A double basic Sr-amino containing MOF as a highly stable heterogeneous catalyst

Article abstract of DOI:10.1039/c9dt01061k

A novel metal-organic framework (MOF) based on strontium alkaline-earth metal and 2-amino-1,4-benzenedicarboxylic acid (NH₂-bdc) has been developed. This material is formed by a linear succession of face-sharing strontium polyhedra bridged by a

Full text of DOI:10.1039/c9dt01061k

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D. Briones Gil, A. Rodríguez Diéguez, D. Choquesillo-Lazarte, G. Calleja and F. Martínez, Dalton Trans.,
2019, DOI: 10.1039/C9DT01061K.
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COMMUNICATION
Douglas W. Stephen et al.
N-Heterocyclic carbene stabilized parent sulfenyl, selenenyl,
and tellurenyl cations (XH , X = S, Se, Te)
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Doubled-basic Sr-Amino containing MOF as highly st
D
a
O
b
I: 1
l
0
e
.1039/C9DT01061K
heterogeneous catalyst
a
b
a
c
Pedro Leo , Gisela Orcajo , David Briones , Antonio Rodríguez-Diéguez ,
d
b
a
Duane Choquesillo-Lazarte , Guillermo Calleja and Fernando Martínez*
a
Department of Chemical and Environmental Technology, Rey Juan Carlos University,
Calle Tulipán s/n, 28933 Móstoles, Spain.
b
Department of Chemical, Energy and Mechanical Technology, Rey Juan Carlos
University, Calle Tulipán s/n, 28933 Móstoles, Spain.
c
Department of Inorganic Chemistry, University of de Granada, Avda. Fuentenueva s/n,
1
8071, Granada, Spain
d
Laboratorio de Estudios Cristalográficos, IACT (CSIC-UGR), Avda. de las Palmeras 4, 18100,
Armilla, Granada, Spain.
ABSTRACT
A novel metal-organic framework (MOF) based on strontium alkaline-earth metal and 2-
amino-1,4-benzenedicarboxylic acid (NH -bdc) has been developed. This material is
2
formed by a linear succession of face-sharing strontium polyhedrons bridged by the
organic ligand molecule to give a three-dimensional network with rhombohedral one-
directional channels. This MOF is stable in polar organic solvents and up to 250 °C. The
basic catalytic activity of both strontium metal nodes and amino groups of the ligand was
tested in Knoevenagel condensation reactions. The influence of the temperature and
reaction solvent over the catalytic preformance of the MOF catalyst was demostrated. The
strontium/amino-containing MOF material evidenced a remarkable activity as compared
to other conventional alkaline oxides typically used as reference basic solid catalysts. The
novel MOF material proved a remarkable activity and structural stability during five
consecutive catalytic runs with no evidence of activity loss at the best reaction conditions
found in this study.
KEYWORDS: Strontium oxide, amino groups, metal-organic framework, basic catalysis,
Knoevenagel condensation.
*
Corresponding author: Tlf: +34 91 488 71 82
e-mail address: fernando.castillejo@urjc.es

