Dehydrogenation of light alkanes to the corresponding olefins is an alternative route for the production of light olefins [1]. Since light alkanes are widely available, cheap and environment friendly, they are potential raw materials for the future chemical industry. In particular, n-butane can be recovered from liquefied petroleum gas (LPG) by distillation [2]. Conversion of n-butane to high value added products has caught much attention from researchers. The catalytic dehydrogenation of n-butane to 1-butene, 2-butene, and 1,3-butadiene is an alternative route for the production of butenes. Among the butenes, 1,3-butadiene is the most important because of its wide use in the manufacture of butadiene-styrene rubber, synthetic rubbers, and plastics with special mechanical properties [3]. Compared to the direct dehydrogenation, the oxidative dehydrogenation (ODH) of n-butane has three advantages: (1) the reaction is exothermic and therefore does not need external heat input; (2) the oxidative dehydrogenation reaction is not limited by thermodynamic equilibrium; (3) there is no catalyst deactivation induced by coking, so frequent regeneration of the catalyst is not needed [4, 5]. However, the low selectivity is still a main issue which blocks the application of the process. Therefore, a high performance catalyst needs to be developed for the catalytic oxidative dehydrogenation of n-butane.
For n-butane oxidative dehydrogenation, supported vanadium oxide catalysts have been reported as the most active and selective catalysts [6, 7, 8]. The variable valence states between V4+ and V5+ makes V the active component for many catalytic reactions. The catalytic activity of supported vanadium catalysts for n-butane oxidative dehydrogenation is related to the structure of the VOx surface species, redox properties of the VOx surface species and acid-base character of the catalyst and support [9, 10, 11, 12, 13, 14]. According to previous works, the oxidative dehydrogenation of n-butane to butene and 1,3-butadiene follows the Mars-van Krevelen mechanism [6, 15, 16, 17]. This indicates that the redox properties of the catalyst play an important role in the oxidative dehydrogenation of n-butane [15, 18, 19, 20]. In addition, the acid-base properties have influences on the activity and selectivity of vanadium catalysts by affecting the adsorption and desorption of the reactants and products [21].
Although MgO is a good support for vanadium catalysts to catalyze n-butane oxidative dehydrogenation [21, 22, 23], its low mechanical strength and low specific surface area indicate that it is not the ideal support for practical application. γ-Al2O3 supported vanadium catalysts have been reported in several studies [1, 24, 25] because γ-Al2O3 with a high specific surface area, suitable pore structure and high mechanical strength is suitable for loading the active components. However, unlike on the alkaline support MgO, it is difficult to get a good dispersion of VOx on the acidic γ-Al2O3 surface, and thus crystalline V2O5 is frequently found at high loading.
Here, alkaline earth metal oxide-doped Al2O3 was synthesized as the support for vanadium catalysts, and the dehydrogenation of n-butane to butenes was conducted to find the relationship between the alkaline earth metal doping and catalyst performance. In addition, catalysts with different amounts of V2O5 loading were prepared to study the relationship between V2O5 loading and catalytic performance.
MO-Al2O3 (M = Mg, Ca, Sr, Ba) supports were prepared by a modified sol-gel method. First, an amount of boehmite powder was dispersed in nitric acid solution (0.1 mol/L). The mixed solution was vigorously stirred for 2 h at room temperature to make the boehmite powder particles fully dispersed. After aging at room temperature for 12 h, an AlOOH sol was formed. After this, an alkaline earth metal nitrate solution was added into the AlOOH sol with an M:Al molar ratio of 1:2. Then the mixture was vigorously stirred at 80 °C for 2 h. Finally, the MO-Al2O3 (M = Mg, Ca, Sr, Ba) support was obtained after drying and calcining at 70 °C for 12 h and 550 °C for 3 h, respectively.
For the preparation of V2O5/MO-Al2O3 catalysts with different V2O5 loading (mass ratio), an aqueous solution containing ammonium metavanadate and oxalic acid (molar ratio = 1:2) was added to the MO-Al2O3 support by the wet impregnation method. The impregnation was performed at 70 °C with continuous stirring. After drying the slurry at 120 °C for 12 h, the resultant powder was pressed into pellets, and then crushed and sieved to 20-40 mesh.
