Robert E. Botto

Cellulose char structure: a combined analytical Py-GC-MS, FTIR, and NMR study

Bulk samples of microcrystalline cellulose were charred in an anaerobic environment, at atmospheric pressure, and temperatures ranging from 190 to 390°C. Curie-point pyrolysis-gas chromatography-mass spectrometry (Cu-Py-GC-MS) was used as an analytical tool for characterization of the resulting chars. Fourier-transform infrared (FTIR) spectroscopy and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy were employed for verification of the results. The abundance of aromatic building blocks (furans, alkylbenzenes, and alkylnaphthalenes) in Cu-Py-GC chromatograms increases with the pretreatment temperature. FTIR and NMR show the breakdown of the pyranose polymer and gradual formation of a new intermediate polymer with an aromatic character which disproportionates further and aromatizes at temperatures above 310°C. A model for the formation of this intermediate polymer is proposed. Cu-Py-GC-MS data are in agreement with the results of FTIR and NMR.

The nature and fate of natural resins in the geosphere—II. Identification, classification and nomenclature of resinites

Abstract A classification scheme for resinites is proposed. Most resinites may be classified, on the basis of structural characteristics, into one of four classes. In order to exclude the effects of structural changes which reflect differences in the relative maturity of different samples, classifications are based on the structural character of the original resin from which the resinite was derived. In all samples characterized to date, this may be reasonably inferred from analytical data concerning the structure and composition of the individual sample. Class I resinites, which are by far the most abundant form of resinite in the geosphere, are derived from resins based primarily on polymers of labdatriene (diterpenoid) carboxylic acids, especially communic or ozic acids. Class II resinites are derived from resins based on polymers of sesquiterpenoid hydrocarbons, especially various isomers of cadinene. Class III resinites are natural polystyrenes. Class IV resinites are the least well characterized of the four resinite classes defined, but appear to be largely non-polymeric materials, dominated by sesquiterpenoids based on the cedrane carbon skeleton. Resinites belonging to Class I are further subdivided into three sub-classes on the basis of details of their composition. Class Ia resinites, which include succinite and related “Baltic ambers”, are derived from resins based primarily on communic acid and incorporate significant amounts of succinic acid into their macromolecular structure. Class Ib resinites are also derived from resins based primarily on communic acid, but do not contain succinic acid. Class Ic resinites are derived from resins based primarily on labdatrienoic acids of the enantio series, especially ozic and/or zanzibaric acids. The structure, origin, and identification of resinites of each class are discussed. The effects of maturation on the structure of Class I resinites, and the consequences of these changes for the recognition and classification of this form of resinite are also briefly discussed.

Quantitative reliability of aromaticity and related measurements on coals by 13C n.m.r. A debate

While solid state 13C n.m.r. has made a major contribution to the characterization of coal and other insoluble carbonaceous materials over the past decade, there has been considerable uncertainty concerning the quantitative reliability of the technique. This debate addresses this important topic and comprises six contributions from authors who are recognized experts in n.m.r. characterization of solid fuels. The principal issue discussed is the accuracy of aromaticity measurements on coals by cross-polarization — magic-angle spinning (CP/MAS) 13C n.m.r., together with additional problems posed by high field measurements and spectral editing, and with some discussion of emerging techniques. There is a consensus that significant errors can arise in CP/MAS 13C n.m.r. measurements of aromaticity due to the unfortunate spin-dynamics of coals, which typically result in only ≈50% of the carbon being observed for bituminous coals. There is clear discrimination against aromatic carbon, but differences of opinion exist over the magnitude of the errors (from 2 to 15 mole carbon %) and whether high field (⩾ 50 MHz) measurements are as accurate as those of low field (< 25 MHz) because either sideband suppression or extremely high speed MAS has to be employed to eliminate sidebands. From the evidence presented, it is suggested that a combination of low field, single pulse excitation with long relaxation delays and the use of a suitable reagent to quench paramagnetic centres is the most satisfactory, albeit time consuming, recipe for obtaining reasonably reliable results on unknown samples. An inter-laboratory exercise is being organized by Argonne National Laboratory to check the precision and to further investigate quantitative reliability of 13C n.m.r. measurements on coals from their Premium Coal Sample Bank.

Structural studies on cellulose pyrolysis and cellulose chars by PYMS, PYGCMS, FTIR, NMR and by wet chemical techniques

Abstract This paper summarizes observations and ideas on cellulose pyrolysis products and structural studies on cellulose chars with a focus on conceptual integration. Information is presented on the larger gas phase oligosaccharides in cellulose pyrolysates, their mechanism of formation and the implications for the processes in the initial stages of char formation. A model for the formation of a new thermally synthesized three-dimensional network polymer is proposed.

The nature and fate of natural resins in the geosphere—iii. re-evaluation of the structure and composition of highgate copalite and glessite

Abstract The results of GC-MS, Py-GC-MS and spectroscopic (solid state 13 C NMR) studies of the structure and composition of two resinites, viz. Highgate Copalite and Glessite , which have been previously described as “fossil elemi” are described. GC-MS analyses of Highgate Copalite establish that this resinite is composed of a simple mixture of diterpenoid resin acids and n -alkyl materials. Based on comparison of these data with similar data obtained by analysis of modern rosin, it is concluded that this resinite is most probably derived from the resin of an undetermined species of pine. In order to accomodate resinites of this general character, it is proposed that an additional class, viz: Class V, be added to the classification system for resinites which we have previously proposed (Anderson et al. , 1992). Py-GC-MS analysis of Glessite establish that this material is a typical Class Ia resinite closely related to Succinite (“normal” or “typical” Baltic Amber). It is concluded that the previously assigned structural character of these two resinites is open to question.

