郑州大学学士论文

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.