Conversion of Methanol to Aromatic-Rich Gasoline over High-Efficiency Bifunctional Catalysts: Green Synthesis of GaZSM-5 Zeolites via Dry-Gel Conversion Strategy

Article abstract of DOI:10.1134/S1070427220010140

Nanoscale ZSM-5 (AlZ5-D) and Ga-substituted ZSM-5 zeolites ((Al,Ga)Z5-D and GaZ5-D) were prepared via green dry-gel conversion strategy. For comparison, ZSM-5 zeolite (AlZ5-T) was also prepared by traditional hydrothermal method. The effects of synthesis method and Ga introduction on the physicochemical property and catalytic behavior of methanol to gasoline (MTG) were investigated. The results showed that the aromatics selectivity decreased while C5+ alkanes selectivity increased for AlZ5-D sample compared with that of AlZ5-T, which was attributed to the significantly reduced density of Br?nsted acid sites and weakened strong acidity. After Ga introduction, the acid strength of samples further weakened due to the formation of Si—OH—Ga bridge hydroxyl group by insertion of Ga into the zeolite framework, inhibiting the cracking reaction and, thus increasing the gasoline selectivity. Also, the extra-framework Ga species, especially in the form of GaO⁺ in GaZ5-D sample, showed high dehydrogenation activity, leading to the highest selectivity of aromatics and gasoline.

Full text of DOI:10.1134/S1070427220010140

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2020, Vol. 93, No. 1, pp. 127−136. © Pleiades Publishing, Ltd., 2020.
CATALYSIS
Conversion of Methanol to Aromatic-Rich Gasoline
over High-Efficiency Bifunctional Catalysts: Green Synthesis
of GaZSM-5 Zeolites via Dry-Gel Conversion Strategy
a
b
c
b
b
b
X. F. Su , Y. L. Zhang , A. L. Maximov , K. Zhang , Q. Xin , C. Q. Feng ,
a,b,d
b,
X. F. Bai , and W. Wu *
a
College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin, 150001 China
b
Key Laboratory of Functional Inorganic Material Chemistry, National Center for International Research on Catalytic
Technology, School of Chemistry and Material Sciences, Heilongjiang University, Harbin, 150080 China
c
Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, 119991 Russia
d
Institute of Petrochemistry, Heilongjiang Academy of Sciences, Harbin, 150040 China
*
e-mail: wuwei@hlju.edu.cn
Received January 12, 2020; revised January 12, 2020; accepted January 13, 2020
Abstract—Nanoscale ZSM-5 (AlZ5-D) and Ga-substituted ZSM-5 zeolites ((Al,Ga)Z5-D and GaZ5-D) were
synthesized by a green dry-gel conversion strategy. For comparison, ZSM-5 zeolite (AlZ5-T) was also prepared
by traditional hydrothermal method. The effects of synthesis method and Ga introduction on the physicochemical
property and catalytic behavior of methanol to gasoline (MTG) were studied in detail. The results show that the
+
aromatics selectivity decreased while C alkanes selectivity increased for AlZ5-D sample compared with that of
5
AlZ5-T, which was attributed to the significantly reduced density of Brønsted acid sites and weakened strong acid-
ity. After Ga introduction, the acid strength of samples further weakened due to the formation of Si–OH–Ga bridge
hydroxyl group by insertion of Ga into the zeolite framework, inhibiting the cracking reaction and thus increasing
+
the gasoline selectivity. Moreover, the extra-framework Ga species, especially in the form of GaO in GaZ5-D
sample, exhibited high dehydrogenation activity, leading to the highest selectivity of aromatics and gasoline.
Keywords: ZSM-5 zeolite, nanocrystals, Ga modification, dry-gel conversion, methanol to gasoline
DOI: 10.1134/S1070427220010140
INTRODUCTION
in MTG reaction. The key technology of catalytic process
is the development of high-efficiency catalyst. Owing
to the high hydrothermal stability, adjustable acidic
property and the shape selective structure for aromatics,
ZSM-5 zeolites with particular 10-ring interconnected
channel system are widely employed in MTG reaction
The conversion of methanol to gasoline (MTG) over
solid-acid catalysts is an alternative important pathway for
the production of clean fuels due to the richly accessible
resources obtained from biomasses, natural gas and coal,
which are particularly significant because of the gradual
depletion of world-wide crude oil deposits [1, 2]. The
gasoline produced by the MTG process mainly consists
of aromatics, isoparaffins, naphthenes and olefins [3].
Among these, high value-added aromatics can be used
either as a blended composition of gasoline or as the basic
feedstock for producing the synthetic materials and fine
chemical products in the modern chemical industry [4, 5].
[6]. However, the mass transfer of reactants, intermediates
and product molecules in the micropores (5.1–5.6 Å) of
ZSM-5 is limited to some extent, which aggravates the
occurrence of secondary reactions and the deactivation
of catalysts [7]. Nanoscale ZSM-5 zeolite with short
diffusion path length, exhibits remarkably enhanced
diffusion ability and larger surface area which expose
more active centers, and thus improving the utilization
of acid centers in zeolite-catalyzed reactions [8].
