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Partial oxidation of methane into C1-oxygenates: role of homogeneous reactions and catalyst surface area
Catalysis Today 71 (2001) 11–19
Partial oxidation of methane into C1-oxygenates: role of homogeneous reactions and catalyst surface area J.A. Barbero, M.A. Bañares, M.A. Peña, J.L.G. Fierro∗ Instituto de Catálisis y Petroleoqu´ımica, CSIC, Campus UAM, Cantoblanco, E-28049 Madrid, Spain
Abstract The influence of the NO concentration (0–1.0 vol.%) in the partial oxidation of methane into C1 -oxygenates (HCHO and CH3 OH) and C2+ hydrocarbons was examined. Activity measurements conducted with reactors of different diameter, empty and loaded with high specific area V/SiO2 and/or Ba/SiO2 catalysts revealed clearly that CH4 is activated by O2 only at moderate temperatures (ca. 823 K) if small amounts of NO are present in the feed stream. The role of NO added is to increase the radicals population in the gas phase and to shift selectivity to oxygen-containing molecules. It was observed that CH3 OH is easily oxidised in the presence of a high specific area V/SiO2 redox catalyst. This was not in the case of the non-redox Ba/SiO2 catalyst, for which HCHO and CH3 OH yields remain at a level only slightly lower than for the homogeneous reaction. On the contrary, higher yields to HCHO (1.7%) and CH3 OH (1.6%) can be achieved at 923 K over a low specific area catalyst. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Methane partial oxidation; Methanol; Formaldehyde; Vanadium catalysts; NO promoter
1. Introduction The partial oxidation of methane into C1 -oxygenates (CH3 OH and HCHO) through a single catalytic step has a tremendous technological importance because it overcomes a first process of syngas production through the energy intensive steam reforming (CH4 + H2 O → CO + 3H2 ). Apart from this, most of the efforts to obtain C1 -oxygenates did not achieve yields beyond 4% [1]. Recently, nitrogen oxides (NOx ) have been used to promote gas-phase reactions with methane [2–9]. Based on thermodynamic considerations, Bromly et al. [4] studied the CH4 + NO + O2 reaction and developed kinetic models to describe experimental data obtained at atmospheric pressure. These authors found ∗ Corresponding author. Tel.: +34-915-854-769; fax: +34-915-854-760. E-mail address: [email protected] (J.L.G. Fierro).
CO as the oxidation product but in no case HCHO or CH3 OH were observed. Conversely, a high yield of CH3 OH and HCHO (up to 7% at atmospheric pressure) was reported when a low specific area V/SiO2 catalyst was used in the presence of NO [8]. It was suggested that the contribution of gas-phase reactions for the formation of C1 -oxygenates and the presence of NO must alter the equilibria in the gas-phase reactions. In a series of recent papers, Tabata et al. have proposed a reaction model for the conversion of CH4 to CH3 OH and HCHO using either NOx (x = 1, 2) [9] or NO2 + O2 [7] as oxidant agents which satisfactorily describes the experimental results of selectivity to C1 -oxygenates. From the above studies, it is clear that the heterogeneous–homogeneous nature of CH4 activation has a great impact on the reaction rate and product distributions. This work was undertaken with the aim to evaluate the role of reaction temperature and NO concentration in the feed stream on high area silica-supported
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vanadium and barium oxide catalysts for the conversion of methane to CH3 OH and HCHO using different reactor configurations. The heterogeneous reaction was studied for different internal configurations of the catalyst bed, including V/SiO2 or Ba/SiO2 or a mix of them. The role of homogeneous reaction was also investigated in a reactor configuration that combines either upstream or downstream dead volumes with a V/SiO2 catalyst. Finally, the influence of surface area of V/SiO2 catalysts was analysed.
2. Experimental 2.1. Catalyst preparation Several V/SiO2 , Ba/SiO2 and VBa/SiO2 catalysts were prepared by impregnation on a high area silica support (Aerosil 200, BET area of 180 m2 /g, particle size of 12 nm). The silica-supported vanadium oxide catalyst (VSi) was prepared by aqueous impregnation of the SiO2 substrate with ammonium metavanadate (NH4 VO3 , Aldrich, reagent grade) solution in the presence of H2 O2 . The excess of water was then removed in a rotary evaporator at 343 K under reduced pressure (1.4 kPa). The impregnate was subsequently dried at 383 K for 15 h and finally calcined in air in two steps: 623 K for 2 h and then at 923 K for 5 h. The low area V/SiO2 catalyst (VSi-L) was prepared similarly using the same silica support treated at 1273 K for 9 h, in order to reduce its initial specific surface. Vanadia loading was adjusted to keep a similar surface coverage on the low specific area SiO2 support. A silica-supported barium oxide (BaSi) was prepared following a similar impregnation procedure, being Ba(NO3 )2 (Merck, reagent grade) the precursor. Drying and calcination were the same as above. A barium–vanadium-containing catalyst (BaVSi) was
