Synergetic Impact of Secondary Metal Oxides of Cr-M / MCM41 Catalyst Nanoparticles for Ethane Oxidative Dehydrogenation Using Carbon Dioxide

: Oxidative dehydrogenation of alkanes to alkenes by a mild oxidant such as carbon dioxide is an active area of research. A series of MCM41-supported bimetallic oxide catalysts containing chromium oxide in addition to metal oxides (Ce, Co, Zn, V, Nb, and Mo) has been prepared. The binary catalysts have Cr metal oxide incorporated into MCM41 structure while the other oxides are either incorporated with Cr or impregnated on the MCM41 surface. The synthesized catalysts were characterized by X-ray di ﬀ raction (XRD), transmission electron microscopy (TEM), N 2 sorption, scanning electron microscopy (SEM), hydrogen temperature programmed reduction (H 2 -TPR), and Di ﬀ use reﬂectance UV–vis spectroscopy (DRS). The catalytic activity of Cr(4)-M(4) / MCM-41 catalysts in the dehydrogenation of ethane with CO 2 was investigated. The textural properties of the synthesized samples showed that the addition of the bimetallic oxides did not disturb the mesoporous structure of MCM41 and the prepared catalysts exhibited a high BET surface area; however, the lowest surface area was recorded for Cr(4)-Mo(4) / MCM41 catalyst at 701 m 2 / g. Among the prepared catalysts, H 2 -TPR proﬁle of Cr(4)-Ce(4) / MCM41 revealed the increase in the concentration of Cr 6 + species which interacted with the framework of siliceous support. On the other hand, H 2 -TPR proﬁles of Cr(4)-Co(4) / MCM41 showed wide reduction peaks centered at 400 ◦ C which is ascribed to reduction of Cr 6 + to Cr 3 + species and Co 3 O 4 to metallic Co. At the same time, Cr(4)-Mo(4) / MCM41 and Cr(4)-V(4) / MCM41 exhibited higher temperature reduction peaks, indicating these two catalysts require higher activation temperatures. The synergy between the Cr with Zn or Nb metals reduced the concentration of Cr 6 + species which is reﬂected in their catalytic performance. Cr(4)-Ce(4) / MCM41 recorded the highest catalytic activity toward ethylene production where the ethane conversion and ethylene yield were 37.9% and 35.1%, respectively. In the present work, the synergetic impact of a secondary metal oxide (M) of Cr-M / MCM41 catalyst system for ethane oxidative dehydrogenation using carbon dioxide was investigated. The e ﬀ ect of the secondary metal oxide (Co, Ce, Zn, Nb, V, and Mo) on the textural, physical, and chemical properties of Cr-M / MCM41 catalysts were evaluated. The main aim of this study is to assess the e ﬀ ect of the secondary metal on the activity behavior of the catalysts for the ODH of ethane. Cr-Zn / MCM41 and Cr-Nb / MCM41 exhibited the highest selectivity to ethylene, their activity much lower than Cr-Ce / MCM41 and Cr-Co / MCM41 catalysts. The results suggested that Cr-Ce / MCM-41 and Cr-Co / MCM-41 possessed promising potentials as catalysts for converting ethane to ethylene; however further studies are still required to optimize their loading content to enhance the catalytic performances.


Introduction
Ethylene, a crucial material in the petrochemical industry, is used in the production of versatile intermediate and final products. Commercially, ethylene is produced by catalytic cracking and steam cracking of hydrocarbon feedstocks like ethane and naphtha [1,2].
Crystals 2020, 10 The cracking processes, Equation (1), have several drawbacks, including high energy requirements because of its endothermicity, severe coke formation, thermodynamic limitation, and inefficient control of ethylene selectivity [3][4][5]. Oxidative dehydrogenation of ethane by oxygen, Equation (2), has been introduced as a good alternative to the thermal cracking process because coke formation is minimized because of the presence of oxygen [6,7].
However, over-oxidation of ethane and ethylene to carbon oxides and the probable formation of hot spots are the major issues of this process [8,9]. On the other hand, utilization of CO 2 as a mild oxidant in ethane dehydrogenation, Equation (3), has received enormous interest in recent years. It has been established that using CO 2 improves the yield and selectivity of ethylene as well as maintains the catalytic activity by reducing cock formation [10,11].
