Oxidation of benzyl alcohol to benzaldehyde over MnOx/sepiolite catalysts - Nguyen Thi Nhu

Vietnam Journal of Chemistry, International Edition, 55(6): 729-733, 2017 DOI: 10.15625/2525-2321.2017-00534 729 Oxidation of benzyl alcohol to benzaldehyde over MnOx/sepiolite catalysts Nguyen Thi Nhu 1,2 , Quach Toan Anh 1 , Nguyen Tien Thao 1* 1Faculty of Chemistry, VNU University of Science, Vietnam National University, Hanoi 2Institute of Environment, Vietnam Maritime University Received 12 June 2017; Accepted for publication 29 December 2017 Abstract MnOx/sepiolite catalysts were synthesized by precipitation method accompanied by the calcination at 410 0C. The prepated solids have been characterized by XRD, SEM, TPR-H2. MnOx particles were deposited on the surface of the sepiolite fibers and act as active sites for the oxidation of benzyl alcohol using tert-butyl hydroperoxide (TBHP) as an oxidizing agent. The catalysts showed a good conversion of benzyl alcohol to benzaldehyde at 60 oC. The influence of the reaction time and reaction temperature was considered. Keywords. MnOx/sepiolite, oxidation, benzyl alcohol conversion, TBHP, benzaldehyde. 1. INTRODUCTION The selective oxidation of benzyl alcohol to benzaldehyde is an important reaction in the pharmaceutical, dyestuff, agrochemical and perfume industries. For a long time ago, benzaldehyde was produced by hydrolyzing benzyl chloride or by oxidizing toluene 0. Product mixture from these reactions has low selectivity to desired product or contaminates chlorine causing drawbacks for environmental influence 2. An other way is to use homogeneous catalysts as CrO3/H + or a complex of transition metal in oxidation of benzyl alcohol that meet difficulties as the separation and recycling catalysts. Therefore, development of heterogeneous catalysts for the selective oxidation of benzyl alcohol became more attractive for many chemists. Recently, a vast number of supported noble-metal catalysts (such as Pt, Pd, Au) have exhibited very low selective oxidation of alkyl benzene at mild conditions 2. However, these materials are high expenditure and difficult presevation. In the present study, manganese oxide supported on sepiolite may be an alternative to noble metal as catalysts for the oxidation benzyl alcohol. Indeed, manganese was reported to be ative for the alkylaromatics with t- BuOOH [6], but was not used for the oxidation of benzyl alcohol up to now. Sepiolite is a clay mineral, which is a hydrated magenesium silicate, its structure consists of 2:1 units linked together by inversion SiO4 tetrahedral along of Si-O-Si bonds; this structural arrangement corresponds to unique framework of nanotunnels 5. This unique fibrous structure gives sepiolite a large specific surface area and high adsorption capacity 7. This is the main reason of the usage of sepiolite to obtain high dispersion of the maganese oxide species which is one of the most important factors in determining the catalytic activity and selectivity [9- 16]. So in this study, the distribution of maganese species on sepiolite for the oxidation of benzyl alcohol was investigated. The preliminary results show that MnOx/sepiolite is a promising catalyst for the oxidation of benzaldehyde. 2. EXPERIMENTAL 2.1. Catalyst preparation and characterization The catalyst was prepared as follows: A quantity of 4 grams sepiolite was put into a 500 mL flask containing 100 mL of distilled water and desired mass ratio of manganese nitrate under stirring and then, precipitated with an excess of NaOH in 2 hours. The precipitate was separated by filtration, washed and dried at 70 oC. After that, the solid was calcined at 410 oC for 4 h, and then it was grinded. The crystalline structure was investigated by X- ray diffraction (XRD) on a D8 Advance-Bruker instrument using CuKα radiation (λ = 1.59 Å). Scanning Electron Microscopy was recorded on Hitachi S-4500 (Japan) with the magnification of 200,000 times. Temperature programmed reduction (TPR) measurements in the range of 20-800 oC were VJC, 55(6), 2017 Nguyen Tien Thao et al. 730 carried out on Thermal conductivity detector Gow- Mac 69-350 with the heat rate of 