19484
J. Phys. Chem. 1996, 100, 19484-19488
Titanium Catalysts Supported on Silica. X-ray Absorption Investigation on Their Structures and Comparison of Their Catalytic Activities in Diels-Alder and Epoxidation Reactions Jose´ M. Fraile, Joaquı´n Garcı´a,* Jose´ A. Mayoral,* M. Grazia Proietti, and Marı´a C. Sa´ nchez Facultad de Ciencias, Instituto de Ciencia de Materiales de Arago´ n, C.S.I.C.sUniVersidad de Zaragoza, 50009 Zaragoza, Spain ReceiVed: May 22, 1996; In Final Form: September 23, 1996X
The correlation between the catalytic activity and the local structure of different silica-titanium supported catalysts prepared from different promoters was studied by means of X-ray absorption spectroscopy (XAS). The treatment of silica with TiCl4 gives rise to a very active catalyst of Diels-Alder cycloaddition and epoxidation of cyclohexene with TBHP, but the selectivity in the latter reaction is not very high. The solid obtained by treatment of silica with Ti(OiPr)4 is very active in epoxidation but not in Diels-Alder. The catalyst obtained by treatment of silica with TiCl2(OiPr)2 displays an intermediate catalytic activity in both reactions, although it is better in epoxidation than in Diels-Alder. The XAS results show how the Ti environment, and hence the catalytic activity of the solid, depends on the Ti compound used as the precursor. Titanium is bonded to four oxygens in the case of Ti(OiPr)4, while chlorine is present in the first coordination sphere when TiCl4 is used. The catalyst prepared from TiCl2(OiPr)2 gives rise instead to the formation of small size anatase particles.
Introduction Silica gel has proved to be a suitable support for the heterogenization of several kinds of catalysts.1 For instance, aluminum,2-6 titanium,3-5 zinc,5-7 and iron8 derivatives supported on silica gel act as acid catalysts. Silica gel is also very useful as a support of titanium epoxidation catalysts.9-12 We have studied several titanium derivatives supported on silica gel as catalysts in Diels-Alder reactions,3-5 and we have shown that the results obtained strongly depend on the nature of the titanium derivative used.3 To understand the origin of these different behaviors we have studied the titanium local structure of these solids by means of X-ray absorption spectroscopy. X-ray absorption spectroscopy provides information on the local structure around the absorbing atom.13,14 EXAFS enables the determination of interatomic distances, coordination numbers, and Debye-Waller factors,13 while XANES gives direct information on the coordination geometry located at the absorbing atom.14 This technique has been employed for a wide range of materials and has been proven to be a valuable tool in the study of titanium catalysts in solution15 and in the solid state.16-18 As reported in the following sections, the results obtained show that clear differences exist among the different catalysts which can explain their different catalytic behavior.
χ(k) )
Experimental Section Preparation of Catalysts. Silica gel, 1 g (Merck silica gel 60, 63-200 nm), was activated by heating under vacuum at 140 °C for 12 h. To a mixture of the activated solid and dry toluene (10 mL), was added 2.5 mL of a 1 M solution of the corresponding titanium derivative (TiCl4, TiCl2(OiPr)2, Ti(OiPr)4). The resulting mixture was heated under reflux in an argon atmosphere for 48 h. Toluene was eliminated via syringe, and the solid was repeatedly washed with dry toluene as previously described.4 The solid was dried and sealed under X
vacuum. TiO2 and Ti(IV)-K10 montmorillonite were prepared as previously described.17 EXAFS and XANES. X-ray absorption experiments at the Ti K-edge were carried out at the Synchroton Radiation Source of Daresbury (U.K.). A double-crystal Si(111) monochromator was used, and harmonic rejection of about 50% was achieved by a small misalignment of the two crystals. The storage ring operated at 2 GeV with an average beam current of 150 mA. The experiments were performed at room temperature in the transmission mode, and the signal was collected by ion chambers. The samples for EXAFS and XANES measurements were pressed to thin pellets, and the thickness of the samples was optimized in order to obtain the best signal-to-noise ratio. The experimental EXAFS signal was extracted from the raw spectra by following standard methods.13 Background removal and the atomic absorption coefficient were determined by a loworder polynomial fit of the spectra. The threshold energy E0, defining the zero wavevector, was taken at the inflection point of the absorption edge. The Fourier transform (FT) of the Ti K-edge k3 χ(k) spectra was calculated using a Gaussian window in the range 3.4-10.5 Å-1. The first-shell contribution was extracted by Fourier filtering the FT spectra between 0.8 and 2.3 Å, and the corresponding structural parameters were obtained by least-squares fitting of the filtered k-weighted spectrum to the EXAFS formula:13
Abstract published in AdVance ACS Abstracts, November 1, 1996.
