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

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.

Journal of Science: Advanced Materials and Devices (2018) 289e295 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Evaluation of catalytic activity of MeOx/sepiolite in benzyl alcohol oxidation Nguyen Tien Thao*, Nguyen Thi Nhu Faculty of Chemistry, Vietnam National University, Hanoi, 19 e Le Thanh Tong ST, Hanoi 100000, Viet Nam a r t i c l e i n f o a b s t r a c t Article history: Received 12 June 2018 Received in revised form July 2018 Accepted 13 July 2018 Available online 20 July 2018 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 © 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Oxidation Sepiolite Benzyl alcohol Benzaldehyde MeOx Introduction Benzaldehyde is a valuable chemical for production of flavors, pharmaceutical compounds, organic dyes, and agricultural chemistry [1,2] In practice, natural benzaldehyde is technically extracted from bitter almonds while the synthetic one is industrially produced by alkali hydrolysis of benzyl chloride and/or the oxidation of alkyl benzene [3e5] However, the traces of chlorine in the product and the low selectivity to benzaldehyde are the major issues in the mentioned industrial processes Thus, the partial oxidation of alkyl benzene or benzyl alcohol (BzOH) using both homogeneous catalysts 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 catalysts in both the vapor- and liquid-phase oxidation of benzyl alcohol to benzaldehyde [9,10] Indeed, noble metals including Pt [11], Pd [12], Au [10,13], Ag [14], Ru [2,15] or bimetallic catalysts of MeePt [9,16,17] are preferentially designed for catalyst systems for the oxidation reaction of benzyl alcohol Nevertheless, noble-metal * Corresponding author Fax: ỵ84 0937898917 E-mail address: ntthao@vnu.edu.vn (N.T Thao) Peer review under responsibility of Vietnam National University, Hanoi catalysts usually require high costs so that alternative heterogeneous 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 transition metal oxides supported on nanofibrous sepiolite in the oxidation reactions In the present work, a series of MeOx/sepiolite was used as catalysts in the liquid-phase-oxidation of benzyl alcohol with tert-butyl hydrogen peroxide (t-BuOOH) The results obtained provide useful information about the designing of oxidation reaction catalyst systems and advance the current understanding of heterogeneous oxidation catalysis Experimental 2.1 Preparation and characterization of the catalysts 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 suspension was magnetically agitated for h at ambient conditions https://doi.org/10.1016/j.jsamd.2018.07.006 2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 290 N.T Thao, N.T Nhu / Journal of Science: Advanced Materials and Devices (2018) 289e295 Afterwards, the transition metal hydroxide-loaded sepiolite was separated by filtration The obtained solid was washed with water several times, and then maintained in oven at 80  C for 24 h prior to calcine at 410  C for h All the prepared catalysts are symbolized as Me/sepiolite samples (Me ¼ Cu, Cr, Co, Ni, Mn, see Table 1) 2.2 Characterization X-ray diffraction (XRD) diagrams were run on a D8 AdvanceBruker 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 BrunauereEmmetteTeller (BET) theory based on the physisorption data of nitrogen performed on an Autochem II 2920 (USA) Energydispersive 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, 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, mL of t-butyl hydrogen peroxide (BuOOH, 70%, SigmaeAldrich) was put into the mixture Simultaneously, the reaction time was counted When the reaction is run out of time, the glass reactor was cooled to the room temperature The catalyst was removed by centrifugation and the filtrate was quantitatively determined by a HP-6890 Plus chromatographyemass spectroscopy 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 In general, the incorporation of metal oxides into a magnesium 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 electron microscopy (SEM) micrographs obtained for some MeOx/sepiolite are presented in Fig 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 Some physical properties of metal oxides/sepiolite samples Catalyst batch SBET (m2/g) Metal ion (wt.