Free radical mechanism investigation of the side-chain alkylation of toluene with methanol on basic zeolites X

Article abstract of DOI:10.1016/S1872-2067(15)60896-8

The side-chain alkylation of toluene represents a novel, environmentally friendly, and low cost route for the production of styrene. However, the yield of styrene produced in this way is currently low, and the mechanism responsible for the side-chain alky

Full text of DOI:10.1016/S1872-2067(15)60896-8

Chinese Journal of Catalysis 36 (2015) 1726–1732
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c h n j c
Article
Free radical mechanism investigation of the side-chain alkylation of
toluene with methanol on basic zeolites X
Huanhui Chen, Xiaoci Li, Guoqing Zhao, Hongbo Gu, Zhirong Zhu *
Department of Chemistry, Tongji University, Shanghai 200092, China
A R T I C L E I N F O
A B S T R A C T
Article history:
The side-chain alkylation of toluene represents a novel, environmentally friendly, and low cost
route for the production of styrene. However, the yield of styrene produced in this way is cur-
rently low, and the mechanism responsible for the side-chain alkylation of toluene is poorly un-
derstood. Furthermore, the reason for the higher catalytic efficiency of CsX over NaX and KX
remains unclear. In this work, the free radical mechanism of the side-chain alkylation of toluene
over basic zeolite X has been elucidated using quantum chemical calculations, together with
isotope tracing experiments and the reaction between p-nitrotoluene and methanol. The adsorp-
tion isotherm of methanol showed that Cs+ ions could block methanol from accessing the β-cage,
which is where the side-chain alkylation reaction occurred. Furthermore, the H–D exchange
results between toluene and deuterated toluene (C6D5CD3) showed that CsX was more efficient as
a catalyst than KX for the conversion of toluene to the corresponding benzyl radical (C6H5CH2•).
These two results therefore explain the higher catalytic activity of CsX towards side-chain alkyla-
tion than KX. Based on the free radical mechanism, the selectivity of styrene could be increased
from 17.4% to 59.4% using CO2 as carrier gas instead of N2.
Received 27 March 2015
Accepted 18 May 2015
Published 20 October 2015
Keywords:
Toluene
Side-chain alkylation
Radical mechanism
Zeolite X
Styrene
Isotope trace
© 2015, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
of toluene because it has the potential to provide a novel route
for the production of styrene while avoiding the limitations
Styrene is an important material that is used for the produc-
tion of a broad range of polymers, which are themselves used in
numerous applications, including medical devices, toys, paper
coatings and food packaging [1]. It is noteworthy that the an-
nual worldwide consumption of styrene monomer is currently
more than 30 million tons. However, there are many disad-
vantages associated with the traditional process for the pro-
duction of styrene, including high costs, as well as high levels of
energy consumption and environmental pollution resulting
from the high reaction temperature and toxic nature of the raw
materials [2]. To address these issues, there has been a recent
increase in research directed towards the side-chain alkylation
associated with the traditional production process [3–11].
Yashima et al. [1] and Sidorenko et al. [12] reported that the
catalytic activities of X-type zeolites for the side-chain alkyla-
tion of toluene with methanol was higher than those of Y-type
zeolites. Freeman et al. [13] reported that the catalytic efficien-
cies of several X-type zeolites increased with the increasing size
of the alkali-metal cations used in their reactions (i.e., NaX < KX
< CsX). However, the precise reason for the higher catalytic
efficiency of CsX compared with NaX and KX remains unclear.
Furthermore, the mechanism of the side-chain alkylation of
toluene is poorly understood. Rep [14] postulated a mechanism
for the side-chain alkylation of toluene. According to this
* Corresponding author. Tel: +86-21-65982563; Fax: +86-21-65982563; E-mail: zhuzhirong@tongji.edu.cn
This work was supported by the National Natural Science Foundation of China (51174277) and Shanghai Key Basic Research (11JC1412500).
