Condensation Reactions of Softwood and Hardwood Lignin Model Compounds Under Organic Acid Cooking Conditions

In order to examine the condensation reactions of softwood and hardwood lignin during organic acid cooking, mixtures of benzyl alcohol type lignin model compounds with guaiacyl and syringyl units as the sources of benzyl cations and creosol and 5-methoxycreosol as the sources of electron-rich aromatic carbons were cooked under acidic pulping conditions.

From the yield of the condensation products in the initial reactions, it was shown that the carbonium cations of guaiacyl nuclei were more reactive than those of syringyl nuclei.

Syringyl type aromatic ring carbons had higher reactivities than guaiacyl type ones.

The cleavage of the benzyl ether bond in syringyl compounds was slower than that of guaiacyl compounds.

The diphenylmethane structures formed by the condensation reaction between veratryl alcohol and 5-methoxycreosol were found to be unstable under the strong acidic cooking conditions.

It is concluded that the condensation reactions between benzylic cations from the guaiacyl compounds and the electron rich aromatic ring carbons of syringyl ones are very fast, but the condensation products are unstable under the strong acidic pulping conditions.

When the guaiacyl nuclei react as an electron-rich aromatic carbon, the reaction is slower but the condensation product is much more stable.

These differences in reactivities, and the stabilities of the condensation products, may contribute to the resistance of softwood toward complete delignification on acid pulping.     

Reactivity of Lignin and Lignin Models Towards UV-Assisted Peroxide

The comparative reactivities of a series of guaiacyl and syringyl lignin model compounds and their methylated analogues towards alkaline peroxide and UV-alkaline peroxide were investigated. The overall reaction was followed by monitoring the reduction of the substrate as a function of time, and in every case, the reaction showed pseudo-first-order kinetics. The reaction rates of most lignin models having identical sidechains with alkaline peroxide and with UV-alkaline peroxide were in the order syringyl > guaiacyl > 3,4,5-trimethoxyphenyl > veratryl. Thus phenols react faster than their methyl ethers, and an extra ortho methoxyl group promotes the reaction. Lignin models possessing electron-donating side-chains had generally higher reaction rates than those with electron-withdrawing sidechains. The reaction rates of the series of benzoic acids were 2–4 times higher at pH 11 than at pH 5. UV-peroxide degradation of a eucalypt kraft lignin was faster than that of a pine kraft lignin, and degradation was 1.4–1.6 times faster at pH 11 than at pH 5. The data are consistent with the formation of higher amounts of reactive radicals under alkaline conditions, and aromatic rings with greater electronegativities promoting reactions with the radicals.     

Liquid products of the alkaline activation of brown coal from the Aleksandriiskoe deposit

The liquid products of the alkaline activation of brown coal from the Aleksandriiskoe deposit (800°C; 1 h; activator, KOH) were studied by thermal analysis, IR spectroscopy, and ¹H and ¹³C NMR spectroscopy. They were formed in ∼30% yield; they consisted of pyrogenic water (∼50%) and a resinous mixture of organic substances with increased hydrogen and oxygen contents and decreased carbon and sulfur contents. On heating, potassium hydroxide completely decomposed the quinoid structures of coal, decreased the concentration of aromatic components in resin, and increased the concentrations of CH₃-, -CH₂-, and OH-groups. This is consistent with the well-known thermally initiated reactions of alkaline degradation and dehydrogenation at high temperatures (400–800°C). The thermal lability of resinous products was evaluated, and the heats of combustion (32.4–32.7 MJ/kg) were determined; the latter values indicate that these products can be used as fuel.     

