Specific surface areas (
Due to the harmful effects of carbon dioxide to the environment, a lot of work has been carried out aiming to find new applications, which can decrease the emissions or to capture and use it. An attractive application for carbon dioxide is the synthesis of chemicals, especially for producing styrene by ethylbenzene dehydrogenation, in which it increases the catalyst activity and selectivity. In order to find efficient catalysts for the reaction, the effect of cerium, chromium, aluminum, and lanthanum on the properties of zirconia‐supported iron oxides was studied in this work. The modified supports were prepared by precipitation and impregnated with iron nitrate. The obtained catalysts were characterized by thermogravimetry, Fourier transform infrared spectroscopy, X‐ray diffraction, specific surface area measurement, and temperature‐programmed reduction. The catalysts showed different textural and catalytic properties, which were associated to the different phases in the solids, such as monoclinic or tetragonal zirconia, hematite, maghemite, cubic ceria, monoclinic or hexagonal lantana, and rhombohedral chromia, the active phases in ethylbenzene dehydrogenation. The most promising dopant was cerium, which produces the most active catalyst at the lowest temperature, probably due to its ability of providing lattice oxygen, which activates carbon dioxide and increases the reaction rate.
- carbon dioxide
- iron oxide
Although greenhouse gas emissions are reaching alarming rates, 80% of the world's energy consumption still comes from fossil fuels, which have been pointed out as the largest source of carbon dioxide emissions . Over the past decade, the global emissions of carbon dioxide from fossil fuels have increased by 2.7% each year and currently are 60% above the levels registered in 1990, which is considered the reference year for the Kyoto Protocol . On the other hand, it is expected that carbon dioxide emissions should reduce by at least 50% to limit the rise of the global average temperature to 2°C by 2050 . Nowadays, the major environmental concerns worldwide, global warming and the acidification of the oceans, are mainly ascribed to the increase of carbon dioxide concentration [4, 5]. Therefore, several alternatives have been proposed to decrease the carbon dioxide concentration and then to mitigate the environment changes. They include demand‐side conservation, supply‐side efficiency improvement, increasing reliance on nuclear and renewable energy, and carbon capture and storage (CCS) systems . Among them, CCS is considered the most practical approach for long‐term carbon dioxide emission reductions, since fossil fuels will continue to be the major source of energy in the next future. However, there are still some technical and economic barriers to be overcome before it can be used on a large scale. One of the main obstacles is the required large capital investment, besides technical difficulties, such as carbon dioxide leakage rates and limited geological storage capacity. Other drawbacks include the costs of transportation and injection when carbon dioxide is only available offshore, such as in United Kingdom, Norway, Singapore, Brazil, and India [7, 8]. Therefore, a more suitable alternative is to capture and use carbon dioxide (carbon capture and utilization [CCU]), changing the waste carbon monoxide emissions into valuable products such as chemicals and fuels, while contributing to climate change mitigation .
Captured carbon dioxide can be used as a commercial product, both directly or after conversion. In food and drink industries, for instance, carbon dioxide is often used as a carbonating agent, preservative, packaging gas, and for extracting flavors, as well as in the decaffeination process. In the pharmaceutical industry, it is used as a respiratory stimulant or for the synthesis of drugs. However, these applications are restricted to high‐purity carbon dioxide, as that obtained in ammonia plants [9, 10]. Moreover, pressurized carbon dioxide has been investigated for wastewater treatment and water disinfection . Other direct applications of carbon dioxide include enhanced oil recovery and coal‐bed methane recovery, where crude oil is extracted from an oil field or natural gas from unminable coal deposits .
