Nickel?alkyne?functionalized metal?organic frameworks: An efficient and reusable catalyst

Article abstract of DOI:10.1016/j.apcata.2021.118216

Electron-donating groups in the robust MOF motif are able to provide an excellent catalytic platform, therefore obtaining site-isolated metal sites. In this study, the terminal alkyne is firstly introduced into UiO-66-type metal-organic frameworks (UiO-66-alkyne). Herein, to further study the application potential of this material, a covalently bonded nickel catalyst based on the alkynyl-tagged UiO-66-alkyne has been prepared and afforded us unprecedented highly dispersed and highly efficient catalytic active species. Meanwhile, this nickel-contained catalyst method for joining metals provided an alternative pathway regarding catalyst designing. With UiO-66-alkyne-Ni, the heterogeneous transformation of homogeneous catalysts is realized. Using benzaldehyde and malononitrile as starting materials, we were able to catalyze the Knoevenagel condensation within 45 min under room temperature with yield (> 99 %). Moreover, the recovery rate of the UiO-66-alkyne-Ni also outperformed previous MOFs in both small-scale and gram-level reactions, which shows UiO-66-alkyne-Ni is a potential contributor to the subsequent industrialization.

Full text of DOI:10.1016/j.apcata.2021.118216

Applied Catalysis A, General 623 (2021) 118216
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Nickelꢀ alkyneꢀ functionalized metalꢀ organic frameworks: An efficient
and reusable catalyst
,
,
,
Hua Chenga 1, Liangmin Ningb 1, Shengyun Liaob c,**, Wei Lia, Siyuan Tanga, Jilin Lie,
Huixin Chenc, Xin Liub, Liming Shaoa
*
,d,
a School of Pharmacy, Fudan University, 826 Zhangheng Road, Zhangjiang Hi-tech Park, Pudong, Shanghai, 201203, China
b College of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (MOE), Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry,
Collaborative Innovation Centre of Chemical Science and Engineering, Nankai University, Tianjin, 300071, China
c Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin,
300384, China
d State Key Laboratory of Medical Neurobiology, Fudan University, 138 Yixueyuan Road, Shanghai, 200032, China
e DezhouDeyao Pharmaceutical Co, Ltd, No.6000 East Dongfanghong Road, Dezhou Economic and Technological Development Zone, Shandong, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Electron-donating groups in the robust MOF motif are able to provide an excellent catalytic platform, therefore
obtaining site-isolated metal sites. In this study, the terminal alkyne is firstly introduced into UiO-66-type metal-
organic frameworks (UiO-66-alkyne). Herein, to further study the application potential of this material, a
covalently bonded nickel catalyst based on the alkynyl-tagged UiO-66-alkyne has been prepared and afforded us
unprecedented highly dispersed and highly efficient catalytic active species. Meanwhile, this nickel-contained
catalyst method for joining metals provided an alternative pathway regarding catalyst designing. With UiO-
66-alkyne-Ni, the heterogeneous transformation of homogeneous catalysts is realized. Using benzaldehyde and
malononitrile as starting materials, we were able to catalyze the Knoevenagel condensation within 45 min under
room temperature with yield (> 99 %). Moreover, the recovery rate of the UiO-66-alkyne-Ni also outperformed
previous MOFs in both small-scale and gram-level reactions, which shows UiO-66-alkyne-Ni is a potential
contributor to the subsequent industrialization.
MOFs
Alkyne modification
Heterogeneous catalyst
Knoevenagel condensation
Industrialization
1. Introduction
environmental pollution due to the product separation issues [10,11].
The heterogeneous transformation of homogeneous catalysts may solve
With an increasing emphasis on environmental protection and green
chemistry [1], it is urgent to eliminate the pollution of industrial cata-
lysts effectively. The Knoevenagel condensation is a fundamental reac-
tion to form carbon-carbon bonds [2–4], which can be used for the
preparation of substituted olefins and coumarin derivatives. In the in-
dustrial field [5,6], Knoevenagel condensation is widely used for the
synthesis of cosmetics, perfumes, polymers, and pharmaceuticals [7,8].
However, basic organic compounds such as amine, pyridine, urea and
piperidine, are used as catalysts for this reaction in the traditional in-
dustrial production process [9]. All of the aforementioned homogeneous
catalysts are not recoverable and are likely to cause serious
these problems, because heterogeneous catalysts not only improve the
stability of catalysts that accompanied without any product separation
issue, but also be able to reduce environmental pollutions [12]. As such,
developing the supported catalysts for Knoevenagel condensation has
become a research hotspot [13,14], and many researches were focused
on finding novel catalytic carriers such as zeolite [15], carbon nanotubes
[16], silica gel [17], graphene [18] and metal ꢀ organic frameworks
(MOFs) [19], etc..
