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Oxidative dehydrogenation of ethane with CO2 over CrOx catalysts supported on Al2O3, ZrO2, CeO2 and CexZr1-xO2
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Oxidative dehydrogenation of ethane with CO2 over CrOx catalysts supported on Al2O3, ZrO2, CeO2 and CexZr1-xO2 Tatiana A. Bugrovaa, Valerii V. Dutova, Valeriy A. Svetlichnyia, Vicente Cortés Corberánb, ⁎ Grigory V. Mamontova, a b
Tomsk State University, 36 Lenin Ave., Tomsk, 634050, Russia Institute of Catalysis and Petroleumchemistry, CSIC, 2 Marie Curie Str., Madrid, 28049, Spain
A R T I C LE I N FO
A B S T R A C T
Keywords: Oxidative dehydrogenation Carbon dioxide Ethane Chromium oxides Support nature
The effect of the support nature on the catalytic activity and stability of CrOx-containing catalysts supported on γ-Al2O3, ZrO2, CeO2 and CexZr(1-x)O2 has been investigated in the oxidative dehydrogenation of ethane with CO2 as a mild oxidant (ODH-CO2). The catalysts were prepared by wetness impregnation and characterized as-prepared and after catalytic tests using N2 adsorption, XRD, UV-Vis diffuse reflectance (DR) and Raman spectroscopies, TPR-H2 and TPO-CO2. The catalysts CrOx/γ-Al2O3 and CrOx/ZrO2 exhibited the highest ethylene formation rates, but the reaction followed different pathways on each: on CrOx/γ-Al2O3 catalyst ethylene is formed by a direct dehydrogenation (DDH) of ethane accompanied by RWGS, while on CrOx/ZrO2 ethylene is formed by selective oxidative dehydrogenation. The nature of the support influences the oxidation state of chromium species. The reaction pathway, catalytic activity and stability of the catalyst depend on both chromium state and support properties, in particular, on its acid-base and red-ox properties.
1. Introduction Currently, high demand in light olefins, such as ethylene and propylene, is caused by continuous growth of polymer industry that invokes high research interest in the field of light alkene production. Moreover, ethylene and propylene are important building blocks in organic and polymeric synthesis for production of valuable chemicals (ethylene oxide, polyethylene, propylene oxide, ethylene glycol, acetaldehyde, etc.) [1]. Steam cracking of naphtha and fluidized catalytic cracking (FCC) are traditional ways of olefin production [2,3]. However, these methods have some disadvantages, including high energy consumption and a wide product distribution, low selectivity and yield. The mixture of products requires the use of a complex of separation methods. In this regard, the alternative ways utilizing non-oil-based feedstock attract more attention due to high level of extraction of shale gas, natural gas and depletion of easily retrievable oil deposits. The oxidative coupling of methane (OCM) [4], methanol-to-olefins technology (MTO) [5], and direct (DDH) [6–9] or oxidative (ODH) [10–12] dehydrogenation of alkanes are the main alternative routes for olefin production. Unfortunately, OCM and MTO technologies still require many improvements due to complexity, CO2 emissions, low ethylene yield and selectivity towards ethylene, while all implementations of propylene and ethylene require high purity of raw materials [3,13].
⁎
Another promising way is a direct transformation of alkanes into olefins by catalytic dehydrogenation. Several processes were commercialized to produce isobutylene and propylene by DDH of isobutane and propane (Catofin, Oleflex), respectively, using modified Cr2O3/Al2O3 and Pt–Sn/Al2O3 catalysts [14,15]. The thermodynamic limitations and coke formation are the main unsolved problems of direct alkane dehydrogenation. Thermodynamic limitations increase in the raw of alkanes: C4 < C3 < C2. Thus, DDH of ethane is difficult to implement at a large scale due to low ethylene yield [16]. ODH reaction is a promising way to produce ethylene from ethane. The ODH of ethane in the presence of oxygen is thermodynamically unrestricted and occurs without coke deposition. However, most catalysts presented in the literature (V- and Mo-containing catalysts, composite oxides, etc.) provide low selectivity towards ethylene due to deep oxidation of ethane that limits their application [17]. The ODH of ethane in the presence of CO2 as a mild oxidant (ODH-CO2) has the advantage of higher selectivity than in case of O2-assisted dehydrogenation. Moreover, ecological problems connected with CO2 utilization may be solved using ODH-CO2 process [18]. The generally accepted point of view on the influence of CO2 addition on the ODH process is as follows. The equilibrium of dehydrogenation reaction in the presence of CO2 is shifted due to reverse water–gas shift reaction (RWGS) that leads to increase of ethylene yield [19]:
Corresponding author. E-mail address: [email protected] (G.V. Mamontov).
https://doi.org/10.1016/j.cattod.2018.04.047 Received 18 December 2017; Received in revised form 14 April 2018; Accepted 21 April 2018 0920-5861/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Bugrova, T.A., Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.04.047
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Table 1 Textural, crystalline and reducibility properties of catalysts and supports. Sample
SBET, m2/ga
Vp, cm3/gb
Dp, nmc
Cr, wt.%
Phase compositiond
H2 consumption, μmol/gcate
H2 consumption after ODH-CO2, μmol/gcate
γ-Al2O3 ZrO2 CeO2 CexZr(1-x)O2 CrOx/γ-Al2O3 CrOx/ZrO2 CrOx/CeO2 CrOx/CexZr(1-x)O2
201 47 84 52 154 47 19 20
0.40 0.24 0.23 0.03 0.31 0.23 0.18 0.04
6.3 16.3 9.1 3.1 6.8 16.2 29.7 5.5
– – – – 8.0 2.0 3.5 2.2
γ-Al2O3 m-ZrO2, t-ZrO2 c-CeO2 c-(Ce,Zr)O2, t-ZrO2 γ-Al2O3 m-ZrO2, t-ZrO2 CeO2, α-Cr2O3 c-(Ce,Zr)O2, t-ZrO2
– 335 335 416 957 936 804 1245
n.d.f n.d. n.d. n.d. 557 422 402 704
a b c d e f
Specific surface area calculated by applying the BET method. Total pore volume (BJH-desorption method). Average pore diameter calculated from the BJH method as 4Vp/Sext. From XRD data; c-: cubic, m-: monoclinic, t-: tetragonal. Calculated from TPR-H2 data. Not determined.
Dehydrogenation:CnH2n+2 ↔ CnH2n + H2
(1)
RWGS: CO2 + H2 ↔ CO + H2O
(2)
Overall: CnH2n+2 + CO2 ↔ CnH2n + CO + H2O
(3)
proposed. Also Cr-containing catalysts tend to deactivate under ODHCO2 conditions due to coke formation [29]. Thus, the use of oxide supports for Cr catalysts with active oxygen is preferable for stable operation of the catalyst. Another cause of catalyst deactivation is sintering of chromium species with formation of the inactive crystalline α-Cr2O3 [30]. Thus, oxide supports with high ability to stabilize amorphous CrOx species in the highly dispersed state may prevent the deactivation of catalyst caused by aggregation of CrOx species. ZrO2, CeO2 and mixed ceria–zirconia oxides are the best candidates as supports for ODH catalysts due to their unique red-ox properties. Different metal oxide catalysts (NbOx, V2O5, etc.) [31–33] supported on CeO2 and ZrO2 are active in this process. However, the CrOx species supported on CexZr(1-x)O2 and CeO2 are poorly studied as catalysts in the ODH of ethane. Therefore, the purpose of the present work is to study the influence of the nature of oxide supports on the performance of CrOx-containing catalysts for ODH of ethane with CO2. The red-ox properties of the supports and catalysts as well as the states of Cr are mainly discussed. The alumina–chromia catalysts with inert support (towards red-ox properties) were used as a reference.
Another point of view is based on direct participation of oxygencontaining sites [O] of the surface of metal oxide in alkane dehydrogenation followed by recovery of oxygen vacancy (re-oxidation) by gaseous CO2 (Mars-van Krevelen mechanism) [20]: [O]s + CnH2n+2 → CnH2n + H2O + []s
(4)
[]s + CO2 → CO + [O]s
(5)
where []s is an oxygen vacancy on the surface of metal oxide. In both cases ODH-CO2 provides higher conversion of alkane with remaining high selectivity towards olefin in comparison with non-oxidative DDH under the same operating conditions (temperature, GHSV, alkane concentration, etc.) [21]. The rate of reaction (4) depends on the concentration of oxygen sites that, in its turn, is affected by the rate of re-oxidation (5) of the oxide surface. The latter is connected with the ability of the surface to activate CO2. Thus, variation of the nature of the metal oxide, namely acid-base and red-ox properties, is an efficient approach to control the re-oxidation rate and catalytic activity in ODHCO2. Due to high temperature of the ODH-CO2 process, deactivation of the catalysts by coke formation is unavoidable under operation conditions. According to the literature data, the higher the density of basic sites on the catalyst surface, the higher the stability of the catalysts in the presence of CO2 is achieved [22]. In this case, a strongly adsorbed CO2 may oxidize carbon deposits on the catalyst surface. The catalytic performance modulation by changing of the nature of support and active component may be used to achieve high conversion of ethane, selectivity to ethylene and stability of the catalyst [23]. Among the supported metal oxides (In2O3, Ga2O3, V2O5) studied for this reaction, chromium oxide is a promising candidate due to its high catalytic activity towards dehydrogenation of alkanes [24]. Chromiaalumina catalysts are commonly used in industry for DDH of light paraffins in Catofin and Catadiene processes. Moreover, supported Crcontaining catalysts show high activity in ODH-CO2 of propane and isobutane [25]. However, the performance of existing catalysts is not enough for industrial application of ODH and strongly depends on the chromia content, type of support and operation conditions (CO2 partial pressure, GHSV, etc.). From this point of view, the research and development of new catalysts for ODH-CO2 is an important challenge. It is noteworthy that the active Cr species in ODH are still under discussion. Red-ox scheme of ODH-CO2 that includes direct participation of Cr6+/Cr3+ or Cr5+/Cr3+ [26,27] and Cr3+/Cr2+ [28] pairs was
2. Experimental 2.1. Sample preparation γ-Al2O3, ZrO2, CeO2 and CexZr(1-x)O2 were used as supports. Aluminum, cerium and zirconium oxides were prepared by thermal decomposition of AlO(OH), Ce(NO3)3·6H2O and ZrO(NO3)2·H2O (Across) at 600 °C for 4 h under static air atmosphere. Mixed CexZr(1x)O2 oxide was synthesized by co-precipitation method. The amounts of Ce(NO3)3·6H2O and ZrO(NO3)2·H2O calculated to get molar ratio of Ce:Zr = 1:1 were dissolved in water. Then, 2 M aqueous solution of ammonia was added dropwise to obtain the precipitate. The precipitate was dried at 90 °C overnight and calcined at 600 °C for 4 h. Chromia containing catalysts were prepared by wetness impregnation of γ-Al2O3, ZrO2, CeO2 and CexZr(1-x)O2 with aqueous solution of CrO3. Chromium loading corresponded to 1 theoretical monolayer (5 atoms Cr per nm2 of the support). The calculated content of Cr in wt. % is presented in Table 1. The catalysts obtained were dried at 95 °C overnight than calcined in air at 600 °C for 4 h. The prepared catalysts were denoted as CrOx/γ-Al2O3, CrOx/ZrO2, CrOx/CeO2 and CrOx/ CexZr(1-x)O2. 2.2. Sample characterization The prepared catalysts were characterized by N2 adsorption, X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), Raman 2
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The catalyst sample (0.25 g with a grain size 0.25–0.42 mm) was diluted by SiC (0.42–0.84 mm) in volume ratio 1:2 and put into the reactor between two quartz wool plugs. Pre- and post-catalytic zones in the reactor were filled with SiC grains to reduce the gas-phase reaction extent. The reaction mixture, 5% C2H6-10% CO2 in He, was passed through the reactor at a total flow rate of 200 cm3/min. Gas flows were controlled by Bronkhorst mass flow controllers. Reaction temperature range was from 500 to 700 °C. The reactor effluents were passed through a condenser and then analyzed by an on-line gas chromatograph Varian MicroGC equipped with a Pora Plot Q capillary column and a TCD. The catalytic tests for each sample were conducted successively at temperatures from 500 to 700 °C in increasing steps of 50 °C, kept for 30 min each. After each temperature setting increase, the new temperature became stabilized after around 10 min. Only measurements made in the isothermic portion of each step were considered valid for analysis and discussion.
