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Catal Lett DOI 10.1007/s10562-016-1710-0 Activity of Molybdate-Intercalated Layered Double Hydroxides in the Oxidation of Styrene with Air Nguyen Tien Thao1 • Nguyen Duc Trung1 • Dang Van Long1 Graphical Abstract Product distribution obtained on Mg–Al–Molybdate like hydrotalcite catalysts in the liquid oxidation of styrene with air 90 80 70 Benzaldehyde 100 60 50 10 tu Te m pe 24 hours 24 hours re 100 110 hours hours 80 90 ) 20 ( oC 30 Styrene Oxide 40 Styrene Oxide Selectivity (%) Abstract Molybdate anions were intercalated into the interlayer spacings of (Mg, Al) like-hydrotalcite compounds as interlayer compensating anions The synthesized samples have been characterized by XRD, FT-IR, Raman, EDS, UV–vis, BET, and XPS The solids possess lamellar structure and uniform platelet particles There is mainly Mo(VI) present in both tetrahedral and octahedral configuration in the samples All the synthesized catalysts have been tested for the liquid oxidation of styrene at mild conditions Under reported conditions, styrene conversion varies with the total amount of molybdate ions Benzaldehyde and styrene oxide were two major components in the product mixture The selectivity to styrene oxide was found to be associated with the nature of oxidants and the amount of tetrahedrally-coordinated Mo species in layered double hydroxides while that to benzaldehyde is related to the overall amount of MoO4- anions in the sample Benzaldehyde Received: February 2016 / Accepted: February 2016 Ó Springer Science+Business Media New York 2016 Keywords Molybdate Á Epoxidation Á Styrene Á Oxidation Á Hydrotalcite Á LDH Introduction Electronic supplementary material The online version of this article (doi:10.1007/s10562-016-1710-0) contains supplementary material, which is available to authorized users & Nguyen Tien Thao ntthao@vnu.edu.vn Faculty of Chemistry, Vietnam National University, Hanoi, 19 Le Thanh Tong ST, Hanoi 10000, Vietnam Olefin oxidation is an important catalytic process in the chemical industry This reaction produces two valuable products, aldehyde and epoxide, which are important chemical feedstock for the production of a wide variety of fine chemicals and pharmaceuticals In tradition, this reaction has been, in general, carried out over both homogeneous and heterogeneous catalyst systems [1–3] However, the use of homogeneous catalysts has recently become less attractive because of the difficulty in the 123 N T Thao et al separation of products and catalysts Thus, heterogeneous or immobilized catalysts have been currently paid much attention Among huge amounts of heterogeneous catalysts used for this oxidation reaction, firstly it should be taken account of titanium-based catalysts, VIIIB-based metals or oxides, noble metals and their derivatives, molecular sieves [2–10] These catalysts are reported to have a good activity in the oxidation of olefinic hydrocarbons Indeed, titana was known as an excellent photocatalyst for the oxidation of organic compounds [2, 4–6] This oxide can be used as supported oxide catalysts (e.g TiO2/SiO2, TiO2/MCM-41) [2, 11, 12], mixed oxides (e.g TiO2–SiO2, TiO2–ZrO2) [13, 14], framework-substituted molecular sieves (e.g TS-1, TS-1, Ti–MCM-41, Ti–MCM-48, Ti-b) [2, 4–6, 15] in the oxidation of unsaturated hydrocarbons In practice, TiO2based photocatalysts are active for the oxidation of olefins, but rapidly deactivated in the presence of water Meanwhile, titanium-substituted molecular sieves can perform the oxidation reaction in aqueous solution because the intra-lattice titanium can make hydroperoxo complexes by hydrolysis of Ti–O–Si connectivities Thus, TS-1 was very selective for the epoxidation of styrene and its catalytic activity is remarkably related to the amount of Ti(IV) in the crystalline framework, catalyst morphology, oxidant, solvent, preparation method [2, 4–6] For instance, Yeung et al [5] observed a high selectivity to phenylacetaldehyde in the oxidation of styrene over intra-framework titanium TS-1 catalyst However, this catalyst possesses small pore size which only allows small reactants accessibly to the active sites Thus, other catalysts developed from the group VIIIB-based metals have been recently reported for the oxidation of olefins [7, 9, 16, 17] The catalytic activity is governed as the appropriate oxidant was used and the oxidation state of transition metal ions is controlled [7–9, 16] In practice, aside from Co-based catalysts, other VIIIB-metal based catalysts seldom catalyze the epoxidation of olefin when oxygen/air is used as oxidant [2, 