Base-Promoted Ruthenium Catalysts

The addition of certain ionic promoters to ruthenium catalytic solutions has been found to dramatically affect the rate and selectivity of CO hydrogenation. Whereas ruthenium solutions do not otherwise produce ethylene glycol as a significant product (except as its derivatives in in reactive solvents), 

Although the ruthenium species observed during CO reduction in the absence of promoters is Ru(CO)s, its concentration can be reduced to unobservable levels by promoters which cause the formation of ionic ruthenium complexes. Because this system differs from unpromoted ruthenium catalysts in as many respects—rates, selectivities, catalytic species observed, and mechanism—it is addressed separately in this section. 

Solvent Notes Pressure (atm) Temp. TO Ethylene glycol rate (hr 1) Methanol rate (hr 1) Ethanol rate (hr1) 

Iodide-promoted reactions in phosphine oxide solvents have been observed under some conditions to produce ethanol from H2/CO with good rates and high selectivities (193-195) (Table XVI, Expts. 1-3). Experimental evidence suggests that the ethanol is a secondary product, although its selectivity is high even after very short reaction times (193). An acid component is believed to be involved in alcohol homologation by this system, which will be described in more detail below. 

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Initial studies showed that Ru3(CO)i2 and [Ru(CO)2(02CCH3)]n were able to promote the addition of carboxylic acids to diphenylacetylene at 145 °C in toluene [29, 30]. Subsequently, a number of catalytic systems based on ruthenium catalysts have been discovered, and these have made possible - under mild conditions - the Markovnikov addition of carboxylic acids to terminal alkynes according to Scheme 8.14 to produce enol esters used as acylating reagents. 

Ruthenium has been investigated by many laboratories as a possible catalyst for ammonia synthesis. Recently, Becue et al. [T. Becue, R. J. Davis, and J. M. Garces, J. Catal., 179 (1998) 129] reported that the forward rate (far from equilibrium) of ammonia synthesis at 20 bar total pressure and 623 K over base-promoted ruthenium metal is first order in dinitrogen and inverse first order in dihydrogen. The rate is very weakly inhibited by ammonia. Propose a plausible sequence of steps for the catalytic reaction and derive a rate equation consistent with experimental observation. 

The early development of catalysts for ammonia synthesis was based on iron catalysts prepared by fusion of magnetite with small amounts of promoters. However, Ozaki et al. [52] showed several years ago that carbon-supported alkali metal-promoted ruthenium catalysts exhibited a 10-fold increase in catalytic activity over conventional iron catalysts under the same conditions. In this way, great effort has been devoted during recent years to the development of a commercially suitable ruthenium-based catalyst, for which carbon support seems to be most promising. The characteristics of the carbon surface, the type of carbon material, and the presence of promoters are the variables that have been studied most extensively. 

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

An alternative generation of a ruthenium catalyst has also emerged which is not based on the benzylidene structural motif. Easily accessible catalyst 8 is typical of a class of cationic catalyst from the groups of Furstner and Dixneuf [17]. This species can promote highly efficient RCM reactions and has the flexibility associated with both thermal and photochemical modes of activation [18]. 

Abstract Ruthenium holds a prominent position among the efficient transition metals involved in catalytic processes. Molecular ruthenium catalysts are able to perform unique transformations based on a variety of reaction mechanisms. They arise from easy to make complexes with versatile catalytic properties, and are ideal precursors for the performance of successive chemical transformations and catalytic reactions. This review provides examples of catalytic cascade reactions and sequential transformations initiated by ruthenium precursors present from the outset of the reaction and involving a common mechanism, such as in alkene metathesis, or in which the compound formed during the first step is used as a substrate for the second ruthenium-catalyzed reaction. Multimetallic sequential catalytic transformations promoted by ruthenium complexes first, and then by another metal precursor will also be illustrated. 

