37. Catalytic oxidation of styrene over Cu doped hydrotalcites

Accepted Manuscript Catalytic oxidation of styrene over Cu-doped hydrotalcites Nguyen Tien Thao, Le Thi Kim Huyen PII: DOI: Reference: S1385-8947(15)00778-0 http://dx.doi.org/10.1016/j.cej.2015.05.090 CEJ 13731 To appear in: Chemical Engineering Journal Received Date: Revised Date: Accepted Date: 14 March 2015 17 May 2015 26 May 2015 Please cite this article as: N.T Thao, L.T.K Huyen, Catalytic oxidation of styrene over Cu-doped hydrotalcites, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej.2015.05.090 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain Catalytic oxidation of styrene over Cu-doped hydrotalcites Nguyen Tien Thao*, Le Thi Kim Huyen Faculty of Chemistry, Vietnam National University, Hanoi 19 Le Thanh Tong ST, Hanoi, Vietnam, 10999 *Corresponding author: Tel (+84) – (04) 3825 3503; Fax: (+84) – (04) 3824 1140 Email: ntthao@vnu.edu.vn (N Tien Thao) Abstract A set of Mg-Cu-Al hydrotalcite–like materials with different Mg/Cu/Al ratios were synthesized by the co-precipitation technique The prepared samples were characterized by physical techniques of XRD, FT-IR, Raman, N2 physisorption, SEM, TEM, XPS, and UV prior to use as catalysts for the liquid oxidation of styrene All synthesized samples possess a well crystallized hydrotalcite structure, medium surface, and uniform particles Cu 2+ ions at octahedral sites stably in the brucite-like sheets catalyze the epoxidation of styrene with tertbutyl hydrogen peroxide oxidant The catalytic activity is related to the amount of copper content and oxidant behavior The Cu-doped hydrotalcite catalysts show a rather good activity and high selectivity to styrene oxide (80-90%) at a high styrene conversion level Key words: oxidation, Cu-doped hydrotalcite, TBHP, styrene oxide, benzaldehyde Introduction Oxidation of styrene is a very important reaction because its products, mainly benzaldehyde and styrene epoxide, are widely used in many application fields [1] Certainly, benzaldehyde is a very valuable chemical that has widespread application in perfumery, pharmaceuticals, dyestuffs, and agrochemicals Styrene oxide is industrially used for the production of epoxy resin diluting agents, ultraviolet absorbents, flavoring agents… [1] It is also a fine chemical for organic synthesis, pharmacochemistry and perfumery However, the oxidation of styrene is somewhat difficulty because it is a terminal olefin Thus, many attempts have been developed to increase in the controllable conversion of styrene to oxygenated products [2-4] First all, different homogeneous catalysts have been used for styrene oxidation using various oxidants, but these reaction systems face a huge challenge of removal of catalyst from the reaction mixture and some cases show rather poor catalytic activity [1,2] Therefore, heterogeneous catalysts are considered as more efficient and environment friendly systems for the greener chemistry principles and thus many new catalysts in this trend have been recently developed [3-10] Indeed, Table summarized a series of typical performances established experimentally for selected representative catalysts This selection of literature data was made for experiments conducted in conditions close to the ones in this work in order to allow some comparisons with our proposed catalyst Among these heterogeneous systems, titanium –containing catalysts are the most available systems reported in literature, but their activity is rather fluctuation and strongly dependent upon the preparation routes, chemical precursors, and laboratory skills (Table 1, Entry # 2-6) [4-8] Hence, many other transition metals have been recently tried in variable ways for the preparation of catalysts, including in the form of oxide film [7,8], nanoxides [9], metals supported catalysts [4,10,17], immobilized systems [2,18-20] The product selectivity is strongly dependent on the catalyst nature, oxidant behavior, solvent… (Table 1) For example, TS-1 film showed a good activity in conversion of styrene into phenylacetaldehyde [7,8] while TiO2/SiO2 produces mainly benzaldehyde from styrene [6] In the case of anhydrous ureahydrogen peroxide used as oxidizing agent, titanium silicate converted styrene to styrene oxide [4,5] For group VIIB elements-containing catalysts, either benzaldehyde or styrene oxide can be a major product; the catalytic activity in