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Ind. Eng. Chem. Res. 2005, 44, 702-706
KINETICS, CATALYSIS, AND REACTION ENGINEERING Effects of Feed Compositions on Oxidative Dehydrogenation of Propane over Mn-P-O Catalyst Baba Y. Jibril* Chemical Engineering Department, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia
Catalytic oxidative dehydrogenation of natural gas components such as propane to propene and other more useful products is technologically relevant, but catalysts proposed so far exhibit moderate performances. A manganese phosphate catalyst was prepared, characterized (BET, FTIR and XPS), and tested for the reaction at atmospheric pressure and a temperature range of 450-550 °C. In the temperature range, the propane conversion changed from 4.1 to 40.7%, and the selectivity to propene showed an insignificant change at 41 ( 3%. In addition to other products, oxygen-containing compounds of about 5% were obtained at high temperatures. Variation of propane/oxygen partial pressures at 450-490 °C elucidated the relevance of gasphase oxygen on the selectivity to propene. At high propane ratios (C3H8/O2 ) 4.0), the catalyst exhibited a propane conversion of 7.0% and selectivity to propene of 78.0% with the balance COx (CO + CO2). At low ratios (C3H8/O2 ) 0.5) and low conversion, only CO was produced. Thus, selectivity to propene could be improved by employing low or moderate oxygen partial pressure. 1. Introduction Catalytic oxidative dehydrogenation is one potential route for converting natural gas components such as propane to their corresponding olefins. These are the linkages between relatively inert alkanes and a vast number of more useful chemicals, intermediates, and petrochemical feedstocks. The long-term objective of adding value to natural gas and increasing propene demand, which is expected to double by 2010,1 serve as motivational factors for conversion of propane to propene. The main sources of propene are steam cracking (69%), refinery FCC (28%), and catalytic dehydrogenation. Propene recovery from FCC has increased from 29% in 1980 to about 80% today.2 High-severity FCC is being optimized to increase the amount of olefins obtainable, but this is constrained by the gasoline demand. The dehydrogenation has a temperature requirement above 600 °C due to the endothermic nature of the reaction. It is limited by thermodynamic equilibrium, and there is a need for catalyst regeneration due to coke deposition. The disadvantages associated with dehydrogenation and the increasing demands of olefins make it important to explore alternative or complementary routes. Oxidative dehydrogenation (OXD) provides a potentially cheaper route, since the reaction is exothermic and the presence of oxygen inhibits coke deposition. Many catalyst compositions have been proposed for propane oxidative dehydrogenation (POD) as reported in reviews.3,4 Phosphate-based catalysts have been shown to be active in oxidative dehydrogenation of ethane,5-7 propane,8-12 and butane.13-14 Physicochemical proper* To whom correspondence should be addressed. E-mail:
[email protected]. Tel: +96614676897. Fax: +96614678770.
ties that improve the performance of the catalysts have been studied. When the reaction was tested on V-PO/TiO2 at 400 °C, propane conversion of 9% and selectivity to propene of 56% were obtained.11 The performance of Mo-P-O was improved by doping with silver. The number of active sites and the reducibility of the catalyst were increased.9 Different levels of promotions were observed with dopings of chromium on Zr-P-O.8-10 One of the catalysts exhibited a propane conversion of 58% and selectivity to propene of 10% at 350 °C. Such low-temperature activity was lost for a catalyst calcined at higher temperature.10 V-P-Obased catalysts have been tested for POD partly because of the high performances of similar compositions in n-butane conversions.15 Despite the research effort, the level of selectivity to propene on the catalysts remains moderate. The activity of the V-P-O was considered to be among the best.13 This implies that all the phosphorus-based catalysts reported in the open literature need further development to improve their performances. The design of catalysts to give high propene yields at propene selectivities higher than 70%, for instance, has been shown to be a challenging task.3,4 One of the challenges is the negative selectivity-conversion trend observed for most of the catalysts proposed. A factor identified for decreasing the selectivity is the catalyst’s reducibility. On most of the promising catalysts, the reaction has been reported to take place through the Mars-Van Krevelen mechanism, where the catalyst donates lattice oxygen to take part in the oxidation reaction.3,15 The gas-phase oxygen reoxidizes the reduced catalyst. The redox character of the catalyst is among the important pillars upon which optimum catalyst activity and selectivity are based. Therefore, the reducibility of the catalyst
