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Effect of magnesium on the catalytic properties of polymetallic zeolite catalysts for conversion of dimethyl ether to light olefins
Microporous and Mesoporous Materials 298 (2020) 110087
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Effect of magnesium on the catalytic properties of polymetallic zeolite catalysts for conversion of dimethyl ether to light olefins N.V. Kolesnichenko , Е.N. Khivrich , T.K. Obukhova , Т.I. Batova *, G.N. Bondarenko A.V.Topchiev Institute of Petrochemical Synthesis Russian Academy of Sciences, Leninsky Prospect, 29, Moscow, 119991, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords: Dimethyl ether Light olefins Zeolite catalysts High-temperature steam treatment of the catalyst
The effects of magnesium modification of mono- and bimetallic zeolite catalysts (La-НZSM-5, Zr-НZSM-5, and La–Zr-НZSM-5) and high-temperature steam treatment of these catalysts on the conversion of dimethyl ether to light olefins were studied. The additional introduction of magnesium resulted in higher catalyst activity and selectivity to light olefins. The high-temperature steam treatment of the catalysts not only increased the catalyst activity, but also resulted in higher selectivity to propylene, which is most likely due to higher contribution of the alkene cycle. The developed magnesium-containing zeolite catalysts can be used for the synthesis of light olefins from dimethyl ether with dilution of dimethyl ether in the feed with steam or synthesis gas of different compositions.
1. Introduction The increasing demand for olefins under conditions of stabilized oil production and growing oil prices initiates the search for new olefin production processes free from the use of oil feedstock. At the current stage of research, production of olefins from synthesis gas, which is obtained from natural gas, coal, biomass, and other carbon feedstock, is best developed. The synthesis of light olefins, first of all, ethylene and propylene, is performed via the intermediate production of oxygenates, that is, methanol and dimethyl ether (DME) [1–6]. Currently, conver sion of methanol to olefins (МТО process) and conversion of methanol to propylene (MTP) have been brought to commercialization [7–9]. The production of С2-С3 olefins from synthesis gas via DME, which can be considered as a key agent for the conversion of non-petroleum feedstock to valuable chemicals, holds great promise, since this process has favorable thermodynamic characteristics [3,10–13]. In the near future, direct DME production from synthesis gas will become the major syn thetic route to DME. Direct synthesis of DME is economically more feasible and the cost of dimethyl ether obtained directly from synthesis gas is 20–30% light than that in the case of production via methanol [14–18]. The decationized ZSM-5 zeolite is the most widely used base of catalysts for olefin synthesis from DME (DTO process) [19–21]. A key issue for the synthesis of light olefins from oxygenates (methanol and/or DME) is to control the product selectivity, which requires fundamental
understanding of the reaction mechanism. Of considerable interest are both further increase in the selectivity to light olefins and control over the ethylene/propylene ratio in the products in order to meet re quirements of production processes of a particular plant. Obviously, this problem is also of theoretical interest, as DME is highly reactive and can be converted along a variety of pathways and, hence, selective pro duction of desired chemical products is a fairly challenging task; furthermore, the mechanism of formation of the carbon skeleton upon the heterogeneously catalyzed reaction in the tight space of ZSM-5 mi cropores is poorly studied. Most often, ZSM-5 zeolites synthesized under hydrothermal condi tions have an Al-rich surface location, meaning that numerous acid sites are distributed over the external surface of the crystals [22–24]. How ever, the active sites on the external surface are not shape-selective and promote the formation of heavy hydrocarbons. The density and strength of acid sites depend on the Al content and distribution in the framework and can be controlled by the conditions of synthesis [25–27]. One more efficient method for controlling the acid properties of ZSM-5 is zeolite modification with active elements. Quite a few publications address the effect of the nature of modifying elements on the properties of zeolite catalysts [28–38]. The introduction of a second metal, which may give rise to a synergistic effect of the introduced elements, is the most effi cient way for designing a zeolite catalyst. Bi- and polymetallic catalysts obtained by successive modification of high-silica ZSМ-5 zeolite behave in different ways in the transformations of oxygenates [15,33,39–41].
* Corresponding author. E-mail address: [email protected] (Т.I. Batova). https://doi.org/10.1016/j.micromeso.2020.110087 Received 25 November 2019; Received in revised form 14 January 2020; Accepted 6 February 2020 Available online 10 February 2020 1387-1811/© 2020 Published by Elsevier Inc.
N.V. Kolesnichenko et al.
Microporous and Mesoporous Materials 298 (2020) 110087
Yet another way to modify the catalytic properties of the zeolite catalyst is high-temperature steam treatment. In both cases, the acidic and textural properties of catalyst systems change, which finally enhances the efficiency of zeolite catalysts and offers new opportunities for catalysis. Physicochemical methods such as XRD, FE-SEM, FT-IR, N2 adsorption/desorption, NH3-TPD, and ICP-AES can be used to evaluate the effect of additional active components introduced into the catalyst. The influence of the nature of active element and high-temperature steam treatment on the catalytic properties of monosubstituted ZSM-5 was reported in previous publications [42,43]. The Mg-НZSM-5 sam ple was found to have high selectivity to propylene and butene, compared with La-НZSM-5 or Zr-НZSM-5, in the DME conversion to light olefins. However, when the synthesis of light olefins was conducted in a steam medium, Mg-НZSM-5 showed a pronounced decrease in the selectivity to olefins and increasing content of methanol in the reaction products, while La-НZSM-5 and Zr-НZSM-5 were characterized by higher stability and low content of methanol in the reaction products. The double modification of H-ZSM-5 by lanthanum and zirconium (La–Zr-НZSM-5) improved the catalytic properties towards the DME conversion to olefins in comparison with monomodified catalysts (La-НZSM-5 and Zr-НZSM-5). In this study, in order to increase the activity and selectivity of the La-НZSM-5, Zr-НZSM-5, and La–Zr-НZSM-5 catalysts, we performed their modification with magnesium, which could permit the additional tuning of the acid-base properties of the catalyst surface as a whole and increase the efficiency of the catalysts towards DME conversion to light olefins.
universal gas sorption analyzer. A sample of the test material (~0.1 g of 0.25–0.50 mm fraction) was placed into a quartz reactor, heated in a helium flow at a rate of 10� С/min up to a temperature of 250� С, annealed at this temperature for 1 h in a helium flow, and cooled down to 60� С. The zeolites were saturated with ammonia in a flow of dry NH3/ N2 mixture (1:1) for 15 min. For the removal of physisorbed ammonia, the sample was kept in a dry helium flow for 1 h at 100� С. Then the samples were cooled down to 60� С in a flow of dried helium (30 mL/min flow rate) and the temperature in the reactor was ramped up to 800� С (8 � C/min rate). The textural characteristics (specific surface area, total pore volume, and pore size distribution) of the samples were determined by lowtemperature adsorption/desorption of molecular nitrogen on an ASAP2010 setup (Micromeritics). All samples were pre-evacuated at 350� С to 4 � 10 1 Pa. The N2 adsorption was carried out at 77 K. Diffuse reflectance infrared spectroscopy (DRIS). The high-temperature spectra under inert atmosphere (Ar) were measured by the DRIS method in situ. The spectra were recorded at 450оС after heat treatment of the catalyst in argon and then at 320оС during cooling in argon in a PIKE Diffus IR high-temperature cell coupled to a VERTEX-70 Bruker FT IR spectrometer. The mathematical processing of the IR spectra was carried out using the OPUS-7 software package. 