Hydrogenation catalyst characteristics

It is apparent that the type and magnitude of these reactions have an impact on the heat balance of the unit. For example, a catalyst with less hydrogen transfer characteristics will cause the net heat of reaction to be more endothennic. Consequently this will require a higher catalyst circulation and, possibly, a higher coke yield to maintain the heat balance,... 

Detailed studies were conducted with ZnO, which is a hydrogenation catalyst and effects isomerization without hydrogen.129,130 Spectroscopic observations130-132 led to the postulation of surface ally lie species and, consequently, a 1,3 hydrogen shift was suggested to account for isomerization. The tt-allylic species is formed and stabilized on adjacent acid-base (Zn2+-02 ) site pairs. The characteristic cis selectivity also supports a mechanism similar to that suggested to be effective in homogeneous systems. 

Activation of hydrogen through oxidative addition is best exemplified by the Wilkinson catalyst. The hydrogenation mechanism characteristic of dihydride complexes was originally suggested by Wilkinson2,109 and was later further... 

The catalysts used are Ni, Ru, Rh, Pd, Ir, Pt and Cu. The characteristic experimental results disclosed in recent publications are summarized in Tables 20 and 21. The majority of hydrogenations were carried out in the presence of Rh, Pt and Cu catalysts. Characteristic examples can be seen in equations 37-41, which also illustrate some exceptional behaviors. 

Supported and coprecipitated nickel catalyst represent an interesting alternative, from the economical point of view, to other selective hydrogenation catalysts such as Pt or Pd, of higher performance but also with a higher price. In the present study, a coprecipitated NiO/NiAl 04 catalyst has been selected to cany out the selective hydrogenation of acetylene to ethylene as a test reaction. One important characteristic of this process is the large amount of coke which may be generated [l]. 

The most important characteristics of a hydrogenation catalyst are, for a particular reaction, its productivity and selectivity to the desired product over economical ranges of temperature and pressure. These ranges in turn depend heavily upon the overall plant context in which the catalyst is to be used. 

The experimental program for the kinetic study comprised only 17 experiments altogether, but the formal program was not started until the ability to obtain quality data had been established. This meant that we had fine-tuned analytical methods and experimental procedures so that good material balances could be obtained routinely at any desired reaction conditions. Also, by the time the formal program was started, the catalyst activity in the autoclave had declined to a relatively constant level from the hyperactivity characteristic of new hydrogenation catalysts. 

A remarkable amount of experimental data show that the Mg/Ti catalysts characteristically provide polymers with lower molecular weight as compared to non-supported catalysts38,91 126,121 128). This is true for ethylene and propylene polymerization and, in principle, may be the result of a considerable increase of the constants for the chain transfer rates with monomer k , hydrogen kf and organoaluminum k 1, although experimental data are rather scarce (see Table 6). Spontaneous P-elimination k,Sp is not considered important at normal polymerization temperatures 126,129,130,131). 

This hypothesis is supported by Lin s studies of the reactions of the Cg to C13 1,2-cycloalkadienes with HRh(PPh3)4 and ClRh(PPh3)3, complexes classified by Collman et al. as monohydride and dihydride hydrogenation catalysts, respectively. The monohydride complex unites with allenes to form TT-allyl compounds which have characteristic NMR spectra in solution. 1,2-Cyclononadiene yields almost the same proportion of cis- and trani-cyclononene with either Pd/Al203 or HRh(PPh3)4 as catalyst little of the trans isomer is formed when ClRh(PPh3)3 is used. The X-ray crystal structure of the complex formed from the reaction of ClRh(PPh3)3 with 1,2-cyclononadiene shows that only one double bond of the diene is coordinated with rhodium. ... 

In another brief examination [ 15] of the impact of monolith supports for mcthanation catalysts, a comparison between nickel and ruthenium catalysts was made utilizing a metal (Fecralloy) support. The conversion tests were run at 673 K, 5400 kPa, 3.47 sec", and with a gas composition of 62% hydrogen, 18% carbon monoxide, and 20% water vapor. A ruthenium pellet catalyst that was run in comparison was approximately twice as active as ruthenium on the monolith. However, the difference in product (methane) selectivity was 97% for the metal monolith catalyst and 83% for the pellet bed. In the comparison between nickel and ruthenium, shown in Fig. 14, the ruthenium was more active and selective. The lack of impact on activity or selectivity as a result of steam addition to the reactant mixture provided useful practical data as well. No further details regarding the catalyst characteristics were provided. 

As a simple example, let us consider a problem arising out of the testing of a hydrogenation catalyst in a differential reactor. Conditions are held constant in two series of runs save that the catalyst is much more finely divided for one series. The same mass of catalyst occupies the same volume in the two cases so that the external voidage of the bed is not significantly changed. There are no great differences in the shape of the particles in the two sizes, so that the normalized Thiele moduli, //I and Zrl, will be proportional to and 2, characteristic dimensions of the particles. The mean observed reaction rates in Ib-mol/hr lb catalyst are r, and a in the two series of runs and the data are as follows ... 

Copper is one of the suitable metals for hydrogenation catalyst and shows characteristic performances in various hydrogenations [1,2]. However, the catalytic performance of the metal itself is not enough, so that various modifications, such as the combination of suitable elements as promoter, have been carried out in order to improve the performance [3-6]. For the promoted copper catalysts, lanthanide elements have been reported as the effective promoter for hydrogenation e.g. the activity of ethene hydrogenation over copper metal catalyst was remarkably improved by adding Yb and Eu, where the catalyst was prepared by Cu metal with Yb or Eu metal dissolved in liquid NH3 [3]. 

All effective catalysts for the asymmetric reduction of prochiral C=N groups are based on complexes of rhodium, iridium, ruthenium, and titanium. Whereas in early investigations (before 1984) emphasis was on Rh and Ru catalysts, most recent efforts were devoted to Ir and Ti catalysts. In contrast to the noble metal catalysts which are classical coordination complexes, Buchwald s a sa-titanocene catalyst for the enantioselective hydrogenation of ketimines represents a new type of hydrogenation catalyst [6]. In this chapter important results and characteristics of effective enantioselective catalysts and are summarized. 

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