Visible light induced green transformation of primary amines to imines using a silicate supported anatase photocatalyst

Article abstract of DOI:10.3390/molecules20021941

Catalytic oxidation of amine to imine is of intense present interest since imines are important intermediates for the synthesis of fine chemicals, pharmaceuticals, and agricultural chemicals. However, considerable efforts have been made to develop efficient methods for the oxidation of secondary amines to imines, while little attention has until recently been given to the oxidation of primary amines, presumably owing to the high reactivity of generated imines of primary amines that are easily dehydrogenated to nitriles. Herein, we report the oxidative coupling of a series of primary benzylic amines into corresponding imines with dioxygen as the benign oxidant over composite catalysts of TiO₂ (anatase)-silicate under visible light irradiation of λ > 460 nm. Visible light response of this system is believed to be as a result of high population of defects and contacts between silicate and anatase crystals in the composite and the strong interaction between benzylic amine and the catalyst. It is found that tuning the intensity and wavelength of the light irradiation and the reaction temperature can remarkably enhance the reaction activity. Water can also act as a green medium for the reaction with an excellent selectivity. This report contributes to the use of readily synthesized, environmentally benign, TiO₂ based composite photocatalyst and solar energy to realize the transformation of primary amines to imine compounds.

Full text of DOI:10.3390/molecules20021941

Molecules 2015, 20, 1941-1954; doi:10.3390/molecules20021941
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Visible Light Induced Green Transformation of Primary Amines
to Imines Using a Silicate Supported Anatase Photocatalyst
Sifani Zavahir and Huaiyong Zhu *
School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty,
Queensland University of Technology, Brisbane QLD 4001, Australia;
E-Mail: fathimasifani.zavahir@hdr.qut.edu.au
*
Author to whom correspondence should be addressed; E-Mail: hy.zhu@qut.edu.au;
Tel.: +61-73138-1581.
Academic Editor: Pierre Pichat
Received: 30 October 2014 / Accepted: 22 January 2015 / Published: 26 January 2015
Abstract: Catalytic oxidation of amine to imine is of intense present interest since imines
are important intermediates for the synthesis of fine chemicals, pharmaceuticals, and
agricultural chemicals. However, considerable efforts have been made to develop efficient
methods for the oxidation of secondary amines to imines, while little attention has until
recently been given to the oxidation of primary amines, presumably owing to the high
reactivity of generated imines of primary amines that are easily dehydrogenated to nitriles.
Herein, we report the oxidative coupling of a series of primary benzylic amines into
corresponding imines with dioxygen as the benign oxidant over composite catalysts of
TiO (anatase)-silicate under visible light irradiation of λ > 460 nm. Visible light response
2
of this system is believed to be as a result of high population of defects and contacts
between silicate and anatase crystals in the composite and the strong interaction between
benzylic amine and the catalyst. It is found that tuning the intensity and wavelength of the
light irradiation and the reaction temperature can remarkably enhance the reaction activity.
Water can also act as a green medium for the reaction with an excellent selectivity. This
report contributes to the use of readily synthesized, environmentally benign, TiO based
2
composite photocatalyst and solar energy to realize the transformation of primary amines
to imine compounds.
Keywords: benzylamine; imine; visible light; anatase; silicate