Bimetallic and multi-metallic catalysts have gained much attention in recent years [50][51][52]. Proper synthesis and characterization of uniformly dispersed supported bi-metallics could provide a synergistic effect that usually enhances the catalytic activity and/or selectivity toward one of the products [52][53][54]. Myint et al. [40] compared the catalytic activity of CoPt/CeO 2 , CoMo/CeO 2 , NiMo/CeO 2 , and FeNi/CeO 2 bimetallic catalysts for the reaction of ethane with CO 2 . They found that the catalytic stability was enhanced in all bimetallic catalysts. In terms of selectivity, the reaction favored the reforming pathway over CoPt/CeO 2 , CoMo/CeO 2 , and NiMo/CeO 2 catalysts while FeNi/CeO 2 showed promising selectivity to ODH pathway [40]. Moreover, Jibril et al. [55] tested MCM41-supported NiMo and CoMo bimetallic catalysts for oxidative dehydrogenation of propane to propylene. Taghavinezhad et al. [56] synthesized Mg-, Al-, Zr-modified VOx/MCM-41 catalyst to assess the impact of modification of vanadium-based MCM41 materials on ODH of ethane with CO 2 . The results revealed that addition of various metal oxides affected the catalytic activity of the modified catalysts by altering their structural properties, such as crystallinity, particle sizes, and dispersity of the modified vanadium species along with their acid/base properties. On the other hand, Le et al. [57], have employed cerium oxide to promote nano-size Na-ZSM-5 after chromium oxide loading; the developed catalyst was tested for the ODH of ethane by using CO 2 . It was found that the introduction of Ce species clearly improves the dispersity of Cr species on the surface of Na-ZSM-5 catalysts which enhanced the catalytic activity and stability of the prepared catalyst, and reduced the amount of carbon formed on the catalyst surface at the same time. Shi et al. [58] showed that the introduction of Ce species to Cr/SBA-15 catalysts has remarkable impact on changing the redox properties which in turn has improved the catalytic activity of chromium oxide species.
In the present work, the synergetic impact of a secondary metal oxide (M) of Cr-M/MCM41 catalyst system for ethane oxidative dehydrogenation using carbon dioxide was investigated. The effect of the secondary metal oxide (Co, Ce, Zn, Nb, V, and Mo) on the textural, physical, and chemical properties of Cr-M/MCM41 catalysts were evaluated. The main aim of this study is to assess the effect of the secondary metal on the activity behavior of the catalysts for the ODH of ethane. O] were ordered from Sigma-Aldrich (Steinheim, Germany). These chemicals were used as precursors of metal oxides in this study. All other reagents were also obtained from Sigma-Aldrich and used as-received without further purification. Moreover, ammonium hydroxide solution [NH 4 OH, 28.0-30.0%] (the precipitating agent) was supplied from Sigma-Aldrich.

Catalyst Preparation
According to precipitation behavior of the metal oxide precursor in the basic media, the secondary metal salts were loaded into Cr-MCM41, either by one-pot co-precipitation approach or through impregnation method [27]. Cr(4)-M(4)/MCM41(M = Ce, Co, and Zn) catalysts were prepared by one pot-synthesis while Cr(4)-M(4)/MCM41(M = Mo, V and Nb) that do not precipitate in basic medium, Cr-MCM41 was formed first by one-pot synthesis and then the secondary metal salts were impregnated on Cr(4)/MCM41 samples. In a typical synthesis, 4.40 ml of CTAC, 52.5 ml of distilled water, and 4 ml of ammonium hydroxide were added to prepare a homogeneous solution, which was stirred (400 rpm) for 10 min. After that, a solution of exact amount of Cr and M (M = Ce, Co, and Zn) precursors was added with stirring for 60 min. Then under a vigorous stirring, 4.94 ml of TEOS was added dropwise for further 60 min. After that the resultant percipient was filtered, washed, and dried at 80 • C overnight. The dried sample was than calcined in air by heating it from ambient temperature to 650 • C over a 5-h period and keeping it at 650 • C for another 12 h. On the other hand, preparation of Cr(4)-M(4)/MCM41 (M = Mo, V and Nb) catalysts started by synthesizing Cr(4)/MCM41 sample using the one-pot synthesis procedure and the secondary metal salt was impregnated on Cr(4)/MCM41 samples. The catalysts were denoted as Cr(4)-M(4)/MCM41, where 4 stands for nominal Cr and M total contents given in wt.% of Cr and M respectively.

