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Laser-Raman spectroscopy of the alumina-supported rhenium oxide metathesis catalyst
.tO~JIEN.\L
OF c \T \t>Ysts
56, 27!F283
(1979)
NOTES Laser-Raman Spectroscopy of the Alumina-Supported Rhenium Oxide Metathesis Catalyst The metathesis of alkenes has attracted considerable attention because of its intercst from a practical and thcowtical point of view (1, 2). A typical example of this react’ion is the conversion of propcne into an cquimolar mixture of 2-butcnc and ethene. A number of catalysts have been rcport,cd to bc actiw in the mc%athcsis reaction (3). Most attention has bow given to silica- and alumina-supportc,d tungstrn, molybdrnum, and rhenium oxide. The inter& in the tungstcxn and molJ.bdenum catalyst’s is not surprising considering the fact chat they aw also uwd in hydrodcsulfurization and oxidation. Spctcial attention has been given to rhwium oxide catalysts which are active wc’n at room tcmpcraturc and at low prwsuw. As the structure of this catalyst is not fully understood, WY have invost,igatcad this system to clucidatc the nature ol the interaction b&wwn promotw and support. Rcwnt publications show that lawrRaman spectroscopy is a valuable tool in catalytic rcwarch. Brown et al. (4) and Medcma et al. (5) have sucwssfully applied laser-Raman spectroscopy in the identification of species prcwnt on molybdenum catalysts. Tungston oxide supported on silica w-as studied by dc Vriw and Pott (6) and by our group (7’). In tho case of silica-supported tungsten oxide WC showed the prcscnw of at least two tungst)cn species : (1) crystalline tungsten trioxidc with main Raman lines at 809 and 715 cm-‘;
(2) a not-yet-idrntifird spw%s with broad band around 970 cm-‘.
a
This second spccicls n-as tentatively identifictd as a polymeric compound consisting of distorted tungstc>n octahedra. It is remarkable that a similar band was obscwcd in the spectrum of a 1%n-t.OjG tungsten-oxide-on-alumina catalyst (7). Also nlodcma et al. (5) obscrvcd a broad band at 950 cm-l in the case of aluminasupported molybdenum oxide. So thr laserRaman spectra of both mrtathcsis catalysts show a broad band around 960 cm-‘. WC wcLr(l intcrcstcd in whcthcr a similar band could be obwrwd in the spwtra of rhenium oxide on alumina. Olsthoorn and Boc:lhou\vw (8) have inwstigatcd this system by in situ infrared spectroscopy. From the abscnct: of characteristic alumina hydroxyl groups thcly conclud(ld that at a high rhc>nium content (20 to 26 wt.7, Rc~O~) the carricir is complctctly covcrcd \\-ith a monolayrr. ‘l’hc nc’\v hydroxyl groups wcrc attribut,cd t’o rhc>nium-bonded hydroxyl groups, We mc>asuwd the laser-Raman spwtra of NH.lRrO,, RcOJ- in aqueous solution and of a number of rhenium-oxide-on alumina catalysts. The catalysts w-cre pwparrd by wet impregnation of y-alumina with aqueous solutions of ammonium pcrrhcnate, drying at 390 K for 16 hr and subwqwnt calcinat’ion in oxygen for 4 hr at 830 K. A dctailcd description of the system is giwn by Kaptclijn et al. (12). The spcetra wow rccordcd on a Ramanor
279 0021-9517/79/020279-05$02.00/0 All
Copyright 0 1979 by Academic Press, Inc. rights of reproduction in any form reserved.
PIG. 1. liarnan
1000
spectrunl
800
of
rMl-
in aqueous
solution.
cm -I
600
Scan
Re C14- (aqueous)
speed 100 cm-‘/min.
400
Slit width 10 cm-l.
200
2Sl
NWlY3S
% Re,07
18
8
6
1
I
1200
1000
1
I
600
800
I
400
I
200
I
0
cm-’ FIG. 2. Raman spectra of alumina-supported Slit width 10 cm-l.
