Oxidative Dehydrogenation of The C4-C5 Paraffins Over Vanadium-Containing Oxide Catalysts

V. CortCs Corberdn and S. Vic Bcllon (Editors), New Developmenis in Seleciive Oxidation I1 0 1994 Elsevier Science B.V. All rights reserved.

125

OXIDATIVE DEHYDROGENATION OF THE C4-Cs PARAFFINS OVER VANADIUMCONTAINING OXIDE CATALYSTS R.G.

Rizayev, R.M. Talyshinskii, J.M. Seifullayeva, Guseinova, Yu.A. Panteleyeva and E.A. Mamedov

E.M.

Catalysis Division, Institute of Inorganic and Physical Chemistry, 29 Azizbekov Avenue, 370143 Baku, Azerbaijan Abstract

A series of binary and ternary vanadium-containing oxides were tested as catalysts for oxidative dehydrogenation of butane and isopentane. The most effective one was found to be aluminasupported Ni-V-Sb oxide system. Its activity and selectivity as well as the reaction kinetics significantly depend on a nature of the support pretreatment which influences the distribution of active components on the surface of the support.

1. INTRODUCTION

Many complex oxides, such as phosphates, molybdates, phosphomolybdates, ferrites and stannates of various metals, are known to accelerate the reactions of oxidative dehydrogenation of the C4-C5 paraffins. These types of catalysts are well studied and repeatedly reviewed [1,2]. Last years it has been shown that some vanadates, like Mg-V [3-51, Nd-V [5,6] and Mo-V [7-91, are also capable of catalyzing the oxidative dehydrogenation of alkanes. On their basis, more complicated catalytic systems were developed [9-131. This paper reports a new family ofvanadium-containing oxide catalysts which demonstrate a good activity and selectivity in oxidative dehydrogenations of n-butane and isopentane. 2.

EXPERIMENTAL

2.1.

Catalyst preparation

All catalysts were prepared by simultaneous impregnation of a commercial y-alumina spheres (1.8-2.0 mm diameter) with solutions of ammonium metavanadate and chlorides or nitrates of other metals in tartaric acid. The alumina had a specific surface area of 200 m2g-’and a pore volume of 0 . 5 0 cm3g-’. The samples were dried at 120 C for 2 h and then calcined in air in three steps at 200, 400 and 800 C f o r 2 h at each temperature. The total amount of supported components did not exceed 28 wt.%. In some cases, before contacting with the impregnating solutions the support was calcined at 900 C for 6 h and/or treated at 50 C for 1 h with a solution of acid (HC1, H3P04! or base (NH,OH) , followed by drying and calcining as described

126

above. The treating procedure as well as the impregnating one were carried out in a specially built installation equipped with a spectrophotometry and pH-metry techniques. Characterization Catalysts were characterized before and after reaction by argon thermodesorption surface area measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM) and infrared spectroscopy (IR) 2.2.

.

Catalytic tests The catalytic activity and kinetic measurements were carried out in a gradientless flow reactor with a vibro-fluidized bed of catalyst at atmospheric pressure. The feed was a mixture of paraffin and oxygen diluted in nitrogen and water vapour with a paraffin/oxygen and paraffin/water molar ratios ranging from 0.2 to 2.0 and from 8 to 30, respectively. The paraffin space velocity (GHSV) ranged from 50 to 800 h-’. The reaction temperature varied from 580 to 650 C. Under these conditions, the empty reactor showed very poor activity; for instance, the hydrocarbon conversion at 650 C did not exceed 5%. The absence of diffusion control was checked by varying the catalyst particle size as well as by changing the linear flow rate. The reaction products were analyzed by on-line gas chromatography. A Chrom-5 gas chromatograph equipped with a thermal conductivity detector was used. Helium was the carrier gas. A switching valve directed a pulse of reaction products into the gas chromatograph. Two columns were used in parallel; a 4-m column packed with 15% Tvin-80/Polysorb-l was used at 90 C to separate the hydrocarbons and carbon dioxide, and a 1.5-m molecular sieve NaX column was used at room temperature to separate oxygen and carbon monoxide. Experiments on catalyst long exploitation were carried out in a pilot scale installation using a vibro-fluidized reactor as well as a fixed-bed reactor with sectional feeding of oxygen. 2.3.

3.

