Two New Approaches to a Better Biodiesel

Munich researchers have developed a more efficient method of converting vegetable oils into alkane hydrocarbons, at relatively low temperatures.

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. _GCC

As a cascade reaction, the complexity of the reaction apparatus is much simplified, and the expense of building and scaling the refiners presumably decreased.

A second new approach to creating a better biodiesel is being developed by researchers at Berkeley's JBEI labs. They are programming E. Coli to produce a novel type of diesel substitute -- bisabolane.

This past fall, JBEI researchers identified bisabolane as a potential new advanced biofuel that could replace D2 diesel. Using the tools of synthetic biology, the researchers engineered strains of bacteria and yeast to produce bisabolene from simple sugars, which was then hydrogenated into bisabolane. While showing much promise, the yields of bisabolene have to be improved for microbial-based production of bisabolane fuel to be commercially viable.

The inefficient terpene synthase enzyme is one of the bottlenecks in the metabolic pathway used by the engineered microbes. Knowing the AgBIS crystal structure will guide us in engineering it for improved catalytic efficiency and stability, which should bring our bisabolene yields closer to economic competitiveness.

—Pamela Peralta-Yahya
Peralta-Yahya and her colleagues determined that the AgBIS enzyme consists of three helical domains, the first three-domain structure ever found in a synthase of sesquiterpenes—terpene compounds that contain 15 carbon atoms. The discovery of this unique structure holds importance on several fronts, according to co-lead author of the Structure paper McAndrew.

That we found the structure of AgBIS to be more similar to diterpene (two carbon terpene compounds) synthases not only provides us with insight into the function of these less well characterized enzymes, it also provides us with clues to the evolutionary heritage as the archetypal three-domain terpenoid synthases became two-domain sesquiterpene synthases in plants.

Furthering our knowledge of the structures and functions of terpenoid synthases may prove to have abundant practical applications aside from advanced biofuels because these enzymes produce a wide variety of specialized chemicals.

—Ryan McAndrew

Solving the three-dimensional crystal structure of AgBIS was made possible by the protein crystallography capabilities of Berkeley Lab’s Advanced Light Source (ALS), a DOE Office of Science national user facility for synchrotron radiation, and the first of the world’s third generation light sources. For this work, the JBEI team used three of the five protein crystallography beamlines operated by the Berkeley Center for Structural Biology (BCSB): beamlines 8.2.1, 8.2.2, and 5.0.3. _GCC

I understand that to most people, solving the 3-D crystal structure of an enzyme protein is as exciting as watching paint dry. But from such seemingly dull discoveries eventually emerge earth-shaking breakthroughs.

Most energy analysts have been far too ready to write off "biofuels," perhaps because they associate the concept too closely to maize ethanol. While it is true that biological processes create energy sources with relatively low energy densities, it is also true that biological processes can create a huge amount of these energy sources. It will be up to humans to devise ways of densifying nature's energy bounty -- either via synthetic biology methods, or in the post-biological stage.

Here is a sad tale about the state of European government energy policy, reflecting a general decline in thinking on the continent. Europe can truly not afford this type of ruinous diversion.