7–10, 16, 17] Fe–Ni—containing catalyst exhibits a good activity in the conversion of styrene with H2O2 or peracids only [1, 2, 7–9] Co–Ni–Cu nanoxides are recently reported as active catalysts for the oxidation of styrene with tertbutyl hydroperoxide (TBHP) [7] Co-based catalysts can oxidize styrene into styrene oxide although a small amount of benzaldehyde is always detected in the product mixture as air is used as oxidant [8, 9] The other catalysts may be designed from transition metals such as copper, silver, vanadium, tungsten, molybdenum, manganese… although the catalytic activity of these catalysts is still modest [2, 10, 18–22] In general, the group VIB–VIIB metals are usually incorporated into complex-derived compounds or to make metal immobilized catalysts [2, 21–24] In latter cases, the active components are present as transition metal oxocomplexes which undergo the selective oxidation of 123 alkenes [22, 25–30] Following this trend, we have incorporated some oxoanions into the interlamellar spaces of hydrotalcites for the purpose of the preparation of oxidation/reduction catalysts It was well known that hydrotalcites are common anionic clays described by the empirical ỵy 3ỵ z formula A2ỵ B OH ị X nH O ; where 1Ày y y=z A2? and B3? are the metal cations, water and exchangeable inorganic or organic charge-compensating anions (Xz-) are present in the interlayer galleries [31, 32] This composition leads to the preparation of numerous hydrotalcite derivatives by substitution of A2? (or B3?) with another transition metal ion or intercalation of foreign anions in the interlayer domains This allows a great flexibility in mixing A2? and B3? cations to obtain several desired reduction/ oxidation properties due to the brucite sheets to accommodate cations of various sizes and valences [19, 27–30] In other ways, substitution of charge-compensating ions with transition metal oxoanions also gives rise to novel redox catalyst systems In reality, a series of layered double hydroxides intercalated with molybdate [21, 23, 33], tungstate [21], chromate [32], and manganate [31] have been prepared and used as catalysts for the oxidation reaction In overall, the conversion and product selectivity were reported to be associated with the position of octahedral sites, nature of interlayer Xz- anion, Mg/Al ratio, basicity of the LDH catalyst [9, 30, 31, 34, 35] It was known that some Mo-containing hydrotalcite like compounds were proved to be active catalysts for different oxidation reactions such as selective olefin oxidation [23, 33] or oxidative dehydrogenation of propane [36] In the present work, a series of Mg–Al molybdate oxoanion-intercalated layered double hydroxides was prepared and expected to have a good reduction–oxidation activity in the tailored oxidation of C=C bond The synthesized catalysts have been tested for the liquid oxidation of styrene with air and the effects of molybdate contents, reaction variables are reported Experimental Section 2.1 Catalyst Preparation Molybdate-intercalated layered double hydroxides were prepared through a modified conventional co-precipitation method Solution A was prepared by dissolving Mg2? and Al3? metal nitrate salts with different Mg2?/Al3? molar ratio into 150 mL distillated water (Table 1S in Supplementary Materials) Solution B was prepared by adding desired amounts of ammonium heptamolybdate ((NH4)6Mo7O24) and NaOH into 100 mL of distillated water Two Activity of Molybdate-Intercalated Layered Double Hydroxides in the Oxidation of Styrene… solutions (A and B) were added simultaneously with a constant flow rate of mL/min under magnetic stirring for h while the pH was adjusted at 9.0 The weighted amounts of starting chemicals are reported in Table 1S (Supplementary Materials) The resultant was then submitted to an aging treatment at 65 °C for 24 h, followed by filtration, washing with hot distilled water, and drying at 70 °C for 24 h The obtained solid was ground into powder In the case of preparation of the Mg/Al intercalated carbonate anion in the free gallery, designated as MAC-00 (Magnesium, Aluminum, Carbon), ammonium heptamolybdate was replaced by sodium carbonate For a mixed oxide (reference) sample with Mg/Al/Mo molar ratio of 6/4/2, three salts, ((NH4)6Mo7O24, Al(NO3)3Á9H2O, Mg(NO3)2Á6H2O were blended together prior to calcinate at 450 °C for h The reference sample is designated as MiOx 2.2 Catalyst Characterization Powder X-ray diffraction (XRD) patterns were recorded on a D8 Advance-Brucker instrument using CuKa radiation (k = 0.1549 nm) Fourier transform infrared (FT-IR) spectra