Hydrogen Auto-transfer Synthesis of Quinoxalines from o-Nitroanilines and Biomass-based Diols Catalyzed by MOF-derived N,P Co-doped Cobalt Catalysts

Article abstract of DOI:10.1002/cctc.202001362

A Co-based heterogeneous catalyst supported on N,P co-doped porous carbon (Co@NCP) is prepared via a facile in-situ doping-carbonization method. The Co@NCP composite features a large surface area, high pore volume, high-density and strong basic sites. Furthermore, doping of P atoms can regulate the electronic density of Co. Therefore, Co@NCP exhibits good performance for the synthesis of quinoxalines from o-nitroanilines and biomass-derived diols under alkali-free conditions.

Full text of DOI:10.1002/cctc.202001362

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Hydrogen auto-transfer synthesis of quinoxalines from o-
nitroanilines and biomass-based diols catalyzed by MOF-derived
N, P co-doped cobalt catalysts
Authors: Kangkang Sun, Dandan Li, Guoping Lu, and Chun Cai
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To be cited as: ChemCatChem 10.1002/cctc.202001362
Link to VoR: https://doi.org/10.1002/cctc.202001362
01/2020

10.1002/cctc.202001362
ChemCatChem
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Hydrogen auto-transfer synthesis of quinoxalines from o-
nitroanilines and biomass-based diols catalyzed by MOF-derived
N, P co-doped cobalt catalysts
Kangkang Sun,[a] Dandan Li,[a] Guoping Lu,*[a] and Chun Cai*[a,b]
[a]
K. Sun, D. Li, Prof. G. Lu and Prof. C. Cai
School of Chemical Engineering
Nanjing University of Science & Technology
Xiaolingwei 200, Nanjing 210094 (P. R. China)
E-mail: c.cai@njust.edu.cn, glu@njust.edu.cn
Prof. C. Cai
[b]
Key Laboratory of Organofluorine Chemistry
Shanghai Institute of OrganicChemistry, Chinese Academy of Sciences
Lingling Lu 345, Shanghai 200032 (P. R. China)
Supporting information for this article is given via a link at the end of the document.
Abstract: A Co-based heterogeneous catalyst supported on N, P co-
doped porous carbon (Co@NCP) is prepared via a facile in-situ
doping-carbonization method. The Co@NCP composite features a
large surface area, high pore volume, high-density and strong basic
sites. Furthermore, doping of P atoms can regulate the electronic
density of Co. Therefore, Co@NCP exhibits good performance for the
synthesis of quinoxalines from o-nitroanilines and biomass-derived
diols under alkali-free conditions.
Introduction
Quinoxaline, an important class of N-containing heterocycles, has
been used in
a
wide range of applications including
pharmaceuticals,[1] agrochemical products[2] and manifold
functional materials.[3] Several drugs containing quinoxaline motif
exhibit significant activity against viruses, such as antineoplastics
(R(+)XK469, Scheme 1A), coccidiostats (Sulfaquinoxaline,
Scheme 1B),[4] or anti-HIV (Echinomycin, Scheme 1C).[5]
Therefore, the development of efficient methods for synthesizing
quinoxalines is of fundamental importance in the chemical
industry.[6]
The classical approaches for quinoxalines synthesis involve the
condensation of 1,2-phenylenediamines with aldehydes or 1,2-
diketones,[7] oxidation-trapping of α-hydroxy ketones with 1,2-
diamines,[8] 1,4-addition of 1,2-diamines to diazenylbutenes,[9]
oxidative coupling of epoxides with ene-1,2-diamines,[10] and
cyclization–oxidation of phenacyl bromides (Scheme 2a).[11]
However, most of these approaches suffer from some
shortcomings including the highly reactive raw materials,
multistep processes, low atomic efficiency and selectivity or
requirement of excess additives. In terms of green and
sustainable chemistry, the synthesis of quinoxalines from diols via
the acceptorless dehydrogenation coupling (ADC) or hydrogen
auto-transfer (HA) strategies is appealing, since diols are derived
from carbohydrates or lignocellulosic biomass,[12] and both ADC
and HA are atom, step and redox economical process.[13]
Scheme 1. Representative pharmaceuticals containing quinoxaline motif in the
molecule.