Catalyst Characterization
X-ray diffraction spectra were obtained for all prepared bimetallic oxide catalysts by using (PANalytical X'Pert PRO MPD) (PANalytical, Netherlands). The isotherms of nitrogen sorption were determined after the samples were evacuated at 200 • C for 3 h to remove adsorbed gases and vapors. The pore size distribution of Cr(4)-M(4)/MCM41 catalysts were analyzed from the desorption profiles of the isotherms by employing the Barrett-Joyner-Halanda (BJH) method. Diffuse reflectance UV-vis spectrum was obtained for the 230-700 nm range by Shimadzu 2550 spectrophotometer (Shimadzu, Kyoto, Japan). The reducibility of the prepared bimetallic oxide catalysts was measured by hydrogen temperature programmed reduction (H2-TPR) by Micromeritics Auto Chem II apparatus(Micromeritics, Norcross, GA, USA) To reveal the surface morphology of the prepared samples, Energy-dispersive X-ray elemental-mapping were preformed using JEOL JEM-7500F scanning electron microscope (SEM) (JEOL, Tokyo, JAPAN) JEOL JEM-2100F transmission electron microscope (TEM) (JEOL, Tokyo, JAPAN) operated at 200 kV was used to determine the internal structure of the samples. Figure 1 depicts the experimental setup used for the ODH of C 2 H 6 with CO 2 over the bimetallic oxide catalysts. The experiments were carried out by using 300 mg of the prepared catalysts sandwiched between two layers of glass wool. The catalyst samples were first pretreated in situ at 600 • C in air flowing at 18 mL/min for 0.5 h. Ethane dehydrogenation with carbon dioxide was performed at atmospheric pressure and reaction temperature range 600-700 • C. The reactants mixture was fed into the reactor with a total flow rate of 75 ml/min. The feed was composed of 7.5 C 2 H 6 , 37.5 CO 2 , and 30 ml/min N 2 respectively. The reaction products as well as unconverted substances were analyzed after 20 min from the beginning of the ODH reaction. The analysis of the various components was conducted by an online gas chromatograph coupled with FID and TCD detectors (Agilent 6890N, Agilent, Santa Clara, CA, USA) and using molecular sieve 5Å hayesepQ and gas pro(Sigma-Aldrich/supelco, Taufkirchen, Germany) as GC columns. Before the analysis the formed water was trapped from the analysis samples. Also, blank tests were performed to confirm that the homogenous reaction is negligible at the experimental conditions. Crystals 2019, 9, x FOR PEER REVIEW 4 of 16 Figure 1 depicts the experimental setup used for the ODH of C2H6 with CO2 over the bimetallic oxide catalysts. The experiments were carried out by using 300 mg of the prepared catalysts sandwiched between two layers of glass wool. The catalyst samples were first pretreated in situ at 600 °C in air flowing at 18 mL/min for 0.5 h. Ethane dehydrogenation with carbon dioxide was performed at atmospheric pressure and reaction temperature range 600-700 °C. The reactants mixture was fed into the reactor with a total flow rate of 75 ml/min. The feed was composed of 7.5 C2H6, 37.5 CO2, and 30 ml/min N2 respectively. The reaction products as well as unconverted substances were analyzed after 20 min from the beginning of the ODH reaction. The analysis of the various components was conducted by an online gas chromatograph coupled with FID and TCD detectors (Agilent 6890N, Agilent, Santa Clara, USA) and using molecular sieve 5Å hayesepQ and gas pro(Sigma-Aldrich/supelco, Taufkirchen, Germany) as GC columns. Before the analysis the formed water was trapped from the analysis samples. Also, blank tests were performed to confirm that the homogenous reaction is negligible at the experimental conditions. Ethane Conversion % F F F X 100 (4) Ethylene Yield % F F X 100 (5) Ethylene Selectivity % F F F X 100 (6) where Fj = No. of moles of component j. The conversion of ethane, yield of ethylene, and selectivity of products were calculated based on Equations (4)-(6);

Catalysts Characterization
Ethylene Selectivity (%) = F C 2 H 4 where F j = No. of moles of component j.

Catalysts Characterization
XRD analysis was performed to study the crystallographic properties of the bimetallic oxide catalysts, and the diffraction patterns are shown in Figure [17]. The intensity of Cr2O3 peaks depends on the secondary element of the bimetallic system. While it is low for Cr-Co/MCM41 and Cr-Zn/MCM41, it is relatively high for Cr-Mo/MCM41. The low intensity of the peaks signifies the formation of small particle size of Cr species and also their fine dispersion on the surface of the catalyst. In contrast, no change was   [60].
