HKUST-1 derived Cu?CuO: X /carbon catalyst for base-free aerobic oxidative coupling of benzophenone imine: High catalytic efficiency and excellent regeneration performance

Article abstract of DOI:10.1039/d0ra06367c

The oxidative coupling of imines to ketazine with molecular oxygen is a green process towards the synthesis of hydrazine or hydrazine hydrate, which could efficiently address the economic and environmental issues of the traditional Raschig or peroxide-ketazine process. Herein, we developed an efficient heterogeneous base-free benzophenone imine oxidative coupling route with O2 catalyzed by Cu/CuOx/carbon materials derived from MOFs under mild conditions. Under optimized conditions, the conversion of BI is up to 98.2% and the selectivity of ketamine is 94.9%. This catalyst has excellent structure stability, recycling, and regeneration performance, owing to the carbonization of organic ligands of MOF at high temperature. More importantly, it is confirmed that the metallic Cu core is essential to improve the catalytic performance of the CuO shell in the BI oxidative coupling reaction, due to the promotion of electron transfer in the CuO surface, making dissolved O2 molecules more easily insert oxygen vacancies. This strategy might open an avenue to the sustainable catalytic synthesis of hydrazine or hydrazine hydrate. This journal is

Full text of DOI:10.1039/d0ra06367c

RSC Advances
View Article Online
View Journal | View Issue
PAPER
HKUST-1 derived Cu@CuOx/carbon catalyst for
base-free aerobic oxidative coupling of
Cite this: RSC Adv., 2020, 10, 36111
benzophenone imine: high catalytic efficiency and
excellent regeneration performance†
Huang Kaimeng,a Chen Siyuan,b Xia Changjiu,a Li Chenhao,a Zhu Bin,a
b
a
b
Peng Xinxin,a Lin Min,a Luo Yibin,
Wang Ge
*
*
Gao Hongyi,
and Shu Xingtiana
The oxidative coupling of imines to ketazine with molecular oxygen is a green process towards the synthesis
of hydrazine or hydrazine hydrate, which could efficiently address the economic and environmental issues
of the traditional Raschig or peroxide-ketazine process. Herein, we developed an efficient heterogeneous
base-free benzophenone imine oxidative coupling route with O2 catalyzed by Cu/CuOx/carbon materials
derived from MOFs under mild conditions. Under optimized conditions, the conversion of BI is up to
98.2% and the selectivity of ketamine is 94.9%. This catalyst has excellent structure stability, recycling,
and regeneration performance, owing to the carbonization of organic ligands of MOF at high
temperature. More importantly, it is confirmed that the metallic Cu core is essential to improve the
catalytic performance of the CuO shell in the BI oxidative coupling reaction, due to the promotion of
electron transfer in the CuO surface, making dissolved O2 molecules more easily insert oxygen
vacancies. This strategy might open an avenue to the sustainable catalytic synthesis of hydrazine or
hydrazine hydrate.
Received 22nd July 2020
Accepted 11th September 2020
DOI: 10.1039/d0ra06367c
rsc.li/rsc-advances
catalytic systems, have been developed (Scheme 1b). But it is
restricted by catalyst separation/recycling and alkaline waste
issues.4,14
1 Introduction
The oxidative coupling of imines (especially for benzophenone
imine, abbreviated as BI) provides a remarkable novel and green
approach to prepare ketazines, which are the key intermediates
for the synthesis of hydrazine (N2H4) or hydrazine hydrate
(N2H4$H2O).1–4 N2H4 and N2H4$H2O are widely used as polymer
blowing agents,5,6 reducing reagents,7,8 precursors of pharma-
ceuticals,9,10 and high-energy fuels.11 Traditionally, N2H4$H2O is
majorly manufactured by the highly polluting Raschig or urea-
based Raschig process (Scheme 1a).1,12 Although H2O2 solu-
tion has been used as a clean oxidant to produce ketazines,13 the
peroxide-ketazine process still suffers severe economic and
environmental issues, due to the high price of H2O2 and the
generation of N-containing wastewater.13 A fascinating alterna-
tive catalytic imine oxidative coupling route with O2 as sole
oxidant, based on Hayashi's CuCl/ZnCl2 or CuCl2/organic base
Recently, metal–organic frameworks (MOFs) as precursors
provide a promising platform for preparing efficient carbon sup-
ported metal or metal oxide catalysts due to their high surface area,
highly ordered characteristics of metal ions, tunable porosity, and
multi-functionality.15,16 Due to their advantages of excellent electron
conductivity, high porosity of carbon-supported metal materials
derived from MOFs, they are introduced with promising applica-
tions in batteries,17 supercapacitors,18,19 electrocatalytic14 and other
reactions.20 Among of them, carbon-supported Cu materials
