Gas phase toluene isopropylation over high silica mordenite

Article abstract of DOI:10.1007/s12039-010-0096-6

Mordenite (HM) catalysts with three different Si/Al ratios were compared for their activity and selectivities in gas phase toluene isopropylation with isopropanol. Catalyst with Si/Al ratio 44·9 offered better cumene selectivity, hence, it was chosen for detailed kinetic investigations. The influence of various process parameters like temperature, time-on-stream, weight hourly space velocity (WHSV), reactant mole ratio on this catalyst activity are discussed. The cymene selectivity was found to increase with reaction temperature and passed through a maximum at 473 K. The deactivation with time-onstream is almost negligible. Lower isopropyl alcohol concentration in the feed improved cymene selectivity. The conversion and selectivity to cymenes were compared with those of the large pore beta catalyst. The rate constant and activation energy were found to be 7·34 m3/kg h and 41·84 kJ/mol, respectively using homogeneous kinetic model. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-010-0096-6

J. Chem. Sci., Vol. 122, No. 4, July 2010, pp. 613–619. © Indian Academy of Sciences.
Gas phase toluene isopropylation over high silica mordenite
SREEDEVI UPADHYAYULA
Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi 110 016
e-mail: sreedevi@chemical.iitd.ac.in
MS received 12 March 2009; revised 11 June 2009; revised 27 July 2009
Abstract. Mordenite (HM) catalysts with three different Si/Al ratios were compared for their activity
and selectivities in gas phase toluene isopropylation with isopropanol. Catalyst with Si/Al ratio 44⋅9 of-
fered better cumene selectivity, hence, it was chosen for detailed kinetic investigations. The influence of
various process parameters like temperature, time-on-stream, weight hourly space velocity (WHSV),
reactant mole ratio on this catalyst activity are discussed. The cymene selectivity was found to increase
with reaction temperature and passed through a maximum at 473 K. The deactivation with time-on-
stream is almost negligible. Lower isopropyl alcohol concentration in the feed improved cymene selecti-
vity. The conversion and selectivity to cymenes were compared with those of the large pore beta catalyst.
3
The rate constant and activation energy were found to be 7⋅34 m /kg h and 41⋅84 kJ/mol, respectively us-
ing homogeneous kinetic model.
Keywords. Mordenite; cymene; p-cymene; kinetics; isopropylation; toluene; silica.
1. Introduction
HY produced only cymenes, HZSM-5 catalysts gave
a very high proportion of n-propyl toluene. Parikh et
4
In recent times, considerable attention is given on
the selective synthesis of para-dialkylbenzenes, like
xylene, ethyl toluene, diethyl benzene via alkylation
reactions catalysed by shape selective zeolites. Iso-
propyl toluene (cymene), in particular, the para-
isomer of it, is one such commercially important
dialkylbenzene. It is used as an important starting
material for the production of a range of intermedi-
ates and end products, such as cresols, isopropyl
phenols, fragrances, pharmaceuticals, herbicides,
heat transfer media, etc.
al studied the same reaction over zeolites varying in
pore system, crystal size after silylation and pro-
posed a more realistic mechanism on ZSM-5. Guo et
5
al investigated the alkylation of toluene by propyl-
ene over HZM-12 zeolite and obtained cymene iso-
mers with a near thermodynamic equilibrium
6
composition. Cejka et al investigated the factors
controlling iso-/n- and para-selectivity in the alkyla-
tion of toluene with isopropyl alcohol on molecular
sieves possessing different acidity (Al- and Fe-
silicates) and structure type (Y, mordenite and MFI
structure). The n-propyl toluene is a result of prod-
uct selectivity on the MFI type catalysts. It comes
out easily, whereas the formation of stable secon-
dary carbocation is favoured, which should give iso-
propyl toluene. However, due to the size constraints,
n-propyl selectivity is increased. Toluene isopropy-
lation on beta zeolites has been reported by Reddy et
The alkylation of toluene over zeolites is normally
considered to follow the well-known Friedel–Crafts
mechanism which evidently proceeds by direct
ortho-para attack of the ring followed by positional
1,2
isomerisation producing the meta isomer. Modifi-
cation of zeolites can alter the chemical properties
of the catalytic center so that it ceases to partially or
completely facilitate isomerisation, thus increasing
the selective yield of the important para-isomer.
