2 (BTC3– = benzeneꢀ
1,3,5ꢀtricarboxylae)26–28 and MgꢀMOFꢀ74.29 However, open metal sites often saturate rapidly upon guest
uptake and thus have a limited role in defining the maximum gas capacity. This is particularly the case at
high pressure, where adsorbate–adsorbate interactions dominate the uptake process. However, the effects of
optimising the configuration and molecular dynamics of the ligand core have been largely overlooked for the
enhancement of overall CH4 adsorption capacity.
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We report here the CH4 adsorption properties in a series of (3,24)ꢀconnected MOFs, MFMꢀ112,22
MFMꢀ11524b and MFMꢀ132 (MFM = Manchester Framework Material replacing the NOTT specification)
incorporating a central phenyl ring, a nitrogen centre at the core of the hexacarboxylate, and anthracene
functionalisation, respectively. Thus, variation of the central part of the hexacarboxylate ligands where three
isophthalate units are covalently connected in a coplanar fashion results in structures with different
functionalities and pore geometries. Significantly, desolvated MFMꢀ115a shows an exceptionally high
deliverable CH4 capacity of 208 cm3 (STP) cm–3 (v/v) between 5 and 80 bar at room temperature, making it
among the best performing porous materials for CH4 storage. In most porous MOF structures, the organic
linker contains mobile aromatic fragments that can rotate around a certain axis within the void. This rotation
creates dynamic disorder in the framework, which effectively changes the inner geometry of the pore, as well
as the positions of possible adsorption sites associated with the fragment. We report also the molecular
dynamics of the rotationallyꢀdynamic aromatic rings in the partially deuterated isostructural series
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(desolvated MFMꢀ112aꢀd12, MFMꢀ115aꢀd12 and MFMꢀ132aꢀd24) as revealed by solidꢀstate H NMR
spectroscopy. We confirm how these dynamics can be controlled in a simple manner by rational synthesis. In
addition, neutron diffraction studies on CD4ꢀloaded MFMꢀ132a and MFMꢀ115a afford precise details of CD4
binding in these two structures at a molecular level. In MFMꢀ132a, a tight pocket of 6 Å diameter formed by
the anthracene rings close to the [(Cu2)3(isophthalate)3] window, and this shows an affinity for CD4 which
surprisingly is stronger than the open Cu(II) sites. Interestingly, CD4 in MFMꢀ115a shows optimum packing
efficiency within this porous structure.
Results and Discussion
Structures and porosity of MFM-112, MFM-115 and MFM-132. The three isostructural MOFs
all have (3,24)ꢀconnected networks, where [Cu2(O2CR)4] paddlewheels are bridged by the isophthalates units
from the hexacarboxylate ligands to form cuboctahedra, which are further connected by the central triangular
ligand core (Fig. 1). In the structure of MFMꢀ132, a [Cu24(isophthalate)24] cuboctahedron (Cage A),
constructed from 12 {Cu2} paddlewheels and 24 isophthalates (from 24 independent BTAT6– units),
[H6BTAT = 5,5',5''ꢀ(benzeneꢀ1,3,5ꢀtriyltris(anthraceneꢀ10,9ꢀdiyl))triisophthalic acid], is heavily shielded by
24 anthracene units from different ligands. Upon removal of coordinated water molecules from the axial sites
of the metal ions, the interior accessible void in Cage A is ~1.3 nm in diameter. This cage possesses a highly
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hydrophobic outer shell (anthracene panels) and an internal hydrophilic cavity [12 open Cu(II) sites in its
activated phase], and is anticipated to show highly confined host–guest binding properties. Cage B, a
truncated tetrahedron comprising four ligands and four [Cu6(isophthalate)3] triangular windows, with 12
anthracene units protruding into the central void, shows a diminished cage size of 6.5 Å (defined as the
diameter of the largest sphere which can be fitted into the cavity taking into account the van der Waals radii
of surface atoms). This is much smaller than that of the phenyleneꢀfunctionalised MFMꢀ112 (Cage B ~14 Å
in diameter). The truncated octahedral Cage C is constructed from eight BTAT6‒ units and six
[Cu8(isophthalate)4] square windows and possess an accessible void 13 Å in diameter despite having 24
anthracene rings protruding into the centre of the cavity. The void between the two closest cuboctahera or the
fused Cages B and C generates the fourth cage, Cage D, which is composed of eight anthracene panels from
four ligands and two [Cu2(O2CR)4] paddlewheels with an enclosed spherical cavity of 10 Å in diameter and
small apertures of 5 Å × 5 Å. Overall, the anthracene functionalisation in MFMꢀ132 yields a rich
combination of metalꢀorganic coordination cages of different geometry, size and pore surface chemistry, thus
representing a unique platform to study the host–guest binding in porous materials.
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The three MOFs show similar thermal stability with the framework decomposition at 350 °C as
confirmed by thermogravimetric analyses (Fig. S9).22,24b The desolvated samples of MFMꢀ112a, MFMꢀ115a
and MFMꢀ132a can be readily prepared by removing the free and coordinated solvent molecules from the
pore at 100 °C under dynamic vacuum. The desolvated materials show retention of the framework structure
as confirmed by PXRD analysis. The solventꢀaccessible void calculated by using PLATON/VOID30 for
MFMꢀ132a is 63%, lower than that of MFMꢀ112a (75%) and MFMꢀ115a (72%) owing to the anthracene
functionalisation. Unsurprisingly, MFMꢀ132a shows a lower Brunauer−Emmett−Teller (BET) surface area
(SBET = 2466 m2 g–1) and total pore volume (Vp = 1.06 cm3 g–1) than that of MFMꢀ112a (SBET = 3800 m2 g–1;
Vp = 1.62 cm3 g–1) and MFMꢀ115a (SBET = 3394 m2 g–1; Vp = 1.38 cm3 g–1). The pore size distribution
calculated by using the NLDFT method indicates a homogeneous distribution of pore size of ~12 Å in MFMꢀ
132a. MFMꢀ112a and MFMꢀ115a show a hierarchical pore system comprising differentꢀsized pores with the
largest pore size of 17 Å and 20 Å, respectively. Because of the presence of bulky aromatic groups, MFMꢀ
132a has a calculated crystal density of 0.650 g cm–3, higher than that of MFMꢀ112a (0.503 g cm–3) and
MFMꢀ115a (0.611 g cm–3).20 Throughout this and other reports, it is common practice for the volumetric gas
uptake to be derived from the bulk material density based upon the single crystal structure, but it should be
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noted that when the efficiency of packing and the polycrystalline nature of the bulk material are considered,
the uptakes will be reduced accordingly; this will be subject to the different compressibility and mechanical
stability of different materials.
