- Email: [email protected]
alumina metathesis catalyst
doumal
of Molecub
Cutdysis.
15 (1982)
14’7 - ‘if36
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STRF_jm AN.5 NATURE OF _4CTZVE SITES OXIDE/ALUMINA METATHESES CATALYST
RYUICKT NAECAMURA and ETSURO Demenf of Chemical Engineering, Meg-w-o-ku, -Tokyo 152 (Japan)
ON AN O,/Rre
ECHIGOYA Tokyo
Irrstituh of Technoiogq.
Ookayama.
The adsorption of oxygen onto a pre-reduced rhenium oxide/alumina catalyst which was prepared by the reduction of Re207/AJ& with H2 or _ CO at 773 K dramatically increases its catalytic activity for the metathesis of propene, yielding a paramagnetic species on its surface. The catalytic activity is proportional to the amount of the radical. The radical was assigned as 0, bound to A13* which is located in the vicinity of the active site, Re(*l)C. The active site, which is a thermaLly meta-stable species, is then formed by a single electron transfer from a relatively stable but cataEyticaHy inactive RenC ion, possibly Re ‘+ to the adsorbing oxygen molecule. Heating the activated cat-a&t in vacuum up to about 523 K causes the into gaseous oxygen and decomposition of the active site, Re ‘“*Iic/A1203/0~, the cztiyticaily inactive site, Re’L*/Al,03, which is returned to the active site by the adsorption of oxygen at room temperature_ It was estimated that about one-fifth of the total rhenium ions in the activated catalyst is active for the metathesis of propene.
Introduction It is of importznce to c’naracterize the active sites for the metathesis of oiefins since the catalytic activity markedly depends on the preparation or activation method of catalysts as well as on their composition [I - 41. This report deals with the structure and properties of the active site which was prepared by the adsorption of oxygen onto a pre-reduced low-loading rhenium oxide/alumina catalyst. It has been known that the catalytic activity of a Re20,/A1203 system for the memthesis of olefins shows an abnormally sharp increase with the content of Rep@ _ The iow-loading catalyst with Re/Al < cz 3/97 (atom ratio) has very Little activity, while the high-loading catalyst with 3/97 < Re/Al < IO/90 is one of the most active cataIysts [ 5 - 81. Recently, inhared studies have reveaIed that the Zess active low-loading catalyst has only monomeric surface rhenium species, [ReCiJaas, and the high0 Ekvier
Sequoh/Printed
in The Netherlands
148
loading active catalyst has ligated dimeric rhenium species, [RG)as shown in a later section [91_ (ReWXMI acts (X = o-OH or aa2-) According to these results we have pointed out the interesting similarities in structure and behaviour between the rhenium species on an alumina support and those in an aqueous solution and/or in a Re,O, crystal unit, indicating a role of the alumina support as a ‘solid solvent’ or an ‘immobile solvent’. We have shown that the monomeric rhenium species is not a suitable precursor of the active species, whiie the dimeric rhenium species is an effective pre-active species if the catalyst is activated in a conventional fashion, i-e. heating in vacuum or in an inert gas atmosphere at elevated temperatures circa: 773 K. One of the interesting and important problems, not only in practice but also in research, is how to increase the activity of the lowloading catalyst. There seem to be, in principle, two different ways to solve this problem although they may not always be independently operated. One is to modify the active site with some effective ligands. In this connection, we have already reported the distinct effect of a wide variety of metal oxide ligands, M,O., in (M,O,-AlsOs)-supported Re20, catalysts on their catalytic activity for the metathesis of olefins and have shown that the logarithmic reaction rate markedly increases almost linearly with increasing electronegative of the M”+ ions. suggesting that the M n* ions change the b&city of the oxide ions bound to active species, probably medium-valent rhenium ion [ 9]_ The other is to increase the number of active sites by adjusting the oxidation state of rhenium. We have not succeeded in the direct reduction of the monomeric rhenium (VII) species to the active site or a more suitable pre-active species. It was found, however, that the adsorption of oxygen at room tempemture to a pre-reduced Re oxide/alumina dramatically increases its catalytic activity, producing a surface paramagnetic species [El ] _ The major purposes of the present work are (i) to characterize the paramagnetic species formed on a pre-reduced low-loading Re oxide/A1209 catalyst (Re/Al = l/199) by oxygen adsorption, (ii) to reveal the role of the paramagnetic species for catalytic activity, and (iii) to discuss the structure and properties of the active site for the metathesis of propene.
Experimental Apparatus A flow system which was connected to a vacuum system was used For reactions. A conventional closed vacuum gas-circulation system (effective
volume 258.5 cm3), used for the pretreatment of the catalysts, was equipped with a calibrated Pirani gauge, mercury manometer, and mass spectrometer. Two different types of EPR measurement tubes were employed in this work. One was a conventional thin quartz tube used only for the assignment of the pammagnetic species on the surface of the catalyst. The other was a quartz tube which could be attached to either the closed system or the flow
system; this was used for oxygen adsorption experiments in the closed system and for the reaction in the flow system in addition to its use in the EPR experiments. Gases Hydrogen was purifkd by passing though a column of Pd/Si02-A1209 (‘deoxo’ cataIyst).zt room temperature, followed by molecular sieves at ‘17 K. Oxygen was dried by repeated distihations over molecukr sieves. Propene (polymerization grade, >99.5%, contain&g propane as a major impurity) was purified by passing through a column of molecular sieves at room temperature. ‘7Ckontaining oxygen (extra dry grade; PO atom% of 170) was obtained from Japan MSD Co., Ltd. and used without further purification. Cafalystpreparatkt Re(VII) oxide/&O3 (Re/Al = l/199, atom ratio) was prepared by impregnating r-Al& (from Nissan Chemicals; surface area ca. 200 m2 g-r) with an aqueous solution of Re,07 (from Mitsuwa Chemicals, >99.99%) and drying at 393 K for 18 h. Another Re(VII) oxide/AlsOs (Re/AI = l/199), which was prepared in a similar manner by the use of AIon-C alumina (from Strem Chemical; ca_ 200 m2 g-r) instead of r-Al,O,, was also studied to test the effect of the alumina source. Re(VII) oxide/Si& (Re/Si = l/199 and Re/Si = 5/95) was prepared in a similar fashion using silica gel (Nissan Chemicals; ca. 200 m2 g-l). These catalysts are denoted as Re-AI, Re-ALC, and Re-Si, respectively. Cata4lystpretreutment The catalysts were pretreated
mainly in the closed system.
