Evaluation of lignins from side-streams generated in an olive tree pruning-based biorefinery: Bioethanol production and alkaline pulping

Lignin and green solvent extracted phenolic compounds from date palm leaves as functional ingredients for the formulation of soy protein isolate biocomposite packaging materials: A circular packaging concept

The current study investigated valorization of lignin nanoparticles (LNPs) and phenolic compounds loaded in chitosan (DLECNPs) extracted from date palm leaves into the soy protein isolate (SPI) biocomposite films. The mechanical, structural, barrier, physiochemical, thermal, optical, antioxidant, and antimicrobial properties of the formulated composite films were investigated. The findings showed that the incorporation of DLECNPs into the SPI films significantly improved the film's antioxidant properties by more than 3 times and showed antibacterial inhibition zone in the range of 10-15 mm against six pathogenic bacteria. Further, incorporating LNPs into SPI-DLECNPs films notably improved the mechanical properties from 4.32 MPa and 29.27 % tensile strength and elongation at break, respectively to 10.13 MPa and 54.94 %, the water vapor permeability from 7.38 g/Pa s m to 5.59 g/Pa s m, and the antibacterial inhibition zone from a range of 10.2 mm to 15.0-21.5 mm as well as making the films more heterogeneous and stronger than control SPI film. Moreover, LNPs changed the initial films' color from light yellow to dark red and reduced the films' transparency. The results indicated that LNPs reinforced SPI composite films showed significant improvements in several properties and thus can be used as a potential ingredient for formulation of biodegradable packaging films.

Bioethanol lignin-rich residue from olive stones for electrospun nanostructures development and castor oil structuring

This work describes the chemical and structural characterization of a lignin-rich residue from the bioethanol production of olive stones and its use for nanostructures development by electrospinning and castor oil structuring. The olive stones were treated by sequential acid/steam explosion pretreatment, further pre-saccharification using a hydrolytic enzyme, and simultaneous saccharification and fermentation (PSSF). The chemical composition of olive stone lignin-rich residue (OSL) was evaluated by standard analytical methods, showing a high lignin content (81.3 %). Moreover, the structural properties were determined by Fourier-transform infrared spectroscopy, nuclear magnetic resonance, and size exclusion chromatography. OSL showed a predominance of β-β' resinol, followed by β-O-4' alkyl aryl ethers and β-5' phenylcoumaran substructures, high molecular weight, and low S/G ratio. Subsequently, electrospun nanostructures were obtained from solutions containing 20 wt% OSL and cellulose triacetate with variable weight ratios in N, N-Dimethylformamide/Acetone blends and characterized by scanning electron microscopy. Their morphologies were highly dependent on the rheological properties of polymeric solutions. Gel-like dispersions can be obtained by dispersing the electrospun OSL/CT bead nanofibers and uniform nanofiber mats in castor oil. The rheological properties were influenced by the membrane concentration and the OSL:CT weight ratio, as well as the morphology of the electrospun nanostructures.

Successive organic solvent fractionation and homogenization of technical lignin for polyurethane foam with high mechanical performance

This work demonstrates an organic solvent fractionation method for lignin homogenization, which can effectively reduce the lignin heterogeneity and use each lignin fraction to prepare polyurethane foams (PUFs) with excellent mechanical properties. Such fractions were fully characterized by GPC, NMR (³¹P, 2D-HSQC), FTIR, and TG to obtain a detailed description of the structures and properties. The properties of PUFs from each lignin fraction showed higher compatibility than that from unfractionated industrial lignin, as studied by morphology and DSC analysis. The improvement of compatibility between the fractionated lignin fractions and polyethylene glycol can effectively enhance the mechanical properties of the prepared PUFs. The hysteresis loss (43.10%-51.85%) and resilience (95.81%-98.81%) of the fractionated lignin polyurethane foams (LPUFs) were better than that from the unfractionated LPUFs (hysteresis loss 41.64%, resilience 94.67%) at the lignin content of 5%. Subsequently, the strong relationships between lignin structures and PUF properties were demonstrated in detail. The suggested approach provides greater possibilities to prepare LPUFs with tunable properties based on real industrial lignin fractions, rather than modified lignin.

