Pathway to ethylbenzene formation in side-chain alkylation of toluene with methanol over cesium ion-exchanged zeolite X

Article abstract of DOI:10.1007/s10562-013-1049-8

To control the styrene to ethylbenzene ratio in the products of side-chain alkylation of toluene with methanol over Cs-X-based catalysts, pathway to ethylbenzene was examined. Styrene underwent transfer hydrogenation with methanol much faster than hydroge

Full text of DOI:10.1007/s10562-013-1049-8

Catal Lett (2013) 143:1025–1029
DOI 10.1007/s10562-013-1049-8
Pathway to Ethylbenzene Formation in Side-Chain Alkylation
of Toluene with Methanol Over Cesium Ion-Exchanged Zeolite X
•
Hideshi Hattori Wahab O. Alabi
•
•
•
B. Rabindran Jermy Abdullah M. Aitani
Sulaiman S. Al-Khattaf
Received: 8 April 2013 / Accepted: 17 June 2013 / Published online: 29 June 2013
Ó Springer Science+Business Media New York 2013
Abstract To control the styrene to ethylbenzene ratio in
the products of side-chain alkylation of toluene with meth-
anol over Cs-X-based catalysts, pathway to ethylbenzene
was examined. Styrene underwent transfer hydrogenation
with methanol much faster than hydrogenation with hydro-
gen to ethylbenzene. Addition of Cs2O and ZrB2O5 to Cs-X
enhanced and suppressed, respectively, the transfer
hydrogenation.
side-chain alkylation takes place over basic catalysts to
styrene and ethylbenzene. It is widely accepted that the
formation of styrene in the side-chain alkylation over basic
catalyst results from dehydrogenation of methanol to
formaldehyde which undergoes aldol-type condensation
with toluene to form styrene [1, 2]. In addition to styrene,
significant quantities of ethylbenzene are also produced in
all the results reported so far. However, the pathway of the
ethylbenzene formation has not been extensively studied.
Elucidation of the pathway to ethylbenzene is important in
controlling the styrene/ethylbenzene ratio in the products in
side-chain alkylation of toluene with methanol. The for-
mation of ethylbenzene is undesirable for an industrial
purpose.
Keywords Ethylbenzene Á Styrene Á Methanol Á Toluene Á
Transfer hydrogenation Á Side-chain alkylation
1 Introduction
The pathways to ethylbenzene proposed so far are (1)
hydrogenation of styrene, (2) direct methylation of toluene
with methanol, (3) hydrogenolysis of 2-phenylethanol
which is formed by aldol-type addition of toluene to
formaldehyde, and (4) transfer hydrogenation of styrene
with methanol as shown below.
Selectivity in alkylation of toluene with methanol is
determined by acid–base properties of the catalyst. While
ring alkylation takes place over acidic catalysts to xylenes,
H. Hattori Á W. O. Alabi Á B. R. Jermy Á
A. M. Aitani Á S. S. Al-Khattaf
Center of Research Excellence in Petroleum Refining and
Petrochemicals, King Fahd University of Petroleum & Minerals,
Dhahran 31261, Saudi Arabia
e-mail: woa792@mail.usask.ca
B. R. Jermy
e-mail: rabindra@kfupm.edu.sa
A. M. Aitani
e-mail: maitani@kfupm.edu.sa
S. S. Al-Khattaf
e-mail: skhattaf@kfupm.edu.sa
The pathway (1) was first proposed by Sidorenko et al.
