Synthesis of ZSM-5 aggregates by a seed-induced method and catalytic performance in methanol-to-gasoline conversion

Article abstract of DOI:10.1016/j.crci.2016.07.005

A seed-induced method was applied to synthesize a series of ZSM-5 aggregates using silicalite-1 (S-1) as seeds. The samples were characterized by several techniques and studied in the methanol-to-gasoline (MTG) conversion reaction in a continuous fixed-bed reactor. The results showed that the molar ratios of Na₂O/SiO₂ and H₂O/SiO₂ in the precursors had great effects on the physicochemical and catalytic properties. The ZSM-5 aggregates with the highest crystallinity, external surface areas, micropore volumes, mesopore volumes and small primary crystal and aggregate sizes, obtained with the molar composition of 0.12Na₂O–1SiO₂–0.02Al₂O₃–25H₂O-0.01Seed in the precursor, exhibited the highest stability.

Full text of DOI:10.1016/j.crci.2016.07.005

C. R. Chimie xxx (2016) 1e10
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Full paper/M eꢀ moire
Synthesis of ZSM-5 aggregates by a seed-induced method and
catalytic performance in methanol-to-gasoline conversion
*
Fanjun Meng, Yaquan Wang , Shougui Wang, Xiao Wang, Shuhai Wang
Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical Engineering & Technology, Tianjin
University, Tianjin 300072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A seed-induced method was applied to synthesize a series of ZSM-5 aggregates using
silicalite-1 (S-1) as seeds. The samples were characterized by several techniques and
studied in the methanol-to-gasoline (MTG) conversion reaction in a continuous fixed-bed
Received 28 April 2016
Accepted 18 July 2016
Available online xxxx
2 2 2 2
reactor. The results showed that the molar ratios of Na O/SiO and H O/SiO in the pre-
cursors had great effects on the physicochemical and catalytic properties. The ZSM-5 ag-
gregates with the highest crystallinity, external surface areas, micropore volumes,
mesopore volumes and small primary crystal and aggregate sizes, obtained with the molar
Keywords:
ZSM-5 aggregates
Na
O/SiO
MTG
2
O/SiO
2
2 2 2 3 2
composition of 0.12Na Oe1SiO e0.02Al O e25H O-0.01Seed in the precursor, exhibited
H
2
2
the highest stability.
©
2016 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
When contacting with ZSM-5 zeolites, methanol is con-
verted into hydrocarbons with the similar fractions to
Gasoline, as the most important transportation fuel in
traditional gasoline. In 1977, Exxon Mobil successfully
commercialized the methanol to gasoline process with
ZSM-5 zeolites as catalysts [12].
the world, is conventionally obtained from the cracking of
crude oil. However, as the increase of energy demands and
the depletion of crude oil reservoirs, the exploration of new
routes to obtain gasoline from some substitutions of crude
oil becomes a hot and endless topic [1,2]. Methanol-to-
gasoline (MTG) has been proven to be an ideal process to
offer a desired solution for the production of gasoline
because methanol can be easily manufactured from
biomass, natural gas and coal [3,4].
Since it was first reported by Mobil in 1972 [13], the
hydrothermal syntheses of ZSM-5 zeolites with one or
more organic templates have been widely studied
[6,14e19]. Many efforts have also been made in attempting
to generate mesopores in the zeolites by using mesoscale
templates, such as cationic polymers, amphiphilic organo-
silane, etc., and many of them have been successfully
applied in the reaction of MTG [5,20e22]. However, the use
of organic templates is not appreciated in the point view of
both the economics and the environmental protection.
Moreover, elevated temperature is needed to remove the
templates, which results in the aggregation of the primary
crystals [23]. To obtain hierarchical ZSM-5 with mesopores,
the method of post-treatment such as dealumination or
desilication was also used [24,25]. However, they are
generally limited by the partial damage of the framework
and sometimes the blockage of the micropores by the
A ZSM-5 zeolite is a kind of crystalline aluminosilicate
which has two interconnected pore systems, i.e. zigzag
channels (0.53 nm ꢀ 0.56 nm) and straight channels
(
[
0.51 nm ꢀ 0.55 nm), creating a three-dimensional network
5]. ZSM-5 zeolites have exhibited excellent catalytic
properties in MTG due to the strong acid sites and the
regular nanochannel system [6e9] and have been proved to
be the most proper catalyst in the MTG process [10,11].