Preparation of lignin modified hyper-cross-linked nanoporous resins and their efficient adsorption for p-nitrophenol in aqueous solution and CO2 capture

A series of lignin modified hyper-cross-linked nanoporous resins (LMHCRs) had been synthesized from lignin, 4-vinylbenzyl chloride, and divinylbenzene by free radical polymerization reaction and following Friedel-Crafts reaction. The results indicated that Brunauer-Emmett-Teller surface area (S_BET) of LMHCRs decreased with different degrees compared with polymeric microspheres (HCRs) without adding lignin. With increasing the feeding amount of lignin, the S_BET of LMHCRs first increased and then decreased, and LMHCR-2 had larger S_BET (968.52 m²/g) and average pore size (D_A: 2.51 nm). Meanwhile, their contact angle continuously decreased from 92.1⁰ to 71.3⁰, indicating the enhanced polarity. Interestingly, the adsorption capacity of p-nitrophenol (PNP) on all LMHCRs were obviously higher than rhodamine B, and LMHCR-2 had the largest capacity ratio (3.780) of PNP to rhodamine B or other organic dyes at 298 K. Specifically, the Q_m of PNP on LMHCR-2 reached the largest value (492.1 mg/g) due to its suitable porosity and favorable surface polarity. LMHCR-2 also displayed excellent CO₂ capture (86.5 mg/g) at 273 K and 1 bar and good reusability. This study provided an efficient route to modify hyper-cross-linked resin by using the residual lignin, and showed the enhanced adsorption performance.

Emulsion Stabilization by Cationic Lignin Surfactants Derived from Bioethanol Production and Kraft Pulping Processes

Oil-in-water bitumen emulsions stabilized by biobased surfactants such as lignin are in line with the current sustainable approaches of the asphalt industry involving bitumen emulsions for reduced temperature asphalt technologies. With this aim, three lignins, derived from the kraft pulping and bioethanol industries, were chemically modified via the Mannich reaction to be used as cationic emulsifiers. A comprehensive chemical characterization was conducted on raw lignin-rich products, showing that the kraft sample presents a higher lignin concentration and lower molecular weight. Instead, bioethanol-derived samples, with characteristics of non-woody lignins, present a high concentration of carbohydrate residues and ashes. Lignin amination was performed at pH = 10 and 13, using tetraethylene pentamine and formaldehyde as reagents at three different stoichiometric molar ratios. The emulsification ability of such cationic surfactants was firstly studied on prototype silicone oil-in-water emulsions, attending to their droplet size distribution and viscous behavior. Among the synthetized surfactants, cationic kraft lignin has shown the best emulsification performance, being used for the development of bitumen emulsions. In this regard, cationic kraft lignin has successfully stabilized oil-in-water emulsions containing 60% bitumen using small surfactant concentrations, between 0.25 and 0.75%, which was obtained at pH = 13 and reagent molar ratios between 1/7/7 and 1/28/28 (lignin/tetraethylene pentamine/formaldehyde).

Different Kraft lignin sources for electrospun nanostructures production: Influence of chemical structure and composition

This work focuses on the structural features and physicochemical properties of different Kraft lignins and how they can influence the electrospinning process to obtain nanostructures. Structural features of Kraft lignins were characterized by Nuclear Magnetic Resonance, Size Exclusion Chromatography, Fourier-transform Infrared Spectroscopy, and thermal analysis, whereas chemical composition was analyzed by standard method. The addition of cellulose acetate (CA) improves the electrospinning process of Kraft lignins (KL). Thus, solutions of KL/CA at 30 wt% with a KL:CA weight ratio of 70:30 were prepared and then physicochemical and rheologically characterized. The morphology of electrospun nanostructures depends on the intrinsic properties of the solutions and the chemical structure and composition of Kraft lignins. Then, surface tension, electrical conductivity and viscosity of eucalypt/CA and poplar/CA solutions were suitable to obtain electrospun nanostructures based on uniform cross-linked nanofibers with a few beaded fibers. It could be related with the higher purity and higher linear structure, phenolic content and S/G ratios of lignin samples. However, the higher values of electrical conductivity and viscosity of OTP/CA solutions resulted in electrospun nanostructure with micro-sized particles connected by thin fibers, due to a lower purity, S/G ratio and phenolic content and higher branched structure in OTP lignin.

