I n d . Eng. Chem. Res. 1994,33, 2935-2941
2936
Activation of Vanadium Phosphorus Oxide Catalysts for Alkane Oxidation. The Influence of the Oxidation State on Catalyst Selectivity? Yves Schuurman and John T. Gleaves’ts Department of Chemical Engineering,Washington University, 1 Brookings Drive, St. Louis, Missouri 63130
The reaction of n-butane with “reactor equilibrated” (V0)2P20~-basedcatalysts activated with different oxygen treatments has been investigated using high speed transient response techniques. It is shown that by maintaining the catalyst in a highly oxidized state the rate of maleic anhydride formation increases, and the n-butane selectivity to maleic anhydride improves. The activation energy for the reaction of n-butane was obtained by a kinetic analysis of the transient responses as a function of the temperature. The value of the activation energy depends on the oxidation state of the catalyst. 1. Introduction
The selective conversion of light alkanes to oxygenates is important to both the petrochemical and energy industries. Oxygenates are used in the preparation of a large number of chemicals, polymers, and plastics and account for approximately 28% of the total quantity of the top 40 organic chemicals currently produced. In addition, oxygenates are employed as octane boosters in gasoline. The continued need t o limit aromatics and other environmentally objectionable compounds is expected to increase the demand for oxygenates, and by the year 2000 it is projected that 750 000 barrels per day of oxygenates will be required for the production of gasoline (National Research Council, 1992). The direct conversion of alkanes to oxygenates is particularly appealing since alkanes are lower-cost feedstocks than their olefin counterparts and are currently underutilized. In general, oxidation processes can be made more environmentally benign by improving selectivity and decreasing the amount of unwanted byproducts and excess heat. In addition, for some processes, more environmentally benign feedstocks might be substituted for current feedstocks. In such cases, a comprehensive approach that uses a more benign feedstock and a new process could simultaneously improve both the frontend and back-end of the process and dramatically lower the overall environmental impact of the process. Strategies for improving selectivity have traditionally focused on developing new or improved catalytic materials for steady-state catalytic processes or reducing product loss by improving heat removal. Another approach, proposed by a number of workers, is to employ a non-steady-state process (Douglas and Rippin, 1966; Bailey, 1973; Boreskov, and Matros, 1983). Non-steadystate operation increases the number of reaction variables such as reactant concentrations, reactant-solid contact times, catalyst states, and reactor temperatures. As a result, non-steady-state operation offers the greatest opportunity to maximize both catalyst and reactor performance. However, although a number of studies have shown the advantages of this approach, little is known about the catalytic properties that maximize performance in non-steady-state modes. This lack of Paper F’resented at the US-Russia Workshop on Environmental Catalysis in Wilmington, DE, Jan 14-16, 1994. E-mail:
[email protected].
