Impregnation vs. coprecipitation dispersion of Cr over TiO₂ and ZrO₂ used as active and stable nanocatalysts in oxidative dehydrogenation of ethane to ethylene by carbon dioxide

Abstract

The catalytic performance of Cr/ZrO₂ and Cr/TiO₂ nanocatalysts, prepared by coprecipitation and impregnation methods, was examined in oxidative dehydrogenation of ethane to ethylene using CO₂ as an oxidant. Physicochemical characterization techniques such as XRD, FESEM, EDX, BET and FTIR were performed to explore the correlation of catalytic performance with properties of the nanocatalysts. The XRD analysis confirmed the formation of crystalline TiO₂ and/or ZrO₂ in the synthesized samples. The FESEM and EDX images revealed the formation of homogeneous spherical agglomerates within the nanometer range and with a uniform dispersion over the surface of all samples, especially Cr/ZrO₂ (P). The BET results proved the high surface area of Cr/ZrO₂ (P) and Cr/TiO₂ (P) nanocatalysts. Among all the samples, the Cr/ZrO₂ (P) nanocatalyst had the highest specific surface area. Catalytic tests showed that Cr-based catalysts prepared by coprecipitation, had higher ethane conversion and ethylene yield in comparison to those prepared by impregnation method. Among all the samples, Cr/ZrO₂ (P) had the highest ethane conversion (48% at 700 °C) and ethylene yield (43% at 700 °C). This could be attributed to the smaller particles, higher surface area, better dispersion of the active phase and uniform morphology of the Cr/ZrO₂ (P) nanocatalyst.

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1. Introduction

Carbon dioxide is one of the major greenhouse gases.^1,2 Emission control and efficient utilisation of CO₂ have attracted considerable attention.^3–5 Various catalytic methods have been used to transform carbon dioxide into valuable products such as CO. Recently, carbon dioxide plays an important role as an oxygen source or oxidant for some oxidative conversions of hydrocarbons.^6–9

Ethylene is one of the most important building blocks for many chemical and petrochemical industries.^10–12 Currently, the major source of ethylene production is thermal cracking of hydrocarbons in the presence of steam^13–15 and some efforts have been made in the methanol to light olefins process.^16–18 Due to the rapidly growing demand for ethylene and limited resources of petroleum in the future, much effort has been devoted to new routes of ethylene production such as dehydrogenation of ethane.^19–21 The inherent obstacles of the ethane dehydrogenation process such as equilibrium limitations regarding ethane conversion, large energy consumption due to endothermic reaction and rapid catalyst deactivation owing to coke formation^10,22,23 cause the ethane oxidative dehydrogenation process to be an alternative route for production of high demand ethylene without these drawbacks of current commercial processes. Different oxidants like O₂, N₂O, and CO₂ can be used in the oxidative dehydrogenation of ethane to ethylene which provide some advantages and disadvantages to this process.^24–29

CO₂ is now considered as a soft oxidant in dehydrogenation of ethane to ethylene due to its role as a diluent in offering high equilibrium conversion, improving the selectivity to ethylene, reducing coke formation and maintaining longer catalyst life time.^10,30,31 The oxidative dehydrogenation (ODH) of ethane to ethylene by carbon dioxide can be described as dehydrogenation reaction coupled with reverse water gas shift (RWGS) reaction. The ethylene production can be improved by the elimination of the hydrogen produced from the dehydrogenation via the RWGS reaction.^32–34

In recent years, several researches have been carried out about the application of chromium-based catalysts in ODH of ethane,^35–37 propane^38–40 and butane.^41–43 According to these investigations, redox mechanism of this process and the activation of reactants are affected by catalyst acidity, reducibility and the degree of the surface active spices dispersion.^7,44 It is known that each of the above-mentioned aspects in the ODH of alkanes depends on the nature of the support and the preparation procedure.^45–47 For example, Wang et al. studied the effect of support on catalytic behaviour in the ODH of ethane with CO₂ over Cr₂O₃-based catalysts.⁴⁶ Among catalysts prepared, Cr₂O₃/SiO₂ exhibited higher conversion, ethylene selectivity and stability. Wu et al.⁴⁸ investigated the effect of temperature over a series of Cr₂O₃–ZrO₂ mixed oxides prepared by hydrothermal method in ODH of propane with CO₂.

