Catalytic Activity of Pt Modified Hierarchical ZSM-5 Catalysts in Benzene Alkylation with Methanol

Article abstract of DOI:10.1007/s10562-014-1458-3

Hierarchical ZSM-5 zeolite showed an improved performance as compared to conventional ZSM-5 catalysts in the alkylation of benzene with methanol. However, ethylbenzene yet remains as a major problem. In this study, we modified hierarchical ZSM-5 with Pt t

Full text of DOI:10.1007/s10562-014-1458-3

Catal Lett
DOI 10.1007/s10562-014-1458-3
Catalytic Activity of Pt Modified Hierarchical ZSM-5 Catalysts
in Benzene Alkylation with Methanol
•
•
•
Hualei Hu Qunfeng Zhang Jie Cen
Xiaonian Li
Received: 3 September 2014 / Accepted: 1 December 2014
Ó Springer Science+Business Media New York 2014
Abstract Hierarchical ZSM-5 zeolite showed an
improved performance as compared to conventional ZSM-
5 catalysts in the alkylation of benzene with methanol.
However, ethylbenzene yet remains as a major problem. In
this study, we modified hierarchical ZSM-5 with Pt to
evaluate the alkylation of benzene with methanol in a
fixed-bed continuous flow reactor. It was found that Pt
modified hierarchical ZSM-5 could successfully combine
the catalytic advantages of hierarchical ZSM-5 and the
high suppression of Pt to ethylbenzene formation. More-
over, employing direct reduction could improve the utili-
zation of Pt by avoiding Pt particles sintering.
modified ZSM-5 could exhibit high suppression ability
towards the formation of ethylbenzene in benzene alkyl-
ation with methanol, which effectively avoided the diffi-
culty of separating ethylbenzene from xylene [11].
However, low conversion of benzene and low selectivity to
xylene was observed over the conventional ZSM-5 and this
might be attributed to the diffusion limitation of microp-
ores [5, 7].
Hierarchical materials can enhance the diffusion of
bulky substrates during the catalytic reactions as the result
of introducing mesopores into zeolite materials without
destroying the catalytic feature of zeolitic micropores [12,
13]. Not surprisingly, hierarchical zeolites could dramati-
cally improve the catalytic performance in the methylation
of toluene and the ethylation of benzene as compared to
conventional zeolite catalysts [14–16]. Recently, Lu et al.
and Deng et al. had reported that hierarchical ZSM-5 dis-
played higher benzene conversion and xylene selectivity in
benzene alkylation with methanol than conventional ZSM-
5 [17, 18]. Unfortunately, hierarchical ZSM-5 still cannot
exterminate the formation of ethylbenzene [19]. Conse-
quently, combining the suppressing ability of Pt towards
the ethylbenzene formation and the catalytic advantages of
hierarchical ZSM-5 would be of great interests. On the
other hand, it is well known that calcination at high tem-
peratures in air causes Pt particle sintering, which decrea-
ses the dispersion of Pt and further influences its
hydrogenation activity [20, 21]. Direct reduction of the
supported Pt precursor in hydrogen flow could reduce the
change of Pt particle caused by calcinations [22]. There-
fore, employing direct reduction method might be able to
improve the utilization efficiency of Pt.
Keywords Hierarchical ZSM-5 ꢀ Pt ꢀ Ethylbenzene ꢀ
Xylene ꢀ Benzene alkylation
1 Introduction
The alkylation of benzene with methanol is a promising
technology in petrochemical industry for the production of
toluene and xylene, which are important chemical inter-
mediates [1–4]. While using conventional ZSM-5 zeolite as
the catalyst for the alkylation of benzene with methanol,
the formation of ethylbenzene was difficult to be elimi-
nated [5–7]. Moreover, the separation or removal of eth-
ylbenzene from C8 aromatic still remains as a major
challenge [8–10]. Recently, researchers found that Pt
H. Hu ꢀ Q. Zhang ꢀ J. Cen ꢀ X. Li (&)
State Key Laboratory Breeding Base of Green Chemistry
Synthesis Technology, Industrial Catalysis Institute of Zhejiang
University of Technology, Hangzhou 310032,
People’s Republic of China
In present work, the catalytic performance of Pt modified
hierarchical ZSM-5 in benzene alkylation with methanol
was investigated. The structural and textural properties of
e-mail: xnli@zjut.edu.cn
123

H. Hu et al.
2.2 Catalyst Characterization
the catalysts were systematically characterized by different
techniques (including SEM, TEM, XRD, BET and TG–
DTA) and the selectivity to ethylbenzene product versus Pt
content was also investigated. We observed that Pt modified
hierarchical ZSM-5 catalysts could retain excellent catalytic
activity of hierarchical ZSM-5, as well as significantly
suppressing the formation of ethylbenzene.
