Ordered hexagonal mesoporous aluminosilicates and their application in ligand-free synthesis of secondary amines

Article abstract of DOI:10.1002/cctc.201402916

A new facile synthesis of ordered mesoporous aluminosilicates has been developed and applied for the efficient catalytic synthesis of N-benzyl secondary amines under ligand and base-free conditions. The MAS(38) catalyst with a well-ordered mesoporous structure and strong acidic sites was used for acid-catalyzed organic transformations and its operational simplicity and ease of its isolation procedure make it an attractive alternative to current methodologies. Get hexed! Mesoporous aluminosilicates were successfully synthesized and applied as ligand, base-free heterogeneous catalysts for C-N bond formation between benzyl alcohol and aniline for various N-benzyl secondary amines with excellent activity and selectivity.

Full text of DOI:10.1002/cctc.201402916

DOI: 10.1002/cctc.201402916
Communications
Ordered Hexagonal Mesoporous Aluminosilicates and
their Application in Ligand-Free Synthesis of Secondary
Amines
Pavuluri Srinivasu,*[a] Dupati Venkanna,[a] Mannepalli Lakshmi Kantam,[a] Jing Tang,[b]
Suresh K. Bhargava,[c] Ali Aldalbahi,[d] Kevin C.-W. Wu,*[e] and Yusuke Yamauchi*[b, d]
A new facile synthesis of ordered mesoporous aluminosilicates
has been developed and applied for the efficient catalytic syn-
thesis of N-benzyl secondary amines under ligand and base-
free conditions. The MAS(38) catalyst with a well-ordered
mesoporous structure and strong acidic sites was used for
acid-catalyzed organic transformations and its operational sim-
plicity and ease of its isolation procedure make it an attractive
alternative to current methodologies.
transition metal complexes have been used as homogeneous
catalysts for synthesizing secondary amines in the presence of
base additives, which retard the commercialization of the pro-
cess because of high cost and environmental concerns.[5–7]
The use of heterogeneous catalysts[8–12] offers several advan-
tages over homogeneous catalysts, including ease of separat-
ing reactants, minimal trace metal in the products, ease of
handling, process control, and reusability of catalysts. These
advantages can thus improve reaction processes. Therefore,
developing an environmentally friendly and recyclable hetero-
geneous catalytic system for synthesizing secondary amines is
of considerable interest. Heterogeneous catalysts such as Fe/
amino acids, Fe3O4, and Fe3O4/tBuOÀ have been used for the
synthesis of secondary amines,[12,13] however, these systems
suffer from complicated synthesis procedures and low activity
compared with precious metal complexes.
N-alkyl secondary amine derivatives linked to herbicides and
pharmaceuticals have been demonstrated as numerous biolog-
ically active compounds for drug discovery.[1] N-benzyl secon-
dary amines, in particular, have attracted considerable atten-
tion from both biological and organic chemists. This is because
they appear as carboxamide derivatives, which are the most
prevalent structural moieties in medicinal chemistry.[2] Howev-
er, despite widespread interest, the most common strategies
for the production of secondary amines such as electrophilic
alkylation, reductive alkylation, and amination of aryl halides,
often exhibit limitations such as low selectivity toward desired
secondary amines, harsh reaction conditions, and stoichiomet-
ric amounts of wasteful salts.[3,4] In addition, various expensive
[Ru3(CO)7] cluster, [Ir(COD)Cl] dimer, rhodium, and platinum
In recent years, mesoporous materials have demonstrated
their potential in solar cells, fuel cells, and drug delivery sys-
tems.[14–21] In addition, mesoporous materials have been in-
creasingly applied as heterogeneous catalysts in various organ-
ic transformations because of their unique structural and tex-
tural properties such as controllable pore size, uniform pore-
size distribution, high thermal stability, and ultrahigh specific
surface area.[22,23] Among various heterogeneous catalysts, alu-
minosilicates have been considered useful solid catalysts be-
cause of their usage as effective catalysts in aldol, Friedel–
Crafts, and Diels–Alder reactions.[24,25] However, based on our
knowledge, no studies have synthesized secondary amines by
using mesoporous aluminosilicate catalysts.
[a] Dr. P. Srinivasu, Dr. D. Venkanna, Dr. M. L. Kantam
Inorganic and Physical Chemistry Division
CSIR-Indian Institute of Chemical Technology
Hyderabad-500007 (India)
E-mail: pavuluri.srini@iict.res.in
We present a facile synthesis of mesoporous aluminosilicate
materials with highly ordered two-dimensional (2D) hexagonal
structures (denoted as MAS) through direct hydrothermal syn-
thesis. The synthesized MAS were further used as solid cata-
lysts for efficiently catalyzing the synthesis of secondary
amines through CÀN bond formation between benzyl alcohol
and aniline (Scheme 1). The MAS catalyst showed an excellent
yield of secondary amines with high reusability of the catalyst
for several cycles.