MOFs, as novel porous organic-inorganic hybrid materials, have
great application potential in catalytic-related programs [20–23].
Dhakshinamoorthy and co-workers reported two Al-MOFs with CAU-1
* Corresponding author at: School of Pharmacy, Fudan University, 826 Zhangheng Road, Zhangjiang Hi-tech Park, Pudong, Shanghai, 201203, China.
** Corresponding author at: Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry and Chemical Engineering, Tianjin
University of Technology, Tianjin, 300384, China.
E-mail addresses: 18202587221@163.com (S. Liao), limingshao@fudan.edu.cn (L. Shao).
1
These authors contributed equally to this work.
https://doi.org/10.1016/j.apcata.2021.118216
Received 8 March 2021; Received in revised form 27 April 2021; Accepted 13 May 2021
Available online 21 May 2021
0926-860X/© 2021 Elsevier B.V. All rights reserved.

H. Cheng et al.
Applied Catalysis A, General 623 (2021) 118216
Metallic platinum (Pt) [38] and metallic palladium (Pd) [39], as well
as non-precious metals such as copper (Cu) [42] and metallic nickel (Ni)
[43] can bond with terminal alkynes. Among them, nickel is widely used
in industry due to its low cost and high catalytic activity, however, it is
the main source of heavy metal pollutions [44–46]. Therefore, if nickel
can be introduced into the MOF material through bonding reactions, the
usage amount of nickel-metal can be reduced, and therefore reducing
the environmental nickel-metal pollutions.
In this work, the synthesis of alkynyl-tagged UiO-66 MOFs is re-
ported for the first time. To be specific, a stable alkynyl-tagged Zr(IV)-
based MOF (UiO-66-alkyne) in water and acidic media was synthesized
by utilizing BDC-alkynyl (2-Alkynyl-1,4-benzene dicarboxylic acid) as
the ligand under relatively mild hydrothermal or solvothermal condi-
tions. Based on the new functionalized MOF motif, the catalytic active
site of nickel is firstly incorporated into UiO-alkyne by bonding with
terminal alkynyl groups (UiO-66-alkyne-Ni). As a heterogeneous cata-
lyst, UiO-66-alkyne-Ni shows excellent catalytic activity, stability, and
industrialization prospects for catalyzing Knoevenagel condensation.
Fig. 1. Alkynyl as a liner to connect MOF and metal.
and CAU-10-type structures and successfully catalyzed the Knoevenagel
condensation reaction under mild conditions (40 ◦C, in ethanol) [24]. In
2019, Gong and co-workers reported a MOF catalyst that based on
UiO-66 (UiO-67-BPY@UiO-66), which could achieve an efficient con-
version of Knoevenagel condensation at room temperature, indicating
the catalytic potential of this types of structure in the Knoevenagel
condensation reaction [25].
2. Experimental section
2.1. Materials and instruments
Functionalization is the distinctive property of MOF materials. Post-
synthesis modification (PSM) represents an attractive functionalization
strategy for modifying pores of MOFs through introducing catalytic
active sites that endows MOFs a huge contribution in the field of cata-
lysts [26–31]. For UiO-66 [32], several organic tagged-groups have been
introduced into UiO-66 including aldehydes [33], amino [34], or nitro
groups [35]. Among them, the introduction of amino groups was
considered able to improve the catalytic performance of UiO-66 signif-
icantly [36,37], whereas other functional groups were not helpful.
Therefore, it is necessary to introduce new functional groups into
UiO-66 to meet the needs of the catalyst field.
Chemicals: 2-Bromoterephthalic acid, trimethylsilyl acetylene,
benzaldehyde, malononitrile, Pd(PPh3)2Cl2, CuI, PPh3, other aldehyde
derivatives were purchased from Sinopharm Chemical Reagent Co., Ltd.
(Shanghai, China), Acros or other suppliers and were used without any
further purifications. All solvents and chemicals used in this work are
analytical grade.
Characterizations: Powder X-ray diffraction (PXRD) were measured
on a D2 PHASER. diffractometer (CuKα, λ =1.54056 Å). The
morphology of MOF materials in this work was obtained by transmission
electron microscope 120 kV (TEM: HT7700 Exalens) and scanning
electron microscope (SEM: MERLIN Compact 6164, ZEISS). X-ray
photoelectron spectra (XPS) related tests such as element content and
valence analysis were all tested by X-ray photoelectron spectroscopy
instrument (PHI 5000 C&PHI 5300) with a dual X-ray anode (Mg and
Al). Fourier transform infrared (FTIR) spectra were obtained on a Bruker
Alpha FTIR spectrometer using potassium bromide as a reference. The
N2 adsorption-desorption isotherm of the catalysts was measured by
fully automatic specific surface and micropore analyser (AUTOSORB-
IQ). NMR data were recorded with a Mercury plus 400 MHz (Varian,
Inc., Palo Alto, CA, USA) or Bruker ASCEND 600 MHz NMR system.
