Oxidative dehydrogenation of n-butane to butadiene

Applied

Catalysis A: General,

87 (1992)

29-43

29

Elsevier Science Publishers B.V., Amsterdam APCAT

A2286

Oxidative dehydrogenation of n-butane to butadiene Effect of different promoters on the performance of vanadium-magnesium oxide catalysts D. Bhattacharyya, Shyamal K. Bej and Musti S. Rao Department

of Chemical

Engineering,

Indian

Institute

of Technology,

Kanpur

208016,

U.P.

(India) (Received 21 November 1991, revised manuscript received 7 April 1992)

Abstract A detailed study has been performed on the effect of different promoters viz. CrZOs, MOO, and TiOz, which are added to vanadium-magnesium oxide (V-Mg-0) catalyst used for the production of butadiene from n-butane via oxidative dehydrogenation. The results are compared with those obtained from unpromoted V-Mg-0 catalyst. It was found that Cr,OB and MOO, promoters enhanced the activity of the catalyst, while TiOp enhanced the selectivity. Finally, a fifth V-Mg-0 catalyst containing both Cr,O, and TiO, as promoters was prepared and the reaction was studied in detail over this catalyst. A maximum yield of butadiene of 35.5 mol-% was obtained with this catalyst. Electron paramagnetic resonance studies showed that V02+ is the active species involved in the reaction. On comparison with the unpromoted V-Mg-0 catalyst, it was found that V-Mg-0, Crz03, TiOe gave a better yield ofbutadiene. Keywords: butadiene, n-butane, oxidative dehydrogenation, nesium oxide.

selectivity (butadiene),

vanadium-mag-

INTRODUCTION

Among the diolefin intermediates used in the petrochemical industry 1,3butadiene is an important one. Its demand has increased tremendously throughout the world due to the vast growth of the polymer industry. Conventionally, n-butane is first dehydrogenated to butenes and then to butadiene over a chromia-alumina catalyst at a high temperature (500-600’ C ) at which undesirable side reactions such as cracking of hydrocarbons and coking of the catalyst may occur. In practice, the chromia-alumina catalyst requires frequent regeneration. A major advance in the production of diolefins occurred when Hearne and Furman [l] showed that these compounds could be made Correspondence to: Prof. MS. Rao, Department of Chemical Engineering, Indian Institute Technology, Kanpur 208016, U.P., India. Fax. ( +91-512)250007, telex no. 325329 IITK IN.

0926-3373/92/$05.00

0 1992 Elsevier Science Publishers

B.V. All rights reserved.

of

30

D. Bhattacharyya

et al./Appl. Catal. A 87 (1992) 29-43

from C, alkanes by oxidative dehydrogenation using oxygen in the presence of some mixed oxide catalysts. Oxidative dehydrogenation, in which water is formed as a by-product instead of hydrogen, does not have the limitation of thermodynamic equilibrium and the catalyst deactivation is usually not a problem since coke and its precursors can be efficiently removed by oxygen. Selective oxidative dehydrogenation of n-butane to 1,3-butadiene is a challengingproblem. At the higher temperatures required to activate n-butane, the dehydrogenated products (viz. butenes and butadiene) react rapidly with oxygen to form stable combustion products like carbon dioxide and water. A suitable catalyst can overcome this limitation. Levin et al. [2] used a number of transition metal oxides and arranged the performance of these oxides. Gasper and Pasternak [3] established the feasibility of the one-step oxidative dehydrogenation of n-butane over molybdic acid-aluminium phosphate catalyst. Later on several researchers [4-61 have studied the effect of the addition of transition metal oxides. Hucknall [7] reviewed the performance of different catalysts used for this reaction. Shenoy and Rao [ 81 used bismuth molybdatealuminium phosphate catalyst for this reaction and obtained a maximum yield of 13 mol.%. Singh et al. [9] used Bi-MO-0 catalyst incorporating iron oxide supported on alumina for this reaction. Most recently, Chaar et al. [lo] reported vanadium-magnesium oxide (V-Mg-0 ) to be a better selective catalyst for the oxidative dehydrogenation of n-butane to butadiene. It is generally accepted that in a selective oxidation reaction, the catalyst gives up lattice oxygen to take part in the oxidation reaction and the reduced catalyst then replenishes oxygen from the gas phase. The evolution of lattice oxygen is usually promoted by the interaction between the metal oxides of the components [ 111. The existing interaction between metal oxides can be changed favourably by the addition of a suitable third metal oxide. Different transition metal compounds are generally added as promoters to improve the performance of catalysts used for various selective oxidation reactions. Hodnett [ 121 and Hutchings [ 131 reviewed the performance of different promoters which are added to vanadium-phosphorus oxide (VPO) catalyst for the selective oxidation of n-butane to maleic anhydride, an industrially important oxidation reaction. Amenomiya et al. [ 14 ] described the effect of the addition of different promoters on various catalysts used for the oxidative coupling of methane. Information on the performance of promoters when added to V-Mg-0, a promising catalyst for the production of butadiene from n-butane is rather scanty. In the present investigation, the effects of the addition of some transition metal oxides (MOO,, Crz03 and TiO,) to V-Mg-0 catalyst as promoters have been studied. A better selective catalyst was then formulated and its performance was investigated thoroughly.

