Environmental Engineering Reference
In-Depth Information
to para -xylene by cycloaddition with ethylene [189-192]. Treatment of CMF
with alcohols in the presence of an N-heterocyclic carbene catalyst gives furoate
esters, which have also been shown to have excellent fuel properties [193]. Longer
carbon chain lengths are required for the engineering of diesel-like hydrocarbons,
which can be achieved by aldol condensation of CMF or CMF-derived HMF with
ketones [194]. Conjugated, electro-active polymers are produced when CMF
reacts with electron-rich aromatics such as pyrrole, furan or thiophene [195].
Finally, oxidation of CMF with nitric acid gives either 2,5-diformylfuran or
2,5-FDCA. Both of these are interesting monomers, in particular the latter which
is considered as a renewable replacement for petroleum-derived terephthalic acid
[175, 186, 196]. The equivalent polymer generated from FDCA, called polyethyl-
ene furanoate (PEF), has been shown to be competitive with PET in terms of
performance.
4.8.3
n-Butanol (Biobutanol): Biological Treatment
Classically, biobutanol has been derived from the anaerobic ABE (acetone-
butanol-ethanol) fermentation of sugars by Clostridium sp., a process that was
first described by Pasteur in 1862 and first industrialised in the UK in the early
part of the twentieth century [197]. But even using the organism of choice for
this process ( Clostridium acetobutylicum ), the production rate is modest and the
solvent titre peaks at around a 2% solution which involves high water usage and
product isolation costs; this is the main reason that biobutanol has not until
recent times attracted stronger market interest. Significant recent advances in the
selection or engineering of strains of Clostridium for higher butanol selectivity
and stress tolerance, as well as the insertion of the butanol pathway into
heterologous organisms such as E. coli and Saccharomyces cerevisiae , are
however leading towards the production of butanol in industrially competitive
yields [198]. In fact, this platform molecule has attracted sufficient attention to
merit the publication of 25 review articles on biobutanol over the past 4 years.
Parallel developments in feedstock selection, reactor management and down-
stream processing are also encouraging further interest in biobutanol
commercialisation. The current movement away from the use of food crops in
favour of lignocellulosic feedstocks is therefore advantageous for Clostridium ,
which can utilise both 5- and 6-carbon sugars, thereby increasing the useable
biomass input into the process over those that can only ferment hexoses.
Production limitations may also be alleviated by transitioning away from batch
fermentation towards continuous processes by immobilisation of organisms on a
support. The application of a two-stage, dual-path procedure that relies on
different species of Clostridium has been shown to mitigate selectivity issues by
decoupling carboxylic acid formation from its reduction to alcohols [199, 200].
Finally, in situ butanol removal during fermentation can improve productivity
and diminish product inhibition. For example, gas stripping is a comparatively
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