Although thefirst diesel engines demonstrated by Rudolf Diesel
were powered by peanut oil the high viscosity of triglycerides
means that they not ideal fuels, and are more commonly converted
to the monoalkyl esters before combustion. This is typically achieved by transesterification to the methyl ester in the presence of
a catalyst.
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The most common method for transesterification
employs a homogeneous Brønstead base catalyst, such as alkali
metal alkoxides or hydroxides. Heterogeneous base catalysts such
as metal oxides, zeolites, and hydrotalcites have also been extensively used, which can improve separation and recycling of catalyst
from reaction mixtures.
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Li and co-workers recently reported the
synthesis of biodiesel from soybean oil, using an N-heterocyclic
carbene (NHC) catalyst generated in situ by thermolysis of
a NHCeCO2adduct.
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This method employs low reaction temperatures and provides high yields of biodiesel when using oils and fats
with low water, and free fatty acid (FFA), content (