Study of carbonaceous mesophase through the e.s.r. spectra of vanadyl chelates

The behaviour of vanadyl chelate complexes in Iranian heavy oil and ethylene tar pitch during the carbonization process has been investigated by the e.s.r. technique to obtain information on the mesophase transformation. Activation energies for rotation of chelates incorporated with aromatic lamellae, and ordering parameters, are estimated from correlation times and intensities of e.s.r. spectra. The molecular plane of the chelates incorporated into the mesophase was found to be oriented parallel to the applied field. The value of activation energy is higher for a mesophase of small size than for a larger one.     

34 GHz e.p.r. study of vanadyl complexes in various asphaltenes: Statistical correlative model of the coordinating ligands

High precision 34 GHz e.p.r. measurements were performed to study the coordination of vanadyl complexes in various asphaltenes. The spin-Hamiltonian parameter analysis indicates that the asphaltenes can be classified into those which have g- and A-parameters nearly identical to those for the vanadyl tetraphenyl porphyrins doped in carbonaceous materials and those for which these parameters are nearly identical to those for the vanadyl etioporphyrins. Quantitative statistical analysis of the parameters for model compounds with coordination N₄, N₂S₂, NS₃, S₄, S₂O₂, N₂O₂ and O₄ shows that these parameters cannot be used to predict the coordination for various asphaltenes. It is found that g₀ is a better parameter than A₀ to predict the coordination, although it cannot resolve the N₄, S₄ and S₂O₂ coordinations. However, the spin-Hamiltonian parameter resolution was sufficient to distinguish between those vanadyl complexes present in porphyrins, humates and minerals. The importance of high precision e.p.r. measurement at 34 GH_z or higher frequencies for model compounds and chromatographically-separated fractions is discussed.     

E.s.r. study of radical polymerization of pentaerythrityl monoacetal dimethacrylate in polystyrene matrix at high pressure

The e.s.r. spectrum of free radicals formed during polymerization of a bifunctional monomer of pentaerythrityl monoacetal dimethacrylate by thermal decomposition of benzoyl peroxide in a polystyrene matrix at 700 MPa is studied. Formation of propagating radicals of cyclopolymerization and crosslinking polymerization is discussed.     

A study of the interaction of atomic oxygen with various carbonaceous materials

A variety of techniques have been used to examine the manner by which atomic oxygen species interact with various carbonaceous solids. In addition, the effect of such a treatment on the physical and surface chemical properties of the carbons has been determined. In situ electron microscopy studies have enabled us to establish that graphite is probably the most reactive form of carbon for this reaction. This behavior is rationalized according to the notion that atomic oxygen reacts preferentially with the delocalized π electrons in the graphite basal plane to create micro-pits. It is shown that careful control of the reaction of atomic oxygen with a carbonaceous surface can lead to certain beneficial effects when such structures are used in a reinforcement application.     

E.s.r. study of formation of coke texture

The optical coke texture is a major factor in the high-temperature properties of metallurgical coke but the mechanism of formation of coke texture is not established yet. By studying the carbonization process during the formation of optically anisotropic coke with an e.s.r. spectrometer fitted with a high temperature cavity a relation between the concentration of the free-spins during carbonization and coke texture has been inferred for some organic compounds used as the model materials and for coals covering a range of coal rank.     

E.s.r. study of pyrolysis vapour of coal

Wyodak and Heatburg subbituminous coals were pyrolysed in-situ in an e.s.r. microwave cavity at ≈510 °C in an evacuated sealed tube. At least two kinds of radicals are detected; first, phenalenyl-like radicals and then, polymerized radicals from the initial ones.     

E.s.r. spectrometric study of directly prepared potassium-coal adducts

E.s.r. spectral parameters (concentrations of paramagnetic centres, g values and linewidths) of two vitrains of different rank (C, 89.2 and 75.8 wt%) and of their potassium adducts prepared by a direct method, as well as of the reductively alkylated high-rank vitrain are reported. The potassium adduct of the high-rank vitrain is characterized by the accumulation of a rather high concentration of unpaired electrons, 7.7 spins per 100 C atoms (a 120-fold enhancement). With the adduct of the low-rank vitrain the concentration is 0.14 spins per 100 C atoms (a 17-fold enhancement). The adduct of the low-rank vitrain is negligibly alkylated by alkyl halides in THF, unlike the adduct of the high-rank vitrain which can be significantly alkylated. The g values and linewidths of the spectra of the vitrains and the corresponding potassium adducts are very similar. The experimental results are interpreted mainly with respect to the formation of ionic potassium-coal adducts of coals of different rank and their alkylation.     

