An in situ methylation of toluene using syngas over bifunctional mixture of Cr2O3/ZnO and HZSM-5

Article abstract of DOI:10.1016/j.apcata.2013.06.025

The in situ methylation of toluene using syngas (in situ methylation hereafter) was studied to produce xylenes over bifunctional catalysts comprising Cr₂O₃/ZnO (CrZ) and HZSM-5 (SiO₂/Al ₂O₃ = 30) in a fixed-bed down-flow reactor at 460 psig. In this study, three basic aspects in the in situ methylation were investigated: (1) the catalytic activity of CrZ as the methanol synthesis catalyst with respect to the equilibrium conversion, (2) the evaluation of catalytic synergies in the in situ methylation over bifunctional catalyst, and (3) the effect of the ratio of catalytic functions regarding the ratio of methanol synthesis catalyst to HZSM-5, and their intimacy. The CrZ catalyst exhibited very low CO conversion to methanol in the methanol synthesis reaction, which is controlled by the equilibrium. However, the bifunctional mixture of CrZ and HZSM-5 catalysts exhibited significant catalytic synergies in the in situ methylation in terms of catalytic activity and product selectivities, with about a 10-times-higher CO conversion rate, and 2- to 3-times-higher toluene conversion and xylene production rates compared to the monofunctional catalysis of methanol synthesis and toluene disproportionation, respectively. The in situ methylation also gave rise to higher catalytic performance than in typical toluene methylation conducted with a molar feed ratio of methanol/toluene of 0.5. The catalytic synergy was manifested when both metallic and acidic functions were brought into close intimacy with a weight ratio of CrZ/HZSM-5 close to one in the in situ methylation.

Full text of DOI:10.1016/j.apcata.2013.06.025

Applied Catalysis A: General 466 (2013) 90–97
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
An in situ methylation of toluene using syngas over bifunctional
mixture of Cr O /ZnO and HZSM-5
2
3
a
a
a
a
b
c
Seulah Lee , Donguk Kim , Jihye Lee , Yeseul Choi , Young-Woong Suh , Changq Lee ,
c
c
a,∗
Tae Jin Kim , Seong Jun Lee , Jung Kyoo Lee
a
Department of Chemical Engineering, Dong-A University, Busan 604-714, Republic of Korea
Department of Chemical Engineering, Hanyang University, Seoul 133-791, Republic of Korea
Catalyst·Process R&D Center, SK Innovation, Daejeon 305-712, Republic of Korea
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The in situ methylation of toluene using syngas (in situ methylation hereafter) was studied to produce
xylenes over bifunctional catalysts comprising Cr2O3/ZnO (CrZ) and HZSM-5 (SiO2/Al2O3 = 30) in a fixed-
bed down-flow reactor at 460 psig. In this study, three basic aspects in the in situ methylation were
investigated: (1) the catalytic activity of CrZ as the methanol synthesis catalyst with respect to the equi-
librium conversion, (2) the evaluation of catalytic synergies in the in situ methylation over bifunctional
catalyst, and (3) the effect of the ratio of catalytic functions regarding the ratio of methanol synthesis
catalyst to HZSM-5, and their intimacy. The CrZ catalyst exhibited very low CO conversion to methanol
in the methanol synthesis reaction, which is controlled by the equilibrium. However, the bifunctional
mixture of CrZ and HZSM-5 catalysts exhibited significant catalytic synergies in the in situ methylation in
terms of catalytic activity and product selectivities, with about a 10-times-higher CO conversion rate, and
2- to 3-times-higher toluene conversion and xylene production rates compared to the monofunctional
catalysis of methanol synthesis and toluene disproportionation, respectively. The in situ methylation also
gave rise to higher catalytic performance than in typical toluene methylation conducted with a molar
feed ratio of methanol/toluene of 0.5. The catalytic synergy was manifested when both metallic and acidic
functions were brought into close intimacy with a weight ratio of CrZ/HZSM-5 close to one in the in situ
methylation.
Received 15 February 2013
Received in revised form 16 June 2013
Accepted 19 June 2013
Available online 28 June 2013
Keywords:
Syngas
Toluene methylation
Methanol synthesis catalyst
Cr2O3/ZnO
HZSM-5
Bifunctional catalyst
Xylenes production
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
conversion catalyst to form a bifunctional catalyst for use in a single
reactor. If both active sites for methanol synthesis (site A in reaction
(1)) and for subsequent methanol conversion (site B) are present in
a bifunctional catalyst, the methanol formed on site A is readily con-
sumed to form the desired product on the nearby site B (see reaction
(1)). This tandem reaction proceeds toward the formation of the
product, and the thermodynamic equilibrium limitation inherent
to methanol synthesis can thereby be circumvented [3,18,20,21]
following a so-called “drain off” mechanism [25]. For example, Inui
et al. [26] showed that the conversion of CO was increased by 6
times over a bifunctional mixture of methanol synthesis catalyst
(Cr O –ZnO–Pd) and ZSM-5 compared to that over monofunctional
Syngas, which can be generated from petroleum alternatives
such as coal, natural gas (including shale gas), and biomass, is a ver-
satile intermediate for fuels and value-added products such as light
olefins and oxygenates [1–3]. In industry, methanol is produced
catalytically from syngas in the presence of Cu–ZnO or Cr O –ZnO-
based catalysts [4–7]. The conversion of syngas to methanol is
limited by a thermodynamic equilibrium that requires high pres-
sure and low temperature for high per-pass conversion [8]. Thus,
copper-containing catalysts are mostly used in industrial processes
2
3
[
4–6].
