Towards inhibition of yellowing of mechanical pulps .2. Water-soluble catalysts for the hydrogenation of lignin model compounds

Water-soluble, polymer-stabilized, colloidal rhodium is an effective catalyst for the hydrogenation of the aromatic rings of the lignin model compounds 4-propylphenol, 2-methoxy-4-propylphenol, and 1,2-dimethoxy-4-propylbenzene under extremely mild conditions (25 degrees C, 1 atm H-2) in a one-phase, aqueous ethanol medium. The water-soluble catalyst is prepared by reducing rhodium trichloride trihydrate with ethanol in the presence of polyvinylpyrrolidone (PVP) and triethylamine, The reactivity of various chromophore precursors in lignin toward hydrogenation catalyzed by water-soluble, colloidal rhodium follows the descending order of carbon-carbon double bond > aromatic ring in 4-propylphenol > carbonyl group and the aromatic rings in 2-methoxy-4-propylphenol and 1,2-dimethoxy-4-propylbenzene, The hydrogenation of 2-methoxy-4-propylphenol and 1,2-dimethoxy-4-propylbenzene is accompanied by some hydrogenolysis and removal of the OMe group.