Nitrobenzene Oxidation Research Articles

Kinetics of Vanillin and Vanillic Acid Production from Pine Kraft Lignin

Vanillin (4-hydroxy-3-methoxybenzaldehyde) is the main component of natural vanilla and a relevant substance in the flavoring and aromatic industries. This study presents a kinetic model to explain both vanillin and vanillic acid concentrations achieved in the alkaline oxidation of pine kraft lignin. Considering that they come from the same precursors, this approach allows an understanding of vanillin production with reaction conditions that minimize the vanillic acid pathway directly from the lignin oligomers, thus maximizing vanillin production. This study involves the effects of oxygen partial pressure, temperature, and the presence or absence of a catalyst (CuSO4 and Fe2(SO4)3 mixture) on the vanillin and vanillic acid yields. An adapted reactor (M/K Systems Inc., Williamstown, MA, USA) with a recirculation and spray liquids system was used in the experiments. The experiments were performed using one liter of a solution of NaOH 2 M and 60 g of lignin. During the lignin oxidation reaction, liquid samples were analyzed at different times (from 0 to 200 min). The oxidation products were quantified by liquid chromatography (HPLC). The catalyzed experiments presented higher maximum vanillin yields than the non-catalyzed ones (39.2–39.6% on nitrobenzene oxidation) achieved at 150 °C. A kinetic model is proposed where the kinetic parameters were estimated using Monte Carlo methods, fitting satisfactorily to the experimental results. The statistical analysis of the kinetic parameters showed that all the studied variables significantly affect the vanillin yield.

Soybean and maize differentially metabolize deuterated ferulic and sinapic acids before polymerizing them into the root cell wall

Hydroxycinnamates induce lignification in young plants, leading to the overproduction of lignin as a defense mechanism. Phenylpropanoids-containing oligosaccharides are thought to be a signal of pathogen attack on the cell wall polysaccharides. However, it is unclear if hydroxycinnamates induce lignification by acting solely as stress elicitors or feeding the phenylpropanoid pathway as lignin precursors. To examine this hypothesis, we added 1 mM deuterated ferulic acid (FA) or sinapic acid (SA) to the nutrient solution in which we cultivated soybean or maize plantlets. After 24 h, we assessed the biometric parameters and the contents of ester-linked FA and SA, total lignin, monolignol ratio, and heavy monolignols. FA treatment increased the content of ester-linked FA, syringyl, and guaiacyl monomers measured by nitrobenzene oxidation, lignin content, and reduced root growth in both soybean and maize plants. p-Coumaric acid content ester-linked to the cell wall increased in soybean but decreased in maize after treatment with FA. Treatment with SA also induced lignification in soybean but not in maize. SA increased S-lignin and sinapoyl esters content in the cell wall polymers in both soybean and maize. Residues of deuterated hydroxycinnamates were detected in the lignin of both plants in both treatments. The assay demonstrated that exogenously applied SA and FA were metabolized through the phenylpropanoid pathway. Furthermore, they were, at least partially, exported to the apoplast, where they were ester-linked to cell wall polymers. This suggests hydroxycinnamic acids are metabolized differently by plants with different types of cell walls.

Phenomenal Insight into Electrochemically Induced Photocatalytic Degradation of Nitrobenzene on Variant Au-Modified TiO2 Nanotubes

TiO2 nanotubes are a prominent type of TiO2-based nanostructure compared to nanorod arrays. A promising way to improve photocatalytic performance is modifying TiO2 nanotubes with metals, either on the surface or inside the tubes. There is a substantial demand for enhancing the conductivity and charge separation of TiO2 nanotubes, with a major focus on gold (Au) modification. Gold (Au) coatings have significantly improved the photocatalytic activity of TiO2 nanotubes, particularly in pollutant oxidation. However, the mechanism underlying the action of Au-modified TiO2 nanotubes in photocatalytic nitrobenzene oxidation under electrochemical induction remains unclear. Therefore, we conducted related experiments to explore the optimal Au concentration under various conditions. Under electric field induction, the maximum removal rate achieved was 54.9%. Lastly, we analyzed the relevant photocatalytic mechanism to elucidate the responses of electrons and holes to a simulated contaminant under a photo-electrochemical field.

