Direct analysis of lignin and lignin-like components from softwood kraft pulp by Py-GC/MS techniques

Analytical pyrolysis combined with gas chromatography/mass spectrometry was used to analyse the structure and quantity of aromatic components, mainly guaiacyl and hydroxyphenyl derivatives, directly from chemical pulps. The quantity of aromatic degradation products was determined using a new external calibration method. The external standard was analyzed similarly to the pulp sample, and the combined area of the degradation products formed, normalized to the sample amount, was used for calibration. The method was sensitive enough to detect aromatics from fully bleached softwood pulps at a concentration level of 0.4 wt.%.The effect of bleaching on lignin structures in softwood pulps was studied by following the changes in guaiacyl-type degradation product distribution. The residual lignin structures that had been modified during cooking were removed during the course of bleaching. The residual lignin in fully bleached pulps therefore was found to bear features characteristic of native lignin in addition to increased oxidation. A striking enrichment of hydroxyphenyl-type aromatic pyrolysis products was observed during bleaching. It is suggested that they are derived not only from lignin but also from other pulp components.     

Radiolysis of nitrate-alcohol binary mixtures at pH 12

Gamma-radiolysis of alkaline binary mixtures of nitrate-alcohol (1-propanol and 1-butanol) has been investigated at a fixed pH of 12. The products of radiolysis, mainly nitrite, aldehyde and hydrogen peroxide were estimated. Also the effect of concentration of each species present in the mixture on the G-values of the products formed has been examined. The G-values of each of the products are found to be lower in basic medium in binary mixtures as compared to those obtained at neutral pH; other conditions being kept constant. However, the yields of products in nitrate solutions show higher values at basic pH in comparison with their counterparts at neutral pH. Results are explained on the basis of reaction mechanism that operates during the process of radiolysis, leading to the formation of the different products.     

Towards quantitative catalytic lignin depolymerization

The products of base-catalyzed liquid-phase hydrolysis of lignin depend markedly on the operating conditions. By varying temperature, pressure, catalyst concentration, and residence time, the yield of monomers and oligomers from depolymerized lignin can be adjusted. It is shown that monomers of phenolic derivatives are the only primary products of base-catalyzed hydrolysis and that oligomers form as secondary products. Oligomerization and polymerization of these highly reactive products, however, limit the amount of obtainable product oil containing low-molecular-weight phenolic products. Therefore, inhibition of concurrent oligomerization and polymerization reactions during hydrothermal lignin depolymerization is important to enhance product yields. Applying boric acid as a capping agent to suppress addition and condensation reactions of initially formed products is presented as a successful approach in this direction. Combination of base-catalyzed lignin hydrolysis with addition of boric acid protecting agent shifts the product distribution to lower molecular weight compounds and increases product yields beyond 85%.     

On the interaction of HBT with pulp lignin during mediated laccase delignification—a study using fractionated pyrolysis-GC/MS

An analytical method using fractionated pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was developed and applied for characterizing the type of interaction between 1-hydroxybenzotriazole (HBT)-mediator and pulp lignin in laccase delignification of pulp. In fractionated pyrolysis, the sample is pyrolyzed at progressively increasing temperatures in order to study particular fractions of the sample and to minimize secondary pyrolysis effects. This makes it possible to determine whether a certain pyrolysis product originates from one chemical moiety or different chemical moieties in one molecule. In the present method, samples were fractionated by thermal desorption at 200 °C followed by pyrolysis at progressively increasing temperatures from 320 to 800 °C. The products formed in each fraction were separated in a capillary GC column and detected and identified using MS. The type of interaction between HBT and pulp lignin was studied by following the formation of nitrogen-containing products during fractionated pyrolysis of a residual lignin isolated from laccase/HBT-treated oxygen-delignified softwood kraft pulp. This residual lignin was found to contain approximately 2% HBT residue. Most (87%) of this residue was covalently linked to the residual lignin. The results also strongly suggest that the HBT residue is present in two chemically different forms.     