[1], and later supported by Yashima et al. [2] on the basis
of the experimental observation that the selectivity for
styrene was reduced in a hydrogen stream as compared to
H. Hattori (&)
Catalysis Research Center, Hokkaido University,
Sapporo 001-0021, Japan
e-mail: hattorih@khaki.plala.or.jp
123

1026
H. Hattori et al.
that in a nitrogen stream over Rb-X and K-X. The H2 used
for the hydrogenation of styrene originated from decom-
position of methanol to acetaldehyde and CO. Decrease in
styrene production in an H2 stream was also observed for
Cs-X modified with Cu and B [3]. This pathway has been
often cited [4–9], though the evidences to support this
pathway were not presented.
exchanged with Cs? ions as described in our preceding
paper [12]. Cesium hydroxide was used as a source of Cs?
according to Engelhardt et al. [6]. The NaX has Si/Al ratio
of 1.24 and surface area of 527 m2 g-1. Into an aqueous
solution of cesium hydroxide (75 ml, 0.5 M), NaX (15 g)
was immersed, stirred for 5 min and stand for ca. 8 h. The
slurry was filtered with Buchner funnel. The filtered cake
was again immersed in the aqueous solution of cesium
hydroxide, stirred and stood for ca. 8 h. The immersion and
filtration procedures were repeated two more times (total 4
times). The resulting slurry was filtered and rinsed with a
75 ml of the aqueous solution of cesium hydroxide. The
resulting filtered cake was immersed in a deionized water
(100 ml) followed by filtering and washing with a small
amount of water (20 ml) to minimize the back ion-
exchange of Cs? with H? during washing with water. The
filtered cake was dried in an oven at 353 K and calcined at
753 K for 3 h in air. The ion exchange percent was 54 %.
The pathway (2) was also proposed by Yashima et al.
[2] to explain the observation that the ethylbenzene/styrene
ratio was higher when methanol was used as an alkylating
reagent than when formaldehyde was used. They claimed
that when methanol is used, the direct methylation of tol-
uene with methanol might occur in addition to the aldol-
type condensation to increase the ethylbenzene/styrene
ratio in the products.
The pathway (3) was proposed by Sooknoi and Dwyer
[9] as one of the possible pathways to styrene without
experimental support.
The pathway (4) was proposed by Garces et al. [10]. They
reported that styrene was readily hydrogenated with meth-
anol, but only minor amount of ethylbenzene was produced
with hydrogen over Cs ion-exchanged X and carbon modi-
fied with Cs and B [10]. However, no experimental data were
given. Although this pathway has not been taken into account
in most of the references on the side-chain alkylation, this
pathway should be examined as a possible one since transfer
hydrogenation with alcohols is frequently observed over
solid base catalysts. In the transfer hydrogenation with
alcohols, alcohols transfer their two hydrogen atoms to other
molecules such as olefins, ketones and nitriles, and the
alcohols convert into aldehydes or ketones [11].
BET surface area was 373 m2 g-1
.
The catalyst that occludes Cs2O in the pores of Cs-X was
prepared by impregnation of the Cs-X with cesium
hydroxide aqueous solution by incipient wetness. The con-
tent of Cs2O occluded in the pores other than ion exchanged
Cs? ions was 4.1 wt% Cs2O which is equivalent to 0.6 Cs
atom per super cage. The catalyst was denoted as Cs2O/Cs-
X. BET surface area of Cs2O/Cs-X was 308 m2 g-1
.
The Cs-X modified with zirconium borate (ZrB2O5) was
prepared by grinding a mixture containing the Cs-X and
ZrB2O5 with mortar for 30 min in a dry state [12]. A
ground mixture was calcined with a slow ramp rate of
2 K min-1 at 773 K for 3 h. The wt% of ZrB2O5 was 10
wt%. The catalyst was denoted as ZrB/Cs-X. Zirconium
borate was prepared according to Knyrim et al. [13] from
aqueous solutions of disodiumtetraborate and zirconium
nitrate. The precipitate was washed with water, dried and
calcined at 623 K in air. BET surface area of ZrB/Cs-X
In the present study, the pathway to ethylbenzene was
examined by carrying out the following reactions (a),
(b) and (c).
(a) The hydrogenation of styrene with hydrogen and with
methanol.
was 286 m2 g-1
.
(b) Toluene side-chain alkylation in a nitrogen stream
and in a hydrogen stream.
2.2 Reaction Procedures
(c) Toluene side-chain alkylation with different toluene/
methanol ratio.
