MgAl2O4 Catalysts

Article pubs.acs.org/IECR

Research on Isobutane Dehydrogenation over Mo/MgAl2O4 Catalysts and Pilot-Scale Evaluation in a Circulating Fluidized-Bed Unit Guowei Wang, Chunyi Li,* Honghong Shan, and Wenlong Wu State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, P. R. China ABSTRACT: In this article, an environmentally friendly non-noble-metal class of Mo/MgAl2O4 catalysts is demonstrated to exhibit excellent performance in isobutane dehydrogenation. Results of activity tests indicated that an induction period is required for the catalysts to develop the active species. Further investigation revealed that high-valence-state Mo species are favorable for the initial oxidative reactions and Mo4+ ions are probably the actual active species for catalytic dehydrogenation. Given the strongly endothermic characteristics and rapid catalyst deactivation of alkane dehydrogenation, a circulating fluidized-bed unit was further applied to evaluate the activity and stability of the catalysts. To avoid undesired oxidative reactions and improve the initial catalytic activity, a flow of hydrogen was introduced into the pilot-scale unit to prereduce the catalysts prior to reaction. In general, the catalysts exhibited highly active and stable performance in the pilot-scale evaluation, with an isobutene yield of up to 35 wt %, demonstrating great potential for industrial applications.

1. INTRODUCTION Given the increasing demand for light alkenes, technologies for the dehydrogenation of the corresponding alkanes have gained renewed attention. In addition, the development of novel catalytic systems that are inexpensive and environmentally friendly represents a major concern.1 Mo-based catalysts, which are safe and relatively inexpensive, have been comprehensively studied in the oxidative dehydrogenation (ODH) of light alkanes such as ethane,2 propane,3−10 and isobutane.11 Highly exothermic ODH eliminates the equilibrium conversion restriction of dehydrogenation and decreases energy consumption.12−14 However, compared with catalytic dehydrogenation (CDH), ODH exhibits lower selectivity to olefins because deep oxidative reactions of alkanes to COx are difficult to control.15−17 The safety of cofeeding paraffins and oxygen is also a cause of concern, as appropriate paraffinto-oxygen ratios must be maintained.15 The aforementioned problems encountered in the ODH process limit the commercial application of Mo-based catalysts. Meanwhile, little research has been done on CDH over Mo-based catalysts. Ledoux and co-workers18,19 reported that carbon-modified MoO3 catalysts are active and selective for the CDH of n-butane, but further improvements are still needed to improve the activity and stability of the catalysts. In addition to the active component, the support of the catalyst also crucially influences the catalytic performance;20,21 therefore, the appropriate choice of the support can effectively improve the performance. ZrO2-supported catalysts induce deeper oxidation than MgO-supported catalysts because of the creation of oxygen on the ZrO2 surface.22 TiO2 is the most selective support for the ODH of isobutane over Mo-based catalysts compared to other supports such as SiO2, Al2O3, and Nb2O5.8 Recently, MgAl2O4 was used as the support for V-based20 and Pt-based23,24 catalysts because of its superior characteristics, namely, moderate acid−base properties, high thermal resistance,25 and excellent catalytic performance.26 Regarding the unit for isobutane dehydrogenation, the circulating fluidized-bed (CFB) reactor might be a good choice. © 2013 American Chemical Society

Isobutane dehydrogenation is a highly endothermic reaction, and high reaction temperatures are necessary to achieve the desired conversion level. However, this leads to high energy consumption, the additional opportunity for side reactions, and rapid catalyst deactivation by coking at the same time. In a CFB unit, with the reaction occurring in the reactor, the spent catalysts can be simultaneously regenerated by burning off the coke and oxidizing the reduced Mon+ to high valence states in the regenerator, realizing continuous reaction and regeneration operations.27,28 Furthermore, the heat consumption for isobutane dehydrogenation can be compensated by recycling high-temperatureregenerated catalysts in a CFB unit, and the high level of mixing of the solid catalysts can also enhance both heat and mass transfer in the reactor.28−30 Therefore, the CFB unit is highly suitable for reactions with obvious heating effects such as dehydrogenation. Nevertheless, because of obvious particle attrition, the catalyst consumption is much higher in a CFB unit than in a fixed-bed reactor; therefore, it is uneconomical to use noblemetal catalysts in fluidized beds. Moreover, because fine catalyst powder (with diameter smaller than 10 or even 20 μm) is likely to be transferred into the separation system and the emission of the fine powder together with flue gas into the atmosphere is inevitable, the catalyst should be harmless to the environment. Fortunately, Mo-based catalysts can meet the environmental requirements and are fully acceptable for CFB unit operations if the attrition resistance is high enough. In this article, the dehydrogenation of isobutane over a novel Mo/MgAl2O4 catalyst system with different Mo loadings in the absence of oxidative gases was studied. The main aim was to determine the optimal Mo loading and the actual active species for isobutane dehydrogenation. Moreover, the stability and activity of the catalysts were also evaluated in a pilot-scale CFB unit. Received: Revised: Accepted: Published: 13297

