Activity of supported Re2O7 catalysts for the metathesis of methyl oleate

Applied Catalysis, 67 (1991) 279-295 Elsevier Science Publishers B.V., Amsterdam

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Activity of supported Re,O, catalysts for the metathesis of methyl oleate M. Sibeijn and J.C. Mel* Department of Chemical Engineering, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WVAmsterdam (The Netherlands), tel. (+31-20)5255265, fax. (+31-20)5255698 (Received 28 May 1990, revised manuscript received 13 July 1990)

Abstract

Catalysts containing 3 wt.-% Re,O, were prepared using a y-alumina support or silica-alumina supports with varying alumina contents and surface areas. The activities of these catalysts in the metathesis of methyl oleate were determined in batch experiments in which tetraethyltin was used as a catalyst promoter. Both the alumina content and the surface area of the catalyst turned out to have a substantial influence on the catalytic activity per unit weight of rhenium. The catalyst with a silica-alumina support containing 25 wt.-% alumina appeared to be most active, while the activity also increased with increasing surface area. Diluting a catalyst with extra support material resulted in a spectacular increase in its specific activity. It appeared that during calcination part of the rhenium was transferred from the original catalyst to the admixed support. This spreading behaviour is either ascribed to a solid-solid wetting process, probably only concerning the ReO, tetrahedra bonded to silicon atoms, or to surface diffusion. An additional role of the extra support material is to remove impurities in the substrate. In accordance with these results, the turnover number of the non-diluted silica-alumina-supported catalysts appeared to increase with decreasing rhenium content. At low catalyst loadings the Si-Al bridging hydroxyl groups on the surface of the support are replaced by ReO; -tetrahedra, resulting in active rhenium centres, while at higher rhenium loadings the hydroxyl groups attached to silicon atoms are also replaced by ReO, tetrahedra. This latter type of rhenium centre is not active in metathesis. In contrast, the turnover number of the alumina-supported catalysts increased with increasing rhenium loading. This can be explained by the fact that on y-alumina the ReO, tetrahedra first replace basic surface hydroxyl groups, resulting in inactive sites. At higher rhenium loadings acidic hydroxyl groups are also replaced by ReO, tetrahedra, which does result in active rhenium centres. Keywords: rhenium oxide/alumina, methyl oleate metathesis, surface area, catalyst preparation (wet impregnation).

INTRODUCTION

The metathesis of alkenes is a challenging reaction. In particular, the metathesis of functionally substituted alkenes affords interesting possibilities for the synthesis of valuable organic compounds which are difficult to obtain by other methods [l-3]. The metathesis of functionalized alkenes is, however, not yet a commercial process, because of the low activity of the relevant catalysts. 0166-9834/91/$03.50

0 1991 Elsevier Science Publishers B.V.

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In the past, only a few catalyst systems had proved to have any prospects. At first only homogeneous catalyst systems, such as WClJSn (CH, ), [ 41, were reported to be active in the metathesis of functionalized alkenes. The discovery that the solid catalyst Re,O,/y-Al,O, containing lo-20 wt.-% Re207, activated with a small amount of tetramethyltin, is active and highly selective for the metathesis of functionalized alkenes at room temperature was a great step forward [ 51. Quite remarkable results were subsequently obtained with modified catalysts, which showed much higher activities than observed for the Rez07/A1203 catalysts. The increase in catalytic activity was accomplished by incorporating a third metal oxide, such as VZ05, MOO, or W03 [6]. Further improvements were achieved by using other supports. Thus, Xu Xiaoding and Mol [ 71 reported that in the metathesis of functionalized alkenes, like methyl oleate (methyl cis-9-octadecenoate ), the use of a silica-alumina support leads to much higher activities for catalysts with low Re207 loadings (3 wt.-% Re,O,) than corresponding alumina supported catalysts. Further investigations showed that other tetraalkyltin compounds and tetraalkyllead compounds are also suitable as promoters [ 71. In the present study we have investigated support material effects in more detail in the metathesis of methyl oleate with a supported Re,O,-catalyst. In this reaction both 9-octadecene and dimethyl 9-octadecenedioate are formed: 2 CHB(CH2)7CH=CH(CHz)7COOCH3~CH3(CHz),CH=CH(CH,),CH, + CH,OOCH (CH,) ,CH=CH ( CH2 ),COOCH, This is a reversible reaction with an equilibrium conversion of about 50%. In this study tetraethyltin was used as a promoter. Several types of silica-alumina supports were used, with varying A1203 contents. Xu Xiaoding et al. [8] had already reported that the activity of a 3 wt.-% Re207 catalyst on a silica-alumina support containing approximately 14 wt.-% A1203 is higher when it has a high surface area than when it has a low surface area. As the surface area of the catalyst appears to be important, we also investigated its influence on the catalytic activity of our catalysts. In addition we studied the effect of adding extra support material, thus increasing the total surface area of the catalyst system. EXPERIMENTAL