derived from Cu-MOFs have been widely concerned, attribute to the
aState Key Laboratory of Catalytic Materials and Reaction Engineering, Research
Institute of Petroleum Processing, SINOPEC, 100083, Beijing, The People's Republic
of China. E-mail: luoyibin.ripp@sinopec.com
bBeijing Advanced Innovation Center for Materials Genome Engineering, School of
Materials Science and Engineering, University of Science and Technology Beijing,
Beijing 100083, The People's Republic of China. E-mail: gewang33@126.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra06367c
Scheme 1 Process for the production of ketazines.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 36111–36118 | 36111

View Article Online
RSC Advances
Paper
valence changing ability between copper and its oxides, as well as 12 h. Then the blue HKUST-1 (Cu3BTC2) product was obtained.35
the absorption and activation ability of Cu and CuOx to H2, O2, CO 1 g HKUST was added into a crucible with a cover. The HKUST-1
and other small molecules. Recently, Wu group obtained a new Cu- was heated to the specic temperatures and kept for 2 h under
MOF derived carbon-supported Cu/Cu2O materials by tuning N2 or H2/Ar atmosphere. Both the heat and cool rate was set as
copper ligands and pyrolytic conditions, which demonstrating
5
ꢀC minꢁ1. For example, when the catalyst is calcined in
excellent catalytic properties in liquid-phase hydrogenation of nitrogen at 400 ꢀC for 2 hours and exposed to air at room
furfural into furfuryl alcohol.21 Lu group22 and Chen group23 inde- temperature for 2 days, the obtained catalyst was labeled as
pendently found that Cu/CuOx/C materials derived from Cu–BTC Cu@CuOx/C–N2-400 + 2d.
can also be applied to catalyze CO oxidation reaction. Cu/CuOx/C
materials with various morphologies or sizes have important 2.3 Characterization of catalysts
inuence on catalytic activity. The signicant increase in electron
density on the Cu/Cu2O and Cu/CuO interface also play a pivotal
role in the enhancement of CO oxidation. Although progress have
been made in the eld of Cu-MOFs derivatives catalysis, the types of
X-ray photoelectron spectroscopic (XPS) experiments were
carried out on a spectrometer (AXIS Supra, Kratos Analytical
Ltd) with Al Ka radiation, the C 1s peak (284.6 eV) was used for
the calibration of binding energy values. Scanning electron
catalytic reactions are extremely limited. In particular, there are few
microscopy (SEM) images were taken from a Hitachi 4800
reports on the synthesis of organic chemicals by MOFs derivatives
microscope (20 kV). Scanning-transmission electron micros-
copy (STEM) images and energy dispersive X-ray spectroscopy
(EDX) of catalysts were recorded on a JEOL JEM-2100 micro-
scope (200 kV). The samples were dispersed in anhydrous
catalytic oxidation in liquid phase.24,25
Herein, to address the drawbacks in the synthesis of hydrazine
hydrate, inspired by homogeneous Cu catalytic systems,14,26
a heterogeneous Cu-MOF derived Cu@CuOx nanoparticles sup-
alcohol using the ultrasonic technique, the suspension ob-
ported on carbon matrix (Cu@CuOx/carbon) catalyst prepared via
tained was added dropwise to a micro-grid membrane and dried
the direct anaerobic pyrolysis of HKUST-1 precursor in N2 or H2/Ar
in air. 1H, 13C-NMR spectra were recorded with a Varian 700 M
atmosphere (see Fig. 1a),20,27–30 which exhibits excellent catalytic
spectrometer. Chemical shis (in ppm) were referenced to
performance in the aerobic oxidative coupling of benzophenone
imine without adding any organic base.24,31–33 Under optimized
conditions, the conversion of benzophenone imine was up to
CDCl3 (d ¼ 7.26 ppm) or TMS (d ¼ 0.00 ppm) as an internal
standard.
98.2% and the selectivity of ketazine was 94.9%. More importantly,
2.4 Catalytic performance tests
this catalyst has excellent structure stability, recycling and regen-
In a typical reaction, to a 25 mL reaction vessel, were added
eration performance, owing to the carbonization of organic
50 mg catalyst and 1.0 mmol benzophenone imine in 2 mL of
ligands of MOF at high temperature.29,34 Consequently, this cata-
solvent. The heterogeneous solution was stirred at 80 ꢀC for 24 h
lytic route unfolds a novel viewpoint on efficient heterogeneous
under 1 O2 balloon. Imine conversion and product selectivity
preparation of ketazines, which has great industrial application
were analyzed by gas chromatography (GC). The product and
potential and academic meaning.
side products were determined by GC-MS and 700 MHz 1H NMR
spectroscopy analysis.36 The performance of the catalyst was
2 Experimental
2.1 Materials
evaluated by imine conversion (x), selectivity (s) and ketazine
formation rate r (mmol gꢁ1 hꢁ1).