Aromatic Products from Reaction of Lignin Model Compounds with UV-Alkaline Peroxide

A series of guaiacyl and syringyl lignin model compounds and their methylated analogues were reacted with alkaline hydrogen peroxide while irradiating with UV light at 254 nm. The aromatic products obtained were investigated by gas chromatography-mass spectrometry (GC-MS). Guaiacol, syringol and veratrol gave no detectable aromatic products. However, syringol methyl ether gave small amounts of aromatic products, resulting from ring substitution and methoxyl displacement by hydroxyl radicals. Reaction of vanillin and syringaldehyde gave the Dakin reaction products, methoxy-1,4-hydroquinones, while reaction of their methyl ethers yielded benzoic acids. Acetoguaiacone, acetosyringone and their methyl ethers afforded several hydroxylated aromatic products, but no aromatic products were identified in the reaction mixtures from guaiacylpropane and syringylpropane. In contrast, veratrylpropane gave a mixture from which 17 aromatic hydroxylated compounds were identified. It is concluded that for phenolic lignin model compounds, particularly those possessing electron-donating aromatic ring substituents, ring-cleavage reactions involving superoxide radical anions are dominant, whereas for non-phenolic lignin models, hydroxylation reactions through attack of hydroxyl radicals prevail.     

31P NMR Spectroscopy in Wood Chemistry. I. Model Compounds

The quantitative reaction in an NMR tube of 1,3,2-dioxaphospholanyl chloride (I) with compounds bearing active hydrogens was explored as a simple method for derivatizing the labile centres known to occur in lignins. Derivatives of phenols, alcohols and carboxylic acids with (I) gave ³¹P chemical shifts which appeared in different ranges of the NMR spectra. Any ortho substitution onto the aromatic ring of phenols significantly affected the magnitude of the ³¹P NMR chemical shifts, while para and/or meta substituents had a much smaller effect. A clear distinction between guaiacyl, syringyl and unsubstituted phenolic hydroxyls can thus be made in mixtures of model compounds. Primary alcohols were clearly distinguished from secondary and tertiary alcohols in their derivatives with (I), while different ³¹P NMR signals for derivatives of erythro and threo forms of lignin-like model compounds were identified. While alcohols, phenols and simple carboxylic acids on reaction with (I) gave derivatives that were substitution products, aldehydes reacted via a distinctly different addition mechanism; ketones did not react at all.     

Steady-state conversion of methane to aromatics in high yields using an integrated recycle reaction system

Methane can be converted in high yields to aromatic products using an integrated recycle system containing both an oxidative coupling (OCM) reactor at 800°C, for conversion of CH₄ to C₂H₄, and a secondary reactor containing Ga/ZSM-5 at 520°C for subsequent conversion of ethylene to aromatics. Using this system, aromatic product yields of >70% at CH₄ conversions of ~100%, based on total CH₄ added, can be obtained. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation rate of benzyl cation intermediate from p-Hydroxyphenyl,guaiacyl, or syringyl nucleus in acidolysis of lignin

The benzyl cation intermediate is believed to be primarily formed in the acid-catalyzed reaction of lignin, and the reaction route of the intermediate determines which reaction products are afforded and whether lignin undergoes depolymerization or condensation. This study aimed to examine the formation rate of the benzyl cation intermediate from phenolic (P) or non-phenolic (N) lignin model compounds with different types of aromatic nuclei, namely p-hydroxyphenyl (H), guaiacyl (G), or syringyl (S). The rate was in the order of H > G > S for both P- and N-type model compounds and of P > N for all H-, G-, and S- type model compounds. The orders were successfully explained by the electron-donating or electron-withdrawing properties of the hydroxy and methoxy groups at the para- and meta- of the benzyl position, which is the reaction center in the formation of the benzyl cation intermediate.     

pKa-Values of Guaiacyl and Syringyl Phenols Related to Lignin

Abstract

The literature relating to the pK_a -values of guaiacyl- and syringyl-derived phenols has been thoroughly surveyed and summarized. In addition, the pK_a -values of a number of guaiacyl, syringyl and other phenols related to lignin have been determined using a spectrophotometric method combined with multivariate evaluation. Differences and similarities between the acidities of a number of substances are extensively discussed. The pK_a -value strongly affects the delignification during pulping, bleaching and leaching of lignin during pulp washing.     

