Selective C-C coupling of terminal alkynes under an air atmosphere without base over Cu-NX-C catalysts

Article abstract of DOI:10.1039/d0nj04801a

The fabrication of a silk fibroin-drived porous, in situ nitrogenated carbon skeleton with highly dispersed Cu-NX-C active sites via a sol-gel and pyrolysis treatment was demonstrated. Cu-NX-C was used to catalyze the oxidative homo-coupling of terminal a

Full text of DOI:10.1039/d0nj04801a

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Selective C-C coupling of terminal alkynes under air condition
without base over Cu-Nx-C catalyst
Xinxin Xiao†a, Yin Xu†a, Sangepu Bhavanarushia, Bin Liu*a, and Xiaomeng Lv*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Fabrication of silk fibroin-driven porous, in-situ nitrogenated carbon skeleton with highly dispersed Cu–Nx–C active sites
via sol-gel and pyrolysis treatment was demonstrated. Cu–Nx–C was used to catalyze oxidative homo-coupling of terminal
alkynes with excellent functional group tolerance and high yields under air condition without base and no additive. The
small amount of catalyst (2 mol%) and proper recyclability further demonstrated its good activity and stability. The
excellent activity of the Cu–Nx–C catalyst can be ascribed to its high surface area, porous structure, small size of highly
dispersed Cu–Nx–C active sites and graphitic N. Possible mechanism was deduced according to the structural and surface
valance analysis of the sample before and after reaction. The report aims to demonstrate that such a nature-based
sustainable catalyst can provide new insights into design of coupling reaction catalysts with outstanding catalytic C-C
coupling performance.
Introduction
Biomass is rich in carbon and heteroatoms (N, P, S), the latter
Trantional metal and nitrogen co-doped carbon materials(M- of which are evenly distributed in the natural materials.
N-C, M: Fe, Co, Ni, Cu, Mn, etc) have aroused great interest as Biomass pyrolysis is considered to be a simple, convenient and
heterogeneous catalysts in photocatalytic degradation,1 green method to prepare porous C-N materials.15 After
electrocatalytic
water
hydrolysis,2-5
oxygen pyrolysis, heteroatoms(N, S, P) uniformly distributed in the
7
reduction,6electrocatalytic CO2 reduction, , electrochemical carbon skeleton thus enabled the preparation of promising
sensor, and batteries9-10 due to its encouraging activity and natural-doped carbon material. Among them, silkworm
8
stability. Various methods have been developed to fabricate calluses made of amino acids containing amino, carbonyl, and
M-N-C materials, among which self-template is important for hydroxyl groups, etc can confine metal ions forming metal
facile design 3D nitrogen and carbon precursor combining with complex, and metal ions were in situ reduced after pyrolysis,
active metal sites. 11Compared with simple pyrolysis mixture of forming M-N-C catalyst.16-17 The present method shed light on
carbon support, nitrogen-rich molecule and metal salt, the the simple, convenient and green method to prepare M-N-C
self-template method using MOF or polymer as precursor materials.
facilitates uniform dispersion, enhanced surface area, high Glaser-Hay coupling is known as one of the most efficient and
porosity, good conductivity, and high density of active sites. practical methods for the construction of conjugated 1, 3-
Zhu et al. prepared different M-N-C(M=Co, Fe, Mn) diynes, which have been present in numerous natural
catalysts by heating meso-tetra phenyl porphyrins (i.e. CoTPP, products18-23 and advanced materials.24-25 Since the pioneering
26-27
FeTPPCl, MnTPPCl) precursors and applied in ethylbenzene work reported by Glaser and Hay,
much efforts has been
oxidation.12 Hou et al. further developed single metal atom devoted to optimize the process with concern on catalyst
bonded by N atoms anchored on carbon(M=Fe, Co, Ni, Cu) via efficiency, substrate generality and environmental friendly
28
in situ pyrolysis of metalloporphyrin molecules.13 Peng et al. oxidant etc.