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1
. INTRODUCTION
Metal-Organic Framework (MOF) materials have emerged as a new class of porous
crystalline hybrid metal-organic polymers built up from metallic ions or clusters
connected through organic linkers by means of coordination chemistry. The topological
richness, tunable pore size and the huge variety of possible combinations between metal
ions and organic linkers make MOF materials excellent candidates for catalytic [1, 2] and
electrocatalytic applications [3, 4]. Metal sites and/or specific functional groups from the
organic linkers can act as active sites. Furthermore, post-synthetic routes can modify
MOF materials in order to incorporate additional active species [5].
Terephthalic acid and its derivatives have been widely used for the synthesis of MOF
materials due to their linear geometry, rigidity and multiple coordination modes of the
carboxylate groups [6]. Likewise, transition metals have been extensively employed due
to their variety of oxidation states [7], that could provide unsaturated open metal sites
with acid and redox properties, able to directly interact with any substrate [8, 9, 10, 11].
In contrast, MOF materials based on s-block metals or rare-earth elements are not so
frequent [12], although networks incorporating magnesium or calcium can also be very
interesting as catalysts [13, 14, 15]. The use of alkaline-earth ions offers interesting
advantages compared to the transition and lanthanide metal ions due to their relatively
larger radius and lower weight, being also more abundant and non-toxic [16, 17, 18].
The assembly of alkaline-earth metals in a MOF scaffold is rather challenging, taking into
account that alkaline-earth metal oxides are classic solid bases. Besides, MOFs can have
different types of basic sites in both metal nodes and organic ligands, leading to design a
range of base catalysts with targeted features. Moreover, these sites are plentiful, site
isolated, and periodically organized in the crystalline material. As a result, MOFs provide
the opportunity of developing new solid bases with characteristic features that are
unachievable by traditional porous materials [19]. In this sense, MOFs with basic
functionalities are able to catalyze an assortment of organic reactions including Aldol-
condensation [20], Knoevenagel condensation [21, 22] and other reactions of interest in
fine chemistry [23].
Knoevenagel condensation between a carbonyl compound and a molecule containing an
activated methylene group have been traditionally used as a test reaction to evaluate the
the catalytic behavior of basic catalysts. This is a two-step reaction with a first nuclephilic
addition of active methylene group to carbonyl group and following dehydration to

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produce an ,-conjugated enone [24]. Some heterogeneous catalysts used so far for
Knoevenagel condensation are zeolites [25, 26], alkali earth oxide supported on alumina
27], magnesium oxide [28] and amino based MOFs [29]. Some examples of amino-
[
containing MOF materials evaluated as basic catalysts for Knoevenagel condensation
have been IRMOF-3, MIL-101(Al)-NH and MIL-53(Al)-NH [29, 30, 31]. All these
2
2
MOF materials were catalytically active due to the non-coordinated primary amines as
Brønsted-basic sites.
Therefore, a new MOF with a crystalline 3D structure based on strontium and 2-amino-
1
,4-benzenedicarboxylate organic ligand (NH -bdc) was synthesized in this work. This
2
material provides alkaline-earth metal oxides and amino groups, both as basic active sites.
The catalytic activity and stability in Knoevenagel condensation reactions were evaluated
under different conditions. The catalytic performance of the new material was compared
to other reference solid base catalysts reported in the literature.
2
2
. EXPERIMENTAL
.1. Synthesis of Sr(NH -bdc)
2
All analytical reagents were commercial products and they were used without further
purification. In a typical synthesis, 27.2 mg (0.150 mmol) of 2-amino-1,4-
benzenedicarboxylic acid and 63.5 mg (0.300 mmol) of strontium (II) nitrate were
dissolved in a mixture of N,N-dimethylformamide (15 mL) and water (1 mL). The
solution was added to a 20 mL scintillation vial and placed in a preheated oven at 90 °C
for 72 hours. Then, yellowish crystals were isolated by decanting the mother liquor and
washed with DMF. The size of crystals was suitable for single X-ray diffraction (section
S1). The synthesis yield was ca. 66% based on the linker content. IR = 3467 (w), 3337
(w), 1643 (m), 1554 (s), 1490 (m), 1377 (m), 1251 (m), 1107 (w), 837 (m), 809 (w), 775
-1
(
m), 667 (w) cm . C, H, N analysis (%): calc. C 36.99, H 3.27, N 7.26; found C 33.57, H
.10, N 6.61.
3
Other catalytic materials used in this work for comparison purposes, such as BaO, SrO
and BETA zeolite, were obtained from Sigma-Aldrich Química S.L. and Zeolyst
International, respectively. BETA zeolite was ion-exhanged with sodium in our
laboratory following the procedure reported by Seung-Tae et al. [32]. Additionally, an
IRMOF-3 material was synthesized according to the method described in literature [33].

