Highly Selective MTO Reaction over a Nanosized ZSM-5 Zeolite Modified by Fe via the Low-Temperature Dielectric Barrier Discharge Plasma Method

Article abstract of DOI:10.1134/S1070427220010152

Nanosized ZSM-5 zeolites were synthesized via in situ seed-induced hydrothermal method, and samples modified with an Fe promoter were prepared by the traditional wet impregnation-thermal decomposition and dielectric barrier discharge plasma (DBD) methods, respectively. The catalytic performance was studied by the methanol-to-olefin (MTO) reaction. The results showed that the acidity of the catalysts, the dispersity of the Fe promoter, and the interaction degree with the ZSM-5 zeolite are closely related to product selectivity in the MTO reaction. Compared with the Fe-NZ5 sample prepared by the traditional impregnation-calcination method, the FeD-NZ5 samples prepared by the DBD method demonstrated the higher selectivity of C₂—C₄ light olefins and the lower coke deposition during long-term evaluation (100 hr), which can be attributed to the weaker acid strength, more uniform Fe promoter dispersion, and strong interaction with the ZSM-5 zeolite.

Full text of DOI:10.1134/S1070427220010152

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2020, Vol. 93, No. 1, pp. 137−148. © Pleiades Publishing, Ltd., 2020.
CATALYSIS
Highly Selective MTO Reaction over a Nanosized ZSM-5 Zeolite
Modified by Fe via the Low-Temperature
Dielectric Barrier Discharge Plasma Method
Yuzong Lia, Xiaofang Sua, A. L. Maximovb, Xuefeng Baia, Yongchao Wanga, Wei Wanga,
N. V. Kolesnichenkob, Z. M. Bukinab, and Wei Wua,*
a 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
b Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, Russia
*e-mail: wuwei@hlju.edu.cn
Received December 16, 2020; revised December 24, 2020; accepted December 26, 2020
Abstract—Nanosized ZSM-5 zeolites were synthesized by an in situ seed-induced hydrothermal method, and
samples modified with an Fe promoter were prepared by the traditional wet impregnation-thermal decomposition
and dielectric barrier discharge plasma (DBD) methods, respectively. The physico-chemical properties of the
catalysts were studied by XRD, SEM, TEM, BET, XPS, H2-TPR, NH3-TPD and Py-IR techniques. The catalytic
performance was eValuated by the methanol-to-olefin (MTO) reaction. The results showed that the acidity of the
catalysts, the dispersity of the Fe promoter and the interaction degree with the ZSM-5 zeolite are closely related to
product selectivity in the MTO reaction. Compared with the Fe-NZ5 sample prepared by the traditional impreg-
nation-calcination method, the FeD-NZ5 samples prepared by the DBD method exhibited the higher selectivity
of C2–C4 light olefins and the lower coke deposition during long-term eValuation (100 h), which can be attributed
to the weaker acid strength, more uniform Fe promoter dispersion and strong interaction with the ZSM-5 zeolite.
The developed Fe-modified catalysts have high potential for application in the MTO reaction.
Keywords: nanosized ZSM-5 zeolite; Fe promoter; modification; dielectric barrier discharge (DBD); methanol-
to-olefin (MTO)
DOI: 10.1134/S1070427220010152
INTRODUCTION
Zeolites and silicoaluminophosphate molecular
sieves, depending on their particular accessible channels,
high surface area, appropriate acidity and excellent
hydrothermal stabilities, have been widely used as
catalysts in petrochemical industrial processes.According
to the literature [3], the SAPO-34 silicoaluminophosphate
with a CHA topological structure and the ZSM-5
zeolite with an MFI framework type both displayed
excellent catalytic performance in the MTO reaction.
For the SAPO-34 catalyst with small 8-ring window (ca.
0.38 nm), high selectivity for light olefins, especially for
ethene, was observed due to its moderate acidity and
appropriate channels [4]. However, SAPO-34 catalysts
have a short catalytic lifetime because of their high
carbon deposition rate. In contrast, nanosized ZSM-5
zeolites exhibited high catalytic stability due to their
Light olefins represented by ethylene and propylene
are important basic organic chemical raw materials that
are mainly produced by the thermal cracking of petroleum
hydrocarbons such as ethane, propane and naphtha [1].
At present, the decreasing reserve of oil resources and the
rapidly increasing requirements for light olefins offer a
strong impetus for the development of new routes for the
production of light olefins. In recent decades, since the
methanol can be easily obtained from richly accessible
resources such as natural gas, coal and biomass, the
methanol-to-olefin(MTO) process has attracted increasing
attention and has provided a promising method to
manufacture light olefins through nonpetrochemical
routes [2].