2.3. Catalytic experiments The catalytic experiments for conversion of DME to light olefins were carried out in a laboratory setup comprising a flow reactor with a fixed catalyst bed. Dimethyl ether with 99.8% purity (manufactured by JSC NAK Azot, Novomoskovsk) served as the starting reactant. Nitrogen, steam, or synthesis gas was used for DME dilution. The DME concen tration in the initial gas mixture was 10–20 vol% and the rest was the diluent. The composition of the fresh synthesis gas of module 1 was (vol %): N2, 5.5; CО, 45,6; СО2, 0.3, and Н2þHBs, 48.6; that of synthesis gas of module 3 was (vol%): N2, 2.9; CО, 16.4; СО2, 4.9; and Н2þHBs, 75.8. The synthesis gas module was calculated by the formula (Н2–СО2)/ (СОþСО2). The catalyst (6 cm3) was charged into a flow reactor. Then the catalyst was activated in N2 flow at 450� С for 1 h. The required reactant flow rates (2000-6000 h 1), temperature (320–380� С), and pressure (0.1 MPa) were set. For setting the specified feed velocity, the gas flow rate was adjusted using RRG-10 gas flow controllers. The temperature in the reactor was controlled by OVEN TRM-210 automated temperature control meter (Russia). After completion of the experiment, the liquid products were poured into receiving vessels, the weight and the volume of the liquid phase were recorded, and the liquid was analyzed. The gas flow was fed to a Kristallux-4000М chromatograph with a flame ionization detector through a sampling valve. The size of the capillary column was 27.5 m � 0.32 mm � 10 μm, the CP-PoraPLOT Q-HT nonpolar phase was used as the adsorbent; this phase proved to be sufficiently efficient to isolate the main groups of reaction products (DME, СН3ОН, С1–С6 hydrocarbons). The analysis was carried out in the temperature programming mode (80–200� С, heating rate of 10� /min) using nitrogen as the carrier gas (30 mL/min). The obtained chromatograms were processed using the NetChromWin software program. The process characteristics were determined from the mass balance. The conversion of DME (Х) was calculated by formula (1):
2. Experimental 2.1. Preparation of the catalyst Catalytically active systems for the synthesis of light olefins from DME were prepared from a high-silica zeolite (ZVM) in the ammonium form, which is a domestic analogue of ZSM-5, with the SiO2/Al2O3 molar ratio of 37 and sodium oxide content of no more than 0.07 wt % (manufactured by public company AZKiOS, Angarsk). The hydrogen (Нþ) form was prepared by annealing the zeolite for 4 h at 500� С. Zeolite-containing systems were fabricated by mixing the ZVM zeolite with alumina suspension as a binder (containing 23 wt % dry Al2O3, manufactured by CJSC Industrial Catalysts, Ryazan) and subsequent pelletizing to form extrudates with 33 wt % content of Al2O3 in the ready catalyst. The extrudates were dried in air and then in a drying oven and annealed at 500� С for 4 h. Magnesium was introduced by residue-free impregnation of ready extrudates, and lanthanum and zirconium were added by residue-free impregnation of the zeolite before mixing with the binder. Aqueous solutions of metal salts served as precursors of the metals. The Mg content in the Mg-HZSM-5/Al2O3 catalyst was 1 wt %, while the La and Zr contents in La–Zr-HZSM-5/Al2O3 were 0.1 and 0.4 wt %, respectively. The high-temperature steam treatment of the catalysts at 500� С was performed in a flow setup by a procedure reported in Ref. [44]. 2.2. Physicochemical analysis Powder X-ray diffraction (PXD). The X-ray diffraction patterns were recorded using a Rotaflex RU-200 X-ray source (Rigaku, Japan) with a rotating copper anode. The source operated at 50 kV voltage and 160 mA current. The source was equipped with a Rigaku D/Max-RC hori zontal wide-angle goniometer; θ-2θ scanning was performed in the Bragg-Brentano geometry. The range of measured 2θ diffraction angles was 3–50� ; the measurement was carried out with continuous scanning at a 1 deg/min rate and a 0.04 deg step. The monochromatic radiation wavelength was 1.542 Å. The acid properties of the samples were studied by temperatureprogrammed desorption (TPD) of ammonia on an USGA-101 (Unisit)
X¼
m0 m ⋅100%; m0
(1)
where m0 and m are the DME weights at the reactor inlet and outlet, respectively, g. The selectivity to olefins (S) was calculated by formula (2): S¼
2
molef ⋅100 wt% mHB
(2)
N.V. Kolesnichenko et al.
Microporous and Mesoporous Materials 298 (2020) 110087
Table 1 Characteristics of high-temperature (500� С) steam-treated and -untreated zeolite catalysts.
where molef and mHB are the weight of olefins and the weight of all hy drocarbons formed, g. 3. Results and discussion
Catalyst
3.1. Effect of magnesium dopant and high-temperature steam treatment on the physicochemical characteristics of zeolite catalysts for DME conversion to light olefins
HZSM-5/Al2O3 Mg-HZSM-5/Al2O3 La-HZSM-5/Al2O3 La–Mg-HZSM-5/Al2O3 La–Mg-HZSM-5/Al2O3 treated with steam at 500� С Zr-HZSM-5/Al2O3 Zr–Mg-HZSM-5/Al2O3 Zr–Mg-HZSM-5/Al2O3 treated with steam at 500� С La–Zr-HZSM-5/Al2O3 La–Zr–Mg-HZSM-5/ Al2O3 La–Zr–Mg-HZSM-5/ Al2O3 treated with steam at 500� С
Fig. 1 shows the isotherms of low-temperature nitrogen adsorption by zeolite catalysts modified by La, Zr, and Mg and those additionally treated with steam at 500 оС. Irrespective of the modifier nature, all catalysts showed classical isotherm inherent in zeolites with micro meter- and submicrometer-sized crystals and small contents of meso pores (Table 1). The presence of hysteresis at p/p0 ¼ 0.4–0.9 and the absence of hysteresis at high relative pressures points to limited connection between the porous structure and the outer surface of the zeolite crystal. Analysis of porous structure parameters of the samples (Table 1) indicates that modification of the zeolite catalyst and, especially, hightemperature steam treatment of the catalysts at 500� С leads to decrease in the external surface area and pore volume. A possible cause is the formation of a certain amount of amorphous compounds in the zeolite bulk, which block the zeolite pores. The introduction of mag nesium into the catalyst also decreases the total pore volume and external surface area. According to powder X-ray diffraction data presented in Fig. 2 and Table 2 for some modified zeolite catalysts, all of the samples corre sponded to the crystallographic type of the ZSM-5 zeolite, irrespective of the nature of the modifying agent or high-temperature steam treatment. The calculated unit cell parameters virtually do not change and are in good agreement with published data [45]. Thus, modification and high-temperature steam treatment at 500� С of zeolite catalysts does not destroy the zeolite crystal lattice, i.e., the zeolite structure is retained. The data on acid properties of modified zeolite catalysts are pre sented in Fig. 3. The high-temperature steam treatment of the catalysts at 500� С leads to a sharp decrease in the total acidity and gives rise to a peak at higher temperature (above 600оС), which is indicative of the appearance of Н3Оþ [46–49]. The strength of the Brønsted acid sites (BAS) in the catalyst samples was evaluated by high-temperature diffuse reflectance infrared
ВЕТ, m2/g
Vtot, cm3/g
Vmicro, cm3/g
Vmeso, cm3/g
Refs
355 286 313 302 278
0.228 0.214 0.214 0.199 0.196
0.163 0.133 0.145 0.132 0.132
0.065 0.081 0.069 0.067 0.064
– [42] [42] – –
324 314 293
0.222 0.200 0.191
0.146 0.130 0.125
0.076 0.070 0.066
[42] – –
305 306
0.224 0.196
0.138 0.124
0.085 0.072
[42] –
281
0.191
0.126
0.065
–
spectroscopy in situ (DRIS) in the 3500–3800 cm 1 range corresponding to the –ОН stretching vibrations at the BAS [50–52]. The spectra were recorded at two temperatures: 450оС (catalyst activation temperature) and 320оС (operating reaction temperature). Fig. 4 shows the spectra for the Zr–Mg-HZSM-5/Al2O3 catalysts treated and untreated with steam at 500� С; the spectra were recorded in situ with heating in argon at 450� С and after cooling in dry argon at 320� С. Similar spectral patterns were also obtained for other catalysts. The intensities changing of the bands corresponding to BAS for all studied samples treated and untreated with steam at 500� С is identical. After annealing, three bands appear in the –ОН range of the spectra for all catalysts: 3725 cm 1 due to isolated –ОН groups in weak BAS; 3673 cm 1 due to associated –ОН groups in medium-strength BAS, and 3595 cm 1 due to –ОН groups in strong BAS. Also, the spectra of the catalysts exhibit broad bands that can be assigned to hydroxonium cations (3328 cm 1) and water molecules adsorbed on the zeolite surface (3500 and 3417 cm 1) [46–48]. During cooling of the Zr–Mg-HZSM-5/Al2O3 catalyst (Fig. 4а) in an argon flow, substantial intensity redistribution for the bands
Fig. 1. Low-temperature nitrogen adsorption-desorption isotherms for zeolite catalysts treated and untreated with steam at high temperature (500� С). 3
N.V. Kolesnichenko et al.
Microporous and Mesoporous Materials 298 (2020) 110087
Fig. 3. TPD of ammonia for zeolite catalysts: (1) La–Mg-HZSM-5/Al2O3, (2) La–Mg-HZSM-5/Al2O3 treated with steam at 500оС, (3) Zr–Mg-HZSM-5/Al2O3, (4) Zr–Mg-HZSM-5/Al2O3 treated with steam at 500оС, (5) La–Zr–Mg-HZSM-5/ Al2O3, (6) La–Zr–Mg-HZSM-5/Al2O3 treated with steam at 500оС.
