Pathways For Formation Of Products Research Articles

A thermal decomposition study of pine wood under ambient pressure using thermogravimetry combined with synchrotron vacuum ultraviolet photoionization mass spectrometry

As pyrolysis is the initial step involved in thermochemical processes of biomass, understanding of the pyrolytic behavior of biomass is crucial to biomass thermochemical conversions. This work presents a new facility for studying the pyrolysis behavior of solid fuels. The experimental setup contains a thermogravimetry system connected with a home-made reflectron time-of-flight mass spectrometer via a single-stage molecular beam sampling interface, and a tunable vacuum ultraviolet (VUV) light generated from synchrotron source is used for soft photoionization. Evolution of the gaseous products of pine wood pyrolysis was examined at a heating rate of 20°C/min in a nitrogen environment at atmospheric pressure. Experiments were performed at the photon energies of 10.0, 10.5 and 11.0 eV, and mass spectra of gaseous products were recorded by the time-of-flight mass analyzer every five seconds. Species with the mass range of m/z 20–220 are presented and possible formation pathways of typical products are discussed.

The mechanism and kinetics of propene ammoxidation over α-bismuth molybdate

Propene ammoxidation over Bi2Mo3O12 was investigated to elucidate product (acrylonitrile, acetonitrile, HCN, acrolein, N2, etc.) formation pathways. Propene consumption rate is first order in propene and zero order in ammonia (for NH3/C3H6=0–2) and oxygen (for O2/C3H6⩾1.5) partial pressures, with an activation energy (Ea=22kcal/mol) comparable to that for propene oxidation, suggesting the same rate-limiting step for both reactions. We propose two N-containing species are relevant at ammoxidation conditions: adsorbed NH3 on surface Bi3+ ions that reacts with a propene derivative to form products with CN bonds, and a few metastable M-NHx (M=Mo, Bi; x=1, 2) groups that are very sensitive to destruction by water, but that are responsible for NH3 oxidation to N2. A proposed reaction mechanism and model that captures the experimental trends in product distribution as a function of partial pressures and temperature are presented.

Effects of reaction time and catalyst on gasification of glucose in supercritical water: Detailed reaction pathway and mechanisms

Supercritical water gasification of glucose as a model compound for biomass was conducted in quartz reactors at 500 °C. The concentration of glucose solution was 5 wt.% and the reaction time was adjusted within the range of 10–1800 s. The effects of reaction time and catalyst on the product distribution were investigated and the formation and degradation pathways of intermediate products with and without catalyst were discussed. The results show that the gas yields increased while the yields of organic intermediates in residual liquid decreased with the reaction time. The organic intermediates in residual liquid were mainly composed of phenols, furans, organic acids, alcohols, arenes and ketones. The Ru/Al2O3 catalyst had a significant influence on the gasification of glucose, which promoted the degradation of intermediates to gaseous products, increased the yield of hydrogen and inhibited the formation of char.

Pyrolysis mechanism of β[sbnd]O[sbnd]4 type lignin model dimer

A βO4 type lignin dimer compound was synthesized, namely 1-(4-methoxyphenyl)-2-(2-methoxyphenoxy) ethanol. To elucidate its pyrolysis mechanism, analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) experiments were performed to reveal the distribution of the pyrolytic products under different temperatures. Concurrently, density functional theory (DFT) calculations were conducted to analyze and verify the thermal decomposition mechanisms of the lignin dimer and the product formation pathways. The results show that the lignin dimer will undergo the CβO bond homolysis to produce 4-methoxystyrene and guaiacol at low pyrolysis temperatures. Whereas at medium pyrolysis temperatures, besides the CβO homolysis, CβO concerted decomposition will also take place to form carbonyl-containing phenolics. At high pyrolysis temperatures, the primary pyrolytic products will undergo secondary decomposition reactions to form a complex variety of products. With the combination of the experimental results and theoretical calculations, the pyrolysis mechanism of the lignin dimer model compound is clearly interpreted in this study.

