Epoxidation of styrene with hydrogen peroxide using hydrotalcites as heterogeneous catalysts

Applied Catalysis A: General 272 (2004) 175–185

Epoxidation of styrene with hydrogen peroxideusing hydrotalcites as heterogeneous catalysts

Ilham Kirma, Francesc Medinaa,∗, Xavier Rodrıgueza,Yolanda Cesterosb, Pilar Salagreb, Jesús Sueirasa

a Department of Chemical Engineering, ETSEQ Universitat Rovira i Virgili, Avda. Pa¨ısos Catalans 26, 43007 Tarragona, Spainb Department of Inorganic Chemistry, Faculty of Chemistry, University Rovira i Virgili, Pl. Imperial Tàrraco 1, 43005 Tarragona, Spain

Accepted 24 May 2004

Available online 4 July 2004

Abstract

Several Mg/Al hydrotalcite-like materials with atomic ratios of 2, 3, 4, 6 and 10 were tested as heterogeneous catalysts for the epoxidationreaction of styrene, using a combined oxidant of hydrogen peroxide and acetonitrile in the presence of acetone and water as solvents. Severalfactors such as the Mg/Al ratio, the presence of the pure hydrotalcite phase in the samples, the reconstruction rate of the hydrotalcite-likephase, carried out during the reaction, and the addition of water, play an important role in the selective epoxidation of styrene.

Several characterization techniques such as inductivity coupled plasma (ICP), powder XRD, TGA, IR spectra, SEM,27Al NMR spectra,TPD of CO2 and N2 physisorption were performed in order to correlate the chemical and textural properties of the as-synthesized and calcinedmaterials with their catalytic behaviors.© 2004 Elsevier B.V. All rights reserved.

Keywords:Hydrotalcite-like materials; Hydrogen peroxide; Epoxidation of styrene

1. Introduction

Hydrotalcites (HTs), or layered double hydroxides(LDHs), are anionic clays. The structure of the hydro-talcite is very similar to that of brucite-like Mg(OH)2where an isomorphous substitution of Mg2+ by a triva-lent element M3+ occurs. In brucite, each magnesiumcation is octahedrally surrounded by hydroxyls. The re-sulting octahedron shares edges to form infinite sheetswith no net charge. When Mg2+ ions are replaced by atrivalent ion, a positive charge is generated in the brucitesheet. The positive charge is compensated by anions in theinterlayer, in the free space of which the water of crys-tallization also finds a place[1–6]. The hydrotalcite-likecompounds are generally described by the empirical for-mula (M1−x

2+M3+x(OH)2)x+(Az−)x/z·mH2O, where M2+

and M3+ are the metal cations (M2+ = Mg2+, Ni2+, . . . ;M3+ = Al3+, Fe3+, . . . ), Az−represents the anion needed

∗ Corresponding author. Tel.:+34 977 55 9787; fax:+34 977 55 9621.E-mail address:[email protected] (F. Medina).

to compensate the net positive charge (Az− = CO32−, NO3

−,

Cl−, . . . ), andm is the number of interlayer water molecules.Anionic clays based on HT have aroused considerable in-terest because the diversity of their chemical compositionsmeans that they have many practical applications, such ascatalysts, catalyst supports, ion exchangers, stabilizers, andadsorbents. Recently, they have been among the most widelyinvestigated catalyst precursors because of the noteworthyproperties of the final catalysts. These include a large surfacearea, basic properties, high metal dispersion, and stabilityagainst sintering even under extreme conditions[6–10].

On the other hand, the selective and efficient oxidation ofolefins to obtain epoxides has become very important be-cause epoxides are versatile intermediates for synthesizingother organic compounds[11,12]. The epoxidation reactionof alkenes can be performed in liquid phase with peracidsand organic hydroperoxides as oxidant agents[13]. How-ever, the use of hydrogen peroxide as the oxidant in thisreaction has other advantages; it is a cheap product, rela-tively inert and easy to handle. So it is more interestingthan other classical oxidants such as organic peroxides andperacids. In this line of research, the selective epoxidation of

0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.apcata.2004.05.039

176 I. Kirm et al. / Applied Catalysis A: General 272 (2004) 175–185

olefins by hydrogen peroxide, heterogeneous catalysts suchas Ti-silicalite[14–17], zeolites[18], or titania–silica mixedoxides[19,20] have also been used.