Comparative study for the separation and depolymerization behavior of lignin from different separation methods

Due to its natural phenolic structure, lignin is the promising feed stock for the production of value-added phenolic monomers. However, the poor understanding of the relationship among separation methods, lignin structure and depolymerization behavior hinders the regulation of phenolic monomers production. In this work, lignin samples were separated by four methods (organic solvent, ionic liquids, deep eutectic solvents, acid-alcohol) using different strategies and their structure difference were characterized and verified by various methods (FTIR, NMR, nitrobenzene oxidation). It was found p-toluenesulfonic acid (PTS) and deep eutectic solvents (DES) were effectively solvents for lignin separation and for structure protection. Highest lignin separation of 99.42% (PTS) and recovery/purity of 92.93%/91.76% (DES) were obtained. The less condensed lignin also showed better performance on mono-phenols production (26.52%). Whereas the condensation C–C bonds formed during separation would hinder the depolymerization and lead to a lower phenolic monomers yield. The research results can provide guidance for the rational selection of lignin separation technology and subsequent depolymerization to produce phenolic monomers.

Lignin Analysis by HPLC and FTIR: Spectra Deconvolution and S/G Ratio Determination.

Fourier transform infrared spectroscopy (FTIR) is a simple nondestructive technique that allows the user to obtain quick and accurate information about the structure of the constituents of wood. Spectra deconvolution is a computational technique, complementary to FTIR analysis, which improves the resolution of overlapped or unobserved bands in the raw spectra. High performance liquid chromatography (HPLC) is an analytical technique useful to determine the ratio of the lignin monomers obtained by the alkaline nitrobenzene oxidation method. Furthermore, lignin content has been commonly determined by wet chemical methods; Klason lignin determination is a quick and accessible method. Here, we detail the procedures for chemical analysis of the wood lignin using these techniques. Additionally, the deconvolution process of FTIR spectra for the determination of the S/G ratio, in lignin isolated by this or other methods, is explained in detail.

Kraft lignin recovery from de-oiled Jatropha curcas seed by potassium hydroxide pretreatment and optimization using response surface methodology

The present study explores resource recovery from the de-oiled Jatropha curcas seed (DJS) using alkaline pretreatment. Potassium hydroxide (KOH) was utilized for the alkaline hydrolysis at a solid loading rate of 10 % w/v for the kraft lignin (KL) recovery. The process variables optimized were reaction time (6.36–73.63 min), temperature (85.68–156.3 °C), and KOH concentration (0.61–12.38 % w/w). Response surface methodology was used to optimize the process parameters. The maximum amount of KL recovered under optimal conditions was observed to be 89.6 %. The functional group changes in treated and untreated DJS were explored through Fourier transform infrared spectroscopy (FTIR) analysis. 1H NMR and FTIR techniques were employed to characterize recovered lignin. Nitrobenzene oxidation of the lignin was performed to understand the lignin monomer profile. The recovered lignin has an S/G ratio of 0.36. The pretreated DJS biomass can be subjected to enzymatic hydrolysis for enhanced sugar yield.

Vanillin Production Pathways in Alkaline Nitrobenzene Oxidation of Guaiacylglycerol-β-guaiacyl Ether.

Alkaline nitrobenzene oxidation (AN oxidation) is a significant method for chemical analysis of lignin. Despite its importance in lignin chemistry, the detailed chemical reactions involved in AN oxidation are not yet fully understood. Surprisingly, there is almost no experimentally supported information available regarding the reaction pathways in the AN oxidation of guaiacyl glycerol-β-guaiacyl ether (GG), a common model compound in lignin chemistry. This study reports the results of our investigation into the formation pathway of vanillin (4-hydroxy-3-methoxybenzaldehyde) in the AN oxidation of GG. Our series of experiments proposed a vanillin formation pathway involving an enol ether with a C2 side chain, 2-methoxy-4-[2-(2-methoxyphenoxy)-ethenyl]-phenol C2EE, as an intermediate, in which C2EE is produced by the non-oxidative degradation of GG by alkali. Another enol ether with a C3 side-chain, Z-4-[3-hydroxy-2-(2-methoxyphenoxy)-1-propen-1-yl]-2-methoxyphenol (C3EE), and the condensation products formed under alkaline conditions were found to be insignificant as vanillin sources. On the other hand, the comparison of the vanillin yields from GG and isolated C2EE (80.7 and 86.5 mol %, respectively) in their AN oxidation to the C2EE yield from GG in the absence of nitrobenzene (69.9 mol %) also suggested that the vanillin formation from GG involved unknown pathways in which C2EE is not an intermediate.

