ARTICLE pubs.acs.org/IECR
Synthesis, Characterization, and Catalytic Behavior of MgAlZn Fe Mixed Oxides from Precursors Layered Double Hydroxide Angelica C. Heredia,† Marcos I. Oliva,‡,^ Carlos I. Zandalazini,§ Ulises A. Agu,†,^ Griselda A. Eimer,†,^ Sandra G. Casuscelli,†,^ Eduardo R. Herrero,† Celso F. Perez,† and Monica E. Crivello*,† †
Universidad Tecnologica Nacional, Facultad Regional Cordoba-CITeQ, Maestro Lopez esq. Cruz Roja Argentina, Ciudad Universitaria (5016) Cordoba, Argentina ‡ IFFAMAF, UNC, Universidad Nacional de Cordoba, Ciudad Universitaria, (5000) Cordoba, Argentina § Centro Laser de Ciencias Moleculares, Departamento de Fisicoquímica, Facultad de Ciencias Químicas, Universidad Nacional de Cordoba, Ciudad Universitaria, (5000) Cordoba, Argentina ^ CONICET, Argentina ABSTRACT: In the present work, MgAlZnFe layered double hydroxides (LDH) were prepared by coprecipitation reaction with hydrothermal treatment. Their corresponding calcinated products (mixed oxides) were used as catalyst for ethylbenzene dehydrogenation to styrene. The characterization of precursors and the mixed oxides was carried out by X-ray diffraction, XPS, TGA-DSC, UVvis-DRS, specific surface area, and magnetic properties. The Fe3þ species were detected by XPS; they exist in two chemical states related to oxides and spinel environment. Zn is found in two possible contributions, such as ZnO or spinel. XRD data indicate that the hydrotalcite phase exists in all precursor samples, except for the sample without magnesium (HT100). In the mixed oxides the ZnO phase increases with the rise of the Zn content. Oxides show a decrease of surface areas with the increase of the Zn content. Ethylbenzene dehydrogenation was carried out with the mixed oxides synthesized. The HT25 sample with Zn/(Zn þ Fe) = 0.25 molar ratio shows a 31.5% conversion and selectivity to styrene superior to 87%, which is directly related to the quantity of oxide, spinel phase and magnesium, as well as the high surface area and the magnetic response obtained.
1. INTRODUCTION Layered double hydroxides (LDH) are represented by the general formula [M2þ(1-x) M3þx (OH)2]xþ (An-x/n) 3 mH2O, where the divalent ion may be Mg2þ, Cu2þ, Zn2þ, Ni2þ, the trivalent ion Al3þ, Fe3þ, Cr3þ, and the anions (A) CO32-, OH, SO42-, NO3, Cl. These materials are known as hydrotalcitelike or anionic clay.1 Their structure consists of brucite-type layers, where the substitution of M2þ with M3þ cations results in a net positive charge, compensated by interlayer anions. There is also water of crystallization in the interlayer region.1 Decomposition of LDH at intermediate temperatures (450500 C) leads to a highly active homogeneous mixed oxide, which have remarkable properties as catalysts and catalyst supports. Interest in LDH is undoubtedly increasing since several kinds of catalytically active metal cations may be accommodated in the brucite-layers, leading to a wide variety of mixed oxide catalysts, and thus their intrinsic properties might be tuned.24 The use of LDH precursors produces an excellent dispersion of metal compounds on a matrix of magnesium and aluminum mixed oxides. These materials are used for a variety of organic transformations, such as aldol condensations,58 epoxidation of limonene and cyclooctene,9,10 oxidative desulfurization,11 epoxidation of cyclohexene and allylic alcohols,12,13 oxidation of methane to synthesis gas,1416 oxidative methanol reforming,1719 and ethylbenzene dehydrogenation.20,21 More than 90% of the world production of styrene is obtained by dehydrogenation of ethylbenzene in the presence of superheated steam, at temperatures between 530 and 650 C, on a catalyst based r 2011 American Chemical Society
on iron(III) oxide and potassium oxide (as K2CO3) activated with different promoters in a two fixed beds adiabatic reactors.2224 The dehydrogenation of ethylbenzene to styrene could be accompanied by several parallel-consecutive reactions,25 as follows: C6 H5 CH2 CH3 T C6 H5 CHdCH2 þ H2
ð1Þ
C6 H5 CH2 CH3 f C6 H6 þ C2 H4
ð2Þ
C6 H5 CH2 CH3 þ H2 f C6 H5 CH3 þ CH4
ð3Þ
C6 H5 CH2 CH3 f 8C þ 5H2
ð4Þ
8C þ 16H2 O f 8CO2 þ 16H2
ð5Þ
2628
