Scandium/Alkaline Metal-Organic Frameworks: Adsorptive Properties and Ionic Conductivity

Article abstract of DOI:10.1021/acs.chemmater.5b03458

Several synthetic approaches have been employed to obtain novel {[ScM(μ₄-pmdc)₂(H₂O)₂]·solv}_n [EHU1(Sc,M)] (where M = Li, Na; pmdc = pyrimidine-4,6-dicarboxylate; solv = corresponding solvent) compounds. The synthesis method is crucial to determine the type of alkaline that could be hosted in the structure as well as the crystallinity, adsorption performance, and ionic conductivity of the resulting materials. Compared with other synthetic methods, a heat-assisted solvent-free procedure has proven to be the most effective route, giving materials with adsorption capacities close to those expected from GCMC (Grand Canonical Monte Carlo) calculations. Despite the presence of alkaline ions in the framework, the pristine materials exhibit rather low conductivity values of ca. 10^-7 S cm^-1. The concentration of charge carriers has been increased by means of a doping approach that incorporates divalent transition metal ions to the structure and forces an increase of the alkaline ions, thus raising the ionic conductivity by 1 order of magnitude. Additionally, soaking the samples in solutions containing alkaline salts led to materials possessing an even higher number of carriers achieving conductivity values among the best results reported for MOFs at room temperature, i.e., 4.2 × 10^-4 and 9.2 × 10^-5 S cm^-1 for EHU1(Sc,Li) and EHU1(Sc,Na) obtained by the solvent-free procedure, respectively.

Full text of DOI:10.1021/acs.chemmater.5b03458

Article
pubs.acs.org/cm
Scandium/Alkaline Metal−Organic Frameworks: Adsorptive
Properties and Ionic Conductivity
Javier Cepeda,*,† Sonia Perez-Yanez,‡ Garikoitz Beobide,‡ Oscar Castillo,*,‡ Eider Goikolea,§
́
́
̃
Frederic Aguesse,§ Leoncio Garrido,∥ Antonio Luque,‡ and Paul A. Wright⊥
†Polymer Science and Technology Department, University of the Basque Country, UPV/EHU, 48940 Donostia, Spain
‡Department of Inorganic Chemistry, University of the Basque Country, UPV/EHU, 48080 Bilbao, Spain
§CIC Energigune, Arabako Parke Teknologikoa, E01510 Minano, Spain
̃
∥Instituto de Ciencia y Tecnología de Polímeros, Consejo Superior de Investigaciones Científicas (ICTP-CSIC), Juan de la Cierva 3,
28006 Madrid, Spain
⊥EaStCHEM, School of Chemistry, University of St Andrews, Purdie Building, North Haugh, St. Andrews, Fife KY16 9ST, United
Kingdom
S
* Supporting Information
ABSTRACT: Several synthetic approaches have been em-
ployed to obtain novel {[ScM(μ4-pmdc)2(H2O)2]·solv}n
[EHU1(Sc,M)] (where M = Li, Na; pmdc = pyrimidine-4,6-
dicarboxylate; solv = corresponding solvent) compounds. The
synthesis method is crucial to determine the type of alkaline
that could be hosted in the structure as well as the crystallinity,
adsorption performance, and ionic conductivity of the resulting
materials. Compared with other synthetic methods, a heat-
assisted solvent-free procedure has proven to be the most
effective route, giving materials with adsorption capacities close
to those expected from GCMC (Grand Canonical Monte
Carlo) calculations. Despite the presence of alkaline ions in the
framework, the pristine materials exhibit rather low con-
ductivity values of ca. 10−7 S cm−1. The concentration of charge carriers has been increased by means of a doping approach that
incorporates divalent transition metal ions to the structure and forces an increase of the alkaline ions, thus raising the ionic
conductivity by 1 order of magnitude. Additionally, soaking the samples in solutions containing alkaline salts led to materials
possessing an even higher number of carriers achieving conductivity values among the best results reported for MOFs at room
temperature, i.e., 4.2 × 10−4 and 9.2 × 10−5 S cm−1 for EHU1(Sc,Li) and EHU1(Sc,Na) obtained by the solvent-free procedure,
respectively.
research plays a key role in this technology expansion, where
the development of new solid electrolytes possessing high
conductivity values at temperatures up to 60 °C would be of
great importance.24 Currently, macroporous membranes
swelled in organic electrolyte are mainly employed as Li-ion
battery separators.25 They lack rigidity and may be pierced by
lithium dendrites causing cell failure.26 In this regard, the use of
MOFs as solid separators could afford the required coexistence
of rigidity and dynamics to avoid the latter problems.27
Additionally, the actual concerns on the global lithium
resources availability are stimulating the battery research
toward the use of other naturally abundant elements, for
instance, Na, which could also lead to inexpensive batteries for
portable electronics and stationary applications.28,29
INTRODUCTION
■
Metal−organic frameworks (MOFs) constitute a rapidly
growing class of materials due to their permanent porosity,
high surface area, large pore volume, adjustable pore size,
uniform distribution, and shape.1−6 This is the reason why they
have emerged as promising candidates for adsorptive
separations and purification purposes, particularly those
concerning the CO2 capture and sequestration technolo-
gies.7−10 Moreover, these materials have been extensively
investigated for possible applications in sensing,11,12 drug
delivery,13,14 catalytic activity,15,16 optoelectronics,17,18 and
magnetism,19 since they benefit from their hybrid metal−
organic nature. However, although lithium-ion conductivity of
these materials has been observed, only few reports concerning
the use of MOFs as Li-ion battery electrolytes have been
published to date.20,21 Characterized by a high specific energy
and efficiency, Li-ion batteries have become the energy storage
systems of choice for consumer electronics.22,23 Electrolyte
Received: September 7, 2015
Revised: March 30, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.chemmater.5b03458
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Synthesis of {[ScNa(μ4-pmdc)2(H2O)2]·DMF·2H2O}n [EHU1-
(Sc,Na)-S2]. A 25 mL amount of a dimethylformamide (DMF)/
methanol (MeOH) (1:1) solution containing 0.300 mmol of ScCl3
(207 μL of 1.45 M solution), 0.600 mmol of H2pmdc (0.1224 g), and
0.300 mmol of NaCl (0.0175 g) was heated by following the above-
mentioned solvothermal procedure at 120−160 °C. A mixture of
polycrystalline sample with few, small, and bad quality single crystals
was achieved. Anal. Calcd for C15H19N5NaO13Sc (545.28 g mol−1): C,
33.04; H, 3.51; N, 12.84; Na, 4.22; Sc, 8.24. Found: C, 32.88; H, 3.65;
N, 13.13; Na, 4.58; Sc, 8.14. FT-IR (KBr, cm−1): 3455 (s), 3225 (sh),
3115 (sh), 3030 (w), 2925 (w), 1650 (vs), 1620 (sh), 1595 (s), 1545
(s), 1465 (w), 1450 (w), 1426 (m), 1380 (vs), 1315 (m), 1285 (w),
1205 (sh), 1190 (m), 1100 (w), 1030 (w), 930 (w), 890 (w), 845 (w),
810 (w), 745 (s), 720 (sh), 700 (s), 585 (w), 535 (w), 460 (w).