In the past years, researchers have made great efforts
to improve the selectivity to gasoline as well as aromatics
127

128
SU et al.
In general, nano-ZSM-5 zeolites were synthesized by
synergy between the active metal species and acid sites
[20]. Additionally, active metal species are easily lost
during the reaction, resulting in the insufficient utilization
of the catalysts. Isomorphous substitution is considered as
an effective method to prepare metal-modified bifunctinal
catalysts. Our previous work [21] found that nanoscale
Ga-modified ZSM-5 zeolites prepared by hydrothermal
isomorphous substitution method showed higher
dispersion of active Ga species compared with the post-
synthesized samples. Moreover, parts of the Ga atoms
were successfully incorporated into the zeolite framework
provided weak Brønsted acidity, and thus inhibited the
cracking reaction in 1-hexene aromatization.
traditional hydrothermal method using large amount of
solvents and organic templates [9]. However, considerable
quantities of nitrogen-containing organic wastewater will
be generated in the crystallization process, and the post-
treatment process is complex, resulting in the energy
consumption and environmental problems. At the same
time, the product yield is relative low. Since Xu et al.
first reported the conversion of aluminosilicate dry gels
into MFI-type zeolites [10], the novel method which
called dry-gel conversion (DGC) has been extensively
applied to synthesize zeolite materials including titanium
substituted silicalite-1 zeolites [11], shaped binderless
ZSM-11 zeolites [12], aluminophosphate molecular
sieves [13], and also mesostructured silica [14]. Unlike
the hydrothermal synthesis, there is no direct contact
between the dry gel and liquid water during DGC process,
which minimizes the waste disposal and makes almost
complete conversion of dry gel powder to zeolites with
a high yield [14]. In light of these merits, DGC approach
is considered as an eco-friendly and cost effective green
route for the synthesis of zeolite nanocrystals. Askari
and co-workers [15] successfully synthesized the
SAPO-34 nanoparticles by DGC method and found that
the formation of nanocrystals in the process of dry-gel
conversion was because of the high nucleation density and
slow crystal growth rate. Niphadkar et al. [16] prepared
the Sn-MFI zeolites with similar SiO /SnO molar ratio
Hence, in this paper, nanoscale ZSM-5 and Ga partially
substituted ZSM-5 zeolite as well as Ga completely
substituted ZSM-5 gallosilicate were synthesized by
DGC method. For comparison, ZSM-5 zeolite was also
prepared by traditional hydrothermal synthesis method.
The catalytic performance of synthesized catalysts was
evaluated in the MTG reaction. The effect of the textural
properties, acidity and the distribution of Ga species on
the catalytic performance of MTG reaction was studied
in detail.
EXPERIMENTAL
Catalyst preparation. Nano-ZSM-5 zeolite was
synthesized by DGC method according to the following
procedures: aluminum isopropoxide (AIP) was first
dissolved in the mixed solution of tetrapropylammonium
hydroxide (TPAOH) and deionized water. Afterwards,
tetraethoxysilane (TEOS) was added to the above
aqueous solution with a molar composition of 1 Al O :
2
2
by hydrothermal crystallization and DGC methods,
respectively. The activation energy of nucleation (En) and
crystallization (Ec) were also measured. Compared to the
conventional hydrothermal synthesis, shorter induction
period and lower values of En and Ec were observed for
DGC method. In addition, Jia et al. [17] demonstrated
that higher utilization of template was obtained for
the synthesis of ZSM-5 zeolites by DGC route and the
prepared catalyst showed higher selectivity to light
aromatics and longer catalytic lifetime in the reaction
of methanol to aromatics than that of the hydrothermal
synthesized samples.
2
3
1
00 SiO : 35.7 TPAOH : 1083.3 H O. The obtained gel
2 2
was kept for 5 h under agitation and the resulting mixture
was transferred to a rundkolben for rotary evaporation
followed by drying at 60°C. The DGC crystallization
was carried out in a specially stainless-steel autoclave
equipped with a self-supporting sand core funnel for
loading the dry gel fine powder, and a certain amount
of water was placed at the autoclave bottom to provide
steam. The autoclave was heated at 180°C for 18 h and
the product was dried at 110°C overnight and calcined
at 550°C for 6 h to remove the template. The obtained
sample was transferred into their H-form by ion exchange
method as described elsewhere [22], denoted asAlZ5-D.
For comparison, nano-ZSM-5 zeolite was also prepared
by traditional hydrothermal crystallization method,
However, the strong acid strength of ZSM-5 zeolite
causes the inevitable coke formation, and thus leads to the
low stability of catalysts. Ga modification is expected to
effectively modulate the catalyst acidity and improve the
selectivity to aromatics [18, 19]. The commonly methods
of metal modification, such as mechanical mixing and
incipient wet impregnation, would result in the surface
enrichment of metal components, and thus exhibit poor
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CONVERSION OF METHANOL TO AROMATIC-RICH GASOLINE
129
named as AlZ5-T. The synthesis process was similar to
the preparation of AlZ5-D, expect for that the initial gel
was directly crystallized in a Teflon-lined stainless-steel
autoclave at 180°C for 24 h.