The catalysts were characterized using a transmission
electron microscopy (TEM) (Tecnai G2 F30 S-Twin) and
scanning electron microscopy (SEM) (Hitachi S-4700(II)).
X-ray diffraction (XRD) (SCINTAG X’’ TRA) was oper-
ated using Cu K-radiation (1,542 A) at 30 mA and 40 kV
high voltage source with scanning angle (2h) from 5° to
80°. BET surface area and porous structure of catalysts
were determined by nitrogen physical adsorption–desorp-
tion at 77 K. Nitrogen adsorption isothermal was measured
using a NOVA 1000e surface area analyzer (Quantachrome
Instruments Corp.). The differential thermal analysis (TG–
DTA) (Netzsch STA 449 C) was evaluated using a tem-
perature ramp from 25 to 850 °C with a heating rate of
5 °C/min in oxygen atmosphere.
2 Experimental
2.1 Catalyst Preparation
Hierarchical ZSM-5 was prepared via solvent evaporation
assisted dry-gel route was reported by the author [17, 18, 23].
Themolar compositionofthereactionmixturewasasfollows:
TEOS
AIP
TPAOH
HTS
EtOH
2.3 Catalytic Activity Test
Molar ratio
1
0.0056
0.2
0.05
15.
All evaluation experiments were carried out in a contin-
uous-flow fixed-bed reactor with a stainless steel tube (8-
mm i.d.) at atmospheric pressure. In each test, 0.5 g
catalyst diluted with inert quartz sand in 10 times volume
as that of catalyst was loaded at the middle of the tube
reactor and a thermocouple was positioned in the center
of the catalyst bed in order to monitor the temperature.
The catalyst was first reduced in situ using a temperature
ramp from ambient temperature to 400 °C at 10 °C/min
and maintained at 400 °C for 3 h in H2 flow having a
space velocity of 2,400 h-1. Then benzene and methanol
mixture (B/M molar ratio = 1:1) was fed into the reactor
(WHSV = 2.0 h-1) at 400 °C with a co-feed H2 flow of
40 mL/min. The effluent from the reactor was analyzed
online by the gas chromatography (Fuli GC9790) with a
DB-1 capillary column (30 m 9 0.25 mm 9 1.00 lm)
and a flame ionization detector. In order to ensure all of
the products were in gas phase, the temperature of the
effluent line was maintained at 200 °C by heating belt.
Conversion of benzene, utilization of methanol and
selectivity to toluene, xylene and ethylbenzene were
defined as follows:
where TEOS tetraethylorthosilicate, AIP aluminum iso-
propoxide, TPAOH tetra-n-propylammonium hydroxide,
HTS hexadecyltrimethoxysilane, EtOH ethanol
Firstly, AIP, 1/2 of the EtOH and TPAOH were mixed
together and stirred until AIP was completely dissolved.
Then TEOS, HTS and the rest of the EtOH were added to
the solution and stirred briefly to obtain a solid gel. Then
the solid gel was placed into stainless steel autoclave
equipped with a PTFE liner and small amount of water.
The synthesis temperature and time were 180 °C and 72 h,
respectively. The resulting solid material was obtained by
filtration, then dried at 110 °C and calcined at 550 °C for
7 h (heating rate of 10 °C/min).
