T. Székely

Thermogravimetry/mass spectrometry study of six lignins within the scope of an international round robin test

Thermogravimetry/mass spectrometry was applied to characterize six lignins prepared for an international round robin test. The sample set includes a mildly isolated lignin, and technical lignins prepared by steam explosion, Alcell, Indulin and Sucrolin processes. The samples were thermally degraded in an argon atmosphere using a heating rate of 20 °C min−1. The weight loss and the evolution profiles of the thermal decomposition products of low molecular mass were monitored. It was found that the intensity and the evolution profile of the products (especially water, formaldehyde, methane and methanol) reflect the severity of the isolation procedure and the origin of the lignin. Correlations have been observed between the abundance of volatile products and the type and amount of functional groups. The terminal CH2OH groups decompose by the release of both water and formaldehyde, as demonstrated by the relationship between the aliphatic hydroxyl group content and the formaldehyde as well as the water evolution. The dependence of the methane yield on the methoxyl group content provided evidence that the scission of methoxyl groups results in the formation of methane as well as methanol. The correlations found allow the assignments of the gaseous products to functional groups.

The effect of cations on the thermal decomposition of lignins

Abstract The thermal decomposition of three lignosulphonates, three sodium salts of a spruce milled wood lignin (MWL) and a mixture of MWL and NaCl was studied by thermogravimetry/mass spectrometry (TG/MS). The TG curves and the evolution profiles of degradation products showed that the lignosulphonate cations (H + , NH + 4 , Na + ) play a key role in the decomposition. The sulphonate groups degrade at low temperature and give rise to a large amount of SO 2 in the case of lignosulphonic acid and NH 4 -lignosulphonate, whereas Na-lignosulphonate releases only 10–20% of the theoretical yield of SO 2 . The catalytic effect of sodium was studied on spruce MWL in detail. Sodium enhances dehydration, demethoxylation, decarboxylation and char formation, whereas it decreases the yield of organic volatiles and CO. Tentative reaction routes are proposed for the rationalization of the influence of sodium on the thermal decomposition of lignins.

Reaction kinetics in thermal analysis. Part 1. The sensitivity of kinetic equations to experimental errors. A mathematical analysis

Abstract The frequent publication of contradicting or meaningless kinetic parameters and the resulting criticism of the “ill-conditioned nature” of non-isothermal reaction kinetics led the authors to an examination of the sensitivity of kinetic parameters to experimental errors. Using simple mathematical deductions, conditions were given at which about 10% precision of the kinetic parameters can easily be achieved. To obtain a graphic picture about the information content of a thermoanalytical curve and the effect of the systematic measurement errors, mathematical relationships were deduced to show the dependence of the kinetic parameters on the formal (geometric) characteristics of the thermoanalytical curves.

Investigation of hydrocarbon constituents of a young sub-bituminous coal by gas chromatography-mass spectrometry

Abstract Hydrocarbon fractions were obtained from a coal extract by open-column chromatographic methods. Compounds amenable to gas chromatography were investigated by computerized gas chromatography-mass spectrometry. Several aliphatic and hydroaromatic biomarkers were observed. Monoaromatic compounds not previously reported in the literature were also detected. They are believed to be intermediates of diagenetic aromatization reactions. The retention indices for several pentacyclic hydroaromatic hydrocarbons are reported.

Natural rubber pyrolysis: Study of temperature-and thickness-dependence indicates dimer formation mechanism

Abstract The technique of pyrolysis—gas chromatography (Py—GC) has been used to study the thermal degradation of natural rubber. The monomer/dimer (M/D) ratio has been measured over the temperature range 300 to 500°C, using sample sizes of the order of 0.3 mg. The M/D ratio has also been measured as a function of sample thickness, using samples in the range 30 nm to 3 μm (0.1 μg to 10 μg). The dependence of M/D on temperature may be interpreted in terms of (a) different activation energies for depropagation versus intramolecular cyclisation, (b) dissociation of dimer to monomer at higher temperatures, or (c) reduced residence time of monomer in the melt at higher temperatures allowing less opportunity for its recombination. The results for the dependence of M/D on thickness (at constant temperature) indicate that interpretation (c) is the most probable. The present work therefore suggests that monomer recombination, possibly by a Diels-Alder mechanism is an important contributor to dimer formation in rubber pyrolysis.

