The catalytic conversion of light alkanes such as ethane and propane into the corresponding value-added alkenes has gained much attention over the past several decades because of the growing demand for light alkenes. The dehydrogenation of alkanes is endothermic and is inevitably controlled by the thermodynamic equilibrium, thus relatively high temperatures are required to obtain high yield of alkenes, resulting in high energy consumption and ready deactivation of the catalyst. For this reason, oxidative dehydrogenation using oxygen has been proposed as an alternative process. However, the over- oxidation of alkanes to carbon dioxide is unavoidable during this process, leading to a decrease in the targeted product selectivity. However, it has been reported that these disadvantages can be overcome by replacing O2 with milder oxidants, such as N2O [1, 2, 3, 4] and CO2 [5, 6, 7, 8, 9, 10, 11].
The dehydrogenation of ethane over In [5], Cr [6, 7], Ga [8, 9], Co [10] and Mn [11]-containing catalysts in the presence of CO2 has been studied intensely for some time now. Cr-based catalysts in particular show excellent activity for the dehydrogenation of ethane, and CO2 can markedly promote the reaction, leading to a significant increase in ethylene yield in the presence of CO2 over Cr-based catalysts. Because of the low surface area of bulk crystalline chromium oxides, Cr species are often dispersed on supports with high surface areas, such as Al2O3 [7], SiO2 [7], ZrO2 [12, 13], TS-1 [14], mesoporous silicas like SBA-1 [15], SBA-15 [16], MSU-x [17], MCM-41 [18] and oxidized diamond [19, 20], so as to prepare a catalyst with an abundance of active sites.
ZSM-5 plays a very important role both in industrial processes and in academic studies as either a catalyst or a catalyst support, owing to its three-dimensional microporous structure, high surface area and high thermal and hydrothermal stability. ZSM-5-supported metal oxide catalysts have been investigated with regard to ethane or propane dehydrogenation using CO2, and H-form ZSM-5 with a Si/Al ratio over 1900 has been reported to be preferred as the support [23]. More recently, Cr supported on Na-type ZSM-5 having a smaller crystal size (ca. 400 nm) was found to be more effective when applied to the dehydrogenation of propane in the presence of CO2, although the stability of the catalyst system was still not satisfactory [24].
In our present work, a series of Cr catalysts supported on submicron H- or Na-form ZSM-5 materials having various Si/Al ratios were prepared and characterized by X-ray diffraction (XRD), nitrogen adsorption, laser Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), diffuse reflectance UV-Vis spectroscopy (DRS) and temperature-programmed reduction (TPR). The catalytic performance of each of these submicron ZSM-5-supported Cr-based catalysts during the dehydrogenation of ethane to ethylene in the presence of CO2 was also investigated. The relationship between catalytic behavior and physicochemical properties is discussed herein on the basis of the experimental results.
Submicron ZSM-5 zeolite was prepared using procedures previously reported in the literature [23], employing tetrapropylammonium hydroxide (TPAOH, 25% aqueous solution, Yixing Dahua) as the template. Typically, NaAlO2 (CP, Sinopharm Chemical) was dissolved in an aqueous TPAOH solution, after which tetraethylorthosilicate (TEOS, AR, Shanghai Lingfeng) was added. The resulting mixture was stirred for 6 h at room temperature, followed by heating at 50 °C with further stirring to evaporate the ethanol resulting from the reaction. A clear gel was obtained with the molar composition 120SiO2:xAl2O3:48TPAOH:3600H2O. This gel was transferred to an autoclave and crystallized by heating at 170°C for 2 d. The obtained product was centrifuged, washed, dried at 110°C overnight and then calcined in air at 600 °C for 6 h to remove the template.
The supported chromium oxide catalysts were prepared by impregnating ZSM-5 with an aqueous solution of Cr(NO3)3×9H2O (AR, Sinopharm Chemical) using the incipient wetness method. The impregnated samples were dried at 110 °C overnight and calcined in air at 650°C for 6 h. The obtained catalysts are denoted as yCr/ZSM-5-c, where y represents the mass fraction of Cr2O3 in the catalysts, and c represents the Si/Al ratio in gel.
