Oxidative dehydrogenation of cyclohexane over heteropolymolybdates

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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Oxidative dehydrogenation of cyclohexane over heteropolymolybdates S. Hocine l, C. Rabia l, M.M. Bettahar2, M. Foumier2 1- Laboratoire de Chimie du Gaz Naturel, Institut de Chimie, BP32, El-Alia, 16111 BabEzzouar, Alger, Alg6rie 2- Laboratoire de catalyse H6t6rog6ne et Homog~ne, URA CNRS N~ 59655Villeneuve D'ASCQ Cedex, France.

B~timent C3,

The oxidation of cyclohexane in presence of molecular oxygen has been studied at 350 and 400~ over heteromolybdates catalysts pretreated at respectively 350 and 400~ under O2 stream. This study examined the relationship between the acid-base and redox properties of Keggin-type heteropolycompounds (HPA) and their catalytic behavior in the oxidation of various C6 hydrocarbons. The obtained results showed that HPA were very selective for the cyclohexane oxidative dehydrogenation to benzene and have pronounced catalytic activity which depends on pretreatment and reaction temperatures, nature of oxometal (Mo,V,Ni) in the primary structure and counter-ion (H30+,Cs+,Fe2+). 1. INTRODUCTION Particular attention is given to most recent developments in the use of Keggin type heteropolycompounds (HPA) as catalysts for the selective oxidation of hydrocarbons [1-4]. This reaction appear to be an alternative way to functionalize low molecular weight paraffins in substitution of the more expensive olefins to obtain oxygenated compounds. One key factor is the surface acidity of both Br6nsted and Lewis type and redox character useful to activate the C-H bond in the secondary carbon atom. The oxidative dehydrogenation of cyclohexane has been studied over various solids. On modified Ni /A1203 catalysts, it was demonstrated that the added metal (Sb, Pb or Cu ) displays an improving effect on the activity, stability and selectivity [5]. It was also shown that the cyclohexane dehydrogenation proceeded on the paired acid-base site over bifunctional catalyst TiO2-ZrO2 [6] and TiO2- ZrO2- V 2 0 5 [7] . On the other hand, Russell and coll suggested that cyclohexene and/or cyclohexadiene are probably intermediates in the dehydrogenation reaction [8]. Only dehydrogenation products (cyclohexene and/or benzene) were observed over vanadate [9] and molybdate [ 1 0 ] based catalyst. The heteropolycompounds (HPA), having well defined structure and the resulting acidic and redox properties, exhibited high oxidation abilities in the selective oxidation of alkanes in according with literature data [1-4] .Also, we have undertaken the study of the oxidative dehydrogenation of cyclohexane to benzene over HPAs. The present paper's results deals with the effect of the nature of central coordination atoms and that of the counter-ion on the catalytic activity of heterophosphomolybdate anions in this reaction.

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2. EXPERIMENTAL The HPA compounds have been prepared according to the literature data [ 11-13] and characterized by XRD and FTIR spectroscopy before and after reaction. Prior to the reaction, 0,2 g of each catalyst was treated in an oxygen stream (21/h) for 1 hour at reaction temperature. The reactions were performed in a continuous flow reactor at atmospheric pressure and in the range of temperature 250-400~ The reaction mixture was obtained by passing a carder gas (N2+ 02) through a saturator containing the cyclohexane and maintained at 280 K (partial pressure of cyclohexane: 40 mmHg ) with a total flow rate of 21/h and N2/O2 =1 ratio. Reactants (cyclohexane and 02) and reaction products (benzene, cyclohexene, cyclohexadiene, n-hexane, methane, CO2) were analyzed online by GC equiped with a column WCOT (glass 20m X 0.3mm coating Carbowax 20M). 3. RESULTS AND DISCUSSION

3.1 Catalysts Characterization The initial surface areas of the HPA catalysts are tabulated in table 1. BET surface areas of salts were l~reater than those of the corresponding acids. They ranged from 50m2/g for salts to around 3m'/g for acids. The XRD and FTIR studies showed that the whole catalysts have Keggin structure [14-15]. This structure was stable up to 350~ under reaction steam but decomposed at 400~ into the parent MOO3, P205, V205 simple oxides for the heteropolyacids as illustrated in Figure 1 for PMol2 anions [16]. In contrast, the corresponding salts were much more stable in the same operating conditions as it can be seen in Fig. 2 for CsFePMol2 solid. Table 1 BET surface areas of HPA Catalysts H4PMOI iVO40

