Light-Alkane Oxidative Dehydrogenation to Light Olefins over

Article pubs.acs.org/IECR

Light-Alkane Oxidative Dehydrogenation to Light Olefins over Platinum-Based SAPO-34 Zeolite-Supported Catalyst Zeeshan Nawaz*,†,‡ and Fei Wei† †

Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China ‡ Chemical Technology Development, SABIC Technology & Innovation Center, Saudi Basic Industries Corporation (SABIC), Riyadh 11551, Saudi Arabia ABSTRACT: The experiments were conducted to study Pt−Sn/SAPO-34 catalyst utilization for oxidative dehydrogenation (ODH) of light paraffins to olefins. In recent times, the interesting concept of similar catalytic bed usage for direct dehydrogenation (PDH) and ODH has gotten attention. The light-paraffin ODH process was enhanced over the Pt−Sn-based catalyst using SAPO-34 as a support. The catalyst demonstrated promising results for propane and butane dehydrogenation, where the initial feed conversion is about 70 wt % at optimum operating conditions of 2:1 feed/oxygen ratio and 550 °C because of lower carbon monoxide and carbon dioxide (COx) formation. A negligible amount of coke formation and a slower deactivation rate was noticed over the proposed catalyst in comparison with the PDH reaction and other ODH processes. Propylene and butene selectivities were obtained above 96 and 90 wt %, respectively. This superior light-olefin selectivity in the ODH reaction was due to superior catalytic control of reaction by providing suitable hydrogen-abstraction platinum sites and intermediate product conversion over SAPO-34. The catalyst performance was analyzed by comparing experimental justification with characterization like X-ray fluorescence, surface area, dioxygen-pulse coke analysis, dihydrogen chemisorption, and transmission electron microscopy.

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INTRODUCTION Naphtha cracking gives a range of products such as ethane, propane, butane, and pentane with some amount of light olefins. The paraffins were further catalytically processed to produce valuable olefinic products. The choice of gas or liquid feedstock largely depends on the availability, cost, and goals of product utilization. As crackers, feeds have become lighter, favoring ethylene production, and fluidized catalytic cracking (FCC) gasoline demand brought “on-purpose” propylene production technologies. Oxidative dehydrogenation (ODH) of propane to propylene has been studied extensively, while the independent process is not promising. Recently, an innovative idea of using a direct dehydrogenation (PDH) catalytic bed for the ODH reaction for equilibrium shifting, reaction coupling, heat management, etc., to develop a more economical process has been of interest. Mobil Oil Corporation introduced the concept of simultaneous equilibrium dehydrogenation of alkane to alkene by combusting hydrogen, which ultimately drives the equilibrium dehydrogenation. Uhde acquired the star dehydrogenation process (reformer type technology) from Phillips Petroleum in 1999. This process was commercialized for the first time in Egypt in 2006. The uniqueness of this process is that they are injecting oxygen at the top of the catalytic tube in order to shift the endothermic reaction toward exothermic. A large number of other catalytic systems were also reported for ODH of propane, but these independent technologies are still far from commercialization. ODH has potential advantages over PDH like exothermic and nonequilibrium-limited reaction. On the other hand, it suffers from lower selectivity because of the production of undesired carbon oxides, flammability of the reaction mixtures, and complexity of control. Despite this © 2012 American Chemical Society

overabundance of innovation in ODH technologies, still more developments are ongoing. In this process, an oxidizing agent, such as O2 or air, is added to the feed, and the reaction is CnH 2n + 2 +

1 O2 → CnH 2n + H 2O 2

ΔH273° = −28 kcal mol−1

In the ODH reaction system, a multistep reaction occurs with two-hydrogen-atom abstraction from propane to a higher C/H ratio product and desorption of water. This reaction is an exothermic reaction and is not limited by equilibrium. Two different concepts were observed in the open literature: (1) The exothermic and endothermic propane dehydrogenation combination was suggested by Schmidt et al. and Forzatti et al. for platinum catalysts and by Buyevskaya et al. for rareearth oxide catalysts.1−3 This process scheme was suggested to overcome thermodynamic limitations and to avoid catalyst regeneration by steam or dioxygen or air. This autothermal reactor operation suffers from product purity and lower desired product selectivity. (2) The second mode is based on redox-type lowtemperature catalytic ODH, where propylene is formed via heterogeneous Marsvan−Krevelen mechanisms. Complete conversion in ODH at lower temperature gives the advantage of engineering economics, while it is difficult to stop Received: Revised: Accepted: Published: 346

