Catalysis Communications 45 (2014) 153–157 Contents lists available at ScienceDirect Catalysis Communications journal homepage: www.elsevier.com/locate/catcom Short Communication Selective oxidation of styrene over Mg–Co–Al hydrotalcite like-catalysts using air as oxidant Nguyen Tien Thao ⁎, Ho Huu Trung Faculty of Chemistry, Vietnam National University, Hanoi, 19 Le Thanh Tong St, Hanoi, Viet Nam a r t i c l e i n f o Article history: Received 16 September 2013 Received in revised form 31 October 2013 Accepted November 2013 Available online 21 November 2013 Keywords: Metal-doped hydrotalcite Styrene oxidation Benzaldehyde Epoxide Mg–Co–Al a b s t r a c t A set of synthesized Mg/Co/Al hydrotalcites was synthesized and characterized by XRD, XPS, BET, SEM, TEM, and FT-IR physical techniques The partial substitution of Mg2+ by Co2+ in brucite layers has not significantly affected the layered double hydroxide structure, but plays a crucial role in the oxidation of styrene in the presence of air The prepared Mg/Co/Al hydrotalcite-like compounds express a good activity and stability in the oxidation of styrene in the free-solvent condition Both styrene conversion and desired product selectivities are strongly dependent on the cobalt substitution content The intra-hydrotalcite lattice Co2+ ions are active sites for the epoxidation of styrene © 2013 Elsevier B.V All rights reserved Introduction Oxidation of styrene is a reaction of great interest because its products act as versatile and useful intermediates [1] Conventionally, this process has been usually carried out by homogeneous catalysts and resulted in a huge amount of toxically corrosive chemical wastes Recently, there has been much interest in solid catalysts and uses of environmentally friendly cheap oxidants [2,3] Several transition metal-containing catalysts based on Ru, Cu, Fe, Mn, V, Ti… have been used in the liquid phase oxidation of olefinic compounds to oxygenates [3–6] Among those, Ru- and Cu-based heterogeneous solids are restricted only to doubly activated alkylaromatics while Fe- and Nicontaining catalysts usually give a rather low yield of oxygenated products [3–7] Therefore, the synthesis of novel easily recyclable catalyst for the oxidation of alkylbenzenes is still a great challenging goal of fine chemical industry Hydrotalcite-like compound is known as a layered double hydroxide 3+ (LDH) mineral with the general formula of [A2+ (1 − x)Bx (OH)2(CO3)0.5x· nH2O] Cations are usually located in coplanar [M(OH)6] octahedra sharing vertices and forming M(OH)2 layers with the brucite structure [8] Partial substitution of divalent cations by trivalent cations leads to the appearance of positive layers which is usually compensated by anions between layers Thus, the complexity of chemical composition in hydrotalcite-like compound makes it be able to act as basic solids and oxidation–reduction catalysts [9–11] For example, Ni-containing basic hydrotalcites were used for the selective oxidation of benzylic C\H bonds of ethyl benzene [11] Mn–MgAl and MoO− /MgAl ⁎ Corresponding author Tel.: +84 3825 3503; fax: +84 3824 1140 E-mail address: ntthao@vnu.edu.vn (N.T Thao) 1566-7367/$ – see front matter © 2013 Elsevier B.V All rights reserved http://dx.doi.org/10.1016/j.catcom.2013.11.004 hydrotalcite-like catalysts present a good activity in the oxidation of alkylbenzenes [9,12] Cobalt-containing hydrotalcites have been used for the steam reforming of ethanol [13] and synthesis of benzoin methyl ether [14] In these cases, transition metal ions in layered structure are the key for the catalytic activity This article provides a novel applicability of Mg–Al hydrotalcites partially substituted by cobalt ions as effective catalyst for the oxidation of styrene under milder conditions Experimental 2.1 Preparation and characterization of the catalysts Mg/Co/Al hydrotalcite-like compounds were