Preparation Of Highly Dispersed Transition Metal In Mesoporous Molecular Sieve And Its Catalytic Performance

Mesoporous molecular sieves have large specific surface area and narrow pore size distribution, which overcome the pore size limits of microporous molecular in the macromolecular reaction and have extensive application value in the catalytic field. Silicon-based mesoporous molecular sieves have many great advantages, such as large specific surface area and pore volume, weak limitation of skeleton atoms. However, its amorphous pore wall can also cause a lot of disadvantages, such as poor thermal stability and weak acid strength. So, the silicon-based mesoporous molecular sieves were seldom applied to the catalytic reaction directly which is caused by the lack of active and acid sites. A lot of research has focused on the chemical modification and functionalization of silicon-based mesoporous molecular sieves and then they would be used as excellent catalysts. Their larger pore diameter is facilitated to the mass and heat transfer, and the large specific surface can greatly improve the number of active site in unit mass materials. Among them, metal modified mesoporous materials were widely used in petrochemical, energy and environmental protection due to their high catalytic activity. In order to improve the utilization of metal component and reduce the catalyst cost, more and more research has been focused on how to improve the dispersion of metal active sites and it has a very important significance.For the metal nanoparticle supported catalysts, the size and dispersion of metal particles is an important factor. However, the number surface unsaturated atoms will increase along with the decrease of metal particle size. Therefore, how to remain the high dispersion and high stability of supported metal nanoparticles is one of the frontier topics in the field of catalyst research. On the other hand, if the highly dispersed metal nanoparticles can load on the surface of mesoporous materials which incorporate heteroatom into the framework, the activity of catalyst will improve greatly.In this paper, we used different synthesis methods to prepare a variety of modified MCM-41 mesoporous materials with highly dispersed metal component. We also investigated many synthesis conditions, and the physical and chemical properties of modified samples were characterized by XRD, SEM, HRTEM, N2 adsorption-desorption, ICP, UV-vis, FT-IR, XPS, H2-TPD, NH3-TPD. In addition, the catalytic performance of different samples was test by the catalytic reaction, such as hydrodechlorination of chlorobenzene, total oxidation of toluene, and hydrocracking of residual oil. The main results are as following:1. Ni@MCM-41 materials in which highly dispersed nickel nanoparticles with an ordered distribution were successfully prepared by a new method. First, the metal ligand was directly implanted into the lattice structure of mesoporous molecular sieves by using the first order structure of the metal (M). Then, the metal iron Mn+ was reduced in situ from the framework of molecular sieve, so that the metal nanoparticles were highly dispersed on the surface of the carrier. The characterization results suggest that the Ni@MCM-41 materials possess highly ordered hexagonal mesostructure of MCM-41 and the highly dispersed nickel nanoparticles with a uniform size smaller than 20 nm are in an ordered configuration in MCM-41 matrix. At the same time, the formation mechanistic of the samples was described. There is a strong interaction between nanoparticles and the carrier. At low temperature, the synthesized samples had very high catalytic activity for the hydrodechlorination of chlorobenzene reaction, and the conversion of the chlorobenzene could achieve 98.2%,82.5% and 97.65%. The reaction product was only benzene and there was no obvious decrease in the catalytic activity after four runs.2. An excellent catalyst Pt@MCM-41 was successfully synthesized by crystal lattice locating method and pH adjusting, in which the platinum small nanoparticles highly dispersed in MCM-41 matrix with an ordered arrangement and uniform size. The reduced Pt atoms gathered in little nanoparticles (about 2nm) and be fixed in the specific sunken position from which these atoms came out. And the total oxidation of toluene could be achieved at 180℃ with the 1wt% metal content. There was no obvious decrease in the catalytic activity after 5 hours reaction, which attribute to the large number of metal active centers and strong interaction between the molecular sieve skeleton and platinum nanoparticles. The HRTEM diagram shows that the metal nanoparticles were still highly dispersed in matrix of MCM-41 after reaction. Koros-Nowak test proved that the rate of the reaction was not influenced by mass and heat transfer.3. By in situ synthesis and selective reduction method, a novel bifunctional mesoporous molecular sieve catalyst Pt@Fe-MCM-41 was prepared successfully. The catalyst contains highly dispersed metal active components and acid carrier with regular structure. The results showed that the platinum nanoparticles with uniform size were highly dispersed in the surface of matrix and the iron ions were tetrahedral coordinated and highly dispersed in the silica framework. The conversion of the residue was 87.8% and the selectivity to gasoline was 74.3% with the high metal content catalyst. The catalytic results showed that highly dispersed metal active sites had an obvious interaction with highly dispersed acidic sites.4. The highly dispersed bifunctional catalyst Pt@M-MCM-41 (M=Ni, Co) was prepared successfully via in-situ synthesis combined co-precipitation method. The catalyst sample has a very regular mesoporous structure with a large specific surface area (600-700 m2·g-1) and a uniform pore size distribution (3-4nm). The platinum nanoparticles with uniform size were highly dispersed in the internal surface of carrier and the Ni, Co ions were highly dispersed in the silica framework. The catalytic results illustrated that incorporated cobalt and nickel into the catalyst framework has a better promotion with the highly dispersed platinum nanoparticles.5. The metal modified catalyst Pt@Fe-Ni-MCM-41 was synthesized successfully by hydrothermal crystallization and selective reduction. The result sample has a relatively regular MCM-41 pore structure. The platinum nanoparticles were well dispersed in the internal surface of carrier and the Ni, Fe ions were tetrahedral coordinated in the silica framework. NH3-TPD test show that the catalyst has moderate strong acid and strong acid sites. In addition, the result sample presented an excellent activity in hydrocracking of residual oil.