Titanium silicalite TPAOH

The isomorphous substitution of T atoms by other elements produces novel hybrid atom molecular sieves with interesting properties. In the early 1980s, the synthesis of a zeolite material where titanium was included in the MFI framework of silicalite, that is, in the aluminum-free form of ZSM-5, was reported. The name given to the obtained material was titanium silicalite (TS-1) [27], This material was synthesized in a tetrapropylammonium hydroxide (TPAOH) system substantially free of metal cations. A material containing low levels (up to about 2.5 atom %) of titanium substituted into the tetrahedral positions of the MFI framework of silicalite was obtained [28], TS-1 has been shown to be a very good oxidation catalyst, mainly in combination with a peroxide, and is currently in commercial use. It is used in epoxidations and related reactions. TS-1, additionally an active and selective catalyst, is the first genuine Ti-containing microporous crystalline material. 

Titanium (TS-1), niobium (NbS-1), and tantalum silicalite-1 (TaS-1) with MFI structure were synthesized by microwave heating of the TPAOH impregnated xerogels which were prepared by sol-gel process. Highly crystalline products were obtained in 30 min to 2 h with yields over 90%. The metallosilicates showed high catalytic performances in Beckmann recirrangement of cyclohexanone oxime to caprolactam. 

The first method uses the controlled concurrent hydrolysis of tetraethoxytitanium (IV) and tetraethoxysilane. This procedure has been labeled the mixed alkoxide method. Frequently, the acronyms TET (tetraethyltitanate) and TEOS (tetraethyl orthosilicate or tetraethoxysilane) are used for the respective reactants these are derived from the alternative names tetraethyl titanate and tetraethylorthosilicate. In examples described in patents, the synthesis involves adding TET to TEOS and then combining the alkoxide mixture with an aqueous solution of a SDA, which is typically tetrapropylammonium hydroxide (TPAOH). The resulting precursor mixture is then heated to a temperature of 175 °C to initiate crystallization. Subsequent washing of the crystallized solid with water, drying, and air calcination produces framework titanium-containing silicalite. 

A dry synthesis procedure was developed by Padovan et at. (90) In this procedure, silica is impregnated with a solution of a titanium precursor in aqueous TPAOH. Pure and well-crystallized Ti silicalites are obtained, except at the longest synthesis times. 

A family of titanium-containing molecular sieves with pentasil-type framework structures is represented by titanivun-silicalite-3 (TS-3) [74], characterized by a framework topology similar to that of TS-2 but with a different degree of stacking faults. In fact, as found by Perego et al. for the boron-substituted MFI/MEL molecular sieves, the frequency of stacking faults may be controlled by choosing the appropriate pair of tetraalkylammonium ions (e.g., TMAOH/TPAOH, TMAOH/TBAOH, TEAOH/TBAOH) [72]. 

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