Showing papers in "Research on Chemical Intermediates in 1995"

New Insights Into the Molecular Architecture of Hardwood Lignins By Chemical Degradative Methods

Abstract: Lignins are composed of phenylpropane units interconnected by labile and resistant bonds. A two-step degradation, thioacidolysis, provides detailed information on these network polymers. The first step involves lignin depolymerization with ethanethiol and BF3 etherate. The determination of the recovered thioethylated monomers provides an estimation of lignin units only involved in labile ether bonds. The ligninderived dimers, representative of resistant interunit bonds, are determined after a further desulfurization step. Results obtained for native and industrial hardwood lignins underline their structural differences. Native hardwood lignins are typified by a high proportion of linear fragments linked bifunctionally by β-O-4 bonds.

VUV photolysis of aqueous solutions of nitrate and nitrite

Abstract: Water homolyses upon vacuum-uv excitation into HO* radicals, hydrogen atoms and with lower efficiency, hydrated electrons. These primary species induce a series of reactions partially depleting nitrate and nitrite from aqueous solutions. Depletion rates depend on the presence of dissolved oxygen and temperature. Nitrate, nitrite, peroxynitrite and N2O were identified as reaction products after irradiation of, either, nitrite and nitrate in aqueous solutions. A reaction mechanism is proposed in accord with the experimental facts and with the evidence given in the literature, where NO2 * and NO* are key intermediates. NO3 −, NO2 −, NO2 *, NO* O2NO2 −, ONO2 − and N2O, seem to be interrelated by many redox reactions and reaction equilibria where pH and the availability of electrons determine their occurrence. The proposed mechanism is supported by a computer program with which the observed experimental behavior could be simulated.

31p nMr in Wood Chemistry: a Review of Recent Progress

Abstract: Over the past few years substantial efforts in our laboratory have been devoted toward the development of novel NMR techniques for the analysis of soluble and solid lignocellulosic substances. These efforts were undertaken in order to expand the frontiers of application of NMR for the detection of functional groups present in such materials. Our methodology involves the selective phosphorus-tagging of a variety of functional groups present in lignin and carbohydrates, followed by solution and solid state31P NMR spectroscopies. This paper attempts to review the status of this technique by discussing its development for the analysis of soluble and solid lignocellulosic samples. Solution31P NMR can be used to examine soluble lignin and carbohydrate samples after phosphitylation with 1,3,2 dioxaphospholanyl chloride. This is a novel and powerful means to determine the three principal forms of phenolic hydroxyls present in ligninsi.e. p-hydroxyphenyl, guaiacyl, and syringyl structures. In addition, primary hydroxyls, carboxylic acids, and the two diastereomeric forms of arylglycerol-beta-aryl ether units (β-O-4 structures) present in lignins can also be determined from a single31P NMR experiment. When applied to carbohydrates, the technique gave characteristic signals for thealpha andbeta anomers and the epimeric forms of monosaccharides. Completely resolved31P NMR spectra were also obtained when lignin-carbohydrate model compounds were examined. Solid state31P NMR can be used to determine quinone chromophore groups present in solid lignocellulosic samples. The method is based on literature accounts that describe adduct formation between trimethyl phosphite and quinones followed by solid state31P NMR. This reaction when reexamined in our laboratory showed that the presence of carboxylic acids in high yield pulps significantly affected the solid state31P NMR signal intensity. This realization permitted the development of an experimental protocol that allowed solid state31P NMR signals to be received from high yield pulps that contain information only onortho-quinones and coniferaldehyde chromophores. It was thus found that about 0.7ortho-quinone groups are present in every 100 C9 units within the lignin of a black spruce refiner mechanical pulp sample, in agreement with previously applied independent techniques.

Photochromism of dinaphthylethene derivatives. Stability of the closed-ring forms

Abstract: Stability of various types of closed-ring forms of dinaphthylperfluorocyclopentenes was investigated from both experimental as well as theoretical points of view. The stability was dependent on the energy differences between the open-ring and the closed-ring forms. When the energy difference was small, the closed-ring form became stabler.

