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Ind. Eng. Chem. Res. 1993,32, 584-587
Effect of Vanadium Oxide Loading on the Selective Oxidation of Methane over VaO~/Si02 Shang Yang Chen'J and David Willcoxt Department of Chemical Engineering, University of Illinois at Chicago, 810 S. Clinton, Chicago, Illinois 60607
The effect of vanadium oxide loading on the selective methane oxidation over V205/Si02 catalysts was investigated. X-ray diffraction (XRD) and temperature-programmed reduction (TPR) were used to characterize these catalysts. The results of XRD and TPR show that the catalysts with 1-8 w t % vanadium oxide loadings had a n uniform structure but no well-defined crystalline phase of vanadium oxide. These catalysts showed very different selectivities for methanol. At the fixed methane conversion, methanol selectivity generally increased with the decrease of the vanadium oxide loading. The size of vanadium oxide ensembles may have significant effect on catalytic selectivity. NzO was found to be much more active than 0 2 for the oxidation of methane. In the presence of both N2O and 0 2 , N2O had to compete with 0 2 for the adsorption sites.
Introduction Many catalytic studies have recently been performed, both industrially and academically, to find a method for converting methane into liquid fuels to allow for easier transportation of remote natural gas reserves, and to create an alternative source of gasoline. Natural gas can currently be utilized in syngas reactions following an endothermic steam reforming step to generate CO and Hz. However, the direct conversion of methane to liquid chemicals would have many economic advantages and allow for greater application of the process at the well head (Edwards and Forster, 1986). One possible method is the direct partial oxidation of methane into methanol. This reaction proceeds under lower reaction temperatures than the oxidative coupling of methane, which historically has allowed more selective catalysts to be developed. There have been a number of studies done recently using vanadium or molybdenum oxides. Some of these used nitrous oxide as the oxidant, and some molecular oxygen. These studies are summarized below. Several recent studies have used nitrous oxide (N2O) and molecular oxygen as the oxidants with silica-supported catalysts based onV205 (Iwamoto, 1983a;Zhen et al., 1985; Spencer, 1988a; Spencer and Pereira, 1989) and Mooa (Iwamoto, 1983b; Kasztelan et al., 1988; Khan and Somorjai, 1985; Liu et al., 1982, 1984; Spencer, 1988b; Spencer et al., 1990). Both V206 and Moo3 had relatively high activity and selectivity for the oxidation of methane by nitrous oxide, oxygen, and steam. Oxygen was found to enhance the conversion, and steam improved the selectivity. In addition, vanadia was found to be approximately twice as active than molybdena for producing methanol (Zhen et al., 1985). Furthermore, vanadia was active a t much lower temperature (460-500 " C ) than molybdena (480-590 "C) (Zhen et al. 1985). The best results yielded a combined selectivity to methanol and formaldehyde of roughly 96 % with 1-2 % methane conversion. Higher conversions have been reported in the patents (Iwamoto, 1983a,b). However, it has also been reported that these results cannot be satisfactorily reproduced by their own researchers (Foster, 1985;Liu et al., 1984). This implies that the reaction may
* To whom correspondence should be addressed. Current address: Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803. Current address: Sandoz Crop Protection, Corporate Headquarters, 1300 East Touhy Avenue, Des Plaines, IL 60018. t
0888-588519312632-0584$04.00/0
be structure sensitive and that different microstructures were obtained from different preparation batches. However, the previous studies were generally based on only one catalyst sample at a time. This makes it extremely difficult to predict what is required for a good catalyst since the results may vary from batch to batch. In this study, a series of Catalysts,containing 1-8 wt 7% vanadium oxide supported on silica, were tested for the partial oxidation of methane with nitrous oxide,molecular oxygen, and steam. These catalysts with different vanadium oxide loadings showed very different selectivities for methanol. It was found that nitrous oxide was more active for the oxidation of methane on these catalysts than molecular oxygen. TPR and X-ray diffraction were used to characterize these catalysts. Methanol selectivity appeared to be only dependent on methane conversion.
