Isopropylation of Benzene over Large Pore Zeolites

Isopropylation of Benzene over Large Pore Zeolites

G . Ohlmann et al. (Editors),Catalysis and Adsorption by Zeolites 0 1991 Elsevier Science PublishersB.V., Amsterdam 341 ISOPROPYLATION OF BENZENE OV...

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G . Ohlmann et al. (Editors),Catalysis and Adsorption by Zeolites 0 1991 Elsevier Science PublishersB.V., Amsterdam

341

ISOPROPYLATION OF BENZENE OVER LARGE PORE ZEOLITES

A.R. PRADHAN, B.S. RAO and V.P. SHIRALKAR Catalysis Group, National (India)

Chemical Laboratory, Pune 411 008

ABSTRACT Isopropylation reaction of benzene is carried out over large pore zeolites with characteristic structural differences (namely La-H-Y, H-mordenite and H-ZSM-12). The activity and deactivation pattern are correlated with structural and acidic properties. The deactivation of La-H-Y is due to blocking of active sites while that o f H-mordenite is due to blocking of channels. The stable activity and selective nature o f H-ZSM-12 for cumene can be attributed to siliceous nature, lower acidity and presence of non-interpenetrating channels. INTRODUCTION benzene using solid phosphoric acid Isopropylation o f (SPA) catalyst and Fridel-Crafts catalysts (refs. 1-3) for the production of cumene is an industrially important reaction. The drawbacks suffered by these processes (environmental and corrosion) can be overcome by using solid acid catalysts like zeolites. Major side products formed in this reaction are isomeric diisopropylbenzenes (DIPB) and at higher temperatures n-propylbenzene (nPB). Even though the medium pore zeolite ZSM-5 is reported (ref. 4) a s a potential catalyst for this reaction, better stability and selectivity were observed over large pore zeolites (ref. 5). In view of this, isopropylation of benzene was carried out over large pore zeolites with characteristic structural differences, like (1) La-H-Y with cubic crystal symmetry and three directional channel system having pore opening of 7.4 k , (2) mordenite with orthorhombic structure and unidirectional dual pore system with 6.7 X 7.0 (12 MR) and 2.9 X 5.7 (8 MR) connected via side pockets of 2.9 i, ( 3 ) ZSM12 with monoclinic symmetry and linear non-interpenetrating channels o f 5.7 X 6.1 "A. The stability, selectivity and deactivation pattern were correlated with structural, acidic

348

and sorption properties of these are reported in this communication.

zeolites and

the

results

EXPERIMENTAL Materials Benzene (

X pure) and propylene (having 4

99.98

% propane)

were used for catalytic studies. Catalysts La-H-Y (SK-500) and H-mordenite from M/s

Carbide, USA

Union

and

(Zeolon 100) were procured Norton, USA, respectively.

ZSM-12 was prepared in this laboratory following the procedure reported (ref. 6 ) earlier in the literature. ation of zeolite beta was avoided.

Co-crystallis-

Characterisation the

Crystalline framework

techniques

phase purity and the state of aluminium in of these samples were characterised by the

XRD,

like

of the samples was ammonia. Adsorption

P/Po

=

0.5

using

sensitivity of

4'

IR

and

MASNMR

spectroscopy.

Acidity

measured by the irreversibly adsorbed studies were carried out at 25°C and

a

McBain

balance

with

silica

spring

of

50.0 cm gm-'.

Catalytic reactions The catalyst was pressed and crushed into 10-20 mesh binder free self supported pellets. Prior to catalytic runs, the catalyst was activated hrs.

Catalytic

bed,

down

Benzene while

was

runs were

flow, fed

propylene

in a flow of dry air at 45OOC for 8

silica

by was

a

carried

out

reactor

at

syringe pump

metered

through

in an integral, fixed atmospheric

(Sage a

pressure.

Instruments, USA)

mass-flow

controller

(Matheson, USA). The products were analysed by gas-chromatography (Shimadzu, Model 15A) using Apiezone L column for liquids and Poropak Q column for gaseous samples. RESULTS AND DISCUSSION The structural features, silicon to aluminium ratio, acidity values in terms of irreversibly retained ammonia and equilibrium sorption capacity for benzene are compared in Table 1.

