Catalysts for Dimethyl Ether Conversion to Lower Olefins: Effect of Acidity, Postsynthesis Treatment, and Steam and Methanol Content in Feedstock

Article abstract of DOI:10.1134/S0965544119040091

Abstract: The most important and interesting results obtained at the Topchiev Institute of Petrochemical Synthesis of the Russian Academy of Sciences in 2008–2015 concerning dimethyl ether conversion to lower olefins in the presence of HZSM-5 zeolite catalysts have been systematized and summarized. The effects of the nature of the modifying element and catalyst steaming on the structural, acidic, and catalytic properties of the zeolite catalyst have been discussed. Some features of the influence of steam present in the feed gas mixture as a function of the nature of the modifying element have been elucidated. Data on the effect of impurities in the feed mixture (methanol and steam) have been described.

Full text of DOI:10.1134/S0965544119040091

ISSN 0965-5441, Petroleum Chemistry, 2019, Vol. 59, No. 4, pp. 427–437. © Pleiades Publishing, Ltd., 2019.
Russian Text © S.N. Khadzhiev, N.V. Kolesnichenko, E.N. Khivrich, T.I. Batova, 2019, published in Neftekhimiya, 2019, Vol. 59, No. 3, pp. 304–314.
Catalysts for Dimethyl Ether Conversion to Lower Olefins:
Effect of Acidity, Postsynthesis Treatment, and Steam
and Methanol Content in Feedstock
a, †
a,
a
a
S. N. Khadzhiev , N. V. Kolesnichenko *, E. N. Khivrich , and T. I. Batova
a
Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, Russia
*e-mail: nvk@ips.ac.ru
Received November 5, 2018; revised November 28, 2018; accepted December 10, 2018
Abstract—The most important and interesting results obtained at the Topchiev Institute of Petrochemical
Synthesis of the Russian Academy of Sciences in 2008–2015 concerning dimethyl ether conversion to lower
olefins in the presence of HZSM-5 zeolite catalysts have been systematized and summarized. The effects of
the nature of the modifying element and catalyst steaming on the structural, acidic, and catalytic properties
of the zeolite catalyst have been discussed. Some features of the influence of steam present in the feed gas
mixture as a function of the nature of the modifying element have been elucidated. Data on the effect of impu-
rities in the feed mixture (methanol and steam) have been described.
Keywords: dimethyl ether, methanol, steam, zeolite catalysts, lower olefins, stability, high-temperature treat-
ment of catalysts
DOI: 10.1134/S0965544119040091
compounds [5–9], which can lead to a change in their
textural and acidic characteristics [10–12]. There are a
large number of reports on studying the effect of the
nature of the modifying element on the properties of
zeolite catalysts [13–17]; however, a concept for
understanding the role of the nature of the modifying
element in the change in the catalytic properties of
these systems has not yet been developed. Therefore, it
Synthesis of lower olefins is a large-scale process;
the production output of these materials is constantly
increasing [1]. The thermal pyrolysis and catalytic
cracking of hydrocarbon feedstocks remain the main
methods used to produce ethylene and propylene. The
rates of growth in the consumption of these valuable
chemicals are far ahead of the scale of their produc-
tion; therefore, it is necessary to search for another—
cheaper and more available—hydrocarbon feedstock. is important to develop a general approach to the
Natural gas can be this feedstock; the production and selection of hydrocarbon conversion catalysts that
large-scale use of natural gas is forecast for a long time would make it possible to differentiate the effect of
period. The best-known and most thoroughly studied modification of zeolites on their acidic, molecular-
method to produce chemicals from natural gas is their sieving, and catalytic properties depending on the
synthesis through the intermediate step of natural gas nature of the modifying element. A change in acidic
reforming to synthesis gas (СО/Н ).
properties can also be achieved by a high-temperature
pretreatment of the catalysts or by changing the water
content in the feedstock. In this context, it is highly
relevant to obtain data on the effect of acidic proper-
ties on the catalytic properties of zeolite catalysts for
DME conversion to olefins. In this case, it is signifi-
cant to determine the effect of the nature of the mod-
ifying element not only on the activity, but also on the
stability of the catalytic properties. These data are par-
ticularly important for reactions running in a steam
atmosphere, which is used in a number of industrial
processes [18–21], because steam can have a signifi-
cant effect not only on the distribution of DME con-
version products, but also on the structural character-
istics of the zeolite catalyst.
2
At present, a process of great interest is the produc-
tion of C –C olefins from synthesis gas through the
2
3
intermediate synthesis of dimethyl ether (DME),
which provides the formation of lower olefins with a
high yield and a higher selectivity [2, 3]. Dimethyl
ether may be thought of as one of the possible key
agents in the conversion of nonpetroleum feedstocks
to valuable chemicals, such as olefins [4]. The most
common base for catalysts for the synthesis of lower
olefins from DME is a high-silica zeolite of the ZSM-
5
type. The targeted impact on the properties of zeolite
catalysts consists in their modification with metal
†
Deceased.