prepared by simultaneous impregnation of the silica with an aqueous solution of NH4 VO3 and Ba(NO3 )2 salts of appropriate concentrations. Drying and calcination were the same as for the VSi and BaSi samples. The chemical composition of calcined samples is summarised in Table 1. 2.2. Catalyst characterisation Chemical analysis of the samples was determined by inductively coupled plasma emission spectroscopy (ICP-AES), Perkin Elmer Optima 3300DV. The solid samples were first digested (in a mixture of HF, HCl and HNO3 ) in a microwave oven, and then aliquots of solution were diluted to 50 ml using deionised water (18.2 m quality). The specific area of the catalysts was calculated by the BET method to the nitrogen adsorption isotherms recorded at 77 K using a Micromeritics ASAP 2000 apparatus. Prior to the adsorption measurements, samples were outgassed at 413 K. BET areas are listed in Table 1. Temperature-programmed reduction (TPR) experiments were carried out in a Micromeritics TPD/TPR 2900 apparatus interfaced to a microcomputer. Prior to reduction, the catalysts (about 0.05 g) were heated at a rate of 10 K/min up to a final temperature of 1023 K and kept at that temperature for 0.5 h in a stream of air. TPR experiments were carried out after sample conditioning and further passing a 10% H2 /Ar flow through the sample. The temperature was increased at a rate of 10 K/min from 323 to 1023 K, and the amount of H2 consumed was determined with a thermoconductivity detector (TCD). The effluent gas was passed through a cold trap before the TCD in order to remove water from the exit stream. Photoelectron spectra of fresh and used samples were recorded on a VG Escalab 200R electron spectrometer equipped with a hemispherical electron
Table 1 Chemical analysis, specific area and XPS atomic ratios of the catalysts Catalyst
ICP V/Si × 102 atom
ICP Ba/Si × 102 atom
BET (m2 /g)
XPS V/Si × 102 atom
XPS Ba/Si × 102 atom
Vsi VSi-L BaVSi BaSi
0.49 0.007 0.33 –
– – 0.93 1.14
175 3 145 142
0.26 0.38 0.20 –
– – 0.25 0.30
J.A. Barbero et al. / Catalysis Today 71 (2001) 11–19
analyser, using a Mg K␣ (hν = 1253.6 eV, 1 eV = 1.603 × 10−19 J) X-ray source. The powder samples were pressed into small Inox cylinders and outgassed at 573 K for 1 h in the pretreatment chamber prior to being transferred to the ion-pumped analysis chamber. The binding energy (BE) of the Si 2p peak of the support at 103.4 eV was taken as an internal standard. The accuracy of the BE values was ±0.1 eV. Each spectral region of the photoelectrons of interest was scanned for a number of times to obtain a good signal-to-noise ratio. The intensities of the peaks were estimated by calculating the integral of each peak after subtracting an S-shaped background and fitting the experimental peak to a combination of Lorentzian/Gaussian lines of variable proportions. 2.3. Catalyst testing The catalyst performance for the partial oxidation of methane was evaluated in two reactors (7 and 10 mm ID, 23 cm length) placed inside a vertical furnace and fed from the top. The catalyst was sandwiched between quartz wool plugs in the middle of the tube. The temperature of the catalyst bed was measured by a thermocouple located inside a coaxially centred quartz sheath whose end was in touch with the catalyst bed. Reactor was designed not only to minimise the dead volume, and hence to decrease the extent of gas-phase reactions, but also to change the upstream and downstream void volume in the reactor. Samples of 0.20 g, GHSV of 6.6 × 104 h−1 , a mixture N2 :CH4 :O2 = 4:2:1 (molar) at a CH4 flow rate of 100 ml/min, atmospheric pressure and temperatures from 803 to 1023 K were used. In order to investigate the effect of NO on both CH4 conversion and product distribution, several NO concentrations (0, 200, 534, 4000, 6000, 10,000 and 14,000 ppm) were used while maintaining constant the O2 and CH4 concentrations. Nitrogen was used to balance the variation of NO%. Four separate lines for CH4 (99.9995 vol.%), N2 (99.995 vol.%), O2 (99.995 vol.%) and N2 /NO (935 ppm NO) mixture were used. Flow rates were adjusted by mass flow controllers, and all gases were mixed prior to reach the reactor. The effluents of the reactor and feed stream were analysed by GC fitted with TCD, using a Porapak Q and 4A molecular sieve columns and He as a carrier gas. The outlet of the reactor to the injection port of GC
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was heated up to 423 K in order to avoid water condensation and formaldehyde polymerisation. On an average, estimated error in replica experiments with fresh catalyst samples and clean quartz reactor was within ±5%.
3. Results 3.1. Catalyst characterisation Table 1 compiles chemical analyses, specific areas and surface atomic ratios, derived from photoelectron spectroscopy analysis, for all the catalysts. Vanadium incorporation leads to a slightly decrease in surface area with respect to the SiO2 substrate. This decrease was even more marked (up to 21%) for the Ba-containing catalysts. Chemical analyses of V and Ba, expressed as total V or Ba atoms per Si atom, differ from XPS surface atom ratios. Although atom ratios at the surface are lower than in the bulk indicating low V dispersion, Ba/Si ratios are even lower than the V/Si ratios. As the binding energy of Ba 3d5/2 is characteristic of BaCO3 , it appears that Ba carbonate accumulates as big aggregates on SiO2 particles. The larger V/Si ratios indicate that vanadium oxide is distributed on VSi catalyst with a higher dispersion degree than in BaVSi catalyst. TPR results showed that SiO2 -supported V and BaV oxides are reduced in a single reduction step with the maximum of H2 -consumption placed around 800–850 K. These profiles also exhibited a small shoulder in the high temperature flank. This peak can be assigned to the reduction of V5+ to V3+ in monomeric or low oligomeric VOx entities [10]. A small proportion of bulk-like V2 O5 could be present as judging from the appearance of the shoulder at high temperatures. Easily reducible V-species can, in part, be responsible for higher activity and also better selectivity toward C1 -oxygenates. BaVSi catalyst is slightly less reducible (Tmax = 850 K) than the VSi counterparts (Tmax = 805 K) indicating that BaO inhibits vanadium oxide reduction to some extent. The same trend is observed in catalytic performance suggesting that the lower activity and C1 -oxygenates selectivity of BaVSi catalyst is associated with a lower reducibility of V-sites, and able to activate oxygen.
