Modulating the basicity of Zn-MOF-74: Via cation exchange with calcium ions

Article abstract of DOI:10.1039/c9dt03332g

The fine-tuning of metal-based molecular building blocks in Zn-MOF-74 through post-synthetic cation exchange with Ca2+ significantly enhances its basicity and corresponding catalytic performance. The resulting material, exemplified by Ca/Zn-MOF-74, is sho

Full text of DOI:10.1039/c9dt03332g

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DOI: 10.1039/C9DT03332G
Modulating the basicity of Zn-MOF-74 via cation exchange
with calcium-ions
Graphical Abstract
Fine-tuning of the metal-based molecular building blocks in Zn-MOF-74 through post-
synthetic cation exchange with Ca2+ significantly enhance the basicity and
corresponding catalytic performance. The resulting material, exemplified by the Ca/Zn-
MOF-74, is shown through Knoevenagel condensation to exhibit improved product
yield over parent Zn-MOF-74 and Ni/Zn-MOF-74.

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Received 00th January
20xx,
Modulating the basicity of Zn-MOF-74 via cation exchange with
calcium-ions
Zhuxiu Zhang,a Yue Xiao,a Mifen Cui, a Jihai Tang,*ab Zhaoyang Fei,ab Qing Liu,a Xian Chena and Xu
Qiao*ab
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Fine-tuning of the metal-based molecular building blocks in Zn- donor molecule (e.g. malononitrile and ethyl cyanoacetate) on
MOF-74 through post-synthetic cation exchange with Ca2+ the Mg-O-Mg species in Mg-MOF-74 exhibit lower energy level
significantly enhance the basicity and corresponding catalytic than other M-MOF-74 iso-structures (M = Ni, Co, Zn). However,
performance. The resulting material, exemplified by the Ca/Zn- it is Ni-MOF-74 rather than Mg-MOF-74 that exhibits best
MOF-74, is shown through Knoevenagel condensation to exhibit catalytic activity. The critical step of Knoevenagel condensation
improved product yield over parent Zn-MOF-74 and Ni/Zn-MOF-74. is the deprotonation of donor molecules, which is highly
associated with the proton affinity of phenolate oxygen in M-O-
M species.14 Unfortunately, proton affinity of phenolate oxygen
in Mg-MOF-74 is lower than in other M-MOF-74 iso-
structures.13 Given the base strength of alkaline earth metal
oxides being in the sequence of BaO > SrO > CaO > MgO, we
infer that the incorporation of alkali earth metal with lower
electronegativity than magnesium in MOF-74 is likely to affords
an opportunity to enhance the electron density of phenolate
oxygen in order to study their impact on the basicity and related
catalytic activity in Knoevenagel condensation.
Currently, there is no report regarding to the one-pot
synthesis of alkaline earth MOF-74 other than Mg-MOF-74.
Attempts to prepare these alkaline earth MOF-74 directly by the
reaction of alkaline earth metal salts with 2,5-dihydroxytere-
phthalate were unsuccessful. Post-synthetic cation exchange is
a well-known approach to bring exogenous metals into MOFs’
MBBs without damaging the structure.15 Nevertheless, the
reported exchange processes often occur between two
transition metals owing to the similarity of electronic
arrangement.16 We herein demonstrate for the first time that
Zn-MOF-74 can be subjected to post-synthetic alkaline earth
metal exchange. The resulting Ca/Zn-MOF-74 exhibits
enhanced basicity and catalytic activity in the Knoevenagel
condensation.
Metal-organic frameworks (MOFs), also knowns as porous
coordination polymers (PCPs), have emerged as a new class of
crystalline materials with diverse compositions and permanent
porosities.1 The crystal engineering approach2, in particular the
pre-selection3 or post-synthetic modification4 of the metal-
based molecular building blocks (MBBs), enables the design of
MOF structures with target-specific catalytic properties for
various reactions.5 Since the seminal work of Kim6 and Fujita7,
most studies of MOF catalysts focused upon the Lewis/Brönsted
acidity8 or redox activity9 of inorganic nodes. MOFs that
function as solid base catalysts remains relatively
underexplored.10
MOFs with intrinsic basicity are exemplified by the way that
alkaline earth metal-based MBBs can be incorporated in MOF
structures, usually as infinite metal-oxygen/hydroxide chain11.
