Titania nanoparticles stabilized HPA in SBA-15 for the intermolecular hydroamination of activated olefins

Article abstract of DOI:10.1002/cctc.201402449

A liquid phase hydroamination (HA) of α,β-ethylenic compounds with amines was investigated with TiO₂ nanoparticles stabilized 12-tungstophosphoric acid (TPA) in SBA-15. The catalysts were prepared by wet impregnation of TPA/TiO₂ nanoparticles into the SBA-15 and calcined at different temperatures. The characterization results reveal that the textural properties and the acidity of the prepared catalysts can be finely controlled with the simple adjustment of the calcination temperature and the structure of the support, decorated with the TiO₂ and TPA nanoparticles, was intact even after the modification. The prepared catalysts were investigated for HA of ethyl acrylate with different aromatic and aliphatic amines over a wide range of reaction conditions to optimize the yield and the selectivity of product. It was found that this process is 100% atom efficient and the catalytic performance depended significantly on the loading of TPA over the catalyst and the calcination temperature. Under optimized reaction conditions, the best catalyst, 15 wt%TPA/22.4 wt%TiO₂/SBA-15 calcined at 1123 K, offered the highest conversion of p-ethylaniline (70%) with 100% chemo-selectivity to the anti-Markovnikov product, i.e., the mono-addition product. The reaction was heterogeneously catalyzed and no contribution from leached TPA into the reaction was observed.

Full text of DOI:10.1002/cctc.201402449

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DOI: 10.1002/cctc.201402449
Titania Nanoparticles Stabilized HPA in SBA-15 for the
Intermolecular Hydroamination of Activated Olefins
Dhanashri Sawant-Dhuri,[a, b] Veerappan V. Balasubramanian,[a] Katsuhiko Ariga,[a] Dae-
Hwan Park,[c] Jin-Ho Choy,[c, d] Wang Soo Cha,[c, d] Salem S. Al-deyab,[e] Shivappa B. Halligudi,[b]
and Ajayan Vinu*[a, d]
A liquid phase hydroamination (HA) of a,b-ethylenic com-
pounds with amines was investigated with TiO2 nanoparticles
stabilized 12-tungstophosphoric acid (TPA) in SBA-15. The cata-
lysts were prepared by wet impregnation of TPA/TiO2 nanopar-
ticles into the SBA-15 and calcined at different temperatures.
The characterization results reveal that the textural properties
and the acidity of the prepared catalysts can be finely con-
trolled with the simple adjustment of the calcination tempera-
ture and the structure of the support, decorated with the TiO2
and TPA nanoparticles, was intact even after the modification.
The prepared catalysts were investigated for HA of ethyl acry-
late with different aromatic and aliphatic amines over a wide
range of reaction conditions to optimize the yield and the se-
lectivity of product. It was found that this process is 100%
atom efficient and the catalytic performance depended signifi-
cantly on the loading of TPA over the catalyst and the calcina-
tion temperature. Under optimized reaction conditions, the
best catalyst, 15 wt%TPA/22.4 wt%TiO2/SBA-15 calcined at
1123 K, offered the highest conversion of p-ethylaniline (70%)
with 100% chemo-selectivity to the anti-Markovnikov product,
i.e., the mono-addition product. The reaction was heterogene-
ously catalyzed and no contribution from leached TPA into the
reaction was observed.
Introduction
The impact of the global warming and the environmental
awareness among the public have made a significant progress
on replacing the homogeneous catalysts that are harmful to
our environment through generation of hazardous wastes and
toxic by-products and used in various petrochemical industrial
processes, with sustainable and environmentally friendly cata-
lysts. Moreover, the restrictions imposed by the waste minimi-
zation laws and economic considerations drive to the develop-
ment of a new catalytic technology which should be safe and
non-hazardous. Modern processes are, in fact, based on solid
type heterogeneous catalysts including acid and bases. Nowa-
days, these catalysts have been widely used in various organic
industrial processes because of their advantages, such as easy
separation from the product and recyclability.