the latter case is related the oxidation state and the coordination geometry of transition metal ions, catalyst morphology [7,11,16,27] Following this trend; we are in an endeavor to design an efficient catalyst based on hydrotalcites for the liquid oxidation of styrene It is well known that hydrotalcite-like compounds are a two-dimensional material and sometimes show good ability to the oxidation reactions [21-29] In essence, isomorphous substitution of transition metal ions in a brucite-like lattice leads to the appearance of oxidation-reduction centers in the sheets which may acts as active sites for the catalytic oxidation reaction [16,25-28] In earlier work, we have successfully modified the hydrotalcite composition as well as hydrotalcite-derived oxides as catalysts for the catalytic applications [16,30,31] A synergetic interaction between transition metal-doped element and the hosting cations in the hydrotalcite lattice gives a great catalytic property in the oxidation and combination reaction [7,16,24,28,32] Hence, this article reports the characteristics and catalytic activity of Mg-Cu-Al hydrotalcite-like materials in the liquid oxidation of styrene The present work also reports the correlation between copper geometry and the product selectivity in the styrene oxidation reaction Experimental 2.1 Preparation and characterization of the catalysts Mg-Cu-Al-CO3 double layered hydroxides were prepared by the coprecipitation method The detailed procedure was described in our previous publications [16,29] In brief, a 150mL-mixed aqueous solution of Mg(NO3)2.6H2O (99%), Cu(NO3)2.3H2O (98%) and Al(NO3)3.9H2O (> 98%) with different molar ratios of Mg2+/Cu 2+/Al3+ was added dropwise to 25 mL of 0.60 M Na2CO3 under vigorous stirring (see Table 1S in Supplementary Materials) The solution pH was adjusted to 9.50 using 1.5 M NaOH and was kept for 24 h Then, the resulting gel-like material was aged at 60-65 ◦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 For the sake of brevity, the prepared catalysts are denoted as TH00 - TH05 and the expected formula compositions are reported in Table The metal composition (Mg, Cu, Al) of catalyst was measured using an ICP-MS Elan 9000 (Perkin Elmer, USA) Powder X-ray diffraction (XRD) patterns were recorded on a D8 Advance-Bruker instrument using CuKα radiation (λ = 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 times with the wavelength of laser beam of 632 nm Energy-dispersive spectroscopy (EDS) data were obtained from Varian Vista Ax X-ray energy-dispersive spectroscope The scanning electron microscopy (SEM) microphotographs were obtained with a JEOS JSM-5410 LV TEM images were collected on a Japan Jeol Jem.1010 The nitrogen physisorption was run on an Autochem II 2920 (USA) The XPS analysis was made on a photoelectron spectrometer (KRATOS Axis 165, Shimadzu, Japan) with Mg Kα radiation (1253.6 eV) Deconvolution of the experimental photopeaks was carried out using a Lorebtzian peak fit procedure 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 2.2 Catalytic performance The catalytic oxidation of styrene in N,N’-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, 8.0 mL of solvent and 0.2 grams of catalyst were loaded into the flask unless some particular tests indicated 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 %) or flow of air (5 mL/min) was conducted into 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 (HP6890 Plus) and a frame ionization detector (FID) is used as a detector The conversion of reactions is calculated based on the molar ratio between the amount of styrene consumed and the initial styrene mole number Results 3.1 Catalyst Characteristics 3.1.1 XRD patterns and physical properties of catalysts Five hydrotalcite-like xCu xAl(OH)2(CO3).mH2O compounds with the nominal formulae of Mg0.7- (x = – 0.35) are prepared by the coprecipitation method and some their physical properties are presented in Table The two samples designated as TH02STH04S are represented as spent catalysts after testing the oxidation of styrene with TBHP in DMF solvent at 70 oC for hours The metal contents of all samples are re-verified by ICP analysis and their molar ratios are indicated in Table Elemental chemical analysis of the samples shows a reasonable correspondence within experimental error limits between atomic ratios of the ternary hydroxides and the starting solutions (Table 