Catalyst preparation The Re,O, catalysts were prepared by pore-volume impregnation of the supports with calculated amounts of an aqueous solution of ammonium perrhenate (Johnson Matthey Chemicals Ltd. JMC 836 Specpure ) , followed by drying overnight in air at 383 K. Catalysts with a low pore volume were impregnated

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TABLE 1 Properties of the support support

Al,O, content (wt.%)*

Specific surface area (m’*g-i)

Pore volume (cm3*g-‘)

y-Al,O,-CK-300 SiO,*Al,O,-KDC SiO,. AI,O,-HA SiOp.A1203-LAL Si02*A1,0,-LA-10 Si02*A1203-LAH SiOZ-D.G.-62

> 99.9 80.8 24.3 15.3 13.7 13.0 0

208 322 380 141 87 464 345

0.51 0.92 0.91 0.46 0.31 0.78 0.89

“Data obtained from the manufacturer. TABLE 2 Properties of the catalysts Support

Specific surface area Pore volume Calculated Re,07 (rn’eg-‘) (cm”*g-‘) content (wt.-%)

y-Al&-CK-300 SiOl*Al,O,-KDC SiOB*AI,03-HA Si02*AI,0,-HA Si02-Al#-LAL Si02*A1&-LA-10 SiOp*A1208-LA-10 SiO~*Al.#-LAH Si02-D.G.-62

208 288 347 339 130 87 87 430 n.d.

0.48 0.90 0.88 0.83 0.42 0.30 0.28 0.73 n.d.

3 3 3 6 3 0.67 3 3 3

Rez07 content after calcination (wt.-%), 3.1 2.9 3.0 5.9 2.7 0.64 2.6 2.8 2.7

“Measured by ICP-AES; accuracy k 0.2 wt.-% Re,O,.

several times to obtain the desired rhenium loading. Between two impregnation steps the catalysts were dried at 383 K for 2 h. Silica was obtained from Davison Grace. The other support materials were obtained from AKZO Chemicals. The supports were ground and sieved, and the 180-250 pm fraction was used. Relevant support and catalyst properties are given in Tables 1 and 2, respectively. The alumina contents of the supports were specified by the manufacturer. The weight percentages of Re,O, in the different catalysts were calculated from the amount of ammonium perrhenate used for the impregnation. Chemicals Methyl oleate was prepared from its urea adduct (Unilever ). The urea adduct (1 kg) was hydrolysed under nitrogen with 0.5 1 of a 10% hydrochloric

