Rhodium nanoparticles embedded on the interior of hollow porous carbon nanospheres, able to sieve monomers from polymers, were used to confirm the precise role of metal catalysts in the reductive catalytic fractionation of lignin. The study provides clear evidence that the primary function of the metal catalyst is to hydrogenate monomeric lignin fragments into more stable forms following a solvent-based fractionation and fragmentation of lignin.