Many studies in the past years have evidenced that at any reaction conditions, the active phase on the Fe2O3K2CO3 catalyst is potassium ferrite, KFeO2. The role of other promoters in the FeKO system is to uphold and support the KFeO2 phase formation. It is necessary to carry out a search for new catalytic systems which have high surface areas and can stabilize the active state of iron, in the absence of potassium. New catalysts for the process of ethylbenzene dehydrogenation to styrene were Received: December 14, 2010 Accepted: April 18, 2011 Revised: April 18, 2011 Published: April 18, 2011 6695
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Table 1. Chemical and Surface Composition of the Studied LDH LDH sample
M2þ
% Zn (with respect to M2þ)
% Fe (with respect to M3þ)
theory
theory
ICP
partially dehydrated precursors
oxides
ICP
surface area (m2/g)
HT0
Mg
0
0
50
45
26
119
HT25
MgZn
25
19
50
43
37
105
HT50
MgZn
50
51
50
46
39
76
HT75
MgZn
75
78
50
42
40
38
HT100
Zn
100
100
50
44
54
20
obtained using vanadium-substituted MgAl hydrotalcite-like compounds as precursors;29,30 FeMgAl31 and FeZnAl32 mixed oxides were also tested. The acidbasic and redox properties of the catalyst, at an optimum configuration in terms of composition, structure, and texture, will influence the conversion and selectivity to styrene (reaction 1). If the surface basic sites are strong enough to abstract β-hydrogen from ethylbenzene, the break of the lateral CC bond is promoted and, therefore, the selectivity to toluene will increase (reaction 3). If the catalyst surface acidity is larger, R-hydrogen can be abstracted from ethylbenzene, and the break of the phenylC bond becomes more probable to happen, therefore, a higher selectivity to benzene will be obtained (reaction 2).25 In the present work, a series of MgAlZnFe mixed oxides was prepared from the corresponding LDH precursors and tested for the dehydrogenation of ethylbenzene. The precursors and the mixed oxides obtained after calcination of MgAlZnFe LDH with different amounts of Zn have been characterized. It is well know that replacement of Mg for Zn in mixed oxides increases the lattice parameter33 due to the Zn2þ ionic radius (0.074 nm) being larger than the Mg2þ ionic radius (0.065 nm). The catalytic performance of the mixed oxides obtained in dehydrogenation of ethylbenzene and how the Zn additions modify both catalytic properties and magnetic responses of mixed oxides have been also reported. We present the preliminary results by analyzing the dehydrogenation with N2 in order to select the most suitable catalyst to continue with the studio at a later stage by varying the carrier gas catalytic and kinetic parameters.
2. EXPERIMENTAL SECTION 2.1. Synthesis of the Samples. The Mg, Al, Zn, Fe LDH precursors were prepared by coprecipitation at low supersaturation method at constant pH (9 ( 0.5), with M2þ/M3þ = 3 and Fe3þ/Al3þ = 1 constant molar ratios.9 The molar ratio Zn2þ/Mg2þ was changed in order to assess the effect of these variables on the properties of the LDH and their calcination products. Two solutions, A and B, were prepared. The A solution contained Mg(NO3)2 3 6H2O, Al(NO3)3 3 9H2O, Zn(NO3)2 3 9H2O, and Fe(NO3)3 3 9H2O dissolved together in distilled water. The amounts of nitrates were selected to obtain the total cation concentration of 0.7 M. The B solution contained 0.085 M of Na2CO3 (according to the relation of [CO32-] = 0.5 [M3þ]); both solutions were added simultaneously to 30 mL of distilled water at a drip rate of 50 mL/h. The pH was maintained constant by adding NaOH 2 M. The coprecipitation was carried out at 60 C. The gel was transferred into Teflon-lined stainless-steel autoclave and kept in an oven at 200 C for 18 h, then the gel was separated by centrifugation at