Copper(0) nanoparticles were demonstrated
synthesized Fe-N-C catalyst via confining [Fe(CN)6]3- in zeolitic capable to promote Glaser-Hay coupling, although high
imidazolate framework-8(ZIF-8) crystals under calcination.14 catalyst loading and base addition were required. Recently,
29
The above reported synthetic strategy open new opportunities the heterogeneous copper nanoparticles, with advantage in
for exploring excellent M-N-C catalysts, yet still limited by the recyclability and stability comparing with their homogeneous
expensive or scarce precursors.
counterparts, received great interests in catalyzing alkyne
homo-coupling reaction. Alonso and Yus reported the copper
nanoparticles on titania as more effective catalyst in alkyne
a. School of Chemistry and Chemical Engineering
Jiangsu University
30
homo-coupling in the presence of base. Cu/C3N4 composite
Zhenjiang, 212013, P.R. China
was utilized as catalyst by Zhu and Yao to achieve homo- and
cross-coupling of terminal alkynes very recently, wherein
additional KOH is necessary to facilitate the formation of
copper acetylides intermediates.31 In another work, amine
E-mail: liub@ujs.edu.cn (B.Liu); laiyangmeng@163.com (X.M.Lv)
† These authors contributed equally to this paper.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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replacing hydroxide worked as base, solvent and coordination hydrogel precursor was pyrolyzed in N2 atmosphere at 100 ℃
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DOI: 10.1039/D0NJ04801A
ligand towards oxidative homo-coupling of terminal alkynes, for 30 min, then 225 ℃ for 2 h and finally 800 ℃ for 2 h with a
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and exhibited high activity.
As exemplified by the key heating rate of 5 ℃ min-1. The final product was labelled as
contributions, it is still of challenge to achieve “green” Cu–Nx–C. Silk fibroin-driven porous carbon (Nx–C) without
processes (avoid additional base or particular oxidant like O2) copper was also prepared.
with consideration on the high efficiency of heterogeneous General Procedure for Glaser-Hay coupling: A mixture of
catalysts. Green approach or methodology has been alkyne 1 (0.2 mmol), catalyst powder (5 mg) (2 mol %) were
developed. The mesoporous Cu/MnOx was reported with taken in DMSO (0.5 mL) and the reaction mixture stirred at 70-
superior activity and stability towards aerobic oxidative 100 ℃ for 8-27 h under air atmosphere, after completion of
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coupling of terminal alkynes via synergistic effect without the reaction (monitored by TLC), reaction mixture was cooled
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base.
Cu-N2 sites on biochar-driven porous carbon were to room temperature, and directly purified by using column
reported high activity and selectivity towards coupling of chromatography. Homo-coupling of phenylacetylene was
terminal alkynes under mild conditions.34
selected to optimize the experimental condition, such as the
Herein, nitrogenated porpous carbon with highly dispersed Cu- reagent, temperature, amount of catalysts, and reaction time.
NX-C active sites was fabricated with cocoon as biomass. The Further exploration was demonstrated to test the substrate
procedure included degum of cocoon, silk fibroin dialysis, gel scope of the reaction. Recovery experiments were also
under CO2 condition, freeze-drying, pyrolysis to get final performed to test catalyst stability.
product and then application for homo-coupling of terminal The structure and texture of the as-prepared samples were
alkynes. Several significant aspects should be concerned: (1) examined by X-ray diffraction (XRD), using D8 Advanced XRD
the hydrophobic and hydrophilic segments in the silk fibroin (Bruker AXS Company, Germany) equipped with Cu Ka
chain can be self-assembled in the solution forming a two- radiation ( = 1.5406 Å). Scanning electron micrograph (SEM)
dimensional nano-sheet structure and further converted into a images was performed with an Hitachi S-4800 II, Japan). X-ray
nitrogen-doped carbon nano-sheet during the pyrolysis photo-electron spectroscopy (XPS) was done using an ESCA
process; (2) the CO2 gas was constantly purged into the silk PHI500 spectrometer. The Brunauer-Emmett-Teller (BET)
fibroin solution, changing the pH from 6-7 to 3.8-4.0 and thus specific surface area and pore size distribution of the samples
reducing the gelation time from days to hours; (3) the strong were characterized by nitrogen adsorption with a Tristar II3020
interaction between the copper ions and the amino groups Instrument. NMR spectra were recorded on a Brucker-400
facilitated the formation of the highly dispersed Cu-NX-C in the MHz spectrometer. HRMS (Bio TOF Q) spectra were recorded
two-dimensional carbon skeleton after pyrolysis. Furthermore, on P-SIMS-Gly of Bruker Daltonics Inc.
the in situ reduction of copper during the pyrolysis process can
promote the graphitization of the nano-carbon; (4) the Results and discussion
composite can catalyze homo coupling reaction of terminal Morphology and structural analysis: Figure 1 is the schematic
alkynes with high yield with small catalyst amount under air synthesis of Cu–Nx–C. The cocoon(Fig.1a) is degummed in
condition without base. Hence, the green synthesis and Na2CO3 solution, and then stripped in LiBr solution and
catalytic process would widen the further application of M-N-C dialyzed to get silk fibroin(Fig.1b), which is composed with
catalyst.
crosslinked peptide chain and β-crystallite(Fig.1c). After
addition of copper nitrate, the sol was bubbled with CO2 gas,
Experimental
changing the pH from 6-7 to 3.8-4.0 and thus reducing the
35
The cocoon was provided from Institute of Life Science, Jiangsu gelation time from days to hours (Fig.1d,e).
