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Effect of various pretreatments on rhenium oxide-alumina catalysts: the structure and activity of active sites for propene metathesis
Journal of Molecular Catalysis, 36 (1986)
79
79 - 89
EFFECT OF VARIOUS PRETREATMENTS ON RHENIUM OXIDE-ALUMINA CATALYSTS: THE STRUCTURE AND ACTIVITY OF ACTIVE SITES FOR PROPENE METATHESIS XU YIDE, WE1 XINGUANG, Dalton Institute (Chma)
SHI YINGZHEN,
ZHANG YIHUA and GUO XIEXIAN
of Chemical Physics, Chmese Academy
of Sciences,
129 Street, Dalian
Summary A series of Re20,-A1203 catalysts containing 3.0 - 15.3 wt.% Re,O, has been studied in a microcatalytic pulse reactor for propene metathesis. The effect of various pretreatments has been examined by catalyst evaluation as well as by the change in surface properties observed in TPR, 02adsorption, ESR and XPS measurements. After N2 pretreatment a linear relationship is obtained between the intensities of the ESR signal, assigned to partially reduced Re-0-Re dimeric species, and the activities of Re20,-Al,Os catalysts. The concentration of the dimeric species strongly depends on the Re20, loading. Hz reduction at 450 “C followed by O2 uptake at reaction temperature (80 “C) is effective in enhancing the activity of low loading catalysts, and the amount of reversibly adsorbed O2 is shown to be closely connected with the catalytic activity. In this case, it is easy to detect the formation of 02- ions on the surface by ESR observation. Surface species of
d 2is very plausibly responsible for the enhanced activity of the low loading catalyst. By pretreatment with propene at 450 “C, the activity of the low loading catalyst is greatly increased. A significant partial reduction of Re207 and simultaneous carbonaceous deposit on the catalyst surface are observed by ESR and XPS examination. It seems probable that monomeric species of Re+6 (or Re+5)-O-A1+3 are highly active and that the formation of such species on the surface is facilitated by treating the low loading catalyst with propene at 400 - 450 “C. It is apparent that the presence of appropriate unsaturated coordinated species of high-valence Re ions on dehydrated alumina is essential for active metathesis catalysts.
@ Elsevier Sequoia/Printed
in The Netherlands
80
Introduction Supported rhenium oxide-alumina catalysts are of great interest in the metathesis of olefins because of their high activity and good selectivity [ 11. Recently, many studies have been directed towards distinguishing the active species which is formed by various pretreatments on the catalyst, and is relevant to the catalytic function. Although it is well established now that the enhanced activity of the catalysts after N2 pretreatment at 550 “C strongly depends on the resulting surface composition, the nature of the active species is still not clearly understood. Kerkhof et al. [Z] suggested on the basis of Raman spectroscopic studies that the active surface species was solely the tetrahedral ReO, ion. However, according to the IR results of Nakamura and Echigoya [3], @e 20 7) ads species are responsible for the high activity in the metathesis of alkenes on high rhenium loading catalysts. For low loading catalysts, as Nakamura and Echigoya pointed out in [4], Hz reduction at 550 “C followed by O2 uptake at room temperature could also enhance their activity. They found that 02- ions could be formed under these conditions, and proposed that partially reduced monomeric species, plausibly in the form of Re’4-O-A1+3, might play the essential role of a precursor formulated as Re+5-O-A1+3 d 2They claimed furthermore that reduction by means of propene, followed by O2 uptake at room temperature, had the same effect. It seems that the oxidation state of Re ions is critical in initiating the active species on the surface of low loading catalysts. Madden [5] reported obvious dependence of the metathesis activity of Re20,-A1203 catalysts on the effect of the support rather than on the oxidation state of Re ions, since an in situ IR cell/reactor examination of the catalyst revealed that the Lewis sites on the catalyst become somewhat stronger than on simple Al2O3, while neither calcination nor reduction affects the rhenium-oxygen stretching vibrational frequency. The diverse results and contradictory conclusions in the literature about the nature of active species stimulated us to further investigate the active species of Re20,-A1203 catalysts for the metathesis of olefins. In this work, TPR, 02-adsorption, ESR and XPS methods were used in combination with activity test runs for characterizing both high and low loading catalysts. Emphasis was placed on the role of the oxidation state of Re ions. Experimental Materials A series of supported rhenium oxide-alumina catalysts with Re207 loading varying from 3.0 to 15.3 wt.% were prepared by impregnating
81
alumina (20 - 40 mesh, BET surface area S = 168 m2 g-i) with an aqueous solution of perrhenic acid which had been prepared by dissolving Re powder with the nominal purity of 99.99% in nitric acid. Samples were dried at 110 “C for 15 h and calcined in air at 550 “C for 4 h. The content of Re20, in the catalysts was determined by using an X-ray fluorescent spectrometer (Table 1). Gases used in the pretreatment and the reaction were purified by conventional methods. To further remove traces of water, a trap (-78 “C) was mounted on the reactor inlet pipe. Propene used in this work was in purity of polymeric degree, and was passed through a zeolite trap to remove traces of water. TPR The TPR set-up used in this work was almost the same as that used by Robertson [6]. Samples were first oxidized at 550 “C in a stream of O2 for 1 h and then flushed with Ar for 0.5 h. The reducing gas was a H2-Ar mixture containing 16.9% of H2 . Typically the TPR experiments were performed at a flow rate of 20 ml min-’ and a heating rate of 8 “C mm-‘. The consumption of H2 was measured by weighing the area under the curve; calibration was executed by using pure Hz. 0, adsorp tton O2 adsorption was carried out in a conventional pulse adsorption system. A sample was reduced at different temperatures and then pulses of O2 were passed over it at room temperature in a stream of Ar until the surface was saturated by the adsorbed 02. Then the sample was heated to 200 “C and purged with Ar for 0.5 h. The second O2 uptake, which was considered tentatively to be the amount of the reversibly adsorbed O2 on the sample, was measured at room temperature. ESR ESR spectra were recorded at X-band frequency on a JEOL-FEBXG ESR spectrometer at room temperature. The relative intensity of a signal was estimated according to the following formula: I
_
(AHPP )2H,PK A
(1)
where AHpp = the horizontal width and Hpp = the vertical height from the bottom to the top in the ESR signal concerned and recorded in the form of the first differential curve, A = the amplitude of the instrument used in the process of measurement and K = the proportionality coefficient. XPS A PHI 550 multifunction photoelectron spectrometer was used to measure and record the XPS spectra concerned. Mg & radiation (1253.6 ev) was selected as the X-ray source and a vacuum of 5 X 10-s torr was maintained during the measurements.
