Abstract:
Nowadays, biomass conversion has become a more attractive process to produce fuels and various high-value chemicals from biological materials. From many kinds of catalysts, the metal-supported gamma-alumina (γ-Al2O3) is a widely used catalyst for various transformation processes, especially in the aqueous phase reaction. Typically, coke formation on γ-Al2O3 causes catalyst deactivation in the thermal reaction process. Moreover, incorporation of aqueous phase medium initiates hydroxylation of γ-Al2O3 and phase transformation of γ-AlOOH, leading to the catalyst deactivation. Not only coke formation and phase transformation are the cause of catalytic deactivation, but the oxygen vacancy, changing the activity of the catalyst, should also be the cause of the deactivation of γ-Al2O3. To understand the interplay between coke formation, oxygen vacancy, and phase transformation, the coke formation on the γ-Al2O3 catalyst has been theoretically investigated using density functional theory (DFT). Starting with the coke formation on the γ-Al2O3 surface, the coke is strongly chemisorbed, and the forming higher coke prefers a cyclic form to the aliphatic on the γ-Al2O3 surface. Afterward, the effects of partial hydroxylation on γ-Al2O3 forming OH/γ-Al2O3 and full hydroxylation until phase transformation forming γ-AlOOH on coke formation were also investigated. Hydroxylation of the γ-Al2O3 surface can suppress the interaction between coke and the surface. However, the covered surface with the hydroxyl group promotes coke polymerization, as evidenced by the relative formation energy. In addition, the effect of oxygen vacancy on coke formation was investigated. The presence of oxygen vacancy has a remarkable impact on the coke formation and makes the elimination more difficult. Moreover, the thermal effect was also investigated at 373 to 773 K. The increasing temperature diminishes the adsorption strength, leading to the weak interaction between coke and the surface. Finally, the coke dimerization has been explored to predict the feasibility of coke evolution on the γ-Al2O3, demonstrating that the dimerization is spontaneous and can occur with the energy barrier of 1.89 eV.