The amide
University, China. Copper nitrate trihydrate (Cu(NO3)2·3H2O), structure of SF coordinated with copper ions after CO2
Sodium carbonate(Na2CO3), Lithium bromide(LiBr) were all diffused into the stagnant solution, facilitating forming micellar
purchased from Sinopharm Chemical Reagent Co. All chemical assembly and quick gelation at the pH of the isoelectric point
reagents were analytic grade and used without further of SF(3.8-4.0). 36After freeze-drying and pyrolysis, the highly
purification. The solvents were used directly without any dispersed Cu was in-situ reduced in the porous nitrogenated
purification.
carbon frame, forming Cu–Nx–C catalyst(Fig.1 f, g).
In a typical procedure, 5 g of silkworm cocoon was dissolved in
0.02 M Na2CO3 solution and boiled for 30 min to degum. The
obtained silk fibroin was dissolved in 9.5 M LiBr solution at 60
℃ for 4 hours. Then, the degummed silk was dialyzed against
deionized water for 48 h using cellulose dialysis membrane
(MWCO 8-14 kDa). The obtained protein solution was
centrifuged and stored at 4 ℃ for further use. 1.488 g of
copper nitrate was dissolved in 20 ml of water at room
temperature. 15 g of silk fibroin was then added to the copper
nitrate solution. The mixture was acidified by purging CO2 (30
sccm) for about 1.5 h. After that, the obtained hydrogel
underwent freeze-drying for 12 h. Finally, the silk fibroin
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Figure 1. Schematic procedure of synthesis of Cu–Nx–C catalyst
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DOI: 10.1039/D0NJ04801A
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Figure 2a is photo of Cu–Nx–C precursor, i.e., mixture of silk
fibroin and copper nitrate solution after CO2 bubbling. The
obtained cyan fluffy sphere suggested that Cu2+ was evenly
dispersed and anchored in the silk fibroin framework,
suggesting potential highly dispersed Cu-Nx-C catalyst. After
freeze-drying and temperature-programmed calcination, the
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as-prepared Cu–Nx–C showed
a porous structure, as
demonstrated by the scanning electron microscopy (SEM)
image (Fig. 2b). The porosity of the Cu–Nx–C was further
investigated using N2 adsorption-desorption and Barrett–
Joyner–Halenda (BJH) pore size distribution curves analysis
(Fig.S1). The result indicated that it had large specific surface
2
area of about 746.54 m g−1, average value of pore size c.a.
Figure 3. a) SEM image of Cu–Nx–C; EDS mapping of Cu–Nx–C with different
element: b) C, c) Cu, and d) N, respectively.
2.41 nm and pore volume of about 0.45 cm3 g−1.
X-ray diffraction pattern of Cu–Nx–C showed a keen-edged
peak at 2θ values of 43.31°, 50.45°, 74.12° , which can be
assigned to Cu(111), Cu(200), and Cu(220) of Cu(0)(JCPDS 85-
1326), respectively (Fig.4a). The chemical state of Cu–Nx–C
was characterized via X-ray photoelectron spectroscopy (XPS)
and the high resolution XPS spectrum of Cu, N, and C were
illustrated in Fig. 4(b-d). Cu 2p with two peaks at 952.7 eV,
932.8 eV contributed to 2p1/2 and 2p3/2 orbital, while peaks at
398.70 eV, 400.70 eV of N1s represented Cu-N and graphitic N,
respectively, indicating that Cu was successfully anchored with
carbon (Fig. 4c). The presence of Cu-N and graphitic N
facilitated the deprotonation of acetylene, which was
promising for efficient Glaser-Hay coupling reaction. The C 1s
peaks at 284.55 eV and 285.46 eV corresponded to C=C, C-C
/C-N bonds, and a partially oxidized carbon peak appeared at
287.46 eV in the XPS spectrum (Fig. 4d).