Euacuafed catalysts These were obtained by heating (<10M5 torr) at 773 K for 1 h_
the original
catalysts
in vacuum
Reduced catalysts These were prepared by reducing the evacuated catalysts with hydrogen (initial pressure 200 torr) at 773 K for 20 min, foILowed by evacuation at 773 K for 30 min. During the course of the reduction, water was removed by the use of a cold trap at 77 K. The average oxidation state of rhenium in the reduced catalyst, which was roughly estimated by the consumption of hydrogen, was between +I and +2. In one experiment carbon monoxide was used as a reducing gas instead of hydrogen; however no significant difference was 0 bserved . Oxygenudsorbed caialysis These were prepared by treating the reduced catalysts with dry oxygen (initial pressure 0.1 - 40.0’ torr) at different temperatures (298* - 773 K) *Stand.zrd
conditions.
for 5 - 3Of; min, followed by outgassing at different temperatures, 298* 773 K, for 20 min. Both reversible and irreversible oxygen uptakes were determined by measuring the variations in the pressure of oxygen in repeated adsorption/evacuation cycles using a calibrated Pirani gauge. The amount of oxygen adsorbed was adjusted by varying adsorption and/or evacuation temperatures as well as adsorption time. Evacuated, reduced, and oxygen-adsorbed catalyst-s are denoted by prefixes, E-, R- and Ox-, respectively, e.g. Ox-Re-Ai. EPR experimen & The EPR spectra were recorded at X-band frequencies on a JEOLPE2X spectrometer at 77 or 298 K. The g-values and absolute number of spins were determined by the use of standard materiais, a freshly prepared benzene solution of DPPH and calibrated Mn2’doped sihcone. Reaction and analysis After the pretreatment of the catalyst, the metathesis of propene was carried out mainly at 298 K in the flow system with the EPR tube as a reactor which could be removed Zom, and attached to, the systems without exposing the catalyst to air. In one case periodic EPR measurements were made during an intermittent flow reaction.
Results and discussion Paramagnetic species on Ox-Re-Al The EPR studies have shown that the adsorption of oxygen at room temperature to R-Re-Al produces a paramagnetic species on its surface. The typical EPR signal is shown in Fig. la. Since neither E-Re-AI nor R-Re--41 has such an EPR signal, the signal must be due to either low-valent rhenium ions such as Rez+, as suggested by Yao and Shelef [II], or to surface oxygen radical species. Et is worth noting that the signal closely resembles that of the 0, radical on 7-A:,0, reported by Losee 1121, who prepared the sample by heating y-Al,O, st 873 K in a stream of N,O. This suggests that the paramagnetic species on R-Re-Al is 0, which is bound to A13+, and not to any Re ion. If this is true, the hyperfme splitting due to the nuclear spin of “Al (I = 5/2) may be observed when the spectrum is recorded at low temperatures. Thus we recorded the signal at 77 K with a low modulation width (0.63 G) and found that the signal was resolved into three sets of six hyperfine lines as shown in Fig. lb. The spectrum was characterized by g, = 2.011 + 0.001, gyy = 2.003 f 0.001, and g, = 2.038 4 0.001 with Ia,1 = 5.0 f 0.5, la,,1 = 3.5 f 0.5 and la,1 = 6.0 + 0.20 G. These values are in fairly good agreement with those reported by Lunsford [13] for the 0; radical
*Skndard condiliom.
151
Fig. I_ EPR spectra OC 0, on R-R+Al; were observed on R-Re-ALC.
a, recorded
at 298
K; b, at 77 K. Similar
spectra
Fig. 2. Variations in the amount of Cz. No. as 2 function of evacuation temperature; 0.0, after R-Re-AI (fresh) was treated with oxygen ai different temperatures. T, for 20 min. followed by evacuation at the same temperature for 5 min (0) or for 30 min (8); *. after R-Re--%I wzs treated with oxygen at 298 K for 20 min, followed by evacuation at 373 K for 30 min; I, after Ox-Re-Al (see text) was evacuated at 473 K for 20 min. then oxygen adsorption at 298 K for 10 min. followed by evacuation at 298 K for 30 min; A, after R-R-Al -as reaxidiaed by OXygeQ at 773 K, then oxygen adsorbed at 298 K for 2.9 min, followed by evacuation at 298 K for 10 min; v , aftc: oxyge= adsomtion OQb the reaxidized catalyst at 298 EL, followed by evacuation at 298 K for 20 min. Initial pressure of oxygen = 40 torr.
