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TAP Vacuum Pulse-Response and Normal-Pressure Studies of Propane Oxidation over MoVTeNb Oxide Catalysts Rebecca Fushimi,† Sergiy O. Shekhtman,† Anne Gaffney,‡ Scott Han,‡ Gregory S. Yablonsky,† and John T. Gleaves*,† Department of Chemical Engineering, Washington University, Campus Box 1198, 1 Brookings Drive, St. Louis, Missouri 63130-4899, and Rohm & Haas Company, 727 Norristown Road, P. O. Box 904, Springhouse, Pennsylvania 19477-0904
The economic advantages of switching to a propane feedstock in the production of acrylic acid have been a motivating force for extensive research studies focused on developing alkane partial oxidation catalysts. While catalyst systems containing MoVTeNb oxides have received considerable attention, the activity of these catalysts has not been systematically characterized over a wide domain of working conditions (different gas compositions, temperatures, etc.) and a detailed kinetic model of the propane-to-acrylic-acid reaction has not been developed yet. Extensive steadystate studies (conversion/product yields vs temperature at different feed compositions and contact times) and non-steady-state studies (pulse-response temporal analysis of products (TAP) experiments) of the propane-to-acrylic-acid reaction were performed using a TAP-2 reactor system under normal and vacuum conditions. TAP pump-probe experiments indicate that there is an optimum time for the introduction of water and that water reacts with an adsorbed intermediate to produce acrylic acid. Under steady-state conditions, the temperature was slowly cycled and the reaction progress was monitored with a temperature hysteresis clearly distinguished. The upper branch of the hysteresis corresponds to a cooling regime and is characterized by complete oxygen conversion. No distinctive hysteresis behavior was observed for propane, while the product yields exhibit clear hysteresis loops, clockwise for acrylic acid and counterclockwise for CO2/CO and acrolein. In this paper, the hysteresis-type dependence is interpreted in terms of a possible detailed mechanism of selective oxidation of propane into acrylic acid. 1. Introduction Catalysts used for the selective oxidation of propane into acrylic acid are multicomponent metal oxides (MMO) containing a number of transition metals.1-2 Effective catalysts based on Mo-V-Te-Nb oxides for converting propane to acrylic acid have been reported by Ushikubo et al.; acrylic acid yields of 48% with a selectivity of 60% have been obtained with this system2. Recent studies by Ushikubo et al. indicate that highly efficient Mo-V-Te-Nb acrylic acid catalysts contain two phases, referred to as M1 and M2.3 The M1 and M2 bulk structures have orthorhombic and hexagonal lattices and formal stoichiometries of (Te2O)M20O56 and (TeO)M3O9 (with M ) Mo, V, and Nb), respectively.4 The role of the two phases is still an issue of debate. Vitry et al.5 prepared both phases separately by hydrothermal synthesis and found that the orthorhombic phase had high activity and selectivity for the conversion of propane into acrylic acid. The hexagonal phase showed poor activity, although it is constructed with Mo and V octahedral complexes, similar to the orthorhombic phase. Baca et al.6 found that mixtures of M1 and M2 are as active as the M1 phase alone, but they are more selective. Physical mixtures of the orthorhombic and hexagonal phases showed no improvement in performance over that of the sum of the two separate phases, suggesting that the cooperation of the phases occurs on the nanoscale.7 * Corresponding author. Tel.: (314) 935-4159. Fax: (314) 935-7211. E-mail:
[email protected]. † Washington University. ‡ Rohm & Haas Company.
