Tuning the adsorption properties of isoreticular pyrazolate-based metal-organic frameworks through ligand modification

Article abstract of DOI:10.1021/ja305267m

Two isoreticular series of pyrazolate-based 3D open metal-organic frameworks, MBDP-X, adopting the NiBDP and ZnBDP structure types [H ₂BDP = 1,4-bis(1H-pyrazol-4-yl)benzene], were synthesized with the new tagged organic linkers H₂BDP-X (X = -NO₂, -NH ₂, -OH). All of the MBDP-X materials have been characterized through a combination of techniques. IR spectroscopy proved the effective presence of tags, while X-ray powder diffraction (XRPD) witnessed their isoreticular nature. Simultaneous TG/DSC analyses (STA) demonstrated their remarkable thermal stability, while variable-temperature XRPD experiments highlighted their high degree of flexibility related to guest-induced fit processes of the solvent molecules included in the channels. A structural isomer of the parent NiBDP was obtained with a sulfonate tagged ligand, H₂BDP-SO₃H. Structure solution from powder diffraction data collected at three different temperatures (room temperature, 90, and 250 °C) allowed the determination of its structure and the comprehension of its solvent-related flexible behavior. Finally, the potential application of the tagged MOFs in selective adsorption processes for gas separation and purification purposes was investigated by conventional single component adsorption isotherms, as well as by advanced experiments of pulse gas chromatography and breakthrough curve measurements. Noteworthy, the results show that functionalization does not improve the adsorption selectivity (partition coefficients) for the resolution of gas mixtures characterized by similar high quadrupole moments (e.g., CO ₂/C₂H₂); however, the resolution of gas mixtures containing molecules with highly differentiated polarities (i.e., N₂/CO₂ or CH₄/CO₂) is highly improved.

Full text of DOI:10.1021/ja305267m

Article
pubs.acs.org/JACS
Tuning the Adsorption Properties of Isoreticular Pyrazolate-Based
Metal−Organic Frameworks through Ligand Modification
Valentina Colombo,*,† Carmen Montoro,‡ Angelo Maspero,† Giovanni Palmisano,†
Norberto Masciocchi,† Simona Galli,† Elisa Barea,‡ and Jorge A. R. Navarro*,‡
†Dipartimento di Scienza e Alta Tecnologia, Universita
‡Departamento de Química Inorgan
ica, Universidad de Granada, Av. Fuentenueva S/N, 18071 Granada, Spain
̀
dell’Insubria, via Valleggio 11, I-22100 Como, Italy
́
S
* Supporting Information
ABSTRACT: Two isoreticular series of pyrazolate-based 3D
open metal−organic frameworks, MBDP_X, adopting the
NiBDP and ZnBDP structure types [H2BDP = 1,4-bis(1H-
pyrazol-4-yl)benzene], were synthesized with the new tagged
organic linkers H2BDP_X (X = −NO2, −NH2, −OH). All of
the MBDP_X materials have been characterized through a
combination of techniques. IR spectroscopy proved the
effective presence of tags, while X-ray powder diffraction
(XRPD) witnessed their isoreticular nature. Simultaneous TG/
DSC analyses (STA) demonstrated their remarkable thermal
stability, while variable-temperature XRPD experiments highlighted their high degree of flexibility related to guest-induced fit
processes of the solvent molecules included in the channels. A structural isomer of the parent NiBDP was obtained with a
sulfonate tagged ligand, H2BDP_SO3H. Structure solution from powder diffraction data collected at three different temperatures
(room temperature, 90, and 250 °C) allowed the determination of its structure and the comprehension of its solvent-related
flexible behavior. Finally, the potential application of the tagged MOFs in selective adsorption processes for gas separation and
purification purposes was investigated by conventional single component adsorption isotherms, as well as by advanced
experiments of pulse gas chromatography and breakthrough curve measurements. Noteworthy, the results show that
functionalization does not improve the adsorption selectivity (partition coefficients) for the resolution of gas mixtures
characterized by similar high quadrupole moments (e.g., CO2/C2H2); however, the resolution of gas mixtures containing
molecules with highly differentiated polarities (i.e., N2/CO2 or CH4/CO2) is highly improved.
the introduction of different functionalities on its skeleton. This
means that, in principle, it is possible to tune and study, within
a single class of materials, those properties that are exclusively
dependent on the modifications introduced on the organic
bridge (i.e., pore size and shape, surface selectivity, uptake
capacity, etc.). However, the number of possibilities of
combining inorganic and organic moieties is immense, and
the almost infinite variation on the nature of the linkers can
provide either a series of isostructural compounds or, without
neglecting the unexpected results of serendipity, even new
structures.4
Among the numerous ligands used in MOFs chemistry,
carboxylate-based ones are a highly studied family, due to their
straightforward availability and to the fact that they can be
easily functionalized with groups of different nature, in terms of
basicity, acidity, or polarity.5 Within the numerous examples
reported in the literature, an exceptional one is the isoreticular
series of IRMOFs 1−16, derived from the prototypic MOF-5
structure.6 Yaghi et al. demonstrated that the organic spacer of
MOF-5 can be functionalized with many different substituents
INTRODUCTION
■
Metal−organic frameworks (MOFs) are a new fascinating class
of materials in which inorganic subunits and organic ligands are
reciprocally linked by strong coordination bonds generating
robust and often porous species featuring extended architec-
tures. The extraordinary pace with which the research is
growing in this field has resulted in an increasing number of
new structures and frameworks compositions with many
different potential applications.1
Structure predictability, an important requirement for the
synthesis of functional materials with tailored properties, has
been recently proved to be viable even in the realm of MOFs.
The concept of rational design was developed by O’Keeffe et
́
al.2 and, as brilliantly described by Ferey in one of his reviews,3
was rooted in the fact that topochemically selected reactions
govern the construction process of metal−organic frameworks
in solvothermal reactions. This suggests that, with adequate
synthetic conditions, targeted inorganic subunits may be
obtained in a systematic way for the construction of a desired
framework. Therefore, as soon as the chemical conditions
associated with the existence of a parent structure are known,
the modulation of pore size and walls decoration can be directly
achieved through the modification of the length of the ligand or
Received: May 31, 2012
Published: July 5, 2012
© 2012 American Chemical Society
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Article
(such as −Br, −NH2, −OC3H7, etc.) and the pore size of the
framework can be expanded with longer molecular struts as, for
example, biphenyl or tetrahydropyrene, with no changes in the
underlying topology.7 A systematic investigation of the effects
of tagging a given carboxylate linker was carried out also in the
case of UiO-66,5g,i MIL-53,5e,h and MIL-88.5f Also, due to such
exceptional examples, porous metal−organic frameworks have
been individuated as promising zeolite-analogous materials,
because they are expected to be more designable in pore size
and shape, as well as to exhibit a wide structural diversity.
However, one of the main weaknesses of carboxylate-based
MOFs is the lack of stability at moderately high temperatures,
in the presence of moisture, in acid or basic media, and,
sometimes, even in air, which limits their utility in industrial
applications.
Scheme 1. Synthesis of Tagged 1,4-Bis(1H-pyrazol-4-
yl)benzene Derivatives and Their Acronyms Used in This
a
Work (H2BDP_X, X = NO2, NH2, OH, SO3H)
Recently, nitrogen-donor ligands have proved to be a valid
alternative to oxygen-based ones.8 In this respect, during the
last years, we have demonstrated that pyrazolate-based ligands
can be successfully exploited for the synthesis of highly robust
metal−organic frameworks.9 Within this class, the sodalite-type
Ni3BTP2 [H3BTP = 1,3,5-tris(1H-pyrazol-4-yl)benzene] mate-
rial9d represents the first example of metal−organic framework
exhibiting high surface area (up to 1900 m2/g) and exposed
metal sites coupled to a high thermal stability (up to 430 °C in
air) and an exceptional resistance to hydrolysis and
decomposition during treatment with hot water, acidic, and
basic solutions.
a
(a) H2SO4/HNO3, 1 h, room temperature, then ice/water; (b)
H2BDP_NO2 + NH4HCO3, Pd/C, N,N′-dimethylformamide, 110 °C,
2 h, then ice/water; (c) H2BDP_NH2 + H2SO4, NaNO2, then H2SO4/
water (1:1), 110 °C, 1 h; (d) H2SO4 (conc.), 1 h, room temperature,
then ice/water.
ascertained by means of X-ray powder diffraction (XRPD).