1
00%
GaZ5-D
Ga partially isomorphous substituted ZSM-5 zeolite
was synthesized as follows: TPAOH, AIP, Ga O ,
9
6%
(Al,Ga)Z5-D
AlZ5-D
AlZ5-T
4 00 5
2
3
and H O were mixed together to obtain the complete
2
dissolution of aluminium source and gallium source.
The subsequent process was consistent with the above
synthesis of AlZ5-D, except that the crystallization
time was 12 h. The molar ratio of the initial gel was
79%
7
9%
3
5.7 TPAOH:100 SiO :0.57 Al O : 0.43 Ga O :1083.3
2 2 3 2 3
H O. The obtained sample was labeled as (Al,Ga)Z5-D.
2
0
01
10
0
2
2 00 3
3 00 4
5
0
0
6
60
0
Ga completely substituted ZSM-5 gallosilicate sample
GaZ5-D with molar composition of 35.7 TPAOH:100
SiO : 1 Ga O :1083.3 H O in the initial gel, was prepared
Fig. 1. XRD patterns of the prepared catalysts.
2
2
3
2
by the same method applied for (Al,Ga)Z5-D but without
the addition of AIP.
into the reactor. The weight feed rate of methanol was
−
1
4
.0 h . The gas effluents and liquid hydrocarbons were
Catalyst characterization. X-ray diffraction (XRD)
analyzed on a gas chromatograph equipped with a
PONA capillary column (50.0 m × 0.2 mm × 0.5 μm).
Unreacted methanol (or DME) and water were analyzed
by gas chromatograph with thermal conductivity
detector and a Porapak Q capillary column (4 m ×
3 mm). The methanol conversion, XMeOH, and product
analysis was performed on on a Bruker D8 powder
diffractometer using CuK radiation (λ = 1.5406 Å). The
α
relative crystallinity was calculated by the integral area
in the 2θ range of 22.5°−25°. The morphology of samples
was recorded on a Hitachi S-4800 scanning electron
microscope (SEM). 27Al, 71Ga and 29Si MAS NMR
spectra were performed using a Bruker AVANCE III
selectivity, S , were calculated according to the formulas:
i
4
00 MHz spectrometer. A selective π/12 pulse was used
for quantitative analysis of 7Al MAS NMR spectra.
2
n(MeOH)in  n(MeOH)out  2n(DME)out
XMeOH

100%,
The N adsorption−desorption isotherms were carried
n(MeOH)in
2
out on a Quantachrome Autosorb-1 automatic surface
and porosity analyzer. Temperature-program desorption
of NH (NH -TPD) were measured in a flow system
ni,outki,C
Si 
100%

ni,outki,C
3
3
with a thermal conductivity detector. FTIR spectra of
pyridine adsorption were performed on an in-situ infrared
spectrometer (Frontier, PerkinElmer). The density of
Brønsted and Lewis acid sites was determined after
desorption at 150°C. Temperature-programmed reduction
where ni,out was the molar quantity of product i and ki,C
was the number of carbon atoms in its molecule.
RESULTS AND DISCUSSION
(
H -TPR) was performed using a chemisorption analyzer
2
The XRD patterns of all samples shown in Fig. 1
revealed typical patterns of 2θ =7.9°, 8.8°, 23.0°, 23.1°,
(AutoChem II 2920, Micromeritics). X-ray photoelectron
spectroscopy (XPS) was measured on an ESCALAB
2
1
2
3.6°, 23.9° and 24.4°, indicating that the synthesized
50 spectrometer with an AlK radiation source (hν =
α
zeolites were pure MFI crystals. The basic structure of
ZSM-5 zeolite was kept after the introduction of Ga
and no diffraction peaks corresponding to Ga O phase
486.6 eV).
Catalytic evaluation. The MTG reaction was run
2
3
in a microscopic fixed-bed reactor at a temperature
of 400°C and a pressure of 1.5 MPa in a nitrogen
atmosphere. The catalyst was pretreated at 450°C for
were observed in the Ga-substituted samples. This
may be attributed to that the Ga species were inserted
into the zeolite framework or were highly dispersed.
Moreover, the relative crystallinities of AlZ5-T and
AlZ5-D samples were comparable, demonstrating that
2
h, and cooled to the reaction temperature followed
by pumping the methanol aqueous solution (50 wt %)
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130
SU et al.
(b)
(a)
2
00 nm
200 nm
(c)
(d)
2
00 nm
200 nm
Fig. 2. SEM images of the prepared catalysts: (a) AlZ5-T, (b) AlZ5-D, (c) (Al,Ga)Z5-D, and (d) GaZ5-D.
the dry-gel conversion method can reach a degree of
crystallization equivalent to the sample prepared by
the traditional hydrothermal method (24 h) in a shorter
crystallization time (18 h). It can also be found that the
relative crystallinity increased for the samples modified
by Ga isomorphous substitution, especially for GaZ5-D
sample, which may be due to the larger particle size of
the Ga-substituted samples. The average crystal sizes
calculated by XRD patterns were ca. 13 nm and ca .17 nm
for the AlZ5-D and GaZ5-D samples, respectively.
larger than that ofAlZ5-D sample, in line with the results
of XRD that higher relative crystallinity was obtained for
Ga-modified samples.