Experiments Catalysts. Several silica-supported samples were prepared with different weight loadings of vanadia (1,2, 4, and 8 wt 5%). These were prepared by impregnating the silica (Cabosil HS-5) with an aqueous solution of ammonium metavanadate at a pH of 10.5. Ammonium hydroxide was used to adjust the pH value of the solution. The water was removed by drying slowly,and the materials were then calcined in air. An unsupported vanadium oxide sample was prepared by calcining the ammonium metavanadate powder in air. For both supported and unsupported catalyst samples, the calcination temperature was raised from 25 to 520 "C at 5 "C/min and was then kept at 520 "C for 20 h. Catalyst Characterization. The BET method was used to measure surface areas of SiOz, V205/SiO~with different vanadium oxide loadings, and unsupported V205 materials. The BET experiments were carried out with the volumetric method at -196 "C. Nitrogen was used as the adsorbate. The BET surface areas were roughly constant at 250 m2/g for Si02 and V205/Si02 with the different vanadium oxide loadings. The surface area of the unsupported V205 was 2.4 m2/g. Temperature-programmed reduction (TPR) was used to characterize these materials. Each TPR experiment was performed with a calcined sample of 0.5 g. The calcined samples (including pure silica) were placed in the reactor and heated in flowing 28% 0 2 in He from room temperature to 500 "Ca t 7.5 "C/min. The samples were maintained in OdHe stream at 500 "C for 1 h. Subse0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No.4,1993 585
quently the samples were cooled to 200 OC in flowing pure He, and were kept at 200 "C in flowing pure He for 1 h so that oxygen was completely removed from the reactor and other dead volumes. Finally the samples were heated to 650 OC at 7.5 "Omin in flowing 28% HZin He. The flow rate of He was 16 mL/min. X-ray diffraction measurements were made on a X-ray powder diffractometer using Cu Ku radiation. Kinetic Experiments. The reaction was carried out at atmospheric pressure in a quartz reactor heated in an isothermal furnace. Usually 0.5 g of catalyst was used in a reaction experiment. The feed composition was kept constant at CH4:N20:02:H20= 2 5 6 5 2 8 except where noted. Water vapor was introduced into the reactor by passing the reactant gas stream through a saturator containing water. The saturator was placed in a thermostated bath. The products were measured online with a gas chromatograph (GC). The lines going to the GC were heated to 120 'C to prevent condensation. Three columns, molecular sieve 5A, Poropak Q, and Poropak R, and two detectors, a thermal conductivity detector and a flame ionization detector, were used in order to detect the followinggases: methane, nitrous oxide, carbon monoxide, carbon dioxide, oxygen, nitrogen, methanol, water, and formaldehyde. The minimum detection limit for formaldehyde was not determined, so the following resulta will not report the formaldehyde which may have been produced in small quantities. The selectivity and conversion werecalculated from the product distribution. For most cases, the GC response to methane did not change significantly due to the small conversion. Typically, the temperature was varied from 440 to 490 "C in order to change the conversion. The contact time was typically 2.2 s. Noevidenceofdesctivationwasfoundafterthecatalysts were used for 10-20 h except at high contact times or temperatures above 490 'C. None of the results shown below were taken a t these conditions and, therefore, are assumed to be free from deactivation. The reaction was allowed to run for 1 h before the first analysis to allow the furnace temperature to stabilize. Repeating the CC analysis every hour, or on subsequent days (after cooling and reheating) showednochange in theactivity or product distribution. The catalyst temperature was not determined in this study. However, thii temperature could be estimated by assuming the worst case (adiabatic conditions) and performing an energy balance with the observed conversion and selectivity. This calculation showed that the maximum temperature rise was only 1-2 "C at the lower conversions, i.e., when the reaction temperature was 440460 OC.
Results and Discussion Catalyst Characterization. The TPR results of the V205/SiOzsamples and pure silica are presented in Figure 1. The bottom spectrum shows the TPR results of the pure silica. Only a single peak was found at around 600 OC. The TF'R spectra of the catalyst samples exhibit two peaks. The first one is at around 500 OC. The second one is at around 600 O C and is identical to the TPR peak in the spectrum of pure silica. It was found that the TPR peak at 600 "C had a roughly constant peak area for all of the VzO,/SiOzsamples. These indicate that the peak at 600 O C for all of the V205, Si02 samples is attributed to the silica support. Only a single peak is attributed to vanadium oxide for all of the catalyst samples, and is at 500 "C.