Recently, it has been found that basic hydroxyl groupsof the hydrotalcite, assisted by H2O2, acts as a catalyst toform a perhydroxyl anion species (HOO−), in similar waythat basic OH− groups[21–24], which in turn nucleophili-cally attacks the nitrile, to generate peroxycarboximidic acid,which is an active intermediate oxidant in the epoxidationof olefins[25–29]. Hydrotalcites are very attractive for thistype of oxidation reaction because of their ability to giveLewis type acid–base bifunctional catalysts or basic cata-lysts with Brönsted type sites, proceeding from the mixedoxide and the meixnerite-like structures, respectively[30].Besides, changing the element ratios of Al to Mg in thebrucite-like layer can tune up the basicity of hydrotalcites[6], obtaining different catalytic behaviors for the selectiveepoxidation of olefins such as cyclohexene and styrene usingH2O2 and benzonitrile[27]. So, along these lines, we focushere on the synthesis, characterization and catalytic activityof several hydrotalcites with different Mg/Al ratios (between2 and 10), for the styrene epoxidation reaction using hydro-gen peroxide as oxidant. The change in the basic propertiesduring the rehydration process of the calcined samples aswell their influence on the catalytic activity is also studied.

2. Experimental

2.1. Synthesis of the hydrotalcites

Five Mg/Al hydrotalcite-like precursors with atomic ra-tios of 2, 3, 4, 6 and 10 were obtained and labeled as HT2,HT3, HT4, HT6 and HT10, respectively (seeTable 1).The samples were obtained by coprecipitation from twoaqueous solutions at a pH between 10.0 and 10.5 ± 0.2.One of these solutions contained appropriate amounts ofMg(NO3)2·6H2O and Al(NO3)3·9H2O dissolved in 400 cm3

Table 1Ideal formula, surface areas, volumes of pores and pore diameter of different hydrotalcite-like materials

Sample Formula SBET (m2 g−1) Volume of pores (nm) Pore diameter (nm)

HT2 nC Mg0.676Al0.324(OH)2(CO3)0.162, 0.35·H2O 157 0.6 15.5HT2 C 220 0.8 14.7HT2 U 76 0.5 16.1HT3 nC Mg0.776Al0.223(OH)2(CO3)0.111, 0.56·H2O 77 0.4 19.6HT3 C 234 0.5 8.5HT3 U 87 0.4 13.4HT4 nC Mg0.827Al0.172(OH)2(CO3)0.086, 0.59·H2O 46 0.2 18.9HT4 C 241 0.3 9.5HT4 U 25 0.5 12.6HT6 nC Mg0.868Al0.131(OH)2(CO3)0.065, 0.35·H2O 41 0.1 15.7HT6 C 144 0.2 16.4HT6 U 98 0.4 16.5HT10 nC Mg0.927Al0.072(OH)2(CO3)0.036, 0.29·H2O 18 0.1 9.6HT10 C 188 0.4 8.9HT10 U 138 0.3 9.3

of water, and the other an aqueous solution of NaOH (1 M)and Na2CO3 (1 M). The two solutions were mixed in a glassreaction vessel (volume 3000 cm3), which initially containeddeionised water 200 cm3. The addition was completed in2 h under vigorous stirring. The precipitated gel was agedfor 18 h at room temperature. It was then filtered, washedseveral times by distilled water and dried at 110◦C. Finally,the solid was calcined at 450◦C for 18 h in order to producea high decomposition of the anions located in the interlayerspace of the hydrotalcite and because the surface basicityincreases when the calcination temperature is in the range of450–500◦C [31–33]. The elemental analyses of Mg and Alin the samples were carried out by emission spectroscopywith inductivity coupled plasma (ICP) in a Perkin-ElmerPlasma 400, after the HT samples had been dissolved in10% HNO3. The structural formulae of the solids were de-termined with thermogravimetric and mass spectra results.