31

D. Bhattacharyya et al./Appl. Catal. A 87 (1992) 29-43 EXPERIMENTAL

Catalyst preparation

The catalysts used in the present study were prepared by the procedure described by Chaar et al. [lo]. 1.235 g of ammonium vanadate was added to an aqueous solution containing 243.3 ml of double distilled water and 5.3 ml of ammonia solution. Ammonium vanadate was dissolved while stirring the mixture. At 70’ C, 3.04 g of magnesium oxide was added to the ammonical solution of ammonium vanadate with continuous stirring. While stirring, the suspension was evaporated to dryness. The resulting solid was calcined at 550’ C for 6 h. The solid was lightly crushed and used as a catalyst. The above procedure was followed for the preparation of unpromoted 24 V-Mg-0 catalyst (containing 24 wt.-% Vz05 and 76 wt.-% MgO). For preparing promoted catalysts, the procedure as described so far was followed except for the addition of the appropriate amount of the respective promoter to the ammonical solution of ammonium vanadate along with magnesium oxide. The added amounts of different promoters are given in Table 1. Catalyst characterization

The catalysts used in the present study were characterized in terms of X-ray diffraction, surface area measurements and electron paramagnetic resonance (EPR) studies. TABLE 1 Amounts of materials used for catalyst preparation Catalyst

24 V-Mg-0 (A) 24 V-Mg-0 promoted MOO, (B) 24 V-Mg-0 promoted Cr@, (C) 24 V-Mg-0 promoted TiO, (D) 24 V-Mg-0 promoted Cr,O, and TiO, (E)

Promoter

by by by by

Molybdic acid Chromoium oxide Titanium oxide Chromium oxide and titanium oxide

Weight ratio

Amounts added (g) Ammonium vanadate

Magnesium oxide

Promoter

V:Mo=lO:l

1.235 1.235

3.04 3.04

0.0928

V:Cr=lO:l

1.235

3.04

0.0786

V:Ti=lO:

1

1.235

3.04

0.0897

V:Cr=lO: V:Ti=lO:l

1

1.235

3.04

0.0786 (Cr&) 0.0897 (I%)

