Study On Hydrodeoxygenation Of Vanillin Over Molybdenum Carbide Based Catalyst

Nowadays, with the exhausting of petroleum reserves and the arising world-wide environmental concerns, it is urgent to search for renewable feedstocks as a replacement for traditional fossil fuels to meet the huge need in the future. Attributed to their amity for environment, world-wide abundance and renewability within a relatively short cycle, bio-oils, as a prospective resource for replacing fossil fuels have attracted more and more attention. Vanillin, as one typical aldehydes compound of pyrolysis oil derived from the lignin fraction, was suggested to be a good candidate for making evaluation of bio-oil upgrade. Hydrodeoxygenation (HDO) as a common hydrotreating method has been proven to be a more promising route for producing economic product by converting O-containing wastes to usable products. Overall the catalysts used in the HDO of bio-oil, Mo2C-based catalysts had been extensively studied for its platinum-like characterization. Due to its ability of activating hydrogen, many researches have pointed out that Mo2C-based catalysts performed excellent catalytic activity in many reactions involved of hydrogen, such as hydrogenation, hydrodesulfurization, hydrocarbon isomerization, methane reforming and so on. Additionally, except for the inexpensive cost, Mo2C-based catalysts possesses the performance of higher coke resistance, selectivity and chemical stability than noble metal catalysts.In the present dissertation, we investigated the highly selective HDO of vanillin over the Mo-based catalysts prepared at different temperature and loaded on different supporters through the exploration of their preparations, characterizations, performance tests and reaction mechanisms. The main conclusions drawn from the work are provided as follows:(1) Loaded on active carbon (AC), nanostructured molybdenum-based catalysts prepared by "carbothermal hydrogen reduction"(CHR) method possessed high specific surface area and dispersed well on the support. The phases on the prepared catalyst were determined by the reductive temperature, while the particle size, pore size and dispersity were depended on the loading. With water as solvent, the optimal reaction conditions catalyzed by Mo2C/AC catalyst were as follow:with catalyst loading of20.0%, at120℃,1.0MPa of H2for3h. Attributing to be recycled for several times with slight loss of activity, the Mo2C/AC catalyst possessed a promising prospect in being used for up-grading bio-oil. Compared with the results catalyzed by Pd/CN and Ru/CNTs in the present researches, the Mo2C/AC catalyst had the advantages in mild reaction conditions and high selectivity to C=O.(2) According to the curves of the concentration of vanillin and products with time, a HDO mechanism involved vanillyl alcohol as intermediate product was proposed. Obtained from the curves of the concentration of vanillin and vanillyl alcohol with time, the reaction rate constants of the first and second step were0.49h-1and0.43h-1, respectively, obviously, the second step of HDO performed as speed control step.(3) Doping of Al2O3to the catalyst could increase the acidity, and lower the specific surface area and dispersity at the same time. The additive of protonic acid will contribute to the activation of vanillin and vanillyl alcohol, which will lead to the reduction of the amount activated by catalyst, as a result, the conversion of vanillin would decrease and instead of the increase of the selectivity of p-creosol. Adding strong acidoid or material possessed pore structure to this HDO systems would induce the competitive adsorption between catalyst and additive, then the relative activity of the catalyst would decrease. The activity was related to the specific surface area, dispersity, acidity, relative acidity to additive and so on.(4) Catalyzed by the Mo2C catalyst supported on carbon nano-tubes (CNTs) and graphite (G) which were constituted by the sp2conjugated structure, the catalytic results of vanillin HDO showed that the support played an important role for the activity. The specific surface area and dispersity were both decided by the supported. The oxide of molybdenum obtained in the process of passivating was showed to be significant for HDO, as it can activate vanillin and donate proton to the activated reagent, which was attributed to its strong Lewis acid sites and Br(?)nsted acid hydroxyl sites. As a promising catalyst to replace noble catalyst, Mo2C-based has a promising prospect. The results of this research were significant for the conversion of bio-based chemicals and the up-grade of bio-oil.