Catalytic activity of MCM-41 and Al grafted MCM-41 for oxidative self and cross coupling of amines

Article abstract of DOI:10.1016/j.molcata.2013.11.001

The catalytic activity of MCM-41 mesoporous silica (MS) and Al grafted MCM-41 (Al-MS) with varied amount of Al were studied for oxidative self and cross coupling of benzyl amines to imines. The role of Al, its amount, acidity of the catalyst, effect of reaction conditions (especially presence of solvent and excess air) and amines nature on the oxidative coupling reactions were investigated. Surface OH groups play an effective role in amine activation by hydrogen bonding, whereas the acid sites generated by incorporation of optimum amount of Al further improve the activity. The Al in optimum amount (Si/Al: 20) showed good catalytic activity in self coupling of benzyl amine as compared to MS and Al-MS with high Al loading. A suitable polar solvent and presence of excess air significantly promote the activity of MS and Al-MS for amine coupling reactions. The MS without Al was also found to be efficient catalyst for oxidative self and cross coupling of amines. The presence of Al in Al-MS (in optimum amount, i.e., Si/Al: 20) made the self coupling of benzyl amine faster than MS, whereas the cross coupling reactions with Al-MS were observed to be slightly slower than MS, which is probably due to competitive adsorption of amine substrate and amine nucleophile over acidic sites. The activity of MS and Al-MS catalysts for self and cross coupling reactions was also dependent of the amines nature (basicity and nucleophilicity).

Full text of DOI:10.1016/j.molcata.2013.11.001

Journal of Molecular Catalysis A: Chemical 382 (2014) 114–125
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Catalytic activity of MCM-41 and Al grafted MCM-41 for oxidative self
and cross coupling of amines
∗
Kamlesh N. Tayade, Manish Mishra
Department of Chemical Engineering and Shah-Schulman Center for Surface Science and Nanotechnology, Faculty of Technology, Dharmsinh Desai
University, College Road, Nadiad 387001, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic activity of MCM-41 mesoporous silica (MS) and Al grafted MCM-41 (Al-MS) with varied
amount of Al were studied for oxidative self and cross coupling of benzyl amines to imines. The role of Al,
its amount, acidity of the catalyst, effect of reaction conditions (especially presence of solvent and excess
air) and amines nature on the oxidative coupling reactions were investigated. Surface OH groups play an
effective role in amine activation by hydrogen bonding, whereas the acid sites generated by incorporation
of optimum amount of Al further improve the activity. The Al in optimum amount (Si/Al: 20) showed
good catalytic activity in self coupling of benzyl amine as compared to MS and Al-MS with high Al loading.
A suitable polar solvent and presence of excess air significantly promote the activity of MS and Al-MS for
amine coupling reactions. The MS without Al was also found to be efficient catalyst for oxidative self and
cross coupling of amines. The presence of Al in Al-MS (in optimum amount, i.e., Si/Al: 20) made the self
coupling of benzyl amine faster than MS, whereas the cross coupling reactions with Al-MS were observed
to be slightly slower than MS, which is probably due to competitive adsorption of amine substrate and
amine nucleophile over acidic sites. The activity of MS and Al-MS catalysts for self and cross coupling
reactions was also dependent of the amines nature (basicity and nucleophilicity).
Received 23 September 2013
Received in revised form 1 November 2013
Accepted 5 November 2013
Available online 19 November 2013
Keywords:
Oxidative coupling of amines
Primary amines
Imines
MCM-41
Al-MCM-41
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
Landge et al. reported the potential of K-10 montmorillonite solid
acid catalyst under microwave irradiation in amine coupling reac-
tions [12]. They stated that acidic sites of K-10 activate the amine
substrates for oxidation by coordinating the NH2 group through
lone pair of electrons.
Imines are important organic compounds for the synthesis
of pharmaceuticals, N-heterocyclics, fine chemicals, etc. [1]. In
recent years, catalytic oxidative coupling (self and cross) of amines
ꢀ
ꢀ
(
1 or/and 1 ) to imines (2 or 2 ) using a suitable catalyst and
We are interested to study the catalytic properties of MCM-41
and Al grafted MCM-41 acidic materials for oxidative coupling of
amines. The interesting feature of the MCM-41 material is that, in
spite of amorphous nature of silica, it possesses an ordered struc-
ture with uniform mesopores arranged into a hexagonal lattice.
Due to the presence of the mesopores, MCM-41 materials display
atmospheric oxygen as oxidant (Scheme 1) has received much
importance as greener route for synthesis of imines [2–12]. Owing
to the benefits of heterogeneous catalysts, amine coupling reac-
tions have been attempted using solid catalysts like bulk copper [3]
and gold powder [4] as well as with supported precious metals like
gold nanoparticles [5] and ruthenium [6]. Some metal free methods
like use of graphite oxide in very high loading (50 wt.%; as cata-
lyst) [7] or prolonged reflux (24–48 h) of amines-water solution [8]
have also been reported as greener routes for imine synthesis. Pho-
tocatalytic oxidation of amines to imines are reported with TiO2
2
a very large specific surface area of approximately >1000 m /g. In
addition, mesopores of MCM-41 materials provide access to large
molecules as well as facilitate diffusion, which is the major problem
with microporous zeolite-like materials. These properties made
MCM-41 materials very popular to be used as a support mate-
rial for development of heterogeneous catalysts. The Al grafted
MCM-41 (Al-MCM-41) have been extensively studied as catalyst
for numerous acid catalyzed reactions like esterification [13a,13b],
alkylation [13c,13d], acetalization [13e], Beckmann rearrangement
[13f], biomass pyrolysis [13g,13h], etc. Therefore, it was of interest
to have the benefit of the surface acidity and the morpholog-
ical properties of MCM-41 materials in oxidative coupling of
amines to imines. The present work was aimed to explore the cat-
alytic property of MCM-41 and Al grafted MCM-41 materials for
under UV irradiation [9], carbon nitride [10] and Nb O3 [11] under
2
visible light. Although the reported methods are attractive strate-
gies for amine coupling, but there is still need of an efficient and
inexpensive catalytic route for activation of amines for oxidative
coupling to have economically viable route for imines synthesis.
∗ Corresponding author. Tel.: +91 268 2520502; fax: +91 268 2520501.
E-mail address: manishorgch@gmail.com (M. Mishra).
1
381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.11.001