A Kinetic Model for Describing the Heterogeneity of Susceptibility to Chemical or Physical Change

Molecules in foods and food ingredients exist in a variety of states, each of which has a different susceptibility to chemical and physical change. A model was proposed for the change in behavior over time based on the assumption that the free energy of activation in relation to the rate constant obeyed a Gaussian distribution. This model could well explain the unique behaviors observed in the thermal inactivation of immobilized enzymes, formation of aldehydes from lignin by alkaline nitrobenzene oxidation, autoxidation processes of microencapsulated lipids, flavor release from microcapsules, and desorption of proteinaceous deposits fouled on stainless steel surfaces.

Characterization and Comparison of Some Kraft Lignins Isolated from Different Sources

Lignin characteristics are significantly affected by kraft processing and isolation conditions. In the studies carried out in this context, commercial lignins or isolated lignins from industrial black solutions are generally preferred. In this study, in order to conduct more comprehensive research, three lignin samples were isolated from kraft black liquor obtained from laboratory cooking trials of pine, poplar, and wheat straw chips, representing softwoods, hardwoods, and annual plants, respectively, according to efficient pulping studies. In addition, another lignin-containing industrial waste was provided from a pulp mill (OBL). The acidification method was applied for isolating lignin from black liquor samples. After isolating the lignin samples from different sources, they were characterized and compared with the commercially available kraft lignin sample (Indulin AT). Total phenolic groups, carboxyl groups, purity, functional groups, nitrobenzene oxidation products, molecular weight, thermal stability, and element contents were analyzed. The isolated lignin samples (except wheat straw) were as pure as commercial lignin. Since the wheat straw was agricultural waste and an annual plant, inorganic elements such as P, K, and Si were more abundant than in the other samples. However, the polydispersity and molecular weight of all of the isolated lignin samples were higher than those of commercial lignin. Because the ash contents of the lignin samples for pine, poplar, OBL, and indulin AT were between 1 and 3%, they can be used for high-value applications. In particular, despite some disadvantages, wheat straw lignin has greater potential for use in extruders than softwood lignins due to their syringyl content.

The effect of oxygen bleaching on reactivity of syringyl and guaiacyl units of Eucalyptus tereticornis pulps lignin

Dioxin lignin of Eucalyptus tereticornis kraft pulps and its oxygen-treated pulps with kappa number 21.32, 12.84; 13.03, 12.18 were isolated and subjected to alkaline nitrobenzene oxidation studies to determine the structural changes during oxygen bleaching. HPLC quantitatively identified eight compounds during nitrobenzene oxidation of dioxin lignin in alkaline media. Lignin obtained from E. tereticornis has syringyl and guaiacyl units. Residual lignin composition in pulps was studied and found that during oxygen delignification, the syringyl unit was degraded more in comparison to guaiacyl and p-hydroxy phenyl unit, i.e., the molar ratio of syringyl unit is lower than guaiacyl and p-hydroxy phenyl unit. The results are adequately described in the research.

Investigation of the Interaction Mechanism between Lignin Structural Units and Enzyme

The effect of lignin structural units on enzymatic hydrolysis of lignocellulosic biomass was investigated, especially the inhibitory role of lignin in non-productive adsorption with enzymes. Milled wood lignin (MWL) was isolated from different hardwoods of poplar, eucalyptus and acacia. The isolated lignin samples were characterized by elemental analysis, gel permeation chromatography, nitrobenzene oxidation and fourier infrared spectroscopy. The mechanism of lignin structural units on enzymatic hydrolysis of cellulose was studied by quartz crystal microbalance (QCM). The results showed that different structural units of lignin had different adsorption capacity for enzymes. The results of nitrobenzene oxidation indicated that the S/G ratio (S: syringyl-like lignin structures; G: guaiacyl-like lignin structures) of lignin of poplar was 0.99, that of eucalyptus was 1.92 and that of acacia was 1.34. According to the results of QCM, the adsorption capacity of the three lignin films was as follows: Poplar MWL (S/G ratio 0.99) < Acacia MWL (S/G ratio 1.34) < Eucalyptus MWL (S/G ratio 1.92). Eucalyptus MWL with higher degree of condensation and S/G ratio showed stronger affinity to enzymes and more non-productive adsorption with enzymes, resulting in less adsorption between enzymes and cellulose, and lower enzymatic hydrolysis efficiency.