prepared by the coprecipitation method The detailed procedure was described in our previous publication [15] In brief, 150 mL-mixed aqueous solution of Mg(NO3)2·6H2O (99%), Co(NO3)2·6H2O (98%) and Al(NO3)3·9H2O (N98%) with Co2 +/(Mg2 + + Al3 +) molar ratios ranging from to 0.44 was added dropwise to 25 mL of 0.6 M Na2CO3 under vigorous stirring The exact amounts of starting materials for each catalyst are given in Supporting information (Table 1S) The solution pH was adjusted to 9.50 using 1.5 M NaOH and was kept for 24 h Then, the resulting gellike material was aged at 65 °C for 24 h The resultant slurry was then cooled to room temperature and separated by filtration, washed with hot distilled water several times, and then dried at 80 °C for 24 h in air The prepared catalysts are denoted as Mg/Co/Al-1, -2, and -3 (Table 1) The elemental composition (Mg, Co, Al) of catalyst was measured using an ICP-MS Elan 9000 (PerkinElmer, USA) and carbon content instrument PE 240 (USA) Powder X-ray diffraction (XRD) patterns were recorded on a D8 Advance-Bruker instrument using CuKα radiation N.T Thao, H.H Trung / Catalysis Communications 45 (2014) 153–157 Table Physical properties of the prepared Mg/Co/Al hydrotalcite-like compounds Catalyst batch Molar ratio of Co2+/(Mg2++Al3+) Mg/Al-0 Mg/Co/Al-1 Mg/Co/Al-2 Mg/Co/Al-3 Mg/Co/Al-2 reacted 0.10 0.24 0.44 0.24 Mg Elemental analysis (wt.%) Co Al C BET surface area (m2/g) Pore volume (cm3/g) 24.74 21.34 14.20 9.18 10.13 – 8.16 12.72 17.89 9.71 11.93 12.30 8.14 8.10 7.24 2.39 1.98 1.87 1.89 2.11 83.4 78.9 74.6 74.5 44.2 0.62 0.60 0.58 0.58 0.49 (λ = 0.1549 nm) Fourier transform infrared (FT-IR) spectra were obtained in 4000–400 cm − range on a FT/IR spectrometer (DX-PerkinElmer, USA) TEM images were collected on a Japan JEOL JEM-1010 The nitrogen physisorption was run on an Autochem II 2920 (USA) The X-ray photoelectron spectra (XPS) of catalysts were recorded with a Thermo K-Alpha The catalytic oxidation of styrene in the absence of solvent was carried out in a 100 mL three-neck glass flask fitted with a reflux condenser For a typical run, 87.28 mmol of styrene and 0.2 g of catalyst were loaded into the flask unless some particular tests were indicated After the reaction mixture was magnetically stirred and heated to the desired temperature, the flow of air (5 mL/min) was bubbled through the vigorously stirred reaction mixture and the reaction time starts recorded After the reaction, the mixture was quenched to room temperature and then catalyst was filtered off The filtrate was analyzed by a GC–MS (HP-6890 Plus) and a frame ionization detector (FID) is used as a detector Results 3.1 Catalyst characteristics The prepared catalyst characteristics and chemical composition are summarized in Table Fig displays the powder X-ray diffraction patterns for all synthesized Mg–Co–Al hydrotalcite-like materials Overall, all samples present a set of reflection lines matching to those characteristics of layered double hydroxide structure [8,10,13,15] Indeed, two sharp and intense peaks at low diffraction angles of 23.2 and 34.4° are ascribed to the diffraction by basal planes (006) and (102), respectively [10,15] Furthermore, broad, less intense peaks at higher angles around 38, 46, and 60° indexed to (105), (108), and (110) planes also confirm the hydrotalcite structure [14,15] The positions of these reflection lines are slightly changed but the signal to noise ratio and full width at half maximum peaks vary with increased cobalt content The latter could possibly be explained by only subtle differences in the octahedral ionic radii of Co2+ (0.74 Å) and Mg2+ (0.72 Å) [13] The XRD patterns (Fig 1) reveal that the cobalt rich-samples are somewhat poorer crystallinity because the affinity of CO2− to Co2+ is less than that to Mg2+ [10,13] No reflection lines corresponding to cobalt oxides are observed, suggesting that cobalt