[b] J. Tang, Prof. Y. Yamauchi
World Premier International (WPI) Research Center
for Materials Nanoarchitectonics
National Institute for Materials Science (NIMS)
1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan)
E-mail: Yamauchi.Yusuke@nims.go.jp
[c] Prof. S. K. Bhargava
Advanced Materials and Industrial Chemistry Group
School of Applied Sciences, RMIT University
Melbourne-3001 (Australia)
[d] Prof. A. Aldalbahi, Prof. Y. Yamauchi
The MAS samples were prepared using a triblock copolymer
Department of Chemistry, College of Science
King Saud University, Riyadh 11451 (Saudi Arabia)
(Pluronic P123, EO20PO70EO20, molecular weight=5800 gmolÀ1
)
as a structure-directing agent.[26,27] Tetraethoxysilane (TEOS)
and aluminum isopropoxide (AlIP) were used as silicon and
aluminum sources, respectively. Typically, P123 (2.0 g) was dis-
solved in a hydrochloric acid solution (72.0 g) and stirred at
room temperature for 10 h. The AlIP in the hydrochloric acid
solution and TEOS were added to the mixture. The resulting
[e] Prof. K. C.-W. Wu
Department of Chemical Engineering
National Taiwan University
No. 1, Sec. 4, Roosevelt Road, Taipei 10617 (Taiwan)
E-mail: kevinwu@ntu.edu.tw
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402916.
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Scheme 1. Schematic representation of MAS-catalyzed synthesis of second-
ary amine in this study.
mixture was stirred at 358C for 20 h. Subsequently, the reac-
tion mixture was heated for 48 h at 1008C under static condi-
tions for the hydrothermal treatment. The solid product was fil-
tered, washed, and then dried at 808C. The obtained product
after calcination at 5508C is denoted as MAS(x), where x repre-
sents the Si/Al mole ratio. The Si/Al mole ratio was determined
from an inductively coupled plasma analysis.
Figure 1. a) Nitrogen adsorption-desorption isotherms of MAS materials with
different Si/Al mole ratios, MAS(93), MAS(54), and MAS(38), b) the corre-
sponding pore size distributions.
The structural characteristics of the MAS materials exhibited
typical X-ray diffraction (XRD) patterns, which were indexed to
the (10), (11), and (20) reflections of the hexagonal 2D pore or-
dering in the p6mm symmetry (data not shown). The nitrogen
sorption measurement results of the MAS materials indicated
type-IV curves with a sharp capillary condensation step and
H1-type hysteresis loop, which indicates large uniform pores
(Figure 1a). The sharp capillary condensation step of the iso-
therm indicates the nitrogen condensation within mesopores,
and the structural ordering of the mesopores remains well
even after the incorporation of Al. Table 1 summarizes the tex-
mesoporous network, indicating that the mesopore diameter
significantly increases from 8.3 to 9.2 nm (Figure 1b). The plau-
sible reason for the increase in the mesopore diameter with
the Al-content increase is that the [AlO4] tetrahedron is slightly
larger than the [SiO4] tetrahedron because the AlÀO bond is
longer compared with that of the SiÀO bond.[28] Furthermore,
the specific surface area and pore volume decreased from
826 m2 gÀ1 and 1.2 cm3 gÀ1, respectively, for MAS(93) to
672 m2 gÀ1 and 0.89 cm3 gÀ1 for MAS(38), as shown in Table 1.
To evaluate the strength and type of acidic sites on the MAS
materials, IR spectra of adsorbed pyridine were studied (data
not shown). The Fourier transform infrared spectra of the
MAS(38) material show the coordinatively bounded pyridine
band at 1445 and 1610 cmÀ1 that is characteristic of pyridine
adsorbed on Lewis acid sites. In addition, the band at
1540 cmÀ1, which is characteristic of pyridine adsorbed on
Brønsted acid sites, exists in both MAS(54) and MAS(38) sam-
ples.[29] The presented results indicate that the MAS(38) materi-
al contains both Lewis and Brønsted acidic sites, whereas the
other MAS materials (i.e., MAS(54) and MAS(93)) contain only
Brønsted acidic sites. Notably, the acidity of the MAS materials
can be tuned by adjusting the Al content in the mesoporous
network.
Table 1. Summary of d10-spacing, surface area, pore volume and pore
size of MAS materials with different Si/Al mole ratios (MAS(93), MAS(54),
and MAS(38) materials).