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acid solution, after which petroleum ether (40-60) was added to prevent further hydrolysis of methyl oleate. The organic layer was separated from the water layer and consecutively washed with a saturated solution of sodium bicarbonate and with water. The organic layer was then dried on sodium sulphate, and any traces of free acids were removed by potassium carbonate. After vacuum distillation, the methyl oleate was stored under dry argon. Tetraethyltin (Aldrich) was used as purchased. Hexane, the solvent, was distilled over sodium under argon before use. Activity measurements The metathesis reactions were carried out in the liquid phase in a glass batch reactor with a capacity of 30 ml. The catalyst was first calcined by heating it to 823 K in an air stream (60 ml/min) at a rate of 20 K/min. After 2 h at this temperature, the catalyst was cooled to room temperature. After calcination the catalyst was transferred to the reaction vessel and placed under an argon atmosphere. Subsequently a solution of tetraethyltin in hexane was added, followed by addition of the methyl oleate. In all experiments the molar ratio of methyl oleate/Re,O,/Sn(C,H,), was 240/l/1.1, unless otherwise mentioned. All metathesis experiments were carried out at room temperature. In a typical experiment 100 mg of a 3 wt.-% catalyst (6.2 pmol Re,O,) was used, to which 1 ml of hexane, containing 1.4 ~1 (7 pmol) of tetraethyltin, and 0.5 ml (1.45 mmol) of methyl oleate were added. The reaction was monitored by GC analysis (Carlo Erba HRGC 5300 Mega Series ) of the liquid phase over a capillary column (Chrompack, WCOT Fused Silica 25 mx0.53 mm I.D., coating CPSil5 CB, wide bore). The GC signals were processed by a Shimadzu C-R5A Chromatopac integrator. The methyl oleate conversions were calculated as (2 - alkene) / (2malkene + methyl oleate) from the corrected peak areas of the GC analyses. The molecular response factors were estimated by Ackman’s method [9,10]. Inductively coupled plasma-atom emission spectrometry Rhenium determinations were carried out by inductively coupled plasmaatom emission spectrometry (ICP-AES) measurements, performed on a Therm0 Jarrell Ash ICAP 61 spectrometer. The rhenium concentration was measured at Re (I ) 228.751 nm. In a typical analysis 100 mg of catalyst was dissolved in circa 3 ml of a mixture of nitric and hydrochloric acids (volumetric ratio l/3). The mixture was then heated to its boiling point and maintained there for 1 h. Water was added to the cooled solution to make it up to 100 ml. The rhenium content of this solution was subsequently measured by ICP-AES. The spectrometer was calibrated with 1 ppm, 10 ppm and 20 ppm solutions

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of rhenium metal. These were prepared from a rhenium AAS standard solution of 1000 ppm rhenium metal (Perstorp Analytical). BET measurements The specific surface areas and the pore volumes of the supports and the catalysts were determined by the BET method on a Carlo Erba Sorptomatic 1800. The samples were measured at 77 K with nitrogen as the adsorbate, after a pretreatment of 1 h in vacuum at 473 K. The results of these measurements are given in Tables 1 and 2. RESULTS

Activity measurements We first investigated the effects of the various supports on the activity of the supported 3 wt.-% Re,O, catalysts. Fig. 1 shows the results of these experiments. It appeared that the silica-alumina-supported catalysts are more active than the catalyst supported on y-alumina alone. In all cases the selectivity was 60

50

ZR

40

. s ‘II

30

iz ;

LAL

20

IO

0 0

1

reaction

2

3

time

/

4

h

Fig. 1. Conversion of methyl oleate for various silica-alumina-supported 3 wt.-% Re,O, catalysts at room temperature as a function of reaction time. Molar ratio methyl oleate/Rez07/ Sn(CZH,)4=240/1/1.1.

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higher than 95%. Rez07/Si02*A1203-HA (3 wt.-% ReZ07), in combination with tetraethyltin, turned out to be the most active catalyst system, which is in agreement with previous results [7]. The 3 wt.-% Re,O,/SiO,-catalyst was least active, showing no activity at all under our experimental conditions. The conversion of methyl oleate after 1 h reaction time is given in Table 3, together with the alumina content of the support for the 3 wt.-% Re,O, catalysts. It appeared that the activity depends on the alumina content of the support. The maximum conversion for the catalysts studied was obtained at approximately 25 wt.-% alumina. The three ‘low-alumina’ (LA) catalysts, viz. LAH, LAL and LA-lo, have silica-alumina supports with different specific surface areas. From Table 3 it also follows that the conversion for these catalysts increases with increasing surface area. One method of increasing the total surface area of a catalyst system is to add extra support material to the catalyst. We compared the activity of the three low-alumina catalysts by diluting the two catalysts based on SiO,*Al,O,-LAL and SiOa-A1203-LA-10 with extra support material before calcination in such a way that the total surface area of the catalyst systems after dilution was equal to that of the LAH-supported catalyst (see Table 4). Note that in all cases the TABLE 3 Conversion after one hour reaction time for the supported 3 wt.-% Re,O, catalysts Support