82
The finely ground sample was fixed onto double-sided adhesive tape or was pressed in the form of a tablet for examining its monolayer structure or reduction behaviour. A PHI 5500 multifunction analog computer was employed for repeating scanning, recording, data processing and drawing. The binding energy was measured by using Ci, (284.6 ev) as the internal standard .
Reaction Propene metathesis was studied in a microcatalytic pulse reactor system. After the catalyst was pretreated, pulses of propene were passed over it with a flow rate of Nz (carrier gas) of 120 ml min-’ at 80 “C. The specific activity of the catalyst was simply expressed as converted molecules per Re ion in a pulse of propene (about 1.34 X 101’ molecules). The three kinds of pretreatment which were mainly involved in this work can be described as follows: (1) Nz pretreatment: the catalyst was pretreated with Nz at 550 “C for 1 h. (2) Hz-O1 pretreatment: the catalyst was reduced with Hz at 450 “C for 15 min and then flushed with Nz, heated to 550 “C and kept at 550 “C for 1 h, then cooled down to the reaction temperature (80 “C) followed by O2 uptake for 15 min. (3) Propene pretreatment: the catalyst was treated with 30 pulses of propene and then purged with Nz for 15 min, heated to 550 “C and kept at that temperature for 1 h.
Results and discussion The monolayer structure of Re207 supported on alumina XPS studies have shown that all of the Re ions of the catalysts mvestigated in this work are in the monolayer state. The XPS results on Re,O,A1203 catalysts are shown in Table 1 and Fig. 1. TABLE 1 Charactenzation Sample
of the monolayer structure of the Rea07-A120s
ReaO, content (wt.%)
R3A R7A RlOA R15A R20A k203
Re NH4ReOd
3 .o 6.8 9.2 13.1 15.3 0.0 100 0 -
catalysts by XPS
Binding energy (ev)
--ZAl(2P)
*Re(4f)
(Re ions nmm2)
O(ls)
AU2p)
Re(4f)
*o(ls)
*W~P)
0.46 1.08 1.51 2.23 2.66 0.0 -
631.1 531 .o 531 .o 531.0 531.4 530.7 -
74 .o 74.2 74.3 74 .l 74 7 74.0 -
45.8 46.2 46.2 46.2 46.2 40.2 46.0
0.13 0.13 0.12 0.13 0.12 0.13 -
0.21 0.47 0.63 0.92 1.28 -
531.3
83
00~ 0
3
1
Ions /nm2 Fig. 1. RelationshIp between the ratio of the relative XPS intensities, Z(Re(4f))lZ(Al(2p)) and the surface density of Re ions. Re
According to Kerkhof and Mouhjn’s model [7], in which a support is regarded as consisting of infinite sheets and a promoter is dispersed on the sheets, it has been shown that a linear relationship should exist between the relative XPS intensities of electrons from the promoter (P) and the support (S) and the corresponding atomic ratios in the bulk. The model leads to the following formula:
where I = the intensity of the XPS peak concerned; (P/S),, = the bulk atomic ratio of the promoter and the support; u = the cross-section of photoelectrons concerned: p a dimensionless factor, /I= t/h; t = the thickness of the sheet and h = the escape depth of electrons from the surface concerned. The value of K obtained from the experiment is equal to 31.6 which is in good agreement with that estimated from the model (K = 31.8) if o(Re,)/a(Al,,) is calculated from the XPS spectra of A1203 and NH4Re04 samples. Aldag et al. [8] reported that the saturated monolayer capacity of RezO,-A1203 catalysts amounted to about 2.7 Re ions nme2 when calcined at 550 “C. The catalysts used in this work have a surface density of Re ions lower than that value (see Table 1) so it is quite reasonable that all Re ions of the catalysts are in a monolayer state. Indeed, the reduction behaviour of the catalyst is strongly modified because of the strong interaction between the highly dispersed phase and the support, as follows from the results of TPR and XPS examination (see below). Propene metathesis over Re20,-Al,O, catalysts after various pretreatments Figure 2 summarizes the results of propene metathesis over Re20,A1203 catalysts after N2, H2-0, or propene pretreatment. Two different relationship trends are clearly observed between the specific activity and the surface density of Re ions on the catalysts. For N2-pretreated Re20,-Al,O, catalysts, the specific activity increases with the surface loading of the Re
oo0
1
2 Re Ions /nm*
Fig. 2. Variations m the specific actiwty of RezO,-Al20s catalyst after various pretreatments as a function of the surface density of Re ions; A N2 pretreatment, q HrOz pretreatment, 0 propene pretreatment.
