One-step selective aerobic oxidation of amines to imines by gold nanoparticle-loaded rutile titanium(IV) oxide plasmon photocatalyst

Article abstract of DOI:10.1021/cs300682d

Among various metal oxide-supported Au nanoparticles, Au/rutile TiO ₂ exhibits a particularly high level of visible-light activity for aerobic oxidation of amines to yield the corresponding imines on a synthetic scale with high selectivity (>99%) at 298 K. Experimental results have suggested that the reaction proceeds via the localized surface plasmon resonance-excited electron transfer from the Au nanoparticle to the TiO ₂.

Full text of DOI:10.1021/cs300682d

Letter
pubs.acs.org/acscatalysis
One-Step Selective Aerobic Oxidation of Amines to Imines by Gold
Nanoparticle-Loaded Rutile Titanium(IV) Oxide Plasmon
Photocatalyst
Shin-ichi Naya,† Keisuke Kimura,‡ and Hiroaki Tada†,‡,*
†Environmental Research Laboratory, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
‡Department of Applied Chemistry, School of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka
577-8502, Japan
S
* Supporting Information
ABSTRACT: Among various metal oxide-supported Au nanoparticles,
Au/rutile TiO2 exhibits a particularly high level of visible-light activity for
aerobic oxidation of amines to yield the corresponding imines on a
synthetic scale with high selectivity (>99%) at 298 K. Experimental results
have suggested that the reaction proceeds via the localized surface plasmon
resonance-excited electron transfer from the Au nanoparticle to the TiO2.
KEYWORDS: localized surface plasmon resonance, visible light photocatalysis, Au nanoparticle, selective oxidation, imine synthesis
mines are important versatile intermediates for fine
chemicals and pharmaceuticals and also are applied for
aerobic oxidation of amines to imines with a maximum at Au
particle size of ∼7 nm at 298 K. The photocatalytic activity can
depend on the Au loading amount as well as its particle size. To
vary the Au particle size (d) with its loading amount maintained
constant, Au NPs were loaded onto various metal oxides
(anatase and rutile TiO2, SrTiO3, ZnO, WO3, In2O3, Nb2O5)
by the heating temperature-varied deposition precipitation
method (Table S1, see Supporting Information).24 Figure 1
shows transmission electron micrographs of several Au NP-
loaded metal oxides (Au/MOs) (see Supporting Information
Figure S1 for the other samples). Apparently, Au NPs are
highly dispersed on the metal oxide surface in every sample.
Table 1 summarizes characterization results of the Au/MOs.
The mean Au particle sizes (d/nm) of Au/MOs increase with
increases in heating temperature (Tc) and time (tc). Figure 2
shows UV−vis absorption spectra of Au/MOs, and that of Au
colloid is shown for comparison. Au/MO has broad absorption
due to the localized surface plasmon resonance (LSPR) of Au
NPs, whose peak position depends on the metal oxide support.
The LSPR peak position greatly depends on the surrounding
permittivity. The significant red-shift of the LSPR peak of Au/
rutile TiO2 results from its very large permittivity (ε = 114) as
compared with that of anatase TiO2 (ε = 48).25 The LSPR
I
mass products, such as dyes.1 Imines can be synthesized by
direct oxidation of amines or condensation of amine with
aldehyde.2 Several procedures use the stoichiometric amount of
harmful or explosive reagents with emission of the correspond-
ing amount of pollutants.3,4 The energy and environmental
issue requires the development of “green” catalytic processes to
oxidize amines efficiently and selectively under mild con-
ditions.5,6 Our basic strategy for the reaction design is to use (i)
molecular oxygen in air as an oxidizing agent, (ii) no solvent or
nontoxic solvent, and (iii) sunlight as an energy source. To
achieve this, a new type of photocatalysts consisting of Au
nanoparticles (NPs) and metal oxide supports, called “plasmon
photocatalysts”, are highly promising7−15 because of the
possible effective utilization of sunlight as an energy source
and their nontoxicity. Although plasmon photocatalysts have
recently attracted much attention,16 studies on organic
synthesis are limited to only the oxidations of alcohols to
carbonyl compounds,17−19 thiols to disulfides,20 and benzene to
phenols.21,22 On the other hand, the rational material design of
highly active plasmon photocatalysts should urge further
development of this hot research field. To increase fundamental
understanding, the correlation between the photocatalytic
activity and the key factors, including the Au particle size17,23
and the kind of metal oxide supports,13,23 should be clarified.
Here, we show that Au/rutile TiO2 plasmon photocatalyst
exhibits a particularly high visible-light activity for selective
Received: October 21, 2012
Revised: November 23, 2012
Published: November 26, 2012
© 2012 American Chemical Society
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Letter
Figure 1. TEM image and size distribution: (a) Au/rutile TiO2 (d = 6.7 nm), (b) Au/anatase TiO2 (d = 7.1 nm), (c) Au/SrTiO3 (d = 8.4 nm), (d)
Au/WO3 (d = 11.8 nm).
