Journal of Molecular Catalysis, 46 (1988) 209 - 228
209
PROPYLENE METATHESIS CATALYSTS PREPARED BY INTERACTION OF Re2(CO),0 WITH r_A120, A. F. DANILYUK, V. L. KUZNETSOV*, V. A. SHMACHKOV, A. L. CHUVILIN and YU. I. YERMAKOV (t) Institute of Catalysis, Novosibirsk 630090
D. I. KOCHUBEY,
(U.S.S.R.)
Summary
In this work new data on the state of Re and on the nature of active sites in Re2(CO)i0 derived catalysts are discussed. Active catalysts for propylene metathesis can be obtained by several treatments of Re2(CO)i0/ A1203: (1) O2 treatment at 393 K of partially decarbonylated samples of Rez(CO)iO/AlzO,. In this case, part of the oxygen reversibly binds to the catalyst, probably in molecular form. The activity of this catalyst increases linearly up to an 0:Re ratio close to 0.03. Subsequent addition of oxygen has no effect on activity. (2) O2 treatment at 300 K of Re2(CO)i0/A1203 samples completely decarbonylated at 773 K. According to the EXAFS and TEM data, completely decarbonylated samples contain associated Re(I1) ions, with Re-Re distances different from those in metallic rhenium. O2 treatment leads to rearrangement in the clusters. Part of the oxygen is adsorbed in the molecular form as 0, on Al ions of the support (TPD and ESR data). The adsorption occurs reversibly and without exchange with the oxygen of the A1203. The activity of the catalysts correlates with their molecular oxygen content. (3) Partial reduction at 473 - 573 K of Re2(CO)i,-,/A1203 preoxidized at 573 K. The concentration of catalytically active Re atoms is assumed to be less than 2 - 3% of the rhenium content in the catalysts. Metal-carbene complexes active in metathesis are formed, via the oxidation of propylene, by rhenium ions in a high oxidation state.
Introduction
Rhenium oxide on alumina is one of the most active and selective catalysts for olefin metathesis [l, 21. These catalysts need different activation conditions, depending on the surface concentration of rhenium: *Author to whom correspondence 0304-5102/88/$3.50
should be addressed. 0 Elsevier Sequoia/Printed in The Netherlands
210
(i) for high rhenium loading catalysts, thermal pretreatment in a flow of N, is the most effective [3] ; (ii) low loading catalysts may be activated via reduction in hydrogen or propylene or via low-temperature adsorption of O2 on previously reduced samples [ 1, 41. Based on the existence of such regularities and the results of catalyst investigation by physical methods, many authors have proposed the existence of different catalytic sites in catalysts with different Re loadings [5 - 71. For high rhenium loading catalysts, (ReZOT)ads species or clusters containing Re-0-Re bonds are thought to be the precursors of the active sites [l, 5, 71. For low rhenium loading catalysts, the formation of active sites from the surface species containing fragments Re”+-0-A13+, has been proposed [7, 81. However, Xu Xiaoding et al. [9] suggested that Briinsted acidity plays a vital role in metathesis catalyzed by rhenium oxide on alumina catalysts. The investigation of rhenium catalysts is complicated because only a small portion of the supported rhenium takes place in metathesis, e.g. less than 0.3% of the rhenium atoms in the monolayer of a 20% Rez07/A1203 catalyst are active for metathesis [lo]. This property appears to be characteristic for all catalysts obtained from oxide compounds of Re [l, 111. Thus, it was of interest to study the formation of olefin metathesis catalysts using other compounds of Re. Recently we have studied the state of Re in the low-loading catalyst, Re,(CO)i0/A1203, and the activity after different pretreatments [12, 131. Scheme 1 shows the state of the majority of the Re in these samples.
vacuum, He, 423 - 473 K
PROM, vacuum,
/A1203 He, Hz, 773 K
Re,*+/Al 0
(III)
02, j:l:K Re6+/A1203
+r;
(IV)
’
Re7+/A1203
(V)
Scheme 1.
This paper discusses new data, characterizing the state of Re and the nature of the active sites in these catalysts.