Revisiting alkaline cupric oxide oxidation method for lignin structural analysis.

Lignin structural analysis is important for the comprehensive utilization of lignin as well as delignification and bleaching during pulping while it is difficult to completely elucidate lignin structure due to its structural complexity and heterogeneity. Depolymerization of lignin into simple monomers via alkaline cupric oxide oxidation (OxCuO) followed by chromatographic analysis of the monomers is an effective method for lignin structural analysis. Here we revisited the OxCuO of lignin model compounds (monomers and dimers) and three representative lignocelluloses (i.e., Eucalyptus, Masson pine, and corn stover) to understand the effects of reaction conditions and lignin sub-structures on oxidation product yields and distributions. The improved OxCuO was found to be effective in oxidatively breaking the robust interunit C-C bonds in the β-β′ and β-5′ moieties of lignin other than β-O-4′ linkages at an elevated temperature (210°C). Further degradation of the monomeric oxidation products could also occur to reduce the monomer yields under a severe condition (i.e., high temperature and long reaction time). In addition, O2 inputs could reduce the monomer yields via nonselective overoxidation, thus having negative effects on accurate structural analysis of lignin. The O2 removal via ultrasonication combined with N2 flushing prior to the oxidation reaction could improve the monomer yield about 1.2 times (compared to that without O2 removal) at a low biomass loading of 5 wt%. By using the improved method of OxCuO, a monomer yield of 71.9% could be achieved from Eucalyptus (hardwood) lignin, which was much higher than conventional nitrobenzene oxidation (59.8%) and reductive depolymerization (51.9%). Considering the low cost, high availability, and low toxicity of CuO, the improved OxCuO could be a convenient and economic method for more accurate lignin structural analysis.

Hardwood and softwood lignins from sulfite liquors: Structural characterization and valorization through depolymerization

This work aims to evaluate the structural characteristics and study the oxidative depolymerization of lignins obtained from hardwood and softwood sulfite liquors. Lignins were obtained after ultrafiltration and freeze-drying of the sulfite liquors and characterized based on inorganic content, nitrobenzene oxidation, 13C NMR, and molecular weight determination. The structural characteristics achieved allow evaluating the potential of each lignin through oxidative depolymerization to produce added-value phenolic monomers. Hardwood and softwood lignins were submitted to alkaline oxidation with oxygen and the reaction conditions optimized to obtain a final oxidation mixture with the maximum yield of phenolic monomers. Through oxidation with O2, hardwood lignin generates mostly syringaldehyde while lignin from softwood biomass mainly produces vanillin; moreover, a lower reaction time and the interruption of O2 admission avoid the degradation of the oxidation products in the final mixture for both lignins, more evidenced to hardwood lignin due to its higher reactivity. From the results, it is possible to conclude that a phenolic aldehyde-rich oxidation mixture could be obtained, confirming the viability of lignin as raw material to produce added-value products as vanillin and syringaldehyde.

Distribution and Chemical Composition of Lignin in Secondary Xylem of Cactaceae.

Cactaceae family has heterogeneity in the accumulation of lignocellulose due to the diversity of shapes and anatomy of the wood. Most studies focus on fibrous and dimorphic species; but the non-fibrous species are poorly studied. The aims of this work were to analyze the syringyl/guaiacyl ratio of lignin and its distribution in secondary xylem, especially in non-fibrous species. The syringyl/guaiacyl (S/G) ratio was quantified from 34 species of cacti by nitrobenzene oxidation of free-extractive wood. The distribution of lignocellulose in wood sections stained with safranin O/fast green was determined with epifluorescence microscopy. The S/G ratio was heterogeneous; most of the non-fibrous species had a higher percentage of syringyl, while the fibrous ones accumulate guaiacyl. Fluorescence emission showed that vessel elements and wide-band tracheids had similar tonalities. It is hypothesized that the presence of a higher percentage of syringyl in most cacti is part of the defense mechanism against pathogens, which together with the succulence of the stem represent adaptations that contribute to survival in their hostile environments.

