J . Chem. SOC., Faraday Trans. 1, 1986,82,2707-2718

Surface Acidity of some Re,O,-containing Metathesis Catalysts

An in situ Fourier Transform Infrared Study using Pyridine Adsorption

Xu Xiaoding,* Johamnes C. Mol and Cornelis Boelhouwer University of Amsterdam, Institute of Chemical Technology, Nieuwe Achtergracht 166,

1018 VW Amsterdam, The Netherlands

Surface acidity of several metathesis catalysts, Re,O,/Al,O,, M, 0, - Re,O,/Al,O, (M, 0, = V20,, MOO, or WO,) and Re,O,/ SiO;Al,O,, have been studied in situ by Fourier transform infrared spectroscopy using pyridine adsorption. A distinct correlation has been found between the activity for metathesis and Bronsted acidity of the catalysts, while their Lewis acidity showed no correlation with the activity.

Surface acidity is an interesting subject in heterogeneous catalysis since it is closely related to the activity of many acid-base catalysed reactions, e.g. isomerization, polymerization and catalytic cracking.l-* Although alkene metathesis is not commonly regarded as an acid-catalysed reaction, there are indications that in some cases surface acidity may have an important impact on the metathesis activity of some heterogeneous catalyst ~ystems.~-l* Treating alumina with strong acids can improve the overall metathesis activity of some catalyst^.^. Low-loading Re,O,/Al,O, catalysts prepared by impregnation with NH,ReO,/HCl show much higher activity in the metathesis of methyl oleate [(Z)-methyloctadec-9-enoate] than the corresponding Re,O,/Al,O, catalysts prepared without HCl.9 Ellison et ~ 1 . ~ 1 ~ reported that 5 wt % Re,O, catalysts supported on various alumina carriers showed negligible activity except for one distinct catalyst that possessed some acidity at that rhenium loading. Several metathesis catalyst systems were reported, viz. Re,O,/SiO, * Al,O,1° and M, 0, * Re,O,/Al,O, (M, 0, = V,O,, MOO, or wo3),73 11-13 which show much higher activities for metathesis than the corresponding Re,O,/Al,O, catalyst systems. For these catalyst systems the high activity has been attributed to their high acidity, especially the Bronsted acidity.1°

Up to now the acidity of Re,O,/Al,O, catalysts has not been studied thoroughly.l*? l5 Our knowledge of the acidity of Re,O,/SiO, - Al,O,1° and M, 0, - Re,O,/A1,O,ll catalysts is practically unexplored. The aim of the present research is to study the acidity of these metathesis catalysts in more detail. The results are discussed in the light of their activity in alkene metathesis.

Various methods have been developed to measure surface acidity, e.g. amine titration using distinct indicators, and i.r. measurements with a base probe.l-* With the amine titration method using different indicators, information about the total acidity as well as about the distribution of acidities of various strengths can be obtained.l7 However, this method cannot distinguish Lewis acidity from Bronsted acidity. 1.r. measurements using a base probe molecule allow determination of both Lewis acid sites and Bronsted acid sites at the same time.16-21 We applied the latter method. Pyridine, a weak base (pK, M 9),16 was chosen as the base probe so that strong acid sites which may be involved in metathesis, could be measured.

Since Parry's article was published,16 the i.r. study of surface acidity by pyridine adsorption has become a classic method to study surface acidity.'-"* Various surface

2707

Dow

nloa

ded

by F

udan

Uni

vers

ity o

n 26

/04/

2013

09:

44:3

4.

