Hydrodeoxygenation of ethyl stearate over Re-promoted Ru/TiO2 catalyst: Rate enhancement and selectivity control by the addition of Re
The hydrodeoxygenation of a model compound, ethyl stearate, of biomass-derived resources into long chain alkanes was investigated with Re-modified Ru/TiO2 catalysts. A monometallic Ru(1)/TiO2 catalyst, in which Ru was loaded in 1 wt %, gave a selectivity of > 95% to the long chain alkanes of n-octadecane and n-heptadecane at almost 100% conversion at 220 °C, while another monometallic Re(10)/TiO2 catalyst was less active in spite of the larger metal loading (10 wt %). It is interesting that the addition of less active Re species to Ru(1)/TiO2 enhanced the rate of the hydrodeoxygenation and increased the ratio of n-octadecane/n-heptadecane while keeping their high total selectivity unchanged. The alkane ratio depended on the content of Re added, increasing significantly from 0.46 for Ru(1)/TiO2 through 3.2 for Ru(1)Re(10)/TiO2. The impacts of the Re addition on the catalytic performance of Ru/TiO2 were discussed on the basis of physicochemical properties characterized for Re-modified and -unmodified Ru/TiO2 catalysts. It is suggested that the improved hydrodeoxygenation activity of Re-modified catalysts depends on the presence of Lewis acid sites, and the weak acid sites are important for determining catalytic selectivity. In addition, reaction pathways in the hydrodeoxygenation of ethyl stearate to n-octadecane and n-heptadecane were examined by control reactions with possible intermediates of 1-octadecanol and stearic acid. The Re-modified Ru/TiO2 catalyst was shown to be recyclable during repeated runs after simple separation (filtration) without other post-treatments.