郑州大学化学与分子工程学院论文 2013年第 12期
郑州大学
学士论文
作者:马长德
指导教师:高镜清教授
院系:化学与分子工程学院
专业:化学
郑州大学化学与分子工程学院论文 2013年第 12期
Novel Redox Photocatalyst PtTiO2 for the Synthesis of
2-Methylquinolines from Nitroarenes
Changde Ma*
Department of Chemistry, Zhengzhou University,
Received March 25, 2011;
An eco-friendly and efficient procedure for synthesis of 2-methylquinoline derivatives from
nitroarenes has been developed by a simple one-pot reaction on the surface of platinum-loaded
TiO2 with neat ethanol under UV irradiation without any harsh reagent according to green chemistry.
TiO2 catalysts with various amounts of Pt loadings were prepared by photodeposition using
chloroplatinic acid solution and characterized by XRD, BET, AFM, HR-TEM, XPS, and DRS.
XRD patterns showed that the crystal structure of PtTiO2 still remained as anatase phase. The
UVvis spectra indicated that Pt promoted the absorption of visible light. The XPS measurements
reveal that platinum particles are present mainly in metallic form. AFM and HR-TEM analysis
revealed the presence of nonspherical shaped platinum nanoparticles of the diameter 312
nm. PtTiO2 on irradiation induces a combined redox reaction with nitroarene and alcohol and this
is followed by condensationcyclization of aniline and oxidation products to give
2-methylquinolines.
In the last few years heterogeneous photocatalysis applied
to synthetic chemistry has become an exciting and rapidly
growing area of research. For a multistep synthesis, illuminated
semiconductors offer unique features. Many studies have
reported photocatalytic organic reactions using mild experimental
conditions.13
To date they are mostly in the categories
of oxidation and oxidative cleavage, reduction, geometric and
valence isomerization, substitution, condensation, and polymerization.
3 A major drawback in TiO2 photocatalyzed
reaction is the recombination of photogenerated electronhole
pair. This energy wasting step can be inhibited by incorporating
noble metals as electron acceptor on TiO2. The deposition of
noble metals, such as Pt, Pd, Ru, and Au onto the surface of
TiO2 can increase the efficiency of photocatalytic reactions
(degradation as well as production of hydrogen gas).46
In previous studies, it has been reported that platinum is one
of the most active metals for photocatalytic enhancement and it
can produce the highest Schottky barrier among the metals that
facilitate electron capture.7 Photogenerated electrons migrate to
the metal, where they become trapped and the electronhole
pair recombination is suppressed. The discovery of the photo-
郑州大学化学与分子工程学院论文 2013年第 12期
Kolbe reaction by Kraeutler and Bard using Pt/TiO2 as a
photocatalyst for the conversion of acetic acid to methane and
CO2 has attracted much attention.8 However in spite of several
semiconductor mediated reactions, examples of combined
redox reactions are rather limited in the literature.912
The fact
that some chemical reactions occur only in photocatalytic
systems is more significant for this application.
Quinoline and its derivatives have been the subject of much
research due to their importance in various applications and
their widespread biochemical significance. A large variety of
quinoline derivatives have been used as antimalarial, antiinflammatory,
antiasthmatic, antibacterial, antihypertensive, and
tyrokinase PDGF-RTK inhibiting agents.1315
Many synthetic
methods such as Skraup, Doebnervon
Miller, Friedländer, and
Combes reactions have been developed for the preparation of
quinolines.1619
However these methods are not fully satisfactory
with regard to operational simplicity, cost of the reagent,
and isolated yield. Thus, the drive continues to find a better and
improved methodology.
Faraway from thermal synthetic routes to quinolines, very
few studies on photocatalytic cyclization of nitrobenzene and
its derivatives have been carried out to yield corresponding
substituted quinolines and tetrahydroquinolines. A number
of photocatalysts such as TiO2, TiO2 with a cocatalyst
(p-toluenesulfonic acid) and others have been used.2022
Recently,
we have reported the use of AuTiO2
for the photocatalytic
conversion of anilines and its derivatives into corresponding
2-methylquinolines under UV light.23 But the method
of photocatalytic conversion of nitroaromatics to 2-methylquinolines
with PtTiO2
has not been investigated. We wish to
report herein our results on a combined redox methodology that
leads to a facile and selective synthesis of 2-methylquinolines
from nitroarene and ethanol under mild reaction conditions
using PtTiO2.
