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PbAl2O4 Catalysts
Guni, L et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congrcss on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
OXIDATIVE COUPLING OF METHANE OVER PbO/PbA1204 CATALYSTS
S. -E. Park and J.4.Chang Catalytic Research Divison, Korea Research Institute of Chemical Technology, 305-606 Taejon, Korea
Abstract The loading of PbO on y -alumina support could make the various phases of spinel between PbZtand Al)' with Pb,O, phase. These spinel phases between Pb2*and A13+were confirmed as magnetoplumbite-likestructure (PbA1,,OI9) and lead aluminate (PbASO,). At the 60wt.% loading of PbO on y -alumina, when it was treated at 750 "C for 4 hrs, it seem like to be mainly lead aluminate-supportedPbO catalyst. And this showed the highest activity and selectivityon C,, in the oxidative coupling of methane. Thus, in the case of PbO-y -alumina, the species responsible for the selective oxidative coupling of methane may be rather a PbO/ PbAl,O, catalyst, perturbed due to such a high loading and thermal treatment. 1. INTRODUCTION
Various kinds of low melting oxides have been reported as active catalysts in the oxidative coupling of methane since the pioneering work of Keller and Bhasin [11. Keller and Bhasin [11 as well as Hinsen and Baerns [2] reported that supported PbO catalysts are also active for the oxidative coupling of methane as oxygen supplier. Baerns' group [3] reported that for supported PbO catalysts, surface acidity influenced the C,-selectivity in oxidative coupling of methane. Also, Fujimoto's group [4] observed that when they used basic oxides as supports they got p a t improvements in methane conversion and C,-selectivity. Besides the effect of surface acidity, they postulated that the selectivity may be dependent on the ability of the catalyst to transfer lattice oxygen, which occurred by interaction with the lattice oxygen of PbO. Wendt et al. [5] ascertained that there would be a key role of reducibility of lead oxides and amorphousPbAl 120,9 phase, which has magnetoplumbite-likesmcture, as an active site. However, Marcelin's group proposed an "isolated site"-type mechanism which means that the isolation of strong oxidizing sites (PbO) on MgO could enable high selectivity [6]. In previous study the following phases on y-alumina-supported PbO catalysts were detected: lead aluminate (PbAl,O,), magnetoplumbite-likephase (PbA1,,OI9),red lead (Pb,O,), and solid solutions of PbO in y -alumina, lead aluminate or/and PbAl,,O,, [7]. At the 60wt.% loading of PbO on y -alumina, when it was treated at 750 "C for 4hrs, PbO orthorhombic and PbAl,O, spinel phases were dominant. The purpose of this study was to investigate the behavior of this PbO/PbAl,O, catalyst in the reaction of oxidative coupling of methane.
2234
PbO/PbAlz04catalyst was prepared by the incipient wetness method using an aqueous solution of lead nitrate. The mixture was dried at 120 "Cand calcined at 750 T for 4hrs.. For the preparation of solid-mixed catalyst, 6Owt.% of PbO and 40wt.% of ')' -alumina were mechanically mixed in an agar m o m and calcined at 750°C for 4hrs. The catalytic experiments were carried out in a conventional fixed-bed reactor (quartz, i.d. 10mm)at atmosphericpressure via cofeed method. The reactants and products were analyzed by an on-lined gas chromatograph (FID, TCD) with Porapak N and MS 5A columns. The Raman spectrawere obtained with a Jobin Yvon UlOOO spectrometerusing a Coherent's INNOVA 70 Argon laser, where the exciting line was typically 514.5 nm. For the ESCA study, binding energies of Pb 4f photoelectron lines, were obtained by VG ESCALAB Mark II photoelectron spectrometerusing A1 K, radiation. Spectra were obtained at 50 eV analyser pass energy and calibrated with the reference of adventitous C 1s at 284.6 eV.