The isotherms of N2 sorption and the distribution of the pore sizes of the bimetallic oxide Cr(4)-M(4)/MCM41 catalysts are presented in Figure 3A,B. The textural properties of these catalysts are displayed in Table 1. The isotherms of N 2 sorption and the distribution of the pore sizes of the bimetallic oxide Cr(4)-M(4)/MCM41 catalysts are presented in Figure 3A,B. The textural properties of these catalysts are displayed in Table 1. The Cr(4)-M(4)/MCM-41 catalysts exhibit type IV isotherm with H1 hysteresis loop which is a standard profile for mesoporous materials according to the IUPAC classification. This confirms that the MCM-41 mesoporous structure was stable and not altered by the addition of the secondary metal oxides ( Figure 3). It can be seen that, the hysteresis loop of Cr-Co/MCM41 that appears at relative pressure P/P0 = 0.2-0.55 clearly proves that its porous feature possess uniform cylindrical pore geometry. On the other hand, the hysteresis loops of Cr(4)-M(4)/MCM41 (Me=Co, Ce, Zn, Nb, V, and Mo) appeared at relatively higher pressures which obviously demonstrated the formation of mesoporous with larger pore diameter [61]. Figure 3 shows the pore size distribution of the synthesized bimetallic oxide catalysts. All Cr(4)-M(4)/MCM41 catalysts showed wide pore size distribution centered at 6 nm for Cr-Nb/MCM41 sample, and at 10 nm for Cr-Mo/MCM41 and Cr-V/MCM41. On the other hand, pore size was 8 nm for Cr-Ce/MCM41, Cr-Co/MCM41, and Cr-Zn/MCM41. Moreover, the synthesized bimetallic oxide catalysts revealed high BET surface area (701-886 m 2 /g) and it was in the order of Ce(4) > Nb(4) > Co(4) > Zn(4) > V(4) > Mo(4). The reducibility of supported MCM41 bimetallic oxide catalysts was investigated using H2-TPR and the comparison between these catalysts is shown in Figure 4A,B. H2-TPR profiles of bimetallic oxide catalysts showed that the reduction peaks of each bimetallic system differ totally from those observed in literature for Cr-monometallic ones [62], suggesting a synergistic interaction between the two elements had taken place. The H2-TPR profile of Cr-Ce/MCM41 displayed a slight shift of  The Cr(4)-M(4)/MCM-41 catalysts exhibit type IV isotherm with H1 hysteresis loop which is a standard profile for mesoporous materials according to the IUPAC classification. This confirms that the MCM-41 mesoporous structure was stable and not altered by the addition of the secondary metal oxides (Figure 3). It can be seen that, the hysteresis loop of Cr-Co/MCM41 that appears at relative pressure P/P0 = 0.2-0.55 clearly proves that its porous feature possess uniform cylindrical pore geometry. On the other hand, the hysteresis loops of Cr(4)-M(4)/MCM41 (Me=Co, Ce, Zn, Nb, V, and Mo) appeared at relatively higher pressures which obviously demonstrated the formation of mesoporous with larger pore diameter [61]. Figure 3 shows the pore size distribution of the synthesized bimetallic oxide catalysts. All Cr(4)-M(4)/MCM41 catalysts showed wide pore size distribution centered at 6 nm for Cr-Nb/MCM41 sample, and at 10 nm for Cr-Mo/MCM41 and Cr-V/MCM41. On the other hand, pore size was 8 nm for Cr-Ce/MCM41, Cr-Co/MCM41, and Cr-Zn/MCM41. Moreover, the synthesized bimetallic oxide catalysts revealed high BET surface area (701-886 m 2 /g) and it was in the order of Ce(4) > Nb(4) > Co(4) > Zn(4) > V(4) > Mo(4).
The reducibility of supported MCM41 bimetallic oxide catalysts was investigated using H 2 -TPR and the comparison between these catalysts is shown in Figure 4A,B. H 2 -TPR profiles of bimetallic oxide catalysts showed that the reduction peaks of each bimetallic system differ totally from those observed in literature for Cr-monometallic ones [62], suggesting a synergistic interaction between the two elements had taken place. The H 2 -TPR profile of Cr-Ce/MCM41 displayed a slight shift of both reduction peaks to higher temperatures with decreasing intensity of the first peak and increasing the second one compared to those of monometallic Cr catalyst [62]. However, these results indicated the decrement of the concentration of free Cr 6+ species which are dispersed on the surface of crystalline α-Cr 2 O 3 along with the increment of concentration of Cr 6+ species which interacted with the MCM-41 structure. Moreover, new broad peak centered on 750 • C arose which are assigned to reduction of ceria oxide that interacted with MCM41 matrix. On the other hand, H 2 -TPR profile of Cr-Co/MCM41 had two wide reduction peaks centered at 400 and 650 • C respectively. The first peak is attributed to the reduction of Cr 6+ to Cr 3+ species and Co 3 O 4 to metallic Co, whereas the second broad one illustrates the reduction of Co 2+ species that is highly dispersed and strongly interacted with the surface of MCM41 [62]. increasing the second one compared to those of monometallic Cr catalyst [62]. However, these results indicated the decrement of the concentration of free Cr 6+ species which are dispersed on the surface of crystalline -Cr2O3 along with the increment of concentration of Cr 6+ species which interacted with the MCM-41 structure. Moreover, new broad peak centered on 750 °C arose which are assigned to reduction of ceria oxide that interacted with MCM41 matrix. On the other hand, H2-TPR profile of Cr-Co/MCM41 had two wide reduction peaks centered at 400 and 650 °C respectively. The first peak is attributed to the reduction of Cr 6+ to Cr 3+ species and Co3O4 to metallic Co, whereas the second broad one illustrates the reduction of Co 2+ species that is highly dispersed and strongly interacted with the surface of MCM41 [62]. In the case of Cr-Zn/MCM41, the intensity of the two peaks decreased compared to those of monometallic-Cr catalyst [62] while the higher temperature peak observed at 700 °C suggested that most of the chromium was formed in bulk chromium oxide.
