Direct Hydroxylation of p-Xylene to 2,5-Xylenol with Hydroxylamine in Ionic Liquids/Molybdenum Catalytic System

Abstract

Direct hydroxylation of p-xylene to 2,5-xylenol with hydroxylamine was carried out in an ionic liquids/molybdenum catalytic system. High 2,5-xylenol selectivity (80-98 %) can be achieved with good p-xylene conversion (5.9-9.9 %) in this catalytic system. The recycling experiments suggested the ionic liquids/molybdenum catalytic system was stable enough to be recycled for the hydroxylation reaction.Keywords: Ionic liquids, Molybdenum, p-Xylene, Hydroxylamine, Hydroxylation, 2,5-Xylenol. Chemistry; Vol. 26, No. 22 (2014), [7527][7528][7529][7530][7531] hydroxylamine 16 . Interestingly, nearly 100 % selectivity was achieved for 2,5-xylenol. However, the yield of 2,5-xylenol was only around 3 % in the process. Asian Journal ofTo improve the yield of 2,5-xylenol, further study was carried out in the present work for p-xylene hydroxylation, using ionic liquids-containing media catalyzed by ammonium molybdate (Mo) catalyst (denoted as ILs/Mo catalytic system). A schematic representation of the reaction is given in Scheme-I. Various reaction parameters such as temperature, time, the amount of hydroxylamine and catalyst were optimized. And higher yield of 2,5-xylenol were obtained in this study. EXPERIMENTAL Preparation of ionic liquids: Ionic liquid [HSO3-bmim][CF3SO3] was prepared as follows 17,18 : equimolar N-methylimidazole and 1,4-butane sultone were charged into a 250 mL round-bottom flask and the mixture was stirred at 40 °C for 10 h. The solid zwitterion Determination of the Hammett acidity function of ionic liquids:The ionic liquids and the indicator 4-nitroaniline were dissolved in double distilled water at concentrations of 10 × 10 -3 mol/L and 1.5 × 10 -4 mol/L, respectively. Then their UVvisible spectra were recorded on a Varian Cary 300 spectrophotometer at room temperature.General procedure and detection method: The hydroxylation reaction was carried out in a 100 mL three-necked flask. Typically, hydroxylamine salts, catalyst and reaction media were charged into the flask. After stirring for about 15 min at 30 °C, p-xylene was introduced and the reaction mixture was heated at 90 °C for 4 h. The resulting mixture was cooled and neutralized to pH = 6-8 with a solution of sodium hydroxide. Then the obtained organic phase was extracted with ether and the concentrations of organic components were analyzed by a SP-3420 gas chromatography. p-xylene conversion (X Xylene ) and product selectivity (S i ) were calculated by the following eqns. 1 and 2:where ni is the number of carbon atoms in each molecule of i component, Wi is the weight percentage of i component and Mi is the molar mass of i component. For recycling of the ILs/Mo catalytic system, the same procedure was used as mentioned above. However, when the hydroxylation was stopped, the reaction mixture was cooled and extracted directly with ether without neutralization process. All the organic compounds including p-xylene, 2,5-xylenol and acetic acid, could be entirely extracted by ether for further analysis. The obtained aqueous mixture was evaporated ...