5430
J . Phys. Chem. 1992,96, 5430-5434
Structural Consideration of Antimony Pentafluoride Deposited on Metal Oxides (SiO,, Ai20s, and Si02-A1,03) by X-ray Absorption (EXAFSIXANES) Spectroscopy Kohki Ebitani, Hideshi Hattori,* Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan
and Tsunehiro Tanaka Department of Hydrocarbon Chemistry and Division of Molecular Engineering, Kyoto University, Kyoto 606-01, Japan (Received: August 27, 1991)
The structural environments around antimony cation in the antimony pentafluoride (SbF,) deposited on the metal oxide (SO2, A1203,and Si02-A1203) were investigated by means of X-ray absorption spectroscopy (EXAFSIXANES) in order to elucidate the mechanism for generation of the strong acid sites on the metal oxides. The antimony K-edge XANES spectra and Fourier transforms (FTs) of k3-weighted EXAFS oscillation of SbF, deposited on the metal oxides at 0 "Care similar to that of liquid SbF,, indicating that the 6-fold coordination of SbF, was retained after the deposition on the metal oxides. However, the curve-fitting analysis of the inverse F T s reveals that the Sb-ligand bond lengths for SbF, on the metal oxides are different from that of SbF, and depend on the type of metal oxide on which SbF, was deposited. For Si02and A1203, the changes in the Sb-ligand bond lengths may be due to distortion of the geometry of the SbF, molecules. For Si02-A1203, the Sb-ligand bond lengths become larger with the increase in the ionic character of the Sb-ligand bonds. On Si02-A1203, the Sb-ligand bond length gradually increases with increase in the deposition temperature up to 300 OC. The electronic interaction between the SbF, molecule and metal oxides is discussed on the basis of the obtained structural parameters of SbF, deposited on metal oxides together with the acidic properties reported previously.
Introduction Liquid super acids such as antimony pentafluoride-fluorosulfuric acids and antimony pentafluoridmulfur dioxide systems have been developed as solvent media for generating stable carbonium ions due to its high acidity and low nucleophilicity.lJ Besides the generating carbonium ions, the liquid super acids exhibit the catalytic activity for the acid-catalyzed reaction such as skeletal isomerization of alkanes a t low temperatures, which is initiated by the addition of proton to alkane^.^-^ Stimulated by the success in the liquid super acids, a number of attempts have been made to prepare the solid super acids because of the great advantages of heterogeneous catalysts. Nafion-H,6 aluminum halides with metal sulfate,' SbF,-treated metal oxides,*-I0 sulfur-treated zirconium oxide,11-13and SbF,/ graphiteI4 have been presented as solid super acids. In mast cases, the modification of the surface states of the solid through the adsorption of molecules results in enhancement of surface acidic properties. In particular, the adsorption of SbFS onto the metal oxides results in the generation of the strong acid sites which catalyze reactions of various alkanes, Le., isomerization and cracking of cyclohexane, hexane, pentane, and butane at room temperature.1° Among the SbF,-treated metal oxides, the SbF,-treated SOzA1203 (SbFS-SiO2-Al2O3) shows the highest acid strengths (-13.75 > Ho > -14.52) which was determined by the indicator method. The characteristic feature of the SbF5-SiO2-Al2O, catalyst is that the type of acid sites (Bronsted or Lewis acid sites) can be controlled by changing the SbF, deposition temperature. By high-temperature deposition (at 300 "C), the acid type is exclusively Lewis, whereas both acid sites are present on SbF,-SiO2-Al2O3 prepared by low-temperature deposition (20-100 OC), which was evidenced by infrared spectroscopic study of adsorbed pyridine. Tracer studies using deuterium revealed that hydrogen atoms presents on the fresh surface of the SbF,-Si02-A1203, prepared by deposition at 300 OC, do not participate in the alkane reactions. Thus, it was concluded that the strong Lewis acid sites, which were created by SbF, adsorption at 300 "C,are the active sites for alkane reactions via H- subtraction from alkanes.I0 The catalytic conversions of alkanes, the reaction mechanism, and the surface acidic properties (by infrared study of adsorbed pyridine, Hammett indicator method) of these SbF,-treated solid
