Catalytic Selective Oxidation - American Chemical Society

Chapter 19

Synergy Effect of Multicomponent Co, Fe, and Bi Molybdates in Propene Partial Oxidation 1

H.Ponceblanc ,J. M. M. Millet, G. Coudurier, and J. C. Védrine

Downloaded by CORNELL UNIV on September 3, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch019

Institut de Recherches sur la Catalyse, Centre National de la Recherche Scientifique, 2 avenue Albert Einstein, 69626 Villeurbanne, France Mechanical mixtures of Bi Mo O and solid solution Fe Co MoO phases of variable relative composition and variablexvalue have been studied for propene oxidation to acrolein. A huge synergy effect was observed when B i , Co and Fe were present and a maximum in selectivity and activity was observed for a mixture of a β phase form of Fe Co MoO with x=0.67, with 15 wt.% Bi Mo O . Electrical conductivity measurements have clearly shown that Fe cations were also present in the solid solution and strongly increased the conductivity (σ x more than 10 ). EDX-STEM analyses showed that the mixtures are composed of the two phases separated but that under catalytic reaction conditions the solid solution particles deposit on the large bismuth molybdate particles while part of the bismuth molybdate spreads over the solid solution deposited on bismuth molybdate large particles. The intimate contact between bismuth molybdate and solid solution, due to the spreading of the former on the latter deposited on large Bi Mo O particles, facilitates electrons and oxygen ions mobility i.e. the redox mechanism involved in the Mars and Van Krevelen mechanism which is known to occur in propene partial oxidation reaction. 2

x

1-x

3

12

x

2

4

3

1-x

4

12 3+

4

2

3

12

The oxidation of propene to acrolein has been one of the most studied selective oxidation reaction. The catalysts used are usually pure bismuth molybdates owing to the fact that these phases are present in industrial catalysts and that they exhibit rather good catalytic properties (1). However the industrial catalysts also contain bivalent cation molybdates like cobalt, iron and nickel molybdates, the presence of which improves both the activity and the selectivity of the catalysts (2,3). This improvement of performances for a mixture of phases with respect to each phase component, designated synergy effect, has recently been attributed to a support effect of the bivalent cation molybdate on the bismuth molybdate (4) or to a synergy effect due to remote control (5) or to more or less strong interaction between phases (6). However, this was proposed only in view of kinetic data obtained on a prepared supported catalyst. 1

Current address: Rhône-Poulenc Recherche, CRA, 52 rue La Haie Coq, 93508 Aubervilliers, France 0097-6156/93/0523-0262$06.00/0 © 1993 American Chemical Society Oyama and Hightower; Catalytic Selective Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

19. PONCEBLANC ET AL.

Multicomponent Co, Fe, and Bi Molybdates 263

In a recent work we were able to show that an electronic effect was detected between B12M03O12 and a mixed iron and cobalt molybdate with an enhancement of the electrical conductivity of the cobalt molybdate with the substitution of the cobaltous ions by the ferrous ions (7). However this effect alone cannot explain the synergy effect and we have investigated the influence of both the degree of subtitution of the cobalt with the iron cations in the cobalt molybdate and the ratio of the two phases (for a given substituted cobalt molybdate) on the catalytic properties of the mixture.We have tried to characterize by XPS and EDX-STEM the catalysts before and after the catalytic reaction in order to detect a possible transformation of the solid. The results obtained are presented and discussed in this study.


