One-pot hydrothermal synthesis of ZSM-5–CeO₂ composite as a support for Cr-based nanocatalysts: influence of ceria loading and process conditions on CO₂-enhanced dehydrogenation of ethane

Abstract

The oxidative dehydrogenation of ethane in the presence of CO₂ was investigated over a series of Cr impregnated ZSM-5–CeO₂ nanocatalysts with the aim of exploring the ceria addition effect. To this aim, ZSM-5–CeO₂ supports varying in ceria content (0, 5, 10, 15, 30 wt%) were synthesized using a one-pot hydrothermal method. The effect of ceria addition and its content on the structural properties and performance of Cr/ZSM-5 was investigated. The as-synthesized nanocatalysts were characterized by XRD, ICP, N₂ adsorption–desorption, FESEM, HRTEM, FTIR, EDX and TPR-H₂ techniques. Based on the characterization results, a decrease in α-Cr₂O₃ formation and metal particles size as well as an observable increase in the Cr dispersion and reducibility were found. However, CeO₂ doping resulted in the decrease of BET surface area. The variation of ceria content indicates that the surface area, metal dispersion, catalyst reducibility and uniformity of surface particles of samples increased with the ceria content up to 10 wt%. However, a decreasing trend and higher number of agglomerations were observed with further increase of the ceria content. The results revealed that CeO₂ addition not only enhances the catalytic activity of Cr/ZSM-5 but also makes it less sensitive to deactivation during the course of running the reaction for about 5 h. It was also observed that there is optimum ceria content in the ZSM-5–CeO₂ support containing 10% ceria for the best catalytic activity, attributable to better textural properties, smaller number of agglomerations, more uniform dispersion and more reducibility. It effectively dehydrogenated ethane to ethylene with CO₂ at 700 °C even after 5 h on-stream operation, giving 58.5% ethylene yield.

---

1 Introduction

Oxidative dehydrogenation (ODH) of light alkanes to olefins is one of the most interesting methods for the production of olefins^1–3 which are the feedstock for the synthesis of a variety of chemicals. Among light olefins, ethylene is one of the most important intermediates in the petrochemical industry.^4,5 The typical current method for the production of ethylene is thermal cracking. High energy cost because of high reaction temperature, significant side reactions and catalyst deactivation owing to coke deposition as the major drawbacks of the typical processes from an industrial standpoint have greatly prompted research on the ODH of ethane.^6,7 It has been proved that employing CO₂ as a mild oxidant in ethane dehydrogenation is an ecological approach that can improve ethylene yield and selectivity in comparison to molecular oxygen.⁸ Nevertheless, catalytic ethane oxidative dehydrogenation for ethylene production has not been commercialized yet and the research toward highly active catalysts in this process is on-going. Among all of the studied catalysts, the supported chromium oxide-based materials have been known as the most promising ones due to their high activity in presence of CO₂.^9–12 According to the ODH reaction mechanism, activity of Cr-based catalysts depends on catalyst reducibility, acid–base properties and chromium dispersion which are mainly influenced by support nature.^12–16 Among different materials used as support of catalyst, microporous crystalline materials such as ZSM-5 because of their unique properties, including tri dimensional micropore structure, high surface area, high thermal and mechanical stability and controllable acidity have gained much attention.^8,11,17,18 Cr/HZSM-5 nanocatalyst especially with SiO₂/Al₂O₃ ratio higher than 190, effectively dehydrogenated ethane to ethylene with CO₂ compared to other Cr/zeolite catalysts.^10,11 However, its ethylene production efficiency is not still sufficient to be used instead of the conventional ethylene production methods. One strategy to circumvent this problem as much as possible is to enhance Cr reducibility and acid–base character of catalyst by applying the ZSM-5-based composites as support in preparing Cr-based catalysts.^19–22 Ceria because of its unique properties such as high oxygen-storage capacity and facile oxidation/reduction of the Ce⁴⁺/Ce³⁺ redox cycle is widely used in different processes either as an effective promoter or as a supporting material.^19,20,23,24 However, CeO₂-based supports in ODH reactions are suffer from the drawback of relatively low surface areas.^14,20,25 Ceria in the role of promoter helps the thermal stability of supports, the dispersion of supported chromium, the catalyst reducibility and the decrease in coke formation on the catalyst surface. Moreover, ceria through increasing surface basicity can accelerate desorption of adsorbed olefins from the catalyst surface and facilitate the reduction–oxidation cycle, resulting in an increase in the stability and final performance. It is worth noting that CeO₂ and supported Ce-based catalysts are well known to be effective for ODH of ethane in the presence of CO₂.^{15,24,26–28} It has been reported that the redox property of ceria would be enhanced by the presence of chromium.^14,15 Therefore, in chromium catalysts supported on ceria-based supports, Cr and Ce species have mutual interaction which can observably enhance their redox ability and lead to promote catalyst activity and stability. Nevertheless, low surface area, basicity and relatively high cost of ceria don't allow ceria to be used as support promoter in any amount.^25,27 Accordingly, it seems that using ZSM-5/CeO₂ composite as support containing appropriate amount of ceria can enhance activity and stability of Cr/ZSM-5 in ODH reactions. To the best of our knowledge, there is no literature about the addition of CeO₂ to ZSM-5 supported Cr catalyst for the oxidative dehydrogenation of ethane.