Since the first introduction of bifunctional catalysts for syngas
2
3
Cr O –ZnO–Pd catalyst, which must have been due to the displace-
2
3
transformation by Chang et al. [9], the direct conversions of syngas
into dimethyl ether (DME) [10–17] and the liquid hydrocarbons of
gasoline range [2,3,18–24] have attracted significant attention. For
this purpose, a methanol synthesis catalyst is coupled to a methanol
ment of the equilibrium in the methanol synthesis step on site A
of reaction (1). Due to potentially lower investment and operating
costs, and the more efficient use of natural gas than conventional
two-step or multi-step processes, the technology based on the
direct conversion of syngas is expected to play an important role in
the future.
∗ Corresponding author. Tel.: +82 51 200 7718; fax: +82 51 200 7728.
E-mail addresses: jklee88@dau.ac.kr, jklducheme@gmail.com (J.K. Lee).
CO + H2 ꢀCH3OH−→ Product
(1)
“A”
“B”
0
926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.06.025

S. Lee et al. / Applied Catalysis A: General 466 (2013) 90–97
91
Most of these examples have focused on the consecutive series
reactions involving methanol formation followed by conversion
through reaction intermediates into oxygenates or liquid hydro-
carbons. Methanol can also be used for petrochemical synthesis in
the presence of secondary reactant. For instance, xylenes can be
produced by the alkylation of toluene with methanol [27,28]. If the
catalytic functions for both methanol synthesis and its alkylation
on toluene are combined in a single reactor with an appropriately
designed bifunctional catalyst, a new process for xylene production
can be developed with a much simpler process than the conven-
tional ones (see reaction (2)). Imai et al. first reported the synthesis
of alkylaromatic compounds with syngas and an aromatic com-
pound (benzene or toluene) over a dual-function catalyst [29,30].
Recently, Ou et al. disclosed a patent for para-xylene production
with syngas and toluene by employing an in situ catalyst selectiva-
tion process [31].
to investigate catalyst structure. Hydrogen temperature-
programmed reduction (H -TPR) was conducted to measure
2
the reducibility of the metal oxide catalyst. In a typical H -TPR
2
◦
experiment, 0.2 g of catalyst was heated up to 600 C in a mixed
gas stream of 15 vol% H2 in Ar (90 ml/min) with a heating rate
◦
of 10 C/min. The effluent gas was passed over a molecular sieve
trap to eliminate the generated water, and then analyzed by a
gas chromatograph (GC) equipped with a thermal conductivity
detector (TCD). The morphology of the catalyst was analyzed by
scanning electron microscopy (SEM) with a JEOL JSM-6700F, and
by transmission electron microscopy (TEM) with a JEOL JEM-2010
operated at 200 kV. The physical properties of the catalyst were
determined by N2 adsorption/desorption measurement at liquid
nitrogen temperature using a volumetric adsorption analyzer
(ASAP 2000, Micromeritics).
2
2
.3. Catalytic activity tests
CO
+
H
2
CH3
.3.1. Methanol synthesis
Methanol synthesis reaction was carried out in a typical fixed-
CH3
Bifunctional Catalyst
CH3
bed down-flow reactor heated by an electrical furnace under a
total pressure of 460 psig. The pressure in the reactor was main-
tained by means of a back pressure regulator, and the feed gas
flow rates were controlled using mass flow controllers. The reactor
effluent lines were heated electrically to prevent condensation of
the reaction products. The reactor effluent gases were analyzed by
an on-line GC equipped with a TCD and a flame ionization detec-
tor (FID). H , CO, and N2 were analyzed using a packed Porapak
Q/MS 5A column connected to a TCD, in which N2 was used as
an internal standard. For every reaction test, a feed gas stream of
(2)
To further elucidate the fundamental catalytic behavior under-
lying the complicated in situ methylation, we report herein a
systematic study on the in situ methylation of toluene using syn-
gas (“in situ methylation” hereafter) on a bifunctional mixture of
Cr O /ZnO (CrZ) and HZSM-5 catalysts. In this study, we inves-
tigated three basic aspects for the in situ methylation: (1) the
catalytic performance of CrZ as the methanol synthesis catalyst
with respect to the equilibrium inherent in the methanol synthesis,
2
3
2
H /CO/N2 was by-passed the reactor and analyzed by TCD for use
2
(
2) the evaluation of catalytic synergies in the in situ methylation
as a reference for the reaction product analysis. Methanol and other
hydrocarbon products were analyzed using a capillary column (Plot
Q, 60 m × 0.32 mm × 0.5 ␮m) connected to an FID.
over bifunctional catalyst in terms of syngas and toluene conver-
sions, and product selectivities, and (3) the effect of ratio of catalytic
functions regarding the ratio of methanol synthesis catalyst to
alkylation catalyst, and their intimacy in the in situ methylation.