Experimental B12M03O12 was prepared by dissolving H2M0O4 and B1ONO3 into water and letting the solution boil under stirring for 2 hours (8). Mixed iron and cobalt molybdate were prepared by adding ammonia to an aqueous solution of (ΝΗ4)6Μθ7θ24·4Η2θ which was boiled for two hours under argon. A solution of FeCl2.4H 0 and Co(N03)2.6H20, in stoichiometric amounts was added to the mixture, which was boiled for one hour (9). The precipitates were filtered, washed with deoxygenated water, (evaporated to dryness under vacuum for the bivalent molybdate) and calcined at 450°C for ten hours under a deoxygenated and dehydrated nitrogen flow. Mechanical mixtures were obtained by mixing the respective powders and hand grinding them for 5 to 10 minutes. Crystal structures of the bismuth molybdate and of the mixed iron and cobalt solid solution molybdate samples were controlled by X-ray diffraction (10). The chemical compositions of the samples were determined by atomic absorption and their surface areas measured by nitrogen adsorption using the BET method. EDX-STEM analyses were performed with a Vacuum Generator VG HB 501 electron microscope with field emission gun and a beam area varying from 0.1 μπι down to 5 nm . XPS measurements were performed on a Hewlett-Packard HP 5950, at room temperature. Qualitative analysis of the peaks, in terms of elemental ratios, was carried out as described previously and the estimated error in such an analysis is approximately 10% (11). Selective oxidation of propene to acrolein was carried out in a dynamic differential microreactor containing 40 to 60 mg of catalyst as described previously (12). Reaction conditions were as follows : propene/02/N2 (diluting gas) = 1/1.69/5; total flow rate 7.2 dm .h ; total pressure 10 Pa; and reaction temperature 380 °C. Results Since a polymorphic transition (α/β) of the mixed iron and cobalt molybdate occurs in the temperature range of the catalytic reaction (10,13,14), and since the high temperature form (β) can metastably be maintained at low temperature, the catalysts were tested directly after heating to 380°C (Feo.67Coo.33MoC>4 is in the α form) and after a subsequent heating to 430°C for several hours and return to 380°C (Feo.67Coo.33MoC>4 is in the β form). The oxidation of propene was conducted at 380°C under the conditions given in the experimental section. The products of the reaction were in all cases exclusively acrolein (ACRO) and C0 . 2

2

2

3

_1

5

2

a) Study of mixtures of B12M03O12 and FexCoj. Mo04 with various iron contents and a fixed phase composition. x

Eight mixtures composed of B12M03O12 and Fe Coi_ Mo04, with a weight content in Bi2Mo30i2of 15% (i.e. 4.3 mol.%) were prepared and tested. The iron content of the solid solution varied from 0 to 100%. x

x

Oyama and Hightower; Catalytic Selective Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

264

CATALYTIC SELECTIVE OXIDATION


The results are given in Table I and the variations of the rates of formation of acrolein and the selectivity to acrolein observed at 380°C on these catalysts as a function of the iron content in the solid solution are presented in Fig. 1 and 2. When the solid solution is in the α form, the activity increased slowly to reach a maximum for an iron content of 70 to 80% whereas it increased sharply to a maximum for an iron content of 67% when it is in the β form. The increase observed was much more important with a β type solid solution than with a α type since the maximum rate obtained in the first case is five times that obtained in the second case. The loss of activity at high iron content could be explained by the decomposition of ferrous molybdate (6FeMo04 + 3/2 O2—> Fe2Û3 + 2Fe2Mo30i2) since both ferric oxide and ferric molybdate have been detected by X-rays diffraction and Mossbauer spectroscopy in the sample after the catalytic run (9). b) Study of mixture ofBiiMoÔn and Feo.67Coo33Mo04 with a fixed iron content and various phase compositions

Four mixtures composed of B12M03O12 and Feo 67C00 33M0O4, with weight contents in B i M o O i o f 7,15,25,and 50% (i.e. 1.8, 4.3, 7.4, 19.3 mol.%) were prepared and tested. The results are presented in Table II and the variations of the rates of formation of acrolein and the selectivity to acrolein observed at 380°C on these catalysts as a function of their bismuth molybdate content are plotted in Fig. 3 and 4. The activity increased slowly to reach a maximum for a mass content of bismuth molybdate of 50% when the solid solution is in the α form, whereas it increased sharply to a maximum for a mass content of bismuth molybdate of 20% when it is in the β form. The increase observed was again much more important with a β type solid solution than with an α type. The maximum rate obtained in the first case is three times that obtained in the second case. The analysis by X-ray diffraction after catalysis showed only the presence of the α or β phase of the mixed iron and cobalt molybdates depending upon heat treatment, 380 or 430°C respectively. No phase suspected to be present in the conditions of the catalysis reaction have been detected. This was confirmed by IR spectroscopy and EPR which did not detected any new ferric species (9). XPS has been used to characterize the three mixtures containing respectively 7,25 ,and 50 weight % of B12M03O12 (Table II samples J,K and L). These samples have been characterized before and after catalytic reaction (table III). Bi, Mo, Fe, Co and Ο have been analyzed. The Mo/O ratio remains equal to 0.25 for all the samples, before and after catalysis which confirms that no new phase was formed since the molybdates suspected to have formed, have a much lower Mo/O ratio (0.17 for B12M0O6 and Bi3FeMo20i2). Concerning the Bi/(Fe+Co) ratio, it can first be observed that before catalysis this ratio was always lower than that calculated from chemical analysis. This can be explained by the difference between the particles size of the bismuth molybdate and the iron and cobalt molybdates which is in a ratio of more than 30 as calculated from differences in surface area values, 0.3 and 9 to 22 m .g . Secondly the Bi/(Fe+Co) ratio increased systematically after catalysis which could be explained by the decrease in size of the bismuth molybdate particles or by the covering of the iron and cobalt molybdate particles by the bismuth molybdate or by both effects. The analysis by EDX-STEM has been focused on a Bi2Mo30i2-Feo.67Coo.33Mo04 mixture containing 15 weight % of B12M03O12 (sample F). Two types of particles have been observed before catalysis. The first particles type with a size of 0.1 to 0.2 μπι only contained mixed iron and cobalt molybdate with an Fe/(Fe+Co) atomic ratio calculated from 16 individual analyses equal to 0.64(±0.03). The second particles type 2