Fig. 2. X-ray diffraction patterns of zeolite catalysts: (1) HZSM-5/Al2O3, (2) La–Mg-HZSM-5/Al2O3, (3) La–Mg-HZSM-5/Al2O3 treated with steam at 500� С, (4) Zr–Mg-HZSM-5/Al2O3, (5) Zr–Mg-HZSM-5/Al2O3 treated with steam at 500� С, (6) La–Zr–Mg-HZSM-5/Al2O3, (7) La–Zr–Mg-HZSM-5/Al2O3 treated with steam at 500� С.
(weak BAS), 3673 cm 1 (medium-strength BAS), and 3595 cm 1 (strong BAS) were quantitatively estimated. The results are summarized in Table 3. As can be seen from the Table, for all of the catalysts the total relative intensity of bands for the weak and medium-strength BAS proves to be markedly higher than the relative intensity of the band at 3595 cm 1 for strong BAS (I/II), which is in good agreement with the TPD data for ammonia (Table 3). The high-temperature steam treatment of the catalysts at 500� С re sults in increasing content of weak BAS and decreasing content of strong and medium-strength BAS. On cooling in argon to 320� С, the numbers of weak and strong BAS sharply decrease, while the number of mediumstrength BAS sharply increases both for the pristine catalyst samples and for the samples steam treated at high temperature. This distribution of BAS of different strengths can be attributed to the presence of water molecules in zeolite pores and their exposure on the surface accessible to DRIS measurements during cooling in an argon flow. Redistribution of the acid sites can occur according to Schemes 1 and 2 presented below. A bridging hydroxy group, which acts as a strong BAS, can be con verted to associated –ОН group (medium-strength BAS) upon coordi nation of a water molecule giving off a proton (Scheme 1). An isolated hydroxy group on aluminum, which acts as a weak BAS (Scheme 2), can be converted to a medium-strength BAS under the same conditions to give off a hydroxide ion. This interaction of weak and strong BAS with isolated Н2О molecules of water vapor lead to increasing content of medium-strength BAS, while the ions formed in parallel can be con verted to water molecules and hydroxonium cations by reactions (3) and (4).
Table 2 Lattice periods and degree of crystallinity of zeolite catalysts. Sample
a, Å
b, Å
c, Å
Unit cell volume, Å3
Degree of crystallinity, %
HZSM-5/ Аl2О3 La–Mg-HZSM5/Al2O3 La–Mg-HZSM5/Al2O3 treated with steam at 500� С Zr–Mg-HZSM5/Al2O3 Zr–Mg-HZSM5/Al2O3 treated with steam at 500� С La–Zr–MgHZSM-5/ Al2O3 La–Zr–MgHZSM-5/ Al2O3 treated with steam at 500� С
20.168 � 0.004 20.153 � 0.004 20.152 � 0.001
19.976 � 0.004 19.986 � 0.004 19.966 � 0.001
13.463 � 0.005 13.417 � 0.005 13.424 � 0.002
5423.8
91
5404.2
92
5400.9
90
20.178 � 0.001 20.160 � 0.005
19.965 � 0.001 19.970 � 0.006
13.450 � 0.003 13.440 � 0.008
5418.5
93
5410.6
91
20.173 � 0.001
19.975 � 0.001
13.453 � 0.002
5420.9
90
20.157 � 0.006
19.970 � 0.006
13.452 � 0.008
5414.8
89
Нþ þ ОН ¼ Н2О þ
þ
Н þ Н2О ¼ Н3О
corresponding to BAS of different strengths takes place. The 3673 cm 1 band due to medium-strength BAS becomes the most intense during cooling. This trend is retained in the spectrum of the catalyst treated with steam at 500� С (Fig. 4b): the intensity of the band at 3673 cm 1 for the medium-strength BAS becomes considerably higher; however, the intensity of the 3725 cm 1 band for weak BAS is still rather high. The spectra of all of the studied catalysts in the –ОН range were brought to identical baselines and normalized to the 1885 cm 1 band, and the integrated intensities of the analytical bands at 3725 cm 1
(3) (4)
3.2. Effect of magnesium and high-temperature steam treatment on the catalytic properties of zeolite catalysts for DME conversion to light olefins All catalysts were tested in the DME conversion to light olefins. The results are presented in Table 4. It can be seen that the pristine catalysts are less active and selective than doubly modified ones and that the introduction of magnesium ¼ enhances the selectivity to ethylene (the С¼ 2 /С3 ratio in the reaction 4
N.V. Kolesnichenko et al.
Microporous and Mesoporous Materials 298 (2020) 110087
Fig. 4. DRIS spectra of the Zr–Mg-HZSM-5/Al2O3 catalyst (a) and Zr–Mg-HZSM-5/Al2O3 catalyst treated with steam at 500� С (b). (1) heating in argon at 450оС, (2) cooling in argon at 320оС.
It is noteworthy that the introduction of magnesium into the cata ¼ lysts followed by high-temperature steam treatment leads to light С¼ 2 /С3 ratio of the reaction products compared with that for the pristine cata lysts, which may be due to a change in the reaction mechanism. In the DME conversion catalyzed by HZSM-5, two reaction cycles are active under the reaction conditions: olefin-based (alkene cycle) and aromatics-based (arene cycle) ones. The arene cycle produces ethylene and propylene with equal selectivity, whereas the alkene cycle is beneficial for the formation of C3þ olefins. The major mechanism of formation of light olefins depends not only on the reaction conditions, but also on the catalyst. More pronounced increase in the selectivity to propylene may be indicative of increasing contribution of the alkene cycle for magnesium-modified high-temperature steam-treated samples. Thus, the additional introduction of magnesium into the pristine zeolite catalysts enhances their activity and selectivity. The hightemperature steam treatment not only promotes the increase in the catalyst activity, with the total selectivity to light olefins being retained, but also leads to a change in the reaction mechanism, thus increasing the contribution of the alkene cycle.
Table 3 Distribution of the composition of Brønsted acid sites of different strength during annealing and cooling in argon according to DRIS data in comparison with the ammonia TPD data. Catalyst
La–Mg-HZSM-5/ Al2O3 La–Mg-HZSM-5/ Al2O3 treated with steam at 500� С Zr–Mg-HZSM-5/ Al2O3 Zr–Mg-HZSM-5/ Al2O3 treated with steam at 500� С La–Zr–MgHZSM-5/Al2O3 La–Zr–MgHZSM-5/Al2O3 treated with steam at 500� С
Т, � С
BAS composition (%)
DRIS
NH3 TPD
Strong 3595 cm 1
I/II
I/II
1.6
Weak 3725 cm 1
Medium strength 3673 cm
450 320 450 320
27 18 42.5 32
33 54 32.5 48
40 28 25 20
1.5 2.6 3.0 4.0
450 320 450 320
26 17 60 37
35 60 15.5 43
39 23 24.5 20
1.6 3.3 3.1 4.0
450 320 450 320
30 16 58 44
33 64 18 34
37 20 24 22
1.7 4.0 3.2 3.5
1
3.2
1.7 2.9
1.5
3.3. Effect of the nature of DME diluent on the catalytic properties of zeolite catalysts in the DME conversion to light olefins
3.4
The conversion of petroleum-based carbon-containing fedstock to other hydrocarbons includes, as a rule, production of synthesis gas (a mixture of СО and Н2), which is then converted to hydrocarbons either directly or via methanol, which is dehydrated to DME. The synthesis of DME affords methanol, water, and СО2. In the case of continuous pro cess, synthesis gas can be fed to the production of light olefins, which also affects the composition of DME conversion products. We studied the effect of the nature of DME diluent (that is, steam or synthesis gas of different compositions) on the catalytic properties of zeolite catalysts of DME conversion to light olefins.
I – sum of weak and medium-strength acid sites. II – strong acid sites.
products increases). After the high-temperature steam treatment at 500� С, the activity of pristine catalysts considerably increases to 95–100%; however, the selectivity to light olefins decreases by 10–20 wt%. The additional introduction of magnesium into the catalysts followed by the hightemperature steam treatment also increases the catalytic activity, with high selectivity to light olefins (up to 80 wt %) being retained.
Scheme 1. Formation of medium-strength BAS from a strong BAS under the action of isolated water molecule. 5
N.V. Kolesnichenko et al.
Microporous and Mesoporous Materials 298 (2020) 110087
Scheme 2. Formation of medium-strength BAS from a weak BAS under the action of isolated water molecule.
for the La–Zr–Mg-HZSM-5/Al2O3 catalyst treated with steam at 500� С. In addition, the fraction of ethylene predominates in the reaction products, especially in the presence of Zr–Mg-HZSM-5/Al2O3 or La–Zr–Mg-HZSM-5/Al2O3. Most probably, under these conditions, methanol does not have enough time to be converted to reaction products, since DME is more reactive and conversion of DME into hydrocarbons required less acti vation energy to proceed as compared with conversion of methanol into hydrocarbons [53–56]. As the temperature increases to 380� С, the ac tivity and selectivity of the steam-treated La–Zr–Mg-HZSM-5/Al2O3 catalyst virtually do not change, but the amount of methanol sharply decreases and some redistribution of the reaction products takes place.
