Microporous Lead-Organic Framework for Selective CO2 Adsorption and Heterogeneous Catalysis

Article abstract of DOI:10.1021/acs.inorgchem.7b02491

A novel microporous metal-organic framework, {[Pb₄(μ₈-MTB)₂(H₂O)₄]·5DMF·H₂O}_n (1; MTB = methanetetrabenzoate and DMF = N,N′-dimethylformamide), was successfully synthesized by a

Full text of DOI:10.1021/acs.inorgchem.7b02491

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Microporous Lead−Organic Framework for Selective CO Adsorption
2
and Heterogeneous Catalysis
†
,†
‡,§
⊥
Miroslav Almas
Maksym V. Opanasenko, Philip L. Llewellyn, and Jirí
́ ̌ ̌ ́ ́
i, Vladimír Zelenak,* Robert Gyepes, Sandrine Bourrelly,
⊥
‡
‡
̌
Cejka
̌
†
̌
Department of Inorganic Chemistry, Faculty of Science, P. J. Safar
‡Department of Synthesis and Catalysis, J. Heyrovsky
Institute of Physical Chemistry of the ASCR, v.v.i., Dolejsk
3 Prague 8, Czech Republic
́
ik University, Moyzesova 11, SK-041 54 Kosi
̌
ce, Slovak Republic
ova 2155/3, CZ-182
́
̌
2
§
Department of Education, University of J. Selye, Bratislavska
́
cesta 3322, SK-945 01 Komar
Aix-Marseille University, CNRS, MADIREL, F-133 97 Marseille Cedex 20, France
S Supporting Information
́
no, Slovak Republic
⊥
*
ABSTRACT: A novel microporous metal−organic framework,
[Pb (μ -MTB) (H O) ]·5DMF·H O} (1; MTB = methanetetra-
{
4
8
2
2
4
2
n
benzoate and DMF = N,N′-dimethylformamide), was successfully
synthesized by a solvothermal reaction and structurally characterized
by single-crystal X-ray diffraction. The framework exhibits a unique
tetranuclear [Pb (μ -COO)(μ -COO) (COO)(H O) ] secondary
4
3
2
6
2
4
building unit (SBU). The combination of the SBU with the
tetrahedral symmetry of MTB results in a three-dimensional network
structure, with one-dimensional jarlike cavities having sizes of about
2
1
4.98 × 7.88 and 14.98 × 13.17 Å and propagating along the c axis.
Due to the presence of four coordinately unsaturated sites per one
metal cluster, an activated form of compound 1 (i.e., desolvated form
denoted as 1′) was tested in gas adsorption and catalytic
2
−1
experiments. The studies of gas sorption revealed that 1′ exhibits a surface area (Brunauer−Emmett−Teller) of 980 m ·g .
This value is the highest reported for any compound from the MTB group. Interactions of carbon dioxide (CO ) molecules with
2
the framework, confirmed by density functional theory calculations, resulted in high CO uptake and significant selectivity of CO
2
2
adsorption with respect to methane (CH ) and dinitrogen (N ) when measured from atmospheric pressure to 21 bar. The high
4
2
selectivity of CO over N is mostly important for capturing CO from the atmosphere in attempts to decrease the greenhouse
2
2
2
effect. Moreover, compound 1′ was tested as a heterogeneous catalyst in Knoevenagel condensation of active methylene
compounds with aldehydes. Excellent catalytic conversion and selectivity in the condensation of benzaldehyde and
cyclohexanecarbaldehyde with malononitrile was observed, which suggests that accessible lead(II) sites play an important role
in the heterogeneous catalytic process.
I. INTRODUCTION
methanetetrabenzoic acid (H MTB) and its germanium- or
4
8
9
silicon-centered derivatives or adamantanetetracarboxylic and
Metal−organic frameworks (MOFs) have attracted tremendous
interest in the recent years because of their unique structural
features and perspective applications in gas storage and
10
adamantanetetrabenzoic acids and others were to date used in
the MOF design.
1
2
3
4
Porous MOFs were shown to be effective sorbents of
separation, catalysis, sensing, drug delivery, and nano-
5
different gases, notably carbon dioxide (CO ), of which
2
particle preparation. This is because of their diversity and
removal from the atmosphere or a flue gas produced in
industry (especially in coal-powered stations) is becoming a
serious environmental problem, causing global warming. The
modularity, which offers the opportunity to systematically fine-
tune and control their properties and performance. Usually
MOFs are constructed from commercially available linear or
bent poly(carboxylic acid)s, which act as linkers between metal
ions or their clusters to give final three-dimensional (3D)
recent studies demonstrate that MOFs are promising CO2
adsorbents with the potential to replace existing materials for
6
CO capture at low concentrations, ambient humidity, and
porous structures. However, the demand to better control the
2
11
moderate temperatures. To enhance the CO adsorption
final framework architecture leads to the design and synthesis of
new linkers with specifically tailored dimensionality. Among
2
7
capacity and selectivity of MOFs, different approaches have
them, the ligands with tetrahedral symmetry have received
considerable interest recently because they are by nature 3D
and extended linkers. Especially, carboxylic acids like
Received: September 27, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02491
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
been reported. The first is based on the building of a porous
activity in condensation reactions and a high selectivity to
desired products, without a significant loss of catalytic activity
even after repetition cycles.
framework with a suitable pore size for CO adsorption either
2
by use of a linker of appropriate dimensionality or by design of
12
the structure with adjusted interpenetration. Another way to
increase the CO adsorption capacity is to enhance the binding
2
II. EXPERIMENTAL SECTION
affinity between the framework and adsorbed CO . This can be
2
Materials. The chemicals needed in the synthesis of the studied
complex and reagents for catalytic reactions were purchased from
Sigma-Aldrich or Acros Organics, respectively, and used without
purification. Methanetetracarboxylic acid [tetraphenylmethane-
done, for example, by the generation of unsaturated
coordination sites on metal ions, generated by the removal of
coordinated terminal ligands from central atoms or by ion
exchange, forming charged frameworks. CO adsorption can be
4,4′,4″,4‴-tetracarboxylic acid (H MTB)] was prepared according to
2
4
3
7
increased also by the incorporation of polar functional groups
the literature procedure. The aldehydes cyclohexanecarbaldehyde
(97%) and benzaldehyde (≥99%) and the active methylene
compounds methyl cyanoacetate (99%), ethyl acetoacetate (≥99%),
and malononitrile (≥99%) were received as commercial samples. In
addition, n-dodecane (≥99%) was used as an internal standard and p-
xylene (≥99%) as the solvent in catalytic experiments. Compound
as accessible strong Lewis basic sites (−OH, −NH , −SH, etc.),
2
which decorate organic linkers, or by postsynthetic modifica-
tion of the final framework, which provides sites for interaction
with CO , thereby improving the affinity of MOFs toward
2
1
3
CO2.
