Environmental Engineering Reference
In-Depth Information
O
OH
O
Pyrolysis
OH
O
OH
O
Hydrolysis
O
OH
O
O
Lignin
OH
OH
OH
HO
Hydrogenolysis
O
Gasification
H
2
, CO
2
, CO, CH
4
Figure 4.8
Example platform molecules derived from lignin.
collected, though gasification followed by Fischer-Tropsch processing is one
avenue to produce small building blocks from lignin. Chemical depolymerisation
covers a broader range of conditions where various temperatures, catalysts (acid,
base, metals and ionic liquids) and reagents (e.g. water and hydrogen) are applied
[83]. Hydrothermal hydrolysis is performed at lower temperatures (<400°C) than
pyrolysis, though higher pressures are often used to increase efficiency of the
process. Hydrothermal treatments can be tailored towards gasification or
liquefaction depending upon temperature, with higher temperatures favouring gas
formation. Hydrogenolysis is performed at similar temperatures and pressures to
hydrothermal hydrolysis but steam is replaced with H
2
, or another source of
hydrogen such as formic acid, and often requires the addition of catalysts. The
milder conditions for hydrolysis and hydrogenolysis hold greater promise for the
production of platform molecules from lignin compared to pyrolysis. However, all
the technologies currently available for the formation of platform chemicals from
lignin are energy intensive and require costly purification of complex mixtures.
Investigations into the controlled enzymatic depolymerisation of lignin have so
far proven ineffective for the formation of platform chemicals. The microbes used
to degrade lignin do so by the enzymatic formation of hydrogen peroxide, and the
products from the initial peroxide-mediated depolymerisation are themselves
further degraded before effective isolation can take place [81].
Several platform molecules derivable from lignin have been highlighted in the
various reviews discussed above, though the molecules with greatest potential are
guaiacol, catechol, vanillin, cresol, syringol, phenol, vanillic acid and syringic
acid (Figure 4.8) [84, 85]. There are a couple of additional interesting candidates
observed occasionally in lignin degradation, including 4-vinylphenol, 4-vinyl-
guaiacol (both via hydrothermal depolymerisation in the presence of ethanol) and
β-ketoadipic acid (by bacterial degradation of phenolics), but more research is
required in all three cases [4, 86, 87]. Of all the molecules mentioned above, only
vanillin production from the lignosulphonate generated during the sulphite
pulping process is currently commercialised. This route is however low yielding