Table 1. Au Loading Amount; Au Particle Size, d; Standard
Deviation, σ; Number of Count; Heating Temperature, Tc;
and Time, tc for the Deposition−Precipitation Method
catalyst
Au mass % d/nm σ/nm count Tc/K tc/h
Au/rutile TiO2
Au/rutile TiO2
Au/rutile TiO2
Au/rutile TiO2
Au/rutile TiO2
Au/anatase TiO2
Au/anatase TiO2
Au/anatase TiO2
Au/anatase TiO2
Au/SrTiO3
0.56
0.56
0.56
0.56
0.56
0.42
0.42
0.42
0.42
0.62
0.61
0.63
0.28
0.31
0.30
0.78
3.5
5.0
0.8
1.1
1.5
1.9
2.9
0.6
1.5
1.6
3.6
0.6
1.2
1.9
2.1
1.7
2.0
1.8
248
250
203
298
198
162
145
120
122
128
111
133
88
773
803
823
873
873
673
773
873
873
673
773
873
873
773
873
873
4
4
6.7
4
9.7
4
14.3
3.6
24
4
7.1
4
9.8
4
12.3
2.7
24
4
Au/SrTiO3
3.9
4
Au/SrTiO3
8.4
24
4
Au/WO3
11.8
7.3
Au/ZnO
169
106
92
4
Au/Nb2O5
6.7
4
Au/In2O3
11.0
4
Figure 2. UV−vis absorption spectra of Au/MOs and Au colloid.
Rutile and anatase TiO2 are abbreviated as R-TiO2 and A-TiO2,
respectively.
absorption of most Au/MOs as well as the Au colloid possesses
asymmetric character, that is, the absorption intensifies at the
shorter wavelength side of the peak. On the other hand, the
LSPR absorption profiles for Au/rutile TiO2, Au/SrTiO3, and
Au/WO3 are relatively symmetric and in good agreement with
that of the solar spectrum.
As shown in Table 2, visible-light irradiation (λ > 430 nm) to
Au/rutile TiO2 in neat benzylamine (PhCH2NH2) yields
benzylidenebenzylamine (Ph−CHN−CH2Ph) as a single
product at 298 K. Intermediate Ph−CHNH reacts with
PhCH2NH2 to generate Ph−CHN−CH2Ph. At an irradi-
ation time of 24 h, the amount of Ph−CHN−CH2Ph reaches
1.06 mmol (4.5%). The turnover number (TON = the
molecule number of the imine generated/the number of Au
surface atoms) exceeds 5000. In addition, Au/rutile TiO2 shows
high levels of visible-light activity for the oxidations of several
other primary and secondary amines to afford the correspond-
ing imines with selectivity >99%.
As described later, photoelectrochemical and adsorption
experiments provide important information about the reaction
mechanism. The oxidation of benzylamine was also carried out
for a 0.1 mM acetonitrile solution. We selected acetonitrile as a
solvent because of its excellent electrochemical stability, large
dielectric constant, and low toxicity. In this case, benzaldehyde
is generated as a major product, with Ph−CHN−CH2Ph as a
minor product due to the hydrolysis of Ph−CHNH by water
involved in acetonitrile (Supporting Information Table S2).
First, the metal oxide support effect on the photocatalytic
activity was examined in the acetonitrile solution. Figure 3A
shows time courses for the benzaldehyde generation.
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Letter
a
This finding suggests that these oxidations proceed via the
electron transfer from the amines to Au/rutile TiO2.
Table 2. Amine Oxidation by Au/Rutile TiO2
To clarify the origin for the striking support effect, the
reaction mechanism was investigated by photoelectrochemical
measurements. Au NP-loaded mesoporous rutile TiO2 nano-
crystalline films were formed on the surfaces of fluorine-doped
SnO2 electrodes (Au/mp-rutile TiO2/FTO), and the electrode
potential (E) change induced by irradiation of monochromatic
light was measured (Supporting Information Figure S3).9
Visible-light irradiation of the electrode in an electrolyte
solution containing benzylamine causes a negative shift in E,
which hardly changed without benzylamine. Figure 4A
amine
X = H
yield/mmol conversion, % selectivity, %
TON
1.06
0.43
0.12
0.49
0.03
0.09
4.5
2.1
0.7
2.7
0.1
0.3
>99
>99
>99
>99
>99
>99
5.7 × 103
2.3 × 103
6.5 × 102
2.6 × 103
1.7 × 102
4.7 × 102
X = Me
X = Cl
X = OMe
R = Me
R = CH2Ph
a
Visible-light irradiation (λ > 430 nm) for 24 h.