7
al. These zeolites were observed to be highly stable
8
with no shape selectivity. Witchterlova et al also
3
Fraenkel and Levy studied toluene isopropylation
studied the selective formation of p-cymene on Al
and Fe silicates.
reaction on medium pore HZSM-5 zeolites differing
in crystal size and morphology and proposed a reac-
tion mechanism. They also compared the product
distribution of the reaction on these medium pore
zeolites with large pore HY and HM. While HM and
From the above discussion, it is evident that
although the reaction has been studied over mor-
denite catalysts for the purpose of comparing with
other catalysts, a detailed study of this reaction over
613

614
Sreedevi Upadhyayula
high silica mordenite has not been reported. Mor- Hence, pseudo first order kinetics were investigated
denite, a large pore zeolite has shown promise in based on isopropyl alcohol. The term ‘conversion of
acid catalysis of many hydrocarbon reactions in- IPA’ is defined as IPA conversion (mol%) = (moles
volving large aromatic molecules. The pore size of of IPA consumed/moles of IPA fed per unit time) ×
mordenite is near to the kinetic diameters of toluene 100.
and cymene molecules. However, it has not been in-
vestigated thoroughly in this reaction by earlier p-cymene selectivity are defined as follows:
workers, especially with varying Si content/acidity.
Hence, this work was taken up to study the kinetics
of this reaction in detail over this zeolite and the toluene in feed] × 100.
The terms toluene conversion, cymene selectivity,
Toluene conversion (wt%) = [(toluene in product)/
effect of various parameters on the yield and cy-
mene selectivity.
Cymene selectivity (wt%) = [(cymenes in product)/
total aromatics (excluding toluene) in product] × 100
p-cymene selectivity (wt%) = [(yield of p-cymene)/
total yield of cymenes] × 100.
2. Experimental
2.1 Materials
Based on the product distribution pattern shown in
table 1, the reaction scheme of toluene isopropyla-
tion with IPA over the HM catalysts may be repre-
sented as
The various mordenite samples used in this investi-
gation were obtained from M/s Süd Chemie AG,
Germany. The beta zeolite used for comparative
study was supplied by PQ Corporation, USA. The
catalyst powder was pelletised, crushed and sieved
to 30–50 mesh particles before loading in the reactor.
All the samples used in this study have been
found to be highly crystalline as revealed by XRD
studies. The Si/Al ratios of the three mordenite cata-
lysts are 9⋅9, 22⋅5 and 44⋅9 and these are designated
as HM-1, HM-2 and HM-3 respectively. The Si/Al
ratio of the beta catalyst was 13.
Main reaction:
toluene + IPA → p-cymene + water.
Secondary reactions:
p-cymene → m-cymene (isomerisation)
p-cymene + IPA → 2,4-diisopropyl
(DIPT) + water (cymene isopropylation)
toluene
2,4-diisopropyl toluene + toluene → 2p-cymene
(transalkylation of DIPT)
p-cymene → toluene + propylene ( cymene deal-
kylation)
Toluene and isopropyl alcohol (IPA) used in the
present study were obtained from E Merck (India)
Ltd. and Qualigens Fine Chemicals, India respec-
tively and were of ‘Analytical Reagent’ grade.
DIPT → p-cymene + propylene (dealkylation of
DIPT)
Toluene → benzene + xylene (toluene dispropor-
tionation)
IPA → propylene + water (isopropyl alcohol de-
hydration)
2.2 Procedure
Propylene → aliphatics (oligomerisation)
The experiments were conducted in a down flow fixed
–3
bed reactor (15 mm i.d.) with 2 × 10 kg of catalyst.
3.1 Effect of Si/Al ratio on activity of HM
Nitrogen was used as the carrier gas. Toluene to
isopropyl alcohol mole ratio in the feed was main-
tained at 6:1 during the runs. A feed flow rate of
Variation of Si/Al ratio is expected to influence the
acidity characteristics of mordenite catalysts, which
in turn relates to their activity in toluene alkylation
with isopropyl alcohol. Table 1 shows the effect of
variation of Si/Al ratio on the product distribution.
