Free radicals in coals and coal conversion. 3. Investigation of the free radicals of selected macerals upon pyrolysis

The nature and concentration of free radicals in natural and pyrolysed samples of several high-purity resinites, vitrinites and fusinites have been determined by electron spin resonance (e.s.r.) spectrometry. Careful measurements of the g-values, linewidths and integrated intensities have been made. The natural concentration of free radicals in the various macerals was measured and found to vary by approximately 5 orders of magnitude. Of the three macerals, the free radical concentration of vitrinites was found to have the greatest dependence upon pyrolysis temperature. The g-values measured offer insight into the chemical nature of the free radicals and how they are altered upon pyrolysis. Interpretation of the data is based upon earlier work on pure macerals and coal pyrolysis. The implications of the nature and behaviour of free radicals in macerals in coal-conversion processes are discussed.     

Free radicals in coal and coal conversions. 6. Effects of liquefaction process variables on the in-situ observation of free radicals

To determine the effects and relative importance of process variables in coal liquefaction, a uniquely designed and fabricated high-pressure/high-temperature electron spin resonance (e.s.r.) apparatus is used to monitor the in-situ formation and behaviour of free radicals, which are generally assumed to be the key factor. It is concluded that the temperature is the most significant single process variable that affects free radical formation; for Powhatan No. 5 coal there is a 9-fold increase in going from 400 to 460 °C. At 460 °C the other process variables tested can affect significantly the free radicals significantly, but at 400 °C these variables have essentially no effect on free radicals formation. The next most significant effect is due to the combination of solvent nature and residence time. Tetralin and the SRC-II heavy distillate quench the free radicals from Powhatan No. 5 to the same extent with one significant difference. In tetralin the maximum concentration is observed shortly after the slurry achieves its highest temperature, whereas in the SRC-II heavy distillate experiments the concentration is still increasing, at 460 °C, even after 1 h of reaction. The heating time, pressures and types of gas used affect the free radical concentration to a much smaller extent. The conversions obtained in the in-situ e.s.r. experiments using SRC-II heavy distillate as the solvent are somewhat lower than those obtained with tetralin as the solvent. The corresponding oil yields with tetralin are considerably higher than with SRC-II heavy distillate.     

Free radicals in coal and coal conversions. 5. Methodology for the in-situ investigation of free radicals in coal depolymerization under SRC-II preheater/reactor conditions

In a continuing series of publications, a systematic investigation of the free radicals during coal depolymerization under SRC-II preheater/reactor conditions has been made. A uniquely designed electron spin resonance (e.s.r.) cavity allows the monitoring of free radicals and how they are affected by residence time, temperature and pressure changes. This paper summarizes the methodology that has been developed to study systematically free radical formation and behaviour in-situ during coal liquefaction. The methodology involved is to be used extensively in subsequent papers that examine in detail the manifold parameters that affect coal depolymerization. Also, results are given on the dependence of free radicals on temperature, pressure and gases in the presence of tetralin. The free radical concentration can be measured 3–4 min after the initiation of the heating process to an accuracy of ± 20%, while the process variables may cause several-fold changes in the free radical concentration.     

Free radicals in coals and coal conversion. 2. Effect of liquefaction processing conditions on the formation and quenching of coal free radicals

Free radicals are generally accepted as playing an important role in the liquefaction of coal. In a continuing series of studies, we are attempting to determine the relation between the nature and properties of free radicals and process variables such as residence time in a reactor, heating rates to achieve highest temperature and the role of various solvents and gases in stabilizing free radicals in lower-molecular-weight moieties. The free radical concentration after thermal treatments correlates well with the propensity of a given solvent to donate its hydrogen.     

Free radicals in coal and coal conversions. 9. Statistical correlative models of the effect of process variables on hydroliquefaction products

Previous papers in this series have reported the degree of coal conversion in autoclave and e.s.r. cavity liquefaction experiments and free radical measurements from the e.s.r. liquefaction experiments. Here a correlative model for the liquefaction products is discussed in terms of temperature,solvent reactor type, pressure and other process variables, similar in form to that used for the free-radical concentration measurements in an earlier paper in this series. Solvent-time interactions were found to be the most significant term in modelling the yields. Based on the strong similarity in the functional form of the radical and product yield models, a model of conversion and the various yields as a fraction of radical concentration and other process variables was constructed. Free-radical concentration and its interaction with solvent were found to be very significant.     