S0022-3654(96)01487-6 CCC: $12.00
∑i (1/k)(Ni/Ri2) Ai(k) e-2σ
i2k2
sin[2kRi + Φi(k)]
The amplitudes Ai(k) and phase shifts Φi(k) used in the best fit were extracted from experimental TiCl4 (solution 1 M in toluene) EXAFS for the Ti-Cl contribution and from experimental Ti(OiPr)4 (solution 1 M in toluene) or TiO2 fine particle17 for the Ti-O contribution. The second-shell contribution was obtained by least-squares fitting of the Fourier-filtered EXAFS signal between 2.1 and 4.0 Å.13 Backscattering amplitudes and phase shifts were obtained from the EXAFS of TiO2 fine particle. For the TiSi contribution, theoretical phases and amplitudes generated from FEFF 3.11 code13e were used. © 1996 American Chemical Society
Titanium Catalysts Supported on Silica
J. Phys. Chem., Vol. 100, No. 50, 1996 19485
TABLE 1: Results Obtained in the Diels-Alder Reaction between Cyclopentadiene (1) and Methyl Acrylate (2)a % conversionb (3n/3x)b 30 min 2h 24 h
mmol of Ti g-1
catalyst TiO2c TiCl4-silicad TiCl2(OiPr)2-silicad Ti(OiPr)4-silicad
-
1 (73:27) 4 (73:27) 13 (93:7) 18 (93:7) 71 (93:7) 85 (93:7) 8 (90:10) 25 (90:10) 2 (82:18) 7 (82:18)
1.21 1.23 0.96
26 (73:27) 23 (92:8) 91 (93:7) 84 (90:10) 36 (82:18)
a In toluene using 0.13 g of catalyst mmol-1 of diene and equimolecular amounts of reagents. b Determined by gas chromatography. c Activated at 120 °C. d Reference 4.
TABLE 2: Best Fit Parameters Obtained for the First Shell Contribution of Silica Modified with Titanium Derivatives titanium derivative
bond
TiCl2(OiPr)2 Ti-O Ti(OiPr)4 Ti-O TiCl4 Ti-O Ti-Cl
N
R (Å)
5.2 ( 0.2 4.2 ( 0.2 3.7 ( 0.2 1.2 ( 0.2
1.89 ( 0.02 1.88 ( 0.01 1.77 ( 0.01 2.39 ( 0.01
∆σ2
2
(Å )
3.6 × 10-3 ((0.004) 8.6 × 10-3 ((0.002) 3.2 × 10-3 ((0.003) 9.3 × 10-3 ((0.005)
Figure 1. EXAFS spectra of silica gel treated with TiCl2(OiPr)2 (B), Ti(OiPr)4 (C), and TiCl4 (D) in comparison with the spectrum of anatase fine particle (A).
SCHEME 2
TABLE 3: Results Obtained in the Epoxidation of Cyclohexene (4) with TBHP (5)a catalyst Ti-K10d TiCl4-silica TiCl2(OiPr)2-silicae TiCl2(OiPr)2-silicae,f Ti(OiPr)4-silicae Ti(OiPr)4-silicae,f
time % conv % select. % conv % select. (h) TBHPb TBHP alkeneb,c alkene 1.0 3.5 1.0 3.5 1.0 3.5 1.0 3.5 1.0 3.5 1.0 3.5
28 47 68 81 17 40 18 35 67 86 33 53
4 6 70 73 100 100 95 100 94 90 92 91
12 20 61 74 17 40 17 35 64 79 31 49
9 10 79 81 100 100 100 100 98 98 98 98
a Using 36 mmol of cyclohexene, 12 mmol of TBHP (3 M solution in isooctane), and 1 g of catalyst. b Determined by gas chromatography. c Referred to the maximum. d Activated at 120 °C. e Activated at 140 °C. f Catalyst recovered by filtration, washed with CH2Cl2, and reactivated at 140 °C.