Organosolv and ionosolv processes for autohydrolyzed poplar fractionation: Lignin recovery and characterization

Biomass fractionation plays a major role in the search for competitive biorefineries, where the isolation and recovery of the three woody fractions is key. In this sense, we have used autohydrolyzed hemicellulose-free poplar as feedstock to compare two fractionation processes, organosolv and ionosolv, oriented to lignin recovery. The recovered lignins were then characterize by different techniques (NMR, GPC, TGA). Both treatments were tested at different temperatures to analyze temperature influence on lignin recovery and properties. The highest lignin recovery was obtained with the ionosolv process at 135 °C, reaching a solid yield of ~70%. Lignin characterization showed differences between both treatments. Lignins enriched in C-O linkages and G units were recovered with the organosolv process, where increasing temperature led to highly depolymerized lignins. However, lignins with higher C-C linkages and S units contents were obtained with the ionosolv process, producing more thermically stable lignins. In addition, increasing temperature caused lignin repolymerization when employing ionic liquids as solvents. Therefore, this work outlines the most important differences between ionosolv and organosolv processes for biomass fractionation, focusing on lignin recovery and its properties, which is the first step in order to valorize all biomass fractions.

Lignin-enriched residues from bioethanol production: Chemical characterization, isocyanate functionalization and oil structuring properties

Lignin-enriched waste products from bioethanol production of agriculture residues were tested as structuring agents in castor oil once functionalized with hexamethylene diisocyanate. Cane bagasse, barley and wheat straw were processed through steam explosion, pre-saccharification and simultaneous saccharification and fermentation (PSSF). Alternatively, cane bagasse was submitted to steam explosion and enzymatic hydrolysis (EH). Several Nuclear Magnetic Resonance techniques were used to characterize both residues and NCO-functionalized counterparts. The β-O-4'/resinol/phenylcoumaran content and hydroxyphenyl/guaiacyl/syringyl distribution depend on biomass source, pretreatment, and enzymatic hydrolysis. Total hydroxyl content (from 1.23 for cane bagasse to 1.85 for wheat straw residues), aromatic/aliphatic hydroxyl ratio (0.78 for cane bagasse and 0.61 and 0.49 for barley and wheat straw residues, respectively) and S/G ratio (ranging from 0.25 to 0.86) influence the NCO-functionalization and oleogel rheological response. Oleogels obtained with barley straw residues exhibited the highest values of the storage modulus; around 2 × 10⁵ Pa and 10⁴ Pa for 25% and 20% contents, respectively. PSSF process showed weaker modification, leading to softer viscoelastic response compared to EH. These oleogels exhibited rheological properties similar to lubricating greases of different NLGI grades. Therefore, we herein show an integrative protocol for the valorization of lignin-enriched residues from bioethanol production as potential thickeners of lubricating greases.

Fungal lignocellulolytic enzymes and lignocellulose: A critical review on their contribution to multiproduct biorefinery and global biofuel research