137

138
YUZONG LI et al.
good resistance to coke deposition attributed to their
unique three-dimensional, interconnected and 10-MR
channel system. However, because of its strong acidity,
the selectivity for light olefins was lower for the ZSM-5
zeolite than for the SAPO-34 samples [5]. Therefore,
recent research has focused on weakening the acidity of
ZSM-5 zeolites and modulating the distribution of acid
centers, thus promoting the formation of light olefins and
prolonging the catalytic lifetimes of catalysts used in the
MTO reaction.
could block the open mouth or channels. Therefore, it is
of great significance to develop an efficient method for
preparing metal/molecular sieve bifunctional catalysts
with high dispersion and high stability of metal species.
Plasma, comprising atoms, electrons, ions and other
excited species, is generally regarded as the fourth
state of matter in addition to solid, liquid, and gas [14].
Depending on the energy, temperature and ion density
of the particles inside the plasma, plasmas are classified
as equilibrium and nonequilibrium plasmas (also called
cold plasmas). Decomposition by cold plasmas is
initiated at room temperature [15]. The combination of
reactivity of plasma species, nonequilibrium state, and
low-temperature operation enables nonthermal plasmas
to be used in unique ways for catalyst preparation to
achieve enhanced reactivity of the surfaces in contact with
the plasma species. Dielectric barrier discharge (DBD),
which is a conventional cold plasma phenomenon, has
been widely used to reduce metal ions [16, 17]. Zhou
[18] and Jiang [19] indicated that DBD-decomposed Ni
and Co catalysts usually show higher activity than those
modified by thermal decomposition.
It has been reported that ZSM-5 zeolites with high
Si/Al ratios are more suitable for MTO reactions with
high selectivity for light olefins [6] due to the decreased
number of acid sites, thus inhibiting side reactions.
Moreover, the modification of ZSM-5 zeolites by metal
promoters is also an effective method to adjust the acidity
of zeolites and thus improve selectivity toward C2~C4
olefins and catalyst stability [7]. The commonly used
methods of metal modification include ion exchange [8],
isomorphous substitution [9] and impregnation [10,11].
Generally, the introduction of active metal species by
ion exchange is limited due to the low degree of ion
exchange. In contrast, the isomorphous substitution
method is considered to be an efficient modification
strategy. Jiang et al. [12] synthesized nanosized ZSM-5
zeolites isomorphously substituted by Fe via an in situ
seed-induced hydrothermal method, and the obtained
[Fe, Al]NZ5 catalyst with appropriate Brønsted acid
sites and improved diffusion properties displayed high
selectivity toward light olefins (82.7%) and low coke
deposition (0.71%) after reaction for 50 h. However,
the preparation process of isomorphous substitution is
complicated, and it is not suitable. for the incorporation
of all metals into the zeolite framework. Compared to
the above methods, the impregnation method has several
advantages, such as simple operation, controllable metal
loading, and good repeatability. M Rostamizadeh et al.
[13] modified HZSM-5 zeolites with an Fe promoter by
the wet impregnation method and investigated the effect
of Fe species on the catalytic performance of the HZSM-5
catalyst in the MTO reaction. The results show that Fe
incorporation decreased coke formation, which resulted
from low acid site density.After Fe modification, the total
light olefin selectivity in the MTO reaction increased to
80%. However, the impregnated samples reported in the
literature usually require high-temperature calcination to
obtain active metal species. During this process, a portion
of the metal oxides agglomerated and sintered, which
Hence, nonthermal DBD plasma treatment operated
at low temperature is a rapid and effective way to prepare
metal-modified bifunctional catalysts. However, to
our knowledge, few studies have focused on the DBD
plasma decomposition of Fe promoters over ZSM-
5 zeolites. In this article, we report the synthesis of
nanosized ZSM-5 zeolites by an in situ seed-induced
method and Fe modification by wet impregnation-thermal
decomposition (calcination at high temperature) and the
low-temperature DBD method. The catalytic performance
of both catalysts in the MTO reaction was eValuated. The
influence of different treatment methods on the product
distribution and the relationship between the light olefin
selectivity and the acidity of zeolites were systematically
investigated to develop appropriate MTO catalysts that
can achieve high light olefin selectivity.
