Unique gas and hydrocarbon adsorption in a highly porous metal-organic framework made of extended aliphatic ligands

Article abstract of DOI:10.1039/b814498b

High and unique gas and hydrocarbon adsorption in a highly stable guest-free microporous metal-organic framework constructed on rigid aliphatic ligands, H₂bodc and ted, is reported in this work. The Royal Society of Chemistry.

Full text of DOI:10.1039/b814498b

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Unique gas and hydrocarbon adsorption in a highly porous metal-organic
framework made of extended aliphatic ligandsw
Kunhao Li, JeongYong Lee, David H. Olson, Thomas J. Emge, Wenhua Bi,
Matthew J. Eibling and Jing Li*
Received (in Austin, TX, USA) 20th August 2008, Accepted 28th September 2008
First published as an Advance Article on the web 15th October 2008
DOI: 10.1039/b814498b
High and unique gas and hydrocarbon adsorption in a highly
stable guest-free microporous metal-organic framework con-
structed on rigid aliphatic ligands, H2bodc and ted, is reported
in this work.
and adamantane6 based carboxylic acids, the synthesis of
H2bodc is quite straightforward and was successfully scaled
up to afford over 5 g/batch of pure H2bodc, which greatly
assisted the subsequent explorative and preparative studies on
crystalline metal-organic framework materials. Other impor-
tant factors, in addition to its rigidity, were also taken into
consideration in the selection of H2bodc as the ligand of
interest. For example, its bulky rod-like bicyclooctane frag-
ment, a structural feature also found in triethylenediamine
(ted), is likely to prevent interpenetration and thus to max-
imize pore space.
Recent progress in the research of microporous metal-organic
frameworks (MMOFs) has demonstrated the great potential
of this new type of multifunctional materials in molecular
storage, separation, catalysis, and sensing.1 Design and synth-
esis of targeted MMOFs through rational selection and com-
bination of organic ligands and single metal centers or metal
clusters (often called secondary building units or SBUs) have
been the main focus and driving force of this relatively new
and fertile research field.2 Aromatic carboxylic acids, such as
benzene-1,4-dicarboxylic acid, naphthalene-2,6-dicarboxylic
acid and biphenyl-4,40-dicarboxylic acid, are examples of the
most common ligands of choice because their excellent che-
mical stability and structural rigidity help sustain the increas-
ingly porous structures resulting from the progressively
extended distances between the ligating carboxylate groups.
In stark contrast, stable MMOFs that are solely built upon
extended aliphatic ligands are rare. This is a direct result of the
scarcity of rigid extended aliphatic carboxylic acids (or the
corresponding carboxylates). Due to the aliphatic nature of
these ligands, the gas and hydrocarbon adsorption behavior
and mechanism are expected to be generally different from
those of the MMOFs based on unsaturated aromatic carboxyl-
ates (and/or nitrogen-containing molecules, e.g. bipyridyl).
Among the few MOF structures based on extended aliphatic
carboxylate ligands,3 no attempts have been made to study
their gas and hydrocarbon adsorption properties. Therefore,
this work on a new type of extended aliphatic ligand based,
guest-free porous [Ni(bodc)(ted)0.5] (1) structure serves, to the
best of our knowledge, as the first of its kind.
When H2bodc was reacted with nickel(II) nitrate hexa-
hydrate and ted under solvothermal conditions,7 a highly
crystalline light-green powder was obtained (1) in excellent yield
(98%). While attempts of growing crystals of 1 with suitable
quality for single-crystal X-ray diffraction were unsuccessful,
single crystal data of the similarly grown [Co2(bodc)2(ted)]ꢀ
3.5DMF (2) crystals were indeed obtained.z In spite of the
difficulty encountered due to the highly disordered and
twinned nature of the crystals of 2 (see ESIw), a three-dimen-
sional framework structure was revealed, which features a
pillared-layer topology similar to that of [Zn(bdc)(ted)0.5].8
The bodc ligands are connected by the paddle-wheel like
Co2(COO)4 SBUs to form distorted square-grid like layers.
The ted ligands coordinate to the paddle-wheels along the
axial directions (coincident with c axis), pillaring the layers
into the overall 3D structure (Fig. 1). The three-dimensionally
interconnected pores, as calculated by PLATON,9 occupy
B49% of the volume of the crystal. Powder X-ray diffraction
study (Fig. 1) confirmed that 1 is iso-structural to 2. Elemental
microanalysis of a freshly made sample of 1, in conformity
with the thermogravimetic analysis results described below,
not only established the proper composition but also uncov-
ered its guest-free nature. A pore volume of 0.47 cm3 gꢁ1, a
BET and Langmuir surface area of 958 and 1244 m2 gꢁ1
,
Bicyclo[2.2.2]octane-1,4-dicarboxylic acid (H2bodc, Scheme
S1 in ESIw), a rod-like dicarboxylic acid, was synthesized via a
4-step synthetic route4 in good overall yields, starting from the
commercially available dimethyl 2,6-diketocyclohexane-1,4-
dicarboxylate. Compared to the syntheses of the cubane5
respectively, and a distribution of pore size around 7.0–7.3 A
(B7.8 A for [Zn(bdc)(ted)0.5]10), were deduced from the argon
adsorption isotherms of 1 (see ESIw).
Thermogravimetric analysis indicated that 1 is highly stable.
No significant weight loss was observed until the compound
began to decompose at around 450 1C, suggesting that no
solvent molecules were encapsulated in the framework during
the crystallization process (see ESIw). The high stability of 1
was also evidenced by the hundreds of the repeated hydro-
carbon adsorption/desorption isotherm measurement cycles at
elevated temperatures (typically, 30 to 150 1C) over an
extended period of more than two months. The stability of 2
Department of Chemistry and Chemical Biology, Rutgers University,
Piscataway, NJ 08854, USA. E-mail: jingli@rutgers.edu;
Fax: (+001)732-4455312
w Electronic supplementary information (ESI) available: Details of the
synthesis of H2bodc, experimental procedures, PXRD patterns, TGA,
gas and hydrocarbon adsorption/desorption results, selected crystal-
lographic data. CCDC 698828. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/b814498b
ꢂc
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Chem. Commun., 2008, 6123–6125 | 6123

