A Study On Novel Synthesis Methods For Titanium Silicalite-1 (TS-1) And Their Catalytic Properties

As an environment benign catalyst, the successful synthesis of titanium silicalite molecular sieve TS-1 has been considered to be a milestone in zeolite catalysis in 1980^th because of its unique catalytic performance in oxidation reaction involving H₂O₂ as an oxidant. However, the application of TS-1 zeolite in the industry was hindered seriously by many defects of the classical synthesis such as the high cost of the raw material and the severe preparation conditions. In last two decades, many improvements have been made, which include decreasing the synthsis costs, enhancing the preparing repetition and stability, decreasing and removing anatase TiO₂ and expanding its industrial application.On the base of the previous achievements, the dissertation was focused on the relationship between the structure and catalytic performance of TS-1 zeolite. A series of new routes for synthesizing TS-1 zeolite were proposed in cheaper ways, such as using new template, cheaper raw materials. At the same time, the mesoporous titanosilicate was been tentatively investigated. The studies on the TS-1 zeolite are of academic and practical values concerning in the industrial application of TS-1 zeolite. The main contents of the dissertation are as follows:In the first part, a lower cost method for synthesizing of TS-1 zeolite was proposed using lower template concentration during gelation process and higher template concentration during crystallization process using tetrapropylammonium hydroxide (TPAOH) as the template. The synthesized TS-1 zeolite exhibited high catalytic activity (phenol conversion was 20.6%) in the hydroxylation of phenol with aqueous hydrogen peroxide in which template concentration was about 8% during gelation process and about 20.5% during crystallization process. Compared to the conventional synthesis method, the cost of TS-1 zeolite synthesized using this new method was lower. To improve the catalytic performance of TS-1 zeolite, a new modification was investigated. That is, as-synthesied TS-1 zeolite is pretreated before calcination with aqueous solution of HNO₃ under 120℃ by oil bath under vigorous stirring for 18 h. UV-Vis spectra verfied that the octahedral Ti species were selectivelyremoved following such a acid treatment procedure, which improves about 10% catalytic activity for TS-1 zeolite with Si/Ti of 30 and 40 in hydroxylation and 1-hexene oxidation than untreated sample. Nevertheless, extraframework Ti species, both octahedral and anatase-like, still remained to a certain level for the samples obtained from the precursors with Si/Ti of 20, because they contained too high a concentration of octahedral Ti. This indicates that once the anatase is formed within the materials upon the calcination, it withstands the acid extraction and is hardly removed.In the second part, the inorganic SiO₂-TiO₂ precursor having Si-O-Ti bonds similar to those present in the titanium silicalite-1 (TS-1) was prepared through spontaneous dispersion of Ti(SO₄)₂ on amorphous porous silica by thermal treatment. The precursor was used as the silica and titanium sources to synthesize TS-1 zeolite using TPAOH as the template under hydrothermal crystallization conditions. The catalytic activity of TS-1 zeolite for the hydroxylation of phenol with dilute hydrogen peroxide showed that the treatment temperature of the SiO₂-TiO₂ precursor obviously influenced the catalyst performance, and the suitable treatment temperature was about 450℃.In the third part, the titanosilicate of TS-1 zeolite is skillfully hydrothermally synthesized from an inorganic silica source and a cheaper structure-directing agent (SDA) of hexamethyleneimine (HMI) with the assistant of active seeds of TS-1 precursor in absence of alkali metal ions. The adding of a precursor gel containing the primary and secondary building units of MFI structure is the key step of the synthesis. The nature of precursor and the preparation conditions have great influence on the incorporation of titanium into the framework, and its effect in this system has also been investigated in detail. Other synthesis parameters such as silica sources, crystallization time, Si/B ratio, H₂O/Si ratio, HMI/Si ratio have been discussed. It is revealed that the crystallinity, crystal size, amount of Ti incorporated in framework and the catalytic properties of the samples were affected greatly by the synthesis conditions.In the fourth part, TS-1 zeolite has been synthesized successfully by a low-costdry gel conversion method from tetraethoxysilane (TEOS) and fumed silica sources, tetrabutylorthotitanate (TBOT) as titanium source and TPAOH as the template. The influence of various factors, such as crystallization time, silica sources, and gel compositions on the formation of TS-1 zeolite has been thoroughly investigated. Some physical and chemical properties and the structure of product were characterized by XRD, FT-IR, UV-Vis, TG/DTG, SEM and ICP-AES techniques. In addition, the adding of the seed and its nature has great influence on the synthesis of TS-1 zeolite. The amount of TPAOH has been greatly reduced in the DGC method by further adding the active seed. The results indicated that TS-1 zeolite synthesized by dry gel conversion technique exhibited higher activity than in the epoxidation of 1-hexene with H₂O₂ than hydrothermally synthesized one because of a much smaller crystal size when the same silica source was used.In the last part, a crystalline MFI type titanosilicate showing mesoporous characteristics has been synthesized by crystallizing the synthesis gel with the assistant of mesoporous carbon (CMK-3) using a dry gel conversion method. The resulting zeolitic mesoporous titanosilicate material, denoted as ZMTS, has been applied to the oxidation of alkenes with H₂O₂ as the oxidant, and its catalytic properties have been compared with that of conventional catalysts such as TS-1 and Ti-MCM-41. ZMTS proves to be superior in catalytic activity.From the above researches, the author have known more about TS-1 zeolite crystallization mechanisms, preparation method and their influencing factors, as well as the correlative theories, which decrease TS-1 synthsis costs and explore the new feasible pathways.