DOI: 10.1016/S1872-2067(15)60896-8 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 36, No. 10, October 2015

Huanhui Chen et al. / Chinese Journal of Catalysis 36 (2015) 1726–1732
1727
mechanism, methanol would be initially dehydrogenated to
give formaldehyde (Eq. (1)), which would react with toluene to
form styrene via an aldol-type condensation reaction (Eq. (2)).
The styrene would then be reduced to ethylbenzene (Eq. (3)).
2.2.2. P-nitrotoluene side-chain alkylation
CsX catalyst (5.0 g) was packed into a stainless steel tubular
fixed-bed reactor with 1.5 cm inner diameter. The reactants
were pumped into the reactor at a rate of 10 mL/h. Side-chain
alkylation was carried out at 425 °C, 1:1:20 p-nitrotoluene:
methanol: benzene molar ratio, and atmospheric pressure with
N2 30 mL/min.
CH3OH → H2CO + H2
(1)
(2)
(3)
2.2.3. H/D exchange experiments
C6H5CH=CH2 + H2 → C6H5CH2CH3
The catalyst (active carbon, CsX, or KX) was packed into a
stainless steel tubular fixed-bed reactor with 1.5 cm inner di-
ameter. The mixture of toluene and toluene-d8 was pumped
into the reactor, giving a feed rate (WHSV) of 2 g g–1 h–1.
Side-chain alkylation was carried out at 380 °C, 1:10 toluene:
toluene-d8 molar ratio, and atmospheric pressure with N2 30
mL/min.
However, the dehydrogenation of methanol on NaCO3 [15]
and the cracking reaction of propylbenzene to toluene [16,17]
is generally considered to proceed via a free radical mecha-
nism, suggesting the possibility that a free radical mechanism
could be responsible for the formation of styrene during the
side-chain alkylation of toluene. Spin trapping-electron spin
resonance (ESR) experiments are normally used to determine
whether a reaction proceeds via a radical mechanism. Unfor-
tunately, this technique cannot be used in this particular case
because the side-chain alkylation of toluene is conducted at
high temperatures (ca. 420 °C), which are too high to use a rad-
ical trapping reagent.
With this in mind, isotope tracing experiments have been
used in the current study together with the reaction between
p-nitrotoluene and methanol to confirm the free radical mech-
anism of the side-chain alkylation of toluene over basic zeolite
X. The higher catalytic activity of CsX towards the side-chain
alkylation compared with KX has been explained in terms of
the adsorption isotherm of methanol and H–D exchange ex-
periments between toluene and deuterated toluene (C6D5CD3).
The use of CO2 as carrier gas instead of N2 led to an increase in
the selectivity for styrene.
2.2.4. CO2 effect experiments
CsX catalyst (5.0 g) was packed into a stainless steel tubular
fixed-bed reactor with 1.5 cm inner diameter. The mixture of
toluene and methanol was pumped into the reactor at a rate of
10 mL/h. Side-chain alkylation was carried out at 420 °C, 1:1
toluene: methanol molar ratio, atmospheric pressure with CO2
(N2) 30 ml/min.
3. Results and discussion
3.1. Mechanism verification
The mechanism of the side-chain alkylation of toluene was
initially investigated using CD3OD as an isotope tracer. The
crude product was analyzed by GC-MS and 2H-NMR. The
2H-NMR spectrum of the reaction mixture (Fig. 1(a)) contained
peaks at 2.23 and 1.18 ppm, which were assigned to the deu-
terated methyl groups of toluene and ethylbenzene, respec-
tively.
2. Experimental
2.1. Materials preparation
Notably, the ion mechanism postulated by Marco Rep (as
shown in Eqs. (1–3)) could not be used to explain why most of
D atoms were located on toluene, whereas very few were at-
tached to the β-C atoms of ethylbenzene. According to Eqs.
(1–3), the β-C atoms of the styrene and ethylbenzene products
resulting from the ion mechanism should contain deuterium.