2.2. BET surfaces areas

BET surface areas were calculated from the nitrogenadsorption isotherms at 77 K with a Micromeritics ASAP2000 surface analyzer and a value of 0.164 nm2 for thecross-section of the nitrogen molecule. The same equipmentautomatically calculates the pore distribution.

2.3. X-ray diffraction

Powder X-ray diffraction (XRD) patterns of the sam-ples were obtained with a Siemens diffractometer D5000by nickel-filtered Cu K� radiation (λ = 1.54056 Å). Thepatterns were recorded over a range of 2θ angles from 5to 85◦ and compared to X-ray powder references to con-firm phase identities using the Joint Committee on PowderDiffraction Standards (JCPDS) files. The patterns of thedetected phases were: Mg6Al2CO3(OH)16·4H2O hydrotal-cite (JCPDS-ICDD 22-700), MgO Periclase (JCPDS-ICDD71-1176), Mg6Al2(OH)18·4.5H2O magnesium–aluminium

I. Kirm et al. / Applied Catalysis A: General 272 (2004) 175–185 177

hydroxide hydrate (JCPDS-ICDD 35-965), Mg2Al(OH)7magnesium–aluminium hydroxide (JCPDS-ICDD 35-1275),Mg4Al2CO3(OH)12·3H2O magnesium-aluminium hydrox-ide carbonate (JCPDS-ICDD 70-2151).

2.4. Temperature-programmed desorption (TPD)

The temperature-programmed desorption of CO2 wasstudied using a TPD/R/O 1100 (ThermoFinnigan) equippedwith a TCD detector and coupled to a mass spectrom-eter QMS 422 Omnistar. Before the TPD, the sample(50± 0.5 mg) was treated at 80◦C with a 3% CO2/He flow(20 cm3 min−1) for 1 h. After that, the desorption processwas carried out from 80 to 900◦C at 10◦C min−1 in He flow.

2.5. SEM pictures

Scanning electronic micrograph (SEMs) measurementswere carried out with a field emission microscope (JEOL,6300F) operated at an acceleration voltage of 40 kV, a dis-tance of 8 and 10 nm, and a range of magnification between25 000× and 50 000×.

2.6. FT-IR spectroscopy

The FT-IR spectra were recorded on a Bruker EQUINOX55 spectrometer in the 7500–370 cm−1 wavenumber rangeusing pressed KBr pellets and a DLATGS detector with KBrwindows.

2.7. Thermogravimetric analysis (TGA)

Thermogravimetric analysis was carried out in a Perkin-Elmer TGA 7 microbalance with an accuracy of 1�gand equipped with a 25–950◦C programmable tempera-ture furnace. Each sample (50 mg) was heated in Ar flow(80 cm3 min−1) from 50 to 900◦C at 5◦C min−1.

2.8. 27Al NMR spectra

27Al NMR spectra were recorded at 78.158 MHz on a Var-ian Unity 300 spectrometer under an external magnetic fieldof 7.05 T. All measurements were made at room tempera-ture. The samples, held in zirconia rotors, were spun at themagic angle (54◦44′ relative to the external magnetic field)at 3.5 kHz. Spectra were recorded at an excitation pulse of45◦ (3 s) and an accumulation interval of 3 s. Al(H2O)63+was used as external standard.

2.9. Catalytic measurements

The epoxidation of styrene was performed in a batchreactor containing styrene (4 mmol) (Aldrich), acetone(0.136 mol) (Aldrich), acetonitrile (0.153 mol) (Aldrich),H2O (10 ml), H2O2 (3 ml at 33% in water) (Aldrich) and0.5 g of hydrotalcite sample (calcined or rehydrated). A

high H2O2/styrene mole ratio (around 8) was used to avoidany effect of the hydrogen peroxide concentration on the re-action rate[28]. The catalytic activity of magnesium oxide(MgO), aluminium oxide (Al2O3) was also studied for pur-poses of comparison and a test was done with no catalyst.

H2O2 efficiency was determined by KMnO4 titration.The reaction was conducted between 0 and 50◦C at atmo-spheric pressure. The reaction mixture was analyzed by aSHIMADZU GC-17 gas chromatograph using an ULTRA 2capillary column and equipped with a FID detector.

