Mesoporous silica-supported copper-catalysts for homocoupling reaction of terminal alkynes at room-temperature

Article abstract of DOI:10.1039/c3nj41006d

Amine-functionalized mesoporous silica-supported copper catalysts SBA-15@amine-Cu and SBA-15@Oamine-Cu were prepared and proved to be efficient and reusable for homocoupling of terminal alkynes at room temperature with air as the oxidant. SBA-15@amine-Cu exhibited better recyclability than SBA-15@Oamine-Cu. The differences in the catalytic performances of the catalysts could be ascribed to catalyst structure and the interaction between copper and the supports. The as-prepared catalysts were systematically characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), high resolution-transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and nitrogen physical adsorption. The analysis indicated that the mesoporous structure of the materials was retained during the immobilization process. XPS results suggested that the as-prepared catalysts were not obtained by a simple physical adsorption of CuCl on the supports. It is noted that, for aliphatic alkynes, the catalytic activity of SBA-15@amine-Cu is even higher than that of the homogeneous copper catalytic system and that of some previously reported heterogeneous systems.

Full text of DOI:10.1039/c3nj41006d

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Mesoporous silica-supported copper-catalysts for
homocoupling reaction of terminal alkynes at
room-temperature†
Cite this: NewJ.Chem., 2013,
37, 1343
Hongling Li, Min Yang,* Xia Zhang, Liang Yan, Jing Li and Yanxing Qi*
Amine-functionalized mesoporous silica-supported copper catalysts SBA-15@amine–Cu and SBA-
15@Oamine–Cu were prepared and proved to be efficient and reusable for homocoupling of terminal
alkynes at room temperature with air as the oxidant. SBA-15@amine–Cu exhibited better recyclability
than SBA-15@Oamine–Cu. The differences in the catalytic performances of the catalysts could be
ascribed to catalyst structure and the interaction between copper and the supports. The as-prepared
catalysts were systematically characterized by inductively coupled plasma atomic emission spectroscopy
(ICP-AES), high resolution-transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray
photoelectron spectroscopy (XPS) and nitrogen physical adsorption. The analysis indicated that the
mesoporous structure of the materials was retained during the immobilization process. XPS results
suggested that the as-prepared catalysts were not obtained by a simple physical adsorption of CuCl on
the supports. It is noted that, for aliphatic alkynes, the catalytic activity of SBA-15@amine–Cu is even
higher than that of the homogeneous copper catalytic system and that of some previously reported
heterogeneous systems.
Received (in Montpellier, France)
6th November 2012,
Accepted 6th February 2013
DOI: 10.1039/c3nj41006d
www.rsc.org/njc
product separation, difficult catalyst recycling and product
contamination caused by the residual components of the
Introduction
1,3-Diyne derivatives are very important building blocks in the
fields of natural products,1 pharmaceuticals,2 optical materials,
molecular devices, and organic conductors.3 Thus, many efforts
have been paid to the development of efficient and selective
methods for the synthesis of diyne derivatives. Homocoupling
reaction of terminal alkynes is one of the most effective
approaches for the preparation of symmetrical diyne compounds.4,5
Although several Pd/Cu(I) co-catalysts have been reported to be
efficient for the oxidative homocoupling of various alkynes,
these systems have the disadvantage of using expensive palla-
dium catalysts.6–9 The Cu(I)/(II)-catalytic system, due to its easy
availability, non-toxicity, ease of handling and low cost, has
been widely employed to carry out the homocoupling reaction
of terminal alkynes first discovered by Glaser in 1869.10–14
However, most of the reported copper-catalyzed systems are
homogeneous,11,12 with shortcomings such as complicated
catalysts. In contrast, the heterogeneous systems with solid
catalysts have significant advantages in catalyst/product(s)
separation and reuse of catalysts.15 A few efficient catalytic
systems for the heterogeneously copper-catalyzed alkyne homo-
coupling reactions have been reported, including the use of
Cu(OH)x/TiO2,14 copper(0) nanoparticles (CuNPs),16 CuAl-LDHs,17
CuI-USY,18 and Cu/PEG.19 However, these protocols still
exhibited some limitations. For example, the catalyst could
not be reused,18 large amounts of catalysts (110 mol% for a
copper-containing hydrotalcite system17 and 10–30 mol% for a
copper-containing zeolite system18) were employed, and high
temperature (66–110 1C)14,16,18 or high pressure of molecular
oxygen (420 atm)20 had to be used. In addition, copper salt-
catalyzed homocoupling reactions of aliphatic alkynes, with
weak acidity of the acetylenic proton, are always slug-
gish.12,13,16,21–24 Thus, the development of efficient, reusable
copper based heterogeneous catalysts is necessary.