The basic site in alkaline earth MOFs is thereby derived from M-
O-M species, similar as the low-coordination sites in alkaline
earth metal oxides12. In this context, Mg-MOF-74 composed of
magnesium oxide strand-like MBBs linked by 2,5-
dihydroxyterephthalate was found to be an active catalyst in
Knoevenagel condensation which is a typical base-catalyzed
reaction.13 Theoretical study reveals that the adsorption of the
Zn-MOF-74 was immersed in a DMF solution of CaCl2. Color
of Zn-MOF-74 changed from light yellow to brown during the
exchange. The exchange process at different temperatures
(25-100 °C) is traced by inductively coupled plasma atomic
emission spectrometer (ICP-AES), from which the quantity of
Ca2+ in solid is consistent with Zn2+ in the exchange reaction
solution (Fig. 1a-b). Zn-MOF-74 is partially exchanged with Ca2+
a. State Key Laboratory of Materials-Oriented Chemical Engineering, College of
Chemical Engineering, Nanjing Tech University, No. 30 Puzhunan Road, Nanjing
211816, China.
b. Jiangsu National Synergetic Innovation Centre for Advanced Materials (SICAM),
No. 5 Xinmofan Road, Nanjing 210009, China.
Electronic Supplementary Information (ESI) available: Experimental section,
elemental mapping, EDX spectrum, TG analysis, CO2-TPD analysis, XRD patterns,
Rietveld refinement, N2 sorption isotherms. See DOI: 10.1039/x0xx00000x
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after 7 days at 75 °C (33%), however, only 20% of Zn2+ in MOF- Ca-O distance (average 2.096 Å). Time-of-flight
74 was exchanged when temperature is reduced to 50 °C. spectrometry (TOFMS) of Ca/Zn-MOF-74DsOhIo: 1w0s.10a3n9/eCw9DsTig0n33a3l2oGf
calcium cations in the positive spectra (Fig. 2c), and there was
I
n
c
r
e
a
s
i
n
g
t
h
e
no signal
Fig. 1 (a) Molar ratio of Ca:(Ca+Zn) in Ca/Zn-MOF-74; (b) Molar ratio of Zn
(substituted):Zn (total) according to Zn2+ concentration in the solution after
exchange; (c) NLDFT pore size distribution of Zn-MOF-74 and Ca/Zn-MOF-74; (d)
SEM images of morphology of Ca/Zn-MOF-74 at 25-100 oC. (insets pictures are
corresponding particle of Ca/Zn-MOF-74 exchanged at 25-100 oC)
Fig. 2 (a) PXRD patterns, (b) N2 sorption isotherms at 77 K, (c) TOFMS positive ions
spectra and (d) TOFMS negative ions spectra of Zn-MOF-74 and Ca/Zn-MOF-74.
Fig. 3 XPS spectra of Zn-MOF-74 and Ca/Zn-MOF-74: (a) survey spectrum; (b) Ca 2p
spectra; (c) Zn 2p spectra; (d) O 1s spectra.
exchange temperature to 100 °C results in a high exchange ratio
of 68%, but the microporous structure is likely to be damaged
as indicated by the presence of mesopores according to pore
size analysis (Fig. 1c). Thermogravimetric analysis in O2
atmosphere verifies that the molar ratios of CaO:ZnO in Ca/Zn-
MOF-74 (25-75 °C) are consistent with the ICP-AES result except
for Ca/Zn-MOF-74 (100 °C) (see the ESI for details). From SEM
images of Ca/Zn-MOF-74 (25-100 °C) (Fig. 1d), there is no
significant change on morphology other than the sample
exchanged at 100 °C. The particle surface of Ca/Zn-MOF-74 (100
°C) became irregular with coarse surface indicating the partial
collapse of MOF structure which is consistent with its pore size
distribution and mismatch between results of TGA and ICP-AES.