SCHEME 1
After background subtraction from the raw spectra, the XANES spectra were normalized taking atomic absorption as unity. Catalytic Reactions. Diels-Alder reactions were carried out in toluene at room temperature under an argon atmosphere using the ratio of reagents and catalysts given in Table 1. They were monitored by gas chromatography.4 Epoxidations of cyclohexene with TBHP were carried out at room temperature under argon atmosphere using the ratio of reagents and catalysts given in Table 3. The reactions were monitored by gas chromatography.12 Results and Discussion The results obtained using these solids as catalysts, in the benchmark Diels-Alder reaction between cyclopentadiene (1) and methyl acrylate (2) (Scheme 1), compared with those obtained using anatase as catalyst, are reported in Table 1. Silica gel modified by treatment with TiCl4 is a very efficient catalyst, which suggests the presence of tetracoordinated
titanium with Cl-Ti bonds. The treatment of diols with TiCl2(OiPr)2 in a homogeneous phase gives rise to iPrOH loss and the titanium diol dichloride formation. We tried to use this methodology with silica gel, but the solid obtained is much less active than that prepared using TiCl4, suggesting the possibility that, under these conditions, the chlorine atoms are eliminated to give a structure similar to that obtained using Ti(OiPr)4. However, both solids display again a different catalytic activity, the catalyst obtained from TiCl2(OiPr)2 being more active. This catalytic study allows us to propose a tentative structural model for the silica gel treated with TiCl4 and with Ti(OiPr)4, but not for the solid obtained using TiCl2(OiPr)2 (Scheme 2). The elemental analysis of the three solids [TiCl4-silica, Cl/ Ti molar ratio 1.21; TiCl2(OiPr)2-silica (does not contain chlorine), iPr/Ti molar ratio 0.5; Ti(OiPr)4-silica, iPr/Ti molar ratio 1.5] agrees with the differences in catalytic activity and shows that, even in the silicas treated with TiCl4 and Ti(OiPr)4, the models proposed (Scheme 2) do not exactly represent the structure of the catalysts. The EXAFS spectra of the modified silicas are shown in Figure 1 and compared with that obtained from fine particle anatase.17 The TiCl2(OiPr)2 silica shows the most structured EXAFS signal with the biggest oscillation amplitude. This signal is very similar to that obtained for anatase. The spectrum of the silica treated with Ti(OiPr)4 is similar but less structured and the signal becomes smoother, indicating an increasing structural disorder. Quite a different EXAFS signal is instead obtained from the TiCl4 treated solid. The corresponding Fourier transforms (FTs) of the EXAFS spectra are shown in Figure 2. The maximum of the first peak appears at different R values, indicating the presence of different titanium environments. The first-shell analysis has been performed by Fourier filtering the EXAFS signal between 0.8 and 2.3 Å. Due to the similarity of the TiCl2(OiPr)2 silica with the TiO2 anatase, we have used
19486 J. Phys. Chem., Vol. 100, No. 50, 1996
Fraile et al.