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2
.2. Characterization techniques
The crystalline structure of Sr(NH -bdc) was determined by single crystal X-ray
2
diffraction. For this purpose, suitable crystals of Sr(NH -bdc) were mounted on a glass
2
fiber and used for data collection using MoK radiation (λ = 0.71073 Å) on a Bruker D8
Venture diffractometer equipped with CMOS Photon100 detector at 100(2) K by
applying the -scan method. The crystal of Sr(NH -bdc) is a two-component
2
nonmerohedral twin (0.59:0.41). The twin law describes a rotation of 180º around the
[1 0 0] direction, given by the matrix (1 0 0 0 -1 0 0 0 -1). Absorption correction was
applied using TWINABS [34]. The structure was solved by direct methods and refined
2
with full-matrix least-squares calculations on F using the program SHELXL [35] and
Olex2 as graphical interface [36]. Anisotropic temperature factors were assigned to all
atoms except for hydrogen atoms, which are riding their parent atoms with an isotropic
temperature factor arbitrarily chosen as 1.2 times that of the respective parent. The
structure exhibits disorder of the coordinated DMF molecule, which was successfully
refined using a two-site model with a 0.50:0.50 occupancy ratio. Moreover, the NH -bdc
2
ligands reside on crystallographically imposed inversion centers and therefore their NH2-
2
groups are equally disordered over two positions. Final R (F), wR (F ) as well as goodness
of fit agreement factors and details on the data collection and analysis can be found in
Table S1. Selected bond lengths and angles are given in Table S2. Powder X-ray
diffraction (XRD) patterns were obtained from a PHILIPS X‘PERT diffractometer using
CuK radiation (1.54056 Å). The data were recorded from 5 to 50° (2) with a resolution
of 0.01°. Fourier transform-infrared spectra (FT-IR) on powdered samples were carried
-1
out on a Varian 3100 Excalibur Series spectrometer with a resolution of 4 cm and 64
scans coupled to an MKII Golden Gate Single Reflection ATR system to acquire spectra
in Attenuated Total Reflectance mode. Nitrogen adsorption–desorption isotherms at −196
°
C were measured using an AutoSorb equipment (Quantachrome Instruments). Samples
-7
were previously evacuated in situ under high vacuum (<10 bar) for 8 h at 150 °C. The
surface area was calculated by using the Brunauer–Emmett–Teller (BET) model [37].
The pore volume and diameter were estimated by non-local DFT calculations, assuming
a kernel model of N at -196 °C on carbon (cylindrical pores, NLDFT equilibrium model)
2
[38]. Simultaneous thermogravimetry and derivative thermogravimetry analyses
−
1
(
TGA/DTG) were carried out under air flow (100 mL min ) at a heating rate of 5 °C/min
up to 900 °C, using a Mettler Toledo TGA/DSC1 thermobalance. Elemental analyses