Synthesis of {[ScNa(μ4-pmdc)2(H2O)2]·5H2O}n [EHU1(Sc,Na)-SF1].
This compound is achieved by means of an oven-heating solvent-free
procedure. To that end, 0.310 mmol of H2pmdc (0.0633 g) was
thoroughly hand ground with 0.155 mmol of sodium chloride (0.0091
g) by means of a mortar until a fine homogeneous powder was
obtained. Then 0.075 mmol of Sc2O3 (0.0103 g) was ground with the
previous mixture, and the resulting powder was sealed into a long-stem
glass vacuole (1 cm × 10.8 cm, 1 mL capacity) and placed in an oven.
There it was heated up to 110 °C, maintained at this temperature for
72 h, and cooled down to room temperature (1.5 °C/h). Anal. Calcd
for C12H18N4NaO15Sc (526.24 g mol−1): C, 27.39; H, 3.45; N, 10.65;
Na, 4.37; Sc, 8.54. Found: C, 27.26; H, 3.33; N, 10.80; Na, 4.28; Sc,
8.39. FT-IR (KBr, cm−1): 3425 (s), 3230 (sh), 3115 (sh), 1655 (vs),
1630 (sh), 1600 (s), 1560 (s), 1470 (w), 1426 (sh), 1385 (s), 1355
(vs), 1305 (sh), 1285 (w), 1205 (m), 1100 (w), 1040 (w), 915 (w),
855 (w), 810 (w), 745 (s), 730 (sh), 695 (s), 535 (w), 495 (w), 460
(w).
Synthesis of {[ScNa(μ4-pmdc)2(H2O)2]·6H2O}n [EHU1(Sc,Na)-SF2].
This compound is obtained following the previously described solvent-
free procedure but using 0.155 mol of sodium nitrate (0.0132 g)
instead of sodium chloride. Anal. Calcd for C12H20N4NaO16Sc (544.25
g mol−1): C, 26.48; H, 3.70; N, 10.29; Na, 4.22; Sc, 8.26. Found: C,
26.42; H, 3.54; N, 10.46; Na, 4.17; Sc, 8.31. FT-IR (KBr, cm−1): 3430
(s), 3235 (sh), 3115 (sh), 1665 (vs), 1635 (s), 1610 (s), 1560 (s),
1470 (w), 1430 (m), 1380 (sh), 1355 (vs), 1310 (sh), 1210 (m), 1100
(w), 1035 (w), 1015(sh), 915 (w), 855 (w), 815 (w), 745 (s), 730
(sh), 690 (w), 635 (w), 595 (w), 530 (w), 480 (w).
Synthesis of {[ScLi(μ4-pmdc)2(H2O)2]·5H2O}n [EHU1(Sc,Li)-SF2].
The compound is obtained following the previously described solvent-
free procedure but using 0.155 mol of lithium nitrate (0.0107 g). Anal.
Calcd for C12H18LiN4O15Sc (510.19 g mol−1): C, 28.25; H, 3.56; Li,
1.36; N, 10.98; Sc, 8.82. Found: C, 28.51; H, 3.45; Li, 1.53; N, 10.77;
Sc, 8.71. FT-IR (KBr, cm−1): 3430 (s), 3235 (sh), 3115 (sh), 1660
(vs), 1630 (s), 1605 (s), 1560 (s), 1480 (w), 1425 (sh), 1385 (s),
1360 (vs), 1305 (sh), 1205 (m), 1100 (w), 1035 (w), 920 (w), 855
(w), 815 (w), 745 (s), 730 (sh), 690 (s), 600 (w), 535 (w), 490 (w).
Synthesis of {[Sc0.95Na1.05Cd0.05(μ4-pmdc)2(H2O)2]·6H2O}n [Cd@
EHU1(Sc,Na)-SF2]. This compound is obtained by the previously
described solvent-free procedure in which 0.071 mmol of Sc2O3
(0.0098 g), 0.162 mmol of NaNO3 (0.0138 g), and 0.008 mmol of
Cd(OH)2·2H2O (0.0015 g) were mixed with previously hand ground
0.310 mmol of H2pmdc (0.0633 g). Anal. Calcd for
C12H20Cd0.05N4Na1.05O16Sc0.95 (548.77 g mol−1): C, 26.26; H, 3.67;
N, 10.21; Cd, 1.02; Na, 4.40; Sc, 7.78. Found: C, 26.02; H, 3.64; N,
10.11; Cd, 1.01; Na, 4.36; Sc, 8.32. FT-IR (KBr, cm−1): 3430 (s), 3235
(sh), 3115 (sh), 1670 (vs), 1640 (s), 1610 (s), 1560 (s), 1475 (w),
1430 (m), 1360 (vs), 1300 (sh), 1280 (sh), 1235 (sh), 1215 (m), 1100
(w), 1035 (w), 925 (w), 855 (m), 815 (w), 745 (s), 730 (sh), 695 (s),
635 (w), 595 (w), 530 (m), 480 (w).