bound to X3+ on the ~-irradiated zeolite but not with those for CF. Therefore, we concluded that the signal is attributed’to the 0, radical bound to A13’ on R-R-Al. This conclusion was also confirmed by the following results. Neither C&-adsorbed R-Re-Si(Ox-Re-Si) nor &,-adsorbed R-Re-41 showed such a characteristic EPR signal at all. These results indicate that both oxygen and Al”+ zre necessary to yield the radical. When oxygen enriched with 10.0 atom% of “;O (1 = 5/2) was employed as an adsorpate instead of naturally occurring oxygen (containing 0.037% of ‘?G), several additional lines, one of which was clearly resolved, which are attributable to the hyperfine splitting of I70 were observed. This result absolutely confirms that the radical is not a Re ion but is the 0; species. Since the adsorption of oxygen onto R-Re-Al, evacuated (at 773 K) A&&Is, and I-Is-treated (zt 773 K) A&O3 did not produce O,, the 0, radical on R-Be-AI should be on an aluminum ion which is located in the vicinity
152
of a low-valent rhenium ion on the surface. This suggestion will be supported by relating the catalytic activity and the quantity of 0, as shown later. Thermal stability of 0; Figure 2 shows that the number of spins for 0, on R-Re-Al decreases with an increase in adsorption and/or evacuation temperatures and 0, is not stable on the catalyst above 523 K. The behavior of the 0, radical resembIes that of 0, on MgO 1141 or on A&O3 [IZ] _ Figure 2 shows that the 0, radical is relatively stable against evacuation at the temperatures at which the oxygen adsorption was carried out, in particular below 373 K; however, the formation of the 0; radical is reversible in the temperature cycle of oxygen adsorption and desorption, suggesting that 0, has little permanent oxidizing effect on the camlyst but mostly desorbs as gaseous oxygen. In fact, the color of the catalyst remained black through the cycles and a reasonable amount of oxygen was observed in the gas phase when the O,carrying catalyst was heated at-higher temperatures. On the contrary, once the ca’dyst was heated in the presence of oxygen at 773 K, its color turned white, indicating that it was fully oxidized to the original catalyst, Re(VII) oxide/&O,, which no longer adsorbs oxygen as 0;. Thus we could prepare Ox-Re-AlzCa with different amounts of 0, on its surface by changing adsorption and/or evacuation temperatures as well as the initial pressure of oxygen _ Comparison of the amount of 0; formed to that of 0, adsorbed In oxygen adsorption experiments, adsorption/evacuation cycles showed that R-Re-Al has two different types of adsorption sites. One. which was denoted as Sl, is the site which adsorbs oxygen only below ca. 473 K and is almost completely recovered by evacuation circa 473 K. The other, S2, is the site which almost completely vanishes on adsorption of oxygen and is not recovered by evacuation even at 773 K, but is reproduced by reduction above cc. 673 K. Figur& 3 compares the amount of 0, formed, N,, with that of 0, adsorbed, Nl OL’Nl + N2 (total), onto fresh R-Re-Al ((0) or line A) having both Sl and S2, or onto E-Ox-Re-Al ((e) or line B) having only Sl, which was prepared by treating fresh R-Re-Al with oxygen (40 torr) at room temperature for 30 mm, followed by evacuation at 523 K for IO min. The amount of oxygen adsorbed was adjusted by varying both adsorption time and initial pressure of oxygen. It is worth noting that the amount of O,, N,,. increases in proportion to that of oxygen adsorbed, Nl, on Sl ahnost along the diagonai line up to 1.8 f 0.3 X 10m5 mol/gca’dyst (Nl = 2.0 2 0.1 X 10M5 mol/gcaMyst). Taking account of the reasonable assumption that the deviation of the line B from the diagonal Me in Fig. 3 could arise from errors in the estimation of the absolute number of spins in EPR experiments, the evidence in Fig. 3 leads us to the conclusion that the adsorption site Sl is the site which stabilizes the 0; radical, namely, the aiuminum ion in the -v-kid@ of low-
valent rhenium ion, as discussed in a previous section, while S2 consists of low-vaIent rheniuk ions such as Rezf and met&k rhenium dispersed on the surface. The stnrctures and catalytic behavior of the active site wilt be discussed lakr.
Fig. 3. The amount of 05, MO us. the reversible, NL, or total N1 + M2, uptake OFoxygen; A, X1 f N2 for fresh R-Re-Al; B, N1 for E-Ox-Re-Al (see Rxt). The amount of oxygen adsorbed was adjusted by changing the initial presure af oxygen.
Metathesis
of propene
over different
catalysts
The metathesis of propene was studied in the flow system mainly at 298 K with different catalysts which had been prepared either iv the flow system or in the closed gascirclilation system. In al.lthe experiments, the reaction producti were ethene and 2-butenes with almost equilibrated ratios of frans/ck; the ratio of ethene/2-butenes was 1.0 f 0.1. Figure 4 compares the catalytic activities of E-R-AI, R-Re-Al for the metathesis of propene at 298 K as a function of time on stream. The adsorp tion of oxygen onto R-Re-Al dramatically increased its catalytic activity, while neither reduction nor oxygen adsorption infiuenced the catalytic activity of E-Re-Al under the conditions used. Unfortunately, the catalytic activity of Ox-R-Al decreased somewhat rapidly in the early stage cf the
I /
h,
Fig. 4. Variations of the conversion of propene, C, and the relativeamount of O,, NdNo, in the mebthesisof propene at 298 K as a EunctionOFtime OP stream; catalyst, 1 g; 0. I, On-Re-Al; A. ELR+Al: 0.0, R-&AI; prssure of propene, 0.50 atm: balance, He (0, m, A, 0) or 02 (I.G%)/He (98.4%) (--C+ -); toM flow rate. 20 cm3 minml.