XRD patterns were acquired with a Persee XD-2 X-ray diffractometer using nickel-filtered Cu Karadiation at 40 kV and 30 mA. The BET surface areas and micropore volumes of the catalysts were determined by N2 adsorption at -196 °C using a Micromeritics ASAP 2000 instrument. Scanning electron microscopy (SEM) images were recorded digitally on a Philips XL 30 microscope operating at 30 kV. Laser Raman spectra were obtained with a Horiba JY XPloRA spectrometer using the 532 nm radiation from an air-cooled solid state laser as the excitation source. The other parameters included a laser power of 25 mW, a data acquisition time of 30 s, an accumulation number of 8 and a spectral resolution of 2 cm-1. The spectra were obtained at room temperature under ambient conditions. DRS spectra were collected on a Shimadzu UV-2450 spectrometer equipped with an integrating sphere attachment. XPS data were acquired using a Perkin-Elmer PHI 5000C spectrometer with Mg Karadiation as the excitation source. All binding energy values were referenced to the C 1s peak at 284.6 eV.
TPR profiles were obtained on a Micromeritics AutoChem II apparatus loaded with 100 mg of catalyst. The TPR experiments were carried out in a 30 mL/min flow of 10% H2-90% Ar with a ramp rate of 10 °C/min. H2 consumption was monitored using a thermal conductivity detector. Thermogravimetric (TG) analysis was performed under an air flow on a Perkin-Elmer 7 Series Thermal Analyzer to determine the amount of coke deposited on the catalyst following the reaction.
Catalytic tests for ethane dehydrogenation with CO2 were performed at 650 °C in a fixed-bed flow microreactor at atmospheric pressure. The catalyst load was 200 mg, and each sample was pretreated at 650°C for 2 h under a nitrogen flow prior to the reaction. The gaseous reactant contained 3% ethane and 15% CO2 with the balance consisting of nitrogen at a total flow rate of 30 mL/min. The hydrocarbon reaction products were analyzed using an on-line gas chromatograph (GC) equipped with a 6 m Porapak Q packed column and a flame ionization detector (FID). The gaseous products were analyzed on-line using a second GC equipped with a thermal conductivity detector (TCD) and a carbon molecular sieve 601 column. The reverse water-gas shift reaction was performed in a fixed-bed flow microreactor at atmospheric pressure using a catalyst load of 200 mg. The H2:CO2:N2 molar ratio was 1:1:1, and the total flow rate of the gaseous reactants was 30 mL/min. The reaction temperature was in the range of 500-650 °C. The amounts of CO2 before and after the reaction were determined by on-line analysis with a GC equipped with a carbon molecular sieve 601 column and a TCD. The reaction data in this work were reproducible, with a variation of less than 5%.
Figure 1 shows the XRD patterns of the various ZSM-5- supported chromium oxide catalysts. All exhibit well-crystallized MFI structures with characteristic reflections at 2θ = 8.0°, 8.9°, 23.1°, 23.4° and 24.0° [27]. No diffraction patterns corresponding to chromium oxide were observed, suggesting that the chromium oxide was well dispersed on all the ZSM-5 supports.
SEM images of the ZSM-5 supports are presented in Fig. 2. All samples were well crystallized without any presence of amorphous materials and exhibit a rod-like morphology with a uniform crystallite size distribution. The average crystallite size was approximately 400 nm for all the ZSM-5 zeolites.
The textural properties of the ZSM-5-supported chromium oxide catalysts are summarized in Table 1. These catalysts, regardless of whether the ZSM-5 support was the Na- or H-form or had a low or high Si/Al ratio, exhibit similar BET surface areas and micropore volumes, showing that the supports had similar crystallinities and that their micropore channels were not blocked by the supported CrOx species.
27Al MAS NMR spectra of the ZSM-5 supports were acquired to characterize the local coordination environment of the aluminum atoms in the zeolites. An intense line at δ = 55, assigned to ZSM-5 framework aluminum atoms in tetrahedral coordination, can be observed in the spectra of all the samples, while no discernable signal at δ = 0, attributed to extra-framework aluminum atoms in octahedral coordination, is evident (Fig. 3). The absence of extra-framework aluminum atoms indicates that all of the Al species have been incorporated into the zeolite framework.
XPS was employed to investigate the oxidation states of Cr species. The Cr 2p2/3 spectra obtained from the catalysts were deconvoluted into two bands at approximately 576.5 and 579.5 eV, assigned to Cr3+ and Cr6+, respectively [22, 25, 26], and the quantitative data after fitting are listed in Table 2. All the catalysts were found to have similar binding energy (BE) values. The Cr6+/Cr3+ ratio evidently decreases with increasing Si/Al ratios and as the Na content is decreased, indicating that Na+ cations may have a positive effect in terms of raising the Cr6+/Cr3+ ratio, similar to the action of K+ ions in K-doped CrOx/Al2O3 catalysts [27]. Compared with the NaZSM-5- supported catalyst, the HZSM-5 material having the same Si/Al ratio exhibited a lower Cr6+/Cr3+ ratio, which may also result from a lower Na content.