In terms of construction, MOFs are set under equilibrium
conditions between metal and ligands, so the variation of the
kinetics by means of the synthetic factors could give profound
changes on the sample crystallinity and, consequently, on the
physical properties of the material. For instance, it has been
stated that the space charge of solid interfaces, besides the
adsorption behavior, exerts a crucial influence on the
conductivity.30 In our research group we extensively employed
pyrimidine-4,6-dicarboxylic acid (H2pmdc) according to its
ability to coordinate to different metal centers by displaying
several coordination modes owing to its large number of donor
atoms.31−35 In fact, architectures from 1D to 3D polymers can
be achieved depending on the ion size and charge of the metal
atoms.36 In particular, this system has us allowed to successfully
give rise to a series of porous MOFs of formula {[CdM(μ4-
pmdc)2(H2O)2]·solv}n (MII = Cd, Mn, Zn) that behave as good
CO2 adsorbents under certain preparation conditions.37 This
framework features two markedly different coordination
environments, which allows selective replacement of the metals
and in turn enables the properties of the material to be tailored.
Taking this fact into consideration, we selected scandium(III)
to partially replace cadmium(II) in the parent MOF due to its
similar ion size and coordination indices (see Supporting
Information).38−40 This trivalent metal also imposes a charge
imbalance that requires monovalent ions to be present in the
crystal structure (M3+ + M+: 2 × pmdc2−). Sc3+ ion is, as well, a
very good candidate to bypass the poorly reversible reactions
observed for most of the MOFs employed as cathode materials
given its null occupation of 3d orbitals. It also features higher
M−O bond stability in the resulting material regarding charge
variations, and it brings about long-range electron delocaliza-
tion by means of the stabilization of mixed-valence states.
Therefore, two new heterometallic MOFs have been
designed employing scandium(III) and alkaline metals as
monovalent ions. These materials provide, in addition to a
high specific surface area, an ordered distribution of alkaline
ions among the crystal structure. Taking advantage of that
structural characteristic, two different procedures have been
designed to endow the insulating material with additional
alkaline ions in an attempt to obtain ionic conductor MOFs.
EXPERIMENTAL SECTION
■
Chemicals. All chemicals were of reagent grade and used as
commercially obtained. The starting material pyrimidine-4,6-dicarbox-
ylic acid was prepared following the previously reported procedure.41
The stock solution of ScCl3 (1.45 M) was prepared dissolving 0.4999 g
of Sc2O3 in 1.8 mL of HCl (37%) and adding water up to 5 mL in a
calibrated flask.
Synthesis of {[ScNa(μ4-pmdc)2(H2O)2]·5H2O}n [EHU1(Sc,Na)-S1].
A 0.300 mmol amount of ScCl3 (207 μL of 1.45 M solution) was
added to 25 mL of a water solution containing 0.600 mmol of H2pmdc
(0.1224 g). A 1 M NaOH solution was added dropwise until a white
precipitate appeared (pH 2.8−4.6). The mixture was sealed into a
Teflon-lined stainless steel autoclave under autogenous pressure and
heated at 170−180 °C for 3 days. Then it was slowly cooled down to
room temperature (2 °C/h), and a polycrystalline sample of
EHU1(Sc,Na)-S1 was achieved. Anal. Calcd for C12H18N4NaO15Sc
(526.24 g mol−1): C, 27.39; H, 3.45; N, 10.65; Na, 4.37; Sc, 8.54.
Found: C, 27.57; H, 3.37; N, 10.42; Na, 4.54; Sc, 8.73. FT-IR (KBr,
cm−1): 3435 (s), 3230 (m), 3115 (m), 1670 (vs), 1645 (sh), 1625 (s),
1595 (sh), 1545 (m), 1475 (w), 1435 (m), 1380 (vs), 1305 (sh), 1285
(w), 1205 (sh), 1185 (w), 1015 (w), 945 (w), 920 (w), 845 (w), 815
(w), 725 (sh), 720 (s), 700 (m), 655 (w), 620 (m), 590 (m), 545 (m),
450 (m).
Synthesis of {[Sc0.95Li1.05Cd0.05(μ4-pmdc)2(H2O)2]·5H2O}n [Cd@
EHU1(Sc,Li)-SF2]. This compound is obtained following the synthesis
of Cd@EHU1(Sc,Na)-SF2 but using 0.162 mmol of LiNO3 (0.0112
g) instead of NaNO3. Anal. Calcd for C12H18Cd0.05Li1.05N4O15Sc0.95
(513.91 g mol−1): C, 28.05; H, 3.53; Cd, 1.09; Li, 1.42; N, 10.90; Sc,
8.31. Found: C, 27.64; H, 3.48; Cd, 1.08; Li, 1.40; N, 10.74; Sc, 9.14.
FT-IR (KBr, cm−1): 3430 (s), 3235 (sh), 3115 (sh), 1660 (vs), 1630
B
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(s), 1610 (s), 1560 (s), 1470 (w), 1430 (m), 1385 (sh), 1355 (vs),
1305 (sh), 1235 (sh), 1205 (m), 1105 (w), 1035 (w), 920 (w), 855
(w), 815 (w), 745 (s), 730 (sh), 695 (s), 630 (w), 600 (w), 530 (w),
450 (w).
rotors. All reported data were acquired at room temperature. A
standard Bruker double-resonance 4 mm cross-polarization (CP)/
magic angle spinning (MAS) NMR probe head was used. The 45Sc
measurements were performed at MAS spinning rates of 11 kHz and
the rest at 7 kHz. All spectra were acquired using a single-pulse
sequence with the exception of the 13C spectra that were acquired with
a CP/MAS pulse sequence, 3 ms CP contact time, and sideband
suppression (seltics), and the 113Cd spectra were acquired with a CP/
MAS sequence, both with proton decoupling (see Supporting
Information for further details). Scanning electron microscopy
(SEM) measurements were performed on a Jeol JSM 6400.
