Kinetics of polyoxometalate (POM) catalyzed hydrolysis of hemicelluloses using (Mo-V-P)-heteropolyacids - Model...

Polyoxometalates (POM), as efficient bifunctional (redox and acid) catalysts, have great potential for biorefining technologies of lignocellulosic biomass. To evaluate the catalytic (acidic) activity of POMs in the conversion of hemicelluloses, the kinetics of POM catalyzed hydrolysis reaction of model commercial birch xylan in much dilute (4 mM) aqueous solutions of mixed-addenda α-Keggin-type (Mo-V-P)-heteropolyacids of series H_(n+3)[PMo_(12-n)V_nO₄₀] or HPA-n, such as HPA-3 and HPA-4, has been studied and compared with all-molybdenum phosphomolybdic acid H₃PMo₁₂O₄₀ (PMA, or HPA-0) and conventional sulfuric acid catalysis. The structural (kinetic) heterogeneity of xylan was accurately assessed using an original kinetic approach based on properties of multi-component reaction systems. In the tested range of 120-140 °C, two kinetically homogeneous xylan fractions (Xyl₁ and Xyl₂) of distinct reactivity (k₁/k₂ of ca. 9) were precisely quantified in proportion of ca. 46.8 % Xyl₁ and 53.3 % Xyl₂ (±0.37 %), supporting the two-phase kinetic model. At equal medium acidity, the catalytic activity of HPAs increased moderately in the order of HPA-0 (PMA) > HPA-3 > HPA-4 and was always substantially higher than the sulfuric acid (ca. 2.4 and 6.6 times, respectively for the Xyl₁ and Xyl₂ fractions). Depending on the HPA species, the apparent activation energy was estimated to be 88.2-95.8 kJ mol^-1 and 101.6-108.2 kJ mol^-1 for the Xyl₁ and Xyl₂ fractions, respectively, compared to 113.5 kJ mol^-1 and 124.8 kJ mol^-1 for sulfuric acid. An extremely high reproducibility of experimental data on xylan hydrolysis by modeled data using the developed new kinetic description was demonstrated.