Modulation Of Active Species And Mass Transfer Of TS-1 Zeolites And Their Catalytic Performance In Alkenes Epoxidation

Zeolite materials have been widely used in catalysis,gas absorption and separations and ion exchange,due to their regular pore system,large specific surface area,great adsorption and regulated active sites.As the demand of zeolite catalysts applications increasing,how to synthesize green and efficient zeolite catalysts have become a hot issue.The introduction of titanium atom into the SiO-Si framework of zeolites,endows pure silica framework with active sites,which paves the way for the use of microporous solid materials as shape-selective catalysts for oxidation reaction.TS-1 zeolite with MFI topology was successfully synthesized in 1983,which was the first time in the history of titanium silicate zeolite catalysts.The TS-1/H2O2 reaction system with TS-1 zeolite as catalyst and hydrogen peroxide(H2O2)as oxidant has the characteristics of high efficiency,mild and environmentally friendly,which meets the basic needs of modern catalytic industry,and can realize several important catalytic reactions such as olefin epoxidation,cyclohexanone amoxime,aromatic hydroxylation and so on.In the nearly 40 years,TS-1 zeolite has attracted more and more researchers to understand and improve its properties.TS-1 zeolite with microporous and relatively narrow aperture caused mass transfer limitation,limits its catalytic applications in macromolecule substrates.One of the methods to improve the above problem is to construct hierarchical structure,which p rovides the possibility for macromolecule substrate to enter into the pore channel and contact with the inner active sites more.Another method is to synthesize nanocrystalline TS-1 to increase the external specific surface area and make the external Ti active site of utilized more fully.On the other hand,titanium species as the active site of TS-1 zeolite,its structure and distribution also affect the catalytic activity of TS-1 wildly.In addition to the traditional framework tetra-coordination titanium species Ti(OSi)4,mononuclear octahedral-coordination titanium species,pentahedral-coordination titanium species and defect tetra-coordination titanium species all have higher catalytic activity.Therefore,the synthesis of TS-1 zeolites with new highly active titanium species is also one of the effective methods to construct efficiency TS-1catalysts.If the above strategies can be combined,it is expected to synthesize TS-1 zeolite with excellent catalytic performance.TS-1 is used as a catalyst for a single catalytic active site,it is also an effective method to introducing another metal and increasing the type of active sites to improve catalytic performance.In this paper,we focus on synthesizing TS-1 zeolite with a large specific surface area(nano-sized TS-1 and hierarchical TS-1),titanium species modulation and one-step synthesis TS-1 with double active site,and their catalytic activity in a series of olefin epoxidation reaction was investigated,the main work and achievements are as follows:1.Nanocrstal(20-40 nm)cumulated morphology TS-1 zeolite(A2d-M-TS-1)was synthesized by combining the synthesis system of titanium-rich concentrated gel with the method of long-time aging and microwave-assisted rapid crystallization.we found that A2d-M-TS-1 had a large external specific surface area and ratio(Sext = 225 m2/g;Sext/SBET = 0.445),and high-activity mononuclear Ti O6 rich on the surface.Compared with the samples synthesized by traditional hydrothermal method,A2d-M-TS-1 showed better catalytic performance in the epoxidation of macromolecular olefin.In addition,through the systematic study of the synthesis conditions,we found that under the conditions of rapid microwave crystallization,with the increase of the concentration of Ti in the synthesis system,the zeolites crystal nucleus and titanium precursor rapidly bind,and the growth of crystal nucleus was gradually blocked.Therefore,the TS-1 zeolite changed from regular morphology to nanocrystal aggregates,and the open mononuclear Ti O6 gradually formed on surface of the TS-1 zeolite.The synergistic regulation of morphology size and active center provides a new idea for the preparation of high ly active TS-1 zeolites.2.Tetrapropyl ammonium hydroxide(TPAOH)and ethylamine(EA)were used to treat microporous parent TS-1 zeolite by different methods,and hierarchical TS-1 zeolite with highly active octahedral-coordinated Ti sites were prepared via the sequential post-treatment method under hydrothermal condition(TS-1-(T-E)).Compared to the untreated conventional TS-1 parent and the post-treated other counterparts,the sample TS-1-(T-E)had extra-large external surface area and highly active octahedral-coordinated Ti species.Such catalyst exhibited an enhanced catalytic performance of 1-octene epoxidation,showed a turnover number up to(114),remarkably higher than that of untreated TS-1 parent(53).Systematic investigation of the post-treatment conditions revealed that,under hydrothermal condition,TPAOH treatment(first step)played an important role in the construction of hierarchical porosity,and EA treatment(second step)took a crucial part in the fabrication of highly active Ti species.3.The tungsten-doped TS-1 zeolite(WTS-1)was synthesized by a one-step hydrothermal method by modulating the addition state of tungsten precursor,aimed to construct a high efficiency double-active site TS-1 zeolite.After optimizing the synthesis parameters,tungsten chloride was selected as the best tungsten source,and its ethanol solution was treated with ultrasound to obtain the hypoxic tungstenoxygen precursor.The WTS-1 synthesized by using the above precursor has nanoscale size and double active sites,which shows higher catalytic activity in the epoxidation of 1-hexene,1-octene and allyl chloride.