Their thermal stability was evaluated by simultaneous TG/DSC
analyses, while the effective presence of the functional sites was
probed by FTIR. Variable-temperature X-ray powder diffraction
experiments allowed one to study their structural response as a
function of temperature and solvent content. Finally, their
potential application in selective adsorption processes for gas
separation and purification purposes of gases of environmental
and industrial interest was investigated by conventional single
component adsorption isotherms, as well as by advanced
experiments of pulse gas chromatography and breakthrough
curve measurements.
Another important challenge in the field of porous polymers
is the development of stable structures including functional
organic sites (i) via direct assembly from inorganic nodes and
tagged-organic linkers or (ii) via postsynthetic modification of
the pores surface.10 However, in the direct-assembly approach,
a functional group may be harder to incorporate, either because
of its thermal instability under MOF synthesis conditions or
due to unpredictable reaction pathways. In the light of the high
stability of N-based ligands incorporated into a MOF structure,
this class of linkers is expected to respond better than their O-
based counterparts to the development of isoreticular series of
MOFs including functional organic sites via direct assembly,
with the final aim of tuning their pore properties (pore size,
shape, volume, and decoration) as a mean of enhancing the
uptake capacity and selectivity toward a targeted guest.
To the best of our knowledge, a systematic investigation
within a nitrogen-based class of isoreticular MOFs is still
missing. Therefore, given the remarkable results obtained with
non functionalized pyrazolate-based linkers, we decided to
investigate, for a given archetypical topology, how different
functionalities on the same ligand can affect the chemical and
physical properties of the corresponding MOF, with particular
attention on estimating the trends of stability and adsorption
properties versus the nature of the substituents. Accordingly,
starting from 1,4-bis(1H-pyrazol-4-yl)benzene (H2BDP), four
new tagged linkers (H2BDP_X, Scheme 1, X = NO2, NH2, OH,
SO3H) were prepared and employed, for the first time, for the
systematic isolation and characterization of the two series of
pyrazolate-based tagged frameworks MBDP_X (M = Ni, Zn),
based on the skeleta of the parent NiBDP and ZnBDP
materials (Figure 1).9a In the following, synthetic strategies
toward the nitro, amino, hydroxyl, and sulfonic acid derivatives
of H2BDP are reported. Moreover, reaction paths toward the
corresponding MBDP_X MOFs, improved with respect to the
original ones leading to NiBDP and ZnBDP, are proposed.
The isostructural nature of NiBDP_X and ZnBDP_X was
EXPERIMENTAL DETAILS
■
Synthesis and Characterization. All chemicals were obtained
commercially and used without further purification. The 1,4-bis(1H-
pyrazol-4-yl)benzene ligand (H2BDP) was prepared as described in
the literature.11
2-Nitro[1,4-bis(1H-pyrazol-4-yl)benzene] (H2BDP_NO2). 1,4-
Bis(1H-pyrazol-4-yl)benzene (1.000 g, 4.76 mmol) was added in
portions to concentrated sulfuric acid (10 mL) while keeping the
reaction mixture cold with an ice bath. To the solution was then added
70% nitric acid (0.255 mL, 5.71 mmol) dropwise while maintaining
the reaction mixture cold. The ice bath was then removed, and the
solution was left at room temperature under stirring for 1 h. Next, 5 g
of crushed ice was added, and the precipitate was filtered off and
washed with 10 mL (2 × 5 mL) of water. The precipitate was
neutralized with aqueous NaHCO3, and the resulting product was
collected by filtration and washed with 10 mL of water (2 × 5 mL),
1
affording the pure ligand as a yellow solid (1.03 g, yield 85%). H
NMR (DMSO-d6): δ 7.63 (d, J = 8.2 Hz, 1H), 7.79 (s, 2H), 7.89 (dd, J
= 8.2, 1.8 Hz, 1H), 8.08 (d, J = 1.8 Hz, 1H), 8.23 (s, 2H). IR (nujol):
3164(br), 1580(s), 1522(vs), 1350(vs), 1256(w), 1175(w), 1151(w),
1041(s), 976(w), 947(s), 894(w), 865(w), 815(s), 741(w), 664(w)
cm−1. Anal. Calcd for C12H9N5O2 (Mw = 255.2 g/mol): C, 56.47; H,
3.55; N, 27.44. Found: C, 55.7; H, 3.26; N, 26.37.
2-Amino[1,4-bis(1H-pyrazol-4-yl)benzene] (H2BDP_NH2). To
a suspension of H2BDP_NO2 (0.300 g, 1.176 mmol) in DMF (5 mL)
was added ammonium formate (0.370 mg, 5.873 mmol) at room
temperature. The reaction mixture was then heated to 120 °C, and
Pd/C (5%, 30 mg) was added in small portions. The final mixture was
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Figure 1. Top: Schematic drawing of the parent structures of ZnBDP and NiBDP [H2BDP = 1,4-bis(1H-pyrazol-4-yl)benzene] highlighting their
square and rhombic lattices, respectively. Bottom: A portion of their polymeric chains evidencing the local sterochemistries at the metal ions.
then kept under stirring at 100 °C for 2 h. After the reaction mixture
became clear, it was filtered through the pad of Celite. The Celite was
then washed with a small amount of DMF, and the filtrate was diluted
with crushed ice (5 g). The obtained precipitate was then filtered off
and washed with water (2 × 5 mL), affording a white powder of pure
for C12H10N4O3S (Mw = 290.3 g/mol): C, 49.65; H, 3.47; N, 19.30.
Found: C, 49.84; H, 3.97; N, 18.64.
General Procedure for NiBDP_X (X = H, NO2, NH2, OH)
Preparation. H2BDP_X (0.5 mmol) was dissolved in DMF (10 mL)
in a 50 mL Schlenk flask and heated to 60 °C. Ni(CH3COO)2·4H2O
(0.5 mmol) was then added to the solution under stirring. The mixture
was allowed to react for 5 h at reflux. After being cooled to room
temperature, the orange solid was collected by filtration, washed with
methanol (3 × 10 mL), and dried under vacuum. Yields typically fall in
the 50−70% range, depending on the amount of entrapped solvent
and on the performances of the filtration processes, heavily
conditioned by the crystal sizes.
Anal. Calcd for Ni(C12H7N5O2)(C3H7NO)2(CH3OH)2 (NiBDP_-
NO2) (Mw = 522 g/mol): C, 46.00; H, 5.60; N, 18.78. Found: C,
45.96; H, 5.45; N, 19.28. IR (nujol): 1676(vs), 1649(vs), 1571(s),
1523(s), 1256(vs), 1179(w), 1136(w), 1107(w), 1062(s), 1006(w),
982(w), 955(w), 869(w), 837(w), 762(w), 738(w), 673(w) cm−1.
Anal. Calcd for Ni(C12H9N5)(C3H7NO)(H2O)(CH3OH)
(NiBDP_NH2) (Mw = 405 g/mol): C, 47.44; H, 5.47; N, 20.75.
Found: C, 47.80; H, 5.95; N, 21.05. IR (nujol): 3437(s), 3355(s),
1654(s), 1620(w), 1573(s), 1346(w), 1262(w), 1169(w), 1134(w),
1064(s), 954(s), 855(w), 810(w), 722(w) cm−1.
Anal. Calcd for Ni(C12H8N4O)(H2O)3(CH3OH) (NiBDP_OH)
(Mw = 369 g/mol): C, 42.31; H, 4.92; N, 15.18. Found: C, 41.99; H,
4.81; N, 15.39. IR (nujol): 3579(s), 1619(w), 1577(s), 1269(w),
1222(w), 1175(w), 1147(w), 1064(s), 955(s), 866(w), 843(w),
813(w), 722(w) cm−1.