RESULTS AND DISCUSSION

A l u m i n a - s u p p o r t e d v a n a d i u m p e n t o x i d e catalyzesthe reactions of one-stage oxidative dehydrogenation of n-butane and isopentane to butadiene and isoprene with a selectivity not exceeding 25 %. Both higher selectivity and yield of these products are characteristic of binary and ternary oxides listed in Table 1. Among these catalysts, the most effective one is aluminasupported Ni-V-Sb oxide which was studied in more detail. Its optimum composition was found [14] to be (wt.%): NiO - 9.0, Sb,O, - 11.0, V05 - 4.5 and A1,0, 75.5. The freshly prepared Ni-V-Sb catalyst is a polyphasic solid, consisting of nickel vanadate, antimony vanadate, nickel antimonate, and nickel and vanadium oxides. Nickel antimonate and antimony vanadate were found to be unstable under the reaction conditions, decomposing to the individual oxides. During the catalytic work, the formation of additional amount of the nickel vanadate was also observed.

-

127

Table 1 Catalytic properties of alumina-supported oxides in oxidative = dehydrogenations of n-butane and isopentane (C,H,,+,/O,/H,O 1/1/20; C,H,,, GHSV = 1 5 0 h-’) Butane, Catalyst

Yield of C4H6

(‘1

650

C

25.0 30.0 27.0 27.0 33.0 35.6

25.0 20.0 27.0 33.0 32.3 31.4

16.4

42.0

30.5

C

C5H8

C5H10

4.8 5.7 6.5 6.6 7.0

27.0 26.0 30.0 30.0 33.0

23.0 21.0 20.0 20.0 20.0

12.6

48.5 47.6

19.7 12.9

C4H8

8.0 9.0 8.0 8.5 12.0 13.6

605

Yield of Selectivity ( % ) C5H, ( % I

Selectivity ( % ) C4H6

Sn-V Mo-V Ni-V co-v Sb-V Bi-V-Sb Sn-V-Sb Co-V-Sb Ni-V-Sb

Isopentane,

11.0

Behaviour of the Ni-V-Sb/A1203 catalyst considerably depends on the state of the support surface before impregnation of active components. This conclusion comes from the data on catalytic properties of the samples supported on differently treated alumina (Table 2 ) . Six catalysts were prepared using the alumina pretreated as follows: - Sample 1: untreated; - Sample 2 : treated with a HC1 solution and not calcined; - Sample 3: untreated and calcined at 600 C; - Sample 4 : treated with HC1 and calcined at 600 C; - Sample 5: treated with H,P04 and calcined at 600 C; - Sample 6: treated with NH40H and calcined at 6 0 0 C. Table 2 Textural and catalytic properties ( 6 2 0 C; CnH10/OZ/H20= 1 / 0 . 5 / 2 0 ; C4HloGHSV = 150 h-’)of the Ni-V-Sb oxide supported on variously treated alumina

Sample

1 2

3 4 5 6

Surf.area (rn2g-’)

90 85 95

ao

95 85

Pore volume ( cm3g-’ )

0.65

0.63 0.68 0.64 0.70 0.65

Yield of C4H6 ( % )

13.3 17.8 14.8 16.7 14.6 13.7

Selectivity

(%)

C4H6

C4H8

36.9 47.1 40.2 45.0 41.2 40.5

43.3 24.9

31.1 29.5 39.8

41.8

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One can see that the alumina pretreatment with a solution of hydrochloric acid (samples 2 and 4) essentially enhances both the yield and selectivity toward butadiene, and practically does not change the extent of butane conversion which varied from 3 4 to 38%. Similar results were obtained for oxidative dehydrogenation of isopentane carried out over the same catalysts. The influence of the support pretreatment on catalytic properties of Ni-V-Sb oxide cannot be explained by a change of its surface area and/or pore volume which, as seen from Table 2, do not show any dependence on the nature of the pretreatment procedure. Also, it cannot be caused by structural changes since XRD and IR analysis did not reveal any difference in phase compositions of the catalysts 1-6, excepting small broadening of the SbVO, and NiV,O, lines observed for samples 2-4 that may be due to a higher surface dispersion of these phases. When using differently pretreated alumina to prepare a Ni-VSb/Al,O, catalyst, there are differences in the distribution of active components on the surface of support. Figure 1 represents the profile distribution, across the diametral section of the alumina sphere, for nickel, vanadium and antimony elements obtained through SEM technique.