Given both exchange progress and the stability of the pore
structure, we decided to choose Ca/Zn-MOF-74 exchanged at
75 °C for subsequent studies and all the Ca/Zn-MOF-74
mentioned later specifically refers to the sample exchanged at
75 °C for 7 days.
of chloride anions in the negative ion spectra (Fig. 2d). It can be
inferred that calcium cations indeed substitute zinc cations in
Zn-MOF-74 rather than reside within the channels of Zn-MOF-
74 as chlorine salt. Uniform distribution of Ca2+ in Ca/Zn-MOF-
74 is confirmed by EDX spectrum, in which both Ca2+ and Zn2+
are highly dispersed in both exterior and internal surfaces of the
crystalline particles (Fig. S7). Thermogravimetric analysis in N2
atmosphere (Fig. S8) reveals that Ca/Zn-MOF-74 and Zn-MOF-
74 exhibit a similar thermal stability with 22.8% and 17.7%
weight loss, respectively, from 30 °C to 200 °C. These weight
losses correspond to the loss of DMF, methanol and H2O, and
followed by a steady plateau from 200 °C to 410 °C and 400 °C
respectively, before decomposition.
Powder X-ray diffraction (PXRD) pattern of Ca/Zn-MOF-74 is
comparable to the parent Zn-MOF-74, and we found a slight
shift of diffraction peak from 2θ value of 7.0° to 6.8° after the
exchange process (Fig. 2a). This phenomenon can be attributed
to the metathesis of smaller Zn2+ by larger Ca2+, giving rise to
the expansion of interplanar distance of (2-10) planes and
corresponding slight increase of the BET surface area (from 688
to 758 m2/g) (Fig. 2b). The structure expansion is confirmed by
Rietveld refinement of PXRD data. Ca/Zn-MOF-74 crystallized in
R-3 with a = b = 26.0556(82), c = 6.8505(23), which is slightly
higher than cell parameters of parent Zn-MOF-74 (a = b =
25.9322(15), c = 6.8365(4)), presumably because of the longer
2 | J. Name., 2012, 00, 1-3
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The chemical composition of Ca/Zn-MOF-74 were furtherly
studied by X-ray photoelectron spectroscopy (XPS). Three main
peaks are observed at 284.4, 532.0 and 1022.2 eV in the XPS
spectrum of Zn-MOF-74, which are ascribed to C 1s, O 1s and Zn
2p respectively (Fig. 3a). After Ca2+ exchange, two new peaks
appear at 350.9 and 347.4 eV corresponding to Ca 2p1/2 and Ca
2p3/2, which indicates the incorporation of Ca2+ (Fig. 3b). With
the appearance of Ca 2p signals, the peak intensities of Zn1/2 and
Zn3/2 decrease significantly (Fig. 3c). This phenomenon is
consistent with the ICP results, indicating that the Zn2+ in the
MOF structure are partially substituted by Ca2+. It is worth
noting that there is a negative shift of binding energy of O 1s
spectra from 532.0 to 531.7 eV after exchange (Fig. 3d). This
shift indicates an increase in electron density around oxygen
atoms due to the lower electron affinity of Ca2+.17 The increase
of the electron density in turn improve the proton affinity of
oxygen, which offers the chance for enhanced the basicity of
Ca/Zn-MOF-74.
MOF-74 has two kinds of basic sites arising from coordination
unsaturated phenolate oxygen atoms and carboxylate oxygen
atoms.13 According to Sanderson’s theory of electronegativity18,
Ca2+ substituted Zn-MOF-74 is supposed to exhibit enhanced
basicity due to the lower electronegativity of calcium. To verify
our hypothesis, CO2-TPD was used to evaluate the basic
strength of the material before and after cation exchange (Fig.
S9). Taking the thermal stability into account, TPD temperature
is only set to 400 °C. Two broad desorption peaks of Ca/Zn-
MOF-74 at approximately 125 °C and 380 °C, which are higher
than Zn-MOF-74 (110 °C and 367 °C), are attributed to weak and
medium basic sites generated from carboxylate oxygen atoms
and phenolate oxygen atoms. The difference on desorption
temperature indicates the enhanced basicity of Ca/Zn-MOF-74,
which is in good agreement with the results of XPS.
