e-Journal of Surface Science and Nanotechnology 23 June 2012 Conference - IWAMN2009 - e-J Surf Sci Nanotech Vol 10 (2012) 263-267 Synthesis and Study on Catalytic Activity of Spinel Metallic Oxides in Styrene Preparation from Ethylbenzene∗ Le Thanh Son,† Hoa Huu Thu, Nguyen Thanh Binh, Tran Thi Nhu Mai, and Nguyen Hong Vinh Department of Petroleum Chemistry, Faculty of Chemistry, Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam (Received December 2009; Accepted 21 December 2011; Published 23 June 2012) A series of spinel oxides AB2−x B’x O4 /γ-Al2 O3 (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−x Fex O4 obtained by coprecipitation method [DOI: 10.1380/ejssnt.2012.263] Keywords: Nano spinel; Ethylbenzene; Dehydrogenation 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 ∗ This paper was presented at the International Workshop on Advanced Materials and Nanotechnology 2009 (IWAMN2009), Hanoi University of Science, VNU, Hanoi, Vietnam, 24-25 November, 2009 † Corresponding author: lethanhson@yahoo.com • 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 AB2−x B’x O4 (A=Ni2+ , Cu2+ , B=Cr3+ , B’=Fe3+ ), 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 AB2−x B’x O4 (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 NiCr2−x Fex O4 (x = 0, 0.5, 1.0, 1.5, 2.0) Spinel NiCr2−x Fex O4 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/cm2 Transfer the granules in a c 2012 The Surface Science Society of Japan (http://www.sssj.org/ejssnt) ISSN 1348-0391 ⃝ 263 Le, et al Volume 10 (2012) TABLE I: Comparison of the catalytic performances for different technique of styrene synthesis from ethylbenzene Techniques Dehydrogenation Dehydrogenation/H2 Oxidative Oxidative Dehydrogenation Oxidative Dehydrogenation Membrane technique Selectivity in styrene (%) 90 90 90 90 94 Reaction temperature (◦ C) 600-650 605 350 480 625 Catalyst Fe/K/Cr/O SMART technology Carbon molsieve V/Mg/O Fe/K/Li/Cr/O States Industrial Commercial Research Research Research scheme (fig.1) TABLE II: Spinel samples obtained by different methods Sample No 10 11 12 13 14 15 Method of preparation NiCr2 O4 NiCr1.5 Fe0.5 O4 Solid-state reaction NiCr0.5 Fe1.5 O4 NiCr2 O4 NiCr2 O4 NiCr1.5 Fe0.5 O4 Coprecipitation NiCr0.5 Fe1.5 O4 NiCr2 O4 CuCr2 O4 CuCr1.5 Fe0.5 O4 Coprecipitation CuCr0.5 Fe1.5 O4 CuCr2 O4 Spinels Spinel formula Sign NC (I) NCF (I) NiCrFeO4 NCF (I) NCF (I) NF 5(II) NC (I) NCF (II) NiCrFeO4 NCF (II) NCF (II) NF 5(II) CC (I) CCF (II) CuCrFeO4 CCF (II) CCF (II) CF 5(II) cup and place this cup in a furnace, heat the electrical furnace at 1300◦ C for hours We cooled the solid obtained in desiccators and cracked them into small grains of 0.1-1.0 mm in diameter as catalyst grains The reaction of spinel formation at 1300◦ C is generally represented after the following equation: 2NiO + xFe2 O3 + (2 − x)Cr2 O3 → 2NiCr2−x FeO4 (1) In the case of x = 0, the reaction is as follows: NiO + Cr2 O3 → NiCr2 O4 (2) As a comparison, we also prepared two series of spinels: NiCr2−x Fex O4 and CuCr2−x Fex O4 by coprecipitation method, using the sources of respective metal nitrates Figure 1: Scheme of formation of ternary spinels NiCr2-xFexO4 at high temperature FIG 1: Scheme of formation of ternary spinels NiCr2−x Fex O4 at high