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3.2. Reaction studies Activity experiments with different concentrations of NO in the feed stream were carried out with the aim to evaluate the role of NO on the CH4 conversion and product distributions at different temperatures. The influence of the size of the reactor, catalysts type, and catalyst bed configuration were also studied. 3.2.1. Homogeneous reaction The homogeneous reaction between CH4 and O2 was studied, using a 10 mm ID reactor with 6 ml empty volume, and feeding simultaneously NO in the 0–10,000 ppm concentration range. Fig. 1 shows CH4 conversion and product distributions at 823 and 923 K as a function of the NO concentration in the feed stream. CH4 conversion reaches ca. 45% at NO concentration in the range 1000–4000 ppm (depending on the temperature) and then remains almost constant at higher NO concentrations. CO yield follows the same trend than CH4 conversion when temperature and NO concentration change. The maximum HCHO and CH3 OH yields are 0.8 and 0.3%, respectively, at the lowest temperature (823 K), remaining almost constant over the whole NO concentration range. The yield to HCHO and CH3 OH decreases with increasing temperature, whereas the C2+ hydrocarbon yield follows the opposite trend. The increase in C2+ hydrocarbon yield with reaction temperature is sharper in the low NO concentration region. 3.2.2. Behaviour of VSi and BaSi catalysts Activity of VSi and BaSi catalysts was studied in the 7 mm ID reactor. CH4 conversion and product distributions at 923 K (data not shown here) as a function of the NO concentration in the feed stream show that CH4 conversion on VSi reaches ca. 3.5% at NO concentration of 4000 ppm and then decreases slightly. CO and HCHO yields follow a similar trend. The maximum HCHO yield is 1.2% and no CH3 OH is observed. For catalyst BaSi, CH4 conversion reaches a 2.8% at 4000 ppm NO and then almost levelled off. A similar trend is observed for CO yield, however HCHO yield increases continuously with NO concentration and attains a maximum value of 1.28% for 10,000 ppm NO. The methane conversion and CO yield on both catalytic systems is ca. 10 times lower than for the homogeneous reaction, studied in the 10 mm ID reactor
Fig. 1. Homogeneous reaction in the 10 mm ID reactor (6 ml dead volume). CH4 conversion and product yield as a function of NO concentration at 823 (a) and 923 K (b) (CH4 — 䊏; CO — ; CO2 — 䉱; HCHO — 䊐; CH3 OH — 䊉; C2+ — 䊊).
(Fig. 1b). This is expected since the decrease of the dead volume, when the catalytic bed is used, produces the diminution of the residence time in the reactor. Concurrently, there is no production of CO2 and C2+ hydrocarbons, but the HCHO yield is almost twofold in comparison with the homogeneous reaction.
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CO production is much higher than CO2 . HCHO yield doubled upon increasing NO from 200 to 4000 ppm NO and then steeply increased until a maximum value of 1.9% at 10,000 ppm NO. C2+ hydrocarbons (yield below 0.3%) were detected near 6000 ppm NO and
Fig. 2. Catalytic reaction in the 7 mm ID reactor. CH4 conversion and product yield as a function of NO concentration at 973 K for VSi (a) and BaSi (b) catalysts (CH4 — 䊏; CO — ; CO2 — 䉱; HCHO — 䊐; CH3 OH — 䊉; C2+ — 䊊).
Fig. 2 displays CH4 conversion and product distributions at 923 K as a function of NO concentration for VSi and BaSi catalysts. For catalyst VSi (Fig. 2a), CH4 conversion reached a maximum ca. 13% at NO concentration of 4000 ppm and then levelled off. A similar pattern was observed for CO and CO2 yields, although
Fig. 3. Catalytic and homogeneous reaction in the 10 mm ID reactor. CH4 conversion and product yield as a function of NO concentration at 923 K for void volume upstream the VSi catalyst (a) and void volume downstream the VSi catalyst (b) (CH4 — 䊏; CO — ; CO2 — 䉱; HCHO — 䊐; CH3 OH — 䊉; C2+ — 䊊).
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then vanished. No CH3 OH was detected on the VSi catalyst. The behaviour of catalyst BaSi is shown in Fig. 2b. For this catalyst, CH3 OH was found in the 4000–6000 ppm NO range, while HCHO increased up to 2.2% at 10,000 ppm NO. In addition, C2+ hydrocarbons were maintained over the whole range of NO concentration explored.
3.2.3. Combined homogeneous and catalytic reactions The catalytic behaviour of sample VSi was studied in the presence or absence of homogeneous reaction upstream or downstream the catalyst bed using the 10 mm ID reactor. Fig. 3 shows the performance of two reactor configurations: void volume upstream the
Fig. 4. Catalytic bed configuration in the 7 mm ID reactor. CH4 conversion and product yield in the 7 mm ID reactor as a function of NO concentration at 973 K: (a) VSi over BaSi; (b) BaSi over VSi; (c) VSi–BaSi mixture; (d) BaVSi (CH4 — 䊏; CO — ; CO2 — 䉱; HCHO — 䊐; CH3 OH — 䊉; C2+ — 䊊).
J.A. Barbero et al. / Catalysis Today 71 (2001) 11–19
VSi catalysts (Fig. 3a) and void volume downstream the VSi catalyst (Fig. 3b), and they are compared with the empty reactor configuration (Fig. 1b). The results presented in Figs. 3 and 1b underline the relevance of homogeneous activation. Both the homogeneous reaction and the catalyst with upstream homogeneous reaction (Figs. 1b and 3a) show similar conversion levels, being CO the main product. The presence of a VSi catalyst increases CO2 production, particularly at low NO concentration and downstream homogeneous configuration (Fig. 3b). In this last configuration, the system affords lower conversion levels and CO yield. Nevertheless, in the absence of NO, this configuration is the most active and the differences between configurations decrease with NO concentration. There are important qualitative differences: no C2+ hydrocarbons are formed when the void volume is downstream the catalyst bed (Fig. 3b) and no methanol is formed when the void volume is upstream the catalyst bed (Fig. 3a). These results further support previous mechanistic reports that suggest that methanol is formed through a different path than that for formaldehyde (HCHO is formed at lower temperature) and that its formation does not necessarily involve a catalytic site. Besides, coupling reaction, that is maximum for the empty reactor, is favoured by the upstream configuration (Fig. 3a), whereas downstream configuration (Fig. 3b) promotes the conversion of methane to methanol and formaldehyde, exhibiting the maximum yield for oxygenates. As shown previously for other reaction configurations (Figs. 1 and 2), the increase in NO concentration enhances the yield to HCHO but not the C2+ hydrocarbon. For type b configuration (Fig. 3b), CH3 OH and HCHO yields reach a maximum for NO concentrations in the range 4000–6000 ppm NO and then slightly decrease. 3.2.4. Bed configurations Fig. 4 shows CH4 conversion and yields to products at 973 K as a function of the NO concentration for different bed configurations: (a) VSi catalytic bed upstream/BaSi catalytic bed downsteam (ratio 1); (b) BaSi catalytic bed upstream/VSi catalytic bed downstream (ratio 1); (c) mixture of VSi and BaSi catalysts (ratio 1) and (d) BaVSi catalyst. In general, no large differences in performance were observed among the three first bed configurations. The HCHO yield was maximum by feeding 6000 ppm NO over the VSi/BaSi
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catalyst bed. However, CH3 OH is observed only for this configuration, with a maximum at 6000 ppm NO, and is not produced in the other cases. BaVSi catalyst showed lower activity than the physical mixture VSi + BaSi, which is consistent with the lower reducibility of the vanadium oxide in the supported Ba–V phase.