350
SW
6b
Temperature (C)
Figure 1. TPR spectra of V,OdSiO~and SiOn.
I
W e i g h t h d i n g of Vanadium Oxide, %
Figure 2. TPR peaL area of vanadium oxide -m vanadium oxide loading.
In previous TPR studies of V20dSiO2, Roozeboom et al. (1980) observed two different peaks attributed to vanadium oxide. The second peak was hardly observable at vanadium content less than 1.4 wt %. However, this peak increased with increasing vanadium content. From Raman spectrum and X-ray diffraction analysis, they indicated thatthe two different TPR peaks were attributed to different structuresof vanadium oxide. However, only a single TPR peak attributed to vanadium oxide was found in this study. The area of this peak linearly increased with increasing vanadium oxide loading (Figure 2). Additionally, XRD measurements in this study gave no evidence showing a well-defined crystalline phase of vanadium oxide for all of the VzOS/SiOz samples, which is in agreement with their XRD results indicating no welldefined crystalline phase of vanadium oxide up to 4.6 w t % V (4.6 wt % V = 8.2 wt % VzOs). Kinetic Measurements. In this study, major products of methane oxidation on VzOs/SiOz were methanol, carbon
886 Ind. Eng. Chem. Res., Vol. 32, No. 4,1993 -I
50
-
440C 40
2 wt% Catalyst
5
490C
40-
tz 30-
30-
v)
I
0
gI
20-
\
0
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4 wt% Catalyst
8 wt% Catalyst
01 0
IO
-
\
00 2
4 6 Methane Conversion, %
8
IO
2
15
0.5
Methane Conversion, %
Figure 3. Methanol selectivityvs methane conversion at a constant contact time 2.2 8.
Figure 4. Effect of temperature and contact time for the 2 w t % catalyst.
Table I. Effect of Temperature on Conversion and Selectivity loading temp CH~conv selectivity ( % ) (wt 7%) (OC) (%) CHaOH CO COz
Table 11. Effect of Contact Time on Conversion and Selectivity at 460 "C contact time CHI conv selectivity ( % ) Catalysts (5) (%) CH30H CO COz
2 2
440 460 490 440 460 490 440 460 490 440 460 490
0.05 0.13 1.16 0.07 0.29 1.68 0.28 0.92 4.63 0.49 1.76 8.77
13.3 27.7 10.1 56.7 31.3 10.2 18.4 8.7 2.7 6.8 3.1 0.2
0.0 23.5 21.2 20.0 54.7 81.0 67.0 82.8 89.0 78.9 84.7 52.3
-86.7 48.8 68.7 23.3 14.0 8.8 14.6 8.5 8.3 14.3 12.2 47.5
monoxide, and carbon dioxide (again, formaldehyde is neglected, but may have been formed in small quantities). The highest methanol selectivity obtained was 56.7% over the 2 wt % catalyst a t 440 O C . The highest methane conversion is 8.77% over the 8 w t % catalyst at 490 "C. Both unsupported V2O5 and pure Si02 were tested. Pure Si02 was inactive a t the temperature range 440-490 "C. COz and CO in trace amounts were found over unsupported V205 a t 490 OC, but no methanol was observed. The results listed in Table I were obtained by changing the reaction temperature (440,460,and 490 "C). It was found that methanol selectivity decreasedwith increasing reaction temperature over all of the catalysts except the 1 w t % catalyst, while the conversion increased. This exceptionwas probably due to the s m d conversion which made the selectivity inaccurate. It is noted that the selectivity to methanol varied with the vanadiunr'oxide loading a t the same reaction conditions. From Figure 3, it can be seen that the selectivity to methanol varied with the vanadium oxide loading a t the same conversion of methane. The 2 wt 5% catalyst performed the best of those tested. The effect of contact time on the conversion of methane and the product distribution was investigated by varying the total gas flow rate. The 2 and 8 wt 7% catalysts were used for this investigation. The reaction temperature was 460 OC. As shown in Table 11,at the shortest contact time (1.4e), the selectivity of methanol was highest for each of these two Catalysts while the conversion of methane was lowest. However, a t the longest contact time (3.4SI, the selectivity of methanol was lowest but the conversion of methane was highest. The activities of N2O and 02 were examined by performing the reaction experimentson the 4wt 7% catalyst a t 490 "C (Table III). The methane conversion was 3.5%
