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Is metallic palladium formed in Wacker oxidation of alkenes?
JOURNAL OF
MOLECULAR CATALYSIS ELSEVIER
Journal of Molecular Catalysis 9 1 ( 1994) 343-352
Is metallic palladium formed in Wacker oxidation of alkenes? Milan Hronec, Zuzana CvengroSov& hefan Holotlk Department of Organic Technology, Faculty of Chemzcal Technology, Slovak Technical University, 812 37 Bratislava, Slovak Republic
(Received November 24, 1993; accepted February 4, 1994)
Abstract The oxidation of cyclohexene with molecular oxygen catalyzed by solid Pd/C or Pd” acetatehydroquinone-iron phthalocyanine gives cyclohexane, benzene and oxygenated products. Oxygen pressure and solvent used influence significantly the distribution of the products and at the pressure above 2 atm no cyclohexane is formed. Over Pd/C catalyst in the absence of oxygen, disproportionation to cyclohexane and benzene (the ratio is nearly 2 : 1) proceeds exclusively. Under comparable conditions 2-cyclohexenol is disproportionated to cyclohexanol and phenol and some of it rearranges to cyclohexanone. The explanation for the disproportionation of cyclohexene under the Wacker conditions is that Pd” centres intermediately formed after stoichiometric oxidation of cyclohexene by Pd” are not completely reoxidized, but depending on the reaction conditions, they can partly aggregate and then, similarly to metallic surfaces, dehydrogenate cyclohexene to benzene. The hydrogen species formed migrate on the palladium surface and hydrogenate cyclohexene or at sufficient oxygen pressure they are oxidized to water. Key words: alkenes; cyclohexene;
liquid phase; oxidation; palladmm
1. Introduction Apart from the production of bulk chemicals e.g., different monomers, palladium catalysts are very active, selective and stereospecific in many hydrogenation, oxidation, carbonylation and other reactions orientated toward preparation of chemical specialities [ 11. Numerous advances have been made in both the homogeneous and heterogeneous catalytic oxidation of various hydrocarbons, alcohols, carbohydrates, where the oxidation state of the catalytically active palladium sites and *Corresponding
author; fax. ( +42-7)495381.
0304.5102/94/$07.00 0 1994 Elsewer Science B.V. All rights reserved SSD10304-5102(94)00043-U
344
M. Hronec et al. /Journal
of Molecular Catalysrs 91 (1994) 343-352
properties of the reaction medium are of primary importance. Heterogeneous carbon supported palladium catalysts are known to be very selective for liquid-phase oxidation of a single hydroxyl group in many polyhydroxylic molecules to corresponding carbonyl compounds [ 2,3], propylene to acrylic acid or ally1 acetate [ 41, cycloalkenes to cycloalkanones [ 5,6], etc. Typical for both heterogeneous and soluble palladium catalysts is the activation of the carbon-hydrogen bond of alkenes [ 41. Vinylic and allylic oxidation products are the result of the influence of several factors (e.g., ligand system), which promote the C-H bond cleavage by surface or solution palladium centres. Very high is also the stereo and regiospecific product selectivity of palladium catalyzed reactions
1731. The process of palladium( II) catalyzed oxidation of acyclic olefins in aqueous media (the Wacker process) combines the stoichiometric oxidation of alkenes by Pd” in aqueous solution (Eq. 1) with the reoxidation of Pd” in situ by molecular oxygen in the presence of copper salt (Eqs. 2 and 3) or the bi-component system hydroquinone and iron( II) phthalocyanine FePc (Eqs. 4 and 5) [9-l 11. RCH=CH:! + HZ0 + PdCl* + R-C( =0)-CH,
+ Pd” + 2HCl
Pd” + 2CuC1, + PdCl* + 2CuCl 2CuCl+
1/202 + 2HCl+
Pd” + benzoquinone hydroquinone
(2)
2CuC12 + H2 0
(3)
+ 2Ht + Pd” + hydroquinone
+ 1/202 + FePc + benzoquinone
(1)
+ H2 0
(4) (5)
Under Wacker conditions cycloalkenes are similarly oxidized to ketone, however, the kinetic expression is not identical with that found for oxidation of acyclic olefins. Recently, the differences in kinetics and mechanism were demonstrated on the oxidation of 2-cyclohexenol [ 121. In this paper is reported the oxidation of cyclohexene in the liquid phase using homogeneous and heterogeneous palladium catalysts.
2. Experimental 2.1. Materials Cyclohexene (99.4%)) I-decene (98.7%), and styrene (99.1%)) were purified by distillation. 2-Cyclohexenol was purchased from Aldrich. All other chemicals and solvents were of reagent grade.
M. Hronec et al. /Journal
of Molecular
Catalysis 91 (1994) 343-352
345
2.2. Catalysts
Palladium(I1) acetate was prepared from PdC& by precipitation with NaOH, dissolving a solid in acetic acid and recrystallization of Pd” acetate in acetic acid. The zero valent Pd, Pt, Ru and Rh catalysts deposited on charcoal (5 wt.% of metal) were prepared by impregnation of charcoal (particle size 0.1-0.315 mm, surface area 1265 rn*. g- ‘) with metal chlorides, followed by reduction with formaldehyde [ 131. Iron phthalocyanine ( FePc) and cobalt tetrasulfophthalocyanine (CoTSPC) were synthesized by the method described by Weber and Busch [ 141. The metal content in the catalysts was determined by polarography.
2.3. Procedure
Oxidation experiments were carried out in a 50 ml glass lined stainless steel reactor connected by a flexible metal capillary to the apparatus for measuring the oxygen consumption at constant pressure. The reactor was filled with the reactants, catalyst and oxygen, heated in a thermostated oil bath and agitated using a shaker. The reaction of alkenes and alcohols with degassed catalyst in the absence of oxygen was examined in glass tubes sealed with a screw cap fitted with a silicone septum. The tube was evacuated and flushed several times with nitrogen and immersed in a silicone oil bath and shaken by using a vibrator. Two to three experiments were done for each run in oxygen and nitrogen atmospheres.