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute
of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P.R. China.
E-mail: minyang1@gmail.com, qiyanxing@gmail.com; Fax: +86 931-4968190;
Tel: +86 931-4968190
Due to its well-defined micro-structure that forms the basis of
uniform active sites of the catalyst and well-controlled steric effects,
mesoporous silica material SBA-1525 has been used as support for
palladium/copper catalysts in recent years.26–29 However, alkyne
homocoupling reactions by the copper catalyst immobilized on
† Electronic supplementary information (ESI) available: Characterization data of
products 2, 3, 4, and 2a–h. See DOI: 10.1039/c3nj41006d
c
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SBA-15 have never been examined. Meanwhile, nitrogen-containing transmission electron microscopy (HRTEM) images were obtained
ligands can form stable complexes with Cu and thus show a broad using a JEM2010 (JEM2010, JEOL) microscope operated at 200 kV.
arena for developing novel silica supported copper catalysts. We The conversion was determined by gas chromatography
have long been interested in developing new catalytic systems for (GC7890II, Techcomp) analysis equipped with a SE-54 column
C–C bond formation.30–33 Triethylenediamine, an effective, inex- (30 m  0.32 mm  0.50 mm film thickness) and a flame
pensive commercially available ligand, was previously reported to ionization detector (FID) using biphenyl as an internal standard.
be immobilized on SBA-15 as an efficient and reusable Pd catalyst The injection temperature, column temperature and detection
for the alkyne homocoupling reactions.33 To further address the temperature were set to 280, 240 and 300 1C, respectively.
issue of whether expensive Pd and/or ammonium salts are essen-
General procedure for the preparation of 1,4-diaza-
bicyclo[2.2.2]octan-2-ylmethanol 4 (Scheme 1)
tial for achieving high catalytic activity and yield, we present herein
the preparation of two kinds of amine-functionalized SBA-15-
copper catalysts and their reusable application on the mild and
efficient heterogeneous alkyne homocoupling reactions with air as
an environmentally benign oxidant.
Firstly, to a stirred solution of acrylic acid ethyl ester 1
(500 mmol, 53.2 mL) in 90 mL of carbon tetrachloride, bromine
(25.9 mL) was added dropwise at room temperature. After being
refluxed for 24 h, the solution was cooled and solvent was
removed under reduced pressure to get 2,3-dibromopropionic
acid ethyl ester 2 in 94% yield.34 Then, 13.5 mL of 2 in 200 mL
of toluene, 8 g of piperazine and 39.2 mL of triethylamine, in
120 mL of toluene, were successively introduced into a 500 mL
round-bottomed flask and the mixture was heated to 80 1C with
magnetic stirring overnight. The mixture was then cooled and
the triethylamine hydrobromide was separated off by filtration.
After being rinsed with ethyl acetate, the solvents were evaporated
from the filtrate under reduced pressure and the residue
was distilled at ambient pressure and the ethyl 1,4-diaza-bicyclo-
[2.2.2]octane-2-carboxylate 3 was obtained. Finally, 1.4 g of
lithium aluminium hydride in a suspension in 15 mL of THF
was introduced into a round-bottomed flask and the suspen-
sion was cooled to 0 1C using an ice-cold bath. Then, 4.1 g of 3
in 20 mL of THF was slowly added with magnetic stirring and
the mixture was stirred at room temperature overnight. The
excess hydride was hydrolysed by the slow addition of 1.4 mL of
water, 1.4 mL of 15% aqueous sodium hydroxide solution and
then a further 1.4 mL of water. The solid was separated off by
filtration. After being rinsed with chloroform, the filtrate was
evaporated under reduced pressure. After distillation, the oily
1,4-diaza-bicyclo[2.2.2]octan-2-ylmethanol 4 product was obtained.35
Experimental section
Reagents and materials
Mesoporous silica SBA-15 was purchased from Fudan University.