K.N. Tayade, M. Mishra / Journal of Molecular Catalysis A: Chemical 382 (2014) 114–125
115
◦
sample was scanned in 2ꢁ range of 1–10 with a scanning rate of
0
◦ −1
27
.02 s . The solid state Al NMR spectra of Al-MS samples were
recorded at room temperature using a Bruker, Avance II (500 MHz)
spectrometer at 27Al frequency of 130 MHz and were spun at
8
kHz. The BET (Brunauer–Emmett–Teller) surface area (SBET) of the
samples was determined from N adsorption–desorption isotherm
2
study at liquid nitrogen temperature (77 K) using Quantachrome
NOVA 1000e surface area analyzer. Temperature programmed
desorption of NH3 (NH3 TPD) for the samples were carried out
for acidity measurement using Micromeritics, USA, taking 50 mg
Scheme 1. Catalytic oxidative self and cross coupling of amines to imines (2: imine
product by self coupling/2 : imine product by cross coupling).
ꢀ
◦
of sample. The sample was heated at 100 C for 1 h under helium
activation of amines for oxidative coupling to imines studying the
influence of catalyst’s surface characteristics (surface OH groups,
Al sites, Al content and acidity), nature of amines and solvent on
the amine coupling reactions. As the benzylic imines are useful
synthetic intermediates possessing C N functionality for variety of
transformations [14], the catalytic capability of the silica materials
was demonstrated for self and cross coupling of different benzylic
amines.
−
1
flow (30 mL h ) as carrier gas. The NH3 was adsorbed on the sam-
◦
ples at 50 C by exposing to a flow of helium mixed NH gas (10%,
3
−
1
20 mL h ) for 1 h. The desorption of NH3 was performed by heat-
ing the sample up to the final temperature of 800 C (12 C min )
◦
◦
−1
and was measured by gas chromatography with thermal conduc-
tivity detector (TCD). The TGA of the samples was carried out
using Mettler thermal analyzer, TGA/DSC 1 SF/752, by heating the
◦
sample from room temperature to 800 C with a heating rate of
◦
−1 −1
2
. Experimental
10 C min
under zero air flow (50 mL min ). FTIR of the sam-
ples were recorded on FT-IR spectrophotometer (IRPrestige-21,
Shimadzu) having a diffuse reflectance scanning disc technique by
mixing the sample with dried KBr (in 1/20 wt. ratio) in the range
2
.1. Materials
−
1
−1
The tetraethyl orthosilicate (TEOS; >99%), 4-methyl benzyl
of 400–4000 cm with a resolution of 4 cm . The type of surface
acidity (Brønsted/Lewis) in the samples was characterized by FTIR
study of the pyridine adsorbed samples. For this, the sample (0.2 g)
amine (98%) and 4-chloro benzyl amine (98%) were purchased from
Aldrich. The toluene (99%), cetyl trimethylammonium bromide
◦
(
CTAB; 98%), sodium hydroxide (NaOH; >97%), aluminium iso-
propoxide (97%), dimethyl sulfoxide (99%), 4-nitro aniline (98%),
-methyl aniline (98%), n-decane (97%), nitrobenzene (98%), con-
was activated at 120 C for 2 h. The activated sample was cooled
in desiccator under vacuum and was exposed to pyridine (25 mL)
vapour for 12 h. The pyridine adsorbed sample was degassed under
vacuum for 15 min to remove physically adsorbed pyridine from
sample.
4
centrated HCl (35%), aniline (99%) and dichloromethane (99%) were
from s.d. Fine Chemicals, India. The benzyl amine (99%), cyclohexyl
amine (99.5%), n-octyl amine (98%), 4-methoxy benzyl amine (98%)
were purchased from Spectrochem. All the chemicals were used
without any further purification.
2.3. Typical procedure for oxidative coupling of amine to imine
with MS and Al-MS
2
.2. Synthesis and characterization of Al grafted mesoporous
The amine was taken in a 50 mL reaction tube of reaction station
silica (Al-MS)samples
(
12 Place Heated Carousel Reaction Station, RR99030, Radleys Dis-
◦
covery Technologies, UK) along with the catalyst activated at 180 C
for 2 h. The reaction was carried out in solvent free or in presence
of solvent at desired temperature under stirring for required reac-
tion time. The reactions were performed in closed condition as well
as under continuous air purging to provide excess oxygen for the
reaction. The detail of reaction conditions are given in the footnote
of the tables and figures. After the reaction, the reaction mixture
was cooled and diluted with dichloromethane (5 mL). The catalyst
was filtered to recover it from the liquid phase. The reaction mix-
ture was analyzed by gas chromatography (Agilent 7890A) having
a HP-5 (60 m) capillary column with a programmed oven temper-
The mesoporous silica (MCM-41; MS) was synthesized by basic
hydrolysis of TEOS and the post synthesis grafting was adopted for
synthesis of Al grafted mesoporous silica (Al-MCM-41; Al-MS) [15].
The CTAB (7.44 g) was dissolved in double distilled water (170 mL)
followed by addition of NaOH (1.22 g) and the mixture was stirred
to get clear solution. To the resulting solution, TEOS (17.68 g) was
added drop wise in 20 min. under vigorous stirring to obtain final
gel composition (0.25 TEOS:0.09 NaOH:27.8 H O:0.06 CTAB). Thus
formed gel was allowed to react for 36 h under stirring at room tem-
perature. The solid mass was then filtered, washed with distilled
2
◦
water and dried at 80 C for 8 h in oven, followed by calcination at
5
◦
3
ature from 50 to 280 C, 0.5 cm /min. flow rate of N as carrier gas
◦
2
50 C for 6 h in static air. For the synthesis of Al grafted MCM-41
and FID detector. The conversion of amine was calculated on the
basis of its weight percent as follows:
samples (Al-MS) with Si/Al molar ratio of 20, 10 and 5; MCM-41
2 g) was added to the solution containing required amount of alu-
(
minium iso-propoxide (0.338 g, 0.679 g and 1.368 g, respectively)
Conversion (wt.%) of amine
dissolved in 150 mL toluene and allowed to stir at room temper-
◦
100 × Initial wt.%
of amine − Final wt.% of amine
ature for 24 h. The solid product was then filtered, dried at 90 C
=
◦
Initial wt.% of amine
for 10 h and calcined at 550 C for 3 h to get Al-MS samples, viz.
Al(5)-MS, Al(10)-MS and Al(20)-MS.
The Al content in the synthesized Al-MS samples was estimated
by inductively coupled plasma atomic emission spectroscopy
The selectivity of the product (imine) was calculated as below:
peak area of product
Selectivity (%) = 100 × ꢀ
(
(
ICP-AES; PerkinElmer Instrument, Optima 200 DV). The sample
100 mg) was dissolved in 40% HF solution (15–20 drops) followed
Total peak area for all the products
by addition of 5 wt.% boric acid solution making up to 100 mL in
volumetric flask, which was analyzed by ICP. The X-ray diffraction
The products formed in the reactions were characterized by
GC–MS analysis and the data were matched with those reported in
the literature. GC–MS analysis was carried out using gas chromato-
graph mass spectrometer (Agilent 5975C GC/MSD with 7890A GC
system) having HP-5 capillary column of 60 m length and 250 ␮m
(
(
XRD) study of samples was carried out using X-ray diffractometer
Bruker, Advanced D8) with Cu K␣ radiation (ꢀ = 1.5418 A˚ ) and Lynx
Eye detector to characterize the structural property of samples. The