June 7, 2012 November 25, 2012 November 29, 2012 November 29, 2012 dx.doi.org/10.1021/ie301422n | Ind. Eng. Chem. Res. 2013, 52, 346−352

Industrial & Engineering Chemistry Research

Article

dehydrogenation formed as a result of chromium−support interactions, with limited information regarding the effect of chromium oxide loading on the ODH of propane.16−19 The platinum-based Al2O3, SiO2, MgO, MCM-41, MCM-48, SBA-15, and ZSM-5 zeolite-supported catalysts were also studied for propane and butane dehydrogenation with highly selective hydrogen combustion catalysts, such as VOx, Te, NiO, Nb, CoNx, Bi2O3, MoOx, Bi2−Mo3O12, In2−Mo3O12, Sb2O4, In2O3, and WO3.20,21 Over a Pt/Na-[Fe]-ZSM5 catalyst, small platinum clusters formed by staged oxygen introduction enhanced selective dehydrogenation.22 It was also discovered that size-preselected platinum clusters stabilized on highsurface-area supports were much more active and selective for the ODH of propane than other platinum and vanadium catalysts.23 The role of the structure and/or acid sites of zeolitesupported catalysts in the ODH process has not yet been fully understood. It is well-known now that strong Brønsted acid sites are undesired for ODH reactions, whereas transition-metal cations enhanced the catalyst activity and selectivity. Likewise, the introduction of vanadium, cobalt, magnesium, or manganese into a medium-pore ALPO-5 structure shows better activity. A comprehensive overview of these catalytic works and how far these processes are from commercialization were nicely discussed by Cavani et al.24 The majority of research focuses on some specific ODH aspects for specific catalyst’s activity, e.g., in the case of supported metal oxide catalysts, morphological aspect modification, and features affecting the performance like the degree of agglomeration of active metal cations (from independent to bulk metal oxide), acid−base characteristics, valence state, collective electronic properties, degree of crystallinity, etc. Experimental evidence was often presented for these aspects and sometimes contradictory results to generated impression of cruciality factor, while the collective interpretation was missing, which would be capable of explaining comprehensive overview. The promising selective ODH of ethane technology was first reported over a platinum-loaded honeycomb catalyst in a millisecond contact-time reactor at high reaction temperature about 850−1000 °C by Huff and Schmidt.25 It was suggested that the optimal catalyst contains a low platinum loading, deposited on a monolithic large-pore support, based on either mullite or barium/manganese aluminate.24 Other systems reported were platinum/tin and platinum/rhodium gauzes, LaMnO3, and LaMnO3 supported on ceramic foam; PtLaMnO3 demonstrates the best performance, i.e., 59% ethylene yield.24 An improved selectivity to ethylene was suggested by feeding CO, dihydrogen (sacrificial fuels), or chlorine, which improves the platinum dispersion. Wolf et al. compared autothermal oxidative propane dehydrogenation over a platinum/alumina catalyst with commercial catalytic dehydrogenation and ODH using a titania-supported vanadium oxide catalyst.26 A key factor in the design of an efficient ODH catalyst is isolation of the active sites.24 Previously, the isomorphous substitution of active metal species, e.g., vanadium, into microporous and mesoporous materials was investigated. Site isolation may be achieved by impregnation over these highsurface-area supports, but the catalyst stability is a big question. To our knowledge, no work has reported small pore zeolites for the ODH process, but they are widely used for methanol-toolefin reactions. Moreover, this idea was first floated by Nawaz et al. for PDH using Pt−Sn/SAPO-34.27 SAPO-34 is an acid silicoaluminophosphate with a chabasite-related structure

the reaction at the desired product because of the presence of oxygen.4 The key to the ODH process is the stable and selective catalyst. Vanadium- and molybdenum-based catalysts were the most widely studied systems for ODH in the temperature range between 450 and 550 °C, and thermodynamic equilibrium conversion is shown in Figure 1.5−9

Figure 1. Thermodynamic equilibrium conversion of propane at 1 bar.

