Development of flow-type high-temperature and high-pressure NMR probe and clarification of chemical reactions in sub-and supercritical water

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The present research aims at the clarification of the mechanism of organic reactions in sub-and supercritical water by a newly developed high-temperature and high-pressure flow-through NMR probe. The first part of this research was the development of the probe equipped with a sample tube, flow line and the connections between them which should be endurable up to 500℃ and 50 MPa. In particular, the temperature control through the flow line was of inevitable importance for the correct determination of the rate constants of the reactions. We devised a temperature controlling system with three heaters along the flow line. Further, the large 1H signal of H_2O should be suppressed for the quantitative determination of the peak areas of the products and reactants whose chemical shifts were overlapped with the water signal. For the suppression, we used the binomial sequence pulse method which enabled the suppression of the water 1H signal by a factor of 400.With the above improvement to the NM … More R probe, we first determined the 1st-order rate constants of the Claisen rearrangement in subcritical water. The straight lines in the 1st-order plots for the decrease of allyl phenyl ether (APE) at several temperatures gave the rate constants of the rearrangement. The activation energy was evaluated to be 25.5 kJ mol-1 in the temperature range of 270-310℃.The decomposition of nitroethane in sub-and supercritical water was then investigated with the probe. The decrease of nitroethane as well as the time dependence of CH_3CHO, CH_3COOH, and CH_3CONH_2 was followed to determine the rate constants and elucidate the reaction mechanism. The reaction intermediates, aci-nitroethane and N-hydroxyacetamide were also detected. These observation indicated the three pathways of the nitroethane decomposition. The major pathway is the formation of CH_3CHO via aci-nitroethane (Nef reaction). The second decomposition pathway forms CH_3COOH, and CH_3CONH_2 via N-hydroxyacetamide and the third minor path generates CH_3CN directly from nitroethane, probably via a radical intermediate. The contribution of the 3rd path increases with increasing temperature.Thus, we successfully demonstrated that the developed high-temperature and high-pressure flow-NMR probe enabled us to determine the rate constants and clarify the reaction mechanism for the chemical reactions occurring in sub-and supercritical temperature. Less