3. RESULTS AND DISCUSSION The PbO loaded y -alumina catalyst contained magnetoplumbite-like(PbAl 12019) and lead aluminatephases mixed with Pb,04 and PbO othorhombicand PbO tetragonalphases depending on the contents of PbO. These phases seemed to be formed by the diffusion of Pb2' and AP* as well as lattice oxygen at the high temperature. Among them, the 60wt.96 loaded catalyst had mainly PbO and lead aluminate, which meant that it didn't have other spinel phases such as y -alumina and megnetoplumbite [7]. The solidmixed catalyst which same composition as the &t.% loaded catalyst showed much lower conversion of methane and selectivity on the C,, hydrocarbons. (Table 1) We already found the volcano-type patterns not only in (3H4conversion but also in C; selectivity were observed at this 60wt.% loaded catalyst among the various loadings [7]. Comparison of the catalytic activities over PbO/PbAl,04 (60% PbO on -alumina) and the solid-mixedcatalystsare shown in Table 1. WO/PbAI,04catalyst gave much higher activity and c,' selectivity. Laser Raman spectrum of this catalyst, coincided with XRD data, showed the large characteristic bands of PbAl,04 at 79,99,209,291, and 372 cm-' with 87,143,288, and 385 cm-I bands of PbO orthorhombicphase.(Figure l(a)) Table 1 Comparison of the activities of catalysts in the oxidative coupling of methane Catalyst PbO/PbAl,04 Solid-mixed
(334
Conv. % 12.9 6.0
co* 32.3 47.7
Selectivity(%) CZH6
Z ' H4
47.2 38.9
16.4 13.4
',' 4.1
2235
104 x 3 (d) after reaction
Solid-mixed
Ub
a f t e r reaction
RAMAN S H l F T ( c 6 ' )
Figure 1. The laser Raman spectra of supported PbO catalysts.
I
I
I
I
I
I
150
146
142
138
134
130
Binding Energy (ev)
Figure 2. XP spectra of Pb 4f core levels in (a) PbO/PbAl,O, and (b) the solid-mixed catalysts.
Bindingenergiesof A1 2p and 0 1son the low PbO loading catalysts were very close to those of 7-alumina [7]. However, they changed to 73.2 and 529.7 eV at the 60% PbO loading, respectively, which values are quite similar to those of CoAl,04 spinel [8]. Also the binding energies of Pb 4f 7R at low loading and high loading catalysts were close to the value of PbO orthorhombic. X-ray photoelectron spectra of Pb 4f in PbO/PbAl,O, and the solid-mixed catalysts are shown in Figure 2. Pb 4f 7R peak in PbO/PbAl,O, catalyst observed at 137.4 eV. It means that supported PbO is not merely PbO itself but strongly interacted with the interacted phase, which is lead aluminate. So, we could call this Owt.96 loaded catalyst as lead aluminatesupported PbO (PbO/PbAl,O,) catalyst. And also, we would say that PbO orthorhombicphase on Pb%04 which was formed by the interactionof PbO with -alumina would be active species in the oxidative couplingof methane contrary to the results of Wendt and co-workers [5],who proposed the active site as an amorphous magnetoplumbite phase. On the other hand, in the case of the solid mixed catalyst PbO phase was major before and after reaction with much smaller content of PbAl,O,. The Raman spectrum of this solid-mixed catalyst showed mainly the bands of PbO due to the poorly scatteringproperty of 7 -alumina.(Figure 1(c)) After the reaction this spectrum had changed abruptly into (d) spectrum,but the Raman spectrum of PbOPbAl,O, catalyst after the reaction seemed to be relatively similar to that of before the reaction. The (d) spectrum showed
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the bands of Pb,04 which is known to behave chemically as a mixture of PbO and PbO, [9],that showed the existence of Pb4+cations. The solid-mixed catalyst could be assumed that the interactions of the components occurred only in the boundaries of the component places, which were mainly between ')'-alumina and PbO instead of PbA1,04 and PbO. That's why his mechanical mixture gave poor activity and selectivity in the oxidative coupling of methane. X-ray photoelectron spectra in the solid-mixed catalyst showed the Pb 4f,, bands at 138.5 eV with the shoulderat 136.5eV.