Biodiesel: Algae burn for you

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  • Published: Jan 15, 2012
  • Author: David Bradley
  • Channels: Atomic
thumbnail image: Biodiesel: Algae burn for you

Micro biodiesel

Atomic absorption spectroscopy is used in a study published in Angewandte Chemie that demonstrates how biodiesel might be produced from microalgae.

Finding sustainable alternatives to fossil fuels that would both solve the problem of dwindling supplies of oil and cut the net carbon dioxide emissions from vehicles running on hydrocarbon fuels is a cause high on the environmental agenda. The use of biomass as a source for fuels compounds has benefits, but the setting aside of the great tracts of land required to "grow" adequate crops for conversion into biodiesel detracts from a parallel agenda of major concern: land use and food security.

Now, Johannes Lercher and his colleagues, Baoxiang Peng, Yuan Yao, Chen Zhao, at the Technische Universität München have developed a new catalytic process that might offer a solution to both problems. Their catalyst can efficiently convert biomass, or more properly biopetroleum, generated by microalgae into diesel fuels for use in suitable vehicles.

Microalgae coming to fruition?

Researchers have spent many years developing reactions that can convert plant oils from sources such as soybean and rapeseed (canola) are promising starting materials for the production of biofuels. These terrestrial crops require land and irrigation whereas microalgae could represent a possible alternative that avoids several of the problems commonly associated with growing crops for fuel. Microalgae are individual cells or short chains of algal cells that grow in water. They can grown in almost any body of water and are usually thought of as problematic if levels get too high as they soak up much of the oxygen and nutrients from the water, damaging pre-existing ecosystems and stifling other organisms. However, their growth can be controlled and they can be readily cultivated.

"They have a number of advantages over oil-containing agricultural products," explains Lercher. "They grow significantly faster than land-based biomass and, unlike the terrestrial cultivation of oilseed plants, their use for fuel production does not compete with food production." Lercher adds that, chemically speaking bipetroleum from microalgae has the distinct advantage of having a very high triglyceride content (60 percent by weight) and of a growth rate of 10 to 200 times that of land-based biofuel crops.

Until now, however, researchers have been unable to find straightforward methods for refining the putative fuel oil from microalgae because of various physical and chemical stumbling blocks. The fuel obtained by these earlier attempts has been shown to have too high an oxygen to carbon ratio. It suffered from poor flow at low temperatures, which is not unlike conventional diesel, which can become waxy in extreme cold. The microalgal biodiesels known to date also required the use of sulfur-containing catalysts, which contaminates the final product. Alternative catalysts have been keenly sought but an efficient compound for the conversion has remained elusive. The Munich scientists have now demonstrated efficacy with a novel process based on a nickel catalyst supported on porous zeolite HBeta. They have shown this catalyst to be able to convert raw, untreated oil from the microalgae under mild conditions (260 Celsius, 4 megapascals, MPa of hydrogen pressure).

"The products are diesel-range saturated hydrocarbons that are suitable for use as high-grade fuels for vehicles," Lercher says. The oil produced by the microalgae is mainly composed of neutral lipids, such as mono-, di-, and tri-glycerides along with unsaturated C18 fatty acids as the primary component (88 percent). The conversion reaction takes about eight hours and generates 78 percent liquid alkanes with octadecane (C18) as the primary component. The team reports that the primary gas-phase side-products are propane and methane. The researchers used atomic absorption spectroscopy (AAS) to show that metal leaching after the reaction was below the technique's detection limit of 1 ppm, parts per million.

Mechanistic cascade

The team has carried out an analysis of the conversion reaction mechanism to ascertain what processes might be involved in producing biodiesel from microalgal biopetroleum. They found that a cascade reaction reaction is involved in which the double bonds of the unsaturated fatty acid chains of the triglycerides are first saturated by hydrogen. These saturated fatty acid intermediates are then split from their glycerin component, which is released as propane gas. In the final step of the cascade, the acid groups in the fatty acids are reduced stepwise to the corresponding alkyl groups to give the final hydrocarbon products.

Atomic absorption spectroscopy is used in a study published in Angewandte Chemie that demonstrates how biodiesel might be produced from microalgae.

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