In order to achieve exclusive propylene production, both oxidative process variants do not reveal an alternative to PDH in terms of conversion and energy. Therefore, a novel selective catalyst is needed for oxidation. Overall, transition-metal oxides, basic oxides such as Li/MgO, and rare-earth oxides have been extensively studied for ODH. Recent works on the development of ODH focuses on vanadium-based catalysts with a number of metal oxides as the support, such as MgO, Al2O3, Nb2O5, and TiO2. Rare-earth orthovanadates of lanthanum, praseodymium, ytterbium, erbium, samarium, cerium, terbium, neodymium, etc., at 400 °C were also studied. Improvement in the ODH of propane was observed over Mg−V−Sb oxide catalysts, i.e., about propylene selectivity 70−77% and ∼68% propane conversion. In previous research work, the influence of molybdenum as an active metal over the Al2O3 support was studied for the ODH of propane.10 Generally, the Al2O3 support was covered by a molybdate layer at less than the monolayer values, while at higher propane conversion than 20%, the propylene selectivity leveled off at around 25% irrespective of the molydenum content of the catalyst. The influence of lithium and/or antimony as a promoter to improve the performance of molybdenum-based catalyst was also studied, which brings changes in texture and support interaction. The overall performances of Mo−Li−Al2O3 and Mo−Sb−Al2O3 were not promising.11,12 Over transition-metal oxide catalysts, ethyl radicals were promoted via a redox cycle at low temperature. Better desired product selectivity with higher conversion over Li/MgO was achieved as a result of rapid desorption of the ethyl intermediate, which reacts with dioxygen. Therefore, the high desired yield may be enhanced by restricting zeolite shape selectivity phenomena. Several recent studies focused on the ODH of alkanes over alumina-supported chromium and/or platinum catalysts.13−15 The studies reported that suitable catalytic sites for 347

dx.doi.org/10.1021/ie301422n | Ind. Eng. Chem. Res. 2013, 52, 346−352


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setup and a TCD. The catalyst (0.2 g) was reduced under flowing pure dihydrogen (5 mL/min) at 400 °C for 2 h and then purged in dinitrogen at 500 °C for 1 h. The samples were saturated by a hydrogen pulse at 25, 300, and 500 °C temperature. The pulse size (2 min at a constant cylinder pressure of 5 bar) was 5 mL/min of 5% (v/v) dihydrogen in a dinitrogen mixture, and the time between pulses was 3 min. The platinum dispersion on the support was obtained using a JEM-2010 high-resolution transmission electron microscope equipped with an energy-dispersive spectrometer (operated at 120.0 kV). The samples of Pt−Sn/SAPO-34 were prepared using a common sonication method, and/or in some cases, thick samples were dispersed in ethanol. Catalyst Performance Evaluation. The ODH reactions were carried out at atmospheric pressure in a continuous fixedbed quartz microreactor with an inner diameter of 6 mm. A measured amount of the catalyst sample was loaded to maintain the desired weight hourly space velocity (WHSV) for the reactions. Catalyst samples were reduced under flowing pure dihydrogen (12 mL/min) at 575 °C for 2.5 h. The reactor was placed in an electrical furnace and temperature-controlled using external and internal thermocouples. Propane (99.95% pure) was used as the feed. Mixtures of propane and air are flammable at propane/oxygen ratios of 0.10−0.49, and pure oxygen is flammable at 0.02−0.72.40 Therefore, the experiments were performed with a propane/oxygen ratio exceeding 1.0, assuring operation outside of the flammability limits. The premixed reactant mixture with varying compositions of propane, hydrogen, and oxygen was fed to the reactor using electronic mass flow controllers. The product gas composition including hydrocarbons C1−C6 and gases O2, CO, and CO2 was continuously monitored by an online gas chromatograph (GC7890II), i.e., equipped with a flame ionization detector and a TCD. From the composition data, the fractional conversion of propane and desired product selectivity were calculated without considering coke. Water and hydrogen in the exit gas also were not analyzed.

