Synthesis Of Mesoporous Zeolite ZSM-5 And Its Catalytic Performance In The Hydrotreating Reactions

The increasingly more stringent environmental regulations and the growing demand for high quality transportation fuels, reduction of the aromatics and sulfur content in diesel fuels has received considerable attention. The conventional metal sulfide catalysts (NiMoS/y-Al2O3 and CoMoS/γ-Al2O3) are widely used in industry for hydroprocessing, but the deep saturation of aromatics over this type of catalysts is difficulty, even at high temperature and high pressure, because of their low hydrogenation activities. Therefore, developing highly active catalysts under low temperature and pressure is of great importance for the cost-effective production of clean diesel fuel. For this purpose, in this thesis, we applied the mesoporous zeolites to prepare highly active metal sulfide and nickel phosphide catalysts for the deep hydrogenation of aromatics. The support and the catalyst are characterized in detail by series techniques as follows:X-Ray diffraction (XRD), nitrogen physisorption, temperature-programmed desorption of ammonia (NH3-TPD), CO-chemisorption, thermogravimetric (TG), Fourier transform infrared spectrum (FT-IR), UV-vis diffuse reflectance spectroscopy (UV-vis DRS), Raman spectroscopy, hydrogen-temperature programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Based on the characterization results, the relationship of the catalytic performance with catalyst properties is obtained. The detailed studies are listed in the following chapters.Firstly, the development of the hydrotreating process in the last decades is briefly outlined. Secondly, the research progress of the various hydrotreating catalysts in terms of preparation, characterization, composition and structure is described in details. The main problems of the various hydrotreating catalysts in terms of catalytic activities are also detailed described. Thirdly, the various routes and synthetic methods of the mesoporous zeolites as well as the potential application of these new type porous materials in the industry are expounded. Finally, based on the literature review, in this thesis, we proposed that the mesoporous zeolites are used as supports to prepare high efficient metal sulfide and nickel phosphide catalysts, and investigated their catalytic performance in deep hydrogenation of phenanthrene.Considering the economic cost of hydrotreating catalyst, a series of highly crystalline mesoporous zeolite ZSM-5 (MZSM-5) with different zeolite morphology and Si/Al ratio is cost-effective synthesized from an aluminosilicate gel containing a random cationic copolymer that contained quaternary ammonium groups. XRD, N2-physisorption and NH3-TPD experiments are performed to obtain the structure, texture parameters and the acidities of the as-synthesized zeolite MZSM-5. In addition, the SEM and TEM experiments are carried out to obtain the morphology and pores structure of the MZSM-5 zeolite.After that, MZSM-5 supported metal sulfide catalysts (NiMoS/MZSM-5 and CoMoS/MZSM-5) are prepared and applied for the hydrogenation of the phenanthrene. For comparison, conventional y-alumina supported metal sulfide catalysts (NiMoS/γ-Al2O3 and CoMoS/γ-Al2O3) are also prepared. The catalyst structure, texture parameters and the acidities are studied by XRD, and N2-physisorption and NH3-STPD experiments, respectively. The surface properties of the supports and the state of the Mo species on the dried catalyst samples are characterized by FT-IR, UV-Vis DRS and Raman spectroscopy. The reduction behavior of Mo and Ni species in the catalyst precursors are studied by H2-TPR experiment. The morphology of MOS2 active phase and the Mo state in the active phase are investigated by TEM and XPS characterization, respectively. The results demonstrate that the Mo species are interacted weakly with the acidic hydroxyl and silica hydroxyl groups existed on the surface of MZSM-5, leading to the formation of easily reducible "free" MoO3 and polymolybdates on the dried NiMo/MZSM-5 catalyst. In addition, these Mo species are easily transformed into