| Maleic anhydride (MA), manufactured by the oxidation of benzene, was the third-largest anhydride material. The hydrogenation of MA can produce succinic anhydride (SAH), y-butyrolactone (GBL),1,4-butandiol (BDO) and tetrahydrofuran (THF), etc. All these products are significant intermediate chemicals and solvents in oil processing, mechanical, war industries, farm chemicals, textile industries and synthetic leather. Recently, with an increase of steel production, the coke production has also been increased. Consequently, the production of benzene is significantly increased. Producing fine chemicals with high values via MA hydrogenation is regarded as a new route for comprehensive utilization of coal resources.Based on the former research of our group, the effect of support including different mono oxide, different zirconia polymorphs and ZrO2-SiO2 mixed oxides on the catalytic performance of Ni-based catalyst for liquid phase hydrogenation of MA was systematically studied. Firstly, the hydrogenation of MA in liquid phase was studied over Ni-based catalysts supported on ZrO2, SiO2 and Al2O3 respectively. The different synergetic effects between Ni°and supports with different surface properties were presumed. Among the tested supports, the synergetic effect between ZrO2 and metallic Ni contributed the hydrogenation of C=O over Ni/ZrO2 catalyts. Consequently, Ni/ZrO2 catalysts exhibited the highest selectivity to GBL. Secondly, the catalytic performance of Ni-based catalyst supported on monoclinic ZrO2 (m-ZrO2) and tetragonal ZrO2 (t-ZrO2) was investigated. The synergetic effect between ZrO2 and Ni°was speculated clearier. ZrO2-SiO2 mixed oxides were prepared and employed as supports for Ni-based catalysts, and the catalytic performance of those catalysts was studied in Chapter 5. All the results demonstrated that there are synergetic effect between Ni°and surface acidity of mixed oxide. The synergetic effect significantly improved the catalytic activity of Ni/ZrO2-SiO2 for C=O hydrogenation. To demonstrate the mechanism for improving metallic disperson and activating C=O in succinic anhydride clearly, the effect of the homogeneity of ZrO2-SiO2 mixed oxide on the metallic disperson and catalytic performance for liquid phase hydrogenation of maleic anhydride were investigated, and the effect of calcination temperature of mixed oxide was also studied. Characterization results proved that the more homogeneous the mixed oxide, the higher disperson of supported metallic Ni. Lewis acidity in mixed oxide was the active site for C=O activation. The synergetic effect between Ni°and Lewis acidity contributed the hydrogenation of C=O over Ni/ZrO2-SiO2 catalyst. The main results were obtained as follows:1. The hydrogenation of MA in liquid phase was studied over Ni-based catalysts supported on ZrO2, SiO2 and Al2O3 respectively. XRD、H2-TPR、H2-TPD and CO-TPR were employed to characterize catalyst. All these results demonstrated that support played an important effect on the metallic Ni disperson and selectivity of catalyst. Metallic Ni was highly dispersed in Al2O3, and Ni/Al2O3 catalyst exhibited the highest amount of chemisorption of H2. However, metallic Ni was existed as Ni crystallizes in Ni/SiO2 and Ni/ZrO2 catalyst. Among all of those catalysts, Ni/ZrO2 catalyst exhibited the highest catalytic acitivity for C=O hydrogenation, while Ni/Al2O3 catalyst exhibited the lowest. Ni/ZrO2 catalyst mainly produced GBL, while Ni/Al2O3 catalyst produced SAH. Over Ni/ZrO2 catalyst,100% conversion of MA and 79.2% selectivity to GBL was obtained after reaction at 483 K,5 MPa H2 pressure for 8 h.100% conversion of MA and 82.2% selectivity to SAH was obtained over Ni/Al2O3 catalyst, under the same reaction condition. Characterization of CO-TPR demonstrated that the differences in selectivities of catalysts resided in the different surface properties of supports. There was a synergistic effect existed between support and metallic Ni during the hydrogenation of MA. Due to special surface properties of ZrO2, it provided additional active sites for C=O adsorption. The synergistic effect between these active sites and metallic Ni