Journal of Morecular Catalysis. 15 (1982) 157 - 172 157

A COMPAJXATIVE STUDY OF yALUMINA SUPPORTED MOLYB- DENUM AKD TUNGSTEN OXIDE; RElXT1ON BETWEEN META- THESIS AC?XVlTY AND REDUCXBLLITY

Lubomtoy forCkemicalTechrrslogy, Uniwtify ofAmsterdam,PtarrtageMrcidergmckt

30. 1018 TVArnsterdam (Tke Netherlands)

A systematic study has been carried out in order to reIate metathesis ectiv-ity and sticture of alumina-supported molybdenum and tungsten oxides. Et has been found that for both series the activity/transition metal content relation is S-shaped. Up to a coverage of 0.5 metal atoms/run2 the activity is very low, while between 0.5 and 2 fxausition met.aI atoms/nm2 the activity sharply increases. At high reaction temperatures the seiectivify decreases because of side reactions. It appears that the cataiytic behaviour of the catalysts correlates with the reducibility. The more easily they are reduced, the higher is the cat&y& activity_ However, when they are reduced too far, side reactions become predominant causing a Eow selectivity. The S-shaped relationship suggests that active sites con‘& preferably several reduced transition metal ions. Alumina supporkd rhenium oxide is &o discussed within this framework.

Introduction

Many papers on metathesis have been pubhshed since the reaction was first described in the open Literature by Banks and Bailey in 1964 El]. Although they discovered tie reaction using sohd camysts Mo(CO),/AE,O,, W(CO)sIAlsOs and Ma0s/CoO/A120s, most mechanistic studies have been performed on homogeneous systems. For the latter type of catalysts it has been weLl documented that carbenoid species are key intermediates in a chain mechanism [2 - 51. An interestig question is how these carbenes are formed. In homogeneous systems a co-catalyst is normally used, and this can act as a source of the initial carbene fragments.

From a study of Grubbs and Swetnick [SI , among others, on a soLid catalyst (MoOs/CoO/AIz03) it can be concluded that on this catalyst aIso the reaction proceeds uti carbenoid species. The work of Farona and Tucker

03~-51@2/52/30Oo-~~oO/S02.‘T5 0 EIsevier Sequoia/Printed in The NetherIands

158

[ 73 , who observed anomalies in product distributions in the alkenes initially formed over a Mo(CO),/A120, catalyst, also supports the carbene mechanism_ Besides the reaction mechanism, it is also interesting to discover which parameters determine the stability and activity of the carbenes. Un- doubtedly the oxidation number of the metal and the nature of its ligancls are important.

A large number of studies has been devoted to the determination of the optimum oxidation number of transition metal ions which are part of the active site in solid metathesis ca*Aysts. In particular MoOs/AlaOs catalysts have been studied. For this type of catalyst Mo6+ [8], Mo5+ [9 - ‘121 as well as Mo4* [13] h ave so far been reported to be active centers in olefin meta- thesis_ In other papers no explicit statement on the extent of reduction of the active sites is given [ld - 19]_

Also Mo(CO)s/A120s catalysts have been studied extensively [29 - 251. MoO,/Ai,O, catalysts are not easily reduced below Mo4*, while Mo(CO)s- based catalysts contain initially Moo compounds, which can be easily oxidized to MO** or MO’+. According to Brenner eS aL [20 - 211 the active catalysts contain species such as Mo(CO), ads, Mo(CO)~O~ ads and ‘(e-O_)sMo’, in which the MO valency is 2+ and 3+. In general ~Mo(CO)~/A~~O~ catalysts are more active than Mo0s/A1203 catalysts. This might imply that species which contain MO ions with an oxidation state below Mo4+ are more active than species which contain MO*+, Mo5+ or Mo6+ ions. Quite interesting in this respect is the observation that O2 can activate [23] as well as deactivate [24] Mo(CO),/y-Al,O, catalysts which have been pretreated at 373 K. Pro- bably in the former cese the oxidation number of the MO ions was not yet optimal, while in the latter case the MO ions were over-oxidized_

For WO,/Al,Os catalysts the amount of reported research is much less than for MoO,/Al,O, catalysts. The reasons are that MO-based catalysts are more extensively applied in catalysis, that the systems are comparable when considered chemically, and that the W-based catalysts are less active in meta- thesis. It has been suggested that W4+ _ IS part of the active site 1261. Also, W(CO)s/AlsOs catalysts behave similarly to MO(CO)6/Ai203 systems.