Pt modified hierarchical ZSM-5 catalysts with a nominal
Pt loading of 0.01–0.1 wt% were prepared by impregnating
method. The hierarchical ZSM-5 was firstly tableted, cru-
shed and screened to the particles with diameters of
0.45–0.90 mm. The catalyst was then impregnated with
quantitative amounts of aqueous solution of H2PtCl6ꢀ6H2O
at ambient temperature for 24 h before dried at 110 °C
overnight. The dried sample was later divided into two
parts. One part was calcined in air at 500 °C for 7 h
(heating rate of 10 °C/min) and reduced in hydrogen at
400 °C for 2 h (heating rate of 10 °C/min). The other part
was directly reduced in hydrogen at 400 °C for 2 h (heat-
ing rate of 10 °C/min).
Conversion of benzene
benzene in feed ꢁ benzene in product
¼
ꢂ 100%
ꢂ 100%
ꢂ 100%
benzene in feed
nðtolueneÞ
nðalkyl aromaticsÞ
nðxyleneÞ
Selectivity of toluene ¼
Selectivity of xylene ¼
Conventional ZSM-5 powder (nSiO2/nAl2O3 = 360)
were purchased from the Catalyst Plant of Nankai Uni-
versity (Tianjin, China). The sample was tableted, crushed
and screened to the size of 0.45–0.90 mm, and then cal-
cined in air at 550 °C for 7 h.
nðalkyl aromaticsÞ
nðethylbenzeneÞ
Selectivity of ethylbenzene ¼
ꢂ 100%
nðalkyl aromaticsÞ
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Catalytic Activity of Pt Modified Hierarchical ZSM-5 Catalysts
hydrolyzed HTS (with hydrophobic tails and hydrophilic
silane head) acted as the surfactant in stabilizing the solu-
tions. With the evaporation of ethanol, the mixture became
more concentrated, accompanying a self-assembly and
condensation process of the moieties, and eventually pro-
duced a solid dry gel which was made of particles separated
by the organic assemblies from HTS. In the hydrothermal
treatment, these organic assemblies inhibited the growth of
crystals as well as created additional porosities via occu-
pying a certain space between the walls of zeolitic.
Figure 3 shows the N2 adsorption/desorption isotherms
and the pore size distribution corresponding to synthesized
hierarchical ZSM-5 zeolites with nSiO2/nAl2O3 ratio of
360. The samples exhibited the characteristic features of
type-IV isotherm, an obvious hysteresis loop which is
associated with capillary condensation taking place in
mesopores and the limiting uptake over a range of high
p/p0 [24] (Fig. 3a). Moreover, the sizes of mesopores were
all about 4 nm according to the pore size distribution
(Fig. 3b).
3 Results and Discussion
3.1 Catalyst Characterization
Figure 1 presents XRD patterns of conventional ZSM-5 and
hierarchical ZSM-5 catalysts. Both types of catalysts dis-
played two distinct diffraction peaks in 8°–10° and 20°–25°
(2h) ranges, all consistent with the typical pattern of MFI
crystal structure. The SEM images in Fig. 2a and b clearly
show the crystal morphology of conventional ZSM-5 zeo-
lites and synthesized hierarchical ZSM-5 zeolites. As shown
in Fig. 2a, the crystals of conventional ZSM-5 are hexago-
nal-shaped crystal rods, while the shape of hierarchical
ZSM-5 crystals is roughly ellipsoidal aggregates with a size
of about 200–300 nm (Fig. 2b). A high-magnification TEM
image (Fig. 2c) indicated that the aggregates were made of
nanoparticles ranging from 10 to 50 nm and a higher reso-
lution micrograph (Fig. 2d) further reveals that the nano-
particles were crystalline in nature. It should be noted that
the nanoparticles in the same aggregate exhibited the same
direction of lattice fringes, indicating that these nanoparti-
cles were crystallographically aligned like a single crystal.
This phenomenon was quite similar to the result reported by
Zhu et al. [23]. The difference morphology of zeolite could
be attributed to different synthesis conditions of ZSM-5
zeolites. It has long been recognized that when different
synthesis conditions were used, the growth orientation of
crystals was different, resulting in the crystals with different
morphologies. For the synthesis of hierarchical ZSM-5, the
formation mechanism had detailed description by Zhu et al.