were obtained in 4000–400 cm-1 range on a FT/IR spectrometer (DX-Perkin Elmer, USA) The Raman spectra of samples were analyzed by a LabRAM HR800 spectroscopy (HORIBA, French) The recorded spectral range was 100–2500 cm-1 and scanned three times with the wavelength of laser beam of 632 nm UV–vis spectra were collected with UV–Visible spectrophotometer, JASCO V-670 BaSO4 was used as a reference material The spectra were recorded at room temperature in the wavelength range of 200–800 nm The XPS analysis was made on a photoelectron spectrometer (KRATOS Axis 165, Shimadzu, Japan) with Mg Ka radiation (1253.6 eV) Deconvolution of the experimental photopeaks was carried out using a Lorentzian peak fit procedure The scanning electron microscopy (SEM) images were obtained with a JEOS JSM-5410 LV Energy-dispersive spectroscopy (EDS) data were obtained from Varian Vista Ax X-ray energy-dispersive spectroscope 2.3 Oxidation of Styrene The catalytic oxidation of styrene in N,N0 -dimethylformide (DMF) solvent was carried out in a 100 mL three-neck glass flask fitted with a reflux condenser For a typical run, 17.4 mmol of styrene, 7.0 mL of solvent and 0.2 g of catalyst were loaded into the flask After the reaction mixture was magnetically stirred and heated to the desired temperature, then t-butyl hydrogen peroxide (TBHP, 70 %, Sigma Aldrich) or hydrogen peroxide solution (H2O2, 30 %) was dropped into the flask As air was used, the flow of air (5 mL/min) was conducted into stirred reaction mixture and the reaction time starts recorded After reaction, the mixture was cooled down to room temperature and then catalyst was filtered off The reaction product mixture was then analyzed by gas chromatography and GC–MS (HP-6890 Plus, capillary column HP-5 MS crosslinked PH % PE Siloxane, 30 m lm 0.32 lm) Results 3.1 Characteristics of the Catalysts The nomenclatures of samples prepared in this study and some typical characteristics are presented in Table Powder-X ray patterns of all samples in Fig display the main reflection planes which are typically characteristics for layered double hydroxide material of sample MAC-00 to MAM-20 [8, 26, 37] Indeed, two sharp and symmetric peaks at low 2-theta of 11.20, 22.49° are essentially assigned to the reflections by the basal planes of (003), (006), respectively The other broad and asymmetric peaks at 2-theta of 34.25, 38.24, 45.53, 60.23, 61.37° correspond respectively to the reflections by the basal planes of (012), (015), (018), (110), and (113), confirming the formation of a crystallized layered double hydroxide structure [23, 33, 34, 36–39] For sample MAM-30, some reflection lines characterizing for Al(OH)3 phase appear in Fig [JCPDS File 01-072-0623], representing the formation of a mixture of products instead of single LDH phase Furthermore, X-ray reflection signals of the molybdate-intercalated samples are somewhat broader and noisier than that of Mg– Al–carbonate hydrotalcite (MAC-00) as seen in Fig [9, 10, 38] The c parameter of the Mo-containing LHD is higher than that of the hydrotalcite reference MAC-00, reflecting a successful intercalation of MoO42- anions into the interlayer spaces between (Mg, Al) brucite-like sheets [23, 33, 36, 37, 39, 40] The value of c parameter slightly decreases from sample MAM-10 to MAM-20 due to a stronger electrostatic interaction between brucite-like sheets and molybdate anions and a decreased molar ratio of Mg/Al [23, 28, 33, 34] In order to investigate the nature of molybdate anions in the synthesized samples, FT-IR, Raman, and UV–vis spectra were recorded FT-IR and Raman spectra of the catalysts are displayed in Fig The FT-IR spectra of MAM-10 and MAM-20 are resembled As shown in Fig 2a, FT-IR spectra of the Mo-containing catalyst show broad adsorption bands suggesting a high disorder of the molecules in the interlayer galleries The broad band is found around 3450 cm-1 due to the stretching mode of the hydroxyl groups present in the metal hydroxide layers, Mooxoanions, and water molecules in the interlayer domain 123 N T Thao et al Table Catalyst characteristics of all samples ˚ )* a (A ˚ )* c (A Batch # Expected formula Mg (wt%) Al (wt%) Mo (wt%) MAC -00 [Mg0.7Al0.3(OH)2](CO3)0.15ÁmH2O 3.047 22.805 24.70 12.39 – MAM-10 [Mg0.8Al0.2(OH)2](MoO4)0.10ÁmH2O 3.078 23.783 23.47 7.10 2.73 MAM-15 [Mg0.7Al0.3(OH)2](MoO4)0.15ÁmH2O 3.074 23.755 20.41 8.94 4.02 MAM-20 [Mg0.6Al0.4(OH)2](MoO4)0.20ÁmH2O 3.044 23.091 18.45 10.57 5.45 MAM-20R [Mg0.6Al0.4(OH)2](MoO4)0.20ÁmH2O 3.036 22.725 18.48 10.32 4.97 MAM-30 [Mg0.4Al0.6(OH)2](MoO4)0.30ÁmH2O – – 21.17 8.05 6.27 MixO MoO3/MgO/Al2O3 – – – – – MixO