ions, especially when the latter is higher than 1.0 Re ion nmm2. H2-02- or propene-pretreated catalysts show a reverse trend, which is more obvious in the case of propene pretreatment as shown in Fig. 2. Another feature which should be noted is that the specific activity of the catalysts is surprisingly low in all cases, not exceeding 0.34 propene molecules converted per Re ion. This seems very incompatible with the fact that the metathesis of olefin is a facile reaction which easily goes to equilibrium. The low specific activity must imply that only a few Re ions on the surface are in the active state. Aldag et al. [S] have worked out that as few as 0.1% of the total Re ions are active on the surface of catalysts of this type. The different correlation between specific activity and surface density shows the possibility of different active species, dependent on different structures and numbers of Re ions as a consequence of the different pretreatments of the same Re20,-Al,Os catalysts. Characterization of the surface species N2 pretreatment ESR studies have shown that N2 pretreatment at 550 “C for 1 h results in a psramagnetic species on the surface of the catalyst. The ESR spectra of N2-treated catalysts exhibit a characteristic signal with g = 2.22 and split into six lines, with a splitting constant A of 800 G (note: one splitting line has a much smaller splitting constant), in addition to the signal with g = 2.00. The relative ESR intensity of the signal with g = 2.22, as well as the specific ESR intensity of the signal, is very sensitive to and increases with the surface density of Re ions (Figs. 3 and 4). Yao and Shelef [9] reported ESR results of a Re20,-Al,O, catalyst with a surface density of 0.21 Re ions nmV2. ESR spectra obtained at room temperature after a so-called ‘dispersion treatment’ (sample degassed in vacuum at 500 “C for 5 h) were characterized by two distinct features: a very strong sharp line at g = 2.00 and a
85
0
Fig. 3. ESR spectra of RezO,-Al203
1
2 3 Re tons /nm2
catalysts after Nz pretreatments.
Fig. 4. Variations in the relative and specific ESR intensities of the signal with g = 2.22 as a function of the surface density of Re ions; o ESR intensity us the surface density of Re ions, A speclflc ESR intensity vs. the surface density of Re ions.
weaker six-line signal with g = 2.25 and a splitting constant A of 780 G. The weaker six-line signal was assigned to dispersed Ree4 ions and the strong sharp line to free electrons trapped in the vacant surface anion sites supposedly produced in the dispersion treatment. The ESR spectra recorded in the present work have features similar to those recorded by Yao and Shelef except that the signal with g = 2.00 is much weaker, and among the six lines of g = 2.22 signal only five lines are of equal splitting constant A. The differences are understandable if we recognize that much higher loading catalysts are used in the present work. As proposed by Nakamura and Echigoya [4] through their IR studies, catalytically active Re-0-Re dimeric species are created on the surface of high loading catalysts. Moreover, a very good linear relationship does exist between the relative ESR intensity of the signal with g = 2.22 and the conversion of propene, as shown in Fig. 5. This lends further evidence to the belief that dimeric species Re-0-Re are responsible for the high activity of the high loading catalysts obtained after Nz pretreatment. Hz-O2 pretreatment The reduction of Re@-A1,03 catalysts has been examined by TPR and XPS. Even when the catalyst is reduced at 550 “C!for 1 h, some Re+n ions are still left on the surface. Figure 6 is a typical XPS spectra of a RlOA catalyst reduced in situ at different temperatures. It is obvious that, for the reduced catalyst, the ratio of Re(U,,,) and Re(4fsJ peaks is different from that of Re metal. This may imply that some Re+” ions still remain on the surface. Therefore it is possible that the XPS spectra are composed of various valence states of Re ions. The TPR experiments lead to the same conclusion since the Hz consumption is far less than the theoretical values.
86
6
5,
iii >Z = 9
25 . 0
20 -
8 & 2 15 ep ; alo
P
/ cl / 0’”
OJ 0
50
100
150
200 IESR
50
Lb
42
Emding energy
30 (eV)
Fig. 5. Average conversion of propene us. the relative ESR mtenslty of the ESR srgnal with g = 2.22. Fig. 6. Re(4f) XPS spectra of RlOA catalysts reduced at different temperatures, (a) fresh RlOA catalyst, (b) metallic powder of Re, (c), (d) and (e) RlOA reduced at 350,400 and 500 “C respectively.
ESR examination of the catalysts after Hz--O, pretreatment exhibited an unsymmetric signal with gll = 2.04 and gl = 2.01, very similar to that assigned to the Ol ions on the surface as reported by Nakamura and Echigoya [4]. The formation of a partially reduced species of Re+“-O-Al+3 (n < +7) via Hz reduction seems indispensible because such a signal of Ol cannot be detected if the sample has not been subjected to any reduction before exposure to Oz. Besides, as shown in Fig. 7, the specific ESR intensity of the signal decreases quickly with the increase of the surface density of Re ions, i.e. with the increase in surface abundance of dimeric species. Therefore it appears that the partially reduced monomeric species on the surface play an important role in the formation and stabilization of Ol ions.
0' 0
1
2
Re lonshm2
0 5 10 ld6 reversible adsorbed molecules of Cl#r? catalyst
Fig. 7. Varratlon m the specific ESR mtenslty of the 0, tion of the surface density of Re ions. Fig. 8. Actrvity of ResO,-Al103 reversible 0, uptake.