A&O,-content of the support (wt.-%)

Specific surface area of the catalyst (m2*g-‘)

Conversion (% )

y-Al&-CK-300 Si02.A120s-KDC Si02.A1203-HA SiO,* A&O,-LAL Si02*A1,0,-LA-10 SiO,* A&O,-LAH SiO,-D.G.-62

> 99.9 80.8 24.3 15.3 13.7 13.0 0

208 288 347 130 87 430

1.6 25.9 47.2 13.8 3.7 21.9 0.0

TABLE 4 Dilution of 100 mg 3 wt.-% Re,0,/Si02*A1,0s Support (100 mn)

Catalyst surface area (m’)

Amount of support added (mg)

Total surface area after dilution (m’)

LA-10 LAL LAH

8.7 13.0 43.0

390 215 none

43 43 43

absolute amount of rhenium is the same. In Fig. 2 it can be seen that the activity of the two catalysts increases dramatically upon dilution. Their activity is even higher than that of the undiluted LAH-supported catalyst, although in all three cases the total surface area of the catalyst system is the same. We investigated next whether all our catalysts show a similar effect when diluted with extra support material. In these experiments we used a catalyst/ support mass ratio of l/l. From Fig. 3 it appears that all catalyst systems with a silica-alumina support containing less than 25 wt.-% alumina showed an increase in activity upon dilution. The activity of the 3 wt.-% RezO, catalysts based on y-alumina and SiO,.Al,O,-KDC, however, remained unchanged. In the foregoing experiments the catalyst and the added support material were calcined together. We also studied the effect of calcining the catalyst and the extra support material separately. This effect was investigated for 3 wt.-% Rez0,/Si0,*A1203-LA-lo, because this catalyst shows a large increase in activity upon dilution. We diluted 100 mg of this catalyst with 350 mg of support material. Fig. 4 shows the results of these experiments. Curve A represents the results with the undiluted catalyst. When the catalyst is calcined together with the admixed support (as in the foregoing experiments), curve C is obtained.

50

I

LA- 10

diluted

LAL

diluted

40

30

20

10

LA-10 n

reaction

time

/

h

Fig. 2. Conversion of methyl oleate for the LA-supported 3 wt.-% Re,O, catalysts as a function of reaction time at room temperature. The catalysts based on LAL and LA-10 have been diluted with such an amount of support material that the total surface area of these catalyst systems is equal to that of the LAH-supportedcatalyst. Molar ratio methyl oleate/Re~O~/Sn(C~H~),=240/1/1.1.

60

60

E

I=

40

0 I!!? 0

1

reaction

2

3

time

4

/

h

‘:L 0

1

reaction

2

3

time

4

/

h

Fig. 3. Conversion of methyl oleate at room temperature for various supported 3 wt.-% Re,07 catalysts (V ) and diluted ones (A ) in a l/l mass ratio. Molar ratio methyl oleate/RepO,/ Sn(C2H,)4=240/1/1.1. A. LAH; B. LA-lo; C. LAL; D. HA; E. KDC; F. A&O,.

When the extra (calcined) support material is added after calcination of the catalyst (Fig. 4, curve B ) , there is also an increase in activity, but this increase is not as high as when the catalyst and the extra support are calcined together. Another method of ‘diluting’ the catalyst with extra support material is to use a catalyst with a lower Re,O, loading. This, in fact, amounts to adding extra support material before impregnation. The overall Re,O, loading of the diluted SiO,-Al,O,-LA-lOsupported catalyst was 0.67 wt.-%. We therefore also prepared a 0.67 wt.-% Re207/Si02-A1203-LA-10 catalyst. The activity of this catalyst (Fig. 4, curve D) turned out to be nearly the same as that of the catalyst system in which the catalyst and the extra support had been calcined together. From the abovementioned experiments it follows that the use of catalysts with a lower Re,O, loading results in a higher activity per rhenium atom (i.e. turnover number). We subsequently tested whether the highly active Re,O,/