Sample
d10-spacing
[nm]
Pore size
[nm]
Surface area
Pore volume
[cm3 gÀ1
]
[m1 gÀ1
]
MAS(38)
MAS(54)
MAS(93)
10.9
10.5
9.2
9.2
8.8
8.3
672
734
826
0.89
0.96
1.20
tural properties such as the d10 value, mesopore diameter, BET
specific surface area, and total pore volume of the MAS materi-
als. The d10 value calculated from the XRD data of mesoporous
silica (no Al incorporation) prepared under similar experimental
conditions is 9.16 nm, which increases up to 10.86 nm for the
MAS(38) material upon incorporating Al in the mesoporous
framework. Notably, the capillary condensation step shifts to
a higher relative pressure with increasing Al content in the
The transmission electron microscopy images of the
MAS(38) material with high Al content clearly show that the
material comprised 2D ordered porous structures with honey-
comb pores and linear pores, as shown in Figures 2a-1 and a-
2, respectively. Moreover, to assess the structure and coordina-
tion of Al species in MAS(38), the material was examined by
using 27Al magic-angle-spinning nuclear magnetic resonance
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Figure 3a. The reaction did not proceed without
a catalyst. Acetonitrile, isopropanol, dimethyl sulfox-
ide, and dimethyl formamide solvents generated
a poor yield of the corresponding product. Unexpect-
edly, no product formation was observed when THF
and DMA were used under similar reaction condi-
tions. We concluded that at 1108C toluene was the
optimal solvent for the synthesis of N-benzyl secon-
dary amines through CÀN bond formation between
benzyl alcohol and aniline when the synthesized
MAS was used as a catalyst.
Furthermore, the reaction was attempted in the
presence of different MAS catalysts with Si/Al mole
ratios of 38, 54, and 93 to obtain excellent yields of
the corresponding N-benzyl secondary amines (Fig-
ure 3b). Among the materials tested, MAS(38) dem-
onstrated a high yield of secondary amines because
of its large pore size and ordered mesoporous struc-
ture. These results demonstrate the significant cata-
lytic activity of MAS catalysts with a high yield of
benzyl secondary amines.
Figure 2. a) TEM images, b) 27Al MAS-NMR spectra, and c) NH3-TPD profile of the MAS(38)
material.
(MAS-NMR). The spectrum of the calcined MAS(38) in Fig-
ure 2b shows two resonance peaks at 0 and 53 ppm. These
peaks are attributed to extra-framework aluminum species in
octahedral coordination and framework Al species in tetrahe-
dral coordination, respectively.[30] These results clearly indicate
that higher numbers of Al species are in the tetrahedral envi-
ronment compared with the octahedral Al species. These re-
sults are also strongly reflected in the MAS(38) acidity data. In
general, the mesoporous siliceous materials possess extremely
low acidic sites. The surface acidity, which plays a vital role in
the coupling reaction of aniline and 4-methoxybenzyl alcohol
and its product selectivity of N-alkyl secondary amines, increas-
Figure 3. a) Optimization of the reaction for the synthesis of N-benzyl
secondary amines over MAS(38), and b) effect of Si/Al mole ratios of MAS
catalyst on the yield of N-benzyl secondary amines.
es when Al is incorporated into the mesoporous siliceous ma-
terials. In this respect, MAS(38) demonstrated acid characteris-
tics (Figure 2c), presenting a profile of the distribution acidic
sites of the MAS(38) catalyst. The profile primarily shows
a broad desorption temperature peak at approximately 2608C,
which is attributed to the NH3 desorption from framework Al
atoms of the weak adsorption site of the MAS(38) material.
The high-temperature peak at approximately 5208C is ascribed
to the desorption of NH3 from strong acidic sites on the extra-
framework Al species; therefore, one can conclude that there
is a strong interaction between ammonia and Al2O3 species.[31]
These results are consistent with the 27Al-MAS-NMR data.
The scope of the reaction was further investigated using dif-
ferent amines and benzyl alcohols with toluene as the solvent
at 1108C and the MAS(38) as the catalyst under optimized con-
ditions. As summarized in Table 2, the reaction between aniline
with unsubstituted benzyl alcohol resulted in a 60% yield of
the corresponding product (Table 2, sample a). Notably, the re-
action proceeded smoothly with electron-donating groups of
benzyl alcohols with aniline (Table 2, samples b, h–k, m). In ad-
dition, other alkyl, alkoxy substituted anilines with methoxy-
benzyl alcohol produced excellent yields (Table 2, samples c–e).
