Ferenc Till

Thermal decomposition of milled wood lignins studied by thermogravimetry/mass spectrometry

Thermogravimetry/mass spectrometry has been applied to characterize 16 milled wood lignins isolated from grasses, softwoods and hardwoods. The samples were thermally decomposed in inert atmosphere using a heating rate of 20°C min−1. The effect of acetylation as well as ZnCl2 and NaCl as catalysts on the thermal decomposition has been studied. The weight loss and evolution profiles of the most abundant degradation products were monitored as a function of temperature. Correlation has been found between the intensity of several products and the compositional parameters of lignins determined by wet chemical methods. The modification of OH groups by acetylation changed the decomposition patterns, but the acetic acid produced by thermolysis had no catalytic effect on the formation of monomers. The evolution profiles of H2O and CO2 from the original and acetylated lignins proved that free OH groups promote the scission of COOH groups. The additives ZnCl2 and NaCl have different influence on the product distribution. NaCl promotes dehydration, demethoxylation and recombination of the primarily formed radicals, although the maximum of the evolution profiles is not shifted significantly. However, water and formaldehyde formation shifts to 60–80°C lower temperature in the presence of ZnCl2 indicating the occurrence of ionic decomposition mechanisms.

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.

Thermogravimetric/mass spectrometric characterization of two energy crops, Arundo donax and Miscanthus sinensis

Abstract Two herbaceous plants, Arundo donax and Miscanthus sinensis with high biomass production potential, were studied by simultaneous thermogravimetry/mass spectrometry (TG/MS) in an inert argon atmosphere at a heating rate of 20 °C min−1. The effect of particle size, the partial removal of the minerals by acid- and water-washing, and the effect of potassium carbonate as catalyst were studied. Experiments with covered sample pans were carried out to investigate the effect of secondary tar cracking on the charcoal production. TG/MS supplied intensity profiles of the low molecular weight volatile products as a function of time or temperature. Their comparison to the weight-loss rate curves (DTG) provided information of the details of the degradation mechanism. The amounts of various volatile pyrolysis products were estimated by the integrals of the corresponding mass spectrometric intensities. The factors influencing the char yield were discussed.

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.

Least squares criteria for the kinetic evaluation of thermoanalytical experiments. Examples from a char reactivity study

Abstract The mathematical modeling of the chemical processes is a crucial problem of the thermal analysis. Simple models with few parameters seldom can describe the real complexity of the phenomena arising during the heating of the various substances. In the case of more complex models the determination of the parameters and the validation of the model require the evaluation of carefully designed experimental series. The one-by-one evaluation of the experiments is a mathematically ill-defined problem at a larger number of unknown parameters. The non-statistical experimental errors of the thermal analysis hinder the determination of a single parameter set by the simultaneous least squares evaluation of the experiments. This paper discusses several evaluation techniques for the handling of the non-statistical errors during the least squares evaluation of experimental series. The methods are illustrated by the evaluation of oxidative thermogravimetric experiments of a lignite and a coal char.

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.

Thermogravimetric-mass spectrometric study on the low temperature oxidation of coals

Abstract Four coal samples of different rank and four coals of extremely high mineral matter content were exposed to air to study low temperature oxidation (weathering) processes under laboratory conditions. The reactivity of the coals was compared by measuring the oxygen consumption and the mass gain at 80°C for 96 hours. It was concluded that both the rank and sulphur content has an influence on the reactivity towards oxygen. Thermogravimetry combined with mass spectrometry (TG-MS) was used to investigate the weathering induced changes in the coal structure. Low temperature oxidation resulted in an increased yield of carbon dioxide, water, acetic acid, carbonyl sulphide and sulphur dioxide during thermal decomposition. Furthermore, decreased intensity of aliphatic hydrocarbons, alkylphenols and hydrogen sulphide was observed in the mass spectra of oxidized samples. These observations are explained by the formation of oxygen-containing functional groups on aliphatic CH groups and by the reaction of phenolic OH groups leading to cross-linking. TG-MS experiments show that both the pyritic and organic sulphide groups are attacked by oxygen resulting in an increased abundance of oxidized forms of sulphur.

The effects of heat and mass transport on the results of thermal decomposition studies: Part 2. Polystyrene, polytetrafluoroethylene and polypropylene

Abstract The thermal decomposition of pure polymer samples was studied at a low heating rate (4°C/min) and with sample masses of 0.3–6 mg in a Perkin-Elmer TGS-2 thermobalance. The various transport effects were distinguished by changing the gas flow from argon to helium, varying the sample mass and geometry and by applying a self-generated atmosphere. Sample masses of 3–6 mg showed marked effects of insufficient heat transfer in a flow of argon. The concentration of the volatile products above the samples did not influence the thermal decomposition. The diffusion problems in the polymer bulk, however, changed the kinetics from first order to zero order in the case of polytetrafluoroethylene, and increased the overall reaction rate of polypropylene. The latter effect was attributed to chain termination caused by the evaporation of small radicals from the thin polypropylene samples.

Pyrolysis-gas chromatographic-mass spectrometric and thermogravimetric-mass spectrometric investigation of brown coals

Abstract Fourteen low-rank coals were subjected to thermogravimetric-mass spectrometric (TG-MS) and pyrolysis-gas chromatographic-mass spectrometric investigations. TG-MS fragment ion curves were used to clarify the thermal decomposition processes taking place during coal pyrolysis. Carbon dioxide production is attributed to the decarboxylation of humic acid content at 300°C and that of other acidic and ester groups up to 500°C. The pyrolytic evolution of alkylarenes seems to be connected with the production of carbon monoxide, through the cleavage of the ketonic carbonyl groups linking the aromatic segments. Methane originates from long-chain alkanes at about 500°C but from alkyl substituents of arenes at higher pyrolysis temperatures. Above 800°C the dehydrogenation process proved to be a consequence of the aromatic ring destruction reaction at aryl ether bonds. The TG-MS curves serve as an adequate basis for the selection of a suitable temperature for fast pyrolysis applied for the characterization of coals. The relative amounts of the isoprenoid and aromatic marker compounds found in the pyrograms at 600°C proved to be related to the rank of the coal. The coal samples investigated have been characterized by the relative amounts of some pyrolysis products considered to be of key importance.

Software for a mass spectrometer-thermobalance system

The questions of the computer control, data acquisition and data processing are treated for a TG-MS system built from a Perkin-Elmer thermobalance (TGS-2), a Balzers quadrupole mass spectrometer (QMG-511) and a small configuration of a DEC PDP-11 minicomputer. The aim of the computer software was to optimize the performance of the system for the requirements needed for the TG-MS analysis of solid fuels and other complex organic materials. The organization of the mass spectrometric measurement, the fast processing of the mass spectrometric peaks, the quick acquisition of time-averaged weight and temperature data and the problems of the concise data storage are treated. The outlined considerations may be applicable to other instruments. A method for the fast digital filtering of the mass spectrometric data is described in a separate Appendix.