The reported procedure for synthesis of
2-methylquinolines is much simpler and more efficient.
Experimental
郑州大学化学与分子工程学院论文 2013年第 12期
Materials and Methods. All chemicals were of the
highest purity available and were used as received without
further purification. Dihydrogen(hexachloroplatinate) dihydrate
(H2[PtCl6]¢2H2O) (Aldrich) was used as Pt source. Nano TiO2
was prepared by the hydrolysis of titanium tetraisopropoxide
(Himedia 98.0%), in isopropyl alcohol solution (1.6 M) by the
addition of distilled water (isopropyl alcoholwater
1:1). The
resulting colloidal suspension was stirred for 4 h. The gel
obtained was filtered, washed and dried in an air oven at 100 °C
for 12 h. The sample was calcinated at 400 °C in a muffle
furnace for 12 h. The metal-loaded TiO2 catalyst was prepared
by photoreduction of metal ions on TiO2 as per the procedure
© 2011 The Chemical Society of Japan
Published on the web September 3, 2011; doi:10.1246/bcsj.20110091
Bull. Chem. Soc. Jpn. Vol. 84, No. 9, 953959
(2011) 953
reported in our previous paper.24 Pt(x)TiO2
samples, containing
different platinum loadings [x (wt%) = Pt/(TiO2 +
Pt)100; x = 0.5, 1.0, 1.5, and 2.0%] were prepared.
Apparatus. X-ray diffraction (XRD) patterns of TiO2 and
PtTiO2
powder samples were obtained using a Model D/Max
2550V with Cu anticathode radiation. The diffractograms were
recorded in 2a range between 10 and 80° in steps of 0.02° with
count time of 20 s at each point. The crystalline phase can be
determined from integration intensities of anatase (101), rutile
(110), and brookite (120) peaks and the average crystallite sizes
were determined according to the DebyeScherrer
equation
using the full width half maximum data of each phase.
D ?K=¢
cos a e1T
Where D is the crystal size of the catalyst, is
the X-ray
wavelength (0.154 nm), ¢ is the full width half maximum
(FWHM) of the catalyst, K = 0.89, and a is the diffraction
angle.
The phase formation, particle size, surface morphology, and
crystallinity of pure and loaded catalysts were examined using
transmission electron microscopy (TEM) (Model JEOL TEM-
3010) operated at 300 keV. The samples for TEM analysis
were prepared by dispersion of the catalysts in ethanol under
郑州大学化学与分子工程学院论文 2013年第 12期
sonication and deposition on a copper grid. High-resolution
TEM (HR-TEM) measurements were carried out using a JEOLJEM-
2010 UHR instrument operated at an acceleration voltage
of 200 kV with a lattice image resolution of 0.14 nm. X-ray
photoelectron spectra (XPS) of the catalysts were recorded with
an ESCA-3 Mark II spectrometer (VG Scientific Ltd., England)
using AlK?(1486.6 eV) radiation as the source. The spectra
were referenced to the binding energy of C(1s) (285 eV).
The DRS of all the catalysts were recorded on a Shimadzu
UV 2450 model UVvisible
spectrophotometer in the range of
800190
nm equipped with an integrating sphere and using
powdered BaSO4 as a reference. Reflectance spectra were
converted to the absorbance spectra using the KubelkaMunk
equation
FeR1eTT
?
e1 R1T2
2R1
e2T
The specific surface areas of the catalysts were determined
using a Micromeritics ASAP 2020 sorption analyzer. The
samples were degassed at 423K for 12 h and analysis was
performed at 77K with N2 gas as the adsorbate. The BrunauerEmmettTeller
(BET) multipoint method least-squares fit
provided the specific surface area.
To study the morphology of catalyst, an atomic force
microscope (AFM) JSPM-5200TM, JEOL model was used.
This AFM uses a silicon tip with a radius of 20mm and a lowresonance
frequency cantilever that has a manufacturer’s spring
constant of 3565Nm
11. Scans of 1.8 © 1.8 ˉm were obtained
for each sample. The images were recorded in the noncontact
mode. All recordings were made in air under ambient
conditions to produce 2D and 3D images.
