Copper(II) ion exchanged AlSBA-15: A versatile catalyst for intermolecular hydroamination of terminal alkynes with aromatic amines

Article abstract of DOI:10.1016/j.jcat.2007.06.007

The hydroamination reaction offers a very attractive route for the synthesis of alkylated amines and their derivatives with no byproduct formation. AlSBA-15 was synthesized by isomorphous substitution of aluminum into the framework of SBA-15, which induce

Full text of DOI:10.1016/j.jcat.2007.06.007

Journal of Catalysis 250 (2007) 274–282
www.elsevier.com/locate/jcat
Copper(II) ion exchanged AlSBA-15: A versatile catalyst for intermolecular
hydroamination of terminal alkynes with aromatic amines
Ganapati V. Shanbhag, Trissa Joseph, S.B. Halligudi ∗
Inorganic Chemistry and Catalysis Division, National Chemical Laboratory, Pune 411008, India
Received 25 April 2007; revised 7 June 2007; accepted 7 June 2007
Available online 31 July 2007
Abstract
The hydroamination reaction offers a very attractive route for the synthesis of alkylated amines and their derivatives with no byproduct for-
mation. AlSBA-15 was synthesized by isomorphous substitution of aluminum into the framework of SBA-15, which induces the Brönsted acid
2+
2+
2+
sites, and these were exchanged with metal ions such as Cu , Zn , and Pd . The catalysts were characterized by XRD, N -sorption, SEM,
2
27
29
TEM, acidity measurements by FT-IR pyridine adsorption, H -TPR, Al MAS NMR, and Si MAS NMR. Hydroamination of phenylacetylene
2
(PhAc) with 2,4-xylidine has been used as a test reaction, which gave N-(1-phenylethylidene)-2,4-dimethylaniline with no byproduct formation.
CuAlSBA-15 and CuAlMCM-41 showed around three times greater activity in hydroamination of PhAc compared with Cu-clay and Cu-beta, due
2+
to the moderate Lewis acidity of Cu present in mesoporous supports. The performance of the CuAlSBA-15 was also determined with different
alkynes and amines to evaluate the catalyst’s general applicability in hydroamination reactions.
© 2007 Elsevier Inc. All rights reserved.
Keywords: Mesoporous; Copper catalyst; AlSBA-15; Heterogeneous; Ion exchange; Hydroamination; Addition; Alkyne; Amine
1. Introduction
transition metal salts, lanthanide and actinide complexes, and
transition metal complexes, have been studied for this seem-
ingly simple but challenging transformation [4–10]. But homo-
geneous methods suffer from their tedious workup procedure
and low catalyst recyclability. There is a need to develop effi-
cient, sustainable, recyclable, and eco-friendly solid catalysts
for hydroamination reactions. Compared with homogeneous
catalysts, only a handful of heterogeneous catalysts have been
reported to date. Penzien et al. developed metal-exchanged ze-
olites for alkyne hydroamination, which favors Markovnikov
addition products [11–14]. But zeolite-based catalysts normally
show low activity toward bulkier substrates due to pore size
restrictions. Pd complex immobilized on silica has been re-
ported for intramolecular cyclization of amino-alkynes [15].
Ionic liquids supported on diatomic earth and silver-exchanged
tungstophosphoric acid also have been reported for heteroge-
neous hydroamination of terminal alkynes [16,17].
The catalytic hydroamination of alkenes and alkynes with
amines is the most desirable transformation leading to C–N
bond formation. It offers a direct route to the synthesis of
nitrogen-containing organics, such as alkylated amines, enam-
ines, imines, and N-heterocycles, which are the building blocks
for wide variety of compounds in the area of natural prod-
ucts, pharmaceuticals, dyes, fine chemicals, polymers, and sur-
factants [1]. The 100% atom economy makes hydroamination
more attractive, because no byproducts, such as salts or water,
are produced. Condensation of ketones with amines is the con-
ventional method for preparing enamines and imines, but this
reaction produces water as a byproduct, which makes it less
atom-efficient and limits its use in certain reactions [2].
The earliest report dates back to 1939, when Loritsch and
Vogt used mercuric oxide and boron fluoride in stoichiometric
amounts for hydroamination of alkynes with anilines [3]. Since
then, various approaches using homogeneous catalysts, such as
Mesoporous materials such as MCM-41 and SBA-15 have
advantages over zeolites due to their larger pore size and higher
surface area. The isomorphous substitution of aluminum into
the mesoporous framework of MCM-41 and SBA-15 induces
Brönsted acidity and can act as ion-exchange sites [18]. Meso-
*
Corresponding author. Fax: +91 20 25902633.
E-mail address: sb.halligudi@ncl.res.in (S.B. Halligudi).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.06.007

G.V. Shanbhag et al. / Journal of Catalysis 250 (2007) 274–282
275
porous solids such as AlSBA-15 and AlMCM-41 have an or-
dered porous structure, and the active components tailored in
their nano-ordered spaces will improve their catalytic perfor-
mance. Transition metal ions can be introduced by ion ex-
change in different states and coordination on their surfaces
as well as inside the pores. Metal cations located at different
ion-exchange positions will have distinct stability as well as
reactivity. Over the past few decades, several catalytic studies
have been made using transition metal ion-exchanged porous
solids and clays [19–24]. Recently, CuMCM-41 was reported
for 1-butene isomerization and hydroxylation of phenol [25,26].
