I. Suelves

Chromatographic separations enabling the structural characterisation of heavy petroleum residues

Abstract Two petroleum residues from European crudes have been fractionated using solvent (heptane) separation and column chromatography. The residues and the separated fractions have been characterised by size exclusion chromatography (SEC) and by UV-fluorescence spectroscopy (UV-F). Matrix assisted laser desorption/ionisation-mass spectrometry of the whole residues and the heptane insoluble fractions indicated that the bulk of the residues covered the mass range m / z 300–2000, while the heptane insolubles (1–2% of the whole) contained material in the mass range from about m / z 300 to 10u2008000. The upper mass ranges indicated by SEC using polystyrene standards were higher; the earliest eluting material from both distillation residues eluted at times corresponding to polystyrene standards of MMs above 1.85 million u. Possible reasons for the different observations are given. Data from UV-F suggests that the heptane solubility separation method was the most successful for the separation of the largest molecular mass and also probably the most polar materials in these residues. However, all three fractionation methods produced similar trends, showing greater polarity of the fractions to correlate with increasing molecular mass. The shift of maximum intensity of fluorescence towards longer wavelengths (in UV-fluorescence) with increasing molecular size, as indicated by SEC, strongly suggests that the fluorescing molecules are large rather than aggregates of small molecules. Differences in comparison with American petroleum residues can be observed.

Characterisation of tars from the co-pyrolysis of waste lubricating oils with coal

Abstract Pyrolysis of a lubricating oil waste with or without coal yields important quantities of valuable products such as C 1 –C 3 alkanes, C 2 –C 4 olefins and BTX. However, information on molecular structures of tars obtained is only available in terms of analyses in the GC–MS ranges. This range corresponds to an upper limit of approximately 300xa0u; aromatics of mass greater than 300xa0u do not normally elute from high-temperature GC columns. For this reason, an oil tar, a coal tar and a tar obtained by the co-pyrolysis of the coal and oil (50:50xa0wt%) have been characterised by an array of techniques: probe-mass spectrometry (probe-MS) (to extend the range of mass to 600xa0u); size exclusion chromatography (SEC) in 1-methyl-2-pyrrolidinone (NMP) solvent; matrix-assisted laser desorption ionisation-mass spectrometry (MALDI-MS); and UV-fluorescence spectroscopy (UV-F) to provide specific information on chemical structures of products. Thin-layer chromatography (TLC) was also carried out for three samples of tar (from oil, coal and mixed oil/coal) and fractions recovered from the plates were analysed by other techniques: SEC, UV-F and probe-MS. In addition, the proportion of the tars amenable to gas chromatography was determined; GC–MS was used to determine the main components of each tar within the GC range. In summary, it can be concluded that Samca coal tar is mainly formed by large polynuclear aromatic ring systems, as well as by heterocyclic structures and alkyl or heteroatom substituents. The tar from the coal/oil mixture is much more similar to the tar from oil than to the tar from coal, reflecting the synergy in the co-pyrolysis reaction.

The unusual properties of high mass materials from coal-derived liquids☆

Abstract This short paper highlights the unusual properties of the high-mass material of coal liquids isolated by their insolubility in pyridine and solubility in NMP. The separation has been achieved by a column chromatography method. One gram quantity have been processed and near quantitative recovery of the sample as fractions has been achieved. This fractionation permitted recourse to a broad range of analytical methods, including some (e.g. 13 C NMR), which require large sample sizes. Multiple macro analyses have been undertaken, using elemental analysis, TGA proximate analysis, NMR and FT-ir in addition to the micro-analytical methods used previously—pyrolysis-gc-ms, SEC, UV–fluorescence, probe-ms and MALDI-ms. The fractions show increasing concentrations of large molecular mass material with increasing polarity of successive eluents used in the fractionation. Evidence from solid-state 13 C NMR and UV–fluorescence spectroscopy show progressive structural changes with increasing apparent molecular mass.

Pyrolysis-gas chromatography/mass spectrometry of a coal extract and its fractions separated by planar chromatography: correlation of structural features with molecular mass