General Procedure for ZnBDP_X (X = H, NO2, NH2, OH)
Preparation. H2BDP_X (0.5 mmol) was dissolved in DMF (10 mL)
in a 50 mL Schlenk flask and heated to 60 °C. Zn(CH3COO)2·2H2O
(0.5 mmol) was then added to the solution under stirring. The mixture
was allowed to react for 5 h at reflux. After being cooled to room
temperature, the white solid was collected by filtration, washed with
methanol (3 × 10 mL), and dried under vacuum. Yields typically fall in
the 50−70% range, depending on the amount of entrapped solvent
and on the performances of the filtration processes, heavily
conditioned by the crystal sizes.
1
H2BDP_NH2 product (250 mg, yield 94%). H NMR (DMSO-d6): δ
4.77 (s, 2H), 6.84 (dd, J = 7.8, 1.7 Hz, 1H), 6.97 (d, J = 1.7 Hz, 1H),
7.15 (d, J = 7.8 Hz, 1H), 7.86 (br s, 4H), 12.88 (br s, 2H). IR (nujol):
3420(s), 3338(s), 3111(br), 1621(s), 1575(s), 1522(w), 1334(w),
1290(w), 1169(vs), 1042(s), 988(w), 961(s), 946(w), 880(w), 864(s),
805(vs), 721(w), 660(w), 627(w) cm−1. Anal. Calcd for C12H11N5
(Mw = 225.2 g/mol): C, 63.99; H, 4.92; N, 31.09. Found: C, 63.31; H,
5.42; N, 31.30.
2-Hydroxo[1,4-bis(1H-pyrazol-4-yl)benzene] (H2BDP_OH).
One gram (4.444 mmol) of H2BDP_NH2 was dissolved in 5 mL of
sulfuric acid. The mixture was stirred until a thick paste was formed.
To this was added about 3 g of crushed ice, and the mixture was then
kept in an ice bath. In a separate beaker, NaNO2 (0.440 g, 5.176
mmol) was dissolved in 4 mL of water. This solution was cooled and
added dropwise, with constant stirring, to the acid amine solution. In a
separate flask, a solution of H2SO4 (3 mL) and water (3 mL) was
heated to 110 °C, and the entire diazonium salt solution was added
dropwise. After the addition was over, the solution was allowed to boil
for another 30 min. It was then cooled with an ice bath, and the
precipitate was filtered off and suspended in a solution of NaHCO3 in
water and stirred for 2 h at 80 °C. The yellowish precipitate was then
filtered off, washed with water (2 × 5 mL), and dried under vacuum
(0.763 g, yield 76%). 1H NMR (DMSO-d6): δ 7.04 (m, 2H), 7.51 (d, J
= 7.8 Hz, 1H), 8.04 (br s, 4H), 9.67 (s, 1H), 12.83 (br s, 2H). IR
(nujol): 3525(s), 3387(br), 3182(br), 1621(w), 1588(s), 1563(w),
1534(w), 1440(s), 1346(w), 1274(w), 1254(w), 1216(w), 1160(vs),
1106(w), 1036(s), 959(s), 948(s), 888(w), 865(w), 834(w), 817(s),
736(w), 670(w) cm−1. Anal. Calcd for C12H10N4O (Mw = 226.2 g/
mol): C, 63.71; H, 4.46; N, 24.76. Found: C, 62.41; H, 5.11; N, 24.50.
2,5-Di(1H-pyrazol-4-yl)benzenesulfonic Acid (H2BDP_SO3H).
1,4-Bis(1H-pyrazol-4-yl)benzene (1 g, 4.76 mmol) was added in
portions to fuming sulfuric acid (10 mL) and was stirred for 1 h at
room temperature. To the resulting solution was added crushed ice
slowly to give a white precipitate that was filtered off, washed
thoroughly with water (2 × 10 mL), and dried under vacuum to give
Anal. Calcd for Zn(C12H7N5O2)(C3H7NO)2(CH3OH)2(H2O)5
(ZnBDP_NO2) (Mw = 619 g/mol): C, 38.81; H, 6.35; N, 15.84.
Found: C, 38.63; H, 6.03; N, 16.06. IR (nujol): 3350(br), 1670(vs),
1578(s), 1527(s), 1532(w), 1257(vs), 1134(w), 1092(w), 1064(s),
1012(w), 982(w), 954(w), 869(w), 847(w), 764(w), 739(w), 722(w),
659(w), 623(w) cm−1.
Anal. Calcd for Zn(C12H9N5)(C3H7NO)2(CH3OH)(H2O)
(ZnBDP_NH2) (Mw = 484.8 g/mol): C, 47.07; H, 6.03; N, 20.22.
Found: C, 47.33; H, 5.74; N, 20.07. IR (nujol): 3409(w), 3341(w),
1675(vs), 1629(w), 1571(s), 1402(w), 1344(w), 1256(s), 1177(w),
1
pure H2BDP_SO3H (0.953 g, yield 69%). H NMR (DMSO-d6): δ
3.16 (s, 1H), 7.48 (d, J = 8 Hz, 1H), 7.61 (dd, J = 8, 1.9 Hz, 1H), 8.11
(s, 2H), 8.15 (d, J = 1.9 Hz, 1H), 8.58 (s, 2H). IR (nujol): 3415(br),
3174(br), 1580(w), 1564(w), 1511(w), 1346(w), 1311(w), 1256(w),
1215(vs), 1176(vs), 1113(s), 1084(s), 1047(s), 1026(s), 969(w),
943(w), 886(w), 831(s), 737(w), 682(w), 656(w) cm−1. Anal. Calcd
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a
Table 1. Unit Cell Parameters of MBDP_X Materials in Their As-Synthesized (30 °C), Evacuated (300 °C), and Back-Solvated
(30 °C + DMF) Forms
30 °C
c, Å
300 °C
c, Å
30 °C + DMF
S.G.
a, Å
b, Å
V, Å3
a, Å
b, Å
V, Å3
a, Å
b, Å
c, Å
V, Å3
NiBDP_NO2
NiBDP_NH2
NiBDP_OH
ZnBDP_NO2
ZnBDP_NH2
ZnBDP_OH
Imma
Imma
Imma
Cccm
Cccm
Cccm
7.06
6.93
22.88
22.32
22.04
19.21
19.12
19.70
13.74
14.32
14.62
7.41
2222.38
2214.68
2190.51
2606.67
2585.68
2535.55
6.88
6.70
23.4
13.25
12.62
13.59
7.18
2135.04
1953.99
2045.01
2232.09
n.a.
7.05
6.93
22.85
22.38
22.19
19.21
19.33
19.68
13.78
14.27
14.56
7.39
2219.30
2213.65
2209.37
2594.52
2587.25
2532.70
23.10
22.68
21.21
n.a.
6.80
6.63
6.83
18.30
18.39
17.82
14.66
n.a.
18.25
18.21
17.88
b
7.35
n.a.
7.34
7.22
17.07
20.55
6.99
2454.19
7.19
a
The unit cell parameter values were retrieved by means of a structureless Le Bail refinement. The corresponding esd’s are on the order of 0.01 Å.
The very low crystallinity of HT ZnBDP_NH2 prevented treatment of its XRPD data.
b
1131(s), 1091(w), 1060(s), 1012(w), 995(w), 951(s), 856(w),
810(w), 724(w), 662(w) cm−1.
respectively. The final Rietveld refinement plots for the three
compounds are shown in Figure S1.
Anal. Calcd for Zn(C12H8N4O)(C3H7NO)2(CH3OH)
(ZnBDP_OH) (Mw = 467.8 g/mol): C, 48.78; H, 5.60; N, 17.96.
Found: C, 48.63; H, 5.98; N, 18.29. IR (nujol): 3470(br), 1675(vs),
1575(s), 1354(w), 1254(s), 1223(w), 1170(w), 1141(w), 1125(w),
1102(w), 1058(s), 1013(w), 955(w), 867(w), 840(w), 814(w),
720(w), 663(w), 603(w) cm−1.
Crystal Data for Ni(H2BDP_SO3)2·2DMF. C30H32N10NiO8S2, Mw =
783.46 g/mol, monoclinic, P21/c, a = 7.526(1) Å, b = 16.343(4) Å, c =
17.631(5) Å, β = 94.07(2)°, V = 2163.1(9) Å3; Z = 2; ρcalc = 1.20 g
cm−3; F(000) = 782; μ(Cu Kα) = 20.0 cm−1. Rp, Rwp, and RBragg are
0.046, 0.062, and 0.051, respectively, for 29 parameters.