1
16
K.N. Tayade, M. Mishra / Journal of Molecular Catalysis A: Chemical 382 (2014) 114–125
Fig. 1. XRD patterns of MS and Al-MS samples.
◦
◦
diameter with a programmed oven temperature from 50 to 280 C,
−1
at 1 mL min flow rate of He as carrier gas and ion source at 230 C.
3
. Results and discussion
3.1. Characteristics of MS and Al-MS samples
The Al loading efficiency in all the samples was found to be
almost 100% as the Si/Al molar ratios of the Al-MS samples (esti-
mated by ICP analysis) were close to the theoretical Si/Al values
(Table 1).
The XRD patterns of the samples (Fig. 1) exhibit the charac-
Fig. 2. 27Al MAS NMR spectra of (i) Al(20)-MS, (ii) Al(10)-MS and (iii) Al(5)-MS
teristic peaks of MCM-41 indicating the hexagonal mesoporous
structure of MS and Al-MS samples [16]. The XRD patterns of the
Al-MS samples are similar to that of the MS, which indicates that
hexagonal order is maintained in Al-MS samples. However, the
intensities of the XRD peaks gradually decreased with the increase
in the loading of Al, which indicates a decrease in the long range
order of the sample by high Al loading. The decrease in XRD peak
intensity attributed to decrease in the long-range order of the
MCM-41 material on increasing the loading of catalytic species has
also been reported by others [17]. The XRD analysis in 2ꢁ range
showing presence of tetra-, penta- and octahedrally co-ordinated Al in samples.
desorption data) was not much changed. But the pore size distribu-
tion of samples (Fig 3ii) shows gradual shifting in size distribution
towards smaller size as well as reduction in the pore volumes after
Al-grafting on MS and on increasing Al loading. The decreased pore
volume with Al loading is indicative of non-framework Al species
in the pores.
The NH3 TPD results (Table 1) revealed that MS and Al(20)-
MS contain weak acidic sites (total acidity: 0.01 mmol/g and
◦
of 10–90 of the Al-MS samples did not show any diffraction peaks
8
8 mmol/g, respectively). The weak sites in MS are mainly silanols,
corresponding to crystallites of alumina phase, which indicates that
the crystallites are very small in size and highly dispersed on the
surface of the support.
which might be adsorbing NH3 through hydrogen bonding. The
grafting of Al in MS with Si/Al ratio of 20 increased the number of
acid sites but the sites were weak. The samples with high loading of
Al, i.e., Al(10)-MS and Al(5)-MS possess weak, medium and strong
acidic sites (total acidity: 1.02 mmol/g and 1.07 mmol/g, respec-
tively). The acidity gradually increases as the amount of framework
The 27Al MAS NMR spectra of the Al-MS samples (Fig. 2) show
three resonance signals centred at 3.9–7.7 ppm, 32.4–37.8 ppm and
5
6.7–58.5 ppm indicating the presence of tetrahedrally, pentahe-
drally and octahedrally co-ordinated Al, respectively. The tetra
co-ordinated Al can be framework Al [15] as well as of extra frame-
work alumina [18]. The octahedral Al is present in extra framework
of alumina [15]. The pentahedrally coordinated Al is indicative of
the defects of loaded alumina phase [19]. It can be seen that the
proportion of tetrahedral Al gradually increases with Al loading.
The BET surface area (SBET) of the samples gradually decreased
(tetrahedral) aluminium increases in the samples [15]. From FTIR
study of pyridine adsorbed MS and Al-MS samples (Fig. 4) it is evi-
dent that the Al-MS samples possess both Brønsted and Lewis acidic
−
1
−1
sites showing bands at 1546 cm and 1446 cm assigned to pyri-
dine bound to Brønsted and Lewis acid sites, respectively. The band
−
1
at 1490 cm
is representative of adsorbed pyridine at both the
acidic sites. In FTIR spectrum of MS the bands at 1596 cm and
446 cm represent the hydrogen bonded pyridine with silanols.
−
1
2
2
from 1230 m /g to 450 m /g with the increase in Al loading
Table 1). The decrease in SBET of Al grafted silica samples in
−
1
1
(
comparison of MS and with increasing Al content indicates the
grafting of Al in internal surface (inside the pores) of MS. The N2
adsorption–desorption isotherms of all the samples were of type IV
showing mesoporous nature of samples (Fig. 3i), and the isotherms
do not change their shape on varying Al content in the samples.
The average pore diameter (DP; obtained by BJH analysis of the
3.2. Catalytic activity of MS and Al-MS for oxidative self coupling
of benzyl amine
Initially the oxidative self coupling of benzyl amine (1a) to imine
◦
(2a) was carried out with MS and Al-MS samples at 160 C for

K.N. Tayade, M. Mishra / Journal of Molecular Catalysis A: Chemical 382 (2014) 114–125
117
Table 1
Properties and catalytic activity of MS and Al-MS samples in oxidative coupling of benzyl amine.
Si/Ala
SBET (m /g)
2
DP (A)
˚
VP (cc/g)
Acidityb
Conv. (wt.%)c
Total amount of adsorbed
benzyl amine (mmol/g)d
Sample
◦
Temp. ( C)
mmol NH3/g
Total (mmol NH3/g)
MS
Al(20)-MS
–
20.5
1230
1156
33.8
33.8
0.767
0.759
99
94
0.01
0.88
0.01
0.88
39
53
0.22
0.38
9
9
0.65
0.26
0.11
Al(10)-MS
Al(5)-MS
10.6
4.7
757
450
33.7
33.7
0.532
0.485
244
10
1.02
1.07
31
23
0.40
0.42
6
9
9
0.51
0.37
0.19
251
36
6
Note: SBET, BET surface area; DP, average pore diameter; VP, pore volume.
a
By ICP analysis.
By NH3 TPD analysis.
b
c
◦
Oxidative self coupling of benzyl amine in solvent free condition; reaction condition: 4.7 mmol benzyl amine, 50 mg catalyst, 160 C, 6 h.
By measuring UV absorbance of supernatant solution of cyclohexane–benzyl amine mixture after liquid phase adsorption over samples (see ESI).
d
6
(
h to evaluate their catalytic activity for amine coupling reaction
Scheme 2). The MS (without Al) sample showed significant activity
for self coupling of benzyl amine giving 39% conversion of amine
1a) to imine (2a) with 100% selectivity (Table 1). The conversion
(
of amine increased to 53% with the sample having Si/Al ratio of 20,
while amine conversion decreased with the samples having high
Al loading (Si/Al: 10 and 5) under similar reaction condition. The
MS
5
4
3
2
1
0
0
0
0
0
(i)
Al(20)-MS
Fig. 4. FTIR spectrum of pyridine adsorbed MS and Al-MS sample (B-pyridine: pyri-
dine bound to Brønsted acid sites; L-pyridine: pyridine bound to Lewis acid sites).
Al(10)-MS
Al(5)-MS
interesting result was that the MS also showed good activity in self
coupling of benzyl amine. Liu et al. [8] proposed that during oxida-
tive coupling of benzylic amines in water under reflux condition,
the amines are activated by water molecules by hydrogen bonding
through amino group and thus amines react with oxygen to form
an imine intermediate via peroxide complexes; the imine interme-
diates finally react with free amines to give the imine products. It
has been reported that the benzyl amine molecules are adsorbed
over silica by forming hydrogen bonds with OH group of surface
silanols through NH2 group [20]. We also observed the benzyl
amine adsorption over MS by hydrogen bonding between NH2
group and surface silanols as shown in Scheme 3; structure (I).
The hydrogen bonding between silanol and NH2 group might be
activating the benzyl amine for the oxidative coupling of amine to
imine. When the reaction was carried out with chromatographic
0.0
0.2
0.4
0.6
0.8
1.0
P/Po
MS
(ii)
0
0
0
0
0
0
.025
.020
.015
.010
.005
.000
Al(20)-MS
Al(10)-MS
Al(5)-MS
2
silica gel (SG; Merck India; mesh size: 100–200; SBET: 496 m /g;
DP: 22 A˚ ), the conversion of benzyl amine to imine was observed
to be reduced to 25%. The reduced activity of SG as compared to
MS is attributed to lesser surface area of SG than MS. The MS will
3
0
40
50
60
70
80
90
MS or Al-MS
160 ºC
NH2
N
2
o
Pore diameter (A )
6
h
(2a)
(
1a)
Fig. 3. (i) N2-adsorption desorption isotherms and (ii) pore size distribution of MS
and Al-MS samples.
Scheme 2. Self coupling of benzyl amine (1a) to imine (2a) over MS and Al-MS.