The cymene selectivity, in general, increased with
the increase in Si/Al ratio until an optimum ratio of
22⋅5, while a simultaneous reduction in aliphatics
content was observed. This shows that side reactions
yielding aliphatics were suppressed, improving the
–3
6⋅8 × 10 kg/h and more was used for the kinetic
study. The products of the reaction were analysed us-
ing flame ionization detector (FID) on a SHIMADZU
15A Gas chromatograph fitted with a ‘CP-Xylenes’
capillary (50 m, 0⋅32 mm i.d.) column.
3 Results and discussion
In the present study, toluene was taken in excess and alkylation selectivity. Product distribution shows
isopropyl alcohol (IPA) was the limiting reactant. that there are considerable amounts of benzene and

Toluene isopropylation over high silica mordenite
615
a
Table 1. Comparison of activities of mordenite samples with varying Si/Al ratio.
Catalyst
HM-1
HM-2
HM-3
Si/Al ratio
9⋅9
22⋅5
44⋅9
Product yields (wt%)
Aliphatics
Trace
Trace
77
Trace
Trace
77
Trace
Trace
77
Benzene
Toluene
C aromatics
8
Trace
Trace
6
Trace
Trace
6
bd*
Trace
10
Cumene
p-Cymene
m-Cymene
o-Cymene
15
15
11
bd*
1
bd*
Trace
21
bd*
2
#
DIPTs
Total cymenes (%)
a
21
21
Cymene sel (%)
b
p-Cymene sel (%)
91
93
92
30
30
48
a
–1
Conditions: temperature = 453 K; IPA/toluene mole ratio = 1 : 6; WHSV = 3 h .
#
*Below detectable limits. Diisopropyl toluenes.
cumene, indicating that the dealkylation/realkylation the catalyst HM-3. The yield and total cymene selec-
reactions cannot be neglected. On the catalyst with tivity was observed to increase with increase in tem-
Si/Al ratio of 44⋅9, the yield of cumene was negligi- perature up to an optimum value of 473 K.
ble. From table 1, it is also observed that when the Thereafter, a slight decrease in the cymene yield and
Si/Al ratio is increased from 22⋅5 to 44⋅9, there is a selectivity was noticed when the reaction tempera-
slight drop in the yield and selectivity of cymenes ture was raised further.
due to increase in the yield of diisopropyl toluene.
At higher temperatures, dealkylation reactions
On catalysts with lower Si content, dealkyla- become pronounced and hence yield and selectivity
tion/transalkylation of DIPT with toluene may not of total cymenes are reduced. Also, as the tempera-
be taking place to the required levels. As a result, ture is increased from 433 to 493 K, the selective
DIPTs are retained in the product. Whereas, DIPTs yield of p-cymene reaches 30% which is nearer to
are converted to cymenes over the catalysts with op- the equilibrium cymene distribution value of around
timum Si content. The selectivity to p-cymene 28⋅5%. At higher temperatures, as a result of isom-
showed a remarkable increase from 30% for Si/Al erisation, the p-cymene is formed in almost equilib-
ratio of 9⋅9 to 48% for the Si/Al ratio of 44⋅9. This rium yield. An interesting observation is that there is
increase in p-selectivity may be attributed to sup- no o-cymene in the product. The reason for this
pression of isomerisation of p-cymene as a result of absence could be that the o-cymene formed inside
low density of acid sites due to low Si/Al ratio. It is the pores of HM may not be able to diffuse out of
reported that during isopropylation of toluene on the unidimensional channels of this large pore zeo-
MCM-41 catalysts, more p-cymene was observed in lite. Similar observations were made by Wichterlova
10
the product mixture as a result of lower acid site and Cejka, and they concluded that zeolite channel
density on the catalysts and also easier diffusion of geometry plays a decisive role in the formation of
9
primary product, i.e. p-cymene. Based on these products.
observations, the catalyst (HM-3) with Si/Al ratio of
44⋅9 was chosen for further study.
