degradation of lignin into phenolic compounds or mono-lignins requires high
energy input. This is one of the reasons why lignin is often regarded to have a
higher value as a fuel compared to its exploitation as feedstock for other uses [4].
There are different methods for lignin isolation, aiming at opening and degrading
the lignin structure well enough to solubilize the resulting fragments in the pulping
media. Once solubilized, these fragments can be separated from the cellulose [4, 8].
However, the pulping process leaves behind a large fraction of highly condensed,
low-quality and low-value lignin. Still from there, different technical and/or
(thermo)chemical processes, i.e., pyrolysis, gasification, hydrothermal liquefaction,
oxidation, and combustion convert lignin into products that can be directly
employed as, or further upgraded into other chemicals [4, 6]. The output from these
processes are bio-oil, char, syngas (CO, H 2 ) and other gases (CO 2 , CH 4 ), where
char is primarily used for power generation, whilst the produced gases can be
channeled into both production of chemicals and fuels (Fig. 1). Besides, combustion produces heat that can be utilized for power production. Both pyrolysis and
oxidation lead to production of liquid bio-oil including chemicals like phenols,
aldehydes and aliphatics, that can be further processed in conventional oil
refineries.
Besides its breakdown, the lignin macropolymer can be chemically modified and
used in the synthesis of polyurethanes, polyesters, epoxide resins and phenolic
resins [4]. Polyurethanes are generated by addition of, e.g., diisocyanates and
polyols to the macropolymer backbone, forming polyurethane groups that provide
the materials with different physical properties dependent on the lignin content used
in the process. Polyesters from lignin are used as the starting material in, e.g.,
different kinds of plastics. Epoxy resins are built from monomers with minimum
one epoxide group and are a type of thermoset polymer typically used in coatings,
electronic materials, and adhesives. Phenolic resins are also thermosets with similar
applications. These are, however, generated by acid- or base-catalyzed polymerization of phenols and aldehydes, whereas the phenolic content of lignin can be
used as the phenol source in this process.
Some processed/technical lignins (such as the lignosulfonates from the sulfite
pulping process) can be used directly in applications such as production of plasticizers in concrete, emulsifiers/emulsion stabilizers, surfactants for pesticides,
drilling muds, resins, fertilizers, and glues [8], and most of the lignin in commercial
application today is used in the form of lignosulfonates [14]. However, they can
also be processed further, resulting in mono-lignins such as phenol, vanillin, guaiacol, ferulate, caffeate and p-coumarate, which in turn have other applications [10,
14] (Fig. 1). They can be used directly as chemicals or further converted by biotransformation and/or fermentation to produce a wide range of chemicals, fuels,
materials, and flavors employing microbial cell factories [4, 8]. It is highly desirable
to utilize the aromatic components of lignin for the production of aromatic solvents,
such as benzene, toluene, and xylene, which are still primarily produced from
petroleum [4, 15]. However, this would require the aromatic monomers to be
deoxygenated and isolated from lignin. Chemical processes for that purpose exist
but have not yet been successfully applied to utilize the bulk of the complex lignin
Recent Advances in Enzymatic Conversion of Lignin …
443
energy input. This is one of the reasons why lignin is often regarded to have a
higher value as a fuel compared to its exploitation as feedstock for other uses [4].
There are different methods for lignin isolation, aiming at opening and degrading
the lignin structure well enough to solubilize the resulting fragments in the pulping
media. Once solubilized, these fragments can be separated from the cellulose [4, 8].
However, the pulping process leaves behind a large fraction of highly condensed,
low-quality and low-value lignin. Still from there, different technical and/or
(thermo)chemical processes, i.e., pyrolysis, gasification, hydrothermal liquefaction,
oxidation, and combustion convert lignin into products that can be directly
employed as, or further upgraded into other chemicals [4, 6]. The output from these
processes are bio-oil, char, syngas (CO, H 2 ) and other gases (CO 2 , CH 4 ), where
char is primarily used for power generation, whilst the produced gases can be
channeled into both production of chemicals and fuels (Fig. 1). Besides, combustion produces heat that can be utilized for power production. Both pyrolysis and
oxidation lead to production of liquid bio-oil including chemicals like phenols,
aldehydes and aliphatics, that can be further processed in conventional oil
refineries.
Besides its breakdown, the lignin macropolymer can be chemically modified and
used in the synthesis of polyurethanes, polyesters, epoxide resins and phenolic
resins [4]. Polyurethanes are generated by addition of, e.g., diisocyanates and
polyols to the macropolymer backbone, forming polyurethane groups that provide
the materials with different physical properties dependent on the lignin content used
in the process. Polyesters from lignin are used as the starting material in, e.g.,
different kinds of plastics. Epoxy resins are built from monomers with minimum
one epoxide group and are a type of thermoset polymer typically used in coatings,
electronic materials, and adhesives. Phenolic resins are also thermosets with similar
applications. These are, however, generated by acid- or base-catalyzed polymerization of phenols and aldehydes, whereas the phenolic content of lignin can be
used as the phenol source in this process.
Some processed/technical lignins (such as the lignosulfonates from the sulfite
pulping process) can be used directly in applications such as production of plasticizers in concrete, emulsifiers/emulsion stabilizers, surfactants for pesticides,
drilling muds, resins, fertilizers, and glues [8], and most of the lignin in commercial
application today is used in the form of lignosulfonates [14]. However, they can
also be processed further, resulting in mono-lignins such as phenol, vanillin, guaiacol, ferulate, caffeate and p-coumarate, which in turn have other applications [10,
14] (Fig. 1). They can be used directly as chemicals or further converted by biotransformation and/or fermentation to produce a wide range of chemicals, fuels,
materials, and flavors employing microbial cell factories [4, 8]. It is highly desirable
to utilize the aromatic components of lignin for the production of aromatic solvents,
such as benzene, toluene, and xylene, which are still primarily produced from
petroleum [4, 15]. However, this would require the aromatic monomers to be
deoxygenated and isolated from lignin. Chemical processes for that purpose exist
but have not yet been successfully applied to utilize the bulk of the complex lignin
Recent Advances in Enzymatic Conversion of Lignin …
443