Hydrothermal treatment and the oxygen functionalities in Wyodak coal

The effects of hydrothermal treatment (liquid water) at 350 °C on Argonne-supplied Wyodak coal previously ‘dried’ at 60 °C under 1 torr for 18 h were studied. Control experiments were conducted in the absence of added water, and with n-undecane replacing water. All runs were conducted in batch reactors with quartz liners for periods of 30 min or 5 h with added N 2 (500 psi cold), and product workups and all manipulations were conducted in an N 2 glove bag. A tar representing 5–7% of the starting coal was deposited on the quartz walls solely for hydrothermal conditions. In all cases the coal lost oxygen, the O-losses being in the order: undecane < no added water (30 min) = added water (30 min) = no added water (5 h) < added water (5 h). Thermal gravimetric and field ionization mass spectrometer (f.i.m.s.) analyses (ambient to 500 °C at 2.5 °C min −1) showed little difference between the experiments with undecane and those with no added water. By contrast, hydrothermal treatment yielded a volatiles fraction that was at the same time more volatile and had a greater weight average molecular weight. Phenol, catechol, and their C 1-, C 2-, and C 3-derivatives were the two most prominent families in the f.i.m.s.-volatiles spectra, and the f.i.m.s. responses showed that under added water conditions the catechols response was increased. With undecane in place of water, the precursors to the f.i.m.s.-volatile arenols engaged primarily in regressive chemistry. The exhaustive (5 h) hydrothermal treatment resulted in virtual elimination of both phenol and catechol signals, demonstrating extensive hydrolysis of the coal structure. The results are consistent with a scheme in which water promotes the production of the tar, removing it from the coal matrix. The organic medium, on the other hand, suppresses the diffusion of thermally generated fragments, and they become irreversibly reincorporated. The O-losses are due primarily to CO 2- and H 2O-evolution. The data suggest that the latter is largely due specifically to catechol dehydroxylation.