Organic chemistry: Creating fuel from algae

Green tides are overgrowths of algae that occur in response to many factors, including excess nutrients in marine water. Up to 10

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tons of a particular algae, E. prolifera, can be collected from a single green tide event. Rather than waste this biomass, researchers attempt to convert the algae into a renewable source of fuel called bio-oil using a process known as hydrothermal liquefaction (HTL). In HTL, water is added to dried algae; the mixture brought to sufficiently elevated temperature and pressure conditions to allow the water to decompose the algal biomass. After the reaction mixture is cooled, it can be separated according to the scheme shown in Figure 1.

Figure 1: Schematic diagram for bio-oil production and separation

Figure 2: Constituents of Bio-Oil. Analysis of the bio-oil indicate that it is composed of over 180 compounds. The top five most common constituent molecules are shown below.

Compound	Name	Formula
1	hexadecanoic acid	CH $_{3}$ ‍(CH $_{2}$ ‍) $_{13}$ ‍COOH
2	oleic acid	CH $_{3}$ ‍(CH $_{2}$ ‍) $_{7}$ ‍CHCH(CH $_{2}$ ‍) $_{7}$ ‍COOH
3	2-methyl-2-cyclopenten-1-one	C $_{6}$ ‍H $_{8}$ ‍O
4	3-methyl-2-cyclopenten-1-one	C $_{6}$ ‍H $_{8}$ ‍O
5	2,3-dimethyl-2-cyclopenten-1-one	C $_{7}$ ‍H $_{10}$ ‍O

Adapted from: Zhou, D., Zhang, L., Zhang, S., Fu, H., & Chen, J. (2010). Hydrothermal liquefaction of macroalgae Enteromorpha prolifera to bio-oil. Energy & Fuels, 24(7), 4054-4061; Zhou, D., Zhang, S., Fu, H., & Chen, J. (2012). Liquefaction of macroalgae Enteromorpha prolifera in sub-/supercritical alcohols: direct production of ester compounds. Energy & Fuels, 26(4), 2342-2351.

The researchers could have used many solvents in extracting the oil from the aqueous reaction mixture. What characteristic of CH $_{2}$ ‍Cl $_{2}$ ‍ makes it a better solvent than hexane for this procedure?

MCAT

Course: MCAT > Unit 3

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