Literature data can be summarized by the conclusion that the oxidation state of the transition metal ions is not very critical in order to obtain active metathesis catalysts, but that the oxidation state is preferably lower than 4+. A complication is, as found often in catalysis, that the portion of active sites is low [27 - 293 _ This has been found for several types of so!id catalysts, and it implies that the major part of structural information and information on oxidation states applies to non-active species.

In most studies on solid metathesis catalysts, thorough attention has not been given to the influence of variation of metal content on activity and selectivity. It is obvious that such a study is important from a practical point of view. Moreover, as will be shown here, information can be obtained on factors which influence catalytic activity, selectivity and stabili+y from the observed catalytic performance at various metal contents,

In this study the effect of variation in transition metal content of MoOsly-AlsO3 and WOs/y-AlaOs catalysts on catalytic activity for m&a-

thesis is described. As a model reaction the conversion of propene to ethene and butene has been chosen. In fact this is the first study in which compar- able series of alumina-supported MO- and W-based catalysts are investigated. The transition metal content range in both series is the same, when this content is expressed as the (calcula’ki) number of transition metal atoms/ nm2 of the support. In such 2 way comparison is mofe appropriate than on basis of the % weight of the metal oxides. The upper coverage limit corresponds to a coverage of approximztely one monolayer of transition metal compounds.

Experimenti

Catalystpreparation Catalysts were prepared by wet impregnation of Ketjen high purity

alumina, type OOO-1.5E, (surface area 213 m2 g-l, particle size 180 - 300~~i-n) with aqueous solutions of ammonium heptamolybdate or ammonium meta- tungstate. Details of the procedure a-e given elsewhere 1301.

Activity measurements Reactions were carried out in a conventional micro-flow reactor system,

as described by Andreini and MOE [31]_ Before tes’hg metathesis activity, the catalysts samples (0.50 g) were activated in a flow of dry nitrogen (Eloekloos, Groenband, purified by molecular sieves and a deoxygenation catalyst) at 825 K during 16 h at a flow of 37 pm01 s-r and at pressure of 0.15 MPa. After cooling in a nitrogen flow of 112 I.rmol s-r to the desired reaction temperature, propene (Matheson CP, purified by alumina, deoxy- genation catalyst and molecular sieves) was fed to the reactor at a flow rate of 143 pm01 s-l. The pressure in the reactor was 0.2 MI%. Reaction products were analyzed by gas chromatography.

Conversions were calculated based on the amount of ethene produced. As a check on the stability of the test system over the period in which activity measuremen& were carried out, control runs were performed before and after both series of test runs with a standard catalyst (3 wt% WOs/Si02). The temperature-programmed reduction (TPR) procedure wiil be described elsewhere [30] .

Results

Activity MoO3 /r-Al& In Fig. 1 conversion data for several MOO&-Al,& catalysts at 480 IS

are shown as a function of process time. The catalysts are satisfactorily stable over a period of 3 h except the catalyst with the highest MO- ccn’knt (14.2 w-t% MOO,). Initially its activity is high (conversion 35% after 7 mm)

160

p 1 i ;_l$L a5 lo is

SW~.3_

hwatarrshm2

Fig. l_ Conversion (Xe) as a function of pmcess time for metathesis of pmpene over some MoO3/y-A1203 catalysts. Mo-at/nm2: (*) O-22;(*) 0.42;(t) 0.95;(O) 1.61;(e) 3.35.