*
Corresponding author.
E-mail address: yqwang@tju.edu.cn (Y. Wang).
extra-framework
residue
generated
during
the
http://dx.doi.org/10.1016/j.crci.2016.07.005
631-0748/© 2016 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1
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2
F. Meng et al. / C. R. Chimie xxx (2016) 1e10
hydrothermal treatment and chemical extraction [26]. To
overcome the disadvantages of the methods for obtaining
hierarchical ZSM-5 mentioned above, several strategies for
preparing ZSM-5 zeolites with a hierarchical structure by
seed-induced methods without organic templates or post-
treatments have been proposed. The ZSM-5 zeolites syn-
thesized by a seed-induced method have been applied in
many reactions, for example, the alkylation of naphthalene
(600 rpm) at ambient temperature for 3 h, and then
transferred into a Teflon-lined autoclave. The hydrothermal
ꢁ
treatment was performed at 105 C for 48 h [36e38]. The
obtained S-1 nanocrystals suspended in the mother liquor
were directly used as the seed.
2.3. Synthesis of ZSM-5 by the seed-induced method
[27],
cyclohexene
hydration
[28],
methanol-to-
The synthesis of ZSM-5 with the molar composition
hydrocarbon [29], methanol-to-olefin [30], propane dehy-
drogenation [31,32], etc. In some reactions above, excellent
catalytic performances of the hierarchical ZSM-5 were
obtained because the ZSM-5 zeolites were aggregates of
nanosized particles which reserve the advantages of low
diffusion limitation, large external surface area, large
xNa
0.12, 0.14, 0.16, y ¼ 25, 35, 45, 55) was started by adding a
3 M NaOH solution to a 0.3 M NaAlO solution, stirred at
2
Oe1SiO
2
e0.02Al
2
O
3
eyH
2
Oe0.01Seed (x ¼ 0.08, 0.10,
2
room temperature for 15 min before the S-1 suspension was
introduced. Then the silica gel solution was slowly dropped
into the mixture under vigorous stirring (600 rpm). The
solution was stirred at room temperature for another 2 h
before being transferred into a Teflon-lined autoclave and
mesopore volume, etc. The influence of alkalinity (Na
2
O/
H
2
O) and aging conditions on the crystal morphology,
ꢁ
crystallinities and elemental component of ZSM-5 in the
seed-induced method has been reported [33,34]. However,
the zeolites were not evaluated for any catalytic reactions.
Although Zhang et al. [35] obtained mesopore-containing
ZSM-5 zeolites by using a coke-deposited spent zeolite
catalyst. In the methanol to propylene reaction, the refab-
ricated ZSM-5 catalyst exhibited much higher propylene
selectivity and a longer catalytic lifetime. To the best of our
knowledge, few have reported the reaction of MTG over
ZSM-5 synthesized by the seed-induced methods.
hydrothermally treated at 170 C for 10 h. After cooling, the
product was recovered by filtration and washed with
deionized water until the pH reached 8 [23,38e40]. Finally,
the as-synthesized samples were ion-exchanged with 0.5 M
ꢁ
HCl solution at 90 C for 2 h, followed by washing, drying
ꢁ
and calcination at 550 C for 6 h. The samples were denoted
as ZM-x-y, where x and y refer to the molar ratios of Na
2
O/
SiO and H O/SiO in the precursors, respectively.
2
2
2
2.4. Characterization
In this work, hierarchical ZSM-5 aggregates were syn-
thesized by a seed-induced method with S-1 crystals as the
The X-ray powder diffraction (XRD) patterns of ZSM-5
seeds. The molar ratios of Na
2
O/SiO
2
and H
2
O/SiO
2
in the
were obtained on a Bruker D8-Focus diffractometer with
ꢁ
synthesis that markedly influenced the properties of the
zeolites were investigated in detail. The samples were
characterized by many techniques and studied in the MTG
reaction. The ZSM-5 aggregates showed excellent catalytic
performance and were correlated with the physicochem-
ical properties.
Cu K
a
radiation at a scanning rate of 8 /min in the 2
q
range
ꢁ
of 5e55 . The relative crystallinities were estimated by
comparing the total intensity of the characteristic peaks
ꢁ
ꢁ
(between 2
q
¼ 22 and 24 ) with the sample with the
highest crystallinity. Scanning electron microscopy (SEM)
images were recorded using a Hitachi S-4800 microscopy.