Raman spectrum showed two main characteristic vibration
modes in the range of 900~2100 cm−1, indicative of D-band
(1360 cm−1) and G-band (1586 cm−1) of carbonaceous
materials (Fig. S2). The value of the intensity ratio (ID/IG) was
0.86, indicating a higher degree of graphitization. The 2D band
appeared at ≈2680 cm−1 indicating few layer graphene, which
was consistent with the smooth surface with layered stacking
graphene structure of the enlarged part in Fig.2b.
Figure 2. a) Photo of Cu–Nx–C precursor. b) SEM image of Cu–Nx-C catalyst.
c) TEM images of Cu–Nx–C catalyst. d) HRTEM images of Cu–Nx–C catalyst
showing the lattice fringes of Cu and graphitized carbon. The inset shows the
particle size distribution.
Transmission electron microscopy (TEM) image (Fig. 2c)
confirmed that Cu–Nx–C with particles size of c.a. 2.07 nm was
evenly distributed throughout the carbon structure. HRTEM
analysis (Fig. 2d) further confirmed the existence of copper
with interlayer lattice distance of 0.217 nm corresponding to
that of Cu (111) plane, and graphitized carbon with lattice
distance of 0.341 nm. Energy-dispersive X-ray (EDX) elemental
mapping of Cu–Nx–C also demonstrated the presence of Cu in
the Cu–Nx–C, and C, N, and Cu elements homogeneously
dispersed(Fig. 3). Inductively coupled plasma optical emission
spectrometry (ICP-OES) analysis was also utilized to determine
the Cu content, revealing the presence of trace amount of Cu
(≈6.012 wt. %).
Figure 4. XRD patterns of Cu–Nx–C (a), and high-resolution XPS spectrum of
Cu2p (b), N1s (c) and C1s (d), respectively.
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With the Cu–Nx–C composite in hand, initial investigation It was worthy to notice that 1-hexyne (1j) failed to participate
commenced with homo-coupling of phenylacetylene in the in the coupling reaction even at 100 ℃ due to the small
View Article Online
DOI: 10.1039/D0NJ04801A
presence of 2 mol% Cu–Nx–C in DMSO without any additives. amount of catalyst. The π stacking interaction between
1, 3-diyne (2a) was obtained in low yield after 7h at 50 ℃ graphene (on copper nanoparticles) and aromatic groups (on
(Table 1, entry 1). To our delight, when conducted at higher substituted aryl-substrates) might also benefit the substrate
temperature, dramatically increased yield was observed (Table approaching to catalyst thus facilitate formation of copper
1, entry 2). Further extending reaction time to 23 h at 70 ℃ acetylide intermediates.
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Table 2. Homocoupling of terminal alkynes catalyzed by Cu-Nx-C.a
led to excellent yield up to 95%. The employment of other
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polar solvents like MeOH or EtOH resulted in decreased yield
as shown in entries 4-5. In contrast with copper nanoparticles,
Cu-Nx-C (2 mol%)
R
R
R
DMSO, air
70oC or 100oC
2
apparently, copper powder was found less efficient even in 10-
fold enhancements of catalyst loading (Table 1, entry 3 vs. 6).
Interestingly, the catalyst loading could be further reduced to
1 mol% when conducted at 100 ℃ (entry 7), while other
copper salts was verified to be inferior to Cu-Nx-C (entry 7 vs.
8-9).
1
Entry
1
Substrate
T (h)
27
Product
Yield(%)b
94
Table 1. Optimization of reaction conditionsa
2
24
10
11
71
91
92
copper catalyst
DMSO, air
3 c
2a
1a
Loading
(mol%)
Temp.
(℃)
Yield
(%)b
4 c
Entry
Catalyst
T (h)
1
2
Cu–Nx–C
Cu–Nx–C
Cu–Nx–C
Cu–Nx–C
Cu–Nx–C
2
2
50
70
70
60
80
70
7
15
52
95
15
35
45
7
5 c
11
23
9
86
63
88
3
2
23
15
22
23
4 c
5 d
6
2
6
2
Cu
20
powder
7 c
7
8
9
Cu–Nx–C
CuI
1
1
1
100
100
100
12
12
12
98
42
40
Cu(OAc)2
8 c
9
72
[a] The reaction of 1a (0.20 mmol) and catalyst was carried out in DMSO (0.5
mL) under air unless otherwise noted. [b] Isolated yield. [c] In MeOH. [d] In EtOH.
9 c
10
n.r.