Very few reports on CuAlSBA-15 are available in the litera-
ture. In one such report, aluminum has been incorporated in the
framework of SiSBA-15 by a postsynthesis method and Cu2+
ions were exchanged to evaluate the ion-exchange property of
AlSBA-15 [27]. In our earlier studies, we have reported metal-
exchanged montmorillonite clay for intermolecular hydroami-
nation of terminal alkynes with aromatic amines [28–30]. The
present study deals with the synthesis, characterization, and ap-
plications of metal ion-exchanged catalysts such as CuAlSBA-
15 in the synthesis of imines from the hydroamination of
alkynes with aromatic amines.
filtered, washed vigorously with dry ethanol, and dried at room
temperature in air. It was calcined in static air at 550 ◦C for 5 h.
AlMCM-41 was synthesized as we described previously [32].
2.2.3. Synthesis of CuAlSBA-15
A known amount of AlSBA-15 (1 g) was stirred with 0.03 M
cupric acetate solution prepared in water (20 ml) at 80 ◦C for
6 h and then cooled to room temperature. Then the exchanged
AlSBA-15 was filtered, washed repeatedly with distilled water,
and dried in air. This procedure was repeated to ensure max-
imum copper exchange. Then it was dried at 120 ◦C for 12 h
and calcined at 500 ◦C for 5 h (room temperature to 500 ◦C;
ramp rate, 4 ◦C min−1; 4 h at 500 ◦C). This procedure was
applied to all of the other catalyst preparations. Similarly, all
other metal-exchanged catalysts with different supports were
prepared by exchanging with the predetermined stock solu-
tions of the corresponding metal acetate by following the above
procedure except for La and Fe, for which LaCl3 and FeCl3 so-
lutions were used. For a comparative study with the exchanged
catalysts, 2 wt% copper was loaded over SBA-15 (hereinafter
CuO/SBA-15) by the wet impregnation method. SBA-15 was
added to aqueous copper acetate solution, stirred well for 10 h,
evaporated to dryness, and calcined at 500 ◦C. These materials
are designated AlSBA-15-(X) and MAlSBA-15 (X), where M is
the exchanged metal ion and X is the Si/Al ratio of the chemical
stoichiometric composition taken in the postsynthesis mixtures.
2. Experimental
2.1. Materials
Amines and solvents were purchased from Merck India Ltd;
alkynes and montmorillonite K-10 were procured from Aldrich,
USA. Zeolite H-beta was obtained from CPP, NCL, Pune. All
of the chemicals were of research grade and were used after
drying following standard procedures.
2.3. Characterization
2.3.1. Structure and texture characterization
Low-angle X-ray diffraction patterns of mesoporous sam-
ples were collected on a Philips X’ Pert Pro 3040/60 diffrac-
tometer using CuKa radiation (l = 1.5418 Å), a nickel filter,
and an X’celerator as a detector, using the real time multiple
strip (RTMS) detection technique. XRD patterns were collected
in the range of 2θ = 0.5◦–5◦.
2.2. Catalyst preparation
2.2.1. Synthesis of SBA-15
A detailed procedure for synthesizing mesoporous silica
SBA-15 was reported by Zhao et al. [31]. In a typical syn-
thesis, 4 g of amphiphilic triblock copolymer, poly(ethylene
glycol)-block-poly(propylene glycol)-block-poly(ethylene gly-
col) (average molecular weight, 5800), was dispersed in 30 g of
water, and 120 g of 2 M HCl solution was added while stirring.
This was followed by the addition of 8 g of tetraethyl orthosili-
cate to the homogeneous solution with stirring. This gel mixture
was continuously stirred at 40 ◦C for 24 h and finally crystal-
lized in a Teflon-lined autoclave at 100 ◦C for 2 days. After
crystallization, the solid product was filtered, washed with dis-
tilled water, and dried in air at room temperature. The material
was calcined in static air at 550 ◦C for 24 h to decompose the
triblock copolymer and obtain a white powder (SBA-15).
The specific surface areas of the catalysts were measured by
N2 physisorption at liquid nitrogen temperature with an Om-
nisorb 100 CX (Coulter). Samples were dried at 300 ◦C in a
dynamic vacuum for 2 h before the N2 physisorption measure-
ments. The specific surface area was determined using the stan-
dard BET method on the basis of adsorption data. The pore size
distributions were calculated from both adsorption and desorp-
tion branches of the isotherms using the BJH method and the
corrected Kelvin equation [33]. Pore volume values were deter-
mined using the t-plot method of De Boer.
TEM photographs were obtained with a JEOL Model 1200
EX microscope operated at an accelerating voltage of 120 kV.