EXPERIMENTAL
Synthesis of nanosized ZSM-5 zeolites and Fe
modification. Nanosized ZSM-5 zeolites were prepared
by an in situ seed-induced method according to ref. [20].
The molar ratio of the mixture gel was Al2O3: 600 SiO2:
216 TPAOH: 6498 H2O. The synthesized sample was
denoted as NZ5.
Modification of the nanosized ZSM-5 zeolite was per-
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HIGHLY SELECTIVE MTO REACTION OVER A NANOSIZED ZSM-5 ZEOLITE
139
formed by incipient wet impregnation using aqueous solu-
tions of Fe(NO3)3‧9H2O to prepare Fe-modified samples
with a loading of 2.2 wt % Fe promoter. The slurry was
sufficiently stirred for 30 min at room temperature, dried
at 100°C overnight and then separated into two portions.
One part was calcined at 550°C for 16 h in a muffle. The
obtained catalyst was denoted as 2.2Fe-NZ5. The other
part was treated by DBD plasma according to the method
reported in the literature [21]. During the decomposition,
the average voltage used was 10 kV. DBD decomposition
was operated for a total of 60 min (3 min of operation
repeated 20 times). After each operation, the sample was
mixed to uniformity. The catalyst prepared by the DBD
method was denoted as 2.2FeD-NZ5.
the acidity of the catalysts. The distribution of Brønsted
acids and Lewis acids in the catalysts was characterized
by means of the infrared spectrum of pyridine adsorption
(Py-IR) at 150°C to calculate the total Brønsted and
Lewis acid contents in the samples. Temperature
programmed reduction (H2-TPR) and H2-chemisorption
characterization were performed using an AutoChem II
2920 chemical analyzer manufactured by Micromeritics,
USA. X-ray photoelectron spectroscopy (XPS) studies
were carried out using an ULTRAAXIS DLD with AlKα
radiation of 1486.7 eV. The amount of coke deposited on
the used catalysts was measured on a thermogravimetric
analyzer (Q50 TGA, TA Instruments).
Catalyst tests. The methanol-to-olefin reaction
was eValuated in a fixed-bed reactor. A total of 1.0 g of
catalyst with a particle size of 20–40 mesh was placed in
the constant-temperature zone of the reaction tube, and both
sides were filled with quartz sand. First, the catalyst was ac-
tivated at 490°C for 1 h under N2 flow (40 mLmin‒1).After
that, the reaction feed of an aqueous solution of methanol
in water (50 wt %) was passed into the reactor through the
LC-01P high-pressure low-pulsation infusion pump. The
gas-phase and liquid hydrocarbon products were analyzed
by gas chromatography with an HP-PLOT/Q capillary
column (30 m × 0.530 mm × 40.0 μm), including FID. The
aqueous-phase products were analyzed by gas chromatog-
raphy with TCD and a Porapak Q capillary column (4 m ×
3 mm). The methanol conversion and product selectivity
were calculated based on the following formulas:
Characterization of the catalysts. X-ray powder
diffraction was performed on a D8 Advance X-ray
diffractometer using CuKα radiation (BRUKER,
Germany, λ = 1.5404 Å) with a scan range of 5° to 55°.
The integration of the peak areas at 22°–25° for the
ZSM-5 zeolite was defined as 100%. The morphology
of the samples was characterized by an S-4800 scanning
electron microscope (SEM) manufactured by Hitachi,
Japan. Energy dispersive X-ray spectroscopy (EDS)
analysis was estimated by transmission electron
microscopy (TEM) on a JEOL JEM 2100 microscope.
The surface area and pore volume of the samples were
analyzed using a Quantachrome Autosorb-1 automatic
surface and porosity analyzer. Temperature-programmed
desorption of NH3 (NH3-TPD) was carried out to analyze
n(Methanol)in  n(Methanol)out  2n(DME)out
XMeOH


100%,
100%
n(Methanol)in
Nn(CxHy )out
SCxH
y
n(Methanol)in  n(Methanol)out  2n(DME)out
where n(Methanol)in is the number of moles of methanol
at the inlet of reactor, n(Methanol)out, n(DME)out
and n(CxHy)out are the number of moles of methanol,
dimethyl ether and hydrocarbons at the outlet of reactor,
respectively. N is the number of carbon atoms in the
hydrocarbon products.
the topological structure of nanosized ZSM-5 zeolites, the
parent and Fe-modified samples prepared by incipient wet
impregnation-thermal decomposition and DBD plasma
methods were characterized by X-ray diffraction, and
the XRD patterns and relative crystallinity are shown
in Fig. 1.
