Stabilisation of low softening point petroleum pitch fibres by HNO3

This paper deals with the stabilisation of low softening point pitch fibres obtained from petroleum pitches using HNO₃ as oxidising agent. This method presents some advantages compared with conventional methods: pitches with low softening point (SP) can be used to prepare carbon fibres (CF), the stabilisation time has been reduced, the CF yields are similar to those obtained after general methods of stabilisation, and the initial treatments to increase SP when low SP pitches are used to prepare CF, are avoided. The parent pitches were characterised by different techniques such as diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), elemental analysis and solvent extraction with toluene and quinoline. The interaction between HNO₃ and the pitch fibres, as well as the changes occurring during the heat treatment, have been followed by DRIFTS.     

Structural characterization of N-containing activated carbon fibers prepared from a low softening point petroleum pitch and a melamine resin

The incorporation of heteroatoms like N in activated carbons is of interest to modify the surface chemistry of the materials and, then, to improve their behavior as catalyst or catalyst support. In this work, N-containing activated carbon fibers have been prepared using a petroleum pitch with a low softening point and an N-containing resin. The novelty of the preparation method is that it involves the steps used in the synthesis of activated carbon fibers, i.e. spinning, stabilization, carbonization and activation. The materials have been characterized with techniques such as XPS and UPS, which allows us to follow the changes in both the chemical state of N species and the valence band structure of the carbon samples during the preparation steps.     

A thermoanalytical study of the co-pyrolysis of coal-tar pitch and petroleum pitch

Four pitch blends were prepared at laboratory scale by mixing a coal-tar pitch and a petroleum pitch in several proportions (CTP:PP 85:15, 70:30, 55:45 and 40:60). Single pitches and blends were characterized by standard procedures, infrared spectroscopy and size exclusion chromatography. Pyrolysis behaviour and interactions between the two pitches in the blends were studied by thermogravimetric analysis (TG/DTG) and differential scanning calorimetry (DSC). The results show that blending does not alter the composition of pitches. However, the TG/DTG curves reveal that coal-tar pitch and petroleum pitch interact actively during pyrolysis, modifying the temperature of initial weight loss and the temperature of the maximum rate of weight loss. Primary quinoline-insoluble particles present in coal-tar pitch and transferable hydrogen seem to be the main factors responsible for these modifications. The DSC curves show that the presence of coal-tar pitch in the blends reduces the reactivity of the petroleum pitch and shifts the exothermic peaks observed at the temperature of the cracking/polymerization reactions (>400 °C) to lower temperatures.     

The correspondence between loss of H-NMR signal intensity and mesophase growth during the thermal maturation of a petroleum pitch

Samples of petroleum pitch, Ashland 240, were heated isothermally at 380, 400, 430 and 450°C and the hydrocarbon evolution and mesophase growth monitored simultaneously by the use of high temperature ¹H-nuclear magnetic resonance (NMR) analysis. For all four temperatures the NMR signal indicated no mesophase formation until the signal intensity had dropped by 30%, which corresponded to 20% weight loss from the pitch. The correlation between the onset and increase in mesophase formation with signal intensity loss was found to be independent of temperature. This temperature independence for the petroleum pitch agrees with the work presented previously by Honda et al. (Carbon 1970, 8, 181). who compared weight loss with the growth of quinoline-insoluble material for a coal-derived pitch. Given the correlation between signal loss and mesophase formation, it is suggested that a relative measure of NMR signal intensity could be used to estimate mesophase content of petroleum pitch, during maturation.     