427

4
28
Table 1. Characteristics of the zeolite catalysts
Sample Crystallinity**, % SВЕТ, m /g V , cm /g V
KHADZHIEV et al.
2
3
3
3
, cm /g V
, cm /g Reference
pore
micropore
mesopore BJH
NH ZSM-5*
100
91
314
311
313
324
305
286
0.201
0.200
0.214
0.222
0.224
0.214
0.154
0.047
0.048
0.069
0.076
0.086
0.081
–
4
HZSM-5/Al O3
0.152
0.145
0.146
0.138
0.133
[30]
[30]
[30]
[30]
[31]
2
La–HZSM-5/Al O3
93
90
90
92
2
Zr–HZSM-5/Al O3
2
La–Zr–HZSM-5/Al O3
2
Mg–HZSM-5/Al O3
2
*
A commercial ZSM-5 zeolite sample in the ammonium form.
*
* Degree of crystallinity was calculated as described in [32].
In this study, results of the development of funda- atmospheric pressure. The catalyst load in the reactor
3
mentally new highly efficient catalyst systems based on was 3–10 cm .
a ZSM-5 zeolite modified with various elements are
The feedstock was DME (weight fraction of DME
summarized and the effect of the nature of the modi-
fying element and a high-temperature steam treatment
on the structural, acidic, and catalytic properties of
the zeolite catalyst for DME conversion to olefins is
discussed.
of 99.99%); the DME diluent was nitrogen and/or
steam and methanol. The concentration of DME and
methanol in the feed gas mixture was 10–75 and 10–
3
0 vol %, respectively. The space velocity of the feed
–1
gas mixture was varied in a range of 2000–15000 h .
The procedure for testing and analyzing the reaction
products is described in [33].
EXPERIMENTAL
A high-temperature pretreatment of the zeolite was
conducted as described in [34, 35] at a temperature of
The studied catalytically active systems for lower
7
50°C in an air atmosphere for 4 h. The catalysts were
olefin synthesis from DME were based on a ZSM-5
treated steaming at 500 and 750°C in a flow reactor as
+
4
zeolite in the ammonium form (NH ) with a
described in [32].
SiO /Al O molar ratio of 37 and a sodium oxide con-
2
2
3
The structural characteristics of the original zeolite
and the modified samples before and after their treat-
ment were studied by infrared (IR) spectroscopy, X-
ray diffraction (XRD) analysis, benzene adsorption,
and solid-state nuclear magnetic resonance (NMR);
the acidic properties of the catalysts were studied by
temperature-programmed desorption of ammonia
tent of at most 0.04 wt % (manufactured by the
Angarsk catalyst and organic synthesis plant, Russia).
The zeolite was transferred into the hydrogen form
+
(
H ) by calcining it at 500°C for 4 h. The catalysts
were synthesized by mixing an HZSM-5 zeolite with
an alumina slurry used as a binder (contains 23 wt % of
dry Al O , manufactured by Promyshlennye Kataliza-
2
3
(
NH -TPD).
3
tory, Ryazan, Russia) and subsequently shaping the
extrudates (Al O content in the finished catalyst is
2
3
3
3–34 wt %). The catalysts were modified with mag-
Diffuse Reflectance Infrared Fourier Transform
nesium, lanthanum, and zirconium compounds.
These active elements were identified as the most
promising for synthesizing industrial catalysts [22–
(DRIFT) Spectroscopy
The DRIFT spectra were recorded in situ at high
temperatures in an inert atmosphere (Ar), as well as in
a stream of DME mixture with nitrogen or steam. The
spectra were recorded in the temperature range of 25–
2
9]. For each of the elements, the most optimum
method for introducing it into the catalyst was deter-
mined. Thus, lanthanum was introduced directly into
the zeolite by a method based on the ion exchange of
the hydrogen form of the zeolite with an aqueous solu-
tion of a metal salt; zirconium was introduced by the
incipient wetness impregnation of the zeolite before
mixing it with the binder. Magnesium was introduced
by impregnating the finished zeolite extrudates mixed
with the binder with an aqueous solution of a metal
salt. Characteristics of the original zeolite and the
modified catalysts based on it are listed in Table 1.
4
50°C using a PIKE Technologies DiffusIR high-
temperature cell coupled with a Bruker VERTEX-70
FTIR spectrometer. The spectrum was recorded in a
continuous mode for 5 min (200 scans/spectrum) at a
–1
–1
resolution of 2 cm in the range of 600–4000 cm .
The mathematical processing of the IR spectra was
conducted using the OPUS-7 software package.
