- Email: [email protected]
Al2O3 metathesis catalysts
Applied Catalysis A: General 167 (1998) 247±256
Evidences for the presence of aluminium perrhenate at the surface of Re2O7/Al2O3 metathesis catalysts F. Schekler-Nahama1*, O. Clause, D. Commereuc, J. Saussey2 Kinetics and Catalysis Division, Institut Franc,ais du PeÂtrole, BP 311, 92506 Rueil-Malmaison Cedex, France Received 17 June 1997; received in revised form 14 October 1997; accepted 15 October 1997
Abstract The nature of the rhenium species present in the Re2O7/Al2O3 metathesis catalysts was investigated by chemical extraction and leaching using various organic salt solutions and water. By means of organic salt solutions, rhenium is extracted as perrhenate salts, and by means of water, rhenium and aluminium are extracted together with a (Re/Al) molar ratio of 3. This result is a ®rst indication for the presence of Al(ReO4)3 (or a decomposition product of Al(ReO4)3) in Re2O7/Al2O3 catalysts. Furthermore, using IR spectroscopy, we pointed out a 1320 cmÿ1 band characteristic of ammonia coordinated on aluminium perrhenate which corroborates our results. We showed that aluminium perrhenate mixed with silica is active in the metathesis reaction. # 1998 Elsevier Science B.V. Keywords: Aluminium perrhenate; Re2O7/Al2O3 metathesis catalyst
1. Introduction In supported heterogeneous catalysts the catalytically active metal can interact with the support material in different ways. One type of interaction involves a solid state reaction between the metal and the support resulting in the formation of a stoichiometric compound. MoO3/Al2O3 and WO3/Al2O3 are catalysts in which compound formation has been demonstrated *Corresponding author. Fax: 00 33 3 20 43 65 01; E-mail: [email protected] 1 Present address: Universite des Sciences et Technologies de Lille, Laboratoire de Catalyse HeÂteÂrogeÁne et HomogeÁne, URACNRS 402 BaÃt. C3, 59655 Villeneuve d'Ascq Cedex, France. 2 Present address: UMR.CNRS 6506-ISMRA, Laboratoire Catalyse et Spectrochimie, 6 boulevard du MareÂchal Juin, 14050 Caen Cedex, France. 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(97)00310-4
[1±3]. For MoO3/Al2O3 samples at high molybdenum content, evidence has been presented for an Al2(MoO4)3 phase [2]. The Laser Raman spectra of all Re2O7/Al2O3, MoO3/Al2O3 and WO3/Al2O3 metathesis catalysts show a broad band around 960 cmÿ1. In this respect Re2O7/Al2O3 catalysts are similar to MoO3/Al2O3 and WO3/Al2O3 catalysts [4]. Since nearly all metal cations form stable perrhenates [5], the formation of aluminium perrhenate Al(ReO4)3 in Re2O7/Al2O3 catalysts is a real possibility. Despite these facts, the possibility of Al(ReO4)3 formation in Re2O7/Al2O3 catalysts has received very little consideration [1]. Usual physico±chemical technologies (X-ray diffraction, transmission electron microscopy) indicated the absence of Re2O7 crystals on the Re2O7/Al2O3 catalyst surface [6±8]. This is not surprising since the
248
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
sublimation temperature of Re2O7 is 2008C, which is far below the calcination temperature required for the activation of the catalyst. We need other methods of approach to characterise the Al±Re mixed surface compound formed during the calcination step. In this study, we report new ways to investigate the Re2O7/Al2O3 catalyst surface. On the one hand, we have tried to extract rhenium from the catalyst surface by organic salt solutions and water. On the other hand, we have studied the surface acidity of Re2O7/Al2O3 by infrared spectroscopy of adsorbed ammonia, which has never been reported before. 2. Experimental 2.1. Preparation of the catalysts Catalysts with different rhenium contents were prepared by incipient wetness impregnation method, using aqueous solutions of commercially available HReO4. The support was g-alumina with a surface area of 195 m2/g. After drying in an oven at 393 K, the support was calcined at 823 K under air atmosphere for 2 h and then cooled in air. Aluminium perrhenate has been synthesised by heating Re2O7 together with dehydrated Al2O3 at 773 K according to the following pathway [9] : Reextr %
acetonitrile and dichloromethane stored over 4A molecular sieves) was placed in a 100 ml ¯ask along with a coated stirring bar. All the experiments were performed under a dry nitrogen atmosphere, at room temperature. The suspension was stirred for 3 h. The suspension was ®ltered on ®lter paper to separate the catalyst from the ®ltrate. The Re and Al concentrations in the ®ltrates were determined by inductively coupled plasma emission spectroscopy (ICPES). 2.2.2. By ionic solutions We followed the same procedure for ReOÿ 4 extraction by cation or anion organic salt solutions. The molar ratio was (salt/Re)1.1. We studied the extraction with solutions of bistriphenylphosphoranylidene ammonium chloride (PPNClÿ), tetraphenylammonium chloride (Ph4NClÿ), tetraethylammonium chloride (Et4NClÿ), pyridinium chloride (PyClÿ), and sodium tetraphenylborate (NaPh4Bÿ). All these salts were dissolved in a convenient solvent. After ®ltration, the catalyst was washed 3 times with 25 ml of the solvent. The catalyst is then dried in an oven at 1208C, to eliminate the solvent remaining in the pore volume. All the ®ltrates were evaporated at rotavapor. The obtained salts were separated, and then analysed for C, H and N. The extracted rhenium was calculated by monitoring the changes of the Re content on the alumina :
Re on the catalyst before extraction ÿ Re on the catalyst after extraction 100 Re on the catalyst before extraction
4
3Re2 O7 Al2 O3 ! Al2 O3 450 C
The Re2O7±Al2O3 mixture (0.33 g of air calcined alumina and 4.75 g of Re2O7) was done in the glove box. 2.2. ReOÿ 4 extraction experiments 2.2.1. By donor solvents The catalysts were activated by calcination at 823 K (with a temperature ramp of 5 K/min) under dry air ¯ow, and then cooled in dry nitrogen ¯ow. 2 g of Re2O7/Al2O3 catalysts (7.9 and 13.7 wt. % Re) and 50 ml of a donor solvent (distilled tetrahydrofuran,
2.2.3. By water We operated in the same conditions as described previously, the extraction solution being distilled water (pH6). When washing was stopped, the suspension was ®ltered to separate the catalyst from the ®ltrate. The ®lter used was a 20 nm pore size membrane. The extracted rhenium was calculated by measuring the rhenium in the water solution after extraction by inductively coupled plasma emission spectroscopy (ICPES). The rate of extracted rhenium was measured as the ratio : Reextr %
Re in the extraction solution=gcata 100 Re on the catalyst before extraction
The aluminium content was measured by ICPES.
