Alkylation of toluene with methanol: The effect of K exchange degree on the direction to ring or side-chain alkylation

Article abstract of DOI:10.1016/j.catcom.2011.12.033

A set of K-exchanged X zeolites with different exchange degree were prepared and investigated in the alkylation of toluene with methanol. The chemical composition, textural properties, crystalline phase and acid-base properties of catalysts were analyzed

Full text of DOI:10.1016/j.catcom.2011.12.033

Catalysis Communications 19 (2012) 90–95
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Alkylation of toluene with methanol: The effect of K exchange degree on the direction
to ring or side-chain alkylation
c
a,
a
Lanlan Song a,b, Zhenrong Li a, , Ruizhen Zhang , Liangfu Zhao , Wen Li
⁎
⁎
a
Laboratory of Applied Catalysis and Green Chemical Engineering, Institute of Coal Chemistry, Taiyuan 030001, PR China
Graduate University of the Chinese Academy of Sciences, Beijing 100039, PR China
b
c
Taiyuan university of technology, Taiyuan 030024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 October 2011
Received in revised form 23 December 2011
Accepted 23 December 2011
Available online 30 December 2011
A set of K-exchanged X zeolites with different exchange degree were prepared and investigated in the alkyl-
ation of toluene with methanol. The chemical composition, textural properties, crystalline phase and acid–
base properties of catalysts were analyzed by ICP, N2 physisorption, XRD, NH3-TPD and XPS. Experimental re-
sults demonstrated that the direction to ring or side-chain alkylation depended on the K exchange degree
(KED) greatly.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Alkylation
Toluene
Methanol
Ion-exchange
X zeolite
1. Introduction
CsX>RbX>KX>NaX>LiX [7,10]. However, there is
about ion-exchange degree being lack of attention.
a problem
Alkylation of toluene with methanol is an attractive synthetic
route to either xylenes or styrene and ethylbenzene compared with
the conventional Friedel–Crafts alkylation [1,2]. The product distribu-
tion is depended on whether ring alkylation or side chain alkylation is
favored. Xylenes are mainly generated from ring alkylation of toluene
with methanol, while styrene and ethylbenzene are the main prod-
ucts of side-chain alkylation (Scheme 1) [2–4].
The direction to ring or side-chain alkylation has been widely
studied over X zeolite and its alkali cations exchanged forms in the
last decades. Giordano et al. [5] demonstrated that side-chain alkyl-
ation occurred at intermediate electronegativity (Sint)b3.6, whereas
ring alkylation was taken place at Sint >3.6. Sivasankar et al. [6]
revealed that the direction to side-chain or ring alkylation lay on
the state of the adsorbed methanol in the zeolite by infrared spectros-
copy and TPD measurements. It has been also reported that relatively
high acidic zeolites like LiX and NaX facilitate the formation of xy-
lenes, but more basic zeolites like KX, CsX and RbX would lead to sty-
rene and ethylbenzene as the main products [3,7–9]. In addition, the
selectivity towards side-chain alkylation is in the order
To the best of our knowledge, CsX, RbX, KX and LiX are obtained
from NaX by ion-exchanged technology and their surface acid–base
properties can be modified by varying the ionic exchange degree
[11]. Therefore, it is reasonably established that the exchange degree
of K can affect on the direction to ring or side-chain alkylation of tol-
uene with methanol.
In addition, a coorperation of acid and base is required for side-
chain alkylation of toluene with methanol [12]. Gradually exchanging
Na with K can provide a fine tuning about acidity and basicity, which
will deliever us more informations about the exact requirements of
acid–base sites density and strength for the side-chain alkylation.
In the present work, a set of K-exchanged X zeolites (KX) were
prepared to have an insight into the effect of KED on the direction to
ring or side-chain alkylation and the exact requirements about acidity
and basicity for the side-chain alkylation.
2. Experimental
2.1. Catalysts Preparation
A set of KX zeilites with KED from 5.71% to 87.15% were prepared
from NaX zeolites (Nankai catalysts company, Si/Al: 1.46) via ion-
exchanged technique. In each case, 10 g of NaX zeolites were added
to a vessel containing 250 ml of an aqueous potassium hydroxide
⁎
Corresponding authors. Tel./fax: +86 351 4041526.
E-mail address: lizr@sxicc.ac.cn (Z. Li).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.12.033