Heteropoly acids (HPAs) are interesting materials with strong
acidity and redox behavior, which make them available as cata-
lysts for various types of organic synthesis, including selective
oxidation, and acid-catalyzed organic reactions.[1–3] However,
they have several disadvantages such as low specific surface
area, poor thermal stability, and a high solubility in polar sol-
vents which inhibit their use in various liquid phase acid cata-
lyzed organic reactions. These disadvantages can be overcome
by various ways including the encapsulation of HPA in porous
silica or zeolite matrix and the formation of acidic salt by inter-
calating the cesium in the Keggin structure.[4–6] It was reported
that the thermal stability of these supported catalysts is signifi-
cantly higher than that of the pure HPA. Therefore, the re-
searchers tried to utilize different supports such as silica, tita-
nia, and zirconia to stabilize the HPA and improve the acidic
properties.[7–9] Zirconia modified with tungstophosphoric acid
(TPA)[7–9] and high surface area nanosized HPA supported zirco-
nia encapsulated in mesoporous materials form a high acidic
catalytic phase with an excellent catalytic activity in a variety
of acid-catalyzed reactions.[10,11] Although sulfated and tung-
stated zirconia are highly acidic catalysts, they suffer from poor
textural characteristics which restrict their use in acid catalyzed
reactions.[12–14] On the other hand, the mesoporous materials
[a] Dr. D. Sawant-Dhuri, V. V. Balasubramanian, Prof. K. Ariga, Prof. A. Vinu
WPI, MANA, National Institute for Materials Science
Tsukuba, Ibaraki 305-0044 (Japan)
[b] Dr. D. Sawant-Dhuri, Dr. S. B. Halligudi
Catalysis Division, National Chemical Laboratory
Pune (India)
[c] Dr. D.-H. Park, Prof. J.-H. Choy, W. S. Cha
Center for Intelligent Nano-Bio Materials (CINBM)
Department of Chemistry and Nano Science
Ewha Womans University
Seoul (Republic of Korea)
[d] Prof. J.-H. Choy, W. S. Cha, Prof. A. Vinu
Australian Institute for Bioengineering and Nanotechnology
The University of Queensland
Brisbane 4073, QLD (Australia)
E-mail: a.vinu@uq.edu.au
[e] Prof. S. S. Al-deyab
Department of Chemistry
King Saud University (Saudi Arabia)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402449.
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used in these studies have poor water stability, owing to thin
walls and a small pore diameter that may easily be blocked by
the HPA and sulfated zirconia or tungstated zirconia nano-
particles.
olefin.[37–41] As previously mentioned, the use of HPAs for this
reaction without proper support significantly reduces the life
of the catalysts and thereby their recyclability.
Consequently, we employed the stable and the most active
TiO2-supported mesoporous SBA-15 nanocomposites with dif-
ferent loadings of TPA prepared in our lab for the HA of acti-
vated olefin, namely ethyl acrylate (EA) with aromatic amines
such as p-ethylaniline to yield anti-Markovnikov product (linear
addition product) (Scheme 1). The effect of the loading of TPA
Mesoporous silica (MS) like SBA-15 has a thick pore walls
and a large pore diameter, and possesses a high specific sur-
face area, large pore volume, and a high thermal and hydro-
thermal stability.[15] As SBA-15 has interesting structural fea-
tures, we have previously employed this as a support to pre-
pare nanosized titania supported TPA composite catalysts.[16] It
has been demonstrated that the SBA-15 has been a good sup-
port and the acidity of the catalysts can be tuned by the
simple adjustment of the calcination temperature and the
loading of the TPA in the mesochannels of the SBA-15. These
nanocomposites were later tried for various organic transfor-
mations including Mannich, hydroamination (HA), and Claisen
rearrangement reactions. In the previous report, we described
the applicability of these fascinating materials in various organ-
ic transformations and presented the preliminary catalytic re-
sults. Encouraged by the initial results, we extended our stud-
ies to investigate the detailed catalytic activity of these cata-
lysts in relation to the aforementioned organic transformations.
In this report, the detailed investigation of the catalysts has
been carried out for the HA of activated olefins.
Scheme 1. Hydroamination (HA) of a,b-ethylenic compounds with amines.
and the calcination temperature affecting the activity of the
catalysts on the HA of both aliphatic and aromatic amines has
been investigated. The textural parameters and the acidity of
the catalyst have been correlated with the performance of the
catalysts and reason for the significant changes in the activity
of the catalysts at different conditions. Finally, the kinetic re-
sults have been interpreted using the initial rate approach
model and the results have been discussed in detail.