1
18
K.N. Tayade, M. Mishra / Journal of Molecular Catalysis A: Chemical 382 (2014) 114–125
Scheme 3. Plausible mechanistic pathway of oxidative self coupling of benzyl amine over MS and Al-MS.
have more surface OH groups due to high surface area for benzyl
amine adsorption and activation for coupling reaction. This clearly
indicates the advantage of high surface area of ordered mesoporous
silica (MCM-41) in the amine coupling reaction.
The enhanced conversion with Al(20)-MS as compared to MS
can be attributed to surface acidic sites (Brønsted and Lewis sites)
generated after Al grafting working as co-ordination site for interac-
tion with amino group of benzyl amine (Scheme 3). The acidic sites
coordinate with the lone pair of electrons of amino group to activate
the amine for oxidation into benzaldehyde [21] or dehydrogena-
tion to form an imine intermediate [12]. The benzaldehyde/imine
intermediate can react with benzyl amine to form imine product.
The Al(10)-MS and Al(5)-MS samples were expected to be more
active for this reaction as these samples possess higher acidity
than Al(20)-MS, however, these samples gave very less conver-
sion. It was anticipated that the reduced activity of these two
Al-MS samples as compared to Al(20)-MS may be attributed to
decreased surface area and pore volume of the samples contain-
ing high Al content. But it was found that the lower activity was
due to the presence of high acidity (strength and number of sites)
in Al(10)-MS and Al(5)-MS samples, which can strongly bind more
number of basic molecules like amine reactant, imine intermediate,
imine product, etc. The strong interaction between sites and basic
molecules will not only block the reaction sites but may also slow

K.N. Tayade, M. Mishra / Journal of Molecular Catalysis A: Chemical 382 (2014) 114–125
119
Fig. 5. (a) TGA profiles of benzyl amine adsorbed MS and Al-MS samples and (b) DTG showing loss of adsorbed benzyl amine from the samples [BA-ads: benzyl amine
adsorbed].
down the product desorption rate from the surface sites reducing
the reaction rate. The adsorption property of the MS and Al-MS
samples for benzyl amine at room temperature (30 C) was studied
may be either for very strongly bound amine or amine derived prod-
ucts formed during thermal treatment (Fig. 5a). The TGA study of
benzyl amine adsorbed samples clearly showed the interaction of
benzyl amine with silanols of MS by hydrogen bonding, and with
OH groups as well as acid sites in Al(20)-MS, while the amine
interaction was very strong with acid sites of high Al containing
samples. The slow reaction with high Al containing samples can
be attributed to high adsorption and strong interaction of basic
molecules (reactant, intermediate and product) with acidic sites.
In the FTIR spectrum of MS (Fig. 6(i)), the broad peak at around
◦
(
see ESI). The order of adsorbed amount of benzyl amine over the
samples was observed to be as follows; MS < Al(20)-MS < Al(10)-
MS < Al(5)-MS (Table 1). It indicates high adsorption of benzyl
amine over Al-MS samples than MS; and in Al-MS samples, the
adsorption increases with increasing Al loading. In spite of high
adsorption of amine on Al(10)-MS and Al(5)-MS, the low activity
of these samples clearly indicates that the reduced activity can be
attributed to strong interaction of basic molecules (amine/imine)
with the acid sites of the samples. The TGA study of benzyl amine
adsorbed MS clearly indicates the presence of adsorbed benzyl
amine by hydrogen bonding with surface silanols showing cor-
−
1
3420 cm is due to O H stretching of adsorbed water; the band
−
1
−1
at 3741 cm and at 3664 cm are assigned to OH stretching of
free silanols and hydrogen bonded silanols, respectively [22]. The
−
1
3664 cm band disappeared in FTIR spectrum of Al(20)-MS show-
ing grafting of Al on MS surface using surface silanols, however
◦
responding weight loss in the range of 160–280 C (Fig. 5a). The
−
1
TGA–DTG profiles of benzyl amine adsorbed Al-MS samples (Fig. 5a
and b) revealed strong interaction of benzyl amine with acidic
sites showing weight loss/decomposition of adsorbed amine at
the band at 3741 cm for free silanols can be seen. This indicates
that Al(20)-MS contains some free silanols, whereas Al(10)-MS and
Al(5)-MS show a broad band in OH region showing no free silanols
probably due to high Al grafting. The FTIR results are revealing
that the samples with high Al loading may not have available free
silanols for hydrogen bonding with amine and therefore amine gets
adsorbed on only acidic sites. The FTIR spectra of amine adsorbed
MS as well as Al(20)-MS samples (Fig. 6(ii)) show disappearance
of the bands for free silanols and increased broadness of the OH
◦
higher temperature (∼340 C). However, in Al(20)-MS both hydro-
gen bonded as well as acid site co-ordinated benzyl amine can be
◦
◦
noticed showing weight loss at ∼220 C and ∼340 C, respectively.
The adsorption of benzyl amine on Al(10)-MS and Al(5)-MS samples
was predominantly on acidic sites showing weight loss at 340 C.
The weight loss above 450 C in BA adsorbed MS or Al-MS samples
◦
◦

