There is growing interest in the valorization of waste organic materials through hydrothermal processing facilitated by catalysts. Significant quantities of waste lipids (e.g., fats, oils, grease residuals) are produced each year, which are often disposed or down-cycled to low value energy products like biogas. Here, we report the conversion of fatty acids and lipids mainly to green diesel (C7 – C18 n-alkanes), a mixture of diesel-like liquid hydrocarbon as fuel, over Pt/C, Rh/C, Pd/C, and Ru/C (nominal 5 wt% metal) under subcritical hydrothermal conditions (330 C, 1 MPa) without addition of external hydrogen gas. Initially, supported metal catalysts were screened using stearic acid as a model/representative fatty acid, and the observed catalytic activity of catalyst follows the order of Pt/C &gt; Ru/C &gt; Pd/C &gt; Rh/C. Application of Ru/C was then examined in detail because of its relatively higher activity and lower cost than the other catalysts. Complete conversion of stearic acid to hydrocarbons over Ru/C was achieved in 12.5 h with a 90% selectivity to C7-C17 n-alkanes, while a similar extent of conversion was observed for a commercial lipid source (1,2-distearoyl-3-palmitoyl-sn-glycerol) in 2.5 h with a 97% selectivity to C7-C17 n-alkanes. The faster kinetics observed for the lipid feedstock resulted from the in-situ formation of H2 via aqueous phase reforming (APR) of the glycerol co-product, which, in turn, accelerated the fatty acid decarboxylation (DOX) process. Increased initial N2 headspace pressure also accelerated the conversion of stearic acid. Conversion products of stearic acid include a series of n-alkanes with heptadecane as the major product. With increasing reaction time, side reactions such as cracking shifted the product distribution towards shorter chain alkanes. Partial deactivation of Ru/C was observed after repeated use, attributed to a combination of (i) Ru leaching from support particles, (ii) sintering of Ru nanoparticles, and (iii) blockage of active sites by the hydrocarbons, fatty acids, lipids or their coking products. These findings provide a path forward for the development of lower-cost catalysts for valorization of waste lipid sources.

Catalytic hydrothermal processing of fatty acids and lipids over Ru/C without external hydrogen addition

2019 Spring National Meeting

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