Several noble catalysts have been reported for the synthesis of
quinoxalines via the ADC or HA reaction.[14] In 2014, an Iridium-
catalyzed
acceptorless
dehydrogenation
synthesis
of
quinoxalines directly from aromatic diamines and diols were
developed by Kempe and co-workers.[4] Subsequently, Wan,[15]
Zhang,[16] and Kundu[17] reported some conceptually similar
strategies using ruthenium or iridium complexes as the
catalysts.[18] Despite the noble metal catalysts show excellent
activity, the development of an earth-abundant-metal catalyst is
highly desirable in terms of economy and sustainability. Indeed,
considerable progress has been made by several groups using
manganese pincer complex or phosphine-free Co(II) complex as
the catalysts.[19] However, these reaction systems usually require
a
stoichiometric amount of base to accelerate the
dehydrogenation of alcohol and mediate the condensation step
(Scheme 2b). Meanwhile, the use of homogeneous catalysts
generally has product-separation issues and limited catalyst
recovery potential. Although a recyclable noble metal catalyst
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(Ru@NC) has been developed for the reaction, the use of base
and noble metal is also necessary.[20] From the viewpoint of green
chemistry, the development of new shortcuts for the construction
of quinoxaline ring from renewable diols using reusable non-
precious metal-based catalysts under base-free condition would
be ideal.
Considering that the Co2+ and Zn2+ have similar coordination
geometry with 2-methylimidazole (2-MeIm) for the formation of
zeolitic imidazole frameworks (ZIFs), we chose synthetic BMZIFs
as templates for carbon materials. During pyrolysis, zinc will
evaporate away at high temperature (> 900°C), which is beneficial
to the increase of specific surface area.[26] The Co@NCP was
prepared in a sequential heteroatom precursor nucleate, in-situ
growth of ZIFs and pyrolysis procedure (Figure 1, see details in
the ESI†).
First, ammonium phosphate nanoparticles were precipitated by
adding a small amount of aqueous ammonium phosphate solution
into a large amount of poor solvents (methanol). The initially clear
methanol solution turned turbid, and SEM results confirmed the
formation of ammonium phosphate nanoparticles (Figure S5 and
Figure S6). Then the growth of the ZIFs shell on ammonium
phosphate particles was carried out by directly adding Co2+, Zn2+
and 2-MeIm, named by BMZIFs-NP. Finally, the Co@NCP was
obtained after the carbonization of BMZIFs-NP. The contents of
Co and P in Co@NCP determined by ICP-MS are 9.7 wt% and
3.5 wt% respectively. (Table S1). For reference, several other Co
catalysts were also prepared (Table 1, more details see ESI).
Scheme 2. Various methods for accessing quinoxalines.
Figure 1. Schematic illustration of the synthetic process for Co@NCP catalysts.
Metal–organic framework (MOF)-derived cobalt–nitrogen–carbon
materials (Co@NCs) have drawn great attention in hydrogen-
transfer reaction because they inherit the structural advantages of
the MOF precursors, such as high surface areas, well-defined
porosity, and good stability.[21] However, the strong
electronegativity of N atom can strengthen the reaction Gibbs free
energy for intermediate adsorption on the surface of the active
cobalt center, which inhibits the kinetic process of the catalytic
reactions.[22] Doping of relatively weak electronegative P atom can
optimize the coordination environment of the active Co center to
control the adsorption and desorption process of the
intermediates, thereby improving the catalytic performance of
these materials.[23] If this is the case, we expect that P doping can
enhance the electronic density of Co in Co@NCs that may
promote the hydrogen transfer of Co-H.[24] In addition, the
phosphorus species can act as a Lewis base to trap positively
charged protons during the hydrogen-transfer process, and
enhance the cyclization route.[25]
All these catalytic materials were investigated using the reaction
1,2-propanediol 2a and o-nitroaniline 1a as the model reaction.
The catalytic reaction was carried out in toluene using 2a as both
hydrogen source and reactant under base-free conditions.
Hydrogen is transferred from vicinal diols to o-nitroamine. The
nitro group of 1a serves as the hydrogen acceptors and facilitates
the removal of hydrogen from alcohols. No hydrogen release was
detected after the reaction. The yield of quinoxaline 3a over
various catalysts are shown in Table 1. The blank run (without any
catalysts) or using NC (derived from ZIF-8) as catalyst gave
essentially no activity in this system (entries 1, 2). To our delight,
the Co-NC was active for the hydrogen auto-transfer synthesis of
quinoxaline (entry 3), and its catalytic activity is higher than Co/NC
(entry 4). These results indicate that cobalt is the active site of the
reaction.
