G . Ohlmann et al. (Editors),Catalysis and Adsorption by Zeolites 0 1991 Elsevier Science PublishersB.V., Amsterdam
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
and sorption properties of these are reported in this communication.
EXPERIMENTAL Materials Benzene (
X pure) and propylene (having 4
were used for catalytic studies. Catalysts La-H-Y (SK-500) and H-mordenite from M/s
(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.
phase purity and the state of aluminium in of these samples were characterised by the
of the samples was ammonia. Adsorption
measured by the irreversibly adsorbed studies were carried out at 25°C and
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.
in a flow of dry air at 45OOC for 8
in an integral, fixed atmospheric
(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
Unit c e l l crystal symmetry Si/Al
5.7 x 7,l 1 2 . 9 X 5.7 ‘A (8 MR)
Equil. sorption capacity for benzene (wt Z)
Unidirectional 8-membered i n t e r connecting
5 . 7 X 6.1
Acidity m mole o f NH3/gm
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)
p r o p e r t i e s of c a t a l y s t s .
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
T a b l e 2. c a t a l y s t s showed
A l l
La-H-Y i n
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
i n La-H-Y
steady activity, over
Influence of temperature In on At
With t h e i n c r e a s e of
are p r e s e n t e d . of p r o p y l e n e i s
350 decrease i n the
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
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
w h 19
* H MORDENITE rLa H Y
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)
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
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
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
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.
> 100t >
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
DIPB again remains constant over the entire
range of WHSV.
t 1 c
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 .
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.
Lo H Y
o FRSH SAhlPLE DEACTIVATED SAMPLE
V , 6
Fig. 4 . Kinetics o f cumene sorpkion on fresh ( 0 ) and coked zeolite catalysts at 25 C (P/P, = 0.5).
( 0 )
The equilibrium sorption capacity for cumene over fresh deactivated samples and the % coke formed for the
Table 4 .
TABLE 4 Physico-chemical studies on fresh and coked catalyst samples Catalyst
Equil. sorption of cumene over fresh sample (wt X)
Equil. sorption of cumene over deactivated sample (wt X)
X sorption capacity
Amount of coke formed (wt X) Time required for deactivation,hrs ( 10 of initial activity)
Amount of coke formed per 100 gm of catalyst per gm of feed (gm)
Although the rate of sorption of cumene is same for fresh coked La-H-Y the equilibrium sorption capacities are
The deactivation of this catalyst may be attributed
product molecules are strongly adherent to the active sites. Also due to dehydrocyclisation reaction,bulkier coke precursors are (ref.
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
initial sorption capacity) for sorption of reactant molecules without any further activity. Thus the deactivation to blocking of the active sites.
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
ivation in the unidirectional
pore system of mordenite leads
to a drastic decrease in sorption capacity in the coked sample. In
coked mordenite only
of the channels. The
sample (0.003 gm of coke formed per gm
100 gm of catalyst
adsorbed ammonia per gm o f catalyst).
of irreversibly In addition, the linear moles
non-interpenetrating unidirectional channel system with virtual absence of larger intra-zeolitic cages (like those in zeolite
of bulkier coke precursors. by
The sorption capacities exhibited
can be explained for H-ZSM-12. CONCLUSIONS H-ZSM-12
alkylation of benzene with propylene. The catalytic activity and stability are dependent on the acidic and structural properties. Coking
H-mordenite, it is due to channel blocking while deactivation of H-ZSM-12 required accelarated ageing. ACKNOWLEDGEMENT We
encouragement throughout this investigation. We also thank Dr. V.G. Gunjikar and Mr. S . P . Mirajkar, for helping in
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
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