In the present study, Cr/ZSM-5–CeO₂ nanocatalysts with different amounts of CeO₂ added during the hydrothermal synthesis of zeolite were prepared and characterized using various techniques for the first time. The effect of CeO₂ addition on the structural and textural properties and catalytic performance of Cr/ZSM-5 for ethane dehydrogenation was investigated, and the optimum amount of CeO₂ loading was determined. Structural properties of as-synthesized samples were investigated by XRD, FESEM, HRTEM, EDX dot-mapping, BET, TPR-H₂ and FTIR analyses, and their catalytic activity for ODH of ethane with CO₂ were assessed. Finally, the influence of some operational parameters such as reaction temperature, CO₂ addition, GHSV and time on stream over the best sample was investigated.

2 Materials and methods

2.1 Materials

Sodium aluminate (Merck), fumed silica (Aldrich, 99.9%), Cr(NO₃)₃·9H₂O (Aldrich, 96%) and Ce(NO₃)₃·6H₂O (Aldrich, 99%) were used as the sources of aluminium, silicon, chromium oxide and ceria, respectively. Also, NH₄OH aqueous solution, NH₄NO₃ aqueous solution, TPABr, NaOH and de-ionized water were consumed as precipitant, ion exchange agent, template, alkaline and reaction medium, respectively in different sections of the synthesis process.

2.2 Nanocatalysts preparation and procedures

Fig. 1 illustrates the schematic flow chart of the procedure used for the synthesis of nanocatalysts. At first, the zeolite-based supports were hydrothermally synthesized from the prepared gel with composition of 0.1TPABr : 0.1Na₂O : 0.0023Al₂O₃ : 1.0SiO₂ : 35H₂O and various amounts of CeO₂ modifier. Typically, weighted sodium aluminate, TPABr and NaOH were dissolved in distilled water under stirring for 1 h, and fumed silica and CeO₂ powder obtained from hydrothermally precipitation of Ce(NO₃)₃·6H₂O aqueous solution, were added in turn. The mixture maintained under vigorous stirring for 24 h. The resulting gel was transferred into autoclave and heated at 150 °C for 144 h. The filtered and washed solid powders were dried at 110 °C for 24 h under air flow, followed by calcination in a static air atmosphere at 550 °C for 12 h to remove organic template and other residuals. To obtain H-form zeolite-based supports, the calcined NaZSM-5-based supports were submitted to the ammonium ion exchange. The NaZSM-5-based supports were refluxed 2 times for 12 h each at 80 °C in 1.0 M NH₄NO₃ solution. After filtration, washing and drying at 110 °C, the products were calcined in air at 500 °C for 4 h. Finally, chromium oxide was deposited on the as-synthesized HZSM-5/CeO₂ composite supports by the wet impregnation method under stirring at 70 °C using the required amount of the aqueous chromium nitrate solution, corresponding to 5 wt% Cr₂O₃ in the final product. All the obtained catalysts were dried at 110 °C for 12 h and then, calcined at 700 °C for 4 h. The resulting Cr-based catalysts depending on the support nature (parent or modified HZSM-5) and ceria content were denoted as Cr/ZSM-5 and Cr/ZSM-5–CeO₂(x), in which the term “x” refers to the nominal weight percent of ceria in the support (x: 5, 10, 15 and 30).

Fig. 1 Synthesis steps of Cr/ZSM-5–CeO₂ nanocatalysts.

2.3 Nanocatalysts characterization

X-ray diffraction (XRD) patterns were obtained on a Bruker D8 advance diffractometer with Cu Kα radiation (λ = 1.54178 Å) to study the crystal structure and crystallinity. The phase identification was made by comparison to the Joint Committee on Powder Diffraction Standards (JCPDSs). The surface morphology of the nanocatalysts was observed by Field Emission Scanning Electron Microscopy (FESEM) analyzer (HITACHI S-4160). The samples studied were covered with a thin film of gold (ion sputtering) to improve conductivity. The surface elemental composition and dispersion of the synthesized catalysts were analysed by energy dispersive X-ray (EDX) on VEGA\\-TESCAN equipped with a BSE detector for elemental analysis. Transmission electron microscopy (TEM) images were taken on a JEOL, JEM-2100 electron microscope operated at 200 kV. Samples were ultrasonically dispersed in ethanol and then, the suspension was deposited on a thin carbon film-coated Cu grid. Characterization of the microstructure parameters, e.g., pore size distribution, specific surface area, and pore volume, of the samples was carried out with N₂ adsorption–desorption isotherms at −196 °C on a Quantachrome ChemBET-3000 instrument. Prior to measurements, the samples were degassed at 200 °C for 30 min. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was applied to determine the content of loaded metals in the synthesized catalysts. The measurements were performed with a Varian Vista-PRO CCD Simultaneous ICP-OES instrument. Infrared spectra of KBr powder-pressed pellets were recorded on a UNICAM 4600 FTIR spectrophotometer. Catalysts reducibility was measured by hydrogen temperature programmed reduction (H₂-TPR) using BELCAT analyser equipped with a TCD detector.