The continuous increase in the global energy demand has diminished fossil fuel reserves and elevated the risk of environmental deterioration and human health. Biorefinery processes involved in producing bio-based energy-enriched chemicals have paved way to meet the energy demands. Compared to the thermochemical processes, fungal system biorefinery processes seems to be a promising approach for lignocellulose conversion. It also offers an eco-friendly and energy-efficient route for biofuel generation. Essentially, ligninolytic white-rot fungi and their enzyme arsenals degrade the plant biomass into structural constituents with minimal by-products generation. Hemi- or cellulolytic enzymes from certain soft and brown-rot fungi are always favoured to hydrolyze complex polysaccharides into fermentable sugars and other value-added products. However, the cost of saccharifying enzymes remains the major limitation, which hinders their application in lignocellulosic biorefinery. In the past, research has been focused on the role of lignocellulolytic fungi in biofuel production; however, a cumulative study comprising the contribution of the lignocellulolytic enzymes in biorefinery technologies is still lagging. Therefore, the overarching goal of this review article is to discuss the major contribution of lignocellulolytic fungi and their enzyme arsenal in global biofuel research and multiproduct biorefinery.

Bioethanol Production and Alkali Pulp Processes as Sources of Anionic Lignin Surfactants

Lignin is an abundant biopolymer with potential value-added applications that depend on biomass source and fractioning method. This work explores the use as emulsifiers of three native lignin-rich product coming from industrial bioethanol production and alkali or Kraft pulping. In addition to their distinctive characteristics, the different molecular organization induced by emulsification pH is expected to interact in various ways at the water-oil interface of the emulsion droplets. Initially, model oil-in-water (O/W) emulsions of a silicone oil will be studied as a function of lignin source, disperse phase concentration and emulsification pH. Once stablished the effect of such variables, emulsion formulations of three potential bitumen rejuvenators (waste vegetable cooking oil, recycled lubricating oil and a 160/220 penetration range soft bitumen). Droplet size distribution, Z-potential and viscous tests conducted on model emulsions have shown that emulsification pH strongly affects stabilization ability of the lignins tested. Regarding bitumen rejuvenators, lignin emulsification capability will be affected by surfactant source, pH and, additionally, by the dispersed phase characteristics. Lower Z-potential values shown by KL at pH 9 and 11 seem to facilitate emulsification of the less polar disperse phases formed by RLUB and bitumen. In any case, lower particle size and higher yield stress values were found for both bioethanol-derived lignins emulsifying RVO and RLUB at pH 13, which are expected to exhibit a longer stability.

Chemical, Thermal and Antioxidant Properties of Lignins Solubilized during Soda/AQ Pulping of Orange and Olive Tree Pruning Residues

Some agroforestry residues such as orange and olive tree pruning have been extensively evaluated for their valorization due to its high carbohydrates content. However, lignin-enriched residues generated during carbohydrates valorization are normally incinerated to produce energy. In order to find alternative high added-value applications for these lignins, a depth characterization of them is required. In this study, lignins isolated from the black liquors produced during soda/anthraquinone (soda/AQ) pulping of orange and olive tree pruning residues were analyzed by analytical standard methods and Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (solid state 13C NMR and 2D NMR) and size exclusion chromatography (SEC). Thermal analysis (thermogravimetric analysis (TGA), differential scanning calorimetry (DSC)) and antioxidant capacity (Trolox equivalent antioxidant capacity) were also evaluated. Both lignins showed a high OH phenolic content as consequence of a wide breakdown of β-aryl ether linkages. This extensive degradation yielded lignins with low molecular weights and polydispersity values. Moreover, both lignins exhibited an enrichment of syringyl units together with different native as well as soda/AQ lignin derived units. Based on these chemical properties, orange and olive lignins showed relatively high thermal stability and good antioxidant activities. These results make them potential additives to enhance the thermo-oxidation stability of synthetic polymers.

