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Propane Oxidative Dehydrogenation over Alumina-Supported Metal Oxides S. M. Al-Zahrani,* B. Y. Jibril, and A. E. Abasaeed Chemical Engineering Department, College of Engineering, King Saud University, PO Box 800, Riyadh 11421, Saudi Arabia
Alumina-supported oxides of V, Cr, Mn, Zr, and Ba were found to catalyze propane oxidative dehydrogenation to propylene at atmospheric pressure, reaction temperatures of 350-450 °C, and a total feed flowrate of 75 cm3/min (26.7% propane, 6.6% oxygen, and the rest helium). Maximum propane conversion (26%) and selectivity to olefin (70%) were achieved with vanadium oxide at 450 °C. The other metal oxides showed lower conversions (9-17%) and olefins selectivities (36-58%). This observation was explained on the basis of the lattice oxygen reactivity as estimated from the reduction potential of the metal cations. Metals whose cations have high potentials (e.g., vanadium oxide catalyst) were found to favor low COx and high selectivity to propene and ethene. This suggests that the metal-oxide bond strength strongly influences the selectivity to olefins in this reaction. A weak linear correlation between the selectivity to propene and the aqueous reduction potential of the cations was found. 1. Introduction
Table 1. Alumina-Supported Metal Oxides, Catalyst Precursors, and Calcination Temperature (°C)
Catalytic oxidative dehydrogenation of alkanes to alkenes is a challenging problem. In principle, it offers a much-needed alternative to pyrolysis as a route to the production of alkenes and intermediate chemicals in the petrochemical industry. This is particularly encouraged by the abundant supplies of liquefied petroleum gas, predominantly propane and butane. Presently, ethylene, propylene, and butylene are mainly produced from their respective alkanes by thermal pyrolysis with the attendant high temperatures necessitated by thermodynamic equilibrium considerations. Capital intensive supply of heat at high temperature is required, and in the reactor, deactivation of the catalyst through coking may occur. It is therefore necessary to find catalysts that can both activate the C-H bond of the alkane and provide suitable, readily available oxygen atoms. These atoms must not result in total-oxidation products. There has been continued interest in finding low-temperature active catalysts in order to avoid the total-oxidation products. Different approaches have been formulated, and new ones are being proposed. Propylene from propane can be produced by oxidative dehydrogenation (ODH) over oxide catalysts1-5 and lithium hydroxide/ lithium iodide.6 Other catalytic systems have also been proposed. Recently, most of the catalysts reported in the literature are metal vanadates7 and metal molybdates.8,9 Metal tungstates are also reported for propane ODH to propylene and acrolein.10 The conversion of propane and the selectivity to olefins vary considerably among systems. This variation is influenced by various factors associated with both the catalyst and the experimental conditions. A number of investigations were directed toward utilizing these factors to obtain high yields of the desired products.1-12 It is suggested that the degree of conversion of the reactant using a particular catalyst is determined by the contact time between the reactant and the surface * Corresponding author. E-mail:
[email protected].
metal oxide on Al2O3
precursor (manufacturer)
calcination temp, °C (time, hrs)
Cr Mn Ba Zr V
Cr(NO3)3‚9H2O (RDH Germany) MnCl2 (Aldrich Chem) (CH3COO)2 Ba (Merck) Zr(NO3)4 (BDH, AnalaR) NH4VO3 (RDH Germany)
600 (2) 600 (2) 800 (2) 600 (2) 600 (2)
and also by the availability and reactivity of the oxygen atom at the active site.13 Some of the indices of the lattice oxygen reactivity are the aqueous reduction potential, standard enthalpy of formation of the metal oxides, ionization potential, and electronegativity of the metal. Also, depending on the types of active sites, the reactivity could be determined by the heat of removal of the lattice oxygen. The reducibility of the cations in the catalysts is also shown to influence the selectivities to alkenes.13 In this work, we report the oxidative dehydrogenation of propane over alumina-supported oxides of Ba, Cr, Mn, V, and Zr. The influences of the respective aqueous reduction potentials of the metals cations on the performance of the catalyst in the reaction are also suggested. 2. Experimental Section 2.1. Catalyst Preparation. The metal oxides were prepared from their respective precursors, as listed in Table 1. A predetermined amount of the precursor was added gradually with stirring to a crystallizing dish containing a predetermined amount of γ-Al2O3 as a support to achieve a metal-to-support weight ratio of 1:10 (i.e., for 100 g of γ-Al2O3, the amounts of the precursors used were Cr(NO3)3‚9H2O, 76.9 g; MnCl2, 23.0 g; (CH3COO)2Ba, 18.6 g; Zr(NO3)4, 37.2 g; and NH4VO3, 22.9 g). The content of the crystallizing dish was continuously stirred while excess deionized water was evaporated. A thick paste was obtained, which was dried in an oven at 100 °C overnight. The oven-dried samples were calcined in air at different temperatures for
10.1021/ie000285o CCC: $19.00 © 2000 American Chemical Society Published on Web 09/29/2000
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Figure 1. Experimental Setup.