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surface is critical in determining the catalyst’s performance.3,4,16 The catalyst reducibility is often coupled with its reoxidizability, both of which could be influenced by the gas-phase oxygen. Recent studies have shown the significance of oxygen/hydrocarbon ratio13 and periodic oxygen control17 toward improving the selectivity to olefins. In a previous study we have investigated the activities of a series of metal pyrophosphate catalysts for POD.18 Among them, manganese pyrophosphate was found to give the most promising performance. The objective of this work was to explore the effect of oxygen/ propane partial pressures on propene yields for the catalyst. Comparison of the propane degree of conversion and product distributions at different partial pressures and reaction temperature elucidate the role of oxygen availability on the performance of the catalyst. This information sheds light on increasing the selectivity to propene without a drastic decrease in propane conversion. 2. Experimental Section The catalyst was prepared by a precipitation method, characterized by BET surface area analysis and Fourier transform infrared spectroscopy (FTIR), and tested as reported earlier.18 Also, survey and multiplex spectra were acquired for the catalyst from XPS analysis. The corresponding binding energies and surface elemental concentrations were obtained. Details on the methods and conditions of the XPS experiment are described elsewhere.19 A sample of the catalyst (1.0 g) was tested using a feed flow rate of 75 cm3/min with ratio C3H8: O2:He of 4:1:10, at reaction temperatures of 450-550 °C. The effects of varying the partial pressure of propane and oxygen in the feed were studied on 0.5 g of catalyst at reaction temperatures of 450-490 °C. Both fuel-rich and fuel-lean sides with respect to flammability limit for the propane/oxygen mixture have been considered. The propane partial pressure (PC3) was varied from 0.07 to 0.53 atm at a fixed oxygen partial pressure (PO2) of 0.07 atm. Then propane pressure was fixed at 0.27 atm while oxygen pressure was varied from 0.07 to 0.20 atm. Triplicate runs of each experiment were conducted. The highest variations between identical runs were about (6%. The values reported in the paper are the average values for each experiment. Carbon balances were typically better than 95%. A gas chromatograph (HP6890) was used for an online analysis of both the feed and product streams. The products flowed directly through a heat-traced line to the GC sampling valve. The hydrocarbons CH4, C2H6, C2H4, C3H8 C3H6, C4H10, and C4H8 were separated by an HP-PLOT column and measured with a flame ionization detector (FID), while O2, CO, and CO2 were separated by MS and HayeSep-R columns and measured with a thermal conductivity detector (TCD). Other oxygen-containing compounds were not calibrated. Peak retention times and carbon balances were used to estimate their production. All GC analyses were performed online by softwaresHP Chemstationsprovided with the equipment. A blank test without a catalyst or with quartz granules in the reactor showed negligible conversion of propane at the reaction conditions. For the partialpressures study, preliminary measures were performed in which the mass and the particle sizes of the catalyst were varied. Conditions were chosen so that the trans-
Table 1. Binding Energies and Surface Atomic Concentration of the Catalyst BE (eV) concn on surface (%)
Mn2p3
P2p
O1s
Na1s
641.8 11.0
133.5 9.5
531.2 45.4
1071.7 8.9
port phenomena did not limit the reaction. Carbon balances were typically better than 95%. The performance of each catalyst is reported on the basis of the following: conversion is defined as the mole fraction of feed carbon present in the reaction products, while selectivity is the fraction of product carbon in a particular product. 3. Results and Discussion The catalyst was characterized by BET, FTIR, and XPS. The BET and FTIR spectra were reported earlier.18 The surface area of 5.6 m2/g, pore volume of 3.5e-8 m3/ g, and average pore diameter of 264.7 Å were obtained. For the FTIR spectrum, a peak that appeared in the range of 900-1200 cm-1 was assigned to the -P2O7 group. This suggests the presence of manganese or other pyrophosphates. Binding energies (BE) and surface percent atomic concentrations as obtained from XPS experiments are given in Table 1. The BE value of 641.8 eV for Mn2p3 is consistent with that of Mn(III), but Mn(II) and Mn(IV) could also be present, as similar BE values were reported for them.20 The BE value of O1s is a composite, as its peak was observed to be broad, suggesting contribution from more than one source. The data indicate the absence of MnP. The BE for Mn in such a compound is 639 eV.21 This is different from the value we obtained. The binding energy for phosphorus (133.5 eV) is consistent with its presence in M-P2O7 or M-PO4. A similar BE was reported for CrPO4.22 M could be either Mn or Na, because a large amount of sodium was present on the catalyst surface, as suggested by the XPS results. Perhaps the sodium was trapped in pores of the catalysts, despite thorough washing of the sample in distilled water before drying. Upon calcinations such trapped species could migrate to the surface. When the reaction was conducted at 450-550 °C, the products obtained were propene, ethene, methane, carbon dioxide, and carbon monoxide. Other oxygencontaining compounds were observed at high temperature. This was based on their peak locations on the chromatograph and our earlier experience with similar
Figure 1. Propane and oxygen conversions and product distributions at different temperatures over Mn-O-P.