32

D. Bhattacharyya

et al./Appl. Catal. A 87 (1992) 29-43

X-ray diffraction studies were carried out using an iso-debyeflex 2002 X-ray diffractometer using Cu Ko! radiation. Surface areas of the catalysts were measured with the help of a BET Quantasorb instrument using nitrogen as adsorbate in a helium carrier. EPR spectra were recorded on a Varian E-109 EPR system with diphenylpicryl-hydrazil (DPPH) as the standard. Catalytic activity measurement The experimental setup used for the measurement of catalytic activity is of a conventional type. Oxygen and nitrogen gases were supplied from respective cylinders. The flow-rates of both these gases were measured by using separate soap-bubble flowmeters. Flow-rates were checked from time to time and the same were found to be constant for a particular setting of the needle valve. Both the gases were then passed through carbon dioxide removal tubes and drying tubes separately. High purity n-butane was also supplied from a cylinder. The flow-rate was measured by a soap-bubble flowmeter. This was then dried and finally mixed with nitrogen and oxygen in a mixing chamber. The mixture of gases was fed to the reactor after preheating it in a preheater. The temperature of the reactor was controlled by using two temperature controllers (supplied by Indotherm Instruments Ltd., Bombay, India). The reactor was a stainless steel tube of 1.35 cm I.D. The approximate length of the reactor was 60 cm including the inlet and outlet calming sections and the reactor section. The desired amount of catalyst (approximately 0.3 g), in the form of a fine powder, was placed in the central portion of the reactor, sandwiched between silica and quartz wool, respectively, at the top and bottom. The effluent gases from the reactor were bubbled through a water bubbler to dissolve any liquid product formed. The temperature of the bubbler was kept at 10°C by passing refrigerated water through its jacket. The gases were then passed through a condenser kept at 6°C by a flow of refrigerated water. The non-condensed gases were then sent to a gas chromatograph for analysis after passing them through a drying tube. The non-condensed gases were analyzed using two different columns, viz. chromatopak (n-octanol on solid Porasil C ) and molecular sieve 5A. The gaseous hydrocarbons and carbon dioxide were separated in a 3-m long chromatopak column and analyzed using a thermal conductivity detector. Oxygen, nitrogen and carbon monoxide were separated in a 2-m long molecular sieve 5A column and analyzed using a thermal conductivity detector. The liquid dissolved in water was analyzed using a Porapak-QS column and detected by using a flame ionization detector. After all the analyses were completed, checks on carbon balance were made. In all cases, negligible amounts of liquid products were formed.

D. Bhattacharyya

et

33

al./Appl. Catal. A 87 (1992) 29-43

RESULTS AND DISCUSSION

Results of characterization X-ray diffraction studies have been carried out for all the five calcined catalysts (viz. catalyst A, B, C, D and E). The values of the “d” spacings and calculated intensities are given in Table 2 along with the values reported by Chaar et al. [lo]. From the results it is clear that the compound formed in all the catalysts is the same as reported by Chaar et al. On the other hand, it can be concluded that the addition of different promoters did not prevent the formation of the main compound. The results of surface area determination are given in Table 3. From the results it appears that all the catalysts have nearly the same surface area. On the other hand, it can be reported that the addition of different promoters did not change the surface area of V-Mg-0 catalyst. An EPR study has been carried out for the unpromoted V-Mg-0 catalyst and V-Mg-0, Cr203, TiO, (catalyst E) . Both fresh as well as spent catalysts TABLE 2 “d” Spacings and relative intensities for different catalysts “d” Spacing (8) (relative intensity) 24 V-MB-0 (A)

24 V-Mg-0, Moo, (B)

24 V-Mg-0, Cr& (C)

24 V-Mg-0, TiOP (D)

24 V-Mg-0, Cr203, TiOz (E)

24 V-Mg-0”

2.106 (100) 1.484(54) 1.214( 15) 1.052(6) 0.942(15) 0.0857(10)

2.106(100) 1.514(40) 1.487(85) 1.215(14) 1.051(7) 0.942 (10)

2.047(100) 1.462(66) 1.208(16) 1.041(8) 0.935(11) 0.855( 15)

2.1016( 100) 1.485(70) 1.214( 15) 1.050(9) 0.942(9)

2.098( 100) 1.487(64) 1.2135(11) 0.941(14) 0.8583 (15)

2.43 (6) 2.10(100) 1.49(53) 1.27(5) 1.215(11) 1.052(5)

“Reported by Chaar et al. [lo]. TABLE 3 Surface areas of different catalysts Catalyst

Surface area (m’/g)