Selective Friedel–Crafts Acylation Reactions of 2-Arylphenoxyacetic Acids: A Simple and Efficient Methodology to Synthesize Dibenzoxepine and Arylcoumaranone Derivatives

Intramolecular Friedel–Crafts acylation reactions of 2-arylphenoxyacetic acids are accomplished using trifluoroacetic anhydride or trifluoromethanesulfonic acid at 0 °C or room temperature. Depending on the reaction conditions, dibenzoxepines or arylcoumaranones are obtained in good yields and with high selectivities (83–100%). Plausible mechanistic pathways for the selective formation of the different reaction products are discussed.

A computational analysis of stoichiometric constraints and trade-offs in cyanobacterial biofuel production.

Cyanobacteria are a promising biological chassis for the synthesis of renewable fuels and chemical bulk commodities. Significant efforts have been devoted to improve the yields of cyanobacterial products. However, while the introduction and heterologous expression of product-forming pathways is often feasible, the interactions and incompatibilities of product synthesis with the host metabolism are still insufficiently understood. In this work, we investigate the stoichiometric properties and trade-offs that underlie cyanobacterial product formation using a computational reconstruction of cyanobacterial metabolism. First, we evaluate the synthesis requirements of a selection of cyanobacterial products of potential biotechnological interest. Second, the large-scale metabolic reconstruction allows us to perform in silico experiments that mimic and predict the metabolic changes that must occur in the transition from a growth-only phenotype to a production-only phenotype. Applied to the synthesis of ethanol, ethylene, and propane, these in silico transition experiments point to bottlenecks and potential modification targets in cyanobacterial metabolism. Our analysis reveals incompatibilities between biotechnological product synthesis and native host metabolism, such as shifts in ATP/NADPH demand and the requirement to reintegrate metabolic by-products. Similar strategies can be employed for a large class of cyanobacterial products to identify potential stoichiometric bottlenecks.

The Role of Oxidative Stress in Diabetic Neuropathy: Generation of Free Radical Species in the Glycation Reaction and Gene Polymorphisms Encoding Antioxidant Enzymes to Genetic Susceptibility to Diabetic Neuropathy in Population of Type I Diabetic Patients.

Diabetic neuropathy (DN) represents the main cause of morbidity and mortality among diabetic patients. Clinical data support the conclusion that the severity of DN is related to the frequency and duration of hyperglycemic periods. The presented experimental and clinical evidences propose that changes in cellular function resulting in oxidative stress act as a leading factor in the development and progression of DN. Hyperglycemia- and dyslipidemia-driven oxidative stress is a major contributor, enhanced by advanced glycation end product (AGE) formation and polyol pathway activation. There are several polymorphous pathways that lead to oxidative stress in the peripheral nervous system in chronic hyperglycemia. This article demonstrates the origin of oxidative stress derived from glycation reactions and genetic variations within the antioxidant genes which could be implicated in the pathogenesis of DN. In the diabetic state, unchecked superoxide accumulation and resultant increases in polyol pathway activity, AGEs accumulation, protein kinase C activity, and hexosamine flux trigger a feed-forward system of progressive cellular dysfunction. In nerve, this confluence of metabolic and vascular disturbances leads to impaired neural function and loss of neurotrophic support, and over the long term, can mediate apoptosis of neurons and Schwann cells, the glial cells of the peripheral nervous system. In this article, we consider AGE-mediated reactive oxygen species (ROS) generation as a pathogenesis factor in the development of DN. It is likely that oxidative modification of proteins and other biomolecules might be the consequence of local generation of superoxide on the interaction of the residues of L-lysine (and probably other amino acids) with α-ketoaldehydes. This phenomenon of non-enzymatic superoxide generation might be an element of autocatalytic intensification of pathophysiological action of carbonyl stress. Glyoxal and methylglyoxal formed during metabolic pathway are detoxified by the glyoxalase system with reduced glutathione as co-factor. The concentration of reduced glutathione may be decreased by oxidative stress and by decreased in situ glutathione reductase activity in diabetes mellitus. Genetic variations within the antioxidant genes therefore could be implicated in the pathogenesis of DN. In this work, the supporting data about the association between the -262T > C polymorphism of the catalase (CAT) gene and DN were shown. The -262TT genotype of the CAT gene was significantly associated with higher erythrocyte catalase activity in blood of DN patients compared to the -262CC genotype (17.8 ± 2.7 × 10(4) IU/g Hb vs. 13.5 ± 3.2 × 10(4) IU/g Hb, P = 0.0022). The role of these factors in the development of diabetic complications and the prospective prevention of DN by supplementation in formulations of transglycating imidazole-containing peptide-based antioxidants (non-hydrolyzed carnosine, carcinine, n-acetylcarcinine) scavenging ROS in the glycation reaction, modifying the activity of enzymic and non-enzymic antioxidant defenses that participate in metabolic processes with ability of controlling at transcriptional levels the differential expression of several genes encoding antioxidant enzymes inherent to DN in Type I Diabetic patients, now deserve investigation.