0-

Radial

position ( m m )

Figure 1. Profile distribution of nickel, antimony and vanadium elements in the sphere of differently pretreated alumina.

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For all catalysts, V element is distributed homogeneously in the support, whereas Ni and Sb elements show some heterogeneities. It seems that the ammonium vanadate mainly impregnates alumina while nickel and antimony chlorides are adsorbed on it competing for the same adsorption sites. This idea has been confirmed by studying the adsorption of these compounds fromtheir solutions in tartaric acid using spectrophotometry and pH-metry techniques. According to the results of these measurements presented in Figures 2 and 3 , active elements have different capability to be adsorbed on y-alumina which changes as follows: Ni2' > Sb3+ >> V5+. So, upon impregnating alumina, vanadium is homogeneously distributed in the support whilst nickel and antimony are preferentially located at the edge of the alumina sphere. Taking into account this circumstance and the XRD data, it is reasonable to assume that in the calcined catalyst vanadium-containing phases, including Vz05, uniformly cover the inner surface whereas the NiSb206 phase is mainly concentrated at the edge of the effect , appearing sphere. The extent of such a llchromatographiclg during the adsorption of active elements on alumina, depends on the nature of support pretreatment being enhanced when treating it with a HC1 solution. One can suggest that this kind of pretreatment gives more optimum distribution of active phases in the support, thus providing higher selectivity toward oxidative dehydrogenation of the C,-C, paraffins to corresponding dienes. Another explanation could be related to the presence of chloride ions in the catalyst which are well known to increase the selectivity of partial oxidation reactions, including the oxidative dehydrogenation of ethane [15]. The enhanced selectivity is eventually lost completely after several hours on stream as a result of the chloride ions consumption. Although we did not determine the chlorine content of the catalyst before and after the reaction, this idea seems to be unlikely because our catalysts showed hardly any change in selectivity during several days of operation. The kinetics of oxidative dehydrogenations of n-butane and isopentane over the samples 1, 4 and 6 listed in Table 2 is described by the same model based on the step-wise scheme of redox mechanism of Mars - van Krevelen type. The rates of paraffin oxidative dehydrogenations to olefin and diene obey the expression ri=kiPlP,0.5 (P1 and P, are the paraffin and oxygen partial pressures), in which the value of the rate constant depends on the catalyst genesis, showing the maximum for the sample 4 . Smaller variations have been found for the kinetic rate constant kj in the equation rj=kjPIP, describing the paraffin total oxidation to carbon oxides. On the basis of kinetic data, mathematical modelling and optimization of processes were carried out. From a reactor design point of view, two modes of operation were predicted to have a good outlook. These are (i) a cascade of Eixed-bed reactors with a separate feeding of oxygen into each reactor, and (ii) a fluidized-bed reactor. Table 3 represents the results of the Ni-V-Sb/Al,O, catalyst activity measurements carried out for oxidative dehydrogenation of n-butane in the three-sectional fixed-bed reactor. This mode of operation provides a 22-23% yield of butadiene per single pass. When using a recycling technology, it increases up to 3 0 % . Similar results were obtained for oxidative dehydrogenation of isopentane in the four-sectional

130

1.7 f.6

-

A l

A

A

L\

1.5 -

k 7.4Q

/"

T 4

0

0.4

I V

I

d

I

-

c,

2

-

- 3

I

I

4

p '

Figure 2. Evolution of the p H of the NH4V03 (1), NiC1, ( 2 ) and S b C 1 3 (3) solutions when contacting to alumina.

Figure 3. Adsorption of NiCIZ (1), S b C 1 3 (2) and NH4V0, (3) on alumina as a function of their concentration in solutions.

131

fixed-bed reactor. Table 3 Oxidative dehydrogenation of n-butane to butadiene in the threesectional adiabatic reactor with a fixed bed of Ni-V-Sb/A1,03 catalyst (C,Hl,/H20 = 1/ (25-30): butane GHSV=300h-') Temperature in sections ( " C ) I 610 610 610 610 615 610

I1

I11

620

625 625

625 625 625

620 625 620 618 620

628

C,H,, conO,/C,H,, molar ratio in sections version ( % ) I1

I11

0.50 0.48

0.40 0.38

0.40

0.31 0.35 0.39

0.31 0.33 0.33

I

0.50

0.44

0.47

0.30

0.28 0.30 0.34

38.3 39.6 40.8 38.1 40.8 38.8

C4H6 selectivity ( % )