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compounds. Figure 1 shows the degree of conversions of propane and oxygen and the products distribution. At 450 °C, the conversions of propane and oxygen are about 4% and 38%, respectively. At these conversions, the selectivities are as follows: propene 41%, ethene 15%, COx about 41%, and the balance methane. When the degree of conversion was increased to about 40% at 550 °C, the selectivity to propene showed insignificant change. The selectivity to ethene more than doubled, to about 34%. A substantial amount of the oxygen in the gas phase was consumed. This leads to production of oxygenate compound rather than COx, whose selectivity decreases drastically to about 7%. Such a decrease in the production of COx at a lower amount of the gasphase oxygen suggests the participation of the latter in the production of the former. Perhaps COx species are produced by direct conversion of propane, as their selectivities are high even as propane conversion decreases. When the relative amount of oxygen decreased due to its conversion (about 78% at 525 °C and 88% at 550 °C), large amounts of ethene (30%) were observed at the expense of COx. About 5% of other oxygencontaining products (Oxy) were observed. The observation of little or no change in selectivity to propene (41 ( 3%) with increase in conversion (4.340.7) is not consistent with negative selectivity-conversion trends familiar with the oxidative dehydrogenations. Generally, the olefin selectivities decrease with an increase in alkane conversions. Such a conversionselectivity relation depends mainly on the catalyst’s reducibility,4 acid-base character (which affects the rate of desorption of propene),3 and oxygen mobility.23 In a previous report, metal pyrophosphates have been shown to desorb no oxygen up to 550 °C. This indicates the absence of electrophilic surface-adsorbed oxygen identified as responsible for degrading propene to COx.24 The selectivity to propene is in line with such observations, since no significant decrease in selectivity to propene was observed. Perhaps there are different types of lattice oxygen responsible for respective production of propene and COx or other oxygen-containing products. As the temperature increased, the reactivity of oxygen species associated with COx production changed, thereby favoring production of oxygenates. There is a corresponding increase in selectivity to ethene. On the basis of the foregoing, it is of interest to directly explore the effects of variation of oxygen to propane ratio in the feed stream. Figures 2 shows the effect of propane partial pressures on the propane conversion and products distributions at 450, 470, and 490 °C. Conversions of propane show an increase with an increase in propane partial pressure (PC3) typical for a reaction between adsorbed species in equilibrium with the gas phase, as shown in Figure 2a. A similar trend may be observed for selectivity to propene, with an insignificant amount at low PC3 (Figure 2b). Figure 2c,d shows that the production of COx decreases drastically with an increase in the partial pressure. At low partial pressure (PC3 < 0.2), only CO was observed. As the pressure increases, the selectivity to CO decreases to as low as 20% at a PC3 value of 0.53 atm. There is a corresponding decrease in the selectivity to CO2. Figure 2b shows that the selectivity to propene increases with PC3. This suggests that a low propane to oxygen ratio leads mainly to formation of CO at the temperatures employed. As the ratio increases, the availability of oxygen decreases, thereby promoting both propene and
Figure 2. Catalyst performance as a function of propane partial pressure (PC3) at an oxygen partial pressure (PO2) of 0.07 atm and 450-490 °C: (a) propane conversion; (b) selectivity to propene; (c) selectivity to carbon monoxide; (d) selectivity to carbon dioxide.