It is well reported that synthesis method can effectively change the catalytic properties and performance.^49–51 Basically, two types of the catalyst preparation procedure are investigated: post synthetic method, like wet impregnation method which is very often used for the deposition of CrO_X species,^{30,34,37,52–55} and direct or one-pot synthesis method. Low degree of dispersion of bulk oxide, the channels blockage by the formation of bulk metal oxide clusters and the synthetic complication related to multiple synthesis and calcination steps are the disadvantages of impregnation method.^56,57 Instead, the catalysts which are prepared in one-pot synthesis method like coprecipitation method offer superior catalytic performance in the oxidative dehydrogenation reaction.^56–59 The surfaces with larger surface area, well-dispersed and more reducible catalytic species are made possible using this synthesis method. Therefore, in this research, two different supports and catalyst preparation methods, coprecipitation and impregnation, were adopted to prepare Cr/ZrO₂ and Cr/TiO₂ catalysts for the comparison in the ODH of ethane by CO₂. The structural evaluation of the catalysts were thoroughly studied using various physiochemical techniques namely, powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX), Brunauer–Emmett–Teller (BET) surface area, and fourier transform infrared analysis (FTIR). The catalytic performance was evaluated for the oxidative dehydrogenation of ethane to ethylene under atmospheric pressure employing CO₂ as a soft oxidant.

2. Materials and methods

2.1. Materials

Zirconyl(IV) nitrate hydrate (Aldrich, Germany, 99%), titanium(IV) chloride (Merck, Germany, 99%), chromium(ІІІ) nitrate (Chem-Lab, Belgium, 96%) were used as the precursors of zirconia, titania and chromia, respectively. Also, ammonium hydroxide solution (Iran, 25%) was supplied as a precipitating agent. All of the materials were used as received without any further purification.

2.2. Nanocatalysts preparation and procedures

Cr/ZrO₂ and Cr/TiO₂ catalysts were synthesized by two different techniques: coprecipitation (P) and impregnation (I). For the coprecipitation method, Fig. 1 illustrates schematically synthesis steps of Cr/TiO₂ (P) and Cr/ZrO₂ (P) nanocatalysts. The required amounts of zirconyl(IV) nitrate hydrate or titanium(IV) chloride and chromium(ІІІ) nitrate corresponding to 5 wt% Cr₂O₃ in the final product were dissolved separately in the deionized water and mixed together for 3.5 h at 40 °C to ensure the formation of homogeneous solution. A diluted aqueous ammonia was added drop wise into this mixture solution under vigorous stirring at 40 °C until the precipitation was completed (pH = 8 ± 0.2), and the stirring was continued further 1 h. The well-formed precipitates were filtered and washed with deionized water several times until the complete removal of chloride ions. The obtained cake was dried at 110 °C for 12 h and finally, calcined at 600 °C for 5 h under air flow. The pure ZrO₂ and TiO₂ supports were also synthesized in the same manner mentioned above.

Fig. 1 Synthesis steps of TiO₂, ZrO₂, Cr/TiO₂ (P) and Cr/ZrO₂ (P) nanocatalysts via coprecipitation method.

For the impregnation method, Cr₂O₃ was deposited over the precipitation synthesized supports by adopting a standard wet impregnation method as indicated in Fig. 2. To achieve this, the desired amount of chromium(ІІІ) nitrate corresponding to 5 wt% Cr₂O₃ in the final product was dissolved in the deionized water for 0.5 h at 50 °C and then the synthesized supports were added under continuous stirring for 1 h at the same temperature. After the impregnation, all these samples were dried at 110 °C for 12 h, and calcined at 600 °C for 5 h in static air atmosphere.

Fig. 2 Synthesis steps of Cr/TiO₂ (I) and Cr/ZrO₂ (I) nanocatalysts via impregnation method.