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CH4 Storage. Adsorption measurements were performed using a gravimetric adsorption analyser for
the desolvated materials at 298 K over the pressure range of 0–90 bar (Fig. 2, S11). The excess CH4 uptakes
in these materials increase with increasing pressure until reaching the maximum in the high pressure region
(80–90 bar). The total CH4 uptake was calculated from the excess uptake by including the amount of
compressed CH4 in the pore using the pore volume as determined from N2 isotherms. Although MFMꢀ112a
shows higher gravimetric CH4 uptakes than MFMꢀ115a in the high pressure range (30–90 bar), the
volumetric uptakes show the opposite trend due to the higher crystal density of MFMꢀ115a. At 35 bar,
MFMꢀ112a and MFMꢀ115a show high total gravimetric CH4 uptakes of 322 and 304 cm3 (STP) g–1,
respectively, comparable to those measured for the bestꢀbehaving MOFs under the same conditions.31–34 In
addition, MFMꢀ115a shows a high volumetric total CH4 capacity of 186 v/v when compared to other highꢀ
performing CH4 uptake materials such as MOFꢀ519,35 PCNꢀ1436 and NUꢀ12524a,31,37 at 35 bar and 298 K. In
comparison, MFMꢀ112a has a lower total volumetric CH4 uptake of 162 v/v at 35 bar. Although MFMꢀ132a
shows lower total gravimetric CH4 uptake of 249 cm3 (STP) g–1 than MFMꢀ112a, it has the same volumetric
uptake of 162 v/v at 35 bar and 298 K owing to the higher crystal density of MFMꢀ132a. At 80 bar, MFMꢀ
115a and MFMꢀ112a both show remarkable increases in their total CH4 adsorption capacities to 419 and 469
cm3 g–1, equivalent to volumetric uptakes of 256 and 236 v/v, respectively. MFMꢀ132a has lower surface
area and pore volume, resulting in lower uptake of 213 v/v compared to MFMꢀ112a and MFMꢀ115a at 80
bar and 298 K.
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The deliverable capacity, defined as the amount of CH4 released from the storage system between a
high pressure (generally 65–80 bar) stage and a low pressure stage (typically 5 bar), is of direct relevance to
the practical performance of a given storage medium. MFMꢀ115a and MFMꢀ112a show high deliverable
CH4 capacities of 191 v/v and 181 v/v between 65 and 5 bar at 298 K, respectively, comparable with best
performing CH4 storage materials such as [Co(bdp)]38 (bdp2– = 1,4ꢀbenzenedipyrazolate) (197 v/v) and
UTSAꢀ76a (196 v/v).39 Significantly, MFMꢀ115a exhibits an exceptionally high deliverable CH4 capacity of
208 v/v between 80 and 5 bar at room temperature, rivalling those reported for the stateꢀofꢀtheꢀart materials
(Fig. 2b, Table 1) such as MOFꢀ905 (203 v/v),40 AlꢀsocꢀMOFꢀ1 (201 v/v)41 and HKUSTꢀ1 (200 v/v).31
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MFMꢀ112a also exhibits an excellent performance in delivering 200 v/v of CH4 between 80 and 5 bar at 298
K. In contrast, compared to both MFMꢀ112a and MFMꢀ115a, MFMꢀ132a shows lower deliverable (to 5 bar)
CH4 capacities of 150 and 162 v/v at 65 and 80 bar, respectively. The isosteric heats of adsorption (Qst) at
low CH4 loading are estimated to be 16.2, 16.3, and 15.7 kJ mol–1 for MFMꢀ112a, MFMꢀ115a and MFMꢀ
132a, respectively. These values are comparable to other MOFs with open Cu(II) sites.26,32
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Comparison of the CH4 adsorption in these three materials reveals interesting findings. MFMꢀ115a,
with moderate porosity across this series of MOFs, displays the highest methane deliverable capacity,
outperforming MFMꢀ112a, which shows slightly higher surface area and pore volume. The only structural
difference between MFMꢀ112a and MFMꢀ115a lies on the central triangular organic moiety in the hexagonal
linker face (Fig. 1). We therefore argued that the pore environment created by the rotational phenylene rings
in the linker can affect and perhaps control the gas adsorption properties of the host materials.
2H NMR Studies. Solidꢀstate 2H NMR technique was applied to investigate the molecular dynamics
of the rotational aromatic rings in this series of MOFs. This is a very powerful experimental technique well
suited to monitor the molecular reorientations over a wide range of characteristic rates (103–107 s–1) and
geometries (angular displacement > 5°) in solid state.44,45 The three MOFs were partially deuterated by
selectively introducing Dꢀatoms on the aromatic rings in the ligands (see Supporting Information Section S4).