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were carried out using an analyzer type Flash 2000 (Thermo Fisher Scientific), equipped
with a thermal conductivity detector (TCD). Carbon, nitrogen and hydrogen are
determined by an oxidation/reduction reactor at a temperature of 900 °C. Oxygen
determination is achieved through an Oxygen-specific pyrolysis reactor heated at 1060
°
C.
2
.3. Catalytic Knoevenagel condensation reaction
The basic catalytic activity of the Sr(NH -bdc) material was assessed in the Knoevenagel
2
condensation of benzaldehyde (BZ) with ethyl cyanoacetate (EC) to produce ethyl trans-
α-cyanocinnamate (ETCN). All the experiments were carried out in a round bottom flask
(50 mL) placed in a silicone bath under nitrogen atmosphere. Typically, 5 mmol of ethyl
cyanoacetate (EC) as methylene compound, 10 mL of solvent and catalyst (5 mmol of
metal) were added at room temperature. Thereafter, the mixture was heated up to the
reaction temperature. Once the desired temperature was achieved, 5 mmol of
benzaldehyde (BZ) was added. The influence of reaction solvent and temperature was
explored according to previous studies [39]. A stirring speed of 1000 rpm was fixed in
order to avoid mass transfer limitations. Aliquots were withdrawn at selected reaction
times ranging from 0 to 4 h. Reactant and product concentrations were quantified by gas
chromatography, using a GC-3900 Varian chromatograph equipped with a DB-5MS Ultra
Inert capillary column (30 m × 0.25 mm, film thickness 0.25 µm) and a flame ionization
detector (FID). Dodecane was used as an internal standard.
3
3
. RESULTS AND DISCUSSION
.1.
Structural characterization of Sr(NH -bdc) material
2
The crystalline structure of the new material Sr(NH -bdc) was elucidated by the single
2
crystal XRD technique. The asymmetric unit comprises a strontium ion linked to two
-2
-2
halves of NH -bdc ligand and one DMF solvent molecule (Figure 1A). The NH -bdc
2
2
ligand is connected to four different strontium ions (Figure 1B). The coordination
geometry of strontium ion can be described as a biaugmented triangular prism, a Johnson
solid (J50) with a distorted C symmetry with eleven faces: one quadrilateral face and
2
v
ten triangular faces, as well as eight vertex and seventeen edges. Similar coordination
geometry for calcium and strontium ions has been previously described in literature [40].
The metallic secondary building unit is composed by lineal chains along [100] direction

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comprised by a succession of face-sharing strontium polyhedrons (Figure 1C), which are
-2
bridges by NH -bdc linkers, giving rise a three-dimensional network, with
2
rhombohedral one-directional channels along the b axis, fully occupied with coordinated
DMF molecules (Figure 1D). More details of the structural unit of Sr(NH -bdc) material
2
were included in Tables S1 and S2.
C(3B)
A)
B)
N(1B)
C(1B)
O(1B)
O(2C)
C(2B)
N(1A)
C(1C)
O(1C)
C(3C)
O(1A)
C(2C)
Sr (1)
C(3A)
C(4A)
C(4C)
C(2A)
O(2A)
N(1C)
C)
D)
-2
Figure 1. (A) Asymmetric unit of Sr(NH -bdc) structure; (B) NH -bdc ligand
2
2
coordination; (C) Metallic secondary building unit of Sr(NH -bdc); (D) View down the
2
b axis of the channels in Sr(NH -bdc) structure. Sr: green, C: grey, N: blue, and O: red.
2
Hydrogen atoms are omitted for clarity.
In order to confirm the purity of the crystalline phase in the bulk sample, the experimental
powder X-ray diffraction pattern of the new material was compared to the simulated one
obtained from its crystallographic data (Figure 2). As observed, the location and intensity
of main reflections match quite well in both patterns, endorsing the crystalline structure
resolved by single crystal XRD.

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Powder-XRD (experimental)
Single crystal XRD (simulated)
5
10
15
20
25
30
35
40
45
50
2
 (°)
Figure 2. XRD patterns of powder and single crystal Sr(NH -bdc) material
2
The thermal stability of the crystalline Sr(NH -bdc) material was evaluated from in-situ
2
powder XRD measurements at different temperatures in inert atmosphere and also using
TG/DTG analyses in air atmosphere, as shown in Figure 3. The in-situ XRD patterns at
different temperatures indicate the presence of the main diffraction reflections of the
structure until 350 ºC, although a significant decay of crystallinity was observed from 250
°
C. The TG/DTG analyses display a strong weight loss at ca. 375-460 °C, caused by the
organic linker decomposition. Thus, the Sr(NH -bdc) material is thermally stable up to
2
2
50 ºC with no evidence of structural damage, but above this temperature the loss of
crystallinity is clear.
A)
4
00 C
3
3
2
50 C
00 C
50 C
2
1
00 C
50 C
1
00 C
0 C
50
3
5
10
15
20
25
30
35
40
45
2
 (°)

