MnOX supported on a TiO2@SBA-15 nanoreactor used as an efficient catalyst for one-pot synthesis of imine by oxidative coupling of benzyl alcohol and aniline under atmospheric air

Article abstract of DOI:10.1039/c6ra14941c

In the present study, a mesoporous silica (SBA-15) encapsulated TiO₂ nanoreactor is used as a support for MnO_x and this MnO_x/TiO₂@SBA-15 acts as a catalyst for the one-pot synthesis of imine by oxidative coupling between benzyl alcohol and aniline in the presence of atmospheric air. To understand the properties, the catalysts were characterized by several analytical techniques, namely, N₂ adsorption-desorption isotherm, small angle X-ray scattering (SAXS), wide angle X-ray diffraction, high resolution transmission electron microscopy (HRTEM), H₂-temperature programmed reduction (H₂-TPR), O₂-temperature programmed oxidation (O₂-TPO) and NH₃-temperature programmed desorption (NH₃-TPD). The pore encapsulation process by SBA-15 causes TiO₂ to be in a highly dispersed state, and this highly dispersed TiO₂ makes maximum contact with the MnO_x species as well as the reactant molecules. The reaction was carried out at atmospheric pressure with equimolar amounts of substrates without additives in the presence of atmospheric air. The yield and selectivity of imines vary with the MnO_x and TiO₂ loading. The 7.5 wt% MnO_x loaded TiO₂@SBA-15 (5 wt% TiO₂) nanoreactor showed the highest catalytic activity. With the increase in weak acid sites and the oxygen activation ability of the prepared catalyst, the conversion and selectivity of the desired product reached 96% and 97%, respectively. The investigation of the reaction mechanism suggests that there is a synergistic effect between highly dispersed TiO₂ and MnO_x, which improves the reactant conversion and the selectivity of the desired product (N-benzylideneaniline) and also the prepared catalyst shows excellent recyclability up to the 10^th cycle. The recyclability and hot filtration study confirms the true heterogeneity of the prepared catalyst during imine synthesis. The heterogeneity of the prepared catalyst, the avoidance of any noble metal and the utilization of air as an oxidizing agent represent an efficient, green reaction pathway for imine synthesis.

Full text of DOI:10.1039/c6ra14941c

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DOI: 10.1039/C6RA14941C
MnOx supported on TiO2@SBA-15 nanoreactor used as an
efficient catalyst for one-pot synthesis of imine by oxidative
coupling of benzyl alcohol and aniline under atmospheric air
Sandip Mandala*, Sudip Maitya*, Sujan Sahaa, Biplab Banerjeeb
a Research Group of Gasification and Liquefaction, CSIRꢀ Central Institute of Mining and Fuel
Research (DC), PO: FRI, Dhanbad ꢀ 828108, India
b Department of Material Science, Indian Association for Cultivation of Science, Jadavpur,
Kolkata ꢀ 700032, India
*Corresponding authors: sandip.chemist09@gmail.com; and sudip_maity@yahoo.com;
Tel: (+91)ꢀ326ꢀ2388363; Fax: (+91)ꢀ326ꢀ2381113
Abstract
In the present study, mesoporous silica (SBAꢀ15) encapsulated TiO2 nanoreactor is used as a
support for MnOx and this MnOx/TiO2@SBAꢀ15 acts as a catalyst for oneꢀpot synthesis of imine
by oxidative coupling between benzyl alcohol and aniline in presence of atmospheric air. To
understand the properties, the catalysts are characterized by several analytical techniques namely
N2 adsorptionꢀdesorption isotherm, small angle Xꢀray scattering (SAXS), wide angle Xꢀray
diffraction, high resolution transmission electron microscopy (HRTEM), H2ꢀ temperature
programmed reduction (H2ꢀTPR), O2ꢀtemperature programmed oxidation (O2ꢀTPO) and NH3ꢀ
temperature programmed desorption (NH3ꢀTPD). The pore encapsulation process by SBAꢀ15
makes TiO2 in highly dispersed state and this highly dispersed TiO2 makes maximum contact
with MnOx species as well as reactants molecules. The reaction is carried out at atmospheric
pressure with equimolar amounts of substrates without additives in presence of atmospheric air.
The yield and selectivity of imine vary with the variation of MnOx and TiO2 loading. 7.5 wt%
MnOx loaded TiO2@SBAꢀ15 (5 wt% TiO2) nanoreactor shows highest catalytic activity. With
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the increase of weak acid site and oxygen activation ability of the prepared catalyst, the
conversion and selectivity of the desired product reach up to 96% and 97% respectively. The
investigation of reaction mechanism suggests that there is a synergistic effect between highly
dispersed TiO2 and MnOx, which improves reactant conversion and selectivity of desired product
(NꢀBenzylideneaniline) and also the prepared catalyst shows excellent recyclability up to 10th
cycle. The recyclability and hot filtration study confirm true heterogeneity of the prepared
catalyst during imine synthesis. The heterogeneity of the prepared catalyst avoiding any noble
metal and utilization of air as an oxidizing agent represent the efficient green reaction pathway
for imine synthesis.
