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Ind. Eng. Chem. Res. 2007, 46, 70-79
Ethane and Propane Oxidation over Supported V2O5/TiO2 Catalysts: Analysis of Kinetic Parameters T. V. Malleswara Rao and Goutam Deo* Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur, 208 016, India
The effect of surface vanadia loading on the alkane (ethane and propane) oxidation reaction kinetics for supported V2O5/TiO2 catalysts is investigated by employing a parallel-sequential Mars-van Krevelen model. Analysis of the kinetic parameters reveals that the preexponential factors for alkene formation and alkene combustion reactions increase with vanadia loading, except for CO2 formation from ethene. Furthermore, the activation energy for ethene formation appears to be greater than that for propene. The values of the rate constant ratios k1/k2 and k1/(k3 + k4), which represent the amount of alkane available and the net alkene formed, are functions of vanadia loading and alkanes oxidized, and are closely related to the ratio of the respective preexponential factors. The highest k1/(k3 + k4) value, which corresponds to the highest alkene yield at isoconversion, is observed for the monolayer catalyst and is higher for propene formation than for ethene formation. 1. Introduction Supported vanadium oxide catalysts form an important class of heterogeneous oxidation catalysts for the production of a wide variety of bulk organic chemicals.1-6 These supported vanadia systems have also emerged as potential catalysts in the alkane oxidative dehydrogenation (ODH) reaction, which is an advantageous route for alkene production.1-6 Specifically, supported V2O5/TiO2 catalysts have shown better catalytic performance in propane ODH than the bulk V2O5 and the vanadia catalysts supported on other metal oxides (Al2O3, SiO2, HfO2).7-16 Supported vanadia catalysts have also been investigated for ethane ODH reaction.16-28 Studies by Banares et al.25 revealed that the structural changes occurring with the increased vanadia loading do not affect the intrinsic ethane oxidation activity (turnover frequency, TOF) of the supported vanadia catalysts. Le Bars et al.21 observed an enhanced selectivity-conversion relationship for ethane oxidation over V2O5/Al2O3 catalysts compared to bulk V2O5 and supported V2O5/SiO2 catalysts.17-20 A comparative study of propane and ethane oxidation over vanadia catalysts supported on titania and alumina at monolayer coverage reveals that propane exhibits higher activity than ethane under the same reaction conditions, irrespective of the support,16 which is due to the difference in C-H bond strength in propane and ethane.29 It was also observed that the supported V2O5/ TiO2 possessed better catalytic properties than the V2O5/Al2O3 catalysts.16 All the above studies suggest that the control of selectivity and activity in alkane oxidation reactions depends on the nature of the oxide support, the vanadia loading, and the specific alkane fed. Several reaction pathways have been considered for kinetic investigation to establish a proper reaction model for explaining the observed alkane ODH over vanadium-based oxide catalysts.7-13,26,27,30-32 Kinetic studies involving isotopic labeling experiments32,33 revealed the involvement of lattice oxygen in alkane oxidation reactions via a Mars-van Krevelen (MVK) or redox reaction model over supported vanadia catalysts. Chen et al.32 reported isotopic tracer findings on propane ODH over V2O5/ZrO2 catalysts and concluded that C3H6 is formed as a * To whom correspondence should be addressed. Tel.: +91-5122597881. Fax: +91-512-2590104. E-mail:
[email protected].
