Synthesis Of Ordered Mesoporous Silica With Extra-Large Pores As Effective Supports For Gold Catalysis

Ordered mesoporous materials have attracted much attention due to their high surface area, large pore volume, well-defined mesostructures and pore size. They have been used in various fields, like adsorption, separation, catalysis, drug delivery, sensors, photonics and nanodevices. Precise tuning of their pore size distribution is a critical issue for scientists. The limitation of some desirable function of ordered mesoporous silica in a particular application due to small pore size, especially involving in large biological molecules, petroleum products. Variation of synthesis temperature and use of large-molecular-weight polymer templates have extended their pore size up to30nm. However, the preparation of ordered mesoporous materials with pore size larger than30nm generally relies on the use of external physical molds,(like polystyrene sphere, silica colloidal and porous alumina membranes) because of dimension limitations of supermolecular templates. So far, the realization of ordered mesoporous materials with periodic30-50nm pores from supermolecular templates is an unfilled gap between the largest ordered mesoporous materials and the smallest ordered macroporous materials, representing a significant synthetic challenge. Moreover, the extra-large mesopores (30-50nm) would have immediate interest for certain fundamental problems, such as diffusion and phase equilibrium in restricted nanoscopic geometries. Catalysis and large-molecule separation processes would also benefit from more uniform and open porous networks that provide optimal mass diffusion and improved efficiency. It provides much lighter future for mesoporous materials.This thesis is mainly related to the synthesis of ordered mesoporous silica with extra-large pores and the heterostructured transition metal oxide-mesoporous silica materials. They are used as effective supports for the gas phase benzyl alcohol selective oxidation reaction and CO oxidation. The thesis is divided into six chapters: the first chapter is the literature review for the research of this thesis; The second chapter is the synthesis of ordered mesoporous silica with extra large pores by using supermolecular as the template; The third chapter is the work about the catalytic activities and life-time tests for supported gold catalysts in the gas phase benzyl alcohol selective reaction. The gold catalysts obtained by using ordered mesoporous silicas with different large pore size as the support; The forth chapter is the surface restructuring of gold nanoparticles by small molecules and their different catalytic behaviors for alcohol oxidation; The fifth chapter is stabilizing gold clusters by heterostructured transition metal oxide-mesoporous silica as the support for enhanced catalytic activities in CO oxidation; The last chapter is summary and outlook. The main conclusions are listing as follows.(1) The second chapter reported that supermolecular templating allows tuning the pore size of ordered mesoporous silica in the once elusive range from30nm to more than60nm through simple control of synthetic variables (salt/F127concentration and hydrothermal temperature). We investigated the effect of synthesis temperature, HC1concentration, and the adding amount of KC1on the physiochemical properties of mesoporous silica. Dynamic light scattering (DLS) was applied to monitor the micellization behavior of F127. DLS experiments revealed that reducing the KC1/F127concentration leads to the structure-breaking of F127micelles and the formation of the unimers. It suggested that the reduced KC1/F127concentration in EP-FDU-12synthesis leads to more efficient1,3,5-trimethylbenzene (TMB) swelling and subsequent pore expansion.(2) The ordered mesoporous silicas with extra large pores were used as the supports for gold nanoparticles in gas phase benzyl alcohol selective oxidation. The supported gold catalysts showed high catalytic activities and stability. Gold nanoparticles supported on ordered mesoporous silica with62nm pore size showed very high activity (more than95%conversion at100%selectivity) and didn’t lose any activity after450h reaction. The turnover number was up to30,070,000according to the surface gold atoms. However, gold nanoparticles supported on ordered mesoporous silicas with smaller pore size (15nm,23nm) will lose their activities sharply in a short time (80h,130h). The coke deposition on the surface of gold active site was contributed to the loss of activities. The ordered mesoporous silica with open porous networks and extra large pore size as the gold support provide optimal mass diffusion and improved efficiency.(3) During the catalytic process, we found that liquid water moleculars adsorbed on the surface of gold nanoparticels can cause a surface sturcture change during the evaporation process, resulting in an obvious activity lose in gas phase benzyl alcohol selective oxidation. The gold nanoparticles with high activity displayed a truncated5-fold twinned spherical morphology, which exposed (111} and{100} facets. The {100} facet at the outer edge and the twinned defects in the non-perfect crystals are supposed to contribute to the high catalytic activity. In situ transmission electron misroscope (TEM) was conducted to observe the morphology change of gold nanoparticles before and after water treatment. After water treatment, the gold nanoparticles lose the edge{100} facet and become more perfect nanocrystals enclosed with{111} facet.(4) Gold clusters, after removal of the protecting ligands via thermal treatment, can be effectively stabilized on a heterostructured mesoporous support that is derived from the surface functionalization of ordered mesoporous silica with a transition metal oxide. A high dispersion of a transition-metal oxide nanophase (e.g., CuO, CO3O4) on mesoporous silica led to a greater stabilizing interaction with gold clusters than that obtained with pure silica or pure transition-metal oxide supports. The heterostructure supported gold clusters can preserve their small size (<2nm) without sintering even under thermal treatment at up to500℃in air. These nascent ultrasmall gold clusters can be used as effective catalysts, and they show enhanced activity in CO oxidation.