The specific surface area of the V2O5/MO-Al2O3 catalyst and V2O5/MgO-Al2O3 catalysts with different V2O5 loading were measured on an OMNISORP 100CX fully automatic physical and chemical adsorption device using the nitrogen adsorption-desorption isotherm method (adsorption data obtained in the relative pressure ranges of 10−6-1 bar for nitrogen at −196 °C). Powder X-ray diffraction (XRD) measurements were used to identify the crystallographic structure of the fresh and used catalysts. The XRD diffraction patterns of the catalyst samples were obtained on a Rigaku D/MAX-RB instrument using Cu Kα radiation source (40 kV, 100 mA) and a graphite monochromatic between 20°-80° at a scan rate of 5°/min. Infrared spectra were recorded on a Nicolet 6700 Fourier Transform Infrared Spectroscopy-Attenuated Total Reflectance (FTIR-ATR, Thermo Scientific Co.) spectrophotometer. Powder samples were placed in contact with an attenuated total reflectance (ATR) multibounce plate of ZnSe crystal at ambient temperature (25 °C). The Raman spectra were recorded at an excitation wavelength of 532 nm and a laser power of 20 mW. H2 temperature programmed reduction (H2-TPR) was performed to observe the reducibility of the catalysts. Prior to the anal ysis, 100 mg of sample was treated in 20% O2-80% N2 (30 mL/min) at 300 °C for 30 min. The reactor temperature was raised to 900 °C at a heating rate of 10 °C/min in 5% H2-95% N2 (30 mL/min). The H2 consumption during the reaction was measured by a thermal conductivity detector (TCD).
The catalytic experiments were performed on a fixed bed quartz tubular reactor (10 mm i.d.) under atmospheric pressure. For each catalytic test, 1.0 g catalyst (20-40 mesh, total bed height of 15 mm) was used. The reactor was heated in a furnace and the temperature was controlled by a microprocessor (Model Al-708, Xiamen Yuguang Electronics Technology Research Institute, China) to within ±1 °C of the set points using a K type thermocouple. The catalysts were calcined under flowing N2 (100 mL/min) for 5 h at 600 °C in the reactor before the catalytic reaction. The catalytic tests were conducted in the temperature range of 500 to 600 °C. The flow rates of n-butane, O2 and N2 (n-butane:O2 ratio = 1.2:1, 1.5:1 and 2:1) were controlled by mass flow controllers. The n-butane volume fraction in the feed gas was 5%. The products of the reactions were analyzed by an online gas chromatograph equipped with a thermal conductivity detector (TCD). Two columns were used for the separation of the products. One was a 30% sebacic dinitrile/Chromosorb packed 9 m 1/8” column for the separation of all the hydrocarbons, and another was a 3 m 1/8” PQ column for the separation of O2, N2 and CO. The conversion of n-butane, selectivity to total butenes (including 1-C4H8, 2-t-C4H8, 2-c-C4H8, 1,3-C4H6) and selectivity to 1,3-butadiene were calculated on the basis of the carbon balance. The yield was calculated by multiplying the conversion of n-butane and selectivity to butenes products as follows:
n-butane conversion, Xn-C4H10 (%) = $\frac{{\sum {Mi \times ni} }}{{4 \times moles of n - butane feeded}} \times 100$
Selectivity to C4=tot, S (%) = $\frac{{Mi \times ni}}{{\sum {Mi \times ni} }} \times 100$
(Mi: the moles of products; ni: the carbon atoms of the product molecules)
Table 1 and Table 2 list the BET specific surface areas of the catalysts before and after the reaction. Of all the catalysts, V2O5/Al2O3 and V2O5 /CaO-Al2O3 have the highest and the lowest surface area, respectively. After the reaction at 600 °C for a set time, all the catalysts showed significant decrease in the specific surface areas compared with before the reaction. The decrease in the specific surface area was attributed to the sintering of the nanoparticles during the catalytic reaction [26]. The surface areas of V2O5/MO-Al2O3 catalysts with different alkaline earth metal dopants showed irregular changes, but the Mg doped sample had the highest specific surface area among the doped samples. In addition, compared to the undoped sample, the MgO-doped sample has a higher surface area after the reaction, which indicated that doping MgO can inhibit the sintering and stabilize the pore structure of the catalyst support although it gave a slightly lower initial surface area. As shown in Table 2, the specific surface area of the V2O5/MgO-Al2O3 catalyst before the reaction decreased slightly with the increase of V2O5 loading. This can be ascribed to the gradual reduction of the volume fraction of the support. After the reaction, the specific surface areas of the MgO-doped catalysts were still higher than 60 m2/g.