The calcined CrZ catalyst was pre-pelletized and sieved into a
50–500 ␮m fraction. The sieved catalyst (0.5–1.5 g) was loaded
2
ꢀ
ꢀ
into a stainless steel reactor (3/8 in OD). Prior to the reaction
test, the CrZ catalyst was dried at 250 C in an He flow for 2 h.
Then, the catalyst was reduced in flowing hydrogen (H /He = 3/2,
GHSV = 4700 cm /h gcat) at 400 C for 3 h. After reduction, the reac-
2
2
2
. Experimental
◦
2
.1. Preparation of catalysts
3
◦
◦
tor temperature was reduced to 120 C, and the feed gas mixture
.1.1. Methanol synthesis catalyst
3
(H /CO/N = 2/1/1, GHSV = 5360 cm /h gcat) was introduced into
2
2
The Cr O /ZnO (Cr:Zn molar ratio = 1:2.3) catalyst was prepared
2
3
the reactor. The reactor pressure was raised to 460 psig, and then
the reactor temperature was raised to the range of 200–500 C to
investigate the catalytic activity of the catalyst in methanol synthe-
sis.
by co-precipitation method [7,32]. Appropriate amounts of metal
◦
nitrates, Cr(NO ) ·9H O and Zn(NO ) ·6H O, were dissolved in DI
3
3
2
3 2
2
water to obtain 100 ml of 1.0 M metal nitrate solution. The metal
nitrate solution was added dropwise into DI water (500 ml) kept at
0 C with stirring while the pH of the solution was maintained at
9 by adding NH OH solution dropwise as a precipitation agent.
After aging for 1 h at 70 C, the precipitate was filtered and washed
with DI water and then dried overnight at 80 C. The dried precipi-
tate was calcined at 500 C for 6 h in air to obtain CrZ catalyst.
◦
7
2
.3.2. In situ methylation
The bifunctional catalyst (CrZ HZ) was pre-pelletized and sieved
∼
4
◦
into a 250–500 ␮m fraction. Typically, 1.0 g of sieved catalyst
CrZ:HZSM-5 = 1:1 weight ratio) was loaded into the reactor. The
upper space of the catalyst bed was filled with glass beads
1–1.25 mm) for preheating and as a distribution zone of the feed
◦
(
◦
(
2
.1.2. Bifunctional catalyst
Bifunctional catalyst was prepared by the physical mixing of
CrZ with a commercial HZSM-5 catalyst (SiO /Al O = 30, Zeolyst)
with a 1:1 weight ratio in a mortar and pestle. Bifunctional CrZ HZ
catalysts (CrZ mixed with HZSM-5) with weight ratios of CrZ to
HZSM-5 of 0.5:1, 1:1, and 2:1 were prepared to investigate the effect
of the catalytic functions of the bifunctional catalysts on the in situ
methylation.
stream. Then, the catalyst was subjected to pretreatment proce-
dures involving drying and reduction with GHSVs of gases based
on the weight of methanol synthesis catalysts described above.
After pretreatment, the reactor temperature was reduced to 120 C,
and the reaction feed mixture (H /CO/N /Toluene = 2/1/1/0.5,
GHSV = 5835 cm /h gcat, Toluene WHSV = 4 h ) was introduced
into the reactor. The feed rate of toluene (Tol) was controlled by
an HPLC pump while gas flow rates were controlled by mass flow
controllers. The reactor pressure was raised to 460 psig, and then
2
2
3
◦
2
2
−1
3
◦
2
.2. Characterization of the catalysts
the reactor temperature was raised to the range of 350–500 C to
investigate the catalytic activities in the in situ methylation. The
reactor effluent was introduced on-line into a gas–liquid separa-
tor with a liquid sampling port and a cooling jacket connected to
X-ray diffraction (XRD) measurement was performed on
a model D/MAX-50 kV system (Cu K␣ radiation, ꢀ = 1.5418 A˚ )

9
2
S. Lee et al. / Applied Catalysis A: General 466 (2013) 90–97
Fig. 2. FESEM image of CrZ catalyst.
a separate Cr O phase, which was consistent with the CrZ catalysts
of similar Cr/Zn ratio [7,19]. The TPR profile of the calcined CrZ cat-
alyst is presented in Fig. 1b. The calcined CrZ catalyst showed a very
2
3
Fig. 1. (a) XRD pattern of CrZ: (᭹) ZnO, (ꢁ) ZnCr2O4, and (b) TPR profile of CrZ.
small TPR profile, but there was hydrogen consumption between
a coolant circulator. The gas products from the separator overhead
were analyzed by an on-line GC equipped with a packed column
◦
2
00 and 275 C. Since ZnO is not reduced under these conditions
[
33], the hydrogen consumption could be attributed to the reduc-
(
Porapack Q/MS 5A) connected to a TCD for H /CO/N2 and light
2
tion of ZnCr O and Cr O in proximity to ZnO particles [17,34].
