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The Structure-Activity Relationship of Re2O7 Metathesis Catalysts Supported on Phosphated Alumina and Silica-Alumina
O d ,L d d.(Editors), New Frontiers in Catalysis Proceedings of thc 10th International Congress on Catalysis, 19-24July, 1992,Budapest, Hungary 6 1993 Elsevier Science Publishers B.V. All righa =sewed
THE STRUCTURE-ACTIVITY RELATIONSHIP OF Re307 MGTATHESIS CATALYSTS SUPPORTED ON PHOSPHATED ALUMINA AND SILICAALUMINA R. Spronk and J. C. Mol Department of Chemical Engineering, University of Amsterdam, Nieuw Achte+cht 1018 WV Amsterdam, The Netherlands
166,
Abstract The structure of the rhenium oxide phase in Re207 metathesis catalysts supported on phosphated alumina and silica-alumina was studied by means of X-ray photoelectron spectroscopy and Raman spectroscopy. It appeared that in phosphated alumina-supported catalysts the rhenium oxide is present as monomeric Re04 species under ambient conditions at any loading. In silica-alumina-supported catalysts, however, the rhenium oxide is predominantly present as monomers at low loadings, but at higher loadings (>3 wt% Re 07)a large fraction of the rhenium oxide is present in three-dimensional cfusters. The activity of the silica-alumina-supported catalysts, measured during propene metathesis, was much higher than the activity of (phosphated) aluminasupported catalysts,
INTRODUCTION Two supported Re207 catalysts which are not so well-known as the industrially-applied Re207/y-A120 catalyst are under investigation in our department, viz Re207/phosphated a l w n a and Re207/silica-alumina. The present study is aimed at determining the structural characteristics that rhenium oxide exhibits when it is brought onto phosphated alumina or silica-alumina. Two complementary techniques were used: X-ray photoelectron spectroscopy ( X P S ) , and laser Raman spectroscopy (LRS). The metathesis of propene, 2 CHzSH-CH,
2 CH2=CH2
+
CH,-CH=CH-CH,
was used to determine the catalyst activity. The relation between the structure and the catalytic activity of these two catalysts was compared with data on alumina-supported catalysts.
EXPERIMENTAL All catalysts were prepared by pore-volume impregnation of the support with an aqueous solution of ammonium perrhenate, followed by overni ht drying in air at 383 K. The supports used were 7-alumina (Akzo CK 380,
2072
S ,=XI8 m2/g) and silica-alumina (Akzo, 24.3 wt% A1203, S ~ 2 = 3 8 0m2/g). 8 e phos hated alumina support was obtained by treating y-alumina with (NH@HPg -as described in ref [l], example 3. T e XP?3 measurements were carried out with a Kratos XSAM 800 s ectrometer using AlKa (1486.6 eV) X-rays. The atomic ratios were &ermined from the Re4f, A1 and SizS signals. The rhenium signal was 5%) for the underfying AlKp signal. experimental set-up consisted of an Ar+ laser (Spectra Physics, Mddel 2016) delivering incident radiation tuned at 514.5 nm. The powdered samples were pressed into self-supporting 13 nm diameter wafers and the spectra were recorded under ambient conditions. The activity measurements were carried out in a micro-catalytic fixed-bed flow reactor as described previously [ 2 , The catalyst samples were calcined in silu in an oxygen stream at 823 K, ollowed by a nitrogen purge. Standard reaction conditions were a contact time (WIF) between 1 and 50 kg(cat).s/mol, a pressure of 1.5 bar and a reaction temperature of 323 K. The propene was diluted with nitrogen (molar ratio 1:2) during the activity measurements. From the conversion data the initial reaction rates were derived, from which the turnover frequencies (TOF) were calculated assuming that all rhenium atoms form active sites.
r
RESULTS
XPS
Three series of catalysts with varying Re207 loading were studied with XPS. The signal ratios of Re4f, Alzs and SizS were plotted against the Re207 loading (Figure 1). 0.08
-I
q0.06
+
c .
--I
0.04
\
J0.02
0.00
Re,O, / wt%
Figure 1. Re4f/(A12s and SizS) signal ratios obtained from X P S as a function of the Re207 content for rhenium oxide supported on y-alumina, phosphated alumina and silica-alumina.