Key Words: Oxidative coupling, Mesoporous catalyst, Nanoreactor, Imine, Atmospheric
oxygen
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1. Introduction
In the present century, tremendous attention has been paid to green and sustainable processes in
diverse fields of our daily life namely energy production, automobile and transportation sector,
agricultural sector as well as chemical sector. In chemical sector oxidation reaction is an
important process for production of different types of industrially important chemicals.
Traditionally these oxidation processes are carried out in presence of different types of harsh
oxidizing agents like chromates, permanganate, peracid etc. which generate environmentally
hazardous byꢀproducts.1 To avoid these types of environmental burden, heterogeneously
catalyzed oxidation process gets a special attention in chemical sector. In case of oxidation or
oxidation related chemical reactions researchers are focused to develop an efficient
heterogeneously catalyzed route by using air as an oxidizing agent. These processes represent a
highly atom efficient, green and sustainable route because oxygen used as an oxidizing agent
which is cheap and readily available and it produces water as a byꢀproduct which is
environmentally safe.2, 3
Oxidative coupling reaction is an industrially important reaction for production of imines
because imines are widely used as Nꢀcontaining compounds which have several applications on
diverse field such as pharmaceutical, agricultural, fertilizer as well as for R&D work.4, 5 In case
of oxidative coupling between alcohol and amine, oxidation process play a vital role because
alcohol gets oxidized to give aldehyde which further reacts with amine to give imine. To avoid
harsh reaction conditions during traditional imine synthesis6, 7, researchers are focusing their
attention to develop heterogeneously catalyzed aerobic oxidation process which gives better
performance towards oxidative coupling reaction and also safe in environmental perspective. An
efficient heterogeneously catalyzed oxidative coupling process strongly depends on the oxygen
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activation ability of the catalyst in presence of molecular oxygen.8 Among different types of
catalysts Au, Pd, Ag, Cu, Pt, Ru based catalysts are mainly used for imines synthesis by
oxidative coupling of alcohol and amine or dimerization of amine.9ꢀ17 As precious noble metal
catalysts are no longer economically acceptable, hence present study aims to design the catalyst
for oxidative coupling reaction by avoiding noble metal and also work under mild condition,
easily separable and reusable. In case of noble metal catalysts mainly pure oxygen is used as an
oxidizing agent for imines synthesis by oxidative coupling of alcohol and amine. Kegnaes et al.9
reported Au/TiO2 catalyst for oxidative coupling of alcohol and amine under ambient condition
but different types of basic agent was added in reaction medium. Chen et al. also reported the use
of MnOx supported on hydroxyapatite as an effective catalyst for oxidative coupling of alcohol
and amine by using air as an oxidizing agent.18
Manganese oxide supported on titania, zirconia, alumina19ꢀ22 is widely used for different types of
oxidation reactions. The synergistic relationship between manganese oxide and titania facilitates
the oxidation reaction. In the present study, SBAꢀ15 mesoporous silica encapsulated TiO2
(TiO2@SBAꢀ15) nanoreactors with variable amount of TiO2 loading (2.5, 5.0 and 7.5 wt%) are
prepared. These TiO2@SBAꢀ15 nanoreactors are used as efficient support for MnOx and these
prepared MnOx/TiO2@SBAꢀ15 materials are used as effective catalysts for oxidative coupling
between alcohol and amine under atmospheric air. It is well known fact that titanium form
hydroperoxo species in presence of oxygen and this hydroperoxo species act as an active
oxidizing agent during oxidation reaction.23, 24 In case of highly dispersed titania, maximium
portion of tatania is in contact with MnOx species as well as atmospheric oxygen molecule
compare to bulk titania. The highly dispersed TiO2 on SBAꢀ15 and gas adsorption property of
SBAꢀ15 facilitate the formation of titaniaꢀperoxo species.
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1. Experimental
2.1. Materials
All chemicals and reagents are of commercial grade and used without further purification;
Pluronic (P123, M.W. ꢀ 5800) [99%, SigmaꢀAldrich], 35% Hydrochloric Acid (Merck India),
Tetraethylorthosilicate (TEOS) [Merck Germany], Tetraethylorthotitanate (Merck Germany),
Mn(NO3)2.4H2O (Merck India), benzyl alcohol (Merck India), Aniline (Merck India), Toluene
(Merck India), Sodium biꢀcarbonate (Merck India). Double purified Millipore water is used
throughout the experimental procedure.
2.2. Preparation of SBAꢀ15 material
The SBAꢀ15 mesoporous silica was prepared by following the procedure reported by Nijhuis et.
al 25
In this procedure, 8 g Pluronic (P123), 288 g deionized water and 10.8 g of conc. HCl (35%)
.
were taken in a 500 mL beaker and stirred continuously until P123 was completely dissolved.