Fig. 2. Relation belhveeen corwersion (Xe) and MO surface cove,-age oE MoO&7-;U20~ catalysts For metathesis of propene.

but after 3 h the conversion decreases to 15% and still no constant activity level is obtained. Similar behaviour was observed for another MOO&-A&O, catalyst with a high Moos content (18.6 wt% Moos prepared by dry im-- - pregnation).

Figure 2 shows the relation between molybdenum content and cataIytic activity after 3 h on stream. Since it is not possible to determine a meaning- ful activity for the 14.2 wt% MOO, catalyst, this catalyst is not taken into account. At high molybdenum levels the reactor clearly does not behave differentially. Conversions are near equilibrium (- 46% conversion). Thus the-obserrred overall reaction rates are lower than the initial reaction rates_ _4n estimate of the jnitial rates can be obtained as follows. For a plug flow resctcr containing W kg of catalyst, thrcugh which a reactant is ffowing at a flow rate of F (mol/s), the fractional conversion x (mol,% kg) are related by the continuity equation:

W/F = = dxfr / 0

- and the reaction rate r

15.11

For the conversion of propene to ethene and butene the overall reaction rate equation, based on the carbene mechanism, can be derived ]2!3].

r’ =

k pp2 {(l -x)2 - x2/4&j

x

1 PeU -x) + Kppp2(l -x)2 + (KE+. &)pp - +

2 - RP

+- cRPE + RE’B)pP2 (1 -x) ; + &#p2

15.21

where : k = overall rate constant. pp = par&d propene pressure in the feed. K = thermodynamic equ.2ibtiu.m constant for overall reaction. g’,‘f K,, e, = adsorption equilibrium constants for propene, ethene and bxtene respectively. K PEI KPBI KEB = equfibrirrm constz~~ts for elementary steps.

For convenience the denominator is designed as &:

The initial rate can be calculated, when the conversion approaches zero (x = 0, r = rob):

kp,’ ro= -

Q

E5.31

f5.41

where it is assumed that the denominator Q remains constant at varying con- version. Substitution of [5.4] in [5.1] gives:

WjF= ‘j dx

4-0 () (l-xp -x2/=,,

or

1 = 1

dx r, = -

W/F o (1 -xl2 -x2/4&,

c5.51

C5.6]

For a given value of x, r. cm be estimated when W, F and KeQ are k&own. In TabIe I the initial reaction rates thus calculated are given, together

with turnover and seiectiviw data. As c-an be seen from Fig. 3 the relation between initial rate and transition meti content is S-shaped. At MO contents below 0.5 auxns/nm2 the rate increases relatively slowly, while it increases sharply between 0.5 and 1.5 atoms/nm2.

In Fig. 4 Eonversion of propene is plotted as a function of process time. Except for the catalyst with the highest W-content, activity decreases con- tinuausly with time. Catiytic activity, again characterized by the conversion level after 3 h, is given in Fig. 5 as a function of W-content, and in Table 2 the initial reaction rates, the turnover frequencies and selectivity data, estimated in the same way as for MoOs/y-Al,Os catalysts, are given. The relation between initial reaction-rate and W surface coverage is S-shaped, as has been found for MoOs/y -Al&Is cat&y* (Fig. 6).

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163

Fig. 3. h&id reaction rate for the metathesis of propene as a functicn of MO surface coverage for Mo03/y-A12Q3 catdgsts_

Fig. 4. Conversion (Xe) as a function ofgrocess time for metathesis of propene over some WO&-AI~OQ catalysts. W-at/am : (f) 0.11; (A) 0.22; (m) 0.47; (e) 0.99; (0) 1.80; (a) and (*) 4.2-L

Fig. 5. Relatiorr between conversion (Xe) and W surface coverage of W0$r-AI@3 catd- y&s for the metathesis of propane.

Fig. 6. Initial reaction rak for metathesis of propane as a function of W surface caver- age for WO&-Al203 catdysts.