LDH Al(OH)3 MoO3 MoO3 LDH LDH Al2O3 Al2O3 MoO3 LDH Al(OH)3 LDH MoO3 Al(OH)3 MgO MoO3 MoO3 Al(OH)3 LDH MoO3 Fig Powder-XRD patterns of catalyst samples MoO3 Al(OH)3 * a = d110 and c = 3/2(d003 ? d006) [26] (MAM-20R: reused sample MAM-20 after a cycle) MAM-30 MAM-20 MAM-15 MAM-10 MAC-00 10 15 20 25 30 35 40 45 50 55 60 65 2-thetra (A) (B) MAM-10 826 1000 MAM-20 296 926 673 856 251 343 385 MixO 479 560 325 359 1658 990 823 894 1050 MAM-30 892 908 940 546 MAM-30 670 798 MAM-15 1370 451 MAM-10 3480 400 700 1000 1300 1600 1900 2200 2500 2800 3100 3400 3700 4000 Wavenumber (cm-1) 200 300 400 500 600 700 800 900 1000 1100 1200 1300 Raman Shift (cm-1) Fig FT-IR (a) and Raman spectra (b) of catalyst samples [34, 37, 40] A broaden band appeared at 920 cm-1 is assigned to the vibrations of Mo=O in polymolybdate Mo7O246- A band at 670 cm-1 with a shoulder at 123 856 cm-1 is typically characteristics for Mo–O–Mo stretching vibration of MoO42- in the interlayer region The lower wave number band at 546 cm-1 and is assigned Activity of Molybdate-Intercalated Layered Double Hydroxides in the Oxidation of Styrene… to the lattice vibrations of Mg–O, Al–O bond The band around 450 cm-1 has been ascribed to a condensed [AlO6]3- group or as single Al–O bonds [34, 35, 39, 40] Figure 2b shows the Raman spectra for molybdate containing samples (MAM-10, 15, 30) and that for the mixed oxide solid (MixO), for comparison The Raman spectrum of MixO exhibits the characteristic sharp bands of bulk MoO3 at 1000 cm-1 (Mo=O symmetric stretch), 826 cm-1 (Mo–O–Mo asymmetric stretching vibrations), 673 cm-1 (Mo–O–Mo symmetric stretching vibrations), 343 cm-1 (bending of terminal Mo=O), 260–220 cm-1 (the bridging Mo–O–Mo deformation) The Raman spectrum of Al2O3 possesses a band at 386 cm-1 For a series of hydrotalcite-like compounds, the Raman spectra show that the band at 560 cm-1 is assigned to the lattice vibration of the brucite octahedral sheets (Mg–O–Al) The strong band at 1050 cm-1 is described for the nitrate vibrations and this peak intensity gradually vanishes with increasing amount of molybdates, indicating the successful replacement of nitrate ions for molybdate anions in the interlayer region [23] Evidently, MoO42- symmetric stretching vibrations of the molybdates in the LDH compounds were described by the appearance of bands at 908 and 892 cm-1 [23, 29] Indeed, the two bands were characteristics for two different species of MoO42- anions, the first one is hydrated and the other one is bonded to the brucite-like hydroxyl surface [34, 37] A broad shoulder at 823 cm-1 is the antisymmetric stretching mode of the MoO4 units A band observed at 325 cm-1 is ascribed to the bending mode of Mo–O [23, 41] Thus, the Raman spectra let us suggest that MoO42 intercalated like-hydrotalcite compounds are prepared at basic preparation conditions of pH = 9.0, in a good agreement with the FTIR, UV–Vis analyses and results reported in the literature [34, 37, 39, 41, 42] For the molybdate-rich sample (MAM30), the bands at 940 and 359 cm-1 are assigned to the Mo–O symmetric stretching mode and a Mo=O bending mode in Mo7O246- anions, respectively [33, 34, 41] Figure presents the UV–vis spectra of three molybdate-containing catalysts in the region of 220–800 nm The absorption bands in the region of 220–280 nm is described as the O2- to Mo6? charge transfer of the isolated MoO42species with Mo in tetrahedral coordination [43, 44] For a higher molybdate contents (MAM-15 and MAM-30), the absorption bands are more broaden and the edge of the bands shifts to a higher wavelength, which is characteristics of the presence of molybdenum polyoxoanions with octahedral coordination although the presence of these polyoxoanions is not expected in the experimental conditions used during catalyst preparation [35] One of the most useful techniques for the investigation of the chemical state of the atoms present in the surface layer region of the heterogeneous catalyst is X-ray MAM-30 MAM-15 MAM-10 180 280 380 480 580 680 780 Wavelength (nm) Fig UV-vis spectra of samples photoelectron spectroscopy (XPS) In the present work, the sample MAM-15 was chosen to record the XPS spectra Figure 4a presents the Mo 3d region which shows Mo 3d3/2–Mo 3d5/2 doublets due to the spin–orbit coupling Furthermore, the peaks are very broad, reflecting molybdenum species having different environments For the fresh MAM-15 sample, the doublet with the main Mo 3d5/2 and 3d3/2 peak at 232.6 and 235.9 eV respectively, is firmly attributed to typical Mo(VI) in MoO42- [35, 45] The additional shoulders at 231.7 and 234.9 eV are