EPR and electron spin echo modulation spectroscopy of Cr^III and Cr^V in CrAPSO-5 molecular sieve: Evidence for framework substitution

Abstract

CrAPSO-5 molecular sieve was synthesized to incorporate Cr^III into the molecular sieve framework using tripropylamine as a structure-directing agent. CrAPSO-5 was characterized by XRD, thermogravimetric analysis and IR spectroscopy. Treatment of as-synthesized CrAPSO-5 with a solution of CH₃OH–HCl is effective in removing the tripropylamine. EPR shows Cr^III with g₁=5.20, g₂=2.00 and g₃=0.98, which is assigned to Cr^III in a distorted octahedral coordination. Spectral simulation indicates a zero-field splitting parameter of 0.50 cm^-1. Cr^III in ion-exchanged (L)Cr-SAPO-5 also shows octahedral coordination and is located in the main channel. After dehydration, Cr^III shows sharper EPR lines with g_⊥=4.00 and g_‖=2.00 which are assigned to Cr^III in tetrahedral coordination. After calcination and dehydration, the EPR of CrAPSO-5 shows Cr^V in tetrahedral coordination, characterized by g_⊥=1.97 and g_‖=1.89. UV–VIS spectra support these coordination assignments. Electron spin echo modulation (ESEM) of ³¹P indicates that Cr^V substitutes for phosphorus in a framework site in calcined CrAPSO-5 and that Cr^V is in a six-ring window in dehydrated, oxidized (L)Cr-SAPO-5. Deuterium ESEM indicates, that, in CrAPSO-5, Cr^V interacts with one molecule of ethylene via a weak σ-bond, while in dehydrated, oxidized (L)Cr-SAPO-5, Cr^V interacts with one molecule of ethylene via a π-bond. These results seem consistent with framework substitution in a phosphorus site in CrAPSO-5.

