Homogeneous catalysts, defined

A catalyst is defined as a substance that influences the rate or the direction of a chemical reaction without being consumed. Homogeneous catalytic processes are where the catalyst is dissolved in a liquid reaction medium. The varieties of chemical species that may act as homogeneous catalysts include anions, cations, neutral species, enzymes, and association complexes. In acid-base catalysis, one step in the reaction mechanism consists of a proton transfer between the catalyst and the substrate. The protonated reactant species or intermediate further reacts with either another species in the solution or by a decomposition process. Table 1-1 shows typical reactions of an acid-base catalysis. An example of an acid-base catalysis in solution is hydrolysis of esters by acids. 

It is interesting to note that using the sol-gel procedure (I) the pre-formation of the rhodium diphosphine complex suppressed the formation of ligand free rhodium-cations on the silica surface. This approach gave rise to a well-defined, very selective hydroformylation catalyst. All immobilised catalysts were 10 to 40 times slower than the homogeneous catalyst under the same conditions, the sol-gel procedure yielding the fastest catalyst of this series. 

A number of potential methods for homogeneous catalyst separation and recovery have been discussed in the preceding chapters. This chapter addresses the separation of homogeneous catalysts by means of advanced filtration techniques. Separation of homogeneous catalysts by size exclusion (ultra- or nanofiltration, defined in detail in Section 4.3.1) offers several advantages ... 

Comparing the different subareas of catalysis, all have common characteristics, but also significant differences are visible. Our area of main interest is homogeneous catalysis based on transition metal catalysts. In general, these homogeneous catalysts (most often the precatalyst) are molecularly defined. 

Abstract This chapter focuses on well-defined metal complexes that serve as homogeneous catalysts for the production of polycarbonates from epoxides or oxetanes and carbon dioxide. Emphasis is placed on the use of salen metal complexes, mainly derived from the transition metals chromium and cobalt, in the presence of onium salts as catalysts for the coupling of carbon dioxide with these cyclic ethers. Special considerations are given to the mechanistic pathways involved in these processes for the production of these important polymeric materials. 

The scope of this chapter will be to focus on well-defined metal complexes that serve as homogeneous catalysts for the production of polycarbonates from epoxides and carbon dioxide. Although there are numerous such well-characterized metal complexes that catalyze this transformation, we will focus this chapter on recent contributions involving metal salicylaldimine (salen) and derivatives thereof [6, 7]. Some of the alternative catalysts systems are very active and selective for copolymer production. Most notably among these are the zinc p-diiminates reported by Coates and coworkers [8, 9]. These systems have been reviewed in detail elsewhere [10]. 

This reaction was one of the first examples of catalysis by a supported organometallic compound. In 1964 it was observed that Mo (CO) 6/ A1203, after activation by heating in vacuo at 120°C, catalyzed the conversion of propylene into ethylene and 2-butene (82). The nature of the active site in this catalyst system is still not fully defined (83). Since the initial discovery many heterogeneous and homogeneous catalyst systems have been reported (84, 85), the latter being more amenable to kinetic and mechanistic studies. 

The mechanisms of hydrogenation of alkenes over finely divided metals such as nickel, platinum, and so on (Section 11-2) now are understood in a general way. However, these reactions are extremely difficult to study because they occur on a metallic surface whose structure is hard to define. In contrast, the mechanisms of hydrogenation with homogeneous catalysts are known in considerable detail and provide insight into their heterogeneous counterparts. 

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