assigned to Mo(V) species due to the possibility of the charge transfer between (Mo6? ? O2-) and (Mo5? ? O-) [24, 34, 46] This is strongly corroborated by the analysis of the O 1s photoline of sample MAM-15 Indeed, the binding energy value of O 1s XPS signals observed at 532.4 eV for the fresh sample MAM-15 is assigned to the O2- (Fig 4b) [35, 38, 45] However, it is likely composed of two overlapping photoelectron lines which can be deconvoluted into two peaks at 532.4 and 531.5 eV (Fig 4b) The BE value at 532.4 eV is attributed to the metal hydroxides while the other signal at 531.6 eV is assigned to oxygen O- in the oxomolybdenum [35, 45, 47] SEM micrographs reveal the morphology of the catalysts SEM images of some representative samples are shown in Figs and 2S The morphology of these catalysts is typical characteristics of hydrotalcite-like materials with platelet thickness ranging from 15 to 30 nm Larger LDH plate was observed on the molybdate-rich sample (MAM15), possibly affected by pH constant during the catalyst preparation (Table 1S) [34, 38, 44, 48–50] Indeed, MAM15 is mainly thin disk-like platelets about 200 nm in diameter and 30 nm in thickness Thus, it is expected to be existence void spaces between catalyst platelets In practice, nitrogen adsorption/desorption measurement for Mocontaining LDH samples shows isothermal curves with a plateau from to 0.5 and a hysteresis loop in the range of 0.62–0.95 (Fig 1S) The patterns are likely classified to the 123 N T Thao et al (A) (B) 232.3 235.4 532.3 531.6 MAM-15 (Spent) MAM-15 (Spent) MAM-15 (Fresh) MAM-15 (Fresh) 215 220 225 230 235 240 245 250 255 526 528 530 532 534 536 538 540 542 544 Binding Energy (eV) Binding Energy (eV) Fig XPS scan of Mo 3d (a) and O1s (b) for MAM-15 samples before and after reactions at 90 °C, h, DMF solvent, air oxidant for spent sample MAM-15 Fig SEM images of MAM-10 (a) and MAM-15 (b) II type and the hysteresis loops are closely to the H3classification, suggesting that these solids are either mesopores or nonporous materials In the present work, the H3-like hysteresis loop is described to the nitrogen condensation/evaporation phenomena in slit-shaped pores created by the agglomeration of uniform plate-like particles [7, 36, 38] The specific surface area of samples is in the range of 5–20 m2/g Scanning electron microscopy and energy-dispersive X-ray spectrometry (SEM–EDS) analysis provides local information of the concentrations of different elements in the outermost layers of the platelet of LDH Alumina, magnesium, molybdenum, and oxygen are clearly identified on the platelet surface of all samples as displayed in Fig No major difference in the percentage of elements on four spots of each sample indicates a good dispersion of elements in the LDH at the micrometer scale [32, 38] Furthermore, molybdenum metal content is close to the theoretical value, but observably minor changes after a reaction cycle Analytical results of the synthesized solids using the EDS technique are reported in Table The EDS 123 O Mg Al Mo Mo N MAM-20R -Spent MAM-30 MAM-20 MAM-15 MAM-10 Energy (keV) Fig EDS compounds spectra of Mg–Al–molybdate hydrotalcite-like analysis clearly shows that traces of nitrogen for nitrate residue are present within the limits of the experiment Thus, it is concluded that the balance of the negative charge must be due to hydroxyl and molybdate anions, in good accordance with results of FT-IR and Raman spectra Activity of Molybdate-Intercalated Layered Double Hydroxides in the Oxidation of Styrene… 3.2 Catalytic Studies The catalytic activity of Mg/Al–molybdates hydrotalcitelike catalysts in the liquid oxidation has been examined at atmospheric pressure and air was led into the reaction mixture without any further purification 3.2.1 Oxidation of Styrene Catalyzed by MolybdateIntercalated LDH Catalysts A series of catalysts with different compositions has been tested at the same conditions Figure presents the catalytic activity of all samples in the styrene oxidation For the purpose of comparison, a blank test (no catalyst) has been also performed, giving null conversion In addition, ammonium heptamolybdate, Mg/Al–CO3 (sample MAC00), and the mixed oxides (sample MixO) each has been tested for the reaction of styrene with air for comparison, but only traces of products were detected after h-reaction time Meanwhile, Mo-containing hydrotalcite-like compounds show a good catalytic activity in the oxidation of styrene under similar experimental conditions Styrene conversion ranges from to 12 %, substantiating that Mospecies in the layered double hydroxides