Evaluation of catalytic activity of MeOx/sepiolite in benzyl alcohol oxidation

A series of MeOx/sepiolite (Me ¼ Cu, Cr, Mn, Co, Ni) catalysts was prepared through the deposition eprecipitation and characterized by means of XRD, EDS, SEM, BET, and H2-TPR. The structural characterization showed a fine distribution of MeOx metal oxides on magnesium silicate nanobars. The sepiolite-loaded-metal oxide catalysts have been screened for the partial oxidation of benzyl alcohol and shown a good conversion and a very high selectivity to benzaldehyde at mild conditions. Under similar experiments, the catalytic activity of the transition metal oxides on sepiolite decreased in the order of Cr > Co > Cu > Mn > Ni. The productivity of benzaldehyde was associated with the behavior of transition metal ions and reaction temperatures.

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se i 10 Keywords: (M teriz dis de c and acti e p This is an open access article under the CC BY license ( ical fo dyes, ehyde hetic o hloride aces of mentioned industrial processes. Thus, the partial oxidation of alkyl MeePt [9,16,17] are preferentially designed for catalyst systems for the oxidation reaction of benzyl alcohol. Nevertheless, noble-metal alcohol with tert-butyl hydrogen peroxide (t-BuOOH). The results the designing of e the current un- s. ysts A weighted amount of sepiolite (Aldrich) was put in 100 mL of aqueous solution of the corresponding nitrate salts including Ni(NO3)2$6H2O, 98.5%; Cu(NO3)2$3H2O, 98%; Cr(NO3)3$9H2O, 99%; Co(NO3)2$6H2O, 98%; Mn(NO3)2$4H2O, 98% (SigmaeAldrich) with a fixed amount of transition metal ions. In order to precipitate all transition metal ions in the mixture, a calculated volume of 0.05 M NaOH aqueous solution was dropped to the mixture. The resultant suspensionwasmagnetically agitated for 2 h at ambient conditions. * Corresponding author. Fax: þ84 0937898917. E-mail address: ntthao@vnu.edu.vn (N.T. Thao). Contents lists availab Journal of Science: Advance .e l Journal of Science: Advanced Materials and Devices 3 (2018) 289e295Peer review under responsibility of Vietnam National University, Hanoi.alcohol to benzaldehyde [9,10]. Indeed, noble metals including Pt [11], Pd [12], Au [10,13], Ag [14], Ru [2,15] or bimetallic catalysts ofbenzene or benzyl alcohol (BzOH) using both homogeneous cata- lysts and heterogeneous systems has been developed, with numerous advanced methods for the selective production of benzaldehyde [1,2,5e8]. Since the homogeneous catalysts always yield a large amount of hazardous wastes, recent great efforts have currently been paid to the use of precious metals as efficient cata- lysts in both the vapor- and liquid-phase oxidation of benzyl obtained provide useful information about oxidation reaction catalyst systems and advanc derstanding of heterogeneous oxidation catalysi 2. Experimental 2.1. Preparation and characterization of the cataland the low selectivity to benzaldehyde are the major issues in the was used as catalysts in the liquid-phase-oxidation of benzylBenzaldehyde MeOx 1. Introduction Benzaldehyde is a valuable chem pharmaceutical compounds, organic istry [1,2]. In practice, natural benzald from bitter almonds while the synt duced by alkali hydrolysis of benzyl c alkyl benzene [3e5]. However, the trhttps://doi.org/10.1016/j.jsamd.2018.07.006 2468-2179/© 2018 The Authors. Publishing services b ( production of flavors, and agricultural chem- is technically extracted ne is industrially pro- and/or the oxidation of chlorine in the product catalysts usually require high costs so that alternative heteroge- neous catalysts using less expensive transition metal oxides for the oxidation processes have generated more attention in recent years. Very recently, we have prepared a high-surface-area sepiolite (magnesium silicate) loaded-chromium-oxide for the oxidation of benzyl alcohol [18]. Thus, it is interesting to screen different tran- sition metal oxides supported on nanofibrous sepiolite in the oxidation reactions. In the present work, a series of MeOx/sepioliteSepiolite