2.3. Nanocatalysts characterization

XRD analysis was performed on Siemens diffractometer D5000 with a Cu Kα radiation (λ = 1.54178 Å) source operating at 40 kV and 30 mA in a scanning range of 2θ = 10–90°. The diffraction peaks of the crystalline phase were compared with those of standard compounds reported in the Joint Committee of Powder Diffraction Standards (JCPDS) database files. The morphology and surface particle size distribution of Cr/TiO₂ and Cr/ZrO₂ nanocatalysts were studied by FESEM (HITACHI S-4160). EDX-Dot Mapping analysis was carried out by VEGA\\TESCAN, BSE DETECTOR for elemental analysis. The specific surface areas of the samples were determined by BET method on Quantachorom CHEMBET-2000 apparatus. FTIR of the powders was recorded on UNICAM 4600 Fourier spectrometer in a range of 400–4000 cm⁻¹ by KBr pellet method in order to identify surface functional groups.

2.4. Nanocatalysts performance test

The experimental set up for the oxidative dehydrogenation of ethane to ethylene by carbon dioxide are shown in Fig. 3. The catalytic experiments were conducted in a quartz tube reactor (6 mm i.d.) under atmospheric pressure. The catalyst loading for the reactor was 500 mg. The reactant stream, consisting of 10% ethane, 50% carbon dioxide and 40% nitrogen, was introduced into the reactor at a flow rate of 75 ml min⁻¹. The reaction temperature ranged between 550 and 700 °C. Blank tests were conducted under the same conditions, by replacing the catalyst with quartz sand particles of the same size. The products and remained reactants were analyzed on-line by a gas chromatograph (GC Chrom, Teif Gostar Faraz, Iran) equipped with a flame ionization detector (FID) using a Carboxen™ 1000 column. Gas stream from the regulators passed through pipes and it was introduced to the mass flow controllers (MFC). Argon was used as the carrier gas for gas chromatography. The ethane conversion (X_C2H6), CO₂ conversion (X_CO2), selectivity (S_C2H4) and yield of ethylene (Y_C2H4) have been defined as follows:

Fig. 3 Experimental setup for testing of catalytic performance of synthesized nanocatalysts used in the oxidative dehydrogenation of ethane to ethylene by carbon dioxide.

3. Results and discussions

3.1. Nanocatalysts characterization

3.1.1. XRD analysis. The XRD patterns of the synthesized supports and Cr/TiO₂ and Cr/ZrO₂ nanocatalysts with different synthesis methods are presented in Fig. 4. According to the JCPDS files, TiO₂ (00-001-0562) and ZrO₂ (01-080-0784) peaks can be detected in the XRD patterns. As can be seen, the XRD analysis reveals the presence of Cr₂O₃ as indicated by the weak diffraction peaks at 2θ = 24.5, 33.6, 36.2°. The diffraction lines of synthesized Cr₂O₃ are typical of the rhombohedral crystal phase (00-006-0504). On the other hand, a comparison between patterns (b) with patterns (d) and (f) reveals that by adding active phase (Cr₂O₃) to support the intensity of zirconia peaks decrease significantly and this observation implies that Cr₂O₃ species were highly dispersed over support surfaces. This hypothesis is going to be supported by BET and EDX analysis. A closer looks reveals that, due to applying coprecipitation method, the diffraction peaks for zirconia in the Cr/ZrO₂ (P) nanocatalyst were broader and weaker than those in the Cr/ZrO₂ (I) nanocatalyst. This trend suggests that employing coprecipitation method might be promising procedure to obtain higher surface area and homogenous distribution. These conclusions are going to be supported by the FESEM and BET analysis. It can be seen just for ZrO₂ and Cr/ZrO₂ (I), intense and broad peaks of monoclinic and tetragonal zirconia were observed but for Cr/ZrO₂ (P), only the characteristic peaks of tetragonal zirconia were observed which is in consistent with Wu et al.'s study.⁴⁸ In the literature,⁶⁰ the better catalytic and mechanical properties of the zirconia have been reported to be associated with the tetragonal phase. Also, Zhang et al.⁶¹ reported that the monoclinic phase of zirconia exhibited higher WGS activity than that on tetragonal phase of zirconia. So, the Cr/ZrO₂ (P) sample will be expected to have more catalytic activity and stability in this process. As seen, the anatase phase of TiO₂ is formed. Comparing pattern (c) and (e) reveals that the diffraction peaks for titania in Cr/TiO₂ (P) nanocatalysts are weaker and broader than those in Cr/TiO₂ (I) nanocatalysts. Therefore, it can be concluded that coprecipitation method creates smaller crystals of them and good dispersion of Cr₂O₃ over the supports.