Electron-withdrawing groups on the aromatic ring of either
benzyl alcohol or aniline influenced the reaction, and the cor-
responding N-benzyl secondary amines were obtained in 20–
45% yields (Table 2, samples f,g). The phenethanol and aniline
reaction also proceeded smoothly, and the corresponding
benzyl secondary amine product was obtained in a 55% yield
under the optimized conditions (Table 2, sample n). The path-
way that we suggest for this process is that the acidic centers
present in MAS(38) were active for dehydration of benzyl alco-
To determine the optimal reaction conditions (i.e., solvent
and temperature) to obtain maximum yield for the one-pot
synthesis of N-benzyl secondary amines, coupling reactions of
aniline (1) and 4-methoxybenzyl alcohol (2) were attempted
using different solvents in the presence of the MAS(38) cata-
lyst. In a typical procedure, a mixture of 1 (1.5 mmol) and 2
(1.0 mmol) as well as MAS(38) (50 mg) in toluene were stirred
for 24 h; product 3 was formed in a yield of 51 and 68% at
808C and reflux temperatures (i.e., around 1108C), respectively,
after column chromatography. The effect of different solvents
on the yield of the final product was compared and shown in
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The reusability test results indicated that because of the
robustness of the MAS(38) catalyst, the high yield of prod-
uct 3 was maintained without much loss of activity up to
four cycles.
Table 2. One-pot synthesis of N-benzyl secondary amines with different sub-
strates over MAS(38) catalyst (Reaction conditions: alcohol, amine, and MAS(38)
at 1108C in toluene). The purity of each reactant is shown in Table S1.
In summary, we have developed a facile synthesis of or-
dered mesoporous aluminosilicate-catalyzed and efficient
synthesis of N-benzyl secondary amines through a CÀN
bond formation between benzyl alcohol and aniline under
ligand and base-free conditions. The MAS(38) catalyst with
a well-ordered mesopore structure and strong acidic sites
can be used to produce excellent catalysts for acid-cata-
lyzed organic transformations. Our MAS catalysts are
highly active and potentially reusable; its operational sim-
plicity and ease of its isolation procedure make it an at-
tractive alternative to current methodologies.
Alcohol
(1)
Amine
(2)
Product
(3)
Yield
[%]
a
b
c
60.0
68.0
71.0
d
70.0
Experimental Section
Typical procedure for the synthesis of secondary amines:
MAS catalyst (50 mg), 1.5 mmol of alcohol and 2.0 mL of tolu-
ene (with purity 99.8%) were placed in a 15 mL sealed tube.
To this mixture, 1.0 mmol of amine was added and stirred at
110 8C for 24 h. After cooling to room temperature, the reac-
tion mixture was filtered and washed with dichloromethane
(with purity 99.8%). The catalyst was recollected after the re-
action. The combined organic layers were concentrated under
reduced pressure, and the resulting crude product was then
subjected to column chromatography using mixture of
hexane and ethyl acetate (9:1) as eluent to afford the pure
product. The obtained secondary amines were characterized
on Bruker 300 MHz NMR (1H and 13C) spectrometer using TMS
as internal standard and chloroform-d (with purity 99.8%) as
a solvent. The yield of each product was calculated based on
e
f
72.0
20.0
g
h
45.0
68.0
i
j
66.0
65.0
1
the result of H and 13C NMR spectra. The calibration curve for
each product was made previously before measuring samples.
All the chemicals were purchased from Sigma Aldrich and
used without further purification. In addition, all reagent
grade solvents were purchased from Sigma Aldrich and used
as received.
k
l
63.0
50.0
61.0
Acknowledgements
m
D. V. thanks the University Grants Commission (UGC) in
India for the award of research fellowship.
n
55.0
Keywords: ligand-free
synthesis
·
mesoporous
aluminosilicate · mesoporous materials · secondary amines
hol through the formation of hydrogen bonds, thereby gener-
ating the corresponding N-benzyl secondary amine and water.
The reusability of the MAS(38) catalyst was studied to deter-
mine the most suitable approach for recycling the catalyst for
the N-alkylation of aniline with 4-methoxybenzyl alcohol in tol-
uene at 1108C. The catalyst was removed through centrifuga-
tion after the reaction, washed with ethyl acetate 3 times (by
sonication for 5 min) to remove organic substances from the
surface, and then later activated. Fresh aliquots of reactants
were added to the recovered catalyst to test the reusability.
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P. Srinivasu,* D. Venkanna, M. L. Kantam,
J. Tang, S. K. Bhargava, A. Aldalbahi,
K. C.-W. Wu,* Y. Yamauchi*
Get hexed! Mesoporous aluminosili-
cates were successfully synthesized and
applied as ligand, base-free heterogene-
ous catalysts for CÀN bond formation
between benzyl alcohol and aniline for
various N-benzyl secondary amines with
excellent activity and selectivity.
&& – &&
Ordered Hexagonal Mesoporous
Aluminosilicates and their Application
in Ligand-Free Synthesis of Secondary
Amines
&
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