A novel enzymatic catalyst for biodiesel production

The results, which have been patented, have just been published in the journal Energy & Environmental Science.

Biofuel production provides an alternative to fossil fuels. Biodiesels, for instance, are processed products based on oils from oleaginous plants such as oilseed rape, palm, sunflower and soybeans. They result from a chemical reaction, catalyzed in either an acidic or preferably a basic medium, between a vegetable oil (90%) and an alcohol (10%). This reaction, known as transesterification, converts the mixture into a methyl ester (the main constituent of biodiesel) and glycerol. A saponification side reaction (methyl ester conversion into the corresponding acid salt) reduces methyl ester yield. To increase the yield, it was therefore necessary to develop alternative catalysts (1).

For this type of reaction, certain enzymatic catalysts such as those belonging to the family of lipases (triglyceride hydrolases) are particularly efficient and selective. However, their high cost and low conformational stability restrict their industrial use, unless they can be irreversibly confined in porous matrices, allowing good accessibility and enhanced mass transport. This has now been achieved by the team led by Professor Rénal Backov (Université Bordeaux 1) at CNRS's Centre de Recherches Paul Pascal (CRPP), in collaboration with researchers from teams led by Dr Hervé Deleuze at the Institut des Sciences Moléculaires in Bordeaux (CNRS/Université Bordeaux 1/Institut Polytechnique de Bordeaux) and Professor Clément Sanchez (2) at the Laboratoire de Chimie de la Matière Condensée in Paris (CNRS/UPMC/ENSCP/Collège de France).

In an initial study, they had already demonstrated the possibility of efficient catalysis, by developing modified silica-based cellular matrices that make it possible to confine lipases (3) in order to obtain exceptional yields for hydrolysis, esterification and transesterification reactions. Their work had also shown that unpurified enzymes could be used in the matrices. The fact that they were unpurified was a first step to significantly reducing the cost of biocatalysts. However, the methodology did not allow continuous biodiesel production. This obstacle has now been overcome.

Researchers have developed a new method that generates the cellular hybrid biocatalyst in situ inside a chromotography column (4). This novel approach makes it possible to carry out continuous, unidirectional flow synthesis over long periods, since catalytic activity and ethyl ester production are maintained at high, practically steady levels during a two-month period of time. These results are amongst the best ever obtained in this field.

Research is continuing into solvent-free conversion of triesters, aimed at minimizing waste production and curbing the use of solvents and metals in chemical transformation processes. This work, which meets current energy and environmental requirements, shows how much chemists are working in the public interest, and confirms the importance of integrative chemistry.

(1) A catalyst is a chemical substance that increases the rate of a chemical reaction but is not itself used up (it is regenerated).

(2) Professor Clément Sanchez holds the Chair of Chemistry of Hybrid Materials at the Collège de France.

(3) a/ Enzyme-based hybrid macroporous foams as highly efficient biocatalysts obtained through Integrative Chemistry. N.Brun, A.Babeau-Garcia, H.Deleuze, F.Duran, C.Sanchez, V.Ostreicher and R.Backov. Chem. Mater., 2010, 22, 4555. b/ Catalyseurs supportés enzymatiques hybrides macrocelluaires et applications. N. Brun, A.Babeau-Garcia, C.Sanchez and R.Backov. French patent 2009, file number FR 09-54634

(4) Chromatography is a technique used to separate the constituents of a mixture, with the aim of identifying or measuring certain constituents of the mixture.

How does it work?

These systems are efficient because a certain number of technical obstacles have been overcome:

• the confinement of the enzymes in macropores (with diameters of a few micrometers) makes them continuously accessible to reactants in solution. The macroporous medium also means that chemical reactions are not slowed down by Fickian diffusion transport, unlike in matrices with a mesoporous surface (diameters of 2-50 nm), where there is little convection.
• the enzymes are used in unpurified form, which contributes to their stability and keeps production costs low.
• hybridization of the surface of the silica support optimizes enzyme/substrate interactions.
• the natural hydration of the silica cellular support enhances enzymatic activity via a lubricating effect.
• the mechanical stability of the silica framework makes it possible to maintain a high inlet pressure and pressure drop (difference in pressure between the inlet and outlet of the reactor under continuous flow) without damage, enabling the use of high reactant flow.