Towards inhibition of yellowing of mechanical pulps. Part III. Hydrogenation of milled wood lignin

Oxygenation of hexadecacarbonylhexarhodium(0), Rh-6(CO)(16), in dimethylfor mamide (DMF) generates nn active catalyst, formulated as Rh-6(mu(3)-O)(4)(O H)(12) (DMF)(n), far the hydrogenation of the aromatic rings of milled wood lignin at similar to 20 degrees C under I atm H-2 in aqueous methyl cellos olve. Partial hydrogenation of the aromatic rings in lignin was confirmed b y H-1 NMR spectroscopy. The ability of the catalyst to hydrogenate the lign in model compound 2-methoxy-4-propylphenol is critically dependent on the r eaction medium: hydrogenation proceeds faster in aqueous ethanol than in aq ueous dioxane, aqueous methyl cellosolve or anhydrous ethanol while no hydr ogenation occurs in aqueous DMF.