Table 4 Effect of magnesium introduction into pristine and high-temperature (500� С) steam-treated zeolite catalysts on the conversion of dimethyl ether to light olefins (feed: 10 vol% DME þ 90 vol% N2, Р ¼ 0.1 MPa, Т ¼ 320 � С, WDME ¼ 0.9 h 1, data over 4 h of operation). Catalyst
ХDME, %
Selectivity to HBs, wt % С¼ 2
С¼ 3
Mg-HZSM72.6 30.2 31.5 5/Al2O3 La–Mg73.8 31.0 31.6 (46.9) (18.6) (38.0) HZSM-5/ Al2O3 Zr–Mg51.3 26.2 38.0 HZSM-5/ (47.3) (22.3) (36.0) Al2O3 La–Zr–Mg60.5 27.8 35.0 HZSM-5/ (50.1) (21.9) (36.1) Al2O3 High-temperature steam treatment at 500� С Mg-HZSM93.4 26.8 24.1 5/Al2O3 La–Mg77.8 28.5 32.2 (99.7) (20.0) (16.0) HZSM-5/ Al2O3 Zr–Mg74.9 26.8 33.6 HZSM-5/ (99.2) (19.0) (16.1) Al2O3 71.4 25.1 33.0 La–Zr–Mg(95.9) (24.8) (20.9) HZSM-5/ Al2O3
ΣС2С¼ 5
С¼ 2/ С¼ 3
22.2
77.8
1.0
12.8 (14.1)
21.9 (25.5)
78.1 (74.5)
1.0 (0.5)
12.9 (13.1)
20.2 (25.4)
79.8 (74.6)
0.7 (0.6)
12.9 (13.0)
21.9 (25.2)
78.1 (74.8)
0.8 (0.6)
14.6
30.1
69.9
1.1
14.3 (14.8)
22.0 (44.5)
78.0 (55.5)
0.9 (1.3)
15.5 (13.5)
20.9 (47.0)
79.1 (53.0)
0.8 (1.2)
14.3 (14.7)
24.0 (35.0)
76.0 (65.0)
0.8 (1.2)
ΣС¼ 4
ΣСþ 1
12.8
3.3.2. Effect of the addition of synthesis gas of different compositions to the feed on the catalytic properties of zeolite catalysts of DME conversion to light olefins The composition of synthesis gas was chosen considering the in dustrial application. The composition of fresh synthesis gas of module 3 corresponds to the composition of synthesis gas produced by steam reforming of methane most widely used in Russia. It can be seen from Table 6 that, when the reaction is carried out in synthesis gas with either low hydrogen content (module 1) or high hydrogen content (module 3), the catalytic properties barely change. For example, excess hydrogen in the feed somewhat decreases the selectivity to olefins, while the ratio СО:Н 2 ¼ 1:1 leads to some decrease in the activity for the Zr–Mg-HZSM-5/Al2O3 catalyst steam-treated at 500� С, with the high selectivity to olefins being retained. Thus, these zeolite-based catalyst systems can be used for the syn thesis of light olefins from DME with synthesis gas of any composition without a significant loss of the activity and selectivity.
*For comparison, results of testing of pristine catalysts without magnesium re ported previously [42] are given in parentheses.
3.3.1. Effect of the addition of steam to the feed on the catalytic properties of zeolite catalysts of DME conversion to light olefins The obtained results on the effect of steam present in the feed on the catalytic properties of magnesium-containing zeolite catalysts are summarized in Table 5. The replacement of nitrogen by steam (Table 5) reduces the catalyst activity, while increasing the selectivity to ethylene and propylene, and sharply increases the methanol content in the reaction products, except
4. Conclusion The additional introduction of magnesium into the pristine zeolite catalysts increases their activity towards the DME conversion and the selectivity to light olefins. The high-temperature steam treatment not only leads to higher ac tivity of the catalysts, but also increases the selectivity to propylene, which is most likely attributable to increasing contribution of the alkene
Table 5 Effect of replacement of nitrogen by steam on the dimethyl ether conversion to light olefins catalyzed by zeolites pretreated with steam at 500� С (Р ¼ 0,1 MPa, Т ¼ 320 � С, WDME ¼ 3.8 h 1, data over 4 h of operation). Catalyst Feed: 20 vol% DMEþ80 vol% N2 La–Mg-HZSM-5/Al2O3 Zr–Mg-HZSM-5/Al2O3 La–Zr–Mg-HZSM-5/Al2O3 Feed: 20 vol% DMEþ80 vol% H2O La–Mg-HZSM-5/Al2O3 Zr–Mg-HZSM-5/Al2O3 La–Zr–Mg-HZSM-5/Al2O3
ХDME, %
Т ¼ 320� С Т ¼ 380� С
Selectivity to HBs, wt %
ΣС2-С¼ 5 (MeOH)
¼ С¼ 2 /С3
С¼ 2
С¼ 3
ΣС¼ 4
ΣСþ 1
78.1 83.1 52.9
22.2 20.1 20.8
25.1 23.2 31.3
17.9 18.9 18.2
26.7 28.7 22.3
73.3 (21.8) 71.3 (17.6) 77.7 (35.0)
0.9 0.9 0.7
28.5 59.4 61.3 65.5
34.6 32.9 31.2 26.2
32.8 25.7 26.7 30.9
9.8 11.4 11.7 10.3
21.0 28.0 27.9 26.7
79.0 (62.2) 72.0 (42.6) 72.1 (37.7) 73.3 (4.8)
1.1 1.3 1.2 0.9
6
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Table 6 Effect of replacement of nitrogen by synthesis gas of different compositions on the conversion of dimethyl ether to light olefins in the presence of zeolite cat alysts pretreated with steam at 500� С (Р ¼ 0.1 MPa, Т ¼ 320 � С, WDME ¼ 1.4 h 1, data over 4 h of operation). Catalyst
ХDME, %
Selectivity to HBs, wt % С¼ 2
С¼ 3
ΣС¼ 4
Feed: 10 vol% DMEþ90 vol% N2 La–Mg-HZSM77.0 22.2 26.1 19.1 5/Al2O3 Zr–Mg-HZSM74.9 22.1 24.8 18.7 5/Al2O3 La–Zr–Mg78.8 21.4 22.7 19.4 HZSM-5/ Al2O3 Feed: 10 vol% DME þ90 vol% (CO þ H2) synthesis gas La–Mg-HZSM74.3 21.2 26.2 18.4 5/Al2O3 Zr–Mg-HZSM71.4 21.8 25.9 18.5 5/Al2O3 La–Zr–Mg77.6 19.7 24.0 19.1 HZSM-5/ Al2O3 Feed: 10 vol% DME þ90 vol% (CO þ H2) synthesis gas 78.1 21.3 26.0 19.2 La–Mg-HZSM5/Al2O3 Zr–Mg-HZSM65.2 19.9 29.0 19.3 5/Al2O3 La–Zr–Mg77.2 21.1 24.4 19.2 HZSM-5/ Al2O3
ΣСþ 1 24.6 26.1 27.8
ΣС2-С¼ 5 (MeOH)
С¼ 2/ С¼ 3
75.4 (20.8) 73.9 (21.2) 72.2 (18.9)
0.9
of module 3 27.1 72.9 (19.1) 26.6 73.4 (25.9) 29.3 70.7 (20.8) of module 1 25.9 74.1 (24.4) 24.3 75.7 (31.1) 27.5 72.5 (19.6)
0.9 0.9
0.8 0.8 0.8
0.8 0.7 0.9
cycle. The decrease in the temperature down to the operating temperature after the catalyst activation and the high-temperature steam treatment of the catalysts lead to increasing fraction of the medium-strength acid sites. The developed zeolite catalysts treated with steam at 500� С are suitable for the synthesis of light olefins from DME using synthesis gas of different compositions for dilution of DME. Declarations of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement N.V. Kolesnichenko: Conceptualization, Methodology, Supervi sion, Writing - review & editing. Е.N. Khivrich: Writing - original draft. T.K. Obukhova: Investigation, Formal analysis, Validation. Т.I. Batova: Investigation, Writing - review & editing. G.N. Bondarenko: Formal analysis. Acknowledgements This work was carried out within the State Program of TIPS RAS. The funding source is Ministry of Science and Higher Education of the Russian Federation. Funding is provided through the ITIPS RAS; there fore, it is not visible. References [1] M. Stoker, Microporous Mesoporous Mater. 29 (1999) 3. [2] C.D. Chang, A.J. Silvestri, J. Catal. 47 (1977) 249. [3] G. Cai, Z. Liu, R. Shi, Ch He, L. Yang, Ch Sun, Y. Chang, Appl. Catal. Gen. 125 (1995) 29. [4] J.Q. Chen, A. Bozzano, B. Glover, T. Fuglerud, S. Kvisle, Catal. Today 106 (2005) 103.