Notably, the P-doped catalysts could further promote the
transformation (entries 5-7). Co@NCP prepared by in-situ
doping-carbonization method, could provide the best result (entry
7). After screening of different solvents, toluene proved to be the
best option (entries 7-10). The catalytic activity of Co with different
content was significantly different (Table S4). It is noted that the
Co content increased from 3.3 mol% to 5.7 mol% and the activity
increased from 35 to 48%. However, further increasing the Co
content, the yield of 3a no longer increased significantly. The
amount of 2a exhibited an impressive influence on the reaction.
As the amount of 2a increased, the yield of 3a gradually increased
(entries 11-13). However, when the molar ratio of 1a:2a increased
to 1:4, the yield did not change significantly. Additionally, a higher
reaction temperature (140°C) was necessary to obtain excellent
Bearing these in mind, we develop a facile and effective in-situ
doping-carbonization method to fabricate highly porous N, P co-
doped carbon materials embedded with tiny Co NPs (Co@NCP)
by thermolysis of bimetallic zeolitic imidazolate frameworks
(BMZIFs). Ammonium phosphate nanospheres are not only the
sacrificial templates to form porous structure, but also the
phosphorus source of the carbon material (Figure 1). The
obtained porous Co@NCP exhibits highly performance in the
base-free hydrogen auto-transfer synthesis of quinoxalines from
diols and o-nitroanilines (Scheme 2c and Table S5).
Results and Discussion
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results (entries 14-16). Finally, the reaction did not proceed using
Co/AC as the catalyst (entry 17).
N and P elements are homogeneously distributed throughout the
whole material. In contrast, the calcination of ZIF-67 or BMZIFs
can only produce non-uniform and larger sized cobalt catalyst
(Figure 2d and Figure 2e). These results suggest that both the
presence of zinc and the addition of phosphorus are beneficial to
the formation of smaller and more uniform cobalt nanoparticles,
which may contribute to the better catalytic activity in the
hydrogen auto-transfer synthesis of quinoxalines.
Table 1. Optimization of the reaction conditions for quinoxalines synthesis.[a]
Entry
Catalyst
Solvent
T(°C)
1a:2a
Yield[b](%)
1
-
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
H2O
110
110
110
110
110
110
110
110
110
110
110
110
110
120
130
140
110
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:1
1:2
1:4
1:3
1:3
1:3
1:3
0
2[c]
3[d]
4[e]
5[f]
6[f]
7
NC
0
Co-NC
15
4
Co/NC
Co@NCP-g
Co@NC-g
Co@NCP
Co@NCP
Co@NCP
Co@NCP
Co@NCP
Co@NCP
Co@NCP
Co@NCP
Co@NCP
Co@NCP
Co/AC
32
23
48
7
8
9
m-xylene
diglyme
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
11
11
19
37
50
56
69
94
1
10
11
12
13
14
15
16
17
Figure 2. SEM images of (a) BMZIFs-NP and (b) Co@NCP, TEM image of (c)
Co@NCP, (d) Co-NC and (e) Co/NC.
[a] Reaction conditions: 1a (0.2 mmol) and Co-catalyst (5.7 mol% Co) in solvent
(1 mL), N2 atmosphere, 12 h. [b] Determined by GC analysis using hexadecane
as the internal standard. [c] The NC was obtained from the carbonation of ZIF-
8. [d] BMZIFs as sacrificial template, catalyst (5.7 mol% Co). [e] ZIF-67 as
sacrificial template, catalyst (5.7 mol% Co). [f] The catalysts were prepared by
solid grinding method.
In order to understand the structural properties of the most active
catalyst, Co@NCP were systematically characterized by various
techniques. The X-ray diffraction (XRD) patterns of as-
synthesized BMZIFs and BMZIFs-NP matched well with the
simulated patterns of ZIF-67 and ZIF-8, confirming that the whole
doping process will not affect the structure of MOFs (Figure S1a).
The scanning electron microscopy (SEM) images (Figure 2a)
show that BMZIFs-NP exhibited a uniform rhombic dodecahedral
morphology with particle sizes of around 240 nm. At the same
time, the particles on the ZIF surface proved the formation of
ammonium phosphate nanostructures (Figure S5).