As a common functional group, the alkyne has good chemical ac-
tivity. On one hand, terminal alkyne, as a weak acid, can react with
alkali metals or heavy metal ions to form metal alkyne. On the other
hand, terminal alkyne can act as two-electron donors to coordinate with
transition metals [38,39]. As such, we considered the terminal alkynyl
groups as a “linker” to connect metallic active sites and MOF material
therefore making the metallic active sites distributed on the surface of
the MOF material evenly (Fig. 1). The introduction of the terminal
alkynyl groups onto the network of MOFs will pave us a new promising
way to obtain site-isolated and coordinatively unsaturated metal sites
for catalytic application [40,41]. In 2015, Li et al. reported an
alkyne-tagged MOF (UiO-68-alkyne) and developed a new MOF material
with an azido functional group [27]. To the best of our knowledge, very
few alkynyl functionalized MOFs have been reported, potentially due to
the difficulties of the synthesis thereof.
2.2. Synthesis of a MOF-ligand
As shown in Scheme 1, MOF-ligand (see the Fig.S5 for the 1H and 13
C
NMR spectra of the MOF-ligand) and UiO-66-C≡CH could be synthesized
by following a similar published procedure [28].
Scheme 1. Synthesis routes of the MOF-ligand, UiO-66-alkyne and UiO-66-alkyne-Ni.
2

H. Cheng et al.
Applied Catalysis A, General 623 (2021) 118216
Compound 1 To a solution of 2-bromoterephthalic acid (7.5 g,
40 mmol) in 250 mL methanol, conc. H2SO4 (15 mL) was added slowly.
Then being stirred at 65 ◦C overnight. After methanol was removed by
evaporation, 300 mL ethyl acetate (EA) was added. The mixture was
washed with K2CO3 solution (1 M) and dried with Na2SO4. Finally, the
crude product 1 was purified by flask silica gel column chromatography
(white solid 7.7 g, yield 92.2 %). 1H NMR (400 MHz, CDCl3) δ 8.30 (d, J
=1.8 Hz, 1 H), 8.12 - 7.93 (m, 1 H), 7.81 (t, J =6.7 Hz, 1 H), 4.00ꢀ 3.93
(m, 6 H). 13C NMR (600 MHz, CDCl3) δ 167.96, 164.36, 137.47, 134.56,
132.18, 129.76, 126.96, 122.04, 47.07.
Compound 2 2-bromoterephthalate (7.7 g, 28.2 mmol), trime-
thylsilyl acetylene (3.1 g, 31.0 mmol), Pd(PPh3)2Cl2 (790 mg,
1.2 mmol), CuI (110 mg, 0.56 mmol) and PPh3 (180 mg, 1.2 mmol) were
dissolved in Et3N, then stirred at 90 ◦C under N2 atmosphere. After
reacting for 9 h, the mixture was filtered with EA and the solvent was
removed. The crude product was purified by flask silica gel column
chromatography to give desired compound 2 (white solid 5.3 g, yield
64.6 %). 1H NMR (400 MHz, CDCl3) δ 8.26 – 8.20 (m, 1 H), 7.96 (ddd,
J = 12.5, 8.4, 4.1 Hz, 2 H), 3.97 – 3.91 (m, 6 H), 0.28 (q, J = 4.8, 3.8 Hz,
9 H). 13C NMR (600 MHz, CDCl3) δ 166.90, 164.40, 136.41, 134.30,
132.16, 130.13, 128.21, 123.78, 101.55, 100.35, 50.62, 1.22.
Fig. 2. XRD patterns of the UiO-66, MOF-ligand, UiO-66-alkyne at different
temperature, UiO-66-alkyne-Ni and UiO-66-alkyne-Ni-recycle.
Compound 3 To the solution of dimethyl 2-((trimethylsilyl)ethynyl)
terephthalate (5.3 g, 17.2 mmol) in 50 mL THF, tetra-n-butylammonium
fluoride THF solution (15 mL, 1 M) was added. Then the mixture was
quenched with H2O and extracted with EA. The mixture was dried with
NaOH and EA was removed by reduced pressure. The crude product was
purified by column chromatography to give the pale red product 3 (solid
3.5 g, yield 90 %). 1H NMR (400 MHz, CDCl3) δ 8.27 (d, J =1.9 Hz, 1 H),
8.06 – 7.94 (m, 2 H), 3.95 (d, J =2.8 Hz, 6 H), 3.46 (d, J =1.6 Hz, 1 H).