Figure1 shows the XRD patterns of the V2O5 /MO-Al2O3 catalysts (before and after the reaction) and V2O5/MgO-Al2O3 catalysts with different V2O5 loading. As shown in Fig. 1(a), orthovanadates and carbonates were formed in the samples doped by Ca, Sr and Ba. The diffraction peaks of the carbonate became stronger after the reaction. Meanwhile the strong orthovanadate diffraction peaks indicated that the Ca, Sr and Ba orthovanadates were stable under the reaction conditions. However, for the MgO doped catalyst (10%-V2O5), no diffraction peaks corresponding to the orthovanadate and carbonate were detected but those for MgAl2O4 spinel and MgO were detected. In addition, the diffraction peaks of the V2O3 and (Mg,V)3O4 phases were detected after the reaction. For the undoped catalyst (10%-V2O5/Al2O3) (Fig. 1(c)), the V2O3 crystalline phase was also detected by XRD, as shown in Fig. 1(b). On the MgO-Al2O3 support, the increase in V2O5 loading made the diffraction peaks of the V2O3 and (Mg,V)3O4 phases stronger, while they disappeared when the loading was decreased to 5%. In other words, the VOx species were well dispersed on the support surface when the V2O5 loading was lowered to 5%. The content of the MgO phase decreased with the V2O5 loading increasing. When the loading was further decreased to 3%, the diffraction peaks of MgO almost disappeared, and all the strong peaks were due to MgAl2O4 spinel.
Figure 2 shows the FTIR spectra of the V2O5/MO-Al2O3 catalysts with different alkaline earth metal dopants before the reaction. The samples doped with Ca, Sr, Ba exhibited clearly visible strong absorption bands at 800-900 and 1400 cm-1. The absorption band at 800-900 cm-1 is the stretching vibration of VO43- [27], while the absorption band at 1400 cm-1 is the antisymmetric stretching vibration of CO32-. The characteristic peak width became narrow gradually in the order of Ca, Sr, Ba. This was ascribed to the increase of the ionic radius in the order of Ca, Sr, Ba (Ca2+: 0.099 nm < Sr2+: 0.112 nm < Ba2+: 0.134 nm). The characteristic peaks were located at 1422.4 (CaCO3), 1480.4 (SrCO3), 1453.0 cm-1 (BaCO3) [28], respectively. Therefore, the FTIR spectra result agreed with the XRD analysis that the orthovanadate and carb onate were present in the Ca, Sr, Ba doped catalysts but not in the Mg doped and undoped catalysts.
The redox properties of the catalysts were characterized by H2-TPR as shown in Fig.3. The reduction peaks shifted to high temperature in the order Mg < Ca < Sr < Ba as the catalysts were doped with alkaline earth oxides. In addition, the intensity of reduction peaks was weaker for the catalysts doped with Ca, Sr, Ba than for the catalysts doped with Mg, which indicated that the orthovanadate crystalline phase in the Ca, Sr, Ba doped catalysts were difficult to reduce. This finding is consistent with the XRD results (Fig. 1(b)). Thus, it can be expected that the Ca, Sr, Ba doped catalysts will have lower catalytic activity than the Mg doped and undoped catalysts.
Figure 4 shows the Raman spectra of the V2O5/MgO-Al2O3 catalysts with different V2O5 loadings after the reaction. The peak intensity at 100-400 and 800-1000 cm-1increased gradually as the V2O5 loading increased from 3% to 20%. The stretching vibration peak of V =O at 995 cm-1 appeared as the V2O5 loading was increased to 10%. This means the V2O5 crystalline phase was formed in the V2O5/MgO-Al2O3 catalysts with 10%, 15%, 20%-V2O5 loading. Han et al. [29] reported that vanadium species achieve the highest monolayer dispersion when the V2O5 loading was 5% for the V2O5/γ-Al2O3 system, and an additional increase in the V2O5 loading led to the appearance of the V2O5 crystalline phase. Here, as the V2O5 loading was decreased to 5%, the stretching vibration peak at 995 cm-1 disappeared, which indicated VOx species were well dispersed on the MgO-Al2O3 support. The peaks at 100-400 cm-1 also belonged to the V2O5 stretching vibration [30]. The wide band vibration at 800-900 cm-1 revealed the polymerization of vanadium oxygen species [31]. The peak strength became gradually stronger with the further increase of v anadium content.