CuCl, (3-chloropropyl) trimethoxysilane (97%), but-3-yn-2-ol
(98%), and phenylacetylene (98%) were obtained from Alfa
Aesar. Triethylenediamine (97%), 4-ethynyltoluene (97%),
4-fluorophenylacetylene (98%), but-3-yn-1-ol (97%), 3,3-
dimethylbut-1-yne (98%), and hex-1-yne (98%) were obtained
from Acros. Ethyl acrylate (98%), sodium hydride (70% in oil),
bromine (98%), piperazine (98%) and biphenyl (CP) were obtained
from Sinopharm Chemical Reagent Co., Ltd. 4-Methoxyphenyl-
boronic acid (98%), 2-methylbut-3-yn-2-ol (98%), and hex-5-yn-
1-ol (98%) were purchased from Lancaster. Bromobenzene
(98%, Shanghai Qingpu Chemical Reagent Company) and
dec-1-yne (98%, International Laboratory, USA) were used as
received without any further pretreatment. Lithium aluminium
hydride (98%) was purchased from Tianjin Chemical Reagent
Co., Ltd. Toluene and dichloromethane were of analytical grade
and dried over CaH2 and distilled before use. All other solvents
were of analytical grade and used without further purification.
Characterization
The C and N content analyses were conducted on a Vario
EL (Elementar). Copper content was analyzed using an IRIS
Advantage ER/S inductively coupled plasma atomic emission
spectrometer (ICP-AES, Thermo Jarrel Ash, Franklin, MA, USA).
X-ray photoelectron spectroscopy (XPS) was performed on a VG
ESCALAB 210 X-ray photoelectron spectrometer, using Mono-
chrome MgKa as the excitation source. X-ray powder diffraction
(XRD) patterns were obtained on a Rigaku D/Max 2400 using
CuKa radiation with the following operating parameters:
60 mA, 40 kV, and 2y scanning from 0.51 to 61 for low-angle
XRD. Nitrogen physical adsorption was carried out on a
Micrometrics ASAP2010 volumetric adsorption analyzer. The
Brunauer–Emmett–Teller (BET) surface area was evaluated from
data in the relative pressure range from 0.05 to 0.20. The total
pore volume of each sample was estimated from the
amount adsorbed at the highest P/P0 (at 0.99). Pore dia-
meters were determined from the adsorption branch using
the Barrett–Joyner–Halenda (BJH) method. High-resolution
Preparation of SBA-15@amine–Cu catalyst
SBA-15@Cl and SBA-15@amine were prepared according to the
previously reported method.28,33 Then, SBA-15@amine (1 g)
was refluxed with CuCl (51.5 mg) in acetone at 50 1C for 5 h.
After filtration, the solid remains were washed three times each
with ethyl ether and methanol, sequentially, and then dried
under air. Finally, the yellow powder was collected and desig-
nated as SBA-15@amine–Cu (Scheme 2).
Scheme 1 Preparation of 1,4-diaza-bicyclo[2.2.2]octan-2-ylmethanol.
c
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Fig. 1 XPS analysis on Cu 2p3/2 of (a) fresh SBA-15@amine–Cu catalyst; (b) SBA-
15@amine–Cu catalyst after the sixth reaction cycle; (c) SBA-15@Oamine–Cu.
and 0.55 mmol gÀ1, and the amount of Cu determined by
ICP-AES is 0.33 and 0.54 mmol gÀ1, suggesting that Cu species
have been successfully loaded onto the functionalized SBA-15.
