- Email: [email protected]
Synthesis, characterization and catalytic application for ethylbenzene dehydrogenation of an iron pillared clay
Microporous and Mesoporous Materials 57 (2003) 219–227 www.elsevier.com/locate/micromeso
Synthesis, characterization and catalytic application for ethylbenzene dehydrogenation of an iron pillared clay Lenin Huerta
a,*
, Alfredo Meyer a, Eduardo Choren
b
a
b
Departamento de Quımica, Facultad de Ciencias, Universidad del Zulia, Maracaibo 526, Venezuela Facultad de Ingenierıa, Centro de Superficies y Cat alisis (CESUC), Universidad del Zulia, Maracaibo 526, Venezuela Received 6 January 2002; accepted 24 October 2002
Abstract New iron pillared clays were synthesized from a smectite sample collected from a Venezuelan clay bed. An oxocentered trinuclear iron (III) acetate aquo complex was chosen as an intercalating cation. The obtained solids were characterized by XRD, nitrogen physisorption (specific surface area and pore size distribution), chemical composition, thermogravimetric analysis, differential thermal analysis, and scanning electron microscopy. The obtained pillared clays . The showed specific surface areas ranging from 118 to 232 m2 /g, and interlayer distances between 17.09 and 18.31 A clays were thermally stable and catalytically active in the dehydrogenation of ethylbenzene to styrene. Ó 2002 Elsevier Science Inc. All rights reserved. Keywords: Iron; Pillared; Clay; Smectite; Dehydrogenation
1. Introduction Microporous solids synthesized by intercalating voluminous metallic complexes between structural layers of swelling clays, such as smectites, are an ongoing subject for research in heterogeneous catalysts, adsorption, separation, and so on. Pillared clays obtained by replacing natural cations with voluminous iron complexes have been the subject of several studies carried out in the past few years [1–6]. An extensive review on recent advances in the synthesis and catalytic applica-
*
Corresponding author. Tel./fax: +58-261-759-8125. E-mail addresses: [email protected], [email protected] (L. Huerta).
tions of pillared clays has been recently published by Gil et al. [7]. It is well-known that iron catalyzes reactions of industrial interest such as ethylbenzene dehydrogenation to styrene [8]. Usual plant operation yields 70% conversion with a 100% selectivity to styrene. In these studies, a mixture of ethylbenzene and steam is fed to the reactor at 630 °C. Steam is added for various purposes: (i) it is the medium for supplying heat to the endothermic dehydrogenation, (ii) it is a diluent of ethylbenzene to increase equilibrium conversion, and (iii) it avoids coke deposition on the catalyst. Recent investigations demonstrated that substitution of steam by CO2 provided a more economic process from an energetic point of view [9]. There is a constant interest in the search of new catalysts or new reactions for obtaining styrene in
1387-1811/02/$ - see front matter Ó 2002 Elsevier Science Inc. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 2 ) 0 0 5 9 3 - 0
220
L. Huerta et al. / Microporous and Mesoporous Materials 57 (2003) 219–227
the polymer industry [10–14]. Cavani and Trifiro [15] have published a detailed review on the existing catalytic alternatives for the production of styrene from ethylbenzene, as well as the possible mechanisms. In a recent study, Weiss and Schlogl [16], based on spectroscopic results over Fe3 O4 (1 1 1) and Fe2 O3 (0 0 0 1) films, determined that ethylbenzene and styrene adsorb onto regular terrace sites with the phenyl rings oriented parallel to the surface, where the p-electron systems interact with exposed Lewis acid iron sites. The ethyl group is dehydrogenated at Br€ onsted basic oxygen sites located at surface defects, and the coupling of the phenyl ring to Fe3þ terrace sites determines the reactant adsorption–desorption kinetics. In the present work a parent clay from a Venezuelan bed was used to synthesize an iron pillared clay which was later characterized, and tested as ethylbenzene dehydrogenation catalyst to obtain styrene.
2. Experimental 2.1. Iron complex The oxo-centered trinuclear iron (III) acetate aquo complex (Fig. 1) was obtained following the procedure described by Yamanaka and Hattori [17]. This complex was previously characterized by infrared spectroscopy and chemical analysis in our previous work [5]. The analyses demonstrated that the synthesized complex is of high purity. In the present work, thermal analysis is also performed. 2.2. Parent clay The clay used for the preparation of the pillared samples came from a bed located in Cojedes, Venezuela. This clay was ground and sieved to obtain a first fraction of 44 lm (labeled as T0), which was later treated with Na2 CO3 , followed by ultrasonic dismembering, filtering, and successive stages of centrifugation and sedimentation until a pure clay fraction was obtained, with a particle size of approximately 2 lm (labeled as T1). The cationic exchange capacity (CEC) of T1 was determined using the micro-Kjeldalhs method [18],
Fig. 1. Schematic illustration of trinuclear ion ½Fe3 OðOCOCH3 Þ6 3H2 OÞþ .