ions are present in LDH structure [13,14] The major photoelectron lines of the elements in a representative Mg/Co–Al-1 are reported in Fig 2A Clearly, magnesium, cobalt, oxygen, carbon and aluminum have photoelectron lines at 1s (1303.93 eV), 2s (88,08); 2p (781.08 eV); 2p (531.90 eV); 2p (289.08 eV) and 2p (74.34 eV), respectively [13] To investigate the oxidation state in the near-surface region, the spectrum corresponding to the Co 2p core level is represented in Fig 2B while Mg 1s and Al 2p scans are elucidated in Supporting information XPS spectrum of Co 2p in Mg/Co/Al sample shows two clear peaks positioned at binding energy values of 781.1 (Co 2p3/2) and 797.1 eV (Co 2p1/2), along with shake-up satellites These binding energy values and the peak separation are essentially ascribed to Co2+ species Furthermore, the high intensities of the satellites are typical characteristics for the cobalt containing layered double hydroxide structure Thus, it is suggested that Co2+ ions locate at octahedral sites in brucite-like layers [13,16] FT-IR spectra of Mg/Co/Al hydrotalcite-like materials present the main band around 3454 cm−1 assigning to the OH stretching mode of water molecules and hydroxyls in the layers [10,12] This band shows a prominent shoulder around 2950 cm−1 ascribed to hydrogen bonding of OHs of layered lattice and/or water molecules with interlayer carbonate anions (see Fig 1S in Supporting information) A sharp band at 1365 cm−1 is firmly assigned to the asymmetric stretching vibration of the CO2− in the hydrotalcite layers A set of bands at 437, 663, 742, and 927 cm− is associated to Al\O, Co\O, Al\OH translation, and doublet Al\OH deformation modes, respectively [13,17] The textural properties of nominal Mg/Co/Al hydrotalcite-like compounds were insignificantly changed with molar ratios of Mg/Co/Al A 420000 O 1s 360000 300000 Counts/s 154 Mg 1s 240000 180000 Co 120000 C1s 60000 Al 2p 1200 1000 800 600 400 200 Binding Energy (eV) B Mg/Co/Al -3 Mg/Co/Al -2 Mg/Co/Al -1 Mg/Al -0 20 25 30 35 40 45 50 2-theta (o) 55 60 65 70 Fig XRD patterns of as-synthesized hydrotalcite-like compounds and the used sample Counts /s Mg/Co/Al -2 - Reacted Co2p scan 781.18 20000 19000 797.0 18000 17000 16000 15000 14000 13000 12000 11000 10000 812 809 806 803 800 797 794 791 788 785 782 779 776 773 770 Binding Energy (eV) Fig Survey scan (A) and Co 2p XPS spectrum (B) of as-synthesized Mg/Co/Al-2 hydrotalcite-like material BET specific surface area of the cobalt-free-sample (Mg/Al-0) is only 83.4 m2/g while that of the others is approximately 74–78 m2/g (Table 1) The nitrogen isothermal curves likely expresses a plateau from to 0.6 and are gradually skewed in the range of 0.6–0.85, reflecting the nitrogen physisorption and condensation in micropores [14,16] Furthermore, the condensation process at relative pressures higher than 0.8 along with a sharp adsorption volume increase is firmly responsible for the physisorption in mesopores (Supporting information) [15] The morphology of Mg/Co/Al-1 LDH is illustrated in Fig and some additional micrographs are depicted in Supporting information Fig 3A shows that the hydrotalcite-like compound particles are regularly hexagonal plates [15] The particle sizes are relatively uniform with the mean crystal domain of 70–100 nm [11,13] More details, the TEM image of Mg/Co/Al-1 LDH shows laminar structure which is an essential characteristic for hydrotalcite mineral and the stacking of the layers (Fig 3C) [17] The flat particles with hexagonal shapes are presented and the grain boundaries are clearly observed The aggregation of uniform particles leads to the formation of voids between primary nanoparticles [13,17] 3.2 Catalytic results The catalytic activity of Mg/Co/Al-hydrotalcite-like catalysts in the liquid oxidation has been examined at atmospheric pressure and air was bubbled into the reaction system without any