2.4. Analysis
Samples of reaction mixtures from the oxidation of cyclohexene or alcohols in aqueous media (after adding 2-hexanone) were extracted (3 X ) with ether and analyzed for products by the GC method (Hewlett Packard 5890-11, column HP20 M 30 m X 0.53 mm X 1,33 pm). To avoid the effect of adsorption of reactants and products on catalyst the same amounts of organic compound and charcoal were mixed in appropriate medium for 24 h under nitrogen, then extracted (3 X ) with ether and after evaporation of ether to ca 2 ml volume analyzed for products. On the basis of these results correlation of conversion and product composition was made in real samples. Samples of homogeneous reaction mixtures were analyzed directly after filtration off a catalyst. The products of 1-decene oxidation were analyzed according to [ 111. Most of the reactants and products in these reactions were identified by comparison of retention times with those from commercially available authentic samples. Otherwise, identification was based on GC-MS (MS-25 RFA Kratos) and NMR measurement (Varian UXR-300).
346
M. Hronec et al. /Journal
of Molecular Catalysis 91 (1994) 343-352
3. Results Oxidation of 1-decene and cyclohexene with molecular oxygen was studied under the same reaction conditions with the three-component catalytic system, palladium( II) acetate-hydroquinone-FePc. Table 1 shows that the oxidation of 1-decene in dimethylformamide and a DMF-H20 mixture produces 2-decanone as the product, but the conversion in aqueous DMF is significantly lower. The conversion of I-decene is low, also in dioxane as a solvent and about 7% of oxidation products are isomeric decanones ( 3- and 4-isomers), probably as a result of isomerization of I-decene prior to the oxidation step. With the identical catalytic system, oxidation of cyclohexene, besides giving oxygenated products, also produces cyclohexane and benzene. The ratio of the products is different in DMF and dioxane. When solid 5% Pd/C catalyst was used instead of soluble Pd” acetate, no oxidation of either alkene was observed in DMF or dioxane. Negative results were obtained also with an ion exchanged Pd-Y zeolite catalyst. Although solid Pd/C catalyst is active at higher temperature, it shows low selectivity of cyclohexene oxidation to oxygenated products (Table 2). In spite of the oxygen atmosphere the main products of the reaction are benzene and cyclohexane, and conversion of cyclohexene is total. The ratio of the products, benzene/cyclohexane is increased with an increase of oxygen pressure. Addition of CuCl, to the reaction system significantly changes the distribution of reaction products. In the presence of CuCl, the oxygenated products are formed preferably, but the conversion of cyclohexene is suppressed. It is known from the literature [9], that under these conditions chlorinated products are formed. Benzene is formed in a lower amount and no cyclohexane was identified among the products. Oxygen pressure has a positive Table 1 Oxidation of 1-decene and cyclohexene Substrate
Solvent
catalyzed by Pd/C or three-component
Conversion
palladium( II) system
Products (wt.%)
(%) 2-decanone 1-decene 1-decene 1-decene cyclohexene cyclohexened cyclohexene’ cyclohexene“,’ cyclohexeneg
DMF DMF + Ha0 dioxane dioxane DMF DMF DMF DMF
92.1 71.4 70.6 14.2 19.6 18.1 28.7 0
92.5 76.1 56.0 _
_
&HI2
C,H,
C-on
C-01
PhOH
Others
2.7 2.4 10.3 3.1 _
5.3 4.8 5.1 10.2 _
4.4 10.6 0.2 10.6 _
0.4 0.1 0.1 0.6 _
1.4 1.3 3.0 _
0.1’ 0.7’ 6.9 0.4 0.4 1.2
“Conditions: 25°C 2 atm 0,. 0.13 g hydroquinone, reaction time 6 h, 0.215 g FePc, 0.085 g Pd acetate, 5 ml solvent, 0.5 ml HzO, 80 ~1 HCIO,, 1 ml substrate. bC-on cyclohexanone, C-01 cyclohexanol, PhOH phenol, DMFdimethylformamide. ‘Mostly isomenc decanones. d70°C, 1.3 atm. ‘Inert atmosphere, ‘0.307 g polyvmylpyrrolidone. gO.O88 g 5% Pd/C
M. Hronec et al. /Journal of Molecular Catalyrs 91 (I 994) 343-352 Table 2 Effect of CuClz on the oxidation” of cyclohexene CUCI,
Conversion
(s)
(%)
0
0 0.13 0.11 0.11
100 100b 40.7 65.8b 94,4b =
in ethanol catalyzed by W/C
Products (wt.%) C,H,z
GH,
c-01
C-on
PhOH
Others
53.2 10.1 0 0 0
42 9 80.9 5.6 10.7 25.5
0.8 0 14.8 5.0 13.2
0. I 3.0 2.6 31.6 14.5
0.1 2.2 5.6 4.8 8.2
2.9 3.8 12.1d 13.7d 34.0d
“Conditions: 90°C 2 atm 0,. 0.25 g 5% Pd/C, ‘0.5 g 5% W/C dMainly chlorinated products.
1 ml cyclohexene,
Table 3 Activity of noble metal catalysts (5 wt.% metal on charcoal) Metal
Converston
341
Products
10 ml ethanol, reaction time 3 h. b6 atm 02,
for reaction” of cyclohexene
in the absence of oxygen
(wt.%)
(%)
Pd Pdb Pdb.’ Pdb.d Ptb Ru Rh
100 100 12.0 100 64.3 0 0
GH,,
C,H,
c-01
C-on
PhOH
Others
66.0 65.5 45.1 64.1 42.4 _
31.7 33.2 23.0 30.7 20.5 _
0.2 0.3 0.8 1.6 0.6 _
1.5 0.8 0.7 2.3 0.8
0.4 0.2 0.3 0.2 0
0.2 0 1.5 1.1 0 _
_
_
“Conditions: 90°C. 1 ml cyclohexene, ‘0.06 g W/C. dReused Pd/C catalyst.