Lignin-Derived Syringol and Acetosyringone from Palm Bunch Using Heterogeneous Oxidative Depolymerization over Mixed Metal Oxide Catalysts under Microwave Heating

Biomass valorization to building block chemicals in food and pharmaceutical industries has tremendously gained attention. To produce monophenolic compounds from palm empty fruit bunch (EFB), EFB was subjected to alkaline hydrothermal extraction using NaOH or K₂CO₃ as a promotor. Subsequently, EFB-derived lignin was subjected to an oxidative depolymerization using Cu(II) and Fe(III) mixed metal oxides catalyst supported on γ-Al₂O₃ or SiO₂ as the catalyst in the presence of hydrogen peroxide. The highest percentage of total phenolic compounds of 63.87 wt% was obtained from microwave-induced oxidative degradation of K₂CO₃ extracted lignin catalyzed by Cu-Fe/SiO₂ catalyst. Main products from the aforementioned condition included 27.29 wt% of 2,4-di-tert-butylphenol, 19.21 wt% of syringol, 9.36 wt% of acetosyringone, 3.69 wt% of acetovanillone, 2.16 wt% of syringaldehyde, and 2.16 wt% of vanillin. Although the total phenolic compound from Cu-Fe/Al₂O₃ catalyst was lower (49.52 wt%) compared with that from Cu-Fe/SiO₂ catalyst (63.87 wt%), Cu-Fe/Al₂O₃ catalyst provided the greater selectivity of main two value-added products, syringol and acetosyrigone, at 54.64% and 23.65%, respectively (78.29% total selectivity of two products) from the NaOH extracted lignin. The findings suggested a promising method for syringol and acetosyringone production from the oxidative heterogeneous lignin depolymerization under low power intensity microwave heating within a short reaction time of 30 min.     

Formation of peroxide compounds on the oxidation of lignin by oxygen in an organic solvent

The results are given of a determination of the steady-state concentrations of peroxide compounds on the oxidation of lignin by oxygen in dioxane. An iodometric method of determining peroxides was used, with spectrophotometric control of the amount of iodine formed. It was established that the achievable steady-state concentration of peroxide compounds on the oxidation of lignin is 0.28–0.44 wt. % of O_act (oxygen pressure 1 atm, temperature 50–80°C). With a rise in the temperature, the steady-state concentration of peroxide compounds decreased. The addition of water (30 vol. %) to the organic solvent led to an increase in the rate of accumulation of peroxide compounds. It has been shown that the presence of alkaline and acidic catalysts exerts no appreciable influence on the achievable steady-state concentration and the rate of accumulation of peroxide compounds during the oxidation of lignin.Bratsk Industrial Institute. All-Union Scientific Production Combine of the Pulp and Paper Industry, Leningrad. Translated from Khimiya Prirodnykh Soedinenii, Nos. 3,4, pp. 413–417, May–August, 1992.     

Photochemically Induced Solid-State Degradation, Condensation, and Rearrangement Reactions in Lignin Model Compounds and Milled Wood Lignin

Abstract— Milled wood lignin produced from alkaline hydrogen peroxide-bleached softwood thermomechanical pulp (TMP) fibers was adsorbed on pure cellulose and irradiated for variable periods of time under oxygen and/or nitrogen. The absolute amounts of β-O-4 ethers, phenolic hydroxyl groups, carboxylic acids and various condensed phenolic units were quantified, nondestructively, using ³¹P NMR spectroscopy. Photoirradiation was found to severely cleave the β-O-4 ethers present in lignin with the concomitant formation of new phenolic units. The rate of this cleavage was found to be faster under oxygen than under nitrogen. The catalytic role of oxygen can be rationalized by invoking the formation of peroxy free radicals that may initiate new sites of radical generation ( e.g. ketyl radicals) within the lignin causing the subsequent breakdown of the β-O-4 linkage. The photoirradiation was found to cause a net increase of the C₅-related condensed phenolic units in lignin. Among these, C_α-C₅ and/ or C_β-C₅ phenolic moieties were found to predominate. Similar irradiation of a binary mixture of model compounds resembling structures present in softwood lignin resulted in 18 products that were identified and quantified using gas chromatography coupled to mass spectrometry. The photochemical products obtained supported the phenacyl and ketyl mechanistic pathways to pho-toyellowing, whereas the detection of β-5 coupling products (common in both the model compound and milled wood lignin studies) may point a new avenue toward the formation of light-induced products that has not been previously considered in the solid state.     