Samples were prepared by placing droplets of a suspension of
the sample in isopropanol on a polymer microgrid supported on
a Cu grid for TEM measurements. SEM was used to character-
ize the surface morphology with a Leica stereoscan Cambridge
440 Microscope (UK) with a Kevex model EDAX system.
2.2.2. Synthesis of AlSBA-15
SBA-15 was used as the parent material to synthesize
AlSBA-15 via a postsynthesis route similar to that reported
elsewhere [27]. Silica SBA-15 (1 g) was combined with 25 ml
of dry ethanol containing various amounts of AlCl3 with mag-
netic stirring at 80 ◦C for 10 h. The solid material was then
2.3.2. Elemental analysis
The Si/Al ratios of the catalyst samples were determined
by XRF spectroscopy using a Shimadzu XRF-1700 sequen-

276
G.V. Shanbhag et al. / Journal of Catalysis 250 (2007) 274–282
tial XRF spectrometer. The amount of exchanged metal ions
on the supports was determined using an atomic absorption
spectrophotometer (AAS-Hitachi Model Z-8000) except for Pd
(determined by ICP-OES) by following standard procedures.
product, the reaction mixture was cooled and filtered to re-
move the catalyst, and the solvent was removed by distillation.
The mixture was dried in vacuo and purified by column chro-
matography on silica gel (petroleum ether/ethyl acetate, 200/1).
N-(1-phenylethylidene)-2,4-dimethylaniline. Light brown oil:
1H NMR (CDCl3, 200 MHz): δ = 7.93–8.02 (m, 2H), 7.43–
7.57 (m, 3H), 7.02 (d, 1H), 6.86 (m, 1H), 6.56 (m, 1H), 2.31
(s, 3H), 2.16 (s, 3H), 2.06 (s, 3H). FTIR (neat, cm−1): 3020,
2951, 1639, 1209, 847, 764, 631. GCMS m/z (relative inten-
sity): 223 (54) [M+], 208 (100), 193 (35), 146 (12), 103 (35),
77 (67), 51 (25).
Hydration of alkyne with water (with the addition of H–OH
over the triple bond and subsequent rearrangement giving a ke-
tone) is the side reaction that may occur in the presence of an
acid catalyst at a faster rate than hydroamination. Hence the
chemicals and the reaction setup must be free of water to avoid
a hydration reaction.
2.3.3. NMR measurements
Magic-angle spinning (MAS) NMR spectra were recorded
on a BRUKER DSX300 spectrometer at 7.05 T (resonance fre-
quency, 59.63 MHz; 29Si: rotor speed, 4000 Hz; number of
scans, 5275 and 78.19 MHz; 27Al: rotor speed, 6000 Hz; num-
ber of scans, 2800). External Al(H2O)36+ was used as a refer-
ence.
2.3.4. FTIR pyridine adsorption measurements
The nature of the acid sites (Brönsted and Lewis) of the
catalyst samples with different loadings were characterized by
in situ Fourier transform infrared (FTIR) spectroscopy with
chemisorbed pyridine in drift mode on an FTIR-8300 Shimadzu
SSU-8000 instrument with 4 cm−1 resolution and averaged
over 500 scans. These studies were performed by heating pre-
calcined powder samples in situ from room temperature to
400 ◦C at a heating rate of 5 ◦C min−1 in a flowing stream
(40 ml min−1) of pure N2. The samples were kept at 400 ◦C
for 3 h and then cooled to 100 ◦C, after which pyridine vapors
(20 µl) were introduced under N2 flow. The IR spectra were
recorded with 4 cm−1 resolution and 500 scans after the sam-
ple was degassed at 200 ◦C for 30 min.
3. Results and discussion
3.1. Physicochemical characterization
3.1.1. X-ray diffraction
Well-defined XRD patterns were obtained for all samples
and were similar to those recorded for SBA-15 materials as de-
scribed by Zhao et al. [31]. XRD patterns of calcined SBA-15,
Al-SBA-15 (10), and CuAlSBA-15 (10) materials with differ-
ent Si/Al ratios (see Figs. 1a, 1b, and 1c, respectively) consisted
of three well-resolved peaks in the 2θ range of 0.8◦–1.8◦ cor-
responding to the (100), (110), and (200) reflections associated
with p6mm hexagonal symmetry in the materials. It is note-
worthy that no obvious change was seen after the isomorphous
substitution of Al into the framework of siliceous SBA-15 by
the postsynthesis method, implying that the hexagonal meso-
porous structure was well preserved. The XRD profile of the
samples demonstrated no other phases or amorphous matter.
In addition, the XRD pattern of CuMCM-41 (15) (Fig. 1d)
shows the appearance of the (100), (110), and (200) reflections
in lower 2θ region, demonstrating the well-ordered nature of
phases in the CuAlMCM-41.
2.3.5. H2-TPR
The reducibility of the calcined copper catalysts was mea-
sured by TPR method using a Micromeritics AutoChem 2910
instrument. About 300 mg of the catalyst was mounted in
a quartz tube and calcined in argon flow at 500 ◦C for 1 h
(temperature-programmed rate, 10 ◦C min−1) with the aim of
removing the substances that were physisorbed. It was then
cooled to ambient temperature in argon before the reduction
by a mixing gas of hydrogen and helium (5% H2 in vol-
ume percentage). In the H2-TPR analysis, the heating rate was
10 ◦C min−1 and the gas flow was 10 ml min−1. The cold trap
consisted of liquid nitrogen and 2-propanol. The hydrogen con-
sumed during the reduction was determined by a thermal con-
ductivity detector.
