Catalytic Conversion of Cyanobacteria Fatty Acids to Alkanes for Renewable Jet Fuel

The overall goal of this project was the catalytic conversion of cyanobacteria-derived fatty acids (CBFAs) to hydrocarbons for renewable jet fuel. A freshwater cyanobacterium, Synechocystis sp. PCC 6803, was genetically modified to secrete free FAs, specifically lauric acid (LA), into the growth medium, by the Vermaas Lab at Arizona State University (ASU). Synechocystis sp. PCC 6803 was grown phototrophically at ASU in bench-top carboys using artificial light and a growth medium that excluded an organic carbon source. Crude CBFAs were adsorbed from the cultures onto polymer resin beads and recovered by elution with methanol and other eluents.;In semi-batch experiments, the model compound LA was deoxygenated via decarbonylation and decarboxylation (DCO) to n-undecane ( n-C11) using a 5 wt. % Pd/C catalyst at 300°C and 15 bar under 1-20% H2 and at initial concentrations of 4-46 mol. %. Gas-phase products were analyzed using an on-line quadrupole mass spectrometer (QMS), and liquid-phase products were analyzed off line by gas chromatography (GC). Higher H2 mole fractions in the purge gas resulted in lower CO + CO 2 and n-C11 yields and CO2 selectivities. The initial decarboxylation rate and batch productivity increased at lower H2 partial pressures. The initial decarbonylation rate was independent of H2 mole fraction in the purge gas. Increasing the initial LA concentration suppressed the initial decarboxylation rate and lowered batch productivity. In contrast, the initial decarbonylation rate increased linearly with LA concentration before saturating at 46 mol. %. Comparison of these data with deoxygenation results for myristic and palmitic acids shows that the initial decarbonylation rate is independent of FA chain length. In contrast, the initial decarboxylation rate increases significantly with FA alkyl chain length; the initial decarboxylation rate quadruples from LA to palmitic acid.;Methyl laurate (ML), ethyl laurate (EL), methyl stearate (MS), and ethyl stearate (ES) were deoxygenated over 5 wt. % Pd/C at 300 °C and 15 bar under 5 vol. % H2. These fatty acid esters (FAEs) were shown to be deoxygenated via a two-step process involving hydrogenolysis to either methane or ethane and the corresponding FA followed by FA deoxygenation (via DCO) to a n-alkane. ML deoxygenation was demonstrated to follow Langmuir-Hinshelwood kinetics. Changing the ester moiety from methyl to ethyl increased the apparent 1st-order rate constant. For ML, this change nearly doubled the apparent rate constant. The increase in FA chain length from ML to MS also caused the apparent 1st-order rate constant to increase.;CBFAs were deoxygenated over 5 wt. % Pd/C at 300 °C and 15 bar under 5 vol. % H2. The CBFAs were recovered by adsorption using an anion exchange resin and a hydrophobic resin. GC analysis of the CBFAs determined that LA was the main constituent (∼80 wt. %). Activated carbon decolorization, alkaline hydrolysis, acid hydrolysis, precipitation, and C18 adsorption chromatography were all used as purification methods for CBFAs. Semi-batch CBFA deoxygenation over Pd/C determined that adsorption chromatography and alkaline hydrolysis were able to reduce the sulfur concentration below 20 ppm---the empirical threshold for achieving >90% CO2 selectivity and >80% n-alkane yield. beta-hydroxymyristic acid (BHMA) was identified as a CBFA constituent using liquid chromatography-mass spectrometry (LC-MS). Semi-batch deoxygenation experiments with reagent-grade material demonstrated that BHMA was converted to n-tridecane (n-C 13), 2-tridecanone (2-C13-ONE), and 2-tridecanol (2-C 13-OH) over 5 wt. % Pd/C. Moreover, addition of BHMA inhibited LA DCO, and this was attributed to preferential adsorption of BHMA on the Pd catalyst.;The model compound n-undecane (n-C11) was hydroisomerized using bifunctional Pt/zeolite catalysts (Pt/HY, Pt/CaY and Pt/NaY) at 300°C and 500 psig (initial H2 pressure) in the liquid-phase. Pt/CaY converted n-C11 with the highest i-C11 yield at moderate to high conversions. For example, at 89.8% n-C11 conversion, the i-C 11 yield was 55.1%. For comparison, Pt/HY produced an i-C 11 yield of only 42.8% at a conversion of 89.6%. Alkanes derived from CBFAs via deoxygenation over Pd/C in semi-batch and fed-batch mode were isomerized over Pt/CaY in the liquid-phase at 300°C. Three out of four isomerization reactions reached conversions between 20 and 30%, with isomer selectivities between 86 and 95%. The successful isomerization of n-alkanes over Pt/CaY was the last step in demonstrating of an integrated catalytic process for producing synthetic paraffinic kerosene (jet fuel sans aromatics and cycloalkanes) from biorenewable CBFAs.