After carbonization at 900°C for 2 h, the resulting Co@NCP
retained the morphology of the BMZIFs precursor, except for
partial shrinkage of the shape (Figure 2b). The TEM image,
HADDF-STEM and the corresponding EDS mappings of
Co@NCP indicated that there are both nano and sub-nano sized
cobalt particles in Co@NCP (Figure 2c and Figure 3). Meanwhile,
Figure 3. HAADF-STEM image and the corresponding EDX maps of Co@NCP.
As shown in Figure S1b, the Co-NC and Co/NC catalysts all
displayed two peaks at 44.2° and 51.5°, indexed to the (111) and
(200) crystalline planes of the metallic cobalt (JCPDS 15-0806).[26]
However, such peaks are not obvious in the sample Co@NCP.
This may be due to the smaller particle size and more uniform
dispersion of cobalt in Co@NCP.[27] Moreover, a new tiny peak at
40.6° is observed in Co@NCP, indicating that the interaction
between Co NPs and heteroatoms in the carbon matrix is likely to
form Co–P or Co–Nx, or their mixture phases.[28]
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The formation of Co-P or Co-Nx was further confirmed by the X-
ray photoelectron spectroscopy (XPS) results (Figure 4). The
elements Co, C, N, O and P were identified in Co@NCP from XPS
survey data in Figure S2a. There is no signal of zinc in Zn 2p XPS
spectrum and XPS survey data, which confirms the evaporation
of zinc during high-temperature treatment (Figure S2). The N 1s
XPS spectra of Co@NCP can be deconvoluted into five modes of
pyridinic N (398.4 eV), Co–Nx (399.1 eV), pyrrolic N (400.5 eV),
graphitic N (401.2 eV), and pyridine N-oxide (403.2 eV) species
(Figure 4a). These N-positions may confer a rich Lewis base
position to the catalyst, which is believed to facilitate the transfer
hydrogenation process.[29] At the same time, the presence of
pyridinic N can serve as anchoring sites for the formation of Co-
Nx sites.[30] The XPS P 2p spectrum in Co@NCP (Figure 4b)
shows deconvoluted peaks at 129.6 eV and 130.5 eV
corresponding to Co-P, and a peak at 133.5 eV could be assigned
to P–C.[31] The two peaks at 130.5 and 129.6 eV indicate that
phosphorus has a partial negative charge, which is due to the
easy transfer of electrons from Co to P, while confirming the
formation of Co-P. The negatively charged P can act as base to
trap positively charged proton. The presence of P-C bonds further
demonstrates the successful introduction of P heteroatoms into
the carbon framework, which is an important factor that changes
the catalyst performance.[28,32] In addition, the XPS spectrum of
Co@NCP (Figure S2c) shows that the C 1s peak can be
deconvoluted into four peaks at 284.7, 285.5, 286.6 and 289.8 eV,
which can be ascribed to C–C, C–P/C–O, C–N and O-C=O,
respectively. The P–C and C–N bonds demonstrate P and N
doping in the carbon matrix.[32] Phosphorus possesses the same
number of valence electrons as nitrogen, but has a larger atomic
radius and higher electron-donating ability, which makes the
doped carbon nanostructure exhibit Lewis basic properties.[25]
In Co 2p3/2 spectra of Co@NCP (Figure 4c), the peaks could be
deconvolved into five main components locating at 778.3 eV,
779.4 eV, 780.6 eV, 782.0 eV, and 784.7 eV, which could be
ascribed to the binding energies of Co0, Co-P, Co-O, Co-Nx, and
satellite, respectively.[27b,33] Compared with the Co 2p3/2 spectra in
Co-NC, a new peak appears at 779.4 eV, confirming the formation
of cobalt-phosphorus bonds in the sample after phosphorus
doping.[28] In addition, the Co0 peak of Co@NCP migrates
negatively to a lower binding energy, indicating that the electronic
interaction within the catalyst after phosphorization. The binding
energy of Co-P is lower than Co-Nx. All of these can reduce the
reaction Gibbs free energy for H adsorption on the surface of the
active cobalt center, thus improving hydrogen transfer of Co-H.[24]
Moreover, the XPS results point out that the content of Co in the
Co@NCP catalyst is 0.62 wt% (Table S3), while the ICP-MS
results show that the content of Co in the catalyst is 9.7 wt%
(Table S1). The difference in atomic content indicates that most
of Co is present in the pores of Co @ NCP, which can effectively
protect the active cobalt nanoparticles from leaching.