N
2
adsorption and desorption isotherms were measured at
ꢁ
2
. Experimental
ꢂ196 C on a Micromeritics TriStar 3000 instrument. SiO
2
/
Al
2
O
3
ratios were measured on a Bruker-axs S4 Explorer X-
-TPD measure-
2
.1. Materials
ray fluorescence spectroscope (XRF). NH
3
ments were carried out by using a TP-5076 instrument
supplied by Tianjin Xianquan Co. to measure the amounts
of acid sites and acid strength of the catalysts. The amounts
of the deposited materials after the reaction were deter-
Tetraethyl orthosilicalite (TEOS, AR) was purchased
from Tianjin Kemiou Chemical Reagent Co., China. Sodium
aluminates (NaAlO , AR), sodium hydroxide (NaOH, AR),
methanol (MeOH, AR) and commercial HZSM-5 (SiO
Al
¼ 50) were purchased from Tianjin Guangfu Chem-
ical Reagent Co., China. Silica gel of 40% SiO in water (AR)
2
2
/
mined with
a Shimadzu TGA-50 thermogravimetric
2 3
O
analyzer. The samples were combusted in the flow of air
ꢁ
ꢁ
2
from room temperature to 700 C with a ramp of 10 C/min
ꢁ
was purchased from Qingdao Haiyang Chemical Reagent
Co., China. Hydrochloric acid (HCl, AR) was purchased from
Tianjin Jiangtian Chemical Reagent Co., China. All pur-
chased chemicals were used without further purification.
Tetrapropyl ammonium hydroxide was prepared through
ion-exchange of tetrapropyl ammonium bromide.
and the weight loss between 300 and 700 C was attributed
to the burning of the deposit [40].
2.5. Catalyst evaluation for MTG
The MTG reaction was carried out at atmospheric
pressure in a quartz tube fixed bed reactor with an internal
diameter of 10 mm and a total length of 370 mm. In a
typical run, the reactor was packed with 0.5 g catalyst
2
.2. Preparation of seed S-1
S-1 was synthesized by a hydrothermal method. TEOS
which was sieved to 425e850 mm and diluted with quartz
was added dropwise into an aqueous solution of 15 wt %
TPAOH. The molar composition was 100SiO
e400EtOHe24TPAOHe2400H O in which EtOH was pro-
duced from the hydrolysis of TEOS. The mixture was stirred
particles of the same sizes in a 1:2 volume ratio. The
ꢁ
2
-
catalyst was pretreated in situ at 400 C for 2 h with a
ꢁ
3
2
heating rate of 10 C/min in the flow of 50 cm /min N
2
.
Then liquid methanol was fed into the reactor at a weight
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F. Meng et al. / C. R. Chimie xxx (2016) 1e10
3
ꢂ1
hourly space velocity (WHSV) of 10 h with a micropump.
As shown in Fig. 1A, with the increase of Na
tios in the precursors, the peak intensities of the obtained
2
O/SiO
2
ra-
ꢁ
The reaction temperature was maintained at 400 C. The
products were separated with an ice-cooled condenser. The
gas products were analyzed on a gas chromatograph
ZSM-5 first increased and then decreased. For the sample of
ZM-0.08-25, the low ratio of Na O/SiO means that less Na
2 2
þ
(
(
GCSP-3420A) equipped with a flame ionization detector
m) capillary
ions were introduced into the precursor species. According
to the literature [34,41], in obtaining MFI type zeolites, Na
ions play a very important role which can direct the MFI
structure, stabilize the formation of structural submits and
favor the formation of secondary building units needed for
the nucleation and the crystallization of ZSM-5 zeolites.
þ
FID) and a KB-PLOT Q (50 m ꢀ 0.32 mm ꢀ 10
m
column. The liquid products were analyzed on another gas
chromatograph (GCSP-3420A) equipped with an FID and an
SE-54 (30 m ꢀ 0.25 mm ꢀ 0.33
mm) capillary column. The
methanol conversion was calculated by the following
equation:
2 2
Due to the low Na O/SiO ratio of 0.08, ZM-0.08-25 did not
crystallize completely under the synthesis conditions (hy-
drothermal treatment at 170 C for 10 h), therefore the
methanol conversionð%Þ ¼ ðMin ꢂ Mout ꢂ 2MDME;out
Þ
ꢁ
ꢀ
(a)
M
in ꢀ 100
intensities of the peaks were weak. At the same time, the
ꢂ
dissolution of silica by OH ions is an important process,
where Min, Mout and MDME,out denoted the amounts of
pumped in, unconverted methanol and the produced
dimethylether during the reaction, respectively.
