| Metal-organic frameworks (MOFs) are a novel class of porous functional materials formed by the self-assembly of metal ions and organic bridging ligands with periodic network structures.Compared with the traditional inorganic porous materials, MOF exhibit extremely abundant chemical and structural diversity, high specific surface area, porosity, as well as tunable properties.As a result of these outstanding properties, MOFs have attracted great attentions in heterogeneous catalysis. In this thesis, MOFs with good hydrothermal stability are used as carriers in the hydrogenation of benzene.Hydrogenation of benzene includes two different parts.Partial hydrogenation of benzene to cyclohexene was studied. Cyclohexene is an important intermediate material,as its reactive double bond facilitates facile conversion to value-added cyclohexanol, adipic acid, and caprolactam via typical olefin reactions. The production of cyclohexene by partial hydrogenation of benzene features exceptional superiority to processes including dehalogenation of cyclohexane halide, dehydration of cyclohexanol, and dehydrogenation of cyclohexane in terms of the low price of feedstock, the atomically economical character, along with the operational simplicity of the process. The Asahi Chemical Industry of Japan carried out manufacturing cyclohexene from the partial hydrogenation of benzene route firstly in 1988. In 1996, the Shenma Group Company of China introduced this technology, and the costs of the catalyst patent are very high. Thus, the development of efficient catalyst system is of great value industrially and academically. Moreover, the complete hydrogenation of benzene to cyclohexane is of significant importance in the petroleum and for environment protection. On one hand, cyclohexane is one of the key intermediates in the synthesis of Nylon-6 and Nylon-66. On the other hand, it is know that the presence of aromatics result into low cetane number and poor ignition quality of diesel. It increases the smoke point of jet fuel and also raises the emission of particulate matters. Thus, removal of aromatic compounds is valuable for environmental protection. In general, the reaction of benzene complete hydrogenation was carried out at high temperature (> 100℃) and/or high hydrogen pressure (> 5 MPa). Therefore, the development of an easily prepared, highly active and reusable catalyst that operates under mild conditions is a paramount challenge. It has been widely recognized that Ru is the most suitable metal for benzene hydrogenation. In this thesis, a series of novel Ru-B/MOF catalysts were prepared and tested for studying their performance in the hydrogenation of benzene. The effects of the MOFs, the modifiers, and the reaction conditions were studied in detail, and the structure-performance relationships were correlated. The experimental results are summaried as follows:1. Partial hydrogenation of benzene to cyclohexene over novel Ru-B/MOF catalystsA series of amorphous Ru-B/MOF catalysts were prepared by the impregnation-chemical reduction method. These materials were subsequently evaluated for the first time as catalysts for the partial hydrogenation of benzene to cyclohexene. The results for the initial hydrogenation rate (r0) for the different catalysts followed the trend of Ru-B/MIL-53(Al)> Ru-B/MIL-53(Al)-NH2> Ru-B/UIO-66(Zr)> Ru-B/UIO-66(Zr)-NH2> Ru-B/MIL-53(Cr)> Ru-B/MIL-101(Cr)>> Ru-B/MIL-100(Fe), whereas the initial selectivity for cyclohexene (S0) was of the order of Ru-B/MIL-53(Al) ≈ Ru-B/MIL-53(Cr)> Ru-B/UIO-66(Zr)-NH2> Ru-B/MIL-101(Cr)>Ru-B/MIL-53(A1)-NH2> Ru-B/UIO-66(Zr)≈ Ru-B/MIL-100(Fe). The Ru-B/MIL-53(Al) catalyst exhibited the highest r0 and S0 values of 23 mmol gcat-1 min-1 and 72%, respectively. The characterization results demonstrated that the Ru-B amorphous alloy nanoparticles were highly dispersed on MIL-53(Al) with the average diameter of 3.2 nm. In contrast, the Ru-B nanoparticles on MIL-100(Fe) had an average diameter of 46.6 nm. The smaller Ru-B nanoparticles not only provided more active sites for the hydrogenation to occur, but could also be beneficial to the formation of cyclohexene. The reaction conditions were further optimized for the Ru-B/MIL-53(Al) catalyst. At 180℃ under a H2 pressure of 5 MPa, a cyclohexene yield of 24% was obtained, highlighting the potential of MOFs as catalyst supports for the partial hydrogenation of benzene.2. Preparation of MIL-53(AlxCr1) with double central coordination metal ions and the partial hydrogenation of benzene to cyclohexene over Ru-B/MIL-53(AlxCr1) catalystsThe MIL-53(AlxCr1)(x=1,2,3,4,5) materials with two kinds of coordination metal ions were successfully prepared. TGA characterization revealed that the thermal stability of the MIL-53(AlxCr1) materials depends on the practical ratio between Al and Cr, and the higher content of Al, the better the thermal stability of the corresponding materials would be. Among the as-synthesized Ru-B/MIL-53(M) catalysts by the impregnation-chemical reduction method, the Ru-B/MIL-53(Al3Cr1) catalyst showed the highest selectivity and yield of cyclohexene, whereas the Ru-B/MIL-53(Cr) catalyst gave the lowest values under the same reaction conditions. The characterization results demonstrated that the Ru-B amorphous alloy nanoparticles were highly dispersed on MIL-53(Al3Cr1), showing the smaller average diameter and higher SRu compared with the Ru-B amorphous alloy nanoparticles on MIL-53(Cr). TGA characterization revealed that there was stronger interaction between Ru-B amorphous alloy nanoparticles and MIL-53(Al3Cr1) than MIL-53(Cr), which meant that the MOF has important influence on the dispersion of the nanoparticles. The smaller Ru-B nanoparticles not only provided more active sites for the hydrogenation to occur, but could also be beneficial to the formation of cyclohexene. The reaction conditions were further optimized for the Ru-B/MIL-53(Al3Cr1 catalyst. At 180℃, H2 pressure of 5 MPa, and using 100μl ethanolamine as the modifier, a cyclohexene yield of 29% was obtained.3. Complete hydrogenation of benzene over Ru-B/MOF catalysts under mild conditionsIn this part, three kinds of MOFs, including MIL-53(Al), MIL-53(Al1Cr1) and MIL-53(Cr), and three kinds of traditional inorganic materials, including SiO2, activated carbon (AC) and HMS, were chosen as catalysts carriers, and the corresponding supported amorphous Ru-B catalysts were prepared by the impregnation-chemical reduction method. These catalysts were subsequently evaluated for the complete hydrogenation of benzene to cyclohexane under mild conditions (30℃ and H2 pressure of 1 MPa). The values of TOFs of the catalysts followed the trend of Ru-B/MIL-53(Al,Cr1)> Ru-B/MIL-53(Cr)≈ Ru-B/MIL-53(Al)> Ru-B/HMS> Ru-B/SiO2> Ru-B/AC.The characterization results demonstrated that the Ru-B amorphous alloy nanoparticles were highly dispersed on MIL-53(Al1Cr1) with the average diameter of 3.2 nm.In contrast, the Ru-B nanoparticles on MIL-53(Cr) and MIL-53(Al) both had an average diameter of 3.7 nm.The smaller Ru-B nanoparticles could be more beneficial to the formation of cyclohexane. The Ru-B/MIL-53(Al1Cr1) catalyst also showed good catalytic performance for the hydrogenation of methyl substituted aromatic compounds. |
PDF Full Text Request