27
71
29
The Al, Ga and Si MAS NMR spectra of samples
were displayed in Fig. 3. It can be seen from Fig. 3a that
an intense signal peak at ca. 54 ppm was observed for all
Al-containing ZSM-5 samples, demonstrating that most
Al atoms were inserted into the zeolite framework in
the form of tetrahedral coordination. A weak resonance
peak at about 0 ppm assigning to hexa-coordinated
extra-framework Al species was also appeared for these
samples. The obvious signal at approximately 160 ppm
of Ga-substituted ZSM-5 samples shown in Fig. 3b was
belong to the tetrahedral coordinated Ga species in the
framework sites. With the increase amount of Ga added
in the gel mixture, the content of framework Ga increased
significantly. No signals of extra-framework Ga species
were observed, which was mainly due to their strong
Figure 2 showed the SEM images of the synthesized
zeolites. The morphologies of ZSM-5 aluminosilicates
prepared by different methods were quite different.
The AlZ5-T sample exhibited rod-like shape consist of
nanocrystals, while the AlZ5-D sample were presented
in the form of cylindrical aggregates accumulated by the
ultra-fine particles. Moreover, the particle size ofAlZ5-D
sample was smaller than that of AlZ5-T, consistent with
the reports that the sample crystallized by DGC method
has a smaller grain size due to its higher nucleation density
and slower crystal growth rate [23].After Ga isomorphous
substitution, the samples exhibited spherical agglomerates
with nanocrytals and the average signal particle size was
2
9
quadrupole effect [24]. The Si MAS NMR spectra in
Fig. 3c showed that all the samples exhibited two signals
at ca. −114 and −104 ppm, respectively. The intense peak
signal at −114 ppm was attributed to Si*(OSi) units.
4
The poorly broad peak at approximately −104 ppm was
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CONVERSION OF METHANOL TO AROMATIC-RICH GASOLINE
a) (b)
131
(
(
Al,Ga)Z5-D
AlZ5-T
AlZ5-D
GaZ5-D
(Al,Ga)Z5-D
1
1
401 20 1008 06 04 02 00
–
20
–
40 4003
00
300
2001
200
Chemical shift, ppm
00
100
0
0
–
100
40 120 100 80 60 40 20 0 ‒20 ‒40 400
Chemical shift, ppm
‒100
(
c)
c-1
c-2
AlZ5-D
AlZ5-T
-104.4
-104.4
–
‒
6
6
0
0
–
‒8
8
0
0
‒
–
1
1
0
0
0
0
‒
–
1
1
2
2
0
0
–
‒
1
1
4
4
0
0
–
‒
6
6
0
0
–
‒8
8
0
0
‒
–
1
1
0
0
0
0
‒
–
1
1
2
2
0
0
–140
‒140
Chemical shift, ppm
Chemical shift, ppm
c-3
c-4
GaZ5-D
(Al,Ga)Z5-D
-104.0
-104.2
‒
–
6
6
0
0–
‒8
8
0
0
‒1
–
0
1
0
00
‒
–
1
1
2
2
0
0
‒
–
1
1
4
4
0
0
‒60
–60–
80
‒80
–100
‒100
–120
‒120
–140
‒140
Chemical shift, ppm
Chemical shift, ppm
Fig. 3. 27Al MAS NMR spectra (a), 71Ga MAS NMR spectra (b) and 29Si MAS NMR spectra (c) of the prepared catalysts.
assigned to (MeO)*Si(OSi) (Q , Me = Al or Ga) sites. which was attributed to the insertion of Ga atom into the
3
3
Additionally, it can also be found that the peak signal zeolite framework resulting in a decrease in chemical
shielding [25].
of Q sites of AlZ5-D sample shifted towards low-field
3
(
high frequency) after Ga isomorphous substitution,
The N adsorption-desorption curves of samples
2
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132
SU et al.
Table 1. Textural properties of the prepared catalysts
2
3
Surface area, m /g
Pore volume, cm /g
Sample
BET
472
505
470
474
micorpore
306
external
166
total
0.701
0.803
0.426
0.604
micorpore
0.122
mesopore
0.579
AlZ5-T
AlZ5-D
Al,Ga)Z5-D
GaZ5-D
311
194
0.141
0.662
(
320
150
0.136
0.290
337
137
0.135
0.469
were shown in Fig. 4a. All isotherms exhibited the
characteristics of type I and type IV isotherms, indicating
the coexistence of micropores as well as mesopores in
the prepared zeolites. Moreover, it can be inferred from
the SEM images that the formation of mesopores was
mainly caused by the accumulation of nanocrystallines.