Conversion (%)
All the reagents and solvents were commercially available and
used for the synthesis without further purication. Copper
x ¼ (1 ꢁ n1) ꢂ 100%
nitrate trihydrate (Cu(NO3)2$3H2O, AR, Sinopharm Chemical
Reagent Co., Ltd.), 1,3,5-benzenetricarboxylic acid (BTC, 98%,
Aladdin), ethanol (EtOH, AR, Sinopharm Chemical Reagent Co.,
Ltd.), and N,N-dimethylformamide (DMF, AR, Sinopharm
Chemical Reagent Co., Ltd.) were used as received.
Benzophenone-azine (ketazine) selectivity (%)
s ¼ (n2/(n2 + n3 + n4)) ꢂ 100%
Ketazine formation rate (mmol gꢁ1 hꢁ1
)
2.2 Catalyst preparation
r ¼ 0.833sx
The HKUST precursor was prepared based on the reported
literature with some modications.28 First, 0.48 g Cu(NO3)2-
$3H2O was added into 24 mL H2O to prepare solution A. And
0.36 g benzene-1,3,5-tricarboxylic acid (BTC) was added into the
mixture of 12 mL ethanol and 12 mL N,N-dimethylformamide to
prepare solution B. Then solution A was added into solution B
with stir constantly for 30 min. The mixed solution was trans-
ni is the mole fraction of component; i is 1, 2, 3 and 4, which
represent the components benzophenone imine, benzophenone-
azine, N-(2-chloroethyl)-1,1-diphenylmethylaniline and benzo-
phenone; 0.833 ¼ 1000 mg/(50 mg$24 h).
2.5 Recycled experiments
ferred into 100 mL Teon-lined stainless steel autoclave and Catalytic stability and regeneration method: add 220 mg of
ꢀ
heated at 85 C for 24 h. The obtained precipitate was washed Cu@CuOx/C–N2-400-20d catalyst, 6 mL of dichloromethane,
ꢀ
with DMF and methanol several times, then dried at 80 C for and 800 mg (5 mmol) of diphenylketimine to the reaction ask
36112 | RSC Adv., 2020, 10, 36111–36118
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
ꢀ
and keep the reaction temperature to 80 C under the oxygen aer the sample was pretreated with ethanol and ultrasonic
balloon. Aer 72 h, dichloromethane adds into the reaction to treatment before TEM experiments. We were surprised to nd
quench. Aer the catalyst was centrifuged twice, it was sepa- that the irregular shaped particles were composed of copper
rated and dried at room temperature for 2 hours, followed by carbon nanospheres with the size of about 40–50 nm, as shown
the next cycle experiment.
in Fig. 2c and S3a.† The high-resolution TEM image demon-
strates that Cu2O (111) and Cu (111) located in the shell and
core of Cu@CuOx/C–400-N2 + 20d (Fig. 2d), which is consistent
with the analysis results of XRD and XPS. According to STEM-
EDS mapping data, the atomic ratio of Cu, C, and O elements
in the Cu@CuOx/C–N2-400 + 20d is 69%, 25%, and 6% (Fig. 2e
and S4†). The N2 adsorption–desorption curves (see Fig. S5†)
show that the specic surface and the external surface area of
the sample is 24.2 m2 gꢁ1 and 11.9 m2 gꢁ1, respectively. The
average pore width of Cu@CuOx/C–400-N2 + 20d is up to
71.6 nm, and therefore the Cu-based active sites have good
substrate accessibility.
3 Results and discussion
As illustrated in Fig. 1a, three sharp diffraction peaks with 2q
peaks located at 43ꢀ, 50ꢀ and 74ꢀ in X-ray powder diffraction
(XRD) spectra are observed, which are indexed to the Cu (111),
(200) and (220) crystalline planes of metallic Cu (JCPDS # 04-
0836), respectively.37 It is inferred that under the effects of high
temperature and reductants (such as ligand derived carbon and
H2), the aggregated Cu particles are transformed from the
framework Cu sites of HKUST-1 during pyrolysis processing.