3

2

2

_1



19

16

19

10

12

0

12

25

36

53

A

Β

C

D

Ε

F

2

40 52

46

12

86

100

G

H

7.5

3 58

97 130 18

80

58

9

67

22

3

96

4.1

6

94 170 280

17 24

80 75

6 93 86

37

60

19

8 30

45

54

6.7

12 91

5.8

40

51

2.7

80

38

x

54

x

_1

0.63

8

selectivities rate of formation % 1 0 mol.s .m-2 co ACRO ACRO with Fe Coi_ Mo04 under the β form

54

x

2

32

x

1

4

0.49

8

selectivities rate of formation % 1 0 mol.s .m co ACRO ACRO with Fe Coi_ Mo04 under the α form

x

41

1

x

46

nAg-

SBET

' specific surface area; ACRO : acrolein

χ %

SBET

sample

(i.e. 4.3 mol.%)) 380°C;

2

TABLE L Catalytic data of the mechanical mixtures of B12M03O12 and Fe Coi. Mo0 (with a weight content in B12M03O12 of 15%



266


20 H

0H 0

«

1

1

20

40

CoMo04

"

1

60

% mol.

·

1

80

'

1 100

FeMo04

Figure 1. Variations of the selectivity (%) in acrolein on Bi^MoÔ^Fe Co M o 0 catalysts at 380 °C, versus iron content of the solid solution (x); a witn α and b with β F e C o M o 0 . x

1

4

x

l x

4

3000

CoMo04

% mol.

FeMo04

Figure 2. Variations of rate of formation of acrolein on Bi^MoÔ^Fe Co M o 0 catalysts at 380 °C, versus iron content of the solid solution (x); a with α and b with β F e C o _ M o 0 . x

1

4

x

1

x

4


Oyama and Hightower; Catalytic Selective Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993. 2

ACRO

%

C0

selectivities

2

0.3

100

M

2 5

98

94

2

98 150

5

50

L

94

25

Κ 100

3

96 280

9

3

96 280

15

83

49

24

75

41

9

3

3.7

3.7

1.8

96

10

15

C0

200

32

F

ACRO

%

selectivities

72

67

13

7

J

2.6 37

17

2

2

with Feo.67Coo.33Mo04 under the β form

ACRO

10" moLs^.m

8

rate of formation

27

with Feo.67Coo.33Mo04 under the α form

ACRO

Ι Ο mol.s^.nr

8

rate of formation

77

!

22

m .g-

2

SBET

0

of B12M03O12

weight %

I

sample

SBET · specific surface area; ACRO : acrolein

TABLE II. Catalytic data of the mechanical mixtures of B12M03O12 and Feo.67Coo.33MoC>4 in the partial oxidation of propene at 380°C;


268


100

-

• v—

90 -

ι

^

»

«

i

L

80 70 60 50 -


40 30 -

< 20 10 -

0 -

ι

' — 1

0

20

40

Feo.67Coo.33Mo0

'

1

•

1

ι

60

100

80

wt %

4

B12M03O12

Figure 3. Variations of the selectivity in acrolein on B i M o 0 Fe C o M o O catalysts at 380 °C, versus B i M o 0 weight content; a with α and b with β Fe Co -,Μο0 . 2

0 6 7

0 3 3

4

2

n

fi7

0

λ

3

3

1 2

1 2

4

300

Figure 4. Variations of rate of formation of acrolein on B i M o 0 F e C o M o O catalysts at 380 °C, versus B i M o 0 weight content; a with a and b with β Fe C o Mo0 . 2