10 ml H20 0.25 g catalyst,
reactton ttme 3 h. b1.25 M aqueous NaOH.
influence on the shift of selectivity to oxygenated products and increases the conversion. The appearance of cyclohexane among the reaction products of cyclohexene oxidation under oxygen pressure has led us to study the transformation of cyclohexene under anaerobic conditions over supported metallic palladium and other noble metals. From Table 3 it is seen, that over Pd and Pt metals, disproportionation of cyclohexene to benzene and cyclohexane proceeds exclusively. Both products are formed in near theoretical ratio 1 : 2. In disproportionation experiments the ratio is not changing with the basicity of the medium, concentration of catalyst (but conversion decreases) and history of catalyst i.e., fresh or reused. The appearance of a small amount of oxygenated products is probably the result of the presence of oxygen sorbed on the catalyst. Under the reaction conditions studied, ruthenium and rhodium catalysts are inactive for cyclohexene disproportionation. As we have mentioned above, the ratio of products of cyclohexene disproportionation over Pd/C catalyst depends on the oxygen pressure in the system. However, it can also be changed by the addition of the sodium salt of cobalt tetrasulfophthalocyanine (CoTSPC) to the reaction mixture (Table 4). It is evident
M. Hronec et al. /Journal of Molecular Catalysis 91 (1994) 343-352
348
Table 4 Effect of CoTSPC on cyclohexene CoTSPC
Conversion
W
(%)
0 0.035 0.035b 0.080b 0.12b 0.1 2c.d “Conditions:
96.1 66.1 86.7 88.4 45.3 100
oxidation catalyzed by Pd/C at 2 atm of oxygen Products (wt.%) C&Z
C,H,
c-01
C-on
PhOH
Others
50.3 17.0 0 0 0 63.9
29.4 31.7 77.7 79.5 40.9 32.2
4.9 4.5 0.5 0.5 0.1 1.2
3.8 4.1 0.4 1.1 0.1 1.4
3.8 7.0 4.0 6.3 2.3 _
3.9 1.8 7.7 1.0 1.9 1.3
see Table 3. “6 atm 0,. ‘1.25 M aqueous NaOH. dNz atmosphere.
from these results that an increase in the CoTSPC concentration under aerobic conditions leads to a decrease in the conversion of cyclohexene. In the absence of oxygen CoTSPC has no influence on the reaction of cyclohexene. Despite a decrease in the conversion of cyclohexene in the presence of CoTSPC, the main products remain benzene and cyclohexane, however, cyclohexane is produced in a much smaller amount. A higher oxygen pressure compensates for a drop in the conversion of cyclohexene caused by CoTSPC and practically suppresses the formation of cyclohexane.
4. Discussion The fact that the main products of cyclohexene oxidation with molecular oxygen catalyzed either by a three-component palladium system or by solid Pd/C are benzene and cyclohexane, requires that cyclohexene dissociates, forming hydrogen species prior to the oxidation step. In a mechanism of cyclohexene oxidation the first step is supposed to be dissociation of cyclohexene on the surface of metallic palladium. GHKW
=CJ&a~
+ I-L(a)
(6)
The experimentally observed ratio of cyclohexane to benzene, which under anaerobic conditions is nearly 2 : 1, supports the fact that disproportionation of cyclohexene proceeds via reactions 6-8. G%a~ CJ-LW
=C&X~~
+ K(a)
+ I-L(a) = C6H~(aj
(7) (8)
The primary product of the reaction 6, cyclohexadiene, rapidly dehydrogenates to benzene and the simultaneously formed hydrogen species migrate on the surface of metallic palladium and hydrogenate cyclohexene. At the temperatures studied, the equilibrium of reactions 7 and 8 is shifted to the right.
M. Hronec et al. /Journal
of Molecular Catalysrs 91 (1994) 343-352
349
The multistep disproportionation process proceeds on metallic surfaces. Unlike solid Pd/C catalyst, in homogeneously catalyzed oxidation of alkenes with Pd” acetate, metallic palladium surfaces active for disproportionation are probably in the form of Pd” aggregates created after stoichiometric oxidation of alkenes by Pd” (Eq. 1). Obviously intermediately formed Pd” centres are oxidized back to Pd” species by the Cu” ions or benzoquinone (Eqs. 2 and 4). Depending on the reaction conditions, type of alkene and solvent, the Pd” centres probably aggregate partly and then react with alkenes, similarly to metallic surfaces [ 41. Stabilization of colloidal palladium by polyvinylpyrrolidene [ 151 could explain the higher conversion of cyclohexene to cyclohexane and benzene. It means that the rate of reoxidation of Pd” species by copper( II) or hydroquinone competes with the rate of palladium( 0) aggregation and instantaneous palladium( 0)-alkene complex formation. In the dissociation step the leaving species is hydrogen, which can (i) migrate on the palladium surface and hydrogenate the adsorbed alkenes, (ii) recombine with molecular hydrogen and desorb from metallic palladium or (iii) in the presence of molecular oxygen react with gaseous oxygen or oxygen chemisorbed on the palladium surface to form water. The first possibility supports the formation of cyclohexane during cyclohexene oxidation in the presence of both homogeneous Pd” acetate and solid Pd/C catalysts (Tables 1 and 2). Another argument in favour of the migration of hydrogen species over the surface of metallic palladium to other coordinated alkenes is the simultaneous reaction of cyclohexene and styrene or 1-decene over Pd/C catalyst (Table 5). Thus, under anaerobic conditions no reaction of 2-butanol, cyclohexanol, benzyl alcohol, 1-decene and styrene was observed. However, in a mixture with cyclohexene, styrene and 1-decene are hydrogenated to ethylbenzene and decane, respectively. The presence of cyclohexanol and 2-butanol in the reaction mixture with cyclohexene has no influence on its reactivity and product distribution. In an inert atmosphere, benzyl alcohol is also unreactive, but similarly to styrene it strongly suppresses the conversion of cyclohexene. This effect is probably the result of the preferential adsorption of aromatic compounds on palladium centres, as compared to cyclohexene. Also, for this reason, under an oxygen atmosphere, conversion of benzyl alcohol to oxygenated products is higher. Useful information concerning the behaviour of cyclic alkenes over metallic palladium catalyst is provided by experiments with 2-cyclohexenol (Table 6). In an inert atmosphere at the same conditions this alcohol is transformed to three main products, cyclohexanone. cyclohexanol and phenol. The product distributions confirm that 2-cyclohexenol is transformed to cyclohexanol and phenol by the same hydrogenation-dehydrogenation mechanism as is observed in the case of cyclohexene disproportion. Cyclohexanone is formed by rearrangement of 2-cyclohexenol on the surface of Pd catalyst containing of hydride species from the previous dissociation process and not by subsequent dehydrogenation of cyclohexanol. It is evident from Table 6, that under experimental conditions in the absence of oxygen there is no reaction by cyclohexanol. Recently, Zaw and Henry [ 121 have published
350
M. Hronec et al. /Journal
of Molecular Catalysis 91 (1994) 343-352
Table 5 Effect of alcohols and oletins on disproportionation” Substrate
Conversion of C-ene’ %
styrene
5.3 5.6’,d O’J
of cyclohexene
over W/C catalyst
Products (wt.%) C-on
GH,,
GH6
0
5.3 5.2
0 0.1
0 _
c-01
PhOH
Others
0
0
01 _
0 _
0.4
0
0.6
0.2 _
_
_
89.9g
51.2
37.8
0
cyclohexanol
100d 100d,h 100 100’
33.0 32.5 32.8 21.7
16.6 16.3 16.5 20.6
1.5 1.6 1.5 4.8
45.4 46.2 45.2 44.4
3.3 2.3 3.0 1.0
0.2 1.1 1.0 1.5
2-butanol
100
64.0
32.3
1.2
1.3
0.4
0.8
0
traces 3.2
0 0.1
0
0
0.2
0
2.0 0.3
00
I-decene
benzyl alcohol
2.v 8.8’,k
5.0
_
“Conditions: 90°C. inert atmosphere, 10 ml 1.25 M NaOH, 0.25 g 5% W/C, 1 ml cyclohexene + 1 ml substrate. bC-ene = cyclohexene. ‘Converston of styrene 12.4%. ethylbenzene 12.2%. d10 ml water. ‘Without cyclohexene. ‘No conversion of 1-decene. “71.7% conversion of I-decene, 52.3% n-decane, 19.7% unidentified products. h0.5 ml cyclohexene + 0.5 ml cyclohexanol. ‘2 atm 0,. ‘No conversion of benzyl alcohol. ‘55% conversion of benzyl alcohol, 24.9% benzaldehyde, 30% benzoic acid. ‘No conversion of styrene.