211
Experimental Rez(CO),,/A120s were prepared as described in [12] by impregnation of y-A120s with hexene solutions of Re2(CO)io. EXAFS
The Re L III X-ray absorption spectra were obtained on the storage ring VEPP-4 of the Institute of Nuclear Physics of the Siberian Branch of the USSR Academy of Sciences. Experimental procedures were as described [14]. Spectra were recorded using a Ge(I1) cut-off crystal monochromator in the range from 200 eV before the absorption edge up to 1500 eV beyond the absorption edge with a step of 4 eV. All samples were measured at room temperature, as powders placed without contact with air in a metallic cell fitted with Be windows. The smooth part of the absorption coefficient was approximated by three cubic spline functions. Fourier transformation (FT) of EXAFS spectra was performed at all experimental points with a varying step of integration over an interval of wavenumbers, 3.8 - 11 A-‘. FT was performed with a window function of the Gaussian form. Functions of the radial distribution of atoms (FRD) are given in R-6 scale, where R is the true distance from the Re atom to the neighbour atom and 6 is the phase shift characteristic for this pair of atoms. Values of 6 depend on the intervals of EXAFS spectra treatment [15], being 0.22 A = 12 A-l, 0.3 A at K,,, = 11 A-l and 0.4 .& at at Kmin = 3.8 and K,,, K max = 10 A-’ for the pair Re-Re. Due to a considerable level of noise, the error in distance estimation was 6 - 8%. IR spectra
Spectra were recorded using a Specord 75 IR spectrometer. Sample preparations were carried out within the glass infrared cell which made possible the treatment of an Al,Os wafer with Rez(CO),, solution and the flow of gases (He, C,H,, CsH, + 0,) without air contact up to 573 K. Temperature-programmable
desorption
(TPD)
TPD of oxygen was investigated using a high vacuum installation with a monopole mass spectrometer MX-7303 (vacuum 1 X lop6 Pa, heating rate 5 K min-l). Preliminary oxygen adsorption (91.2%, ‘so*) was carried out at 67 Pa.
Transmission
electron
microscopy
(TEM)
Transmission electron micrographs were obtained using a JEOL-100X apparatus with 3 A resolution. After dispersion in alcohol by ultrasonic vibration, catalyst samples were supported on a perforated carbon film and placed in the operational chamber of the microscope. Due to the chamber
212
construction it was possible to treat the samples at high temperature without contact with the atmosphere. Catalytic
tests
Testing was performed in a flow differential reactor at 298 - 523 K at atmospheric pressure of propylene. To remove traces of OZ, propylene (polymerisation purity grade) was passed through a U-tube with 20% Mn(N03),/SiOz reduced in Hz at 643 K. Products were analysed gas-chromatographically, using 0.3 cm X 3 m A1203 and 0.3 cm X 2 m graphitized carbon columns. The rate of propylene metathesis was calculated from the yield of ethylene and butenes. A mixture of 0, with He was added by using a glass four-way port doser.
Results and discussion
Re2(CO)1,JA1203 catalysts, pretreated under different conditions (Scheme 1, samples I, II, III IV) and thoroughly investigated earlier using XPS, ESR, IR spectroscopy and other [ 121 methods, were now studied using EXAFS, TEM, IR and TPD methods. Sample I
Figure 1 shows the Re L III absorption spectra for I at 298 K, the normalized EXAFS data extracted from it and the modulus of the Fourier transform of the EXAFS data (the function of the radial distribution of atoms (FRD). There are three maxima on the FRD, which may be related to the distances between the Re atoms and the atoms in the first two coordination spheres (Table 1). The peak with maximum intensity appears to correspond to the distance between the atoms of Re. Taking into account the phase shift (6) value, this peak corresponds to the distance 2.98 A. This value R differs insignificantly (in the error limits) from the Re-Re distance, estimated using X-ray methods and neutron scattering (3.02 and 3.04 A, respectively), for Re2(CO)i0 [16]. A peak at 1.6 A (phase-shift: corrected value 2.0 A) can be related to the Re-C distances, characteristic for the Re carbonyl (2.0 a [16]). The relatively low intensity of this peak can be explained as follows. The amplitude of photoelectron backscattering on the carbon atom falls to almost zero in the range of wavenumbers 6 8 A-‘. For this reason the effective interval for analysis is 3.8 - 8 A-l with a length of -4 A-i. The G aussian window function in the FT of the EXAFS data decreases the effective interval length to 2 - 3 A-r. Thus, a small interval of analysis and a high noise level leads to a situation in which the peak corresponding to the Re-C distance has an only marginal intensity. The second reason for the low intensity of this peak may be due to the existence of Re-C distances in adsorbed carbonyl Re,(CO),, which differ somewhat from each other. This is similar to the broadening of the IR
213
, 0.0
, _ .., 2.0
4.0
6.0
6.0
10.0
R-S (8) Fig. 1. X-ray absorption spectrum (L III region) of rhenium (A), normalized EXAFS data (B), with associated Fourier transform (C), for I.