After formation of homogeneous solution, 16.8 g tetraethylorthosilicate (TEOS) was added to it
and stirred continuously for 24 h at 35 °C temperature. The mixture was further transferred in a
Teflon autoclave bottle and kept at 100 °C for 24 h under static condition. The resulting
precipitate was filtered by using Whatman No. 42 filter paper, washed with deionized water and
dried at 80 °C for 16 h and then the dried mass was calcined at 540 °C temperature for 16 h with
1 °C heating ramp. The prepared solid mass was taken and stored in a desicator.
2.3. Preparation of TiO2@SBAꢀ15 catalyst
In this procedure 0.4 g tetraethylorthotitanate was dissolve in 20 mL anhydrous ethanol and 1 g
SBAꢀ15 was added to it. Now the whole content was stirred at room temperature until ethanol
was evaporated. After initial evaporation of 20 mL of ethanol, further 5 mL of ethanol was added
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twice and stirred until whole of the ethanol was evaporated at each stage. The prepared solid
mass was collected and dried at 80 °C for 16 h. After drying the solid mass was taken in a quartz
crucible and calcined at 500 °C for 4 h with 1 °C heating ramp. The prepared solid mass was
taken and stored in a desicator.
2.4. Preparation of MnOx/TiO2@SBAꢀ15 catalyst
In this procedure 0.1154 g of Mn(NO3)2.4H2O was dissolve in 50 mL deionized water and the
solution was heated at 50 °C temperature. Now the pH of the solution was maintained at 7.5 by
adding (N/20) aqueous NaHCO3 solution with continuous stirring [300 rpm]. After that 0.4 g
TiO2@SBAꢀ15 was added to it and pH of the solution was maintained at ~7.5 by adding (N/20)
aqueous NaHCO3 solution with continuous stirring [300 rpm]. This whole content was stirred
[300 rpm] for 24 h by maintaining temperature ~50 °C and pH ~7.5. After stirring the whole
content was cooled down to room temperature and the solid material was filtered by Whatman
No. 42 filter paper and washed thoroughly by deionized water. The prepared solid material was
dried at 80 °C for 16 h and then calcined at 300 °C for 4 h with 1 °C heating ramp. The prepared
solid mass was taken and stored in a desicator.
2.5. Catalyst Characterization
The N2ꢀadsorption desorption isotherm of the prepared catalyst was measured at liquid nitrogen
temperature with Micromeritics TriStar instrument. The surface areas of the catalysts were
calculated by BraunerꢀEmmettꢀTeller (BET) equation and pore size distributions were calculated
by BJH method. Small angle Xꢀray scattering (SAXS) were carried out by using a Bruker AXS
Dꢀ8 Advance diffractometer operated at a 40 kV voltage and 40 mA current and calibrated with a
standard silicon sample, using Niꢀfiltered Cu Kα (λ = 0.15406 nm) radiation. Wide angle Xꢀray
diffraction pattern was measured by using D8 ADVANCE (BRUKER AXS, Germany)
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diffractometer using CuKα radiation with parallel beam (Gobel Mirror). Peaks are identified by
search match technique using DIFFRACplus software (BRUKER AXS, Germany) with reference
to the JCPDS database. The software TOPAS 3.0 from Bruker AXS (2005) is used for
refinement of crystalline phase to determine the average crystallite size making all of the
corrections and refinement processes using the fundamental parameter approach (FPA). The
High resolution transmission electron microscopy with elemental mapping was done by JEOL
JEM 2100 microscope operated at 200 kV acceleration voltage using a lacy carbon coated Cu
grid of 300 mesh size. The Fourier Transform Infrared (FTꢀIR) spectroscopic analysis is carried
out by using Cary 660 FTIR spectrometer (Agilent Technology Cary 600 Series) using KBr
pellet technique. The temperature programmed reduction (TPR), temperature programmed
oxidation (TPO) and temperature programmed desorption (TPD) profiles of the catalysts were
measured by using Chemisorb 2720 (Micrometrics, USA) instrument equipped with a TCD
detector. The H2ꢀTemperature programmed reduction profiles were measured in presence of 10%
H2 in Ar at a flow rate of 20 mL/min and the temperature was increased from ambient to 900 °C
at a rate of 10 °C/min. The TPO profiles of the samples were measured in presence of 4.2 % O2
in He with a flow rate of 20 mL/min at temperature range between ambient to 700 °C at a rate of
10 °C/min. The NH3ꢀTPD of the prepared catalysts were measured by absorbing 0.4 % NH3 in
He at room temperature and then the adsorbed gas was desorbed in presence of He at a flow rate
of 20 mL/min and the temperature was increased from ambient to 900 °C at a rate of 10 °C/min.