2.4 Nanocatalysts performance test

The C₂H₆/CO₂ oxidative dehydrogenation reaction was carried out in steady-state conditions using a flow type quartz micro-reactor (i.d., 6 mm) packed with 500 mg of the catalyst diluted with quartz chips under atmospheric pressure. The loaded reactor was placed inside a temperature controlled electric furnace. Prior to the reaction, the catalyst was activated in situ at 600 °C in an air stream (15 cm³ min⁻¹) for at least 0.5 h. Catalytic reactions were carried out at temperature range of 600–700 °C with a feed flow rate of 50 ml min⁻¹. The reactant feed consisted of 10 vol% C₂H₆, 50 vol% CO₂, and balance N₂. For the tests without CO₂, it was replaced by nitrogen to keep the same total flow of 50 ml min⁻¹. The reactants and products were analyzed by an online gas chromatograph (GC Chrom, Teif Gostar Faraz, Iran) equipped with TCD and FID detectors and a methanizer, using a Carboxen™ 1000 column (Agilent Co.). Before analysing, the effluent stream was passed through an ice-cooled trap to condensate and separate H₂O. Blank tests performed under the experimental conditions of this work excluded the occurrence of thermal dehydrogenation of ethane. Carbon balances were found to be 100 ± 3%. The catalyst behavior was evaluated in terms of the ethane conversion (X_C2H6), ethylene selectivity (S_C2H4) and ethylene yield (Y_C2H4) calculated as follows:where, F_i is the molar flow rate of each component. Moreover, the reaction data in the work were reproducible with a precision of less than 3%.

3 Results and discussions

3.1 Nanocatalyst characterization

3.1.1 XRD analysis. The XRD patterns of the as-synthesized nanocatalysts are shown in Fig. 2. All catalysts present the characteristic peaks of HZSM-5 zeolite in the tetragonal phase (JCPDS: 00-044-0002). Sharp and intense peaks in the ranges of 8–9° and 22–25° in all of the samples feature the highly crystalline ZSM-5 phases. Moreover, diffraction peaks at 2θ = 28.8, 33.2, 47.7, 56.6, 59.4, 69.8, 77.1, 79.5, 88.9° are related to CeO₂ in the cubic phase (JCPDS: 01-075-0076). Existence of these peaks together with the characteristic peaks of HZSM-5 observed in CeO₂ containing samples confirms successfully synthesis of ZSM-5/CeO₂ composites. A glancing over the XRD patterns shows that with increasing CeO₂ content in the composite supports, the intensity of CeO₂ peaks increases and in contrast, the intensity of ZSM-5 peaks decreases. In spite of the weak intensity, the diffraction peaks at about 2θ = 36.2, 54.9 and 63.4° assigned to the rhombohedra ordered structure of Cr₂O₃ (JCPDS: 00-006-0504) can be observed for all the samples. As can be seen, the diffraction peaks intensity of Cr species decreases after loading of CeO₂. This result suggests that ZSM-5/CeO₂ composite support markedly enhances the dispersion of Cr₂O₃ active phase in the synthesized nanocatalysts in comparison to parent ZSM-5. Enhanced dispersion of Cr₂O₃ over the composite supports can be also proved by the TPR-H₂ and EDX analyses in the next sections.

Fig. 2 XRD patterns of the synthesized nanocatalysts: (a) Cr/ZSM-5, (b) Cr/ZSM-5–CeO₂(5), (c) Cr/ZSM-5–CeO₂(10), (d) Cr/ZSM-5–CeO₂(15) and (e) Cr/ZSM-5–CeO₂(30).

3.1.2 FESEM analysis. Fig. 3 shows the surface morphology of the synthesized samples. It can be observed from the recorded images that the surface of coffin-like micro-particles related to the ZSM-5 structure has been covered with great amount of small nano-scale particles. This observation confirms that all samples have nanometric surface particles. Nanoparticles provide more reactive and reducible sites which result in a superior catalytic performance of the catalyst. In the FESEM images of the CeO₂ containing samples, irregular aggregations of nanoparticles are observed over the ZSM-5 particles. Qualitatively comparison of the FESEM images indicates that, the size and number of the particles agglomeration increase with gradually increasing the ceria content, which make it difficult to sight the coffin-like morphology of the ZSM-5 particles. Detail examination of the images reveals that the agglomerated particles on the surface of the Cr/ZSM-5–CeO₂(10) nanocatalyst have uniform distribution and smaller size in comparison to others. In addition, a better contact between the surface particles and zeolite support can be realized from the FESEM images of the Cr/ZSM-5–CeO₂(10) sample.

Fig. 3 FESEM images of synthesized nanocatalysts: (a) Cr/ZSM-5, (b) Cr/ZSM-5–CeO₂(5), (c) Cr/ZSM-5–CeO₂(10), (d) Cr/ZSM-5–CeO₂(15) and (e) Cr/ZSM-5–CeO₂(30).

3.1.3 ICP analysis. The chemical composition of the as-synthesized samples obtained by ICP analysis is included in Table 1. As can be seen, the content of loaded metals is near to their nominal contents. This result, together with access to the required crystalline phases is an evidence for the successful synthesis.

Table 1 Chemical composition of the synthesized samples

Nanocatalyst	Synthesis method	CeO2 content in support (wt%)	Si/Al (mol/mol)	Ce content (wt%)		Cr content (wt%)
				Nominal	ICP	Nominal	ICP
Cr/ZSM-5	Impregnation/hydrothermal	0	220	—	—	3.4	3.2
Cr/ZSM-5–CeO2(5)	Impregnation/one-pot hydrothermal	5	220	3.8	3.4	3.4	3.3
Cr/ZSM-5–CeO2(10)	Impregnation/one-pot hydrothermal	10	220	7.7	7.5	3.4	3.2
Cr/ZSM-5–CeO2(15)	Impregnation/one-pot hydrothermal	15	220	11.6	11.3	3.4	3.3
Cr/ZSM-5–CeO2(30)	Impregnation/one-pot hydrothermal	30	220	23.1	22.7	3.4	3.3