Crystal Data for Ni(H2BDP_SO3)2·DMF. C27H25N9NiO7S2, Mw
=
Synthesis of Ni(C12H9N4SO3)2(C3H7NO)2, NiBDP_SO3H.
H2BDP_SO3H (0.050 g, 0.172 mmol) was dissolved in DMSO (3
mL) in a 50 mL Schlenk flask, and DMF (3 mL) was added to this
solution. The mixture was heated to 60 °C, and Ni(CH3COO)2·4H2O
(0.085 g, 0.5 mmol) was added under stirring. The mixture was then
allowed to react for 5 h at 160 °C. After being cooled at room
temperature, the yellowish solid was collected by filtration, washed
with DMF (3 × 10 mL), and dried under vacuum. Anal. Calcd for
Ni(C12H9N4SO3)2(C3H7NO)2 (Mw = 785.48 g/mol): C, 45.87; H,
4.36; N, 17.83. Found: C, 46.34; H, 4.85; N, 17.35. IR (nujol):
3442(s), 3159(w), 1666(s), 1590(w), 1572(w), 1251(s), 1158(s),
1080(w), 1053(w), 1022(s), 961(w), 854(w), 790(w), 721(w),
691(w), 823(w) cm−1.
710.37 g/mol, monoclinic, P21/c, a = 7.4533(4) Å, b = 15.005(1) Å, c
= 17.611(3) Å, β = 91.293(6)°, V = 1969.0(4) Å3; Z = 2; ρcalc = 1.21 g
cm−3; F(000) = 732; μ(Cu Kα) = 21.1 cm−1. Rp, Rwp, and RBragg are
0.019, 0.028, and 0.009, respectively, for 52 parameters.
Crystal Data for Ni(H2BDP_SO3)2. C24H18N8NiO6S2, Mw = 637.27
g/mol, monoclinic, P21/c, a = 7.4594(5) Å, b = 15.589(2) Å, c =
17.151(3) Å, β = 91.648(8)°, V = 1993.6(4) Å3; Z = 2; ρcalc = 1.06 g
cm−3; F(000) = 652; μ(Cu Kα) = 20.2 cm−1. Rp, Rwp, and RBragg are
0.0245, 0.033, and 0.011, respectively, for 37 parameters. Crystallo-
graphic data (excluding structure factors) for the structures reported in
this Article have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication nos. CCDC 884528−
884530. Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax,
+44 1223 336 033; e-mail, deposit@ccdc.cam.ac.uk).
Variable-Temperature X-ray Powder Diffraction. Thermodif-
fractometric experiments were performed on the as-synthesized
NiBDP_SO3H·2DMF sample to highlight its “structural” response
to temperature changes. A powdered batch of the sample was
deposited in the hollow of an aluminum sample holder, assembled by
Officina Elettrotecnica di Tenno, Ponte Arche, Italy; diffractograms
were acquired in air from 30 °C to the loss of crystallinity, with steps
of 20 °C, in a significant low-angle 2θ range (7−22°). Le Bail
refinements were carried out to derive the lattice constants, later
employed in the rationalization of the thermal properties. By means of
the same apparatus, another set of experiments was performed on all
series of MBDP_X materials (M = Ni2+, Zn2+; X = NO2, NH2, OH):
for each species, starting from a freshly prepared batch, three
diffractograms were acquired in isothermal conditions at 30, 300,
and back to 30 °C with further solvation of the material with N,N′-
dimethylformamide. The data were then treated by the Le Bail
refinement method to derive the corresponding unit cell parameters
(Table 1).
Gas Adsorption Measurements. Conventional adsorption
isotherms were measured using a Micromeritics Tristar 3000
volumetric instrument under continuous adsorption conditions.
Brunauer−Emmet−Teller (BET) and Langmuir analyses were carried
out to determine the total specific surface areas for the N2 isotherms at
77 K. In addition, CO2 isotherms at 273 K were measured to evaluate
the micropore region by means of the Dubinin−Radushkevich
equation (Table 2). Prior to measurement, powdered samples were
heated at 453 K for 12 h and outgassed to 10−6 Torr.
Variable-Temperature Pulse Gas Chromatography. Gas-
phase adsorption at zero coverage surface was studied using the
pulse chromatographic technique,13 employing a gas chromatograph
(Varian 450-GC) equipped with a 15 cm-column (0.4 cm internal
X-ray Powder Diffraction Structural Analysis (XRPD). A
powdered, microcrystalline sample of DMF-solvated NiBDP_SO3H
(NiBDP_SO3H·2DMF) was deposited in the hollow of an aluminum
sample holder equipped with a zero-background plate. As the
compound is poorly stable at ambient conditions, due to quick
solvent release, numerous attempts were preliminary carried out to
find proper measurement settings: with our setup, solvent loss was
invariably faster than the time scale of the XRPD acquisition. The best
compromise between a limited solvent loss and data of sufficient
quality to allow a structural analysis was found collecting only in the
6−55° 2θ range (Δ2θ = 0.02°, 1 h scan). Data collections were
performed on a Bruker AXS D8 Advance diffractometer, equipped
with a linear position-sensitive Lynxeye detector, primary beam Soller
slits, and Ni-filtered Cu Kα radiation (λ = 1.5418 Å). Generator
setting: 40 kV, 40 mA. Standard peak search, followed by indexing
with TOPAS,12 allowed the detection of approximate unit cell
parameters: monoclinic P, a = 7.52 Å, b = 16.32 Å, c = 17.60 Å, β =
94.17°, V = 2155.63 Å3, and GOF = 17.5. The space group P21/c was
assigned on the basis of the systematic absence conditions and was
later confirmed by a Le Bail refinement. Structure solution was
performed by the simulated annealing technique, as implemented in
TOPAS, using for H2BDP_SO3H and DMF rigid, idealized models.
The final refinement was carried out by the Rietveld method,
maintaining the rigid bodies introduced at the structure solution stage.
The background was modeled by a polynomial function of the
Chebyshev type. One, refined, isotropic thermal parameter was
assigned to the metal atoms, augmented by 2.0 Å2 for lighter atoms.
For the other two structures reported below (the NiBD-
P_SO3H·DMF and NiBDP_SO3H species), an identical procedure
was applied to diffractograms collected at 90 and 250 °C, for which the
same apparatus used for the variable-temperature experiments was
adopted. For these experiments, overnight data collections were
performed in the ranges 6−70° and 7−35° 2θ (Δ2θ = 0.02°),
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under a pure He flow (30 mL min−1) at 453 K for 7 h and then used
for evaluating the CO2/N2, CO2/CH4, and C2H2/CH4 separation
performances of the materials. The desired gas mixture (10 mL min−1)
was prepared via mass flow controllers. For instance, CO2/N2
(0.14:0.86) mixtures were prepared to simulate the emission of flue
gas from a power plant employing 1.4 mL min−1 of CO2 and 8.6 mL
min−1 of N2 flows, and the breakthrough experiments were carried out
at 273, 303, and 323 K by step changes from He to CO2/N2 flow
mixtures. The relative amounts of gases passing through the column
were monitored on a mass spectrometer gas analysis system (Pfeiffer
Vacoon) detecting ion peaks at m/z 44 (CO2), 28 (N2), and 4 (He).
Other Physical Measurements. Infrared (IR) spectra were
acquired in transmission mode on a FTIR Shimadzu Prestige-21
spectrometer. The materials under study were mixed with nujol to
form a mull and recorded with KBr plates in the spectral range 4000−
600 cm−1. Simultaneous TG and DSC analyses were performed in a
N2 stream on a Netzsch STA 409 PC Luxx with a heating rate of 10 °C
min−1 to 900 °C. Elemental analyses were carried out on a Perkin-
Elmer CHN Analyzer 2400 Series II at the facility of the Universita
dell’Insubria in Como.
