Pyrolysis in the presence of oxygen

Oxygen can be a participant in the pyrolysis in two different ways it can be present as atmospheric oxygen, or it may be already reacted with part of the sample in an autoxidation process by exposure of the sample to air and light over a period of time. 

Polymers exposed to air and light may contain oxidized groups such as peroxides ( OOR). The 0-0 bond is weak (30-50 kcal/mol) and, upon heating, dissociates to form free RO- radicals and radical chains. These radicals may influence the composition of the pyrolysis products. Autoxidation may take place in food, paint, rubber, etc. It is important, therefore, to consider this possibility when evaluating the composition of the pyrolysis products of a material that was exposed to air and light although the pyrolysis is performed in an inert gas. 

---

Natural rubber was also studied regarding pyrolysis in the presence of oxygen. Thermal oxidation of natural rubber is assumed always to be associated with scission, although photo-oxidation at low temperature may involve peroxide formation without scission. 

For the oxidative pyrolysis (pyrolysis in the presence of oxygen around 100° C to 150° C, namely below the ignition temperature), it was shown that in contrast to cellulose, substituted celluloses degrade oxidatively [23a]. This was evaluated in more depth for ethyl cellulose [51]. The reaction starts probably with the initiation step at the free aldehyde groups ... 

Some PBBs are considered possible precursors of toxic poly-brominated dibenzo-p-dioxins (12-138) and dibenzofurans (12-139), which are formed during combustion. The formation of degradation products of PBBs depends generally on the temperature, the amount of oxygen present and certain other factors. A study of the pyrolysis of the commercial mixture FireMaster BP-6 in the absence of oxygen at 600-900 °C showed the formation of bromobenzenes and lower brominated biphenyls, but polybromi-nated furans did not result. Pyrolysis in the presence of oxygen at 700-900 °C, however, yielded di- to heptabromodibenzofurans. 

Pyrolysis is a thermal degradation in the absence of oxygen. In this case it is carried out to determine the carbon black content of a compound and is supplemented as a matter of routine by pyrolysis in the presence of oxygen to determine the ash content in accordance with DIN 53585. 

Mercury-sensitized irradiation of 1,2,3-triphenylisoindole (65) in the presence of oxygen gives a peroxide (103). This peroxide is relatively stable compared with the peroxide (104) derived from similar oxidation of 1,3-diphenylisobenzofuran and can be reconverted to the isoindole (65) by pyrolysis or by treatment with zinc and acetic acid. Reduction of 103 under mild conditions affords o-dibenzoylbenzene (46) and aniline. Aerial oxidation of 47 gives 46 and methylamine, presumably via a peroxide intermediate similar to 103. °... 

Hexafluorinated Dewar benzene oxide 1 is prepared in 7% yield by the irradiation of hexafluorobenzene in the presence of oxygen,12 research undertaken in the hope of gaining access to the valence isomers perfluorobenzene oxide (3) and perfluorooxepin (4). Even gentle heating of 1 (50 C, 7d), however, leads only to perfluorocyclohexa-2,4-dien-l-one (2). Note that cyclohexadienone 2 is also the only product obtained from the pyrolysis of per-fluorobicyclo[2.2.0]hex-5-en-2-otie hydrate (see Section 5..3.2.1.). 

Another method for the determination of oxygen in coal involves reduction of the coal by pyrolysis in the presence of hydrogen, whereupon the oxygen is converted catalytically to water. However, the procedure is relatively complex and the catalyst may be poisoned by sulfur and by chlorine. 

The isothermal pyrolysis in the presence of air proceeds at a much faster rate and higher weight losses are obtained as compared to vacuum pyrolysis at the same temperature. The first order rate constant obtained is linearly related to the expression [%LOR + o-(% crystallinity)]//o with a degree of correlation r = 0.923, where a is the accessible surface fraction of the crystalline regions according to Tyler and Wooding [501], and / is the orientation factor. No correlation could be found with DP due to very rapid depolymerization. The fact that the rate is inversely proportional to the orientation and that it decreases with the increase in the thickness of the fibers indicates that the rate of the diffusion of the oxygen into the fibers controls the kinetics and that oxidation is the predominant process in air pyrolysis. 