It can be seen from Fig. 1 that the Fe-modified
ZSM-5 samples show a characteristic diffraction peak
corresponding to the MFI topological structure (JCPDS
No. 42-0024), and there is no diffraction peak attributed to
the iron oxide crystal phase, indicating that the modified
RESULTS AND DISCUSSION
Structure, morphology and textural properties. To
investigate the effect of the Fe modification method on
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140
YUZONG LI et al.
different degrees, which may be due to the formation of
skeletal defects caused by the dealumination of the zeolite
framework during impregnation by an Fe(NO3)3 solution
with weak acidity. Compared with that of the 2.2Fe-
NZ5 catalyst prepared by thermal decomposition after
impregnation, the relative crystallinity of the 2.2FeD-
NZ5 catalyst prepared by the room-temperature plasma
treatment method (DBD) did not decrease significantly,
indicating that modification by the DBD method can
better maintain the zeolite skeleton structure.
94%
3
2
86%
Figure 2 shows SEM images of the nanosized ZSM-5
zeolite and Fe-modified samples. The well-crystallized
nanosized ZSM-5 zeolite exhibited an ellipsoidal
aggregate morphology, and each crystallite was 50-
80 nm in size. The morphology of the 2.2FeD-NZ5
catalyst did not change significantly, but the surface of
the 2.2Fe-NZ5 sample became irregular, and more small
particles adhered to the surface of the sample. This may
be attributed to the destruction of the zeolite structure
and the formation of amorphous species deposited on
the surface of the zeolite during the modification process,
which is consistent with the more significant decrease in
the relative crystallinity of the 2.2Fe-NZ5 sample.
1
100%
10
20
30
40
50
Fig. 1. XRD patterns of the parent and Fe-modified nanosized
ZSM-5 zeolite catalysts: (1) NZ5; (2) 2.2Fe-NZ5; and
(3) 2.2FeD-NZ5.
samples maintain the topological structure of MFI and that
the deposited iron oxide is highly dispersed on the surface
of the ZSM-5 zeolite. However, the relative crystallinity
of the samples after Fe modification is decreased to
The HAADF-STEM micrograph and corresponding
EDX mapping images of the Fe-modified samples are
shown in Fig. 3. It can be seen that Si and Al are eVenly
(a)
(b)
100 nm
(c)
100 nm
100 nm
Fig. 2. SEM images of the parent and Fe-modified nanosized ZSM-5 zeolite catalysts: (a) NZ5; (b) 2.2Fe-NZ5; (c) 2.2FeD-NZ5.
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HIGHLY SELECTIVE MTO REACTION OVER A NANOSIZED ZSM-5 ZEOLITE
141
(e)
(f)
(a)
(b)
(h)
(c)
(d)
(g)
Fig. 3. STEM image of (a) the 2.2Fe-NZ5 catalyst and (b) the corresponding EDX mapping of Si; (c) Fe and (d) Al. STEM image of the
(e) 2.2FeD-NZ5 catalyst and (f) the corresponding EDX mapping of Si; (g) Fe and (h) Al.
in the micropore surface area and the pore volume was
observed for the 2.2Fe-NZ5 sample, which can be
attributed to the destruction of micropores or the blocking
of pores during the impregnation-calcination process. The
external surface area and mesoporous pore volume of the
samples after Fe modification were slightly increased.
This may be due to the formation of secondary mesopores
caused by partial dealumination after Fe modification.
distributed on the two samples. In addition, compared
with the 2.2Fe-NZ5 sample, the 2.2FeD-NZ5 sample
exhibited a more uniform distribution of Fe species,
indicating that the Fe promoter was highly dispersed
on the surface of the zeolite. The uneven distribution of
the Fe promoter in the 2.2Fe-NZ5 sample was mainly
attributed to the formation of large particles due to the
aggregation of iron oxide.
To investigate the effect of Fe modification methods
on the textural properties of the nanosized ZSM-5 zeolite,
the parent ZSM-5 zeolite and Fe-modified samples were
characterized by the N2 physical adsorption method. The
adsorption-desorption isotherms are shown in Fig. 4,
and the surface area and pore volume data are shown in
Table 1.
