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Additive telluromolybdates as catalyst for vapor-phase selective oxidation of ethyl lactate to pyruvate
ii~iii!ii~i~i~!~i~i~i~i~iii~i~i!~ii~i~!~ii!i! applied surface science ELSEVIER
Applied Surface Science 121/ 122 (1997) 378-381
Additive telluromolybdates as catalyst for vapor-phase selective oxidation of ethyl lactate to pyruvate Hiromu Hayashi a,*, Shigeru Sugiyama a, Nobuhiro Kokawa h, Kichiro Koto b Department of Chemical Science and Technology, Faculty of Engineering, UniL,ersity of Tokushima, Minamijosanjima, Tokushima 770, Japan b Faculo" of Integrated Arts and Science, Unicersity of Tokushima, Minamijosanjima, Tokushima 770, Japan
Received 9 October 1996; accepted 12 February 1997
Abstract Additive telluromolybdates MnTeMoO6( = M nO. TeO 2 • MoO 3; M ii = Co, Mn, Zn) showed excellent activities in the selective oxidation of ethyl lactate to pyruvate at 250-300°C in an order of M IX= Co > Mn > Zn, and even for M H = Zn the telluromolybdate was active comparable with c~-Te2MoO7, while the component ZnMoO4 and TeO 2 were inactive. The catalysts were characterized in reference to a-Te2MoO 7, an active species in binary oxides of TeOz-MoO 3. It appears of benefit to locate tellurium adjacent to molybdenum in the present oxides for composing the active site. © 1997 Elsevier Science B.V. Keywords: Catalyst; Selective oxidation; Ethyl lactate; a-Te,MoOT; M]ITeMoO6 (M II= Co, Mn. Zn)
1. Introduction Binary oxides, T e O 2 - M o O 3, converted ethyl lactate selectively to pyruvate in a vapor-phase fixed-bed flow system [1]. A synergy in activity was observed, showing a sharp maximum at a composition of M o O 3 . 2 T e O 2, and c~-Te2MoO 7 was suggested as the active species [2]. The phase transition of crystalline ce-TezMoO 7 to the vitreous /3-form and regeneration of c~-phase by the recalcination were demonstrated in the previous paper [3] with evidence of powder XRD. In the present paper, excellent activities of additive telluromolybdates of ternary oxides M IITeMoO6
* Corresponding author. Fax: +81-886-557025; e-mail: [email protected].
( = M ' O . T e O 2 • MOO3; M I I = C o , Mn, Zn) in lactate oxidation are described with catalyst characterization in reference to o~-TezMoO 7.
2. Experimental Telluromolybdates were prepared by the solidphase reaction of molybdates with TeO~ at 500 ° a n d / o r 620°C. Powder XRD was measured by a MXP system of MAC Science. Infrared spectra were recorded for catalyst powder tabletted with KBr by JASCO FTIR-3. TG-DTA was measured by Rigaku TAS-100. X-ray photoelectron spectra (XPS) of Te 3d5/2 and M o 3d5/2 core electrons were measured by Shimadzu ESCA-1000. X-ray absorption spectra near K-edges of Mo, Co, Mn and Zn were measured
0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0169-4332(97)00337- 1
Ol Havashi et al. /Applied Surface Science 121 / 122 (1997) 378-381
by the transmission method for the boron nitride disk at the Photon Factory (BL-7c or 10B) of the National Laboratory for High Energy Physics, Tsukuba. Apparatus and procedure for reaction studies were described in the previous paper [2].