TABLE 1 S t r u c t u r a l and physico-chemical Catalyst

La-H-Y

Channel structure

H-ZSM-12

i

7.4

Unit c e l l crystal symmetry Si/Al

Cubic

total

5.7 x 7,l 1 2 . 9 X 5.7 ‘A (8 MR)

Monoclinic

Orthorhombic

60.5

1.338

Equil. sorption capacity for benzene (wt Z)

6.4

0.535

0.067

9.8

13.2

20.3

is

acidity

Unidirectional 8-membered i n t e r connecting

5 . 7 X 6.1

2.3

Acidity m mole o f NH3/gm

H-mordenite

Unidirectional l i n e a r non-inte r p en e t r at i n g

Three directional with interconnecting channels

P o r e opening (12-membered ring)

The

p r o p e r t i e s of c a t a l y s t s .

to

found

be

in

accordance

with

the

aluminium c o n t e n t o f t h e z e o l i t e , w h i l e t h e s o r p t i o n of b e n z e n e does

not

the

follow

same

trend

due

to

structural

the

differences. The

catalytic

propylene

over

performance the

all

in

three

alkylation

zeolites

has

of

benzene

been

with

compared

in

T a b l e 2. c a t a l y s t s showed

A l l

But

the

and

higher

and

impurities

of

in

than

200

catalytic

aliphatics,

fractions

catalyst.

after

even

hrs

(Fig.

cumene

to

fast

reaction

Due

were

studies

are is

conversions.

C8

toluene,

(H.B.F)

A

the

1).

propylene

aromatics

more very

deactivation

in high

was

La-H-Y i n

the

noticed

w h i l e s t e a d y a c t i v i t y was o b s e r v e d

and H-mordenite,

H-ZSM-12

X)

( )99

Selectivity

H-ZSM-12

i n La-H-Y

like

boiling

H-mordenite.

case

high

to

its

performed

was

performed

for

steady activity, over

H-ZSM-12

more

further

catalyst.

Influence of temperature In on At

Table

the

3

product

temperatures

not completed.

the

results

on

distribution below

200°C,

the over

the

influence H-ZSM-12

conversion

With t h e i n c r e a s e of

of

temperature

are p r e s e n t e d . of p r o p y l e n e i s

temperature,

a continuous

350 decrease i n the

DIPB formation

is noticed,

while

appreciable

q u a n t i t i e s o f nPB a r e o b s e r v e d a b o v e 230°C.

TABLE 2 I s o p r o p y l a t i o n o f benzene o v e r z e o l i t e c a t a l y s t s R e a c t i o n t e m p e r a t u r e = 230OC; P r e s s u r e = Atmospheric;-TOS = 3 h r s ; Benzene t o p r o p y l e n e molar r a t i o = 6 . 5 ; WHSV = 2 . 5 h r Catalyst

La-H-Y

H-ZSM-12

Product d i s t r i b u t i o n (wt % ) Aliphatics Benzene T o l u e n e t C8 a r o m a t i c s Cumene nPB C9-C11 a r o m a t i c s DIPB H.B.F

0.74 77.10 0.97 18.52 0.40 0.29 1.75 0.18

0.06 77.80 0.04 20.50 0.02 0.04 1.55 0.01

0.30 78.30 0.27 18.10 0.74 0.12 1.78 0.26

C3 = c o n v e r s i o n Cumene s e l e c t i v i t y S e l e c t i v i t y (cumene t DIPB)

99.4 81.1 88.5

99.9 92.3 99.3

99.8 82.9 91.6

1./F

H-mordenite

t,

w h 19

2

a

H.ZSM-I2

* H MORDENITE rLa H Y

4

6 TIME

ON STREAM (hra)

F i g . 1 . C a t a l y t i c p e r f o r m a n c e o f t h e wide p o r e z e o l i t e s i n t h e i s o p r o p y l a t i o n of b e n z e n e w i t h t i m e o n s t r e a m (TOS). R e a c t i o n temp. = 230OC; WHSV = 2 . 5 h r - 1 ; Benzene t o p r o p y l e n e m o l a r r a t i o = 6 . 5 .

