Catalytic Properties of Zeolites in Oxidation and Ammoxidation Reaction

T. Inui (Editor), Successful Design of Catalysts © 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

247

CATALYTIC PROPERTIES OF ZEOLITES IN OXIDATION AND AMMOXIDATION REACTION G. CENTI 1, P. JIRU 2 and F. TRIFIRO,l 1 Department of Industrial Chemistry and Materials, V.le Risorgimento 4, 40136 BOLOGNA (Italy) 2 The Heyrovsky Institute of Physical Chemistry and Electrochemistry, 12138 Prague 2, Chechoslovakia

ABSTRACT The catalyt ic oxidat ion propert ies of HY, HZSM5 and HZSMll zeolites modified by vanadium-phosphorus oxide or vanadium-oxide deposition and of catalysts obtained from V-silicalite precursors are analyzed in the selective conversion of butadiene to furan and maleic anhydride and para- and meta-xylene ammoxidation, and pure ZSM5 zeolites with different Si/AI ratios are tested in the propane conversion to aromatics in the presence of 02' In the HY and HZSM5 zeolites the deposited PV clusters interact with protondonor centers giving rise to an inhibition of the maleic anhydride formation from butadiene. This effect is not present in the HZSMll zeoli te. An analogous change in the catalytic behavior is found for the ammoxidation of xylenes. The insertion of V in the framework of silicalite leads to, after activation, the formation of selective sites for furan synthesis from butadiene. Selectivity increases with Si/V atomic ratio. The presence of oxygen in the feed increases the formation of aromatics from propane on the pure zeolites. The oxygen effect is a function of the concentration of OH sites and is attributed to the formation of radical-like surface sites. INTRODUCTION The behavior of zeolites in the presence of gaseous oxygen and/or ammonia

has

been

investigated

propert ies of these materials. in

the

literature

that

less

than

There are,

indicate

the

other

however,

interest

for

catalytic

some examples this

field

of

research. Tagiyev and Minachev [1] have shown that Na-zeolites in the

presence

of

02

are

active

and

selective

in

dehydrogenation of cyclohexane and ethylbenzene. O2 to a pentasyl

propane

feed

increases

type

zeolites

[2].

catalyzes

the

reaction

of

In

the

aromatization

the

presence

and

of

activity air,

of

ZSM-5

pyridines,

whereas in anaerobic condi t ions the react ion product s

are amines

in the presence of oxygen,

ammonia

oxidative

to

[3]. Therefore,

ethanol

the

The addition of

zeolitic materials may

exhibit new catalytic properties. The nature of the zeolitic cage

248

influences the nature of the oxidative properties selectivity

effects.

This

last

point

is

inducing shape

particularly

important

when the zeolite is modified by transition metal ions or oxides. Their the

introduct ion catalytic

example,

in cat ion

performances

or of

framework the

posi ti on s

zeolite.

can

improve

V-silicalite,

for

showed higher selectivity than ZSM-5 in the formation of

aromatics

from

olefins

[4].

On the

other

hand,

a

complementary

aspect is the change of reactivity of the transition metal (V, for example)

induced

typical

component

expected

that

by of

its

stabilization of

the

zeolitic

selective

catalytic

different

structure

oxidation

behavior

[5].

Vanadium

catalysts.

can

be

It

can

changed

coordinations or valence

is

by

states.

a be

the Very

stable V( IV)

forms by solid state reaction of V with zeolites 20 5 [6]. The ions, which migrate from the outer surface of the zeolite

crystals are coordinated in the cationic positions of the zeolite, creating

vanadium

sites

with

new

oxidation

properties.