CO2. This indicates that CO is produced from direct combustion of propane with a type of oxygen species. Perhaps, as the amount of oxygen decreases, the dy-
Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 705 Table 2. Effect of Oxygen Partial Pressures (PO2) at a Propane Partial Pressure of 0.27 atm on Conversions and Product Distributions at 510 °C PO 2 conversn (%) C3H8 O2 selectivity (%) C3H6 C2H4 CO2 CO
0.07
0.10
0.13
0.17
2.9 11.1
5.5 11.7
8.1 10.0
8.0 5.9
79.0 0.0 0.0 21.0
68.7 7.4 6.0 17.9
65.2 11.5 5.4 18.0
65.3 8.2 6.0 20.5
When the propane partial pressure was fixed at 0.27 atm and that of oxygen (PO2) was varied from 0.07 to 0.20 atm, the propane conversions increase, especially at 490 °C, as shown in Figure 3a. PO2 shows a relatively lower effect on the selectivity to propene than that of COx. Figure 3b shows that the selectivity decreases from about 50 to 40% in the PO2 range at 490 °C. At lower temperature and PO2, only COx was produced, again with CO appearing to be produced from direct combustion of propane at low conversion, as shown in Figure 3c. As both PO2 and temperature increase, the selectivity to COx shows insignificant changes (Figure 3c,d). This suggests the participation of lattice oxygen in their production. Perhaps gas-phase oxygen is involved in the electrophilic attack on propene to produce surface intermediate species that may be further oxidized to COx by lattice oxygen. This indicates the participation of different types of oxygen species. At low temperature and conversion there could be enough easily removable oxygen to lead to overoxidation of propene to mainly CO.25 As PO2 increases, CO could be oxidized to CO2, thereby forming both CO and CO2 at 450 °C. At higher temperature and especially at lower PO2, the oxygen mobility might increase.23 This may decrease the relative amount of oxygen species responsible for COx production, hence promoting propene with considerable selectivity. The effect of oxygen partial pressure was further elucidated at 510 °C, as shown in Table 2. The propane conversion increases from 3 to 8%, while selectivity to propene decreases from 79 to 65% by doubling the PO2 value. There is a corresponding production of ethene, maintaining selectivity to olefins of about 75%. The propene yield increases by almost 2-fold. Further increase in PO2 showed no significant change in conversion and selectivity. Even at such a temperature, the selectivity to COx shows no dependence on PO2. The results show that the selectivity to propene could be improved by a judicious choice of operating conditions. 4. Conclusion
Figure 3. Catalyst performance as a function of oxygen partial pressure (PO2) at a propane partial pressure (PC3) of 0.27 atm and 450-490 °C: (a) propane conversion; (b) selectivity to propene; (c) selectivity to carbon monoxide; (d) selectivity to carbon dioxide.
namic of the redox cycle changes with a concomitant decrease in rate of release of lattice oxygen, thereby increasing production of propene.
Mn-P-O is active and selective as a catalyst in oxidative dehydrogenation of propane. Propane and oxygen conversions increased with increasing temperature. Selectivity to propene showed no significant change, but that of COx decreased with a corresponding increase in selectivity to ethene. The catalyst performance depends on the propane and oxygen partial pressures. When the propane partial pressure was changed, the conversion and selectivity to propene showed a trend typical of an adsorbed species in equilibrium with the gas phase. A change in oxygen partial pressure, at a fixed propane partial pressure,
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resulted in a different behavior. Propane conversion increased, especially at high temperature. The selectivity to propene decreased, while that to CO and CO2 showed less dependence. Perhaps different types of oxygen species are involved in electrophilic degradation of propene and production of COx. At low temperature and oxygen partial pressure, CO was the only product, indicating a fully oxidized catalyst with high availability of oxygen, leading to overoxidation to yield mainly CO. The results suggest that there are different types of oxygen species that determine the conversion and products distributions, depending on propane/oxygen partial pressure. Literature Cited (1) http://pep.sric.sri.com/Public/Reports/Phase_84/RP178/ RP178.html. (2) Marcilly, C. Present status and future trends in catalysis for refining and petrochemicals. J. Catal. 