Reactivity of Lignin Diphenylmethane Model Dimers Under Mild Alkali-O2 Conditions

A series of phenolic lignin-model dimers of the guaiacyl diphenylmethane (DPM) type has been shown to be quite reactive under mild alkali-O₂ conditions (0.5 N NaOH, 55°C) giving significant amounts of monomeric products. The reactivity of these dimers varies significantly and decreases in the order of α-5, 5,5′, α-6, and α-1 types. Interestingly, the oxidative cleavages of these DPM units, except the α-6 type, all gave the benzyl alcohol products e.g. vanillyl alcohol.     

Study on combustion mechanism of asphalt binder by using TG-FTIR technique

The combustion mechanism of asphalt binder was investigated by using thermogravimetric analyzer coupled with Fourier transform infrared spectrometer (TG-FTIR) in a mixed gas environment of 21% oxygen and 79% nitrogen. The results show that the combustion process of asphalt binder consists of three main consecutive stages at a low heating rate. The combustion reaction becomes more and more intense from the 1st to 3rd stage. The release of volatiles occurs mainly at 300-570 °C, and the gaseous products in each stage are different. The main products in the 1st stage are CO₂, CO, H₂O, hydrocarbons, formaldehyde, tetrahydrofuran, formic acid, aromatic compounds, etc. In the next stage, the combustion products mentioned above keep on increasing, but some new volatiles such as alcohols, phenols, styrene, etc. are present. In the last stage, the CH and CO bonds continue to fracture and aromatization reaction occurs, and the release amount of CO₂, CO, and H₂O reaches the maximum. But the content of other products decreases or even disappears due to burning. Among the above volatiles, CO₂ is the dominant gaseous product in the whole combustion process. The concentration of CO₂ and CO keeps increasing, and reaches the maximum intensity at about 520 °C. The evolution of H₂O, CH₄, and formic acid exhibits the trend of increase first, and then decrease. Over 570 °C, there are few products released at the end of the combustion process. Asphalt binder combustion process includes two modes of complete and incomplete combustion, and the latter may be main combustion mode of asphalt binder.     

Photo-electro catalytic oxidation of aromatic alcohols on visible light-absorbing nitrogen-doped TiO2

Visible light illumination of nitrogen-doped TiO₂ brings about the selective oxidation of benzyl and cinnamyl alcohol to the corresponding aldehydes. The photocatalyst was prepared by a sol-gel method and characterised mainly by XRD, UV-vis diffuse reflectance and Raman spectroscopy. The conditions limiting the observation of visible light photoactivity are the use of dry nitriles as nonaqueous solvents and aromatic alcohols as the substrates. No visible light oxidation takes place in an aqueous medium. The efficiency of benzyl and cinnamyl alcohol photo-oxidation in nitrile solvents follows the order: CH₃CN > CH₃CH₂CN > CH₃(CH₂)₂CN. Conversely, since alcohol photo-oxidation occurs with 100% selectivity on electrodes in O₂ saturated solutions at potentials close to E_fb or under open-circuit conditions, suspensions of the photocatalyst can be advantageously employed. The process involves a relatively weak adsorption of the alcohol substrates which, however, do not readily capture the photogenerated holes. On the basis of the electrochemical and photoluminescence data, it appears that the solvent (e.g. acetonitrile), in addition to O₂ has an active role in the reaction mechanism.     

Review of the kinetics of pyrolysis and hydropyrolysis in relation to the chemical constitution of coal

Kinetic data show that the pyrolysis reactions of hard coal may be interpreted in terms of parallel first order reactions, relating to the coal functional groups which can be considered as predecessors of the pyrolysis products. This relation is confirmed by C-, H- and O-balances of pyrolysis products and those of the corresponding predecessor structural groups of the coal. The activation energies measured are of the same order of magnitude as the bonding energies of bridge CC bonds between the aromatic ring-systems in the coal molecule. These observations suggest a mechanism of coal pyrolysis, consisting of the following steps: rupture of CC bridges; formation of radical groups; and recombination of radicals to stable molecules part of which, i.e., those of low molecular weight, diffuse out of the solid matter whereas the rest (whose diffusion in the pore system of the solid is prevented due to their higher molecular weight) react with each other at higher temperatures to give coke, releasing elementary hydrogen. In the presence of hydrogen > 500 °C, additional reactions of partial hydrogenation of polynuclear aromatics with subsequent hydrocracking will occur, leading to increased formation of highly aromatic tar, BTX, CH₄ and H₂O.     