Figure 2. FT spectra of silica gel treated with TiCl2(OiPr)2 (B), Ti(OiPr)4 (C), and TiCl4 (D) in comparison with the spectrum of anatase fine particle (A).
the signal from experimental TiO2 fine particle (Ti-O interatomic distance 1.8 Å, N ) 4) as a reference signal for the Ti-O pair. The first-shell analysis of silica modified with TiCl4 and Ti(OiPr)4 was performed using the experimental spectra of Ti(OiPr)4 and TiCl4 as a reference for the Ti-O (d ) 1.84 Å, N ) 4) and Ti-Cl (d ) 2.19, N ) 4) pairs, respectively. These two model compounds present the characteristic that all the firstshell atoms are at the same distance from the Ti absorber. The best fit parameters obtained for the different solids are reported in Table 2, and the comparison between the experimental and fitted spectra are shown in Figure 3. We found that titanium in silica modified with TiCl2(OiPr)2 is surrounded by five oxygens at a distance of 1.89 Å. The low value of the coordination number can be explained as in the case of fine size anatase and titanium supported K10 montmorillonite17 as due to the existence of mixed titanium oxides with different geometries. Silica modified Ti(OiPr)4 is characterized by titanium surrounded by oxygen atoms at about 1.9 Å with a coordination number of 4. Silica modified with TiCl4 shows a smaller Ti-O distance of R ) 1.8 Å and the presence of chlorine atoms at a distance of R ) 2.39 Å. The overall Ti coordination is around 5 with a ratio between oxygen and chlorine coordination numbers N(O)/N(Cl) ) 3. The first conclusion was that the solid obtained from TiCl4 is the only one having chlorine atoms in the first coordination shell and that is in agreement with the observed higher catalytic activity. To get a more detailed characterization, we also undertook a second-shell analysis of the EXAFS spectra of the different samples. The EXAFS signal filtered between 2.1 and 4. Å is shown in Figure 4. The filtered spectra are clearly different, indicating that the catalysts have different second-shell environments. The comparison between the second shell of TiCl2(OiPr)2 modified silica with TiO2 fine particle (Figure 4A) shows the same structure except for the region 5-6 Å-1. This difference is related to the third-shell contribution, represented by the shoulder on the high R-values side of the second peak, that is not resolved enough to be filtered out. It strongly depends both on the TiO2 particle size and on the presence of different Ti(IV) oxides. Moreover, the fitting of the first shell gives, for the silica modified with TiCl2(OiPr)2, an average coordination value, N ) 5.2, smaller than that expected for oxide particles. As in the case of Ti(IV)-exchanged K10 montmorillonite,17 this could be explained by the presence, in the silica modified with TiCl2(OiPr)2, of small particles of anatase and a mixed amorphous Ti(IV) oxo-ion species. The particle size of the TiO2 formed in the modified silica must be very small because it cannot be observed by XRD. The filtered second shell of silica modified with Ti(OiPr)4 is represented in Figure 4B. The structure is different from the
Figure 3. Comparison between experimental and fitted EXAFS spectra of silica gel treated with TiCl2(OiPr)2, Ti(OiPr)4, and TiCl4.
Figure 4. Second-shell filtered spectra of silica gel treated with TiCl2(OiPr)2 (in comparison with that of anatase fine particle) (A), Ti(OiPr)4 (B), and TiCl4 (in comparison with the fitted spectrum) (C).
anatase fine particle. High-resolution solid state NMR studies showed the presence of Ti-OiPr species,11 indicating that also the carbon should be present in second shell. The second shell should therefore contain carbon, titanium, silicon, and/or oxygen atoms. The mixing of at least two different elements having
Titanium Catalysts Supported on Silica
J. Phys. Chem., Vol. 100, No. 50, 1996 19487 SCHEME 3
Figure 5. Normalized XANES spectra of silica gel treated with TiCl2(OiPr)2 (B), Ti(OiPr)4 (C), and TiCl4 (D) in comparison with that of anatase fine particle (A).