0888-5885/94/2633-2935$04.50/0
knowledge, includes, for example, the relationship between reaction selectivity and reactant-induced catalyst modifications that can occur during a non-steadystate heterogeneous process. In most selective oxidation processes, lattice oxygen is the active and selective agent, and the rate of oxygen transfer from the catalyst to the reactant is a key factor (Weng and Delmon, 1992). In general, the rate of oxygen transfer and the oxygen availability will depend on the state of the catalyst surface and the rate of oxygen diffusion from the bulk (Haber, 1982). These factors can, in turn, depend strongly on the reactive environment in which the catalyst is operated. Non-steady-state operation is likely to involve dramatically different reactive environments and thus may require a catalyst with significantly Werent properties than steady-state operation (Boreskov and Matros, 1983). The continued development of non-steady-state technologies is hindered by the absence of details of the fundamental kinetic processes. Similarly, increased mechanistic details are essential for the development of higher selectivity processes based on complex heterogeneous catalytic systems. One important class of catalysts active and selective for alkane oxidation are those based on vanadium phosphorus oxides (VPO). VPO catalysts are used industrially in the conversion of n-butane to maleic anhydride (MA). Currently, this process is the only industrial process involving the heterogeneous selective oxidation of an alkane. Recently, Contractor and coworkers demonstrated that the n-butane to MA reaction can be performed in a non-steady-state process using a riser reactor (Contractor et al., 1987; Contractor et al., 1990; Lerou and Mills, 1993). Their results indicate that a non-steady-state approach can have significant advantages over conventional fixed bed technologies including higher selectivity. In fmed-bed processes, an important feature of the VPO formation process is an extended catalyst breakin period (typically 200-500 h) (Centi et al., 1988; Stefani et al., 1990; Ebner and Gleaves, 1988; Centi et al., 1989). During this time the “non-equilibrated” catalyst is exposed to an air-butane reaction mixture and undergoes changes in oxidation state and in the relative concentrations of different VPO phases (Centi et al., 1988; Centi, 1993). Another feature of VPO fixed-bed processes is that “equilibrated” catalysts show temporary improvements in performance if the butane feed is terminated for a short period, and the catalyst 0 1994 American Chemical Society
2936 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994
is held at reaction temperatures in air before the butane feed is restarted. It is also observed that higher selectivity is achieved at lower butane to oxygen feed ratios. The above results indicate that the air-butane mixture plays an important role in the catalyst formation process and in determining the performance of an equilibrated catalyst. It is not well understood, however, how the reactants actually influence the catalytic properties of VPO during break-in or during catalyst operation. In light of new non-steady-state processing approaches to butane oxidation (Contractor et al., 1987; Contractor et al., 1990; Lerou and Mills, 1993)it is also of interest to investigate the process by which the VPO system stores oxygen and if stored oxygen is utilized selectively. In a previous study (Schuurman et al., 1994) it was shown that (VO)zP207 has a unique “storage-supply” system capable of adsorbing oxygen and efficiently channeling it to the active catalytic site. It was proposed that storage occurs through the transformation of (V0)2P207 into V+5 compounds, and the supply mechanism involves the reverse reaction. The purpose of this paper is to examine how exposure of a catalyst t o different amounts of gas phase oxygen influences the rate of butane conversion and to determine if the reaction selectivity can be enhanced by different oxygen treatments. 2. Experimental Section 2.1. Catalyst Preparation. Vanadium phosphorus oxide catalysts were prepared by refluxing a mixture of 7340 cm3 of isobutyl alcohol, 513.5 cm3 of V205, and 663.97 cm3of H3P04 (100%)for 16 h t o give a light blue precipitate. Upon cooling, the precipitate was filtered and dried at ambient temperature under vacuum. The dried precipitate was washed with isobutyl alcohol, dried for 2.5 h at 145 “Cand calcined in air for 1h at 400 “C. The resulting powder was then charged to a 122 cm long, 2.1 cm i.d. fixed-bed tubular reactor for performance testing. Tests were conducted at a fixed set of reactor conditions of 1.5%butane, 15 psig reactor inlet pressure, and 2000 GHSV. After a sufficient break-in period (-500 h) the catalyst gave steady-state selectivities to MA of approximqtely 66% at 78% conversion. The catalyst samples used in this study were operated a t these conditions for approximately 3000 h and were designated “reador-equilibrated”. XRD analysis of the reactor-equilibrated samples showed that they were monophasic (VO)ZPZO~. Chemical analysis gave a PN ratio of 1.01 and vanadium oxidation state of 4.02. The samples had a BET surface area of 16.5 m2/gm. 2.2. TAP-2Multifunctional Reactor System. The transient response and oxygen activation experiments reported in this paper were performed with a TAP-2 multifunctional reactor system. A simplified schematic of the TAP-2 system is shown in Figure 1. The system is comprised of (1)a high-throughput, liquid nitrogentrapped, ultrahigh vacuum system, (2) a microreactoroven assembly with temperature controller, (3) a heatable gas manifold with five input ports containing four pulse valves and one manual bleed valve, (5) a valve control module for actuating the pulse valves, (6) a gas blending station for preparing reactant mixtures from gases and liquids, (7) a pressure transducer oven, (8)a quadrupole mass spectrometer (QMS), (9) a 486 PC computer-based control and data acquisition system, and (10) a slide valve assembly with heated exhaust line (the exhaust line can be attached t o a GC or other external analytical device if desired).