X-ray Diffraction Analysis
Tests of the catalysts were conducted on a system
The XRD data were obtained on a Stoe STADT P
with a flow reactor at a temperature of 320–380°C and powder diffractometer using CuK radiation at a wave-
α
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CATALYSTS FOR DIMETHYL ETHER CONVERSION TO LOWER OLEFINS
Table 2. Acidic properties of studied zeolite catalysts
429
Number of sites
Number of sites
d
Sample
Total acidity, μmol/g
I/II*
with Е < 130 kJ/mol (I) with Е > 130 kJ/mol (II)
d
HZSM-5
HZSM-5/Al O *
778
757
634
880
534
586
227 (30%)
551 (70%)
482 (64%)
376 (59%)
497 (56%)
278 (52%)
325 (55%)
0.4
0.6
0.7
0.8
0.9
0.8
275 (36%)
258 (41%)
383 (44%)
256 (48%)
261 (45%)
2
3
La–HZSM-5/Al O3
2
Zr–HZSM-5/Al O3
2
Mg–HZSM-5/Al O3
2
La–Zr–НZSM-5/Al O3
2
High-temperature treatment of zeolite (Т)
La–Zr–НZSM-5(Т)/Al O3
461
367
265 (58%)
242 (66%)
196 (42%)
125 (34%)
1.4
1.9
2
Mg–HZSM-5(Т)/Al O3
2
Steaming of catalyst
Mg–HZSM-5/Al O (500°С)
491
197
319
170
191 (39%)
156 (79%)
172 (54%)
137 (81%)
300 (61%)
41 (21%)
147 (46%)
33 (19%)
0.6
3.8
1.2
4.2
2
3
Mg–HZSM-5/Al O (750°С)
2
3
La–Zr–HZSM-5/Al O (500°С)
2
3
La–Zr–HZSM-5/Al O (750°С)
2
3
*
I/II is the ratio of the fraction of medium-strength acid sites to the fraction of strong acid sites.
length of λ = 1.5418 Å, a voltage of 40 kV, and a current
of 30 mA.
Textural Characteristics
The textural characteristics of the samples (specific
surface area, total pore volume, and pore size distribu-
tion) were studied by low-temperature adsorption–
desorption of molecular nitrogen on a Micromeritics
ASAP-2010 instrument. Prior to analysis, all the sam-
ples were evacuated at a temperature of 350°C to 4 ×
Solid-State NMR
The solid-state NMR spectra were recorded on an
1
1.7 T Varian Unity Inova 500 instrument. The Lar-
27
mor frequencies were 130 and 99 MHz for Al and
–1
29
27
10 Pa. Nitrogen adsorption was run at a temperature
of 77 K.
Si, respectively. The Al spectra were recorded using
a 3.2-mm probe at a spinning rate of 15 kHz, a π/2
pulse length of 1 μs, an acquisition time of 0.015 s, and
29
an interval between the scans of 0.5 s. The Si magic
angle spinning (MAS) NMR spectra were recorded
using a 7.5-mm probe at a spinning rate of 4.5 kHz, a
π/6 pulse length of 2 μs, an acquisition time of 0.01 s,
and an interval between the scans of 30 s. The stan-
RESULTS AND DISCUSSION
The crucial factor of the effectiveness of zeolite cat-
alysts in various versions of oxygenate conversion to
hydrocarbons is the acidic properties of the catalysts,
which can be varied by modifying the zeolites by post-
synthesis high-temperature treatment of the original
HZSM-5 zeolite or hydrothermal treatment (steam-
ing) of the zeolite catalyst. The latter treatment leads
not only to a change in the acidic properties of the cat-
alysts, but also to the formation of additional acid
sites, thereby affecting the catalytic and physicochem-
ical properties of the zeolite catalyst. The strength dis-
tribution of acid sites as a function of the nature of the
introduced element and the method of postsynthesis
treatment of zeolite catalysts is shown in Table 2.
29
27
dards for determining the positions of the Si and Al
spectral lines were tetramethylsilane and aluminum
chloride, respectively.
Benzene Adsorption
These measurements were made in a dynamic
mode at 200°C by gas–solid chromatography as
described in [36]. The calculation of isotherms was
performed by the Glueckauf method [37].
Ammonia Temperature-Programmed Desorption
The modification of the samples with a magnesium
Ammonia TPD data for the synthesized samples compound and the double modification with lantha-
were obtained as described in [32] and processed by num and zirconium lead to a significant decrease in
fitting the shapes of the experimental and calculated the total acidity; at the same time, the fraction of
curves as described in [38].
medium-strength acid sites (I/II) slightly increases.
PETROLEUM CHEMISTRY Vol. 59 No. 4 2019

4
30
KHADZHIEV et al.
