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XAFS study of Ru(II)Sn(II) cluster supported on NaY zeolite
Physica B 208&209 (1995) 685-686
ELSEVIER
XAFS study of Ru(II)-Sn(II) cluster supported on NaY zeolite Y. Udagawa a'*, T. Yamakawa b, H. Hatanaka b, Li-Chang Yang h, S. Shinoda b aResearch Institute for Scientific Measurements, Tohoku University, Katahira, Sendal 980, Japan blnstitute of Industrial Science, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan
Abstract Complexes having Ru(II)-Sn(II) bond(s) are very effective for acetic acid synthesis from methanol alone. In particular the catalyst shows a very long life if supported on Y-type zeolite, suggesting that the Ru-Sn cluster species are encapsuled in zeolite supercages. Local structures around Ru were studied by XAFS at several stages of the catalyst preparation procedure, and it has been clearly shown that Ru are coordinated by Sn atoms in the catalyst.
1. Introduction
2. Experimental
Acetic acid is one of the very importnat chemicals in chemical industry. Recently it has been reported that heteronuclear cluster complexes having Ru-Sn bond(s) are effective for a one-step conversion of methanol to acetic acid [1-4]. They can be used in the form of homogeneous as well as heterogeneous (supported) catalyst. In particular, a catalyst prepared by ion exchange of NaY zeolite with [Ru(NH3)6] 3+ [5] followed by a treatment with a methanol solution of SnCI2'2H20 and NaC1 is found to be not only efficient but also very resistant to deactivation [6]. The reason of the long life and efficiency may be attributed to the fact that a species having Ru-Sn bond(s) is formed inside the zeolite supercage. It is also noteworthy that an induction period is observed before a stationary catalytic activity is attained. The purpose of the present paper is to study with XAFS spectroscopy whether and when Ru-Sn bond is formed.
The catalyst preparation procedure is described elsewhere [6] and is summarized in Fig. 1. Ru XAFS spectra were obtained with a double crystal XAFS spectrometer reported previously [7].
*Corresponding author.
3. Results and discussion As reference compounds, EXAFS analysis was carried out for [Ru(NH3)6]C13 (t) and (NEt4)4 [Ru(SnC13)6] (g). As is evident from Fig. 2(0 and (g), each has a simple EXAFS oscillation due to the six equivalent nearest neighbor atoms, resulting in a single large peak in the Fourier transform. The oscillation of the former, however, decays at around k = 12, while that of the latter extends to at least k = 17, reflecting whether the nearest neighbor atom is light or heavy. It is thus straightforward from EXAFS to prove whether Sn atom is coordinated to Ru or not. Structural parameters obtained for these compounds as well as those of catalyst precursors are listed in Table 1. XAFS spectra were observed at each stage of the catalyst preparation procedure. EXAFS oscillation and
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 7 8 9 - 6
686
Y. Udagawa et al./Physica B 208&209 (1995) 685-686
NaY-Zeolite (4.1g(dry), SIO2/A1203=5.5 )
Table 1 Structural parameters of the catalysts at several stages of the catalyst preparation procedure and of reference compounds
[Ru(NH3)6]CI 3 (38% Exchange) dry(a) SnCI 2 • 2H20/NaCl/MeOH(Contacted for 24 h with the 2 1 solution with [Ru]:[Sn]:[NaCI]=l:25:160) precursor 1 (b) He flow (200°C, 2h) precursor 2 (c) MeOH (5 mol %)/ He (200~C, 15 h) Catalyst (d) MeOH (5 mol %) / He (200~C, 285 h) Catalyst (e) Fig. 1. Catalyst preparation procedure. 200
5
100
0 100
o 200
0
1/A
I
Compounds (see text and Fig. 1)
Ru Sn
e (~) (a) (b) (c) (d) (e) [Ru(NHa )6]C13 (1) (NEt4)4 [Ru(SnCI3 )6] (g)
Ru-N
N
2.60 2.65
2.4 2.6
2.56
6
R (~)
N
2.12 2.12 2.11
5.5 4.6 4.5
2.09
6
evidently Sn is not coordinated at this stage. Although not shown here, precursor 2(c) also showed a similar EXAFS oscillation and accordingly almost the same structural parameters were obtained. During the induction period, that is, after the flow of M e O H (5 m o l % ) / H e for about 15 h, the local structure of Ru shows a significant change (Fig. 2(d)); a long oscillation with a shorter period, which is characteristic of heavy backscatterer, takes place. A standard analysis employing F T followed by inverse F T clearly indicates that the nearest neighbors of Ru at this stage include Sn and the corresponding coordination number is about 2.5; for this sample the valency of Ru(II) is indicated by XPS analysis. It is notable that the R u - S n bond persists after the time on stream of 300 h((e) in Table 1), which would correspond to the comparatively long life of the catalyst. XAFS study undoutedly showed that R u - S n bond exists in the zeolite supercage, the coordination number being about 2.5. The coordination of Sn to Ru takes place not immediately after the treatment with SnC12.2H20/NaC1/MeOH solution but after passing M e O H for several hours, together with reduction of Ru(III) to Ru(II).