%) Sepiolite Co/sepiolite Mn/sepiolite Cu/sepiolite Ni/sepiolite Cr/sepiolite 166.2 191.1 133.9 127.4 122.5 129.6 e 7.19 7.44 6.74 6.09 6.73 X-ray diffractograms for all metal oxide/sepiolite catalysts are presented in Fig 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 appears 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 dispersed MeOx 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 shows the signals of transition metals (Cr, Co, Ni, Mn, Cu) in addition to Si, Al, Mg, O in the EDS spectra of the corresponding MeOx/sepiolite samples The weight percentage of transition metal of each specimen was listed in Table 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 asprepared 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 Cu2ỵ to Cu0, in good accordance with the literature [24] It is noted no other reaction peaks in H2TPR 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 temperature peak is ascribed to the reduction of Cr3ỵ to Cr2ỵ 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 Mn4ỵ to Mn3ỵ and the maximum peak can be ascribed to the reductive conversion of Mn3O4 to MnO, in agreement with the literature [7,9,23,24,29] Moreover, the intensity and broad shape of H2-TPR peaks for the N.T Thao, N.T Nhu / Journal of Science: Advanced Materials and Devices (2018) 289e295 291 Fig SEM micrographs for MeOx/sepiolite samples sepiolite 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 deposited 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 reaction 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 was 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] Therefore, the enhanced conversion observed over MeOx/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 substrate 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 and therefore, a comparative catalytic activity can be withdrawn 292 N.T Thao, N.T Nhu / Journal of Science: Advanced Materials and Devices (2018) 289e295 Fig XRD patterns for metal oxides/sepiolite Fig EDS spectra of the metal oxides/sepiolite catalysts Fig Comparative activity of the metal oxide/sepiolite in 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 recorded All the catalysts show similar curves in activities with the progress of the reaction (Fig 6) Conversion of benzyl alcohol increases 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 H2-TPR profiles of the metal oxides/sepiolite solids Obviously, the bell-shaped curve in Fig 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 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 N.T Thao, N.T Nhu / Journal of Science: Advanced Materials and Devices (2018) 289e295 Fig Effect of reaction temperature on the conversion in the oxidation of benzyl alcohol over metal oxides/sepiolite catalysts, h, BzOH/BuOOH ¼ 1/1.5, solvent-free reaction 293 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 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 and 8) and the activity order is in good agreement with what is observed in Fig 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 Effect of reaction temperature on benzaldehyde selectivity (a) and productivity (b) in the oxidation of benzyl alcohol over metal oxides/sepiolite catalysts, h, BzOH/ BuOOH ¼ 1/1.5, solvent-free reaction 294 N.T Thao, N.T Nhu / Journal of Science: Advanced Materials and Devices (2018) 289e295 of toluene and benzyl benzoate were observed at higher temperatures due to the occurrence of the deep oxidation, esterification, and disproportionation reaction of benzyl alcohol [2,8,9,32,33] These side-reactions are presumably accelerated at elevated reaction temperatures, giving rise to a significant change in the product distribution in the reaction mixture (Fig 8a) [9,12,13,23,34] Since a variety of products formed at higher temperatures, the mere comparison of specific activity of such a catalyst series cannot be done on the basis of the conversion alone in the entire reaction temperature range In this case, the benzaldehyde productivity reported in Fig 8b becomes more meaningful for the investigation of temperature effect on the oxidation of benzyl alcohol It is very interesting to observe that the curve of benzaldehyde productivity reaches a climax around 65  C over Cr/sepiolite catalyst As seen in Fig 8b, the climax shifts systematically to 75 and 80  C in the trends of benzaldehyde productivity obtained on Co and Cu/sepiolite catalysts, 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 two oxides may be more efficient for the oxidation of benzyl alcohol at higher reaction temperatures, in a good agreement with the data reported for the oxidation of benzyl alcohol over MnNiOx/carbon nanotube catalysts at 100  C [35] and on Ni-promoted Pd catalyst at 120  C [31] or on AgeNi fibers in gas phase oxidation of PhCH2OH [34] Therefore, the lowest reaction temperature for the production of highest productivity of benzaldehyde was found on Cr/sepiolite catalyst [29,31,35] Conclusion A series of MeOx/sepiolite (Me ¼ Cr, Co, Ni, Mn, Cu) catalysts was prepared by the precipitation method The as-prepared solids are mainly composed of a high distribution of MeOx clusters on the magnesium silicate nanorods The sepiolite loaded metal oxide catalysts have exhibited the selective oxidation of benzyl alcohol with 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 preparation and treatments The benzaldehyde selectivity and productivity are strongly dependent on the behavior of transition metals and reaction temperature, while the conversion varies with temperature 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 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investigation of temperature effect on the oxidation of benzyl alcohol It is very interesting