/ APPLIED CATALYSIS A: GENERAL

ELSEVIER Applied Catalysis A: General 155 (1997) 15-26

A modified Wacker catalysis using heteropolyacid: Interaction of heteropolyanion with Cu(II) in

cyclohexene oxidation

Youngjin Kim, Hyungrok Kim*, Jungho Lee, Kyuseon Sim, Yohan Han, Hangnam Paik

Catalysis Division, Korea Research Institute of Chemical Technology, PO. Box 107, Yusung Taejon 305-606, South Korea

Received 13 February 1996; received in revised form 11 September 1996; accepted 23 September 1996

Abstract

Palladium(II)-copper(II)-heteropolyacid catalyst system for the Wacker type oxidation of cyclohexene to cyclohexanone was studied. Addition of heteropolyacid to chloride ion-free Pd(II)- Cu(II) catalyst system resulted in great increase of catalytic activity in cyclohexene oxidation. A catalyst system of Pd(NO3)2-CuSOg-H3PMOl2040 combination showed much nigher activity and selectivity compared with Pd(NO3)2-CuSO4 or Pd(NO3)2-H3PMol2040 combination. The spectroscopic changes of the heteropolyanion species in the catalytic systems were monitored by IR, UV-Vis and ESR methods. The spectroscopic data supported the existence of the interaction between Cu species and heteropolyanion. The interaction was proposed to be related with the electron transfer and di-oxygen reduction for the facile re-oxidation of Pd(0) to Pd(II) in the catalytic reaction cycle.

Keywords: Palladium; Heteropolyacid; Wacker; Cyclohexene; Oxidation

1. Introduction

The researchers of the vast heteropolyanion(HPA) chemistry have mainly focused on the preparation, structure characterization, and analytical applications of these compounds [ 1,2]. Only recently has the potential use of heteropolyanions as catalysts gained recognition, both as BrCnsted acid and oxidation catalysts [3- 6]. Heteropolyanions are attractive oxidation catalysts because of the unique redox

* Corresponding author. Fax: 82-42-861-4245

0926-860X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. P I I S0926-860X(96)00353-5

16 Y. Kim et al./Applied Catalysis A: General 155 (1997) 15-26

properties and inherent stability toward decomposition under extreme oxidation conditions. The Wacker type di-oxygen oxidation of olefins to aldehydes or ketones catalyzed by Pd-heteropolyacid system has been the subject of several interesting studies [7,8]. Some of them utilized the redox reactions in which the role of heteropolyanions were assigned as electron-transfer as described in the following equations.

Substrate + [HPA]o x + H20 ~ Product + [HPA]red q- 2H +.

[HPAlred + 2H + + 102 --~ [HPA]ox + H20.

Substrates may be of an inorganic nature as Pd(0) (product is Pd(II)) as in the Wacker-type oxidation of ethylene to acetaldehyde or acetoxylation of aromatics. In this type of application of heteropolyanions, a kind of transition metal sub- stituted heteropolyanions have also been utilized to modify redox properties of catalytic systems [9-12].

Various catalytic reactions have recently been proposed based on the redox properties of heteropolyanions. Grate group at Catalytica (Mountain View, CA) developed homogeneous catalyst systems for olefin oxidations using the optimized catalytic composition of ,-~ 2mMPdSO4 ~ 0.2M mixture of H(3+x)PMo0e_x) Vx04O [13,14]. Izumi et al. also reported that (PMo6W6040) 3- was a very effective heteropolyanion and that the slow step of the reaction was the re-oxidation of Pd ° to Pd 2+ in the Wacker type 1-butene oxidaton [15]. Saito et al. claimed a representative olefin oxidation process using the catalyst system of a transition metal compound containing Cu, Fe, Mn, etc. and Pd(II)-heteropolyacid with acetonitrile [16,17]. It was proposed that the above combinations were effective for the transformations of linear olefins (C2~C4) to the corresponding carbonyl compounds. In their studies, the enhancement in the catalytic activity was not significant when compared to others, probably due to the differences in catalytic compositions and chain length of the linear olefins used.