and results are expressed as cation meq/100 g of clay. The chemical composition of the clay was determined by atomic absorption spectroscopy (AA) and energy dispersive X-ray microanalysis (EDX). For AA, a Perkin Elmer spectrometer model 3100, was used. The clay was first dried for 1 h at 110 °C. Then a 0.1 g sample was mixed with a 0.5 g sample of lithium metaborate. This mixture was placed on a platinum melting dish and melted at 1000 °C for 20 min. The pearl thus obtained was dissolved in 25 ml of nitric acid (1:25), filtered and diluted with water up to 100 ml. After that a adequate dilution of a mixture sample was performed to reach readable concentrations by the equipment used. 2.3. Synthesis of pillared clays Three samples of pillared clays from T1 were prepared. Each sample was dispersed (1%) in distilled water for 30 min. A solution containing the cationic ion complex was then slowly added in a quantity equal to 1, 5, and 10 times the CEC of the clay. The solutions were stirred for about 3 h at 25 °C, which was long enough to reach an exchange equilibrium. Once the cationic exchange
L. Huerta et al. / Microporous and Mesoporous Materials 57 (2003) 219–227
had ended, a flocculating agent (NaCl) was added to the system. The intercalated clay was filtered by suction, washed with distilled water, and dried in an air flow at 40 °C for approximately 12 h. Finally, the clay was heated from room temperature to 450 °C at 7 °C/min, and maintained at this temperature for 4 h. These solids were labeled PT11, PT12, and PT13 respectively. 2.4. Solid characterization 2.4.1. X-ray diffraction XRD analyses were carried out with a Philips PW 1130 diffractometer using a Co Ka radiation source. The fine powder sample was dispersed in acetone and placed on a glass slide. The analysis was performed from 2° to 40° 2H. 2.4.2. Area and pore size distribution The specific surface area and the pore size distribution were determined by nitrogen adsorption and desorption isotherms at 77 K using a Cahn microbalance connected to a vacuum line. BET and Conway Pierce equations were applied to determine specific surface area and pore size distribution respectively. 2.4.3. Thermal studies Thermogravimetric analysis (TGA) was performed using a Cahn 200 microbalance coupled to a glass line. A 150 mg sample was placed on the dish and heated from 25 to 500 °C at 10 °C/min with a nitrogen flow of 30 ml/min. Differential thermal analysis (DTA) was carried out using a Texas Instruments thermal analyzer, with a coupled DTA module. Approximately 7 mg of both the sample and the reference (a-alumina) were weighed and placed on platinum vials. The samples were then heated from 25 to 500 °C at 10 °C/ min with a helium flow of 30 ml/min. The parent clay was heated to 1200 °C using the same conditions. 2.4.4. Scanning electron microscopy and energy dispersive X-ray microanalysis Scanning electron microscopy (SEM) was performed using a Philips XL 30 electronic mi-
221
croscope with an EDAX energy dispersive X-ray microanalyzer (EDX) coupled to it. Microscopy test conditions are shown in each microphotograph. EDX analyses were carried out using the least possible magnification that would allow visualization of the maximum amount of sample being studied, on 100 s. 2.4.5. Catalytic test Catalytic activity of T1 and PT12 for the dehydrogenation of ethylbenzene reaction was studied. PT12 was chosen due to its higher ratio of iron to amount of intercalating complex needed during synthesis, when compared to the other two pillared clays, the saturation of the clay was reached with this sample. The reaction was carried out at 410 °C in a glass microreactor, using 160 mg of catalyst. The reactor was purged with N2 during preheating for 30 min at reaction temperature before running the test. Ethylbenzene was then fed to the microreactor from a saturator kept at 65 °C using nitrogen as a carrier gas (W =F ¼ 0:01 g min/ml) over 60 min. Reaction products of the reaction were injected into a Perkin Elmer AutoSystem XL gas chromatograph, which was equipped with a flame ionization detector and a Carbowax 20 M fused silica capillar column (30 m 0:53 mm), every 5 min, via a gas valve. The oven temperature was 70 °C and nitrogen flow was 25 ml/min.
3. Results and discussion 3.1. Textural and chemical characterization of parent clays and PILCs Table 1 shows the textural characteristics of T0 and T1 clay fractions, and CEC. Both fractions have a very similar texture, varying only in the particle size and CEC. CEC variation is explained because T0 fraction contains non-argillaceous impurities (mainly quartz), which diminished the CEC of a specific sample weight, when compared to an equivalent weight of T1, which is a pure clay. During the CEC process, chemical analysis of the remnant solution showed that Mg2þ is the main
222
L. Huerta et al. / Microporous and Mesoporous Materials 57 (2003) 219–227
Table 1 Textural characteristic of the solids, iron complex and flocculating agent concentration in the synthesis Solid
Particle size (lm)
CEC (meq/100g)
T0 T1 PT11 PT12 PT13
44 2
70.80 76.05
Complex concentration (times CEC)
1 5 10
interlayer cation in T1. Traces of Ca2þ and Naþ were also detected. Table 1 also shows the textural properties of synthesized PILCs in regard to the concentrations of intercalating complex. There is an increase of the specific surface area following increases of complex concentration, reaching up to threefold increase of the parent clay area. The basal space was doubled when the complex concentration was increased to 5 CEC, which implies the formation of interlayer space of approximately . An additional increase in the complex 8.7 A concentration from 5 to 10 CEC resulted in an 80 m2 /g area increase, although the basal space remained basically the same. This last result indicates a slight increase in pillar formation. Table 2 shows the chemical composition of T1 clay fraction, as determined by EDX and AA, and the chemical composition of T0 fraction by EDX in a study with a backscattering electron (BSE) detector.
Table 2 Normalized chemical composition (wt.%) of T1 fraction (AA and EDX) and T0 fraction (only EDX). BSE characteristics of T0 fraction is also included T1 fraction
T0 fraction
Element
AA
EDX
BSE gray zone (EDX)
BSE clear zone (EDX)
Mg Al Si Ca Fe Cr Ti Na Si/Al
4.81 15.93 50.79 0.06 27.39 ND 0.92 0.09 3.19
5.15 15.45 53.12 – 26.27 Traces Traces Traces 3.44
5.51 14.76 60.00 – 18.42 0.48 0.84 – 4.07
4.75 9.85 29.80 – 53.40 1.16 1.04 – 3.03
NaCl (wt.%)
Area (m2 /g)
d(0 0 1) ) (A
0.25 0.25 0.50
73 75 118 150 232
9.94 9.94 17.09 18.65 18.31
Differences in the AA and EDX values for T1 fraction are due to the fact that AA shows the composition of total mass of the solid, whereas EDX is a semi quantitative analysis that shows the composition in a specific region at a depth . between 5000 and 10 000 A The Si/Al ratio encountered is typical of silica aluminates. The T1 fraction displayed a high percentage of iron, which must be part of the solid structure by isomorphic substitution of Al because it was not found in the remnant solution of cationic exchange performed for CEC determination. With the aid of the BSE detector, it was possible to perform a study on the chemical composition of different particles observed by SEM in the T0 fraction. The corresponding microphotograph is shown later in the SEM section. As shown in Table 2, the clear area observed in T0 has high percentages of iron, titanium and chromium, which indicates that this is a non-argillaceous zone. The Si/Al relationship in this zone is typical of clay materials. This relationship could be due more to the influences of the surrounding area than of the zone itself. The gray area observed in T0 shows a composition similar to that found in T1, which reveals its argillaceous nature. Table 3 shows the chemical composition of pillared clays obtained by different iron complex concentrations. It is clearly shown that with an iron complex concentration equal to the CEC, the amount of iron present in the solid increase 1.7 times in relation to the initial iron content of T1. The clay was practically saturated with the complex at a concentration of 5 times the CEC and would not incorporate any more iron, even though the concentration was increased up to 10 times the CEC.
