Isostructural antimonates in olefin oxidative dehydrogenation.

257

Applied Catalysis, 25 (1986) 257-264 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

ISOSTRUCTURAL

ANTIMONATES

IN OLEFIN OXIDATIVE DEHYDRO-

GENATION. G.I.Straguzzi,

K.B.Bischoff,

Center for Catalytic Engineering,

Science

University

T.A.Koch and G.C.A.Schuit and Technology,

of Delaware.

Dept.

Newark,

of Chemical

Del. 19716.

U.S.A.

ABSTRACT. Compounds with the rutile or trirutile structure and the composition MSb04 or MSbz06, were investigated as catalysts for the oxidative &hydrogenation of i-&me and the oxidation of CO; their activities were compared to those of Sb204 and Fe203. The reagentswere studied separately but also as mixtures. Sb204, Al- and CrSb04 were hardly active.The Fe-r-utile was fairly active and also selective for the oxidation of butene to butadiene; its selectivity increased by an impregnation with Sb203 but at the cost of a loss .in activity. The trirutile, CoSb206, was active but unselective. For all catalysts, except RhSbO.+, the oxidation of CO was inhibited in the presence of butene. RhSbO, was very active for both reactions but entirely unselective for the butene oxidation; the presence of butene did not inhibit its CO oxidation. These results are discussed in terms of the ensemble theory with its parameters “ensemble size” and ” electronic factor”. We suggest that two reactions with different mechanims are involved, INTRODUCTION Catalysts

for

the

selective

oxidation

of hydrocarbons

are

binary oxides such as Bi2M03012 or FeSbO., for the special oxidation cation are

of olefins.

and the other

generally

hydrocarbon consists

Of the two cations, a cation

believed withdraws

in a reoxidation

from

to proceed

usually

case of the

one is often a transition

group VE3. The oxidation in two

steps.

oxygen atoms from the catalyst, of the reduced catalyst

In the

metal

reactions first,

the

while the second

by molecular

oxygen

258

(Mars

- van Krevelen

different

a sequence

selectivity

of

encountered selective

two

in

locally

their

metal

than single

reaction

different

reactions

St.&a-S

may

will present

number

differ

selective.

the average

by inert

improve model

Limiting

metals

the catalyst’s is rarely

the

is not common.

exclusive,

we planned to investigate

In alloy

Since

a simple

were

selected

for

this

purpose.

In one,

such

as

A1Sb04,

CrSb04,

oxidations transition

were

of butene

to be investigated

possible

models.

as to their

and of CO, to ascertain

and

the sequential where the

to be mutually

oxidation

Two lines

FeSb04,

non-

part of the

catalysis,

catalysts

of

metal

reactions

catalysis,

metal cation at the same bulk structure.

Ws to be impregnated

Single

by covering

catalytic

of the two competing

CoSb,O,(trirutile)

to need

sized ensembles

the two seem

the viewpoint

,

explained

ensembles

of sites.

to oxidation

from

structure

is often

more

they will be intrinsically

restrict

model

are

sites;

size

ensemble

phenomena

of contiguous

ensemble

contrary

often

frequently

for differently

the

are

is supposed

reactions;

selectivity.

considered,

effect

in the number

shall

oxides

are

the

with

Each reaction

many possibilities of different

should then

connected

Similar

this

that

However,

binary

alloys

and where

that allow catalysis

surface

where

theory”.

with a typical

also

components,

metals

with help of the “ensemble an ensemble

seems

of two

suggests

process

reactions.

cations;

catalysis,

presence

cations

the overall

separated

of different

than

The

and of two different

overall

presence

selective

mechanism).

its own reaction;

of the

simultaneous more

redox

types of reactions

each cation catalyzes be

or

with

reaction of attack

the

rutile

RhSbO4

and

activity

for the

influence

of the

In the other,

FeSbO4

the

with Sb203 or Fe203. to vary the ensemble

size.

259

EXPERIMENTAL The catalyst

samples

were

prepared

mixtures

of oxides in flowing air.

prepared

by ammoniacal

solution

of metal-nitrates

calcination samples

composition surface

of a slurry

[Saia et al (l)].

characterized

mixture

of Sbz03 in an aqueous

RhSb04

of the oxides

by gravimetric

and structure,

of stoichiometric

The Fe, Al, Cr, and Co oxides were

precipitation

of a dry ground were

by calcination

was prepared [Brandt

analysis,

(Z)].

TABLE 1 *Characterization

All solids

All

and XRD as to

and by BET and XPS measurements

area and composition.

had the (tri)rutile

as to

structure.

of catalysts

Compound

A1Sb04

CrSb04

FeSb04

CoSbx06

RhSb09

Calc.T,*C

1050

1100

750

850

950

Surf.Area,m2/g The surface the metal

7.5

51.0

34.0

or Sb-tartrate

and carried

volume technique.