The application of the Pd-heteropolyacid catalytic system to the Wacker type oxidation of cyclohexene can be described in Scheme 1 and has not been successful in terms of catalytic activity [3]. In applying the catalytic system, however, combination effects of the transition metal compound and other components of Pd-heteropolyacid were realized and the small improvements on the catalytic activity with the addition of the transition metal compound were explained by the simple synergy effect between the redox metal and acetonitrile ligand. This type of catalytic systems for cyclohexene oxidation still presented problems associated with the precipitation of catalyst components during the reaction, thereby lowering the reaction rate to a remarkably low level after few hours of reaction and reducing the possibility of commercial application of the catalytic system. Thus, there has been a need to understand roles of the redox metal compounds and heteropolyacid in the heteropolyacid containing Wacker type catalytic system to manipulate the problems.

Y Kim et aL/Applied Catalysis A: General 155 (1997) 15-26 17

0 II

O2' H20 .

Pd(II)-HPA, Transition Metal Compound

Scheme 1. Wacker type cyclohexene oxidation using Pd-heteropolyacid with transition metal compound.

In this paper, we report the composition dependencies of catalytic activity in the oxidation of cyclohexene using heteropolyacid containing Wacker type catalyst systems. We also trace spectroscopic changes of heteropolyacid containing Wacker type catalyst systems and propose a catalytic cycle in which Cu species interacts with heteropolyanion for the facile re-oxidation of Pd(0) species in Wacker type oxidation of cyclohexene to explain remarkably high activity of the Pd(NO3)2- CuSOa-H3PMo12040 catalyst system.

2. Experimental

2.1. Materials

All chemicals were of reagent grade and were used as received. Pd (NO3)2 " H20 and CuSO4 • 5H20 were purchased from Aldrich (Milwaukee, WI) H3PMol:O4o and other heteropolyacids were purchased from Fluka (Buchs, Switzerland). Cyclohexene (99%) was purchased from Lancaster (Eastgate, UK) and was used after simple distillation.

2.2. Oxidation of cyclohexene

The reactions were performed in a 100 cm 3 stirred pressure reactor. The batch reactor was enclosed in an electric furnace equipped with an internal thermocouple and an MTCP-1 temperature controller. A typical example of the oxidation of cyclohexene is illustrated. Pd(NO3)2.H20 (2.8x10 -4 mol), CuSO4-5H20 ( l l .2x 10 -4 mol) and H3PMo12040 (11.2× 10 -4 mol) were dissolved in water (30 ml, 3% H2504 solution) and was charged in the reactor with acetonitrile (15 ml) and cyclohexene(10 ml). The reactor was flushed three times with oxygen gas, and then heated to 80°C in 20 min. Reaction was started by supplying oxygen at the reaction temperature. The operating pressure for the oxidation was typically 10 atm. The reaction mixture was sampled during the reaction for analysis. A GC(Hewlett Packard Ser. II) equipped with FID detector and a fused silica capillary column(Supelco, SPBawl-35) was used for the quantitative analysis. The identification and standardization of GC peaks were carried out with authentic samples.

18 Y Kim et al./Applied Catalysis A: General 155 (1997) 15-26

2.3. Isolation and component identification of green crystalline heteropolyanion species

After oxidation of cyclohexene catalyzed by Pd(NO3)2-CuSO4-H3PMol204o system for 3 h, green-colored homogeneous reaction mixture was transferred into a tightly sealed bottle. Green-colored crystals were formed three weeks later at room temperature. The crystals were dried in air after washing with cold ethanol(ca. 30 ml) and diethylether(ca. 80 ml). Qualitative component analysis of the green crystal was carded out by XPS.

2.4. Instrumental measurements

IR spectra were obtained on Polaris FT-IR spectrometer over the range of 4000-400 cm -1. UV-Vis absorption spectra were recorded on a Schimadzu UV-265 UV-Vis spectrophotometer. ESR measurements were carried out with an ESP-300S Bruker spectrometer operating at X-band frequen- cy(9.1~9.5 GHz). The external magnetic field was calibrated with micro- wave bridge Model ER 032MR Brucker wavemeter and temperature was controlled with Model ER 4111 VT Brucker device with the accuracy of +I°C.

