M.-J. Lazaro

Structural characterization of biomass pyrolysis tars/oils from eucalyptus wood waste: effect of H2 pressure and sample configuration

Changes in yields and structural characteristics of tars/oils produced in the first stage of a two-stage fixed-bed (‘hot-rod’) reactor were determined as a function of sample configuration and pressure. The work included a critical evaluation of analytical methods for monitoring structural changes in biomass-derived liquids. Total volatile and tar/oil yields decreased and significant structural changes were observed with increasing sample bed height and pressure, leading to ‘lighter’ tars/oils. Products were observed to become more aromatic and less oxygenated. Less intense degradation reactions were observed under hydrogen than under helium. 1-Methyl-2-pyrrolidinone (NMP) was used as eluent in size-exclusion chromatography; solubility limitations were found with tetrahydrofuran as eluent. Good agreement was obtained between structural changes inferred from size-exclusion chromatography and u.v. fluorescence and u.v. absorption spectroscopies. The characterization of liquid products by 1H n.m.r. was found to be less informative than expected, due to the multiplicity of structural assignments in relatively narrow bands. The work suggests that mild hydropyrolysis in deeper beds may be considered as a process step more likely to produce lighter, less oxygenated and more stable tars/oils than liquids produced at atmospheric pressure. However, the reforming process achieved by extraparticle reactions and by the effect of pressure gave rise to significant loss of liquid product.

Molecular mass determinations in coal-derived liquids by MALDI mass spectrometry and size-exclusion chromatography

Abstract This study examined the effect of changes in instrument-related parameters on mass spectra obtained by MALDI-m.s.: ion-extraction voltage, laser power and sample loading. Results were compared with those of size-exclusion chromatography (s.e.c.) using 1-methyl-2-pyrrolidinone as mobile phase. A coal tar pitch and its pyridine-soluble and pyridine-insoluble fractions were used as samples. MALDI-mass spectra were obtained with no added matrix, since the sample absorbed strongly at the laser wavelength, 337 nm. Higher extraction voltages (up to 30 kV) led to the observation of higher masses in the whole pitch and its pyridine-insoluble fraction, where the presence of higher-molecular-mass material was indicated by s.e.c. Increasing the ion extraction voltage served only to accelerate species already ionized, without otherwise disturbing the sample: the use of higher ion extraction voltages must be therefore be considered as providing a more complete inventory of already ionized species. Increasing the laser power level and sample loading on the target led to similar changes in the spectra, although there were indications that for both these parameters, the results might no longer improve (and might eventually deteriorate) beyond optimum values, which appear to be sample-dependent. The low-power spectra revealed more structural information at low masses, where series of homologous peaks could be observed, compared with the spectra at high power.

Planar chromatographic separation of petroleum residues and coal-derived liquids

Abstract Vacuum distillation residues from two petroleum crudes, a coal liquefaction extract and a coal tar pitch have been fractionated by planar chromatography (PC) using two solvent sequences: pyridine–acetonitrile and tetrahydrofuran (THF)–toluene. Fractions recovered from PC were examined by UV-fluorescence spectroscopy (UV-F), size-exclusion chromatography (SEC). UV-F and SEC of the whole samples showed differences in aromatic cluster size and molecular mass (MM) ranges which could be related to the different origins of the samples. The MM ranges indicated by SEC were greater for the vacuum residues than for coal-derived materials. However, the UV-F spectra of the fractions indicated that the petroleum residue fractions contained similar aromatic types, whereas the fractions from coal liquids contained significantly different aromatic types. SEC profiles of the fractions indicated a separation of coal-derived samples by increasing molecular size with increasing immobility in PC, whereas for petroleum fractions, the same trend was not apparent. MALDI-mass spectra of the set of original samples showed broadly similar ranges of MM distributions but additional work is necessary to identify appropriate matrices and procedures in order to improve the MALDI spectra.

Comparison of the fractionation of a coal tar pitch by solvent solubility and by planar chromatography

Abstract Two methods of fractionation of a coal tar pitch, one based on solvent solubility separation and the other on preparative thin layer chromatography were compared for effectiveness of the separation. The fractions were characterised using size exclusion chromatography (SEC) and UV-fluorescence spectroscopy (UV-F). Fractionation by planar chromatography was found to give a better separation and to use less solvent than the solvent separation method. Both SEC and UV-F clearly showed less overlap in fractions separated by planar chromatography, compared to solvent fractionation. The cleaner separation of the heaviest fraction is particularly relevant for the mass spectroscopic investigation of large molecular mass materials, where the presence of lighter materials was observed to suppress high-mass signal. The close agreement observed between SEC and UV-F is consistent with previously reported correlations for coal tar pitches. However, the relevant SEC work was performed using pyridine as the mobile phase. We have found that 10%–15% of coal tar pitches cannot be dissolved in pyridine whilst total dissolution of coal tar pitch may be achieved using NMP. The calibration of analytical columns operated with NMP would be expected to lead to more accurate evaluations of molecular masses in these complex mixtures.