Photocatalytic degradation of nitrobenzene in wastewater by persulfate integrated with Ag/Pb3O4 semiconductor under visible light irradiation

Nitrobenzene oxidation was executed utilizing an innovative method, in which Ag/Pb3O4 semiconductors irradiated by visible light were used for activation of persulfate into sulfate radicals. Batch mode experiments were accomplished to elucidate the effect of persulfate concentrations and Ag/Pb3O4 dosages on the nitrobenzene oxidation behaviors. The physicochemical properties of original and reacted Ag/Pb3O4 were illustrated by X-ray diffraction analyses, UV-Vis diffuse reflectance spectra, FE-SEM images, EDS analyses, photoluminescence spectra and X-ray photoelectron spectra, respectively. The main oxidant was hypothesized to be sulfate radicals, induced from persulfate caused by photocatalysis of Ag/Pb3O4. It was clearly reflected on the scavenging experiments with addition of benzene, ethanol and methanol individually. As far as degradation pathways concerned, nitrobenzene was essentially transformed into hydroxycyclohexadienyl radicals, and sequentially converted to 2-nitrophenol, 3-nitrophenol or 4-nitrophenol simultaneously. Denitration of nitrophenols gave rise to synthesis of phenol, followed with generation of hydroquinone and p-benzoquinone.

Syringyl ratio and its relation to the erythro ratio of β-O-4-structure in leaf cell walls

In case of wood cell walls, there is a very clear relationship between the aromatic structure of lignin and the stereo configuration of β-O-4-structures; the syringyl ratio (S ratio, syringyl/(syringyl + guaiacyl)) obtained by the nitrobenzene oxidation method is linearly correlated to the erythro ratio (E ratio) of β-O-4 structure (erythro/(erythro + threo)) obtained by the ozonation method. In this study, 21 leaf samples and 9 stem samples were analyzed to examine whether such relationships can be found in the lignin of leaf and stem cell wall samples. Although some correlation between the S and E ratios was found in leaf and stem cell wall samples, the correlation was not as clear as in the case of woods (correlation coefficient, R2= 0.988 for 21woods and R2= 0.653 for all the leaf, stem and wood samples). In addition, the E ratios of some leaf samples were lower than 0.5, which is the lowest limit of the E-ratio for wood lignin. It was demonstrated that wood, stem and leaf samples are roughly distinguished by the yield of ozonation and nitrobenzene oxidation products from the sample (not from lignin), but not by the apparent lignin content obtained by the Klason method. The presence of syringyl nuclei in the leaves of Ginkgo, which is classified as gymnosperm, was confirmed by nitrobenzene oxidation.

Changes of Meranti, Padauk, and Merbau Wood Lignin during the ThermoWood Process.

Thermal modification is an environmentally friendly process in which technological properties of wood are modified using thermal energy without adding chemicals, the result of which is a value-added product. Wood samples of three tropical wood species (meranti, padauk, and merbau) were thermally treated according to the ThermoWood process at various temperatures (160, 180, 210 °C) and changes in isolated lignin were evaluated by nitrobenzene oxidation (NBO), Fourier-transform infrared spectroscopy (FTIR), and size exclusion chromatography (SEC). New data on the lignins of the investigated wood species were obtained, e.g., syringyl to guaiacyl ratio values (S/G) were 1.21, 1.70, and 3.09, and molecular weights were approx. 8600, 4300, and 8300 g·mol−1 for meranti, padauk, and merbau, respectively. Higher temperatures cause a decrease of methoxyls and an increase in C=O groups. Simultaneous degradation and condensation reactions in lignin occur during thermal treatment, the latter prevailing at higher temperatures.