The solidꢀstate 2H NMR spectra for MFMꢀ112aꢀd12, MFMꢀ115aꢀd12 and MFMꢀ132aꢀd24 were collected over
a range of temperatures (100–525 K, up to the temperature limit of the NMR magnet probe) to follow their
structural dynamics (Figs. 3, 5, and 6). The molecular motion was analyzed by the evolution of the 2H NMR
line shape arising from the perdeuterated fragments in the frameworks in a given temperature range. The
temperature evolution of the 2H NMR line shape typically involves three stages: (i) static, when k < 103 s–1
(the rotational rate constant) with the line shape dependent solely on the electronic configuration of the C–D
bond; (ii) intermediate exchange with 103 < k < 107 s–1 with the line shape reflecting both the rate and the
geometry of the motion; and (iii) fast limit when k > 107 s–1 with an averaged line shape reflecting the final
geometry but not the rate of the molecular orientation (Fig. S20). We probed as wide a temperature range as
possible to study the most informative, intermediate exchange regime for each material.
MFMꢀ112aꢀd12: As shown in Fig. 3, the numerical line shape analysis indicates that the rotation of the
phenyl fragment in MFMꢀ112aꢀd12 is realized by a 4ꢀsite jump exchange between four positions given by the
2
following axial angles: φ1 = 40°, φ2 =140°, φ3 =220°, φ4 = 320° (Fig. 4 and Fig. S21). Interestingly the H
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NMR data reveals two dynamically different states for rotation of the linker in MFMꢀ112aꢀd12. In each state
the geometry of the rotation is the same, but the rate at each temperature is different. These two dynamic
phases (states) of MFMꢀ112aꢀd12 coexist between 133 K and 163 K. Below 133 K the material is the phase
p1 (rate k1), while above 163 K it fully switches to phase p2 (rate k2). In both states the corresponding rate
constants k1 and k2 obey the simple Arrhenius law (Fig. 3g) and are characterized by the same activation
barrier (E = 8.6 kJ mol–1). The only difference is in the preꢀexponential (collision) factor, which differs by ~6
times: k1,0 = 3 x 108 s–1, k2,0 = 18 x 108 s–1. The nature of these two states requires additional investigation,
and the change of the dynamic state for the mobile fragment in the linker in MFMꢀ112ꢀd12 is most likely
associated with the change of the steric interactions between the rotating phenylene rings and the central
immobile phenyl ring at different temperatures.46
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Line shape analysis for MFMꢀ112aꢀd12 loaded with CH4 (10 bar of equilibrium pressure above the
MOF at 298 K in a sealed glass NMR cell) (Fig. 3c, d) shows that the linker rotation mechanism remains
unchanged, but the actual rates are affected as the CH4 guest molecules partially block the linker rotation.47
The corresponding rates and temperature dependences are shown in Fig. 3h. It follows that for the low
temperature phase p1 the effect of CH4 presence is reflected in the collision factor, which is smaller by a
factor of ~0.62 relative to the guestꢀfree material. The second phase p2 is also characterized by a smaller
rotation rate compared to the guestꢀfree material, but the barrier is slightly increased up to E~10 kJ mol–1 (k2,0
= 29 x 108 s–1). However, it should be noted that this small increase in activation energy is most likely related
to the partial desorption of CH4 from the pores above 200 K.
MFMꢀ115aꢀd12: As shown in Fig. 5 the line shape develops from a static pattern to a dynamically averaged
one between 203 to 315 K. However, in the static case, the pattern is characterized by a nuclear quadrupolar
coupling constant Q0 = 184 KHz and an asymmetry parameter η0 = 0.06, which is unusual and shows how
strongly the electronic state of the mobile phenyl groups is distorted by the nitrogen core of the ligand. The
averaged pattern is given by a narrowed line shape, as expected for axial rotation of the phenyl groups
around the C2 symmetry axis (Q1 ~20 kHz~Q0/8).48 However, the narrowed patterns are characterized by an
even larger asymmetry parameter η~1, indicating that the geometry of the rotation is different to that in
MFMꢀ112aꢀd12. As observed in the Arrhenius plot (Fig. 5e), the temperature dependence of the exchange
rate constant k is characterized by two regions: below 283 K the motion is characterized by a barrier E = 14
kJ mol–1 and a collision factor k0 = 2 x 108 s–1; above 283 K the barrier increases to E = 40 kJ mol–1 and a
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collision factor k0 = 5 x 1012 s–1. The mechanism of rotation is given by a 4ꢀsite jump rotation, but above 283
K the 6ꢀsite jump rotation gives slightly better agreement with the experimental patterns (Fig. S21). The
positions for the 4ꢀsite rotation are given by the following axial angles: φ1 = 42°, φ2 = 138°, φ3 = 222°, φ4 =
318°; for the positions for the 6ꢀsite rotation: φ1 = 42°, φ2 = 90°, φ3 = 138°, φ4 = 222°, φ5 = 270°, φ6 = 318°.
Such drastic changes in both activation barrier and the preꢀexponential factor indicate that there are
two possible rotational stochastic mechanisms that compete, with the 2H NMR line shape reflecting the faster
motion. In MFMꢀ115aꢀd12 the three rotating phenyl groups are bound to a single Nꢀcentre and hence their
steric interaction is expected to be notably stronger than in MFMꢀ112aꢀd12. Thus, the individual rotation of
each phenyl ring is expected to be characterized by a high activation energy and a normal preꢀexponential
factor of ~1012 s–1. However, since the three phenyl groups create a molecular “gearꢀlike” mechanism, we
might expect potential correlated rotation. In such a case the overall activation barrier might be expected to
be much lower compared to individual rotations. However, the chance for such cooperative motion to occur,
(i.e., the number of attempts) is much lower as well. Therefore, the lowꢀenergy mode is characterized by a
much lower preꢀexponential factor of ~108 s–1. To our knowledge, this is the first example of direct
observation of how the cooperative molecular rotations in MOFs are switched to an individual motif.