Synthesis of {[Sc0.95Na1.05Mn0.05(μ4-pmdc)2(H2O)2]·6H2O}n [Mn@
EHU1(Sc,Na)-SF2]. The same solvent-free procedure was employed
grinding 0.071 mmol of Sc2O3 (0.0098 g), 0.162 mmol of NaNO3
(0.0138 g), and 0.008 mmol of Mn(acetate)2·4H2O (0.0020 g) over
previously ground 0.310 mmol of H2pmdc (0.0633 g) to give rise to
M n @ E H U 1 ( S c , N a ) - S F 2 . A n a l . C a l c d f o r
C12H20N4Mn0.05Na1.05O16Sc0.95 (545.90 g mol−1): C, 26.40; H, 3.69;
Mn, 0.50; N, 10.26; Na, 4.42; Sc, 7.82. Found: C, 25.98; H, 3.63; Mn,
0.50; N, 10.10; Na, 4.35; Sc, 8.75. FT-IR (KBr, cm−1): 3430 (s), 3235
(sh), 3115 (sh), 1670 (vs), 1635 (s), 1615 (s), 1560 (s), 1465 (w),
1435 (m), 1360 (vs), 1310 (m), 1280 (sh), 1240 (w), 1210 (m), 1110
(w), 1040 (w), 925 (w), 855 (m), 815 (w), 745 (s), 730 (sh), 695 (s),
620 (w), 585 (w), 530 (m), 470 (w), 435 (w).
Synthesis of {[Sc0.95Li1.05Mn0.05(μ4-pmdc)2(H2O)2]·5H2O}n [Mn@
EHU1(Sc,Li)-SF2]. This compound is obtained following the synthesis
of Mn@EHU1(Sc,Na)-SF2 but using 0.162 mmol of LiNO3 (0.0112
g). Anal. Calcd for C12H18Li1.05Mn0.05N4O15Sc0.95 (511.03 g mol−1): C,
28.20; H, 3.55; Li, 1.43; Mn, 0.54; N, 10.96; Sc, 8.36. Found: C, 27.69;
H, 3.49; Li, 1.43; Mn, 0.53; N, 10.76; Sc, 9.39. FT-IR (KBr, cm−1):
3430 (s), 3235 (sh), 3115 (sh), 1670 (vs), 1640 (sh), 1605 (s), 1565
(s), 1470 (w), 1430 (m), 1385 (s), 1360 (vs), 1305 (sh), 1235 (sh),
1205 (m), 1105 (w), 1040 (w), 1015 (sh), 915 (w), 855 (w), 815 (w),
745 (s), 730 (sh), 695 (s), 635 (w), 530 (w), 445 (m).
Excess amounts of ligand and alkaline salts have been employed in
all solvent-free syntheses to ensure a complete reaction of Sc2O3, such
that there is no more nonreacted reagent in the resulting samples.
Moreover, samples were washed with a water/ethanol mixture to
ensure that starting H2pmdc and alkaline salts are removed and dried
in open atmosphere (X-ray powder patterns confirm the purity of the
samples).
X-ray Diffraction Data Collection. The X-ray powder diffraction
(PXRD) patterns were collected on a Phillips X’PERT powder
diffractometer with Cu Kα radiation (λ = 1.5418 Å) over the range 5°
< 2θ < 50° with a step size of 0.02°, a variable automatic divergence
slit, and an acquisition time of 2.5 s per step at 25 °C. Indexation of
the diffraction profiles was made by means of the FULLPROF
program (pattern-matching analysis).42 Variable-temperature X-ray
powder diffraction measurements were run under ambient atmosphere
with heating rates of 5 °C min−1; a complete diffraction pattern was
measured every 20 °C.
Computational Details: GCMC Calculations. Force-field based
Grand Canonical Monte Carlo (GCMC) simulations of single-
component (N2 and CO2) adsorption were carried out using the
SORPTION module included in the Accelrys “Materials Studio”
package.47 The theoretical background of GCMC simulations is
described in detail elsewhere.48 For comparison with experimental
data, simulations of single-component isotherms were carried out
under the same conditions for each adsorbate (P < 1 bar; N2 at 77 K,
CO2 at 196, 273, and 298 K). The simulations of adsorption isotherms
involved 4 million equilibration steps and 6 million production steps.
Models of Fluid Molecules. In all simulations, dispersive and
electrostatic interactions were taken into account. Dispersive
interactions were modeled using a Lennard−Jones (LJ) 12-6 potential.
Parameters to represent the interaction between different atom types
were calculated using Lorentz−Berthelot mixing rules. A cutoff radius
of 12.5 Å was employed for dispersive interactions. Electrostatic
interactions were modeled by assigning point charges to the atomic
sites, and an Ewald summation was used to account for the periodicity
of the simulation box. The representation of the models and
parameters used for the fluid molecules can be found elsewhere.37
The selection of the models and parameters used to define the fluid
molecules has been done based on previous studies. For the N2
molecule, the LJ parameters were taken from the TraPPE model.49
This model simulates the quadrupolar moment of the N2, placing two
negative charges (−0.482) in the positions of the nitrogen atoms and a
positive one in the center of mass (+0.964). The LJ parameters used to
́
represent the CO2 interactions were taken from the work of Garcia-
San
́
chez and co-workers50 and consist of a modified version of the
TraPPE potential model. The combination of these parameters with
the distribution of the charges calculated in a previous work has
proved to be suitably adjusted to experimental data of different
MOFs.51
Physical Measurements. Elemental analyses (C, H, N) were
performed on an Euro EA Elemental Analyzer, whereas the metal
content was determined by inductively coupled plasma (ICP-AES)
and absorption spectrometry performed on a Horiba Yobin Yvon
Activa spectrometer and a PerkinElmer Analyst 100, respectively. The
IR spectra (KBr pellets) were recorded on a FTIR 8400S Shimadzu
spectrometer in the 4000−400 cm−1 spectral region. Thermal analyses
(TG/DTA) were performed on a TA Instruments SDT 2960 thermal
analyzer in a synthetic air atmosphere (79% N2/21% O2) with a
heating rate of 5 °C min−1. All samples were activated in vacuo for 6 h
at temperatures ranging from 100 to 200 °C depending on the sample.
Nitrogen physisorption data were recorded with a Micromeritics
Tristar II 3020. The specific surface area was calculated from the
adsorption branch in the relative pressure interval using the Brunauer−
Emmett−Teller (BET) method43 and the consistency criteria
proposed by Walton and Snurr that is commonly applied for
MOFs,44,45 while the micropore volume was estimated by fitting the
measured N2 isotherms with the t-plot method.46 The volumetric
carbon dioxide physisorption data were recorded in a Micromeritics
ASAP 2020 porosity analyzer at 298 and 273 K and in a Hiden IGA
automatic gravimetric porosimeter at 196 K (ethanol/dry ice mixture).