L. Song et al. / Catalysis Communications 19 (2012) 90–95
91
Scheme 1. The two paths of the toluene alkylation with methanol.
solution with stirring at 80 °C. The obtained sample was filtered, dried
2.3. Catalytic evaluation
overnight at 100 °C and calcinated in air at 550 °C for 3 h. The relevant
parameters are summarized in Table 1.
Reactions were carried out in a fixed-bed reactor. About 4 g cata-
lysts were loaded in the middle of the reactor, and heated in situ at
500 °C in flowing N2 for 2 h before cooling to the reaction tempera-
ture of 425 °C. A mixture of toluene and methanol at a molar ratio
of 1:2 was delivered to the reactor at a rate of 2.4 cm3 h−1 . The reac-
tion was carried out under atmospheric pressure with a space
velocity of 0.5 h−1. The liquid samples were analyzed by gas chroma-
tography with a HP-FFAP capillary column.
The KED was calculated according to the following formula:
W
Na;i ꢀWNa;t
WNa;i
KEDð%Þ ¼
ꢁ 100
where WNa, i and WNa, t are, respectively, the mass percentage of Na in
the initial NaX zeolite and the treated zeolite.
The conversion of toluene (XTol), the selectivity (Si) and yield (Yi)
to the various products are defined as follows:
2.2. Catalysts characterization
The elemental compositions of catalysts were determined by in-
ductively coupled plasma-optical emission spectroscopy (Perkin-
Elmer ICP OPTIMA-3000). BET surface areas were measured by N2
physisorption (Micromeritics Tristar 3000). The crystalline phases
were characterized by a Rigaku D-Max/RB X-ray diffractometer. The
acid concentration and strength were determined by NH3-TPD. The
desorption of NH3 was monitored on-line by a thermal conductivity
detector.
∑yi; o
XTolð%Þ ¼
Sið%Þ ¼
ꢁ 100
yTol; o þ ∑yi; o
yi; o
ꢁ 100
∑yi; o
yi; o
yTol; o þ ∑yi; o
Yið%Þ ¼
ꢁ 100
O1s binding energies were detected by X-ray photoelectron spec-
troscopy (PHI Quantum 2000 Scanning ESCA Microprobe, monochro-
matic Al-Kα radiation) to determine the strength of surface basic
sites.
where ∑yi,o and yTol,o are the molar fractions of aromatic products
and the outlet molar fraction of toluene, respectively. Time on stream
was 10 h.
Table 1
Physicochemical characteristics and experimental parameters of KX.
Sample
KOH
Chemical composition (wt.%)
KED (%)
Sg (m2/g)
Vgc (cm3/g)
δ−od
concentration (mol/L)
Na
K
Si
Al
NaX
–
12.30
11.60
10.96
10.36
9.12
7.08
6.92
6.52
6.10
5.29
4.68
3.88
3.21
2.46
2.36
2.34
1.58
–
22.06
22.32
22.58
21.85
22.01
21.02
21.18
21.65
21.92
22.23
22.58
22.35
21.32
21.45
21.31
21.57
21.28
14.62
14.52
14.38
14.35
14.30
14.24
14.01
14.20
14.06
13.89
13.65
13.72
13.70
13.65
13.85
13.71
13.47
–
633.78
582.14
580.31
572.14
564.32
538.26
545.47
535.12
525.24
503.45
519.15
516.25
488.84
491.26
489.53
469.18
464.98
0.31
0.29
0.29
0.27
0.28
0.28
0.28
0.27
0.27
0.26
0.27
0.26
0.27
0.27
0.26
0.25
0.25
−0.395
−0.395
−0.393
−0.397
−0.398
−0.409
−0.409
−0.408
−0.409
−0.408
−0.409
−0.411
−0.421
−0.431
−0.431
−0.431
−0.430
aKX1
aKX2
KX3
0.02
0.03
0.03
0.1
0.3
0.3
0.4
0.5
0.5
1
0.5
3
4
3
4
0.96
1.63
2.58
4.31
7.70
7.81
8.56
9.27
10.38
11.42
12.52
14.02
16.45
16.58
16.65
17.11
5.71
10.89
15.81
25.85
42.44
43.74
47.12
50.40
57.02
61.95
68.46
73.90
80.01
80.84
81.03
87.15
aKX4
aKX5
KX6
aKX7
KX8
aKX9
KX10
a,bKX11
KX12
KX13
bKX14
bKX15
bKX16
5
All samples were obtained by exchanging once for 2 hours except those symbolised by a and b.
a
Exchange for 1 hour.
Exchange twice.
Total volume at P/P0 =0.986.
Oxygen charge.
b
c
d