Influence of ferrocene addition on the morphology and structure of carbon from petroleum residue

Nano-sized iron particle/carbon composites have been synthesized from a petroleum residue by heating at 420 °C with ferrocene under pressure. The morphologies and structural features of the composites were investigated using TEM, HREM and XRD measurements. The effect of ferrocene addition on the development of turbostratic carbon from the petroleum residue was discussed. It was found that, by increasing the amount of ferrocene added from 3% to 20%, the size of the nano-iron particles tended to increase from 20–80 nm to 30–180 nm. The iron particles pyrolyzed from ferrocene mainly exist in the forms Fe–O and Fe_1−xS when the ferrocene content was low (3% and 6%), and α-Fe when ferrocene contents were high (10% and 20%). Upon further heat treatment in the range 500 to 900 °C, the iron particles tended to aggregate and Fe–O and Fe_1−xS were transformed into α-Fe and austenite. In comparison with the carbon formed without ferrocene addition, the resulting carbon exhibited a turbostratic structure as shown by HREM, in which the d₀₀₂ spacing decreased with the increase of ferrocene loading and increasing temperature, suggesting the carbonization was promoted by the catalysis of the nano-iron particles.     

Analysis of the carbonization and formation of coal tar pitch mesophase under dynamic conditions

Thermokinetic analysis of three pitch samples was carried out: coal tar pitch obtained from light coke oven tar (P), mesophase pitch after 10.5 h (MP1), and mesophase pitch after 12 h (MP2) thermopreparation at 410 °C. The process was realized in a continuous system with a 10 kg mass being charged to the reactor. It was demonstrated using Kissinger’s law that the temperature criterion, the first-order thermokinetics and the calculated Arrhenius law parameters fulfill the isokinetic effect when the classical routes of thermokinetic analysis of the samples prepared under dynamic conditions (at three heating rates) are used, which makes the qualitative interpretation of differences between these samples difficult. An alternative solution was proposed using the relative rate of thermal decomposition. The temperature ranges of the chemical reactions leading to the formation of mesophase structures, as well as the temperature ranges of the coking processes of the Fixed Carbon phase, were determined.     

Chemistry of the co-pyrolysis of an aromatic petroleum residue with a pyridine-borane complex

Synthesis of boron-doped mesophase by controlled co-pyrolysis of an aromatic petroleum residue and a pyridine-borane complex, PB, as a boron source, was carried out. Pyrolysis was performed at temperatures ranging from 400 to 440 °C, under 1 MPa nitrogen atmosphere. Soaking time was varied between 1.5 and 6 h, and the boron concentration in the pyrolysis mixtures ranged from 0 to 1 wt.%. The effect of the presence of boron upon solid yield, insoluble content, mesophase development and microstructure was studied. PB reacts with molecules of the petroleum residue and creates B-C bonds, B substituting C in the polyaromatic molecules, with sp² hybridisation, and probably also creating B crosslinking between polyaromatic units. The enhanced reactivity caused by B addition is reflected in an increment of viscosity; as a result, the development of mesophase is strongly increased, but the size of the mesophase structures is decreased to a mosaic texture; B also causes an increase of the interlayer spacing and a noticeable enhancement of insoluble material.     

Chemical, microstructural and thermal analyses of a naphthalene-derived mesophase pitch

A detailed characterisation of a synthetic naphthalene-derived mesophase pitch, in its as-received state and during pyrolysis, has been performed. The study has been conducted by means of various techniques and with a particular attention to Raman microspectroscopy. The Raman spectra show features in common with the naphthalene precursor, i.e., a broad and complex band at 1150-1500 cm⁻¹ and a multicomponent G band at 1600 cm⁻¹. These features correspond to the vibration modes of the molecules of the pitch and more especially to the non-aromatic C-C bonds involved in alkyl groups, aryl-aryl bonds or naphthenic rings. The pyrolysis of the pitch into coke takes place within a narrow temperature range (480-500 °C) through the elimination of hydrogen and light alkanes resulting from the breaking of homolytic C-H bonds and naphthenic cycles, respectively. This process initiates a swelling of the pitch. The analysis of the Raman features shows that the structure of the pitch is only slightly affected within this temperature range. Conversely, significant structural changes of the material (as shown by the vanishing of the multicomponent bands at 1600 and 1150-1500 cm⁻¹) are evidenced beyond 750 °C, simultaneously with a hydrogen release and an increase of the true density. This phenomenon corresponds to the extension of the graphene layers of the coke and the formation of a distorted carbon network.     