Homo- and hetero-nuclear 2D NMR techniques: Umambiguous structural probes for non-cyclic benzyl aryl ethers in soluble lignin samples

Abstract: A number of two-dimensional NMR experiments are evaluated with respect to their ability to provide unambiguous evidence for the presence or absence of non-cyclic benzyl aryl ether (α-O-4) structures in soluble lignin samples. The most suitable techniques, in terms of both sensitivity and structural information content were the homonuclear Hartmann-Hahn (HOHAHA) and heteronuclear multiple quantum coherence (HMQC) techniques. By spiking an acetylated Pinus radiata MWL sample with an oligomeric α-O-4 ether model compound, it was possible to determine a detection limit of <0.3 structures per 100 lignin C9 units for both the HOHAHA and HMQC techniques. From this, and other work, it can be shown that if α-O-4 structures are present in MWL samples, they are present at a level below the detection limit of these experiments. The implications of these results are discussed in terms of lignin biosynthesis and reactivity.

Observation of quinonoid groups during the light-induced yellowing of softwood mechanical pulp

Abstract: Unbleached and alkaline hydrogen peroxide bleached black spruce (picea mariana) stone groundwood samples were irradiated with a weak light source over a period of six days in an effort to probe the early photochemical events occurring during the light-induced brightness reversion. Samples withdrawn at daily intervals were subjected to oxyphosphorylation with trimethyl phosphite to labelortho-quinones and other chromophoric species with phosphorus. The solid-state31P NMR spectra of these samples revealed the rapid formation ofortho-quinones during the early phase of irradiation, followed by a dramatic reduction of these species at latter phases. The data suggests that during the early photochemistry,ortho-quinones are produced in high yield pulps, which subsequently react creating other more complex chromophores which do not possess the quinonic character.

Flash photolysis study on reversible addition reactions of thiyl radicals

Abstract: Obtaining the rate constants in the absolute values affords many advantages compared with the relative rates. It had been believed that the absolute values obtained by the flash photolysis method were not precise enough to examine substituent effects such as the Hammett equation. In this review, however, one can recognize that we can examine substituent effects and solvent effects with the absolute rate constants obtained by the flash photolysis. Especially, comparison among the reactivities of different attacking radicals becomes possible with measurements of the absolute rate constants.

Molecular variability of lignin fractions isolated from wheat straw

Abstract: Three lignin fractions were isolated from wheat straw internodes and leaves: alkali lignin (LA), milled lignin (LM) and enzyme lignin (LE). Thioacidolysis and alkaline nitrobenzene oxidation were used for the characterisation of the lignin monomeric composition. Esterified and etherifiedp-coumaric and ferulic acids were characterised by alkali hydrolysis and acidolysis, respectively. Lignin fractions from leaves are more condensed than those from internodes. The syringyl/guaiacyl (S/G) ratio is higher in the case of internode lignins. Among the three lignin fractions, LA is the less condensed whereas LM and LE do not present significant structural differences according to their monomeric composition. Finally, the lignin fractions studied differ in their contents and proportions of associated phenolic acids.

The effect of dark recovery time on the photoefficiency of heterogeneous photocatalysis by TiO2 suspended in non-aqueous media

Abstract: Variation of dark recovery time influences the efficiency of a heterogeneous photocatalytic oxidation on irradiated TiO2 particles suspended in non-aqueous media, but does not affect the efficiency of a parallel photocatalyzed reduction. The efficiency of the photocatalyzed oxidation of 1-octanol on Degusa P-25 TiO2 in oxygenated acetonitrile nearly doubled when the dark interval between excitation pulses was increased from 0.1 s to 1 s. No improvement in photoefficiency could be detected in the photocatalytic reduction of p-nitroacetophenone under the same conditions.