92
L. Song et al. / Catalysis Communications 19 (2012) 90–95
Fig. 1. XRD patterns of NaX and K-exchanged zeolites.
Fig. 3. The conversion of toluene and the selectivity of products versus KED
.
3. Results and discussion
3.1. Textural characterization
The chemical composition, surface area and pore volume of cata-
lysts are listed in Table 1. It can be seen that the concentrations of
Si and Al decreases slightly with KED increasing, which is most likely
because that a small amount of Si and Al are dissolved out of the
framework of zeolite due to the existence of KOH in the exchange so-
lution [13]. In addition, there is a reduction in the total pore volume
and the total surface area. This may be due to the zeolite pore access
being partially blocked when K ions replaced Na ions within zeolite
pores [14].
3.2. XRD characterization
Fig. 1 shows the XRD patterns of samples. These samples exhibit
the characteristic peaks of the faujasite framework [15] and no new
significant diffraction peak is observed for KX, indicating that K+
ions dispersed well in the zeolite framework. In addition, with the
increase of KED there is an obvious decrease in the intensities of the
diffraction peaks, which indicates the crystallinity of samples gradu-
ally decreases and the framework of zeolites is partially damaged.
3.3. The acidity characterization
The results of NH3-TPD are presented in Fig. 2a. As shown in
Fig. 2a, all samples present two major peaks at relatively low
Table 2
Products yield of NaX and K-exchanged zeolite.
Sample
KED
(%)
XTol
(%)
Yield (%)
Eba
Styb
Xyc
C9d
Eb+Sty
NaX
KX4
KX8
KX10
KX14
KX16
0
10.78
10.33
6.63
4.74
5.70
8.32
0.52
0.51
1.87
1.56
3.62
6.47
0.03
0.06
0.43
0.89
1.31
1.28
7.82
7.53
3.58
1.82
0.34
0.21
2.40
2.23
0.73
0.47
0.43
0.36
0.55
0.57
2.30
2.45
4.93
7.75
25.85
50.40
61.95
80.84
87.15
a
Ethylbenzene.
Styrene.
Xylenes.
b
c
Fig. 2. The acidity and basicity of catalysts: (a) results of NH3-TPD, Inset plot: acid concen-
d
tration versus KED; (b) The oxygen charge versus KED; (c) O1s binding energy versus KED
.
Trimethylbenzene.

Fig. 4. The relationships of the yield of alkylation with acidity and basicity (a):KED b40%; (b): 40%bKED b58%; (c): KED >58%.