Publ

ishe

d on

01

Janu

ary

1986

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/F

1986

8202

707

View Article Online / Journal Homepage / Table of Contents for this issue

2708 Re,O,-containing Metathesis Catalysts

species, viz. hydrogen-bonded pyridine (HPy), physisorbed pyridine, coordinatively bonded pyridine (LPy) and pyridinium ions (BPy) can be determined from an analysis of i.r. bands in the ‘ring’ region (1400-1700 ~m-l).l-~9 l6 The i.r. bands of LPy sites appear at ca. 1620, 1580, 1490 and 1450 cm-l, while those of the BPy sites appear at ca. 1640, 1620, 1540 and 1490 cm-l.16 The band at ca. 1540 cm-1 is characteristic of Bronsted acid sites, while the one at ca. 1450 cm-l is characteristic of Lewis acid sites, provided there is no interference from other surface The exact band positions, however, depend among other factors on the pretreatment, the conditions during the measurement and the nature of the samples. From the band heights or integrated areas of the i.r. bands, quantitative data of these species can be ~btained.l-~ Evacuation at elevated temperatures (> 423 K) can remove the H-bonded and physi- sorbed pyridine.l6> 1 7 9 2o By varying the temperature of evacuation after pyridine adsorp- tion and by varying the method of pretreatment of the samples, information about the strength of Lewis acidity can be obtained from the ease with which the adsorbed species can be pumped off and from the frequency shift of the band at ca. 1450 cm-l.l-*

Experiment a1

Fourier transform infrared (FTIR) spectra in the region 400-4000 cm-l were recorded on a Nicolet 7199 FTIR spectrophotometer with a liquid-nitrogen cooled HgCdTe detector. The resolution used was 2cm-l and the number of scans was 32 for each spectrum. The i.r. cell has been described el~ewhere.~ A pair of NaCl windows (35 mm) was used and silicon-rubber rings were fixed between the cell and the windows at both sides of the windows to ensure that the metal cell was vacuum tight under the conditions used. The self-supporting wafer was mounted on a metal sample holder which was located at the centre of the cell. A thermocouple was inserted inside the cell and positioned near the wafer. During the measurements water flowed through the cooling jacket of the cell to prevent the window part from overheating. The temperature of the cooling water was maintained by a thermostat at a temperature sufficiently high to prevent moisture from condensing on the windows (303-323 IS). The heating of the cell was monitored by a proportional temperature controller. The cell could be moved vertically in order to measure the background spectra and the sample spectra, separately. By ratioing the sample energy spectrum to the background energy spectrum, recorded under the same conditions, the proper i.r. spectrum was ~btained.,~

The i.r. cell was connected with a conventional gas and vacuum system, which provided the possibility of evacuation (up to ca. 0.1333 Pa), the admittance of various gases (He, 0,) and substrates (e.g. pyridine) and the introduction of a liquid reactant via the septum of an injection port. The oxygen used was dried by 5A molecular sieves. The helium was dried and deoxygenated by activated molecular sieve 5A and a heated Cu/A1,0, catalyst. The pyridine used was distilled under reduced pressure after being dried by molecular sieves and only the middle-cut was used for the experiments. The catalysts used to prepare the self-supporting wafers were prepared by a wet incipient impregnation method, as described earlie~,~-ll and were ground to a particle size suitable for i.r. purposes.

The i.r. wafers were pressed at 5 ton (on the ram) for a period of 1 min. The diameter of the wafers was 13 mm and their weights ca. 10-20 mg.

In pretreating the catalyst wafers, they were first heated at 773 K in a dry oxygen stream overnight, followed by heating at the same temperature for 2 h in a dry and deoxygenated helium stream. After cooling to 423 K and evacuation, a small amount of pyridine (up to a pressure of 4666Pa) was admitted into the system at that temperature. The sample was kept in the pyridine atmosphere at 423 K for 1 h and the system was then evacuated at 423 K for 1 h. The temperature was raised stepwise (steps of 50 or 100 K) during evacuation till 773 K was reached. Spectra were recorded after

Dow

nloa

ded

by F

udan

Uni

vers

ity o

n 26

/04/

2013

09:

44:3

4.

Publ

ishe

d on

01

Janu

ary

1986

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/F

1986

8202

707

View Article Online

X . Xiaoding, J . C . Mol and C . Boelhouwer 2709 stabilizing the conditions at each temperature for 0.5 h. Integration of the peak areas was conducted by using either the integration program of the FTIR, according to the usual tangent baseline method,17 or, in the case of overlapping peaks, by a cut-and-weigh method. The peak positions of the i.r. bands were picked up via a computer program of the FTIR spectrometer.

In some cases distinct amounts of water were admitted to the system via the injection port after the sample was heated in vacuum to 773 K and cooled down to 423 K to see the effect of adding water.

The activity for the metathesis of the catalysts were measured either in the liquid phase in glass batch reactors (for the metathesis of methyl oleate9-11), or in a fixed-bed microcatalytic flow reactor system at 323 K at a propene pressure of 0.05 MPa (for the metathesis of propeneg* 24) as described earlier.