Benzyl alcoholOxidation metal ions and reaction temperatures. © 2018 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.Original Article Evaluation of catalytic activity of MeOx/ oxidation Nguyen Tien Thao*, Nguyen Thi Nhu Faculty of Chemistry, Vietnam National University, Hanoi, 19 e Le Thanh Tong ST, Hano a r t i c l e i n f o Article history: Received 12 June 2018 Received in revised form 9 July 2018 Accepted 13 July 2018 Available online 20 July 2018 a b s t r a c t A series of MeOx/sepiolite eprecipitation and charac terization showed a fine sepiolite-loaded-metal oxi shown a good conversion experiments, the catalytic Cr > Co > Cu >Mn > Ni. Th journal homepage: wwwy Elsevier B.V. on behalf of Vietnam0000, Viet Nam e ¼ Cu, Cr, Mn, Co, Ni) catalysts was prepared through the deposition ed by means of XRD, EDS, SEM, BET, and H2-TPR. The structural charac- tribution of MeOx metal oxides on magnesium silicate nanobars. The atalysts have been screened for the partial oxidation of benzyl alcohol and a very high selectivity to benzaldehyde at mild conditions. Under similar vity of the transition metal oxides on sepiolite decreased in the order of roductivity of benzaldehyde was associated with the behavior of transitionpiolite in benzyl alcohol le at ScienceDirect d Materials and Devices sevier .com/locate/ jsamdNational University, Hanoi. This is an open access article under the CC BY license Afterwards, the transition metal hydroxide-loaded sepiolite was separated by filtration. The obtained solid was washed with water several times, and thenmaintained in oven at 80 C for 24 h prior to calcine at 410 C for 2 h. All the prepared catalysts are symbolized as Me/sepiolite samples (Me ¼ Cu, Cr, Co, Ni, Mn, see Table 1). 2.2. Characterization glass reactor was cooled to the room temperature. The catalyst was microscopy (SEM) micrographs obtained for some MeOx/sepiolite Some physical properties of metal oxides/sepiolite samples. N.T. Thao, N.T. Nhu / Journal of Science: Advance290Catalyst batch SBET (m2/g) Metal ion (wt.%) Sepiolite 166.2 e Co/sepiolite 191.1 7.19 Mn/sepiolite 133.9 7.44 Cu/sepiolite 127.4 6.74 Ni/sepiolite 122.5 6.09are presented in Fig. 1. We observed the existence of magnesium silicate nanorods with the width and length of 50e60 nm and several microns. The macroscopic structure of sepiolite is not significantly changed by the addition of transition metal oxides (MeOx) (Fig. 1) and no agglomeration of metal oxide particles is observed for all samples. Meanwhile, Cr/sepiolite possesses smooth magnesium silicate nanorods (Fig. 1). Thus, it is suggested that MeOx can be finely distributed both on the external surface and in the tunnels of the sepiolite [9,18,19]. Table 1removed by centrifugation and the filtrate was quantitatively determined by a HP-6890 Plus chromatographyemass spectroscopy. 3. Results and discussion 3.1. Catalyst characteristics The transition metal ions on the catalyst surface and the specific surface area (SBET) of all as-prepared catalysts are tabulated in Table 1. In general, the incorporation of metal oxides into a mag- nesium silicate support leads to a small decrease of specific surface area of the support from 166 m2/g to 122e133 m2/g except for that of Co/sepiolite (SBET ¼ 191 m2/g). Furthermore, scanning electronX-ray diffraction (XRD) diagrams were run on a D8 Advance- Bruker apparatus with CuKa radiation (l ¼ 1.549 Å). The scanning electron microscopy images were recorded on a JEOS JSM-5410 LV. The specific surface area of all catalysts was calculated using Bru- nauereEmmetteTeller (BET) theory based on the physisorption data of nitrogen performed on an Autochem II 2920 (USA). Energy- dispersive spectroscopy of the samples was collected on Varian Vista Ax X-ray instrument. H2-TPR profiles were monitored on a GOW-MAC 69-350 flow system. 