This was followed

at a temperature,

SO* lower than the original

Catalytic

1.6

Sb/M ratio was changed by impregnation nitrate

activities

were

with an amount of catalyst

with a solution

of

to the pore

by drying at 125 *C and calcination

determined

calcination

temperature.

using a fixed bed tubular

having approximately

area; this

conditions.

The flow

of reactants

gascompositions

was 120 cm3 per minute;

+ 7 O2 + 86.5 He or 2.5 CO + 2.8 O2 + 94.7

were 6.5 butene

He. Both reactions

Gas chromatographic

applied to both feed and product streams.

reactor

5 m2 surface

isothermal

run over the range of 350-SOO*C.

9.1

out according

was diluted with Sg. of Sic to achieve

analysis

by

Surface

were checked before and after the reaction

area,

analysis

were was

XPS, XRD and

260

RESULTS Table II gives the data obtained for surface compositions, selectivities

for butene oxidation and activities

TABLE II Activities

Catalysts

and selectivities

activities

for the CO oxidation.

of the MSb04 -catalysts.

1-butene

Sb/M XPS

Activity

co Selectivity

mmol/m2,h

(2~ of Ml

and

Activity

%

mmol/m2, h-

A1Sb04

1.2

1

75

1

CrSb04

3.1

2

77

7

FeSbO.,

3.2

21

84

36

CoSb206

3.2

6

47

117

RhSb04

4.2

15

17

100

Sb204

3

74

Fe203

13

33

XPS Sb 3d signal The conversions been in operation all catalysts

corrected

for 0 1s eontribution.

quoted in the table were obtained when the catalyst for at least one hour. With the exception

gave very high conversions

one hour to a level that remained before

and after

shown by fig.2.

7

this

first

for FeSb04

at the start;

constant

thereafter.

The XP- spectra considerably

as

When only exposed to O2 in l-le, two bands

at 543 and 534 EV, belonging to Sbst, were observable. reaction

of RhSbO.,,

these decayed after

hour were found to differ .

had

After the

these bands were much weaker while a new system

around 540 and 530 EV appeared.

A fresh sample,

prepared

of bands by heating

261

the precursor

mixture

another,obtained

of FeSbO., in He at 7.50 OC

by adding 2% Sbz04 to the precursor

in He (sample II), showed the

heating

stronger(seeFig. 1) while regeneration eliminated

second

set

the XRD diagrams

remained

the incipient

of bands.

It

therefore

certain that the second set belonged to Sb3t, indicating surface

became

the surface

partly

reduced

could be re-oxidized

during operation.

in He ; (D) id.with

oxidation

and very selective

seems

and

reasonably

that the catalyst’s if reduced,

temperature.

before and after reaction.

2 % Sbz03.

butene or CO oxidation.

high activity

A

Fig .2: XPS of FeSbO_, catalyst

Table II shows that Sbz04, A1Sb04, and CrSbO,, either

and again

the same.

Moreover,

by O2 at the reaction

Fig. 1: XPS of FeSb04 (C) calcined

mixture

secondset of bandsto be much

by 7% 02 in He restored the

( sampleCl and

FeSb04

for butadiene;

were hardly

active

for

was quite active for the butene it was not particularly

262

for the oxidation

active very

active

for

both reactions

Activities

oxidation.

of CO. CoSb206 and especially and quite

RebQ

unselective

for the two reactions

for

was almost

in the presence of butene for all catalysts,

except RhSbO+

In table

111 a m-vey

It was found that changing

manner, caused activity

butene parallel

entirely

it-ddited

is given of the results of experiments in which

the s&ace of’the FeSbO, -catalyst was impregnated antimony.

the

ran approximately

but it is noteworthy that the CO - oxidation

WEE

the surface

and selectivity

with either

iron or

Sb/Fe ratio

in this

of the catalyst

to be altered

considerably. TABLE III Activity

(mmole/nP,h)

XPS, Sb/Fe

3.0

2.0

activity selectivity,

8 selectivity

%

of impregnated

3.2

FeSbO+

3.3

3.8

3.9

25

22

21

17

13

9

77

a2

a4

a4

aa

92

DISCUSSION It is well known that FeSbO., has to possess 2.5 in order to be maximally containing

catalysts

USb30t0 - catalyst

exhibit by Grasselli

et al (5), explained Urlich

selective similar

(

characteristics,

et al (4).

this by assuming

importance

of the simultaneous

to activate

some oxygen anions because as observed

presence

from quadrupole

ratio Sb/Fe

Yoshino et al (3)).

Z

Other Sb-

as shown for the

Some authors,

such as Trifiro

that Sb5+ is the active

et al (6), Aso et al (7) and Burriesci

surrounding

a surface

species.

et al (8) emphasize

the

of Fe and Sb; this is supposed of a distortion shifts

of the octahedral

in Mossbauer

spectra.

