Hydrogenation of citral over Pd/alumina: comparison of “supercritical” CO2 and conventional solvents in continuous and batch reactors Original Research Article

Hydrogenation of citral (3,7-dimethyl-2,6-octadienal) over a 5 wt.% Pd/alumina catalyst was carried out in a continuous fixed bed reactor and a batch autoclave in the pressure range 30 to 90 bar. The catalytic performance in two kinds of solvents (dense carbon dioxide and conventional organic solvents) was compared. Pd was highly selective to the saturation of the CC bonds and afforded around 95% selectivity to the corresponding aldehydes {3,7-dimethyl-6-octenal (citronellal), 3,7-dimethyl-2-octenal, and 3,7-dimethyl-octanal (dihydrocitronellal)} at high conversion. The reaction rate and product distribution were strongly influenced by the type of reactor, solvent, total pressure, and feed composition. Striking changes in the rate of hydrogenation of the allylic and isolated CC bonds of citral in dense carbon dioxide could be well interpreted by the variations in the phase behavior analyzed in a high-pressure view cell. The rate of citral hydrogenation in carbon dioxide under single-phase conditions (TOF: 26 500 h−1) was about two orders of magnitude higher than in organic solvents.