Phosphomolybdic acid (H3PMo12O40) as a catalyst for the vapour-phase oxidative dehydrogenation of isobutyric acid : kinetic parameters of supported and unsupported catalysts. Role of water.

167

C&a&is Today, l(1987) 167-180 Elaevier Science Pubiishers B.V., Amsterdam - Printed in The Netherhknds

PHOSPHOMOLYBDIC OXIDATIVE

ACID

(H~PMo~~O~~)

DEHYDROGENATION

AND UNSUPPORTED

CATALYSTS.

AS A CATALYST

OF ISOBUTYRIC

FOR THE VAPOUR-PHASE

: KINETIC PARAMETERS

ACID

OF SUPPORTED

ROLE OF WATER.

yincent ERNSTI, Yolande BARBAUX2 and Pierre COURT~NE' Departement

de Gc%ie Chimique,

B.P. 233, 60206 Compiegne 2C.D.F,

UniversitC

Cedex

Chimie S.A., C.R.N.C.,

de Technologie

de Compiegne,

(France)

BP 57, 62630 Bully-les-Mines

(France)

ABSTRACT In order to determine the catalytic properties of molybdophosphoric acid used recently in the oxydebydrogenation of isobutyric acid, a systematic kinetic study of the formation of three main products has been undertaken. For each of these three products, propene, acetone and methacrylic acid, the results are explained in terms of a steady-state mechanism of the oxidised sites. Related to the morphology of these compounds, we show that these three products result from three different specific sites. The selectivity of the catalyst can be explained by these results. The important role played by water on these compounds has not been studied in any previous work concerning the catalytic oxydehydrogenation of isobutyric acid. The adsorption equilibrium constants of water on each site have been determined from steady state kinetic readings. This confirms the presence of three types of different sites, and explains the dependence of activity and selectivity on the partial pressure of water.

INTRODUCTION 12-Heteropoly-acids units of molybdate,

and their salts result

tungstate

or vanadate

from the condensation

with a "heteroion"

silicate, germanate or arsenate, forming the Kegging structure. have been reviewed

of twelve

such as phosphate, These compounds

(ref.l-3).

In the solid state, this family shows strong and attractive properties for the industrial heterogeneous oxydehydrogenationof isobutyric acid (IBA) to methacrylic

acid (MAA). Previous

work on molybdovanadophosphoricacid and its

salts has shown that the latter are highly active and selective for this reaction (ref.&6).

In order to determine the catalytic properties of this

family, a systematic kinetic study has been undertaken for the three main products, MAA, acetone and propene,formed A specific

study in the absence

during the reaction.

of oxygen

nature of the active oxygen on the surface

has revealed of the

ments have now been made which show a decrease water.

Further,

0920-5~1/87/$~.~

an adsorption

mechanism

(ref.7) the exact

catalyst.

in activity

is proposed

0 1987 IQevier Science Publishers B.V.

Kinetic

measure-

in the presence

to explain

of

the role of water

168 on different specific sites, EXPERIMENTAL Phosphomolybdicacid, HsPM01~04~.15H~0,was prepared by addition of concentrated hydrochloric acid to a solution of disodium hydrogenophosphateand

disodium ~o~ybda~ followed by extraction with diethylether. This c~~o~nd been conditioned in two forms : (1) The powder was calcined under air at 300°C and c~pacted into pellets

has

2 mm in diameter and 2 mm thickness. (2) The phosphomolybdicacid was then supported on diatomaceous earth with a granularity of 1.5-2.5 mn by the "incipient-wetness"method. We obtained a catalyst of composition 1 :

1 (w/w) phosphomolydicacid-support.

Catalytic oxidation of IDA was establish in a stainless-steelflow reactor at atmospheric pressure. IDA (Merck, synthesis grade) was injected into a nitrogen and oxygen mixture at 130%. The catalyst volume ranged from 3 to 25 ml

and the total

flow-rate

from 35 to 160 1 h

,

In order to characterize the effect of water, the 7atter was injected into the gas flow at 90°C. The concentrationof water as steam varied from 1 to I.0 mole% of the total mixture, The products formed were analysed on three chromatographson-line with the reactor. Four columns, LAC 446

(IBA, MAA, acetone), BB'-oxydipropionitrile

(propene), Porapak D and molecular sieve (02, NQ, CO, COz) were used for the separation of the products. RESULTS AND

DISCUSSION

The phosph~lybdic

acid prepared in this work shows the same infrared

spectrum as indicated previously (ref. 2-9). TBA and DTA analyses showed that the last mole of water of hydration was removed between 260 and 34OT. Catalytic results are shown in Table 1. The conversion and selectivity of the main products, MAA and acetone, are comparable to those found in previous work (ref. 4).