1
20
K.N. Tayade, M. Mishra / Journal of Molecular Catalysis A: Chemical 382 (2014) 114–125
◦
Fig. 6. FTIR spectra of (i) MS and Al-MS samples after drying at 180 C (showing OH region); (ii) and benzyl amine adsorbed MS and Al-MS samples showing OH region;
−
1
−
1
(
iii) neat benzyl amine and benzyl amine adsorbed samples in the range of 1300–1700 cm and (iv) in the range of 780–680 cm [BA ads.: benzyl amine adsorbed samples;
BA: benzyl amine].
band in hydroxyl region indicating the hydrogen bonding between
adsorbed benzyl amine molecules and surface silanols. The charac-
teristic peaks of benzyl amine (Fig. 6(iii); at 1603 and 1495 cm for
towards high frequency. It clearly shows that the adsorption of ben-
zyl amine altered the aromatic nature of phenyl group and also
the nature of benzylic carbon was changed. In addition, the bands
−
1
−
1
−1
−1
−1
aromatic ring, 1583 cm for N H bending and 1452 cm for ben-
zylic C H bending [23]) can be seen to be significantly changed in
the spectrum of benzyl amine adsorbed samples. The broad band
(736 cm and 698 cm in benzyl amine) for out of plane vibra-
tion of aromatic hydrogens are also shifted to higher frequencies
−
1
−1
( 751 cm and 700 cm , respectively in adsorbed benzyl amine)
(Fig. 6(iv)). Granqvist et al. [20] reported that one of the hydro-
gen atoms (ortho) in the phenyl ring lies near (on the distance
of only 269 pm) to the oxygen atom in silanol whose hydrogen
atom forms a hydrogen bond to amine’s nitrogen. Therefore, it
is possible that one of ortho hydrogen of benzyl amine can form
hydrogen bonding with surface OH groups as shown in the struc-
ture II, Scheme 3. Based on the IR results of benzyl amine adsorbed
samples, we propose that the spectral change may be attributed
−
1
−1
,
centred at ∼1517 cm may consist of shifted bands (1603 cm
−1
−1
1
583 cm
and 1495 cm ) for adsorbed benzyl amine and also
some new bands (for protonated or acid site co-ordinated amino
group of benzyl amine), which are merged with each other. The
−
1
band at 1463 cm is for benzylic C–H bending of adsorbed ben-
−1
zyl amine, which is at 1452 cm for free benzyl amine. It can be
observed that the frequencies for aromatic and amino group were
lowered while that for benzylic C H bending was slightly shifted

K.N. Tayade, M. Mishra / Journal of Molecular Catalysis A: Chemical 382 (2014) 114–125
121
100
90
80
70
60
50
BA ads. Al(20)-MS
40
30
20
10
0
SF
DMF
DMSO
DEC
NB
Solvent
Al(20)-MS
Fig. 8. Conversion of benzyl amine in presence of different solvents [reaction con-
◦
dition: 4.7 mmol benzyl amine, 50 mg catalyst (Al(20)-MS), 1 mL solvent, 160 C,
150
100
50
0
-50
-100
-150
6
h] (SF: solvent free; DMF: dimethyl formamide; DMSO: dimethyl sulfoxide; DEC:
ppm
decane; NB: nitrobenzene).
Fig. 7. 27Al MAS NMR spectra of Al(20)-MS and benzyl amine adsorbed Al(20)-
MS showing conversion of octahedrally co-ordinated Al to tetra co-ordinated Al
in Al(20)-MS.
◦
be achieved with Al(20)-MS at 160 C and with 10 wt.% catalyst
loading (ESI, Figs. S3 and S4). In order to promote the efficiency of
MS and Al-MS for oxidative self coupling of benzyl amine, reactions
were also carried out in excess air and in different solvents studying
the effect of solvent nature (polar/non-polar) under similar reaction
condition.
to the structure II formed at the surface. The structure II shows
diminished aromaticity of phenyl group lowering the related IR
frequencies and strengthened benzylic C H bond as C H bend-
ing frequency was slightly increased. The hydrogen bonded or acid
sites co-ordinated (quaternized) NH2 group and hydrogen inter-
action of ortho aromatic hydrogen might be activating the amine
molecule for oxidative dehydrogenation to imine intermediate. The
O2 can react with thus activated amine complex (i.e., II) either at
ortho position in aromatic ring or at benzylic position forming their
hydroperoxide intermediates IIIa or III . The IIIa or III can elim-
The reactions were performed with Al(20)-MS in polar
nitrobenzene, DMSO, DMF) and non-polar (n-decane) media. The
(
amine conversion in non-polar medium (n-decane) was decreased
as compared to solvent free reaction (Fig. 8). The nitrobenzene was
found to be effective polar solvent for the reaction giving 70–80%
conversion of amine (100% imine selectivity) with MS and Al-MS
samples (Fig. 8 and Table 2). The highest conversion in presence
of nitrobenzene may be attributed to its polar nature, which can
facilitate the product desorption form the surface sites by solu-
bilizing the product. In addition the presence of polar solvent can
also assist the bulk and pore diffusion of the reactant and product.
b
b
inate H O and gives an imine intermediate to react with benzyl
2
2
amine giving imine product (2a). The similar mechanism for oxida-
tive coupling of benzylic amine in water has also been proposed [8]
showing the activation (by hydrogen bonding of benzylic amines
with water molecules) of ortho position of aromatic ring and ben-
zylic position for O addition.
2
2
7
The Al MAS NMR (Fig. 7) of benzyl amine adsorbed Al(20)-MS
shows the increased proportion of tetrahedral Al clearly indicating
the conversion of octahedrally co-ordinated Al to tetra co-ordinated
Al after amine adsorption. This indicates that some of the octahe-
dral Al are existing due to co-ordination of OH groups (of Si OH
and/or Al OH) to Al having less than six co-ordination (i.e., tri,
tetra and penta co-ordinated Al), which may get converted to tetra
co-ordinated Al during the reaction of amine with co-ordinated
OH groups breaking their co-ordination with Al. In addition, this
will also make the Al sites free for co-ordination with benzyl
amine to form adsorbed amine complex (II, Scheme 3). Thus FTIR
Table 2
Activity of MS and Al-MS for benzyl amine self coupling reaction in solvent free
condition and in presence of polar solvent and effect of excess air.
Reaction in solvent
free (SF)/in
nitrobenzene (NB)
Reaction in excess air
(AIR)/in closed vessel
(CV)
Sample
Conv. of amine
(wt.%)
SF
SF
SF
SF
SF
SF
SF
SF
NB
NB
NB
NB
NB
NB
NB
NB
CV
CV
CV
CV
AIR
AIR
AIR
AIR
CV
CV
CV
CV
AIR
AIR
AIR
AIR
MS
39
53
31
23
42
57
62
77
70
80
78
77
83
100
99
96
Al(20)-MS
Al(10)-MS
Al(5)-MS
MS
Al(20)-MS
Al(10)-MS
Al(5)-MS
MS
Al(20)-MS
Al(10)-MS
Al(5)-MS
MS
Al(20)-MS
Al(10)-MS
Al(5)-MS
27
and Al NMR studies show that the surface OH groups and Al
sites are facilitating the amine adsorption and activation (through
hydrogen bonding and Al sites co-ordination) for the coupling
reaction.
3.3. Effect of solvent and excess air on catalytic performance of
MS and Al-MS in oxidative self coupling of benzyl amine
◦
After optimizing the reaction parameters like reaction tempera-
ture and catalyst loading, the highest amine conversion (53%) could
Solvent free: 4.7 mmol benzyl amine, 50 mg catalyst, 160 C, 6 h. In solvent: 4.7 mmol
benzyl amine, 1 mL NB, 50 mg catalyst, 160 C, 6 h.
◦