3. Results and discussion

In Fig. 1, the composition dependencies of catalytic activity of the Pd(NO3)2- CuSOa-H3PMol2040 catalyst system for the Wacker type cyclohexene oxidation are shown. The conversion rates of cyclohexene within the experimental con- centration ranges are dependent on the catalyst compositions. With the increase of Pd concentration, cyclohexene conversion rate increases with almost the same ratio. At a given Pd concentration, much excess CuSO4 and H3PMo1204o con- centrations are required to maintain the stable and maximum conversion rates. Especially in the composition mol ratio of 1 : 3 : 3 or higher for Pd, CuSO4 and H3PMo1204o, almost maximum cyclohexene conversion rate can be obtained even though catalytic systems were not optimized both in anion components and reaction conditions. In Table 1, cyclohexene conversions of the Pd(II)-Cu(II)- heteropolyacid and related Wacker type catalytic systems of specific composition ratios are compared. In all runs of the reactions, a major product of cyclohexanone and small quantities of side-products, i.e. cyclohexenone were formed as expected in Wacker type oxidation of cyclohexene. The combination of Pd(NO3)a-CuSOa- H3PMo1204o with mol ratio of 1 : 2 : 2 and 1 : 4 :4 were found to be very effective for the reaction judging from cyclohexene conversions. The relatively low cyclohexene conversions and cyclohexanone yields of Pd(NO3)2---fuSO4 catalytic combination and Pd(NO3)2-H3PMol204o catalytic combination of 1 : 2 mol ratios

Y Kim et aL/Applied Catalysis A: General 155 (1997) 15-26 19

120 , = 100t (a) ~ ~

0 0.05 0.1 0.15 0.2 0.25 0.3 Pd Concentration (retool)

= ~°° I (b)

~ 6o

o , -

2 0

0 ! ~

0 0.2 0.4 0.6 0.8 1 1.2 H~teropolyacid Concentration (retool)

o ¢ 90 '= 80 (c) .-.m--

70 / <~_ 60

o 5 0 , o

~ - - 30 • ~ 20

~J

0.5 1 1.5 Cu Concentration (retool)

Fig. 1. Composition dependencies of cyclohexene conversion rate in the Pd(NO3)2-CuSO4-H3PMol2040 catalyst system; reaction conditions; Temp.: 80°C, 02 Pressure: 10 atm, Time: 1 h. Cyclohexene: 20 ml (198 retool), Acetomtrile: 20 ml, Water: 30 ml (2% H2SO,,). (a): Pd concentration dependency (Cu: 0.6 mmol, HPA: 0.6 retool), (b): HPA concentration dependency (Pd: 0.2 retool, Cu: 0.6 retool), (c): Cu concentration dependency (Pd: 0.2 retool, HPA: 0.6 retool).

present the strong synergetic effect of Cu species and heteropolyacid on the catalytic activity.

In describing the catalytic properties of the Pd(NO3)2-CuSO4-H3PMol204o} catalytic system in cyclohexene oxidation, the effect of anion and redox behaviors of Cu and heteropolyacid were considered because these two aspects were believed to be the main factors in differentiating the catalytic activity from that of the classical PdClE-CuC12 Wacker catalyst system. Under our cyclohexene oxidation condition, PdC12-CuC1E-H3PMol2040 catalyst system prepared by the addition of H3PM012040 to the PdC12-CuC12 combination showed conversion of 8% with 90% of cyclohexenone selectivity. However, without the addition of the heteropolyacid, very low cyclohexene conversion(3%) with 43% of cyclohexanone selectivity was observed. The low cyclohexenone selectivity resulted from the portion of many unidentified by-products through non-Wacker type oxidation pathways at the low cyclohexene conversion rate. When we compare this result with the case of chloride ion-free Pd(NO3)E-CuSO4-HaPMOl2040 catalyst system which showed