Free radicals in coal and coal conversions. 10. Kinetics and reaction pathways in hydroliquefaction

The development is reported of a point rate model for the solubility fractions obtained from Powhatan No.5 coal liquefaction in terms of free-radical concentration. Stepwise regression procedures were used to determine rate constants for general hypothetical rate models. Chemical principles were used at each stage to eliminate terms and to fix others. Rate constant expressions were obtained by fitting In k to the inverse absolute temperature for each solvent. Generalized reaction paths were determined from these individual reactions by combining various reactions. These reactions described the conversion of material between solubility fractions via free-radical interactions. In all three solvents progressive liquefaction reactions that did not involve measured radicals (conventional reaction terms) predominated in the rapid break-up of coal, while free-radical reactions are predominant in retrogressive changes. At higher radical concentrations retrogressive reactions predominated. More reactions of all types became significant with increasing temperature.     

Generation of free radicals in partial oxidation of coal

Pyrolytic and oxidative treatments of two coals, Illinois No. 6 and Montour, have been performed between 140 and 400 °C. Free radical concentrations were followed by electron paramagnetic resonance as a function of time of treatment at a given temperature. Initial increases in the organic free radical concentrations were observed at all temperatures, but at the higher temperatures termination reactions caused the increase to be transient. Variations in the free radical concentrations of weathered and unweathered coal showed that the former produced a larger number of radicals. Possible effects due to native mineral matter are suggested.     

E.p.r. study of radicals produced mechanically in PGMA and their interaction with oxygen

The vibrational grinding of poly(ethyleneglycol methacrylate) (PGMA) in vacuo at the liquid nitrogen temperature gives rise to polymer radicals in high concentrations. Changes in the radical concentration as a function of temperature in the presence and absence of oxygen were followed by means of electron paramagnetic resonance. It was found that polymer radicals reacted at very low temperatures with oxygen with simultaneous formation of polymer peroxy radicals and of a non-paramagnetic polymer tetroxide. This polymer tetroxide, which has been proved indirectly, can decompose to yield polymer peroxy radicals and non-paramagnetic products; the observed anomalies on the curve of the thermal decomposition of radicals may be thus elucidated. The relative participation of polymer tetroxide depends on the oxygen concentration, on the temperature of the sample in contact with oxygen and on the concentration of polymer radicals arising by grinding predominantly on the surface of polymer particles.     

Changes in free radicals in coal-derived pyrites upon heating in N2, H2, and vacuum: Role of pyrite—pyrrhotite conversion

Through electron spin resonance (e.s.r.) studies, three different kinds of free radicals have been identified in pyrite samples containing coal impurities. Changes in the e.s.r. parameters (g-value, linewidths, and concentration n) of free radicals have been monitored as the coal-derived pyrites are heated in different atmospheres (vacuum, N₂ and H₂) from room temperature to 500°C in 50°C steps for a residence time of 30 min at each temperature. Changes in n (concentration/g) and other parameters begin to occur at 300°C in vacuum, at 200°C in N₂, and at 100°C in H₂ amtospheres. At these temperatures conversion of pyrite to pyrrhotite is also observed to occur. It is argued that the enhancement of n in these samples is related to the conversion mechanism of pyrite to pyrrhotites. The maximum relative increase in n follows the sequence n (vacuum)>n(N₂)>n(H₂). At high temperature n decreases due to recombination of free radicals formed so that at 500°C, all three types of radicals have small but equal concentrations.     

基于Py-GC/MS研究热解反应中自由基的捕获反应

在500℃下对类煤模型化合物1-萘甲醇与同位素示踪剂的共热解进行了机理研究。利用Py-GC/MS进行快速产物检测,同时联合自由基捕获剂来推断自由基的反应过程。结果表明,在500℃和0.2 min的条件下,1-萘甲醇的热解产物相对单一,但是自由基反应的引发、交换和湮灭极其快速,现有检测方法是对某一时间节点的平衡反应结果的分析。通过D₂O和H₂¹⁸O同位素示踪,成功捕获到了自由基交换后的D取代产物和¹⁸O取代产物。揭示了1-萘甲醇经过1-萘甲基自由基转化为1-萘甲醛的转化过程,发现萘环上的取代基在1-萘甲醇的热解过程中起着重要作用。同时对自由基及产物进行了定性和半定量分析,发现在加入自由基捕获剂后产物的丰度较之前降低了一个数量级,说明了自由基捕获剂抑制了产物的生成。     