2073
For catalysts supported on phosphated alumina a strai ht line through the origin he slope of the line was observed, indicating monola er behaviour. corres on& to the slope observed or -alumina-supported catalysts. However, for t e silica-alumina-supported catarysts a bent line was observed. The dispersion of the Re207/nlica-alumina catalysts was calculated from the XPS signal ratios [3]. The degree of dispersion remained constant at nearly 100% in the range 1-3 wt% Re20 loading and decreased strongly above a 3 wt% loading to a value of roughfy 30 %.
%
H
l
LRS Re207/phosphated alumina ave the same Raman spectrum as Re 0 7 . on ?-alumina [4]. We could not ietect any Raman bands on the silica-afummasu ported catalysts under our experimental conditions. However, Vuurman et al. [5 did find Raman signals with a weak intensity under ambient conditions for si ica-alumina-supported rhenium oxide catalysts which could be assigned to a hydrated Re04 species.
P
Activity measurements Figures 2 and 3 show the initial reaction rates and the TOF's calculated from these initial rates as a function of the Re207 loading for three catalyst systems.
0
I
10
16
0
10
Figure 3. Turnover frequencies as a function of Re 0 7 content for 0 ) alumina, (o)phospfnted alumina an (A)silica-alumina supported catalysts.
Fi ure 2. Initial reaction rates as a unction of Re 0 content for 0 ) alumina, (o)phosp&zed alumina an @)silica-alumina supported catalysts.
!
6
Re207 I wt%
Re207 I wt%
6
6
For the silica-alumina-supported catalysts the TOF was highest in the range 1-3 wt% Re207 loading and decreased above a 3 wt% loading to a constant value, whereas for the catalysts sup orted on phosphated alumina the TOF increased almost linearly. The data or alumina-supported catalysts are for comparison; these data are in good agreement with those measurep;; Kapteijn et al. [6].
P
2074
DISCUSSION Structure
The following picture of the rhenium oxide phase emerges from the results of the two spectroscopic techniques. On phosphated alumina the rhenium oxide is present as a monomeric Re04 s ecies under ambient conditions at any loading. On silica-alumina, however, t e rhenium oxide is present as monomers at low loadings ( L 3 wt%), but at higher Re20 loadin s, a large fraction of rhenium oxide is present in three-dimensional clusters. ince cluster formation does not occur on -,-alumina, it seems likely that these clusters reside on the silica component of silica-alumina.
g
8
Catalyst activity Treatment of 7-alumina with phosphate results in a more active catalyst for Re207 contents up to 12 wt%. The relationship between the TOF and the rhemum oxide loading for catalysts supported on phosphated alumina can be explained on the basis of four different types of hydroxyl groups present on the surface of the support which can react with ReO4- ions during the preparation of the catalyst [7]. The activity of the silica-alumina-supported Re207 metathesis catalysts is much higher than that of the phosphated alumina- or y-alumina-supported catalysts, which is attributed to their much stronger Bransted acidity [2]. The highest TOF’s for silica-alumina-supported catalysts are found at low loadings indicating that here, in contrast to alumina-supported catalysts [7], rhenium oxide has an affinity for those sites on the surface which will enable it to become active sites for metathesis. However, the TOF of these silica-alumina-supported catalysts (and thus the fraction of active sites) decreases above a 3 wt% loading to a constant value, as does the dispersion.
ACKNOWLEDGEMENTS We would like to thank Dow Benelux N.V. for financial support. Prof. J.A.R. van Veen is gratefully acknowledged for the X P S measurements as are M.N.H. Kieboom and M.J. Vaas for their experimental assistance. This work was supported by the Netherlands Foundation for Chemical Research (SON), with financial aid from the Netherlands Organization for Scientific Research
(NWO).
REFERENCES 1 2 3 4
5 6 7
M.J. Lawrenson (to B.P.), US Patent No. 3 974 233 (1976). R. Spronk, A. Andreini and J.C. Mol, J. Mol. Catal., 65 (1991) 219. H.P.C.E. Kuipers, H.C.E. van Leuven and W.M. Visser, Surf. Interface Anal., 8 (1986) 235. F.D. Hardcastle, I.E. Wachs, J.A. Horsley and G.H. Via, J. Mol Catal., 46 (1988) 15. M.A. Vuurman, I.E. Wachs and J.C. Mol, unpublished results. F. Kapteijn, L.H.G. Bredt and J.C. Mol, Recl. Trav. Chim. Pays-Bas, 96 (1977) Ml39. M. Sibeijn, R. Spronk, J.A.R. van Veen and J.C. Mol, Catal. Lett., 8 (1991) 201.