Current results indicate that the M1 phase is responsible for activation of propane and its dehydrogenation to propene. Both M1 and M2 are active and selective for propene conversion, but the M2 phase is more selective and allows an increase in acrylic acid yields. Several studies suggest that all four elements are necessary to obtain both high propane conversion and high selectivity to acrylic acid.2,8,9 On the contrary, other research indicates that Nb is not necessary for the formation of the hexagonal and orthorhombic phases, and in quaternary systems, the Nb is present as a substitution element for V forming Mo5+. The presence of the Mo5+ gives rise to an oxygen storage-supply mechanism, and the addition of Nb to the catalysts appears to support a surface enrichment of Te.5,10 Recent studies by Ueda et al. with a hydrothermally synthesized Mo-V-Te oxide catalyst found respectable propane conversion (33%) and acrylic acid selectivity (51%) when only the orthorhombic phase was present. The hexagonal phase was found to be inactive, and mixtures of the two phases resulted in a lower overall yield.10 When comparing Mo-V, Mo-V-Te, and Mo-V-TeNb oxide catalysts, similar propane conversion was observed over a large temperature range (280-400 °C); the acrylic acid selectivity, however, was highly dependent on the constituents: the highest selectivity was found for the quaternary oxide, followed by the ternary oxide. This evidence suggests that the Mo and V octahedral complexes form the active site for propane activation.10
10.1021/ie049162k CCC: $30.25 © 2005 American Chemical Society Published on Web 07/12/2005
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Figure 1. Oxidation pathways for reaction of propane over MoV-Te-Nb oxide catalysts.1
Lin1 proposed an oxidation pathway for reaction over a Mo-V-Te-Nb-O catalyst that consists of at least three independent routes of product generation (Figure 1). According to Lin,1 this pathway utilizes molecular oxygen to partially oxidize propane to acrylic acid by way of propylene and acrolein. The selectivity toward acrylic acid is then dependent on the oxidative dehydrogenation of propane to propylene and the allylic oxidation of propylene.1 Waste products such as acetic acid are produced via an acetone pathway.1 It is worth indicating that water plays a significant role in this oxidation reaction. According to Bettahar et al.,11 Ai,12 and Centi et al.,13, the presence of water should be beneficial, detrimental, or both to the desired valuable products (acrylic acid and acrolein) in many systems of propane oxidation. Water addition increases acrylic acid at the expense of acrolein yield. Lin has hypothesized that the concentration of surface hydroxyl groups may increase with the amount of water and, thus, enhance the reaction between surface intermediates and hydroxyl groups forming acrylic acid.1 The desorption of acrylic/acetic acids from the surface may be assisted by water, preventing the overoxidation of these acids to waste products. To date, the activity of Mo-V-Te-Nb catalysts has not been systematically characterized over a wide domain of working conditions, i.e., different gas compositions, temperatures, etc. As a result, a detailed kinetic model of the propane-to-acrylic-acid reaction has not been developed yet. Essential information about the kinetics and the mechanism of this reaction can be obtained in systematic experiments where reaction parameters (e.g., temperature, gas composition, and contact time) are cycled in a controlled way. Such experiments may result in a significant difference in kinetic dependencies (hysteresistype dependence). In this paper, such dependencies are obtained and interpreted in terms of a possible detailed mechanism of the selective oxidation of propane into acrylic acid. 2. Experiment Description All studies were performed in a TAP-2 experimental system using a three-zone reactor configuration at normal and vacuum conditions. The details of the TAP-2 reactor system can be found elsewhere.14-16 In brief, the system comprises three basic components: a valve manifold, a reactor, and a mass spectrometer. The mass spectrometer is contained in a high-vacuum system that can easily accommodate low-intensity fast transient response experiments and that can handle high-volume continuous flows as a result of a specially designed slide valve that permits the reactor to operate at vacuum or high-pressure conditions (10-8 to 7000 Torr). The reactor configuration parameters were as follows: reactor length, 33 mm; reactor diameter, 5 mm; length of two inert zones (packed with 730 mg of quartz particles), 12 mm; and length of catalyst zone, 3.3 mm for normal-pressure studies (packed with 120 mg of the catalyst) and 1.0 mm
for low-pressure studies (packed with 32 mg of the catalyst). The signals of oxygen (AMU32), propane (AMU29), propene (AMU42), water (AMU18), argon (AMU40), or krypton (AMU84) and those of product molecules CO (AMU28), CO2 (AMU44), acrylic acid (AMU72 & 56), acrolein (AMU56), and acetic acid (AMU60) were monitored with the mass spectrometer. Normal-Pressure Conditions. In the normal-pressure mode, reactant gas mixtures of propane, oxygen, CO2, and argon were passed through a heated water bubbler before being admitted to the reactor through a continuous flow valve at 1 atm; the reaction conditions were as follows: temperature range, 20-400 °C; feed composition, propane 6.2-40%, oxygen 15-50%, CO2 0-50%, and balance argon (propane-to-oxygen ratio is maintained at ∼1/2) passed through a fritted, heated water bubbler (T ) 65 °C); contact time, 10-1 s; and heating rates, varied from 0.5 to 20 °C/min. The outlet composition measurements were performed by passing a small portion of the flow into the mass spectrometer chamber through a needle valve located between the reactor exit and the vacuum chamber. Carbon and hydrogen mass balances of the effluent gas were within 5-7%, indicating that a bias toward lighter molecules passing over the needle valve was not a factor. Vacuum Conditions. The amount of catalyst was decreased to 32 mg in the vacuum pulse-response experiments to ensure a thin zone (1 mm) relative to the overall reactor length. Prior to the TAP vacuum pulse-response experiments being performed, each catalyst sample was operated at atmospheric pressure at 350°C using a typical steady-state feed (Pr/O2/Ar, 7/14/ 79 molar ratio) and contact time (1.2 s) for 30 min. After 30 min, the propane feed was stopped and the catalyst was exposed to the oxygen-argon mixture until the propane signal disappeared to achieve an oxidized state. In some cases, the catalyst was exposed to a longer oxidation period, with the maximum period being ∼12 h. After oxidation, the catalyst was maintained at 350 °C, and the reactor was evacuated while the reactor effluent was monitored. The principal product observed during the evacuation was water. After the water signal decreased to the vacuum-system background level, TAP vacuum pulse-response experiments and TAP pumpprobe experiments were performed using different mixtures of oxygen, propane, propene, and water. Krypton, argon, and helium (∼20%) were used as internal standards to determine gas transport though the bed. Pulsing experiments were typically performed at 20-350 °C with an input pulse width of 300 µs and ∼1013 molecules per pulse. The exit flow was detected by the mass spectrometer and used to calculate the moments (0th, 1st, and 2nd), or time-weighted areas under the different response curves, in order to determine conversion, selectivity, and kinetic parameters.16-17 The advanced thin-zone TAP reactor (TZTR) proposed by Shekhtman et al.17,19 was used where the thickness of the catalyst zone is small compared to the total length of the reactor, thus making the change in the gas concentration across the catalyst zone small compared to its average value. A key advantage of the TZTR configuration, aside from the simplicity of the mathematical model, is that the catalyst bed can be changed uniformly by exposing the catalyst to a long series of small pulses.