2
4
2
3
hydrocarbon products, and with a capillary column (HP-INNOWax,
The morphology of the calcined CrZ catalyst investigated by SEM
is shown in Fig. 2. The calcined CrZ catalyst exhibited irregular
6
0 m × 0.32 mm × 0.5 ␮m) connected to an FID for hydrocarbon
products. Liquid product sampled from the bottom of the separator
was analyzed by an off-line GC equipped with a capillary column
(
HP-INNOWax, 60 m × 0.32 mm × 0.5 ␮m) connected to an FID. To
investigate the effect of catalytic functions on the in situ methyl-
ation, bifunctional catalyst mixtures with three different weight
ratios of CrZ catalyst to HZSM-5 were tested (0.5:1, 1:1 and 1:0.5).
For a control experiment, a stacked catalyst bed system (a methanol
synthesis catalyst on top of an HZSM-5 bed) was employed to inves-
tigate the catalytic synergy in the in situ methylation in comparison
to the standard bed of well-mixed bifunctional catalyst. Product
samples were analyzed after the initial stabilization with time-on-
stream (TOS) of 1.5–2 h.
2.3.3. Toluene disproportionation and toluene methylation
As control tests, toluene disproportionation and methyla-
tion with methanol were carried out over a monofunctional
HZSM-5 (SiO /Al O = 30) with reaction temperature in the
2
2
3
◦
range of 350–450 C under a total pressure of 460 psig. After
the pre-treatment procedure involving catalyst drying, toluene
disproportionation was conducted with a molar feed composi-
−
1
tion of H /Tol = 1.0/0.5 (GHSV = 3890 ml/h gcat, Tol WHSV = 8 h ).
2
Toluene methylation was conducted with a molar feed compo-
sition of H /Tol/MeOH = 1.0/0.5/0.25 (GHSV = 3890 ml/h gcat, Tol
2
−1
WHSV = 8 h ). For both tests, the toluene feed rate was set to be
equivalent to that for the in situ methylation based on the HZSM-
5
weight of the bifunctional catalyst. The reaction products were
analyzed by the same procedure with the in situ methylation test.
3
. Results and discussion
3.1. Properties of the catalysts
Fig. 1a shows the XRD pattern of the calcined CrZ catalyst.
ZnO (JCPDS #05-664) and ZnCr O (JCPDS #22-1107) phases were
2
4
clearly observed in the XRD pattern of calcined CrZ catalyst without
Fig. 3. HRTEM images of CrZ catalyst.

S. Lee et al. / Applied Catalysis A: General 466 (2013) 90–97
93
Fig. 5. Catalytic performance of CrZ HZ catalyst on the in situ methylation; (a) CO
conversion, (b) toluene conversion and xylene yield.
Fig. 4. Catalytic performance of CrZ catalyst in methanol synthesis; (a) CO conver-
sion to methanol. Lines depict the equilibrium conversion of CO to methanol. (b)
Product selectivity.
equilibrium conversion of CO to methanol in conditions [35,36]
similar to those in this study are presented in Fig. 4a. The CO con-
version to methanol over CrZ catalyst remained very low (<4.6%) at
spherical aggregates 100–200 nm in size. The structure of the cal-
cined CrZ catalyst was further investigated using the TEM images
shown in Fig. 3. As observed by SEM, the TEM image of CrZ
revealed aggregates of primary spherical particles of 10–30 nm
◦
reaction temperatures between 250 and 500 C and 460 psig (i.e.,
3
MPa), which is consistent with the equilibrium CO conversion to
methanol. This clearly implies that the CO conversion to methanol
is strongly controlled by the equilibrium and CrZ, as the methanol
synthesis catalyst requires much higher reaction pressure to obtain
(
Fig. 3a). The particles showed planes corresponding to ZnCr O4
2
(
d2 2 0 = 0.29 nm and d1 1 1 = 0.49 nm) as well as ZnO (d1 0 1 = 0.25 nm
a CO conversion level comparable to that of Cu/ZnO/Al O3 cat-
and d1 0 2 = 0.19 nm), and they were arranged in an alternating fash-
ion to form larger aggregates (Fig. 3b). The physical properties of
the calcined CrZ catalyst were determined by BET measurement.
The BET surface area and pore volume of the calcined CrZ catalyst
2
alyst [36]. The methanol selectivity over CrZ catalyst remained
◦
at almost 100% up to 300 C, and then decreased rapidly due to
by-product (mainly C –C3 gases) formation at the reaction tem-
1
◦
2
3
peratures greater than 350 C, as shown in Fig. 4b. Overall, the
were measured to be 46.4 m /g and 0.17 cm /g, respectively.
CrZ catalyst exhibited very low methanol yield and selectivity at
◦
reaction temperatures of 350 C and above, which is the required
3.2. Catalytic activity for methanol synthesis
reaction temperature range for toluene alkylation with methanol.