Table 6 Anaerobic
reactiona of cyclohexene,
Substrate
cyclohexanol 2-cyclohexenol 2-cyclohexenol 2-cyclohexenol
2-cyclohexenol
and cyclohexanol
over 5% Pd/C at 90°C
Converston of C-ene
Products (wt.%)
(%)
&HI>
GH6
C-on
C-01
PhOH
2-Cyclohexenone
100b 100’ O’.d 0Ur.e
33.0 33.0 0 _
16.5 16.8 0.6 _
1.5 16.1 29.5 45.0
45.2 19.4 43.7 -
3.3 13.2 26.1 -
19.0
“Catalyst 0.25 g, 10 ml HzO, 1 ml substrate + 1 ml cyclohexene. cyclohexenol 100%. dWithout cyclohexene. “See [ 121.
‘No cyclohexanol
3-Hydroxycyclohexanone _
0.5 1.5 0.1 _
36.0 conversion.
Others
‘Converston
of 2-
results on the oxidation of 2-cyclohexenol by Pd” under Wacker conditions. The product distribution (Table 6) does not consist of cyclohexanol and phenol, and only cyclohexanone is the common product in both types of reaction. 2-Cyclohexenone and 3-hydroxycyclohexanone are the other main products. An almost theoretical ratio of products of cyclohexene disproportionation, observed experimentally over Pd/C under anaerobic conditions, suggests that intermediately formed hydrogen very rapidly hydrogenates cyclohexene and cyclo-
M. Hronec et al. /Journal
of Molecular Catalysis 91 (1994) 343-352
351
hexadiene and does not desorb before catalytic hydrogenation. However, we have experimental evidence that during the reaction of other substrates over Pd/C catalyst, e.g., in the case of the oxidation of alcohol with molecular oxygen, hydrogen is accumulated in the gas phase. The important point of the study is that the presence of molecular oxygen and its partial pressure suppress the hydrogenation processes, i.e., the ratio of cyclohexane to benzene (Tables 2 and 4). There are two explanations for this fact: (i) oxidation of hydrogen to water by oxygen chemisorbed on palladium centres or to a lesser extent with gaseous oxygen competes with the rate of hydrogenation, ( ii) desorption of hydrogen proceeds by competitive adsorption of oxygen on the metallic surface. The effect of oxygen is supported by the addition of CoTSPC to the system. It is known that this metal complex readily coordinates molecular oxygen [ 161 and is adsorbed on solid surfaces. Adsorption of the complex is probably the reason for the lower conversion of cyclohexene. In addition to these effects, the concentration of oxygen on the Pd surface is probably increased and thus the rate of hydrogen oxidation is favoured. The possibility of hydrogen desorption by competitive adsorption of oxygen cannot be totally excluded, though in this experiment only a trace amount of hydrogen was detected in the gaseous phase. Under the reaction conditions used, no activity for cyclohexene disproportionation was found for the Rh and Ru catalysts. The platinum catalyst is less active, but both products of the disproportionation are formed in the almost theoretical ratio. The lower reactivity of the Pt and unreactivity of the Ru and Rh catalysts with cyclohexene on the basis of the present studies are unclear. However, the Rh catalyst is active for the oxidative dehydrogenation of alcohols in the liquid phase [ 171 and the Ru catalyst in the gas phase disproportionation of cyclohexene [ 181. Differences can be linked with the auto-reductive removal of oxygen by cyclohexene as reducing agent from the noble metal catalysts, which in the initial stage are more or less covered with oxygen, but the effect of metal particle size also cannot be excluded [ 191. Therefore it is obvious that for a detailed mechanistic analysis, further research is needed.
References [ 1 I Y. Fujiwara, T. Jintoku and K. Takaki, CHEMTECH, October ( 1990) 636. [21 Y. Schuurman, B.F.M. Kuster, K. van der Wtele and G.B. Marin, in P. Ruiz and B. Delmon (Eds.), Studies in Surface Science and Catalysis, Vol. 72, , Elsevier, 1992, pp. 43-45. [31 T. Tsujino, S. Ohigasht, S. Sagiyama, K. Kawashiro and H. Hayashi, I. Mol. Catal., 71 ( 1992) 25. [41 J.E. Lyons, Catal. Today, 3 ( 1988) 245. [51 T. Kumazawa and M. Kanzawa, Jpn. Kokai Tokkyo Koho JP 63.280039, 17 Nov. 1988 (Chem. Abstr., 110 (1989) 231169).
[61Sumitomo Chem. Co. Ltd., Jpn. Kokai Tokkyo Koho JP 60, 92 236, 23 May 1985 (Chem. Abstr., 103 (1985)
141517).
[71 J. Nokami, H. Ogawa, S. Miyamoto, 5181.
