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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-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 93 No. 1 2020