Since the term “lignin” refers to a heterogenous material and not a single compound,
it is useful to group lignins based on some shared characteristics. The process by which lignins are obtained is an acceptable approach for this purpose. However, it should be noted that many differences will remain among lignins of the same group. For instance, the so called
“native lignin” shares some properties, e.g. a relatively high number of β-O-4’ linkages, but lignins from wheat straw, spruce or eucalyptus, have been proven to be essentially different from each other, even though all of them were produced as native lignins [19].
Kraft lignin process: This is the main chemical pulping process and consists in the treatment of lignin at high pressure and temperatures of 150 to 180 °C, at a high pH conditions and in presence of NaOH, Na2SO4 and polysulfide. Lignin is solubilized and then recovered by precipitation, lowering the pH. The product is known as kraft lignin, it shows an average molecular weight between 1000 and 3000, and about 70% to 75% is chemically sulfonated, which makes it soluble in alkali and confers it with surfactant properties. In addition, it shows increased phenolic hydroxyl groups and biphenyl structures and it is bound to some residual carbohydrates. Kraft lignin represent the largest source of available lignin for the biorefinery [6, 11, 20].
Lignosulfonate process: Lignosulfonate is a by-product of wood pulp production when sulphite pulping has been used as the extract process. It consists in treatment of lignin at high temperatures and pressure, in presence of various salts of sulphurous acid, with a pH varying from 2 to 12, depending on the cationic compositions of the pulping liquor, though many processes are carried out in acidic conditions. The product, sulfonated lignin, contains many sulfonate groups in the aliphatic side chains and is negatively charged. A pH shift is not enough to isolate it, so desulfonation steps are needed before its precipitation, normally in presence of Ca. Its molecular weight ranges from 5000 to 20000 (higher than kraft lignin) and it is very soluble in water in the whole range of pH [6, 20, 21].
Organosolv process: Organosolv pulping is a process by which components of wood are separated by using organic solvents at high pressures and temperatures, giving separate streams of cellulose, hemicellulose and lignin. A list of some solvents used in this process include acetic acid, formic acid, peroxy-organic acids and ethanol. Some processes have been commercially registered and used at industrial scale, e.g. the Acell process, which uses a mixture of ethanol – water, or CIMV process, which uses a mixture of acetic acid – formic acid – water. Organosolv lignin is isolated easily by solvent removal or precipitation with water. This mean of extraction makes it possible to obtain a less modified lignins at a higher
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purity. The average molecular weight is less than 1000, making it the lignin with the lower MW, and it is insoluble in water, though some organosolv lignins could be soluble in alkali conditions and may contain phosphorous groups [6, 20, 21].
Steam explosion process: In this process, lignin is treated for a short time, from 1 to 20 minutes, with steam at high temperature and pressure. Temperature ranges from 180 to 230 °C, while pressures can be between 200 to 500 psi. Then, the pressure is released rapidly,
hence the name “steam explosion”. Lignin can be recovered from the resulting product, by alkali washing or solvent extraction and its molecular weight is lower than kraft lignin, while its solubility in alkali is higher. Some modifications of product stream include a reduction of
β-O-4’ linkages and increase of C-C bonds, resulting in a more condensed lignin [20, 22]. Processes resulting in native lignin: processes such as cellulolytic treatment or milling, consisting in an enzymatic degradation and wood grounded in a ball mill, respectively, followed by a solvent extraction; have been reported to give similar lignin products. These techniques are carried out normally at room temperature and pressure. The
resulting lignins show a relatively high number of β-O-4’ linkages and are structurally
identical from each other, showing some carbohydrates residues. The final product resembles the previously described native lignin, also called protolignin [23, 24].
Other processes, e.g. pyrolysis, ionic liquid extraction or acidic hydrolysis, are not discussed, as their utilization is restricted, or not well developed [21].
The field that seeks for a better understanding of lignin properties is called lignin characterization; and comprises diverse analysis and laboratory techniques. Powerful imaging, spectroscopy, functional and chemical characterization methods have been developed with this purpose. The characteristics that have been studied include elemental analysis and empirical formula, determination of molecular weight, analysis of functional groups and frequencies of linkage types and functional groups in lignin. In addition, chemical and biochemical characterization and modification of lignin, including biodegradability and enzyme-based oxidation, have also been studied. The appropriate techniques to conduct this type of research varies, depending on the characteristic under study. For instance, gel permeation chromatography, light scattering, vapor-pressure osmometry and ultrafiltration can be used to determine the lignin molecular weight [3, 25]. Considering that no single method could stand alone as the supreme method for lignin characterization, techniques involving nuclear magnetic resonance (NMR), specially 13C NMR, which provides structural and functional information with high sensitivity and precision; imaging; mass spectrometry;
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and vibrational spectroscopy; have proven to be very valuable for lignin characterization [25].