7
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Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Effect of magnesium on the catalytic properties of polymetallic zeolite catalysts for conversion of dimethyl ether to light olefins N.V. Kolesnichenko , Е.N. Khivrich , T.K. Obukhova , Т.I. Batova *, G.N. Bondarenko A.V.Topchiev Institute of Petrochemical Synthesis Russian Academy of Sciences, Leninsky Prospect, 29, Moscow, 119991, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords: Dimethyl ether Light olefins Zeolite catalysts High-temperature steam treatment of the catalyst
The effects of magnesium modification of mono- and bimetallic zeolite catalysts (La-НZSM-5, Zr-НZSM-5, and La–Zr-НZSM-5) and high-temperature steam treatment of these catalysts on the conversion of dimethyl ether to light olefins were studied. The additional introduction of magnesium resulted in higher catalyst activity and selectivity to light olefins. The high-temperature steam treatment of the catalysts not only increased the catalyst activity, but also resulted in higher selectivity to propylene, which is most likely due to higher contribution of the alkene cycle. The developed magnesium-containing zeolite catalysts can be used for the synthesis of light olefins from dimethyl ether with dilution of dimethyl ether in the feed with steam or synthesis gas of different compositions.
1. Introduction The increasing demand for olefins under conditions of stabilized oil production and growing oil prices initiates the search for new olefin production processes free from the use of oil feedstock. At the current stage of research, production of olefins from synthesis gas, which is obtained from natural gas, coal, biomass, and other carbon feedstock, is best developed. The synthesis of light olefins, first of all, ethylene and propylene, is performed via the intermediate production of oxygenates, that is, methanol and dimethyl ether (DME) [1–6]. Currently, conver sion of methanol to olefins (МТО process) and conversion of methanol to propylene (MTP) have been brought to commercialization [7–9]. The production of С2-С3 olefins from synthesis gas via DME, which can be considered as a key agent for the conversion of non-petroleum feedstock to valuable chemicals, holds great promise, since this process has favorable thermodynamic characteristics [3,10–13]. In the near future, direct DME production from synthesis gas will become the major syn thetic route to DME. Direct synthesis of DME is economically more feasible and the cost of dimethyl ether obtained directly from synthesis gas is 20–30% light than that in the case of production via methanol [14–18]. The decationized ZSM-5 zeolite is the most widely used base of catalysts for olefin synthesis from DME (DTO process) [19–21]. A key issue for the synthesis of light olefins from oxygenates (methanol and/or DME) is to control the product selectivity, which requires fundamental
understanding of the reaction mechanism. Of considerable interest are both further increase in the selectivity to light olefins and control over the ethylene/propylene ratio in the products in order to meet re quirements of production processes of a particular plant. Obviously, this problem is also of theoretical interest, as DME is highly reactive and can be converted along a variety of pathways and, hence, selective pro duction of desired chemical products is a fairly challenging task; furthermore, the mechanism of formation of the carbon skeleton upon the heterogeneously catalyzed reaction in the tight space of ZSM-5 mi cropores is poorly studied. Most often, ZSM-5 zeolites synthesized under hydrothermal condi tions have an Al-rich surface location, meaning that numerous acid sites are distributed over the external surface of the crystals [22–24]. How ever, the active sites on the external surface are not shape-selective and promote the formation of heavy hydrocarbons. The density and strength of acid sites depend on the Al content and distribution in the framework and can be controlled by the conditions of synthesis [25–27]. One more efficient method for controlling the acid properties of ZSM-5 is zeolite modification with active elements. Quite a few publications address the effect of the nature of modifying elements on the properties of zeolite catalysts [28–38]. The introduction of a second metal, which may give rise to a synergistic effect of the introduced elements, is the most effi cient way for designing a zeolite catalyst. Bi- and polymetallic catalysts obtained by successive modification of high-silica ZSМ-5 zeolite behave in different ways in the transformations of oxygenates [15,33,39–41].
* Corresponding author. E-mail address: [email protected] (Т.I. Batova). https://doi.org/10.1016/j.micromeso.2020.110087 Received 25 November 2019; Received in revised form 14 January 2020; Accepted 6 February 2020 Available online 10 February 2020 1387-1811/© 2020 Published by Elsevier Inc.
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Microporous and Mesoporous Materials 298 (2020) 110087
Yet another way to modify the catalytic properties of the zeolite catalyst is high-temperature steam treatment. In both cases, the acidic and textural properties of catalyst systems change, which finally enhances the efficiency of zeolite catalysts and offers new opportunities for catalysis. Physicochemical methods such as XRD, FE-SEM, FT-IR, N2 adsorption/desorption, NH3-TPD, and ICP-AES can be used to evaluate the effect of additional active components introduced into the catalyst. The influence of the nature of active element and high-temperature steam treatment on the catalytic properties of monosubstituted ZSM-5 was reported in previous publications [42,43]. The Mg-НZSM-5 sam ple was found to have high selectivity to propylene and butene, compared with La-НZSM-5 or Zr-НZSM-5, in the DME conversion to light olefins. However, when the synthesis of light olefins was conducted in a steam medium, Mg-НZSM-5 showed a pronounced decrease in the selectivity to olefins and increasing content of methanol in the reaction products, while La-НZSM-5 and Zr-НZSM-5 were characterized by higher stability and low content of methanol in the reaction products. The double modification of H-ZSM-5 by lanthanum and zirconium (La–Zr-НZSM-5) improved the catalytic properties towards the DME conversion to olefins in comparison with monomodified catalysts (La-НZSM-5 and Zr-НZSM-5). In this study, in order to increase the activity and selectivity of the La-НZSM-5, Zr-НZSM-5, and La–Zr-НZSM-5 catalysts, we performed their modification with magnesium, which could permit the additional tuning of the acid-base properties of the catalyst surface as a whole and increase the efficiency of the catalysts towards DME conversion to light olefins.
universal gas sorption analyzer. A sample of the test material (~0.1 g of 0.25–0.50 mm fraction) was placed into a quartz reactor, heated in a helium flow at a rate of 10� С/min up to a temperature of 250� С, annealed at this temperature for 1 h in a helium flow, and cooled down to 60� С. The zeolites were saturated with ammonia in a flow of dry NH3/ N2 mixture (1:1) for 15 min. For the removal of physisorbed ammonia, the sample was kept in a dry helium flow for 1 h at 100� С. Then the samples were cooled down to 60� С in a flow of dried helium (30 mL/min flow rate) and the temperature in the reactor was ramped up to 800� С (8 � C/min rate). The textural characteristics (specific surface area, total pore volume, and pore size distribution) of the samples were determined by lowtemperature adsorption/desorption of molecular nitrogen on an ASAP2010 setup (Micromeritics). All samples were pre-evacuated at 350� С to 4 � 10 1 Pa. The N2 adsorption was carried out at 77 K. Diffuse reflectance infrared spectroscopy (DRIS). The high-temperature spectra under inert atmosphere (Ar) were measured by the DRIS method in situ. The spectra were recorded at 450оС after heat treatment of the catalyst in argon and then at 320оС during cooling in argon in a PIKE Diffus IR high-temperature cell coupled to a VERTEX-70 Bruker FT IR spectrometer. The mathematical processing of the IR spectra was carried out using the OPUS-7 software package. 