H3PMoI204o (NH4)6HPMollNi04o

Cs2,sFeo,osHl,26PMollV040 Cs2,5Feo,osHo,26PMo1204o

"2'0 ...... 3'0....... 4'0....... 50 .... 60'

70 '(20)

Fig. 1" XRD patterms of H3PMol2040 (under air): a) 25 ~ b) 350~ e)400~

Surface areas (m2/g) 5 2,8

2,78 51 51

~l~-zu~ 304050

-~;~0::-----2060 ( )

Fig.2 9XRD patterms of CsFePMol204o (under air): a) 25~ b) 400~

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3.2 Catalytic activity 3.2.1Cyclohexane dehydrogenation The influence of the reaction temperature. The obtained results show that benzene was the major product at low conversion levels, n-hexane, cyclohexene, cyclohexadiene, methane, carbon dioxide and heavy product corresponding likely to a carbon chain rupture and polycondensation of several cyclohexane molecules were also observed. The main testing experiments are reported in Figures 3, 4 and 5 and Table 2. At steady-state, the obtained results showed that the catalytic activity of the solids was very sensitive to the reaction temperature and to the nature of the coordination ion (MoVl,vV, Ni u ) and to the counter-ion " (H30 §,Cs + , Fe 1I, N1.I1 ) (Table 2). A general trend was the increase of the conversion with the reaction temperature and the decrease in the selectivity of benzene. Thus, the conversion and benzene selectivity passed from 8.2 to 55.5% and from 90.2 to 34.3% respectively when the reaction temperature was increased from 250 to 350~ for PMol2. The substitution of a Mo ion by V ion did not influence the conversion (about 50% at 350~ but increased slightly the benzene selectivity (from 34 to 46%) (Table 2). This shows that the protons in anhydrous HPA are accessible to the reactant molecules, thus the high catalytic activity of HPA may be related to high acid strength and high mobility of protons which are responsible to the dehydrogenation of cyclohexane. This agrees with Chang and all. [7] that the dehydrogenation of cyclohexane was due to Br6nsted acid sites on the surface. Also, the substitution of Mo ion by Ni ion lead to a substancial decrease of conversion (from 8,2 to 0,3% ) and benzene selectivity (from 90,2 to 50%) at low temperature (250~ inversely at high reaction temperature (350~ the conversion and benzene selectivity increased from 55,5 to 72,0% and decreased 34,2 to 30,5% respectively (Table 2). In presence of PMo~Ni, heavy products of polycondensation are detected, but not identified, with a selectivity of around 40% at high reaction temperature. It can be result of the different degrees of oxidation of the catalysts. Thus at steady state, the surface of PMollNi is more reduced than PMolIV and PMol2. PMollNi will have a higher tendency to oxidize cyclohexane beyon benzene. PMol2, PMollVand PMollNi catalysts show difference, which could be related to their rates of reduction and reoxidation. More important effects were observed with the HPA salts. Thus,the salts showed higher activity at higher reaction temperature compared with the parent acids. For example, in the case of CsFePMol2, the conversion passes from 2,7 to 80,1% and the selectivity of benzene from 90,1 to 73,1% when the temperature rises from 300 to 400~ It demonstrate that the amount of Lewis acid sites are much higher than the amount of to Br6nsted acid sites on the HPA salts. It suggest that the Lewis acid sites are the active sites on acid-base bifunctional catalysts in the dehydrogenation of cyclohexane reaction as reported in a earlier paper [6]. We conclude that the acidic sites of catalysts play the most important role in the dehydrogenation of cyclohexane. Cyclohexene selectivity was more strongly decreased for the HPA salt than for the parent acid: it was about less than 5% against 22% at 350~ (Table 2). For all solids, in addition to benzene and cyclohexene, methane is also formed and the trend of the methane selectivity against conversion parallels the benzene selectivity (for T> 350~ the methane formation is attributed to the cracking of cyclohexane at high temperature. Besides to these