287 60

50

8

40

. 5 *2

30

: ii 0

20

10

A a

0

0

1

reaction

2

3

time

/

4

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Fig. 4. Conversion of methyl oleate for several ReP07/Si02*A1,0,-LA-10 catalysts as a function of reaction time at room temperature. Molar ratio methyl oleate/ReP07/Sn(C2H,),=240/1/1.1. A. 3 wt.-% Re207/Si0,.A1,0,-LA-lo; B. 3 wt.-% Rez07/Si02.A1203-LA-lo, 100 mg catalyst diluted with 350 mg LA-10 support after calcination; C. 3 wt.-% Re,O,/SiO,*AleOs-LA-lo, 100 mg catalyst diluted with 350 mg LA-10 support before calcination; D. 0.67 wt.-% Re,O,/SiO,* Al,O,LA-lo.

SiO,.Al,O,-HA catalyst also shows an increase in turnover number with decreasing Re,O, loading. We studied three catalysts with Re,O, loadings of 1 wt.-%, 3 wt.-% and 6 wt.-%, respectively. The results of these experiments are shown in Fig. 5. In all these experiments the molar ratio methyl oleate/Re,07/ Sn(C,H,)4 was kept constant at 240/l/1.1. It did indeed appear that in this case catalysts with low loadings also have a higher activity per rhenium atom. However, in a separate experiment in which we used double the amount of methyl oleate, we did not find any difference in activity between 1 wt.-% and 0.5 wt.-% Rez07 supported on SiOssA1203-HA. The same kind of experiment was performed with Re207/y-A1203 catalysts, with Re,07 loadings of 3 wt.-%, 6 wt.-% and 12 wt.-%, respectively (molar ratio methyl oleate/Re,O,/Sn (C,H,), = 120/l/1.1.). Fig. 6 shows that in this case the catalyst activity increases with increasing loading, the opposite of the results obtained with silica-alumina supported catalysts.

288

40

6 wt% 30

reaction

time

/

h

Fig. 5. Conversion of methyl oleate for Re,O,/SiO,* Al,O,-HA catalysts with different Fk207 loadings at room temperature as a function of reaction time. Molar ratio methyl oleate/Re,07/ Sn(CzH5)4=240/1/1.1.

ICP-AES The Re207 content of the supported 3 wt.-% Re207 catalysts was measured by ICP-AES. The accuracy of the method for this type of catalysts is k 0.2 wt.% Re,O,. The results are included in Table 2. It appears that most of the rhenium remains on the catalysts during calcination. A slight rhenium loss is only observed for Si02*A1203-LA-lo, Si02*A1203-LAL and SiO,. We also investigated whether rhenium was transferred from the diluted catalyst to the admixed support during calcination. This was possible by using different particle sizes for the original catalyst and the admixed support, and separating them after calcination by sieving. In Table 5 the ICP-AES results are given for several supported 3 wt.-% Re,O, catalysts which were diluted with equal amounts of support (mass ratio l/l). From this table it is clear that for y-alumina nearly all the rhenium remained on the original catalyst particles. For Si0:!-A1208-KDC only a very small amount of the rhenium was transferred to the admixed support. In the case of the other silica-alumina supports, however, a larger percentage of the rhe-

289

20

15

10

5

0

0

1

2

reaction

time

3

/

4

h

Fig. 6. Conversion of methyl oleate for RexO,/y-A&O, catalysts with different RezO, loadings at room temperature as a function of reaction time. Molar ratio methyl oleate/Re,O,/ Sn(C,H5),=120/1/1.1.