spectroscopy (RS), temperature-programmed reduction (TPR) and oxidation in CO2 (TPO-CO2), thermogravimetric analysis and differential scanning calorimetry coupled with mass spectrometry (TGA-DSC-MS). The specific surface area (SSA) and pore diameter of the samples were measured by nitrogen sorption at −196 оC using a TriStar II 3020 analyzer (Micromeritics, USA). Prior to experiments, all samples were degassed at 200 °C up to constant mass for 2 h. SSA was determined by the BET method. The pore size distribution was calculated from the desorption branch of the adsorption–desorption isotherm by the Barrett-Joyner-Halenda (BJH) method. X-ray diffraction (XRD) patterns were recorded in the range 2θ from 10 to 90° with a scanning rate of 2°/min on a MiniFlex 600 (Rigaku, Japan) diffractometer using monochromatized Cu-Kα radiation (λ = 1.5418 Å) with a power setting of 40 kV and 15 mA. The phase identification was made according to PCPDFWIN database. UV–vis disffuse reflectance (DR) spectra were recorded using a Cary 100 SCAN spectrophotometer with DRA-CA-30I, Labsphere attachment (Varian, Australia) in the wavelength range of 200–900 nm, with a step of 1 nm, and a spectral resolution up to 0.1 nm. MgO was used as a standard sample. Raman spectra were obtained at ambient conditions using inVia confocal Raman microscope (Renishaw, UK) with a spectral resolution of 2 cm−1. The samples were excited with the 532 nm line and a laser power of 4 mW (5% from initial laser power). The spectra were recorded at an exposure time of 1 s, an accumulation number of 50, and an objective with magnification x20 was used. The red-ox properties of the catalysts were investigated by TPR-H2 using a ChemiSorb 2750 chemisorption analyzer (Micromeritics) equipped with thermal conductivity detector (TCD). The TPR experiments were performed in the temperature range from room temperature (RT) to 650 °C at a heating rate of 10 °C/min using a 10 vol. % H2/ Ar mixture (flow rate of 20 cm3/min). To avoid the effect of water formed, a cold trap with a mixture of liquid nitrogen and isopropanol (ca. −90 °C) was set prior to the detector. Prior to the TPR experiments, the catalysts were pre-oxidized in a TPO mode in an air flow (20 cm3/ min) with a heating rate of 10 °C/min up to 650 °C. TPO-CO2 experiments were performed in a U-shaped quartz reactor using a ChemiSorb 2750 analyzer (Micromeritics, USA) coupled with a quadrupole mass spectrometer (MS) UGA 300 (Stanford Research Systems, USA). Prior to experiment the sample (∼0.2 g) was reduced in 10%H2/Ar flow (20 cm3/min) in a TPR mode up to 650 °C and kept at this temperature for 20 min. After cooling to RT in the 10%H2/Ar mixture, the sample was purged with He flow (27 cm3/min). Then the flow was switched to 10%CO2/He mixture (30 cm3/min), and sample was heated from RT to 950 °C with a rate of 10 °C/min. Concentrations of CO2 (m/z = 44), CO (m/z = 28), H2O (m/z = 18) and O2 (m/ z = 32) were monitored by MS. To remove the trace amounts of oxygen and water from the 10%CO2/He mixture, two traps filled with reduced Cu/SiO2 (maintained at 340 °C) and calcined alumina (maintained at RT) were used, respectively. The TG-DSC-MS analysis of the samples was performed using a STA 449 F1 Jupiter analyzer (Netzsch, Germany) coupled with quadrupole mass spectrometer QMS 403D Aëolos (Netzsch, Germany). The samples (5–10 mg) were placed in Al2O3 crucible and heated from 25 to 900 °C at a heating rate of 10 °C/min in air flow (10 cm3/min Ar + 80 cm3/ min air).
3. Results 3.1. Catalytic properties It is known that dehydrogenation of alkanes in the presence of CO2 is accompanied by a wide variety of side reactions due to high process temperature [12]. The following reactions take place: selective ODH: C2H6 + CO2 ↔ C2H4 + CO + H2O
(6)
DDH: C2H6 ↔ C2H4 + H2
(7)
non-selective reaction: C2H6 + 5 CO2 ↔ 7 CO + 3 H2O
(8)
water–gas shift reaction: CO + H2O ↔ CO2 + H2
(9)
cracking: C2H6 + H2 ↔ 2 CH4
(10)
disproportionation: 2 C2H6 ↔ C3H8 + CH4
(11)
coke formation: C2H4 ↔ 2 C + 2H2
(12)
C2H6 ↔ 2 C + 3H2
(13)
Boudouard reaction: C + CO2 ↔ 2 CO
(14)
dry reforming: C2H6 + 2 CO2 ↔ 4CO + 3H2
(15)
Moreover, gas-phase processes (homogeneous reactions) cannot be excluded, especially above 650 °C [35]. Thermodynamic calculations of ΔGr of these reactions are presented as a function of temperature in Fig. 1. One can see that all side reactions are thermodynamically more favorable especially at high temperatures than the target reactions (ODH and DDH of ethane). Dry reforming and non-selective reactions take place only at high temperatures. Therefore, the selectivity toward ethylene strongly depends on the properties of the active sites of the catalyst. It is known that high acidity of the catalyst surface provokes cracking and coke formation leading to selectivity decrease. Coke formation via ethylene decomposition is more favorable than via ethane decomposition due to higher polymerization ability of the former. This means that a higher formation rate of ethylene (higher activity of catalyst) will lead to more rapid deactivation due to coke deposition. However, oxidation of coke via Boudouard reaction is possible for supports with active oxygen species. The temperature dependencies of ΔGr for DDH and selective ODH reactions are similar due to low ability of CO2 to oxidize H2. Equilibrium of the RWGS shifts to CO formation (K° > 1) only at 1100 K. Therefore, both processes have thermodynamic limitations. High ratio of CO2/C2H6 (2 or above) is required to shift the equilibrium and improve the ethylene yield (Fig. 2). The advantage of the ODH-CO2 of ethane in comparison with the DDH may be connected with the kinetic reasons rather than thermodynamic ones. Ethane dehydrogenation in the presence of surface oxygen formed
2.3. Catalytic test The catalytic activity in the ODH-CO2 of ethane was tested at Microactivity-Reference unit (PID Eng&Tech, Spain). The experiments were carried out in a stainless steel, down flow reactor (305 mm long, 9 mm i.d.) operating at atmospheric pressure [34]. During the experiment the reactor was placed in electrical furnace located inside a hot box at 175 °C. Reaction temperature was controlled by coaxial K-type thermocouple inserted in the center of catalyst bed without thermowell. 3
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catalyst is characterized by similar rates of CO and ethylene formation at 500–550 °C, which means that the reaction occurs via selective ODH. When the temperature increases, the formation rate of CO rises in a higher extent than the one for ethylene due to non-selective reaction and/or dry reforming of ethane. Both CrOx/ZrO2 and CrOx/Al2O3 are characterized by fast deactivation that occurs mainly at 650-700 °C. The CrOx/CeO2 catalyst exhibits the lowest rate of C2H4 formation, and, hence, higher stability among all studied catalysts. The CO formation rate is fourfold that of C2H4, which may be associated with nonselective reaction or dry reforming of ethane. The rate of C2H4 formation is higher for CrOx/CexZr(1-x)O2 in comparison with CrOx/CeO2. The CrOx/CexZr(1-x)O2 is characterized by slightly faster deactivation than CrOx/CeO2 and similar rate of CO formation. The difference between the rates of CO and C2H4 formation gradually increases with the temperature increase for CrOx/CexZr(1-x)O2 and CrOx/CeO2, which means that both catalysts intensively accelerate the non-selective reactions of ethane transformation. One of the possible reasons for low rate of catalyst deactivation is the oxidation of carbon deposits on the catalyst surface. Due to the big differences in SSA of the catalysts, the surface-specific rates of ethylene and CO formation (i.e., per m2 of catalyst SSA) were also calculated and are plotted in Fig. 3(c,d). Indeed, the ratios between the formation rates of CO and C2H4 are the same as those calculated per catalyst mass. However, in this case the CrOx/Al2O3 and CrOx/CeO2 catalysts exhibit the lowest surface-specific rate of C2H4 formation at 500–650 °C. This means that the highest activity of CrOx/Al2O3 is mainly caused by the higher SSA of this catalyst (Table 1). The CrOx/ ZrO2 catalyst showed the higher ethylene formation rate. The rate of ethylene production at 500–650 °C for CrOx/CexZr(1-x)O2 catalysts is slightly lower than for CrOx/ZrO2. The surface-specific rates of CO formation for CrOx/ZrO2, CrOx/CeO2, and CrOx/CexZr(1-x)O2 at 500–650 °C are similar, but differed significantly at 700 °C. The evolution of ethane conversion and C2H4 selectivity with reaction temperature and time is shown in Fig. 4. One can see that CrOx/ CeO2 and CrOx/CexZr(1-x)O2 possess the lowest ethane conversion (2–10%) and selectivity towards ethylene at 550–700 °C. This selectivity noticeably decreases from 95 to 45% as the reaction temperature increases. These catalysts are characterized by the highest ratios of CO-to-ethylene formation rates, which indicate non-selective and/or dry reforming side reactions. The highest selectivity was observed for CrOx/Al2O3 and CrOx/ZrO2 catalysts, for which it slowly decreased to 90 and 80%, respectively, during the reaction time. CrOx/ Al2O3 was the most selective catalyst due to the absence of non-selective oxidation and/or dry reforming reactions. The main side reactions for this sample are cracking and coke deposition. The highest conversion of ethane (5–21%) was observed for CrOx/ZrO2 at 550–650 °C. Rapid deactivation of catalyst at temperatures above 600 °C can be associated with its active oxide sintering. Thus, the deactivation of all catalysts is mainly connected with coke deposition and possibly with agglomeration of active component. The rate of deactivation gradually increased with the temperature and depended on the ethylene formation rate. High activity of CrOx/ZrO2 and CrOx/Al2O3 catalysts toward ethylene production leads to their rapid deactivation.
Fig. 1. Thermodinamic calculations of ΔGr of the possible reactions as a function of temperature.
Fig. 2. Dependence of equilibrium conversion of ethane on CO2/C2H6 ratio.
through dissociation of CO2 probably occurs more easily (with lower activation energy). The specific rates of ethylene and CO formation per catalyst mass unit for all catalysts during the ODH-CO2 are presented in Fig. 3(a,b). It can be seen that the rate of C2H4 formation was higher than the one of CO for CrOx/Al2O3 catalyst, though the difference was small, even at 700 °C. Thus, the process consists of two reactions: direct dehydrogenation and RWGS. The equilibrium of the latter reaction is shifted to CO and H2O and is accompanied by a temperature increasing, which leads to reduction in the difference between the C2H4 and CO formation rates. The rate of ethylene production at 500–650 °C for CrOx/ZrO2 catalysts is slightly higher than for CrOx/Al2O3. However, the rates of CO formation for these catalysts differ significantly. The CrOx/ZrO2
3.2. Textural properties and phase composition of the samples The porous structure of the supports and catalysts was studied by low-temperature N2 sorption. The presence of a hysteresis loop on the isotherms at relative pressures from 0.45 to 1.0 for all supports (not shown) indicates formation of mesoporous structure. According to pore size distributions (Fig. 5a), the porous structure of the supports consisted of pores with sizes 2–20, 3–50, 2–20 and 2–6 nm for γ-Al2O3, ZrO2, CeO2 and CexZr(1-x)O2, respectively. The values of SSA, average pore size and total pore volume of supports and catalysts are presented in Table 1. SSA of γ-Al2O3 is much higher than those of ZrO2, CeO2 and 4
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Fig. 3. Specific rates of ethylene (a) and CO (b) formation and Area-specific rates of ethylene (c) and CO (d) formation in heterogeneous ODH-CO2 over Crcontaining catalysts.
The most dramatic changes in the porous structure are observed for CrOx/CeO2 and CrOx/CexZr(1-x)O2 catalysts: in comparison with CeO2 and CexZr(1-x)O2 supports (Fig. 5), their SSAs decrease by 77 and 62%, respectively. Pore size distributions for catalysts CrOx/CeO2 and CrOx/ CexZr(1-x)O2 in the range of higher pore diameter was shifted from average pore size 9.1 and 3.1 nm to 29.7 and 5.5 nm, respectively (Table 1). The described changes in porous structure may be caused by the filling of the smaller pores of ceria with chromium oxide during the impregnation with chromic acid solution or possible sintering after the second calcination. The phase composition of supports and catalysts is also presented in Table 1. The absence of reflections of Cr-containing phases indicates the stabilization of CrOx in an amorphous state on the surface of Al2O3, ZrO2 and CexZr(1-x)Ox supports. Reflections of α-Cr2O3 phase (2θ = 24.50, 33.61 and 36.20°) are observed only for CrOx/CeO2 catalysts. CeO2 particle sizes grow from 5.9 nm (reflection of CeO2(111) at 28.6°) for CeO2 support to 18.9 nm for CrOx/CeO2 catalyst. Thus, sintering of ceria together with agglomeration of chromia in α-Cr2O3 phase is observed for CrOx/CeO2 catalyst.
Fig. 4. ODH-CO2 of ethane on Cr-containinig catalysts: conversion of ethane and selectivity to ethylene as a function of reaction temperature and time.