Ultrafiltration and Pyrolysis Gas Chromatography Mass Spectrometry of Chlorolignins in Pulp Mill Effluent

Abstract

Investigations on the effluent of a German pulp mill on the river Rhine using ultrafiltration and pyrolysis gas chromatography mass spectrometry have shown the presence of several chlorinated 2-methoxyphenols, which probably originate from chlorinated lignin or lignosulfic acid. Chlorinated phenolic pyrolysis products identified were 2-methoxy-6-chlorophenol, 2-methoxy-4-methyl-6-chlorophenol, 2-methoxy-dichlorophenol, 2-methoxy-4-vinyl-6-chlorophenol, 2-methoxy-4-(chloropropyl)phenol, 2-methoxy-4-(prop-2-enyl)-6-chlorophenol, 2-methoxy-4-(propan-2-one)-6-chlorophenol, 2-methoxy-3, 5, 6-trichlorophenol, 2-methoxy-4-vinyl-3, 5, 6-trichlorophenol. Monochlorinated 2-methoxyphenols were the dominant chlorinated pyrolysis products, smaller amounts of di- and trichloromethoxyphenols were also detected. 2-Methoxyphenols (guaiacols) were the dominant lignin pyrolysis products, only small amounts of 2, 6-dimethoxyphenols (syringols) were detected. This indicates the origin from soft wood. A Py-(GC)-(MS)-TIC-chromatogram of reference spruce milled wood lignin is comparable with the TIC-chromatogram of pulp mill effluent MW-fraction > 10,000, showing many similar compounds. The ultrafiltration fraction 1000 < MW < 10,000 showed a remarkably empty TIC-chromatogram, with 2-methoxyphenol and 2-methoxy-6-chlorophenol as the only significant phenolic peaks. This may indicate a high degree of oxidation/chlorination of lignin in this fraction.     

Precipitation of lignin and extractives on kraft pulp: effect on surface chemistry, surface morphology and paper strength

The effect of pH on the formation of precipitates (lignin, extractives and metals) on kraft pulp surfaces was examined by electron spectroscopy for chemical analysis, time-of-flight secondary ion mass spectrometry and atomic force microscopy (AFM). A softwood kraft pulp slurry from an oxygen delignification stage was diluted to 3% consistency with water or an acidic Z filtrate. After heating to 70 °C the pH was lowered from 11 to 2–5, using sulphuric acid. Lignin and extractives precipitated at pH values below 6, and their amounts increased with decreasing pH. Most of the precipitated lignin was found on the pulp surface after sheet forming, whereas the main part of the precipitated extractives could be easily washed away with water. The layer of precipitated lignin was apparently thicker than the layer of extractives. AFM showed the precipitated material as a granular phase. Neither surface morphology nor surface coverage depended on the addition of Z filtrate. The amount of metals ID the pulp and on the pulp surface decreased when pH was lowered to 2. More metals, such as Ca and Mg, were detected ID the pulps as well as on the sheet surfaces when the pulp was diluted with Z filtrate. Strength and bonding properties of the pulp sheets were slightly impaired by the precipitated material. Acidification appears to be the main reason for the precipitation of both lignin and extractives on the pulp surfaces. This should be taken into account when filtrates are recycled ID the bleaching or washing of pulps.     

Reactions of lignin in chlorine dioxide bleaching of kraft pulps

The reactions between chlorine dioxide and the residual lignin in oxygen-bleached softwood kraft pulps have been studied. In a first series, isolated lignin samples have been subjected to chlorine dioxide oxidation at different pH values and subsequently analysed by oxidative degradation and elemental analysis. Different analytical techniques have also been employed to follow the gradual chemical changes in lignins isolated from kraft pulps after each of the bleaching stages in the OD(EOP)DD sequence. The results demonstrate that, in order to minimize chlorination of the lignin, the first chlorine dioxide stage should be carried out at a pH around or above three. At this pH level, a high degree of lignin oxidation is also achieved. A certain (mono)-chlorination of the lignin in the first D stage cannot be avoided, but this chlorine is to a large extent removed in the later bleaching stages. The efficient and non-selective oxidation of the various phenolic lignin end groups by chlorine dioxide is clearly illustrated by the analytical data. Moreover,¹³C NMR reveals that reduced lignin structures formed during the kraft cook survive the oxidative bleaching stages to a large extent.     