1S in Supplementary Materials) [17,18,20] A small difference in the initial ratios of cations between starting materials and the copper-richer solids (x > 0.28) is probably due to the different solubility products of ionic solids at a constant pH during the precipitation course [28,30,32-35] Powder X-ray diffraction patterns for all samples (TH00 - TH05) displayed in Figure show a hydrotalcite-like structure, in good accordance with the literature [18,32-34] Indeed, three sharp and symmetric reflections at 2-theta of 11.57, 23.45, 34.86o are firmly ascribed to the diffraction by the basal planes of (003), (006), and (012), respectively The other broad asymmetric peaks at higher 2-theta angles of 39.5, 47.1, 60.9, and 62.3 o are presented as the diffraction by (015), (018), (110), and (113) planes, confirming the formation of a wellcrystallized hydrotalcite structure with a hexagonal lattice with R3m rhombohedral symmetry [16,25-27,33,36] Thus, the cell parameters may be estimated by using equations of a = × d (110) and c = × (d ( 003) + × d ( 006) ) in which the parameter a corresponds to the cation-cation distance within the brucite-like sheet and c refers to the thickness of the brucite-like layer and the interlayer spaces [27,33,35-38] The crystallographic data from XRD powder patterns of all samples are tabulated in Table Both a and c cell parameter are only slightly changed with the amount of copper content in samples due to the radius resemblance between magnesium (II) and copper (II) ions in octahedral environment geometry [23,26,31,33] A minor deviation in a and c parameters with increasing the copper content is probably related to the distorted octahedral geometry of Cu2+ in the brucite-like sheets and the weaker interaction between Cu2+ and CO32anions A declined lattice parameters at a higher copper content (x > 0.28) may be correlated with the amount of copper-intra lattice Figure displays an increased signal to noise ratio and the twine peak at 2-theta of 60.9 and 62.3o is very broaden, reflecting poorer crystallinity for the copper rich-samples [33,39,40] 3.1.2 Infrared and Raman spectra Since XRD is a good technique to analyze the catalyst structure, IR is a sensitive method to investigate the presence of anions in the hydrotalcite samples Figure 2A presents IR spectra of three fresh samples (TH00, TH02 and TH04) and two spent catalysts (TH02S and TH04S) As seen in Fig 2A, a broad and strong band at 3480 cm-1 is probably assigned to the stretching mode of hydrogen-bonded hydroxides from the brucite-like layers and from the interlayer water molecules [16,33,41,42] A visible shoulder at 3040 cm-1 is firmly attributed to the vibration of water molecules hydrogen-bonded to carbonate ions in the interlamellar layer [33,41] The position of this band slightly shifts to lower frequency region as the amount of copper increase, implying an affinity difference between CO32- to Cu2+ and to Mg2+ The weak absorption at 1635 cm-1 is generally assigned to the bending mode of interlayer water molecules while the strong band at 1357 cm-1 is solely described to the asymmetric stretching mode of the carbonate species [16,33,34] In the region below 1000 cm-1, a broad band at 775 cm-1 is ascribed to translation modes of hydroxyl groups mainly affected by trivalent aluminum and a weak shoulder around 870 cm-1 is usually attributed to a doubly degenerate bending motion of the interlayer carbonate groups [34, 36-38] The bands at 667, 570, and 440 cm-1 are assigned to metal oxygen modes in the hydrotalcite lattice [42] These observations are in good accordance with the X-ray analysis and Raman spectra Raman spectra of three representative specimens were collected to identify the nature of the interlayer anion and its interactions with the rest of the structure, particularly the hydrogen-bond network Figure 2B shows Raman spectra of the two selected sample with a band 148 cm-1 assigned to oxygen-metal-oxygen bending modes [36,38,42] The bands in the range of 500-555 cm-1 result from the interlayer carbonate-water molecules, where the two hydrogen atoms of the H2O molecule are bridged to two oxygen atoms of the carbonate anions [43,44] Furthermore, the band around 1061 cm−1 corresponds to interlayer carbonate anions in the brucite-type layers 3.1.3 Energy-Dispersive Spectrometry (ESD) Scanning electromicroscopy and energy-dispersive X-ray