Model compound studies on the cleavage of glycosidic bonds by ozone in aqueous solution

Abstract: Kinetic expressions have been developed for the initial phase of the reactions between ozone and methyl-β-D-glucopyranoside, a model compound for cellulose, in aqueous solutions at moderately low pH (pH 3) and ambient temperature Ozone is reasonably stable under these conditions The reaction is first order in the glucoside and ozone, second order overall Kinetic and analytical data suggest that the glucoside is directly attacked by ozone to form polyoxide intermediates The α-glucoside was more stable and was attacked by ozone exclusively at the acetal oxygen in the initial phase of the reaction In contrast, the β-anomer was ozonized nearly twice as fast, and was attacked by ozone mainly at the C-H bond of the anomeric carbon The latter reaction resulted in the formation of δ-gluconolactone, probably by way of a hydrotrioxide intermediate The fragmentation of the intermediate could be assisted by properly oriented non-bonding orbitals of the two acetal oxygens These stereoelectronic rules were previously proposed by Deslongchamp and coworkers on the ozonolysis of acetals in non-aqueous media The activation energy for the reaction was 695 kJ/mole in both acetic and sulfuric acid solutions at pH 3 The presence of transition metal-ion, in particular Co(II) and Fe(II) ions, resulted in enhancement of both ozone decomposition and degradation of the glycoside, apparently caused by free radical reactions In some cases, addition of acetic acid led to substantial suppression of the degradation of the glucoside

Effect of supports on copper Catalysts for Methanol Synthesis from CO2 + H2

Abstract: Methanol was synthesized from carbon dioxide and hydrogen over various supported copper catalysts. A support was selected from the viewpoint of its acid-base properties. Acid supports showed high selectivity but low activity. Neutral and basic supports produced only carbon monoxide. Amphoteric supports showed high activities. Among them, TiO2 supports suppressed the reverse water gas shift reaction (CO formation). The role of supports was discussed on the basis of in situ infrared results.

Factors Controlling the Efficiencies of Photoinduced Electron-Transfer Reactions

Abstract: The overall efficiencies of photoinduced electron transfer reactions in polar solvents are usually determined by the efficiency with which separated radical ions are formed from the initially formed geminate radical-ion pairs. These separation efficiencies are determined by the competition between retum electron transfer and separation within the geminate pairs. A method is described for determining whether variations in the quantum yields for formation of separated radical ions are due to changes in the reorganization parameters for the return electron transfer reactions, or to other factors. The use of the method is illustrated in studies of the effects of varying steric bulk and molecular size of the donors, and also in studies of the effect of using a charged sensitizer.

Homolytic cleavage of the β-ether bond in phenolic β-O-4 structures in wood lignin and in guaiacylglycerol-β-guaiacyl ether

Abstract: The degradation of guaiacylglycerol-β-guaiacyl ether and lignin in spruce wood was studied in order to investigate the extent of homolytic reactions on heating with dioxane: water media (“mild hydrolysis”). The degradation reactions were studied at different temperatures, reaction times and pH-values. The results clearly show that extensive cleavage of phenolic β-ethers occur under the conditions prevailing during heating in dioxane: water media. Homolytic cleavage of phenolic β-ethers should therefore be considered in interpretations of results from “mild hydrolysis” of lignin.

The GC-MS Identification of Some Aliphatic Intermediates From the Tio2 Photocatalytic Degradation of Dimethoxybenzenes in Water

Abstract: In an attempt to answer the as yet unsolved question of the structures of the aliphatic compounds resulting from the ring opening of aromatic pollutants in water during a photocatalytic treatment, we have carried out a GC-MS study (with or without silylation) of aqueous solutions of each of the dimethoxybenzenes (DMBs) UV-irradiated in the presence of powder TiO2. The aliphatic intermediates are esters or acids which were identified either by comparison with authentic compounds (as is indicated by an asterisk in the following list) or by interpretation of the mass spectra (EI and CI). From 1,4-DMB, they are: (E)* and (Z)* H3COOC-CH =CH-COOCH3. From 1,3-DMB, they are: H3COOC-CH=CH-CO-COOH3; H3COOC-CO-CH(OCH3)-CH2OH; H3COOC-CH=CH-CHO; H3COOC-CH=CHOCH3. From 1,2-DMB, they are: (E,E) H3COOC-CH =CH-CH=CH-COOCH3*; HOOC-CH=CH-CH=CH-COOCH3 isomers; H3COOC-CH=CH-CHO; (E) HOOC-CH=CH-COOH*; H3COOC-CHOH-CHO; H3COOC-COOCH3*, In the case of 1,2-DMB, additional analyses showed the presence of CH3OH. Its formation was suggested to result from the cleavage of the methoxy groups of DMBs. These identifications show the complexity of the degradation pathways. The dimethyl esters of (E,E) hexadienedioic acid and of (E) and (Z) butenedioic acids could result from the opening of 1,2- or 1,4-DMB, respectively, before these pollutants are hydroxylated. This suggests that hydroxyl radicals are not the only oxidizing species involved. The fact that all aliphatics contain a number of C atoms ≤ to that in DMBs bears witness to the bond cleavages which lead to the mineralization.