Setectiviiy

At all L&IO zontenk, the seiectitity to ethene and butene is good and the ethene/butene ratio is close tu 1.0. The SlightIy higher ethene/butene ratio at low conversion levels is probably due to an underestimation of the amount of butene in comparison to ethene. This is caused by the larger width of the butene GLC peaks. The E/Z-2-butene ratio is constant with fkne for aU catalysts except for the caktiysts with the highest MO contint-(Fig. 7)_ For this cataEy.st, it decreases continuously fiorn 1.8 to 1.4. The cataIyst with

164

I E bulene-2 Z butene-2

1s

1.3-

I I I 1 2 3 time ---

hr

Fig. 7. Relation between amounts of E-2-butene and Z-2-butene formed during~etathesis of propene over Mo03/y-Al203 catalysts as function of process time. Mc-WU.m : (0) 0.10; (A) 0.22; (f) i-61; (“) 3.35.

18.6 wt% Moo, showed knilar behavior:; a decrease was observed &om 1.6 to 1.2. Besides 2-butene small amounts of other butenes are formed. In Table 1 the sum of 1-butene and isobutene is given as these were not separated by GLC analysis.

ks the selectivity is lowest at low W-contents, it is probable that the carrier itself is at least partly responsible for the low selectivity. This can be seen from Table 2, where the activity and selectivity data for y-AE,O, are also given. Besides ethene and butene, substantial amounts of propane and higher-boiling hydrocarbons are also formed at this reaction temperature. Aithough the amount of higher-boiling products has not been determined exactly, it can be estimated that after reaction approximately 97 - 99% cf all carbon atoms otiginally present in propene is recovered as Ca- to C,-alkenes. A closer analysis of the butenes formed over alumina revealed that, besides the 2-butenes, a substantial amount of isobutene and a trace of l-butene are present.

Temperature-programmed reduction Besides qualitative information on reducibility of catalysts, TPR also

provides quantitative information on the amount of oxygen which can be removed from a sample. Thus from TPR studies it can be determined whether catalysts are reduced during the activation procedure prior to a metathesis- activity test. In Fig. 8 the TPR patterns are shown for the 14.2 WEGO MaOJ yiU20z and the 25.9 wt% W0,/~-,4120, ca4%lysB after oxidation in air (773 K, L h) and. after activation in nitrogen (823 K, 16 h), As can be seen, the reduction patterns do not change due to the Nz-treatment in comparison with the oxidized catalysts. Also for both catalysts, the area of the paf&erIls is independent of the pretreatment.

165

Fig. 8. TPR patterns of 14.2 w t?G Ma03/-y-&03 and 25.9 w-t% W03/~-1u&?3 cafAysts, zfter activation in air or nitrogen (heating rate 10 K/min).

The relation between molybdenum content and metathesis activity for the present MOO&-A&Cl, cataEy.sts agrees well with the data pubLished in theliterature. According to Ismayel-Mitanovic ef CT-L [8l catalysts with molyb- denum contents of 6 - 9 we% MO (9 - 12 wt.% MOO,) are the most active catalysts for the disproportionation of Z-2-pentene. Nakamura and Echigoya [9] obtained maximum activity at a MO-content of 5 atom% MO (7 w-t% MOO,). At higher Mo contents the activity was much lower. Giordano et al. [12] obtained maximum activity for propene metathesis between 8 and 15 w-t% MO&. Fcr W0s/~-A120s catalysts no information has been reported up to now in the literature concerning the reIation between ‘W-content and metathesis activity. In this study MOO&-A&O, and WO&-A&C& cai;alysts give comparabIe conversion leveIs at 480 and 675 K respectively; however, it cannot be stated that MOO&-_AI,O, cataIysts are therefore more active, since the reIation between activity and reaction temperature is compkx for both systems [12,26,31].