2.6. Catalytic activity study
A series of experiments were performed to investigate the catalytic activity of the prepared
catalysts and in each experiment, catalyst was used after drying at 100 °C temperature. In these
experiments benzyl alcohol (1 mmol), aniline (1 mmol), anhydrous toluene solvent (2 mL) and
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catalyst (0.1 g) were mixed together in 50 mL round bottomed flask fitted with water condenser
and air balloon. Now this reaction mixture containing solvent, reactants and catalysts in air
atmosphere (moisture free) was kept in a thermostatic oil bath and stirred at 300 rpm speed for
24 h at 80 °C temperature. After completion of each reaction, the product mixture was
centrifuged and supernatant liquid was used for GC analysis (Make: Chemito, Model: GCꢀ1000)
equipped with SEꢀ30 packed column and FID detector.
3. Results & Discussions
3.1. N2 AdsorptionꢀDesorption Isotherm
The N2 adsorptionꢀdesorption isotherms (Fig. 1) of prepared catalysts exhibit typeꢀIV (IUPAC
classification) isotherms with a hysteresis loop characteristic of cylindrical mesopores.26 A sharp
capillary condensation in relative pressure range 0.58ꢀ0.76 is observed in case of SBAꢀ15
material which indicates presence of uniform mesopores with narrow pore size distributions.
After TiO2 incorporation into SBAꢀ15 material, it is observed that the isotherm pattern exhibits
almost similar features (Fig. 1) which indicates the maintenance of structural characteristic
which is also observed from HRTEM analysis however the adsorption pattern is not parallel to
the desorption pattern. This kind of deviation suggests that there is pore blocking of SBAꢀ15
material by TiO2 incorporations27. The tailing of the desorption branch is observed in case of
TiO2@SBAꢀ15 (5 wt% TiO2, Fig 1c), TiO2@SBAꢀ15 (7.5 wt% TiO2, Fig 1d) and MnOx/
TiO2@SBAꢀ15 (7.5 wt% MnOx & 5 wt% TiO2, Fig 1e) catalysts, which indicates that some of
the pores are not perfectly cylindrical after TiO2 incorporations.28 Distribution of pore diameter
(Fig. 2) reveals that there is almost uniform pore size in prepared catalysts. The parent SBAꢀ15
sample exhibited a maximum pore diameter (5.13 nm) and surface area (628 m2/g) (Table 1).
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Fig. 1: N2 adsorptionꢀdesorption isotherm of (a) SBAꢀ15, (b) TiO2@SBAꢀ15 (2.5 wt% TiO2), (c)
TiO2@SBAꢀ15 (5 wt% TiO2), (d) TiO2@SBAꢀ15 (7.5 wt% TiO2), (e) MnOx/ TiO2@SBAꢀ15
(7.5 wt% MnOx & 5 wt% TiO2)
Fig. 2: Pore diameter distribution of (a) SBAꢀ15, (b) TiO2@SBAꢀ15 (2.5 wt% TiO2), (c)
TiO2@SBAꢀ15 (5 wt% TiO2), (d) TiO2@SBAꢀ15 (7.5 wt% TiO2), (e) MnOx/ TiO2@SBAꢀ15
(7.5 wt% MnOx & 5 wt% TiO2)
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Table 1: Surface Area and Pore Diameter of different catalysts
Catalyst
Surface Area Pore Diameter
(m2/g)
(nm)
SBAꢀ15
628
5.82
4.88
5.36
4.72
6.35
TiO2@SBAꢀ15 (2.5 wt% TiO2)
TiO2@SBAꢀ15 (5 wt% TiO2)
TiO2@SBAꢀ15 (7.5 wt% TiO2)
MnOx/ TiO2@SBAꢀ15 (7.5 wt% MnOx & 5 wt% TiO2)
543
526
486
302
Incorporation of TiO2 into mesoporous silica results in a shift of the pore maximum to smaller
diameters and a decrease in surface area (Table 1) furthermore it is also observed that the pore
diameter of MnOx/TiO2@SBAꢀ15 (7.5 wt% MnOx & 5 wt% TiO2) is increased compared to
other catalysts and may be due to formation of MnOx particles on silica grain.29
3.2. XꢀRay Scattering Patterns
Fig. 3: SAXS patterns of (a) SBAꢀ15, (b) TiO2@SBAꢀ15 (2.5 wt% TiO2), (c) TiO2@SBAꢀ15
(5 wt% TiO2), (d) TiO2@SBAꢀ15 (7.5 wt% TiO2), (e) MnOx/TiO2@SBAꢀ15 (7.5 wt% MnOx &
5 wt% TiO2)
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Fig. 4: Wide angle XRD pattern of (a) SBAꢀ15, (b) TiO2@SBAꢀ15 (2.5 wt% TiO2), (c)
TiO2@SBAꢀ15 (5 wt% TiO2), (d) TiO2@SBAꢀ15 (7.5 wt% TiO2), (e) MnOx/TiO2@SBAꢀ15
(7.5 wt% MnOx) (5 wt% TiO2), (f) MnOx/TiO2@SBAꢀ15 (7.5 wt% MnOx) (5 wt% TiO2) (after
10 times recycling), (g) MnOx/TiO2 (7.5 wt% MnOx)
Small angle Xꢀ ray scattering patterns of the prepared catalysts (Fig. 3) shows a wellꢀresolved
pattern with a prominent peak at 2θ of 0.9° accompanied by two weak peaks at 1.6° and 1.9°.