contributed to the hydrogenation of C=O on the surface of Ni/ZrO2 catalyst. Then, Ni/ZrO2 catalyst exhibited higher selectivity for GBL. However, the Al2O3 support interacted strongly with C=O forming some stable intermediates. These intermediates strongly held on the surface of Ni/Al2O3 catalyst and acted as inhibiting entities. Consequently, Ni/Al2O3 catalyst maintained low activity for C=O hydrogenation and exhibited the lowest selectivity to GBL (highest selectivity to SAH).2. To explore the mechanism for the synergetic effect between Ni°and support, the liquid phase hydrogenation of MA was further carried out over Ni-based catalyst supported on different ZrO2 polymorphs. Monoclinic ZrO2 (m-ZrO2) supported catalyst exhibited the larger surface area and smaller Ni crystallize size. After reaction at 483 K,5 MPa H2 presure for 8 h, tetragonal ZrO2 (t-ZrO2) supported catalyst exhibited higher catalytic activity for C=O hydrogenation. The selectivity to GBL of Ni/t-ZrO2 was as high as 75.4%, while that of Ni/m-ZrO2 was 25.3%. So, the ZrO2 polymorphs strongly influenced the catalytic activity of Ni/ZrO2 catalyst for liquid phase hydrogenation of MA to GBL. The major reason is the different Zr4+ density of different ZrO2 polymorphs. Characterizations of NH3-TPD and Py-FTIR of ZrO2 demonstrated that t-ZrO2 exhibited higher surface acidity than m-ZrO2. Only Lewis acidity sites were found over t-ZrO2, while both Bronsted and Lewis acidity sites existed on m-ZrO2 surface. This phenomenon proved that the Zr4+ density of t-ZrO2, derived from their preferential planes, are largely higher than m-ZrO2. Zr4+ was the active site for adsorption of C=O. Then, Ni/t-ZrO2 catalyst, whose support had higher Zr4+ density, exhibited higher catalytic activity for MA hydrogenation to GBL.3. The catalytic performance of Ni/ZrO2-SiO2 catalyst for liquid phase hydrogenation of MA was investigated in Chapter 5. FT-IR, NH3-TPD, XRD, H2-TPR and H2-TPD were employed to characterize the support and catalyst. All these results demonstrated that the formation of Zr-O-Si hetero-linkages in ZrO2-SiO2 mixed oxide effectively improved the dispersion of metallic Ni. Further more, the generated new acidity sites in mixed oxide effectively activated the C=O in SAH. The synergetic effect between smaller Ni°and acidity sites significantly contributed the hydrogenation of C=O over Ni/ZrO2-SiO2 catalyst. The selectivity to GBL showed volcano-shaped curves with respect to ZrO2 content in mixed oxide. Among the catalysts tested, the Ni/10-ZrO2-SiO2 catalyst showed the highest yield of GBL. After reaction at 483 K,5 MPa H2 pressure for 8 h,82% yield of GBL was obtained over Ni/10-ZrO2-SiO2 catalyst.4. To speculate the mechanism for the improvement of disperson of metallic Ni and yield of GBL over more clearly, ZrO2-SiO2 mixed oxides with different homogeneity were prepared by varying the prehydrolysis time of TEOS. The catalytic performances of Ni/ZrO2-SiO2 catalysts are then evaluated by liquid phase hydrogenation of MA in Chapter 6. All the results proved that the more homogeneous the mixed oxide, i.e. the more amounts of Zr-O-Si hetero-linkages, the higher dispersion of metallic Ni. Lewis acidity formed in ZrO2-SiO2 mixed oxide was the active site for C=O adsorption. It can accept the lone electron pair forming "C---O---Lewis acidity" transient formation and the C=O was attacked easily by hydrogen adsorbed on the Ni particles.5. Based on the results of Chapter 6, the effect of heat treatment on the evolution of the amounts of Zr-O-Si hetro-linkages and the surface acidity of ZrO2-SiO2 mixed oxide was investigated. The influence of these evolutions on the disperson of metallic Ni and catalytic performace of Ni/ZrO2-SiO2 catalyst are then studied in Chapter 7. The results are in accordance with that of Chapter 6. The more homogeneous the mixed oxide, the higher dispersion of metallic Ni. Lewis acidity formed in ZrO2-SiO2 mixed oxide was the active site for C=O adsorption. The synergetic effect between Ni°and Lewis acidity effectively contributed to the hydrogenation of C=O over Ni/ZrO2-SiO2 catalyst. |
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