Since reduction of MO=+ and W6+ ions probably is a prerequisite for metathesis, it is obvious that the reduction properties of the catalysts are important. From TPR data, obtained on the catalysts with the highest Mo- and W-contents, activated by nitrogen at 823 K (Fig, 8). it can be concluded that the catalysts are stiLI furry oxidized ~J%JX this activation procedure. Since it has been found, also by TPR, that the other catalysts containing lower amounts of transition met.aI XC less easily reduced, it is pIausibIe that these catalysts are also not reduced uciz the nitrogen activation. Akhough it is diffr-

166

cult to draw definite conclusions because the concentration of active .sites might be very low, it is concluded thet 2fter ‘activation’, reduction is required to generate active sites. Under the present conditions, the only way to genera& active catalysts is ui4 reduction by propene. In principle, it is to be expected that activity will increase with the degree of reduction and thus with process time. However, as can be seen from Figs. 1 and 4, this is not the case. This can easily be understood by assuming that the main reduction has been per- formed in the first minutes of the test. Due to reactor stabilization problems it was not useful to analyze reaction products during this period.

Since in 5 min 100 - 25GO propene molecules per MO/W atom (depending on the metal oxide content) have passed the reactor, it is certainly possible that the active sites have been formed in this period. Moreover, according to Lombard0 ef aL 1161 metathesis activity of Mo0,/-r-1U203 catalysts is neady invariant with extent of reduction (0.3 - 1.5 e/Me). This implies also that after the initial reaction period the cataiysts can be reduced further, while this is not manifested by an increase in catalytic activity.

From TPR studies it has been established that reducibility increases the higher the metal oxide content of the ca’tal@% It can therefore be expected that reduction by propene will also be more extensive the higher the metal oxide content. The relation between the initial reaction rate for propece met&he& and the reducibility of the MoO,/r-Al=& and WO,/rAl,Oa catalysts is given in Figs. 9 and 10. The reducibility is expressed by the temperature, where under TPR conditions half of the amount of the oxidic transition metal species had been reduced. Figures 9 and 10 show that the higher the reducibility, the higher the reaction rate. The catalyst with the highest MO content has not been taken into account, as no reliable value of r. could be calculated.

It is striking that especially for this catalyst two reduction bands are present in the TPR pattern. One band is found at a relatively low temperature, which may imply that reduction by propene leads to the formation of more strongly reduced MO ions. Such ions will also be able to catalyze other reactions, e.g. polymerization and cracking. An indication of such side reactions is the re!ative!y strong coloration of this catiyst after reaction due to coke formation. This explains why these catiysts show a pronounced decline in activity. It has been reported for other metathesis catalysts that stronger reduction destroys catalytic activity [ 14,321, so it is to be expected that this catalyst will be more stable when the reaction is carried out at a lower temperature. Ln that case, reductio? will be less severe while sites can still be formed which are a&h-e in metathesis.

The data for the WC&/r-AlsO3 catalyst with the highest W content are not present in Fig_ IO. The timeconversion relation (Fig. 4) of this catalyst differs from the catalysts with lower W content. For the catalyst with the highest W-content the conversion goes through a maximum while the other catalysts show a slowly decreasing conversion. This cart be understood when the TPR-data for this catalyst are taken into account. According to TPR the catiyst with the highest W content is reduced more easily than the other

167

1 43

llrml s-b-’

I IOO-

\ l-&l-

1032 la0

reduc(on lmpera~ure

K

Fig. 9. Relation bekeen initial reaction rate For mekathesis OF propene and average reduction temperature in TPR Ear Mo03/“/-1U203 catdyds.

Fig_ IO. ~Iationbe~~n~tiaEreadiourateformetath~sis of pmpene andaverage reductionteemperatureiuTFRfor WO3/7-AI~O3cataIysts.

168

W03;r-Al,0s cataly_* ( Tmd = 970 K; for the other catalysts TEd = 1170 - 1340 K). So it is possible that in the first period of the metathesis activity test a larger amount of propene oxidation products (e.g. acrolein, COa, H,O) is formed which is adsorbed on the catalyst, leading to a tempo- rary, partly reversible, poisoning. For MoO,/SiO, catalysts it has also been reported that, in the initial stage of the reaction, products are formed that are poisons for the metathesis reaction [33] _ Simultaneously a slow irrevers- ible poisoning also occurs, as can be seen from the hehaviour of the other catalysts and is probably caused by coke formation at the relatively high reaction temperature. Combination of the two processes leads to a situation in which activity at fast increases due to desorption of oxidation products and then decreases due to deposition of coke.