In order to investigate the synergistic effects between CrZ and
HZSM-5 catalysts in the in situ methylation, in which a bimolec-
ular reaction of toluene with methanol formed in situ proceeds
as depicted in reaction (2), the CrZ catalyst was tested first for
the methanol synthesis with syngas under a moderate pressure of
3.3. In situ methylation of toluene using syngas
Fig. 5 shows the results from the in situ methylation over CrZ HZ
catalyst. In contrast to the methanol synthesis reaction over CrZ
catalyst, the CO conversion was enhanced considerably over CrZ HZ
4
60 psig.
The CO conversion to methanol as a function of reaction tem-
◦
catalyst at reaction temperatures of 350–450 C (Fig. 5a). For exam-
◦
perature is given in Fig. 4a. For comparison, curves depicting the
ple, at 400 C, the CO conversion in the in situ methylation over
Table 1
Product distributions from toluene disproportionation over HZSM-5 and in situ methylation over bifunctional CrZ HZ catalyst.
Tol disproportionation
In situ methylation
Stacked bed
Well-mixed bed
350
◦
Temperature ( C)
350
400
–
450
–
350
400
450
18.2
400
40.4
450
43.4
Conversion (%)
CO
–
3.8
13.8
8.4
34.7
25.6
19.1
Toluene
4.1
17.2
46.4
50.5
41.2
44.7
Hydrocarbon products (wt%)
C1–C5
Bz
EB
1.0
45.1
0.5
1.5
42.9
0.3
3.7
42.9
1.1
0.3
42.0
0.6
4.7
38.3
0.7
7.3
43.9
1.1
2.1
12.3
1.0
5.5
9.4
0.9
14.0
12.0
0.9
Xylenes
C9 Aro
Yield of xylenes (wt%)
53.4
0.0
2.2
54.0
1.3
9.3
47.1
5.2
21.8
57.1
0.0
7.9
52.9
3.3
18.4
43.5
4.2
22.0
72.2
12.5
13.8
69.1
15.0
28.5
62.3
10.8
27.8
+

9
4
S. Lee et al. / Applied Catalysis A: General 466 (2013) 90–97
4
3
2
1
0
0
0
0
0
50
(
a)
(a)
40
30
20
10
well mixed bed
stacked bed
CrZ_HZ
CrZ
X 10
0
3
00
350
400
450
500
3
00
350
400
450
500
o
Temperature, C
Temperature,oC
5
4
3
2
1
0
0
0
0
0
(
b)
8
0
0
0
0
(
b)
6
4
2
X 2.4
CrZ_HZ
HZ
well mixed bed
stacked bed
450
0
3
00
350
400
450
500
Temperature,oC
300
350
400
500
30
20
10
0
Temperature,oC
(c)
3
3
2
20
1
1
5
0
5
(c)
X 3.0
CrZ_HZ
HZ
5
0
well mixed bed
3
00
350
400
450
500
5
0
stacked bed
Temperature,oC
300
350
400
450
500
Fig. 6. Catalytic synergies of bifunctional CrZ HZ catalyst in the in situ methylation
over monofunctional catalysis; (a) CO conversion, (b) toluene conversion and (c)
xylene production rates.
Temperature,oC
Fig. 7. Effect of intimacy of catalytic functions in bifunctional CrZ HZ catalysts
stacked bed vs. well mixed bed); (a) CO conversion, (b) xylene selectivity and (c)
(
xylene yield.
CrZ HZ was 40.4%, while it was as low as 5.6% in the methanol syn-
thesis over CrZ catalyst. The toluene conversion was greater than
◦
the equilibrium in the methanol synthesis step on site A of reaction
4
0%, and the xylene yield was on the order of 28% at 400–450 C
(1) over bifunctional catalyst.
in the in situ methylation over the bifunctional CrZ HZ catalyst
Fig. 5b and Table 1). These results support that the in situ methyl-
The toluene conversion rates from the in situ methylation over
(
CrZ HZ and those from toluene disproportionation over monofunc-
tional HZSM-5 catalyst are compared in Fig. 6b. The rates in the
in situ methylation over CrZ HZ were much faster than those in the
toluene disproportionation over HZSM-5 at the reaction tempera-
ation in which a bimolecular reaction occurs between toluene and
methanol formed in situ is highly feasible catalysis. It is also notice-
able that the bifunctional CrZ HZ clearly shows catalytic synergy
with respect to the CO conversion in the in situ methylation in com-
parison to that in the monofunctional methanol synthesis reaction
over CrZ catalyst.
◦
tures of 350 and 400 C. The rates in both reactions were about the
◦
same at 450 C, mainly due to the increased side reactions at high
reaction temperature. As shown in Fig. 6c, the enhanced toluene
conversion rates in the in situ methylation resulted in higher
xylene production rates than in the monofunctional toluene dis-
proportionation. Overall, the CO and toluene conversion rates over
bifunctional catalyst were increased by factors of approximately
10 and 2.4, respectively, compared to those in the corresponding
monofunctional catalysts, which clearly demonstrates the catalytic
synergies in the in situ methylation.
