Catalysis by mixed-metal oxide systems

T.S.R. Prasada Rao and G. Murali Dhar (Editors) Recent Advances in Basic and Applied Aspects of Industrial Catalysis Studies in Surface Science and Catalysis, Vol. 113 9 1998 Elsevier Science B.V. All rights reserved

CATALYSIS BY MIXED-METAL

991

OXIDE SYSTEMS

S.S. Lokegaonkar, A. P. Sathe and V. S. Darshane. The Institute of Science, Madam Cama Road, Mumbai 400 032

Mixed-metal oxides viz. ferrites, ferrochromites, and chromites have been studied in order to see if there exists a correlation between bulk physical properties and catalytic performance using different alcohols as probe molecules. We have observed a reasonably good relationship between electronic activation energy, mobility of charge carriers, Curie temperature and catalytic activity of various oxidic spinel systems. A good correlation between nature of charge carriers and dehydration / dehydrogenation selectivity was also noted in the systems investigated. 1.

INTRODUCTION

Mixed-metal oxides have been exhibited considerable interest because of their ability to show remarkable transport and magnetic properties and their ability to impart extra stabiliy to the catalyst under various reaction conditions [1-5 ]. Mixed - metal oxides (MgFe204 , Z n F e 2 0 4 ) have been used as catalysts for industrially important reactions like conversion of butene to butadiene [6], hydrodesulphurisation of petroleum crude [7], and ( C o F e 2 0 4 , NiFe204 ) in treatment of automobile exhaust gases [8]. Further CuCr204 has been reported as good catalyst for oxidation of carbon monoxide [9] and for conversion of p-ethyl toluene to pmethyl styrene [10]. We are reporting herewith bulk physical properties and catalytic decomposition of alcohols over some mixed-metal oxidic system.

2

EXPERIMENTAL

The various compositions of catalyst were synthesized using citrate precursor technique to get ultrafine particles. For example to get ZnFe204 the precursor Zn3Fe6(C6HsO7)8 12H20 was calcined at 550 o C for 5h to get final product. A Zn3Fe6(C6H507)8 ,>3 ZnFe204 + 48 C 0 2 nt- 20 H20 (1) 550~ The average agglomerate size of ZnFe2Q obtained using a Horiba LA-500 particles size analyzer was around 0.65 gm. Thermal decomposition of Zn3Fe6(C6HsO7)8 involves seven steps which were investigated (11) using thermogravimetry, IR and XRD techniques. Other compositions were prepared by similar method by using necessary salts in appropriate molar proportions.

992 3 3.1

C H A R A C T E R I S A T I O N OF C A T A L Y S T Structural Analysis X-ray powder diffraction patterns were recorded on Siemens D-500 Kristalloflex diffractometer with a microprocessor controller using Cu K~ radiation (~=1.5405 A ~ with Nickel filter. The X-ray patterns of all the compositions indicated the formation of a single phase having spinel structure. To calculate the relative intensity, I of a given hkl reflection, the following formula, I = (Fhkl) P ( 1 + cos220 / sin20cos0 ) was used.

3.2

Transport Studies DC electrical resistivity measurements were carried out using LCR Macroni bridge with two probe conductivity arrangement. The end faces of each pellet were coated with a thin layer of conducting silver paste which was activated in an oven for 5h. Resistivity was measured from room temperature to 773 K. An electric field of 20 V cm l was applied across the pellet for these measurements. 3.3

Magnetic Measurements Initial susceptibility measurements were carried out from room temperature to 873 K for the compositions, using a field of 0 . 5 0 e by a double coil method. From the plots of Z/7~L vs T, the Curie temperatures (To) of the compounds were determined.

3.4

Infrared studies Infrared spectra of all the catalysts were recorded on spectrophotometer (Perkin-Elmer 1600 FTIR) in the frequency range 1000-400 cm ~

Each compound of the system was studied for its catalytic activity in the temp. range 200 to 4000 C using a fixed bed flow reactor as described earlier [12]. The reactor made up of pyrex glass contained 3.0 gms of powdered catalyst held between two glass wool plugs surrounded by glass beads to facilitate heat exchange. The products were collected and analysed on a gas chromatograph with FID. A standard 10% carbowax 20M column was used with N2 as a carrier gas (30dm 3 minl). 4

R E S U L T S AND DISCUSSION

Indexed X-ray diffraction patterns of all the compositions indicated in Table 1 showed formation of single spinel phase. In all the compositions the plots of log pvs 103/T showed a linear nature without any break or inflection obeying Wilsons law. The activation energy values obtained from the plots varied between 0.16 and 0.71 eV. It is seen that in all the series there is increase in activation energy which is due to the substitution of Zn 2§ and Cr 3§ ions having stable oxidation states thereby decreasing number of hopping pairs. Thermo electric power measurements were carried out to determine nature of charge carriers. The compositions viz CuFe204, ZnFe204, MgFe204, MgFeCrO4 and CoCr204 showed -ve Seebeck coefficient while rest of the compositions showed p-type behaviour. The mobility of charge carriers was calculated using the relation p = ed 2 t~exp (-AE/kT)/kT. In all the series there is decreasing trend of the mobility values with the substitution of Cr 3§ or Zn 2§ in the series. Such a low mobility values have also been reported

