(592c) Catalytic Hydropyrolysis of Rice Husk over a Hierarchical Micro-Mesoporous Composite Catalyst

Catalytic fast pyrolysis (CFP) has been widely used in research to convert biomass into liquid hydrocarbons. It consists of thermal decomposition of the organic molecules from biomass in inert atmosphere and in the presence of a heterogeneous catalyst. Unfortunately, there are critical problems in this traditional thermal cracking method, such as catalyst coking and low carbon efficiency, since large fractions of carbon initially present in the biomass end up in the coke and carbon-containing gases, resulting in a liquid phase product with relatively low hydrocarbons content. In contrast, catalyst hydropyrolysis (CFP) uses hydrogen as the reaction medium, which potentially may provide higher yields of aromatic hydrocarbons.

This work presents a comparison between the products from catalytic fast pyrolysis (CFP) and catalytic hydropyrolysis (CHP) of rice husk over a hierarchical catalyst. The results suggest that the addition of hydrogen inhibits polycondensation reactions, leading to a 2.3 wt. % reduction in coke yield at 400 °C and 35 bar. In addition, the presence of the hydrogen at these conditions increases the relative content of hydrocarbons in both liquid (by 16 %) and gas (by 2.7 %) products, suggesting that hydrodeoxygenation, decarbonylation, and decarboxylation reactions are promoted. The gaseous hydrocarbons promoted in the presence of hydrogen include CH₄ and C₂-C₃. In addition, we report the effect of catalyst characteristics on the products from catalytic hydropyrolysis of rice husk. Alkali-treatment of the HZSM-5 improves the catalytic activity and coking resistance of catalyst. The addition of an extra external MCM-41 layer to the alkali-treated HZSM-5 further improves the catalytic activity, resulting in a relative hydrocarbon content of 48.9%, which is higher than that obtained with alkali-treated HZSM-5 (36.8%) and pristine HZSM-5 (28.6%). We also report the effect of pressure on the products. As the pressure increases from 1 to 35 bar, the relative content of hydrocarbons increases from 39.9 % to 48.9 %, coke yield decreases from 3.4 wt. % to 1.9 wt. %, and CH₄ and C₂-C₃ yields decrease from 10.3 wt. % and 5.1 wt. % to 14.2 wt. % and 6.7 wt. %, respectively. As the temperature increased from 300 to 400°C, the relative content of hydrocarbons increased from 38.2% to 48.9%. Both CH₄ and C₂-C₃ yields reach the highest values of 14.2 wt. % and 6.7 wt. % at 400 ℃. The results from this work highlight the effect of the hydrogen atmosphere for the thermal decomposition of rice husk over the hierarchical catalyst.