L. Huerta et al. / Microporous and Mesoporous Materials 57 (2003) 219–227
223
Table 3 Iron complex concentration and chemical composition of obtained solids (wt.% by EDX) Solid
Complex concentration
Mg
Al
Si
Ti
Cr
Fe
PT11 PT12 PT13
1 CEC 5 CEC 10 CEC
4.13 4.19 2.31
12.85 11.82 11.27
34.43 31.86 34.08
Traces Traces Traces
Traces Traces Traces
48.60 52.14 52.34
3.2. Thermal analyses 3.2.1. Iron complex Fig. 2 shows TGA and DTA of the complex. The TGA diagram presents three decomposition ramps with very high and increasing slopes. Minima in the first derivative appear at around 160, 240, and 280 °C. However, the almost absence of an intermediate plateaux, along with the known tendency of iron complexes to decompose at low temperature, suggests an almost steady complex decomposition, with no generation of stable intermediate compounds. DTA only shows two im-
Fig. 2. TGA and DTA of the iron complex.
portant peaks: one endothermic, which might correspond to evaporation of solvent that could have been occluded in the crystals of the solid, and the other exothermic at 305 °C, which corresponds to final decomposition of the complex to iron oxyhydroxides and was signaled by a reddish NO2 vapor emission. Eventual temperature differences for both the DTA and the TGA diagrams are most likely attributable to mass effects, whereas about 150 mg were used for TGA, the mass employed for DTA tests was just 4–7 mg, generating different condition for mass and heat transfer [19]. 3.2.2. Clay and pillared clays Fig. 3 shows TGA and DTA diagrams of T1 fraction, with the temperature differences by the mass effect already cited. In the TGA diagram, the first weight loss appears between 90 and 200 °C with a minimum in the first derivative occursing at 145 °C. This minimun corresponds to the loss of adsorbed water by the mineral and solvation water from interlayer cations, mainly Mg2þ , as already noted. These losses correspond to an endothermic peak at 90 °C in the DTA. At 200 °C, the slope (TGA) becomes less pronounced, hygroscopic water has evaporated and the main source of weight loss is due to water formation by reaction among structural hydroxyl groups. A negative slope (endothermic) in the DTA occurs at 480 °C accompanies this loss. In the DTA high temperature region, an exothermic peak associated with crystalline reordering and interlamellar space contraction is observed at 775 °C [20]. The DTA diagram shows two other pronounced endothermic peaks at 975 and 1030 °C, attributed to the complete destruction of clay layers with a vitreous structure formation [21]. Fig. 4 shows TGA and DTA diagrams for noncalcined (PT13 precursor) and calcined (PT13)
224
L. Huerta et al. / Microporous and Mesoporous Materials 57 (2003) 219–227
Fig. 3. TGA and DTA of the T1 clay fraction.
pillared clay. Complex breakdown inside the clay layers now occurs between 300 and 400 °C. This higher temperature needed to decompose the iron complex is explained by the ‘‘sheltering’’ effect on this complex due to surrounding clay layers. The TGA for the calcined solid has a single weight loss between 90 and 150 °C, accompanied by a drop in the DTA base line, which shows a small endothermic shoulder this is due to loss of adsorbed water. A similar endothermic shoulder, which is also observed in the DTA of the non-calcined solid. 3.2.3. Pore size distribution Figs. 5–7 show nitrogen physisorption isotherms at 77 K and pore size distribution for T1, PT12 and PT13, respectively. For calculations Dollimore Heal, Crunston Inkley and Conway Pierce methods were used, with the aid of a software specifically designed for this purpose [22]. The Conway Pierce method was selected because it showed less deviation from specific surface area, calculated by the sum of pore volumes, in comparison to the surface area calculated by the BET method. In the inserts of these three figures it can be seen that these isotherms are type IV and present type B hysteresis curves, as expected for parallel-wall layered structures with open groove [21].
According to results obtained by XRD and ), knowing the thickness of the clay layer (9.60 A would be expected an interlayer distance of 5.26 A for PT12 for non-calcined T1 (14.86–9.6), 9.05 A for PT13 (18.31–9.6). Al(18.65–9.6) and 8.71 A though the pore size interval here determined is higher than the expected from the XRD analysis, this result is a good indication of the porosity of these solids and how the pillaring process affects it. It can be observed that, for all the three solids, the largest amount of pores is in the micropore range. The T1 fraction present a portion of meso. Solids pores of approximately 23, 32, and 60 A PT12 and PT13 present a pore distribution similar to T1, but with a significant increase in the amount of micropores (up to 5 times) for PT12. By reviewing the adsorption isotherm for PT13 (high adsorption at low pressures), it can be seen that this solid has a very high surface area (232 m2 / g) due to its great amount of micropores (Figs. 4–6). 3.2.4. SEM Fig. 8 shows a BSE microphotograph of the T0 fraction. Easily observable are the numerous gray grains of clay along with clear grains that proved to be iron as presented in Table 3.
L. Huerta et al. / Microporous and Mesoporous Materials 57 (2003) 219–227
225
Fig. 5. T1 clay fraction pore size distribution.
Fig. 6. PT12 pillared clay pore size distribution.
Fig. 4. TGA and DTA of the non-calcined and calcined PT13 pillared clay.
