DSpace at VNU: Synthesis and study on catalytic activity of spinel metallic oxides in styrene preparation from ethylbenzene

Synthesis and Study on Catalytic Activity of Spinel Metallic Oxides in StyrenePreparation from Ethylbenzene∗ Le Thanh Son,† Hoa Huu Thu, Nguyen Thanh Binh, Tran Thi Nhu Mai, and Nguyen H

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 6 December 2009; Accepted 21 December 2011; Published 23 June 2012)

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−xFexO4obtained by coprecipitation method

[DOI: 10.1380/ejssnt.2012.263]

Keywords: Nano spinel; Ethylbenzene; Dehydrogenation

I INTRODUCTION

Recent years, the styrene quantity consumed is

increas-ing 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

ethyl-benzene and (ii) as a by-product in the epoxidation of

propene with ethylbenzene hydroperoxide and

Molybnum complex-based catalysts [1] The ethylbenzene

de-hydrogenation is similar to the de-hydrogenation of

alka-nes The actual ethylbenzene dehydrogenation process

is highly endothermic, reversible and needing reactant

re-cycle, high steam-to-ethylbenzene ratios So, it needs the

presence of catalysts The traditional catalysts for

ethyl-benzene dehydrogenation are iron oxides promoted by

al-kali metal ions [3, 4, 6] However, it is observed a slight

irreversible deactivation of the catalysts with usage

be-cause of migration of potassium from the styrene to the

bulk [4, 7] That is why, catalyst research for

ethylben-zene dehydrogenation has been of interest to many

chem-ical 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

oxida-tion of hydrogen in order to furnish the heat of

reac-tion to the former and shift the reacreac-tion equilibrium

toward the right, styrene formation

• Oxidative dehydrogenation in order to realize an

exothermic reaction and shift the reaction

equilib-rium toward the product formation and to carry out

the reaction at lower temperature

∗This paper was presented at the International Workshop on

Ad-vanced 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

equilib-rium and to carry out the reaction at lower tem-perature [1]

Table I summarizes the catalytic performances obtained with the different techniques, inside the ethylbenzene de-hydrogenation is the only process widely used at a com-mercial level [8]

In recent years, the spinel metallic oxide having move activity for dehydrogenation and oxidative

dehydrogena-tion of ethylbenzene to styrene is reported [5? ] Spinel

oxides having cation distribution in the planes of (110) and (111) showed high catalytic activity for dehydrogena-tion of hydrocarbon, isopropanol, cyclohexanol [1] In the present investigation, we have prepared several series

of spinel oxides AB2−xB’xO4 (A=Ni2+, Cu2+, B=Cr3+, B’=Fe3+), determined textural and structural character-istics and evaluated their catalyst ability for oxidative de-hydrogenation 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−xB’xO4 (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

1 Solid-state reaction method

In this method, iron (III) oxide, chromium (III) nickel oxide all in PA, were used as sources of metal-lic irons in spinel ternary structure NiCr2−xFexO4 (x =

0, 0.5, 1.0, 1.5, 2.0). Spinel NiCr2−xFexO4 was prepared

as following: first, the quantities calculated in advance of the oxides above were mixed carefully in a porcelain mor-tar for 30 minutes Then, the powder mixture granulated

at pressure of 2.000 N/cm2 Transfer the granules in a

TABLE I: Comparison of the catalytic performances for different technique of styrene synthesis from ethylbenzene Techniques Selectivity in styrene (%) Reaction temperature (◦C) Catalyst States

TABLE II: Spinel samples obtained by different methods

No preparation Spinel formula Sign

2 NiCr1.5Fe0.5O4 NCF 2 (I)

3 Solid-state reaction NiCrFeO4 NCF 3 (I)

4 NiCr0.5Fe1.5O4 NCF 4 (I)

7 NiCr1.5Fe0.5O4 NCF 2 (II)

8 Coprecipitation NiCrFeO4 NCF 3 (II)

9 NiCr0.5Fe1.5O4 NCF 4 (II)

12 CuCr1.5Fe0.5O4 CCF 2 (II)

13 Coprecipitation CuCrFeO4 CCF 3 (II)

14 CuCr0.5Fe1.5O4 CCF 4 (II)

cup and place this cup in a furnace, heat the electrical

furnace at 1300◦C for 4 hours We cooled the solid

ob-tained 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 + xFe2O3+ (2− x)Cr2O3→ 2NiCr2−xFeO4 (1)

In the case of x = 0, the reaction is as follows:

NiO + Cr2O3→ NiCr2O4 (2)

As a comparison, we also prepared two series of spinels:

NiCr2−xFexO4 and CuCr2−xFexO4 by coprecipitation

method, using the sources of respective metal nitrates

2 Coprecipitation method

This is a simple method and very favorable in

mak-ing the ternary spinels Here, we have used the

source of metallic irons under from of their nitrates:

Ni(NO3)3·6H2O, Cu(NO3)2·6H2O Cr(NO3)3·9H2O and

Fe(NO3)3·9H2O, all in PA (Aldrich) The spinel

NiCr2−xFexO4 and CuCr2−xFexO4 were prepared as

fol-lows: first, the quantities of metallic salts after the

gen-eral 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

4OH 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,

4

scheme (fig.1)

Figure 1: Scheme of formation of ternary spinels NiCr 2-x Fe x O 4 at high temperature