{
[Ni (μ -MTB) (μ -H O) (H O) ]·10DMF·11H O} , used in the
4 6 2 2 2 4 2 4 2 n
The MOFs are also very interesting for applications in
catalysis and liquid-phase reactions because MOFs combine the
advantages of heterogeneous (recycling, stability, ease of
separation, etc.) and homogeneous (selectivity, high efficiency,
controllability under mild reaction conditions, etc.) catalysts.
The possibility of tailored synthesis and organization of metal
nodes and organic linkers in MOFs (playing the role of active
catalytic sites), and their functionalization on the nanoscale
level, opens the potential to build up MOFs particularly
adsorption experiments, was prepared by the synthetic procedure
described in ref 38.
Synthesis of {[Pb (μ -MTB) (H O) ]·5DMF·H O} (1). The
4
8
2
2
4
2
n
mixture of Pb(NO ) (10 mg, 0.03 mmol) and H MTB (20 mg,
3
2
4
0.04 mmol) in 3 mL of N,N′-dimethylformamide (DMF), 3 mL of
water (H O), and 3 mL of ethanol (EtOH) were placed in a 20 mL
2
glass Ace tube. The mixture was slowly heated to 353 K (at a heating
−1
rate of 1 K·min ) and kept at 353 K for 24 h. Then the mixture was
−1
cooled to room temperature at a cooling rate of 0.1 K·min . The
colorless crystals of 1 were filtered off and washed several times with a
14
modified for catalysis. To date, MOFs have been used as solid
mixture of DMF/H O/EtOH in the ratio 1/1/1 (v/v/v). Elem anal.
2
−1
catalysts in a variety of organic transformations such as
Calcd for Pb C H N O (2269.21 g·mol ): C, 38.64; H, 3.42; N,
4
73 77
5
26
1
5
16
17
20
−
1
Knoevenagel and Friedlander condensations, Pechmann,
3.09. Found: C, 38.77; H, 3.51; N, 3.12. FT-IR (KBr technique, cm ):
ν(OH) 3440(m, br); ν(CH)ar 3063(vw), 3036 (vw); ν(CH
1
8
19
Henry, and Prins reactions, Huisgen cycloaddition,
Beckmann rearrangement, and Friedel−Crafts alkylation/
acylation. In this study, we were particularly interested in the
Knoevenagel reaction. The Knoevenagel reaction of active
methylene compounds with aldehydes has been widely used in
the synthesis of fine chemicals as well as heterocyclic
compounds of biological importance. The Knoevenagel
)
3 asym
21
2
1
928(w); ν(CH ) 2975(w); ν(CO) 1659(s); ν(CC) 1601(s),
435(m); ν(COO ) 1588(s), 1532(s); ν(COO ) 1389(s);
as s
3
s
ar
−
−
2
2
−
δ(CCH) 1187(w), 1138(w), 1101(w); γ(CCH) 844(m); δ(COO )
ar
ar
7
77(m); γ(CO) 662(w) (s = strong, m = medium, w = weak, br =
broad, asym = asymmetric, sym = symmetric, and ar = aromatic).
Characterization. Elemental analysis was performed with a Vario
MICRO CHNOS elemental analyzer from Elementar Analysensys-
teme GmbH. IR spectra were recorded with an Avatar FT-IR 6700
2
3
reaction is conventionally catalyzed by ionic liquids,
2
4
25
26
−1
surfactants, alkali metals, organic amines, and polyox-
spectrometer in the range 4000−400 cm . The sample was prepared
2
7
ometalates under homogeneous conditions. Recently, differ-
in the form of KBr pellets with a complex/KBr mass ratio of 1/100.
Before IR measurements, KBr was dried at 973 K for 3 h in an oven
and cooled in a desiccator. The room temperature spectrum was
recorded after the preparation of a pellet of complex 1. The same
pellet was then heated at 373, 473, 573, and 673 K for 15 min in an
oven, and after this time, the IR spectrum of the sample was measured
again. The thermal behavior of compound 1 was studied by TGA with
simultaneous differential thermal analysis (DTA) and EGA in the
region of 300−1200 K (TGA/DTG−DTA−EGA). The sample with a
weight of approximately 30 mg was placed in an aluminum oxide
ent solid catalysts have been used to study this reaction such as
2
8
29
zeolites, surface-modified mesoporous silica, covalent−
30
organic frameworks (COFs), zeolitic imidazolate frameworks
34
3
1
32
33
(
ZIFs), and MOFs: MIL-100, MIL-101, IRMOF-3,
35 36
UiO-67, MOF-76, and others.
In the present study, a new MOF compound of the MTB4−
series with the composition {[Pb (μ -MTB) (H O) ]·5DMF·
4
8
2
2
4
H O} (1) was successfully prepared and characterized. Single-
2
n
crystal X-ray diffraction (XRD) showed a 3D polymeric
framework with a one-dimensional jarlike cavity and active
metal nodes. An activation study of the compound was
monitored by a combination of IR measurements, obtained
after heating of the compound to different temperatures, and
thermogravimetric analysis (TGA) coupled with simultaneous
evolved gas analysis (EGA) measurements (TGA−EGA) and
powder X-ray diffraction (PXRD). After activation, 1 exhibits a
stable porous framework with a specific Brunauer−Emmett−
(Al O ) crucible and heated under a dynamic atmosphere of argon (50
2 3
3
−1
−1
cm ·min ) at a heating rate of 9 K·min using a Netzsch 409-PC
STA apparatus. The residues evolved during thermal decomposition
̈
were monitored by an Aeolos QMS mass spectrometer coupled with
the STA apparatus in the region of 1−300 amu. The crystallinities of
the samples (as-synthesized, activated, and after stability tests in H O
2
at different pH values and after catalytic runs) were determined by
PXRD measurements on a Bruker AXS D8 ADVANCE diffractometer
in the Bragg−Brentano geometry using Cu Kα (λ = 1.54056 Å)
radiation with a graphite monochromator and a positron-sensitive NaI
dynamic scintillation detector (Vantec-1) in the 2θ range 4−40°.