The synthesis of many important personal care products or
pharmaceutical chemicals requires amines as starting com-
pounds. Considerable effort, therefore, has been devoted on
the development of efficient protocols for the construction of
carbon–nitrogen bonds in the organic molecule, which is of in-
terest owing to the fundamental importance of amines as nat-
ural products, pharmacological agents, fine chemicals, and
dyes.[17,18] Compared to the other methods for the synthesis of
amines, enamines, and imines,[12] HA offers one of the most at-
tractive pathways to such molecules. Although the addition of
alcohol HÀOR bonds to alkenes and alkynes has been known
for a long time, additions of amines to alkenes and alkynes are
rare. The direct addition of amine to an olefin, leading to
a new amine, which is particularly attractive because the de-
sired nitrogen containing products are formed in a single reac-
tion step from inexpensive alkenes without the intrinsic forma-
tion of side products.[13,14] However, this transformation is
weakly exergonic and its reaction entropy is highly negative.[19]
These thermodynamic characteristics make the difficulties for
the HA of olefins to catalyze which in fact brings no generally
applicable solution for this system.[17] Significant effort has,
therefore, been made to develop metal catalyzed HA process-
es.[20,21] Various ion exchanged zeolites and clays[22–25] and im-
mobilized zinc salts[26] have been successfully used for the cyc-
lization of 6-aminohex-1-yne and intermolecular HA of phenyl-
acetylene with aniline. Immobilized transition metal complexes
were also used in the addition of 4-isopropylaniline to phenyl-
acetylene.[27] A broad spectrum of the reactions performed on
the addition of activated olefins to aliphatic amines are avail-
able in the literature.[28–32] However, the reports on the selec-
tive HA of aromatic amines using different solid-acid catalysts
are quite limited.[34–36] HPAs or metal ion exchanged HPAs have
also been used as catalysts for the intermolecular HA of
Results and Discussion
The detailed structural information of the prepared catalysts
has been described in our previous report.[16] However, to get
a clear insight into the relation between the textural parame-
ters and the acidity of the catalysts with their performances,
some of the results are presented. Low-angle XRD patterns of
xT-22.4TO-S-15 with different TPA loadings and calcined at
1123 K reveal that the meso-structure of the materials is intact
up to 50 wt% of TPA loading whereas the wide-angle XRD pat-
terns of the samples show that the pure anatase phase of TiO2
which is critical for the performing organic transformation is
changed to rutile phase when the loading of TiO2 is increased
to more than 15 wt% (Figure S1—see the supporting informa-
tion). In addition, the decomposition of TPA into WO3 crystalli-
tes was observed when the loading of TPA was increased
above 15 wt%. These results reveal that the sample with a TPA
loading of 15 wt% is the best condition that can keep both
the structure of the support as well as the active sites of TPA
which is crucial for the catalytic studies. It can be seen from
Figure S1 that the structure of the samples is affected after the
calcination at different temperature. The meso-structure and
anatase phase of TiO2 were intact up to the calcination tem-
perature of 1123 K but collapsed/changed as the temperature
was increased above 1123 K.
The effects of TPA and TiO2 loading on SBA-15, and the calci-
nation temperature on the textural parameters of the catalysts
have been shown Figure 1 and Figure 2, respectively. It is evi-
dent from the data that the specific pore volume and the spe-
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However, the value of the acidity decreases drastically when
the calcination temperature was increased above 1123 K,
which is mainly due to the decomposition of TPA into WO3
crystallites. All the samples were also characterized by using
different characterization techniques such as 31P MAS NMR,
HRTEM, FT-IR of adsorbed pyridine, UV/vis. Analysis of the
spectroscopy, XPS, and FT-Raman and their conclusions were
discussed in detail in our previous report.[16] As the sample
containing 15 wt%TPA, 22.4 wt%TiO2 on SBA-15 calcined at
1123 K (1-1123) showed the optimum surface area, with a mon-
olayer coverage of TPA and small TiO2 nanoparticles on the
surface and a high total acidity (Figure 1), it was chosen for the
HA of activated olefins with a series of aliphatic and aromatic
amines (Scheme 1).
!
Figure 1. Variation of physico-chemical properties such as ( ) surface area,
~
(
&
*
) pore volume, ( ) pore diameter and ( ) total acidity with a variation of
TPA loading (x) in xT-22.4TO-S-15 catalyst calcined at 1123 K. For 1 x=15.
The catalytic activity of 1-1123 for the addition of aliphatic
amines with EA at room temperature has been investigated
and the results are presented in Table 1. All the reactions were
exclusively regioselective and only the product of anti-Markov-
nikov addition to amine to CC double bonds is formed. The
electron rich carbonyl group of EA combines with the electron
deficient centers in the 1-1123 (mainly Brønsted acidic), which
makes b-carbon electron deficient and attacks the electron rich
center in the amine. This is followed by 1,3-proton shift in ste-
p III with the release of anti-Markovnikov product in step IV to
regenerate the catalyst (Scheme S1).[42] In the absence of any
solvent, the catalyst gave excellent conversion for all the ali-
phatic amines to the selective product at the amine/acrylate
molar ratio of 1. The reaction of n-propylamine and n-butyla-
mine with EA gave a conversion of ꢀ99% with a selectivity of
90% and 87% for mono-substituted product with 10% and
13% for di-substituted product, respectively. Bulkier molecules
like dicyclohexylamine also gave higher yield (85%) with 100%
selectivity for mono-addition product, which indicates that the
bulky amines can easily pass through the mesopores of 1-
1123. The catalyst offered 86.83% conversion and a yield of
96% for the mono-addition product and 4% for the di-addi-
tion product when a long chain primary amine, n-octylamine
was used.