263

We want to propose, and Gates

as an extension

(9) that antimony

improved

the size of the Fe - 0 ensembles Sb5+ to be almost

inactive,

of an earlier

suggestion

selectivity

because

whether present

ensemble

theory.

with increasing

reduction

FeSb04

its activity

decreased

Sb/Fe ratio in conformity

important

function

during operation:

of Sb”+

was

revealed

by its

because the to

of O2 into to two 02- anions. However, replenishment

by a two-dimensional

analogon of the reaction

+ Sb205 -

The work of Ulrich

catalyst

in the

and anion vacancies

of. spent oxygen anions can also occur by migration

reduction

partial

above, direct

by O2 may be impossible

may not have enough electrons

allow a conversion

2 Fe0

with the

it appeared to act as an intermediate

of the Fe -complexes

reduced complex

and

at the surface.

re - oxidation of a reduced site . In a model, as presented oxidation

with

Hence, one of the functions of Sb - oxide is to decrease

the size of the Fe-oxide complexes Another

We found

in Sb204, A1Sb04 or CrSb04.

Sb205 on top was less active but selective; increased

it decreased

that are the active agents.

Fez03, on the other hand, was quite active but unselective.

selectivity

by Schuit

) Fez03

:

+ Sb204

et al (6) and our

- 52

results

of Sb5+ and I-e-oxidation of Sb3+ occurred is therefore

bi-functional;

and Sb of oxygen introduction

from antimony oxide

kcal

showed

( 1 )

indeed

,that

during operation.

The

Fe takes care of the olefin oxidation

and limiting

the size of the ensemble.

TABLE IV Redox enthalpies Transition

Me0 + Sb205 and electrochem

metal cation

pot. Me2+ - Me3+

Ti

V

Cr

Mn

Fe

Co

Ni

Redox enth. ,kcal/mole

- 100

-70

-70

- 26

-52

-27

+ 13

Electrochem.Pot.

+ 2.0

+.26

t.41

-1.51

-0.77

V

-1.84

?

264

Table v presents

thermodynamic

in eq.

1 ad

cations

on the left,

therefore

the electrochemical

inactive.

difficult

data,such potentials

as Cr3+ are

such

On the right,

to re - oxidize;

as redox enthalpies

also

too stable

bivalent these

Me2+ - Me3+.

preferential position of Fe as an oxidation understood. Co ad

Rh while

type of reaction

the butene

such as Co2+, are

not be very active. The is

catalyst

emerged,

branching

oxidation,

hence

readily

at a decreased r-ate,

although

To explain this observation, a thermal

at the surface,

we suggest that a new

chain reaction

in the gas phase with

that could be written

as follows:

Co2+ + H202 -> Co3+ + OH- + OH*

(2)

Co3+ t H,O, ->

(3)

where * stands

for radicals

chain branching

during

as “afterburning”

Co2+ + H+ t HC&*

and HzOz for hydroperoxides.

homogeneous

al (10); Daniel and Keulks occurred

and

However, the oxidation of CO turned out to be very fast over

became nonselective.

catalytic

Trivalent

to be reduced

cations,

should

as used

(i

catalytic

1) have suggested

during

catalytic

oxidation

For catalytic see Sheldon

that thermal

oxidation

over

et

oxidation

Bi2Mo06.

REFERENCES 1 Sala, F and Trifiro F., J.Catal. fl, 1, 1 (1976) 2 Bra& K. ,Arkiv for Kemi, Mineralogi och Geologi, B& 1 (1943) 3 Yoshh,T. ,%ito,S. ad Sobukova,B., Japan. Patent 7,103,438 (197 1) 4 Grasse1liR.K. and Suresh,D.D. J.Catal. 25, 273,(1972) 5 Trifim, F.,Carbucichio,M and Centi, C.J.,J.Catal.,s, 85 (198.5) 6 Urich, F. J. ,Kriegsmann,H., Ohimann, G.and Scheve, J., Proc.Int.Congr.Catal.,6th., London,V2, p.836.,(19?7) 7 As4 ,Yamazoe,N. ,Amamoto,T. and Seiyama,T., Proc. Int.Congr.Cat. 7th. Tokio (1981) p.1239 8 Burriesci, N., Garbassi, F. ,Petrera,M., and Petrini,G., J.Ckm.&c. Faraday Trans. i , 78, (1982) 8 f 7. 9 SchWG.C.A. and Gates,B.C. Chemtech,Nov.,693 (1983) 10 Sheldon,R.A. and Kochi,J.K., Oxid.Comb.Rev.5,135(1973). 11 Daniel,C.and Keulks,G.W. J.Catal. 24, 529 (1972).