---

3.1.4 EDX analysis. The elemental composition and dispersion of Cr/ZSM-5 and Cr/ZSM-5–CeO₂ with different CeO₂ loadings were examined by energy dispersive X-ray spectroscopy (EDX). The obtained results (see Fig. 4) demonstrate the origination of well-defined peaks related to Cr, Al, Si, Ce and O elements. No other peak related with any impurity was detected in the EDX spectra within the detection limit of EDX. Moreover, the Si/Al ratio in the prepared samples and corresponding initial gels is close to each other. These results are additional evidences for the successful synthesis. By comparing the EDX dot-mappings of Cr element for the ceria containing samples with that of the Cr/ZSM-5 sample, a higher degree of Cr dispersion can be realized, which is in good agreement with the XRD results. This reflects more ability of the ZSM-5-based supports to uniformly disperse Cr species in comparison to parent ZSM-5 support. Among the ZSM-5/CeO₂ composites, one containing 10 wt% ceria enables an extremely uniform dispersion of deposited Cr species on the surface which is going to affect the catalyst reducibility and thereupon, catalytic performance. This feature could be explained by better contact between Cr species and CeO₂ modifier as a result of the appropriate content and apt dispersion of ceria as evidenced by EDX dot-mappings of Ce element. By gradually increasing CeO₂ content from 10 to 30 wt%, accumulation of Ce elements in some regions and thus, a decrease in dispersion can be clearly found which was expected based on the high content of CeO₂ loading and low surface area of the composite support.

Fig. 4 EDX analysis of synthesized nanocatalysts: (a) Cr/ZSM-5, (b) Cr/ZSM-5–CeO₂(5), (c) Cr/ZSM-5–CeO₂(10), (d) Cr/ZSM-5–CeO₂(15) and (e) Cr/ZSM-5–CeO₂(30).

3.1.5 HRTEM analysis. TEM images of the Cr supported on bare ZSM-5 and the ZSM-5/CeO₂ composites are shown in Fig. 5. The black spots on the bright background are particles of the metals deposited on the surface of the support. It is obvious that these metals particles are chromia in the Cr/ZSM-5 sample while they can be chromia or ceria in the case of ceria containing sample. Comparing with the captured TEM images of Cr/ZSM-5, much smaller and uniform spherical nanoparticles containing CrO_X which are homogeneously distributed on the support, are observed in the TEM images of Cr/ZSM-5–CeO₂(10%) sample. This observation implies the remarkable synergetic effect of composite support on the distribution and size of metallic particles. The presence of CeO₂ modifier can increase the interaction between chromium and the support and suppress the migration of metallic particles, thus enhancing metal dispersion effectively and decreasing the size of loaded metallic particles. The obtained results are in highly consistent with EDX results in which a better dispersion of loaded metals was achieved by CeO₂ addition.

Fig. 5 TEM images of synthesized nanocatalysts: (a) Cr/ZSM-5 and (b) Cr/ZSM-5–CeO₂(10).

3.1.6 N₂ adsorption–desorption analysis. The nitrogen physisorption isotherms of the synthesized catalysts are depicted in Fig. 6 and the detailed textural properties derived from these isotherms are summarized in Table 2. It can be seen that all samples exhibit typical reversible type-I isotherms (according to the IUPAC classification), which are characteristic of microporous materials. Regarding to the textural properties given in Table 2, the ceria containing samples exhibited lower surface areas and pore volumes in comparison to the Cr/ZSM-5 sample. This can be addressed by mainly worse textural properties of ceria modifier compared to ZSM-5 and partly, the blockage of some the ZSM-5 pores by the surface CeO₂ particles. The gradually variation of CeO₂ content indicates that the textural properties enhance with the ceria content up to 10 wt%. However, a decreasing trend is observed with further increase of CeO₂ content. This can be justified by a decrease in the dispersion of loaded metals and also an increase in the number and size of conglutinated particles, as indicated by FESEM and EDX results. Among the Cr/ZSM-5–CeO₂ samples, Cr/ZSM-5–CeO₂(10) nanocatalyst possesses the highest BET and external surface areas and pore volume. Moreover, a detail examination of the textural data reveals that the fraction of external surface area is maintained in high level with ceria addition only up to 10 wt%. This feature helps to the decrease of the diffusion resistance and enhancement of mass and heat transfer, resulting in a high catalytic performance.

Fig. 6 Nitrogen adsorption–desorption isotherms of the synthesized nanocatalysts: (a) Cr/ZSM-5, (b) Cr/ZSM-5–CeO₂(5), (c) Cr/ZSM-5–CeO₂(10), (d) Cr/ZSM-5–CeO₂(15) and (e) Cr/ZSM-5–CeO₂(30).

Table 2 Textural characteristics of the synthesized samples

Nanocatalyst	Surface area (m^2 g^−1)			Pore volume (cm^3 g^−1)
	BET	External	Micropore
Cr/ZSM-5	395	228	167	0.21
Cr/ZSM-5–CeO2(5)	367	202	165	0.191
Cr/ZSM-5–CeO2(10)	376	217	159	0.20
Cr/ZSM-5–CeO2(15)	334	166	168	0.184
Cr/ZSM-5–CeO2(30)	270	136	134	0.179

