The Study Of Preparation And Catalytic Performances Of Gold Catalysts In Functionalized Periodic Mesoporous Organosilica

Gold nanoparticles have been extensively investigated in optics, electronics,and catalysis etc.since the pioneer works of Haruta,in which gold particles were demonstrated to possess unusually high catalytic activities for CO oxidation at low temperature provided that the particle size is less than 5 nm.The potentialities of nanoparticles are mainly due to the quantum size effect,which is derived from a dramatic reduction of the number of free electrons.Therefore,how to control carefully the particle size,distribution and stability of the nanoparticles becomes very important.It is well known that the catalytic and adsorption properties are influenced by gold dispersion,structure and interaction between gold nanoparticles(GNPs) with the support as well as the type of supports.For supported gold catalysts,the agglomeration of Au particles is a major cause of catalyst deactivation.Usually, functional organic moieties containing polar head groups like -SH,-NH₂, etc.are incorporated into mesoporous silicas to stabilize nanoparticles and to prevent particles agglomeration.In this dissertation,GNPs stabilized with the disulfide-functionalized ionic liquids,thioether or tetra-sulfide groups immobilized in mesoporous silicates were synthesized.The main contents discussed were as follows:(1) The mesoporous organosilicas(PMO-2S-IL-SBA- 15, 4S-PMO-SBA-15 and SH-SBA-15) were synthesized via a one-pot condensation process involving the assembly of ionic liquid silsesquioxane organic precursor containing disulfide group, (1,4)-bis(triethoxysilyl)propane tetrasufide(BTESPTS) or 3-mercaptopyltrimethoxy silane(MPTS) and tetraethoxysilane in the presence of P123.Aqueous chloroaurate ions were in-situ reduced by the silanol groups presented on the channel surface of the mesoporous organosilicas to form GNPs that were subsequently captured by these sulfide group to obtain Au-containing mesoporous organosilicas (Au-PMO-2S-IL-SBA-15,Au-4S-PMO-SBA-15 and Au-SH-SBA-15). They were characterized by N₂ adsorption-desorption measurements, XRD,TEM,UV-Vis and silid ²⁹Si MAS-NMR.It was found that both marerials and catalysts have biggish surface area,pore volume and the decreased pore size.But the TEM observations indicated that the GNPs uniformly dispersed in the channel of Au-PMO-2S-IL-SBA-15,and the other two GNPs have aggregated in some grade.(2)The three nano-gold catalysts we prepeared were used as the catalyst in the epoxidation of styrene.The catalytic activity follows the order of Au-PMO-2S-IL-SBA-15＞Au-4S-PMO-SBA-15＞Au-SH-SBA-15.The reaction conditions of solvents,temperature, amount of oxidant,pH value,Au loading and amount of catalyst were investigated.It is found that 99.6%of styrene conversion with 93.3%of selectivity to styrene epoxide could be obtained provided that the reaction was conducted in acetonitrile at 333 K for 8h over 50 mg Au-PMO-2S-IL-SBA-15 with 2.0 wt%gold loading.Furthermore,the Au-PMO-2S-IL-SBA-15 could be conveniently recovered for recycled use without significant loss of catalytic activity and selectivity.(3) The catalytic performances of a series of mesoporous organosilica (PMO-2S-IL-SBA-15)-supported gold nanocatalysts with varying gold loading were studied in the solvent-water selective oxidation of benzyl alcohol by aqueous t-butyl hydroperoxide.It is found that 29.1%of benzyl alcohol conversion with 100%of selectivity to benzaldehyde could be obtained provided that the reaction was conducted at 353 K for 5h over the Au-PMO-2S-IL-SBA-15 with 4.5 wt%gold loading. Furthermore,the Au-PMO-2S-IL-SBA-15 could be conveniently recovered for recycled use without significant loss of catalytic activity and selectivity.