Photodissociation and geminate dynamics in solution phase: Picosecond transient absorption studies of tetraphenylhydrazines and diphenyl disulfides

Abstract: Photodissociation of aromatic molecules and the following geminate dynamics of the photochemically produced fragment radical pair in liquid phase are discussed in relation to the results of ultrafast transient absorption measurements. Photodissociation of tetraphenylhydrazine occurs from the unequilibrated excited singlet state competing with the relaxation to the fluorescence state. In addition to the rapid fragmentation, the N−N bond rupture is clearly demonstrated to take place in the fluorescence state of tetraphenyl-and tetratolyl-hydrazine in tens of picoseconds time scale. In the case of di-p-aminophenyl disulfide in nonpolar solvents, geminate recombination of the p-aminophenylthiyl radical and the formation of the radical dimer were observed in tens of picoseconds. The observed slow geminate recombination can be explained by the repulsion between the large dipole of the fragment radical preventing the reformation of the S−S bond and by the restricted conformation required for the dimer formation at the encounter of radicals.

Inclusion polymerization of Isoprene in the channels of sepiolite

Abstract: We have tried an inclusion polymerization in the channels of sepiolite, and studied the structure of polymer formed inside and outside the channels. Isoprene was polymerized not only on the outer surface but also in the channels of the sepiolite at 25 °C. Strong Bronsted acid sites in the outer and inner surfaces of the sepiolite intitiate a polymerization of isoprene and styrene. Double bonds of polyisoprene formed in the channels were protonated by the strong Bronsted acid sites of the sepiolite. The interaction between the protonated double bonds and the acid sites changed the color to violet. The outer surface of the folded sepiolite whose channels were collapsed formed a usual polyisoprene which caused no color change.

High pressure nitration of toluene using nitrogen dioxide on zeolite catalysts

Abstract: Nitration of toluene by nitrogen dioxide in the presence of zeolite catalysts was carried out in a high pressure reaction system. Different zeolites (viz. H-ZSM-5, H-mordenite, HY etc.) were used as catalysts. The effects of NO2/toluene molar ratio and the total pressure on the formation of mono- and di-nitrotoluenes were also investigated. The N2-sorption capacity, X-ray diffraction analysis and catalytic activity measurements of the regenerated HY catalyst revealed that the HY catalyst can be reused.

Reactions ofOrtho-quinones and the model compound 4-t-butyl-1,2-benzo-quinone in alkaline peroxide

Abstract: The literature on the occurence and reactions of quinone structures in mechanical pulps has been reviewed. A study on the reactions of a quinone model compound (4-t-butyl-1,2-benzoquinone) in alkaline hydrogen peroxide has been reported, with detailed analysis of reaction products and kinetic phenomena. The study reveals that the diversity of products formed is much more complex than that obtained using an α,β-unsaturated aldehyde as a model, a simple second-order expression can be used to describe the kinetics. The kinetics representing chromophore removal for the two classes of model compound are compared with previously reported studies of kinetic phenomena during bleaching of mechanical pulps.

Ir and uv spectroscopic study of borohydride reduced mechanical pulp during monochromatic and wide band irradiation

Abstract: The fate of aromatic carbonyl groups during light-induced yellowing of peroxide-bleached mechanical pulps was investigated using infrared and uv-visible reflectance spectroscopies. If the interference of existing carbonyl groups is eliminated by borohydride reduction, both techniques indicate clearly that aromatic carbonyl groups are formed during irradiation. Methoxylated and unmethoxylatedortho-quinones were introduced into the pulp by treatment with sodium periodate or Fremy’s radical. Comparison with pulps which were discoloured by monochromatic near-uv irradiation (λ=320 nm) indicates that both types of quinones are formed during irradiation. The methoxylated quinones were bleached by 420 nm irradiation, but the unmethoxylated quinones seemed instead to sensitize destruction of aromatic groups.

