Homogeneous base catalysts

A new homogeneous process for hydroformylation of olefins using a water-soluble catalyst has been developed (40). The catalyst is based on a rhodium complex and utilizes a water-soluble phosphine such as tri(M-sulfophenyl)phosphine. The use of an aqueous phase simplifies the separation of the catalyst and products (see Oxo process). 

A new process developed by Institut Francais du Petrole produces butene-1 (1-butene) by dimerizing ethylene.A homogeneous catalyst system based on a titanium complex is used. The reaction is a concerted coupling of two molecules on a titanium atom, affording a titanium (IV) cyclic compound, which then decomposes to butene-1 by an intramolecular (3-hydrogen transfer reaction. ... 

Fatty acid methyl esters (FAME) are currently manufactured mainly by trans-esterification with an alcohol, using a homogeneous base catalyst (NaOH/KOH). Methanol is more suitable for biodiesel manufacturing, but other alcohols can in principle also be used, depending on the feedstock available. The... 

The process has been commercially implemented in Japan since 1977 [1] and a decade later in the U.S., Germany and Austria. The catalysts are based on a support material (titanium oxide in the anatase form), the active components (oxides of vanadium, tungsten and, in some cases, of molybdenum) and modifiers, dopants and additives to improve the performance, especially stability. The catalyst is then deposited over a structured support based on a ceramic or metallic honeycomb and plate-type structure on which a washcoat is then deposited. The honeycomb form usually is an extruded ceramic with the catalyst either incorporated throughout the stmcture (homogeneous) or coated on the substrate. In the plate geometry, the support material is generally coated with the catalyst. 

Natural products and common industrial chemicals in massive form are seldom useful as catalysts because they have low specific surface areas, may contain various amounts of impurities that have deleterious effects on catalyst performance, do not usually have the exact chemical composition desired, or are too expensive to use in bulk form. The preparation of an industrial catalyst generally involves a series of operations designed to overcome such problems. Many catalysts can be produced by several routes. The actual choice of technique for the manufacture of a given catalyst is based on ease of preparation, homogeneity of the final catalyst, stability of the catalyst, reproducibility... 

Biodiesel is a mixture of methyl esters of fatty acids and is produced from vegetable oils by transesterification with methanol (Fig. 10.1). For every three moles of methyl esters one mole of glycerol is produced as a by-product, which is roughly 10 wt.% of the total product. Transesterification is usually catalyzed with base catalysts but there are also processes with acid catalysts. The base catalysts are the hydroxides and alkoxides of alkaline and alkaline earth metals. The acid catalysts are hydrochloride, sulfuric or sulfonic acid. Some metal-based catalysts can also be exploited, such as titanium alcoholates or oxides of tin, magnesium and zinc. All these catalyst acts as homogeneous catalysts and need to be removed from the product [16, 17]. The advantages of biodiesel as fuel are transportability, heat content (80% of diesel fuel), ready availability and renewability. The... 

Even though the base-catalyzed process seems operator friendly and economically possible, it suffers from a key limitation only refined oils and pretreated fats with low concentrations of FFAs be used to produce biodiesel using homogeneous base catalysts. FFAs can react with the base catalyst giving... 

Current biodiesel technologies are in many aspects simple, operator friendly and, within their boundaries, efficient. Homogeneous base catalysts, such as NaOH and MeONa (most commonly used in biodiesel synthesis), are fast and produce high biodiesel yields in relatively short reaction times. However, only... 

Despite numerous screening studies, the literature contains little evidence that homogeneous catalyst systems based on metals other than Co, Rh, or Ru have significant activity for catalytic CO reduction. As seen for the known active catalytic systems, however, properties of solvents and additives or promoters can have enormous effects on catalytic activities. Solvents and additives can serve many roles in these catalytic systems. One important function of promoters in the Rh and Ru systems appears to be that of stabilizing metal oxidation states involved in catalytic chemistry. Other... 

One class of smart catalysts is based on homogeneous rhodium-based polytalkcnc oxidos, in particular those with a poly(cthylcnc oxide) backbone. Traditionally chemical catalyzed reactions proceed in a manner in which the catalysts become more soluble and active as the temperature is raised. This can lead to exothermal runaw ays. thus, posing both safety and yield problems. The behavior of these smart catalysts is different from that of traditional catalysts. As the temperature increases, they become less soluble, thus precipitating out of solution and becoming inactive. As the reaction mixture cools down, a smart catalyst redissolves and becomes active again. 

Many homogeneous catalysts are based on a (transition) metal atom that is stabilized by a ligand. The ligand is usually an organic molecule that attaches to the metal atom. By changing this ligand, one can change the catalyst s properties. 

In contrast to heterogeneous Ziegler-Natta catalysts, homogeneous catalysts based on biscyclopentadienyl derivatives of group 4 transition metals, which contain cationic metallocene species of formally d° 14-electronic structure, hardly promote the polymerisation of conjugated dienes, since the diene can act as a donor of four electrons rather than of two electrons as in monoolefin polymerisation (let us recall that the polymerisation of conjugated dienes is catalysed by half-sandwich metallocene-based catalysts). However, it has been reported [162] that statistical copolymers of ethylene and butadiene were obtained with the Cp2ZrCl2— [Al(Me)0]x catalyst. 

From an economic viewpoint it would be desirable to develop efficient chiral homogeneous catalyst systems based on metals other than those from the expensive noble group. Ohgo and co-workers (45a-c) have made some progress with chiral cobalt catalysts, but much remains to be done in this area. 

Until recently when a number of these homogeneous catalysts were heterogenized (171), the only known heterogeneous catalysts were based on metal ion-exchanged zeolites. 

Carbonylation of methanol has in recent years become a commercially important route for the production of acetic acid and methyl acetate. Industrial catalysts are at present homogeneous, based on cobalt and more recently rhodium compounds. The cobalt catalysts are less active 195) and require more severe operating conditions (i.e., 250°C, 650-750 atm) than the rhodium-based catalysts 196) (170-250°C, 7-14 atm). 

For homogeneous base catalyzed processes, reaction conditions are generally at ambient or slightly higher pressure, and a temperature of 65 °C-70 °C, in the presence of approximately 0.5% catalyst, with a 6 1 molar ratio of methanol to oil (Freedman et al, 1986). The process can be operated continuously (Noureddini et al, 1998) or in batch mode. Examples of continuous industrial processes include Ballestra, Connemann CD, and the Lurgi PSI process. 

---