3. Results and discussion

3.1. BET areas

Table 1shows the results of the BET and the porosimetrydeterminations, which include the BET surface areas, thepore volumes, and the average pore diameter (Å) for thenon-calcined (nC), calcined (C) and used (U) samples.

The chemical composition of the sample has a stronginfluence on the surface areas. The BET areas of the mate-rials range between 18 and 157 m2 g−1 for the non-calcinedsamples, and between 144 and 241 m2 g−1 for the calcinedsamples. This increase in surface area after calcination cor-relates with a decrease in the average pore diameter and anincrease in the pore volume values. This behavior is sim-ilar to that observed for other hydrotalcite-like compoundsand their homologous calcination products[6,32]. The cal-cination of hydrotalcite-like samples at 450◦C results inmesoporous mixed oxides with high surface areas, whichcorresponds with the disappearance of the HT-XRD pattern[33] (see below in XRD spectra). The calcined sample HT4(with Mg/Al 4/1) has the highest surface area (241 m2 g−1),while HT6 and HT10 calcined samples have the lowestsurface areas. This difference may be attributed to the ex-istence of other side phases in the HT6 and HT10 samplesdetected by XRD (seeFig. 2).

It is important to mention that the HT2, HT3 and HT4 usedsamples have the lowest surface area (76, 87, 25 m2 g−1, re-spectively). The surface area decreases because the rehydra-tion process performed during the reaction in the presenceof water leads to the reconstruction of the lamellar struc-ture characteristic of the hydrotalcite-like materials. How-ever, for HT6 and HT10 used samples, the decrease in thesurface area is lower (98, 138 m2 g−1, respectively), both bya less effective rehydration process, due to the high Mg/Alratios and by the presence of other side phases.

3.2. X-ray diffractions

Figs. 1 and 2show the X-ray powder diffraction pat-terns of the HT4 and HT10 samples (non-calcined (A),calcined (B) and used (C)), respectively. The XRD patternsof the non-calcined samples, with Mg/Al ratios between2 and 4, show the typical pattern of a well-crystallized


Fig. 1. XRD patterns for hydrotalcite HT4: (A) non-calcined; (B) calcined;(C) used.

hydrotalcite-like material. However, HT6 and HT10non-calcined samples with Mg/Al ratios of 6 and 10 haveother phases[34] (seeFig. 2). The calcination process ofthe samples produces the loss of the hydrotalcite structureand provides a mixed oxide phase (Mg(Al)Ox). During theepoxidation reaction of styrene with hydrogen peroxideand because water is the solvent, the layered structure isrecovered for the HT2, HT3 and HT4 samples. These solidsare transformed into a meixnerite-like material with OH−as the compensating anions in the interlayer space[35].The reconstruction of the lamellar structure considerablydecreases the surface area (from 241 to 25 m2 g−1 for theHT4 sample). For the HT6 and HT10 used samples, thelayered structure, probably the meixnerite-like phase is alsodetected by XRD. However, other phases such as brucite,magnesite and gibbsite are not detected.

Fig. 2. XRD patterns for hydrotalcite HT10: (A) non-calcined; (B) cal-cined; (C) used.

3.3. Thermogravimetric analysis (TGA)

Fig. 3 shows the thermogravimetric analysis (TGA) ofthe HT4 and HT10 samples. HT2 and HT3 behave in asimilar way to HT4. They are characterized by two weightlosses, which is typical for hydrotalcite-like materials[36].HT4 shows a first weight loss, between 120 and 270◦C(16.3 wt.%), which is attributed to the removal of weaklybonded water (probably the hydration water located in the in-terlayer space of the hydrotalcite phase). The second weightloss, between 270 and 500◦C (22.5 wt.%), is connected tothe dehydroxylation of the OH− from the brucite-like lay-ers, and the decomposition of the anions (mainly carbonate)[6]. On the other hand, for the HT6 and HT10 samples, threezones of weight losses are detected. For HT10, the first isbetween 120 and 250◦C (11 wt.%), the second between 250and 430◦C (19 wt.%) and the third between 430 and 560◦C(16 wt.%). This additional third zone can be attributed to thedecomposition of the side phases mentioned above.

