A Schiff Base Modified Pd Catalyst for Selective Hydrogenation of 2-Butyne-1,4-diol to 2-Butene-1,4-diol

Article abstract of DOI:10.1007/s10562-020-03125-y

Abstract: Schiff-base modified Pd nanoparticles (NPs) supported on silica with an average size of ca. 2?nm have been synthesized via a one-pot aldimine condensation followed by impregnation-reduction of a palladium precursor, and the sample exhibits an excellent catalytic activity and selectivity in hydrogenation of 2-butyne-1,4-diol (BYD) to 2-butene-1,4-diol (BED). Under the mild reaction conditions (50?°C, 2?MPa H₂, and 4?h) and additive-free, 95.2% BYD conversion has been achieved with ca. 100% BED selectivity over the Pd/SiO₂–Schiff catalyst, and the Pd/SiO₂–Schiff catalyst presents an excellent catalytic stability. The above results are much better than that of commercial Lindlar catalyst, and the improved catalytic performance is attributed to the strong metal–support interaction derived from the coordination of nitrogen sites (Schiff-base) to Pd NPs, based on catalyst characterization results. Graphic Abstract: Featured by additive-free and high selectivity, hydrogenation of 2-butyne-1,4-diol to 2-butene-1,4-diol over the Pd/SiO₂–Schiff catalyst has been developed, aiming for green production of fine chemicals.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-020-03125-y

Catalysis Letters
https://doi.org/10.1007/s10562-020-03125-y
A Schif Base Modifed Pd Catalyst for Selective Hydrogenation
of 2‑Butyne‑1,4‑diol to 2‑Butene‑1,4‑diol
Haomeng Li1 · Xinkui Wang1 · Xiao Chen1 · Chuang Li1 · Min Wang1 · Yanhui Yi1 · Min Ji1 · Huilong Wang1 ·
Zhengfeng Shao2 · Changhai Liang1
Received: 6 October 2019 / Accepted: 27 January 2020
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
Schif-base modifed Pd nanoparticles (NPs) supported on silica with an average size of ca. 2 nm have been synthesized via
a one-pot aldimine condensation followed by impregnation-reduction of a palladium precursor, and the sample exhibits an
excellent catalytic activity and selectivity in hydrogenation of 2-butyne-1,4-diol (BYD) to 2-butene-1,4-diol (BED). Under
the mild reaction conditions (50 °C, 2 MPa H2, and 4 h) and additive-free, 95.2% BYD conversion has been achieved with
ca. 100% BED selectivity over the Pd/SiO2–Schif catalyst, and the Pd/SiO2–Schif catalyst presents an excellent catalytic
stability. The above results are much better than that of commercial Lindlar catalyst, and the improved catalytic performance
is attributed to the strong metal–support interaction derived from the coordination of nitrogen sites (Schif-base) to Pd NPs,
based on catalyst characterization results.
Graphic Abstract
Featured by additive-free and high selectivity, hydrogenation of 2-butyne-1,4-diol to 2-butene-1,4-diol over the Pd/SiO2–
Schif catalyst has been developed, aiming for green production of fne chemicals.
Keywords Schif base · Selective hydrogenation · 2-Butyne-1,4-diol · 2-Butene-1,4-diol · Palladium catalyst
1 Introduction
2-Butene-1,4-diol (BED), an important chemical interme-
Electronic supplementary material The online version of this
diate, is not only widely used for producing endosulfan [1],
vitamins A and B6 [2, 3], but also used as plasticizer of
alkyd resin, fungicide of synthetic resin and crosslinking
agent [4]. Among all of the synthetic methods for BED,
selective hydrogenation of 2-butyne-1,4-diol (BYD) is an
efective strategy [5, 6]. In BYD molecule, two groups,
C–OH and C≡C, can be hydrogenated. Therefore, it is vital
to develop a high efcient catalyst achieving the chem-
oselective hydrogenation of BYD to BED and inhibiting
article (https://doi.org/10.1007/s10562-020-03125-y) contains
supplementary material, which is available to authorized users.
* Xinkui Wang
wangxinkui@dlut.edu.cn
1
State Key Laboratory of Fine Chemicals, Department
of Chemistry, Dalian University of Technology,
Dalian 116024, China
2
Institute of Materials, China Academy of Engineering
Physics, Mianyang 621907, China
Vol.:(0123456789)
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H. Li et al.
the generation of by-products (1,4-butanediol (BDO),
n-butanol, and 2-hydroxytetrahydrofuran, etc.) [7]. The
commercial Lindlar catalyst [8] presents a high selectivity
to BED in the liquid phase hydrogenation of BYD to BED.