References
0
2
4 6 8 DISTANCE / ~
10
Fig. 2. Extracted EXAFS oscillation and the associated Fourier transforms at several stages of the catalyst preparation procedure and of reference compounds. See Table 1 and Fig. 1 for notation of compounds. the associated F T of [Ru(NH3)6] 3+ after inclusion into zeolite are almost identical to those of the starting material which are shown in Fig. 2(0. As is clear from a comparison of Fig, 2(0 with Fig. 2(b), EXAFS oscillation does not change even after the treatment with SnC12.2H20/NaCI/MeOH;
[1] S. Shinoda and T. Yamakawa, J. Chem. Soc. Chem. Commun. (1990) 1511. I-2] T. Yamakawa, P. Tsai and S. Shinoda, Appl. Catal. A. 92 (1992) LI. [3] H. Einaga, T. Yamakawa and S. Shinoda, J. Coord. Chem. 32 (1994) 117. [4] T. Yamakawa, M. Hiroi and S. Shinoda, J. Chem. Soc. Dalton Trans. (1994) 2265. [5] J.H. Lunsford, Rev. Inorg. Chem. 9 (1987) 1. [6] T. Yamakawa, M. Hiroi, H. Hatanaka and S. Shinoda, Shokubai 35 (1993) 418. [7] K. Tohji, Y. Udagawa, T. Kawasaki and K. Mieno, Rev. Sci. lnstrum. 59 (1988) 1127.
ELSEVIER
XAFS study of Ru(II)-Sn(II) cluster supported on NaY zeolite Y. Udagawa a'*, T. Yamakawa b, H. Hatanaka b, Li-Chang Yang h, S. Shinoda b aResearch Institute for Scientific Measurements, Tohoku University, Katahira, Sendal 980, Japan blnstitute of Industrial Science, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan
Abstract Complexes having Ru(II)-Sn(II) bond(s) are very effective for acetic acid synthesis from methanol alone. In particular the catalyst shows a very long life if supported on Y-type zeolite, suggesting that the Ru-Sn cluster species are encapsuled in zeolite supercages. Local structures around Ru were studied by XAFS at several stages of the catalyst preparation procedure, and it has been clearly shown that Ru are coordinated by Sn atoms in the catalyst.
1. Introduction
2. Experimental
Acetic acid is one of the very importnat chemicals in chemical industry. Recently it has been reported that heteronuclear cluster complexes having Ru-Sn bond(s) are effective for a one-step conversion of methanol to acetic acid [1-4]. They can be used in the form of homogeneous as well as heterogeneous (supported) catalyst. In particular, a catalyst prepared by ion exchange of NaY zeolite with [Ru(NH3)6] 3+ [5] followed by a treatment with a methanol solution of SnCI2'2H20 and NaC1 is found to be not only efficient but also very resistant to deactivation [6]. The reason of the long life and efficiency may be attributed to the fact that a species having Ru-Sn bond(s) is formed inside the zeolite supercage. It is also noteworthy that an induction period is observed before a stationary catalytic activity is attained. The purpose of the present paper is to study with XAFS spectroscopy whether and when Ru-Sn bond is formed.
The catalyst preparation procedure is described elsewhere [6] and is summarized in Fig. 1. Ru XAFS spectra were obtained with a double crystal XAFS spectrometer reported previously [7].
*Corresponding author.