However, the results showed that only a small amount of D was
incorporated on to the β-C atoms of ethylbenzene and that
most of D were observed on the toluene. This result therefore
suggested that the H–D exchange reaction between the methyl
groups of CD3OD and toluene was occurring by a free radical
reaction (Eqs. (4–7)).
NaX zeolite was obtained from Tong Xing Corp (Shanghai,
China). The CsX and KX zeolites were prepared from NaX fol-
lowing three ion exchange processes with aqueous solutions (1
mol/L) of CsOH and KOH, respectively. The exchanged samples
were dried at 85 °C and calcined in air at 520 °C for 3 h before
being rehydrated and rinsed with distilled water. Deuterated
methanol-d4 (CD3OD) and deuterated toluene-d8 (C6D5CD3)
were provided by Aladdin Industrial Corporation (Shanghai,
China).
2.2. Catalytic reaction
C6H5CH3 ⇄ C6H5CH2• +H•
CD3OD ⇄ •CD2OD + D•
C6H5CH2 + D• ⇄ C6H5CH2D
(4)
(5)
(6)
(7)
2.2.1. Isotope trace experiments
•
A specific amount of each catalyst was packed into stainless
steel tubular fixed-bed reactors with an inner diameter of 1.5
cm. A mixture of toluene and deuterated methanol-d4 was then
pumped into the reactors at a rate of 10 mL/h. The isotope
tracing side-chain alkylation reactions were carried out at 425
°C using a 10:1 (mol/mol) mixture of toluene and CD3OD under
atmospheric pressure with N2 30 mL/min.
•CD2OD +H• ⇄ CHD2OD
As indicated in the above equations, the toluene would be
cracked to give C6H5CH2• and H• and CD3OD would dissociate to
give •CD2OD and a deuterium radical (D•). The C6H5CH2• and D•
radicals would then combine to form deuterated toluene-d1
•
(C6H5CH2D), whilst CD2OD would react with H• to give trideu-

1728
Huanhui Chen et al. / Chinese Journal of Catalysis 36 (2015) 1726–1732
(a)
(b)
CH2
H2C
D
D
8
6
4
2
0
 (ppm)
(c)
(d)
10000
tol-d
70
60
50
40
30
20
10
0
1
tol-d
tol-d
tol-d
tol-d
tol-d
tol-d
tol-d
2
3
4
5
6
7
8
8000
6000
4000
2000
0
NaX
KX
CsX
0
40
80
p/bar
120
160
KX
Active carbon
CsX
Fig. 1. (a) 2H-NMR spectrum of the reaction mixture; (b) Representation of the faujasite structure showing only the tetrahedral atoms (SI, SI', SII, SII',
SIII, and SIII' are the positions of the cationic sites); (c) Adsorption isotherms of methanol; (d) Deuterium distribution density.
teromethanol (CHD2OD).
Seemingly, H–D exchange between methyl group of CD3OD
and toluene also could proceed by ion reaction (Eqs. (8–11)).
evidenced by the fact that the methyl hydrogen of NO2-Ph-CH3
(pKa = 20.4) dissociates much more readily to give H+ than that
of toluene (pKa = 43), with the resulting p-nitro-benzyl anion
C6H5CH3 ⇄ C6H5CH2– + H+
CD3OD ⇄ CD2OD– + D+
(8)
(9)
(NO2-Ph-CH2 ) being much more stable than the benzyl anion
–
–
(Ph-CH2 ). If the side-chain alkylation of toluene did proceed
C6H5CH2– + D+ ⇄ C6H5CH2D
CD2OD– + H+ ⇄ CHD2OD
(10)
(11)
via an ionic mechanism, then the yield of the product resulting
from the side-chain alkylation of p-nitrotoluene would increase
compared with that of toluene. Furthermore, nitrobenzene
compounds generally behave as radical free radical scavenger
because aromatic nitro groups can react with active radicals to
form more stable radicals (Eq. (12)).
