Hydrogenation reaction, scope

Drawing heavily from prior experience in hydrogenation of nitriles (7-10) and of ADN to ACN and/or HMD (11), in particular, we decided to restrict the scope of this investigation to Raney Ni 2400 and Raney Co 2724 catalysts. The hydrogenation reactions were initially carried out in a semi-batch reactor, followed by continuous stirred tank reactor to study the activity, selectivity, and life of the catalyst. 

Furthermore, it is far beyond the scope of this chapter to provide any detailed insight into the materials science aspects of ionic liquids. However, it should be clearly stated that at least some understanding of the ionic liquid material is a prerequisite for its successful use as a catalyst layer in hydrogenation reactions. Therefore, the interested reader is strongly encouraged to explore the more specialized literature [39]. 

The feverish interest in hydrido complexes has, as its main cause, the tremendous potential of these reactions in catalytic systems. In a relatively short span of time, hydrido complexes have been found to play a role in a significant number of catalytic processes (6, 9)—e.g., oligomerization of olefins (rhodium), decarboxylation reactions (rhodium), and hydrogenation reactions (ruthenium, osmium, rhodium, iridium, and platinum). Discussion of these applications would go beyond the scope of the present treatment. 

Practically, all of the above reactions have been realized, with different metals and conditions. In determining the scope of this review, we have attempted to focus our attention on the nature of the transformations at the metal center, especially with regard to oxidation state and formation of the initial alkyl-, alkoxy-, or carboalkoxy-metal bond from saturated precursors. Therefore, while it appears that hydrocarboxylation reactions make some contribution to the total reactivity in a variety of alcohol carbonylation systems, we feel that the mechanistic aspects of this topic would be better covered separately. So, except for noting where this chemistry makes probable contributions, it will not be discussed here. Similarly, homologation reactions, which are believed to usually proceed by way of aldehyde intermediates, will be discussed only as they pertain to the incorporation of the CO into the metal-carbon bonds, that is, the factors governing the subsequent hydrogenation reactions will not be covered. 

The study by Hitzler et al. has illustrated the broad scope of potential hydrogenation reactions in SC-Some of the substrates investigated included m-cresol, benzaldehyde, acetophenone, 1-octene, and cyclohexene. Reactions were performed in 5 and 10 ml packed bed reactors and space times of up to 300 hr were achieved. Residence times of this magnitude render the issue of scale-up largely irrelevant for small (kilogram) quantities of product in a continuous process. This is advantageous for the commercialization of SCF reaction processes because it minimizes development and capital costs. 

Most crystalline aluminosilicates have little intrinsic catalytic activity for hydrogenation reactions. However, a considerable amount of data has recently accumulated on the use of zero-valent metal-containing zeolites in many hydrocarbon transformations. Thus noble and transition metal molecular sieve catalysts active in hydrogenation (7,256-760), hydroisomerization (161-165), hydrodealkylation (157, 158,165-167), hydrocracking (168,169), and related processes have been prepared. Since a detailed discussion of this class of reactions is beyond the scope of this review, only a few comments on preparation and molecular-shape selectivity will be made. 

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