act as active components for the selective oxidation of styrene In detail, the styrene conversion varies with the overall amount of molybdate anions in the catalysts as follows of MAM20 [ MAM-15 [ MAM-10 [ MAM-30 The highest molybdate-containing sample shows a lowest styrene conversion (Fig 7) This is not surprising as considering the characterization of the sample MAM-30 since it is constituted of oxides, hydroxides and has a low Mg/Al ratio (Fig 1) Indeed, a low activity of sample MAM-30 is explained by a poorer crystalline hydrotalcite-like structure 90 Styrene conversion Benzaldehyde Sel Styrene oxide Sel 80 Percent, % 70 60 50 40 30 and a smaller amount of Mo species and a lower basicity (Mg/Al ratio) [9, 23, 42, 51, 52] The two main products are benzaldehyde and styrene oxide in the product mixture With the exception of two reference samples (MAM-00 and MixO), the other catalysts show a high selectivity to the two main products of 99 % (Fig 7) The selectivity to styrene oxide decreases linearly from the sample MAM-15 to MAM-30 At a similar conversion of styrene, the selectivity to styrene oxide over MAM-10 is much higher than that over sample MAM-30 3.2.2 Effect of Oxidant Nature Effect of nature of oxidants in the oxidation of styrene was studied to improve the yield of products Three oxidants including aqueous H2O2 solution (30 %), tert-butyl hydroperoxide (70 % in water), and air have been used for the oxidation of styrene over selected samples The results collected in Table give important information Under the same reaction conditions, H2O2 was the most active oxidant for the oxidation of styrene over molybdate-containing catalysts The conversion of styrene reaches to 90–99 % but a broad spectrum of products including benzoic acid, 1-phenylethane-1,2-diol, and some unidentified polymer compounds is detected, clearly indicating that the oxidation of styrene with hydro peroxide is a less selected reaction (Table 2) [26] Once air was used, styrene conversion remains about 10–15 %, but varies observably with the amount of molybdate contents in the catalysts In this case, two desired products were obtained with the total selectivity of 99 % This reaffirms an important role of tetrahedrallycoordinated molybdate anions in activating oxygen molecules to oxidize styrene into aldehyde or epoxide As air is substituted by tert-butyl hydrogen peroxide, the styrene conversion decreases to 6–7 % and the oxidation reaction leads to the formation of styrene oxide only, in good consistent with the results of oxidation of olefins over Mo(VI)-containing catalysts reported in the literature [2, 23, 27, 35] It is noteworthy that TBHP is an expensive organic reagent and always accompanies by a large amount of organic waste in the product Thus, Table indicates that Mg/Al-molybdate intercalated LDHs are promising catalysts for the oxidation of styrene with air 20 3.2.3 Effect of Reaction Time 10 MAC-00 MAM-10 MAM-15 MAM-20 MAM-30 MixO Catalyst Batch Fig Catalytic activity of molybdate-containing hydrotalcite-like catalysts in the oxidation of styrene with air at 90 °C, h, DMF solvent In order to enhance the conversion of styrene at friendly mild reaction conditions, an increased reaction time is a wise choice A series of oxidation reactions has been carried out for different reaction times at 90 °C The catalytic activity of MAM-15 is displayed in Fig and that of some 123 N T Thao et al Table Effect of oxidant nature on catalytic activity of the catalysts in the oxidation of styrene at 90 °C, h, DMF solvent Oxidant Batch # H2O2 Air TBHP Nominal formula Styrene conversion (%) Product selectivity (%) BA SO Others MAM-10 [Mg0.8Al0.2(OH)2](MoO4)0.1ÁmH2O 94.0 14.2 2.3 83.5 MAM-15 [Mg0.7Al0.3(OH)2](MoO4)0.15ÁmH2O 99.0 29.8 – 70.2 MAM-20 [Mg0.6Al0.4(OH)2](MoO4)0.20ÁmH2O 95.4 21.2 – 78.8 MixO MoO3/MgO/Al2O3 71.8 8.6 – 91.4 MAM-10 [Mg0.8Al0.2(OH)2](MoO4)0.1ÁmH2O 10.7 58.0 42.0 – MAM-15 [Mg0.7Al0.3(OH)2](MoO4)0.15ÁmH2O 12.7 48.6 51.4 – MAM-20 MixO [Mg0.6Al0.4(OH)2](MoO4)0.20ÁmH2O MoO3/MgO/Al2O3 15.0 0.6 61.5 – 36.5 – – – MAM-10 [Mg0.8Al0.2(OH)2](MoO4)0.1ÁmH2O 7.3 – 99.0 1.0 MAM-15 [Mg0.7Al0.3(OH)2](MoO4)0.15ÁmH2O 6.5 – 99.0 1.0 MAM-20* [Mg0.6Al0.4(OH)2](MoO4)0.20ÁmH2O 6.0 1.0 98.0 1.0 MixO MoO3/MgO/Al2O3 0.4 – – – Others: benzoic acid, 1-phenylethane-1,2-diol, phenyl acetaldehyde, unidentified compounds; (-): Traces; BA benzaldehyde, SO styrene oxide other catalysts are presented in Fig 3S (Supplementary Materials) Figure shows that the conversion of styrene remains about 12 % at 90 °C for h, but monotonically increases to 74 % for 20 h It is noteworthy that