Isomerization and metathesis of 2-methyl-1-alkenes over Re2O7—Al2O3 catalysts

Journal of Molecular Catalysis, 76 (1992) 2499261

249

M2966

Isomerization and metathesis of 2-methyl-l-alkenes Re,O, -Al,O, catalysts

over

Tadashi Kawai, Takuji Okada and Tomomichi Ishikawa Department of Industrial Chemistry, Faculty of Technology, Tokyo Metropolitan University, Tokyo 192-03 (Japan)

Abstract The double-bond shift isomerization and the metathesis of 2-methyl-1-alkenes were investigated over Re,O,-Al,O, catalysts in the liquid phase at atmospheric pressure using a continuous-flow system. When the amount of rhenia was increased, both isomerization and metathesis activity increased in a similar manner. Over a wide range of reaction temperatures ( -35 “C to 60 “C), activation temperatures (50 “C to 500 “C) and reduction temperatures (100 “C to 500 “C), isomerization occurred very readily and seemed to reach equilibrium. In contrast, the metathesis activity was greatly affected by the reaction conditions: it was very low at reaction temperatures lower than 0 “C and at activation temperatures lower than 150 “C, and was destroyed at reduction temperatures higher than 200 “C. This indicates that the active sites of double-bond isomerization are not necessarily directly associated with those for metathesis. As skeletal isomerization and polymerization did not occur, the acid strength is probably fairly weak. The active sites for the double-bond isomerization are thought to be both Bronsted and Lewis acid sites, but Bronsted acidity is more important than Lewis acidity for isomerization.

Introduction The Re,O,-Al,O, catalyst is well known for its high activity and selectivity in the metathesis of a variety of olefins. We have shown, for example, that a CsNO,-Re,O,-A&O, catalyst is very active for the metathesis of various kinds of alkenes, such as n-alkenes [ 1,2], cc,w-dienes 13, 41 and 1,4-alkadienes 14-61, with a metathesis selectivity of >94% without double-bond shift isomerization or polymerization. The catalytic metathesis of methyl-1-alkenes showed homometathesis with high selectivity, except for 2-methyl-1-hexene (2-Me-l-C;) [ 71. Alkylsubstituted vinylenes have exhibited anomalous reaction behavior, without exception, using a CsNO,-Re,O,-Al,O, catalyst f8]. Here, the catalyst promoted migration of the double bond to the resulting isomer and subsequent co-metathesis [ES].However, neither homometathesis nor rearrangement of the carbon skeleton occurred. The metathesis of methyl-substituted vinylenes has received relatively little attention. Doyle reported that 3-Me-2-C; is homometathesized over (nBu)*N[Mo(CO)~Cl] ---MeAlC!l, 191, while a West German patent described

0304-5102/92/$5.00

0 1992 ~ Elsevier Sequoia. All rights reserved

250

the homometathesis of 2-Me-l-C: over WO( OPh), -AlCl,-E&Al [lo]. An intermediate in the synthesis of musk perfume, 3,3-dimethyl-I-butene (3,3DMe-1-C:) is obtained by co-metathesis of 2,4,4-trimethyl-2-pentene (2,4,4TMe-2-C;) with ethylene over a WO,-SiO,-MgO catalyst [ll]. 2,5DMe-2,Chexadiene is homometathesized over Re,O, -Al,O, -&Me, with high selectivity [ 121. However, there are no reports of the isomerization of reactants in the literature. Studies on the isomerization of double bonds using metathesis catalysts are limited. Most studies are for l-C& [13-161 and 2-C:, [16-B] using Re,O,-A&O, [14, 151, Re,O,-SiO, [14], MOO,-Al,O, [15, 17, 181, WO,not Al,O, [ 151 and WO,-SiO, [ 13, 161 in the vapor phase. Although employing metathesis catalysts, the vapor-phase isomerization of 2-Me-lCi with Cu-SiO, [19] and 2-Me-l-C;, 3-Me-l-C; and 1-C; with various oxide catalysts [20, 211 have been reported. As mentioned above, the study of the isomerization of alkenes with alkyl-substituted vinyl groups using metathesis catalysts has been reported only in our papers [7, 81. Experiments in this laboratory have shown that alkenes with alkyl-substituted vinyl groups, such as 2-Me-lC&, 2-Me-l-C;, 2-Me-l-C;, 2-Me-2-C;, 2-Me-2-C;, 2-Et-l-C; and 3-Me-2-C;) undergo rapid double-bond isomerization to the more highly substituted olefins, and subsequent co-metathesis with the starting material using a CsNO, -Re,O, -Al,O, catalyst [ 7, 81. The double-bond isomerization of isoalkenes was 5-10 times faster than that of other alkyl-substituted vinylenes. The aim of this work is to clarify the active sites of double-bond migration on Re,O,-Al,O, catalysts for olefin metathesis. For this purpose, the effects of reaction temperature, activation temperature, amount of rhenia on Y-Al,O, and reduction temperature were investigated in detail, mainly using 2-Me-l-C& as the starting material.