Space-

constrains can further modify the catalytic oxidation behavior of a

transition metal oxide

simple supportation.

inside a

zeol i tic cage with respect

to

In this paper we discuss these concepts and

report some examples from our work on the oxidative properties of zeolites. aspects materials

The of

aim

the and

of

is

to

study

of

their

evidence the

use

some

oxidative

as

supports

problems behavior

for

and

critical

of

zeoli tic

transition

metal

oxides. EXPERIMENTAL The pure starting zeolites used were commercial reas the depositions of the PVO or lized by

impregnation with

samples, whe-

va active components were rea-

PV-heteropolyacids

or with ammonium

vanadate, respectively. V-silicalites were prepared using a slightly modified method for

2SM-type zeolite synthesis,

using tetra-

buthylammonium hydroxide or hexamethylene as template agents.

All

details on the preparation as well on the physico-chemical characterization have been reported previously [7-9,13-15].

All cataly-

tic tests were carried out in flow type reactors after attainment of the steady-state catalytic behavior, except for the aromatization of propane where the data refer to the behavior after 1 hour. Experimental conditions are also reported in the original papers. RESULTS AND DISCUSSION PV- and V- interaction with the zeoli tic support. the possibility of modifying zeolites without

In studies on

destruction of the

249

structure

we

have

tried

to

prepare

v-p-o

zeolitic cavities [7-9].

V-P

oxide

clusters

inside

mixed oxides are specific catalysts

for the selective oxidation of C 4 hydrocarbons, in particular of to furan and maleic anhydride [10] and of n-butane to

butadiene maleic

anhydride

transformation industrial

of

possibility

of

an

is

be selectively formed on oxygen

clusters

oxide

on

constrains

4

of

the

anhydride.

The

therefore

the

intermediate

to

maleic

expensive

and

oxidation

process

for

the

hydrocarbons is of interest. Furan can vanadium-phosphorus

concentration

inside the

very

heterogeneous

formation of furan from C conversion and

an

olefins

the

synthesis

is

Furan

[11] .

formation

cages

the

of

The

[10] .

zeolitic

the

oxides only at insertion

may

larger

induce

maleic

of

low V-P

steric

anhydride

molecule, with a decrease in the rate of furan oxidation to maleic anhydride and an increase in the yield of furan. b.c

electivity. %

a 25

.-< I .-<

I

OD

Temperatures (K) and relative butadiene conversions (%) of the maximum selectivities: PVO [(a) 683,33; (b) 706,98]; HY-PVO [(a) 597,8], HZSMS-PVO [(a) 651,12],HZSMII-PVO [(a) 600,30; (b) 650,98]

40

17


...:I

9

20

O"l

e-

III

.>:

I1ZSMll-

-pya

FIG 1 (a) First order rate constant of butadiene depletion at 573 K for pure PVO and PVO supported on zeolite carrier catalysts [activity per g of V active component in the catalyst] and maximum selectivities to fur an (b) and maleic anhydride (c) on the same catalysts [7-10]. The

results

are

summarized

in

Figure

1.

Supporting

the

PVO

compound on zeolites increases the activity by about two orders of magnitude,

indicating

dispersed.

However,

that

the

active

component

is

really

well

a marked change in the selective behavior is

observed. ZSMll-PVO behaves in a similar way to pure (unsupported) PVO compound, forming furan at low conversion and maleic anhydride at high conversion. On the contrary, and HZSl-15 conversion.

zeolites An

leads

analysis

only to of

the

the deposition of PVO on HY

the

formation

change

of

of

fur an

accessible

at

low

(large

cavities) OR groups after interaction of the zeolites with the PVO

250 component

[Table

I]

shows

that

in

the

Y and

Z5M5

zeolites

both

phosphorus and vanadium interact with these proton-donor centers, as confirmed by the

zeolites only impregnated with vanadium.

parallel change of the

The

initial rates of oligomerization confirms

the decrease in OH concentrat ion after V-

and PV-deposi tion on Y

and

due

Z5M5.