Fig. 9 shows a SEM microphotograph of T1 fraction. Although it is unusual to refer to particle
Fig. 7. PT13 pillared clay pore size distribution.
size using this technique, it could be said that for T1, the fraction size range is between 2 and 20 lm. Fig. 10 shows a SEM microphotograph of PT13, in which flake formation and particle size
226
L. Huerta et al. / Microporous and Mesoporous Materials 57 (2003) 219–227
Fig. 10. PT13 solid microphotography by SEM. Fig. 8. T0 clay fraction microphotography by BSE.
Fig. 9. T1 clay fraction microphotography by SEM.
distribution similar to that of the original clay can be observed. 3.2.5. Catalytic test Fig. 11 shows the catalytic results obtained during the ethylbenzene dehydrogenation on T1 and PT12. Convertion of ethylbenzene to styrene can be calculated with this figure by: 100%–% ethylbenzene, and selectivity to styrene can be calculated by: % styrene/(% styrene þ % benzene). T1 showed a maximum styrene production of approximately 12 wt.% at 18 min of reaction, whereas pillared clay (PT12) showed a 18 wt.% at 22 min. PT12 also showed a higher production of benzene from the cracking of ethylbenzene than T1 fraction.
Fig. 11. Catalytic results of T1 clay fraction and PT12 pillared clay for ethylbenzene dehydrogenation.
With the purpose to determine if the increase in styrene production was due only to a surface area effect or to the iron oxide pillars, it was synthesized a sample of PILC-Al from the same smectite T1 by the method reported in our previous work [23]. This sample of PILC-Al showed a basal space of and a specific surface area of 152 m2 /g, 17.99 A very similar to the area obtained for PT12. Catalytic test for this sample of PILC-Al, at the same experimental conditions used for T1 and PT12, yielded a maximum styrene production of 3.78
L. Huerta et al. / Microporous and Mesoporous Materials 57 (2003) 219–227
wt.% at 28 min of reaction. Thus, it can be concluded that iron oxide pillars in PT12 are responsible for the increase of catalytic activity in ethylbenzene dehydrogenation. In this way, the production of styrene in both solids is achieved by oxidative dehydrogenation in accordance with the results obtained in previous work by Perez et al. [8]. Since the actual industrial process for ethylbenzene dehydrogenation is fed with a mixture of steam and ethylbenzene, it will be necessary for further studies under conditions similar to those used industrially.
4. Conclusions An iron pillared clay with five more times the number of micropores and twice the specific surface area existing in the original clay was synthesized. The interlayer distance was also increased. Thermal studies showed that the solid is stable and therefore may be used as a heterogeneous catalyst in reactions in which iron oxide is the catalyst. Pillaring process with iron significantly increased (50%) the capacity of the clay for obtaining styrene through ethylbenzene dehydrogenation. An increase in the production of benzene as by product was also observed, due to the solidÕs higher cracking activity.
Acknowledgements The authors would like to thank Consejo de Desarrollo Cientıfico y Humanıstico (CONDES) of Universidad del Zulia for supporting this project, Centro de Corrosi on of the Facultad de Ingenierıa for their assistance with electron microscopy and EDX studies, and the company
227
Schlumberger from Ciudad Ojeda, for their collaboration in the XRD analyses.
References [1] M. Occelli, J. Stencel, S. Suib, J. Mol. Catal. 64 (1991) 221. [2] E. Rightor, M.S. Tzou, T. Pinnavaia, J. Catal. 130 (1991) 29. [3] G. Vitale, Preparaci on y caracterizaci on de pilares de hierro y su uso en la hidrogenaci on de mon oxido de carbono, Trabajo Especial de Grado de Licenciatura en Quımica, Facultad de Ciencias, Universidad Central de Venezuela, Caracas, Venezuela, 1992, pp. 56–60. [4] R. Yang, J. Chen, E. Kikkinides, L. Cheng, J. Chichanowicz, Ind. Eng. Chem. Res. 31 (1992) 1440. [5] A. Meyer, L. Huerta, C. De La Cruz, Ciencia 2 (1994) 97. [6] B. Choudary, M. Kanton, M. Satessh, K. Rao, P. Santhi, Appl. Catal. A 149 (1997) 257. [7] A. Gil, L. Gandıa, M. Vicente, Catal. Rev. Sci. Eng. 42 (2000) 145. [8] G. Perez, A. De Stefanis, A. Tomlinson, J. Mater. Chem. 7 (1997) 351. [9] N. Mimura, I. Takahara, M. Saito, T. Hattori, K. Ohkuma, M. Ando, Catal. Today 45 (1998) 61. [10] C. Franco, J. Angulo, M. Maury, Rev. Soc. Ven. Cat. 2 (1988) 34. [11] M. Sugino, H. Shimada, T. Turuda, H. Miura, N. Ikenaga, T. Suzuki, Appl. Catal. A 121 (1995) 125. [12] W. Oganowski, J. Hanuza, L. Kepiski, Appl. Catal. A 171 (1998) 145. [13] R. Newman, I. Dror, Appl. Catal. A 172 (1998) 67. [14] Jr. Hamilton, D. Morris, US Patent 5,689,023, 1997. [15] F. Cavani, F. Trifiro, Appl. Catal. A 133 (1995) 219. [16] W. Weiss, R. Schlogl, Topics Catal. 13 (2000) 75. [17] S. Yamanaka, M. Hattori, Catal. Today 2 (1988) 261. [18] R. Dinauer, Methods of Soil Analysis, American Society of Agronomy, 1965, pp. 897–902. [19] M. Schilling, Clays Clay Miner. 38 (1990) 556. [20] E. Choren, A. Moronta, G. Varela, A. Arteaga, J. Sanchez, Clays Clay Miner. 45 (1997) 213. [21] R. Grim, Clay Mineralogy, McGraw Hill, New York, 1953, p. 378. [22] E. Contreras, Area Especıfica y distribuci on de tama~ no de poro en s olidos porosos, Trabajo Especial de Grado de Maestrıa en Ingenierıa Quımica, Facultad de Ingenierıa, Universidad del Zulia, Maracaibo, Venezuela, 1992, pp. 72–78. [23] J. Ochoa, A. Meyer, Ciencia 6 (1998) 51.