3.3. The state of supports and active component CexZr(1-x)O2. Incorporation of chromium oxide into chromia-containing catalysts caused an important decrease of the surface area and pore volume (Table 1) excepting for CrOx/ZrO2 catalyst which maintained the properties of ZrO2 support. This may be explained by stabilization of mononuclear chromia species on the surface of zirconia and additional stabilization of zirconia towards sintering by chromia at high temperatures [36].
Fig. 6 shows the UV–vis DR and Raman spectra of supports and catalysts. UV–vis spectra of all catalysts (Fig. 6(a)) two main absorption bands at ∼275 and ∼376 nm attributed to CrVI species [37]. The absorption at 200–500 nm observed for CeO2 and CexZr(1-x)O2 supports can be attributed to the presence of cubic CeO2 and mixed Ce-Zr oxide [38]. The two bands at ∼460 and ∼582 nm observed for CrOx/CeO2 catalyst indicate the presence of CrIII species in the octahedral 5
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Fig. 5. Pore size distributions for supports (a) and catalysts (b).
symmetry (α-Cr2O3) [37,39]. The presence of α-Cr2O3 in CrOx/CeO2 catalyst is also confirmed by XRD. According to Raman spectra (Fig.6(b)), the CrOx/γ-Al2O3 is only characterized by those bands originating from vibrations of chromium oxides. The spectrum contains a weak band at ∼630 cm−1, which probably corresponds to υs(CreOeCr). Two bands and shoulders observed at 865, 1003 cm−1 and at ∼787, ∼920 cm−1 are assigned to the υs(OeCreO), υs(Cr]O), υas(CreOeCr) and υs of CrO2 units that terminate the surface polymeric species, respectively [40,41]. All these bands and shoulders can be attributed to poly-, mononuclear CrVI species. In the case of ZrO2 and CrOx/ZrO2, the bands at 145, 178, 219, 266, 330, 380, 476, 537, 556, 615 and 634 cm−1 correspond to tetragonal and monoclinic zirconia [42]. These bands are slightly shifted for the
CrOx/ZrO2 catalyst. The bands at 780, 817, 854, 890, 1009 and 1028 cm−1 attributed to CrVI species are found in the range from 650 to 1400 cm−1. The most intense Raman band at 854 cm−1 may be assigned to ‘isolated’ tetrahedral surface-mononuclear (CrVI) species distorted by the interaction with the support surface [43], while the peaks at 780, 817 and 890 cm−1 as shoulders to the main band probably correspond to CreO vibrations in higher polymerization degree species (polynuclear CrVI). Two bands at 1009 and 1028 cm−1 are attributed to mono- and dioxo Cr]O vibrations [44]. The Raman spectra of CeO2 contain strong F2g band of the fluorite phase at 460 cm−1 and weak bands at 260, 600 and 1170 cm−1 related to the second-order transverse acoustic mode (2 TA), defect-induced mode (D) and a second overtone band (2LO), respectively [45]. The spectra of the CrOx/CeO2 catalyst differ significantly from those of
Fig. 6. UV–vis DRS (a) and Raman (b) spectra for catalysts and supports. 6
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Fig. 8. TPR-H2 profiles of supports, as-prepared catalysts and catalysts after ODH-CO2 (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 7. Raman spectra of catalysts before (as prepared) and after ODH-CO2 (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Cr2O3 state and formation of mixed chromia–alumina phases [30]. Both processes lead to irreversible catalyst deactivation. The H2 consumption peak at temperatures above 450 °C for the ZrO2 support is attributed to partial reduction of Zr4+ to Zr3+ in the structure of zirconia [49]. The TPR peak above 400 °C for CeO2 support is associated with the reduction of the surface Ce4+ to Ce3+ [50–52]. Two peaks at temperatures above 350 °C for the CexZr(1-x)O2 can be attributed to reduction of Zr4+ and Ce4+. The reducibility of mixed ceria–zirconia oxides is known to be higher of than those of individual CeO2 and ZrO2 [53]. These results are in agreement with increased number of defects of CexZr(1-x)O2 observed during the Raman studies. The TPR profile for the CrOx/ZrO2 catalysts is characterized by an intensive peak with maximum at ∼340 °C, which can be attributed to reduction of Crn+ from the high valence state to Cr2+/3+ [54]. The shift of this peak in comparison with the CrOx/Al2O3 catalyst may indicate the stabilization of isolated CrV and/or CrVI species on the ZrO2 surface [23,37] that is confirmed by the Raman spectroscopy. The H2 consumption at 400–600 °C for the CrOx/ZrO2 catalyst may be attributed to reduction of CrOx species strongly bonded with the support or reduction of zirconia catalyzed by chromia [25,55]. The decrease of this peak intensity is observed for the catalysts after the catalytic process, while the peak area at 350 °C did not change significantly. This indicates higher stability of red-ox chromia species on the zirconia surface (in comparison with the CrOx/Al2O3 catalyst). The same results are observed for CrOx/CeO2 and CrOx/CexZr(1-x)O2 catalysts. The difference between the maximum of peak reduction of CrV/VI may be attributed to both, different interaction of chromia with the support and different nuclearity of CrV/VI species. TPO-CO2 experiments were carried out to investigate the ability of catalyst for re-oxidation by CO2 (Figs. 9 and 10). One can see that no reoxidation of CrOx/Al2O3 catalyst by CO2 was detected. This phenomenon may be associated with poor basicity of CrOx/Al2O3 catalyst. According to literature data [18], the active sites for CO2 activation
CrOx/γ-Al2O3 and CrOx/ZrO2. Besides the bands of CeO2, weak bands at 537, 837 and 1018 cm−1 are observed for the CrOx/CeO2 catalyst. The bands at 837 and 1018 cm−1 can be attributed to CreO and Cr]O vibrations of CrVI species, while the bands at 537 and 600 cm−1 are characteristic for the crystalline Cr2O3 nanoparticles (CrIII octeO bond) [39,44]. These results are in agreement with the DRS and XRD data. The bands for CexZr(1-x)O2 support are the same as those for CeO2 support that confirms formation of mixed CeeZr oxide with cubic phase. The bands of monoclinic and tetragonal ZrO2 are not observed. In comparison with spectrum of pure CeO2 the increased intensity of the bands at 230 and 615 cm−1 attributed to oxygen vacancies [46] indicates the growth of number of defects of the mixed oxide. The bands at 834, 863, 1008 and 1023 cm−1 can be attributed to poly(mono-) nuclear CrVI species as described above. It is noteworthy that the bands of vibrations of CrIII species (537 cm−1) are not observed. After ODH-CO2 (Fig. 7), two strong bands at 1598 and 1343 cm−1 assigned to the structured carbon [47] are observed for CrOx/γ-Al2O3, CrOx/ZrO2 and CrOx/CexZr(1-x)O2 catalysts. These bands are also present in the spectra of the CrOx/CeO2, but their intensity is low. The vibrations of chromium species are not found for CrOx/γ-Al2O3 and CrOx/ZrO2 due to carbon deposits on the catalyst surface and are weak for the CrOx/CexZr(1-x)O2 and CrOx/CeO2 possibly due to reduction of CrVI species. The TPR profiles for supports, as-prepared catalysts and catalysts after catalytic test are shown in Fig. 8. The amount of H2 consumed is summarized in Table 1. The intensive H2 consumption peak with a maximum at 379 °C observed for the as-prepared CrOx/Al2O3 catalyst is attributed to reduction of CrV/VI to CrII/III species [48]. The decrease of intensity of this peak and a shift in a high-temperature region are observed for the used catalyst. This indicates the decreased amount of redox chromia (i.e. CrOx that can be reversibly oxidized-reduced (CrV/ VI ↔CrII/III)) due to its agglomeration, with stabilization in inactive α7
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Fig. 9. TPO-CO2 profiles of supports CeO2 and CexZr(1-x)O2: Evolution of mass spectra of CO (m/z = 28), CO2 (m/z = 44) and water (m/z = 18) emission (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
CrOx/CexZr(1-x)O2 are responsible for CO2 activation. ZrO2 possess not only strong acid sites (Zr4+), but also strong Lewis basic sites (O2−) and Bronsted basic sites (terminal OH-groups) [56]. According to TPO-CO2 results, small peak of CO2 consumption is observed at 760 °C, while CO emission starts at 800 °C.
should possess electron donor properties (basic properties). After electron acceptance, a linear CO2 molecule transforms into bended charged active species CO2‾ on the oxide surface and can dissociate with filling of oxygen vacancy and CO formation. Similarly the absence of re-oxidation was observed for CrOx/CeO2, though the re-oxidation of pure CeO2 took place (Fig. 9). This means that addition of chromia to CeO2 leads to a significant change of electronic and acid-base properties of cerium oxide. For CrOx/CexZr(1-x)O2 catalyst, a peak of CO emission at 535 °C together with two peaks of CO2 desorption at 150 and 450 °C were observed. One may conclude that basic sites on the surface of
3.4. Investigation of coke deposits on the catalyst surface after ODH-CO2 The TG-DSC data for the catalysts after ODH-CO2 with mass spectrum of CO2 and H2O emission are shown in Fig. 11. The first step of
Fig. 10. TPO-CO2 profiles of Cr-containing catalysts: Evolution of mass spectra of CO (m/z = 28), CO2 (m/z = 44) and water (m/z = 18) emission (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 8
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Fig. 11. TG-DSC data for CrOx/γ-Al2O3 (a), CrOx/ZrO2 (b), CrOx/CeO2 (c), CrOx/CexZr(1-x)O2 (d) catalysts after ODH-CO2 with mass spectrum of CO2 and H2O emission.
weight loss in the low-temperature region (from RT to 150–200 °C) is connected with desorption of the adsorbed water. The second step, accompanied by exothermic effect and CO2 emission, was observed in the temperature range of ∼150–300 °C for CrOx/CeO2 and ∼200–400 °C for CrOx/γ-Al2O3, CrOx/CexZr(1-x)O2 and CrOx/ZrO2 catalysts. This indicates the combustion of carbon deposits on the catalyst surface. The difference in temperature maxima of the DSC-signal and CO2 emission indicates differences in the nature of the carbon deposits and/or in the ability of the catalyst surface toward carbon oxidation. The supports with active surface oxygen (CeO2, CexZr(1-x)O2, ZrO2) oxidize carbon deposits at lower temperatures. In addition, an increase of the sample mass was observed in the range of 350–500 °C for CrOx/CeO2 and CrOx/CexZr(1-x)O2 due to reoxidation of the catalysts by oxygen. At the same time, the mass loss at the second step differs greatly for CrOx/γ-Al2O3 and for CrOx/ZrO2, CrOx/CeO2, CrOx/CexZr(1-x)O2 catalysts (7.46 and 0.36, 0.39, 0.8%, respectively). Accumulation of large amounts of coke on the surface of CrOx/γ-Al2O3 is connected with high rate of ethylene formation and low reaction ability of surface oxygen of alumina. Strong acid sites are known to facilitate coke formation on the alumina surface. The small amount of carbon deposits on the surface of CrOx/ZrO2 (0.36% wt.) may be associated with the removal of coke by gaseous CO2 during the ODH process. It was shown [57] that the using of ZrO2 as a modifier for Ni/Al2O3 catalyst significantly improved its coke resistance in dry reforming of methane. Carbon deposits may be oxidized by CO2 at the Ni/ ZrO2 interface. The CrOx/CeO2 and CrOx/CexZr(1-x)O2 catalysts are characterized by low amounts of coke. One of the possible reasons is a low rate of ethylene formation. Another reason is connected with carbon removal by CO2 during the ODH as in the case of ZrO2 support. Moreover, DFT calculations show that CeO2 with high concentration of oxygen vacancies on the surface may be re-oxidized during the CO2
dissociation [58]. So oxygen mobility in CeO2 and CexZr(1-x)O2 network may favor the removal of coke precursor on their surfaces. 4. Discussion Our results show that the state of chromium depends on the nature of support used. The reaction pathway and catalytic activity depend on both chromium state and support properties, in particular, on acid-base and red-ox properties. Thus, CrOx/γ-Al2O3 catalyst is characterized by formation of ethylene during the DDH accompanied by the RWGS. This is a ‘classical’ catalyst for the dehydrogenation processes. The CrOx/γAl2O3 catalyst contains amorphous chromium oxide in both CrV/VI and CrII/III states. Chromium red-ox cycle CrV/VI ↔ CrII/III on the catalyst surface triggers DDH process by activation of C2H6 molecule. According to our results, chromia on the surface of γ-Al2O3 is not re-oxidized by gaseous CO2 as confirmed by the TPO-CO2 data (Fig. 10). This limits the possibility of CO2 involvement in the reactions (ODH, non-selective reactions). High surface acidity triggers an intensive coke deposition on the surface and deactivation of the catalyst, while the absence of strong CO2 adsorption leads to impossibility of carbon removal. According to DSC-TG results, the amount of carbon after ODH-CO2 was 7.46% wt, one order of magnitude higher than on the other, non-acidic support catalysts. As a consequence, the rate of deactivation of CrOx/γ-Al2O3 is faster than for other catalysts (Figs. 3, 4). Both selective ODH and DDH are the possible reaction pathways over CrOx/ZrO2 catalyst. In this catalyst, chromium is stabilized mainly as poly-, mononuclear Cr V/VI species that provides the increased activity. The higher amount of CrVI in CrOx/ZrO2 sample (∼66% of total Cr content) in comparison with the one in CrOx/γ-Al2O3 (∼35% of total Cr content) may be caused by a better distribution of chromium on the surface of ZrO2 due to stronger interaction with the support. The ability 9
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References
of ZrO2 support to activate CO2 over basic sites improves the activity in the ODH process. Besides, it is possible to conclude that side reactions over CrOx/ZrO2 catalyst comprise non-selective oxidation and/or dry reforming reactions, since the ratio between the CO and C2H4 formation rates is 1.5–2. According to TG-DSC results, low amount of coke (0.36% wt.) remains after the catalytic experiment, which indicates the possible oxidation of deposited coke on the catalyst surface by CO2 (Boudouard reaction). The relatively low activity of CrOx/CeO2 catalyst is connected with the formation of α-Cr2O3 phase (low activity in DDH) and a small amount of reducible Cr V/VI. The re-oxidation ability of CeO2 support is observed by TPO-CO2. This, together with the high oxygen mobility in the ceria nertwork, makes non-selective oxidation and/or dry reforming reactions (ratio CO/C2H4 is up to 5) the side reactions characteristic for this catalyst. The low amount of coke observed (0.39% wt.) is caused by both low dehydrogenation activity of CrOx/CeO2 catalyst and Boudouard reaction. Unlike CrOx/CeO2 catalyst, the catalyst supported on CexZr(1-x)O2 contains chromium mainly in a high valence state (Cr V/VI) according to TPR-H2 and Raman results, and no α-Cr2O3 phase is detected. And, in parallel, the activity of CrOx/CexZr(1-x)O2 catalyst is higher than the one of CrOx/CeO2 (Fig.3(c)). Ethylene formation takes place by selective ODH-CO2. CexZr(1-x)O2 is more reducible than ZrO2. The side reactions over CrOx/CexZr(1-x)O2 catalyst are the same as for CrOx/ZrO2 and CrOx/CeO2, though the ratio between the CO and C2H4 formation rates amounts to 3 and is lower than the one for CrOx/CeO2. Thus, the increase of CrV/VI species concentration leads to increase in catalyst activity in ethane dehydrogenation. The possibility of CO2 activation on the support leads to increased activity in ODH for CrOx/ ZrO2 and CrOx/CexZr(1-x)O2 catalysts. CeO2 support provides activity of catalysts in non-selective oxidation. Thus, the control of red-ox properties of the support to provide stabilization of CrV/VI species on its surface is a key factor determining the activity of chromia catalysts in oxidative dehydrogenation of ethane with CO2 as a mild oxidant.