different periods of time depending on the catalyst, as shown in Table 1. The calcined catalysts were pressed into pellets, crushed, and sieved into 20-40 mesh granules. In the subsequent discussions, the catalysts are represented as M-Al-O. 2.2. Catalyst Testing. The catalysts were tested in a fixed-bed, quartz laboratory reactor, operated at atmospheric pressure and a temperature range of 350-450 °C. The feed was composed of 26.7% propane, 6.6% oxygen, and the rest helium. The high-purity gases (obtained from Linde) were passed through microfilters for additional purification and delivered to the reactor preheat zone by metering through Omega electronic mass flow controllers. The experimental setup is shown in Figure 1. The reactor was a 70 mm long quartz tube of 7 mm internal diameter (id) tapered to a 2 mm id to remove the reaction gases from the reaction zone as fast as possible. This was done to minimize possible gas-phase reactions. One gram of the 20-40 mesh size granules of each catalyst was placed in the reaction zone of the reactor and supported on quartz wool directly above the junction of the reaction zone with the bottom of the reactor. The temperature of the catalyst bed was measured by a thermocouple placed on the reactor wall from outside. A temperature controller (Omega) was used to monitor the temperature. The actual temperature of the catalyst bed was calibrated in a separate experiment using a second thermocouple positioned in the center of the catalyst bed. This is the reaction temperature reported in this work. In each case, the catalyst was pretreated in a stream of a mixture of oxygen and helium for 30 min at 450 °C. Then, helium alone was passed over the catalyst for about the same time. Thereafter, the reactant gases (a 75 cm3/min mixture of C3H8, O2, and He in a ratio of 4:1:10) were passed through the reactor at the desired reaction temperature. 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, and C3H6, were separated by HP-plot column and analyzed with an FID detector, while O2, CO, and CO2 were separated by MS and Hayesep columns and analyzed with a TCD detector. 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%. 3. Results and Discussion When the alumina-supported metal oxides prepared in this work were tested as catalysts for propane ODH, propylene, ethylene, and carbon oxides were generally obtained as the main products. Oxygenates were not found to have appreciable quantities under the conditions of the experiments. The propane conversion (X) and product i selectivity (Si) are as defined below. The yield is a mathematical product of conversion and selectivity. The data reported here were obtained in the temperature range of 350-450 °C. The background reactivity of the propane with oxygen was found to be insignificant when measured in a reactor filled with quartz granules of the same amount and mesh size as the actual catalysts at the reaction conditions.
X)
ni npi - npo and Si ) npi npi - npo
where npi, npo, and ni are the number of moles (based on C3) of propane at the inlet and outlet and of the product i in the exit stream, respectively. Figure 2 shows the total conversion of propane as a function of reaction temperature in the range 350-450 °C. It is clear that V-Al-O gave the highest propane conversion for all reaction temperatures. The maximum propane conversion was 25.6%, and the maximum propylene yield was 17.0%. In a previous investigation, a V-containing system was shown to have similar performance (conversion of 20% and yield of 12%).2 Barium oxide was found to have the least average propane conversion. It is also observed that the V-Al-O catalyst had the highest rate of change in activity with temperature. The positive gradient of activity with temperature rise could be explained on the basis of a redox mechanism. An increase in the reaction temperature activates the lattice oxygen; therefore, as the temperature increases the catalyst activity (hence, degree of conversion) also increases. The Ba-Al-O, CrAl-O, and V-Al-O catalysts show increases in propane conversion with reaction temperature. Zr-Al-O seems to have attained a constant value that did not change with further increase in temperature. Initially, MnAl-O maintained an average constant activity, which
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Figure 2. Propane conversion as a function of reaction temperature.
Figure 4. Product selectivity as a function of propane conversion for Cr-Al-O.
Figure 3. Product selectivity as a function of propane conversion for V-Al-O.