2.3. Catalytic experiments The catalytic experiments for conversion of DME to light olefins were carried out in a laboratory setup comprising a flow reactor with a fixed catalyst bed. Dimethyl ether with 99.8% purity (manufactured by JSC NAK Azot, Novomoskovsk) served as the starting reactant. Nitrogen, steam, or synthesis gas was used for DME dilution. The DME concen tration in the initial gas mixture was 10–20 vol% and the rest was the diluent. The composition of the fresh synthesis gas of module 1 was (vol %): N2, 5.5; CО, 45,6; СО2, 0.3, and Н2þHBs, 48.6; that of synthesis gas of module 3 was (vol%): N2, 2.9; CО, 16.4; СО2, 4.9; and Н2þHBs, 75.8. The synthesis gas module was calculated by the formula (Н2–СО2)/ (СОþСО2). The catalyst (6 cm3) was charged into a flow reactor. Then the catalyst was activated in N2 flow at 450� С for 1 h. The required reactant flow rates (2000-6000 h 1), temperature (320–380� С), and pressure (0.1 MPa) were set. For setting the specified feed velocity, the gas flow rate was adjusted using RRG-10 gas flow controllers. The temperature in the reactor was controlled by OVEN TRM-210 automated temperature control meter (Russia). After completion of the experiment, the liquid products were poured into receiving vessels, the weight and the volume of the liquid phase were recorded, and the liquid was analyzed. The gas flow was fed to a Kristallux-4000М chromatograph with a flame ionization detector through a sampling valve. The size of the capillary column was 27.5 m � 0.32 mm � 10 μm, the CP-PoraPLOT Q-HT nonpolar phase was used as the adsorbent; this phase proved to be sufficiently efficient to isolate the main groups of reaction products (DME, СН3ОН, С1–С6 hydrocarbons). The analysis was carried out in the temperature programming mode (80–200� С, heating rate of 10� /min) using nitrogen as the carrier gas (30 mL/min). The obtained chromatograms were processed using the NetChromWin software program. The process characteristics were determined from the mass balance. The conversion of DME (Х) was calculated by formula (1):
2. Experimental 2.1. Preparation of the catalyst Catalytically active systems for the synthesis of light olefins from DME were prepared from a high-silica zeolite (ZVM) in the ammonium form, which is a domestic analogue of ZSM-5, with the SiO2/Al2O3 molar ratio of 37 and sodium oxide content of no more than 0.07 wt % (manufactured by public company AZKiOS, Angarsk). The hydrogen (Нþ) form was prepared by annealing the zeolite for 4 h at 500� С. Zeolite-containing systems were fabricated by mixing the ZVM zeolite with alumina suspension as a binder (containing 23 wt % dry Al2O3, manufactured by CJSC Industrial Catalysts, Ryazan) and subsequent pelletizing to form extrudates with 33 wt % content of Al2O3 in the ready catalyst. The extrudates were dried in air and then in a drying oven and annealed at 500� С for 4 h. Magnesium was introduced by residue-free impregnation of ready extrudates, and lanthanum and zirconium were added by residue-free impregnation of the zeolite before mixing with the binder. Aqueous solutions of metal salts served as precursors of the metals. The Mg content in the Mg-HZSM-5/Al2O3 catalyst was 1 wt %, while the La and Zr contents in La–Zr-HZSM-5/Al2O3 were 0.1 and 0.4 wt %, respectively. The high-temperature steam treatment of the catalysts at 500� С was performed in a flow setup by a procedure reported in Ref. [44]. 2.2. Physicochemical analysis Powder X-ray diffraction (PXD). The X-ray diffraction patterns were recorded using a Rotaflex RU-200 X-ray source (Rigaku, Japan) with a rotating copper anode. The source operated at 50 kV voltage and 160 mA current. The source was equipped with a Rigaku D/Max-RC hori zontal wide-angle goniometer; θ-2θ scanning was performed in the Bragg-Brentano geometry. The range of measured 2θ diffraction angles was 3–50� ; the measurement was carried out with continuous scanning at a 1 deg/min rate and a 0.04 deg step. The monochromatic radiation wavelength was 1.542 Å. The acid properties of the samples were studied by temperatureprogrammed desorption (TPD) of ammonia on an USGA-101 (Unisit)
X¼
m0 m ⋅100%; m0
(1)
where m0 and m are the DME weights at the reactor inlet and outlet, respectively, g. The selectivity to olefins (S) was calculated by formula (2): S¼
2
molef ⋅100 wt% mHB
(2)
N.V. Kolesnichenko et al.
Microporous and Mesoporous Materials 298 (2020) 110087
Table 1 Characteristics of high-temperature (500� С) steam-treated and -untreated zeolite catalysts.
where molef and mHB are the weight of olefins and the weight of all hy drocarbons formed, g. 3. Results and discussion
Catalyst
3.1. Effect of magnesium dopant and high-temperature steam treatment on the physicochemical characteristics of zeolite catalysts for DME conversion to light olefins
HZSM-5/Al2O3 Mg-HZSM-5/Al2O3 La-HZSM-5/Al2O3 La–Mg-HZSM-5/Al2O3 La–Mg-HZSM-5/Al2O3 treated with steam at 500� С Zr-HZSM-5/Al2O3 Zr–Mg-HZSM-5/Al2O3 Zr–Mg-HZSM-5/Al2O3 treated with steam at 500� С La–Zr-HZSM-5/Al2O3 La–Zr–Mg-HZSM-5/ Al2O3 La–Zr–Mg-HZSM-5/ Al2O3 treated with steam at 500� С
Fig. 1 shows the isotherms of low-temperature nitrogen adsorption by zeolite catalysts modified by La, Zr, and Mg and those additionally treated with steam at 500 оС. Irrespective of the modifier nature, all catalysts showed classical isotherm inherent in zeolites with micro meter- and submicrometer-sized crystals and small contents of meso pores (Table 1). The presence of hysteresis at p/p0 ¼ 0.4–0.9 and the absence of hysteresis at high relative pressures points to limited connection between the porous structure and the outer surface of the zeolite crystal. Analysis of porous structure parameters of the samples (Table 1) indicates that modification of the zeolite catalyst and, especially, hightemperature steam treatment of the catalysts at 500� С leads to decrease in the external surface area and pore volume. A possible cause is the formation of a certain amount of amorphous compounds in the zeolite bulk, which block the zeolite pores. The introduction of mag nesium into the catalyst also decreases the total pore volume and external surface area. According to powder X-ray diffraction data presented in Fig. 2 and Table 2 for some modified zeolite catalysts, all of the samples corre sponded to the crystallographic type of the ZSM-5 zeolite, irrespective of the nature of the modifying agent or high-temperature steam treatment. The calculated unit cell parameters virtually do not change and are in good agreement with published data [45]. Thus, modification and high-temperature steam treatment at 500� С of zeolite catalysts does not destroy the zeolite crystal lattice, i.e., the zeolite structure is retained. The data on acid properties of modified zeolite catalysts are pre sented in Fig. 3. The high-temperature steam treatment of the catalysts at 500� С leads to a sharp decrease in the total acidity and gives rise to a peak at higher temperature (above 600оС), which is indicative of the appearance of Н3Оþ [46–49]. The strength of the Brønsted acid sites (BAS) in the catalyst samples was evaluated by high-temperature diffuse reflectance infrared
ВЕТ, m2/g
Vtot, cm3/g
Vmicro, cm3/g
Vmeso, cm3/g
Refs
355 286 313 302 278
0.228 0.214 0.214 0.199 0.196
0.163 0.133 0.145 0.132 0.132
0.065 0.081 0.069 0.067 0.064
– [42] [42] – –
324 314 293
0.222 0.200 0.191
0.146 0.130 0.125
0.076 0.070 0.066
[42] – –
305 306
0.224 0.196
0.138 0.124
0.085 0.072
[42] –
281
0.191
0.126
0.065
–
spectroscopy in situ (DRIS) in the 3500–3800 cm 1 range corresponding to the –ОН stretching vibrations at the BAS [50–52]. The spectra were recorded at two temperatures: 450оС (catalyst activation temperature) and 320оС (operating reaction temperature). Fig. 4 shows the spectra for the Zr–Mg-HZSM-5/Al2O3 catalysts treated and untreated with steam at 500� С; the spectra were recorded in situ with heating in argon at 450� С and after cooling in dry argon at 320� С. Similar spectral patterns were also obtained for other catalysts. The intensities changing of the bands corresponding to BAS for all studied samples treated and untreated with steam at 500� С is identical. After annealing, three bands appear in the –ОН range of the spectra for all catalysts: 3725 cm 1 due to isolated –ОН groups in weak BAS; 3673 cm 1 due to associated –ОН groups in medium-strength BAS, and 3595 cm 1 due to –ОН groups in strong BAS. Also, the spectra of the catalysts exhibit broad bands that can be assigned to hydroxonium cations (3328 cm 1) and water molecules adsorbed on the zeolite surface (3500 and 3417 cm 1) [46–48]. During cooling of the Zr–Mg-HZSM-5/Al2O3 catalyst (Fig. 4а) in an argon flow, substantial intensity redistribution for the bands
Fig. 1. Low-temperature nitrogen adsorption-desorption isotherms for zeolite catalysts treated and untreated with steam at high temperature (500� С). 3
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Microporous and Mesoporous Materials 298 (2020) 110087
Fig. 3. TPD of ammonia for zeolite catalysts: (1) La–Mg-HZSM-5/Al2O3, (2) La–Mg-HZSM-5/Al2O3 treated with steam at 500оС, (3) Zr–Mg-HZSM-5/Al2O3, (4) Zr–Mg-HZSM-5/Al2O3 treated with steam at 500оС, (5) La–Zr–Mg-HZSM-5/ Al2O3, (6) La–Zr–Mg-HZSM-5/Al2O3 treated with steam at 500оС.
Fig. 2. X-ray diffraction patterns of zeolite catalysts: (1) HZSM-5/Al2O3, (2) La–Mg-HZSM-5/Al2O3, (3) La–Mg-HZSM-5/Al2O3 treated with steam at 500� С, (4) Zr–Mg-HZSM-5/Al2O3, (5) Zr–Mg-HZSM-5/Al2O3 treated with steam at 500� С, (6) La–Zr–Mg-HZSM-5/Al2O3, (7) La–Zr–Mg-HZSM-5/Al2O3 treated with steam at 500� С.
