Production Of Monophenolic Compounds By Oxidative Degradation Of Alkaline Lignin

The production of fuels or chemicals from renewable biomass has become a research hotspot in recent decades due to increasing concerns on the world energy crisis and the environmental pollution from fossil fuel burning. Lignin is the largest renewable natural aromatic macromolecule composed of three main phenylpropane units, namely guaiacyl, syringyl, and p-hydroxyphenyl, which are linked together through ether bond and C–C bond. Accordingly, monophenolic, dimers or oligomic aromatics can be obtained via lignin depolymerization and it has been of great significance in promoting the high efficient utilization of lignin.In this work, the oxidative degradation of lignin for producing monophenols was carried out with H2O2 as oxidant under alkaline condition. The experimental results show that the yield of monophenols can be improved with the addition of copper ferrite, the optimized process variables for the oxidative degradation are as follows: 0.3 g of alkaline lignin, 20.0 m L of 2.0 mol/L Na OH solution(the solvent is consisted of 18 m L H2 O and 2 m L THF), 2.0 m L of H2O2, and 0.06 g of Cu Fe2O4 were run the depolymerized at 150℃ for 60 min. Under such conditions, the yield of monophenols and the degradation rate of lignin reach 15.40% and 79.6%, respectively.Modified HZSM-5 catalysts were prepared by using ion-exchange method under microwave irradiation and their impacts on the depolymerization of lignin were also studied. It is found that the modified HZSM-5 as a catalyst can obtain a higher yield of monophenols compared with HZSM-5. When the ion-exchange concentration of Cu2+ is 0.5 mol?L-1, and the La3+/Ce3+ is 0.2 mol?L-1, Cu/HZSM-5, Cu-La/HZSM-5 and Cu-Ce/HZSM-5 have better catalytic properties, and the corresponding yields of monophenols are 14.63%, 15.69% and 15.73%, respectively. Zeolite samples were investigated by using XRD, IR and BET, and the results demonstrate that ion exchange does not change the zeolite crystal structure, but it improves the acidity of Lewis and decreases the BET surface area.The degradation products and oxidation residues were characterized by using 1H-NMR, HRMS, IR and elemental analysis. The results show that the linkages of ether bond and C-C bond among the three basic structural units of lignin are destroyed after the oxidation degradation. The most of the degradation products are monomers and dimers with the molecular weight ranging of 140-500, and the oxidation residues are incompletely degraded lignin with a higher C/H ratio, indicating that partial dehydration and carbonization take place in the lignin oxidation process.