Continuous Semihydrogenation of a Propargylic Alcohol over Amorphous Pd81Si19 in Dense Carbon Dioxide: Effect of Modifiers

The influence of chemical modifiers on the semihydrogenation of a propargylic alcohol, dehydroisophytol, was studied in a continuous-flow reactor over amorphous Pd81Si19, employing dense carbon dioxide as solvent. All organic modifiers were able to improve the selectivity, however, at the expense of a lowered conversion. The extent of the effect depended strongly on the number of heteroatoms and on the specific structure of the modifier applied. For maximum isophytol yield, a compromise between high selectivity and reasonable conversion proved to be beneficial, with propyl disulfide and isopropyl disulfide representing the best choices under the applied conditions. Apart from choosing the appropriate modifier, selectivity could also be tuned by adjusting the modifier concentration. Our observations are consistent with a model including competitive adsorption of reactants and the modifier. Different effects of structural isomers imply that, at least partly, steric effects are involved. Compared to the organic modifiers, the use of a lead-modified catalyst could hardly improve the selectivity at a given conversion under the applied conditions.