A flow type reactor (Model # 401C-0286, Autoclave
Engineers) was employed for carrying out three types of
reactions; toluene side-chain alkylation with methanol,
hydrogenation of styrene with hydrogen, and transfer
hydrogenation of styrene with methanol. A catalyst sample
(400 mg) was placed in the reactor and pretreated in a
nitrogen stream at 723 K for 1 h prior to the reaction.
For the side-chain alkylation, a mixture containing tol-
uene and methanol in 6:1 molar ratio was fed at the rate of
0.12 ml min-1 into a nitrogen or hydrogen flowing at
40 ml min-1 (STP). The partial pressures of nitrogen,
toluene (or hydrogen) and methanol were 378, 326 and
55 Torr (1 Torr = 133.3 Pa), respectively. For some
Based on the results, it is concluded that ethylbenzene is
mainly formed through the transfer hydrogenation of sty-
rene with methanol, though hydrogenation of styrene with
hydrogen occurs to a smaller extent.
2 Experimental
2.1 Preparation of Catalysts
The base catalyst of cesium ion-exchanged X (Cs-X) was
prepared from NaX (Junsei Chemicals, Japan) by ion-
123

Ethylbenzene Formation in Side-Chain Alkylation of Toluene
1027
experiments, a mixture containing toluene and methanol in
1:1 molar ratio was used, where the partial pressures of N2,
toluene and methanol were 324, 218 and 218 Torr,
respectively.
Hydrogenation of styrene was carried out using different
hydrogenating agents; hydrogen and methanol. For the
hydrogenation of styrene with hydrogen, a benzene solution
containing 5 mol% styrene was fed at the rate of
0.12 ml min-1 into
a hydrogen carrier flowing at
40 ml min-1 (STP). The partial pressures of hydrogen,
benzene and styrene were 392, 368 and 20 Torr, respec-
tively. For the hydrogenation of styrene with methanol
(transfer hydrogenation), a mixture containing styrene
(4.3 mol%), methanol (17 mol%) and benzene (78.7 mol%)
was fed at the rate of 0.12 ml min-1 into an N2 carrier
flowing at 40 ml min-1 (STP). The partial pressures of
nitrogen, benzene, methanol and styrene were 348, 324, 70
and 18 Torr, respectively. The products were analyzed by
on-line gas chromatography. All the reactions were carried
out at 683 K.
Fig. 1 Conversions of styrene to ethylbenzene in hydrogenation with
H2 (filled square) and in transfer hydrogenation with methanol
(unfilled square) over Cs-X, ZrB/Cs-X and Cs2O/Cs-X
3 Results
3.2 Side-Chain Alkylation of Toluene with Methanol
in Nitrogen and in Hydrogen
3.1 Reactions of Styrene with Hydrogen
and with Methanol
Table 1 shows the results of the reaction of toluene with
methanol in nitrogen and in hydrogen over Cs-X and Cs2O/
Cs-X. For all catalysts, the toluene conversion did not
change much, and the ethylbenzene selectivity decreased.
The increase in the ethylbenzene selectivity indicated that
hydrogenation of styrene with hydrogen was taking place
in the reaction in hydrogen.
In the reaction of styrene with hydrogen or methanol, only
ethylbenzene was formed, no other products being detec-
ted. Since cyclohexane and cyclohexene were not pro-
duced, hydrogenation of diluent benzene did not occur.
Formaldehyde, CO and hydrogen which might be formed
as products of transfer hydrogenation by methanol and the
following decomposition could not be detected by gas
chromatography.
The last column in Table 1 lists the methanol conver-
sion. As already reported in the preceding paper [12],
methanol consumed in the reaction exceeded the toluene
converted, indicating that methanol decomposition to CO
and hydrogen occurred to a considerable extent.
Figure 1 shows the conversions of styrene in the
hydrogenation with hydrogen and the transfer hydrogena-
tion with methanol over Cs-X, ZrB/Cs-X and Cs2O/Cs-X.