Results Fig. 1 shows the i.r. spectra of catalysts containing 0, 3, 6, 12 and 18 wt % Re,O, after activation in pyridine at 423 K for 1 h. The positions of the i.r. bands are given in table 1. It appears that the 12 and 18 wt % catalysts show a peak at 1539 cm-l and a shoulder peak at 1637 cm-l. The band at ca. 1540 cm-l remained on the surface even at 773 K in vacuum. Although from the spectra in fig. 1 no peak at 1539 cm-l can be observed for the 6 wt % Re,07/A1,0, catalyst, the subtracted spectrum (the spectrum after

1701 1611 1521 1431 1341 v/cm-'

Fig. 1. 1.r. spectra of Re,O,/Al,O, catalysts after activation and pyridine adsorption at 423 K.

Dow

nloa

ded

by F

udan

Uni

vers

ity o

n 26

/04/

2013

09:

44:3

4.

Publ

ishe

d on

01

Janu

ary

1986

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/F

1986

8202

707

View Article Online

2710 Re,O,-containing Metathesis Catalysts

Table 1. Positions of the i.r. bands for various Re,07/A1,0, catalysts at 423 K after pyridine adsorption

wt% Re207 band positions/cm-l

0 - 1623, 1614, 1596,a 1577, - 1493, 1451. 3 - 1622, 1616, 1592,a 1577, - 1494, 1451. 6 - 1622, 1616, 1592,a 1576, 1539,b 1494, 1455.

12 18

1637, 1623, 1617, 1594,a 1575, 1539," 1494, 1455. 1636, 1622, 1616, 1593,a 1575, 1539,c 1494, 1455.

a Only observed on spectra in pyridine. Obtained from the subtracted spectrum at 423 K in vacuum after and before pyridine adsorption. This i.r. band was observed at 773 K in vacuum at 1544 and 1545 cm-', for 12 and 18 wt "/, Re,O,/Al,O, catalysts, respectively .

I I I I 1701 1633 1565 1497 1429

v/crn-'

Fig. 2. 1.r. spectra of 3 wt % Re,O, catalysts on (a) HA-, (b) LAH-, (c) LAL-silica-alumina carriers and ( d ) on y-alumina.

pyridine adsorption and evacuation minus the spectrum before pyridine adsorption in vacuum) did show a small peak at 1539 cm-l. The integrated areas of the ca. 1540 and ca. 1450 cm-1 bands were used to calculate Bronsted and Lewis acidity, respectively.20

A 6 wt % Re20,/A1203 catalyst prepared by HCl impregnation was also measured. A weak peak at ca. 1550 cm-l was observed at 423 K in vacuum. The addition of water ( 5 x cm3) to the sample increased the intensity of this peak.

Dow

nloa

ded

by F

udan

Uni

vers

ity o

n 26

/04/

2013

09:

44:3

4.

Publ

ishe

d on

01

Janu

ary

1986

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/F

1986

8202

707

View Article Online

X . Xiaoding, J . C. Mol and C. Boelhouwer 271 1

Table 2. Band positions" of SA-supported Re207 catalysts

SA

LAH LAL HA HA HA HA HA

wt% Re207 i.r. band positions/cm-l

1636 1622 - 1544 1491 1636 1622 - 1545 1491 1638 1622 - 1545 1491 1637 1622 - 1545b 1491 1637 1621 1545d 1490 1637 1622 1612c 1545e 1491 1640 1622c 1610 1545 1489

1455 1455 1455 1454 1454 1454 1454

a The spectra were recorded at 423 K after pyridine adsorp- tion and evacuation for 1 h at 423 K. 1451 cm-l at 673 K.

Shoulder. 1546 cm-l at 773 K. 1547 cm-l at 773 K.

Table 3. Weight normalized peak areas of Lewis acid sites, Bronsted acid sites and the ca. 3740 cm-l band of SA-supported catalysts with different Re207 wt % at 423 K in vacuum after

pyridine adsorptiona

SA wt% Re207

LAH LAL HA HA HA HA HA

Bronsted acidity

A/cm-l g-l

46.3 29.0 65.6 99.8

100.2 142.3 158.0

Lewis acidity

A/crn-l g-l

OH (3740 cm-l) Alcm-l g-l

148.6 100.7 205 262 236 171 122

-

952 862 710 555 375

a All obtained by an FTIR integration program.

