Experimental Study On Catalytic Hydrogenation Of Model Compounds For Microalgae-based Bio-oil

The hydro-upgrading catalysts were optimized for hydrotreating of stearic acid and hexadecanamide, which were respectively selected as the oxygen-containing and nitrogen-containing model compounds for microalgae-based bio-oil, and the effect of catalytic deoxygenation and denitrification for upgrading of microalgae-based bio-oils was also investigated.A series of Mo-based catalysts supported on CNTs were prepared by controlling the reduction temperature of catalyst synthesis. The properties of the obtained catalysts were systematically characterized by XRD, TEM, N2 adsorption,XPS and NH3/H2-TPD techniques, and the catalytic activity for the hydrodeoxygenation of stearic acid was evaluated. The results showed that Mo species on the surface of catalyst were reduced with the increase of reduction temperature, and the reduction process was as followed: MoO3â†'MoO2â†'Moâ†'Mo2C. The active phase of MoO2 had been found when reduction temperature is below 550 oC; further elevating the reduction temperature to 600 oC, a mixed phase of MoO2/Mo/β-Mo2 C existed. β-Mo2 C was main active phase when reduction temperature was over 650 oC, and it exhibited higher activity of hydrodeoxygenation compared to the active phase of MoO2 catalysts. In addition, the MoO2/Mo/β-Mo2 C mixed phase catalyst of 600 oC showed the highest catalytic activity in hydrodeoxygenation(HDO) of stearic acid due to its large specific surface area, more acid sites and stronger H2 adsorption ability.The optimizing reaction condition and reaction mechanism for deoxygenation of stearic acid were explored over β-Mo2C/CNTs catalyst, which exhibited higher activity in the screening process of catalyst, and the results indicated that the optimized reaction conditions were as followed: reaction temperature of 180 oC, reaction pressure of 4 Mpa, reaction time of 3 h and ratio of reactant to catalyst of 2:1. The conversion of stearic acid reached the maximum of 100%, and the main product was octadecane with the highest yield of 91.24% under the optimal reaction conditions, which showed that the β-Mo2C/CNTs exhibited higher activity at low temperature and favored the pathway of hydrogenation-dehydration-hydrogenation with keeping the economy of carbon atoms. The reaction mechanism of HDO of stearic acid over β-Mo2C/CNTs catalyst was as followed: the reactant of stearic acid(R-COOH) firstly converted into octadecanal(R-CHO) by catalytic hydrogenation, then the intermediate of octadecanal(R-CHO) followed by either decarbonylation to heptadecane and carbon monoxide(minor route) or hydrogenation to another intermediate octadecanol(R-CH2OH). Subsequently, the obtained octadecanol(R-CH2OH) underwent sequential hydrogenation-dehydrationhydrogenation to obtain the final product of octodecane(major route). In addition, the equilibrium between octadecanal and octadecanol was existed. The recycle tests demonstrated that the β-Mo2C/CNTs exhibited excellent stability, and can be reused for seven times consecutively without reduction of catalytic stability. When the cycle increased to the twelfth, the catalytic stability of β-Mo2C/CNTs was obviously decreased. The structural destruction of CNTs under acidic condition is the main reason for reduction of catalytic stability by the XRD and TEM analysis.In the hydrotreatment of stearic acid and hexadecanamide over β-Mo2C/CNTs catalyst, the effect of reaction tempreture on conversion of stearic acid and hexadecanamide and yields of products were investigated, and the results showed β-Mo2C/CNTs was a potential alternative catalyst to effectively convert heteroatomic model compounds into n-C15~C18 alkanes under a mild condition(≤ 200 oC), which opened a new approach to upgrade microalgae-based bio-oils into high-grade transportation bio-fuels.