Figure 4. (a) N 1s, (b) P 2p, and (c) Co 2p XPS spectra of Co@NCP.
Raman measurements of Co@NCP and Co-NC presented in
Figure S3 reflected there were more defects in Co@NCP than in
sample Co-NC. The two characteristic peaks are located at 1339
and 1587 cm-1, which correspond to the D-band (defect) and G-
band (graphitic), respectively. However, the peak intensity ratio of
the D-band intensity to G band intensity (ID/IG) increased from
0.97 for sample Co-NC to 1.01 for sample Co@NCP, suggesting
that the doping of P atoms increases the structural disorder of
Co@NCP.[34] The nitrogen adsorption-desorption isotherm shown
in Figure 5 demonstrate that materials Co@NCP display a type-
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IV hysteresis loop, possess micro- and mesoporous structure.
The BET surface area of Co@NCP and Co-NC reached 934.3
m2g-1 and 502.2 m2g-1, respectively, and the total pore volumes
are 0.64 and 0.37 cm3g-1 (Table S2), indicating that in-situ
phosphorus doping is conducive to the generation of high surface
area and large pore volume during the annealing at high
temperature. This unique porous structure is of great significance
for the absorption and transport of reactant molecules and the
exposure of the largest active center in the hydrogen auto-transfer
reaction. The average pore size of Co@NCP is calculated to be
2.74 nm (Table S2). Generally, the molecular diameter is less
than 1 nm, so the reactants can enter the pore. To investigate the
adsorption behaviour of substrate on catalysts, the UV adsorption
test were performed. The signals of 1a disappeared after
Co@NCP was added (Figure S6). These results suggest that
highly porous structure of Co@NCP can absorb the reactants
efficiently, which shortens diffusion pathway and provides
sufficient interface area accessible to reactants, thereby making
the reactants fully contact with the catalysts, thus playing a better
catalytic role.
catalyst Co-NC only has a peak at about 630°C, and the total
number of basic sites is lower than Co@NCP. These results show
that the doping of N and P atoms can enhance the Lewis alkalinity
of this carbon material.[35] The high density and strong of basic
sites on the surface of Co@NCP could enhance the adsorption
and dehydrogenation of alcohols and cyclization during the
synthesis of quinoxalines.
Figure 6. CO2-TPD profiles of Co@NCP and Co-NC.
In summary, the excellent catalytic performance of the Co@NCP
is believed to be due to that: (1) the introduction of ammonium
phosphate nanospheres as the source of phosphorus and
sacrificial template is the key to form uniformly dispersed and
smaller Co nanoparticles and carbon material with large specific
surface areas; (2) The formation of Co-P bonds can optimize the
coordination environment of the active cobalt center, thus
improving hydrogen transfer of Co-H; (3) The Lewis basicity of
Co@NCP is increased by doping N and P atoms, which is
conducive to the dehydrogenation of 1,2-propanediol and
cyclization, thus allowing the reaction to proceed under base-free
conditions.
Table 2. Synthesis of quinoxalines derivatives[a]
Figure 5. Nitrogen adsorption–desorption isotherms and (b) the pore size
distributions of Co@NCP and Co-NC.
Basicity are reported to play an important role in promoting the
activity of hydrogen auto-transfer synthesis of quinoxalines. We
used temperature-programmed CO2 desorption (CO2-TPD) to
characterize the surface basicity of the catalysts (Figure 6). The
CO2-TPD profiles of Co@NCP shows that a small CO2 desorption
peak appears at 350°C, and two obvious desorption peaks
appear around 600 and 700°C, indicating that Co@NCP has both
weak and strong Lewis basicity. Compared with Co@NCP, the
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Scheme 3. Synthesis of imine and benzoimidazole.
Several control experiments were also carried out to investigate
the mechanism for this reaction. At first, o-phenylenediamine (1a’)
was treated with 1, 2-propanediol under standard reaction
conditions (Scheme 4a). Because there is no nitro group as
hydrogen acceptor, only trace of 3a was formed. Then,
methylglyoxal (2a’) was used instead of the 1, 2-propanediol (2a)
to reaction with o-nitroanilines (1a). No quinoxaline product (3a)
was detected since the nitro group cannot be reduced without
hydrogen-donating alcohol, (Scheme 4b). Finally, when the pre-
reduced 1a’ was reacted with the pre-oxidized 2a’, the yield of 3a
reached 76% at standard reaction condition (Scheme 4c). These
results show that o-phenylenediamine and methylglyoxal might
be the key intermediates of this reaction.
