Rare earth catalyst systems

Both polybutadiene and polyisoprene with high cis-1,4 content can be obtained by the same rare earth catalyst. This is one of the characteristics of the rare earth catalyst system. As shown in table 2, about 97% cis-1,4 content is achieved for PBd prepared by any active rare earth catalyst system, except in the case of Er and Tm which give a somewhat lower ds-1,4 content. For polyisoprene, about 95% cis-1,4 content is obtained by La, Ce, Pr, and Nd catalyst catalysts prepared from the remaining elements can yield polyisoprene with a slightly higher cis-1,4 content but these are weaker in activity. 

The yields of the polymerization of three alkynes by all rare earth catalyst systems in chlorobenzene are given in table 18. The scandium and neodymium systems show the highest activities. Clearly, these new Ziegler-Natta catalysts are capable of promoting high-molecular-weight, cis-cisoid polymers in high yields. 

The patenting activity in the field of rare earth catalysts during 1970 and 1985 is illustrated in Fig. 12.3a. The vertical scale is arbitrary and is based on a total of 580 publications in 1985. As compared to 1970, the total published papers increased by four times and the patents by three times. Further, the patent activity showed a shift in emphasis from petroleum refining to pollution control activity. Other commercial catalyst systems are ammoxidation and dehydrogenation in which rare earths play a crucial role. 

Transition metal catalysts from across the periodic table have been investigated for this transformation. [56b, 57] Early transition metal catalysts [58] are of particular interest due to their high reactivities, with reduced air and moisture sensitivity compared with the rare earth metal systems, and lower cost and toxicity compared with the late transition metal catalysts. The A,0-ligands generating tight four-membered metallacycles described above have been studied as precatalysts for hydroamination methodologies that display promising substrate scope and reactivity. 

Abstract. Three types of polymer-supported rare earth catalysts, Nafion-based rare earth catalysts, polyacrylonitrile-based rare earth catalysts, and microencapsulated Lewis acids, are discussed. Use of polymer-supported catalysts offers several advantages in preparative procedures such as simplification of product work-up, separation, and isolation, as well as the reuse of the catalyst including flow reaction systems leading to economical automation processes. Although the use of immobilized homogeneous catalysts is of continuing interest, few successful examples are known for polymer-supported Lewis acids. The unique characteristics of rare earth Lewis acids have been utilized, and efficient polymer-supported Lewis acids, which combine the advantages of immobilized catalysis and Lewis acid-mediated reactions, have been developed. 

Table 9 lists the ds contents of PA obtained by all rare earth catalysts composed of naphthenate and phosphonate with aluminum alkyl. It can be seen that the cis content of PA obtained by a R(P204)3 system is somewhat lower than that of R(naph)3 catalyzed PA. Rare earth naphthenates combined with Al(i-Bu)3 systems are the best catalysts to synthesize easily and conveniently high-cis PA with metallic appearance at room temperature. 

The number of active species of the Nd(naph)3 and NdCl3 systems in Bd polymerization, determined by tritiated methanol quenching, kinetic and retarding agent methods, amounts to 0.6-10 mol% of Nd, while the Ti catalyst systems are generally only about 0.5%. Hu et al. (1982), and Hu and Ouyang (1983) proposed that the polymerization of conjugated diene with rare earth catalysts was the same as that of d-orbital transition metal catalysts such as Ti and Co and could be described as follows ... 

Although the copolymerization of propylene oxide with C02 takes place effectively with organozinc additives or the (tetraphenyl) porphyrin-AlCl system [61], the copolymerization of epichlorohydrin with C02 seldom occurs with these catalysts. Shen et al. [62] showed that a rare earth metal catalyst such as the Nd(2-EP)3/AliBu3 (Al/Nd = 8) system was very effective for the copolymerization of epichlorohydrin with C02 (30-40 atm) at 60 °C (Scheme 16). The content of C02 in the copolymer reached 23-24 mol % when 1,4-dioxane was used as solvent. 