3.3 Effect of time-on-stream on toluene conversion
and cymene selectivity over HM
3.2 Effect of temperature on cymenes yield and
From figure 1, it is evident that the activity of HM-3
catalyst is almost steady during the 4 h of time-on-
selectivity over HM
Table 2 summarizes the effect of temperature on the stream. The toluene conversion and total cymene
yield and selectivity of cymenes, and product distri- selectivity showed negligible variation. Only a slight
bution in the temperature range of 433–493 K for fall in m-cymene yield was noticed with a simulta-

616
Sreedevi Upadhyayula
a
Table 2. Effect of temperature on product distribution.
Temperature (K)
433
453
473
493
Product distribution (wt%)
Aliphatics
Trace
Trace
77
Trace
Trace
77
bd*
Trace
78
bd*
Trace
78
Benzene
Toluene
C aromatics
8
bd*
bd*
9
bd*
Trace
10
Trace
Trace
8
Trace
Trace
6
Cumene
p-Cymene
m-Cymene
8
11
13
14
o-Cymene
bd*
bd*
6
bd*
2
bd*
Diisopropyl toluenes
Trace
Trace
17
Total cymenes (%)
21
92
48
52
21
96
38
62
21
92
30
70
Cymene sel (%)
74
p-Cymene sel (%) (28⋅22)**
m-Cymene sel (%) (56⋅44)**
54
46
a
–1
Conditions: IPA/toluene mole ratio = 1 : 6; WHSV = 3 h .
*Below detectable limits.
**Equilibrium distribution of isomers in the temperature range of 400–500 K.
Figure 1. Effect of time on stream on yield and selec-
tivity of cymenes over HM-3.
Figure 2. Variation of activities of three mordenite
neous rise in the yield of p-cymene, which may be
attributed to pore blockage due to coke.
catalysts with time on stream.
Figure 2 shows the activity of the three HM cata-
lysts with respect to time-on-stream. It is observed
that toluene conversion and cymene yield on HM-1,
with the lowest Si/Al ratio, is comparatively low and
decreases rapidly with time-on-stream. HM-2 cata-
lyst showed high initial toluene conversion and
cymene yield, however, this catalyst was also found
to deactivate fast after the first hour. HM-3 showed
stable activity as evident from the toluene conver-
sion and cymene yield curves in the figure. This
shows that an optimum acidity is important to main-
tain the stability of the mordenite catalysts.
3.4 Effect of feed mole ratio on toluene conversion
and cymene selectivity over HM
Figure 3 shows the effect of feed mole ratio on tolu-
ene conversion, cymene yield and p-cymene selec-
tivity. At lower toluene to isopropyl alcohol feed
ratios, although toluene conversion is high, the
cymene yield is low due to the formation of undesir-
able side products. At higher toluene to isopropyl
alcohol mole ratios, these undesirable side products
are reduced showing an increase in the total cymene

Toluene isopropylation over high silica mordenite
617
a
Table 3. Effect of weight hourly space velocity (WHSV) on product distribution.
–1
WHSV (h )
3
6
7⋅2
9⋅6
12
Product distribution(wt%)
Aliphatics
bd*
Trace
78
bd*
Trace
78
bd*
Trace
79
bd*
Trace
80
Trace
Trace
83
Benzene
Toluene
C aromatics
8
Trace
Trace
8
bd*
Trace
9
bd*
Trace
10
bd*
Trace
9
bd*
bd*
8
Cumene
p-Cymene
m-Cymene
13
12
10
8
6
o-Cymene
bd*
Trace
21
bd*
Trace
21
bd*
1
bd*
2
bd*
4
DIPTs
Total cymenes
Cymene sel (%)
p-Cymene sel (%)
20
18
13
96
95
93
88
79
38
45
49
53
58
a
Conditions: temperature = 473 K; IPA/toluene mole ratio = 1 : 6.
*Below detectable limits.
observed to decrease with increased space velocity
(decrease in residence time).
It is clear from the table that there is a clear cut
link between yield of DIPTs and total cymene selec-
tivity. When the yield of the former increased with
decreasing space velocity, the yield of cymene
decreased. However, the cymene selectivity, in gen-
eral, and p-cymene selectivity, in particular, is
observed to increase at higher space velocities.
Improvement in p-cymene selectivity is probably
due to the suppression of isomerisation of para- to
meta-isomer.