1898 products, n-hexane is detected with a selectivity of around 5% or less and cyclohexadiene is observed specially in the presence of acids at 350~ its selectivity is inferior than 8%. Carbon dioxide was the alone combustion product detected in presence of CsFePMoIIV and PMol iNi, with a selectivity less than 10%. The presence of cyclohexene and cyclohexadiene in small quantities among the reaction products means that they are produced as an intermediates in the dehydrogenation of cyclohexane to benzene, as indicated by other authors [8-9-10]. It appears that, selectivity for alkenes decreases with increasing conversion. This behavior agrees with the fact that alkenes are the primary products so their selectivity declines at higher conversion on account of the secondary reactions. Table 2 Oxidation of cyclohexane in ~resence of molecular oxygen over HPA. catalyst T(~ time (h) conv selectivity of product (%)

(%)

Ic .,o Icon. H3PMoI204o

H4PMo~IVO4o Cs2.5Feo.ogPMol2

CsFe0.0sPMollV (NH4)6HPMol tNi

250 350 250 350 300 350 400 350 400 250 350 400

8 8 8 8 8 8 8 8 8 8 8 8

8.2 55.4 2.5 56.1 2.7 10.5 80.0 4.9 56.9 0.3 72.0 90.0

90.2 34.2 86.2 46.0 90.1 89.7 73.1 86 59.5 50 30.5 42.9

22.2 21.2 0.5 3.4 8.6 43.4 4.0 5.5

7.8 5.2 -

[cr lco I o* 4.4 2.0 0.5 -

-

14.1 -

7.8 -

14.2 -

22.8 -

17.5 15.7

9.8 17.3 13.8 17.8 9.9 9.8 9.8 2.9 10.1 9.1 6.6 4.3 43.7 12.4 23.5 -

-

* Pc "polycondensation product no identified T h e

i n f l u e n c e

of

the

time

on

stream.

The variation of conversion and benzene selectivity were studied as a function of the reaction time (Fig 3, 4, 5). Steady-state activities were obtained within a period of few minutes or hours depending on the nature of catalyst. In presence of heteropolyacid, the steady-state activity of cyclohexane and benzene selectivity is reached after 4h. We observed a trend of increasing activity with decreasing benzene selectivity in initial period and then remains nearly constant. In the case ofheteropolysalts, at 350~ the conversion is lower (<11%) and seemed to be more stable during reaction time, likewise the benzene selectivity also stable, it is higher (>86%). However, pretreated and tested at 400~ (fig. 5), the heteropolysalts showed a different behavior. The conversion and benzene selectivity are higher and decreased slightly versus the time. These results revealed that the presence of Cs-Fe as counter-ions display an improving effect on the activity, stability and the benzene selectivity of HPA in the cyclohexane dehydrogenation.

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Fig.4: Selectivity of benzene as a function of time reaction over HPA catalysts. Reaction conditions: T=350~ N2/O2=1.

Fig.3" Cyclohexane conversion as a function of time reaction over HPA catalysts. Reaction conditions: T=350~

3.3 Cyclohexene dehydrogenation The dehydrogenation of cyclohexene on CsFePMol2 at 350~ showed very high conversion and benzene selectivity values (conv. =100%, Sben=90%) (fig. 6). The first step in the dominant pathway for further reaction of alkenes is the loss of an allylic hydrogen to from a surface allylic species. In the case of cyclohexene loss of another hydrogen from the surface allyl to form benzene occurs rapidely [9]. This result suggests that cyclohexene is the intermediate product for benzene formation. This observation correlates with those made with vanadate and molybdate alumina-supported catalysts [ 1-5-6].

120 120

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Fig.5: Cyclohexane conversion and selectivity of benzene as a function of time reaction over Cs2.sFeo.08PMol2 and Cs2.sFeo.08PMol~V catalysts. Reaction conditions: T=400~ N2/O2=1.

i

L

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1O0 200 conversion -~ benzene selectivity !

r

300 time(mn)

Fig.6: Cyclohexene conversion and selectivity of benzene as a function of time reaction over Cs2.sFeo.08PMol2 catalyst. Reaction conditions: T=400~ N2/O2=1.

1900 4. CONCLUSION In conclusion, the HPA exhibit high catalytic activities for dehydrogenation of cyclohexane. On these catalysts, the product distribution depends on operating conditions and the composition of the catalyst, which suggest that the nature of counter-ion and oxometal affects the properties of the catalyst. Heteropolyacid and heteropoysalts catalysts show interesting differences, which could be related to the different redox and acid-basic properties. The higher benzene selectivities on HPA can be the result of the simultanous presence of several metals (FeMo, FeMoV or MoNi ..... ) in the catalyst at different oxidation degrees. REFERENCES

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