Experimental evaluation on the ionic liquid BMIMCl and H₅PV₂Mo₁₀O₄₀ dual function catalytic degradating wheat straw alkali lignin

The H₅PV₂Mo₁₀O₄₀ heteropoly acid was prepared by the conventional methods and was characterized with FT-IR, XRD and SEM. InH₅PV₂Mo₁₀O₄₀+BMIMCl dual function catalytic system and BMIMCl single function catalytic system, the effects of reaction time, reaction temperature and the amount of H₅PV₂Mo₁₀O₄₀ on the lignin degradation process were studied, and the degradation mechanism was also discussed preliminarily. The results indicated that the contents of P, V, Mo in the synthesis H₅PV₂Mo₁₀O₄₀ heteropoly acid accorded with the relationship of 1:2:10, and the H₅PV₂Mo₁₀O₄₀ heteropoly acid had the typical Keggin structure.Under the optimal reaction condition, i.e., 140 ℃, 3 hours, the ionic liquids were used as the catalysts, the contents of total hydroxyl of alkali lignin increased from 3.49 to 10.92 mol/kg after reaction, and the mass fraction of vanillin and syringaldehyde were 2.00% and 2.15% in liquid products, respectively. When 20% H₅PV₂Mo₁₀O₄₀ was added, the optimal reaction time and temperature were 6 hour and 120 ℃, and the total hydroxyl increased from 3.49 to 14.12 mol/kg in solid degradation products. The molar mass reduced from 7.691 to 1.589 kg/mol. Only vanillin was found in liquid products, and its mass fraction was 6.97%. It was believed that H⁺ attacked oxygen atom on the β-O-4 bond in lignin firstly when BMIMCl catalyzed lignin alone. At the same time, the positive charge was transferred to the β position, and then the oxygen atom in the water molecule attacked the β position which had positive charge. As a result, catalyst H⁺ was eliminated and β-O-4 bond generated guaiacol due to the hydrolytic cleavage. Then the side chain generated vanillin and syringaldehyde by the oxidation reaction. When H₅PV₂Mo₁₀O₄₀ was added, PV₂Mo₁₀O₄₀^5- released VO₂⁺ and its valencestate was changed from V(V) to V(IV). The single electron oxidation model compounds were obtained and the lignin was demethylated. It was thought that the lignin demethoxy group was changed from lignin-type lignin to guaiacol-type lignin, not only without any clove aldehydes were found in the liquid product but also the contents of vanillin were increased.