3.1.2. Nitrogen sorption studies
Textural properties of porous solids are typically obtained
from low-temperature (−196 ◦C) nitrogen adsorption isotherms,
which allow computation of the specific surface area, spe-
cific pore volume, and mesopore size distribution. The nitro-
gen adsorption–desorption isotherms of AlSBA-15 (10) and
CuSBA-15 samples of different Si/Al ratios are shown in Fig. 2,
and the textural properties of all samples are presented in Ta-
ble 1. All isotherms were of type IV, as defined by IUPAC, and
exhibited an H1-type broad hysteresis loop typical of large-
pore mesoporous solids. As the relative pressure increased
(p/p0 > 0.6), all isotherms exhibited a sharp step charac-
teristic of capillary condensation of nitrogen within uniform
mesopores, where the p/p0 position of the inflection point is
correlated with the diameter of the mesopore. The pore size
2.3.6. Catalyst testing
In a typical reaction, the reaction mixture consisting of
0.59 g of phenyl acetylene (PhAc), 1.41 g of 2,4-xylidine, 2 ml
of toluene, and 0.1 g of catalyst (activated at 300 ◦C for 4 h
before use) was placed in a 50-ml round-bottomed flask in an
oil bath and refluxed at 110 ◦C under N2 atmosphere. Sam-
ples were withdrawn at regular intervals and analyzed using
a Shimadzu 14B gas chromatograph equipped with a flame
ionization detector using a capillary column. Reactions at tem-
peratures above the boiling point of toluene or substrate were
performed in a 50-ml Parr autoclave. To determine the general
applicability of the catalyst, the reactions were carried out with
different alkynes and aromatic amines. Conversion of PhAc
was expressed in mol%. To determine the authenticity of the

G.V. Shanbhag et al. / Journal of Catalysis 250 (2007) 274–282
277
Table 1
Structural characteristics of different catalysts and their catalytic activities
a
b
Catalyst
Si/Al (XRF)
Surface area BET
Pore volume
(cm /g)
Average pore
diameter
Copper conc.
(mmol/g)
PhAc convn.
(mol%)
TOF
2
−1
3
(m /g
)
SBA-15
∞
17
26
36
43
15
15
–
–
–
–
–
–
–
–
–
–
–
745
685
730
720
620
962
530
230
664
685
603
652
685
820
208
480
–
1.03
1.31
1.16
0.99
0.98
0.80
0.28
–
1.09
0.95
0.98
1.1
0.95
0.72
–
66.6
76.7
66.3
63.5
63.4
33.6
–
–
–
–
–
–
–
–
–
0.21
0.15
0.11
0.09
0.31
0.26
0.22
0.25
–
1.2
2.0
1.8
1.5
1.3
2.1
1.5
1.2
48
37
27
21
8
–
–
–
–
–
–
–
–
22
24
25
23
3
21
9
7
AlSBA-15 (10)
AlSBA-15 (20)
AlSBA-15 (30)
AlSBA-15 (40)
AlMCM-41 (15)
H-beta
Clay
–
CuAlSBA-15 (10)
CuAlSBA-15 (20)
CuAlSBA-15 (30)
CuAlSBA-15 (40)
CuO/SBA-15
CuAlMCM-41 (15)
Cu-clay
67.9
65.8
67.7
67.6
66.0
33.1
–
–
–
–
52
28
31
37
0
Cu-beta (15)
–
–
–
Cu(CF SO )
17
–
3
3 2
CuO
–
–
a
◦
Conditions: temperature = 110 C, 2,4-xylidine to PhAc mole ratio = 2, catalyst wt = 0.15 g, total reactants wt = 2 g, time = 6 h.
TOF = mole of PhAc converted per mole copper per hour.
b
Fig. 1. Low angle XRD patterns of (a) SBA-15, (b) AlSBA-15, (c) CuAlSBA-15
(10), (d) CuAlMCM-41.
Fig. 2. N adsorption and desorption isotherms of nitrogen on samples: (a) Al-
2
SBA-15 (10), (b) CuAlSBA-15 (10), (c) CuAlSBA-15 (30), (d) CuAlSBA-15
(40); Inset picture: pore size distribution of CuAlSBA-15 (10).
distribution was calculated from the Kelvin equation and is pre-
sented as a BJH plot (Fig. 2, inset) exhibiting a narrow pore size
distribution with an average mesopore size of 91 Å and a high
surface area ABJH of 685 m2/g. The N2 adsorption isotherms
of all CuAlSBA-15 materials were quite similar to the isotherm
of AlSBA-15. The overall N2 adsorption decreased depending
on the aluminum loading, although no particular trend was ob-
served. The calculated BET specific surface areas, total pore
volume, and average mesopore parameters based on BJH plots
are listed in Table 1. Low alumination of SBA-15 did not affect
the original pore structure of the parent SBA-15, but the surface
area decreased slightly with increased aluminum loadings. Fur-
ther modification of AlSBA-15 materials with copper exchange
and calcination reduces the surface areas slightly, as expected.