Moreover, by employing Scherrer's formula, it is conrmed that
the calculated aggregation degree of Cu particles is along with
the increase of pyrolysis temperature, as shown in Fig. S2.†
When the as-made samples exposed to air for a specic dura-
tion (the corresponding samples are referred to as Cu@CuOx/C–
400-N2 + 2d and Cu@CuOx/C–400-N2 + 20d, with the exposure
time of 2 days and 20 days, respectively), a signicant 2q peak at
around 36.5ꢀ ascribing to Cu2O (111) plane (JCPDS # 78-2076)
formed. And the peak strength is correlated to the exposure
time, suggesting the slow aerobic oxidation of metallic Cu
particles by O2 in the air.38–40 Meanwhile, this oxidation process
was directly veried by X-ray photoelectron spectroscopy (XPS).
As shown in Fig. 1b, the peak intensity of CuO increases with
the extension of exposure time, indicating that the surface Cu2O
(binding energy of 932.6 eV) is transformed into CuO (binding
energy of 934.4 eV), which can be powerfully supported by Cu
Auger spectroscopy (Cu LMM). It is demonstrated that the
oxidation Cu-containing particles occur from external surface to
inner core, resulting in the formation of core–shell Cu@CuOx
composite material.37–41
In Table 1, benzophenone-azine A is the target product of the
BI oxidative coupling, while two side products, benzophenone B
and N-(2-chloroethyl)-1,1-diphenylmethanimine C are caused
by the hydrolysis of BI and proposed imine N-alkylation with
dichloroethane, respectively. Worthy to note that all the prod-
1
ucts were determined by GC-MS and H, 13C NMR analysis in
Fig. S6.†36 It is conrmed that B and C are two competitive
products in benzophenone-azine A synthesis process, as shown
in Table 1. It should be noted that BI is easy to hydrolyze to
produce ammonia and benzophenone.42 To restrict the hydro-
lysis of BI, the pH value of the mixture of BI and various solvents
is set at 8.0 in our experiments (Fig. S7†). When different
homogeneous Cu salt catalysts are used, the generation of Cu-
imine complexes as Lewis acid may facilitate the hydrolysis of
BI, with side product B selectivity up to 47.6% and 58.4%,
respectively, as entries 1 and 2 in Table 1. In the contrary,
various kinds of heterogeneous Cu@CuOx/carbon catalysts
possess more excellent benzophenone-azine
A selectivity,
higher than 85%, and up to 94.2% in maximum, as entries 3–6
in Table 1.
While under the same conditions, the selectivity of product A
is only 3.4% for the HKUST-1 catalyst,43 see entry 7 in Table 1. As
a result, it is suggested that Cu oxides are promising active sites
for the oxidative coupling reaction of BI with O2, rather than
CuII in alkaline solution or organic solvent.
Aer calcination and carbonization of trimethylbenzoic acid
ligand, copper aggregates to form octahedral and irregular
derivative particles (Fig. 2a). Due to the large volume of octa-
hedron, it is hard to well characterized by TEM analysis.
However, the irregular shape particles were partially dissociated
Fig. 1 (a) XRD and in situ XRD patterns of HKUST-1 derivatives; (b) XPS spectrums of HKUST-1 derivatives and CuO.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 36111–36118 | 36113

View Article Online
RSC Advances
Paper
Fig. 2 (a) SEM analysis of Cu@CuOx/C–N2-400 + 20d material; (b–d) HR-TEM spectrum of Cu@CuOx/C–N2-400 + 20d; (e–h) STEM-EDS
mapping images of Cu@CuOx/C–N2-400 + 20d.
Moreover, core–shell Cu@CuOx/carbon catalysts generally reaction process, and without signicant impact on the catalytic
perform higher BI conversion and better selectivity than performance Cu@CuOx/C–N2-400 + 20d, as shown in Table S1,†
commercial CuO and Cu2O powders. More in detail, Cu@CuOx/ indicating that lattice oxygen species involved in the oxidative
C–N2-400 + 2d as a catalyst, the 83.5% BI conversion, and the reaction.45
94.0% product A selectivity, which is much higher than that of
Interestingly, owing to the difference of solubility between
CuO powder (38.1% BI conversion, 86.3% product A selectivity), organic solvents and water, chlorinated solvents, e.g., dichlo-
as shown in entries 8 and 9, Table 1. It is reasonably attributed roethane, trichloromethane and 1-chloropropane, perform
to the smaller particle size and unique induced effect of Cu core remarkably enhanced both BI conversion and A selectivity than
on CuOx shell,44 which is conducive to the adsorption of BI and those of other chloride-free solvents, e.g., acetonitrile and
the H abstraction by Lewis acid site to promote the imine dioxane, as shown in entries 10–13 in Table 1. In more detail, BI
dehydration oxidative coupling reaction. Moreover, it is found and product A completely dissolve in hydrophobic chlorinated
that increasing the exposure time and pyrolysis temperature solvents to preventing BI hydrolysis, while acetonitrile and
(corresponding to Cu@CuOx/C–N2-400 + 20d and Cu@CuOx/C– dioxane can make organic subtracts/products and water
N2-850 + 20d, respectively) have a slight negative impact on BI homogeneously dissolved. More than that, probably due to the
conversion. While the selectivity was maintained at 94%, the BI number and position of chlorine atoms in these two solvents
conversion decreased by 5.8% and 8.3%, respectively. When decrease the activity of alkylation reaction, when chloroform or
using H2 as reductant, the Cu@CuOx/C–H–Ar-400 + 2d shows 1-chloropropane was used as solvents, no corresponding N-
even higher BI conversion (reaching 87.7%), but the selectivity alkylation of imines was detected by GC or GC-MS (Fig. S8†).
