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HYDROGENATION OF 4-OXOISOPHORONE IN SUPERCRITICAL CO2 MEDIUM OVER ALUMINA SUPPORTED PALLADIUM CATALYST
Citation:
Pillai**, U R. AND E SahleDemessie*. HYDROGENATION OF 4-OXOISOPHORONE IN SUPERCRITICAL CO2 MEDIUM OVER ALUMINA SUPPORTED PALLADIUM CATALYST. Presented at AIChE, San Francisco, CA, 11/16-21/2003.
Description:
Ketoisophorone or 4-oxoisophorone (2,6,6-trimethyl-cyclohex-2-ene-1,4-dione) is a cyclic enedione of tremendous industrial importance due to its use as a key intermediate in the synthesis of various carotenoids and flavoring agents. The catalytic hydrogenation and enzymatic reduction of this compound lead to different products some of which are of immense industrial importance. For example, (S)-stereo isomer of 4-hydroxy isophorone, a volatile constituent of saffron, is an intermediate in the synthesis of pharmaceuticals, natural pigments and flavoring agents. Earlier studies on the selective hydrogenation of a,b-unsaturated aldehydes and ketones are focused on molecules such as crotonaldehyde, cinnamaldehyde and citral. Solvent effects on the selectivity of a reaction are well known eventhough there is only a limited understanding about the mechanism. Variation of the dielectric constants of the solvents affects the product selectivity and non-polar solvents tend to prefer the hydrogenation of C=O bond than that of C=C. Supercritical carbon dioxide (sc-CO2), which is aprotic in nature, could be beneficial for hydrogenations. This paper presents the details of 4-oxoisophorone hydrogenation over 1 % Pd/Al2O3 impregnated catalysts under sc-CO2 medium. The effects of temperature, pressure and reaction medium on the conversion and product selectivity are discussed. Phase behavior studies have also been carried out to determine the conditions at which a supercritical phase is formed as a function of various experimental parameters as well as the miscibility behavior of different reactant mixtures. A comparison of the supercritical CO2 medium reaction with the conventional liquid phase hydrogenation in organic solvents as well as CO2-organic co-solvents have also been made. The reaction rate in supercritical CO2 medium is found to be comparable to that in commonly employed organic solvents. Reaction in supercritical CO2 has resulted in product selectivities different from liquid phase reactions and much lower catalyst deactivation. The reaction mechanism leading to different products is discussed