Re20,/Si02.A120, (SA) catalysts of 0, 1, 3, 6 and 9 wt % Re20, supported on HA-SA (24.3 wt % Al,O,, 374 m2 g-l) and 3 wt % Re,O, on LAH- (13.0 wt % Al,O,, 460 m2 g-l) and LAL- (15.3 wt % A120,, 144 m2 g-l) silica-aluminal* were also measured as described above.

Fig. 2 shows the i.r. spectra at 423 K in vacuum of 3 wt % Re207 catalysts supported on HA-SA, LAH-SA, LAL-SA and on y-alumina carriers. It appears that all the silica-alumina-supported catalysts contain a band at 1545 cm-l and a peak at ca. 1636 cm-l, while the alumina-supported one does not. The areas of the ca. 1545 cm-l band decrease in the order of HA-SA, LAH-SA, LAL-SA and alumina ( ~ 0 ) .

The band positions of silica-alumina-supported Re,O, catalysts are given in table 2. Table 3 gives the integrated peak areas of i.r. bands at ca. 1540 and 1450 cm-l of some

silica-alumina-supported Re207 catalysts, representing their respective Bronsted and Lewis acidities and the integrated peak area of the i.r. band at ca. 3740cm-l, representing the isolated OH groups on the catalyst surfaces.

Fig. 3 shows the OH stretching region of a 1 wt % Re,O,/SA (HA) catalyst at various temperatures after pyridine adsorption in vacuum. It appears that the adsorption of pyridine decreases the band at 3741 cm-l and the evacuation at increasing higher temperatures gradually restores the OH band.

cm3 of water was added to a 1 wt % Re,O,/SA (HA) catalyst after pyridine adsorption and evacuation at 423 K. The intensity of the peak at 1544 was greatly

5 x

Dow

nloa

ded

by F

udan

Uni

vers

ity o

n 26

/04/

2013

09:

44:3

4.

Publ

ishe

d on

01

Janu

ary

1986

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/F

1986

8202

707

View Article Online

2712 Re@,-containing Metathesis Catalysts

I I I I

3801 3711 3621 3531 3431 v/crn-'

Fig. 3.1.r. spectra in the OH stretching region of a 1 wt % Re20,/Si02~A1203 (HA) catalyst. (a) Before pyridine adsorption in vacuum at 423 K, (b) in pyridine at 423 K, (c)-cf) after pyridine

adsorption in vacuum: at 423 K (c), 573 K (d) , 673 K (e) and 773 K (f).

Table 4. FTIR results of some mixed-oxide catalysts (based on a 3 wt % Re20,/A1203 catalyst) and some related samples

catalysta Bronsted acidity Lewis acidity

A/cm-l g-l A/cm-l g-l

Mo/Re 0 Mo/Re 2 Mo/Re 4 Mo/Re 6 Mo/Re 8 W/Re 6 V/Re 6 V/Re 2 6 wt % Re20,/A1203b 18 wt % W03/A1203 18 wt % Mo03/A1203

0 4.6

10.8 80.7

106.0 73.0 31.1 8.4

30.1 191.6 216.7

0

306.2 394.2 419.6 149.7 245.3 470.6 328.6 219.5 531.4 45 1.4 240.2 265.6

a The numbers after M/Re are the metal to rhenium atomic ratios of various mixed-oxide catalysts (Re : A1 = 1 : 154). Prepared by HC1 impregnation.

Dow

nloa

ded

by F

udan

Uni

vers

ity o

n 26

/04/

2013

09:

44:3

4.

Publ

ishe

d on

01

Janu

ary

1986

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/F

1986

8202

707

View Article Online

X . Xiaoding, J . C . Mol and C. Boelhouwer

0 2 4 6 8 10 Mo/Re

2713

Fig. 4. Bronsted acidity (a) and conversion/wt % Re,O, (v) for methyl oleate as a function of Mo: Re atomic ratio for a series of MOO,. Re,O,/Al,O, (Re : A1 = 1 : 154) catalysts.

increased, while the peak at 1453 cm-l decreased. The band at 3741 cm-l slightly changed, while a peak at 1648 cm-l appeared.

Table 4 shows the FTIR results of various mixed-oxide catalysts. The conditions used for the measurements and the method to calculate the acidity were the same as for table 3.