Fig. 4 XRD patterns of synthesized nanocatalysts: (a) TiO₂, (b) ZrO₂, (c) Cr/TiO₂ (I), (d) Cr/ZrO₂ (I), (e) Cr/TiO₂ (P) and (f) Cr/ZrO₂ (P).

3.1.2. FESEM analysis. Fig. 5 displays the surface morphology of synthesized bare supports and Cr/TiO₂ (I), Cr/TiO₂ (P), Cr/ZrO₂ (I), and Cr/ZrO₂ (P) nanocatalysts. According to FESEM micrographs, it can be seen that the different synthesis methods used in this work produce materials with different morphologies and grain sizes. The particles of Cr/TiO₂ (I) nanocatalyst were larger and the surface was comparatively rougher. In contrast, spherical particles of Cr/TiO₂ (P) nanocatalyst were small with a uniform dispersion. A continuous structure with no clear border between the particles exists in the Cr/ZrO₂ (I) sample but in Cr/ZrO₂ (P) a uniform morphology with small and homogenous particles can be detected. The border between the particles in Cr/ZrO₂ (P) is clear. Smoother particles with large contacting interfaces are observed in this catalyst. The superior effect of the one step coprecipitation synthesis method in creating smaller catalyst particles with well-defined morphologies can be concluded from a comparison of the Cr/ZrO₂ (I) and Cr/ZrO₂ (P) FESEM images. As a result, utilizing coprecipitation method to synthesize Cr/ZrO₂ (P) and Cr/TiO₂ (P) samples made the morphology of surface material more homogeneous and spherical type agglomerates with small sizes within the nanometer range and higher surface area compared to impregnation synthesized samples which this can cause the catalytic sites to be more available for the reactants and thereby, to better catalytic performance.

Fig. 5 FESEM images of synthesized nanocatalysts: (a) TiO₂, (b) ZrO₂, (c) Cr/TiO₂ (I), (d) Cr/ZrO₂ (I), (e) Cr/TiO₂ (P) and (f) Cr/ZrO₂ (P).

3.1.3. EDX analysis. In order to confirm the presence of different components and also, distribution of the desirable elements in the nanocomposite structure, elemental analysis was performed by EDX dot-mapping and the results are illustrated in Fig. 6. All of the materials used in the preparation of nanocatalysts (Zr, Ti, and Cr), can be observed in the EDX dot-mapping pictures. These results at first, proved the presence of Cr species as the active phase. This is important since, in the XRD analysis due to similar and weak patterns, it was somewhat difficult to identify some phases. Secondly, chemical compositions are in agreement with the expected values especially in Cr/ZrO₂ (P) sample, confirms the validity of preparing procedure. So, Cr is well-dispersed over Cr/ZrO₂ (P) and Cr/TiO₂ (P) and no sintering or agglomeration of particle was observed in Cr species. This observation confirms the XRD results which are previously presented. Therefore, these nanoparticles provide more reactive and reducible Cr species over the support and lead to high catalytic performance. To sum up, EDX analysis reveals significant effect of coprecipitation method to synthesize catalysts specially Cr/ZrO₂ (P) nanocatalyst on dispersion of active phase.

Fig. 6 EDX dot mapping analysis of synthesized nanocatalysts: (a) TiO₂, (b) ZrO₂, (c) Cr/TiO₂ (I), (d) Cr/ZrO₂ (I), (e) Cr/TiO₂ (P) and (f) Cr/ZrO₂ (P).

