Selective oxidation of isobutene over bismuth molybdate catalyst

Applied Catalysis, 66 (1990) 383-393 Elsevier Science Publishers B.V., Amsterdam

383

Selective oxidation of isobutene over bismuth molybdate catalyst F. Benyahia*” and A.M. Mearns Department

and Process Engineering, University of Newcastle-upon-Tyne, NE1 7RU(U.K.), tel. (+44-91)2227270

of Chemical

Newcastle-upon-Tyne

(Received28 June 1990 1

Abstract Initial rate data were used to study the kinetic behaviour of the selective oxidation of isobutene to methacrolein over y-bismuth molybdate at low conversion levels. The investigation was carried out in a differential flow reactor over the temperature range 623 to 729 K. The data were fitted to a number of models based on Mars-van Krevelen and Langmuir-Hinshelwood mechanisms, and also to the empirical model known as the power rate law model. The preferred model for the formation of methacrolein is found to be the redox model with non-dissociated oxygen involved in the reoxidation step. The activation energy for catalyst reduction was found to be 52 kJ/mol while the value for catalyst reoxidation was 142 kJ/mol. Keywords: Isobutene oxidation, bismuth, molybdate, methacrolein, kinetics, redox model.

INTRODUCTION

Catalytic oxidation of alkenes has been extensively reviewed by Voge and Adams [ 11, Morgolis [2] and Hucknall [3], and specific publications on the mechanistic aspects of catalytic oxidation can be found in Sampson and Shooter’s [4] and Sachtler’s [5] reviews. Kinetic studies of the selective oxidation of propene over bismuth molybdate catalysts have been carried out by Krenzke and Keulks [ 61 and Monnier and Keulks [ 71. The propene dependency was reported to change with temperature from 0 to 1 while that of oxygen varied from 0.4 to 0 as temperature increased. The Arrhenius plot showed a break and two values of activation energy were found. These range from 63 to 180 kJ/mol for the selective oxidation to acrolein, depending on the range of temperatures studied. A more recent study by Kremenic et al. [ 81 of the oxidation of propene over a Mo-Pr-Bi catalyst revealed that the data were best fitted by equations derived form the redox Mars-van Krevelen mechanism and the stationary state “Present address: Department of Chemical Engineering, University of Leeds, U.K., tel. ( + 44532)332410, fax. ( +44-532)332405.

0166-9834/90/$03.50

0 1990 Elsevier Science Publishers B.V.

384

of the adsorption model, assuming in both cases non-dissociative adsorption of oxygen. They also found nearly first order dependency on propene and a fractional order of 0.4 decreasing to 0.1 for oxygen as temperature varied from 593 to 653 K. Activation energy values for the selective oxidation of propene to acrolein have been found to be from 50 to 250 kJ/mol by various workers [g-12]. In contrast to propene, the kinetics of the catalytic oxidation of isobutene have received considerably less attention. Mann and Ko [ 131 studied the oxidation of isobutene over a copper promoted bismuth molybdate catalyst in the range 573-833 K. The rate of formation of methacrolein was satisfactorily correlated by a Langmuir-Hinshelwood mechanism which assumes the rate-controlling step to be the surface reaction between adsorbed isobutene and oxygen species. Ray and Chanda [ 141 oxidised isobutene over bismuth molybdate in the temperature range 623-773 K and found that methacrolein and carbon oxides formation rates were independent of oxygen partial pressure and first order in isobutene when the oxygen: isobutene ratio was higher than 6: 1. They also found that methacrolein and carbon oxides were formed by parallel routes. Vinogradova et al. [ 151 studied the oxidation of isobutene in the temperature range 593-673 K, over a molybdenum-cobalt oxide catalyst modified with bismuth and iron. They found that methacrolein, methacrylic acid and carbon oxides are formed according to a parallel-consecutive scheme and that the total oxidation was first order in oxygen, while selective oxidation is first order in oxygen for oxygen: isobutene ratios below 3 and zero order for higher oxygen: isobutene ratios. The activation energy for methacrolein formation was found to be 138 kJ/mol. In this study, the kinetics of the selective oxidation of isobutene over y-phase bismuth molybdate were investigated and various kinetic models were tested for goodness of fit and agreement with observed data. As well as correlation coefficients, Exner’s t,~[ 161 was used to discriminate between models. EXPERIMENTAL