primary product and carbon oxides (CO and CO2) mainly form as secondary propene oxidation products via the MVK reaction model. The direct propane oxidation to CO2 was also observed. Information on the kinetic analysis of supported V2O5/TiO2 catalysts for ethane and propane ODH is sparse. The supported V2O5/TiO2 catalytic system was studied by Khodakov et al.7 to analyze the effect of vanadia loading on the kinetic parameters associated with propane ODH using pseudo-first-order reaction rates. Grabowski and co-workers9-11 and Routray et al.12 examined the propane ODH reaction kinetics specifically over monolayer V2O5/TiO2 catalyst using a Eley-Rideal steady-state adsorption model and MVK model, respectively. Despite the importance of the supported V2O5/TiO2 system, to the best of our knowledge, no information regarding the kinetic analysis of ethane ODH over supported V2O5/TiO2 catalysts exists in the open literature. The objective of the present study is to compare the alkane (ethane and propane) oxidation activities over supported V2O5/ TiO2 catalysts and to understand the effect of vanadia loading through detailed kinetic analysis. To achieve this objective, the ethane and propane oxidation reactions are performed over synthesized V2O5/TiO2 catalysts with various vanadia loadings for which kinetic parameters are estimated. The kinetic parameters are analyzed with respect to the specific alkane oxidized and the vanadia loading. 2. Experimental Section 2.1. Catalyst Preparation. A series of supported V2O5/TiO2 (VTi) catalysts were prepared by the incipient-wetness impregnation technique on pretreated TiO2 (Degussa, P-25) support.12 The precursor used was a solution of ammonium metavanadate (NH4VO3) in oxalic acid. Incipient volumes of solutions containing predetermined amounts of precursor and the pretreated support were intimately mixed in order to prepare the catalysts with different loadings of vanadia. The mixture was kept in a desiccator overnight followed by drying at 383 K for 6 h, and at 473 K for another 6 h. Finally the samples were calcined at 723 K for 6 h. The prepared catalysts were denoted as xVTi, where x is the weight percent loading corresponding to V2O5. 2.2. Physical Characterization. 2.2.1. Surface Area. The surface areas of the samples were obtained by a multipoint BET
10.1021/ie060715w CCC: $37.00 © 2007 American Chemical Society Published on Web 12/06/2006
Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007 71
method using N2 adsorption data at 77 K. Degassing of the samples was realized by heating the samples at 423 K in flowing helium. The apparatus used for the measurements was a COULTER SA 3100 analyzer equipped with SA-VIEW software. 2.2.2. X-ray Diffraction (XRD). Powder XRD patterns of the prepared catalysts were measured with a Seifert ISODebyeflex 2002 using Ni filtered KR radiation from a Cu target (λ ) 1.440 56 Å). 2.2.3. Raman Spectroscopy. The ambient Raman spectra of the prepared catalysts were obtained using a Raman spectrometer system (Horbia-Jobin Yvon LabRam-HR). The samples were excited with a 532 nm (visible) YAG double diode pumped laser, and the spectral resolution was ∼2 cm-1. The scattered photons were directed into a single monochromator (Jobin Yvon, LabRam-HR). A LN2 CCD detector (Jobin Yvon CCD-3000V) was used to collect the signal. Sample weights of ∼0.01 g were placed onto a glass slide below the confocal microscope. Additional details are given elsewhere.34 2.3. Temperature-Programmed Reduction Using Hydrogen (H2-TPR). The H2-TPR measurements were performed in a Micromeritics Pulse Chemisorb 2705 apparatus. A sample weight of ∼0.018-0.06 g was taken in a U-shaped quartz reactor and pretreated. The pretreatment was achieved by heating the sample in a He gas flow (30 mL/min) at 473 K for 30 min. After the sample was cooled to 300 K, a 5% H2/Ar mixture flowing at 30 mL/min was introduced into the reactor for reduction measurements. The catalyst sample was heated to a final temperature of 1150 K at a ramping rate of 10 K/min. The hydrogen consumption was measured by a thermal conductivity detector. Known amounts of 5% H2/Ar mixture were used for quantification purpose. 