The structural characterisation of a coal liquefaction extract and its three fractions separated by planar chromatography has been described. Size exclusion chromatography showed the molecular mass distributions to become progressively larger with decreasing mobility on the plate. UV-fluorescence spectroscopy of the fractions indicated parallel increases in the sizes of polynuclear aromatic ring systems. Analysis by probe-mass spectrometry of the whole coal extract showed the expected array of small polynuclear aromatic groups extending to m/z 450. The probe mass spectra of the lightest fraction (mobile in pyridine and acetonitrile) showed similar features, except for effects due to vacuum drying to remove solvent. In sharp contrast, the two heaviest fractions (mobile in pyridine and immobile in acetonitrile and immobile in pyridine) showed no significant ions other than those from residual NMP solvent (m/z 98 and 99). Pyrolysis-gas chromatography/mass spectrometry of these two heaviest fractions showed only traces of aromatic compounds or fragments. The aromatic pyrolysis products of these fractions were too large and involatile to pass through the GC column. The major components observed in the pyrolysis-gas chromatography/mass spectrometry of the two heavy fractions were alkanes and alkenes, ranging between C10-C25. Since none of the samples contained free alkanes, alkenes or cycloalkanes before pyrolysis, they were generated during the pyrolysis step. The shifts of UV-fluorescence spectral intensity to shorter wavelengths with decreasing size indicated by size exclusion chromatography (SEC) provide direct evidence of differences in structure with changing molecular mass. This evidence strongly suggests that species identified as being of large molecular mass in this extract sample are not composed of molecular aggregates. It remains difficult to establish whether and when it would be legitimate to invoke molecular aggregates to explain the large molecular masses (MMs) identified here and in other work. Copyright 2000 John Wiley & Sons, Ltd.

Pyrolysis of Baltic amber in a wire-mesh pyrolysis reactor: structural comparison of the tars with amber extracts in NMP

Abstract A sample of Baltic amber believed to be about 40 million years old, has been pyrolysed in a wire-mesh reactor. Nearly all (99%) of the sample was found to volatilise; the condensable tar yield was about 82%. These tars and extracts in 1-methyl-2-pyrrolidinone (NMP), ∼20% of the original amber sample, have been characterised by size exclusion chromatography (SEC) and UV-fluorescence spectroscopy (UV-F). Size exclusion chromatograms of the soluble fraction of the amber gave relatively low intensity signal, indicating the lack of strongly absorbing chromophores. The profiles of the pyrolysis tar were more intense, suggesting that the pyrolytic process promoted aromatisation of the original structures. SEC of the amber extracts showed the presence of apparently large molecular mass material, possibly up to several million units. The SEC of the tar showed lower molecular mass material than that of the extract, with less absorbance at longer wavelengths, suggesting the presence of smaller polynuclear aromatic groups. Synchronous UV-fluorescence spectra suggest that the tar consisted mainly of fragmentation products of larger molecular mass material, deriving mostly from the insoluble, probably partly cross-linked part of the original sample.

Structural features of large molecular mass material in coal-derived liquids: catalytic hydrocracking of the pyridine-insoluble fraction of a coal-tar pitch

The pyridine-insoluble fraction of a coal-tar pitch has been catalytically hydrocracked. The starting sample contained no material that could be observed by gas chromatography/mass spectrometry (GC/MS) or by heated-probe mass spectrometry. The aim of the study was to generate structural information on this narrow cut of large-molecular-mass material, which consisted mainly of compounds boiling above 450°C. A much broader boiling-point distribution was found for hydrocracked products; evidence from all analytical techniques used (size-exclusion chromatography, UV-fluorescence spectroscopy and heated-probe mass spectrometry) indicated a significant reduction of the molecular mass range. A wide variety of aromatic groups was identified in hydrocracked products by heated-probe mass spectrometry, ranging from phenanthrene (m/z 178) to beyond dibenzocoronene (m/z 400). Ions corresponding to alkyl (m/z 43, 57, 71 and 85) and alkenyl groups (m/z 41, 55, 69 and 83) were detected. The results reflect the ability of the hydrocracking process to cleave bonds within large pitch molecules, releasing the structural units which form the molecules. The structure of the original pyridine-insoluble material may thus be seen as a series of aromatic groups linked by short bridges or small groups, but with some longer aliphatic bridges (C12 and C17) as shown by pyrolysis-GC/MS. Clearly, some molecular species were reduced in mass but still lay above the range of detection by heated-probe mass spectrometry and could not be identified. Although the extent to which large polycyclic aromatic entities themselves have been cracked (or otherwise reduced to smaller polycyclic aromatic groups) cannot be quantitatively discerned from the present data, the hydrocracking of isolated fractions of intractable coal-derived material appears to offer a useful method for probing their structural features.

