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 °C by simple recrystallization at room temperature without the need of an organic solvent.
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