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
Blanks were conducted in order to determine the alumina solubility and the ®nal pH of the washing solution as a function of time. 2.3. IR spectroscopy: in situ static infrared cell In situ Fourier Transform Infrared measurements were carried out on a NICOLET MAGNA-750 Instrument. The infrared spectra were recorded at room temperature. The sample was pressed into self-supporting disks 16 mm in diameter 0.2 mm thick, weighing 20 mg. The catalyst was activated at 823 K (with a ramp temperature of 5K/min) for 2 h under dry oxygen atmosphere. Regular replacements of oxygen were performed to remove desorbed water. The sample is then cooled in 45 min, and oxygen is evacuated. Dry ammonia (3.102 Pa at equilibrium) was introduced at room temperature, then evacuated under a dynamic vacuum 10ÿ3 Pa at room temperature and at 423 K to eliminate physisorbed species. The desorption of ammonia was analysed after 30 min at 50 K intervals up to 573 K. Subtraction of spectra after ammonia desorption from those obtained before ammonia adsorption evidences bands due to adsorbed ammonia species. All the IR spectra have been normalised to the same amount of solid (10 mg). 3. Results and discussion 3.1. ReOÿ 4 extraction experiments 3.1.1. By donor solvents Re2O7 is known to react with donor solvents to form adducts of the general formula O3ReOReO3(2L) (Lmonodendate ligand site) [10]. Treatments with donor solvents (THF, CH2Cl2, CH3CN) of the air calcined samples of the catalyst indicate that there is no rhenium extracted under the operating conditions. In this respect, no colour change of the extraction solution has been noticed during the experiment. This is a new proof for the absence of Re2O7 crystals on the Re2O7/Al2O3 catalyst surface. 3.1.2. By ionic solutions Among the various hypothesis regarding the nature of the surface species, formation of rhenium com-
249
plexes could be a real possibility. A cationic rhenium complex would result from an electroplilic attack from a carrier proton, whereas an anionic rhenium complex would result from a nucleophilic attack from surface hydroxyl groups. In the following, these two workinghypothesis have been tested. In the assumption of a cationic rhenium complex, we tried to extract rhenium by washing calcined Re2O7/Al2O3 samples by means of a NaPh4Bÿ/ CH3CN solution. ICP analysis shows that no rhenium is extracted. On the basis of this result, it can be concluded that rhenium does not exist under a cationic complex form ReO 3 . However, it is possible to extract rhenium by cationcontaining organic salt solutions. The amount of extraction of rhenium as a function of the number of extractions by PPNClÿ/CH2Cl2 is illustrated in Fig. 1. Between each extraction, 1 g of the Re2O7/ Al2O3 catalyst is collected to quantify the rhenium content. Two rhenium contents (7.9 and 14.9 wt % Re) were studied. First we observe that the rate of rhenium extracted is quite similar (about 50%) during the ®rst extraction for the two rhenium contents studied. The C and H elementary analysis of the residual rhenium salt after separation from the excess of PPN prove that rhenium is extracted as a bistriphenylphosphoranylidene ammonium perrhenate salt (analysis found (calculated for PPN ReOÿ 4 788:0) :C, 54.7 (54.8); H, 4.0 (3.8)). It is important to emphasise that PPNClÿ/ CH2Cl2 extraction is exclusively due to the PPN cation, as previous donor solvent experiments show. On the other hand, as the extracted amount of rhenium increases with the number of extractions, it can be assumed that increasing washings lead to the removal of most of the perrhenate. Consequently, rhenium does not exist as mixed 50% anionic ReOÿ 4 and 50% cationic ReO3 forms as Nakamura et al. suggested [11]. This is supported by the extraction experiments by organic anionic salt solutions. The effect of molar ratio (extraction salt/Re) on the amount of the extracted rhenium was also investigated. The evolution of the extracted rhenium as a function of the molar ratio (extraction salt/Re) is shown in Fig. 2. The amount of extracted rhenium for the two studied rhenium contents is simultaneously increasing with molar ratio (extraction salt/Re). This suggests that there is an equilibrium between the
250
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
Fig. 1. Effect of the increasing number of extraction by PPNClÿ on the amount of extracted rhenium. The amount of the extracted rhenium is calculated on the basis of the initial rhenium content on the Re2O7/Al2O3 catalysts (before the first extraction). The organic salt solution used is PPNClÿ/CH2Cl2, (PPNClÿ/Re)1.1 M. Experiments are conducted under nitrogen atmosphere, at room temperature. The solution was stirred for 3 h.
Fig. 2. Effect of increasing the molar ratio (extraction salt/Re) on the rate of the extracted rhenium. Organic salt solution is PPNClÿ/CH2Cl2, (PPNClÿ/Re) has been studied between 0.6 and 3.0 M. Experiments are conducted under nitrogen atmosphere, at room temperature. The solution was stirred for 3 h.
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
extracted rhenium and the rhenium remaining on the catalyst surface. In further experiments, the proportion of the extracted rhenium, about 50%, is not changed by varying the rhenium content and the steric hindrance of the organic cation (Ph4P, Et4N, Py). Consequently, it can be assumed that the Al±O±Re surface bond strength is the same whatever the rhenium content on the Re2O7/Al2O3 catalyst is. This result is in agreement with a perrhenate homogeneous coverage of the rhenium oxide supported on alumina, as Kapteijn et al. [8], Kerkhof et al. [12], and Turek et al. [13], suggested. Regarding extractions by other organic cations (Ph4P, Et4N, Py), we succeeded in isolating and identifying pyridinium, and tetraphenylphosphonium perrhenate salts. 3.1.3. By water When calcined samples of Re2O7/Al2O3 are put in contact with distilled water, some additional data about the nature of the structure formed are provided. Our main results are gathered in Fig. 3. It is clear that rhenium can be extracted from the Re2O7/Al2O3 catalyst surface by washing with water. As seen in Fig. 3, the amount of extracted rhenium is increased by increasing the initial rhenium content. In addition, the aluminium concentration measurements in the extraction solutions indicate that rhenium is removed together with alumina with a (Re/Al) molar ratio of 3. According to Mulcahy et al. [14], rinsing of catalysts prepared by equilibrium adsorption indeed removes most of the perrhenates deposited by adsorption.