The catalytic reaction was conducted in the temperature range of 500-600 °C. To account for the gas phase oxidation dehydrogenation and thermal cracking effects, a blank experiment was performed in the empty reactor (without catalyst) with the same feed gas composition. n-Butane conversion and butene selectivity in the reactor were 3.36% and 16.6% (9% 1-C4H8, 5.2% 2-t-C4H8, 2.4% 2-c-C4H8, none 1,3-C4H6) at 600 °C, respectively. Based on these results, the influence of the gas phase conversion was insignificant for the evaluation of the active catalysts. The catalytic performance of the various V2O5/MO-Al2O3 catalysts for n-butane oxidative dehydrogenation experiments was measured under different conditions, i.e., reaction temperature, V2O5 loading, n-C4H10/O2 ratio, and reaction space velocity. The main carbon-containing products were CO2, CO, C2, C3H6, i-C4H10, 1-C4H8, 2-t-C4H8, 2-c-C4H8, 1,3-C4H6.
Table 3 shows the n-butane conversion and selectivity to C4=,tot and COx at a low space velocity (GHSV = 7500 mL/(g∙h)) and at a high space velocity (GHSV = 51000 mL/(g∙h)), respectively. From Table 3, the samples doped with different alkaline earth metals exhibited different activity, resulting in 27.2%, 26.3%, 25%, 25.7% n-butane conversion and 61.6%, 36.1%, 59.1%, 57.5% C4=,tot selectivity for the Mg, Ca, Sr, Ba doped catalysts at the low space velocity; 26%, 3.8%, 13.4%, 13% n-butane conversion and 61.4%, 38.7%, 50.2%, 51.1% C4=,tot selectivity were achieved at the high space velocity. The significant decrease in conversion at the higher space velocity revealed that the Ca, Sr, Ba doped catalysts have lower catalytic activity than the Mg doped catalyst. In addition, the Mg doped catalyst was better than the undoped one in terms of both n-butane conversion and C4=,tot selectivity. Both at the low space velocity and high space velocity, the catalyst doped with Mg exhibited the highest catalytic activity and selectivity among these catalysts. Therefore, the catalytic activity order was: V2O5/MgO-Al2O3 > V2O5/Al2O3 > V2O5/SrO-Al2O3 ≈ V2O5/ BaO-Al2O3 >> V2O5/CaO-Al2O3. The low activit y and selectivity of the Ca, Sr, Ba doped catalysts were related to their low specific surface areas (Table 1) and the stable orthovanadate (see Fig. 1(a) and (b)) where V5+ was difficult to reduce to V4+ under the reaction conditions, as shown in Fig. 3. Thus the redox cycle was difficult to establish.
The conversion and product selectivity for the n-butane ODH reaction are affected by both the vanadium loading and vanadium surface density [32, 33, 34, 35]. As shown in Table 4, at the low space velocity, there was no large change in n-butane conversion and C4=,tot selectivity for the catalysts with 3%, 5%, 10%, 15% V2O5 loadings, but the 1,3-C4H6 selectivity gradually decreased with increased V2O5 loading after it reached the maximum at 5%. At the high space velocity, n-butane conversion, C4=,tot selectivity, 1,3-C4H6 selectivity and the STY all gradually decreased when the V2O5 loading was more than 5%. The high space velocity was more favorable for the ODH reaction compared to the low space velocity in terms of the conversion, butene selectivity and STY. It can be inferred that the monolayer coverage was achieved when the V2O5 loading reached 5%. As the V2O5 loading was increased to 10%, the V2O5 crystalline phase appeared which was reduced to V2O3 under the reaction conditions (see Fig. 1(c)). However, as the V2O5 loading was decreased to 3%, the MgO crystalline phase disappeared, which was accompanied by a decrease in conversion and selectivity. It has been proved by other researchers that the MgO crystalline phase is necessary to achieve high catalytic activity and selectivity for V-Mg-O catalysts [32]. Therefore, the high performance was dependent on the well dispersed VOx species and the existence of the MgO crystalline phase, which wer e both fulfilled at a V2O5 loading of 5%.
