Multimetal catalysts

Rasmussen, S.C., Richter, M.M., Yi, E., Place, H. and Brewer, KJ. (1990) Synthesis and characterization of a series of novel rhodium and iridium complexes containing polypyridyl bridging ligands Potential uses in the development of multimetal catalysts for carbon dioxide reduction. Inorg. Chem., 29, 3926—3932. 

The important discovery that supported bimetal and multimetal catalysts when used under severe industrial conditions often display a stability distinctly superior to that of their monometal counterparts. 

We have used the CPAP technique in two unrelated deactivation investigations. The experimental procedures and analyses are tailored to the individual processes. We show the effect of temperature and coke level in the relative magnitudes of site suppression and pore choking in deactivation by coking during cumene cracking over a zeolite catalyst, and relate results to a simple physical model. For HT of CDL over a supported multimetal catalyst, we show that site suppression is the predominant mode of deactivation by deposition of carbonaceous and metallic materials, at least at large times on stream. 

Chapter 4 contains the background of the development of effective modified Ni catalysts, discusses the methods of preparation of different types of stable and active metal catalysts, and discusses the selection of effective modifiers and the most suitable substrate molecules having practical interests. On the basis of these studies a reaction mechanism for the new effective catalytic systems was suggested and experimentally examined. The Chapter discusses the preparation variables for the development of this new type of effective chiral modified Ni catalyst, the supported metal catalysts, the chiral modified bimetal and multimetal catalysts including rare earth metals, and the new chiral modified nickel-ruthenium and palladium catalysts. Attempts are undertaken to elucidate the mechanism of enantioselectivity and to reveal the general regularities of asymmetric actions. 

Table 5-45 lists the advantages and disadvantages of the adsorbates. It can be seen that the adsorbates are not specifically suited to just one type of metal, so that chemisorption on multimetal catalysts has only Umited applicability. 

Although the mechanistic details of this system are not yet clear, it certainly represents a significant step toward the design of Fischer-Tropsch catalysts capable of functioning under mild conditions of temperature and pressure. Furthermore, it lends some credence (vide infra) to the suggestion that multimetal systems may be more useful than monometallic systems for the reductive polymerization of CO. 

In analyzing more than 250 publications Chapter 4 covers effective metal catalysts, mainly nickel, but also bimetal- and multimetal-systems, and their best modifiers, amino acids and tartaric acids. It is noted that it took more than 25 years to improve the modified nickel catalysts from their original poorly efficient systems into the modem excellent heterogeneous catalysts that hydrogenate carbonyl compounds with enantioselectivities of 96-98%. 

The LDOs, i.e., calcination derivatives of LDHs, are often used as catalyst supports. As mentioned previously, LDOs are mixed multimetal oxides with the cations well dispersed. Due to the flexibility of compositions (nature and ratios of cations) and the good cationic dispersal, the basic strength of LDOs as supports can be finely tuned. As a consequence, the activity and selectivity of loaded active components can be adjusted or controlled, at least to some degree, to meet the requirement for the specific catalytic reactions. 

The control can be achieved by using just one of these means or by combining two or more of them, providing thereby a wide variety of possibilities to achieve the metal concentration needed for a specific catalytic reaction or surface modification. These means are not restricted to one metal but can be used to control the concentration of two or several metals in the catalyst. The more complex the multimetal structures we want to prepare the clearer the advantage of using ALE becomes, as demonstrated in section 5 below. 

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