multi-stacked Type II MoS2 active phases during sulifidation, which should enhance the hydrogenation activity of the NiMoS/MZSM-5 catalyst. The activity test results show that NiMoS/MZSM-5 and CoMoS/MZSM-5 catalysts exhibit high activities in the deep hydrogenation of bulky aromatic phenanthrene under mild conditions (280 ℃ and 5 MPa), as comparison to conventional NiMoS/γ-Al2O3 and CoMoS/y-Al2O3 as well as Pd supported on MZSM-5 catalyst (Pd/MZSM-5). In addition, the intrinsic activity of the NiMoS/MZSM-5 catalyst is also much higher than the NiMoS/γ-Al2O3 catalyst under the absence of mass and heat transfer condition.Based on the above research results, the NiMoS/MZSM-5 catalysts with different Mo loading (3.0,6.0 and 12.0 wt.%) are prepared to further investigate the relationship between morphology of active phase and catalytic performance. XRD, N2-physisorption and NH3-TPD experiments are used to analyze the influence of Mo loading on the structure, texture parameters and acidity of the catalyst. The XPS and TEM techniques are employed to investigate the change in the sulfidation degree of Mo species and morphology of MoS2 active phase in the NiMoS/MZSM-5 catalysts with different Mo loadings. It is found that the average length and stacking of the MoS2 slabs decreased with the decrease of Mo loading, while the MoS2 dispersion increased with the decrease of Mo loading, leading to the formation of more Type II active phase and NiMoS phase on the NiMoS/MZSM-5 catalyst, resulting in the enhancement of hydrogenation activity.The catalytic performance of the MZSM-5 supported nickel phosphide catalyst (Ni2P/MZSM-5) in phenanthrene hydrogenation is also studied. For comparison, the ordered mesoporous molecular sieves (MCM-41) and high surface area of silica (SiO2) supported nickel phosphide catalysts (Ni2P/MCM-41 and Ni2P/SiO2) are also prepared. The texture parameters and acidities of the catalysts are characterized with N2-physisorption and NH3-TPD techniques, respectively. The interaction of the Ni with support is studied with IR spectroscopy and H2-TPR. The dispersion of active phase on the catalysts is estimated by combining the XRD, CO-Chemisorption and TEM experiments. The electron state of the Ni in the Ni2P active phase is analyzed by XPS and CO-adsorbed IR. The conclusion is that the Ni species are interacted strongly with the acidic hydroxyl on the MZSM-5, resulting in the formation of the highly dispersed Ni2P active phase with smaller size, which enhances the catalytic activity of the Ni2P/MZSM-5 catalyst, as comparison to Ni2P/MCM-41 and Ni2P/SiO2 catalysts. In addition, the electron-rich Ni in the Ni2P active phase on the Ni2P/MZSM-5 catalyst further improves its catalytic performance.The MZSM-5 supported-Ni2P catalyst (Ni2P-CA/MZSM-5) is further prepared by addition of the citric acid (CA) in the impregnation solution of the metal salt. The catalytic performance of the Ni2P-CA/MZSM-5 is investigated in the phenanthrene hydrogenation, and the influence of CA content on the catalytic activity of the catalyst is also examined. The structure, texture parameters and the acidities of the catalyst are characterized by XRD, N2-physisorption and NH3-TPD, respectively. The coordination states of the Ni species of the dried and the calcined catalyst samples are analyzed by FT-IR and UV-Vis DRS techniques, and the electron valence state of Ni and P in the catalyst is analyzed by XPS technique. The dispersion and particle size of the Ni2P active phase are obtained by TEM, and further examined by CO-chemisorption. The characterization results show that CA could coordinate with metal Ni, and change the coordination state of metal Ni after the catalyst calcination. In this manner, the isolated tetrahedral Ni species in strong interaction with the support could be formed, resulting in the formation of highly dispersed M2P active phase with smaller size during reduction. The phenanthrene hydrogenation results show tha the activity of the catalyst is increased with usage amount of CA in catalyst preparation. When-the mole ratio of Ni/CA is 3, the obtained catalyst shows the highest activity.