The nature and fate of natural resins in the geosphere. IX Structure and maturation similarities of soluble and insoluble polylabdanoids isolated from Tertiary Class I resinites.

Abstract Soluble polylabdanoids were isolated from five Tertiary Class I resinites by sequential extraction and were characterized by 13 C and 1 H NMR spectroscopy and Py–GC–MS. The structure and maturation characteristics of soluble extracts were strikingly similar to those observed for polylabdanoids in the native resinite. Soluble and insoluble materials undergo parallel geotransformation processes similar to those observed previously, including (i) exomethylene and total olefinic carbon depletion, (ii) double bond redistribution as indicated by ∑C14/∑C15 pyrolysis product ratios and (iii) A-ring defunctionalization. Resonances at 138 and 127 ppm in 13 C NMR spectra were shown to be those of aromatic and olefinic structures in mature samples, indicating that double bond content had been overestimated in the past. Proton NMR analyses have revealed several structural features previously unobserved. Resonances at 5.3 ppm of mature polymers were assigned to trisubstituted olefins in cyclized/cross-linked polylabdanoids. Further evidence in support of this pathway is derived from 13 C NMR spectra, from which an increase in aliphatic carbon content paralleling the loss of olefinic structures was observed.

The application of soft X-ray microscopy to the in-situ analysis of sporinite in coal

Abstract Soft X-ray imaging and carbon near edge absorption fine structure spectroscopy (C-NEXAFS) has been used for the in-situ analysis of sporinite in a rank-variable suite of organic-rich sediments extending from up to high volatile A bituminous coal. The acquisition of chemically based images (con trast based on the 1s−1π∗ transition of unsaturated carbon), reveals a homogeneous chemical structure in the spore exine. C-NEXAFS microanalysis indicates a chemical structural evolution of the sporopollenin bio/geopolymer with increasing maturation. The most significant change in the C-NEXAFS spectrum is an increase in unsaturated carbon, presumably aromatic, with rank. The rate of “aromatization” in sporinite in coal exceeds that of the associated vitrinite. Increases in the concentration of unsaturated carbon are paralleled by losses of aliphatic and hydroxylated aliphatic carbon components. Carboxyl groups are present in low and variable concentrations. Absorption due to carboxyl persists in the most mature specimen in this series, a high volatile A bituminous coal. The reactions that drive sporopollenin chemical structural evolution during diagenesis presumably involve sequential dehydration, Diels-Alder cyclo-addition, and dehydrogenation reactions that ultimately lead to a progressively aromatized bio/geopolymer.

Artificial coalification study: Preparation and characterization of synthetic mecerals

Abstract It was found that lignin heated with clay minerals at 150°C for 2–8 months in the absence of oxygen was readily transformed into an insoluble material resembling low rank coals. The H/C and O/C ratios were in the natural evolutionary range found for vitrinites with the samples from longer reaction times resembling the vitrinites of higher rank. The chemical and physical characterization of the artificial products indicated that their basic chemical structure closely resembles that of vitrinite macerals. Simple pyrolysis of only the lignin at 350–400°C yielded very different products with a substantialy lower H/C ratio and a higher O/C ratio than either transformed lignin with clay or vitrinites of any rank. Such products were found to correspond to fusinite in chemical structure. Macromolecules similar to alginites or type I kerogens were produced by heating fatty acids at 200°C with clays. The present study suggests that coal macerals could have been produced directly from the biological source material via catalytic thermal reactions.

Thermal catalytic transformation of pentacyclic triterpenoids: Alteration of geochemical fossils during coalification

Abstract To elucidate the possible natural evolutionary pathways for the transformation of pentacyclic triterpenoids, three terpenoid samples, Δ 2 -allobetulene, tetranormethylallobetulheptaene and fernenes were heated independently at 150°C for 7 weeks with montmorillonite clay. Common products from these reactions consisted of di, tri-, tetra-, and pentacyclic hydroaromatic and aromatic hydrocarbons, which are commonly found in higher rank coals. C-ring cleaved (8,14-seco) aromatic terpenoid derivatives were also found in the thermal catalytic reaction products of Δ 2 -allobetulene. Such compounds could be important intermediates in the formation of bicyclic hydrocarbons from pentacyclic triterpenoids.

The nature and fate of natural resins in the geosphere - VIII - NMR and Py-GC-MS characterization of soluble labdanoid polymers isolated from holocene class I resins.

Soluble polylabdanoids isolated by sequential solvent extraction have been characterized by liquid-state {sup 13}C- and {sup 1}H NMR and {sup 13}C-{sup 1}H HMQC (heteronuclear correlation) NMR spectroscopy in addition to solid-state NMR and Py-GC-MS techniques. Two Holocene resins originating from Santander, Colombia and Mombasa, Kenya were analyzed. Soluble polymers were isolated by extraction with a 1:1 (v/v) methylene chloride-methanol mixture following sequential extractions with methylene chloride and methanol. The molecular weight of polymer extracts was shown by GPC analyses to exceed that of non-polymeric occluded terpenoids. Py-GC-MS, solid-state {sup 13}C CP/MAS and {sup 13}C cross-polarization/depolarization NMR spectroscopy results indicated that chemical compositions of soluble polymers isolated from immature resins are highly representative of the structure of corresponding insoluble polymers, i.e. polylabdatrienes. These data provide evidence for cross-linking or cyclization of side-chain olefinic carbons during or shortly after polymerization. Generally, the characterization of soluble resin polymers by liquid-state NMR spectroscopy has proven to be an excellent means for investigating the maturation mechanism of polylabdanoid resinites, and has potential for furthering the application of Class I resinites as geothermal indicators.