Experimental Catalyst Four kinds of Re,O, -Al,O, catalysts (40-60 mesh) were prepared by impregnation, as described in previous papers [ 1, 31. The amount of Re,O, was determined, after calcination at 550 “C, to be 4.6, 9.5, 13.4 and 17.4 wt.%, respectively, by atomic absorption spectroscopy. Re,O,-Al,O, catalysts modified with alkali hydroxide (NaOH, KOH, RbOH, CsOH) or CsNO, were also prepared by impregnating 17.4 wt.% Re,O,-Al,O, catalyst with the relevant aqueous solution. After drying at 110 “C for 24 h, the modified catalysts were calcined at 500 “C for 2 h under oxygen flow. The amount of alkali metal was calculated to be 7.52 x lop5 mol (g cat) -‘, approximating to 1 wt.% as metal ion, for each. SiO,-Al,O,, obtained from Catalysts and Chemicals Industries (IS-28, Al,O, 29.07%, Na,O O.Ol%, SO, 0.28%, 420 m2 g-l), was milled and sieved, and 40660 mesh was used.

251

Reagents Chlorobenzene, used as the solvent, and toluene and o-xylene, used as internal standards, were purified by rectification over P,O,. 2-Me-1-C; was obtained from Aldrich, 2-Me-l-C& was obtained from Wako Pure Chemical Industries and 2-Me-l-C; and 1-C; were obtained from Tokyo Kasei Kogyo. All reactants were dried over 5A molecular sieves, but used without further purification, since GC analysis confirmed a purity of >99% for each. Proced we The reactions were carried out in the liquid phase at atmospheric pressure in a single-path, continuous-flow tubular Pyrex glass reactor with a fixed catalyst bed. The detailed experimental procedure and the analysis of the reaction products were practically the same as previously described [ 81. The data obtained were analyzed using the following definitions: 2-/l- = I/R = [2-Me-2-Cb]/[2-Me-l-C;i] where 2-/l- indicates the isomer ratio, taken to be the amount of isomer remaining (I) relative to the amount of starting material remaining (R). This equation illustrates the case of the reaction of 2-Me-l-C;. % conversion where

A is the number

% selectivity where

= [ 1 - (I + R)/A]

x 100

of moles of reactant

used.

= 2 x N, x lOO/(A - I - R)

Ni is the number

of moles of a reaction

ISO( %) = (I + C + H + 0)

product

(i) produced.

x 100/A

where IS0 is the degree of isomerization. C is the number of moles of co-metathesis products formed, H the number of moles of homometathesis products of the isomer formed, and D the number of moles of byproducts formed. meta.( O/o) = (C;i + CL + 2 x Cg + 2 x C;,)

x 100/A

where ‘meta.’ means the degree of metathesis reaction. tion illustrates the case of the reaction of 2-Me-l-C!:. The mass balance was >97% for each.