It can be seen from Fig. 4 that for all catalysts, the N2
adsorption-desorption isotherms have the characteristics
of both type I and type IV isotherms, and the H4 type
hysteresis loop appeared in the range of P/P0 of 0.5–1.0,
indicating that the samples not only have microporous
properties but also have intercrystalline mesoporous
structures due to the aggregation of nanoparticles in the
interparticle space. Table 1 shows that impregnation
by an Fe(NO3)3 solution does not change the texture of
the ZSM-5 zeolite. The BET surface area, micropore
surface area and pore volume of the 2.2FeD-NZ5 sample
decreased slightly compared to those of the parent sample
NZ5, and this difference is attributed to minor damage to
the zeolite framework. In contrast, a significant reduction
3
2
1
0.00
0.0
.2
0.2
0.40
0.4
.6
0.6
P/P0
0.81
0.8
.0
1.0
Fig. 4. N2 adsorption-desorption isotherms of the parent and
Fe-modified nanosized ZSM-5 zeolite catalysts: (1) NZ5;
(2) 2.2Fe-NZ5; and (3) 2.2FeD-NZ5.
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142
YUZONG LI et al.
(a)
429°C
Fe2p1/2
642°C
Fe2p3/2
2
401°C
1
Fe2p1/2
(b)
200
400
Temperature, °C
800
Fe2p3/2
600
Fe3+
Fe2+
Fig. 6. H2-TPR profiles of the parent and Fe-modified nanosized
ZSM-5 zeolite catalysts: (1) 2.2Fe-NZ5; and (2) 2.2FeD-NZ5.
Fe3+
Fe2+
To better understand the function of the Fe promoter
in modified ZSM-5 zeolites in the MTO reaction, XPS
analysis was used to characterize the Fe species on the
surface of the samples. The Fe 2p XPS spectra of the
iron oxides of the sample prepared by the precipitation
method and the Fe-modified ZSM-5 zeolite samples are
compared in Fig. 5. The XPS spectrum in the Fe2p region
for the 2.2FeD-NZ5 is weaker than the corresponding
spectrum of 2.2Fe-NZ5. However, the iron oxide loading
is equal in the two samples (2.2 wt %). Therefore, for
the 2.2FeD-NZ5 sample, a large fraction of the iron ions
likely diffused into the zeolite channels or deposited
on the pore mouth during the impregnation and DBD
treatment processes. The Fe 2p3/2 and Fe 2p1/2 peaks
were deconvoluted into two components by assuming a
Gaussian shape of the component peaks. Table 2 lists the
(c)
7407 35 7307 25 7207 15 7107 05 700
740 735 730 725 720 715 710 705 700
Binding energy, eV
Fig. 5. Deconvoluted Fe2p XPS spectra for the Fe2O3 and Fe-
modified ZSM-5 zeolite catalysts: (a) Fe2O3; (b) 2.2Fe-NZ5;
and (c) 2.2FeD-NZ5.
Table 1. The textural properties of the parent and Fe-modified nanosized ZSM-5 zeolite catalysts
Surface area, m2 g–1
Pore volume, cm3 g–1
Sample
a
b
c
d
b
c
SBET
Smicro
Sext
Vtotal
Vmicro
Vmeso
NZ5
455
422
449
372
306
325
83
0.336
0.327
0.331
0.169
0.154
0.162
0.167
0.173
0.169
2.2Fe-NZ5
116
124
2.2FeD-NZ5
a BET method. b t-plot method. c Sext = SBET − Smicro. (Vmeso. = Vtotal − Vmicro.). d Vtotal = Vads at p/p0 = 0.995.
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HIGHLY SELECTIVE MTO REACTION OVER A NANOSIZED ZSM-5 ZEOLITE
Table 2. The binding energies and relative amounts of Fe2+ and Fe3+ on the surfaces of the samples
Binding energy, eV
143
The surface
Fe2+/(Fe2++Fe3+) ratio
Sample
Fe2p1/2
Fe2p3/2
Fe3+
Fe2+
Fe3+
Fe2+
Fe2O3
726.1
725.9
–
712.7
712.6
–
0
2.2Fe-NZ5
724.4
711.1
0.41
2.2FeD-NZ5
725.8
724.1
712.6
710.6
0.57
results of the deconvolution of the XPS spectra calculated
according to the area by Gaussian fitting, corresponding
to the Fe2+ and Fe3+ species.
respectively), indicating greater charge transfer from the
zeolite to the surface iron oxides of the catalyst, which
is due to the stronger interaction between the zeolite
surface hydroxyl group and Fe species with more uni-
form dispersion on the catalyst prepared by the DBD
method, which will help to inhibit the aggregation and
loss of the Fe promoter. The 2.2FeD-NZ5 catalyst has
a larger Fe2+/(Fe2+ + Fe3+) ratio than 2.2Fe-NZ5, which
is attributed to the stronger reduction of DBD treatment
and thus the decreased formation of Fe2O3 particles [23].