similar phase shifts and backscattering amplitudes makes it difficult to perform a reliable best fit of the spectrum. It is shown in any case for comparative purposes. The EXAFS signal corresponding to the second-shell contribution of TiCl4-silica is reported in Figure 4C. A best fit has been performed using theoretical phases and amplitudes for the Ti-Si pair calculated by means of FEFF 3.11 code.13e No reasonable fits were obtained including Ti-Ti or Ti-O contributions. The Ti-Si distance obtained from the best fit is 3.5 Å, typical of titanium silicalites corresponding to a Ti-OSi angle of 159°.18 Due to the small amplitude of the secondshell oscillations and the high correlation between coordination and Debye-Waller, we cannot give a quantitative coordination value. Nevertheless, the interaction between titanium and support is observed only in this case. The normalized XANES spectra of silica modified with the different titanium derivatives are shown in Figure 5. We can observe that the XANES structure is different depending on the Lewis acid used to modify the silica. The differences in the pre-edge structures obtained are clearly remarkable. As already observed19 the position and intensity of the pre-edge structure provide information about the coordination geometry of titanium. A three-peak structure is obtained for octahedral coordination, and the intensity of the central peak increases with distortion of the octahedra while the side peaks disappear. It also increases while changing from square pyramidal to tetrahedral coordination. Silica modified with TiCl2(OiPr)2 shows a XANES spectrum (Figure 5B) with three peaks very similar to the XANES spectrum of TiO2 fine particle (Figure 5A). This result confirms the previous EXAFS results: fine particle TiO2 is formed when TiCl2(OiPr)2 is used to modify silica. Besides, the small shift of the central peak at lower energies suggests the presence of titanium in different coordination geometries. Silica modified with Ti(OiPr)4 (Figure 5C) shows a XANES spectrum with a broad prepeak shifted at lower energies, indicating that different environments of titanium coexist in this solid (distorted tetrahedral and perhaps pentacoordinated titanium) in agreement with the EXAFS results. The XANES spectrum of silica modified with TiCl4 (Figure 5D) shows a prepeak which suggests a dominant distorted tetrahedral structure. However, the presence of some small anatase particles cannot be discarded.
The results of EXAFS, which are an average of all the different configurations present on the solid, together with the XANES spectra and the previously mentioned elemental analysis, indicate that these catalysts are not homogeneous materials. The silica treated with TiCl4 is a mixture of small particles of anatase and chlorine containing supported titanium. The likely dominant structure is shown in Scheme 2. The silica treated with Ti(OiPr)4 contains supported titanium atoms with different numbers of iPrO groups, and the structure containing two groups would be the most important. Finally, the silica modified with TiCl2(OiPr)2 is composed essentially of very small anatase particles, probably with isopropoxy groups in the external titanium atoms. Nevertheless, the presence of some isolated titanium isopropoxide cannot be discarded. The formation of anatase could be explained considering that the HCl, formed during the preparation of the catalyst, partially dehydrates the silica and the resulting water hydrolyses the titanium derivative to give TiO2. Titanium isopropoxy groups are more easily hydrolyzed than titanium chlorine. The lack of chlorine in Ti(OiPr)4 accounts for the lack of anatase in the silica treated with this titanium derivative. These structures explain the differences in the catalytic activity of the different compounds. The