20 Y. Kim et al./Applied Catalysis A: General 155 (1997) 15-26

Table 1 Oxidations of cyclohexene with Wacker type catalyst systems a

Catalyst composition (mmol) Cyclohexene conversion (%) Cyclohexanone selectivity (%)

Pd(NO3)2-H3PMo1204o 6 96 (0.28/0.56) Pd(NO3)2~CuSO4 13 50 (0.28/0.56) Pd(NO3)2"-CuSO4-H3PMoI2040 47 94 (0.28/0.56/0.56) Pd(NO3)2-CuSO4-H3PMo12040 49 97 (0.28/1.12/1.12) Pd(NO3)2~2uSO4-H3PW 12040 14 86 (0.28/0.56/0.56) PdCI2-CuCI2-HaPMo1204o 8 90 (0.28/0.56/0.56) PdCI2-CuC12 3 43 (0.28/0.56)

a Reaction conditions; Temp.: 80°C, 02 Pressure: 10 atm, Time: 1 h. Cyclohexene: 10 ml (99 mmol), AcetoniWile: 15 ml, Water: 30 ml (3% H2SO4).

cyclohexene conversion of 47% with 94% of cyclohexenone selectivity, it is obvious that the catalytic properties of the heteropolyacid containing Wacker type systems are also strongly affected by chloride ion. Thus, chloride ion significantly suppresses the rate of Wacker type cyclohexene oxidation even in the presence of the heteropolyacid. Studies on the effects of anions in the Pd(II)-heteropolyacid type Wacker catalyst system in cyclo-olefin oxidation also revealed strong activity lowering effect of chloride ion compared with NO3-, or 5042- [18,19]. The effective coordination of chloride ion to Pd(II) ion may prevent cyclohexene from coordinating which may change the rate determining step of the reaction from the generally accepted Pd(0) re-oxidation step to the cyclohexene coordination step [20,21]. The relatively low cyclohexene conversion of 13%, however, in chloride ion-free Pd(NO3)2-fuSO4 catalyst combination in which cyclohexene coordina- tion is expected to be more effective than in the chloride ion containing system still implies that the other limiting factor may operate in the reaction system. Probably the low conversion of cyclohexene in this case can be attributed to the loss of re- oxidation rate of reduced Pd(0) to Pd(II) species. In this aspect of cyclohexene oxidation using the Pd(NO3)2-CuSOa-H3PM012040 catalyst system, the addition of heteropolyacid may be justified as a supplementary oxidant for the improvement of low Pd(0) re-oxidation ability of the chloride-free Pd(NO3)2--CuSO4 catalyst system. Many approaches using supplementary oxidants such as, Fe(III) ion, MnO2, or quinones have been proved to be somewhat effective and the catalytic cycle involving the roles of the supplementary oxidants has been proposed as reaction Scheme 2 [21]. However, when remarkably high activity of the Pd(NO3)2- CuSOa-H3PMo12040 catalyst system and activity improvement effect of PdClz- CuCI2 catalytic system by the addition of H3PMo1204o are considered, it may be

Y Kim et al./Applied Catalysis A: General 155 (1997) 15-26 21

~ , , /pd(ti)~ x f Cu(1)\ / [Supplementary ~, ¢H20

H20

+ ~ ~ NPd(O) / \ CU(II) Supolemen / \ 2H ÷ ~ x~{ Oxidant ta~Re d O2,2H ÷

ii O

Scheme 2. Three redox cycles of chloride ion-free catalyst system involved in supplementary oxidant.

suggested that the heteropolyanion participates some interaction with other catalytic components by which the transition state in the catalytic cycles are modified by utilizing the unique Lewis base property of heteropolyanion.

