Synthesis Of High Quality TS-1 Zeolites For Catalytic Application In Olefin Epoxidation

Catalytic oxidation systems based on TS-1 zeolite and H₂O₂ have been widely used in the epoxidation of olefin,ammoxidation of cyclohexanone,hydroxylation of phenol and other important industrial processes due to its nice features such as mild reaction conditions,easy operation and environmental benignity.The synthesis of TS-1 zeolite is usually performed under hydrothermal conditions and requires the use of large amounts of expensive tetrapropylammonium hydroxide（TPAOH）template.The isolated tetrahedral framework Ti species（Ti O₄）has been generally recognized as the main active site for various H₂O₂-mediated oxidation reactions.Due to the difficulty in matching the hydrolysis/condensation rates of the titanium and silicon sources,some undesirable Ti species like oligomeric octahedral coordination Ti and anatase Ti O₂ are commonly present in the resultant TS-1 zeolites.These extra-framework Ti species could adversely affect the activity,selectivity and stability of the catalyst,and lead to invalid decomposition of H₂O₂ and low utilization rate.To solve the above problems,great efforts have been continuously devoted to exploring and optimizing the synthesis methods and conditions for getting more efficient TS-1 zeolites with eligible morphology,composition and titanium species.Recent literature works demonstrated that the formation of non-framework titanium species could be effectively inhibited by using diol-based polymers containing titanium and silicon as precursors,thus providing a new effective way to synthesize TS-1 zeolites with higher framework titanium contents.Moreover,the addition of an appropriate amount of the second component in the initial sol-gel synthesis system may modulate the surface properties and the active center properties of TS-1 zeolites within a certain range,possibly leading to the formation of modified TS-1 catalysts with enhanced catalytic performance.According to the above situation,this thesis mainly focuses on the synthesis of high-quality TS-1 zeolite catalysts through the improvement and optimization of the polymer-precursor strategy.In addition,a series of tungsten-doped TS-1 zeolite catalysts were synthesized by steam-assisted crystallization method.The catalytic performance of the resultant zeolite catalysts were investigated by the epoxidation of1-hexene and propylene with H₂O₂ as oxidant.The physicochemical properties of the zeolite catalysts,including the morphology,porosity,state of titanium and tungsten species were characterized by a variety of characterization techniques,and were correlated with their catalytic performance for olefin epoxidation.The main results of the thesis are as follows:1.A series of anatase-free TS-1 zeolites were effectively synthesized by hydrothermal crystallization of a composite precursor composed of diol-based polymer（containing titanium and silicon）and tetrapropylammonium bromide（TPABr）.Addition of a certain amount of low-cost TPABr into the polymer precursor could keep high degree of Ti incorporation into the zeolitic framework when a lower concentration of tetrapropylammonium hydroxide（TPAOH）template is adopted in the following hydrothermal crystallization process.The coordination state of active titanium species and its efficiency of entering the framework can be effectively regulated.The condition-optimized TS-1 zeolite with smaller particle size and higher framework Ti content exhibits very high catalytic activity and selectivity for the epoxidation of 1-hexene with H₂O₂ as oxidant,showing a turnover frequency of 149h^-1 at the reaction temperature of 60°C,higher than the conventional TS-1 zeolite(109 h^-1)and those reported in literatures.The relatively small particle size of the resultant TS-1 crystals may enhance the accessibility of the catalytically active framework Ti species to reagents,and the absence of extra-framework Ti species like anatase Ti O₂ could avoid the ineffective decomposition of H₂O₂,thus bringing high activity and selectivity for H₂O₂-mediated olefin epoxidation.2.Tungsten doped nano-sized TS-1 zeolite aggregates（TS-1＿x W,x=W/Si ratio）were synthesized by steam-assisted crystallization（SAC）method.By optimizing the synthesis parameters such as the ratio of the initial gel and the addition amount of tungsten source,TS-1＿x W zeolite aggregates with high crystallinity and small particle size could be obtained.Compared with the traditional hydrothermal crystallization method,the SAC method can lead to the formation of W-doped TS-1zeolites with higher titanium content and tungsten content.The representative sample of TS-1＿0.05W zeolite shows a morphology of nano-sized polycrystalline aggregates（around 200 nm）,which are assembled from irregular small nanocrystals（10⁴0 nm）.The presence of some intercrystalline voids may create a portion of mesopores,leading to the relatively high specific surface area and large mesoporous volume in the resultant zeolites.The SAC-synthesized TS-1＿0.05W catalyst exhibited excellent catalytic activity for the epoxidation of propylene and 1-hexene.Under the tested conditions,the conversions of propylene and 1-hexene reached 97.5%and 49.2%,respectively,much higher than those of conventional TS-1 and other W-doped TS-1zeolites.The smaller particle size and the larger mesoporous volume of TS-1＿0.05W zeolite facilitate the diffusion and mass transfer of the reactants;the introduced W species may have positive role in adjusting the surface polarity of the zeolites,and may also act as assisted active sites to participate in the activation of reactants,thus leading to an enhanced catalytic performance for olefin epoxidation.