Solid state NMR measurements were performed in a Bruker Avance
400 spectrometer equipped with a 89 mm wide bore, 9.4 T
superconducting magnet (Larmor frequencies of 400.14 (1H),
376.51 (19F), 155.51 (7Li), 100.61 (13C), 97.20 (45Sc), and 88.80
Models of Adsorbents. In all cases 2 × 2 × 2 supercells were
employed. The LJ parameters for all atoms of the adsorbents were
taken from the universal force field (UFF).52 The partial charges to
represent the electrostatic potential inside the pores were derived from
DFT calculations using the ESP method as described by Singh and
Kollman,53 which is implemented in the DMOL3 code.54 For this
calculation the DNP basis set and the PBE exchange-correlation
functional were selected.55
Conductivity Measurements. Electrochemical impedance spec-
troscopy (EIS) was performed using a Solartron 1260 gain phase
analyzer from 1 MHz down to 1 Hz. The collected materials were
pressed into 5 mm diameter pellets by uniaxial pressing at 3 T for 1
min. The pellets were placed in a Swagelok cell firmly closed to ensure
the good contact between the electrolyte and the stainless steel
plungers. The cell assembly was performed in air, exposing the pellet
to atmospheric moisture. Once sealed the system was isolated from the
atmosphere. Z-view software was used to fit the EIS data using a
parallel RC compound in series with an open Warburg function.
EHU1(Sc,Li)·(LiBF4) and EHU1(Sc,Na)·(NaPF6) samples were
soaked in 1 M solutions of LiBF4 and NaPF6 salts, respectively, in
dimethyl carbonate (DMC) and stirred overnight. The resulting
mixtures were then filtered, rinsed with an excess of DMC, and dried
at 60 °C under vacuum. Arrhenius plots were obtained from the
measurements at different temperatures from 20 to 100 °C.
(
113Cd) MHz). Powdered samples were placed in 4 mm zirconia
C
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Table 1. Summary of the Explored Synthesis Conditions for EHU1(Sc,Na) Compounds
code
formula
synthetic route
solvent/metal source
T (°C)
pH
EHU1(Sc,Na)-S1
EHU1(Sc,Na)-S2
EHU1(Sc,Na)-SF1
EHU1(Sc,Na)-SF2
{[ScNa(μ4-pmdc)2(H2O)2]·5H2O}n
{[ScNa(μ4-pmdc)2(H2O)2]·DMF·2H2O}n
{[ScNa(μ4-pmdc)2(H2O)2]·5H2O}n
{[ScNa(μ4-pmdc)2(H2O)2]·6H2O}n
hydrothermal
solvothermal
solvent-free
solvent-free
H2O/ScCl3/NaOH
(DMF:MeOH)/ScCl3/NaCl
−/Sc2O3/NaCl
170−180
120−160
110
2.8−4.6
1.5
−/Sc2O3/NaNO3
110
RESULTS AND DISCUSSION
Comments on the Synthesis of EHU1(Sc,M) Com-
■
pounds. The synthesis method plays a key role in determining
not only the type of alkaline ion that can be hosted at the
structure of EHU1(Sc,M) but also the crystallinity of the
resulting material, a fact that affects the adsorption performance
and ionic conductivity. Thus, prior to any analysis of the
physical properties of these compounds, several synthetic
routes and conditions were examined for EHU1(Sc,Na) (Table
1). The synthetic study is divided into two major routes, such
as solvothermal (S) and solvent free (SF), each of which have
been explored through two alternative procedures (S1/S2 and
SF1/SF2, respectively). Overall, an almost proportional
improvement of the samples crystallinity is observed along
the S1 → S2 → SF1 → SF2 sequence, which is inferred from
PXRD (see Supporting Information).
Figure 1. Representation of MBBs of EHU1(Sc,Na) obtained from
the DFT structure optimization. Color coding: blue (oxygen), green
(nitrogen), pink (hydrogen), red (metal).
The good performance of the solvent-free synthesis56−60 can
be attributed to the slow diffusion of the reactants within the
melt formed by the hydrated nitrate/chloride salts (given their
hygroscopic nature). Consequently, this method allowed us to
obtain EHU1(Sc,Li)-SF2, although the pure potassium
counterpart could not be isolated in spite of all synthetic
efforts. SEM micrographs on EHU1(Sc,M)-SF2 (M = Li, Na)
samples show the presence of small twinned cubes with a
somewhat regular size distribution of ca. 10 μm (Supporting
Information). Notwithstanding the latter assertions, the great
dissolving capacity of the DMF/MeOH mixture61−63 leads to
low-quality single crystals along with the polycrystalline powder
of EHU1(Sc,Na)-S2, which afford reliable unit cell parameters
and identification of metal positions in the structure.
dodecahedron. Therefore, this building block can be regarded
as a tetrahedral node from the topological point of view. The
MBB in Figure 2b is built up by the coordination of four
nonchelating atoms and two coordination water molecules,
displaying a distorted octahedral geometry. The connectors are
arranged around alkaline ions (M) describing a pseudosquare-
planar building unit. Hence, the pmdc ligand exhibits the μ4-
κ2N,O:κ2N′,O′:κO″:κO‴ coordination mode. Geometry opti-
mizations by means of periodic dispersion-corrected DFT
(density functional theory) support the metal distribution over
the two crystallographically independent centers present in the
crystal structure (energy difference of around 18−19 kcal mol−1
between the two alternative distributions). Moreover, 45Sc solid
state NMR measurements show the presence of a unique and
quite symmetrical environment (ηQ of 0.15 and 0.06 for lithium
and sodium counterparts) in concordance with the octacoordi-
nated site. The comparison between the experimental and the
DFT-simulated diffractograms shows an acceptable agreement
for both compounds (Figure 2).
Structural Description of EHU1(Sc,M) Compounds.