[1] L. Kniel, O. Winter, K. Stork, Ethylene, keystone to the petrochemical industry, M. Dekker, New York, 1980. [2] Z. Nawaz, Rev.Chem. Eng. 31 (2015) 413–436. [3] I. Amghizar, L.A. Vandewalle, K.M. Van Geem, G.B. Marin, Eng. Lond. 3 (2017) 171–178. [4] W. Shen, Acta Phys. Chim. Sin. 33 (2017) 2321–2322. [5] M. Sedighi, M. Ghasemi, A. Jahangiri, Korean J. Chem. Eng. 34 (2017) 997–1003. [6] T.P. Otroshchenko, U. Rodemerck, D. Linke, E.V. Kondratenko, J. Catal. 356 (2017) 197–205. [7] H. Lee, W.-I. Kim, K.-D. Jung, H.L. Koh, Korean J. Chem. Eng. 34 (2017) 1337–1345. [8] T.P. Otroshchenko, V.A. Kondratenko, U. Rodemerck, D. Linke, E.V. Kondratenko, Catal. Sci. Technol. 7 (2017) 4499–4510. [9] T.A. Bugrova, N.N. Litvyakova, G.V. Mamontov, Kinet. Katal. 56 (2015) 758–763. [10] F. Rahmani, M. Haghighi, S. Mahboob, Ultrason. Sonochem. 33 (2016) 150–163. [11] S. Al-Ghamdi, M. Volpe, M.M. Hossain, H. De Lasa, Appl. Catal. A Gen. 450 (2013) 120–130. [12] M.A. Atanga, F. Rezaei, A. Jawad, M. Fitch, A.A. Rownaghi, Appl. Catal. B Environ. 220 (2018) 429–445. [13] W. Liang, S. Sarsani, D. West, A. Mamedov, I. Lengyel, H. Perez, J. Lowrey, Catal. Today 299 (2018) 60–66. [14] H.N. Pham, J.J.H.B. Sattler, B.M. Weckhuysen, A.K. Datye, ACS Catal. 6 (2016) 2257–2264. [15] V.Z. Fridman, Appl. Catal. A Gen. 382 (2010) 139–147. [16] M. Bender, Chem. Biochem. Eng. Rev. 1 (2014) 136–147. [17] C.A. Gartner, A.C. van Veen, J.A. Lercher, ChemCatChem 5 (2013) 3196–3217. [18] D. Mukherje, S.-E. Park, B.M. Reddy, J. CO2 Util. 16 (2016) 301–312. [19] P. Michorczyk, P. Kuśtrowski, A. Kolak, M. Zimowska, Catal. Commun. 35 (2013) 95–100. [20] R. Yuan, Y. Li, H. Yan, H. Wang, J. Song, Z. Zhang, W. Fan, J. Chen, Z. Liu, Z. Liu, Z. Hao, Chin. J. Catal. 35 (2014) 1329–1336. [21] F. Rahmani, M. Haghighi, M. Amini, J. Ind. Eng. Chem. 31 (2015) 142–155. [22] H. Tsuneoka, K. Teramura, T. Shishido, T. Tanaka, J. Phys. Chem. 114 (2010) 8892–8898. [23] F. Rahmani, M. Haghighi, B. Mohammadkhani, Microporous Mesoporous Mater. 242 (2017) 34–49. [24] J.J.H.B. Sattler, J. Ruiz-Martinez, E. Santillan-Jimenez, B.M. Weckhuysen, Chem. Rev. 114 (2014) 10613–10653. [25] G. Sun, Q. Huang, H. Li, H. Liu, Z. Zhang, X. Wang, Q. Wang, J. Wang, Chin. J. Catal. 32 (2001) 1424–1429. [26] N. Mimura, I. Takahara, M. Inaba, M. Okamoto, K. Murata, Catal. Commun. 3 (2002) 257–262. [27] X. Ge, M. Zhu, J. Shen, React. Kinet. Catal. Lett. 77 (2002) 103–108. [28] Y. Ohishi, T. Kawabata, T. Shishido, K. Takaki, Q. Zhang, Y. Wang, K. Takehira, J. Mol. Catal. A Chem. 230 (2005) 49–58. [29] P. Michorczyk, P. Pietrzyk, J. Ogonowski, Microporous Mesoporous Mater. 161 (2012) 56–66. [30] V.Z. Fridman, R. Xing, Ind. Eng. Chem. Res. 56 (2017) 7937–7947. [31] M.V. Martinez-Huerta, G. Deo, J.L.G. Fierro, M.A. Banares, J. Phys. Chem. 111 (2007) 18708–18714. [32] R. You, X. Zhang, L. Luo, Y. Pan, H. Pan, J. Yang, L. Wu, X. Zheng, Y. Jin, W. Huang, J. Catal. 348 (2017) 189–199. [33] A. Talati, M. Haghighi, F. Rahmani, RSC Adv. 6 (2016) 44195–44204. [34] M. Guío, J. Prieto, V.Cortes Corberán, Catal. Today 112 (2006) 148–152. [35] R.X. Valenzuela, G. Bueno, V. Cortés Corberán, Y. Xu, C. Chen, Catal. Today 61 (2000) 43–48. [36] H.R. Mahmoud, J. Mol. Catal. A Chem. 392 (2014) 216–222. [37] S. De Rossi, M.P. Casaletto, G. Ferraris, A. Cimino, G. Minelli, Appl. Catal. A Gen. 167 (1998) 257–270. [38] A. Bensalem, F. Bozon-Verduraz, M. Delamar, G. Bugli, Appl. Catal. A Gen. 121 (1995) 81–93. [39] G.E. Bekmukhamedov, A.N. Mukhamed’yarova, S.R. Egorova, A.A. Lamberov, Catalysts 6 (2016) art. no. 162. [40] T.J. Dines, S. Inglis, Phys. Chem. Chem. Phys. 5 (2003) 1320–1328. [41] D.S. Kim, I.E. Wachs, J. Catal. 142 (1993) 166–171. [42] F. Adar, Spectrosc. (Santa Monica) 29 (9) (2014). [43] V.Z. Fridman, R. Xing, M. Severance, Appl. Catal. A. Gen. 523 (2016) 39–53. [44] E.L. Lee, I.E. Wachs, J. Phys. Chem. 111 (2007) 14410–14425. [45] Z. Wu, M. Li, J. Howe, H.M. Meyer III, S.H. Overbury, Langmuir 26 (2010) 16595–16606. [46] B.M. Reddy, G.K. Reddy, L. Katta, J. Mol. Catal. A Chem. 319 (2010) 52–57. [47] A. Simon, M. Seyring, S. Kamnitz, H. Richter, I. Voigt, M. Rettenmayr, U. Ritter, Carbon 90 (2015) 25–33. [48] D. Shee, A. Sayari, Appl. Catal. A Gen. 389 (2010) 155–164. [49] D. Hoang, H. Lieske, Catal. Lett. 27 (1994) 33–42. [50] E. Aneggi, D. Wiater, C. Leitenburg, J. Llorca, A. Trovarelli, ACS Catal. 4 (2014) 172–181. [51] M.V. Grabchenko, G.V. Mamontov, V.I. Zaikovskii, O.V. Vodyankina, Kinet. Katal. 58 (2017) 654–661. [52] A.M. Venezia, G. Pantaleo, A. Longo, G. Di Carlo, M. Pia Casaletto, F.L. Liotta, G. Deganello, J. Phys. Chem. B 109 (2005) 2821–2827. [53] A.C. Sajeevan, V. Sajith, Fuel 183 (2016) 155–163. [54] T.P. Otroshchenko, U. Rodemerck, D. Linke, E.V. Kondratenko, J. Catal. 356 (2017) 197–205. [55] D. Hoang, H. Lieske, Thermochim. Acta 345 (2000) 93–99. [56] K. Pokrovski, K.T. Jung, A.T. Bell, Langmuir. 17 (2001) 4297–4303. [57] S. Therdthianwonga, C. Siangchina, A. Therdthianwong, Fuel Proc. Technol. 89 (2008) 160–168. [58] X. Lu, W. Wang, S. Wei, C. Guo, Y. Shao, M. Zhang, Z. Deng, H. Zhu, W. Guo, RSC Adv. 5 (2015) 97528–97535.