3.1.4. BET analysis. The total specific surface areas and the crystalline phases are reported in Table 1 for the synthesized samples. As can be noted from this table, the BET specific surface areas of the Cr/ZrO₂ (P) and Cr/TiO₂ (P) nanocatalysts were 121 and 73.2 m² g⁻¹, respectively while the specific surface areas of the Cr/ZrO₂ (I) and Cr/TiO₂ (I) were 57.3 and 28.6 m² g⁻¹, respectively. This result suggested that the coprecipitation method could keep larger specific surface area and is in excellent agreement with FESEM analysis which was previously presented, confirming that smaller particles of the catalyst would result in higher specific surface area. Due to penetration of the deposited active oxide into the pores of the supports (ZrO₂ and TiO₂), the slight differences in the specific surface areas from 67.6 and 35 m² g⁻¹ to 57.3 and 28.6 m² g⁻¹ for the Cr/ZrO₂ (I) and Cr/TiO₂ (I), are observed, respectively. This observation confirms the XRD results which were previously presented. However, it seems that among these nanocatalysts, Cr/ZrO₂ (P) sample has higher specific area (121 m² g⁻¹) than the others. Higher surface area helps the accessibility of reactant to active sites. Also, higher surface area can enhance adsorption of ethane.

Table 1 Composition and BET specific surface area of synthesized nanocatalysts

Nanocatalyst/support	Cr2O3 (%)	Synthesis method	SBET (m^2 g^−1)	Crystallographic phase in XRD patterns
				Cr2O3	TiO2	ZrO2
TiO2	—	Precipitation	35	Rhombohedral	Anatase	—
ZrO2	—	Precipitation	67.6	Rhombohedral	—	Tetragonal/monoclinic
Cr/TiO2 (I)	5	Precipitation/impregnation	28.6	Rhombohedral	Anatase	—
Cr/ZrO2 (I)	5	Precipitation/impregnation	57.3	Rhombohedral	—	Tetragonal/monoclinic
Cr/TiO2 (P)	5	Coprecipitation	73.2	Rhombohedral	Anatase	—
Cr/ZrO2 (P)	5	Coprecipitation	121	Rhombohedral	—	Tetragonal

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3.1.5. FTIR analysis. FTIR spectra of the synthesized samples were recorded in the 400–4000 cm⁻¹ range and are shown in Fig. 7. The spectrum of the synthesized catalysts showed peaks of stretching vibration of structural O–H at wave numbers about 3450 cm⁻¹, and stretching vibration around 1635 and 1400 cm⁻¹ can be attributed to physically adsorbed water.^62–64 Due to the ability of the OH groups in removing the formed coke, the presence of these groups is very important.⁶⁵ This allows us to suggest, better coking resistance and subsequently improved performance of coprecipitation catalysts may be slightly related to the higher intensity of OH groups compared to impregnation synthesized samples. The peak at 2370 cm⁻¹ is attributed to residual atmospheric CO₂ (ref. 66–68) or C–H stretching mode of atmospheric hydrocarbons on the surface of the catalysts.⁶⁹ Absorption peaks around 465 cm⁻¹ are related to the anatase phase Ti–O metal oxides infrared vibrations.⁷⁰ Also, the peaks around 400–600 cm⁻¹ are likely due to the metal–oxygen–metal bond (M–O–M; M = Ti, Zr, and Cr) or the stretching vibration adsorption spectrum of Zr–O.^71–73

Fig. 7 FTIR spectra of synthesized nanocatalysts: (a) TiO₂, (b) ZrO₂, (c) Cr/TiO₂ (I), (d) Cr/ZrO₂ (I), (e) Cr/TiO₂ (P) and (f) Cr/ZrO₂ (P).