In the production of chemicals and fuels, carbon dioxide has attracted increasing attention over several decades, for the synthesis of various fine and bulk chemicals. It has already been used in the industrial production of urea, cyclic carbonates, salicylic acid, and methanol . It is expected that carbon dioxide can produce feedstock for chemical, pharmaceutical, and polymer industries by carboxylation reactions to obtain organic compounds, such as carbonates, acrylates, and polymers, or by reduction reactions, where the C=O bonds are broken to produce chemicals such as methane, methanol, syngas, urea, and formic acid [9, 13]. Carbon dioxide can have several other applications, both as carbon or oxygen sources, for the synthesis of chemicals by several processes, as solvent and/or as reactants. It has potential applications in supercritical conditions, in direct carboxylation reactions, in the conversion of natural gas to liquid (GTL technology), and in methanol synthesis . Carbon dioxide can also act as an oxidant in the dehydrogenation of ethane , propane , isobutene , and ethylbenzene [18–20], as well as in methane dry reforming  and oxidative coupling of methane . It is expected that the 115 million metric tons of carbon dioxide, currently consumed every year as feedstock in a variety of synthetic processes, can be triplicated by the use of new technologies . In addition, carbon dioxide can overcome several drawbacks of the processes, especially in the case of dehydrogenation reactions.
In industrial processes, the dehydrogenation of hydrocarbons is often carried out at high temperatures to increase the conversion because of its reversibility and limitation by thermodynamic equilibrium. Besides being an energy‐consuming process, the high temperatures cause the hydrocarbons cracking, decreasing the selectivity. On the other hand, by the oxidation of the produced hydrogen or by using an oxidant in the presence of a catalyst, these difficulties can be overcome, since the oxidative dehydrogenation is exothermic and can be performed at low temperatures, making negligible the formation of cracking products. Therefore, the use of an oxidant increases the catalyst selectivity and decreases the undesirable products, besides other advantages. Among the oxidizing agents, carbon dioxide has proven to be the most promising one for dehydrogenation reactions . In the ethylbenzene dehydrogenation, for instance, the use of carbon dioxide can provide a route, which represents an elegant and promising alternative to the conventional process of styrene production.
Currently, the ethylbenzene dehydrogenation in the presence of overheated steam [Eq. (1)] is the main commercial route to produce styrene, one of the most used intermediate for organic synthesis. It is the main building block for several polymers, such as polystyrene, styrene‐butadiene rubber, styrene‐acrylonitrile, acrylonitrile‐butadiene‐styrene, and other high‐value products. The ethylbenzene dehydrogenation supplies 90% of the global production of styrene, which was around 30 × 106 t in 2010 .
In spite of this fact, the commercial process still has several drawbacks, such as the high consumption of energy, the reaction endothermicity (Δ
The use of carbon dioxide includes other advantages such as being an inexpensive, nontoxic, and renewable feedstock, which provides a positive impact on the global carbon balance. In addition, it can accelerate the reaction rate, improve styrene selectivity, decrease the thermodynamic limitations, suppress the total oxidation, increase the catalyst life, and avoid hotspots . Therefore, the ethylbenzene dehydrogenation with carbon dioxide has been studied over several different catalysts, including iron oxide, vanadium oxide, antimony oxide, chromium oxide, cerium oxide, zirconium oxide, lanthanum oxide, perovskites, and the oxide catalysts promoted with alkali metals supported on several oxides [16, 19, 20, 24, 26–33]. In addition, several works have shown that carbon‐based catalysts are active and selective to produce styrene through ethylbenzene dehydrogenation with carbon dioxide. Activated carbons [34, 35], carbon nanofibers , onion‐like carbons , diamonds and nanodiamonds [37, 38], graphites , and multiwalled carbon nanotubes (MWCNTs) , among others, have been evaluated in ethylbenzene dehydrogenation.