December 16, 2012 March 31, 2013 August 27, 2013 August 27, 2013 dx.doi.org/10.1021/ie303487q | Ind. Eng. Chem. Res. 2013, 52, 13297−13304

Industrial & Engineering Chemistry Research

Article

ratio of 1:6 was fed into the microreactor at a total flow rate of 14 mL/min; that is, the reaction was carried out at a gas hourly space velocity (GHSV) of 54.5 h−1 for isobutane. The effluent gaseous products were analyzed using a gas chromatograph (Bruker 450) equipped with a flame ionization detector (FID) to determine the composition of hydrocarbons, as well as two thermal conductivity detectors (TCDs) to analyze the contents of hydrogen, carbon monoxide, and carbon dioxide. The conversion and product selectivity were calculated as follows

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. MgAl2O4 was prepared from pseudoboehmite and magnesium nitrate. First, pseudoboehmite powder was mixed with distilled water in a vessel to obtain a suspended solution. Then, HCl solution (36−38 wt %) was added dropwise under vigorous stirring at 70 °C. After a gel-like mixture had formed, an appropriate amount of magnesium nitrate solution was added. The mixture was stirred for 4 h, dried at 140 °C for 24 h, and calcined at 700 °C for 4 h in air atmosphere. Finally, the support was crushed and sieved to 80− 180 μm for later use. The pore properties and specific surface area of the as-prepared support are listed in Table 1. Table 1. BET Surface Areas and Pore Properties of Mo/MgAl2O4 Catalysts catalyst

specific surface area (m2/g)

pore volume (cm3/g)

pore size (nm)

MgAl2O4 5Mo/MgAl2O4 10Mo/MgAl2O4 20Mo/MgAl2O4 30Mo/MgAl2O4

116 102 71 55 38

0.27 0.24 0.22 0.17 0.13

7.9 7.9 9.6 11.5 12.1

conversion of isobutane (wt %) isobutane,in − isobutane,out = × 100 isobutane,in

(1)

selectivity to product i (wt %) i , out = × 100 isobutane,in − isobutane,out

(2)

2.3.2. Isobutane Dehydrogenation in a Pilot-Scale Circulating Fluidized-Bed Unit. A schematic of the pilot-scale CFB unit is presented in Figure 1. The unit mainly consists of a reactor (with a length of 1 m and an inner diameter of 92 mm) coupled with a disengager, a regenerator, a reduction section,

Subsequently, a series of Mo/MgAl2O4 catalysts with various Mo loadings was prepared by the incipient wetness impregnation of the as-prepared MgAl2O4 with corresponding amounts of ammonium heptamolybdate solution. Then, the wetness impregnation samples were dried for 24 h and calcined for 2 h at 700 °C. The MoO3 content of the catalysts was varied from 5 to 30 wt %, and corresponding samples are denoted hereafter as xMo/MgAl2O4, where x is the nominal MoO3 content by weight. In addition, the catalysts used in the pilotscale evaluation were prepared by spray granulation. 2.2. Catalyst Characterization. The specific surface areas and pore structure properties of the samples were determined by nitrogen adsorption−desorption measurements at liquidnitrogen temperature using a Quadrasorb SI instrument. X-ray diffraction (XRD) patterns of the samples were obtained using an X’Pert PRO MPD diffractometer system with Cu Kα radiation at 40 kV and 40 mA operated from 5° to 75° at a speed of 10°/min. The redox behaviors of the samples were examined by the temperature-programmed reduction (TPR) of hydrogen. About 100 mg of sample was loaded into the apparatus, pretreated under helium flow at 200 °C for 0.5 h, brought into contact with a H2/N2 mixture after being allowed to cool to 80 °C, and then heated at a rate of 10 °C/min to 1000 °C. X-ray photoelectron spectra were recorded using a Thermo Fisher K-Alpha apparatus with an Al Kα X-ray excitation source. The binding energy (BE) values were calibrated against the C 1s peak at 284.6 eV. The data processing involved background subtraction using the Shirley method, spectrum deconvolution using a Gaussian−Lorentzian mixed-line-shape function, and peak-area determination by integration. 2.3. Catalytic Activity Tests. 2.3.1. Isobutane Dehydrogenation in a Fixed-Bed Microreactor. Isobutane dehydrogenation was conducted in a fixed-bed microreactor at atmospheric pressure and 560 °C. About 4 g of catalysts with a size of 80− 180 μm was loaded into the reactor. The catalysts were initially degassed at reaction temperature under a nitrogen flow to remove adsorbed oxygen and water before reaction. During the test, a flow of isobutane diluted by nitrogen with a fixed molar

