Vapour phase ethylation of benzene over ZSM-5 synthesized in fluoride medium

Article abstract of DOI:10.14233/ajchem.2013.13638

ZSM-5 zeolite (Si/Al ratio 50, 75 and 100) was hydrothermally synthesized in fluoride medium. Their crystal size and crystallinity were high. SEM images of synthesized material showed large elongated prismatic crystals. Their catalytic activity was examin

Full text of DOI:10.14233/ajchem.2013.13638

Asian Journal of Chemistry; Vol. 25, No. 5 (2013), 2679-2682
http://dx.doi.org/10.14233/ajchem.2013.13638
Vapour Phase Ethylation of Benzene Over ZSM-5 Synthesized in Fluoride Medium
*
S. HARIHARAN, M. PALANICHAMY and V. MURUGESAN
Department of Chemistry, Anna University, Chennai-600 025, India
*Corresponding author: E-mail: palani_annachem@yahoo.com, igooha1@gmail.com
(Received: 21 February 2012;
Accepted: 19 November 2012)
AJC-12427
ZSM-5 zeolite (Si/Al ratio 50, 75 and 100) was hydrothermally synthesized in fluoride medium. Their crystal size and crystallinity were
high. SEM images of synthesized material showed large elongated prismatic crystals. Their catalytic activity was examined for vapour
phase ethylation of benzene between 200 and 400 ºC. The ethylbenzene conversion was high at 300 ºC compared with other temperatures.
The selectivity of ethylbenzene was high at low ethanol content in the feed, but decreased with an increase in ethanol content. Among the
diethyl benzene isomers the selectivity of p-diethyl benzene was higher than others. High selectivity of p-diethyl benzene was due to large
crystal size and high crystallinity. Hence synthesis of ZSM-5 in fluoride medium could catalyze para selective ethylation of ethylbenzene.
The same catalyst could also be applied to other selective alkylations.
Key Words: ZSM-5, Fluoride media, Benzene, Ethylation.
(nafion-H) catalyst. The reaction was studied between 125 and
210 ºC. The lifetime of aluminum halide catalysts was limited
due to their leaching cause a sharp decline in the catalytic
activity with time. But sulphonic acid resign catalyst showed
better stability and good activity.
INTRODUCTION
ZSM-5 zeolite is the largely exploited catalyst in the petro-
chemical industries. Medium pore size, thermal and hydro-
thermal stabilities and strong acidity are some of the impor-
tant characteristics for their main catalytic applications. The
acidity can be tuned to desired extents altering by Si/Al ratios.
This zeolite has been synthesized in alkaline medium and
subjected to ion exchange to get H-form. Alkaline medium
was reported to yield reduced crystallinity and surface area1.
In contrast, synthesis of ZSM-5 in fluoride medium was
reported to yield large crystal size and high crystallinity2. High
crystallinity precludes external acid sites which are commonly
observed over ZSM-5, synthesized in alkaline medium.
Generally ZSM-5 zeolite with external acid sites promotes
isomerization of p-diethyl benzene to meta-diethyl benzne
(m-DEB) at high temperatures. Hence it could be logical that
ZSM-5 synthesized in fluoride medium, can be ideal for para
selective ethylation of benzene to p-diethyl benzene suppressing
its isomerization. ZSM-5 synthesized in fluoride medium was
also reported to possess weak and medium acid sites without
strong acid sites. It is an advantage such acid sites might not
promote much isomerization. Hence in the present study,
ethylation of benzene was carried out in the vapour phase with
this interest of acquiring high selectivity to p-diethyl benzene
using ethanol as alkylating agent. Olah et al.3 reported alkylation
and trans alkylation of benzenes over graphite-intercalated
Lewis acid halides and per fluorinated resign sulphonic acid
Vijaya ragavan et al.4 also reported AlPO-5, MAPO-5,
ZnAPO-5 and MnAPO-5 catalyst were used the products of
ethyl benzene, p-diethyl benzene, m-diethyl benzene and poly
alkyl benzene. Mn-AOP5 was more effective catalyst than
others. Conversion was maximum (47 %) at 400 ºC. ZSM-5
has been used in the mobile Badger process for the vapour
phase alkylation of benzene with ethylene5-7. The same reaction
was also studied over MCM-228,9. However such reactions
could not much importance in para selective product, but ZSM-
5 synthesized in fluoride medium can be avoid further isomer-
ization.
EXPERIMENTAL
Tetraethylorthosilicate (TEOS), aluminium sulphate,
ammonium fluoride and phosphoric acid were purchased from
Merck. Structural directing agent (tetrapropylammonium
bromide) was purchased from Sigma Aldrich.
The gel-composition used for the synthesis of ZSM-5 is
given below:
0.07 C12H28NBr: 1.0 SiO2: 0.01Al2O3 :0.01P2O5:1.6 NH4F: 80 H2O
ZSM-5 (50, 75 and 100) samples were synthesized in
hydrothermal fluoride medium, but only difference is by their
aluminium ratios alteration. The ZSM-5 (50) zeolite synthesized