Results

The above

equa-

and discussion

Effect of the amount of rhenia on alumina The effect of the amount of rhenia on alumina was investigated. First, four kinds of catalysts with various amounts of rhenia were applied to 1-C; as a representative n-alkene at reaction conditions of 40 “C and

252

W/F = 4.6 (g cat) h mol-‘. When the amount of rhenia was increased, the metathesis reaction increased by 14.3% for 4.6 wt.% Rez07, 36.3% for 9.5 wt.% Re,O, and up to 39.4% for 17.4 wt.% Re,O,. The reaction product was 6-dodecene by homometathesis, with > 94% selectivity irrespective of the amount of rhenia. The degree of isomerization was less than 1.8%, showing that the double-bond isomerization of 1-C; to 2-C; is very limited. Figures 1 and 2 show the results obtained for 2-Me-l-CL. The changes in the activity for the metathesis of 2-Me-l-C: with various amounts of rhenia show similar tendencies to those for 1-C; and to those reported for various kinds of olefins [22-261. Scheme 1 summarizes the different

0

20

10 Re20,

CONTENT

(~1%)

Fig. 1. Effect of Re,O, content. W/F = 6.5 (g cat) 550 “C, 1.5 h, N,, reactant 2-methyl-l-hexene.

h mol-I,

react.

temp. 40 “C, activation

hmol-‘,

react.

temp. 40 “C, activation

100

so_ 3

::

60

-

5 F: :: 40i m 20.

A : C,’ O:C, n : Cl,’ cl:C14’

0

-I

10

0 Re20,

CONTENT

20 (wt%)

(g cat) Fig 2. Effect of Re,O, content. W/F=63 550 ‘C, 1.5 h, N,, reactant 2-methyl-1-hexene.

253

C

c-L=c-c-c-c-c +

c=c-c-c-c

:: (Cq’ )

CC,‘)

&

::

(C(j’ )

CC,‘)

c-Yc=c-c-c C Scheme

1. Reaction

routes

of reaction

products

in metathesis

of 2-methyl-1-hexene.

reaction pathways leading to the metathesis products in the reaction of 2-Me-l-C&. The major reaction products were 5Me-4-C; (C;,) and 2-Me-lC; (Ci) by co-metathesis, but the other co-metathesis products, 1-C; (C;) and 2,3-DMe-2-C; (Cg), were not detected. GC-MS analysis showed that there were small amounts of C,,H,, (C;,), C&H,, (C&J and C,,H,, (C;,) at higher rhenia contents, but their structures and the reaction pathways have not yet been confirmed. The homometathesis product, 5,6-DMe-5-C;, (C;,), the rearrangement of the carbon skeleton and the isomerization to 2-Me-3-C; were not detected under the reaction conditions. The amount of isomerization of 2-Me-l-C; to 2-Me-2-C; increased when the amount of rhenia was increased to 15 wt.%, and reached an almost constant value above 15 wt.%, as shown in Fig. 1. The shape of the curve for the degree of isomerization was similar to that for the metathesis activity, and this seemed to suggest that the active sites were the same. However, this presumed relationship between metathesis and isomerization activity was found to be misleading, as will be described below. Most of the 2-Me-2-C: produced by the isomerization of 2-Me-l-C; was consumed by co-metathesis below 10 wt.% Re,O, loading, but the 2-/l- ratios were ~10 above 15 wt.% Re,O, loading, showing that unreacted 2-Me-2-C; remained largely in the reaction mixtures. This induced the homometathesis of 2-Me-2-C&, producing 2,3-DMe-2-C& (CA) and 4-C; (Cg). The relatively small extent of homometathesis of 2-Me-2-C& in spite of the abundance of 2-Me-2-C; at higher rhenia levels, is thought to reflect the steric hindrance encountered when the metallacyclobutane intermediates are formed. This shows that isomerization occurs much faster than cometathesis at higher loadings of rhenia.

254

Similar results were obtained for 2Me-1-C; and 2-Me-l-CL, although the absolute product ratios were somewhat different. Reaction temperature The effect of the reaction temperature was investigated for the 17.4% Re,07-Al,O, catalyst. As shown in Figs. 3 and 4, the conversion and metathesis activities were dependent on the reaction temperature at low temperatures, but metathesis activities became independent of the reaction temperature above 40 “C.