T. Mandai, S. Wakabayashi
and J. Tsuji, Tetrahedron
Lett., 29 ( 1988)
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[Sl L.El Firdoussi, A. Benharref, S. Allaoud, A. Karim, Y. Castanet, A. Mortreux and F. Petit, J. Mol. Catal., 72 (1992) Ll. 191 P.M. Henry, Palladium Catalyzed Oxidation of Hydrocarbons, Reidel, Dordrecht, 1980. [ 101 J.E. Blckvall, m M. Guisnet et al. (Eds.), Heterogeneous Catalysis and Fine Chemicals, Studies in Surface Science and Catalysis, Vol. 41, Elsevier, 1988, p. 105. [ 111 S. Srinivasan and W.T. Ford, J. Mol. Catal., 64 ( 1991) 291. [ 121 K. Zaw and P.H. Henry, Organometalhcs, 11 ( 1992) 2008. [ 131 M. Hronec, Z. CvengroSova and J. Kizlink, J. Mol. Catal., 83 ( 1993) 75. [ 141 J.H. Weber and D.H. Busch, Inorg. Chem., 4 ( 1965) 469. [ 151 F. Porta, F. Ragamr and S. Cenini, Gazz. Chim. Ital., 122 ( 1992) 361. [ 161 R.A. Sheldon and J.K. Kochi, Metal Catalyzed Oxidations of Organic Compounds, Academic Press, New York, 1981. [ 171 H.E. van Dam, L.J. Wrsse and H. van Bekkum, Appl. Catal., 61 (1990) 187. [ 181 M.C. Schoenmaker-Stolk, J.W. Verwrjs, J.A. Don and J.J.F. Scholten, Appl. Catal., 29 (1987) 73. [ 191 A. Rochefort, F. Le Peltrer and J.P. Boitiaux, J Catal., 138 ( 1992) 482.
MOLECULAR CATALYSIS ELSEVIER
Journal of Molecular Catalysis 9 1 ( 1994) 343-352
Is metallic palladium formed in Wacker oxidation of alkenes? Milan Hronec, Zuzana CvengroSov& hefan Holotlk Department of Organic Technology, Faculty of Chemzcal Technology, Slovak Technical University, 812 37 Bratislava, Slovak Republic
(Received November 24, 1993; accepted February 4, 1994)
Abstract The oxidation of cyclohexene with molecular oxygen catalyzed by solid Pd/C or Pd” acetatehydroquinone-iron phthalocyanine gives cyclohexane, benzene and oxygenated products. Oxygen pressure and solvent used influence significantly the distribution of the products and at the pressure above 2 atm no cyclohexane is formed. Over Pd/C catalyst in the absence of oxygen, disproportionation to cyclohexane and benzene (the ratio is nearly 2 : 1) proceeds exclusively. Under comparable conditions 2-cyclohexenol is disproportionated to cyclohexanol and phenol and some of it rearranges to cyclohexanone. The explanation for the disproportionation of cyclohexene under the Wacker conditions is that Pd” centres intermediately formed after stoichiometric oxidation of cyclohexene by Pd” are not completely reoxidized, but depending on the reaction conditions, they can partly aggregate and then, similarly to metallic surfaces, dehydrogenate cyclohexene to benzene. The hydrogen species formed migrate on the palladium surface and hydrogenate cyclohexene or at sufficient oxygen pressure they are oxidized to water. Key words: alkenes; cyclohexene;
liquid phase; oxidation; palladmm
1. Introduction Apart from the production of bulk chemicals e.g., different monomers, palladium catalysts are very active, selective and stereospecific in many hydrogenation, oxidation, carbonylation and other reactions orientated toward preparation of chemical specialities [ 11. Numerous advances have been made in both the homogeneous and heterogeneous catalytic oxidation of various hydrocarbons, alcohols, carbohydrates, where the oxidation state of the catalytically active palladium sites and *Corresponding
author; fax. ( +42-7)495381.
0304.5102/94/$07.00 0 1994 Elsewer Science B.V. All rights reserved SSD10304-5102(94)00043-U
344
M. Hronec et al. /Journal
of Molecular Catalysrs 91 (1994) 343-352
properties of the reaction medium are of primary importance. Heterogeneous carbon supported palladium catalysts are known to be very selective for liquid-phase oxidation of a single hydroxyl group in many polyhydroxylic molecules to corresponding carbonyl compounds [ 2,3], propylene to acrylic acid or ally1 acetate [ 41, cycloalkenes to cycloalkanones [ 5,6], etc. Typical for both heterogeneous and soluble palladium catalysts is the activation of the carbon-hydrogen bond of alkenes [ 41. Vinylic and allylic oxidation products are the result of the influence of several factors (e.g., ligand system), which promote the C-H bond cleavage by surface or solution palladium centres. Very high is also the stereo and regiospecific product selectivity of palladium catalyzed reactions
1731. The process of palladium( II) catalyzed oxidation of acyclic olefins in aqueous media (the Wacker process) combines the stoichiometric oxidation of alkenes by Pd” in aqueous solution (Eq. 1) with the reoxidation of Pd” in situ by molecular oxygen in the presence of copper salt (Eqs. 2 and 3) or the bi-component system hydroquinone and iron( II) phthalocyanine FePc (Eqs. 4 and 5) [9-l 11. RCH=CH:! + HZ0 + PdCl* + R-C( =0)-CH,
+ Pd” + 2HCl
Pd” + 2CuC1, + PdCl* + 2CuCl 2CuCl+
1/202 + 2HCl+
Pd” + benzoquinone hydroquinone
(2)
2CuC12 + H2 0
(3)
+ 2Ht + Pd” + hydroquinone
+ 1/202 + FePc + benzoquinone
(1)
+ H2 0
(4) (5)
Under Wacker conditions cycloalkenes are similarly oxidized to ketone, however, the kinetic expression is not identical with that found for oxidation of acyclic olefins. Recently, the differences in kinetics and mechanism were demonstrated on the oxidation of 2-cyclohexenol [ 121. In this paper is reported the oxidation of cyclohexene in the liquid phase using homogeneous and heterogeneous palladium catalysts.
2. Experimental 2.1. Materials Cyclohexene (99.4%)) I-decene (98.7%), and styrene (99.1%)) were purified by distillation. 2-Cyclohexenol was purchased from Aldrich. All other chemicals and solvents were of reagent grade.