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B) 100
DOI: 10.1039/C9DT01061K
9
8
7
6
5
4
3
2
0
0
0
0
0
0
0
0
1
00 200 300 400 500 600 700 800 900
Temperature (C)
Figure 3. In-situ XRD patterns at different temperatures (a) and TG/DTG analyses
b) of Sr(NH -bdc) material.
(
2
Nitrogen adsorption/desorption at 77 K of Sr(NH -bdc) material shows a type-IV
2
isotherm (Figure S3), typical of nonporous or macroporous materials, according to the
IUPAC classification [41]. The low porosity suggests a strong interaction of coordinated
solvent molecules, which are not evacuated after the degasification treatment, occupying
a significant volume of the voids. The profiles of TG/DTG analyses seem to confirm that
these solvent molecules strongly linked to the crystalline framework are released just
before the organic ligand decomposition.
The structural stability of MOF materials in different chemical media is also a crucial
issue for their application as heterogeneous catalysts. Thus, the chemical stability of
Sr(NH -bdc) material was tested in several solvents of different polarities, ranging the
2
polar indexes from 2.3 (toluene) to 10.2 (water). Figure 4 shows the powder XRD patterns
after dispersing the material in the corresponding solvent for 24 h at room temperature
and subsequent drying. It was observed that the crystalline structure of Sr(NH -bdc)
2
collapsed in water. However, it was maintained for all the other organic solvents, keeping
the typical diffraction peaks of the characteristic crystalline framework.

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H O
DOI: 10.1039/C9DT01061K
2
EtOH
MeOH
THF
Toluene
DMF
Non-solvent treatment
5
10
15
20
25
30
35
40
45
50
2
 (°)
Figure 4. Powder XRD patterns of the Sr(NH -bdc) material after suspension in
2
different solvents.
The remarkable structural stability of Sr(NH -bdc) material can be rationally explained
2
by the ionic nature and high charge density of alkaline-earth metal ions that make the M–
O bonding interactions much stronger [42]. Moreover, the large ionic size of the alkaline-
earth metals allows a higher coordination number ranging from six to ten that leads to a
higher connectivity of the alkaline-earth metal nodes of the framework, and consequently
an improved stability. These two effects have been already reported in literature for other
MOF materials based on alkaline-earth metal ions [43].
3
.2.
Catalytic activity of Sr(NH -bdc) material
2
Figure 5 shows the catalytic activity of Sr(NH -bdc) in terms of benzaldehyde (BZ)
2
conversion and ETCN product yields in Knoevenagel condensation of benzaldehyde and
ethyl cyanoacetate (EC) using DMF as solvent, at 80°C, equimolar ratio of BZ/EC, and
catalyst concentration of 5 mmol of strontium. Additionally, several experiments were
included: i) blank reaction, in absence of Sr(NH -bdc); ii) amino homogeneous reaction,
2
in absence of Sr(NH -bdc) but 2-amino-1,4-benzenedicarboxylic acid as basic organic
2
ligand; and iii) homogeneous reaction test, in presence of Sr(NH -bdc) for the first 15
2
min and later removal by hot filtration. The catalytic activity of the heterogeneous
Sr(NH -bdc) catalyst is clearly evidenced taking into account the results of the additional
2
experiments. The BZ conversion of Sr(NH -bdc) is 66% after 15 min, and 94% after 240
2