4

40

z v

20

ii s+ 20 15 10

0 -40

-20

0

REACTION

20

40

TRMPERATURE

60

L r; d, is rj

80

("C)

Fig. 3. Effect of reaction temperature. Catal. 17.4wt.% Re,O,-Al,O,, h mob.‘, activation 550 “C, 1.5 h, N,, reactant Z-methyl-1-hexene.

W/F=6.5

(g cat)

60

-40

-20

0

REACTION

20

40

TEMPERATURE

60

80

("C)

Fig. 4. Effect of reaction temperature. Catal. 17.4 wt.% Re,O,-Al,O:,, h mol-‘, activation 550 “C, 1.5 h, N,, reactant 2-methyl-l-hexene.

W/F = 6.5 (g cat)

255

The 2-/l- ratio decreased with increasing reaction temperature. The degree of isomerization was independent of the reaction temperature, suggesting that the isomerization was very fast and reached equilibrium even at temperatures as low as -40 “C. The activity changes in metathesis and isomerization differed greatly. This suggests that the active sites for double-bond isomerization are not necessarily directly associated with those for metathesis. C;, was the main reaction product at lower temperatures, but its selectivity decreased with increasing reaction temperature. On the other hand, the selectivity toward co-metathesis increased up to a reaction temperature of 20 “C and then decreased above 20 “C. The amounts of C& and CL, homometathesis products of the isomer, and Cl,, increased gradually with increasing reaction temperature.

Reduction

temperature

After the normal activation treatment under nitrogen flow at 500 “C for 1.5 h, the catalyst was cooled to reduction temperatures under nitrogen flow and subsequently regenerated under hydrogen flow for 1 h. After reduction, the catalyst was cooled to the reaction temperature under nitrogen flow. As shown in Figs. 5 and 6, the metathesis activity and conversion decreased drastically with increasing reduction temperature and the active sites for metathesis were completely destroyed above 200 “C. In contrast, the isomerization activity and 2-/l- ratios behaved differently than the metathesis activity, and were independent of the reduction temperature. At higher reduction temperatures, the main reaction product was C;,.

100

0

. 0

*. 100

200

REDUCTION

300

400

TEMPERATURE

500

600

(“C)

Fig. 5. Effect of reduction temperature. Catal. 17.4 wt.% h mob’, react. temp. 40 “C, reactant 2-methyl-1-hexene.

Re,O,-Al,O,,

W/F = 6.5 (g cat)

256 100

0

loo

200

REDUCTION

300

400

TEMPERATURE

500

600

("C)

Fig. 6. Effect of reduction temperature. Catal. 17.4 wt.% h molk’, react. temp. 40 “C, reactant 2-methyl-1-hexene.

Re,O,-Al,O,,

W/F = 6.5 (g cat)

It has been reported that Re(VI1) exists below 150 “C, Re(V1) between 50” to 350 “C, Re(IV) above 100 “C and Re(0) above a 450 “C reduction temperature [14]. The active valence of Re for metathesis has been reported to be Re(VII), and this agrees with the results obtained in this work. According to a report by Edreva-Kardjieva and Andreev [14], the active species in the isomerization of 1-C: to 2-C: is Re(IV). However, the degree of isomerization was independent of the valence state of Re and isomerization occurred even below the reduction temperature of 100 “C, and under this condition Re(IV) does not exist. The differences in the behaviors toward metathesis and isomerization suggest that the active sites differ from each other. 2-Me-l-C& and 2-Me-l-C& showed similar reaction behaviors to 2-Me-lCh under the same reaction conditions, although the absolute product ratios were somewhat different. Activation temperature It has been suggested that double-bond isomerization occurs on acid sites. The adsorbed water on solid catalysts influences the nature of the catalysts, and the acid properties in particular. The 17.4 wt.% Re,O,Al,O, catalyst was activated at various temperatures under nitrogen flow for 1.5 h in order to control the concentration of surface hydroxyls. The results are shown in Figs. 7 and 8. The metathesis activity increased greatly up to an activation temperature of 200 “C, and reached an almost constant value above 300 “C. In contrast, the degree of isomerization and the 2-/lratio were almost independent of the activation temperature, and seemed to reach equilibrium values. This shows that the adsorbed water poisons the active sites for metathesis, but does not poison those for

257

60

_

Cl

100

200

ACTIVATION

300

400

500

TEMPERATURE

600

(“C)

Fig. 7. Effect of activation temperature. Catal. 17.4 wt.% Re,O,--Al,O,, W/F = 6.5 (g cat) h mol-‘, react. temp. 40 “C, activation 1.5 h, N,, reactant 2-methyI-l~~exene.