2680 Hariharan et al.
Asian J. Chem.
1600
1400
1200
1000
800
procedure as follows: 39 g of water was taken in a 100 mL of
beaker, weighed accurately 0.08 g of phosphoric acid in it.
0.66 g of structural directing agent was added and allowed to
constant stirred for an hour. The pH was raised to 5.5 0.1
used 1:1 ammonia solution. The solution was kept an hour for
aging, then follow by the additions 7.3 g of tetraethyl
orthosilicate and 2.1 g of ammonium fluoride added in it. The
gel was transferred into 75 mL Teflon lined auto clave at 170 ºC
in a hot over for 3 days hydrothermal process. A complete
crystalline material was obtained after filtration followed by
dried in an oven at 110 °C for 12 h.
Detection methods: X-ray diffraction spectra of ZSM-5
catalysts were recorded on X-ray diffractometer (PAN alytical
X' pert Pro) with CuKα radiation source (λ = 1.54 Å) used
liquid nitrogen-cooled germanium solid state detector. The
diffractograms were recorded in the 2θ range 5-80º in step of
1.2º with count time of 10 sec at each point.
Si/Al=100
Si/Al=75
Si/Al=50
600
400
200
1600
1400
1200
1000
800
600
400
200
1600
1400
1200
1000
800
The surface morphology of the catalysts was viewed
under high resolution scanning electron microscope (SEM-S-
3400 N HITACHI) with an ion-sputter gold coated particles
approximately 50-20 µm thick. Elemental composition was
obtained with EDX of the same instrument. .
600
400
200
Vapour phase ethylation of benzene was carried out in
down-flow vertical fixed bed reactor. 0.3 g of catalyst was
placed in mid of the glass reactor and the catalyst supported
by glass wool. Equal proportions of the reactants were fed
into the reactor using infusion pump with proper flow time
interval. The products were confirmed by GC-MS (gas chroma-
tograph/mass spectrometer) (Perkin-Elmer Clarus 500).
0
5
10
15
20
25
30
35
40
45
50
2θ
(º)
Fig. 1. XRD patterns of ZSM-5 Zeolites
RESULTS AND DISCUSSION
The XRD patterns of ZSM-5 (50, 75 and 100), 3 days
synthesized samples are shown in Fig. 1. All these synthesized
materials were showed, their resultant peaks similar with
reported10, but their relative intensity of crystallinity was higher
than alkaline medium synthesized1. The high crystallinity was
due to more silica and alumina can complete soluble in acidic
pH of between 4 and 6. Hence, more atom transport to the gel
composition for high crystallinity.
The SEM images of ZSM-5 (50, 75 and 100) zeolites are
shown in Fig.2. All of them seem to be apt elongated prismatic
crystals, but few tiny crystals were also present in it. These
tiny crystals are commonly available during the ZSM-5
synthesis11. The relative tiny crystals very low compared with
alkaline medium synthesized, and these tiny crystals were also
established the optimum limitation of Si/Al ratio.
Catalytic activity
Effect of temperature: Ethylation of benzene was carried
out over ZSM-5 (50, 75 and 100) at 200, 250, 300, 350 and
400 ºC with the feed ratio 1:1 (benzene : ethanol) and WHSV
2.775 h-1. The products were mainly ethyl benzene with the
minor products of o-diethyl benzene (o-DEB) and m-diethyl
benzene. The results of ethyl benzene conversion and product
selectivity are presented in Table-1. The benzene conversion
over ZSM-5 (50) increased with the increase in temperature
from 200 to 300 ºC and then decreased. The decrease in conver-
sion was due to coke formation. The formation of coke was
verified by physical observation. The following reaction