2800 rpm, and washed with distilled water until a sodium content lower than 0.13 wt %. Finally, the solid was dried overnight at 90 C in open air. All the samples were calcined in atmosphere of air at 550 C for 9 h. Compositions of the catalysts are presented in Table 1, where they are named according to the Zn content in the synthesis (keeping in mind only cations M2þ). 2.2. Catalyst Characterization. Inductively coupled plasma (ICP) optical emission spectroscopy was used for the determination of the metal content in the oxides. The measurements were performed with a Varian Spectra AA. The XRD powder patterns were collected on a Rigaku diffractometer, using monochromatized Cu KR radiation (λ = 1.54 A) at a scan speed of 1/4 min in 2θ and interfaced to a DACO-MP data acquisition microprocessor provided with Diffract/AT software. The diffraction pattern was identified by comparison with those included in the JCPDS (Joint Committee of Powder Diffraction Standards) database. The UVvis diffuse reflectance (DRUVvis) spectra of the LDH precursors were recorded using an Optronicss OL 750427 spectrometer in the wavelength range of 2001000 nm. Termogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out in a TA Instruments SDT Q600 (01500 C) instrumentation apparatus in a flowing air atmosphere. Approximately 30 mg of the sample was loaded and heated at a rate of 10 C/min to 550 C. X-ray photoelectron spectroscopy (XPS) analyses were carried out using an ESCA (VG microtech) spectrometer with a nonmonochromatic Mg KR radiation (υ = 1253.6 eV) as the excitation source. High-resolution spectra were recorded in the constant pass energy mode at 20 eV, using a 720-mm diameter analysis area. Under these conditions, the Au 4f7/2 line was recorded with 1.16 eV fwhm (full width at half maximum) at a binding energy (BE) of 84.0 eV. The spectrometer energy scale was calibrated using Cu 2p3/2, Ag 3d5/2 and Au 4f7/2 photoelectron lines at 932.7, 368.3, and 84.0 eV, respectively. Charge referencing was done against adventitious carbon (C 1s, 284.8 eV). The pressure in the analysis chamber was maintained lower than 109 Torr. PHI ACCESS ESCA-V6.0 F software package was used for acquisition and data analysis. A Shirley-type background was subtracted from the signals. The specific surface area was determined by the BET method, which was recorded on a Micromeritics ASAP 2000 instrument. To eliminate the water physically adsorbed, the precursors and the calcined materials were degassed at 100 and 390 C, respectively, both for 60 min. The magnetic measurements were performed in a conventional vibrating sample magnetometer (Lakeshore 7300) at room temperature varying the applied field between (15000 Oe. The powder samples were compacted applying 5 Tn to make a disk 6696
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Figure 1. X-ray diffraction patterns of the LDH precursors with different Zn content: (9) LDH, (Δ) zinc oxide ZnO, and (1) magnesium carbonate MgCO3.
shaped sample of 5-mm diameter and 2-mm height to measure magnetization as function of applied field (M vs H). 2.3. Catalytic Activity. The ethylbenzene dehydrogenation was carried out in a fixed-bed flow-type reactor consisting of a tubular glass flow reactor (8 mm I.D. and 35 mm length). The temperature of the reactor was measured by a thermocouple placed in the center of the catalyst bed. The reactions were conducted at atmospheric pressure and 550 C using 0.1 g of catalyst. To determine the conditions of reaction in which the intraphase concentration gradients are absent, the experiments were carried out maintaining constant contact time, varying the diameter of the particle used from 0.46 to 0.164 mm. The experiments were performed at 550 C and 0.0019 mol/h of ethylbenzene. It was determined that below a particle diameter of 0.33 mm the conversion remained constant, indicating that the system is under kinetic control in this zone, whereas, above this value, the conversion varied with the size of the particle (transfer of restrictive mass). Thus, the size average of the particle chosen was 0.335 mm, according to the limits by internal diffusion. The minimum molar flow rate that guarantees the absence of gradients is superior to 0.0019 mol/h. The reagent was introduced in the reactor through a gas mixture of ethylbenzeneN2 so ethylbenzene was fed by bubbling a gas of N2 at 40 mL/h through liquid ethylbenzene held at 50 C in a thermostat. The gas hourly space velocity (GHSV) was 9600 h1 and the volume of the catalyst bed was 0.25 cm3. The reaction products (styrene, toluene, and benzene) and ethylbenzene were liquefied in a cold trap, while the gaseous products were vented. The reaction mixture was analyzed by a Hewlett-Packard 5890 gas chromatography equipped with a cross-linked methylsilicone column and FID detector.