5. Conclusion Cr-containing catalysts supported on γ-Al2O3, ZrO2, CeO2, CexZr(1have been investigated in the ODH-CO2. The nature of the support influences the oxidation state of chromium species, and the activity and stability of the catalyst. CrOx/γ-Al2O3 and CrOx/ZrO2 exhibit the higher activity in oxidative dehydrogenation of ethane, but the reaction pathways are different on them. The main route for CrOx/γ-Al2O3 catalyst is a direct dehydrogenation (DDH) of ethane accompanied by the RWGS, with cracking and coke formation as side reactions. On CrOx/ ZrO2 ethylene is formed by selective oxidative dehydrogenation and the side reactions are non-selective oxidation or/and dry-reforming reactions. The rapid deactivation of the CrOx/ZrO2 at high temperatures is caused by possible sintering of the catalyst and/or formation of inactive chromium species. CrOx/CeO2 is the less active, but the most stable catalyst due to formation of α-Cr2O3 phase that is inactive in DDH. Small contribution of selective ODH on this catalyst is connected with difficulties of surface re-oxidation by CO2. Relatively high stability of CrOx/CeO2 and CrOx/CexZr(1-x)O2 is caused by their low activity for ethylene formation and removal of carbon deposits by CO2 due to basic sites of the supports. The side reactions for CrOx/CeO2 and CrOx/ CexZr(1-x)O2 are similar to CrOx/ZrO2, i.e., non-selective oxidation or/ and dry-reforming reactions. x)O2
Acknowledgments The authors thank Dr. E.D. Fakhrutdinova for UV–vis DRS studies and Dr. M.A. Salaev for English review. This work was supported by Tomsk State University competitiveness improvement program (grant no. 8.2.31.2018, Russia) and project CTQ2015-71823-R of MINECO (Spain). 10
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Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Oxidative dehydrogenation of ethane with CO2 over CrOx catalysts supported on Al2O3, ZrO2, CeO2 and CexZr1-xO2 Tatiana A. Bugrovaa, Valerii V. Dutova, Valeriy A. Svetlichnyia, Vicente Cortés Corberánb, ⁎ Grigory V. Mamontova, a b
Tomsk State University, 36 Lenin Ave., Tomsk, 634050, Russia Institute of Catalysis and Petroleumchemistry, CSIC, 2 Marie Curie Str., Madrid, 28049, Spain
A R T I C LE I N FO
A B S T R A C T
Keywords: Oxidative dehydrogenation Carbon dioxide Ethane Chromium oxides Support nature
The effect of the support nature on the catalytic activity and stability of CrOx-containing catalysts supported on γ-Al2O3, ZrO2, CeO2 and CexZr(1-x)O2 has been investigated in the oxidative dehydrogenation of ethane with CO2 as a mild oxidant (ODH-CO2). The catalysts were prepared by wetness impregnation and characterized as-prepared and after catalytic tests using N2 adsorption, XRD, UV-Vis diffuse reflectance (DR) and Raman spectroscopies, TPR-H2 and TPO-CO2. The catalysts CrOx/γ-Al2O3 and CrOx/ZrO2 exhibited the highest ethylene formation rates, but the reaction followed different pathways on each: on CrOx/γ-Al2O3 catalyst ethylene is formed by a direct dehydrogenation (DDH) of ethane accompanied by RWGS, while on CrOx/ZrO2 ethylene is formed by selective oxidative dehydrogenation. The nature of the support influences the oxidation state of chromium species. The reaction pathway, catalytic activity and stability of the catalyst depend on both chromium state and support properties, in particular, on its acid-base and red-ox properties.
1. Introduction Currently, high demand in light olefins, such as ethylene and propylene, is caused by continuous growth of polymer industry that invokes high research interest in the field of light alkene production. Moreover, ethylene and propylene are important building blocks in organic and polymeric synthesis for production of valuable chemicals (ethylene oxide, polyethylene, propylene oxide, ethylene glycol, acetaldehyde, etc.) [1]. Steam cracking of naphtha and fluidized catalytic cracking (FCC) are traditional ways of olefin production [2,3]. However, these methods have some disadvantages, including high energy consumption and a wide product distribution, low selectivity and yield. The mixture of products requires the use of a complex of separation methods. In this regard, the alternative ways utilizing non-oil-based feedstock attract more attention due to high level of extraction of shale gas, natural gas and depletion of easily retrievable oil deposits. The oxidative coupling of methane (OCM) [4], methanol-to-olefins technology (MTO) [5], and direct (DDH) [6–9] or oxidative (ODH) [10–12] dehydrogenation of alkanes are the main alternative routes for olefin production. Unfortunately, OCM and MTO technologies still require many improvements due to complexity, CO2 emissions, low ethylene yield and selectivity towards ethylene, while all implementations of propylene and ethylene require high purity of raw materials [3,13].
⁎
Another promising way is a direct transformation of alkanes into olefins by catalytic dehydrogenation. Several processes were commercialized to produce isobutylene and propylene by DDH of isobutane and propane (Catofin, Oleflex), respectively, using modified Cr2O3/Al2O3 and Pt–Sn/Al2O3 catalysts [14,15]. The thermodynamic limitations and coke formation are the main unsolved problems of direct alkane dehydrogenation. Thermodynamic limitations increase in the raw of alkanes: C4 < C3 < C2. Thus, DDH of ethane is difficult to implement at a large scale due to low ethylene yield [16]. ODH reaction is a promising way to produce ethylene from ethane. The ODH of ethane in the presence of oxygen is thermodynamically unrestricted and occurs without coke deposition. However, most catalysts presented in the literature (V- and Mo-containing catalysts, composite oxides, etc.) provide low selectivity towards ethylene due to deep oxidation of ethane that limits their application [17]. The ODH of ethane in the presence of CO2 as a mild oxidant (ODH-CO2) has the advantage of higher selectivity than in case of O2-assisted dehydrogenation. Moreover, ecological problems connected with CO2 utilization may be solved using ODH-CO2 process [18]. The generally accepted point of view on the influence of CO2 addition on the ODH process is as follows. The equilibrium of dehydrogenation reaction in the presence of CO2 is shifted due to reverse water–gas shift reaction (RWGS) that leads to increase of ethylene yield [19]:
Corresponding author. E-mail address: [email protected] (G.V. Mamontov).
https://doi.org/10.1016/j.cattod.2018.04.047 Received 18 December 2017; Received in revised form 14 April 2018; Accepted 21 April 2018 0920-5861/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Bugrova, T.A., Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.04.047
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Table 1 Textural, crystalline and reducibility properties of catalysts and supports. Sample
SBET, m2/ga
Vp, cm3/gb
Dp, nmc
Cr, wt.%
Phase compositiond
H2 consumption, μmol/gcate
H2 consumption after ODH-CO2, μmol/gcate
γ-Al2O3 ZrO2 CeO2 CexZr(1-x)O2 CrOx/γ-Al2O3 CrOx/ZrO2 CrOx/CeO2 CrOx/CexZr(1-x)O2
201 47 84 52 154 47 19 20
0.40 0.24 0.23 0.03 0.31 0.23 0.18 0.04
6.3 16.3 9.1 3.1 6.8 16.2 29.7 5.5
– – – – 8.0 2.0 3.5 2.2
γ-Al2O3 m-ZrO2, t-ZrO2 c-CeO2 c-(Ce,Zr)O2, t-ZrO2 γ-Al2O3 m-ZrO2, t-ZrO2 CeO2, α-Cr2O3 c-(Ce,Zr)O2, t-ZrO2
– 335 335 416 957 936 804 1245
n.d.f n.d. n.d. n.d. 557 422 402 704
a b c d e f
Specific surface area calculated by applying the BET method. Total pore volume (BJH-desorption method). Average pore diameter calculated from the BJH method as 4Vp/Sext. From XRD data; c-: cubic, m-: monoclinic, t-: tetragonal. Calculated from TPR-H2 data. Not determined.
Dehydrogenation:CnH2n+2 ↔ CnH2n + H2
(1)
RWGS: CO2 + H2 ↔ CO + H2O
(2)
Overall: CnH2n+2 + CO2 ↔ CnH2n + CO + H2O
(3)
proposed. Also Cr-containing catalysts tend to deactivate under ODHCO2 conditions due to coke formation [29]. Thus, the use of oxide supports for Cr catalysts with active oxygen is preferable for stable operation of the catalyst. Another cause of catalyst deactivation is sintering of chromium species with formation of the inactive crystalline α-Cr2O3 [30]. Thus, oxide supports with high ability to stabilize amorphous CrOx species in the highly dispersed state may prevent the deactivation of catalyst caused by aggregation of CrOx species. ZrO2, CeO2 and mixed ceria–zirconia oxides are the best candidates as supports for ODH catalysts due to their unique red-ox properties. Different metal oxide catalysts (NbOx, V2O5, etc.) [31–33] supported on CeO2 and ZrO2 are active in this process. However, the CrOx species supported on CexZr(1-x)O2 and CeO2 are poorly studied as catalysts in the ODH of ethane. Therefore, the purpose of the present work is to study the influence of the nature of oxide supports on the performance of CrOx-containing catalysts for ODH of ethane with CO2. The red-ox properties of the supports and catalysts as well as the states of Cr are mainly discussed. The alumina–chromia catalysts with inert support (towards red-ox properties) were used as a reference.
Another point of view is based on direct participation of oxygencontaining sites [O] of the surface of metal oxide in alkane dehydrogenation followed by recovery of oxygen vacancy (re-oxidation) by gaseous CO2 (Mars-van Krevelen mechanism) [20]: [O]s + CnH2n+2 → CnH2n + H2O + []s
(4)
[]s + CO2 → CO + [O]s
(5)
where []s is an oxygen vacancy on the surface of metal oxide. In both cases ODH-CO2 provides higher conversion of alkane with remaining high selectivity towards olefin in comparison with non-oxidative DDH under the same operating conditions (temperature, GHSV, alkane concentration, etc.) [21]. The rate of reaction (4) depends on the concentration of oxygen sites that, in its turn, is affected by the rate of re-oxidation (5) of the oxide surface. The latter is connected with the ability of the surface to activate CO2. Thus, variation of the nature of the metal oxide, namely acid-base and red-ox properties, is an efficient approach to control the re-oxidation rate and catalytic activity in ODHCO2. Due to high temperature of the ODH-CO2 process, deactivation of the catalysts by coke formation is unavoidable under operation conditions. According to the literature data, the higher the density of basic sites on the catalyst surface, the higher the stability of the catalysts in the presence of CO2 is achieved [22]. In this case, a strongly adsorbed CO2 may oxidize carbon deposits on the catalyst surface. The catalytic performance modulation by changing of the nature of support and active component may be used to achieve high conversion of ethane, selectivity to ethylene and stability of the catalyst [23]. Among the supported metal oxides (In2O3, Ga2O3, V2O5) studied for this reaction, chromium oxide is a promising candidate due to its high catalytic activity towards dehydrogenation of alkanes [24]. Chromiaalumina catalysts are commonly used in industry for DDH of light paraffins in Catofin and Catadiene processes. Moreover, supported Crcontaining catalysts show high activity in ODH-CO2 of propane and isobutane [25]. However, the performance of existing catalysts is not enough for industrial application of ODH and strongly depends on the chromia content, type of support and operation conditions (CO2 partial pressure, GHSV, etc.). From this point of view, the research and development of new catalysts for ODH-CO2 is an important challenge. It is noteworthy that the active Cr species in ODH are still under discussion. Red-ox scheme of ODH-CO2 that includes direct participation of Cr6+/Cr3+ or Cr5+/Cr3+ [26,27] and Cr3+/Cr2+ [28] pairs was
2. Experimental 2.1. Sample preparation γ-Al2O3, ZrO2, CeO2 and CexZr(1-x)O2 were used as supports. Aluminum, cerium and zirconium oxides were prepared by thermal decomposition of AlO(OH), Ce(NO3)3·6H2O and ZrO(NO3)2·H2O (Across) at 600 °C for 4 h under static air atmosphere. Mixed CexZr(1x)O2 oxide was synthesized by co-precipitation method. The amounts of Ce(NO3)3·6H2O and ZrO(NO3)2·H2O calculated to get molar ratio of Ce:Zr = 1:1 were dissolved in water. Then, 2 M aqueous solution of ammonia was added dropwise to obtain the precipitate. The precipitate was dried at 90 °C overnight and calcined at 600 °C for 4 h. Chromia containing catalysts were prepared by wetness impregnation of γ-Al2O3, ZrO2, CeO2 and CexZr(1-x)O2 with aqueous solution of CrO3. Chromium loading corresponded to 1 theoretical monolayer (5 atoms Cr per nm2 of the support). The calculated content of Cr in wt. % is presented in Table 1. The catalysts obtained were dried at 95 °C overnight than calcined in air at 600 °C for 4 h. The prepared catalysts were denoted as CrOx/γ-Al2O3, CrOx/ZrO2, CrOx/CeO2 and CrOx/ CexZr(1-x)O2. 2.2. Sample characterization The prepared catalysts were characterized by N2 adsorption, X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), Raman 2
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The catalyst sample (0.25 g with a grain size 0.25–0.42 mm) was diluted by SiC (0.42–0.84 mm) in volume ratio 1:2 and put into the reactor between two quartz wool plugs. Pre- and post-catalytic zones in the reactor were filled with SiC grains to reduce the gas-phase reaction extent. The reaction mixture, 5% C2H6-10% CO2 in He, was passed through the reactor at a total flow rate of 200 cm3/min. Gas flows were controlled by Bronkhorst mass flow controllers. Reaction temperature range was from 500 to 700 °C. The reactor effluents were passed through a condenser and then analyzed by an on-line gas chromatograph Varian MicroGC equipped with a Pora Plot Q capillary column and a TCD. The catalytic tests for each sample were conducted successively at temperatures from 500 to 700 °C in increasing steps of 50 °C, kept for 30 min each. After each temperature setting increase, the new temperature became stabilized after around 10 min. Only measurements made in the isothermic portion of each step were considered valid for analysis and discussion.