Since 1957 and the discovery of the Speir s catalyst H2PtCl6/ PrOH, considerable efforts have been made to find new catalysts with high activity and selectivity. Along with the platinum-based catalysts, the Wilkinson s complex [103] Rh(Ph3P)3Cl is one of the most popular hydrosilylation catalysts. Ruthenium catalysts are also able to promote the addition of silanes to unsaturated carbon-carbon bonds, and several reports have shown during the past decade that the well-defined ruthenium complexes of type Ru(H)(Cl)(CO)L can provide excellent activity and selectivity [104—... 

The high activity of a ruthenium-promoted iridium catalyst has improved productivity in plants that previously used rhodium catalysts [123], For example, a 75% increase in throughput was achieved at the Samsung-BP plant in Ulsan, South Korea. Another benefit of the iridium catalyst is higher selectivity, with smaller amounts of both gaseous and liquid by-products. The WGS reaction does occur, but at a lower rate than for rhodium, resulting in reduced formation of C02 and CH4. Since the process is less sensitive to CO partial pressure, the reactor can operate with a lower rate of bleed of recycle gas which, in combination with the secondary reactor, results in an increase in CO conversion from 85% (Rh) to >94% (Ir). Selectivity to acetic acid is >99% based on methanol with reduced propionic acid by-product formation relative to the process with the rhodium catalyst. This, along with the lower water... 

Rarog-Pilecka W, Szmigeel D, Kowalczyk Z, Jodis S, Zielinski J (2003), Ammonia decomposition over the carbon-based ruthenium catalyst promoted with barium or cesium , J. Catal., 218, 465 69. 

We improved the WGS activity of cobalt- and ruthenium-based eatalysts while suppressing the methanation aetivity by adding a promoter. As shown in Figures 5 and 6, these catalysts are more aetive than commereial iron-chrome at temperatures >300°C. Our results indicate that the cobalt and ruthenium catalysts would be suitable replacements for iron-chrome as an HTS catalyst. 

It has been claimed that carbon-supported ruthenium-based catalysts for ammonia synthesis show some important drawbacks, such as high catalyst cost and methanation of the carbon snpport under industrial reaction conditions. This has stimnlated the research for alternative catalysts, although the use of carbon snpports is a common feature. One example of these new catalysts is provided by the work of Hagen et al. [61], who reported very high levels of activity with barinm-promoted cobalt catalysts snpported on Vulcan XC-72. It was demonstrated that althongh cobalt had received little attention as a catalyst for ammonia synthesis, promotion with barium and the nse of a carbon support resulted in very active catalysts with very low NH3 inhibition. 

In this paper, we postulate that the primary role of an alkali promoter is to reduce the mobility of the chemisorbed hydrogen on the ruthenium surface based on the data obtained from NMR spectroscopy. We will examine the active hydrogen adsorption states (a and P) in supported ruthenium catalysts and report the effects of the alkali promoters on the population and the mobility of the adsorbed states. 

Other ruthenium catalysts have also been studied. In one study, ((IPrH2) (PCy3)(Cl)2Ru=CHPh) promoted ADMET faster than [Ru]2, although it too promoted olefin isomerization [28b]. The initiation rate of [Ru]3 was reported to be 1/30 that of [Ru]l, but the propagation rate was found to be four times faster [16]. The activity of [Ru]4 has even been shown to surpass that of Schrock s molybdenum-based catalysts [39]. 

The reaction can proceed through the formation of metal hydride species. Moreover, it was found that in case of ruthenium catalysts, the reaction rate can be significantly improved by addition of a base to the reaction mixture [20], The base promotes the formation of ruthenium alkoxide that further undergoes a -elimination to give, in sequence, a mono- and a dihydride complex (Scheme 18.8) [21], the latter being assumed as an active form of the catalyst [22]. 

Ru-based catalysts have been identified as highly active catalysts for ammonia synthesis [156]. The catalytic activity of an Ru-based catalyst can be improved by the addition of alkali or earth-alkali promoters [157, 158]. Many kinetic and theoretic studies have been carried out in order to understand the role of these promoters in ammonia synthesis. Guraya et al. [159] investigated the alkali- and earth-alkali-promoted ruthenium eatalysts supported on graphitized carbon by... 

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