100C/min. 2.2. Catalytic performance Liquid phase oxidation of benzyl alcohol (BA) has been carried out in a 100 mL three-neck glass flask fitted with a reflux condenser and a thermometer, 3 ml of benzyl alcohol and 0.2 grams of catalyst were added into the flask. After the reaction mixture was magnetically stirred and heated to the desired temperature, tert-butyl hydroperoxide solution (TBHP, 70 %) was dropped into stirred reaction mixture and the reaction is initiated. The three-neck glass flask was cooled to room temperature and then catalyst was separated by filtration. The filtrate was quantitatively analyzed by a gas chromatography (GC-MS, HP-6890 Plus). The conversion was calculated as the follows: 100 initial [Alcohol] final Alcohol][ initial [Alcohol] (%) Conversion 3. RESULTS AND DISCUSSION 3.1. Catalyst characterization 3.1.1. XRD patterns Figure 1 shows the XRD diagram of sepiolite and MnOx/sepiolite (calcined at 410 oC). In which, 2θ = 7.4, 20.2, 28.5, 39.2o are the characteristic peaks for sepiolite while the values of 2θ = 32.8, 36.0, 50.9o are essentially characteristic peaks for Mn3O4 phase, indicating the presence of Mn3O4 oxide on the carrier [8, 9, 13, 16, 17]. 5 15 25 35 45 55 65 TNK04-10Mn TNK04-7Mn Sepiolite 7.4 10.8 20.2 28.5 36.0 39.232.8 50.9 Figure 1: X-ray diffraction patterns of sepiolite, TNK04-7Mn, and TNK04-10Mn It is noted that the intensity of the latter reflection signals disappear for the lower Mn- content sample (TNM-4-7Mn) due to the high dispersion of manganese oxide particles on the sepiolite matrix. 3.1.2. SEM and specific surface area The SEM of sepiolite and 10 wt.%Mn2+/sepiolite (TNK04-10Mn) is showed in figure 2. As seen in Fig. 2a, the sepiolite had a fibrous morphology with smooth surface and clear boundary grains. The fibers have the length of microns and the width of hundreds of nanometers. After loading manganese oxides, the sepiolite morphology has slightly modified. The surface of the fibers became rougher and the fibrous length is reduced as shown in Fig. 2b. Figure 2: SEM micrographs of sepiolite (a) and TNK04-10Mn (b) Furthermore, there are existence of numerous uniformly rounded particles with the diameter of 100 b a VJC, 55(6), 2017 Oxidation of benzyl alcohol to benzaldehyde 731 nm. Thus, the specific surface of MnOx/sepiolite is expected lower than that of sepiolite parrent. Indeed, the specific surface area of sepiolite was 166.2 m2/g. while that of the MnOx–loaded sample (TNK04- 10Mn) was about 133.9 m2/g as mesured by N2 adsorption–dersorption method (not shown here). The decrease of specific surface area of in the latter case could be attributed to the incorporation of manganese oxide species [16, 17]. 3.1.3. H2-TPR analysis The oxidation-reduction property is usually interpreted from the H2 temperature – programmed reduction (H2-TPR). Figure 3 presents a H2-TPR profile for a representative sample of MnOx/sepiolite recorded from room temperature to 800 oC. The stages of the reduction has been explained as the phase evolution accompanied with valence development of manganese 8. As shown in Fig. 3, H2-TPR profile displays a couple of hydrogen consumption signals. The low temperature reduction peak (< 200 oC) corresponds to the oxidation of the absorbed oxygen species on the catalyst surface without decomposition of the material 9. The peak at 41 0oC is firmly ascribed as the reduction of Mn3O4 to MnO, in good accordance with the data reported in the literature 12. It is noted that in some reports, Mn3O4 may also be reduced in a two-stage reduction instead of one step because the reduction of MnOx was also dependent on the different manganese precursors and catalyst preparation method 14. H2-TPR for TNK04-10Mn -0.002 0 0.002 0.004 0.006 0.008 0.01 0 100 200 300 400 500 600 700 800 900 Temperature ( o C) T C D s ig n a l ( a .u ) 410 206 Figure 3: H2-TPR profile of TNK04-10Mn sample Thus, H2-TPR analysis reaffirmed the existance of Mn3O4 phase in the synthesized MnOx/sepiolite sample, in good argreement with XRD results shown in Fig. 1. 