Q 6 7

0 3 3

4

2

Q6 7

Q

3 3

3

1 2

4


3

1 2


7

25

50

Κ

L

of B12M03O12

weight %

J

sample

1 2

Q 6 7

Q 3 3

0.17

0.55 0.58 0.52

0.012 0.062

0.44 0.36

0.25 0.25

before catalysis

after catalysis

chemical analysis

0.72

0.14 0.22 0.49

0.54 0.54

0.056 0.082 0.15

0.44 0.38 0.31

0.25 0.26 0.25

after catalysis

chemical analysis

0.67

0.63

0.67

0.16 0.50

0.067

0.41

0.25

0.48

0.57

before catalysis

0.03

0.67 0.51

0.018

0.48

0.25

chemical analysis

0.63 0.038

0.60

Fe /(Fe+Co)

0.089

0.56

0.036

0.41

0.25

after catalysis

0.0039

Bi /(Fe+Co)

0.0018

0.55

Mo /Xcat.

0.45

Bi /Ecat.

4

0.26

Mo/O (Fe+Co) /Icat.

3

before catalysis

2

TABLE ΠΙ. Comparison of the surface elementary ratios of the cations in the mechanical mixtures of B i M o 0 and Fe C o M o 0 (B, D, and E) calculated from XPS analysis data before and after catalytic' reaction and those calculated from chemical analysis data before catalysis; Σ cat.: sum of all the cations



270

with a larger size (2 to 3 μπι) contained only the bismuth molybdates. No cobalt and iron was detected in the analyses (table IV and Fig. 5a). After catalysis the two types of particles were always present. The smaller particles were exactly the same composition and no bismuth was detected. The larger particles had a size reduced by a factor 2 and were not any more composed exclusively of bismuth and molybdenum. The presence of cobalt and iron, in the same proportions was also detected whatever the size of the analyzed surface (from 5 nm to Ιμπι ). These results show that the large bismuth molybdates particles are covered with a number of small particles of the solid solution (Table IV and Fig. 5b). 2

2

Discussion The synergy effect observed for the intimate B 1 2 M 0 3 O 1 2 and Fe Coi. Mo04 mixtures may partly be explained by the fact that the presence of iron in the solid solution increases its electric conductivity as it was observed by electrical conductivity measurements (σ multiplied by a factor of 10 ) (7). The electrons stemming from the oxidation of the olefin on the bismuth molybdate are more easily transfered to the solid solution which should play an important role in the redox mechanism. Such synergy effect is possible only if intimate contacts between the two phases exist. It is difficult to imagine that only a mechanical mixture of the two solid phases can give rise to these intimate contacts and a change in the morphology of the sample has to occur. This is what is observed in the case of the study of the B12M03O12 and Feo.67Coo.33Mo04 mixtures. Before catalysis the two phases are separated while after catalysis the large particles of bismuth molybdates are covered with a number of small particles of solid solution as shown by EDX-STEM. These small particles were themselves covered by bismuth molybdate since the Bi/(Fe+Co) ratio as observed by XPS, increased instead of decreasing after catalysis. It can be seen that this ratio becomes for certain catalysts even larger than the chemical ratio (Table III). Such a phenomenon can only be explained by the covering of particles of the solid solution by the bismuth molybdate. A schematic representation of the catalyst particles is shown in Fig. 6. Note that only part of the solid solution particles was deposited on bismuth molybdate particles since some solid solution crystallites were observed without bismuth by EDX-STEM analysis. Such particles have obviously négligeable importance in catalytic activity. The change of morphology related to the spreading of the bismuth molybdate on the solid solution could be temperature dependent. As a matter of fact the activation of the catalysts was observed but it is difficult to determine whether it is due to the α/β phase transition of the solid solution or to the spreading which occurs in the range of temperature of the polymorphic transition. We may conclude that the observed activation is due to both transformations. The maximum of activity observed for a relative mass ratio of B12M03O12 and Feo.67Coo.33MoC>4 may correspond to the maximum of covering of the bismuth molybdate by the solid solution particles. The difference of the maximum between the α and the β phase could be explained by the fact that the spreading may occur more easily on the β phase. It is not possible to determine if it is the type of polymorph or the heat treatment which is related to this phenomenon. These results and the comparison between the catalyst particles before and after catalytic run point out the ability for these particles both to exchange electrons and oxygen anions and to change morphology under the conditions of the catalytic reaction with spreading of the oxides one over the other. These two phenomena should be at the basis of the explanation of synergy effect in molybdates based catalysts. The fact that some Fe Coi_ MoC>4 particles remain free (i.e. not deposited on bismuth molybdate particles) show that even more active and selective catalysts may be obtained in more reliable preparation conditions.


x

x

4

x

x


19.

Multicomponent Co, Fe, and Bi Molybdates

PONCEBLANC ET AL.

