Nickel acetylene hydrogenation

An a priori choice of an optimum acetylene hydrogenation catalyst is not always easy. For instance, hydrogenation of octadeca-3,6-diynol over P-2 nickel gave the corresponding (Z,Z) dienol satisfactorily (37), but when the... 

Bilimoria, M. R., and Bailey, J. E. Dynamic studies of acetylene hydrogenation on nickel catalysts. ACS Symp. Ser. 

Acetylene hydrogenation. Selective hydrogenation of acetylene to ethylene is performed at 200°C over sulfided nickel catalysts or carbon-monoxide-poisoned palladium on alumina catalyst. Without the correct amount of poisoning, ethane would be the product. Continuous feed of sulfur or carbon monoxide must occur or too much hydrogen is chemisorbed on the catalyst surface. Complex control systems analyze the amount of acetylene in an ethylene cracker effluent and automatically adjust the poisoning level to prepare the catalyst surface for removing various quantities of acetylene with maximum selectivity. 

The nature of the unsaturated hydrocarbon has a very important role in the sulfur action Berenblyum et al. (83) have reactivated a palladium catalyst, poisoned with thiols, through the interaction with phenylacethyl-ene the presence of acetylenics together with low levels of sulfur even activate the nickel sites activity for acetylene hydrogenation (84, 85). 

The ciystalJine structure of the catalyst and the oxidation state of surface nickel are specially relevant in this case due to the fact that coke deposition as well as acetylene hydrogenation occur on the metallic nickel sites [2]. Therefore, the pretreatments carried out on the catalyst with the aim of obtaining the active species have a great influence on the relationship between coke deposition and the main reaction kinetics. 

Paraffins, of course, cannot have an associative mechanism and do not adsorb on hydrogen-covered nickel. When adsorbed on bare nickel they behave in a manner similar to ethylene. Acetylene, on the other hand, exhibits a unique behavior. Regardless of the pretreatment of the nickel, acetylene produces the spectrum assigned to chemisorbed ethyl radicals. This indicates extensive self-hydrogenation and the formation of a surface carbide. The idea of a surface carbide is supported by a very large increase in spectral intensity when hydrogen is added to chemisorbed acetylene. 

Dynamic Studies of Acetylene Hydrogenation on Nickel Catalysts... 

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. 

Hydrogenation of Acetaldehyde. Acetaldehyde made from acetylene can be hydrogenated to ethanol with the aid of a supported nickel catalyst at 150°C (156). A large excess of hydrogen containing 0.3% of oxygen is recommended to reduce the formation of ethyl ether. Anhydrous ethanol has also been made by hydrogenating acetaldehyde over a copper-on-pumice catalyst (157). 

Hydrazoic acid Hydrides, volatile Hydrogen cyanide (unstabilized) Hydrogen (low pressure) Hydrogen peroxide (> 35% water) Magnesium peroxide Mercurous azide Methyl acetylene Methyl lactate Nickel hypophosphite Nitriles > ethyl Nitrogen bromide... 

Highly stereospecific hydrogenations of acetylenes to cis olefins have been achieved also with nickel (P 2) catalysts in the presence of ethylenediamine as prorrtoter (37 8 55 58,72). The catalyst is prepared by reduction of nickel acetate in ethanol with sodium borohydridefi ). Despite successes (44), the use of nickel is relatively infrequent (51). 

The stereochemistry of electrochemical reduction of acetylenes is highly dependent upon the experimental conditions under which the electrolysis is carried out. Campbell and Young found many years ago that reduction of acetylenes in alcoholic sulfuric acid at a spongy nickel cathode produces cis-olefins in good yields 126>. It is very likely that this reduction involves a mechanism akin to catalytic hydrogenation, since the reduction does not take place at all at cathode substances, such as mercury, which are known to be poor hydrogenation catalysts. The reduction also probably involves the adsorbed acetylene as an intermediate, since olefins are not reduced at all under these conditions and since hydrogen evolution does not occur at the cathode until reduction of the acetylene is complete. Acetylenes may also be reduced to cis olefins in acidic media at a silver-palladium alloy cathode, 27>. 

The cluster catalyzes hydrogenation (20°C and 3 atm) of dialkyl- and diarylacetylenes to the c/s-olefins via unsaturate routes, likely involving Ni4(CNR)6(RC=CR) and Ni4(CNR)4(RC==CR)3 (391, 392). The acetylenes in the latter complex bridge three nickel centers, and increase of the acetylenic carbon-carbon bond distance is considered to enhance reduction by hydrogen (392, 393). 

Practical systems based on Eq. (66), but using Ni(COD)2/RNC mixtures, were reported. Catalysis via Ni(CNR)3 or Ni(CNR)2 intermediates could not be completely ruled out in these cluster systems. The nickel isonitrile and acetylene clusters did not effect hydrogenation of the triple bond in nitrogen (394). 

Recently, Kumada et al. (49) have published a report on what they refer to as dehydrogenative, stereoselective cis double silylation of internal acetylenes. This appears to be a variation of Eq. (53), with diethyl bipyridyl nickel(11) as the catalyst, in which hydrogen is liberated instead of being added to an alkene to form a saturated product. 

Another highly active non-pyrophoric nickel catalyst is prepared by reduction of nickel acetate in tetrahydrofuran by sodium hydride at 45° in the presence of tert-amyl alcohol (which acts as an activator). Such catalysts, referred to as Nic catalysts, compare with P nickel boride and are suitable for hydrogenations at room temperature and atmospheric pressure, and for partial reduction of acetylenes to civ-alkenes [49]. 

In acetylenes containing double bonds the triple bond was selectively reduced by controlled treatment with hydrogen over special catalysts such as palladium deactivated with quinoline [565] or lead acetate [56], or with triethylam-monium formate in the presence of palladium [72]. 1-Ethynylcyclohexene was hydrogenated to 1-vinylcyclohexene over a special nickel catalyst (Nic) in 84% isolated yield [49]. 

Triple bonds in side chains of aromatics can be reduced to double bonds or completely saturated. The outcome of such reductions depends on the structure of the acetylene and on the method of reduction. If the triple bond is not conjugated with the benzene ring it can be handled in the same way as in aliphatic acetylenes. In addition, electrochemical reduction in a solution of lithium chloride in methylamine has been used for partial reduction to alkenes trans isomers, where applicable) in 40-51% yields (with 2,5-dihydroaromatic alkenes as by-products) [379]. Aromatic acetylenes with triple bonds conjugated with benzene rings can be hydrogenated over Raney nickel to cis olefins [356], or to alkyl aromatics over rhenium sulfide catalyst [54]. Electroreduction in methylamine containing lithium chloride gives 80% yields of alkyl aromatics [379]. 

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