Sonochemical oxidation of phenol and three of its intermediate products in aqueous media: Catechol,hydroquinone, and benzoquinone. Kinetic and mechanistic aspects

The sonochemical oxidation of phenol has been examined in airequilibrated aqueous media at various pH’s and at various insonation powers. Its disappearance follows zero-order kinetics at [phenol]_initial ~ 30 to 70 μM Three principal intermediate species formed at pH 3: catechol (CC), hydroquinone (HQ), and p-benzoquinone (BQ); at natural pH (5.4–5.7) only catechol and hydroquinone formed. No intermediate species were detected at pH 12 under the conditions used. The sonochemical fate of CC, HQ, and BQ was also examined at pH 3 and at natural pH’s. At pH 3, BQ is the major species formed during insonation of HQ, while HQ is produced during insonation of BQ. In both cases, an additional intermediate formed in trace quantities that is identified as hydroxy-p-benzoquinone. These same intermediate species have been identified in the heterogeneous photocatalyzed oxidation of phenol in irradiated titania suspensions. The present results confirm the important role of ’OH radicals in degradation processes. Although CO₂ is the ultimate product in heterogenous photocatalysis, irradiation of a phenolic aqueous solution by ultrasounds showed no loss of total organic carbon (TOC) after several hours, even though the aromatic substrate and the intermediates had degraded. A simple kinetic model/scheme is described to account for the events in the conversion of the substrates to products. It is concluded that the hydrophobic benzoquinone reacts with ¹OH and H¹ radicals at the hydrophobic gas bubble/liquid interface, while the hydrophilic species (phenol, CC, and HQ) react, to a large extent, with the ¹ OH radicals in the solution bulk.     

On-line product analysis of pine wood pyrolysis using synchrotron vacuum ultraviolet photoionization mass spectrometry

The pyrolysis process of pine wood, a promising biofuel feedstock, has been studied with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry. The mass spectra at different photon energies and temperatures as well as time-dependent profiles of several selected species during pine wood pyrolysis process were measured. Based on the relative contents of three lignin subunits, the data indicate that pine wood is typical of softwood. As pyrolysis temperature increased from 300 to 700 °C, some more details of pyrolysis chemistry were observed, including the decrease of oxygen content in high molecular weight species, the observation of high molecular weight products from cellulose chain and lignin polymer, and potential pyrolysis mechanisms for some key species. The formation of polycyclic aromatic hydrocarbons (PAHs) was also observed, as well as three series of pyrolysis products derived from PAHs with mass difference of 14 amu. The time-dependent profiles show that the earliest products are formed from lignin, followed by hemicellulose products, and then species from cellulose.

木质素酚类单体化合物制备烷烃燃料

通过引入中间小分子化合物,采用傅克烷基化反应,实现了从木质素酚类单体化合物制备长链烷烃燃料.考察了催化剂、醛酮类小分子化合物、反应时间、反应温度、物料比、底物等条件对从木质素酚类单体制备二聚体反应结果的影响,并对得到的木质素酚类二聚体产物进一步加氢还原,得到C_13~19烷烃燃料.结果表明,当物料比n(木质素酚类单体)/n(醛酮类中间小分子)为15:3,以Amberlyst-15为酸性催化剂,在100 ℃的条件下,反应24 h,可以得到68%产率的二聚体化合物(当底物是愈创木酚和丙醛时).将得到的二聚体化合物在270 ℃,4 MPa H₂的不锈钢反应釜中进行加氢反应,3 h后,二聚体化合物完全转化为液体烷烃.提出从木质素单体出发通过引入中间小分子,实现C-C链增长来制备烷烃燃料的合成路线,为木质素的开发和应用提出了新思路与实验基础.     