Reactant gases can be introduced into the microreactor as steady flows or transient inputs with pulse widths (FW”)ranging from -150 ,us to seconds. Reaction products are analyzed in real-time using a quadrupole mass spectrometer. Pulses from separate valves can be introduced as sets of pulses of predetermined length or in a pump-probe format alternating between two valves. The vacuum system can be isolated from the microreactor using a slide valve that has a full open, full closed, or pin-hole leak position for operation at elevated pressures. When operated in the full open position all of the reactor effluent exits into the vacuum system. In the pin-hole leak position a portion of the effluent exits through the pin-hole leak and the remainder escapes through an external vent that contains an adjustable pressure regulator. The high pumping speed of the vacuum system and the near proximity of the quadrupole to the microreactor insure that pulses measured by the quadrupole reflect the true microreactor transient response. All key experimental parameters can be entered from the computer including the reactor temperature program, the pulse valve operating sequence, the pulse valve intensity, the data collection time, and the mass spectrometer sensitivity. The data acquisition software has a SCAN mode for collecting conventional mass spectra and a TRANSIENT mode for collecting high speed transients. The TRANSIENT mode allows sequential collection of up t o 5 different mass peaks so that a series of product peaks can be collected in a single experimental run. Both formats can be operated in a temperature programmed mode. Data is stored directly on the computer hard drive during transient experiments, and each product pulse is stored separately. After an experiment is completed the data can be averaged, or each pulse can be viewed individually. 2.3. Procedures. Reaction studies were performed with oxygen-treated reactor-equilibratedcatalyst samples. Various procedures have been used to oxygen treat catalysts and are detailed in a previous paper (Schuurman et al., 1994). In the oxidation procedure used in this study, the microreactor was operated at high pressure (84-108 kPa), and the catalyst was heated to 723 K and exposed to a continuous flow of oxygen for 5 and 10 min. Immediately after an oxidation treatment the reactor was cooled to reaction temperatures and opened t o vacuum. Subsequently, low pressure, isothermal pulse response experiments were carried out at a variety of temperatures (535-698 K), pulse intensities, and pulse formats. Typical pulse intensities were in the range of 10l2 to 1014molecules per pulse. A standard microreactor charge consisted of 0.1-0.2 g of catalyst with an average particle size of 350 pm. Anaerobic pulse experiments, using a 311 C4H16Ne mixture were used to determine the amount of active oxygen. Pump-probe experiments using a 3/1 CJ31dNe mixture and a 4/102/ Ne mixture were used to determine the relative product pulse shapes after each oxygen treatment. The pump-probe experimental format was used in acquiring data for the kinetic analysis of n-butane conversion. An important advantage of this format is that it permits the catalyst to be maintained at a fixed oxidation state while anaerobic experiments are performed. Figure 2 illustrates the input pulse sequence and pulse spacing and the resulting transient responses in a typical pump-probe experiment. Alternating the oxygen and butane inputs cycles the catalyst between an oxidizing and reducing feed. As a result, the average
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 2937
I Gas Rlendine Slalisn and Ccinlinuous Feed S y s l m
Figure 1. Simplified schematic of the TAPd multifunctional reaetor system.
oxidation state of the surface remains constant. However, because the pulses are separated in time the butane kinetics can be investigated in the absence of gas phase oxygen. Moreover, since the pulse intensities can be controlled independently the catalyst can easily be maintained in a high oxidation state with intense oxygen pulses, while the butane kinetics are investigated a t Knudsen flow conditions.