Compared with Fe2O3, both the 2.2Fe-NZ5 and
2.2FeD-NZ5 catalysts not only show characteristic
Fe3+ p1/2 and Fe3+2p3/2 peaks at ~725.9 eV and ~712.6 eV,
respectively, but also give rise to peaks at ~724.4 eV
and ~711.1 eV that correspond to the formation of Fe2+
species, which can be attributed to electron transfer to the
Fe species and partial reduction of Fe3+ to Fe2+ because of
the interaction between the deposited Fe promoters and
the ZSM-5 zeolite. In addition, a broad satellite peak of
Fe2p appears near 718 eV [22].
To study the interaction between Fe species and the
zeolite surface of two Fe-modified samples, temperature-
programmed reduction in a hydrogen atmosphere (H2-
TPR) was performed, and the results are shown in Fig. 6.
Sample 2.2Fe-NZ5 showed a broad reduction peak at
350–500°C (peak temperature was 401°C), attributed to
Moreover, it is worth noting that the binding ener-
gies of both Fe2+ 2p3/2 and Fe2+ 2p1/2 of the 2.2FeD-NZ5
catalyst shifted to a lower position (710.6 and 724.1 eV,
1543
3
2
3
1543
2
1542
1
1
100
200
300
400
°C
500
1400
1450
1500
1550
1600
Temperature,
Wave number, cm–1
Fig. 7. NH3-TPD curves of the parent and Fe-modified
nanosized ZSM-5 zeolite catalysts: (1) NZ5; (2) 2.2Fe-NZ5;
and (3) 2.2FeD-NZ5.
Fig. 8. Py-IR spectra of the parent and Fe-modified nanosized
ZSM-5 zeolite catalysts: (1) 2.2FeD-NZ5; (2) 2.2Fe-NZ5; and
(3) NZ5.
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144
Table 3. The acidity of the parent and Fe modified nanosized ZSM-5 zeolite catalysts
Peak temperature, oCa Acid amount,a mmol g–1
YUZONG LI et al.
Acid amount,b μmol g–1
Sample
Brønsted
acidity
LT-peak
HT-peak
total
weak
strong
Lewis acidity
NZ5
173
171
171
381
373
362
0.322
0.287
0.254
0.120
0.122
0.125
0.202
0.165
0.128
27.2
8.3
8.6
8.9
2.2Fe-NZ5
24.1
2.2FeD-NZ5
19.8
a The LT and HT peaks represent the desorption peaks at low and high temperatures, respectively, as determined by the NH3-TPD method.
b Determined by the Py-IR method at a desorption temperature of 150°C.
(a)
(b)
60
55
50
45
40
90
87
84
81
78
75
72
3
2
1
3
2
1
0
20
40
60
80
100
02
204
406
6008
800
100
Time on stream, h
Time on stream, h
(c)
(d)
100
80
60
40
20
0
100
H2O
80
60
40
20
0
=
=
nCH3OH
H2O
CH3OH
C4
C2
CH3OH
nH2O
(CH3)n+1
CH3OH
=
C3
Higher
alkenes
(CH3)n
H2O
Alkanes
=
C2
(CH3)n+2
H2O
=
CH3OH
C4
02
2004
4006
6008
80
100
Time on stream, h
Fig. 9. Catalytic performance of the synthesized catalysts in the MTO reaction: (a) Selectivity for propylene; and (b) selectivity for C2~C4
light olefins: (1) NZ5; (2) 2.2Fe-NZ5; and (3) 2.2FeD-NZ5; (c) conversion of methanol and product distribution over the 2.2FeD-NZ5
catalyst; (d) proposed mechanism of the MTO reaction [28].