A B C D E

81 85 83 80 89

34

D. Bhattacharyya et al./Appl. Catal. A 87 (1992) 29-43

(after the reaction) were used as samples for EPR studies. Spent catalysts were collected after stopping the reaction at 560’ C and immediately transferring the catalysts in an atmosphere of nitrogen. The EPR spectra (not shown here) were taken for both fresh and spent, promoted and unpromoted V-Mg-0 catalysts. All combinations of the catalysts used for EPR investigation at room temperature under an identical experimental setup show the spectra (having a g value of 2.00) which closely match with those of V02+ species [ 151. Since the formation of butadiene from n-butane is a redox reaction, the possibility of the formation of V5+ or V3+ or both during the course of the reaction can be expected. V5+ is EPR inactive and, therefore, will not show any EPR spectra. If V3+ (which is paramagnetic) is formed, it should remain in the oxide lattice and may interact magnetically with the adjacent V02+ centre. If this is the case, then the EPR line shape in the spent catalyst should have changed. Since there was no difference in the line shape for V02+ signals between fresh and spent catalysts, one may conclude that V3+ is not present in the catalytic system. However, two more possibilities exist. During the course of the reaction, there may be a possibility of V3+ generation in situ which could only have been observed by monitoring the catalyst at that time at high temperature using EPR. Secondly, if the magnetic interaction between V3+ and V02+ centres are faster than the EPR time scale, then EPR can not also resolve this possible interaction. Comparison of the catalytic performance of the promoted and unpromoted V-Mg-0 catalyst All experiments were carried out in the presence of negligible intra- and inter-particle resistances. The absence of intra-particle resistance was verified by comparing the rates of butadiene formation and n-butane consumption using the catalyst in the form of pellets and fine powder. To make sure that pore diffusional resistances are eliminated, subsequent runs were taken with finely powdered catalysts. The inter-particle transport effect was checked by changing the feed rate at a constant contact time of the reactants. The results are plotted in Fig. 1. From the figure it was noted that beyond a total flow-rate of 60 ml/min the results are insensitive to changes in flow-rate. Thus in all subsequent experimental runs, a flow-rate of 100 ml/min was used. This study has been carried out with a view to determine the effects of different transition metal oxide promoters when added to V-Mg-0 catalyst. This may throw some light on the development of a better catalyst for this reaction. From this point of view, initially three promoted catalysts (Catalyst B, C and D ) were prepared by adding MOO, (added as molybdic acid), Crz03 and TiO, respectively to V-Mg-0 catalyst. However, the exact role of these oxides with V-Mg-0 was not fully known. To investigate the performance of the promoted catalysts for the reaction of n-butane to 1,3-butadiene, experiments were con-

D. Bhattacharyya et al./Appl. Catal. A 87 (1992) 29-43

0

20

40

60

80

100

35

120

Total tlow rato(cc/min)

Fig. 1. Effect of flow-rate on conversion, selectivity and yield. Symbols: ( q ) conversion, selectivity and (0 ) yield.

(A )

ducted and the activity and selectivity were measured with respect to three process variables viz., reaction temperature, contact time of feed stream (W/J’) and n-butane feed concentration. The unpromoted V-Mg-0 catalyst was used as a reference catalyst in this study. The following important dependent variables were considered for this study. Conversion percentage = mol of butane consumed/mol of butane fed. 100. Selectivity percentage = mol of (butenes + butadiene) formed/m01 of butane converted* 100. Effect of reaction temperature on conversion and selectivity Experiments were carried out at five different temperatures in the range of 510-63O”C, keeping the n-butane concentration at 4% and a constant n-butane-to-oxygen ratio of 1: 2. The contact time of the feed stream was kept constant at 73.85 g gmol-’ min-l. Fig. 2 shows the effect of reaction temperature on conversion of n-butane and selectivity for butenes plus butadiene. The conversion of n-butane gradually increased for each catalyst with increase in temperature. The selectivity was found to decrease with increasing temperature for all the catalysts except for catalyst C for which it increased first and then decreased. The drop in selectivity for catalyst B was steeper compared to the drop in selectivity for the other catalysts. A was more selective than catalysts B and C while catalyst D seems to have better selectivity for the dehydrogenated products along the entire range of temperatures. It was observed that the yield (defined as: percentage yield = percentage conversion multiplied by percentage selectivity/lOO) of dehydrogenated products

D. Bhattacharyya et al.JAppl. Catal. A 87 (1992) 29-43

36

-70

-60 F -505

$50-

E

E 4.

