SELECTIVE LIQUID-PHASE HYDROGENATION
53
nied by side reactions which, together with steric hindrance,
decrease the rate at which the substrate can access active
sites.
The hydrogenation of the E (trans) isomer is favored over
that of the Z (cis) isomer as a result of the adsorption of the
latter on the catalyst surface being sterically hindered.
The presence of Lewis acid additives such as FeCl2 in-
duces an electron transfer between the two metals (Fe
and Pd) arising from their differential electronegativity;
==
the effect favours adsorption of citral via the C O group
and, hence, increases the selectivity towards geraniol and
nerol.
ACKNOWLEDGMENTS
The authors gratefully acknowledge funding of this research by the
Consejer´ıa de Educacio´n y Ciencia de la Junta de Andalucı´a and the Di-
reccio´n General de Investigacio´n Cientı´fica y Te´cnica (DGICyT) in the
framework of Project PB92-0816.
REFERENCES
1. Zorau, A., Sasson, Y., and Blum, J., J. Mol. Catal. 26, 321 (1984).
2. Vannice, M. A., and Sen, B., J. Catal. 115, 65 (1989).
3. Hutchings, G. J., King, F., Okoye, I. P., and Rochester, C. H., Appl.
Catal. 83, L7–L13 (1992).
4. Ueshima, M., and Shimasaki, Y., Catal. Lett. 15, 405 (1992).
5. Giroir-Fendler, A., Richard, D., and Gallezot, P., Catal. Lett. 5, 175
(1990).
6. Blackmond, D. G., Oukaci, R., Blanc, B., and Gallezot, P., J. Catal.
131, 401 (1991).
7. Poltarzewki, Z., Galvagno, S., Pietropaolo, R., and Staiti, P., J. Catal.
102, 190 (1986).
8. Touroude, R., J. Catal. 65, 110 (1980).
9. Padley, M. D., Rochester, C. H., Hutchings, G. J., Okoye, I. P., and
King, F., in “Proc. 10th Int. Congr. Catal., Budapest, 1992,” p. 329.
10. Waghray, A., Wang, J., Okadi, R., and Blackmond, D. G., J. Phys.
Chem. 96, 5954 (1992).
11. Ravasio, N., Antenori, M., Gargano, M., and Mastrorilli, P., Tetrahe-
dron Lett. 37, 3529 (1996).
12. Galvagno, S., Milone, C., Donato, A., Neri, G., and Pietropaolo, R.,
Catal. Lett. 17, 55 (1993).
13. Richard, D., Ockelford, J., Giroir-Fendler, A., and Gallezot, P., Catal.
Lett. 3, 53 (1989).
14. Schro¨der, U., and De Verdier, L., J. Catal. 142, 490 (1993).
15. Nagase, Y., Hattori, H., and Tanabe, K., Chem. Lett., 1615 (1983).
16. Poltarzewski, Z., Galvagno, S., Pietropaolo, R., and Staiti, P., J. Catal.
102, 190 (1986).
17. Noller, H., and Lin, W. M., J. Catal. 85, 25 (1984).
18. Chen, Y. Z., and Wei, S. W., Appl. Catal. 99, 85 (1996).
19. Johnstone, R. A. W., Wilby, A. H., and Entwisth, I. D., Chem. Rev. 85,
129 (1985).
20. Makouangou, R., Dauscher, A., and Touroude, R., in “Proc. 10th Int.
Congr. Catal., Budapest, 1992,” p. 2475.
FIG. 7. Selectivity versus conversion profiles for the catalysts Pd3/
PS400 (a), Pd3/PM2 (b), and Pd3/PM2 with FeCl2 (relation molar Fe2+/Pd =
1) (c). Reaction conditions: amount of catalyst (Pd3/PM2), 50 mg; solvent,
dioxane; temperature, 303 K; citral concentration, 0.5 M (ꢀ, citronelal;
᭝, dihydrocitronelal; ᭜, citronelol; ᭡, unsaturated alcohol (geraniol +
nerol)).
citronelal at any conversion and the catalyst doped with
FeCl2 the most selective towards the unsaturated alcohol
(geraniol + nerol).
CONCLUSIONS
The reduction of citral with a single-metal Pd catalyst
yields the saturated aldehyde; the selectivity towards cit-
ronelal is especially high at low pressures and temperatures.
The solvent has a marked effect on the reduction rate; thus,
nonpolar solvents lead to greater reduction rates. The use
of alcohols gives rise to the formation of acetals between
the solvent and citral, the reduction process being accompa-
21. Grass, F., Grosselin, J. M., and Mernier, C., in “Proc. 3rd Int. Symp.,
Poitiers, 1993,” p. 129.
22. Court, J., Janati-Idrissi, F., Vidal, S., and Wierzchowski, P. T., J. Chim.
Phys. 87, 379 (1990).
23. Koros, R. M., and Novak, E. J., Chem. Eng. Sci. 22, 470 (1967).
24. Madon, R. J., and Boudart, M., Ind. Eng. Chem. Fund. 21, 438 (1982).
25. Aramend´ıa, M. A., Borau, V., Jime´nez, C., Marinas, J. M., and Rodero,
F., Colloids Surf. 8, 599 (1984).