Reaction of carbon tetrachloride with methane in a non-equilibrium plasma at atmospheric pressure, and characterisation of the polymer thus formed

In this paper we focus on the development of a methodology for treatment of carbon tetrachloride utilising a non-equilibrium plasma operating at atmospheric pressure, which is not singularly aimed at destroying carbon tetrachloride but rather at converting it to a non-hazardous, potentially valuable commodity. This method encompasses the reaction of carbon tetrachloride and methane, with argon as a carrier gas, in a quartz dielectric barrier discharge reactor. The reaction is performed under non-oxidative conditions. Possible pathways for formation of major products based on experimental results and supported by quantum chemical calculations are outlined in the paper. We elucidate important parameters such as carbon tetrachloride conversion, product distribution, mass balance and characterise the chlorinated polymer formed in the process.

Photophysical and photochemical properties of 4-thiouracil: time-resolved IR spectroscopy and DFT studies.

Intensified research interests are posed with the thionucleobase 4-thiouracil (4-TU), due to its important biological function as site-specific photoprobe to detect RNA structures and nucleic acid-nucleic acid contacts. By means of time-resolved IR spectroscopy and density functional theory (DFT) studies, we have examined the unique photophysical and photochemical properties of 4-TU. It is shown that 4-TU absorbs UVA light and results in the triplet formation with a high quantum yield (0.9). Under N2-saturated anaerobic conditions, the reactive triplet undergoes mainly cross-linking, leading to the (5-4)/(6-4) pyrimidine-pyrimidone product. In the presence of O2 under aerobic conditions, the triplet 4-TU acts as an energy donor to produce singlet oxygen (1)O2 by triplet-triplet energy transfer. The highly reactive oxygen species (1)O2 then reacts readily with 4-TU, leading to the products of uracil (U) with a yield of 0.2 and uracil-6-sulfonate (U(SO3)) that is fluorescent at ~390 nm. The product formation pathways and product distribution are well rationalized by the joint B3LYP/6-311+G(d,p) calculations. From dynamics and mechanistic point of views, these results enable a further understanding for 4-TU acting as reactive precursors for photochemical reactions relevant to (1)O2, which has profound implications for photo cross-linking, DNA photodamage, as well as photodynamic therapy studies.

Density functional theory study on bond dissociation enthalpies for lignin dimer model compounds

In order to enhance understanding of the mechanism of lignin pyrolysis, homolytic bond dissociation enthalpies (BDEs) in ten lignin dimer model compounds were calculated by using density functional theory methods at B3LYP/6-31G++(d,p) level and B3P86/6-31G++(d,p) level. The BDE values calculated at B3LYP/6-31G(d,p) level are about 20 kJ/mol lower than that at B3P86/6-31G(d,p) level, but the variation trends of BDE values at two levels are the same and the B3P86 functional was found to yield accurate BDEs. The calculation results show that the order of the BDE values for the β-O-4 type of linkage is as follows: Cβ-O < Cα-Cβ < C4-O < C1-Cα, and the substituents (methoxyl, carbonyl, and hydroxyl) on both the aromatic and alkyl groups in model compounds 1 have an important effect on the BDEs. It is found that the BDEs of Caromatic(1,4,5)-C(or O) in lignin model compounds are higher, but the BDEs of Calkyl(α, β)-Calkyl(α, β)(or O) are lower. Therefore, the initial step in pyrolysis of lignin is the homolytic cleavage of the Calkyl(α, β)-Calkyl(α, β)(or O) bond. The possible formation pathways of major products in the pyrolysis process of model compounds have been analysed.