154
reaction and then slowly with time; however, an injection of a pulse of oxygen onto the catalyst during the course of the reaction increased its catalytic activity _ Role
of 0; in metathesisuctiuity Figure 4 also compze~ the variation in the catalytic activity of Ox-Re-
A with that in the amount of 0, on the catiyst. A fairly good correspondence between the catalytic activity and the amount of 0, indicati that the 0, has an impo,mt role in the enhancement of catiytic activity, suggestig that the adsorption site of 05, SI or Al”+, is adjacent to the active site of metathesis and stabilizes a me-table rhenium ion, possibly Re5+, by a single electron transfer from a relatively stable Re& _ It was found that the decay of the catalytic activity, or of the amount of O,, accompanied the evolution of oxygen into the gas phase. The evolution of oxygen should leave electrons on the catalyst, reducing the active Ret-l’+ to more stable-but much less active, or almost inactive, Re”*; and the injection of oxygen reproduces the active site. These phenomena prompted us to study the metathesis of prapene in the presence of gaseous oxygen. The result is also shown in Fig. 4 (- - -)_ The reason for the deactivation of the catalyst in the reacticn with flowing oxygen is still unknown. Numerous additional flow experiments were independently carried out at 298 K with Ox-Re-Al having different amounts of 0;. In these experiments, the conversion of propene was kept less than 10% by adjusting the amount of catalyst used. The initial reaction rate, r. , was obtained by extrapolation of reaction rates, P, at different times, r, to~~ds f = 0. Figure 5 shows that the initial rate r. varies in proportion to the amount of O,, No, or the number
of the active
sites on the catalyst,
suggesting
that the ackorp-
rater;, ‘0, us. the amount oE 0,. No; reaction temperature. 298 K; pressure of propene, 1 atm; flow rate, 20 cm3 g-’ -, amount of 0, was adjusted by changing the atir@ion/evacua.tion temperature, 0, or the evacuation temperakre after oxygen adsorption at 298 K, 0; 0, after the puke of oxygen (1 cm”) was injected onto R-R-RI before the reaction. Fig. 5. Initial
155
tion of oxygen as 0, has an impo&nt roLein the formation of the active site for the metathesis of propene. Since one gram of Ox-Re-Al (standard) has 9.5 X IO-' mol of Re and CQ. 2.0 X low5 moE 'ofOz. abont one-fifth of the total Re atoms in the catalyst appeers to be working as active sites for metathesis. An estimated turnover frequency was about 033 see-’ per single active site. A possible smtcfrcre of the active site It is know-n that there is [Real , 1, on the surface of E-Re-AI [93 _ The present Kork has shown that the reduction of E-Re-AI with hydrogen under the standard conditiorLs yields a mixture of rhenium species with average oxidation number between +I and +2. Probably the mixture consists of Re4*, ReZ” , and metaUk Re. The variations in the amount of 0; in the O,-adsorption/desorption (evacuation) cycles have indicated that the active .s&e is a meta-stable rhenium ion, Re(“+‘-‘f/Al,O,/O~ formed by a single electron transfer from a relatively stable rhenium ion, Ren*/M20,. Since, in general, Re(IV) oxide is known to be a relatively stable species and Re(V) oxide is me&stable, and both metallic rhenium and Rezf should adsorb oxygen irreversibly, we assumed that R = 4. In fact IR sttrdies have shown that both Ox-Re-Al and E-Ox-Re-Al, which was prepared by evacuation of Ox-Re-Al at 523 K, have an absorption band in the vicinity of 915 cm- 1 due to the Re=O stretching vibration, in&.cating the presence of Re** on their surfaces. A possible scheme for the formation of the active site is illustrated below.
c Re
AL._o_H+
H2,)
AL-& _ gzRef<
673K
a O2.
AIF
673K
1 H
Al-O, Al-OReym Al-O’ A+ 0, L
,
G .,, -298K L
metal
A 5673K
Oads/Re
metal
0, _ L523K
A> in vacuum 523K
AZ
References 1 R. L. Banks snd G. C. Bailey, Ind. Eng. Chem. Prod. Res_ Develop.. 3 (1964) 170. 2 R. L. Banks. Fortschr. Cherm Foxch. 25 (1972) 39_
156 3 4 5 6 ‘7 8 9 18 12 12 13 14
J. C. MO! and 9. A. MouIijn, Adu. CaEaL. 2# (1975) 131. R. Nzk.amura ad E. Eehigoya. BlrlL J-pit PefroL Inst.. I4 (1972) 187. R. Ns!mruura imd E. E&igoye, Chem Leti_., (1977) 1227s R. Nakamura md E. Echigoya, J. Roycl Netiei-kmd Chem. Sac.. 96 (1977) 279. F. Kapteijn. L. I-i. G. Bredt and J. C=.Mol. Ruceedings fSffM-2. (1977) M138. R. Nakamura. K. Iida and E. Ecbigoya. Ckem. Left., (1972) 273. R. Nakamum, Fumio Abe xnd E. Echigoye, CFrem. Left., (1981) 51. R. Nekamurx, K. Ichikawn and E. Echigoya, Cfzem. Leti., (1978) 813. H. C. Yao and M. Shdel. J. Catal.. 44 (1972) 392. D. B. Losee. J. ChtuL. 50 (1977) 545. K. M. Wang and IX Lunsfmd. J_ Pkys. Cizem.. 73 (1969) 2069. A. J. Tenth and P. J. Kokoyd, J_ Chem. See.. Chem. COIIZITIKR..(1968) 471.
of Molecub
Cutdysis.