At the same time, H2-TPR profiles of the catalysts prepared with the impregnation method is shown in Figure 4B. In the case of Cr-Mo/MCM41, the peak at low temperature (250 °C) disappeared [62]; the second reduction of monometallic Cr overlapped with the reduced Mo and a new peak centered at 850 °C was detected. The second peak shape indicates that there are multiple peaks overlapping at temperatures above 500 °C. These multiple peaks reflected the complex interaction of Mo species with different oxidation states on the MCM41 matrix. Arnoldy and his colleagues [63] reported that the low temperature peak is formed due to the reduction of octahedrally Mo 6+ multilayer while the peak at higher temperature (850 °C) is related to the reduction of tetrahedrally and octahedrally Mo 6+ monolayer species. However, the pronounced peak at around 460 °C has been associated with the reduction of Cr 6+ to Cr 3+ species and octahedral Mo species.
H2-TPR profile of Cr-V/MCM-41 bimetallic catalyst showed the increment of the second peak temperature (460 °C) along with a broader peak centered around 720 °C. The main peak is related to the reduction of the dispersed Cr 6+ species as well as the tetrahedral vanadium species, while the broader one could be related to the reduction of polymeric V 5+ species [64]. The Cr-Nb/MCM-41 bimetallic oxide catalyst showed a slight shift of both peaks toward higher reduction temperature with a large drop in the peaks intensity, indicating the domination of unreducible chromium. H2-TPR profiles of Cr-Ce/MCM41 and Cr-Co/MCM41 exhibited the highest intensity of C 6+ peaks which suggested the formation of high concentration of interacted C 6+ with silica matrix that acts as active sites for alkane dehydrogenation with CO2 [27,62].
The presence of different oxidation states of Cr (Cr 6+ and Cr 3+ ) on the surface of bimetallic catalysts was investigated with UV-Vis diffuse reflectance spectroscopy and the spectra are shown In the case of Cr-Zn/MCM41, the intensity of the two peaks decreased compared to those of monometallic-Cr catalyst [62] while the higher temperature peak observed at 700 • C suggested that most of the chromium was formed in bulk chromium oxide.
At the same time, H 2 -TPR profiles of the catalysts prepared with the impregnation method is shown in Figure 4B. In the case of Cr-Mo/MCM41, the peak at low temperature (250 • C) disappeared [62]; the second reduction of monometallic Cr overlapped with the reduced Mo and a new peak centered at 850 • C was detected. The second peak shape indicates that there are multiple peaks overlapping at temperatures above 500 • C. These multiple peaks reflected the complex interaction of Mo species with different oxidation states on the MCM41 matrix. Arnoldy and his colleagues [63] reported that the low temperature peak is formed due to the reduction of octahedrally Mo 6+ multilayer while the peak at higher temperature (850 • C) is related to the reduction of tetrahedrally and octahedrally Mo 6+ monolayer species. However, the pronounced peak at around 460 • C has been associated with the reduction of Cr 6+ to Cr 3+ species and octahedral Mo species.
H 2 -TPR profile of Cr-V/MCM-41 bimetallic catalyst showed the increment of the second peak temperature (460 • C) along with a broader peak centered around 720 • C. The main peak is related to the reduction of the dispersed Cr 6+ species as well as the tetrahedral vanadium species, while the broader one could be related to the reduction of polymeric V 5+ species [64]. The Cr-Nb/MCM-41 bimetallic oxide catalyst showed a slight shift of both peaks toward higher reduction temperature with a large drop in the peaks intensity, indicating the domination of unreducible chromium. H 2 -TPR profiles of Cr-Ce/MCM41 and Cr-Co/MCM41 exhibited the highest intensity of C 6+ peaks which suggested the formation of high concentration of interacted C 6+ with silica matrix that acts as active sites for alkane dehydrogenation with CO 2 [27,62].