having eight-membered rings (3.8 Å) and silicon-based acid sites. The SAPO-34 support has high thermal stability and a suitable cage structure to enhance the propylene selectivity. During dehydrogenation/cracking, catalyst coking occurs because of reversible endothermic processes and frequent regeneration is required; therefore, the ODH of the propane reaction is considered to overcome this problem and enhance the equilibrium conversion. The main objective of this work is to use highly selective PDH catalyst Pt−Sn/SAPO-34 for the ODH of propane. So, the Pt−Sn/SAPO-34 catalyst can be used in the development of promising coupling, heat-integrated, and/or mixed phenomena processes. The Pt−Sn/SAPO-34 catalyst has been extensively studied and characterized using a number of physicochemical techniques in a number of studies by authors.27−30 In this paper, we will discuss oxidative propane dehydrogenation to produce light olefins over Pt−Sn/SAPO34. The catalyst was parametrically characterized in order to understand the ODH reaction dynamics using a platinum-based small-pore zeolite catalyst.

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EXPERIMENTAL SECTION Catalyst Preparation. The SAPO-34 catalyst support was first prepared by mixing Al2O3/P2O5/SiO2/TEA/H2O in a molar ratio of 1:1:0.5:2:100.31−33 The solution was stirred, then aged for 24 h at 25 °C temperature, and later autoclaved at 200 °C under autogenous pressure. After 24 h, the product was filtered, dried, and calcined at 550 °C [Brunauer−Emmett− Teller (BET) surface area 478 m2/g]. The Pt−Sn/SAPO-34 catalyst was prepared by a sequential impregnation method.27,28,34−39 The support was impregnated first with promoter tin (with 0.16 M SnCl2·2H2O aqueous solution at 80 °C) and Sn/SAPO-34 prepared with 1 wt % tin content. After, the promoter’s impregnation catalyst was dried at 105 °C for 3 h and calcined at 520 °C for 4 h. Later, Sn/SAPO-34 was impregnated again with platinum (0.03 M H2PtCl6 aqueous solution) at 70 °C. Afterward, the catalyst was dried at 105 °C for 3 h and calcined accordingly. The catalysts were used in pure form without pelletization, and active metal contents were confirmed using X-ray fluorescence (XRF). Catalysis Characterization. The prepared catalyst was characterized by the physicochemical techniques XRF, BET, and transmission electron microscopy (TEM). The metallic content of the prepared sample was verified using XRF on a Shimadzu XRF 1700 fluorimeter. The BET surface area of the Pt−Sn/SAPO-34 catalyst was measured using a dinitrogen adsorption/desorption isotherm and determined using an automatic analyzer (Autosorb-1-C). The surface area of the catalyst was calculated using the BET equation. The amount of coke deposited over the catalyst was measured quantitatively by the dioxygen-pulse chemisorption technique. The dioxygen-pulse experiments were carried out at 700 °C by injecting pulses of pure dioxygen (99.99%) into used catalysts (0.03 g), which were maintained under flowing pure dinitrogen between two successive pulses (nitrogen dilution will increase the accuracy of the measurement). The CO2 generated was continuously monitored with a thermal conductivity detector (TCD) detector. Pulses of pure oxygen were continued until the deposited carbon was entirely removed (converted to CO2). Then the amount of coke formed was calculated from the generated CO2. The dihydrogen chemisorption of the catalysts was determined by the pulse chemisorption technique using a conventional setup equipped with a temperature-controlled

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RESULTS AND DISCUSSION Partial oxidation systems like ODH, methane to synthesis gas, partial oxidation of ethanol/propane/propylene, etc., are very complex, where numerous reactions may take place and oxygen plays an important role in the reaction rates for products and byproducts. Blank experimental tests without catalyst were also performed under operation conditions identical with those for the Pt−Sn/SAPO-34-catalyzed system. This demonstrates the significance of homogeneous thermal pyrolysis reactions at temperatures between 400 and 600 °C in the empty reactor using a gas mixture with or without oxygen. Below 435 °C, no activity was observed. At temperature levels of 450, 500, and 600 °C, the conversion of propane was about 0.25%, 1%, and 5%, respectively. Therefore, the possibility of mass-transfer limitation on the experimental conditions has been ruled out. The effect of oxygen introduction on the performance of a platinum-based SAPO-34 zeolite-supported catalyst was examined. It can be seen that the presence of oxygen changes the catalytic behavior, and the results are shown in Table 1. The conversion of propane is directly affected by changes in the propane/oxygen ratio; on the other hand, with an increase in conversion, the selectivity starts to decrease. With an excess of oxygen, the catalyst remains in a highly oxidizing state, enabling fast oxidation of propane to carbon oxides, instead of propylene. 348