However, lead acetate and organic bases such as quinoline
are necessary to obtain a high selectivity, and thus the tox-
icity of lead acetate limits its application in fne chemical
synthesis and pharmaceutical production [9]. Therefore,
the design of lead-free catalysts and bases-free process
for the chemoselective hydrogenation of BYD to BED is
attracting increasing interests.
XRD, and XPS) of the structural and electronic properties
of the Pd NPs.
2 Materials and Methods
2.1 Catalyst Synthesis
SiO2–Schif: The SiO2–Schif powder was prepared accord-
ing to our previously reported method [16]. Firstly, 7 mL of
starting material APTES ((3-aminopropyl)-triethoxysilane)
was hydrolyzed at 40 °C. Then 10 mL of HCHO (37%) was
added into the hydrolyzed APTES aqueous solution, and
the reaction mixture was stirred for 2 h. After flter opera-
tion, the white precipitate was washed by deionized water at
room temperature. Finally, the sample was dried in vacuum
at 120 °C overnight to give terminal Schif base functional-
ized SiO2 support (SiO2–Schif).
Generally, the combination of Pd with one or more met-
als, i.e., Zn, Cu, Cd, Ca, etc., to form alloys or intermetal-
lic compounds can signifcantly improve the selectivity
of BED in hydrogenation of BYD due to the active-sites
isolation and electronic modifcation [10–12]. In addi-
tion, tuning the dispersion and electronic properties of
metal active sites through modifying the support is a
good strategy to enhance the selectivity towards target
product through regulating the adsorption and desorption
of feedstock and reaction intermediate. Therefore, some
researchers are working on developing novel supports to
tune the catalytic activity and selectivity in hydrogenation
of BYD. For instance, Liang and co-workers reported a
polyvinyl-pyrrolidone (PVP)-protected Pd nano-sol cata-
lyst supported on ZIF-8, which shows a good catalytic
performance, i.e., almost 100% BYD conversion and 94%
BED selectivity at 120 °C, 3 MPa H2 [13], attributing to
the edge sites of Pd blocking by ZIF-8 and the assistance
of the N containing organic ligand in ZIF-8. Francová
et al. reported a Pd catalyst using layered double hydrox-
ides (LDH) as support, which exhibits a high selectivity
towards BED up to 90% without any additives [14].
Schif base (C=N), serving as an active group, is exten-
sively used in various homogeneous processes [15, 16].
Additionally, metal–Schif base complexes have been used
as catalysts in many reactions such as hydrogenation [17]
and Heck reaction [18], with good catalytic activity. We pre-
viously reported a high dispersed gold catalyst using Schif
base modifed silica as a support, which shows an excellent
catalytic activity in dehydrogenation of formic acid to H2, as
well as CO2 hydrogenation to formic acid [16, 19]. Accord-
ingly, functionalization supported metal nanocatalysts by
Schif base is potential in designing a promising catalyst in
BYD hydrogenation to BED.
Pd/SiO2–Schif: The Pd/SiO2–Schif sample was prepared
through an impregnation–reduction method. Typically, 0.5 g
SiO2–Schif sample was used as the support, and 200 mL of
0.12 mol L−1 PdCl2 ethanol solution was used as impregna-
tion liquid. After stirring for 15 min at 80 °C, 10 mL NaBH4
(20 mg) aqueous solution was introduced into the mixture,
and was stirred for 1 h at 80 °C again. After flter operation,
the sample was washed with an excess amount of deionized
water. Finally, the wet sample was dried in vacuum at 120 °C
for 12 h.
Pd/SiO2: As mentioned above, the Pd/SiO2 was synthe-
sized by the similar impregnation–reduction method using
NaBH4 as reductant, the starting material was replaced by
commercially available SiO2 (with a BET surface area of
596 m2 g−1) as support.
2.2 Catalyst Characterization
The loadings of Pd in the prepared samples were determined
by an inductively coupled plasma-atomic emission spectros-
copy (ICP-AES). The materials were examined by a Bruker
tensor27 equipped with a DLATGS detector to provide
Fourier transform infrared (FT-IR) spectra. The high-angle
annual dark-feld scanning transmission electron microscopy
(HAADF-STEM) images of samples were recorded with a
FEI Tecnai G2 F30 S-Twin microscope operated at 300 kV.