Related to these catalysts are the systems based on lanthanide metal systems or rare earth metal complexes [46, 47]. The main problem with these catalyst systems is their instability. When the catalyst solution is prepared by reachng a metallocene with an organolithium compound in a polar solvent, the prepared catalyst soluhon is unstable and decomposes quickly, even under a nitrogen atmosphere. The activity of these catalysts can be high only if the catalyst is added to the polymer soluhon immediately after preparation. Attempts have been made to overcome the stability problem by using an additive in the system to improve the stability and the activity of the catalyst [33-35, 41, 57, 58, 61]. Re-... 

Lewis acids as water-stable catalysts have been developed. Metal salts, such as rare earth metal triflates, can be used in aldol reactions of aldehydes with silyl enolates in aqueous media. These salts can be recovered after the reactions and reused. Furthermore, surfactant-aided Lewis acid catalysis, which can be used for aldol reactions in water without using any organic solvents, has been also developed. These reaction systems have been applied successfully to catalytic asymmetric aldol reactions in aqueous media. In addition, the surfactant-aided Lewis acid catalysis for Mannich-type reactions in water has been disclosed. These investigations are expected to contribute to the decrease of the use of harmful organic solvents in chemical processes, leading to environmentally friendly green chemistry. 

Characterization techniques continue to develop and will impact their application to zeolitic systems. Aberration corrected electron microscopes are now being used to improve our understanding of catalysts and other nano-materials and will do the same for zeolites. For example, individual Pt atoms dispersed on a catalyst support are now able to be imaged in the STEM mode [252]. The application of this technique for imaging the location of rare-earth or other high atomic number cations in a zeolite would be expected to follow. Combining this with tomography... 

Alumina, alkaline-earth oxides, mixed oxides (spinels), rare-earth oxides, and lanthanide ores are known additives capable of sorbing S-impurities. The properties of these materials can be manipulated to produce catalysts capable of reducing up to -80% S-emissions and meet the refiner needs. It is, however, unlikely that these systems will be capable of satisfying the more stringent environmental S-emission standards expected in the future. Details of the reaction mechanism by which additives and promoters catalyze the oxidative sorption of S-impurities and details of catalyst deactivation have not yet been proposed. This work could provide useful information to help design more efficient S-transfer catalysts. The catalytic control of S-emissions from FCC units has been described in detail in two papers appearing in this volume (46,47) and in the references given (59). 

Table III compares the gasoline composition from three steam deactivated catalyst systems. The first contains 10% rare earth exchanged faujasite (RE FAU) in an inert silica/clay matrix at a cell size of 2.446 nm the second contains 20% of an ultra stable faujasite (Z-14 USY) at a unit cell size of 2.426 nm in inert matrix. The third contains 50% amorphous high surface area silica-alumina (70% AI2O3 30% Si02) and 50% clay the nitrogen BET surface area of this catalyst after steam deactivation is 140 m /g. All three catalysts were deactivated for 4 hrs. at 100% steam and at 816°C.

A lean NOx trap (LNT) (or NOx adsorber) is similar to a three-way catalyst. However, part of the catalyst contains some sorbent components which can store NOx. Unlike catalysts, which involve continuous conversion, a trap stores NO and (primarily) N02 under lean exhaust conditions and releases and catalytically reduces them to nitrogen under rich conditions. The shift from lean to rich combustion, and vice versa, is achieved by a dedicated fuel control strategy. Typical sorbents include barium and rare earth metals (e.g. yttrium). An LNT does not require a separate reagent (urea) for NOx reduction and hence has an advantage over SCR. However, the urea infrastructure has now developed in Europe and USA, and SCR has become the system of choice for diesel vehicles because of its easier control and better long-term performance compared with LNT. NOx adsorbers have, however, found application in GDI engines where lower NOx-reduction efficiencies are required, and the switch between the lean and rich modes for regeneration is easier to achieve. 

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