94
L. Song et al. / Catalysis Communications 19 (2012) 90–95
temperature, which are probably resulted from alkali ions (Lewis
acid) located at different sites in NaX zeolite [11,16,17]. In addition,
40% to 57%, the Yside-chain presents an increasing trend. On the other
hand, Fig. 4b shows that the concentration of acid decreases while
the oxygen charge is remained at −0.409. This indicates that the
acidity plays an important role in the side-chain alkylation, while ba-
sicity is no more the key factor for the reaction when the basicity of
frameoxygen is sufficient.
the total amounts of desorbed NH3 decreases with increasing KED
,
which reveals that the concentration of acid in zeolites are lessened
due to the incorporation of K+
.
3.4. The basicity analysis
Fig. 4c shows that with the rising of KED from 57% to 80%, the Yring
decreases, while the Yside-chain increases obviously accompaning with
the reducing acidity and the enhancing basicity. To further clarify the
roles of acid and base on the side-chain alkylation, which requires a
cooperative action of acid–base pairs [22–24], the growth rate of the
It has been reported that the base sites of alkali-exchanged X zeo-
lites mainly generated from the frameoxygen [18], the basicity of
which could be associated with the oxygen charge (δ−o) and O1s
binding energy [19,20]. These results of oxygen charge and O1s bind-
ing energies are displayed in Fig. 2.
Yside-chain versus KED is listed in Fig. 5.
It can be seen from Fig. 5 that the growth rate exhibits an increas-
It is well noticed that the oxygen charge increases gradually
(Fig. 2b) and O1s binding energy shifts to a lower value (Fig. 2c)
with the KED rising. The higher oxygen charge and the lower binding
energy are associated with higher basic strength [19,20]. In addition,
the two curves both show a flatform when KED is between 40% and
57%. Partial oxygen atoms may be dissolved out of framework accom-
panying with Si and Al in order to maintain electroneutrality, which
plays a negative role on the basicity, while Na+ exchanged by K+
can lead to the increase of basicity. Thus, the cooperation between
the loss of oxygen atoms and K-exchange may result in the formation
of flatform.
ing trend with KED rising from 40% to 70%. However, this curve begins
to decline when KED is greater than 70%. This may be due to the in-
creasing basicity of samples. It has been reported that too basic cata-
lysts can result in excessive formation of CO and H2 by methanol
decomposition [1,8,25], which is negative for side-chain alkylation
of toluene with methanol [26]. That ought to have leaded to the
Yside-chain decreasing. However, in our results, the Yside-chain still ex-
hibit an increasing trend (Fig. 4c). This may because that the decrease
of acidity promotes the side-chain alkylation to a certain degree.
In addition, when KED is less than 57%, the activity of ring alkyl-
ation is higher than that of side-chain alkylation and the toluene con-
version decreases with the acidity of samples reducing. On the
contrary, for samples with KED greater than 57%, the side-chain alkyl-
ation is the major reaction and the toluene conversion increases with
the KED increasing. Thus, a minimum value of toluene conversion can
be observed when KED is about 57% (Fig. 3).
Considering the alkylation of toluene with methanol, the alkali
metal cations (Lewis acid) mainly weaken the bond between the ox-
ygen and the methyl group of methanol yielding a highly activated
methyl group which can alkylate the toluene ring to produce xylenes
[27]. The framework oxygen (Lewis base) is claimed to dehydroge-
nate methanol to formaldehyde as the actual side-chain alkylating
agent [7,24]. When the acidity of samples is decreased, more metha-
nol may be dehydrogenated to formaldehyde so that the activity of
side-chain alkylation will be enhanced. Therefore, relatively weak
acidity can promote the side-chain alkylation when the basicity is
sufficient.
3.5. Catalytic results
Fig. 3 shows the conversion of toluene and the selectivity of prod-
ucts versus KED. Product yield of partial samples is summarized in
Table 2. As can be seen from Fig. 3, when KED is less than 40%, the se-
lectivity of xylenes (SXy) is stable at about 75%, while the selectivity of
ethylbenzene and styrene (SEb+Sty) is only 5%. As KED further increas-
ing, SEb+Sty increases. When KED is greater than 57% (KX5), ethylben-
zene and styrene become the major reaction products. The highest
SEb+Sty is up to 93.13% over KX16. These results indicate that the di-
rection to ring alkylation or side-chain alkylation of toluene with
methanol depends on KED greatly.
3.6. Catalytic performance and surface acid–base properties
Fig. 4 shows that the relationships of the yields of alkylation with
acidity and basicity. In Fig. 4a, with KED increasing to 40%, the concen-
tration of acid declines and the basicity of sample is raised, which has
few effect on the yields of both ring alkylation and side-chain alkyl-
ation (Yring ≈7%, Yside-chain b1%). The Yside-chain begins to increase
when KED is greater than 40% (Fig. 4b,c). This may be due to the suf-
ficient strong basicity in samples [12,21]. With the KED raising from
4. Conclusions
The suface acidity and basicity of zeolites were modified by K ions
exchange and the direction to ring alkylation or side-chain alkylation
of toluene with methanol depends on KED greatly. Zeolites of KED
lower than about 40% form essentially ring alkylation products. How-
ever, zeolites exhibits high activity for the side-chain alkylation when
KED is greater than 57%. It was found that the Yside-chain presents an in-
creasing trend with the decreased acidity and the comparable basicity
when KED is between 40% and 57%. In addition, when KED is beyond to
70%, the the Yside-chain exhibits a rising trend with the descresing acid-
ity in strong basic environment. All results suggest that the relatively
weak acidity can promote the side-chain alkylation of toluene with
methanol when the basicity is sufficient to ensure the reaction to
take place.
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