The presence of different oxidation states of Cr (Cr 6+ and Cr 3+ ) on the surface of bimetallic catalysts was investigated with UV-Vis diffuse reflectance spectroscopy and the spectra are shown in Figure 5. All spectra showed two main absorption bands at 270 and 370 nm. These two bands originate from O 2-→Cr 6+ charge transfer transition of chromate species in tetrahedral coordination [65][66][67]. A shoulder band observed at around 415 nm, in Cr(4)-M(4)/MCM41 (M = Ce, Co, V, Nb, and Mo) samples, is assigned to dichromates or poly-chromates on the catalysts surface [68,69]. This shoulder had disappeared in case of Cr-Zn/MCM41 and overlapped with the band at 470 nm for Cr-Zn/MCM41. On the other hand, the absorption bands at 470 and 600 nm are referred to d-d transitions of Cr 3+ species in the octahedral symmetry [12,68,[70][71][72][73]. It can be observed that the Cr 3+ bands disappeared in the Cr-Zn/MCM41 and Cr-Nb/MCM41 samples. As it has already been established that Cr +3 and Cr 6+ coexist on the surface of Cr/silica catalysts and the highly dispersed Cr 6+ species is believed to constitute the active sites in ODH of alkanes [18,71,[74][75][76].  SEM observation as well as EDX elemental mapping were conducted for two selected samples, Cr(4)-Ce(4)/MCM41 and Cr(4)-Co(4)/MCM41 to estimate the state of dispersion in both catalysts and the results are presented in the Figure 6. SEM images for Cr-Ce (Figure 6a) and Cr-Co (Figure 6b) showed the irregular structure of the formed nanoparticles agglomerates; no pronounced effect was observed when changing the secondary metal oxide from Ce to Co. Elemental mapping for Cr (blue) showed homogenous distribution over the surface of mesoporous silica nanoparticles (red color). However, Cr-Ce sample showed a relatively bigger size of the formed CrOx species than Cr-Co sample. On the other hand, Ce (green) and Co (cyan) mapping suggested that both oxides have good and homogeneous distribution. The good distribution of CrOx, CeO2 and Co3O4 suggested that these oxides diffused into the mesochannels of mesoporous silica along with their formation onto the outer surface as well. SEM observation as well as EDX elemental mapping were conducted for two selected samples, Cr(4)-Ce(4)/MCM41 and Cr(4)-Co(4)/MCM41 to estimate the state of dispersion in both catalysts and the results are presented in the Figure 6. SEM images for Cr-Ce (Figure 6a) and Cr-Co (Figure 6b) showed the irregular structure of the formed nanoparticles agglomerates; no pronounced effect was observed when changing the secondary metal oxide from Ce to Co. Elemental mapping for Cr (blue) showed homogenous distribution over the surface of mesoporous silica nanoparticles (red color). However, Cr-Ce sample showed a relatively bigger size of the formed CrOx species than Cr-Co sample. On the other hand, Ce (green) and Co (cyan) mapping suggested that both oxides have good and homogeneous distribution. The good distribution of CrOx, CeO 2 and Co 3 O 4 suggested that these oxides diffused into the mesochannels of mesoporous silica along with their formation onto the outer surface as well.  The TEM results complement SEM elemental mapping results that showed more homogenous distribution of Cr, Ce, and Co on the surface of mesoporous silica. The results agree with the previous findings that the catalysts will fill first the mesochannels of mesoporous silica and thereafter they begin to form onto outer surface [62]. This mechanism provided the rod-like Cr and irregular Ce nanoparticles in case of Cr-Ce sample and rhombohedral Cr and round Ce nanoparticles as in case of Cr-Co sample.
The selected area electron diffraction (SAED) have been conducted for Cr(4)-Ce(4)/MCM41 and Cr(4)-Co(4)/MCM41 and are displayed in Figure 7Bb and 7Bb', respectively. It is clear that Cr-Ce/MCM41 in Figure 7Bb possess poly-crystallinity with spot pattern because of the co-existence of poly-crystalline CrOx and CeO2 nanoparticles. On the other hand, Cr-Co/MCM41 in Figure 7Bb' reveal that the samples has rhombohedral spot pattern that can be attributed to the formation of large size rhombohedral mono-crystalline CrOx. Because of the small sizes of Co3O4 nanoparticles, they did not appear into SAED pattern.

Catalytic Activity of Cr(4)-M(4)/MCM41 Catalysts
The catalytic performance of Cr(4)-M(4)/MCM41 bimetallic oxide catalysts were investigated in ODH of C2H6 with CO2 at different reaction temperatures and the results are presented in Figure 8. Generally, it can be observed that for all bimetal oxide catalysts, both the C2H6 conversion and the C2H4 yield increased with rising of reaction temperature because of the endothermicity of the reaction. On the other hand, the ethylene selectivity dropped slightly with temperature, indicating that hydrocracking of ethane and ethylene are favorable at high reaction temperatures. Also it can be noted that Cr-Ce/MCM41 and Cr-Co/MCM41 catalysts have the highest catalytic performance among the prepared samples. 37.9% C2H6 conversion and 35.1% C2H4 yield were obtained for the Cr-Ce/MCM41 catalyst and 36.1% of ethane conversion with 33.7% ethylene yield was recorded for the Cr-Co/MCM41 catalyst. The TEM results complement SEM elemental mapping results that showed more homogenous distribution of Cr, Ce, and Co on the surface of mesoporous silica. The results agree with the previous findings that the catalysts will fill first the mesochannels of mesoporous silica and thereafter they begin to form onto outer surface [62]. This mechanism provided the rod-like Cr and irregular Ce nanoparticles in case of Cr-Ce sample and rhombohedral Cr and round Ce nanoparticles as in case of Cr-Co sample.