3.4. SEM analysis

SEM pictures for non-calcined samples with Mg/Al be-tween 2 and 4 show the lamellar morphology which ischaracteristic of the hydrotalcite phase. For the HT6 andHT10 samples, however, other morphologies are detected(seeFig. 4A and Bfor HT4 and HT10 samples, respec-tively). This is in agreement with the XRD results men-tioned above. The calcination of the materials up to 450◦Cmakes the layered structure disappear, as can be concludedfrom the XRD results. However, for the hydrotalcite phase,the general morphology is preserved after calcinationsat this temperature[37]. Dehydroxylation and interlayercarbonate decomposition lead to the release of water andcarbon-dioxide during the calcination process. At high tem-peratures, water and CO2 leak out from the interlayer spaceand increase the BET area values of the sample.

3.5. TPD of CO2 analysis

The TPD of CO2 analysis for calcined materials shows theexistence of different basic sites (seeFig. 5). The sampleswith a Mg/Al ratio higher than 4 shows two broad desorp-tion peaks of CO2 at temperatures around 300 and 600◦C.Nevertheless, samples with Mg/Al between 2 and 4, presentan additional desorption peak of CO2 at higher temperatures(around 850◦C), which shows the presence of strong Lewisbasic sites.Fig. 6shows the TPD of CO2 for HT2, HT3 andHT4 rehydrated samples (a, b and c, respectively). As canbe seen, there is an important change in the basic propertiesof these materials during the rehydration process. First theamount of CO2 desorbed is higher for the rehydrated samplesthan for the calcined ones (1550, 1350 and 1210�mol g−1

for HT2, HT3 and HT4 rehydrated samples, instead of 310,525 and 709�mol g−1, for the calcined ones, respectively).This indicates that after the rehydration process, numerous


Fig. 3. Thermogravimetric analysis of samples HT4 and HT10.

Fig. 4. SEM pictures of different calcined samples: (A) HT4; (B) HT10.

Brönsted basic sites are produced. It is important to mentionthat the main desorption peaks of CO2 for the HT2, HT3,and HT4 rehydrated samples are detected at temperaturesaround 360, 390 and 450◦C, respectively (seeFig. 6). Thisindicates that the basic sites for the pure hydrotalcite-like

Fig. 5. TPD spectra of CO2 of calcined samples: (A) HT4; (B) HT10.

samples HT2, HT3 and HT4 are stronger when the Mg/Alratio increases.

3.6. IR spectra

The presence and nature of the charge-compensating an-ions were investigated with infrared spectroscopy (IR). TheIR spectra for samples HT3, HT4, HT6 and HT10 calcinedat 450◦C are shown inFig. 7. A broad absorption band canbe seen at around 3456 cm−1, which may be assigned tothe stretching mode of hydrogen-bonded hydroxyl groupsfrom the brucite-like layer and from the interlayer watermolecules. This indicates that a considerable number of hy-droxyl groups are present in the brucite layer and a bandcorresponding to the deformation mode (δHOH) appearedat ∼1636 cm−1. The absorption bands of CO3

2− ions areobserved at∼1385 cm−1 and attributed to the asymmetric


Fig. 6. TPD of CO2 of rehydrated hydrotalcites: (a) HT2; (b) HT3; (c)HT4.

stretching vibration of interlayer CO32− ions[29,38,39]. Thepresence of water and carbonate in the calcined sample canbe attributed to the adsorption of water and CO2 when thesample is handled to make the KBr pellet.

Fig. 8shows the IR spectra for non-calcined (A), calcined(C), and used (B) HT4 samples. The broad absorption bandlocated at∼3480 cm−1 and the band located at 1644 cm−1

increase for the used samples. This indicates that the pres-ence of water molecules and hydroxyl groups on the inter-layer space of the used material increase due to the recon-struction of the lamellar structure.