3. Results and discussion
Telluromolybdates are classified into three groups [4] of the Anderson-type heteropoly [ T e M o 6 0 2 4 ] 6 - , substitutive wolframite [T%Mo 1_,,O4]2- and additive [ T e M o O 6 ] 2 - telluromolybdate. The chemical composition of an additive telluromolybdate MnTe M o O 6 is equimolar for the component oxides ( = M ~ O . T e O 2 .MoO 3) and tellurium is tetravalent, while hexavalent in heteropoly and substitutive telluromolybdates. The binary oxide Te2MoO v is also an additive telluromolybdate. 3.1. Formation. phase transition and structure of Te 2 MoO 7 A mixture of MoO3.2TeO 2 kneaded with an appropriate amount of water, showed a sharp exotherm at 450°C without change in the weight, suggesting the solid phase reaction to afford crystalline a - T % M o O 7. However, the reaction at 500°C for 5 h revealed unreacted component oxides in the powder XRD pattern as shown in Fig. l(a). Melting the a-phase at 600°C, followed by the rapid cooling to room temperature gave an orange glass (/3-phase),
I
o
•
•
l il t
• I
,
(e) i
;o
2'0
3'o 20
,'o
,o
(°)
Fig. 1. Powder XRD patterns for crystalline ol-Te2MoO7: (a) calcined MoO3.2TeO 2 at 500°C in air for 5 h, (b) melting at 600°C, cooled to afford vitreous /3-form, and recalcined at 500°C, (c) Rietveld deviation for (b); C) MOO3, • TeO:.
379
of which recalcination at 500°C and the Rietveld analysis showed regeneration of a-Te2MoO 7 with high purity as shown in Fig. l(b, c). The EXAFS data provide evidence that the transition of crystalline a-Te2MoO 7 to the vitreous /3-form occurred without appreciable change in the metal-oxygen distances [3]. The crystalline a-Te2MoO 7 is monoclinic with a = 0 . 4 2 8 6 , b = 0 . 8 6 1 8 , c = 1.5945 nm, /3= 95.67 °, Z = 4 and space group P21/c [5]. 3.2. Enriched Te-content at surface and actici~.' of. catalysts XPS depth-profile revealed enriched Te-content at surface, accounting for the regenerated a-phase to be less active [2]. The activity level of fresh c~Te2MoO 7 was reproduced on exposure of active crystalline a-phase to the surface by grinding the regenerated a-form [3]. 3.3. Molybdates and telluromolybdates An aqueous molybdic acid or ammonium molybdate was mixed with an equimolar aqueous solution of Co(NO3) 2 • 6H:O, MnCI 2 - 4 H 2 0 or Zn(NO3) 2 • 6H20, neutralizing with aqueous ammonia, aging at 80-85°C, filtered, dried, and finally calcined at 500°C in air for 5 h. The powder XRD patterns agreed well with JCPDS data [6], showing the preparations in the present work to be a mixture of a- and /3-phase for CoMoO 4, and c~-MnMoO4 with a high purity. Telluromolybdates were obtained by calcination of kneaded paste of an equimolar mixture of the corresponding molybdate and TeO 2 at 500°C in air for 5 h as given in the literature [7,8]. Powder XRD patterns and IR spectra of additive MIITeMoO6 for M II = Co, Mn, Zn, were similar together, signifying their structures to be isotypic. However, differential thermal analysis showed sharp indications of solid-phase reaction at 551 and 592°C for CoMoO4-TeO 2 and ZnMoO4-TeO 2, respectively, suggesting the calcination temperature of 500°C taken in literature [7] was not enough for the present solid-phase reaction. Unreacted components, CoMoO 4 and TeO2, were found, as anticipated, in the powder XRD pattern for CoTeMoO 6 calcined at 500°C, but disappeared at 620°C as compared in Fig. 2(a, b). Endotherms without decrease in weight observed at 662°C for
H. Hayashi et al./Applied Surface Science 121 / 122 (1997) 378-381
380
110
2()
I
t
I
I
30
40
50
60
20( ° ) Fig. 2. Powder XRD patterns for CoTeMoO6: (a) CoMoO4-TeO 2 calcined at 500°C, (b) recalcined at 620°C, (c) melting at 700°C and cooled; © CoMoO 4, • TeO 2.