1
22
K.N. Tayade, M. Mishra / Journal of Molecular Catalysis A: Chemical 382 (2014) 114–125
1
00
MS or
NH2 Al(20)-MS
N
2
9
8
7
6
5
4
3
2
0
0
0
0
0
0
0
0
o
R
160 C, 6 h,
R
R
(
1a-d)
air, NB
(2a-d)
1
a: benzyl amine
1
1
1
b: p -methoxy benzyl amine
c: p-methyl benzyl amine
d: p-chloro benzyl amine
Scheme 4. Oxidative self coupling of benzyl amine derivatives with MS and Al(20)-
MS.
MS
3.4. Self coupling of different benzyl amine derivatives with MS
and Al(20)-MS
Al(5)-MS
Al(10)-MS
Al(20)-MS
As among Al grafted samples, Al(20)-MS showed good activity
in benzyl amine self coupling reaction, the catalytic performance
of Al(20)-MS was also tested for self coupling of the derivatives
of benzyl amine (1a–d) and its activity was compared with MS
(Scheme 4).
1
2
3
4
5
6
Reaction time (h)
Fig. 9. Reaction kinetics of benzyl amine self coupling with MS and Al-MS samples
in nitrobenzene and in excess air (4.7 mmol benzyl amine, 50 mg catalyst, 1 mL NB,
in excess air, 160 C).
Both the samples, i.e., MS and Al(20)-MS gave good to excellent
conversion (83–100%) of substrates to respective imine products
◦
(98–100%) under similar reaction condition showing the versatil-
ity of the MCM-41 and Al grafted MCM-41 materials for catalysis
of self coupling of benzylic amines (Table 3). However, it can be
seen that the Al(20)-MS shows higher activity (obviously due to
presence of acidic sites) than MS giving 100% conversion. Further-
The nitrobenzene might also be reducing the possibility of surface
sites blockage (complexation) by removing the adsorbed species
(
reactant, intermediate and product molecules) by solubilization.
more, the amines with electron donating groups ( OCH and CH3)
The polar solvents like DMF and DMSO were observed to be reacting
with benzyl amine forming number of by-products. Furthermore,
3
were found to be more reactive than benzyl amine and p-chloro
benzyl amine giving high conversion with MS. This may be due
to the comparatively higher basicity of p-methoxy benzyl amine
1
–2 mL volume of solvent (nitrobenzene) was found to be opti-
mum amount for better conversion of amine in the reaction (see ESI,
Fig. S5) and further increase in solvent volume reduces the conver-
sion, which may be due to slow bulk diffusion. In the present study,
nitrobenzene was selected as polar solvent due to its high dielectric
(pKa = 9.5) and p-methyl benzyl amine (pKa = 9.51) than benzyl
amine (pKa = 9.34) and p-chloro benzyl amine (pKa = ∼8.8) resulting
into higher surface adsorption for oxidation to imine intermedi-
ate. The adsorption extent of amines over silica surface has been
reported to be dependent on basicity of amines; the adsorption
amount increases as the basicity of amine increases [25]. In addi-
tion, the higher nucleophilicity of p-methoxy benzyl amine and
p-methyl benzyl amine (due to electron donating groups) would
also facilitate the nucleophilic reaction of amine with imine inter-
mediate (IV; Scheme 3).
◦
constant (34.8), high boiling point (205 C) and un-reactive nature,
which has already been used as polar solvent in high temperature
heterogeneous catalytic reactions [24]. This study reveals that a
suitable (greener) high boiling point polar solvent can be explored
to promote the efficiency of MCM-41 catalysts for amine coupling
reactions.
The solvent free reactions carried out in presence of excess air
(
by purging the atmospheric air in the reaction tube from air com-
3
.5. Cross coupling of benzyl amine with different amines using
pressor) gave slight enhancement in amine conversion with MS
and Al(20)-MS. Whereas benzyl amine conversion was significantly
increased with Al(10)-MS and Al(5)-MS samples as compared to the
reactions in closed vessel under solvent free condition (Table 2). It
can be seen that the conversion is increasing with increasing Al
content, which may be due to increase in acidity and the amine
adsorption. The presence of excess oxygen can speed up the oxi-
dation of adsorbed amine to imine avoiding the sites blockage by
complexation. This result also indicates that the amine activation
is via oxidative dehydrogenation.
MS and Al(20)-MS
The catalytic activity of MS and Al(20)-MS samples were evalu-
ated for cross coupling of benzyl amine (1a) with different amines
ꢀ ꢀ
1 a–e) to cross imines (2 a–e) under similar reaction conditions
Scheme 5). Aniline (1 a) and its derivatives (p-nitro aniline (1 b)
(
(
ꢀ
ꢀ
ꢀ
ꢀ
and p-methyl aniline (1 c)), cyclohexyl amine (1 d) and n-octyl
amine (1 d) were taken as substrates (aromatic, alicyclic and
aliphatic, respectively) to study the effect of substrate nature on
ꢀ
From these studies, it was evident that the presence of polar sol-
vent (nitrobenzene) and excess air in the reaction could promote
the catalysts activity. In nitrobenzene, the presence of excess air
significantly enhanced the conversion of amine (83–100%) with all
the samples showing that the presence of polar solvent and excess
air facilitates the reaction (Table 2). The Al-MS samples showed
higher activity than MS. Furthermore, it was observed that the reac-
tion with Al(20)-MS was faster than high Al containing samples in
nitrobenzene and in excess air (Fig. 9). This shows the higher activ-
ity of Al(20)-MS than Al-MS samples with high Al loading, which
is attributed to its high surface area, presence of finely dispersed
optimum amount of Al and acidity, and also presence of accessible
free surface OH groups.
MS or
R
NH2
Al(20)-MS
N
R
NH2
+
o
1
60 C, 6 h,
air, NB
(
1a)
(1'a-e)
(2'a-e)
1
'a: aniline
1'b: p -nitro aniline
1'c: p-methyl aniline
1
1
'd: cyclohexyl amine
'e: n-octyl amine
ꢀ
Scheme 5. Cross coupling of benzyl amine (1a) and different amines (1 a–e) to cross
ꢀ
imines (2 a–e) with MS and Al(20)-MS.