The naming conversions were used in calculating the
product selectivities: C1e4 represented the gaseous hydro-
carbons with carbon numbers ranging between 1 and 4;
TMB referred to trimethylbenzene; C5þ was the sum of
hydrocarbons with carbon numbers ꢃ5 (benzene, toluene,
xylene and TMB were excluded). The product selectivities
were calculated using the following equation:
ꢂ
and more OH ions in the precursors lead to the easy for-
mation of aluminasilicate and thus the growth of crystals
[
42]. In the precursors of ZM-0.08-25, also due to the low
ꢂ
concentration of OH ions, silica might not have been
completely dissolved by OH ions under the given condi-
ꢂ
tions, which contributed to the appearance of amorphous
materials in the end products. So, with the increase of the
Na O/SiO ratios in the precursors, the peak intensities of
2 2
the obtained ZSM-5 first increased. However, too many
ꢂ
OH ions make a deeper depolymerization of alumi-
nasilicate occur [42]. Thus the degree of the crystallization
product selectivityð%Þ ¼ M
i
=MHC ꢀ 100
(b)
decreased with further increase of the Na
shown in Fig. 1B, the peak intensities of the samples
decreased gradually with increasing the H O/SiO ratios in
the precursors. This should be ascribed to the decrease of
2 2
O/SiO ratios. As
where M was the amount of the product to be calculated
i
and MHC was the total amount of hydrocarbon products.
2
2
þ
ꢂ
Na and OH ion concentrations by adding more H
synthesis.
2
O in the
3
. Results and discussion
SEM was applied to investigate the morphology and
particle sizes of the samples as well as the crystals
combined in the aggregates. Fig. 2A shows that the seed
S-1 had an irregular sphere morphology with the crystal
sizes of ca. 150 nm. As seen in Fig. 2BeF, typical shapes of
both the aggregates and the crystals strongly depended
3
.1. Characterization of samples with different ratios of Na O/
2
2 2 2
SiO and H O/SiO
XRD patterns of ZSM-5 with different ratios of Na
2
O/
SiO
2
and H O/SiO in the synthesis mixture are presented
2
2
in Fig. 1. Five typical diffraction peaks of the MFI framework
appeared without other peaks, confirming a high purity of
the products. The relative crystallinities, with the highest of
ZM-0.12-25 being taken as 100%, are shown in Table 1.
on the Na
increasing the Na
aggregate size decreased from 1
the primary crystal size first decreased from ca. 700 nm
2
O/SiO
2
ratios in the synthesis mixture. With
O/SiO ratios from 0.08 to 0.16, the
m to 0.6 m; however,
2
2
m
m
2 2 2 2
Fig. 1. XRD patterns of ZSM-5 with different ratios of Na O/SiO and H O/SiO in the synthesis mixture.
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4
F. Meng et al. / C. R. Chimie xxx (2016) 1e10
Table 1
Textural properties of ZSM-5 with different ratios of Na
2
O/SiO
2
and H
2
2
O/SiO .
2
3
Sample
Relative crystallinity
Surface area (m /g)
Pore volumes (cm /g)
a
b
b
c
b
d
S
BET
S
Micro
S
Ext
V
Total
V
Micro
V
Meso
ZM-0.08-25
ZM-0.10-25
ZM-0.12-25
ZM-0.14-25
ZM-0.16-25
ZM-0.12-35
ZM-0.12-45
ZM-0.12-55
56
79
100
82
70
88
76
65
287
321
330
310
305
322
310
262
224
248
252
246
251
249
246
215
63
74
78
64
54
72
64
47
0.158
0.179
0.193
0.180
0.162
0.184
0.165
0.145
0.103
0.113
0.117
0.113
0.116
0.118
0.113
0.099
0.055
0.066
0.076
0.067
0.046
0.066
0.052
0.046
a
Calculated by the BET method in the p/p
Calculated using the t-plot method.
