Benzene, an aromatic hydrocarbon, is a natural component of crude oil and is one of the elementary petrochemicals. It is used as a precursor to the manufacture of more complex chemicals, and, with a high octane number, is an important component of gasoline.
Researchers at the University of Konstanz in Germany have now developed a two-step, one-pot process for the synthesis of benzene from the five-fold unsaturated fatty acid eicosapentaenoic acid (EPA), a component of microalgae oils. Their paper is published in the RSC journal Green Chemistry.
The two step-process—olefin metathesis and the catalytic dehydrogenation of the resulting 1,4-cyclohexadiene—produces two equivalents of benzene per EPA substrate molecule. The only major by-products are 5-octenoic acid and 5-decenedioic acid.
Further, performing the dehydrogenation step under hydrogen pressure results in the formation of their saturated analogues—sebacic acid and octanoic acid, both desirable products—while the simultaneous dehydrogenation step to benzene is not hampered.
Aromatics are essential key compounds of the chemical industry. In particular, benzene is the starting material for the production of numerous agrochemical and pharmaceutical actives, dyes and other chemicals. In view of the finite nature of petrochemical sources and the environmental impact of fossil fuel recovery, schemes that provide access to aromatics from renewable sources are desirable in the long term.
In principle, aromatics occur in large amounts in lignin. However, lignin is a complex mixture containing numerous oxygenated substituted benzene motifs linked to a high molar mass biopolymer. Efforts to produce single clean aromatic compounds by breaking down lignin efficiently have not been successful to date. Thus, lignin is used as a low value fuel for the very largest part. Possible schemes for the efficient generation of benzene utilizing suitable renewable molecules are lacking.
We now report on the catalytic generation of benzene from eicosapentaenoic acid (EPA). EPA is contained in substantial amounts in various microalgae or can also be produced by yeasts. … Towards this aim, we pursued ring closure by olefin metathesis with subsequent dehydrogenation to yield benzene.—Pingen et al.
Olefin metathesis was carried out in the presence of a solvent. Without any further purification, a solid dehydrogenation catalyst was added and the reaction mixture was heated to optimum dehydrogenation conditions.
The team investigated whether both reactions could be combined into a single step; however, the metathesis step was hampered under that condition, and did not reach full conversion.
The researchers noted that while the production of benzene from fatty acids is not economically viable yet, their scheme does provide a perspective to access this important building block from non-fossil sources efficiently.
Dennis Pingen, Julia Zimmerer, Nele Klinkenberg and Stefan Mecking (2018) “Microalgae lipids as a feedstock for the production of benzene” Green Chemistry doi: 10.1039/C8GC00423D