!
Figure 2. Variation of physico-chemical properties such as ( ) surface area,
~
(
&
*
) pore volume, ( ) pore diameter and ( ) total acidity of 1 calcined at dif-
ferent calcination temperatures.
cific surface area of the modified samples are much lower than
those of the parent SBA-15, and decrease with increasing TPA
or TiO2 loadings, which is mainly due to the coverage of the
surface of the support with the well-dispersed TPA or TiO2
nanoparticles. It can be seen from Figure 2 that the increasing
the calcination temperature from 923 to 1273 K, the specific
surface area and pore volume of the support decrease from
462 to 202 m2 gÀ1 and from 0.61 to 0.31 cm3 gÀ1, respectively.
The total acidity in terms of the
amounts of NH3 in mmolgÀ1 for
Table 1. HA of EA with aliphatic amines over 1-1123 at room temperature.[a]
the catalysts with different load-
ings of TPA and different calcina-
tion temperatures is also pre-
sented in Figure 1 and Figure 2,
respectively. It is evident from
the data in Figure 1 that the
total acidity of the catalyst in-
creases with increase in TPA
loading from 5 to 15 wt% and
then decreases with further in-
crease in TPA loading. On the
other hand, total acidity increas-
es with increasing the calcina-
tion temperature from 923 to
1123 K and reaches to a maxi-
No. Aliphatic
Amines
Conv.[b] Selectivity [%][c] MA product
TOF RC, ka
[sÀ1 [ꢁ10À4 sÀ1
]
[%]
MA
DA
]
1.
2.
3.
4.
5.
6.
7.
8.
9.
morpholine
piperidine
dicyclohexylamine 85.00
dipropylamine
cyclohexylamine
n-octylamine
benzylamine
propylamine
butylamine
95.00
87.01
100
100
100
100
100
96
99.7
90.0
87.0
0
0
0
0
0
ethyl-3-morpholinopropanoate
ethyl-3-(piperidin-1-yl) propanoate
ethyl-3-(dicyclohexylamino) propanoate 0.48 5.27
ethyl-3-(dipropylamino) propanoate
ethyl-3-(cyclohexylamino) propanoate
ethyl-3-(octylamino) propanoate
ethyl-3-(benzylamino) propanoate
ethyl-3-(propylamino) propanoate
ethyl-3-(butylamino) propanoate
0.80 8.32
0.75 5.67
93.35
91.68
86.83
93.37
98.8
0.74 7.53
0.73 6.91
0.60 5.63
0.72 7.54
0.98 12.29
0.91 12.53
4[d]
0.3[e]
10[f]
13[g]
98.9
[a] Reaction conditions: Amine to EA ratio=1, Amount of catalyst=0.15 g (5 wt% of total reaction mixture),
time=1 h; [b] Conversion determined by GC analysis with respect to amine; [c] The anti-Markovnikov product
selectivity was determined by GC analysis. MA: monoaddition; DA: di-addition; [d] diethyl 3,3’-(octylazanediyl)
dipropanoate; [e] diethyl 3,3’-(benzylazanediyl) dipropanoate; [f] diethyl 3,3’-(propylazanediyl) dipropanoate;
[g] diethyl 3,3’-(butylazanediyl) dipropanoate.
mum value of 0.38 mmolgÀ1
.
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The influence of TPA loading
(5 to 70 wt%) on the conversion
and selectivity was studied at
a reaction temperature of 383 K,
the reaction time of 2 h and p-
ethylaniline to EA ratio of 1. It
was observed that the conver-
sion of amines was highly de-
pendent on the loading of TPA
on the catalyst. The conversion
was increased from 35.49 to
47.53% with increasing the TPA
loading from 5 to 15 wt%. How-
ever, the activity of the catalyst
Table 2. HA of EA with aromatic amines over 1-1123 at 383 K.[a]
No. Aromatic
Amines
Conv.[b] Select.[c] MA product
TOF
RC, ka
[%]
[%]
[sÀ1ꢁ10À1
]
[ꢁ10À5 sÀ1
]
1.