The CH4ꢀloaded sample of MFMꢀ115aꢀd12 (10 bar of equilibrium pressure above the MOF at 298 K
in a sealed glass NMR cell) showed identical line shape and temperature dependence compared to the guestꢀ
free material. The corresponding rates constants are shown on Fig. 5f. Thus, the effect of CH4 for the
phenylene ring rotation in MFMꢀ115aꢀd12, even at elevated pressure is negligible, probably reflecting the
higher temperatures required for rotational mobility in MFMꢀ115aꢀd12 compared with MFMꢀ112aꢀd12.
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MFMꢀ132ꢀd24: The H NMR spectrum is composed of two signals, which correspond to geometrically
different C–D groups on the anthracene (Fig. 6e, f), the line shape (Fig. 6) of which does not change from
203 K up to 525 K. However, these bulky anthracene groups are also different in terms of their capability to
sense dynamics. The angle between the C–D bond and the rotational axis (along the bonding axis) is 60
degrees for H at positions 2, 3, 6 and 7, i.e., the same as for a typical phenyl group, and is 0 degree for the C–
D groups at positions 1, 4, 5, and 8. The latter will not sense any motion around the rotational axis due to its
angular configuration.48 Therefore, the line shape splits into a pattern with Q1 = 153 KHz and an asymmetry
parameter η1 = 0.08 for the C–D groups at 2, 3, 6 and 7 positions, with pattern with typical static parameters
Q2 = 176 KHz and η2 = 0.0 from C–D groups at 1, 4, 5, and 8 positions. Since the line shapes do not change
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within this temperature range, no large amplitude motions are present. But the slightly averaged pattern of
the terminal C–D group indicates that the anthracene fragments are in fact involved in fast but limited (ꢁφ =
32°) angular librations. Similar librations of organic groups in porous materials has been observed recently
for ZIFꢀ8.49 The CH4ꢀloaded sample of MFMꢀ132aꢀd24 (10 bar of equilibrium pressure above the MOF at
298 K in a sealed glass NMR cell), as for MFMꢀ115aꢀd12, shows identical line shape and temperature
dependence as the guestꢀfree material.
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NMR summary
Comparison of the NMR results shows that the least stericallyꢀhindered phenylene rings in MFMꢀ
112aꢀd12 are involved in full 360° rotation around the C2 symmetry axis and are static only for T < 100 K; the
intermediate region occurs around 153 K and these rings show fast motion at T > 203 K. In the case of
MFMꢀ115aꢀd12, the full axial rotation of the deuterated phenylene rings persists, but the whole dynamic
range is shifted to higher temperature by ~150 K. Thus, there is no rotation at 200 K, with the intermediate
stage occurring around 300 K with the fast limit reached above 403 K. The greater steric confinement of the
branched phenylene rings in the hexacarboxylate linkers in MFMꢀ115aꢀd12 compared to MFMꢀ112aꢀd12 is
well described by the activation barriers of the rotation: 40 kJ mol–1 for MFMꢀ115aꢀd12 versus 8.6 kJ mol–1
for MFMꢀ112aꢀd12. This is also consistent with the geometric environment of the branched phenylene rings
in MFMꢀ112 and MFMꢀ115 obtained from the singleꢀcrystal Xꢀray structural analysis. Two distances are
thus defined: d1 is the distance between the closest hydrogens of the mobile fragment and the nonꢀrotating
central core in the ligand face, while d2 is the distance between the closest hydrogens of two neighboring
mobile fragments (Fig. 4). When the central phenyl core in the hexacarboxylate unit in MFMꢀ112 is replaced
with a nitrogen atom in MFMꢀ115, the shortest possible distance between the phenylene hydrogens decreases
from 1.9 Å (d1 in MFMꢀ112) to 0.8 Å (d2 in MFMꢀ115), thereby creating a considerably tighter confinement
for the phenylene ring rotation in MFMꢀ115.
In contrast, the NMR analysis of MFMꢀ132aꢀd24 confirms that no full rotation of the anthracene
rings is possible due to the steric hindrance of neighbouring anthracene rings in this (3,24)ꢀconnected
network. Indeed, no notable motion was detected up to the thermal decomposition of the material at T > 567
K. However, the observed line shapes show that the anthracene rings are not completely static, being
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involved in fast but limited angular librations. Thus, for MFMꢀ132a, the steric restriction is sufficient to
render the anthracene group almost immobilized in a wellꢀdefined position.
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In situ 2H NMR spectra for the three samples loaded with CH4 were collected to probe the effect of
CH4 adsorption on the rotational molecular dynamics of the frameworks. The line shape analysis indicates
that the ligand rotation in MFMꢀ112aꢀd12 is affected by the adsorption of CH4, revealing a reduced rate of
rotation. This indicates that CH4 interacts with linkers by random collision, and inhibits the rotation in the
range T = 123ꢀ203 K. In contrast, addition of CH4 to MFMꢀ115aꢀd12 (or to MFMꢀ132aꢀd24, where there is
only libration) causes no observed changes to the rotation of the phenyl groups. Rotation for MFMꢀ115aꢀd12
occurs over the range T= 203ꢀ403 K, much higher than for MFMꢀ112aꢀd12, and it appears that rotation of
phenyl groups at this elevated temperature will therefore not be effected by the relatively low energy
collisions with CH4 molecules.