Molecules 2015, 20
. Introduction
Imines are a group of N-containing compounds, with a pivotal role as chemically and biologically
1942
1
useful intermediates in various cycloaddition, condensation and reduction reactions [1,2]. These
compounds play a major role in pharmacophores, fragrances and numerous biologically active
compounds [3]. For a long time, traditional condensation of amines with carbonyl compounds was
regarded as the simplest way to prepare imines [4–6]. Highly reactive nature of aldehydes made
handling difficult. This was overcome later by replacing aldehydes with alcohols and temporarily
producing aldehydes in-situ within the reaction mixture and the subsequent reaction with an amine
yield the imine compound [7,8]. Wang and co-workers have recently developed organosilicon
supported TiO catalyst for this reaction at 160 °C with added base to achieve a good imine yield [9].
2
However, these processes yield range of by-products and greatly affect the selectivity to the
desirable product.
Amines are easily accessible compounds that can also be attractive precursors to synthesize imine
by controlled oxidation. Until recently, several methods for oxidation of secondary amines to imines
have been developed, while little attention has been devoted to the oxidation of primary amines. This
is probably because of the generated imines, in which α-amino hydrogen is present, are generally
intermediate products that are rapidly dehydrogenated to nitriles [10–12]. Subsequent research in the
area has been dominated by the development of new catalytic processes that allow the aerobic
oxidation of primary amines to imines under mild conditions. Angelici and co-workers reported aerobic
oxidative homocoupling of primary amines to give imines, catalyzed by gold powder (50 µm size) and
gold nanoparticles supported on alumina (5% Au/Al
2
3
O ) in toluene at 100 °C [13]. Following this
study Au/C and CuCl catalysts have been employed in this reaction at 100 °C under molecular oxygen
atmosphere [14,15]. It is becoming increasingly important to look for new materials that can catalyze
reactions under moderate conditions (relatively low temperature and pressure). In this regard, the
utilization of sunlight as an energy source to reduce the working temperature has recently attracted
much attention [16–18]. Wang and co-workers have attained imines in excellent yields using
mesoporous graphite carbon nitride photocatalyst at 80 °C [19]. In general, unavailability of
structurally diverse amines has hampered the synthetic scope of oxidative coupling of benzylic amines
to yield corresponding imines, yet this method is highly selective for imines.
Over the last few years, many efforts have been extended to organic redox-transformation reactions
using TiO
2
photocatalysis [20–25]. However, to date, most of the reported reactions for the synthetic
photocatalysts were carried out under UV irradiation and were usually
transformations using TiO
2
associated with low selectivity [26,27]. Performing visible light induced selective transformations by
photocatalysts is a challenge that has gained increasing attention. Recent discoveries demonstrated the
surface modification of TiO with noble metal complexes or nanoparticles rather than bulk doping
2
might be a better strategy in light of new visible light responsive photocatalysts that could enhance the
design of efficient redox reactions under visible light irradiation. Zhao and co-workers achieved this
conversion of primary amine to imine with TiO
they also found it is also possible for this reaction to be initiated by visible light irradiation of
λ > 420 nm [29]. According to them, amine molecules adsorbed onto TiO forms a surface complex
that could absorb visible light and so initiate electron transfer and ensuing reactions. Because the
2
under UV light irradiation (>300 nm) [28] and later
2

Molecules 2015, 20
1943
reaction takes place on the TiO
2
surface, we envisioned that ultrafine TiO
2
powders with large specific
surface areas should exhibit better catalytic activity. A feasible approach to stabilizing TiO
nanocrystals is to disperse them in an inorganic medium, such as layered clays creating porous
composite structures, while ensuring that most of the surface of TiO crystals is accessible to various
2
2
molecules [30]. Nevertheless, the structure of the composite solids has a profound impact on their
catalytic performance [31]. The mesoporous composites of anatase nanocrystals and silicate are the
catalyst materials of the optimal structure for high photocatalytic activity. Synthetic layered clay,
laponite, can be used in the synthesis of the composite as silicate source [30].
Here in we report TiO nanocrystal-silicate composite, prepared using laponite, as feasible
2
photocatalyst for the selective oxidation of benzylamine to N-benzylidene benzylamine with excellent
conversion and selectivity under the irradiation of visible light (λ > 460 nm). It is found that in the
TiO
under identical conditions TiO
activity to pure TiO (anatase) powder. Nitrogen adsorption data confirms the large surface area of the
2
-silicate composite catalyst TiO
2
is in anatase phase. Reference reactions with anatase show that
2
-silicate composite catalyst exhibited a much more superior catalytic
2
composite catalyst. Furthermore, we also found that water can be used as the solvent. This catalyst
could be employed for heterocoupling of two structurally diverse amines in the synthesis of imines as
well as homocoupling of benzylic amines to imines, and the catalyst can be recycled up to five rounds
without any significant loss of activity.
2
. Results and Discussion
The aerobic photocatalytic oxidation of benzylamine to N-benzylidene benzylamine was chosen as
the model reaction to optimize the reaction system. Reactions are carried out using 500 W halogen
lamps where the light emitted is in 400–800 nm range. According to the data given in Table 1 it is
apparent that TiO
2
-silicate (abbreviated as TiO -S hereafter) is the most photocatalytically active
2
photocatalyst towards this transformation. Catalyst samples were also prepared by loading Au and Pd
nanoparticles (NPs) (3% by weight) and another sample with AuPd alloy NPs (1.5% weight of each
metal) loaded on to TiO
lower imine product yield of 60% with Au@TiO
enhanced light absorption by Au NPs in the visible region due to localized surface plasmon resonance
LSPR) effect which is characterized by an intense band around 520 nm (Figure S1, Supplementary
2
-S composite material (Characterization is provided in SI). We observed a
2
-S, compared to 82% by TiO -S, despite the
2
(
Information) [16–18]. This observation also serves as an example to support the fact, light absorption
by a material is not the sole governing factor deciding catalysts ability to drive a particular chemical
reaction under light irradiation. It appears that the reaction takes place on the surface of anatase, the
loaded Au NPs lower the exposed surface area of TiO
lowering the accessibility to the reactants. Pd@TiO -S catalyst had similar activity to that of TiO
whereas AuPd@TiO -S was slightly sluggish. Results further indicate the unique potential of TiO
based materials towards oxidation reactions and importance of evaluation of surface modifications of
TiO for activity improvements. In control experiments, the reaction did not proceed without a
photocatalyst or in the dark.
2
, the catalytically active sites of this system,
2
2
-S,
2
2
2