Antifungal Activities of Wood and Non-Wood Kraft Handsheets Treated with Melia azedarach Extract Using SEM and HPLC Analyses

The main objective of this work was to evaluate pulp produced by kraft cooking for wood materials (WMT) (Bougainvillea spectabilis, Ficus altissima, and F. elastica) and non-wood materials (NWMT) (Sorghum bicolor and Zea mays stalks) and to study the fungal activity of handsheets treated with Melia azedarach heartwood extract (MAHE) solutions. Through the aforementioned analyses, the ideal cooking conditions were determined for each raw material based on the lignin percentage present. After cooking, pulp showed a decrease in the Kappa number produced from WMT, ranging from 16 to 17. This was in contrast with NWMT, which had Kappa numbers ranging from 31 to 35. A difference in the optical properties of the pulp produced from WMT was also observed (18 to 29%) compared with pulp produced from NWMT (32.66 to 35.35%). As for the evaluation of the mechanical properties, the tensile index of the pulp ranged from 30.5 to 40 N·m/g for WMT and from 44.33 to 47.43 N·m/g for NWMT; the tear index ranged from 1.66 to 2.55 mN·m²/g for WMT and from 4.75 to 5.87 mN·m²/g for NWMT; and the burst index ranged from 2.35 to 2.85 kPa·m²/g for WMT and from 3.92 to 4.76 kPa·m²/g for NWMT. Finally, the double fold number was 3 compared with that of pulp produced from pulp, which showed good values ranging from 36 to 55. In the SEM examination, sheets produced from treated handsheets with extract from MAHE showed no growth of Aspergillus fumigatus over paper discs manufactured from B. speclabilis pulp wood. Pulp paper produced from Z. mays and S. bicolor stalks was treated with 1% MAHE, while pulp paper from F. elastica was treated with 0.50% and 1% MAHE. With the addition of 0.5 or 1% MAHE, Fusarium culmorum showed no increase in growth over the paper manufactured from B. speclabilis, F. altissima, F. elastica and Zea mays pulps with visual inhibition zones found. There was almost no growth of S. solani in paper discs manufactured from pulps treated with 1% MAHE. This is probably due to the phytochemical compounds present in the extract. The HPLC analysis of MAHE identified p-hydroxybenzoic acid, caffeine, rutin, chlorogenic acid, benzoic acid, quinol, and quercetin as the main compounds, and these were present in concentrations of 3966.88, 1032.67, 834.13, 767.81, 660.64, 594.86, and 460.36 mg/Kg extract, respectively. Additionally, due to the importance of making paper from agricultural waste (stalks of S. bicolor and Z. mays), the development of sorghum and corn with high biomass is suggested.

Olive Leaf Waste Management

Olive trees are the oldest known cultivated trees in the world and present-day cultivation is widespread, with an estimated magnitude of 9 million hectares worldwide. As the olive oil industry has continued to grow, so has the environmental impact of olive oil production, such as the energy and water consumption, gas emissions and waste generation. The largest contributor to waste generation are the olive leaves, an abundant and unavoidable byproduct of olive-oil production due to the necessity of tree-pruning. It is estimated that an annual 1.25 million tons of olive leaf waste are generated in Spain alone, around 50% of the total world production. The leaves are currently used for biomass production or animal feed. However, because of their polyphenolic composition, olive leaves have potential in numerous other applications. In this review we analyze the chemical composition of olive leaves, and discuss current processing methods of the olive leaf waste, including thermochemical, biochemical, drying, extraction and condensation methods. We also examine current applications of the treated olive leaves in sectors relating to cattle feed, fertilizers, novel materials, energy generation, and food and pharmaceutical products. The aim of this review is to provide a resource for producers, policy makers, innovators and industry in shaping environmentally sustainable decisions for how olive leaf waste can be utilized and optimized.

Properties versus application requirements of solubilized lignins from an elm clone during different pre-treatments

Kraft pulping, organosolv process and acid hydrolysis were applied on an elm clone. The solubilized lignins were recovered and analyzed. Kraft pulping and acid hydrolysis led to lignins with higher phenolic OH content as result of extensive cleavage of β-O-4' linkages, as revealed by ¹³C solid state and ¹³C-¹H heteronuclear single quantum coherence nuclear magnetic resonance. This depolymerization also yielded lower molecular weight lignins inferred by size exclusion chromatography. Contrarily, organosolv process gave rise to a lignin with a more preserved structure, maintaining a large number of β-O-4' linkages. Consequently, organosolv lignin presented lower phenolic OH content and higher molecular weight. Moreover, the high content of the labile native β-O-4' linkages in organosolv lignin resulted in a lower thermostability as compared to the kraft and acid lignins. On the other hand, the solubilized lignins from kraft and acid processes displayed an enrichment of S-units, whereas lignin from organosolv process was slightly enriched in G-units, containing all of them different native as well as pre-treatment derived units. These results could help to increase the inventory of lignin sources available for future lignin-based products, for which knowledge of the lignin properties versus application requirements is crucial.