On

concentration interact

ZSMll, and

with

on

perhaps

a

consequence,

contrary,

different

proton-donor

absorbance and of the As

the

centers

nature, (no

to

the

lower

the

PVO

does

change

of

OH

OH not

infrared

initial rate of ethylene oligomerization).

in

the

oxidation

of

butadiene

to

maleic

anhydride this catalyst behaves as the pure (unsupported) PVO, the only difference being that it is more active TABLE I Change in the conc~1tration of OH grouJls in the large cavities [IR band at 3640 cm in Y and 3610 cm in Z5M] after deposition of V- or P-V active components and initial rates of ethylene oligomerization at 353 K [Ro] [8] Zeolite

%wt of PV or V

Si/Al

HY HY-PVO HY-V HZSM-5 HZSM5-PVO HZSM-ll HZSMll-PVO

2.1 2.1 2.1 13.6 13.6 49.0 49.0

[OH] , mol 103/ g

Ro 102/ mi n

2.48 0.98 1. 52 1.10 0.63 0.06 0.06

0.72 0.28 0.36 18.00 5.50 0.18 0.19

5.3 2.2 4.2 4.1

Similar effects are present in the ammoxidation of para- and meta-xylenes on V supported on HY, HZSM5, and HZSMll catalysts. In this case only a V-compound was deposited on the zeolite support, since generally the active phase for this reaction is supported Voxide [12]. para-

and

The selectivities to nitriles at meta-xylene

impregnation support

are

ZSMll behaves

and

on

an

compared

on

three

analogous

in

Tabl e

II.

zeolites preparat ion As

low conversion from prepared

found

using for

by

NH

Ti0

as 2 butadiene,

4V03 the the

like the active/selective VTiO catalyst. ZSM5 and Y

zeolites show (i) lower selectivity to nitriles and (ii) an higher ratio higher

of

the

selectivity

conversions

[13]

from

the

para-

and

selectivity

from

meta-xylenes.

decreases.

The

loss

At of

selectivity is mainly due to dealkylation reactions in the case of ZSMll

(formation

of

toluene,

benzene,

benzonitrile)

and

to

the

251 high rate of carbon oxide formation the

contrary,

dealkylation

for is

pure

in the case of Y and ZSM5. On

zeolites

observed

in

(not

modified

p-xylene

by

V),

ammoxidation

only

and

the

activity is proportional to the surface acidity (ZSMll is the less act i ve in dealkylation)

[13].

TABLE I I Selectivity to nitriles (mono- and di-nitriles) and m-xylene for V-supported on different zeolites, ratio selectivity to nitriles for p- and m-xylene ammoxidation and specific rate (per g of V) of hydrocarbon depletion various samples [13].

Selectivity to nitriles at 20% of conversion para-xylene meta-xylene

Zeolite

HZSMll-V HZSM5-V HY-V Ti0 2-V

70

45 91

A possible

interpretat ion

vanadium interacts the

specific the

in different

interaction

selective sites, to

absence

of

of

1.0 2.0 1.5 1.0

these

15.5 14.1 6.2 19.6

observations

ways with V

3 rate 10 at693 K moles /s.g

S /S p m

87 35 30 90

89

with

the

the

is

that

zeolites.

zeolite

the

On

ZSMII

forms

very

possibly not localized in the zeolitic cages due of

shape

selectivity effects.

On

vanadium is

deposited inside

the void structure,

of

zeolite-vanadium

interaction

specific

from pof the (S /S ) orr tWe

leads

ZSM5 and

Y the

but the absence

to

a

decrease

in

selectivity.

A possible tentative interpretation of these effects

(increase

the

in

rate

of

the

formation

of

carbon

oxides

in

the

order ZSMll-V < ZSM5-V < HY-V and of the formation of dealkylation products in the opposite order) may be as follows. The hydrocarbon activated by

Br on s t.e d

sites

may

react with

giving rise to unselective oxidation. OH concentration centers,

this

and possibly

effect

is

separate

reduced with

the

adjacent

V sites

On ZSMII which has a localization

final

lower

of V and OH

higher selectivity to

nitriles and higher formation of products of dealkylation. Such an interpretation must be confirmed but suggests the important aspect of

the

influence

of

the

relative

localization

of

acid

and

oxidation sites on selectivity in oxidation reactions. An analysis of these

problems

is

fundamental

for

the

development

made zeolite catalysts for oxidation reactions.

of

tailor-

252 Creation of

selective sites by framework insertion of vanadium in The substitution of Al 3+ with v 3+ in

a silicalite-type structure.

