Ti-containing molecular sieves

Similar to Ti-containing molecular sieves, an IR band at ca. 960 cm was also reported for [Si,V]MFI structures (cf., e.g., [340] and Volume 1, Chapter 7, pages 208-209, of the present series and references therein). Thus, Hongbin Du et al. [341] observed by the FTIR/KBr technique in synthesized large pore V-contain-ing zeolite Beta a band at 960 cm" and took the appearance of this signal as an indication of vanadium incorporation into the framework to produce H-[Si,Al,V]Beta. 

Camblor, M.A., A. Corma, P. Esteve, A. Martinez, and S. Valencia, Epoxidation of Unsaturated Fatty Esters Over Large-Pore Ti-Containing Molecular Sieves as Catalysts Important Role of the Hydrophobic-Hydrophilic Properties of the Molecular Sieve, J. Chem. Soc. Chem. Commun., 795-796 (1997). 

Table 3. Structural and spectroscopic techniques used for characterizing Ti-containing molecular sieves ...

In spite of the large interest in Ti-containing molecular sieves, the application of theoretical methods to the problem of Ti siting (i. e., Ti site geometry and preferential substitution sites) has been made only recently. In fact, most of the studies concerning MFI-type molecular sieves have concerned the aliuninum-containing parent structure (ZSM-5). 

Table 5. Ti-containing molecular sieves prepared by secondary synthesis ...

As for Ti-containing molecular sieves, the incorporation of V has been mainly attempted in pentasil-type frameworks (MFI and MFI/MEL), while little information is available about the possibility to synthesize vanadosilicates with the framework topology of other zeolites. Table 6 summarizes the data reported in the literature. Hydrothermal synthesis is the route used in most cases. 

Similarly to Ti-containing molecular sieves (Sect. 3.2), incorporation of V is supported by spectroscopic and structural evidence. In calcined samples, a band located at ca. 960 cm was observed in the IR region, not present in the silica... 

Nevertheless, several aspects of the synthesis need to be investigated. In particular, it seems crucial to achieve more detailed information about the phenomena involved in the stabilization of the monomeric Ti species in solution and their interaction with silica. A deeper investigation of the chemistry of tita-nia-sihca gels might open the way for synthesizing other Ti-containing molecular sieves. 

A very comprehensive review has been published on TS-1 and other titanium-containing molecular sieves [26]. TS-1 was synthesized by the hydrothermal crystallization of a gel obtained from Si(OC2H5)4 and Ti(OC2H5)4 [10, 27] (Enichem method, hereafter named method A). The incorporation of Ti into the framework of... 

Post-synthesis modifications have been successful in preparing titanium-containing molecular sieves active in oxidation. The method employed for the post-synthesis incorporation of Ti to the zeolite beta was also applied to the incorporation of Ti into MOR and FAU [60]. 

The effect of zeolite porosity on the reaction rate was also well demonstrated in liquid-phase oxidation over titanium-containing molecular sieves. Indeed, the remarkable activity in many oxidations with aqueous H2O2 of titanium silicalite (TS-1) discovered by Enichem is claimed to be due to isolation of Ti(IV) active sites in the hydrophobic micropores of silicalite.[42,47,68 69] The hydrophobicity of this molecular sieve allows for the simultaneous adsorption within the micropores of both the hydrophobic substrate and the hydrophilic oxidant. The positive role of hydrophobicity in these oxidations, first demonstrated with titanium microporous glasses,[70] has been confirmed later with a series of titanium silicalites differing by their titanium content or their synthesis procedure.[71] The hydrophobicity index determined by the competitive adsorption of water and n-octane was shown to decrease linearly with the titanium content of the molecular sieve, hence with the content in polar Si-O-Ti bridges in the framework for Si/Al > 40.[71] This index can be correlated with the activity of the TS-1 samples in phenol hydroxylation with aqueous H2C>2.[71] The specific activity of Ti sites of Ti/Al-MOR[72] and BEA[73] molecular sieves in arene hydroxylation and olefin epoxidation, respectively, was also found to increase significantly with the Si/Al ratio and hence with the hydrophobicity of the framework. 

The oxidation of aniline was carried out in the liquid phase over a series of transition metal - substituted molecular sieves. For low oxidant/aniline ratios, azoxybenzene (AZY) was the major product formed over Ti-containing catalysts, the reaction was limited by diffusion for medium pore zeolites like TS-l and mesoporous silicas were preferred as they permitted the use of both H2O2 and tert-butyl hydroperoxide as oxidants. Higher oxidant/aniline ratios (>2) led to the formation of nitrobenzene (NB), whose selectivity was proportional to the catalyst concentration. In contrast, vanadium containing molecular sieves were only active with TBHP and aniline was converted very selectively into nitrobenzene for all oxidant concentrations. 