Vol. 25, No. 5 (2013)
Vapour Phase Ethylation of Benzene Over ZSM-5 Synthesized in Fluoride Medium 2681
HO
H3C
OH
H3C
CH+2
Al-
+
+
Si
(ZSM-5)
CH3
C2H5
H3C
CH+2
CH3
Scheme-I: Ethylation of benzene and ethyl benzene isomers
Effect of feed ratio: The effect of feed ratios (benzene:
ethanol) was studied at 300 ºC over ZSM-5 (50). The results
are presented in Table-2. The ethyl benzene conversion
increased from 1:1 to 1:2, but decreased at 1:3 and 1:4. The
high conversion at 1:2 was due to increase the number of ethyl
cations for ethylation. The decreased at 1:3 and 1:4 was due
to dilution of benzene. Decrease in conversion with an increase
in the content of alkylating agent was also reported in the
literature13. The selectivity of ethylbenzene decreased with
increase in alcohol content of the feed established consecutive
alkylation. The selectivity of m-diethyl benzene was very low
due to large crystal size and high crystallinity. The observation
suggested that the route of converting benzene to diethyl benzene
directly. The high selectivity of ethylbenzene compared to all
other products even at high alcohol content in the feed.
Fig. 2. SEM pictures of ZSM-5 (a) Si/Al = 50; (b) Si/Al = 75; (c) Si/Al = 100
TABLE-1
EFFECT OF TEMPERATURE. REACTION CONDITION: FEED
RATIO 1:1, Wt OF THE CATALYST 0.3 g, WHSV = 0.2775 h-1
Product selectivity (%)
Temp. Conv.
Catalysts
p-
m-
o-
(ºC)
(%)
EB
Others
DEB DEB DEB
200
250
300
350
400
200
250
300
350
400
200
250
300
350
400
31
34
39
31
25
23
28
35
29
20
11
13
19
16
10
87
89
91
82
78
89
90
91
87
85
89
93
94
89
87
1
2
2
3
3
1
2
3
4
4
1
0
0
3
4
6
6
5
7
9
8
5
4
6
8
9
6
6
7
7
2
1
2
1
1
1
2
1
1
1
1
1
0
0
0
4
2
0
7
9
1
1
1
2
2
0
0
0
1
2
ZSM-5
(50)
ZSM-5
(75)
TABLE-2
EFFECT OF FEED RATIOS. REACTION CONDITIONS:
CATALYST = ZSM-5 (50); CATALYST 0.3 g; TEMP. = 300 ºC
Product selectivity (%)
Feed Conv.
Catalysts
p-
m-
o-
ratios
(%)
ZSM-5
(100)
EB
Others
DEB DEB DEB
1:1
1:2
1:3
1:4
39
46
38
29
91
76
69
61
5
2
4
3
2
2
2
1
2
0
2
1
2
ZSM-5
(50)
16
26
33
EB = Ethyl benzene, DEB = Diethyl benzene.
EB = Ethyl benzene; DEB = Diethyl benzene
Scheme-I illustrates ethylation of benzene. Ethanol is chemi-
sorbed on Bronsted acid sites to form ethyl cation which make
electrophilic attack on benzene to form mono and diethyl
benzenes. Although in the channel intersection of the zeolite
(9 Å), all the three diethyl benzene isomers could be formed
but EB was the major product since the alcohol content 1:1
feed ratio. The selectivity of the products showed a higher
value for ethyl benzene than the diethyl benzene isomers. The
selectivity of ethyl benzene increased with an increase in
temperature slowly but at 350 and 400 ºC it decreased steadily.
The decrease was due to its conversion of di and poly alkyl
benzene. Consecutive alkylation of ethyl benzene to diethyl
benzene is also on endothermic process12. Formation of di and
poly alkyl benzene was favour at high temperature.
The same reaction was also studied over ZSM-5 (75 and
100) and the results are presented in the same Table. These
catalysts also showed an increase in conversion with an increased
in temperature from 200 to 300 ºC and then decreased there-
after. The product selectivity also followed the same trend that
of ZSM-5 (50): the selectivity of p-diethyl benzene was higher
than diethyl benzene isomers due to high crystallinity and large
crystal size.
Effect of weight hourly space velocity (WHSV): The
effect of weight hourly space velocity on the ethylbenzene
conversion and product selectivity was studied at 2.832, 5.455,
8.183 and 10.917 h-1 and the results are presented in Table-3.
With increase in weight hourly space velocity the conversion
was decreased. The selectivity of the ethyl benzene gradually
increased due to suppression of consecutive alkylation. It was
also confirmed by decreased in the selectivity of p-diethyl
benzene and m-diethyl benzene.
TABLE-3
EFFECT OF WEIGHT HOURLY SPACE VELOCITY. REACTION
CONDITIONS: CATALYST = ZSM-5(50); CATALYST
Wt = 0.3 g; TEMP. = 300 ºC; FEED RATIO 1:2
Product selectivity (%)
Feed
ratios
Conversion
(%)
p-
m-
o-
DEB
2
1
1
0
EB
Others
DEB
DEB
2.832
5.455
8.183
10.970
46
33
29
23
76
79
83
89
16
15
13
10
4
3
2
1
2
2
1
0
EB = Ethyl benzene; DEB = Diethyl benzene

2682 Hariharan et al.
Asian J. Chem.
Effect of time on stream: The effect of time on stream at
6 h with feed ratio 1:1 and WHSV 2.74 h-1 at 300 °C was
studied. The results are presented in Table-4. With increased
in time on stream a slight decrease conversion was noted at
the end of 6 h. Hence coke might be form at very slow rate.
The selectivity of EB remained the same throughout the time
on stream. Similarly the selectivity of m and p-diethyl benzene
also remained the same. Hence as said above the reaction did
not promote coke formation.
high crystallinity and large crystal size. Hence the same
catalyst could be very active for para selective ethylation of
ethylbenzene. Even for para selective alkylation of other alkyl
aromatics this catalyst could be convenient.
ACKNOWLEDGEMENTS
The authors thank to UGC-JRF fellowship and DST-FIST
for providing instrument facilities.
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TABLE-4
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Conversion
(%)
p-
m-
o-
EB
Others
Stream (h-1)
DEB DEB DEB
1
2
3
4
5
6
41
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3
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Hence ethylation of benzene could be made to give
ethylbenzene to high selectivity with the feed ratio 1:2. The
selectivity of diethyl benzene could also be enhanced by
increase the alcohol content in the feed. Suppression of o-
diethyl benzene and m-diethyl benzene was essentially due to