Catalyst preparation

and gases

Bismuth molybdate catalyst was prepared by the co-precipitation method of Batist [ 171. A mixture of solutions of bismuth nitrate and ammonium heptamolybdate (BDH, Analar) held at pH 5 with ammonium hydroxide, was evaporated and dried at 383 K for 24 h. The mass obtained was calcined at 793 K in air for 2 h to give a bright yellow cake from which sieve fractions of 0.7 to 1.2 mm were obtained. The catalyst was characterised by surface area measurement (0.54 m”/g) using a Micromeritics (Rapid Surface Area Analyser and by powder X-ray

385

diffraction which identified the catalyst as essentially the y-phase of bismuth molybdate (Bi,O,.MoO,). The gases used were isobutene C.P. grade (99% ), high purity oxygen and nitrogen (99.97% ) from British Oxygen. Kinetic measurements and analysis system The kinetic measurements were carried out in a flow system employing a Pyrex-glass differential tubular reactor (1.3 cm I.D.) with a concentric thermowell (0.8 cm O.D.) to monitor the temperature in the catalytic bed (6.26 cm3 ). The reactor was heated in a Tecam (model SBL-2) fluidised sand bath, supplied by Techne (Cambridge). An on-line gas chromatograph [ Perkin-Elmer model F30 fitted with a flame ionisation detector (FID ) ] was used to analyse isobutene and methacrolein separated by a column packed with 3% XF1150 + 10% Octoil S on Chromosorb P.AW, 80/100 mesh. A Perkin-Elmer model 452 gas chromatograph fitted with a thermistor detector was used to analyse the permanent gases (N,+O,, CO,) separated on a column packed with Poropak Q, 80/100 mesh. The catalyst was conditioned for several hours until steady activity was observed. Runs with dummy catalyst (glass chips of similar granular size as the catalyst ) did not produce products in measurable quantities. Conversion levels were kept low by adjustment of space time and did not exceed 20% at the highest temperature studied. The rates of formation of methacrolein and depletion of isobutene were calculated from the differential reactor equation after measuring the appropriate reactant conversion and product concentration. The external heat and mass transfer limitations were investigated and were found to be negligible. This was carried out by calculating the value of the nonisothermal effectiveness factor [18] in terms of experimental observable quantities and physical and chemical data of the reactants and catalyst. In the absence of heat and mass transfer limitations, the external effectiveness factor should be equal to unity. The isobutene dependency was obtained from runs whereby the concentration of oxygen was kept constant (0.45 atm) and that of isobutene was allowed to vary (0.25-0.05 atm, 1 atm = 101 kPa) . Similarly, the oxygen dependency was obtained from runs by keeping the isobutene constant (0.09 atm) and varying the oxygen concentration (0.47-0.09 atm). The temperature interval studies was 623-729 K. Kinetic analysis Four widely used kinetic models have been tested. Model 1 is the power rate law model. It is widely used in reactor design for its simplicity, but does not provide information about possible mechanisms for the reaction. Its mathematical expression is eqn. (1)

387

niently transformed into a linear form and the associated parameters were estimated by a linear regression analysis. The general linear forms of the models discussed in the previous section are as follows: Model 1: lnr, =A lnp+B

(5)

Model 2 and 3: (I/r,)

=A(I/p)

+B

(6)

Model 4: (7)

(p/~-,)*.~=Ap+ll

where r, is the initial rate of methacrolein formation expressed per unit area of catalyst and p is the partial pressure of the reactant under consideration. A and B are the slope and intercept of the linear plot of the dependent variable versus independent variable of eqns. (5 ) to ( 7). The data were reasonably well correlated by models 1,2,3 and 4 and further discrimination between these can be made using Exner’s statistic I,Y[16] (smaller values of I,Uare for better fits). With this technique, the following ranked order of goodness of fit is obtained, 1> 2a > 3a > 4a. The calculated values of the kinetic parameters of these models are shown in Tables 1,2,3 and 4 respectively. Models 3b and 4b, based on the Langmuir-Hinshelwood mechanism for dissociative adsorption constants and were thus discarded. Model 2b produced the highest values of I,Yand will not be considered further. Fig. 1 shows the variation of methacrolein rate of formation with isobutene pressure and Fig. 2 shows the dependence on oxygen pressure. The x- and yaxis are in logarithmic scale. It can be seen from Table 1 that for model 1 (the power rate law) the apparent orders of reaction for isobutene and oxygen change with temperature. For the isobutene dependency, there is an increase with temperature from approximately zero at 623 K to 0.8 at 729 K suggesting that a first order might be obtained at higher temperatures. The apparent order in TABLE