2.4. Reaction Studies. The prepared catalysts were examined for the alkane (ethane and propane) oxidation reactions at atmospheric pressure in a vertical down-flow quartz reactor mounted in a tubular furnace. Details of the reactor are given elsewhere.13 Prior to the reaction studies the catalyst samples were heated in a muffle furnace for 30 min at 673 K. A physical mixture of 0.006-0.12 g of the catalyst and the required amount of inert quartz particles to form a bed height of 1 cm was loaded into the reactor. The temperature of the catalyst bed was measured by a thermocouple located just above the catalyst bed and was controlled by a PID temperature controller (FUJI Microcontroller X Model PXZ 4). The flow rates of reactants (alkane and O2) and inert gas (N2) were adjusted through separate thermal mass flow controllers (Bronkhost Hi-Tec, Model F-201D FAC-22-V) to maintain the specified alkane-to-oxygen ratio and total flow rate. The reactant gases were mixed prior to sending to the reactor. Nitrogen was used as a diluent, and its amount was fixed corresponding to inlet air composition. The exit gases were sent for online analysis to a gas chromatograph (AIMIL-NUCON 5700) equipped with a methanizer. The carbon oxides (CO and CO2) and hydrocarbons (C3H8 and C3H6 for propane oxidation or C2H6 and C2H4 for ethane oxidation) were separated using a Hysep-Q column and analyzed in FID mode. The effect of contact time on alkane conversion was studied by changing the total reactant flow rate between 30 and 120 mL/min at 653 K and an alkane:O2 ratio of 2:1. For determining kinetic parameters, the catalyst was initially heated in the reactor to 613 K in flowing O2. After 30 min the required flow rate of alkane, O2, and N2 was introduced at an alkane:oxygen ratio of 1:1 and the temperature was varied from 613 to 673 K. After the reaction data were collected at the various temperatures,
Scheme 1 . Reaction Sequence for Alkane Oxidation, Where n ) 2 for Ethane and n ) 3 for Propane
the inlet gas stream was changed to pure O2 and then catalyst was heated at 613 K. After 30 min the inlet gas stream was changed and the alkane oxidation reaction was carried out at an alkane:oxygen ratio of 2:1 by varying the temperatures from 613 to 673 K. Similarly, the reaction data were obtained at an alkane:oxygen ratio of 3:1 after treatment of the catalyst at 613 K in flowing O2 for 30 min. After the data were collected at all specified alkane:oxygen ratios and temperatures, the reproducibility of some runs was verified. A constant total flow rate of 75 mL/min was maintained. Low alkane conversions ( 2VTi > 1VTi, irrespective of the alkane. Thus, the results indicate that the alkane conversion increases up to monolayer coverage (4VTi) and is then relatively constant for the 6VTi sample, for which an incipient amount of bulk V2O5 is formed.
74 Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007
Figure 5. (A) Alkane conversion as a function of contact time; (B) variation of alkene yield with alkane conversion. At T ) 653 K and alkane:O2 ) 2:1.
Based on the conversion data the TOFs for the VTi catalysts are calculated at specific contact times for ethane and propane oxidation reactions and are presented in Table 2. It is observed from Table 2 that the TOFs for ethane and propane oxidation increase slightly up to the values for the 4VTi sample and then decrease for the 6VTi catalyst, where bulk V2O5 is observed. Previous studies have observed relatively invariant TOF values with an increase in loading for monolayer or lower loadings of V2O5/Al2O342,43 and V2O5/ZrO2 catalysts44 during propane ODH, and for vanadia catalysts supported on ZrO2, TiO2, Al2O3, and SiO2 during ethane ODH.24,25 The invariant TOFs with loading suggest that the alkane ODH reaction is structure insensitive. Comparing the two oxidation reactions, it is observed that the TOF for propane is an order of magnitude higher than that for the ethane oxidation reaction, which is expected. The variations of alkene (ethene and propene) selectivity with alkane (ethane and propane) conversion for each catalyst are shown in Figure 4. It is observed from Figure 4 that the alkene selectivity and alkane conversion are inversely related for each catalyst. However, the exact nature of the selectivity-conversion curve depends on the specific surface vanadia loading. The alkene selectivity at isoconversion varies as 4VTi g 2VTi > 6VTi > 1VTi. On comparing the specific alkene selectivity, it is observed that the variations in ethene selectivity are not as pronounced as variations in propene selectivity. Furthermore,
Figure 6. (A) Parity plot for the 4VTi catalyst in ethane oxidation; (B) parity plot for the 4VTi catalyst in propane oxidation.