3.2. Catalytic performance of Cr-based nanocatalysts toward ethane to ethylene

3.2.1. C₂H₆ conversion. Fig. 8 depicts C₂H₆ conversion over synthesized nanocatalysts at temperatures ranging from 550 to 700 °C. In this section, all reactions were carried out at constant feed composition and gas hourly space velocity (GHSV = 9000 h⁻¹). The blank test shows little activity in the oxidative dehydrogenation of ethane to ethylene by CO₂ below 650 °C. It exhibits only approximately 10% ethane conversion at 700 °C, indicating that the homogeneous reaction and quartz sand will not make significant contributions to the heterogeneous reaction. It is seen that catalytic conversion increases with increasing temperature. That is because the ODH of ethane occurs more at higher temperatures due to endothermic nature of reactions. As can be seen, Cr/ZrO₂ (P) sample which according to XRD patterns, intense and broad peaks of tetragonal zirconia were observed, have more catalytic activity in RWGS reaction and ethane conversion in this process compared to Cr/ZrO₂ (I). However, the catalytic performance of Cr/ZrO₂ (P) and Cr/TiO₂ (P) is higher than Cr/ZrO₂ (I) and Cr/TiO₂ (I) nanocatalysts. This observation could be attributed to higher surface area, well-defined morphologies, and well-dispersed active phase over supports of these nanocatalysts. As expected, the Cr/ZrO₂ (P) nanocatalyst is more successful in ethane conversion compared to other nanocatalysts. Improved morphology of this nanocatalyst resulted from coprecipitation synthesis method caused the highest ability in ethane conversion.

Fig. 8 C₂H₆ conversion over synthesized nanocatalysts at various temperatures.

As presented in Fig. 8, deposition of 5 wt% Cr₂O₃ as an active phase over the ZrO₂ and TiO₂ supports causes to dramatic increase in ethane conversion (increase from 15% and 12% to 32% and 28% at 700 °C, respectively) which is probably due to the redox property of Cr₂O₃. Based on this result and other proposed reaction pathway, the overall reactions can be described as follows:^46,74

C₂H₆ → C₂H₄ + H₂ (5)

3C₂H₆ + 2CrO₃ → 3C₂H₄ + Cr₂O₃ + 3H₂O (6)

Cr₂O₃ + 3CO₂ → 3CO + 2CrO₃ (7)

H₂ + CO₂ → CO + H₂O (8)

C₂H₆ + 2CO₂ → 4CO + 3H₂ (9)

C₂H₆ + H₂ → 2CH₄ (10)

3.2.2. C₂H₄ yield. The C₂H₄ yields over the synthesized nanocatalysts at temperatures ranging from 550 to 700 °C are presented in Fig. 9. The yield of C₂H₄ was dramatically increased with the increasing operation temperature due to the endothermicity of the reaction. It must be underlined that the ethylene yield over Cr/ZrO₂ (P) is remarkably higher than that of the other nanocatalysts which reveals that its good physicochemical properties significantly enhance the ethylene yield and will improve the catalyst stability for the ethane dehydrogenation in CO₂.

Fig. 9 C₂H₄ yield over synthesized nanocatalysts at various temperatures.

3.2.3. Time on stream performance. The catalytic activity of Cr/ZrO₂ (P) sample was tested for 300 min time on stream (TOS) at constant temperature (700 °C), feed ratio and GHSV and the results are shown in Fig. 10. The result depicts the high and stable catalytic activity during the reaction up to 300 min for the Cr/ZrO₂ (P) nanocatalyst. This can be addressed by utilizing coprecipitation method in this research, due to excellent catalytic properties such as small particle size, uniform distribution and enhanced interaction between active sites and the support helped to prevent deactivation of the catalyst aside from enhancing catalytic activity.

Fig. 10 Time on stream behaviour of Cr/ZrO₂ (P) nanocatalyst in the oxidative dehydrogenation of ethane to ethylene by carbon dioxide.