Molecules 2015, 20
1944
Table 1. Photocatalytic oxidation of benzylamine to N-benzylidene benzylamine over
different catalyst materials and solvents. a
Entry
Catalyst
Solvent
Acetonitrile
DMSO
Conv. (%) b
Sel. (%) b
Yield (%)
1
2
3
4
5
6
7
8
9
TiO
TiO
TiO
TiO
2
2
2
2
-S
-S
-S
-S
88
18
94
74
65
88
89
0
92
100
73
97
93
90
96
--
81
18
69
72
60
79
85
0
THF
Toluene
Au@TiO
AuPd@TiO
Pd@TiO -S
Laponite
2
-S
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
2
-S
2
TiO
2
(anatase)
51
73
100
97
51
71
1
0
H-titanate
a
Reaction conditions: 50 mg catalyst, 0.5 mmol benzylamine, 5 mL solvent, 500 W halogen lamp (cut off
2
, 24 h. b Determined by GC analysis.
wavelength below 400 nm) intensity 0.36 W/cm , 1 atm O
DMSO = dimethyl sulfoxide, THF = tetrahydrofuran.
2
As can be seen in Table 1, activity of TiO
anatase) as the photocatalyst material. In order to understand this change in behavior we closely
studied the light absorption abilities of both TiO (anatase) and TiO -S, in the presence and absence of
benzylamine. UV-Visible diffuse reflectance spectra of benzylamine adsorbed TiO (anatase) and
TiO -S shows increased absorbance compared to solitary TiO (anatase) and TiO -S, particularly in the
visible region. This observation agrees well with previous reports, where electron rich molecules like
amines make a charge transfer complex with TiO and respond to visible light illumination [29]. It is
also notable, the absorption of benzylamine adsorbed on TiO -S is significantly high compared to
benzylamine adsorbed on TiO (anatase) as shown in the Figure 1A. Even though TiO is present in
anatase phase in both TiO and TiO -S photocatalysts used in the current study, the distribution of
anatase particles is different in TiO
2
-S is superior to that of an equivalent amount of TiO
2
(
2
2
2
2
2
2
2
2
2
2
2
2
2
-S. During TiO
2
-S preparation, layered clay structure of precursor
material laponite clay is lost as a result of the acidic titanium sol solution reacting with hydroxyl
groups in the clay layers that are bound to magnesium ions within the layer [31]. Most of the
magnesium in the clay was leached out in this way. Composition of the catalyst estimated by energy
dispersive X-ray (EDX) confirms high weight percentage of silicate in the composite catalyst despite
the leaching of Mg units. During preparation Si:Mg ratio (by weight) decreased from 1:0.58 to 1:0.25,
this together with TEM image is a clear indication that ordered layer structure is damaged. Thus, TiO
in this TiO -S composite catalyst exists as discrete anatase crystals on fragmentized pieces of silicate.
Correspondingly this composite structure restrains agglomeration of anatase particles leading to high
exposed surface area of TiO . Brunauer-Emmett-Teller (BET) surface area of initial laponite clay
2
2
2
2
−1
2 −1
changed from 330.6 m g to 518.3 m g in the final TiO
2
-S catalyst material. The composite has
3
porosity of about 0.4 cm /g and a mean pore size of 5 nm. Finally, in the obtained composite catalyst
silica particles and anatase crystals exist as inter-dispersed phases in nanometer scale with a highly porous