CuAlMCM-41 (15) had a higher surface area but lower aver-
age pore diameter than CuAlSBA-15 (10) (820 vs 664 m2/g
and 33.1 vs 67.9 Å, respectively. The surface areas of Cu-beta
(480 m2/g) and Cu-clay (207 m2/g) were lower than that of
CuAlSBA-15.
3.1.3. Microscopic analysis
SEM images of CuAlSBA-15 (Fig. 4) reveal that the micro-
morphology remained the same, with rope-like structures, even
after modification of SBA-15 with postsynthesis Al incorpora-
tion and Cu2+ exchange. TEM measurements were carried out
to study the morphology of the AlSBA-15 and CuAlSBA-15
catalysts (Fig. 3). TEM images of these catalysts demonstrated
retention of the periodic structure of parent SBA-15 precursor
(not shown), confirming that the hexagonally arranged meso-
pores of SBA-15 were retained after modification with alu-
minum and copper.

278
G.V. Shanbhag et al. / Journal of Catalysis 250 (2007) 274–282
Fig. 3. SEM photographs of (a) and (b) CuAlSBA-15.
27
Fig. 5. Al MAS NMR profile of (a) AlSBA-15 (10), (b) CuAlSBA-15 (10),
(c) ZnAlSBA-15 (10). T = tetrahedral Al sites and O = octahedral Al sites.
Fig. 4. TEM photographs of (a) and (b) AlSBA-15 (10), (c) and (d) CuAlSBA-
15.
3.1.4. Nuclear magnetic resonance
Solid-state 27Al MAS NMR (Fig. 5) was used to investigate
the local environment of aluminum species. The sharp peak ob-
served at 53 ppm, corresponding to tetrahedrally coordinated
framework aluminum in which Al is bound covalently to four
Si atoms via oxygen bridges, indicates that most of the alu-
minum was incorporated into the framework. The peak at 0 ppm
can be attributed to distorted or octahedrally coordinated alu-
minum species in extra-framework positions. In addition, the
presence of distorted tetrahedral or five-coordinated aluminum
cannot be discarded due to the low-intensity signals observed
at around 30 ppm [34]. Compared with the ZnAlSBA-15 (10)
sample, CuAlSBA-15 (10) had a larger line width and lower-
intensity 27Al NMR signal at 53 ppm, indicating the presence
of paramagnetic Cu2+ ions near the aluminum sites in AlSBA-
15. The intensity of octahedral Al signal at 0 ppm was reduced
after metal ion exchange, possibly due to the removal of extra-
framework aluminum during the ion exchange of AlSBA-15
with the metal acetate solution. The material retained its struc-
ture after the metal ion exchange and calcination.
29
Fig. 6. Si MAS NMR profile of (a) SBA-15, (b) AlSBA-15 (10), (c) CuAl-
SBA-15.
the shoulders at −90, −107, and −110 ppm are attributed
to Si(OSi)2(OH)2, Si(3Si, 1Al), and Si(OSi)4 structural units.
There are no significant differences between 29Si MAS NMR
spectra of CuAlSBA-15, AlSBA-15, and SBA15.
3.1.5. Acidity measurements
Pyridine adsorption in situ FTIR spectroscopy was per-
formed for AlSBA-15 and Cu-incorporated AlSBA-15 catalysts
and the spectra recorded after outgassing at 200 ◦C are repre-
sented in Fig. 7. Adsorption of pyridine on the parent AlSBA-15
resulted in absorption bands at 1542 and 1451 cm−1, which can
be assigned to pyridine molecules interacting with Brönsted (B)
and Lewis (L) acid sites, respectively. Incorporation of copper
led to an increase in Lewis acidity. CuAlSBA-15 (10) catalysts
prepared by exchanging with different copper acetate concen-
29Si MAS NMR spectra of each sample (Fig. 6) demon-
strate a broad signal at −100 ppm and shoulders at −90, −107,
and −110 ppm. According to Kolodziejski et al. [35], the
main signal at −100 ppm is due to Si(OSi)3OH sites, whereas

G.V. Shanbhag et al. / Journal of Catalysis 250 (2007) 274–282
279
Table 2
Effect of copper acetate concentrations on Cu exchange and their acidities
2+
Catalyst
Copper acetate
solution (mol/L)
Total exchange
time (h)
Copper concentration
(mmol/g)
Brönsted acidity
Lewis acidity
B/L
I /I
1542 1451
I
I
1542
1451
AlSBA-15 (10)
–
–
–
2.23
1.68
1.05
1.14
1.56
0.79
2.44
3.23
5.01
4.56
3.47
3.91
0.91
0.52
0.21
0.25
0.45
0.20
CuAlSBA-15 (10)
CuAlSBA-15 (10)
CuAlSBA-15 (10)
CuAlSBA-15 (10)
CuSBA-15 (10)
0.01
0.03
0.06
0.1
18
18
18
18
40
0.14
0.21
0.26
0.33
0.40
0.03
Fig. 8. H -TPR profiles of (a) CuO/SBA-15, (b) CuAlSBA-15 (40), (c) Cu-
2
AlSBA-15 (30), (d) CuAlSBA-15 (20), (e) CuAlSBA-15 (10).