spectrometry (SEM-EDS) analysis both provide regional concentration of different elements in the outermost layers of the platelet of samples EDS spectra of three representative specimens shown in Figure clearly indicate the presence of all components [16,30] The atomic percent of the elements are reported in Table 2S (Supplementary Materials) Their atomic percent is average values collected randomly from four different spots of Mg-Cu-Al hydrotalcite-like materials The percentages of elements at different positions are almost unchanged, reflecting good distribution of metal elements in both fresh and spent samples at micrometer scale (Fig and Table 2S) [34] 3.1.4 Nitrogen physisorption Figure displays the N2 adsorption-desorption isothermal profiles at -196 oC for all Mg-CuAl hydrotalcite-like materials In all cases, the patterns correspond likely to the II type and the hysteresis loops fall closely into the H3-classification, interpreting that these samples are either mesopores or nonporous materials Although the distance between layers is in the range of micro-porosity, but nitrogen molecules are unable physically to penetrate in the interlayer spaces of hydrotalcites Thus, the H3-like hysteresis loop is attributed to the presence of slit-shaped pores created by the agglomeration of plate-like particles [16,23,24] The specific surface area of samples is in the range of 60-85 m2/g, but slightly lowers upon increasing the Cu2+ content in the sample (Table 1) For the highest Cu2+ content sample (TH05), the N2 absorption-desorption isothermal curve is somewhat different from the others because of a small alternation of the microscopic morphology of TH05 As seen in Table 2, a slightly decreased surface of the copper-richer sample is probably associated with the catalyst morphologies [16,25,36,37] 3.1.5 Microscopic Investigation Both SEM and TEM images of the two representative Cu-doped samples are illustrated in Figures and SEM micrographs of TH02 and TH04 show that the hydrotalcite-like compound particles are roughly hexagonal plates [16,31] The particle sizes are relatively uniform with the mean crystal domain of 80-100 nm [12,40] In other context, TEM image of Mg-Cu-Al like hydrotalcite sample of TH02 presents laminar structure which is essentially characteristic for hydrotalcite mineral and the stacking of the layers (Fig 6) [16,30,31] The flat particles with hexagonal shapes are presented and the grain boundaries are clearly observed However, Figure also indicates an observable difference in the lateral platelet sizes between TH02 and TH04 The crystal domain of the copper-richer sample (TH04) is slightly bigger than that of the copperlower one Agglomeration of bigger crystallites yield larger voids between interparticles which result in a decreased surface area for the copper-richer sample as indicated in N2 physisorption experiments (Fig and Table 1) [41,43,45,46] 3.1.6 XPS analysis X-ray photoelectron spectroscopy of the samples is usually used to identify the nature of the interested components present in MgCuAl isostructural hydrotalcites Figure displayed XPS spectra of Cu, O, C while that of Al is presented in Supplementary Materials (Fig 2S) The binding energies of copper ions in samples TH02 and TH02S (Fig 7A) are 935.4 and 955.5 eV The Cu 2p3/2 peak at 935.4 eV is much higher than the binding energy of Cu2+ in CuO (932.5 eV) and very close to that in the lattice of copper containing-minerals [39,46,47] For a higher copper content sample (TH04), XPS spectra appear more two shake-satellite signals at 944.8 and 963.8 eV as shown in Fig 7B Therefore, these binding energy values are essentially assigned to Cu2+ in octahedral positions and predominantly present on the surface, in good agreement with the literature [26,38,39,46,47] To understand the geometrical arrangement of octahedral Cu2+ in the synthesized product, we survey the XPS of O 1s due to the fact that oxygen may be present in different environments Figure 7C shows O 1s region for samples TH02 and TH04 (fresh and spent catalyst) The O 1s photoline shape indicates that the oxygen signal is rather complicated and typically composed of some overlapping peaks Therefore, these photolines have been deconvoluted into some contributions positioned at different binding energies Firstly, a small deconvoluted peak at 530.4 eV could be characteristic of cation-hydroxyl-carbonate [41] The