This might be attributed to the easy access of Cu species to
anchored limited steric hindrance triethylenediamine, with
chelating nitrogen atoms at its ends. Fig. 1 exhibits the XPS
analysis of the SBA-15@Oamine–Cu and SBA-15@amine–Cu
catalyst before and after the sixth reaction cycle. The binding
energy (BE) of Cu 2p3/2 was located at 933.7 eV corresponding to
Cu(I) for fresh SBA-15@amine–Cu and SBA-15@Oamine–Cu
catalysts (Fig. 1a and c). The BE value of Cu 2p3/2 of the catalyst
(933.7 eV, Fig. 1b) almost remained unchanged after the sixth
reaction cycle. In addition, compared with the BE value of Cu
2p3/2 of CuCl (932.6 eV, reported by Wagner),36 the relatively
large negative BE shift of Cu(I) indicated that the SBA-
15@amine–Cu and SBA-15@Oamine–Cu catalysts were not
obtained by a simple physical adsorption of CuCl on the
amine-functionalized SBA-15. Thus we speculated that triethylene-
diamine may act as a ligand stabilizing the Cu species with a
strong chelate interaction.
To get more accurate details of the structural change in
the catalyst preparation, the SBA-15 backbone of each
synthetic step was monitored by N2 sorption. The N2 sorption
isotherms and pore size distributions of modified SBA-15 at the
different stages are shown in Fig. 2 and 3 respectively. A
summary of surface area, pore volume and pore size is listed
in Table 1.
Based on these results, SBA-15 exhibited a type-IV isotherm
pattern with an H1 hysteresis loop, which is characteristic
of the mesoporous structure. Similar to the parent SBA-15,
SBA-15@amine showed a type-IV isotherm with an H1 hysteresis
loop, indicating that the mesoporous structure was maintained
after modification, while the values of the surface area and
pore volume became smaller with introduction of the silane
((3-chloropropyl) trimethoxysilane) and triethylenediamine as
expected. The pore size of SBA-15@amine–Cu and SBA-
15@Oamine–Cu catalysts decreased after anchoring of Cu,
Scheme 2 Procedures for the preparation of SBA-15@amine–Cu and SBA-
15@Oamine–Cu.
Preparation of SBA-15@Oamine–Cu catalyst
9.95 mmol of sodium hydride in a suspension in 30 mL of THF
was introduced into a 50 mL round-bottomed flask and the
suspension was cooled to 0 1C using an ice-cold bath under an
argon atmosphere, then 7.65 mmol of 1,4-diaza-bicyclo-
[2.2.2]octan-2-ylmethanol in 10 mL of THF was slowly intro-
duced into the above flask and the mixture was stirred at room
temperature for 2.5 h. The obtained reaction mixture was
introduced into another flask in which 2.5 g of SBA-15@Cl
and 25 mL of THF already existed. After being refluxed at 65 1C
for 24 h, the mixture was cooled to room temperature, and then
washed with THF, water, CH2Cl2 and methanol respectively.
Finally, the cake was dried under vacuum at 60 1C overnight.
The obtained solid was designated as SBA-15@Oamine. The
preparation approach of SBA-15@Oamine–Cu was similar to
that of the SBA-15@amine–Cu catalyst (Scheme 2).
Typical procedure for the homocoupling reactions of terminal
alkynes
A mixture of alkyne (0.2 mmol), catalyst (5 mol%), and piper-
idine (1 mL) was stirred at room temperature under an air
atmosphere for the desired time until complete consumption
of starting material as judged by TLC and GC analysis, which
usually took 4–24 h. After the first reaction cycle, the catalyst
was recovered simply by centrifugation and filtration. After
being washed with ethyl acetate three times and dried under
vacuum, the recovered catalyst was directly used for the
next cycle.
Results and discussion
Characterization of the catalyst
The amount of triethylenediamine in SBA-15@amine–Cu and suggesting the presence of the complex inside the channels of
SBA-15@Oamine–Cu evaluated by the nitrogen content is 0.34 SBA-15 in some extent. In addition, the mesoporous structure for
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Fig. 4 Low-angle XRD patterns of (a) SBA-15; (b) SBA-15@amine–Cu; (c) SBA-
15@Oamine–Cu; and (d) SBA-15@amine–Cu after the third use.
Fig. 2 N2 sorption isotherms: (a) SBA-15; (b) SBA-15@amine; (c) SBA-15@amine–Cu;
(d) SBA-15@amine–Cu after the sixth use; (e) SBA-15@Oamine–Cu.
