Gas-phase pyrolysis of heterocyclic compounds, part 1 and 2: flow pyrolysis and annulation reactions of some sulfur heterocycles: thiophene, benzo[b]thiophene, and dibenzothiophene. A product-oriented study

• Published in: Journal of analytical and applied pyrolysis 2002, Vol.62 (1), p.123-141
• Main Authors: Winkler, Jens K, Karow, Werner, Rademacher, Paul
• Format: Article
• Language: English
• Subjects: Annulation; Polycycles; Pyrolysis; Sulfur heterocycles
• ISSN: 0165-2370; 1873-250X
• DOI: 10.1016/S0165-2370(00)00218-7

Continuous flow pyrolysis (CFP) of thiophene (1), benzo[b]thiophene (2), and dibenzothiophene (3) was investigated. 1 and 2 were studied in the temperature range 500-1100°C. Up to about 20 products were identified by GC-MS and their temperature profiles determined. 3 was pyrolysed at 900°C. All together, more than 30 products were identified in the pyrolysates, the largest number was found for 2. A distinct tendency to form polycyclic aromatic hydrocarbons (PAHs) and sulfur-containing polycyclic hetarenes (PHAs), even with more than one sulfur atom, is observed. At temperatures below 800°C, mainly ‘oligomers’ of the educts arise by radical recombination reactions. Above 800°C, fragmentation of the skeleton becomes important, and products are generated from the starting molecules and their fragments. From 1, PHAs such as benzo[b]thiophene (2, 29%), dibenzothiophene (3, 11%), and thianthrene (22, 4%), and from 2, PHAs such as benzo[b]naphtho[2,1-d]thiophene (32, 13%) and [1]benzothieno[2,3-b][1]benzothiophene (21, 5%) are formed in highest amounts at 850–900°C. Minor amounts of highly annulated thiophenes such as triphenyleno[1,1a,1b,2-b,c,c]thiophene (28) and benzo[1,2-b:3,4-b′:6,5-b″]tri(benzo[b]thiophene) (36), are detected. Formation of PAHs such as benzene (4), naphthalene (5), phenanthrene (7), fluoranthene (8), and pyrene (9) increases rapidly above 850°C. Product formation is explained by annulation reactions of the educts with their fragments (PHAs) or by exclusive combination of fragments (PAHs). Benzo-annulation of thiophene increases the thermal stability.