next deconvoluted peaks at 532.2 eV is assigned to metal hydroxides and the other peak at and 532.9 eV is essentially assigned to the oxygen in carbonate groups which are in good harmony with IR and Raman results (Fig 4) and C 1s XPS scan [47-49] Indeed, the binding energy of C 1s is presumably composed of two overlapping peaks at 284.5 and 285.8 eV The first peak is assigned from the carbons in contaminated organic product of spent samples while the higher binding value is characteristically assigned to carbonate anions (C = O, C - O bond) in the interlayered spaces [29,36,47,49,50] 3.1.7 UV-vis spectra UV–vis spectra were acquired in order to further demonstrate the coordination environment of Cu species Thus, two samples with different copper contents were recorded UV-vis spectra in the wavelength range of 200-800 nm (Fig 8) The UV-vis spectra of both TH02 and TH04 samples appear two main bands at 210-240 and 700-800 nm accompanied by a visible shoulder at 295 nm The first band can be attributed to the O2- → Cu2+ ligand to metal charge transfer transition [46,51] The other broad band between 700-800 nm is attributed to d–d transitions of Cu2+ ions in the distorted octahedron, in good agreement with XPS analysis [46,47,51] 3.2 Catalytic activity in the liquid oxidation of styrene 3.2.1 Effect of copper contents in Cu-doped hydrotalcites The catalytic activity of all synthesized Mg-Cu-Al hydrotalcite-like material in the liquid oxidation has been examined at atmospheric conditions It is noted that blank test (no catalysts) showed no styrene conversion in presence of different oxidants (air, BuOOH, H2O2) Furthermore, Cu-doped hydrotalcite catalysts also show no observable activity as air was flown into the reaction mixture at a rate of mL/min after hours at 70 oC in DMF solvent When air (oxidant) is replaced by tert-butyl hydrogen peroxide or hydrogen peroxide solution, the conversion of styrene increases sharply and the product selectivity significantly changes with copper amount and oxidant nature Indeed, Figure indicates that the Cu-based samples are active and selective for the epoxidation of styrene with TBHP oxidant, but are very less selective with H2O2 (Fig 9B) Meanwhile, Cu-free catalyst exhibit a rather low activity in conversion of styrene to benzaldehyde since the basic sites in hydrotalcites are known to slightly active for the oxidation of styrene with H2O2 [35,39] A significant increase in styrene conversion observed on Cu-doped hydrotalcite catalysts indicates a strong synergistic effect of copper ions and basic sites on the oxidation of styrene (Fig 9A) [25,36,38] However, the copper-free sample shows no catalytic activity as TBHP was used as an oxidant (Fig 9A) As Cu2+ ions are introduced into hydrotalcite lattice, the styrene conversion increases monotonically with the amount of copper Interestingly, the selectivity to styrene oxide almost remains about 80-90% under reported conditions in Fig 9B, suggesting that copper ions in hydrotalcite-like catalysts act as active sites for the selective oxidation of styrene to epoxide [39,41] On the contrary, no styrene oxide is detected in the mixture of products and the products are benzaldehyde and other oxygenated compounds such as phenyl acetaldehyde, benzoic acid, and polymerized products… as the H2O2 solution oxidant is used In the latter case, the styrene conversion gradually decreases with increasing copper content, as consequence the selectivity to benzaldehyde shows a better value on the copper-richer catalysts This indicates that Cu2+ ions has not only shown a good ability to oxidize styrene to benzaldehyde, but also exhibited the parallel competitive catalytic destruction of H2O2 [20,51,52] 3.2.2 Effect of oxidant/substrate molar ratio In order to avoid the overoxidation of styrene, we have carried out the liquid oxidation at different TBHP/substrate molar ratios over sample TH04 and the catalytic results are presented in Figure 10 It is interesting to note that no significant changes in the catalytic activity within TBHP/styrene of 2-5, demonstrating that copper ions in the brucite-like sheets are active for the decomposition of TBHP in to intermediated radicals (Fig 10) An increased oxidant/reactant ratio leads to the facilitation of styrene conversion, but simultaneously appears some oxygenricher products such as benzoic acid, glycol and phenyl acetaldehyde in