Structure Determination and Refinement. Compound 1
crystallizes as colorless prismlike crystals. Diffraction data were
collected on a Nonius Kappa CCD diffractometer equipped with an
APEX II detector (Mo Kα radiation, λ = 0.71073 Å) at 150(2) K. Data
2
−1
Teller (BET) surface area of 980 m ·g and high CO2
adsorption capacity and high selectivity compared to those of
methane (CH ) and dinitrogen (N ) at 303 K and pressure up
4
2
to 21 bar. The catalytic properties of 1 were tested in the
Knoevenagel condensation of cyclohexanecarbaldehyde and
benzaldehyde with active methylene compounds (methyl
cyanoacetate, ethyl acetoacetate, and malononitrile). The
results confirmed that the compound shows high catalytic
39
were processed by the diffractometer software. The phase problem
was solved by direct methods (SHELXS 2013) and the structure
refined by full-matrix least squares on F2 (SHELXL 2013).
Diffraction maxima observed in the cavities could not be refined
40
B
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Inorganic Chemistry
Article
Figure 1. (a) Atomic labeling diagram of the [Pb (μ -COO)(μ -COO) (COO)(H O) ] cluster with corresponding coordination polyhedra of
4
3
2
6
2
4
lead(II) ions. (b) View of the framework of 1 along the c crystallographic axis, after subtraction of solvent molecules.
using a chemically plausible model and have thus been discarded using
experiments, N of 99.9995% purity was used, which was obtained
from Linde Gas.
2
41
the SQUEEZE procedure in PLATON. Hydrogen atoms were
refined isotropically and all other atoms anisotropically. Hydrogen
atoms residing on aromatic carbon atoms were included in an ideal
The measurements of the gas adsorption up to 21 bar were realized
at 303 K with gases CO , CH , and N using a homemade high-
2
4
2
4
7
position with the C−H bond fixed to 0.95 Å and U (H) assigned to
throughput instrument. Gas adsorption was measured via a
manometric gas dosing system on six samples in parallel. The
amounts of gas adsorbed were calculated by an equation of state using
the Reference Fluid Thermodynamic and Transport Properties (RE-
FPROP 8.0) software package of the National Institute of Standards
iso
1
.2U of the adjacent carbon atom. The absorption correction was
eq
semiempirical on symmetry equivalents. The structure figures were
42
drawn using DIAMOND software. Crystal data for 1: crystal size 0.52
×
0.16 × 0.13 mm, monoclinic, space group C2/c, a = 41.582(2) Å, b
4
8
=
20.2045(10) Å, c = 28.205(3) Å, β = 131.6580(10)°, V = 17 704(2)
and Technology. For the measurements, around 100 mg of the
sample was used. The sample was thermally activated in situ under
vacuum at 483 K overnight. The gases for measurements were
3
−3
−1
Å , Z = 8, D
= 1.758 g·cm , μ = 7.660 mm . Of the 74859 total
calcd
reflections collected (−51 ≤ h ≤ 51, −20 ≤ k ≤ 25, −35 ≤ l ≤ 23),
1
8252 were unique (R = 0.0418) and used to solve the crystal
obtained from Air Liquide. The purity of N and CH was 99.9995%
2 4
int
structure. On the basis of these data and 748 refined parameters, final
R indices [I > 2σ(I)], R = 0.0659, wR = 0.2021; R indices (all data),
(N55), and the purity of CO was 99.995% (N45).
2
To evaluate the regenerability of the sample, two measurements
with the same parameters were performed on the sample for each gas
1
2
2
R = 0.1018, wR = 0.2158, and the goodness-of-fit on F is 1.147.
1
2
Molecular Modeling. Density functional theory (DFT) compu-
(CO , CH , and N ). Between the two experiments, the sample was
2 4 2
tations of 1 have been performed on the bose cluster at the J.
Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the
Czech Republic, v.v.i,. using Gaussian 09, revision D.01.
submitted to evacuation at 303 K and under a primary vacuum for 80
min. Using this procedure, from the second gas adsorption
measurement, the regenerability/recovery of the sample was checked.
́
4
3
Computations were conceived as spin-unrestricted DFT using
The selectivity of CO
adsorbed solution theory (IAST) in the range of pressures 1−10 bar.
Adsorption selectivities for equimolar CO /N and CO /CH
4
2 2 4
over N and CH was calculated by ideal
Grimme’s B97D functional including a long-range dispersion
4
4
45
correction and the LANL2DZ core potential employed for all
atoms. The structure motif of the MOF was represented by 28 atoms
(
2
2
2
mixtures were predicted using pure-component isotherm fits, defined
by
four Pb2 ions and eight COO groups), which were kept fixed
+
−
during geometry optimization. The coordinates of all other atoms were
allowed to vary. The interaction enthalpy between the MOF and its
adsorbate molecules was computed assuming their infinite separation
xi/xj
Si/j
=
yi/yj
46
and employing the Counterpoise approach.
Gas Adsorption Measurements and Analysis. The N2
adsorption isotherm was measured using NOVA 1200e (Quantach-
rome) and ASAP 2020 (Micromeritics) automatic adsorption
analyzers at 77 K. Before the adsorption studies, the sample was
activated by evacuation at 483 K for 24 h to remove the guest
molecules from the cavities. Compound {[Ni (μ -MTB) (μ -
where Si/j is the selectivity toward component i in a binary mixture
with j (i stands for CO and j for CH or N ), and x and y denote the
equilibrium mole fraction for component i in the adsorbed and gas
2
4
2
i
i
phases, respectively. The IAST is fully explained in the original paper
4
9
by Myers and Prausnitz.
4
6
2
2
Catalysis. The Knoevenagel condensation of active methylene
compounds and aldehydes was realized in a liquid phase under
atmospheric pressure at temperature 403 K on a multiexperiment
workstation StarFish. Before catalytic experiments, single crystals of
the catalyst were gently ground to increase their active sites for
catalytic experiments and then activated at 473 K for 90 min in a
H O) (H O) ]·10DMF·11H O} , used in adsorption experiments,
2
4
2
4
2
n
was activated by the procedure described in ref 38. The N adsorption
2
isotherm for desolvated sample 1 (further denoted as 1′) was collected
in a relative pressure range from p/p = 0.005 to 1. From the N
0
2
adsorption isotherm, the BET specific surface area (SBET) and pore
3
−1
volume (V ) of the sample were evaluated. In the adsorption
stream of air (50 cm ·min ). Typically, 4 mmol of aldehyde, 0.4 g of
p
C
DOI: 10.1021/acs.inorgchem.7b02491
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Inorganic Chemistry
Article
dodecane (internal standard), 50 mg of catalyst, and 10 mL of p-xylene
2.382(11)−2.714(10) Å for Pb2, 2.379(10)−2.667(12) Å, for
Pb3 and 2.44371)−2.757(8) Å for Pb4 (Table S1). The
distances of Pb2−O17 and Pb4−O5 [2.714(10) and 2.757(8)
Å] are obviously longer than the others, indicating weak
interaction (semicoordination). Anyway, the average bond
lengths are comparable to those of other reported lead(II)
(
solvent) were added to the 25 mL three-necked vessel, equipped with
a condenser and a thermometer. The final suspension was stirred and
heated. After temperature 403 K was reached, 6 mmol of the
methylene compound was added to the reaction vessel through a
syringe.