2.
3.
4.
5.
6.
7.
p-anisidine
p-ethylaniline
2,4-xylidene
N-methylaniline 48.6
aniline
68
63.3
55.8
100
100
100
100
100
nil
ethyl-3-(4-methoxyphenylamino) propanoate
ethyl-3-(4-ethylphenylamino) propanoate
ethyl-3-(2,4-dimethylphenylamino) propanoate 0.33
ethyl-3-(methyl(phenyl)amino) propanoate
ethyl-3-(phenylamino) propanoate
nil
0.40
0.38
5.28
4.64
3.78
3.08
3.96
nil
0.31
0.39
nil
57.5
nil
2
o-nitroaniline
p-ethylaniline[d]
100
ethyl-3-(4-ethylphenylamino) propanoate
–
–
[a] Reaction conditions: Amine to EA ratio=1, toluene=1 mL, Amount of catalyst=0.2 g (10 wt% of total reac-
tion mixture), time=6 h; [b] Conversion determined by GC analysis with respect to amine; [c] The mono-addi-
tion anti-Markovnikov product selectivity was determined by GC analysis. [d] No Catalyst.
was
significantly
decreased
(32.3%) with the further increase
To gain more insight into the scope and limitation of our
method, we carried out the reaction with a series of aromatic
amines using 1-1123. It was found that the catalyst was not
active at room temperature when aromatic amines were used
as the reactants. However, the aromatic amines reacted with
EA to yield mono-addition product in good conversion and se-
lectivity as the reaction temperature is increased. When the re-
action was tried without the catalyst, only 2% of mono-addi-
tion product was obtained, revealing the active role of the cat-
alyst in this particular reaction. In addition, the influence of
electronic effects of the substituents in the 2 and 4-position of
amine on the yield and selectivity of the reaction has been in-
vestigated using 1-1123 and the results are given in Table 2. It
is found that the substitution on the aromatic amine signifi-
cantly influences the outcome of the catalytic reaction. The
aniline derivatives having electron-donating groups at the
ortho- and para- positions reacted smoothly with EA to give
corresponding HA products (Table 2). It is interesting to note
that the p-anisidine having a strongly electron donating group
gave higher yield than 2,4-xylidene (Table 2, Entry 1, 3). This is
mainly due to the fact that the methoxy group of p-anisidine
is strongly electron donating and the 2-methyl groups of 2,4-
xylidene cause some steric hindrance. The role of the electron
withdrawing groups in the amines on the activity of the cata-
lyst was also investigated. As expected, the reaction of amines
with electron withdrawing substituents like nitro group did
not undergo HA reaction (Table-2, Entry 6). The feasibility of
secondary amines to undergo this addition was demonstrated
using N-methylaniline and EA (Table 2, Entry 4). These results
reveal that the reactivity of amine depends on their basicity. It
should be noted that aliphatic amines are more basic than aro-
matic amines and amines with electron donating substituents
are more basic than amines with electron withdrawing sub-
stituents. As it strongly influences the adsorption on the sur-
face of the catalysts with acid functional groups, the difference
in the activity of the catalyst was expected. From these results,
it can be concluded that the activity of the catalyst increases
with the basicity of the amines for the HA reactions of a,b-eth-
ylenic compounds with amines using this particular catalyst, 1-
1123.
of TPA loading to 70 wt.%. Among the catalysts with different
TPA loading studied, 1-1123 registered the highest conversion
for p-ethylaniline (47.53%) for the reaction time of 2 h (Fig-
ure 3A). These results indicate that the structural features of
the catalysts play a significant role in controlling the activity of
the catalysts. It was previously discussed that the Keggin struc-
ture of the TPA was collapsed when the TPA loading in the
samples is higher than 15 wt%. Therefore, it may be concluded
that the higher catalytic activity of 1-1123 is mainly due to the
Figure 3. Influence of A) TPA loading (catalysts calcined at 1123 K) and B) cal-
*
cination temperature (1) on the HA of EA with p-ethylaniline ( ) Conversion
!
in (%) and ( ) TOF. Reaction conditions: p-ethylaniline/EA ratio=1, tempera-
ture=383 K, 10 wt% catalyst, Toluene=2 mL and time=2 h. Anti-Markovni-
kov mono-addition product selectivity (100%) was determined by GC
analysis.