13C NMR (600 MHz, CDCl3) δ 166.53, 164.81, 141.23, 134.41, 132.39,
129.71, 128.23, 122.28, 84.00, 79.60, 50.69.
catalyst, and methanol (0.3 mL) into the flask. The reaction was carried
out at room temperature. After the reaction was completed, 100 μL
dibromomethane was added, and the yield was determined by 1H NMR.
2.5. Recycling test
In order to determine the stability of the catalyst UiO-66-alkyne-Ni
after reuse, the catalyst was separated by centrifugation, washed with
DCM and TEA, and then the catalyst was dried for the next use, and the
reaction was catalyzed under the same conditions. Then the yield was
determined by 1H NMR.
Compound 4 To the solution of compound 3 (3.5 g, 16.1 mmol) in
THF (50 mL), aqueous KOH solution (4 %, 50 mL) was added. Then
stirred at room temperature overnight, the THF was removed and the
reaction mixture was acidified with 1.0 M HCl aqueous solution. The
crude product was collected by filtration and washed with water
(2 × 50 mL). Then the compound 4 was obtained by reduced pressure
(solid 2.7 g, yield 90 %). 1H NMR (400 MHz, DMSO) δ 13.50 (s, 2 H),
8.04 (s, 1 H), 8.00 (d, J =8.1 Hz, 1 H), 7.93 (d, J =7.8 Hz, 1 H), 4.50 (s,
1 H). 13C NMR (600 MHz, DMSO) δ 167.28, 166.48, 138.98, 135.30,
134.00, 130.72, 129.33, 122.25, 86.73, 81.23.
3. Results and discussion
3.1. Preparation and characterization of the UiO-66-alkyne and UiO-66-
alkyne-Ni
Previous work showed that the formation process of tagged UiO-66
can be modulated through varying reaction temperature [33]. There-
fore, the influence of temperature on the formation of MOF in the syn-
thesis of UiO-66-alkyne was explored. It can be seen from Table S1 that
the UiO-66-alkyne cannot be formed normally at 60 ◦C, and MOF
appeared at 80 ◦C. Compared with entries under 100 ◦C (83 %) or 120 ◦C
(72 %), the yield of UiO-66-alkyne at 80 ◦C (89 %) was the highest under
the premise of the same feeding amount. The PXRD patterns suggested
that the as-synthesized UiO-66-alkyne and UiO-66 were isomorphic. The
diffraction peaks of UiO-66-alkyne synthesized at different temperatures
(Fig. 2) were overlapped, showing the synthesis temperature that ranged
from 80 ◦C to 120 ◦C could afford us UiO-66-alkyne with pure phase. The
SEM images of the UiO-66-alkyne synthesized at 80 ◦C and 120 ◦C
exhibited that UiO-66-alkyne under lower temperature had more uni-
form particle size and morphology (Fig. 3 and Fig S1 in Supporting In-
formation). Therefore, 80 ◦C was chosen as the best synthesis
temperature according to the analysis results.
2.3. Preparation of UiO-66-alkyne and UiO-66-alkyne-Ni
UiO-66-alkyne To a solution of compound 4 (300 mg, 1.57 mmol),
ZrCl4 (262.8 mg, 1.12 mmol) in DMF (30 mL) and HCl (2 mL) was added,
the mixture was placed in a 50 mL vial. After all solids were dissolved by
ultrasound, the mixture was heated to 80 ◦C and kept at this temperature
for 12 h. The crude product was isolated by centrifugation and washed
three times with DMF and MeOH, respectively. The residual solvents in
the centrifuged microcrystalline powders were removed under vacuum.
The as-obtained powder was dried to give the white solid (solid
424.5 mg, yield 89 %).
UiO-66-alkyne-Ni UiO-66-alkyne-Ni was synthesized with the
similar published procedure used to synthesize small molecule [43].
UiO-66-alkynyl (100 mg) was added into the solution of Ni(PPh3)2Cl2
(300 mg), and CuI (13 mg) in a 10 mL mixed solvent (CH2Cl2:
TEA = 1:1), then stirred for about 12 h. The catalyst powders were
collected through being filtered and washed DCM (3 × 50 mL) and TEA
(3 × 50 mL). Then the solid was dried under reduced pressure to give the
product (solid 245 mg, yield 80 %).