Selectivity

The ethene/butene ratio is as expected for metathesis. As can be seen from Table 1 the E/Z ratio is 1.7, which is close to equilibrium 129, 371. The amount of 1-butene is much lower than expected for an equilibrium mixture of butenes. The equilibrium ratio is 0.16 [37] , while the values. observed are between 0.01 and 0.03. For all catalysts, except those with the highest MO contents, the ethene/butene, E/Z, and 1-b/2-b ratios are constant over the fest period of 3 h. As l-b/2-b equilibrium is not reached, we conclude that the E/Z equilibrium is not due to isomerization e.g. uia a carbonium ion mechanism. Evidently the MO compounds sre involved in the rapid E/Z interconversion. According to Kapteijn [29] the E/Z isomerization can be described as a degenerate metathesis reaction, which is much faster than the productive metathesis of propene.

An explanation for this difference in reaction rate can be found in the adsorption strength of butene in comparison with propene. On the average the residence time of butene on the surface and active sites will be higher than for propene, leading to a high E/Z interconversion rate.

The ethene/butene ratio is close to 1.0 for the catalysts with inter- mediate W content (Table 2). At lower W contents the ratio is influenced by the formation of by-products on the carrier. At higher W contents part of the l-butene is metathesized with propene to ethene and 2-pentene. For all catslysts the E/Z-2-butene ratio corresponds to equilibrium [34]. The high (l-b + i-b)/2-b ratio atlowW contents is duet-o the form&ion of relatively

larg% amounts of isobutene on alumina itself. At higher W contents the conve.rsion levels are higher and the small amount of isobutene hardly influences the (l-b l i-b)/2-b ratio, since the amount of I-butene is then relatively high.

169

Relation between metathesis a&i&y and redrtcibifr’ty of the catalysts As has been showm in Figs. 9 and IO, a relation exists between reduci-

bi?ity (average reduction temperature in TPR) and initial reaction rate in metathesis for MoO,/y-AlzOa as well as WC&/y-AEzOs. It is interesting that, aIso for silica-supported tungsten oxide and molybdenum oxide, it has been found that easy reducibility is a prerequisite for catalytic activity [33]. To understand the importance of such a relation in terms of the nature of the active sites in metathesis, it is necessary to know what causes the observed differences in reducibility of the catalysts. A possible explanation for the variation in reducibility can be derived from heterogeneity of the ahrmina surface. This implies that the fmt ions are adsorbed at the strongest adsorption sites. Such ions are relatively more numerous in catalysts with low metal oxide content.

At higher contents ions will also be adsorbed at weaker sites. It is reasonable to expect that strongly adsorbed molybdate/tungstate compounds are less susceptible to reduction in TPR as well as under metathesis conditions. An indirect result of the mean decreasing adsorption strength between molybdate or tungstaate ions and the carrier can be an increase in the degree of polymerization of these ions. The first ions adsorb separately at strong adsorption sites, while subsequent ions can choose between adsorption at relatively weaker sites of the carrier or on metal ions already present at the carrier surface. The latter can be described as a polymerization process. In fact even on a homogeneous surface an increase in degree of aggregation is expected, the higher the metal oxide content. The chance for each ion to find another, already adsorbed ion increases with surface coverage. The

conclusion that the degree of aggregation for MO&/~-AlzO, as well as WO,/ y-AlaO, catalysts increases with metal oxide content is supported by Raman spectroscopic data [30]_

The shift of the TPR patterns to an average lower reduction temperature with surface coverage can also be due to an autocatalytic effect. Reduced metal species will then accelerate the reduction of other species, e.g. by hydrogen spill-over. Spill-over is a localized process, so its contribution to the reduction process will only be noticed when the distance between reducible compounds is small, ie. at high surface coverage. However, from the observation that activity and reducibility are related it can be concluded that this effect is less probable. The relation between reducibility and meta- thesis activity thus suggests that an active site consists preferably of more than one MO/W-atom. For Re207/y-A1203 no positive correlation can be found between initial reaction rate and reducibility. Only at low Re contents