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DOI: 10.1039/C9DT03332G
Table 1 Catalytic properties of Ca/Zn-MOF-74 in the Knoevenagel condensation
Reaction condition: aldehyde (1 mmol), malononitrile (1 mmol), catalyst (50 mg),
toluene (2 mL), temp.: 70 oC, time: 12h. a Zn-MOF-74 as catalyst. b Ni/Zn-MOF-74
as catalyst. c Ca/Zn-MOF-74 as catalyst.
evenagel condensation reactions are shown in Table 1. When
Ca/Zn-MOF-74 was used as catalyst, the reaction of
benzaldehyde with malononitrile gave a high yield of 88%,
which was twice of the parent Zn-MOF-74. The effect of
substituent groups on benzaldehyde were investigated as
shown in entry 2-6. It is clear that the reaction of benzaldehyde
with electron-withdrawing groups (e.g. -NO2, -CN, -Cl) gives a
higher yield of the product than benzaldehyde with electron-
donating groups such as -CH3 and -C2H5. In order to further
explore the effect of Ca2+ on the catalytic activity of Ca/Zn-MOF-
74, we also synthesized Ni2+-exchanged Ni/Zn-MOF-74, of which
the exchange ratio (48%) is even larger than Ca/Zn-MOF-74
(33%). In the MOF-74 family, Ni-MOF-74 exhibits the best base
catalytic activity. However, Ca/Zn-MOF-74 exhibits much higher
yield than Ni/Zn-MOF-74 for all benzaldehyde derivatives. The
higher catalytic activity for Ca/Zn-MOF-74 compared with
Ni/Zn-MOF-74 could be presumably attributed to the stronger
proton affinity of structural phenolate oxygen in Ca/Zn-MOF-74,
which facilitate the activation of donor molecules, followed by
the attack of benzaldehyde.
In summary, we have successfully synthesized a rare Ca/Zn-
MOF-74 via post-synthetic Ca2+ exchange of Zn-MOF-74. The
exchange process is monitored by ICP-AES and the resulting
Ca/Zn-MOF-74 exhibits permanent porosity that is comparable
to the parent Zn-MOF-74. The enhanced basicity of Ca/Zn-MOF-
74 is confirmed by XPS and CO2-TPD. The catalytic property of
Ca/Zn-MOF-74 are highly desirable in that they maximise the
product yield compared to parent Zn-MOF-74 and Ni/Zn-MOF-
74 in the context of Knoevenagel condensations. The promising
features of Ca/Zn-MOF-74 indicate that alkaline metal-
incorporated MOF-74 can serve as platforms for the generation
of MOF with intrinsic basicity that would otherwise be
impossible to realize.
The catalytic activities of Ca/Zn-MOF-74 for the Knoevenagel
condensation, a classic reaction catalyzed by base, was
evaluated. Different benzaldehyde derivatives were used with
O
CN
H
+
R
+
R
H2O
NC
CN
CN
We would acknowledge National Key R&D Program of China
(2017YFB0307304), National Natural Science Foundation of
China (21676141, 21808104), Natural Science Foundation of
Jiangsu Province (BK20170989) and Natural Science Foundation
of Jiangsu Higher Education Institutions of China
(17KJB530005). The authors are grateful for the helpful
discussions and advices given by Prof. Dr. D. Yang.
Yielda
(%)
Yieldb
(%)
Yieldc
(%)
Entry
1
Aldehyde
product
O
CN
H
42
31
24
47
46
60
55
49
30
61
53
65
88
82
74
93
90
96
CN
O
CN
H
H
2
3
4
5
6
CN
H3
C
H3
C
O
O
CN
CN
C2H5
C2H5
Conflicts of interest
There are no conflicts to declare.
CN
H
H
CN
O2
N
O2
N
O
CN
Notes and references
CN
CN
NC
NC
O
H
1
(a) S. Batten, S. Neville and D. Turner, Coordination Polymers:
Design, Analysis and Application, Royal Society of Chemistry,
Cambridge, UK, 2009; (b) L. R. MacGillivray, Metal-Organic
Frameworks: Design and Application, John Wiley & Sons,
Hoboken, 2010.
CN
Cl
Cl
malononitrile under the same condition. The catalytic results of
Kno-
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