M. Hronec et al. /Journal
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345
2.2. Catalysts
Palladium(I1) acetate was prepared from PdC& by precipitation with NaOH, dissolving a solid in acetic acid and recrystallization of Pd” acetate in acetic acid. The zero valent Pd, Pt, Ru and Rh catalysts deposited on charcoal (5 wt.% of metal) were prepared by impregnation of charcoal (particle size 0.1-0.315 mm, surface area 1265 rn*. g- ‘) with metal chlorides, followed by reduction with formaldehyde [ 131. Iron phthalocyanine ( FePc) and cobalt tetrasulfophthalocyanine (CoTSPC) were synthesized by the method described by Weber and Busch [ 141. The metal content in the catalysts was determined by polarography.
2.3. Procedure
Oxidation experiments were carried out in a 50 ml glass lined stainless steel reactor connected by a flexible metal capillary to the apparatus for measuring the oxygen consumption at constant pressure. The reactor was filled with the reactants, catalyst and oxygen, heated in a thermostated oil bath and agitated using a shaker. The reaction of alkenes and alcohols with degassed catalyst in the absence of oxygen was examined in glass tubes sealed with a screw cap fitted with a silicone septum. The tube was evacuated and flushed several times with nitrogen and immersed in a silicone oil bath and shaken by using a vibrator. Two to three experiments were done for each run in oxygen and nitrogen atmospheres.
2.4. Analysis
Samples of reaction mixtures from the oxidation of cyclohexene or alcohols in aqueous media (after adding 2-hexanone) were extracted (3 X ) with ether and analyzed for products by the GC method (Hewlett Packard 5890-11, column HP20 M 30 m X 0.53 mm X 1,33 pm). To avoid the effect of adsorption of reactants and products on catalyst the same amounts of organic compound and charcoal were mixed in appropriate medium for 24 h under nitrogen, then extracted (3 X ) with ether and after evaporation of ether to ca 2 ml volume analyzed for products. On the basis of these results correlation of conversion and product composition was made in real samples. Samples of homogeneous reaction mixtures were analyzed directly after filtration off a catalyst. The products of 1-decene oxidation were analyzed according to [ 111. Most of the reactants and products in these reactions were identified by comparison of retention times with those from commercially available authentic samples. Otherwise, identification was based on GC-MS (MS-25 RFA Kratos) and NMR measurement (Varian UXR-300).
346
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3. Results Oxidation of 1-decene and cyclohexene with molecular oxygen was studied under the same reaction conditions with the three-component catalytic system, palladium( II) acetate-hydroquinone-FePc. Table 1 shows that the oxidation of 1-decene in dimethylformamide and a DMF-H20 mixture produces 2-decanone as the product, but the conversion in aqueous DMF is significantly lower. The conversion of I-decene is low, also in dioxane as a solvent and about 7% of oxidation products are isomeric decanones ( 3- and 4-isomers), probably as a result of isomerization of I-decene prior to the oxidation step. With the identical catalytic system, oxidation of cyclohexene, besides giving oxygenated products, also produces cyclohexane and benzene. The ratio of the products is different in DMF and dioxane. When solid 5% Pd/C catalyst was used instead of soluble Pd” acetate, no oxidation of either alkene was observed in DMF or dioxane. Negative results were obtained also with an ion exchanged Pd-Y zeolite catalyst. Although solid Pd/C catalyst is active at higher temperature, it shows low selectivity of cyclohexene oxidation to oxygenated products (Table 2). In spite of the oxygen atmosphere the main products of the reaction are benzene and cyclohexane, and conversion of cyclohexene is total. The ratio of the products, benzene/cyclohexane is increased with an increase of oxygen pressure. Addition of CuCl, to the reaction system significantly changes the distribution of reaction products. In the presence of CuCl, the oxygenated products are formed preferably, but the conversion of cyclohexene is suppressed. It is known from the literature [9], that under these conditions chlorinated products are formed. Benzene is formed in a lower amount and no cyclohexane was identified among the products. Oxygen pressure has a positive Table 1 Oxidation of 1-decene and cyclohexene Substrate
Solvent
catalyzed by Pd/C or three-component
Conversion
palladium( II) system
Products (wt.%)
(%) 2-decanone 1-decene 1-decene 1-decene cyclohexene cyclohexened cyclohexene’ cyclohexene“,’ cyclohexeneg
DMF DMF + Ha0 dioxane dioxane DMF DMF DMF DMF
92.1 71.4 70.6 14.2 19.6 18.1 28.7 0
92.5 76.1 56.0 _
_
&HI2
C,H,
C-on
C-01
PhOH
Others
2.7 2.4 10.3 3.1 _
5.3 4.8 5.1 10.2 _
4.4 10.6 0.2 10.6 _
0.4 0.1 0.1 0.6 _
1.4 1.3 3.0 _
0.1’ 0.7’ 6.9 0.4 0.4 1.2
“Conditions: 25°C 2 atm 0,. 0.13 g hydroquinone, reaction time 6 h, 0.215 g FePc, 0.085 g Pd acetate, 5 ml solvent, 0.5 ml HzO, 80 ~1 HCIO,, 1 ml substrate. bC-on cyclohexanone, C-01 cyclohexanol, PhOH phenol, DMFdimethylformamide. ‘Mostly isomenc decanones. d70°C, 1.3 atm. ‘Inert atmosphere, ‘0.307 g polyvmylpyrrolidone. gO.O88 g 5% Pd/C
M. Hronec et al. /Journal of Molecular Catalyrs 91 (I 994) 343-352 Table 2 Effect of CuClz on the oxidation” of cyclohexene CUCI,
Conversion
(s)
(%)
0
0 0.13 0.11 0.11
100 100b 40.7 65.8b 94,4b =
in ethanol catalyzed by W/C
Products (wt.%) C,H,z
GH,
c-01
C-on
PhOH
Others
53.2 10.1 0 0 0
42 9 80.9 5.6 10.7 25.5
0.8 0 14.8 5.0 13.2
0. I 3.0 2.6 31.6 14.5
0.1 2.2 5.6 4.8 8.2
2.9 3.8 12.1d 13.7d 34.0d
“Conditions: 90°C 2 atm 0,. 0.25 g 5% Pd/C, ‘0.5 g 5% W/C dMainly chlorinated products.