variations of CO and CO2 selectivities with propane conversion suggest that the carbon oxides are primarily formed from secondary combustion of propene. However, in the case of ethane oxidation, the CO appears to be formed from secondary combustion of ethene, while the CO2 selectivity profile suggests that CO2 is primarily formed from the direct combustion of ethane. The CO and CO2 selectivity data are not shown for brevity. For comparing the ethane and propane oxidation reactions, the alkane conversions as a function of contact time are presented in Figure 5A for the 4VTi catalyst. The data in Figure 5A reveal that, at isocontact time, the propane conversion is higher than that of ethane conversion, suggesting that the breaking of a C-H bond for ethane is more difficult than that for propane.29 Furthermore, the data in Figure 5B reveal that at isoconversion the 4VTi catalyst exhibits higher propene than ethene yield. Similar results are also observed for other VTi catalysts and are not shown. Thus, the above results suggest that the product distribution and the catalytic alkane oxidation activity are strongly influenced by the specific alkane oxidized and the vanadia loading used. To obtain additional insight into the differences between ethane and propane oxidation and the effect of vanadia loading, the kinetic parameters were estimated for both reactions. The parameter values, the units, and the standard error associated
Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007 75 Table 2. Activity and Turnover Frequency (TOF) Values of Supported VTi Catalysts Experimental contact time (kg m-3 s)
reaction ethane ODH ethane ODH ethane ODH ethane ODH propane ODH propane ODH propane ODH propane ODH
catalyst
100
1VTi 2VTi 4VTi 6VTi
15
1VTi 2VTi 4VTi 6VTi
activity (× 106 g-mol/(g s))
TOF (s-1) × 102
predicted TOF (s-1) × 102
0.41 0.86 2.30 2.70
0.37 0.39 0.52 0.41
0.33 0.40 0.55 0.42
2.3 4.1 4.8 3.1
2.0 4.0 5.3 3.5
2.4 9.0 21.0 21.2
Table 3. Kinetic Parameters for Supported VTi Catalystsa Ethane ODH
Propane ODH
parameter
1VTi
2VTi
4VTi
6VTi
1VTi
2VTi
4VTi
6VTi
k10 (mL STP min-1 (g of catalyst)-1 atm-1) k20 (mL STP min-1 (g of catalyst)-1 atm-1) k30 (mL STP min-1 (g of catalyst)-1 atm-1) k40 (mL STP min-1 (g of catalyst)-1 atm-1) k50 (mL STP min-1 (g of catalyst)-1 atm-1)
1.12 (0.1) 0.05 (0.007) 465 (15) 193 (10) 182 (18)
3.28 (0.1) 0.1 (0.006) 766 (42) 186 (6) 338 (42)
9.24 (0.4) 0.6 (0.03) 1231 (45) 123 (8) 969 (39)
10.5 (0.1) 1.15 (0.005) 1352 (5) 79 (1) 654 (5)
8.07 (0.1) 0.53 (0.03) 369 (7) 98 (9) 108 (2)
40.6 (0.4) 0.8 (0.01) 618 (5) 262 (3) 349 (3)
137 (1) 1.88 (0.02) 1486 (33) 394 (5) 965 (16)
169 (1.2) 2.51 (0.05) 4770 (90) 1660 (58) 751 (7)
E1 (kJ mol-1) E2 (kJ mol-1) E3 (kJ mol-1) E4 (kJ mol-1) E5 (kJ mol-1)
63 (2) 58 (2) 22 (4) 21 (1) 334 (21)
77 (3) 52 (4) 21 (7) 15 (3) 223 (27)
81 (2) 60 (2) 12 (4) 25 (3) 216 (29)
82 (1) 58 (1) 19 (1) 35 (1) 110 (2)
72 (1) 88 (2) 33 (1) 13 (1) 118 (2)
68 (1) 86 (2) 38 (1) 31 (1) 135 (4)
51 (1) 90 (2) 39 (2) 30 (1) 155 (2)
57 (1) 106 (2) 48 (1) 35 (1) 106 (2)
a
The standard errors are given in parentheses. Tm ) 643 K.