of product A drops to 91.4%. Consequently, by combining with The inuence of halogen-containing solvents on this oxidative
characterization results, it conrms that higher Cu and Cu2O coupling reaction needs to be further explored and summa-
content is in favor of more promising catalytic performance in rized, which will be further explained in the follow-up research.
oxidative coupling reaction, which may be accelerated by oxygen
The catalytic performance of Cu@CuOx/C–N2-400 + 20d
radical species and lattice oxygen species in principle. There- becomes gradually decreased along with the increasing of
fore, in order to ruling out possible oxygen radical species, recycle times, with BI conversion and ketazine selectivity
several free radical scavengers were added to the general reducing from 94.5% and 87% at initial stage to 46.2% and
36114 | RSC Adv., 2020, 10, 36111–36118
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Table 1 Catalyst screening and reaction condition optimization for oxidative coupling of benzophenone iminea
Selectivity (%)
Entry
Catalyst
Solvente
Conversion (%)
A
B
C
1b
2b
3
4
5
Cu(OAc)2$H2O
CuCl2$2H2O
1,2-Dichloroethane
1,2-Dichloroethane
1,2-Dichloroethane
1,2-Dichloroethane
1,2-Dichloroethane
1,2-Dichloroethane
1,2-Dichloroethane
1,2-Dichloroethane
1,2-Dichloroethane
Trichloromethane
1-Chloropropane
Acetonitrile
97.5
96.4
83.5
77.7
75.2
87.7
30.1
38.1
46.2
95.0
91.0
27.5
42.9
51.2
40.7
94.0
94.1
94.2
91.4
3.4
86.3
90.5
86.0
94.8
42.3
30.7
47.6
58.4
4.9
4.6
4.6
6.3
75.4
7.5
4.8
14.0
5.2
1.2
0.8
1.1
1.3
1.2
2.3
21.2
6.2
4.7
0
Cu@CuOx/C–N2-400 + 2d
Cu@CuOx/C–N2-400 + 20d
Cu@CuOx/C–N2-850 + 20d
Cu@CuOx/C–H–Ar-400 + 2d
HKUST-1
6
7
8c
9c
10d
11e
12
13
CuO
Cu2O
Cu@CuOx/C–N2-400 + 20d
Cu@CuOx/C–N2-400 + 20d
Cu@CuOx/C–N2-400 + 20d
Cu@CuOx/C–N2-400 + 20d
0
0
0
57.7
69.3
1,4-Dioxane
a
Reaction conditions: 2 mL solvent, 50 mg HKUST-1 or HKUST-1 derived catalyst, 1 mmol BI, 80 ꢀC, 24 h, 1 O2 balloon. Yields are determined by GC
spectroscopy. All the product and by-products were determined by 1H, 13C NMR and GC-MS, see ESI for more information. b 20% mmol metal salts
as the catalyst. c 1 mmol Cu catalyst. d No corresponding imine alkylation product was detected. e No corresponding imine alkylation products were
detected.
76.5% aer six recycles, respectively (Fig. 3). Corresponding, in planes (JCPDS # 80-1916), corresponding to 2q of 32.5ꢀ, 35.6ꢀ,
accordance with XRD analysis, deactivated catalyst possesses 38.8ꢀ, and 48.9ꢀ (blue line in Fig. 4a). Meanwhile, it can be seen
crystalline CuO phases with (110), (ꢁ111), (111) and (ꢁ202) from the Cu 2p3/2 XPS spectrum that the surface Cu species of
the deactivated catalyst mainly exists in the form of CuO. The
peak with binding energy of 934.4 eV and satellite peak are
almost the same as that of commercial CuO powder, indicating
that Cu and Cu2O of Cu@CuOx/C–N2-400 + 20d are completely
oxidized to CuO (Fig. 4b).46 To recover the initial catalytic
activity, the deactivated catalyst was regenerated by H2/Ar (5%/
95%) gas mixtures at 400 ꢀC for 1 hour. Interestingly, the BI
conversion and ketazine selectivity of regenerated catalyst is up
to 98.2% and 94.9%, respectively, even higher than those of
fresh Cu@CuOx/C–N2-400 + 2d catalyst. In Fig. 4a, three sharp
diffraction peaks of metallic Cu (JCPDS # 04-0836) were detected
in the XRD patterns of regenerated catalyst (red line), suggest-
ing that CuO can be quickly reduced to Cu2O and metallic Cu by
H2/Ar atmosphere at high temperature through a facial reduc-
tion method.47,48 This transformation was further conrmed by
the appearance of the peak at 932.6 eV in Cu 2p3/2 XPS (Fig. 4b).