The dispersion from STEM images was calculated based on
the following equation [20]:
Herein, we report a Pd nanocatalyst modifed by Schif
base, which shows an excellent catalytic performance in
selective hydrogenation of BYD to BED. The conversion
of BYD was as high as 95.2% with ca. 100% BED selectiv-
ity under the mild reaction conditions (50 °C, 2 MPa H2,
and 4 h) and additive-free. Furthermore, the roles of Schif
base in improving catalytic activity, selectivity and stability
have been revealed by the detailed characterization (STEM,
1.12
D(Pd dispersion) =
d(mean diameter of Pd nanoparticles, nm)
The X-ray difraction (XRD) patterns were recorded by
a Lab X XRD-6000 (Shimadzu) difractometer. X-ray pho-
toelectron spectroscopy (XPS) profles of the samples were
1 3

A Schif Base Modifed Pd Catalyst for Selective Hydrogenation of 2‑Butyne‑1,4‑diol to…
operated under a vacuum pressure of 10–9 Torr by a VG
2.3 Catalyst Test
ESCALAB 250 equipped with a monochromated Al–Ka
radiation source (1486.6 eV). The binding energy was cali-
brated by the binding energy of C1s (284.6 eV). The Pd-
dispersion was also measured by pulse adsorption of CO
(10% CO in Ar) performed at 40 °C via AutoChem 2920
(Micromeritics, USA). Before the measurements, the sample
was reduced in a fow of H2/Ar (40 mL/min) at 100 °C for
2 h followed by Ar purge at 100 °C for 2 h. A stoichiometry
of CO/Pd=1 was used for calculations.
The catalytic performance of the catalysts in selective hydro-
genation of BYD was examined in a 25 mL autoclave. Gen-
erally, 20 mg catalyst was added into the autoclave, which
contained 10 mmol BYD feedstock, 0.5 mmol 1,2-propan-
ediol as an internal standard, and 10 mL H2O as a solvent.
Before the reaction, the autoclave was fushed with H2 for
three times to exhaust air and pressurized to the desired
pressure. Typically, the reaction was carried on at mild con-
dition (50 °C, 2 MPa) under stirring. Liquid product was
quantitative analyzed by a gas chromatography (Shang-
hai Tianmei, GC7890GC) equipped with a FFAP column
(30 m×0.32 mm×0.25 μm) and a fame ionization detector.
3 Results and Discussion
3.1 Catalyst Characterization
Figure 1 shows the XRD patterns of diferent samples. For
the fresh Pd/SiO2 catalyst (Fig. 1a), there are two refec-
tion peaks, 2θ value of 40° and 46°, which correspond to
Pd (111) and Pd (200) facets (ICDD PDF No. 46-1043),
respectively. However, compared with Pd/SiO2 catalyst,
only a weaker and broader peak of Pd at 40° was detected
for the fresh Pd1.0/SiO2–Schif catalyst (Fig. 1c), implying
high dispersion of Pd NPs on the SiO2–Schif support with
smaller size. To further clarify the particle size, the STEM
was employed. Figure 2a, c show that average size of ca.
3.2 0.7 nm and 2.0 0.3 nm for the Pd NPs were observed
Fig. 1 XRD patters of Pd1.0/SiO2 (a), Pd1.0/SiO2-Schif (b), Spent-
Pd1.0/SiO2 (c) and Spent-Pd1.0/SiO2-Schif (d)
Fig. 2 STEM images and Pd particle size distributions of Pd1.0/SiO2 (a), Spent-Pd1.0/SiO2 (b), Pd1.0/SiO2–Schif (c), Spent-Pd1.0/SiO2–Schif (d)
1 3

H. Li et al.
Fig. 3 FT-IR spectra of Pd1.0/SiO2 (a), Spent-Pd1.0/SiO2 (b), Pd1.0
/
Fig. 4 Pd3d core level XPS profle of Pd1.0/SiO2 (a) and Pd1.0/SiO2–
SiO2–Schif (c) and Spent-Pd1.0/SiO2–Schif (d)
Schif (b) catalysts
in the Pd1.0/SiO2 and Pd1.0/SiO2–Schif samples, respec-
tively. The above XRD and STEM results reveal that owing
to the presence of Schif base groups, high dispersion of Pd
NPs have been achieved.