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Neutron powder diffraction (NPD) studies. The locations of adsorbed molecules within MFMꢀ
132a and MFMꢀ115a were determined by in situ NPD as a function of CD4 loading. NPD patterns were
recorded at 10 K for the desolvated materials and at loadings of 0.25 and 0.5 CD4/Cu for MFMꢀ132a and of
1.0 and 1.5 CD4/Cu for MFMꢀ115a (see Supporting Information Section 6). MFMꢀ132a shows highly
hindered cage geometry and restricted molecular dynamics owing to the anthracene functionalisation, and
was therefore selected for the investigation of CH4 binding at low loading where adsorption is dominated by
pore geometry and surface chemistry. In parallel, CH4 binding at higher surface coverage was studied in
MFMꢀ115a, which displays the record high CH4 storage capacity, in order to probe the distribution of CH4
within the structure to a wider extent. Fourier difference map analysis of the NPD data of the desolvated
MOFs indicates no residual nuclear density peak in the pore, thus confirming the effective activation and
structural stability of the desolvated samples. Upon loading of the targeted amount of CD4 into the
desolvated MOFs, sequential Fourier difference map analysis revealed the position of the centre of mass of
the adsorbed CD4 molecules, which were further developed by Rietveld refinement of these data. Analysis of
lattice parameters of gasꢀloaded MOFs confirms the absence of notable structural changes.
For MFMꢀ132a at 0.25 CD4/Cu, Rietveld analysis revealed four distinct CD4 binding sites, the
strongest of which was located in the [(Cu2)3(isophthalate)3] window pocket (denoted site A1; Fig. 4). The
cavity formed by three isophthalate rings and the enclosed anthracene moieties shows an internal void with a
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diameter of only ~6 Å, which can accommodate only one CD4 molecule; this molecule sits on the C3ꢀaxis of
the pocket with a distance of 3.88(1) Å from the centre of the isophthalate ring. The three anthracene units at
the bottom of the pocket further stabilize the CD4 molecule via strong space confinement with a close
distance of 2.41(1) Å between D atoms and H atoms on anthracene rings. Significantly, nearly 50% of
loaded CD4 is found on site A1, confirming the presence of strongly geometricalꢀhindered binding sites in
MFMꢀ132a. In addition, ~40% of loaded CD4 is found equally distributed on the two open Cu(II) sites with
one protruding into the cuboctahedral cage while the other pointing outside the cage, denoted as the second
(A2) and third (A3) binding sites, respectively. Thus, in the case of MFMꢀ132a, a tight pocket constructed
solely from aromatic rings that matches the symmetry of a CD4 molecule shows higher affinity for CD4 than
open Cu(II) sites. Importantly, the open Cu(II) centres in MOFs are found not to be the strongest binding site
for adsorbed methane molecules, and this may be in part due to the introduction of steric hindrance within
the pores of the MOF.
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The fourth adsorption site (A4) is located inside the cuboctahedron and near the
[(Cu2)3(isophthalate)3] window with a distance of 3.56(6) Å to the A1 site and accounts for ~10% of loaded
CD4. The CD4 molecule on this site also sits on the 3ꢀfold axis of the triangular window. A4 also shows close
contacts [CD4 (centroid)•••CD4 (centroid) = 3.84(6) Å] with the CD4 molecules on site A2. The CD4 on site
A4 therefore occupies the pocket created by the three CD4 molecules adsorbed on the three closest open
Cu(II) sites of the triangular window and the encapsulated CD4 within the small 6 Å cavity (site A1),
indicating the presence of intermolecular adsorbate–adsorbate interaction that stabilises the packing of
methane molecules in the structure. No additional site was identified at the higher loading of 0.5 CD4/Cu,
where the occupancies for all four binding sites increase proportionately. Notably, the A1 site occupancy
remains the highest of the four sites identified, further confirming the strong affinity between CD4 molecules
and this tight pocket.
The NPD study of MFMꢀ115a at both 1.0 and 1.5 CD4/Cu loadings revealed four binding sites
(denoted as A1’, A2’, A3’ and A4’ in the order of decreasing occupancy; Fig. 5). A1’ and A2’ are found
within the triangular [(Cu2)3(isophthalate)3] window, and A3’ and A4’ are located on the open Cu(II) sites.
At the first loading, site A1’ is located on the 3ꢀfold axis of the [(Cu2)3(isophthalate)3] window. This is not
surprising because site A1’ sits in the tight pocket created by three CD4 molecules on site A3’ and one CD4
on site A2’, indicating the presence of strong intermolecular dipole interaction. Site A2’ also resides on the
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3ꢀfold axis of the small triangular window, having a similar location as the strongest adsorption site A1
observed in MFMꢀ132a, but with a longer distance of 4.34(3) Å to the centre of the isophthalate ring. The
reduction in binding affinity of A2’ in MFMꢀ115a is a direct result of the absence of a tight cavity formed by
organic moieties as found in MFMꢀ132a. To our surprise, the two types of open Cu(II) sites in MFMꢀ115a
have distinct occupancies, with the one located inside the cuboctahedral cage showing almost five times of
the occupancy as the other one outside this cage. This observation indicates that 12 methane molecules inside
the cuboctahedral cage show a compact geometry in MFMꢀ115a. The two opposite [Cu2(O2CR)4]
paddlewheels in cage D are separated by a shorter distance in MFMꢀ115a than in MFMꢀ132a due to the
smaller size of the hexacarboxylate ligand in the former. This effectively accounts for the difference of CD4
binding behaviour on the open Cu(II) sites in these two structures. Thus, the distance between two opposite
Cu(II) sites in Cage D in MFMꢀ115a is 3.95(5) Å, much shorter than that [6.83(5) Å] observed in MFMꢀ132a.
At the 1.5 CD4/Cu loading, additional CD4 molecules are mainly populated across sites A1’, A2’ and A3’,
further indicating a unique and optimized CD4 packing geometry in MFMꢀ115a.