251
Andreev et al. [15] established that after cold water treatment about 35 wt % of Re was extracted. However, the literature data indicate that the extraction of aluminium has never been proven before [14,15]. The ®nal pH of the washing solution is plotted as a function of the contact time on Fig. 4. Blank experiment was checked with alumina. It can be seen that the ®nal pH of the washing solutions of Re2O7/Al2O3 catalysts with 7.9 and 14.9 wt % Re decreases from 6, initial pH value of distilled water, to about 3.5 in the ®rst minute of contacting the catalyst with water. In the blank experiment, the ®nal pH slightly increases from 6 to 7. In fact, it approximates to the isoelectric point of alumina (7.3). Strong differences are observed in aluminium concentrations of the water washings which were found to be about 200 ppm in the case of water washing of Re2O7/Al2O3 14.9 wt % Re, against less than 1 ppm in the case of Al2O3. It is noticeable that the decrease is more important in the case of Re2O7/Al2O3 (14.9 wt % Re) than Re2O7/Al2O3 (7.9 wt % Re) (Fig. 3). After 1 h contact time, ®nal pH and molar ratio (Re/Al) remain constant. It can be concluded that an equilibrium between the extracted rhenium and the rhenium remaining at the catalyst surface is reached within the ®rst hour of extraction. As for extraction by means of organic cationic salt solutions, the extracted rhenium increases by increasing the extraction number, and it can be assumed that most of the rhenium may be removed by this method. We checked this hypothesis by performing a second water washing of an already once washed Re2O7/ Al2O3 catalyst.
Fig. 3. Distilled water washing of various Re2O7/Al2O3 catalysts. Experiments are conducted at room temperature. The solution was stirred for 3 h.
252
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
Fig. 4. Evolution of the final pH as a function of the contact time.
Fig. 5. Two methods to extract rhenium from the Re2O7/Al2O3 catalyst surface.
Based on our results, Fig. 5 visualises the different methods to extract rhenium from the Re2O7/Al2O3 catalyst surface. Contacting with organic cationic salts, rhenium is extracted with the formation of perrhenate salts, whereas contacting with water, rhenium and aluminium are extracted at the same time with a (Re/Al) molar ratio of 3. Excluding the hypothesis of a coverage of the carrier surface with rhenium heptoxide which may be responsible for some acidity of the extraction
solution, and given that perrhenate anion is an anion of a strong acid, the decrease of the ®nal pH observed during rhenium and aluminium extraction can be attributed to the extraction of Al3 ions. Furthermore, the (Re/Al) molar ratio of 3 in the extracts corresponds to the Re/Al stoichiometric ratio in aluminium perrhenate. Given this salt is water soluble [9] and decomposed above 573 K, our results strongly suggest that the surface compound of the Re2O7/Al2O3 calcined materials is a decomposition product from aluminium perrhenate. Besides, it is interesting to notice that Al3 ions included in the surface compound does not get back into alumina, even after calcination at 773 K. 3.2. IR spectroscopy We have studied ammonia adsorption±thermodesorption on the Re2O7/Al2O3 calcined catalyst by IR spectroscopy. Fig. 6 shows the IR spectra of Re2O7/Al2O3 catalysts with various rhenium contents after activation and ammonia adsorption±thermodesorption in vacuum at 423 K. The spectrum of alumina (a) shows a broad and poorly de®ned band centred around
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
253
Fig. 6. Infrared spectra of Re2O7/Al2O3 with varying Re loadings after ammonia adsorption± thermodesorption in vacuum (10ÿ3 Pa) at 423 K. (a) 0.0 wt % Re; (b) 1.8; (c) 4.5; (d) 7.9; (e) 13.7.
Fig. 7. Effect of rhenium content on the intensity of the 1320 cmÿ1 band. n represents the difference between the wavenumber of the 1320 cmÿ1 band and the wavenumber of the 1295 cmÿ1 band.
1280 cmÿ1 and one band at 1623 cmÿ1 corresponding to ammonia coordinated to Lewis acid sites. Besides, it can be noticed that the intensity and the wavenumber of the band at 1295 cmÿ1 for alumina increase on increasing the rhenium content. At 13.7 wt % Re, this band reaches 1340 cmÿ1. Moreover, there is a linear variation between the shift of the 1320 cmÿ1 band and the rhenium content (Fig. 7). The evolution, with desorption temperature, of this `1320 cmÿ1 band' of adsorbed ammonia is reported in Fig. 8. First, all
the spectra possess two bands at 1456 and 1496 cmÿ1 which characterise amide species. Their constant intensity with increasing thermodesorption temperature rules out the possibility of ammonia being protonated to BroÈnsted sites. As regards the 1320 cmÿ1 band, it is decreasing with increasing thermodesorption temperature. This observation has never been reported before. According to KnoÈzinger et al. [16], coordinated ammonia to the Lewis acid sites of alumina is characterised by a 1260±1285 cmÿ1 band. The higher the wavenumber, the stronger the Lewis acidity of alumina. To investigate the origin of this new band, we tested other rhenium systems (Fig. 9). Al(ReO4)3/ SiO2 15 wt % Re was prepared by mechanical mixture of Al(ReO4)3 and SiO2, and activated at 573 K with a temperature ramp of 5 K/min under dry oxygen atmosphere. The calcination temperature has been limited to 573 K, since above 573 K, aluminium perrhenate begins to decompose. As follows from Fig. 9, the band at 1320 cmÿ1 is also found in a mixture of Al(ReO4)3 and SiO2, but not in Re2O7/SiO2, submitted to ammonia adsorption. Then it should be typical of ammonia coordinated to aluminium perrhenate.
254
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
Fig. 8. Evolution, with desorption temperature, of the 1320 cmÿ1 band of adsorbed ammonia on the Re2O7/Al2O3 8.0 wt % Re. T (K) under vacuum (10ÿ3 Pa) (a) 298; (b) 423; (c) 473; (d) 523; (e) 573.