Additionally, more intense and steeper hysteresis loop was
observed forAlZ5-D sample than that for Ga-substituted
ZSM-5 zeolites, which may be attributed to the smaller
particle size of AlZ5-D. The BJH pore size distributions
of the samples were shown in Fig. 4b.As seen, an obvious
pore distribution between 30 and 300 nm was observed for
all samples, which came from the capillary condensation
within the intergranular mesopores [26].
by Ga isomorphous substitution via dry-gel conversion
route, leading to the relative large nanoparticles.
Figure 5 illustrated the NH -TPD curves of the
3
synthesized zeolites. All samples exhibited two peaks
in the low-temperature (L.T.) of ca. 200°C and the high-
temperature (H.T.) of ca. 370°C, which was attributed
to the adsorption of NH on weak and strong acid sites,
3
respectively. As observed in Table 2, there was obvious
shift towards lower temperature for both the L.T. and the
H.T. desorption peak of theAlZ5-D sample, demonstrating
that the acid strength of AlZ5-D was weaker than that
of AlZ5-T. After Ga introduction, the samples show a
further lower temperature of H.T. desorption peak. It
should be mentioned that the strong acid sites in zeolites
were mainly Brønsted acid sites. Therefore, the reduction
of strong acidity was ascribed to the lower Δ(χO−χGa)
compared withΔ(χO−χAl), where χAl, χGa, and χO were
the electronegativity of the framework Al, Ga, and O,
respectively. Because the smaller difference between χO
and χGa, the binding of Ga−O is weaker compared with
The corresponding textural data of samples was listed
in Table 1. The mesopore volume and external area of
AlZ5-D sample were larger than that of AlZ5-T, which
was attributed to the smaller particle size of AlZ5-D
zeolite. After Ga introduction, the mesopore volume
and external area of the zeolites obviously decreased
compared with that ofAlZ5-D sample, which may be due
to the rapid growth after nucleation of nanoscrystallines
+
that ofAl−O bond. Therefore, the Si−O(H )−Ga bridging
+
hydroxyl cannot easily give up H , resulting in the weaker
(
b)
(
a)
1
1
1
0
0
0
.4
.2
.0
.8
.6
.4
AlZ5-T
AlZ5-D
GaZ5-D
(
Al,Ga)Z5-D
GaZ5-D
(
Al,Ga)Z5-D
AlZ5-D
AlZ5-T
0.2
.0
0
05
0
50
0
100 150 200 250 300
100 150 200 250 300
0
0
.
.
0
00 0.
.
2
2
0
0
.
.
4
40 0.
.
6
6
0
0
.
.
8
8
1
1.
.
0
0
Pore diameter, nm
P/P0
Fig. 4. N adsorption-desorption isotherms (a) and BJH pore size distributions (b) of the prepared catalysts.
2
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CONVERSION OF METHANOL TO AROMATIC-RICH GASOLINE
133
Brønsted acidity of Ga-substituted ZSM-5 zeolites [27].
Table 2. Acidic properties of the prepared catalysts
Py-IR measurement was estimated to investigate the
density of Brønsted and Lewis acid sites of the samples,
and the corresponding data was listed in Table 2. Both
the density of Brønsted and Lewis acid sites decreased
for the samples synthesized by DGC method compared
with the AlZ5-T. This may be due to the fact that parts of
extra-framework Al species which did not insert into the
zeolite framework during the dry-gel conversion process,
was washed away in the subsequent ion exchange process.
Moreover, Ga-substituted ZSM-5 zeolites exhibited
higher density of Lewis acid sites compared with that of
AlZ5-D sample, which can be attributed to the formation
of extra-framework Ga species. It can also be found that
the density of Brønsted acid sites of (Al,Ga)Z5-D sample
was lower than that of GaZ5-D, suggesting the lower
total content of metal atoms (Al and Ga) inserted in the
skeleton sites because of the competition of Ga and Al.
Tpeak,a °C
Acid amount , μmol g
b
‒1
Sample
weak strong Brønsted acid Lewis acid
AlZ5-T
AlZ5-D
204
381
113.25
43.25
47.17
54.61
84.50
35.73
56.09
71.76
1
96
373
364
(Al,Ga)Z5-D 184
GaZ5-D 192
368
a
Determined by NH -TPD.
3
b
Calculated by Py-IR.
AlZ5-T
AlZ5-D
(Al,Ga)Z5-D
GaZ5-D
Figure 6a showed the H -TPR profiles of the prepared
2
samples.Asmall reduction peak appeared in both samples
at ca. 200°C, demonstrating that parts of the low dispersed
gallium oxides on the outer surface can be reduced at
rather low temperature. Moreover, another characteristic
peak at approximately 420°C was also observed, attrib-
uting to the reduction of well-dispersed Ga O particles
2
3
formed in the samples. Compared with (Al,Ga)Z5-D, an
obvious reduction signal at a higher temperature of ca.
1
1
0
0
0
02
2
0
0
0
3
3
0
0
0
04 40
0
0
0
5
5
0
0
0
0
6
60
0
0
0
700
700
Temperature, °C
Temperature (°C)
7
00°C was obtained for GaZ5-D. This was mainly attrib-
+
uted to the formation of cationic GaO species at charge
compensation sites by the replacement of acid protons in
Fig. 5. NH -TPD curves of the prepared catalysts.