with each kinetic parameter value are listed in Table 3. It is observed from Table 3 that the standard error values are very small compared to the corresponding parameter value. Furthermore, a comparison of the actual and predicted concentrations for all the catalysts (for example, see Figure 6 for the 4VTi catalyst) reveals a close correspondence suggesting a proper representation of the ethane and propane oxidation reactions over these supported VTi catalysts. Detailed analysis of the kinetic parameters presented in Table 3 reveals that the preexponential factor associated with alkene formation or the alkane ODH step, k10, increases significantly as the loading is increased. The rapid increase in k10 values occurs up to monolayer coverage (4VTi), after which the increase is gradual for both oxidation reactions. The gradual increase in k10 values for above monolayer coverage appears to be related to the low oxidation activity to alkene of crystalline V2O5 present in the sample. Furthermore, for each catalyst the k10 values for propane are significantly higher than those for ethane oxidation, which is related to the higher alkane ODH activity for propane. The preexponential factor for alkane combustion, k20, increases with an increase in vanadia loading for both oxidation reactions, suggesting that bulk V2O5 also contributes to the primary combustion reaction. The variations in the preexponential factors for alkene combustion, k30 and k40, with vanadia loading are different for ethane and propane oxidation. The k30 and k40 values for propane oxidation increase gradually until monolayer coverage (4VTi) and then increase rapidly for the 6VTi sample. It appears that the presence of bulk V2O5 facilitates the formation of carbon oxides relative to the surface vanadia species. Similar results were also observed for propane ODH over V2O5/Al2O3 catalysts.43 In contrast to propene, the k30 value for ethene oxidation increases with an increase in vanadia loading up to 6VTi sample, whereas the k40 value decreases gradually as the loading is increased. The decrease in the k40 value suggests that the oxidation of ethene to CO2 is due to the exposed titania sites. Thus, the surface vanadia species are responsible for the propene combustion to
CO and CO2; however, the surface vanadia species only appear to combust ethene to CO. Comparison of the different activation energies presented in Table 3 reveals that the activation energy for alkene formation, E1, is relatively independent of vanadia loading. It also appears that the E1 values (51-72 kJ/mol) for propene formation are lower than those for ethene formation (63-82 kJ/mol), as expected. The activation energy for CO2 formation from direct alkane oxidation, E2, is relatively constant with respect to loading and is higher for propane combustion. The E2 value is always greater than E1 for propane oxidation, and is always less than E1 for ethane oxidation. The activation energies for CO and CO2 formation from secondary combustion of alkene, E3 and E4, are relatively constant with respect to loading and are always less than E1. The differences between E1 and the E3 and E4 values are generally larger for ethane oxidation than for propane oxidation. The influence of vanadia loading on kinetic parameters associated with reoxidation of catalyst (k50 and E5) over VTi catalysts for alkane (ethane and propane) oxidation is also observed in Table 3. It is observed for both oxidation reactions that the k50 values increase with an increase in vanadia loading up to monolayer coverage (4VTi), and then decrease for 6VTi catalyst, where bulk V2O5 is observed. The activation energy for reoxidation, E5, however, decreases with vanadia loading for ethane oxidation, whereas for propane oxidation the E5 value increases up to 4VTi and then decreases for the 6VTi sample. Additionally, the E5 values are relatively higher for ethane oxidation than those for propane oxidation. Thus, the reoxidation rates of VTi catalysts appear to depend on the vanadia loading and specific alkane molecule. Our previous studies reveal that knowledge of the degree of reduction, β, is essential for the quantitative description of the conversion or turnover frequency of propane ODH reaction over supported vanadia catalysts and the effects of applying firstorder kinetics.43 The contribution of β is implicitly given in eqs 6 and 7, which describes the conversion of the alkane molecules. To appreciate the contribution of the degree of
76 Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007
Figure 7. (A) Degree of reduction as a function of contact time for ethane oxidation; (B) degree of reduction as a function of contact time for propane oxidation. At T ) 653 K and alkane:O2 ) 2:1.
Figure 8. (A) Variation of k1/k2 ratio with temperature for ethane oxidation; (B) variation of k1/k2 ratio with temperature for propane oxidation.