These three peaks are indexed as (100), (110), and (200) reflections corresponding to p6mm
hexagonal symmetry of SBAꢀ15 material.30 The SAXS patterns (Fig. 3) also indicate that the
incorporation of TiO2 into the SBAꢀ15 material does not change the mesoscopic order of SBAꢀ
15. Wide angle Xꢀ ray diffraction study of the present catalysts does not show any crystalline
phase (Fig 4: a – f) for MnOx and TiO2 deposited on SBAꢀ15. Hence it is evident that both the
MnOx and TiO2 phases are in highly dispersed state. However, MnOx/TiO2 (7.5 wt% MnOx)
catalyst is prepared by using commercial TiO2, hence this catalyst is having highly crystalline
anatase phase (Crystallite size: 164.3 nm; Fig 4 g). This study proves the fact that metal – metal
(MnꢀTi) interaction is much higher for the highly dispersed MnOx/TiO2@SBAꢀ15 catalysts than
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that of MnOx/TiO2 catalyst. From the SAXS and XRD studies, it is well understood that TiO2 is
present within the hexagonal pores of SBAꢀ15 material by pore encapsulation process. SAXS
patterns also indicate that the intensity of (100) peak is decreased in case of TiO2@SBAꢀ15 (7.5
wt% TiO2, Fig 3d) and MnOx/TiO2@SBAꢀ15 (7.5 wt% MnOx & 5 wt% TiO2, Fig 3e) catalyst
which is due to the introduction of TiO2 on SBAꢀ15 material, resulting decrease of scattering
contrasts between pore wall and pore space.31 The low intensity of higher order diffraction peak
of TiO2@SBAꢀ15 (7.5 wt% TiO2, Fig. 3d) and MnOx/TiO2@SBAꢀ15 (7.5 wt% MnOx & 5 wt%
TiO2, Fig. 3e) catalyst is due to high ratio of wall thickness versus pore size.32, 33 It is clearly
visualized that the relative intensity of (100), (110), (200) peaks are larger in case of
TiO2@SBAꢀ15 (2.5 wt% TiO2, Fig. 3b) and TiO2@SBAꢀ15 (5 wt% TiO2, Fig. 3c) compared to
SBAꢀ15 material. This observation clearly indicates that there is a formation of homogeneous
layer of TiO2 on the silicious pore wall of SBAꢀ15 material.34 These above experimental results
support the formation of TiO2@SBAꢀ15 nanoreactor.
3.3. HRTEM image of prepared catalysts:
HRTEM images of prepared catalysts are shown in Fig. 5 where the mesoporous channels are
clearly visible in every catalyst. In case of SBAꢀ15 material, clear mesoporous channeling is
observed (Fig. 5, A & A1) and after titanium source is added into the SBAꢀ15 material, it is
observed that TiO2 is incorporated into the mesoporous channels of SBAꢀ15 material (Fig. 5, B
& B1). This pore encapsulated TiO2 gives the long term activity of TiO2 material because this
encapsulation process prevents the agglomeration of TiO2 material. After depositing MnOx into
SBAꢀ15 encapsulated TiO2 material (TiO2@SBAꢀ15), some dark spots are observed on
MnOx/TiO2@SBAꢀ15 material (Fig. 5, C & C1). From HAADFꢀSTEM image with elemental
analysis (Supporting information, Fig. S1), it is observed that Ti and Mn are homogeneously
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distributed on SBAꢀ15 material. This homogeneous distribution of Ti and Mn makes maximum
numbers of TiꢀMn contact area which may facilitate the proposed reaction. FTꢀIR spectroscopic
(Supporting information, Fig. S2) analysis does not show any characteristic peak for Mn–O–Ti
bonds hence individual phases of MnOx & TiO2 are present in the catalysts.