signal wrth g = 2.01 as a func-
catalysts after HTOs pretreatment
us the amount of
87
Figure 8 shows a linear relation between the amount of reversible O2 uptake and the activity of the catalysts after Hz-O, pretreatment. The amount of O2 uptake provides an estimation of active sites of about 2 - 6% of the total Re ions on the catalyst. This value is about one order of magnitude higher than that obtained by Aldag et al. and somewhat smaller than that estimated by Nakamura and Echigoya. Quantitative coincidence, however seems very improbable on account of the large differences in surface state for catalysts of different preparation and pretreatment. However, the conclusion that 02- ions are actively involved m the reaction is justified and quantitatively substantiated both by the present authors as well as by Nakamura and Echigoya. Propene pretreatment The results of m situ XPS measurements exclude any severity of reduction occurring on the catalyst surface during propene pretreatment, since no significant changes in XPS spectra are observed before and after the pretreatment, as shown in Fig. 9. It is important to note that the results indicate, on the other hand, that a small but detectable part of Re ions is reduced during propene pretreatment. If the surface Re ions are completely intact, no 0, signal should have been detected. However, for catalyst treated with propene at 450 “C and subsequent O2 uptake at reaction temperature (80 “C), an ESR signal similar to that observed after Hz-O, pretreatment has been recorded. Furthermore, because of the minute population of propenereducible Re ions and the required higher temperature for treatment and for O2 uptake to produce the 02- signal, it is anticipated that the propenetreated catalyst is accommodated with reduced Re ions in a high oxidation state; in other words, the presence of Re ions as Re+6 or Re+5 might be expected in these catalysts after propene pretreatment. The ESR spectra of the catalysts after propene pretreatment also possess a clearly detectable signal characterized by g = 2.004. This signal is simply due to carbonaceous deposits as referred to in [9]. Although active carbon may be influential for electron-donating or -accepting processes of a catalyst surface, no satisfactory correlation can be drawn from the intensity of the ESR signal of these carbonaceous deposits with respect to the specific activity of the catalysts (Fig. 10). There are two factors imposed on the catalyst by the pretreatment with propene which might influence the catalytic performance in an opposite way: the active Re ions in the state of Re+6/Re+5 and the inhibiting carbonaceous deposits which make the active surface less accessible. As a consequence of the counter-effect of both factors, the effective temperature of a propene pretreatment will be restricted to a certain range near 450 “C. Creation of activity in Re207-A1203 catalysts for the metathesis of propene The above discussion stresses the fact that each pretreatment mentioned in this work will cause a certain reduction of Re,O,-Al,Os catalysts and concludes that two different kinds of active species, monomeric and
52
La BIndIng
LL energy
LO (eV)
350
450 550 Temperature PC)
Frg. 9. Re(4f) XPS spectra of RlOA catalysts treated wrth propene at different temperatures; (a) fresh RlOA catalyst, (b), (c) and (d) RlOA treated wrth propene at 350, 450 and 550 “C, respectively. Frg. 10. Dependence of the specific activity and the amount of carbonaceous (Zn~n by g = 2.004) of R3A on the temperature of propene pretreatment.
deposit
dimeric, exist on the surface. The former exists on low loading catalysts, the latter may become important in high loading catalysts. It has been shown that Nz pretreatment facilitates the formation of active dimeric species such as Re+” -O-Re+4 (the exact oxidation state of Re+” in the dimeric species is at present uncertain) in high loading catalysts. In low loading catalysts it is not possible to create any significant activity, because most Re ions strongly interact with the support, and the monomeric species in the form of Re+7-O-Al+3 are not feasible for deoxidation by simple N2 pretreatment. Hz is a strong reducing reagent, and dimeric species Re+“-O-Re+4 are unstable in a H, atmosphere at high temperatures. Hz pretreatment at high temperature leads to reduction of the dimeric species to metallic Re, or to interaction with the surface and formation of non-reducible surface complexes of Re ions such as Re+4- 0-Al+3. However, this does not result in an increase in activity, which means that surface species of this type are almost inert for metathesis, and O2 uptake at room (or reaction) temperature is necessary to bring about activity through reoxidation. Due to the ability of Re+4-O-A1+3 species to transfer electrons, 02- ions can be formed while the Re+4 ions are reoxidized to Re+’ ions. In other words, a new and active species is generated in the form of Re+5-O-A1’3 during O2 uptake, and the higher activity of low loading catalysts after this Hz-O2 pretreatment indicates their relatively higher abundance of active species and/or stronger interaction between Re ions and surface in comparison with higher loading catalysts. Propene reduction is a milder process than H2 reduction. According to our XPS and ESR results we propose the formation of some less reduced
89
or states: Re+6 and/or Re+5. Monomeric species such as Re+S-O-A1+3 Re+6-O-A1+3 in this case play the role of precursor for the reaction: such precursors are presumably characteristic in being highly active and needing no further oxidation for activation,
Conclusion It is apparent that the oxidation state of Re ions on the surface is crucial in the metathesis of propene for both high and low loading catalysts. For high loading catalysts, the active species are in a dimeric form or more definitely in the form of Re+“- 0-Re+4. For low loading catalysts, the active species is supposedly in the form of Re+5-O-A1+3 and/or Re+6-O-A1+3. To create such species on the catalyst surface, Hz reduction goes too far and reoxidation at room or reaction temperature is effectively needed for the ‘renaissance’ of Re+’ and/or Re+6 species. Propene reduction at 450 “C is surprisingly satisfactory in creating the inherently active species of Re ions in low loading catalysts. References 1 R. L. Ranks, Catalysis, Vol. 4, Special Periodical Report, The Chemical Society, London, 1980,~. 101. 2 F_ P. J. M. Kerkhof, J. A. Moulijn and R. Thomas,J. Catal., 56 (1979) 279. 3 R. Nakamura and E. Echlgoya, Chem. Lett., 109, (1981) 51, zdem, Reck Trau Chim. Pays-Bas., 96 (1977) M31. 4 R. Nakamura and E. Echigoya, J. Mol. Catal., 15 (1982) 147. 5 M. P. Madden, Ph. D. Dissertatzon, University of Oklahoma, 1983, J. C. Hsu, Ph. D. Dissertation, University of Oklahoma, 1980. 6 S. R. Robertson, J. Catal., 37 (1975) 424. 7 F. P. J. M. Kerkhof and J. A. Moulijn, J. Phys. Chem., 83 (1979) 1612. 8 A. W. AIdag, C. J. Lin and A. Clark, J. Catal., 51 (1978) 278. 9 H. I. Yao and M. Shelef, J. Catal., 44 (1976) 392.