2
2
2
2
2
800
600
400
200
000
4
1
800
600
400
200
000
1
1
3
2
1
1
1
8
00
00
00
00
0
6
4
2
0
10
20
30
2
40
50
60
70
θ
Fig. 1. X-ray diffraction pattern of zeolite catalysts: (1) HZSM-5/Al O , (2) Mg–HZSM-5/Al O , (3) La–Zr–HZSM-5/Al O ,
2
3
2
3
2 3
and (4) La–Zr–HZSM-5/Al O steamed at 500°C.
2
3
A high-temperature treatment of the original zeo-
This effect can be attributed to the dealumination
lite leads to a significant decrease in the total acidity of the zeolite by steaming the catalysts. For the sam-
and an increase in the fraction of medium-strength ples modified with lanthanum and zirconium com-
29
acid sites. Steaming at 500°C also leads to a decrease pounds and with magnesium, the Si reflection
in the total acidity; however, the fraction of strong acid shifted from −108.5 to −110 ppm, indicating a change
sites increases in the case of Mg–HZSM-5/Al O and in the local environment of silicon atoms in the crystal
2
3
decreases in the case of La–Zr–HZSM-5/Al O . structure of the zeolite.
2
3
Steaming at 750°C leads to an abrupt decrease in the
More significant changes were observed in the
total acidity and the fraction of strong acid sites.
27
27
Al NMR spectrum (Fig. 3). The Al reflection at
Steaming of zeolite catalysts can lead to partial deg- 58.5 ppm corresponding to tetragonally coordinated
radation of their structure. To verify the effect of aluminum atoms in the zeolite structure was shifted to
steaming on the structural properties of zeolites, the 56 ppm by treatment with modifier compounds and to
La–Zr–HZSM-5/Al O and Mg–HZSM-5/Al O
3
53.6 ppm after steaming the Mg–HZSM-5 catalyst. In
2
3
2
catalysts samples were subjected to XRD analysis addition, the spectrum exhibited a broad signal, which
before and after treatment to compare with original coincided with the resonance of the standard in the
HZSM-5/Al O .
region of 0 ppm. Thus, it can be concluded that some
of the aluminum atoms were withdrawn from the zeo-
2
3
According to XRD data shown in Fig. 1 for some of
the modified zeolites, all the studied samples, regard-
less of the nature of the modifying agent, the modifi-
cation method, and the treatment temperature, corre-
3+
lite structure and transformed into Al ions during
modification and steaming.
27
An interesting feature exhibited by the Al NMR
spond to the crystallographic type of the ZSM-5 zeo- spectra is a significant decrease in the signal intensity
lite. The calculated unit cell parameters are as follows: at 56 ppm for the La–Zr–HZSM-5 catalyst after
a = 20.09, b = 19.96, and c = 13.40 Å (according to steaming, especially at 750°C. Taking into account the
published data, a = 20.02, b = 19.89, c = 13.38 Å [39]). fact that the acquisition of signals is the same in all
Thus, steaming did not lead to the complete degrada- cases, the observed effect can be attributed to a
tion of the zeolite crystal lattice. The unit cell param- decrease in zeolite crystallinity, including that due to
eters of the zeolite also remained almost unchanged.
dealumination. Comparison of the intensity of
27
29
Al NMR signals for these catalysts suggests that the
Figure 2 shows the Si NMR spectra of the La–
Zr–HZSM-5 and Mg–HZSM-5 samples before and
after steaming. Comparison of the positions of the sig-
nals with published data for the unmodified zeolite
presence of magnesium oxide in the sample contrib-
utes to some stabilization of the structure during
steaming.
[
40] allow us to attribute the reflection at −108.5 ppm
Figure 4 shows the results of IR spectroscopy study
29
in the Si NMR spectrum to the Si(OSi) moiety and of the La–Zr–HZSM-5/Al O crystal structure
4
2 3
the signal at −104.2 ppm, to the Si(OH)(OSi)
before and after steaming. The spectrum of the origi-
nal HZSM-5/Al O exhibits absorption bands at 720–
3
hydroxylated groups. After steaming at 500°С, the
shoulder at −104.5 ppm disappears for La–Zr– 780 cm , which characterize the Al–O bond with the
HZSM-5 and decreases for Mg–HZSM-5. tetrahedral aluminum environment. In the case of the
2
3
–1
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CATALYSTS FOR DIMETHYL ETHER CONVERSION TO LOWER OLEFINS
a) (b)
431
(
–111.5
–110.5
–110
–111
4
7
–108.5
3
2
–
106
6
5
–104.5
–
104.5
1
1
–88
–96 –104 –112 –120 –128
–88
–96
–104 –112 –120 –128
Fig. 2. 29Si MAS solid-state NMR spectra for (a) La–Zr–HZSM-5 and (b) Mg–HZSM-5 catalysts: (1) HZSM-5, (2) La–Zr–
HZSM-5, (3) La–Zr–HZSM-5 steamed at 500°C, (4) La–Zr–HZSM-5 steamed at 750°C, (5) Mg–HZSM-5, (6) Mg–
HZSM-5 steamed at 500°С, and (7) Mg–HZSM-5 steamed at 750°С.