(weak BAS), 3673 cm 1 (medium-strength BAS), and 3595 cm 1 (strong BAS) were quantitatively estimated. The results are summarized in Table 3. As can be seen from the Table, for all of the catalysts the total relative intensity of bands for the weak and medium-strength BAS proves to be markedly higher than the relative intensity of the band at 3595 cm 1 for strong BAS (I/II), which is in good agreement with the TPD data for ammonia (Table 3). The high-temperature steam treatment of the catalysts at 500� С re sults in increasing content of weak BAS and decreasing content of strong and medium-strength BAS. On cooling in argon to 320� С, the numbers of weak and strong BAS sharply decrease, while the number of mediumstrength BAS sharply increases both for the pristine catalyst samples and for the samples steam treated at high temperature. This distribution of BAS of different strengths can be attributed to the presence of water molecules in zeolite pores and their exposure on the surface accessible to DRIS measurements during cooling in an argon flow. Redistribution of the acid sites can occur according to Schemes 1 and 2 presented below. A bridging hydroxy group, which acts as a strong BAS, can be con verted to associated –ОН group (medium-strength BAS) upon coordi nation of a water molecule giving off a proton (Scheme 1). An isolated hydroxy group on aluminum, which acts as a weak BAS (Scheme 2), can be converted to a medium-strength BAS under the same conditions to give off a hydroxide ion. This interaction of weak and strong BAS with isolated Н2О molecules of water vapor lead to increasing content of medium-strength BAS, while the ions formed in parallel can be con verted to water molecules and hydroxonium cations by reactions (3) and (4).
Table 2 Lattice periods and degree of crystallinity of zeolite catalysts. Sample
a, Å
b, Å
c, Å
Unit cell volume, Å3
Degree of crystallinity, %
HZSM-5/ Аl2О3 La–Mg-HZSM5/Al2O3 La–Mg-HZSM5/Al2O3 treated with steam at 500� С Zr–Mg-HZSM5/Al2O3 Zr–Mg-HZSM5/Al2O3 treated with steam at 500� С La–Zr–MgHZSM-5/ Al2O3 La–Zr–MgHZSM-5/ Al2O3 treated with steam at 500� С
20.168 � 0.004 20.153 � 0.004 20.152 � 0.001
19.976 � 0.004 19.986 � 0.004 19.966 � 0.001
13.463 � 0.005 13.417 � 0.005 13.424 � 0.002
5423.8
91
5404.2
92
5400.9
90
20.178 � 0.001 20.160 � 0.005
19.965 � 0.001 19.970 � 0.006
13.450 � 0.003 13.440 � 0.008
5418.5
93
5410.6
91
20.173 � 0.001
19.975 � 0.001
13.453 � 0.002
5420.9
90
20.157 � 0.006
19.970 � 0.006
13.452 � 0.008
5414.8
89
Нþ þ ОН ¼ Н2О þ
þ
Н þ Н2О ¼ Н3О
corresponding to BAS of different strengths takes place. The 3673 cm 1 band due to medium-strength BAS becomes the most intense during cooling. This trend is retained in the spectrum of the catalyst treated with steam at 500� С (Fig. 4b): the intensity of the band at 3673 cm 1 for the medium-strength BAS becomes considerably higher; however, the intensity of the 3725 cm 1 band for weak BAS is still rather high. The spectra of all of the studied catalysts in the –ОН range were brought to identical baselines and normalized to the 1885 cm 1 band, and the integrated intensities of the analytical bands at 3725 cm 1
(3) (4)
3.2. Effect of magnesium and high-temperature steam treatment on the catalytic properties of zeolite catalysts for DME conversion to light olefins All catalysts were tested in the DME conversion to light olefins. The results are presented in Table 4. It can be seen that the pristine catalysts are less active and selective than doubly modified ones and that the introduction of magnesium ¼ enhances the selectivity to ethylene (the С¼ 2 /С3 ratio in the reaction 4
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Microporous and Mesoporous Materials 298 (2020) 110087
Fig. 4. DRIS spectra of the Zr–Mg-HZSM-5/Al2O3 catalyst (a) and Zr–Mg-HZSM-5/Al2O3 catalyst treated with steam at 500� С (b). (1) heating in argon at 450оС, (2) cooling in argon at 320оС.
It is noteworthy that the introduction of magnesium into the cata ¼ lysts followed by high-temperature steam treatment leads to light С¼ 2 /С3 ratio of the reaction products compared with that for the pristine cata lysts, which may be due to a change in the reaction mechanism. In the DME conversion catalyzed by HZSM-5, two reaction cycles are active under the reaction conditions: olefin-based (alkene cycle) and aromatics-based (arene cycle) ones. The arene cycle produces ethylene and propylene with equal selectivity, whereas the alkene cycle is beneficial for the formation of C3þ olefins. The major mechanism of formation of light olefins depends not only on the reaction conditions, but also on the catalyst. More pronounced increase in the selectivity to propylene may be indicative of increasing contribution of the alkene cycle for magnesium-modified high-temperature steam-treated samples. Thus, the additional introduction of magnesium into the pristine zeolite catalysts enhances their activity and selectivity. The hightemperature steam treatment not only promotes the increase in the catalyst activity, with the total selectivity to light olefins being retained, but also leads to a change in the reaction mechanism, thus increasing the contribution of the alkene cycle.
Table 3 Distribution of the composition of Brønsted acid sites of different strength during annealing and cooling in argon according to DRIS data in comparison with the ammonia TPD data. Catalyst
La–Mg-HZSM-5/ Al2O3 La–Mg-HZSM-5/ Al2O3 treated with steam at 500� С Zr–Mg-HZSM-5/ Al2O3 Zr–Mg-HZSM-5/ Al2O3 treated with steam at 500� С La–Zr–MgHZSM-5/Al2O3 La–Zr–MgHZSM-5/Al2O3 treated with steam at 500� С
Т, � С
BAS composition (%)
DRIS
NH3 TPD
Strong 3595 cm 1
I/II
I/II
1.6
Weak 3725 cm 1
Medium strength 3673 cm
450 320 450 320
27 18 42.5 32
33 54 32.5 48
40 28 25 20
1.5 2.6 3.0 4.0
450 320 450 320
26 17 60 37
35 60 15.5 43
39 23 24.5 20
1.6 3.3 3.1 4.0
450 320 450 320
30 16 58 44
33 64 18 34
37 20 24 22
1.7 4.0 3.2 3.5
1
3.2
1.7 2.9
1.5
3.3. Effect of the nature of DME diluent on the catalytic properties of zeolite catalysts in the DME conversion to light olefins
3.4
The conversion of petroleum-based carbon-containing fedstock to other hydrocarbons includes, as a rule, production of synthesis gas (a mixture of СО and Н2), which is then converted to hydrocarbons either directly or via methanol, which is dehydrated to DME. The synthesis of DME affords methanol, water, and СО2. In the case of continuous pro cess, synthesis gas can be fed to the production of light olefins, which also affects the composition of DME conversion products. We studied the effect of the nature of DME diluent (that is, steam or synthesis gas of different compositions) on the catalytic properties of zeolite catalysts of DME conversion to light olefins.
I – sum of weak and medium-strength acid sites. II – strong acid sites.
products increases). After the high-temperature steam treatment at 500� С, the activity of pristine catalysts considerably increases to 95–100%; however, the selectivity to light olefins decreases by 10–20 wt%. The additional introduction of magnesium into the catalysts followed by the hightemperature steam treatment also increases the catalytic activity, with high selectivity to light olefins (up to 80 wt %) being retained.
Scheme 1. Formation of medium-strength BAS from a strong BAS under the action of isolated water molecule. 5
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Microporous and Mesoporous Materials 298 (2020) 110087
Scheme 2. Formation of medium-strength BAS from a weak BAS under the action of isolated water molecule.
for the La–Zr–Mg-HZSM-5/Al2O3 catalyst treated with steam at 500� С. In addition, the fraction of ethylene predominates in the reaction products, especially in the presence of Zr–Mg-HZSM-5/Al2O3 or La–Zr–Mg-HZSM-5/Al2O3. Most probably, under these conditions, methanol does not have enough time to be converted to reaction products, since DME is more reactive and conversion of DME into hydrocarbons required less acti vation energy to proceed as compared with conversion of methanol into hydrocarbons [53–56]. As the temperature increases to 380� С, the ac tivity and selectivity of the steam-treated La–Zr–Mg-HZSM-5/Al2O3 catalyst virtually do not change, but the amount of methanol sharply decreases and some redistribution of the reaction products takes place.