Fig. 4 shows the change of Bronsted acidity as a function of Mo: Re atomic ratios for a series of MOO,. Re,O,/Al,O, catalysts, based on 3 wt % Re,O,/Al,O,, together with their activity for the metathesis of methyl oleate using SnMe, as co-catalyst, expressed in conversion/wt % Re,O, at 90 min reaction time.ll

Fig. 5 shows Bronsted and Lewis acidity as a function of wt % Re,O, of Re,O,/Al,O, catalysts together with their activity for the metathesis of propene, expressed in initial reaction rate (mol g-l s-l), reported by Kapteijn.,,

Fig. 6 shows Lewis and Bronsted acidity of HA-SA-supported catalysts as a function of rhenium loading, together with their activities for the metathesis of propene, expressed in initial reaction rate (mol g-l s - ~ ) , ~ calculated according to the formula of Kapteijn.,,

Repeated i.r. experiments using the same samples deviated within less than ca. 10% of the reported values based on measurement of the integrated peak areas. Data for the peak areas calculated by the cut-and-weigh method or by the computer program are the same within f 2 % .

90 FAR 1

Dow

nloa

ded

by F

udan

Uni

vers

ity o

n 26

/04/

2013

09:

44:3

4.

Publ

ishe

d on

01

Janu

ary

1986

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/F

1986

8202

707

View Article Online

2714 Re20 -con tain ing Metathesis Catalysts

B

80

- I OD d I

4 0 2 0 0

0 5 10 15 wt% Re20,

Fig. 5. Bronsted (+) and Lewis (m) acidity and the initial reaction rate for propene metathesis (0) of various Re,O,/Al,O, catalysts.

Discussion On solid surfaces, several species can be observed after pyridine adsorption, viz. hydrogen-bonded pyridine (HPy), physisorbed pyridine, coordinated pyridine (LPy) and pyridinium ions (BPy).lP4 Since evacuation at temperatures higher than 423 K results in desorption of the physisorbed and hydrogen-bonded pyridine,l6? 1 7 9 2o the integrated areas obtained after evacuation at 423 K are representative for the acidity of these surfaces.

The spectra of Re,07/A120, show two peaks (or a peak with a shoulder) at ca. 1620 and two peaks at 1450 cm-l. The intensities of the peaks (or shoulders) with the lower wavenumbers decreased after evacuation. It appears that there are two types of Lewis acid sites on the Re207/A1203 catalysts under investigation. This phenomenon was also observed with pure aluminas.lq l6* l9 Moreover, the intensities of the peaks with higher wavenumbers seem to increase with the rhenium loading. This indicates that rhenium oxide is deposited preferentially on the weak Lewis sites and this deposition turns the weak Lewis sites into stronger ones. A similar phenomenon was also observed in the case of Mo03/A120, with the formation of protonic sites.20 The frequency of the ca. 1450 cm-1 band gives an indication of the strength of Lewis acidity.l69 l8 From table 1 it can be seen that the frequency shifts to a higher value for rhenium-alumina catalysts with an increasing rhenium loading, indicating that stronger Lewis acid sites are formed

Dow

nloa

ded

by F

udan

Uni

vers

ity o

n 26

/04/

2013

09:

44:3

4.

Publ

ishe

d on

01

Janu

ary

1986

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/F

1986

8202

707

View Article Online

2715

200

r(

I en

I H

1

T

100

0

X . Xiaoding, J . C . Mol and C. Boelhouwer

/ 0 2 4 6 8

wt% Re20,

Fig. 6. Bronsted (m) and Lewis (+) acidity and the initial reaction rate for propene metathesis (a) of various Re,O,/SiO, .A1,0, (HA) catalysts.

on Re,O,/AI,O,. That the ca. 1594 cm-l band was only observed in pyridine and not in the spectra after evacuation shows that pyridine at these sites is easy to desorb and we attribute this band to H-bonded or physisorbed pyridine.16 From fig. 1 and 5, it appears that Bronsted acidity was developed on Re,O,/AI,O, catalysts at Re,O, loadings higher than ca. 6 wt % . The activity for metathesis increases with an increasing Bronsted acidity. Although Lewis acidity also increases with rhenium loading, the pattern of the change of activity agrees better with that of Bronsted acidity (fig. 5). That a 6 wt % Re,O,/Al,O, catalyst prepared by HCI impregnation shows a weak peak at ca. 1550 cm-l indicates that HCI impregnation increases Bronsted acidity.,~