53.6
(a)
(b)
5
6
7
4
3
58.5
6
5
58.5
2
1
1
80
70 60 50 40 30 20 10
0
–10 80 70 60 50 40 30 20 10
0
–10
Fig. 3. 27Al MAS solid-state NMR spectra for (a) La–Zr–HZSM-5 and (b) Mg–HZSM-5 catalysts: (1) HZSM-5, (2) La–Zr–
HZSM-5, (3) La–Zr–HZSM-5 steamed at 500°C, (4) La–Zr–HZSM-5 steamed at 750°C, (5) Mg–HZSM-5, (6) Mg–
HZSM-5 steamed at 500°С, and (7) Mg–HZSM-5 steamed at 750°С.
HZSM-5/Al O zeolite catalyst modified with La and siderable changes were found in the intensity and posi-
2
3
tion of the absorption bands due to bending vibrations
in the region of the structural vibrations of the zeolite
framework at 450 and 550 cm . Thus, steam treat-
ment of the catalysts leads to dealumination; in addi-
tion, the remaining aluminum partially loses the tetra-
hedral environment [32].
Zr (spectrum 2), the band is shifted from 795.5 to
–1
7
86.9 cm ; the shift indicates the appearance of het-
–1
eroatoms in the zeolite composition. Steaming the
La–Zr–HZSM-5/Al O catalyst at 500°С leads to a
2
3
shift of this absorption band toward short wavelengths
–1
to 790.3 cm (spectrum 3), and steaming the catalyst
at 750°С leads to an even more significant shift of this
The adsorption properties of the La–Zr–HZSM-
–1
band, namely, to 804.7 cm (spectrum 4). This shift 5/Al O and Mg–HZSM-5/Al O catalysts before and
2
3
2
3
of the absorption band indicates a decrease in the after steaming were studied. For the La–Zr–HZSM-
number of aluminum atoms in the tetrahedral config- 5/Al catalyst, the amount of adsorbed benzene
O
3
2
uration and an increase in the coordination number of decreases with an increase in steaming temperature;
the remaining aluminum. At the same time, no con- for the Mg–HZSM-5/Al
O sample, this parameter
3
2
PETROLEUM CHEMISTRY Vol. 59 No. 4 2019

4
32
KHADZHIEV et al.
4
912
3
2
804.7
925
790.3
9
38 919
891
5
50.4
1
4
4
47.7
48.9
5
49.8
786.9
795.5
5
46.4
451.7
5
46.4
452.5
9
00
800
500
400
–
1
cm
Fig. 4. Infrared spectra of catalyst samples: (1) HZSM-5/Al O , (2) La–Zr–HZSM-5/Al O , and La–Zr–HZSM-5/Al O
2 3
2
3
2
3
steamed at (3) 500 and (4) 750°C.
increases (Table 3). It is unlikely that this effect is probably lead to an overestimated limiting adsorption
caused by different patterns of degradation of the zeo- value for Mg–HZSM-5/Al O . Steaming at 750°C
2
3
lite crystal lattice, because lanthanum-containing leads to partial degradation of the zeolite crystal struc-
forms are more stable. In this case, a crucial role is ture, as evidenced by a significant decrease in ammo-
apparently played by cation migration, which vigor-
ously occurs during the hydrothermal treatment of the
catalysts. Apparently, lanthanum and zirconium are
nia adsorption according to NH -TPD data (Table 2).
3
Characteristic energies of adsorption E for benzene
mostly concentrated in the zeolite and thereby were calculated from the experimental data by the
decrease the pore volume, while magnesium is con- Dubinin equation [41]. It is evident from Table 3 that
centrated in the binder—alumina—and thereby emp- the characteristic energies of adsorption for benzene
ties the zeolite pores. At the same time, it is necessary differ depending on the nature of the modifier and the
to take into account the different characters of ben- calcination temperature. A nearly antibatic change in
zene adsorption on these catalyst samples, which the characteristic energy of adsorption and the limit-
Table 3. Calculated values of limiting adsorption, characteristic energies, and differential molar heats of adsorption
Q, kJ/mol ±1.0
а ,
μmol/g
0
Sample
Е, kJ/mol ±0.5
а = 10
μmol/g
а = 100
μmol/g
а = 200
μmol/g
HZSM-5/Al O3
1219
1121
987
19.6
16.9
16.1
20.6
19.9
17.3
16.0
112
94
67
56
51
54
59
55
51
52
43
39
38
44
48
39
2
La–Zr–HZSM-5/Al O3
2
La–Zr–HZSM-5/Al O steamed at 500°С
87
2
3
La–Zr–HZSM-5/Al O , steamed at 750°С
590
813
103
104
94
2
3
Mg–HZSM-5/Al O3
2
Mg–HZSM-5/Al O , steamed at 500°С
997
2
3
Mg–HZSM-5/Al O steamed at 750°С*
1050
88
2
3,
*
Benzene adsorption on this sample lies in the Henry region; therefore, the estimate of the limiting a value according to the Dubinin
0
equation is overestimated.