271

TABLE IV. EDX-STEM analyses of particles of sample F (a Bi2Mo30i2-Feo.67Coo.33Mo0 mixture 4

containing 15 weight % of

B12M03O12)

before (A) and after catalysis (B) shown respectively in Fig. 5a

and 5b analysis

nm

Bi

Co

Fe

Mo

analysed area

% mol.

2


A before catalysis 31.7

1 (total)

1.6 10

7

61.6

4.1

2.6

2

3.0 10

4

52.4

35.3

12.3

0

3

1.0 10

3.8

2.7

33.0

4

3.0 10

4

60.5 54.4

30.2

15.5

0

5

1.0 10

4

29.6

16.4

0

6

120

59.5

0.7

0.9

39.0

7

120

51.8

30.0

18.2

0

16.7

9.6

16.0 0

4

54.0

Β after catalysis 1

1.0 10

56.7

4

2

120

54.1

28.5

17.4

3

48

51.8

31.3

16.9

4

48

54.6

27.6

17.8

0 0

5

12

58.7

24.3

17.0

0

6

300

10.2

5.7

22.5

7

300

61.5 62.0

9.3

5.2

23.5

8

300

57.0

16.6

9.5

16.9

9

300

54.9

21.0

12.3

11.8

10

300

56.9

20.0

11.3

11.8

* *r

1

6

fl

6

1

7

V

8

Pv_

9

400

^S^Sr

nm

200

•

nm

Figure 5. Electron micrograph schematic representation of a particle of sample F (a B i M o 0 - F e C o M o 0 mixture containing 15 weight % of B i M o 0 ) . The numbers correspond to the EDX-STEM analyses which results are given in Table IV: a) sample before and b) after catalytic reaction. 2

2

3

3

12

() 6 7

Q 3 3

4

12


272


Feo.67Coo.33Mo0

4

particle

panicles

spreading of


B12M03O12

Figure 6. Schematic representation of the catalysts particles for explaining synergy effect for mixtures of B L M o 0 and Fe C o M o 0 . 3

1 2

Q6 7

Q 3 3

4

Acknowledgments Financial support for this work by RHONE POULENC company is gratefully acknowledged. Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)

M. El Jamal, M. Forissier, G. Coudurier and J.C.Vedrine,in Proceed, of the 9th Intern. Congress on Catalysis, M.J. Phillips and M. Ternan (Ed.), Chem. Soc. of Canada, Ottawa, 4, 1617 (1988). J.L. Callahan, R.W. Foreman and F. Veatch, US Patent 2,941,007 (1960). J.C. Daumas, J.Y. Derrien and F. Van den Bussche, Fr. Patent 2,364,061 (1976). Y. Moro-oka, D.H. He and W. Ueda, in "Symposium on Structure-Activity Relationships in Heterogeneous Catalysis", R.K. Grasselli and A.W. Sleight (Ed.) Stud, in Surf. Sci. and Catal., Elsevier, Amsterdam, 67, 57 (1991). L.T. Weng and B. Delmon, Appl. Catal. A, 81, 141 (1992). O. Legendre, Ph. Jaeger and J.P. Brunelle, in "New Developments in Selective Oxidation by Heterogeneous Catalyis", P. Ruiz and B. Delmon (Ed.), Stud, in Surf. Sci. and Catal., Elsevier, Amsterdam, 72, 387 (1992). H. Ponceblanc, J.M. M. Millet, G. Coudurier, J.M. Hermann and J.C. Védrine, submitted to J. Catal. august 1992. P.A.Batist, J. Chem. Tech. Biotechn., 29 451 (1979). H. Ponceblanc, Thesis Lyon n° 259-90 (1990). H. Ponceblanc, J.M. M. Millet, G. Coudurier, O. Legendre and J.C.Védrine, J. Phys. Chem. in press november 1992. J.C.Vedrineand Y. Jugnet, in "Les techniques physiques d'étude des catalyseurs", Β. Imelik and J.C.Vedrine(Ed.), Technip Paris, 365 (1988) Chap.10. M. Forissier, A. Larchier, L. De Mourgues, M. Perrin and J.L. Portefaix, Rev Phys. Appl.,11639 (1976). A.W. Sleight and B.L. Chamberland, Inorg. Chem., 7, 1672 (1968). H.Ponceblanc, J.M.M.Millet, G.Thomas, J.M.Herrmann and J.C. Védrine, J. Phys. Chem. in press november 1992.

RECEIVED December 2, 1992


Catalytic Selective Oxidation - American Chemical Society

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