Aqueous High-Temperature Chemistry. 24. Nitrogen-Containing Heterocycles in Supercritical Water at 460 .degree.C

Abstract

Seven nitrogen-containing heterocycles were chosen to complement a previous investigation (Energy Fuels 1994, 8, 990) of aqueous hydrogenation and denitrogenation of fossil fuel model compounds: 5,6,7,8-tetrahydroquinoline, pyrrole, 2,5-dimethylpyrrole, 1-methylindole, 2,4,6-trimethylpyridine, 1,2,3,4-tetrahydrocarbazole, and 4-propylpyridine. They were heated at 460 °C for 7 min and for 1 h in four different solvents: (i) cyclohexane, (ii) water, (iii) 15% aqueous formic acid, and (iv) 15% aqueous sodium formate. Two of the compounds, pyrrole and 2,5-dimethylpyrrole, were previously studied after reaction at 250 °C in 10% phosphoric acid for various periods of time (Energy Fuels 1992, 6, 450). They were included in this study for comparison of the results with different temperatures and solvents. The additional five compounds have not been studied previously under aquathermolysis conditions and complement an earlier study under supercritical aqueous conditions. 5,6,7,8-Tetrahydroquinoline (55) showed its highest conversion when reacted in 15% aqueous formic acid to give a wide product slate containing some denitrogenated and ring-opened products which are differentiable from the thermal control in cyclohexane. Pyrrole (6) showed its highest conversion when reacted in 15% aqueous formic acid to give mainly products of formylation and reduction; very little denitrogenation or ring opening was observed. 2,5-Dimethylpyrrole (23) was moderately reactive in 15% aqueous formic acid as solvent, showing mainly demethylation and also methyl group transfer to give some isomeric methylated derivatives; only traces of denitrogenation were observed. 1-Methylindole (62) was, again, most reactive in 15% aqueous formic acid predominantly hydrogenating to 1-methylindoline. 2,4,6-Trimethylpyridine (21) was only slightly reactive in all solvents. The highest conversion of 2,4,6-trimethylpyridine to demethylated products was observed in 15% aqueous formic acid. 1,2,3,4-Tetrahydrocarbazole (77) was also most reactive in 15% aqueous formic acid, undergoing mainly reduction after 7 min. Ring opening to form mainly indoles was the major route observed after 1 h. 4-Propylpyridine (32) showed little conversion in all solvents; no demethylation or ring opening was observed. In this study hydrogenation, ring opening and denitrogenation of N-heterocycles was again most significant under aqueous/CO reducing conditions in 15% aqueous formic acid. As in the previous study, denitrogenation via aqueous chemistry was not substantial.