Highly efficient hydrolysis of plant hemicelluloses by mixed-addenda Keggin-type (Mo-V-P)-heteropolyacids in diluted aqueous solution.

Polyoxometalate (POM) catalyzed hydrolysis of wood and agro-crop hemicelluloses by mixed-addenda molybdic α-Keggin-type heteropolyacids (HPAs), such as molybdovanadophosphate heteropolyacids (Mo-V-P)-HPAs of series H(n+3)[PMo(12-n)VnO40] with n = 3 and 4 (so-called HPA-3 and HPA-4), have been assessed in comparison with conventional sulfuric acid catalysis. In very diluted (20 mM) aqueous solutions of (Mo-V-P)-HPAs, up to 90% of total biomass xylan was readily converted into soluble sugars (mono and oligosaccharides) after 60 min hydrolysis reaction at 130 °C, vs only ca. 25% conversion (as a max.) for sulfuric acid solutions. Low cellulose degradation in (Mo-V-P)-HPA solutions resulted in a significant improvement in the selectivity of hydrolysis (by up to 70%) compared with sulfuric acid. The oxidative capacity of (Mo-V-P)-HPAs caused increased dissolution of lignin during the hydrolysis reaction (by up to 13% over the sulfuric acid), thus providing favorable conditions for the sustainable recovery of cellulose fibers from the xylan-free residues in subsequent biorefining stages.