Free radicals in coals and coal conversion. 4. Investigation of the free radicals in selected macerals upon liquefaction

This study of the nature and role of free radicals in coals and coal conversion considers free radicals from selected macerals in the liquefaction process. High-purity macerals were liquefied to different extents at high temperature (425–480 °C) in the presence of naphthalene or tetralin and hydrogen or nitrogen. The initial concentrations of free radicals as well as the residual concentrations have been measured. The dependence of free radical concentrations on solvents, gases, and temperature is reported. Resinites show the least number of free radicals. They also liquefy almost completely under all conditions used. Fusinites have the highest concentration of residual free radicals and they are unaffected by solvent, gas or temperature changes: The vitrinites show intermediate free radical concentrations that depend very strongly on the solvent used, and on the temperature, but they are unaffected by the gas used. A correlation has been established between free radical concentration and degree of conversion.     

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. 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.     

E.p.r. study of organic free radicals in Victorian brown coal

The signals at g≈2 in the e.p.r. spectra of Victorian brown coals are attributed to organic free radicals. Mathematical lineshape analysis and variation of linewidth with frequency suggested that the observed signals at low power levels are mainly homogeneously broadened. Power saturation experiments, however, resolved the signals to show the presence of from one to three resonances due to different free radical types.     

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.     

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.     

Observation of transient free radicals during coal pyrolysis

Electron spin resonance studies of the early stages of carbonization of a high-volatile A bituminous coal have been used to detect and quantify transient free radicals present in the coal pyrolysate. Free radical (spin) populations are converted from susceptibilities measured at elevated temperatures assuming that the population present after ≈1 h of pyrolysis does not change when the coal pyrolysate is cooled to 20 °C. Co-carbonization of 9–10 dihydroanthracene and anthracene with the coal suggest that disproportionation pathways may be preferred over combination pathways as temperatures of pyrolysis and of liquefaction processes are increased.     

Relationship of coal extraction with free radicals and coal macerals

Extraction and de-ashing of coals with N-methyl pyrrolidone (NMP) under mild conditions has been reported recently. In this work, we have studied about a dozen American coals, along with some of their residues and extracts after treatment with NMP using in situ ESR (Electron Spin Resonance) spectroscopy of free radicals. At room temperature, N-₈ (number of free radials/g) decreases whereas the percentage extraction increases with atomic H/C of coals. Also N₈ for extract (residue) is lower (higher) than that for coals, ESR spectra of the extract exhibits only the single broad component and the weighted average of N₈ for the extract and the residue nearly equals that of the parent coal. These results have led to the hypothesis that the treatment with NMP is an extraction process in which exinites and to a lesser degree vitrinites are favored, whereas the inertinites are nearly rejected by NMP.     

Free radicals trapped in polyethylene matrix: 1. Location of the alkyl radicals in irradiated polyethylene

Two types of alkyl radicals were found to be trapped in irradiated crystals grown from polyethylene solution. One of them corresponds to the broad sextet pattern of the e.s.r. spectrum and the other corresponds to the sharp sextet pattern. The free radicals attributed to the broad sextet began to disappear at a lower temperature than the temperature at which the free radicals attributed to the sharp sextet disappeared. When butadiene molecules were brought into contact with the specimen, the decay of the free radicals corresponding to the broad sextet was accelerated. When the specimen was subjected to fuming nitric acid treatment, no broad sextet was observed. The mat of the crystals, was aligned so that the c-axes of its crystallites were perpendicular to its surface. The broad sextet showed no anisotropy when the angle between the direction of applied magnetic field and that of the c-axis of the crystallite was varied. On the other hand, the sharp component of the spectrum showed apparent anisotropy. Consequently, it can be concluded that the broad component comes from the free radicals trapped in the lamellar surface and the sharp component is attributed to the free radicals trapped in the inner part of the crystallite. Hence, the locations of these two types of free radicals have been clarified with much more certainty than before.     

Electron spin resonance studies of γ-irradiated cellulose. II. Free radicals in accessible regions to water in cellulose I and cellulose II

The types of free radicals produced in the water-accessible regions of cellulose I and cellulose II fibers by γ-irradiation in nitrogen atmosphere at room temperature were studied by ESR spectroscopy. The ESR spectra of the irradiated cellulose I and II change by contacting the fibers with water, and after immersion in water the spectral shape depends on the orientation of the fiber axes to the magnetic field. These spectra are probably related to the free radicals generated in the highly ordered regions inaccessible to water in irradiated cellulosic fibers. The ESR spectrum of free radicals generated in decrystallized cellulose after irradiation consists of a singlet and a doublet. When the ESR spectra of free radicals formed in the highly ordered regions of cellulose I and II and the singlet and the doublet are combined in adequate ratio, the constructed spectra are similar to those of the radicals scavenged by water in the irradiated cellulose I and II fibers. From these facts, the spectra due to the free radicals in the water-accessible regions in irradiated cellulose I and II are considered to consist of the singlet and the doublet formed by free radicals in the typical amorphous regions and the spectra of other types of radicals generated in the semicrystalline regions.     