Figure 5. Product selectivity as a function of propane conversion for Zr-Al-O.
appeared to increase slightly at higher temperatures. Compared to the other catalytic systems, Cr-Al-O is second to V-Al-O (although still much lower), with a propane conversion of 16.7% at 450 °C. Figures 3-7 display the variations in the selectivities of various products versus the conversion of propane at various reaction temperatures. Values for olefins (propylene and ethylene), methane, and COx (x ) 1, 2) are shown. Figure 3 shows the values for V-Al-O, for which there is an increase in propylene selectivity with conversion until a conversion of 17%. Above this value, the increase in conversion has no observable effect on the selectivity. The average propylene selectivity is about 62%, whereas that of CO2 is 20%, which is also constant with increase in conversion. It is interesting to observe that the selectivity to CO drops with an increase in propane conversion. Selectivity to ethylene is very low in this case and about the same with methane, indicating probable formation through the splitting of propane. Both have negligible quantities at low temperature, but as the temperature increases to a maximum of 450 °C, an increase of about 4% of each
component was observed. It is, therefore, concluded that the V-Al-O catalyst is highly selective to propylene. The other catalysts also showed various degrees of propane conversion and propene selectivities, but their values are much lower than those of the V-Al-O and Cr-Al-O, as indicated in Figures 4-7. Figure 4 shows the product distribution as a function of conversion for the reaction on Cr-Al-O. This is second to V-Al-O in performance. The other interesting observation is that this catalyst produced significant amounts of CO2 at lower propane conversions. However, as conversion increased, the amount of CO2 decreased. Figure 5 shows selectivity versus conversion for Zr-Al-O. The C3H6 selectivity is about half and the COx selectivity about twice the values obtained in the case of V-Al-O. The combined olefin selectivity is about 44% at 450 °C. The CO and CO2 maintained relatively high and constant selectivity values, with the former slightly decreased as the conversion increased. Figures 6 and 7 show the selectivity versus conversion plots for Ba-Al-O and Mn-Al-O, respectively. Both have COx as major products, with Ba-Al-O being the most selective to COx
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Figure 6. Product selectivity as a function of propane conversion for Ba-Al-O.
Figure 7. Product selectivity as a function of propane conversion for Mn-Al-O.
(59% selectivity) at 450 °C. The olefin selectivity was about 36%, resulting mainly from enhanced ethylene formation at 450 °C. At the same temperature, MnAl-O has an olefin selectivity of 34%, and that COx of about 51%. It is clear from Figure 8 that V-Al-O has the highest propylene selectivity, whereas Ba-Al-O has the lowest. The general ranking of the catalysts, with respect to C3H6 selectivity is V-Al-O > C-Al-O > Zr-Al-O > Mn-Al-O > Ba-Al-O. The constant or increasing selectivity to olefins with increasing conversion observed in Figure 8 is rather unusual. This could be explained on the basis of the reducing environment on the surface of the catalysts. The low oxygen supply limited the possible overoxidation in the reaction. Because of this limitation, there were high oxygen consumptions (more than 95% at lower reaction temperatures), and complete consumption at 425 °C and above. Therefore, the selectivity to olefins was increased because the tendency to form a C-O bond, responsible for reducing the selectivity, was decreased at low oxygen supply. It is significant that this result was obtained at temperatures lower than the values for typical dehydrogenation reactions.
Figure 8. Propene selectivity as a function of conversion for all the catalysts.
Figure 9. Propene selectivity (at 10% propane conversion) as a function of reduction potential of the cation (metal) in the catalysts.
