Search

The main goal of this study was the elucidation of structures of products of oxidation of industrial hydrolysis lignin (HL) obtained in the system H2O2-H2SO4-H2O. It was found that the obtained oxidized hydrolysis lignin (OHL) contained structural fragments of muconic acid dilactone. The latter appeared as a result of the oxidation of aromatic ring of HL into unstable structures of 3,4-dihydroxyadipic acid, which are spontaneously cyclized into the corresponding fragments of muconic acid dilactone in acidic oxidative medium. Consequent reaction of OHL with aqueous sodium hydroxide (NaOH) led to the formation of sodium salt of OHL (Na-OHL), in which the muconic acid dilactone units of OHL opened into fragments of disodium salts of 3,4-dihydroxyadipic acid. The Na-OHL was transformed into the series of novel derivatives of lignins. Thus, the reaction of Na-OHL with thionyl chloride proceeded as a substitution of hydroxyl groups into chlorine atoms in the structure of OHL, that afforded chloro-derivative of OHL. The latter was transformed into amino-amide and ester-ether derivatives of OHL by nucleophilic substitution of chlorine atoms with amines and alcohols correspondingly.

Potentiometric titration, FTIR spectroscopy, 13C solid-state NMR spectroscopy and chemical analysis were used to study the gelation of technical lignins (oxidized hydrolysis, kraft, soda and lignosulfonates) and Pepper lignin in interaction with metal salts (CaCl2, Mg(NO3)2, FeSO4, Co(NO3)2, Pb(NO3)2, Fe(NO3)3, CuSO4, AgNO3). It has been found that compounds that are salts of weak base and strong acid possess an ability to gelation; the metal cation has a standard potential above a certain value, has moderate hydrolysis ability and the cation hydroxide formed in the hydrolysis reaction has a relatively low solubility. At the gel formation, a compounds of the composition R–COO–Mez +–OOC–R, z = 2, 3 were formed. A scheme of intermolecular bonds at the point of lignin hydrogel formation was proposed, according to which, adjacent lignin molecules bound to each other by three types of bonds: ionic between carboxyl groups of lignin and metal cation, coordination between metal cation and water molecules included in the coordination sphere, and hydrogen bonds, binding all components of the system into a single whole. Lignin hydrogels have a high sorption ability with respect to heavy metals such as lead, iron and copper, that, depending on the content of acid groups in lignin and the molar mass of the sorbate, is approximately 25–50% by weight of lignin. This opens up the possibility of using a new type of lignin hydrogels for wastewater treatment of chemical plants.

Lignin valorization is strongly dependent on a right strategy for converting lignin into value-added products. Conversion of lignin into monomeric degradation products is one of the important avenues. In this study, chemical mechanisms and monomeric product compositions in hydrolysis (acidic and alkaline), hydrogenolysis, catalytic oxidation, electrochemical oxidation and reduction, photochemical and enzymatic degradation of native and technical lignins, and lignin model compounds are comparatively analyzed. The effect of the structure of a phenylpropane unit in lignin on the chemical reactivity of α-O-4 and β-O-4 bonds in cleavage reactions is also described. Published experimental data suggest some form of activation to be a necessary prerequisite for the splitting of a β-O-4 bond in all chemical reactions under consideration, the nature of which is dependent on the reagents and reaction conditions. Thus, in catalytic oxidation processes, a benzyl hydroxyl group is converted into carbonyl group at the first stage. Chemical transformation involving the α-position in a phenylpropane unit is a usual trigger of further lignin depolymerization. The yield of monomeric products of hydrogenolysis of native lignin is close to the theoretical one, reaching 23% in softwood and 51% in hardwood; in alkaline hydrolysis as well as oxygen and nitrobenzene oxidation of native lignin, the trend is the same that is explained in this study. On the other hand, the yield of monomeric products from isolated samples and technical lignins is much lower. A loss of arylether bonds in the process of lignin separation from wood and in wood pulping explains this difference.

This is a review of historical and modern literature data on the structure versus properties of wood lignin in view of the concepts developed by the authors based on their own research. Changes in the structure of lignin and related changes in its chemical reactivity during alkaline wood pulping are assessed based on the comparison of the structures of lignin at three kinetically distinct stages of delignification: initial, bulk and final. Lignin gradually moves from a solid to a liquid phase during the pulping process; therefore, structures of native, dissolved and residual lignin are elucidated and compared. The emphasis is on changes in the molecular weight distribution and content of alkylarylether bonds, and functional groups, in particular phenolic hydroxyls. For comparison, splitting rates for α- and β-alkylarylether bonds in both phenolic and non-phenolic lignin model compounds are analyzed. Based on the comparative analysis of the experimental data, it is suggested that native lignin in wood consists mainly of three distinct fractions that are different in chemical reactivities of alkylarylether bonds. This phenomenon results in three kinetically distinct stages of the pulping process. Wood delignification is essentially a process of lignin functionalization followed by its dissolution. The functionalization, i.e., formation of additional functional groups in the macromolecule, continues until it reaches the level sufficient for lignin dissolution under chosen conditions, and then, delignification occurs. At the bulk stage of pulping, the rate of delignification is directly proportional to the degree of functionalization. The data characterizing the effect of redox reactions on the structure and chemical reactivity of lignin in alkali–anthraquinone pulping are analyzed in detail in view of their general importance for our understanding of the chemical reactivity of lignin. Results of polarographic studies of numerous representative lignin model compounds (> 70 samples) and lignin samples, including chemically changed lignins, are compiled, and a diagram of reduction potentials of polarographically active functional groups in lignin is drawn. From a comparison of redox properties of lignin and 32 pulping additives, criteria for selection of potential alkaline pulping catalysts are derived. Solubility belongs to basic properties of polymers, lignin included, and all methods of delignification of plant materials can be essentially reduced to solid polymer functionalization followed by its dissolution. Factors contributing to lignin solubility are analyzed, including such characteristics as molecular weight, temperature, liquid–solid ratio and ionic strength. Based on the analyzed data, a uniform scale of solubility for different lignin types is proposed, and formulae for calculating lignin solubility in alkaline media are derived.

