Conference-IWAMN 2009-Synthesis and Study on Catalytic Activity of Spinel Metallic Oxides in Styrene Preparation from Ethylbenzene

A series of spinel oxides AB2−xB’xO4/γ-Al2O3 (A: Ni, Cu; B: Cr; B’: Fe and x = 0, 0.5, 1, 1.5, 2) were synthesized by two methods: solid-state reaction and coprecipitation. The oxides obtained were characterized by XRD, SEM and BET to determinate their textural and structural properties. Their catalytic activity was evaluated by reaction of oxidative dehydrogenation of ethylbenzene to styrene. The XRD showed the spinel phase formed for all oxides synthesized by two methods. However, the coprecipitation method seems to be more favorable for formation of spinel phase. All samples showed a high catalytic activity and selectivity for oxidative dehydrogenation of ethylbenzene to styrene, especially, in the case of NiCr2−xFexO4 obtained by coprecipitation method. [DOI: 10.1380/ejssnt.2012.263]


I. INTRODUCTION
Recent years, the styrene quantity consumed is increasing as start materials to synthesize the polymers and copolymers.The worldwide capacity for production of stryrene is approximately 15.106 t/year [9].Stryrene is produced by two processes: (i) dehydrogenation of ethylbenzene and (ii) as a by-product in the epoxidation of propene with ethylbenzene hydroperoxide and Molybdenum complex-based catalysts [1].The ethylbenzene dehydrogenation is similar to the hydrogenation of alkanes.The actual ethylbenzene dehydrogenation process is highly endothermic, reversible and needing reactant recycle, high steam-to-ethylbenzene ratios.So, it needs the presence of catalysts.The traditional catalysts for ethylbenzene dehydrogenation are iron oxides promoted by alkali metal ions [3,4,6].However, it is observed a slight irreversible deactivation of the catalysts with usage because of migration of potassium from the styrene to the bulk [4,7].That is why, catalyst research for ethylbenzene dehydrogenation has been of interest to many chemical manufactures, at the same time, many techniques have been proposed to find out a best solution producing styrene.These techniques are the following alternative ones: • Ethylbenzene dehydrogenation followed by oxidation of hydrogen in order to furnish the heat of reaction to the former and shift the reaction equilibrium toward the right, styrene formation.
• Oxidative dehydrogenation in order to realize an exothermic reaction and shift the reaction equilibrium toward the product formation and to carry out the reaction at lower temperature.
• Membrane catalysis in order to shift the equilibrium and to carry out the reaction at lower temperature [1].
Table I summarizes the catalytic performances obtained with the different techniques, inside the ethylbenzene dehydrogenation is the only process widely used at a commercial level [8].
In recent years, the spinel metallic oxide having move activity for dehydrogenation and oxidative dehydrogenation of ethylbenzene to styrene is reported [5? ].Spinel oxides having cation distribution in the planes of ( 110) and (111) showed high catalytic activity for dehydrogenation of hydrocarbon, isopropanol, cyclohexanol [1].In the present investigation, we have prepared several series of spinel oxides AB 2−x B' x O 4 (A=Ni 2+ , Cu 2+ , B=Cr 3+ , B'=Fe 3+ ), determined textural and structural characteristics and evaluated their catalyst ability for oxidative dehydrogenation of ethylbenzene to styrene.

II. EXPERIMENTAL A. Spinel preparation
There have been a lot of methods to prepare spinel materials.Here, we have used two methods of preparing the spinels AB 2−x B' x O 4 (x = 0, 0.5, 1.0, 1.5, 2.0), which are described in the following.Details of samples used in the present study are summarized in Table II.

Solid-state reaction method
In this method, iron (III) oxide, chromium (III) nickel oxide all in PA, were used as sources of metallic irons in spinel ternary structure NiCr 2−x Fe x O 4 (x = 0, 0.5, 1.0, 1.5, 2.0).Spinel NiCr 2−x Fe x O 4 was prepared as following: first, the quantities calculated in advance of the oxides above were mixed carefully in a porcelain mortar for 30 minutes.Then, the powder mixture granulated at pressure of 2.000 N/cm 2 .Transfer the granules in a  cup and place this cup in a furnace, heat the electrical furnace at 1300 • C for 4 hours.We cooled the solid obtained in desiccators and cracked them into small grains of 0.1-1.0mm in diameter as catalyst grains.
The reaction of spinel formation at 1300 • C is generally represented after the following equation: In the case of x = 0, the reaction is as follows: As a comparison, we also prepared two series of spinels: NiCr 2−x Fe x O 4 and CuCr 2−x Fe x O 4 by coprecipitation method, using the sources of respective metal nitrates.