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Figure 2. Propane and oxygen conversion as well as product yields versus temperature: contact time ) 3.5 s; oxygen ) 19.1%, propane ) 9.2%, balance argon; passed through a water bubbler at 65 °C.
Catalyst Preparation and Characterization. A catalyst was prepared in a manner similar to a previously published procedure.18 A nominal composition of Mo1.0V0.3Te0.23Nb0.17Ox was prepared in the presence of nitric acid in the following manner: 200 mL of an aqueous solution containing ammonium heptamolybdate tetrahydrate (1.0 M Mo), ammonium metavanadate (0.3 M V), and telluric acid (0.23 M Te), formed by dissolving the corresponding salts in water at 70 °C, was added to a 2000 mL rotavap flask. Then, 200 mL of an aqueous solution of ammonium niobium oxalate (0.17 M Nb), oxalic acid (0.155 M), and nitric acid (0.24 M) were added thereto. After removal of the water via a rotary evaporator with a warm water bath at 50 °C and 28 mm Hg, the solid materials were further dried in a vacuum oven at 25 °C overnight and then calcined. (Calcination was effected by placing the solid materials in an air atmosphere, heating them to 275 °C at 10 °C/ min, and then holding them under the air atmosphere at 275 °C for 1 h; the atmosphere was then changed to argon, and the material was heated from 275 °C to 600 °C at 2 °C/min and then held under the argon atmosphere at 600 °C for 2 h.) XRD analysis revealed diffraction peaks at the following angles ((0.3°) of 2θ: 22.1°, 36.2°, 45.2°, and 50.0°. 3. Results and Discussion 3.1. Steady-State Kinetic Dependencies under Normal-Pressure Conditions. 3.1.1. Hysteresis. Extensive steady-state studies (conversion/product yields vs temperature at different feed compositions and contact times) and non-steady-state studies (pulseresponse TAP experiments) of the propane-to-acrylicacid reaction were performed using a TAP-2 reactor system with a three-zone reactor configuration at normal and vacuum conditions. As the catalyst was slowly heated from 20 to 400 °C and then cooled back to 20 °C, the reaction was monitored at the mass spectrometer. The following process conditions were investigated: temperature range, 20-400 °C; feed
composition, propane 6.2-19%, oxygen 15-38%; contact times, 0.1-10 s; and catalyst heating rate, 1-10 °C/min. Because many experiments indicate the significant influence of water on the acrylic acid yield, the reaction mixture was passed through a heated water bubbler maintained at a constant temperature of 65 °C in all experiments. At atmospheric pressures in a steady-state temperature cycle experiment, the following results were obtained (plotted in Figure 2): (1) A temperature hysteresis is observed at oxygen/ propane ratios ranging from 1.5 to 4 (“upper” and “lower” branches of hysteresis can be clearly distinguished). (2) The upper branch of the hysteresis corresponds to a cooling regime and is characterized by complete oxygen conversion. (3) No distinctive hysteresis behavior for propane conversion was observed. (4) Complete oxygen conversion is accompanied by an increase in acetic acid (AcA) and CO/CO2 yields and a drop in the acrylic acid (AA) yield (i.e., a dramatic drop in AA selectivity). (5) The maximum AA yield is observed on the lowtemperature edge of the hysteresis domain. (6) The temperature domain of hysteresis shifts toward the lower temperatures as the contact time increases. A typical temperature-hysteresis is presented in Figure 2. A comparison of Figure 2 with Figure 3 shows how hysteresis shifts to higher temperatures as contact time decreases (from 3.5 to 3.0 s, respectively). A comparison of Figures 2 and 4 indicates a hysteresis shift to lower temperatures as contact time increases (from 3.0 to 8.9 s, respectively). At high propane-tooxygen ratios, no hysteresis behavior is observed, as illustrated by Figure 5. According to the traditional terminology, observed hystereses for oxygen, CO2/CO, and AcA are counterclockwise, while hysteresis for AA is clockwise.