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Conclusions
A family of isostructural (3,24)ꢀconnected frameworks, MFMꢀ112a, MFMꢀ115a and MFMꢀ132a,
show interesting and distinct CH4 adsorption properties. The overall structures of these materials are
constructed by alternate packing of four types of metal–organic coordination cages with varying geometry
and sizes. Specifically, MFMꢀ132 contains a type of highly geometricallyꢀhindered cages (diameter of ~6 Å)
because of the anthracene functionalisation. MFMꢀ112a, MFMꢀ115a and MFMꢀ132a possess high, moderate
and relatively low porosity, respectively. Significantly, MFMꢀ115a displays exceptionally high deliverable
CH4 storage capacity [208 v/v (5–80 bar)] at room temperature in comparison to other best performing
porous solids reported to date. MFMꢀ112a also reveals excellent CH4 adsorption capacity at high pressure
owing to its high surface area and pore volume. In contrast, MFMꢀ132a shows relatively low CH4 storage
capacity. Thus, there is a direct correlation between the structure design and materials function across this
series of MOFs.
The molecular motions of the rotational aromatic rings in the corresponding partially deuterated
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materials, MFMꢀ112aꢀd12, MFMꢀ115aꢀd12 and MFMꢀ132aꢀd24 were investigated using in situ solidꢀstate H
NMR spectroscopy. The results reveal that the branched phenylene ring rotation in MFMꢀ115aꢀd12 shows an
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energy barrier which is almost five times higher than that in MFMꢀ112aꢀd12. The anthracene ring in MFMꢀ
132aꢀd24 shows very limited angular librations within its confinement, which allows the anthracene rings to
form stable wellꢀdefined cavities within the framework. The CH4 loading affects the ligand rotation in MFMꢀ
112aꢀd12 slightly, but does not pose any notable hindrance for the ligand mobility in MFMꢀ115aꢀd12 and
MFMꢀ132aꢀd24. The NMR study confirms that MFMꢀ112a, MFMꢀ115a and MFMꢀ132a have fast, medium
and slow molecular dynamics, respectively. Investigations on the binding sites for CD4 within MFMꢀ132a
and MFMꢀ115a reveal that the primary binding site is located within the small pocket enclosed by the
[(Cu2)3(isophthalate)3] window and three anthracene/phenyl panels. This pocket shows strong van der Waals
interactions with CD4 due to the small cavity size, and has an excellent size and geometric match for a single
CD4 molecule. The open Cu(II) sites are the secondary/tertiary adsorption sites in these structures. Thus,
direct experimental evidence has been obtained showing that a tight cavity can generate a stronger binding
affinity to gas molecules than open metal sites, which are widely reported as the primary binding sites in
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various MOFs. Thus, the NPD coupled with H NMR confirm that a combination of optimum molecular
dynamic and pore geometry/size leads to the interesting CH4 adsorption performance in these materials.
Experimental Section
Synthesis of MFM-112 and MFM-115. MFMꢀ112 and MFMꢀ115 were synthesized following the
methods adapted from previous reports.22,24b Specifically, a solution of H6TDBB (1,3,5ꢀtris(3',5'ꢀ
dicarboxy[1,1'ꢀbiphenyl]ꢀ4ꢀyl)benzene) (or H6NTBD = 4',4'',4'''ꢀnitrilotribiphenylꢀ3,5ꢀdicarboxylic acid, 0.1
mmol, 1.0 equiv.) and Cu(NO3)2•2.5H2O (93 mg, 4.0 equiv.) in N,Nꢀdimethylformamide (DMF, 40 mL) and
H2O (4 mL) was placed in a 100 mL pressure flask. Then 0.5 mL 2M HCl was added to the mixture and the
resultant solution was sealed and heated at 90 °C in an oil bath for 24 h. The obtained crystals were filtered
and washed with DMF (20 mL × 2). The asꢀsynthesized materials were exchanged in acetone for 3 days
before being activated for gas sorption experiments.
Synthesis of [Cu3(BTAT)(H2O)3]•9DMF (MFM-132). H6BTAT (50 mg, 0.045 mmol) (see
Supporting Information Section S1) and Cu(NO3)2•2.5H2O (63 mg, 0.27 mmol) were dissolved in a mixture
of DMF (8.0 mL) and H2O (0.5 mL) and the solution placed in a pressure tube (15 mL). Upon addition of 6
M HCl (15 ꢂL), the tube was capped and heated at 90 °C for 16 h. A large amount of microcrystalline
product precipitated: the blue crystals were collected by filtration and washed with warm DMF and dried in
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air. Yield: 77 mg (85%). Selected FTIR (neat, cm−1): 1655 (vs), 1632 (vs), 1587 (w), 1493 (w), 1434 (s),
1366 (vs), 1299 (w), 1253 (m), 1195 (w), 1149 (w), 1095 (s), 1060 (w), 1027 (w), 942 (w), 922 (w), 864 (w),
774 (vs), 734 (m), 701 (m), 660 (s), 648 (m). Anal. Calcd (%) for C99H105Cu3N9O24: C, 59.59; H, 5.30; N,
6.32. Found (%): C, 60.38; H, 5.31; N, 6.78.
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High-Pressure CH4 Adsorption Measurements. CH4 sorption measurements (0–90 bar) were
performed using a Xemis gravimetric adsorption apparatus (Hiden Isochema, Warrington, UK) equipped
with a clean ultrahigh vacuum system. The pressure in the system is accurately regulated by mass flow
controllers. All measurements were made with ultraꢀhighꢀpurity grade (99.999%) CH4 or He, the latter being
used for skeletal volume measurements. Sample containers of a known weight were loaded with ~100 mg of
desolvated sample under an argon atmosphere and the samples were further degassed at 100 °C for 16 h
before adsorption of CH4.