Fig. 9. Desorption spectra at 298 K after ammonia adsorption for various rhenium based systems. (a) SiO2 ; (b) Re2O7/SiO2 15 wt % Re; (c) Al(ReO4)3/SiO2 15 wt % Re.
Furthermore, it can be assumed that this band is to be attributed to a Lewis acidity of a surface aluminium perrhenate on the Re2O7/Al2O3 calcined catalyst. This Lewis acidity originates from the Al3 ions initially
engaged in Al(ReO4)3, the decomposition of which starts above 573 K. On the other hand, this compound seems to generate some BroÈnsted acidity as proven by the 1417 cmÿ1
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
255
Fig. 10. Propylene metathesis in the static mode catalyzed by Al(ReO4)3/SiO2 15 wt % Re: gaseous reactants IR spectra. Evolution of the IR spectra with contact time: (a) 0 min; (b) 15 min; (c) 30 min; (d) 1 h.
band. However, this is only a weak BroÈnsted acidity since ammonium species are entirely decomposed after ammonia thermodesorption at 523 K. Regarding our previous results concerning the occurrence of a decomposition product from aluminium perrhenate on the Re2O7/Al2O3 catalyst surface, we evidence now a new 1320 cmÿ1 band which is characteristic of ammonia coordinated to aluminium perrhenate. 3.3. Activity measurements Now, the question is, does aluminium perrhenate, which is proposed to be present at the Re2O7/Al2O3 catalyst surface, take part in metathesis? To verify the occurrence of a metathesis reaction, Al(ReO4)3/SiO2 has been contacted with 3.103 Pa of propylene gas at 348 K. Fig. 10 shows the evolution of gaseous reactants spectra. The increase of the 950 cmÿ1 !(CH2) band intensity, characteristic of ethylene formation and the decrease of the 912 cmÿ1 !(CH2) band intensity, characteristic of propylene reaction, are observed. Consequently, Al(ReO4)3/SiO2 15 wt % Re is active in metathesis in our experimental conditions, whereas Re2O7/SiO2 does not. In agreement with Banks [17], a Re2O7/ TiO2/SnMe4/AlEt3 catalyst with 4 wt % Re calcined at 823 K for 60 min in ¯owing dry air then for 30 min in
dry nitrogen, is active in metathesis. At 823 K, Al(ReO4)3 is formed [9]. Therefore, aluminium perrhenate or a decomposition product, would be the active species in ole®n metathesis. Thus, the presence of the 1320 cmÿ1 band in rhenium-based systems after NH3 adsorption±thermodesorption seems to be correlated with metathesis activity. It can be assumed that the higher the 1320 cmÿ1 band intensity and wavenumber, the more active the catalyst. 4. Conclusions Up to now, physico±chemical measurements (X-ray diffraction, transmission electron microscopy) did not allow to evidence a mixed Al±Re compound on the Re2O7/Al2O3 catalyst surface. Rhenium extraction experiments from the calcined Re2O7/Al2O3 catalyst surface indicate that rhenium exists in the ReOÿ 4 anion state. Furthermore, the water washings strongly suggest that the surface compound of the calcined Re2O7/ Al2O3 materials is a decomposition product of aluminium perrhenate Using infrared spectroscopy, we pointed out by ammonia adsorption±thermodesorption a band at 1320 cmÿ1 which has never been shown before. The 1320 cmÿ1 band characterises Lewis acidity of
256
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
aluminium perrhenate, probably brought by the Al3 ions of the perrhenate. This result is a second indication of the presence of an aluminium perrhenate (or a decomposition product) on the Re2O7/Al2O3 catalyst surface. For further reading ± Ref. [18] Acknowledgements We express our thanks to Mrs. J. Boussard, N. Dos Santos, E. Leplat, V. Poitrineau and Mr. B. Leze for Xray ¯uorescence and inductively coupled plasma emission spectroscopy, and to Mr. Chauvin for fruitful discussions. References [1] G.T. Pott, W.H.J. Stork, in B. Delmon, P.A. Jacobs, G. Poncelet (Eds.), Preparation of Catalysts, Elsevier, Amsterdam, Netherlands, 1976, p 537. [2] N. Giordano, J.C.J. Bart, A. Vaghi, A. Castellan, G. Martinotti, J. Catal. 36 (1975) 81. [3] G.T. Pott, W.H.J. Stork, J.G.F. Coolegem, J. Catal. 32 (1974) 497.
[4] J.M. Kerkhof, J.A. Moulijn, R. Thomas, J. Catal. 56 (1979) 279. [5] G. Rouschias, Chem. Rev. 74 (1974) 531. [6] P. Arnoldy, E.M. Van Oers, O.S.L. Bruinsma, V.H.J. de Beer, J.A. Moulijn, J. Catal. 93 (1985) 231. [7] A. Ellison, A.K Coverdale, P.F Dearing, J. Mol. Catal. 28 (1985) 141. [8] F. Kapteijn, L.H.G. Bredt, J.C. Mol, Rev. Trav. Chim. PaysBas 96(97) (1977) 139. [9] G. Baud, M. Capestan, Bull. Soc. Chim. Fr. 11 (1966) 3608. [10] W.A. Herrmann, F.P. Kipro, F.E. KuÈhn, W. Scherer, M. Kleine, M. Elison, H.V. Volden, K. Rypdal, S. Gundersen, A. Haaland, Bull. Soc. Chim. Fr. 129 (1992) 655. [11] R. Nakamura, E. Echigoya, F. Abe, Chem. Lett., 51 1981 . [12] J.M. Kerkhof, J.A. Moulijn, J. Phys. Chem. 83(12) (1979) 1612. [13] A.M. Turek, I.E. Wachs, E. De Canio, J. Phys. Chem. 96 (1992) 5000. [14] F.M. Mulcahy, M.J. Fay, A. Proctor, M. Houalla, D.M. Hercules, J. Catal. 124 (1990) 231. [15] A.A. Andreev, R.M. Edreva-Kardjieva, J. Catal. 94 (1985) 97. [16] H. KnoÈzinger, P. Ratnasamy, Catal. Rev. ± Sci. Eng. 17(1) (1978) 31. [17] M.B. Varfolomeev, N.B. Yakovleva, V.E. Plyushchev, Russ. J. Inorg. Chem. 14(1) (1969) 54. [18] R.L. Banks, Phillips Petroleum Company, Brevet ameÂricain, 4 454 368 (1984).