---

3.1.7 FTIR analysis. FTIR spectra of the Cr-based catalysts in a frequency range of 400–4000 cm⁻¹ are depicted in Fig. 7. The vibration frequency peaks at 450, 560, 640, 810, 1110, 1240, 1640 and 3450 cm⁻¹ can be recognized from FTIR spectra of all the nanocatalysts. The absorption bands appearing at about 450, 560, 810, 1110 and 1240 cm⁻¹ are typical characteristic vibrations in MFI type zeolites.^21,29–32 In detail, IR band at 470 cm⁻¹ are ascribed to the vibrations of internal bonds (T–O) of SiO₄ and AlO₄ tetrahedra.³³ The vibrations located at about 560 cm⁻¹ are ascribed to external bonds of double five member rings.³⁴ The bands observed at about 810 cm⁻¹ are related to symmetric stretching of external bonds between tetrahedral.^21,30 The strongest absorption peak appeared around 1110 cm⁻¹ corresponds to the internal asymmetric stretching of Si–O–T bonds.^30,35 Moreover, there is an asymmetric stretch vibration of the T–O bond at 1240 cm⁻¹ which has been assigned to external linkages between TO₄ tetrahedral.³⁶ Appearance of these peaks in FTIR spectra of all the synthesized samples is indicative that ZSM-5 structure remains intact after modification and Cr impregnation. This is in line with XRD results. The existence of IR peak around 640 cm⁻¹ which is a characteristic for α-Cr₂O₃ phase of chromium oxide^37,38 confirms the presence of Cr₂O₃ crystallites and is in good consistency with XRD results. It should be pointed that the recorded peaks at around 450, 560 and 1110 cm⁻¹ could be also related to M–O stretching, Cr–O distortion and M–OH stretching vibrations, respectively,³⁸ which have overlaps with zeolitic vibrations. The peak at 1649 cm⁻¹ is assigned to the O–H bending vibrating mode of the interlayer water molecules, caused by the physically adsorbed water molecules.^20,39,40 At last, the broad peak at 3450 cm⁻¹ characterizes the stretching vibrations of bridging hydroxyl groups.^41–43

Fig. 7 FTIR spectra of synthesized nanocatalysts: (a) Cr/ZSM-5, (b) Cr/ZSM-5–CeO₂(5), (c) Cr/ZSM-5–CeO₂(10), (d) Cr/ZSM-5–CeO₂(15) and (e) Cr/ZSM-5–CeO₂(30).

3.1.8 TPR-H₂ analysis. As well known, the ODH of alkanes over supported transition-metal oxides follows a redox mechanism, in which the reducibility serves as a key factor determining the catalytic performance in the reaction.^9,14 TPR-H₂ profiles and H₂ consumptions of the as-prepared catalysts are illustrated in Fig. 8. The Cr/ZSM-5 sample shows only two reduction peaks, centred at 411 and 547 °C which can be assigned to the reduction of Cr⁶⁺ (ref. 8 and 11) and Cr³⁺ (ref. 44) species directly attached to the zeolite material, respectively (Cr⁶⁺/ZSM-5 and Cr³⁺/ZSM-5). Upon CeO₂ addition to HZSM-5 support, the low temperature reduction peak of the Cr/ZSM-5 sample shifts to lower temperature and the weak peak corresponding to Cr³⁺ reduction disappears, indicating a significant improvement in the dispersion of Cr species. This is in line with XRD and EDX results which confirm the enhancement of Cr dispersion over the composite supports. Moreover, a shoulder and an additional weak reduction peak in the temperature range of 200–500 °C (Zone-I) appear on the profiles of samples with ceria content more than 5 wt%, corresponding to the reduction of surface Ce particles^14,15,25 and most probably, Cr⁶⁺ species dispersed on CeO₂ (Cr⁶⁺/CeO₂), respectively. This observation confirms the presence of at least two types of the Cr⁶⁺ species with different reducibility in the CeO₂ containing samples. The Cr⁶⁺/CeO₂ species are harder to reduce compared to Cr⁶⁺/ZSM-5 species. At low CeO₂ loading, the Cr⁶⁺ species directly attached to ZSM-5 dominate, whereas at higher CeO₂ contents, both Cr⁶⁺/ZSM-5 and Cr⁶⁺/CeO₂ species co-exist. One sharp peak at 500–800 °C (Zone-II) are also observed which can be assigned to the bulk ceria reduction.^14,15,25 As a result of more uniform dispersion of metal species in the Cr/ZSM-5–CeO₂(10) sample, all of the reduction peaks corresponding to surface Ce and Cr species appear in lower temperatures in comparison to other ceria containing samples, indicating improvement in the reducibility of loaded Ce and Cr species in this sample. The above results also indicate that the appropriate addition of Ce species can further increase catalyst reducibility, which can well account for their different initial catalytic activities in the ODH of ethane.

Fig. 8 TPR-H₂ analysis of synthesized nanocatalysts: (a) Cr/ZSM-5, (b) Cr/ZSM-5–CeO₂(5), (c) Cr/ZSM-5–CeO₂(10) and (d) Cr/ZSM-5–CeO₂(15).