bands of the CO groups of adsorbed Rez(CO)io compared with the bands for this complex in solution [ 121. The peak at 3.64 A in the FRD may correspond to a partial decomposition of the adsorbed Re2(CO)ia. To account for the high intensity, one may suppose that this peak may be related to the distance between two Re ions (-3.94 A) bonded to each other via an oxygen ion. Analysing the IR [12] and EXAFS data for the initial samples of one concludes that the majority of the Re exists as Re&O)io/ALO3, Re2(CO)io. At the same time the EXAFS and XPS [123 data indicate that some of the adsorbed Re2(CO)i0 is oxidized. A peak corresponding to the distance Re-Re -3.9 A is also evident in the FRD. Finally the peaks for the Re 4f level from the XPS data for adsorbed Rez(CO)io were broadened
214 TABLE 1 Values of interatomic (EXAFS data) Sample
I II
distances for the coordination
sphere of Re in Re*(CO)~~Al~O~
R-6 a ta,
Eb
6 a)
RC (A)
1.6 2.68 3.64
C Re Re
0.46 0.3 0.3
2.06 + 0.16 2.98 k 0.24 3.94 rt 0.32
1.68 2.66
C(O) Re
0.46 0.3
2.14 + 0.16 3.96 1: 0.24
aMaximum position on FRD without phase correction (6). bSupposed scattering atom. Yf’rue interatomic distance.
and shifted to a higher binding energy compared with that of Re2(CO),, itself 1121. Sample II According to IR, XPS, ESR and TPD data, there are subcarbonyl complexes existing in sample II (Scheme 1) containing RefCO), fragments with an average oxidation state of Re2+ [ 121. Figure 2 shows the results of an investigation of these samples by EXAFS. The two most important peaks on the FRD appear at 2.66 and 1.6 A. The peak at 2.66 a is most likely to be attributed to the Re-Re distance of 2.96 A (Table 1) because of its high intensity. The Re-Re distances for hydridocarbonyl complexes of Re (r(Re-Re) = 2.90 A [ 161) are close to this value. The peak maximum at 1.6 a on the FRD for II is related to the Re-C bond distance giving the value 2 a (Table l), which is close to the analogous value for the complexes H2Re2(CO)s and H4Re4(C0)i2 [ 161. It should be noted that the EXAFS method does not allow oxygen and carbon scattering atoms to be distinguish~ from each other. Therefore, according to the data for the Re oxidation state in II [12] and to account for the high intensity of the FRD peak at 1.6 A, one may also suppose that the Re-0 bonds make a considerable contribution to the peak. Clusters of Re ions in II were not observed using electron microscopy (Fig. 5a). This may be the consequence of the small sixes of these clusters and their low contrast compared with the support background [ 171. Oxygen pretreatment of II at 393 K does not alter their EXAFS spectra. This agrees with the IR data (see below) relating to the stability of the position and the intensity of the carbonyl bands for Re(CO), fragments, when II is treated by oxygen and/or propylene. Samples III and IV Figure 3 shows the EXAFS data for sample III. For these cases, the EXAFS oscillation amplitude is significantly (more than 2 times) lower
215
)1(E) 2.293 2.198
Energy
(eV )
&i+WbW”““‘.‘....‘..“‘.‘.‘iHfl 0.0
2.0
Fig. 2. X-ray absorption spectrum Fourier transform (C), for III.
6.0
5.0
(A),
normalized
**O R-S(d9.0
EXAFS
data
(B),
with
compared with those for I and II, leading to a considerable decrease in the signal/noise ratio and to difficulties in estimating the true distances from the FRD. Similar spectra were obtained for IV (Fig. 4). Spectral acquisition for the Re catalysts was carried out under the same conditions, so that the decrease in the intensity of the EXAFS oscillation amplitudes seems to correspond to the existence of Re ions in the samples, which differ considerably in their environments.