Quantum chemical calculations were performed at the
B3LYP/6-31+G level using the Gaussian 09 program system to
verify the route of the H–D exchange reaction. Scheme 1 pro-
vides a visual insight into the energies associated with the dif-
ferent reactants, as well as the energies of the transition states
and products. The transition state of the radical mechanism
was found to be about 0.149 Hartree higher than that of meth-
anol. In contrast, the transitions state of the ion mechanism was
about 0.657 Hartree. The energy barrier associated with the
radical route was therefore shown to be much lower than that
of the ion route, which provided a strong indication that the
H–D exchange reaction between the methyl group of CD3OD
and toluene would have proceeded via the radical route.
(12)
However, if the side-chain alkylation did proceed via a free
radical reaction mechanism, then the methyl side-chain of
p-nitrotoluene would not be alkylated. In practice, the results of
this experiment revealed that there was no product resulting
from the side-chain alkylation of p-nitrotoluene, which indi-
cated that the side-chain alkylation of toluene proceeded via a
free radical reaction.
Although the results of the H–D exchange reaction can be
used to confirm the presence of free radicals during the reac-
tion, they cannot be used to demonstrate that the side-chain
alkylation of toluene proceeds via a free radical reaction. To
examine whether the side-chain alkylation of toluene is a free
radical reaction, we investigated the reaction of p-nitrotoluene
(NO2-Ph-CH3) with methanol instead of toluene. Aromatic ni-
tro-groups generally behave as strong electron acceptors, as
Overall, the isotope trace experiments demonstrated that a
D atom from the methyl group of CD3OD was exchanged with a
hydrogen atom from the methyl group of toluene. Furthermore,
the results of the quantum chemical calculations showed that
the energy barrier for the H–D exchange reaction was 0.148640
Hartree for the radical route and 0.656555 Hartree for the ionic
route. Taken together, these results provided strong evidence
in support of the H–D exchange reaction between the methyl

Huanhui Chen et al. / Chinese Journal of Catalysis 36 (2015) 1726–1732
1729
(b)
+
+
-CD2OD
D
(a)
+
CD2OD
D
·
·
E ≈ -115.027090
Hartree
E ≈ -115.535005
Hartree
E ≈ +0.656555
Hartree

E ≈ +0.14864
CHD2OD
CD3OD
Hartree
CD3OD
CHD2OD
E ≈ -115.683645
E ≈ -115.683645
E ≈ -115.687145
E ≈ -115.687145
Hartree
Hartree
Hartree
Hartree
H+
-
+
CH2
+
H
CH2
· ·
E ≈ -270.854924 Hartree
E ≈ -271.302942 Hartree
E ≈ +0.606599
Hartree

CH2D

E ≈ +0.137517
Hartree
CH2D
E ≈ -271.461523
E ≈ -271.458023
E ≈ -271.461523
E ≈ -271.458023
Hartree
Hartree
Hartree
Hartree
Scheme 1. (a) Relative energies of H/D exchange between toluene and methanol (Hartree) in radical mechanism; (b) Relative energies of H/D ex-
change between toluene and methanol (Hartree) in radical mechanism.
groups of CD3OD and toluene proceeding via a radical mecha-
nism. Further proof of this process occurring via a radical
mechanism came from the observation that the side chain of
p-nitrotoluene was not alkylated with methanol. Instead, the
C6H5CH3/C6H5CH2 reacted with CH3 /CH3OH to generate
2-phenyl-ethanol (C6H5CH2CH2OH), which underwent a dehy-
dration to produce styrene (Eqs. (13) and (14)). Furthermore,
faujasite and β cage structures are around 0.74 and 0.25 nm,
respectively. The diameter of toluene is about 0.65 nm, which is
much bigger than the diameter of the 6-ring orifice of the β cage
structure of the NaX. With this in mind, toluene should be able
go inside the faujasite cages, but not inside the β cages. Basler
et al. [18] demonstrated that NH3 can enter the β cages of NaX.