15 (1982)
14’7 - ‘if36
147
STRF_jm AN.5 NATURE OF _4CTZVE SITES OXIDE/ALUMINA METATHESES CATALYST
RYUICKT NAECAMURA and ETSURO Demenf of Chemical Engineering, Meg-w-o-ku, -Tokyo 152 (Japan)
ON AN O,/Rre
ECHIGOYA Tokyo
Irrstituh of Technoiogq.
Ookayama.
The adsorption of oxygen onto a pre-reduced rhenium oxide/alumina catalyst which was prepared by the reduction of Re207/AJ& with H2 or _ CO at 773 K dramatically increases its catalytic activity for the metathesis of propene, yielding a paramagnetic species on its surface. The catalytic activity is proportional to the amount of the radical. The radical was assigned as 0, bound to A13* which is located in the vicinity of the active site, Re(*l)C. The active site, which is a thermaLly meta-stable species, is then formed by a single electron transfer from a relatively stable but cataEyticaHy inactive RenC ion, possibly Re ‘+ to the adsorbing oxygen molecule. Heating the activated cat-a&t in vacuum up to about 523 K causes the into gaseous oxygen and decomposition of the active site, Re ‘“*Iic/A1203/0~, the cztiyticaily inactive site, Re’L*/Al,03, which is returned to the active site by the adsorption of oxygen at room temperature_ It was estimated that about one-fifth of the total rhenium ions in the activated catalyst is active for the metathesis of propene.
Introduction It is of importznce to c’naracterize the active sites for the metathesis of oiefins since the catalytic activity markedly depends on the preparation or activation method of catalysts as well as on their composition [I - 41. This report deals with the structure and properties of the active site which was prepared by the adsorption of oxygen onto a pre-reduced low-loading rhenium oxide/alumina catalyst. It has been known that the catalytic activity of a Re20,/A1203 system for the memthesis of olefins shows an abnormally sharp increase with the content of Rep@ _ The iow-loading catalyst with Re/Al < cz 3/97 (atom ratio) has very Little activity, while the high-loading catalyst with 3/97 < Re/Al < IO/90 is one of the most active cataIysts [ 5 - 81. Recently, inhared studies have reveaIed that the Zess active low-loading catalyst has only monomeric surface rhenium species, [ReCiJaas, and the high0 Ekvier
Sequoh/Printed
in The Netherlands
148
loading active catalyst has ligated dimeric rhenium species, [RG)as shown in a later section [91_ (ReWXMI acts (X = o-OH or aa2-) According to these results we have pointed out the interesting similarities in structure and behaviour between the rhenium species on an alumina support and those in an aqueous solution and/or in a Re,O, crystal unit, indicating a role of the alumina support as a ‘solid solvent’ or an ‘immobile solvent’. We have shown that the monomeric rhenium species is not a suitable precursor of the active species, whiie the dimeric rhenium species is an effective pre-active species if the catalyst is activated in a conventional fashion, i-e. heating in vacuum or in an inert gas atmosphere at elevated temperatures circa: 773 K. One of the interesting and important problems, not only in practice but also in research, is how to increase the activity of the lowloading catalyst. There seem to be, in principle, two different ways to solve this problem although they may not always be independently operated. One is to modify the active site with some effective ligands. In this connection, we have already reported the distinct effect of a wide variety of metal oxide ligands, M,O., in (M,O,-AlsOs)-supported Re20, catalysts on their catalytic activity for the metathesis of olefins and have shown that the logarithmic reaction rate markedly increases almost linearly with increasing electronegative of the M”+ ions. suggesting that the M n* ions change the b&city of the oxide ions bound to active species, probably medium-valent rhenium ion [ 9]_ The other is to increase the number of active sites by adjusting the oxidation state of rhenium. We have not succeeded in the direct reduction of the monomeric rhenium (VII) species to the active site or a more suitable pre-active species. It was found, however, that the adsorption of oxygen at room tempemture to a pre-reduced Re oxide/alumina dramatically increases its catalytic activity, producing a surface paramagnetic species [El ] _ The major purposes of the present work are (i) to characterize the paramagnetic species formed on a pre-reduced low-loading Re oxide/A1209 catalyst (Re/Al = l/199) by oxygen adsorption, (ii) to reveal the role of the paramagnetic species for catalytic activity, and (iii) to discuss the structure and properties of the active site for the metathesis of propene.
Experimental Apparatus A flow system which was connected to a vacuum system was used For reactions. A conventional closed vacuum gas-circulation system (effective
volume 258.5 cm3), used for the pretreatment of the catalysts, was equipped with a calibrated Pirani gauge, mercury manometer, and mass spectrometer. Two different types of EPR measurement tubes were employed in this work. One was a conventional thin quartz tube used only for the assignment of the pammagnetic species on the surface of the catalyst. The other was a quartz tube which could be attached to either the closed system or the flow
system; this was used for oxygen adsorption experiments in the closed system and for the reaction in the flow system in addition to its use in the EPR experiments. Gases Hydrogen was purifkd by passing though a column of Pd/Si02-A1209 (‘deoxo’ cataIyst).zt room temperature, followed by molecular sieves at ‘17 K. Oxygen was dried by repeated distihations over molecukr sieves. Propene (polymerization grade, >99.5%, contain&g propane as a major impurity) was purified by passing through a column of molecular sieves at room temperature. ‘7Ckontaining oxygen (extra dry grade; PO atom% of 170) was obtained from Japan MSD Co., Ltd. and used without further purification. Cafalystpreparatkt Re(VII) oxide/&O3 (Re/Al = l/199, atom ratio) was prepared by impregnating r-Al& (from Nissan Chemicals; surface area ca. 200 m2 g-r) with an aqueous solution of Re,07 (from Mitsuwa Chemicals, >99.99%) and drying at 393 K for 18 h. Another Re(VII) oxide/AlsOs (Re/AI = l/199), which was prepared in a similar manner by the use of AIon-C alumina (from Strem Chemical; ca_ 200 m2 g-r) instead of r-Al,O,, was also studied to test the effect of the alumina source. Re(VII) oxide/Si& (Re/Si = l/199 and Re/Si = 5/95) was prepared in a similar fashion using silica gel (Nissan Chemicals; ca. 200 m2 g-l). These catalysts are denoted as Re-AI, Re-ALC, and Re-Si, respectively. Cata4lystpretreutment The catalysts were pretreated
mainly in the closed system.
