Olefins hydrogenation, sulfur poisoning

Deactivation by sulfur has been explained by the withdrawing of electrons from the catalyst surface. It has also been shown that sulfur inhibits the dissociation of CO on iron surfaces l]. The deliberate partial poisoning of iron/manganese cataly.sts with sulfur has been used to shift the product selectivity towards short-chain hydrocarbons. At higher sulfur concentrations (0.7 mg S/g catalyst) the activity is significantly decreased and the olefin selectivity increased [82]. Sulfur poisoning of nickel catalysis has recently been shown to inhibit the chemisorption of hydrogen 83.84). 

Palladium and promoted palladium catalysts are used for the hydrogenation of small amounts of acetylenic compounds from purified olefin streams. These catalysts are sensitive to sulfur poisoning and, therefore, the feed gas must be free of sulfur prior to hydrogenation. The noble metal catalysts are more active than the base metal types and can therefore operate at lower temperatures—as low as 80°F—and are capable of producing effluent purities below 1 ppmv acetylene under optimum conditions. An improved catalyst, consisting of promoted palladium on alumina, was introduced by United Catalysts, Inc., in 1988. The catalyst... 

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. 

Unlike boron fluoride, titanium tetrachloride does not catalyze the liquid phase polymerization of isobutylene under anhydrous conditions (Plesch et al., 83). The addition of titanium tetrachloride to a solution of the olefin in hexane at —80° failed to cause any reaction. Instantaneous polymerization occurred when moist air was added. Oxygen, nitrogen, carbon dioxide, and hydrogen chloride had no promoting effect. Ammonia and sulfur dioxide combined with the catalyst if these were added in small quantity only, subsequent addition of moist air permitted the polymerization to occur. Ethyl alcohol and ethyl ether, on the other hand, prevented the polymerization even on subsequent addition of moist air. They may be regarded as true poisons. 

In the normal oxo reaction a certain amount of hydrogenation occurs, a minor amount of olefins being converted to paraffins in the case of certain olefinic compounds hydrogenation indeed occurs to the exclusion of hydroformylation. It is a remarkable fact that this catalytic reaction occurs in the presence of carbon monoxide and also of sulfur compounds, although cobalt metal is notoriously poisoned by traces of these compounds. The significance of this was pointed out by Adkins and Krsek (23) and Wender, Orchin, and Storch (25) in terms of the concept that the hydroformylation catalyst is a homogeneous one, not sensitive to carbon monoxide or sulfur compounds and in this respect different from usual solid cobalt catalysts. 

As an example of low-temperature catalytic reactions, hydrogenation of unsaturated hydrocarbons is the most important industrial application. Chemical industrial needs are mainly for unsaturated hydrocarbons, which have reactivities that enable polymer or petrochemical product development. All the processes developed for the production of olefins, diolefins, and aromatics give a mixture of unsaturated hydrocarbons, which are not valuable as such further hydrogenations are necessary to obtain usable products for refining and chemical industry. Sulfur is generally considered to be a poison of hydrogenation catalysts. But in the case of hydrodehydrogenation reactions, this compound can also be used as a modifier of selectivity or even, in some cases, as an activator. 

Saturation of a carbohydrate double bond is almost always carried out by catalytic hydrogenation over a noble metal. The reaction takes place at the surface of the metal catalyst that absorbs both hydrogen and the organic molecule. The metal is often deposited onto a support, typically charcoal. Palladium is by far the most commonly used metal for catalytic hydrogenation of olefins. In special cases, more active (and more expensive) platinum and rhodium catalysts can also be used [154]. All these noble metal catalysts are deactivated by sulfur, except when sulfur is in the highest oxidation state (sulfuric and sulfonic acids/esters). The lower oxidation state sulfur compounds are almost always catalytic poisons for the metal catalyst and even minute traces may inhibit the hydrogenation very strongly [154]. Sometimes Raney nickel can... 

Poisoning can affect the selectivity as well as the rate of conversion, and mild poisoning may be beneficial. The oxidation of ethylene is carried out using silver catalysts that are deliberately poisoned with chlorine compounds, and the selectivity is improved, because the total oxidation reaction is suppressed more than the rate of ethylene oxide formation [14]. The presence of sulfur compounds changes the selectivity for competitive hydrogenation, such as the hydrogenation of acetylenes or diolefins in the olefins [15]. 

Other partially poisoned catalysts have long been used in the laboratory. Supported palladium catalysts, poisoned with lead (Lindlar catalysts), sulfur, or quinoline, are used for the hydrogenation of acetylenic compounds to cis-olefins. Another... 

---