408
Y. Bolbukh et al.
As lignin has different functional groups in its structure, it has become an interesting material with prospects of application in many areas, including adsorption
of heavy metals [6], as a substrate for the synthesis of low molecular organic
compounds [7] and, in particular, as an active component of biocomposites [8, 9].
Moreover, lignin strongly participates in water economy and helps with water and
nutrient transport. The presence of various functional groups (aliphatic and aromatic
OH groups) makes lignin a suitable substance for chemical syntheses. The amounts
of functional groups depend on the source of the lignin, the process used to extract
it, and/or posttreatment. Kraft lignin is a by-product of kraft pulping of wood.
Kraft lignin contains a greater amount of phenolic groups, due to the extensive
cleavage of b-aryl bonds during kraft pulping, some biphenyl units, as well as
other condensed structures as a result of the severe cooking conditions [10]. The
phenolic and aliphatic hydroxyl groups present in the lignin structure can be used
for chemical modifications. Based on the reaction parameters and the reactants
used, esterification is one of the easiest methods of the chemical modification
of lignin. Depending on the type of new introduced groups, the properties of
lignin, such as hydrophobicity, solubility, and thermal behavior, can be significantly
changed. Esterification reactions have also been applied to increase lignin reactivity
by introducing new active sites into the lignin macromolecule that are able
to copolymerize with other monomers [11]. The lignin derivatives with acrylic
functionality and containing reactive vinyl groups through esterification reactions
have been successfully copolymerized with styrene (St) and divinylbenzene (DVB)
to produce functional porous microspheres for use as specific sorbents in solid-phase
extraction [12].
With an esterification reaction when a carboxylic acid is treated with an alcohol
and an acid catalyst, an ester is formed (along with water). However, this reaction
can be used not only to introduce new active sites into the polymer structure
but also to immobilize the polymer on the inorganic carrier. Organic/inorganic
hybrid materials combine most applicable properties for practical using. One of
the most popular inorganic components for such materials is silica. There are a
lot of studies for lignin/silica hybrid materials, but it used few main methods to
obtain the samples, such as sol–gel synthesis with tetraethoxysilane (TEOS) [13–
15] and precipitation in a nonpolar or polar medium using sodium silicate [16].
An advanced functional silica/lignin material was obtained when silica was initially
modified with N-2-(aminoethyl)-3-aminopropyltrimethoxysilane [5] to activate the
carrier surface. In order to activate the lignin, an oxidation process was carried
out using sodium iodate. The activation of lignin and initial functionalization of
silica made possible the chemical combination of the precursors to produce a
functional SiO 2 /lignin sorbent. In [17] the silica/lignin interaction was achieved due
to silica surface functionalization with aminosilane and then modified with kraft
lignin preliminary activated with sodium periodate solution. In other work the key
parameter of silica/lignin interaction was lignin modification with periodate [18].
So, the production of novel inorganic/organic composites with specific application possibilities has recently become a pressing need. The use of organic products
of natural origin, including biomass in the broad sense of the word, is of particular
Y. Bolbukh et al.
As lignin has different functional groups in its structure, it has become an interesting material with prospects of application in many areas, including adsorption
of heavy metals [6], as a substrate for the synthesis of low molecular organic
compounds [7] and, in particular, as an active component of biocomposites [8, 9].
Moreover, lignin strongly participates in water economy and helps with water and
nutrient transport. The presence of various functional groups (aliphatic and aromatic
OH groups) makes lignin a suitable substance for chemical syntheses. The amounts
of functional groups depend on the source of the lignin, the process used to extract
it, and/or posttreatment. Kraft lignin is a by-product of kraft pulping of wood.
Kraft lignin contains a greater amount of phenolic groups, due to the extensive
cleavage of b-aryl bonds during kraft pulping, some biphenyl units, as well as
other condensed structures as a result of the severe cooking conditions [10]. The
phenolic and aliphatic hydroxyl groups present in the lignin structure can be used
for chemical modifications. Based on the reaction parameters and the reactants
used, esterification is one of the easiest methods of the chemical modification
of lignin. Depending on the type of new introduced groups, the properties of
lignin, such as hydrophobicity, solubility, and thermal behavior, can be significantly
changed. Esterification reactions have also been applied to increase lignin reactivity
by introducing new active sites into the lignin macromolecule that are able
to copolymerize with other monomers [11]. The lignin derivatives with acrylic
functionality and containing reactive vinyl groups through esterification reactions
have been successfully copolymerized with styrene (St) and divinylbenzene (DVB)
to produce functional porous microspheres for use as specific sorbents in solid-phase
extraction [12].
With an esterification reaction when a carboxylic acid is treated with an alcohol
and an acid catalyst, an ester is formed (along with water). However, this reaction
can be used not only to introduce new active sites into the polymer structure
but also to immobilize the polymer on the inorganic carrier. Organic/inorganic
hybrid materials combine most applicable properties for practical using. One of
the most popular inorganic components for such materials is silica. There are a
lot of studies for lignin/silica hybrid materials, but it used few main methods to
obtain the samples, such as sol–gel synthesis with tetraethoxysilane (TEOS) [13–
15] and precipitation in a nonpolar or polar medium using sodium silicate [16].
An advanced functional silica/lignin material was obtained when silica was initially
modified with N-2-(aminoethyl)-3-aminopropyltrimethoxysilane [5] to activate the
carrier surface. In order to activate the lignin, an oxidation process was carried
out using sodium iodate. The activation of lignin and initial functionalization of
silica made possible the chemical combination of the precursors to produce a
functional SiO 2 /lignin sorbent. In [17] the silica/lignin interaction was achieved due
to silica surface functionalization with aminosilane and then modified with kraft
lignin preliminary activated with sodium periodate solution. In other work the key
parameter of silica/lignin interaction was lignin modification with periodate [18].
So, the production of novel inorganic/organic composites with specific application possibilities has recently become a pressing need. The use of organic products
of natural origin, including biomass in the broad sense of the word, is of particular