Synthesis, characterization and catalytic application for ethylbenzene dehydrogenation of an iron pillared clay Lenin Huerta
a,*
, Alfredo Meyer a, Eduardo Choren
b
a
b
Departamento de Quımica, Facultad de Ciencias, Universidad del Zulia, Maracaibo 526, Venezuela Facultad de Ingenierıa, Centro de Superficies y Cat alisis (CESUC), Universidad del Zulia, Maracaibo 526, Venezuela Received 6 January 2002; accepted 24 October 2002
Abstract New iron pillared clays were synthesized from a smectite sample collected from a Venezuelan clay bed. An oxocentered trinuclear iron (III) acetate aquo complex was chosen as an intercalating cation. The obtained solids were characterized by XRD, nitrogen physisorption (specific surface area and pore size distribution), chemical composition, thermogravimetric analysis, differential thermal analysis, and scanning electron microscopy. The obtained pillared clays . The showed specific surface areas ranging from 118 to 232 m2 /g, and interlayer distances between 17.09 and 18.31 A clays were thermally stable and catalytically active in the dehydrogenation of ethylbenzene to styrene. Ó 2002 Elsevier Science Inc. All rights reserved. Keywords: Iron; Pillared; Clay; Smectite; Dehydrogenation
1. Introduction Microporous solids synthesized by intercalating voluminous metallic complexes between structural layers of swelling clays, such as smectites, are an ongoing subject for research in heterogeneous catalysts, adsorption, separation, and so on. Pillared clays obtained by replacing natural cations with voluminous iron complexes have been the subject of several studies carried out in the past few years [1–6]. An extensive review on recent advances in the synthesis and catalytic applica-
*
Corresponding author. Tel./fax: +58-261-759-8125. E-mail addresses: [email protected], [email protected] (L. Huerta).
tions of pillared clays has been recently published by Gil et al. [7]. It is well-known that iron catalyzes reactions of industrial interest such as ethylbenzene dehydrogenation to styrene [8]. Usual plant operation yields 70% conversion with a 100% selectivity to styrene. In these studies, a mixture of ethylbenzene and steam is fed to the reactor at 630 °C. Steam is added for various purposes: (i) it is the medium for supplying heat to the endothermic dehydrogenation, (ii) it is a diluent of ethylbenzene to increase equilibrium conversion, and (iii) it avoids coke deposition on the catalyst. Recent investigations demonstrated that substitution of steam by CO2 provided a more economic process from an energetic point of view [9]. There is a constant interest in the search of new catalysts or new reactions for obtaining styrene in
1387-1811/02/$ - see front matter Ó 2002 Elsevier Science Inc. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 2 ) 0 0 5 9 3 - 0
220
L. Huerta et al. / Microporous and Mesoporous Materials 57 (2003) 219–227
the polymer industry [10–14]. Cavani and Trifiro [15] have published a detailed review on the existing catalytic alternatives for the production of styrene from ethylbenzene, as well as the possible mechanisms. In a recent study, Weiss and Schlogl [16], based on spectroscopic results over Fe3 O4 (1 1 1) and Fe2 O3 (0 0 0 1) films, determined that ethylbenzene and styrene adsorb onto regular terrace sites with the phenyl rings oriented parallel to the surface, where the p-electron systems interact with exposed Lewis acid iron sites. The ethyl group is dehydrogenated at Br€ onsted basic oxygen sites located at surface defects, and the coupling of the phenyl ring to Fe3þ terrace sites determines the reactant adsorption–desorption kinetics. In the present work a parent clay from a Venezuelan bed was used to synthesize an iron pillared clay which was later characterized, and tested as ethylbenzene dehydrogenation catalyst to obtain styrene.
2. Experimental 2.1. Iron complex The oxo-centered trinuclear iron (III) acetate aquo complex (Fig. 1) was obtained following the procedure described by Yamanaka and Hattori [17]. This complex was previously characterized by infrared spectroscopy and chemical analysis in our previous work [5]. The analyses demonstrated that the synthesized complex is of high purity. In the present work, thermal analysis is also performed. 2.2. Parent clay The clay used for the preparation of the pillared samples came from a bed located in Cojedes, Venezuela. This clay was ground and sieved to obtain a first fraction of 44 lm (labeled as T0), which was later treated with Na2 CO3 , followed by ultrasonic dismembering, filtering, and successive stages of centrifugation and sedimentation until a pure clay fraction was obtained, with a particle size of approximately 2 lm (labeled as T1). The cationic exchange capacity (CEC) of T1 was determined using the micro-Kjeldalhs method [18],
Fig. 1. Schematic illustration of trinuclear ion ½Fe3 OðOCOCH3 Þ6 3H2 OÞþ .