spectroscopy (RS), temperature-programmed reduction (TPR) and oxidation in CO2 (TPO-CO2), thermogravimetric analysis and differential scanning calorimetry coupled with mass spectrometry (TGA-DSC-MS). The specific surface area (SSA) and pore diameter of the samples were measured by nitrogen sorption at −196 оC using a TriStar II 3020 analyzer (Micromeritics, USA). Prior to experiments, all samples were degassed at 200 °C up to constant mass for 2 h. SSA was determined by the BET method. The pore size distribution was calculated from the desorption branch of the adsorption–desorption isotherm by the Barrett-Joyner-Halenda (BJH) method. X-ray diffraction (XRD) patterns were recorded in the range 2θ from 10 to 90° with a scanning rate of 2°/min on a MiniFlex 600 (Rigaku, Japan) diffractometer using monochromatized Cu-Kα radiation (λ = 1.5418 Å) with a power setting of 40 kV and 15 mA. The phase identification was made according to PCPDFWIN database. UV–vis disffuse reflectance (DR) spectra were recorded using a Cary 100 SCAN spectrophotometer with DRA-CA-30I, Labsphere attachment (Varian, Australia) in the wavelength range of 200–900 nm, with a step of 1 nm, and a spectral resolution up to 0.1 nm. MgO was used as a standard sample. Raman spectra were obtained at ambient conditions using inVia confocal Raman microscope (Renishaw, UK) with a spectral resolution of 2 cm−1. The samples were excited with the 532 nm line and a laser power of 4 mW (5% from initial laser power). The spectra were recorded at an exposure time of 1 s, an accumulation number of 50, and an objective with magnification x20 was used. The red-ox properties of the catalysts were investigated by TPR-H2 using a ChemiSorb 2750 chemisorption analyzer (Micromeritics) equipped with thermal conductivity detector (TCD). The TPR experiments were performed in the temperature range from room temperature (RT) to 650 °C at a heating rate of 10 °C/min using a 10 vol. % H2/ Ar mixture (flow rate of 20 cm3/min). To avoid the effect of water formed, a cold trap with a mixture of liquid nitrogen and isopropanol (ca. −90 °C) was set prior to the detector. Prior to the TPR experiments, the catalysts were pre-oxidized in a TPO mode in an air flow (20 cm3/ min) with a heating rate of 10 °C/min up to 650 °C. TPO-CO2 experiments were performed in a U-shaped quartz reactor using a ChemiSorb 2750 analyzer (Micromeritics, USA) coupled with a quadrupole mass spectrometer (MS) UGA 300 (Stanford Research Systems, USA). Prior to experiment the sample (∼0.2 g) was reduced in 10%H2/Ar flow (20 cm3/min) in a TPR mode up to 650 °C and kept at this temperature for 20 min. After cooling to RT in the 10%H2/Ar mixture, the sample was purged with He flow (27 cm3/min). Then the flow was switched to 10%CO2/He mixture (30 cm3/min), and sample was heated from RT to 950 °C with a rate of 10 °C/min. Concentrations of CO2 (m/z = 44), CO (m/z = 28), H2O (m/z = 18) and O2 (m/ z = 32) were monitored by MS. To remove the trace amounts of oxygen and water from the 10%CO2/He mixture, two traps filled with reduced Cu/SiO2 (maintained at 340 °C) and calcined alumina (maintained at RT) were used, respectively. The TG-DSC-MS analysis of the samples was performed using a STA 449 F1 Jupiter analyzer (Netzsch, Germany) coupled with quadrupole mass spectrometer QMS 403D Aëolos (Netzsch, Germany). The samples (5–10 mg) were placed in Al2O3 crucible and heated from 25 to 900 °C at a heating rate of 10 °C/min in air flow (10 cm3/min Ar + 80 cm3/ min air).
3. Results 3.1. Catalytic properties It is known that dehydrogenation of alkanes in the presence of CO2 is accompanied by a wide variety of side reactions due to high process temperature [12]. The following reactions take place: selective ODH: C2H6 + CO2 ↔ C2H4 + CO + H2O
(6)
DDH: C2H6 ↔ C2H4 + H2
(7)
non-selective reaction: C2H6 + 5 CO2 ↔ 7 CO + 3 H2O
(8)
water–gas shift reaction: CO + H2O ↔ CO2 + H2
(9)
cracking: C2H6 + H2 ↔ 2 CH4
(10)
disproportionation: 2 C2H6 ↔ C3H8 + CH4
(11)
coke formation: C2H4 ↔ 2 C + 2H2
(12)
C2H6 ↔ 2 C + 3H2
(13)
Boudouard reaction: C + CO2 ↔ 2 CO
(14)
dry reforming: C2H6 + 2 CO2 ↔ 4CO + 3H2
(15)
Moreover, gas-phase processes (homogeneous reactions) cannot be excluded, especially above 650 °C [35]. Thermodynamic calculations of ΔGr of these reactions are presented as a function of temperature in Fig. 1. One can see that all side reactions are thermodynamically more favorable especially at high temperatures than the target reactions (ODH and DDH of ethane). Dry reforming and non-selective reactions take place only at high temperatures. Therefore, the selectivity toward ethylene strongly depends on the properties of the active sites of the catalyst. It is known that high acidity of the catalyst surface provokes cracking and coke formation leading to selectivity decrease. Coke formation via ethylene decomposition is more favorable than via ethane decomposition due to higher polymerization ability of the former. This means that a higher formation rate of ethylene (higher activity of catalyst) will lead to more rapid deactivation due to coke deposition. However, oxidation of coke via Boudouard reaction is possible for supports with active oxygen species. The temperature dependencies of ΔGr for DDH and selective ODH reactions are similar due to low ability of CO2 to oxidize H2. Equilibrium of the RWGS shifts to CO formation (K° > 1) only at 1100 K. Therefore, both processes have thermodynamic limitations. High ratio of CO2/C2H6 (2 or above) is required to shift the equilibrium and improve the ethylene yield (Fig. 2). The advantage of the ODH-CO2 of ethane in comparison with the DDH may be connected with the kinetic reasons rather than thermodynamic ones. Ethane dehydrogenation in the presence of surface oxygen formed
2.3. Catalytic test The catalytic activity in the ODH-CO2 of ethane was tested at Microactivity-Reference unit (PID Eng&Tech, Spain). The experiments were carried out in a stainless steel, down flow reactor (305 mm long, 9 mm i.d.) operating at atmospheric pressure [34]. During the experiment the reactor was placed in electrical furnace located inside a hot box at 175 °C. Reaction temperature was controlled by coaxial K-type thermocouple inserted in the center of catalyst bed without thermowell. 3
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catalyst is characterized by similar rates of CO and ethylene formation at 500–550 °C, which means that the reaction occurs via selective ODH. When the temperature increases, the formation rate of CO rises in a higher extent than the one for ethylene due to non-selective reaction and/or dry reforming of ethane. Both CrOx/ZrO2 and CrOx/Al2O3 are characterized by fast deactivation that occurs mainly at 650-700 °C. The CrOx/CeO2 catalyst exhibits the lowest rate of C2H4 formation, and, hence, higher stability among all studied catalysts. The CO formation rate is fourfold that of C2H4, which may be associated with nonselective reaction or dry reforming of ethane. The rate of C2H4 formation is higher for CrOx/CexZr(1-x)O2 in comparison with CrOx/CeO2. The CrOx/CexZr(1-x)O2 is characterized by slightly faster deactivation than CrOx/CeO2 and similar rate of CO formation. The difference between the rates of CO and C2H4 formation gradually increases with the temperature increase for CrOx/CexZr(1-x)O2 and CrOx/CeO2, which means that both catalysts intensively accelerate the non-selective reactions of ethane transformation. One of the possible reasons for low rate of catalyst deactivation is the oxidation of carbon deposits on the catalyst surface. Due to the big differences in SSA of the catalysts, the surface-specific rates of ethylene and CO formation (i.e., per m2 of catalyst SSA) were also calculated and are plotted in Fig. 3(c,d). Indeed, the ratios between the formation rates of CO and C2H4 are the same as those calculated per catalyst mass. However, in this case the CrOx/Al2O3 and CrOx/CeO2 catalysts exhibit the lowest surface-specific rate of C2H4 formation at 500–650 °C. This means that the highest activity of CrOx/Al2O3 is mainly caused by the higher SSA of this catalyst (Table 1). The CrOx/ ZrO2 catalyst showed the higher ethylene formation rate. The rate of ethylene production at 500–650 °C for CrOx/CexZr(1-x)O2 catalysts is slightly lower than for CrOx/ZrO2. The surface-specific rates of CO formation for CrOx/ZrO2, CrOx/CeO2, and CrOx/CexZr(1-x)O2 at 500–650 °C are similar, but differed significantly at 700 °C. The evolution of ethane conversion and C2H4 selectivity with reaction temperature and time is shown in Fig. 4. One can see that CrOx/ CeO2 and CrOx/CexZr(1-x)O2 possess the lowest ethane conversion (2–10%) and selectivity towards ethylene at 550–700 °C. This selectivity noticeably decreases from 95 to 45% as the reaction temperature increases. These catalysts are characterized by the highest ratios of CO-to-ethylene formation rates, which indicate non-selective and/or dry reforming side reactions. The highest selectivity was observed for CrOx/Al2O3 and CrOx/ZrO2 catalysts, for which it slowly decreased to 90 and 80%, respectively, during the reaction time. CrOx/ Al2O3 was the most selective catalyst due to the absence of non-selective oxidation and/or dry reforming reactions. The main side reactions for this sample are cracking and coke deposition. The highest conversion of ethane (5–21%) was observed for CrOx/ZrO2 at 550–650 °C. Rapid deactivation of catalyst at temperatures above 600 °C can be associated with its active oxide sintering. Thus, the deactivation of all catalysts is mainly connected with coke deposition and possibly with agglomeration of active component. The rate of deactivation gradually increased with the temperature and depended on the ethylene formation rate. High activity of CrOx/ZrO2 and CrOx/Al2O3 catalysts toward ethylene production leads to their rapid deactivation.
Fig. 1. Thermodinamic calculations of ΔGr of the possible reactions as a function of temperature.
Fig. 2. Dependence of equilibrium conversion of ethane on CO2/C2H6 ratio.
through dissociation of CO2 probably occurs more easily (with lower activation energy). The specific rates of ethylene and CO formation per catalyst mass unit for all catalysts during the ODH-CO2 are presented in Fig. 3(a,b). It can be seen that the rate of C2H4 formation was higher than the one of CO for CrOx/Al2O3 catalyst, though the difference was small, even at 700 °C. Thus, the process consists of two reactions: direct dehydrogenation and RWGS. The equilibrium of the latter reaction is shifted to CO and H2O and is accompanied by a temperature increasing, which leads to reduction in the difference between the C2H4 and CO formation rates. The rate of ethylene production at 500–650 °C for CrOx/ZrO2 catalysts is slightly higher than for CrOx/Al2O3. However, the rates of CO formation for these catalysts differ significantly. The CrOx/ZrO2
3.2. Textural properties and phase composition of the samples The porous structure of the supports and catalysts was studied by low-temperature N2 sorption. The presence of a hysteresis loop on the isotherms at relative pressures from 0.45 to 1.0 for all supports (not shown) indicates formation of mesoporous structure. According to pore size distributions (Fig. 5a), the porous structure of the supports consisted of pores with sizes 2–20, 3–50, 2–20 and 2–6 nm for γ-Al2O3, ZrO2, CeO2 and CexZr(1-x)O2, respectively. The values of SSA, average pore size and total pore volume of supports and catalysts are presented in Table 1. SSA of γ-Al2O3 is much higher than those of ZrO2, CeO2 and 4
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Fig. 3. Specific rates of ethylene (a) and CO (b) formation and Area-specific rates of ethylene (c) and CO (d) formation in heterogeneous ODH-CO2 over Crcontaining catalysts.
The most dramatic changes in the porous structure are observed for CrOx/CeO2 and CrOx/CexZr(1-x)O2 catalysts: in comparison with CeO2 and CexZr(1-x)O2 supports (Fig. 5), their SSAs decrease by 77 and 62%, respectively. Pore size distributions for catalysts CrOx/CeO2 and CrOx/ CexZr(1-x)O2 in the range of higher pore diameter was shifted from average pore size 9.1 and 3.1 nm to 29.7 and 5.5 nm, respectively (Table 1). The described changes in porous structure may be caused by the filling of the smaller pores of ceria with chromium oxide during the impregnation with chromic acid solution or possible sintering after the second calcination. The phase composition of supports and catalysts is also presented in Table 1. The absence of reflections of Cr-containing phases indicates the stabilization of CrOx in an amorphous state on the surface of Al2O3, ZrO2 and CexZr(1-x)Ox supports. Reflections of α-Cr2O3 phase (2θ = 24.50, 33.61 and 36.20°) are observed only for CrOx/CeO2 catalysts. CeO2 particle sizes grow from 5.9 nm (reflection of CeO2(111) at 28.6°) for CeO2 support to 18.9 nm for CrOx/CeO2 catalyst. Thus, sintering of ceria together with agglomeration of chromia in α-Cr2O3 phase is observed for CrOx/CeO2 catalyst.
Fig. 4. ODH-CO2 of ethane on Cr-containinig catalysts: conversion of ethane and selectivity to ethylene as a function of reaction temperature and time.