100

0

100 ACTIVATION

200

300

400

TEMPERATURE

500

600

(“C)

Fig. 8. Effect of activation temperature. Catal. 17.4 wt.% Re,O,-Al,O,, W/F = 6.5 (g cat) h mol-‘, react. temp. 40 “C, activation 1.5 h, N,, reactant 2-methyl-l-hexene.

isomerization. The different behaviors in isomerization and metathesis suggest that their active sites differ from each other. The product distributions changed greatly between 100 “C and 250 “C. At temperatures lower than 150 “C, the main product was C;, and metathesis products were few. On the other hand, the product distributions were completely reversed at temperatures higher than 300 “C. The adsorbed water is helpful in the formation of Ci, and harmful to metathesis. The main active species in C;, formation is thought to have Brernsted acidity.

258

2-Me-1-C; showed quite similar reaction behavior to 2-Me-l-C; under the same reaction conditions, although the absolute values were somewhat different. The effect of activation temperature on the metathesis of 1-C; was also investigated, in order to make a tiomparison with the behavior of 2-Me-l-C;. The metathesis activity increased greatly, showing a tendency similar to that of 2-Me-l-C;, and homometathesis occurred predominantly at temperatures higher than 300 “C, The isomerization of 1-C; to 2-C; and the subsequent co-metathesis to produce 5-C;,, l-C!&and 2-C; occurred mostly at activation temperatures lower than 200 “C, although the conversion was less than 14%. The degree of isomerization and the 2-/l- ratio were less than 4% and 1, respectively, and these values were extremely low compared with those for 2-Me-l-C;. Active sites of iso~eri~ution The values of the degree of isomerization and the 2-/l- ratio for the 17.4 wt.% Re,O,-Al,O, catalyst were ca. 80% and ca. 13, respectively, over a wide range of reaction temperatures (Fig. 3), reduction temperatures (Fig. 5) and activation temperatures (Fig. 7). These data show that the isomerization occurs very easily and reaches equilibrium. It has been reported that Re,O,---Al,O, catalysts have both Lewis and Bronsted acidity [26, 271. The isomerization increased when the amount of rhenia was increased, as shown in Fig. 1, and this must be associated with the increase in both Bronsted and Lewis acidity with increasing rhenia loading of Re,O,-Al,O,, as reported by Xu Xiaoding et al’. [26]. Reactions over modified Re,O,-Al,O, catalysts with alkaline hydroxides or CsNO, were carried out, and the results are shown in Table 1. Compared with the 17.4 wt.% Re,O,-Al,O, catalyst, over the modified catalysts the 2-/l- ratio decreased greatly, from 13 to about 1, and the degree of isomerization also

TABLE 1 Effect of addition of alkaline salt to 17.4 wt.% Re,O,-Al,O, Alkali

Conv.

2-/l-

WJ) none NaOH KOH RbOH CsOH &NO,

51.8 45.3 38.6 38.5 43.3 49.5

13.4 1.7 1.2 0.8 1.8 1.0

IS0

Meta.

Selectivity of product (%)

(%f

(%)

C6

e*

c,

c;,

c;,

c;,

79.0 54.5 48.3 51.0 56.1 52.7

47.0 44.0 36.6 34.3 37.1 48.5

7.0 2.1 1.7 1.1 2.2 1.8

11.9 2.9 2.1 1.0 3.2 2.1

1.4 1.9 0.8 0.5 1.4 1.6

62.3 91.5 89.8 86.0 87.2 92.4

9.1 1.2 2.9 4.3 4.5 0.8

12.4 1.6 3.6 3.1 3.2 1.6

Conditions: activation at 500 “C for 1.5 h, N,; react. temp. 40 “C; W/F = 6.5 (g cat) h mol-‘; substrate = 2-Me-l-C&.