selectivity have been obtained using Pd/SiO2 catalyst (entry
2). In contrast to the Pd/SiO2, however, 95.2% BYD con-
version and ca. 100% BED selectivity have been achieved
(entry 3) using Pd1.0/SiO2–Schif catalyst, indicating that
the activity and selectivity have been greatly enhanced by
Schif base in the support. To understand the intrinsic activ-
ity of Pd1.0/SiO2–Schif catalyst, the metal dispersion has
been measured by pulse adsorption of CO and calculated
by STEM images. As seen in Table S1, the Pd dispersion
of Pd1.0/SiO2–Schif is 7.6% based on the CO adsorption,
which is better than that of Pd1.0/SiO2 (4.1%). The disper-
sions of Pd over Pd1.0/SiO2 and Pd1.0/SiO2–Schif samples
are calculated to be 35.0% and 56.0% based on the STEM
images, respectively, which are higher than the data obtained
by CO chemisorption. The various adsorption models (lin-
ear adsorption and bridge adsorption models) for the CO
molecule on the Pd sites lead to the dispersion lower than
the calculated value. Furthermore, the prohibiting of Schif
base toward CO chemisorption on Pd sites [23–25] is also
supposed to lower the capacity of CO adsorption on Pd sites
in CO chemisorption measurement. The turnover frequency
(TOF) value over Pd1.0/SiO2–Schif (4002 h−1) is about
twice of Pd1.0/SiO2 (2154 h−1) based on conversion of BYD
less than 10% and the metal dispersion measured by pulse
adsorption of CO. The signifcant improvement on intrinsic
activity might attribute to the modifcation of Schif base
(–N=C) group on electronic property of Pd, which has been
demonstrated by XPS results (Fig. 4).
To clarify the surface modifcation of the catalysts, the
FT-IR characterization of the fresh Pd1.0/SiO2 and Pd1.0
/
SiO2–Schif has been performed. As shown in Fig. 3, for
the Pd1.0/SiO2 catalyst, the bands at 1099 cm−1 and 801 cm−1
have been observed, and they are assigned to the asymmet-
rical stretching and symmetrical stretching modes of the
Si–O–Si bond, respectively [21]. For the Pd1.0/SiO2–Schif
catalyst, additional bands at 2872 and 2936 cm−1 can be
assigned to the symmetric and asymmetric vibrations of the
–CH2 groups. In addition, the presentation of the Schif base
(–N=C) band at 1691 cm−1 confrms the successful prepara-
tion of functionalized silica with Schif base [16].
Figure 4 shows the XPS profles of the samples. For the
fresh Pd1.0/SiO2 sample (Fig. 4a), the binding energy cen-
tered at 340.6 eV and 335.3 eV, assigned to Pd 3d3/2 and Pd
3d5/2 of Pd0 [22], respectively, have been observed. For the
Pd1.0/SiO2–Schif catalyst (Fig. 4b), however, the binding
energy of Pd 3d3/2 and Pd 3d5/2 of Pd0 shifted to 337.7 and
333.4 eV, respectively. Such a shift towards lower binding
energy is attributed to the electron donation from nitrogen
atom to Pd, indicating a strong interaction between the Pd
NPs and organic groups on SiO2–Schif.