titanium atoms with tetrahedral coordination and containing chlorine must be indeed the most active catalytic centers. The catalytic activity of the silica treated with TiCl2(OiPr)2 is similar instead to that observed for the anatase fine particle,17 but the lack of water and, as a consequence of Brønsted acidity, reduces the polymerization of the diene and increases the final yield despite an initially slower reaction. Finally, the silica treated with Ti(OiPr)4 promotes the reaction but with a considerably lower catalytic activity. The presence of titanium with a low coordination and coordinated to chlorine or isopropoxy, which can be easily removed, suggested the possibility of using these solids as catalysts for the epoxidation of alkenes. In fact, the silica modified with Ti(OiPr)4 is an efficient catalyst for epoxidation of alkenes with tert-butylhydroperoxide (TBHP).11,12 The catalytic performance of these solids, in comparison with Ti(IV)exchanged K10 montmorillonite, was tested in the epoxidation of cyclohexene (4) with TBHP (5) (Scheme 3). The silica supported catalysts are more active and more selective than Ti-K10 montmorillonite. This clay has a very high content of titanium, and it is essentially anatase with small particle size.17 The acidity of this solid can promote several side reactions which justifies the low selectivity. The catalysts obtained from TiCl4 and Ti(OiPr)4 show similar activity, the former being less selective. It can be suggested that TBHP reacts with the titanium with the subsequent formation of iPrOH or HCl. The alkene coordinates this titanium and is oxidized. The remaining tert-butoxy is a worse leaving group which justifies the decrease in catalytic activity (Scheme 4). The formation of HCl in the case of silica treated with TiCl4 accounts for the decrease in selectivity given that this HCl promotes the opening of the epoxide to yield 2-chlorocyclohexanol. The formation of this byproduct supports the mechanism proposed. The lower catalytic activity of the solid obtained from TiCl2(OiPr)2 agrees with the observation of anatase structure by X-ray absorption spectroscopy. However, it still has some
19488 J. Phys. Chem., Vol. 100, No. 50, 1996 SCHEME 4
catalytic activity due to the small size of the particles. It is not easy to prepare anatase with this activity and selectivity. In fact, if water is not eliminated the residual acidity promotes the opening of the epoxide and other side reactions, but at the same time, this water cannot be eliminated by heating because this treatment would cause the increase of the particle size and, consequently, a decrease in the catalytic activity. This treatment of silica gel with TiCl2(OiPr)2 allows anatase with a small particle size to be obtained, and without water and residual Brønsted acidity, so that it promotes the epoxidation of cyclohexene (4) with high selectivity in both reagents. Furthermore, the catalyst is recovered without loss of selectivity. To sum up, the treatment of silica gel with TiCl4, TiCl2(OiPr)2, and Ti(OiPr)4 results in three solids with different catalytic properties in Diels-Alder cycloaddition and in epoxidation of alkenes with TBHP. X-ray absorption spectroscopy allows us to obtain direct structural information and to propose models which would account for the differences observed in the catalytic activity. Acknowledgment. This work was made possible by the generous financial support of the Comisio´n Interministerial de Ciencia y Tecnologı´a (Projects MAT93-0224 and PB92-1077). Supporting Information Available: Comparison between experimental and fitted EXAFS spectra of silica gel treated with TiCl2(OiPr)2, Ti(OiPr)4, and TiCl4 shown in the R space (1 page). Ordering information is given on any current masthead page. References and Notes (1) Butters, M. In Solid Supports and Catalysts in Organic Synthesis; Smith, K., Ed.; Ellis Harwood: Chichester, U.K., 1992; p 65. (2) (a) Drago, R. S.; Getty, E. E. J. Am. Chem. Soc. 1988, 110, 3311. (b) Gagnieu, C.; Groiller, A. Carbohydr. Res. 1980, 84, 61. (3) (a) Cativiela, C.; Garcı´a, J. I.; Mayoral, J. A.; Pires, E.; Royo, A. J.; Figueras, F. Appl. Catal. 1995, 131, 159. (b) Cativiela, C.; Garcı´a, J. I.;
