Titanium oxide reaction with

New plants are successfully operating with clean and efficient process technologies. This is the most significant development made possible by the discovery, in 1983, of titanium-containing crystalline silicas and their unique catalytic properties, especially in selective oxidation reactions with H202 as the oxidant. 

The synthesis of Ti-mordenite has been conducted by reaction of TiCl4 with dealuminated mordenite or by hydrothermal synthesis (Section IV.F). The evidence for the incorporation of titanium is limited. The UV-visible spectra show that, in addition to the transition at 48,000 cm- , assigned to isolated Tiiv in tetrahedral coordination, there is also an absorption at 35,000 cm-, indicating extra-framework Ti02. The catalytic properties in oxidation reactions with H202 are significantly different from those of Ti02 deposited on mordenite, but they are limited to the hydroxylation of benzene and the oxidation of w-hexane (Kim and Cho, 1993). 

Many different oxidation reactions with peroxides and H202 as the oxidants and titanium silicates as the catalysts have been reported. 

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. 

The most smdied O-bonded transition metal enolates are titanium enolates . The reason for their success has beeu recognized in the fact that titanium enolates show an enhanced stereochemical control in C—C bond-forming reactions over simple lithium enolates and the possibility of incorporating chiral ligands at the titanium centre, a possibility which has lead to enantioselective aldol reactions with excellent enantiomeric excess. Moreover, titanium euolates have been used in oxidation reactions with remarkable diastereoselectivity. 

The old method of heating the calcium salts of formic and a second carboxylic acid for aldehyde formation has been modified by the use of a catalytic decomposition technique. By this scheme, the acid vapors are passed over thorium oxide, titanium oxide, or magnesium oxide at 300° or the acids are heated under pressure at 260° in the presence of titanium dioxide. In the latter procedure, non-volatile acids can be used. With aliphatic acids over titanium oxide, reaction occurs only when more than seven carbon atoms are present, the yields increasing with increase in the molecular weight (78-90%). Aromatic-acids having halo and phenolic groups are converted in high yields to aldehydes, e.g., salicylaldehyde (92%) and p-chlorobenzaldehyde (8S>%). Preparation of a thorium oxide catalyst has been described (cf. method 186). 

Binary vanadium-titanium oxide catalysts with various ratios of vanadium oxide and titania, as well as individual oxides of vanadium and titanium were examined in oxidation of P-picoline. Nicotinic acid, 3-pyridinecarbaldehyde, and CO2 were the reaction products over all the catalysts. The binary catalysts and individual vanadium oxide were highly selective for nicotinic acid, the most effective in P-picoline oxidation were the samples containing 20% and more of vanadium pentoxide. A regular stacking of crystallites of V2O5 and Ti02 was found to be the characteristic feature of the structure of the most effective compositions. 

Previously, Pasini [27] and Colonna [28] had described the use chiral titani-um-Schiff base complexes in asymmetric sulfide oxidations, but only low selec-tivities were observed. Fujita then employed a related chiral salen-titanium complex and was more successful. Starting from titanium tetrachloride, reaction with the optically active C2-symmetrical salen 15 led to a (salen)titani-um(IV) dichloride complex which underwent partial hydrolysis to generate the t]-0x0-bridged bis[(salen)titanium(IV)] catalyst 16 whose structure was confirmed by X-ray analysis. Oxidation of phenyl methyl sulfide with trityl hydroperoxide in the presence of 4 mol % of 16 gave the corresponding sulfoxide with 53% ee [29]. 

The presence of titanium in the silicalite structure gave to the TS-1 original properties in oxidation reactions with hydrogen peroxide [6-13]. 

Finally, titanium silicates have also been extensively investigated for the epoxida-tion of olefins. The reaction of ethylene over a silver-supported catalyst to ethylene oxide is one of the few large-scale industrial oxidation reactions with molecular oxygen as the oxidant. Numerous studies have shown TS-1 to be effective at selectively forming propylene oxide (PO) from propylene using hydrogen peroxide as the oxidant. This is a more environmentally friendly route to PO than the currently used chlorhydrin route, and it is likely that this process will see commercialization in the near future. 