(Figure 2(b)) This indicatesthat two phases existedseparately. It means that there was little possibility of interaction between PbO and PbAl,04. So, these experiments on the solid-mixed catalyst indicate that there should exist an interaction between PbO and PbA1,04 for obtaining high activity and C,-selectivity in methane coupling reaction. The formation of spinel phases such as lead aluminate and magnetoplumbite could be the evidence of the diffusion of both cations and lattice oxygen during not only calcination but also methane coupling reaction at such a high reaction temperature. Thus,lead specieson the surface could be interactingwith supportedmaterials during the oxidativecoupling of methane. It means that the movement of lead cataions and lattice oxygens of the support are possible on the surface and PbO on lead aluminate would be the preferable phase for the oxidativecoupling of methane. And lead aluminate would be adequate for the diffusion of oxide ion to PbO, which would be the site of hydrogen abstraction. This also indicated that ')' -alumina is characterized by fast bulk diffusionof oxygen [ 101.Lead aluminate is known as a spinel which is preferably formed at such high PbO composition and high temperature [ 111. Therefore, lead aluminate would be appropriate in the oxygen mobility for getting good catalytic activity and C,, hydmcarbon selectivity with the low CO, selectivity. And lead aluminate-supported PbO catalyst is a more proper expression for the 60wt.95 PbO loaded catalyst than ')' -alumina-supported PbO catalyst. 4. REFERENCES
1. G.E. Keller and M.M. Bhasin, J. Catal., 73 (1982) 9. 2. W. Hinsen, W. Bytyn and M. Baerns, Proc. 8th Int. Congr. Catal., 3 (1984)581. 3. W. Bytyn and M. Baems, Appl. Catal., 28 (1986)199. 4. K. Asami, S.Hashimoto, T. Shikada, K. Fujimoto and H. Tominaga, Chem. Lett., 1233 (1986). 5. G. Wendt, C.-D. Meinecke and W. Schmitz, Appl. Catal., 45 (1988)209. 6. S.S.Aganval, R.A. Migone and G. Marcelin, J. Catal., 121 (1990)110. 7. S.-E. Park and J.-S. Chang, Catal. Sci. Tech., 1 (1991)435. 8. C. D.Wagner, W. M. Riggs, L. E. Davis and J. F. Moulder, "Handbook of X-ray Photoelectron spectroscopy." Perkin-Elmer Cop., USA, 1979. 9. A.F.Wells, "Structural Inorganic Chemistry", Clarendon Press., Oxford, 1984,p.558. 10. G.I. Golodets, "HeterogeneousCatalyticReactionshvolving Molecular Oxygen." (translated by J.R.H. Ross), Chap. 3;Elsevier, Amsterdam, 1983. 11. K. Torkar, H.Krischner and H. Moser, Ber. Deut. Keram. Ges., 43 (1966)259.
OXIDATIVE COUPLING OF METHANE OVER PbO/PbA1204 CATALYSTS
S. -E. Park and J.4.Chang Catalytic Research Divison, Korea Research Institute of Chemical Technology, 305-606 Taejon, Korea
Abstract The loading of PbO on y -alumina support could make the various phases of spinel between PbZtand Al)' with Pb,O, phase. These spinel phases between Pb2*and A13+were confirmed as magnetoplumbite-likestructure (PbA1,,OI9) and lead aluminate (PbASO,). At the 60wt.% loading of PbO on y -alumina, when it was treated at 750 "C for 4 hrs, it seem like to be mainly lead aluminate-supportedPbO catalyst. And this showed the highest activity and selectivityon C,, in the oxidative coupling of methane. Thus, in the case of PbO-y -alumina, the species responsible for the selective oxidative coupling of methane may be rather a PbO/ PbAl,O, catalyst, perturbed due to such a high loading and thermal treatment. 1. INTRODUCTION
Various kinds of low melting oxides have been reported as active catalysts in the oxidative coupling of methane since the pioneering work of Keller and Bhasin [11. Keller and Bhasin [11 as well as Hinsen and Baerns [2] reported that supported PbO catalysts are also active for the oxidative coupling of methane as oxygen supplier. Baerns' group [3] reported that for supported PbO catalysts, surface acidity influenced the C,-selectivity in oxidative coupling of methane. Also, Fujimoto's group [4] observed that when they used basic oxides as supports they got p a t improvements in methane conversion and C,-selectivity. Besides the effect of surface acidity, they postulated that the selectivity may be dependent on the ability of the catalyst to transfer lattice oxygen, which occurred by interaction with the lattice oxygen of PbO. Wendt et al. [5] ascertained that there would be a key role of reducibility of lead oxides and amorphousPbAl 120,9 phase, which has magnetoplumbite-likesmcture, as an active site. However, Marcelin's group proposed an "isolated site"-type mechanism which means that the isolation of strong oxidizing sites (PbO) on MgO could enable high selectivity [6]. In previous study the following phases on y-alumina-supported PbO catalysts were detected: lead aluminate (PbAl,O,), magnetoplumbite-likephase (PbA1,,OI9),red lead (Pb,O,), and solid solutions of PbO in y -alumina, lead aluminate or/and PbAl,,O,, [7]. At the 60wt.% loading of PbO on y -alumina, when it was treated at 750 "C for 4hrs, PbO orthorhombic and PbAl,O, spinel phases were dominant. The purpose of this study was to investigate the behavior of this PbO/PbAl,O, catalyst in the reaction of oxidative coupling of methane.