further purification 3.2.1 Oxidation of styrene catalyzed by cobalt ions in hydrotalcites Fig presents the reaction results of three Mg/Al/Co hydrotalcitelike materials in the oxidation of styrene By comparison, a blank test and the cobalt-free-sample (Mg/Al-0) have been also performed (%) N.T Thao, H.H Trung / Catalysis Communications 45 (2014) 153–157 155 100 90 80 70 60 50 40 30 20 10 Conversion (%) Benzaldehyde Sel Styrene oxide Sel Other Product Sel Mg/Co/Al = 6/1/3 Mg/Co/Al =5/2/3 Mg/Co/Al = 4/3/3 Hydrotalcite catalysts Fig The correlation between catalytic activity in the oxidation of styrene and cobalt contents in Mg/Co/Al hydrotalcite-like catalysts (other products: phenyl acetaldehyde, benzoic acid, styrene glycol, benzyl benzoate, and polymerized products) under the same reaction conditions The former test shows a null conversion of styrene while the cobalt-free sample (Mg/Al-0) converts a negligible amount of styrene (b1%) over Mg/Al-hydrotalcite-like material basic sites to benzaldehyde [10,19] Meanwhile the cobaltlow-sample (Mg/Co/Al-1) selectively oxidizes about 6% styrene to benzaldehyde [18,19] Furthermore, styrene conversion reaches 54% after h of reaction time over Mg/Co/Al-3 sample (Fig 4) and two major products are styrene oxide and benzaldehyde in addition to small amounts of phenylacetaldehyde, benzoic acid, styrene glycol, and benzyl benzoate… Therefore, it is suggested that the presence of Co2+ in LDH structure (Fig 2) has a synergetic effect on the formation of aldehyde and yielded a major amount of styrene oxide [11,19–22] Indeed, the intra-hydrotalcite lattice cobalt ions are more stable and avoided the oxidation to higher oxidation states (e.g Co3O4, Co2O3), in accordance with those observed for Co2+ exchanged in zeolites [20,21] A B C D Fig SEM micrographs of as-synthesized (A) and the used (B) and TEM images of as-synthesized (C) and the used (D) Mg/Co/Al-1 hydrotalcite-like compound N.T Thao, H.H Trung / Catalysis Communications 45 (2014) 153–157 3.2.3 Effects of reaction time The influence of the reaction time on the reaction over Mg/Co/Al hydrotalcite-like materials is represented in Fig The conversion of reactant gradually increases from beginning time to h and reaches a plateau after h Overall, the styrene conversion over the cobalt-rich sample is always higher than the cobalt-low-catalyst Fig also indicates that the selectivity to styrene oxide slightly increases with reaction time whereas that to benzaldehyde decreases from 70% to 46% over Mg/Co/Al-3 catalyst (Fig 5B) [14,21,22,24] It is noted that no significant change in structural feature and morphology during the oxidation reaction although the specific surface area of spent hydrotalcite catalysts slightly decreases (74.6 to 44.2 m2/g for Mg/Co/Al-2) The Co/(Mg + Al) molar ratio of the reacted sample is almost unchanged after h, demonstrating that the intra-LDH lattice cobalt ions are the key for the oxidation of styrene The catalytic tests reported in Fig were lasting for h with no significant changes in conversions In the case of sample Mg/Co/Al-1, the catalyst was recorded infrared bands and its IR spectrum was reported in Fig 1S Table Catalytic activity of Mg/Co/Al-hydrotalcite-like compounds at different reaction temperatures after h Catalysts Mg/Co/Al-1 Mg/Co/Al-2 Mg/Co/Al-3 Reaction temperature (°C) Styrene conversion (%) Product selectivity (%) Benzaldehyde Styrene oxide Othersa 65 75 85 95 65 75 85 95 65 75 85 95 2.6 3.3 4.5 12.1 5.0 13.4 32.3 38.1 18.7 46.2 52.6 93.0 91 98 91 82 93 71 57 59 99 64 55 45 – 11 24 27 36 30 38 36 1 16 19 a Other products: phenyl acetaldehyde, benzoic acid, styrene glycol, benzyl benzoate, polymerized products A100 90 Conversion Benzaldehyde Sel Styren oxide Sel Other product Sel 80 70 Percent (%) 3.2.2 Effect of reaction temperatures Table describes the variation of activity and main product selectivities