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selectivity for propylene increased as the degree of reduction of the catalyst increased. The results supported the fact that the selectivity is affected by the binding energy of the lattice oxygen. A higher oxidation state of platinum, where the lattice oxygen is loosely bound, therefore, it is more likely to promote a deep oxidation of hydrocarbons to carbon oxides. Oxidative nbutane dehydrogenation to light olefins under operating conditions identical with those used for oxidative propane dehydrogenation is shown in Table 2.

Table 1. Conversion and Selectivity in Oxidative Propane Dehydrogenation over Pt−Sn/SAPO-34 at Different Propane/Oxygen Ratios, 525 °C, and TOS 5 min selectivity (%) propane/oxygen ratio

propane conversion

CO

CO2

C1−C2

C3H6

3:1 2:1 1:1 1:2

49.4 57.2 59.3 60.8

0.5 0.7 0.3 0.8

1.2 1.4 2.1 2.9

1.9 2.4 3.7 5.2

96.4 96.5 93.9 91.1

Table 2. Oxidative n-Butane Dehydrogenation under Operating Conditions Identical with Those in Figure 2

The presence of oxygen largely promotes the initial conversion of propane to 69.89%, and the final conversion is 62.94% at TOS 12 h, while the propylene selectivity is 97 ± 2 with COx in the range of 0.5−2.1, as shown in Figure 2.

butane/ oxygen ratio

n-butane conversion (%)

COx selectivity (%)

C1−C2 selectivity (%)

propylene + butene selectivity (%)

2:1

71.13

4.41

5.32

90.37

The product distribution represented in Figure 2b may be affected by the oxygen concentration, but we proceed with 2:1 as the optimum. With lower oxygen molar ratio, the concentration of water will also increase to some extent, which was not measured in this study. However, at lower oxygen concentration, a high propane conversion indicates that steam formed may participate in the reaction of propane dehydrogenation by producing hydroxyl groups, which participate in the propane dehydrogenation reaction by hydrogen elimination. Moreover, the presence of hydrogen may help to maintain the stability of the catalyst by inhibiting the oxidation of platinum and reduce sintering, while it is also a fact that small platinum clusters over a zeolite support produce water by consuming more than 90% of the dioxygen introduced, even with hydrocarbons as predominant available reactants.24 The effect of the contact time on the performance of the catalyst was investigated by varying the WHSV by changing the catalyst amount and keeping the flow rate constant as above. It is noticed that an increase in the WHSV initially improves conversion but then it decreases, while the selectivity of propylene improves constantly. In other words, we can say that propane conversion increases rapidly with increasing contact time from 0.2 to 0.5 s, but further increases show almost stable conversion. With increasing contact time, significant activation of the hydrogenolysis phenomenon was observed. Table 3 shows the effects of temperature on the activity and selectivity in oxidative propane dehydrogenation over the Pt− Sn/SAPO-34 catalyst with time. At 550 °C, the performance of Table 3. Conversion and Selectivity in Oxidative Propane Dehydrogenation over Pt−Sn/SAPO-34 at Different Temperatures and a Propane/Oxygen Ratio of 2:1, Where the Initial Value Was at 5 min and the Final Values Were Noted at 12 h Figure 2. Propane conversion and product selectivity during ODH over Pt−Sn/SAPO-34.

propane conversion

Platinum produces a notable increase in the activity, and the support stabilizes the overall catalyst in oxygen atmospheres. The selectivity of propylene strongly depends on the stereochemical control of the catalyst. Over the Pt−Sn/ SAPO-34 catalyst, oxygen does not bring a large number of COx molecules. Obviously, the presence of tin can evidently reduce the formation of COx and coke formation. The 349