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Figure 3. Propane and oxygen conversion as well as product yields versus temperature: contact time ) 3.0 s; oxygen ) 42.7%, propane ) 21.4%, balance argon; passed through a water bubbler at 65 °C.
Figure 4. Propane and oxygen conversion as well as product yields versus temperature: contact time ) 8.9 s; oxygen ) 48%, propane ) 32%, balance argon; passed through a water bubbler at 65 °C.
Estimation of mass and heat transfer criteria indicates that, in our case, the process is limited by kinetic factors. The hysteresis of reaction rate observed in this domain for different substances indicates chemistry is the main contributor to the complex behavior. Hysteresis is a complex phenomenon caused by both chemical kinetic factors and physical (e.g., thermal) factors, and the occurrence of opposite hysteresis trends for different substances (i.e., clockwise hysteresis for acrylic acid and counterclockwise hystereses for CO2, CO, and AA) indicates that the main cause is chemical, i.e., competition between different substances on the catalyst surface. Such a difference is very typical for surface adsorption mechanisms with competition of different
substances on the catalyst surface. Thus, we consider that the origin of this hysteresis is chemical. 3.1.2. Influence of CO2. The influence of the CO2 feed percentage on AA and CO production was clearly observed in a special set of steady-state experiments carried out using a contact time of 3.3 s and different CO2 feed percentages: 0%, 10%, 20%, 30%, 40%, and 50%. The propane and oxygen percentages were kept the same for all experiments, 9.2% and 19.1%, respectively. In the temperature range 375-385 °C, there was an increase in the AA yield when the CO2 feed percentage changes from 0% to 50% as presented by Figure 6. This increase was estimated to be as high as 10% relative to the yield with no CO2 in the feed and took
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Figure 5. Propane and oxygen conversion as well as product yields versus temperature: contact time ) 8.0 s; oxygen ) 38%, propane ) 6.2%, balance argon; passed through a water bubbler at 65 °C. No hysteresis behavior observed.
Figure 6. AA yield versus CO2 feed percentage for different temperatures: “Upper” hysteresis branch.
place in the left boundary of hysteresis curve. The influence of CO2 addition on the reaction rate may result from CO2 adsorption on a special active site and/or from the generation of special intermediates that participate in one of the reaction routes. 3.2. Step-Transient Experiments under NormalPressure Conditions. Step-transient response experiments were performed at atmospheric pressure and at different contact times by introducing propane or water as a step function. Prior to each step input, the catalyst was maintained in an O2/Ar flow (8/92 molar ratio) of 4.3 cm3/min (contact time 1.4 s) for 5 min at 350 °C. A portion of the flow was diverted to the mass spectrometer, and masses 0-90 were monitored. The first 49 scans recorded the steady-state oxidation response. At scan 50, a mixture of propane and krypton was rapidly pulsed into the reactor to create a step-transient input of propane. The amount of propane was balanced so that the clear production of products could be monitored, yet the Pr/Kr mixture did not replace the O2/Ar feed. After 50 additional scans, the Pr/Kr mixture was shut off and the catalyst was allowed to return to the steady-state oxidation conditions.
At different contact times, different kinetic regimes with different reactant and product transient responses are observed as shown in Figure 7. It can be noticed that, as propane enters the feed, water has the most immediate response, along with an increase in oxygen conversion. Figure 8 presents a number of dependencies versus contact time. 3.3. TAP Vacuum Pulse-Response Experiments. 3.3.1. Oxygen Uptake Experiments. Oxygen uptake experiments are performed to determine how the number of oxygen adsorption sites on a catalyst surface changes after the catalyst is exposed to different reaction or treatment conditions. We examined samples exposed to steady-state reaction feeds and oxidizing feeds at atmospheric pressure and to pulsed treatments with propane and propene. In a typical uptake experiment, the catalyst sample was heated in a vacuum at reaction temperature until the water peak due to desorption decreased to the background level. The catalyst was then exposed to a series of oxygen pulses to measure the oxygen uptake. Figure 9 shows a typical oxygen uptake curve after the initial water loss and after being exposed to 900 propane pulses. During oxygen uptake, a small amount of CO and CO2 was produced (estimated to be