During the catalytic experiments, 0.1 mL of the reaction suspension
was sampled after 10, 30, 60, 120, 240, 360, and 1440 min. After the
sampling, the mixture was immediately centrifuged for 5 min at 4000
rpm. Then, the reaction products were analyzed by gas chromatog-
raphy (GC) using an Agilent 6850 chromatograph with a flame
ionization detector equipped with a nonpolar HP1 column (diameter
52
carboxylate complexes.
In 1, the discrete Pb
cluster is surrounded by eight different
4
MTB4 ligands and the secondary building unit can be
described as a distorted hexagonal bipyramid (Figure S1).
Topological analysis of 1 shows an unusual structure that does
not imitate simple natural minerals like previously published
−
0.25 mm, thickness 0.2 μm, and length 30 m). The products were
identified using GC/mass spectrometry (MS) analysis (ThermoFinni-
gan FOCUS DSQ II single quadrupole gas chromatograph/mass
spectrometer). Moreover, the identity of the reaction products was
4−
compounds containing the MTB ligand, prepared via a
4
−
37
combination of MTB with zinc(II) (PtS and DIA-
5
3
54
38
1
MOND ), copper(II) (PtS ), nickel(II) (CaF
and
also confirmed by H NMR spectroscopy (Varian Unity 400
2
5
5
56
1
DIAMOND ), cobalt(II) (CaF ), and cadmium(II)
spectrometer). H chemical shifts in CDCl3 were given with
2
57
tetramethylsilane as the internal standard. The observed spectra
(CaF ) as metal centers.
2
50
corresponded to the published data.
Figure 1b depicts complex 1 exhibiting a 3D non-
interpenetrated framework that contains open channels
propagating along the c crystallographic axis. The channels
Recyclability of the catalyst was tested in the reaction of
cyclohexanecarbaldehyde with malononitrile in 10 repetition cycles.
After each test, the catalyst was separated by centrifugation, stirred in
solvents with different polarities (EtOH, acetone, and n-hexane),
filtered off, reactivated at 473 K for 90 min, and then reused in the next
run.
To evaluate a leaching of the active species from 1 during catalytic
experiments, a fraction of the reaction mixture was separated and
immediately centrifuged. The obtained liquid phase was reinvestigated
in Knoevenagel condensation under the same reaction condition.
are formed by repeated jarlike cavities, with a narrow entrance
2
(
14.98 × 7.88 Å ) and a wider inside pocket (14.98 × 13.17
2
Å ). Because the guest solvent molecules could not be
determined by XRD data to be due to the high thermal
disorder, they were proven by elemental analysis, TGA−EGA,
and IR measurements at different temperatures. After removal
of the guest molecules, the void space in the total crystal
3
volume, determined by the PLATON program, is 8 913 Å (cell
3
III. RESULTS AND DISCUSSION
volume 17704 Å ) and corresponds to 50.3% of the crystal
volume.
Description of the Crystal Structure. In the crysta−l
structure of 1, two crystalographically independent MTB4
molecules and four independent lead(II) ions exist, which have
different coordination numbers and geometries. The coordina-
tion polyhedron of each lead ion in the [Pb (μ -COO)(μ -
Framework Stability. The thermal robustness of the
framework and solvent removal process was studied by a
combination of Fourier transform infrared (FT-IR) spectros-
copy measured step by step at selected temperatures (HT-IR;
Figure S2), TGA−EGA (Figure 2), and PXRD (Figure 3).
TGA−EGA (Figure 2) indicates a mass loss of 19.8% in the
temperature range 370−570 K, corresponding to the removal
4
3
2
COO) (COO)(H O) ] cluster is strongly distorted because of
6
2
4
2
the stereochemically active 6s lone-pair electrons on the
central lead(II) atoms, which has also been reported in other
51
of nine H O and five DMF molecules (calcd 20.1%) in one step
polynuclear clusters bridged by oxygen atoms. The Pb1 atom
has a distorted tetragonal-bipyramidal geometry formed by six
oxygen atoms (O7, O8, O10, O13, O17, and O18) from four
different carboxylates. The carboxylate O10, O13, and O17
atoms make a bridge between the Pb1 and Pb2 atoms (see
Figure 1a). The Pb2 atom is coordinated by seven oxygen
atoms (O1, O2, O9, O10, O13, O14, and O17), forming a
polyhedron with a distorted monocapped trigonal-prism shape.
The carboxylate group, including the carbonyl C28 atom, is
coordinated in a chelate−anti−anti fashion and makes a bridge
between the Pb1, Pb2, and Pb3 atoms. The Pb3 atom is also 7-
connected with a distorted monocapped trigonal-prism
geometry formed by five oxygen atoms (O5, O6, O9, O15,
and O16) from four carboxylate MTB ligands and two
2
(
endothermic effect at 390 K on DTA). The MS spectra of the
gaseous products of the first stage of thermal decomposition
showed signals with 18 and 73 amu, which were assigned to
coordinated H O oxygen atoms (O11 and O12). The
2
environment around the Pb4 atom can be described as eight-
coordinated distorted bicapped trigonal prism, and it is
coordinated with six oxygen atoms (O1, O2, O3, O4, O5,
4
−
and O15) from four different MTB anions and two
coordinated H O molecules (O19 and O20).
2
The lead(II) ions are connected in the sequence of Pb1−
4
−
Pb2−Pb3−Pb4 by the carboxylate oxygen atoms of the MTB
molecules, and the distances of Pb1−Pb2, Pb2−Pb3, Pb3−Pb4,
and Pb4−Pb2 within the cluster unit are 4.0205(2), 4.1642(2),
4
.1400(2), and 4.1520(2) Å, respectively. The Pb−O bond
Figure 2. Thermoanalytical (TGA/DTG−DTA−EGA) curves of the
thermal decomposition of 1 in an argon atmosphere.
lengths fall in the range of 2.435(9)−2.661(8) Å for Pb1,
D
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The stability of the prepared MOF was also tested after
treatment of the compound in H O at different pH values, and
2
the stability of the framework was studied using PXRD and N2
adsorption measurements (Figures S3 and S4). As can be seen
from Figures S3 and S4, a 7-day treatment of the sample in
H O at pH 2, 7, and 14 had no effect on the porosity and
2
structural integrity of the sample (Figures S3 and S4). From the
obtained results, it could be concluded that the prepared MOF
is H O-stable, which is an important property of the materials
2
for further application in various industrial fields.