TABLE 5 ICP-AES measurements of calcined supported 3 wt.-% Re,O, catalysts, diluted with support material in a l/ 1 mass ratio support

Re,O, content of the catalyst (wt.-%)*

Re,O, content of the admixed support (wt.-%))*

y-A&O,-CK-300 SiO,*Al,O,-KDC SiO,. Al&-HA Si0,*A1203-LAL SiO,.Al&-LA-10

3.2 2.9 2.1 1.7 1.7 1.9 1.6

0.05 0.1 1.1 1.0 0.9 0.7 1.2

SiOP* A&O,-LAH

SiO,-D.G.-62 “Accuracyf0.2

wt.-% Rez07.

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nium, viz. 27-37 %, was transferred from the catalyst to the admixed support. With silica alone, 43% of the rhenium was transferred. DISCUSSION

The effect of the support The type of support appears to be an important factor for the activity of supported Re,O, catalysts. Both Xu Xiaoding et al. [8] and Ellison et al. [ 111 report a distinct correlation between the metathesis activity of ReZ07 catalysts and the Bronsted acidity of the support. Ellison et al. [ 111 studied Re,O, catalysts with various alumina supports; the activity of these catalysts for the metathesis of unsaturated esters increased with increasing Brensted acidity of the alumina. Xu Xiaoding et al. [8] studied the activity of several silica-alumina-supported Rez07 catalysts in the metathesis of methyl oleate. They also observed an increase in activity with increasing Bronsted acidity of the support. The present results seem to support this correlation. Fig. 1 shows a maximum in activity for the silica-alumina-supported catalyst with an alumina content of approx. 25 wt.-%. It is well known that silica-alumina with approx. 25 wt.-% alumina has the highest Brcansted acidity [12]. However, from our results it cannot be explicitly concluded whether the maximum activity of silica-alumina supported catalysts is actually reached for an alumina content of approx. 25 wt.-% or at higher alumina percentages, because commercial silicaalumina supports with an alumina content between 25 and 80 wt.-% were not available to us. In what way does the Bronsted acidity of the support correlate with the activity of the Re,O, catalysts? The Brransted acidity of the silica-aluminas with an alumina content of up to 25 wt.-% alumina is due to two types of hydroxyl groups [ 141. One type consists of hydroxyl groups attached to a silicon atom and the other type consists of bridging hydroxyl groups attached to both a silicon atom and an aluminium atom. The latter type of hydroxyl group is more electropositive - thus more Bronsted acidic -than the hydroxyl groups attached to a single silicon atom [ 12,141. During calcination the ReO,- ions are bonded to sites which were previously occupied by the bridging hydroxyl groups. The reaction of ReO,- ions with the bridging hydroxyl groups results in electropositive rhenium centres (ReO; tetrahedra), which will easily accept the complexation of the electron-rich carbon-carbon double bond of the alkene. This might explain why silica-alumina-supported Re,O, catalysts are already very active at low rhenium loadings. For y-alumina-supported catalysts, however, the activity per rhenium atom increases with increasing Re,O, loading. This can be explained by taking a closer look at the surface composition. On y-alumina there are not only acidic