Reactions of lignin in chlorine dioxide bleaching of kraft pulps

Abstract: 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.

Solvent and temperature effects on the fluorescence and competitive photoreactions of cis-9-styrylanthracene

Abstract: A fluorimetric and photochemical study of cis-9-styrylanthracene as a function of temperature has been carried out in different solvents with the aim of answering some open questions about the behavior of its first excited singlet state, S,. In non-polar solvents, a parallel photoreactive pathway, leading to a cyclization adduct, was found to compete with fluorescence and isomerization already at 200 K, its contribution increasing markedly with temperature. The cis → trans photoisomerization occurs prevalently by a triplet mechanism, a detectable contribution of diabatic and adiabatic isomerization in S, being operative in these solvents from room temperature upwards. In polar solvents, the main deactivation pathway competitive with fluorescence is isomerization to trans, which occurs prevalently through a mixed singlet mechanism with a major diabatic and a minor adiabatic components.

A high-power X-band pulsed microwave induced catalytic decomposition of methane with integral acoustic measurements

Abstract: A high-power microsecond pulsed microwave system operating in the X-band region was used for the catalytic conversion of methane. Microwave microsecond pulses at repetition rates of 50 and 80 Hz were used to initiate chemical reactions. The Microwave-Induced Acoustic technique was employed in combination with gas chromatography for on-line detection of chemical products. Methane was converted to C2 and C3 hydrocarbons. The selectivity of ethane in hydrocarbon products can be >90 %. The contribution of hydrogen evolution and carbon deposition has been shown to be important. A comparison between previous experiments performed with millisecond pulse durations is given.

Kinetic and spectral properties of the intermediates formed in the pulse radiolysis of 2-mercaptobenzimidazole

Abstract: The reactions of primary species of water radiolysis such as e- aq, H* and *OH, and some specific one electron reductants and oxidants with 2-mercaptobenzimidazole have been studied at various pHs. *OH radical reaction with MBZ at pH 7 gave a transient species having absorption maxima (λmax) at 330 and 590 nm. The transient species (pKa = 3.6) was found to be neutral at this pH and was a mild oxidant. The initial transient species formed by the reaction of MBZ with e- aq at pH 7 and with H atom at pH 0 were found to react with the parent molecule to form another transient species which has an absorption spectrum similar to that obtained by *OH radical reaction with λmax at 590 nm. However the reaction is not quantitative. The kinetic, spectral, acid-base and redox properties of the transient species are reported.

Characteristic changes in the fluorescence spectra of 1-naphthol at the gelation point during the sol-gel-xerogel transitions

Abstract: The fluorescence spectra of 1-naphthol were observed during the sol-gel-xerogel transitions of two different systems as a function of time; one is in the silicon and titanium (Si:Ti = 4500:1) binary systems involving no catalysts and the silicon and lithium (Si:Li = 99:1) binary systems involving HC1 as the catalyst. During the first stage of the sol-gel reaction of the 1-naphthol system, the fluorescence spectra mainly originated from the broad ’L2 state. The fluorescence spectrum originating from the anionic species at around 470 nm increased as the reaction proceeded. It was found that the fluorescence spectra originating from the anionic species of 1-naphthol drastically decreased in both systems just after gelation. These findings indicate that it becomes difficult for the dissociation of the excited state of 1-naphthol to give a dissociative proton to the surrounding matrix. The fluorescence-excitation spectra for the Si/Ti system indicated that the main route for the excited state of 1-naphthol to form a dissociative proton is through the excited state of the contact ion pair, while the main route in the Si/Li system is via the direct excitation of the neutral 1-naphthol and its dissociation. The observed changes in the fluorescence spectra of 1-naphthol in these sol-gel systems provide a sensitive means to monitor changes during the sol-gel transition process.