The effect of solvent on postcuring in free radical photopolymerization

During free radical photopolymerization, with an increasing degree of conversion, the viscosity increased and vitrification occurred. The unreacted double bonds had difficulty diffusing to the reactive site in the vitrifying polymer. However, the free radicals generated in the vitrifying matrix could survive for a long time. Thus, the postcuring process occurred. In this study, free radical photopolymerization kinetics of methacrylate with or without solvent was investigated by real‐time FTIR, which was combined with a heating device. Results showed that without solvent, postcuring occurred only by heating at a higher temperature than the glass transition temperature of the polymer. However, with solvent, the postcuring occurred even at room temperature, where only a small amount of solvent was enough to cause postcuring. The lifespan of the free radicals inside the polymer was over 10 h. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44223.     

Use of in-situ electron paramagnetic resonance to assess formation of free radicals and their role in the hydroliquefaction of coal

The depolymerization of coal is generally presumed to proceed via a free radical mechanism. A unique electron spin resonance cavity that allows the observation of free radical formation and behaviour under hydroliquefaction conditions (500 °C and 12.4 MPa) was designed to facilitate the study of the effect of process variables on the formation of free radicals. Parallel experiments were carried out in autoclaves, and the degree of conversion and quality of coal-derived liquids produced was assessed. Correlative statistical models were established that allow the quantitative determination of the effect of process variables, and their interactions, on the production and behaviour of free radicals and of coal-derived liquids produced. Strong evidence was obtained that free radical chemistry plays a key role in determining the degree of coal conversion and quality of products. The kinetics and mechanism of hydroliquefaction are addressed with the explicit inclusion of experimental free radical concentration in the kinetic expressions. Progressive and regressive reactions are discussed. Optimum combinations (‘processing windows’) of process variables can be identified.     

Approach for promoting liquid yield in direct liquefaction of Shenhua coal

Single and multi-stage liquefaction of Shenhua (SH) bituminous coal and re-liquefaction of its liquefaction residue (SHLR) were carried out in an autoclave reactor to investigate the essential approach for promoting oil yield and conversion in SH coal direct liquefaction (SHDL). The multi-stage liquefaction includes pretreatment, keeping the reactor at 250 °C for 40 min before heating up to the reaction temperature, and two-stage liquefaction processes consisting of low temperature stage, 400 °C, and high temperature stage, 460 °C. The results show that the pretreatment has slight effect on oil yield and conversion of SHDL, especially for liquefaction at 460 °C. There is a positive function of two-stage liquefaction in shortening reaction time at high temperature. Increasing ratio of solvent to SHLR can promote the oil yield and abate reaction condition in SHLR re-liquefaction, that is, it can promote the conversion from preasphaltene and asphaltene to oil. The primary factor to inhibit coal liquefaction is the consumption of hydrogen free radical (H·) from solvent or H₂ and condensation of free radicals from coal pyrolysis after a period of reaction. So the essential approach for increasing oil yield and conversion of SHDL is to provide enough H· to stabilize the free radicals from coal pyrolysis.     

Plasma polymerization of tetrafluoroethylene in a field-free zone

Plasma polymerization of tetrafluoroethylene by itself and mixed with inert gases has been studied in the field free zone inside a Faraday cage. The chemical structure was analyzed by ESCA, revealing both linear and branched products. Linear products are formed by less energetic plasmas and at low monomer residence times. Lower energy plasmas result from the use of lower powers and lower percentages of helium in the feed. Production of linear products under these conditions is probably due to lower rates of free radical and metastable formation in the plasma. Through a combination of kinetic and mass transfer effects, shorter monomer residence times under high flow rates and within short distances from the front edge of the electrode give rise to a lower concentration of free radicals at the electrode surface, producing a more linear polymer. Linear products were also formed at very high powers. This latter result is quite unexpected and is probably due to rapid gas phase polymerization. The chemical structure was not affected significantly by the substrate temperature or by hydrodynamics in this work. All of the evidence indicates that the gases were well mixed in the reaction zone for the range of process variables and for the gases studied. The deposition rate was also studied as a function of the reaction conditions. It was affected by the concentration of free radicals, the concentration of the monomer, and the substrate temperature. The observed deposition rate profile across the electrode is consistent with mass transfer and kinetic considerations governing free radical and monomer concentrations on the electrode surface. The deposition rate is greater at the lower substrate temperature used, probably due to enhanced adsorption process.     