.‘,40r : s30-

: .? t w

-302

20 -

-

10 -

-10

01 500

I

I

550

600

Reaction

temperature

20

, 62 (‘C)

Fig. 2. Effect of reaction temperature on conversion and selectivity. Symbols: (0 ) catalyst A, (A ) catalyst B, (0 ) catalyst C and (0 ) catalyst D.

for catalysts A, B and C increased with increase in temperature and then decreased after passing through a maximum. It may be the result of further oxidation of butadiene to oxides of carbon at higher temperatures. The maxima occurred between temperatures of 570 and 600°C. However, with catalyst D, the yield gradually decreased with increase in temperature. Effect of contact time on conversion, selectivity and yield In order to study the effects of contact time on conversion, selectivity and yield further experiments were conducted at four different values of contact times ranging from 49.23-123.08 g gmol-’ min-‘. The reaction temperature and n-butane feed concentration were kept constant at 570°C and 4 mol-%, respectively. The n-butane-to-oxygen ratio was kept at 1: 2 for all the experiments. Fig. 3 shows the effect of contact time on conversion and selectivity for all the catalysts. In each case, with increasing contact time, the conversion also increased reflecting the irreversible nature of the reaction. The order of activity can be arranged for the different catalysts as C > B > A > D. In all cases the selectivity passed through a maximum. For all catalysts the maximum selectivity occurred at a contact time of 73.85 g/gmol. The maxima may be attributed to the complex (consecutive and parallel) nature of the reactions.

D. Bhattacharyya

37

et al./Appl. Catal. A 87 (1992) 29-43

60-

-60

50-

-50 2

2 z E&O-

--1o 2 B ._ >

: .? !g30-

-30

t 74 u-l

5 " -20

20-

-10

20

LO

60 Contact

80 timo(

g/g

100 mall

120

140

min)

Fig. 3. Effect of contact time on conversion and selectivity. Symbols are the same as in Fig. 2.

Effect of n-butane feed concentration on conversion and selectivity To understand the effect of n-butane concentration in the feed on the activity and selectivity of the different catalysts, experiments were carried out at a temperature of 570” C at four different levels of n-butane concentrations. The values of contact time and oxygen-to-n-butane ratio were kept constant at 73.85 g gmol-l min-l and 2 : 1, respectively. The oxygen-to-n-butane ratio was kept at 2 : 1, which enabled a comparison between experimental yield with that of literature value [lo]. Fig. 4 shows the effect of n-butane concentration on conversion and selectivity. The conversion was found to increase with an increase in the n-butane concentration. Catalyst A was more active compared-to catalyst B at lower concentrations but the reverse behaviour was observed at concentrations greater than 3%. At higher concentrations, the activities of catalysts B and C tend to level off. Here also catalyst D was found to be the least active while catalyst C was the most active one. It is noticed that the selectivity decreased steeply with an increase in nbutane concentration. The overall yield increases gradually with a tendency to level off at higher n-butane concentrations. Possibly the rise in conversion with increase in n-butane concentration is a common feature observed in the oxidation of paraffinic hydrocarbons. Alkanes are poor in their affinity for the catalyst due to their inherent low reactivity

D. Bhattacharyya et ul.fAppl. Cutul. A 87 (1992) 29-43

38

70-

60-

60

a = 50-

-502

0 E

z E -4o.g

.:40-

.? ; w

r w z -30

s30-

20-

-20

IO-

-10

0' 1

2

3

4

5

6

&

7O

Cont. of n-butane(mol%)

Fig. 4. Effect of n-butane Fig. 2.

concentration

on conversion

and selectivity.

Symbols are the same as in

and so the amount of adsorbed and/or activated molecules may increase pro’ portionally to the concentration in bulk gas phase. Formulation of the final catalyst and study of its performance Having looked carefully into the results of the comparative study of the promoted catalysts it is obvious that Cr,O,-promoted catalyst (catalyst C) was the most active one and TiO,-promoted catalyst (catalyst D) was the most selective one. The higher selectivity with catalyst D was achieved with a penalty in the conversion of n-butane. The activities and selectivities in the oxidation reactions are possibly governed by the acid-base properties of the catalyst and the reactant [ 161. The dehydrogenated products, butenes and butadiene, are considered to be basic compounds because of their double bonds. The basicity of butadiene is greater than that of butenes. A necessary condition for an effective catalyst for the production of dehydrogenated basic compounds is the possession of sufficient basic prcperty. V-Mg-0, Ti02 catalyst is basic enough to produce basic compounds. But V-Mg-0, Moos catalyst is sufficiently acidic. As a result, the basic compounds produced on V-Mg-0, MOO, catalyst are easily activated by