Fig. 7. FT-IR pyridine adsorption of (a) AlSBA-15 (10) and (b) CuAlSBA-15
(10) catalysts with Brönsted (B) and Lewis acid (L) peaks.
copper–oxygen phase (not necessarily CuO) (reaction (1)).
The peaks at >210 and >330 ◦C indicate the reduction of
isolated Cu2+ cations in reactions (2) and (3), respectively.
The results indicate that the copper ions are located mostly
in extra-framework positions in the exchanged form. The dif-
ferent shoulders observed for these solids indicate that they
contain Cu2+ species present in small clusters in different en-
vironments. It is also seen that as the Si/Al decreased, the
temperature at the peak maximum of the first peak shifted
to higher temperatures. The CuO/SBA-15 catalyst (Fig. 8a)
showed a weak TPR profile, suggesting that it contains very-
low-exchanged copper sites and that all of the isolated Cu2+
sites observed for CuAlSBA-15 catalysts are due to the ex-
change with the Brönsted acid sites in AlSBA-15 samples.
trations and the B/L ratios obtained by intensity measurements
are listed in Table 2. It is seen that the B/L ratio increased as
copper concentration increased in mesoporous support up to
0.21 mmol/g of Cu and then decreased thereafter. This suggests
that optimum copper exchange occurs at lower concentrations
of copper acetate (0.03 M). For overexchanged catalysts, the
extra copper present in the catalysts is due to the formation of
CuO clusters inside the pores and the catalyst surface [23].
3.1.6. TPR
H2-TPR studies were performed for the identification of
copper species in AlSBA-15 materials modified with copper.
According to the literature [26,36,37] the following reduction
processes can be considered:
3.2. Hydroamination reaction
CuO + H2 → Cu0 + H2O,
Cu2+ + 1/2H2 → Cu+ + H+,
Cu+ + 1/2H2 → Cu0 + H+.
Reactions (1) and (2) occur at lower temperatures than reac-
tion (3). TPR profiles of CuAlSBA-15 catalysts with different
Si/Al ratios and CuO/SBA-15 are depicted in Fig. 8. The TPR
graph of pure CuO shows a peak maximum at 207 ◦C (in-
set). The Cu2+-exchanged AlSBA-15 samples show different
peaks at 160–650 ◦C. The low-temperature peaks below 210 ◦C
for copper-containing samples may be due to reduction of the
(1)
(2)
(3)
The hydroamination of phenylacetylene with 2,4-xylidine
to give an enamine as an intermediate (not observed), which
rearranges itself to form N-(1-phenylethylidene)-2,4-dimethyl-
aniline (Scheme 1), was used as a model reaction for testing
the catalytic activities of various catalysts. The reaction was
highly selective, with only the preferred Markovnikov addition
product observed. The different transition metal ions, such as
Cu2+, Zn2+, Pd2+, Co2+, Mn2+, La3+, and Fe3+, were ex-
changed with AlSBA-15 (10), and the catalytic activities in the
hydroamination of PhAc with 2,4-xylidine were compared (Ta-
ble 3). CuAlSBA-15 showed the highest activity, with 44 mol%

280
G.V. Shanbhag et al. / Journal of Catalysis 250 (2007) 274–282
Scheme 1. Hydroamination of phenylacetylene with 2,4-xylidine.
Table 3
n+
Catalytic activities of M AlSBA-15 (10) in hydroamination of PhAc with
2,4-xylidine
n+
2+
2+
2+
2+
2+
3+
3+
+
M
Cu
Zn
31
Pd
10
Co
5
Mn
4
La
7
Fe
4
H
4
PhAc conversion 44
(mol%)
◦
Conditions: temperature = 110 C, 2,4-xylidine to PhAc mole ratio = 2, catal-
yst wt = 0.1 g, total reactants wt = 2 g, time = 8 h.
conversion of PhAc after 4 h of reaction, with the others follow-
ing in the decreasing order of CuAlSBA-15 > ZnAlSBA-15 >
PdAlSBA-15 > LaAlSBA-15 > CoAlSBA-15 > MnAlSBA-
15 > FeAlSBA-15.
As reported in the literature, the catalysts with Cu2+ and
Zn2+ sites are highly active for hydroamination of alkynes by
amines compared with other metal ions due to their moderately
hard Lewis acidic properties [12,28]. Cu2+ ions exchanged with
different supports, such as AlSBA-15, AlMCM-41, H-beta, and
montmorillonite K-10, and the catalytic activities in the hy-
droamination of PhAc were compared (Table 1). The unex-
changed catalysts (supports) showed very low catalytic activ-
ity. The turnover frequencies (TOF = mol of PhAc converted
per mol of Cu2+ per hour) were the highest with CuAlSBA-
15 (10) (TOF = 22) and CuAlMCM-41 (15) (TOF = 21) in
the hydroamination of PhAc by 2,4-xylidine compared with
clay- and zeolite-supported catalysts. The higher activities of
the mesoporous-supported catalysts can be attributed to their
higher surface areas and larger pore sizes.