ZSM type zeolite synthesis leads to the formation of v-silicalite precursors

with

(silicalite-2)

crystalline

or

ZSM48

structures

(14].

of

either

Physico-chemical

ZSMl1

characterization

suggests that the V atoms are at least partially inserted in the silicalite structure. The remaining part is probably present as v 3 + in the cation sites from where it can be removed by ion exchange.

The

generally

precursor

(calcination

into the H-form) structure. in

some

used

in air

oxidative at

773

activation

cases

of

the

K followed by ion exchange

lead in most cases to a collapse of the zeolite

the resulting catalysts consist of

non-oxidative

mode

of

a mixture

activation

of

of

crystobalite the

zeolitic

a-crystobalite,

or

and silicalite-2. precursor,

on

A

the

contrary, preserves the original pentasyl structure. Extra-lattice vanadium ions are removed by the ion exchange treatment, with an increase in the Si/AI ratio in the act i vated catalysts.

In order

to investigate the catalytic properties of these samples we have utilized as test reactions the butadiene oxidation and p- and mxylene ammoxidation .


.;, ,,,...,

.~:

30

o

,,,>

..., 20

oQ)

,..;

Q)

U1

<: 10 rll

'"

::l

'"

0 10

?

~o/

0

.

100

•• SI/V

.0: <,

'"

0

,..;

><

0.

------0.

2 0_-------0

til

0------- 0 _

Q)

0

0 _______

,..;

0 E

e '"

0 Z

1000

0

0

5

FIG 2 Selectivity to furan at 35% ( 8 ), 50% ( 0 ) of total butadiene conversion vs. the atomic catalysts prepared by activation of V-silicalite (open symbols) and comparison with the behavior vanadium on ZSM (full symbols).

10

and 65% ( a ) ratio Si/V in precursors (14] of a supported

FIG 3 Effect of oxygen concentration on the normalized formation of benzene (D), toluene ( 8 ) and xylenes ( 0 ) from propane at 693 K on HZSM5 with Si/Al=35 (15].

253 The

select i vi ty

reported

in

to

furan

Figure

2

activated catalysts are

obtained

conditions

for

very

from

as

a

[14].

butadiene

function

High

high

specific

of

of

these

catalysts

selectivities and yields

values

of

active

sites

the

is

s i zv ratio in the

the

ratio.

SijV

form.

The

to

furan

In

these

comparison with

the simple impregnation of HZSM with the same very small amount of V

(full

symbols)

stresses

the

specificity

of

sites. As the V content increases, however, V selective form that decrease formation

of

cases

maleic

no

probable

the

nature

selectivity to furan

unselective

anhydride

the

due

was

V

to the

extra-lattice V sites.

formation

of

sites which are not

detected.

It

In

is

all

worth

noting that these samples are not acid due to the absence of AI. This suggests that the low selectivity of the HZSM-V sample with a high SijV ratio

(Fig.

2,

full

symbols)

can be

correlated to the

detrimental presence of acid sites. As shown in Figure 1, in fact, these

catalysts

conversion)

have

when

higher

selectivities

P-V compounds

are

to

furan

deposited

in

(at

very

greater

low

amounts

(the Sijv ratio in the catalysts of Figure 1 is around 30). of

Modification

zeolite

surface

in

properties

the

presence

of

oxygen. In

the

oxidation of bUtadiene on pure

ZSM and Y zeolites we

have observed that the unmodified zeolites are already very active in

hydrocarbon

only

oxidation,

reaction

product

giving

[8].

