Effect of reaction temperature on Mo/Al_2O_3 catalytic hydrodeoxygenation of alkali lignin bio-oils

Alkali lignin, an important source of industrial lignin, can be used for producing aromatic components in biofuels while retaining its aromatic ring structure through hydrothermal liquefaction technology. The temperature during the reaction process significantly influences the bio-oil yield and composition. This study investigates the impact of reaction temperature on the oil yield and aromatics yield in the preparation of bio-oil through hydrothermal liquefaction of alkali lignin catalyzed by Mo/Al_2O_3. The study′s findings revealed that the oil yield initially increased and then decreased with increasing temperature, reaching its highest value at 280 ℃. As the temperature increased, the O/C ratio of the bio-oil continuously decreased to a minimum of 0.22. In contrast, the H/C ratio steadily increased to a maximum of 1.14. Additionally, the high calorific value steadily increased to a maximum of 30.41 MJ/kg. These results indicated that higher temperatures contributed to improving the fuel properties of the bio-oil. However, increasing the temperature was not conducive to the enrichment of aromatic components in the bio-oil. The higher the temperature, the lower the relative content of aromatic compounds and the higher the content of phenolic compounds in the bio-oil. The study observed that the relative content of aromatics reached its peak value of 34.1% at 260 ℃ when 15% Mo/Al_2O_3 was added as a catalyst. At this temperature, the selectivity to toluene in the aromatics product was as high as 97.7%, indicating the optimal enrichment of aromatics. In summary, the study demonstrates that the temperature during the hydrothermal liquefaction process of alkali lignin affects both the bio-oil yield and composition. While higher temperatures enhance the oil yield and fuel properties of the bio-oil, they also reduce the relative content of aromatic compounds. The addition of Mo/Al_2O_3 as a catalyst at an appropriate temperature can promote the enrichment of aromatics, where toluene is the dominant product.