Figure 1. Schematic diagram of the pilot-scale CFB unit. 13298

dx.doi.org/10.1021/ie303487q | Ind. Eng. Chem. Res. 2013, 52, 13297−13304


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at 23.2°, 26.4°, and 27.1° attributed to the crystalline structure of MgMoO4 emerged, and the intensity of the MgMoO4 peaks progressively increased with increasing Mo content. The presence of MgMoO4 suggests that the impregnation of Mo species affects the composition and structure of the support because of the aggregation and crystallization of the new phase MgMoO4. Xie and Tang31 reported that a MoO3 molecule occupies 20 Å2. Hence, to form a closed-packed MoO3 monolayer on the catalyst surface, the specific surface areas of 5Mo/MgAl2O4 and 10Mo/MgAl2O4 should be at least 41.8 and 83.7 m2/g, respectively. Previous N2 adsorption−desorption measurements indicated that samples with Mo loadings larger than 5 wt % have insufficient surface areas, further leading to the formation of crystalline MgMoO4. Therefore, Mo species can be better dispersed over 5Mo/MgAl2O4 catalysts than over the other asprepared catalysts. The reducibility of the Mo species in the Mo/MgAl2O4 catalysts was investigated by TPR experiments. Figure 3 shows

and a delivering riser. During the tests, isobutane was introduced into the bottom of the reactor through a pump at a flow rate of 5 kg/h, flowing upward and contacting the prereduced catalysts (mean diameter of 70 μm) with a catalyst-to-isobutane weight ratio about 4 at 560 °C (exit of the reactor with 20 °C gradient inside) and atmospheric pressure for 10 s. After the reaction, the mixture of products and catalysts was separated in the disengager; the gaseous products were collected, and their composition was determined by a Bruker 450 gas chromatograph. The spent catalysts were transferred into the regenerator through the delivering riser to burn off the coke and oxidize the reduced Mo to high valence state in the air for about 15 min at 700 °C, and the flow rate of the main air was 550 L/h. Then, to shorten the induction period and improve the initial activity, the regenerated catalysts were delivered into the next section for prereduction. A flow of hydrogen (120 L/h) was injected into the bottom of the reduction section operated at approximately 700 °C, and the catalysts coming from the regenerator contacted with the hydrogen for about 10 min. Finally, the prereduced catalysts were transported back into the reactor for continuous operation.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. N2 adsorption−desorption measurements were carried out to determine the specific surface areas and pore structures of catalyst samples. As listed in Table 1, the bare support had the largest specific surface area, and the surface area decreased from 116 to 38 m2/g as the Mo content increased. This variation trend is similar to those reported by other researchers2,5 and is believed to be caused by the obvious plugging effect of the progressively aggregated Mo species in the partial pore channels of the support, as evidenced by the decrease in the pore volume with increasing Mo loading. Such a reduction in the surface area further results in poor dispersion of Mo species on the support. Phase analysis by XRD was carried out to investigate the phase composition of the various Mo/MgAl2O4 catalysts. The diffraction pattern of 5Mo/MgAl2O4 catalyst in Figure 2 presents a well-crystallized phase characteristic of the bare support MgAl2O4, indicating the highly dispersed nature of the Mo species on this sample. As the Mo loading increased, additional diffraction peaks