[a] Reaction conditions: o-nitroanilines (0.2 mmol), vicinal diol (0.6 mmol),
catalyst (5.7 mol% of Co), toluene (1 mL), 140°C, 12 h. Yields were determined
by GC with hexadecane as an internal standard.
To further confirm the general applicability of this catalytic, other
kinds of vicinal diols and o-nitroanilines with different substituents
were selected as reactants for the hydrogen auto-transfer
synthesis of quinoxalines (Table 2). As can be seen there, the
reaction of o-nitroaniline with different substituted diols provided
excellent yields for the expected products (3a-3e). However,
when the long-chain vicinal diol was used as reactant, the yield
slightly decreased (3h). In the case of 1-phenylethane-1,2-diol,
89% of the corresponding quinoxaline derivatives were afforded
(3l). We also examined the hydrogen auto-transfer reactions of
vicinal diols with a variety of substituted o-nitroaniline. Both
electron-rich (-Me and -OMe) and electron-deficient (-Cl)
substituents on o-nitroaniline yielded the desired products. In
general, o-nitroanilines with electron-rich groups have higher
reactivity than o-nitroanilines with electron-donating substituents.
Notably, challenging 2-nitro-3-pyridinamine also afforded 2,3-
dimethylpyrido[2,3-b]pyrazine (3i) with moderate yields.
Scheme 4. Mechanistic Studies.
Next we investigated the extension of our discoveries to other
similar reactions. The imine and benzoimidazole were
synthesized successfully through the route of the reductive
coupling of nitroarenes and alcohols over Co@NCP under
standard reaction conditions (Scheme 3a and 3c). Furthermore,
the reactions of aniline and o-phenylenediamine with benzyl
alcohols were investigated. The results showed that the yield was
not ideal without nitro group as hydrogen acceptor (Scheme 3b
and 3d).
Based on the above-described results as well as the related
processes,[36] a possible reaction mechanism is shown in Scheme
5. The overall mechanism can be divided into three-parts: (1)
vicinal diols dehydrogenation to methylglyoxal; (2) reduction of o-
nitroamine to o-phenylenediamine; (3) at last, the desired product
3a was obtained by imidization and cyclization reaction (Scheme
5).
6
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and redox economy. In addition, the catalyst can be easily
recovered for successive reuse without significant loss in activity.
The large surface area, high pore volume, high-density and strong
basic sites, and the adjustment of cobalt charge by P doping are
primarily responsible for the superior activity and stability. This
work not only demonstrates a new strategy for heteroatom
dopants but also provides an efficient and sustainable method for
the synthesis of quinoxalines.
Experimental Section
Materials
Scheme 5. Proposed Mechanism.
All chemicals such as 2-Methylimidazole (2-MeIm), Co(NO3)2·6H2O,
Zn(NO3)2·6H2O, o-nitroanilines, 1, 2-propanediol and ammonium
phosphate were purchased from commercial suppliers and used directly
without further purification.
Reusability of a heterogeneous catalyst is key to sustainable and
practical applications. To investigate the durability of Co@NCP,
the used catalyst was collected, washed, and submitted to
another reaction cycle under the same conditions. As shown in
Figure 7, the catalytic activity remained high, and the yield change
of 3a after the six recovery was negligible, indicating that the high
durability of this catalyst. The XRD spectra of recovery Co@NCP
are not obvious different from the fresh samples (Figure S1b).
Meanwhile, it can be seen from the TEM image (Figure S5c) that
there is no change in morphology and the Co nanoparticles did
not agglomerate. After six cycles, the BET surface area of
Co@NCP is 824 m2g-1 and the pore volume is 0.52 cm3g-1, which
is similar to the fresh sample (Table S2). Moreover, the Co
content in Co@NCP hardly changed after six cycles due to the
anchoring effects of N and P atoms (Table S1). These results
indicate that Co@NCP is highly stable for auto-transfer synthesis
of quinoxalines from o-nitroanilines and biomass-based diols.