Fig. 5 HRTEM images of the SBA-15@amine–Cu.
periodic arrangement of channels in a hexagonal geometry
(Fig. 4),37 and this is the direct evidence that the mesoporous
structure was kept well after introduction of triethylenediamine
and Cu. HRTEM images of SBA-15@amine–Cu show clearly the
regular meso-structure (Fig. 5). It reveals that the hexagonal
symmetry of the pore arrays is still conserved after the immo-
bilization (Fig. 5). This result is consistent with the conclusion
given by other techniques shown above.
Fig. 3 Pore size distribution: (a) SBA-15; (b) SBA-15@amine; (c) SBA-15@amine–Cu;
(d) SBA-15@Oamine–Cu.
Homocoupling reaction of terminal alkynes
Phenylacetylene (1a) was chosen as a model substrate for
screening the reaction conditions (Table 2). The efficiency of
Table 1 N2 sorption results of SBA-15, SBA-15@amine, SBA-15@amine–Cu,
SBA-15@amine–Cu after the sixth use and SBA-15@Oamine–Cu
Pore
Decrease in
Table
2
Effects of reaction parameters on the homocoupling reaction of
phenylacetylene (1a)a
Samples
Sa/m2 gÀ1 Vb/cm3 gÀ1 sizec/nm pore sized/nm
SBA-15
SBA-15@amine
SBA-15@amine–Cu 389.4
SBA-15@amine–Cu 270.0
after the sixth use
554.7
392.7
0.83
0.60
0.59
0.43
6.0
5.8
5.7
5.6
—
Entry Solvent
Base
Oxidant Time (h) Yieldb (%)
0.2
0.3
0.4
1
2
CH3CN
CH3CN
Triethylenediamine O2
Triethylenediamine Air
K2CO3
Et3N
NMP
24
24
24
24
24
24
15
15
15
15
15
15
60
58
0
0
0
94
82
99
75
78
76
99
3
4
CH3CN
CH3CN
O2
O2
O2
O2
O2
Air
Air
Air
Air
Air
SBA-15@Oamine–Cu 383.9
0.61
5.9
0.1
5
6
CH3CN
CH3CN
a
b
BET surface area. Single point pore volume calculated at a relative
Piperidine
pressure P/P0 of 0.99. BJH method from the adsorption branch. The
decreased value of pore diameter, compared with the pore size of the
parent SBA-15.
c
d
7
8
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
—
—
—
—
—
—
9c
10d
11e
12f
the aforementioned catalysts was still well maintained, as evi-
denced by its type-IV isotherm with an H1 hysteresis loop.
XRD analysis of SBA-15 before and after the modification
both show three peaks, a strong peak at 0.81 and two weak
peaks at around 1.451 and 1.651, which were assigned to 100,
110 and 200 reflections. These results indicated the presence of
a
Reaction conditions: 1a (0.2 mmol), base (1 eq.), SBA-15@amine–Cu
b
(1 mol%), and solvent (1 mL) at room temperature. Determined by GC
using biphenyl as internal standard. The mixture of SBA-15@amine/
CuCl as the catalyst. The mixture of SBA-15-Cl/triethylenediamine/
CuCl as the catalyst. CuCl as the catalyst. SBA-15@Oamine–Cu as
the catalyst.
c
d
e
f
c
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Table 3 SBA-15@amine–Cu catalyzed homocoupling reactions of terminal
the as-prepared SBA-15@amine–Cu catalyst was firstly evalu-
ated in the presence of CH3CN and triethylenediamine at room
temperature. When 1 mol% of SBA-15@amine–Cu was used,
either O2 (1 atm) or air as the oxidant, the corresponding
product 2a was obtained in moderate yield (entries 1 and 2).
Next, different bases were evaluated in the presence of O2
(1 atm) (entries 3–5). Piperidine as the base gave a 94% yield
of desired product 2a, while no product 2a was found when
K2CO3, Et3N or 1-methyl-2-pyrrolidinone (NMP) was used (entry
5 vs. entries 1, 3–5). Interestingly, when piperidine was chosen
as both solvent and base, 82% yield of desired product 2a could
still be obtained at a relatively shorter reaction time (entry 7 vs.
entry 6). It was noteworthy that better efficiency could be
observed by using air as the oxidant with piperidine as both
solvent and base (entry 8 vs. entry 7). According to the reported
possible reaction mechanism of homocoupling, which involves
the formation of alkynyl-cuprate(I) and then smooth oxidative
dimerization with oxygen to the corresponding 1,3-diyne pro-
duct,24 the reason for the higher activity observed under air
with respect to O2 might be that molecular oxygen from air as
the oxidant was more beneficial to the catalytic cycle of the
homocoupling than pure O2. Moreover, with either the mixture
of SBA-15@amine and CuCl, or the mixture of SBA-15@Cl,
triethylenediamine and CuCl, or CuCl as the catalyst, lower
yield of desired product 2a would be obtained (entries 9–11).