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Solid-State H NMR spectroscopy. To prepare samples for the NMR experiments, 50–70 mg of
partially deuterated MOF was loaded as a fine powder into a 5 mm (o.d.) glass tube and connected to a high
vacuum line. The sample was heated at 100 °C for 24 h under vacuum to a final pressure above the sample of
10–2 Pa to ensure removal of any remaining traces of guest molecules. The neck of the tube was then flameꢀ
sealed, while the sample was maintained in liquid nitrogen in order to prevent the heating from the flame.
The sealed sample was then transferred into an NMR probe for analysis with 2H NMR spectroscopy.
2
ω
z/2π
= 61.42 MHz on a Bruker
H NMR experiments were performed at the Larmor frequency
Avanceꢀ400 spectrometer using a high power probe with 5 mm horizontal solenoid coil. All 2H NMR spectra
were obtained by Fourier transformation of quadratureꢀdetected phaseꢀcycled quadrupole echo arising in the
pulse sequence
, where τ1 = 20 ꢂs, τ2 = 21 ꢂs and t is a repetition time of
the sequence during the accumulation of the NMR signal.50 The duration of the π/2 pulse was 1.6 ꢂs. Spectra
were typically obtained with 1000–20000 scans with a repetition time ~0.4 seconds. The temperature of the
samples was controlled with a flow of N2 gas using a variableꢀtemperature unit BVTꢀ3000 with a precision
2
of ~1 K. The H NMR spectra lineꢀshape simulations were performed using a homemade FORTRAN
program package based on the general formalism given in the Supporting Information Section S5.
Neutron Powder Diffraction (NPD). NPD measurements were performed on the bare MOF and the
same sample loaded with CD4 using the highꢀresolution powder diffractometer WISH at the ISIS pulsed
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neutron source, Rutherford Appleton Laboratory, UK. Prior to NPD experiments, the desolvated sample of
MFMꢀ132a (1.4 g) or MFMꢀ115a (1.8 g) was loaded into a cylindrical vanadium sample container with an
indium ring vacuum seal and connected to a gas handling system. The sample was further degassed at 10–7
mbar and 100 °C for 24 h to remove any remaining trace guest solvents. The temperature during data
collection was controlled using a helium cryostat (10 ± 0.2 K). The loadings of CD4 were performed by
volumetric method at 150 K in order to ensure that CD4 was present in the gas phase when not adsorbed and
also to ensure sufficient mobility of CD4 inside the crystalline structure. After collecting the NPD data for the
bare material, target amounts of CD4 were introduced from the gasꢀpanel system. The sample was then
slowly cooled (over a period of 2 h) to 10 K to ensure that CD4 was completely adsorbed. Sufficient time
was allowed to achieve thermal equilibrium before data collection. Diffraction data as a function of neutron
timeꢀofꢀflight were collected in five detector banks centred at 2θ = 27.1, 58.3, 90.0, 121.7, and 152.9°.
Due to the large unit cell of these two frameworks, rigid bodies were applied to the organic ligand
and the CD4 molecules in the Rietveld refinements. Differential nuclear scattering Fourier maps were used to
locate the adsorbed CD4 molecules. The refinements on all the parameters including fractional coordinates,
occupancies for the adsorbed CD4 molecules, and background/profile coefficients yielded satisfactory
agreement factors. The total occupancies of CD4 molecules obtained from the refinement are also in good
agreement with the experimental values for the CD4 loading. The refined structural parameters including the
refined positions and orientations of the CD4 molecules are detailed in the Supporting Information Section
S6.
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Supporting Information
Supporting information is available free of charge on the ACS Publications website. Synthetic details for
2
organic linkers and partiallyꢀdeuterated compounds; Gas adsorption data; Solidꢀstate H NMR and neutron
powder diffraction analysis. CIF files can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif (CCDC 1499823–1499831).
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Author Information
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Corresponding Author
*Sihai.Yang@manchester.ac.uk
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*M.Schroder@manchester.ac.uk
Notes
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The authors declare no competing financial interests.
Acknowledgements
We thank the EPSRC, the Universities of Manchester and Nottingham for funding. We are grateful to STFC
and the ISIS Neutron Facility for access to Beamline WISH and to Diamond Light Source for access to
Beamlines I11 and I19. M.S. gratefully thanks the ERC for an Advanced Grant (AdG 226593) and EPSRC
(EP/I011870) for funding and acknowledges the Russian Ministry of Science and Education for the award of
a Russian Megagrant (14.Z50.31.0006). M.S. and D.I.K. gratefully acknowledge receipt of the Royal Society
International Exchanges Scheme grant ref. IE150114. D.I.K. and A.G.S. acknowledge financial support
by Russian Academy of Sciences (budget project No. 0303ꢀ2016ꢀ0003 for Boreskov Institute of Catalysis).
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Figure 1. View of the chemical structure of the ligands and crystal structures of the isostructural MFMs series. (a) The
three hexacarboxylate ligands for the construction of MFMꢀ112, MFMꢀ115 and MFMꢀ132, respectively; (b) In the
(3,24)ꢀconnected network, the linker with C3ꢀsymmetry is connected to three cuboctahedra (Cage A). The MFMꢀ132
case is shown as an example. (c) The different cage structures (Cage B, C and D) in the above three frameworks. The
colours for the spheres which fit into the void of different types of cages are orange for Cage B, yellow for Cage C and
green for Cage D.
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a
b
Figure 2. Comparison of highꢀpressure CH4 adsorption data for the bestꢀperforming MOFs. (a) Highꢀpressure
volumetric CH4 adsorption isotherms for MFMꢀ115a, MFMꢀ112a and MFMꢀ132a in the pressure range 0–90 bar at 298
K. (b) Comparison of the deliverable CH4 capacity for a range of MOFs at 298 K.
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Table 1. Comparison of CH4 adsorption data for a variety of MOFs at 298 K.