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CATALYSTS FOR DIMETHYL ETHER CONVERSION TO LOWER OLEFINS
433
ing adsorption of benzene is observed. For the La– in the olefin selectivity, and a decrease in the alkane
Zr–HZSM-5/Al O catalyst samples, the characteris- selectivity, which is most probably due to partial deg-
2
3
tic energy of adsorption increases with a decrease in radation of the zeolite crystal structure [32].
the limiting adsorption of benzene; this finding can be
Lower olefins are typically synthesized in an inert
attributed to lanthanum and zirconium cations con-
gas atmosphere at a DME concentration of at most
centrated in the zeolite. In contrast, for the Mg–
3
0 vol %. If the process is run in a steam atmosphere,
HZSM-5/Al O catalyst sample, steaming decreases
2
3
which is used in a number of industrial processes [18–
1], the steam/feedstock ratio can have a significant
effect on distribution of DME conversion products
43]. A change in this ratio can lead to both an increase
the characteristic energy of benzene adsorption and
increases the limiting adsorption; this effect is associ-
ated with the concentrating of magnesium in the
binder.
2
[
and a decrease in the yield of lower C –C olefins. In
2
4
At all the given a values, the maximum heat of
addition, in the synthesis of lower olefins from CO and
adsorption was obtained for the original HZSM-
H through the intermediate production of DME, the
2
5
/Al O zeolite. With an increase in the a value, the Q
2 3
production of DME from CO and H is accompanied
2
values of adsorbed benzene regularly decrease for all
the samples.
by the formation of water, which can be involved in the
synthesis of lower olefins.
Kitaev et al. [42] observed that in the case of non-
specific adsorption of n-paraffins in pentasil channels,
the characteristic energy of adsorption does not exceed
The effect of the DME/steam ratio on the distribu-
tion of the reaction products in the absence of nitrogen
was studied at a constant DME load. The substitution
of steam for nitrogen has a considerable effect on the
catalytic properties of the zeolite catalysts; this effect
also significantly depends on the nature of the modi-
fying elements (Fig. 5). Thus, after the addition of
steam to the feedstock, at a DME concentration of
~
22 kJ/mol. In this study, we found that the E value
for benzene is 16–21 kJ/mol. Thus, the adsorption of
benzene molecules is most probably nonspecific and
the observed changes in the characteristic energy of
adsorption are apparently due to diffusion factors.
The synthesized catalysts modified with different
elements and subjected to different postsynthesis
treatments were studied in DME conversion to lower
olefins at two reaction temperatures (320 and 380°C),
which correspond to the temperatures at the inlet and
outlet of the reaction zone of an industrial adiabatic
reactor (Table 4).
2
0 vol %, in the presence of the La–Zr–HZSM-
/Al O catalyst, the total selectivity for C –C olefins
5
2
3
2
4
remains almost unchanged and weakly depends on the
steam content in the feedstock (25–90 vol %). Meth-
anol was not detected in the reaction products; the
alkane content decreased with an increase in the steam
concentration in the feedstock. In the presence of the
The introduction of an active element into a zeolite magnesium-modified zeolite catalyst in a steam atmo-
catalyst leads to a change not only in the acidic char- sphere, the selectivity for lower olefins abruptly
acteristics, but also in the catalytic properties of the decreases. However, with an increase in the steam
sample. In all the cases, except for magnesium, modi- content, the olefin selectivity increases, while the
fication leads to a decrease in catalytic activity at T = alkane selectivity remains almost unchanged; in this
3
20°C. The selectivity for ethylene and propylene case, the methanol content in the reaction products is
changes symbatically with a change in the ratio of high. Although the content of methanol decreases
medium-strength acid sites (I) to strong acid sites (II). with an increase in the steam content, the fraction of
It is medium-strength acid sites that are involved in this product remains dominant.
the catalytic formation of ethylene and propylene,
The in situ DRFIT spectroscopy study showed that
in the case of passing DME through Mg–HZSM-
while DME conversion occurs on all acid sites [32].
An increase in the reaction temperature from 320
to 380°C leads to an increase in activity and a decrease
in the olefin yield, which is associated with an increase
in the rate of secondary reactions.
The high-temperature treatment of the original
zeolite leads to an increase in the olefin selectivity and
provides a high DME conversion that is stable over
time and significantly exceeds the DME conversion
observed for the original catalyst [32].