Evidences for the presence of aluminium perrhenate at the surface of Re2O7/Al2O3 metathesis catalysts F. Schekler-Nahama1*, O. Clause, D. Commereuc, J. Saussey2 Kinetics and Catalysis Division, Institut Franc,ais du PeÂtrole, BP 311, 92506 Rueil-Malmaison Cedex, France Received 17 June 1997; received in revised form 14 October 1997; accepted 15 October 1997
Abstract The nature of the rhenium species present in the Re2O7/Al2O3 metathesis catalysts was investigated by chemical extraction and leaching using various organic salt solutions and water. By means of organic salt solutions, rhenium is extracted as perrhenate salts, and by means of water, rhenium and aluminium are extracted together with a (Re/Al) molar ratio of 3. This result is a ®rst indication for the presence of Al(ReO4)3 (or a decomposition product of Al(ReO4)3) in Re2O7/Al2O3 catalysts. Furthermore, using IR spectroscopy, we pointed out a 1320 cmÿ1 band characteristic of ammonia coordinated on aluminium perrhenate which corroborates our results. We showed that aluminium perrhenate mixed with silica is active in the metathesis reaction. # 1998 Elsevier Science B.V. Keywords: Aluminium perrhenate; Re2O7/Al2O3 metathesis catalyst
1. Introduction In supported heterogeneous catalysts the catalytically active metal can interact with the support material in different ways. One type of interaction involves a solid state reaction between the metal and the support resulting in the formation of a stoichiometric compound. MoO3/Al2O3 and WO3/Al2O3 are catalysts in which compound formation has been demonstrated *Corresponding author. Fax: 00 33 3 20 43 65 01; E-mail: [email protected] 1 Present address: Universite des Sciences et Technologies de Lille, Laboratoire de Catalyse HeÂteÂrogeÁne et HomogeÁne, URACNRS 402 BaÃt. C3, 59655 Villeneuve d'Ascq Cedex, France. 2 Present address: UMR.CNRS 6506-ISMRA, Laboratoire Catalyse et Spectrochimie, 6 boulevard du MareÂchal Juin, 14050 Caen Cedex, France. 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(97)00310-4
[1±3]. For MoO3/Al2O3 samples at high molybdenum content, evidence has been presented for an Al2(MoO4)3 phase [2]. The Laser Raman spectra of all Re2O7/Al2O3, MoO3/Al2O3 and WO3/Al2O3 metathesis catalysts show a broad band around 960 cmÿ1. In this respect Re2O7/Al2O3 catalysts are similar to MoO3/Al2O3 and WO3/Al2O3 catalysts [4]. Since nearly all metal cations form stable perrhenates [5], the formation of aluminium perrhenate Al(ReO4)3 in Re2O7/Al2O3 catalysts is a real possibility. Despite these facts, the possibility of Al(ReO4)3 formation in Re2O7/Al2O3 catalysts has received very little consideration [1]. Usual physico±chemical technologies (X-ray diffraction, transmission electron microscopy) indicated the absence of Re2O7 crystals on the Re2O7/Al2O3 catalyst surface [6±8]. This is not surprising since the
248
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
sublimation temperature of Re2O7 is 2008C, which is far below the calcination temperature required for the activation of the catalyst. We need other methods of approach to characterise the Al±Re mixed surface compound formed during the calcination step. In this study, we report new ways to investigate the Re2O7/Al2O3 catalyst surface. On the one hand, we have tried to extract rhenium from the catalyst surface by organic salt solutions and water. On the other hand, we have studied the surface acidity of Re2O7/Al2O3 by infrared spectroscopy of adsorbed ammonia, which has never been reported before. 2. Experimental 2.1. Preparation of the catalysts Catalysts with different rhenium contents were prepared by incipient wetness impregnation method, using aqueous solutions of commercially available HReO4. The support was g-alumina with a surface area of 195 m2/g. After drying in an oven at 393 K, the support was calcined at 823 K under air atmosphere for 2 h and then cooled in air. Aluminium perrhenate has been synthesised by heating Re2O7 together with dehydrated Al2O3 at 773 K according to the following pathway [9] : Reextr %
acetonitrile and dichloromethane stored over 4A molecular sieves) was placed in a 100 ml ¯ask along with a coated stirring bar. All the experiments were performed under a dry nitrogen atmosphere, at room temperature. The suspension was stirred for 3 h. The suspension was ®ltered on ®lter paper to separate the catalyst from the ®ltrate. The Re and Al concentrations in the ®ltrates were determined by inductively coupled plasma emission spectroscopy (ICPES). 2.2.2. By ionic solutions We followed the same procedure for ReOÿ 4 extraction by cation or anion organic salt solutions. The molar ratio was (salt/Re)1.1. We studied the extraction with solutions of bistriphenylphosphoranylidene ammonium chloride (PPNClÿ), tetraphenylammonium chloride (Ph4NClÿ), tetraethylammonium chloride (Et4NClÿ), pyridinium chloride (PyClÿ), and sodium tetraphenylborate (NaPh4Bÿ). All these salts were dissolved in a convenient solvent. After ®ltration, the catalyst was washed 3 times with 25 ml of the solvent. The catalyst is then dried in an oven at 1208C, to eliminate the solvent remaining in the pore volume. All the ®ltrates were evaporated at rotavapor. The obtained salts were separated, and then analysed for C, H and N. The extracted rhenium was calculated by monitoring the changes of the Re content on the alumina :
Re on the catalyst before extraction ÿ Re on the catalyst after extraction 100 Re on the catalyst before extraction
4
3Re2 O7 Al2 O3 ! Al2 O3 450 C
The Re2O7±Al2O3 mixture (0.33 g of air calcined alumina and 4.75 g of Re2O7) was done in the glove box. 2.2. ReOÿ 4 extraction experiments 2.2.1. By donor solvents The catalysts were activated by calcination at 823 K (with a temperature ramp of 5 K/min) under dry air ¯ow, and then cooled in dry nitrogen ¯ow. 2 g of Re2O7/Al2O3 catalysts (7.9 and 13.7 wt. % Re) and 50 ml of a donor solvent (distilled tetrahydrofuran,
2.2.3. By water We operated in the same conditions as described previously, the extraction solution being distilled water (pH6). When washing was stopped, the suspension was ®ltered to separate the catalyst from the ®ltrate. The ®lter used was a 20 nm pore size membrane. The extracted rhenium was calculated by measuring the rhenium in the water solution after extraction by inductively coupled plasma emission spectroscopy (ICPES). The rate of extracted rhenium was measured as the ratio : Reextr %
Re in the extraction solution=gcata 100 Re on the catalyst before extraction
The aluminium content was measured by ICPES.