2.3. Catalytic performance All oxidation reaction experiments were performed in a bath reaction with 100 mL three-neck glass flask equipped with a condenser. In an experiment, 3 mL of benzyl alcohol and 0.20 g of powdered catalyst were loaded into the glass reactor, and then the mixture was stirred until the temperature reached to a setting value. Afterwards, 6 mL of t-butyl hydrogen peroxide (BuOOH, 70%, SigmaeAldrich) was put into the mixture. Simultaneously, the re- action time was counted. When the reaction is run out of time, theCr/sepiolite 129.6 6.73X-ray diffractograms for all metal oxide/sepiolite catalysts are presented in Fig. 2. First, the XRD patterns of all catalysts display a set of observable reflections at corresponding angles of 7.3, 20.6, 23.8, 26.7, 28.1, 35.2, 40.1 (JCPDS # 01-075-1597) [18,19]. The presence of metal oxides on the support is also evident from XRD. In detail, the Cr2O3 crystals show some strong reflection angles at 24.8, 32.8 and 36.7 (JCPDS # 00-038-1479) [8,18,20]. Co3O4 ap- pears to have two weak reflection lines at 2-theta of 36.9 and 44.8 (JCPDS # 01-078-1970) [21,28]. NiO crystals show some visible signals at 2-theta of 37.1 and 43.4 (JCPDS # 00-044-1159) [17,22]. For the Mn-containing catalyst, the XRD pattern displays typical lines of Mn3O4 at 2-theta of 28.8, 36.0, 44.5 (JCPDS # 00-016-0154) [9,23,24]. The copper-containing sample has low signals at 2-theta of 35.5, 38.9, and 48.9 (JCPDS # 01-089-5895) [6,24]. These signals are rather weak due to a low amount of transition metal oxides and highly dispersedMeOx particles on the high surface area support as well, although some parts of MeOx may also be in an amorphous phase [9,18,21,25]. To confirm the presence of the transition metal ions on the support, EDS spectra were recorded. Energy-dispersive X-ray spectrometry (EDS) analysis gives us more information of the elemental composition in the catalyst surface. Fig. 3 shows the signals of transition metals (Cr, Co, Ni, Mn, Cu) in addition to Si, Al, Mg, O in the EDS spectra of the corre- sponding MeOx/sepiolite samples. The weight percentage of tran- sition metal of each specimen was listed in Table 1. In addition, the surface composition of recorded spots is quite close to each other, indicating a good dispersion of metal ions on the large surface scale of the support. However, the chemical composition of the as- prepared catalysts is slightly different from the theoretical values due to the deposition of tiny particles of metal oxides in the channels and tunnels of the sepiolite support [6,9,18,25,26]. Both XRD and EDS spectra strongly substantiate the presence of transition metal oxides dispersed on magnesium silicate nanorods. H2-TPR analysis will give more important information about the oxygen defect, location and coordination number of transition metal ions in the oxides (Fig. 4). As a consequence, the H2-TPR profile of Cu/sepiolite presents a single reduction peak at 224 C with onset temperature from 170 C. The main temperature peak is attributable to the reduction of Cu2þ to Cu0, in good accordance with the literature [24]. It is noted no other reaction peaks in H2- TPR traces, suggesting the existence of single CuO phase on the large surface sepiolite. Meanwhile, the H2-TPR profile of the Ni/ sepiolite sample displays a strong reduction peak at 323 C along with the broad tail at higher temperatures, ascribing to the reaction of Ni(II) to Ni(0) at this temperature [17,22]. The temperature-tailed signal may be related to the reduction process of large particles on the support. A slight symmetric temperature peak was observed in the H2-TPR curve of the Cr/sepiolite sample, the reduction tem- perature peak is ascribed to the reduction of Cr3þ to Cr2þ in uniform Cr2O3 clusters highly distributed on the porous support [18,20,27]. For the Co/sepiolite sample, H2-TPR signal displays three reductive singles at 275, 345, and 395 C, indicating a multiple-reaction step. According to the literature, the low temperature peak around 275 C is assigned to the reduction of CoO(OH) to Co3O4 although no CoO(OH) phase was detected by XRD technique. The reduction of Co3O4 to CoO occurred at 345 C while the final reaction step of CoO to metallic cobalt performed around 400 C [21,28,29]. The reduction of cobalt oxides occurred