3.7. 27Al NMR spectra

Fig. 9 shows the27Al NMR results for HT4 and HT10samples. The spectrum for the non-calcined samples HT4nC (Fig. 9A) and HT10 nC (Fig. 9C) has a single signal,centred at 6.7 and 7.5 ppm, respectively, which was assignedto Al octahedrally coordinated to hydroxyl groups in thehydrotalcite-like structure. This is in agreement with pre-

Fig. 7. IR spectra of different calcined samples: (A) HT10; (B) HT6; (C) HT4; (D) HT3.

viously reported results[40] and with the assumption thatMg2+ cations are isomorphically substituted by Al3+ ionsin the hydrotalcite structure. As noted earlier, calcinationof the hydrotalcite yields a mixture of Mg and Al oxides.For the calcined sample HT4C (Fig. 9B), the27Al spectrumshows two signals at 15.2 and 71.4 ppm that are assigned tooctahedrally and tetrahedrally coordinated Al, respectively[41]. Furthermore, the HT10C sample (seeFig. 9D) alsoshows two signals at 15.3 and 82.1 ppm, which can also beassigned to octahedrally and tetrahedrally coordinated Al,respectively. The spectrum of the HT4 sample after reactionhas a single signal, centred at 8.9 ppm, which suggests thatduring the reaction, and because water is used as the sol-vent, the tetrahedral Al is transformed into octahedral Al.The HT10 sample behaves in a similar way.

The 27Al NMR technique shows that during the reactionthe lamellar structure is reconstructed. We used this tech-nique to study the reconstruction process in more detail.To this end, several HT4 samples were taken out from thereactor during the reaction and analyzed as a function oftime. The results inFig. 10shows that the peak detected at71.4 ppm for the calcined sample, assigned to tetrahedrallycoordinated Al, disappears after 5 min of reaction. This in-dicates that under our reaction conditions (in the presenceof water) the reconstruction rate of the lamellar structureis a very fast process. This is also supported by XRD (seeFig. 11), which shows that for 2 min during the reaction onlybroad peaks assigned to the hydrotalcite-like phase are de-tected. An increase in the reaction time (to 10 min) producesa more crystalline hydrotalcite-like phase.

3.8. Catalytic activity

The epoxidation reaction rate of several olefins usinghydrotalcite-like materials as heterogeneous catalysts andhydrogen peroxide as the oxidant at atmospheric pressure,


Fig. 8. IR spectra of HT4: (A) non-calcined; (B) used; (C) calcined.

can be influenced by such factors as the Mg/Al ratio in thesamples, the activation condition of the hydrotalcite-like ma-terials, the nature of the solvent, the reaction temperature, therehydratation process of calcined samples, etc.[25,27–29].

In this respect, several catalytic experiments were per-formed in order to elucidate the effect of these differentvariables on the epoxidation reaction of styrene (Scheme 1).Firstly, a blank test (with no catalyst) was performed lead-ing to null conversion (seeTable 2), which showed that acatalyst is essential for this reaction.

3.9. Effect of Mg/Al ratio

Table 2shows the catalytic activity for the epoxidationof styrene using several hydrotalcites with different Mg/Alratios (HT2, HT3, HT4, HT6 and HT10) calcined at 450◦C.The effect of the Mg/Al ratio was investigated. The catalystsobtained from the calcination of hydrotalcite-like materials

Table 2Influence of the Mg/Al ratio on the catalytic activity

Samplea Reactiontime (h)

Styreneconversion(%)

Styrene oxideselectivity (%)

H2O2

efficiency(%)

HT2 23 46 99.8 63HT3 6 81 99.7 75HT4 0.25 88 99.8 85HT4b 7 96 99.6 77HT4c 2 97 99.7 83HT6 2 80 95.9 65HT10 4 84 96.1 70MgO 9 65 95.7 78�-Al2O3 24 0 0 –Without catalyst 24 0 0 –

a The samples were calcined at 450◦C before reaction.b The reaction conditions were similar than in reference[27].c The reaction conditions were similar than in reference[27] with the

addition of 10 ml of water.

and magnesium oxide (MgO) are active catalysts for this re-action. However, null activity is observed for�-Al2O3, indi-cating that the basic sites play an important role in the epox-idation reaction. The selectivity to styrene oxide is higherthan 95% for all the catalysts, which shows that the effi-ciency of the hydrogen peroxide is always higher than 60%.