The conversion of ethylbenzene Xeb and the selectivity to styrene (Sst), benzene (Sben), and toluene (Stol) were calculated according to eqs 1 and 2 X eb ð%Þ ¼
Sð%Þ ¼
F eb Initial F eb Unreacted 100 F eb Initial
F Component in the product 100 F eb Initial F eb Unreacted
ð1Þ
ð2Þ
where F is the molar low rate.
3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction. For X-ray diffraction, in all precursor samples the hydrotalcite phase was detected, except in the sample without magnesium (HT100) which presents poor crystallinity. Most of the peak positions were matched with the ICDD (International Centre for Diffraction Data) PCPDFWIN data. Powder X-ray diffraction patterns of precursors with different (Mg2þ/Zn2þ) molar ratios are shown in Figure 1. The hydrotalcite phase (PCPDFWIN 70-2151) and the secondary phase MgCO3 (magnesium carbonate PCPDFWIN 83-1761) were observed in all samples except in the sample without magnesium (HT100). Two peaks located near 2θ of 11.62 and 23.38 are associated to diffraction by (003) and (006) planes characteristic of the hydrotalcite phase. The peak centered at 2θ of 36.2 in all the XRD patterns is associated with the ZnO phase (PCPDFWIN 80-0075); this phase is formed by hydrothermal aging of the coprecipitated products under relatively rough conditions (200 C for 18 h). The relative intensity of the reflection of ZnO peaks decreases with the increase Mg2þ content. 6697
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Table 2. Results of the TGADSC for Different Samples sample temperature range
observed weight loss
DSC maximum
(C)
(%)
(C)
HT0 until 100
1.09
63.9
100250
8.50
151.97
250550
27.17
373.43
total weight loss
36.76
HT25 until 100
0.81
65.64
100250 250550
8.07 21.12
207.84 389.1
total weight loss
30.00
HT50
Figure 2. X-ray diffraction patterns of calcined samples: (O) spinel MgFe2O4, (b) spinel ZnFe2O4, (0) periclase MgO, (þ) hematite Fe2O3, and (Δ) zinc oxide ZnO.
until 100
1.27
100250
5.07
193.4
67.08
250550
13.67
390.29
total weight loss
20.01
HT75 until 100 100250
0.51 2.04
65.3 138
250550
5.88
383
total weight loss
8.43
HT100
Figure 3. UVvis diffuse reflectance spectra of the LDH precursors with different Zn content.
For a given series of precursors, the sharpness of the peaks (directly related to crystallinity) increases with the Mg content in the sample. After calcinations at 550 C, the (003) and (006) reflections disappeared (Figure 2). The MgO phase (periclase PCPDFWIN 78-0430) was detected, except for the sample HT100. The XRD pattern of HT0 showed that the Fe3þ ion crystallizes in two structures, MgFe2O4 (magnesium iron oxide PCPDFWIN 711232) and Fe2O3 (hematite PCPDFWIN 79-1741). In all the samples with Zn content, it was observed ZnO (zinc oxide PCPDFWIN 80-0075). The zinc iron oxide (ZnFe2O4 PCPDFWIN 01-071-5149) was detected in all the samples except in the HT0 and HT100. The relative intensity of ZnFe2O4 peaks was apparently reduced while the intensity of ZnO increases with an increase in the Zn content. 3.2. UVvis-DR Spectroscopy. The UVvis-DR spectra of the LDH precursors are shown in Figure 3. The absorption edge 380 nm in the samples HT25, HT50, HT75, and HT100 is attributed
until 100
0.25
100250
0.71
250550
1.65
total weight loss
2.60
to ZnO particles formed on the structure of the LDH34 during the synthesis process in a Teflon-lined stainless-steel autoclave at 200 C for 18 h. This absorption band increases with the Zn content which corresponds well with the intensity of the ZnO peak assigned by X-ray diffraction patterns. The HT0 exhibits a band at ∼210 nm and ∼260 nm assigned to the charge transfer of isolated Fe3þ ions octahedrally coordinated in brucite layered structure.35,36 3.3. Thermal Analyses (TGA and DSC). Thermal properties of the samples have been assessed by TGA and DSC; both these studies were carried out in air. To understand the decomposition procedure of the MgAlZnFe LDH the TGADSC profiles were analyzed in detail. Results of the weight losses, in the all the ranges analyzed, and the temperatures of the maximum endothermic peaks in the DSC profiles are reported in Table 2. Two weight loss stages were observed on the TGA curves, corresponding to the two endothermic peaks on the DSC profiles, demonstrating that the decomposition proceeded in two steps. All the samples show a weight loss below ∼100 C due to the water physically adsorbed, which corresponds to a shoulder in the DSC profiles. The latter was not observed in the HT100 sample. The region between 100 and 250 C shows a weight loss which corresponds to the loss of interlayer water, with a first maximum endothermic peak in the DSC profiles centered at approximately 180 C. The dehydroxylation of the brucite-like sheets and the loss of carbonates take place between 250 and 550 C. This 6698
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Figure 5. (a) XPS Zn 2p1/2 and 2p3/2 spectra of the calcined samples. (b) XPS Zn 2p1/2 spectra at BE around 1020 eV.