2003, 216, 47. (3) Mamedov, E. A.; Corte´s Corbera´n, V. Oxidative dehydrogenation of lower alkanes on vanadium oxide-based catalysts: The present state of the art and outlooks. Appl. Catal. 1995, 127, 1. (4) Cavani, F.; Trifiro`, F. The oxidative dehydrogenation of ethane and propane as an alternative way for the production of light olefins. Catal. Today 1995, 24, 307. (5) Ciambelli, P.; Galli, P.; Lisi, L.; Massucci, M. A.; Patrono, P.; Pirone, R.; Ruoppolo, G.; Russo, G. TiO2 supported vanadyl phosphate as catalyst for oxidative dehydrogenation of ethane to ethene. Appl. Catal. 2000, 203, 133. (6) Miller, J. E.; Gonzales, M. M.; Evans, L.; Sault, A. G.; Zhang, C.; Rao, R.; Whitwell, G.; Maiti, A.; King-Smith, D. Oxidative dehydrogenation of ethane over iron phosphate catalysts. Appl. Catal. 2002, 231, 281. (7) Loukah, M.; Coudurier, G.; Vedrine, J. C.; Ziyad, M. Oxidative dehydrogenation of ethane on V- and Cr-based phosphate catalysts. Microporous Mater. 1995, 4, 345. (8) Alcantara-Rodriguez, M.; Rodriguez-Castellon, E.; JimenezLopez, A. Propane Dehydrogenation on Mixed Ga/Cr Oxide Pillared Zirconium Phosphate Materials. Langmuir 1999, 15, 1115. (9) Zhang, X.; Wan, H.; Weng, W.; Yi, X. Effect of Ag promoter on redox properties and catalytic performance of Ag-Mo-P-O catalysts for oxidative dehydrogenation of propane. Appl. Surf. Sci. 2003, 220, 117. (10) Jimenez-Lopez, A.; Rodriguez-Castellon, E.; SantamariaGonzalez, J.; Braos-Garcia, P.; Felici, E.; Marmottini, F. Insertion of Porous Chromia in γ-Zirconium Phosphate and Its Catalytic Performance in the Oxidative Dehydrogenation of Propane. Langmuir 2000, 16, 3317. (11) Savary, L.; Saussey, J.; Costentin, G.; Bettahar, M. M.; Gubelmann-Bonneau, M.; Lavalley, J. C. Propane oxydehydroge-
nation reaction on a VPO/TiO2 catalyst. Role of the nature of acid sites determined by dynamic in-situ IR studies. Catal. Today 1996, 32, 57. (12) Lindblad, T.; Rebenstorf, B.; Yan, Z.-G.; Andersson, S. L. T. Characterization of vanadia supported on amorphous AlPO4 and its properties for oxidative dehydrogenation of propane. Appl. Catal. 1994, 112, 187. (13) Marcu, I.; Sandulescu, I.; Millet, J. M. Oxidehydrogenation of n-butane over tetravalent metal phosphates based catalysts. Appl. Catal. 2002, 227, 309. (14) Takita, Y.; Sano, K.; Kurosaki, K.; Kawata, N.; Nishiguchi, H.; Ito, M.; Ishihara, T. Oxidative dehydrogenation of iso-butane to iso-butene I. Metal phosphate catalysts. Appl. Catal. 1998, 167, 49. (15) Kung, H. H.; Kung, M. C. Oxidative dehydrogenation of alkanes over vanadium-magnesium-oxides. Appl. Catal. 1997, 157, 105. (16) Grasselli, R. K. Fundamental principles of selective heterogeneous oxidation catalysis. Top. Catal. 2000, 21, 79. (17) Creaser, D.; Andersson, B.; Hudgins, R. R.; Silveston, P. L. Oxygen partial pressure effects on the oxidative dehydrogenation of propane. Chem. Eng. Sci. 1999, 54, 4365. (18) Jibril, B.Y.; Al-Zahrani, S. M.; Abasaeed, A. E. Propane Oxidative Dehydrogenation over Metal Pyrophosphates Catalysts. Catal. Lett. 2001, 74, 145. (19) Al-Zahrani, S. M.; Jibril, B. Y.; Abasaeed, A. E. Selection of optimum chromium oxide-based catalysts for propane oxidehydrogenation. Catal. Today 2003, 81, 507. (20) http://srdata.nist.gov/xps/ (NIST X-ray Photoelectron Spectroscopy Database). (21) Myers, C. E.; Franzen, H. F.; Anderegg, J. W. X-ray photoelectron spectra and bonding in transition-metal phosphides. Inorg. Chem. 1985, 24, 1822. (22) Watson, I. M.; Connor, J. A.; Whyman, R. Noncrystalline chromium, molybdenum and tungsten phosphate films prepared by metal organic chemical vapour deposition. Thin Solid Films 1991, 201, 337. (23) Centi, G.; Cavani, F.; Trifiro, F. Selective Oxidation by Heterogeneous Catalysis; Kluwer Academic/Plenum: London, 2001. (24) Pantazidis, A.; Bucholz, S. A.; Zanthoff, H. W.; Schuurman, Y.; Mirodatos, C. A TAP reactor investigation of the oxidative dehydrogenation of propane over a V-Mg-O catalyst. Catal. Today 1998, 40, 207. (25) Mazzocchia, C.; Aboumrad, C.; Diagne, C.; Tempesti, E.; Herrmann, J. M.; Thomas, G. On the Nickel Molybdate (NiMoO4) Oxidative Dehydrogenation of Propane to Propene: Some Physical Correlation with Catalytic Activity. Catal. Lett. 1991, 10, 181.
Received for review March 17, 2004 Revised manuscript received October 12, 2004 Accepted November 8, 2004 IE040087N