216
217
Based on the EXAFS data obtained, it is only possible to state that there are no detectable amounts (-10%) of metallic Re particles in the samples. Otherwise, by virtue of the high packing density of the Re atoms in the metal, the EXAFS oscillation amplitude would be considerably higher, even for very small particles. Particles less than 25 A in size were observed in the electron micrographs of III (Fig. 5b). According to the results of oxygen titration of these samples, the average oxidation state of Re was estimated to be 2+ [ 121. Therefore, the difficulty encountered in the reduction of such particles in H, at 773 K may testify to their two-dimensional structure, together with the existence of chemical bonds between Re and the support oxygen, as proposed by Yao and Shelef [ 181.
Fig.
5.
catalysts. treatment
Transmission
electron
micrographs
(A) after thermal treatment under vacuum at 773 K (III);
For the same region of Rez(CO)10/A1203 under vacuum at 473 K (II); (B) after thermal (C) after O2 adsorption on sample III (IV).
Increasing the surface concentration of Re leads to the formation of three-dimensional Re particles, which can be readily reduced to metal [ 181. The existence of Re clusters in high-loading Re catalysts was demonstrated in [19]. Oxygen pretreatment of III leads to a considerable decrease in the contrast of the Re clusters in the electron micrographs until the smallest clusters completely disappear (Fig. 5~). This may show the rearrangement of their structure accompanied by disaggregation and the resulting decrease in the particle-contrast in electron micrography.
218
In accordance with the XPS, ESR and oxygen titration data, oxygen adsorption on III at 298 K to form IV leads to only partial oxidation of Re to, on average, the oxidation state 6+. Part of the oxygen is adsorbed as Ol on A13+ ions [12]. The TPD i80Z data for IV will be presented together with the results of studies of their catalytic properties. Catalytic properties of rhenium catalysts obtained from Re,(C0)20/A1,03 Active catalysts for propylene metathesis can be obtained from Re,(CO),,/A1203 after the following pretreatments: (i) adsorption of O2 at 393 K on the partially decarbonylated Re,(C0)i0/A1203 samples at 423 - 473 K (II): (ii) adsorption of O2 at 300 K on completely decarbonylated Re2(C0)&A1203 samples at 773 “C (IV); and (iii) partial reduction at 473 - 573 K of Re,(CO)i0/A1203 samples, preoxidized at 573 K [I31 W). Pretreatment (i) Immediately after preparation, Re,(CO),,/A120s (sample I) is not active in propylene metathesis. Sample II, heated in a flow of purified inert gas or in vacuum at 423 - 473 K, also does not show any catalytic properties. Only after pretreatment with small pulses of O2 at 393 K do they acquire catalytic activity. Using a glass flow cell, the interaction of II with propylene and oxygen was studied by IR spectroscopy. The IR spectra of II, containing clusters of Re ions with Re(CO), fragments (absorption bands 2020 and 1900 cm-‘) were found to be practically unaltered after treatment of the sample with oxygen, propylene and propylene-oxygen mixture (0.5% 0,) at 393 - 453 K. This shows that surface subcarbonyl complexes of Re containing Re(CO), fragments are inactive in propylene metathesis and are not precursors of the active sites of this reaction. Pretreatment of II with O2 at 393 K renders them rather active in propylene metathesis. Therefore one may suppose that, in addition to surface complexes containing Re(CO)* fragments, there are some species on the surface of II which are capable of interaction with 0, to form metathesis-active sites (or their precursors). It should be noted that at temperatures lower than 390 K oxygen does not activate II. However, as reported previously [12], low oxidation state Re ions without CO groups can be readily oxidized at room temperature (sample IV). It appears that during pretreatment (i), O2 oxidizes subcarbonyl species of Re, designated here as Re(CO),/A1203. The stability of such species to the action of O2 at room temperature may be explained by the presence of carbonyl ligands in their composition. Figure 6 shows the dependence of initial catalytic activity of II for propylene metathesis on the temperature of their preliminary thermal treatment in a flow of He. Since the clusters with Re(C0)2 fragments are inactive in propylene metathesis, the curve in Fig. 3 represents mainly the conditions of formation and the thermal stability of the surface species Re(CO),/A120,. At temperatures lower than
219
350
550
450
Thermal
treatment
temperature,
K
Fig. 6. Dependence of the initial activity of Re2(CO)ie/A1,0s catalysts in propylene metathesis on the thermal treatment temperature under vacuum (after thermal treatment catalysts were activated by 0s at 393 K, reaction temperature 393 K).