Occurrence of surface polysulfides during the interaction between ferric (hydr)oxides and aqueous sulfide.

Polysulfides are often referred to as key reactants in the sulfur cycle, especially during the interaction of ferric (hydr)oxides and sulfide, forming ferrous-sulphide minerals. Despite their potential relevance, the extent of polysulfide formation and its relevance for product formation pathways remains enigmatic. We applied cryogenic X-ray Photoelectron Spectroscopy and wet chemical analysis to study sulfur oxidation products during the reaction of goethite and lepidocrocite with aqueous sulfide at different initial Fe/S molar ratios under anoxic conditions at neutral pH. The higher reactivity of lepidocrocite leads to faster and higher electron turnover compared to goethite. We were able to demonstrate for the first time the occurrence of surface-associated polysulfides being the main oxidation products in the presence of both minerals, with a predominance of disulfide (S2(2-)(surf)), and elemental sulfur. Concentrations of aqueous polysulfide species were negligible (<1%). With prior sulfide fixation by zinc acetate, the surface-associated polysulfides could be precipitated as zerovalent sulfur (S°), which was extracted by methanol thereafter. Of the generated S°, 20-34% were associated with S2(2-)(surf). Varying the Fe/S ratio revealed that surface polysulfide formation only becomes dominant when the remaining aqueous sulfide concentration is low (<0.03 mmol L(-1)). We hypothesize these novel surface sulfur species, particularly surface disulfide, to act as pyrite precursors. We further propose that these species play an overlooked role in the sulfur cycle.

Characterization of skin sensitizers from autoxidized citronellol – impact of the terpene structure on the autoxidation process

Citronellol is a frequently used fragrance compound in consumer products. It is present in fragrance mix II, which is used for screening of contact allergy to fragrances. Because of its chemical structure, citronellol could be susceptible to autoxidation. To compare the behaviour of citronellol with that of the structurally similar compounds linalool and geraniol, in terms of ability to autoxidize, the products formed, and the sensitization potencies of these. Citronellol was exposed to air, and autoxidation was followed by gas chromatography-mass spectrometry (GC-MS) analysis after derivatization of thermolabile compounds. The sensitizing potencies of the oxidation mixture and its major oxidation compounds were examined with the local lymph node assay. The concentration of citronellol decreased while the sensitization potency increased in air-exposed samples over time, with hydroperoxides being identified as the major oxidation products and main skin sensitizers. The present study shows the impact of the absence of the 2,3-double bond in the citronellol structure on the oxidation pathways for formation of oxidation products. The study also shows the usefulness of our new GC-MS method for quantification of the citronellol oxidation products, especially the hydroperoxides. The investigated citronellol hydroperoxides could be important allergens, owing to the high concentrations detected and frequent exposure to citronellol in the population.

Molecular dynamic simulation study on pyrolytic behaviour of xylan

Xylan is the most relevant component in hemicellulose, and in order to understand the mechanism of thermal decomposition of hemicellulose, the pyrolysis process of xylan as a hemicellulose model compound has been simulated by molecular dynamics method. The simulation results show that the pyrolysis process of xylan can mostly be divided into three stages: low, intermediate and high temperature pyrolysis stage. The hydroxyl bonds begin to break down when temperature rises to about 400 K. The glycosidic bonds on side groups begin to break down at about 550 K, and those on main chain begin to break down at about 600 K. Thus, the whole molecule depolymerises and all kinds of fragments are formed. Based on the related experimental results in references, the possible formation pathways of major products have been analysed.

