Heterogeneous oxidation catalysts

Enichem made one of the most important steps forward in the development of general heterogeneous oxidation catalysts in the early 1990s with the commercialization of titanium silicate (TS-1) catalysts. TS-1 has a structure similar to ZSM-5 in which the aluminium has been replaced by titanium it is prepared by reaction of tetraethylorthosilicate and tetra-ethylorthotitanate in the presence of an organic base such as tetrapropy-lammonium hydroxide. This catalyst is especially useful for oxidation reactions using hydrogen peroxide (Scheme 4.11), from which the only byproduct is water, clean production of hydroquinone being one of the possibilities. 

Phillips (1) A process for polymerizing ethylene and other linear olefins and di-olefins to make linear polymers. This is a liquid-phase process, operated in a hydrocarbon solvent at an intermediate pressure, using a heterogeneous oxide catalyst such as chromia on silica/ alumina. Developed in the 1950s by the Phillips Petroleum Company, Bartlesville, OK, and first commercialized at its plant in Pasadena, TX. In 1991, 77 reaction fines were either operating or under construction worldwide, accounting for 34 percent of worldwide capacity for linear polyethylene. 

Anhydrous RuO has a tetragonal rutile structure with a slightly distorted octahedral structure, there being two sets of Ru-0 distances at 1.917(8) and 1.999(8) A [645] and has an extensive chemistry as a heterogeneous oxidation catalyst, a topic beyond the scope of this book. It is rarely used as a precursor for Ru oxidations, the hydrated form RuO. nH O (for brevity written below simply as RuO ) being much more effective in this respect. A procedure for converting inactive RuO (presumably the anhydrous form) to the hydrated RuO used in catalytic oxidations has been described [243]. 

TS-1 has unique properties as heterogeneous oxidation catalyst for the oxidation of organic compounds with H2O2 very high selectivities are obtained and this parallels the behaviour of Ti(IV) based homogeneous catalysts. 

The incorporation of vanadium(V) into the framework positions of silicalite-2 has been reported by Hari Prasad Rao and Ramaswamy469. With this heterogeneous oxidation catalyst the aromatic hydroxylation of benzene to phenol and to a mixture of hydroquinone and catechol could be promoted. A heterogeneous ZrS-1 catalyst, which has been prepared by incorporation of zirconium into a silicalite framework and which catalyzes the aromatic oxidation of benzene to phenol with hydrogen peroxide, is known as well in the literature. However, activity and selectivity were lower than observed with the analogous TS-1 catalyst. 

Metal-Substituted Zeolites as Heterogeneous Oxidation Catalysts... 

A rather interesting application of zeolite-based alkene oxidation catalysis has been demonstrated by Japanese workers (46, 47). In particular, a Pd2 +, Cu2 +Y zeolite was shown to be an active and stable heterogeneous oxidation catalyst which is analogous to the well-known homogeneous Wacker catalyst system containing PdCl2 and CuCl2 (48). Under Wacker conditions (i.e., alkene/02/H20) the zeolite Y catalyst was shown to convert ethylene to acetaldehyde and propylene to acetone with selectivities in excess of 90% with C02 as the major by-product. 

As heterogeneous oxidation catalyst, 5% Pt, 1% Bi/C has been identified as an efficient catalyst for the conversion of 2-octanol to 2-octanone and 1-octanol to octanoic acid (see Fig. 4.72) [210]. Also manganese-substituted octahedral mo-... 

Another approach to designing shape-selective heterogeneous oxidation catalysts was to use redox metal oxides as the pillaring agents in the preparation of pillared clays. These redox pillared clays have been used for a number of selective oxidations. Chromium pillared montmorillonite (Cr-PILC) is an effective catalyst for the selective oxidation of alcohols with tert-butyl hydroperoxide. 7 Primary aliphatic and aromatic alcohols are oxidized to the aldehydes in very good yields. Secondary alcohols are selectively oxidized in the presence of a primary hydroxy group of a diol to give keto alcohols in excellent yields (Eqn. 21.12). 2... 

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. 

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