Thus the weakly Bronsted acidic boron zeolites allow acid-catalyzed reactions to be carried out with high selectivity. Gallium substitution gives effective, sulfur-resistant catalysts for the synthesis of aromatics from lower alkanes, without the need for noble metal doping [8], The nonacidic titanium siUcalite exhibits very interesting properties in selective oxidation reactions with H2O2 [T32]. 

The invention of TS-1 created significant scientific interest. The material s unique properties and the resulting catalytic performance are beheved to be associated with the isolated titanium sites, which are highly active and selective in oxidation reactions with hydrogen peroxide as the oxidant. The scientists from the ENI group (3e,86,89) have since published more details about the material s synthesis and its physical and catalytic properties. However, as more than 600 papers were obtained when using TS-1 as a search keyword, we restrict our summary to only a small fraction of them, which we regard as essential papers from the early years (i.e., 1983-1995) and others that appeared in the subsequent 10-15 years. 

Titanosilicate Catalysts. Oxidation Reactions with Titanosilicate (Titanium Silicalite). Titanium-substituted silicalite, with the Ti substituting for the Si, can be prepared with a homogeneous distribution of Ti ions in the crystal, vide infra. The Ti ions seem to be all in Ti valence, and ESR shows the absence of TP. Also assumed is the fact that the TP ions are surrounded by four Si ions, and thus, the catalytic site is a single TP ion. ... 

Reaction with porous titanium oxide coated with polymerized 1,3-diaminobenzene... 

It is carried out in the Hquid phase at 100—130°C and catalyzed by a soluble molybdenum naphthenate catalyst, also in a series of reactors with interreactor coolers. The dehydration of a-phenylethanol to styrene takes place over an acidic catalyst at about 225°C. A commercial plant (50,51) was commissioned in Spain in 1973 by Halcon International in a joint venture with Enpetrol based on these reactions, in a process that became known as the Oxirane process, owned by Oxirane Corporation, a joint venture of ARCO and Halcon International. Oxirane Corporation merged into ARCO in 1980 and this process is now generally known as the ARCO process. It is used by ARCO at its Channelview, Texas, plant and in Japan and Korea in joint ventures with local companies. A similar process was developed by Shell (52—55) and commercialized in 1979 at its Moerdijk plant in the Netherlands. The Shell process uses a heterogeneous catalyst of titanium oxide on siHca support in the epoxidation step. Another plant by Shell is under constmction in Singapore (ca 1996). 

Many other reactions of ethylene oxide are only of laboratory significance. These iaclude nucleophilic additions of amides, alkaU metal organic compounds, and pyridinyl alcohols (93), and electrophilic reactions with orthoformates, acetals, titanium tetrachloride, sulfenyl chlorides, halo-silanes, and dinitrogen tetroxide (94). 

For the construction of oxygen-functionalized Diels-Alder products, Narasaka and coworkers employed the 3-borylpropenoic acid derivative in place of 3-(3-acet-oxypropenoyl)oxazolidinone, which is a poor dienophile in the chiral titanium-catalyzed reaction (Scheme 1.55, Table 1.24). 3-(3-Borylpropenoyl)oxazolidinones react smoothly with acyclic dienes to give the cycloadducts in high optical purity [43]. The boryl group was converted to an hydroxyl group stereospecifically by oxidation, and the alcohol obtained was used as the key intermediate in a total synthesis of (-i-)-paniculide A [44] (Scheme 1.56). 

The reaction is limited to allylic alcohols other types of alkenes do not or not efficiently enough bind to the titanium. The catalytically active chiral species can be regenerated by reaction with excess allylic alcohol and oxidant however the titanium reagent is often employed in equimolar amount. 

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