Silica-alumina (SA) is well known for its strong a~idity.l-~ Acid sites with H, -= - 8.2, even H , < - 12.0" have been reported on SA.l* 3 9 25 The acidity is influenced by the alumina content, and the method of preparation and measured data of the acidity depend also on the method of measurement.l SA with 25 wt % A1,0, has been reported to contain maximum acidity.l? 26 Fig. 2 shows that among the four 3 wt % Re,O, catalysts the one supported on HA-SA contains the highest Bronsted acidity; however, the LAH-SA-supported catalyst has a higher surface area. Alumina has no Bronsted acidity, in accordance with literature data.14 The frequency of the band at ca. 1450 cm-l of

* Where H , is the Hammet index.

90-2

Dow

nloa

ded

by F

udan

Uni

vers

ity o

n 26

/04/

2013

09:

44:3

4.

Publ

ishe

d on

01

Janu

ary

1986

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/F

1986

8202

707

View Article Online

2716 Re,O,-containing Metathesis Catalysts

(a) n . n

Re20,/A1203 Re207/Si02 A1203

Fig. 7. Distribution of Bronsted acid sites (a) and rhenium species (0) on the surface of Re20,/A120, and of Re20,/Si02.A1,0, catalysts with (a) low, (b) medium and (c) high Re,O,

loadings.

silica-alumina (1455 cm-l) is higher than the one of alumina (145 1 cm-l), indicating a higher strength of the Lewis acidity of the silica-alumina-supported catalysts. Although the Lewis acidity of the alumina-supported catalyst is higher than that of all the silica-alumina-supported ones, its activity is the lowest in the metathesis of methyl oleatelo and of p r ~ p e n e . ~ The activity for metathesis decreases in the order of decreasing Bronsted a ~ i d i t y . ~ ~ lo It thus appears that there is a distinct correlation between Bronsted acidity and metathesis activity, but not between Lewis acidity and the activity.

A similar correlation has been observed with the activity for metathesis of the series of Re,O,/SA (HA) catalysts with different rhenium loadings (fig. 6).

In the spectra of silica-alumina and Re,O,/SiO, . Al,O, catalysts, a sharp band at ca. 3745 cm-l and a broad one at a lower frequency were observed in the OH region (fig. 3). The broad peak is attributed to bridged OH groups27 and the 3745 cm-1 band to isolated surface OH g r o ~ p s . ~ ~ - ~ ~ Although some authors suggest that there is only one type of isolated OH group on silica-alumina, due to its similarity to corresponding Si-OH groups on silica, others think that there are two types of isolated surface OH groups on SA with different acidities.l-*? 26-30 The integrated areas of the 3745 cm-l band of the series of HA-SA-supported Re,O, catalysts after pyridine adsorption and evacuation decrease with increasing rhenium loading. That this band does not totally disappear, even after a long time in pyridine, indicates that not all the OH groups on Re,O,/SiO, * Al,O, (HA) react with pyridine, which in turn indicates that the OH groups corresponding to the band at ca. 3745 cm-l have different acidities. It appears that the deposition of rhenium oxide on silica-alumina initiates the acidity of distinct surface OH groups, hence decreasing the amount of non-acidic OH groups. Bronsted acidity increases with rhenium loading of the catalysts, Lewis acidity passes through a maximum at ca. 1 wt % Re,O,, while the reaction rate of the catalysts increases with an increasing rhenium oxide loading (fig. 6).9910 This again indicates that Lewis acidity does not correlate with metathesis activity, while Bronsted acidity does.

It is known that the addition of certain metal oxides, e.g. MOO,, WO, or V,O,, to Re,07/A1,0, catalysts increases the metathesis a ~ t i v i t y . ~ ~ 11-13 The deposition of these oxides on alumina also generates Bronsted acidity.l79 2o From fig. 4 and table 4 it is obvious that the addition of these metal oxides also increases the Bronsted acidity of catalysts with low rhenium loading and their activity indeed increases with increasing

Dow

nloa

ded

by F

udan

Uni

vers

ity o

n 26

/04/

2013

09:

44:3

4.