temperature the precipitate was dried at 120◦ C for 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 hours By these ways, we have obtained the following spinel B 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 ˚ A 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 Coprecipitation method C 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: Ni(NO3 )3 ·6H2 O, Cu(NO3 )2 ·6H2 O Cr(NO3 )3 ·9H2 O and Fe(NO3 )3 ·9H2 O, all in PA (Aldrich) The spinel NiCr2−x Fex O4 and CuCr2−x FexO4 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% NH4 OH solution was added in the last solution until pH=7 This one was maintained at 80◦ C for hours in order to precipitate completely the desired solid The precipitate was filtered and washed with distillated water until absence of NO− ions Then, 264 Characterization 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 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/) e-Journal of Surface Science and Nanotechnology   Volume 10 (2012) (a)of splitting hydride on metallic sites Fe or Cr (Me ): Process Break of C-C bond: (b) FIG 2: XRD patterns of spinels NiCr2−x Fex O4 (x = 0, 0.5, 1.0, 1.5, 2.0) obtained by solid-state reaction method - (c) (a) (b)   FIG 4: Schemes of the monomolecular reaction after the mechanism of Langmuir-Hinshelwood to form the reaction Figure3: SEM images of spinel samples: (a) sample NiCr O obtained by solid-stateproducts (a) Process of splitting hydride on metallic sites Fe3+ or Cr3+ (Me3+ ); (b) Breaking of C–C bond; (c) OxidaFIG 3: SEM images of spinel samples: (a) sample NiCr2 O4 tive dehydrogenation of intermediate obtained by solid-state reaction method; (b) sample NiCr2 O4 obtained by coprecipitation method 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◦ C 2NiO + xFe2 O3 + (2 − x)Cr2 O3 → 2NiCr2−x Fex O4 (x = 0, 0.5, 1.0, 1.5, 2.0) (3) In these reactions, NiO existing at solid state with body-centered cubic structure coordination number of Ni2+ , O2− ions being 6; Cr2 O3 and Fe2 O3 having hexadiTABLE III: Characteristic absorption bands in IR region of rection structure, while the spinels NiCr2−x Fex O4 represpinel samples sented face-centered cubic structure So, the formation Absorption bands in the IR of samples (cm−1 ) of spinels is easy because of their structure being apSpinels Vibration of Vibration of Presence Presence proachable although the reaction temperature 1300◦ C was tetrahedral octahedral of nitrate of water far from their fusion temperature In the reaction prometal-oxygen metal-oxygen NO− ∼1670 [3] cess, the ions consisting of anion O2− and cation Ni2+ , bond ∼620 [3] bond ∼530 [3] ∼1390 [3] Cr3+ and Fe3+ at different phase interface of the oxNC (I) 600 447 — — ides NiO, Cr2 O3 , Fe2 O3 diffuse one an other resulting NCF (I) 638 495 — — spinel structure This can be imagined after the scheme NCF (I) 627 466 — — shown in Fig Thus, all XRD patterns of five samNCF (I) 633 433 — — ples NC 1(I), NCF2(I), NCF3(I), NCF4(I) and NF5(I) NF 5(II) 627 423 — — (Fig 2), demonstrated that the spinels NiCr2−x Fex O4 NC (I) 610 517 — 1634 NCF (I) 604 507 1378 1664 (x = 0, 0.5, 1.0, 1.5, 2.0) were formed NCF (I) 664 554 1344 1638 This solid-state reaction process can be analogous to NCF (I) 621 514 1439 — crystallization