New Pathways for Formation of Acids and Carbonyl Products in Low-Temperature Oxidation: The Korcek Decomposition of γ-Ketohydroperoxides

We present new reaction pathways relevant to low-temperature oxidation in gaseous and condensed phases. The new pathways originate from γ-ketohydroperoxides (KHP), which are well-known products in low-temperature oxidation and are assumed to react only via homolytic O-O dissociation in existing kinetic models. Our ab initio calculations identify new exothermic reactions of KHP forming a cyclic peroxide isomer, which decomposes via novel concerted reactions into carbonyl and carboxylic acid products. Geometries and frequencies of all stationary points are obtained using the M06-2X/MG3S DFT model chemistry, and energies are refined using RCCSD(T)-F12a/cc-pVTZ-F12 single-point calculations. Thermal rate coefficients are computed using variational transition-state theory (VTST) calculations with multidimensional tunneling contributions based on small-curvature tunneling (SCT). These are combined with multistructural partition functions (Q(MS-T)) to obtain direct dynamics multipath (MP-VTST/SCT) gas-phase rate coefficients. For comparison with liquid-phase measurements, solvent effects are included using continuum dielectric solvation models. The predicted rate coefficients are found to be in excellent agreement with experiment when due consideration is made for acid-catalyzed isomerization. This work provides theoretical confirmation of the 30-year-old hypothesis of Korcek and co-workers that KHPs are precursors to carboxylic acid formation, resolving an open problem in the kinetics of liquid-phase autoxidation. The significance of the new pathways in atmospheric chemistry, low-temperature combustion, and oxidation of biological lipids are discussed.

On the Use of Metabolic Control Analysis in the Optimization of Cyanobacterial Biosolar Cell Factories

Oxygenic photosynthesis will have a key role in a sustainable future. It is therefore significant that this process can be engineered in organisms such as cyanobacteria to construct cell factories that catalyze the (sun)light-driven conversion of CO2 and water into products like ethanol, butanol, or other biofuels or lactic acid, a bioplastic precursor, and oxygen as a byproduct. It is of key importance to optimize such cell factories to maximal efficiency. This holds for their light-harvesting capabilities under, for example, circadian illumination in large-scale photobioreactors. However, this also holds for the "dark" reactions of photosynthesis, that is, the conversion of CO2, NADPH, and ATP into a product. Here, we present an analysis, based on metabolic control theory, to estimate the optimal capacity for product formation with which such cyanobacterial cell factories have to be equipped. Engineered l-lactic acid producing Synechocystis sp. PCC6803 strains are used to identify the relation between production rate and enzymatic capacity. The analysis shows that the engineered cell factories for l-lactic acid are fully limited by the metabolic capacity of the product-forming pathway. We attribute this to the fact that currently available promoter systems in cyanobacteria lack the genetic capacity to a provide sufficient expression in single-gene doses.

Theoretical studies on pyrolysis mechanism of O-acetyl-xylopyranose

In order to understand the pyrolysis mechanism of hemicellulose and to identify the formation pathways of key products during pyrolysis, the pyrolysis processes of O-acetyl-xylopyranose are investigated by using density functional theory methods at B3LYP/6–31 G ++ (d, p) level. In the pyrolysis, O-acetyl-xylopyranose firstly decomposes to form acetic acid and IM1 with an energy barrier of 269.4 kJ/mol, and then IM 1 is converted to acyclic carbonyl isomer IM2 with a low energy barrier of 181.8 kJ/mol. IM2 further decomposes to form all sorts of small molecules through four possible pyrolytic reaction pathways. The equilibrium geometries of the reactants, transition states, intermediate and products were fully optimized, and the standard thermodynamic and kinetic parameters of every reaction pathway were calculated. The calculation results show that reaction pathways (2) and (4) are the major reaction channels in pyrolysis of O-acetyl-xylopyranose and the major products are low molecular products such as acetic acid, acetaldehyde, glycolaldehyde, acetone, CO, CO2 and CH4, which is according with related analysis of experimental results.