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Fig. 5: HRTEM image of SBAꢀ15 (A & A1), TiO2@SBAꢀ15 (5 wt% TiO2) (B & B1) and
MnOx/ TiO2@SBAꢀ15 (7.5 wt% MnOx & 5 wt% TiO2) (C & C1)
3.4. H2ꢀTemperature programmed reduction (H2ꢀTPR)
Fig. 6: H2ꢀTPR profile of (a) TiO2@SBAꢀ15 (2.5 wt% TiO2), (b) TiO2@SBAꢀ15 (5 wt% TiO2),
(c) TiO2@SBAꢀ15 (7.5 wt% TiO2), (d) MnOx/TiO2 (7.5 wt% MnOx), (e) MnOx/TiO2@SBAꢀ15
(7.5 wt% MnOx & 2.5 wt% TiO2), (f) MnOx/TiO2@SBAꢀ15 (7.5 wt% MnOx & 5 wt% TiO2), (g)
MnOx/TiO2@SBAꢀ15 (7.5 wt% MnOx & 7.5 wt% TiO2)
H2‐TPR is used to investigate the reducibility of MnOx on TiO2@SBAꢀ15 surface and H2
consumption is compared by using the areas of the TPR profiles (Fig. 6). From this analysis, it is
observed that MnOx/TiO2@SBAꢀ15 (7.5 wt% MnOx & 5 wt% TiO2, Fig: 6f) has a higher H2
consumption than other catalysts. In case of MnOx/TiO2 (7.5 wt% MnOx, Fig: 6d) catalyst two
peaks are observed, one at 305 °C and another at 570 °C which indicate two steps reduction of
the prepared catalyst. MnOx reduced in the following sequence MnO2→ Mn2O3→ Mn3O4→
MnO, hence the low temperature reduction peak correspond to easily reducible MnO2 and high
temperature peak correspond to Mn3O4 or bulk MnOx species.35 From this H2ꢀTPR profile, it is
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observed that when MnOx is deposited on TiO2@SBAꢀ15 surface, the reduction temperature is
dramatically shifted to lower temperature region which indicate the easily reducible nature of the
MnOx/TiO2@SBAꢀ15 catalyst (240 °C and 260 °C, Fig. 6f). The reduction temperatures of the
dispersed metal oxides depend on the amount of dispersion of metal components where
reduction temperature of highly dispersed metal oxide occur at lower temperature region than
that of its aggregate counterpart.36 These results are implying that highly dispersed TiO2 on
mesoporous SBAꢀ15 material i. e. TiO2@SBAꢀ15 nanoreactor plays a vital role on reducibility
of MnOx species. Here both reduction peaks correspond to MnO2 & Mn2O3 (Fig. 6 e, f, g) shifted
to lower temperature region with respect to MnOx/TiO2 (Fig. 6 d). The higher dispersion of
MnOx on TiO2@SBAꢀ15 material and decrease in reduction temperature would increase the
oxygen mobility on the catalyst suface.37ꢀ40
3.5. NH3ꢀTemperature programmed desorption (NH3ꢀTPD):
The amount and strength of acid site of the prepared catalysts are determined by NH3ꢀTPD
experiment. Fig.7 shows the ammonia desorption patterns of catalysts. The total acidity of each
catalyst is summarized in Table 2. For MnOx/TiO2 (7.5 wt% MnOx) catalyst three desorption
peaks at 400, 415 and 562 °C are observed which correspond to the strong acid site in the
catalysts, no weak acid site is observed in case of MnOx/TiO2 (7.5 wt% MnOx) catalyst. In case
of TiO2@SBAꢀ15 and MnOx deposited TiO2@SBAꢀ15 material, a peak is observed at 88 – 90
°C temperature which corresponds to weak acid site of the catalysts. The presence of weak acid
site facilitates the reaction because weak acid site strongly interact with aniline which is a weak
base. It is well known fact that the weak acidꢀweak base interaction is strong compared to strong
acidꢀweak base or weak acidꢀstrong base interactions. For this kind of interaction, aniline may
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reside on the catalyst surface and readily reacts with benzaldehyde which is immediately formed
by the oxidation of benzyl alcohol.
Fig. 7: NH3ꢀTPD of (a) TiO2@SBAꢀ15 (2.5 wt% TiO2), (b) TiO2@SBAꢀ15 (5 wt% TiO2), (c)
TiO2@SBAꢀ15 (7.5 wt% TiO2), (d) MnOx/TiO2@SBAꢀ15 (7.5 wt% MnOx & 2.5 wt% TiO2), (e)
MnOx/TiO2@SBAꢀ15 (7.5 wt% MnOx & 5 wt% TiO2), (f) MnOx/TiO2@SBAꢀ15 (7.5 wt%
MnOx & 7.5 wt% TiO2), (g) MnOx/TiO2(7.5 wt% MnOx)
Tableꢀ2: Total acidity of the prepared catalysts
Catalyst
Peak maxima
Total acidity
(µmol/g)
19
(°C)
80
TiO2@SBAꢀ15 (2.5 wt% TiO2)
TiO2@SBAꢀ15 (5 wt% TiO2)
88
88
81
90
86
65
90
36
84
80
TiO2@SBAꢀ15 (7.5 wt% TiO2)
MnOx/TiO2@SBAꢀ15 (7.5 wt% MnOx & 2.5 wt% TiO2)
MnOx/TiO2@SBAꢀ15 (7.5 wt% MnOx & 5 wt% TiO2)
MnOx/TiO2@SBAꢀ15 (7.5 wt% MnOx & 7.5 wt% TiO2)
MnOx/TiO2 (7.5 wt% MnOx)
400
415
562
8
1
15
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3.6. Catalytic activity studies
The oxidative coupling reaction between benzyl alcohol and aniline (1:1 molar ratio) was
performed over a series of supported catalysts (Table – 3) under aerobic condition. The results of
catalytic activity is summarized in Table – 3 along with the general equation of imine formation
and shows that MnOx/TiO2@SBAꢀ15 (7.5 wt% MnOx & 5 wt% TiO2) catalyst exhibit superior
activity compared to other catalysts. No reaction is observed in case of SBAꢀ15 and TiO2@SBAꢀ
15 (2.5 wt% TiO2) catalysts but very low yield of Nꢀbenzylideneaniline (C) is observed in case
of TiO2@SBAꢀ15 (5 wt% TiO2), TiO2@SBAꢀ15 (7.5 wt% TiO2) and MnOx/SBAꢀ15 (7.5 wt%
MnOx) catalysts. These experimental results suggest that not only MnOx or TiO2 acts as an active
component for this reaction but also their optimum compositions in combination act as an active
component in this reaction. So there is a synergistic relation between MnOx and TiO2.
