A team from the Technische Universität München led by Dr. Johannes Lercher, who is also Director of the Institute for Integrated Catalysis at the Pacific Northwest National Laboratory, has introduced a new catalytic process that allows the effective conversion of diesel-range alkanes from microalgae oils under mild conditions. A paper on the work appears in the journal Angewandte Chemie.
Plant oils are promising starting materials for the production of biofuels. Microalgae are attractive feedstock resources in that context, as they feature high triglyceride contents (up to 60 wt %); rapid growth rates that are 10–200 times faster than terrestrial oil crops such as soybean and rapeseed; and do not compete directly with edible food/oil production. There are currently three approaches used for microalgae oil refining, Lercher and his team note:
Transesterification of triglycerides and alcohol into fatty acid alkyl esters (FAAEs) and glycerol—i.e., first-generation biodiesel. Such esters, however, have a relatively high oxygen content and poor flow property at low temperatures, limiting their application as high-grade fuels.
Conventional hydrotreating catalysts such as sulfided NiMo and CoMo, for upgrading. However, these sulfide catalysts contaminate products through sulfur leaching, and deactivate because of its removal from the surface.
Supported noble and base metal catalysts for decarboxylation and decarbonylation of carboxylic acids to alkanes at 300–330 °. These catalysts, however, show low activities and selectivities for C15–C18 alkanes. Contributions addressing microalgae oil upgrading using sulfur-free catalysts have not been reported, according to the team.
Herein, we report for the first time a novel and scalable catalyst, that is, Ni supported on and in zeolite HBeta, to quantitatively convert crude microalgae oil under mild conditions (260 °C, 40 bar H2) to diesel-range alkanes as high-grade second-generation transportation biofuels.—Peng et al.
The microalgae oil in the study comprised unsaturated C18 fatty acids (88.4 wt %), saturated C18 fatty acids (4.4 wt %), as well as some other C14, C16, C20, C22, and C24 fatty acids (7.1 wt %) in total.
The researchers directly hydrotreated the microalgae oil in batch mode with 10 wt % Ni/HBeta (Si/Al = 180) at 260 °C and 40 bar H2; after 8 hours, they obtained 78 wt % yield of liquid alkanes containing 60 wt % yield of C18 octadecane)—very close to the theoretical maximum liquid hydrocarbon yield of 84 wt %. Propane (3.6 wt %) and methane (0.6 wt %) were the main products in the vapor phase.
Analysis of the reaction mechanism showed that this is a cascade reaction. First the double bonds of the unsaturated fatty acid chains of the triglycerides are saturated by hydrogen. Then, the now-saturated fatty acids take up hydrogen and are split from their glycerin component, which reacts to form propane. In the final step, the acid groups in the fatty acids are reduced stepwise to the corresponding alkane.
In summary, we have shown that microalgae oil can be nearly quantitatively hydrodeoxygenated to alkanes by cascade reactions on bifunctional catalysts based on Ni and an acidic zeolite. Ni catalyzes efficiently the hydrogenolysis of the fatty acid ester, the decarbonylation of aldehyde inter- mediates, as well as the hydrogenation of –COOH, –CHO and C=C double bonds in reactants and intermediates. The acid function catalyzes the dehydration of alcohol intermediates and the hydroisomerization and hydrocracking of the alkane products. The knowledge of the individual reaction rates allows balancing these rates by adjusting the concentration of catalytically active sites to design tailored and stable catalysts for selectively converting crude microalgae oil to diesel-range alkanes.
The approach opens new possibilities to produce sulfur-free high-grade green transportation fuels from microalgae at large scale.—Peng et al.
Baoxiang Peng, Yuan Yao, Chen Zhao, and Johannes A. Lercher (2012) Towards Quantitative Conversion of Microalgae Oil to Diesel-Range Alkanes with Bifunctional Catalysts. Angewandte Chemie International Edition doi: 10.1002/anie.201106243