The data plotted were taken at a time on stream of 90 min.
For all catalysts, the transfer hydrogenation proceeded
faster than the hydrogenation. In particular, Cs2O/Cs-X
exhibited a high activity for the transfer hydrogenation, the
formation of ethylbenzene being almost saturated (91.8 %).
The hydrogenation of styrene was slow, but was not so
slow as to be excluded from the reaction mechanisms of the
side-chain alkylation.
3.3 Effects of Toluene to Methanol Ratio
on the Styrene Selectivity in Side-Chain Alkylation
The toluene/methanol mole ratio exerted significant effect
on both the activity and the ethylbenzene selectivity. The
results of the reactions carried out with different toluene/
methanol mole ratios over Cs-X and Cs2O/Cs-X in nitrogen
are given in Table 1 (entry 1 and 5 for Cs-X, and entry 3
and 6 for Cs2O/Cs-X). As the toluene/methanol ratio
decreased from 6/1 to 1/1, the ethylbenzene selectivity
increased both for Cs-X (from 19.9 to 28.6 %) and for
Cs2O/Cs-X (87.8 to 90.2 %). At the same time, the toluene
conversion increased by about twice for both Cs-X and
Cs2O/Cs-X (from 1.1 to 2.7 % and 3.3–7.4 %,
respectively).
Addition of ZrB2O5 to Cs-X suppressed the transfer
hydrogenation to a significant extent (15.9–11.9 %) but
increased the hydrogenation to a small extent (3.4–3.9 %).
On the other hand, addition of Cs2O to Cs-X caused an
increase in activity for both the hydrogenation and the
transfer hydrogenation. In particular, an increase in the
transfer hydrogenation was large.
123

1028
H. Hattori et al.
increased for both Cs-X and Cs2O/Cs-X. The effect of the
toluene/methanol ratio on the styrene selectivity can be
understood if the formation of ethylbenzene is mainly
through the transfer hydrogenation with methanol. The
partial pressure of methanol was higher in the reaction with
1/1 toluene/methanol mixture (218 Torr) than with 6/1
mixture (55 Torr). Increase in the methanol partial pressure
would facilitate the transfer hydrogenation of styrene with
methanol, resulting in an increase in the formation of
ethylbenzene. The observed results suggest that the transfer
hydrogenation with methanol is the main pathway to eth-
ylbenzene in the side-chain alkylation. The higher styrene
selectivity for higher toluene/methanol ratio was also
reported for Rb-X [2].
Table 1 Results of side-chain alkylation of toluene with methanol in
H2 and in N2
Catalyst
Carrier Tol/
gas
Tolconv EBselect MeOHconv(%)
MeOH (%) (%)
Cs-X
Cs-X
N2
H2
6/1
6/1
6/1
6/1
1/1
1/1
1.0
1.2
3.3
3.0
2.7
7.4
19.9
26.2
87.8
91.8
28.6
90.2
36.3
40.5
74.2
74.0
36.4
45.0
Cs2O/Cs-X N2
Cs2O/Cs-X H2
Cs-X
N2
Cs2O/Cs-X N2
Tol/MeOH molar ratio of toluene to methanol in reactant, Tolconv
toluene conversion, EBselect ethylbenzene selectivity = [ethylbenzene]/
[ethylbenzene ? styrene] 9 100, MeOHconv methanol conversion
An increase in the ethylbenzene selectivity in the reac-
tion of toluene with methanol under a hydrogen stream was
reported by Yashima et al. [2] and Lacroix et al. [3]. On the
basis of the results, Yashima et al. proposed that ethyl-
benzene was produced primarily by the hydrogenation of
styrene with the hydrogen formed from decomposition of
methanol. Since then, the hydrogenation of styrene with
hydrogen has been most frequently cited as a main route to
ethylbenzene formation. In the present study too, the
increase in the ethylbenzene selectivity was observed under
a hydrogen stream. Contribution of the hydrogenation of
styrene with hydrogen to the ethylbenzene formation
should exist. The contribution, however, is estimated to be
small as compared to that of the transfer hydrogenation
with methanol because the hydrogenation of styrene with
hydrogen is slow as compared with the transfer hydroge-
nation with methanol.
