Citral, hydrogenation

Unsaturated aliphatic aldehydes were selectively reduced to unsaturated alcohols by specially controlled catalytic hydrogenation. Citral treated with hydrogen over platinum dioxide in the presence of ferrous chloride or sulfate and zinc acetate at room temperature and 3.5 atm was reduced only at the carbonyl group and gave geraniol (3,7-dimethyl-2,6-octadienol) [59], and crotonaldehyde on hydrogenation over 5% osmium on charcoal gave crotyl alcohol [763]. 

Enklaar has showil that when citral is reduced by metals in a stream of hydrogen, it yields not only reduced aliphatic compounds, but that the ring is also closed, and a series of cyclic compounds is also formed. 

According to Skita, the reaction proceeds in a different manner if the reduction be effected with palladium chloride and hydrogen. In this case the citral in alcoholic solution is mixed with an aqueous solution of palladium chloride and the whole thickened with gum-arabic. Hydrogen gas is then forced into this solution under pressure. The products of the reduction include citronellal and citronellol and a di-molecular aldehyde, C Hj O, which probably has the following constitution —... 

Hydrogenation reactions, particularly for the manufacture of fine chemicals, prevail in the research of three-phase processes. Examples are hydrogenation of citral (selectivity > 80% [86-88]) and 2-butyne-l,4-diol (conversion > 80% and selectivity > 97% [89]). Eor Pt/ACE the yield to n-sorbitol in hydrogenation of D-glucose exceeded 99.5% [90]. Water denitrification via hydrogenation of nitrites and nitrates was extensively studied using fiber-based catalysts [91-95]. An attempt to use fiber-structured catalysts for wet air oxidation of organics (4-nitrophenol as a model compound) in water was successful. TOC removal up to 90% was achieved [96]. 

Nanostructured Pt(0) catalysts supported on cross-linked macromolecular matrices (Figure 5) have recently been evaluated in the hydrogenation of the a,P-unsaturated aldehyde, ( , Z)-3,7-dimethyl-2,6-octadienal (citral) (Scheme 10) [25]. 

Selective hydrogenation of citral Rh/SiO, Geraniol, nerol Fragrances... 

Tuning Selectivity through the Support in the Hydrogenation of Citral over Copper Catalysts... 

Here we wish to report that the support acidity, investigated through IR spectroscopy of adsorbed CO, allows one to tune the selectivity towards different products in the hydrogenation of citral over Cu catalysts. 

Results obtained in the hydrogenation of citral by using Cu catalysts on these two supports are reported in Figure 9.3 and Figure 9.4. 

Determination of the acidic sites through IR spectroscopy of adsorbed CO is a valuable tool for the choice of the support when selective or multifunctional processes are to be set up. This technique allowed to identify a particular kind of silica as the support of choice for the selective hydrogenation of citral to citronellal and sepiolite as a Lewis acid support able to promote the one-step transformation of citral into menthol. 

Figure 13.3. Hydrogenation of CAL over Ir catalysts supported over mixed oxides (a.) furfural, (b.) cinnamaldehyde, (c.) citral.

The reactor system works nicely and two model systems were studied in detail catalytic hydrogenation of citral to citronellal and citronellol on Ni (application in perfumery industty) and ring opening of decalin on supported Ir and Pt catalysts (application in oil refining to get better diesel oil). Both systems represent very complex parallel-consecutive reaction schemes. Various temperatures, catalyst particle sizes and flow rates were thoroughly screened. 

To evaluate the performance of the reactor system, the catalytic hydrogenation of citral to citronellal and citronellol in ethanol was nsed as a sample reaction. The reaction scheme is displayed below. 

The bed void fraction and the Reynolds number were determined with the experimental procedures reported in literature [7]. Inprehminaty experiments, citral hydrogenation was investigated in six parallel reactors under identical reaction conditions, i.e., at 25°C and 6.1 bar hydrogen with the residence time of 156 s. The... 

Citral hydrogenation was carried out in parallel reactor tubes at six different temperatures to screen the temperature effect. The initial hydrogenation rates increased with increasing temperature until 45°C, thereafter the rates were about the same due to extensive catalyst deactivation, which was more prominent at higher temperatures, especially above 60°C. The results were well reproducible. 

Figure 47.2. (a) Effect of residence time 156 s, fresh catalyst (solid symbol) 80 s, catalyst used once (open symbol) and (b) effect of catalyst particle size in citral hydrogenation at 25°C, 6.1 bar total pressure, residence time 156 s, solvent ethanol, 0.1 g catalyst Ni/Si02, initial citral concentration 0.02 M. 

In general, the results point to the edges and/or corners (small particles) favoring hydrogenation of the C=C bond whereas the planes (large particles) favor hydrogenation of the C=0 bond. This seems to be true for all compounds on Pt (see Table 2.6, lines 10-27, and 29-30) and for cinnamaldehyde on Ru and Rh (see Table 2.6, lines 33, 34, 37, and 38) however, citral on Ru did not exhibit this effect (see Table 2.6, lines 35 and 36), according to the authors statement. The reasons for this latter result are not clear. Why, for example should other alkyl-substituted a,P-unsaturated aldehydes exhibit this structure sensitivity and citral not Clearly, other factors are also at play. 

---