Metallic nickel is non-precious therefore is widely used in industrial
catalysis. In view of the mild reaction of terminal acetylene with nickel
chloride [43], a facile chemical method has been developed to incor-
porated the nickel centers into the network of UiO-66 through bonding
reactions between the terminal alkynyl groups and NiCl2(PPh3)2 in a
weakly basic solvent under the catalysis of CuI (Scheme 1). Here, it is
expected to synthesize highly stable non-noble metal heterogeneous
catalysts with active site separation using the stable chemical bond.
2.4. Catalytic reaction
Knoevenagel condensations were all carried out in 5 mL small round
bottom flasks. Put benzaldehyde (1 mmol), malononitrile (1.1 mmol),
3

H. Cheng et al.
Applied Catalysis A, General 623 (2021) 118216
Fig. 3. SEM images: UiO-66-alkyne synthesized at 80 ◦C (a-b); UiO-66-alkyne-Ni (c-d); UiO-66-alkyne-Ni-recycle (e-f).
3.2. Characterization UiO-66-alkyne and UiO-66-alkyne-Ni
simulated patterns of UiO-66 from the crystallographic data, therefore
suggesting that the as-synthesized UiO-66-alkyne and UiO-66 are
isomorphic, which means that, the introduction of nickel doesn’t destroy
the crystal structure of MOF. Meanwhile, the diffraction peaks of UiO-
66-alkyne synthesized at different temperatures (Fig. 2) were over-
lapped and the results, showed the synthesis temperature ranging from
80 ◦C to120 ◦C could afford us UiO-66-alkyne with pure phase. In
addition, FT-IR spectra analysis was very important that could be used to
confirm the chemical bonding effect between the MOF materials and
metallic nickel. In the FT-IR spectra of UiO-66-alkyne and the ligand, the
The morphology and crystal size of MOF materials can be determined
via field-emission scanning electron microscopy (FE-SEM) (Fig. 3). The
HRSEM images of UiO-66-alkyne illustrated an octahedral shape
morphology with particle size ~140 nm and its particle size uniform
(Fig. 3(a) and (b)). Compared with UiO-66-alkyne, UiO-66-alkyne-Ni is
slightly changed regarding morphology, however, overall morphology
and particle size were still well-maintained after the introduce of nickel
into the frameworks (Fig. 3(c) and (d)). A same conclusion could be
drawn from the TEM images too (Fig. 4). Similar to UiO-66-alkyne
(Fig. 4(a) and (b)), the TEM images of UiO-66-alkyne-Ni (Fig. 4(c) and
(d)) show that the novel catalyst has a uniform structure, indicating that
the introduction of nickel did not cause UiO-66-alkyne to collapse. The
uniformly distribution of Ni elements in the material can also be seen
from Fig. 5.
absorption peak at 3301 cmꢀ 1 could be assigned to the C H stretching
–
band of the alkynyl group (Fig. 6). However, no absorption peak of the
–
C
H alkynyl group was observed from the spectrum of UiO-66-alkyne-
Ni, indicating that the terminal alkyne was reacted with Ni(PPh3)2Cl2.
The N2 adsorption and desorption isotherms of that UiO-66-alkyne,
UiO-66-alkyne-Ni and UiO-66 at 77 K were both isotherms of type I,
indicating that the MOF materials were microporous (Fig. 7). The BET
surface area of UiO-66 was 1097.230 m2 gꢀ 1, larger than UiO-66-alkyne
which is attributed to increased overall weight and reduced free space
available of UiO-66-alkyne [35]. However, the BET surface area of
PXRD was used to evaluate the crystal structure of the materials.
Fig. 2 presented the XRD patterns of MOF-ligand, UiO-66-alkyne, UiO-
66-alkyne-Ni, and recycled UiO-66-alkyne-Ni. The XRD patterns of
UiO-66-alkyne and UiO-66-alkyne-Ni matched perfectly with the
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H. Cheng et al.
Applied Catalysis A, General 623 (2021) 118216
Fig. 4. TEM images: UiO-66-alkyne (a-b); UiO-66-alkyne-Ni (c-d); UiO-66-alkyne-Ni-recycle (e-f).
Fig. 5. Elemental mapping: UiO-66-alkyne-Ni (a-c); UiO-66-alkyne-Ni-recycle (d-f).
UiO-66-alkyne (852.618 m2 gꢀ 1) was larger than UiO-66-alkyne-Ni
(438.849 m2 gꢀ 1), potentially due to the gap shrinkage when metal
was introduced. In order to validate the hypothesis, non-local density
functional theory analysis of the N2 sorption isotherm was used to study
the pore size distributions of the two MOF materials. As is shown in the
Fig. 7, two mode diameters for UiO-66-alkyne (9 and 11 Å) and UiO-66
also have two mode diameters (4 and 5 Å). In addition, only one sig-
nificant peak was observed in UiO-66-alkyne-Ni, indicating that the
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H. Cheng et al.