In summary, the presented results of spectral, thermal, and
PXRD studies confirm the stability of the framework after
58
thermal treatment. On the basis of Kitagawa’s classification,
compound 1 could be classified in the second MOF generation
with a stable and robust porous framework, showing permanent
porosity after the removal of guest molecules from the pores.
Gas Adsorption Properties. The relatively large solvent-
3
accessible volume in 1 (50.3% of the cell volume 17704 Å )
made it challenging to investigate the gas adsorption properties
of 1 using different gases (N , CO , and CH ) and different
2
2
4
temperatures and pressures (from vacuum up to 21 bar). The
solvents in the cavities of the as-synthesized sample 1 were
removed by evacuation under dynamic vacuum and heating at
4
83 K for 24 h. By such a treatment, the desolvated compound
{
[Pb (MTB) ]} (denoted as 1′ from now on) was obtained.
4
2
n
After the activation procedure, a weight loss of 18.9% was
observed, and it is in good agreement with the thermogravim-
etry results.
The N adsorption measurements at 77 K (Figure S5)
2
revealed a type I isotherm according to IUPAC classification,
which is typical for microporous materials. The small difference
in the shapes of the ideal (type I isotherm) and measured N2
Figure 3. PXRD patterns of 1 (a) calculated from single-crystal XRD,
isotherms for 1′ could be explained by the presence of N
2
(
b) experimentally measured on an as-synthesized sample, and (c)
weaker attractive forces, corresponding to fluid−pore wall
after 24 h of activation at 483 K.
attraction, and N interaction with a variety of surface active
2
sites (e.g., formed after coordinated H O elimination from the
2
H O (18) and DMF (73) molecules. Next, heating of the
sample caused thermal decomposition of the polymeric
framework accompanied by an increase of the intensity of the
central ions) and other vacant sites (dislocation and defects in
crystals) formed during the activation procedure.
2
At relative pressure p/p
reaches 278 cm ·g , corresponding to an experimental pore
= 0.95, the N
uptake capacity
0
2
3
−1
EGA signals with 18 amu (H O), 28 amu (CO), 44 amu
2
3
−1
(
CO ), and 78 amu (benzene) with significant exothermic
volume of around 0.43 cm ·g , and is close to the total
3
2
−1
effects at 633, 708, 788, and 873 K on DTA. The organic part
of the framework did not decompose completely upon heating
to 1200 K because the decomposition of 1 was studied in an
inert gas atmosphere (argon). The carbonized product was still
present at 1200 K (residual mass 55.3%).
microporous volume of 0.67 cm ·g calculated by DFT. The
experimental pore volume (V ) was calculated by eq 1 in
Supporting Information. Evaluation of the N isotherm using
p
2
2
the BET equation gave a specific surface area (SBET) of 980 m ·
−
1
g . It is of note that the observed SBET is the highest compared
FT-IR spectra of 1 also confirm the gradual release of solvent
with other reported MOFs constructed from the methanete-
molecules, H O and DMF, by decreasing the intensities of their
trabenzoate linker such as {[Zn
2
(μ
8
-MTB)(H
2
O)
O)
-MTB)(κ -CYC)
(μ
2
]·3DMF·
2
2
−1 37
characteristic vibrations in the spectra. Detailed information on
the stability and vibrational features are depicted in Figure S2
and reported in the Supporting Information.
3H
2
O}
n
(248 m ·g ), {[Cu
2
(μ
(μ
8
-MTB)(H
2
2
]·6DEF·
]·2DMF·
2
2
−1 54
4
2H
7H
O}
O}
(526 m ·g ), {[Zn
2
n
2
4
(644 m ·g−1),
4
2
53
and {[Ni
O}
-M38TB)
(μ
-
2
n
4
6
2
2
2
−1
A comparison of the PXRD pattern of 1 with the simulated
pattern derived from the single-crystal XRD data of 1 is shown
in Figure 3. A good correspondence between the simulated and
experimentally measured patterns was found, which shows the
phase purity of the as-synthesized compound. In the case of the
desolvated sample 1′, the PXRD pattern is almost identical with
the simulated as well as the experimentally measured pattern
with only some loss of intensity. This indicates that the
activated sample after heating at 483 K for 24 h is highly
crystalline and retains its original framework after removal of
the lattice solvent molecules without disruption of its structural
integrity.
H
2
O)
4
(H
2
O) ]·10DMF·11H
2
n
(700 m ·g ). For the
complexes {[Co (μ -MTB) (μ -H O) (H O) ]·13DMF·
4
6
2
2
2
4
2
4
56
4
55
11H O}
and {[Ni (μ -MTB)(κ -CYC) ]·4DMF·8H O} ,
2 4 2 2 n
3
2
n
−1
adsorbing a limited amount of N [21.6 cm ·g for cobalt(II)
2
3
−1
and 9.2 cm ·g for nickel(II)], their surface areas, 356 and 141
2
−1
m ·g , respectively, were estimated from the CO adsorption
2
isotherms measured at 195 K using the Dubinin−Radushkevich
model.
The other frequent probe to investigate the sorption
properties of MOFs is CO . The adsorption/desorption
2
isotherm of CO measured at 273 K is shown in Figure 4.
2
The maximal storage capacity of CO at 1 bar and 273 K for 1′
2
E
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Inorganic Chemistry
Article
Figure 4. CO adsorption/desorption isotherm measured over 1′ at
2
273 K up to 1 bar.
Figure 5. Repeated high-pressure adsorption measurements of N2,
CH , and CO performed at 303 K up to 21 bar (N adsorption up to
is 2.1 mmol·g−1 (9.3 wt %). As can be seen from Figure 4, the
4
2
2
1
7 bar) on activated compounds 1 and 2 (activated compounds are
adsorption and desorption branches of the isotherm did not
close, and the irreversibility is observed even at the lowest
equilibrium pressures. Such behavior can arise from a strong
interaction between the adsorbate and adsorption sites. A
comparison of the adsorption capacity with the literature data
marked by single primes).
(3.3 Å). The higher quadrupole moment of CO might induce
2
has shown that the CO adsorption capacities were reported for
stronger interactions with the framework of 1′, predominantly
with coordinately unsaturated lead(II) centers, in comparison
to N and CH . Moreover, the observed adsorption trend could
2
four compounds containing the MTB ligand, namely, {[Zn (μ -
2
8
−
1
MTB) (H O) ]·3DMF·3H O} (1.3 mmol·g , 5.67 wt %, 1
2
2
2
n
2
4
3
7
bar, 273 K), {[Co (μ -MTB) (μ -H O) (H O) ]·13DMF·
also be explained by the different boiling points of the gases:
4
6
2
2
2
4
2
4
−
1
56
1
1H O} (1.6 mmol·g , 7.02 wt %, 1 bar, 273 K), {[Zn (μ -
CO , 195 K; CH , 112 K; N , 77 K.