351 The increase in selectivity to cumene with temperature Above is a result of transalkylation of DIPB with benzene. 230°C, the selectivity towards cumene decreases on account of the formation of nPB. Also unwanted products like aliphatics, C9-C11 aromatics and higher boiling fractions increase with the increase in temperature as a result of cracking of higher alkylbenzenes (ref. 7). TABLE 3 Influence of temperature on product distribution Catalyst = H-ZSM-12; Benzene to propylene molar ratio WHSV = 2.5 hr-1 Temperature ("C)

170

190

210

Product distribution (wt X ) Aliphatics 0.03 0.03 Benzene 84.42 82.38 0.07 Tol. t C8 arom. 0.26 Cumene 11.47 13.39 nPB 0.01 C9-C11 arom. 0.06 DIPB 4.01 3.86 H.B.F -

0.05 80.65 0.11 16.68 0.03 0.04 2.41 0.02

0.06 80.00

99.8 86.2 97.1

C3 = conversion 85.3 Select. to cumene 73.6 Select (cumene 99.4 + DIPB)

.

95.3 76.0 97.8

230

=

6.8;

250

270

0.18 18.24 0.11 0.06 1.35

0.12 79.71 0.22 18.46 0.31 0.10 1.05 0.03

0.14 78.87 0.23 18.74 0.63 0.17 1.03 0.09

99.8 91.2 98.0

99.6 90.9

99.4

-

96.2

88.7 93.4

Influence o f mole ratio With increase in benzene to propylene molar ratio, selectivity to cumene increases, even though the total selectivity to cumene and DIPB remained almost constant during present investigation (Fig. 2). This is due to high propylene concentration at lower mole ratios, resulting in the successive alkylations of cumene. Relative amounts of unwanted byproducts are less at higher mole ratios.

352

> 100t >

F

_-

95-

0

w

d

90.

s

85-

xCUMENE*UPB CUMENE

v)

5

20

IS

10

MOLE RATIO

Fig. 2. Influence of reactant mole ratio on the isopropylation of benzene over H-ZSM-12 catalyst. Reaction temp. = 2 3 0 ° C ; WHSV = 2 . 5 hr-l Influence of weight hourly space velocity (WHSV) Low space velocities are found to be favourable selective to

formation

cumene

and

of

(Fig.

cumene

3).

Total

for

selectivity

DIPB again remains constant over the entire

range of WHSV.

100

t

t 1 c

95 90

; 0

. '

CUMENE *DIP6

:\

.

.

CUMENE

85.

fn

s

80

1

2

3

4

5

6

7

SPACE VELOCITY ( W H S V I h i l

F i g . 3.

Influence o f weight hourly space velocity (WHSV) in the isopropylation of benzene over H-ZSM-12 catalyst. Reaction temp. = 2 3 0 ° C ; Benzene to propylene molar ratio = 6 . 5 .

353

Thus

the

optimised condition for propylation of benzene over H-ZSM-12 catalyst are at temperature 230°C, WHSV, 2.5 hr-l, and reactant mole ratio, 6-8. As already mentioned, the ageing studies indicated faster deactivation o f La-H-Y and H-mordenite catalysts. Whereas H-ZSM-12 catalyst did not deactivate even after 200 hrs of time on stream (TOS), therefore faster deacpivation was carried out by accelarated ageing. All the three coked samples were subjected to thermal and sorption studies to find out the Fig. 4 presents the cumene probable cause of deactivation. sorption kinetics on fresh and coked samples.

r

I CATALYST

Lo H Y

o FRSH SAhlPLE DEACTIVATED SAMPLE

0

I= -=+/

1

V , 6

n

a w

2

w

5u

t

l4

10

2

I

f

r

5t

= I

10

30

20

TIME

(

'

120

min 1

Fig. 4 . Kinetics o f cumene sorpkion on fresh ( 0 ) and coked zeolite catalysts at 25 C (P/P, = 0.5).