Applied Catalysis A, General 623 (2021) 118216
Fig. 6. The FT-IR spectra of the MOF-ligand, UiO-66-alkyne at different temperature, UiO-66-alkyne-Ni and UiO-66-alkyne-Ni-recycle.
Fig. 7. (a-b) N2 sorption isotherm of UiO-66-alkyne and UiO-66-alkyne-Ni obtained at 77 K. (c-d) Pore size distribution of UiO-66-alkyne and UiO-66-alkyne-Ni.
of UiO-66-alkyne-Ni depended on the reaction time and catalyst amount
in Fig. 8 (a) and (b), and the yield of the reaction was measured by 1H-
NMR. UiO-66-alkyne-Ni showed superior catalytic activity. The con-
version could reach 93.5 % using 10 mg UiO-66-alkyne-Ni after 15 min
and when the reaction time was prolonged to 45 min, the yield was up to
more than 99 %. In order to confirm whether the metal Ni leaches, a
leaching test was done and shown in Fig. S2. We removed the catalyst at
5 min, the reaction the reaction almost stopped. As a comparison, a
blank test without any catalysts was performed to investigate the reac-
tivity of the substrates themselves. It was found that the conversion
distributions were 6.5 %, 11.2 %, and 16 % after 15, 30 and 45 min
respectively, indicating that there was a lower conversion rate between
reaction substrates without catalysts. In addition, in order to clarify the
real active species, UiO-66-alkyne, NiCl2, PPh3, and Ni(PPh3)2Cl2 were
used as catalysts. The results showed that UiO-66-alkyne not only not
promote the reaction but also shows a certain inhibitory effect on the
reactions (Fig. 8 (a)). To explore the reason, the experiment that UiO-66
catalyzed the Knoevenagel condensation reaction was added and got a
Scheme 2. Knoevenagel condensation of the benzaldehyde and malononitrile.
introduction of metal had further caused the reduction of the pore size.
3.3. Catalytic performance
The Knoevenagel condensation reaction was selected as a template
reaction to evaluate the catalytic activity of UiO-66-alkyne-Ni, and
benzaldehyde and malononitrile were used as reaction substrates
(Scheme 2). As a heterogeneous catalyst, UiO-66-alkyne-Ni showed
excellent catalytic activity that could catalyze the Knoevenagel
condensation reaction at room temperature. The catalytic performance
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H. Cheng et al.
Applied Catalysis A, General 623 (2021) 118216
Fig. 8. (a) The conversions of benzaldehyde after 15, 30, 45 min ((UiO-66-alkyne-Ni (10 mg), UiO-66-alkyne (10 mg), Ni(PPh3)Cl2 (7 mg), PPh3 (7 mg), NiCl2
(7 mg)). (b) The effect of the amounts of UiO-66-alkyne-Ni (0, 4, 7, 10 mg) on the conversions of benzaldehyde (reaction time: 45 min).
Table 1
Table 2
Reported MOFs for the Knoevenagel condensation of benzaldehyde and malo-
nonitrile (RT).
The catalytic activity of UiO-66-alkyne-Ni using different aldehydes.a
Entry
Substrate
Product
Yieldb
Time (min)
Entry
Catalyst
Solvent
Substrate
(mmol)
t /
Yield
Ref.
1
>99 %
45
(Amount)
min
2
3
>99 %
>99 %
30
45
1
2
3
ZIF-9 (15 mol%)
ZIF-8 (5 mol%)
DETA-MIL-101
(50 mg)
Water
PhCH3
PhCH3
0.75
1.9
240
180
60
99 %
100%
97%
[47]
[8]
1.9
[48]
4
5
6
7
Zn@ZIF-67
(2.5 mol%)
UiO-66-NH-
RNH2 (50 mg)
UiO-66-NH-
RNH2 (50 mg)
UiO-67-
DMSO
PhCH3
EtOH
3.8
1.9
1.9
1
90
50%
97%
53%
98 %
[3]
[2]
[2]
[25]
4
5
>99 %
>99 %
30
30
120
120
60
6
7
>99 %
>99 %
45
45
DMSO
BPY@UiO-66
(18 mg)
8
9
UiO-66-alkyne
(10 mg)
EtOH
EtOH
1
1
45
45
12.9%
This
8
9
>99 %
>99 %
45
45
work
This
UiO-66-alkyne-
Ni (10 mg)
>99
%
work
10
> 99 %
30
yield between UiO-66-alkyne and blank test. It indicated that the
inhibitory effect of Knoevenagel condensation may be attributed to the
acidic property of both terminal alkyne and UiO-66 itself. Furthermore,
NiCl2, PPh3 and Ni(PPh3)2Cl2 also showed limited catalytic activity in
the Knoevenagel condensation reaction. Among them, Ni(PPh3)2Cl2
could afford 82 % yield after 45 min, so the synergistic effect between
PPh3 and NiCl2 may be responsible for the excellent performance of UiO-
66-alkyne-Ni.