E.s.r. measurements on asphaltene and resin fractions from various separation methods

Asphaltenes and resins obtained from Athabasca and Cold Lake bitumens (Alberta, Canada) have been fractionated by adsorption, ion-exchangers, and gel permeation chromatography. These fractions have been investigated by electron spin resonance (e.s.r.) of their organic paramagnetic components and vanadium (IV) content. The correlation between number of spins, vanadium content and fractions sequence is discussed.     

E.s.r. study of γ-irradiated isotactic and atactic polypropylene

An e.s.r. study of γ-irradiated isotactic (i-) and atactic (a-) polypropylene (PP) was carried out. Both the 17 line (or 9 line) spectrum for i-PP and the 6 line spectrum for a-PP are attributed to the tertiary carbon radical, CH₂C(CH₃)CH₂. The difference in the hyperfine structure between the spectrum of i-PP and that of a-PP can be explained by different radical conformations owing to the stereoregularity. Hyperfine coupling and the radical conformation for the tertiary carbon radical in i- and a-PP were determined from the cos² ? rule. It is shown that the δ methylene proton as well as the β methylene proton contribute to the slight anisotropy in the stretched sample. The spectral change from 9 lines to 17 lines with elevation of temperature, observed at low temperature in i-PP, could be explained by the hindered oscillation of the β methylene proton.     

E.s.r. spectroscopic study on the chemistry of coal oxidation

A better understanding of the mechanisms of pre-oxidation could lead to improved methods for prevention of coal weathering, which has a deleterious influence on liquid yield during pyrolysis. Previous Morgantown Energy Technology Center studies demonstrated that the extractable fractions of coal are more susceptible to oxidative weathering than the residue. To better understand the chemistry of coal weathering and the influence of pre-oxidation on subsequent devolatilization, in situ electron spin resonance (e.s.r.) spectroscopic studies were performed on coal, weathered coal and on the corresponding untreated and preoxidized fractions (extract and residue). In this technique, pyrolysis is performed in an e.s.r. cavity and the concentration of free radicals is followed as the sample is pyrolysed. Elemental analysis of the pre-oxidized and untreated samples indicated that H/C ratio of the extract and the coal sample was reduced slightly during 48 h of oxidation at 150 °C in air whereas the H/C ratio of the residue was essentially unchanged. The influence of pre-oxidation (48 h at 150 °C) was to increase the e.s.r. spin concentration of the weathered coal slightly compared to the raw coal when devolatilized at 460 °C. The e.s.r. experiments performed at 460 °C also showed that pre-oxidation significantly enhanced free radical concentration in the extract but had a lesser effect on the residue. The enhanced free radical formation of the pre-oxidized extract relative to the untreated extract implies that mild pre-oxidation affects hydrogen-rich components (extract) more than the primarily hydrogen-poor components (residue) of coal.     

A DRIFTS study of the heterogeneous reaction of NO2 with carbonaceous materials at elevated temperature

Commercial carbon black, spark generator soot, Diesel soot from passenger car and high-purity graphite were used for the investigation of the reaction of carbonaceous materials with NO₂ applying diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The occurrence of infrared bands was analyzed as they were depending on the reaction temperature. The focus of interest was particularly on the conversion at an elevated temperature of 400 °C. The formation of oxidation products and the adsorption of NO₂ on the surface of the samples were observed. Infrared bands could be attributed to C(O)OR, RNO₂, and RONO as the main functionalities. The comparison of the results from the different samples revealed that different infrared signals appear when NO₂ is adsorbed either on aliphatic or graphitic domains of soot. However, the formation of characteristic bands for an acidic functional group did not occur. This supports the assumption, made in a prior temperature programmed desorption mass spectroscopy (TPD-MS) study, that this group is a transition state.     

Formation of carbonaceous mesophase at lower temperature

The effects of heat-treatment at lower temperature on the formation of carbonaceous mesophase were studied. Heat-treatments of QS pitch below 340°C and of KF asphalt below 370°C lead to the formation of mesophase spherules having a different structure from those found previously. For QS pitch, the lowest temperature of mesophase formation was 290°C but more than one month was required for heat-treatment. The structure of the fourth-type spherule is proposed from the microscopic observations and its orientational behavior in a magnetic field. The structure of this type is very similar to that of the Brooks-Taylor-type one, and it is believed that the fourth-type spherule is a meta-stable phase of the Brooks-Taylor-type one.     