5
6
7
8
9
deliverable
total CH4
deliverable
deliverable
CH4 capacity
(5 to 80 bar)
BET
total uptake
at 35 bar
total uptake
at 80 bar
pore
crystal
density
(g/cm3)
CH4 capacity uptake at 65 CH4 capacity
(5 to 35 bar)
surface
area
material
volume
bar
(5 to 65 bar)
(cm3/g)b
(m2/g)
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v/v
v/v
v/v
v/v
v/v
v/v
MFM-115a
MFM-112a
MFM-132a
Co(bdp)38
MOF-90540
Al-soc-MOF-141
3394
3800
2466
2911a
3490
5585
1.38
1.62
1.06
0.611
0.503
0.65
186
138
238
191
256
208
162
162
125
109
218
201
181
150
236
213
200
162
1.02
1.34
2.3
0.774
0.537d
0.34
161
145
127
155
120
106
203
206
197
197
181
176
ꢀ
ꢀ
228
221
203
201
MOF-51935,40
UTSA-76a39
HKUST-17,31
PCN-147,31,36
2400
2820
1850
2000
0.938
1.09
0.78
0.85
0.953
0.699
0.883
0.829
200c
211
227
195
151c
151
150
128
260c
257
267
230
209c
196
190
157
279c
230c
ꢀ
ꢀ
272
250
200
178
NU-125
(NOTT-
3286
1.41
0.589
182
135
232
183
ꢀ
ꢀ
122a)24a,31,37
Ni-MOF-
747,29,31
1350
3971
0.51
1.12
1.206
0.595
228
151
106
110
251
199
129
158
267
216
152
175
Cu-tbo-MOF-
542
MOF-17743
MOF-21043
4500
6240
1.89
3.6
0.427
0.25
122
82
102
69
ꢀ
ꢀ
205
166
185
154
141
128
aLangmuir surface area. bpore volume was measured by N2 adsorption isotherms at 77 K. cThese values are likely overestimated due to difficulty in controlling composition of MOFꢀ
519.40 dCrystal density determined from pycnometer density data.
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Figure 3. Temperatureꢀdependent 2H NMR line shapes for the deuterated phenyl fragments in guestꢀfree MFMꢀ112aꢀd12
– (a) experimental and (b) simulation; MFMꢀ112aꢀd12 loaded with CH4 (10 bar at 298 K) – (c) experimental and (d)
2
simulation. (e) The H NMR spectrum for the guestꢀfree MFMꢀ112aꢀd12 comprising of (f) two dynamic phases at T
=153 K. (g) Arrhenius plots of the rotation rate constants k1 (□) and k2 (○) for the two corresponding phases of the guestꢀ
free MFMꢀ112aꢀd12 and (h) k1 (∇) and k2 (ꢁ) for MFMꢀ112aꢀd12 loaded with CH4 at 10 bar and 298 K.
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Figure 4. View of the rotational models probed by solidꢀstate H NMR spectroscopy for the partially deuterated MFM
series in this study: (a) MFMꢀ112aꢀd12, (b) MFMꢀ115aꢀd12 and (c) MFMꢀ132aꢀd12.
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Figure 5. Temperatureꢀdependent H NMR line shapes for the deuterated phenyl fragments in: guestꢀfree MFMꢀ115aꢀ
d12 – (a, c) experimental and (b, d) simulation. (e) Arrhenius plots of the rotation rate constant k1 (□) of the guestꢀfree
MFMꢀ115aꢀd12 and (f) k1 (ꢁ) for the MFMꢀ115aꢀd12 framework loaded with CH4 at 10 bar and 298 K.
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Figure 6. Variable temperature H NMR line shapes for MFMꢀ132ꢀd24 – (a) experimental and (b) simulation. The H
NMR spectrum of MFMꢀ132ꢀd24 (c) experimental (d) simulation is composed of two signals (e and f), which
correspond to geometrically different C–D groups on the anthracene fragment. The green spheres represent deuterium
atoms.
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Figure 7. CD4 adsorption sites revealed by Rietveld analysis of the NPD data for MFMꢀ132a with CD4 dosing at
0.25/0.5 CD4/Cu. (a) The strongest CD4 binding site is located in the small pocket created by [(Cu2)3(isophthalate)3]
window and three anthracene rings. There are four of this type of pockets in a truncated tetrahedron (Cage B); (b) The
four binding sites within the partial structure showing the Cage A and Cage B, both sharing the triangular
[(Cu2)3(isophthalate)3] window; (c) Side and (d) topꢀdown view of the tight pocket created by the [(Cu2)3(isophthalate)3]
window and three anthracene rings. The two Cu(II) ions on the same [Cu2(O2CR)4] paddlewheel show no discrepancy
for CD4 binding. Colour scheme: C, teal; H, grey; Cu, aqua; site A1, violet; site A2 and A3, pink; site A4, orange.
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Figure 8. CD4 sites in MFMꢀ115a revealed by neutron powder diffraction at loading of 1.0 CD4/Cu. (a) CD4 sites in the
cuboctahedral cage showing CD4 molecules are compacted within this cage. (b) CD4 sites within the partial structure of
a tetrahedral cage. Due to the shorter distance between the two opposite [Cu2(O2CR)4] paddlewheels in cage D (on the
edge of the tetrahedron) in MFMꢀ115a compared with that in MFMꢀ132a, the two CD4 sites (A4’) show a close contact
of 3.95(5) Å. (c) A close view of the four binding sites. Site A1’ sits within the tight pocket creates by three CD4 on site
A3’ and one CD4 on site A2’, indicating an optimum packing geometry of CD4 molecules in MFMꢀ115a. (d) Topꢀdown
view of the small triangular window. Colour scheme: C, teal; H, grey; Cu, aqua; site A1’, orange; site A2’, green; site
A3’ and A4’, pink.
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