over all hydrotalcite-like samples in the reaction temperature range of 60–95 °C At higher temperatures the reaction becomes quite complicated because some side reactions like overoxidation and polymerization occurring simultaneously [3,19,22–24] In general, styrene conversion varies dramatically with the temperatures from 60 to 95 °C, but the selectivity to epoxide approaches a highest value at 80–90 °C while that to benzaldehyde reaches a maximum level at lower temperatures of 60–70 °C Moreover, the overall conversion of styrene was found to decrease in order of Mg/Co/Al-3 N Mg/Co/Al-2 N Mg/Co/Al-1 in a whole range of reaction temperatures (Table 2) Since the reaction has a negligible activity over the cobalt-free sample, this order indicates a strong relation between catalytic activity and the surface density of cobalt ions [20,22] Table also presents that the selectivity towards phenyloxirane significantly increases with the Co/(Mg + Al) molar ratio order of 0.44 N 0.24 N 0.10 N The possible incorporation of Co(II) into the brucite layers of hydrotalcite-like materials provides available sites for the epoxidation of styrene It is well known that Co(II) complexes can activate molecular oxygen to form a transition complex of (Co–O)* [2,22] In the present work, Co(II) octahedral sites in hydrotalcite structure are responsible for the formation of the (Co3 +–O− ) species which further generate radical oxygen species for the initiation of the oxidation reaction under mild conditions [2,20–22] 60 50 40 30 20 10 Reaction time (h) B Conversion Benzaldehyde Sel Styren oxide Sel Other product Sel 100 90 80 Percent (%) 156 70 60 50 40 30 20 10 Reaction time (h) Fig Catalytic activity in the oxidation of styrene over Mg/Co/Al-1 sample (A) and Mg/ Co/Al-3 catalyst (B) 85 °C (other products: phenyl acetaldehyde, benzoic acid, styrene glycol, benzyl benzoate, and polymerized products) Conclusion Three Mg/Co/Al materials show good characteristics of layered double hydroxides: the presence of carbonate ions between the layers, homogeneous and laminar structure, and a medium surface area The catalysts were tested for the oxidation of styrene in solvent free conditions using air as a friendly cheap oxidant All synthesized Mg–Co–Al catalysts exhibit good activity and relative stability in the selective oxidation of styrene to benzaldehyde and epoxide Both reactant conversion and product selectivities are dependent on the surface area of cobalt ions and reaction variables Co2+ in octahedral sheets is suggested to be acting as active sites for the oxidation of styrene into styrene oxide while both Co2 + intra-lattice ions and basic sites in hydrotalcite are responsible for the formation of benzaldehyde The conversion of styrene reaches about 70–90% and selectivity to desired products (benzaldehyde + styrene oxide) is about 92–99% Acknowledgment This research is funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.99-2011.50 Appendix A Supplementary data Supplementary data to this article can be found online at http://dx doi.org/10.1016/j.catcom.2013.11.004 N.T Thao, H.H Trung / Catalysis Communications 45 (2014) 153–157 References [1] G Centi, F Cavani, F Trifiro, Selective Oxidation by 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Catalysis Communications 45 (2014) 153–157 Table Physical properties of the prepared Mg/ Co/ Al hydrotalcite- like compounds Catalyst batch Molar ratio of Co2 +/ (Mg2 + +Al3 +) Mg/ Al- 0 Mg/ Co/ Al- 1 Mg/ Co/ Al- 2... 4/3/3 Hydrotalcite catalysts Fig The correlation between catalytic activity in the oxidation of styrene and cobalt contents in Mg/ Co/ Al hydrotalcite- like catalysts (other products: phenyl acetaldehyde,... amount of styrene (b1%) over Mg/ Al- hydrotalcite- like material basic sites to benzaldehyde [10,19] Meanwhile the cobaltlow-sample (Mg/ Co/ Al- 1) selectively oxidizes about 6% styrene to benzaldehyde