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Fig. 1 Top: the distorted square-grid like layer of [Co(bodc)] viewed
along the c axis (left) and the overall 3D structure of 2 with one cage
shown in ball-and-stick model and the remainder of the framework in
gray stick model (right). The space inside one cage is highlighted by the
yellow sphere (Co: green square pyramids; C: gray; N: blue; O: red;
only one set of disordered carbon atoms of bodc and ted are shown;
hydrogens were omitted for clarity). Bottom: a comparison of the
calculated pattern of 2 (black), and the observed patterns of as-
synthesized 2 (green) and 1.
Fig. 2 Top: comparison of methanol, ethanol and propanol adsorp-
tion isotherms at 30 1C. Bottom: cyclohexane adsorption isotherms.
methanol, ethanol and propanol at 30 1C (Fig. 2, top) clearly
shows that with each added CH2 group, the increased organo-
philic attraction results in higher adsorption at lower pressure.
The isosteric heats of adsorption, Qst, for this series are 52–52,
49–57 and 64–64 kJ molꢁ1 (from low to high loading), respec-
tively, increasing as each additional CH2 group is added (see
ESIw). The results also indicate that these alcohols could be
separated (methanol from ethanol and propanol) by 1 in an
equilibrium-based process such as Pressure Swing Adsorption.12
In contrast to the classical Langmuir adsorption/desorption
behavior for benzene, p-xylene and n-hexane (see ESIw), the
isotherms of cyclohexane (c-hexane) sorption shows particu-
larly interesting features between 50 and 80 1C (Fig. 2, bot-
tom). We propose that the c-hexane isotherms may be
considered as having two regions, pore filling, occurring at
lower temperatures (below 50 1C) and a monolayer region
(above 80 1C). The ‘‘abnormal’’ features observed between
these two temperatures are due to the transition from mono-
layer adsorption to pore filling. The calculated isosteric heats
of adsorption support this description, with Qst linear up to
about 130 mg gꢁ1 with a value of 52 kJ molꢁ1 and then 66 to
73 kJ molꢁ1 in the pore filling region, 145 to 230 mg gꢁ1 (see
ESIw). In the latter region, Qst gradually increases as adsorbate–
adsorbate interactions increase.
(decomposing quickly in air), however, is significantly inferior;
therefore the gas and hydrocarbon sorption properties are
attainable only for 1.
1 exhibits very interesting and unique gas adsorption prop-
erties. It takes up significant amounts of hydrogen at 77 K
(1.29 wt%) and 87 K (0.78 wt%) at 1 atmosphere. At 77 K and
60 bar, the hydrogen amount reaches B2.76 wt% (see ESIw).
This behavior is comparable with MMOFs based on aromatic
ligands. If the differences of their BET surface areas and
pore volumes are taken into consideration (958 m2 gꢁ1 and
0.47 cm3 gꢁ1 vs. 1450 m2 gꢁ1 and 0.75 cm3 gꢁ1), 1 actually
adsorbs comparable amount of hydrogen per unit surface area
(or per unit pore volume) as [Zn(bdc)(ted)0.5] does,10 sugges-
ting that the aliphatic nature of 1 does not diminish its capability
of taking up hydrogen. It is also of interest to note that the
calculated isosteric heats of hydrogen adsorption (Qst, based
on the isotherms at 77 and 87 K, see ESIw) are between 5.7 to
6.5 kJ molꢁ1, somewhat (B1 kJ molꢁ1) higher than those
observed in [Zn(bdc)(ted)0.5] (5.0–5.3 kJ molꢁ1).10 This stronger
adsorbent–adsorbate interaction could be attributed to the
smaller pores found in 1, consistent with similar Qst values found
in other small-pore metal-organic frameworks.11
In summary, a highly stable guest-free microporous metal-
organic framework constructed on rigid aliphatic ligands,
H2bodc and ted, is reported in this work. The use of rigid,
bulky aliphatic ligands effectively prevented the structure from