Calculated by measuring the amount of adsorbed nitrogen at p/p
0
range of 0.01e0.1.
b
c
0
¼ 0.99.
d
Calculated using VTotalꢂVMicro
.
Fig. 2. SEM images of (A) S-1, (B) ZM-0.08-25, (C) ZM-0.10-25, (D) ZM-0.12-25, (E) ZM-0.14-25, (F) ZM-0.16-25, (G) ZM-0.12-35, (H) ZM-0.12-45, and (I) ZM-0.12-
5.
5
of ZM-0.08-25 to ca. 200 nm of ZM-0.14-25, and then
increased to ca. 500 nm of ZM-0.16-25. With increasing
200e400 nm nanocrystals were formed in ZM-0.12-25
without impurities.
Apart from the contribution of Na ions to the nucle-
ation and crystal growth [34,41,43], OH ions could
þ
the Na
2
O/SiO
2
ratios, the coffin crystals in ZM-0.08-25
ꢂ
gradually disappeared and became small spherical nano-
crystals. At the same time, the crystallization did not
complete and there were some amorphous materials be-
tween the zeolite crystals. The sample with the smallest
nanocrystals of 80 nm appeared in ZM-0.14-25. The ag-
gregates with uniform sizes of ca. 800 nm composing of
dissolve the surfaces of S-1 seeds and release some in-
gredients which could be the nuclei for the formation of
small crystals [42]. At the end, the seeds were attached by
small particles due to the high surface Gibbs free energy
ꢂ
[27]. With the continuous increase of OH ions, some of the
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F. Meng et al. / C. R. Chimie xxx (2016) 1e10
5
small crystals might be dissolved and larger crystals were
obtained as shown in the SEM images of ZM-0.14-25 and
ZM-0.16-25. This phenomenon could be ascribed to the
reason that zeolite crystallization, being similar to other
crystallization processes, was accompanied by Ostwald
ripening, which is a spontaneous and natural process, and
can minimize the surface free energy of crystals by dis-
solving small crystals and contribute to large crystal growth
The N
2
adsorption/desorption isotherms of the samples
O/SiO and H O/SiO are pre-
with different ratios of Na
2
2
2
2
sented in Fig. 3, and the corresponding textural properties
are also listed in Table 1. All the samples exhibited type IV
isotherms with hysteresis loops. The hysteresis loops of
ZM-0.08-25 and ZM-0.16-25 were slightly discernable. The
hysteresis loops appearing in the isotherms at the relative
0
pressure of p/p > 0.5 were usually associated with the
[
22,44,45]. In addition, it should be recalled that the
capillary condensation taking place in the mesopores. The
shapes of the hysteresis loops displayed an H3 type (loosely
packed pores), confirming the formation of mesopores
resulted from the voids constructed upon the aggregation
of individual crystals [27,28,31].
mechanism of the crystallization was different from that
suggested by Ren et al. who synthesized well faceted ZSM-
5
and put forward a seed surface crystallization (SSC)
mechanism [38]. In this work, the seeds might be dissolved
releasing some ingredients which induced the growth of
small crystals attached on the seeds, and the mechanism
was similar to that of Xue et al. [26]. To track the structural
changes in the synthesis, the sample with the same com-
ponents of ZM-0.12-25 was hydrothermally crystallized for
different times. The SEM images (in Fig. S1) showed that
after crystallization for 5 h, there were some amorphous
materials attached on the surfaces of S-1. With the pro-
longing of the crystallization time, the aluminasilicate
transformed into a zeolite and after 7 h the aggregates
appeared. After crystallization for 10 h, the products were
dominated by zeolite aggregates consisting of clear primary
crystals, while no amorphous phase could be observed.
The SEM images of samples with different molar ratios
2 2
With increasing the Na O/SiO ratios, the BET specific
surface areas, micropore surface areas, mesopore surface
areas, total volumes, micropore volumes and mesopore
volumes generally first increased and then decreased, and
2 2
at a Na O/SiO ratio of 0.12, the peak values appeared. The
variations of the values were in accordance with their N
2
adsorption/desorption isotherms and could be ascribed to
the differences of the sample morphologies. With the in-
crease of the Na O/SiO ratios, the primary crystal sizes
2 2
decreased and then increased; therefore the amounts of
voids constructed upon the aggregation of individual
crystals first increased and then decreased, which led to the
variations of isotherms and the corresponding textural
2 2
properties. With the increase of H O/SiO ratios, all the
of H
2
O/SiO
2
(25, 35, 45 and 55) are presented in Fig. 2D, G,
O/SiO ratios,
samples exhibited isotherms of type IV with H3 type hys-
teresis loops; the hysteresis loops became smaller and
smaller; the BET specific surface areas, micropore surface
areas, mesopore surface areas, total volumes, micropore
volumes and mesopore volumes all decreased gradually. All
the results above could be ascribed to the increase of the
aggregates and primary crystal sizes in the four samples.