3.6 Comparison of activity of HM with Hbeta in
toluene isopropylation reaction
Table 4 shows a comparison of product distribution
and cymene selectivities over HM and Hbeta cata-
–1
Figure 3. Effect of reactants mole ratio on toluene con-
lyst at 473 K, 3⋅5 h WHSV, using a toluene to iso-
version and cymene selectivity over HM-3.
propyl alcohol feed mole ratio of 8 : 1. The cymene
yield was almost the same (about 17%) on both the
catalysts, however, HM shows a better selectivity to
7
yield. At the same time, the isomerisation of para- to
meta- isomer seems to increase and hence a decrease
in para selectivity was observed with increasing
toluene to isopropanol feed mole ratio. Toluene
conversion fell, as expected, with lesser availability
of the alkylating agent in the feed.
cymenes and p-cymene, in particular. Reddy et al
found that HM (Si/Al ~ 10) did not give a better
cymene selectivity compared to Hbeta but a higher
Si/Al ratio of HM enhanced the selective formation
of cymenes as a result of narrow distribution of
acidity, which is suited for alkylation selectivity.
Higher reaction temperatures and high Si/Al ratios
help in accelerating the transalkylation of diisopro-
pyl toluenes. This is clearly reflected in the table as
catalyst HM-3 yields less amount of DIPTs. It is
also to be noted that on HM-3 catalyst, small
3.5 Effect of weight hourly space velocity (WHSV)
Table 3 summarizes the effect of space velocity on
product distribution. The yield of cymenes was

618
Sreedevi Upadhyayula
a
Table 4. Comparison of activities of HM with Hbeta in toluene isopropylation.
Catalyst
Si/Al
Hbeta
13
HM-3
44⋅9
Product distribution (wt%)
Aliphatics
Trace
Trace
81
bd*
Trace
83
Benzene
Toluene
C aromatics
8
Trace
Trace
5
Trace
Trace
6
Cumene
p-Cymene
m-Cymene
11
11
o-Cymene
Trace
Trace
bd*
Diisopropyl toluenes(DIPTs)
Trace
Total cymenes (%)
17
90
29
66
5
17
97
36
64
0
Cymene sel (%)
p-Cymene sel (%) (28.22)**
m-Cymene sel (%) (56.44)**
o-Cymene sel (%) (15.34)**
a
–1
Conditions: temperature = 473 K; IPA/toluene mole ratio = 1 : 8; WHSV = 3⋅5 h .
*Below detectable limits. **Equilibrium distribution of isomers.
amounts of benzene are produced indicating sup-
pression of dealkylation reactions. Cymene selecti-
vity on HM-3 is also enhanced.
3.7 Kinetic study
To determine the reaction rate parameters for the
conversion of isopropyl alcohol, a differential plug
flow reactor was assumed since the conversions of
isopropyl alcohol in the temperature range studied
were below 30%. The first order rate equation for a
differential plug flow reactor can be written as:
–r = dX /dτ = dX /d(W/F ) = kC ,
A A A Ao A
(1)
where W is the weight of the catalyst in kilograms,
is the feed isopropyl alcohol flow rate in
F
AO
kgmol/h, τ is the space time in kg h/kg mol, k is the
3
Figure 4. First order kinetic plot for toluene isopropyla-
pseudo-first order rate constant in m /kg h and C is
A
3
tion at 403 K over HM-3.
the concentration of isopropyl alcohol in kg mol/m .
Integrating (1) yields,
The rate of reaction of isopropyl alcohol is, there-
fore, first order with respect to IPA which is the limit-
ing reactant and is of zero order with respect to
toluene which is in excess. From the slope of the
–ln(1 – X ) =kC (W/F ),
A Ao AO
(2)
–4
3
line and with C as 5⋅32 × 10 kg mol/m , the rate
Ao
where X is the fractional conversion of isopropyl
A
3
constant (k) was calculated to be 7⋅34 m /kg h at
alcohol.