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hydroxyl groups, but also basic and neutral hydroxyl groups [ 13,151. IR data indicated that the ReO,- ions react preferentially with the most basic hydroxyl groups [ 161. These rhenium centres are electronegatively charged, and the complexation of an electron-rich carbon-carbon double bond is less likely. Therefore these rhenium centres will not be active in metathesis. At higher Re,O, loadings the number of neutral and acidic surface hydroxyl groups that have reacted with ReO,- ions increases, and consequently the activity of these catalysts increases. There is an apparent discrepancy in that the ReO,- ions react with the acidic hydroxyl groups of the silica-alumina supports but with the basic hydroxyl groups of the alumina support. In fact the ReO; tetrahedra prefer to bind to aluminium containing sites, either during impregnation or calcination, as inferred from the dilution experiments: for the diluted 3 wt.-% Re207/A1203 catalyst no rhenium transfer is observed, while for the 3 wt.-% Re,O,/SiO, catalyst almost half of the rhenium transfers to the admixed support. On silica-aluminas with a higher alumina content ( > 25 wt.-% ), an alumina phase is present as well as the silica-alumina phase [ 131. Not only bridging hydroxyl groups and hydroxyl groups attached to silicon atoms are present, but also various other types of Al-OH groups. From our results with the KDCsupported catalyst it seems that the ReO; ions react preferentially with the bridging hydroxyl groups, since the activity of this catalyst is much higher than that of the y-alumina-supported catalyst. The effect of the Re207 loading It has already been discussed that the specific activity of y-alumina supported catalysts increases with increasing rhenium loading or, in other words, the turnover number of these catalysts increases with increasing Re,O, loading. The turnover number of the silica-alumina-supported catalysts with an alumina content of up to 25 wt.-% A1203, however, decreases with increasing Re,07 loading (see Fig. 5), due to the formation of inactive rhenium centres at higher rhenium loadings. These inactive centres are created by the formation of rhenium clusters or by the replacement of hydroxyl groups attached to silicon atoms by ReO, tetrahedra. The latter type of rhenium centre is not active, as no activity was observed in experiments with 3 wt.-% Re,07 on silica, although most of the rhenium remained on the catalyst (Table 2 ) . We observed no difference in turnover number between catalysts with Re,O, loadings below 1 wt.-%. Xu Xiaoding et al. [ 81 also reported an increase in turnover number with a decreasing Rez07 loading on Si02-A1208-HA. In their experiments a 0.5 wt.-% Re207/Si02.A1203 catalyst appeared to have a higher specific activity than the corresponding 1 wt.-% catalyst. However, in their case the methyl oleate/Re,O, molar ratio was not kept constant, but rather the methyl oleate/ catalyst weight ratio.

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The activity of the low-alumina catalysts increases with increasing surface area. This increase in activity might be explained by the fact that a higher surface area means that the total number of favourable acid sites per unit weight of support increases. Xu Xiaoding et al. [ 81 reported that the silica-alumina support with the highest specific surface area, LAH, is indeed more acidic than the LAL support per unit weight of support material. Therefore more ReO,ions can react with favourable acidic sites on the LAH support than on the LAL support.

The effects of diluting the catalyst

The addition of extra support material also increases the total surface area of a catalyst system. The activities of the catalysts with an alumina content of less than 25 wt.-% (Fig. 3 ) increase upon dilution. From Table 2 it follows that during combined calcination of the catalyst and the admixed support part of the rhenium is transferred to the added support material. In the diluted 3 wt.% Re,O, catalysts based on the silica-alumina supports ( < 25 wt.-% Al,O,) one third of the rhenium atoms is transferred from the original catalyst to the admixed support particles. However, when the 3 wt.-% Re,O, catalysts supported on Si02*A1208-KDC and y-alumina are diluted, scarcely any rhenium is transferred to the extra support material. This is in agreement with the fact that we did not observe any increase in activity upon dilution for these catalysts. Thus, the transfer of rhenium atoms from the catalyst to the admixed support particles appears to have important consequences for the catalytic activity of these systems: the increase in activity indicates that more active rhenium centres have been created. If the three low-alumina catalysts are diluted in such a way that the total surface areas of the catalyst systems are equal, the diluted LAL- and LA-lOsupported catalysts will have a higher activity than the undiluted LAH-supported catalyst. XPS measurements showed that the surface alumina concentrations of these three supports are higher than the bulk alumina concentrations [ 171. Moreover, the surface alumina concentrations of both LA-10 and LAL are higher than that of LAH. The acidity of a support is more likely to be determined by its surface composition than its bulk composition. Thus more bridging hydroxyl groups are available in the LA-10 and LAL supports. This means that more active rhenium centres can be formed during calcination if part of the rhenium is transferred to the extra support material. The question remains as to how rhenium is transferred from the original catalyst to the admixed support. Leyrer et al. [18] report the spreading of metal oxides on the surface of support oxides, such as y-alumina and titania,