Copper triflate was tested for its catalytic activity as the cor-
responding homogeneous counterpart by keeping the amount
of copper the same as for CuAlSBA-15 (10). This catalyst was
less active than CuAlSBA-15 (10) by ∼10%. This finding is
contrary to the expectation that porous catalysts are often as-
sumed to be less active than the corresponding homogeneous
catalysts, because diffusion within the pores may slow the over-
all reaction. The higher activity of porous catalysts may be due
to the presence of some Brönsted acid sites left unexchanged,
which may act as promoters in hydroamination, which is absent
in copper salts [38]. In contrast to the Cu2+ ion-exchanged ma-
terials, CuO showed no catalytic activity in the hydroamination
reaction, which may be attributed to its basic nature. However,
CuO/SBA-15 showed some activity (TOF = 3), possibly due to
some exchangeable protons present in SBA-15. H2-TPR results
showed that the CuAlSBA-15 catalysts contained an insignifi-
cant amount of CuO, present mostly in the Cu2+ state, the active
site for the hydroamination reaction in the present study.
Among the CuAlSBA-15 catalysts with different Si/Al ra-
tios (as given in parentheses), the activity increased with de-
2+
Fig. 9. Influence of Cu concentration on catalytic activity conditions: catal-
◦
yst = CuAlSBA-15 (10), temperature = 110 C, 2,4-xylidine to PhAc mole
ratio = 2, catalyst wt = 0.1 g, total reactants wt = 2 g.
creasing Si/Al ratio (Table 1). Brönsted acidity increased with
decreasing Si/Al ratio, which in turn increased the ion-exchange
capacity and subsequently enhanced the number of exchanged
Cu2+ ions. To determine the effect of Cu2+ concentration on
catalytic activity, catalysts with different Cu2+ concentrations
(0.14–0.40 mmol/g) were tested in the hydroamination of PhAc
with 2,4-xylidine. The results, shown in Fig. 9, indicate that
conversion of PhAc increased at copper concentrations up to
0.21 mmol/g and decreased at higher concentrations. These
findings are in agreement with the acidity data given in Table 2.
Brönsted acidity decreased with increasing Cu2+ concentration
up to 0.21 mmol/g and then decreased with further increases.
The nature of the copper species depends on the concentra-
tion of copper acetate solution as well as the time of exchange.
Thus in overexchanged catalysts, copper oxide clusters may be
formed inside the channels as well as on the surface, affecting
the activity.
CuAlSBA-15 (10) was used to study the effect of tempera-
ture (80–120 ◦C ranges) in the hydroamination of phenylacety-
lene with 2,4-xylidine. The results indicate that temperature
has a remarkable effect on the conversion of PhAc (Fig. 10a).
The conversion was only 10 mol% at 90 ◦C and increased to
52 mol% at 110 ◦C after 12 h. Following the initial rate ap-
proach, the graph of ln(rate) versus 1/T was plotted and from
the slope of the straight line (slope = −Ea/R), the activation
energy calculated was found to be 65 kJ/mol.
The effect of substrate concentrations on PhAc conversion
was studied at 100 ◦C by keeping the total weight of the reaction

G.V. Shanbhag et al. / Journal of Catalysis 250 (2007) 274–282
281
Table 4
Hydroamination of different alkynes and amines catalyzed by CuAlSBA-15
a
b
Entry
Amine
Conversion (mol%)
Entry
Alkynes
Conversion (mol%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Aniline
o-Toluidine
p-Toluidine
2,4-Xylidine
61
38
52
48
37
35
38
45
1
2
3
4
5
6
7
8
1-Hexyne
1-Heptyne
3-Hexyne
Phenylacetylene
Diphenylacetylene
4-Ethynyltouene
4-Ethynylanisole
1-Ethynylnaphthalene
15
31
NR
61
NR
52
55
46
2,4,6-Trimethylaniline
2-Isopropylaniline
4-Isopropylaniline
4-Methoxyaniline
2-Chloroaniline
4-Bromoaniline
4-Nitroaniline
1-Naphthylamine
N-Methylaniline
Cyclohexylamine
15
21
No reaction
58
No reaction
No reaction
a
◦
Hydroamination of PhAc with amines. Conditions: catalyst = CuSBA-15 (10), amine to PhAc mole ratio = 2, toluene = 2 ml, temperature = 110 C, catalyst
wt = 0.15 g, total reactants wt = 2 g, time = 6 h, 100% selectivity for Markovnikov product.
b
Hydroamination of alkynes with 2,4-xylidine. Conditions: catalyst = CuSBA-15 (10), 2,4-xylidine to alkyne mole ratio = 2, toluene = 2 ml, temperature =
◦
110 C, catalyst wt = 0.15 g, total reactants wt = 2 g, time = 6 h, NR = no reaction, 100% selectivity for Markovnikov product.
To determine whether any active species of the catalyst are
leaching into the reaction mixture, hydroamination of pheny-
lacetylene with 2,4-xylidine to give an imine was carried out
at optimized conditions with CuAlSBA-15 (10) catalyst. After
2 h (20 mol% conversion), the reaction was stopped and fil-
tered hot, and then the reaction was continued with the filtrate
under identical conditions. Conversion remained the same even
after 20 h, indicating no leaching of any active species of the
catalyst. The recyclability of CuAlSBA-15 catalyst was tested
in the hydroamination of phenyl acetylene and 2,4-xylidine by
conducting five runs (∼80 mol% conversion after 20 h) us-
ing optimized reaction conditions. After each run, the catalyst
(brown in color) was repeatedly washed with toluene, dried at
120 ◦C for 2 h, and calcined at 500 ◦C in air for 4 h (turned
white). It was then used in the hydroamination reaction with a
fresh reaction mixture, and it was found that the conversion of
PhAc was practically the same in all the five cycles.