PtSnNi/C nanoparticle electrocatalysts for the ethanol oxidation reaction: Ni stability study

This work describes the use of Pt3Sn/C, Pt3Ni/C and Pt3SnNi/C nanoparticle electrocatalysts with a 20% metal loading on carbon prepared using the polymeric precursor method for the ethanol oxidation reaction (EOR). XRD measurements revealed the presence of segregated Pt and NiO phases in the Pt3Ni/C electrocatalysts, whereas for Pt3SnNi/C, there was some evidence that Ni and Sn atoms are incorporated into the Pt structure with the presence of segregated SnO2 and NiO phases. The mean crystallite sizes were 3.6, 5.7 and 7.2nm for Pt3Sn/C, Pt3Ni/C, and Pt3SnNi/C, respectively. The onset oxidation potential obtained for the EOR using Pt3SnNi/C was close to 0.22V. Chronoamperometric measurements revealed that the highest current densities for the EOR were obtained using the Pt3SnNi/C nanoparticle electrocatalysts (16mAmgPt−1). Based on the Ni accelerated stress tests, this element was more stable in the ternary material. In contrast, there was a change in the product formation pathways before (acetaldehyde and acetic acid were the primary products) and after the accelerated stress tests (acetaldehyde was the primary product) for the Pt3SnNi/C catalyst. The experimental results indicate that the Pt3SnNi/C electrocatalysts exhibited better electrocatalytic activity compared to the other electrocatalysts for the EOR. It is suggested that this activity is related to the presence of Ni, which can modify the electronic structure of Pt and combine with Sn to facilitate the removal of adsorbed CO on the surface of the Pt, thereby promoting the EOR.

Reaction of Li/Cl phosphinidenoid complexes with a phosphite substituted ketone: access to complexes with a novel mixed-valence polycyclic P,C-ligand system

Reaction of Li/Cl phosphinidenoid pentacarbonyltungsten(0) complexes 2a,b (R = CH(SiMe3)2, Cp*) with bifunctional phosphite-substituted ketone 3 yielded tungsten complexes 4a,b having a novel mixed-valence polycyclic P,C-cage ligand with a P–P bond. DFT calculations provide insight into an unusual product formation pathway.

Biocatalytic characteristics, product formation and putative pathway ofP-coumaric acid oxidation by a crude peroxidase from onion

Onion peroxidase (POD) has been known to act on some o-diphenol acids, such as caffeic and hydrocaffeic acid, but data on monophenols are rather scarce. This study is aimed to investigate the use of a crude peroxidase preparation from onion solid wastes for oxidising p-coumaric acid, a widespread phenolic acid, various derivatives of which may occur in foods. The highest enzyme activity was observed at a pH value of 5, but considerable activity was also retained for pH up to 6. Favourable temperatures for increased activity varied between 30 and 50 °C, 40 °C being the optimal. Liquid chromatography/mass spectrometry analysis of a POD-treated p-coumaric acid solution showed the formation of two major products, which were identified as a p-coumaric acid dimer and a decarboxylated dehydrotetramer. The putative oxidation pathway proposed shares common features with those reported for other hydroxycinnamates.

Aerobic epoxidation of propene over silver (1 1 1) and (1 0 0) facet catalysts

Catalytic performance of single-crystal Ag(100) and Ag(111) catalysts for propene oxidation has been investigated by means of Raman spectroscopy and mass spectrometry, and the energy profile has been obtained at the DFT-D level. It has been theoretically found and confirmed by Raman spectroscopy that Ag(100) surface is more reactive toward O2 dissociation than Ag(111) and that adsorbed oxo-species are more strongly bonded to the Ag(100) facet. The higher selectivity toward propylene oxide (PO) observed for the Ag(100) catalyst can be rationalized from theoretical calculations indicating that the activation barriers for formation of the allylic intermediate – precursor of combustion products – are rather similar on both silver surfaces, whereas energy barriers on the reaction pathways for formation of PO and carbonylic products (acetone and propanal) are systematically smaller on the Ag(100) surface.