These studies have shown that the effect of carbon dioxide on the activity, selectivity, and stability of the catalysts for ethylbenzene dehydrogenation depends on the kind of the catalyst, as well as on the reaction conditions. For zirconia‐based catalysts, the positive effect of carbon dioxide was found to be highly dependent on the crystalline phase at 550°C. It was noted that the tetragonal phase showed high activity and selectivity to styrene, a fact that was related to differences in specific surface area of the solids and their affinity with carbon dioxide associated with the surface basic sites [41, 42]. In a previous work , we have found that zirconia was the most active and selective catalyst to produce styrene through ethylbenzene dehydrogenation with carbon dioxide, as compared to metal oxides such as lantana (La2O3), magnesia (MgO), niobia (Nb2O5), and titania (TiO2). This finding was related to the highest intrinsic activity of zirconia.
In spite of the numerous studies on the catalyst properties for the dehydrogenation of ethylbenzene in the presence of carbon dioxide, no satisfactory catalyst was found yet, requiring new developments. In the present work, the effect of cerium, chromium, aluminum, and lanthanum on the properties of zirconia‐supported iron oxides was studied aiming to find efficient catalysts for the reaction.
2.1. Catalysts preparation
The precursor of zirconium oxide was obtained by hydrolysis of zirconium oxychloride (1 mol/l) with an ammonium hydroxide solution (30% w/v). The obtained gel was rinsed with an ammonium hydroxide solution (1% w/v) eight times up not to detect chloride ions by Mohr's method anymore. The gel was then dried in an oven at 120°C, for 12 h. The solid was calcined at 600°C, for 4 h, under airflow (50 ml/min).
The metal‐doped zirconia samples were prepared by the same method, using solutions of zirconium oxychloride and of metal nitrates (Zr/
The modified zirconium oxides were subsequently impregnated with an iron nitrate solution (0.17 mol/l), at room temperature, to obtain the catalysts.
2.2. Catalysts characterization
After iron impregnation, the samples (catalyst precursors) were analyzed by thermogravimetry (TG) and Fourier transform infrared spectroscopy (FTIR).
After calcination, the catalysts were characterized by Fourier transform infrared spectroscopy, X‐ray diffraction (XRD), specific surface area measurement, and temperature‐programmed reduction.
The experiments of thermogravimetry (TG) were performed on a Mettler Toledo TGA/SDTA 851 equipment. The sample (0.02 g) was placed in a platinum crucible and heated (10°C/min) from room temperature to 1000°C, under airflow (50 ml/min).
The presence of nitrate species in the samples was detected by FTIR, using a Perkin Elmer, Model––Spectrum One, equipment, in the range of 400–4000 cm-1. The samples were prepared as potassium bromide discs, in a 1:10 proportion.
The experiments of X‐ray diffraction (XRD) were carried out in a Shimadzu model XD3A apparatus, using CuKα radiation generated at 30 kV and 20 mA and nickel filter.
The specific surface areas were measured in a Micromeritics ASAP 2020, using the sample (0.2 g) previously heated at 300°C, under nitrogen flow.
The curves of temperature‐programmed reduction were obtained on a Micromeritics model TPR/TPD 2900 equipment, utilizing 0.3 g of the sample, and heating the solid with a rate of 10°C/min, under flow of a mixture of 5% hydrogen in nitrogen up to 1000°C.
2.3. Catalysts evaluation in ethylbenzene dehydrogenation with carbon dioxide
The catalysts were evaluated in ethylbenzene dehydrogenation in the presence of carbon dioxide in a fixed bed reactor, using 0.3 g of catalyst, at several temperatures (530, 550, 570, 590, 610, and 630°C) under atmospheric pressure. A carbon dioxide to ethylbenzene molar ratio of 10 was used for all experiments.
The reaction products were analyzed by online gas chromatography, using a Varian Star 3600 Cx equipment with a flame ionization detector. A commercial catalyst for the ethylbenzene dehydrogenation with steam, based on iron and chromium oxides, was also evaluated in the same conditions, for comparison.