Figure 3. H2 TPR profiles of Mo/MgAl2O4 catalysts with different Mo loadings: (a) 5Mo/MgAl2O4, (b) 10Mo/MgAl2O4, (c) 20Mo/MgAl2O4, (d) 30Mo/MgAl2O4.

that all profiles of the investigated samples exhibit two distinct reduction peaks from 400 to 750 °C and from 750 to 1000 °C. This result is consistent with the research of Kumar et al.32 Previous studies33,34 also reported that the reduction pattern of bulk or supported MoO3 follows the stepwise path Mo6+ → Mo4+ → Mo0 and that the low-temperature peak is due to the reduction of Mo6+ whereas the high-temperature peak is due to the reduction of Mo4+. As shown in Figure 3, the hydrogen consumption area increased significantly with increasing Mo loading for both lowand high-temperature peaks because of the increasing amount of reducible species, and the temperatures of the two wellresolved peaks also increased. These results combined with the XRD patterns jointly indicate that the well-dispersed Mo species on the support are more reducible than the highly aggregated Mo species. 3.2. Effect of Mo Loading on the Catalytic Performance. Isobutane dehydrogenation over Mo/MgAl2O4 catalysts, as well as over pure MoO3 and MgAl2O4, was investigated in the absence of oxidants in a fixed-bed microreactor at 560 °C. The conversion and selectivity data (1.5 h on stream) of the investigated catalysts are plotted in Figure 4. As the Mo loading increased, the isobutane conversion increased, reached a

Figure 2. XRD patterns of Mo/MgAl2O4 catalysts with different Mo loadings: (a) 5Mo/MgAl2O4, (b) 10Mo/MgAl2O4, (c) 20Mo/MgAl2O4, (d) 30Mo/MgAl2O4. 13299



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result of the plugging of intergranular pores. All of these defects restricted the contact between the active species and isobutane, resulting in decreased reactivity. Therefore, well-dispersed Mo species are indispensable for achieving high reactivity of Mo/MgAl2O4 catalysts. On the other hand, the selectivity toward isobutene increased with Mo loading. Mo/MgAl2O4 catalysts with low Mo loadings contain abundant pore channels, which are unfavorable for product desorption and provide more opportunities for undesired secondary reactions. On the contrary, increasing the Mo loading decreases the pore volume, facilitating desorption and diffusion of the product and finally increasing the isobutene selectivity. 3.3. Determination of the Active Species. For the excellent performance of 5Mo/MgAl2O4 catalysts, isobutane dehydrogenation over the catalysts was further carried out at 560 °C for 8 h. The results are shown in Figure 5, and detailed product distributions in the initial period of the reaction are listed in Table 2. For the fresh catalysts, both the isobutane conversion and the selectivity toward isobutene were relatively low in the initial stage and increased as the reaction progressed.

Figure 4. Conversions of isobutane and selectivities to isobutene obtained at 560 °C for Mo/MgAl2O4 catalysts with different Mo loadings.

maximum at 5% loading, and then decreased at higher loadings. MgAl2O4 is almost inactive for the reaction; therefore, catalysts with Mo loadings lower than 5% are probably not sufficiently active because they do not have enough active sites on the catalyst surface. MoO3 also demonstrates very low isobutane conversions under the reaction conditions. Figure 4 further shows that the selectivity to isobutene increased with Mo content, and the yield of isobutene achieved a maximum at a Mo loading of 5 wt %. According to Kumar et al.,32 a Mo loading of 8 wt % is monolayer coverage of MoO3 on the MgO−Al2O3 support surface. Considering the XRD results, 5 wt % Mo could be well-dispersed on the surface of the support, whereas crystalline MgMoO4 formed on the catalyst surface as the loading increased to 10 wt %, indicating that the dispersion worsened with increasing Mo content. Precisely because of the good dispersion of active components at low Mo loading, the contact between active species and isobutane was facilitated, and the catalyst reactivity was further increased. However, decreasing surface area and crystal growth were observed at high Mo loadings, and the pore volume also decreased markedly as a

Table 2. Initial Product Distributions of Different 5Mo/MgAl2O4 Catalysts parameter

without pretreatment

H2 prereduction

selectivity (wt %) methane ethane ethene propane propene n-butane n-butene i-butene H2 CO2 CO othersa conversion (wt %) mass balance (wt %)

3.65 0.20 0.84 0.33 5.69 0.07 2.51 71.67 2.10 8.68 2.23 2.03 18.06 97.5

3.40 0.83 0.65 1.31 4.66 1.15 8.01 73.18 4.10 0.00 1.03 1.68 43.17 98.1

a

Others refers to some hydrocarbons, such as 3-methyl-1-butylene, 2-pentene, 2-methyl-1-butylene, and 2-methyl-2-butylene.