[23]. Organic aluminum source, organic silicon source, HTS
and TPAOH were dissolve in ethanol to hydrolyze to form
small hydrophilic moieties (such as AlOx- and SiOx-). The
3.2 Catalytic Tests
3.2.1 Catalytic Alkylation of Benzene with Methanol
over Conventional ZSM-5 and Hierarchical ZSM-5
The catalytic performance of synthesized hierarchical
ZSM-5 was investigated in benzene alkylation with meth-
anol and the results were compared to that of conventional
ZSM-5 with same silica-alumina ratio (nSiO2/nAl2O3 =
360). The presence of alkane and alkene in the reaction
products indicated that the side reaction of methanol had
occurred in methanol alkylation, which was attributed to
the effect of strong acid sites on the external surface of
conventional ZSM-5 [25]. As shown in Table 1, the pre-
sence of methanol was not observed in the products means
that methanol has been completely converted. Moreover,
hierarchical ZSM-5 gave higher contents of alkyl aromatics
and fewer contents of light alkenes and alkanes, suggesting
that hierarchical ZSM-5 could effectively reduce the side
reaction of methanol. Many literatures had investigated the
parameters of conventional ZSM-5 and hierarchical ZSM-5
and compared their catalytic properties for the reactions
such as the alkylation of benzene and the aromatization of
methanol [17, 18, 23, 25–27]. Rownaghi et al. [25] had
observed that as compared to conventional ZSM-5, Nano-
ZSM-5 catalyst gave the highest activity which could be
attributed to the lower diffusion limitation and higher
concentration of strong acid sites on the surface, while
Meso-ZSM-5 gave higher yields of alkyl aromatics and
paraffins and fewer light olefins due to the channel struc-
ture of mesoporos ZSM-5 crystals provided easier access to
the active sites in micropores. Zhu et al. [23] had reported
Fig. 1 XRD patterns of conventional ZSM-5 and hierarchical ZSM-5
catalysts
123

H. Hu et al.
Fig. 2 SEM image of conventional ZSM-5 zeolites (a) and hierarchical ZSM-5 zeolites (b), TEM image of the hierarchical ZSM-5 zeolites
(c, d)
Fig. 3 N2 adsorption/desorption isotherms (a) and pore size distribution (b) of hierarchical ZSM-5 zeolites
that the mesoporosity of hierarchical ZSM-5 was formed
from the voids between crystallites and aggregates and the
hierarchical ZSM-5 exhibited better performance due to the
high connectivity between meso- and micropores which
would facilitate mass transfer and provide more accessible
acidic sites in the zeolites. Deng et al. [18] had found that
with increasing the Si/Al ratios of catalyst, the xylene
selectivity over conventional ZSM-5 did not show
remarkable change while the benzene conversion and
xylene selectivity over hierarchical ZSM-5 increased. They
proposed that the intracrystal diffusion no longer limits the
mass transport process due to the presence of additional
porosity. According to these literatures, we could conclude
that although the parameters (including the primary parti-
cle size, the extra-framework Al species, the properties of
the external surface and so on) were different between
hierarchical ZSM-5 and conventional ZSM-5, the excellent
catalytic activity and xylene selectivity to hierarchical
ZSM-5 mainly attributed to the presence of mesopore
which promoted the diffusion of reactants and products
and provided easier access to the active sites in
micropores.
123

Catalytic Activity of Pt Modified Hierarchical ZSM-5 Catalysts
benzene and ethylene. This result suggested that sup-
pressing the side reaction of methanol would help to reduce
the formation of ethylbenzene.
Table 1 Products content of benzene alkylation with methanol over
conventional ZSM-5 catalyst and hierarchical ZSM-5 catalyst
Components
Content (wt)/%
The stability of hierarchical ZSM-5 and conventional ZSM-
5 was also investigated. As seen in Fig. 5, the stability of
hierarchical ZSM-5 (320 h) is obviously higher than that of
conventional ZSM-5 (80 h). There are two reasons to explain
the higher stability of hierarchical ZSM-5. One relates to the
coke location of hierarchical ZSM-5. Schmidt had reported that
the coke distribution for the microporous ZSM-5 showed a
gradient over the particle and the carbonaceous species mainly
accumulated in the outer particle layers which lead to the rapid
deactivation of the catalyst, while hierarchical ZSM-5 showed
a homogeneous distribution of coke residuals over the whole
particle [28]. The other reason relates to the coke formation
from alkenes by polymerization, so the decease of alkenes
could reduce the coke deposit rate [20]. In addition, comparing
the TG–DTA profiles (Fig. 6) of the hierarchical ZSM-5 cat-
alyst after 320 h reaction on stream with that of the conven-
tional ZSM-5 catalyst after 80 h reaction on stream, the amount
of coke deposit (removed within 200–800 °C) of the hierar-
chical ZSM-5 is obvious lower than that of the conventional
ZSM-5 and this further proves the inference above.