Table 2. Langmuir, BET (N2, 77 K), and Dubinin−
Radushkevich (CO2, 273 K) Specific Surface Areas for the
Two Series NiBDP_X and ZnBDP_X (X = H, NO2, NH2,
OH, SO3H) Desolvated under Reduced Pressure at 180 °C
for 12 h
SABET/m2 g−1
SALangmuir/m2 g−1
SADR/m2 g−1
NiBDP
1066(18)
1131(17)
1305(22)
1103(18)
819(16)
1350(9)
1423(9)
1645(8)
1394(10)
1044(6)
2857(2)
2358(5)
1788(78)
1484(7)
1480(10)
2350(4)
990(2)
NiBDP_NO2
NiBDP_NH2
NiBDP_OH
NiBDP_SO3H
ZnBDP
1980(7)
1220(9)
1220(11)
710(3)
2288(41)
1875(52)
1345(21)
1170(22)
ZnBDP_NO2
ZnBDP_NH2
ZnBDP_OH
670(9)
1420(4)
̀
diameter) packed with ca. 1 g of pelletized sample (particles size 0.5
mm). To avoid pressure drops in the chromatographic column, the
particles were aggregated by emulsifying them in an aqueous
suspension (1 mL) of starch (0.1 g) at 343 K for 1 min. The solvent
was then removed under reduced pressure, and the resulting solid was
grounded through a 0.5 mm sieve to give the pelletized samples. For
the activation of the material, a He flow (30 mL min−1) was passed
through the column at 453 K for 7 h. Later, 1 mL of an equimolecular
gas mixture of C2H2, H2, CH4, and CO2 was injected at 1 bar on an He
flow (15 mL min−1). The separation performance of the chromato-
graphic column was then examined at different temperatures (273−
323 K) by means of a mass spectrometer gas analysis system (Pfeiffer
Vacoon), detecting ion peaks at m/z 44 (CO2), 26 (C2H2), 16 (CH4),
4 (He), and 2 (H2). The dead volume of the system was calculated
using the retention time of hydrogen as a reference.
On the other hand, the retention time of H2O was determined at
variable temperature (363−473 K) by using a thermal conductivity
detector (TCD) of the gas chromatograph equipment, configured for
this determination using an on-column injector (0.2 μL).
Breakthrough Experiments for Gas Separation. The 15 cm
chromatographic column, prepared as detailed above, was activated
RESULTS AND DISCUSSION
■
Syntheses. The 1,4-bis(1H-pyrazol-4-yl)benzene ligand
(H2BDP, Scheme 1) was chosen as parent skeleton for the
preparation of a new series of functionalized linkers
(H2BDP_X, Scheme 1) due to its easy availability from
synthesis, in good yields and in gram scale.11 The first
functional group introduced on the aromatic spacer was the
nitro group: to isolate 2-nitro[1,4-bis(1H-pyrazol-4-yl)-
benzene], H2BDP_NO2, a simple nitration in sulfuric and
nitric acid mixtures was performed.14 Next, reduction of the
nitro group yielded 2-amino[1,4-bis(1H-pyrazol-4-yl)benzene],
H2BDP_NH2.15 Because of the low solubility of H2BDP_NO2,
the reduction was carried out in N,N′-dimethylformamide
rather than in an alcoholic solution.16 Subsequently, by
exploiting the reactivity of diazonium salts, readily available
through primary aromatic amines, the amino group was
Figure 2. (a) Schematic drawing of the crystal structure of Ni(H2BDP_SO3)2·2DMF viewed down a, with two DMF molecules highlighted in one
of its cavities; and (b) magnification of the stereochemistry at the Ni(II) ions.
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found to afford the whole series of MBDP_X (M = Ni2+, Zn2+;
X = NO2, NH2, OH) frameworks, in a polycrystalline and pure
form, differs with respect to the original ones. Indeed, acetate
salts were reacted with the organic linkers in boiling N,N′-
dimethylformamide, in the absence of extra base, such as
triethylamine, in the reaction system. Interestingly, the very
same procedure was proved to be viable also for the parent
frameworks NiBDP and ZnBDP, providing polycrystalline and
pure DMF solvates. Worthy of note, this synthetic procedure is
reproducible and could be scaled up, making these MOFs
available on a large scale.
Unfortunately, any attempt to produce a MOF containing
the ligand bearing the sulfonic acid functionality
(H2BDP_SO3H, see Scheme 1) and isostructural to the
other MBDP_X materials invariably failed. However, by
employing different synthetic conditions, a structural isomer
of the parent NiBDP species was obtained (vide infra), in the
following labeled as NiBDP_SO3H. On the contrary, repeated
attempts at obtaining a pure phase with Zn(II) ions, by varying
the synthesis parameters, such as time, temperature program,
stoichiometry, solvents, and pH, were unsuccessful.
Crystal Chemistry. The bulk crystallinity and structural
topology of each tagged MOF was evaluated by X-ray powder
diffraction (XRPD). Their diffraction patterns are reported in
Figure S2, revealing that all materials are crystalline, although
no single crystals were ever formed. The comparison between
the diffraction patterns of the parent materials and those of the
tagged MOFs demonstrates the formation of pure, topologi-
cally equivalent, phases for both the NiBDP_X and the
ZnBDP_X (X = NO2, NH2, OH) series. Thus, like NiBDP, the
NiBDP_X compounds crystallize in the orthorhombic space
group Imma and feature a porous framework in which the metal
chains are arranged in a rhombic disposition, governed by the
square-planar geometry of the Ni(II) ions. On the contrary, in
ZnBDP and ZnBDP_X, linear chains of tetrahedrally
Figure 3. FTIR spectra of NiBDP_SO3H recorded as nujol mull. The
inset highlights the IR region around 3500 cm−1 in which the two
absorption bands of a desolvated material are visible.
converted into a hydroxo functionality,17 leading to 2-
hydroxo[1,4-bis(1H-pyrazol-4-yl)benzene], H2BDP_OH. Fi-
nally, the electrophilic aromatic substitution for the preparation
of benzene sulfonic acids was exploited to obtain the 2,5-di(1H-
pyrazol-4-yl)benzenesulfonic acid derivative, H2BDP_SO3H.
Later, for the preparation of isostructural series of porous
MOFs based on the H2BDP_X spacers, we focused our
attention on nickel(II) and zinc(II) ions, which generally result
in stable frameworks of easy handling in air, as properly
witnessed, for example, by the NiBDP and ZnBDP
archetypes.9a NiBDP was originally obtained by reacting the
H2BDP ligand with Ni(II) acetate in acetonitrile at 80 °C in the
presence of triethylamine. The isolated product was invariably
contaminated by unreacted H2BDP, which is extremely
insoluble in the reaction solvent, implying the necessity of an
additional washing procedure in dimethylsulfoxide at 60 °C. On
the other hand, ZnBDP was isolated as a pure product, by
reacting Zn(ClO4)2 with H2BDP in benzonitrile and triethyl-
amine.18 Notably, in this work, the more suitable preparation
●
Figure 4. FTIR spectra of NiBDP_X (X = H, NO2, NH2, OH) recorded as nujol mull. The symbol “ ” denotes residual DMF.
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Figure 5. Simultaneous DSC (- - -) and TG (−) analyses for NiBDP_NH2 (blue), NiBDP_NO2 (orange), NiBDP_OH (green), and
NiBDP_SO3H (red).
Figure 6. Powder X-ray diffraction patterns for each tagged MBDP_X framework (M = Ni2+, Zn2+, X = NO2, NH2, OH) collected, in the same
experiment, at 30, 300, and back to 30 °C with further solvation with DMF.
coordinated Zn(II) ions are arranged to form a square lattice
(see Figure 1). Unit cell parameters for each framework,
derived by structureless Le Bail refinements, are reported in
Table 1.
