Alan A. Herod

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.

Fractionation by planar chromatography of a coal tar pitch for characterisation by size-exclusion chromatography, UV fluorescence and direct-probe mass spectrometry

Abstract A coal tar pitch, extensively studied by other techniques, has been fractionated by planar chromatography using successive development in tetrahydrofuran, chloroform-methanol (4:1, v/v), toluene and pentane. Pitch fractions were distinguished by relative retention. The fractions were characterised in solution by size-exclusion chromatography, UV-fluorescence emission spectroscopy and as the solid on silica, by direct solid-probe mass spectrometry. The method has led to structural information not readily available by direct characterisation of the original mixture itself: (i) A progressive shift of UV-fluorescence spectral maximum intensity and molecular mass with decreasing mobility of the fractions on the planar chromatographic plate, suggesting progressively larger polynuclear aromatic systems; (ii) Nitrogen containing species in the pitch separated from the polynuclear aromatic hydrocarbons which form the bulk of the sample. Neutral and basic nitrogen fractions were defined relative to standards and molecular mass numbers in probe mass spectrometry; (iii) Mass ranges of nitrogen components in these fractions exceed those found previously by GC-MS of the total pitch or its fractions. (iv) Size-exclusion chromatography of the fractions suggests that polar materials are not simply separated by size.

Investigation of nitrogen compounds in coal tar products. 2. Basic fractions

Abstract Gas chromatography (g.c.), in conjunction with nitrogen-selective alkali flame detection (AFD) and mass spectrometry (m.s.), has been used for the detailed examination of nitrogen compounds in basic fractions from an anthracene oil and a low-temperature coal tar. The compounds identified were mainly aza species, but some di-nitrogen and mixed heteroatomic compounds were detected in the anthracene oil bases. Nitrogen-selective g.c.—AFD chromatograms of the extraction residues showed complex mixtures of non-basic nitrogen compounds; the major peaks were identified, but further g.c.-m.s. investigation of these compounds will require the preparation of hydrocarbon-free fractions.

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.

Identification of large molecular mass material in high temperature coal tars and pitches by laser desorption mass spectroscopy

Abstract Molecular masses of up to 12 000 Da have been identified by laser desorption mass spectroscopy (l.d.-m.s.) in a number of coal tars and extracts. Observed molecular masses were greater than any hitherto detected in coal-derived liquids; fractions of high temperature coke oven tars have been used for verification of initial findings. Low molecular mass materials have been analysed using similar experimental procedures, to confirm the absence of laser or other apparatus derived high molecular mass clusters. Spectra extending to 12 000 Da show an envelope between 1000 and 3000 Da and a low molecular mass envelope between 12 and 400 Da, the latter apparently indicating the presence of carbon clusters. At high laser power densities, the 1000 to 3000 Da envelope was observed to lose intensity, with parallel gain in intensity of the low mass carbon cluster envelope, suggesting increased breakdown of substrate molecules under increasing laser power density. Comparisons with results from g.c.-m.s. and probe m.s. are presented, showing masses up to 600 Da. Limited structural information about the large molecular mass materials can be inferred from the present results, suggesting the presence of small aromatic groups with mass differences corresponding to methyl groups, ethylene bridges and benzo groups.

Absolute calibration of size exclusion chromatography for coal derivatives through MALDI-m.s.

This paper reports the fractionation and subsequent examination of two, very different coal derived materials, a coal tar pitch and a commercial liquefaction extract. These were separated on the basis of their retention in a typical preparative size exclusion chromatography apparatus using THF as the eluent. Matrix assisted laser desorption mass spectrometry, using the small molecules of the sample as matrix, has been used to examine the various fractions from preparative s.e.c. and the peak mass intensities have been used to define an absolute calibration of the s.e.c. separation. Limitations of the methods are discussed. The u.v.-fluorescence spectra of the fractions were recorded and trends in the spectra are described. The fractions were also analyzed by size exclusion chromatography employing N-methyl-2-pyrrolidinone as the chromatographic eluent. The main advantages of this system over traditional solvents is the avoidance of surface adsorption effects observed with THF and the more complete solution of coal-derived material.

Coal tar analysis by mass spectrometry — a comparison of methods

Abstract The analyses of coal-derived hydropyrolysis tar fractions by a variety of mass spectrometric methods are compared, to identify common ground in the analytical results. Methods compared include high performance liquid chromatography interfaced with mass spectrometry (m.s.) by moving belts and a particle beam, low and high voltage solids injection probe to 350 °C, desorption chemical ionization (d.c.i.) using isobutane, field ionization (f.i.), field desorption (f.d.) and fast atom bombardment (f.a.b.). Aromatic and asphaltenic fractions of two hydropyrolysis tars are compared. The tars were produced at relatively low temperatures under gentle conditions. The results show that for the aromatics, the probe methods, solids injection, d.c.i. and f.i. all show similar masses and mass ranges for the aromatics ; differences reflect differences in sensitivity and conditions of ionization. F.d. involves ionization at a surface followed by desorption of the ion, and signals of different components are seen. For the asphaltenes, some components, mainly multi-hydroxyl derivatives of aromatics, are identified by probe methods (solids injection and f.i.m.s.) but f.d. again detects different material. F.a.b. produces evidence of the structure of the molecules detected — small aromatic and heterocyclic groups loosely linked together, rather than the very large, condensed aromatic structures expected of a high temperature coal tar. Further definition of the molecular structure of asphaltenes requires an improvement in separation techniques.

Identification of sulphur heterocycles in coal tar and pitch

Abstract The direct application of gas chromatography with a flame photometric sulphur detector and mass spectrometry to the identification of sulphur heterocycles in coal tar and pitch without the need for prior separation of a sulphur-rich fraction is described. Accurate mass measurement has allowed the assignment of atomic compositions to the sulphur heterocycles with little ambiguity, despite their presence as minor components in a complex aromatic mixture. Single ion chromatograms and retention indices calculated relative to aromatic hydrocarbons have been used to correlate the mass spectral identifications with the sulphur-selective gas chromatograms.

N-methyl-2-pyrrolidinone as a mobile phase in the size-exclusion chromatography of coal derivatives

Abstract This paper reviews the current state of molecular mass determination for fossil fuel materials by chromatographic methods and describes improvements which can be made to both the chromatographic separation and the detection of eluting material. Most significantly, N-methyl-2-pyrrolidinone (NMP) offers a number of advantages as a mobile phase in the size-exclusion chromatography (SEC) of coal derivatives. Much more coal-derived material dissolves in NMP and solute-column packing interactions common with SEC solvents such as tetrahydrofuran are much reduced. NMP is compatible with UV absorption and UV fluorescence detection

Investigation of nitrogen compounds in coal tar products. 1. Unfractionated materials

Abstract The direct application of gas chromatography (g.c.) with a nitrogen-selective alkali-flame detector (AFD) and mass spectrometry to the identification of nitrogen compounds in coal tar products without the need for prior separation of nitrogen-rich fractions is described. Accurate mass measurement has allowed the assignment of atomic compositions to many nitrogen heterocycles despite their presence as minor components in complex aromatic mixtures, but some interference from hydrocarbons was encountered. Boiling point characteristics were used to assist in distinguishing between the various possible isomeric compound types. The AFD was used successfully for the quantitative g.c. determination of a range of nitrogen heterocycles in the unfractionated samples.