Figure 5. (a) Conversions of isobutane and (b) selectivities to isobutene obtained at 560 °C for 5Mo/MgAl2O4 catalysts. 13300



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At 1.5 h, the maximum conversion was achieved, and the activity decreased slightly with time on stream because of coke formation and the consumption of active sites. This phenomenon indicates that an induction period is required to develop the actual active species, namely, a period for the in situ formation of active sites from well-dispersed Mo species under the reaction conditions, as confirmed by the subsequent study. Similar induction periods have also been reported for n-butane and propane dehydrogenation over MoAl and InAl catalysts.35−38 In general, the selectivity to isobutene increased with time on stream: The actual active species that appeared after the induction period benefited the initial increase, and the latter increase was probably caused by the decreased acidity because of coke coverage. The appearance of the induction period indicates that Mo species on the catalyst surface probably undergo some kind of transformation during the reaction. Therefore, a comprehensive study was further conducted to determine the actual active species responsible for the CDH of isobutane. To verify whether the reduced Mo species or the molybdenum carbide formed during the reaction is the actual active species, H2 prereduced catalysts were also subjected to the activity test, and the results are included in Figure 5 and Table 2. As expected, the initial conversion was much higher than the case without pretreatment and then declined with time on stream, suggesting that the actual active species are probably the reduced Mo species generated on the catalyst surface after H2 prereduction, rather than molybdenum carbide. With regard to the H2 prereduced catalysts, the selectivity to COx in the initial stage was about one-tenth that of the untreated catalysts, whereas the initial selectivity to H2 was significantly higher, indicating that the prereduction of the catalysts consumes the lattice oxygen and absorbed oxygen species, which are the origin of the undesired oxidative reactions, and facilitates catalytic dehydrogenation of isobutane. The selectivity to isobutene showed a trend similar to that of the fresh catalysts, namely, steadily increasing with progress of the reaction, but at a much higher value. Moreover, a TPR study of 5Mo/MgAl2O4 catalysts subjected to H2 prereduction at 560 °C for 1.5 h was also performed. Compared with the TPR profile of the untreated catalysts, the low-temperature peak shifted toward a lower reduction temperature, and the corresponding area evidently decreased, whereas the high-temperature peak remained almost unchanged. Given the aforementioned fact that the low-temperature peak is due to the reduction of Mo6+ whereas the high-temperature peak is due to the reduction of Mo4+, it can be concluded that the reduction of Mo4+ species is not initiated for the prereduced catalysts, and the valence of the Mo species on the reduced catalyst is presumably between +4 and +6, as later confirmed by XPS study. XPS study was also applied to gain insight into the valence variation of Mo species during the reaction and after H2 reduction, with results displayed in Figure 6. Table 3 provides more specific information on the deconvoluted spectra in the Mo 3d region of different 5Mo/MgAl2O4 catalysts. In the case of the fresh 5Mo/MgAl2O4 sample (Figure 6a), not all valence states of the Mo species on the catalyst surface were the highest. Two Mo 3d doublets with high BEs at 232.8 and 231.9 eV, assigned to Mo(VI) and Mo(V), respectively,35,39−41 can be observed. Based on the deconvolution analysis (Table 3), Mo(V) species account for 78% of all Mo species. After 1.5 h of reaction at 560 °C, additional features corresponding to the formation of Mo4+ species (BE Mo 3d5/2 = 230.2 eV)40,41

Figure 6. XPS results for different 5Mo/MgAl2O4 catalysts: (a) fresh 5Mo/MgAl2O4, (b) 5Mo/MgAl2O4 reacted for 1.5 h, (c) 5Mo/ MgAl2O4 reacted for 5 h, (d) 5Mo/MgAl2O4 prereduced in H2.