HG2S and a Codcrg PHO spectrometer. The samples were measured in capillary tubes. The output power of the laser (Coherent Radiation CR8 argon ion laser) was 700-1000 mW (.514.5 nm line). To check a possible decomposition of the
Rhenium
oxide catalysts.
Scan speed 100rm-l/min.
samples, duplicate runs were made with rotating samples on a Jeol JRS-1 spectrometer. Excellent agreement with published rcsults for KH4ReOd (9, IO) and ReOb- in aqueous solution (11) was obtained. The
2s2
NOTES
major Raman lines were found at 965 (Ye RcO), 911 and 890 (v,, RcO), and 339 and 332 (6 ORcO) cm-l for the solid pclrrhcnate and 970 ( yb RcO), 916 (v,, RcO), and 332 (6 OReO) cm-’ for RcO,- in solution (Fig. 1). Figure 2 shows the spectra of the catalysts. In spite of the low signal/ noise ratio a number of bands can be observed. The main lines are located at 968 and 328 cm-‘, whereas in most samples a weak broad band is observed at 910 cm-‘. Recording the spectra at 20 cm-‘/ min inst,cad of 100 cm-‘/min did not result in a significantly better signal. Because the same frequencies are observed for the catalysts as for Re04- in compounds with a tetrahedral coordination we conclude that on the catalyst surface rhenium is present as Re04- tetrahedra. The broadening of the band at 916 cm-l points to a dynamic distortion of the tetrahedron just as has been observed for Rc04- in aqueous solution. This distortion, which can be caused by interaction with the carrier and surface hydroxyl groups, is obviously not static because in that case shifts of Raman frequencies are cxpectcd. The fact that alumina-supported rhenium oxide is easily reduced compared to, e.g., tungsten oxide on alumina (8) confirms the smaller interaction between carrier and promoter. The low signal/noise ratio for the catalyst samples is caused by fluorcscencc. In case of a 3-v&Y’ rhenium oxide catalyst it was even impossible to record a reasonable spectrum because of the fluorescence. Furthermore, the abscncc of a correlation and rhenium between peak intensity loading is probably caused by a slight variation of the expcrimcntal conditions which could hardly be avoided. It is interesting to compare our results with the conclusions of Mcdema et al. (5) and Giordano et al. (13) with respect to molybdenum on alumina. At low promoter contents this system contains only tetrahedral molybdenum species. At higher
molybdenum contents (above 10 wt.Ojo) othcbr compounds wore found, viz., octahodrally-coordinated molybd(>num species:, aluminum molybdatc and free molybdenum trioxidc. However, r&her from X-ray diffraction measurrmcnts nor from laserRaman spectroscopy is thcrc an indication that other rhenium species, e.g., octahcdrally coordinated rhenium or Al/Re/O compounds, are formed. This diffcrencc with the molybdenum catalysts can partly bc explained by the fact that rhenium has a much higher atomic number than molybdenum and, thcreforc, at the same weight percentage the surface density for rhenium is lower (10 wt.% molybdenum oxide on alumina can bc compared with 17 wt.% rhenium oxide). Summarizing, we conclude that laserRaman spectroscopy confirms the presence of a single rhenium species on the alumina surface. This species consists of tetrahedral Re04- ions which are dynamically distorted by, for example, the carrier or surface hydroxyl groups. ACKNOWLEDGMENTS We are indebted to Dr. J. Medema, Dr. V. H. J. de Beer, Dr. I). C. Koningsberger and Dr. 1). J. Stufkens for helpful discussions on the Raman spectra. The spectra were recorded by courtesy of the Laboratory of Inorganic Chemistry, University of Amsterdam. This study was supported by the Netherlands Foundation for Chemical Research (SON), with financial aid from the Netherlands Organization for the advancement of Pure Research (ZWO). REFERENCES 1. Mel, J. C., and Moulijn, J. A., in “Advances Vol. 24, p. 131. Academic in Catalysis,” Press, New York, 1975. 2. Proceedings, International Symposium on Metathesis, Rec. Trav. Chim. Pays-Ras 96, MlMl41 (1977). 3. Banks, R. L., Fortschr. Chem. Forsch. 25, 39 (1972). 4. Brown, F. It., Makovsky, L. E., and Rhee, K. H., J. Catal. 50, 162 (1977). 5. Medema, J., van Stam, C., de Beer, V. H. J., Konings, A. H. J., and Koningsberger, D. C., J. Catal. 53, 386 (1978).
