Study On Selective Hydrogenation Of Isophorone Catalyzed By Palladium Nanoparticles

Isophorone(IP)is a kind of prochiral compounds,with conjugated unsaturated ketone in molecular structure.The product of selective hydrogenation of C=C bond is 3,3,5-trimethyl cyclohexanone(TMCH),which is a colorless,low viscosity solvent and an important fine chemical intermediates,widely usded in pesticide,paint,medicine and other fields.The chiral enantiomer from the asymmetric hydrogenation of IP or the resolution of the racemic TMCH is the intermediate of cyclandelate and hormone,which is of great research and economic value.In this paper,the selective hydrogenation of isophorone and the kinetic resolution of TMCH were studied,furthermore,the mechanism of hydrogenation was preliminarily discussed.The palladium nanoparticles were reduced and stabilized in the presence of poly(ethylene glycol)(PEG),a green solvent,in the selective hydrogenation of isophorone.The influences of reduction temperature,polymerization degree of PEG,types of solvents,reaction conditions on the catalytic effects were evaluated in autoclave reactor.The results showed that the chemical reduction method is a kind of green synthesis method,the palladium nanoparticles could be effectively stabilized by the long-chains in PEG,with small averge particle size distribution of 3 nm.Besides,The palladium nanoparticles stabilized by PEG showed good catalytic effect in the water-oil biphase system in the selective hydrogenation of isophorone,the optimized conversion of IP was 93.07%,and the selectivity of TMCH could reach up to 99.64%.The asymmetric hydrogenation of IP and the kinetic resolution of racemic TMCH were studied in the presence of natural S-proline as a chiral modifier with the palladium nanoparticles prepared by the ultrasonic chemical reduction.The results show that the ultrasonic chemical reduction method is a highly efficient preparation method for nano-catalyst.Different preparation method of chiral catalyst were compared,when the nano palladium and chiral modifier dispersed in the ethanol solution system directly,the catalytic effect was good.The conversion of IP was as high as 99.59% in the asymmetric hydrogenation,the ee % of product can reach up to 93.07%,besides,the ee % was 91.03% in the kinetic resolution of TMCH.The mechanism of asymmetric hydrogenation of IP was preliminarily discussed,the results show that the enanitomer configurations is not only under the kinetic control of the hydrogenation of the complex,but also associated with the particle size of palladium catalyst;the stereodifferentiation not merely arises from the initial homogeneous reaction,but also relates to the surface of the catalyst.The adsorption of proline produces a chiral environment in the catalyst surface is open to discussion.In fact,proline is an reactant,and it takes part in the dehydration reaction in stoichiometric ratio,then the reaction route of S-TMCH and R-TMCH will be changed and the kinetic resolution of racemic TMCH will come true.