2234
PbO/PbAlz04catalyst was prepared by the incipient wetness method using an aqueous solution of lead nitrate. The mixture was dried at 120 "Cand calcined at 750 T for 4hrs.. For the preparation of solid-mixed catalyst, 6Owt.% of PbO and 40wt.% of ')' -alumina were mechanically mixed in an agar m o m and calcined at 750°C for 4hrs. The catalytic experiments were carried out in a conventional fixed-bed reactor (quartz, i.d. 10mm)at atmosphericpressure via cofeed method. The reactants and products were analyzed by an on-lined gas chromatograph (FID, TCD) with Porapak N and MS 5A columns. The Raman spectrawere obtained with a Jobin Yvon UlOOO spectrometerusing a Coherent's INNOVA 70 Argon laser, where the exciting line was typically 514.5 nm. For the ESCA study, binding energies of Pb 4f photoelectron lines, were obtained by VG ESCALAB Mark II photoelectron spectrometerusing A1 K, radiation. Spectra were obtained at 50 eV analyser pass energy and calibrated with the reference of adventitous C 1s at 284.6 eV.
3. RESULTS AND DISCUSSION The PbO loaded y -alumina catalyst contained magnetoplumbite-like(PbAl 12019) and lead aluminatephases mixed with Pb,04 and PbO othorhombicand PbO tetragonalphases depending on the contents of PbO. These phases seemed to be formed by the diffusion of Pb2' and AP* as well as lattice oxygen at the high temperature. Among them, the 60wt.96 loaded catalyst had mainly PbO and lead aluminate, which meant that it didn't have other spinel phases such as y -alumina and megnetoplumbite [7]. The solidmixed catalyst which same composition as the &t.% loaded catalyst showed much lower conversion of methane and selectivity on the C,, hydrocarbons. (Table 1) We already found the volcano-type patterns not only in (3H4conversion but also in C; selectivity were observed at this 60wt.% loaded catalyst among the various loadings [7]. Comparison of the catalytic activities over PbO/PbAl,04 (60% PbO on -alumina) and the solid-mixedcatalystsare shown in Table 1. WO/PbAI,04catalyst gave much higher activity and c,' selectivity. Laser Raman spectrum of this catalyst, coincided with XRD data, showed the large characteristic bands of PbAl,04 at 79,99,209,291, and 372 cm-' with 87,143,288, and 385 cm-I bands of PbO orthorhombicphase.(Figure l(a)) Table 1 Comparison of the activities of catalysts in the oxidative coupling of methane Catalyst PbO/PbAl,04 Solid-mixed
(334
Conv. % 12.9 6.0
co* 32.3 47.7
Selectivity(%) CZH6
Z ' H4
47.2 38.9
16.4 13.4
',' 4.1
2235
104 x 3 (d) after reaction
Solid-mixed
Ub
a f t e r reaction
RAMAN S H l F T ( c 6 ' )
Figure 1. The laser Raman spectra of supported PbO catalysts.
I
I
I
I
I
I
150
146
142
138
134
130
Binding Energy (ev)
Figure 2. XP spectra of Pb 4f core levels in (a) PbO/PbAl,O, and (b) the solid-mixed catalysts.