Lignin-based carbon fibers: Oxidative thermostabilization of kraft lignin

The thermostabilization of lignin fibers used as precursors for carbon fibers was studied at temperatures up to 340 °C at various heating rates in the presence of air. The glass transition temperature (T_g) of the thermally treated lignin varied inversely with hydrogen content and was found to be independent of heating rate or oxidation temperature. A continuous heating transformation (CHT) diagram was constructed from kinetic data and used to predict the optimum heating rate for thermostabilization; a heating rate of 0.06 °C/min or lower was required in order to maintain T_g > T during thermostabilization. Elemental and mass analyses show that carbon and hydrogen content decrease during air oxidation at constant heating rates. The hydrogen loss is sigmoidal, which is consistent with autocatalytic processes. A net increase in oxygen occurs up to 200-250 °C; at higher temperatures, oxygen is lost. Spectroscopic analyses revealed the oxidation of susceptible groups within the lignin macromolecule to ketones, phenols and possibly carboxylic acids in the early stage of the reaction; the later stage involving the loss of CO₂ and water and the formation of anhydrides and possibly esters. Slower heating rates favored oxygen gain and, consequently, higher glass transition temperatures (T_g) as opposed to faster heating rates.     

Bulk quantity and physical properties of boron nitride nanocapsules with a narrow size distribution

A new synthesis route for the formation of boron nitride (BN) nanocapsules by means of a substitution process using single wall carbon nanotubes as templates, with yields of >95% is presented. It is also shown that these BN nanocapsules can act as ideal reference samples for the determination of the relative sp² to sp³ configuration in BN species, a value that is crucial for the physical properties of these nanostructures.     

Preparation and modification of activated carbon fibres by microwave heating

Thermal treatment of activated carbon fibres (ACF) has been carried out using a microwave device, instead of a conventional furnace. The results show that microwave treatment affects the porosity of the ACFs, causing a reduction in micropore volume and micropore size. More importantly, the results also show that microwave treatment is a very effective method for modifying the surface chemistry of the ACFs with the production of pyrone groups, detected by FTIR. As a result very basic carbons, with points of zero charge approximately equal to 11, are readily obtained.     

EPR study on petroleum cokes annealed at different temperatures and used in lithium and sodium batteries

EPR spectroscopy was used for characterisation of petroleum cokes heat-treated in the temperature range of 480-2800 °C and after their electrochemical reactions with lithium and sodium. Unpaired spins with localised character were detected for cokes treated below 1000 °C, while delocalised electrons were found to contribute to the EPR profile of cokes treated between 1400 and 2800 °C. After electrochemical interaction of cokes with lithium, the EPR signal due to delocalised electrons undergoes a strong line narrowing combined with an increase in signal intensity. Localised paramagnetic centres interact with Li and Na, as a result of which there is a line narrowing and decrease in the signal intensity. A new narrow signal is detected for low-temperature cokes after reaction with lithium. The origin of this signal could be associated with ‘near-metallic’ lithium, which is accommodated in non-ordinary carbon sites such as microcavities. The intensity of this signal is higher for samples treated at 600 °C, where a higher hysteresis in the voltage versus composition curve is observed.     

UV stabilization route for melt-processible PAN-based carbon fibers

Ultraviolet radiation-based stabilization routes were explored to produce carbon fibers from melt-processible PAN-based copolymers. An acrylonitrile/methyl acrylate (AN/MA) copolymer was melt-spun into fibers that were crosslinked using UV radiation. The fibers could then be stabilized by oxidative heat treatment, and subsequently carbonized. Physical and mechanical testing was performed to determine the degree of stabilization and the properties of the stabilized and carbonized fibers.     