Study of aromatic biomarkers in brown coal extracts

Abstract Gas Chromatographic mass spectrometric (g.c.-m.s.) analysis or aromatic hydrocarbon fractions of 10 brown coal samples has shown the predominance of pentacyclic compounds. Some tetracyclic compounds were also detected. They appeared to be diagenetically related to triterpenoids naturally occurring in the biosphere. Diagenetic aromatization of triterpenoids having a six-membered E ring mainly follow a pathway in which dehydrogenation proceeds from ring A to E. A set of reactions involving C ring cleavage is also an omnipresent process. Aromatization including an A ring cleavage played a minor role.

Thermal decomposition of aryl—alicyclic polyimides studied by thermogravimetry/mass spectrometry and pyrolysis—gas chromatography/mass spectrometry

Abstract The thermal decomposition of four aryl—alicyclic polyimides was studied by thermogravimetry/mass spectrometry and pyrolysis—gas chromatography/mass spectrometry. The effect of the structure of aliphatic segments on the thermal stability and decomposition mechanism of polyimides is discussed. The thermal stability is highly affected by the ring strain of the aliphatic cycles, but the chemical structure of aliphatic units has only a minor effect on the decomposition temperature of aromatic segments. However, the yield of pyrolysis products of both aromatic and aliphatic origin is substantially affected by the structure of aliphatic segments, showing that the primary decomposition of alicycles plays a key role in the decomposition mechanism. The fast pyrolysis and thermogravimetric analysis result in different product yields. However, in both cases the charring process is influenced by the hydrogen-content of the samples indicating the importance of hydrogen-transfer processes.

Kinetics of γ-alumina chlorination by carbon tetrachloride

Abstract The kinetics of the reaction between γ-Al2O3 and gaseous CCl4 has been studied by isothermal TG measurements in the temperature range 700—1123 K. The reaction starts with a weight gain which can be attributed to the chemisorption of the reactive gas. The weight loss vs. time curves at relatively high temperatures can be described by the contracting cylinder equation and at relatively low temperatures by first-order kinetics. The dependence of the initial reaction rate on the CCl4 partial pressure follows the Langmuir—Hinshelwood rate expression. At 700—723 K, chemical control is thought to be predominant and an apparent activation energy of 212 kJ mole−1 is found for the chlorination process.

Kinetics of γ-alumina chlorination by phosgene

Abstract The kinetics of the reaction between γ-Al2O3 and COCl2 have been studied by isothermal TG measurements in the temperature range 585–1085 K. The influence of the partial pressure of COCl2 on the reaction rate was investigated at 648, 673 and 698 K in the 104–105 Pa range, using N2 as a diluting gas. The reaction seems to proceed in the chemical control region below 675 K. Above this temperature, however, diffusional processes and decomposition of COCl2 are considered to affect the reaction rate. In the chemical control region an apparent order of reaction of 0.75 in respect of COCl2 and an apparent activation energy of 128–135 kJ mole−1 were found for the chlorination process.

Identification of aliphatic biological markers in brown coals

Abstract Ten coal samples were subjected to solvent extraction and aliphatic hydrocarbon fractions were obtained by open column chromatography. The fractions were analysed by gas chromatography-mass spectrometry : nalkanes, di- and triterpenoid hydrocarbons and (in lesser amounts) sterenes were found. The distribution of the biological markers found indicated the low rank of the samples, but contradictions in the composition of hopanoids were observed. Some compounds not previously reported in the literature were also present.