the product mixture These oxygenated products result probably from the consecutive oxidation of styrene oxide [11,24,36,53,54] 3.2.3 Effect of reaction temperature The effect of reaction temperature on the styrene oxidation over Cu-doped hydrotalcite catalysts are investigated in the range of 50-80 oC Figure 11 shows a variation in conversion of styrene as a function of reaction temperature At low temperatures of 50-60 oC, styrene conversion is negligible over all examined catalyst samples (Fig 11A), but exponentially increases at higher temperatures (> 65 oC) Among the three examined catalysts, the styrene conversion, arranged in descending orders, is as follows: TH05 > TH04 > TH02 in entire temperature range, re-asserting that the styrene conversion is directly correlated to the copper content [53,55] Meanwhile, the bell-shaped like curves of product selectivity with the climax at 65 oC is observed over all three Cu-doped catalysts (Fig 11B) At higher temperature, benzaldehyde may be formed at the expense of styrene oxide especially on the copper-richer samples [6,12,52,53] 3.2.4 Effect of reaction time Effect of reaction time on the catalytic activity of Mg-Cu-Al hydrotalcite-like catalysts is shown in Figure 12 In general, the conversion of styrene linearly increases with increasing reaction time and the Cu2+-doped amount (Fig 12 A) [5,51,54] and the conversion trends are arranged in order of those reported in Fig 11A The main product selectivity remains more than 80% after hours and then gradually decreases about 10 % after h As consequence, the selectivity to byproducts increases slightly Among these byproducts, benzaldehyde is a majority component (4-10%) after h (Fig 12B) This may be suggested that the main product may be converted to secondary product due to the hydrolysis and opening of the oxirane ring [20,52,54] The side reactions of epoxide ring cleavage are suggested occurring on the acid sites of the catalysts The present water in the reaction mixture may compete with the reactant and be adsorbed on the Lewis sites of the catalyst generating Bronsted acid sites which facilitates the side reactions [3,51,55] Discussion Copper -doped hydrotalcites exhibit a good catalytic activity in the oxidation of styrene The overall activity is related to the copper content while the activity in the oxidation is correlated to the geometry and concentration of copper ions in a well-ordered two-dimensional lattice of hydrotalcite-like materials [24,26,30,51,56] Thus, the textural feature of hydrotalcite is vital to the oxidation process and Cu2+ ions stabilized at octahedral sites play as active sites for the selective oxidation although the synergistic interaction between copper ions and basic sites for the overall oxidation process are not ruled out Furthermore, the textural structure of the catalysts are still well preserved (Supplementary Materials) The surface area of spent catalysts are higher than that of the fresh one (Table 2) and the crystallinity of the catalysts likely improved (Fig.1 S in Supplementary Materials) while a small change in Mg/Al and Cu/Al was observed on the spent sample This observation indicates that amorphous oxides on samples were leached after reaction Indeed, both SEM and TEM images of the spent samples clearly show a better visualization as compared with the fresh ones The average particle sizes seem smaller with grain borders These phenomena may result from the “memory effect” of hydrotalcites [27,28,37,39,40] Under similar conditions, Cu-doped hydrotalcites are rather active for the oxidation of styrene and quite selective for the formation of epoxide product as compared with other heterogeneous catalysts reported in literature (Table 1) In the present work, the correlation between the conversion and product distribution and the catalytic characteristics would suggest that Cu2+ lattice is prerequisite for the epoxidation of styrene with TBHP The product selectivity is also dependant on the reaction conditions and the nature of oxidants H2O2 is a reactive agent for the production of benzaldehyde and a synergistic effect of copper ions and basic sites has facilitated the oxidation of styrene [27,30] However, higher copper content showed a negative effect on the styrene conversion On the other hand, the oxidation reaction steers into the epoxidation as H2O2 agent was substituted by TBHP A significant