one of spinels through reorganization of NF 5(II) — — — — metallic cation Cr3+ and Fe3+ in the octagonal sites and CC (I) 630 528 — 1644 Ni2+ in the tetragonal sites of face-centered cubic strucCCF (II) 620 456 1342 1634 ture This favor formation of big crystals SEM image of CCF (II) 625 529 1351 1656 sample NC1(I) illustrated our explication (see Fig 3) CCF (II) 625 449 1341 — The size of NC1(I) crystal is bigger than sample CF 5(II) 595 443 1338 — NCF3(II) The IR results were represented in Table III http://www.sssj.org/ejssnt (J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) 265 Le, et al Volume 10 (2012) TABLE IV: X-ray d-spacing for the series of samples: NiCr2−x Fex O4 and CuCr2−x Fex O4 obtained by coprecipitation method compared with NiCr2−x Fex O4 obtained by solid-state reaction method and reference [3] Plane (hkl) 111 220 311 222 400 422 Plane (hkl) 111 220 311 222 400 422 Plane (hkl) 111 220 311 222 400 422 dref (˚ A) 4.81 2.93 2.40 2.40 2.08 1.70 dref (˚ A) 4.81 2.93 2.40 2.40 2.08 1.70 dref (˚ A) 4.81 2.93 2.40 2.40 2.08 1.70 NC 1(II) 4.81 2.95 2.50 2.40 2.08 1.70 CC (II) 4.81 2.95 2.51 2.41 2.08 1.70 NC 1(I) 4.80 2.95 2.51 2.41 2.08 1.70 d-spacing for NiCr2−x Fex O4 obtained by coprecipitation NCF (II) NCF (II) NCF (II) 4.80 4.80 4.79 2.95 2.95 2.95 2.51 2.51 2.51 2.41 2.41 2.40 2.08 2.08 2.08 1.69 1.70 1.69 d-spacing for CuCr2−x Fex O4 obtained by coprecipitation CCF (II) CCF (II) CCF (II) 4.79 4.80 4.79 2.93 2.94 2.94 2.51 2.51 2.51 2.41 2.31 2.42 2.06 2.07 2.09 — — — d-spacing for NiCr2−x Fex O4 obtained by solid-state reaction NCF (I) NCF (I) NCF (I) 4.79 4.79 4.81 2.90 2.93 2.93 2.48 2.50 2.50 — — 2.40 2.07 2.07 2.07 1.69 1.69 1.69 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 NiCr2−x Fex O4 type (A = Ni2+ , Cu2+ ; Br = Cr3+ and Bf=Fe3+ ) at lower temperature, 750◦ C than the reaction temperature of spinels preparation NiCr2−x Fex O4 by solid-state reaction, 1300◦ C and the size of spinel grain is smaller with the specific surface around 20 m2 /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 NiCrFeO4 3(I), NiCrFeO4 3(II) and CuCrFeO4 3(II) (x = 1) represent the highest catalytic activity and selectivity in styrene In oxidative dehydrogenation of ethylbenzene to styrene on spinel catalyst CuCr2−x Fex O4 , 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 266 CCF (II) 4.79 2.95 2.49 4.42 2.09 — 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 ): 3+ 1/2O2 + Me2+ → (O− ad ) + Me (4) And then, the monomolecular reaction went on after mechanism of Langmuir-Hinshelwood to form the reaction products (see Fig 3): (a) Process of splitting hydride on metallic sites Fe3+ or Cr3+ (Me3+ ); (b) Breaking of C–C bond; (c) Oxidative dehydrogenation of intermediate; (d) Process repeated: 2OH− → H2 O + O2− net Me2+ + 1/2O2 → Me3+ + O− ad , (5) where O2− net is oxygen of network crystalline of spinel and O− oxygen adsorbed Here, Me3+ can be either Cr3+ or ad 3+ Fe The both these cations are Lewis acid cites and represent catalytic possibility of hydride elimination analogue IV CONCLUSION 1) Three series of catalyst spinel samples were synthesized by solid-state reaction and coprecipitation method The coprecipitation method has revealed http://www.sssj.org/ejssnt (J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology Volume 10 (2012) 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 Reaction temperature (◦ C) Catalyst NC (I) NCF (I) NCF (I) NCF (I) NF 5(I) NC (II) NCF (II) NCF (II) NCF (II) NCF 5(II) CC (I)* CCF (II)* CCF (II)* CCF (II)* CF 5(II)* 350 400 450 Overall Selectivity Toluene Overall Selectivity Toluene Overall Selectivity Toluene conversion in styrene + benzene conversion in styrene + benzene conversion in styrene + benzene of ethyl(%) yield (%) of ethyl(%) yield (%) of ethyl(%) yield (%) benzene (%) benzene (%) benzene (%) 12.31 45.31 54.69 17.71 39.52 60.48 33.79 36.80 63.20 14.15 66.29 32.71 21.32 61.30 38.70 38.14 53.20 45.19 19.20 92.90 6.18 34.47 92.40 6.54 61.43 91.21 8.70 18.71 78.07 21.41 31.38 70.07 28.16 47.65 64.30 35.40 16.25 78.29 21.45 28.69 69.45 30.06 46.44 59.12 35.22 21.72 56.06 40.15 22.52 57.63 41.23 32.37 40.76 55.58 26.45 72.60 20.33 34.15 65.15 30.75 40.26 64.47 33.51 21.17 92.14 6.74 43.43 85.60 13.21 58.82 82.60 16.79 16.67 87.58 11.56 38.43 71.13 24.43 43.52 69.63 30.04 15.71 79.83 18.81 28.36 72.52 24.63 40.08 60.13 37.69 35.27 93.69 — 47.03 70.72 — 98.21 47.76 — 30.42 92.78 — 51.62 89.43 — 70.14 57.15 — 40.01 78.27 — 70.23 90.17 — 70.23 69.20 — 39.56 81.17 — 65.42 72.06 — 78.95 70.45 — 29.94 95.19 — 38.18 96.72 — 51.12 78.74 — *reaction conditions on these catalysts are identical to reaction above, but in addition with water presence 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 [1] F Cavani and F Trifigo, Appl Catal A: General 133, 219 (1995) [2] R M Gabr, M M Girgis, A M El-Awad, and B M Abou-Zeid, Mater Chem Phys 39, 53 (1994) [3] T Inui, M Anpo K Izui, S Yanagida, and T Yamaguchi (Eds.), Advances in Chemical Conversions for Mitigating Carbon Dioxide (Studies in Surface Science and Catalysis, Vol 114) (Elsevier Science, 1998), p 415 [4] N J Jebarathinam, M Eswaramoorthy, and V Krishnasamy, Appl Catal A: General, 145, 57 (1996) [5] H H Kung and M C Kung, Appl Catal A: General, 157, spinels NiCr2−x Fex O4 (I) and (II) was complicated beside the main product, styrene there was secondary reaction influencing quality of styrene obtained Acknowledgments the authors gratefully acknowledge financial support from the National Foundation for Science and Technology Development of Vietnam (NAFOSTED) 105 (1997) [6] E H Lee, Catal Rev 285 (1974) [7] A Mihajlova, A Andreev, D Shopor, et al., in Proc Vith Int Symp Heterogeneous Catalysis (Sofia, 1987), Part 2, p 436 [8] R Perrin and J P Scharff, Chimie Industrielle, Vol.1 (Masson, Paris-Milan-Barcelone-Born, 1993) [9] M Tuyen, in Proc Conf Refinery and Petrochemical Technol in the XX-XXI Century (Hanoi, 20-21 Dec., 2002) http://www.sssj.org/ejssnt (J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) 267 ... temperature, reaction medium in uence their catalytic capacity in oxidative dehydrogenation of ethylbenzene to styrene Table V represents the results of catalytic activity evaluations of spinels in. .. 1300◦ C and the size of spinel grain is smaller with the specific surface around 20 m2 /g (see Fig 3) That’s catalysts composition, catalyst preparation method, conditions of catalyst preparation as... 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