E.s.r. study of free radicals in poly(vinyl carbazole)

Poly(N-vinylcarbazole) was γ-irradiated at 77 K. The analysis of e.s.r. spectra showed that cation radicals centred at N were produced in the system. Hyperfine splitting constants are: a^N=0.57 mT and a^H_β=0.68 mT. The dependence of the rate constant of free radical decay on pressure (1–800 MPa) was studied at various temperatures (90–170°C). Activation volumes were determined for two characteristic pressure and temperature ranges. The results of e.s.r. measurements are compared with those obtained from dynamic-mechanical measurements.     

An e.s.r. study of some coals under rapid passage condition at high temperatures and pressure

The e.s.r. spectra of Miike and Taiheiyo coals were measured under rapid passage conditions in the temperature range 150–450 °C. A parameter, R, was defined by a ratio of out-of-phase and in-phase spectral amplitudes and the ratios for Miike and Taiheiyo coals were estimated in this temperature range. It was found that the temperature dependence for R was different for the two coals examined, and the reason for the difference was discussed. Further, the ratios for those coals mixed with pyrene were estimated in the temperature range 150–400 °C at atmospheric and 1.1 MPa nitrogen pressure. Based on the temperature dependence of the ratio the coal liquefaction ability of pyrene for those coals was discussed.     

E.s.r. studies of peroxy radicals in polyethylene: 2. Peroxy radicals in urea — polyethylene complexes

Temperature dependent e.s.r. spectra of peroxy radicals in urea—polyethylene complex materials (UPEC) were discussed. Peroxy radicals in UPEC were much more stable than in normal polyethylene. Change of anisotropic g-values of the radicals with temperature were investigated and it was found that the g₁-axis, which was assigned to be parallel to the chain axis, seemed to be almost unchanged throughout the temperature range in the present study, but the g₂- and g₃-axes changed drastically. Using these results, it was concluded that the main motion of the radical sites was rotation around the chain axis and this motion was found to be much faster than in normal polyethylene, which we have already studied and which we reported recently. This is a reflection of the properties of the materials studied, i.e. interaction between molecular chains in UPEC must be much weaker than in normal polyethylene because of the presence of the wall of urea molecules in the former.     

High temperature and high pressure H n.m.r. and e.s.r. studies on coal liquefaction reactions

Changes in chemical reactions during coal liquefaction of Yallourn brown coal and Akabira bituminous coal were monitored by using high temperature and high pressure ¹H n.m.r. A simple high temperature and high pressure e.s.r. cell was constructed and the variation in concentration of free radicals during coal liquefaction reactions was monitored under the same experimental conditions as for n.m.r. With the liquefaction and coking reaction, typical spectra of n.m.r. and e.s.r. were obtained and could distinguish the reaction steps: swelling; liquefaction; coking. The combination of high temperature and high pressure n.m.r. and e.s.r. is promising to the chemistry of coal liquefaction reactions.     

E.s.r. spectra of eastern oil shales

E.s.r. spectra of several oil shales from eastern and western USA have been measured at 9 and 25 GHz. The spectra of the western shales were similar to previously reported spectra and consist of a Mn²⁺ signal and a featureless organic signal. The eastern spectra consist of a V⁴⁺ signal and a complex and unusual organic signal consisting of at least four component signals. The organic signal has been characterized in terms of g factor, line-width and saturation characteristics. The effect of oxygen on the signal has also been examined. Electron nuclear double resonance (ENDOR) signals have been obtained from the organic signals of the eastern oil shales.     

Formation of anisotropic mesophase from various carbonaceous materials in early stages of carbonization

The low-temperature carbonization of pitches, reduced crudes, synthetic polymers and various ranks of coals has been studied by polarized-light microscopy, X-ray diffraction and solvent extraction. Formation of anisotropic mesophase is a function of heat-treatment temperature and residence time for those materials that pass through a fusion stage in the temperature range from 350 to 500 °C. The nucleation of spherules from the pitches and reduced crude during low-temperature carbonization is proportional to the amount of β-resin, which is benzene-insoluble and quinoline-soluble, and the rate of its formation. Addition of sulphur to the pitch turns the texture of mesophase spherules to that of a fine mosaic when the mixtures are heat-treated in the temperature range of 350–500 °C. Methylation of the hydroxyl groups in 3,5-dimethyl phenol formaldehyde resin, by reacting with dimethyl sulphate, gives an anisotropic texture which consists actually of tiny spherules together with parent matrix; the original resin gives the typical mosaic texture. The molecular forces in the decomposing carbonaceous materials during low-temperature carbonization are among important factors governing the mesophase transformation. A common mechanism of formation of various anisotropic textures can explain the behaviour of coal-cokes, pitch, crudes and polymers.