3. Results and discussion
The TG curves for the catalyst precursors (before calcination) are displayed in Figure 1. For all cases, there was a weight loss in two stages: the first at around 200°C, related to loss of volatiles adsorbed on the solids; the second stage at higher temperatures, in the range of 200–450°C, can be assigned to the decomposition of iron hydroxide to produce hematite and/or maghemite [43, 44]. It can be noted that the kind of the support affected hematite formation, probably due to different interactions of the iron oxide precursor with the support. The process was easier over lanthanum‐doped zirconia (225°C), followed by cerium‐doped zirconia (250°C). On the other hand, for aluminum‐doped zirconia (292°C) and for chromium‐doped zirconia (300°C), the process was delayed, suggesting that iron hydroxide was more strongly bonded to these supports.
3.2. Fourier transform infrared spectroscopy
The FTIR spectra for the precursors (Figure 2a) show two bands at 3400 and 1600 cm-1, assigned to the bending vibrations of OH groups in iron hydroxides and in adsorbed water . The absorption at 1384 cm-1 is related to the nitrate species , from iron nitrate. In the low‐frequency region, a broad band was observed, in the range of 800–400 cm-1, attributed to the Fe–O bond . For the catalysts (Figure 2b), it can noted that the band at 1384 cm-1 decreased for the samples, except for chromium‐doped catalyst, indicating that the calcination was effective for the removal of nitrate species.
3.3. X‐ray diffraction
From the X‐ray diffractograms of the solids (Figure 3), different phases were found for all samples, related to the different oxides. However, for most cases, it was not possible to assure the presence of isolated phases of iron, zirconium, and of the dopants. Therefore, hematite, α‐Fe2O3 (JCPDS 871166), maghemite, γ‐Fe2O3 (JCPDS 251402), or zirconium oxide, ZrO2 (monoclinic, JCPDS 830944 and tetragonal, JCPDS 881007), as well as lanthanum oxide, La2O3 (monoclinic, JCPDS 220641 or hexagonal, JCPDS 401279), aluminum oxide, Al2O3 (orthorhombic, JCPDS 880107), or chromium oxide, Cr2O3 (rhombohedral, JCPDS 841616), cannot be detected, because of the coincidence of the diffraction peaks of these phases. Only maghemite and the cubic phase of ceria, CeO2 (JCPDS 780694), were detected as isolated phases for the chromium and cerium‐doped samples, respectively.
3.4. Specific surface areas
Table 1 shows the specific surface areas of the catalysts, as well as of pure and doped supports. It can be noted that pure oxides showed different values, which are typical of the nature of each oxide. Zirconia showed the highest values, while chromium showed the lowest one. In addition, the dopants changed the specific surface area of zirconia (73 m2/g), depending on the kind of dopant. These different behaviors are related to the size of the ions, the possibility of the ion to enter into zirconia lattice, and the formation of mixed compounds. According to previous studies [47–49], it would be expected that these dopants would increase the specific surface areas of zirconia, because of the differences in ionic radius of cerium (0.97 Å), chromium (0.615 Å), aluminum (0.54 Å), and lanthanum (1.16 Å), as compared to zirconium (0.84 Å). These differences often cause stresses in zirconia lattice, favoring the production of smaller particles, since they decrease the stress to surface ratio. However, only for the aluminum‐doped zirconia the specific surface area increased, suggesting that most of the dopants did not enter into the lattice but rather remain as a segregated phase, as detected for cerium‐doped zirconia.
The impregnation of iron on the supports also changes the specific surface areas, as shown in Table 1. For the chromium‐doped and lanthanum‐doped samples, the addition of iron caused an increase in specific surface area, suggesting a contribution of the iron oxides to these values. On the other hand, the other samples showed a decrease in the specific surface area, indicating that they went on sintering during the calcination step, after iron impregnation. The chromium‐based catalyst showed the highest value, while the cerium‐based catalyst showed the lowest one.