Details of the PXRD refinement of EHU1(Sc,M) (M = Li, Na)
compounds are summarized in Table 2. These compounds are
Table 2. Crystallographic Data and Structure Refinement
Details of EHU1(Sc,M)-SF2 Compounds
The junction of four tetrahedral and two square-planar MBBs
gives rise to a bigger building unit (β-cage, in which the metal
centers are arranged into truncated octahedra) that is further
connected through the whole crystal structure by sharing solely
the M1 vertices (Figure 3). The resulting packing generates an
nbo topological network with the (64.82) point symbol,64−66
which leaves an intersecting 3D channel system of voids filled
with solvent molecules. The accessible volume varies slightly
according to the alkaline atom and represents 42−47% of the
unit cell volume.67 Additionally, the pore system of these
compounds is modulated according to the solvent and synthetic
route employed. A deep analysis of the porous structure by a
Monte Carlo procedure described elsewhere68 concluded that
there are three main contributions to the unit cell voids: a large
cubic cavity surrounded by six β-cages (diameter of 9.38 Å),
eight smaller cubes delimited by six pyrimidine rings, and
bottleneck tunnels interconnecting the previous cavities with
diameters ranging from 5.85 to 6.15 Å. It is worth noting that
atoms occupying the M site lie on the faces of the largest cubic
pore, so they could behave as open-metal sites once the
EHU1(Sc,Li)-SF2
EHU1(Sc,Na)-SF2
empirical formula
fw
C12H18N4LiO15Sc
510.05
C12H20N4NaO16Sc
544.03
space group
a (Å)
V (Å3)
Im-3m
Im-3m
18.742(2)
6584(1)
18.878(2)
6728(1)
RF/RB
RP/Chi2
1.26/1.06
4.75/1.49
1.34/1.46
4.88/1.23
isostructural to a family of porous compounds with {[CdM(μ4-
pmdc)2(H2O)2]·solv}n (MII = Cd, Mn, Zn) formula.37 Their
crystal structure consists of a 3D neutral metal−organic
framework that is the result of the junction of two different
molecular building blocks (MBB, Figure 1) established by Sc3+
and alkaline (M+) ions coordinated to the pmdc ligands. The
MBB in Figure 2a consists of four chelating N/O moieties of
pmdc that are coordinated to the scandium atom (Sc),
rendering a N4O4 donor set that resembles a triangular
D
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Figure 3. Structural analysis of EHU1(Sc,M): (a) arrangement of the
β-cage fraction, (b) junction of β-cages, and (c) pore-size distribution.
Table 3. Comparison between Experimental and Simulated
Accessible Surface Areas of EHU1(Sc,M) Compounds
a
b
c
compound
SBET
mol/unit cell
V
EHU1(Sc,Li)_Simulation
EHU1(Sc,Li)-SF2
1090
762
1046
848
703
631
207
52
39
52
43
36
32
10
6713
6384
7349
6755
EHU1(Sc,Na)_Simulation
EHU1(Sc,Na)
SF2
SF1
S2
S1
a
b
BET specific surface area (m2 g−1). N2 molecules adsorbed per unit
c
cell. Unit cell volume (Å3).
Figure 2. Experimental diffractograms and those simulated from DFT-
optimized models together with pattern-matching analyses.
the crystallinity envisaged from the PXRD patterns of the
samples.
coordination water molecules are released (see TG analyses in
the Supporting Information). However, the examination of the
coordinatively unsaturated (cus) metal environments on the
DFT-optimized anhydrous structures reveals that they are
recessed into the surface in an attempt to maximize their
coordination to both carboxylate oxygen atoms (Figure S6).
Gas Adsorption Properties on EHU1(Sc,M) Com-
pounds. Gas adsorption and ionic mobility on these porous
materials are closely related because of the need of a highly
accessible network of channels so that ionic charge carriers can
better diffuse through it. Therefore, routine gas adsorption
measurements become a good test to evaluate the viability of a
porous material in terms of ionic conductivity, since a well-
ordered channel system (characteristic of highly crystalline
materials) is required. Permanent porosity of EHU1(Sc,M)
compounds was studied by means of N2 adsorption isotherms
at 77 K. All adsorption curves show a sharp knee at relatively
low pressures (P/P0 < 0.01) followed by a plateau, indicative of
a rapid filling of the micropores (see Supporting Information).
Table 3 gathers the results of BET analysis, which shows a clear
trend for the experimental surface area values that fit rather well
The surface areas obtained from the optimized crystal
structures for each system can be used as a reference value to
get insight into the accessibility of the pore channel system.
Experimental surface areas for solvent-free samples, although
slightly below, are close to the simulated values, which clearly
indicates that solvent-free route (i.e., EHU1(Sc,M)-SF2
samples) provides the best results.
CO2 adsorption isotherms were conducted at 298, 273, and
196 K, covering the low-pressure range (0−1 bar) (Figure 4),
since it is a molecular probe that provides complementary
information on the pore network due to its different features. In
fact, at 196 K the simulations yield a different CO2 uptake for
lithium and sodium compounds (37.0 vs 47.1 molecules/cell),
with these values being somewhat below those corresponding
to N2 adsorption (52 molecules/cell), which can be explained
by the different size of the probes together with the presence of
narrow adsorption cavities (Table 4). Close to saturation, the
deviations between the experimental and the GCMC-simulated
isotherms (21.1% and 16.1% at ca. P = 0.9 bar for
EHU1(Sc,Li) and EHU1(Sc,Na)) are similar to that found
for N2 (25.0% and 17.3%), which stands not only for the
E
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Figure 4. Experimental and simulated CO2 adsorption isotherms at (a) 298, (b) 273, and (c) 196 K for EHU1(Sc,M) compounds.
Electrochemical Properties: Ionic Conductivity. As
previously stated, the presence of lithium and sodium cations
in a porous ScIII−organic framework makes EHU1(Sc,Li) and
EHU1(Sc,Na) compounds potential candidates for battery
electrolyte applications. The mechanism of ion conduction in
solids depends, among others, on factors such as concentration,
mobility, and charge of the conductive ions. Given the porous
nature of MOFs, charge carriers mobility may involve not only
an intrinsic (ion diffusion through the channels) but also an
extrinsic (migration through interparticle phases) mechanism.
In fact, a recent work by Tominaka and Cheetham79 points out
that proton conduction in many MOFs seems to be dominated
by hydrated interparticle phases when pressed into pellets.
