| Ordered mesoporous materials as a new type of nanometer material have attracted extensive attention due to their highly uniform channel, large surface area and pore volume, narrow pore-size distribution, and tunable pore size and acidity, which make them possessing much more excellent properties for the use in macromolecule catalytic cracking, host-guest assembly, adsorption and separation, nanoreactor, optics and magnetism. However, mesoporous materials reported up to date generally show poor stability, relatively low acidity, and absent catalytic active sites, which severely hinder their practical application in numerous catalytic reactions. A large number of studies have shown that the introduction of metal heteroatoms into mesoporous walls could modify the physical and chemical properties of mesoporous materials, which could be attributed to the geometric and electronic effect of heteroatoms. As a result, highly ordered mesoporous materials with extremely high stability and large surface area could be synthesized by the selectively incorporating heteroatoms into mesoporous framework of silica or alumina. In addition, the highly homogeneous incorporation of metal heteroatoms and the formation of metal-O-Al bonding could be advantageous for the generation of acidic centers or catalytic active sites, and their content could be adjusted by changing the incorporation method and the amount of introduced metal heteroatoms. Therefore, how to develop a facile and easy to repeat synthesis strategy to effectively incorporate metal heteroatoms into mesoporous framework of silica or alumina to improve the mesostructural ordering, enhance the stability, and increase the surface area and pore volume, has become a trend to prepare functionalized mesoporous materials and transition-metal-doped mesoporous catalysts, taking advantage of both the properties of ordered mesoporous materials and the enhanced catalytic activity of transition metal.Herein, we aimed to design a new and controllable approach to prepare highly ordered mesoporous materials with highly ordered mesostructure, high stability and tunable acidity sites by selectively introducing metal heteroatoms into mesoporous framework. Mesoporous silica and alumina were respectively selected as the research object to investigate the effect of metal heteroatoms introduced on structural ordering, stability, acidity, and catalytic activity of resultant mesoporous materials. By improving the understanding of the interaction between inorganic species(including Si, Al, and metal heteroatom species) and organic template micelle molecules, the acting mechanisms of metal heteroatoms existing within mesoporous framework on mesostructural modification, skeleton support, and porous walls properties(such as the coordination state of inorganic species and acidic sites) were explained. Based on various characterization techniques, the effect of metal heteroatoms precursor type, introducing way, and introducing amount on the structure, wall properties and stability of mesoporous materials were comprehensively studied, which remarkably affected the generation of catalytic active centers and distribution. We have achieved the following results:(1) As we all known, it is difficult to directly introduce Al into the siliceous skeleton of SBA-15 because Al3+ only exists in aqueous cationic form under strongly acidic condition during the synthesis of SBA-15. As a result, SBA-15 prepared with the traditional method usually exhibited weak acidity and low hydrothermal stability under steam conditions at high temperature. To improve the hydrothermal stability and acidity of SBA-15, we proposed one-step hydrothermal grafting approach, and successfully synthesized highly ordered mesoporous Al-SBA-15 materials with high aluminum content and highly hydrothermal stability under the weak acid medium produced only by the hydrolysis of inorganic aluminum salt. By selectively introducing inorganic aluminum salt in the synthesis system, and further optimizing hydrothermal treatment temperature and the amount of introduced inorganic aluminum salt, the hydrolysis of Al ion and the condensation of silicon hydroxyl(Si-OH) species within the mesoporous walls could be efficiently controlled to adjust the relative amount of Al-OH and Si-OH groups and their interaction. As a result, the ordered mesostructure of Al-SBA-15 could be assembly at the mesoscale and the coordination status/the acidic sites could be controlled at the atomic scale. The characteristic results from XRD, N2 adsorption, TEM, EDX, SEM, FTIR, and NMR techniques showed that when aluminium nitrate was used as the aluminum precursor, and hydrothermal treatment temperature and initial molar ratio of silicate to aluminium were adjusted as 140 ℃ and 10, respectively, the synthesized sample exhibited highly ordered hexagonal mesostructure and extremely high hydrothermal stability. The highly ordered mesostructure can be maintained even after steam treatment at 800 ℃ for 2 h with decreasing only by 18.82 % and 16.67 % in the Brunauer-Emmett-Teller surface area and total pore volume, respectively. Moreover, the hydrothermal grafting process at 140 ℃ was advantageous for the homogeneous incorporation of Al into the silica mesoporous framework to form the Si-OH-Al bond, leading to the production of Br?nsted and Lewis acid sites with relatively strong acidity on the mesoporous walls. The reaction results of isopropyl benzene catalytic cracking further manifested that the resultant Al-SBA-15 prepared by our method showed outstanding activity and stability.