change in product selectivity in these cases is associated with the reaction pathways As TBHP is used, Cu2+ ions plays as active sites for the formation of tert-butylhydroxy (tBuO•) or tert-butylhydroperoxy radicals (tBuOO•) from TBHP The attack of t-BuO•/t-BuOO• radicals to the ethylenic group leads to the selective formation of epoxide [39,52,55] On the other hand, Cu2+ species interact with H2O2 to form Cu+•O2H intermediates which further couple with another H2O2 molecule to give Cu2+, O2, H2O, and •OH species The •OH radical attacks a C=C bond to form aldehyde or other oxygenated products [51,53] Conclusions MgCuAl ternary hydrotalcite–like materials with different Mg/Cu/Al ratios were prepared and used as the catalysts for the oxidation of styrene with different oxidants The results indicated that almost Cu2+ ions are present in the brucite-like sheets at lower content and carbonate anions intercalated between the interlayer spaces The MgCuAl solids showed a moderate surface area and uniform crystal domains The substitution of Mg2+ by Cu2+ in the brucite sheets makes Mg-Cu-Al hydrotalcite-like compounds become an efficient catalyst for the oxidation of styrene The selectivity to products was dependent on the copper ions in the catalysts and the nature of oxidant The Cu2+ ions in octahedral sites are necessary for the selective formation of styrene oxide with tert-butyl hydrogen peroxide oxidant under mild conditions The selectivity to styrene oxide reaches to 80-85% at styrene conversion of 90-95% Acknowledgment This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.05-2014.01 Table Comparison of catalytic activity of different catalysts in the oxidation of styrene Reaction conditions No Catalyst Temperature (oC) 10 11 12 13 14 15 16 17 Cr-Silica TS-1 TS-1 TiO2/SiO2 TS-1 Film TS-1 zeolite CuNiCoAl 3%V2O5/SBA-15 Fe-ZSM-5 Fe-MCM-41 Mn-MCM-48 V-MCM-48 Cr-MCM-48 Au/LDH Au/MgO W-Silica MgCoAl-LDH 80 60 40 100 80 70 55 25 73 73 80 80 80 80 80 80 85 Oxidant H2O2 H2O2 H2O2 O2 H2O2 H2O2 TBHP H2O2 H2O2 H2O2 TBHP TBHP TBHP THBP TBHP H2O2 Air Time (h) 10 12 5 2 24 24 24 Styrene Solvent Acetonitrile Dimethylformamide Acetonitrile None Acetone Acetone Acetonitrile Acetone Dimethylformamide Dimethylformamide Acetonitrile Acetonitrile Acetonitrile Benzene None Acetonitrile None conversion (%) 99.9 53.6 51 51.7 16 23 81.0 55.5 5.0 13.8 58 84 98 79.4 62 79.7 52.6 Product selectivity (%) Benzaldehyde 62.5 19.2 93.5 9.0 16 39.0 38.6 63.1 37.3 59 81 47 24.3 14 84.4 55 Styrene oxide 61.7 82 0.2 0.4 16.0 8.9 36.9 41.8 31 24 74.4 52 38 Others 37.5 19.1 12 6.5 90.6 84 45.0 20.9 10 15 29 1.3 34 15.6 Ref [3] [4] [5] [6] [7] [8] [9] [10] [11] [11] [12] [12] [12] [13] [14] [15] [16] Table Characteristics of the synthesized Mg-Cu-Al catalysts Batch # Nominal formula Surface Lattice area parameter (Å) (m2/g) a c Atomic ratio Mg/Al Cu/Al TH00 Mg0.7Al0.3(OH)2(CO3)0.15.mH2O 84 3.047 22.805 2.4 - TH01 Mg0.63Cu0.07 Al0.3(OH)2(CO3)0.15.mH2O 78 3.048 22.926 2.18 0.23 TH02 Mg0.56Cu0.14 Al0.3(OH)2(CO3)0.15.mH2O 84 3.047 22.789 1.96 0.46 TH03 Mg0.49Cu0.21 Al0.3(OH)2(CO3)0.15.mH2O 73 3.054 22.817 1.87 0.83 TH04 Mg0.42Cu0.28 Al0.3 (OH)2(CO3)0.15.mH2O 63 3.052 22.742 1.59 0.94 TH05 Mg0.35Cu0.35 Al0.3(OH)2(CO3)0.15.mH2O 64 3.050 22.677 1.23 1.05 TH02S Mg0.56Cu0.14 Al0.3(OH)2(CO3)0.15.mH2O 102 3.048 22.773 1.87 0.50 TH04S Mg0.42Cu0.28 Al0.3 (OH)2(CO3)0.15.mH2O 73 3.053 22.774 1.51 1.03 15 (003) (006) (012) (015) (110) (113) (018) TH05 TH04 TH03 TH02 TH01 TH00 10 15 20 25 30 35 40 45 50 55 2-theta (°) Figure XRD patterns for all Mg-Cu-Al hydrotalcite –like compounds 60 65 TH00 A TH02 TH04 TH02S 400 TH04S 1635 570 667 440 3480 1357 800 1200 1600 2000 2400 2800 3200 3600 4000 -1 Wavenumber (cm ) 148 B 1061 555 500 TH02 TH04 100 400 700 1000 1300 1600 1900 2200 Raman shift (cm-1) Figure FT-IR (A) and Raman spectra (B) of Mg-Cu-Al hydrotalcite –like compounds before (TH00, TH02, TH04) and after reaction at 70oC, hours (TH02S, TH04S) O(K) Mg(K) Cu(L) Al(K) Spent samples Cu (Kα) C Cu(Kβ) TH04S TH02S TH05 TH04 TH03 TH02 TH01 10 Energy (keV) Figure EDS spectra of Mg-Cu-Al hydrotalcite –like compounds before (TH01-TH05 and after reaction at 70 oC, hours (TH02S, TH04S) spent catalysts TH04S TH02S TH05 TH04 TH03 TH02 TH01 TH00 0.05 0.15 0.25 0.35 0.45 0.55 0.65 0.75 0.85 0.95 Relative Pressure (P/Po) Figure Nitrogen absorption-desorption isotherms of Mg-Cu-Al hydrotalcite –like compounds