3
(a)
(b)
1120.3 eV
GaZ5-D
1118.7 eV
GaZ5-D
(
Al,Ga)Z5-D
(
Al,Ga)Z5-D
2
2
003 00 4005 00 6007 00 8009 00 1105 1110 1115 1120 1125 1130 1135 1140
00 300 400 500 600 700 800 900
Te ra
ur
Biding energy (eV)
mpe ture (°C)
Temperat e,
°C
,
Fig. 6. H -TPR profiles (a) and Ga 2p3/2 spectra (b) of the prepared catalysts.
2
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134
SU et al.
Table 3. Catalytic performance of MTG reaction over the prepared catalysts
Product selectivity, %
CMeOH
%
,
Sgasoline
,
C4
Sample
%
HTI
C -C alkanes C -C olefins C5+ alkanes C5+ olefins
Aromatics
30.38
1
4
2
4
AlZ5-T
AlZ5-D
97.6
97.7
97.2
97.5
34.85
8.59
22.11
33.73
23.74
20.14
4.07
0.38
7.09
7.42
56.56
56.62
61.54
66.74
0
0
0
0
.93
.91
.82
.85
26.20
17.29
17.91
17.18
21.17
15.35
22.51
30.71
39.18
(Al,Ga)Z5-D
GaZ5-D
GaZ5-D sample, thus leading to the reduction of GaO+
at higher temperatures because of the strong interaction
between the zeolite framework and GaO species [28].
AlZ5-D sample had a lower density of Brønsted acid
sites with weaker acid strength, inhibiting the hydrogen
transfer reaction and thus resulting in the lower selectivity
+
to aromatics. Accordingly, the selectivity to C alkanes
produced by the competitive reaction was increased.
XPS spectra were recorded to characterize the Ga
species on the zeolite surface. As seen in Fig. 6b, no
signal peak was observed for (Al,Ga)Z5-D sample,
demonstrating that the content of surface Ga species
was very low. In contrast, two types of Ga species were
distinguished in GaZ5-D sample with binding energies
of ca. 1118.7 and 1120.3 eV, corresponding to Ga O
5+
The C hydrogen transfer index (C HTI) calculated by
4
4
C alkanes divided by the sum of C alkanes and C
4
4
4
olefins [33], which was used to describe the extent of
the hydrogen transfer process, and the selectivity to low-
carbon C –C alkanes as by-products of the hydrogen
1
4
2
3
+
transfer reaction also decreased. The higher selectivity
particles and active GaO species, respectively [29],
=
=
to light olefins C –C of AlZ5-D sample was attributed
in line with the results from H -TPR characterization.
2
4
2
+
to the promotion of olefin-based cycle. Although both
cycles process were catalyzed by Brønsted acid sites, the
reduction of Brønsted acidity had a more significant effect
on the inhibition of the aromatic-based cycle, which in
turn promoted the olefin-based cycle.
The higher binding energy of active GaO species was
attributed to the stronger covalent character of the GaO /
+
Brønsted acid coordination center [30]. Because partial
+
electron was transferred from GaO species to the more
2
−
electronegative lattice O of the zeolite framework, and
+
thus decrease the electron density of GaO and result in
After Ga introduction, the selectivity to gasoline and
to aromatics of Ga-substituted ZSM-5 samples obviously
increased compared with that of AlZ5-D. This was due
to the fact that although the hydrogen transfer reaction
was inhibited by the reduction of Brønsted acidity,
the extra-framework Ga species formed in (Al,Ga)
Z5-D and GaZ5-D samples substantially promoted the
dehydrogenation reaction. Moreover, it can also be found
that the selectivity to C5+ alkanes decreased while C5+
olefins selectivity increased due to the improvement
of dehydrogenation process. The highest selectivity to
gasoline and aromatics was obtained for GaZ5-D sample
prepared by Ga completely isomorphous substitution.
This was mainly attributed to the co-existence of extra-
framework Ga species including Ga O and cationic
the higher BEs of Ga 2p3/2 peak.
The catalytic performance over the synthesized
zeolites was studied for the MTG reaction and the result
was listed in Table 3. The methanol conversion over all
the catalysts was above 97%. It is known that there are
two distinct but dependent catalytic cycles involved in
MTG reaction over ZSM-5 zeolites, which are olefin- and
aromatic-based cycles, respectively [31, 32]. The olefin-
based cycle is to obtain C3+ olefins via the methylation
and cracking reaction of low-carbon olefins, while the
aromatic-based cycle mainly produce aromatics through
continuous methylation and dealkylation reactions of
methylbenzene intermediates, which formed by the
cyclization and hydrogen transfer reaction catalyzed on
Brønsted acid sites. Therefore, the product distributions of
MTG on ZSM-5 zeolites can be affected by the different
acidity of catalysts. Compared with AlZ5-T zeolite,
2
3
+
active GaO species with higher dehydrogenation activity,
which significantly promoted the dehydrogenation
reaction and thus increased the aromatics selectivity.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 93 No. 1 2020

CONVERSION OF METHANOL TO AROMATIC-RICH GASOLINE
135
CONCLUSIONS
vol. 361, pp. 193−203.