(2) Compared to silica mesoporous materials, mesoporous alumina is more popular in catalysis area for its broad applications as industrial catalysts and catalyst supports employed in petroleum refinement, automobile emission control, and others. Unfortunately, typical recipes for the preparation of ordered mesoporous silica do not work effectively for the synthesis of alumina analogues, due to the fast hydrolysis and condensation rates of aluminum precursor and phase transitions accompanied by the thermal breakdown of the structural integrity during the calcination process to remove organic templates. As a result, mesoporous alumina prepared by the traditional soft-template method exhibited disordered wormhole-like mesostructure, low surface area and pore volume, and poor thermal stability. Based on the improving understand of synthesis mechanism for ordered mesoprous alumina and the hydrolytic process of aluminum alkoxide, we proposed an evaporation-induced triconstituent cooperative co-assembly method, and successfully synthesized a series of highly ordered mesoporous cobalt-alumina catalysts(denoted as OMCA-x) with a variety of nAl/nCo ratios, which were applied to the liquid phase selective oxidation of styrene. The characteristic results from XRD, N2 adsorption, TEM, EDX, NMR, UV-vis, XPS, and H2-TPR confirmed that using our method, almost all Co species have been highly homogeneously incorporated into alumina mesoporous framework at the atomic level. The formation of framework Co-O-Al bond can effectively increase the relative population of tetra-coordinated and penta-coordinated framework aluminum species. Compared with mesoporous alumina prepared without introducing Co species during the synthesis, the resultant OMCA-x catalysts exhibited much more highly ordered 2D hexagonal mesostructure, narrower pore-size distribution, and higher BET surface area and pore volume. The catalytic results further confirmed that catalysts OMCA-x displayed much higher catalytic activity and selectivity to styrene oxide than those of Co/Al2O3 prepared by the traditional incipient wetness impregnation method with the mesoporous alumina as support. More significantly, the strong interaction between the cobalt and aluminum species in the mesoporous framework can effectively inhibit the leaching of active cobalt species. As a result, catalysts OMCA-x showed excellent recycling stability, without significant loss of activity and selectivity within three successive runs. Taking catalyst OMCA-10 for example, the highly ordered mesostructure can be well maintained even after the third epoxidation of styrene with only a 5.8 % decrease in its BET surface area, and the retained content of cobalt species in the reused catalyst was as high as 6.2 wt%, with only a 1.59 wt% decrease compared to the as-prepared OMCA-10.(3) By correlating the cobalt state and mesotructural properties of cobalt-alumina catalysts with activity and selectivity for styrene oxide, we believed that a synergetic effect of the highly homogeneously dispersed Co2+ in the tetrahedral position and the amount of Ia type Al-OH sites resulted in the remarkably high conversion of styrene and epoxide productivity over catalysts OMCA-x. The mechanism of catalytic epoxidation of styrene with O2 as oxidant over OMCA-x was not completely understood in our present work; however, we proposed a tentative mechanism. The four-coordinated Co2+ cations presenting at surface or near-surface of mesopores in catalysts OMCA-x can firstly bind and activate oxygen to form Co(III)-(O2-) species, which undergo reactions to generate peroxo and superoxo active oxygen species with a radical nature. The generated radical-type active oxygens could be easily received by solvent 1,4-dioxane molecules coordinated to the cobalt cations through oxygen atoms, and further be transferred to active sites Ia type Al-OH species to create the superficial hydroperoxides(Al-OOH), at which the styrene molecules are selectively oxidized to styrene oxide. While, the reactant styrene molecules reacted directly with Co(III)-OO˙ superoxo type complex rather than superficial Al-OOH are mainly oxidized into benzaldehyde with a small quantity of styrene oxide. The catalytic performance of OMCA-10 catalyst was compared with the earlier reported containing-cobalt heterogeneous catalysts for epoxidation of styrene using O2 as oxidant without adding any initiator. Interestingly, catalyst OMCA-10 showed the relatively higher TOF value for catalytic epoxide production or comparable to the earlier reports, and the styrene conversion and yield of styrene oxide over OMCA-10 were better than those of catalysts reported. The comparison results further corroborated that catalyst OMCA-10 prepared with our method acted as an efficient catalyst for epoxidation of styrene using O2 as oxidant. Our achievements provided new contributions to understanding the preparation of active transition-metal-doped ordered mesoporous alumina with high stability via a facile and controlled method, taking advantage of both the properties of mesoporous alumina and the enhanced catalytic activity of transition metal, which may further extend the catalytic application for various reactions in petroleum and petrochemical industry. |
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