Partial hydrogenation of a C18:3-rich FAME mixture over Pd/C

The study focuses on the partial hydrogenation of a C18:3-rich FAME mixture (α-linolenate/linoleate: ∼3.5:1 mol/mol) taken as a model of FAME materials obtainable from alternative non-edible oils deriving from non-food biomass, which are suitable feedstocks for 2nd generation biodiesel. High C18:1 yield and selectivity required an accurate control of reaction time and temperature because of stearate generation through consecutive reactions C18:3→C18:2→C18:1→C18:0. In n-heptane, under mild conditions (50 °C, 0.1 MPa H2, 1 h), the FAME mixture was converted over Pd(5%)/C into C18:1 selectively (97.8%), with high yield (81.1%). The final mixture was C18:3-free (0.1%) and showed a low content of stearate (2.0%). Also the C18:2 component was markedly reduced (21.5%→16.7%). The content of trans-C18:1-monoenes was 39.0%. A lower content of trans-C18:1 (16.4%) was attained at 0 °C (1.5 h). Under the latter conditions, stearate and C18:3 in the final product were, respectively, 3.3% and 1.5%. Remarkably, Pd/C can act as a tunable catalyst being able to promote effectively the full hydrogenation of the FAME mixture to stearate under uncommon very mild conditions (0 °C, 0.1 MPa) within a relatively short time (6 h). Under solventless conditions or in polar solvents (MeOH, THF, dimethyl carbonate), hydrogenation was slower and less selective than in n-heptane.