reduction on the reaction rates the degree of reduction, β, defined by eq 11, is evaluated and plotted as a function of contact time in Figure 7 at 653 K and a C3H8:O2 ratio of 2:1 for all the VTi catalysts. Figure 7 illustrates the nonlinear increase in β with contact time for all VTi catalysts, and at isocontact time β increases as the loading increases, irrespective of the alkane oxidized. Additionally, it is observed that the β values of VTi catalysts for propane oxidation are higher than those for ethane oxidation. At a common contact time of 15 kg m-3 s for propane ODH and 100 kg m-3 s for ethane ODH, the predicted conversion data, which include β, are converted as TOFs and presented in Table 2 for the VTi catalysts. Comparison of the experimental TOF and predicted TOF reiterates the quantitative representation of the alkane ODH reaction and the need to include β for quantitative representation of the data. Of primary interest in the oxidation of ethane and propane in the present study is the formation of alkenes. Analysis of the MVK model discussed above reveals that two lumped parameters, k1/k2 and k1/(k3 + k4), are important to describe the alkene yield at isoconversion values. The lumped parameter k1/ k2 is required to describe the relative contribution of alkane oxidation to alkene and CO2 and is related to the amount of alkane available for the ODH reaction. The lumped parameter k1/(k3 + k4) describes the relative rates of alkene formation and alkene oxidation to COx (CO and CO2) and is related to the yield of alkene at isoconversion. Each of these lumped
parameters is a function of temperature. The variations of k1/k2 with temperature for the VTi catalysts are shown in parts A and B of Figure 8 for ethane and propane oxidation reactions, respectively. The k1/k2 ratio increases with temperature for ethane oxidation and decreases with temperature for propane oxidation. These k1/k2 ratio variations with temperature are related to the difference in E1 and E2, which is positive for ethane and negative for propane. Consequently, the direct alkane oxidation to CO2 is favored at lower temperatures for ethane, and is favored at higher temperatures for propane. The variation of k1/k2 with loading and specific alkane is similar to the variation in k10/k20. The k10/k20 ratio varies as 2VTi > 1VTi > 4VTi > 6VTi for ethane oxidation and as 4VTi > 6VTi > 2VTi > 1VTi for propane oxidation. Furthermore, for ethane oxidation and propane oxidation the k1/k2 ratio ranges from 7 to 105. Specifically, for ethane oxidation the k1/k2 ratio ranges from 7 to 40 and for propane oxidation the k1/k2 value ranges from 13 to 105, suggesting that the contribution of direct propane oxidation to CO2 is much smaller in comparison with the contribution of ethane oxidation. The variations of the k1/(k3 + k4) ratio with temperature are given in parts A and B of Figure 9 for ethane and propane oxidation, respectively. It is observed that the k1/(k3 + k4) ratio increases with an increase in temperature irrespective of the alkane. The increase in the k1/(k3 + k4) ratio occurs since E1 is greater than E3 and E4 for both reactions. The increase in the
Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007 77
Figure 9. (A) Variation of k1/(k3 + k4) ratio with temperature for ethane oxidation; (B) Variation of k1/(k3 + k4) ratio with temperature for propane oxidation.
k1/(k3 + k4) ratio suggests an increase in alkene yield at isoconversion at higher temperatures.13,37 The variation of k1/ (k3 + k4) values is similar to those of the k10/(k30 + k40) values, and the highest value is observed for the monolayer catalyst, 4VTi. The k1/(k3 + k4) values vary with loading as 6VTi ≈ 4VTi > 2VTi > 1VTi for ethane oxidation and 4VTi > 2VTi > 6VTi > 1VTi for propane oxidation. The trends in the k1/(k3 + k4) values are consistent with the experimental trends in alkene yields at isoconversion, suggesting that the effect of direct combustion of alkanes is small, which is in accordance with the analysis of the k1/k2 values above. Based on the parameter values presented in Table 3, the predicted variations of alkene yields with alkane conversion are presented in Figure 10for the different VTi catalysts. The variations in alkene yields are consistent with the trends observed in k1/(k3 + k4) values for the VTi catalysts suggested before.13,37,45 Additionally, the k1/(k3 + k4) values observed are higher for propane oxidation than those for ethane oxidation in accordance with the observed propene yield being higher than the ethene yield at isoconversion values (see, e.g., Figure 5). It is also observed from Figure 10 that as the propane conversion increases the propene yield increases continuously in the conversion range considered, whereas for ethane oxidation, the ethene yields appear to have reached optimum values on all the VTi catalysts. Analysis of the oxygen concentrations reveals that oxygen depletion has not taken place and that the ethene
Figure 10. (A) Variation of ethene yield with ethane conversion; (B) variation of propene yield with propane conversion. At T ) 653 K and alkane:O2 ) 2:1.