In addition, from SEM images of deactivated catalyst (Fig. S9†),
the morphology and size of nanoparticles are well maintained.
Fig. 3 Recycling experiments by the catalysis of Cu@CuOx/C–N2-
400 + 20d.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 36111–36118 | 36115

View Article Online
RSC Advances
Paper
Fig. 4 (a and b) XRD pattern and XPS analysis of regenerated and recycled catalyst; (c) correlation between Cu2O ratio and ketazine formation
rates; (d) correlation between the total area of lattice oxygen plus defect oxygen and ketazine formation rates.
However, the original octahedral carbon matrix becomes
irregular. Nevertheless, ICP-OES analysis demonstrates that the corresponding catalytic activities,
On the basis of the study of surface composition and the
synergetic catalytic
a
Cu leaching is negligible aer several recycles, therefore mechanism was proposed where O2 molecules adsorbed on
showing its excellent application potential (Fig. S10†). Totally, it Cu2O to form Cu(II)–O2–Cu(II) intermediate, and then the
strongly infers that signicant increase in the density of low- oxygen activated by copper split into oxygen adatoms and
valence Cu species (including metallic Cu and Cu2O) favors transfer to ll the oxygen vacancies on CuO.53 Imine adsorbed
the enhanced catalytic performance for BI oxidative coupling on CuO is oxidatized dehydrogenation into Cu-imine species
reaction, as shown in Fig. 2d and S11.† Specically, when the by the lattice oxygen of CuO. It is suggested that two redox
Cu2O area ratio rises from 14% to 50%, the ketazine formation semi-reactions involved in the heterogeneous BI oxidative
rate increased linearly from 0.24 mmol gꢁ1 hꢁ1 to 0.77 mmol coupling: (1) oxidative BI coupling by the active oxygen species
gꢁ1 hꢁ1, which suggests that Cu2O and metallic Cu species have of CuO, with the formation of ketazine (containing N–N
induction effects on CuO and then facilitate the generation and bonds), water and Cu2O; (2) the aerobic oxidation of Cu2O to
transformation of active oxygen species.49–51 Furthermore, the O CuO, making oxygen adatoms insert the oxygen vacancy sites,
1s XPS spectra directly proved that the high content of lattice as illustrated in Scheme 2.26 In a general procedure, the
oxygen and defect oxygen in the catalyst was benecial to the surface CuII species acted as Lewis acid sites make the H–N
high yield of ketazine (see Fig. 4d and S12†). Owing to the bonds of BI more active, through the charge transfer between
enhancement of electron transfer on CuO surface induced by Cu sites and N atoms in imine groups. Then, the activated H
low valence Cu and Cu2O core, which makes the formation and atom is oxidized by the surface lattice oxygen species to
donation of lattice oxygen much more accessible.52 Once the produce water and oxygen vacancies, and the divalent copper
metal Cu is nally oxidized to CuO, the promoting electron is reduced to monovalent copper; in the meantime, the N–N
effect would be weakened, and its catalytic performance is bond is oxidized to ketazine. Aer that, Cu2O is re-oxidized by
similar to that of commercial CuO powder.
O2 to CuO, with the nishing of one catalytic recycle. The
Lewis acidity and redox capacity of Cu-containing sites are
36116 | RSC Adv., 2020, 10, 36111–36118
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
2 E. W. Schmidt, in Hydrazine and Its Derivatives: Preparation,
Properties, Applications, Wiley-Blackwell, New York, 2001, p.
2122.
3 E. Rothgery, in Hydrazine and Its Derivatives, 2004, vol. 13.
4 H. Hayashi, A. Kainoh, M. Katayama, K. Kawasaki and
T. Okazaki, Prod. R&D, 1976, 15, 299–303.
5 A. S. Dutta, in Recycling of Polyurethane Foams, ed. S. Thomas,
A. V. Rane, K. Kanny, V. K. Abitha and M. G. Thomas,
William Andrew Publishing, 2018, pp. 17–27.