3.2 Catalytic performance toward C₂H₆/CO₂ oxidative dehydrogenation

3.2.1 Effect of CeO₂ loading. The ethane conversion and ethylene yield over the as-synthesized nanocatalysts at different reaction temperatures are depicted in Fig. 9(a) and (b), respectively. The equilibrium conversions were also calculated from the thermodynamics estimation of the process described in literature⁴⁵ under same operational conditions and depicted in Fig. 9(a). All the experiment conversions are below the equilibrium ones as maximum possible conversion which can be verification for the authenticity of the measured data. The catalytic activity of the nanocatalysts is found to be strongly dependent on the reaction temperature and support composition. As can be seen, the ethane conversion and the ethylene yield are close to each other in whole temperature range, implying the high ethylene selectivity. This can be attributed to the favorable acidity behavior as a result of high Si/Al ratio applied and ceria loading. A comparison of catalytic performance of samples shows that employing composite supports can significantly increase the initial catalytic activity of Cr/ZSM-5 sample over all the examined temperatures. This improvement in catalytic activity can be explained by the enhanced Cr dispersion, smaller surface particle size and improved reduction behaviour in Cr/ZSM-5–CeO₂ nanocatalysts which were proved by EDX dot mapping, HRTEM and TPR-H₂ results, respectively. In addition, higher catalytic activity of Cr/ZSM-5–CeO₂ nanocatalysts may also be induced by the presence of cerium species, which are effective for the ODH of ethane with CO₂. Apart from improving the structural and textural properties of Cr/ZSM-5 catalyst, CeO₂ because of its redox properties can participate in the activation of CO₂ and produce active oxygen species for the ODH reaction leading to enhanced catalytic activity. After CeO₂ loading from 5 to 10%, the catalytic activity has an obvious enhancement. The ethane conversion of 70.4% and C₂H₄ yield of 61.1% can be achieved over the Cr/ZSM-5–CeO₂(10) nanocatalyst. By further increasing ceria content of the composite support, the catalytic activity exhibits decreasing trend. The different initial catalytic activities of the samples in the ODH of ethane can well be attributed to the difference in their Cr dispersion and reducibility behaviour. These results are agreement with the apparent activation energy (E_a) values of the samples as presented in Table 3 which were determined from Arrhenius-plots for the ethane conversion data over the Cr/ZSM-5–CeO₂ nanocatalysts. The apparent activation energy characterizes the temperature dependence of the rate limiting step for the ethane conversion to ethylene. It can be seen that the activation energy of the Cr/ZSM-5–CeO₂ nanocatalysts is in the range of 88–116 kJ mol⁻¹ and the lowest activation energy (88 kJ mol⁻¹) belongs to the Cr/ZSM-5–CeO₂(10) nanocatalyst. These values are in good agreement with previously published values from Mimura,^10,11 who found energy barriers for the oxidative dehydrogenation of ethane with CO₂ over Cr/HZSM-5 catalysts in the range of 90–110 kJ mol⁻¹. The difference in the activation energy suggests that both the number and nature of activity sites have changed after adding ceria and altering its content in the Cr/ZSM-5 catalyst.

Fig. 9 Effect of temperature and CeO₂ loading on catalytic performance of synthesized nanocatalysts: (a) C₂H₆ conversion and (b) C₂H₄ yield.

Table 3 Kinetic data and initial activity results of the Cr/ZSM-5–CeO₂ nanocatalysts^a

Catalyst	Rate of ethane conversion (molC2H6 g^−1 s^−1)	TOF (h^−1)	Apparent activation energy Ea (kJ mol^−1)	Productivity (gprod gcat^−1 h^−1)
a Reaction conditions: CO2 : C2H6 : N2 volume ratio = 5 : 1 : 4; GHSV = 6000 h^−1; T = 700 °C.
Cr/ZSM-5	36.61 × 10^−7	21.42	116	0.34
Cr/ZSM-5–CeO2(5)	43.65 × 10^−7	24.76	107	0.41
Cr/ZSM-5–CeO2(10)	52.39 × 10^−7	30.64	88	0.46
Cr/ZSM-5–CeO2(15)	47.38 × 10^−7	26.88	97	0.42
Cr/ZSM-5–CeO2(30)	44.54 × 10^−7	25.26	103	0.40

---

Based on the initial activity data, reaction rate normalized per catalyst weight, the turnover frequency (TOF) and productivity of the synthesized nanocatalysts were also obtained and summarized in Table 3. As expected, the results clearly show a similar trend to that observed for the ethane conversion and ethylene yield with increasing the ceria content in the composite support of Cr-based catalysts. A plausible explanation to this observation could be related to the different amount of suitable and accessible CrO_X species which are crucial for high activity. As can be seen, the TOF number of Cr/ZSM-5–CeO₂(10) is higher than the other catalysts in the range of measured temperatures, suggesting that active Cr sites in Cr/ZSM-5–CeO₂(10) is more effective to build ethylene. Considering the above, it can be concluded that 10 wt% of ceria is an appropriate content in ZSM-5–CeO₂ composite as support for the best catalytic activity.

3.2.2 Effect of CO₂ addition on catalytic performance. It was shown in literature that CO₂ can enhance or depress the dehydrogenation activity depending on catalyst nature. Fig. 10 displays the results of the dehydrogenation reaction over the Cr/ZSM-5–CeO₂(10) catalyst depending on the presence or the absence of carbon dioxide. As can be seen, in the presence of CO₂ the ethane conversion and ethylene yield are significantly higher than those without CO₂ and only in N₂ atmosphere. CO₂ improves the catalytic activity mainly via a redox mechanism in which the catalyst undergoes reduction (by ethane) and reoxidation (by CO₂) cycles. Moreover, reduction of coke deposition, the promotion of equilibrium conversion by diluting feed and/or reverse water–gas shift reaction, and poisoning the non-selective sites have been also proposed for explaining the promoting effect of CO₂ on the dehydrogenation reactions over various catalysts in literature.^9,11,13–15

Fig. 10 Effect of CO₂ addition on C₂H₆ conversion and C₂H₄ yield over Cr/ZSM-5–CeO₂(10) nanocatalyst.

