Catalytic conversion of cyanobacteria-derived fatty acids to alkanes for biorenewable synthetic paraffinic kerosene

Abstract

A two-step catalytic process for converting cyanobacteria-derived fatty acids (CBFAs) to linear and branched alkanes for biorenewable synthetic paraffinic kerosene was demonstrated. Fatty acids synthesized and secreted into the growth medium by an engineered strain of the cyanobacterium Synechocystis sp. PCC 6803 were recovered from 20 liter photobioreactor cultures by adsorption on hydrophobic resin beads. By design, lauric acid (LA, C12:0, ∼80% w/w) was the main CBFA constituent; however, myristic acid (MA, C14:0, 6–10% w/w), palmitic acid (PA, C16:0, 2–6% w/w), and β-hydroxymyristic acid (BHMA, 2–3% w/w) also were produced. LA and MA model compounds were deoxygenated over a 5 wt% Pd/C catalyst to n-undecane and n-tridecane, respectively, with high yields and CO₂ selectivities. Major products of BHMA deoxygenation over Pd/C were n-tridecane and 2-tridecanone. BHMA concentrations typical of the CBFA samples were found to inhibit LA deoxygenation. Because Pd sites responsible for fatty acid decarboxylation are poisoned at sulfur concentrations [S] typical of crude CBFA samples (100–150 ppm), post-recovery purification procedures were developed and evaluated based on their efficacy in reducing S-containing impurities. Deoxygenation of CBFAs was most effective when purification procedures limited [S] to <15 ppm, as evidenced by >80% n-alkane yield and ∼90% CO₂ selectivity. The n-alkane products of CBFA deoxygenation were hydroisomerized in the liquid phase (with added n-dodecane) over a 0.70 wt% Pt/CaY catalyst. The resultant mixtures had isoalkane/normal alkane ratios of 0.25–0.50.