An analytical calculation method for the estimation of the contents of alkyl aryl ether bonds (α-O-4 and β-O-4) in lignin was developed. In the framework of the method, Alkyl–O–Aryl type bonds are described as coupled phenolic hydroxyls (OHphen). The method is based on the balance equation including the free and coupled OHphen contents in dissolved and residual lignins, on the one hand, and their respective contents in native lignin, on the other. The free OHphen content is calculated on the basis of the OHphen contents of dissolved and residual lignin, determined by the aminolysis method in the course of kraft cooking of softwood. The calculation results for soluble lignin preparations are in good agreement with the 13C NMR (nuclear magnetic resonance) spectral data for the solutions. The content of Alkyl–O–Aryl bonds in native softwood (pine, spruce) lignin was estimated at 79/100 PPU (phenylpropane unit). In isolated lignin preparations, the contents of these bonds decrease in the sequence: Freudenberg...

The structure of hydrolysis lignin oxidized with hydrogen peroxide in acid medium was studied by NMR spectroscopy and by matrix-assisted laser desorption/ionization and electrospray ionization mass spectrometry. The sorption, ion-exchange, and surfactant properties of oxidized hydrolysis lignin were studied, and possible application areas of this substance were suggested.

A method is developed to assess the limits of selective depolymerization, that is, splitting of aryl ether bonds to yield monomeric aromatic products, of native and technical lignins. The method and respective formula is based on measuring the difference between a sum of aryl ether bonds (α-О-4 and β-О-4) and sum of 5-5 and 4-O-5 bonds. The values calculated based on the proposed formula are correlated with published experimental data on catalytic degradation of lignin. In all cases, wood species and experimental conditions notwithstanding, the yields of monomeric aromatic products are found to be close to theoretically calculated for both softwood (23%) and hardwood (51%). Notice that in the actual experiments the yield in hardwood is twice as much as in softwood. In technical lignins (kraft, organosolv) the yields of monomeric products are lowed due to splitting of α-О-4 and β-О-4 bonds during pulping. The developed method is helpful in evaluation of technologies aiming to bring value-added products from lignin through selective degradation.

Several lignin preparations (Freudenberg lignin, Björkman lignin, and Pepper lignin), technical lignins (soda, soda-AQ, Kraft, Kraft-AQ, and hydrolysis), dimeric lignin model compounds, and different polysaccharides (galactoglucomannan, arabinogalactan, xylan, and arabinan) were analyzed by means of solid-state 13C CP/MAS NMR. Signals assignment in solid-state NMR lignin spectra was performed on the basis of the conducted studies and earlier published data. It was established that there exists strong linear correlation (r = 0.985) between Alkyl-O-Aryl inter-unit bond content in lignin and integral signals intensity in NMR spectra in the range of chemical shifts of 96–68 ppm. The integral signals intensity was measured in correlation with the reference integral signals in the range of chemical shifts of 162–102 ppm, typical for aromatic carbon atoms. To eliminate the effect, caused by carbohydrates contained in lignin, the correction factor of 0.67% of the area of integration per 1% of carbohydrates was determined. It was shown that the solid-state 13C CP/MAS NMR method allowed to determine Alkyl-O-Aryl bond content in both soluble and insoluble lignin preparations, and also to determine methoxyl groups content in soluble preparations.

Products of oxidation of industrial acid hydrolysis lignin in the H2O2-H2SO4 system were studied using 13C NMR (in solution and solid state), MALDI-MS, and MS(ESI) techniques. Oxidation of hydrolysis lignin leads to the opening of aromatic rings of lignin, yielding carboxylic groups. Alkyl aryl ether linkages (β-O-4-bonds) between lignin phenyl propane units are not significantly affected by the oxidation. The structure of oxidized hydrolysis lignin is proposed. The basic structural unit of oxidized hydrolysis lignin is a muconic acid derivative.

Lignin model dimers representing all typical interunit linkages in lignin (β‐O‐4, α‐O‐4, α‐O‐γ, β–5, 5–5, β–β, stilbene, enol ether) were characterized by differential pulse polarography (DPP) and cyclic voltammetry (CV) in DMSO. Polarographic activity was registered for α‐(C˭O)‐β‐O‐4 structures and conjugated double bonds in stilbene and enol ether structures. A structure–chemical reactivity correlation diagram is drawn based on the studies of one‐ and two‐unit model structures of lignin. Peaks in differential pulse polarograms of lignin were assigned to certain functional groups and bond types based on the comparison between the data obtained on model compounds and selectively chemically modified [NaBH4, LiAlH4, CH2N2, (CH3O)2SO2] lignin samples.

Electrochemical properties of compounds modeling typical structural units of lignin were determined by differential pulse polarography in dimethylsulfoxide with tetrabutylammonium perchlorate as a supporting electrolyte. p-Quinonemethides; derivatives of cinnamyl aldehyde and cinnamic acid; aromatic aldehydes, ketones, diketones, and carboxylic acids; isoeugenol; and coniferyl alcohol were studied. Structural effects on the reduction potentials of lignin units were estimated. The potentials, which correspond to the first polarographic peak, varied from -0.8 (p-quinonemethide) to -2.8 V (coniferyl alcohol). The compounds containing nonconjugated carbonyl or carboxyl groups were not polarographically active. The autoprotonation reaction had a profound effect on the electrochemical reduction of compounds containing hydroxyl groups.