2
n
2
4
2
4
2
4
−1
MTB)(κ -CYC) ]·2DMF·7H O} (2.4 mmol·g , 10.5 wt %, 1
Furthermore, all isotherms are completely reversible and
repeatable after primary vacuum treatment at 303 K, indicating
that a simple regeneration process for this porous solid is
possible. This property of the materials is important and
desirable for their potential application in practice.
2
2
n
5
3
bar, 273 K), and {[Ni (μ -MTB) (μ -H O) (H O) ]·
4
6
2
2
2
4
2
4
−
1
1
0DMF·11H O} (3.2 mmol·g , 12.36 wt %, 1 bar, 273
2
n
3
8
K). As is evident, the adsorption capacity of 1′ is similar to
4
that of the {[Zn (μ -MTB)(κ -CYC) ]·2DMF·7H O} com-
2
4
2
2
n
plex. Anyway, the adsorption uptake of 1′ is rather high
For comparison and evaluation of the results obtained for 1,
high-pressure adsorption measurements for another compound
of the MTB family, namely, {[Ni (μ -MTB) (μ -
compared to those of other MOFs previously proposed for
−
1
CO separation purposes, such as Nd-RPF9 (0.5 mmol·g , 1
2
4
6
2
2
2
−1 59
−1
bar, 303 K, S = 13 m ·g ), Sc BDC (0.7 mmol·g , 1 bar,
H O) (H O) ]·10DMF·11H O} (2), were also performed. 2
BET
2
3
2 4 2 4 2 n
6
0
−1
61
3
03 K), UiO-66-2COOH (1 mmol·g , 1 bar, 303 K), and
was selected because of its similarity to 1. Both compounds are
built from a methanetetrabenzoate linker and contain
tetranuclear clusters with four unsaturated coordination sites
after the desolvation process. Repeated high-pressure isotherms
measured on activated compound 2′ are depicted in Figure 5. It
is evident that 2′ has a slightly lower adsorption capacity of
−1
62
Sc BDC -NO (1.1 mmol·g , 1 bar, 303 K).
2
3
2
Further, we were interested in exploration of the adsorption
potential of 1′ at high pressures for N , CH , and CO as probe
2
4
2
molecules. The repeated adsorption isotherms for 1′ were
measured at temperature 303 K up to 21 bar. The excess uptake
adsorption isotherms on compound 1′ after three repeated
measurements are depicted in Figure 5. The absence of district
plateaus in the isotherms, indicated that the maximal capacity
was still not achieved and compound 1′ is not saturated by the
gases at 21 bar. The amounts adsorbed for CH and N are
−
1
CO , 4.1 mmol·g (21 bar), in comparison to 1′. The higher
2
adsorption capacity of 1′ compared to 2′ could be explained by
2
−1
2
−1
the different surface areas (980 m ·g for 1′ and 700 m ·g for
2′) and the dimensionality of the pore system.
Moreover, when we normalized the high-pressure adsorption
isotherms from the original parameter (the adsorbed amount of
the probe in millimoles per gram of the activated adsorbent) to
the adsorbed amount in moles per 1 mol of the support (Figure
6), the adsorption capacity of compound 1′ showed even better
adsorption properties because lead(II) is much heavier
compared to nickel(II). After recalculation, 1′ adsorbs 8.1
−1
4
2
relatively low, with saturation capacities of approximately 1.9
mmol·g− (21 bar) and 0.8 mmol·g (17 bar), respectively.
These adsorption results indicate the absence of specific
adsorption sites for these two probes at the MOF surface. On
the other side, the activated sample has a better adsorption
1
−1
−1
capacity for CO molecules, being ∼4.5 mmol·g at 21 bar.
2
mol·mol− (CO ), 3.5 mol·mol (CH ), and 1.5 mol·mol
1
−1
The sample 1′ adsorbs 2−4 times more CO than CH and N .
2
4
2
2
4
The enhanced affinity of the framework of 1′ toward CO
(N ) at maximal measured pressures and 303 K. These values
2
2
−1
could be assigned to different quadrupole moments (−Q ) of
are higher in comparison to those of 2′: 5.0 mol·mol (CO2),
ij
−40
−40
2
−1 −1
used gas molecules, 14.3 × 10 C·m (CO ) > 4.7 × 10 C·
2.6 mol·mol (CH ), and 1.1 mol·mol (N ). On the basis of
4 2
2
2
2
m (N ) > 0 C·m (CH ), and their kinetic diameters, which
the presented results, it could be concluded that 1′ shows
2
4
have the opposite trend: CH (3.8 Å) > N (3.64 Å) > CO
higher affinity and storage capacity of CO compared to 2′.
4
2
2
2
F
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Inorganic Chemistry
Article
molecules were all “pulled” from nearby lead(II) sites. The
result coincides with our anticipation of the strong adsorption
capacity of lead. Further studies showed that the geometries of
the optimized structures for the same types of gas molecules
were almost identical. The optimized structures with the
highest adsorption energies are shown in Figure 7c,d. The
average binding energy of the optimized configuration is
1
−1
−
28.01 kJ·mol− for CO and −12.49 kJ·mol for CH . The
2
4
low affinity of CH suggests the absence of specific interactions
4
between CH and the cluster. The average distance between the
4
oxygen atom of the CO molecule and the closest lead(II) atom
2
is 3.081 Å, while the average distance between the hydrogen
atom of CH and the lead(II) ions is longer, 3.467 Å. The
4
calculated affinities and distances using DFT clearly pointed to
a significant interaction of CO over CH to the lead cluster in
2
4
1
′.
To evaluate the gas adsorption ability of 1′, the IAST was
employed for the prediction of the adsorption selectivities from
Figure 6. High-pressure adsorption measurements of N , CH , and
2
4
the experimental pure-gas adsorption isotherms fitted for this
CO at 303 K up to 21 bar on activated (′) compounds 1 and 2 after
2
63,45
purpose with a triple-site Langmuir equation.
Figure 8
recalculation of the adsorbed amount per 1 g of activated adsorbent
−
1
(
(
mmol·g ) to the adsorbed amount per 1 mol of activated support
1
−
mol·mol ).
On the other hand, a slightly higher affinity of 2′ toward CH
4
2.1 mmol·g− (21 bar)] compared to that of 1′ was observed.