Added-Value Chemicals from Lignin Oxidation

Lignin is the second most abundant component, next to cellulose, in lignocellulosic biomass. Large amounts of this polymer are produced annually in the pulp and paper industries as a coproduct from the cooking process—most of it burned as fuel for energy. Strategies regarding lignin valorization have attracted significant attention over the recent decades due to lignin’s aromatic structure. Oxidative depolymerization allows converting lignin into added-value compounds, as phenolic monomers and/or dicarboxylic acids, which could be an excellent alternative to aromatic petrochemicals. However, the major challenge is to enhance the reactivity and selectivity of the lignin structure towards depolymerization and prevent condensation reactions. This review includes a comprehensive overview of the main contributions of lignin valorization through oxidative depolymerization to produce added-value compounds (vanillin and syringaldehyde) that have been developed over the recent decades in the LSRE group. An evaluation of the valuable products obtained from oxidation in an alkaline medium with oxygen of lignins and liquors from different sources and delignification processes is also provided. A review of C₄ dicarboxylic acids obtained from lignin oxidation is also included, emphasizing catalytic conversion by O₂ or H₂O₂ oxidation.     

Structural and physico-chemical characterization of lignins solubilized during alkaline peroxide treatment of barley straw

In this paper, the seven acid-insoluble lignin preparations from barley straw were first extracted with alkaline hydrogen peroxide in order to study how the delignification and degradation of the lignin is influenced by aqueous 1.5% H₂O₂ extractant to straw ratios. The results showed that treatment of dewaxed barley straw with 1.5% H₂O₂ at 45°C for 14 h (pH 12.0) under the extractant to straw ratios of 10:1, 13:1, 15:1, 18:1, 20:1, 25:1, and 30:1 resulted in dissolution of 65.8%, 68.4%, 68.4%, 69.0%, 69.7%, 71.6%, and 72.3% of the original lignin and 78.7%, 79.8%, 82.3%, 83.4%, 84.8%, 85.3%, and 85.3% of the original hemicelluloses, respectively. The degraded seven lignin samples were analyzed with respect to their chemical compositions, content of chemically linked polysaccharides, molecular weights and structural changes. It was found that the alkaline peroxide treatment under the conditions given led to a noticeable increase in a amount of carboxyl groups due to the oxidation. The results from 13C-NMR analyses showed that the treatment was extremely effective for isolation of highly pure lignins from the straw, and the treatment under the conditions used did not affect the overall structure of lignin. The β-O-4 ether bond and β-β carbon-carbon linkage were found to be the major linkages between lignin units. Hydroxycinnamic acids, such as p-coumaric and ferulic acids, appeared to be strongly linked to lignin molecules, in which p-coumaric acid was found to be bonded to lignin by ester linkage, while ferulic acid was linked by its phenolic group via ether bond to lignin and also principally linked by its carboxyl group via ester bond to lignin and/or hemicelluloses.     

Kinetic Study and Optimization of Catalytic Peroxide Delignification of Aspen Wood

It is established that the main regularities of the peroxide delignification of aspen wood in the temperature range of 70–100°С in the presence of dissolved (H₂SO₄) and solid (TiO₂) catalysts are similar. With an increase of the temperature, the concentration of hydrogen peroxide and acetic acid, and the hydromodule (HM) values, as well as the duration of the process and the content of cellulose in the cellulose products, increase, while the content of the residual lignin decreases. Simultaneously, the total yield of cellulose products decreases independently of the nature of the catalyst. Delignification processes are satisfactory described by the first-order equation. A sufficiently high activation energy (88 kJ/mol in the presence of H₂SO₄ and 75 kJ/mol in the presence of TiO₂) indicates the absence of significant external diffusion constraints in the selected conditions. The optimal conditions of obtaining cellulose products with a low content of residual lignin from aspen wood are found by the calculation methods. It is shown by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM) that the structure of cellulose products obtained corresponds to the structure of industrial microcrystalline cellulose. In the optimal conditions, a high-quality cellulose product can be obtained in mild conditions (the temperature is 100°С, atmospheric pressure) by using a safer and technological TiO₂ catalyst instead of a sulfuric acid catalyst.     