COx selectivity (%)

C3H6 selectivity (%)

reaction temperature (°C)

initial

final

initial

final

initial

final

500 525 550 575 600

45.33 57.20 69.89 71.76 74.19

37.85 50.06 62.94 63.89 64.67

4.36 2.1 1.99 1.84 1.78

2.97 1.62 0.61 1.43 1.36

95.38 96.50 96.01 95.43 95.37

96.65 97.74 97.45 96.27 96.13



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the catalyst was observed to be superior as lower byproducts. The initial conversion of propane increases from about 45 to 75% with an increase in the reaction temperature from 500 to 600 °C. Coke is one of the inherent factors that influence the performance of dehydrogenation catalysts, in particular platinum-based catalysts. Coke deposited over a catalyst at different temperatures was quantitatively analyzed by dioxygenpulse chemisorption. Table 4 shows the results of coke

It is well-known that tin in an oxygen atmosphere also plays an important role in promoting the propylene selectivity and favoring hydrogen adsorption. The results of the measurements are shown in Table 5. The dihydrogen uptake was measured at Table 5. Hydrogen Chemisorption Analysis of Pt−Sn/ SAPO-34 Catalysts dihydrogen uptake (mL of H2/g of Pt)

Table 4. Coke Analysis after Oxidative Propane Dehydrogenation at Different Temperatures, with a Propane/Oxygen Ratio of 2:1 and without Oxygena temperature (°C)

De

cokeb

500 525 550 575 600 585 (without oxygen after PDH)43

0.165 0.125 0.099 0.111 0.128 15.011

0.101 0.087 0.074 0.081 0.092 0.412

catalyst

25 °C

300 °C

500 °C

total

fresh spent

9.6 7.9

21.5 18.6

14.1 12.3

45.2 38.9

500, 300, and 25 °C. Here, the total amount of dihydrogen uptake of the sample at low and high temperatures indicates the interactions between platinum and the promoter or support. The hydrogen absorption ability of the catalyst is mainly influenced by coke deposition and platinum sintering. The smallest decrease in the dihydrogen uptake over spent Pt−Sn/ SAPO-34 could be related to the coke. These chemisorption results are inconsistent with the literature and the above explanation of the catalyst activity and stability in the ODH reaction. The dispersion of platinum over the SAPO-34 support with platinum sites can be seen in the TEM picture in Figure 3.

a

Deactivation of the Pt (0.5 wt %)−Sn (1 wt %)/SAPO-34 catalyst in the ODH reaction. Deactivation was defined as De = [(X0 − Xf)/X0 × 100], where X0 is the initial conversion at 5 min and Xf is the final conversion at 12 h. bDioxygen-pulse analysis.

measurements. During catalytic dehydrogenation, two side reactions take place, which are hydrogenolysis and coke formation. Most of the side reactions were significant over a monoplatinum catalyst (catalyst without promoter) because of the presence of large platinum ensembles as hydrogenolysis and coke precursors.29,41−43 Moreover, these platinum sites were modified during the ODH process, giving small ensembles of platinum. These modifications in the electronic properties lead to a more stable catalyst. It is known now that the addition of tin to a platinum-based catalyst further modifies the platinum ensembles and reduces coke formation. Also, the presence of oxygen helps to stabilize the active sites (Pt−O support). The rate of coke production is much faster than the rate of coke elimination, while in an oxygen and/or a steam medium, a negligible amount of coke is detected, as observed for the results presented in Table 4. The high active oxygen species on a modified platinum-based catalyst could participate in the elimination of coke, which was verified by previous temperature-programmed oxidation experiments. Another possible reason for this lower coke formation in ODH may be that the steam formed helps to reduce coke. Therefore, the deactivation rate in the ODH reaction is much slower than deactivation without oxygen (PDH). The superior stability of the Pt−Sn/ SAPO-34 catalyst was observed because the support acidity is silicon-based and Al2O3 or ZSM-5-supported catalysts have aluminum-based acidity. Aluminum-based acidity has a problem with oxygen or steaming because coke formation in ODH is not the main reason for deactivation. The sintering of platinum particles and destruction of their interaction with tin and support are important factors for catalyst deactivation in an oxygen or steam atmosphere. Hydrogen chemisorption analysis was conducted to analyze the nature of the active platinum sites in the ODH reaction. The metal surface area and platinum dispersion are other important factors in the catalyst activity and are measured by the corresponding dihydrogen uptake while it is somehow interfered with by the interaction of platinum with tin oxides.37

Figure 3. TEM picture of platinum dispersion over the SAPO-34 support.