3.2. Catalytic activity The oxidation reaction of benzyl alcohol over MnOx/sepiolite catalysts with tert-butyl hydroperoxide solution was performed at atmospheric pressure and the temperature range of 50-90 oC. It is well known that the oxidation reactions were strongly dependant on reaction conditions as catalyst dosage, temperature, nature of oxidizing agent, solvent [19, 20]. In this work, we are interested in the effect of reaction time and temperature on the selectivity of the desired products. For the sake of comparison, a blank experiment was made using sepiolite calcined at 410 oC. The conversion of benzyl alcohol was observed only 2 % at 70 oC temperature for 4h while TNK04-10Mn exhibited 18 % of benzyl alcohol conversion in the same reaction conditions. These prove that an introduction of MnOx on sepiolite has promoted the catalytic oxidation of of benzyl alcohol to benzaldehyde [9, 15, 18]. Therefore, we continued to carry out in the reaction temperature range of 50-90 oC. The results were illustrated in Fig. 4. 0 10 20 30 40 50 60 70 80 90 100 50 60 70 80 90 Reaction temperature ( o C) P e rc e n t (% ) Benzaldehyde Sel Benzoic acid Sel Conversion Figure 4: Effect of reaction temperature on catalytic activity of sample TNK04-10Mn (10 wt.% Mn2+/sepiolite) for 4 h, TBHP/BA = 1.5 mol As being expected, Fig. 4 displays a significant influence of reaction temperature on benzyl alcohol conversion. Although the catalyst likely produced a single product at lower reaction temperature of 50- 60 oC, but the yield for benzaldehyde is somewhat small due to a moderate conversion of benzyl alcohol obtained at these conditions. An increased reaction temperature gave rise to higher conversion of benzyl alcohol, but there is appearance of small VJC, 55(6), 2017 Nguyen Tien Thao et al. 732 amounts of benzoic acid as a secondary product. The latter acid was possibly resulted from the over- oxidation of benzaldehyde [4, 19]. Therefore, it is suggested that 70 o C is the most appropriate temperature for the selective oxidation of benzyl alcohol to benzaldehyde product over MnOx/catalysts in the present work. Another process to approach a better conversion of benzyl alcohol is to prolong the reaction at a low temperature. Thus, a series of experiments have carried out at 60oC and kept the reaction mixture in a batch reactor for periods of 2-10 h. 0 20 40 60 80 100 2 4 6 8 10 Reaction time (h) P e rc e n t (% ) Conversion Benzaldehyde Sel Benzoic acid Sel Figure 5: Effect of reaction time on catalytic activity of sample TNK04-5Mn (5 wt.%Mn2+/sepiolite) at 60 oC, TBHP/Benzyl alcohol = 1.5 mol, none solvent Figure 5 presents the cataylytic activity for a longer reaction time. It is clearly observed that the an increased both benzyl alcohol conversion and benzaldehyde selectivity with increasing reaction time [5, 9, 19]. Obviously, benzyl alcohol conversion continuously increases linearly from 8 to 22 % while selectivity for benzaldehyde was almost remained constant. There is only small amount of benzoic acid (< 3 %) formed after 8-hour-reaction, reflecting a high selective activity of MnOx/sepiolite catalysts in the oxidation of benzyl alcohol. 4. CONCLUSION With XRD pattern of sepiolite and MnOx/sepiolite analyzed; manganese oxide existed as Mn3O4 on sepiolite nanofibers after the precipitation from nitrate salt. The morphology of sepiolite was slightly modified after calcincation process. The Mn3O4 particles were well distributed on the surface of the fibrous sepiolite and caused slight decrease in specific surface area of the support. Under hydrogen flowrate, Mn3O4 was reduced into MnO in a single step and well dispersed on carrier. The MnOx/sepiolite catalyst showed a good ability to conversion benzyl alcohol into benzaldehyde. 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Mahdavi V., Hasheminasab H. R. Vanadium phosphorus oxide catalyst promoted by cobalt doping for mild oxidation of benzyl alcohol to benzaldehyde in the liquid phase, Appl. Catal. A, 482, 189-197 (2014). Corresponding author: Nguyen Tien Thao Faculty of Chemistry, VNU University of Science Vietnam National University Hanoi 19, Le Thanh Tong street, Hoan Kiem district, Hanoi, Viet Nam E-mail: ntthao@vnu.edu.vn; Telephone: 093789891.