259

decreased, from 75% to 50%. There were no large differences between the results obtained using the various metal ions as modifiers, but CsNO, appeared to be the most effective salt for depressing the isomerization activity and for improving the metathesis selectivity. As the degree of isomerization and the 2-/l- ratio were lowered by the addition of a small amount of alkaline salt, it is clear that alkali poisoned active sites for isomerization, probably Brsnsted acid sites. However, isomerization occurs at a fairly high percentage (about 50%) for alkali-treated catalysts. This must be the result of the ‘isomerization activity caused by rhenia as a Lewis acid. These findings suggest that Brransted acid sites have a greater isomerization activity than Lewis acid sites. In order to clarify this point, reactions were run using ~-Al,0~, SiO,-Al,O, and 10 wt.% Re,O,-SiO,-Al,O, as catalysts. The results are summarized in Table 2 together with other data. y-Al,O,, which shows mostly Lewis acidity and has a few weak Brernsted acid sites, did not give any products, but had isomerization activity. SiO,-A&O,, which has strong Bronsted acid sites, showed very high activity toward isomerization and C;, formation even under the milder reaction conditions, such as low temperature ( -28 “C) and low contact time (0.6 (g cat) h molll). With the addition of 10 wt.% Re,O, to the SiO,-Al,03, the isomerization activity was depressed and metathesis activity appeared, but the catalyst had much higher isomerization activity than the 10 wt.% Re,O,-A&O, catalyst. This may be caused by Bronsted acidity on the surface of SiO,-Al,O, not covered with rhenia [25]. Double-bond isomerization and Cl, formation occurred over the 17.4% Re,O,-Al,O, catalyst reduced even at temperatures higher than 100 “C, although the activities were lower, as shown in Figs. 5 and 6.

TABLE Reaction

2 behavior

Catalyst

over various

catalysts

Temp

W/F

Conv.

IS0

(“C)

((g cat) h mol-‘)

(%)

(%)

40 -28.0 25.0

6.5 0.6 0.6

5.4 73.5 84.7

20.5 67.3 51.5

Re,O,-Al,O, (9.5% Re,O,)

40.0

6.5

29.2

Re,O,-AlzO, (17.4% Re,O,)

40.0

6.5

MOHPRe,O,-Al,O,” (17.4% Re,O,)

40.0

6.5

Y-&O, SiO, -Al,O, Re,O, GGiO, PA1,O, (10% Re,O,)

Note: reactant = 2-Me-l-CA. “M = Na, K, Rb, Cs.

2-/l-

Meta.

Select.

(%)

(%)

0.3 22.4 11.7

0 0 16.6

0 x 100 60.2

47.5

0.8

28.5

0

52.4

79.0

13.5

47.0

10.2

38-45

48-56

0.8-1.7

34-44

of C;,

1.6-3.6

260

Taking into account that only Lewis acidity was found for a Re,O,-Al,O, catalyst reduced at 270 “C! for 1 h in flowing H, [27], double-bond isomerization and C;, formation occurred on Lewis acid sites. Migration of the double bond undoubtedly occurs on both Bronsted and Lewis acids. The isomerization activity of Brsnsted acid sites is greater than that of Lewis acid sites. As the skeletal isomerization and polymerization did not occur over Re,O,-Al,O, catalysts, the acid strength must be fairly weak. Various mechanisms of double-bond isomerization have been reported in the literature [16, 19-21, 281. Lewis acid sites, Bronsted acid sites, Lewis aciddLewis base site pairs and Bronsted acid-Lewis base site pairs are proposed to be active sites of the isomerization. While the above mechanisms are compatible with our data, further studies are needed to reveal the detailed mechanisms and the nature of the active sites of double-bond isomerization over Re,O,-Al,O, catalysts.