Effect of dilution on branching and gelation in living copolymerization of monomer and divinyl cross-linker: Modeling using dynamic lattice liquid model (DLL) and Flory-Stockmayer (FS) model

The effect of dilution on random living copolymerization of vinyl- and divinyl monomers with fast initiation and slow propagation was simulated and compared with the experiments. Two Monte Carlo simulation methods have been used: one, purely statistical, based on the Flory-Stockmayer (FS) theory and the second using the dynamic lattice liquid model (DLL). The results were compared with experiments using atom transfer radical polymerization (ATRP), one of controlled/living radical polymerization methods. Molecular weights, polydispersities and the crosslinking/cyclization of macromolecules were analyzed as a function of conversion and time for various solvent concentrations and initiator crosslinker/monomer ratios. The results obtained by the DLL method are in good agreement with the experiment, showing an increase of conversion and time at gel point with dilution, i.e., with the increase of solvent concentration. The FS theory was found to be unable to reproduce even qualitatively the dilution effect on gelation. It is shown that intrachain crosslinking becomes more and more important with increasing solvent concentration, preventing macroscopic gelation at high dilutions, even if the number of crosslinks per primary chain is much greater than one (Flory criterion).DLL simulations give an insight into the mechanism of crosslinking processes near the gel point (leading to gelation) and/or microgel formation when solvent concentration is too high. At very high solvent concentration polymerization is slow and side effects like deactivation of radicals become important. Taking such effects into account, the simulation gives better agreement with the experimental data.     

Importance of surface free radicals in the reactivity of granular activated carbon under water treatment conditions

Free aqueous HOC1 reacts with phenols adsorbed on granular activated carbon (GAC) to produce a series of oxidation products, not formed in absence of carbon. When GAC is treated with a free radical quencher, BHT, however, the formation of these products is inhibited. Experimental data and measurements with ESR (electron spin resonance) show that surface free radicals are produced from the oxidation of GAC with aqueous HOC1. The radicals enter radical chain reactions and produce the observed products.Some of the free radicals on the GAC surface are associated with oxygen-containing functional groups. Outgassing of GAC at high temperatures results in reduction of surface oxygen and free radical concentration, and ability of GAC to promote the surface reactions.The ash content of the GACs studied does not correlate well with their ability to promote surface reactions. A surface reaction mechanism is postulated to explain the observed products.     

In-situ monitoring of the stabilization with hydrogen of free radicals thermally induced from coal using high pressure e.s.r., d.t.a. and p.d.a. equipment

High temperature, high pressure e.s.r. measurements of the hydrogenation reaction of Taiheiyo coal in the presence of catalysts were carried out to understand the stabilization of thermally and/or catalytically induced free radicals. A decrease in free radical concentration with increasing temperature was observed for ZnCl₂ and SnCl₂ · 2H₂O catalysts at 10MPa under hydrogen gas. High pressure modified single-cell d.t.a. and p.d.a. equipment augmented the uniquely designed high temperature, high pressure e.s.r. cell. The hydrogenation reaction was monitored under the same experimental conditions as for e.s.r. From the results of the combination of high temperature, high pressure e.s.r. with high pressure d.t.a. and p.d.a., it was established that H₂ molecules can react efficiently with free radicals from coal molecules created by the presence of ZnCl₂ and SnCl₂ · 2H₂O catalysts.     

Decay of free radicals in irradiated PMMA at high pressures studied by ESR

The decay of free radicals in irradiated PMMA subjected to high pressures has been studied by the method of ESR. The rate constants and activation energies has been determined for the decay of free radicals at the pressures of 5000 and 10,000 atm over the temperature range of 100–170°C. It has been found that the rate of decay was reduced essentially by the effect of high pressure. The mechanism of the decay of free radicals in solid polymers is being discussed. The important influence of the motion of macromolecule segments on the course of radical reactions in solid phase is printed out.     

Conversion of alkyl radicals to allyl radicals in irradiated single crystal mats of polyethylene

The decay of alkyl radicals, the conversion of alkyl radicals to allyl radicals and the trapping of allyl radicals in irradiated single crystal mats of polyethylene have been studied by electron spin resonance (e.s.r.). It has been suggested that in the crystal core alkyl radicals react with trans-vinylene double bonds and are converted into trans-vinylene allyl radicals; at the crystal surface, alkyl radicals react with vinyl end groups and are converted into allyl radicals with vinyl end groups. The decay of radical pairs and the formation of trans-vinylene double bonds are discussed.