A plot of –ln(1 – X ) against W/F
403 K. Employing the power law equation from
over HM
AO
A
11
Power law kinetics, the rate of reaction for the pre-
gave a straight line passing through the origin as
illustrated in figure 4 at 403 K.
sent system would be,

Toluene isopropylation over high silica mordenite
619
until an optimum of 22⋅5. However, the catalyst with
Si/Al ratio of 44⋅9 showed good stability with time-
on-stream. Also, p-cymene selectivity over this cata-
lyst was significantly higher than the other two HM
catalysts studied. The yield of cymenes went through
a maximum as the reaction temperature was increased
from 433 to 493 K. The highest yield and cymene
selectivity was observed at 473 K. Stable catalyst
activity was observed throughout the experiments. A
change in toluene to isopropyl alcohol feed ratio
from 2 to 10 resulted in enhanced cymene yield and
decreased p-cymene selectivity. High silica mor-
denite (HM) showed similar conversion as Hbeta,
but HM proved to be comparatively a better cymene
selective catalyst under similar reaction conditions.
Acknowledgements
Figure 5. Arrhenius plot with HM-3 as the catalyst.
–r = Aexp(–E /RT)C ,
The author is grateful to Council for Scientific and
Industrial Research (CSIR), New Delhi for a Senior
Research Fellowship. The author thanks Dr C V
Satyanarayana, Catalysis Division, National Chemi-
cal Laboratory (NCL), Pune, India for his invaluable
help with the catalyst and experimental facilities in
his laboratory. The author is also grateful to Prof. N
C Pradhan, Chemical Engineering Division Indian
Institute of Technology (IIT) Kharagpur, for his
constant advice and support.
(3)
A
a
A
where E is the apparent activation energy and A is
a
the Arrhenius frequency factor. Since, for a reaction
in a differential plug flow reactor, –r = (F /W)X ,
A AO A
(3) can be modified as
(F /W)X = Aexp(–E /RT)C .
AO A a A
(4)
writing C in terms of initial concentration and frac-
A
tional conversion of IPA and rearranging the equa-
tion, we obtain,
References
1. Allen R H and Yats L D 1961 J. Amer. Chem. Soc. 83
2799
ln(X /1 – X ) = lnA – E /RT,
A A o a
(5)
2. Olah G A, Kuhn S J and Flood S H 1962 J. Amer.
Chem. Soc. 84 1688
where A is the apparent Arrhenius frequency factor
o
3. Fraenkel D and Levy M 1989 J. Catal. 118 10
4. Parikh PA, Subrahmanyam N, Bhat Y S and Halgeri
AB 1992 Appl. Catal. A 90 1
which includes a constant such as W/F as well as
AO
C
AO
(initial concentration of IPA) and E is the appar-
a
ent activation energy. From (5), it is clear that a plot
of ln(X /1 – X ) against 1/T would give a straight line
5. Guo H, Han C and Wang X 1993 Shiyou Xuebao Shi-
you Jiagong 9 41
A
A
from which E and the A can be obtained.
a o
6. Čejka J, Kapustin G A and Witchterlová B 1994
Appl. Catal. A108 187
The Arrhenius plot for the IPA conversion em-
7. Reddy K S N, Rao B S and Shiralkar V P 1995 Appl.
Catal. A121 191
ploying (5) is shown in figure 5. The value of E
a
4
from the plot is 41⋅84 kJ/mol and A is 8⋅93 × 10 .
o
8. Witchterlová B, Čejka J and Zilkova N 1996 Micro-
porous Mater. 6 405
The apparent activation energies evaluated is com-
parable to the activation energies reported by earlier
workers for isopropyl alcohol conversion on large
9. Medina-Valtierra J, Sanchez M A, Montoya J A,
Navarrete J and De Los Reyes J A 1997 Appl. Catal.
A158 11
12,13
pore molecular sieves for benzene alkylation.
10. Witchterlová B and Čejka J 1994 J. Catal. 146 523
11. Yue P L and Olaofe O 1984 Chem. Eng. Res. Des. 62
167
4. Conclusions
12. Palekar M G and Rajadhyaksha R A 1986 Catal. Rev.
Sci. Eng. 28 371
The isopropylation of toluene conducted on HM
catalysts with varying Si/Al ratio showed an increase
in toluene conversion with increasing Si/Al ratio
13. Parikh P A, Subrahmanyam N, Bhat Y S and Halgeri
A B 1991 Chem. Eng. J. 54 79