Heterogeneous catalysis titanium oxide

Heterogeneous Catalysis. The main discovery of the 1980s was the use of titanium sihcaUte (TS-1) a synthetic zeoHte from the ZSM family containing no aluminum and where some titanium atoms replace siUcon atoms in the crystalline system (Ti/Si = 5%) (33). This zeoHte can be obtained by the hydrolysis of a siUcate and an alkyl titanate in the presence of quaternary ammonium hydroxide followed by heating to 170°C. Mainly studies have been devoted to the stmcture of TS-1 and its behavior toward H2O2 (34). The oxidation properties of the couple H2O2/TS-I have been extensively developed in... 

One of the exciting results to come out of heterogeneous catalysis research since the early 1980s is the discovery and development of catalysts that employ hydrogen peroxide to selectively oxidize organic compounds at low temperatures in the liquid phase. These catalysts are based on titanium, and the important discovery was a way to isolate titanium in framework locations of the inner cavities of zeolites (molecular sieves). Thus, mild oxidations may be run in water or water-soluble solvents. Practicing organic chemists now have a way to catalytically oxidize benzene to phenols alkanes to alcohols and ketones primary alcohols to aldehydes, acids, esters, and acetals secondary alcohols to ketones primary amines to oximes secondary amines to hydroxyl-amines and tertiary amines to amine oxides. 

In the field of heterogeneous catalysis using H2O2 as oxygen source, examples of the use of titanium-silicalite (TS-1) or Ti-beta in the oxidation of selected alcohols, with formation of a Ti-peroxo species, have been reported... 

In this paper, we will review the chemical behaviour of transition metal oxides which is of crucial importance for heterogeneous catalysis, adhesion and many technological applications. Among them, MgO(lOO) is the simplest surface, with a square unit-cell containing two ions with opposite charges titanium oxides represent another important class of systems used for their catalytic properties either directly as catalyst or indirectly as support for other catalysts (metals such as Ni, Rh for the Fischer-Tropsch reaction or V2O5 for the reduction of NOx) or as promotors[l]. The most stable surface for rutile is the (110) face. 

Another good example of greener chemistry through the use of heterogeneous catalysis is the use of TSl, a titanium siHcate catalyst for selective oxidation reactions [21] such as the 4-hydroxylation of phenol to the commercially important hydroquinone (Scheme 1.1-2). 

In this context, the development of the heterogeneous titanium silicalite (TS-1) catalyst, by Enichem in the mid-1980s was an important milestone in oxidation catalysis. TS-1 is an extremely effective and versatile catalyst for a variety of synthe-... 

In the catalysis community, it is generally accepted that there are two types of support materials for heterogeneous oxidation catalysts [84]. One variety is the reducible supports such as iron, titanium, and nickel oxide. These materials have the capacity to adsorb and store large quantities of molecules. The adsorbed molecules diffuse across the surface of the support to the catalyst particle where they are activated to a superoxide or atomically bound state. The catalytic reaction then takes place between the reactant molecules and the activated on the catalyst particle. Irreducible supports, in contrast, have a very low ability to adsorb O. Therefore, can only become available for reaction through direct adsorption onto the catalyst particle. For this reason, catalysts deposited on irreducible supports generally exhibit turnover frequencies that are much lower than those deposited on reducible supports [84]. More recent efforts in our laboratory are focused on characterizing catalyst support materials that are commonly used in industry. These studies are aimed at deciphering how specific catalyst and support material combinations result in superior catalytic activity and selectivity. 

Heterogeneous catalysts which are active for the catalysis of the MPVO reactions include amorphous metal oxides and zeolites. Their activity is related to their surface basicity or Lewis acidity. Zeolites are only recently being developed as catalysts in the MPVO reactions. Their potential is related to the possibility of shape-selectivity as illustrated by an example showing absolute stereoselectivity as a result of restricted transition-state selectivity. In case of alkali or alkaline earth exchanged zeolites with a high aluminium content (X-type) the catalytic activity is most likely related to basic properties. For zeolite BEA (Si/Al=12), however, the dynamic character of those aluminium atoms which are only partially connected to the framework appear to play a role in the catalytic activity. Similarly, the Lewis acid character of the titanium atoms in aluminium free [Ti]-BEA explains its activity in the MPVO reactions. 

The epoxide is used as a monomer for polymer production. The byproduct ethylbenzene alcohol can be dehydrated to styrene, also a monomer for the production of polymers. If isobutane is used, iso-butylhydroperoxide replaces ethylbenzene-hydroperoxide as the oxidant. The byproduct tert-butanol can be converted with methanol to an ether that is an important additive in new environmental friendly gasolines. Complexes of Mo, V, or Ti are used in homogeneous epoxidation catalysis, while heterogeneous Ti02/Si02 catalysts can be used also. The active sites consist of a titanium ion with a fourfold coordination of oxygen in a tetrahedral geometry. Titanium acts essentially as a Lewis acid to activate the 0-0 bond in the hydroperoxide. 

---