3.2.3 Effect of GHSV on catalytic performance. From an application point of view, it is crucial to study parameters such as GHSV. Fig. 11 depicts the effect of GHSV on the catalytic activity of Cr/ZSM-5–CeO₂(10) in terms of C₂H₆ conversion and C₂H₄ yield. As can be seen, both C₂H₆ conversion and C₂H₄ yield decrease by increasing GHSV. With increasing space velocity, residence time or contact time between feed molecules and catalyst surface decreases and some of the feed remains unreacted which leads to decrease in catalytic activity. However even in high 9000 h⁻¹ GHSV, Cr/ZSM-5–CeO₂(10) with 52% ethane conversion and 47% ethylene yield still exhibits good performance.

Fig. 11 Effect of GHSV on C₂H₆ conversion and C₂H₄ yield over Cr/ZSM-5–CeO₂(10) nanocatalyst.

3.2.4 Time on stream performance. Apart from the catalyst activity, stability is a key factor for the catalyst from application point of view. To this aim, the time on stream performance of the best catalyst, Cr/ZSM-5–CeO₂(10), was continuously evaluated for 5 h under the optimum reaction conditions and illustrated in Fig. 12, together with that of Cr/ZSM-5 for comparison and further evaluation of the CeO₂ loading effect on the performance. Over the Cr/ZSM-5 catalyst, the ethane conversion and ethylene yield drop gradually during the 300 min-on-stream, suggesting some deactivation of the catalyst. It is well known that both the coke formation and reduction of redox Cr species are responsible for the deactivation of dehydrogenation catalysis. On the other hand, the activity of the ceria promoted catalyst, Cr/ZSM-5–CeO₂(10), is stable at least for 5 h time on stream (TOS) and no noticeable deactivation is detected during the reaction time. This is evidently indicating the promoting effect of ceria on sustaining the activity in the ethane dehydrogenation with CO₂. The fact that the ZSM-5–CeO₂(10%) composite is characterized by probably an appropriate acid–base character, redox property and high potential of metal dispersion. These features appear to be the key factors for achieving the high stability in the titled reaction. The presence of CeO₂ can help to suppress the carbon deposition by the decrease of the surface acidity, easy desorption of adsorbed ethylene from the surface and the gasification of carbon formed during the reaction. Additionally, the redox Ce⁴⁺/Ce³⁺ of CeO₂ can be used for the activation of CO₂ and facilitate the re-oxidation of reduced redox Cr species, alleviating the deactivation caused by reduced redox sites. The extremely uniform dispersion of Cr species, much smaller and uniform deposited particles and superior Cr-support interaction, in accordance with HRTEM and EDX images are other reasons for the maintenance of catalytic stability. The deposition of very small Cr particles in close contact with the support assists to facile oxygen mobility of catalyst which is extremely detrimental for the stable catalytic activity during the reaction.

Fig. 12 Time on stream behaviour of synthesized nanocatalysts: (a) Cr/ZSM-5 and (b) Cr/ZSM-5–CeO₂(10).

4 Conclusions

With the aim of improving Cr/ZSM-5 catalyst structural properties and consequently its performance toward ODH reaction, a series of Cr/ZSM-5–CeO₂ catalysts were synthesized with different ceria loadings to find an appropriate CeO₂ content. CeO₂ loading resulted in a significant modification of the redox and acid–base properties of the impregnated Cr/ZSM-5 catalyst. According to the characterization results, ceria addition can enhance the active phase dispersion, uniformity of surface particles, metallic particles size and reducibility effectively.

Moreover, the results reveal that the level of synergetic effect of ceria addition strongly depends on its content. The uniformly dispersed and highly reducible metal nanoparticles on the surface of composite support containing 10 wt% CeO₂ were obtained. However, with the excessive addition of CeO₂ promoter, agglomerations of metallic particles on the catalyst surface and a significant decrease in the surface area and metals dispersion were observed. Considering the features induced by CeO₂ addition, employing composite supports can significantly increase not only the catalytic activity but also the stability of the Cr/ZSM-5 sample. The Cr/ZSM-5–CeO₂ catalysts exhibit excellent catalytic activity at the ODH of ethane with CO₂. It was found that there is the optimum ceria content in Cr/ZSM-5–CeO₂(10) for the best catalytic activity. The 61.1% ethylene yield can be obtained over Cr/ZSM-5–CeO₂(10) catalyst at 700 °C.

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.