Table 2 describes the detail comparison of the results from
the in situ methylation and a typical toluene methylation with
methanol. Both tests were conducted under the same temperature,
pressure, and toluene WHSV. In the liquid product analyses given
in Table 2, the in situ methylation yielded higher toluene conver-
sion, xylene yield, and selectivity with a lower yield of C9+ heavy
aromatic compounds than a conventional toluene methylation con-
ducted with the feed molar ratio of MeOH/Tol = 0.5. Without the
exact information on the CO conversion to methanol in the in
situ methylation, the results indicate a highly efficient catalysis
3
.4. In situ methylation versus monofunctional catalysis
To further evaluate the catalytic synergies in the in situ meth-
ylation over bifunctional CrZ HZ catalyst, its catalytic performance
was compared to those obtained with monofunctional catalysts,
with methanol synthesis over CrZ, and toluene disproportionation
and methylation with methanol over HZSM-5.
The catalytic performance is compared in terms of the reaction
rates in Fig. 6. The CO conversion rates in the in situ methyla-
tion over CrZ HZ were higher by almost an order of magnitude
than those in methanol synthesis over CrZ catalyst in the reaction
temperature range of 350–450 C. For example, the CO conver-
sion rate in the in situ methylation over CrZ HZ was as much as
3
functional methanol synthesis over CrZ catalyst at 400 C (Fig. 6a).
This clearly shows the catalytic synergy due to the displacement of
◦
3.2 mmol/gcat h, while it was only 3.3 mmol/gcat h in the mono-
◦

S. Lee et al. / Applied Catalysis A: General 466 (2013) 90–97
95
Table 2
Comparison of toluene methylation with methanol over HZSM-5 and in situ methylation over bifunctional CrZ HZ catalyst.
Toluene methylation
In situ methylation
◦
Reaction temperature ( C)
350
460
400
460
450
460
350
460
400
460
450
460
Pressure (psig)
Feed, molar ratio
MeOH/Tol
H2/CO/ToI
Catalyst
1/2
–
HZ
8
1/2
–
HZ
8
1/2
–
HZ
8
–
–
–
2/1/0.5
CrZ + HZ
8
2/1/0.5
CrZ + HZ
8
2/1/0.5
CrZ + HZ
8
Tol WHSV (h−1) (based on ZSM-5)
Conversion (%)
Toluene
17.2
–
28.2
–
36.6
–
19.0
25.6
40.2
40.4
41.9
43.4
CO
Liq. product distributions (wt%)
NonAro. compounds
Benzene
Toluene
Ethylbenzene
P-xylene
0.2
0.0
82.8
0.0
2.4
2.6
4.9
7.1
0.1
2.5
71.8
0.4
3.9
8.4
3.8
9.0
0.0
5.5
63.4
0.5
5.4
12.0
5.1
8.1
0.0
2.4
81.0
0.2
3.3
7.6
3.1
2.4
0.4
3.9
59.8
0.4
6.6
15.6
6.7
6.5
1.1
5.6
58.1
0.4
6.7
15.6
6.9
5.5
M-xylene
O-xylene
+
C9 Aro. compounds
Xylene yield (wt%)
Xylene selectivity (%)
9.9
57.4
16.1
57.1
22.5
61.4
14.0
73.7
28.9
72.0
29.3
69.8
involved in the in situ methylation. However, a rough estimation
monofunctional toluene disproportionation was in the range of
47–54%, which was not improved much in the in situ methyla-
tion over the stacked-bed due to the high yield of undesirable
can be made as follows. The CO conversions were 25.6, 40.4, and
◦
4
3.4% at 350, 400, and 450 C, respectively, in the in situ methyla-
tion. The feed molar ratio of CO/Tol was 2.0. If it is assumed that CO
is converted solely to methanol, the molar ratios of MeOH/Tol on
the catalyst would be 0.51, 0.81, and 0.87 at 350, 400, and 450 C,
by-products, including C –C5 and benzene. However, the selec-
1
tivity to xylenes in the in situ methylation over the well-mixed
catalyst bed was increased up to 62–72%, giving a xylene yield of
approximately 28 wt%.
◦
respectively, which are all higher than the ratio of 0.5 used in the
toluene methylation test. Thus, the higher molar ratio of MeOH/Tol
in the in situ methylation could give rise to higher toluene conver-
sion and xylene yield, as typically observed in toluene methylation
with methanol [27]. Further study is required to obtain more pre-
cise information on the CO conversion to methanol in the in situ
methylation.