2wt % 2wt% 2wt % 8wt % 8wt %
1.4 2.2 3.4 1.4 3.4
0.15 0.27 0.52 0.77 2.52
46.6 34.0 19.4 5.2 1.4
31.2 50.5 67.4 89.3
84.0
22.2 15.5 13.2 5.5 14.6
Table 111. Effect of Oxidant on Conversion over the 4 wt % Catalyst at 490 "C feed composition (mole fraction) CHI conv CH4 HzO NzO 0 2 He (%) 0.37 0.37 0.37
0.09 0.09 0.09
0.47 0.47 0.0
0.0 0.07 0.47
0.07 0.0 0.07
3.5 2.6 0.06
when NzO was used as the only oxidant. Under the same conditions but with 0 2 alone as the oxidant, methane conversion of 0.06% was obtained. Obviously, N20 was more active for the oxidation of methane on V205/SiO~ catalyst than 0 2 , contradicting a previous report in the literature (Zhen et al., 1985). This implies that oxygen species generated by the adsorption of N2O could be different from those generated by the adsorption of 0 2 , or the activation energy for the adsorption of N20 could be lower than that of 02.When both N20 and 02 were used as the oxidants,methane conversion of 2.6% was obtained, which was higher than that obtained with 0 2 alone as the oxidant but lower than that obtained by using N2O. Since NnO was more active than 02for the oxidation of methane, it is understandable that the conversion of methane obtained by using both NzO and 0 2 was higher than that by using 0 2 alone. It is interesting that the methane conversion in the presence of N20 alone was higher than that in the presence of both NzO and 0 2 . A possible explanation is that NzO had t o compete with 0 2 for the adsorption sites in the presence of both N2O and 0 2 . Some sites were occupied by N20 and some by 02. However, all of these sites were available for the adsorption and dissociation of N20 in the absence of 0 2 to generate a maximum of more active oxygen species. From the above discussion,a fact is noted the selectivity of methanol increases when the conversion of methane decreases. The selectivitiesof methanol from the reaction experimentsat the 2 and 8wt % catalysts have been plotted as a function of methane conversion in Figures 4 and 5. These data apparently lay on a single "curve" for a given catalyst. This implies that the effect of temperature and contact time are equivalent under the conditions studied.
Ind. Eng. Chem. Res., Vol. 32, No. 4,1993 587
'I i 0
Time Contact: I 4, 3.4 sec Temperature: 440.460, 490C
j 4 j u!
oi
0
+
\
+ 0.159 g of catalyst instead of 0.5 g (440, 460 and 490C 2 . 2 sec.)
2
6 Methane Conversion, % 4
8
I
Methanol selectivity generally increased with decreasing vanadium oxide loading at a constant methane conversion. NzO was more active than 02 for the oxidation of methane on these catalysts, This is explained by different oxygen species generated by N20 and 0 2 , respectively. In the presence of both N20 and 0 2 , N20 had to compete with 02 for the adsorption sites. It is suggested that larger ensembles of vanadium oxide are at higher loadings. The size of V2O5 ensembles may have a significant effect on catalytic selectivity because a methane molecule adsorbed on the larger V2O5 ensemble may be surrounded by more active oxygen species than on the smaller VzO5 ensemble.
10
Figure 5. Effect of temperature and contact time for the 8 wt % catalyst.
