Study On The Green Oxidation Of Alkylaromatics

The oxidation of alkylaromatics is of major industrial importance, and improving its efficiency and selectivity for the value-added products remains a challenge. Traditional approaches for the reaction were based on stoichiometric reagents (potassium hypermanganate) oxdiation, but the disadvantages of low products selectivity could not be avoided. In recent years, green oxidation of alkylaromatics is attracting continuous attention due to the development of oxidation technology.The research mainly concentrates on the oxidative coupling of nitrotoluenes, the oxidative dehydrogenation of nitro derivatives of bibenzyl and the aerobic selective oxidation of alkylaromatics to acids in ionic liquid.Trinitrotoluene (TNT) is oxidized to trinitrobenzyl chloride (TNBCl) and hexanitrobibenzyl (HNBB) with sodium hypochlorite. The yields of HNBB and TNBCl are both above80%. The solvent, the base, and available chlorine of the sodium hypochlorite are the three key factors of the oxidation. Acetic ether is replaced to THF as solvent for the production of HNBB. The mechanism is discussed and the function of the alcohol that it can connect base with TNT is studied.A simple and effective oxidative dehydrogenation of HNBB to hexanitrostilbene (HNS) is developed using a mixture of2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO) or N-hydroxyphthalimide (NHPI) and metal salts (FeCl2) as catalyst in DMSO under an atmospheric pressure of oxygen. The yield of HNS is80%with TEMPO, while that is70%with NHPI. A possible mechanism of this catalytic process is proposed, and the catalyst system TEMPO/FeCl2is successful used in the dehydrogenation of other nitro derivatives of bibenzyl. With the C-H and C-N bond dissociation energy of HNBB, the comparison of catalyst activities between TEMPO and NHPI is discussed. And the efficient and more environmentally friendly method is proved to be a better selection when compared to other previous methods, such as Shipp and Kaplan’s method.PEG1000-based functional dicationic acidic ionic liquid (PEG1000-DAIL) is used for the first time as the reaction solvent for NHPI/Co(OAc)2or N’, N", N’",-trihydroxyisocyanuric acid (THICA)/Co(OAc)2catalyzed aerobic oxidations of alkylaromatics to the corresponding acids. Several alkylaromatics are efficiently oxidized to their corresponding acids. With NHPI,99.9%conversion of toluene with99.5%selectivity for benzoic acid could be obtained, and the xylene is oxidized to the corresponding methylbenzoic acid. The oxidation of toluene is catalyzed by THICA/Co(OAc)2benzoic acid in99.0%selectivity at99.5%conversion. Using xylenes as the substrates is obtained the diacids, and nitrotoluene can be oxidized. This procedure includes many advantages such as mild reaction conditions, simplicity of operation, high yields, easy isolation of products by a simple decantation and excellent recyclability of the catalytic system.The novel and efficient catalytic systems THICA/dimethylglyoxime (DMG) and THICA/manganese binoxide (MNO2) are described for the selective oxidation of toluene derivatives with dioxygen in PEG1000-DAIL, respectively. With THICA/DMG system, the conversion of toluene is99.0%and the selectivity of benzoic acid is99.0%. When THICA/MnO2system is used, the conversion is99.8%and the selectivity is99.0%, respectively. Several toluene derivatives with electron-rich or electron-deficient substituted group are efficiently oxidized to corresponding acids under mild conditions. Based on the experimental observations, a possible mechanism is proposed. Both the catalyst and PEG1000-DAIL could be reused at least eight times without significantly decreasing the catalytic activity.