79
79 - 89
EFFECT OF VARIOUS PRETREATMENTS ON RHENIUM OXIDE-ALUMINA CATALYSTS: THE STRUCTURE AND ACTIVITY OF ACTIVE SITES FOR PROPENE METATHESIS XU YIDE, WE1 XINGUANG, Dalton Institute (Chma)
SHI YINGZHEN,
ZHANG YIHUA and GUO XIEXIAN
of Chemical Physics, Chmese Academy
of Sciences,
129 Street, Dalian
Summary A series of Re20,-A1203 catalysts containing 3.0 - 15.3 wt.% Re,O, has been studied in a microcatalytic pulse reactor for propene metathesis. The effect of various pretreatments has been examined by catalyst evaluation as well as by the change in surface properties observed in TPR, 02adsorption, ESR and XPS measurements. After N2 pretreatment a linear relationship is obtained between the intensities of the ESR signal, assigned to partially reduced Re-0-Re dimeric species, and the activities of Re20,-Al,Os catalysts. The concentration of the dimeric species strongly depends on the Re20, loading. Hz reduction at 450 “C followed by O2 uptake at reaction temperature (80 “C) is effective in enhancing the activity of low loading catalysts, and the amount of reversibly adsorbed O2 is shown to be closely connected with the catalytic activity. In this case, it is easy to detect the formation of 02- ions on the surface by ESR observation. Surface species of
d 2is very plausibly responsible for the enhanced activity of the low loading catalyst. By pretreatment with propene at 450 “C, the activity of the low loading catalyst is greatly increased. A significant partial reduction of Re207 and simultaneous carbonaceous deposit on the catalyst surface are observed by ESR and XPS examination. It seems probable that monomeric species of Re+6 (or Re+5)-O-A1+3 are highly active and that the formation of such species on the surface is facilitated by treating the low loading catalyst with propene at 400 - 450 “C. It is apparent that the presence of appropriate unsaturated coordinated species of high-valence Re ions on dehydrated alumina is essential for active metathesis catalysts.
@ Elsevier Sequoia/Printed
in The Netherlands
80
Introduction Supported rhenium oxide-alumina catalysts are of great interest in the metathesis of olefins because of their high activity and good selectivity [ 11. Recently, many studies have been directed towards distinguishing the active species which is formed by various pretreatments on the catalyst, and is relevant to the catalytic function. Although it is well established now that the enhanced activity of the catalysts after N2 pretreatment at 550 “C strongly depends on the resulting surface composition, the nature of the active species is still not clearly understood. Kerkhof et al. [Z] suggested on the basis of Raman spectroscopic studies that the active surface species was solely the tetrahedral ReO, ion. However, according to the IR results of Nakamura and Echigoya [3], @e 20 7) ads species are responsible for the high activity in the metathesis of alkenes on high rhenium loading catalysts. For low loading catalysts, as Nakamura and Echigoya pointed out in [4], Hz reduction at 550 “C followed by O2 uptake at room temperature could also enhance their activity. They found that 02- ions could be formed under these conditions, and proposed that partially reduced monomeric species, plausibly in the form of Re’4-O-A1+3, might play the essential role of a precursor formulated as Re+5-O-A1+3 d 2They claimed furthermore that reduction by means of propene, followed by O2 uptake at room temperature, had the same effect. It seems that the oxidation state of Re ions is critical in initiating the active species on the surface of low loading catalysts. Madden [5] reported obvious dependence of the metathesis activity of Re20,-A1203 catalysts on the effect of the support rather than on the oxidation state of Re ions, since an in situ IR cell/reactor examination of the catalyst revealed that the Lewis sites on the catalyst become somewhat stronger than on simple Al2O3, while neither calcination nor reduction affects the rhenium-oxygen stretching vibrational frequency. The diverse results and contradictory conclusions in the literature about the nature of active species stimulated us to further investigate the active species of Re20,-A1203 catalysts for the metathesis of olefins. In this work, TPR, 02-adsorption, ESR and XPS methods were used in combination with activity test runs for characterizing both high and low loading catalysts. Emphasis was placed on the role of the oxidation state of Re ions. Experimental Materials A series of supported rhenium oxide-alumina catalysts with Re207 loading varying from 3.0 to 15.3 wt.% were prepared by impregnating
81
alumina (20 - 40 mesh, BET surface area S = 168 m2 g-i) with an aqueous solution of perrhenic acid which had been prepared by dissolving Re powder with the nominal purity of 99.99% in nitric acid. Samples were dried at 110 “C for 15 h and calcined in air at 550 “C for 4 h. The content of Re20, in the catalysts was determined by using an X-ray fluorescent spectrometer (Table 1). Gases used in the pretreatment and the reaction were purified by conventional methods. To further remove traces of water, a trap (-78 “C) was mounted on the reactor inlet pipe. Propene used in this work was in purity of polymeric degree, and was passed through a zeolite trap to remove traces of water. TPR The TPR set-up used in this work was almost the same as that used by Robertson [6]. Samples were first oxidized at 550 “C in a stream of O2 for 1 h and then flushed with Ar for 0.5 h. The reducing gas was a H2-Ar mixture containing 16.9% of H2 . Typically the TPR experiments were performed at a flow rate of 20 ml min-’ and a heating rate of 8 “C mm-‘. The consumption of H2 was measured by weighing the area under the curve; calibration was executed by using pure Hz. 0, adsorp tton O2 adsorption was carried out in a conventional pulse adsorption system. A sample was reduced at different temperatures and then pulses of O2 were passed over it at room temperature in a stream of Ar until the surface was saturated by the adsorbed 02. Then the sample was heated to 200 “C and purged with Ar for 0.5 h. The second O2 uptake, which was considered tentatively to be the amount of the reversibly adsorbed O2 on the sample, was measured at room temperature. ESR ESR spectra were recorded at X-band frequency on a JEOL-FEBXG ESR spectrometer at room temperature. The relative intensity of a signal was estimated according to the following formula: I
_
(AHPP )2H,PK A
(1)
where AHpp = the horizontal width and Hpp = the vertical height from the bottom to the top in the ESR signal concerned and recorded in the form of the first differential curve, A = the amplitude of the instrument used in the process of measurement and K = the proportionality coefficient. XPS A PHI 550 multifunction photoelectron spectrometer was used to measure and record the XPS spectra concerned. Mg & radiation (1253.6 ev) was selected as the X-ray source and a vacuum of 5 X 10-s torr was maintained during the measurements.