Molecules 2015, 20
1945
structure as can be seen in Figure 1B. X-ray diffraction (XRD) pattern of the catalyst only exhibit peaks
responsible for the anatase phase of TiO with no peaks related to silicate units or laponite clay, this
indicates silica is present in the amorphous phase, and anatase particles of mean crystal size 4.22 nm
estimated by Debye-Scherrer equation using the broadening of the highly intense (101) XRD peak at
θ = 25.3°) have homogeneously crystallized over amorphous silica moiety. This TiO -S structure
2
(
2
2
obtained in the present study, offers high thermal and chemical stability, also provides ample
opportunity for the reactant molecules to interact with energetic charge carriers. Smaller anatase
particles reduce the possibility of charge recombination, since charge carriers are generated at the close
proximity of surface and efficiently captured by benzylamine and oxygen molecules on the surface.
B
D
Figure 1. (A) UV-Visible diffuse reflectance spectra; (B) transmission electron
microscopy (TEM) image; (C) X-ray diffraction (XRD) peak patterns indexed for
*
—silicate phase and **—anatase phase; (D) energy dispersive X-ray (EDX) spectra of
TiO -S composite catalyst.
2
To further investigate the contribution from light in this reaction, we conducted a series of reactions
at variable intensities (Figure 2A). The conversion rate of benzylamine on TiO -S catalyst increased
gradually as the intensity increased, with the other reaction conditions unchanged (Experimental
2

Molecules 2015, 20
1946
section). Selectivity to the product imine had a little influence on the intensity; however, overall imine
yield (conversion rate x selectivity) increased with the intensity. Such a tendency reveals a strong
dependence on the intensity for the light induced oxidative coupling of benzylamine, because in
general light incident with a higher intensity is able to generate more energetic charge carriers (holes
and electrons). Such conditions favor stronger interaction between benzylamine and the catalyst, and
positively influence the reaction.
Figure 2. The effect of (A) light intensity and (B) temperature influence of the irradiation
on the oxidative coupling of benzylamine.
Impact of the temperature on the yield of the reaction was studied by conducting the experiments at
two different temperatures slightly above room temperature (40 °C and 50 °C). As shown in Figure 2B,
observed yield was always high for the reaction under the study, oxidative coupling of benzylamine to
imine at higher temperature for a given constant intensity within 40 to 50 °C temperature range. For
−
2
instance, at 40 °C and 0.26 W·cm intensity, 58% of benzylamine was converted to imine product
whereas at 50 °C it was 83%. At high temperature however, the enhancement in the yield by increasing
the intensity lessens since the selectivity to the imine product starts to decline, though reaction
proceeds at a higher conversion rate. As the intensity was increased, the difference between the yields
at 40 and 50 °C finally decreased, even though higher conversion rate was observed for 50 °C
compared to that at 40 °C in all cases.
The dependence of yield on the irradiation wavelength was studied using five monochromatic light
emitting diodes (LEDs) and it shows that higher photocatalytic yields are achieved under irradiation of
short wavelengths (<460 nm). Anatase phase of TiO
as in this case TiO -S is highly active up to 460 nm. Figure 3 demonstrates the apparent quantum yield
A.Q.Y) dependence on the incident wavelength; A.Q.Y. is a measure of imine yield per photon of
energy absorbed per unit time. This finding indicates that composite TiO -S catalyst structure has a
broad light response below 460 nm in the visible region due to the collective effects of benzylamine
2
exhibits a band gap of 3.2 eV (387.5 nm) where
2
(
2