XRF analyses of all the samples are shown in Table 2.
H and I, respectively. With increasing the H
2
2
the aggregate sizes increased and the small spherical in-
dividual crystals were replaced by bigger coffin or thick
sheet shaped individual ones. At the same time, there were
some amorphous materials appeared between the aggre-
gates at the H
2
O/SiO
2
molar ratio ꢃ35. Increasing the ratios
þ
of H
2
O/SiO
2
reduced the concentrations of both Na and
ꢂ
OH ions, decreasing the crystal growth rate and gradually
preventing the dissolution of S-1 and/or ZSM-5 crystals;
therefore well-defined large crystals were formed on the
surfaces of the seeds.
With the increase of Na
Al decreased from 48.7 to 37.5. There are two main
factors affecting the SiO
2 2 2
O/SiO ratios, the ratios of SiO /
2 3
O
2
/Al
2
O
3
ratios, i.e. the concentration
ꢂ
þ
ꢂ
of OH ions and Na ions. The OH ions can contribute to
Fig. 3. N
2
adsorption/desorption isotherms of samples with different ratios of Na
2
O/SiO
2
and H
2
O/SiO
2
at ꢂ196 ꢁC.
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6
F. Meng et al. / C. R. Chimie xxx (2016) 1e10
Table 2
ratios in the precursors increased the amounts of both the
weak and the strong acid sites. The numbers of the total
acid sites decreased with the increase of the H O/SiO ra-
tios. The acid sites are caused by the incorporation of the
aluminum into zeolites; therefore the change of the
aluminum contents in the products led to the variations of
the amounts of the total acid sites [46].
Elemental component and acid amounts of the samples.
Samples
Elemental
component
Acid amounts (mmol/g)b
2
2
)a
Total acidity Weak acidity Strong acidity
(
2 2 3
SiO /Al O
ZM-0.08-25 48.7
ZM-0.10-25 46.1
ZM-0.12-25 43.5
ZM-0.14-25 40.4
ZM-0.16-25 37.5
ZM-0.12-35 44.0
ZM-0.12-45 44.2
ZM-0.12-55 46.1
0.602
0.629
0.664
0.716
0.817
0.721
0.503
0.588
0.405
0.419
0.431
0.447
0.553
0.433
0.292
0.433
0.197
0.210
0.233
0.259
0.264
0.288
0.211
0.155
3.2. Catalyst performance in MTG
The methanol conversions and yields of light hydro-
a
carbons (LHs) as a function of time on stream over the
catalysts with different Na O/SiO ratios are given in Fig. 5.
Determined by XRF.
b
Calculated by NH
3
-TPD.
2
2
It should be reiterated that in order to compare the cata-
lysts quickly, the reactions were conducted at high WHSV
(10 h ) [10]. ZM-0.08-25 had the lowest stability. With
the dissolution of amorphous silica in the synthesis
ꢂ
ꢂ1
mixture. With the increase of Na
2
O/SiO
2
ratios, more OH
ions were introduced into the mixture, which facilitated
the dissolution of more amorphous silica and then the
crystallization rate. Hence the SiO /Al O ratios of the
2 2 3
increasing the Na
bility of the samples increased. At a Na
the sample ZM-0.12-25 had the highest stability. When the
Na O/SiO ratios were increased further, the stability of the
samples decreased. The characterization results showed
that compared with the other catalysts, ZM-0.12-25 had
relatively small aggregates and primary crystals, the high-
est external surface areas and the largest mesopore vol-
umes, which could provide easy accessibility of the
reactants to the acid sites [47e52] and reduce the diffusion
limitations for large molecules, avoiding severe coking.