Coprecipitation method
This is a simple method and very favorable in making the ternary spinels.
Here, we have used the source of metallic irons under from of their nitrates: The spinel NiCr 2−x Fe x O 4 and CuCr 2−x FexO 4 were prepared as follows: first, the quantities of metallic salts after the general formula of spinel and weighed were dissolved in the 10% salts solution.The obtained solution was mixed and heated at 80 • C.Then, 5% NH 4 OH solution was added in the last solution until pH=7.This one was maintained at 80 • C for 5 hours in order to precipitate completely the desired solid.The precipitate was filtered and washed with distillated water until absence of NO − 3 ions.Then, the precipitate was dried at 120 • C for 6 hours in order to eliminate the adsorbed water and form links of metaloxygen-metal existing in the solid mass obtained.Finally, the solid was calcined at 750 • C for 4 hours.By these ways, we have obtained the following spinel.

B. Characterization
X-ray diffraction (XRD) patterns were recorded for all samples of spinel obtained on a SIEMENS D5000 diffractometer single X-ray with wavelength of 1.5406 Å. Scanning electron microscope, SEM image were performed several samples representative.Infrared (IR) spectra for all samples were measured on a Fourier transform IR spectrometer (Nicolet 760 Magara, Japan).Specific surface of samples was determined by nitrogen adsorptiondesorption at −196 • C on Autosorb01 equipment.

C. Reaction system and analysis of liquid products obtained
The reaction of oxidative dehydrogenation was carried out in the vapor phase in a fixed bed flow type reactor consisting of a quarts tube in which the catalyst bed was placed in the middle of the tube.The reactor was heated by electricity and controlled by digital temperature controller.The temperature was measured by thermocouple placed in the center of the catalyst bed.The reactants were fed into the catalyst bed by a syringe infusion pump following the ethylbenzene flow rate desired.The liquid products collected for the first 30 min were discarded and analyzed on Gas Chromatography-Mass spectroscopy (GC-MS HP 6890).http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/)