Measurement of anisotropic development in a low-rank coal using e.s.r. spectroscopy and optical texture

Anisotropic development in a low-rank coal (Cerrejón coal) has been induced using natural heavy oil or anthracene oil as additives in a cocarbonization system at 500 °C. The heavy oil showed better modifying activity. To monitor the anisotropic development optical microscopy and e.s.r. spectroscopy have been used. The concentration of stabilized free radicals, measured at room temperature (trapped free radicals) increased with anisotropic development. The concentration of stabilized free radicals allows interaction in the carbonization system to be followed even when the microcrystallinity of the anisotropic phase is too small to be observed under the microscope.     

Regulation of radicals by hydrogen-donor solvent in direct coal liquefaction

Radicals are important intermediates in direct coal liquefaction. Certain radicals can cause the cleavage of chemical bonds. At high temperatures, radical fragments can be produced by the splitting of large organic molecules, which can break strong chemical bonds through the induction pyrolysis of radicals. The reaction between the formation and annihilation of coal radical fragments and the effect of hydrogen-donor solvents on the radical fragments are discussed in lignite hydrogenolysis. Using the hydroxyl and ether bonds as indicators, the effects of different radicals on the cleavage of chemical bond were investigated employing density functional theory calculations and lignite hydrogenolysis experiments. Results showed that the adjustment of the coal radical fragments could be made by the addition of hydrogen-donor solvents. Results showed that the transition from coal radical fragment to H radical leads to the variation of product distribution. The synergistic mechanism of hydrogen supply and hydrogenolysis of hydrogen-donor solvent was proposed.     

Structural aspects of sub-bituminous coal deduced from solvation studies. 1. Anthracene-oil solvents

In order to study coal structure indirectly and the role of hydrogen donors, an investigation of the major parameters involved in the solvation of a Wyoming sub-bituminous coal has been made. This study utilized the catalytic and non-catalytic hydrogenation of anthracene oil and coal-solvent slurries. Indirect evidence concerning major structural units in the coal was obtained and the net contribution of coal to liquefaction products was estimated. The significance of each parameter to the degradation of coal molecules was also estimated. Data support the concept that coal liquefaction follows a solid → asphaltene → resin → oil route. This stepwise dissociation of the solid is directly related to the breaking of CO, CN, and to a lesser degree CC bonds, resulting in the formation of free radicals of relatively low molecular weight. These free radicals are stabilized by hydrogen transfer from hydroaromatic solvent molecules. A lack of significant quantities of high-molecular-weight hydrocarbons derived from coal solvation implies the prevalence of small molecular units in the coal structure.     

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.     

Relation between diffusion controlled decay of radicals and α-relaxation in polyethylene and polyoxymethylene

Diffusion controlled process theory was applied to the analysis of the data concerning decay reactions of free radicals in irradiated polyethylene and polyoxymethylene in detail. Diffusion constants of free radical sites were estimated from the data of the decay reactions and their time constants were calculated by using Stokes-Einstein relation. These time constants were plotted against the inverse temperature in the relaxation maps, which were arranged by Wada. In the case of polyethylene, the time constants of decay reactions of alkyl radicals trapped in the lamellar surface and the inner part of crystallites were in good agreement with the relaxation times of α′- and α-processes, respectively. However, it was found that the time constants of main chain radicals trapped in polyoxymethylene were closely related to α-relaxation processes. The activation energies of the decay reactions of free radicals were in good agreement with those of the corresponding relaxation processes in both cases. From these results, it was concluded that the decay reactions of the free radicals reflect the molecular motions associated with the respective relaxation processes. Concerning the decay reaction of free radicals in the inner crystallite of polyethylene, effects of lamellar thickness and crystal surface were also studied by using solution grown crystals, which were crystallized at various temperatures, and the fuming nitric acid treated materials. It was found that the diffusion constant of free radical sites and its activation energy were strongly dependent on the lamellar thickness and half-life of free radicals was a function of the amount of the fold surface; i.e., the reaction rate was much depressed as the lamellar thickness was increased and the amount of the fold surface was decreased.     

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.     

The role of free radicals and molecular size in mesophase pitch

The melting behavior of polynuclear aromatic hydrocarbons and hydrocarbon free radicals was studied with differential scanning calorimetry and hot-stage microscopy. Mesophase was not observed during melting of large aromatic compounds, including some containing as many as nine condensed rings. Synthetic aromatic free radicals derived from benzanthrene and naphthanthrene did not induce the formation of mesophase when melted with other polynuclear hydrocarbons. The development of mesophase in pitch is attributed to an increase in molecular size as a result of thermal polymerization.