Euacuafed catalysts These were obtained by heating (<10M5 torr) at 773 K for 1 h_
the original
catalysts
in vacuum
Reduced catalysts These were prepared by reducing the evacuated catalysts with hydrogen (initial pressure 200 torr) at 773 K for 20 min, foILowed by evacuation at 773 K for 30 min. During the course of the reduction, water was removed by the use of a cold trap at 77 K. The average oxidation state of rhenium in the reduced catalyst, which was roughly estimated by the consumption of hydrogen, was between +I and +2. In one experiment carbon monoxide was used as a reducing gas instead of hydrogen; however no significant difference was 0 bserved . Oxygenudsorbed caialysis These were prepared by treating the reduced catalysts with dry oxygen (initial pressure 0.1 - 40.0’ torr) at different temperatures (298* - 773 K) *Stand.zrd
conditions.
for 5 - 3Of; min, followed by outgassing at different temperatures, 298* 773 K, for 20 min. Both reversible and irreversible oxygen uptakes were determined by measuring the variations in the pressure of oxygen in repeated adsorption/evacuation cycles using a calibrated Pirani gauge. The amount of oxygen adsorbed was adjusted by varying adsorption and/or evacuation temperatures as well as adsorption time. Evacuated, reduced, and oxygen-adsorbed catalyst-s are denoted by prefixes, E-, R- and Ox-, respectively, e.g. Ox-Re-Ai. EPR experimen & The EPR spectra were recorded at X-band frequencies on a JEOLPE2X spectrometer at 77 or 298 K. The g-values and absolute number of spins were determined by the use of standard materiais, a freshly prepared benzene solution of DPPH and calibrated Mn2’doped sihcone. Reaction and analysis After the pretreatment of the catalyst, the metathesis of propene was carried out mainly at 298 K in the flow system with the EPR tube as a reactor which could be removed Zom, and attached to, the systems without exposing the catalyst to air. In one case periodic EPR measurements were made during an intermittent flow reaction.
Results and discussion Paramagnetic species on Ox-Re-Al The EPR studies have shown that the adsorption of oxygen at room temperature to R-Re-Al produces a paramagnetic species on its surface. The typical EPR signal is shown in Fig. la. Since neither E-Re-AI nor R-Re--41 has such an EPR signal, the signal must be due to either low-valent rhenium ions such as Rez+, as suggested by Yao and Shelef [II], or to surface oxygen radical species. Et is worth noting that the signal closely resembles that of the 0, radical on 7-A:,0, reported by Losee 1121, who prepared the sample by heating y-Al,O, st 873 K in a stream of N,O. This suggests that the paramagnetic species on R-Re-Al is 0, which is bound to A13+, and not to any Re ion. If this is true, the hyperfme splitting due to the nuclear spin of “Al (I = 5/2) may be observed when the spectrum is recorded at low temperatures. Thus we recorded the signal at 77 K with a low modulation width (0.63 G) and found that the signal was resolved into three sets of six hyperfine lines as shown in Fig. lb. The spectrum was characterized by g, = 2.011 + 0.001, gyy = 2.003 f 0.001, and g, = 2.038 4 0.001 with Ia,1 = 5.0 f 0.5, la,,1 = 3.5 f 0.5 and la,1 = 6.0 + 0.20 G. These values are in fairly good agreement with those reported by Lunsford [13] for the 0; radical
*Skndard condiliom.
151
Fig. I_ EPR spectra OC 0, on R-R+Al; were observed on R-Re-ALC.
a, recorded
at 298
K; b, at 77 K. Similar
spectra
Fig. 2. Variations in the amount of Cz. No. as 2 function of evacuation temperature; 0.0, after R-Re-AI (fresh) was treated with oxygen ai different temperatures. T, for 20 min. followed by evacuation at the same temperature for 5 min (0) or for 30 min (8); *. after R-Re--%I wzs treated with oxygen at 298 K for 20 min, followed by evacuation at 373 K for 30 min; I, after Ox-Re-Al (see text) was evacuated at 473 K for 20 min. then oxygen adsorption at 298 K for 10 min. followed by evacuation at 298 K for 30 min; A, after R-R-Al -as reaxidiaed by OXygeQ at 773 K, then oxygen adsorbed at 298 K for 2.9 min, followed by evacuation at 298 K for 10 min; v , aftc: oxyge= adsomtion OQb the reaxidized catalyst at 298 EL, followed by evacuation at 298 K for 20 min. Initial pressure of oxygen = 40 torr.