In the present oxidative coupling reaction, SBAꢀ15 material plays a vital role in reactant
conversion and product distribution. In case of MnOx/TiO2 (7.5 wt% MnOx) catalysts 68%
conversion and 91% selectivity of NꢀBenzylidineaniline (C) is observed but when 7.5 wt% of
MnOx and 5 wt% of TiO2 is dispersed on SBAꢀ15 material conversion has reached up to 96%
and selectivity was 97%. Haruta et al.41 reported that the electroꢀnegativity of metal ion play a
vital role in alcohol conversion and also present a volcano type relationship between benzyl
alcohol conversion and electroꢀnegativity of different metal ion and titanium acts as a very active
metal for benzyl alcohol conversion reaction. Several groups of researchers have carried out
oxidative coupling study of alcohol and amine but most of them carried out using noble metals.
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Table 3: Imine formation by oxidative coupling of benzyl alcohol and aniline
O
OH
Catalyst
Toluene, 80 oC
24 h, air balloon
+
H2N
+
N
H
N
A
B
C
D
Catalyst
MnO2 loading
(wt%)
TiO2 loading
(wt%)
Conversion Selectivity
Yield*
of C (%)
(%)
of C (%)
SBAꢀ15
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2.5
5
No Reaction
TiO2@SBAꢀ15
TiO2@SBAꢀ15
TiO2@SBAꢀ15
No Reaction
7
60
62
97
96
93
95
97
97
94
96
91
96
91
4
7.5
2.5
2.5
2.5
5
11
36
34
15
78
96
88
74
92
63
26
68
7
MnOx/TiO2@SBAꢀ15
MnOx/TiO2@SBAꢀ15
MnOx/TiO2@SBAꢀ15
MnOx/TiO2@SBAꢀ15
MnOx/TiO2@SBAꢀ15
MnOx/TiO2@SBAꢀ15
MnOx/TiO2@SBAꢀ15
MnOx/TiO2@SBAꢀ15
MnOx/TiO2@SBAꢀ15
MnOx/SBAꢀ15
5
7.5
10
5
35
33
14
74
93
85
70
89
58
25
62
7.5
10
5
5
5
7.5
7.5
7.5
ꢀ
7.5
10
7.5
7.5
MnOx/TiO2
ꢀ
Reaction condition: Anhydrous toluene, 2 mL; Benzyl alcohol, 1 mmol; Aniline, 1 mmol;
Catalyst, 100 mg; moisture free air atmosphere; Reaction time, 24 h. (* GC Yield)
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Tayade et al and Huang et al. have achieved ~ 99 % conversion but their study was conducted at
high temperature or high pressure conditions.42,43 Soule et al, Zhang et al and Kang and et al. 10,
11, 44
carried out experiments at ambient pressure in presence of noble metal catalysts
(gold/palladium, copper) and achieved 100%, 77% conversion and ~ 90% yield respectively but
45
in their case they used different alkali e.g. NaOH, KOH in reaction medium. Xu et al.
conducted oxidative coupling of alcohol and amine in absence of any metal at ambient pressure
and 110 °C temperature and achieve ~ 90 yield but they added different types of base (KOH,
CsOH, NaOH, tꢀBuONa, tꢀBuOK) into the reaction medium. In the present study oxidative
coupling reaction between benzyl alcohol and aniline is carried out at milder temperature
(~80oC) in atmospheric condition with air as oxidizing agent and also no noble metal as well as
base has been used for this coupling reaction.