3. Results Figure 3 shows the oxygen-uptake as a function of time when a reactor-equilibrated catalyst is heated to 723 K and a fixed volume of oxygen with a n initial pressure of 86 kF'a is introduced into the microreactor. The decrease in oxygen pressure from 86 kPa to 44 kPa corresponds to a total uptake of lozooxygen molecules. Similar experiments indicated that the rate of oxygen uptake is a function of oxygen pressure and catalyst temperature. Maintaining a catalyst in a constant oxygen pressure of ~ 1 0 kPa 8 for 8 h at 723 K gave a yellow compound indicative of a V5 phase and a n oxygen uptake corresponding to one oxygen atom for every vanadium atom. Chemical analysis of a catalyst exposed t o ~ 1 0 kPa 8 and 723 K for 3 min gave a bulk vanadium oxidation state of 4.14,corresponding to an increase from 2 to 14% in Vf5 content.
Figure 4 shows the transient responses for neon (mle = 20),n-butane (mle = 58),and maleic anhydride (mle = 98)when a 30l1 CdHldNe mixture is pulsed over an oxygen-treated catalyst sample maintained a t 698 K. The curves are plotted to show the relative intensities of the parent ions of each species. The absolute pulse intensity was 010'~ moleculedpulse. Increasing the pulse intensity by 1order of magnitude did not change the pulse shape of the inert gas pulse. The independence of the inert gas pulse shape with respect to the pulse intensity indicates that gas transport through the microreactor can be described by Knudsen diffusion (Svoboda et al., 1992). From continuous flow experiments using pure reagents it was determined that the QMS signals at mle = 20, 58,and 98 were unique to neon, n-butane, and maleic anhydride, respectively. Therefore, the area of the transient responses a t these mass numbers is directly proportional to the amount of each species. Figure 5 shows a series of transient responses for n-butane at reactor temperatures of 544,575,600,and 629 K, respectively, when a CIHldNe mixture is pulsed over an oxygen-treated catalyst sample. The decrease in the n-butane pulse intensity reflects an increased n-butane conversion with temperature. By determining the area ratio of the transient responses for n-butane
2938 Ind. Eng. Chem. Res., Vol. 33,No. 12,1994
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Time (-1
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Time (.seconds)
Figure 4. Transient responses at 698 K of neon ( d e = 20). n-butane (m/e = 58), and maleic anhydride ( d e = 98) from a n n-butandneon mixture pulsed over a VPO catalyst sample previously oxidized for 10 min.
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Figure 2. Depiction of TAP pump-pmbe experiment showing alternating oxygen-butane pulse sequence, closeup of the microreactor, valve manifold, and QMS, and product transient responses.
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Figure 5. Transient responses a t different reaction temperatures of n-butane ( d e = 58) pulsed over a VPO catalyst sample previously oxidized for 10 min. The n-butane conversions (XIare: 544 K, X = 0.08 575 K X = 0.22; 600 K X = 0.33; 629 K. X = 0.44.
Time (kilmmmdrl
Figure3. Oxygen uptake a t 723 K by a 0.2 g reactorequilibrated catalyst sample a t a pressure of86-44 kPa versus time.
at reaction temperatures and n-butane a t 473 K (no reaction), the absolute conversion can be determined. The areas of the plotted curves correspond to conversions of 0.08,0.22, 0.33,and 0.44,respectively. Figure 6 shows a corresponding series of transient responses for MA a t reactor temperatures of 575,600,and 629 K. Figure 7a.b shows the MA transient responses from pulsing n-butane a t 585 K over the same catalyst sample that has been exposed to different oxygen
0
Figure 6. Transient responses a t different reaction temperatures ofmaleie anhydride from an n-butandneon mixture pulsed over a VPO catalyst sample previously oxidized for 10 min.