Wheat straw components fractionation, with efficient delignification, by hydrothermal treatment followed by facilitated ethanol extraction

Lignocellulosic biomass fractionaion into its three major components is critically important for efficient feedstock utilization. The hydrothermal-ethanol method has broad application as its first step, hydrothermal treatment, provides high hemicellulose separation efficiency. However, it severely inhibits the delignification on the subsequent ethanol extraction. In this study, the second step, ethanol extraction, was facilitated by the addition of 3% NaOH and 3% H₂O₂, resulting in a significant improvement of lignin separation (by 48.2%). SEM, AFM, XPS, and XRD were used to characterize the surface composition of the remaining solids (crude cellulose) while the structure of isolated lignin was characterized by FT-IR, CP/MAS 13C NMR, GPC and TGA. The lignin samples isolated with both facilitated and non-facilitated ethanol extraction were compared to elucidate the lignin removal mechanism. The results showed that lignin degradation and crosslinking/polymerization occur in parallel during both the hydrothermal treatment and ethanol extraction.

Production of Cellulose Nanofibers from Olive Tree Harvest—A Residue with Wide Applications

With the aim of identifying new sources to produce cellulose nanofibers, olive tree pruning biomass (OTPB) was proposed for valorization as a sustainable source of cellulose. OTPB was subjected to a soda pulping process for cellulose purification and to facilitate the delamination of the fiber in the nanofibrillation process. Unbleached and bleached pulp were used to study the effect of lignin in the production of cellulose nanofibers through different pretreatments (mechanical and TEMPO-mediated oxidation). High-pressure homogenization was used as the nanofibrillation treatment. It was observed that for mechanical pretreatment, the presence of lignin in the fiber produces a greater fibrillation, resulting in a smaller width than that achieved with bleached fiber. In the case of TEMPO-mediated oxidation, the cellulose nanofiber characteristics show that the presence of lignin has an adverse effect on fiber oxidation, resulting in lower nanofibrillation. It was observed that the crystallinity of the nanofibers is lower than that of the original fiber, especially for unbleached nanofibers. The residual lignin content resulted in a greater thermal stability of the cellulose nanofibers, especially for those obtained by TEMPO-mediated oxidation. The characteristics of the cellulose nanofibers obtained in this work identify a gateway to many possibilities for reinforcement agents in paper suspension and polymeric matrices.

Production and characterization of lignin and cellulose fractions obtained from pretreated vine shoots by microwave assisted alkali treatment

This work deals with the optimization of the second stage of a biorefinery scheme to separate simultaneously cellulose and lignin from hydrothermally pre-treated vine shoots. For this, the suitability of the microwave-assisted alkaline delignification was assessed and optimized through a Box-Wilson experimental design. The optimum conditions (150 °C, 6 wt% NaOH, 30 min) allowed maximizing the lignin removal (82 wt%) and minimizing the loss of the cellulose (35 wt%) present in the pre-treated vine shoots. A thorough characterization of the two fractions obtained at optimum conditions was performed: the cellulose rich solid was analyzed by XRD and FTIR and the lignin was subjected to HPSEC, Py/GC-MS, ¹³C- and ¹H NMR. This purposed second stage would allow performing an integral biorefinery with low energy requirements and environmentally friendly conditions. This approach aligns with the circular economy and the zero waste production philosophies, promoting the sustainable development.