Table 4 Effect of magnesium introduction into pristine and high-temperature (500� С) steam-treated zeolite catalysts on the conversion of dimethyl ether to light olefins (feed: 10 vol% DME þ 90 vol% N2, Р ¼ 0.1 MPa, Т ¼ 320 � С, WDME ¼ 0.9 h 1, data over 4 h of operation). Catalyst
ХDME, %
Selectivity to HBs, wt % С¼ 2
С¼ 3
Mg-HZSM72.6 30.2 31.5 5/Al2O3 La–Mg73.8 31.0 31.6 (46.9) (18.6) (38.0) HZSM-5/ Al2O3 Zr–Mg51.3 26.2 38.0 HZSM-5/ (47.3) (22.3) (36.0) Al2O3 La–Zr–Mg60.5 27.8 35.0 HZSM-5/ (50.1) (21.9) (36.1) Al2O3 High-temperature steam treatment at 500� С Mg-HZSM93.4 26.8 24.1 5/Al2O3 La–Mg77.8 28.5 32.2 (99.7) (20.0) (16.0) HZSM-5/ Al2O3 Zr–Mg74.9 26.8 33.6 HZSM-5/ (99.2) (19.0) (16.1) Al2O3 71.4 25.1 33.0 La–Zr–Mg(95.9) (24.8) (20.9) HZSM-5/ Al2O3
ΣС2С¼ 5
С¼ 2/ С¼ 3
22.2
77.8
1.0
12.8 (14.1)
21.9 (25.5)
78.1 (74.5)
1.0 (0.5)
12.9 (13.1)
20.2 (25.4)
79.8 (74.6)
0.7 (0.6)
12.9 (13.0)
21.9 (25.2)
78.1 (74.8)
0.8 (0.6)
14.6
30.1
69.9
1.1
14.3 (14.8)
22.0 (44.5)
78.0 (55.5)
0.9 (1.3)
15.5 (13.5)
20.9 (47.0)
79.1 (53.0)
0.8 (1.2)
14.3 (14.7)
24.0 (35.0)
76.0 (65.0)
0.8 (1.2)
ΣС¼ 4
ΣСþ 1
12.8
3.3.2. Effect of the addition of synthesis gas of different compositions to the feed on the catalytic properties of zeolite catalysts of DME conversion to light olefins The composition of synthesis gas was chosen considering the in dustrial application. The composition of fresh synthesis gas of module 3 corresponds to the composition of synthesis gas produced by steam reforming of methane most widely used in Russia. It can be seen from Table 6 that, when the reaction is carried out in synthesis gas with either low hydrogen content (module 1) or high hydrogen content (module 3), the catalytic properties barely change. For example, excess hydrogen in the feed somewhat decreases the selectivity to olefins, while the ratio СО:Н 2 ¼ 1:1 leads to some decrease in the activity for the Zr–Mg-HZSM-5/Al2O3 catalyst steam-treated at 500� С, with the high selectivity to olefins being retained. Thus, these zeolite-based catalyst systems can be used for the syn thesis of light olefins from DME with synthesis gas of any composition without a significant loss of the activity and selectivity.
*For comparison, results of testing of pristine catalysts without magnesium re ported previously [42] are given in parentheses.
3.3.1. Effect of the addition of steam to the feed on the catalytic properties of zeolite catalysts of DME conversion to light olefins The obtained results on the effect of steam present in the feed on the catalytic properties of magnesium-containing zeolite catalysts are summarized in Table 5. The replacement of nitrogen by steam (Table 5) reduces the catalyst activity, while increasing the selectivity to ethylene and propylene, and sharply increases the methanol content in the reaction products, except
4. Conclusion The additional introduction of magnesium into the pristine zeolite catalysts increases their activity towards the DME conversion and the selectivity to light olefins. The high-temperature steam treatment not only leads to higher ac tivity of the catalysts, but also increases the selectivity to propylene, which is most likely attributable to increasing contribution of the alkene
Table 5 Effect of replacement of nitrogen by steam on the dimethyl ether conversion to light olefins catalyzed by zeolites pretreated with steam at 500� С (Р ¼ 0,1 MPa, Т ¼ 320 � С, WDME ¼ 3.8 h 1, data over 4 h of operation). Catalyst Feed: 20 vol% DMEþ80 vol% N2 La–Mg-HZSM-5/Al2O3 Zr–Mg-HZSM-5/Al2O3 La–Zr–Mg-HZSM-5/Al2O3 Feed: 20 vol% DMEþ80 vol% H2O La–Mg-HZSM-5/Al2O3 Zr–Mg-HZSM-5/Al2O3 La–Zr–Mg-HZSM-5/Al2O3
ХDME, %
Т ¼ 320� С Т ¼ 380� С
Selectivity to HBs, wt %
ΣС2-С¼ 5 (MeOH)
¼ С¼ 2 /С3
С¼ 2
С¼ 3
ΣС¼ 4
ΣСþ 1
78.1 83.1 52.9
22.2 20.1 20.8
25.1 23.2 31.3
17.9 18.9 18.2
26.7 28.7 22.3
73.3 (21.8) 71.3 (17.6) 77.7 (35.0)
0.9 0.9 0.7
28.5 59.4 61.3 65.5
34.6 32.9 31.2 26.2
32.8 25.7 26.7 30.9
9.8 11.4 11.7 10.3
21.0 28.0 27.9 26.7
79.0 (62.2) 72.0 (42.6) 72.1 (37.7) 73.3 (4.8)
1.1 1.3 1.2 0.9
6
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Table 6 Effect of replacement of nitrogen by synthesis gas of different compositions on the conversion of dimethyl ether to light olefins in the presence of zeolite cat alysts pretreated with steam at 500� С (Р ¼ 0.1 MPa, Т ¼ 320 � С, WDME ¼ 1.4 h 1, data over 4 h of operation). Catalyst
ХDME, %
Selectivity to HBs, wt % С¼ 2
С¼ 3
ΣС¼ 4
Feed: 10 vol% DMEþ90 vol% N2 La–Mg-HZSM77.0 22.2 26.1 19.1 5/Al2O3 Zr–Mg-HZSM74.9 22.1 24.8 18.7 5/Al2O3 La–Zr–Mg78.8 21.4 22.7 19.4 HZSM-5/ Al2O3 Feed: 10 vol% DME þ90 vol% (CO þ H2) synthesis gas La–Mg-HZSM74.3 21.2 26.2 18.4 5/Al2O3 Zr–Mg-HZSM71.4 21.8 25.9 18.5 5/Al2O3 La–Zr–Mg77.6 19.7 24.0 19.1 HZSM-5/ Al2O3 Feed: 10 vol% DME þ90 vol% (CO þ H2) synthesis gas 78.1 21.3 26.0 19.2 La–Mg-HZSM5/Al2O3 Zr–Mg-HZSM65.2 19.9 29.0 19.3 5/Al2O3 La–Zr–Mg77.2 21.1 24.4 19.2 HZSM-5/ Al2O3
ΣСþ 1 24.6 26.1 27.8
ΣС2-С¼ 5 (MeOH)
С¼ 2/ С¼ 3
75.4 (20.8) 73.9 (21.2) 72.2 (18.9)
0.9
of module 3 27.1 72.9 (19.1) 26.6 73.4 (25.9) 29.3 70.7 (20.8) of module 1 25.9 74.1 (24.4) 24.3 75.7 (31.1) 27.5 72.5 (19.6)
0.9 0.9
0.8 0.8 0.8
0.8 0.7 0.9
cycle. The decrease in the temperature down to the operating temperature after the catalyst activation and the high-temperature steam treatment of the catalysts lead to increasing fraction of the medium-strength acid sites. The developed zeolite catalysts treated with steam at 500� С are suitable for the synthesis of light olefins from DME using synthesis gas of different compositions for dilution of DME. Declarations of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement N.V. Kolesnichenko: Conceptualization, Methodology, Supervi sion, Writing - review & editing. Е.N. Khivrich: Writing - original draft. T.K. Obukhova: Investigation, Formal analysis, Validation. Т.I. Batova: Investigation, Writing - review & editing. G.N. Bondarenko: Formal analysis. Acknowledgements This work was carried out within the State Program of TIPS RAS. The funding source is Ministry of Science and Higher Education of the Russian Federation. Funding is provided through the ITIPS RAS; there fore, it is not visible. References [1] M. Stoker, Microporous Mesoporous Mater. 29 (1999) 3. [2] C.D. Chang, A.J. Silvestri, J. Catal. 47 (1977) 249. [3] G. Cai, Z. Liu, R. Shi, Ch He, L. Yang, Ch Sun, Y. Chang, Appl. Catal. Gen. 125 (1995) 29. [4] J.Q. Chen, A. Bozzano, B. Glover, T. Fuglerud, S. Kvisle, Catal. Today 106 (2005) 103.
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