Photocatalytic Synthesis of 2-Methylquinolines. In a
typical experimental run, 50 mg 1.5%PtTiO2
was suspended
in 25mL of an absolute ethanolic solution containing 25mM of
the nitrobenzene and irradiated by a 365 nm medium-pressure
mercury lamp (Sankyo Denki, Japan; intensity I = 1.381 ©
1016 einsteinL11 s11) after purging with N2 for 30min. N2
bubbling (flow rate = 6.1mLs11) and magnetic stirring of the
郑州大学化学与分子工程学院论文 2013年第 12期
suspension were continued throughout the reaction while the
temperature was maintained at 30 « 1 °C. Progress of the
reaction was monitored by TLC. Product analysis was
performed by GC analysis, Perkin-Elmer GC-9000 with a
capillary column of DB-5 and flame ionization detector was
used. GC/MS analysis was carried out using a Varian 2000
Thermo with the following features: capillary column VF5MS
(5% phenyl95%
methylpolysiloxane), 30m length, 0.25mm
internal diameter, 0.25 ˉm film thickness, temperature of
column range from 50 to 280 °C (10 °Cmin11), and injector
temperature 250 °C, attached to mass spectrometer model SSQ
7000. The isolation was performed by column chromatography
on a silica gel column by eluting with a cosolvent of hexane
and ethyl acetate (volume ratio: 8:2).
Results and Discussion
Catalyst Characterization. Pt-loaded TiO2 (1.5 wt% Pt)
catalyst was characterized by X-ray diffraction (XRD), BET
surface area, atomic force microscopy (AFM), transmission
electron microscopy (TEM), and diffuse reflectance spectroscopy
(DRS). The XRD pattern of prepared TiO2 is identical
with the standard pattern of anatase (JCPDS 01-078-2486 C),
and rutile lines (01-089-0553 C) are absent. The peaks at 25.43,
37.92, 48.03, 53.97, 55.05, 62.70, 68.80, 70.39, and 75.05° are
the diffractions of the (101), (004), (200), (105), (211), (204),
(116), (200), and (215) crystal planes of anatase TiO2,
respectively (Figure S1). These results confirm that photodeposition
of the platinum does not modify the basic crystal
structure of the TiO2 used (tetragonal, a = 0.37845 nm,
c = 0.95143 nm, body centered). The diffraction patterns of
the PtTiO2
samples do not show XRD peaks of metallic
platinum at 39.76, (111); 67.71°, (220); etc. This may be
because of either homogeneous dispersion of discrete platinum
deposits in the nanoscale on the surface of the TiO2 or because
of very low platinum content.
In Figure S2 an AFM 3D image of a dense Pt-loaded TiO2
nanoparticle is shown. PtTiO2
particles are well dispersed
without any trace of coalescence. Consequently we can be
assured that Pt particles in TiO2 layer are either overcoated or
sandwiched.
TEM images at two different regions permit easy differentiation
of Pt loading in TiO2 crystallites (Figures 1a and 1b).
郑州大学化学与分子工程学院论文 2013年第 12期
Pt nanocrystals are seen on the surface of the TiO2 particle as
dark dots. The particle size of PtTiO2
nanoparticles has been
analyzed in HR-TEM and shown in Figure 1c. It can be seen
that the size of PtTiO2
particles are in the range from 5 to
25 nm with an average particle size of 14.8 nm. Pt metal
particles are randomly dispersed on the TiO2 surface. Further
observations from high-resolution TEM micrograph show that
the Pt particles diameters are in the range of 312
nm and
randomly located on the crystal surface. Figures 1d and 1e
show the lattice fringes of Pt and TiO2 and SAED pattern of
PtTiO2.
The distance between the fringes is measured with
analysis software included in the instrument. The statistical
d-value is 0.225nm for the Pt particles and 0.351nm for the
TiO2 crystals, indicating that the observed fringes are for the
Pt(111) plane and TiO2(101) anatase facets, respectively.