K.N. Tayade, M. Mishra / Journal of Molecular Catalysis A: Chemical 382 (2014) 114–125
123
Table 3
Conversion and imine selectivity in oxidative self coupling of different benzylic amines with MS and Al(20)-MS.
a
Sample
MS
Amine (1a–d)
Conv. (wt.%)
83
Imine (2a–d) selectivity (%)
100
NH2
Al-MS(20)
MS
100
96
100
100
NH2
Al-MS(20)
MS
H CO
3
100
93
100
99
NH2
Al-MS(20)
H C
100
100
3
MS
Al-MS(20)
NH2
86
100
98
100
Cl
a
◦
Reaction condition: 4.7 mmol amine, 50 mg catalyst, 1 mL NB, in excess air, 160 C, 6 h.
Table 4
ꢀ
a
Conversion of benzyl amine and products (self and cross imine) selectivity in cross coupling of benzyl amine (1a) and different amines (1 a–e) with MS and Al(20)-MS.
ꢀ
Amine (1 a–e)
Time (h)
Conv./select. with MS
Conv./select. with Al(20)-MS
ꢀ
ꢀ
Conv. (wt.%)
100
Self imine (2a)
Cross imine (2 a–e)
Conv. (wt.%)
100
Self imine (2a)
Cross imine (2 a–e)
NH2
12
12
88
20
80
2
1
1
0
2
2
100
1
99
100
99b
2
98
18
NH2
O N
2
100b
40
47
54
NH2
H C
3
100
85
14
32
86
68
99
79
10
35
90
65
NH2
12
2
4
2
97
91
9
91
79
100
83
10
20
90
80
1
21
NH2
7
24
100
13
82
99
15
85
a
ꢀ
◦
Reaction condition: 4.7 mmol benzyl amine, 4.7 mmol amine (1 a–e), 50 mg catalyst, 1 mL NB, in excess air, 160 C.
b
The byproduct was benzaldehyde (13% with MS and 21% with Al(20)-MS).
cross coupling reaction. The results (Table 4) show the excellent
activity of MS and Al(20)-MS in cross coupling reactions giving
almost complete conversion of 1a in 12–24 h with good selectiv-
ity to cross imines (except p-nitro aniline). However, the difference
reactive giving highest cross imine selectivity with 99–100% con-
version of benzyl amine. The good reactivity of aniline may be due
to its comparatively less adsorption over the surface reactive sites
ꢀ
ꢀ
than aliphatic amines (1 d and 1 e) keeping the aniline molecules
free for nucleophilic reaction with self imine (2a). The cross reac-
ꢀ
in reactivity of amines (1 a–e) in the MS and Al(20)-MS catalyzed
ꢀ
cross coupling reactions was observed, which can be explained on
the basis of the basicity and nucleophilicity of the amines. The high
tion of benzyl amine with cyclohexyl amine (1 d) and n-octyl amine
ꢀ
(1 e) was observed to be slow giving less conversion and selectiv-
ꢀ
selectivity of a cross imine (2 a–e) in the reaction indicates high
ity of respective cross imine as compared to aniline and aniline
ꢀ
ꢀ
ꢀ
reactivity of the 1 a–e amine. It was found that the cross imine
2 a–e) formation takes place via nucleophilic reaction of amines
1 ) with self imine product (2a) as shown in Scheme 6. The sim-
derivatives. Owing to the higher basicity of 1 d and 1 e than ani-
ꢀ
ꢀ
(
line, these molecules will be more keen to be adsorbed on surface
ꢀ
(
reaction sites (silanols/acidic). The adsorption of amine (1 d and
ꢀ
ilar mechanism for cross coupling of benzyl amine with different
amines has also been proposed by others [5b,12].
1 e) will not only slow down the reactions (i.e., self coupling of
ꢀ
ꢀ
1a as well as cross coupling with 1 d and 1 e) by blocking the reac-
tion sites but also reduce the availability of free amine nucleophiles
for cross reaction. If we compare the reactivity of aniline and its
ꢀ
Aniline (1 a) being less basic (pKa: 4.6) than cyclohexyl amine
ꢀ
ꢀ
(
1 d; pKa: 10.7) and n-octyl amine (1 d; pKa: 10.6) is more
R
NH
2
Self coupling
R
(
1'a-e)
NH2
of 1a
N
N
NH2
2
+
Nucleophilic
(
1a)
(2a)
reaction of 1'a-e
(2'a-e)
(1a)
ꢀ
ꢀ
Scheme 6. Formation of cross imine (2 a–e) by nucleophilic reaction of amine (1 a–e) with self imine product (2a) formed by self coupling of benzyl amine (1a).