(up to - 3 wt’% Re207) reducibility increases with Re content; at higher Re loadings reducibility is essentiaLly independent of Re content ]30.35] _ By Raman spectroscopy it has been established that, between 6 and 18 wt% Rea07, only single Re species consisting of tetrahedral ReOi ions are present.& the catiyst surface 1361. This implies that catalysts consisting of non-polymerized transition metal ions czn also be active as metathesis catalysts,

i70

In view gf the relationship between initial reaction rate and Re content it is also plausible that for Re,07/~-Al,0s catalysts the dispersion of the non-polymerized transition metal ions is important with respect to catalytic activity. The degree of aggregation will increase inevitably with Re conkent, so apparently the more ReO, species present in zn aggregate. the higher the activitv per Re atom. In contrast to MoOs,+AI20, and WC&/ y-A&0, c2talyst.5, the imrease in the degree of aggregation is not manifested in changes in Raman spectra or TPR patterns of the catalysts. This can be understood from the fact that, in contrast to rhenium, molybdates and lungstates often exist as polyanions. It can be concluded that reducibility is a common favorable property of Re,O,/r-AlsO,, MOO&-Al,G3 and WO,/ y-Al20, catalysts with respect to metathesis activity. This holds for the relative activity within each series of MoO,/y-Al,03 and WO,/r-Al,03 catalysts as well as for the relative activity of the three systems, since ResO,i r-Al,03 catalysts are much more active in metathesis than MoO,/y-Ala03 2nd WO,/r-AI303 catalysts. The oxidation state of MO and W ions is important with respect to met&hesis activity. According to Brenner et al_

[20,21], MO’+ and Mo3+ species are more active in metathesis than MO++ and Mof5. The former species were obtained uk oxidation of Mo’(CO), or Mo’(~~-(C~H~))~ species in mild conditions. Starting with Mo+~ or W6+ compounds, the only way to generate catalysts which ;ue active in meta- thesis is by mild reduction to Most or Mo4+. The activity of these reduced catalysts will be much lower than catalysts containing MO=+ or Mo3* species. It may be possible to obtain MO=* and Mo3+ oxidation states from Mo6+ when severe reduction conditions are applied. However, it is to be expected that such a treatment will also lead to damaging of the catalyst (loss in surface srea, sintering erc.). In general it is importtt lo carry out the reaction at such low temperatures that reactions other than metathesis do not noticeably occur. At higher temperatures, highly r=luced ions (initially present in the catalysts or formed during reaction ukz reduction by alkenes) will increasingly catalyze side reactions better than met&hesis, leading to a decrease in selectivity.

Conclusions

(i) MOO3ir-AlsO3 2s well as WO,/y-AI303 cetalysts are relativeiy inactive metathesis catalysts up to a surface coverage of - Cl.5 metal atoms/ nm2_ Between 0.5 and 2.0 atoms/nm2 activity increases sharply with surface coverage, and m2xirnum turnover frequencies are found at - 2 met& atoms/ nm2.

(ii) A positive correlation exists between reducibility of the catalysts (TPR) and catalytic activity for metathesis of propene which confirms that reduction is a fundzu-nental step in the genesis of active sites.

(iii) When the reaction temperature is too high, the selectivity for meta- thesis decreases, due to the formation of highly reduce5 ions, which also catalyze other reactions.

171

iv) tMet&besis sites in MoO,iyA&&, WO,/yAI,O, a& Re,C&/ r-Al,O, catalysts conskt of species cordairing preferably several metal ions.

Acknowledgements

Thanks are due to A. Andreini and J_ C. Mel for the use of the micro flow-reactor system, to E. Rus for performing the TPR measurements and to F. Kapteijn for helpful discussion of kinetics.

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