Structural characterization of tar from a coal gasification plant : Comparison with a coke oven tar and a crude oil flash-column residue

Residual tar from a coal gasification plant was characterized using a wide array of analytical techniques. Structural comparisons were made with a coke oven tar and a crude oil flash-column residue. The gasification tar residue was found to have a carbon aromaticity >94%, with extensively dealkylated aromatic structures and only small amounts of oxygenates, compared with the 56% carbon aromaticity and significant occurrence of alkyl substitution in the coke oven tar. The proportion of material boiling > 450°C as well as the elemental carbon and ‘fixed carbon’ contents of the gasification tar residue were also higher than those of the other samples. As expected, the petroleum flash-column residue was mainly aliphatic (fa = 0.12), with small aromatic ring systems attached to long alkyl chains. For all three samples, MALDI mass spectra showed a signal at molecular masses > 20 000 u, with the gasification tar residue showing the narrowest range of molecular masses. Taken together, structural differences between the gasification tar and coke oven tar reflect the more severe thermal treatment and chemical history of the gasification tar, leading to the survival of low amounts (∼ 1 wt% of coal feed) of an apparently, chemically, very stable residue.

Effect of Polydispersity on the Characterization of Coal-derived Liquids by Matrix-assisted Laser Desorption/Ionization Time-of-flight Mass Spectrometry: Inferences from Results for Mixtures of Polystyrene Molecular Mass Standards

Seven polystyrene molecular mass standards have been run in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) singly, in pairs, in groups of three and as a mixture of the seven distinct samples. All standards had polydispersities of near unity. When run singly or in pairs, MALDI-spectra of all standard polymer samples could be observed clearly and showed approximately equal areas. Mixtures of multiple standards have been run at variable ion extraction voltages: when the voltage was reduced below 25 kV, low molar mass samples tended to predominate in the spectra, at the expense of the higher molar mass standards. Peaks due to the high mass standards were still observable above the noise level, but (for equimolar mixtures) areas under the peaks were no longer comparable. With the full set of seven samples, discrimination against intensities of the higher mass standards could be clearly observed, even at the highest ion extraction voltage available (30 kV). With increasing ion extraction voltage the MALDI-spectra of a coal tar pitch and its tetrahydrofuran-insoluble fraction showed increasing intensities of higher mass ranges. Due to the wide polydispersity of coal derived samples, MALDI-spectra would be expected to severely underestimate the proportion of high mass materials present. Further work on mixtures of different types of polymer standards would clearly be useful; for coal derived liquids, more extensive fractionation than that used here would seem advisable in order to reduce sample polydispersity.

Fractionation of a wood tar pitch by planar chromatography for the characterisation of large molecular mass materials

Abstract A commercial tar pitch derived from pine wood – Massen Pine ( Pinus Massonia ) and sold as Stockholm tar has been fractionated by planar chromatography with examination of the fractions by size exclusion chromatography in NMP eluent, by UV-fluorescence and by matrix assisted laser desorption mass spectrometry. The relatively small molecules, mobile in planar chromatography, are shown to be non-polar. Large molecules were found in each fraction, corresponding in SEC elution times up to polystyrenes of molecular mass of at least 1.8 million. Size exclusion chromatography profiles by UV light absorbance showed differences in relative absorbance of different wavelengths for large and small molecules, implying differences in structures. MALDI mass spectra indicated molecules of mass of several thousand mass units with the upper limit of mass not defined. Planar chromatography provides a fast, cheap method of isolating large molecular mass fractions of this biomass tar.

Changes in sample reactivity and catalyst deactivation during early stages of the hydrocracking of a coal extract

Abstract Changes in sample reactivity and catalyst deactivation during the early stages of the hydrocracking of coal liquefaction extracts, using presulphided supported NiMo/Al 2 O 3 and a dispersed Mo catalyst, have been investigated. The characterisation of the liquefaction extract and the hydrocracking products has been made using size exclusion chromatography (SEC), UV-fluorescence spectroscopy (UV-F), matrix assisted laser desorption ionisation (MALDI-ms) and thermo-gravimetric analysis (TGA). A complex sequence of events has been resolved involving the simultaneous development of catalyst activity with parallel deactivation due to carbon deposition alongside thermal cracking of the extract. The balance of evidence suggests that the sharp reduction in the rate of reaction early in the hydrocracking process reported in earlier work is primarily caused by loss of sample reactivity.

Catalytic activity of fullerenes for hydrocracking coal extracts

A liquefaction extract of Point of Ayr coal (UK) was hydrocracked using a mixture of fullerenes as dispersed catalyst and tetralin as solvent. Structural characteristics of the hydrocracking products were examined as a function of increasing reaction time and compared with those of products prepared in the absence of catalyst. Comparison of t.g.a.-derived boiling point distributions showed that the proportion of material boiling at temperatures below 450°C progressively increased with reaction time up to 120 min. Size exclusion chromatograms (s.e.c.) of products indicated a similar steady reduction in the proportion of material observed at the exclusion limit of the column, at up to 90 min reaction. However, reaction between 90 and 120 min appeared to have the reverse effect. Comparison with hydrocracking in the absence of catalyst gave no indication that the fullerenes enhance reductions in boiling point distributions or in apparent molecular masses. MALDI-m.s. largely confirmed findings by s.e.c. The evidence strongly suggests that fullerenes used in these experiments were unable to retain their integrity during the process.

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.