3.2 Catalytic Performances
We conclude that the dispersion of Pd has been improved
through modifcation of Schif base. That is, the active num-
ber over Pd1.0/SiO2–Schif catalyst has been enhanced com-
pared with Pd1.0/SiO2 catalyst, which is also favorable to
improve apparent activity (BYD conversion). In addition, the
Without any base additives, the catalytic performance of
the as-prepared Pd nanocatalysts in BYD hydrogenation was
studied under mild condition (50 °C and 2 MPa H2). As
shown in Table 1, 42.7% BYD conversion and 65.3% BED
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A Schif Base Modifed Pd Catalyst for Selective Hydrogenation of 2‑Butyne‑1,4‑diol to…
Table 1 Results of selective hydrogenation of 2-butyne-1,4-diol over diferent catalysts
Entry
Catalyst
Conversion (%)
Selectivity (%)
BED
BDO
Other
1
2
3
SiO2
Pd1.0/SiO2
Pd1.0/SiO2–Schif
–
42.7
95.2
–
65.3
>99
–
25.3
–
–
9.4
–
Reaction conditions: 10 mmol BYD, 20 mg catalyst, 10 mL H2O (solvent), 50 °C, 2 MPa H2
Table 2 Efect of solvent on selective hydrogenation of BYD over Pd/
SiO2–Schif catalyst
Entry Solvent
Conversion (%) Selectivity (%)
BED BDO Other
1
2
3
4
5
6
H2O
95.2
>99
76.7 23.3
39.9 48.4 11.7
–
–
–
–
–
–
H2O/ethanol (8/2) 100
H2O/ethanol (5/5) 100
H2O/ethanol (2/8) 100
Ethanol
Methanol
89.1 10.9
90.4
91.6
100
100
9.6
8.4
Reaction conditions: 10 mmol BYD, 20 mg catalyst, 4 h, 50 °C,
2 MPa H2
Fig. 5 Efect of temperature on BYD conversion and product selectiv-
ity
presence of Schif base group is benefcial to the selectivity
of BYD because the amount of acidic active sites on the
catalyst are reduced by the Schif base group modifcation,
prohibiting the isomerization of BED to other by-products
[10]. Thus, we believe that the presence of Schif base func-
tional groups is the key to catalyst with high catalytic activ-
ity and selectivity.
hydrogenation reactions [4, 27]. Therefore, we consider that
a large amount of ethanol served as the source of hydrogen
to participate in the reaction, which may cause over-hydro-
genation reaction. Using methanol as the solvent, however,
BED was not formed and the yield of BDO reached 91.6%
after 4 h, similar with the yield in ethanol solvent.
The infuence of solvents on hydrogenation performance
has been investigated. As shown in Table 2, water solvent
showed a better catalytic performance than some other sol-
vents, with ca. 100% selectivity to BED at a 95.2% BYD
conversion (entry 1). With the increase of ethanol content in
the solvent, the selectivity of BED decreased rapidly, while
the selectivity of BDO increased. One conceivable reason
is that ethanol will change the solubility and dissociation
of H2 [21]. Similar to acetylene hydrogenation, signifcant
dissociative adsorption of hydrogen on the Pd surface may
detriment the adsorption of BYD. And then excess Pd–H
may eventually favor over-hydrogenation at the expense of
BED selectivity [26]. In addition to being a solvent, etha-
nol is usually used as a hydrogen transfer agent in many
The infuence of temperature was also investigated and
the result was displayed in Fig. 5. The conversion of BYD
increased from 51.1% to ca. 100% with the increasing tem-
perature from 40 to 100 °C. However, the selectivity to
BED decreased to 86.2% at 60 °C with the BYD converted
completely. With further increase of temperature, the selec-
tivity of BED declined continuously It could be concluded
that increasing the temperature afects the reaction kinetics
before the BYD is completely converted, thereby improving
the catalytic performance.
The efect of pressure on the activity and selectivity for
Pd/SiO2–Schif catalyst was investigated and the result was
displayed in Table 3. When the reaction was carried on at
1 3

H. Li et al.
Table 3 Efect of H2 pressure
on selective hydrogenation
of BYD over Pd/SiO2–Schif
catalyst
Entry
p/MPa
t/h
Conversion (%)
Selectivity (%)
BED
BDO
Other
1
2
3
4
5
1
1
2
3
3
4
8
4
3
5
13.2
87.4
95.2
93.9
100
>99
>99
>99
80.5
50.1
–
–
–
19.5
49.9
–
–
–
–
–
Reaction conditions: 10 mmol BYD, 20 mg catalyst, 10 mL H2O (solvent), 50 °C
consumed, meanwhile, the selectivity of BYD still remained
above 95%. It is worth mentioning that only a few amounts
of BDO and other by-products were detected, meaning that
the BED intermediate product is stable under the reaction
conditions and does not undergo further hydrogenation. We
attribute this result to the competitive adsorption of uncon-
verted BYD and generated BED.
The amount of acidic active sites was reduced due to the
Schif base modifcation on the carrier. Considering that
the generation of BDO was attributed to the larger amount
of Lewis acid sites, the strong adsorption of BYD inhib-
its further hydrogenation of BED, similar to the process of
acetylene’s selective hydrogenation [11]. Meanwhile, the
electronic regulation of nitrogen on Pd increased the den-
sity of electron clouds, which may enable rapid desorption
of intermediate BED.