bound to X3+ on the ~-irradiated zeolite but not with those for CF. Therefore, we concluded that the signal is attributed’to the 0, radical bound to A13’ on R-R-Al. This conclusion was also confirmed by the following results. Neither C&-adsorbed R-Re-Si(Ox-Re-Si) nor &,-adsorbed R-Re-41 showed such a characteristic EPR signal at all. These results indicate that both oxygen and Al”+ zre necessary to yield the radical. When oxygen enriched with 10.0 atom% of “;O (1 = 5/2) was employed as an adsorpate instead of naturally occurring oxygen (containing 0.037% of ‘?G), several additional lines, one of which was clearly resolved, which are attributable to the hyperfine splitting of I70 were observed. This result absolutely confirms that the radical is not a Re ion but is the 0; species. Since the adsorption of oxygen onto R-Re-Al, evacuated (at 773 K) A&&Is, and I-Is-treated (zt 773 K) A&O3 did not produce O,, the 0, radical on R-Be-AI should be on an aluminum ion which is located in the vicinity
152
of a low-valent rhenium ion on the surface. This suggestion will be supported by relating the catalytic activity and the quantity of 0, as shown later. Thermal stability of 0; Figure 2 shows that the number of spins for 0, on R-Re-Al decreases with an increase in adsorption and/or evacuation temperatures and 0, is not stable on the catalyst above 523 K. The behavior of the 0, radical resembIes that of 0, on MgO 1141 or on A&O3 [IZ] _ Figure 2 shows that the 0, radical is relatively stable against evacuation at the temperatures at which the oxygen adsorption was carried out, in particular below 373 K; however, the formation of the 0; radical is reversible in the temperature cycle of oxygen adsorption and desorption, suggesting that 0, has little permanent oxidizing effect on the camlyst but mostly desorbs as gaseous oxygen. In fact, the color of the catalyst remained black through the cycles and a reasonable amount of oxygen was observed in the gas phase when the O,carrying catalyst was heated at-higher temperatures. On the contrary, once the ca’dyst was heated in the presence of oxygen at 773 K, its color turned white, indicating that it was fully oxidized to the original catalyst, Re(VII) oxide/&O,, which no longer adsorbs oxygen as 0;. Thus we could prepare Ox-Re-AlzCa with different amounts of 0, on its surface by changing adsorption and/or evacuation temperatures as well as the initial pressure of oxygen _ Comparison of the amount of 0; formed to that of 0, adsorbed In oxygen adsorption experiments, adsorption/evacuation cycles showed that R-Re-Al has two different types of adsorption sites. One. which was denoted as Sl, is the site which adsorbs oxygen only below ca. 473 K and is almost completely recovered by evacuation circa 473 K. The other, S2, is the site which almost completely vanishes on adsorption of oxygen and is not recovered by evacuation even at 773 K, but is reproduced by reduction above cc. 673 K. Figur& 3 compares the amount of 0, formed, N,, with that of 0, adsorbed, Nl OL’Nl + N2 (total), onto fresh R-Re-Al ((0) or line A) having both Sl and S2, or onto E-Ox-Re-Al ((e) or line B) having only Sl, which was prepared by treating fresh R-Re-Al with oxygen (40 torr) at room temperature for 30 mm, followed by evacuation at 523 K for IO min. The amount of oxygen adsorbed was adjusted by varying both adsorption time and initial pressure of oxygen. It is worth noting that the amount of O,, N,,. increases in proportion to that of oxygen adsorbed, Nl, on Sl ahnost along the diagonai line up to 1.8 f 0.3 X 10m5 mol/gca’dyst (Nl = 2.0 2 0.1 X 10M5 mol/gcaMyst). Taking account of the reasonable assumption that the deviation of the line B from the diagonal Me in Fig. 3 could arise from errors in the estimation of the absolute number of spins in EPR experiments, the evidence in Fig. 3 leads us to the conclusion that the adsorption site Sl is the site which stabilizes the 0; radical, namely, the aiuminum ion in the -v-kid@ of low-
valent rhenium ion, as discussed in a previous section, while S2 consists of low-vaIent rheniuk ions such as Rezf and met&k rhenium dispersed on the surface. The stnrctures and catalytic behavior of the active site wilt be discussed lakr.
Fig. 3. The amount of 05, MO us. the reversible, NL, or total N1 + M2, uptake OFoxygen; A, X1 f N2 for fresh R-Re-Al; B, N1 for E-Ox-Re-Al (see Rxt). The amount of oxygen adsorbed was adjusted by changing the initial presure af oxygen.
Metathesis
of propene
over different
catalysts
The metathesis of propene was studied in the flow system mainly at 298 K with different catalysts which had been prepared either iv the flow system or in the closed gascirclilation system. In al.lthe experiments, the reaction producti were ethene and 2-butenes with almost equilibrated ratios of frans/ck; the ratio of ethene/2-butenes was 1.0 f 0.1. Figure 4 compares the catalytic activities of E-R-AI, R-Re-Al for the metathesis of propene at 298 K as a function of time on stream. The adsorp tion of oxygen onto R-Re-Al dramatically increased its catalytic activity, while neither reduction nor oxygen adsorption infiuenced the catalytic activity of E-Re-Al under the conditions used. Unfortunately, the catalytic activity of Ox-R-Al decreased somewhat rapidly in the early stage cf the
I /
h,
Fig. 4. Variations of the conversion of propene, C, and the relativeamount of O,, NdNo, in the mebthesisof propene at 298 K as a EunctionOFtime OP stream; catalyst, 1 g; 0. I, On-Re-Al; A. ELR+Al: 0.0, R-&AI; prssure of propene, 0.50 atm: balance, He (0, m, A, 0) or 02 (I.G%)/He (98.4%) (--C+ -); toM flow rate. 20 cm3 minml.
