Titanium post-synthesis treatment

Even better results are obtained by a post-synthesis treatment of TS-1 with both hydrogen peroxide and ammonium hydrogen fluoride, NH4HF2. Upon such a treatment (H202/F/Ti = 10 2.5 1 60 °C 4h), a substantial amount of titanium (up to 75% of the initial value) is removed. Nevertheless, the crystalline structure of the zeolite remains unchanged and the catalytic activity does not decrease. On the contrary, it actually increases since the turnover frequency of residual titanium atoms rises from 31 to 80 h . Even more importantly, at 8.6% benzene conversion the selectivities, both on benzene and on hydrogen peroxide, also increase from 83 to 94% and from 67 to 83% respectively, with formation of catechol (4%) and hydroquinone (2%) as the only by-products, without any evidence of further oxidation reactions [19]. 

The catalysts used in the aforementioned studies were always titanium silicates of MFI structure prepared by hydrothermal synthesis. Ti can, however, be inserted in the silica lattice by post-synthesis treatment of a dealuminated H-ZSM-5 with TiCl4 vapor [11]. Titanium silicalite-2 (TS-2), with the MEL structure of ZSM-11, was prepared shortly after the first synthesis of TS-1 [15]. Both catalysts have been used for the hydroxylation of phenol. Kraushaar-Czarnetzki and van Hooff showed that no major catalytic differences resulted from the method of synthesis of TS-1 [11]. The slow rate of reaction they observed was probably the result of large crystal size and low titanium content [7]. Tuel and Ben Taarit demonstrated there was no perceptible difference between the catalytic activity of TS-2 and TS-1 [8]. This was predictable, because of the close similarity of the Ti-site structure, chemical composition, and pore dimensions of the two titanium silicates. 

Niobium and titanium incorporation in a molecular sieve can be achieved either by hydrothermal synthesis (direct synthesis) or by post-synthesis modification (secondary synthesis). The grafting method has shown promise for developing active oxidation catalyst in a simple and convenient way. Recently, the grafting of metallocene complexes onto mesoporous silica has been reported as alternate route to the synthesis of an active epoxidation catalyst [21]. Further the control of active sites, the specific removal of organic material (template or surfactant) occluded within mesoporous molecular sieves during synthesis can also be important and useful to develop an active epoxidation catalyst. Thermal method is quite often used to eliminate organic species from porous materials. However, several techniques such as supercritical fluid extraction (SFE) and plasma [22], ozone treatment [23], ion exchange [24-26] are also reported. 

Titanium Silicalite-2 (TS-2), structurally similar to TS-1, could be prepared likewise using tetrabutylammonium hydroxide as the template [13, 14]. Titanium aluminum Beta (Ti,Al-[3) was prepared by hydrothermal synthesis from amorphous silica, sodium aluminate, tetraethyltitanate and tetraethylammonium hydroxide [15]. The presence of A1 was necessary for the crystallization of the product. Al-free Titanium Beta (Ti-[3) could be obtained in the presence of particular templates, such as dibenzyldimethylammonium hydroxide [16]. Titanium Mordenite (Ti-MOR), conversely, was obtained by post-synthesis insertion of Ti to dealuminated Mordenite [17]. Ti-MWW (Ti-MCM-22) was obtained by the synthesis of the lamellar precursor of Ti,B-MCM-22 followed by acid treatment to remove most of the boron and extra-framework Ti and finally calcination to burn out the template and bring about the condensation of lamellae into the three-dimensional MWW structure [18]. Ti is present in a number of different environ-... 

Recently [211], a post-synthesis modification of zeolite beta consisting of separate dealumination and titanation steps has been reported. First hydroxyl nests were formed by removal of up to 90% of the aluminum by leaching with oxalic or nitric acid, than up to 2 wt.% titanium was inserted into the lattice vacancies without formation of Ti02 as a second phase by treatment with gaseous TiCl4 at 500 °C. 

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