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min, which are much higher than the values observed by the blank reaction. The amino
homogeneous reaction test shows a certain activity but significantly lower than that
obtained by the heterogeneous Sr(NH -bdc) catalyst. This fact clearly proves the
2
important benefit of Sr(NH -bdc) material with double basic sites, alkaline-earth metal
2
oxides and amine groups. Note that concentration of the amino-organic ligand was
equivalent in both experiments (heterogeneous and amino homogeneous reaction tests).
Moreover, the Sr(NH -bdc) material evidenced a higher selectivity to ethyl trans-α-
2
cyanocinnamate (ETCN) than that displayed by the amino homogeneous organic ligand.
1
13
H and C NMR analyses confirmed the formation of ETCN product for Knoevenagel
condensation of benzaldehyde and ethyl cyanoacetate with Sr(NH -bdc) catalyst (Figure
2
S4a and S4b, respectively). The homogeneous reaction test, which was performed in order
to assess the potential activity of species leached-off by Sr(NH -bdc) following the
2
procedure used in literature [44], shows no further conversion of BZ after removing the
Sr(NH -bdc) solid material (at 15 minutes). This fact indicates that there is not
2
contribution of homogeneous active species. The absence of strontium in the liquid phase
of reaction (measured by ICP-OES analysis) confirmed this hypothesis.
blank reaction
amino homogeneous reaction
homogeneous reaction test
Sr(NH -bdc)
2
1
00
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
1
5 min
120 min
240 min
Figure 5. Catalytic activity of Sr(NH -bdc) material, blank and homogeneous
2
experiments for Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate.
Normal columns, benzaldehyde (BZ) conversion (left vertical axis); filled columns,
ethyl trans-α-cyanocinnamate (ETCN) product yield (right vertical axis).

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3
.3 Catalytic activity of Sr(NH -bdc) material versus other solid catalysts
2
The catalytic performance of Sr(NH -bdc) was compared to different basic materials such
2
as another NH containing-MOF material (IRMOF-3; Zn-NH -bdc), alkaline-earth metal
2
2
oxides (MgO, SrO and BaO) and Na-exchanged BETA zeolite. These catalytic
experiments were carried out at 80 °C, equimolar ratio of benzaldehyde/ethyl
cyanoacetate and DMF as solvent. The catalyst loading of IRMOF-3, metal oxides and
Na-exchanged BETA zeolite was set to amounts corresponding to the same concentration
of NH -bdc (IRMOF-3) and alkaline-earth metal content (metal oxides and Na-
2
Exchanged zeolite) which were 0.25 mmol of NH -bdc ligand and 5 mmol of alkaline-
2
earth metal. Figure 6 shows the BZ conversion and ETCN product yield for all tested
materials.
MgO
SrO
BaO
IRMOF-3
Sr(NH -bdc)
Na-Exchanged zeolite
2
1
00
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
1
5 min
120 min
240 min
Figure 6. Catalytic activity of different catalysts for the Knoevenagel condensation of
benzaldehyde and ethyl cyanoacetate. Normal columns, benzaldehyde conversion; filled
columns, ETCN product yield.
The alkali-earth oxides (MgO, SrO, BaO) were significantly less active than the
Sr-(NH -bdc). The increasing basicity of the alkaline-earth metal oxides with its atomic
2
weight leads to a slight enhanced of its catalytic performance. It can be seen that the
Sr(NH -bdc) material, with both strontium metal oxide and amine basic sites, allows
2
increasing significantly the catalytic activity of the SrO as itself. Likewise, the ETCN
selectivity of Sr(NH -bdc) material is slightly higher than that obtained by the SrO.
2
Regarding the amino-containing IRMOF-3 material (Zn-NH -bdc), a very good catalytic
2
activity, particularly at shorter reaction times, was observed. However, according to the

