Ni-Based Catalyst System

The catalytic system described by Kealy is expected to undergo a series of interactions among its various components before generating 

If the second pathway is possible, then one should be able to use a Ni° 

The essential steps in the nickel-catalyzed 1 1 codimerization reaction, which involve hydride addition to butadiene and ethylene coordination to the metal atom, were first proposed by Kealy, Miller, and Barney (35) and were later demonstrated by Tolman (40) using a model complex. Tolman prepared the complex H—Ni+L PFe [L = (EtO)3P] and showed that, after prior dissociation to form H—NiL3, it can react with butadiene to form a 7r-crotyl complex 19. 

4 Assume the ir-crotyl group occupies two coordination sites. 

It should be noted here again that the catalytic reaction does not involve a change of valence of the metal. In general, catalytic olefin addition reactions that involve a hydride transfer do not require change of valence in the metal catalyst. On the other hand, carbon-carbon bond formation by coupling reactions which involve electron shifts, such as in Wilke s Ni°-catalyzed butadiene oligomerization reaction [Eq. (1)], requires a valence change on the metal. 

---

In the polymerization of BD by Ti-, Co- and Ni-based catalyst systems the polymerization has to be shortstopped at a specific monomer conversion in order to avoid the formation of gel. In contrast, polymerization catalysis by Nd catalysts does not need control of monomer conversion. As gel formation is particularly low with Nd catalysts full monomer conversion can be accomplished [427,428]. 

Most reaction models which describe the mechanism of diene polymerization by Nd catalysts have been adopted from models developed for the polymerization of ethylene and propylene by the use of Ti- and Ni-based catalysts systems. A monometallic insertion mechanism which accounts for many features of the polymerization of a-olefins has been put forward by Cossee and Arlman in 1964 [624-626]. Respective bimetallic mechanisms date back to Patat, Sinn, Natta and Mazzanti [627,628]. The most important and generally accepted mechanisms for the polymerization of dienes by Nd-based catalysts are discussed in the following. 

Replacement of the traditionally used Pd with less expensive Ni-based catalyst systems can significantly reduce costs for cross-couplings of this type, especially... 

In this work, the catalytic reforming of CH4 by CO2 over Ni based catalysts was investigated to develop a high performance anode catalyst for application in an internal reforming SOFC system. The prepared catalysts were characterized by N2 physisorption, X-ray diffraction (XRD) and temperature programmed reduction (TPR). 

The complete steam reforming of acetic acid can be achieved over commercial Ni-based catalysts [79]. The operating temperature of these systems is aWays higher than 650 °C. The robustness of the catalysts based on Ni guarantees operation over thousands of hours, but this metal leads to extensive coke formation. In order to improve the stability, La203 vas introduced in the catalyst formulation [258]. 

In early 1997, Schuurman, et al. reported Ni-based catalysts for the oxidative dehydrogenation of ethane to ethylene [12], but with only a limited ability to explore composition space the results were not compelling, and the real potential for Ni-based systems was missed. Tab. 1.1 shows secondary screening performance data for pure Ni oxide and binary and ternary Ni compositions containing Ta and Nb. The pure Ni catalyst is poor in terms of both conversion (11%) and selectivity (54%). The Ni catalyst containing 12% Ta was essentially the same (12% conversion and 55% selectivity). Increasing the Ta concentration in the binary to 38%... 

With the well-established Ti-, Ni- and Co-based catalyst systems molar mass regulation is achieved by the addition of appropriate amounts of hydrogen, 1,2-butadiene or cyclooctadiene. In Nd-catalyzed BD polymerizations these molar mass control agents are not effective [82,206,207]. 

As can be seen in Table 30 Co-, Ni-, Ti- and Nd-based catalyst systems yield BR with cis- 1,4-contents >90%. A direct consequence of high cis-1,4-contents is strain-induced crystallization of raw rubbers as well as of the respective vulcanizates. As the cis- 1,4-content is extraordinary high in Nd-BR, strain-induced crystallization is particularly pronounced for this BR-grade. As strain-induced crystallization beneficially influences the tack of raw rubbers as well as tensile strength and resistance of vulcanizates to abrasion and fatigue the high cis- 1,4-content makes Nd-BR particularly useful for tire applications. 

The difference in the preferred binding mode observed for the Pd- and Ni-based catalysts can be the crucial factor determining activity/inactivity of these two systems in polar copolymerization. However, the question arises about the stability of the alternative binding modes at finite temperature. If the minima were separated by relatively low barriers and fast interconversion between the two isomer complexes could occur, then this difference would be of minor importance. In order to check the stability of the two modes and get the insight into the mechanism of possible interconversions, a series of molecular dynamics simulations was performed. 

But, 1,2- or vinyl BR can be polymerised in the atactic, the syndiotactic or the isotactic form. Hence, five different configurations can be obtained by polymerisation reactions with butadiene (CH2=CH-CH=CH2) as monomer. The product obtained depends on the catalyst system used but is usually a mixture of 1,4 cis-, 1,4 trans- and atactic 1,2-BR. The commercial processes using Co-, Ni- or Ti-based catalyst systems, for instance, produce BR with a 1,4 cis-BR content higher than 90 %wt. But butyllithium initiated homopolymerisation of butadiene results in a product with 1,4 trans-BR contents up to 60 %wt. 

---