393 K, such species are not formed and the system is catalytically inactive. Rather active catalysts are obtained after treating Re2(CO)i0/Al,03 in a flow of He or in vacuum at 423 - 453 K. Further increase in the decarbonylation temperature appears to result in the decomposition of Re(CO),/Al,Os complexes, followed by a decrease in the catalytic activity of the samples. Figure 7 shows the dependence of the initial activity of sample II (decarbonylated at 453 K) on the amount of O2 added at 393 K. Up to an O/Re ratio -0.035 a linear increase in activity is observed, but further addition of O2 does not significantly influence the reaction rate. Taking into account the fact that the formation of active sites may require more than 1 atom of oxygen per Re ion and that oxygen can be consumed in side reactions, the amount of active sites may be less than 3.5% of the total amount of rhenium in the catalyst. This may explain
0.025
0.05
O/Re Fig. 7. Initial for activation
1.0
2.0
ratio
activity of II in the propylene (at 393 K).
metathesis
at 393
K us. amount
of 01 used
220
% Gt
I 50 Time.
100 min
Fig. 8. Activity of II in propylene metathesis us. time of treatment in gas flow after 02 activation at 393 K: (0) CsHs; (0) He (reaction temperature 393 K).
the observation that complexes of Re(CO),/Al,Os are not observed in the IR spectra of II in the dominant background absorption of Re(C0)2. Further study of the activating pretreatment of II by 0, demonstrated the reversible character of the interaction with oxygen. Figure 8 shows the dependence of the initial catalytic activity of II, activated in 02, on the exposure time to a flow of He or C3H, at 393 K. The catalytic activity is seen to decrease in a flow of both propylene and inert gas. Nevertheless after a fresh pulse of 02, the catalyst activity is completely restored. It should be noted that if the initial activation in O2 occurs at only 393 K, catalyst activation could be restored by pretreatment in O2 even at room temperature. When samples are activated in 02, oxidation of the surface complexes of Re(CO),/Al,Os appears to be followed by the removal of CO from the coordination sphere of Re. In this case, part of the oxygen is reversibly bonded to the catalyst forming metathesis-active sites or their precursors. When a catalyst is exposed to a flow of He, oxygen is lost, leading to a decrease in the catalyst activity (Fig. 8). The addition of a fresh portion of 0, leads to its adsorption on the catalyst, followed by regeneration of active sites. Re ions which have reacted with oxygen contain no CO ligands, and therefore reactivation in O2 can be carried out at room temperature. Pretreatment (ii) Completely decarbonylated samples Re,(CO),s/A1203 (III calcined in vacuum or in a flow of H, at 773 K) do not exhibit any activity towards propylene metathesis; only at room temperature after pretreatment with O2 do they become active as IV. As mentioned above, adsorption of O2 at 300 K on completely decarbonylated samples leads to the partial oxidation of clusters of Re to the higher oxidation state. In this process, part of the O2 is adsorbed on A13+ ions as O,-. In accordance with TPD data, IV, containing ion-radicals 02-, is capable of desorbing molecular oxygen into the gas phase at 340 - 370 K.
,n-\ \ //
:
\
\
N
\
425
Tempetoture.
Ed, k&&l
\
325
Tmax
31097
-2
1,--l
I
221
2
360
103
3
335
95
4
320
91
525
K
Fig. 9. TPD patterns of ‘802 from IV with different rhenium loadings (wt.% Re): (1) 0.5; (2) 0.82; (3) 2.0; (4) 3.0 (adsorption of ‘*02 at 298 K, 3 torr, 20 min).
Figure 9 shows TPD patterns of O2 for samples IV, cont~n~g different amounts of supported Re. It should be emphasized that the adsorption of ‘802 molecules on IV leads to the desorption of isOZ molecules only (in accordance with the enrichment of the O2 used). The absence of exchange between isOZ and oxygen ‘60 of the supp ort indicates that desorbing-oxygen has been adsorbed on the catalyst in molecular form. Comparatively low activation energies of desorption (-90 - 100 kJ mol-‘) also testify to the molecular nature of the adsorbed oxygen [20]. The adsorption-desorption cycles are reversible. When cooled, the samples can adsorb O2 and then desorb it at the elevated temperature, showing the existence of the following equilibrium (Scheme 2):
Scheme
2.