the styrene conversion increases with the progress of the reaction time while the product distribution remains almost constant within h This indicates that both benzaldehyde and styrene oxide may be simultaneously produced under reported reaction conditions [6, 7, 9, 25] Furthermore, benzaldehyde becomes a major product as the reaction is kept for longer reaction time (Fig 8) The change in product selectivity may be associated with the instability of oxygenate intermediate products and secondary reactions occurring in the product mixture as the reaction was lasted for a long period of time [9, 11, 16, 17, 20, 38] 100 Benzaldehyde Sel Other produtcs Styrene Oxide Sel Styrene conversion Percent, % 80 60 40 Effects of reaction temperature on catalytic activity have been investigated using the sample MAM-10 in the range of 80–120 °C Some additional data were provided in Fig 4S (Supplementary Materials) Figure shows that styrene conversion slightly increases with increasing reaction temperature as the reaction was kept for h, but in overall fraction of styrene converted is still modest under these conditions Figure shows another trend in styrene conversion varied with reaction temperature as the reaction mixture is kept for 24 h Clearly, the styrene conversion approaches almost 99 % at 110 °C while the total selectivity to benzaldehyde and styrene oxide is very high ([90 %) Figure 9b presents the effect of temperature on products selectivity The selectivity to products changes with the reaction temperature of 80–100 °C This is explained by the fact that cleavage of C=C considerably happens at lower temperatures and the epoxidation competes more favorably against C=C cleavage at higher temperatures [32, 50] At a reaction temperature of 110 °C, more benzaldehyde was found in the product mixture may result from the decomposition of styrene oxide though hydrolysis reaction [2, 9, 35, 52] 3.2.5 Reusability of Catalysts 20 12 20 Reaction time (h) Fig Effect of reaction time on the catalytic activity over MAM-15 at 90 °C, DMF solvent, 0.2 g of catalyst, DMF solvent (others: benzoic acid, styrene glycol, phenyl acetaldehyde) 123 3.2.4 Effect of Reaction Temperatures The catalyst was recovered from the reaction mixture by filtration, washed with ethanol, dried at room temperature prior to reuse for the oxidation reaction under the same reaction conditions Figure 10 shows the results of the recycling experiments of sample MAM-15 The styrene conversion remains about 29–33 % after two recycles and then decreases to % in the fourth cycle 10 hours 80 85 90 95 100 Reaction temperature 105 110 115 100 hours 24 hours 110 24 hours ) Te 75 80 90 re 20 tu 30 hours 40 pe 24 hours 50 m 60 Styrene Oxide Selectivity (%) 70 100 90 80 70 60 50 40 30 20 10 Styrene Oxide Styrene conversion (%) 80 (o C (B) 90 Benzaldehyde (A) 100 Benzaldehyde Activity of Molybdate-Intercalated Layered Double Hydroxides in the Oxidation of Styrene… (oC) Fig Effects of reaction temperature on styrene conversion (a) and product selectivity (b) over MAM-10 after (a) and 24 h (b), DMF solvent, 0.2 g of catalyst 100 Benzaldehyde Sel Other Sel Styrene Oxide Sel Conversion 90 80 Percent, % 70 60 50 40 30 20 10 Number of Cycle Fig 10 Oxidation of styrene with air over reused MAM-15 sample at 90 °C, h, DMF solvent, 0.2 grams of catalyst It is noted that the styrene conversion changes insignificantly while the selectivity to styrene oxide decreases sharply after two cycles This is postulated by the phenomenon of migration of MoO42- anions from the interlayer spacing to the external surface Thus, overall amount of tetrahedrally-coordinated Mo-species is almost preserved, but the content of MoO4- in the interlamellar spaces of hydrotalcites decreases remarkably An increased number of cycles result in the leaching MoO4- out of the samples, evidenced by an observably decreased Mo component in the reused samples (Table 1) Thus, both conversion and styrene oxide selectivity decreases with the number of cycles Discussion Molybdenum complexes are well-known catalysts for the oxidation of styrene with alkyl peroxides In all cases, the active species are usually associated with Mo(VI) compounds although the reaction mechanism of Mo(VI) catalyzed oxidation of unsaturated compounds still remains a subject of debate [22, 24, 25, 35, 53] It is known that the molybdate anion MoO42- maintains a tetrahedral configuration in neutral and alkali solutions This anion is easily changed into polymolybdate salts as separated from solution at neutral conditions [23, 34] Thus, incorporation of MoO42- in the interlayer