Molecules 2015, 20
1947
adsorbed TiO
composite photocatalyst structure. This reveals that TiO
edge (460 nm) compared to 420 nm cut-off for solitary TiO
2
(anatase) charge transfer surface complex and high population of defects in the
-S catalyst can function at a lower cut-off
(anatase) system reported by Zhao and
2
2
co-workers [29]. It is noteworthy that, in the composite structure of the catalyst there are contacts
between silicate and anatase crystals. At these sites, the anatase surface is similar to the silica doped
anatase surface that exhibits light absorption and visible light photocatalytic activity [32].
Figure 3. The effect of wavelength of the irradiation on the oxidative coupling of benzylamine.
According to the results summarized in Table 1, the solvent has important impact on the catalytic
activity. Acetonitrile serves as the best solvent for this reaction while the poorest performance was
observed in DSMO. Water is a viable solvent for organic reactions, and it is interesting to study the
impact of water as the reaction medium. For some organic reactions, water exhibits special reactivity
or selectivity due to its unique physical properties. In the present study, benzylamine oxidation
reaction proceeded in water at a moderate conversion rate of 45.6% under the illumination of 500 W
halogen lamp (400–800 nm) for 24 h, but with an excellent selectivity where the sole product being
N-benzylidene benzylamine with an overall yield of 46%. Doubling the amount of catalyst from 50 mg
to 100 mg of TiO -S enhanced the reaction yield to 62% without compromising the selectivity.
2
Motivated by this result, we expanded the scope of the substrates for the oxidation of amines to
imines. Table 2 summarizes the photocatalytic oxidation of the benzylic amines to corresponding
imines with their conversion rate and selectivity. Oxidation of primary benzylic amines substituted
with an electron donating group (Table 2, entry 2–3) proceeded efficiently under visible light
irradiation with good to high conversion rates and high selectivity for the imine product. Substituent
group influences the conversion rate of the reaction than the selectivity to the imine product. The
relatively low conversion rate for the oxidative coupling of 4-chlorobenzylamine (Table 2, entry 4)
into its corresponding imine might be caused by inductive effects of C-Cl σ-bond polarity. No change
was observed when aniline was subjected to the reaction, this is consistent with our hypothesis that the

Molecules 2015, 20
1948
presence of a –H, bonded to the α-carbon is important for this transformation to take place.
Furthermore, non-aromatic cyclic amines (Table 2, entry 7–8) did not yield the desired imine product.
Control experiments carried out using cyclohexylamine (consist of a single α-hydrogen) produced the
corresponding oxime (cyclohexanone oxime) instead of the imine. Benzaldehyde oxime was one of the
products observed during the time course of the reaction of benzylamine.
Table 2. Aerobic oxidation of primary benzylic amines photocatalyzed by TiO -S under
2
visible light irradiation. a
Entry
Substrate
Product
Con. (%) b
Select. (%) b Yield (%)
1
88
92
92
96
81
88
96
2
3
4
96
100
57
100
>99
35
57
c
c
c
8
8
87
5
7
91
51
74
32
10
36
20
48
8
a
Reaction condition: 50 mg catalyst, 0.5 mmol amine substrate, 5 mL acetonitrile, 500 W halogen lamp
2
, 24 h. b Determined by GC analysis.
(
c
cut-off wavelength below 400 nm) intensity 0.36 W/cm , 1 atm O
Reaction time 36 h.
2
This reaction goes through the widely known intermediate benzaldehyde and a tentative
mechanistic pathway is given in Scheme 1 based on the products observed. The photocatalysts
contribution is mainly in the step of benzylamine oxidation to benzaldehyde, whereas the condensation
of benzaldehyde with a benzylamine molecule leading to the imine product is faster. In the oxidation
2
step, TiO -benzylamine surface complex absorb visible light (400–800 nm) and excite electrons. These
excited electrons are then captured by oxygen molecules adsorbed on TiO
2
surface, then in the
proceeding steps oxygen interacts with benzylamine and the substrate molecules lose the H bonded to
the α-carbon atom, and oxidized to aldehyde. Thus, it is rational that under visible light irradiation, the
oxygen molecules adsorbed on the catalyst capture the light excited electrons, and react with the H at
the α-carbon. Role of oxygen is further confirmed, when the reaction was carried out in the air
atmosphere benzylamine exhibit a relatively lower observed conversion rate of 51% and a selectivity
of 96%, yielding 49% of imine after 24 h. This mechanism agrees well with the observed product
selectivity results. At higher conversions of benzylamine, a decrease in the selectivity for the imine