Nevertheless, the low conductive values achieved by electro-
chemical impedance spectroscopy (EIS) for the pristine
EHU1(Sc,M) compounds (in the order of 10−7 S cm−1)
dismisses such a proton conduction mechanism (Table 5). In
Table 4. Experimental and Simulated CO2 Uptakes (mmol
g−1) of EHU1(Sc,M) Compounds at Different Temperatures
experimental
DFT
a
b
a
b
compound
T (K) uptake
molecules
uptake
molecules
EHU1(Sc,Li)
298
273
196
298
273
196
2.65
3.87
6.08
2.54
3.51
8.23
12.2
17.8
29.2
12.2
16.9
39.5
2.83
4.46
8.02
2.54
4.13
9.8
13.0
20.5
37.0
12.2
19.8
47.1
EHU1(Sc,Na)
a
b
CO2 uptake at ∼0.9 bar. CO2 molecules per unit cell.
suitability of the selected computational method but also
indicates that both adsorbates probe a similar fraction of the
accessible pore. At higher adsorption temperatures (273 and
298 K) a better fitting is observed between the experimental
and the simulated uptakes. As far as we are aware, the reported
capacities at 273 K (17.0% and 15.4% for EHU1(Sc,Li) and
EHU1(Sc,Na), respectively) can be considered as moderately
high since they outperform those obtained for many well-
known MOFs, such as MIL-53(Al, Cr), MOF-5, MOF-177, or
even most of ZIF materials,69 which makes these compounds of
relevance to postcombustion flue gases purification.70−72 It is
worth noting that at low loadings (see isotherms at 273 and
298 K) the experimental curve for EHU1(Sc,Na) exceeds the
simulated one, which can be related with the presence of
structural defects that diminish the total porosity but increase
the adsorbate−adsorbent interactions. In this context, CO2
adsorption enthalpies (Qst) of lithium and sodium counterparts
were also calculated using the modified Clausius−Clapeyron
equation by fitting adsorption isotherms at 273 and 298 K (see
Supporting Information).73,74
Table 5. Room Temperature Results of ac Impedance
Spectroscopy for as-Synthesized and Doped EHU1(Sc,M)-
SF2 Compounds
a
b
compound
R (Ω)
R (Ω cm) thckness σ (S cm−1
)
EHU1(Sc,Li)
5.0 × 10−5
2.6 × 10−6
5.7 × 10−5
8.8 × 10−5
9.1 × 10−6
2.9 × 10−6
0.74
0.27
1.01
0.72
0.71
3.8 × 10−7
Cd@EHU1(Sc,Li)
Mn@EHU1(Sc,Li) 2.3 × 10−5
EHU1(Sc,Na)
4.0 × 10−4
1.7 × 10−6
9.5 × 10−7
1.1 × 10−7
3.4 × 10−7
1.7 × 10−6
5.3 × 10−5
Cd@
EHU1(Sc,Na)
Mn@
EHU1(Sc,Na)
1.3 × 10−5
5.2 × 10−5
0.93
2.0 × 10−6
a
b
Thickness (mm). Ionic conductivity.
the intrinsic conduction, the occurrence of an ordered and
noncollapsed porous structure is a key factor to allow ion
diffusion, a fact that is governed by the crystallinity of the
sample. Taking into account the targeted conductivity value for
a battery electrolyte (above 10−3 S cm−1),80 different strategies
have been envisaged to enhance the mobility of charge carriers
in MOFs.81−84 All these approaches are based on postsynthetic
modifications of the compounds in order to attain structures
with a significantly higher number of free charge carriers.
However, we focused on this issue through a one-pot
synthetic approach through the doping of EHU1(Sc,M) with
divalent metal ions. To that end, we took advantage of the
flexibility demonstrated by this MOF to replace metal atoms,
The Qst values at zero coverage approach 28 and 36 kJ mol−1,
respectively, to subtly decay to an almost constant value of ca.
25 kJ mol−1 during the whole loading range, in good agreement
with those estimated from the simulated isotherms (ca. 24 kJ
mol−1). These values, although being rather high, lie far below
those attributed to open-metal sites,4,75−78 in good agreement
with the hindering experimented by the metal atom in the cus
M position. In concordance to the latter, the CO2 average
occupation maps provided by GCMC calculations on the DFT-
optimized EHU1(Sc,Na) reveal that the smaller cubes and
bottleneck tunnels are, in fact, the preferred adsorption sites
along the accessible surface (Figure S18).
F
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Chemistry of Materials
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i.e., isostructural EHU1(Cd) and EHU1(Mn) compounds.37
Hence, four new materials were successfully prepared by
doping EHU1(Sc,Li)-SF2 and EHU1(Sc,Na)-SF2 samples
with a 5% of CdII and MnII salts (see Experimental Section) to
render Cd@EHU1(Sc,M) or Mn@EHU1(Sc,M) samples.
PXRD, elemental, and thermogravimetric analyses confirmed
that doped samples preserve the initial MOF structure.
Interestingly, an increase by 1 order of magnitude on the
conductivity by EIS spectroscopy was observed for the doped
samples. Taking Cd@EHU1(Sc,Li) as a reference 113Cd CP/
MAS NMR spectra indicate that these doping atoms occupy a
unique site in the structure, in good agreement with the
assumption of a partial replacement of Sc(III) by Cd/Mn(II)
atoms in the crystal structure, and consequently the presence of
an excess of lithium ions to neutralize the charge imbalance.
These hypotheses are confirmed by the broadening of the main
peaks in all NMR spectra in addition to the occurrence of a
Figure 5. Nyquist plots of the ac impedance data for soaked
EHU1(Sc,M) compounds.
7
minor component in the Li spectrum at a chemical shift of
−1.6 ppm that could be assigned to the somewhat unbound
aquo lithium complexes (Supporting Information), which
points out that conductivity improvement could come from
the mobility of these ions. Moreover, minor components are
also detected in the 45Sc and 13C spectra owing to local
structural distorsions taking place in the surroundings of the
exchanged metal sites. However, the decrease of the mean
particle size observed in the SEM images of the doped samples
does not allow discarding an extrinsic contribution to the
overall enhancement of the conductivity (Supporting Informa-
tion).
In light of these results, we opted for a soaking procedure
previously reported by Wiers et al.,8 which has proven to be an
adequate way to boost the conductivity of gel polymer
electrolytes. According to it EHU1(Sc,Li)-SF2 and EHU1-
(Sc,Na)-SF2 materials were soaked overnight in solutions
containing LiBF4 or NaPF6 salts in DMC, giving rise to the
ionic conductivities shown in Table 6.