( 0 )

354

and

The equilibrium sorption capacity for cumene over fresh deactivated samples and the % coke formed for the

cummulative

feed

passed

over

each

catalyst

is

presented

in

Table 4 .

TABLE 4 Physico-chemical studies on fresh and coked catalyst samples Catalyst

La-H-Y

H-mordenite

H-ZSM-12

Equil. sorption of cumene over fresh sample (wt X)

17.6

4.5

12.55

Equil. sorption of cumene over deactivated sample (wt X)

11.16

1.4

6.36

X sorption capacity

63

31

51

7.48

5.80

8

10

250

0.37

0.15

0.003

retained

14.78

Amount of coke formed (wt X) Time required for deactivation,hrs ( 10 of initial activity)

X

Amount of coke formed per 100 gm of catalyst per gm of feed (gm)

and

Although the rate of sorption of cumene is same for fresh coked La-H-Y the equilibrium sorption capacities are

different. to

high

The deactivation of this catalyst may be attributed

acid

irreversibly structure

site

density

adsorbed

wherein

the

(ref.

ammonia)

8)

in

reactants

(as the

revealed three

(especially

by

total

dimentional

propylene)

and

product molecules are strongly adherent to the active sites. Also due to dehydrocyclisation reaction,bulkier coke precursors are (ref.

readily

9).

(14.78 %

formed

in

the

large

intra-zeolite

cavities

This results in higher amount of coke formation calculated b y thermogravimetric methods) in the

catalyst. In spite o f this, the coked sample shows more than half the void volume still available ( 6 3 % o f

case of La-H-Y

355

initial sorption capacity) for sorption of reactant molecules without any further activity. Thus the deactivation to blocking of the active sites.

is due

The phenomenon of deactivation in H-mordenite is relatively slower on account of lower acid site density compared to La-H-Y (ref. 10) (Si/A1 = 6 . 4 and 0.535 m moles of irreversibly adsorbed

ammonia

per

gm

of

catalyst).

ivation in the unidirectional

However,

the

deact-

pore system of mordenite leads

to a drastic decrease in sorption capacity in the coked sample. In

fact,

capacity 30

X)

the in

sorption

the

suggest

kinetics

coked mordenite only

surface

and

equilibrium

(retained

adsorption

sorption

sorption

capacity

indicating

blocking

of the channels. The

least

coking

tendency

is

observed

sample (0.003 gm of coke formed per gm

of

feed

density

passed)

(Si/A1

again

on

60.5 and

=

in

the

H-ZSM-12

100 gm of catalyst

account

0.067 m

adsorbed ammonia per gm o f catalyst).

of

lowest

acid

per site

of irreversibly In addition, the linear moles

non-interpenetrating unidirectional channel system with virtual absence of larger intra-zeolitic cages (like those in zeolite

Y)

and

highly

siliceous nature

of bulkier coke precursors. by

ZSM-12

activity

zeolite for

not

favour the

formation

The sorption capacities exhibited

support

prolonged

do

these

period

of

findings. time

Thus

(more

the

than

stable

200 hrs)

can be explained for H-ZSM-12. CONCLUSIONS H-ZSM-12

is

a

superior

and

selective

catalyst

in

the

alkylation of benzene with propylene. The catalytic activity and stability are dependent on the acidic and structural properties. Coking

of

La-H-Y

is

due

to

acid

site

blocking,

in

H-mordenite, it is due to channel blocking while deactivation of H-ZSM-12 required accelarated ageing. ACKNOWLEDGEMENT We

sincerely

thank

Dr.

P. Ratnasamy,

for

his

constant

encouragement throughout this investigation. We also thank Dr. V.G. Gunjikar and Mr. S . P . Mirajkar, for helping in

356

the

thermal and s o r p t i o n s t u d i e s .

T h e w o r k was p a r t l y f u n d e d

by t h e UNDP. REFERENCES

2 3

4 5

6 7 8

9

10

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