and results are expressed as cation meq/100 g of clay. The chemical composition of the clay was determined by atomic absorption spectroscopy (AA) and energy dispersive X-ray microanalysis (EDX). For AA, a Perkin Elmer spectrometer model 3100, was used. The clay was first dried for 1 h at 110 °C. Then a 0.1 g sample was mixed with a 0.5 g sample of lithium metaborate. This mixture was placed on a platinum melting dish and melted at 1000 °C for 20 min. The pearl thus obtained was dissolved in 25 ml of nitric acid (1:25), filtered and diluted with water up to 100 ml. After that a adequate dilution of a mixture sample was performed to reach readable concentrations by the equipment used. 2.3. Synthesis of pillared clays Three samples of pillared clays from T1 were prepared. Each sample was dispersed (1%) in distilled water for 30 min. A solution containing the cationic ion complex was then slowly added in a quantity equal to 1, 5, and 10 times the CEC of the clay. The solutions were stirred for about 3 h at 25 °C, which was long enough to reach an exchange equilibrium. Once the cationic exchange
L. Huerta et al. / Microporous and Mesoporous Materials 57 (2003) 219–227
had ended, a flocculating agent (NaCl) was added to the system. The intercalated clay was filtered by suction, washed with distilled water, and dried in an air flow at 40 °C for approximately 12 h. Finally, the clay was heated from room temperature to 450 °C at 7 °C/min, and maintained at this temperature for 4 h. These solids were labeled PT11, PT12, and PT13 respectively. 2.4. Solid characterization 2.4.1. X-ray diffraction XRD analyses were carried out with a Philips PW 1130 diffractometer using a Co Ka radiation source. The fine powder sample was dispersed in acetone and placed on a glass slide. The analysis was performed from 2° to 40° 2H. 2.4.2. Area and pore size distribution The specific surface area and the pore size distribution were determined by nitrogen adsorption and desorption isotherms at 77 K using a Cahn microbalance connected to a vacuum line. BET and Conway Pierce equations were applied to determine specific surface area and pore size distribution respectively. 2.4.3. Thermal studies Thermogravimetric analysis (TGA) was performed using a Cahn 200 microbalance coupled to a glass line. A 150 mg sample was placed on the dish and heated from 25 to 500 °C at 10 °C/min with a nitrogen flow of 30 ml/min. Differential thermal analysis (DTA) was carried out using a Texas Instruments thermal analyzer, with a coupled DTA module. Approximately 7 mg of both the sample and the reference (a-alumina) were weighed and placed on platinum vials. The samples were then heated from 25 to 500 °C at 10 °C/ min with a helium flow of 30 ml/min. The parent clay was heated to 1200 °C using the same conditions. 2.4.4. Scanning electron microscopy and energy dispersive X-ray microanalysis Scanning electron microscopy (SEM) was performed using a Philips XL 30 electronic mi-
221
croscope with an EDAX energy dispersive X-ray microanalyzer (EDX) coupled to it. Microscopy test conditions are shown in each microphotograph. EDX analyses were carried out using the least possible magnification that would allow visualization of the maximum amount of sample being studied, on 100 s. 2.4.5. Catalytic test Catalytic activity of T1 and PT12 for the dehydrogenation of ethylbenzene reaction was studied. PT12 was chosen due to its higher ratio of iron to amount of intercalating complex needed during synthesis, when compared to the other two pillared clays, the saturation of the clay was reached with this sample. The reaction was carried out at 410 °C in a glass microreactor, using 160 mg of catalyst. The reactor was purged with N2 during preheating for 30 min at reaction temperature before running the test. Ethylbenzene was then fed to the microreactor from a saturator kept at 65 °C using nitrogen as a carrier gas (W =F ¼ 0:01 g min/ml) over 60 min. Reaction products of the reaction were injected into a Perkin Elmer AutoSystem XL gas chromatograph, which was equipped with a flame ionization detector and a Carbowax 20 M fused silica capillar column (30 m 0:53 mm), every 5 min, via a gas valve. The oven temperature was 70 °C and nitrogen flow was 25 ml/min.
3. Results and discussion 3.1. Textural and chemical characterization of parent clays and PILCs Table 1 shows the textural characteristics of T0 and T1 clay fractions, and CEC. Both fractions have a very similar texture, varying only in the particle size and CEC. CEC variation is explained because T0 fraction contains non-argillaceous impurities (mainly quartz), which diminished the CEC of a specific sample weight, when compared to an equivalent weight of T1, which is a pure clay. During the CEC process, chemical analysis of the remnant solution showed that Mg2þ is the main
222
L. Huerta et al. / Microporous and Mesoporous Materials 57 (2003) 219–227
Table 1 Textural characteristic of the solids, iron complex and flocculating agent concentration in the synthesis Solid
Particle size (lm)
CEC (meq/100g)
T0 T1 PT11 PT12 PT13
44 2
70.80 76.05
Complex concentration (times CEC)
1 5 10
interlayer cation in T1. Traces of Ca2þ and Naþ were also detected. Table 1 also shows the textural properties of synthesized PILCs in regard to the concentrations of intercalating complex. There is an increase of the specific surface area following increases of complex concentration, reaching up to threefold increase of the parent clay area. The basal space was doubled when the complex concentration was increased to 5 CEC, which implies the formation of interlayer space of approximately . An additional increase in the complex 8.7 A concentration from 5 to 10 CEC resulted in an 80 m2 /g area increase, although the basal space remained basically the same. This last result indicates a slight increase in pillar formation. Table 2 shows the chemical composition of T1 clay fraction, as determined by EDX and AA, and the chemical composition of T0 fraction by EDX in a study with a backscattering electron (BSE) detector.
Table 2 Normalized chemical composition (wt.%) of T1 fraction (AA and EDX) and T0 fraction (only EDX). BSE characteristics of T0 fraction is also included T1 fraction
T0 fraction
Element
AA
EDX
BSE gray zone (EDX)
BSE clear zone (EDX)
Mg Al Si Ca Fe Cr Ti Na Si/Al
4.81 15.93 50.79 0.06 27.39 ND 0.92 0.09 3.19
5.15 15.45 53.12 – 26.27 Traces Traces Traces 3.44
5.51 14.76 60.00 – 18.42 0.48 0.84 – 4.07
4.75 9.85 29.80 – 53.40 1.16 1.04 – 3.03
NaCl (wt.%)
Area (m2 /g)
d(0 0 1) ) (A
0.25 0.25 0.50
73 75 118 150 232
9.94 9.94 17.09 18.65 18.31
Differences in the AA and EDX values for T1 fraction are due to the fact that AA shows the composition of total mass of the solid, whereas EDX is a semi quantitative analysis that shows the composition in a specific region at a depth . between 5000 and 10 000 A The Si/Al ratio encountered is typical of silica aluminates. The T1 fraction displayed a high percentage of iron, which must be part of the solid structure by isomorphic substitution of Al because it was not found in the remnant solution of cationic exchange performed for CEC determination. With the aid of the BSE detector, it was possible to perform a study on the chemical composition of different particles observed by SEM in the T0 fraction. The corresponding microphotograph is shown later in the SEM section. As shown in Table 2, the clear area observed in T0 has high percentages of iron, titanium and chromium, which indicates that this is a non-argillaceous zone. The Si/Al relationship in this zone is typical of clay materials. This relationship could be due more to the influences of the surrounding area than of the zone itself. The gray area observed in T0 shows a composition similar to that found in T1, which reveals its argillaceous nature. Table 3 shows the chemical composition of pillared clays obtained by different iron complex concentrations. It is clearly shown that with an iron complex concentration equal to the CEC, the amount of iron present in the solid increase 1.7 times in relation to the initial iron content of T1. The clay was practically saturated with the complex at a concentration of 5 times the CEC and would not incorporate any more iron, even though the concentration was increased up to 10 times the CEC.