For the HT2, HT3 and HT4 catalysts (obtained from thepure hydrotalcite phase), the activity reaches a maximumfor the HT4 sample (with a Mg/Al ratio of about 4), whichshows a conversion of 88% in 15 min and a selectivity tostyrene oxide of 99.8% and an efficiency for the H2O2 of85%. The HT2 sample shows the lowest activity, around46% in 23 h and the lowest efficiency for the H2O2 (around63%). It seems that for these catalysts an increase in theMg/Al ratio increases the activity. Besides the highest ef-ficiency observed for the HT4 sample can be explainedby their considerable reaction rate. The BET area of thecalcined samples increases at higher Mg/Al ratios (220,234 and 241 m2 g−1 for HT2, HT3 and HT4 samples, re-spectively). However, these BET values make it difficult toexplain the different catalytic behavior observed for thesesamples. However, the TPD of CO2 for the HT2, HT3, andHT4 rehydrated samples (seeFig. 6) showed that Brönstedbasic sites are stronger when the Mg/Al ratio increases. Ithas been said above that during the reaction, and due to thepresence of water, the layered structure of the hydrotalcite isrecovered. This rehydration process leads to the formationof Brönsted basic sites, and it is well-known that these Brön-sted basic sites are derived from surface hydroxyl groupson hydrotalcites and might play an important role in thisreaction[25]. The most active sample (HT4) has strongerbasic sites, which may be the reason for its high activity.

The samples with Mg/Al ratios higher than 4 are notsingle phases, and HT6 and HT10 probably consist of asurface layer of amorphous magnesia supported on hydro-talcite. Their catalytic behaviour is probably between thatof a pure hydrotalcite phase and pure MgO[40].


Fig. 9. 27Al NMR spectra for the samples: (A) HT4 nC; (B) HT4 C; (C)HT10 nC; (D) HT10 C (* denotes spinning side bands).

3.10. Effect of the activation conditions

The effects of the activation conditions were investigatedusing the HT4 sample (seeFig. 12). The conversion ofnon-calcined samples (as-synthesized) was lower than 20%after 3 h. This result is in agreement with previously re-ported results for the epoxidation of 2-cyclohexen-1-onewith TBHP as oxidant and methanol as solvent[29]. How-ever, when a calcined sample was used as a catalyst, conver-

Fig. 10.27Al NMR spectra for the sample HT4, calcined (A), rehydratedfor 5 min (B) rehydrated for 1 h (C) (* denotes spinning side bands).

Fig. 11. XRD patterns for rehydrated HT4: (A) 2 min; (B) 5 min; (C)10 min.


Scheme 1. Chemical reaction of the epoxidation of styrene.

sion was 88% in 15 min. This result seems to be differentfrom the result obtained by Figueras and co-workers[29].However, taking into account that our reaction experimentsused some water as solvent and that during the reaction thereconstruction of the lamellar structure is very fast (as hasbeen shown by XRD and27Al NMR), we can consider thatthe rehydrated material, not the calcined material, is respon-sible for the high activity. In order to study the effect of wateron the catalytic activity, several experiments were performed(seeFig. 12). At first, an ex situ rehydrated HT4 sample wasobtained using the following procedure: after calcination at450◦C the sample was rehydrated in liquid water (1 g ofsample in 100 ml of water) for 1 h in the absence of CO2 atroom temperature and under mechanical stirring. Then it wasfiltered, washed with ethanol, and dried under argon, yield-ing rehydrated HT4. Conversion was 50% after 15 min ofreaction with this catalyst. The fact that the catalytic activi-ties of the calcined sample and the ex situ rehydrated sampleare different can be attributed to the loss of BET area de-tected after the rehydration process (241 and 25 m2 g−1, re-spectively). Besides, another experiment was performed thatdid not use water as solvent. The result shows that styreneconversion is lower than 60% after 6 h (seeFig. 12). Thisindicates that the addition of water as solvent considerablyincreases activity[29] and has a strong effect on the recon-struction of mixed oxides to give the meixnerite structureand strong Brönsted basic sites, which are necessary for theepoxidation reaction.

