Titanium silicates synthesis

Appendix C. Synthesis of Titanium Silicate Molecular Sieves. 143... 

Acid catalysis by titanium silicate molecular sieves another area characterized by recent major progress. Whereas only two categories of acid-catalyzed reactions (the Beckmann rearrangement and MTBE synthesis) were included in the review by Notari in 1996 (33), the list has grown significantly since then. In view of the presence of weak Lewis acid sites on the surfaces of these catalysts, they can be used for reactions that require such weak acidity. 

The review of Notari (33) covers the synthesis methodologies of titanium silicate molecular sieves available up to 1996. The reviews of Corma (279) and subsequently of Biz and Occelli (280) describe the synthesis of mesoporous molecular sieves. An informative article on the preparation of TS-1 was reported recently by Perego et al. (68). In this section we list some of the recent developments in the synthesis of micro and mesoporous titanosilicate molecular sieves. 

Compositions of the synthesis gel and other physical characteristics of titanium silicate materials obtained in various synthesis methodologies are listed in Table Cl. 

R. Meiers, U. Dingerdissen, and W. F. Holderich, Synthesis of propylene oxide from propylene, oxygen and hydrogen catalyzed by palladium—platinum-containing titanium silicate, J. Catal. 176, 376-386(1998). 

Chapman, D.M., and Roe, A.L. (1990) Synthesis, characterization and crystal chemistry of microporous titanium-silicate materials. Zeolites, 10, 730. 

Kuznicki, S.M., Trush, K.A., Allen, F.M., Levine, S.M., Hamil, M.M., Hayhurst, D.T., and Mansom, M. (1992) Synthesis and adsorptive properties of titanium silicate molecular sieves, in Synthesis of Microporous Materials, Molecular Sieves, vol. 1 (eds M.L. Ocelli, and H.E. Robson), Van Nostrand Reinhold, New York,... 

Titanium containing pure-silica ZSM-5 (TS-1) materials are synthesized using different methods. The activity of the titanium containing catalysts for the oxidation of alkanes, alkenes and phenol at temperatures below 100 °C using aqueous H2O2 as oxidant is reported. The relationships between the physicochemical and catalytic properties of these titanium silicates are discussed. The effects of added duminum and sodium on the catalytic activity of TS-1 are described. The addition of sodium during the synthesis of TS-1 is detrimental to the catalytic activity while sodium incorporation into preformed TS-1 is not. The framework substitution of aluminum for silicon appears to decrease the amount of framework titanium. 

The Ti02 used here was made by hydrolyzing TNBT in distillated H2O with subsequent calcination at 500 °C. ETS-10, which is a titanium silicate molecular sieve with titanium in octahedral coordination, was provided by Engelhard, Co. For comparison, pure-silica ZSM-5 was also synthesized in the absence of alkali metal cations. Its synthesis involves the use of tetrapropylammonium bromide (TPABr) and piperazine. 

Octahedral coordination of Tiiv is also present in the titanium silicates ETS-4 and ETS-10. The structure of these materials is reported to be similar to that of zorite, and they can be described as microporous crystals with uniform pores similar in dimensions to classical small- and large-pore zeolites. In ETS-4 and ETS-10, there are two monovalent cations or one divalent cation for each Tilv ion (Kuznicki, 1989, 1990 Kuznicki et al., 1991a, 1991b, 1991c, 1993 Deeba et al., 1994). A recent report of the synthesis of ETS-10 with tetramethyl-ammonium chloride indicates a ratio of monovalent cations to Tilv of 1.6 (Valtchev et al., 1994). The acidic properties of these materials have not been reported. A material modified by the addition of Al3+ has been obtained, ETAS-10, which, after exchange with NH4 salts, exhibits acidic properties but these are due to the presence of Al3+ and not to the Tilv (Deeba et al., 1994). 

The synthesis of these titanium-substituted zeolites has been described to occur by a secondary synthesis process involving the reaction of [NH4]2TiF6 with the preformed corresponding zeolite (Section IV.G). The chemical and physicochemical properties described are not sufficient to establish the presence of Tiiv ions in framework positions. The titanium concentrations reported are much higher than the maximum values observed in titanium silicates for which isomorphous substitution has been demonstrated. The possible presence of Ti02 particles has not been investigated. No indication of the properties of these materials as catalysts in reactions typical of titanium silicates has been provided. It is therefore very doubtful that real isomorphous substitution has been obtained (Skeels et al., 1989 Skeels, 1993). 

The high-temperature synthesis from strongly alkaline suspensions of salts of Tilv and Silv produces crystalline microporous materials in which Tiiv is present in octahedral coordination. These materials do not exhibit the catalytic properties typical of the other titanium silicates in which TiIV is in tetrahedral coordination (Kuznicki, 1989, 1990 Kuznicki et al., 1991a, 1991b, 1991c, 1993 Deeba et at., 1994). The acidic properties of these materials have been discussed (Section II.B). 

The synthesis of crystalline microporous titanium silicate requires conditions that are specific to each product, such as the chemical nature of the structure directing agent, reactant concentrations, temperatures, and times of crystallization. For brevity, only a short outline of the basic rules that apply to the synthesis of all titanium silicates will be given, with reference to a few selected papers in which clear, detailed descriptions of the syntheses can be found. 

The discovery of the new titanium silicates and of their catalytic properties in H2O2 oxidation reactions has had a major impact in catalytic science and its industrial applications. One 10,000 ton/year plant for the production of catechol and hydroquinone has been operating since 1986 with excellent results. Moreover, successful tests conducted on a 12,000-ton/year pilot plant for cyclohexanone ammoximation (Notari, 1993b) could be followed soon by an industrial-size plant that would greatly simplify the synthesis of caprolactam. Both these examples are clear indications of the potentials of the new oxidation chemistry made possible by the new materials. 

DEVELOPMENT OF THE WET METHODS FOR SYNTHESIS OF MESOPOROUS TITANIUM-SILICATE BASED COMPOSITES... 

The synthesis of titanium silicates by basic and acidic hydrolysis in an organic-water media were carried out at 80-150 C in an autoclave with stirring (150-200 rpm) at autogenic pressure (2.5-4.5 at.). As silicon sources tetraethoxysilane and silicic acid have been used. As metal sources the metal-organic compounds alcoholates, stearates, acetylacetonates, acetates or mineral salts have been used. Hexamethylenetetramine, N,N-dimethylocteIamine, monoethanolamine were used as the structure-directing agents. 

Further interesting developments in this field are the discovery of the large pore titanium silicate molecular sieve ETS-10 [77] and the synthesis of mesoporous materids containing titanium and vanadium [78-81], Much systematic work will be required to elucidate how useful these new materials are as catalysts in selective oxidation reactions. 

A. Corma, M.T. Navarro, and J.P. Pariente, Synthesis of an Ultralarge Pore Titanium Silicate Isomorphous to Mcm-41 and Its Application as a Catalyst for Selective Oxidation of Hydrocarbons. J. Chem. Soc. Chem. Commun., 1994, 147-148. 

M. Crocker, R. H. M. Herold, A. G. Open, M. T. A. Overgaag, Synthesis and characterisation of titanium silasesquioxane complexes Soluble models for the active site in titanium silicate epoxi-dation catalysts, /. Chem. Soc., Dalton Trans. (1999) 3791. 

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