References

1. A. Qiao, V. N. Kalevaru, J. Radnik and A. Martin, Catal. Today, 2016, 264, 144–151.
2. L. Kong, J. Li, Z. Zhao, Q. Liu, Q. Sun, J. Liu and Y. Wei, Appl. Catal., A, 2016, 510, 84–97.
3. A. H. Elbadawi, M. S. Osman, S. A. Razzak and M. M. Hossain, J. Taiwan Inst. Chem. Eng., 2016, 61, 106–116.
4. E. V. Ishchenko, T. Y. Kardash, R. V. Gulyaev, A. V. Ishchenko, V. I. Sobolev and V. M. Bondareva, Appl. Catal., A, 2016, 514, 1–13.
5. A. H. Elbadawi, M. S. Ba-Shammakh, S. Al-Ghamdi, S. A. Razzak, M. M. Hossain and H. I. de Lasa, Chem. Eng. Sci., 2016, 145, 59–70.
6. S. Deng, S. Li, H. Li and Y. Zhang, Ind. Eng. Chem. Res., 2009, 48, 7561–7566.
7. R. Wu, P. Xie, Y. Cheng, Y. Yue, S. Gu, W. Yang, C. Miao, W. Hua and Z. Gao, Catal. Commun., 2013, 39, 20–23.
8. Y. Cheng, F. Zhang, Y. Zhang, C. Miao, W. Hua, Y. Yue and Z. Gao, Chin. J. Catal., 2015, 36, 1242–1248.
9. P. Michorczyk, J. Ogonowski and K. ZeÅ„czak, J. Mol. Catal. A: Chem., 2011, 349, 1–12.
10. N. Mimura, M. Okamoto, H. Yamashita, S. T. Oyama and K. Murata, J. Phys. Chem. B, 2006, 110, 21764–21770.
11. N. Mimura, I. Takahara, M. Inaba, M. Okamoto and K. Murata, Catal. Commun., 2002, 3, 257–262.
12. F. Rahmani, M. Haghighi and M. Amini, J. Ind. Eng. Chem., 2015, 31, 142–155.
13. P. Michorczyk, P. Pietrzyk and J. Ogonowski, Microporous Mesoporous Mater., 2012, 161, 56–66.
14. X. Shi, S. Ji and K. Wang, Catal. Lett., 2008, 125, 331–339.
15. X. Shi, S. Ji, K. Wang and C. Li, Catal. Lett., 2008, 126, 426–435.
16. F. Rahmani and M. Haghighi, J. Nat. Gas Sci. Eng., 2015, 27, 1684–1701.
17. Z. Shen, J. Liu, H. Xu, Y. Yue, W. Hua and W. Shen, Appl. Catal., A, 2009, 356, 148–153.
18. F. Zhang, R. Wu, Y. Yue, W. Yang, S. Gu, C. Miao, W. Hua and Z. Gao, Microporous Mesoporous Mater., 2011, 145, 194–199.
19. S. J. Hassani Rad, M. Haghighi, A. Alizadeh Eslami, F. Rahmani and N. Rahemi, Int. J. Hydrogen Energy, 2016, 41, 5335–5350.
20. F. Rahmani, M. Haghighi and P. Estifaee, Microporous Mesoporous Mater., 2014, 185, 213–223.
21. F. Rahmani, M. Haghighi, Y. Vafaeian and P. Estifaee, J. Power Sources, 2014, 272, 816–827.
22. K. N. Rao, B. M. Reddy and S.-E. Park, Appl. Catal., B, 2010, 100, 472–480.
23. P. Estifaee, M. Haghighi, N. Mohammadi and F. Rahmani, Ultrason. Sonochem., 2014, 21, 1155–1165.
24. Y. Xu and V. C. Corberan, Prog. Nat. Sci., 2000, 10, 22–26.
25. Z. Abbasi, M. Haghighi, E. Fatehifar and S. Saedy, J. Hazard. Mater., 2011, 186, 1445–1454.
26. M. Guío, J. Prieto and V. C. Corberán, Catal. Today, 2006, 112, 148–152.
27. R. X. Valenzuela, G. Bueno, V. C. Corberán, Y. Xu and C. Chen, Catal. Today, 2000, 61, 43–48.
28. X. Ge, S. Hu, Q. Sun and J. Shen, J. Nat. Gas Chem., 2003, 12, 119–122.
29. Y. Vafaeian, M. Haghighi and S. Aghamohammadi, Energy Convers. Manage., 2013, 76, 1093–1103.
30. S. Allahyari, M. Haghighi, A. Ebadi and S. Hosseinzadeh, Ultrason. Sonochem., 2014, 21, 663–673.
31. S. Sadeghi, M. Haghighi and P. Estifaee, J. Nat. Gas Sci. Eng., 2015, 24, 302–310.
32. I. Othman Ali, Mater. Sci. Eng., A, 2007, 459, 294–302.
33. M. A. Ali, B. Brisdon and W. J. Thomas, Appl. Catal., A, 2003, 252, 149–162.
34. J. C. Jansen, F. J. van der Gaag and H. van Bekkum, Zeolites, 1984, 4, 369–372.
35. I. C. L. Barros, V. S. Braga, D. S. Pinto, J. L. de Macedo, G. N. R. Filho, J. A. Dias and S. C. L. Dias, Microporous Mesoporous Mater., 2008, 109, 485–493.
36. Y. Cheng, L.-J. Wang, J.-S. Li, Y.-C. Yang and X.-Y. Sun, Mater. Lett., 2005, 59, 3427–3430.
37. T. Ivanova, K. Gesheva, A. Cziraki, A. Szekeres and E. Vlaikova, J. Phys.: Conf. Ser., 2008, 113, 1–5.
38. H. R. Mahmoud, J. Mol. Catal. A: Chem., 2014, 392, 216–222.
39. R. Khoshbin and M. Haghighi, Catal. Sci. Technol., 2014, 4, 1779–1792.
40. E. Aghaei and M. Haghighi, Microporous Mesoporous Mater., 2014, 196, 179–190.
41. S. M. Sajjadi, M. Haghighi, A. A. Eslami and F. Rahmani, J. Sol–Gel Sci. Technol., 2013, 67, 601–617.
42. S. M. Sajjadi, M. Haghighi and F. Rahmani, J. Sol–Gel Sci. Technol., 2014, 70, 111–124.
43. E. Aghaei and M. Haghighi, Powder Technol., 2015, 269, 358–370.
44. Y. Ramesh, P. Thirumala Bai, B. Hari Babu, N. Lingaiah, K. S. Rama Rao and P. S. S. Prasad, Appl. Petrochem. Res., 2014, 1–6.
45. M. A. Botavina, G. Martra, Y. A. Agafonov, N. A. Gaidai, N. V. Nekrasov, D. V. Trushin, S. Coluccia and A. L. Lapidus, Appl. Catal., A, 2008, 347, 126–132.

---