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upon heating physical mixtures of Moos, V,O, or WO, and a support material in a stream of oxygen. The spreading behaviour can either be ascribed to solidsolid wetting, where the gradient of the surface free energy is the driving force, or to transport by surface diffusion caused by a concentration gradient. This might also apply to our silica-alumina-supported ( ~25 wt.% A1203) Re,O, catalysts: the rhenium might be spread over the surface by either of these mechanisms. Gas phase transport, i.e. sublimation as Re207 and redeposition on the extra support is less likely, because it follows from Table 2 that no substantial rhenium losses occur during calcination of the undiluted 3 wt.-% RezO, catalysts. For Re,07 on y-alumina and on Si02.Alz08--KDC the driving force is apparently not strong enough to initiate spreading of the rhenium. During calcination of a diluted 3 wt.-% Rez07/SiOz catalyst (mass ratio l/l), 43% of the total amount of rhenium was transferred from the original catalyst to the silica diluent. Therefore it is likely that on silica-aluminas only rhenium transfer occurs for the ReO, tetrahedra bonded to silicon atoms, and not for those bonded to aluminium atoms. This means that on these types of silica-alumina at low rhenium loadings, when all the ReO; tetrahedra are bonded to aluminium atoms, no rhenium transfer will be observed. The fact that rhenium transfer is also not observed for SiOz.A1,O,--KDC (80.8 wt.-% Al,O,) supports this suggestion, as the number of aluminium atoms in this support material is rather high. The increase in activity cannot be entirely explained by the creation of extra active rhenium centres due to the transfer of rhenium to the extra support material. In Fig. 5 we see an increase in the activity of the catalyst system when extra support material is added both before or after calcination; however, in the latter case the increase in activity is lower. It is unlikely that rhenium is transferred from the original catalyst to the admixed support during the metathesis reaction which takes place at room temperature. Nevertheless, an increase in activity is observed when the extra support is added after calcination. A possible explanation is that the added support material acts as a trap for impurities, such as peroxides present in the methyl oleate. If this is correct it would appear from the dilution experiments that y-A1203 and Si02*A10,-KDC are unable to trap impurities. CONCLUSIONS

The metathesis activity of supported Re,O, catalysts is strongly dependent on the alumina content of the support which effects its Brsnsted acidity. The catalytic activity increases with increasing Bronsted acidity of the support as ReO, tetrahedra replace Bronsted-acid hydroxyl groups and become electropositively charged. These rhenium centres will accept the complexation of an electron-rich carbon-carbon double bond more easily than electronegativelycharged rhenium centres.

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During the preparation of the catalysts supported on silica-aluminas with an alumina content of less than 25 wt.-% the available acidic hydroxyl groups are replaced by ReO, tetrahedra, resulting in very active catalysts at low rhenium loadings. However, the turnover number of silica-alumina-supported catalysts decreases with increasing RezO, loading, because at higher rhenium loadings inactive rhenium centres are also formed. On y-alumina the basic hydroxyl groups are first replaced by ReO, tetrahedra, resulting in a low catalyst activity at low rhenium loadings. At higher RezO, loadings electropositively-charged rhenium centres are also formed; the specific activity of this catalyst thus increases with increasing rhenium loading. The activity of the catalysts with silica-alumina supports containing less than 25 wt.-% alumina also increases with increasing surface area, as the number of acidic hydroxyl groups increases and more active rhenium centres can be formed. This is also the case when extra support material is added to these catalysts. During combined calcination of the catalyst and the admixed support, part of the rhenium is transferred from the catalyst to the extra support material, increasing the number of rhenium centres which are active for metathesis. The transfer of rhenium to the admixed support material probably only concerns ReO; tetrahedra that are bonded to single silicon atoms of the silica-aluminas. An additional effect of the dilution of Re,O, catalysts supported on silica-aluminas ( c 25 wt.-% A1203), is the trapping of impurities in the substrate. ACKNOWLEDGEMENTS

We would like to thank Unichema International for financial support. Thanks are also due to G.C. Hoogendam for experimental assistance, to M.C. Mittelmeijer-Hazeleger for the BET measurements and to Dr. F. Maessen and H. Balke for the ICP-AES measurements. We also thank AKZO Chemicals B.V. for the silica-alumina samples.

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