Fig. 12. Effects of the activation conditions on the catalytic properties ofHT4 hydrotalcite for the epoxidation of styrene.

3.11. Effect of the solvent

The effects of the solvent were investigated by comparingacetone, methanol and ethanol in the presence of acetonitrile,distilled water, calcined sample HT4 and H2O2 as oxidant, atroom temperature and atmospheric pressure.Fig. 13showsthat the activity of acetone and ethanol is higher than thatof methanol. This indicates that it is important to choosesolvents with low dielectric constants, as expected for anionic mechanism.

It is well-known that in the absence of nitriles, no activityis observed. Consequently, a combined effect between theoxidant (H2O2), acetonitrile and the hydroxyl groups of thehydrotalcite sample is necessary to improve the epoxidationreaction (Scheme 2) [26,41,42].

A comparison of two different nitriles such as acetonitrileand benzonitrile shows that acetonitrile gives higher conver-sion (seeFig. 14), in agreement with the literature resultsgiven for the oxidation reaction of dibenzothiophene by hy-drogen peroxide[40].

3.12. Influence of the reaction temperature

Fig. 15 shows the effect of the reaction temperature onthe catalytic properties for the HT4 calcined sample atthree different temperatures between 0 and 50◦C. As isexpected the catalytic activity was highest at the highesttemperature (50◦C). After 5 min of reaction the conver-sion was around 92%. This conversion is higher than thatobserved by Kaneda and co-workers[27], where a con-version of 97% was reported using Mg10Al2(OH)24CO3as catalyst, for 24 h at 60◦C. In order to compare our re-sults with the Kaneda et al. results, two catalytic tests wereperformed using the HT4 calcined sample (seeTable 2).The reaction conditions were[27]: styrene (3.9 mmol),benzonitrile (10.5 mmol), HT4 (0.05 g), MeOH (10 ml),30% aqueous H2O2 (2.4 ml) and a reaction temperatureof 60◦C. A 96% of conversion is achieved after 7 h,while Kaneda et al. reported that a 97% of conversionis obtained after 24 h. This fact could be explained tak-ing into account the lower Mg/Al ratio in our sample (4),instead of 5 for Kaneda et al. Besides, we introduce thecatalyst as calcined form while Kaneda et al. was in therehydrated form. Furthermore, when 10 ml of water wasadded to the reaction (see footnote c inTable 2) a 97%of conversion is achieved in 2 h. This indicates that theaddition of water produces a strong effect in the catalyticactivity.


Fig. 13. Effect of the solvent on the catalytic properties of HT4 hydrotalcite for the epoxidation of styrene.

Scheme 2. Possible mechanism.

Fig. 14. Comparison between acetonitrile and benzonitrile in the epoxidation of styrene using HT4 hydrotalcite.


Fig. 15. Effect of reaction temperature in the epoxidation of styrene.

4. Conclusion

The results of our work reveal that calcined hydrotalcite-like materials are good catalysts for the epoxidation reactionof styrene when hydrogen peroxide is used as the oxidant,and acetone and water are used as the solvent. The presenceof nitrile is essential to perform the reaction. As well, theaddition of water to the reaction plays an important rolein order to obtain a quickly reconstruction of the lamellarstructure (from mixed oxides) and so, increasing the reactionrate.

The Mg/Al ratio in the hydrotalcite also plays an impor-tant role in the reaction. For mixed oxides obtained fromthe pure hydrotalcite phase, the increase in the Mg/Al ratioincreases the catalytic activity. The increases in the reactionrate with the Mg/Al ratio have been correlated by TPD ofCO2and it has been seen that the Brönsted basic sites arestronger when the Mg/Al ratio increases.

For samples HT6 and HT10 the range of activity is be-tween HT4 and MgO samples. This indicates the presenceof MgO supported on the hydrotalcite phase. The solventalso plays an important role in this reaction.

Acknowledgements

Ministerio Ciencia y Tecnologıa (REN2002-04464-CO2-01, PETRI No. 95-0801.OP) and Destilaciones Bordas S.A.

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Epoxidation of styrene with hydrogen peroxide using hydrotalcites as heterogeneous catalysts

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