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retention of the Keggin structure of TPA which offers a high
acidity. High acidity is important parameter for obtaining
better performance on the HA of EA with p-ethylaniline. On
the other hand, the partial decomposition of TPA is responsible
for the lower activity of the catalyst loaded with the higher
amount of TPA wherein the acidity of the catalyst is lower as
compared to that of the samples loaded with 15 wt% of
TPA.[16]
The influence of calcination temperature on the catalytic ac-
tivity for HA of EA with p-ethylaniline over 1 calcined at differ-
ent temperatures (923–1273 K) was investigated and the re-
sults are shown in Figure 3B. The 1-923 gave a conversion of
only 13.24%, which is increased to 47.53% for 1-1123 (Fig-
ure 3B). This clearly shows the relationship between the cata-
lytic activity of 1 and the calcination temperature. The conver-
sion of p-ethylaniline decreases to as low as 15% when the cal-
cination temperature of the catalyst was increased to 1273 K. It
was found that the calcination temperature 1123 K was found
to be the best which is mainly due to the availability of the
highest Brønsted acidity together with the perfect monolayer
coverage of TPA on the surface of the catalytic support
(15 wt%).[16]
In order to optimize the yield of the catalyst with a maximum
conversion and selectivity for mono-addition product in the
HA of EA with p-ethylaniline, the reaction parameters have
been varied (Scheme 2). The best catalyst, 1-1123, was chosen
Figure 4. Influence of reaction parameters on the HA of EA with p-ethylani-
line: A) Influence of reaction temperature (reaction conditions: p-ethylani-
line/EA molar ratio=1, catalyst=1-1123 (10 wt% of total reaction mixture),
Time=2 h. B) Influence of reactant molar ratio (reaction conditions: temper-
ature=383 K, catalyst=1-1123 (10 wt% of total reaction mixture),
Time=2 h. C) Influence of catalyst wt% (Reaction conditions: p-ethylaniline
*
to EA molar ratio=1, Temperature=383 K, Time=2 h with ( ) conversion in
!
(%) and ( ) TOF. Anti-Markovnikov mono-addition product selectivity
Scheme 2. HA of ethyl acrylate with p-ethylaniline on titania stabilized over
(100%) was determined by GC analysis.
HPA supported SBA-15.
for these studies. Initially, the effect of temperature on the ac-
tivity of the catalyst in the HA at the temperature range of
363–403 K was investigated and the results are displayed in
Figure 4A. It was found that the increase of the reaction tem-
perature has a profound effect on the conversion and selectivi-
ty for mono-addition product. The conversion was increased
with increasing the reaction temperature from 363 to 403 K for
the reaction time of 2 h. The catalyst offered only 27.4% con-
version of p-ethylaniline at 363 K, which further increased to
68.9% when the reaction temperature was increased to 403 K.
However, the selectivity of the mono-addition product was de-
creased slightly at a high reaction temperature (403 K) as
a function of time. Interestingly, at a high reaction tempera-
ture, the trace amount of di-addition product is formed which
indicates that harsh condition is required for the formation of
the Markovnikov adduct.
The influence of p-ethylaniline to EA molar ratio (2–0.25) for
HA of EA with p-ethylaniline was studied over 1-1123 at a reac-
tion temperature of 383 K and a reaction time of 2 h but keep-
ing the total weight of the reaction mixture constant (Fig-
ure 4B). At a molar ratio of 1, the conversion was 33.38% with
100% selectivity for mono-addition product i.e. ethyl-3-(4-eth-
ylphenyl amino) propanoate. When the p-ethylaniline to EA
molar ratio decreased from 1 to 0.25, the conversion is found
to increase from 33.38 to 67.15%. The increase of the conver-
sion may be attributed to the availability of more amount of
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EA which may enhance its adsorption over the surface of the
highly active sites of the catalysts. As a result, the conversion
of p-ethylaniline is increased as the ratio increases. It was also
expected that the availability of excess amount of EA actually
leads to the addition of another molecule of EA for the di-addi-
tion product. However, only trace amount of di-addition prod-
uct (0.3%) is formed even at the p-ethylaniline to EA of 0.25.
This indicates that the mesoporosity of the catalyst plays an
important role and the reaction indeed takes place inside the
mesopores which favors the formation of the mono-addition
product with almost 100% selectivity and 100% atom efficient
process.
first cycle of the reaction was separated by filtration, washed
three times with 1, 2-dichloromethane, dried in an oven at
373 K for 24 h and activated at 773 K for 4 h in an air. The acti-
vated catalyst was used for HA of EA with p-ethylaniline under
selected reaction conditions. Same procedure is repeated for
second cycle and the data on the conversion of aniline are pre-
sented in Table S1. From the result, it is concluded that there is
no appreciable loss in the catalytic activity and product selec-
tivities in the two cycles and catalyst could be reused.