Given that the diameter of CH3OH is similar to that of NH3, it
was envisaged that CH3OH would also be able to access to the β
cages of NaX.
•
•
•
•
the reaction of C6H5CH2 with CH3OH and the reaction of CH3
with C6H5CH3 would give ethylbenzene (Eqs. (15) and (16)).
Ethylbenzene was also produced by the hydrogenation of sty-
rene with H• (Eq. (17)).
The adsorption isotherms of CH3OH were measured to de-
termine the validity of this hypothesis and the results are
shown in Fig. 1(c). The results revealed that NaX had a similar
adsorption capacity (ca. 8500 molecules per unit cell) to KX.
The adsorption capacity of NaX was found to be much greater
than that of CsX (ca. 6300 molecules per unit cell). This result
indicated that the β cages would be blocked when SII and SII′
were occupied by Cs+ instead of K+ because the radius of Cs+ is
larger than that of K+ and CH3OH would therefore be prevented
-H-
•
C6H5CH2 +CH3OH
C6H5CH3+•CH2OH
C6H5CH3CH2OH ⇄ C6H5CH =CH2 (13)
-H-
C6H5CH3CH2OH ⇄ C6H5CH=CH2 (14)
-HO-
•
C6H5CH2 + CH3OH
C6H5CH2CH3
(15)
-H-
•
C6H5CH3 + CH3
C6H5CH2CH3 ΔH = 50.6 kJ/mol (16)
C6H5CH =CH2 + 2H• ⇄ C6H5CH2CH3
(17)
(18)
(19)
CH3OH + H• ⇄ CH3 + H2O
CH3OH + H• ⇄ H2 + •CH2OH
•
•
from entering the β cages. In this case, the CH3 radical would
Toluene could also react with CH3• to generate ethylbenzene
(Eq. (16)), and the CH3• radical could react with CH3OH to gen-
erate CH4 (Eq. (20)), CO (Eq. (21)), and several other
by-products. The radical mechanism could also be used to ex-
plain the formation of CO and CH4 (Eqs. (20) and (21)). The
enthalpy changes associated with Eqs. (16), (20), and (21) were
50.6, −27.9, and −27.0 kJ/mol, respectively, and therefore very
only be able to react with CH3OH/CH2O to form side-products
•
in the β cages. Furthermore, the CH3 radicals would collide
with toluene molecules to generate styrene and ethylbenzene
in the faujasite cages. The blocking of the β cages with Cs+ ions
would therefore improve the probability of collisions between
•
•
the CH3 radicals and toluene (or C6H5CH2 ). Taken together,
these results show that CsX is more efficient than KX for cata-
lyzing the reaction of CH3OH with toluene to give styrene and
ethylbenzene.
•
different. Based on the theory of thermodynamics, CH3 would
be better suited to form CH4 and CO than styrene and
ethylbenzene. To improve the usage of CH3OH, it would there-
fore be necessary to reduce the probability of a collision be-
tween CH3• and CH3OH/formaldehyde (CH2O), while improving
3.3. Activation ability of toluene on different X zeolites
•
the probability of a collision between CH3 and C6H5CH3 (or
•
As mentioned above, the side-chain alkylation of toluene
occurs via a free radical mechanism. With this in mind, the
process responsible for the formation of the free radicals rep-
resents an important step in this reaction. The activation of
C6H5CH2 ).
CH3• + CH3OH ⇄ CH4 + •CH2OH ΔH = –37.9 kJ/mol (20)
CH3• + CH2O ⇄ CH4 + CO + H• ΔH = –27.0 kJ/mol (21)
•
3.2. β cages constraint on methanol
C6H5CH3 to give the corresponding radical species C6H5CH2
was evaluated with a variety of different catalysts based on the
H/D exchange reaction of a 10:1 (mol/mol) mixture of C6H5CH3
and C6D5CD3. The resulting distribution of D in the products
X-type zeolites are composed of faujasite and β cages (Fig.