Fig. 5. CH4 conversion and product yield in the 10 mm ID reactor as a function of NO concentration on low area VSi-L catalyst at 873 (a) and 923 K (b) (CH4 — 䊏; CO — ; CO2 — 䉱; HCHO — 䊐; CH3 OH — 䊉; C2+ — 䊊).
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3.2.5. Role of surface area of VSi catalysts A low specific area silica-supported vanadium oxide catalyst (VSi-L) was used with the aim to evaluate the roles of reaction temperature and NO concentration. Fig. 5 displays the activity of this catalyst as a function of NO concentration at 873 and 923 K in the 10 mm ID reactor without void volume. VSi-L catalyst becomes very active at either temperatures when NO concentration increases. Yield to different oxidation (CO, HCHO, CH3 OH, CO2 ) and coupling (C2+ ) products also increase with reaction temperature. HCHO and CH3 OH yield resulted particularly high at 923 K with values of 1.4 and 0.9%, respectively.
4. Discussion Methane activation by oxygen is strongly promoted by NO through complex homogeneous–heterogeneous reactions. The role of NO added to the feed stream is to increase the radicals population in the gas phase and to shift selectivity to oxygen-containing molecules. The very low activity in the absence of NO is due to the lack of any source of radicals. The sharp effect on CH4 conversion upon adding 200 ppm NO in the feed must be due to the chain propagation of radical reactions. Oxygen conversion increases in parallel and oxygen-limiting conditions dominate above 823 K for NO concentrations above 4000 ppm (cf. Fig. 1). For a fixed CH4 :O2 ratio in the feed, the channels for C1 -oxygenates and C2+ hydrocarbon formation appear to be influenced by reaction temperature and NO concentration. Thus, high yields to oxygenate products (1.3%) were reached at 823 K in a purely homogeneous process. At higher temperature, the yield to oxygenates decreased whereas it was compensated by the formation of C2+ hydrocarbons. The incorporation of NO in the feed stream up to the concentrations of 10,000 ppm promoted gas-phase reactions between CH4 and O2 as already documented in the previous contributions [7–9]. The strong dependence of CH4 activation on the dimension and configuration of the reactor and catalytic bed again confirm the radical nature of the activation of CH4 molecule. However, the appearance of CH3 OH among the reaction products is very sensitive to the presence of the VSi redox catalyst. On the vanadia surface, the CH3 OH produced through homogeneous reactions
became completely oxidised to CO and CO2 . This is not for the case of the non-redox BaSi catalyst, for which HCHO and CH3 OH yields are slightly higher. Methane activation also occurs to a significant extent on the catalyst surface although high reaction rates are obtained at temperatures above than that of the homogeneous process [8] even over high specific area samples. On the contrary, high specific area is detrimental for methanol production. The effect of specific area on CH3 OH and HCHO is different. The CH3 OH/HCHO ratio decreases with specific area. Thus, surface redox sites appear to degrade CH3 OH species but they are responsible for HCHO production. The data reported in Fig. 5 for a low specific area VSi catalyst show that CH4 conversion reached more than 30% at 923 K, and yields to HCHO and CH3 OH resulted particularly high. Thus, the use of a low specific area redox catalyst and adding NO in the feed stream is the choice option to circumvent deep oxidation reactions, which yield non-selective oxidation products, while maintaining very high activity. 5. Conclusions Partial oxidation of CH4 by O2 into C1 -oxygenates (and C2+ hydrocarbons) takes place at moderate temperatures (823 K) only if variable amounts of NO (200–10,000 ppm) are added into the feed stream. In the absence of NO, CH4 activation also occurs on high specific area VSi and BaSi catalysts, although high rates are only achieved at temperatures substantially higher (above 923 K) than that of the pure homogeneous process. The strong dependence of CH4 activation on the size and configuration of reactor and catalyst bed points to the participation of both homogeneous and heterogeneous processes. It has been observed that high specific area VSi catalysts are detrimental for C1 -oxygenates selectivity, and particularly for CH3 OH. However, higher yields to HCHO and CH3 OH can be obtained by using a low specific area VSi redox catalyst. Acknowledgements The financial support for this work from the Research Institute of Innovative Technology for the Earth, RITE (Japan) is greatly acknowledged.
J.A. Barbero et al. / Catalysis Today 71 (2001) 11–19
References [1] C. Batiot, B.K. Hodnett, Appl. Catal. A 137 (1996) 179, and references therein. [2] S. Irusta, E.A. Lombardo, E.E. Miró, Catal. Lett. 29 (1994) 339. [3] L.B. Han, S. Tsubota, M. Haruta, Chem. Lett. (1995) 931. [4] J.H. Bromly, F.J. Barnes, X.Y. Muris, B.S. Haynes, Comb. Sci. Technol. 115 (1996) 259. [5] K. Otsuka, R. Takahashi, K. Amakawa, I. Yamanaka, Catal. Today 45 (1998) 23.