Molecules 2015, 20
1949
occurs and benzaldehyde appears in the products. This is due to the fact that, increased consumption of
benzylamine in the solution could not ensure the complete condensation of aldehyde and the amine.
Ph
NH2
OH
Ph
NH2
Ph
Ph
N
O
Ph
N
Ph
TiO -S, hv (400-800 nm), O
2
2
Scheme 1. Tentative reaction pathway.
Ability of TiO
2
-S photocatalyst to catalyze the oxidative cross-coupling of two benzylic amines
with different substituent groups to yield a heterocoupled imine product was also studied using
benzylamine, 4-methylbenzylamine and 4-methoxybenzylamine (two at a given reaction). Results
demonstrated a poor selectivity since all four possible imines were observed in relatively similar yields
after 24 h. Oxidative coupling of benzylamine with 4-methylbenzylamine had 95% of imine product
yield. Self-coupling products of benzylamine (28%) and 4-methylbenzylamine (22%) were observed
together with the two heterocoupled imines (50%), the product distribution of heterocoupling of
benzylamine with 4-methylbenzylamine is as desired (~1:1:1:1) since difference in the
nucleophilicities of “H” and methyl group is not significant. In order to evaluate the product
distribution over the time span of this heterocoupled imine synthesis, we chose benzylamine and
4
-methoxybenzylamine as the two benzylic amine substrates and the reaction profile is given in the
Table 3, this reveals both the precursor imines produce the corresponding aldehydes as per the
oxygenation step shown in Scheme 1, and then reacts with a free amine molecule to yield the final
imine. Aldehyde of more electro deficient nucleus reacts faster with the more electron rich amine (P3)
at early stages of the reaction and then with either amine as the reaction is progressing. Rate of
aldehyde formation is slower in electron rich benzene nucleus, benzylamine in this system and it acts
as the nucleophile (amine half), while 4 methoxybezylamine is easier to oxidize and preferentially be
the aldehyde half. In the product distribution more P3 and P4 are observed during the whole cause of
the reaction indicating high formation and reactivity of 4-methoxybenzaldehyde. Dual amine systems of
benzylamine/aniline and 4-methoxybenzylamine/aniline yield only the self-coupled imines of
benzylamine (92%) and 4-methoxybenzylamine (96%) respectively. Amount of aniline introduced in
the reaction system remained unchanged even after the reaction, portraying its inert role in this
photocatalyzed oxidative coupling reaction, anilne with a –NH
2
unit in its structure failed to participate
in this heterocoupling reactions at least as the amine half.
Reusability of the catalyst is an important parameter in heterogeneous catalysis. The composite
TiO -S photocatalyst studied in this system can be recovered readily from aqueous or organic solutions
2
by simple filtration or sedimentation. The anatase nanocrystals in these composite samples are linked
to silicate pieces such that grains in the µm scale are formed. Operational life of this catalyst examined
over five consecutive rounds (Figure 4) revealed no apparent activity loss after five rounds. This
further confirms the thermal and chemical stability of the catalyst. However selectivity towards the
imine product was gradually decreased during each cycle lowering the overall product yield.

Molecules 2015, 20
1950
Table 3. Time conversion plot for oxidative coupling of benzylamine with 4-methoxybenzylamine a.
Conversion (%)
Selectivity (%)
Entry
Time (h)
Benzylamine 4-Methoxybenzylamine P1 P2
P3
100
40
P4
0
1
2
3
4
5
2
4
13
41
84
96
96
13
57
90
97
97
0
0
12
15
19
21
12
16
16
16
36
32
26
24
8
38
17
20
40
40
a
Reaction Conditions: 25 mg catalyst, 0.25 mmol amine substrates, 2 mL acetonitrile, 500 W halogen lamp
2
(
cut-off wavelength below 400 nm) intensity 0.36 W/cm , 1 atm O
2
.
P1
P2
P3
P4
Figure 4. Reusability data of TiO -S catalyst for the oxidative coupling of benzylamine.
2
3
. Experimental Section
3.1. General Information and Materials
The laponite clay was supplied from Fernz specialty chemicals Australia, all other chemicals were
purchased from Sigma Aldrich (Castle Hill NSW, Australia) and used as received without further
purification. Water used in all experiments was milli-Q water passing through an ultra-purification system.
3
.2. General Procedure for the Synthesis of TiO -S Composite
2
TiO
2
precursor was prepared by hydrolyzing Ti(OCH
3
4
) in HCl for 3 h following a slightly
modified method proposed by J. Sterte [30,31,33].
Initially 1.0 g of laponite was slowly dispersed in 50 mL of deionized water and kept stirring until it
was transparent. Then 4.0 g of polyethylene glycol (FW 585) surfactant and the metal precursor
solution was added drop wise with continuous stirring. Mixture was then transferred to teflon covered
autoclaves and heated at 100 °C for 2 days. The solid was then recovered from centrifugation, followed