appear. After 5 h, the signals characteristic of Mo6+ disappear, and the features assigned to Mo2+/3+ emerge with a BE of Mo 3d5/2 at 229.0 eV,40,41 indicating that the reduction of Mo species gradually proceeds with increasing reaction time, as expected. With regard to the BE ranges of the Mo 3d electrons in the spectrum of the sample prereduced by H2, Figure 6d shows that two doublets typical of Mo5+ and Mo4+ species are contributed by the consumption of Mo6+ and some of the Mo5+ species during the reduction process, which agrees with the H2 TPR results. Determination of the actual active species was done with the combined analyses of the results obtained in the activity tests and XPS studies. As the reaction proceeds, the CDH of isobutane is initially enhanced with the consumption of Mo6+ species, indicating that Mo6+ ions are possibly not the active species. Assuming that Mo5+ ions are the unique active species, 78% of the Mo5+ species present on the fresh catalyst surface should result in a high proportion of CDH at the very beginning of the reaction, which is contrary to the preceeding discussion. Hence, the Mo5+ species are also not active enough for the CDH of isobutane. The activity of the catalysts for isobutane dehydrogenation achieves a maximum after 1.5 h of reaction, and the corresponding valence distribution (Table 3) is 8% Mo6+, 79% Mo5+, and 13% Mo4+. Because of the exclusion of both Mo6+ and Mo5+, the Mo4+ ions or a combination of Mo4+ and other ions (Mo6+ or Mo5+) are probably the active species. Given the high initial performance of the prereduced catalysts and the fact that Mo6+ ions do not exist on the catalyst surface after prereduction, the Mo6+ ions are not indispensable to isobutane dehydrogenation. Thus, the possibility of a combination with Mo6+ is eliminated. Moreover, the isobutane conversion decreases dramatically from 1.5 to 5 h, and the ratio of the Mo4+ species also shows a declining trend. By contrast, during the declining period of isobutane conversion, Mo5+ ions still increase. 13301



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Table 3. Peak Deconvolution Results of Mo 3d Photoelectron Signals for Different 5Mo/MgAl2O4 Catalysts catalyst fresh (after calcination) after 1.5 h of reaction after 5 h of reaction after H2 reduction oxidation state

Mo 3d5/2 232.8 eV 22% 233.0 eV 8% − − − − Mo(VI)

average oxidation state − − 230.2 eV 13% 230.5 eV 11% 230.3 eV 14% Mo(IV)

231.9 eV 78% 232.2 eV 79% 232.2 eV 85% 232.1 eV 86% Mo(V)

− − − − 229.0 eV 4% − − Mo(II,III)

5.22 4.95 4.79 4.86

Figure 7. Results of the catalytic dehydrogenation of isobutane in the CFB unit.

Therefore, the Mo4+ ions are more likely to be the actual active species than a combination of Mo4+ and Mo5+ ions. In the presence of abundant oxygen species on the fresh catalyst, the reaction begins with relatively intense unselective oxidative reactions and ODH under the catalysis of Mo6+ species. However, as the consumption of surface oxygen species and the appearance of Mo4+ species proceed, CDH dominates the reaction during the steady period. The possible reactions in these two stages are as follows

catalytic dehydrogenation and probably facilitate the initial unselective oxidative reactions to COx, regenerated catalysts after air combustion would be inappropriate for isobutane dehydrogenation. Prereduction of the catalysts with hydrogen prior to reaction seems to be a suitable approach to avoid oxidative reactions and improve the initial catalytic performance. A pilot-scale CFB unit patented by our research group44 was applied to evaluate the commercial potential of Mo/MgAl2O4 catalysts for isobutane dehydrogenation under reaction conditions similar to those used for the activity tests in the fixed-bed microreactor. Based on the earlier discussion, to reduce Mo species to the active phase, a flow of hydrogen was introduced into a separate section for prereducing the regenerated catalysts prior to reaction (as displayed in Figure 1). Because of the variation of the valence states of the Mo species as the reduction proceeds, it is difficult to obtain the active Mo species even if hydrogen is introduced. However, in the pilot-scale circulating fluidized-bed unit, it is impossible and unnecessary to transform all of the Mo species on the catalysts to Mo4+ because of the fact that catalytic performance can be maintained at a relatively high level with the valence state of most Mo species being controlled effectively. Consequently, the operating conditions of the reduction section, such as reduction temperature, flow rate of introduced hydrogen, and average residence time of the catalyst, should be adjusted appropriately. Through comprehensive investigations into the relationship between catalytic performance and reduction conditions, the optimal reduction conditions were obtained as a temperature of approximately 700 °C, a hydrogen flow rate of 120 L/h, and a mean catalyst residence time of 10 min. Because of the high regenerated catalyst temperature of 700 °C and the exothermic characteristics of the reduction process, a high