2x3
NOTES 6. de Vries, S. L. K. I?., and Pott, G. T., Rec. Trav. Chim. Pays-Bus 96, Ml15 (1977). 7. Thomas, R., Moulijn, J. A., and Kerkhof, F. P. J. M., Rec. Truv. Chim. Pays-Bus 96, Ml34 (1977). 8. Olsthoorn, A. A., and Boelhouwer, C., J. Cc&Z. 44, 197 (1976). 9. Ulbricht, K., and Kriegsmann, H., Z. Chem. 6, 232 (1966); Ulbricht, K., and Kriegsmann, II., Z. Anorg. Allg. Chem. 358, 193 (1968). 10. Busey, R. II., and Keller, 0. L., J. Chem. Phys. 41, 215 (1964). 11. Woodward, L. A., and Roberts, H. L., Trans. Furaday Sac. 52, 615 (1956). 18. Kaptcijn, F., Bredt, L. H. G., and Mel, J. C., Rec. Trav. Chlm. Pays-l?as 96, Ml39 (1977). 13. Giordano, N., Bart, S. C. J., Vachi, A., Cas-
tcllan, A., and Martinotti, G., J. Catal. 36, 81 (1975). Spectra of Inorganic 14. Nakomoto, K., “Infrared and Coordination Compounds.” Wiley, New York, 1970. 15. Yao, FT. C., and Hhelef, RI., .I. Catul. 44, 3!X (1976).
F. 1'. J. M. KERKHOF, J. !I. ~VO.[JLIJN ASD R. THOMAS Institute for Chemical Technology University of Amsterdam Plantage Muidergracht $0 1018 TV Amsterdam The Xeihedands Received April
S’, i978
OF c \T \t>Ysts
56, 27!F283
(1979)
NOTES Laser-Raman Spectroscopy of the Alumina-Supported Rhenium Oxide Metathesis Catalyst The metathesis of alkenes has attracted considerable attention because of its intercst from a practical and thcowtical point of view (1, 2). A typical example of this react’ion is the conversion of propcne into an cquimolar mixture of 2-butcnc and ethene. A number of catalysts have been rcport,cd to bc actiw in the mc%athcsis reaction (3). Most attention has bow given to silica- and alumina-supportc,d tungstrn, molybdrnum, and rhenium oxide. The inter& in the tungstcxn and molJ.bdenum catalyst’s is not surprising considering the fact chat they aw also uwd in hydrodcsulfurization and oxidation. Spctcial attention has been given to rhwium oxide catalysts which are active wc’n at room tcmpcraturc and at low prwsuw. As the structure of this catalyst is not fully understood, WY have invost,igatcad this system to clucidatc the nature ol the interaction b&wwn promotw and support. Rcwnt publications show that lawrRaman spectroscopy is a valuable tool in catalytic rcwarch. Brown et al. (4) and Medcma et al. (5) have sucwssfully applied laser-Raman spectroscopy in the identification of species prcwnt on molybdenum catalysts. Tungston oxide supported on silica w-as studied by dc Vriw and Pott (6) and by our group (7’). In tho case of silica-supported tungsten oxide WC showed the prcscnw of at least two tungst)cn species : (1) crystalline tungsten trioxidc with main Raman lines at 809 and 715 cm-‘;
(2) a not-yet-idrntifird spw%s with broad band around 970 cm-‘.