ZnTeMoO 6 and 676°C for CoTeMoO 6, respectively, show the melting of the additive telluromolybdates. Amorphous material was obtained, similarly as in TezMoO 7 [2,3], by quenching at room temperature after melting CoTeMoO 6 at 700°C as shown in Fig. 2(c). Metal-oxygen distances, MH-O and M o - O , in the nearest core obtained by EXAFS analysis were similar to those in molybdates MHMoO4 and in Te2MoO 7 as shown in Table 1.
active species a-Te2MoO v. A pair of MoO 6 octahedra are linked by edge sharing to the [Mo20~0] unit and the double chains of distorted molybdenum octahedra connected at comers along the a-direction are linked by tetrahedral oxotellurium, Te Iv, chains to build up the three dimensional arrangement [5]. The active site for oxidation would be Mo = O, similarly as in the single oxide MoO 3 with layered structure [9]. It appears of benefit to locate tellurium adjacent to molybdenum for composing the active site. Entrapping the substrate as alcoxide on the basic TeO 4 site followed by the oxidation at C - H on M o = O would proceed cooperatively, leading to the synergy in activity for the binary oxide system. Additive telluromolybdates MnTeMoO6 (Mn = Co, Mn, Zn) are orthorhombic with Z = 2 and space group of P 21212 [7,8], and showed excellent activities in the vapor-phase selective oxidation of ethyl
s°I O
3.4. Structure of telluromolybdates and actiui~ in lactate oxidation
~150
I 200
250
300
350
I 400
200
250
300
350
400
250
300
350
400
250
300
350
400
Activity of binary oxides, TeO2-MoO 3, appears to be closely related to the structure of crystalline ~=
o
-
150
I
Table 1 M e t a l - o x y g e n distances (,~) for additive telluromolybdates and related components M n -0
Mo-O o ~
XRD MoO 3 a-Te2 MoO 7 /3-Te 2 MoO 7 ZnO ZnMoO 4 ZnTeMoO 6 CoMoO 4 CoTeMoO 6 MnMoO 4 MnTeMoO 6
2.018 ~ 2.095 d
EXAFS
2.00 2.07 2.02 2.03 2.13 2.09 -
XRD
EXAFS
1.700 ~ 1.722 ~' -
1.73 1.68 1.75
1.723 c
1.79 1.68 1.74 1.70 1.78
1.728 d 1.73
Average distances of the nearest two oxygens: ~ Ref. [9], b Ref. [5], c Ref. [11], d Ref. [12].
L
_
150
200
150
200
I
Temperature (°C)
Fig. 3. Vapor-phase oxidation of ethyl lactate to pyruvate over telluromolybdates (circles) and their component molybdates (triangles): reaction conditions, 5% ethyl lactate, 30% 0 2 , space velocity 3600 h t; open symbols, lactate conversion; filled symbols, pyruvate yield.
H. Hayashi et al. /Applied Surface Science 121 / 122 (1997) 378-381
lactate to pyruvate at 250-300°C in an order of M l I = Co > Mn > Zn, as shown in Fig. 3. An estimated structure of ZnTeMoO 6 based on the vibrational spectra (IR and Raman) has been proposed [10], but there remain some doubts about the unit cell and atomic site of Zn. However, tellurium would be again located adjacent to molybdenum as in ozTe2MoO 7. Thus even for M ~1= Zn the ternary telluromolybdate was active comparable with the binary a-Te2MoO 7 [2] as illustrated in Fig. 3, while the component ZnMoO 4 and TeO 2 were inactive. X-ray photoelectron spectra [13] of Te 3d5/2 and Mo 3d5/2 core electrons of a binary oxide of TeO zMoO 3 provided an evidence for reduction of Te to the metallic state in the presence of gas-phase oxygen in the feed, while the component Mo was rather stable under reductive environment, in the lactate oxidation at 300°C. Lattice oxygen was supplied to make up for the oxygen-deficit at the surface, and the catalyst should be used under oxidative, oxygenrich conditions [13]. It is not yet clear why Co enhances the catalyst activity, but the results are reminiscent of multi-component B i - M o - M H - M HI catalysts. When bismuth molybdate is supported on the core of a divalent metal molybdate such as C o M o O 4 having enough concentration of lattice vacancies by doping trivalent Fe 3+ cation, active oxygen species required for the reaction would be easily
381
supplied to the active site through the bulk diffusion of oxide ion [14], leading to highly active oxide catalysts.