993 in number of oxidic spinels[ 13]. IR spectra of all the compositions showed two strong bands ol and o2 around 600 and 400 ~ 500 cm l. The band positions are in good agreement with the values reported in literature [ 14]. The catalytic performance data for the decomposition of different alcohols at 350~ is summerised in Table 1. We could observe a good relationship between electronic activation energy and catalytic activity in all series. As the catalysis involves trasfer of electrons/holes from the surface of the catalyst to substrate molecule and the process being reversible i.e. greater the activation energy, the greater will be the energy required for electronic transition resulting in decreased activity of the catalyst. Copper ferrite having lowest activation energy showed maximum % conversion (Table 1) in the series CuFez.xCrxO4while MgCr204 having highest activation energy showed minimum conversion in the series MgFez.xCrxO4 (Fig. 1). Table 1 indicates the effect of charge carriers on dehydrogenation and dehydration selectivities. It is observed that in the series CuFe2_xCrxO4 and MgFe2.xCrxO4 dehydrogenation selectivity increases with increase in value of Seebeck coefficient. It is well known that the ptype semiconductors are active and selective towards dehydrogenation owing to rapid migration of holes while dehydration is selectively favoured by n-type semiconductors owing to rapid migration of electrons[15]. From Table 1 it can be concluded that CuFeCrO4, CuMnFeO4 and CoFe204 show better dehydrogenation selectivity while CuFe204, MgFe204 and CoCr204 exhibit better dehydration selectivity(fig.2). From Table 1 it is observed that the mobility values showed decreasing trend with increase in concentration of Zn 2+ or Cr 3+ ions in the lattice. Further since catalytic activity depends on the exchange of electrons/holes from the surface to the probe molecule, as the mobility values decreased exchange of electrons/holes will be more difficult resulting in decrease in % conversion of alcohol e.g. in the series CuFez_xCrXO4,CuFe204 (~t=l.2 x/10 .7 m 2/V-sec) gave 92% conversion while CuCr204 (/a=3.8 x 10-l~ mZ/V-sec) gave 82% conversion at 350~ (Fig.3). The parallelism between the magnetic behaviour and performance of a catalyst has been explained by Panchenkov and Lehdev [16]. Last composition of each series showed antifferromagnetic behaviour. Further from table 1 it is observed that with decreasing Curie temperature % conversion of alcohols also decreased. This is due to the fact that with decreasing Tc, magnetic ordering temperature decreases which might be supressing the catalytic activity.

4.1

Stability of the catalysts Stability test for each composition of the series showed good stability under reaction conditions. Conversion levels were steady and did not fall even after 15h. process time. X-ray diffraction patterns and IR spectra of catalysts did not show any change indicating that there is no bulk reduction of the catalysts (Fig.4).

994

TABLE 1

Sam- Compound pie No.

a A~

AE eV

tx kt Tc % Con(~tV/K) (m2/V.sec) (K) version

% Probe Selectivity molecule -one/ -ene -aid

1

CuFe204

8.37

0.16

-333.0

1.2x10 7

490 92

06

73

BENZYL

2

CuFeCrO4

8.33

0.25

+100.0 3.8x10 -9

417 88

70

04

ALCOHOL

3

CuCr204

8.27

0.31

+080.0 3.8x10 -1~

82

54

17

4

CuMnFeO4

8.40

0.42

+070.0 5.3x10 -12

575 72

83

11

5

Zno.aCuo.6 8.41 Mno.6Fel.40

0.46

+005.0 1.1xl0 12

517 49

62

34

6

ZnFe204

8.42

0.59

-055.0 7.1x10 -15

23

45

49

7

MgFe204

8.37

0.39-171.0

1.7x10 -ll

680 47

16

79

CYCLO-

8

MgFeCrO4

8.35

0.49

-083.0 3.8x10 13

653 36

31

65

HEXANOL

9

MgCr204

8.33

0.71

+250.0 7.7x10 17

22

55

41

10

CoFe204

8.36

0.27

+267.0 6.6x10 l ~

785 36

71

28

BENZYL

11

CoFeCrO4

8.34

0.47

+093.0 2.1x10 -14

538 26

54

42

ALCOHOL

12

CoCr204

8.32

0.67

-132.0 3.6x10 16

28

60

-

HEPTANOL

4

-

-

-

11

995

100

t'-

8O

3

0

~

t/I

=-

4 5

60

>

o

U

7

40

8

10

u

20 i

!

0

l

I

0.4

0:2

!

12 I.

0.6

I

I

I

0.8

1.0

A E ----------~

F i g . l : Plot of activation energy vs %conversion

Dehydrogenation

100

c

m

Dehydration

==

rn

9 9 w

o Co & M9 oo C u M n F r

Cu

-I00

8O

-80

60

- 6O

._o

01 0

0

"~

40

-40

"13 ~" c-

20-

-20

0 ~

0

c

C:

I,..

i__

O

I.

-~(io

Fig.2:Plot

I -200

1

I 0

~

of Seebeck coefficient

| _ + 200

I ~ 400

vs % selectivity

996

100

._.-------0 ~

t- 8O _ O ul "

o,~z

&

3

60

> to

5

7

40

u

10

2(1 I

112

I

1616

Fig.3"

I

I

I

1614

I

ld lz

Plot of mobility

(a)

I

I

10-~0

vs %

i

1(~e'

conversion

(b) 311

311

440 511 220

400

30

40

I

50

220 ~

60

70

29

30

400

40

511

50

440

60

29

Fig. 4" XRD pottern of CoFe204 (ol Before Cotalysis and (b) After Catalysis

70

997 5

CONCLUSIONS

The average agglomerate size of catalysts using citrate precursor technique was around 0.65 ~tm. X-ray diffraction analysis indicated single spinel phase formation for all the compositions. Electrical resistivity-temperature behaviour obeyed Wilson's Law, 9 =9 0 exp (-AE/kT), indicating semiconducting nature of all compounds. Further from Seebeck measurements it is observed that copper ferrochromite, cobalt ferrite and copper ferromanganite show greater tendency towards dehydrogenation while copper ferrite, magnesium ferrite and cobalt chromite show fairly good dehydration selectivity. REFERENCES

1. 2. 3. 4.

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