1(b)). The diameters of the 12- and 6-ring orifices of the

1730
Huanhui Chen et al. / Chinese Journal of Catalysis 36 (2015) 1726–1732
was analyzed by GC-MAS and the results are shown in Fig. 1(d).
The deuterium distribution density results (Fig. 1(d))
showed that C6D5CD3 existed as the major species in the reac-
tion mixture when the catalytic process was conducted with
active carbon. This result indicated that it was difficult to con-
vert C6H5CH3 to the corresponding C6H5CH2• radical using acti-
vated carbon as a catalyst. Furthermore, the experiments in-
volving the side-chain alkylation of toluene showed that the
conversion of toluene was about 0.1% when the reaction was
conducted over active carbon (Table 1). Taken together, these
results show that the catalytic effect of activated carbon to-
wards the side-chain alkylation of C6H5CH3 was in agreement
its catalytic effect towards the conversion of C6H5CH3 to
Fig. 2. The captodative effect of metal cation and anion on the frame-
work.
•
of CsX towards the conversion of C6H5CH3 to C6H5CH2 was
critical to its higher catalytic activity towards the side-chain
alkylation of toluene compared with KX.
•
C6H5CH2 . In contrast, when the reaction was conducted in the
3.4. CO2 effect on the selectivity of styrene
presence of a X-type zeolite (KX or CsX), the amount of C6D5CD3
decreased sharply, whereas the amount deuterated toluene-d1
increased considerably. This result demonstrates that the zeo-
lite was having a significant effect on activating toluene to-
wards free radicals. The effect of the zeolite could be attributed
to a captodative effect involving the synergistic effect of elec-
tron-withdrawing (captor) and electron-releasing (donor)
groups attached to the radical center, which could make the
radical much more stable [19]. As the toluene molecules en-
tered the cavities of the zeolite, the cations would form com-
plexes with the aromatic rings, and the resulting complexes
would attract electrons. At the same time, the anions within the
zeolite framework would donate an electron to the methyl
group of toluene (Fig. 2). Based on these interactions, the zeo-
lite would be able to stabilize the C6H5CH2• radical.
Several experiments were designed to improve the selectiv-
ity of the side-chain alkylation reaction of toluene based on the
radical reaction mechanism. Notably, CO2 was used as a carrier
gas instead of N2. Under an atmosphere of N2, there are three
possible reaction routes for H• (Fig. 3(a)), including: (1) the
reaction of H• with CH3OH following route S to generate
•CH2OH, which would be subsequently converted to styrene;
(2) the reaction of H• with CH3OH according to route E1 to gen-
•
erate CH3 , which would subsequently react with toluene to
give C6H5CH2CH3; and (3) the reaction of H• with C6H5CH=CH2
following route E2 to give C6H5CH2CH3. Under an atmosphere of
CO2, H• would react with CO2 to form •COOH. The high bonding
energy of the C–H bond in formic acid (ca. 468 kJ/mol) indi-
•
cates that the COOH radical could readily abstract a H atom
Compared with the KX catalyst, more of the deuterium at-
oms present in the mixture existed as toluene-d1 following the
CsX-catalyzed process. The fact that more deuterium atoms
were exchanged from deuterated toluene to toluene using the
CsX catalyst demonstrated that the H/D exchange process oc-
curred to a much greater extent over the CsX catalyst than it did
from the methyl group of CH3OH to form HCOOH as an inter-
mediate, which could decompose to give H2 and CO2 at the end
of the reaction (Fig. 3(b)). It would be much more difficult for
the •COOH radical to obtain an OH moiety from CH3OH to form
H2CO3 because of the low bonding energy of the C–OH bond in
H2CO3. CH3OH would therefore be more inclined to form sty-
rene following route S because the reaction of CH3OH with
•COOH would be relative facile and energetically favorable. The
presence of CO2 would lead to a decrease in the concentration
of H•, which would lead to a further decrease in the production
of C6H5CH2CH3 from the reaction between styrene and H•. Thus,
the selectivity of the reaction for styrene would be increased.