11
12
13
>99 %
>99 %
>99 %
45
45
60
a
Reaction conditions: aldehyde (1 mmol), malononitrile (1.1 mmol), UiO-66-
alkyne-Ni (10 mg) EtOH (0.3 mL), room temperature (RT).
The amount of catalyst was another key factor that affected the re-
action. As shown in Fig. 8(b), with increased amounts of catalyst, the
yield per unit time constantly turned better. But the amount of catalysts
showed less significant impact on the yield.
b
The conversions of aldehydes were determined by 1H NMR analysis using
the dibromoethane as the internal standard (see the Fig.S6 for the 1H and 13
NMR spectra).
C
For Knoevenagel condensation, most MOF catalysts require high
temperature or harsh condition [7,36,49–52]. The catalytic perfor-
mance of UiO-66-alkyne-Ni for the Knoevenagel condensation reaction
using benzaldehyde and malononitrile as substrates was compared with
the other MOF-based catalysts (Table 1). Though most of Knoevenagel
condensation reactions can be carried out at room temperature, more
equivalents of catalyst or longer reaction time (more than or equal to
1 h) were required, whereas, benzaldehyde was completely converted
within a very short time with a very small amounts of UiO-66-alkyne-Ni
(10 mg) under mild reaction conditions. Such results indicated that
UiO-66-alkyne-Ni outperforms other reported MOF-based catalysts in
terms of catalytic performance.
The adaptability of the substrate is an important performance index
of catalyst. The scope of the substrates was further expanded with UiO-
66-alkyne-Ni as the catalyst (Table 2). The structure information of the
corresponding product was characterized by H and 13C NMR spectra
1
(Supporting Information). It was found the catalytic performance of
UiO-66-alkyne-Ni towards Knoevenagel condensation of the benzalde-
hydes with different substituents such as steric, electron-withdrawing or
electron-donating were all good (Table 2, Entries 1–9). In addition, the
7

H. Cheng et al.
Applied Catalysis A, General 623 (2021) 118216
or 6 and 7, or 8 and 9).
In order to study the reusability, a loop experiment has been per-
formed and the results were shown in Fig. 9. The yield was slightly
dropped after 5 cycles. Subsequently, the properties of the recycled
catalyst were characterized by PXRD, XPS, SEM, TEM, and IR. The PXRD
patterns showed that the crystal structure of UiO-66-alkyne-Ni after 5
cycles was still intact, whose diffraction peaks was completely consistent
with the fresh UiO-66-alkyne-Ni (Fig. 2). In addition, TEM and SEM
images showed that the morphology and size of the catalyst kept almost
unchanged before and after recycling (Figs. 3 and 4).
The element distribution pictures of the catalyst before and after the
reaction were shown in Fig. 5. It was found that the content of Ni ele-
ments maintained a very high level and dispersed uniformly on the
catalyst after the catalytic reaction, which was mainly due to the co-
existence of acetylene and metal. The bonding effect endowed the
good stability of the catalyst. The extra evidence for this conclusion was
the analysis results of XPS data (Fig. S3 in Supporting Information). The
elemental analysis of XPS showed that the Ni metal content before and
after cycling was 11.2 % and 9.17 %, respectively. This result also
verified the Ni metal content on the surface of the MOF remained almost
unchanged after cycle. The hypothesis that the alkynyl group acts as a
“linker” to connect Ni and MOF through bonding effect was further
supported by the aforementioned results.
Fig. 9. The recycling performance of UiO-66-alkyne-Ni.
benzaldehyde substrate with the electron-withdrawing group (Entries 2,
3, and 5) showed better reactivity (30 min). Other aromatic aldehydes
could also be converted completely under mild conditions (Table 2,
Entries 10–12). When the substrate was replaced by cyclohexane car-
boxaldehyde, the reaction time of the product was slightly increased
(60 min). In addition, the substitution positions of the substituents on
the benzene ring showed no effect on the yield (Table 2, Entries 3 and 4,
The analysis of XPS spectra of fresh and used UiO-66-alkyne-Ni was
shown in Fig. 10. The best-fitted peaks of fresh UiO-66-alkyne-Ni
(857.4 eV, 874.9 eV) and UiO-66-alkyne-Ni-recycle (857.2 eV,
Fig. 10. XPS spectra of UiO-66-alkyne: (a) fresh UiO-66-alkyne; (b) used UiO-66-alkyne.