A study of pitch-based precursors for general purpose carbon fibres

The isotropic phase isolated from a thermally treated coal-tar pitch was studied as a possible precursor for carbon fibres. Extraction with different solvents was performed in order to increase its softening point and so enable higher stabilisation temperatures to be used, with a significant reduction in time. The extraction conditions were selected studying the softening temperatures of the residues, the results of their thermogravimetric analysis and reactivity in air studied by means of differential scanning calorimetry. The residue obtained with a mixture of 40% acetone-60% acetonitrile was found to be the most suitable precursor for the fibres. The carbonised fibres showed a homogeneous surface and diameter, and had tensile properties comparable to other isotropic fibres described in the literature.     

Templated pyrolytic carbon: the effect of poly(ethylene glycol) molecular weight on the pore size distribution of poly(furfuryl alcohol)-derived carbon

Poly(ethylene glycol), which has a negligible carbon yield upon pyrolysis, was used as a template to study the controlled formation of mesoporosity in pyrolytic carbons. A series of carbons was produced from mixtures of poly(ethylene glycol) and poly(furfuryl alcohol) with 25, 50 and 75% composition by weight and an M_n of 300 to18?500 g/mol of template. Polydisperse dextran adsorption reveals a maximum in uptake for 8000 g/mol and 50% templated carbons, while materials from 75% mixtures or those from less than 2000 g/mol template yielded negligible dextran uptake. These results correlated well with the intensity ratio of a broad peak between 7 and 11° 2θ in the X-ray diffraction spectrum and the 002 diffraction peak and also qualitatively with micrographs of the internal microstructure of the carbons. The results suggest a templating process dominated by both the molecular size of the template and the rate of expulsion of decomposed template material during the formation of the solid.     

Determining the anisotropic content in a petroleum pitch - Comparison of centrifugation and optical microscopy techniques

The anisotropic content of a pitch is one of the most important parameters for characterizing such materials. Polarized light optical microscopy is the technique most commonly employed (ASTM D 4616 standard procedure) to measure this pitch parameter. However, this standard procedure is limited to pitches with mesophase contents only up to 20%. An alternative technique for determining the anisotropic content of a pitch is high-temperature centrifugation, which can be used without limitation for pitches with up to 100% anisotropic content. In this work, the two techniques have been compared; samples of four pitches with mesophase contents lower than 20% have been analyzed by both techniques and the results have been compared. The high-temperature centrifugation technique showed good repeatability, and the results that it yielded matched those obtained from optical microscopy when the anisotropic content of the pitch was higher than around 5%. The centrifugation technique is always faster, simpler, and possibly more accurate than optical microscopy for pitches with mesophase contents higher than 20%.     

Synthesis of submicrometer-sized β-SiC particles from the precursors composed of exfoliated graphite and silicone

Utilizing micro-spaces of exfoliated graphite (EG), a process to synthesize fine β-SiC particles was developed. Two types of low molecular weight silicone and a catalyst were impregnated into EG by sorption, and then heated in air to cure the compounds. Formed precursors were black flakes of a few millimeters in diameter. Only by the heat treatment of precursors at 1500 °C for 5 h or at 1550-1600 °C for 1 h in Ar, β-SiC of a few tens to hundreds nanometers in size was obtained with the yield around 40 mass%. In the present process, EG play two important roles; one is as reaction spaces and the other is as a reductant that functions at elevated temperatures. Initially the cured silicone is coating the graphite sheets of EG as thin films of less than 1 μm, and above 1300 °C they start to decompose and form small particles of a few tens to 100 nm in diameter on the graphite sheets. These intermediate particles are composed of the Si-C-O composites and SiO₂ and they spontaneously decompose to β-SiC from around 1400 °C, and between 1400 and 1500 °C the reduction of remaining Si-C-O intermediates by graphite sheets occur and form well-crystallized fine β-SiC particles. The process is simple and raw materials are not expensive so that it is promising for industrial applications.