Under the optimum conditions, we next surveyed the
substrate scope of this homo-coupling reaction. A range of
substituted aryl-acetylenes were examined and both electron-
withdrawing or electron-donating groups could be tolerated,
although high temperature was needed in some cases to
provide the products with satisfactory yields ranging from 71%
to 94% (Table 2, entries 1-5). Moreover, substituents on ortho-
, meta- and para-positions can well participate in the reaction
without steric interference affording the 1, 3-diynes 2e-2f in
92% and 86% yields, respectively. When amide substituted
alkyl-acetylene 1g was subjected to the standard conditions,
2g was generated in 63% yield. Other functionalized alkyl-
acetylenes 1h-1i could also undergo the coupling reaction
smoothly, leading to the desired products 2h-2i in good yields.
[a] The reaction of alkyne (0.20 mmol) and Cu-Nx-C (2 mol%) was carried out in
DMSO (0.5 mL) at 70 ℃ under air unless otherwise noted. [b] Isolated yield. [c]
At 100 ℃.
The structural and surface valance analysis of Cu–Nx–C after
the coupling reaction were examined and illustrated in Fig. 5.
After reaction, peaks observed at 42.32°, and 61.41° appeared
in the XRD pattern (Fig. 5a), attributing to Cu2O (PDF: 78-
2076). Furthermore, the Cu 2p3/2 and Cu 2p1/2 at 932.3 and
953.12 eV were slightly red-shifted to 933.04 and 953.12 eV,
which again identified the existence of a small amount Cu2O
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(Fig. 5b). The XPS spectra of N1s and C1s agreed basically with Furthermore, the low amount of catalyst also paves the way of
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DOI: 10.1039/D0NJ04801A
that before the reaction (Fig. 5c, 5d).
sustainable development.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
The work was supported by National Natural Science
Foundation of China (21801099)
Notes and references
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Figure 5. XRD patterns of Cu-Nx-C after reaction(a), and high-resolution XPS
spectrum of Cu2p(b), N1s(c), and C1s(d) for Cu-Nx-C after reaction,
respectively.
Based on preliminary results and reported mechanism
studies31-32, an individualized mechanism is illustrated in
Scheme 1. Cu (0) was initially oxidized into Cu (I) by oxygen
thus triggered the coordination of acetylenes to copper center.
Acetylenic protons could be abstracted by nitrogen on nano-
sheet to facilitate the formation of copper acetylide (C).
Following oxidation would generate Cu (III) species (D) as
tracked by Cyclic Voltammetry (see Fig. S3). Reductive
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elimination of complex
D produced acetylenic coupling
product 2a and Cu (II) species (E). After the capture of another
acetylene, secondary reductive elimination of Cu (II) complex
(F) release diacetylene and water, meanwhile regenerate
intermediate (A) for the next catalytic cycle.
Ph
Ph
H
H
Ph
Ph
H
H
O2
N
N
N
N
Cu(0)
H2O
Cu(I)
(B)
Cu(I)
(C)
Cu(I)
(A)
1/2 O2
Ph
Ph
1/2 Ph
Ph
HO
OH
OH2
Cu(II)
(F)
N
N
N
Cu(II)
(E)
Cu(III)
(D)
1/2 Ph
Ph
Scheme 1. Plausible mechanism of coupling reaction under Cu-Nx-C catalyst.
Conclusions
In summary, we have developed a novel silk fibroin-driven Cu–
Nx–C composite for oxidative coupling of terminal alkynes
under air condition without bases. The obtained composite
with high surface area, porous pore structure, small size of
highly dispersed Cu–Nx–C active sites and graphitic N exhibited
high performance towards Glaser-Hay coupling reaction with
excellent functional group tolerance and high yields.
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This journal is © The Royal Society of Chemistry 20xx
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Graphical abstract
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Selective C-C coupling of terminal alkynes under air condition
without base over Cu-Nx-C catalyst
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XinXin Xiao†, Yin Xu†, Sangepu Bhavanarushi, Bin Liu*, and XiaoMeng Lv*
aSchool of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, P.R. China
*E-mail: liub@ujs.edu.cn(B.Liu); laiyangmeng@163.com(X.M.Lv)
†The authors contribute equally to the work
Highly dispersed copper nanoparticles supported on mesoporous nitrogenated carbon (Cu-NX-C)
were synthesized. The material exhibited superior catalytic activity towards aerobic oxidative
coupling of terminal alkynes with low catalyst amount, no base, and no additive.