1

Kinetic parameters

T

x

and Exner’s

Y

y for model 1

k, x lo6

VI

(mol s-l m-‘)

(K) 623

0.03

0.70

643 663

0.35 0.33

0.71 0.70

683

0.47

707 729

0.63 0.80

0.79

0.55 k 0.07 2.33 kO.18 2.82 f 0.29 10.00 k 1.40

0.18

0.67 0.37

23.60 k 5.80 37.10 k 7.60

0.31 0.26

0.16 0.09 0.12

388 TABLE Kinetic

2 parameters

and Exner’s

T

k,X106

W)

(mol atm-’

y for model 2a

lz,, x lo6 5-l mP2)

(mol atm-‘sP1

v

rnm2)

623

8.88 + 2.80

0.92 k 0.05

0.29

643

16.82 f 3.60

1.72+0.10

0.18

663

26.60f21.00

3.47 I!Y0.08

0.24

683 707

32.20 k4.10 37.00 k 5.30

7.23 + 0.25 18.90 + 2.40

0.14 0.27

729

39.74_+ 13.40

54.80 + 11.50

0.23

TABLE Kinetic

3 parameters

T

kx 10’

(K)

(mol s-l

and Exner’s y for model 3a

m-“)

h, (atm-‘)

ko

623 643

0.60+0.10 2.08 k 0.25

257.60 + 35.35 16.43 i 1.52

663

2.16+ 1.97

25.58 i 12.40

2.03 i 0.09 1.65 i 0.16 2.25 i 0.04

683

9.17?

11.01 i 0.15

1.47io.10

707 729

TABLE

1.32

13.50f3.10 31.20+6.30

6.45 i 0.63 1.74 2 3.60

‘i/

(atm-‘)

3.40 * 0.21 12.62 k 1.52

0.19 0.14 0.10 0.17 0.44 0.29

4

Kinetic parameters

and Exner’s

y for model 4a

T

kx106

(K)

(mol sP1 rn-‘)

623

2.83 i 0.51

13.09 rir1.48

1.60 i0.35

0.22

643

8.56 i 1.75 20.40 + 4.58

5.73 i 1.10 6.29 i 2.47

0.90 i 0.35 1.30 k 0.32

0.23 0.53

45.60 k 7.23 58.36 k 16.97

4.16 i 0.98 3.63 k 1.23

0.76 i 0.23 1.38 i 0.54

0.17 0.29

75.60 k 15.40

2.84 2 2.94

3.28 i 1.08

0.23

663 683 707 729

Kh

K

(atm-‘)

(atm-‘)

ry

oxygen remained fractional at all temperatures. This fairly complex kinetic behaviour is not easily related to any mechanism proposed in the literature for selective oxidation of hydrocarbons. Similar trends in reaction orders for propene oxidation have been reported by Krenzke and Keulks [6] and Monnier and Keulks [ 7 1. Model 3a however, where surface reaction is assumed controlling and where

389

P,=O.45

atm

iK iK

/+

I 04

0.06

0.08 0.10

I

0.20

P, in Atm Fig. 1. Rate of methacrolein

formation

P, = 0.09

as a function

of isobutene

partial pressure.

atm

200 7

E 100

i VI80 zm Lo c ._

L!! 20

10

a 8

I

’ 0.08

I

I

I

I

I

I

0.10

0.20

I

0.40

P, in atm Fig. 2. Rate of methacrolein

formation

as a function

of O2 partial pressure.

there is rapid adsorption of isobutene and oxygen on different sites, provides kinetic parameters which are chemically acceptable when we observe the trends with temperature (Table 3 ) . The isobutene equilibrium adsorption constants follow the expected decreasing trend as temperature increases while the values for oxygen remain virtually constant and increase at temperatures above 707 K, suggesting that chemisorption of oxygen becomes important as the temperature increases. Adsorption experiments on the propene-oxygen system carried out by Kremenic et al. [8] show that oxygen is adsorbed almost irreversibly on a Mo-Pr-Bi catalyst and that preadsorption of propene has little effect on the amount of reversible oxygen adsorption. They also found that the interaction of oxygen with the catalyst surface is significantly stronger than that of propene. Model 2a, based on the redox mechanism assumes that catalyst reduction and reoxidation occur simultaneously the hydrocarbon being oxidised when the catalyst is reduced. There is substantial evidence for redox mechanism involvement with bismuth molybdate catalysts [ 19,231. The rate constants for catalyst reduction and reoxidation for the redox model can be seen in Table 2. Their values follow the expected trend with temperature. In order to select the most probable model between the rival models 1,2a and 3a, a plot of calculated versus experimental initial rates of methacrolein formation is shown in Fig. 3. It can be seen that model 2a, based on the redox mechanism produces the closest agreement with experimental rate data. It is also of interest to notice *lo-'

70 r

65 E

i

50

m55

0

0

5

10

15

20

25

EXPERIMENTAL

30

35

40

45

50

rm in mol s-’

55

60

65

rn-’

Fig. 3. Calculated vs. experimental initial rates of methacrolein formation.