Fig. 6A depicts the thermalgravity analysis (TGA) profiles of
2
O/SiO
2
ratios in the synthesis, the sta-
2
O/SiO ratio of 0.12,
2
samples increased. However, enhancing the Na
tios means that more Na ions are also introduced into the
synthesis mixture. As charge compensators, Na ions can
2
O/SiO
2
ra-
2
2
þ
þ
keep the balance of the charges for the framework of ZSM-5
and facilitate the introduction of aluminum into the
framework [34]. Therefore, with the increase of Na
ratios, the amounts of aluminum in the end products
should increase. In general, as the Na O/SiO ratios
increased, the ratios of SiO /Al decreased. The higher
the H O/SiO ratios in the precursors, the higher the SiO
Al ratios of the samples. It can be seen in the SEM im-
ages (Fig. 2) that, with the increase of H O/SiO ratios, there
2 2
O/SiO
2
2
2
2 3
O
2
2
2
/
the samples with different Na
action time of 12 h. With increasing the Na
2
O/SiO
2
ratios after the re-
O/SiO ratios,
2
O
3
2
2
2
2
the amounts of coke formed in the catalysts first decreased
and then increased. ZM-0.12-25 had the highest resistance
to coking, which could be attributed to the highest stability.
Moreover, its highest crystallinity might also contribute to
the best performance.
were increasing amounts of amorphous materials between
the aggregates, which might be amorphous silicate;
therefore the SiO
slightly.
2 2 3
/Al O ratios detected by XRF increased
NH
3
-TPD diagrams of the samples are presented in
With increasing the Na
ratios decreased slightly. However, the variation of the
SiO /Al ratios in the zeolite synthesized under the
conditions for ZM-0.12-25 had meager influence on the
performance in MTG (see the Supporting information). So,
the excellent performance of ZM-0.12-25 was not caused by
2 2 2 2 3
O/SiO ratios, the SiO /Al O
Fig. 4, and the related data are listed in Table 2. The TPD
desorption peaks were in the ranges of 193e208 C and
20e440 C which were attributed to the chemical
desorption of NH
ꢁ
2
2 3
O
ꢁ
4
3
from weak and strong acidic sites,
O/SiO
respectively. As shown in Table 2, the increase of Na
2
2
3 2 2 2 2
Fig. 4. NH -TPD spectra of the samples with different ratios of Na O/SiO and H O/SiO .
Please cite this article in press as: F. Meng, et al., Synthesis of ZSM-5 aggregates by a seed-induced method and catalytic per-
formance in methanol-to-gasoline conversion, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.07.005

F. Meng et al. / C. R. Chimie xxx (2016) 1e10
7
Fig. 5. Catalytic performance of the prepared catalysts with different Na
2
O/SiO
2
ratios. (A) The methanol conversions and (B) the LH yields. Reaction conditions:
ꢁ
ꢂ1
.
400 C, 1 atm, and WHSV ¼ 10 h
Fig. 6. TGA profiles of the catalyst samples after undergoing 12 h under the same reaction conditions: 400 C, 1 atm, and WHSV ¼ 10 hꢂ1
ꢁ
.
Fig. 7. Total selectivities of light olefins versus time on stream. Reaction conditions: 400 C, 1 atm, and WHSV ¼ 10 hꢂ1
ꢁ
.
Please cite this article in press as: F. Meng, et al., Synthesis of ZSM-5 aggregates by a seed-induced method and catalytic per-
formance in methanol-to-gasoline conversion, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.07.005

8
F. Meng et al. / C. R. Chimie xxx (2016) 1e10
Fig. 8. Catalytic performance of the prepared catalysts with different H
2
O/SiO
2
ratios. (A) The methanol conversions and (B) the LH yields. Reaction conditions:
ꢁ
ꢂ1
.
400 C, 1 atm, and WHSV ¼ 10 h
the variation of SiO
Na O amount in the synthesis.
As shown in Fig. 5B, all the samples exhibited constant
2
/Al
2
O
3
ratios with the change of the
samples were enough for the reaction at the initial time,
but then decreased due to coking. The total selectivities of
the light olefins (C ], C ] and C ]) with time on stream
2 3 4
2
yields of LHs (ca. 32%) during the stable time on stream, but
decreased when the conversions decreased. The total se-
over the samples are shown in Fig. 7B. The selectivities of
the light olefins over ZM-0.12-25 were the lowest, which
might result from the reason that most of the methanol was
converted to gasoline.
lectivities of the light olefins (ethylene C
2 3
], propylene C ]
and butylene C ]) with time on stream over the samples
4
are shown in Fig. 7A. The selectivities of the light olefins
over ZM-0.12-25 were the lowest, which might result from
the reason that most of the methanol was converted to
gasoline.