The catalytic synergy observed in the in situ methylation was
further manifested in the tests with different catalyst bed configu-
rations: a well-mixed mixture of CrZ and HZSM-5, versus a stacked
bed of CrZ on top of HZSM-5 catalyst under identical reaction con-
ditions. The two different catalyst bed configurations represent the
two extremes of intimacy between metallic and acidic functions
in the bifunctional catalysts. As expected, the stacked-bed config-
uration exhibited much worse performance indices in terms of CO
and toluene conversions, xylene selectivity, and xylene yield than
the well-mixed catalyst mixture (Fig. 7a–c). Table 1 depicts the
detailed product distributions from the in situ methylation over
a well-mixed catalyst bed and a stacked-bed, and from the toluene
disproportionation as a control test. The selectivity to xylenes from
These results suggest that the enhanced catalytic performance
in the in situ methylation originated from a strong catalytic synergy
between metallic and acidic functions in the bifunctional catalyst,
and that it can best be achieved when metallic and acidic functions
are brought into close intimacy to each other in the bifunctional
catalyst. Fig. 8 depicts the catalytic synergy observed in the in situ
methylation. The methanol synthesis is strongly affected by equi-
librium limitation, and CrZ catalyst possesses very low activity (low
k1) in monofunctional methanol synthesis under a moderate pres-
sure of 460 psig. However, in the in situ methylation, as soon as
methanol is formed on the CrZ catalyst, methanol is readily con-
sumed in the methylation of toluene (k ) on the HZSM-5. Thus, the
3
equilibrium limitation of methanol synthesis reaction is circum-
vented, and the reaction proceeds toward the formation of xylenes.
Furthermore, the undesirable side reactions (k ) such as methanol
2
decomposition and oligomerization at relatively high reaction tem-
peratures are substantially suppressed by subsequent methylation
reaction (k ).
3
Fig. 8. Reaction pathways for the in situ methylation over bifunctional CrZ HZ catalyst.

9
6
S. Lee et al. / Applied Catalysis A: General 466 (2013) 90–97
catalyst in the bifunctional mixture exhibited limited CO conver-
sion to methanol in methanol synthesis under a moderate pressure
of 460 psig due to the inherent equilibrium limitation in methanol
synthesis. (2) The bifunctional CrZ HZ catalyst, however, exhibited
strong catalytic synergies in terms of CO and toluene conversion
rates, and the xylene production rate, compared to monofunc-
tional catalysts. In the in situ methylation over CrZ HZ catalyst at
◦
4
00 C, the CO conversion rate was around 10 times higher than
that of methanol synthesis over CrZ catalyst, and toluene conver-
sion and xylene production rates were about 2–3 times higher than
those of toluene disproportionation over monofunctional HZSM-5.
The in situ methylation also gave rise to higher toluene conver-
sion and xylene yield than a typical toluene methylation conducted
with a feed molar ratio of MeOH/Tol = 0.5. (3) The catalytic syner-
gies were manifested in the bifunctional catalytic mixture of CrZ
and HZSM-5, in which metallic and acidic functions were brought
into close contact with a CrZ/HZSM-5 weight ratio of around one.
The results also suggest that syngas can be directly converted into
chemicals in the presence of other reactants over properly designed
bifunctional catalysts, in which the thermodynamic equilibrium
limitation inherent in methanol synthesis can be effectively cir-
cumvented.
Acknowledgment
The authors gratefully acknowledge the financial support of SK
Innovation for this study.
References
[
[
[
1] R. Horn, K.A. Williams, N.J. Degenstein, A. Bitsch-Larsen, D.D. Nogare, S.A. Tupy,
L.D. Schmidt, J. Catal. 249 (2007) 380–393.
2] Q.G. Yan, P.T. Doan, H. Toghiani, A.C. Gujar, M.G. White, J. Phys. Chem. C 112
(
2008) 11847–11858.
3] J. Erena, J.M. Arandes, J. Bilbao, A.T. Aguayo, H.I. de Lasa, Ind. Eng. Chem. Res.
7 (1998) 1211–1219.
3
[
[
[
4] Y.W. Suh, S.H. Moon, H.K. Rhee, Catal. Today 63 (2000) 447–452.
5] M. Behrens, J. Catal. 267 (2009) 24–29.
6] X.K. Phan, H. Bakhtiary-Davijany, R. Myrstad, P. Pfeifer, H.J. Venvik, A. Holmen,
Appl. Catal. A 405 (2011) 1–7.
Fig. 9. Effect of catalyst composition in bifunctional CrZ HZ catalyst on the in situ
methylation; (a) CO conversion, (b) toluene conversion and (c) xylene yield.
[
7] M.C.J. Bradford, M.V. Konduru, D.X. Fuentes, Fuel Process. Technol. 83 (2003)
1
1–25.
3.5. Effect of CrZ/HZSM-5 ratio on in situ methylation
[8] F. Marschner, F.W. Moeller, Methanol synthesis, in: Applied Industrial Catalysis,
vol. 2, Elsevier Inc., 1983.
[
9] C.D.L. Chang, W.H. Lang, A. Silvestri, US Patent (1978).
Finally, the effect of catalyst ratios of CrZ to HZSM-5 on the in
situ methylation was studied with three different weight ratios of
[
[
10] Q.J. Ge, Y.M. Huang, F.Y. Qiu, S.B. Li, Appl. Catal. A 167 (1998) 23–30.
11] J.H. Kim, M.J. Park, S.J. Kim, O.S. Joo, K.D. Jung, Appl. Catal. A 264 (2004) 37–41.
CrZ/HZSM-5 (0.5, 1.0 and 2.0) at reaction temperatures of 350 and
[12] J. Erena, R. Garona, J.M. Arandes, A.T. Aguayo, J. Bilbao, Catal. Today 107-108
(2005) 467–473.