Photocatalytic 954 Bull. Chem. Soc. Jpn. Vol. 84, No. 9 (2011) Synthesis of 2-Methylquinolines
from Nitrobenzenes
The electronic state of Pt in the catalysts was analyzed by
XPS. The XPS survey spectrum (Figure 2) of the PtTiO2
indicates the peaks of elements Ti, O, C, and Pt (trace). The
carbon peak is attributed to the residual carbon from the sample
and adventitious hydrocarbon from the XPS instrument itself.
The Ti2p peak at 461 eV, O1s peak at 534 eV, and Pt4f peak at
70.3 eV reveal the presence of Ti, O, and Pt elements in
PtTiO2.
A high-resolution XPS spectrum (inset Figure 2) from
62 to 80 eV confirms the existence of Pt. Figure 2 shows the
binding energies of Pt4f7/2 (70.3 eV) and Pt4f5/2 (74.0 eV) and
the splitting of the 4f doublet (3.7 eV). These binding energies
indicate that platinum is present in metallic state.
N2 adsorption and desorption studies show the isotherms as
type II (Figure S3). This type of isotherm is indicative of
nonporous materials. Surface area measurements, made by the
BET method, provide the specific surface areas of PtTiO2
and
prepared TiO2 as 64 and 74m2 g11, respectively. The specific
pore volume of PtTiO2
sample (0.995: 0.243 cm3 g11) shows
that the PtTiO2
is nonporous.
郑州大学化学与分子工程学院论文 2013年第 12期
The diffuse reflectance spectrum of 1.5% Pt-doped shows
an increased absorption in the visible region (Figure S4). The
reflectance data reported as F(R) values have been obtained by
application of the KubelkaMunk
algorithm. The band gap of
the doped oxide has been deduced from the Tauc plot of
[F(R)hˉ]1/2 versus photon energy (Figure S5) as 3.02 eV.
Photocatalytic Synthesis of 2-Methylquinoline from
Nitrobenzene. Initial experiments were carried out with
ethanolic solutions of nitrobenzene containing TiO2 nanoparticles
under different conditions. Irradiation of nitrobenzene
and TiO2 in ethanol with 365 nm UV light produced cyclized
product of 2-methylquinoline. Neither the irradiation of
ethanolic solution of nitrobenzene alone nor the solution of
nitrobenzene and the catalyst without light gave any product.
This indicates that both light and TiO2 are essential for the
formation of 2-methylquinoline. The catalytic activity of
PtTiO2
for the synthesis of 2-methylquinoline 2 was investigated
for the reaction of nitrobenzene 1 with ethanol.
The changes in the concentrations of 1 and the product,
2-methylquinoline 2 during the photocatalytic reaction were
determined at different times. With pure TiO2 (Figure 3a), 6 h
photoirradiation was required to achieve >99% consumption
of 1, affording 2 in only ca. 60% yield. The yield was
determined by comparison with the retention times of authentic
samples and by coinjection with the authentic compounds. The
main product identified was 2-methylquinoline. In contrast,
PtTiO2
promoted the rapid and selective production of 2 by
achieving 99% consumption of 1 with only 5h irradiation to
afford 2 in 70% yield (Figure 3b). This indicates that PtTiO2
promotes rapid and selective 2-methylquinoline production.
The higher efficiency and selectivity of PtTiO2
was also
shown by the formation of products from 3-methylnitrobenzene,
4-methylnitrobenzene, and 4-methoxynitrobenzene.
PtTiO2
produced 74, 76, and 68% of substituted 2-methylquinolines
but bare TiO2 gave only 68, 47, and 41.0% from
3-methylnitrobenzene, 4-methylnitrobenzene, and 4-methoxynitrobenzene
respectively (data not shown). This indicates that
PtTiO2
promotes rapid and selective production of substituted
郑州大学化学与分子工程学院论文 2013年第 12期
2-methylquinolines.
The higher conversion of nitrobenzene with PtTiO2
is due to
the trapping of electrons by Pt on the excited TiO2. The electrons
excited from the valence band of TiO2 to the conduction band
are efficiently transferred to Pt (charge separation effect) and
accumulated therein (electron pool effect). The oxidant (alcohol)
and the reductant (NB) are abundantly supplied to the
oxidation (TiO2), and reduction sites (Pt), respectively (reasonable
delivery effect) facilitating a combined redox reaction.