L. Huerta et al. / Microporous and Mesoporous Materials 57 (2003) 219–227
223
Table 3 Iron complex concentration and chemical composition of obtained solids (wt.% by EDX) Solid
Complex concentration
Mg
Al
Si
Ti
Cr
Fe
PT11 PT12 PT13
1 CEC 5 CEC 10 CEC
4.13 4.19 2.31
12.85 11.82 11.27
34.43 31.86 34.08
Traces Traces Traces
Traces Traces Traces
48.60 52.14 52.34
3.2. Thermal analyses 3.2.1. Iron complex Fig. 2 shows TGA and DTA of the complex. The TGA diagram presents three decomposition ramps with very high and increasing slopes. Minima in the first derivative appear at around 160, 240, and 280 °C. However, the almost absence of an intermediate plateaux, along with the known tendency of iron complexes to decompose at low temperature, suggests an almost steady complex decomposition, with no generation of stable intermediate compounds. DTA only shows two im-
Fig. 2. TGA and DTA of the iron complex.
portant peaks: one endothermic, which might correspond to evaporation of solvent that could have been occluded in the crystals of the solid, and the other exothermic at 305 °C, which corresponds to final decomposition of the complex to iron oxyhydroxides and was signaled by a reddish NO2 vapor emission. Eventual temperature differences for both the DTA and the TGA diagrams are most likely attributable to mass effects, whereas about 150 mg were used for TGA, the mass employed for DTA tests was just 4–7 mg, generating different condition for mass and heat transfer [19]. 3.2.2. Clay and pillared clays Fig. 3 shows TGA and DTA diagrams of T1 fraction, with the temperature differences by the mass effect already cited. In the TGA diagram, the first weight loss appears between 90 and 200 °C with a minimum in the first derivative occursing at 145 °C. This minimun corresponds to the loss of adsorbed water by the mineral and solvation water from interlayer cations, mainly Mg2þ , as already noted. These losses correspond to an endothermic peak at 90 °C in the DTA. At 200 °C, the slope (TGA) becomes less pronounced, hygroscopic water has evaporated and the main source of weight loss is due to water formation by reaction among structural hydroxyl groups. A negative slope (endothermic) in the DTA occurs at 480 °C accompanies this loss. In the DTA high temperature region, an exothermic peak associated with crystalline reordering and interlamellar space contraction is observed at 775 °C [20]. The DTA diagram shows two other pronounced endothermic peaks at 975 and 1030 °C, attributed to the complete destruction of clay layers with a vitreous structure formation [21]. Fig. 4 shows TGA and DTA diagrams for noncalcined (PT13 precursor) and calcined (PT13)
224
L. Huerta et al. / Microporous and Mesoporous Materials 57 (2003) 219–227
Fig. 3. TGA and DTA of the T1 clay fraction.
pillared clay. Complex breakdown inside the clay layers now occurs between 300 and 400 °C. This higher temperature needed to decompose the iron complex is explained by the ‘‘sheltering’’ effect on this complex due to surrounding clay layers. The TGA for the calcined solid has a single weight loss between 90 and 150 °C, accompanied by a drop in the DTA base line, which shows a small endothermic shoulder this is due to loss of adsorbed water. A similar endothermic shoulder, which is also observed in the DTA of the non-calcined solid. 3.2.3. Pore size distribution Figs. 5–7 show nitrogen physisorption isotherms at 77 K and pore size distribution for T1, PT12 and PT13, respectively. For calculations Dollimore Heal, Crunston Inkley and Conway Pierce methods were used, with the aid of a software specifically designed for this purpose [22]. The Conway Pierce method was selected because it showed less deviation from specific surface area, calculated by the sum of pore volumes, in comparison to the surface area calculated by the BET method. In the inserts of these three figures it can be seen that these isotherms are type IV and present type B hysteresis curves, as expected for parallel-wall layered structures with open groove [21].
According to results obtained by XRD and ), knowing the thickness of the clay layer (9.60 A would be expected an interlayer distance of 5.26 A for PT12 for non-calcined T1 (14.86–9.6), 9.05 A for PT13 (18.31–9.6). Al(18.65–9.6) and 8.71 A though the pore size interval here determined is higher than the expected from the XRD analysis, this result is a good indication of the porosity of these solids and how the pillaring process affects it. It can be observed that, for all the three solids, the largest amount of pores is in the micropore range. The T1 fraction present a portion of meso. Solids pores of approximately 23, 32, and 60 A PT12 and PT13 present a pore distribution similar to T1, but with a significant increase in the amount of micropores (up to 5 times) for PT12. By reviewing the adsorption isotherm for PT13 (high adsorption at low pressures), it can be seen that this solid has a very high surface area (232 m2 / g) due to its great amount of micropores (Figs. 4–6). 3.2.4. SEM Fig. 8 shows a BSE microphotograph of the T0 fraction. Easily observable are the numerous gray grains of clay along with clear grains that proved to be iron as presented in Table 3.
L. Huerta et al. / Microporous and Mesoporous Materials 57 (2003) 219–227
225
Fig. 5. T1 clay fraction pore size distribution.
Fig. 6. PT12 pillared clay pore size distribution.
Fig. 4. TGA and DTA of the non-calcined and calcined PT13 pillared clay.