To examine the leaching of MnOx species hot filtration study has been carried out and it is
observed that 32% conversion of reactant has occurred after 2 h and 41% conversion has been
observed after 24 h. From this hot filtration study it can be inferred that almost negligible amount
of MnOx species was leached out from reaction medium (Supporting Information, S3). From
hot filtration study and O2ꢀTPO results (Supporting Information, Fig. S4), reaction pathway of
this reaction can be elucidated (Scheme 1). Oxidative coupling reaction proceeds through two
consecutive steps via oxidation and coupling. From H2ꢀTPR (Fig. 6) result two peaks correspond
to MnO2 and Mn2O3 for MnOx/TiO2@SBAꢀ15 (7.5 wt% MnOx & 5 wt% TiO2) was shifted to
lower temperature region compare to other catalysts and simultaneously peak intensity of MnO2
ꢀ
and Mn2O3 is increased. The oxygen molecule adsorbed on Mn3+ site to give O2 species without
oxygen splitting.46
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H
(Mn3+/Mn4+)
MnOx
O
Ti
O2
4+
)
e
+
O
O
2
/
1
+
(Mn3+___O2/Mn4+)
MnOx
O
OH
OHꢀ
H
(
ꢀ
Ti
N
O
O
H
OH
Ti
ꢀ
(Mn4+/Mn4+)
MnOx
OH
HO
ꢀ
_ O2
O
l
a
n
i
m
a
i
H
O-
C
H
m
e
H
(Mn4+/Mn4+)
MnOx
OH
OH
O
H2N
O
Ti
O
_
C H
ꢀ H+/+H+
H
C
H
H
(Mn4+/Mn4+)
MnOx
OH
Ti
O
ꢀ
O
O
O
Scheme 1. Reaction pathway for oxidative coupling between benzyl alcohol and aniline over
MnOx/TiO2@SBAꢀ15 catalyst under atmospheric air
ꢀ
This O2 species adsorbed on Ti4+ gives rise to TiꢀOꢀOꢀ species.23 This TiꢀOꢀOꢀ species acts as an
active oxidizing species in case of oxidation reactions. The MnOx species deposited on highly
dispersed TiO2 on SBAꢀ15 material makes maximum number of MnOx/TiO2 contact area which
facilitates the formation of TiꢀOꢀOꢀ species. In this study, benzyl alcohol is used as a reactant and
the –OH group of benzyl alcohol gets oxidized in presence of TiꢀOꢀOꢀ species and produce
benzaldehyde as an intermediate product. Once benzaldeyde produced, it immediately reacts
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with aniline to produce coupling product. The surface silanol group (SiꢀOH), which is weak
acidic in nature, makes aniline and surface –OH interaction facilitating imine formation via
hemiaminal intermediate.41 In case of MnOx@TiO2/SBAꢀ15 catalyst, the weak acid site is
increased which induces a weakꢀweak interaction between catalyst surface and aniline.41,47 The
interaction of aniline on weak acid site of the catalyst lead to excellent conversion as well
required product selectivity.41
3.7. Catalyst recycling: Catalyst recyclability is an important phenomenon for its industrial
Fig. 8: Recyclability test of MnOx/TiO2@SBAꢀ15 (7.5 wt% MnOx & 5 wt% TiO2) towards
oxidative coupling of benzyl alcohol and aniline
applications. After completion of reaction the catalyst is separated by centrifugation and washed
several times by anhydrous toluene and then by anhydrous ethanol. This washed catalyst is dried
at 100 °C for overnight and the calcined at 300 °C for 4 h with 1 °C/min heating ramp and this
calcined catalyst is further reused for oxidative coupling reaction. From this activity studies it is
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observed that up to 5th cycle the catalyst gives almost similar activity and after 5th cycle it is
observed that it lose around 8 – 10% of its original activity. These experimental results suggest
the long term activity and robustness of the catalyst. The 8 – 10% activity lose may be due to the
agglomeration of MnOx nanoclaustures. Recyclability test of MnOx/TiO2@SBAꢀ15 (7.5 wt%
MnOx & 5 wt% TiO2) towards oxidative coupling of benzyl alcohol and aniline is shown in Fig.
8.
4. Conclusion
Highly selective imine synthesis by oneꢀpot process between oxidative coupling of benzyl
alcohol and aniline over MnOx/TiO2@SBAꢀ15 catalyst is carried out under atmospheric air.
Present study also reports high conversion as well as selectivity of desired product by avoiding
noble metal as well as addition of base into the reaction medium and it also shows high metal
utilization due to dispersion metal oxide on SBAꢀ15 material. This highly dispersed metal oxide
favors oxidative coupling reaction by facilitating the oxidation of benzyl alcohol. The robustness
of the catalyst may be further useful for industrial application for synthesis of much needed
imines.
Acknowledgement
S. Mandal acknowledges the funding of Start – up Research Grant (No.: SB/FT/CSꢀ100/2014) by
SERB, Department of Science & Technology, Govt. of India. We thank the Director, CSIRꢀ
CIMFR, Dhanbad for allowing to publish this manuscript.
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165x110mm (107 x 106 DPI)