According to Le Chatelier’s principle, with a decrease in the hydrogen pressure, the thermodynamic equilibrium is broken through and the reaction is driven toward the product side. The steam formed could eliminate coke and further promote the catalyst performance. Oxygen can directly react with hydrogen and produce water. The increase in the rate of hydrogen elimination promotes the dehydrogenation reaction over platinum sites and then transfer to the support. The hydrogen atoms adsorb on the anionic sites of SAPO-34, yielding hydroxyl groups and releasing free electrons to support. These hydroxyl species also take part in the promotion of hydrogen abstraction from the feed. Therefore, ODHs by oxygen and steaming have almost similar reaction performances and trends. The observations demonstrate that, for platinum-based catalysts, light olefin yield can be substantially improved by suppression of the secondary reactions of deep oxidation and cracking.

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CONCLUSION The promising results of light paraffin dehydrogenation to olefins were obtained over the Pt−Sn/SAPO-34 catalyst. The presence of a small quantity of oxygen can remarkably promote conversion of theparaffins. The use of the Pt−Sn/SAPO-34 catalyst bed for both ODH and PDH reactions simultaneously, 350



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(12) Abello, M. C.; Gomez, M. F.; Rivarola, J. B.; Ferretti, O. 4th World Congress on Oxidation Catalysis, Potsdam, Germany, Sept 2001; Springer: Berlin, 2001; p 87. (13) Flick, D. W.; Huff, M. C. Oxidative dehydrogenation of ethane over supported chromium oxide and Pt modified chromium oxide. Appl. Catal., A 1999, 187, 13. (14) Grzybowska, B.; Sloczynki, J.; Grabowski, R.; Wcislo, K.; Kozlowska, A.; Stoch, J.; Zielinski, J. Chromium Oxide/Alumina Catalysts in Oxidative Dehydrogenation of Isobutane. J. Catal. 1998, 178, 687. (15) Al-Zahrani, S. M.; Elbashir, N. O.; Abasaeed, A. E.; Abdulwahed, M. Catalytic Performance of Chromium Oxide Supported on Al2O3 in Oxidative Dehydrogenation of Isobutane to Isobutene. Ind. Eng. Chem. Res. 2001, 40, 781. ́ (16) Grzybowska-Swierkosz, B. Thirty years in selective oxidation on oxides: what have we learned? Top. Catal. 2000, 11−12, 23. (17) Hoang, M.; Mathews, J. F.; Pratt, K. C.; Xie, Z. A kinetic study of oxidative dehydrogenation of isobutane to isobutylene over chromium oxide supported on lanthanum carbonate. Kinet. Catal. 2010, 51, 398. (18) Al-Zahrani, S. M.; Jibril, B. Y.; Abaseed, A. E. Propane Oxidative Dehydrogenation over Alumina-Supported Metal Oxides. Ind. Eng. Chem. Res. 2000, 39, 4070. (19) Grzybowska, B.; Sloczynki, J.; Grabowski, R.; Keromnes, L.; Wcislo, K.; Bobinska, T. Liquid-Phase Hydrodechlorination of CCl4 to CHCl3 on Pd/Carbon Catalysts: Nature and Role of Pd Active Species. J. Catal. 2001, 209, 279. (20) Grasselli, R. K.; Stern, D. L.; Tsikoyannis, J. G. Catalytic Dehydrogenation (DH) of Light Paraffins Combined with Selective Hydrogen Combustion (SHC): I. DH−SHC−DH Catalysts in Series (Co-Fed Process Mode). Appl. Catal., A 1999, 189, 1. (21) Grasselli, R. K.; Stern, D. L.; Tsikoyannis, J. G. Catalytic Dehydrogenation (DH) of Light Paraffins Combined with Selective Hydrogen Combustion (SHC): II. DH + SHC Catalysts Physically Mixed (Redox Process Mode). Appl. Catal., A 1999, 189, 9. (22) Waku, T.; Biscardi, J. A.; Iglesia, E. Catalytic Dehydrogenation of Alkanes on Pt/Na-[Fe]ZSM-5 and Staged O2 Introduction for Selective H2 Removal. J. Catal. 