Steaming the catalysts at 500°C leads to a signifi-
cant increase in activity at both 320 and 380°C, while
the selectivity slightly decreases; this effect is mostly
due to the change in acidity (Table 4). Steaming the
catalyst at 750°C leads to an abrupt decrease in the
−1
5
/Al O , the spectrum exhibits a band at 1084 cm
2 3
due to the C–O bonds in the methoxy groups [44];
however, in the case of passing DME with H O, the
2
spectrum of Mg–HZSM-5/Al O recorded under the
2
3
same conditions does not exhibit the above band; at
−1
the same time, the intensity of the band at 1047 cm ,
which is the strongest band in the spectrum of metha-
nol, abruptly increases. The formation of methoxy
groups on the zeolite surface can occur during the
interaction of DME with the base sites on the zeolite
surface (absorption band at 3720 cm ) to release
methanol, as shown in [45].
–1
In many cases of synthesis of lower olefins from
DME conversion (from 99.8 to 2.9 wt %), an increase DME, the formation of methanol is observed; metha-
PETROLEUM CHEMISTRY Vol. 59 No. 4 2019

4
34
KHADZHIEV et al.
Table 4. Dimethyl ether conversion to lower olefins in the presence of zeolite catalysts (P = 0.1 MPa; feedstock: 10 vol %
−1
−1
DME + 90 vol % N ; W
= 0.9 h , T = 320°C, W = 3.7 h , T = 380°C, the data for 4 h on stream
DME
2
DME
Hydrocarbon selectivity, wt %
DME
Т, °С
Σ
Catalyst
conversion, %
=
=
=
+
С –С olefins
2
5
C2
C3
ΣC4
C paraffins
1
HZSM-5/Al O3
320
80
320
80
320
80
320
80
320
80
67.4
94.3
72.6
97.3
46.9
96.6
47.3
87.2
50.1
91.5
9.2
14.9
23.3
31.5
31.6
38.0
25.9
36.0
25.8
36.1
27.1
19.5
11.5
12.8
17.4
14.1
13.9
13.1
11.5
13.0
13.0
55.3
36.9
22.2
31.6
25.5
43.0
25.4
44.6
25.2
42.6
44.7
63.2
77.8
68.4
74.5
57.0
74.6
55.4
74.8
57.4
2
3
21.6
30.2
13.7
18.6
13.0
22.3
14.3
21.9
13.0
Mg–HZSM-5/ Al O3
2
3
La–HZSM-5/ Al O3
2
3
Zr–HZSM-5/ Al O3
2
3
La–Zr–HZSM-5/Al O3
2
3
High-temperature treatment of zeolite (Т)
La–Zr–HZSM-5(T)/Al O3
320
80
65.0
95.4
25.1
20.9
30.0
29.2
8.4
8.9
26.7
34.4
73.3
65.6
2
3
Steaming at 500°С
Mg–HZSM-5/Al O3
320
80
320
80
320
80
320
80
93.4
93.2
99.7
98.4
99.2
95.0
95.9
90.8
26.8
10.4
20.0
11.9
19.0
13.1
24.8
24.1
36.0
16.0
29.2
16.1
29.8
20.9
33.6
14.6
17.2
14.8
15.7
13.5
14.3
14.7
14.8
30.1
30.5
44.5
38.0
47.0
38.5
35.0
34.3
69.9
69.5
55.5
62.0
53.0
61.5
65.0
65.7
2
3
La–HZSM-5/Al O3
2
3
Zr–HZSM-5/Al O3
2
3
La–Zr–HZSM-5/Al O3
2
3
12.1
Steaming at 750°С
8.7
La–Zr–HZSM-5/ Al O3
380
2.9
36.1
13.4
19.6
80.4
2
nol in turn can affect the composition of the DME presence of methanol in the reaction zone does not
conversion products.
significantly affect the catalytic properties of La–Zr–
HZSM-5/Al O ; however, at a DME concentration of
2
3
Data on the effect of methanol on the catalytic
properties of La–Zr–HZSM-5/Al O are shown in
up to 30 vol %, the presence of methanol leads to a
2
3
decrease in the selectivity for ethylene and propylene.
Table 5. It is evident from the table that the addition of
methanol to DME hardly affects the total olefin selec-
tivity; with an increase in the methanol concentration
from 10 to 30 vol %, the ethylene content increases
from 25.8 to 30.9 wt %, while the amount of propylene
slightly decreases; in this case, the products contain
many alkanes in which the proportion of methane is
about 15 wt %. With an increase in the DME concen-
tration in the feedstock to 30 vol % in the presence of
methanol, the selectivity for olefins, in particular, for
ethylene, decreases, while the alkane content
increases. A decrease in the space velocity of the feed
Upon the substitution of steam for nitrogen
(
Table 6), in the presence of methanol, the total olefin
selectivity remained almost unchanged; the use of an
equilibrium mixture of oxygenates with steam as the
feedstock leads to an increase in the proportion of eth-
ylene in the products.