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[6] K. Otsuka, R. Takahashi, I. Yamanaka, J. Catal. 185 (1999) 182. [7] K. Tabata, Y. Teng, Y. Yamaguchi, H. Sakurai, E. Suzuki, J. Phys. Chem. 104 (2000) 2648. [8] M.A. Bañares, J.H. Cardoso, G.J. Hutchings, J.M. Correa-Bueno, J.L.G. Fierro, Catal. Lett. 56 (1998) 149. [9] Y. Teng, H. Sakurai, K. Tabata, E. Suzuki, Appl. Catal. A 190 (2000) 283. [10] M.A. Bañares, J.H. Cardoso, F. Agulló-Rueda, J.M. Correa-Bueno, J.L.G. Fierro, Catal. Lett. 64 (2000) 191.
Partial oxidation of methane into C1-oxygenates: role of homogeneous reactions and catalyst surface area J.A. Barbero, M.A. Bañares, M.A. Peña, J.L.G. Fierro∗ Instituto de Catálisis y Petroleoqu´ımica, CSIC, Campus UAM, Cantoblanco, E-28049 Madrid, Spain
Abstract The influence of the NO concentration (0–1.0 vol.%) in the partial oxidation of methane into C1 -oxygenates (HCHO and CH3 OH) and C2+ hydrocarbons was examined. Activity measurements conducted with reactors of different diameter, empty and loaded with high specific area V/SiO2 and/or Ba/SiO2 catalysts revealed clearly that CH4 is activated by O2 only at moderate temperatures (ca. 823 K) if small amounts of NO are present in the feed stream. The role of NO added is to increase the radicals population in the gas phase and to shift selectivity to oxygen-containing molecules. It was observed that CH3 OH is easily oxidised in the presence of a high specific area V/SiO2 redox catalyst. This was not in the case of the non-redox Ba/SiO2 catalyst, for which HCHO and CH3 OH yields remain at a level only slightly lower than for the homogeneous reaction. On the contrary, higher yields to HCHO (1.7%) and CH3 OH (1.6%) can be achieved at 923 K over a low specific area catalyst. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Methane partial oxidation; Methanol; Formaldehyde; Vanadium catalysts; NO promoter
1. Introduction The partial oxidation of methane into C1 -oxygenates (CH3 OH and HCHO) through a single catalytic step has a tremendous technological importance because it overcomes a first process of syngas production through the energy intensive steam reforming (CH4 + H2 O → CO + 3H2 ). Apart from this, most of the efforts to obtain C1 -oxygenates did not achieve yields beyond 4% [1]. Recently, nitrogen oxides (NOx ) have been used to promote gas-phase reactions with methane [2–9]. Based on thermodynamic considerations, Bromly et al. [4] studied the CH4 + NO + O2 reaction and developed kinetic models to describe experimental data obtained at atmospheric pressure. These authors found ∗ Corresponding author. Tel.: +34-915-854-769; fax: +34-915-854-760. E-mail address: [email protected] (J.L.G. Fierro).
CO as the oxidation product but in no case HCHO or CH3 OH were observed. Conversely, a high yield of CH3 OH and HCHO (up to 7% at atmospheric pressure) was reported when a low specific area V/SiO2 catalyst was used in the presence of NO [8]. It was suggested that the contribution of gas-phase reactions for the formation of C1 -oxygenates and the presence of NO must alter the equilibria in the gas-phase reactions. In a series of recent papers, Tabata et al. have proposed a reaction model for the conversion of CH4 to CH3 OH and HCHO using either NOx (x = 1, 2) [9] or NO2 + O2 [7] as oxidant agents which satisfactorily describes the experimental results of selectivity to C1 -oxygenates. From the above studies, it is clear that the heterogeneous–homogeneous nature of CH4 activation has a great impact on the reaction rate and product distributions. This work was undertaken with the aim to evaluate the role of reaction temperature and NO concentration in the feed stream on high area silica-supported
0920-5861/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 5 8 6 1 ( 0 1 ) 0 0 4 4 5 - X
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J.A. Barbero et al. / Catalysis Today 71 (2001) 11–19
vanadium and barium oxide catalysts for the conversion of methane to CH3 OH and HCHO using different reactor configurations. The heterogeneous reaction was studied for different internal configurations of the catalyst bed, including V/SiO2 or Ba/SiO2 or a mix of them. The role of homogeneous reaction was also investigated in a reactor configuration that combines either upstream or downstream dead volumes with a V/SiO2 catalyst. Finally, the influence of surface area of V/SiO2 catalysts was analysed.