Fraile et al. Mayoral, J. A.; Pires, E.; Royo, A. J.; Figueras, F. Tetrahedron 1995, 51, 1295. (c) Cativiela, C.; Figueras, F.; Garcı´a, J. I.; Mayoral, J. A.; Pires, E.; Royo, A. J. Tetrahedron: Asymmetry 1993, 4, 621. (d) Cativiela, C.; Garcı´a, J. I.; Mayoral, J. A.; Pires, E.; Royo, A. J.; Figueras, F. Appl. Catal. 1995, 131, 159. (e) Fraile, J. M.; Garcı´a, J. I.; Mayoral, J. A.; Pires, E.; Tarnai, T.; Figueras, F. Appl. Catal. 1996, 136, 113. (4) Cativiela, C.; Fraile, J. M.; Garcı´a, J. I.; Mayoral, J. A.; Pires, E.; Royo, A. J.; Figueras, F.; de Me´norval, L. C. Tetrahedron 1993, 49, 4073. (5) Cativiela, C.; Garcı´a, J. I.; Mayoral, J. A.; Pires, E.; Brown, D. R. Tetrahedron 1995, 51, 9217. (6) Garcı´a, J. I.; Mayoral, J. A.; Pires, E.; Brown, D. R.; Massam, J. Catal. Lett. 1996, 37, 261. (7) (a) Rhodes, C. N.; Brown, D. R. J. Chem. Soc., Faraday Trans. 1992, 88, 2269. (b) Rhodes, C. N.; Brown, D. R. J. Chem. Soc., Faraday Trans. 1993, 89, 1387. (8) (a) Keinan, E.; Mazur, Y. J. Org. Chem. 1978, 43, 1020. (b) Fadel, A.; Salaun, J. Tetrahedron 1985, 41, 413. (c) Fadel, A.; Salaun, J. Tetrahedron 1985, 41, 1276. (9) Sheldon, R. A. In Heterogeneous Catalysis and Fine Chemicals II; Guisnet, M., Barrault, J., Bouchoule, C., Duprez, D., Pe´rot, G., Maurel, R., Montassier, C., Eds.; Stud. Surf. Sci. Catal. 1991, 59, 33. (10) (a) Wulff, H. P. U.S. Patent 3,923,843, 1975. (b) Wulff, H. P.; Wattimena, F. U.S. Patent 4,021,454, 1977. (c) Wulff, H. P.; Wattimena, F. U.S. Patent 4,367,342, 1983. (11) Fraile, J. M.; Garcı´a, J. I.; Mayoral, J. A.; de Me´norval, L. C.; Rachdi, F. J. Chem. Soc., Chem. Commun. 1995, 539. (12) Cativiela, C.; Fraile, J. M.; Garcı´a, J. I.; Mayoral, J. A. J. Mol. Catal., in press. (13) (a) Konisberger, D. C.; Prins, R. X-Ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES; Wiley: New York, 1988. (b) X-Ray Absorption Fine Structure; Hasnain, S. S., Ed.; Ellis Horwood: Chichester, U.K., 1988. (c) Teo, B. K. EXAFS: Basic Principles and Data Analysis; Springer: Berlin, 1986. (d) Appendix to the report on the International Workshop on Standards and Criteria in Absorption X-Ray Spectroscopy, March 7-9, 1988; Bookhaven National Laboratory. Physica B 1989, 158, 701. (e) Rehr, J. J.; Mustre de Leo´n, J.; Zabinsky, S. I.; Albers, R. C. J. Am. Chem. Soc. 1991, 113, 5135. (14) Bianconi, A.; Garcı´a, J.; Benfatto, M. In Synchroton Radiation in Chemistry and Biology I (Mandelkow, E., Ed.). Topics Curr. Chem. 1988, 145, 29. (15) (a) Sa´nchez, M. C.; Garcı´a, J.; Mayoral, J. A.; Proietti, M. G.; Chaboy, J.; Ruiz-Lo´pez, M. F. Jpn. J. Appl. Phys. 1993, 32, 512. (b) Assfeld, X.; Garcı´a, J.; Garcı´a, J. I.; Proietti, M. G.; Ruiz-Lo´pez, M. F.; Sa´nchez, M. C. J. Chem. Soc., Chem. Commun. 1994, 2165. (c) Assfeld, X.; Garcı´a, J.; Garcı´a, J. I.; Mayoral, J. A.; Proietti, M. G.; Ruiz-Lo´pez, M. F.; Sa´nchez, M. C. J. Org. Chem. 1996, 61, 1636. (16) Sa´nchez, M. C.; Garcı´a, J.; Mayoral, J. A.; Blasco, J.; Proietti, M. G. Physica B 1995, 208, 702. (17) Sa´nchez, M. C.; Garcı´a, J.; Mayoral, J. A.; Blasco, J.; Proietti, M. G. J. Mol. Catal. 1994, 92, 311. (18) Sankar, G.; Rey, F.; Thomas, J. M.; Greaves, G. N.; Corma, A.; Dobson, B. R.; Dent, A. J. J. Chem. Soc., Chem. Commun. 1994, 2279. (19) (a) Greegor, R. B.; Lythe, F. W.; Sandstrom, D. R.; Wong, J.; Schultz, P. J. Non-Cryst. Solids 1983, 55, 27. (b) Wong, J.; Lythe, F. W.; Messmer, R. P.; Maylotte, D. H. Phys. ReV. B 1984, 30, 5596.
JP961487U