Fig. 10. (a) Effect of reaction temperature. Conditions: catalyst = CuAlSBA-15
(10), 2,4-xylidine to PhAc mole ratio = 2, catalyst wt = 0.1 g, total reac-
tants wt = 2 g. (b) Effect of 2,4-xylidine to PhAc mole ratio. Conditions:
To check the general applicability of the catalyst, two sets
of reactions were carried out and the conversions were ex-
pressed as the conversion of alkyne. In the first set, different
amines were reacted with phenyl acetylene (Table 4). The na-
ture of substituents on the aromatic ring of aniline derivatives
has significant effect over the reactivity. The electron-donating
groups at the -ortho and -para positions gave higher conver-
sions to yield the corresponding imines, which are comparable
to that of aniline reaction. The sterically hindered amines, such
as 2,4,6-trimethylaniline (37%) and 2-isopropylaniline (35%)
and large-molecule 1-naphthylamine (58 mol%), gave higher
conversions, which indicates that the bulky amines can eas-
ily pass through the mesopores of CuAlSBA-15. The amines
with electron-withdrawing substituents, such as 2-chloroaniline
(15%) and 4-bromoaniline (21%), were least reactive. No reac-
tivity was observed when phenylacetylene was reacted with a
secondary aromatic amine (N-methylaniline) and an aliphatic
amine (cyclohexylamine), suggesting that CuAlSBA-15 is suit-
able only for aromatic primary amines for the hydroamination
of alkynes.
◦
catalyst = CuAlSBA-15 (10), temperature = 100 C, catalyst wt = 0.1 g, total
reactants wt = 2 g. (c) Effect of catalyst concentration. Conditions: catalyst =
◦
CuAlSBA-15 (10), temperature = 100 C, 2,4-xylidine to PhAc mole ratio = 2,
total reactants wt = 2 g.
mixture constant (Fig. 10b). As the molar ratio of 2,4-xylidine
to PhAc was increased from 1 to 2, the PhAc conversion in-
creased from 11 to 16% after 4 h. But further increases in the
molar ratio of 2,4-xylidine to PhAc decreased the PhAc conver-
sion. Higher PhAc conversions were observed at a 2,4-xylidine
to PhAc molar ratio of 2.
The effect of catalyst concentration on PhAc conversion
studied at 100 ◦C with a molar ratio of 2 (Fig. 10c) showed
increasing PhAc conversion with increasing catalyst concentra-
tion. A linear increase in conversion occurred for up to 6 h,
and the reaction slowed with only marginal increases at 6–12 h.
At optimized reaction conditions (110 ◦C, 2,4-xylidine/PhAc 2,
catalyst weight 0.15 g and total reactant weight 2 g with 2 ml of
toluene as a solvent), PhAc conversion was 88 mol% after 20 h
reaction.

282
G.V. Shanbhag et al. / Journal of Catalysis 250 (2007) 274–282
In the second set, the reactions were carried out by reacting
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different alkynes with 2,4-xylidine (Table 4). Aromatic alkynes
were more reactive than aliphatic alkynes, whereas terminal
alkynes gave the higher yields. Internal alkynes (entries 3 and 5)
did not undergo the reaction, indicating that this catalyst is suit-
able only for the hydroamination of terminal alkynes. The ac-
tivated alkynes with electron-donating substituents, CH3– and
CH3O–, gave better yields (entries 6 and 7), whereas the larger
molecules, like 1-ethynylnaphthalene, showed high conversion
(46%).
4. Conclusion
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Cu2+ ion-exchanged AlSBA-15 catalysts were effectively
used in intermolecular hydroamination of phenylacetylene with
2,4-xylidine. The state and behavior of copper in AlSBA-15
materials depend strongly on the method used for Cu inclu-
sion. The reaction was catalyzed by Cu2+ ions obtained by
exchanging with protons of AlSBA-15, not by the unexchanged
copper present as its oxide. AlSBA-15 and AlMCM-41 were
the best supports compared with zeolite beta and clay, whereas
Cu- and Zn-exchanged AlSBA-15 catalysts showed higher ac-
tivity compared with other metals. The alkynes and amines with
electron-donating substituents showed higher activity, whereas
the presence of electron-withdrawing groups on the aromatic
ring retards the reaction. The catalyst did not catalyze hydroam-
ination reactions involving internal alkynes, aromatic secondary
amines, and aliphatic amines. These findings indicate that the
CuAlSBA-15 catalyst is a versatile and reusable catalyst for in-
termolecular hydroamination of terminal alkynes with aromatic
primary amines.
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Acknowledgments
[30] G.V. Shanbhag, S.M. Kumbar, T. Joseph, S.B. Halligudi, Tetrahedron.
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Support was provided by CSIR, New Delhi (a Senior Re-
search Fellowship to G.V.S.) and DST, New Delhi.
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