Aufklärung der Molekülstruktur von Rohölinhaltstoffen durch NMR-Spektroskopie. III. Quantitative Analyse von Erdölfraktionen im Siedebereich über 220°C

Elucidation of the Molecular Structure of Petroleum Constituents by N.M.R. Spectroscopy. III. Quantitative Analysis of Crude Oil Fractions in the Boiling Range above 220°C A method is presented which permits a quantitative analysis of crude oil products boiling > 220°C by means of the data of the ¹H n.m.r. spectrum. Besides of the quantitative analysis (aromatics, paraffins/naphthenes), further structural informations for both of these groups are given, concerning the number of carbon atoms in aromatic rings, the degree of substitution of the aromatic rings, and the CH₃-share of paraffins/naphthenes.     

Design of active NiCo2O4‐δ spinel catalyst for abatement of CO‐CH4 emissions from CNG fueled vehicles

Increasing number of CNG vehicles on road emits considerable amount of CO, a poisonous gas and CH₄, a greenhouse‐gas. Highly active and oxygen‐deficient NiCo₂O_4‐δ spinel and its individual metal‐oxides were synthesized by calcination of precipitated/co‐precipitated basic‐carbonates followed by calcination under different strategies of stagnant air(s), flowing air(f) and reactive calcination(RC) for total oxidation of CO‐CH₄ mixture. The catalysts were characterized by XRD, XPS, BET surface‐area, SEM‐EDX and TEM. The performance order of the catalysts for the oxidation of CO‐CH₄ mixture was as follows: NiCo_RC>NiCo_f>NiCo_s>Co_RC>Co_f>Co_s>Ni_RC> Ni_f>Ni_s. The pairing of Ni and Co in spinel‐structure together with RC produced catalyst was oxygen‐deficient highly active for total oxidation of the mixture at the lowest temperature of 350°C. The NiCo_RC was found stable under reaction‐conditions for 50h at 350°C and after four successive heating (350°C)‐cooling (35°C) cycles besides accelerated‐aging tests up to 600°C. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2632–2646, 2018     

Substituent Effects on 13C Chemical Shifts of Aromatic Carbons in β-O-4 and β-5 type Lignin Model Compounds

Abstract

Substituent effects on the chemical shifts of aromatic carbons in lignin model compounds have been elucidated from ¹³C NMR spectra of guaiacyl and syringyl type monomerio and β-O-4 model compounds and guaiacyl type β-5 model compounds. Evaluation of the observed values of substituent chemical shift (SCS) for the aromatic carbons leads to elucidation of a generalized SCS additivity rule, for estimation of the chemical shifts of aromatic carbons in ring A of β-O-4 and β-5 type substructures in model compounds and in ring B of β-O-4 substructures in lignin preparations, with errors of less than 1 ppm. The rule is applicable to substructures of both guaiacyl and syringyl types, using an appropriate parent compound as reference instead of benzene. Signals in the aromatic region of the ¹³C NMR spectra of β-O-4 and β-5 type model compounds are reassigned on the basis of the observed SCS's as well as APT spectra of the compounds.     

Electron Transfer Reactions in Pulping Systems VIII. Reactions of Syringyl Alcohol in Aqueous Alkali

Syringyl alcohol, a simple lignin model compound, has been heated in lM NaOH at 135°. Five products - three monomers and two dimers - were identified from the reaction mixture; two of the products were presumably formed by radical processes. One of the radical products, 4-methylsyringol, was shown by deuterium labeling to incorporate a benzylic hydrogen from syringyl alcohol. Alkaline reactions of syringyl alcohol in the presence of radical initiators and inhibitors were not able to establish if the condensation reactions proceeded by a radical mechanism.     