154
reaction and then slowly with time; however, an injection of a pulse of oxygen onto the catalyst during the course of the reaction increased its catalytic activity _ Role
of 0; in metathesisuctiuity Figure 4 also compze~ the variation in the catalytic activity of Ox-Re-
A with that in the amount of 0, on the catiyst. A fairly good correspondence between the catalytic activity and the amount of 0, indicati that the 0, has an impo,mt role in the enhancement of catiytic activity, suggestig that the adsorption site of 05, SI or Al”+, is adjacent to the active site of metathesis and stabilizes a me-table rhenium ion, possibly Re5+, by a single electron transfer from a relatively stable Re& _ It was found that the decay of the catalytic activity, or of the amount of O,, accompanied the evolution of oxygen into the gas phase. The evolution of oxygen should leave electrons on the catalyst, reducing the active Ret-l’+ to more stable-but much less active, or almost inactive, Re”*; and the injection of oxygen reproduces the active site. These phenomena prompted us to study the metathesis of prapene in the presence of gaseous oxygen. The result is also shown in Fig. 4 (- - -)_ The reason for the deactivation of the catalyst in the reacticn with flowing oxygen is still unknown. Numerous additional flow experiments were independently carried out at 298 K with Ox-Re-Al having different amounts of 0;. In these experiments, the conversion of propene was kept less than 10% by adjusting the amount of catalyst used. The initial reaction rate, r. , was obtained by extrapolation of reaction rates, P, at different times, r, to~~ds f = 0. Figure 5 shows that the initial rate r. varies in proportion to the amount of O,, No, or the number
of the active
sites on the catalyst,
suggesting
that the ackorp-
rater;, ‘0, us. the amount oE 0,. No; reaction temperature. 298 K; pressure of propene, 1 atm; flow rate, 20 cm3 g-’ -, amount of 0, was adjusted by changing the atir@ion/evacua.tion temperature, 0, or the evacuation temperakre after oxygen adsorption at 298 K, 0; 0, after the puke of oxygen (1 cm”) was injected onto R-R-RI before the reaction. Fig. 5. Initial
155
tion of oxygen as 0, has an impo&nt roLein the formation of the active site for the metathesis of propene. Since one gram of Ox-Re-Al (standard) has 9.5 X IO-' mol of Re and CQ. 2.0 X low5 moE 'ofOz. abont one-fifth of the total Re atoms in the catalyst appeers to be working as active sites for metathesis. An estimated turnover frequency was about 033 see-’ per single active site. A possible smtcfrcre of the active site It is know-n that there is [Real , 1, on the surface of E-Re-AI [93 _ The present Kork has shown that the reduction of E-Re-AI with hydrogen under the standard conditiorLs yields a mixture of rhenium species with average oxidation number between +I and +2. Probably the mixture consists of Re4*, ReZ” , and metaUk Re. The variations in the amount of 0; in the O,-adsorption/desorption (evacuation) cycles have indicated that the active .s&e is a meta-stable rhenium ion, Re(“+‘-‘f/Al,O,/O~ formed by a single electron transfer from a relatively stable rhenium ion, Ren*/M20,. Since, in general, Re(IV) oxide is known to be a relatively stable species and Re(V) oxide is me&stable, and both metallic rhenium and Rezf should adsorb oxygen irreversibly, we assumed that R = 4. In fact IR sttrdies have shown that both Ox-Re-Al and E-Ox-Re-Al, which was prepared by evacuation of Ox-Re-Al at 523 K, have an absorption band in the vicinity of 915 cm- 1 due to the Re=O stretching vibration, in&.cating the presence of Re** on their surfaces. A possible scheme for the formation of the active site is illustrated below.
c Re
AL._o_H+
H2,)
AL-& _ gzRef<
673K
a O2.
AIF
673K
1 H
Al-O, Al-OReym Al-O’ A+ 0, L
,
G .,, -298K L
metal
A 5673K
Oads/Re
metal
0, _ L523K
A> in vacuum 523K
AZ
References 1 R. L. Banks snd G. C. Bailey, Ind. Eng. Chem. Prod. Res_ Develop.. 3 (1964) 170. 2 R. L. Banks. Fortschr. Cherm Foxch. 25 (1972) 39_
156 3 4 5 6 ‘7 8 9 18 12 12 13 14
J. C. MO! and 9. A. MouIijn, Adu. CaEaL. 2# (1975) 131. R. Nzk.amura ad E. Eehigoya. BlrlL J-pit PefroL Inst.. I4 (1972) 187. R. Ns!mruura imd E. E&igoye, Chem Leti_., (1977) 1227s R. Nakamura md E. Echigoya, J. Roycl Netiei-kmd Chem. Sac.. 96 (1977) 279. F. Kapteijn. L. I-i. G. Bredt and J. C=.Mol. Ruceedings fSffM-2. (1977) M138. R. Nakamura. K. Iida and E. Ecbigoya. Ckem. Left., (1972) 273. R. Nakamum, Fumio Abe xnd E. Echigoye, CFrem. Left., (1981) 51. R. Nekamurx, K. Ichikawn and E. Echigoya, Cfzem. Leti., (1978) 813. H. C. Yao and M. Shdel. J. Catal.. 44 (1972) 392. D. B. Losee. J. ChtuL. 50 (1977) 545. K. M. Wang and IX Lunsfmd. J_ Pkys. Cizem.. 73 (1969) 2069. A. J. Tenth and P. J. Kokoyd, J_ Chem. See.. Chem. COIIZITIKR..(1968) 471.