Regarding the assignment of catalytic function of H3PMo1204 o in the Pd(NO3)2- CuSOa-H3PMo12040} catalyst system, spectroscopic methods were applied to trace the chemical changes of the catalytic system. Fig. 2 shows IR absorption spectra of heteropolyacid (H3PMol2040), catalytic solution and the "green" species isolated as a crystal form from the reaction mixture after releasing oxygen pressure and standing at room temperature. Each bands can be assigned according to the literature as follows: 1065 cm -1 to P-O stretching, 964 cm -1 to Mo=O stretching, 779, 864 cm -1 to Mo-O-Mo vibration [22]. During the oxidation reaction, the intensities of the Vv-o band and the VMo-O-Mo band decreased significantly, while the intensity of the VMo=O band was almost unchanged. Similar behaviors of IR absorption spectra of the heteropolyacid were reported by other researchers and the decreases in intensity were assigned as a characteristic pattern of the reduced Keggin type heteropolyacid compounds [23-25]. The IR spectra of the heteropolyacid in the reaction mixture are clearly different from those of MoO 3 moiety, thereby the decomposition of Keggin structure during the oxidation reaction should be ruled out. Thus, at least some portion of total heteropolyanion in the reaction mixture can be assigned to maintain Keggin structure of reduced state.

UV-Vis absorption spectra of the Pd(II)--Cu(II)-H3PMo12040 catalyst system with the variation of reaction time are shown in Fig. 3. The H3PMo1204o in the catalytic mixture exhibits charge-transfer transition band(at<470 nm) and a strong d-d transition band at 720 nm region in the beginning of the reaction (Fig. 3b). The band at 720 nm shifted toward 700 nm and absorbence of the d-d band decreased with reaction time. The isolated "green" species exhibits a d-d band at 706 nm region which is about the same band position of the d-d transition band of the reduced form of the Keggin type poly-oxoanion [26]. In the reaction mixture, the d- d transition band corresponding to the band position of the reduced Keggin species was also observed, which suggests that reduced Keggin species was formed during the oxidation reaction. Moreover, the d-d transition band of H3PMo1204o in the Cu-free Pd(II)-H3PMo1204o catalyst system did not change with reaction time both in band position and intensity. Therefore, it is very probable that

22 Y. Kim et al./Applied Catalysis A: General 155 (1997) 15-26

!

4o00.0 3ooo.o 2ooo.o 15oo.o 10oo.o 4oo.0

WAVENUMBER(cm- 1 )

Fig. 2. Infrared spectra of (a) heteropolyacid (H3PMo1204o), (b) Heteropolyanion fomed after 1 h reaction and (c) "green" crystal of isolated from catalytic reaction mixture (KBr pellet, cm-1).

(PMo1204o) 3- anion in the Pd(II)-Cu(II)-HaPMo1204o catalyst system was modified by Cu species resulting the changes in the d-d transition energy. The absorption peak at 706 nm of the isolated "green" species suggests that the reduced Keggin anion species was crystallized from reaction mixture after releasing di-oxygen pressure.

ESR spectra for the Pd(II)--Cu(II)-H3PMol204o catalyst and the isolated "green" species in frozen glass of acetonitrile and toluene are shown in Fig. 4. The ESR spectrum of the Pd(II)--Cu(II)-HaPMOl2040 catalyst system in the reaction mixture shows four weak line hyperfine splitting pattern which con'e-

E Kim et al./Applied Catalysis A: General 155 (1997) 15-26 23

3.00

2.25.

~ 1.50.

0.75'

0 i i

300 500 700 900.

m

Fig. 3. Absorption spectral changes of Pd(U)-Cu(I1)--H3PMOl204o catalyst with varied oxidation time in cyclohexene oxidation by dioxygen; (a) catalyst solution before oxidation reaction, (b)30 rain, (c)60 min, (d)90 rain, (e)180 rain.

(a)

g, =2.4m / I

_ox,o. I f

2,400 2600 2600 3000 3200 3400 3600

[6]

(b)

/ -

. ) . )

~600 3000 3.¢00 3600 " 2 0 0 [G]

Fig. 4. X-band ESR spectra of (a) Pd(I/)--Cu(U)-H3PMol204o catalyst system after 1 h reaction, (b) isolated "green" species from reaction mixture in toluene at 77 K.