References

1. S. Thiele, K. Hoffmann, R. Vetter, F. Marlow and S. Radaev, Zeolites, 1997, 19, 190.
2. S. Radaev, W. Joswig and W. Baur, J. Mater. Chem., 1996, 6, 1413.
3. D. Demuth, K. Unger, F. Schuth, V. Srdanov and G. Stucky, J. Phys. Chem., 1995, 99, 479.
4. (a) M. McDaniel, Adv. Catal., 1985, 33, 47; (b) M. McDaniel, Ind. Eng. Chem. Res., 1988, 27, 1559.
5. B. Wan, K. Huang, T. Yang and C. Tai, J. Chin. I. Ch. E., 1991, 22, 17.
6. (a) J. Chen, J. Dakka and R. A. Sheldon, Appl. Catal. A, 1994, 108, L1; (b) J. Chen, M. Haanepen, J. van Hooff and R. A. Sheldon, Stud. Surf. Sci. Catal., 1994, 84, 973; (c) J. Chen and R. A. Sheldon, J. Catal., 1995, 153, 1; (d) J. D. Chen, E. B. Lempers and R. A. Sheldon, J. Chem. Soc., Faraday Trans., 1996, 92, 1807.
7. D. Escalante, L. Giraldo, M. Pinto, C. Pfaff, V. Sazo, M. Matjushin, B. Mendez, C. Lopez, F. Machado, J. Goldwasser and M. Ramirez De Agudelo, J. Catal., 1997, 169, 176.
8. E. M. Flanigen, B. M. Lok, R. L. Patton and S. T. Wilson, U.S. Pat., 4, 738, 837, 1988.
9. N. Rajic, D. Stojakovic, S. Hocevar and V. Kaucic, Zeolites, 1993, 13, 384.
10. (a) B. M. Weckhuysen and R. A. Schoonheydt, Zeolites, 1994, 14, 360; (b) B. M. Weckhuysen, R. A. Schoonheydt, F. E. Mabbs and D. Collison, J. Chem. Soc., Faraday Trans., 1996, 92, 2431.
11. L. Giraldo, C. Pfaff, C. Lopez, F. Machado, B. Mendez, J. Goldwasser, M. Ramirez De Agudelo, S. Rondon, M. Houalla and D. Hercules, Surf. Interface Anal., 1996, 24, 863.
12. Z. Zhu, T. Wasowicz and L. Kevan, J. Phys. Chem. B, 1997, 101, 10763.
13. B. Notari, Adv. Catal., 1996, 41, 253.
14. G. Brouet, X. Chen, C. Lee and L. Kevan, J. Am. Chem. Soc., 1992, 114, 3720.
15. E. M. Flanigen, B. M. Lok, R. L. Patton and S. T. Wilson, Eur. Pat., 158 976, 1985; U.S. Pat., 4, 759, 919, 1988.
16. P. Malla and S. Komarneni, Zeolites, 1995, 15, 324.
17. K. Kohler, C. Schlapfer, A. Zelwsky, J. Nicki, J. Engweiler and A. Baiker, J. Catal., 1993, 143, 201.
18. L. Kevan, in Time Domain Electron Spin Resonance, ed L. Kevan and R. N. Schwartz, Wiley, New York, 1979, ch. 8.
19. S. A. Dikanov and Yu. D. Tsvetkov, Electron Spin Echo Envelope Modulation Spectroscopy, CRC, Boca Raton, FL, 1992.
20. C. P. Keijzers, E. J. Reijerse, P. Stam, M. F. Dumont and M. C. M. Gribnau, J. Chem. Soc., Faraday Trans. 1, 1987, 83, 3493.
21. D. Brown, J. Chem. Soc. Chem. Commun., 1962, 3322.
22. M. Huang, Z. Deng and Q. Wang, Zeolites, 1990, 10, 272.
23. A. Munoz-Paez, R. Pappalardo and E. Marcos, J. Am. Chem. Soc., 1995, 117, 11710.
24. L. Kevan, in Electron Spin Resonance, ed. M. C. R. Symons, The Royal Society of Chemistry, Cambridge, 1991, vol. 12B, ch. 3.
25. P. Kasai and R. Bishop, J. Phys. Chem., 1977, 81, 1527.
26. J. Pearce, D. Sherwood, M. Hall and J. Lunsford, J. Phys. Chem., 1980, 84, 3215.
27. B. Wichterlova, Z. Tvaruzkova and J. Novakova, J. Chem. Soc., Faraday Trans. 1, 1983, 79, 1573.
28. V. Atanasova, V. Shvets and V. Kazanskii, Kinet. Catal., 1977, 18, 848.
29. D. O'Reilly and D. MacIver, J. Phys. Chem., 1962, 66, 276.
30. R. Landry, J. Fournier and C. Young, J. Chem. Phys., 1967, 46, 1285.
31. J. Fournier and R. Landry, J. Chem. Phys., 1971, 55, 2522.
32. T. Chapus, A. Tuel, Y. Taarit and C. Naccache, Zeolites, 1994, 14, 349.
33. O. Nakamura, J. Mambrim, H. Pastore, E. Vichi, F. Gandra, E. Silva, H. Vargas and J. Pelzl, J. Chem. Soc., Faraday Trans., 1992, 88, 2071.
34. N. Ulagappan and C. N. R. Rao, J. Chem. Soc., Chem. Commun., 1996, 1047.
35. W. Zhang and T. J. Pinnavaia, Catal. Lett., 1996, 38, 261.
36. A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, Clarendon, Oxford, 1970, p. 403.
37. J. Henning, J. Liebertz and R. Van Stapele, J. Phys. Chem. Solids, 1967, 28, 1109.
38. B. Figgis, Introduction to Ligand Field Theory, Wiley, New York, 1964.
39. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Wiley, New York, 4th edn., 1988, p. 688.
40. W. Mowat, A. Shortland, G. Yagupsky, N. Hill, M. Yagupsky and G. Wilkinson, J. Chem. Soc., Dalton. Trans., 1972, 533.
41. Y. Chen and J. Liu, Polyhedron, 1996, 15, 3433.