The results might be attributed to the following reasons:
(1) SBA-15 with well-defined micro-structure benefits formation
of uniform active sites of the catalyst; (2) triethylenediamine, a
spindle-shaped molecule with two nitrogen atoms located at
the both tips, was used as a ligand to minimize hindrance for
reactants approaching the copper active sites, which facilitates
the reaction. Therefore, the copper active species immobilized
onto the triethylenediamine ligand functionalized SBA-15 is
more favourable for the catalytic cycle of the homocoupling of
terminal alkynes. Furthermore, all of the three above mentioned
catalysts could not be reused any more. When SBA-15@Oamine–
Cu was used as the catalyst, a considerable yield could also be
obtained (entry 12).
alkynesa
Entry Substrate
1
Product
Time (h) Yieldb (%)
4
4
4
100 (97)c
100 (95)c
94 (89)c
2
3
4
5
18
100 (94)c
24
24
87 (83)c
6
7
100 (90)c
24
24
91 (88)c
83 (80)c
8
a
Reaction conditions: 1 (0.2 mmol), SBA-15@amine–Cu (5 mol%) and
b
piperidine (1.0 mL) at room temperature under air. Determined by GC
using biphenyl as internal standard. Isolated yield.
c
the corresponding diyne (2h) in high yield, which gives an
opportunity for further functionalization at the intact chloride
group in the diyne product (Table 3, entry 8). Overall, the
present SBA-15@amine–Cu–air catalytic system proved to have
broad substrate scopes, both aliphatic and aromatic alkynes
underwent oxidative homocoupling reactions to give the
desired 1,3-diynes in good to excellent yields (83%–100%).
And several functional groups, including –OH, –F, –Cl and
–CH3, are all compatible with the reaction conditions.
To evaluate the substrate scope, we examined the protocol
by employing various terminal alkynes under the optimized
conditions (Table 3). The reactions of aromatic alkynes with
both an electron-withdrawing group (–F) and an electron-
Recycling of SBA-15@amine–Cu and SBA-15@Oamine–Cu
catalysts
donating group (–CH3) provided the corresponding 1,3-diynes Next, we turned our attention to the reusability of the present
in high yields in the presence of 5 mol% SBA-15@amine–Cu for Cu-supported catalysts. Both the SBA-15@amine–Cu and SBA-
4 h (entries 2 and 3). No dehalogenation proceeded in the case 15@Oamine–Cu catalysts could be easily separated from the
of halogen-substituted ethynylbenzene derivative 1b, providing reaction mixture by simple filtration. The recovered catalyst was
an opportunity for further functionalization of the product then reused for the oxidative dimerisation of phenyl acetylene
diyne (entry 2). The reaction of the OH-containing aliphatic under the same conditions as those of the first run (Table 4).
alkyne (1d) also proceeded smoothly to give the corresponding After the third run, the SBA-15@amine–Cu catalyst still exhib-
diyne (2d) (Table 3, entry 4). It is noteworthy that less reactive ited high activity with a slight decrease in the yield, while the
aliphatic C6–C10 alkynes16,18,21,22 of 3,3-dimethylbut-1-yne (1e), yield dropped to 81% for 4 h and good yield was still obtained
1-decyne (1f), and 1-hexyne (1g) were successfully converted with longer reaction time (12 h) in the 6th run. Meanwhile, the
into the corresponding diynes (2e–g) in high yields (87%–100%, activity of the SBA-15@Oamine–Cu catalyst was less than that
entries 5–7). Furthermore, the present system could be of the SBA-15@amine–Cu catalyst and only 65% yield was
applied to the homocoupling of the halogen-containing linear obtained for 8 h after the fifth run. As shown in Fig 4d, the
aliphatic alkyne 5-chloropent-1-yne (1h), selectively affording XRD of the recovered SBA-15@amine–Cu after the third run
c
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Table 4 Recycling study of the SBA-15@amine–Cu catalysta
yield of the desired product was obtained with a longer reaction
time (12 h). This experimental result further proved that the
catalyst could be recycled.