1
24
K.N. Tayade, M. Mishra / Journal of Molecular Catalysis A: Chemical 382 (2014) 114–125
NH2
NH2 Al(20)-MS
N
N
+
+
+
o
R
R
R
160 C, 6 h,
(1a)
(1b, 1c, 1d)
NB
(2a)
(2b, 2c, 2d)
air,
1b: p -methoxy benzyl amine
1c: p-methyl benzyl amine
1d: p -chloro benzyl amine
N
N
+
R
R
(
2ba', 2ca', 2da')
(
2ab', 2ac', 2ad')
Scheme 7. Competitive self and cross coupling reactions of benzyl amine (1a) and its derivatives (1b–d) over Al(20)-MS.
Table 5
Conversion and product distribution in competitive self and cross coupling reactions of benzyl amine (1a) and its derivatives (1b–d) over Al(20)-MS.
a
Entry
Amine
1a)
Conv. of 1a
Conv. of 1b–d
Selectivity (%) of
ꢀ
ꢀ
ꢀ
ꢀ
(
(1b–d)
2a
(2ab –2ad )
(2ba –2da )
2b–d
NH2
NH2
NH2
NH2
H CO
3
1
2
7
84
7
13
46
34
NH2
H C
3
15
77
83
86
15
23
45
30
18
32
11
NH2
Cl
3
27
a
◦
Reaction condition: 4.7 mmol benzyl amine, 4.7 mmol amine (1b–d), 50 mg catalyst, 1 mL NB, in excess air, 160 C, 6 h.
ꢀ
ꢀ
ꢀ
derivatives (1 a–c), we find that the nucleophilicity of aromatic
as substrate and 2b–d as nucleophiles; (iv) 2ba –2da : cross imines
formed by reaction of 2b–d as substrates and 1a as nucleophile.
This is evident from the results (Table 5; entry 1 and 2) that
the 1b and 1c benzylic substrates having electron donating groups
amines might be influencing the cross reaction. The electron
donating CH3 group in p-methyl aniline (1 c) will increase the
nucleophilicity of 1 c as compared to aniline giving 90% cross imine
ꢀ
ꢀ
selectivity in 12 h (which was 80% with aniline after 12 h). The
decreased nucleophilicity of p-nitro aniline (1 b) due to electron
( OCH and CH ) are more reactive as compared to benzyl amine
3 3
ꢀ
(1a) giving higher conversion (82% and 83%, respectively) and high
selectivity to self of 1b and 1c as well as respective cross imines
withdrawing NO2 group may reduce its reactivity for cross reac-
ꢀ
ꢀ
ꢀ
ꢀ
tion in comparison of 1 a and 1 c. In the cross reaction of benzyl
amine with p-nitro aniline, significant amount of benzaldehyde
2ba and 2ca formed by nucleophilic reaction of benzyl amine
(1a). The probable reason is the higher adsorption as well as nucle-
ophilicity of 1b and 1c as compared to 1a due to slightly higher
basicity of 1b (pKa = 9.5) and 1c (pKa = 9.51) than 1a (pKa = 9.34).
The electron withdrawing Cl decreases the basicity (pKa = ∼8.8) of
p-chloro benzyl amine, which may reduce the surface adsorption as
compared to benzyl amine. The less surface availability of p-chloro
(
12% with MS and 21% with Al(20)-MS) was found in the reaction
mixture. In this reaction, benzaldehyde is possible to be formed by
ꢀ
acid catalyzed hydrolysis of cross product (2 b; C N bond hydrol-
ysis) as the amount of benzaldehyde was more (21%) in Al(20)-MS
catalyzed reaction decreasing cross product selectivity. The nitro
ꢀ
group in cross imine product (2 b) can polarize the C N bond facil-
itating the acid hydrolysis. The cyclohexyl amine and n-octyl amine
having ␣-hydrogen did not undergo self coupling as no respec-
tive self imine products were formed in the reaction. It shows that
the cyclohexyl amine and n-octyl amine are less reactive towards
oxidative dehydrogenation with MS and Al-MS as compared to ben-
zyl amine (may be due to formation of complex II for benzyl amine;
Scheme 3) and therefore benzyl amine always reacts as substrate
to produce self imine.
100
9
8
7
6
0
0
0
0
Conv. with MS
The reaction kinetics (Fig. 10) of cross coupling reaction of ben-
Self imine with MS
ꢀ
Cross imine with MS
Conv. with Al(20)-MS
Self imine with Al(20)-MS
Cross imine with Al(20)-MS
zyl amine (1a) and aniline (1 a) with Al(20)-MS was observed to
50
be slower as compared to MS under similar reaction condition
4
3
2
1
0
0
0
0
0
◦
(
at 160 C, in presence of solvent and in excess air). This may be
attributed to the interaction of aniline molecules with acidic sites
reducing the number of adsorption and reaction sites for cross
coupling reaction of self imine product (2a) and also reducing the
concentration of free nucleophile for the reaction.
The competitive coupling reactions of benzyl amine (1a) and
substituted benzyl amine (1b–d) were carried out in the mixture
with Al(20)-MS to study the influence of the substituent on the
reactivity of benzylic amines for self and cross coupling reactions
2
4
6
8
10
12
14
16
18
20
22
24
Time (h)
(
Scheme 7). In this type of reaction, four products are possible to be
Fig. 10. Reaction kinetics of benzyl amine cross coupling with aniline over MS and
formed; (i) 2a: self imine of 1a; (ii) 2b–d: self imines of second ben-
zylic amines; (iii) 2ab –2ad : cross imines formed by reaction of 1a
Al(20)-MS samples in nitrobenzene and in excess air [reaction condition: 4.7 mmol
ꢀ
ꢀ
◦
benzylamine, 4.7 mmol aniline, 50 mg catalyst, 1 mL NB, in excess air, 160 C].

K.N. Tayade, M. Mishra / Journal of Molecular Catalysis A: Chemical 382 (2014) 114–125
125
benzyl amine will give less self imine product (2d) as well as less
cross imine (2da ) formation by reacting as substrate (Table 5; entry
(o) Y. Maeda, T. Nishimura, S. Uemura, Bull. Chem. Soc. Jpn. 76 (2003)
2399–2403;
ꢀ
(
p) M. Shimizu, H. Orita, T. Hayakawa, K. Suzuki, K. Takehira, Heterocycles 41
1995) 773–779.
3
). It is evident that the reactivity of benzylic amines in the MS and
(
Al-MS catalyzed self and cross coupling reactions is dependent of
basicity and nucleophilicity of amines.
[3] R.D. Patil, S. Adimurthy, RSC Adv. 2 (2012) 5119–5122.
[
[
4] B. Zhu, R.J. Angelici, Chem. Commun. (2007) 2157–2159.
5] (a) B. Zhu, M. Lazar, B.G. Trewyn, R.J. Angelici, J. Catal. 260 (2008) 1–6;
(
b) A. Grirrane, A. Corma, H. Garcia, J. Catal. 264 (2009) 138–144;
4
. Conclusions
(c) L. Aschwanden, B. Panella, P. Rossbach, B. Keller, A. Baiker, ChemCatChem 1
(
(
(
2009) 111–115;
d) M.-H. So, Y. Liu, C.-M. Ho, C.-M. Che, Chem. Asian J. 4 (2009) 1551–1561;
e) H. Miyamura, M. Morita, T. Inasaki, S. Kobayashi, Bull. Chem. Soc. Jpn. 84
The MCM-41 and Al grafted MCM-41 were demonstrated to
be good catalysts for oxidative self and cross coupling of benzyl
amines to imines. The self and cross amine coupling reactions are
influenced by presence of Al, its amount, acidity of catalyst, amines
nature (basicity and nucleophilicity), presence of polar solvent and
excess air. The surface OH groups and acid sites play an effec-
tive role in amine activation, but high acidity reduces the activity.
The MCM-41 without Al also showed good activity in oxidative self
and cross coupling of amines. The Al in optimum amount (Si/Al:
(2011) 588–599.
[6] K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed. 42 (2003) 1480–1483.
7] H. Huang, J. Huang, Y.-M. Liu, H.-Y. He, Y. Kao, K.-N. Fan, Green Chem. 14 (2012)
30–934.
[8] L. Liu, S. Zhang, X. Fu, C.-H. Yan, Chem. Commun. 47 (2011) 10148–10150.
[9] (a) X. Lang, H. Ji, C. Chen, W. Ma, J. Zhao, Angew. Chem. Int. Ed. 50 (2011)
[
9
3
(
5
934–3937;
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