Wet air and related metal ion-catalysed oxidation reactions of methylpyridines.

The Wet Air Oxidation process has considerable attractions for the disposal of toxic organic wastes. In this thesis, a fundamental study is made of the mechanism of oxidation under wet air and related conditions of a series of well-defined substances known to occur as components of industrial wastes, and which are known to present difficulties in the Wet Air Oxidation process. In the initial stages, the oxidation of a series of simple alkylpyridines, namely 2-, 3-, and 4-methylpyridines, has been studied under simulated Wet Air Oxidation plant conditions in a laboratory autoclave operating at 250°C and 250 atmospheres. The progress of the oxidation was followed by withdrawing samples at intervals and subjecting these to chromatographic analysis, using Gas Chromatography-Mass Spectrometry and High Performance Liquid Chromatography, so as to establish the nature of the oxidation products. In the autoclave oxidation of 2-, and 4-methylpyridine, a wide range of oxidation products was detected, including a number of compounds which appeared to be derived from the reactions of pyridylalkyl radicals formed from the parent substance, implying that a free radical mechanism was occurring under Wet Air Oxidation conditions. Under these conditions, 3-methylpyridine appeared to be more resistant to oxidation, the only significant oxidation product being the related aldehyde. The literature suggests that the formation of the hydroxyl radical (OH) under Wet Air Oxidation conditions may be responsible for the initiation of the above reactions, and thus the possibility of catalysis of the above systems by reagents known to generate hydroxyl radicals has been explored. The literature suggests that Fenton's reagent, which is a mixture of iron(ll) and hydrogen peroxide, provides a source of hydroxyl radicals. Thus, the oxidation of the methylpyridines using Fenton's reagent at ambient temperature and atmospheric pressure was carried out and it was also used as a catalyst in the autoclave oxidation reactions. The effectiveness of other metal ion/hydrogen peroxide mixtures was explored, e.g. involving iron(lll), copper(ll), copper(l), titanium(lll), and vanadium(IV), as there is considerable evidence from the literature of their involvement in oxidation chemistry. In all of the oxidation reactions investigated, both under autoclave conditions, and at room temperature, evidence of destructive oxidation of the heteroaromatic ring has been gained for all three methylpyridines. However, in addition to ring destruction products, a range of intermediate oxidation products was observed and similarities were found between those products formed in the autoclave and those reactions carried out in the laboratory. However, recent literature has questioned the formation of hydroxyl radicals by Fenton and related reagents, and so the Fenton catalysed oxidation of each of the methylpyridines was explored further. This was done by the incorporation of appropriate radical trapping agents and complexing agents such as 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and ethylenediaminetetraacetic acid (EDTA) respectively. In each of the oxidation reactions studied, attempts have been made to identify as many as possible of the products observed by comparison with known substances. However, it has been necessary to develop procedures for the preparation of some of these compounds, notably a range of dimeric structures derived from the simple alkylpyridines, e.g. dipyridylethenes, dipyridylethanes, and dipyridylmethanes.