Hydrolytic hydrogenation of cellulose with hydrotreated caesium salts of heteropoly acids and Ru/C

We recently demonstrated excellent yields of hexitols for the reductive splitting of highly concentrated cellulose mixtures using a combination of commercially available heteropoly acids, viz. H3PW12O40 (H3PW) and H4SiW12O40 (H4SiW), and Ru on carbon (Ru/C) (J. Geboers et al., Chem. Commun., 2010, 46, 3577-3579). Despite their excellent catalytic performance, recovery and reuse of these soluble acid catalysts remains challenging. This contribution evaluates the potential of the so-called insoluble non-stoichiometric caesium salts of HPAs, viz. CsPW and CsSiW, in combination with Ru/C for the conversion of cellulose. As-synthesized CsHPA salts are shown to be even more active than native HPAs and their hydrolytic activity is improved even further by increasing the calcination temperature during synthesis, followed by a hydrotreating procedure. Reactions with CsSiW are highly selective due to suppression of hydrogenolysis side reactions: yields of up to 90% hexitols under mild reaction conditions are achieved using ball-milled avicel cellulose. Whereas recovery of commercial HPAs was not successful, CsPW salts were fully retained after reaction at 170 degrees C by simple recrystallization at room temperature without the need of an organic solvent.