treatments (5 and 10 min, respectively) corresponding to the addition of -5 x l O I 9 and 4 x lozn oxygen molecules. These values were estimated from comparable pressure drop experiments. Figure 7a shows the
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 2939 be more accurately determined by comparing the areas of the two MA transient responses plotted in Figure 7a. To correct for the higher n-butane conversion on the sample oxidized for 10 min, the area of curve a was decreased by 11%relative to its measured area. After correcting for this difference in conversions, the ratio of the pulse areas becomes 1.36. The n-butane to MA selectivity on the sample oxidized 10 min is assumed to be 96%, which would indicate that the n-butane to MA selectivity on the sample oxidized 5 min is 71%. 4. Discussion
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In previous TAP studies (Centi et al., 1988;Ebner and Gleaves, 1988; Gleaves et al., 1988) butene, butadiene, furan, maleic anhydride, and carbon oxides have been detected when n-butane was pulsed over a VF'O catalyst under anaerobic conditions. Similar results were obtained in this study, but it was found that the relative amounts of partial oxidation products were strongly influenced by the oxygen treatment procedure. Catalysts exposed to 108 W a oxygen treatments for 5 and 10 min gave only MA and carbon oxides with very high selectivities ('95% in the temperature range of 553 to 683 K for samples oxidized for 10 min) to MA. Because the transient response experiments were performed at very low pulse intensities molecules/ pulse), gas transport can be accurately described as Knudsen diffusion, and the conversion of n-butane can be modeled using the following equation:
0.2
.
0.0
i5
io Time (seconds)
Figure 7. (a) Transient responses at 585 K of maleic anhydride from an n-butandneon mixture pulsed over a VPO catalyst sample exposed to different length oxidation treatments. (b) Same data height normalized.
relative intensities and Figure 7b the height-normalized intensities. The area of curve a corresponds to a conversion of 0.27. The n-butane to MA selectivity was estimated by first determining the n-butane to COZselectivity. Calibrated blends were used to determine the relative magnitudes of the QMS signals at mle = 20,44, and 58 for known concentrations of neon, COz, and n-butane. For the reaction product spectrum, the peak at mle = 44 can be equated with the COZ concentration provided small contributions from MA and n-butane are subtracted out. Under reaction conditions, the areas of the transient responses a t m/e = 98 and 58, in conjunction with the ratios of those peaks to the mass 44 peak in the pure substances, can be used to determine the contributions of MA and n-butane t o the total mass 44 peak. After determining the COZselectivity, the MA selectivity was estimated by assuming that all of the n-butane not converted t o C 0 2 was converted to MA. This assumption is most valid for the more oxidized sample. For the less oxidized sample, the formation of carbon residues on the catalyst surface may lead to an overestimate of the n-butane to MA selectivity. For the sample oxidized 10 min, the n-butane to MA selectivity was estimated to be >96% a t 27% conversion. For the sample oxidized 5 min, the n-butane to MA selectivity was estimated t o be 80% a t 24% conversion. The relative difference in selectivity between the sample oxidized 10 min and the one oxidized 5 min can
where C = concentration of butane, De is the effective Knudsen diffusion coefficient, €b is the bed porosity, k, is the reaction rate constant, and z is the reactor axial coordinate. It is assumed that n-butane chemisorption involves fissure of a CH bond and that the reverse reaction can be neglected. Using the above model with appropriate boundary conditions, the series of n-butane transient responses that were obtained after treating catalysts with 108 kPa oxygen for 5 and 10 min were modeled at various temperatures. The experimental data and model fits for the 10 min sample are shown in Figure 8. Similar results were obtained for the 5 min sample. The curves were modeled using k r as the only parameter to be estimated. The diffisivity was obtained by modeling the n-butane transient response data at zero conversion, and the porosity was calculated from the mean particle diameter. Figure 