The middle lamella remainders on the surface of various mechanical pulp fibres

The surfaces of various mechanical pulp fibres, including thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), and alkaline peroxide mechanical pulp (APMP) fibres, were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), and X‐ray photoelectron spectroscopy (XPS). With SEM and AFM, the middle lamella material was observed to be non‐fibrillar and patch‐like, while the fibre secondary wall was observed to have a micro‐fibrillar structure. It was found that after the first‐stage refiner, lignin‐rich middle lamella remainders were present on the fibre surface of all three pulps, although most of the fibre surfaces exhibited a micro‐fibrillar structure. After the final‐stage refining, large amounts of granules were present on the TMP fibre surface. In contrast, most middle lamella remainders were still visible on the surface of CTMP fibres after the final‐stage refining and even after peroxide bleaching. XPS results have confirmed that the non‐fibrillar surface material is the lignin‐rich middle lamella remainder that contribute to the high surface lignin concentration. Copyright © 2006 John Wiley & Sons, Ltd.     

Antioxidant activity of lignin phenolic compounds extracted from kraft and sulphite black liquors

The antioxidant activity of the phenolic compounds present in industrial black liquors obtained from the two cooking processes (kraft and sulphite) used in Portugal to produce Eucalyptus globulus pulp was evaluated. The black liquors treated at several pH values were extracted with ethyl acetate. Phenolic fractions were further separated by liquid chromatography of the crude extracts of kraft liquor at pH = 6 and sulphite liquor at the original pH. Total phenolic content was determined in terms of gallic acid equivalents (Folin-Ciocalteu colorimetric method), and the antioxidant activity in the crude extracts at several pH values and in the separated fractions was measured using the DPPH test for radical scavenging capacity. The total phenolic content of crude extracts and separated fractions ranged from 92.7 to 181.6 and from 91.6 to 1,099.6 mg GAE/g, respectively, while the antioxidant activity index (AAI) ranged from 2.20 to 3.41 and from 2.21 to 11.47 respectively, showing very strong antioxidant activity in all studied cases. The fractions separated by column chromatography were submitted to mass spectrometry analysis and the results were compared to others in the literature of natural products, mainly from Eucalyptus, and the characteristic bands of functional groups were identified by 1H-NMR and FTIR. These methods allowed the identification of 17 phenolic compounds.     

Pyrolysis-GC/MS and TG/MS study of mediated laccase biodelignification of Eucalyptus globulus kraft pulp

Biobleaching studies using laccase mediator system (LMS) were carried out, under optimized conditions, on two unbleached Eucalyptus globulus kraft pulps, one produced by conventional way, with kappa number of 16.1, and another with kappa number of 14.5, obtained by modified kraft procedure with a high liquor/wood ratio and with black liquor replacement in the middle of the cooking. The pulp properties before and after LMS and alkaline extraction were evaluated in terms of kappa number, hexeneuronic acid content, viscosity, brightness and acid insoluble lignin content.The original milled wood sample and the kraft pulps were characterized by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and thermogravimetry/mass spectrometry (TG/MS). Eucalypt wood lignin produces guaiacol and syringol derivatives during pyrolysis. These lignin products can be detected with high sensitivity using the selected ion chromatograms even in the bleached pulp of low lignin content (about 0.5%). Py-GC/MS revealed that the lignin moieties were similarly altered during biobleaching as during pulping, which is exemplified by the preferential removal of aldehyde groups from the alkyl side groups. Semi-quantitative analysis of the pyrograms indicates that the lignin content of the biobleached pulps is reduced by about half in comparison with the unbleached pulps. The TG/MS results show that the hemicellulose content of wood was strongly modified during pulping resulting in higher thermal stability.     

Analysis of lignin degradation products by capillary electrophoresis

Summary A method for the quantitative analysis of phenolic lignin degradation products by capillary electrophoresis (CE) with on-column UV detection has been developed. The liquid biomass solutions contain low molecular hemicellulosic sugars and phenolic lignin degradation products with various degrees of polymerization. Special attention has been paid to the monomeric phenolic components of lignin degradation fragments, e.g. derivatives of phenolic acids, aldehydes, and alcohols. Uncoated fused silica capillaries and borate-phosphate buffer systems at moderate pH conditions were used in order to separate the compounds of interest. To provide validation of the method, the same samples were analyzed independently by HPLC using gradient elution on a RP-C18 column. As sugar degradation fragment, furan-2-carboxylic acid was detected.