24 Y Kim et al./Applied Catalysis A: General 155 (1997) 15-26

sponds to the pattern of partially reduced Cu(I) species(I=3/2). Anisotropic ESR parameters of catalytic solution at high cyclohexene conversion are g11=2.419 and g±=2.085 with AI1=126 G and A±--10.4 G, respectively. The ESR parameters for the isolated "green" species of heteropolyanion in toluene at 77 K are <g> = 1.946 and <A> =47 G. These values are again identical to the mixed-valence pattern of reduced (PMo1204o) 4- species [27]. The absence of characteristic ESR signal of (PMo12040) 4- anion in the reaction mixture suggests that the reduced form of [PM01204o] anion in the Pd(II)-Cu(II)-H3PM012040 catalyst system is not the simple (PMo1204o) 4- type anion. The reduced form of (PMo1204o) 4- ion possibly interacts with Cu(II) species which also has one unpaired electron. On the surface of the Keggin anion, the strong electronic coupling between the reduced (PMo12040) 4- ion and Cu(II) ion could result in magnetically diluted electronic state which is silent in ESR measurement. The appearance of the ESR peak corresponding to (PMo12040) 4- anion from the green species which does not contain Cu component might be explained by the loss of the electronic coupling of (PMo12040) 4- ion with Cu(II). In this aspect, it could also be proposed that the reduced form of the heteropolyanion is oxidized by molecular .oxygen to form (PMo12040) 3- anion and maintains its oxidation state as observed in the copper free Pd(II)-H3PMoa2040 catalytic system. The great increase of catalytic activity in the Pd(II)-Cu(II)-H3PM012040 system, together with the fact that the decrease of (PMo12040) 3- characteristic absorption peak in the presence of copper species as observed on UV-Vis spectrum and absence of ESR signal of (PMo1204o) n- anion in the reaction system lead us to propose the existence of strong electronic interaction between Cu species and the heteropolyanion. This type of Lewis acid- base interaction of heteropolyanion with cationic species have been also proposed to explain the improved catalytic reaction rates by the formation of stabilized transition state in the many heteropolyanion catalyzed reactions [28].

Considering the remarkably high activity and spectroscopic data of the Pd(NO3)2-CuSOa-H3PMo1204o catalyst system in cyclohexene oxidation, it is reasonable to propose that one of the important functions of H3PM012040 is to interact with Cu species to participate facile re-oxidation of Pd(0) species and reduction of di-oxygen as shown in Scheme 3 because electron transfer between

HzO

+ ~ ' ] ~" ~Pd(O) [Cu-HPA]o x # "~ H20 2H + h O

Scheme 3. Redox cycles proposed in the oxidation of Pd(0) in Wacker type reaction with the action of the Cu- HPA interaction.

Y Kim et al./Applied Catalysis A: General 155 (1997) 15-26 25

Cu ion and the heteropolyanion is expected to be faster and Cu(II) ion with the heteropolyanion is more stable due to the acid-base interaction than in the separated system where Cu(II) and the heteropolyanion function independently. Even though changes of specific properties of the catalytic system with the Cu- HPA type interaction are not revealed, the variation in the electron mediation step resulting from the interaction should be crucial in the Wacker type catalytic cycles of di-oxygen oxidation of cyclohexene.

4. Conclusion

Palladium(II)-copper(II)-heteropolyacid (especially H3PMo12040 for hetero- polyacid) combinations were found to be an efficient modified-Wacker type catalyst system for the oxidation of cyclohexene to cyclohexanone. ESR, UV- Vis and IR spectroscopic data of the catalytic system revealed that Cu(I) species in mixed valence state with Cu(II) and reduced form of heteropolyanion were involved in the reaction cycles, but not in the form of simple (PMo1204o) 4- anion. We proposed a catalytic reaction cycle which may elucidate the remarkably high catalytic activity of the Pd(NO3)2-CuSO4-H3PM012040 catalyst system. In the catalytic cycle, a kind of Cu-heteropolyanion interaction operates to facilitate Pd(0) oxidation, instead of direct Pd(0) oxidation by Cu(II) or heteropolyanion species alone.