https://doi.org/10.1016/j.jcat.2018.03.001
In this paper, a green synthesis route via dry-gel
conversion method was successfully explored to prepare
the nanoscale ZSM-5 (AlZ5-D) and Ga-subtituted
ZSM-5 zeolites ((Al,Ga)Z5-D and GaZ5-D) in a shorter
crystallization time compared with the traditional
hydrothermal method.After Ga introduction, the Brønsted
acid strength decreased due to the formation of Si-OH-
Ga bridge hydroxyl group, which inhibited the hydrogen
transfer reaction. In contrast, the dehydrogenation
reaction of cycloolefin intermediates was significantly
promoted by the formation of extra-framework Ga
species compared with AlZ5-D sample. Ga completely
substituted ZSM-5 gallosilicate sample GaZ5-D exhibited
the highest aromatics selectivity of 39.18% and gasoline
selectivity of 66.74%, respectively, which was attributed
5. Ma, L. and Zou, X.Q., Appl. Catal. B: Environ., 2019,
vol. 243, pp. 703−710.
https://doi.org/10.1016/j.apcatb.2018.11.014
6. Wan, Z.J., Wu, W., Li, G.K., and Wang, Ch., Appl. Catal.
A: Gen., 2016, vol. 523, pp. 312–320.
https://doi.org/10.1016/j.apcata.2016.05.032
7
8
9
. Meng, L., Mezari, B., Goesten, M.G., and Hensen, E.J.M.,
Chem. Mater., 2017, vol. 29, pp. 4091–4096.
https://doi.org/110.1021/acs.chemmater.7b00913
. Firoozi, M., Baghalha, M., Asadi, M., Catal. Commun.,
2
009, vol. 10, pp. 1582–1585.
https://doi.org/10.1016/j.catcom.2009.04.021
. Chen, H.B., Wang, Y.Q., Meng, F.J., Chao, S., Li, H.,
Wang, Zh., Fei, G., Wang X., and Wang, Sh., Microporous
Mesoporous Mater., 2017, vol. 244, pp. 301–309.
https://doi.org/10.1016/j.micromeso.2017.02.014
+
to the formation of active GaO species with high
dehydrogenation activity.
1
0. Xu, W.Y., Dong, J.X., Li, J.P., et al., Chem. Commun.,
990, vol. 10, pp. 755–756.
1. Ke, X.B., Zeng, C.F., Yao, J.F., Zhang, L., and Xu, N.,
1
1
FUNDING
Mater. Lett., 2008, vol. 62, pp. 3316–3318.
https://doi.org/10.1016/j.matlet.2008.02.040
This work was supported by the National Natural
Science Foundation of China (no. 21676074),
Intergovernmental International Science and Technology
Innovation Cooperation Key Project (2018YFE0108800),
National Ministry of Education “Silk Road 1+1” Research
Cooperation Project, the Transformation for Science and
TechnologyAchievements in University of Department of
Education of Heilongjiang Province (TSTAU-C2018015).
1
1
1
2. Zhang, W.S., Zhang, S., Xin, W.J., Hui, L., Yongchen, Sh.,
Xiangxue, Zh., Shenglin, L., and Longya, X., J. Energy
Chem., 2017, vol. 26, pp. 380–389.
https://doi.org/10.1016/j.jechem.2016.12.008
3. Li, M., Zhang, J.M., Liu, X.Y., Wang, Y., Liu, G., Hu, D.,
Zeng, G., Zhang, Y., Wei, W., and Sun, Y., Microporous
Mesoporous Mater., 2016, vol. 225, pp. 261–271.
https://doi.org/10.1016/j.micromeso.2015.11.056
4. Shen, S.C., Chen, F.X., Chow, P.S., Phanapavudhikul, P.,
CONFLICT OF INTEREST
Zhu, K., and Tan, R.B.H., Microporous Mesoporous Mater.,
2
006, vol. 92, pp. 300–308.
https://doi.org/10.1016/j.micromeso.2006.01.019
The authors declare that they have no conflict of
interest in this work.
15. Askari, S., Sedighi, Z., and Halladj, R., Microporous
Mesoporous Mater., 2014, vol. 197, pp. 229–236.
https://doi.org/10.1016/j.micromeso.2014.06.028
REFERENCES
1
6. Niphadkar, P.S., Tangale, N.P., Joshi, P.N., and Awate,
S.V., Microporous Mesoporous Mater., 2013, vol. 182,
pp. 73–80.
1
. Olah, G.A., Angew. Chem. Int. Ed., 2005, vol. 44, pp. 2636–
639. https://doi.org/10.1002/anie.200462121
2
https://doi.org/10.1016/j.micromeso.2013.08.026
2
. Fu, T.J., Chang, J.W., Shao, J., and Li, Zh., J. Energy
Chem., 2017, vol. 26, pp. 139–146.