Fig. 6 Kinetics for hydrogenation of BYD on Pd1.0/SiO2–Schif cata-
Finally, we have tested the durability of the two catalysts.
The results are depicted in Fig. 7. No remarkable decrease in
the catalytic activity of the Pd/SiO2–Schif catalyst has been
observed. After six cyclic tests, the selectivity to BED still
remains 96.4% with a BYD conversion of 97.3%. In contrast,
for the Pd/SiO2 catalyst, there was a remarkable decrease of
selectivity from 65.3 to 53.8% and the conversion from 42.7
to 33.7% after two cyclic tests.
lyst
1 MPa, only 13.2% of the reactants were converted after
4 h (entry 1) and 87.4% conversion was obtained after pro-
longing the reaction time to 8 h. As H2 pressure increasing
to 2 MPa, the conversion of BYD reached 95.2% after 4 h
under the same conditions with the BED as the only prod-
uct (entry 3). However, further increase of H2 pressure to
3 MPa leads to over hydrogenation of BED. The formation
of BDO is possible either directly from the activated BYD
intermediate or via BED hydrogenation. Formation of BDO
is more favored at high H2 pressures which is similar with
the hydrogenation of acetylene over Pd-based catalyst [25].
In order to further understand the reaction mechanism,
a series of kinetics experiments were carried on. Figure 6
To uncover the reason for the excellent catalytic perfor-
mance of the Pd/SiO2–Schif with special structure, we tried
our best to compare the characterization of the two samples.
The ICP-AES was employed to certifed the eminent durabil-
ity and the results showed that the Pd content of the Pd/SiO2
catalyst decreased remarkably from 0.93 to 0.81% after one
time catalytic test. However, that of the Pd/SiO2–Schif cata-
lyst remained almost constant at 0.85% after one time cata-
lytic test, which may be attributed to the role of electron-rich
Schif-base in preventing the leaching of palladium during
the reaction process through strong metal-support interac-
tion, as demonstrated by XPS result. In addition, no aggrega-
tion of Pd nanoparticles over the spent Pd/SiO2–Schif sam-
ple has been observed by XRD (Fig. 1) pattern and STEM
image (Fig. 2). The FT-IR spectrum (Fig. 3) of spent Pd/
SiO2–Schif catalyst is also nearly the same as that of the
fresh Pd/SiO2–Schif sample, further proving an excellent
catalytic stability of Pd/SiO2–Schif catalyst. For the Pd/
showed the kinetics for hydrogenation of BYD on Pd1.0
/
SiO2–Schif catalyst carried on at 50 °C in water. From the
kinetic data, the kinetic of BYD hydrogenation is dominated
by zero order since the curve of BYD concentration with
reaction time is a straight line at frst, which has also been
observed by some other researchers [28, 29]. In the initial
reaction process, the concentration of BED increased gradu-
ally with the progress of the reaction until it became stable
at 4 h, and the yield of BED reached to 95.2% with sin-
gle selectivity. After 5 h reaction, the BYD was completely
1 3

A Schif Base Modifed Pd Catalyst for Selective Hydrogenation of 2‑Butyne‑1,4‑diol to…
Fig. 7 Catalytic performance of Pd1.0/SiO2–Schif (a) and Pd1.0/SiO2 (b) catalysts in cyclic tests
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mance for BYD selective hydrogenation to BED in aqueous
solution without any base additives. 95.2% BYD conversion
has been achieved with ca. 100% BED selectivity over Pd/
SiO2–Schif catalyst. In addition, the Pd/SiO2–Schif catalyst
presents robust stability in this liquid reaction. The signif-
cantly improved catalytic performance can be attributed to
the presence of Schif base on the support. The Schif base
group is benefcial to the high dispersion of Pd by modifying
the electronic density, thus enhance the catalyst activity and
stability. The amount acidic active sites on the catalyst are
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Acknowledgements This work was supported by the National Natu-
ral Science Foundation of China (Grant Nos. 21978032, 21676045,
21603198, 21377018, 21770643 and 21872135), the Science and Tech-
nology Innovation Fund in Dalian City (Grant No. 2019J12GX028),
and the Fundamental Research Funds for the Central Universities
(Grant No. DUT18LK34).
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