Conformational control in previtamin d photochemistry by heterogeneous reaction media

Abstract: The possibility of conformational control realization in the reaction of previtamin D photosynthesis in vitro was examined. The sharp changes in the kinetics of provitamin D photoisomerization in some heterogeneous media were attributed to the removal of previtamin D conformational equilibrium by the restriction of its intramolecular rotation. It was found that the quantitative evaluation of the reaction medium effect on the specific previtamin D reaction channels requires consideration of the irreversible photoconversions.

Fluorescence, fluorescence-excitation and absorption spectra of trans-1-(2-anthryl)-2-phenylethene conformers

Abstract: Fluorescence spectra of trans-1-(2-anthryl)-2-phenylethene (APE) obtained under varying conditions of excitation wavelength and oxygen concentration in toluene are resolved into two distinct components by application of principal component analysis with self-modeling (PCA-SM). Self-modeling is guided by the constraint that Stem-Volmer quenching plots for the individual conformers be independent of excitation wavelength. The same process applied on a matrix of fluorescence-excitation spectra leads to resolved conformer-specific fluorescence-excitation spectra. Consistency between the fluorescence and the fluorescence-excitation spectrum of each conformer is established. The fluorescence-excitation spectr and literature fluorescence quantum yields are used to resolve the UV absorption spectrum of APE. The pure conformer spectra obtained in this work are compared with those from earlier PCA-SM treatments in which self-modeling procedures were based on the Lawton and Sylvestre nonnegativity constraint and on a maximal spectral dissimilarity constraint.

Generation of Acidity of Silica Monolayer By Network of Si-O-Si On Alumina

Abstract: Structures of materials deposited from various Si alkoxides on alumina were determined based on gravimetry, product analysis and coverage measurement. The activity for double-bond isomerization of l-butene was observed over the silica monolayer with a network of siloxanes (Si-O-Si) deposited from Si(OCH3)4 at 493 and 593 K. However, the isolated silica species deposited from Si(OCH3)4 at low temperatures (≤ 423 K) and those deposited from Si(OCH3)n(CH3)4-n (n ≥ 3) showed almost no activity. Based on these findings, the species — Al-O-SiOH in the network of Si-O-Si was suggested to be the Bronsted acid site.

The role of stilbenes in bleaching and colour stability of mechanical pulps. i. the reaction of lignin model stilbenes with alkali and oxygen

Abstract: The reactions of lignin model hydroxystilbenes and TMP with alkaline hydrogen peroxide and with alkali and oxygen have been studied to determine the susceptibility of stilbenes, to oxidative degradation. Oxygen in the presence of alkali readily destroys lignin model stilbenes via cleavage of the olefinic double bond. The stilbene must contain a ring hydroxyl substituent for the reaction to occur. The reaction is fastest if the stilbenes havep,p′-dihydroxyl substituents. Stilbenes of the type likely to be attached to the lignin in pulps have only one hydroxyl group and are more stable. Hydrogen peroxide, by itself, is unreactive with stilbenes but the presence of oxygen as a peroxide decomposition product should limit the accumulation of stilbenes during alkaline peroxide bleaching of mechanical pulps. Alkaline peroxide bleaching of mechanical pulps under a pressurised atmosphere of oxygen gas leads to a slight increase in brightness and brightness stability. Guaiacyl-substituted stilbenes are more stable to alkali and oxygen than are syringyl-substituted stilbenes. This may partially explain why it is easier to bleach hardwood than softwood mechanical pulps.

The influence of intramolecular hydrogen bonding on the structure and E ↔ Z photoisomerization of urocanic acid derivatives

Abstract: The molecular structures and E ↔ Z photoisomerization reactions of methyl urocanate and several urocanamides were investigated. All of the Z isomers possess intramolecular hydrogen bonds. The intramolecular hydrogen bonds may influence the efficiency of photoisomerization but do not totally inhibit its occurrence. The relative energies of the electronic absorption of the E and Z isomers depends on the mode of hydrogen bonding and are accurately predicted in the case of methyl urocanate using INDO/S-CI calculations. The solvent dependence of the absorption spectra of methyl urocanate can be related to the effects of inter- and intramolecular hydrogen bonding.