GC-MS chromatograms recorded at different reaction times of
the photocatalytic conversion of nitrobenzene in ethanol are
Figure 1. HR-TEM analysis: (a and b) images of PtTiO2,
(c) particle size distribution of PtTiO2,
(d) lattice fringes
of PtTiO2,
and (e) SAED pattern of PtTiO2.
Figure 2. X-ray photoelectron spectra of PtTiO2.
K. Selvam et al. Bull. Chem. Soc. Jpn. Vol. 84, No. 9 (2011) 955
presented in Figure 4. These chromatograms reveal the formation
of nitrosobenzene, aniline, N-hydroxyaniline, 4-anilino-
2-methyl-1,2,3,4-tetrahydroquinoline as intermediates.
Earlier it was reported that the irradiation of nitrobenzene
(2mmol) and TiO2 (0.5 g) in nitrogen-saturated ethanol solution
for 812
h gave ethoxytetrahydroquinolines as the major
product. We obtained 2-methylquinoline with continues purging
of nitrogen for 6 h irradiation. Our results reveal that the
oxidation power of TiO2 is well controlled under these
experimental conditions.
Analysis of the effect of catalyst concentration revealed that
the percentage conversion of nitrobenzene increased from 49 to
70% with increasing amount of PtTiO2
from 0.5 to 1 g L11. As
the conversion efficiency decreases above 1 g L11, the optimum
catalyst concentration is 1gL11. The decrease in efficiency at
concentrations above 1 g L11 may be due to the light scattering
by the catalyst particles. It is also found that the increase of
substrate concentration from 15 to 65mM decreases the
formation of 2-methylquinoline from 80 to 42% in this
reaction.
Figure 3. Time-dependent change in the concentrations of substrate and products during
photoirradiation of nitrobenzene in neat
EtOH with a) TiO2 and b) PtTiO2
郑州大学化学与分子工程学院论文 2013年第 12期
catalyst. Reaction conditions: 25mM of nitrobenzene in ethanol, Catalyst suspended: 50 mg.
Figure 4. GC-MS chromatograms at different reaction times for the photocatalytic conversion of
nitrobenzene.
Photocatalytic 956 Bull. Chem. Soc. Jpn. Vol. 84, No. 9 (2011) Synthesis of 2-Methylquinolines
from Nitrobenzenes
This process is facile for the synthesis of various substituted
2-methylquinolines. Photoirradiation of alcohol solutions that
contained various substituted nitrobenzenes and the PtTiO2
catalyst successfully afforded the corresponding 2-methylquinolines
(Table 1). 3-Methylnitrobenzene and 4-methylnitrobenzene
gave 2,7-dimethylquinoline (76%) and 2,6-dimethylquinoline
(71%) respectively. 2-Methylquinoline yield for 3-
methylnitrobenzene is higher than for 4-methylnitrobenzene.
In the synthesis of 2-methylquinoline, reduction of nitro is
followed by condensation with aldehyde and cyclization. It
seems that the electron-donating group at p-position inhibits
the condensation of amino group with aldehyde. This is also
revealed by the formation of only 68% 6-Methoxy-2-methylquinoline
from 4-methoxynitrobenzene which has a strong
electron-donating group at p-position. In the case of 3,5-
dimethylnitrobenzene, the cyclization reaction is hindered due
to steric effect and this decreases the product yield (68%)
when compared to 3-methylnitrobenzene (76%). In the case of
4-chloro- and 4-fluoronitrobenzenes, the yield of product
was very low. This is attributable to photoinduced dehalogenation.
Dehalogenated anilines have been identified in
GC-MS analysis.
Mechanism. Scheme 1 provides a tentative overview of
plausible reaction mechanisms leading to the cyclization
products identified by GC-MS. The initial steps in the reaction
which are necessary for all the reactions are the photocatalytic
reduction of nitroaromatic compound and oxidation of the
alcohol. The nitroaromatic compound is reduced to aniline by
photogenerated conduction band electron on the semiconductor
surface. Simultaneously the alcohol is oxidized to the corresponding
aldehyde consuming the photogenerated valance
band holes on TiO2. The photocatalytic formation of an
aldehyde from alcohol was well established.