Conclusions
The weight of the catalyst was tuned to investigate its influ-
ence on the conversion and product selectivity in the HA of EA
with p-ethylaniline at a reaction temperature of 383 K, p-ethyl-
aniline to EA molar ratio of 1 and the reaction time of 2 h and
the results are shown in Figure 4C. The weight of the 1-1123
was varied from 2.5–10 wt% of the total reaction mixture. The
activity of the catalyst was significantly increases as the weight
of the catalyst is increased. The conversion of p-ethylaniline in-
creases from 13.5 to 47.53% with increasing the weight per-
centage of the catalyst from 2.5 to 10. This could be due to
the increase of more number of active sites for the reactant
molecule when higher amount of catalyst was supplied, and
thereby the conversion increases significantly. It should be also
noted that the selectivity of the mono-addition product was
almost 100% and was not at all affected upon increasing the
weight of the catalyst in the reaction mixture.
The liquid phase HA of EA with p-ethylaniline was carried out
with a series of nanosized titania supported 12-tungstophos-
phoric acid (TPA) in SBA-15 composite prepared by vacuum
impregnation of TPA/TiO2 nanoparticles inside the mesoporous
channels of support. It was found that the catalytic activity is
mainly related with the textural parameters and the acidity of
the catalyst which solely depend on the TPA coverage on the
surface of the catalyst and the calcination temperature. The ef-
fects of various reaction parameters affecting the activity of
the catalyst in the HA of EA with amines were investigated.
Under the optimized reaction conditions, the most active cata-
lyst 1-1123, gave a maximum conversion of 70% over a period
of 8 h with a high regio- and chemo-selectivity to anti-Markov-
nikov product, i.e. mono-addition product. The catalyst also
showed higher activities for HA of EA with aliphatic and aro-
matic amines with electron withdrawing substituents, and sec-
ondary amine. Among the catalysts studied, 1-1123 was found
to be highly active, recyclable and heterogeneously catalyzed
for intermolecular HA of activated olefins with amines under
optimized reaction conditions. This process was found to be
100% atom efficient and gave only single mono-addition prod-
uct. As the prepared catalysts have excellent textural parame-
ters and a high acidity, these materials can be used for various
other acid catalyzed organic transformation and open the door
for the synthesis of various fine chemicals under mild reaction
conditions.
The influence of time on stream on conversion and selectivi-
ty of products for the HA reaction over 1-1123 for the time
period of 1 to 8 h at a p-ethylaniline to EA molar ratio 1 and
reaction temperature of 383 K was investigated and the results
are given in Figure-5. With the increasing the reaction time,
the conversion of p-ethylaniline also increases to a maximum
of 69.35% with a clean selectivity for mono-addition product
under the selected reaction conditions. These results clearly in-
dicate that the catalyst is quite stable and its activity is re-
tained. The availability and the stability of the active sites on
the surface of the SBA-15 support are responsible for the sus-
tainability and the higher activity of the catalyst. In addition,
the coke was not formed on the surface of the catalyst even
after the reaction time of 8 h.
Experimental Section
Tetraethylorthosilicate (TEOS), triblock copolymer of ethylene oxide
(EO) and propylene oxide (PO), EO20PO70EO20 (Pluronic P123)
(Mavg =5800), titanium chloride (TiCl4) were purchased from Aldrich
and 12-tungstophosphoric acid (TPA) was obtained from Merck. p-
Ethylaniline was also procured from Aldrich. EA, acrylonitrile and
acrylic acid were purchased from Loba Chemicals, Mumbai. All the
amines and toluene were purchased from S. D. Fine chem. Ltd.,
Mumbai. Toluene used in the reaction was distilled over sodium
wire before use.
The study of heterogeneity of the catalyst in the reaction
media is important because of the solubility of TPA in EA
(polar media). In order to study the heterogeneity of the cata-
lyst in the reaction mixture, the reaction was carried out for
30 min under selected reaction conditions using fresh 1-1123.