yields are indeed optimum values. The values of k1/(k3 + k4) are indeed small, and consequently optimum ethylene yields are expected as per prescribed criteria.36 5. Conclusions The alkane (ethane and propane) oxidation reactions were performed over supported V2O5/TiO2 (VTi) catalysts in order to understand the effect of surface vanadia loading on the alkene yields in terms of the kinetic parameters associated with a particular reaction model. Characterization results reveal that only dispersed surface vanadia species are present below monolayer coverage (4VTi) and incipient amounts of crystalline V2O5 along with surface vanadia species are present in the above monolayer coverage sample (6VTi). The vanadia surface density of the monolayer catalyst corresponds to ∼6.3 V atoms/nm2 for the VTi catalysts. The kinetic parameters were successfully estimated for a parallel-sequential Mars-van Krevelen model in alkane oxidation reactions over VTi catalysts, and it was determined that the parameter values provided a proper representation of the reaction. This reaction model considers the alkene and CO2 as primary products, CO and CO2 as secondary products, and a reoxidation step. Analysis of estimated kinetic parameters provides several insights. The preexponential factors for all the
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reaction steps increase with vanadia loading up to monolayer coverage (4VTi), except for the preexponential factor corresponding to CO2 formation as a secondary product from ethene, which decreases with vanadia loading. Above monolayer coverage, the increase in k10 is less rapid, suggesting the ineffectiveness of bulk V2O5 for alkene formation, and the preexponential factors related to reoxidation decrease. The activation energy value, E1, for ethene formation appears to be greater than that for propene, suggesting that the activation of the C-H bonds is more difficult in ethane than in propane. Furthermore, the E2 values are lower than the E1 values for ethane oxidation, while for propane the E2 values are higher than the E1 values. However, the E3 and E4 values corresponding to alkene combustion reactions are always less than the E1 values for both oxidation reactions. The degree of reduction of the catalyst, which is required for completely describing alkane oxidation reactions using the Mars-van Krevelen model, increases with vanadia loading and is higher for propane than for ethane. Two lumped parameters, k1/k2 and k1/(k3 + k4), were considered for understanding the effects of loading and alkane oxidized on the alkene yields. The value of k1/k2 was related to the amount of alkane available for the ODH reaction, and the value of k1/(k3 + k4) was related to the net alkene formed by the ODH reaction. Both lumped parameters varied with loading and alkane oxidized, and primarily depended on the ratio of the preexponential factors k10/k20 and k10/(k30 + k40). Relatively large values of k1/k2 suggested that a majority of alkane was available for the ODH reaction. Furthermore, the k1/(k3 + k4) value appeared to follow the trends in alkene yield at isoconversion, with a higher k1/(k3 + k4) value corresponding to higher alkene yield. The highest k1/(k3 + k4) value was observed for the monolayer catalyst at the highest reaction temperature considered and for propane oxidation. Acknowledgment The authors gratefully acknowledge support from the Department of Science and Technology (DST), India. The authors also acknowledge Prof. Israel E. Wachs, Lehigh University, for allowing the Raman data to be obtained and Mr. D. Shee of IIT Kanpur for assistance with the TPR experiments. The authors also acknowledge helpful discussions with Prof. A. A. Lemonidou, Aristotle University of Thessaloniki. Literature Cited (1) Kung, H. H. Oxidative Dehydrogenation of Light (C2 to C4) Alkanes. AdV. Catal. 1995, 40, 1. (2) Weckhuysen, B. M.; Keller, D. E. Chemistry, Spectroscopy and the Role of Supported Vanadium Oxides in Heterogeneous Catalysis. Catal. Today 2003, 78, 25. (3) Mamedov, E. A.; Corberan, V. C. Oxidative Dehydrogenation of Lower Alkanes on Vanadium Oxide-Based Catalysts. The Present State of the Art and Outlooks. Appl. Catal., A 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) Blasco, T.; Nieto, J. M. L. Oxidative Dehydrogenation of Short Chain Alkanes on Supported Vanadium Oxide Catalysts. Appl. Catal., A 1997, 157, 117. (6) Wachs, I. E.; Weckhuysen, B. M. Structure and Reactivity of Surface Vanadium Oxide Species on Oxide Supports. Appl. Catal., A 1997, 157, 67. (7) Khodakov, A.; Olthof, B.; Bell, A. T.; Iglesia, E. Structure and Catalytic Properties of Supported Vanadium Oxides: Support Effects on Oxidative Dehydrogenation Reactions. J. Catal. 1999, 181, 205. (8) Boisdron, N.; Monnier, A.; Jalowiecki-Duhamel, L.; Barbaux, Y. Oxydehydrogenation of Propane on V2O5/TiO2 Catalyst: Kinetic and Mechanistic aspects. J. Chem. Soc., Faraday Trans. 1995, 91, 2899.
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ReceiVed for reView June 6, 2006 ReVised manuscript receiVed September 12, 2006 Accepted October 31, 2006 IE060715W
