Algal biomass has gained attention recently as an alternative carbon resource, due to its high oil production efficiency and lack of use in foodstuffs. The algal residue left over after oil production is normally discarded. These residues could be used to produce chemicals and increase the value of using algal biomass instead of using fossil fuels. A team of Japanese researchers have utilised algal residues as a new carbon resource to produce two important chemicals: methyl levulinate and methyl lactate.
Many important chemicals produced today are derived from fossil fuels. One common example is petroleum oil, which is currently used as fuel, heating oil and raw materials for plastic production. Various hydrocarbons are currently obtained from natural gas sources, but the dependence on both of these carbon sources present an issue - they are finite and rapidly depleting.
As an alternative, many methods to convert carbohydrates from woody and crop biomass into important chemicals have been developed with great success.
However, with an ever-growing population, the amount of farmland required to produce a significant amount of crop-biomass is simply unrealistic. This is epecially true when any spare agricultural land will be preferentially developed for the production of food. As such, an alternative has been required for quite some time.
Microalgae has gathered interest of late for the production of biomass, as the productivity per unit time is very high. There is no dependence on farmland with no competitive interests, unlike other methods. Algae has been used to produce both oil and carbohydrates for the production of biofuels, but currently, the algal residues are discarded. The usefulness of converting their starch molecules into other chemicals has been overlooked.
Cellulose has previously been used to produce alkyl lactate or alkyl levulinate, and the Japanese researchers developed a similar method to produce methyl lactate and methyl levulinate.
The researchers employed a one-pot synthesis method using an algae-methanol suspension at 160 °C in the presence of various selective catalysts for 24 hours. The algae were cultivated, centrifuged, dried, freezed and suspended into the methanol solvent through sonication methods. This process released both carbohydrates, including starch, and oil.
The researchers tried and tested different catalysts to produce the two chemicals, with a high yield using all of the catalysts towards methyl levulinate production. A small amount of methyl lactate was also produced but not in the same quantities.
The two products were produced through the degradation of both the starch molecules and the algae itself. A glucose monomer was generated through the dissociation of the glycosidic bond, which was then converted into tetrose and glycoaldehyde through a [4+2] retro-aldol reaction, or into 1,3-dihydroxyacetone and glyceraldehyde through a [3+3] retro-aldol reaction.
The [3+3] reaction was found to be the most thermodynamically stable reaction and led to the formation of methyl levulinate. The lower stability [4+2] reaction produced the methyl lactate product. The thermodynamic stability of the reaction was the driving force for the higher yields obtained.
Depending on the catalyst used, the production of methyl levulinate varied between 33% and 53%, whereas the highest yield seen for methyl lactate was only 2%. TfOH was found to be the most effective catalyst for methyl levulinate production.
The results are vastly different to similar methods using cellulose. During the production process using cellulose, most catalyst did not produce many significant results, apart from two. One of these was preferable for methyl levulinate and the other for methyl lactate. The highest yields were 48% and 27% for methyl levulinate and methyl lactate, respectively.
The differences between the production of these chemicals from different starting materials is an interesting discovery. It shows that the reactivities of cellulose and starch are quite different, even though cellulose and starch are chemically similar, with both being composed of glucose monomers.
The researchers have shown that algae can not only be used for oil production, but as a carbon source for the production of important chemicals. This lends itself to the possibility that algae biomass could be used in future instead of fossil fuels as a carbon resource, and alleviate some of the environmental and resource strain the world is currently experiencing.
"Development of New Carbon Resources: Production of Important Chemicals from Algal Residue" - S. Yamaguchi et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-00979-y
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