Page 195 - Lignocellulosic Biomass to Liquid Biofuels

Lignocellulosic biomass to biodiesel 157

lignocellulosic hydrolysates. As a matter of facts, acid catalysts are able to
promote simultaneously the triglyceride transesterification and the fatty
acid esterification.
Currently, heterogeneous alkaline catalysts are considered very promis-
ing, as they ensure relatively high reaction rates, reusability, easier separa-
tion from the products of reaction by filtration, lower cost, and require
less energy as compared to acid catalysts. The main limitation of heteroge-
neous catalysts stems from diffusional resistances, leading to slower reac-
tion rates [165].
Heterogenous acid catalysts offer a lower reaction rate in comparison
to their alkaline counterpart, though they are specifically used with tri-
glyceride feedstock containing higher concentrations of free fatty acids.

4.10 Genetic and metabolic engineering of microbes
Due to the large interest raised form the use of microalgae for biodiesel
production, an increasing research activity is currently concerned at the
metabolic and genetic engineering of microalgae. The green alga
Chlamydomonas reinhardtii has been the most studied, being its genome the
best known. The current research is mainly aimed at overcoming the bot-
tlenecks that limit the two main pathways leading to the production of
TAGs. These pathways is the acyl CoA-dependent pathway allowing the
acylation of diacylglycerol, and the acyl CoA-independent pathway
allowing the recycle of membrane lipids. Recent studies have elucidated
the role of the enzymes involved in these pathways. In particular the
enzymes acetyl CoA carboxylase (involved in the synthesis of malonyl-
CoA) and type-II fatty acid synthase (involved in the fatty-acid chain
elongation) have been found to be rate-limiting in fatty acid synthesis
[158]. Consequently, metabolic engineering strategies are being developed
to increase the activity of these enzymes [163].
Another potential target of the research is the improvement of the dis-
tribution of fatty acid residues in cellular lipids. As a matter of facts, a bal-
anced proportion between saturated and unsaturated fatty acids
encourages the use of the biodiesel as automotive fuel, leading to a mod-
erate tendency to autoxidation and to a reduced tendency to gelification,
thus improving the performance of biofuel at cold temperatures.
Nevertheless, the results so far obtained in this field are not yet satisfac-
tory, and much study is still required. Consequently, it can be said that in
the near future, the production of microalgal biodiesel is not likely to be