1
[
The N adsorption isotherm for 2′ is similar to that measured
2
−1
for 1′, with a maximal capacity of 0.8 mmol·g (17 bar) for 2′.
For a better understanding of the enhanced affinity of CO2
over CH4 in 1′, DFT calculations, which are quite well
established in the modeling of gas adsorption in MOFs, were
performed. For the computational study, only the tetranuclear
cluster resolved from the single-crystal X-ray data was chosen as
a model (Figure 7a). From the cluster, terminal H O molecules
2
Figure 8. IAST selectivities predicted between 0.5 and 15 bar for
equimolar binary gas mixtures of CO /N (red circles) and CO /CH
2
2
2
4
(black squares) on 1′ at 303 K.
shows the predicted adsorption selectivities for equimolar
CO /N and CO /CH mixtures as a function of the pressure
2
2
2
4
(
between 0.5 and 15 bar), which shows that the material is
selective toward CO . The CO /CH adsorption selectivity
2
2
4
remains almost constant, around a value of 7, as the pressure
increases, despite an increase at low pressure. It can be seen
that the predicted CO /N selectivities are not constant as a
Figure 7. (a) View of the lead(II) cluster with coordinated H O
2
2
2
molecules (blue ellipsoids), solved from single-crystal X-ray data. (b)
Initial model used in the DFT calculation and the final calculated
geometries of (c) CO and (d) CH adsorbing onto an unsaturated
function of the pressure. The CO /N selectivity at 0.5 bar is
2
2
approximately 28 and drops to 17 at 15 bar. The high initial
2
4
[
Pb (μ -COO)(μ -COO) (COO)(H O) ] cluster using DFT calcu-
selectivity can be explained by the adsorption of CO on the
2
4
3
2
6
2
4
lations.
open metal sites at low loading, which are the lead(II) clusters
acting as Lewis acid sites. When these strongest specific
adsorption sites are occupied, the CO2 molecules cannot
interact anymore with them and, accordingly, adsorb less
strongly on the surface, leading to a weaker selectivity. This
were removed and vacant orbitals served as a source of the
primary adsorption sites (Figure 7b). To achieve the most
stable configuration of the adsorbate−adsorbent structure,
geometry optimization on several initial positions of adsorbed
molecules was performed. The investigated gas molecules were
first placed randomly with different distances and directions
from lead(II) metal sites, and subsequently geometry
optimization followed. Then we found out that the gas
64,65
effect was also observed in other works.
The higher selectivities of CO /N can be explained by the
2
2
lower uptake of pure N2 compared to CH . The same
4
tendencies in the selectivities as a function of the pressure could
be expected because CH and N are both similarly neutral
4
2
G
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Inorganic Chemistry
Article
adsorbates, showing no particularly strong interactions with the
coordinately unsaturated metal sites present in the material.
However, the CO /CH selectivity is constant in the whole
2
4
pressure range. This is surprising, but maybe we can explain this
result by a higher interaction of CH4 for the adsorbent
compared to N . Indeed, the high polarizability of CH (2.59 ×
2
4
−3
3
−3
3
10
nm > 1.74 × 10 nm for N ), combined with a possible
affinity with the organic part of the material, could balance
2
somehow the strong adsorption of CO on the lead(II) CUS,
2
resulting in a moderate constant selectivity.
It is of note that this CO /CH predicted selectivity for 1′ is
2
4
comparable to those of previously studied well-known MOF
materials such as MIL-100(Cr) (selectivity 6−8, S
= 1720
BET
2
−1
64
m ·g , two unsaturated sites per cluster), MIL-53(Al)
Figure 9. (a) Scheme of Knoevenagel condensation with experimental
conditions. (b) Estimated diameters of the active methylene
compounds: malononitrile, methyl cyanoacetate, and ethyl acetoace-
tate. (c) Aldehydes used in Knoeveneagel condensation: cyclo-
hexanecarbaldehyde and benzaldehyde.
2
−1
(
selectivity 7−8, S = 950 m ·g , without unsaturated active
BET
66
2 −1
sites), and HKUST-1 (selectivity 6−9, S
two unsaturated sites per cluster). It is more delicate to
compare the selectivities toward CO in N because, to the best
= 2210 m ·g ,
BET
67
2
2
of our knowledge, no experimental CO /N binary isotherms
2
2
68
Table 1. Catalytic Results of the Knoevenagel Condensation
of Cyclohexanecarbaldehyde and Benzaldehyde with
Different Methylene Substrates Performed at 403 K
have been reported yet. The review of Sumida et al. provides
a table of selectivity for selected MOFs, calculated at low
pressures (directly from the pure-component isotherms and not
predicted by using the IAST). The value obtained at low
pressure on 1′ in this study, around 28, is in a comparable order
of magnitude with selectivities predicted under the same
a
b
c
aldehyde
PhCHO
methylene compound
MNT
run
1st
conversion
selectivity
100
100
100
100
100
100
100
100
68
100
97
2
3
4
6
8
1
nd
rd
65
conditions by Wiersum, for HKUST-1 (selectivity ∼20),
98
UiO-66-NH (selectivity ∼33), MIL-100(Fe) (selectivity ∼38),
2
th
98
and MIL-101(Cr) (selectivity ∼40). Good selectivities may
indicate that 1′ has great potential in the adsorptive separation
of CO /N and CO /CH gas mixtures. Mainly, the high
th
99
th
97
2
2
2
4
0th
98
selectivity of CO over N is important for the capture of CO
2
2
2
MCA
EAA
100
87
from the atmosphere to decrease the greenhouse effect on the
environment.
ChCHO
MNT
MCA
EAA
100
61
100
100
55
Knoevenagel Condensation Reactions. Knoevenagel
condensation is an important C−C bond coupling reaction in
organic chemistry, in which a carbonyl group (aldehyde) reacts
with the methylene group activated by two electron-with-
drawing groups. This reaction is widely used in the synthesis of
57
a
ChCHO = cyclohexanecarbaldehyde, PhCHO = benzaldehyde, MNT
malononitrile, MCA = methyl cyanoacetate, and EAA = ethyl
acetoacetate; The yield is calculated by GC using n-dodecane as the
=
30
b
c
pharmaceuticals and fine chemicals. The condensation can be
internal standard. Conversion after 8 h of reaction. Selectivity after 8
h of reaction.
acid-catalyzed (Lewis acids) or base-catalyzed, abd it can be
15,23−36
homogeneous or heterogeneous.