2. Experimental 2.1. Catalyst preparation Several V/SiO2 , Ba/SiO2 and VBa/SiO2 catalysts were prepared by impregnation on a high area silica support (Aerosil 200, BET area of 180 m2 /g, particle size of 12 nm). The silica-supported vanadium oxide catalyst (VSi) was prepared by aqueous impregnation of the SiO2 substrate with ammonium metavanadate (NH4 VO3 , Aldrich, reagent grade) solution in the presence of H2 O2 . The excess of water was then removed in a rotary evaporator at 343 K under reduced pressure (1.4 kPa). The impregnate was subsequently dried at 383 K for 15 h and finally calcined in air in two steps: 623 K for 2 h and then at 923 K for 5 h. The low area V/SiO2 catalyst (VSi-L) was prepared similarly using the same silica support treated at 1273 K for 9 h, in order to reduce its initial specific surface. Vanadia loading was adjusted to keep a similar surface coverage on the low specific area SiO2 support. A silica-supported barium oxide (BaSi) was prepared following a similar impregnation procedure, being Ba(NO3 )2 (Merck, reagent grade) the precursor. Drying and calcination were the same as above. A barium–vanadium-containing catalyst (BaVSi) was
prepared by simultaneous impregnation of the silica with an aqueous solution of NH4 VO3 and Ba(NO3 )2 salts of appropriate concentrations. Drying and calcination were the same as for the VSi and BaSi samples. The chemical composition of calcined samples is summarised in Table 1. 2.2. Catalyst characterisation Chemical analysis of the samples was determined by inductively coupled plasma emission spectroscopy (ICP-AES), Perkin Elmer Optima 3300DV. The solid samples were first digested (in a mixture of HF, HCl and HNO3 ) in a microwave oven, and then aliquots of solution were diluted to 50 ml using deionised water (18.2 m quality). The specific area of the catalysts was calculated by the BET method to the nitrogen adsorption isotherms recorded at 77 K using a Micromeritics ASAP 2000 apparatus. Prior to the adsorption measurements, samples were outgassed at 413 K. BET areas are listed in Table 1. Temperature-programmed reduction (TPR) experiments were carried out in a Micromeritics TPD/TPR 2900 apparatus interfaced to a microcomputer. Prior to reduction, the catalysts (about 0.05 g) were heated at a rate of 10 K/min up to a final temperature of 1023 K and kept at that temperature for 0.5 h in a stream of air. TPR experiments were carried out after sample conditioning and further passing a 10% H2 /Ar flow through the sample. The temperature was increased at a rate of 10 K/min from 323 to 1023 K, and the amount of H2 consumed was determined with a thermoconductivity detector (TCD). The effluent gas was passed through a cold trap before the TCD in order to remove water from the exit stream. Photoelectron spectra of fresh and used samples were recorded on a VG Escalab 200R electron spectrometer equipped with a hemispherical electron
Table 1 Chemical analysis, specific area and XPS atomic ratios of the catalysts Catalyst
ICP V/Si × 102 atom
ICP Ba/Si × 102 atom
BET (m2 /g)
XPS V/Si × 102 atom
XPS Ba/Si × 102 atom
Vsi VSi-L BaVSi BaSi
0.49 0.007 0.33 –
– – 0.93 1.14
175 3 145 142
0.26 0.38 0.20 –
– – 0.25 0.30
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analyser, using a Mg K␣ (hν = 1253.6 eV, 1 eV = 1.603 × 10−19 J) X-ray source. The powder samples were pressed into small Inox cylinders and outgassed at 573 K for 1 h in the pretreatment chamber prior to being transferred to the ion-pumped analysis chamber. The binding energy (BE) of the Si 2p peak of the support at 103.4 eV was taken as an internal standard. The accuracy of the BE values was ±0.1 eV. Each spectral region of the photoelectrons of interest was scanned for a number of times to obtain a good signal-to-noise ratio. The intensities of the peaks were estimated by calculating the integral of each peak after subtracting an S-shaped background and fitting the experimental peak to a combination of Lorentzian/Gaussian lines of variable proportions. 2.3. Catalyst testing The catalyst performance for the partial oxidation of methane was evaluated in two reactors (7 and 10 mm ID, 23 cm length) placed inside a vertical furnace and fed from the top. The catalyst was sandwiched between quartz wool plugs in the middle of the tube. The temperature of the catalyst bed was measured by a thermocouple located inside a coaxially centred quartz sheath whose end was in touch with the catalyst bed. Reactor was designed not only to minimise the dead volume, and hence to decrease the extent of gas-phase reactions, but also to change the upstream and downstream void volume in the reactor. Samples of 0.20 g, GHSV of 6.6 × 104 h−1 , a mixture N2 :CH4 :O2 = 4:2:1 (molar) at a CH4 flow rate of 100 ml/min, atmospheric pressure and temperatures from 803 to 1023 K were used. In order to investigate the effect of NO on both CH4 conversion and product distribution, several NO concentrations (0, 200, 534, 4000, 6000, 10,000 and 14,000 ppm) were used while maintaining constant the O2 and CH4 concentrations. Nitrogen was used to balance the variation of NO%. Four separate lines for CH4 (99.9995 vol.%), N2 (99.995 vol.%), O2 (99.995 vol.%) and N2 /NO (935 ppm NO) mixture were used. Flow rates were adjusted by mass flow controllers, and all gases were mixed prior to reach the reactor. The effluents of the reactor and feed stream were analysed by GC fitted with TCD, using a Porapak Q and 4A molecular sieve columns and He as a carrier gas. The outlet of the reactor to the injection port of GC
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was heated up to 423 K in order to avoid water condensation and formaldehyde polymerisation. On an average, estimated error in replica experiments with fresh catalyst samples and clean quartz reactor was within ±5%.
3. Results 3.1. Catalyst characterisation Table 1 compiles chemical analyses, specific areas and surface atomic ratios, derived from photoelectron spectroscopy analysis, for all the catalysts. Vanadium incorporation leads to a slightly decrease in surface area with respect to the SiO2 substrate. This decrease was even more marked (up to 21%) for the Ba-containing catalysts. Chemical analyses of V and Ba, expressed as total V or Ba atoms per Si atom, differ from XPS surface atom ratios. Although atom ratios at the surface are lower than in the bulk indicating low V dispersion, Ba/Si ratios are even lower than the V/Si ratios. As the binding energy of Ba 3d5/2 is characteristic of BaCO3 , it appears that Ba carbonate accumulates as big aggregates on SiO2 particles. The larger V/Si ratios indicate that vanadium oxide is distributed on VSi catalyst with a higher dispersion degree than in BaVSi catalyst. TPR results showed that SiO2 -supported V and BaV oxides are reduced in a single reduction step with the maximum of H2 -consumption placed around 800–850 K. These profiles also exhibited a small shoulder in the high temperature flank. This peak can be assigned to the reduction of V5+ to V3+ in monomeric or low oligomeric VOx entities [10]. A small proportion of bulk-like V2 O5 could be present as judging from the appearance of the shoulder at high temperatures. Easily reducible V-species can, in part, be responsible for higher activity and also better selectivity toward C1 -oxygenates. BaVSi catalyst is slightly less reducible (Tmax = 850 K) than the VSi counterparts (Tmax = 805 K) indicating that BaO inhibits vanadium oxide reduction to some extent. The same trend is observed in catalytic performance suggesting that the lower activity and C1 -oxygenates selectivity of BaVSi catalyst is associated with a lower reducibility of V-sites, and able to activate oxygen.