Conventional ZSM-5
Hierarchical ZSM-5
Methane
Ethylene
Ethane
0.40
0.17
0.18
2.45
0.90
0.32
48.22
26.43
3.10
10.3
3.25
4.28
0.10
0.12
0.08
0.59
0.22
0.20
41.84
22.05
1.50
17.86
5.46
9.98
C3
C4
C5
Benzene
Toluene
Ethylbenzene
Xylene
Trimethylbenzene
Other
Reaction conditions: 400 °C, H2, 1 atm, WHSV = 2.0 h-1, methanol
to benzene molar ratio = 1:1
As shown in Fig. 4, the benzene conversion and xylene
selectivity over hierarchical ZSM-5 (48.4 and 33.4 %,
respectively) are relatively higher than those of conven-
tional ZSM-5 catalysts (43.1 and 20.4 %, respectively),
indicating that the acid sites for catalysing the alkylation of
benzene were retained after introducing the mesopores into
zeolite materials. On the other hand, the increase of xylene
selectivity indicated that the existence of mesopores could
promote the diffusion of bulky aromatics in the reactions.
The same phenomenon could be observed in trimethyl-
benzene selectivity (5.6 % for conventional ZSM-5 and
9.0 % for hierarchical ZSM-5). It is noteworthy that the
selectivity to ethylbenzene dropped from 6.2 to 2.8 %
possibly due to the decrease of ethylene in the reaction
products as ethylbenzene was formed by the alkylation of
Above all, hierarchical ZSM-5 catalyst performed
excellently in the alkylation of benzene with methanol.
However, it still could not solve the problem of ethylben-
zene formation.
3.2.2 Pt Modified Hierarchical ZSM-5 Catalysts
for Suppressing the Formation of Ethylbenzene
in Benzene Alkylation with Methanol
The effect of Pt modification on the catalytic performance
of hierarchical ZSM-5 catalysts in benzene alkylation with
Fig. 4 Catalytic performances of conventional ZSM-5 and hierar-
chical ZSM-5 in benzene alkylation with Methanol
Fig. 5 Stability of conventional ZSM-5 and hierarchical ZSM-5 in
benzene alkylation with methanol
123

H. Hu et al.
Fig. 8 Catalytic performance of Pt modified hierarchical ZSM-5
prepared by direct reduction method
Fig. 6 TG-DTA profiles of the catalysts after successive reaction
time: conventional ZSM-5, 80 h; hierarchical ZSM-5, 320 h
noted that the selectivity to ethylbenzene gradient decrease
from 2.8 to 0.15 % with the increasing content of Pt
(0–0.1 wt%) in modified hierarchical ZSM-5 by calcina-
tions (Fig. 7). As for the modified hierarchical ZSM-5
catalysts prepared by direct reduction method, loading with
0.01 wt% Pt could notably decrease the selectivity to eth-
ylbenzene to 0.18 % (Fig. 8). Obviously, direct reduction
method was helpful to reduce the required amount of Pt on
the modified catalyst. In addition, although many litera-
tures [29–32] had reported that Pt/ZSM-5 could catalyze
the hydrogenation of aromatic hydrocarbons, the hydro-
genation of benzene was not occurred due to that the high
temperature and low pressure used in this work was
unfavorable for this reaction.