Initial stage i‐C4 H10 + (13 − x)[O] → xCO + (4 − x)CO2 + 5H 2O

i‐C4 H10 + [O] → i‐C4 H8 + H 2O

Steady stage i‐C4 H10 → i‐C4 H8 + H 2

Studies on active species are extensive. Haber and Lalik35 pointed out that reduced Mo4+ ions on the catalyst surface can act as the active species for hydrocarbon molecule activation by hydrogen abstraction. However, Abello et al.42 reported that the ODH activity of propane is related to Mo5+ ion formation. The active species for n-butane dehydrogenation are also possibly produced by the reduced oxidation of Mo, Mo5+, or Mo4+, as previously reported by Chen et al.43 Because of the discrepancy in the catalytic systems used, these conclusions from previous studies are not fully consistent with our results, but our efforts can still greatly contribute to the development of the investigated catalytic system. 3.4. Pilot-Scale Evaluation in a CFB Unit. Considering that high-oxidation-state Mo species are not the active phase for 13302



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Notes

temperature in the reduction section can easily be achieved. As for the prereduction process, although the possibility of oxidative reaction during isobutane dehydrogenation is reduced, over-reduction also has a negative influence on the catalytic performance, such as insufficient activity of Mo species with a much lower valence state. In addition, the reaction temperature can be adjusted by either changing the temperature of the regenerated catalysts or manipulating the catalyst circulation rate (i.e., higher regenerated catalyst temperature and higher catalyst circulation rate lead to higher reaction temperature). Accordingly, both the catalyst circulation rate and the conditions of the reduction and regeneration processes should be appropriately regulated for isobutane dehydrogenation in this pilot-scale CFB unit. The pilot-scale evaluation in this CFB unit lasted for 200 h, and the results are illustrated in Figure 7. It is clearly shown that, because of the prereduction of the catalysts, the induction period disappeared and the isobutane conversion remained at a high level, varying slightly from 42 to 48 wt % for the 200-h test, further confirming that reduced Mo ions are the actual active species. In addition, the selectivity to isobutene fluctuated steadily on the whole, even slightly higher than that in the fixedbed reactor. In general, the isobutene yield remained relatively constant at about 35 wt %, indicating an outstanding potential for commercial applications of the catalysts. To improve the economics of a practical production process, the hydrogen introduced into the reduction section in this pilot-scale unit can be replaced by coproduced hydrogen-rich dry gas or purified hydrogen obtained by pressure swing adsorption (PSA). Furthermore, because of the low coke yield in isobutane dehydrogenation, the heat released in the regenerator through the combustion of coke deposited on the catalyst is insufficient for the heat supply, and additional fuel is still needed to satisfy the heat requirements of this strongly endothermic reaction.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National 973 Program of China (No. 2012CB215006) and the Fundamental Research Funds for the Central Universities (No. 12CX06040A).

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4. CONCLUSIONS Isobutane dehydrogenation over a novel high-potential class of Mo/MgAl2O4 catalysts was investigated in this research. Activity test results revealed that an induction period is necessary for the catalysts to develop the active species. As further evidenced by the XPS study, Mo species with high valence states were favorable for the initial oxidative reactions, and Mo4+ species were probably responsible for catalytic dehydrogenation. Because isobutane dehydrogenation is strongly endothermic and the catalysts deactivate quickly, a CFB unit was employed for isobutane dehydorgenation over the Mo-based catalysts to achieve continuous reaction and regeneration. The evaluation of the activity and stability of the catalysts was carried out in a pilot-scale CFB unit. To maintain the active valence state of Mo species and avoid the induction period of the catalysts, a flow of hydrogen, which could be replaced by hydrogen-rich dry gas or purified hydrogen obtained by PSA, was introduced into the pilot-scale unit for prereducing the catalysts prior to reaction. In general, the catalysts exhibited excellent activity and stability, with an isobutene yield of up to 35 wt %, and demonstrated great potential for commercial applications.

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MgAl2O4 Catalysts

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