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3.2. Reaction studies Activity experiments with different concentrations of NO in the feed stream were carried out with the aim to evaluate the role of NO on the CH4 conversion and product distributions at different temperatures. The influence of the size of the reactor, catalysts type, and catalyst bed configuration were also studied. 3.2.1. Homogeneous reaction The homogeneous reaction between CH4 and O2 was studied, using a 10 mm ID reactor with 6 ml empty volume, and feeding simultaneously NO in the 0–10,000 ppm concentration range. Fig. 1 shows CH4 conversion and product distributions at 823 and 923 K as a function of the NO concentration in the feed stream. CH4 conversion reaches ca. 45% at NO concentration in the range 1000–4000 ppm (depending on the temperature) and then remains almost constant at higher NO concentrations. CO yield follows the same trend than CH4 conversion when temperature and NO concentration change. The maximum HCHO and CH3 OH yields are 0.8 and 0.3%, respectively, at the lowest temperature (823 K), remaining almost constant over the whole NO concentration range. The yield to HCHO and CH3 OH decreases with increasing temperature, whereas the C2+ hydrocarbon yield follows the opposite trend. The increase in C2+ hydrocarbon yield with reaction temperature is sharper in the low NO concentration region. 3.2.2. Behaviour of VSi and BaSi catalysts Activity of VSi and BaSi catalysts was studied in the 7 mm ID reactor. CH4 conversion and product distributions at 923 K (data not shown here) as a function of the NO concentration in the feed stream show that CH4 conversion on VSi reaches ca. 3.5% at NO concentration of 4000 ppm and then decreases slightly. CO and HCHO yields follow a similar trend. The maximum HCHO yield is 1.2% and no CH3 OH is observed. For catalyst BaSi, CH4 conversion reaches a 2.8% at 4000 ppm NO and then almost levelled off. A similar trend is observed for CO yield, however HCHO yield increases continuously with NO concentration and attains a maximum value of 1.28% for 10,000 ppm NO. The methane conversion and CO yield on both catalytic systems is ca. 10 times lower than for the homogeneous reaction, studied in the 10 mm ID reactor
Fig. 1. Homogeneous reaction in the 10 mm ID reactor (6 ml dead volume). CH4 conversion and product yield as a function of NO concentration at 823 (a) and 923 K (b) (CH4 — 䊏; CO — ; CO2 — 䉱; HCHO — 䊐; CH3 OH — 䊉; C2+ — 䊊).
(Fig. 1b). This is expected since the decrease of the dead volume, when the catalytic bed is used, produces the diminution of the residence time in the reactor. Concurrently, there is no production of CO2 and C2+ hydrocarbons, but the HCHO yield is almost twofold in comparison with the homogeneous reaction.
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CO production is much higher than CO2 . HCHO yield doubled upon increasing NO from 200 to 4000 ppm NO and then steeply increased until a maximum value of 1.9% at 10,000 ppm NO. C2+ hydrocarbons (yield below 0.3%) were detected near 6000 ppm NO and
Fig. 2. Catalytic reaction in the 7 mm ID reactor. CH4 conversion and product yield as a function of NO concentration at 973 K for VSi (a) and BaSi (b) catalysts (CH4 — 䊏; CO — ; CO2 — 䉱; HCHO — 䊐; CH3 OH — 䊉; C2+ — 䊊).
Fig. 2 displays CH4 conversion and product distributions at 923 K as a function of NO concentration for VSi and BaSi catalysts. For catalyst VSi (Fig. 2a), CH4 conversion reached a maximum ca. 13% at NO concentration of 4000 ppm and then levelled off. A similar pattern was observed for CO and CO2 yields, although
Fig. 3. Catalytic and homogeneous reaction in the 10 mm ID reactor. CH4 conversion and product yield as a function of NO concentration at 923 K for void volume upstream the VSi catalyst (a) and void volume downstream the VSi catalyst (b) (CH4 — 䊏; CO — ; CO2 — 䉱; HCHO — 䊐; CH3 OH — 䊉; C2+ — 䊊).
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then vanished. No CH3 OH was detected on the VSi catalyst. The behaviour of catalyst BaSi is shown in Fig. 2b. For this catalyst, CH3 OH was found in the 4000–6000 ppm NO range, while HCHO increased up to 2.2% at 10,000 ppm NO. In addition, C2+ hydrocarbons were maintained over the whole range of NO concentration explored.
3.2.3. Combined homogeneous and catalytic reactions The catalytic behaviour of sample VSi was studied in the presence or absence of homogeneous reaction upstream or downstream the catalyst bed using the 10 mm ID reactor. Fig. 3 shows the performance of two reactor configurations: void volume upstream the
Fig. 4. Catalytic bed configuration in the 7 mm ID reactor. CH4 conversion and product yield in the 7 mm ID reactor as a function of NO concentration at 973 K: (a) VSi over BaSi; (b) BaSi over VSi; (c) VSi–BaSi mixture; (d) BaVSi (CH4 — 䊏; CO — ; CO2 — 䉱; HCHO — 䊐; CH3 OH — 䊉; C2+ — 䊊).