Regarding the ZnBDP_X series, as evidenced by independ-
ent indexing processes followed by Le Bail refinements, the new
preparation carried out in DMF as the solvent leads to an
orthorhombic deformation (Cccm space group) of the parent
tetragonal square-grid framework, possibly due to the actual
nature and distribution of the solvent within the network pores
(unit cell transformation from the tetragonal lattice requires
that the a+b and a−b diagonals are taken as the new
orthorhombic axes). This observation is not surprising, because
lowering of the P42/mmc symmetry was also observed for the
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Figure 7. Variation of the unit cell axes upon thermally induced desolvation (30−300 °C) and subsequent solvent readsorption (back to 30 °C, see
text). Left, NiBDP_X; right, ZnBDP_X. Orange, NO2; blue, NH2; green, OH. Unit cell parameters for ZnBDP_NH2 are not available, due to its
very low crystallinity at 300 °C, preventing data treatment.
solution from powder diffraction allowed one to disclose a new
framework, of Ni(H2BDP_SO3)2·2DMF formulation, which
can be considered as an isomeric form of NiBDP. The local
coordination geometry of this heteroleptic framework, in the
following NiBDP_SO3H·2DMF, can be appreciated in Figure
2b, while the overall structure is shown in Figure 2a. This
crystal phase is monoclinic, space group P21/c; the asymmetric
unit contains one crystallographically independent Ni(II) ion
(−1 position, Wyckoff site a) and one H2BDP_SO3 molecule
(general position, Wyckoff site e). Each metal center is
coordinated, in a trans-NiN4O2 octahedral stereochemistry, by
four nitrogen atoms of four distinct ligand moieties and two
oxygen atoms of the −SO3 groups of other two distinct ligands.
The overall packing results in a 3D porous framework with
monodimensional channels of almost square shape running
Figure 8. Variation of the unit cell parameters of NiBDP_SO3H·DMF
(PT) normalized to the corresponding 90 °C values (P90) as a function
parallel to a. The pore size generated by this arrangement is
of the temperature, in the range 90−350 °C. a, green; b, orange; c,
blue; and β, red.
about 9 × 7 Å2 (van der Waals radii corrected) with two DMF
molecules (per cell height) accommodated into the void
volumes. Intriguingly, this new material preserves the flexible
isostructural CoBDP derivative, isolated either as a N,N′-
nature of NiBDP, as it displays a breathing of the framework
related to the amount of solvent molecules located in its pores
(see thermal behavior paragraph). Finally, it is worth noting
that the ligand is deprotonated at the −SO3 functionality, thus
featuring two crystallographically different N−H groups: while
one interacts through a hydrogen bond with an oxygen atom of
the adjacent −SO3 moiety (N−H···O = 2.92 Å), the second
diethylformamide solvate belonging to the orthorhombic space
23a
group P2221
or as a DMF solvate crystallizing in the
monoclinic space group P21/c.19
In Figure S2, the powder diffraction pattern of the nickel-
based framework bearing a sulfonic acid functionality on the
benzene ring is reported. As is visually evident, its diffraction
data differ from those of the other Ni(II) species. Structure
Figure 9. Left: N2 adsorption isotherms measured at 77 K for NiBDP_NH2 (blue), NiBDP_NO2 (orange), NiBDP_OH (green), NiBDP (black),
and NiBDP_SO3H (red). Filled and empty symbols represent adsorption and desorption, respectively. Right: N2 adsorption isotherms measured at
77 K for ZnBDP_NH2 (blue), ZnBDP_NO2 (orange), ZnBDP_OH (green), and ZnBDP (black). Filled and empty symbols represent adsorption
and desorption, respectively.
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Figure 10. CO2 adsorption isotherms measured at 273 K for the MBDP_X (M = Ni, Zn, X = H, NH2, NO2, OH, SO3H) systems. MBDP_NH2
(blue), MBDP_NO2 (orange), MBDP_OH (green), MBDP (black), and NiBDP_SO3H (red).
Table 3. Isosteric Heats (Qst), Henry Constants (KH), and Adsorption Selectivities (α) of the NiBDP_X Systems Obtained from
Pulse Gas Chromatographic Studies
adsorbate
(A)
partition coefficient
partition coefficient
partition coefficient
partition coefficient
a
a
a
a
a
Qst (kJ mol−1
)
KH (cm3 m−2
)
αCH4/A
αCO2/A
αC2H2/A
αH2O/A
NiBDP
C2H2
CO2
CH4
H2O
−25.05(2)
−20.02(9)
0.098(6)
0.052(3)
0.005(1)
5.30(2)
0.072
0.097
0.74
10
54
101
1.35
13.8
0.02
1000
−42.5(6)
0.001
0.01
NiBDP_NH2
C2H2
CO2
CH4
H2O
−32.50(5)
−28.2(1)
0.267(1)
0.0894(4)
0.005(1)
10.3(7)
0.02
0.35
38
115
0.056
2.80
50
17.85
0.01
2052
−51.3(2)
0.0005
0.03
NiBDP_NO2
NiBDP_OH
C2H2
CO2
CH4
−23.3(2)
−25.1(3)
0.0620(9)
0.12(1)
0.09
0.12
0.81
8.13
0.77
3.60
1.23
10
0.005(1)
C2H2
CO2
CH4
−26.10(5)
−21.20(7)
0.19(1)
0.26
0.33
0.0680(2)
0.005(1)
1.29
4.66
a
Calculated from KH relation at 298 K.
one protrudes into the channels. This is further evidenced by
means of IR spectroscopy where, in the region relevant for N−
H stretching modes, two absorption bands, a sharp one at 3440
cm−1 and another one, of a broad shape, at 3150 cm−1, are
found (Figure 3).
MOFs and both the electronic and steric nature of the
functional groups introduced on the organic linkers. However,
here, by considering the electronic effects of each functional
group, a simple correlation between thermal stability and
Hammett σ values of the substituents can be envisaged.20
Indeed, the NH2-tagged ligand is electron rich and the NO2-
tagged one is electron poor, while the −H, −OH, and
−SO3H21 substituents have Hammett σ values somewhere
between. For the Ni(II) series, there is a good correlation
between the tabulated Hammett σ values and the decom-
position temperatures obtained by STA, the highest thermal
stability being observed for the amino-tagged MOF (Td ca. 470
°C). Decomposition temperatures nicely follow the trend NO2
< SO3H < OH < H < NH2 (400 < Td < 470 °C). However, this
is not true for the ZnBDP_X series, along which all samples
(but the amino one, with Td ca. 470 °C) undergo
decomposition at rather similar temperatures (Td ca. 420 °C,
in line with that previously observed for ZnBDP).9a
IR Spectroscopy Characterization. Because each func-
tional group introduced on the organic linkers shows
characteristic signals in the IR region, both series of
MBDP_X were studied by means of IR spectroscopy. Figure
4 shows the comparison between the IR spectra of the parent
NiBDP and the tagged nickel-based MOFs: in the spectra of
the latter, typical absorption modes of the NO2, NH2, OH, and
−
SO3 functional groups are found, further confirming the
effective functionalization of the pore surfaces of each MOF.
The Supporting Information provides a complete description of
the IR spectroscopy characterization.
Thermal Behavior. Simultaneous thermogravimetric and
differential scanning calorimetry analyses (STA) performed on
the two series of nickel(II) and zinc(II) materials demonstrated
the high thermal stability of their pyrazolate-bridged frame-
works, even in the presence of substituents on the organic
linkers (see Figures 5 and S5).
In the TG curves of the as-synthesized forms of all of the
MBDP_X materials (Figures 5 and S5 for NiBDP_X and
ZnBDP_X, respectively), each weight loss step that occurs
below the decomposition temperature can be assigned to the
removal of the occluded guest molecules. Worthy of note, in
the DSC curves the thermal events due to solvent loss and
Lillerud et al. reported5g the difficulty in finding a simple
correlation between the thermal stability of tagged UiO-66
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Table 4. Isosteric Heats (Qst), Henry Constants (KH), and Adsorption Selectivities (α) of the ZnBDP_X Systems Obtained from
Pulse Gas Chromatographic Studies
adsorbate
(A)
partition coefficient
partition coefficient
partition coefficient
partition coefficient
a
a
a
a
a
Qst (kJ mol−1
)
KH (cm3 m−2
)
αCH4/A
αCO2/A
αC2H2/A
αH2O/A
ZnBDP
C2H2
CO2
CH4
H2O
−20.2(2)
−19.7(2)
0.015(6)
0.0093(7)
0.005(1)
2.1(9)
0.85
0.84
1.02
137
228
412
0.98
2.35
0.007
2.39
−46.7(3)
0.0025
0.004
ZnBDP_NO2
ZnBDP_NH2
ZnBDP_OH
C2H2
CO2
CH4
H2O
−23.51(9)
−21.4(1)
0.042(3)
0.022(4)
0.005(1)
2.0(6)
0.11
0.13
0.89
49
93
1.13
18.7
16.6
401
−42.8(6)
0.0025
0.011
0.021
C2H2
CO2
CH4
H2O
−31.82(4)
−28.1(1)
0.16(1)
0.083(3)
0.005(1)
9.4(2)
0.31
0.35
0.90
59
113
1.11
6.39
0.017
5.74
2000
−51.2(6)
0.0005
0.009
C2H2
CO2
CH4
−29.11(8)
−22.9(2)
0.073(3)
0.026(8)
0.005(1)
0.18
0.54
0.34
2.9
12.1
35.7
a
Calculated from KH relation at 298 K.