6 G. Wypych, in Handbook of Foaming and Blowing Agents, ed.
G. Wypych, ChemTec Publishing, 2017, pp. 123–177.
7 A. Furst, R. C. Berlo and S. Hooton, Chem. Rev., 1965, 65, 51–
68.
8 D. Formenti, F. Ferretti, F. K. Scharnagl and M. Beller, Chem.
Rev., 2019, 119, 2611–2680.
9 C. H. Zhou and Y. Wang, Curr. Med. Chem., 2012, 19, 239–
280.
10 J. P. Schirmann and P. Bourdauducq, Hydrazine, Wiley-VCH
Verlag GmbH & Co. KGaA, 2001.
11 G. D. Byrkit and G. A. Michalek, Ind. Eng. Chem. Res., 1950,
42, 1862–1875.
12 F. Raschig, Ber. Dtsch. Chem. Ges., 1907, 40, 4580–4588.
13 P. Nikhitha and K. B. S. Saibabu, Chem. Eng. Technol., 2010,
33, 1543–1551.
14 A. Laouiti, M. M. Rammah, M. B. Rammah, J. Marrot,
F. Couty and G. Evano, Org. Lett., 2012, 14, 6–9.
15 W. Yang, X. Li, Y. Li, R. Zhu and H. Pang, Adv. Mater., 2019,
31, 1804740.
16 W. Chaikittisilp, K. Ariga and Y. Yamauchi, J. Mater. Chem. A,
2013, 1, 14–19.
Scheme 2 Plausible catalytic mechanism.
apparently synergistical to boost both BI activation and active
oxygen species transfer, thus exhibiting excellent BI conver-
sion and ketazine selectivity.
4 Conclusions
In summary, a novel and efficient heterogeneous oxidative BI
coupling route catalyzed by Cu@CuOx/carbon composites
without the assistance of organic base, has been developed.
Under the optimized conditions, the Cu@CuOx/carbon catalyst
not only offers 98.2% BI conversion and 94.9% ketazine selec-
tivity, but also possess exciting regeneration ability. More
importantly, it is suggested that the metallic Cu core is essential
to improve the catalytic performance of CuO shell in BI oxida-
tive coupling reaction, because it promotes the electron transfer
on CuO surface and makes the dissolved O2 molecules more
easily inserted into the oxygen vacancies. The enhanced redox
cycle between CuI and CuII plays an essential role in BI activa-
tion, the dehydrated coupling of BI and the regeneration of
active sites. Therefore, the low-valence Cu content is closely
relevant with BI conversion and ketazine selectivity. With the
increase of catalyst recycling times, Cu@CuOx/carbon catalyst
was slowly deactivated but could be entirely regenerated by
a facial hydrogenation process. Hence, this heterogeneous
oxidative coupling method has ultra-important potential for
sustainable catalytic synthesis of hydrazine hydrate.
17 X. Zhang, A. Chen, M. Zhong, Z. Zhang and X. H. Bu,
Electrochem. Energy Rev., 2019, 2, 29–104.
18 R. R. Salunkhe, Y. V. Kaneti, K. Jeonghun, J. Ho and
Y. Yamauchi, Acc. Chem. Res., 2016, 49, 2796–2806.
19 J. Wang, M. Rao, C. Ye, Y. Qiu, W. Su, S. R. Zheng, J. Fan,
S. L. Cai and W. G. Zhang, RSC Adv., 2020, 10, 4621–4629.
20 K. Shen, X. Chen, J. Chen and Y. Li, ACS Catal., 2016, 6,
5887–5903.
21 K. Chen, J. L. Ling and C. D. Wu, Angew. Chem., 2020, 59,
1925–1931.
22 S. Zhang, H. Liu, C. Sun, P. Liu, L. Li, Z. Yang, X. Feng, F. Huo
and X. Lu, J. Mater. Chem. A, 2015, 3, 5294–5298.
23 R. Zhang, L. Hu, S. Bao, R. Li, L. Gao, R. Li and Q. Chen, J.
Mater. Chem. A, 2016, 4, 8412–8420.
24 C. Bai, A. Li, X. Yao, H. Liu and Y. Li, Green Chem., 2016, 18,
1061–1069.
25 R. Fang, R. Luque and Y. Li, Green Chem., 2016, 18, 3152–
3157.
26 M. C. Ryan, Y. J. Kim, J. B. Gerken, F. Wang, M. M. Aristov,
J. R. Martinelli and S. S. Stahl, Chem. Sci., 2020, 11, 1170–
1175.