III. RESULTS AND DISCUSSIONS
The solid solution of metallic oxides mixture having spinel structure or the ceramic materials are often prepared to suit their applications.Generally, the spinel solid solutions are formed at different temperatures according  to chemical precursors used for preparing spinels desired.
In the solid-state reaction method, the precursors are all metallic oxides, the reaction temperature is used being 1300 In these reactions, NiO existing at solid state with body-centered cubic structure coordination number of Ni 2+ , O 2− ions being 6; Cr 2 O 3 and Fe 2 O 3 having hexadirection structure, while the spinels NiCr 2−x Fe x O 4 represented face-centered cubic structure.So, the formation of spinels is easy because of their structure being approachable although the reaction temperature 1300 • C was far from their fusion temperature.In the reaction process, the ions consisting of anion O 2− and cation Ni 2+ , Cr 3+ and Fe 3+ at different phase interface of the oxides NiO, Cr 2 O 3 , Fe 2 O 3 diffuse one an other resulting spinel structure.This can be imagined after the scheme shown in Fig. 1.Thus, all XRD patterns of five samples NC 1(I), NCF2(I), NCF3(I), NCF4(I) and NF5(I) (Fig. 2), demonstrated that the spinels NiCr 2−x Fe x O 4 (x = 0, 0.5, 1.0, 1.5, 2.0) were formed.
This solid-state reaction process can be analogous to crystallization one of spinels through reorganization of metallic cation Cr 3+ and Fe 3+ in the octagonal sites and Ni 2+ in the tetragonal sites of face-centered cubic structure.This favor formation of big crystals.SEM image of sample NC1(I) illustrated our explication (see Fig. 3).
The size of NC1(I) crystal is bigger than sample NCF3(II).The IR results were represented in Table III Volume 10 (2012) Le, et al. affirming spinel structure of our products.The solid samples obtained by coprecipitation method were affirmed to be expected ternary spinels by XRD results represented in Table IV.
As all what we have represented above, the coprecipitation method permit to prepare ternary spinels of NiCr 2−x Fe x O 4 type (A = Ni 2+ , Cu 2+ ; Br = Cr 3+ and Bf=Fe 3+ ) at lower temperature, 750 • C than the reaction temperature of spinels preparation NiCr 2−x Fe x O 4 by solid-state reaction, 1300 • C and the size of spinel grain is smaller with the specific surface around 20 m 2 /g (see Fig. 3).That's catalysts composition, catalyst preparation method, conditions of catalyst preparation as temperature, reaction medium influence their catalytic capacity in oxidative dehydrogenation of ethylbenzene to styrene.Table V represents the results of catalytic activity evaluations of spinels in oxidative dehydrogenation of ethylbenzene to styrene.
The results presented in the Table V showed catalytic performance of these spinels in oxidative dehydrogenation of ethylbenzene to styrene.These data also showed that when the reaction temperature was increasing, the ethylbenzene conversion increasing, the selectivity in styrene decreasing.In the reaction temperature range from 350 • C to 450 • C, the catalyst NFC3(I), NFC3(II) and CCF3(II) or the NiCrFeO 4 3(I), NiCrFeO 4 3(II) and CuCrFeO 4 3(II) (x = 1) represent the highest catalytic activity and selectivity in styrene.In oxidative dehydrogenation of ethylbenzene to styrene on spinel catalyst CuCr 2−x Fe x O 4 , the role of water was very important.The presence of water has eliminated secondary reactions as deakylation ethybnezene molecular.While these secondary reactions took place styrene at the same time with the reaction, oxidative dehydrogenation of ethylbenzene.
Basing on several publications in recent years and the results represented in Table V, the main reaction and the secondary reactions in the oxidative dehydrogenation of ethylbenzene can be explained after the following steps: First, oxygen that comes from the air was adsorbed on the hole vacant catalyst surface to form oxygen adsorbed (O − ad ): And then, the monomolecular reaction went on after mechanism of Langmuir-Hinshelwood to form the reaction products (see Fig.
where O  to be more favorable with formation of spinels at lower temperature.
2) It was used physical method to verify structural characteristics of the spinel products obtained.The data obtained have affirmed the structure of spinels synthesized.
3) Generally, the spinel materials showed a high catalytic activity and selectivity in styrene in the oxidative dehydrogenation of ethylbenzene to styrene.
4) The oxidative dehydrogenation of ethylbenzene on spinels NiCr 2−x Fe x O 4 (I) and (II) was complicated beside the main product, styrene there was secondary reaction influencing quality of styrene obtained.

FIG. 4 :
FIG. 4: Schemes of the monomolecular reaction after the mechanism of Langmuir-Hinshelwood to form the reaction products.(a) Process of splitting hydride on metallic sites Fe 3+ or Cr 3+ (Me 3+ ); (b) Breaking of C-C bond; (c) Oxidative dehydrogenation of intermediate.

TABLE I :
Comparison of the catalytic performances for different technique of styrene synthesis from ethylbenzene.

TABLE II :
Spinel samples obtained by different methods.

TABLE III :
Characteristic absorption bands in IR region of spinel samples.

TABLE IV :
[3]ay d-spacing for the series of samples: NiCr2−xFexO4 and CuCr2−xFexO4 obtained by coprecipitation method compared with NiCr2−xFexO4 obtained by solid-state reaction method and reference[3].
2−net is oxygen of network crystalline of spinel and O − ad oxygen adsorbed.Here, Me 3+ can be either Cr 3+ or Fe 3+ .The both these cations are Lewis acid cites and represent catalytic possibility of hydride elimination analogue.

TABLE V :
Composition of liquid product obtained in oxidative dehydrogenation of ethylbenzene to styrene, at different temperatures, air flow of 1.0 l/min, special velocity 0.6 h −1 .