References [1] S. Sugiyama, N. Shigemoto, N. Masaoka, S. Suetoh, H. Kawami, K. Miyaura, H. Hayashi, Bull. Chem. Soc. Jpn. 66 (1993) 1542. [2] H. Hayashi, S. Sugiyama, N. Masaoka, N. Shigemoto, Ind. Eng. Chem. Res. 34 (1995) 135. [3] H. Hayashi, S. Sugiyama, T. Moriga, N. Masaoka, A. Yamamoto, 4th Int. Symp. Heterog. Catal. and Fine Chem., Sept. 8-12, 1996, Basel, Switzerland. [4] J, Slocynsky, B. Sliwa, Z. Anorg. Allg. Chem. 438 (1978) 295. [5] Y. Arnaud, M.T. Averbuch-Pouchet, A. Durif, J. Guidot, Acta Cryst. B 32 (1976) 1417. [6] JCPDS, 25-1434(a-Co); 20-868(/3-Co); 27-1280(Mn); 35765(Zn). [7] P. Forzatti, F. Triffiro, P.L. Villa, J. Catal. 55 (1978) 52. [8] G. Tieghi, P. Forzatti, J. Appl. Cryst. ll (1978) 291. [9] L. Kihlborg, Ark. Kem. 21 (1963) 357. [10] E.J. Baran, I.L. Botto, L.L. Fornier, Z. Anorg. Allg. Chem. 476 (1981) 214. [11] G.W. Smith, J.A. Ibers, Acta Cryst. 19 (1965) 269. [12] S.C. Abraham, J.M. Reddy, J. Chem. Phys. 43 (1965) 2533. [13] H. Hayashi, N. Shigemoto, S. Sugiyama, N. Masaoka, K. Saitoh, Catal. Lett. 19 (1993) 273. [t4] Y. Moro-oka, W. Ueda, Adv. Catal. 40 (1994) 233.
Applied Surface Science 121/ 122 (1997) 378-381
Additive telluromolybdates as catalyst for vapor-phase selective oxidation of ethyl lactate to pyruvate Hiromu Hayashi a,*, Shigeru Sugiyama a, Nobuhiro Kokawa h, Kichiro Koto b Department of Chemical Science and Technology, Faculty of Engineering, UniL,ersity of Tokushima, Minamijosanjima, Tokushima 770, Japan b Faculo" of Integrated Arts and Science, Unicersity of Tokushima, Minamijosanjima, Tokushima 770, Japan
Received 9 October 1996; accepted 12 February 1997
Abstract Additive telluromolybdates MnTeMoO6( = M nO. TeO 2 • MoO 3; M ii = Co, Mn, Zn) showed excellent activities in the selective oxidation of ethyl lactate to pyruvate at 250-300°C in an order of M IX= Co > Mn > Zn, and even for M H = Zn the telluromolybdate was active comparable with c~-Te2MoO7, while the component ZnMoO4 and TeO 2 were inactive. The catalysts were characterized in reference to a-Te2MoO 7, an active species in binary oxides of TeOz-MoO 3. It appears of benefit to locate tellurium adjacent to molybdenum in the present oxides for composing the active site. © 1997 Elsevier Science B.V. Keywords: Catalyst; Selective oxidation; Ethyl lactate; a-Te,MoOT; M]ITeMoO6 (M II= Co, Mn. Zn)
1. Introduction Binary oxides, T e O 2 - M o O 3, converted ethyl lactate selectively to pyruvate in a vapor-phase fixed-bed flow system [1]. A synergy in activity was observed, showing a sharp maximum at a composition of M o O 3 . 2 T e O 2, and c~-Te2MoO 7 was suggested as the active species [2]. The phase transition of crystalline ce-TezMoO 7 to the vitreous /3-form and regeneration of c~-phase by the recalcination were demonstrated in the previous paper [3] with evidence of powder XRD. In the present paper, excellent activities of additive telluromolybdates of ternary oxides M IITeMoO6
* Corresponding author. Fax: +81-886-557025; e-mail: [email protected].