27 D. Farrusseng, S. Aguado and C. Pinel, Angew. Chem., Int. Ed.,
2009, 48, 7502–7513.
Conflicts of interest
There are no conicts to declare.
28 H. C. Zhou and S. Kitagawa, Chem. Soc. Rev., 2014, 43, 5415–
5418.
29 A. Corma, H. Garcia and F. X. Llabres i Xamena, Chem. Rev.,
2010, 110, 4606–4655.
Notes and references
1 H. Hayashi, Catal. Rev., 1990, 32, 229–277.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 36111–36118 | 36117

View Article Online
RSC Advances
Paper
ˇˇ ´
30 P. A. Julien, C. Mottillo and T. Friscic, Green Chem., 2017, 19, 43 L. Mitchell, B. Gonzalez-Santiago, J. P. S. Mowat, M. E. Gunn,
2729–2747.
P. Williamson, N. Acerbi, M. L. Clarke and P. A. Wright,
31 X. Zhang, W. Dong, Y. Luan, M. Yang, L. Tan, Y. Guo, H. Gao,
Catal. Sci. Technol., 2013, 3, 606–617.
Y. Tang, R. Dang, J. Li and G. Wang, J. Mater. Chem. A, 2015, 44 C. Sarkar, S. Pendem, A. Shrotri, D. Q. Dao, P. Pham Thi Mai,
3, 4266–4273.
T. Nguyen Ngoc, D. R. Chandaka, T. V. Rao, Q. T. Trinh,
M. P. Sherburne and J. Mondal, ACS Appl. Mater. Interfaces,
2019, 11, 11722–11735.
32 Y. Luan, Y. Qi, H. Gao, R. S. Andriamitantsoa, N. Zheng and
G. Wang, J. Mater. Chem. A, 2015, 3, 17320–17331.
33 W. Zhong, H. Liu, C. Bai, S. Liao and Y. Li, ACS Catal., 2015, 45 J. J. Varghese, Q. T. Trinh and S. H. Mushrif, Catal. Sci.
5, 1850–1856. Technol., 2016, 6, 3984–3996.
34 A. Dhakshinamoorthy, A. M. Asiri and H. Garcia, Chem. 46 R. P. Vasquez, Surf. Sci. Spectra, 1998, 5, 262–266.
Commun., 2014, 50, 12800–12814.
35 K. Schlichte, T. Kratzke and S. Kaskel, Microporous
Mesoporous Mater., 2004, 73, 81–88.
47 S. D. Pike, E. R. White, A. Regoutz, N. Sammy, D. J. Payne,
C. K. Williams and M. S. Shaffer, ACS Nano, 2017, 11,
2714–2723.
36 K. Li, X. Shao, L. Tseng and S. J. Malcolmson, J. Am. Chem. 48 A. P. LaGrow, M. R. Ward, D. C. Lloyd, P. L. Gai and
Soc., 2018, 140, 598–601. E. D. Boyes, J. Am. Chem. Soc., 2017, 139, 179–185.
37 Y. J. Chen, M. H. Li, J. C. Huang and P. Chen, Sci. Rep., 2018, 49 S. Yadav, A. Jain and P. Malhotra, Green Chem., 2019, 21,
8, 7646. 937–955.
38 Q. Zhu, L. Zou, G. Zhou, W. A. Saidi and J. C. Yang, Surf. Sci., 50 T.-J. Huang, Catal. Lett., 2003, 87, 173–178.
2016, 652, 98–113.
51 L. Wan, Q. Zhou, X. Wang, T. E. Wood, L. Wang,
39 I. Platzman, R. Brener, H. Haick and R. Tannenbaum, J.
Phys. Chem. C, 2008, 112, 1101–1108.
40 Z.-J. Zuo, J. Li, P.-D. Han and W. Huang, J. Phys. Chem. C,
2014, 118, 20332–20345.
P. N. Duchesne, J. Guo, X. Yan, M. Xia, Y. F. Li, A. A. Jelle,
U. Ulmer, J. Jia, T. Li, W. Sun and G. A. Ozin, Nat. Catal.,
2019, 2, 889–898.
¨
52 T. Kangas, K. Laasonen, A. Puisto, H. Pitkanen and
41 M. C. Biesinger, Surf. Interface Anal., 2017, 49, 1325–1334.
42 R. W. Layer, Chem. Rev., 1963, 63, 489–510.
M. Alatalo, Surf. Sci., 2005, 584, 62–69.
53 B. Wei, N. Yang, F. Pang and J. Ge, J. Phys. Chem. C, 2018,
122, 19524–19531.
36118 | RSC Adv., 2020, 10, 36111–36118
This journal is © The Royal Society of Chemistry 2020