The effect of temperature on n-butane ODH reaction was investigated in the temperature range of 500-600 °C. Table 5 lists the conversion and selectivity for the n-butane ODH reaction performed on the 5%-V2O5/MgO-Al2O3 catalyst at different temperatures. n-Butane conversion and C4=,tot selectivity increased with the reaction temperature both at the low and high space velocity. The 1,3-C4H6 selectivity increased from 18.7% to 24.1% at the low space velocity and from 23.1% to 32.0% at the high space velocity as the reaction temperature was increased from 500 to 600 °C, which was accompanied by a decrease in COx (CO + CO2) selectivity. This result indicated that both high space velocity and high temperature are beneficial to the formation of 1,3-C4H6. A similar phenomenon was also reported by other researchers on V2O5/MgO catalyst [36, 37].
The ODH reactions under different n-butane/O2 ratios (i.e. 1.2/1, 1.5/1, 2/1) were conducted to know its effects on the catalytic performance of the 5%-V2O5/MgO-Al2O3 catalyst. Table 6 lists the reaction results at 600 °C. The COx selectivity decreased with the increase in n-butane/O2 ratio. This was due to the decrease in the deep oxidation of n-butane and butenes when the O2 concentration in the feed gas was decreased. The n-butane conversion decreased as expected, while the selectivity to C4=,tot increased significantly with the increase in n-butane/O2 ratio. As shown in the table, the STY was higher than 1500 g/(kg∙h) when the n-butane/O2 ratio was higher than 2/1 at the GHSV of 51000 mL/(g∙h).
The influence of reaction space velocity is shown in Table 4 to 6 for the V2O5/MgO-Al2O3 catalysts. At a fixed n-butane/O2 ratio and reaction temperature, the selectivity to 1,3-C4H6 and C4=,tot increased with the space velocity, while that of COx decreased. This was attributed to that the reaction contact time was shortened from ~0.48 to ~0.07 s when the space velocity was increased from 7500 to 51000 mL/(g∙h). As a result, deep oxidation was inhibited. Since the ODH reaction was conducted at a higher n-butane/O2 ratio of 1.5/1, which is much higher than that used in the combustion reaction (n-butane/O2 ratio of 1/6.5) and that used by other researchers (n-butane/O2 ratio of 1/2) [22], the oxygen conversion was 100% for all the experiments. More oxygen was consumed by the combustion reaction than the selective oxidation reactions, thus the increase of space velocity not only improved the selectivity of 1,3-C4H6 and C4=,tot but also increased the n-butane conversion, as shown in Table 6.
Table 7 gives a comparison of the catalytic performance of the 5%-V2O5/MgO-Al2O3 catalyst in this study compared with those reported in the literature at similar n-butane conversion. It can be found that the catalyst developed in this study has both higher1,3-C4H6 and C4=,tot selectivity than most of the reported catalysts [38, 39, 40]. The high performance was related to that the well dispersed VOx species and the existence of MgO crystalline phase, which were both fulfilled at 5% V2O5 loading.
V2O5/MO-Al2O3 (M = Mg, Ca, Sr, Ba) catalysts were synthesized by wet impregnation. Their catalytic performance was tested with the n-butane ODH reaction. For the catalysts doped with Ca, Sr or Ba, the orthovanadate phase was difficult to reduce under the reaction conditions, so the redox cycle of the catalysts was difficult to be established. The catalyst doped with Mg that showed high catalytic activity was easier to reduce than those with the other alkaline earth dopants. The higher specific surface area of the Mg modified support gave high dispersion of VOx species, while 5% V2O5 loading allowed the existence of the MgO crystalline phase. Therefore, the 5%-V2O5/ MgO-Al2O3 catalyst exhibited high catalytic activity and butene selectivity. Under the optimal operation conditions, a n-butane conversion of 30.3%, C4=,tot selectivity of 64.3%, and STY of 1530 g/(kg∙h) were achieved at 600 °C.
The authors gratefully acknowledge financial support from the Yanchang Petroleum.
2015, Vol. 36 Issue (7): 1060-1067
PDF (726 KB)
2015, Vol. 36 Issue (7): 1060-1067
PDF (726 KB)