Catalysts modified forms

Dicyclopentadiene has produced some interesting results. With rhodium catalyst at 115°C in tetrahydrofuran (THF), the dialdehyde was produced in good yield at 180°C that reaction proceeds further to form the diol in 67% yield (67). With a rhodium catalyst modified by excess triphenyl phosphite, the unsaturated monoaldehyde was obtained in a rapid reaction under very mild conditions (68, 69). The nonstrained 5-membered ring olefin required more strenuous conditions for hydroformylation. Either compound could be obtained in good yield by proper choice of conditions. 

This pore size distribution was shown to provide an optimum pore structure for platinum reforming catalyst activity, selectivity and catalyst life. This catalyst eventually was commercialized and became the famous RD-150 still in use in only slightly modified form. 

Rhodium catalysts modified with carboxylated phosphines 45 (Table 3 n=5, n=7)229 and phosphonium phosphines 103 (Table 5 n=2,3,6,10)255 form very active catalytic systems for the hydrogenation of olefins in aqueous/organic two phase systems. 

In our discussion of micellar catalysts in Chapter 8, we noted that effective catalysts have two features the ability to accelerate the rate of a reaction and the ability to do so selectively. Chemistry students are familiar with the general notion that catalysts modify the mechanistic path of a reaction in such a way as to lower the activation energy and make the conversion of reactants to products more probable. One of the easiest places to see this is in reactions of diatomic gas molecules. In the gas phase the mechanism for the reaction of hydrogen and oxygen to form water involves the following steps, among others ... 

Conventional Ziegler catalysts are not suitable for use with acrylonitrile because, among other reasons, the monomer reacts with the catalyst or forms complexes with it. Recently, modified catalysts have been developed in Natta s laboratory (106), using such combinations as Chromium acetylacetone plus dibutyl zinc,... 

The above results were reviewed in 1974 (5). Since then the main advances in the field have been the achievement of asymmetric hydro-carbalkoxylation (see Scheme I, X = -OR) using palladium catalysts in the presence of (-)DIOP (6), the use of other diphosphines as asymmetric ligands in hydroformylation by rhodium (7), and the achievement of the platinum-catalyzed asymmetric hydroformylation (8, 9). Further work in the field of asymmetric hydroformylation with rhodium catalysts has been directed mainly towards improving optical yields using different asymmetric ligands (10), while only very few efforts were devoted to asymmetric hydroformylation catalyzed by cobalt or other metals (11, 12) and it will be discussed in a modified form in this chapter. 

Tishchenko (79), using a modified form of Raney nickel, obtained a 95.7 % yield of camphor from the dehydrogenation of borneol. Rutovskii, (80) received a 93.5% yield of camphor with Raney alloy. Reeves and Adkins (81), studying the dehydrogenation of primary alcohols, removed the hydrogen with ethylene. It was found that, though Raney nickel could be used for a catalyst for the reaction, the yields were low and, in general, the Raney nickel was inferior to a catalyst composed of copper, zinc, nickel, and barium chromite. 

ESMS was employed to identify reactive intermediates in the enantioselective hydrogenation of ethyl pyruvate on Pt-alumina, Pt black, and Pt black+alumina catalysts modified by dihydrocinchonidine in acetic acid [56]. The ESMS spectra of the raw product revealed a large number of species which fell into four groups (1) dihydrocinchonidine and its hydrogenated derivatives (2) the adducts of ethyl pyruvate and its oligomers (3) (R)-ethyl lactate, the product of the enantioselective hydrogenation, and its adducts and (4) oxonium compounds formed from alumina. The latter most likely play a decisive role in the development of the chiral environment of the catalyst surface. As suggested by the authors, these oxonium cations could make the so-called electrostatic catalysis [57],based on electrostatic acceleration, possible. 

Clay minerals or rocks are used as catalysts in both natural and chemically modified forms. Their very first application dates back to the 1960s when acid-treated clays were used for cracking in oil industry (Franz et al. 1959). In the last decades, several reviews have been published on the catalytic effects of clay minerals (Theng 1974, 1982 Solomon and Hawthorne 1983 Laszlo 1986a, 1986b Adams 1987 McCabe 1996 Adams 1987 Adams and McCabe 2008). 

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