Conclusions Reactions of 2-methyl-l-alkenes were carried out over selective Re,O,-Al,O, metathesis catalysts. However, homometathesis did not occur at all, while the double-bond isomerization to 2-methyl-2-alkene occurred readily. The selectivities toward the reaction products (2-methyl-2-alkenes, co-metathesis products between the starting material and the resulting varied greatly with the rhenia isomer, and unknown Ci, compounds) loading and the activation and reduction conditions. Metathesis activity was lower over catalysts reduced with hydrogen above 100 “C or treated at activation temperatures lower than 200 “C. Double-bond shift isomerization of 2-Me-1-alkene to 2-Me-2-alkene was very facile. The active sites of isomerization differed from those of metathesis. Both Lewis and Bronsted acids are active sites for the isomerization and C!;, formation, but Bronsted acidity is more important than Lewis acidity for these reactions. C;, is produced selectively over catalysts having Bronsted acidity and over Re,O,-Al,O, catalyst reduced at temperatures higher than 200 “C.

References 1 2 3 4 5

T. T. T. T. T.

Kawai, Kawai, Kawai, Kawai, Kawai,

Y. H. Y. Y. N.

Yamazaki and A. Tokumura, Sekiyu Gakkai Shi, 26 (1983) 332. Goto, Y. Yamazaki and T. Ishikawa, J. Mol. Cutal., 46 (1988) 157. Yamazaki, T. Taoka and K. Kobayashi, J. Cutal., 89 (1984) 452. Yamazaki and M. Nishikawa, Sekiyu Guklzui Shi, 27 (1984) 378. Maruoka and T. Ishikawa. J. Mol. Cutal., 39 (1987) 369.

261 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

T. Kawai, H. Goto and Y. Yamazaki, Se&u Gakkai Shi, 29 (1986) 212. T. Kawai, N. Maruoka, M. Goke and T. Ishikawa, J. Mol. Cutal., 49 (1989) 261. T. Kawai, N. Maruoka and T. Ishikawa, J. Mol. Cutal., 60 (1990) 209. G. Doyle, J. Catal., 30 (1973) 118. Get-. Offen 2024835 (1970) to Entreprise de Recherches et d’Activit6.s Petrol&es (ERAP); Chem. A&r., 74 (1971) 44118. R. L. Banks, D. S. Banasiak, P. S. Hudson and J. R. Norell, J. Mol. Cutal., 15 (1982) 21. E. F. G. Woerlee, R. H. A. Bosma, J. M. M. Van Eiji and J. C. Mol, Appl. Cutal., lO(1984) 219. T. Takahashi, Nippon Kugaku Kaishi, 9 (1976) 1454. R. M. Edreva-Kardjieva and A. A. Andreev, J. Mol. CutaZ., 46 (1988) 201. J. Engelhardt, Actu Chim. Acud., 111 (1982) 465. A. J. Van Roosmalen and J. C. Mol. J. Cutal., 78 (1982) 17. J. Engelhardt, J. Mol. Catal., 8 (1980) 119. J. Engelhardt, React. Kinet. Cutal. Lett., 11 (1979) 229. A. Molnar, J. T. Kiss and M. Bartbk, J. Mol. Cutal., 51 (1989) 361. G. G&i and G. Resofszki, J. Mol. Catal., 51 (1989) 295. G. Resofszki, G. G&i and I. Halasz, AppZ. Cutal., 19 (1985) 241. A. Ellison, A. K. Coverdale and P. F. Dearing, AppZ. Cutal., 2 (1983) 109. R. Nakamura, H. Iida and E. Echigoya, Nippon Kuguku Kuishi, 2 (1976) 221. R. Nakamura, K. Ichikawa and E. Echigoya, Nippon Kuguku Kuishi, 1 (1978) 36. Xu Xiaoding, C. Boelhouwer, D. Vonk, J. I. Benecke and J. C. Mol, J. Mol. Cutal., 36 (1986) 47. Xu Xiaoding, J. C. Mol and C. Boelhouwer, J. Chem. Sot., Faraday Trans. 1, 82 (1986) 2707. K. Segawa and W. K. Hall, J. Catal., 76 (1982) 133. A. J. Van Roosmalen, M. C. G. Hartmann and J. C. Mol, J. Cutal., 66 (1980) 112.