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powder DRX patterns, the IRMOF-3 material clearly evidenced signs of structural
collapse with a significant loss of crystallinity after reaction. In contrast, the Sr(NH -bdc)
2
material kept its crystalline structure unchanged (Figure S5). These results indicate that
Sr(NH -bdc) material is chemically more stable than IRMOF-3 under the operation
2
conditions of Knoevenagel condensation reaction. Additionally, FT-IR spectra of the
Sr(NH -bdc) material before and after reaction revealed the characteristic bands of
2
functional organic groups of Sr(NH -bdc) material (Figure S6). This fact also proves the
2
chemical stability of this material for Knoevenagel condensation reactions. Finally, the
Sr(NH -bdc) material showed a similar catalytic performance than Na-exchanged BETA
2
zeolite after 4 hours of reaction, with BZ conversions and ETCN yields above 90 and
8
0%, respectively.
3
.4. Influence of substrates and reaction conditions on the catalytic performance of
Sr(NH -bdc)
2
Table 1 displays the catalytic performance of Sr(NH -bdc) for Knoevenagel condensation
2
reactions using different substrates, solvents and temperatures. Concerning the substrates,
4
-methybenzaldehyde and 4-nitrobenzaldehyde were used instead of benzaldehyde
(entries 1-3). The 4-nitrobenzaldehyde exhibited the highest conversion of all the tested
carbonyl compounds with a remarkable conversion of 91% at 15 minutes. 4-
methylbenzaldehyde showed the lowest conversion with values of 61 and 84 % at 15min
and 120 min, respectively. The largest conversion for 4-nitrobenzaldehyde is attributed
to the strong electron withdrawing effect of the nitro substituent. This fact favors the
nucleophilic attack at the carbonyl group of benzaldehyde molecule by the methylene
group [45]. This behavior has been also reported in literature by other amino-containing
MOF materials [39]. Additionally, malononitrile was studied instead of ethyl
cyanoacetate as active methylene molecule (entry 4). When using malononitrile, a clear
acceleration of the aldehyde conversion was observed, being complete not only at 120
min but also at 90 min (result not shown). These results are due to the higher reactivity
of the methylene group as its acid strength is increased, being higher for malononitrile
(pKa  11) as compared to ethyl cyanoacetate (pKa  13) [39, 45].
The effect of the solvent polarity was assessed for solvents with different dielectric
constants: DMF (36.7), methanol (32.7), ethanol (24.3), THF (7.6) and toluene (2.4). The
results (Table 1, entries 5-9) evidenced a clear relationship between the BZ conversion

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and the solvent polarity. The high polarity of DMF, methanol and ethanol led to higher
conversions. This fact is due to the ability of polar solvents to produce protonated forms
of the amino groups of the MOF linker, which favors the formation of bezaldimine
intermediates by a proton transfer mechanism. The higher conversion achieved by
methanol despite of having a lower polarity than DMF, is related to the amphiprotic
character of methanol [29], which induces the electrophilic polarization of the carbonyl
group of benzaldehyde, and consequently increasing the reaction rate [46, 47]. In contrast,
toluene, as less polar solvent showed the lowest BZ conversion and ETCN yield.
Table 1. Catalytic activity of Sr(NH -bdc) at different reaction conditions.
2
Conv. (%)
Yield. (%) Conv. (%) Yield. (%)
Variable
Nº
Value
1
5 min
61
66
91
83
71
66
49
39
33
80
71
63
52
38
15 min
37
44
63
57
54
44
23
5
120 min
84
120 min
77
1
2
3
4
5
6
7
8
9
4-methylbenzaldehyde
benzaldehyde
93
80
SUBSTRATE a
4-nitrobenzaldehyde
100
100
96
89
malononitrile
MeOH
DMF
EtOH
THF
87
84
93
80
SOLVENT b
65
47
54
10
Toluene
100
2
47
8
1
1
0
1
64
54
49
34
22
100
96
88
80
84
TEMPERATURE (°C) c
12
60
87
79
1
1
3
4
40
80
74
25
72
60
Rest of reaction conditions:
a
Temperature: 80 °C, [Sr] = 5 mmol, DMF as solvent.
b
c
Temperature: 80 °C, [Sr] = 5 mmol, substrate: benzaldehyde.
Sr] = 5 mmol, MeOH as solvent, substrate: benzaldehyde.
[