in consecutive stages, reflecting a multiple-phase of this oxide in the catalyst. For the Mn/sepiolite sample, the H2-TPR profile also contains a band reduction peak from 300 to 460 C with a maximum at 407 C. The first signal at 205 C is assigned to the reaction of Mn4þ to Mn3þ and the maximum peak can be ascribed to the reductive conversion of Mn O to MnO, in agreement with the literature [7,9,23,24,29]. d Materials and Devices 3 (2018) 289e2953 4 Moreover, the intensity and broad shape of H2-TPR peaks for the anceN.T. Thao, N.T. Nhu / Journal of Science: Advsepiolite loaded metal oxides are firmly associated with different coordination numbers and the locations of MeOx clusters on the support [9,18,22,23,29,30]. Apparently, the MeOx particles depos- ited on the external surface or channels are more reducible than those stayed in the tunnels [9,18,28]. The distribution of these oxide clusters also affect the activity of the liquid phase oxidation of benzyl alcohol. These studies indicate that the reducibility of the metal oxides/sepiolite depends on the nature as well as the dispersion of transition metal oxides on the support. A high reac- tion temperature indicates a high reducibility of metal oxide on the support, which prevents the oxidationereduction cycles during the catalytic oxidation process at mild reaction conditions [28]. 3.2. Catalytic evaluation of MeOx/sepiolite The oxidation of benzyl alcohol with t-BuOOH in the absence of catalysts shows no conversion of alcohol. When pure sepiolite was added into the reaction mixture, a small amount of benzyl alcohol Fig. 1. SEM micrographs ford Materials and Devices 3 (2018) 289e295 291was converted into benzaldehyde (Fig. 5), but the addition of MeOx (Me ¼ Ni, Co, Cr, Cu, Mn) increased significantly alcohol conversion and benzaldehyde was formed as a major product [1,8,18]. There- fore, the enhanced conversion observed overMeOx/sepiolite (Fig. 5) clearly substantiates the synergistic effect between the metal oxides and sepiolite support on the decomposition of t-BuOOH into active species that further react with benzyl alcohol [9,18,24,31e33]. To clarify this issue, additional reaction tests of benzyl alcohol over MeOx/sepiolite catalysts using oxidant have been performed, which show a very small conversion of the sub- strate at high temperatures. Thus, it is suggested that the lattice oxygen in the small amounts of metal oxides or oxygen molecules (in air) insignificantly contributed to the oxidation reaction of the alcohol in the present experimental conditions, evidenced by a high reducibility of transition metal oxides in H2-TPR experiments. It is noted that the benzaldehyde selectivity was obtained about 99% over all catalysts under reaction conditions reported in Fig. 5 and therefore, a comparative catalytic activity can be withdrawn. MeOx/sepiolite samples. anced Materials and Devices 3 (2018) 289e295N.T. Thao, N.T. Nhu / Journal of Science: Adv292Obviously, the bell-shaped curve in Fig. 5 corresponds to an activity changing with the nature of transition metal ions in the catalysts. The catalytic activity of the compared catalysts decreases as the following order of Cr > Co > Cu >Mn > Ni. A higher activity on the Cr-containing catalyst is associated with a higher dispersion and locations of chromium oxide particles. Another reason is related to the flexible reducibility of Cr(III) ions that are easily oxidized to higher oxidation states which are known as effective components in the oxidation reaction [18,20]. For confirming the comparative activity at high product selectivity (Fig. 5), the catalytic evaluation of the MeOx/sepiolite catalysts at different reaction times was also Fig. 2. XRD patterns for m Fig. 3. EDS spectra of the metal oxides/sepiolite catalysts. Fig. 4. H2-TPR profiles of the metal oxides/sepiolite solids.etal oxides/sepiolite.recorded. All the catalysts show similar curves in activities with the progress of the reaction (Fig. 6). Conversion of benzyl alcohol in- creases 2e3 times after 10 h-on-time, notably the selectivity to