Titanium-beta catalytic activity

Investigation of mechanisms of reactions catalyzed by titanium silicates has been limited to oxidation reactions with H202 as the oxidant, as described below. As was previously discussed, elements different from titanium and silicon in the catalyst materials change their properties. Catalytic activity of doubly substituted materials such as Ti-beta, H[Al,Ti]-MFI and -MEL, and H[Fe,Ti]-MFI and -MEL is considered separately because the acidic properties associated with the added element affect the composition of the reaction products. 

A very high stereoselectivity was observed in the reduction of 4-tert-butylcyclohexanone to the m-alcohol (> 95%), which is the industrially relevant product. The observed high selectivity to the thermodynamically unfavorable cis-alcohol was explained by a restricted transition-state for the formation of the trans-alcohol within the pores of the zeolites (Scheme 5). This reaction was found not only to be catalysed by Al-Beta, van der Waal et al. reported the catalytic activity of aluminum-free zeolite titanium beta (Ti-Beta) in the same reaction.74 Again, a very high selectivity to the cis-alcohol was observed indicating similar steric restrictions on the mechanism. Kinetically restricted product distributions were also reported for the 2-,3- and 4-methylcyclohexanone the cis, trans- and ds-isomers being the major products, respectively. In this case the tetrahedrally coordinated Ti-atom was assumed to behave as the Lewis acid metal center. Recent quantum-chemical calculations on zeolite TS-1 and Ti-Beta confirm the higher Lewis acidic nature of the latter one.75... 

Titanium modified beta zeolite (Ox-Ti-p) was prepared by treating aluminum containing beta zeolite (Na-K-, NH4- or H-form) with ammonium titanyl oxalate solutions of different concentrations. The obtained catalysts were characterized by atomic absorption, XRD, IR, UV-Vis and XPS. The incorporation of Ti into the zeolite framework was evidenced by (i) the increase in the [302] interplanar d-spacing, (ii) the increase in the intensity of the 960 cm IR band, (iii) the presence of an absorption band at 220 nm and the absence of absorption band due to TiOa in UV-Vis, (iv) the high binding energy of Ti 2pa/2 (459.9 eV) and (v) the catalytic activity in epoxidation reactions. The amount of Ti in the samples was found to depend on the concentration of ammonium titanyl oxalate solutions and also on the nature of the counter-cations of the parent beta zeolite. 

The incorporation of Ti into various framework zeolite structures has been a very active research area, particularly during the last 6 years, because it leads to potentially useful catalysts in the oxidation of various organic substrates with diluted hydrogen peroxide [1-7]. The zeolite structures, where Ti incorporation has been achieved are ZSM-5 (TS-1) [1], ZSM-11 (TS-2) [2] ZSM-48 [3] and beta [4]. Recently, mesoporous titanium silicates Ti-MCM-41 and Ti-HMS have also been reported [5]. TS-1 and TS-2 were found to be highly active and selective catalysts in various oxidation reactions [6,7]. All other Ti-modified zeolites and molecular sieves had limited but interesting catalytic activities. For example, Ti-ZSM-48 was found to be inactive in the hydroxylation of phenol [8]. Ti-MCM-41 and Ti-HMS catalyzed the oxidation of very bulky substrates like 2,6-di-tert-butylphenol, norbomylene and a-terpineol [5], but they were found to be inactive in the oxidation of alkanes [9a], primary amines [9b] and the ammoximation of carbonyl compounds [9a]. As for Ti-P, it was found to be active in the epoxidation of alkenes and the oxidation of alkanes and alcohols [10], even though the conversion of alkanes was very low. Davis et al. [11,12] also reported that Ti-P had limited oxidation and epoxidation activities. In a recent investigation, we found that Ti-P had a turnover number in the oxidation of propyl amine equal to one third that of TS-1 and TS-2 [9b]. As seen, often the difference in catalytic behaviors is not attributable to Ti sites accessibility. 

The commonly used MPVO catalysts consist of metal alkoxides, which are easily hydrolysed to inactive oxides in the presence of water. Since the proposed catalytic species for the MPVO reaction also consists of an alkoxide intermediate [6,9] the influence of water and strong Lewis bases on the catalytic activity and selectivity was investigated. As already reported for the liquid-phase reaction [9], Ti-beta has a high tolerance for water due to its hydrophobic interior. As can be seen from Fig. 6a the presence of water is not detrimental to the activity of Ti-beta in the MPV reduction of 4-methylcyclohexanone. The temperature of 110 °C, at which a ketone conversion of 50% is measured, is identical to the temperature required for 50% conversion in the absence of water (Fig. 3), Le. water has no effect whatsoever on the overall MPVO activity of the titanium site. 

Dartt, C.B., and M.E. Davis, Characterization and Catalytic Activity of Titanium Containing SSZ-33 and Aluminum-Free Zeolite Beta, Appl. Catal. A General 143 53-73... 

Titanium. In recent years, titanium-modified open-framework materials have been extensively stndied. Tetrahedral titanium has been incorporated into several zeolites silicalite-1 (169), silicalite-2 (170), MCM-41 (171), zeolite-Beta (172), and ZSM-5 (173). These titan-modified materials show remarkable reactivity in the oxidation reactions using diluted aqueous hydrogen peroxide nn-der mild conditions. The catalytic activity is mostly related to the tetrahedrally coordinated titanium incorporated in the framework (174-179). It is worth noticing that the titanium-incorporated silicalite-1 (TS-1) has been one of the most relevant industrial catalysts in the past two decades. These facts have directed research to investigate the possibility of the incorporation of titanium into the aluminophosphate framework. 

The discovery in the early 80 s of titanium silicalites [62-64] opened the new application perspective of zeolitic materials as oxidation catalysts. Several reactions of partial oxidation of organic reactants using dilute solutions of hydrogen peroxide could for the first time be performed selectively in very mild conditions. Other elements inserted in the lattice of silicalites have since been shown to have similarly interesting catalytic properties including, vanadium, zirconium, chromium and more recently tin and arsenic [65]. Titanium silicalites with both MFI (TS-1) and MEL (TS-2) structures have however been the object of more attention and they still seem to display unmatched properties. Indeed some of these reactions like the oxyfunctionalization of alkanes [66-69] by H2O2 are not activated by other Ti containing catalysts (with the exception of Ti-Al-Beta [70]). The same situation... 

Based on our experimental data, the following conclusions may be drawn (i) the incorporation of titanium into the framework of beta zeolite was achieved by treating Al-beta zeolite with ammonium titanyl oxalate solution and calcining the resultant material at 833 K for 6 h, (ii) the presence of Ti in tetrahedral framework positions was evidenced by various techniques, particularly UV-Vis, XPS and catalytic properties and (iii) Ti-P and Ox-Ti-P samples were active in the epoxidation of olefins. 

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