( = M ' O . T e O 2 • MOO3; M I I = C o , Mn, Zn) in lactate oxidation are described with catalyst characterization in reference to o~-TezMoO 7.
2. Experimental Telluromolybdates were prepared by the solidphase reaction of molybdates with TeO~ at 500 ° a n d / o r 620°C. Powder XRD was measured by a MXP system of MAC Science. Infrared spectra were recorded for catalyst powder tabletted with KBr by JASCO FTIR-3. TG-DTA was measured by Rigaku TAS-100. X-ray photoelectron spectra (XPS) of Te 3d5/2 and M o 3d5/2 core electrons were measured by Shimadzu ESCA-1000. X-ray absorption spectra near K-edges of Mo, Co, Mn and Zn were measured
0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0169-4332(97)00337- 1
Ol Havashi et al. /Applied Surface Science 121 / 122 (1997) 378-381
by the transmission method for the boron nitride disk at the Photon Factory (BL-7c or 10B) of the National Laboratory for High Energy Physics, Tsukuba. Apparatus and procedure for reaction studies were described in the previous paper [2].
3. Results and discussion
Telluromolybdates are classified into three groups [4] of the Anderson-type heteropoly [ T e M o 6 0 2 4 ] 6 - , substitutive wolframite [T%Mo 1_,,O4]2- and additive [ T e M o O 6 ] 2 - telluromolybdate. The chemical composition of an additive telluromolybdate MnTe M o O 6 is equimolar for the component oxides ( = M ~ O . T e O 2 .MoO 3) and tellurium is tetravalent, while hexavalent in heteropoly and substitutive telluromolybdates. The binary oxide Te2MoO v is also an additive telluromolybdate. 3.1. Formation. phase transition and structure of Te 2 MoO 7 A mixture of MoO3.2TeO 2 kneaded with an appropriate amount of water, showed a sharp exotherm at 450°C without change in the weight, suggesting the solid phase reaction to afford crystalline a - T % M o O 7. However, the reaction at 500°C for 5 h revealed unreacted component oxides in the powder XRD pattern as shown in Fig. l(a). Melting the a-phase at 600°C, followed by the rapid cooling to room temperature gave an orange glass (/3-phase),
I
o
•
•
l il t
• I
,
(e) i
;o
2'0
3'o 20
,'o
,o
(°)
Fig. 1. Powder XRD patterns for crystalline ol-Te2MoO7: (a) calcined MoO3.2TeO 2 at 500°C in air for 5 h, (b) melting at 600°C, cooled to afford vitreous /3-form, and recalcined at 500°C, (c) Rietveld deviation for (b); C) MOO3, • TeO:.