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
Blanks were conducted in order to determine the alumina solubility and the ®nal pH of the washing solution as a function of time. 2.3. IR spectroscopy: in situ static infrared cell In situ Fourier Transform Infrared measurements were carried out on a NICOLET MAGNA-750 Instrument. The infrared spectra were recorded at room temperature. The sample was pressed into self-supporting disks 16 mm in diameter 0.2 mm thick, weighing 20 mg. The catalyst was activated at 823 K (with a ramp temperature of 5K/min) for 2 h under dry oxygen atmosphere. Regular replacements of oxygen were performed to remove desorbed water. The sample is then cooled in 45 min, and oxygen is evacuated. Dry ammonia (3.102 Pa at equilibrium) was introduced at room temperature, then evacuated under a dynamic vacuum 10ÿ3 Pa at room temperature and at 423 K to eliminate physisorbed species. The desorption of ammonia was analysed after 30 min at 50 K intervals up to 573 K. Subtraction of spectra after ammonia desorption from those obtained before ammonia adsorption evidences bands due to adsorbed ammonia species. All the IR spectra have been normalised to the same amount of solid (10 mg). 3. Results and discussion 3.1. ReOÿ 4 extraction experiments 3.1.1. By donor solvents Re2O7 is known to react with donor solvents to form adducts of the general formula O3ReOReO3(2L) (Lmonodendate ligand site) [10]. Treatments with donor solvents (THF, CH2Cl2, CH3CN) of the air calcined samples of the catalyst indicate that there is no rhenium extracted under the operating conditions. In this respect, no colour change of the extraction solution has been noticed during the experiment. This is a new proof for the absence of Re2O7 crystals on the Re2O7/Al2O3 catalyst surface. 3.1.2. By ionic solutions Among the various hypothesis regarding the nature of the surface species, formation of rhenium com-
249
plexes could be a real possibility. A cationic rhenium complex would result from an electroplilic attack from a carrier proton, whereas an anionic rhenium complex would result from a nucleophilic attack from surface hydroxyl groups. In the following, these two workinghypothesis have been tested. In the assumption of a cationic rhenium complex, we tried to extract rhenium by washing calcined Re2O7/Al2O3 samples by means of a NaPh4Bÿ/ CH3CN solution. ICP analysis shows that no rhenium is extracted. On the basis of this result, it can be concluded that rhenium does not exist under a cationic complex form ReO 3 . However, it is possible to extract rhenium by cationcontaining organic salt solutions. The amount of extraction of rhenium as a function of the number of extractions by PPNClÿ/CH2Cl2 is illustrated in Fig. 1. Between each extraction, 1 g of the Re2O7/ Al2O3 catalyst is collected to quantify the rhenium content. Two rhenium contents (7.9 and 14.9 wt % Re) were studied. First we observe that the rate of rhenium extracted is quite similar (about 50%) during the ®rst extraction for the two rhenium contents studied. The C and H elementary analysis of the residual rhenium salt after separation from the excess of PPN prove that rhenium is extracted as a bistriphenylphosphoranylidene ammonium perrhenate salt (analysis found (calculated for PPN ReOÿ 4 788:0) :C, 54.7 (54.8); H, 4.0 (3.8)). It is important to emphasise that PPNClÿ/ CH2Cl2 extraction is exclusively due to the PPN cation, as previous donor solvent experiments show. On the other hand, as the extracted amount of rhenium increases with the number of extractions, it can be assumed that increasing washings lead to the removal of most of the perrhenate. Consequently, rhenium does not exist as mixed 50% anionic ReOÿ 4 and 50% cationic ReO3 forms as Nakamura et al. suggested [11]. This is supported by the extraction experiments by organic anionic salt solutions. The effect of molar ratio (extraction salt/Re) on the amount of the extracted rhenium was also investigated. The evolution of the extracted rhenium as a function of the molar ratio (extraction salt/Re) is shown in Fig. 2. The amount of extracted rhenium for the two studied rhenium contents is simultaneously increasing with molar ratio (extraction salt/Re). This suggests that there is an equilibrium between the
250
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
Fig. 1. Effect of the increasing number of extraction by PPNClÿ on the amount of extracted rhenium. The amount of the extracted rhenium is calculated on the basis of the initial rhenium content on the Re2O7/Al2O3 catalysts (before the first extraction). The organic salt solution used is PPNClÿ/CH2Cl2, (PPNClÿ/Re)1.1 M. Experiments are conducted under nitrogen atmosphere, at room temperature. The solution was stirred for 3 h.
Fig. 2. Effect of increasing the molar ratio (extraction salt/Re) on the rate of the extracted rhenium. Organic salt solution is PPNClÿ/CH2Cl2, (PPNClÿ/Re) has been studied between 0.6 and 3.0 M. Experiments are conducted under nitrogen atmosphere, at room temperature. The solution was stirred for 3 h.
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
extracted rhenium and the rhenium remaining on the catalyst surface. In further experiments, the proportion of the extracted rhenium, about 50%, is not changed by varying the rhenium content and the steric hindrance of the organic cation (Ph4P, Et4N, Py). Consequently, it can be assumed that the Al±O±Re surface bond strength is the same whatever the rhenium content on the Re2O7/Al2O3 catalyst is. This result is in agreement with a perrhenate homogeneous coverage of the rhenium oxide supported on alumina, as Kapteijn et al. [8], Kerkhof et al. [12], and Turek et al. [13], suggested. Regarding extractions by other organic cations (Ph4P, Et4N, Py), we succeeded in isolating and identifying pyridinium, and tetraphenylphosphonium perrhenate salts. 3.1.3. By water When calcined samples of Re2O7/Al2O3 are put in contact with distilled water, some additional data about the nature of the structure formed are provided. Our main results are gathered in Fig. 3. It is clear that rhenium can be extracted from the Re2O7/Al2O3 catalyst surface by washing with water. As seen in Fig. 3, the amount of extracted rhenium is increased by increasing the initial rhenium content. In addition, the aluminium concentration measurements in the extraction solutions indicate that rhenium is removed together with alumina with a (Re/Al) molar ratio of 3. According to Mulcahy et al. [14], rinsing of catalysts prepared by equilibrium adsorption indeed removes most of the perrhenates deposited by adsorption.
