Organosolv lignin is a type of lignin recovered from biomass in the absence of
sulfur through the treatment with organic solvent such as ethanol, methanol, acetic
acid or ketone, at low pH [11]. The use of organic solvents for lignin extraction has
the benefit of generating lignin with high purity and quality with minimal amounts
of cellulose and the ether bonds largely intact. In addition, absence of sulfur
compounds prevents the formation of organosulfur compounds, that in the case of
lignins from kraft and sulfite pulping can inhibit subsequent (bio)catalytic processing [12]. The prevalence of native lignin and the absence of sulfur compounds
renders this process advantageous for a greater biomass utilization towards
high-value products. Usually, the cellulose and the hemicellulose fraction can be
more efficiently employed in further processes, whilst the lignin is more accessible
for enzymatic depolymerization into low molecular weight compounds for direct
transformation into products or as a feedstock in fermentation (Fig. 1).
Hydrolysis lignin is a result of inefficient enzymatic processing of lignocellulose
for bioethanol production and generally contains lignin, cellulose and oligosaccharides [7]. It often contains nitrogen but is free from sulfur and in principle
represents native lignin. Fast pyrolysis [7, 9, 10] converts lignin (or lignocellulose)
into bio-oil. Upon addition of water to bio-oil, a water-insoluble fraction is formed,
called pyrolytic lignin, which in turn also can be recovered and further purified
using organic solvents to remove ash and other inorganic compounds. In contrast to
the other types of technical lignin, pyrolytic lignin is generated by repolymerization
of lignin oligomers during the pyrolysis process.
1.3 Lignin Valorization and Added Value of Lignin Derived
Products
Lignin is a polymer with high energy content due to its high C/O ratio and the
largest reservoir of bio-based aromatic building blocks on earth. Despite of this,
lignin is still an underutilized resource, mainly due to challenges in its valorization
as value-added chemical linked to the poor qualities provided from the current bulk
industrial processes that are targeting the cellulose portion of lignocellulose as the
main product. Instead, lignin is typically used for heat and power generation [9, 13],
although there exist options for the thermochemical conversion of lignin into
syngas and bio-oil for deriving other products (Fig. 1). Controlled depolymerization of lignin is challenging due to its heterogeneity. In addition, the products from
lignin depolymerization are highly diverse, which further complicates their processing and fractionation into homogenous products of marketable volumes. The
most common chemical linkages between aromatic subunits in native lignin are
b-O-4 ether linkages (representing almost 50% of all linkages), together with b-5,
b-1, b-b, 5–5, and 4-O-5 bonds, which are all challenging to break through traditional processing since radicals are formed that lead to C–C bond formation and
repolymerization [4]. In addition, in technical lignins recovered from pulping
processes, most of the b-O-4 ether linkages are broken and fragments recombined
with highly inert C–C bonds. Due to the high energy content of the polymer,
442
G.-S. Nguyen et al.
sulfur through the treatment with organic solvent such as ethanol, methanol, acetic
acid or ketone, at low pH [11]. The use of organic solvents for lignin extraction has
the benefit of generating lignin with high purity and quality with minimal amounts
of cellulose and the ether bonds largely intact. In addition, absence of sulfur
compounds prevents the formation of organosulfur compounds, that in the case of
lignins from kraft and sulfite pulping can inhibit subsequent (bio)catalytic processing [12]. The prevalence of native lignin and the absence of sulfur compounds
renders this process advantageous for a greater biomass utilization towards
high-value products. Usually, the cellulose and the hemicellulose fraction can be
more efficiently employed in further processes, whilst the lignin is more accessible
for enzymatic depolymerization into low molecular weight compounds for direct
transformation into products or as a feedstock in fermentation (Fig. 1).
Hydrolysis lignin is a result of inefficient enzymatic processing of lignocellulose
for bioethanol production and generally contains lignin, cellulose and oligosaccharides [7]. It often contains nitrogen but is free from sulfur and in principle
represents native lignin. Fast pyrolysis [7, 9, 10] converts lignin (or lignocellulose)
into bio-oil. Upon addition of water to bio-oil, a water-insoluble fraction is formed,
called pyrolytic lignin, which in turn also can be recovered and further purified
using organic solvents to remove ash and other inorganic compounds. In contrast to
the other types of technical lignin, pyrolytic lignin is generated by repolymerization
of lignin oligomers during the pyrolysis process.
1.3 Lignin Valorization and Added Value of Lignin Derived
Products
Lignin is a polymer with high energy content due to its high C/O ratio and the
largest reservoir of bio-based aromatic building blocks on earth. Despite of this,
lignin is still an underutilized resource, mainly due to challenges in its valorization
as value-added chemical linked to the poor qualities provided from the current bulk
industrial processes that are targeting the cellulose portion of lignocellulose as the
main product. Instead, lignin is typically used for heat and power generation [9, 13],
although there exist options for the thermochemical conversion of lignin into
syngas and bio-oil for deriving other products (Fig. 1). Controlled depolymerization of lignin is challenging due to its heterogeneity. In addition, the products from
lignin depolymerization are highly diverse, which further complicates their processing and fractionation into homogenous products of marketable volumes. The
most common chemical linkages between aromatic subunits in native lignin are
b-O-4 ether linkages (representing almost 50% of all linkages), together with b-5,
b-1, b-b, 5–5, and 4-O-5 bonds, which are all challenging to break through traditional processing since radicals are formed that lead to C–C bond formation and
repolymerization [4]. In addition, in technical lignins recovered from pulping
processes, most of the b-O-4 ether linkages are broken and fragments recombined
with highly inert C–C bonds. Due to the high energy content of the polymer,
442
G.-S. Nguyen et al.