Activity for hydrogenation of ethylene

C. also decreases proportionally to the decrease in activity for hydrogenation of ethylene at room temperature. Thus both the relative chemisorption of CO at room temperature and the van der Waal s adsorption of krypton are reliable measurements for the surface available for hydrogenation of ethylene. The same is true for the fast adsorption of hydrogen at — 196°C., so that the latter presents a third criterion by which it is possible to determine the catalytically active surface of sintered films. 

Magnesia may also be activated by molecular H2 at 430°C, without the need of a Pt/AI203 activator. However, the sites active for hydrogenation of ethylene, unlike those on alumina or silica, were destroyed by oxidation at 430°C. Spiltover hydrogen (but not molecular H2) activates MgO already at 200°C and the active sites are insensitive to 02 treatment but are blocked by NH3. The active sites are therefore not the same as those created on Si02 and... 

Catalytic tests show (Fig. 3) that nickel and palladium dispersed on carbon supports by this method are active for hydrogenation of ethylene. These tests, performed between 30°C and 80°C after H2 reduction (320°C), allowed to measure apparent activation energies Ea= 53 kJ/mol and Ea = 28 kJ/mol were found for nickel and palladium respectively. These values are in good agreement with those found in the literature [20]. 

Heterogeneous Pt catalysts were prepared by controlled thermolysis of salts of [Pt3(CO)6ls, which had been deposited in the hexagonal channel of the mesoporous zeolite FSM-16. The catalysts were active for hydrogenation of ethylene at 300 K. With a partially decarbonylated catalyst, 1,3-butadiene was selectively hydrogenated to 1-butene complete decarbonylation above 463 K gave a Pt catalyst which completely hydrogenated butadiene to butane. ... 

Preparation of metal oxide thin film by means of stepwise absorption of metal alkoxide has been carried out in the past for the activation of heterogeneous catalysts [13]. For example, Asakura et al. prepared one-atomic layer of niobium oxide by repeating chemisorption of Nb(OEt)5 on silica beads. The catalyst obtained by immobilizing platinum particles on a niobum oxide layer showed improved reactivity for hydrogenation of ethylene in comparison with... 

Co2(CO)6(HCs CH)] has catalytic activity for hydrogenation of acetylene to ethylene. On increasing the acetylene concentration, however, cyclotrimerization to benzene becomes the dominant process. ... 

Olefinic bonds also undergo, hydrogenation in the presence of copper-chromium oxide catalyst but usually require a temperature above 150-200°C. The latter catalyst is useful if it is desired to reduce certain other groups in the molecule at the same time. Zinc chromium oxide is even less active for hydrogenating an ethylenic linkage. 

If the production of vinyl chloride could be reduced to a single step, such as dkect chlorine substitution for hydrogen in ethylene or oxychlorination/cracking of ethylene to vinyl chloride, a major improvement over the traditional balanced process would be realized. The Hterature is filled with a variety of catalysts and processes for single-step manufacture of vinyl chloride (136—138). None has been commercialized because of the high temperatures, corrosive environments, and insufficient reaction selectivities so far encountered. Substitution of lower cost ethane or methane for ethylene in the manufacture of vinyl chloride has also been investigated. The Lummus-Transcat process (139), for instance, proposes a molten oxychlorination catalyst at 450—500°C to react ethane with chlorine to make vinyl chloride dkecfly. However, ethane conversion and selectivity to vinyl chloride are too low (30% and less than 40%, respectively) to make this process competitive. Numerous other catalysts and processes have been patented as weU, but none has been commercialized owing to problems with temperature, corrosion, and/or product selectivity (140—144). Because of the potential payback, however, this is a very active area of research. 

A selective poison is one that binds to the catalyst surface in such a way that it blocks the catalytic sites for one kind of reaction but not those for another. Selective poisons are used to control the selectivity of a catalyst. For example, nickel catalysts supported on alumina are used for selective removal of acetjiene impurities in olefin streams (58). The catalyst is treated with a continuous feed stream containing sulfur to poison it to an exacdy controlled degree that does not affect the activity for conversion of acetylene to ethylene but does poison the activity for ethylene hydrogenation to ethane. Thus the acetylene is removed and the valuable olefin is not converted. 

The kinetics of ethylene hydrogenation on small Pt crystallites has been studied by a number of researchers. The reaction rate is invariant with the size of the metal nanoparticle, and a structure-sensitive reaction according to the classification proposed by Boudart [39]. Hydrogenation of ethylene is directly proportional to the exposed surface area and is utilized as an additional characterization of Cl and NE catalysts. Ethylene hydrogenation reaction rates and kinetic parameters for the Cl catalyst series are summarized in Table 3. The turnover rate is 0.7 s for all particle sizes these rates are lower in some cases than those measured on other types of supported Pt catalysts [40]. The lower activity per surface... 

Solutions of Ru3(CO)i2 in carboxylic acids are active catalysts for hydrogenation of carbon monoxide at low pressures (below 340 atm). Methanol is the major product (obtained as its ester), and smaller amounts of ethylene glycol diester are also formed. At 340 atm and 260°C a combined rate to these products of 8.3 x 10 3 turnovers s-1 was observed in acetic acid solvent. Similar rates to methanol are obtainable in other polar solvents, but ethylene glycol is not observed under these conditions except in the presence of carboxylic acids. Studies of this reaction, including infrared measurements under reaction conditions, were carried out to determine the nature of the catalyst and the mechanism of glycol formation. A reaction scheme is proposed in which the function of the carboxylic acid is to assist in converting a coordinated formaldehyde intermediate into a glycol precursor. 

The anion vacancy sites have acid properties. As stated above, the catalytic activity and the hydrogenolytic behavior correlated with the acid properties of the catalyst, as well as the extent of reduction. Therefore, the adsorption of an aryl group will occur on the coordinatively unsaturated molybdenum sites generated during reduction. According to the reaction scheme for the hydrogenation of ethylene over a reduced MoOj-AljOj catalyst, ethylene becomes it-bonded at a second vacant ligand position of a coordinatively unsaturated Mo species and inserts to form the [Pg.267]

M/AI2O3 (M = Cr, Mo, W) catalysts were ultra-active for the hydrogenation of ethylene when compared with others prepared by traditional methods from appropriate salts-a low oxidation state of M is of crucial importance for these catalysts to be active in the hydrogenation of olefins. 

These catalysts are also active for the hydrogenation of ethylene (Hindin... 

The application of Absolute Rate Theory to the interpretation of catalytic hydrogenation reactions has received relatively little attention and, even when applied, has only achieved moderate success. This is, in part, due to the necessity to formulate precise mechanisms in order to derive appropriate rate expressions [43] and, in part, due to the necessity to make various assumptions with regard to such factors as the number of surface sites per unit area of the catalyst, usually assumed to be 10 5 cm-2, the activity of the surface and the immobility or otherwise of the transition state. In spite of these difficulties, it has been shown that satisfactory agreement between observed and calculated rates can be obtained in the case of the nickel-catalysed hydrogenation of ethylene (Table 3), and between the observed and calculated apparent activation energies for the... 

The hydrogenation of ethylene has been extensively studied over a wide variety of metal catalysts. In this section we review some of the results obtained for the kinetics and activation energies and from the use of deuterium as a tracer. 

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