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VSi catalysts (Fig. 3a) and void volume downstream the VSi catalyst (Fig. 3b), and they are compared with the empty reactor configuration (Fig. 1b). The results presented in Figs. 3 and 1b underline the relevance of homogeneous activation. Both the homogeneous reaction and the catalyst with upstream homogeneous reaction (Figs. 1b and 3a) show similar conversion levels, being CO the main product. The presence of a VSi catalyst increases CO2 production, particularly at low NO concentration and downstream homogeneous configuration (Fig. 3b). In this last configuration, the system affords lower conversion levels and CO yield. Nevertheless, in the absence of NO, this configuration is the most active and the differences between configurations decrease with NO concentration. There are important qualitative differences: no C2+ hydrocarbons are formed when the void volume is downstream the catalyst bed (Fig. 3b) and no methanol is formed when the void volume is upstream the catalyst bed (Fig. 3a). These results further support previous mechanistic reports that suggest that methanol is formed through a different path than that for formaldehyde (HCHO is formed at lower temperature) and that its formation does not necessarily involve a catalytic site. Besides, coupling reaction, that is maximum for the empty reactor, is favoured by the upstream configuration (Fig. 3a), whereas downstream configuration (Fig. 3b) promotes the conversion of methane to methanol and formaldehyde, exhibiting the maximum yield for oxygenates. As shown previously for other reaction configurations (Figs. 1 and 2), the increase in NO concentration enhances the yield to HCHO but not the C2+ hydrocarbon. For type b configuration (Fig. 3b), CH3 OH and HCHO yields reach a maximum for NO concentrations in the range 4000–6000 ppm NO and then slightly decrease. 3.2.4. Bed configurations Fig. 4 shows CH4 conversion and yields to products at 973 K as a function of the NO concentration for different bed configurations: (a) VSi catalytic bed upstream/BaSi catalytic bed downsteam (ratio 1); (b) BaSi catalytic bed upstream/VSi catalytic bed downstream (ratio 1); (c) mixture of VSi and BaSi catalysts (ratio 1) and (d) BaVSi catalyst. In general, no large differences in performance were observed among the three first bed configurations. The HCHO yield was maximum by feeding 6000 ppm NO over the VSi/BaSi
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catalyst bed. However, CH3 OH is observed only for this configuration, with a maximum at 6000 ppm NO, and is not produced in the other cases. BaVSi catalyst showed lower activity than the physical mixture VSi + BaSi, which is consistent with the lower reducibility of the vanadium oxide in the supported Ba–V phase.
Fig. 5. CH4 conversion and product yield in the 10 mm ID reactor as a function of NO concentration on low area VSi-L catalyst at 873 (a) and 923 K (b) (CH4 — 䊏; CO — ; CO2 — 䉱; HCHO — 䊐; CH3 OH — 䊉; C2+ — 䊊).
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3.2.5. Role of surface area of VSi catalysts A low specific area silica-supported vanadium oxide catalyst (VSi-L) was used with the aim to evaluate the roles of reaction temperature and NO concentration. Fig. 5 displays the activity of this catalyst as a function of NO concentration at 873 and 923 K in the 10 mm ID reactor without void volume. VSi-L catalyst becomes very active at either temperatures when NO concentration increases. Yield to different oxidation (CO, HCHO, CH3 OH, CO2 ) and coupling (C2+ ) products also increase with reaction temperature. HCHO and CH3 OH yield resulted particularly high at 923 K with values of 1.4 and 0.9%, respectively.
4. Discussion Methane activation by oxygen is strongly promoted by NO through complex homogeneous–heterogeneous reactions. The role of NO added to the feed stream is to increase the radicals population in the gas phase and to shift selectivity to oxygen-containing molecules. The very low activity in the absence of NO is due to the lack of any source of radicals. The sharp effect on CH4 conversion upon adding 200 ppm NO in the feed must be due to the chain propagation of radical reactions. Oxygen conversion increases in parallel and oxygen-limiting conditions dominate above 823 K for NO concentrations above 4000 ppm (cf. Fig. 1). For a fixed CH4 :O2 ratio in the feed, the channels for C1 -oxygenates and C2+ hydrocarbon formation appear to be influenced by reaction temperature and NO concentration. Thus, high yields to oxygenate products (1.3%) were reached at 823 K in a purely homogeneous process. At higher temperature, the yield to oxygenates decreased whereas it was compensated by the formation of C2+ hydrocarbons. The incorporation of NO in the feed stream up to the concentrations of 10,000 ppm promoted gas-phase reactions between CH4 and O2 as already documented in the previous contributions [7–9]. The strong dependence of CH4 activation on the dimension and configuration of the reactor and catalytic bed again confirm the radical nature of the activation of CH4 molecule. However, the appearance of CH3 OH among the reaction products is very sensitive to the presence of the VSi redox catalyst. On the vanadia surface, the CH3 OH produced through homogeneous reactions
became completely oxidised to CO and CO2 . This is not for the case of the non-redox BaSi catalyst, for which HCHO and CH3 OH yields are slightly higher. Methane activation also occurs to a significant extent on the catalyst surface although high reaction rates are obtained at temperatures above than that of the homogeneous process [8] even over high specific area samples. On the contrary, high specific area is detrimental for methanol production. The effect of specific area on CH3 OH and HCHO is different. The CH3 OH/HCHO ratio decreases with specific area. Thus, surface redox sites appear to degrade CH3 OH species but they are responsible for HCHO production. The data reported in Fig. 5 for a low specific area VSi catalyst show that CH4 conversion reached more than 30% at 923 K, and yields to HCHO and CH3 OH resulted particularly high. Thus, the use of a low specific area redox catalyst and adding NO in the feed stream is the choice option to circumvent deep oxidation reactions, which yield non-selective oxidation products, while maintaining very high activity. 5. Conclusions Partial oxidation of CH4 by O2 into C1 -oxygenates (and C2+ hydrocarbons) takes place at moderate temperatures (823 K) only if variable amounts of NO (200–10,000 ppm) are added into the feed stream. In the absence of NO, CH4 activation also occurs on high specific area VSi and BaSi catalysts, although high rates are only achieved at temperatures substantially higher (above 923 K) than that of the pure homogeneous process. The strong dependence of CH4 activation on the size and configuration of reactor and catalyst bed points to the participation of both homogeneous and heterogeneous processes. It has been observed that high specific area VSi catalysts are detrimental for C1 -oxygenates selectivity, and particularly for CH3 OH. However, higher yields to HCHO and CH3 OH can be obtained by using a low specific area VSi redox catalyst. Acknowledgements The financial support for this work from the Research Institute of Innovative Technology for the Earth, RITE (Japan) is greatly acknowledged.
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