a
This second spccicls n-as tentatively identifictd as a polymeric compound consisting of distorted tungstc>n octahedra. It is remarkable that a similar band was obscwcd in the spectrum of a 1%n-t.OjG tungsten-oxide-on-alumina catalyst (7). Also nlodcma et al. (5) obscrvcd a broad band at 950 cm-l in the case of aluminasupported molybdenum oxide. So thr laserRaman spectra of both mrtathcsis catalysts show a broad band around 960 cm-‘. WC wcLr(l intcrcstcd in whcthcr a similar band could be obwrwd in the spwtra of rhenium oxide on alumina. Olsthoorn and Boc:lhou\vw (8) have inwstigatcd this system by in situ infrared spectroscopy. From the abscnct: of characteristic alumina hydroxyl groups thcly conclud(ld that at a high rhc>nium content (20 to 26 wt.7, Rc~O~) the carricir is complctctly covcrcd \\-ith a monolayrr. ‘l’hc nc’\v hydroxyl groups wcrc attribut,cd t’o rhc>nium-bonded hydroxyl groups, We mc>asuwd the laser-Raman spwtra of NH.lRrO,, RcOJ- in aqueous solution and of a number of rhenium-oxide-on alumina catalysts. The catalysts w-cre pwparrd by wet impregnation of y-alumina with aqueous solutions of ammonium pcrrhcnate, drying at 390 K for 16 hr and subwqwnt calcinat’ion in oxygen for 4 hr at 830 K. A dctailcd description of the system is giwn by Kaptclijn et al. (12). The spcetra wow rccordcd on a Ramanor
279 0021-9517/79/020279-05$02.00/0 All
Copyright 0 1979 by Academic Press, Inc. rights of reproduction in any form reserved.
PIG. 1. liarnan
1000
spectrunl
800
of
rMl-
in aqueous
solution.
cm -I
600
Scan
Re C14- (aqueous)
speed 100 cm-‘/min.
400
Slit width 10 cm-l.
200
2Sl
NWlY3S
% Re,07
18
8
6
1
I
1200
1000
1
I
600
800
I
400
I
200
I
0
cm-’ FIG. 2. Raman spectra of alumina-supported Slit width 10 cm-l.
HG2S and a Codcrg PHO spectrometer. The samples were measured in capillary tubes. The output power of the laser (Coherent Radiation CR8 argon ion laser) was 700-1000 mW (.514.5 nm line). To check a possible decomposition of the
Rhenium
oxide catalysts.
Scan speed 100rm-l/min.
samples, duplicate runs were made with rotating samples on a Jeol JRS-1 spectrometer. Excellent agreement with published rcsults for KH4ReOd (9, IO) and ReOb- in aqueous solution (11) was obtained. The
2s2
NOTES
major Raman lines were found at 965 (Ye RcO), 911 and 890 (v,, RcO), and 339 and 332 (6 ORcO) cm-l for the solid pclrrhcnate and 970 ( yb RcO), 916 (v,, RcO), and 332 (6 OReO) cm-’ for RcO,- in solution (Fig. 1). Figure 2 shows the spectra of the catalysts. In spite of the low signal/ noise ratio a number of bands can be observed. The main lines are located at 968 and 328 cm-‘, whereas in most samples a weak broad band is observed at 910 cm-‘. Recording the spectra at 20 cm-‘/ min inst,cad of 100 cm-‘/min did not result in a significantly better signal. Because the same frequencies are observed for the catalysts as for Re04- in compounds with a tetrahedral coordination we conclude that on the catalyst surface rhenium is present as Re04- tetrahedra. The broadening of the band at 916 cm-l points to a dynamic distortion of the tetrahedron just as has been observed for Rc04- in aqueous solution. This distortion, which can be caused by interaction with the carrier and surface hydroxyl groups, is obviously not static because in that case shifts of Raman frequencies are cxpectcd. The fact that alumina-supported rhenium oxide is easily reduced compared to, e.g., tungsten oxide on alumina (8) confirms the smaller interaction between carrier and promoter. The low signal/noise ratio for the catalyst samples is caused