D. Bhattacharyya

et al./Appl. Catal. A 87 (1992) 29-43

39

the acidic catalyst and subsequently reoxidized to oxides of carbon. V-Mg-0, Cr,O, being intermediate in nature, plays a role in between the above two extremes, and thereby an intermediate selectivity was observed. Coupling of highly’ selective promoter TiO, and highly active promoter Cr,O, together may lead to a better catalyst having appreciable activity and selectivity. Accordingly, a final catalyst (catalyst E ) comprising of V-Mg-0, Cr,O, and TiOa with V : Cr : Ti weight ratio of 10 : 1: 1 was prepared and its performance was studied with respect to reaction temperature, contact time of reactants, n-butane concentration and oxygen-to-n-butane ratio in the feed respectively. Effect of reaction temperature on the performance of catalyst E With the final catalyst V-Mg-0, Cr203, TiOz (catalyst E) experiments were conducted at five different temperatures, using 4 vol.-% n-butane and 8 vol.-% oxygen (the rest being nitrogen) in the feed. The total flow-rate was 100 ml per min and 0.3 g of catalyst was taken. The effects of reaction temperature on conversion, selectivity and yield are shown in Fig. 5. An increase in conversion was observed with an increase in temperature. However, starting from 59 mol-%, the selectivity slightly increased to 63.5 mol-% after which it decreased. A similar kind of behaviour was observed for yield. A maximum yield of 33.8 mol-% was observed at about 570°C.

550 Reaction

600 temperature

650

(‘C)

Fig. 5. Effect of reaction temperature on conversion, selectivity and yield for catalyst E. Symbols are the same as in Fig. 1.

D. Bhattacharyya et al./Appl. Catal. A 87 (1992) 29-43

40

Effect of contact time on the performance of catalyst E The effect of contact time on conversion, selectivity and yield was investigated by carrying out experiments at four different values of W/F. Total flowrate was kept constant at 100 ml per min, while the weight of catalyst was varied. The feed stream comprised of 4 vol.-% n-butane, and 8 vol.-% oxygen, the rest being nitrogen. Fig. 6 shows the effect of contact time on conversion, selectivity and yield. A maximum yield of 36 mol-% was obtained at a contact time of 90 g gmol-’ min-‘. Effect of n-butane feed concentration on the performance of catalyst E The reaction was conducted by changing n-butane feed concentration from 2-6 vol.-% while oxygen-to-n-butane ratio was kept constant at 2.0. Reaction temperature was chosen to be 570°C since at this temperature the yield was maximum. Fig. 7 shows the conversion, selectivity and yield at different nbutane feed concentrations. The conversion increased almost proportionally with increase in n-butane concentration. At the same time the selectivity decreased steadily with an increase in the n-butane concentration.

20

40

60 contact

80

time(glg

100

mol lmin

120

140

1

Fig. 6. Effect of contact time on conversion, same as in Fig. 1.

selectivity

and yield for catalyst E. Symbols are the

D. Bhattachatyya

41

et al./Appl. Catal. A 87 (1992) 29-43

s 0 E 50-

0 1

I

I

I

I

I

2

3

4

5

6

Cont.

ot

n-butane

(ml

Fig. 7. Effect of n-butane concentration bols are the same as in Fig. 1

0* 0

0.5

1.0 Oxygen

1.5 to

butane

Fig. 8. Effect of oxygen-to-n-butane bols are the same as in Fig. 1.

7

“I.1

on conversion,

2.0

selectivity

and yield for catalyst E. Sym-

2.5

ratio

ratio on conversion,

selectivity

and yield for catalyst E. Sym-

Effect of oxygen-to-n-butane ratio in feed on the performance of catalyst E The oxygen-to-n-butane ratio is an important variable in the study of oxidative dehydrogenation reactions. The choice of oxygen-to-n-butane ratio is