Additional information on the state of Cr species was obtained from UV-Vis diffuse reflectance measurements, and the results are summarized in Fig. 4. Two bands are observed for each catalyst, at approximately 272 and 370 nm, assigned to the O2-→Cr6+ charge transfer transition of chromate species in tetrahedral coordination [25, 28, 29, 30, 31, 32]. Bands at 468 and 605 nm, corresponding to octahedral Cr3+ species in Cr2O3 or CrOx clusters, are not present, indicating that all Cr species were well dispersed on the submicron ZSM-5 support.
Laser Raman spectra were acquired to identify the molecular nature of the Cr species dispersed on the ZSM-5 supports, and the results are depicted in Fig. 5. A band at 551 cm-1, assigned to crystalline Cr2O3, appears in the spectra of the 3Cr/HZSM-5-160 catalyst [18, 22, 25, 30, 31, 32, 33], while the band at the same position is much weaker in the case of the NaZSM-5 supported catalysts. The intensity of the 551 cm-1 band decreases in the order 3Cr/HZSM-5-160 > 3Cr/NaZSM-5-60 > 3Cr/NaZSM-5-100 » 3Cr/NaZSM-5-160. These results demonstrate that the use of a high Si/Al ratio and the Na-form of the zeolite favor CrOx dispersion.
Two intense bands at approximately 980 and 1000 cm-1 are present in the spectrum of each of the catalysts, and can be ascribed to the symmetric vibrational modes of the terminal Cr=O bonds of monochromates and polymeric chromates, respectively [18,22,25,30,31,32,33]. Meanwhile, a weak, broad band at 810 cm-1, attributed to the bending mode of the Cr-O-Cr linkage of polymeric chromates, also appears. The intensity of the band at 1000 cm-1 increases as the Si/Al ratio is increased, indicating that the dispersed Cr(VI) species on the catalyst surfaces gradually transition from monochromates to polymeric chromates with increasing Si/Al ratios.
H2-TPR was carried out to characterize the redox ability of Cr species on the catalysts, which has a very important effect on the dehydrogenation activity. The results are presented in Fig. 6 and Table 2. It is evident that there is only one reduction peak at 370 °C, with a shoulder at 270 °C, that can be attributed to the reduction of Cr6+ to Cr3+ (and/or Cr2+) [30,31,32]. H2 consumption is higher over the NaZSM-5-supported catalysts as compared with HZSM-5 material, and increases as the Si/Al ratio is increased, indicating increasing quantities of reducible surface Cr6+. All these findings are consistent with the data obtained from laser Raman studies, showing that high Si/Al ratios and the Na-form are favorable for CrOx dispersion.
The prepared Cr/ZSM-5 catalysts were evaluated during ethane dehydrogenation in the presence of CO2, and the results are shown in Table 3. For each catalyst, the ethane conversion drops with reaction time while the selectivity for ethylene increases. There are only minimal differences in the initial activities as well as the initial ethylene yields among the Cr/NaZSM-5 catalysts, although the stability is improved with increasing Si/Al ratios in the ZSM-5 support, resulting in a slight increase in the plateau value for the yield of ethylene. While all the submicron particle-supported catalysts exhibit superior performance, the 3Cr/NaZSM-5-160 shows higher activity than the 3Cr/HZSM-5-160.
Cr-based catalysts have been reported to represent one of the most promising catalysts for light alkane dehydrogenation reactions because of their high catalytic efficiency both in the absence and presence of CO2. There are two types of coordinatively-unsaturated Cr(III) in chromium species, Cr(III) ions formed from the reduction of Cr(VI) and dispersed on fresh catalysts. Both are generally considered as active sites during non-oxidative dehydrogenation and oxidative dehydrogenation reaction processes. However, several groups have reported that coordinatively unsaturated Cr(III) formed from the reduction of higher-valence states (i.e. Cr(VI)) is more active, whether applied to the non-oxidative dehydrogenation reaction or oxidative dehydrogenation with CO2. As a result, the amount of reducible Cr(VI) on the calcined samples is crucial for the dehydrogenation reaction [17,22]. H2 consumption results allow one to estimate the amount of redox-active Cr(VI) species, and it can be seen from Table 2 that the amount of reducible Cr(VI) on the freshly calcined samples increases with the Si/Al ratio, which is in agreement with their increasing dehydrogenation activity. These results confirm that the reducible Cr(VI) makes an important contribution to the dehydrogenation reaction. This is also the reason why the activity of the 3Cr/NaZSM-5 is higher than that of the 3Cr/HZSM-5.