169 TABLE 1

Catalytic

activity

in comparison with previous work at 300°C.

and selectivity

Contact

SELECTIVITY W3PMo12040 Conversion Propene Acetone mole %

mole %

UNSUPPORTEO

mole %

MAA

CO t CO, Time

mole% mole %

Specific Surface A;ea m fg

46

8.5

27.7

49.6

14.2

0.21 5

2.9

43

9.4

31.7

50.1

8.8

0.44 s

8.3

32

20.3

28.1

50.9

0.6

6.757 gllcatal st. _r g mole

2-5

IBA, 3%

0s

(af SUPPORTED (a) REFERENCE 4 (b)

Feed : (a) 1% (b) 2.1%

XEA, 3.2% O2

Kinetic study The evolution of the selectivity of the main products (MAA, acetone, propene, CO and CO,) with increasing conversion is shown in figure 1. At low conversion (G%),the

selectivities of the three main products (MAA, acetone

and propene) are constant. At higher conversion, the selectivities for acetone and MAA decrease, with increased CO and CO2 formation ; this indicates that MAA, acetone and propene are formed by a parallel mechanism such as

r-

1 oz------+ 2

Propene co2 + H20

1

+ CO2 + H20

170

selectivity

20%

40%

60%

80%

100%

conversion

Fig. 1. Variations in the catalytic selectivity of supported 12-molybdophosphoric acid with conversion in the oxidative deshydrogenationof IBA. Reaction temperature 300°C ; feed, IBA l%, 02 3,9%, Ns diluent.

For values lower than 40%, the conversion is proportional to the contact time (Fig. 2). This proportionality,together with the constant selectivities observed for conversions lower than 151, indicate that kinetic studies are valuable in the range explored.

. W

*

.

0,s

Contact

.

‘$4

‘,O time

.

.

,

‘78

,

272

,

,

25

seconds

2. Variation of total conversion with contact time Conditions as in 1.

171 Kinetic model of the steady For the catalytic 3OO'C. we propose

state of oxidized

oxidation

sites

of IBA to MAA over molybdophosphoric

for each product

a model based on the following

acid at equations

(ref. 10, 11) : k

IBA + KOred

Kti 2 where

Product

k 02 5

t K

KO

KO and K are the active centres

At the steady state reduction

rate

oxidation

rate

oxidised

or reduced.

: =

k

=

kox

red

'Ko'PIBA IKI

/PO2 ;I

IS’ = ‘KO’+ ‘K’ where

(SI is the total number of active

and catalytic

rate = oxidation

can thus be expressed

sites (constant,):

rate = reduction

rate = r. The catalytic

rate

as

r

k

P ox

For fixed values of P

' 02

I

and PIBA, a linear relationship

is obtained

between

02

1 1 _ and 7Z

P

or

_

1 1.

, respectively.

P2 02

IBA

for each product

on an unsupported

catalyst.

To verify the model on a supported reaction

was plotted

pressure

of oxygen

are straight parallel

against

constant.

parallel

confirms

(ref. 5), we propose

Kinetic

constant

of the rate of reaction

of reaction

obtained.

of the rate of

of IBA, keeping

to the complex

are summarized

in Table 2.

the

of oxygen

of IBA, straight

against

the reciprocal

(fig.L5). Contrary

to Akimoto

that the order in oxygen is $ instead

(ref. 12-15).

parameters

pressures

of oxygen were obtained

the orders

(fig. 3). This order corresponds the catalyst

the reciprocal

of the pressure

The curves drawn for various pressures

lines. For various

lines of the reciprocal

This model

catalyst,

the reciprocal

of the square root of pressure

al.'s work

Fig. 4 shows the linear transformation

dissociation

of 1

of dioxygen

on

et

172

10

20

30

=Fd

40

-1

atm

Fig. 3. Transformationof Akimoto et al.'s results (ref.5) on acetone. Catalyst, H3PMo120Lt~(unsupported); reaction temperature, 3OOY ; contact time, 0.15 s. TABtE 2 Kinetic parameters for unsupportedand supported H~PMo~~O,+~at 300°C.