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HIGHLY SELECTIVE MTO REACTION OVER A NANOSIZED ZSM-5 ZEOLITE
Samples
145
NZ5
2.2Fe-NZ5
2.2FeD-NZ5
506
504
525
633
627
621
0.5
0.4
0.3
0.2
0.1
0.0
(a)
99
98
97
96
95
94
3
3
2
1
2
1
Coke
Samples
content, %
3.02
1.03
0.64
NZ5
2.2Fe-NZ5
2.2FeD-NZ5
0
02
200
00
400
4006
600
00
800
200
400
600
800
T, °C
T, °C
Fig. 10. (a) TGA and (b) DTG curves of the parent and Fe-modified nanosized ZSM-5 zeolite catalysts: (1) NZ5; (2) 2.2Fe-NZ5; and
(3) 2.2FeD-NZ5.
the reduction of isolated iron oxide particles deposited
on the surface of the sample [24]. The 2.2FeD-NZ5
sample exhibited two zones of hydrogen consumption,
i.e., 350–500°C (peak temperature was 429°C) and
500–700°C (peak temperature was 642°C). The first
peak, according to the reduction of Fe species on the
surface of the 2.2FeD-NZ5 sample, was shifted to a
higher temperature (429°C). This can be explained by the
stronger interactions between the hydroxyl groups on the
surface of the ZSM-5 zeolite and the Fe species of the
sample prepared by the DBD method, which inhibited
the reduction of iron oxide particles to Fe [25]. The
second hydrogen consumption peak for the 2.2FeD-NZ5
sample obtained at a high temperature of 642°C can be
assigned to the reduction of highly dispersed Fe species
deposited on the pore mouth or in the zeolite channel
[26]. In addition, the larger total areas of two peaks for
the 2.2FeD-NZ5 sample indicated that greater quantities
of Fe species presented on the zeolite surface and within
the channel. To further compare the dispersion of Fe
species on the two samples modified by different methods,
H2 chemisorption characterization was carried out. The
amounts of desorbed hydrogen from the 2.2Fe-NZ5
and 2.2FeD-NZ5 samples were 13.0 and 16.7 μmol g–1,
respectively. This indicates that the 2.2FeD-NZ5 sample
has more Fe species exposed on the zeolite surface,
consistent with the H2-TPR characterization results.
Acidity. The acidity of the nanosized ZSM-5 zeolite
and Fe-modified samples was characterized by NH3-
TPD and FTIR measurements of adsorbed pyridine.
The results are shown in Figs. 7, 8 and Table 3. It can be
seen from Figs. 7 and Table 3 that for the Fe-modified
samples, two peaks appeared at 120–260°C and 260–
450°C, attributed to the weak and strong acid sites in the
catalysts, respectively. There was no significant change
in the temperature of NH3 desorption on the weak acidic
sites of the synthesized samples, but the high-temperature
desorption peak of Fe-modified samples significantly
shifted to lower temperatures than that of the parent
ZSM-5 sample. The acid strength of strong acid sites
of the samples decreases in the following order: NZ5 >
2.2Fe-NZ5 > 2.2FeD-NZ5. In addition, fewer strong acid
sites were observed for the 2.2FeD-NZ5 catalyst. These
results indicated that the highly dispersed Fe promoter of
the 2.2FeD-NZ5 sample has a stronger interaction with
the bridging hydroxyl group on the surface and in the
channels of the zeolite. The slight increase in the number
of weak acid sites can be attributed to the formation of
new acid sites provided by the deposited iron oxide or
the presence of nonskeletal aluminum species caused by
dealumination during the modification process.
Figure 8 shows the FTIR spectra of adsorbed pyri-
dine. Absorption peaks at ~1542 and ~1455 cm–1 were
observed for all samples, corresponding to the Brønsted
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YUZONG LI et al.
Table 4. MTO reaction results over the parent and Fe modified nanosized ZSM-5 zeolite catalysts
Product distribution, mol %
Catalysts
C2–C4
olefin
+
C2H4
C3H6
C4H8
C1–C4 paraffin
C5
arom
NZ5
16.4
15.6
15.2
45.3
47.7
51.5
15.5
16.7
17.0
8.8
8.8
8.9
8.13
5.88
2.75
5.87
5.32
4.65
77.2
80.0
83.7
2.2Fe-NZ5
2.2FeD-NZ5
C5+ : C5+ products except for aromatics.
and Lewis acid sites, respectively. Moreover, Table 3
shows that the number of Brønsted acid sites in the Fe-
modified samples decreased, and the number of Lewis
acid sites increased. The 2.2FeD-NZ5 catalyst had the
fewest Brønsted acid sites and the most Lewis acid sites.
This is attributed to the reaction of more highly dispersed
iron species with the bridging hydroxyl group of the
zeolite, thus reducing the number of Brønsted acid sites
and forming more new Lewis acid sites during the DBD
treatment process. These results are consistent with the
results obtained by NH3-TPD.
for both propylene and total light olefins over the Fe-
modified catalysts obviously increased in the following
order: NZ5 < 2.2Fe-NZ5 < 2.2FeD-NZ5.