Fig. 11. Proposed mechanisms for the Knoevenagel condensation of benzaldehyde with malononitrile over UiO-66-alkyne-Ni.
8

H. Cheng et al.
Applied Catalysis A, General 623 (2021) 118216
reaction, in which a yield of 99 % in 45 min at room temperature was
obtained, and the high yield value remains even after 5 cycles. In
addition, UiO-66-alkyne-Ni also shows the promising industrialization
characteristics. To be more specific, it maintains high catalytic activity
after the reaction being amplified 10 times and 500 times and showed
strong reusability stability when being magnified by 10 times. The
developed efficiently and environmentally friendly nickel-alkyne-
functionalized catalyst provides valuable insights into the usage of the
functional MOF materials with alkyne as a linker.
CRediT authorship contribution statement
L.M. Shao, S.Y. Liao, and X. Liu conceived the conceptualization. L.
M. Ning and H. Cheng did the data curation and formal analysis. J.L. Li,
S.Y. Tang and H.X. Chen did the data curation and formal analysis. S.Y.
Liao and X. Liu carried out the investigation of XPS measurements. H.X.
Chen and W. Li performed the investigation of HRTEM and HRSEM
analyses. The manuscript was primarily written by H. Cheng and L.M.
Ning, and revised by L.M. Shao, S.Y. Liao, and X. Liu. S.Y. Tang helped
improve the language and corrected minor language errors in the article.
All authors discussed the results of the manuscript.
Fig. 12. The recycling performance of UiO-66-alkyne-Ni (10 mmol).
874.6 eV) should be assigned as the binding energy of Ni(PPh3)2Cl2
(Ni2p3/2 and Ni2p1/2) [53], which demonstrated the existence form of
Ni is Ni(II). This conclusion further confirmed that the valence state of
nickel does not change in the process of bonding with terminal alkyne
and the catalytic reaction.
Declaration of Competing Interest
A possible reaction mechanism for Knoevenagel condensation with
UiO-66-alkyne-Ni as catalyst is proposed (Fig. 11). Firstly, benzaldehyde
is activated by the catalytic center Ni(II) at the alkynyl end of the MOF
material [54]. Triphenylphosphine acts as a Lewis base to promote the
deprotonation of malononitrile, thereby generating carbanion in-
termediates [36]. The formed carbanion intermediate attacks the acti-
vated the carbonyl carbon to generate the intermediate (III), which is
converted to the product (2-benzylidene malononitrile) through dehy-
dration, and the catalyst returns to the initial state in order to enter the
next cycle. This reaction mechanism can be further proved by the
resolved compounds of the synthetic catalyst raw material Ni(PPh3)2Cl2.
As shown in Fig. 8(a), both PPh3 and NiCl2 increase the yield of Knoe-
venagel condensation, which coincides with the activation of aldehyde
groups by metals in the reaction mechanism [54] and the activation of
malononitrile by Lewis bases. Meanwhile, using Ni(PPh3)2Cl2. to cata-
lyze the reaction could further improve the yield, potentially because
the two substrates are activated at the same time. Furthermore, the use
of MOF as the carrier maximizes the performance of the catalyst.
In order to explore the potential application of the as-synthesized
catalyst in the industry, 10-fold (10 mmol) and 500-fold (500 mmol)
scale reaction were studied (Fig. S4 in Supporting Information). In the
two amplification experiments, the amount of benzaldehyde was
10 mmol (1.06 g) and 500 mmol (53.00 g), and the amount of catalyst
was 100.00 mg and 5.00 g, respectively. Without increasing the catalyst
equivalent and extending the time, both reactions can be completed
under the regulated conditions (RT) that selected from pilot experi-
ments, and the yields of the 10 mmol experiment (1.50 g, 98 %) and
500 mmol experiment (76.50 g, 99 %) are almost unchanged. Cyclicity
of the 10 mmol experiment reaction was further analysed. The results
showed that the gram-level reaction still maintains a good cyclicity
(Fig. 12), which indicates UiO-66-alkyne-Ni is a promising industriali-
zation prospect catalyst.
The authors report no declarations of interest.
Acknowledgments
This work was supported by the National Natural Science Foundation
of China (21601135).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2021.118216.
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