Bindingenergiesof A1 2p and 0 1son the low PbO loading catalysts were very close to those of 7-alumina [7]. However, they changed to 73.2 and 529.7 eV at the 60% PbO loading, respectively, which values are quite similar to those of CoAl,04 spinel [8]. Also the binding energies of Pb 4f 7R at low loading and high loading catalysts were close to the value of PbO orthorhombic. X-ray photoelectron spectra of Pb 4f in PbO/PbAl,O, and the solid-mixed catalysts are shown in Figure 2. Pb 4f 7R peak in PbO/PbAl,O, catalyst observed at 137.4 eV. It means that supported PbO is not merely PbO itself but strongly interacted with the interacted phase, which is lead aluminate. So, we could call this Owt.96 loaded catalyst as lead aluminatesupported PbO (PbO/PbAl,O,) catalyst. And also, we would say that PbO orthorhombicphase on Pb%04 which was formed by the interactionof PbO with -alumina would be active species in the oxidative couplingof methane contrary to the results of Wendt and co-workers [5],who proposed the active site as an amorphous magnetoplumbite phase. On the other hand, in the case of the solid mixed catalyst PbO phase was major before and after reaction with much smaller content of PbAl,O,. The Raman spectrum of this solid-mixed catalyst showed mainly the bands of PbO due to the poorly scatteringproperty of 7 -alumina.(Figure 1(c)) After the reaction this spectrum had changed abruptly into (d) spectrum,but the Raman spectrum of PbOPbAl,O, catalyst after the reaction seemed to be relatively similar to that of before the reaction. The (d) spectrum showed
2236
the bands of Pb,04 which is known to behave chemically as a mixture of PbO and PbO, [9],that showed the existence of Pb4+cations. The solid-mixed catalyst could be assumed that the interactions of the components occurred only in the boundaries of the component places, which were mainly between ')'-alumina and PbO instead of PbA1,04 and PbO. That's why his mechanical mixture gave poor activity and selectivity in the oxidative coupling of methane. X-ray photoelectron spectra in the solid-mixed catalyst showed the Pb 4f,, bands at 138.5 eV with the shoulderat 136.5eV.(Figure 2(b)) This indicatesthat two phases existedseparately. It means that there was little possibility of interaction between PbO and PbAl,04. So, these experiments on the solid-mixed catalyst indicate that there should exist an interaction between PbO and PbA1,04 for obtaining high activity and C,-selectivity in methane coupling reaction. The formation of spinel phases such as lead aluminate and magnetoplumbite could be the evidence of the diffusion of both cations and lattice oxygen during not only calcination but also methane coupling reaction at such a high reaction temperature. Thus,lead specieson the surface could be interactingwith supportedmaterials during the oxidativecoupling of methane. It means that the movement of lead cataions and lattice oxygens of the support are possible on the surface and PbO on lead aluminate would be the preferable phase for the oxidativecoupling of methane. And lead aluminate would be adequate for the diffusion of oxide ion to PbO, which would be the site of hydrogen abstraction. This also indicated that ')' -alumina is characterized by fast bulk diffusionof oxygen [ 101.Lead aluminate is known as a spinel which is preferably formed at such high PbO composition and high temperature [ 111. Therefore, lead aluminate would be appropriate in the oxygen mobility for getting good catalytic activity and C,, hydmcarbon selectivity with the low CO, selectivity. And lead aluminate-supported PbO catalyst is a more proper expression for the 60wt.95 PbO loaded catalyst than ')' -alumina-supported PbO catalyst. 4. REFERENCES
1. G.E. Keller and M.M. Bhasin, J. Catal., 73 (1982) 9. 2. W. Hinsen, W. Bytyn and M. Baerns, Proc. 8th Int. Congr. Catal., 3 (1984)581. 3. W. Bytyn and M. Baems, Appl. Catal., 28 (1986)199. 4. K. Asami, S.Hashimoto, T. Shikada, K. Fujimoto and H. Tominaga, Chem. Lett., 1233 (1986). 5. G. Wendt, C.-D. Meinecke and W. Schmitz, Appl. Catal., 45 (1988)209. 6. S.S.Aganval, R.A. Migone and G. Marcelin, J. Catal., 121 (1990)110. 7. S.-E. Park and J.-S. Chang, Catal. Sci. Tech., 1 (1991)435. 8. C. D.Wagner, W. M. Riggs, L. E. Davis and J. F. Moulder, "Handbook of X-ray Photoelectron spectroscopy." Perkin-Elmer Cop., USA, 1979. 9. A.F.Wells, "Structural Inorganic Chemistry", Clarendon Press., Oxford, 1984,p.558. 10. G.I. Golodets, "HeterogeneousCatalyticReactionshvolving Molecular Oxygen." (translated by J.R.H. Ross), Chap. 3;Elsevier, Amsterdam, 1983. 11. K. Torkar, H.Krischner and H. Moser, Ber. Deut. Keram. Ges., 43 (1966)259.