3.3. The state of supports and active component CexZr(1-x)O2. Incorporation of chromium oxide into chromia-containing catalysts caused an important decrease of the surface area and pore volume (Table 1) excepting for CrOx/ZrO2 catalyst which maintained the properties of ZrO2 support. This may be explained by stabilization of mononuclear chromia species on the surface of zirconia and additional stabilization of zirconia towards sintering by chromia at high temperatures [36].
Fig. 6 shows the UV–vis DR and Raman spectra of supports and catalysts. UV–vis spectra of all catalysts (Fig. 6(a)) two main absorption bands at ∼275 and ∼376 nm attributed to CrVI species [37]. The absorption at 200–500 nm observed for CeO2 and CexZr(1-x)O2 supports can be attributed to the presence of cubic CeO2 and mixed Ce-Zr oxide [38]. The two bands at ∼460 and ∼582 nm observed for CrOx/CeO2 catalyst indicate the presence of CrIII species in the octahedral 5
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Fig. 5. Pore size distributions for supports (a) and catalysts (b).
symmetry (α-Cr2O3) [37,39]. The presence of α-Cr2O3 in CrOx/CeO2 catalyst is also confirmed by XRD. According to Raman spectra (Fig.6(b)), the CrOx/γ-Al2O3 is only characterized by those bands originating from vibrations of chromium oxides. The spectrum contains a weak band at ∼630 cm−1, which probably corresponds to υs(CreOeCr). Two bands and shoulders observed at 865, 1003 cm−1 and at ∼787, ∼920 cm−1 are assigned to the υs(OeCreO), υs(Cr]O), υas(CreOeCr) and υs of CrO2 units that terminate the surface polymeric species, respectively [40,41]. All these bands and shoulders can be attributed to poly-, mononuclear CrVI species. In the case of ZrO2 and CrOx/ZrO2, the bands at 145, 178, 219, 266, 330, 380, 476, 537, 556, 615 and 634 cm−1 correspond to tetragonal and monoclinic zirconia [42]. These bands are slightly shifted for the
CrOx/ZrO2 catalyst. The bands at 780, 817, 854, 890, 1009 and 1028 cm−1 attributed to CrVI species are found in the range from 650 to 1400 cm−1. The most intense Raman band at 854 cm−1 may be assigned to ‘isolated’ tetrahedral surface-mononuclear (CrVI) species distorted by the interaction with the support surface [43], while the peaks at 780, 817 and 890 cm−1 as shoulders to the main band probably correspond to CreO vibrations in higher polymerization degree species (polynuclear CrVI). Two bands at 1009 and 1028 cm−1 are attributed to mono- and dioxo Cr]O vibrations [44]. The Raman spectra of CeO2 contain strong F2g band of the fluorite phase at 460 cm−1 and weak bands at 260, 600 and 1170 cm−1 related to the second-order transverse acoustic mode (2 TA), defect-induced mode (D) and a second overtone band (2LO), respectively [45]. The spectra of the CrOx/CeO2 catalyst differ significantly from those of
Fig. 6. UV–vis DRS (a) and Raman (b) spectra for catalysts and supports. 6
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Fig. 8. TPR-H2 profiles of supports, as-prepared catalysts and catalysts after ODH-CO2 (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 7. Raman spectra of catalysts before (as prepared) and after ODH-CO2 (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Cr2O3 state and formation of mixed chromia–alumina phases [30]. Both processes lead to irreversible catalyst deactivation. The H2 consumption peak at temperatures above 450 °C for the ZrO2 support is attributed to partial reduction of Zr4+ to Zr3+ in the structure of zirconia [49]. The TPR peak above 400 °C for CeO2 support is associated with the reduction of the surface Ce4+ to Ce3+ [50–52]. Two peaks at temperatures above 350 °C for the CexZr(1-x)O2 can be attributed to reduction of Zr4+ and Ce4+. The reducibility of mixed ceria–zirconia oxides is known to be higher of than those of individual CeO2 and ZrO2 [53]. These results are in agreement with increased number of defects of CexZr(1-x)O2 observed during the Raman studies. The TPR profile for the CrOx/ZrO2 catalysts is characterized by an intensive peak with maximum at ∼340 °C, which can be attributed to reduction of Crn+ from the high valence state to Cr2+/3+ [54]. The shift of this peak in comparison with the CrOx/Al2O3 catalyst may indicate the stabilization of isolated CrV and/or CrVI species on the ZrO2 surface [23,37] that is confirmed by the Raman spectroscopy. The H2 consumption at 400–600 °C for the CrOx/ZrO2 catalyst may be attributed to reduction of CrOx species strongly bonded with the support or reduction of zirconia catalyzed by chromia [25,55]. The decrease of this peak intensity is observed for the catalysts after the catalytic process, while the peak area at 350 °C did not change significantly. This indicates higher stability of red-ox chromia species on the zirconia surface (in comparison with the CrOx/Al2O3 catalyst). The same results are observed for CrOx/CeO2 and CrOx/CexZr(1-x)O2 catalysts. The difference between the maximum of peak reduction of CrV/VI may be attributed to both, different interaction of chromia with the support and different nuclearity of CrV/VI species. TPO-CO2 experiments were carried out to investigate the ability of catalyst for re-oxidation by CO2 (Figs. 9 and 10). One can see that no reoxidation of CrOx/Al2O3 catalyst by CO2 was detected. This phenomenon may be associated with poor basicity of CrOx/Al2O3 catalyst. According to literature data [18], the active sites for CO2 activation
CrOx/γ-Al2O3 and CrOx/ZrO2. Besides the bands of CeO2, weak bands at 537, 837 and 1018 cm−1 are observed for the CrOx/CeO2 catalyst. The bands at 837 and 1018 cm−1 can be attributed to CreO and Cr]O vibrations of CrVI species, while the bands at 537 and 600 cm−1 are characteristic for the crystalline Cr2O3 nanoparticles (CrIII octeO bond) [39,44]. These results are in agreement with the DRS and XRD data. The bands for CexZr(1-x)O2 support are the same as those for CeO2 support that confirms formation of mixed CeeZr oxide with cubic phase. The bands of monoclinic and tetragonal ZrO2 are not observed. In comparison with spectrum of pure CeO2 the increased intensity of the bands at 230 and 615 cm−1 attributed to oxygen vacancies [46] indicates the growth of number of defects of the mixed oxide. The bands at 834, 863, 1008 and 1023 cm−1 can be attributed to poly(mono-) nuclear CrVI species as described above. It is noteworthy that the bands of vibrations of CrIII species (537 cm−1) are not observed. After ODH-CO2 (Fig. 7), two strong bands at 1598 and 1343 cm−1 assigned to the structured carbon [47] are observed for CrOx/γ-Al2O3, CrOx/ZrO2 and CrOx/CexZr(1-x)O2 catalysts. These bands are also present in the spectra of the CrOx/CeO2, but their intensity is low. The vibrations of chromium species are not found for CrOx/γ-Al2O3 and CrOx/ZrO2 due to carbon deposits on the catalyst surface and are weak for the CrOx/CexZr(1-x)O2 and CrOx/CeO2 possibly due to reduction of CrVI species. The TPR profiles for supports, as-prepared catalysts and catalysts after catalytic test are shown in Fig. 8. The amount of H2 consumed is summarized in Table 1. The intensive H2 consumption peak with a maximum at 379 °C observed for the as-prepared CrOx/Al2O3 catalyst is attributed to reduction of CrV/VI to CrII/III species [48]. The decrease of intensity of this peak and a shift in a high-temperature region are observed for the used catalyst. This indicates the decreased amount of redox chromia (i.e. CrOx that can be reversibly oxidized-reduced (CrV/ VI ↔CrII/III)) due to its agglomeration, with stabilization in inactive α7
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Fig. 9. TPO-CO2 profiles of supports CeO2 and CexZr(1-x)O2: Evolution of mass spectra of CO (m/z = 28), CO2 (m/z = 44) and water (m/z = 18) emission (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
CrOx/CexZr(1-x)O2 are responsible for CO2 activation. ZrO2 possess not only strong acid sites (Zr4+), but also strong Lewis basic sites (O2−) and Bronsted basic sites (terminal OH-groups) [56]. According to TPO-CO2 results, small peak of CO2 consumption is observed at 760 °C, while CO emission starts at 800 °C.
should possess electron donor properties (basic properties). After electron acceptance, a linear CO2 molecule transforms into bended charged active species CO2‾ on the oxide surface and can dissociate with filling of oxygen vacancy and CO formation. Similarly the absence of re-oxidation was observed for CrOx/CeO2, though the re-oxidation of pure CeO2 took place (Fig. 9). This means that addition of chromia to CeO2 leads to a significant change of electronic and acid-base properties of cerium oxide. For CrOx/CexZr(1-x)O2 catalyst, a peak of CO emission at 535 °C together with two peaks of CO2 desorption at 150 and 450 °C were observed. One may conclude that basic sites on the surface of
3.4. Investigation of coke deposits on the catalyst surface after ODH-CO2 The TG-DSC data for the catalysts after ODH-CO2 with mass spectrum of CO2 and H2O emission are shown in Fig. 11. The first step of
Fig. 10. TPO-CO2 profiles of Cr-containing catalysts: Evolution of mass spectra of CO (m/z = 28), CO2 (m/z = 44) and water (m/z = 18) emission (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 8
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Fig. 11. TG-DSC data for CrOx/γ-Al2O3 (a), CrOx/ZrO2 (b), CrOx/CeO2 (c), CrOx/CexZr(1-x)O2 (d) catalysts after ODH-CO2 with mass spectrum of CO2 and H2O emission.
weight loss in the low-temperature region (from RT to 150–200 °C) is connected with desorption of the adsorbed water. The second step, accompanied by exothermic effect and CO2 emission, was observed in the temperature range of ∼150–300 °C for CrOx/CeO2 and ∼200–400 °C for CrOx/γ-Al2O3, CrOx/CexZr(1-x)O2 and CrOx/ZrO2 catalysts. This indicates the combustion of carbon deposits on the catalyst surface. The difference in temperature maxima of the DSC-signal and CO2 emission indicates differences in the nature of the carbon deposits and/or in the ability of the catalyst surface toward carbon oxidation. The supports with active surface oxygen (CeO2, CexZr(1-x)O2, ZrO2) oxidize carbon deposits at lower temperatures. In addition, an increase of the sample mass was observed in the range of 350–500 °C for CrOx/CeO2 and CrOx/CexZr(1-x)O2 due to reoxidation of the catalysts by oxygen. At the same time, the mass loss at the second step differs greatly for CrOx/γ-Al2O3 and for CrOx/ZrO2, CrOx/CeO2, CrOx/CexZr(1-x)O2 catalysts (7.46 and 0.36, 0.39, 0.8%, respectively). Accumulation of large amounts of coke on the surface of CrOx/γ-Al2O3 is connected with high rate of ethylene formation and low reaction ability of surface oxygen of alumina. Strong acid sites are known to facilitate coke formation on the alumina surface. The small amount of carbon deposits on the surface of CrOx/ZrO2 (0.36% wt.) may be associated with the removal of coke by gaseous CO2 during the ODH process. It was shown [57] that the using of ZrO2 as a modifier for Ni/Al2O3 catalyst significantly improved its coke resistance in dry reforming of methane. Carbon deposits may be oxidized by CO2 at the Ni/ ZrO2 interface. The CrOx/CeO2 and CrOx/CexZr(1-x)O2 catalysts are characterized by low amounts of coke. One of the possible reasons is a low rate of ethylene formation. Another reason is connected with carbon removal by CO2 during the ODH as in the case of ZrO2 support. Moreover, DFT calculations show that CeO2 with high concentration of oxygen vacancies on the surface may be re-oxidized during the CO2
dissociation [58]. So oxygen mobility in CeO2 and CexZr(1-x)O2 network may favor the removal of coke precursor on their surfaces. 4. Discussion Our results show that the state of chromium depends on the nature of support used. The reaction pathway and catalytic activity depend on both chromium state and support properties, in particular, on acid-base and red-ox properties. Thus, CrOx/γ-Al2O3 catalyst is characterized by formation of ethylene during the DDH accompanied by the RWGS. This is a ‘classical’ catalyst for the dehydrogenation processes. The CrOx/γAl2O3 catalyst contains amorphous chromium oxide in both CrV/VI and CrII/III states. Chromium red-ox cycle CrV/VI ↔ CrII/III on the catalyst surface triggers DDH process by activation of C2H6 molecule. According to our results, chromia on the surface of γ-Al2O3 is not re-oxidized by gaseous CO2 as confirmed by the TPO-CO2 data (Fig. 10). This limits the possibility of CO2 involvement in the reactions (ODH, non-selective reactions). High surface acidity triggers an intensive coke deposition on the surface and deactivation of the catalyst, while the absence of strong CO2 adsorption leads to impossibility of carbon removal. According to DSC-TG results, the amount of carbon after ODH-CO2 was 7.46% wt, one order of magnitude higher than on the other, non-acidic support catalysts. As a consequence, the rate of deactivation of CrOx/γ-Al2O3 is faster than for other catalysts (Figs. 3, 4). Both selective ODH and DDH are the possible reaction pathways over CrOx/ZrO2 catalyst. In this catalyst, chromium is stabilized mainly as poly-, mononuclear Cr V/VI species that provides the increased activity. The higher amount of CrVI in CrOx/ZrO2 sample (∼66% of total Cr content) in comparison with the one in CrOx/γ-Al2O3 (∼35% of total Cr content) may be caused by a better distribution of chromium on the surface of ZrO2 due to stronger interaction with the support. The ability 9
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References
of ZrO2 support to activate CO2 over basic sites improves the activity in the ODH process. Besides, it is possible to conclude that side reactions over CrOx/ZrO2 catalyst comprise non-selective oxidation and/or dry reforming reactions, since the ratio between the CO and C2H4 formation rates is 1.5–2. According to TG-DSC results, low amount of coke (0.36% wt.) remains after the catalytic experiment, which indicates the possible oxidation of deposited coke on the catalyst surface by CO2 (Boudouard reaction). The relatively low activity of CrOx/CeO2 catalyst is connected with the formation of α-Cr2O3 phase (low activity in DDH) and a small amount of reducible Cr V/VI. The re-oxidation ability of CeO2 support is observed by TPO-CO2. This, together with the high oxygen mobility in the ceria nertwork, makes non-selective oxidation and/or dry reforming reactions (ratio CO/C2H4 is up to 5) the side reactions characteristic for this catalyst. The low amount of coke observed (0.39% wt.) is caused by both low dehydrogenation activity of CrOx/CeO2 catalyst and Boudouard reaction. Unlike CrOx/CeO2 catalyst, the catalyst supported on CexZr(1-x)O2 contains chromium mainly in a high valence state (Cr V/VI) according to TPR-H2 and Raman results, and no α-Cr2O3 phase is detected. And, in parallel, the activity of CrOx/CexZr(1-x)O2 catalyst is higher than the one of CrOx/CeO2 (Fig.3(c)). Ethylene formation takes place by selective ODH-CO2. CexZr(1-x)O2 is more reducible than ZrO2. The side reactions over CrOx/CexZr(1-x)O2 catalyst are the same as for CrOx/ZrO2 and CrOx/CeO2, though the ratio between the CO and C2H4 formation rates amounts to 3 and is lower than the one for CrOx/CeO2. Thus, the increase of CrV/VI species concentration leads to increase in catalyst activity in ethane dehydrogenation. The possibility of CO2 activation on the support leads to increased activity in ODH for CrOx/ ZrO2 and CrOx/CexZr(1-x)O2 catalysts. CeO2 support provides activity of catalysts in non-selective oxidation. Thus, the control of red-ox properties of the support to provide stabilization of CrV/VI species on its surface is a key factor determining the activity of chromia catalysts in oxidative dehydrogenation of ethane with CO2 as a mild oxidant.