1 ml cyclohexene,
Table 3 Activity of noble metal catalysts (5 wt.% metal on charcoal) Metal
Converston
341
Products
10 ml ethanol, reaction time 3 h. b6 atm 02,
for reaction” of cyclohexene
in the absence of oxygen
(wt.%)
(%)
Pd Pdb Pdb.’ Pdb.d Ptb Ru Rh
100 100 12.0 100 64.3 0 0
GH,,
C,H,
c-01
C-on
PhOH
Others
66.0 65.5 45.1 64.1 42.4 _
31.7 33.2 23.0 30.7 20.5 _
0.2 0.3 0.8 1.6 0.6 _
1.5 0.8 0.7 2.3 0.8
0.4 0.2 0.3 0.2 0
0.2 0 1.5 1.1 0 _
_
_
“Conditions: 90°C. 1 ml cyclohexene, ‘0.06 g W/C. dReused Pd/C catalyst.
10 ml H20 0.25 g catalyst,
reactton ttme 3 h. b1.25 M aqueous NaOH.
influence on the shift of selectivity to oxygenated products and increases the conversion. The appearance of cyclohexane among the reaction products of cyclohexene oxidation under oxygen pressure has led us to study the transformation of cyclohexene under anaerobic conditions over supported metallic palladium and other noble metals. From Table 3 it is seen, that over Pd and Pt metals, disproportionation of cyclohexene to benzene and cyclohexane proceeds exclusively. Both products are formed in near theoretical ratio 1 : 2. In disproportionation experiments the ratio is not changing with the basicity of the medium, concentration of catalyst (but conversion decreases) and history of catalyst i.e., fresh or reused. The appearance of a small amount of oxygenated products is probably the result of the presence of oxygen sorbed on the catalyst. Under the reaction conditions studied, ruthenium and rhodium catalysts are inactive for cyclohexene disproportionation. As we have mentioned above, the ratio of products of cyclohexene disproportionation over Pd/C catalyst depends on the oxygen pressure in the system. However, it can also be changed by the addition of the sodium salt of cobalt tetrasulfophthalocyanine (CoTSPC) to the reaction mixture (Table 4). It is evident
M. Hronec et al. /Journal of Molecular Catalysis 91 (1994) 343-352
348
Table 4 Effect of CoTSPC on cyclohexene CoTSPC
Conversion
W
(%)
0 0.035 0.035b 0.080b 0.12b 0.1 2c.d “Conditions:
96.1 66.1 86.7 88.4 45.3 100
oxidation catalyzed by Pd/C at 2 atm of oxygen Products (wt.%) C&Z
C,H,
c-01
C-on
PhOH
Others
50.3 17.0 0 0 0 63.9
29.4 31.7 77.7 79.5 40.9 32.2
4.9 4.5 0.5 0.5 0.1 1.2
3.8 4.1 0.4 1.1 0.1 1.4
3.8 7.0 4.0 6.3 2.3 _
3.9 1.8 7.7 1.0 1.9 1.3
see Table 3. “6 atm 0,. ‘1.25 M aqueous NaOH. dNz atmosphere.
from these results that an increase in the CoTSPC concentration under aerobic conditions leads to a decrease in the conversion of cyclohexene. In the absence of oxygen CoTSPC has no influence on the reaction of cyclohexene. Despite a decrease in the conversion of cyclohexene in the presence of CoTSPC, the main products remain benzene and cyclohexane, however, cyclohexane is produced in a much smaller amount. A higher oxygen pressure compensates for a drop in the conversion of cyclohexene caused by CoTSPC and practically suppresses the formation of cyclohexane.
4. Discussion The fact that the main products of cyclohexene oxidation with molecular oxygen catalyzed either by a three-component palladium system or by solid Pd/C are benzene and cyclohexane, requires that cyclohexene dissociates, forming hydrogen species prior to the oxidation step. In a mechanism of cyclohexene oxidation the first step is supposed to be dissociation of cyclohexene on the surface of metallic palladium. GHKW
=CJ&a~
+ I-L(a)
(6)
The experimentally observed ratio of cyclohexane to benzene, which under anaerobic conditions is nearly 2 : 1, supports the fact that disproportionation of cyclohexene proceeds via reactions 6-8. G%a~ CJ-LW
=C&X~~
+ K(a)
+ I-L(a) = C6H~(aj
(7) (8)
The primary product of the reaction 6, cyclohexadiene, rapidly dehydrogenates to benzene and the simultaneously formed hydrogen species migrate on the surface of metallic palladium and hydrogenate cyclohexene. At the temperatures studied, the equilibrium of reactions 7 and 8 is shifted to the right.
M. Hronec et al. /Journal
of Molecular Catalysrs 91 (1994) 343-352
349
The multistep disproportionation process proceeds on metallic surfaces. Unlike solid Pd/C catalyst, in homogeneously catalyzed oxidation of alkenes with Pd” acetate, metallic palladium surfaces active for disproportionation are probably in the form of Pd” aggregates created after stoichiometric oxidation of alkenes by Pd” (Eq. 1). Obviously intermediately formed Pd” centres are oxidized back to Pd” species by the Cu” ions or benzoquinone (Eqs. 2 and 4). Depending on the reaction conditions, type of alkene and solvent, the Pd” centres probably aggregate partly and then react with alkenes, similarly to metallic surfaces [ 41. Stabilization of colloidal palladium by polyvinylpyrrolidene [ 151 could explain the higher conversion of cyclohexene to cyclohexane and benzene. It means that the rate of reoxidation of Pd” species by copper( II) or hydroquinone competes with the rate of palladium( 0) aggregation and instantaneous palladium( 0)-alkene complex formation. In the dissociation step the leaving species is hydrogen, which can (i) migrate on the palladium surface and hydrogenate the adsorbed alkenes, (ii) recombine with molecular hydrogen and desorb from metallic palladium or (iii) in the presence of molecular oxygen react with gaseous oxygen or oxygen chemisorbed on the palladium surface to form water. The first possibility supports the formation of cyclohexane during cyclohexene oxidation in the presence of both homogeneous Pd” acetate and solid Pd/C catalysts (Tables 1 and 2). Another argument in favour of the migration of hydrogen species over the surface of metallic palladium to other coordinated alkenes is the simultaneous reaction of cyclohexene and styrene or 1-decene over Pd/C catalyst (Table 5). Thus, under anaerobic conditions no reaction of 2-butanol, cyclohexanol, benzyl alcohol, 1-decene and styrene was observed. However, in a mixture with cyclohexene, styrene and 1-decene are hydrogenated to ethylbenzene and decane, respectively. The presence of cyclohexanol and 2-butanol in the reaction