Among all the catalysts, ZM-0.12-25 exhibited the best
performance in MTG. Compared with the commercial ZSM-
5 (Fig. S2), the lifetime of ZM-0.12-25 increased by a factor
of 2.
The methanol conversions with time on stream over the
samples with different H O/SiO ratios are shown in Fig. 8A.
2 2
All the samples showed almost the same initial methanol
conversions of 100%. The methanol conversion over ZM-
Table 3 lists the methanol conversions and hydrocarbon
selectivities for the MTG reactions over the synthesized
catalysts at a TOS of 6 h. The product distributions of the
2 2
reactions over the samples with different Na O/SiO and
0
.12-25 remained the highest during the time on stream,
i.e. after 28 h of the reaction the conversion remained at
8%. The increase of the H O/SiO ratios in the synthesis
H
2
O/SiO ratios did not show much significant difference.
2
This might result from the fact that in the first several hours
of the reaction, the active sites, i.e. acid sites, on the surface
of the catalysts were enough for the conversion and were
not the rate-determining factors. The selectivities of the
different light olefins compared at a TOS of 6 h are shown in
Fig. 9. The components of the light olefins were similar, but
the total amount of the light olefins over ZM-0.12-25 was
the lowest, which might result from the reason that most of
the methanol was converted to gasoline.
9
2
2
resulted in the faster deactivations as certified by TGA in
Fig. 6B, which might result from the increase of the zeolite
aggregate and the primary crystal sizes as well as the
decrease of the total surface areas, external surface areas
and mesopore volumes.
As shown in Fig. 8B, during the initial reaction period,
the four samples had the same LH yields (ca. 32%) at the
initial time, but then decreased with the time on stream.
This might result from the fact that the acid sites of the
In the synthesis of the hierarchical ZSM-5 aggregates by
the seed-induced method, samples with different molar
Table 3
Methanol conversion and hydrocarbon distribution (TOS ¼ 6 h, 400 ꢁC, 1 atm, and WHSV ¼ 10 hꢂ1).
Sample
Methanol conversion (%)
Selectivity (%)
Yield of LH (%)
C
1e4
C
5þ
C
bene
C
toluene
C
xylene
C
TMB
ZM-0.08-25
ZM-0.10-25
ZM-0.12-25
ZM-0.14-25
ZM-0.16-25
ZM-0.12-35
ZM-0.12-45
ZM-0.12-55
98.7
99.5
99.9
99.9
99.9
99.9
99.7
99.6
46.7
47.2
46.9
47.6
46.6
47.5
44.5
46.1
26.4
30.7
31.5
31.1
31.9
31.4
31.3
31.1
1.1
1.2
1.5
1.5
1.5
1.4
1.3
1.2
7.7
7.6
7.6
7.2
7.6
7.4
7.2
7.6
12.5
13.1
12.4
12.7
12.3
12.3
12.6
11.3
5.6
6.5
6.6
5.6
5.1
6.2
6.0
5.5
31.3
31.8
31.9
31.5
29.3
31.3
31.8
31.4
Please cite this article in press as: F. Meng, et al., Synthesis of ZSM-5 aggregates by a seed-induced method and catalytic per-
formance in methanol-to-gasoline conversion, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.07.005

F. Meng et al. / C. R. Chimie xxx (2016) 1e10
9
Fig. 9. Selectivities toward the different light olefins compared at TOS ¼ 6 h for the samples. Reaction conditions: 400 ꢁC, 1 atm, and WHSV ¼ 10 hꢂ1
.
ratios of SiO
2
/Al
2
O
3
and seed/SiO
2
had also been studied.
Appendix A. Supplementary data
The results in Fig. S3e6 in the supplements showed that
the samples with different molar ratios of SiO
seed/SiO
in the MTG reaction. However, the ratios of Na
O/SiO
discussed above.
2
/Al
2
O
3
and
and
Supplementary data related to this article can be found
at http://dx.doi.org/10.1016/j.crci.2016.07.005.
2
had meager effects on the catalytic performance
2
O/SiO
2
H
2
2
had pronounced effects on the final products as
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formance in methanol-to-gasoline conversion, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.07.005