The type of Cr(VI) species is considered to be another possible factor in the dehydrogenation reaction. Kumar et al. [34]reported that isolated chromium species are more active for the dehydrogenation reaction than crystalline α-Cr2O3, whereas oligomeric chromium species are more active than isolated chromium species. We can see from Fig. 5 that dispersed Cr(VI) species are present in the form of polymeric chromates and monochromates on the surface of Cr/NaZSM-5 while crystalline Cr2O3 was formed on the Cr/HZSM-5. This may be the reason why 3Cr/HZSM-5-160 exhibits relatively low activity even though it has a higher amount of reducible Cr(VI) as compared with 3Cr/NaZSM-5-60.
The effect of the Cr loading on the dehydrogenation activity was also investigated. The ethane conversion was found to increase with increasing extents of Cr loading and then decreased as the Cr content was further increased, such that mass fraction of 3% Cr2O3 is optimal.
To investigate the stability of the 3Cr/NaZSM-5-160 catalyst, the dehydrogenation reaction was run continuously for 50 h, with the results shown in Fig. 7. The catalyst is relatively stable; the ethylene yield over the catalyst is maintained at about 46% without any obvious deactivation over the 50 h, although the activity does drop slowly. This is quite different from the behavior of other commonly studied Cr-containing catalysts, over which the ethylene yields are reported to have dropped quickly within 6 h.
The effect of CO2 partial pressure on the dehydrogenation of ethane was also investigated, and the results are summarized in Table 4. The promotional effect of CO2 on the reaction is quite evident. The initial ethane conversion increases quickly from 20.4% with increasing CO2/C2H6 ratios until it reaches its peak at 65.5% when the CO2/C2H6 ratio equals 5, after which the conversion decreases slightly with further increases in the CO2/C2H6 ratio. The effect of CO2 can be attributed to the reverse water-gas shift reaction, which accelerates the formation of the dehydrogenation products by transforming H2 and CO2 into CO and H2O. This is demonstrated by the results of the H2/CO2 reaction over 3Cr/NaZSM-5-160. Obviously, the catalyst is very active for the reverse water-gas shift reaction, with a CO2 conversion of 22.6% at 650 °C.
The stability of the catalysts was also improved greatly by the addition of CO2. About 40% of the initial activity was lost within 6 h in the absence of CO2. However, when 15% CO2 was introduced, only 5% loss was observed during the same period. The enhanced stability can be explained primarily by two effects: (1) CO2 promotes the Cr6+/Cr3+ reaction through the oxidation of reduced Cr species, Cr(III)Ox-1 + CO2 → Cr(VI)Ox + CO, and regenerates the active species [6,7,17,35,36,37], and (2) CO2 eliminates the formation of coke by the Boudouard reaction, CO2 + C → 2CO, and thus exposes more active sites to the reactants [6,7,17,35,36,37]. This can be confirmed by thermogravimetric tests that show that the amount of coke deposited on the catalyst after 6 h is 3.4% in the absence of CO2, a value that is higher than the 3.0% amount obtained in the presence of CO2. The fact that the Boudouard reaction proceeds is also evident from the observation that the molar ratio of CO formed to CO2 converted is approximately 1.4 during the reaction, since this ratio should be equal to unity if the Boudouard reaction does not occur.
Catalysts composed of submicron ZSM-5-supported chromium oxide particles were prepared by an incipient wetness method, and their catalytic performance for ethane dehydrogenation in the presence of CO2 was compared. The results show that chromium oxide supported on submicron Na-type ZSM-5 with a high Si/Al ratio is an excellent catalyst for the oxidative dehydrogenation of ethane with CO2. High activity as well as high stability can be obtained over this catalyst. The promotional effect of CO2 on dehydrogenation can also be observed on this catalyst and is attributed to the reverse water-gas shift reaction. Characterization by laser Raman, UV-Vis DRS, XPS and H2-TPR revealed that improved dispersion of Cr species can be achieved in submicron catalysts, resulting in a greater quantity of the reducible Cr(VI) species that play a key role in the ethane dehydrogenation reaction. The type of Cr(VI) species present is another possible factor affecting the dehydrogenation reaction.
2015, Vol. 36 Issue (8): 1242-1248
PDF (25010 KB)
2015, Vol. 36 Issue (8): 1242-1248
PDF (25010 KB)