Figure 4. XPS Fe 2p1/2 and 2p3/2 spectra of the calcined samples.
Table 3. XPS Characteristics of the Fe 2p3/2 and the Zn 2p1/2 Regions in the Calcined Samples samples calcined Fe3þ
a
peak IIa/Fe3þ
binding energy (eV) peak II satellite Fe3þ Zn 2p1/2
HT0
710.6
714.27
718.64
HT25
710.07 713.83
717.99
1019.5
0.45 0.32
HT50
710.07 713.87
718.10
1020.0
0.17
HT75
709.84 713.92
717.85
1019.9
0.41
HT100
710.55 714.09
718.53
1020.8
0.38
Intensity ratio
weight loss is accompanied by a maximum endothermic peak in the DSC profile centered at approximately 370 C. Final thermal decomposition products have shown to be metal oxides as well as mixed oxides and spinel-like species. Endothermic peaks were not detected in the HT100 sample, but an increasing absorption of heat in the whole temperature range analyzed was assigned to the loss of water adsorbed on the surface and to other substances. This indicates the absence of hydroxyl groups in the brucite-like sheets and carbonates in the interlayer, therefore the absence of the LDH phase correlated with the XRD patterns. A decrease in the total weight loss with an increase in the Zn content in the samples was observed (Table 2). 3.4. XPS Analysis. The nature of the active species present on the surface is important for establishing the properties of the catalyst. For this purpose, the XPS analysis has been done in order to obtain information about the surface composition of the MgAlZnFe mixed oxides derived from LDH precursors. Figure 4 shows the Fe 2p XPS spectra for the oxides with equal iron content. All the spectra show the main Fe 2p3/2 peak at BE of around 710.2 ( 0.38 eV, accompanied by a satellite line visible at
BE of around 718.24 ( 0.39 eV, only indicating the presence of Fe3þ cations.3739 Whereas the contribution at 723.9 eV is assigned to Fe 2p1/2. This signal (710 eV) can be decomposed in two contributions, which indicate that the Fe3/2 species exist in more than one chemical state. Most probably, the two chemical states may be related to different coordination environments of the Fe3þ, its tetrahedral or octahedral environment of Fe3þ cations in structure: Fe3þ (tetrahedral sites) at higher binding energy and Fe3þ (octahedral sites) at lower binding energy.23 The peak at 714.05 ( 0.22 eV (peak II) is related to the coordination environment of the Fe3þ cations in spinels phase (tetrahedral sites), the intensity ratio Peak II/Fe3þ is modified with the Zn content (table 3).40 This variation could be attributed to that, when Mg2þ is substituted by Zn2þ ions, this occupies the tetrahedral sites displacing Fe3þ ions toward the octahedral sites of inverse spinel. This phase was not detected by XRD in the HT100 sample; this could be due to their small size (this was also detected by magnetic properties). If Zn continues increasing, the ZnO formation is favored and a fraction of the Fe3þ ions occupies the tetrahedral sites again.41 As it is observed in the HT100 sample, Fe does not compete against Zn for the tetrahedral sites because the latter forms preferably ZnO. These results are the same as the ones observed by XRD. Figure 5 shows the Zn 2p spectra for the catalysts HT25, HT50, HT75, and HT100. The spectra show two bands corresponding to transitions Zn 2p3/2 and Zn 2p1/2 which can be assigned to two possible contributions, such as ZnO or spinel. All the catalysts exhibited a peak centered at approximately 1020.15 ( 0.32 eV. A shift of this peak toward higher binding energy was observed with the increase of Zn content in the samples (Figure 5), indicating a bigger formation of ZnO (1022 eV);42 this was also detected by XRD. 3.5. Measurement of Specific Surface Areas. The specific surface area was determined by the BET method. The values obtained are summarized in Table 1. The specific surface area is directly related with the Zn content; in the partially dehydrated precursor this area increases with the Zn content. This can be associated to the structures observed by DRX; when the Zn content increases, the LDH phase decrease is observed. 6699
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Table 4. Ethylbenzene Dehydrogenation over Different Catalysts at 310 min Time on the Stream samples
Zn (% mol)
performances (%) Xeb
Sst
Sben
Stol
HT0
0
54.7
12.9
0.05
0.13
HT25 HT50
25 50
31.5 24.6
87.1 26.0
0.31 0.13
0.90 0.36
HT75
75
16.3
43.5
0.62
0.56
HT100
100
19.1
12.8
0.10
0.36
a Molar ratio N2/ethylbenzene = 48.6, T = 550 C, GHSV = 9600 h1, 0.1 g of the catalyst. Xeb = ethylbenzene conversion; Sst, Sben, Stol = selectivity in styrene, benzene, and toluene.