[1] L. Kniel, O. Winter, K. Stork, Ethylene, keystone to the petrochemical industry, M. Dekker, New York, 1980. [2] Z. Nawaz, Rev.Chem. Eng. 31 (2015) 413–436. [3] I. Amghizar, L.A. Vandewalle, K.M. Van Geem, G.B. Marin, Eng. Lond. 3 (2017) 171–178. [4] W. Shen, Acta Phys. Chim. Sin. 33 (2017) 2321–2322. [5] M. Sedighi, M. Ghasemi, A. Jahangiri, Korean J. Chem. Eng. 34 (2017) 997–1003. [6] T.P. Otroshchenko, U. Rodemerck, D. Linke, E.V. Kondratenko, J. Catal. 356 (2017) 197–205. [7] H. Lee, W.-I. Kim, K.-D. Jung, H.L. Koh, Korean J. Chem. Eng. 34 (2017) 1337–1345. [8] T.P. Otroshchenko, V.A. Kondratenko, U. Rodemerck, D. Linke, E.V. Kondratenko, Catal. Sci. Technol. 7 (2017) 4499–4510. [9] T.A. Bugrova, N.N. Litvyakova, G.V. Mamontov, Kinet. Katal. 56 (2015) 758–763. [10] F. Rahmani, M. Haghighi, S. Mahboob, Ultrason. Sonochem. 33 (2016) 150–163. [11] S. Al-Ghamdi, M. Volpe, M.M. Hossain, H. De Lasa, Appl. Catal. A Gen. 450 (2013) 120–130. [12] M.A. Atanga, F. Rezaei, A. Jawad, M. Fitch, A.A. Rownaghi, Appl. Catal. B Environ. 220 (2018) 429–445. [13] W. Liang, S. Sarsani, D. West, A. Mamedov, I. Lengyel, H. Perez, J. Lowrey, Catal. Today 299 (2018) 60–66. [14] H.N. Pham, J.J.H.B. Sattler, B.M. Weckhuysen, A.K. Datye, ACS Catal. 6 (2016) 2257–2264. [15] V.Z. Fridman, Appl. Catal. A Gen. 382 (2010) 139–147. [16] M. Bender, Chem. Biochem. Eng. Rev. 1 (2014) 136–147. [17] C.A. Gartner, A.C. van Veen, J.A. Lercher, ChemCatChem 5 (2013) 3196–3217. [18] D. Mukherje, S.-E. Park, B.M. Reddy, J. CO2 Util. 16 (2016) 301–312. [19] P. Michorczyk, P. Kuśtrowski, A. Kolak, M. Zimowska, Catal. Commun. 35 (2013) 95–100. [20] R. Yuan, Y. Li, H. Yan, H. Wang, J. Song, Z. Zhang, W. Fan, J. Chen, Z. Liu, Z. Liu, Z. Hao, Chin. J. Catal. 35 (2014) 1329–1336. [21] F. Rahmani, M. Haghighi, M. Amini, J. Ind. Eng. Chem. 31 (2015) 142–155. [22] H. Tsuneoka, K. Teramura, T. Shishido, T. Tanaka, J. Phys. Chem. 114 (2010) 8892–8898. [23] F. Rahmani, M. Haghighi, B. Mohammadkhani, Microporous Mesoporous Mater. 242 (2017) 34–49. [24] J.J.H.B. Sattler, J. Ruiz-Martinez, E. Santillan-Jimenez, B.M. Weckhuysen, Chem. Rev. 114 (2014) 10613–10653. [25] G. Sun, Q. Huang, H. Li, H. Liu, Z. Zhang, X. Wang, Q. Wang, J. Wang, Chin. J. Catal. 32 (2001) 1424–1429. [26] N. Mimura, I. Takahara, M. Inaba, M. Okamoto, K. Murata, Catal. Commun. 3 (2002) 257–262. [27] X. Ge, M. Zhu, J. Shen, React. Kinet. Catal. Lett. 77 (2002) 103–108. [28] Y. Ohishi, T. Kawabata, T. Shishido, K. Takaki, Q. Zhang, Y. Wang, K. Takehira, J. Mol. Catal. A Chem. 230 (2005) 49–58. [29] P. Michorczyk, P. Pietrzyk, J. Ogonowski, Microporous Mesoporous Mater. 161 (2012) 56–66. [30] V.Z. Fridman, R. Xing, Ind. Eng. Chem. Res. 56 (2017) 7937–7947. [31] M.V. Martinez-Huerta, G. Deo, J.L.G. Fierro, M.A. Banares, J. Phys. Chem. 111 (2007) 18708–18714. [32] R. You, X. Zhang, L. Luo, Y. Pan, H. Pan, J. Yang, L. Wu, X. Zheng, Y. Jin, W. Huang, J. Catal. 348 (2017) 189–199. [33] A. Talati, M. Haghighi, F. Rahmani, RSC Adv. 6 (2016) 44195–44204. [34] M. Guío, J. Prieto, V.Cortes Corberán, Catal. Today 112 (2006) 148–152. [35] R.X. Valenzuela, G. Bueno, V. Cortés Corberán, Y. Xu, C. Chen, Catal. Today 61 (2000) 43–48. [36] H.R. Mahmoud, J. Mol. Catal. A Chem. 392 (2014) 216–222. [37] S. De Rossi, M.P. Casaletto, G. Ferraris, A. Cimino, G. Minelli, Appl. Catal. A Gen. 167 (1998) 257–270. [38] A. Bensalem, F. Bozon-Verduraz, M. Delamar, G. Bugli, Appl. Catal. A Gen. 121 (1995) 81–93. [39] G.E. Bekmukhamedov, A.N. Mukhamed’yarova, S.R. Egorova, A.A. Lamberov, Catalysts 6 (2016) art. no. 162. [40] T.J. Dines, S. Inglis, Phys. Chem. Chem. Phys. 5 (2003) 1320–1328. [41] D.S. Kim, I.E. Wachs, J. Catal. 142 (1993) 166–171. [42] F. Adar, Spectrosc. (Santa Monica) 29 (9) (2014). [43] V.Z. Fridman, R. Xing, M. Severance, Appl. Catal. A. Gen. 523 (2016) 39–53. [44] E.L. Lee, I.E. Wachs, J. Phys. Chem. 111 (2007) 14410–14425. [45] Z. Wu, M. Li, J. Howe, H.M. Meyer III, S.H. Overbury, Langmuir 26 (2010) 16595–16606. [46] B.M. Reddy, G.K. Reddy, L. Katta, J. Mol. Catal. A Chem. 319 (2010) 52–57. [47] A. Simon, M. Seyring, S. Kamnitz, H. Richter, I. Voigt, M. Rettenmayr, U. Ritter, Carbon 90 (2015) 25–33. [48] D. Shee, A. Sayari, Appl. Catal. A Gen. 389 (2010) 155–164. [49] D. Hoang, H. Lieske, Catal. Lett. 27 (1994) 33–42. [50] E. Aneggi, D. Wiater, C. Leitenburg, J. Llorca, A. Trovarelli, ACS Catal. 4 (2014) 172–181. [51] M.V. Grabchenko, G.V. Mamontov, V.I. Zaikovskii, O.V. Vodyankina, Kinet. Katal. 58 (2017) 654–661. [52] A.M. Venezia, G. Pantaleo, A. Longo, G. Di Carlo, M. Pia Casaletto, F.L. Liotta, G. Deganello, J. Phys. Chem. B 109 (2005) 2821–2827. [53] A.C. Sajeevan, V. Sajith, Fuel 183 (2016) 155–163. [54] T.P. Otroshchenko, U. Rodemerck, D. Linke, E.V. Kondratenko, J. Catal. 356 (2017) 197–205. [55] D. Hoang, H. Lieske, Thermochim. Acta 345 (2000) 93–99. [56] K. Pokrovski, K.T. Jung, A.T. Bell, Langmuir. 17 (2001) 4297–4303. [57] S. Therdthianwonga, C. Siangchina, A. Therdthianwong, Fuel Proc. Technol. 89 (2008) 160–168. [58] X. Lu, W. Wang, S. Wei, C. Guo, Y. Shao, M. Zhang, Z. Deng, H. Zhu, W. Guo, RSC Adv. 5 (2015) 97528–97535.
5. Conclusion Cr-containing catalysts supported on γ-Al2O3, ZrO2, CeO2, CexZr(1have been investigated in the ODH-CO2. The nature of the support influences the oxidation state of chromium species, and the activity and stability of the catalyst. CrOx/γ-Al2O3 and CrOx/ZrO2 exhibit the higher activity in oxidative dehydrogenation of ethane, but the reaction pathways are different on them. The main route for CrOx/γ-Al2O3 catalyst is a direct dehydrogenation (DDH) of ethane accompanied by the RWGS, with cracking and coke formation as side reactions. On CrOx/ ZrO2 ethylene is formed by selective oxidative dehydrogenation and the side reactions are non-selective oxidation or/and dry-reforming reactions. The rapid deactivation of the CrOx/ZrO2 at high temperatures is caused by possible sintering of the catalyst and/or formation of inactive chromium species. CrOx/CeO2 is the less active, but the most stable catalyst due to formation of α-Cr2O3 phase that is inactive in DDH. Small contribution of selective ODH on this catalyst is connected with difficulties of surface re-oxidation by CO2. Relatively high stability of CrOx/CeO2 and CrOx/CexZr(1-x)O2 is caused by their low activity for ethylene formation and removal of carbon deposits by CO2 due to basic sites of the supports. The side reactions for CrOx/CeO2 and CrOx/ CexZr(1-x)O2 are similar to CrOx/ZrO2, i.e., non-selective oxidation or/ and dry-reforming reactions. x)O2
Acknowledgments The authors thank Dr. E.D. Fakhrutdinova for UV–vis DRS studies and Dr. M.A. Salaev for English review. This work was supported by Tomsk State University competitiveness improvement program (grant no. 8.2.31.2018, Russia) and project CTQ2015-71823-R of MINECO (Spain). 10