2004, 222, 480. (23) Vajda, S.; Pellin, M. J.; Greeley, J. P.; Marshall, C. L.; Curtiss, L. A.; Ballentine, G. A.; Elam, J. W.; Catillon-Mucherie, S.; Redfern, P. C.; Mehmood, F.; Zapol, P. Subnanometre platinum clusters as highly active and selective catalysts for the oxidative dehydrogenation of propane. Nat. Mater. 2009, 8, 213. (24) Cavani, F.; Ballarini, N.; Cericola, A. Oxidative dehydrogenation of ethane and propane: How far from commercial implementation? Catal. Today 2007, 127, 113. (25) Huff, M.; Schmidt, L. D. Ethylene formation by oxidative dehydrogenation of ethane over monoliths at very short contact times. J. Phys. Chem. 1993, 97, 11815. (26) Wolf, D.; Dropka, N.; Smejkal, Q.; Buyevskaya, O. Oxidative dehydrogenation of propane for propylene productioncomparison of catalytic processes. Chem. Eng. Sci. 2007, 56, 713. (27) Nawaz, Z.; Tang, X. P.; Zhang, Q.; Dezheng, W.; Wei, F. A highly selective Pt−Sn/SAPO-34 catalyst for propane dehydrogenation to propylene. Catal. Commun. 2009, 10, 1925. (28) Nawaz, Z.; Shu, Q.; Jixian, G.; Tang, X. T.; Wei, F. Effect of Si/ Al ratio on performance of Pt−Sn-based catalyst supported on ZSM-5 zeolite for n-butane conversion to light olefins. J. Ind. Eng. Chem. 2010, 16, 57. (29) Nawaz, Z.; Wei, F. Pt−Sn-based catalyst’s intensification using Al2O3-SAPO-34 as a support for propane dehydrogenation to propylene. J. Ind. Eng. Chem. 2011, 17, 389. (30) Nawaz, Z.; Chu, Y.; Yang, W.; Tang, X. P.; Wang, Y.; Wei, F. Study of Propane Dehydrogenation to Propylene in an Integrated Fluidized Bed Reactor Using Pt−Sn/Al-SAPO-34 Novel Catalyst. Ind. Eng. Chem. Res. 2010, 49, 4614. (31) Nawaz, Z.; Tang, X. P.; Zhu, J.; Wei, F.; Naveed, S. Catalytic cracking of 1-hexene to propylene using integrated SAPO-34 catalysts topologies. Chin. J. Catal. 2009, 30, 1049.

by varying the operation strategy, is an opportunity to enhance production. In order to maintain a high desired product selectivity and restrain COx generation, the amounts of feed/ oxygen ratio are kept high (lower oxygen than the amount necessary for a stoichiometric reaction with the available hydrogen). This will further control water production, while the steam formed during the reaction also helps to eliminate coke and further promote the catalyst activity. The OH groups may also participate in hydrogen elimination of the adsorbed feed and further enhance the rate of dehydrogenation. The catalytic performance of a weak surface acid SAPO-34 support was found to be superior in activity and selectivity, where conversion was above 70 wt % and selectivities of light olefins were above 90 wt %. These interesting results suggest that the intermediate product conversion over/within the chabasite cages enlightens the potential of a platinum-based catalyst for ODH phenomena. However, hydrogen chemisorption results indicated that the steam somehow reduced the metal−support interaction and promoted sintering of platinum.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This experimental work was conducted at the Department of Chemical Engineering, Tsinghua University, Beijing, China. REFERENCES

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Light-Alkane Oxidative Dehydrogenation to Light Olefins over

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