A study on the aliphatic energetic plasticizers containing nitrate ester and nitramine

The aliphatic energetic plasticizers with three and four –CH₂ between nitrate ester and nitramine were synthesized to obtain a plasticizer that is more stable than N-butyl-N-nitratoethyl nitramine (BuNENA) with two –CH₂. First, amino alcohol compounds such as propylamino propanol (PAP) and ethylamino butanol (EAB) as a precursor of energetic plasticizer were synthesized. However, unlike in BuNENA synthesis, various side reactions occurred in the nitration of amino alcohols. Fortunately, it was possible to considerably suppress the formation of side products in the nitration of PAP by using a solvent and an appropriate concentration of nitric acid. In addition, an energetic plasticizer with higher oxygen content was obtained through the nitration of intermediate, amide alcohol, which was formed in the synthesis of EAB.     

The electrochemical oxidation of dimethyl disulfide—Anodic methylsulfanylation of phenols and aromatic ethers

The electrochemical oxidation of dimethyl disulfide was investigated in acetonitrile and dichloromethane. The nature of the oxidation strongly depends on the nucleophilicity of the solvent. In acetonitrile a one-electron oxidation is observed and the consecutive species did not exhibit any reactivity towards aromatic compounds subject to electrophile substitution. On the contrary, the oxidation of dimethyl disulfide in methylene chloride afforded a two-electron process with the formation of a species consistent with CH₃S⁺. Its reactivity towards phenols and aromatic ethers was confirmed and showed a selective monomethylsulfanylation in most of the cases.     

The reactions of alkylene oxides with various butyl and other alcohols

Acid-catalyzed additions of ethylene oxide to various isomeric butyl alcohols were observed to agree with the Poisson equation and the products were relatively unaffected in composition by changes in the reactants or reaction conditions. Base-catalyzed reactions yielded products whose composition could be more closely approximated by Weibull and Nycander’s equations. The product composition, although largely independent of reaction conditions, varied with the structure of the starting alcohol. Good correlation was found between the relative acidities of the starting alcohols and their reaction products and Weibull-Nycander distribution constants for base-catalyzed reactions of ethylene oxide and propylene oxide. It is believed that the relative acidities of the alcohol and its epoxide adduct influence the product composition through the equilibrium reaction: ROH+RO (CH₂CH₂O)=_X→RO⁻+RO(CH₂CH₂O)_xH Presented at the AOCS meeting in Toronto, Canada, 1962. An erratum to this article is available at .     

Electrolyte effects on oxygen reduction kinetics at platinum: A rotating ring-disc electrode analysis

A comparative study of the electrode kinetics of oxygen reduction of platinum in perchloric, phosphoric, sulfuric, trifluoromethanesulfonic acids (all at pH = 0) and in potassium hydroxide (pH = 14) was made at 25°C using rotatating ring-disc electrode techniques. The platinum electrode was first characterised in these electrolytes using the cyclic voltammetric method. The results showed that in the potential region from 0.8 to 0.6 V/rhe, the kinetics of oxygen reduction in these electrolytes decreases in the order KOH > H₂SO₄ ～ CF₃SO₃H > H₃PO₄ > HClO₄. This order of activity is reflected in the effects of the electrolytes, in respect to specific adsorption of anions, on the platinum oxide formation reaction. The role of anion adsorption is also apparent in the dependence of the rate constant for oxygen reduction to water or to hydrogen peroxide and of hydrogen peroxide reduction to water on potential. The superior behavior of oxygen reduction in KOH is due to minimal adsorption of the OH^? ion. The more complex adsorption behavior of the oxyanions in the investigated acid electrolytes than that of simple anions like the halide ions presents difficulties in drawing detailed correlations between oxygen reduction kinetics and adsorption behavior of oxyanions of platinum.