Frank-Dieter Kopinke

Thermal decomposition of biodegradable polyesters-II. Poly(lactic acid)

Abstract The thermal decomposition of the biologically degradable polymer poly(lactic acid) (PLA) was investigated by means of several thermoanalytical techniques: thermogravimetry, differential scanning calorimetry, time resolved pyrolysis-MS and pyrolysis-GC/MS. The results mainly confirm reaction mechanisms proposed in the literature. The dominant reaction pathway is an intramolecular transesterification for pure PLA ( T max = 360 ° C ), giving rise to the formation of cyclic oligomers. In addition, acrylic acid from cis-elimination as well as carbon oxides and acetaldehyde from fragmentation reactions were detected. PLA samples contaminated with residual Sn from the polymerization process show a preceding selective depolymerization step ( T max = 300 ° C ) which produces lactide exclusively. The GC analysis of the oligomers gives insight into the stereochemistry of the original polymer chain with respect to the configuration of the asymmetric C atoms, as well as into the stereochemistry of decomposition reactions. Other experimental findings, which do not fit the proposed reaction mechanisms, are also discussed.

Thermal decomposition of biodegradable polyesters—I: Poly(β-hydroxybutyric acid)☆

The thermal decomposition of the biologically degradable polymer poly(β-hydroxybutyric acid) (PHB) was investigated by means of several thermoanalytical techniques: thermogravimetry, differential scanning calorimetry. time resolved pyrolysis-MS and pyrolysis-GC/MS. The results conform with reaction mechanisms proposed in the literature. The dominant reaction pathway is a stereoselective cis-elimination giving rise to the formation of trans-crotonic acid and its oligomers. Crude PHB embedded in the original biomass is significantly less stable (Tmax = 235 °C) than the purified polymer (Tmax = 290 °C). An apparent activation energy of EΛ = 235 kJ · mol−1 was derived from DSC measurements. The data from dynamic thermogravimetric and pyrolysis-MS experiments are not consistent with first order random cleavage kinetics, as would be expected from the reaction mechanism. Thermogravimetry offers a convenient tool for analyzing quantitatively the PHB content of biomasses.

Rearrangement reactions in the thermal formation of aromatics from cycloolefins. 14C-labelling studies

Abstract The thermal conversion of non-aromatic cyclic hydrocarbons into aromatics involves the change of exocyclic α-C atoms of the non-aromatics into C atoms of the aromatic ring system. To gain a better understanding of the largely unknown details of this reaction path [methyl- 14 C]methylcyclopentadienes, [methyl- 14 C]methyl-1,3-cyclohexadienes, [7- 14 C]methylenecyclohexane, and [methyl- 14 C]methylcyclohexane were synthesized and thermally converted into benzene and toluene in a stainless steel flow reactor at 600–750° C. By means of radio gas chromatographic analysis and, in part, by determining the position of the 14 C label in the products formed, an extensive C scrambling, including ring opening reactions prior to aromatization, could be proved in all cases. The results of these and some additional experiments on a Pt catalyst are discussed with respect to other experimental work and some speculative assumptions on possible intermediate species of the observed skeletal reorganizations are made.

New results about the mechanism of TLE fouling in steam crackers

The deposition of coke-like products was studied in the postcracking zone ( < 500°C) of a laboratory pyrolysis reactor coupled with a thermobalance. By means of 14C-labelled hydrocarbons, data on the contribution of individual feedstock components to the coke formation were obtained. Reactive, volatile hydrocarbons, like toluene and hexyne, do not contribute significantly to the deposits if they pass the postcracking zone undecomposed. The results favour as a dominant mechanism in transfer line heat exchanger fouling the condensation of heavy components of the fuel oil fraction and precipitation of tar droplets. At a catalytically active steel surface the coke formation from a cracker effluent at 500°C is strongly influenced by steam dilution.

Thermoanalytical methods for characterizing hydrocarbon—sludge—soil mixtures

Hydrocarbon sludges from petrochemical plants and soils contaminated with these sludges were characterized by applying thermoanalytical methods such as pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and thermogravimetry/mass spectrometry (TG-MS). In combination with conventional and supercritical fluid extraction (SFE) followed by GC/MS analysis, more than 100 constituents were identified. The Py-GC/MS technique is suitable for a qualitative rather than a quantitative analysis of organic pollutants in sludge-soil mixtures. The TG-MS method was used for describing interactions between pollutants and solid matrices (sludges and soils). It was found that a mild thermal treatment of samples (thermodesorption) did not release other compounds or higher quantities than those obtained by solvent extraction. Consequently, these organic pollutants must be fixed on soil organic matter (SOM) by sorption rather than by covalent binding. The TG-MS results permit us to distinguish between the release of a component by thermodesorption and by pyrolysis.

Reactions of hydrocarbons during thermodesorption from sediments

Thermal reactions of hydrocarbons, which were loaded onto a river sediment, were investigated at 300°C in an open and in a closed reactor. The results give evidence that the thermodesorption of PAHs and hydroxylated aromatics is accompanied by their chemical conversion to non-volatile products, which can be called charring. Quartz powder as an inert carrier does not effect such reactions. The rank in reactivity observed for charring of PAHs is consistent with their reactivity in radical reactions. The sediment ash is more active in hydrocarbon conversion reactions than the native sediment. This points to the mineral matrix of the sediment as the actual carrier of the catalytic activity rather than the sediment organic matter.

14C polyaromatics as radio isotope tracers for grafting analysis during heavy oil pyrolysis

Abstract Labelled polyaromatics, such as 14 C chrysene and 14 C acenaphthylene, have been used as tracers during pyrolysis of a heavy oil residue. Prior to this approach the thermal behaviour of 1:1 mixtures of chrysene and Safaniya vacuum residue (VR) were examined using mass spectrometry and ultraviolet (u.v.) analysis. Chrysene was found to be highly reactive, producing dimers and alkylated chrysene materials. From feedstocks with 14 C chrysene and 14 C acenaphthylene as radio tracers, the distribution of polyaromatics in the pyrolysed heavy oil fractions were determined. In this series, acenaphthylene was found to be more reactive than chrysene in grafting reactions on heavy oil components. The radical quenching propensity of polyaromatic additives towards small radicals has been confirmed, supporting their anticoke effect when used at high concentration. Interestingly, the diluted polyaromatic tracers did not accumulate in the coke fractions. This result points to weak and reversible bonding reactions of the polyaromatic additives onto the heavy oil components.