Fuels and Chemicals from Hemicellulose Sugars - Biotechnology in China: Biofuels and Bioenergy - page 210

hydrolyzates as carbon sources for efficient 2,3-BD production. Aiming at simulta-
neous consumption of glucose and xylose, Ji et al. [ 84 ] further constructed an
engineered K. oxytoca harboring the CRP(in) phenotype which could metabolize
glucose and xylose mixtures simultaneously. As a consequence of the co-metabolism
of glucose and xylose, the total sugars could be consumed faster, leading to higher
growth rate and 2,3-BD productivity by the recombinant compared to the parent
strain. This recombinant has the potential to produce 2,3-BD directly from the
hemicellulose hydrolyzates.
The above biologically produced 2,3-BD from the hemicellulose may also be
further converted to methyl ethyl ketone (MEK, also called 2-butanone), which is
an industrial solvent and may find use as a liquid fuel additive by acid-catalyzed
dehydration [ 88 ]. This reaction can be carried out using catalysts such as alumina or
sulfuric acid, resulting in a[95% yield [ 89 , 90 ]. This reaction mechanism involves
a hydride shift which could also be made possible by an enzyme catalyzing process.
Biosynthesis of MEK from 2,3-BD using a diol dehydratase (EC 4.2.1.28) obtained
from Lactobacillus brevis has been previously reported [ 91 ]. This method would be
a promising approach for MEK biosynthesis in the future. In a more recent paper of
Wang et al. [ 92 ], dehydration of pretreated 2,3-BD fermentation broth to MEK
using dilute sulphuric acid as catalyst was conducted. The[95% yield of MEK was
then distilled from the broth. As the boiling point of 2,3-BD was greater than that of
MEK (180 vs. 79C), MEK could be distilled from the broth more easily. The
designed 2,3-BD-MEK coupling production process would reduce the energy
consumption for the downstream processing of 2,3-BD fermentation, and achieve
the production of the 2,3-BD value-added derivative at the same time. Recently,
besides the homogeneous catalysis process for MEK production from 2,3-BD, great
attention has been paid to the heterogeneous catalysis process using solid acid as
catalyst, such as the zeolites of NaY and ZSM-5, and also the metal-modified
zeolites, as this means reduced consumption of sulphuric acid and its possible
pollution [ 93 - 95 ].
To further improve the added value of the hemicellulose-derived 2,3-BD, the
coupled MEK produced could be further hydrogenated to 2-butanol, a novel
biofuel. Butanol, which has four isomers, n-butanol, 2-butanol, iso-butanol, and
t-butanol, contains more energy, is more hydrocarbon-like, and blends easier with
gasoline than ethanol. Furthermore, butanol does not absorb moisture from air;
without moisture, butanol causes no corrosion. Butanol and butanol-gasoline
blends can be transported through existing pipelines, without expensive trucking.
Major oil companies are now showing more interest in butanol than ethanol [ 96 ].
Among the four butanol isomers, n-butanol is one of three products of acetone-
butanol-ethanol (ABE) fermentation that predated the petrochemical industry.
Acetone, butanol and ethanol are produced in the ratio of 3:6:1. The high toxicity of
butanol towards the bacterial cells that produce it limits the final concentration of
butanol to 14 g/L [ 97 ]. The industry disappeared after petro-processing became
popular. The process suffers from low yield, high cost of separation of co-products,
and handling in strictly anaerobic conditions. Recent interest in biofuels has