THE STRUCTURE-ACTIVITY RELATIONSHIP OF Re307 MGTATHESIS CATALYSTS SUPPORTED ON PHOSPHATED ALUMINA AND SILICAALUMINA R. Spronk and J. C. Mol Department of Chemical Engineering, University of Amsterdam, Nieuw Achte+cht 1018 WV Amsterdam, The Netherlands
166,
Abstract The structure of the rhenium oxide phase in Re207 metathesis catalysts supported on phosphated alumina and silica-alumina was studied by means of X-ray photoelectron spectroscopy and Raman spectroscopy. It appeared that in phosphated alumina-supported catalysts the rhenium oxide is present as monomeric Re04 species under ambient conditions at any loading. In silica-alumina-supported catalysts, however, the rhenium oxide is predominantly present as monomers at low loadings, but at higher loadings (>3 wt% Re 07)a large fraction of the rhenium oxide is present in three-dimensional cfusters. The activity of the silica-alumina-supported catalysts, measured during propene metathesis, was much higher than the activity of (phosphated) aluminasupported catalysts,
INTRODUCTION Two supported Re207 catalysts which are not so well-known as the industrially-applied Re207/y-A120 catalyst are under investigation in our department, viz Re207/phosphated a l w n a and Re207/silica-alumina. The present study is aimed at determining the structural characteristics that rhenium oxide exhibits when it is brought onto phosphated alumina or silica-alumina. Two complementary techniques were used: X-ray photoelectron spectroscopy ( X P S ) , and laser Raman spectroscopy (LRS). The metathesis of propene, 2 CHzSH-CH,
2 CH2=CH2
+
CH,-CH=CH-CH,
was used to determine the catalyst activity. The relation between the structure and the catalytic activity of these two catalysts was compared with data on alumina-supported catalysts.
EXPERIMENTAL All catalysts were prepared by pore-volume impregnation of the support with an aqueous solution of ammonium perrhenate, followed by overni ht drying in air at 383 K. The supports used were 7-alumina (Akzo CK 380,
2072
S ,=XI8 m2/g) and silica-alumina (Akzo, 24.3 wt% A1203, S ~ 2 = 3 8 0m2/g). 8 e phos hated alumina support was obtained by treating y-alumina with (NH@HPg -as described in ref [l], example 3. T e XP?3 measurements were carried out with a Kratos XSAM 800 s ectrometer using AlKa (1486.6 eV) X-rays. The atomic ratios were &ermined from the Re4f, A1 and SizS signals. The rhenium signal was 5%) for the underfying AlKp signal. experimental set-up consisted of an Ar+ laser (Spectra Physics, Mddel 2016) delivering incident radiation tuned at 514.5 nm. The powdered samples were pressed into self-supporting 13 nm diameter wafers and the spectra were recorded under ambient conditions. The activity measurements were carried out in a micro-catalytic fixed-bed flow reactor as described previously [ 2 , The catalyst samples were calcined in silu in an oxygen stream at 823 K, ollowed by a nitrogen purge. Standard reaction conditions were a contact time (WIF) between 1 and 50 kg(cat).s/mol, a pressure of 1.5 bar and a reaction temperature of 323 K. The propene was diluted with nitrogen (molar ratio 1:2) during the activity measurements. From the conversion data the initial reaction rates were derived, from which the turnover frequencies (TOF) were calculated assuming that all rhenium atoms form active sites.
r
RESULTS
XPS
Three series of catalysts with varying Re207 loading were studied with XPS. The signal ratios of Re4f, Alzs and SizS were plotted against the Re207 loading (Figure 1). 0.08
-I
q0.06
+
c .
--I
0.04
\
J0.02
0.00
Re,O, / wt%
Figure 1. Re4f/(A12s and SizS) signal ratios obtained from X P S as a function of the Re207 content for rhenium oxide supported on y-alumina, phosphated alumina and silica-alumina.
2073
For catalysts supported on phosphated alumina a strai ht line through the origin he slope of the line was observed, indicating monola er behaviour. corres on& to the slope observed or -alumina-supported catalysts. However, for t e silica-alumina-supported catarysts a bent line was observed. The dispersion of the Re207/nlica-alumina catalysts was calculated from the XPS signal ratios [3]. The degree of dispersion remained constant at nearly 100% in the range 1-3 wt% Re20 loading and decreased strongly above a 3 wt% loading to a value of roughfy 30 %.