As for Ti- and V-containing molecular sieves, the synthesis conditions must assure the suppression of the polymerization/precipitation reactions of iron oxide/hydroxide species. Though the aquo-chemistry of Fe + is well documented (see [133] and references therein), little information is available on the behavior of Fe sources in the presence of silica, particularly under the hydrothermal conditions necessary for the crystallization of molecular sieves. Monomeric octahedral aquo-hydroxocomplexes of iron are stable only at pFl < 2. At higher... 

Allylic alcohols can be converted to epoxy-alcohols with tert-butylhydroperoxide on molecular sieves, or with peroxy acids. Epoxidation of allylic alcohols can also be done with high enantioselectivity. In the Sharpless asymmetric epoxidation,allylic alcohols are converted to optically active epoxides in better than 90% ee, by treatment with r-BuOOH, titanium tetraisopropoxide and optically active diethyl tartrate. The Ti(OCHMe2)4 and diethyl tartrate can be present in catalytic amounts (15-lOmol %) if molecular sieves are present. Polymer-supported catalysts have also been reported. Since both (-t-) and ( —) diethyl tartrate are readily available, and the reaction is stereospecific, either enantiomer of the product can be prepared. The method has been successful for a wide range of primary allylic alcohols, where the double bond is mono-, di-, tri-, and tetrasubstituted. This procedure, in which an optically active catalyst is used to induce asymmetry, has proved to be one of the most important methods of asymmetric synthesis, and has been used to prepare a large number of optically active natural products and other compounds. The mechanism of the Sharpless epoxidation is believed to involve attack on the substrate by a compound formed from the titanium alkoxide and the diethyl tartrate to produce a complex that also contains the substrate and the r-BuOOH. ... 

Effect of trimethylsilylation on catalytic activity of Ti-containing mesoporous molecular sieves... 

Since the first synthesis of TS-1 in 1983 [1], considerable efforts have been devoted to the synthesis of titanium-containing zeolites [2, 3]. Recently, Ti-beta, a large-pore molecular sieve, has been extensively studied [4, 5]. Owing to its unique large-pore channel system, Ti-beta seems to be more active than the medium-pore TS-1 catalyst for the oxidation of cyclic and branched alkenes with aqueous hydrogen peroxide. Under the usual synthesis conditions, however, Ti-beta crystallizes with some Al as a framework constituent [4], This leads to the presence of acid centers, which may have a detrimental effect on the activity or selectivity of this type of catalyst. Since 1992, the discovery of a new family of mesoporous molecular sieves has received much attention [6,7], Because of their mesoporous nature (20-100A), the Ti-MCM-41 zeolites may be useful as oxidation catalysts for larger molecules [8], In this... 

MCM-41 and silicalite-1 can be synthesized in the presence of niobium- and tantalum-containing compounds. The results indicated that Nb(V) and Ta(V) are well dispersed in the framework of silicalite-1 and in the amorphous walls of MCM-41 y-irradiation of activated niobium and tantalum molecular sieves show two radiation induced hole centers (V centers) located on Si-O-Si and M-O-Si (M = Nb, Ta) units. True isomorphous substitution as suggested in the literature for Ti(IV), however, is unlikely to be present Nevertheless, interesting chemical and catalytic properties can be expected from these systems and are subject to further studies... 

The use of heterogeneous catalysts in the synthesis of urethanes from aliphatic and aromatic amines, C02 and alkyl halides has been explored only recently. Titanosilicate molecular sieves [60a], metal phthalocyanine complexes encapsulated in zeolite-Y [60a], beta-zeolites and mesoporous silica (MCM-41) containing ammonium cations as the templates [60b, c], and adenine-modified Ti-SBA-15 [60d, e] each function as effective catalysts, even without any additional base. 

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. 

We consider first the textural properties of a typical wormhole Ti-HMS in comparison to well ordered hexagonal Ti-MCM-41 and Ti-SBA-3 analogs prepared by S+I and S+XT electrostatic assembly pathways. Table 1 provides the surface areas and pore volumes that characterize the framework mesoporosity (Vfr) and textural porosity (Vtx). The total mesoporosity (Vtotai) is the sum of these two values. Each mesostructure contains 2 mole % Ti and exhibits a HK pore size near 2.8 nm. The values in parenthesis in the table are for the corresponding pure silicas. Note the very high ratio of textural to framework mesoporosity for the HMS molecular sieves (Vtx/Vfr = 1.06) compared to the hexagonal molecular sieves (Vtx/Vfr = 0.03). As will be shown below, the textural porosity of HMS catalysts can improve catalytic activity by facilitating substrate transport to the active sites in the mesostructure framework. 

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