Pleasingly, the experimental results of this study were found to
be consistent with the design strategy, as shown in Table 1,
with the conversion of toluene decreasing slightly and the se-
lectivity for styrene increasing from 17.4% to 59.4% when the
reaction was conducted under CO2.
•
over the KX catalyst and that the C6H5CH2 radical was much
more stable over CsX. Cs can lose an electron more readily than
K, which means that the CsX zeolite has a weaker Cs+ cation
(electron-withdrawing) and stronger framework anion (elec-
tron-releasing) than the KX zeolite. The cooperativity of the
acceptor and donor moieties of the CsX zeolite therefore pos-
sesses better captodative effects to increase the stability of the
•
C6H5CH2 radical. Furthermore, CsX exhibited higher catalytic
activity towards the side-chain alkylation of toluene than KX.
The levels of toluene conversion using the CsX and KX zeolites
were found to be 16.6% and 3.6% (Table 1), respectively. This
result therefore demonstrates that the higher catalytic activity
Table 1
Toluene side-chain alkylation data.
Conversion of
toluene (%)
Yield of
styrene (%)
Yield of EB
(%)
Selectivity of
styrene (%)
Conversion of
methanol (%)
Yield of
methane (%)
Yield of CO
(%)
Catalyst
C
KX
CsX
0.75
3.60
16.60
13.80
0.15
1.32
2.89
8.20
0.15
1.37
12.67
7.80
—
—
17.4
59.4
—
99.4
98.2
—
—
43.5
34.2
—
—
49.3
48.2
—
CsX (CO2 atmosphere)
YCO = NCO, out /NMeOH, in, Ymethane = Nmethane, out /NMeOH, in, where Ni is calculated from the GC response signal (area of the FID or TCD peak) using the response
factors for individual components and converted into mols.

Huanhui Chen et al. / Chinese Journal of Catalysis 36 (2015) 1726–1732
1731
Reaction pathway E
2
Reaction pathway E
2
H
·
CO
2
H
COOH
2
·
H·
HCOOH
H CO
H
2
2
3
CH OH
3
H O
CH OH
2
3
CH OH
CH
2
3
·
·
CH OH
CH
2
3
·
·
H
·
H
·
H
HO
·
H
·
HO
(a)
(b)
Fig. 3. (a) Reaction in atmosphere of N2; (b) Reaction in atmosphere of CO2.
of the strong interaction between CO2 and H•, the use of CO2 as
carrier gas led to significant improvements in the selectivity for
styrene.
4. Conclusions
The reaction mechanism of the side-chain alkylation of tol-
uene has been fully elucidated using isotope tracing experi-
ments, as well as investigative reactions between p-nitrotolu-
ene and methanol. H/D exchange reactions between CD3OD
and C6H5CH3 provided experimental proof of the presence of
free radicals in the reaction process. The results for the reac-
tion between p-nitrotoluene and methanol further validated
the idea that the side-chain alkylation of toluene proceeded via
a free radical reaction mechanism. Cs+ ions blocked CH3OH
from going inside the β cages, which consequently reduced the
occurrence of side reaction. Furthermore, CsX was found to be
more effective than KX for activating toluene towards free rad-
icals, which was in agreement with the results of our experi-
ments for the catalytic side-chain alkylation of toluene. In light
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Graphical Abstract
Chin. J. Catal., 2015, 36: 1726–1732 doi: 10.1016/S1872-2067(15)60896-8
Free radical mechanism investigation of the side-chain
alkylation of toluene with methanol on basic zeolites X
Huanhui Chen, Xiaoci Li, Guoqing Zhao, Hongbo Gu, Zhirong Zhu*
Tongji University
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