D. Bhattacharyya et al./Appl. Catal. A 87 (1992) 29-43

42

governed by the explosion range of the n-butane-oxygen-nitrogen mixture. Again at a very low oxygen concentration in the feed, the catalyst is likely to get deactivated faster. In view of the above facts, the lower and upper limits of the oxygen-to-n-butane ratio were kept at 0.5 and 2.0 respectively for this study. n-Butane feed concentration was kept at 4 vol.-% and the reaction temperature at 570°C. Fig. 8 shows the dependence of conversion, selectivity and yield on the oxygen-to-n-butane ratio in the feed stream. The enhanced catalytic activity at higher oxygen concentrations may be attributed to the continuous removal of coke from the catalyst site. CONCLUSIONS

The aim of the present investigation was to study the effects of different promoters added to V-Mg-0 for the selective oxidation of n-butane to butadiene. Accordingly, four catalysts with different promoters, viz., 24 V-Mg-0 (A), 24 V-Mg-0, MOO, (B), 24 V-Mg-0, Cr,Os (C) and 24 V-Mg-0, Ti02 (D ) were prepared. The detailed experimental investigation of these catalysts with respect to three variables, viz., the reaction temperature, contact time and n-butane feed concentration indicated the order of activity to be: 24 V-Mg-0, CrzO, > 24 V-Mg-0, MOO, > 24 V-Mg-0 > 24 V-Mg-0, TiOz and the order of selectivity as 24 V-Mg-0, TiO, > 24 V-Mg-0, Cr,Os > 24 V-Mg-0 > 24 V-Mg-0, Moos. It is likely that butenes and butadiene, being basic in nature, interact more strongly with acidic catalysts than with relatively basic catalysts. A fifth catalyst 24 V-Mg-0, Cr203, TiOz (E) incorporating both Cr,OB and TiOa showed not only higher activity but also better selectivity. Its performance was investigated in detail. A maximum yield of 35.6 mol-% was obtained with this catalyst. The yield obtained with 24 V-Mg-0, Cr203, TiOz was higher than that reported in the literature [lo],with simple 24 V-Mg-0 catalyst. EPR investigation suggests a very low chance of the operation of V4+ eV3+ redox cycle for this oxidation reaction. ACKNOWLEDGEMENT

The authors acknowledge with thanks the fruitful discussions they had with Professor S. Sarkar, Department of Chemistry, IIT, Kanpur on the EPR studies.

REFERENCES 1 2

G.W. Hearne and K.W. Furman, U.S. Patent 2 991320 (1961). V.A. Levin, T.P. Vernova, T.P. Tuktarova and A.L. Tsailingold, Kinet. Katal., 13 (1972) 454.

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N.J. Gasper and I.S. Pasternak, U.S. Patent 3 320 331 (1967). S.A. Venyaminov, G.B. Barannik, A.N. Pitaeva, N.N. Sazonova and L.M. Plyasova, Kinet. Katal., 18 (1977) 462. G.A. Stepanov, A.L. Tsailingold, V.A. Levin and F.S. Pilipenk, in T. Seiyama and K. Tanabe (Editors), New Horizons in Catalysis: Part B, Stud. Surf. Sci. Catal. Vol. 7, Elsevier, Amsterdam, 1981, p. 1293. V.A. Doroshenko, L.P. Shapovalova and D.N. Tmenov, J. Prik. Khim., 55 (1982) 80. D.J. Hucknall, Selective Oxidation of Hydrocarbons, Academic Press, London, 1974, Chapter 4. S.C. Shenoy and M.S. Rao, J. Chem. Tech. Biotech., 36 (1986) 110. D. Singh, J.K. Gehlawat and M.S. Rao, J. Chem. Tech. Biotech., 47 (1990) 127. M.A. Chaar, D. Patel, MC. Kung and H.H. Kung, J. Catal., 105 (1987) 483. R.K. Grasselli and J.D. Burrington, Adv. Catal., 30 (1981) 133. B.K. Hodnett, Catal. Rev. Sci. Eng., 27 (1985) 373. G.L. Hutchings, Appl. Catal., 72 (1991) 1. Y. Amenomiya, V.I. Birss, M. Goledzinowski, J. Galuszka and A.R. Sanger, Catal. Rev. Sci. Eng., 32 (1990) 163. B.A. Goodman and J.B. Raynor in H.J. Emeleus and A.G. Sharpe (Editors), Advances in Inorganic Chemistry and Radiochemistry, Vol. 13, Academic Press, New York, 1970, p. 136. M. Ai and S. Suzuki, J. Catal., 26 (1972) 202.