In order to better evaluation the synthesized catalysts' performance, the catalytic activity was compared with those reported in the literature. Table 2 shows the catalytic performance of the various supported Cr₂O₃ catalysts evaluated in the oxidative dehydrogenation of ethane in the presence of CO₂. Ethylene yield is an appropriate factor to compare the results obtained to that of investigated in cited literature in order to clarify the effect of the one step synthesis method (coprecipitation) adopted in this study. On the other hand, catalytic stability and durability is a very important factor aside from activity of nanocatalyst. T indicates the temperature at which highest value of catalytic activity has been obtained. As can be seen, Cr₂O₃ (5 wt%)/TiO₂, reported in the literature, exhibits the lowest yield but the Cr/TiO₂ (P) catalysts in this investigation in spite of higher GHSV show higher yield due to the smaller particles, higher surface area, better dispersion of active phase, and uniform surface morphology. According to the activity information and taking into account the process conditions and catalyst composition given in Table 2, Cr/ZrO₂ (P) seems to have the appropriate performance towards ethylene production. Notable activity was also found on Cr₂O₃ (8%)/SiO₂, Cr (10%)–Fe (5%)/ZrO₂, and Cr (5%)/SBA-15 catalysts. However, the different order in activity is probably due to the different reaction system, the Cr content, and the support employed. Besides, the former two catalysts were operated at relatively low GHSV. Therefore, it can be concluded that higher activity of these catalysts compared to Cr/ZrO₂ (P) is partly attributed to lower GHSV or different feed composition or various length of catalytic bed in the tubular reactor used as well as catalyst composition. But, Cr/ZrO₂ (P) allocated the highest TOS among the studied catalysts in this table. Therefore, the above discussion and comparison offer to utilize coprecipitation method to synthesize Cr/ZrO₂ (P) and Cr/TiO₂ (P) samples in making the morphology of surface material more homogeneous and spherical type agglomerates with small sizes within the nanometer range and higher surface area compared to impregnation synthesized samples which this can cause the catalytic sites to be more available for the reactants and thereby, to better catalytic performance.

Table 2 Catalytic performance comparison of various supported Cr₂O₃ catalysts in ethane oxidative dehydrogenation by CO₂

Sample	T (°C)	Catalytic activity (%)			Reaction conditions		Cr deposition method	TOS (min)	Ref.
		Ethane conversion	CO2 conversion	Yield	Catalyst weight (g)	GHSV (ml gcat^−1 h^−1)
Cr (5%)/ZrO2 (P)	700	47.56	26.05	43.17	0.5	9000	Co-precipitation	300	Present study
Cr (5%)/TiO2 (P)	700	41.08	21.40	39.43	0.5	9000	Co-precipitation	—	Present study
Cr (10%)-Fe (5%)/ZrO2	650	≈54	≈16	≈50	0.2	4500	Co-precipitation	100	54
Cr2O3 (5%)/TiO2	650	5.90	1.60	5.80	1	3600	Impregnation	—	46
Cr2O3 (8%)/SiO2	650	61.40	≈15	55.50	1	3600	Impregnation	240	46
Cr2O3 (5%)/Al2O3	650	19.20	5.50	18.50	1	3600	Impregnation	—	46
Cr2O3 (15%)/ZrO2	650	42.20	37.4	15.19	1	3600	Impregnation		10
Cr (5%)/SBA-15	700	46.30	≈20	43.80	0.2	3600	Impregnation	100	55
Cr (5%)/SiO2	700	30.70	2.60	29.60	0.5	9600	Impregnation	—	27
Cr2O3 (5%)/ZSM-5	650	≈33	—	≈29	0.3	20 000	Impregnation	—	75

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4. Conclusions

This investigation has explored the effects of support and catalyst preparation procedure on catalytic properties and performance over Cr/TiO₂ and Cr/ZrO₂ nanocatalysts. This study confirms that the Cr/ZrO₂ (P) catalyst prepared by the coprecipitation method exhibits the highest ethane conversion and ethylene yield among all the samples and stable catalytic performance during the time on stream for 300 min due to the higher physicochemical properties of Cr/ZrO₂ (P) nanocatalyst and these favoured the better dispersion of active phase and the activation of reactants.

Acknowledgements

The authors gratefully acknowledge Sahand University of Technology for the financial support of the project as well as Iran Nanotechnology Initiative Council for complementary financial support.

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