(652c) Catalytic Deoxygenation of Cyanobacteria-Derived Fatty Acids to Hydrocarbons

A phototrophic cyanobacterium, Synechocystis sp. PCC6803, was genetically modified to secrete fatty acids (FAs), predominantly lauric acid (LA) into the growth medium.¹ Previous research demonstrated that LA can be deoxygenated using a Pd/C catalyst with high selectivity to n-undecane and CO₂.² In contrast, cyanobacteria-derived fatty acids (CBFAs) recovered from the growth medium by adsorption on a hydrophobic resin were deoxygenated much less efficiency and with lower CO₂ selectivity. Herein, we demonstrate that the decarboxylation activity of the catalyst is related inversely to the sulfur concentration in the CBFA sample. Since this is likely due to contaminants such as sulfolipids, β-hydroxy FAs and proteins, effective removal of sulfur-containing compounds from the FA feedstock is essential to achieving high alkane yield and CO₂ selectivity. Activated carbon decolorization, C18 adsorption chromatography, acid hydrolysis, and alkaline hydrolysis were each employed as purification methods for the raw CBFA samples. Samples were analyzed for sulfur and phosphorus by inductively coupled plasma-optical emission spectroscopy (ICP-OES). CBFA samples were deoxygenated in a semi-batch reactor at 300°C under 5% H₂ using a 5 wt.% Pd/C catalyst. Deoxygenation yield and CO₂ selectivity were found to be highly correlated to sulfur concentration. A CBFA sulfur concentration of <20 ppm was required to achieve results equivalent to those obtained previously with LA.

CBFAs also contain β-hydroxy fatty acids that are constituents of an endotoxin produced by gram-negative bacteria such as Synechocystis sp. PCC6803.^{3, 4} Deoxygenation experiments with 10% β-hydroxy myristic acid (BHMA) and 90% LA under standard semi-batch conditions evidenced that the presence of BHMA does not significantly decrease the final alkane yield or CO₂ selectivity of LA deoxygenation; however, it does inhibit the deoxygenation kinetics. Deoxygenation of BHMA proceeded with high CO₂+CO yield (92%), liquid product yield (86%) and CO₂ selectivity (85%). The reaction also produced a strong initial H₂ evolution peak. Major liquid-phase products were n-tridecane, 2-tridecanone and 2-tridecanol. Both 2-tridecanone and 2-tridecanol also were reacted over 5 wt.% Pd/C. Conversion of 2-tridecanol proceeded mainly via dehydrogenation to 2-tridecanone, which explains the H₂ evolution observed during BHMA deoxygenation. Neither 2-tridecanol nor 2-tridecanone was deoxygenated to n-tridecane in significant yield. BHMA deoxygenation to n-tridecane most likely proceeds via a myristic acid intermediate with 2-tridecanol and 2-tridecanone as byproducts.

References

1.         X. Liu, J. Sheng and R. Curtiss, III, PNAS 108 (17), 6899-6904 (2011).

2.         J. P. Ford, J. G. Immer and H. H. Lamb, Top. Catal. 55 (3-4), 175-184 (2012).

3.         B. Szponar, L. Krasnik, T. Hryniewiecki, A. Gamian and L. Larsson, Clin. Chem. 49 (7), 1149-1153 (2003).

4.         M. Asayama, Appl. Microbiol. Biotechnol. 95 (3), 683-695 (2012).