Publ

ishe

d on

01

Janu

ary

1986

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/F

1986

8202

707

View Article Online

X . Xiaoding, J . C . Mol and C. Boelhouwer 2717

Bronsted acidity. This holds for Mo:Re ratios up to ca. 6; higher amounts of Mo apparently are an excess, covering active rhenium sites. BET data9 show that the catalyst with a Mo: Re ratio of 8 has a smaller surface area (192 m2 g-l) than the one with the ratio of 6 (220 m2 g-l), and this appears to reinforce the view mentioned above.

Alumina-boria, a well known solid a ~ i d ~ l - ~ * with H, < - 8.2l~ 25 (its acidity being mainly of the Bronsted type1v34), is also a good carrier for rhenium oxide metathesis catalyst^.^^ 35 This again proves the vital role of Bronsted acidity in metathesis. Furthermore, the results of hexamethyldisilazane (HMDS) poisoning experiment^,^^ 3 5 7 36

in which poisoning of Bronsted acid sites by HMDS leads to a decrease and eventually to a total termination of metathesis activity of all the Re20,-containing catalysts studied, is a convincing proof of this correlation.

Based on the acidity of the catalysts observed, a model is proposed (fig. 7 ) to illustrate the distribution of Bronsted acid sites and rhenium species on the surfaces of rhenium oxide-alumina and rhenium oxide-SA catalysts. The addition of a suitable third metal oxide to low rhenium loading Re20,/A1203 catalysts causes a transition from the situation in the left to the right in fig. 7(a) . With this simple picture in mind and with the model presented in ref. (36) and (37), in which the formation of the metallacarbene intermediates includes the function of a reduced rhenium species and a neighbouring Bronsted acid site, we can explain the activity of the Re20,-containing catalysts studied so far. Nevertheless, this model is not necessarily applicable to other catalyst systems. For example, WO,/SiO, catalysts show completely different b e h a v i o ~ r , ~ ~ indicating that other types of active sites might be operating.

Conclusions Bronsted and Lewis acidity increase with increasing rhenium loading of Re20,/A1203 catalysts. The former starts to appear at ca. 6 wt % Re,O,. HCl impregnation leads to the formation of Bronsted acidity of catalysts with lower rhenium loading.

The addition of a suitable third metal oxide to Re20,/A120, catalysts with low rhenium loading leads to the formation and increase of Bronsted acidity.

Addition of rhenium oxide to silica-alumina increases its Bronsted acidity, while its Lewis acidity passes through a maximum.

The metathesis activity of all the rhenium oxide-containing catalysts studied shows a distinct correlation with their Bronsted acidity. No clear correlation with Lewis acidity of these catalysts could be established. Very active rhenium oxide-containing metathesis catalysts are characterized by a pronounced Bronsted acidity. The key to the increase of the activity of low rhenium loading catalysts, therefore, lies in the increase of their Bronsted acidity by proper modifications.

Xu Xiaoding (on leave of absence from the Department of Chemistry, Fudan University, Shanghai) is the recipient of a fellowship on the basis of an exchange programme between the Netherlands and the People’s Republic of China. We thank G. C. N. van den Aardweg for his technical help in building the equipment for the measurements.

References 1 K. Tanabe, Solid Acids and Bases (Kodansha, Tokyo; and Academic Press, New York, 1970). 2 K. Tanabe, Catalysis, Science and Technology, ed. J . R. Anderson and M. Boudart (Springer-Verlag,

3 H. A. Benesi and B. H. C. Winquist, Adv. Catal., 1978, 27, 97. 4 H. Knozinger, Adv. Catal., 1976, 25, 184. 5 A. Ellison, A. K. Coverdale and P. F. Dearing, Appl. Catal., 1983, 8, 109. 6 A. Ellison, A. K. Coverdale and P. F. Dearing, J. Mol. Catal., 1985, 28, 1 . 7 R. Nakamura and E. Echigoya, Recl. Trav. Chim., Pays-Bas, 1977, 96, M31.

Berlin, 1981), vol. 2, pp. 232-271.

Dow

nloa

ded

by F

udan

Uni

vers

ity o

n 26

/04/

2013

09:

44:3

4.