Fig. 9 shows a SEM microphotograph of T1 fraction. Although it is unusual to refer to particle
Fig. 7. PT13 pillared clay pore size distribution.
size using this technique, it could be said that for T1, the fraction size range is between 2 and 20 lm. Fig. 10 shows a SEM microphotograph of PT13, in which flake formation and particle size
226
L. Huerta et al. / Microporous and Mesoporous Materials 57 (2003) 219–227
Fig. 10. PT13 solid microphotography by SEM. Fig. 8. T0 clay fraction microphotography by BSE.
Fig. 9. T1 clay fraction microphotography by SEM.
distribution similar to that of the original clay can be observed. 3.2.5. Catalytic test Fig. 11 shows the catalytic results obtained during the ethylbenzene dehydrogenation on T1 and PT12. Convertion of ethylbenzene to styrene can be calculated with this figure by: 100%–% ethylbenzene, and selectivity to styrene can be calculated by: % styrene/(% styrene þ % benzene). T1 showed a maximum styrene production of approximately 12 wt.% at 18 min of reaction, whereas pillared clay (PT12) showed a 18 wt.% at 22 min. PT12 also showed a higher production of benzene from the cracking of ethylbenzene than T1 fraction.
Fig. 11. Catalytic results of T1 clay fraction and PT12 pillared clay for ethylbenzene dehydrogenation.
With the purpose to determine if the increase in styrene production was due only to a surface area effect or to the iron oxide pillars, it was synthesized a sample of PILC-Al from the same smectite T1 by the method reported in our previous work [23]. This sample of PILC-Al showed a basal space of and a specific surface area of 152 m2 /g, 17.99 A very similar to the area obtained for PT12. Catalytic test for this sample of PILC-Al, at the same experimental conditions used for T1 and PT12, yielded a maximum styrene production of 3.78
L. Huerta et al. / Microporous and Mesoporous Materials 57 (2003) 219–227
wt.% at 28 min of reaction. Thus, it can be concluded that iron oxide pillars in PT12 are responsible for the increase of catalytic activity in ethylbenzene dehydrogenation. In this way, the production of styrene in both solids is achieved by oxidative dehydrogenation in accordance with the results obtained in previous work by Perez et al. [8]. Since the actual industrial process for ethylbenzene dehydrogenation is fed with a mixture of steam and ethylbenzene, it will be necessary for further studies under conditions similar to those used industrially.
4. Conclusions An iron pillared clay with five more times the number of micropores and twice the specific surface area existing in the original clay was synthesized. The interlayer distance was also increased. Thermal studies showed that the solid is stable and therefore may be used as a heterogeneous catalyst in reactions in which iron oxide is the catalyst. Pillaring process with iron significantly increased (50%) the capacity of the clay for obtaining styrene through ethylbenzene dehydrogenation. An increase in the production of benzene as by product was also observed, due to the solidÕs higher cracking activity.
Acknowledgements The authors would like to thank Consejo de Desarrollo Cientıfico y Humanıstico (CONDES) of Universidad del Zulia for supporting this project, Centro de Corrosi on of the Facultad de Ingenierıa for their assistance with electron microscopy and EDX studies, and the company
227
Schlumberger from Ciudad Ojeda, for their collaboration in the XRD analyses.
References [1] M. Occelli, J. Stencel, S. Suib, J. Mol. Catal. 64 (1991) 221. [2] E. Rightor, M.S. Tzou, T. Pinnavaia, J. Catal. 130 (1991) 29. [3] G. Vitale, Preparaci on y caracterizaci on de pilares de hierro y su uso en la hidrogenaci on de mon oxido de carbono, Trabajo Especial de Grado de Licenciatura en Quımica, Facultad de Ciencias, Universidad Central de Venezuela, Caracas, Venezuela, 1992, pp. 56–60. [4] R. Yang, J. Chen, E. Kikkinides, L. Cheng, J. Chichanowicz, Ind. Eng. Chem. Res. 31 (1992) 1440. [5] A. Meyer, L. Huerta, C. De La Cruz, Ciencia 2 (1994) 97. [6] B. Choudary, M. Kanton, M. Satessh, K. Rao, P. Santhi, Appl. Catal. A 149 (1997) 257. [7] A. Gil, L. Gandıa, M. Vicente, Catal. Rev. Sci. Eng. 42 (2000) 145. [8] G. Perez, A. De Stefanis, A. Tomlinson, J. Mater. Chem. 7 (1997) 351. [9] N. Mimura, I. Takahara, M. Saito, T. Hattori, K. Ohkuma, M. Ando, Catal. Today 45 (1998) 61. [10] C. Franco, J. Angulo, M. Maury, Rev. Soc. Ven. Cat. 2 (1988) 34. [11] M. Sugino, H. Shimada, T. Turuda, H. Miura, N. Ikenaga, T. Suzuki, Appl. Catal. A 121 (1995) 125. [12] W. Oganowski, J. Hanuza, L. Kepiski, Appl. Catal. A 171 (1998) 145. [13] R. Newman, I. Dror, Appl. Catal. A 172 (1998) 67. [14] Jr. Hamilton, D. Morris, US Patent 5,689,023, 1997. [15] F. Cavani, F. Trifiro, Appl. Catal. A 133 (1995) 219. [16] W. Weiss, R. Schlogl, Topics Catal. 13 (2000) 75. [17] S. Yamanaka, M. Hattori, Catal. Today 2 (1988) 261. [18] R. Dinauer, Methods of Soil Analysis, American Society of Agronomy, 1965, pp. 897–902. [19] M. Schilling, Clays Clay Miner. 38 (1990) 556. [20] E. Choren, A. Moronta, G. Varela, A. Arteaga, J. Sanchez, Clays Clay Miner. 45 (1997) 213. [21] R. Grim, Clay Mineralogy, McGraw Hill, New York, 1953, p. 378. [22] E. Contreras, Area Especıfica y distribuci on de tama~ no de poro en s olidos porosos, Trabajo Especial de Grado de Maestrıa en Ingenierıa Quımica, Facultad de Ingenierıa, Universidad del Zulia, Maracaibo, Venezuela, 1992, pp. 72–78. [23] J. Ochoa, A. Meyer, Ciencia 6 (1998) 51.