Figure 11. Pulse gas chromatograms for an equimolecular mixture of C2H2, H2, CH4, and CO2, collected at 273 K. For the sake of simplicity, the
curves are reported only for NiBDP and NiBDP_NH2. Chromatograms for other tagged frameworks are reported in the Supporting Information.
Table 5. Amount of CO2 (mmol g−1) Retained in Dynamic
Conditions As Retrieved from CO2/N2 Breakthrough
Experiments at Different Temperatures for the MBDP_X
Systems
MOF
ZnBDP
273 K
298 K
323 K
0.34
0.63
1.02
0.50
0.69
1.07
1.34
0.89
1.04
ZnBDP_NO2
ZnBDP_NH2
ZnBDP_OH
NiBDP
0.27
0.57
0.20
0.16
0.28
0.15
0.17
0.24
0.32
0.23
a
0.32
Figure 12. CH4/CO2/C2H2 separation breakthrough experiments
performed on NiBDP at 273 K.
NiBDP_NO2
NiBDP_NH2
NiBDP_OH
NiBDP_SO3H
0.38
0.54
a
0.35
material decomposition22 are well separated (by ca. 200 °C),
further confirming the high stability of these frameworks (see
the Supporting Information).
a
Calculated at 303 K.
Variable-temperature X-ray powder diffraction experiments
were also performed to assess the thermal and structural
stability of these metal−organic frameworks. In a typical
experiment, carried out in air, three diffractograms for each
sample were collected at (i) 30 °C, on the solvated material, (ii)
300 °C, and (iii) back to 30 °C, with further addition of solvent
(N,N′-dimethylformamide, DMF) to the sample. As reported
in Figure 6, while preserving the unit cell symmetry, the
desolvation process brings about definite changes in the
positions and intensities of the XRPD peaks. In the NiBDP_X
series, as was already observed for NiBPD,9a the a axis, which
corresponds to the metal chains propagation direction, is the
least affected by the thermal treatment. On the contrary, b and c
undergo higher and inversely correlated variations that well
witness the flexible nature of this 3D framework. Indeed, the
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Figure 13. N2/CO2 separation breakthrough experiments at 273 K, showing the effect of tag nature within the ZnBDP_X series. Breakthrough
curves for other tagged frameworks are reported in the Supporting Information.
rhombic channels deflate with increasing temperature and
release of solvent, leading to progressively smaller unit cell
volumes (Figure 7 and Table 1).
Adsorption Properties. One of the areas of highest
interest in the field of MOFs research is the potential
application of their selective adsorption properties for gas
separation and purification purposes.25 Particularly, the possible
application of these systems in greenhouse gas capture
processes is one of the most challenging research areas.26 To
examine the suitability of the MBDP_X species for this
application, we carried out different studies for characterizing
their porous nature, conventional single component adsorption
isotherms, as well as advanced experiments (pulse gas
chromatography and breakthrough curve measurements).
Static Adsorption Experiments. To evaluate the
permanent porosity of the MBDP_X materials, N2 adsorp-
tion/desorption isotherms at 77 K (see Figure 9) were
collected, and Langmuir specific surface areas were con-
sequently calculated (Table 2). CO2 isotherms at 273 K
(Figure 10) were used to evaluate the micropore region by
means of the Dubinin−Radushkevich equation (Table 2). All of
the tagged MOFs were found to retain porosity to both N2 and
CO2 despite the presence of different functional groups on the
linker.
In our previous study,9a the parent ZnBDP framework was
found to be rather stiff when heated in the diffractometer
cradle. This rigid behavior was attributed to the stereochemical
rigidity of the ZnN4 tetrahedron and of its polyconnected
tetragonal lattice. Here, however, the tagged ZnBDP_X series
shows a higher degree of flexibility promoted by solvent release
(Figure 6). This is particularly evident for ZnBDP_NH2, whose
HT phase cannot be easily traced back to the solvated one due
to profound changes in the structure. On the contrary, the HT
phases of the NO2- and OH-tagged MOFs can still be related to
their solvated counterparts with smaller unit cell volumes
(Table 1). Surprisingly, even for ZnBDP_NH2, after exposure
of the HT phases to DMF, the original diffraction patterns are
regenerated, as a further confirmation of the high stability and
flexible nature of our tagged pyrazolate-based MOFs. For both
series, these observations are consistent with an accordion-type
flexing behavior that closes and opens the channel pores as a
function of the external stimulus, as previously observed, for
example, not only for NiBDP,9a but also for Long’s CoBDP,23
The Langmuir specific surface areas were found to range
between 1044 and 1645 m2/g and 1484 and 2860 m2/g for
NiBDP_X and ZnBDP_X, respectively. Moreover, they follow
Ferey
́
’s MIL-53 and MIL-88 families,5f and Yaghi’s ZIF-7.24
Another surprising behavior was detected on the NiBD-
P_SO3H material with a thermodiffractometric experiment.
Diffraction data were collected on the DMF-solvated
NiBDP_SO3H·2DMF in the range 30−350 °C with steps of
20 °C. By raising the temperature to 90 °C, NiBD-
P_SO3H·2DMF loses solvent with a fast kinetics (less than
30 min) leading to another metastable solvate, NiBD-
P_SO3H·DMF, with a lower solvent content. Structure
solution on data collected at this temperature shows that,
while the overall stereochemistry of the Ni(II) ions remains
intact, the lower solvent content induces a deformation of the
channels to a rhombic shape, with a concomitant unit cell
volume shrinking of about 9%. Indeed, in this crystal structure,
only one DMF molecule (per cell height) is present in each
cavity, disordered on two, crystallographically equivalent, sites.
Further heating of NiBDP_SO3H·DMF produces further cell
parameter changes in the same direction as those described
above for the NiBDP compound, with limited cell volume
variation. The results of a structureless Le Bail refinement of
the TXRPD data in the range 90−350 °C are reported in
Figure 8. Again, a is only marginally affected, while b and c
show significant and inversely correlated variations. Moreover,
at 250 °C, a structure solution process revealed that the
framework maintains the same stereochemistry at the Ni(II)
ions, with empty rhombic voids of about 6 × 8 Å2 (van der
Waals radii corrected).
different trends within the two series, NiBDP_NH2
NiBDP_NO2 > NiBDP_OH > NiBDP > NiBDP_SO3H for
the nickel(II) materials, and ZnBDP > ZnBDP_NO2
>
>
ZnBDP_NH2 > ZnBDP_OH along the zinc(II) series. The
decrease in surface areas for the zinc(II) tagged frameworks, as
compared to the parent ZnBDP structure, follows the V/Z
trend and can be largely attributed to both the reduced free
space available and an overall increased weight of the MOFs as
a result of introducing substituents on the organic linker.
Within the tagged materials, ZnBDP_NO2 shows the highest
surface area, concomitant to the presence of a hysteretic loop.
This feature is observed also in the isotherm of the amino-
tagged MOF, although to a lesser extent. By contrast,
ZnBDP_OH shows the lowest N2 uptake, with a classical
type I isotherm. For flexible-type MOFs with hydroxyl groups
decorating the pores walls, the formation of intraframework
interactions can be envisaged to explain the low uptake.5e The
occurrence of hysteresis loops along the N2 isotherm has been
previously observed in Long’s CoBDP.22 As was already
pointed out, ZnBDP_X and CoBDP are isostructural, and,
consequently, a similar flexible behavior can be expected.