super acids have been well demonstrated.I0 However, not much attention has been paid to the investigation of the structure of the modifier (SbFS)and the mechanism for the generation of the strong acid sites. Since the adsorption states on solid surface are considered to have a different structure from that of the corresponding bulk one, the long-range structure would not be expected. X-ray absorption spectroscopy (EXAFS/XANES) is particularly suitable to study the near-neighbor (short-range) en~ironment'~*'~ and the structural symmetryl7-l9 around the absorbing atom. The present work represents the structural consideration of the antimony pentafluoride (SbFS) complex deposited on the metal oxides (Si02, A1203, and SiOZ-AlzO3) by means of EXAFS/XANES techniques. The interactions between the SbF, complex and the metal oxides are discussed on the basis of the structural parameters of the S b cation together with the acidic and catalytic properties reported previousIy.*-l0
Experimental Section Sample Preparation. The SbFSwas deposited on three kinds of metal oxides ( S O 2 , A1203, and SiO2-AI2O3). Silica was prepared by hydrolysis of an aqueous solution of ethyl orthosilicate with aqueous ammonia, followed by washing with deionized water and calcination at 500 OC in air. Alumina was AE-I 1 purchased from Nishio Ind. Co., Ltd. Silicaalumina was N 631L obtained from Nikki Chemicals Co., Ltd. The metal oxides were outgassed at 500 OC prior to exposure to SbF, vapor. The evacuated metal oxide was exposed to gaseous SbF, with the vapor pressure of SbFS(liquid with high viscosity) at 0 OC and then evacuated a t the same temperature to remove excess SbF, on the surface. For the SO2-A1203 sample, the SbF, deposition-evacuation temperature was varied from 0 OC to 300 "C in 100-deg increments. All treatments were performed in the in situ EXAFS Pyrex glass cell with a suitable optical path length. X-ray Absorption. The antimony K-edge X-ray absorption spectra were obtained in a transmission mode at EXAFS facilities installed on BL-1OB line of the Photon Factory at National Laboratory for High Energy Physics (KEK-PF), Tsukuba, Japan, with a ring energy of 2.5 GeV and storage current 185-250 mA. Si(311) channel-cut monochromator was used. The X-ray beam height was 1.0 mm and the distance from the X-ray source is 25 m, corresponding to the resolution of 10 eV without considering
0022-365419212096-5430$03.00/00 1992 American Chemical Society
X-ray Absorption (EXAFSIXANES) Spectroscopy
The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5431
TABLE I: Results of Curve-Fitting Analyses for Sb-F and/or Sb-0 Shells of Reference Compounds curve-fitting results compd SbF, Sb203
crystallographic data"
N
RIA
N
RIA
2.5 2.5
1.89 1.98
3 3
1.88 2.00
w 6 M
.-
..$
"Thcse data are taken from ref 25.
I I
x
I I
I
0 0
1
2
3
4
5
6
0
1
2
3
4
5
6
liL
I
4
2
00
/A
1
2
3
4
5
6
Distance Distance /A Figure 2. Fourier transforms of k3-weighted antimony K-edge EXAFS. (a) SbF,, (b) SbFSdeposited on the S i 0 2 at 0 "C, (c) SbF, deposited on AI2O3at 0 OC, and (d) SbFS deposited on Si02-A1203 at 0 OC.
0
50
100
0
50
100
Photon energy /eV Figure 1. Near edge structure of antimony on the S b K-edge. (a) SbF,, (b) SbF3, (c) Sb203. (d) SbF, deposited on S i 0 2 at 0 OC, (e) SbFS deposited on A1203at 0 O C , (f)SbFs deposited on SiO2-AI2O3 at 0 "C, and (9) SbF, deposited on Si02-A1203at 300 OC. Energy offset is taken to be 30045 eV.
the size of X-ray source. Energy calibration was done with Ag K-edge absorption (25 514 eV). Data Analysis, The oscillatory part of absorption coefficient as a function of the X-ray photon energy (E) was extracted as described elsewhere.z0 Normalization of EXAFS oscillation was performed by fitting the background absorption coefficient (po) around energy region 35-50 eV higher than the absorption edge with the smoothed absorption of an isolated atom (McMaster type function, CE-2.75zl), This normalization methodzz has been reported previou~ly.~~ Fourier transformation of k3-weighted normalized EXAFS oscillation was performed with the 3.8 < k < 14.0 A-1 range to obtain the radial structure function.24 The N (coordination number of scatterers), R (distance between an absorbing atom and scatterers), and Debye-Waller factor were estimated by curvefitting analysis with the inverse FT (0.6 < R
