Hydrogenation of a,P unsaturated aldehydes

A detailed physico-chemical study of the catalyst was performed. It showed that tin was present as two species  

When the hydrogenation of citral is performed with supported nanoparticles of rhodium metal, for example Rh/Si02 under classical conditions [liquid phase, rhodium dispersion 80% (particles in the range of 1-2nm), citral/Rhs = 200, P(ti2) = 80bar, T = 340 K], the catalytic activity is very high but most of the above products are obtained and the reaction is totally non-selective, even if the major product was citronellal. 

A detailed physicochemical study of the catalyst showed that tin was present in the form of two species  

Adatoms (a very small amount), a situation corresponding to tin complexes having reacted with the most hydrogenolyzing sites. Those sites are thus selectively poisoned (see below). 

Several examples showing the effects of adatoms on activity and selectivity of a given catalytic reaction were observed. In most cases, this effect can be rationalized as a selective poisoning of undesirable sites. Usually, the presence of adatoms leads to a simultaneous decrease of the global activity and to a significant increase of selectivities in favor of the desired products. We describe here two examples. 

Claus recently reviewed the hydrogenation of a,P-unsaturated aldehydes, a process that displays very high selectivity by supported gold catalysts [215]. 

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. 

One more step was proposed after it was found that the addition of a second metal to the supported gold catalyst could refine the selectivity of the process. 

An example of the usefulness of the methodology was the new industrial route to cyclohexanone oxime developed by Corma and Serna, a process which is based on the selective hydrogenation of 1-nitro-l -cyclohexene by gold nanoparticles supported on Ti02 or Fe203 [220]. 

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The selective hydrogenation of a,P-unsaturated aldehydes to allylic alcohols (desired products) and/or saturated aldehydes is of commercial relevance, as mentioned in the... 

Figure 6. Reaction network of the hydrogenation of a,p-unsaturated aldehydes.

Hydrogenation of a,p-Unsaturated Aldehydes over Ir Catalysts Supported on Monolayers of Ti-... 

Ir catalysts supported on binary oxides of Ti/Si and Nb/Si were prepared and essayed for the hydrogenation of a,P-unsaturated aldehydes reactions. The results of characterization revealed that monolayers of Ti/Si and Nb/Si allow a high metal distribution with a small size crystallite of Ir. The activity test indicates that the catalytic activity of these solids is dependent on the dispersion obtained and acidity of the solids. For molecules with a ring plane such as furfural and ciimamaldehyde, the adsorption mode can iirfluence the obtained products. SMSI effect (evidenced for H2 chemisorption) favors the formation of unsaturated alcohol. 

Choppin GR (2003) Actinide speciation in the environment. Radiochim Acta 91 645-649 Claus P (1998) Selective hydrogenation of a,P-unsaturated aldehydes and other C = O and C = C bonds containing compounds. Top Catal 5 51-62 Colon D, Weber EJ, Anderson JL, Winget P, Suarez LA (2006) Reduction of nitrobenzenes and N-hydroxylanilines by Fe(II) species Elucidation of the reaction mechanism. Environ Sci Technol 40 4449-4454... 

Concerning the hydrogenation of a,P-unsaturated aldehydes, monometallic systems based on transihon metals readily achieve the reachon, leading to saturated aldehydes (SALs), saturated alcohols (SOLs) and, to a lesser extent, unsaturated alcohols (UOLs) (Scheme 6.2). To improve the selechvity towards UOL, which is generally the desired product, diverse alternahves have been studied, such as the use of different supports, increased metal parhcle size and promotion of the metallic phase by the addition of other metals [73, and references therein]. 

Ir/tppts catalysts exhibit almost the same selectivity as Ru/tppts in the hydrogenation of a,p-unsaturated aldehydes albeit with approximately 70 times lower rates.485 In sharp contrast to the ruthenium and iridium based tppts catalysts, RhJ tppts complexes catalyse the chemoselective hydrogenation of a,fl-unsaturated aldehydes to the corresponding saturated aldehydes (Figure 14, III).54-485... 

Hydrogenation of a, p- unsaturated aldehydes Oxidation Catalysis Pt/Al203 (unmodified)... 

Iridium,204,205 together with osmium, has been not widely used in catalytic hydrogenation. Recently, however, iridium or iridium-based catalysts have been shown to be effective in various hydrogenations, such as in selective hydrogenation of a,P-unsaturated aldehydes to allylic alcohols (Section 5.2), of aromatic nitro compounds to the corresponding hydroxylamines (Section 9.3.6), of halonitrobenzenes to haloanilines without loss of halogen (Section 9.3.2), and in the stereoselective hydrogenation of carbon to carbon double bonds (see, e.g., eqs. 3.25-3.27 and Table... 

Osmium and iridium catalysts have been shown to be highly selective for the formation of unsaturated alcohols by hydrogenation of a,P-unsaturated aldehydes without any additive. Good yields of allyl alcohol (73%), crotyl alcohol (90%), and cinnamyl alcohol (95%) (eq. 5.26) were obtained by the hydrogenation of acrolein, crotonaldehyde, and cinnamaldehyde, respectively, over 5% Os-C catalyst both with and without solvent.63... 

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