Noteworthy, as was already pointed out in the thermal behavior
paragraph, although ZnBDP behaved only as a rigid frame-
work,9a the introduction of tags is responsible for an increased
flexibility.
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CO2 isotherms at 273 K were also collected to investigate the
adsorption properties of each tagged framework with respect to
this gas of environmental significance.27 Indeed, the results
show that the introduction of tags on the BDP linkers gives rise
to a general increase of the steep of the adsorption isotherm in
the low pressure region, which can be attributed to a narrowing
of the pore size with a concomitant increase of the adsorbate−
adsorbent interactions. This fact is particularly evident in the
case of the −NH2 substituents, in the presence of which the
formation of H-bonding interactions with the quadrupolar CO2
molecules should be claimed to explain the enhancement of the
adsorption energy.28 These results are further confirmed on the
basis of variable-temperature pulse gas experiments (see
below).
results show that CH4 is not retained in the column in the
essayed temperature range (273−323 K). The highest
interactions of the material take place with the quadrupolar
CO2 and C2H2 gases, which are efficiently retained in the
essayed experimental conditions (Figure 12). For both gases,
the interaction is similarly strong, and, consequently, in CH4/
CO2/C2H2 mixtures, both CO2 and C2H2 are jointly retained,
although there is a higher retention of the latter.
Later, a much simpler experiment was carried out on the
whole series of tagged MBDP_X to simulate an emission flue
gas from a power plant employing 1.4 mL min−1 of CO2 and
8.6 mL min−1 of N2 flows.30 Indeed, the breakthrough
experiments were carried out by step changes from He to
CO2/N2 flow mixtures. Again, the results show that N2 is not
retained in the column, while, in the essayed temperature range
(273−323 K), a high interaction takes with CO2. Framework
functionalization plays an important role also in this separation;
as a matter of fact, according to the low pressure region of the
CO2 adsorption isotherms collected at 273 K, the highest
interaction was achieved for the MBDP_NH2 systems (see
Table 5 and Figure 13). Noteworthy, the quantities of CO2
retained at 323 K, which is the typical condition of flue gas
emission from a fossil fuel power plant, are 0.28 and 0.32 mmol
g−1 for ZnBDP_NH2 and NiBDP_NH2, respectively.
Pulse Gas Chromatographic Studies. To assess the
possible application of these materials on gas purification
processes, a chromatographic column was constructed and
packed with each tagged MOF. The samples were first
pelletized to avoid pressure drops over the column. Afterward,
the adsorption selectivity of the materials was studied by
injecting a complex gas mixture (1 mL), composed of
equimolecular amounts of C2H2, H2, CH4, and CO2, on an
inert He flow (15 mL min−1) as carrier gas. To quantify the
strength of the interaction of these guest molecules with the
tagged materials, their zero-coverage adsorption heats (ΔHads)
were derived from the variation of the retention volumes (Vg)
as a function of temperature, according to the Clausius−
Clapeyron type equation ΔHads = −Rδ(ln Vg)/δ(1/T).29 The
direct relation between the retention volume (Vg) and Henry
constant (KH) also permits one to calculate the Henry constant
values at 298 K (Tables 3 and 4). The results for the NiBDP_X
series show that the retention volumes follow the trend H2O ≫
C2H2 > CO2 ≫ CH4 > H2 (see Figure 11; water is not shown
because it has been measured separately). The interaction of H2
with the frameworks is almost negligible, and, consequently, it
has been taken as reference value for the column dead volume.
Noteworthy, the introduction of polar tags in the framework
is responsible for an enhancement of the interaction with polar
adsorbates (higher adsorption energies), and a concomitant
increase in the partition coefficients between apolar (CH4) and
polar (CO2, C2H2) adsorbates (Tables 3 and 4). However, the
adsorption selectivity (partition coefficients) between CO2/
C2H2 is not significantly increased upon functionalization,
which should be attributed to the closely related nature of these
two types of adsorbates (possessing quadrupole moments,
similar size, and similar physical properties). From these results,
it can be concluded that the tagged materials are adequate for
gas purification purposes, particularly for the removal of small
impurities of polar gases and polar vapors (e.g., moisture, CO2,
C2H2) from natural gas.
CONCLUSIONS
■
The foregoing results confirm that pyrazolate-based ligands can
successfully respond to the challenge of constructing
isoreticular families of MOFs bearing organic functionalities,
as a mean to tune pore size, shape, and volume, hence the
adsorption capacity and selectivity toward specific guests.
Specifically, we exploited the versatility of the H2BDP ligand for
the straightforward introduction, on its benzene moiety, of
different tags (−NO2, −NH2, −OH, −SO3H). By applying the
appropriate reaction conditions, the tagged H2BDP_X spacers
were successfully employed in the isolation, on the gram scale,
of the two series of NiBDP_X and ZnMBDP_X materials. To
the best of our knowledge, they represent the first families of
robust, isoreticular pyrazolate-based MOFs with tagged ligands
whose thermal behavior and adsorption properties have been
systematically studied.
XRPD experiments, in isothermal conditions or varying the
temperature, demonstrated the flexibility of their frameworks,
existing in open- or closed-pore forms in response to stimuli
such as solvent content or temperature modification. Despite
the thermally induced flexibility of their frameworks, all of the
materials not only maintain their functionality, but also exhibit a
remarkable thermal stability, decomposing, in air, only at 400
°C or higher temperatures. Conventional single component
adsorption isotherms and advanced experiments of pulse gas
chromatography and breakthrough curve measurements
allowed one to rationalize the effect of linker functionalization
on the adsorption selectivity and capacity toward gases of
environmental interest. Noteworthy, the adsorption selectivities
(partition coefficients) for the resolution of mixtures of apolar
and polar gases are highly improved (i.e., N2/CO2 or CH4/CO2
mixtures) with the introduction of polar tags. Nevertheless, this
strategy fails in the improvement of the separation of polar gas
mixtures (e.g., CO2/C2H2): in this case, the adsorption
selectivity is not significantly increased upon functionalization,
which should be attributed to the closely related nature of these
two adsorbates (in terms of quadrupole moments, size, and
physical properties). The reported results collectively evidence
Gas Separation Breakthrough Experiments. In the case
of bulk separation of gases, breakthrough experiments are the
adequate experiments to assess the gas separation performances
of a material. Thus, gas mixture adsorption breakthrough
experiments were performed on columns packed with each
tagged MOF. As for the above-described experiments, the
samples were first pelletized to avoid pressure drops over the
column, and the desired gas mixture was flowed through the
material.
On the nonfunctionalized NiBDP system, a preliminary
breakthrough experiment with a complex gas mixture of CH4,
CO2, and C2H2 was performed to assess, qualitatively, the
interaction of the framework with these gases. As expected, the
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that our tagged materials are adequate for gas purification
purposes, particularly for the removal of small impurities of
polar gases and polar vapors (e.g., moisture, CO2, C2H2) from
low polar ones (e.g., N2, CH4, H2).
(6) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.;
O’Keeffe, M.; Yaghi, O. M. Science 2003, 300, 1127.
ASSOCIATED CONTENT
* Supporting Information
Additional figures and experimental details, as well as CIF files.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
(7) Deng, H.; Doonan, C. J.; Furukawa, H.; Ferreira, R. B.; Towne, J.;
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AUTHOR INFORMATION
Corresponding Author
valentina.colombo@uninsubria.it; jarn@ugr.es
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
V.C. gratefully acknowledges the Regione Lombardia (Progetto
DOTE, COMIX Challenges in OrganoMetallic Investigations
by X-rays) for partial funding. The Spanish MCINN
(CTQ2011-22787/PPQ) is gratefully acknowledged for
generous funding.
(13) Diaz, E.; Ordonez, S.; Vega, A. J. Colloid Interface Sci. 2007, 305,
̃
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procedure for the same material where they were reacting
Zn(CF3SO3)2 and the ligand in N,N-diethylformamide in a boron
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