AIR OXIDATION OF TURKISH BEYPAZARI LIGNITE .3. CHANGE IN THE STRUCTURAL CHARACTERISTICS OF THE RESIDUE IN OXIDATION REACTIONS AT 150-DEGREES-C

Beypazari lignite was subsequently demineralized with HCl/HF and extra cted by pyridine under supercritical conditions. The extracted residue was oxidized in air at 150-degrees-C for up to 120 h in a ventilated oven. Elemental analysis, diffuse reflectance Fourier transform infrar ed (DRIFT), solid-state C-13 CP/MAS/TOSS NMR, and pyrolysis mass (Py-M S) spectroscopic techniques were used for the structural characterizat ion of the oxidized and unoxidized samples. It was found that the oxid ation of the extracted residue proceeded in two consecutive stages. Th e rate of oxidation, as measured by changes in the concentrations of c arboxyl and aliphatic CH2 and CH3 groups and in the O/H atomic ratio, was much faster in the initial than in the second stage. The removal o f 90.1% of the mineral matter and 39.7% of the soluble material from t he parent lignite facilitated the access of oxygen to the network stru cture which was relatively enriched in aromatic units by the supercrit ical fluid extraction. It was therefore easier to form aryl esters, al kyl esters, and anhydrides. Since aliphatic structures were relatively less in concentrations than aromatics, there was little change in the intensities of alkylbenzenes, alkylnaphthalenes, alkyldihydroxybenzen es, alkylphenols, and alkanes during oxidation. The higher intensities of CO2+ (m/z 44) and CH3COOH+ (m/z 60) molecular ions observed in Py- MS than CO+ (m/z 28) molecule ion indicated that more carboxyl-contain ing functional groups were formed by oxidation than carbonyl-containin g groups. Aromatic structures in the organic network were not affected during oxidation. The oxidation pathway of the extracted residue was considered to lie between of the oxidation pathways of the parent and the demineralized Beypazari lignites.