Chemical and thermal analysis of lignin streams from Robinia pseudoacacia L. generated during organosolv and acid hydrolysis pre-treatments and subsequent enzymatic hydrolysis

Lignin streams produced in biorefineries are commonly used to obtain energy. In order to increase the competitiveness of this industry, new lignin valorization routes are necessary, for which a depth characterization of this biological macromolecule is essential. In this context, this study analyzed lignin streams of Robinia pseudoacacia L. generated during organosolv and acid hydrolysis pre-treatments and during the subsequent enzymatic hydrolysis. These lignins included dissolved lignins from pre-treatment liquors and saccharification lignins from pre-treated materials. Chemical composition and structural features were analyzed by analytical standard methods and Fourier Transform Infrared spectroscopy (FTIR), size exclusion chromatography (SEC), ¹³C solid state nuclear magnetic resonance (¹³C NMR) and ¹H-¹³C two-dimensional nuclear magnetic resonance (2D NMR); while thermal characterization included thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). In general, all studied lignins contained a predominance of β-O-4' aryl ether linkages, followed by resinol (β-β') and phenylcoumaran (β-5'), with a predominance of syringyl over guaiacyl and hydroxyphenyl units. Nevertheless, the dissolved lignins revealed a removal of linkages, especially β-O-4', leading to an enrichment of phenolic groups. Moreover, high thermal stability and good thermoplasticity were characteristics of these lignins. Contrary, the saccharification lignins exhibited a more intact structure, but with an important remaining carbohydrates content.

Characterization of lignins from Populus alba L. generated as by-products in different transformation processes: Kraft pulping, organosolv and acid hydrolysis

The complexity and heterogeneity of lignin requires a detailed understanding in order to decide about more efficient lignin valorization approaches. This study deals with the characterization of lignins from Populus alba L. generated as by-products in different transformation processes: kraft pulping, organosolv and dilute acid hydrolysis. In addition to the composition, the chemical and structural features of the different lignins were investigated by Fourier Transform infrared spectroscopy (FTIR), solid-state ¹³C nuclear magnetic resonance (¹³C NMR), two-dimensional nuclear magnetic spectrometry (2D NMR), size exclusion chromatography (SEC), and thermal analysis. Organosolv lignin showed noticeably different characteristics compared to kraft and acid hydrolysis lignins; higher molar mass, higher amount of side-chain linkages (mainly aryl-β ether and resinol) together with lower phenolic content. On the contrary, kraft and acid hydrolysis lignins presented an extensive elimination of lateral chains and therefore a higher phenolic content, which suggests a much stronger lignin depolymerization (lower molar mass) during these processes. Moreover, thermal analysis results revealed that the thermal stability of kraft and acid hydrolysis lignins was higher than that of organosolv lignin, especially in the case of acid hydrolysis lignin. According to all these characteristics, several valorization pathways for studied lignin are discussed.

Alternatives for Chemical and Biochemical Lignin Valorization: Hot Topics from a Bibliometric Analysis of the Research Published During the 2000–2016 Period

A complete bibliometric analysis of the Scopus database was performed to identify the research trends related to lignin valorization from 2000 to 2016. The results from this analysis revealed an exponentially increasing number of publications and a high relevance of interdisciplinary collaboration. The simultaneous valorization of the three main components of lignocellulosic biomass (cellulose, hemicellulose, and lignin) has been revealed as a key aspect and optimal pretreatment is required for the subsequent lignin valorization. Research covers the determination of the lignin structure, isolation, and characterization; depolymerization by thermal and thermochemical methods; chemical, biochemical and biological conversion of depolymerized lignin; and lignin applications. Most methods for lignin depolymerization are focused on the selective cleavage of the β-O-4 linkage. Although many depolymerization methods have been developed, depolymerization with sodium hydroxide is the dominant process at industrial scale. Oxidative conversion of lignin is the most used method for the chemical lignin upgrading. Lignin uses can be classified according to its structure into lignin-derived aromatic compounds, lignin-derived carbon materials and lignin-derived polymeric materials. There are many advances in all approaches, but lignin-derived polymeric materials appear as a promising option.