391

that at low temperatures (these are indicated by the lower values of rate data in Fig. 3)) the values of methacrolein formation rates, as calculated by the three models, are very close to the actual values. But, as the temperature increases, the rate values as obtained from model 1 and 3a start to deviate from the diagonal, and only model 2a (redox model) produces values close enough to the diagonal. Fig. 4 shows an Arrhenius plot of the variation of the rate constant for catalyst reoxidation with temperature. The linear fit is fairly good. In Fig. 4, the constant k is Iz,, for catalyst reoxidation (Table 2 ) . The activation energy for catalyst reduction is 52 kJ/mol and the value for catalyst reoxidation is 142 kJ/mol (Table 5 ). These values indicate that under the conditions employed in this study, the catalyst reoxidation may have been the rate determining step in the overall reaction. This is supported by comparing the values of rate constants for catalyst reduction Fz,and catalyst reoxidation k,, in Table 2. Only at the highest temperature (729 K) does the rate -9

1.25

1.30

1.35

1.40

1.45

1.50

1.55

1.60

1.65

1.70

l/T, T in K Fig. 4. Kinetic constant for catalyst reoxidation of model 2a as a function of temperature. TABLE 5 Energies of activation computed from various models Model

1

2a

3a

E (kJ/mol)

149 * 19

142 + 14” 52i 13b

11’7+23

“For catalyst reoxidation ‘For catalyst reduction

392

constant for catalyst reoxidation become greater than the rate constant for catalyst reduction. Uda et al. [ 191 studied the kinetics of reduction and reoxidation of y-phase bismuth molybdate using propene as hydrocarbon feed. They found that the activation energy for catalyst reduction is about 58 kJ/mol. This vale agrees well with the value obtained from model 2a for catalyst reduction (Table 5). Uda et al. [ 191 observed a more complex reoxidation pattern, as evidenced by the appearance of two peaks in their temperature-programmed reoxidation (TPR) experiments. For the low TPR peak, the Arrhenius plot showed a change in slope at 443 K. The activation energy below 443 K was found to be 96 kJ/ mol and above 443 K the value was approximately zero. For the high TPR peak, a similar Arrhenius plot was obtained, this time the change in slope occurred at 673 K. The activation energy below 673 K was reported to be 192 kJ/mol and above 673 K the value was approximately zero. From Table 5, we can see that the value of activation energy for catalyst reoxidation obtained in the present work is different from both values reported by Uda et al. [ 191. This could be explained by the fact that Uda et al. performed their experiments at fixed degrees of catalyst reduction, whereas in this investigation, the partial pressure of isobutene was allowed to vary over a wider range. CONCLUSIONS

The kinetics of the selective oxidation of isobutene to methacrolein at low conversion levels over y-bismuth molybdate are fairly complex. The kinetic dependency of isobutene varied with temperature from approximately zero at 623 K to 0.8 at 729 K, which is in agreement with the results of Krenzke and Keulks [ 61 and Monnier and Keulks [ 71 who studied the oxidation of propene over bismuth molybdate. The kinetic dependency of oxygen remained fractional at all temperatures. The kinetic data obtained were best represented by the redox model in which non-dissociated oxygen was involved in the reoxidation step. From the values of the activation energies for catalyst reduction (52 kJ/mol) and reoxidation (142 kJ/mol), it seems that catalyst reoxidation may have been the rate controlling step in the overall reaction. The value of activation energy for catalyst reduction obtained in this study is in agreement with that obtained by Uda et al. [ 191 who studied the separate reduction and reoxidation kinetics of bismuth molybdate with feeds of propene and oxygen. However, the value of activation energy for catalyst reoxidation differed from their reported values and it is thought that this may be due to the different pressure ranges employed. ACKNOWLEDGEMENT

We would like to thank the technical team under the supervision of Erick Horseley for their assistance in constructing the experimental set-up.

393

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