before (TH01-TH05) and after reaction at 70 oC, hours (TH02S, TH04S) TH02-Fresh TH04-Fresh TH02-Spent TH04-Spent Figure SEM micrographs of TH02 ( Mg0.56Cu0.14Al0.3(OH)2(CO3)0.15.xH2O) (left) and TH04 (Mg0.42Cu0.28Al0.3(OH)2(CO3)0.15.xH2O) (right) before and after reaction at 70 oC, h TH02-Fresh TH02-Spent Figure TEM images of TH02 ( Mg0.56Cu0.14Al0.3(OH)2(CO3)0.15.xH2O) before (left) and and after reaction (right) at 70 oC, h 2p3/2 A 2p1/2 Cu 2p TH02-Fresh TH02-Spent 935.38 955.56 920 925 930 935 940 945 950 955 960 965 970 975 Binding Energy (eV) Fig 7A 2p3/2 B Cu 2p 2p1/2 TH04S-Spent 935.38 955.56 TH04-Fresh 920 925 930 944.88 935 940 963.79 945 950 955 960 Binding Energy (eV) Fig 7B 965 970 975 980 980 C O 1s 532.2 530.4 532.9 HT02S-Spent HT02-Fresh HT04S-Spent HT04-Fresh 518 522 526 530 534 538 542 546 Binding Energy (eV) Fig 7C D C 1s 285.8 284.5 TH02S-Spent TH02-Fresh TH04S-Spent TH04-Fresh 275 280 285 290 295 300 305 310 315 Binding Energy (eV) Fig 7D Figure XPS Cu2p (A and B), C 2p (C), and O 1s (D) spectra of TH02 and TH04 (Mg0.56Cu0.14Al0.3(OH)2(CO3)0.15.xH2O) before and after reaction at 70 oC, h (TH02S and TH04S) TH04 TH02 200 250 300 350 400 450 500 550 600 650 700 750 800 Wavelength (nm) Figure UV-Vis spectra of TH02 ( Mg0.56Cu0.14Al0.3(OH)2(CO3)0.15.xH2O) and TH04 (Mg0.42Cu0.28Al0.3(OH)2(CO3)0.15.xH2O) Styrene conversion (%) A 80 H2O2 oxidant TBHP oxidant 70 60 50 40 30 20 10 TH00 TH01 TH02 TH03 TH04 TH05 Catalyst Benzaldehyde -H2O2 Others-H2O2 Product selectrivity (%) B Styren oxide-TBHP Others-TBHP 100 90 80 70 60 50 40 30 20 10 TH00 TH01 TH02 TH03 TH04 TH05 Catalyst Figure Conversion (A) and product selectivity (B) over Mg-Cu-Al hydrotalcites at 70 oC, h, DMF solvent with different oxidants (Other products: phenyl acetaldehyde, benzoic acid, and styrene glycol, benzyl benzoate, polymerized products) 100 Styrene oxide Sel Coversion Other Sel 80 (%) 60 40 20 TBHP / Styrene Figure 10 Correlation between catalytic activity and TBHP/styrene molar ratio over TH04 at 70 oC, hours, DMF solvent Conversion (%) A TH05 TH04 TH02 100 75 50 25 45 50 55 60 65 70 75 80 85 90 95 o Reaction temperature ( C) Styrene oxide Selectivity (%) B TH02 TH04 TH05 100 90 80 70 60 50 40 30 20 10 45 50 55 60 65 70 75 80 o Reaction temperature ( C) 85 90 95 Figure 11 Correlation between catalytic activity and reaction temperatures over TH02-TH05 catalysts after hours, DMF solvent A 100 TH05 TH04 TH02 Conversion (%) 80 60 40 20 Reaction time (h) 100 25 90 80 20 70 60 15 TH02 50 TH04 TH05 40 10 30 20 Benzaldehyde Selectivity (%) Styrene oxide Selectivity (%) B 10 0 Reaction time (h) Figure 12 Correlation between catalytic activity and reaction time over TH02-TH05 catalysts at 70 oC , DMF solvent Catalytic oxidation of styrene over Cu-doped hydrotalcites Nguyen Tien Thao*, Le Thi Kim Huyen Faculty of Chemistry, Vietnam National University, Hanoi 19 Le Thanh Tong, Hanoi, Vietnam, 10999 *Corresponding author: Tel (+84) – (04) 3825 3503; Fax: (+84) – (04) 3824 1140 Email: ntthao@vnu.edu.vn (N Tien Thao) Graphical Abstract Cu2+ ions in the brucite-like sheet play a crucial role in the oxidation of styrene at 60-90 oC The catalytic activity is related to the overall amount of Cu2+ ions and the nature of oxidants 27 Catalytic oxidation of styrene over Cu-doped hydrotalcites Nguyen Tien Thao*, Le Thi Kim Huyen Faculty of Chemistry, Vietnam National University, Hanoi 19 Le Thanh Tong, Hanoi, Vietnam, 10999 *Corresponding author: Tel (+84) – (04) 3825 3503; Fax: (+84) – (04) 3824 1140 Email: ntthao@vnu.edu.vn (N Tien Thao) Highlights The successful substitution of Mg2+ by Cu2+ in Mg/Al hydrotalcite/ Cu2+ in octahedral sites of hydrotalcite is prerequisite for the oxidation of styrene/ Styrene conversion depends on the copper content/ Selectivity to products is dependant on the nature of oxidants/ 28 ... analysis [46,47,51] 3.2 Catalytic activity in the liquid oxidation of styrene 3.2.1 Effect of copper contents in Cu- doped hydrotalcites The catalytic activity of all synthesized Mg -Cu- Al hydrotalcite-like... probably from the consecutive oxidation of styrene oxide [11,24,36,53,54] 3.2.3 Effect of reaction temperature The effect of reaction temperature on the styrene oxidation over Cu- doped hydrotalcite.. .Catalytic oxidation of styrene over Cu- doped hydrotalcites Nguyen Tien Thao*, Le Thi Kim Huyen Faculty of Chemistry, Vietnam National University, Hanoi