3.5. Temperature‐programmed reduction
The catalysts showed different reduction profiles, as displayed in Figure 4. The cerium‐doped catalyst showed a peak beginning at 192°C and others in the range from 398 to 931°C. The first peak can be assigned to the reduction of Fe+3 to Fe+2 species, while the latter is due to the reduction of Fe+2 to Fe0 species , as well as to the reduction processes related to the support [32, 33]. On the other hand, the chromium‐doped zirconia sample showed a reduction peak beginning at 182°C, with a shoulder at around 274°C, as well as another peak in the range 501–929°C. The first peak can be associated to the reduction of Cr+6 to Cr+3  species and of Fe+3 to Fe+2 species, while the latter one is due to the reduction of Fe+2 to species Fe0 . The lanthanum‐doped sample showed a peak beginning at 225°C and other ones at 327, 393, and 500°C, attributed to the reduction of Fe+3 species in different interactions with the support. A broad peak in the range of 600–781°C is related to the reduction of Fe+2 to Fe0 species and to the processes related to the support. For the aluminum‐doped sample, two reduction peaks beginning at 200 and 332°C were noted associated to the reduction of Fe+3 to Fe+2 species in different interactions with the support. A broad peak in the range of 406–704°C can be assigned to the reduction of Fe+2 to Fe0 species. The easiness of the reduction decreased with the dopants in the order: Cr > Ce > Al > La.
3.6. Activity and selectivity of the catalysts
Figure 5 shows the values of ethylbenzene conversions as a function of temperature during the dehydrogenation with carbon dioxide. It can be noted that the samples were more active than a commercial catalyst, for all temperature ranges. Also, the catalysts showed different performances, depending on the reaction temperature. At low temperatures, the cerium‐based sample led to the highest conversion that, however, decreased with the temperature increase. This can be related to the ability of cerium oxide (detected by X‐ray diffraction) for providing lattice oxygen, which activates the carbon dioxide molecule and then increases the reaction rate [32, 33]. The chromium‐doped catalyst was the second most active one, leading to conversions of around 46%, which increased with temperature, a fact that can be associated to the high dehydrogenation activity of chromium compounds . The aluminum‐doped and lanthanum‐doped samples showed similar behaviors, leading to low conversions that increased with temperature.
The selectivity of the catalysts to styrene (Figure 6) also changed with the kind of the dopant and with temperature. The aluminum‐doped catalyst was the most selective one, but the selectivity decreased as the temperature increased. A similar behavior was noted for the commercial catalyst. On the other hand, the selectivity of cerium‐doped sample showed a maximum at around 570°C, while the selectivity of lanthanum‐based and chromium‐based solids almost did not change with temperature. These findings can be related to the kind of the dopants and their different interactions with the support, as well as to the reaction temperature.
Figure 7 shows the yields obtained over the catalysts. One can see that the yield largely depends on the reaction temperature and on the kind of the dopant. The highest value was obtained at 530 and 560°C, over the cerium‐doped catalyst. However, the yield decreased with temperature increase, suggesting the catalyst deactivation at high temperatures. While the other catalysts showed low yields for all temperature ranges, the chromium‐doped catalyst led to a yield of around 35% at 590°C.
Catalysts based on iron oxides (hematite and/or maghemite), supported on zirconium oxide doped with cerium, chromium, aluminum, or lanthanum, show different textural and catalytic properties in ethylbenzene dehydrogenation with carbon dioxide. These findings can be related to the different phases of the supports, such as zirconia (monoclinic or tetragonal), iron oxides (hematite or maghemite), cerium oxide (cubic), lantana (monoclinic or hexagonal), and chromium oxide (rhombohedral), which are also active in the reaction.
The most promising sample was the cerium‐doped solid, which led the highest yield (46%) at the lowest temperature. This was assigned to the role of cerium oxide in providing lattice oxygen, which activates carbon dioxide and increases the reaction rate.
The catalysts have proven to provide another alternative to use carbon dioxide, one of the main greenhouse gas and then to contribute to the environment protection.
SBL and SMSB acknowledge CAPES and CNPq for their fellowships. The authors thank CNPq and FINEP for the financial support.