379
of which recalcination at 500°C and the Rietveld analysis showed regeneration of a-Te2MoO 7 with high purity as shown in Fig. l(b, c). The EXAFS data provide evidence that the transition of crystalline a-Te2MoO 7 to the vitreous /3-form occurred without appreciable change in the metal-oxygen distances [3]. The crystalline a-Te2MoO 7 is monoclinic with a = 0 . 4 2 8 6 , b = 0 . 8 6 1 8 , c = 1.5945 nm, /3= 95.67 °, Z = 4 and space group P21/c [5]. 3.2. Enriched Te-content at surface and actici~.' of. catalysts XPS depth-profile revealed enriched Te-content at surface, accounting for the regenerated a-phase to be less active [2]. The activity level of fresh c~Te2MoO 7 was reproduced on exposure of active crystalline a-phase to the surface by grinding the regenerated a-form [3]. 3.3. Molybdates and telluromolybdates An aqueous molybdic acid or ammonium molybdate was mixed with an equimolar aqueous solution of Co(NO3) 2 • 6H:O, MnCI 2 - 4 H 2 0 or Zn(NO3) 2 • 6H20, neutralizing with aqueous ammonia, aging at 80-85°C, filtered, dried, and finally calcined at 500°C in air for 5 h. The powder XRD patterns agreed well with JCPDS data [6], showing the preparations in the present work to be a mixture of a- and /3-phase for CoMoO 4, and c~-MnMoO4 with a high purity. Telluromolybdates were obtained by calcination of kneaded paste of an equimolar mixture of the corresponding molybdate and TeO 2 at 500°C in air for 5 h as given in the literature [7,8]. Powder XRD patterns and IR spectra of additive MIITeMoO6 for M II = Co, Mn, Zn, were similar together, signifying their structures to be isotypic. However, differential thermal analysis showed sharp indications of solid-phase reaction at 551 and 592°C for CoMoO4-TeO 2 and ZnMoO4-TeO 2, respectively, suggesting the calcination temperature of 500°C taken in literature [7] was not enough for the present solid-phase reaction. Unreacted components, CoMoO 4 and TeO2, were found, as anticipated, in the powder XRD pattern for CoTeMoO 6 calcined at 500°C, but disappeared at 620°C as compared in Fig. 2(a, b). Endotherms without decrease in weight observed at 662°C for
H. Hayashi et al./Applied Surface Science 121 / 122 (1997) 378-381
380
110
2()
I
t
I
I
30
40
50
60
20( ° ) Fig. 2. Powder XRD patterns for CoTeMoO6: (a) CoMoO4-TeO 2 calcined at 500°C, (b) recalcined at 620°C, (c) melting at 700°C and cooled; © CoMoO 4, • TeO 2.
ZnTeMoO 6 and 676°C for CoTeMoO 6, respectively, show the melting of the additive telluromolybdates. Amorphous material was obtained, similarly as in TezMoO 7 [2,3], by quenching at room temperature after melting CoTeMoO 6 at 700°C as shown in Fig. 2(c). Metal-oxygen distances, MH-O and M o - O , in the nearest core obtained by EXAFS analysis were similar to those in molybdates MHMoO4 and in Te2MoO 7 as shown in Table 1.
active species a-Te2MoO v. A pair of MoO 6 octahedra are linked by edge sharing to the [Mo20~0] unit and the double chains of distorted molybdenum octahedra connected at comers along the a-direction are linked by tetrahedral oxotellurium, Te Iv, chains to build up the three dimensional arrangement [5]. The active site for oxidation would be Mo = O, similarly as in the single oxide MoO 3 with layered structure [9]. It appears of benefit to locate tellurium adjacent to molybdenum for composing the active site. Entrapping the substrate as alcoxide on the basic TeO 4 site followed by the oxidation at C - H on M o = O would proceed cooperatively, leading to the synergy in activity for the binary oxide system. Additive telluromolybdates MnTeMoO6 (Mn = Co, Mn, Zn) are orthorhombic with Z = 2 and space group of P 21212 [7,8], and showed excellent activities in the vapor-phase selective oxidation of ethyl
s°I O
3.4. Structure of telluromolybdates and actiui~ in lactate oxidation
~150
I 200
250
300
350
I 400
200
250
300
350
400
250
300
350
400
250
300
350
400
Activity of binary oxides, TeO2-MoO 3, appears to be closely related to the structure of crystalline ~=
o
-
150
I
Table 1 M e t a l - o x y g e n distances (,~) for additive telluromolybdates and related components M n -0
Mo-O o ~
XRD MoO 3 a-Te2 MoO 7 /3-Te 2 MoO 7 ZnO ZnMoO 4 ZnTeMoO 6 CoMoO 4 CoTeMoO 6 MnMoO 4 MnTeMoO 6
2.018 ~ 2.095 d
EXAFS
2.00 2.07 2.02 2.03 2.13 2.09 -
XRD
EXAFS
1.700 ~ 1.722 ~' -
1.73 1.68 1.75
1.723 c
1.79 1.68 1.74 1.70 1.78
1.728 d 1.73
Average distances of the nearest two oxygens: ~ Ref. [9], b Ref. [5], c Ref. [11], d Ref. [12].