82
The finely ground sample was fixed onto double-sided adhesive tape or was pressed in the form of a tablet for examining its monolayer structure or reduction behaviour. A PHI 5500 multifunction analog computer was employed for repeating scanning, recording, data processing and drawing. The binding energy was measured by using Ci, (284.6 ev) as the internal standard .
Reaction Propene metathesis was studied in a microcatalytic pulse reactor system. After the catalyst was pretreated, pulses of propene were passed over it with a flow rate of Nz (carrier gas) of 120 ml min-’ at 80 “C. The specific activity of the catalyst was simply expressed as converted molecules per Re ion in a pulse of propene (about 1.34 X 101’ molecules). The three kinds of pretreatment which were mainly involved in this work can be described as follows: (1) Nz pretreatment: the catalyst was pretreated with Nz at 550 “C for 1 h. (2) Hz-O1 pretreatment: the catalyst was reduced with Hz at 450 “C for 15 min and then flushed with Nz, heated to 550 “C and kept at 550 “C for 1 h, then cooled down to the reaction temperature (80 “C) followed by O2 uptake for 15 min. (3) Propene pretreatment: the catalyst was treated with 30 pulses of propene and then purged with Nz for 15 min, heated to 550 “C and kept at that temperature for 1 h.
Results and discussion The monolayer structure of Re207 supported on alumina XPS studies have shown that all of the Re ions of the catalysts mvestigated in this work are in the monolayer state. The XPS results on Re,O,A1203 catalysts are shown in Table 1 and Fig. 1. TABLE 1 Charactenzation Sample
of the monolayer structure of the Rea07-A120s
ReaO, content (wt.%)
R3A R7A RlOA R15A R20A k203
Re NH4ReOd
3 .o 6.8 9.2 13.1 15.3 0.0 100 0 -
catalysts by XPS
Binding energy (ev)
--ZAl(2P)
*Re(4f)
(Re ions nmm2)
O(ls)
AU2p)
Re(4f)
*o(ls)
*W~P)
0.46 1.08 1.51 2.23 2.66 0.0 -
631.1 531 .o 531 .o 531.0 531.4 530.7 -
74 .o 74.2 74.3 74 .l 74 7 74.0 -
45.8 46.2 46.2 46.2 46.2 40.2 46.0
0.13 0.13 0.12 0.13 0.12 0.13 -
0.21 0.47 0.63 0.92 1.28 -
531.3
83
00~ 0
3
1
Ions /nm2 Fig. 1. RelationshIp between the ratio of the relative XPS intensities, Z(Re(4f))lZ(Al(2p)) and the surface density of Re ions. Re
According to Kerkhof and Mouhjn’s model [7], in which a support is regarded as consisting of infinite sheets and a promoter is dispersed on the sheets, it has been shown that a linear relationship should exist between the relative XPS intensities of electrons from the promoter (P) and the support (S) and the corresponding atomic ratios in the bulk. The model leads to the following formula:
where I = the intensity of the XPS peak concerned; (P/S),, = the bulk atomic ratio of the promoter and the support; u = the cross-section of photoelectrons concerned: p a dimensionless factor, /I= t/h; t = the thickness of the sheet and h = the escape depth of electrons from the surface concerned. The value of K obtained from the experiment is equal to 31.6 which is in good agreement with that estimated from the model (K = 31.8) if o(Re,)/a(Al,,) is calculated from the XPS spectra of A1203 and NH4Re04 samples. Aldag et al. [8] reported that the saturated monolayer capacity of RezO,-A1203 catalysts amounted to about 2.7 Re ions nme2 when calcined at 550 “C. The catalysts used in this work have a surface density of Re ions lower than that value (see Table 1) so it is quite reasonable that all Re ions of the catalysts are in a monolayer state. Indeed, the reduction behaviour of the catalyst is strongly modified because of the strong interaction between the highly dispersed phase and the support, as follows from the results of TPR and XPS examination (see below). Propene metathesis over Re20,-Al,O, catalysts after various pretreatments Figure 2 summarizes the results of propene metathesis over Re20,A1203 catalysts after N2, H2-0, or propene pretreatment. Two different relationship trends are clearly observed between the specific activity and the surface density of Re ions on the catalysts. For N2-pretreated Re20,-Al,O, catalysts, the specific activity increases with the surface loading of the Re
oo0
1
2 Re Ions /nm*
Fig. 2. Variations m the specific actiwty of RezO,-Al20s catalyst after various pretreatments as a function of the surface density of Re ions; A N2 pretreatment, q HrOz pretreatment, 0 propene pretreatment.