benzaldehyde is almost unchanged at this reaction temperature [18,30]. Furthermore, in this experiment series, the Cr/sepiolite also gave the highest catalytic performance in term of activity value (38%) while the Ni/sepiolite showed the lowest conversion of benzyl alcohol under the same controlled reaction conditions. The Fig. 5. Comparative activity of the metal oxide/sepiolite in the benzyl alcohol con- version over all as-prepared catalysts at 60 C, BzOH/BuOOH ¼ 1/1.5, 0.20 g of catalyst, solvent-free reaction. Fig. 6. Effect of reaction time on the benzyl alcohol conversion over all as-prepared catalysts at 60 C, BzOH/BuOOH ¼ 1/1.5, 0.20 g of catalyst, solvent-free reaction. low activity of some MeOx/sepiolite samples is possibly associated with the low dispersion and poor crystallinity of transition metal oxides, as in line with the XRD analysis [26,32e34]. In order to shed more light on the catalytic activity of such a catalyst series, additional sets of experiments have been carried out at different reaction temperatures. Fig. 7 presents the conversion of benzyl alcohol and the product distribution over MeOx/sepiolite catalysts was reported in Fig. 8a. In general, the conversion of alcohol increases with increasing reaction temperatures. In the temperature range of 50e70 C, MeOx/sepiolite catalysts converted selectively benzyl alcohol to benzaldehyde (Figs. 7 and 8) and the activity order is in good agreement with what is observed in Fig. 5. However, at higher reaction temperatures (80e90 C), this catalyst activity order has been slightly modified and the oxidation reaction did not proceed selectively to the formation of benzaldehyde. In fact, a small amount of benzoic acid and dibenzyl ether with traces Fig. 7. Effect of reaction temperature on the conversion in the oxidation of benzyl alcohol over metal oxides/sepiolite catalysts, 4 h, BzOH/BuOOH ¼ 1/1.5, solvent-free reaction. Fig. 8. Effect of reaction temperature on benzaldehyde selectivity (a) and productivity (b BuOOH ¼ 1/1.5, solvent-free reaction. N.T. Thao, N.T. Nhu / Journal of Science: Advanced Materials and Devices 3 (2018) 289e295 293) in the oxidation of benzyl alcohol over metal oxides/sepiolite catalysts, 4 h, BzOH/ temperature range. In this case, the benzaldehyde productivity re- of benzaldehyde productivity obtained on Co and Cu/sepiolite cat- catalysts have exhibited the selective oxidation of benzyl alcohol ancewith TBHB to benzaldehyde. Under similar conditions, chromium oxide yielded a highest conversion of benzyl alcohol, while the NiO/ sepiolite exhibited a poorest activity as the comparative activity order of Cr > Co > Cu > Mn > Ni under the same catalyst prepa- ration and treatments. The benzaldehyde selectivity and produc- tivity are strongly dependent on the behavior of transition metals and reaction temperature, while the conversion varies with tem- perature only. The maximum productivity reached a highest value at the lowest examined temperature on Cr/sepiolite. The highest conversion of benzaldehyde is about 60e70% and the selectivity to benzaldehyde remains at 99% in solvent-free conditions. Acknowledgements This research is funded by Vietnam's National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.05-2017.04. References [1] T. Mallat, A. Baiker, Oxidation of alcohols with molecular oxygen on solid catalysts, Chem. Rev. 104 (2004) 3037e3058. [2] R. Ciriminna, V. Pandarus, F. Beland, Y.-J. Xu, M. Pagliaro, Heterogeneously catalyzed alcohol oxidation for the fine chemical industry, Org. Process Res. Dev. 19 (2015) 1554e1558. [3] J. Wu, T. Su, Y. Jiang, X. Xie, Z. Qin, H. Ji, Catalytic ozonation of cinnamaldehyde to benzaldehyde over CaO: experiments and intrinsic kinetics, AIChE J. 63 (2017) 4403e4417. [4] G.D. Yadav, P.H. Mehta, B.V. Haldavanekar, Capsule membrane phase transferalysts, respectively. For the Mn/sepiolite sample, the benzaldehyde productivity profile likely approaches a plateau in the temperature range of 85e90 C while that for Ni/sepiolite increases gradually with reaction temperatures, reflecting that the