FIG 1: Scheme of formation of ternary spinels NiCr2−xFexO4

at high temperature

the precipitate was dried at 120◦C for 6 hours in order

to eliminate the adsorbed water and form links of metal-oxygen-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 diffrac-tometer single X-ray with wavelength of 1.5406 ˚A Scan-ning electron microscope, SEM image were performed sev-eral samples representative Infrared (IR) spectra for all samples were measured on a Fourier transform IR spectrometer (Nicolet 760 Magara, Japan) Specific sur-face of samples was determined by nitrogen adsorption-desorption 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 con-troller 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 liq-uid products collected for the first 30 min were discarded and analyzed on Gas Chromatography-Mass spectroscopy (GC-MS HP 6890)

FIG 2: XRD patterns of spinels NiCr2−xFexO4 (x = 0, 0.5,

1.0, 1.5, 2.0) obtained by solid-state reaction method

Figure3: SEM images of spinel samples: (a) sample NiCr O obtained by solid-state

FIG 3: SEM images of spinel samples: (a) sample NiCr2O4

obtained by solid-state reaction method; (b) sample NiCr2O4

obtained by coprecipitation method

III RESULTS AND DISCUSSIONS

The solid solution of metallic oxides mixture having

spinel structure or the ceramic materials are often

pre-pared to suit their applications Generally, the spinel solid

solutions are formed at different temperatures according

TABLE III: Characteristic absorption bands in IR region of

spinel samples

Absorption bands in the IR of samples (cm−1)

Spinels Vibration of Vibration of Presence Presence

tetrahedral octahedral of nitrate of water

metal-oxygen metal-oxygen NO−3 ∼1670 [3]

bond∼620 [3] bond ∼530 [3] ∼1390 [3]

(a) Process of splitting hydride on metallic sites Fe or Cr (Me ):

Break of C-C bond: (b)

- (c)



 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

Fe3+ or Cr3+ (Me3+); (b) Breaking of C–C bond; (c) Oxida-tive dehydrogenation of intermediate

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 + xFe2O3+ (2− x)Cr2O3 → 2NiCr2−xFexO4

(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; Cr2O3and Fe2O3having

hexadi-rection structure, while the spinels NiCr2−xFexO4 repre-sented face-centered cubic structure So, the formation

of spinels is easy because of their structure being ap-proachable although the reaction temperature 1300◦C was

far from their fusion temperature In the reaction pro-cess, the ions consisting of anion O2− and cation Ni2+,

Cr3+ and Fe3+ at different phase interface of the ox-ides NiO, Cr2O3, Fe2O3 diffuse one an other resulting spinel structure This can be imagined after the scheme shown in Fig 1 Thus, all XRD patterns of five sam-ples NC 1(I), NCF2(I), NCF3(I), NCF4(I) and NF5(I) (Fig 2), demonstrated that the spinels NiCr2−xFexO4

(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 Cr3+ and Fe3+ in the octagonal sites and

Ni2+ in the tetragonal sites of face-centered cubic struc-ture 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

TABLE IV: X-ray 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]

Plane dref d-spacing for NiCr2−xFexO4 obtained by coprecipitation

Plane dref d-spacing for CuCr2−xFexO4obtained by coprecipitation

Plane dref d-spacing for NiCr2−xFexO4 obtained by solid-state reaction

affirming spinel structure of our products The solid

sam-ples 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

copre-cipitation method permit to prepare ternary spinels of

NiCr2−xFexO4 type (A = Ni2+, Cu2+; Br = Cr3+ and

Bf=Fe3+) at lower temperature, 750◦C than the

reac-tion temperature of spinels preparareac-tion NiCr2−xFexO4by

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

prepara-tion method, condiprepara-tions of catalyst preparaprepara-tion as

tem-perature, reaction medium influence their catalytic

ca-pacity in oxidative dehydrogenation of ethylbenzene to

styrene Table V represents the results of catalytic

activ-ity evaluations of spinels in oxidative dehydrogenation of

ethylbenzene to styrene

The results presented in the Table V showed

cat-alytic performance of these spinels in oxidative

dehydro-genation of ethylbenzene to styrene These data also

showed that when the reaction temperature was

increas-ing, the ethylbenzene conversion increasincreas-ing, the

selec-tivity in styrene decreasing In the reaction

tempera-ture range from 350◦C to 450◦C, the catalyst NFC3(I),

NFC3(II) and CCF3(II) or the NiCrFeO43(I), NiCrFeO4

3(II) and CuCrFeO4 3(II) (x = 1) represent the

high-est catalytic activity and selectivity in styrene In

ox-idative dehydrogenation of ethylbenzene to styrene on

spinel catalyst CuCr2−xFexO4, the role of water was very

important The presence of water has eliminated

sec-ondary 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):

1/2O2+ Me2+→ (O −

ad) + Me3+ (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− → H2O + O2−

net

Me2++ 1/2O2 → Me3+

+ O−

ad, (5) where O2−

netis oxygen of network crystalline of spinel and

O−

adoxygen adsorbed Here, Me3+ can be either Cr3+ or

Fe3+ The both these cations are Lewis acid cites and represent catalytic possibility of hydride elimination ana-logue

IV CONCLUSION

1) Three series of catalyst spinel samples were syn-thesized by solid-state reaction and coprecipitation method The coprecipitation method has revealed

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

temperature (◦C)

Catalyst 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 (%)

*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

cat-alytic activity and selectivity in styrene in the

ox-idative dehydrogenation of ethylbenzene to styrene

4) The oxidative dehydrogenation of ethylbenzene on

spinels NiCr2−xFexO4 (I) and (II) was complicated beside the main product, styrene there was sec-ondary reaction influencing quality of styrene ob-tained

Acknowledgments

the authors gratefully acknowledge financial support from the National Foundation for Science and Technol-ogy Development of Vietnam (NAFOSTED)

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