The dependence of the amount of oxygen desorbed on the content of Re in the catalysts (Fig. 10) may show that the same coord~atively unsaturated ions Ai3+ (A1203 centres) take part in oxygen adsorption and in the bonding of the surface Re compounds with the support. Reversible adsorption of oxygen is not observed for the catalysts containing more than 0 .4 X 101'Re atom m-* on the Alz03, This maximum value, corresponding to the surface saturation of A1203 by Re compounds, blocking the 02 adsorption sites, is close to that calculated by Yao and Shelef [18] for an A120s surface occupied by two-dimensional Re species. They proposed that increasing the surface concentration of Re leads to the formation of threedimensional compounds of Re.
222
1 Re
2 content,
3
4
wt.%
Fig. 10. Quantity of reversibly adsorbed 02 US.rhenium loading of IV.
The high-temperature pretreatment of A1203 removes the surface water to form coordinatively unsaturated A13+ ions [21], which are the oxygenadsorbing sites in Re2(CO)iO/A1203 catalysts. In fact, after prolonged pretreatment of IV in vacuum (1.3 X 10m5 Pa) at 773 K, the amount of reversibly adsorbed oxygen was found to increase by 25%. These observations seem important because the catalytic activity of samples IV depends upon the amount of reversibly adsorbed oxygen. Figure 11 shows the dependence of the initial rate of propylene metathesis for IV on the amount of O2 desorbing. These data demonstrate that the catalyst activity increases non-linearly with the amount of oxygen adsorbed. Such a non-linearity may
Fig. 11. Initial activity of IV in propylene metathesis at 298 K us. quantity of reversibly adsorbed O2 (catalysts with different rhenium loadings were used).
223
be the consequence of a relatively wide binding energy distribution of the adsorbed 0, molecules. The results on oxygen thermal desorption (Fig. 9) show that the more oxygen adsorbed by the sample, the higher the activation energy of oxygen desorption. The distribution of binding energy of oxygen molecules with the catalyst may reflect the environmental heterogeneity of the Re ions in (0,~)Re w +i)+/A1203. The differences which occur in the Recn + ‘)+ ion environment may influence their ability to form metathesis-active sites or the activity of the sites formed. These reasons may explain the non-linear dependence of the initial catalyst activity on the content of molecularly adsorbed O2 In studies of the Rez07/A1203 system, Nakamura and Echigoya [8] obtained a simple linear dependence for the rate of the metathesis reaction on the amount of oxygen adsorbed. These authors, however, do not give any data concerning the energy characteristics of the adsorbed oxygen, and investigate a catalyst having the same content of Re. In consequence, the deviation from the linear dependence of the reaction rate on the oxygen content may be not as obvious as observed in this present study. The variation in the catalyst properties of Re catalysts as a result of adding transition element admixtures [5,9] may be explained by the changes in oxygen-bond strength in the coordination sphere of Re. This change, in its turn, may influence the formation of carbene complexes which are active in metathesis. Similar assumptions have been made previously [ 51. Taking into account the participation of oxygen in the formation of active sites, their amount does not seem to exceed 0.5% of the total number of Re ions in sample IV. Catalysts for propylene metathesis, obtained by oxygen activation of II at 393 K, show little difference in their properties in comparison with catalysts IV. The close similarity in the catalytic properties and in the conditions of the formation of metathesis-active sites suggests that both catalysts II and IV contain the same type of active sites. Processes of oxygen adsorption, which may lead to activation of these samples, are presented in Scheme 3.
Scheme 3.