The reaction was stopped and catalyst was separated by filtra-
tion and then the filtrate was stirred for another 4 h under
same reaction conditions. It was found that there was no fur-
ther increase in the conversion of p-ethylaniline in the absence
of the catalyst, which indicated the absence of leaching of TPA
into the reaction medium (Figure S2). In addition, the leaching
of TPA (dissolution of P or W) into the hot filtrate was tested
by inductively coupled plasma-optical emission spectroscopy
(ICP-OES), which did not show any trace of P or W in the reac-
tion mixture. For reusing the catalyst, the catalyst used in the
Catalyst preparation: Synthesis of mesoporous silica SBA-15: SBA-
15 was synthesized with the following gel composition: 0.041
TEOS: 0.24 HCl: 6.67H2O.[43] In a beaker, 4 g of P123 was dispersed
in 30 g of water and stirred for 4 h. Then, 120 g of 2m HCl solution
was added and stirred for 2 h. To this homogeneous solution,
8.54 g of TEOS was added under stirring. The resulting gel was
aged at 313 K for 24 h and finally heated to 373 K for 48 h. Then,
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the solid was filtered, washed with distilled water and dried in an
oven at 373 K in air for 5 h followed by calcination at 813 K to
remove the triblock-copolymer.
cross-linked 5% diphenyl-95% dimethylpolysiloxane. The products
were also identified by GC-MS (HP-5973) equipped with an identi-
cal column and a mass detector. Conversion of the various reac-
tions conducted using the catalysts was calculated based on
amines. The reaction mixture was cooled to room temperature
after the completion of the reaction and filtered to remove the cat-
alyst whereas the solvent was removed by distillation. The product
was separated by column chromatography using neutral alumina
as the stationary phase and petroleum ether/ethyl acetate (95:5) as
the eluent and characterized.
Mesoporous silica embedded TiO2 supported TPA: The mesoporous
silica SBA-15 (2 g) was first wet-impregnated along with an aque-
ous solution of titanium tetrachloride (TiCl4) under vacuum condi-
tion using a turbo pump. An aqueous solution of the TiCl4 solution
was prepared in an ice-water bath under air-tight condition. Differ-
ent amount of this solution was kept in contact with the mesopo-
rous silica SBA-15 for 6 h under vacuum. An excess aqueous solu-
tion of TiCl4 (22.4 wt%) was introduced and kept in contact for 6 h
under vacuum. After the volatiles were pumped out, the solid was
dried at 373 K in air. Similar procedure was followed for the wet-
impregnation of TPA (10, 15, 30, 50, 70, and 90 wt%) over an opti-
mized 22.4 wt of TiO2 loaded on SBA-15. 15 wt%TPA over SBA-15
samples loaded with different amount of TiO2 were also prepared.
All the catalysts were dried at 373 K and further calcined at 1123 K.
The nanocomposites were calcined at different calcination temper-
atures. The samples were named as xT-22.4TO-S-15-z, where xT, TO,
S-15, and z denote the wt% of TPA, the wt% of TiO2, SBA-15, and
the calcination temperature, respectively.
Acknowledgements
Dhanashri Sawant-Dhuri thanks the NIMS for offering a NIMS
postdoctoral researcher award. One of the authors, Ajayan Vinu
is grateful to ARC for the award of the future fellowship and the
University of Queensland for the start-up grant. The research
grant from King Saud University to NIMS was also greatly ac-
knowledged.
Keywords: heteropoly acids · hydroamination · mesoporous
materials · nanocatalysts
Characterization methods: Inductively coupled plasma-optical emis-
sion spectroscopy (ICP-OES) and EDAX were used to determine the
Ti, W, and P contents in the samples. XRD patterns were deter-
mined with a Bruker instrument equipped with a general area de-
tector diffraction system (GADDS) using CuKa radiation at a step
size of 0.018, the generator operating at 40 kV and 40 mA. The
wide angle X-ray diffraction patterns were detected using Rigaku
Model D/MAXIII VC, Japan, l=1.5418 ꢂ with CuKa radiation. The
specific surface area, pore volume and pore size distribution of
samples were calculated from the nitrogen adsorption isotherms
measured using an Omnisorb 100CX (Coulter, USA). The total
amount of acidity of the catalysts was obtained using temperature
programmed desorption (TPD) of NH3 on Micromeritics AutoChem
2910. The experiments were conducted initially by dehydrating
0.1 g of the catalyst sample at 773 K in dry air for 1 h and purging
with helium for 0.5 h. The temperature was decreased to 398 K
under the flow of helium and then 0.5 mL pulses of NH3 were sup-
plied to the samples until no more uptake of NH3 was observed.
Then, the NH3 was desorbed in helium flow by increasing the tem-
perature to 813 K with a heating rate of 108CminÀ1 and measured
using TCD detector. Thermo-gravimetric measurements (TG-DTG)
were performed on a Setaram TG-DTA 92 apparatus from room
temperature to 1273 K in flowing dry air (ꢀ50 mLminÀ1). For each
experiment, 25–30 mg of the sample was used with a heating rate
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Received: June 13, 2014
Published online on September 1, 2014
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ChemCatChem 2014, 6, 3347 – 3354 3354