Run
1
2
3
4
5
6
Conclusions
Yieldb (%)
100
100
97
91
90
55
81 (85d)
76 (82d)
68 (73e)
61 (65e)
63 (68f)
—
Yieldb,c (%)
In summary, two air-stable mesoporous silica SBA-15-supported
amine–copper catalysts, SBA-15@amine–Cu and SBA-15@Oamine–
Cu, have been synthesized and well characterized. SBA-15@amine–
Cu, an efficient heterogeneous catalyst, readily available from
inexpensive reagents, was successfully applied on dimerization
of a variety of alkynes (including aromatic alkynes and demand-
ing aliphatic alkynes) affording corresponding diynes at
room temperature, with air as the oxidant. The advantage of
the heterogeneous catalysts lied in the ease of separation
and recyclability provided by the catalyst support. The
SBA-15@amine–Cu catalyst could be reused at least four times
with a slight decrease in activity for the homocoupling reaction.
It is noted that, for aliphatic alkynes, the catalytic activity of
SBA-15@amine–Cu is even higher than that of the homo-
geneous copper catalytic system and that of the previously
reported heterogeneous systems.11,12,18 Efforts to extend the
applications of this catalyst to other transformations are under-
way in our laboratory.
a
Reaction conditions: 1a (0.5 mmol), SBA-15@amine–Cu (5 mol%) and
b
piperidine (2.5 mL) at room temperature under air. Determined by GC
using biphenyl as internal standard. SBA-15@Oamine–Cu (5 mol%) as
the catalyst. 6 h. 8 h. 12 h.
c
d
e
f
still exhibited three well-resolved diffraction peaks which
were similar to that of the fresh SBA-15@amine–Cu catalyst,
indicating that the well-defined hexagonally ordered structure
of the catalyst was not destroyed during the reusability test.
In addition, after the third run, the copper content of the
SBA-15@amine–Cu catalyst decreased from 0.33 to 0.25 mmol gÀ1
as determined by ICP-AES, indicating that the catalyst used was
somewhat unstable, but still active. Moreover, the mesoporous
structure of SBA-15@amine–Cu was still maintained after the
6th run, as evidenced by its type-IV isotherm with an H1
hysteresis loop (Fig. 2), which explicated the fact that
the SBA-15@amine–Cu catalyst could be reused with stable
catalytic activity. In contrast, SBA-15@Oamine–Cu showed
worse reusability than the SBA-15@amine–Cu catalyst, and
after its fifth use, the homocoupling yield of phenyl acetylene
decreased to 65%. The differences in the catalytic performances
of the catalysts could be ascribed to catalyst structure and
the interaction between copper and the supports. For the
SBA-15@Oamine–Cu catalyst, the oxygen atom might also be
coordinated with copper, and thus hindered the substrate near
the active copper species. In addition, the amounts of copper
coordinated with oxygen increased as the balance exchange of
Cu with the ligand proceeds, which further made it less
effective. Therefore, the activity of the SBA-15@Oamine–Cu
catalyst is less effective than that of the SBA-15@amine–Cu
catalyst. From the perspective of sustainable development, the
SBA-15@amine–Cu which could be reused at least four times
would be the best choice.
Acknowledgements
This work is supported by Chinese Academy of Science and
Technology Project in Support of Gansu (XBLZ-2011-013) and
Technologies
R & D Program of Gansu Province (No.
1104FKCA156).
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1
2
3
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Substrate
Yieldb (%)
1a
100
1c
99
1b
97
1a
86c
a
Reaction conditions: 1 (0.5 mmol), SBA-15@amine–Cu (5 mol%)
and piperidine (2.5 mL) at room temperature under air for 4 h.
b
c
Determined by GC using biphenyl as internal standard. 12 h.
c
1348 New J. Chem., 2013, 37, 1343--1349
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