4 Discussion
All the results obtained in the present study strongly sug-
gest that the formation of ehylbenzene in the side-chain
alkylation of toluene with methanol results mainly from the
transfer hydrogenation of styrene with methanol. Although
the hydrogenation of styrene with H2 could not be exclu-
ded, its contribution to ethylbenzene formation is small as
compared to the transfer hydrogenation with methanol.
As shown in Fig. 1, the transfer hydrogenation of styrene
with methanol proceeded faster than the hydrogenation of
styrene with hydrogen. This result coincides with what
Garces et al. [10] described for Cs ion exchanged zeolite X
and carbon containing Cs and B, though they did not present
the data. The rates of the transfer hydrogenation with
methanol and the hydrogenation with hydrogen varied with
the type of catalyst. The rate of the transfer hydrogenation
was particularly high for Cs2O/Cs-X. The high ethylben-
zene selectivity observed over Cs2O/Cs-X in the side-chain
alkylation of toluene with methanol is suggested to be due
to the fast transfer hydrogenation of styrene with methanol.
The results shown in Fig. 1 were obtained under a
methanol partial pressure of 70 Torr for the hydrogen
transfer, and a hydrogen partial pressure of 392 Torr for the
hydrogenation. In the side-chain alkylation of toluene with
methanol (toluene/methanol = 6/1) in a nitrogen stream,
the partial pressures of methanol and hydrogen were 55 and
less than 103 Torr, respectively. The value 103 Torr was
calculated on the assumption that all methanol converted to
CO and hydrogen. The actual hydrogen pressure in the
side-chain alkylation should be less than 103 Torr, and was
much smaller than that used in the hydrogenation of sty-
rene. Therefore, the ratio of the hydrogenation to the
transfer hydrogenation in the side-chain alkylation should
be smaller than those for the reactions from which the data
shown in Fig. 1 were obtained.
The possibility of direct methylation of toluene to eth-
ylbenzene seems to be low. The direct methylation was
proposed by Yashima et al. [2] as one of the possible ways
to explain the observation that the ethylbenzene/styrene
ratio was higher when methanol was used as an alkylating
reagent than when formaldehyde was used. The possible
mechanism for the direct methylation would involve the
reaction of benzyl anions formed from toluene with methyl
cations formed from methanol. If methyl cations were
formed, the ring methylation of toluene would have
occurred to form xylene isomers. The formation of xylene
isomers was not observed. Accordingly, the direct meth-
ylation is not plausible over Cs-X based catalysts. The high
ethylbenzene/styrene ratio observed when methanol was
used as an alkylating agent can be explained by the
occurrence of the transfer hydrogenation of styrene with
methanol.
Addition of Cs2O to Cs-X caused an increase in the
toluene conversion and the ethylene selectivity, which was
also reported by Lacroix et al. [3], Engelhardt et al. [6] and
Hathaway and Davis [14]. In the present study, it was
shown that the activity for the transfer hydrogenation of
As the toluene/methanol ratio in the reaction mixture
decreased from 6/1 to 1/1, the ethylbenzene selectivity
123

Ethylbenzene Formation in Side-Chain Alkylation of Toluene
1029
3. The addition of zirconium borate to Cs-X suppresses
the transfer hydrogenation of styrene with methanol.
styrene with methanol was markedly increased by the
addition of Cs2O to Cs-X as shown in Fig. 1. It is suggested
that a high ethylbenzene selectivity (low styrene selectiv-
ity) in the side-chain alkylation results from a high activity
of Cs2O/Cs-X for the transfer hydrogenation. As addition
of Cs2O to Cs-X was reported to result in the increase in
the strength of basic sites [15], the increase in the transfer
hydrogenation is likely to result from the increase in the
basic strength.