251
Andreev et al. [15] established that after cold water treatment about 35 wt % of Re was extracted. However, the literature data indicate that the extraction of aluminium has never been proven before [14,15]. The ®nal pH of the washing solution is plotted as a function of the contact time on Fig. 4. Blank experiment was checked with alumina. It can be seen that the ®nal pH of the washing solutions of Re2O7/Al2O3 catalysts with 7.9 and 14.9 wt % Re decreases from 6, initial pH value of distilled water, to about 3.5 in the ®rst minute of contacting the catalyst with water. In the blank experiment, the ®nal pH slightly increases from 6 to 7. In fact, it approximates to the isoelectric point of alumina (7.3). Strong differences are observed in aluminium concentrations of the water washings which were found to be about 200 ppm in the case of water washing of Re2O7/Al2O3 14.9 wt % Re, against less than 1 ppm in the case of Al2O3. It is noticeable that the decrease is more important in the case of Re2O7/Al2O3 (14.9 wt % Re) than Re2O7/Al2O3 (7.9 wt % Re) (Fig. 3). After 1 h contact time, ®nal pH and molar ratio (Re/Al) remain constant. It can be concluded that an equilibrium between the extracted rhenium and the rhenium remaining at the catalyst surface is reached within the ®rst hour of extraction. As for extraction by means of organic cationic salt solutions, the extracted rhenium increases by increasing the extraction number, and it can be assumed that most of the rhenium may be removed by this method. We checked this hypothesis by performing a second water washing of an already once washed Re2O7/ Al2O3 catalyst.
Fig. 3. Distilled water washing of various Re2O7/Al2O3 catalysts. Experiments are conducted at room temperature. The solution was stirred for 3 h.
252
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
Fig. 4. Evolution of the final pH as a function of the contact time.
Fig. 5. Two methods to extract rhenium from the Re2O7/Al2O3 catalyst surface.
Based on our results, Fig. 5 visualises the different methods to extract rhenium from the Re2O7/Al2O3 catalyst surface. Contacting with organic cationic salts, rhenium is extracted with the formation of perrhenate salts, whereas contacting with water, rhenium and aluminium are extracted at the same time with a (Re/Al) molar ratio of 3. Excluding the hypothesis of a coverage of the carrier surface with rhenium heptoxide which may be responsible for some acidity of the extraction
solution, and given that perrhenate anion is an anion of a strong acid, the decrease of the ®nal pH observed during rhenium and aluminium extraction can be attributed to the extraction of Al3 ions. Furthermore, the (Re/Al) molar ratio of 3 in the extracts corresponds to the Re/Al stoichiometric ratio in aluminium perrhenate. Given this salt is water soluble [9] and decomposed above 573 K, our results strongly suggest that the surface compound of the Re2O7/Al2O3 calcined materials is a decomposition product from aluminium perrhenate. Besides, it is interesting to notice that Al3 ions included in the surface compound does not get back into alumina, even after calcination at 773 K. 3.2. IR spectroscopy We have studied ammonia adsorption±thermodesorption on the Re2O7/Al2O3 calcined catalyst by IR spectroscopy. Fig. 6 shows the IR spectra of Re2O7/Al2O3 catalysts with various rhenium contents after activation and ammonia adsorption±thermodesorption in vacuum at 423 K. The spectrum of alumina (a) shows a broad and poorly de®ned band centred around
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
253
Fig. 6. Infrared spectra of Re2O7/Al2O3 with varying Re loadings after ammonia adsorption± thermodesorption in vacuum (10ÿ3 Pa) at 423 K. (a) 0.0 wt % Re; (b) 1.8; (c) 4.5; (d) 7.9; (e) 13.7.
Fig. 7. Effect of rhenium content on the intensity of the 1320 cmÿ1 band. n represents the difference between the wavenumber of the 1320 cmÿ1 band and the wavenumber of the 1295 cmÿ1 band.
1280 cmÿ1 and one band at 1623 cmÿ1 corresponding to ammonia coordinated to Lewis acid sites. Besides, it can be noticed that the intensity and the wavenumber of the band at 1295 cmÿ1 for alumina increase on increasing the rhenium content. At 13.7 wt % Re, this band reaches 1340 cmÿ1. Moreover, there is a linear variation between the shift of the 1320 cmÿ1 band and the rhenium content (Fig. 7). The evolution, with desorption temperature, of this `1320 cmÿ1 band' of adsorbed ammonia is reported in Fig. 8. First, all
the spectra possess two bands at 1456 and 1496 cmÿ1 which characterise amide species. Their constant intensity with increasing thermodesorption temperature rules out the possibility of ammonia being protonated to BroÈnsted sites. As regards the 1320 cmÿ1 band, it is decreasing with increasing thermodesorption temperature. This observation has never been reported before. According to KnoÈzinger et al. [16], coordinated ammonia to the Lewis acid sites of alumina is characterised by a 1260±1285 cmÿ1 band. The higher the wavenumber, the stronger the Lewis acidity of alumina. To investigate the origin of this new band, we tested other rhenium systems (Fig. 9). Al(ReO4)3/ SiO2 15 wt % Re was prepared by mechanical mixture of Al(ReO4)3 and SiO2, and activated at 573 K with a temperature ramp of 5 K/min under dry oxygen atmosphere. The calcination temperature has been limited to 573 K, since above 573 K, aluminium perrhenate begins to decompose. As follows from Fig. 9, the band at 1320 cmÿ1 is also found in a mixture of Al(ReO4)3 and SiO2, but not in Re2O7/SiO2, submitted to ammonia adsorption. Then it should be typical of ammonia coordinated to aluminium perrhenate.
254
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
Fig. 8. Evolution, with desorption temperature, of the 1320 cmÿ1 band of adsorbed ammonia on the Re2O7/Al2O3 8.0 wt % Re. T (K) under vacuum (10ÿ3 Pa) (a) 298; (b) 423; (c) 473; (d) 523; (e) 573.
Fig. 9. Desorption spectra at 298 K after ammonia adsorption for various rhenium based systems. (a) SiO2 ; (b) Re2O7/SiO2 15 wt % Re; (c) Al(ReO4)3/SiO2 15 wt % Re.
