Cyclohexane Oxidation Over Supported Ionic Liquid-Metal Molecular Sieve(IL-M/ZSM-5) Catalysts

The oxidation of cyclohexane to cyclohexanone and cyclohexanol(KA-oil)has been gathered much attentionin the field of saturated C-H oxidation.Due to the high stability of cyclohexane and the high reactivity of KA oil,low conversion and selectivity of products are the urgent problems for this process.Therefore,the development of efficient and stable catalyst is great significant to obtain breakthrough.Ionic liquid is one kind of clean,efficient and mild solvent medium with unique properties,such as steam pressure,strong ability to dissolve polar substances,good thermal stability,high polarity and good electrical conductivity etc.ZSM-5 molecular sieve is widely used as a solid acid catalyst for hydrocarbon oxidation due to its unique crystal structure,suitable acidity,good shape selection and thermal stability.However,due to its small pore size,the large moleculeproducts formed in the pore cavity are not easy to diffuse and further result in deactivate by carbon deposition.Therefore,ionic liquid and transition metal supported on ZSM-5 simultaneouslywas proposed in this dissertation.The catalytic protocal was applied in the selective oxidation of cyclohexane,aiming atreducing the deep oxidation of products and preventing the formation of carbon deposition in the molecular sieve pore based on the ionic liquid layer,which results in the improvement of KA oil selectivity.The major researchhighlights are presented as follows:1.The IL-Co/ZSM-5 catalysts were prepared and characterized by XRD,BET,XPS,FT-IR,TGA,TG,DSC,FESEM and HRTEM etc.Followed,the catalytic performance of catalysts were tested in the catalytic oxidation of cyclohexane.The results showed that ionic liquids and metal cobalt(Co3O4)successfully supportedon the ZSM-5 zeolite.Compared with ZSM-5 and Co/ZSM-5 as catalyst,the C6mimHSO4-Co/ZSM-5 catalyst(1-hexy1-3-methyl imidazole salt-Co/ZSM-5,abbreviated as C6mimHSO4-Co/ZSM-5)with supported ionic liquid did not significantly improve the cyclohexane conversion rate,but the selectivity of KA oil reached higher than 90%,especially the selectivity of cyclohexanol increased significantly.The catalytic activity did not decrease significantly after 5 cycles.Mechanism studiesshowed that the ionic liquid had good solubility and strong polarity,which not only increased the active position,but also made the product more easily diffused without deep oxidation,resulting in improving the selectivity of KA oil.2.Effects of metal types on catalytic performance was investigated towards four series of ionic liquids(C6mimHSO4,C6mimH2PO4,C6mimNO3,C6mimTsO).The results showed that the order of the catalytic activity was as follows:Co(?)>Mn(?)>Zn(?)>Fe(?)>Ni(?).High-catalytic activities of Co and Mn were due to the presence of Co(?)and Mn(?)with high-valence,which were easier to accept electrons.While Zn(?),Fe(?)and Ni(?)were relatively difficult to accept any electrons.Compared with Co and Mn,the catalytic activity of divalent(Mn(?)?OMn(?))of Mn(?)was lower than that of Co(?).Therefore,the optimized metal for supporting on ZSM-5 cobalt.3.By using Co as the active metal,the influence of anion types of ionic liquids on catalytic performance(that is,ionic liquids composed of different anions with the same cation)was investigated.The investigated ionic liquids included five series of imidazoles with different carbon chain lengths(C4,C5,C6,C7,C8)as cations.The results showed that the order of its catalytic activity was as follows:[HSO4-1]>[H2PO4-1]>[NO3-1]>[TsO-1].The different catalytic activity could be related with the acidity and oxidation of anions.4.By using Co as the active metal,the influence of cation chain length of imidazoles on catalytic performance(that is,ionic liquids composed of different cation with the same anions)was investigated.The investigated inonic liquids included four series of different anions([HSO4-1],[H2PO4-1],[NO3-1],[TsO-1]).The resultsindicated that C5 and C6 chain lengths of imidazoles were proved to be more efficient than that of chain lengths it could be ascribed to their better solubility and structural hindrance.5.The catalytic mechanism of cyclohexane oxidation catalyzed by IL-Co/ZSM-5 was studied.Firstly,IL-modified ZSM-5-supported metal oxide catalysts presented excellent selectivity for KA oil and suitable activity in catalytic aerobic oxidaiton of cyclohexane.The anionic group of the IL not only improved and stabilized the catalytic activity of Co/ZSM-5,but also inhibited the deep oxidation of KA oil.Secondly,according to the electron paramagnetic resonance(EPR)spectroscopy and Haber-Weiss cycle,the detail mechanism of cyclohexane oxidation was proposed.The first step of the cyclohexane catalytic aerobic oxidation was initiated by the abstraction of a H atom,leading to the formation of a carbon-centered parent radical(C6H11·).Due to its high reactivity,it immediately reacted with O2 to generate cyclohexyl peroxyl radical(C6H11OO·).As the reaction progressed,cyclohexanone and cyclohexanol could be generated by rearrangement of two cyclohexyl peroxyl radicals(C6H11OO·).