Metal-ion pillared clays as hydrocracking catalysts (II): effect of contact time on products from coal extracts and petroleum distillation residues☆ ☆

Abstract Novel catalysts have been prepared, based on montmorillonite (a natural clay) and laponite (a synthetic clay) pillared with tin, chromium and aluminium pillars as well as layered double hydroxides based on polyoxo-vanadate and -molybdate as previously described. These novel catalysts were compared initially with a standard Ni/Mo catalyst supported on alumina and a dispersed catalyst, Mo(CO) 6 in hydrocracking a coal extract for a short contact time of 10 min at 440 °C in a microbomb reactor with tetralin solvent and hydrogen at a pressure of 190 bar. In the present work, the best of the novel catalysts, chromium montmorillonite calcined at 500 °C and tin laponite, have been compared with the supported catalyst and a dispersed catalyst (Mo(CO) 6 ) in the repeated hydrocracking of fresh coal extract over three sequential periods of 1 h. Also, the chromium montmorillonite calcined at 500 °C has been used in the hydrocracking of primary coal extracts, prepared in the flowing solvent liquefaction rig from Pittsburgh #8 and Illinois #6 coals, for reaction times of 10 min and 2 h. Further, the chromium montmorillonite calcined at 500 °C and tin laponite, have been compared with the supported catalyst and in the absence of a catalyst, in the hydrocracking of a petroleum distillation residue with 10 min and 2 h reaction times. Results were compared by size exclusion chromatography in NMP solvent and by UV-fluorescence and evaluated by the extent of the shift of the SEC profile to small molecules and by the shift of the synchronous UV-fluorescence profiles to shorter wavelengths. The performances of both catalysts at short, long or repeated reaction times are seen to be better than that of the conventional NiMo catalyst for the hydrocracking of coal-derived materials and a petroleum residue. Trials on a longer time scale are necessary in the next level of evaluation.

Metal-ion pillared clays as hydrocracking catalysts (I): Catalyst preparation and assessment of performance at short contact times

Abstract A set of pillared clay catalysts based on montmorillonite (a natural clay) and laponite (a synthetic clay) have been prepared. The new catalysts have been pillared with tin, chromium and aluminium pillars as well as layered double hydroxides based on polyoxo-vanadate and -molybdate. The activities of these novel catalysts have been compared with that of a commercial supported NiMo/Al 2 O 3 catalyst and with sulphided Mo(CO) 6 during short (10xa0min) contact runs. A coal extract sample was reacted at 440xa0°C in a microbomb reactor in the presence of tetralin and 19xa0MPa hydrogen. Products were compared by size exclusion chromatography, using NMP as eluent, and by UV–fluorescence. Boiling point distributions of hydrocracked products were determined by a TGA based method; ‘conversions’ were defined as the decrease in the fraction of material with boiling points >450xa0°C during the reaction. Previous work at 440xa0°C and 19xa0MPa H 2 indicates extensive thermal (pyrolytic) cracking during the first 10xa0min; in the absence of catalyst recombination reactions rapidly take over. Results with several of the new catalysts did not show any improvement compared to the absence of catalyst with ∼39% conversion. The highest conversion (∼70%) was obtained with the Sn laponite pillared clay. The Cr montmorillonite catalyst, pre-calcined at 500xa0°C, gave the greatest overall shift to smaller molecular masses even though the observed conversion of >450xa0°C boiling material was relatively poor.

A comparative study of the composition of anthracene oil polymerized by different treatments

Abstract An industrial anthracene oil (AO) was polymerized by thermal treatment with the aid of AlCl3, sulphur and air. The composition of the reaction products was investigated by size exclusion chromatography (SEC) and synchronous UV-fluorescence spectroscopy. The SEC profiles show significant differences in the composition of the polymerized products. This suggests that the reactivity of the AO components and the mechanisms of polymerization are different. With sulphur and air, the polymerization involves a low number of AO components, which selectively react to form intermediate and large polymers. AlCl3 causes a more extensive polymerization, giving rise to a product in which the fraction of oligomers of intermediate size (500–700xa0u) is predominant. Synchronous UV-fluorescence spectroscopy corroborates and supports the SEC results.

Structure and composition of coal tars: An attempt to correlate molecular structure with increasing molecular mass

Near-burner-zone combustion rates in pulverized-fuel (PF) combustors depend primarily on the amount and the structure/composition of volatiles released from the fuel. Mathematical simulations of PF flames often assume that volatiles combustion takes place at rates similar to those of light hydrocarbons. However, tars constitute 60-70% of volatiles released by most power station coals; tar molecules are far larger and oxidize more slowly. The problem is usually ignored. This article describes the characterization of structures and compositions of molecules in a low-temperature coal tar, typical of material combusted in the near-burner zone. Several techniques have been used in combination. The procedure relies on the estimation of molecular mass distributions by size exclusion chromatography coupled with bulk structural characterization by several established techniques (e.g., 13 C-NMR, UV-fluorescence spectrometry). Sample fractionation by polarity and molecular mass have been found necessary for enhancing the resolution of the analytical tools, as well as for allowing meaningful correlations to be established between changing molecular mass ranges of successive fractions of the tar and the structural features of each fraction. The approach, described in some detail, extends the range of molecular masses amenable to examination to levels far above ceilings imposed by limitations of conventional GC, GC-MS, and probe-MS. The distribution of structures and molecular masses found in the tar suggests reaction pathways for the formation of soot during PF combustion.