The active sites for metathesis, or their precursors, are shown in the scheme as (0,) Re(" + ‘)+/Al2O3, where O2- is bonded to an A13+ ion which is close to the reversibly-oxidizing Re ion in a high oxidation state. Dioxygen seems not to be the only possible activating-agent for Re2(CO)i0/A1203 catalysts. N,O and NO molecules may also act as activators,
224
TABLE 2 Rez(CO)&A120s catalyst activity in the reaction C$-Ie, differential reactor)
of propylene
metathesis (760 torr
Catalyst
Pretreatment conditions
Temperature (K)
Reaction rate mol C3H6 x 102 i mol Re s
I II II II III IV V V V
-
298 - 393 393
0 0 0.9
02, 393 K
298 393 393 298 298 393 298
02, 393 K III, 02, 298 K Hz, 523 K
4.5 0 4.8 0.067 0.25 0.2
but it should be emphasized that, in addition to their oxidizing properties, they possess the ability to accept electrons on adsorption on the catalyst. Catalytic activity of the samples obtained by oxidation at high temperatures with oxygen (pretreatment iii) Sample V, oxidized by oxygen at high temperature, demonstrates low activity in propylene metathesis at room temperature (Table 2). Hydrogen pretreatment at increased temperature however increases their catalytic activity. The data presented in Fig. 12 show that there is an optimum degree
m
3 -5 t Cl
c
E” 2
F
0” c
-?I f
m
2 373
473
Reduction
573
773
673
temperature
( K)
Fig. 12. Initial activity of V in propylene metathesis at 393 K us. reduction temperature in H2 flow and temperature-programmed reduction (TPR) pattern of this catalyst.
225
of reduction for the Re in active metathesis catalysts. On raising the reduction temperature, the activity increases and reaches a maximum value at the reduction temperature 523 K. A further rise in the reduction temperature leads to a decrease in the catalyst activity. Hydrogen reduction, even at 773 K, does not substantially increase the size of the particles in the low Re-loading samples. Therefore, the change in catalyst activity appears to depend not on the change in dispersion of the active component, but on the change in the oxidation state of the Re ions in the catalyst. Comparing the curve for the dependence of activity on the reduction temperature of V with the data for temperature-programmed reduction (TPR, Fig. 12), one may conclude that the highest catalyst activity is achieved at that reduction temperature at which no absorption of hydrogen is observed on the TPR curve. The amount of hydrogen absorbed by V at 523 K lies within the accuracy limits of the procedure used for its measurement. Therefore, it is not possible to estimate precisely the number of active sites formed in the reduction. It can only be supposed that the amount of active sites is very small and that an average oxidation state of Re in the active (IV) is close to 7+. The active sites for metathesis, or their precursors, in these catalysts probably contain Re ions in a high oxidation state (6+ or 5+).
Supposed structure and formation of the active sites for propylene metathesis catalysts The number of catalytically active metathesis sites is rather small, and the data, obtained by physical methods, characterize only the state of the majority of the supported Re. Therefore, we can only draw conclusions based upon the results of TPD studies and the catalytic tests of Re2(CO)ia/ Al,Os catalysts. The carbene chain mechanism for metathesis is now generally accepted [ 1, 21, so that the formation of the initiating metal carbene has received considerable attention. Isomerization of coordinated olefins or transformation of o-bonded alkyl groups (e.g. a-hydrogen abstraction) are mainly considered to be steps in the formation of the metal carbene. These reactions do not require a change in the oxidation state of the transition metal ion. The results obtained here force the conclusion that catalytic activity towards propylene metathesis is associated with the presence of species containing Re ions in a high oxidation state. It is reasonable to suppose therefore that alkene interaction with high oxidation state Re proceeds via the redox type of reaction. Terminal 02- ions in the first coordination sphere of the Recn + I)+ ion may be considered to be carbene analogues. A consideration of some biradicals as carbene analogues (:SiR,, :NR, :S) allows similarities in the mechanisms of their reactions to be found [22 - 241. Thus, the alkene interaction with oxygen-containing Re species and the formation of the coordinated carbene may proceed in a fashion similar to the steps of chain growth in the metathesis process:
226
0
0
>F!e (n + l)+ A
+
CH,-_CH=CH,
-
H I CHJ
=RiJe(“+L)+C/
b
0 -
CH 2
H
>I!,=&
‘CH3 +
O=CH2 Scheme 4.