The selected area electron diffraction (SAED) have been conducted for Cr(4)-Ce(4)/MCM41 and Cr(4)-Co(4)/MCM41 and are displayed in Figure 7Bb and 7Bb', respectively. It is clear that Cr-Ce/MCM41 in Figure 7Bb possess poly-crystallinity with spot pattern because of the co-existence of poly-crystalline CrOx and CeO 2 nanoparticles. On the other hand, Cr-Co/MCM41 in Figure 7Bb' reveal that the samples has rhombohedral spot pattern that can be attributed to the formation of large size rhombohedral mono-crystalline CrOx. Because of the small sizes of Co 3 O 4 nanoparticles, they did not appear into SAED pattern.

Catalytic Activity of Cr(4)-M(4)/MCM41 Catalysts
The catalytic performance of Cr(4)-M(4)/MCM41 bimetallic oxide catalysts were investigated in ODH of C 2 H 6 with CO 2 at different reaction temperatures and the results are presented in Figure 8. Generally, it can be observed that for all bimetal oxide catalysts, both the C 2 H 6 conversion and the C 2 H 4 yield increased with rising of reaction temperature because of the endothermicity of the reaction. On the other hand, the ethylene selectivity dropped slightly with temperature, indicating that hydrocracking of ethane and ethylene are favorable at high reaction temperatures. Also it can be noted that Cr-Ce/MCM41 and Cr-Co/MCM41 catalysts have the highest catalytic performance among the prepared samples. 37.9% C 2 H 6 conversion and 35.1% C 2 H 4 yield were obtained for the Cr-Ce/MCM41 catalyst and 36.1% of ethane conversion with 33.7% ethylene yield was recorded for the Cr-Co/MCM41 catalyst. For the Cr-based catalytic system it is well-known that Cr 6+ is the key factor to higher catalytic activity in the oxidative dehydrogenation of ethane [71,77]. In this study, the characterization results show that the addition of Ce or Co species increased the reducibility of Cr 6+ and the ratio of interacted Cr 6+ with silica matrix. In addition, the higher catalytic activity of Cr-Ce/MCM41 and Cr-Co/MCM41 catalysts may also be caused by the presence of Ce and Co species respectively, which enhanced their performance because of the redox property of CeO2 and Co3O4 as well as synergy. Valenzuela et al. [35] and Sharma et al. [78] reported that the redox Ce 4+ /Ce 3+ of CeO2 can activate CO2 to produce active oxygen species for the ODH of ethane reaction. On the other hand, Cr-V/MCM41 showed low catalytic activity in the oxidative dehydrogenation of ethane with CO2 where lower values of ethane conversion and ethylene yield were observed over this bimetallic oxide catalyst. Moreover, the selectivity to ethylene showed a decay trend over Cr-V/MCM41, suggesting further oxidation of the produced ethylene.
It was found that the reaction temperature has a strong influence on the catalytic performance of vanadium oxide-based catalytic system and it is mainly related to the reducibility of the active sites (V atoms) [64,[79][80][81]. However, Cr-V/MCM41catalyst may need much higher temperature to be active for ODH of ethane reaction. This assumption is consistent with H2-TPR measurement where the reduction temperatures of this catalyst have shifted to higher temperature. The same trend can be concluded for the other three catalysts, Cr(4)-M(4)/MCM41 (M = Zn, Nb, Mo), which showed much lower catalytic activities compared to Cr-Ce/MCM41 and Cr-Co/MCM41 catalysts. Finally, the synergy between the Cr and Zn or Nb oxides led to a drop in the catalytic activity toward ethylene For the Cr-based catalytic system it is well-known that Cr 6+ is the key factor to higher catalytic activity in the oxidative dehydrogenation of ethane [71,77]. In this study, the characterization results show that the addition of Ce or Co species increased the reducibility of Cr 6+ and the ratio of interacted Cr 6+ with silica matrix. In addition, the higher catalytic activity of Cr-Ce/MCM41 and Cr-Co/MCM41 catalysts may also be caused by the presence of Ce and Co species respectively, which enhanced their performance because of the redox property of CeO 2 and Co 3 O 4 as well as synergy. Valenzuela et al. [35] and Sharma et al. [78] reported that the redox Ce 4+ /Ce 3+ of CeO 2 can activate CO 2 to produce active oxygen species for the ODH of ethane reaction. On the other hand, Cr-V/MCM41 showed low catalytic activity in the oxidative dehydrogenation of ethane with CO 2 where lower values of ethane conversion and ethylene yield were observed over this bimetallic oxide catalyst. Moreover, the selectivity to ethylene showed a decay trend over Cr-V/MCM41, suggesting further oxidation of the produced ethylene.
