Titanium mixed alkoxide” method

Taramasso et al. (5) had originally reported two methods for the hydrothermal synthesis of TS-1. The first method (mixed alkoxide method) involves the preparation of a solution of mixed alkoxides of titanium and silica (preferably ethoxides) followed by hydrolysis with alkali-free solution of tetrapropylammonium hydroxide (TPAOH), distillation of the alcohol and crystallization of the resulting gel at 448 K. In the second method (dissolved or hydrolyzed titanium method) a soluble tetrapropylammonium peroxo-titanate species was prepared initially and then colloidal SiC>2 (Ludox AS-40) was added. This entire operation had to be carried out at 278 K. The TS-1 samples obtained by these two synthesis routes differed, particularly because of the presence of impurities such as Al3+ usually present in colloidal silica (33). 

Similar procedures adopted for the synthesis of TS-1 (the mixed alkoxide method, dissolved titanium method, pre-hydrolysis method, wetness impregnation method, and promoter induced synthesis method) were also used for the synthesis of TS-2. Tetrabutylammonium hydroxide (TBAOH) instead of TPAOH was used as the template (6,7,305-308). 

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. 

The preparation method, firstly reported by Taramasso et al. [10] in 1983, is based on the controlled hydrolysis of an aqueous solution containing tetraethyl-orthosilicate (TEOS), tetraethylorthotitanate (TEOT) and tetrapropylammonium hydroxide (TPAOH) as template. This procedure is known as the mixed alkoxide method. Another procedure has been proposed, known as the dissolved titanium method, which makes use of colloidal siUca instead of TEOS [11]. The mixed alkoxide method allows the best control of the synthesis parameters, assuring the crystallization of high quaUty products. 

This method is more complicated than the mixed alkoxide technique (Fig. 1). Hydrogen peroxide is added to the titanium-containing solution in order to form stable peroxo-complexes able to release slowly the metal ion during the hydrothermal treatment. The gel composition, the hydrothermal conditions and the characteristics of the products are similar to those described for the mixed alkoxide method. 

The preparation first reported is based on the controlled hydrolysis of aqueous solutions contaiiung tetraethylorthosilicate, tetraethylorthotitanate and tetrapropy-lammoiuum hydroxide this procedure, known as mixed alkoxide , allows a fine control of synthesis parameters giving crystallization of high quality. Many other methods are reported in the literature. One of the last most important improvements, from the point of view of application, was a new process which, by operating at a particularly low dilution, with suitable molar ratios between water and titanium... 

The addition of a base, typically ammonia, to mixtures of transition metal halides and alcohols allows the synthesis of homoleptic alkoxides and phenoxides for a wide range of metals. Anhydrous ammonia was first used in the preparation of titanium alkoxides where the reaction is forced to completion by the precipitation of ammonium chloride.41 Although useful for the synthesis of simple alkoxides and phenoxides of Si, Ge, Ti, Zr, Hf, V, Nb, Ta and Fe, as well as a number of lanthanides,42-47 the method fails to produce pure /-butoxides of a number of metals.58 Presumably, secondary reactions between HC1 and Bu OH take place. However, mixing MC14(M = Ti, Zr) with the Bu OH in the presence of pyridine followed by addition of ammonia proves successful, giving excellent yields of the M(OBul)4 complexes.59... 

Titanium salts nicely promote acetalization, transacetalization, and deacetalization, etc. Acetals and related compounds are prepared from the parent carbonyl compounds or other acetals in the presence of a titanium salt. In addition to ordinary acetals, N,Oy or (5, 5)-acetals could be prepared by this method. Equation (234) illustrates the preparation of a mixed acetal with different alkoxide groups [533]. Table 20 shows the preparation of different acetals in the presence of titanium salts. 

If mixed-metal alkoxide precursors can be prepared and their reactivity can be controlled, ordered macroporous mixed metal oxides can be produced by a sol-gel method using a colloidal crystal template. Alkoxides of Ti and Zr with other metals are easily prepared by mixing titanium alkoxide and zirconium alkoxide with alkoxides of other metals or metal salts, and the formed mixed metal alkoxides can infiltrate the voids of templates. Therefore, production of mixed metal oxides with Ti and Zr is straightforward. 

In the second synthesis method, the precursor is obtained by mixing colloidal silica with a solution containing TPAOH, hydrolyzed titanium alkoxide, and hydrogen peroxide. This procedure has been labeled the dissolved titanium method. 

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