interpenetration, thus, resulting in a highly porous framework
with three-dimensional pores. [Ni(bodc)(ted)0.5] (1) exhibits
1 also adsorbs significant amounts of selected aromatic and
aliphatic hydrocarbons (Table 1). This capacity, as exemplified
by the n-hexane adsorption at 30 1C (0.41 cm3
g
ꢁ1), is
substantially higher than the 0.26 cm3 gꢁ1 of the widely used
zeolite Y. A comparison of the sorption isotherms for
ꢂc
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Soc. Rev., 2007, 36, 770–818; J. L. C. Rowsell and O. M. Yaghi,
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Table 1 A summary of the hydrocarbon adsorption properties of 1
Weight
adsorbed/mg gꢁ1
Volume
adsorbed/cm3 gꢁ1
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and O. M. Yaghi, J. Am. Chem. Soc., 2000, 122, 11559–11560;
M. Eddaoudi, J. Kim, D. Vodak, A. Sudik, J. Wachter,
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2002, 99, 4900–4904; J. Kim, B. Chen, T. M. Reineke, H. Li,
M. Eddaoudi, D. B. Moler, M. O’Keeffe and O. M. Yaghi, J. Am.
Chem. Soc., 2001, 123, 8239–8247.
Adsorbate
Qst/kJ molꢁ1
n-Hexane
c-Hexane
Benzene
p-Xylene
Methanol
Ethanol
268
254
268
319
260
300
296
0.41
0.33
0.31
0.37
0.33
0.38
0.37
54–73
52–73
44–58
64–88
52–52
49–57
64–64
Propanol
increasingly stronger affinity towards hydrocarbons with de-
creasing polarity. As a consequence, 1 can take up significant
amounts of selected aromatic and aliphatic hydrocarbons,
B50% higher than those observed for the large-pore zeolite
Y. Unique features in the adsorption isotherms of c-hexane
were observed, which indicate a transition from monolayer
adsorption to pore filling. This is the first time, to the best of
our knowledge, that such phenomenon is observed in metal-
organic frameworks. It appears that this kind of behavior is
closely related to the pore size, shape and nature (aliphatic or
aromatic). In depth study and understanding of similarly
unique hydrocarbon adsorption behavior in a few MMOFs
are currently under way and will be reported in the near future.
The authors are grateful to the Department of Energy
(DOE) for the financial support through Grant No. DE-
FG02-08ER46491. J. L. would like to thank Rutgers Univer-
sity for an Academic Excellence Fund (AEF) award for the
purchase of a high pressure gas analyzer.
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7 Solvothermal reactions of Ni(NO3)2ꢀ6H2O (99.9%, 0.185 g, 0.64
mmol) with H2bodc (0.114 g, 0.58 mmol) and ted (97%, 0.163 g,
1.41 mmol) in N,N-dimethylformamide (DMF, 20 mL/ 0.1 mL
conc. HNO3 added) at 100 1C for 4 days afforded a light-green
crystalline powder of Ni-bodc-ted. The product was vacuum
filtered, and rinsed with DMF (10 mL) and diethyl ether (10 mL)
(0.177 g, 98% yield based on H2bodc). Elemental analysis (QTI,
NJ): Calcd. for [Ni2(bodc)2(ted)] or C26H36ON2Ni2O8: C, 50.23%;
H, 5.83%; N, 4.50%; Found: C, 49.95%; H, 5.90%; N, 4.35%.
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Notes and references
z Crystal data for 2: Co2C36.5H60.5N5.5O11.5, Mr = 878.26, tetragonal,
space group P42212 (no. 94): a = 15.2597(6), c = 19.092(1) A,
V = 4445.7(4) A3, Z = 4, Dc = 1.312 g cmꢁ3, m(Mo-Ka) = 0.806.
All measurements were made on a SMART CCD area detector at
296(2) K with graphite-monochromated Mo-Ka radiation. A total of
47 325 reflections were collected (5522 unique, R(int) = 0.0368)
between a y of 1.711 to 28.281 (99.5% completeness). R1 = 0.053
(I 4 2s(I)), wR2 = 0.185 (all data) and GOF = 1.002 (all data).
Largest diff. peak and hole 1.050 and ꢁ1.202 e Aꢁ3 (see ESIw for
details on the handling of psudosymmetry).
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ꢂc
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Chem. Commun., 2008, 6123–6125 | 6125