Research On The Oxidation Of Aromatic Compounds Over Metal-organic Frameworks Catalysts

Direct hydroxylation of aromatic compounds to the corresponding phenols is of great interest as green approaches for the high atom economy.However,the high activating energy of C-H bond in aromatic ring causes the low efficiency of the selective hydroxylation of the aromatic s,and the hydroxylated products usually undergo further oxidation which results in the low selectivity.In this paper,the metal-organic frameworks catalysts were used in the selective hydroxylation of benzene and naphthalene under mild conditions with H₂O₂ as an oxidant.MOF-5,HKUST-1,MOF-74 and MOF-177 were prepared by ultrasonic assisted synthesis or stirring method in room temperature.The MOFs were used as catalysts for the hyroxylation of benzene.The results showed that the catalytic activity of the four MOFs from high to low was: HKUST-1,MOF-177,MOF-5,MOF-74.According to the cost of raw materials,the yield,stability and catalytic activity of MOFs,HKUST-1 was used as a catalyst or support for the following catalytic reactions of benzene and naphthalene.Pretreatment of HKUST-1 catalyst with water significantly accelerated the catalytic hydroxylation of benzene.In a suitable condition,the corresponding oxygenates had a yield of 32.3%,and the selectivity to phenol and hydroquinone was 56.2% and 32.1%,respectively.The turnover frequency?TOF?was 34.7h-1.Comparatively,the product yield was only 4.5% over the original HKUST-1,and the TOF was 5.3h-1.Water pretreated HKUST-1 was used for the catalytic hydroxylation of benzene.The water molecules existing in the pores of the HKUST-1 or surrounding around the HKUST-1 interacting with benzene via ?HB impelled benzene to contact with the C u2+ sites,and accelerated the reaction of benzene with H₂O₂.Moreover,water treatment protected HKUST-1 from decomposition due to formation of a new oxidation mode.Therefore,the catalytic syste m in the presence of water opened a new door towards a facile and efficient preparation of phenol and hydroquinone.Four cupric salts,C u?CH3COO?2·H2O,CuSO4·5H2O,CuCl2·2H2O and Cu?NO3?2·3H2O were treated with benzenetricarboxylic acid respectively to the formation of metal-organic framework HKUST-1.The effect factors of cupric salts,synthetic methods,triethylamine on the yield,crystal structure,surface area and pore size,as well as crystal morphology of HKUST-1 were investigated.The yield and selectivity of benzene oxygenates were obviously different in the hydroxylation of benzene over the 12 kinds of HKUST-1 samples.The catalytic activity of small crystal particles was higher because of the more metal sites exposure.HK UST-1/UN were synthesized under ultrasound using C u?NO3?2·3H2O and benzenetricarboxylic acid as the raw materials,and used for hydroxylation of benzene.The corresponding oxygenates had a yield of 32.3 %,and the selectivity to phenol and hydroquinone was 54.3 % and 45.7 %,respectively.In all HKUST-1 samples,HKUST-1/UN showed a certain industrial application prospect because of higher catalytic activity and rapid synthesis.The Fe@HKUST-1?iron supported on HKUST-1?was formed owing to the catalytic activity of iron complexes in the catalytic reaction of benzene to phenol.The yield of Fe@HKUST-1 decreased with the increase of ferric chloride addition amount.Using copper sulfate and copper acetate as raw metal materials achieved more Fe@HKUST-1.The Fe@HKUST-1 yields were relatively low with cupric nitrate and cupric chloride as raw materials.The thermal stability and catalytic activity of Fe@HKUST-1 was enhanced by the addition of iron.Layered silicate as adsorbent could rapidly adsorb phenol and naphthol in aqueous solution.Some reaction parameters such as the catalytic activity,temperature,reaction time,which could affect the hydroxylation activity of the catalyst were investigated for benzene and naphthalene hydroxylation with layered silicate.The higher phenol and naphthol selectivity was achieved,but corresponding a lower benzene and naphthalene conversion.It is necessary to further optimize reaction conditions systematically.