The methand dectivity appeared to be independent of temperature-contact time combination. However, the selectivity-conversion curve for the 2 wt % catalyst (Figure 4) is very different from that for the 8 w t % catalyst (Figure 5). The 8 wt 9% catalyst of 0.159 g was tested with the normal feed composition at contact time 2.2 s. The data points of methanol selectivity vs methane conversion match the selectivity-conversion curve for the 8 w t 5% catalyst well but do not fit the curve for the 2 w t % catalyst, although 0.159 g of the 8 w t % catalyst and 0.5 g of the 2 w t % catalyst have a similar amount of vanadium oxide. This means that the 2 wt 9% catalyst could not be equivalently substituted by the 8 wt % catalyst and they had very different activities and selectivities. Assuming 0.103 nm2 per VO2.5 unit (Roozeboom et al., 1980), a complete monolayer coverage of V Z O on ~ Si02 would occur at 26.5 wt % catalyst loading at 100% dispersion. The amount of vanadium oxide in the 2 wt % catalyst is far below the amount required for a complete monolayer coverage, and hence it is suggested that vanadium oxide in the 2 wt 9% catalyst is spread broadly on the silica support with isolated sites and small ensembles. However, the ensembles of vanadium oxide possibly become larger when vanadium oxide is increased from 2 to 8wt % without a significant change in catalyst structure. This means that a methane molecule adsorbed on the 8 w t % catalyst could be surrounded by more active oxygen than on the 2 wt % catalyst so that excessive oxidation leading to the formation of CO and COZtakes place on the 8 wt 9% catalyst more easily than on the 2 wt % catalyst. Perhaps this is one reason why catalysts with different vanadium oxide loadings had very different selectivities.
Summary and Conclusions The VzO5/SiO2 catalysts with 1-8 w t % vanadium oxide loadings had a stable and uniform structure but had no well-defined crystalline phase of vanadium oxide. These catalysts showed very different selectivities for methanol.
Nomenclature BET = BrunauerEmmett-Teller GC = gas chromatograph TPR = temperature-programmed reduction XRD = X-ray diffraction Literature Cited Edwards, J. H.; Foster, N. R. The Potential for Methanol Production from Natural Gas by Direct Catalytic Partial Oxidation. Fuel Sci. Technol. Znt. 1986,4, 365. Foster, J. H. Direct Catalytic Oxidation of Methane to Methanol. Appl. Catal. 1985, 19, 11. Iwamoto, M. Japanese Patent 58 92,629 1983a. Iwamoto, M. Japanese Patent 58 92,639, 1983b. Kasztelan, S.; Payen, E.; Moffat, J. B. The Formation of Molybdewilic Acid on Mo/SiOZ Catalysts and Its Relevance to Methane Oxidation. J. Catal. 1988, 112, 320. Khan, M. M.; Somorjai, G. A. A Kinetic Study of Partial Oxidation of Methane with Nitrous Oxide on a Molybdena-Silica Catalyst. J. Catal. 1985, 91, 263. Liu, R. S.; Iwamoto,M.; Lunsford, J. H. Partial Oxidation of Methane by Nitrous Oxide over Molybdenum Oxide Supported on Silica. J. Chem. Soc., Chem. Commun. 1982, 78. Liu, H. F.; Liu, R. S.; Liew, K. Y.;Johnson, R. E.; Lunsford, J. H. Partial Oxidation of Methane by Nitrous Oxide over Molybdenum on Silica. J. Am. Chem. SOC.1984,106,4117. Roozeboom, F.; Mittelmeijer-Hazeleger, M. C.; Moulijn, J. A.; Medema, J.; V. H. J. de Beer; Gellings, P. J. Vanadium Oxide Monolayer Catalysts. 3. A Raman Spectroscopic and Temperature-Programmed Reduction Study of Monolayer and CrystalType Vanadia on Various Supports. J. Phys. Chem. 1980,84, 2783. Spencer, N. D. U.S.Patent 4727198, 1988a. Spencer, N. D. Partial Oxidation of Methane to Formaldehyde by Means of Molecular Oxygen. J. Catal. 19888,109, 187. Spencer, N. D.; Pereira, C. J. VzOs-SiOz-CatalyzedMethane Partial Oxidation with Molecular Oxygen. J. Catal. 1989,116, 399. Spencer, N. D.; Pereira, C. J.; Graaselli, R. K. The Effect of Sodium on the MoOz-SiOz-CatalyzedPartial Oxidation of Methane. J. Catal. 1990,126,546. Zhen, K. J.; Khan, C. H.; Lewis, K. B.; Somorjai, G. A. Partial Oxidation of Methane with Nitrous Oxideover VzO&iOz Catalyst. J. Catal. 1985, 94, 501. Received for reuiew September 21, 1992 Reuised manuscript receiued December 29, 1992 Accepted January 14, 1993