In the present study, Knoevenagel condensation of cyclo-
hexanecarbaldehyde and benzaldehyde with different active
methylene compounds was performed. The general scheme of
the condensation reaction with reaction conditions and
structures of the substrates with corresponding molecular
sizes are depicted in Figure 9. The results obtained from the
catalytic experiments are listed in Table 1.
In the case of other reactions, the conversions ranged from
57 to 68%, with yields of 100% of the desired products in the
reactions with methyl cyanoacetate. On the other hand,
selectivities of below 87% (benzaldehyde) and 55% (cyclo-
hexanecarbaldehyde) were achieved in the reactions with ethyl
acetoacetate. The observed trend could be explained by the
sizes of the molecules: the molecule of cyclohexanecarbalde-
hyde is bulkier compared to benzaldehyde (Figure 9c), and
reactions are limited by the pore/cavity size of the catalyst
In the presence of the catalyst, the reactions of
benzaldehyde/cyclohexanecarbaldehyde with malononitrile
2
(
1/1.5 molar ratio) occurred smoothly and gave 2-benzylide-
(14.98 × 7.88 and 14.98 × 13.17 Å ). The decreased reactivity
nemalononitrile and 2-(cyclohexylmethylene)malononitrile,
respectively, as the only products, with a yield of 100% with
complete conversion after 4 h. The catalytic properties of 1′ for
the Knoevenagel condensation of aldehydes with methyl
cyanoacetate and ethyl acetoacetate were also investigated.
We observed a decrease in the activity of 1′ in cyclo-
hexanecarbaldehyde condensation with methyl cyanoacetate,
especially with ethyl acetoacetate, in comparison to benzalde-
hyde. As can be seen from Figure 10 and Table 1, the 100%
conversion of methyl cyanoacetate or ethyl acetoacetate with
selected aldehydes after 8 h was observed only in the reaction
of cyclohexanecarbaldehyde with methyl cyanoacetate.
in the order ethyl acetoacetate < methyl cyanoacetate <
malononitrile well agrees with the increase in the acid
69
dissociation constant of these active methylene substrates. It
should be noted that this sequence of substrate reactivities is in
36,38
good agreement with those in our previous reports.
The conversion of benzaldehyde and malononitrile in
Knoevenagel condensation was only slightly dependent on
the weight of 1′ (Figure 11a). In the corresponding
condensation reaction, 100% conversion of the targeted
product after 60 min of reaction was achieved when using
150 mg of 1′, while conversions equal to 94% and 89% were
observed in the presence of 100 and 50 mg of 1′, respectively.
H
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Figure 10. Conversion (a) and selectivity (b) curves of the desired
products between cyclohexanecarbaldehyde (ChCHO), benzaldehyde
(
(
PhCHO), and different methylene compounds: malononitrile
MNT), methyl cyanoacetate (MCA), and ethyl acetoacetate (EAA).
On the other side, the selectivity to 2-benzylidenemalononitrile
was almost 100%, independent of the weight of the catalyst.
On the other hand, conversion of the respective product,
formed in the presence of 1′ (50 mg) in the benzaldehyde
condensation with malononitrile, was dependent on the molar
ratio of aldehyde/malononitrile used. As can be seen from
Figure 11b, when the molar ratio of the reaction substances was
Figure 11. (a) Effect of the catalyst weight (50, 100, and 150 mg) in
Knoevenagel condensation of benzaldehyde and malonotrile over 1′
performed at 403 K and a molar ratio of 1/1.5 (benzaldehyde/
malononitrile). (b) Effect of the substance molar ratio (aldehyde/
active methylene compound) on conversion of the corresponding
product for condensation of benzaldehyde catalyzed by 1′ at the
reaction temperature 403 K.
1/1, after 30 min of reaction, the conversion of the desired
product was 54%. When the molar reaction of the reactants
increased to 1/1.5 and 1/2, the trends of the conversion curves
were similar and the corresponding yields at the same reaction
time were comparable: 82 and 83%, respectively.
[Pb (μ -COO)(μ -COO) (COO)(H O) ] clusters bridged by
4
3
2
6
2
4
MTB linkers and exhibits a noninterpenetrated 3D framework
with repeated jarlike cavities. The stable desolvated framework
with permanent porosity can be prepared by heating the
compound to 550 K. The porous nature of the complex was
confirmed by various gas adsorption measurements up to 21
bar, showing the highly selective adsorptive separation of CO2
over N and CH . DFT calculations identified directly the
To clarify whether the catalysis in the presence of 1′ is
heterogeneous or homogeneous, the catalyst was removed by
“hot filtration” after 30 min of reaction of malonitrile with
benzaldehyde. The filtrate was kept under the same conditions
and monitored by GC. As can be seen from Figure S6, after
filtration, the conversion of benzaldehyde stops at ∼50% after
removal of the catalyst. These results show that the catalysis is
heterogeneous and no leaching of the catalytic species from the
framework into the liquid phase occurs.
The recyclability of 1′ was tested for the reaction of
cyclohexanecarbaldehyde with malononitrile. The activity of the
catalyst after 10 catalytic runs remains at high levels
2
4
functional binding sites located on the lead cluster after removal
of terminal H O molecules. We demonstrated that the open
2
lead(II) coordination sites play an important role in the
adsorption process. The results of the catalytic tests clearly
confirm the excellent catalytic activity of 1′ for Knoevenagel
condensation reactions. Thus, the sustainability and high
potential of 1 for application in gas separation and catalysis
was demonstrated in multiple experiments, while the origin of
such an efficiency was explained using computational
approaches. We plan to further investigate compound 1 in
Knoevenagel condensation of different bulky aldehydes with
active methylene compounds and, in this way, to study more in
(
conversion 100%; selectivity 97−100%; Figure S7).
IV. CONCLUSIONS
We have synthesized and characterized a novel lead metal−
organic framework, {[Pb (μ -MTB) (H O) ]·5DMF·H O}
4
8
2
2
4
2
n
(
1). Compound 1 is built by asymmetric tetranuclear
I
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depth the effect of the pore size of the catalyst on its catalytic
performance.
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ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02491.
(4) (a) Su, H.; Sun, F.; Jia, J.; He, H.; Wang, A.; Zhu, G. A Highly
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Corresponding Author
E-mail: vladimir.zelenak@upjs.sk.
ORCID
Vladimír Zelenak: 0000-0002-6118-1269
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Philip L. Llewellyn: 0000-0001-5124-7052
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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This work was supported by the Scientific Grant Agency of the
Slovak Republic (VEGA) under Project 1/0745/17, by the
Slovak Research and Development Agency under Contract
APVV-15-0520, and by Project VVGS-2016-249 from P. J.
Safarik University. J.C. acknowledges the Czech Science
Foundation for Project P106/12/G015, and R.G. thanks OP
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