%
H
l
LRS Re207/phosphated alumina ave the same Raman spectrum as Re 0 7 . on ?-alumina [4]. We could not ietect any Raman bands on the silica-afummasu ported catalysts under our experimental conditions. However, Vuurman et al. [5 did find Raman signals with a weak intensity under ambient conditions for si ica-alumina-supported rhenium oxide catalysts which could be assigned to a hydrated Re04 species.
P
Activity measurements Figures 2 and 3 show the initial reaction rates and the TOF's calculated from these initial rates as a function of the Re207 loading for three catalyst systems.
0
I
10
16
0
10
Figure 3. Turnover frequencies as a function of Re 0 7 content for 0 ) alumina, (o)phospfnted alumina an (A)silica-alumina supported catalysts.
Fi ure 2. Initial reaction rates as a unction of Re 0 content for 0 ) alumina, (o)phosp&zed alumina an @)silica-alumina supported catalysts.
!
6
Re207 I wt%
Re207 I wt%
6
6
For the silica-alumina-supported catalysts the TOF was highest in the range 1-3 wt% Re207 loading and decreased above a 3 wt% loading to a constant value, whereas for the catalysts sup orted on phosphated alumina the TOF increased almost linearly. The data or alumina-supported catalysts are for comparison; these data are in good agreement with those measurep;; Kapteijn et al. [6].
P
2074
DISCUSSION Structure
The following picture of the rhenium oxide phase emerges from the results of the two spectroscopic techniques. On phosphated alumina the rhenium oxide is present as a monomeric Re04 s ecies under ambient conditions at any loading. On silica-alumina, however, t e rhenium oxide is present as monomers at low loadings ( L 3 wt%), but at higher Re20 loadin s, a large fraction of rhenium oxide is present in three-dimensional clusters. ince cluster formation does not occur on -,-alumina, it seems likely that these clusters reside on the silica component of silica-alumina.
g
8
Catalyst activity Treatment of 7-alumina with phosphate results in a more active catalyst for Re207 contents up to 12 wt%. The relationship between the TOF and the rhemum oxide loading for catalysts supported on phosphated alumina can be explained on the basis of four different types of hydroxyl groups present on the surface of the support which can react with ReO4- ions during the preparation of the catalyst [7]. The activity of the silica-alumina-supported Re207 metathesis catalysts is much higher than that of the phosphated alumina- or y-alumina-supported catalysts, which is attributed to their much stronger Bransted acidity [2]. The highest TOF’s for silica-alumina-supported catalysts are found at low loadings indicating that here, in contrast to alumina-supported catalysts [7], rhenium oxide has an affinity for those sites on the surface which will enable it to become active sites for metathesis. However, the TOF of these silica-alumina-supported catalysts (and thus the fraction of active sites) decreases above a 3 wt% loading to a constant value, as does the dispersion.
ACKNOWLEDGEMENTS We would like to thank Dow Benelux N.V. for financial support. Prof. J.A.R. van Veen is gratefully acknowledged for the X P S measurements as are M.N.H. Kieboom and M.J. Vaas for their experimental assistance. This work was supported by the Netherlands Foundation for Chemical Research (SON), with financial aid from the Netherlands Organization for Scientific Research
(NWO).
REFERENCES 1 2 3 4
5 6 7
M.J. Lawrenson (to B.P.), US Patent No. 3 974 233 (1976). R. Spronk, A. Andreini and J.C. Mol, J. Mol. Catal., 65 (1991) 219. H.P.C.E. Kuipers, H.C.E. van Leuven and W.M. Visser, Surf. Interface Anal., 8 (1986) 235. F.D. Hardcastle, I.E. Wachs, J.A. Horsley and G.H. Via, J. Mol Catal., 46 (1988) 15. M.A. Vuurman, I.E. Wachs and J.C. Mol, unpublished results. F. Kapteijn, L.H.G. Bredt and J.C. Mol, Recl. Trav. Chim. Pays-Bas, 96 (1977) Ml39. M. Sibeijn, R. Spronk, J.A.R. van Veen and J.C. Mol, Catal. Lett., 8 (1991) 201.