methanol was shown in Figs. 7 (using calcination method)
and 8 (using direct reduction method). We could see that Pt
modified catalysts prepared by calcination or direct
reduction method both could effectively reduce the selec-
tivity to ethylbenzene and improve the selectivity to
xylene. According to our previous report, the hydrogena-
tion of ethylene to ethane could restrain the alkylation of
benzene with ethylene to form ethylbenzene [11]. The
change of ethylene and ethane content in the products
(Fig. 9) clearly showed that Pt modified catalysts could
effectively decrease the content of ethylene and increase
the content of ethane. Therefore, we concluded that the
high suppressing of ethylbenzene formation on Pt modified
hierarchical ZSM-5 could be attributed to the hydrogena-
tion of ethylene to ethane on Pt clusters. It should be noted
that the decrease of ethane content in the products of 0.05
and 0.1 wt% Pt modified catalysts prepared by direct
reduction was due to the decreasing of benzene conversion
improved the content of benzene in the products. We also
As mentioned earlier, calcinations could influence the
size of Pt metal particle, which in turn influenced the
hydrogenation activity of Pt [20]. When the supported
Fig. 7 Catalytic performance of Pt modified hierarchical ZSM-5
prepared by calcination method
Fig. 9 Content of ethylene and ethane in the products of Pt modified
hierarchical ZSM-5 prepared by different method
123

Catalytic Activity of Pt Modified Hierarchical ZSM-5 Catalysts
precursor was calcined in air and then reduced in hydrogen
(calcination method), aggregated large particles were
observed in TEM (Fig. 10a, b). Moreover, the degree of
sintering increased with increasing the content of Pt from
0.01 to 0.1 wt%. In contrast, when the supported precursor
was directly reduced (direct reduction method), aggregated
large particles was no longer observed even when
increasing the content of Pt to 0.1 wt% (Fig. 10c, d). This
outcome could be ascribed to that chloroplatinic acid pre-
cursor after being calcined in air formed Pt oxides which
enabled the growth of platinum particles through vapor
phase transport [33, 34]. While employing direct reduction,
the hydrogen atmosphere suppressed the generation of
volatile Pt oxides and formed Pt particles without the
migration of Pt species [22]. In addition, removing the
chlorine is necessary as chlorine would cause metal poi-
soning and surface corrosion [35]. However, compared to
direct reduction in hydrogen, chlorine was retained at much
higher temperatures during calcinations in air [33]. More-
over, high temperatures will cause more particles sintering
due to that small metal particles have lower melting point
than bulk metal. Considering the suppressing ability of Pt/
ZSM-5 for the formation of ethylbenzene and the content
of Pt, we can draw a safe conclusion that direct reduction
could improve the utilization efficiency of Pt by avoiding
the Pt particle sintering and the resulted Pt particles with
smaller sizes were conducive to hydrogenate ethylene into
ethane. On the other hand, the well dispersed Pt particles
would cover more acid sites of the catalyst, which caused
the decrease of benzene conversion over the directly
reduced catalyst (48.4–39.0 %) higher than that over the
calcined catalyst (48.4–45.5 %).
Although the reaction temperature (400 °C) was lower
than the calcination temperature (500 °C), the Pt particles
sintering of Pt modified hierarchical ZSM-5 prepared by
direct reduction might still occur which could further
influence the hydrogenation activity of Pt. Thus, investi-
gating the stability of Pt modified hierarchical ZSM-5
prepared by direct reduction is necessary. 0.02 wt% Pt
modified hierarchical ZSM-5 could reduced the selectivity
to ethylbenzene to a relatively low amount and at the same
times remained high activity (Fig. 8), so we investigated it
in the following experiment. It could be seen that the
selectivity to ethylbenzene maintains under 0.1 %, benzene
conversion maintains 48–51 % and xylene selectivity
maintains about 35.5 % during 50 h reaction on stream
(Fig. 11). The hold of the performance for suppressing the
formation of ethylbenzene indicated that the Pt particles
Fig. 10 TEM images of Pt modified catalysts hierarchical ZSM-5 by different method: a 0.01 wt% Pt, calcination, b 0. 1 wt% Pt, calcination;
c 0.01 wt% Pt, direct reduction, d 0.1 wt% Pt, direct reduction
123

H. Hu et al.
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Pt modified hierarchical ZSM-5 successfully combined the
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Employing direct reduction method could improve the
utilization of Pt by avoiding the migration of Pt particles
causing particle sintering. These technologies would help
to improve the commercial viability of the process of
benzene alkylation with methanol into toluene and xylene.
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Acknowledgments This work was financially supported by
National Natural Science Foundation of China (NSFC-21476207 and
211360001), National Basic Research Program of China (973 Pro-
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