Comparison of methodologies used to determine aromatic lignin unit ratios in lignocellulosic biomass

BackgroundMultiple analytical methods have been developed to determine the ratios of aromatic lignin units, particularly the syringyl/guaiacyl (S/G) ratio, of lignin biopolymers in plant cell walls. Chemical degradation methods such as thioacidolysis produce aromatic lignin units that are released from certain linkages and may induce chemical changes rendering it difficult to distinguish and determine the source of specific aromatic lignin units released, as is the case with nitrobenzene oxidation methodology. NMR methods provide powerful tools used to analyze cell walls for lignin composition and linkage information. Pyrolysis-mass spectrometry methods are also widely used, particularly as high-throughput methodologies. However, the different techniques used to analyze aromatic lignin unit ratios frequently yield different results within and across particular studies, making it difficult to interpret and compare results. This also makes it difficult to obtain meaningful insights relating these measurements to other characteristics of plant cell walls that may impact biomass sustainability and conversion metrics for the production of bio-derived fuels and chemicals.ResultsThe authors compared the S/G lignin unit ratios obtained from thioacidolysis, pyrolysis-molecular beam mass spectrometry (py-MBMS), HSQC liquid-state NMR and solid-state (ss) NMR methodologies of pine, several genotypes of poplar, and corn stover biomass. An underutilized approach to deconvolute ssNMR spectra was implemented to derive S/G ratios. The S/G ratios obtained for the samples did not agree across the different methods, but trends were similar with the most agreement among the py-MBMS, HSQC NMR and deconvoluted ssNMR methods. The relationship between S/G, thioacidolysis yields, and linkage analysis determined by HSQC is also addressed.ConclusionsThis work demonstrates that different methods using chemical, thermal, and non-destructive NMR techniques to determine native lignin S/G ratios in plant cell walls may yield different results depending on species and linkage abundances. Spectral deconvolution can be applied to many hardwoods with lignin dominated by S and G units, but the results may not be reliable for some woody and grassy species of more diverse lignin composition. HSQC may be a better method for analyzing lignin in those species given the wealth of information provided on additional aromatic moieties and bond linkages. Additionally, trends or correlations in lignin characteristics such as S/G ratios and lignin linkages within the same species such as poplar may not necessarily exhibit the same trends or correlations made across different biomass types. Careful consideration is required when choosing a method to measure S/G ratios and the benefits and shortcomings of each method discussed here are summarized.

Pulping and bleaching potential of banana pseudo stem, banana leaf and banana peduncle

The chemical, morphological and anatomical characteristics of banana pseudo stem, banana leaf (midrib) and banana peduncle were determined. Banana wastes were characterized with good amount of α-cellulose (40–45%), moderate amount of lignin (20–24%) and medium to long fiber length (1.22–1.92 mm). Alkaline nitrobenzene oxidation (ANO) showed that the main lignin unit of banana stem and leaf was p-hydroxybenzaldehyde and that of banana peduncle was syringaldehyde. The yield of ANO was very low (17–24%). Soda-anthraquinone (AQ) and formic acid/peroxyformic (FA/PFA) acid pulping processes of these banana wastes were evaluated. In soda-AQ process, pulping at 20% alkali charge, pulp yields were 30.0%, 34.55% and 39.12%, for banana stem, leaf and peduncle, respectively, with kappa number below 20. Higher pulp yield was found in FA/PFA process than soda-AQ process due to very high ash content in the samples. The banana waste pulps showed excellent papermaking properties.

Impact of Fractionation Severity of Ethanol Organosolv Lignin on Structural Characteristics and Depolymerization of Lignin

In this study, ethanol organosolv lignin (EOL) yield and structural characteristics of EOL depending on the production condition were evaluated to understand the lignin fractionation behavior and to determine the optimal condition for the production of phenolic compounds. EOL yield was enlarged until combined severity factor (CSF) reached 1.2, while molecular weight and nitrobenzene oxidation (NBO) yield of EOL were declined more than a half compare to those from CSF 0 to 0.6. Both EOL and NBO yields were predictable as a function of CSF with very high coefficient of determination. Concentration of phenolic compounds released from pyrolysis GC/MS was relatively stable until reaching CSF 1.5. However, it was significantly affected by not only CSF but also the concentration of acid used for the fractionation of EOL. Therefore, in order to produce phenolic compounds from EOL, production condition should be controlled between CSF 1.2 and 1.5 with low acid concentration.