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 propargyl ic, alcohol, dehydroisophytol, was studied in a continuous-flow reactor ove r amorphous Pd81Si19, employing dense carbon dioxide as solvent. All organi c modifiers were able to improve the selectivity, however, at the expense o f a lowered conversion. The extent of the effect depended strongly on the n umber of heteroatoms and on the specific structure of the modifier applied. For maximum isophytol yield, a compromise between high selectivity and rea sonable conversion proved to be beneficial, with propyl disulfide and isopr opyl disulfide representing the best choices under the applied conditions. Apart from choosing the appropriate modifier, selectivity could also be tun ed by adjusting the modifier concentration. Our observations are consistent with a model including competitive adsorption of reactants and the modifie r. Different effects of structural isomers imply that, at least partly, ste ric effects are involved. Compared to the organic modifiers, the use of a l ead-modified catalyst could hardly improve the selectivity at a given conve rsion under the applied conditions.