Environmental Engineering Reference
In-Depth Information
made more accessible [39]. However, as the processes involved for the production
of ethanol from lignocellulose are by the hydrolysis of cellulose to the free
monosaccharide sugars, technological advancements here could benefit any
platform molecules produced via biological, chemical or thermal treatment of
monosaccharides.
Hemicellulose is more hydrolytically sensitive than cellulose and is therefore
typically hydrolysed first when lignocellulose is being processed. The residual
cellulose is treated under harsher conditions once the hemicellulose sugars are
removed, or recovered and used such as in the pulp and paper industry [40, 41].
Several catalysed hydrolytic methodologies are applied to hemicellulose including
enzymatic, mineral acids and bases [42], and supported acids and bases [43, 44]
and metal catalysts. Both enzymatic and mineral acid and base catalysed hydroly-
sis suffers from uneconomical catalyst recovery, and residual mineral acid or base
may inhibit subsequent fermentation [45]. Supported catalysts offer benefits in the
ease of recovery (via filtration), but reduced rates of molecular diffusion of
the polymeric constituents of biomass (e.g. cellulose and lignin) into the pores of
the catalyst can reduce their suitability [46]. Hemicellulose is a branched polymer
and may contain several different sugar monomers, both pentoses and hexoses,
though xylose is usually present in the largest quantity. This diversity of sugars
within hemicellulose results in several enzymes being required for hydrolysis to
free monosaccharide monomers [38]. Hexoses recovered from the hemicellulose
can be used in the same manner as those isolated from starch and cellulose hydrol-
ysis, while the pentoses are ideally applied to the formation of furfural [47, 48], as
shown in Figure 4.3. Indeed, furfural is currently the platform molecule with
the largest global production from lignocellulose, annually in excess of 200,000
tonnes, and has the potential to lead to a diverse set of higher-value chemicals such
as 2-methylTHF, ethyl levulinate and furoic acid [47, 49]. Ethanol and butanol
have also been produced by fermentation of monosaccharides derived from hemi-
cellulose, and other higher-value platform molecules can also be readily derived
from the sugars including xylitol, levulinic acid and itaconic acid [40, 50, 51].
The conditions for the hydrolysis of cellulose to glucose are more severe than
those required for the hydrolysis of starch and hemicellulose. Such conditions
often lead to unwanted by-products, including isomeric sugars (fructose,
mannose), HMF and levoglucosan [52]. Some of these by-products, especially
furan derivatives such as HMF and furfural, are known inhibitors in some
fermentations; improved selectivity and/or post-hydrolysis detoxification are
therefore needed. As for hemicellulose, enzymes, mineral acid and base-supported
acids and metals have also been applied to the hydrolysis of cellulose [53, 54].
The milder conditions involved in enzyme-catalysed hydrolysis avoid the issues
of by-product formation and are typically high yielding, but cellulose containing
biomass requires pretreatment to improve accessibility of the molecular structure
of this robust polysaccharide [55]. Enzymatic hydrolysis of the cellulose also
typically leads mainly to oligomers rather than monomer sugar production, though