L
_
150
200
150
200
I
Temperature (°C)
Fig. 3. Vapor-phase oxidation of ethyl lactate to pyruvate over telluromolybdates (circles) and their component molybdates (triangles): reaction conditions, 5% ethyl lactate, 30% 0 2 , space velocity 3600 h t; open symbols, lactate conversion; filled symbols, pyruvate yield.
H. Hayashi et al. /Applied Surface Science 121 / 122 (1997) 378-381
lactate to pyruvate at 250-300°C in an order of M l I = Co > Mn > Zn, as shown in Fig. 3. An estimated structure of ZnTeMoO 6 based on the vibrational spectra (IR and Raman) has been proposed [10], but there remain some doubts about the unit cell and atomic site of Zn. However, tellurium would be again located adjacent to molybdenum as in ozTe2MoO 7. Thus even for M ~1= Zn the ternary telluromolybdate was active comparable with the binary a-Te2MoO 7 [2] as illustrated in Fig. 3, while the component ZnMoO 4 and TeO 2 were inactive. X-ray photoelectron spectra [13] of Te 3d5/2 and Mo 3d5/2 core electrons of a binary oxide of TeO zMoO 3 provided an evidence for reduction of Te to the metallic state in the presence of gas-phase oxygen in the feed, while the component Mo was rather stable under reductive environment, in the lactate oxidation at 300°C. Lattice oxygen was supplied to make up for the oxygen-deficit at the surface, and the catalyst should be used under oxidative, oxygenrich conditions [13]. It is not yet clear why Co enhances the catalyst activity, but the results are reminiscent of multi-component B i - M o - M H - M HI catalysts. When bismuth molybdate is supported on the core of a divalent metal molybdate such as C o M o O 4 having enough concentration of lattice vacancies by doping trivalent Fe 3+ cation, active oxygen species required for the reaction would be easily
381
supplied to the active site through the bulk diffusion of oxide ion [14], leading to highly active oxide catalysts.
References [1] S. Sugiyama, N. Shigemoto, N. Masaoka, S. Suetoh, H. Kawami, K. Miyaura, H. Hayashi, Bull. Chem. Soc. Jpn. 66 (1993) 1542. [2] H. Hayashi, S. Sugiyama, N. Masaoka, N. Shigemoto, Ind. Eng. Chem. Res. 34 (1995) 135. [3] H. Hayashi, S. Sugiyama, T. Moriga, N. Masaoka, A. Yamamoto, 4th Int. Symp. Heterog. Catal. and Fine Chem., Sept. 8-12, 1996, Basel, Switzerland. [4] J, Slocynsky, B. Sliwa, Z. Anorg. Allg. Chem. 438 (1978) 295. [5] Y. Arnaud, M.T. Averbuch-Pouchet, A. Durif, J. Guidot, Acta Cryst. B 32 (1976) 1417. [6] JCPDS, 25-1434(a-Co); 20-868(/3-Co); 27-1280(Mn); 35765(Zn). [7] P. Forzatti, F. Triffiro, P.L. Villa, J. Catal. 55 (1978) 52. [8] G. Tieghi, P. Forzatti, J. Appl. Cryst. ll (1978) 291. [9] L. Kihlborg, Ark. Kem. 21 (1963) 357. [10] E.J. Baran, I.L. Botto, L.L. Fornier, Z. Anorg. Allg. Chem. 476 (1981) 214. [11] G.W. Smith, J.A. Ibers, Acta Cryst. 19 (1965) 269. [12] S.C. Abraham, J.M. Reddy, J. Chem. Phys. 43 (1965) 2533. [13] H. Hayashi, N. Shigemoto, S. Sugiyama, N. Masaoka, K. Saitoh, Catal. Lett. 19 (1993) 273. [t4] Y. Moro-oka, W. Ueda, Adv. Catal. 40 (1994) 233.