Synthesis of BMZIFs-NP
BMZIFs-NP was synthesized with the following steps. Typically, 0.13 g
Co(NO3)2·6H2O and 1.33 g Zn(NO3)2·6H2O were added to 260 mL
methanol under ultrasonic. Ammonium phosphate (dissolved in H2O) was
then added into the solution gradually and stirred for 10 min. Then, 3.24 g
2-MeIm was dissolved in 60 mL methanol and added to the above mixture
under vigorous stirring at room temperature for 24 h. Finally, the BMZIFs-
NP was separated, washed (methanol) and dried at 60℃ under vacuum.
Synthesis of Co@NCP, NC, Co-NC and Co/NC.
The pyrolysis of BMZIFs-NP was carried out in a tubular furnace under a
nitrogen atmosphere. The sample was heated from room temperature to
900°C at a heating rate of 5 °C/min and kept at this temperature for 2 h.
The resulting catalyst was denoted as Co@NCP. The NC, Co-NC and
Co/NC were prepared by a similar method using ZIF-8, BMZIFs and ZIF-
67 instead of BMZIFs-NP, respectively.
Characterization
1H NMR and 13C NMR spectra were recorded on an AVANCE III Bruker
spectrometer operating at 500 MHz and 125 MHz in CDCl3, respectively,
and chemical shifts were reported in ppm relative to the center of the
singlet at 7.26 ppm for CDCl3. GC-MS was performed on an ISQ Trace
1300 in the electron ionization (EI) mode. GC analyses are performed on
an Agilent 7890A instrument (Column: Agilent 19091J-413: 30 m × 320 μm
× 0.25 μm, carrier gas: H2, FID detection. XRD analysis was performed on
Shimadzu X-ray diffractometer (XRD-6000) with Cu Kα irradiation.
Transmission electron microscopy (TEM) images were taken using a
PHILIPS Tecnai 12 microscope operating at 120 kv. CO2-TPD data were
obtained on a Micromeritics AutoChem II 2920 instrument. Scanning
electron microscopy (SEM) images were performed using a Hitachi S-4800
apparatus on a sample powder previously dried and sputter-coated with a
thin layer of gold. X-ray photoelectron spectroscopy (XPS) was performed
on an ESCALAB 250Xi spectrometer, using an Al Kα X-ray source (1350
eV of photons) and calibrated by setting the C 1s peak to 284.80 eV.
Inductively coupled plasma mass spectrometry (ICP-MS) was analyzed on
Optima 7300 DV. The concentration of reactants was recorded on a UV-
Vis spectrometer (Evolution 220) within the range of 300-800 nm. BET
surface area and pore size measurements were performed with N2
adsorption/desorption isotherms at 77 K on a Micromeritics ASAP 2020
instrument. Before measurements, the samples were degassed at 150°C
for 12 h.
Figure 7. Recyclability of Co@NCP.
Conclusion
In conclusion, we developed a facile in-situ doping-carbonization
method to synthesize a reusable heterogeneous earth-abundant
cobalt nanostructured catalyst encapsulated with N, P co-doped
carbon shell derived from BMZIFs. The resultant Co@NCP
demonstrated excellent catalytic activity for the synthesis of
quinoxalines from diols and o-nitroanilines. The process was
carried out under base-free conditions with excellent step, atom
Catalytic Reactions
7
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10.1002/cctc.202001362
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Typically, a calculated amount of Co@NCP (5.7 mol% of Co) and o-
nitroanilines (0.2 mmol) in toluene (1 mL) were placed in a 25 mL sealed
tube, and 1, 2-propanediol (0.6 mmol) was added to the mixture under
nitrogen atmosphere with a magnetic to initiate the reaction at 140°C for
12 h. After the reaction completed, the mixtures were cooled to ambient
temperature and EtOAc (5 mL) was added to the sealed tube. After the
catalyst was separated by magnetic force, the organic solution was
collected and concentrated in vacuum. Then, the obtained residue was
analyzed by GC/MS and purified by column chromatography. The yield of
the product was determined by GC with hexadecane as an internal
standard. The recyclability of the Co@NCP was investigated under the
same reaction conditions by using the recovered catalysts. After each
cycle, the catalyst was isolated from the solution by external magnet,
washed three times with methanol, and dried under vacuum to remove the
residual solvent and then reused for another reaction cycle.
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Entry for the Table of Contents
Hydrogen auto-transfer synthesis of quinoxalines from o-nitroanilines and biomass-based diols catalyzed by MOF-derived N, P co-
doped cobalt catalysts
9
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