The suggested scheme includes alkene coordination, formation of a fourmembered metallocycle, and its subsequent decomposition with the formation of a metathesis-active site. In the catalyst activation process, the role of reversibly adsorbed molecular oxygen (according to Scheme 2) is to form Re(“+ ‘)+/A1203 ions, which are capable of reaction, from Re”+/A1,03 ions. Such active Re species may also arise from the pretreatment with hydrogen of completely oxidised Re2(CO)i0/A1203. However, there were no indications in this work of the coincidence of Re oxidation states in catalysts activated by different pretreatments. It is known that olefin oxidation on some catalysts which contain transition metal oxides may proceed with rupture of the C=C bond of the olefin [25]. A catalytic reaction for the formation of formaldehyde from ethylene and oxygen in the presence of an alumina-molybdenum catalyst has been described [4] and named by the authors as the cometathesis of ethylene and oxygen. Indeed, the important role of oxygen in the activation of some olefin metathesis catalysts was highlighted by the investigators. Nevertheless, the mechanism of catalyst activation with oxygen is still not clearly understood. An assumption is made that the role of the oxygen is limited by the oxidation of the metal ion to some intermediate oxidation state [ 271. Scheme 4 may be valid for the majority of cases of the formation of metathesis catalysts via the interaction of olefins with transition metal oxide compounds in a high oxidation state. Based on this, there is no need to assume initially that the metal ions are reduced by the olefin to some intermediate oxidation state, with subsequent absorption and isomerisation of the olefin into the active metal carbene. According to Scheme 4, the carbene centre is formed immediately in an olefin redox reaction with an oxygen-containing species of the transition metal. It is also possible that 02- ions in a coordination sphere of the active carbene complex =Re(” + ‘)+(=O)(=CHCHs) are essential for metathesis (see Scheme 4). In accordance with the theoretical energy calculation of Rappe and Goddard for Cr and MO compounds, an oxo-ligand directs the conversion of the coordinated olefin precisely to that step in which the metathesis products form [ 28,291. The promoting effect of oxygen can be observed in real catalytic systems, when compounds containing no metal-oxygen bonds are used as the initial reactants. For example, ReCl, + AlEts, W(CO)+rene, RuC12-
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(PPh& systems exhibit their high activity in metathesis only in the presence of catalytic amounts of oxygen [27, 30, 311. Analyzing Scheme 4 for the formation of metathesis-active sites, it can be seen that formaldehyde is a byproduct of this reaction. Considering the complexity of the problem, two methods were used to attempt to discern the presence of formaldehyde in the products of the reaction of propylene with a catalyst: (i) the adsorption of water vapour on a catalyst pretreated with propylene (to remove CH,O from the A120s surface), with subsequent freezing of the products and chemical microanalysis [ 321; (ii) thermal desorption of the products of the reaction of propylene with a catalyst in high vacuum and subsequent mass spectrometric analysis. Unfortunately, direct evidence for the presence of formaldehyde in the products of interaction of O*-pretreated catalysts with propylene was not obtained by the methods used. It is likely that the very low expected concentration of formaldehyde (1 X lo-’ mol g-’ catalyst) and its high reactivity, especially in the presence of Al*Os with a large surface area together with Re oxides, are responsible for the failure. Nevertheless CO (m/e = 28) desorption was detected in the TPD spectra of catalysts (IV) used in propylene metathesis. This may testify indirectly to the presence of formaldehyde in such catalysts, since CO is one of the products of the thermal decomposition or partial oxidation of formaldehyde. However, CO can also be formed in the reaction of olefins with surface oxygen. Based on the facts that the number of metathesis-active sites in catalysts obtained from Rez(CO)i0/A1203 is very small, and that their properties are very similar to those of low Re-loading catalysts prepared using Re*O, or NH4Re04, it will be of interest in future to study the formation of carbene complexes from oxygen-containing complexes of Re; and to search for new Re compounds which would make possible the synthesis of active sites for olefin metathesis on the surface of solid supports. The latter effort may lead to more effective rhenium catalysts for olefin metathesis.
Conclusion The results obtained show that for the catalysts prepared using Rez(C0)iO/A1203, the number of active sites for propylene metathesis does not exceed 1 - 2% of the total amount of Re in the sample. These catalysts are similar in their properties to those investigated previously and prepared using RezO, or NH4Re0+ It seems that carbonyl ligands are not present in the structure of the carbene complexes which are active in metathesis. These complexes are thought to be formed via the oxidation of propylene by Re ions in a high oxidation state. Such Re ions may occur as a result of low-temperature oxygen pretreatment of the samples containing low-valent Re ions, or from the partial reduction of samples previously oxidised at high temperature.
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