| Zeolites have been widely used in catalysis,ion-exchange,adsorption and separation and other fields due to their high specific surface area,uniform microporous structure and tunable compositions.Among them,the successful synthesis of MFI-type titanosilicate zeolites（TS-1）with highly catalytic framework Ti and hydrophobic characteristics has been considered as a breakthrough in the field of green chemistry.However,restricted by the small pore size（0.54 nm in pore width）of the microporous structures,conventional microporous TS-1 materials show limited accessibility to the active titanium species located within the frameworks especially for bulky molecules.In order to tackle these problems,two major strategies,i.e.,decreasing zeolite particle sizes to nanoscale（nanozeolites）and introducing auxiliary mesoporous structures（hierarchical zeolites）,have been developed and great progresses have been made in recent years.Unfortunately,synthesis of nanozeolites often suffers from the low solid yields and the consumption of a large amount of microporous structure-directing agents（SDAs）,greatly limiting its potential application.However,additional mesopore templates are necessarily adopted in the conventional complex synthesis process of hierarchically structured zeolites（HSZs）.A novel,green and highly efficient synthetic strategy for preparation of high-surface-area TS-1 material is highly desired.Therefore,in this thesis,we focus on the design,synthesis and catalytic properties of newly structured TS-1 materials based on the dry-gel steam-assisted crystallization（SAC）and following akali-etching process.The main results are outlined as follows:（1）By a simple sequent process of dry-gel steam-assisted crystallization process and following top-down alkali-etching treatment,hierarchically structured TS-1nanozeolites（nanoTS-1D）with abundant micro/mesopores have been synthesized for the first time.The effects of alkali-etching process on the micro/mesoporous structure,morphology and catalytic performance were investigated.The resultant nanocatalyst featured with nanoscale particle sizes and hierarchically micro/mesoporous structures possesses remarkably high specific surface area of 606 m2g-1 and pore volume of 0.86cm3g-1.In the epoxidation of 1-hexene,the conversion of 1-hexene in 4 h was 40.9%over nanoTS-1D,which increased by 108%compared to 19.7%of conventional microporous TS-1（microTS-1）.And the high stability of nanoTS-1D was verified by the 6 recycling-regeneration tests.In the epoxidation of cyclohexene,the conversion of cyclohexene in 2 h over nanoTS-1D achieved 15.6%,which increased by 260%compared to 4.3%of microTS-1.Meanwhile,nanoTS-1D also showed a high conversion of 23.6%in the hydroxylation of aromatics,which was close to 20.8%of microTS-1.Unfortunately,at present it is unsuccessful when micrometer-sized TS-1zeolite was adopted as precursors for preparation of TS-1 HSZs by the same post-synthesis alkali-etching procedure.The resultant materials demonstrated surface corrosion,but not the formation of interconnected mesoporous structures.Based on these phenomena,a new“crystal size mediated alkali-etching”mechanism is explored.（2）PtSn-incorporated TS-1-based nanocatalysts had been prepared by the incipient wetness impregnation method and their catalytic performance on propane dehydrogenation reactions had been characterized.Research interests were focused on the effect of Si/Ti molar ratios,alkali-etching process and metal pre-treatment conditions on the materials crystallinity,micro-/mesoporous structures and catalytic performance on propane dehydrogenation reactions.The results proved that all nanometer-sized TS-1-based catalysts exhibited high reactivity with anti-deactivation ability and the Ti content had no significant influence on their catalytic performances.Typically,their initial conversions were above 53%with the deactivation rate constant（Kd）of about 0.008 h-1;In contrast,the initial conversion for microTS-1 was only 31.2%with Kd as high as 0.073 h-1.Moreover,alkali-etched nanoTS-1D-based catalyst exhibited an even higher conversion of 55.6%and maintained at 51.7%after 7.5 h reaction.It is obvious that the high specific surface area and rich porous structure of nanoTS-1 and nano TS-1D can effectively promote the dispersion of active Pt-Sn species,shorten the molecular diffusion paths.As a result,the synthesized TS-1-based nanocatalysts achieved the improved high reactivity,anti-deactivation ability and capability to accommodate coke formed in the reaction. |