IBA ORDER

i

SUPPORTED

1

-2

molh m 0.085

UNSUPPORTED PROPENE SUPPORTED

_I

atm

k+,, mol h

-1

m

-2

_I atm

3

l/2

3.5.10-

0.018

l/2

o.42*10-3

1

0.061

l/2

2.1.10-3

1

0.022

w

0.29.10

1

0.013

l/2

2.2.10-3

I

0.0017

l/Z

0.031*lo-3

MM

UNSUPPORTED ACETONE SUPPORTED

ORDER

'Slkr, -1

UNSUPPORTED

OXYGEN

-3

173

loo

Reaction

temperature,

Fig, 5. Verification

--

300,

400

of Mars and Van Krevelen.

500

-1 atm

300°C ; catalyst, H~PMo~~OI+~ (supported).

of the mechanism

PUBA)

200

2.0

P (024

30

See explanations

1.0

MAA

41,

5.0

in the text.

atm1

176 On the catalyst surface, the extent of oxidation of the different sites leading to the primary products obeys the law

1

ISI

k

l+

red

‘IBA I

At different partial pressures of IBA, with a constant partial pressure of oxygen of 3%, the calculated values show that the propene site (Table 3) is more oxidized than the acetone and MAA sites. The fact that the extents of oxidation for MAA and acetone are similar leads to the conclusion that acetone and MAA are probably produced on nearly the same sites.This indicates that H~PMo~~O~~ is the oxidized phase, This difference confirms (ref.4) that propene is probably catalysed on the cationic site, which is H+.

TABLE 3 Calculated extent of oxidation for different partial pressures of IBA under atmospheric pressure at 300°C with a constant P

= 3%.

02

pIBA

KO propene

= 0.2%

pIBA

= 0.8%

P

YEA

= 1.8%

0.89

0.67

0.48

0.78

0.43

0.25

0.75

0.47

0.28

Isl KO acetone ISI KO AMA Is/

The results of the above calculation do not reveal the exact nature of the oxidizing oxygen on the surface of the catalyst. The active oxygen can be activated adsorbed oxygen or lattice oxygen. If the latter is involved in the reaction, a "Mars and Van Krevelen" mechanism occurs. To confirm this assumption, the activity of the catalyst was determined after removal of 02 from the reactor (ref. 7, 13, 14, 15).

176 Nature of oxygen The results

are shown in Fig. 6 for the supported

of the number of layers consumed, oxygen

atoms per m2

we assumed

catalyst.

that a maximum

For evaluation 19 of 10

density

1s reached and that initially all the sites are completely

oxidized.

ii

iii

m

/

300

200 1

400

500

600

win

ime

Fig. 6. Conversion of IBA over HsPMo 120 40 (supported) in the absence of oxygen. Reaction temperature, 3OO'C ; catalyst surface area, 35m2 ; each i corresponds to the consumption of a layer of 0 ; diluent. N2 (high purity grade) tpOi3

ppm) ; P15A' 0.0025 atm

; contact time, 0.17 s. Symbols as used

previously. The curve shown in Fig. 6 might (i) a rapid decrease oxygen

be explained

in activity

during the reduction

of these sites

(ii) the lattice oxygen diffuses -stationary occurs

state" between

as follows

corresponds

:

to the consumption

of surface

;

from the bulk to the surface and a "pseudo-

the reduction

; the rate is nearly constant

;

and the oxidation

of the active

sites

(iii) the rate of diffusion of oxygen from the bulk to the surface decreases slowly, and thus the activity decreases. During the first and second steps, the initial activity and selectivity are restored when oxygen is added again. If the reduction is more important (third step), the catalyst could be partially restored but with a decrease in selectivity in with respect to MAA. The lattice oxygen is thought to participate in the reaction. We thus can conclude that oxidation of IBA obeys the "Mars and Van Krevelen" mechanism for each primary product, as bridging oxygen atoms are known to be easily removed in heteropoly c~pounds, and we propose that these bridges are the different active sites for the production of MAA and acetone (ref.5, 9). Activation energies were determined for the oxidation an reduction steps on supported molydophosphoricacid. Table 4 confirms the presence of three different specific sites. The activation energy of the IBA to MAA step is, nearly the sa;: as 338.2 kJ mole

the energy_yequiredto break the bond (C-H in alkanes,

(80.9 kcal mole

).

TABLE 4 Activation energies for oxidation_Tnd reduction steps for the different products (activationenergies in kcal mole

-1

are given in parentheses).