9a,b shows Arrhenius plots of In k versus 1000/ T. Linear plots are observed for both samples, but the resulting activation energies are significantly different. The activation energy for the reaction of n-butane over the 10 min sample is 53.8 kJ/mol, and over the 5 min sample is 98.8 kJ/mol. Table 1 shows activation energies determined by other workers. As indicated in Table 1, previous studies have found activation energies ranging from 45 to 115 kJ/mol. Our study indicates that dramatic differences in activation energies can be obtained by changes in the catalyst oxidation state. Similarly, the differences in the activation energies observed by other workers may be due to different catalyst oxidation states although other variables such as variations in catalyst composition, promoters, etc. may also contribute. Comparison of the pulse shapes of the MA responses from the 5 and 10 min samples shown in Figure 7b
2940 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994
Figure 8. Experimental (open circles) and model-predicted (solid line) transient responses at different temperatures of n-butane from a n-butaneheon mixture pulsed over a VPO catalyst sample previously oxidized for 10 min. Table 1. Activation Energy for n-Butane Conversion over VPO Catalysts
10 minute oxidation trestment
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On the catalyst exposed to the 10 min oxygen treatment, selectivities of greater than 90% were obtained at conversions greater than 70%. In contrast, selectivities in steady-state experiments at similar conversions were about 20 points lower. Also, samples receiving 10 min oxygen treatments showed significant increases in selectivity over those receiving 5 min oxygen treatments. These results indicate that the selectivity is strongly influenced by the oxygen treatment and that selectivity increases with the oxygen availability. The results also confirm those presented in a previous study (Schuurman et al., 1994) indicating that (V0)2P207 is capable of adsorbing oxygen and efficiently channeling it to the active catalytic site. Since the VPO oxidation state in a reactive environment will be determined by the rates of catalyst reduction and reoxidation, the most selective surface may not be available in a steady-state mode.
Literature Cited 0.00
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1000/Temperature Figure 9. Arrhenius plots for n-butane conversion over a VPO catalyst sample exposed to different length oxidation treatments.
shows that the response from the more highly oxidized surface is more narrow. This narrowing of the response curve indicates that the combined rate of MA formation and desorption is faster on the more oxidized surface.
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L., Delmon, B., Derouane, E., Eds.; D. Reidel Publishing Co: Dordrecht, Holland, 1982;pp 1-45. Lerou, J. J.; Mills, P. L. Du Pont Butane Oxidation Process. In Precision Process Technology; Kluwer Academic Publishers: Amsterdam, 1993;pp 175-195. National Research Council. Catalysis Looks To The Future; National Academy Press; Washington D.C., 1992. Schnider, P.; Emig, G.; Hofmann, H. Kinetic Investigation and Reactor Simulation for the Catalytic Gas-Phase Oxidation of n-Butane to Maleic Anhydride. Ind. Eng. Chem. Res. 1987,26, 2236-2241. Schuurman, Y.; Gleaves, J. T.; Ebner, J. R.; Mummey, M. J. Activation of Vanadium Phosphorus Oxide Catalysts for Alkane Oxidation-Oxygen Storage and Catalyst Performance. In New Developments in Selective Oxidation IZ; Corberan, V. C., Bellon, S. V., Eds.; Elsevier: Amsterdam, 1994;pp 203-212. Stefani, G.; Budi, F.; Fumagalli, C.; Suciu, G. D. Fluidized Bed Oxidation of n-Butane: A New Commercial Process for Maleic Anhydride. In New Developments in Selective Oxidation; Centi, G., W i r o , F., Eds.; Elsevier Pub.: Amsterdam, 1990;pp 537552. Svoboda, G. S.;Gleaves, J. T.; Mills, P. L. New Method for Studying the Pyrolysis of VPE/CVD Precursors under Vacuum Conditions. Application to Trimethylantimony and Tetramethyltin. Ind. Eng. Chem. Res. 1992,31,19-29. Weng, L.T.; Delmon, B. Phase Cooperation and Remote Control Effects in Selective Oxidation Catalysts. Appl. Catal. 1992,81, 141. Received for review April 1,1994 Revised manuscript received August 3, 1994 Accepted August 12, 1994@ Abstract published in Advance ACS Abstracts, November 1, 1994. @