◦
4
00 C, and the results are shown in Fig. 9. All the catalytic perfor-
[
13] D.S. Mao, W.M. Yang, J.C. Xia, B. Zhang, Q.Y. Song, Q.L. Chen, J. Catal. 230 (2005)
40–149.
[14] J.W. Bae, S.H. Kang, Y.J. Lee, K.W. Jun, Appl. Catal. B 90 (2009) 426–435.
mance indices, such as CO and toluene conversions and the xylene
yield, are maximized at the CrZ/HZSM-5 ratio of 1.0 at both tem-
peratures. In the conventional alkylation of toluene with methanol,
the toluene conversion and xylene yield are significantly reduced
with increases in toluene/methanol molar ratio from 1 to 8, since
methanol is a limiting reagent [27]. The same is true for the low
CrZ/HZSM-5 ratio of this study, in which methanol formation is
the limiting step in the in situ methylation. At a high ratio of
CrZ/HZSM-5, however, toluene WHSV with respect to HZSM-5 cata-
1
[
15] G.H. Yang, N. Tsubaki, J. Shamoto, Y. Yoneyama, Y. Zhang, J. Am. Chem. Soc. 132
2010) 8129–8136.
16] A. Garcia-Trenco, A. Martinez, Appl. Catal. A 411 (2012) 170–179.
(
[
[17] A. Venugopal, J. Palgunadi, K.D. Jung, O.S. Joo, C.H. Shin, Catal. Lett. 123 (2008)
42–149.
18] F. Simard, A. Mahay, A. Ravella, G. Jean, L.H. De, Ind. Eng. Chem. Res. 30 (1991)
448–1455.
[19] F. Simard, U.A. Sedran, J. Sepulveda, N.S. Figoli, H.I. Delasa, Appl. Catal. A 125
1995) 81–98.
1
[
1
(
−1
[20] J. Erena, J.M. Arandes, J. Bilbao, M. Olazar, H.I. de Lasa, J. Chem. Technol. Bio-
technol. 72 (1998) 190–196.
[21] J. Erena, J.M. Arandes, J. Bilbao, A.G. Gayubo, H.I. De Lasa, Chem. Eng. Sci. 55
(2000) 1845–1855.
lyst increases to as high as 16 h , in which the alkylation kinetics is
the limiting step in the in situ methylation. Therefore, a CrZ/HZSM-5
ratio around 1.0 would be close to the optimum ratio for the in situ
◦
[22] J. Bao, J. He, Y. Zhang, Y. Yoneyama, N. Tsubaki, Angew. Chem. Int. Ed. 47 (2008)
methylation at moderate reaction temperature (350–400 C) and
353–356.
pressure (460 psig).
[
[
23] P.C. Zonetti, A.B. Gaspar, F.M.T. Mendes, E.V. Sobrinho, E.F. Sousa-Aguiar, L.G.
Appel, Fuel Process. Technol. 91 (2010) 469–475.
24] P. Mohanty, K.K. Pant, J. Parikh, D.K. Sharma, Fuel Process. Technol. 92 (2011)
4
. Conclusion
600–608.
[
[
[
25] P.B. Weisz, Adv. Catal. 13 (1962) 137.
26] Y.T.T. Inui, ACS Symp. Ser. 27 (1982) 982.
27] W.W. Kaeding, C. Chu, L.B. Young, B. Weinstein, S.A. Butter, J. Catal. 67 (1981)
Xylenes can be efficiently produced by the in situ methylation
of toluene with syngas over a bifunctional mixture of Cr O /ZnO
2
3
159–174.
and HZSM-5 catalysts. The three basic aspects revealed in this
study were as follows: (1) CrZ catalyst as the methanol synthesis
[28] J.P. Breen, R. Burch, P.J. Collier, S.E. Golunski, US Patent 7,321,072 B2 (2008).
[29] T. Imai, US Patent 4,487,984 (1984).

S. Lee et al. / Applied Catalysis A: General 466 (2013) 90–97
97
[
[
30] T. Imai, P.T. Barger, US Patent 4,665,238 (1987).
31] J.D.Y. Ou, Z. Hong, S. Tan, T.E. McMinn, US Patent 2008/0033067 A1
[34] N. Liu, Z.S. Yuan, C.W. Wang, S.D. Wang, C.X. Zhang, S.J. Wang, Fuel Process.
Technol. 89 (2008) 574–581.
(
2008).
[35] K. Omata, M. Hashimoto, Y. Watanabe, T. Umegaki, S. Wagatsuma, G. Ishiguro,
M. Yamada, Appl. Catal. A 262 (2004) 207–214.
[36] J.G. van Bennekom, R.H. Venderbosch, J.G.M. Winkelman, E. Wilbers, D. Assink,
K.P.J. Lemmens, H.J. Heeres, Chem. Eng. Sci. 87 (2013) 204–208.
[
[
32] C. Baltes, S. Vukojevic, F. Schuth, J. Catal. 258 (2008) 334–344.
33] P. Kurr, I. Kasatkin, F. Girgsdies, A. Trunschke, R. Schlogl, T. Ressler, Appl. Catal.
A 348 (2008) 153–164.