MAA

Ea kJ. mole

Propene

acetone

151 koX

230 (54.8)

125 (30.0)

334 (80.0)

"I 'red

194 (46.4)

206 (49.2)

344 (82.5)

Role of water Heteropoly compounds have a strong affinity for water. The replacement (ref.8) of nitrogen by steam in the flow shows that water strongly inhibits the catalytic activity and modifies the selectivitiesat 300aC. This fact leads us to propose an adsorption-desorptionmechanism on the oxidized sites. For each product, the stationary state can be expressed by the following equations : KO + HzO&

(KO H,Oadsl

k IBA + KOredK

tlO, 2

Product t K

k &

KO

178

rate = oxidation rate = reduction rate =

K' = equilibrium constant =

On the surface

1

k

ox

fKO.H,Oj

KOl + IKI + IKO X,Oadsl = S

1

Ii1

*

K’P~ o 2

At 300'C a linear relationship between :

ind partial pressure of water for

each product is obtained (fig. 7) at constant P

02

and

P

IBA ’

In addition, we verified that the intercept of the curve with the ordinate confirmed the calculated values obtained. For each product such phenomena confirm that adsorbed water does not change the oxidation and reduction rates on the catalytic sites. The different equilibrium constants were deduced (Table 5). TASLE 5 Equilibrium constants and intercepts with ordinate for the three main products on unsupported molybdophosphoricacid ; Catalytic conditions : TR = 3OOY IBA partial Pressure, 1% ; 02 partial = Pressure, 3%.

Propene 133

K'

Acetone

211

FIAA

360

intercept with or-finat! (measured) (mol.

h.m )

1.1o4

0.5&+

0.6.103

o.3.104

o.4.103

intercept with orffnatt (calculated)

(mol.

hm )

o.7.104

;

179

1

t

1

1

l

,

,

t

t

2,O S-0 6,O 8,0 lO,O ld*x(P H20’

atm

Fig. 7. Variations in catalytic activities with steam pressure for the main products over unsupported HaPMo12040 catalyst. Reaction temperature,3OO*C ; feed, 1% IBA 3% O2 ; contact time , 0.15 s. Symbols as used previously. CONCLUSION We have shown that oxy-dehydrogenationof IBA obeys the "Mars and Van Krevelen" mechanism at the surface of the catalyst. The kinetic measurements confirm that the catalytic production of propene is due to acid or cationic sites, and that acetone and MAA are probably formed on the bridge oxygens. This method allows the intercomparisonof different heteropoly compounds. A spectroscopic study in the temperature range of the catalytic reaction should be of substantial help in the determination of the localization of the sites. REFERENCES 1 32 4 :

Pope M.T., Heteropoly and Isopoly Oxometalates, Springer-Verlag, Berlin, Heidelberg, New York, Tokyo, 1983, 170 pp. Tsigdinos G.A., Ind. Eng. Chem. Prod. Res. Develop., 13 (4), 1974, 267 Souchay P., Ions Mineraux Condenses, Masson, Paris, 1969, 83-107 Akimoto M., Isuchida Y, Sato K., Echigoya E., J. Catal., 72 (1981), 83-94. Akimoto M., Shima K, Ikeda H., Echigoya E., J. Catal., 86 (1984), 173-186. Akimoto M., Ikeda H., Okabe A., Echigoya E., J. Catal., 89 (1984), 196-208.

180 7 8 9 :‘: :5 14 15

Bradzif J.F., Suresh D.D., Grasselli R.K., J. Cata?., 66 (1980}, 247-367. Konishi Y., Sakata K., Misono M., Yoneda Y., J. Catal., 77 (19821, 169-179 * Rocchiccioli-DeltcheffC,, Thouvenot R,, Franck R., Spectrochimica Acta, 32 (A) (1976), 587-598. Germain J.E., J. Chim. Phys., 70 (19731, ~048-~052 Mars P., Van Krevelen D.W., Spec. suppl. to them. Eng. Sci., 3 (1954)s 41-59. Bielanski A,, Haber J., Catal. Rev, Sci. Eng., 19 (1) (1979), 1-41 Grasselli R.K., Burrington J.D., Adv. Catal., 30 (1981), 133-163 Dadyburjor D.B., Jewur S.S., Ruckenstein E., Catal. Rev. Sci. Eng., 19 (2) (1979), 293-350. Cullis C.F., Hucknall D.J., Catalysis, 5 (1981), 273-307.