Figure 7. Magnetization as function of applied field after the calcination.
Figure 6. Conversion (% mol) of ethylbenzene vs time on stream (min) using catalysts with different Zn content. T = 550 C, P = 1 atm. GHSV = 9600 h1, 0.1 g of the catalyst. (9) HT0, (b) HT25, (2) HT50, (1) HT75, (() HT100.
Samples calcined at 550 C for 9 h showed a decrease in the areas with the increase of the Zn content, which is attributed to the decrease of the Mg content. Therefore it leads to a decrease in the content of MgO responsible for a high surface area together with an amorphous phase which contains aluminum ions.43 This shows that the biggest areas were obtained in the HT0 and HT25, while in the sample without magnesium the smallest area was obtained. In general, the surface area of oxides is higher than their partially dehydrated precursor. However, oxides with a higher Zn content have smaller areas than their partially dehydrated precursor; this can be attributed to the formation of clusters and to the decrease of the Mg content, responsible for a good dispersion of the mixed oxides. 3.6. Inductively Coupled Plasma (ICP). Zn and Fe were determined by inductively coupled plasma (ICP). The Zn percent (keeping in mind only cations M2þ) and Fe (keeping in mind only cations M3þ) of the bulk were calculated and are summarized in Table 1. It is worth noting that the Zn and Fe percent of calcined samples obtained by ICP were similar to the values of the composition synthesis; except in the HT75 that presented an increase of the Zn content and a decrease of the Fe content in the bulk.
3.7. Catalytic Activity. The catalytic activity of the samples calcined at 550 C was assessed in the dehydrogenation of ethylbenzene to styrene in presence of nitrogen. Styrene was the main product of the reaction; however toluene and benzene were also detected in the byproduct condensed; while other products not condensed were deduced based on the stoichiometry of the dehydrogenation (not shown in table 4). Coke also was observed, as it is described later in the text. The results obtained are summarized in Table 4. All the samples tested show a low selectivity toward toluene and benzene. The HT0 sample without Zn content presented the highest ethylbenzene conversion, and after introducing different Zn percentages into the MgAlFe mixed oxide, an increase of the selectivity to styrene was observed except in the HT100 sample. Figure 6 shows the conversion versus time on stream using catalysts with different percentages of Zn in their structure. The initial conversion was similar in all cases; but in HT75 and HT100 the conversion falls after a short time of use. This decrease is presumably due to a spontaneous formation of coke (as indicated by the darker color of reaction mixture). To verify the formation of coke, in one of our experiments we separated the catalyst, and it was dried and heated in air at 550 C. A significant difference in the weight was noted between the catalysts before and after heated. In this manner we corroborated the formation of coke, which oxidized after heating in air. Similar observations were also made earlier in CuMgAl ternary hydrotalcites.44 If the yield of styrene as function of the Zn content is analyzed; the HT25 sample shows the best performance. Comparing these data with those of characterization, one concludes that the presence of Zn as ZnO and zinciron forming part of a spinel-like structure ZnFe2O4 were necessary to improve the activity, but magnesium transmits stability and dispersion to the catalysts. The conversion is strongly dependent on the extent of Zn substitution. 3.8. Magnetic Behavior. The magnetic behavior of mixed oxides as a function of the Zn amount incorporated are quite different. All the samples show a paramagnetic behavior but HT0 and HT25 (samples without Zn and with the least amount of Zn incorporated) show a similar superparamagnetic behavior. The M vs H curves are shown in Figure 7, samples HT50, HT100, and HT75 are typical curves for paramagnetic systems where 6700
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paramagnetic response decreases according with the observed formation of zinc oxide.