Okamura and co-workers (18) have taken air-cured PCS polymer and, through pyrolysis in the presence of ammonia, prepared essentially carbon-free silicon oxynitride fibers (equation 13). However, if the PCS polymer fiber is cured by electron beam radiation (to prevent oxygen addition), the same ammonia pyrolysis conditions provide nearly stoichiometric quantities of silicon nitride fibers (equation 14). 

In addition to the initiations previously indicated, the traces of oxygen molecules, hydroperoxide side groups, and residual peroxide catalysts that are still present in a polymer structure may be sites for the pyrolysis initiation reaction. These initiations usually do not affect the final composition of the pyrolysates, but they may affect the temperature where the pyrolysis process starts. For example, pyrolysis in the presence of traces of air (oxygen) typically occurs at temperatures 50° C to 100° C lower than pyrolysis in pure He. 

Pyrolysis of synthetic polyisoprene in the presence of oxygen also is expected to be identical to that of natural rubber. Thermal oxidation of natural rubber is assumed always to be associated with scission, although photo-oxidation at low temperature may involve peroxide formation without scission. The effect of oxygen is to increase the reaction rate of scission and therefore to decrease the temperature where the scission starts. The oxidation may take place after the initial formation of a free radical that reacts with oxygen ... 

S.7.6.2.2 Pyrolysis Processes in the Presence of Oxygen Furnace Black Process... 

The chemical nature of the surface of carbon black is crucial to its applications-related behavior and in the first instance is a function of the manufacturing process. In addition to physically adsorbed organic substances, chemically combined surface oxygen is present on the surface, which is formed upon pyrolysis in the presence of sufficient oxygen and accounts for the acidity of gas blacks. Furnace blacks produced in oxygen-poor conditions with feedstocks with low sulfur contents have an excess of basic metal oxides on their surfaces and have neutral to alkaline properties. The content of physically and chemically bonded species is known as the volatile content, since it can be removed by heating to 950°C in the absence of air. 

Ethylene dichloride (EDC) is used to manufacture vinyl chloride monomer (VCM), which is one of the largest commodity chemicals produced in the world. EDC may be produced by the direct chlorination of ethylene or oxychlorination of ethylene in the presence of oxygen and hydrogen chloride. Pyrolysis of EDC produces VCM and an equal amount of hydrogen chloride as a co-product. This hydrogen chloride produced in the pyrolysis reactor is utilized by the oxychlorination process as one of the reactants. Therefore, the component processes of direct chlorination, EDC pyrolysis and oxychlorination are combined to develop a balanced process for the production of VCM with no net consumption or production of hydrogen chloride ... 

The effect of potassium in the form of potassium carbonate or potassium silicate on reduction of NO, on coal chars was also investigated [108-111]. The best materials were prepared by pyrolysis of coal at 1300 K with high KOH/coal ratio [108]. On these adsorbents, at temperature smaller than 473 K, physical adsorption is predominant while the true NO, reduction by char occurs at T> 473 K with formation of Nj and COj. The results indicated that a material with the high surface area should be used to promote adsorption of NO, and potassium remaining in chars catalyzes NO reduction in the presence of oxygen [109]. 

Photolysis of 4,5-diphenyl-l,2,3-selenadiazoIe in the presence of oxygen has been found to yield, besides diphenylacetylene in high yield, small amounts of tetraphenyl-l,4-diselenafulvene, benzil, and benzophenone. A series of 1,2,3-selenadiazoles substituted with arylsulfonyl moieties (139-141) have been prepared they could not be converted into the corresponding acetylenes by pyrolysis. Arylsulfonylacetylenes 142 and 143 could, however, be prepared by the photolysis of 1,2,3-selenadiazoles. ... 

One overall approach currently receiving much attention is the direct coupling of methane to yield higher hydrocarbons. This coupling may be carried out catalytically in the presence of oxygen or other oxidizing agent or by the pyrolysis of methane over a suitable catalyst. Should such a route prove successful the likely products will be rich in lower olefins. 

The formation of block copolymers was demonstrated by an increase in weight that did not take place either with homogeneous solution polymers or with heterogeneous solution polymers in the presence of oxygen. That these high-yield products were not mixtures of homopolymers was obvious from their solubility characteristics. However, this conclusion was also verified by solvent extraction and pyrolysis of the solvent fractions. The presence of more than one monomer in the pyrolyzate was demonstrated by characteristic gas-chromatographic retention times. 

---