Furthermore, it can be assumed that this band is to be attributed to a Lewis acidity of a surface aluminium perrhenate on the Re2O7/Al2O3 calcined catalyst. This Lewis acidity originates from the Al3 ions initially
engaged in Al(ReO4)3, the decomposition of which starts above 573 K. On the other hand, this compound seems to generate some BroÈnsted acidity as proven by the 1417 cmÿ1
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
255
Fig. 10. Propylene metathesis in the static mode catalyzed by Al(ReO4)3/SiO2 15 wt % Re: gaseous reactants IR spectra. Evolution of the IR spectra with contact time: (a) 0 min; (b) 15 min; (c) 30 min; (d) 1 h.
band. However, this is only a weak BroÈnsted acidity since ammonium species are entirely decomposed after ammonia thermodesorption at 523 K. Regarding our previous results concerning the occurrence of a decomposition product from aluminium perrhenate on the Re2O7/Al2O3 catalyst surface, we evidence now a new 1320 cmÿ1 band which is characteristic of ammonia coordinated to aluminium perrhenate. 3.3. Activity measurements Now, the question is, does aluminium perrhenate, which is proposed to be present at the Re2O7/Al2O3 catalyst surface, take part in metathesis? To verify the occurrence of a metathesis reaction, Al(ReO4)3/SiO2 has been contacted with 3.103 Pa of propylene gas at 348 K. Fig. 10 shows the evolution of gaseous reactants spectra. The increase of the 950 cmÿ1 !(CH2) band intensity, characteristic of ethylene formation and the decrease of the 912 cmÿ1 !(CH2) band intensity, characteristic of propylene reaction, are observed. Consequently, Al(ReO4)3/SiO2 15 wt % Re is active in metathesis in our experimental conditions, whereas Re2O7/SiO2 does not. In agreement with Banks [17], a Re2O7/ TiO2/SnMe4/AlEt3 catalyst with 4 wt % Re calcined at 823 K for 60 min in ¯owing dry air then for 30 min in
dry nitrogen, is active in metathesis. At 823 K, Al(ReO4)3 is formed [9]. Therefore, aluminium perrhenate or a decomposition product, would be the active species in ole®n metathesis. Thus, the presence of the 1320 cmÿ1 band in rhenium-based systems after NH3 adsorption±thermodesorption seems to be correlated with metathesis activity. It can be assumed that the higher the 1320 cmÿ1 band intensity and wavenumber, the more active the catalyst. 4. Conclusions Up to now, physico±chemical measurements (X-ray diffraction, transmission electron microscopy) did not allow to evidence a mixed Al±Re compound on the Re2O7/Al2O3 catalyst surface. Rhenium extraction experiments from the calcined Re2O7/Al2O3 catalyst surface indicate that rhenium exists in the ReOÿ 4 anion state. Furthermore, the water washings strongly suggest that the surface compound of the calcined Re2O7/ Al2O3 materials is a decomposition product of aluminium perrhenate Using infrared spectroscopy, we pointed out by ammonia adsorption±thermodesorption a band at 1320 cmÿ1 which has never been shown before. The 1320 cmÿ1 band characterises Lewis acidity of
256
F. Schekler-Nahama et al. / Applied Catalysis A: General 167 (1998) 247±256
aluminium perrhenate, probably brought by the Al3 ions of the perrhenate. This result is a second indication of the presence of an aluminium perrhenate (or a decomposition product) on the Re2O7/Al2O3 catalyst surface. For further reading ± Ref. [18] Acknowledgements We express our thanks to Mrs. J. Boussard, N. Dos Santos, E. Leplat, V. Poitrineau and Mr. B. Leze for Xray ¯uorescence and inductively coupled plasma emission spectroscopy, and to Mr. Chauvin for fruitful discussions. References [1] G.T. Pott, W.H.J. Stork, in B. Delmon, P.A. Jacobs, G. Poncelet (Eds.), Preparation of Catalysts, Elsevier, Amsterdam, Netherlands, 1976, p 537. [2] N. Giordano, J.C.J. Bart, A. Vaghi, A. Castellan, G. Martinotti, J. Catal. 36 (1975) 81. [3] G.T. Pott, W.H.J. Stork, J.G.F. Coolegem, J. Catal. 32 (1974) 497.
[4] J.M. Kerkhof, J.A. Moulijn, R. Thomas, J. Catal. 56 (1979) 279. [5] G. Rouschias, Chem. Rev. 74 (1974) 531. [6] P. Arnoldy, E.M. Van Oers, O.S.L. Bruinsma, V.H.J. de Beer, J.A. Moulijn, J. Catal. 93 (1985) 231. [7] A. Ellison, A.K Coverdale, P.F Dearing, J. Mol. Catal. 28 (1985) 141. [8] F. Kapteijn, L.H.G. Bredt, J.C. Mol, Rev. Trav. Chim. PaysBas 96(97) (1977) 139. [9] G. Baud, M. Capestan, Bull. Soc. Chim. Fr. 11 (1966) 3608. [10] W.A. Herrmann, F.P. Kipro, F.E. KuÈhn, W. Scherer, M. Kleine, M. Elison, H.V. Volden, K. Rypdal, S. Gundersen, A. Haaland, Bull. Soc. Chim. Fr. 129 (1992) 655. [11] R. Nakamura, E. Echigoya, F. Abe, Chem. Lett., 51 1981 . [12] J.M. Kerkhof, J.A. Moulijn, J. Phys. Chem. 83(12) (1979) 1612. [13] A.M. Turek, I.E. Wachs, E. De Canio, J. Phys. Chem. 96 (1992) 5000. [14] F.M. Mulcahy, M.J. Fay, A. Proctor, M. Houalla, D.M. Hercules, J. Catal. 124 (1990) 231. [15] A.A. Andreev, R.M. Edreva-Kardjieva, J. Catal. 94 (1985) 97. [16] H. KnoÈzinger, P. Ratnasamy, Catal. Rev. ± Sci. Eng. 17(1) (1978) 31. [17] M.B. Varfolomeev, N.B. Yakovleva, V.E. Plyushchev, Russ. J. Inorg. Chem. 14(1) (1969) 54. [18] R.L. Banks, Phillips Petroleum Company, Brevet ameÂricain, 4 454 368 (1984).