Figure 6. SEM images of EHU1(Sc,Li) before and after soaking on
LiBF4.
particles over the surface is noticed. ICP-AES analyses of
EHU1(Sc,Li)·(LiBF4) and EHU1(Sc,Na)·(NaPF6) show a
remarkable increase of the Li/Na:Sc ratio (1.8:1 vs 1:1 and
1.7:1 vs 1:1, respectively) compared to the pristine material.
Elemental and thermogravimetric analyses give the {[ScLi(μ4-
pmdc)2(H2O)2]·2H2O·0.8LiBF4}n and {[ScNa(μ4-
pmdc)2(H2O)2]·2H2O·0.7NaPF6}n formulae for EHU1-
(Sc,Li)·(LiBF4) and EHU1(Sc,Na)·(NaPF6). In fact, PXRD
patterns of soaked samples confirm that pristine structures are
maintained although the relative intensity of some maxima have
been clearly modified, which supports the inclusion of LiBF4
Table 6. Results of ac Impedance Spectroscopy for Soaked
EHU1(Sc,M) Compounds and Best Results Found for
Alkaline-Based MOF Conductors
and NaPF6 ionic species into the voids of the structure (see 19
F
NMR and FTIR spectra in the Supporting Information). This
fact is reinforced by the lower content of lattice water molecules
per formula in the soaked samples compared to the pristine
materials. Paying attention to the NMR spectra it can be
concluded that the soaking procedure significantly modulates
the crystal structure. Overall, all peaks corresponding to the
a
b
c
d
e
compound
ion
Li+
σ
T
T.S.
ref
f
EHU1(Sc,Li)·(LiBF4)
EHU1(Sc,Na)·(NaPF6)
[Zr6O4(OH)2(bdc)6]-UiO-66
4.2 × 10−4
9.2 × 10−5
1.8 × 10−5
3.1 × 10−4
25
25
r.t.
27
270
310
540
380
f
Na+
Li+
Li+
g
1
85
20
studied nuclei are broadened, particularly for the H (peak
h
7
Mg2(dobdc)-MOF-74
centered at 3.5 ppm of ca. 2.0 kHz) and Li spectra (peak at
a
bdc = 1,4-benzenedicarboxylate; dobdc = 1,4-dioxido-2,5-benzenedi-
0.65 ppm of ca. 1.4 kHz), which is indicative of a major
heterogeneity of environments. However, this fact prevents a
further study of their mobility through, for instance, the pulse
b
c
carboxylate. Conductive ion. Ionic conductivity (S cm−1).
d
e
f
Temperature (°C). Thermal stability (°C). LiBF4 and NaBF4 are
g
h
the electrolyte salts. The sample was grafted with LiOtBu; The
7
gradient field (PFG-NMR) technique. The Li peak can be
sample was grafted with LiiOPr and soaked in a LiBF4 solution.
related with a diverse and disordered minor population of Li
atoms in addition to those occupying the M site. Therefore, the
increase of the alkaline-ion concentration after soaking appears
to be responsible for the enhancement of the conductivity,
although it should not be excluded that the alteration of the
external surface of the crystallites could also play an important
role.
Varying the temperature of the pellets during the measure-
ments revealed Arrhenius-type-activated behavior in both cases
(Figure 7). According to the fitting, soaked EHU1(Sc,Li)·
(LiBF4) presents a lower activation barrier than the sodium
counterpart (0.25 vs 0.64 eV, respectively). Taking into account
the conductivity (10−4 S cm−1) and the activation energy lower
The room-temperature conductivities achieved for EHU1-
(Sc,Li) and EHU1(Sc,Na) after the soaking are 3 and 2 orders
of magnitude greater than those of the pristine materials
(Figure 5). Indeed, that of the lithium compound is, as far as we
are concerned, the best result reported for Li-containing MOFs
materials. Moreover, EHU1(Sc,Na)·(NaPF6) represents the
first MOF to show ionic conductivity in a Na-ion-based MOF.
SEM images of the EHU1(Sc,Li)·(LiBF4) sample show that
particles retain their initial morphology and size, although some
of them seem to have been etched as a consequence of the
soaking procedure (Figure 6). Moreover, deposition of small
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knowledge, the first MOF to show ionic conductivity provided
by the Na ion.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemma-
ter.5b03458.
FTIR spectra, chemical analysis, thermal behavior, PXRD
data, analysis of porosity based on experimental and
simulated adsorption isotherms, SEM images, NMR
spectra, and additional Nyquist plots (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: javier.cepeda@ehu.eus.
*E-mail: oscar.castillo@ehu.eus.
■
Funding
́
This work was funded by Ministerio de Economia y
Figure 7. Arrhenius plots of ionic conductivity data obtained for
EHU1(Sc,M) compounds soaked in 1 M LiBF4 and NaPF6 DMC
solutions.
Competitividad (MAT2013-46502-C2-1-P). J. C. and S. P. Y.
thank the Universidad del Pais Vasco/Euskal Herriko
Unibertsitatea for their postdoctoral fellowships.
́
Notes
than 0.4 eV, the EHU1(Sc,Li)·(LiBF4) material can be referred
The authors declare no competing financial interest.
to as a promising superionic conductor.86,87
ACKNOWLEDGMENTS
■
CONCLUSIONS
■
The authors are thankful for the technical and human support
provided by SGIker of UPV/EHU. J.C. also gratefully
acknowledges Dr. Jesus M. Ugalde for his guidance and
support along the postdoctoral period. Dr. Leire Ruiz Rubio is
acknowledged for the thermogravimetric measurements.
In the present work, two new heterometallic 3D porous
scandium(III)/alkaline/pyrimidine-4,6-dicarboxylato frame-
works, namely, EHU1(Sc,Li) and EHU1(Sc,Na), have been
structurally and chemically characterized, while their permanent
porosity has been confirmed by means of adsorption of gases. A
thorough synthetic study has allowed achieving materials with
the best performance in terms of crystallinity and porosity, both
of which are highly correlated given their intricate micropore
system. These compounds have proven a high capacity to
capture CO2 at room temperature owing to the strong CO2···
host interactions established specifically at two sites of the
crystal structure, as inferred from the computed CO2 average
occupation maps. Experimental adsorption performances have
been well contrasted with GCMC simulations.
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