region of Mg1-xAlx(OH)2 (MoO4)x/2ÁmH2O hydrotalcite-like compounds is to stabilize its tetrahedral configuration Analysis of XRD data indicates that no additional diffraction peaks appeared as x = 0–0.2, suggesting the formation of single LHD phase with the intercalation of tetrahedrally-coordinated Mo species in the interlayer domains As a consequence, a slight shift in ‘c’ or interlayer thickness was observed An intercalation of MoO42tetrahedral ions into interlayer free gallery dramatically modifies the reduction–oxidation characteristics of layered double hydroxides [22, 23, 32–35] However, the molybdate-rich samples contain both Mo in tetrahedral and octahedral coordination The amount of octahedrally coordinated Mo-species is prevailing in the molybdatericher samples because the interlayer spacings can only allow a certain amount of MoO4- accessible [34] As molybdate-intercalated LDHs were used as catalysts for the oxidation of styrene with oxygen, the styrene conversion varies with the molybdate amount, reaction conditions The analysis of the product selectivity allows to shed light on the role of the molybdate locations in the production of the main products From the results in Figs 1, 7, and 10, it was suggested that the product selectivity is strongly dependant on the location of tetrahedral MoO4- anions while styrene conversion depends on the total amount of molybdate anions although the role of basic sites from hydrotalcites are not ruled out Indeed, the role of Mg/Al brucite sheets could be stabilization of 123 N T Thao et al radical oxygen species generated through interaction with basic sites [38, 51] As discussed above in Sect 3.2.1, the changes in molar ratios (Mg/Al/Mo) would vary the basicity/acidity of the catalysts An increased Mo-content in LDHs (from sample MAC-00 to MAM-20) leads to a remarkable augmentation of styrene conversion, but a decrease in the styrene oxide selectivity (Fig 7) due to an increased the acidity of higher Mo-content samples A low styrene conversion on sample MAM-30 and MixO indicates a synergistic interaction between MoO4- anions and the Mg–Al hydroxide layers in the oxidation of styrene Benzaldehyde and styrene oxide are postulated to produce parallel to each other (Figs and 3S) The tetrahedral MoO4- intercalated anions in the interlayer free gallery are responsible for the epoxidation of styrene while benzaldehyde can produce over both tetrahedral MoO4intercalated anions and molybdate species on the external surface (Fig 10) [53] In addition, all MoO4- ions would only produce styrene oxide as TBHP was used as oxidant This may be due to the presence of TBHP to steer the oxidation reaction mechanism into another pathway Styrene oxide is suggested to be produced over molybdate ions though the formation of a Mo(VI) butyl peroxide and transfer of the distal oxygen atom of butyl peroxide [53] Conclusions Mg–Al–molybdate hydrotalcite-like samples with different ratio of Mg/Al/Mo were synthesized at pH constant value Molybdate anions were introduced in the interlayer regions of Mg–Al hydrotalcite as interlamellar anions and present in a number of oxoanions The catalysts possess the hydrotalcite structure with layered structure, but have low external surface area The crystallinity of the sample decreased with increasing molybdate content The Mg–Al– molybdates catalysts were found to have a good activity in the oxidation of styrene with air The styrene conversion depends on the nature of oxidant, total amount of molybdate anions; reaction variables while the product selectivity was found to be related to the position of molybdates between interlayer spacing or external surface of the solids The preliminary investigation results indicated that air was found to be a promising oxidant for the selective conversion of styrene into styrene oxide and benzaldehyde Mg/ Al/MoO4-like hydrotalcite catalyst showed 74–90 % styrene conversion with 70 % benzaldehyde and 26 % styrene epoxide selectivity at 90 °C in the presence of DMF solvent at 90 °C after 20–24 h Acknowledgments This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under Grant Number 104.05-2014.01 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Mospecies in the layered double hydroxides act as active components for the selective oxidation of styrene In detail, the styrene conversion varies with the overall amount of molybdate anions in the. .. good reduction? ?oxidation activity in the tailored oxidation of C=C bond The synthesized catalysts have been tested for the liquid oxidation of styrene with air and the effects of molybdate contents,