Figure 8. Hysteresis loop after subtracting the paramagnetic contribution. Inset graph is a zoom view showing coercitivities less than 100 Oe.
Figure 9. Paramagnetic contribution of the hysteresis loop. Inset graph shows susceptibility as function of Zn content in the samples.
M = χ 3 H. The HT0 and HT25 curves, also exhibit a paramagnetic contribution, since the curves do not saturate. Figure 8 shows the ferromagnetic behavior of HT0 and HT25 samples after subtraction of paramagnetic contribution from the whole hysteresis loop. Both of them have similar ferromagnetic response mainly due to the contribution of spinel MgFe2O4, ZnFe2O4 particles which were detected by XRD diffraction (Figure 2). The last magnetization measurements obviously show a typically superparamagnetic behavior, indicating a small particle size, those samples show the highest surface areas, which could be indicating a good dispersion of the particles on the MgAl matrix. However, the saturation magnetization decreases with increasing the Zn content.45 Therefore, the paramagnetic contribution of the hysteresis loop is shown in Figure 9; in the inset graph susceptibility as function of the Zn content in the samples is shown and the corresponding susceptibility (χ). The addition of a small quantity of Zn increases the value of paramagnetic response (χ) but overcoming a threshold quantity of Zn incorporated, the
4. CONCLUSION MgAlZnFe LDH were successfully prepared by coprecipitation reaction with hydrothermal treatment. Their mixed oxides have good catalytic effect on ethylbenzene dehydrogenation. In all the cases the hydrotalcite phase was detected by XRD, except in the sample without magnesium (HT100), which is consistent with the low weight loss and weak intensity of endothermic peaks shown in TGA and DSC profiles. By XRD and UVvis-DR of precursors, the ZnO phase was detected which was increasing with the Zn content. When the samples were calcined at 550 C, MgO, MgFe2O4, ZnFe2O4, Fe2O3, and ZnO phases were observed, the last phase is increased with Zn content in the samples. The XPS analysis shows that the Fe3þ ion forms can be found as two coordination environment as mixed oxides, and as spinel like-structure. The synthesized samples show a high selectivity to styrene, decreasing with the increase of Zn content. The catalytic activity can be attributed to the Zn content in each sample; the best catalytic activities were obtained with the sample that contained 25% of Zn (19% by ICP). This sample presents a greater percentage of Fe3þ octahedral sites; showing that the Mg substituted by Zn favors thus Fe3þ as octahedral sites more than Fe3þ in tetrahedral sites. It can be proposed that a magnesium percent contributes in a significant extent to the dispersion of entities of ZnO and ZnFe2O4 phase on the surface of the calcined samples. The magnetic behavior of the mixed oxide was evaluated by conventional hysteresis measurement at room temperature. The samples HT0 and HT25 show similar paramagnetic and ferromagnetic contributions. The samples HT50, HT75, HT100, show only paramagnetic response. Analyzing each of the samples, an improvement on the ethylbenzene conversion with an increase in the spinel phase and the surface area was observed while the superparamagnetic behavior is favored. Therefore, the superparamagnetic response of the samples may be correlated with the catalytic conversion which provides a fast and inexpensive method to use as a first step for selecting possible materials with good catalytic performance. ’ AUTHOR INFORMATION Corresponding Author
*Tel./Fax: 054-351-4690585. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the UTN-FRC of Argentina. We thank UTN for doctoral fellowship for A.C.H. We thank geol. Julio D. Fernandez (UTN-FRC, Cordoba, Argentina) for help in recording specific surface area data. ’ REFERENCES (1) Cavani, F.; Trifiro, F.; Vaccari, A. Hydrotalcite-type anionic clays: Preparation, properties and applications. Catal. Today 1991, 11, 173. (2) Rives, V.; Prieto, O.; Dubey, A.; Kannan, S. Synergistic effect in the hydroxylation of phenol over CoNiAl ternary hydrotalcites. J. Catal. 2003, 220, 161. 6701
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