Figure 9 shows the selectivity to propene at 10% propane conversion (the data for V-Al-O was extrapolated to 10% conversion) against the aqueous reduction potential of the metal cations. Although there is an upward trend, depicting an increase in selectivity with an increase in the reduction potential, it is not sufficient to explain the activity/selectivity of the catalysts. The behavior of V-Al-O is relatively far off from the other catalysts, which makes any attempt to propose a strong correlation difficult. However, the simple trend suggests some mapping of the behavior of the metals with the selectivity. It indicates that the higher the reduction potential, the lower the selectivity to COx within the range of potential used in this work. This is contrary to the earlier finding in butane,13 which indicated an opposite behavior, although, in that work, the metals were thought to play a different role. The aqueous reduction potential (RP) of a cation (the metal in the catalyst) indicates the ease of removal of oxygen from the metal oxide. The higher its value, the lower the attraction of the lattice oxygen to the metal in the catalyst. A reaction path initiated by breakage of the first C-H bond in the propane molecule is considered to be the rate-determining step in this
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reaction.13 The selectivity is determined by the nature of the reaction of the propyl radical. Dehydrogenation products are obtained if primary C-H bonds are broken, whereas the rupture of C-C or secondary C-H bonds leads to oxygenates or combustion products. Implicit in this explanation is the assumption that the oxygen inserted in the propyl species is the lattice oxygen. The lower the value of the RP, the higher the attraction of oxygen to the metal. This can be assumed to lead to instantaneous polarity between the metal and oxygen. Therefore, the tendency of the oxygen to attack an electron-rich center such as the C-C or secondary C-H bonds increases with the decrease in the reduction potential of the cation in the metal oxide. This leads to combustion products. On the other hand, when the metal has a high RP (i.e., less attraction to the oxygen) the lattice oxygen in the metal oxide has less of a tendency to attack the electron-rich species. In addition, it is easier to desorb electron-rich propene from the surface, which explains the observation of higher selectivity to propene on the catalyst whose cation has a higher reduction potential. 4. Conclusion The supported-metal (V, Cr, Zr, Mn, and Ba) oxides tested as catalysts for propane oxidative dehydrogenation were found to be active. V-Al-O showed promising results in oxidative dehydrogenation of propane and selectivity toward olefin production. It shows better performance than some of the proposed systems in the literature.7 The order of the catalysts with respect to propene selectivity at 450 °C is V (66.1%) > Cr (54.1%) > Zr (37.3%) > Mn (34.5%) > Ba (14.1%); the opposite trend was observed for the COx selectivity. The order of selectivity was explained on the basis of the metaloxygen bond strength as indicated by the aqueous reduction potential of the metal cation. There was a weak linear correlation between propene selectivity and reduction potential. This could not lead to a categorical conclusions on the structure-selectivity relationship. There are other factors that could influence the selectivity. This is clearly indicated by the behavior of V-AlO, which is radically different from the other catalysts in both conversion and selectivity.
Literature Cited (1) Sheshan, K.; Swaan, H. M.; Smits, R. H. H.; van Omens, J. G.; Ross, J. R. H. In New Development in Selective Oxidation; Centi, G., Trifiro, F., Eds.; Elsevier: Amsterdam, The Netherlands, 1990; pp 505-512. (2) Nguyen, K. T.; Kung, H. H. Generation of Gaseous Radicals by a V-Mg-O Catalyst during Oxidative Dehydrogenation of Propane. J. Catal. 1990, 122, 415. (3) Buyevskaya, O. V.; Baerns, M. Catalytic Selective Oxidation of Propane. Catal. Today 1998, 42, 315. (4) Seshan K.; Smits R. H. H.; Ross J. R. H. The Selective Oxidative Dehydrogenation of Propane to Propylene over V2O5/ Nb2O5 Catalysts; In Selective Hydrogenation and Dehydrogenation; DGMK Conference, Nov 11-12, 1993, Kessel, Germany. (5) Burch, R.; Crabb, E. M. Homogeneous and Heterogeneous Contributions to the Oxidative Dehydrogenation of Propane on Oxide Catalysts. Appl. Catal. A 1993, 100. (6) Dahl, I. M.; Grande, H.; Jens, K. J.; Rytter, E.; Slagtern, Oxidative Dehydrogenation of Propane in Lithium Hydroxide/ Lithium Iodide Melts. Appl. Catal. A 1991, 77, 163. (7) 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. (8) Stern, D. L.; Grasselli, R. K. Propane Oxydehydrogenation over Molybdate-Based Catalysts. J. Catal. 1997, 167, 550. (9) Stern, D. L.; Grasselli, R. K. Catalysts Reaction Network and Kinetics of Propane Oxydehydrogenation over Nickel Cobalt Molybdate. J. Catal. 1997, 167, 560. (10) Stern, D. L.; Grasselli, R. K. Propane Oxydehydrogenation over Metal Tungstates. J. Catal. 1997, 167, 570. (11) Kung, H. H. Transition Metal Oxides: Surface Chemistry and Catalysis; Elseveir Science: Amsterdam, The Netherlands, 1989. (12) Bielanski, A.; Haber, J. Oxygen in Catalysis; Marcel Dekker: New York, 1991. (13) Kung, H. H.; Michalakos, P.; Owens, L.; Kung, M.; Andersen, P.; Owen, O.; Jahan, I. Factors That Determine Selectivity for Dehydrogenation in Oxidation of Alkanes. In Catalytic Selective Oxidation; Oyama, S. T., Hightower, J. W., Eds.; ACS Symposium Series 523; American Chemical Society: Washington, D.C., 1992; pp 389-408.
Received for review March 2, 2000 Revised manuscript received July 11, 2000 Accepted August 10, 2000 IE000285O
