Nature of catalyst deactivation during citral hydrogenation: a catalytic and ATR-IR study

Deactivation of a 5 wt% Pd/Al2O3 catalyst during hydrogenation of citral (1) to citronellal (2), 3,7-dimethyl-2-octenal (3), and dihydrocitronellal (4) has been studied in a continuous-flow fixed-bed reactor. The reactions were carried out at 40 °C and 190 bar in hexane, “supercritical” CO2, or ethane as solvents. ATR-IR spectroscopic analysis of the solid/liquid interphase under reaction conditions at low pressure revealed that 1 and 3 decarbonylate on the Pd surface resulting in strongly adsorbed CO and image-type hydrocarbon fragments. The absence of decarbonylation products from 2 and 4 and the excellent chemoselectivity for the hydrogenation of the Cdouble bond; length as m-dashC bonds are interpreted by adsorption of the molecules in a tilted position on Pd, π-bonded via one Cdouble bond; length as m-dashC bond (2) or di-π-bonded via the Cdouble bond; length as m-dashCsingle bondCdouble bond; length as m-dashO fragment (1 and 3). A practically important observation is that a short reoxidation by air of the used Pd/Al2O3 under very mild conditions (40 °C, 6 bar) can circumvent further deactivation, but the original activity cannot be regained. All these observations indicate that the irreversible deactivation of Pd/Al2O3 cannot be traced to CO poisoning. We propose that site blocking caused by heavier oligomeric surface products is the major reason for the observed catalyst deactivation.