however

The

addition

carbon of

oxides

the

PV-

as

the

compound

generally only slightly affects the activity, changing principally the formation of products of partial oxidation such as furan. order

to

investigate better

the

nature of

the

change of

In

surface

reactivity of the zeolite in the presence of gaseous 02 we studied the

oxygen

particular,

effect we

using

analyzed

a

less

the

reactive

conversion

hydrocarbon.

of

light

In

alkanes

to

aromatics using HZSM-5 zeolites with different acidities. Figure 3 shows that the presence of small amounts of gas phase 02 increases the

formation

propane

due

activation. (olefin

of to

The

and

aromatics the

(benzene,

formation

dependence

aromatics

of

of

toluene specific

the

formation)

oxidative and

activity on the acidity of the HZSM-5 sites alkane

are

involved both

activation

and

in

in

the

and

[15]

formation

unselective

xylenes)

centers

total

of

from alkane

dehydrogenation oxidation

(COx)

suggests that Bronsted of

sites

activation.

of

The

selective former

is

probably related to the formation of localized free radicals by 02

254

interaction with OH sites; may

directly or indirectly,

species,

give

rise

to

these highly energetic radical species via the

selective

forming the corresponding olefin OH

groups

can

directly

activate

formation

activation

of oxygen radical of

the

paraffin

[15]. On the other hand, hydrocarbons

on

which

strong oxygen

interaction gives rise to unselective oxidation to COx' In

conclusion,

the

interaction

of

oxygen

with

the

zeolite

alters the surface reactivity creating centers with a radical-like nature that can modify overall

catalytic behavior.

The formation

of these centers is both a function of the nature and pretreatment of the zeolite and of its surface acidity. ACKNOWLEDGEMENTS Financial support for this work was provided by the Ministero Pubblica Istruzione (Italy). REFERENCES [ 1]

[2 ] [3 ]

[4 ]

[5 ] [6 ] [7 ] [8 ] [9 ]

[10 ] [11]

[12 ] [13]

[14] [15]

D.B. Tagiyev, K.M. Minachev, In "New Developments in Zeolite Science and Technology", Y. Murakami et a I , Eds., Elsevier Pub.:Amsterdam 1986; p. 981. S.S. Shepelev, K.G. lone, React.Kinet.Catal.Lett., 29 (1985) 203. F.J.van der Gaag, F. Louter, H. van Bekkum, In "New Developments in Zeolite Science and Technology", Y. Murakami et al. Eds., Elsevier Pub.:Amsterdam 1986; p. 763. T. Inui, O. Yamase, K. Fukuda, A. Itoh, J. Tarumoto, M. Morinaga, T. Hagiwara, T. Takagami, In "Proceedings, 8th Int. Congress on Catalysis", Dechema Pub.:Frankfurth A.M. 1984, III-569. G. Centi, K. Habersberger, P. Jiru, F. Trifiro', Z. Tvaruzkova, Chern. Expr., 1 (1986) 717. A.V. Kucherov, A.A. Slinki, Zeolites, 7 (1987) 38. G. Centi, Z. Tvaruzkova, F. Trifiro', P. Jiru, L. Kubelkova, App1. Cata1., 13 (1984) 69. Z. Tvaruzkova, G. Centi, P. Jiru, F. Trifiro', App1. Catal., 19 (1985) 307. P. Jiru, G. Centi, Z. Tvaruzkova, F. Trifiro', React. Kinet. Cata1. Lett., 31 (1986) 259. G. Centi, F. Trifiro', J. Mo1ec. Cata1., 35 (1986) 255. G. Centi, F. Trifiro', J. Ebner, V. Franchetti, Chern. Rev., 88 (1988) 55 P. Cavalli, F. Cavani, I. Manenti, F. Trifiro', Catal. Today, 1 (1987) 245. F. Cavani, F. Trifiro', P. Jiru, K. Habersberger, Z. Tvaruzkova, Zeolites, 8 (1988) 12. K. Habersberger, P. Jiru, Z. Tvaruzkova, F. Cavani, G. Centi, F. Trifiro', Zeolites, submitted. G. Centi, G. Go1inel1i, J. Cata1., submitted.