Publ

ishe

d on

01

Janu

ary

1986

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/F

1986

8202

707

View Article Online

2718 Re@, -containing Me tat hesis Catalysts

8 N. Calderon, The Chemistry of Double-bonded Functional Groups, ed. S . Patai (John Wiley, London,

9 Xu Xiaoding, Ph.D. Thesis (University of Amsterdam, 1985). 1977), part 2, pp. 913-963.

10 Xu Xiaoding and J. C. Mol, J. Chem. Soc., Chem. Commun., 1985,631. 11 Xu Xiaoding, P. Imhoff, G. C. N. van den Aardweg and J. C. Mol., J. Chem. SOC., Chem. Commun.,

12 E. I . Bogolepova. R. A. Fridman and A. N. Bashkiev, Izv. Akad. Nauk. SSSR, Ser. Khim., 1979, 7 ,

13 R. Nakamura and E. Echigoya, Chem. Lett., 1977, 1227. 14 A. A. Olsthoorn and C. Boelhouwer, J. Catal., 1976, 44, 207. 15 P. F. Dearing, Ph.D. Thesis (Humberside College of Higher Education, Hull, 1984). 16 E. P. Parry, J. Catal., 1963, 2, 371. 17 F. E. Kiviat and L. Petrakis, J. Phys. Chem., 1973, 77, 1232. 18 M. R. Basila, T. R. Kantner and K. H. Rhee, J. Phys. Chem., 1964,68, 3197. 19 M. R. Basila and T. R. Kantner, J. Phys. Chew., 1966, 70, 1681. 20 Koh-Ichi Segawa and W. K. Hall, J. Catul., 1982, 76, 133. 21 T. R. Hughes, H. M. White and R. J. White, J. Catal., 1969, 13, 58. 22 E. A. Pankshtis, R. I. Soltanov and E. N. Yurchenko, React. Kinet. Catal. Lett., 1982, 19, 105. 23 G. Visser-Luirink, E. R. A. Matulewicz, J. Hart and J. C. Mol., J. Phys. Chem., 1983, 87, 1470. 24 F. Kapteijn, Ph.D. Thesis (University of Amsterdam, 1980). 25 H. A. Benesi, J. Am. Chem. SOC., 1956,78, 5490. 26 P. Andreu, G. Martin and H. Noller, J. Catal., 1971, 21, 255. 27 R. E. Sempels and P. G. Rouxhet, J. Colloid Interface Sci., 1976, 55, 263. 28 M. Takahashi, Y. Iwasawa and S. Ogasawara, J. Catal., 1976,45, 15. 29 H. Boehm and H. Knozinger, Catalysis, Science and Technology, ed. J. R. Anderson and M. Boudart

(Springer-Verlag, Berlin, 1983), vol. 4, pp. 4&195. 30 J. M. Guil, J. E. Herrero and A. R. Paniego, J. Colloid Interface Sci., 1984, 102, 1 1 1 . 31 A. Ozaki and K. Kimura, J. Catal., 1964, 3, 395. 32 M. Sato, T. Aonuma and T. Shiba, Proc. 3rd Int. Congr. Catul., ed. W. ha. H. Sachtler, G. C. A. Schuit

and P. Zwietering (North-Holland, Amsterdam, 1964), vol. 1, pp. 396407. 33 V. A. Dzisko, Proc. 3rd Int. Congr. Catal., ed. W. M. H. Sachtler, G. C. A. Schuit and P. Zwietering

(North-Holland, Amsterdam, 1964), vol. 1, pp. 422432. 34 Y. Izumi and T. Shiba, Bull. Chem. Soc. Jpn, 1964, 37, 1797. 35 Xu Xiaoding, C. Boelhouwer, J. I. Benecke, D. Vonk and J. C. Mol, J. Chem. Soc., Faraday Trans. 1,

36 Xu Xiaoding, C. Boelhouwer, D. Vonk, J. I. Benecke and J. C. Mol, J. Mol. Catal., in press. 37 D. T. Laverty, J. J. Rooney and A. Stewart, J. Catal., 1976, 45, 110. 38 A. J. Van Roosmalen and J. C. Mol, J. Cutal., 1982, 78, 17.

1985,273.

1623.

1986,82, 1945.

Paper 5 / 18 13 ; Received 18th October, 1985

Dow

nloa

ded

by F

udan

Uni

vers

ity o

n 26

/04/

2013

09:

44:3

4.

Publ

ishe

d on

01

Janu

ary

1986

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/F

1986

8202

707

View Article Online