ions, especially when the latter is higher than 1.0 Re ion nmm2. H2-02- or propene-pretreated catalysts show a reverse trend, which is more obvious in the case of propene pretreatment as shown in Fig. 2. Another feature which should be noted is that the specific activity of the catalysts is surprisingly low in all cases, not exceeding 0.34 propene molecules converted per Re ion. This seems very incompatible with the fact that the metathesis of olefin is a facile reaction which easily goes to equilibrium. The low specific activity must imply that only a few Re ions on the surface are in the active state. Aldag et al. [S] have worked out that as few as 0.1% of the total Re ions are active on the surface of catalysts of this type. The different correlation between specific activity and surface density shows the possibility of different active species, dependent on different structures and numbers of Re ions as a consequence of the different pretreatments of the same Re20,-Al,Os catalysts. Characterization of the surface species N2 pretreatment ESR studies have shown that N2 pretreatment at 550 “C for 1 h results in a psramagnetic species on the surface of the catalyst. The ESR spectra of N2-treated catalysts exhibit a characteristic signal with g = 2.22 and split into six lines, with a splitting constant A of 800 G (note: one splitting line has a much smaller splitting constant), in addition to the signal with g = 2.00. The relative ESR intensity of the signal with g = 2.22, as well as the specific ESR intensity of the signal, is very sensitive to and increases with the surface density of Re ions (Figs. 3 and 4). Yao and Shelef [9] reported ESR results of a Re20,-Al,O, catalyst with a surface density of 0.21 Re ions nmV2. ESR spectra obtained at room temperature after a so-called ‘dispersion treatment’ (sample degassed in vacuum at 500 “C for 5 h) were characterized by two distinct features: a very strong sharp line at g = 2.00 and a
85
0
Fig. 3. ESR spectra of RezO,-Al203
1
2 3 Re tons /nm2
catalysts after Nz pretreatments.
Fig. 4. Variations in the relative and specific ESR intensities of the signal with g = 2.22 as a function of the surface density of Re ions; o ESR intensity us the surface density of Re ions, A speclflc ESR intensity vs. the surface density of Re ions.
weaker six-line signal with g = 2.25 and a splitting constant A of 780 G. The weaker six-line signal was assigned to dispersed Ree4 ions and the strong sharp line to free electrons trapped in the vacant surface anion sites supposedly produced in the dispersion treatment. The ESR spectra recorded in the present work have features similar to those recorded by Yao and Shelef except that the signal with g = 2.00 is much weaker, and among the six lines of g = 2.22 signal only five lines are of equal splitting constant A. The differences are understandable if we recognize that much higher loading catalysts are used in the present work. As proposed by Nakamura and Echigoya [4] through their IR studies, catalytically active Re-0-Re dimeric species are created on the surface of high loading catalysts. Moreover, a very good linear relationship does exist between the relative ESR intensity of the signal with g = 2.22 and the conversion of propene, as shown in Fig. 5. This lends further evidence to the belief that dimeric species Re-0-Re are responsible for the high activity of the high loading catalysts obtained after Nz pretreatment. Hz-O2 pretreatment The reduction of Re@-A1,03 catalysts has been examined by TPR and XPS. Even when the catalyst is reduced at 550 “C!for 1 h, some Re+n ions are still left on the surface. Figure 6 is a typical XPS spectra of a RlOA catalyst reduced in situ at different temperatures. It is obvious that, for the reduced catalyst, the ratio of Re(U,,,) and Re(4fsJ peaks is different from that of Re metal. This may imply that some Re+” ions still remain on the surface. Therefore it is possible that the XPS spectra are composed of various valence states of Re ions. The TPR experiments lead to the same conclusion since the Hz consumption is far less than the theoretical values.
86
6
5,
iii >Z = 9
25 . 0
20 -
8 & 2 15 ep ; alo
P
/ cl / 0’”
OJ 0
50
100
150
200 IESR
50
Lb
42
Emding energy
30 (eV)
Fig. 5. Average conversion of propene us. the relative ESR mtenslty of the ESR srgnal with g = 2.22. Fig. 6. Re(4f) XPS spectra of RlOA catalysts reduced at different temperatures, (a) fresh RlOA catalyst, (b) metallic powder of Re, (c), (d) and (e) RlOA reduced at 350,400 and 500 “C respectively.
ESR examination of the catalysts after Hz--O, pretreatment exhibited an unsymmetric signal with gll = 2.04 and gl = 2.01, very similar to that assigned to the Ol ions on the surface as reported by Nakamura and Echigoya [4]. The formation of a partially reduced species of Re+“-O-Al+3 (n < +7) via Hz reduction seems indispensible because such a signal of Ol cannot be detected if the sample has not been subjected to any reduction before exposure to Oz. Besides, as shown in Fig. 7, the specific ESR intensity of the signal decreases quickly with the increase of the surface density of Re ions, i.e. with the increase in surface abundance of dimeric species. Therefore it appears that the partially reduced monomeric species on the surface play an important role in the formation and stabilization of Ol ions.
0' 0
1
2
Re lonshm2
0 5 10 ld6 reversible adsorbed molecules of Cl#r? catalyst
Fig. 7. Varratlon m the specific ESR mtenslty of the 0, tion of the surface density of Re ions. Fig. 8. Actrvity of ResO,-Al103 reversible 0, uptake.