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Finally, the influence of the reaction temperature was evaluated in the interval of
2
5-100 °C using methanol as solvent, catalyst concentration of 5 mmol of strontium and
equimolar ratio between benzaldehyde and ethyl cyanoacetate (Table 1, entries 10-14).
The effect of temperature is quite relevant in the activity of Sr(NH -bdc), increasing the
2
BZ conversion and the ETCN product yield. This effect is particularly noticeable at early
reaction times (15 min). Remarkable results of BZ conversion (96%) and ETCN yield
(84%) were achieved at 80ºC after 120 min of reaction (entry 11). The increase of
temperature up to 100ºC did not significantly enhance the BZ conversion and the ETCN
product yield (entry 10).
3
.5 Catalyst reusability and stability
The reusability of Sr(NH -bdc) catalyst was also studied at 80 °C, catalyst concentration
2
of 5 mmol of strontium, benzaldehyde/ethyl cyanoacetate equimolar ratio and methanol
as solvent. The results of BZ conversion and ETCN yield for five consecutive catalytic
runs are shown in Figure 7A. As observed, the values of BZ conversion and ETCN yield
remain practically constant (around 94% and 84%, respectively). Interestingly, the solid
recovery after each reaction was almost total (above 97% in all the cases). Moreover, the
characteristic crystalline phase of the Sr-(NH -bdc) material is maintained along the
2
succesive cycles, as it can be attested from the X-ray diffraction patterns of fresh and
recovered Sr(NH -bdc) catalyst after the different cycles (Figure 7B). These results
2
indicate the excellent stability of the catalyst under the studied reaction conditions.
A)100
100
B)
th
5
4
cycle
8
6
4
2
0
0
0
0
0
80
60
40
20
0
th
cycle
cycle
rd
3
nd
2
cycle
cycle
st
1
st
nd
rd
th
th
5
10
15
20
25
30
35
40
45
50
1
cycle
2
cycle
3
cycle
4
cycle
5 cycle
2
 (°)
Figure 7. (A) Catalytic performance of Sr(NH -bdc) in several Knoevenagel
2
condensation reaction cycles. Normal columns, benzaldehyde conversion; fill columns,

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ETCN product yield; and (B) X-ray diffraction patterns of fresh and recovered Sr(NH2-
bdc) catalyst after different cycles.
4
. CONCLUSIONS
A novel metal-organic framework, named Sr(NH -bdc) has been synthesized and
2
characterized by different physico-chemical techniques. This material, based on the
2
-
linkage of 2-aminobencenedicarboxilate (NH -bdc ) with strontium ions, exhibits a 3D
2
crystallographic structure with 1-D secondary building lineal chains of face-sharing
strontium polyhedrons in the [100] direction. This material provides strong basic catalytic
properties with the presence of strontium metal oxides and amino containing groups. This
material showed an excellent thermal and chemical stability. The catalytic performance
of Sr(NH -bdc) for Knoevenagel condensation reactions is significantly better than those
2
of alkaline-earth oxides, typically used in basic-catalyzed reactions. These results
evidence the important catalytic role of strontium metal sites and amino-containing ligand
in a crystalline MOF network. In addition, this material showed a better stability than
IRMOF-3 as well-known amino-containing MOF material. The catalytic activity of
Sr(NH -bdc) material increases for benzaldehyde derivatives containing electron
2
withdrawing groups and activated methylene compounds of higher acid strength for
favoring the nuclephilic addition. The polarity of the solvent and the reaction temperature
also play an important role in the catalytic performance. Remarkable results of
benzaldehyde conversion (96%) and ethyl trans-α-cyanocinnamate product yield (84%)
were achieved after 120 min of reaction using 80ºC for condensation of benzaldehyde
and ethyl cyanoacetate. Moreover, the Sr(NH -bdc) exhibited a notable structural stability
2
without loss of the crystalline phase and also maintaining its catalytic activity under the
optimal conditions during five consecutive catalytic runs.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgments
The authors wish to thank Spanish Ministry of Science and Innovation for the financial
support to the CICYT Project (CTQ2015-64526-P).

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Table of Contents
Sr(NH -bdc) is new strontium-based MOF material that exhibits high activity in the
2
Knoevenagel condensation which can be facilely separated and be reused without
significant degradation.