Acknowledgements

This work was partially supported by MOST (South Korea).

References

[1] H.T. Pope, Heteropoly and Isopoly Oxometallates: Springer, New York, 1983. [2] V.W. Day and W.G. Klemperer, Science, 228 (1985) 533. [3] C.L. Hill and C.M. Prosser-McCartha, Coordination Chemistry Reviews, 143 (1995) 407. [4] I.V. Kozhevnikov and K.I. Matveev, Russ. Chem. Rev., 51 (1982) 1075. [5] I.V. Kozhevnikov and K.I. Matveev, Appl. Catal., 5 (1983) 135. [6] I.V. Kozhevnikov, Russ. Chem. Rev., 56 (1987) 1417. [7] N. Mizuno and M. Misono, J. Mol. Catal., 86 (1994) 319. [8] G.W. Parshall, S.D. Ittel, Homogeneous Catalysis. The Applications and Chemistry of Catalysis by Soluble

Transition Metal Complexes (2nd edn.), Wiley-Interscience, New York, 1992, pp. 138-142. [9] C.L. Hill and R.B. Brown, J. Am. Chem. Soc., 108 (1986) 536.

[10] M. Faraj and C.L. Hill, J. Chem. Soc. Chem. Commun., (1987) 1487. [11] D.E. Katsoulis and M.T. Pope, J. Chem. Soc. Dalton Trans., (1989) 1483. [12] R. Neumann and C. Abu-Gnim, J. Chem. Soc. Chem. Commun., (1989) 1324. [13] A. Selling, I. Andersson, L. Pettersson, C.M. Schramm, S.L. Downey and J.H. Grate, Inorg. Chem., 33

(1994) 3141.

26 Y Kim et aL /Applied Catalysis A: General 155 (1997) 15-26

[14] J.R. Grate, D.R. Harem and S. Mahajan, in M.T. Pope and A. Muller (Eds.), Polyoxometalates; From Platonic Solids to Anti-retroviral Activity, Kluwer, Dordrecht, The Netherlands, 1993, p. 281.

[15] K. Urabe, E Kimura and Y. Izumi, Proc. 7th Int. Congr. Catal., 1980, Kodansha Elsevier, Tokyo and Amsterdam, 1981, p. 1478.

[16] Y. Saito, M. Tsuzuki, H. Ishii, Patent, EP 0 498 305 A1(1992). [17] Y. Saito, M. Tsuzuki, Patent, EP 0 418 764 A2(1990). [18] J.E. Backvall and R.B. Hopkins, Tetrahedron Lett., 29 (1988) 2885. [19] H. Ogawa, H. Fujimann, K. Taya and S. Tertani, Bull. Chem. Soc. Jpn., 57 (1984) 1908. [20] J. Smidt, W. Hafner, R. Jira, J. Sedlmeier, R. Sieber, H. Kojer, Angew. Chem. 71 (1959) p. 176, 626. [21] P.M. Maitlis, The Organic Chemistry of Palladium, Vol. 2, Academic Press, New York, 1971, p. 76.. [22] H. Ogawa, H. Fujinami and K. Taya, J. Chem. Soc. Chem. Commun., (1981) 1274. [23] K. Katayama, T. Nakamura, M. Misono and Y. Yoneda, Chem. Lett., (1981) 89. [24] K. Eguchi, Y. Toyozawa, K. Furuta, N. Yamazoe and T. Seiyama, Chem. Lett., (1981) 1253. [25] M. Akimoto and E. Echigoya, Chem. Lea., (1981) 1759. [26] M.T. Pope, in D.B. Brown (Ed.), Mixed-valence Compounds, D. Reidel, Dordrecht, The Netherlands,

1980, p. 365. [27] C. Sanchez, J. Livage, J.P. Launary, M. Fournier and Y. Jeannin, J. Am. Chem. Soc., 104 (1982) 3194. [28] Y. Izumi, K. Urabe, M. Onaka, Zeolite, Clay, and Heteropoly Acid in Organic Reactions, Kodansha,

Tokyo, Japan, 1992, p. 105-111.