3. Results and discussion As reference compounds, EXAFS analysis was carried out for [Ru(NH3)6]C13 (t) and (NEt4)4 [Ru(SnC13)6] (g). As is evident from Fig. 2(0 and (g), each has a simple EXAFS oscillation due to the six equivalent nearest neighbor atoms, resulting in a single large peak in the Fourier transform. The oscillation of the former, however, decays at around k = 12, while that of the latter extends to at least k = 17, reflecting whether the nearest neighbor atom is light or heavy. It is thus straightforward from EXAFS to prove whether Sn atom is coordinated to Ru or not. Structural parameters obtained for these compounds as well as those of catalyst precursors are listed in Table 1. XAFS spectra were observed at each stage of the catalyst preparation procedure. EXAFS oscillation and
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 7 8 9 - 6
686
Y. Udagawa et al./Physica B 208&209 (1995) 685-686
NaY-Zeolite (4.1g(dry), SIO2/A1203=5.5 )
Table 1 Structural parameters of the catalysts at several stages of the catalyst preparation procedure and of reference compounds
[Ru(NH3)6]CI 3 (38% Exchange) dry(a) SnCI 2 • 2H20/NaCl/MeOH(Contacted for 24 h with the 2 1 solution with [Ru]:[Sn]:[NaCI]=l:25:160) precursor 1 (b) He flow (200°C, 2h) precursor 2 (c) MeOH (5 mol %)/ He (200~C, 15 h) Catalyst (d) MeOH (5 mol %) / He (200~C, 285 h) Catalyst (e) Fig. 1. Catalyst preparation procedure. 200
5
100
0 100
o 200
0
1/A
I
Compounds (see text and Fig. 1)
Ru Sn
e (~) (a) (b) (c) (d) (e) [Ru(NHa )6]C13 (1) (NEt4)4 [Ru(SnCI3 )6] (g)
Ru-N
N
2.60 2.65
2.4 2.6
2.56
6
R (~)
N
2.12 2.12 2.11
5.5 4.6 4.5
2.09
6
evidently Sn is not coordinated at this stage. Although not shown here, precursor 2(c) also showed a similar EXAFS oscillation and accordingly almost the same structural parameters were obtained. During the induction period, that is, after the flow of M e O H (5 m o l % ) / H e for about 15 h, the local structure of Ru shows a significant change (Fig. 2(d)); a long oscillation with a shorter period, which is characteristic of heavy backscatterer, takes place. A standard analysis employing F T followed by inverse F T clearly indicates that the nearest neighbors of Ru at this stage include Sn and the corresponding coordination number is about 2.5; for this sample the valency of Ru(II) is indicated by XPS analysis. It is notable that the R u - S n bond persists after the time on stream of 300 h((e) in Table 1), which would correspond to the comparatively long life of the catalyst. XAFS study undoutedly showed that R u - S n bond exists in the zeolite supercage, the coordination number being about 2.5. The coordination of Sn to Ru takes place not immediately after the treatment with SnC12.2H20/NaC1/MeOH solution but after passing M e O H for several hours, together with reduction of Ru(III) to Ru(II).
References
0
2
4 6 8 DISTANCE / ~
10
Fig. 2. Extracted EXAFS oscillation and the associated Fourier transforms at several stages of the catalyst preparation procedure and of reference compounds. See Table 1 and Fig. 1 for notation of compounds. the associated F T of [Ru(NH3)6] 3+ after inclusion into zeolite are almost identical to those of the starting material which are shown in Fig. 2(0. As is clear from a comparison of Fig, 2(0 with Fig. 2(b), EXAFS oscillation does not change even after the treatment with SnC12.2H20/NaCI/MeOH;
[1] S. Shinoda and T. Yamakawa, J. Chem. Soc. Chem. Commun. (1990) 1511. I-2] T. Yamakawa, P. Tsai and S. Shinoda, Appl. Catal. A. 92 (1992) LI. [3] H. Einaga, T. Yamakawa and S. Shinoda, J. Coord. Chem. 32 (1994) 117. [4] T. Yamakawa, M. Hiroi and S. Shinoda, J. Chem. Soc. Dalton Trans. (1994) 2265. [5] J.H. Lunsford, Rev. Inorg. Chem. 9 (1987) 1. [6] T. Yamakawa, M. Hiroi, H. Hatanaka and S. Shinoda, Shokubai 35 (1993) 418. [7] K. Tohji, Y. Udagawa, T. Kawasaki and K. Mieno, Rev. Sci. lnstrum. 59 (1988) 1127.