by fluorcscencc. In case of a 3-v&Y’ rhenium oxide catalyst it was even impossible to record a reasonable spectrum because of the fluorescence. Furthermore, the abscncc of a correlation and rhenium between peak intensity loading is probably caused by a slight variation of the expcrimcntal conditions which could hardly be avoided. It is interesting to compare our results with the conclusions of Mcdema et al. (5) and Giordano et al. (13) with respect to molybdenum on alumina. At low promoter contents this system contains only tetrahedral molybdenum species. At higher
molybdenum contents (above 10 wt.Ojo) othcbr compounds wore found, viz., octahodrally-coordinated molybd(>num species:, aluminum molybdatc and free molybdenum trioxidc. However, r&her from X-ray diffraction measurrmcnts nor from laserRaman spectroscopy is thcrc an indication that other rhenium species, e.g., octahcdrally coordinated rhenium or Al/Re/O compounds, are formed. This diffcrencc with the molybdenum catalysts can partly bc explained by the fact that rhenium has a much higher atomic number than molybdenum and, thcreforc, at the same weight percentage the surface density for rhenium is lower (10 wt.% molybdenum oxide on alumina can bc compared with 17 wt.% rhenium oxide). Summarizing, we conclude that laserRaman spectroscopy confirms the presence of a single rhenium species on the alumina surface. This species consists of tetrahedral Re04- ions which are dynamically distorted by, for example, the carrier or surface hydroxyl groups. ACKNOWLEDGMENTS We are indebted to Dr. J. Medema, Dr. V. H. J. de Beer, Dr. I). C. Koningsberger and Dr. 1). J. Stufkens for helpful discussions on the Raman spectra. The spectra were recorded by courtesy of the Laboratory of Inorganic Chemistry, University of Amsterdam. This study was supported by the Netherlands Foundation for Chemical Research (SON), with financial aid from the Netherlands Organization for the advancement of Pure Research (ZWO). REFERENCES 1. Mel, J. C., and Moulijn, J. A., in “Advances Vol. 24, p. 131. Academic in Catalysis,” Press, New York, 1975. 2. Proceedings, International Symposium on Metathesis, Rec. Trav. Chim. Pays-Ras 96, MlMl41 (1977). 3. Banks, R. L., Fortschr. Chem. Forsch. 25, 39 (1972). 4. Brown, F. It., Makovsky, L. E., and Rhee, K. H., J. Catal. 50, 162 (1977). 5. Medema, J., van Stam, C., de Beer, V. H. J., Konings, A. H. J., and Koningsberger, D. C., J. Catal. 53, 386 (1978).
2x3
NOTES 6. de Vries, S. L. K. I?., and Pott, G. T., Rec. Trav. Chim. Pays-Bus 96, Ml15 (1977). 7. Thomas, R., Moulijn, J. A., and Kerkhof, F. P. J. M., Rec. Truv. Chim. Pays-Bus 96, Ml34 (1977). 8. Olsthoorn, A. A., and Boelhouwer, C., J. Cc&Z. 44, 197 (1976). 9. Ulbricht, K., and Kriegsmann, H., Z. Chem. 6, 232 (1966); Ulbricht, K., and Kriegsmann, II., Z. Anorg. Allg. Chem. 358, 193 (1968). 10. Busey, R. II., and Keller, 0. L., J. Chem. Phys. 41, 215 (1964). 11. Woodward, L. A., and Roberts, H. L., Trans. Furaday Sac. 52, 615 (1956). 18. Kaptcijn, F., Bredt, L. H. G., and Mel, J. C., Rec. Trav. Chlm. Pays-l?as 96, Ml39 (1977). 13. Giordano, N., Bart, S. C. J., Vachi, A., Cas-
tcllan, A., and Martinotti, G., J. Catal. 36, 81 (1975). Spectra of Inorganic 14. Nakomoto, K., “Infrared and Coordination Compounds.” Wiley, New York, 1970. 15. Yao, FT. C., and Hhelef, RI., .I. Catul. 44, 3!X (1976).
F. 1'. J. M. KERKHOF, J. !I. ~VO.[JLIJN ASD R. THOMAS Institute for Chemical Technology University of Amsterdam Plantage Muidergracht $0 1018 TV Amsterdam The Xeihedands Received April
S’, i978