signal wrth g = 2.01 as a func-
catalysts after HTOs pretreatment
us the amount of
87
Figure 8 shows a linear relation between the amount of reversible O2 uptake and the activity of the catalysts after Hz-O, pretreatment. The amount of O2 uptake provides an estimation of active sites of about 2 - 6% of the total Re ions on the catalyst. This value is about one order of magnitude higher than that obtained by Aldag et al. and somewhat smaller than that estimated by Nakamura and Echigoya. Quantitative coincidence, however seems very improbable on account of the large differences in surface state for catalysts of different preparation and pretreatment. However, the conclusion that 02- ions are actively involved m the reaction is justified and quantitatively substantiated both by the present authors as well as by Nakamura and Echigoya. Propene pretreatment The results of m situ XPS measurements exclude any severity of reduction occurring on the catalyst surface during propene pretreatment, since no significant changes in XPS spectra are observed before and after the pretreatment, as shown in Fig. 9. It is important to note that the results indicate, on the other hand, that a small but detectable part of Re ions is reduced during propene pretreatment. If the surface Re ions are completely intact, no 0, signal should have been detected. However, for catalyst treated with propene at 450 “C and subsequent O2 uptake at reaction temperature (80 “C), an ESR signal similar to that observed after Hz-O, pretreatment has been recorded. Furthermore, because of the minute population of propenereducible Re ions and the required higher temperature for treatment and for O2 uptake to produce the 02- signal, it is anticipated that the propenetreated catalyst is accommodated with reduced Re ions in a high oxidation state; in other words, the presence of Re ions as Re+6 or Re+5 might be expected in these catalysts after propene pretreatment. The ESR spectra of the catalysts after propene pretreatment also possess a clearly detectable signal characterized by g = 2.004. This signal is simply due to carbonaceous deposits as referred to in [9]. Although active carbon may be influential for electron-donating or -accepting processes of a catalyst surface, no satisfactory correlation can be drawn from the intensity of the ESR signal of these carbonaceous deposits with respect to the specific activity of the catalysts (Fig. 10). There are two factors imposed on the catalyst by the pretreatment with propene which might influence the catalytic performance in an opposite way: the active Re ions in the state of Re+6/Re+5 and the inhibiting carbonaceous deposits which make the active surface less accessible. As a consequence of the counter-effect of both factors, the effective temperature of a propene pretreatment will be restricted to a certain range near 450 “C. Creation of activity in Re207-A1203 catalysts for the metathesis of propene The above discussion stresses the fact that each pretreatment mentioned in this work will cause a certain reduction of Re,O,-Al,Os catalysts and concludes that two different kinds of active species, monomeric and
52
La BIndIng
LL energy
LO (eV)
350
450 550 Temperature PC)
Frg. 9. Re(4f) XPS spectra of RlOA catalysts treated wrth propene at different temperatures; (a) fresh RlOA catalyst, (b), (c) and (d) RlOA treated wrth propene at 350, 450 and 550 “C, respectively. Frg. 10. Dependence of the specific activity and the amount of carbonaceous (Zn~n by g = 2.004) of R3A on the temperature of propene pretreatment.
deposit
dimeric, exist on the surface. The former exists on low loading catalysts, the latter may become important in high loading catalysts. It has been shown that Nz pretreatment facilitates the formation of active dimeric species such as Re+” -O-Re+4 (the exact oxidation state of Re+” in the dimeric species is at present uncertain) in high loading catalysts. In low loading catalysts it is not possible to create any significant activity, because most Re ions strongly interact with the support, and the monomeric species in the form of Re+7-O-Al+3 are not feasible for deoxidation by simple N2 pretreatment. Hz is a strong reducing reagent, and dimeric species Re+“-O-Re+4 are unstable in a H, atmosphere at high temperatures. Hz pretreatment at high temperature leads to reduction of the dimeric species to metallic Re, or to interaction with the surface and formation of non-reducible surface complexes of Re ions such as Re+4- 0-Al+3. However, this does not result in an increase in activity, which means that surface species of this type are almost inert for metathesis, and O2 uptake at room (or reaction) temperature is necessary to bring about activity through reoxidation. Due to the ability of Re+4-O-A1+3 species to transfer electrons, 02- ions can be formed while the Re+4 ions are reoxidized to Re+’ ions. In other words, a new and active species is generated in the form of Re+5-O-A1’3 during O2 uptake, and the higher activity of low loading catalysts after this Hz-O2 pretreatment indicates their relatively higher abundance of active species and/or stronger interaction between Re ions and surface in comparison with higher loading catalysts. Propene reduction is a milder process than H2 reduction. According to our XPS and ESR results we propose the formation of some less reduced
89
or states: Re+6 and/or Re+5. Monomeric species such as Re+S-O-A1+3 Re+6-O-A1+3 in this case play the role of precursor for the reaction: such precursors are presumably characteristic in being highly active and needing no further oxidation for activation,
Conclusion It is apparent that the oxidation state of Re ions on the surface is crucial in the metathesis of propene for both high and low loading catalysts. For high loading catalysts, the active species are in a dimeric form or more definitely in the form of Re+“- 0-Re+4. For low loading catalysts, the active species is supposedly in the form of Re+5-O-A1+3 and/or Re+6-O-A1+3. To create such species on the catalyst surface, Hz reduction goes too far and reoxidation at room or reaction temperature is effectively needed for the ‘renaissance’ of Re+’ and/or Re+6 species. Propene reduction at 450 “C is surprisingly satisfactory in creating the inherently active species of Re ions in low loading catalysts. References 1 R. L. Ranks, Catalysis, Vol. 4, Special Periodical Report, The Chemical Society, London, 1980,~. 101. 2 F_ P. J. M. Kerkhof, J. A. Moulijn and R. Thomas,J. Catal., 56 (1979) 279. 3 R. Nakamura and E. Echlgoya, Chem. Lett., 109, (1981) 51, zdem, Reck Trau Chim. Pays-Bas., 96 (1977) M31. 4 R. Nakamura and E. Echigoya, J. Mol. Catal., 15 (1982) 147. 5 M. P. Madden, Ph. D. Dissertatzon, University of Oklahoma, 1983, J. C. Hsu, Ph. D. Dissertation, University of Oklahoma, 1980. 6 S. R. Robertson, J. Catal., 37 (1975) 424. 7 F. P. J. M. Kerkhof and J. A. Moulijn, J. Phys. Chem., 83 (1979) 1612. 8 A. W. AIdag, C. J. Lin and A. Clark, J. Catal., 51 (1978) 278. 9 H. I. Yao and M. Shelef, J. Catal., 44 (1976) 392.