Hydrogenation selectivity promoters

Bailie et al. were the first to mention alcohol formation from aldehydes by supported gold-catalyzed selective hydrogenation. The reaction of the formation of crotyl alcohol from crotonaldehyde showed high selectivity (up to 81%) at conversions of 5-10%, with preferential hydrogenation of C=0 rather than the C=C bond [216]. The addition of thiophene promoted this selective hydrogenation. This promotional effect was also observed in similar situations for Cu and Ag, but it was not very common for gold. 

The metallic function and the acid-base properties of the catalysts used, play a central role in the reforming reaction of ethanol. This is illustrated by the Cu/Ni/K/y-AUCU catalyst, which exhibits acceptable activity, stability and hydrogen selectivity at relatively low temperature (573 K) and at atmospheric pressure.171 In this catalyst, copper is the active agent, nickel promotes C-C bond rupture and increases hydrogen selectivity, and potassium neutralises the acidic sites of the 7-alumina substrate and improves the general performance of the catalyst. 

With Raney Nickel catalyst we performed the hydrogenations first without and then with selectivity promoters. 

A new selectivity promoter, thiourea is efficient, for selective hydrogenation of halogenonitrobenzenes on Raney Nickel. This promoter stays on the catalyst which can be reused without loss of activity or selectivity. The activity of the thiourea doped Raney Nickel catalyst is good enough (0.1 - 0.5 x 103 mole h"1 kg"1 of catalyst, depending on the halogenonitrobenzene) for an industrial application. 

Complete hydrogenation of naphthalene in conventional processes produces mixtures of cis- and trans-decalin. The motivation for selective naphthalene hydrogenation comes from our accidental finding on zeolite-catalyzed isomerization of cis-decalin and from the need to tailor the formation of desired isomers from two-ring compounds. In our previous studies on naphthalene hydrogenation, certain catalysts show higher selectivity towards cis-decalin or trans-decalin. More recently, we found that zeolite-supported catalysts selectively promote the formation of cis-decalin or trans-decalin [Schmitz et al., 1996], as shown in Scheme 10. 

Literature on the use of promoters is voluminous, and all claim enhancement of semi-hydrogenation selectivity. One of the more successful commercial catalysts for ethyne conversion to ethene, uses a silver promoted alumina-supported palladium catalyst [23]. Other promoting metals have been used, including rhodium and gold [24,25], copper [26-28], zinc (shown to inhibit oligomerization) [29-... 

The geometric and promoter effects on the selectivity to furfuryl alcohol were studied in the hydrogenation of furfural over various series of platinum catalysts. The results obtained with the PtCu catalysts indicate that the size of the active site does not affect the selectivity. Promotion of the platinum catalyst can lead to a considerable rise in the selectivity to furfuryl alcohol. The hydrogenation over the series of PtSn catalysts showed the influence of the reaction conditions. The experiments under isothermic conditions resulted in up to 80% selectivity for furfuryl alcohol, while the selectivity dropped to approximately 40% if non-isothermic conditions were applied. This change in selectivity is attributed to the self-poisoning of the catalyst at high temperatures. 

A catalyst has recently been developed (Cordier et al., 1994) based on the use of thiourea as a new selectivity promoter, which gives almost 100% hydrogenation selectivity. A few other promoters such as thiophene and sodium sulfide also give good selectivities, but thiourea is by far the best. The azo and azoxy compounds that are usually the main side products are hardly formed at all (< 10 ppm). 

Pt group metals can activate alcohols and molecular oxygen under close to ambient conditions, producing the corresponding carbonyl or carboxylic acids in high yields. Enhanced selectivity and activity has been obtained by the use of bi- and multimetallic catalysts. " However, as was the case with selective hydrogenation, the optimum catalysts developed to date have involved a combination of supported metals and selectivity promoters. The most commonly used catalysts consist of either Pt or Pd as the active metal combined with Bi or Pb as promoters, commonly on carbon or alumina supports. Other promoters reported include Cd, Co, Cu, Se, ° Ce, Te, Sn, Au and Ru. The catalysts can be prepared by simultaneous deposition and reduction of the metal precursors onto a suitable support. However, more commonly, preparation involves a... 

Gold has been used as a promoter in CuO/ZnO catalysts. The Au/CuO/ZnO catalysts are more active and exhibit higher hydrogen selectively with a smaller amount of CO compared to the CuO/ZnO counterparts. The enhanced activity of Au containing catalysts is due to the strong interaction between Au and CuO... 

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