1
7. Jia, Y.M., Wang, J.W., Zhang, K., Chen, G., Yang, Yu., Liu,
Sh., Ding, Ch., Meng, Y., and Liu, P., Powder Technol.,
https://doi.org/10.1016/j.jechem.2016.09.011
2
018, vol. 328, pp. 415–429.
3
4
. Olsbye, U., Svelle, S., Bjørgen, M., Beato, P.,
Janssens, T.V.W, Joensen, F., Bordiga, S., and Lillerud, K.P.,
Angew. Chem. Int. Ed., 2012, vol. 51, pp. 5810–5831.
https://doi.org/10.1002/anie.201103657
https://doi.org/10.1016/j.powtec.2018.01.022
18. Kuzmin, I.V., Zhidomirov, G.M., and Hensen, E.J.M.,
Catal. Lett., 2006, vol. 108, pp. 187–191.
https://doi.org/10.1007/s10562-006-0043-9
. Li, K., Chang, Q., Yin, J.Q., Zhao, C., Huang, L., Tao, Zh.,
Yun, Y., Zhang, Ch., Xiang, H., and Li, Y., J. Catal., 2018,
19. Chen, Y.Y., Chang, C.J., Lee, H.V., Juan, J.C., and Lin, Yu-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 93 No. 1 2020

136
SU et al.
C., Ind. Eng. Chem. Res., 2019, vol. 58, pp. 7948−7956.
https://doi.org/10.1021/acs.iecr.9b00726
26. Thommes, M., Kaneko, K., Neimark, A.V., et al., Pure
Appl. Chem., 2015, vol. 87, pp. 1051−1069.
2
7. Xiao, F.S., Xu, Y.H., Feng, S.H., et al., Chin. J. Catal.,
987, vol. 4, pp. 358−363.
8. Al-Yassir, N., Akhtar, M.N., Ogunronbi, K., and Al-
Khattaf , S., et al., J. Mol. Catal. A: Chem., 2012, vol. 360,
pp. 1–15. https://doi.org/10.1016/j.molcata.2012.04.005
2
0. Hensen, E.J.M., Garcıá-Sánchez , M., Rane, N., Magusin,
P.C.M.M., Liu, P.-H.,Chao, K.-J., and Santen, R.A.van,
Catal. Lett., 2005, vol. 101, pp. 79–85.
1
2
https://doi.org/10.1007/s10562-004-3753-x
2
2
2
1. Su, X.F., Wang, G.L., Bai, X.F., Wu, W., Xiao, L., Fang, Y.,
2
3
3
9. Ausavasukhi, A., Sooknoi, T., Catal. Commun., 2014,
vol. 45, pp. 63–68.
and Zhang, J., Chem. Eng. J., 2016, vol. 293, pp. 365–375.
https://doi.org/10.1016/j.cej.2016.02.088
https://doi.org/10.1016/j.catcom.2013.10.026
2. Fang, Y.J., Su, X.F., Bai, X.F., Wu, W., Wang, G., Xiao, L.,
and Yu, A., J. Energy Chem., 2017, vol. 26, pp. 768–775.
https://doi.org/10.1016/j.jechem.2017.03.014
0. Rane, N., Overweg,A., Kazansky, V., Santen, R.A.van, and
Hensen, E.J.M. J. Catal., 2006, vol. 239, pp. 478−485.
https://doi.org/10.1016/j.jcat.2006.03.004
3. Hirota, Y., Murata, K., Tanaka, S., Nishiyama, N.,
Egashira, Y., and Ueyama, K., Mater. Chem. Phys., 2010,
vol. 123, pp. 507–509.
1. Ilias, S. and Bhan, A., J. Catal., 2012, vol. 290, pp. 186–
1
92. https://doi.org/10.1016/j.jcat.2012.03.016
https://doi.org/10.1016/j.matchemphys.2010.05.005
32. Wang, N., Hou, Y.L., Sun, W.J., Cai, D., Chen, Zh., Liu, L.,
Ge, B., Hu, L., Qian, W., and Wei, F., Appl. Catal. B:
Environ., 2019, vol. 243, pp. 721−733.
2
2
4. Bayense, C.R., Kentgens, A.P.M., De Haan, J.W., Van de
Ven, L.J.M., and Van Hooff, J.H.C., J. Phys. Chem., 1992,
vol. 96, pp. 775−782.
https://doi.org/10.1016/j.apcatb.2018.11.023
https://doi.org/10.1021/j100181a047
33. Bjørgen, M., Joensen, F., Spangsberg Holm, M., Olsbye,
U., Lillerud, K.-P., and Svelle, S., Appl. Catal. A: Gen.,
5. Hsieh, C.Y., Chen, Y.Y., and Lin, Y.C., Ind. Eng. Chem.
Res., 2018, vol. 57, pp. 7742−7751.
2
008, vol. 345, pp. 43−50.
https://doi.org/10.1016/j.apcata.2008.04.020
https://doi.org/10.1021/acs.iecr.8b00126
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 93 No. 1 2020