mixture with cyclohexene has no influence on its reactivity and product distribution. In an inert atmosphere, benzyl alcohol is also unreactive, but similarly to styrene it strongly suppresses the conversion of cyclohexene. This effect is probably the result of the preferential adsorption of aromatic compounds on palladium centres, as compared to cyclohexene. Also, for this reason, under an oxygen atmosphere, conversion of benzyl alcohol to oxygenated products is higher. Useful information concerning the behaviour of cyclic alkenes over metallic palladium catalyst is provided by experiments with 2-cyclohexenol (Table 6). In an inert atmosphere at the same conditions this alcohol is transformed to three main products, cyclohexanone. cyclohexanol and phenol. The product distributions confirm that 2-cyclohexenol is transformed to cyclohexanol and phenol by the same hydrogenation-dehydrogenation mechanism as is observed in the case of cyclohexene disproportion. Cyclohexanone is formed by rearrangement of 2-cyclohexenol on the surface of Pd catalyst containing of hydride species from the previous dissociation process and not by subsequent dehydrogenation of cyclohexanol. It is evident from Table 6, that under experimental conditions in the absence of oxygen there is no reaction by cyclohexanol. Recently, Zaw and Henry [ 121 have published
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of Molecular Catalysis 91 (1994) 343-352
Table 5 Effect of alcohols and oletins on disproportionation” Substrate
Conversion of C-ene’ %
styrene
5.3 5.6’,d O’J
of cyclohexene
over W/C catalyst
Products (wt.%) C-on
GH,,
GH6
0
5.3 5.2
0 0.1
0 _
c-01
PhOH
Others
0
0
01 _
0 _
0.4
0
0.6
0.2 _
_
_
89.9g
51.2
37.8
0
cyclohexanol
100d 100d,h 100 100’
33.0 32.5 32.8 21.7
16.6 16.3 16.5 20.6
1.5 1.6 1.5 4.8
45.4 46.2 45.2 44.4
3.3 2.3 3.0 1.0
0.2 1.1 1.0 1.5
2-butanol
100
64.0
32.3
1.2
1.3
0.4
0.8
0
traces 3.2
0 0.1
0
0
0.2
0
2.0 0.3
00
I-decene
benzyl alcohol
2.v 8.8’,k
5.0
_
“Conditions: 90°C. inert atmosphere, 10 ml 1.25 M NaOH, 0.25 g 5% W/C, 1 ml cyclohexene + 1 ml substrate. bC-ene = cyclohexene. ‘Converston of styrene 12.4%. ethylbenzene 12.2%. d10 ml water. ‘Without cyclohexene. ‘No conversion of 1-decene. “71.7% conversion of I-decene, 52.3% n-decane, 19.7% unidentified products. h0.5 ml cyclohexene + 0.5 ml cyclohexanol. ‘2 atm 0,. ‘No conversion of benzyl alcohol. ‘55% conversion of benzyl alcohol, 24.9% benzaldehyde, 30% benzoic acid. ‘No conversion of styrene.
Table 6 Anaerobic
reactiona of cyclohexene,
Substrate
cyclohexanol 2-cyclohexenol 2-cyclohexenol 2-cyclohexenol
2-cyclohexenol
and cyclohexanol
over 5% Pd/C at 90°C
Converston of C-ene
Products (wt.%)
(%)
&HI>
GH6
C-on
C-01
PhOH
2-Cyclohexenone
100b 100’ O’.d 0Ur.e
33.0 33.0 0 _
16.5 16.8 0.6 _
1.5 16.1 29.5 45.0
45.2 19.4 43.7 -
3.3 13.2 26.1 -
19.0
“Catalyst 0.25 g, 10 ml HzO, 1 ml substrate + 1 ml cyclohexene. cyclohexenol 100%. dWithout cyclohexene. “See [ 121.
‘No cyclohexanol
3-Hydroxycyclohexanone _
0.5 1.5 0.1 _
36.0 conversion.
Others
‘Converston
of 2-
results on the oxidation of 2-cyclohexenol by Pd” under Wacker conditions. The product distribution (Table 6) does not consist of cyclohexanol and phenol, and only cyclohexanone is the common product in both types of reaction. 2-Cyclohexenone and 3-hydroxycyclohexanone are the other main products. An almost theoretical ratio of products of cyclohexene disproportionation, observed experimentally over Pd/C under anaerobic conditions, suggests that intermediately formed hydrogen very rapidly hydrogenates cyclohexene and cyclo-
M. Hronec et al. /Journal
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351
hexadiene and does not desorb before catalytic hydrogenation. However, we have experimental evidence that during the reaction of other substrates over Pd/C catalyst, e.g., in the case of the oxidation of alcohol with molecular oxygen, hydrogen is accumulated in the gas phase. The important point of the study is that the presence of molecular oxygen and its partial pressure suppress the hydrogenation processes, i.e., the ratio of cyclohexane to benzene (Tables 2 and 4). There are two explanations for this fact: (i) oxidation of hydrogen to water by oxygen chemisorbed on palladium centres or to a lesser extent with gaseous oxygen competes with the rate of hydrogenation, ( ii) desorption of hydrogen proceeds by competitive adsorption of oxygen on the metallic surface. The effect of oxygen is supported by the addition of CoTSPC to the system. It is known that this metal complex readily coordinates molecular oxygen [ 161 and is adsorbed on solid surfaces. Adsorption of the complex is probably the reason for the lower conversion of cyclohexene. In addition to these effects, the concentration of oxygen on the Pd surface is probably increased and thus the rate of hydrogen oxidation is favoured. The possibility of hydrogen desorption by competitive adsorption of oxygen cannot be totally excluded, though in this experiment only a trace amount of hydrogen was detected in the gaseous phase. Under the reaction conditions used, no activity for cyclohexene disproportionation was found for the Rh and Ru catalysts. The platinum catalyst is less active, but both products of the disproportionation are formed in the almost theoretical ratio. The lower reactivity of the Pt and unreactivity of the Ru and Rh catalysts with cyclohexene on the basis of the present studies are unclear. However, the Rh catalyst is active for the oxidative dehydrogenation of alcohols in the liquid phase [ 171 and the Ru catalyst in the gas phase disproportionation of cyclohexene [ 181. Differences can be linked with the auto-reductive removal of oxygen by cyclohexene as reducing agent from the noble metal catalysts, which in the initial stage are more or less covered with oxygen, but the effect of metal particle size also cannot be excluded [ 191. Therefore it is obvious that for a detailed mechanistic analysis, further research is needed.
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