Table 4 shows that the selectivity for ethylene and
aromatics decreased with increasing propylene and butene
selectivity over the Fe-modified NZ5 zeolite catalysts.
This result can be explained by the dual-cycle reaction
mechanism of the MTO reaction (Fig. 9d).According to this
mechanism, ethylene and aromatics are mainly produced
by the dealkylation process of multymethylbenzene in
the aromatic-based cycle, while propylene and butene
are formed by the cracking of high-carbon olefins in the
olefin-based cycle. The decreased density and weakened
acid strength of the Brønsted acid sites inhibit the aromatic-
based cycle, thereby promoting the olefin-based cycle. For
this reason, for the 2.2FeD-NZ5 catalyst, which has the
fewest Brønsted acid sites and weakest acid strength due
to the strong interaction between the highly dispersed iron
species and the bridging hydroxyl group of the zeolite, the
lowest selectivity for ethylene and aromatics was obtained
because of the inhibition of the aromatic-based cycle,
while the selectivity for propylene and butene reached a
maximum due to effective promotion of the olefin-based
cycle, and thus, the maximum selectivity of the total C2–C4
light olefins of 83.7% reached.
Catalytic performance. It is known that the strong
acid sites of the catalyst are necessary for dehydration
of methanol and formation of the first C–C bonds in
the MTO reaction, but the strong acidity also leads to
the hydrogen transfer, alkylation and polycondensation
reactions, which convert light olefins into paraffins,
aromatics, cycloalkanes and higher olefins [27].
Therefore, in this work, weakening the acid strength
and decreasing the strong acid content of the catalysts
are beneficial for inhibiting these associated reactions to
improve the selectivity for C2=–C4= light olefins.
To study the effect of Fe modification by different
methods on the catalytic performance of the MTO
reaction, the parent and Fe-modified nano-ZSM-5 zeolites
were eValuated in a fixed-bed reactor under identical
conditions (T = 490°C, P = 0.1 MPa, WHSV = 1.0 h–1,
N2 flow rate = 40 mL/min). All catalysts achieved full
methanol conversion (100%) for a long time period
(100 h). The MTO reaction results over all catalysts during
the long-term investigation and the proposed mechanism
are shown in Fig. 9, and the product distribution of the
MTO reaction at TOS = 100 h is listed in Table 4.
The TG and DTG curves of the catalysts after reaction
for 100 h are shown in Figs. 10a and 10b, respectively.
The weight loss peak below 100°C and the weight loss
peak between 300 and 700°C correspond to desorption of
the physically adsorbed water and the decomposition of
carbonaceous species, respectively. The carbon content of
the spent catalysts was calculated from the weight loss in
the range of 300 to 700°C. It can be clearly seen that the
carbon content on the Fe-modified 2.2Fe-NZ5 and 2.2FeD-
NZ5 catalysts is markedly lower than that on the NZ5
sample, which can be attributed to the reduced number of
Brønsted acid sites and the weakened acid strength of the
catalysts. Moreover, the 2.2FeD-NZ5 sample has the lowest
carbon content of 0.64 wt % due to its significantly reduced
Figures 9a and 9b show that the selectivity for
propylene and C2–C4 light olefins during the long-term
reaction of 100 h over all the catalysts remains constant,
which indicates that all catalysts had excellent stability.
Compared with that of the NZ5 catalyst, the selectivity
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HIGHLY SELECTIVE MTO REACTION OVER A NANOSIZED ZSM-5 ZEOLITE
147
International Science and Technology Innovation Cooperation
Key Project (2018YFE0108800), the Transformation for Science
and Technology Achievements in University of Department of
Education of Heilongjiang Province (TSTAU-C2018015)
number of Brønsted acid sites and weak acid strength, and
these characteristics inhibit the formation of carbon deposi-
tion precursors produced by the aromatic-based cycle. As
seen from the DTG curve in Fig. 10b, in addition to the
desorption peak corresponding to the physically adsorbed
water below 100°C, three weight loss peaks are observed
at approximately 325, 505, and 630°C, corresponding to
the decomposition of carbon deposits resulting from acid
centers with different acid strengths. In addition, decom-
position of the carbon deposit on the 2.2FeD-NZ5 catalyst
occurred at a lower temperature due to the moderate acidity
of this catalyst, indicating that the formed carbon deposit
species are more easily eliminated.
CONFLICT OF INTEREST
The other authors declare that they have no conflict
of interest.
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