CONCLUSIONS
The nature of the active element has a significant
effect on the catalytic and acidic properties of the
modified zeolite catalyst for DME conversion to lower
olefins. The selectivity for lower olefins changes sym-
batically with an increase in the ratio of medium-
−1
mixture to 4000 h leads to a decrease in the alkane
content and an increase in the olefin selectivity.
Thus, in the case of the reaction run in a nitrogen strength acid sites to strong acid sites with a decrease in
atmosphere, at a DME concentration of 20 vol %, the the total acidity. Thus, medium-strength acid sites are
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2019

CATALYSTS FOR DIMETHYL ETHER CONVERSION TO LOWER OLEFINS
435
80
70
60
50
40
30
20
ΣС –C = La-Zr
2
4
ΣС –C = Mg
2
4
MeOH Mg
ΣС + La-Zr
1
ΣС + Mg
1
10
0
2
0
40
60
80
100
Steam content in the feed mixture, vol %
Fig. 5. Effect of feedstock dilution with steam on the catalytic properties of La–Zr–HZSM-5/Al O and Mg–HZSM-5/Al O .
2
3
2 3
Table 5. Effect of methanol addition to the feed mixture on the catalytic properties of La–Zr–HZSM-5/Al O (P = 0.1 MPa,
2
3
T = 340°C, the data for 4 h on stream)
Hydrocarbon selectivity, wt %
Feed mixture, vol %
Parameters
Σ С –С olefins
2
5
=
=
=
+
C2
C3
ΣC4
C paraffins
1
Vmix = 6000 h–1
WDME = 4.1 h–1
Vmix = 6000 h–1
WDME = 4.1 h–1
WМеOH = 1.6 h–1
Vmix = 6000 h–1
WDME = 4.1 h–1
WМеOH = 3.2 h–1
Vmix = 6000 h–1
WDME = 4.1 h–1
WМеOH = 4.8 h–1
Vmix = 6000 h–1
WDME = 6.8 h–1
WМеOH = 4.8 h–1
Vmix = 4000 h–1
WDME = 4.6 h–1
WМеOH = 3.2 h–1
DME(20) + N (80)
26.4
25.8
34.8
33.8
9.4
8.2
21.6
78.4
2
DME(20) + МеOH(10) + N (70)
25.1
74.9
76.2
75.8
69.5
74.3
71.8
2
DME(20) + МеOH(20) + N (60)
28.1
30.9
25.4
27.5
25.8
33.2
32.5
31.1
33.3
23.1
7.9
7.1
7.2
7.6
7.5
23.8
24.2
30.5
25.7
38.7
2
DME(20) + МеOH(30) + N (50)
2
DME(30) + МеOH(30) + N (40)
2
DME(30) + МеOH(30) + N (40)
2
Vmix = 6000 h–1
WМеOH = 3.2 h–1
МеOH(20) + N (80)
2
PETROLEUM CHEMISTRY Vol. 59 No. 4 2019

4
36
KHADZHIEV et al.
Table 6. Oxygenate conversion to lower olefins in the presence of the La–Zr–HZSM-5/Al O catalyst (P = 0.1 MPa, T =
2
3
−1
3
40°C, V = 4000 h , the data for 4 h on stream)
mix
Hydrocarbon selectivity, wt %
Σ С –С
olefins
2
5
Feed mixture
DME(30) + МеOH(30) + N (40)
Parameters
=
=
=
+
C2
C3
ΣC4
C paraffins
1
WDME = 4.6 h–1
WМеOH = 3.2 h–1
WDME = 4.0 h–1
WМеOH = 3.2 h–1
WH2O = 2.0 h–1
27.5
31.9
33.3
33.4
7.6
6.1
25.7
74.3
2
DME(30) + МеOH(30) + H O(40)
25.3
74.7
2
involved in the catalytic formation of ethylene and
propylene, while the DME conversion occurs on all
acid sites.
A postsynthesis high-temperature treatment of the
HZSM-5 zeolite and steaming the catalyst at 500°C
lead to a significant increase in the catalyst stability.
9. R. V. Dmitriev, D. P. Shevchenko, E. S. Shpiro, et al.,
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10. D. Freeman, R. P. K. Wells, and G. J. Hutschings, J.
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1. N. V. Kolesnichenko, T. I. Goryainova, E. N. Biryu-
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2. M. N. Mikhailov, V. B. Kazansky, and L. M. Kustov,
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3. G. M. Zhidomirov, A. A. Shubin, V. B. Kazansky, and
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ACKNOWLEDGMENTS
and C. F. van Egmond, US Patent No. 7989669 (2011).
This work was performed at the Topchiev Institute
of Petrochemical Synthesis, Russian Academy of Sci-
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