Surprisingly, among the various Au/MOs, Au/rutile TiO2
shows a particularly high level of visible-light activity for the
amine oxidation. Zhao and co-workers have recently reported
the TiO2-photocatalyzed oxidation of amines to imines under
visible-light irradiation via a surface complex mechanism.26 The
activity of Au/rutile TiO2 is much greater than that of TiO2
under the same conditions. Au NP loading, visible-light
irradiation, and O2 were necessary for the reaction to be
sustained (Supporting Information Figure S2).
Second, the Au particle size effect on the amine oxidation in
the acetonitrile solution was studied for the Au/rutile TiO2,
Au/anatse TiO2, and Au/SrTiO3 systems. Regardless of the Au
particle size, the aldehyde and imine were selectively produced.
Figure 3B shows plots of the turnover number (TON = the
molecule number of the sum of aldehyde and imine generated/
the number of Au surface atoms) at an irradiation time of 1 h as
a function of d. The plot for the Au/rutile TiO2 system shows a
volcano-shaped curve with a maximum at d ≈ 6.7 nm, whereas
in the Au/anatase TiO2 and Au/SrTiO3 systems, the TONs
slightly increase with increasing d.
In addition, the visible-light activity of Au/rutile TiO2 was
examined for the oxidations of several other primary and
secondary amines in the acetonitrile solutions (Supporting
Information Table S2). The oxidation for every amine yields
the corresponding aldehyde and imine with 86−99%
conversion at an irradiation time of 24 h. Interestingly, the
plot of the initial reaction rate (v0) vs the oxidation potential of
the primary amines (Eox/V vs standard hydrogen electrode,
SHE) a shows linear relation with a negative slope (Figure 3C).
Figure 4. (a) Action spectrum of (dE/dt)0 (violet circle) and diffuse
reflectance UV−vis spectrum of Au/mp-rutile TiO2 (solid line). (b)
Adsorption isotherms of 4-methoxybenzylamine (MBA) on rutile
TiO2 (open square) and Au/rutile TiO2 (d = 5.0 nm, blue triangle) at
275 K.
compares the action spectrum of the initial rate of the
electrode potential change (dE/dt)0 and the absorption
spectrum of Au/mp-rutile TiO2/FTO. The good resemblance
of the profiles indicates that the excitation of the Au NP-LSPR
is the driving force for the amine oxidation. The potential
upward shift can result from the electron transfer from
benzylamine to TiO2 via LSPR-excited Au NPs. The surface
Figure 3. (a) Time course of [PhCHO] generated. (b) TON at tp = 1 h for amine oxidation as a function of Au size d: Au/ZnO (open circle), Au/
Nb2O5 (open rhombus), Au/In2O3 (open triangle), Au/WO3 (open square). (c) Initial oxidation rate of 4-substituted benzylamine (X−C6H4−
CH2NH2) as a function of their oxidation potential (Eox).
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Letter
complex between benzylamine and TiO2 has very weak
featureless absorption in the visible region.26 Consequently,
the Au/rutile TiO2-photocatalyzed amine oxidation is mainly
induced not by the surface complex excitation but probably by
the LSPR-excited electron transfer.
(Figure S2), amine oxidation (Table S2), and PCP curves
(Figure S3). This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: h-tada@apch.kindai.ac.jp.
■
The effect of Au NP loading on the adsorption property of
rutile TiO2 for amine was also examined. Figure 4B compares
the adsorption isotherms for 4-methoxybenzylamine on rutile
TiO2 and Au/rutile TiO2 at 275 K: Y denotes the equilibrium
adsorption amount per unit mass of the solid. Rutile TiO2 has
good adsorptivity for 4-methoxybenzylamine. Diffuse reflec-
tance infrared spectroscopy using pyridine as a probe molecule
has suggested the presence of acid sites on the surface of rutile
TiO2.27 4-Methoxybenzylamine would be adsorbed on the
rutile TiO2 surface by the acid−base interaction. In addition,
loading a small amount of Au NP further increases the
adsorption amount by ∼3 μmol g−1. At a Au loading amount of
0.56 mass % and d of 5.0 nm, the surface area of Au NPs on
rutile TiO2 occupies only ∼2% of the total surface area. This
fact means that the amine is adsorbed on the surface of the Au
NP in preference to rutile TiO2. A recent density functional
theory study has shown that amine is adsorbed on under-
coordinated sites of the Au(111) surface via a donation/back-
donation mechanism.28
Notes
The authors declare no competing financial interest.
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ASSOCIATED CONTENT
* Supporting Information
Experimental details, properties of the supports and Au NPs
(Table S1, Figure S1), time course for the amine oxidation
■
S
(31) Scaife, D. E. Sol. Energy 1980, 25, 41.
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