It was found that the reaction temperature has a strong influence on the catalytic performance of vanadium oxide-based catalytic system and it is mainly related to the reducibility of the active sites (V atoms) [64,[79][80][81]. However, Cr-V/MCM41catalyst may need much higher temperature to be active for ODH of ethane reaction. This assumption is consistent with H 2 -TPR measurement where the reduction temperatures of this catalyst have shifted to higher temperature. The same trend can be concluded for the other three catalysts, Cr(4)-M(4)/MCM41 (M = Zn, Nb, Mo), which showed much lower catalytic activities compared to Cr-Ce/MCM41 and Cr-Co/MCM41 catalysts. Finally, the synergy between the Cr and Zn or Nb oxides led to a drop in the catalytic activity toward ethylene production. This drop is related to the reduction of C 6+ on the surface of these two catalysts. The catalytic activity of synthesized bimetallic oxide catalysts can be arrange in the following sequence: Ce(4) > Co(4) > V(4) > Zn(4) > Nb(4) > Mo(4).
The stability of Cr(4)-M(4)/MCM41 bimetallic oxide catalysts was investigated by running continuously the ODH of ethane with CO 2 reaction for more than 8 h. The results are presented in Figure 9. It can be seen that, the catalytic activity of all synthesized bimetallic oxide catalysts, in terms of C 2 H 6 conversion and C 2 H 4 yield, decreased slowly with increasing reaction period. On the other hand, the C 2 H 4 selectivity almost remains stable above 91% even after 8 h. Drop of highly reducible Cr species in addition to coke formation are the expected reasons for such a decline in catalytic performance. However, Cr-Ce/MCM41 and Cr-Co/MCM41 catalysts still showed acceptable catalytic properties after 8 hours on stream reaction. The reasonable stability of these catalysts can be referred to the high reducible Cr species on the surface of these samples. The stability of Cr(4)-M(4)/MCM41 bimetallic oxide catalysts was investigated by running continuously the ODH of ethane with CO2 reaction for more than 8 h. The results are presented in Figure 9. It can be seen that, the catalytic activity of all synthesized bimetallic oxide catalysts, in terms of C2H6 conversion and C2H4 yield, decreased slowly with increasing reaction period. On the other hand, the C2H4 selectivity almost remains stable above 91% even after 8 h. Drop of highly reducible Cr species in addition to coke formation are the expected reasons for such a decline in catalytic performance. However, Cr-Ce/MCM41 and Cr-Co/MCM41 catalysts still showed acceptable catalytic properties after 8 hours on stream reaction. The reasonable stability of these catalysts can be referred to the high reducible Cr species on the surface of these samples.

Conclusion
MCM41 supported bimetallic oxide catalysts containing chromium oxide along with a secondary metal oxide (Ce, Co, Zn, V, Mo, or Nb) were prepared. The prepared catalysts have been characterized and evaluated for ODH of ethane by using CO2 as an oxidant. Cr-Ce/MCM41 and Cr-Co/MCM41 exhibited catalytic activity which can be correlated to the high population of redox sites on their surfaces as indicated by H2-TPR along with good dispersion of Cr and Ce and Co oxide species and high textural properties. On the other hand, Cr-V/MCM41 and Cr-Mo/MCM41 showed a significant drop in the activity and the selectivity to ethylene. While Cr-Zn/MCM41 and Cr-Nb/MCM41 exhibited the highest selectivity to ethylene, their activity was much lower than Cr-Ce/MCM41 and Cr-Co/MCM41 catalysts. The results suggested that Cr-Ce/MCM-41 and Cr-Co/MCM-41 possessed promising potentials as catalysts for converting ethane to ethylene; however further studies are still required to optimize their loading content to enhance the catalytic performances.

Conclusions
MCM41 supported bimetallic oxide catalysts containing chromium oxide along with a secondary metal oxide (Ce, Co, Zn, V, Mo, or Nb) were prepared. The prepared catalysts have been characterized and evaluated for ODH of ethane by using CO 2 as an oxidant. Cr-Ce/MCM41 and Cr-Co/MCM41 exhibited catalytic activity which can be correlated to the high population of redox sites on their surfaces as indicated by H 2 -TPR along with good dispersion of Cr and Ce and Co oxide species and high textural properties. On the other hand, Cr-V/MCM41 and Cr-Mo/MCM41 showed a significant drop in the activity and the selectivity to ethylene. While Cr-Zn/MCM41 and Cr-Nb/MCM41 exhibited the highest selectivity to ethylene, their activity was much lower than Cr-Ce/MCM41 and Cr-Co/MCM41 catalysts. The results suggested that Cr-Ce/MCM-41 and Cr-Co/MCM-41 possessed promising potentials as catalysts for converting ethane to ethylene; however further studies are still required to optimize their loading content to enhance the catalytic performances.