Acknowledgments The financial support of King Abdulaziz City
for Science & Technology (KACST) under Project #AR-30-253 is
acknowledged. The authors appreciate the support from the Ministry
of Higher Education, Saudi Arabia in establishment of the Center of
Research Excellence in Petroleum Refining and Petrochemicals at
KFUPM. The authors thank Mr. Khurshid Alam for technical assis-
tance in setting up the reactor system and analytical works. H. H.
thanks the Japan Cooperation Center, Petroleum (JCCP) for financial
support under the High-level Researcher Dispatching Program, JCCP.
Effects of the addition of B components to the catalyst
on the activity for side-chain alkylation have been reported
in both literature and patents [16–19]. It was reported that
addition of B to catalyst suppressed the decomposition of
methanol, and, therefore, increased the total yield of sty-
rene and ethylbenzene [18, 19]. In our previous paper, we
reported that ZrB2O5 was the best additive in increasing the
activity and the styrene selectivity [12]. In the present
study, we observed that the conversions of styrene to eth-
ylbenzene in the transfer hydrogenation of styrene with
methanol were 15.9 % over Cs-X and 11.9 % over ZrB/Cs-
X. The addition of ZrB2O5 to Cs-X suppressed the transfer
hydrogenation of styrene with methanol, which resulted in
the high styrene selectivity in the side-chain alkylation of
toluene with methanol, though the reason of the suppres-
sion effect of ZrB2O5 addition is not clear at present.
References
1. Sidorenko YN, Galich PN, Gutrya VS, Ill’in VG, Neimark IE
(1967) Dokl Akad Nauk SSSR 173:132
2. Yashima T, Sato K, Hayasaka T, Hara N (1972) J Catal 26:303
3. Lacroix C, Deluzarche A, Kinnemann A, Boyer A (1984) Zeo-
lites 4:109
4. Palomares AE, Rder-Mirth G, Lercher LA (1997) J Catal 168:442
5. Wang X, Wang G, Shen D, Fu C, Wei M (1991) Zeolites 11:254
6. Engelhardt J, Szanyi J, Valyon J (1987) J Catal 107:296
7. Hunger M, Schenk U, Weitkamp J (1998) J Mol Catal A 134:97
8. Jian J, Lu G, Miao C, Wu X, Wu W, Sun Q (2013) Micro
Mesopor Mater 167:213
9. Sooknoi T, Dwyer J (1995) Stud Surf Sci Catal 97:423
10. Garces JM, Vrieland GE, Bates SI, Scheidt FM (1985) Stud Surf
Sci Catal 20:67
11. Ono Y, Hattori H (2011) Solid base catalysis (Springer series in
chemical physics 101. Springer, Tokyo Institute of Technology
Press, Berlin
5 Conclusions
12. Tope KB, Alabi WO, Aitani AM, Hattoti H, Al-Khattaf SS
(2012) Appl Catal A 443–444:214
13. Knyrim JS, Huppertz H (2008) Naturforsch Z B Chem Sci 63:707
14. Hathaway PE, Davis ME (1989) J Catal 119:497
15. Yagi F, Tsuji H, Hattori H (1997) Micropor. Mater 9:237
16. Unland ML (1978) J Phys Chem 82:580
17. Unland ML, Baker GE (1978) U.S. Pat. 4,115,424, Monsanto
18. Wieland WS, Davis RJ, Garces JM (1996) Catal Today 28:443
19. Barthomeuf D, de Quirillic VQ (1991) US Patent 5,068,483,
CNRS
1. Ethylbenzene is mainly formed through the transfer
hydrogenation of styrene with methanol in the side-
chain alkylation of toluene with methanol over Cs-X
based catalysts; the contribution of the hydrogenation
of styrene with hydrogen to ethybenzene formation is
small.
2. The addition of Cs2O to Cs-X enhances the activity for
the transfer hydrogenation of styrene with methanol,
which results in a high ethylbenzene selectivity in the
side-chain alkylation.
123