Molecules 2015, 20
1951
by washing with water until no more chloride ions left (confirmed by a test with AgNO
3
). Product was
−1
then dried in air and finally calcined at 500 °C for 20 h with the step being 2 °C·min .
.3. Characterization of TiO -S Composite
The diffuse reflectance UV/Vis (DR-UV/Vis) spectra were recorded on a Cary 5000 UV/Vis-NIR
3
2
Spectrophotometer (Agilent, Santa Clara, CA, USA). X-ray diffraction (XRD) patterns of the samples
were recorded on a Philips PANalytical X’Pert PRO diffractometer (PANalytical, Sydney, Australia)
using CuKα radiation (l = 1.5418 Å) at 40 kV and 40 mA. Transmission electron microscopy (TEM)
images were taken with a Philips CM200 Transmission electron microscope (Philips, Eindhoven,
The Netherlands) employing an accelerating voltage of 200 kV. The specimens were fine powders
deposited onto a copper micro grid coated with a holey carbon film. Nitrogen physisorption isotherms
were measured on the Tristar II 3020 (Micromeritics, Norcross, GA, USA). Prior to the analysis,
sample was degassed at 110 °C overnight under high vacuum. The specific surface area was calculated
by the Brunauer-Emmett-Teller (BET) method from the data in a P/P° range between 0.05 and 0.2. The
compositional data was determined by energy-dispersive X-ray spectroscopy (EDS) (EDAX, Mahwah,
NJ, USA ) attached to an FEI Quanta 200 scanning electron microscope (SEM, Quanta, OR, USA).
3
.4. General Procedure for the Photocatalytic Reactions
Benzylic amine compound 0.5 mmol, 5 mL of solvent were measured to a clean dry reactor tube.
0 mg of the catalyst was added and finally the reactor was purged with oxygen gas. These reactors
5
were kept magnetically stirring in front of a 500 W halogen lamp (except for dark and wavelength
experiments) for 24 h at 40 °C. At the end of the reaction 1 mL samples were collected in to small
glass vials after filtering out the solid catalyst using 0.2 µm milli pore filter. We tested for the products
using a gas chromatograph (GC, Agilent, Santa Clara, CA, USA) equipped with a DB 5 column. For
wavelength experiments, 5 monochromatic light emitting diodes (LEDs) of 390–410 nm, 460–462 nm,
5
15–517 nm, 587.5–560 nm or 620–625 nm was used.
4
. Conclusions
We have successfully applied TiO -S composite photocatalyst in the oxidative coupling of
2
benzylamine to imine under visible light irradiation. The numerous contacts between the anatase
crystals and silicate and high population of defects in the composite photocatalyst are the possible
reasons behind the enhanced visible light activity. The formation of imines proceed via an oxidation
pathway: under visible light irradiation, the oxygen molecules adsorbed on the catalyst capture the
light excited electrons, and react with the H bonded to the α-carbon of the substrate molecules, which
is oxidized to aldehyde. The condensation of the aldehyde with amine yields the product imine. This
photocatalyst has a very high activity in the region λ > 460 nm. This range is much broader compared
to previously reported results for anatase materials (λ > 420 nm). Intensity, wavelength and reaction
temperature can be tuned to optimize the reaction rate of TiO
2
-S catalyzed oxidative coupling of
benzylic amines. Water can be used as a solvent giving moderate conversion rate but sole product.

Molecules 2015, 20
1952
These findings encourage us to further study the surface modified titania based materials for selective
organic synthesis.
Supplementary Materials
Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/20/02/1941/s1.
Acknowledgments
This project was supported by Australian Research Council (ARC DP110104990).
Author Contributions
S.Z. and H.Z. designed the research; S.Z. performed the research, analysed the data and wrote the
paper. Both authors read and approved the final manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
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