59.0

55.9 57.4 56.9 57.6 56.7

When using one-sectional fluidized-bed reactor, a higher total selectivity to butenes and butadiene at considerably lower C,H,,/H,O molar ratios was observed. In this case, however, there were some problems with a mechanical strength of catalyst spheres. As for the stability of catalytic activity, it did not change during a 500 h pilot scale run. 4. CONCLUSION

Vanadium, antimony and nickel (cobalt) mixed oxides supported on alumina seem to be promising catalytic systems for the oxidative dehydrogenation of alkanes. Their performance considerably depends on the nature of the support pretreatment which effects the distribution of active phases in the support sphere. The concentration of gas-phase oxygen is another key parameter affecting the selectivity since the intrinsic reaction rates exhibit different dependences on this parameter. To keep it optimal along the catalyst bed, a sectional feeding of oxygen can be used. This mode of operation has provided a 22-25% yield of butadiene (isoprene) per single pass. ACKNOWLEDGMENT

We gratefully acknowledge the help of Dr A.Shkarin during the SEM measurements. REFERENCES 1 2 3

V.K. Skarchenko, Russian Chem. Rev., 46 (1977) 1411. T.G. Alkhazov and A.E. Lisovskii, Oxidative Dehydrogenation of Hydrocarbons, Khimiya, MOSCOW, 1980. D. Patel, M.C. Kung and H.H. Kung, in M . J . Philips and M.

132

4

5 6 7 8 9 10 11 12 13

Ternan (eds.), Proc. 9th Inter. Congr. Catal., v01.4, p.1555, Chemical Institute of Canada, Ottawa, 1988. M.A. Chaar, D. Patel and H.H. Kung, J. Catal., 109 (1988) 463 D. Patel, P.J. Andersen and H.H. Kung, J. Catal., 125 (1990 132.

M.C. Kung and H.H. Kung, J. Catal. , 128 (1991) 287. E.M. Thorsteinson, T.P. Wilson, F.G. Young and P.H. Kasai, J Catal., 52 (1978) 116. R. Burch and R. Swarnakar, Appl. Catal., 70 (1991) 129. Y. Han, W. Lu, X. Gao and C. Hui, Shiyou Huagong (Chinese Petrochem. Technol.), 20 (1991) 309. F.G. Young and E.M. Thorsteinson, USA Patent 4250346 (1981). J.H. McCain, USA Patent 4524236 (1985). J.H. McCain, USA Patent 4568790 (1986). R.M. Manyik, J.L. Brockwell and J.E. Kendall, USA Patent

.

4899003 (1990) 14 V.S. Aliyev, R.G. Rizayev, R.M. Talyshinskii et al., USA Patent 4198586 (1980). 15 R. Burch, G.D. Squire and S.C. Tsang, Appl. Catal., 46 (1989) 69.

F . T R I F I R O ( U . of Bologna, Bologna, Italy): Is it possible that your catalysts are active in pure (non-oxidative)dehydrogenation and they oxidize molecular hydrogen, thus shifting the equilibrium? E . MAMEDOV (I. of Inorganic and Physical Chemistry, Baku, Azerbaijan): Our catalysts show poor activity in pure dehydrogenation which rapidly decreases as the reaction proceeds. As for the oxidation of molecular hydrogen, its rate is much lower than the rate of oxidative dehydrogenation of paraffin.

B.

DELMON (Catholic University of Louvain, Louvain-la-Neuve, Belgium): Why did you select Y-Al,O, as a support (here NiO segregates, and its action impairs the selectivity of your catalyst)? Did you compare with other supports?

E . MAMEDOV: We checked a series of commercial supports, and the best one was found to be y - A 1 2 0 3 . Its pretreatment with a HC1

solution reduces the segregation of NiO and facilitates the formation of nickel vanadate and nickel antimonate.

(I. of Chemical Physics, Moscow, Russia): What do you think about the possibility of gas-phase radical-chain reactions initiated by the catalyst surface, and about possible influence of porous structure?

0 . KRYLOV

MAMEDOV: Under the testing conditions, the empty reactor showed very poor activity (the paraffin conversion did not exceed 5%). As for the heterogeneous-homogeneous reactions and the influence of catalyst porous structure, we did not study these questions.

E.